Environmentally friendly and earth-abundant colloidal chalcogenide nanocrystals for photovoltaic applications

Huiying Fu
Department of Materials Science, Fudan University, Shanghai 200433, China. E-mail: hyfu@fudan.edu.cn

First published on 21st December 2017

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Huiying Fu

Huiying Fu obtained her PhD in Materials Science from Fudan University. She was with National Research Council Canada and the Hong Kong University of Science and Technology before her current appointment with Fudan University. Her research activities focus on organic conjugated materials, colloidal semiconducting nanocrystals and perovskite for photovoltaic applications.

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Introduction

The global energy situation is turning progressively more serious and prompting that the worldwide dependence on exhaustible carbon-based resources must change towards further supporting the implementation of secure and sustainable energy for the purpose of providing copious energy universally in a practically economic way. The development of photovoltaic (PV) devices, which harvest incident photons from the sun and transform the infinite solar energy into electricity, represents one of the most fascinating long-term solutions for clean and renewable energy. Current PV technology relies upon single crystal silicon, the processing of which, however, suffers from high cost and high energy consumption during the manufacturing of PV devices. Thin-film PV technology based on cadmium telluride (CdTe) and copper indium gallium selenide (CuInGaSe₂, CIGS) offers a noteworthy cost improvement, but the energy conversion efficiency of these materials needs to be enhanced to make them practically viable. Furthermore, CdTe and CIGS thin-film PV technology is vulnerable to the environmental incompatibility of cadmium and the possible tellurium and indium scarcity (mainly as 70% of the global production is used for the fabrication of transparent conductive films of indium tin oxide, ITO). The emergence of highly efficient solution-processable PV devices based on organic conjugated materials, colloidal semiconducting nanocrystals (NCs), also known as quantum dots (QDs), and organometal halide perovskite makes it conceivable to produce PV cells via cost-effective manufacturing approaches. To date, organic PV (OPV) devices with an air mass of 1.5 global (AM 1.5G) and power conversion efficiency (PCE) approaching 12%, PV devices based on colloidal lead sulfide (PbS) NCs with a PCE over 11% and perovskite solar cells fabricated with organometal halide as the light absorber with a certified PCE of up to 22% have been reported.^1–3

Colloidal semiconducting NCs are considered to be exceptional candidates for usage in solar cells due to their broad absorption spectra covering the range from the visible to the infrared, narrow emission spectra, high charge-carrier mobility and multiple exciton generation (MEG) capability.^4–9 Among these, chalcogenide semiconductors have been proposed as excellent absorber materials for PV applications on account of their unique high absorbance and broad-band absorption. Over the past two decades, tremendous progress has been made with respect to synthesizing new families of colloidal semiconducting chalcogenide NCs with different sizes, shapes and dimensions, producing monodisperse colloidal semiconducting NCs, and employing colloidal semiconductor NCs as functional materials in technological applications. The size- and shape-controlled syntheses of II–VI and IV–VI NCs, cadmium sulfide (CdS), cadmium selenide (CdSe), CdTe, PbS, lead selenide (PbSe) and ternary chalcogenide alloys have been intensively investigated to achieve the tuneable optical and electronic properties of the materials.^10–27 This has resulted in a number of reports on colloidal semiconducting NCs-based solar cells with high energy conversion efficiencies and good air-stability.^2,28–34

Nevertheless, the intrinsic toxicity of cadmium and lead elements and the potential exposure of the materials to the ecosystem may pose momentous threats to the environment. Alternatively, earth-abundant, non-toxic and environmentally friendly Cd- and Pb-free materials that can be employed in PV devices have drawn considerable attention and are considered as two of the most credible targets for PV applications. Several distinctive binary light-absorptive materials that fit these specifications have been proposed, including iron disulfide (FeS₂), copper(I) sulfide (Cu₂S), tin sulfide (SnS), tin selenide (SnSe) and bismuth sulfide (Bi₂S₃).^35–40 Among these semiconductors, FeS₂, Cu₂S, SnS and SnSe are typically p-type semiconductors, while Bi₂S₃ is an n-type. Apart from binary semiconducting chalcogenide materials, multinary chalcogenide alloys, such as copper zinc tin sulfide (Cu₂ZnSnS₄, CZTS), offer remarkable substitutes for environmentally friendly and earth-abundant PV materials. CZTS is similar to CIGS in terms of the crystal structure and optical properties (near optimal bandgap and high absorption coefficient) but with an earth-abundant chemical composition.^41–45 While the above-mentioned materials for solar cells have already been studied to a great extent, the number of reports on the syntheses of environmentally friendly and earth-abundant colloidal semiconducting chalcogenide NCs in the literature remains comparatively small. Additionally, compared with the great advances that have been achieved in colloidal synthetic control over the size and shape of cadmium and lead chalcogenide NCs, the syntheses of high-quality environmentally friendly and earth-abundant colloidal semiconducting NCs are far less developed.

A number of key themes and topics have emerged in the field of environmentally friendly and earth-abundant colloidal chalcogenide NCs, such as the effort required to synthesize monodisperse and stable colloidal semiconducting NCs and how to exploit them in the production of inexpensive and highly efficient PV devices through scalable solution-processable procedures. In this feature article, we review the recent developments in the area of the syntheses and PV applications of environmentally friendly and earth-abundant colloidal chalcogenide NCs, including Fe₂S, Cu₂S, SnS, SnSe, Bi₂S₃ and CZTS. We highlight the major progress and achievements in the preparation of high-quality environmentally friendly and earth-abundant colloidal semiconducting NCs, focusing on the syntheses of colloidal NCs with a well-controlled size and morphology. An outlook about the future research challenges in this area is also provided.

Iron disulfide

Iron disulfide (FeS₂), also called iron pyrite or fool's gold, is a distinctive absorbent material because of its high absorption coefficient, which is of the order of 10⁵ cm⁻¹ in the visible region of the solar spectrum, and its chemical composition of non-toxic and earth-abundant elements.⁴⁶ The high absorption coefficient of FeS₂ offers an exceptional example among inorganic semiconductors to integrate a thin absorbing layer (less than 100 nm) in a PV device to capture most of the incident photons from solar radiation. FeS₂ can be used as a PV absorber material with an indirect energy transition at 0.95 eV, which is very close to that of silicon, followed by a direct transition at 1.03 eV, indicating the realization of the ideal bandgap of 1.34 eV of FeS₂ where the Schockley–Queisser efficiency limit reaches its highest level, is attainable due to the quantum confinement of NCs.³⁵ The estimated highest achievable energy conversion efficiency of FeS₂ PV film has been reported to be as high as that for single crystal silicon.⁴⁷ Despite the attractive promise that FeS₂ holds for PV applications, only limited progress in producing highly efficient FeS₂-based solar cells has been attained. This underdevelopment may be partially due to the complex crystal structures and physical properties of iron sulfide.⁴⁸

The solution-based approaches investigated for the production of iron sulfide crystals or thin films include the hydrothermal process, chemical bath deposition (CBD), sol–gel chemistry, electrodeposition (ED), solvothermal method, spray pyrolysis and colloidal synthesis.^49–57 However, only a few approaches have offered iron sulfide NCs with a pure pyrite phase.^55–57 In most cases, impurities, such as FeS (troilite) and Fe_1−xS (pyrrhotite), exist in the prepared products, which could greatly hinder the performance of the materials. Post-treatment, like the annealing of FeS or Fe_1−xS in a sulfur vapour atmosphere, have been employed to produce iron pyrite NCs or thin films with good quality.^58–60 The development of a facile and scalable system to synthesize pure FeS₂ NCs with well-defined sizes and shapes has become one of the most challenging issues for researchers working in the field of iron sulfide NCs. It was discussed in Habas’ review that the use of nonaqueous solvents may help to facilitate the successful solution deposition of FeS₂, which arose due to the difficulty of making FeS₂ NCs from aqueous solution medium.⁶¹

The emergence of the colloidal synthetic approach has made it possible to prepare phase-pure iron pyrite NCs with a precisely controlled composition and size, as demonstrated with the growth of cadmium and lead chalcogenide NCs. The thermal reactions of an iron complex with a sulfur precursor via hot-injection or non-hot-injection procedures and the thermal decomposition of a single-source precursor have both been developed for the preparation of phase-pure colloidal iron sulfide NCs. Nonetheless, iron sulfides with the structures of pyrrhotite, greigite (Fe₃S₄) or a mixture of pyrrhotite and greigite were synthesized via a single-source precursor-decomposition approach.^62–66 Phase-pure pyrite FeS₂ NCs with sizes below 100 nm were produced via hot-injection approaches by injecting a solution of sulfur into an iron precursor (or the other way around, by injecting iron complex into the sulfur solution).^{56,57,67–87} The crystal structure of pyrite FeS₂ is illustrated in Fig. 1. The endeavours made in the colloidal syntheses of iron pyrite NCs through the hot-injection procedure are summarized in Table 1. A non-hot-injection approach involving heating up iron and sulfur precursors in a hexadecylamine and oleylamine solution also provided iron pyrite nanocubes with an average edge length of ∼37 nm.⁸⁸

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For the applications of colloidal iron pyrite NCs in highly efficient PV devices, the long-term maintained stability and low surface trap density of the synthesized NCs are critical parameters for producing a thin absorbing layer of FeS₂ with minimum recombination loss and maximum charge mobility. The pioneering work in the colloidal synthesis of FeS₂ NCs was accomplished by injecting the sulfur–diphenyl ether solution into the iron–octadecylamine complex at a temperature of 220 °C.⁵⁶ The colloidal NCs attained were phase-pure pyrite and single-crystalline, but the as-obtained FeS₂ NCs gradually aggregated. The basic structural and absorption spectroscopic characterization data are presented in Fig. 2. PV devices with the architectures of a Schottky junction, heterojunction or hybrid organic/NCs fabricated with FeS₂ NCs were synthesized following the procedures reported by Puthussery et al. as the absorber layer, but showed no PV response.⁷² The aggregation of as-synthesized NCs also occurred when oleylamine was employed as the surfactant for iron to synthesize colloidal FeS₂ NCs.^68,74,79 It was demonstrated by Ge et al. that the aggregation of fine initial FeS₂ nanoparticles could lead to the formation of FeS₂ nanoplates with a side length of 20 nm.⁷⁹

Kirkeminde et al. reported a growth method involving a second injection of iron precursor into pre-formed FeS₂ QDs, based on a procedure where octadecylamine functions as the surfactant for iron.^70,75 By tuning the ageing time of the reaction after the second injection, colloidal FeS₂ nanocubes with sizes ranging from ∼80 to ∼120 nm were obtained.⁷⁰ FeS₂ nanocubes with an average size of 80 nm (bandgap of 1.1–1.2 eV and Fe/S molar ratio of 1:1.83) were employed as the absorbing material in a colloidal FeS₂:CdS QDs (volume ratio of 1:1) bulk heterojunction PV device with an active layer thickness of 500 nm. The FeS₂:CdS layer was layer-by-layer ligand-exchanged with 1,2-ethanethiol and hydrazine sequentially. A moderate PCE of 1.1% and a large open-circuit voltage (V_OC) of 0.79 V under simulated AM 1.5G solar irradiation (100 mW cm⁻²) were achieved, which indicated the formation of a well-defined percolation network of FeS₂:CdS film. The current density–voltage curves of the devices with different volume ratios of FeS₂ and CdS are shown in Fig. 3. The authors also demonstrated the fabrication of devices with the structures of a Schottky and FeS₂/CdS bilayer; although this resulted in an inferior performance compared to the structure of the bulk heterojunction. Truong et al. modified the procedure with octadecylamine functioning as the ligand for iron by increasing the Fe to S molar ratio to 1:3, instead of the 1:6 ratio used by Puthussery et al.⁸⁷ Quasi-rod-shaped NCs were obtained with diameters of about 8 nm and lengths of about 21 nm. By introducing the as-synthesized FeS₂ NCs blended with the polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) into a bulk-heterojunction solar cell, a PCE of 0.45% was demonstrated. Compared to bulk-heterojunction devices based on PbS NCs and polymers, the efficiency was low, possibly owing to the poor charge collection, which was due to the non-optimized shape and size of FeS₂ NCs and the poor interpenetrating network between the polymer and FeS₂. Oleylamine-capped FeS₂ NCs with a cubic shape were also developed as a successful light absorber for PV devices.⁸⁰ The devices were fabricated with a structure of ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate)/FeS₂ NCs:CdSe NCs/Au, and yielded a PCE of 0.50%. The authors also studied the performance of a device based on an FeS₂ NCs:CdSe NCs bulk heterojunction without the hole-transporting material PEDOT:PSS, but this showed no PV response.

Compared with amines, which have been extensively studied for the syntheses of colloidal FeS₂ NCs, phosphine oxides are slightly stronger bases and have been proven to successfully coordinate the surface of NCs, such as cadmium chalcogenides (CdS, CdSe, and CdTe), indium phosphide (InP) and zinc selenide (ZnSe).^89–93 By introducing trioctylphosphine oxide as the co-surfactant for the reaction of iron and sulfur in oleylamine, Bi et al. demonstrated the preparation of phase-pure FeS₂ NCs with notable air-stability.⁵⁷ A high carrier mobility of 80 cm² V⁻² s⁻¹ with p-type behaviour and a strong photoconductivity were observed for the thin films of FeS₂ NCs capped with trioctylphosphine oxide at room temperature. The presence of photoconductivity in the FeS₂ NCs films indicated not only the good quality of the as-prepared FeS₂ NCs but also highlighted the promising PV applications of the as-synthesized phase-pure colloidal pyrite FeS₂ NCs.

It was reported that the employment of 1,2-hexadecanediol and oleylamine as the surfactants for the synthesis of colloidal iron pyrite NCs also provided FeS₂ NCs with superb surface passivation.⁶⁷ A PCE of 0.16% (AM 1.5G) for a hybrid solar cell fabricated with poly(3-hexylthiophene) (P3HT)/as-obtained FeS₂ NCs (diameters about 10 nm) as the photoactive layer was demonstrated. The active P3HT/FeS₂ NCs layer was deposited by spin-coating and was then annealed at 110 °C for 30 min. The poor performance was partially due to the poor quality of the NCs as an unsatisfying size uniformity and inadequate surface passivation of the surfactants of the as-synthesized NCs were observed. In an attempt to improve the conductivity of the photodiodes with the heterojunction of ZnO/FeS₂ NCs, cross-linking of the FeS₂ NCs with 1,2-ethanedithiol was deployed to remove the long-chain ligands from the surface of the NCs.⁶⁹ The new photodiode showed an advanced photocurrent/dark current ratio of up to 8000 at −1 V under AM 1.5G illumination and an encouraging spectral response ranging from visible to the NIR (near infrared). This sterling optoelectronic property of the FeS₂ NCs indicated their substantive potential for the advancement of highly efficient solution-processable PV devices.

Copper(I) sulfide

Similar to iron sulfides, copper sulfides are also stoichiometrically and structurally complex. There are several stable phases of Cu_xS at room temperature, with x ranging from 2 to 1, such as Cu_2–1.997S (chalcocite), Cu_{1.934–1.965}S (djurleite), Cu_1.8S (digenite), Cu_1.75S (anilite or roxbyite) and covellite (CuS). The energy bandgap of copper sulfide varies from 1.2 eV for Cu_2–1.997S and 1.5 eV for Cu_1.8S to 2 eV for CuS. Among all these, Cu_2–1.997S (referred to as Cu₂S in the following text) is an ideal candidate for PV applications because its indirect bandgap of 1.2 eV is close to the energy bandgap for an optimum solar absorbing material (1.1 eV).⁹⁴

Due to the copper vacancies within the crystal lattice, Cu₂S is a noteworthy p-type semiconductor. Its combination with n-type CdS in the fabrication of thin-film heterojunction solar cells has been widely investigated in the past as long ago as the ‘70s and PCEs of up to 9% have been reported.^36,95 In the field of thin-film solar cells, the Cu₂S/CdS heterojunction has shown many good benefits, but the diffusion of copper into the CdS layer and the resulting doping of copper in CdS led to long-term performance degradation and ultimately the abandonment of further developing this system.⁹⁶ However, with the progressive advances in nanotechnology, a new chapter of Cu₂S-based solar cells has been launched as the integration of nanoscale substances for device applications is seemingly leading to a continuous unveiling of new worthwhile characteristics of the materials.

The employment of a mild and controllable preparation process for colloidal NCs, such as the thermolysis of single-source precursors and hot-injection methods, has shown great promise in producing semiconducting Cu₂S NCs with a well-controlled composition and morphology.^97–100 The synthetic procedures for colloidal Cu₂S NCs are summarized in Table 2. The first example of thermolysis, which was specified as solventless synthesis, of copper-thiolate-derived precursor to produce Cu₂S NCs was published by Larsen et al.⁹⁷ Cu₂S nanorods with a narrow size and shape distributions were generated in a decomposition process of a molecular copper alkylthiolate complex with dodecanethiols as the capping ligands. Modification of this solventless procedure led to the synthesis of monodisperse Cu₂S nanodisks, with thicknesses of 3 to 12 nm and diameters of 3 to 150 nm, and nanoplatelets.⁹⁸ Later on, Bryks et al. carried out the solventless thermolysis of copper alkanethiolate precursors with alkyl backbone chain lengths of n = 4, 8, 12 or 16 carbons as the ligands.¹⁰¹ It was revealed that the precursors with short chains produced nanosheets of Cu₂S, while the long-chain precursors produced nanodisks. Fig. 4 shows the TEM images and XRD patterns of Cu₂S nanodisks synthesized from different copper alkanethiolates. Chen and co-workers studied the optical properties of Cu₂S nanodisks synthesized from solventless thermolysis.¹⁰² The Cu₂S nanodisk samples showed increases in the bandgap and blueshifts of the absorption edge compared to those of bulk Cu₂S, indicating the quantum confinement effect of NCs.

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On the other hand, the thermal decomposition of single-source precursors in solvents like oleylamine or dodecanethiol provided Cu₂S nanoparticles with sizes of less than 10 nm as well.^99,103,104 Here, as-synthesized Cu₂S nanoparticles with a uniform size of 6.5 nm presented a broad absorption band in the UV-Vis-NIR range, indicating potential for the employment of Cu₂S NCs for PV applications.¹⁰⁴ Monodisperse Cu₂S nanodisks and nanoplates with a well-controlled morphology could also be synthesized through this solution-phase thermolysis.^105,106 By tuning the experimental conditions, nanodisks with thicknesses of 4 to 6 nm and diameters of 8 to 12 nm and uniform shape were reported.¹⁰⁶ Al-Shakban et al. reported the synthesis of monodisperse Cu₂S nanorods through the hot-injection of a series of novel copper(I) xanthate single-source precursors into oleylamine.¹⁰⁷

Instead of using single-source precursors, heating up a mixture of copper complexes, such as copper acetylacetonate or copper oleate, with sulfur precursors in high-boiling-point organic solvents, like dodecanethiol, 1-octadecene, octyl ether, oleylamine, dioctylamine, trioctylamine or oleic acid, has been extensively exploited to produce Cu₂S nanoparticles with well-controlled sizes.^108–115 Freymeyer et al. investigated the influence of the applied solvents on the obtained phase, size and morphology of Cu_2−xS nanostructures¹¹⁵ and found that the ratio of dodecanethiol to oleic acid in the reaction system affected the solid-state structure and stoichiometry of Cu_2−xS. The influence of the oleic acid/dodecanethiol solvent ratio on the copper sulfide crystal phases is shown in Fig. 5. Study of the UV-Vis-NIR absorption spectra of the as-synthesized Cu_2−xS NCs demonstrated the existence of a broad absorption band in the NIR region, which was ascribed to the plasmonic behaviour of the NCs, arising from the transformation of stoichiometric Cu₂S into its nonstoichiometric counterpart Cu_2−xS (x > 0).^116,117 This plasmon-enhanced size- and shape-dependant absorption of Cu_2−xS nanomaterials needs to be taken into consideration in respect to their potential applications in solar cells.

In the case of the hot-injection approach, Li et al., Kruszynska et al. and Wang et al. developed an approach involving the injection of dodecanethiol into hot solutions containing copper salt, a surfactant (or without a surfactant) and the high-boiling-point solvent 1-octadecene, which provided monodisperse Cu₂S NCs with well-controlled sizes and shapes.^100,119 The Cu₂S nanodisks synthesized from the procedure presented in Wang's work showed a tuneable bandgap from 1.36 to 1.53 eV, indicating the prospective applications of the products in solar cells.¹¹⁹ Wu et al. reported the synthesis of single-crystalline Cu₂S nanoparticles via a hot-injection procedure, followed by the integration of the as-synthesized NCs into solar cells through a spin-casting process.¹²⁰ The combination of colloidal CdS nanorods and Cu₂S nanoparticles into the heterojunction PV devices yielded a PCE of 1.6% and a V_OC of 0.574 V (AM 1.5G, 100 mW cm⁻²). The Cu₂S layer was spin-cast first and then heated for 10 min at 150 °C to remove the excess solvent. The current-density–voltage characteristics and the external quantum efficiency spectrum of the device are shown in Fig. 6a and b. The short-circuit current density were determined as a function of the illumination intensity (Fig. 6c). The results showed a near-linear relationship between the light intensity and the short-circuit current density, indicating that only minor charge-carrier recombination occurred in the PV devices. It is notable that an encapsulated device showed a practically constant performance over a 4-month measurement period (Fig. 6d).

The technique of oriented attachment of NCs has also proved itself as an efficacious method for providing colloidal Cu₂S nanorods with a precisely controlled morphology.¹²¹ By employing monodisperse quasi-spherical dodecanethiol-capped Cu₂S NCs as the seeds, the oriented attachment of NCs into nanorods took place when a crystal-bound ligand 1,2-hexadecanediol was introduced into the attachment process. It is believed that the quantum confinement of Cu₂S nanorods can be attained if small seed Cu₂S NCs are exploited; thus the realization of PV devices based on Cu₂S nanorods can be anticipated.

Tin(II) sulfide

Tin sulfide (SnS) works as a potential non-toxic candidate material for the light absorber in solar cells because of its appropriate bandgap (E_g = ∼1.1–1.4 eV) and its high absorption coefficient in the visible wavelength (a > 10⁴ cm⁻¹).^122–124 Single-crystalline SnS showed a hole mobility, perpendicular to the (010) direction, as high as 98 cm² V⁻¹ s⁻¹ at room temperature.¹²⁵ Given the estimated theoretical conversion efficiency of a single-junction SnS-based solar cell (assuming a bandgap of 1.1 eV) is 32%, substantial attempts have been made towards the development of thin-film solar cells based on SnS. Nonetheless, the record energy conversion efficiency of SnS-based heterojunction solar cells has only reached 4.36% thus far, using an absorbing layer of SnS fabricated by atomic layer deposition (ALD).¹²⁶ Meanwhile, a p–n homojunction solar cell fabricated with single-crystalline orthorhombic SnS nanowires using both boron and phosphorus as the dopants yielded a relatively high PCE of up to 1.95%.¹²⁷ Additionally, a SnS-based thin-film solar cell with the SnS structure of cubic crystalline was reported, achieving an energy conversion efficiency of 1.28%.¹²⁸

SnS was preferentially crystallized in the structure of the orthorhombic phase, with lattice parameters of a = 11.210 Å, b = 3.987 Å and c = 4.334 Å.¹²⁹ It was composed of double layers of tightly bonded Sn and S atoms, with weak van der Waals forces between the layers, which intrinsically provided a chemically inert surface without dangling bonds and surface density of states and thereby no Fermi level pinning at the surface.¹³⁰ There is also a metastable zinc blende structure of SnS, which has been reported in particles and thin films.^131–133 The crystal structure of zinc blende SnS is shown in Fig. 7. Various techniques have been reported for the controlled fabrication of crystalline SnS thin films, among which pulsed chemical vapour deposition (CVD), thermal evaporation and ALD have been demonstrated as the most effective methods to construct SnS thin-film solar cells with a moderate performance to date.^{126,134–145} Even so, the production of high-quality SnS thin films with favourable optical and electrical properties remains critical. In the effort to engineer SnS nanostructures possessing superior and practical qualities for optoelectronic applications, colloidal synthetic methods have been employed. We summarize the reported synthetic advances of colloidal SnS NCs in Table 3.

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The most generally explored approach for the production of colloidal SnS NCs is the hot-injection procedure. Hickey et al. developed the synthesis of monodisperse SnS NCs via the hot-injection of thioacetamide–oleylamine solution into a mixture of bis[bis(trimethylsisyl)amino]tin(II), trioctylphosphine, oleic acid and 1-octadecene.¹⁴⁶ The crystalline structure of the as-synthesized spherical and triangular SnS NCs were indexed to the orthorhombic phase. The optoelectronic behaviour of the NCs was studied by depositing the NCs onto conductive mercaptopropionic acid-derivated ITO substrates and by then probing their photoelectrochemical response. Photocurrent values in the range of 6–8 nA cm⁻² were achieved for monolayers/submonolayers of SnS NCs.

Modifications of the synthetic approach developed by Hickey et al. led to the preparation of SnS nanostructures with well-controlled sizes and shapes.^147–151 Zhang et al. reported the synthesis of monodisperse SnS nanoparticles with different sizes by introducing tin(II) acetate as the tin precursor and ammonium sulfide as the sulfur precursor.¹⁴⁷ The PV properties of the as-obtained SnS nanoparticles were studied by building a heterojunction of SnS nanoparticles and n-type ZnO NCs, which showed a strongly increased current density under white illumination compared to in the dark.¹⁴⁸ In regard to shape control, Biacchi et al. demonstrated a high-yield synthesis of colloidal SnS NCs with a diverse range of shapes, including ∼12 nm nanocubes, ∼10 nm spherical nanopolyhedra and ∼270 nm square nanosheets.¹⁵⁰ The UV-Vis absorption spectra of the as-synthesized SnS nanocubes and nanopolyhedra were broad, with an onset starting at about 850–900 nm and extending throughout the visible range; therefore the integration of SnS nanocubes and nanopolyhedras into PV devices can be anticipated. De Kergommeaux et al. reported the synthesis of monodisperse square SnS nanoplatelets in a orthorhombic phase, showing an edge length in the range of 15–100 nm and a thickness of 4–15 nm.¹⁵¹ The small-sized nanoplatelets showed long-term storage stability (2 years in chloroform at ambient temperature), which is particularly important in view of introducing SnS NCs into PV applications. When SnCl₂ was used as the tin precursor instead of SnCl₄·5H₂O for the synthesis of SnS nanoplatelets, spherical/faceted SnS nanoparticles were obtained, as shown in Fig. 8.

SnS NCs with crystalline structures of orthorhombic and zinc blende have both been reported via synthetic methods employing oleylamine and hexamethyldisilazane as the surfactants.^133,152 By injecting a hot elemental sulfur–oleylamine solution into a mixture of tin(IV) iodide, oleylamine and hexamethyldisilazane at a temperature of 100 °C, long single-crystalline SnS nanoribbons with thicknesses down to 10 nm were obtained, which then grew through a metastable-to-stable phase transition from zinc blende nanospheres to orthorhombic nanoribbons.¹⁵² Optical characterization showed the onset of the absorption of the initial zinc blende nanospheres at 760 nm, while the absorption of the final orthorhombic nanoribbons began at 1015 nm. The photoconductive behaviours of SnS single nanoribbons were investigated, which showed a highly sensitive and rapid response to illumination by visible light at room temperature. Switching between photocurrent generation and annihilation was complete within one ms, accompanied by high photoconductivity gains (up to 2.3 × 10⁴) and good stability. The synthetic approach using oleylamine only as the capping ligand for SnS also provided monodisperse and size-tuneable SnS nanoparticles.¹⁵³

SnS nanoparticles with sizes as small as 5 nm and square SnS nanosheets with sizes down to 40 nm were prepared via a colloidal synthesis by introducing both oleylamine and oleic acid as the surfactants.¹⁵⁴ Later work demonstrated by Liu et al. showed that deploying alkylthiols along with oleylamine and oleic acid as the ligands also provided SnS nanosheets with a well-controlled morphology.¹⁵⁵ With proper selection of alkylphosphine and alkylacid ligands as the selective facet binders in company with alkylamine, monodisperse SnS nanocubes and nanotetrahedrons with a narrow size distribution were achieved within 5 s of the reactions.¹⁵⁶ It was discussed that the size of the nanocubes could be tuned in a wide range by tuning the density of nucleation, which could make the assembly of SnS nanocubes with suitable sizes into PV devices feasible.

Herron et al. reported a heating-up process using oleylamine as the ligand for the synthesis of SnS NCs, which provided colloidal SnS nanosheets with a high aspect ratio and a thickness below 5 nm.¹⁵⁷ The SnS thin-film fabricated from the nanosheets ink exhibited lamellar stacking to a large extent. Optical investigation of the film demonstrated high reflectivity and an indirect bandgap of 1.23 eV for SnS. The SnS nanosheets film achieved an in-plane mobility of 5.7 cm² V⁻¹ s⁻¹, which is comparable to SnS film fabricated from ALD.¹⁵⁸ Rabkin et al. reported the synthesis of highly symmetric SnS nanotetrahedra by employing the decomposition of a single-source precursor tin ethylxanthate in octadecylamine.¹⁵⁹ Electron diffraction was used to study the crystalline structure of the nano-sized SnS crystals and revealed the studied phase to be a cubic structure. The optical and electrical properties of this cubic SnS were expected to be significantly different from orthorhombic SnS. By using the single-source precursor tin diethyldithiocarbamate-1,10-phenanthroline, Yang et al. synthesized uniform rectangular nanosheets of single-crystalline SnS.¹⁶⁰ The SnS nanosheets could be turned into spherical SnS₂ nanoplates by tuning the reaction temperature and additionally introducing oleic acid as a surfactant. Dibutyl-bis(piperidinedithiocarbamato)tin(IV) can also be used as a single-source precursor for the production of colloidal SnS nanosheets.¹⁶¹

The initial attempts to incorporate colloidal SnS NCs into PV applications were demonstrated by Stavrinadis et al.¹⁶² The fabrication of SnS NCs (sized 3.6 nm)/PbS NCs (first exciton peak at 920 nm) heterojunction solar cells yielded a PCE of 0.5% and a V_OC of 0.35 V under AM 1.5G illumination (80 mW cm⁻²). SnS NCs films were fabricated by employing the 1,2-ethanedithiol dip-coating method to replace the long-chain ligand used in the NCs synthesis and by additional washing using a mixture of ethyl acetate and acetonitrile to remove excess long-chain ligands after the initial deposition. The impact of the additional washing was noticeable as the value for J_SC (short-circuit current density) was doubled compared to the device without additional washing. But still, the performance was poor, which was partially due to the unsatisfactory quality of the NCs as the ligands on the surface of the as-synthesized NCs could be easily washed away by alcohol during the precipitation. Recently, Truong et al. reported the production of SnS-based bulk-heterojunction solar cells by blending SnS nanospheres with a mean size of 3–4 nm with conjugated polymers to build the photoactive layer with a subsequent annealing of the SnS nanospheres at high temperature in nitrogen.¹⁶³ The SnS nanospheres underwent a solid-state morphological transformation after annealing, where the size of the SnS nanospheres increased to 5–6 nm. A moderate PCE of 0.71% (under AM 1.5G illumination) was achieved for the device with the structure of a SnS/PTB7 (poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})) bilayer. The authors also studied the PV behaviour of a device without thermal annealing, which resulted in a poorer performance compared with devices with annealing. Meanwhile, the deployment of colloidal SnS NCs in solid-state quantum-dot sensitized solar cells (QDSSCs) resulted in inferior performance.¹⁶⁴

Tin(II) selenide

Tin(II) selenide (SnSe), also known as stannous selenide, is an important environmentally friendly narrow bandgap semiconductor with great advantages in PV applications. Bulk SnSe has an indirect bandgap of 0.9 eV and a direct bandgap of 1.3 eV with a high optical absorption coefficient of 10⁵ cm⁻¹.¹⁶⁵ SnSe works as a p-type semiconducting material with a reported hole mobility of up to 7 × 10³ cm² V⁻¹ s⁻¹ at 77 K and a carrier concentration of 10¹⁷ cm⁻³.^166,167 As one of the essential layered metal chalcogenides, SnSe crystallizes in the orthorhombic structure with a cell unit of a = 11.501 Å, b = 4.153 Å and c = 4.455 Å.¹⁶⁸ Similar to SnS, SnSe has been expected to possess distinctive electronic structures that would exhibit superb physical properties on account of their special geometric structures with weak interlayer van der Waals interactions.

Thin-film solar cells based on SnSe deposited by flash evaporation, thermal deposition, ED and CBD have been studied.^169–174 Among these reported methods, thermal deposition has proved itself an effective route to produce SnSe-based solar cells with appealing efficiency. El-Rahman et al. reported thermal deposition of a SnSe film onto an n-Si single crystal to form a p-SnSe/n-Si heterojunction solar cell.¹⁷¹ The cell exhibited promising PV performance with a PCE of 6.44%, a V_OC of 425 mV and a J_SC of 17.23 mA cm⁻² under light illumination with an input power density of 50 mW cm⁻².

Compared with other semiconducting materials from the IV–VI family, progress towards the colloidal syntheses of SnSe NCs has remained tardy. The first attempt to prepare colloidal SnSe nanoparticles was demonstrated by using bis[bis(trimethylsilyl)amino]tin(II) as the Sn precursor and trioctylphosphine selenide as the Se precursor in the presence of oleylamine, as reported by Kovalenko et al.¹⁷⁵ Nevertheless, the as-synthesized SnSe NCs showed a tendency to aggregate. In another approach, Baumgardner et al. developed an approach involving the injection of bis[bis(trimethylsilyl)amino]tin(II) in oleylamine into a mixture of trioctylphosphine selenide and oleylamine and a subsequent injection of oleic acid into the reaction system to quench the nucleation.¹⁷⁶ The sizes of NCs could be tuned from 4 to 10 nm by manipulating the injection and reaction temperature for the synthesis. However, the shapes of the as-obtained NCs turned out to be irregular and pseudospherical and the size distribution of the NCs was large. The optical study showed that the energy bandgap of SnSe was tuneable, ranging from 0.9 to about 1.3 eV, which covered the ideal range for single-junction solar energy conversion.

Franzman et al. accomplished a synthetic approach employing dialkyl selenide as the selenium precursor through hot-injection for the preparation of high-quality SnSe NCs and pioneered the integration of SnSe NCs into PV devices.¹⁷⁷ The as-obtained elongated NCs showed widths of 19.0 ± 5.1 nm, whereas the lengths were more polydisperse. SnSe NCs-based PV devices with a structure of a hybrid were fabricated by deploying a blend of SnSe NCs and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (PPV) as the absorbing layer. A PCE of 0.06% and a J_SC of 0.39 mA cm⁻² were reported for the hybrid devices, while the analogous polymer-only devices yielded a PCE of 0.03% and a J_SC of 0.20 mA cm⁻², whereas the V_OCs and FFs (fill factors) of both devices were comparable. The enhanced efficiency of the hybrid device was attributed to the improved quantum efficiency near 500 nm due to the efficient electron transfer from PPV to SnSe, as the absorption coefficients of the hybrid film and neat PPV were nearly equivalent near 500 nm. Compared with PV devices based on FeS₂, Cu₂S and SnS, the PCE for SnSe NCs-based devices was relatively low, which was ascribed to no post-treatment (like ligand-exchange or annealing) being introduced into the device fabrication process and as the long-chain ligands on the surface of the NCs hindered the exciton dissociation and charge transport at the heterojunction.

Liu et al. demonstrated the synthesis of single-crystalline SnSe nanosheets through the hot-injection of an alkylphosphine–selenium solution into a mixture of tin chloride, oleic acid and 1-octadecene.¹⁵⁵ By replacing alkylphosphine for oleic acid or oleylamine, the formation of monodisperse dot-like SnSe NCs was also reported. The successful employment of oleic acid and oleylamine as the coordinating solvents to form organo-Se precursors paves the way for the development of a phosphine-free approach for the production of high-quality selenide NCs. The exploitation of tin(II) oxide hydroxide Sn₆O₄(OH)₄ as the Sn precursor and selenourea as the Se precursor, which was phosphine-free as well, could provide SnSe NCs in a variety of shapes, including nanocubes, nanoparticles and nanosheets.¹⁷⁸Fig. 9 shows the TEM images of SnSe NCs produced at different reaction temperatures and Sn/Se molar ratios. All the obtained NCs showed a narrow size distribution.

Instead of using hot-injection, researchers have also developed a heating-up process for the synthesis of SnSe NCs, which produce SnSe with the morphology of nanosheets mostly. Vaughnll et al. described the colloidal synthesis of SnSe nanosheets by heating up tin(II) chloride and trioctylphosphine selenide in the presence of hexamethyldisilazane and oleylamine.¹⁷⁹ The thickness of the resulting nanosheets could be tuned ranging from 10 to 40 nm through control of the concentrations of tin(II) chloride and trioctylphosphine selenide. Drop-cast films of the as-obtained nanosheets showed an absorption onset in the NIR region at about 1350 nm. Later work synthesizing single-crystalline SnSe nanosheets with a four-atomic thickness of about 1.0 nm via heating-up was realized by Li et al.¹⁸⁰ Here, it was reported that 1,10-phenanthroline played a dominant role in tuning the growth kinetics of the SnSe nanosheets since uniform SnSe nanoflowers were obtained when no 1,10-phenanthroline was employed in the reaction system. By introducing borane-tert-butylamine and trioctylphosphine as the ligands and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone as the solvent and a weak oxidant, Zhang et al. also developed the synthesis of ultrathin colloidal SnSe nanosheets through the heating-up approach.¹⁸¹ The thickness of the as-obtained SnSe nanosheets was revealed to be about 2 nm by atomic force microscopy (AFM) measurement.

By using selenido-bridged stannene dimer as the single-source precursor, SnSe nanoplates were prepared and the oriented-attachment of the as-synthesized nanoplates led to the formation of SnSe nanocolumns.¹⁸² A study of the reflectance spectra of the SnSe NCs showed a bandgap of 0.96 eV for the nanoplates and a red-shifted bandgap of 0.93 eV for the nanocolumns. An optoelectronic study of the SnSe nanocolumns showed an increased photocurrent under visible light irradiation compared with that in the dark state, indicating that the SnSe nanocolumns can act as promising absorbent materials for PV applications.

We summarize the reported synthetic routes for colloidal SnSe NCs in Table 4.

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Bismuth sulfide

Bismuth sulfide (Bi₂S₃), also known as bismuthinite, has been identified as a major n-type semiconducting material for PV applications on account of its low toxicity and its direct bandgap located in the NIR region. It has been referred in the literature that the bandgap of Bi₂S₃ varies with the crystallinity and/or stoichiometry of the material. Bandgaps of 1.1 eV for a natural Bi₂S₃ crystal and 1.1–1.3 eV for bulk Bi₂S₃ were reported, while polycrystalline Bi₂S₃ thin-film exhibited a lower bandgap of 1.3–1.7 eV, depending on the synthetic approaches for the film.¹⁸³ In addition, Bi₂S₃ possesses strong absorptivity with a high absorption coefficient in the order of 10⁵ cm⁻¹ in the visible light region.¹⁸⁴ Reports on the growth of nanostructured Bi₂S₃ have addressed that Bi₂S₃ crystallizes in a lamellar structure in which Bi–S atom chains form parallel to the preferred growth direction (c-axis), while Bi₂S₃ layers are held together through van der Waals interactions.^185–187 The rapid growth of Bi₂S₃ crystals along the c-axis often brings about dimensional structures, such as nanowires, nanorods and nanosheets.^186–189

Significant progress has been achieved in the production of Bi₂S₃ nanostructures through colloidal synthetic procedures and the incorporation of as-synthesized Bi₂S₃ NCs into PV applications. The initial work of synthesizing colloidal Bi₂S₃ NCs via a hot-injection method was reported by Christian et al.¹⁹⁰ injecting an elemental sulfur–octylamine solution into a mixture of bismuth acetate in octylamine at a high temperature to afford Bi₂S₃ with a rod-like morphology with diameters of around 19 nm. Instead of using bismuth acetate, Du et al. reported the colloidal synthesis of Bi₂S₃ NCs based on bismuth chloride as the bismuth precursor and deploying oleylamine and octylamine as the surfactants.¹⁹¹ Ultrathin Bi₂S₃ nanowires with diameters of 1.2 nm were attained. A feature worth noting of this BiCl₃-S-oleylamine-octylamine synthetic process is that it is a general procedure for producing various kinds of high-quality ultrathin metal sulfide NCs with a high yield.

Malakooti et al. demonstrated the shape-controlled synthesis of Bi₂S₃ NCs by employing oleylamine as the ligand, bismuth chloride as the bismuth precursor and elemental sulfur as the sulfur source.¹⁹² Bi₂S₃ nanodots with diameters of 9.5 ± 0.5 nm and Bi₂S₃ nanorods with diameters of 4.6 ± 0.5 nm and aspect ratios of 7 ± 3 were prepared, as shown in Fig. 10. By replacing bismuth chloride with bismuth citrate, Bi₂S₃ nanowires with thicknesses less than 2 nm and a necklace architecture were achieved.¹⁹³ Study of the absorption spectra and second derivative analysis of the Bi₂S₃ samples revealed the occurrence of three excitonic transitions within the band-edge absorption of Bi₂S₃ nanowires. By exploiting bismuth acetate as the bismuth precursor for this oleylamine-based approach, Calzia et al. established the synthesis of Bi₂S₃ nanowires with diameters down to 3 nm, lengths exceeding 300 nm and a coherence length of about 30 nm, which indicated the appropriate electronic conductivity and attractive optical properties of the material.¹⁹⁴

Instead of using amines, deploying oleic acid as the ligand for the preparation of Bi₂S₃ NCs also presented Bi₂S₃ with various morphologies, including nanoparticles, nanorods and split nanostructures.^195,196 Aresti et al. described the preparation of Bi₂S₃ nanoparticles by using bismuth acetate as the bismuth precursor and hexamethyldisilazane as the sulfur source based on oleic acid as the ligand in the presence of 1-octadecene.¹⁹⁷ By tuning the injection temperature of sulfur and the reaction time, Bi₂S₃ nanoparticles with sizes ranging from 3 nm to 30 nm were attained. The time-integrated differential transmission spectrum of the Bi₂S₃ nanoparticles revealed that only photoexcited holes were captured by the intragap states, which was consistent with the assumption that the surfaces of Bi₂S₃ nanoparticles were incompletely passivated by oleic acid.

Konstantatos et al. reported the synthesis of Bi₂S₃ nanorods through the hot-injection of a hexamethyldisilazane–octadecene solution into a mixture of bismuth acetate, oleic acid and 1-octadecene.¹⁹⁸ The nanorods showed an average width of about 7 nm with a distribution of about 34% and an average length of about 22 nm with a distribution of about 16%. Optical characterization of the materials revealed that the Bi₂S₃ nanorods exhibited a bandgap of 1.3 eV, similar to that of bulk Bi₂S₃. Modifications of this oleic acid-based colloidal procedure offered Bi₂S₃ nanorods with average dimensions of 18 nm × 10 nm.¹⁹⁹ PV devices fabricated with Bi₂S₃ nanorods as an n-type semiconductor and PbS NCs with a first exciton peak at ∼860 nm as a p-type semiconductor and a structure of ITO/PbS/Bi₂S₃/Ag showed a PCE of 1.61%, a V_OC of 0.44 V and an FF of 41% (Fig. 11a). PV devices with an inverted structure of ITO/Bi₂S₃/PbS/Au showed an inferior PCE (1.02%) but a superior FF of 51% (Fig. 11b), which could be attributed to the higher quality metal–semiconductor interfaces between PbS and Au as evidenced by the lower series resistance (100 Ω) compared to the normal structure (250 Ω). The authors studied the size-effect of PbS NCs on the performance of PV devices by employing PbS NCs with a first exciton peak at ∼1300 nm. The current density–voltage characteristics for the normal and inverted structures are shown in Fig. 11c and d. The obtained V_OC in these structures was about 0.35 V as a result of the lower Fermi level of the PbS NCs and thereby the lower build-in field with Bi₂S₃. Subsequently, Rath et al. reported the fabrication of bulk-heterojunction devices based on Bi₂S₃ nanorods and PbS NCs with a first exciton peak at ∼920 nm, yielding a PCE as high as 4.87%, a V_OC of 0.40 V and a J_SC of 24.2 mA cm⁻².²⁰⁰ The formation of a bulk heterojunction at the nanoscale allowed efficient charge separation and transport, which led to an over 3-fold enhancement in the PCE and external quantum efficiency compared to bilayer p–n junction devices made of the same materials (Fig. 12). The performance comparison between the bilayer and bulk heterojunction revealed that nanoscale charge separation in the bulk heterojunction was essential for the prolonged carrier lifetime, which eliminated the poor minority carrier transport in Bi₂S₃.

The integration of Bi₂S₃ nanorods and poly(3-hexylthiophene) (P3HT) into polymer-NCs-based solar cells was demonstrated by Martinez et al.^201,202 A PCE of 0.46% was obtained for a Bi₂S₃ nanorods/P3HT PV device with a bilayer structure, while a PCE of 1% was achieved for the Bi₂S₃:P3HT PV device with a hybrid bulk-heterojunction structure. The authors investigated the impact of different thicknesses of active Bi₂S₃:P3HT layer on the performance of bulk-heterojunction devices. It was revealed that the J_SC of the device with a thicker active layer (350 nm) was increased due to the increased light absorption, while V_OC and FF retained minimal losses compared with a device with a thinner active layer (150 nm). Further increasing the thickness of the active layer led to a significant reduction in V_OC and FF, indicating that the recombination process had taken over from charge transport.

The employment of oleic acid and oleylamine as the surfactants for the synthesis of colloidal Bi₂S₃ nanoarchitectures led to Bi₂S₃ NCs with the morphologies of nanodots and nanorods. Ibáñez et al. demonstrated the production of Bi₂S₃ nanorods with thicknesses of ∼5 nm and lengths of ∼50 nm by injecting an elemental sulfur–oleylamine solution into a mixture of bismuth acetate, oleic acid and 1-octadecene at high temperatures.²⁰³ Han et al. reported the preparation of Bi₂S₃ nanodots and nanorods through an oleic acid and oleylamine-based approach by using bismuth neodecanoate as the bismuth precursor and thioacetamine as the sulfur source.²⁰⁴ The morphology-controlled synthesis of Bi₂S₃ NCs was accomplished by hot-injecting a sulfur source into bismuth solution under different injection temperatures, and by varying the precursor ratios of Bi to S and the overall reaction times. Absorption characterization of the Bi₂S₃ NCs revealed that Bi₂S₃ nanodots with diameters of 3–4 nm showed a bandgap of 2.04 eV, while nanorods with diameters of 7–8 nm showed a bandgap of 1.89 eV.

The introduction of ammonium sulfide as the sulfur source into the colloidal synthesis of Bi₂S₃ NCs also provided Bi₂S₃ nanoparticles with a narrow size distribution.¹⁴⁷ By mixing ammonium sulfide–oleylamine solution with a solution of bismuth dodecanethiolate complexes and dichlorobenzene at room temperature, Zhang et al. prepared Bi₂S₃ quantum dots with average sizes of about 4.5 ± 0.5 nm, which showed an onset of absorption at around 620 nm. Smaller-sized Bi₂S₃ nanoparticles were also synthesized through this ammonium sulfide process by employing bismuth oleate as the bismuth precursor and oleic acid as the surfactant.

The exploitation of single-source precursors, including bismuth dialkyldithiophosphate, bismuth thiobenzoate and bismuth diethyldithiocarbamate, for the colloidal synthesis of Bi₂S₃ NCs has been reported. For instance, the thermal decomposition of bismuth di-n-octyl-dithiophosphate in the presence of oleylamine provided single-crystalline Bi₂S₃ nanorods with diameters of 7–21 nm and lengths of several hundred nanometres.²⁰⁵ The reaction between bismuth thiobenzoate, dodecanethiol and trioctylphosphine oxide generated Bi₂S₃ nanorods with an average diameter of 21.3 ± 2.6 nm and a length of 300 ± 30 nm.²⁰⁶ The thermolysis of bismuth diethyldithiocarbamate in 1-dodecylamine presented discrete Bi₂S₃ nanosheets, which were formed through the attachment and in situ recrystallization of the initially shaped metastable nanorods.²⁰⁷ It was reported that 1-dodecylamine functioned better as a surfactant, reaction medium and electron donor to produce Bi₂S₃ nanosheets with good quality and a high yield compared with oleylamine.

Sigman et al. demonstrated the preparation of Bi₂S₃ nanorods and nanowires with well-defined surfaces and small diameters through the solventless thermolysis of bismuth alkylthiolate.¹⁸⁷ The solventless synthesis was carried out in air at about 225 °C in the presence of octanoate as the ligand to provide high aspect ratio nanowires, while lower aspect ratio nanorods were synthesized by the same procedure but with the addition of sulfur at a lower temperature of 160 °C. It was deduced that the addition of elemental sulfur to the reaction system increased the rate of Bi₂S₃ formation and depleted the accessible Bi needed to extend the nanostructures to a large aspect ratio, which thus quenched the nanowire elongation. This procedure provides a potential method for achieving a generic shape control over NCs through colloidal synthetic routes.

The integration of nano-heterostructures based on colloidal Bi₂S₃ NCs into PV devices was reported by Saha et al., who reported the colloidal synthesis of metal–semiconductor Schottky junctions formed through gold nanoparticles attached to Bi₂S₃ nanorods.²⁰⁸ Incorporation of the Bi₂S₃/Au Schottky diodes into a P3HT matrix to fabricate hybrid bulk-heterojunction solar cells yielded a PCE of 2% and a V_OC of 0.67 V (under one sun illumination). Such hybrid bulk-heterojunction devices performed more efficiently than devices based on only Bi₂S₃ nanorods in a P3HT matrix or gold nanoparticles added separately to the heterojunction, indicating the efficient charge separation in the metal–semiconductor Schottky diodes.

The colloidal synthetic routes developed for Bi₂S₃ NCs are summarized in Table 5.

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The performances of PV devices based on FeS₂ NCs, Cu₂S NCs, SnS NCs, SnSe NCs and Bi₂S₃ NCs are summarized in Table 6.

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Copper zinc tin sulfide

Copper zinc tin sulfide (Cu₂ZnSnS₄, CZTS) is considered to be one of the most promising environmentally friendly and earth-abundant absorber materials for solar cells due to its pertinent bandgap ranging from 1.45 eV to 1.5 eV and absorption coefficient of over 10⁻⁴ cm⁻¹.²⁰⁹ The PV effect of CZTS was first demonstrated in 1988 by Ito et al.²¹⁰ Following this, numerous vacuum and non-vacuum deposition methods have been employed to develop CZTS solar cells with high efficiency, including thermal evaporation, sputtering, pulsed laser deposition, spray pyrolysis, sol–gel route, spin-coating and ED.^211–217 The latest recorded energy conversion efficiency of CZTS-based solar cells was up to 9.1%.²¹⁸ This value has, however, been surpassed by the related compound Cu₂ZnSnS_xSe_4−x (CZTSSe)-based PV devices, of which an efficiency of 12.6% has been reported.²¹⁹

The investigation of CZTS NCs has attracted a great amount of attention in recent years because it makes possible ex situ control of the crystal phase, stoichiometry and morphology of the materials prior to their processing into thin films, which is a critical factor for the construction of highly efficient solar cells based on quaternary CZTS. Several methods have been employed to synthesize CZTS and related CZTSSe and Cu₂ZnSnSe₄ (CZTSe) nanostructures, including hot-injection, heating-up and the thermal decomposition of organometallic precursors. Developments in the colloidal syntheses of CZTS and CZTSe have been reviewed.^220–223 Here we focus on only the development of solar cells based on colloidal CZTS and CZTSe NCs.

Agrawal's group were the first to demonstrate the formation of kesterite CZTS NCs and the integration of CZTS NCs into solar cells in 2009.²²⁴ The synthesis approach was based on the hot injection of a sulfur–oleylamine solution into a mixture of oleylamine and metal acetylacetonates. The CZTS NCs synthesized showed mild polydispersity, with the sizes of most of the NCs being in the range 15–25 nm, and had a slightly copper-rich composition of Cu_2.12Zn_0.84Sn_1.06S₄. The fabrication of PV devices was accomplished by drop-casting the CZTS NCs onto Mo-coated soda lime glass substrates and then selenizing the as-prepared CZTS thin films to form CZTSSe (crystal structure of kesterite CZTSSe shown in Fig. 13) afterwards. Devices based on CZTSSe as the absorber material showed a PCE of 0.80% based on the nonshadowed area under AM 1.5G illumination. By modifying the feeding molar ratio of cation precursors to form copper-poor and zinc-rich compositions, which are commonly employed in literature, Agrawal's group successfully achieved CZTS NCs with the composition of Cu_1.31±0.02Zn_0.91±0.03Sn_0.95±0.02S₄ in 2010.^225–227 The incorporation of as-synthesized CZTS NCs into PV devices produced a PCE as high as 7.2% after light soaking. In a separate report, subsequent modifications of the synthesis recipe were made to provide CZTS NCs with a better uniformity of average composition and a narrower size distribution.²²⁸ Optimization of the selenization conditions was also performed in order to fabricate CZTSSe thin CZTSSe thin films with micron-sized grains and minimal fine-grains and MoSe₂ thickness, as can be seen in Fig. 14. The improvement of PV performance in CZTSSe solar cells was achieved with a PCE of 9.0%, a J_SC of 35.1 mA cm⁻² and an FF of 63.7%. Further adjustment of the selenization process, wherein a rapid thermal processing (RTP) furnace capable of delivering a constant supply of Se vapour to CZTS film was introduced, led to an enhanced performance of 9.3% PCE.²²⁹ It was reported that the RTP processed films contained a minimal residue of sulfur in the crystal lattice after selenization and that a reduction in the absorber's bandgap had been achieved.

Apart from selenization for CZTS, the cation substitution of Sn with germanium (Ge) has also been investigated to tune the bandgap of CZTS to improve the PV performance. Agrawal's group reported the substitution of Sn with Ge in CZTSSe PV devices by controlling the feed molar ratio of Ge/Sn for the synthesis of Cu₂Zn(Sn_1−xGe_x)S₄ (CZTGeS) NCs.²³⁰ The bandgap of the as-synthesized NCs was observed to increase with increasing the Ge amount of the synthesis precursors. PV devices fabricated based on CZTGeS NCs with a Ge/(Ge + Sn) ratio of 0.7 yielded a PCE of 6.8%. By further tuning the Ge/(Ge + Sn) ratio to 0.25 in CZTGeS NCs, Agrawal's group succeeded in reaching a PCE of 8.4%.²³¹ In the subsequent work by this group, tuning the Ge content of 30 at% in the CZTGeSSe material system was performed by controlling Ge loss from the bulk of the absorber film during high-temperature selenization.²³² An enhanced device performance of CZTGeSSe NCs solar cells was achieved with a PCE up to 9.4%. The investigation of the impact of bulk Ge loss in CZTGeSSe on the device performance was carried out by fabricating solar cells with various [Ge]/([Ge] + [Sn]) ratios using both GeCl₄ and GeI₄ precursors in the NCs preparation. The study showed that V_OC did not increase monitonically with the Ge content for devices experiencing considerable Ge loss with respect to the precursor composition (GeCl₄), while a monotonic increase was observed for the optimized absorber layers that did not show any bulk Ge loss with respect to the precursor composition (GeI₄), as shown in Fig. 15a. Contrariwise, the absorber bandgaps for all the devices turned out to be independent of this Ge loss (Fig. 15b).

Ag-Alloyed CZTS solar cells were also explored by Agrawal's group, with the purpose of modifying the defect properties and tuning the bandgap of (Ag,Cu)₂ZnSnSe₄ (ACZTSe) to enhance device performance.²³³ By substituting the Cu precursor with a Ag precursor in the CZTS NCs synthesis, (Ag,Cu)₂ZnSnS₄ (ACZTS) NCs with Ag-alloy concentrations of Ag/(Ag + Cu) at 0%, 5% and 50% were obtained. Selenization of the NCs films of 5%-ACZTS and 50%-ACZTS was carried out through RTP processing. Tuneable bandgap of the ACZTSe absorbers were demonstrated. An improvement in the average device performance (7.1% PCE) was also accomplished for ACZTSe (5%-Ag) absorbers compared with CZTSe (6.6% PCE) absorbers with a similar bandgap. The device performance and optoelectronic properties for CZTSe and ACZTSe devices are summarized in Table 7. The enhanced average V_OC and average PCE for the 5%-ACZTSe device relative to CZTSe with a similar bandgap was ascribed to the improved grain growth, which led to an increased minority carrier lifetime, reduced defect growth and reduced band-tailing, as evidenced in the cross-sectional and plain-view scanning electron microscopy (SEM) images for the selenized CZTSe and ACZTSe absorbers in Fig. 16. The inferior device performance of 50%-ACZTSe was due to the poor p–n heterojunction band alignment. Optimization of the band alignment through n-type materials may lead to improved device performance for 50%-ACZTSe since it exhibited superior optoelectronic properties related to the band-tailing and carrier lifetime compared to CZTSe and 5%-ACZTSe.

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Based on the consideration of inducing an efficient microstructural evolution without the involvement of organic residues in CZTS NCs-derived solar cells, the molecular metal chalcogenide complex (MCC) ligand Sn₂S₆⁴⁻ was employed to replace the long-chain ligand oleylamine on the surface of the as-synthesized Cu₂ZnGeS₄ (CZGeS) NCs.²³⁴ CZTGeS films were obtained by annealing the Sn₂S₆⁴⁻-capped CZGeS ligand-exchanged material with different concentrations of Sn₂S₆⁴⁻. The bandgap of the CZTGeS films could be easily tuned by varying the amount of Sn₂S₆⁴⁻ absorbed on the surface of the CZGeS NCs. A bandgap-graded CZTGeS was developed by successive coatings of CZGeS NCs inks with different Sn₂S₆⁴⁻ concentrations. Compared with ungraded CZTGeS solar cells, the bandgap grading configuration brought about a higher J_SC and V_OC, resulting in a PCE of 6.3% (4.8% for ungraded devices).

Steinhagen et al. also reported the synthesis of CZTS NCs in 2009, based on copper(II) acetylacetonate, zinc acetate and tin(II) chloride dihydrate as the cation precursors.²³⁵ The as-prepared NCs exhibited an average diameter of 10.6 ± 2.9 nm and had an average composition of Cu_2.08Zn_1.01Sn_1.20S_3.70. PV devices based on CZTS NCs were fabricated by spray-coating a CZTS NCs–toluene solution without annealing. A PCE of 0.23% was obtained for the devices. The poor device performance indicated that post-annealing and selenization for CZTS NCs are essential for the fabrication of CZTS PV devices.

Other than employing a selenization process for kesterite CZTS NCs films, researchers have also developed a sulfurization treatment to transform metastable wurtzite CZTS NCs into kesterite CZTS for highly efficient pure-sulfide solar cells. Liu et al. demonstrated the synthesis of wurtzite CZTS NCs by using trioctylamine, dodecanethiol and tert-dodecylmercaptan as the surfactants.²³⁶ The annealing of spin-coated wurtzite CZTS NCs films in a sulfur atmosphere was carried out after a thin layer of sodium fluoride (NaF) was deposited on top of the CZTS. It was revealed that the sulfurized CZTS film consisted of two regions: a large-grained top layer and a small-grained bottom layer, which was consistent with selenized kesterite CZTS films. By exploiting the prepared CZTS films in PV devices, the authors achieved an impressive PCE of 4.83% for pure-sulfide CZTS solar cells. Later, the same group reported an enhanced PV performance of 6% PCE for pure-sulfide CZTS devices by improving the crystalline quality of the active large-grained layer and reducing the fine-grained sublayer, which were accomplished by modifying the Na-doping process and by introducing SnS powder into the sulfurization treatment.²³⁷

Studies about the Na-doping in CZTS solar cells revealed the significance of Na-doping in increasing the grain size, defects and traps passivation and in boosting the carrier concentration and/or mobility. Zhou et al. reported the preparation of homogeneous CZTS:Na NCs by incorporating Na directly onto the surface of CZTS NCs in solution.²³⁸ Solar cells based on CZTS:Na NCs on borosilicate glass exhibited a PCE of 6.1%, which was 50% higher than that of CZTS NCs devices. Capacitance–voltage (CV) measurements of CZTS:Na devices showed a higher carrier concentration and longer minority carrier lifetime, compared with CZTSSe devices with Na doped by evaporation.

Hsu et al. reported the composition-dependent properties of CZTSSe PV devices.²³⁹ Based on the synthesis of CZTS NCs with a wide window of composition, Cu/Sn ranging from 1.5 to 2.0 and with Zn/(Cu + Zn + Sn) varying from 28% to 40%, the engineering of spatial composition distributions in the bulk CZTSSe and especially near the surface of the film was demonstrated. The correlation between the device performance and the composition of the NCs is shown in Fig. 17. Optimized devices starting from CZTS NCs with a Zn amount 1.6 times more than that of the reported stoichiometry showed higher PCEs. The best device efficiency attained was 8.6%, as fabricated through full solution processing from the absorber layer to the transparent top electrode.

Qu et al. studied the impact of the synthesis parameters, including reaction time, temperature and cooling rate, of the CZTS NCs on the PV performance.²⁴⁰ The authors noted that prolonging the reaction time provided a new means to increase the concentration of acceptor levels in the CZTSSe absorbers and led to an enhanced device performance compared to CZTSSe with a shorter reaction time. It was stated that elevating the reaction temperature of the CZTS NCs’ synthesis also improved the device performance, with a PCE of 6.3% obtained, which could be ascribed to the existence of secondary phase copper tin sulfide (Cu₂SnS₃), which was obtained during the synthesis along with CZTS.

Cao et al. took a different approach to prepare CZTSSe thin films with pure crystallinity, which involved annealing an appropriate mixture of binary and ternary sulfide NCs in the presence of selenium.^241,242 The composition of the CZTSSe films could be effortlessly manipulated by adjusting the ratio of the binary and ternary NCs deployed. It was observed that the CZTSSe film comprised a fine-grained sublayer and a large-grained outer layer consisting of densely packed micrometre-sized crystals. Introducing CZTSSe films fabricated by this method into solar cells led to achieving a PCE up to 9.02% and a J_SC as high as 32.01 mA cm⁻².²⁴²

In the interest of avoiding long-chain ligands on the surface of CZTS NCs, Kim et al. developed a process for the synthesis of colloidal single-crystalline CZTS NCs by employing triphenylphosphate as the capping ligands, which can be easily decomposed at low temperature.²⁴³ The as-obtained CZTS NCs facilitated the production of a dense and crack-free film with large grains through annealing under a nitrogen and hydrogen sulfide atmosphere. The incorporation of triphenylphosphate-capped CZTS NCs into PV devices was investigated without further selenization for CZTS films. The PV performance of pure-sulfide NCs-based solar cells exhibited a promising efficiency of 3.6% and a J_SC as high as 23.28 mA cm⁻².

On account of concerns that the dispersion of CZTS nanocrystals involves the utilization of toxic and environmentally unfriendly solvents, such as toluene and hexanethiol, van Embden et al. developed the fabrication of high-efficiency CZTSSe NCs solar cells processed from a benign polar solvent system.²⁴⁴ The authors used 5-amino-1-pentanol to replace the long-chain ligand oleylamine on the surface of CZTS NCs and anhydrous 1-propanol to disperse the 5-amino-1-pentanol-capped NCs to provide a polar CZTS ink. By blending the polar NCs ink with optimized selenization processing, CZTSSe NCs solar cells were successfully constructed with the best cell efficiency of up to 7.68%.

It was reported that CZTS solar cells suffered from a V_OC that was lower than 60% of the V_OCmax expected from the Shockley–Queisser limit.²⁴⁵ To address the issue that NCs surface-related recombination leads to a large V_OC deficit for CZTS solar cells, Korala et al. presented comprehensive research on clarifying the optimal ligand-exchange strategies for CZTS NCs to fabricate ligand-passivated and uniform NC films.²⁴⁶ A ligand-exchange approach was devised to passivate the surface metal ions by employing both organic and inorganic ligands and to bind the surface anionic chalcogen atoms with inorganic ligands. It was demonstrated that CZTS NCs solar cells fabricated with CZTS NCs passivated by ethylenediamine, iodide and zinc chloride showed the highest V_OC (686 ± 6 mV) among CZTS NCs passivated by organic and inorganic ligands, which was also the best V_OC value reported to date, to the best of our knowledge, for CZTS NCs solar cells. However, the device exhibited a low current density, which led to very poor performance. The reason behind this was the high series resistance caused by the non-optimized window layer, the back contact issues and the possible intrinsic bulk defects in CZTS NCs.

We summarize the performance of PV devices based on CZTS NCs in Table 8. The synthetic routes for the CZTS and CZTS-related NCs employed in the PV devices are summarized in Table 9.

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Other chalcogenide NCs

There are a number of less researched colloidal chalcogenide compounds that fit in the environmentally friendly and earth-abundant PV absorber family, such as Cu₂SnS₃, copper antimony sulfide (CuSbS₂), silver bismuth sulfide (AgBiS₂) and a range of composition tuneable alloyed semiconductors. The ternary Cu-IV–VI semiconductor Cu₂SnS₃ shows enormous potential for application in PVs owing to its high optical absorption coefficient (∼10⁴ cm⁻¹) and suitable bandgap (0.93 eV).²⁴⁷ Cu₂SnS₃-Based solar cells with a maximum PCE of 4.63% have been reported by annealing a coevaporation-deposited stacked precursor with a stacking order of NaF/Cu/Sn in a S/Sn vapour atmosphere.²⁴⁸ Meanwhile, solution-processed PV devices based on Cu₂SnS₃ nanoflakes, which were synthesized through a polyol-mediated hot-injection approach, exhibited a promising PCE of 2.77% and excellent long-term stability.²⁴⁹ The progression of the colloidal synthesis of Cu₂SnS₃ NCs towards hot-injection and heating-up routes has allowed for accurate control of the size, shape and phase of the NCs.^{241,250–255} Nonetheless, PV devices based on colloidal Cu₂SnS₃ NCs still remain unexplored.

CuSbS₂ has been recognized as a new candidate for p-type absorbent materials for solar cells in recent times, based on first-principle calculations using density functional theory (DFT).^256,257 It was reported that CuSbS₂ possesses significant features useful for PV applications, such as a strong absorption coefficient with an extinction coefficient of more than 10⁵ cm⁻¹ at 1.8 eV and an indirect bandgap of 1.4–1.5 eV. Over the past five years, CuSbS₂ NCs synthesis has seen moderate progress but the reports on CuSbS₂ NCs-based PV devices remain rare.^258–262 Suehiro et al. were the forerunners to demonstrate the deployment of colloidal CuSbS₂ NCs in PV applications.²⁶² The colloidal synthesis of CuSbS₂ involved the hot-injection of sulfur–oleylamine solution into a mixture of Cu(II) acetylacetonate, Sb(III) acetate and oleylamine. The reaction provided rod-shaped CuSbS₂ NCs with diameters of ∼50 nm and lengths of ∼1 μm, where the nanorods showed an absorption onset at about 1150 nm and a bandgap of 1.5 eV. PV devices prepared with CuSbS₂ NCs spin-coated as the absorber layer with no post-treatment showed a PV response with a PCE of 0.01%, indicating the feasible applications of CuSbS₂ NCs in solar cells.

Ternary I–V–VI compound AgBiS₂ is also an attractive candidate as a light-absorbent material with a favourable estimated absorption coefficient of 10⁵ cm⁻¹ and a calculated bandgap of 1.5 eV.²⁶³ A few reports on AgBiS₂ NCs synthesized from colloidal approach have been presented.^264–266 Bernechea et al. took the first step in the integration of colloidal AgBiS₂ NCs into PV devices²⁶⁶ when they synthesized AgBiS₂ NCs through a low-temperature hot-injection procedure with oleic acid as the surfactant, where the NCs crystallized into a cubic rock salt structure with a narrow size-distribution (diameters of 4.6 ± 1 nm). Devices based on AgBiS₂ NCs ligand-exchanged with tetramethylammonium iodide and thin AgBiS₂ layer annealed in air at low temperature (≤100 °C) showed a certificated PCE of 6.3% and a J_SC of ∼22 mA cm⁻². The PV devices did not exhibit hysteresis when the voltage was scanned in the forward and reverse directions (Fig. 18a). The light-biased EQE spectrum of the device showed a strong photocurrent response over a broad spectral range from 300 to 1100 nm (Fig. 18b). The results achieved represented a breakthrough in the production of low-temperature solution-processed environmentally friendly solar cells.

The method of alloying two semiconductors with different bandgaps allows the preparation of alloyed NCs with properties distinct from those of the parent semiconductors. The bandgaps of alloyed NCs can be fine-tuned by adjusting the constituent stoichiometries. The reported colloidal synthetic procedures for environmentally friendly and earth-abundant chalcogenide NCs so far include the preparation of Zn_xFe_1−xS₂, SnS_xSe_1−x, Sn_xGe_1−xSe, Sn_xSb_1−xSe, Cu₂Sn(S_3−xSe_x), Cu₂Ge(S_3−xSe_x) and Cu₂(Ge_xSn_1−x)(S_ySe_3−y).

Mao et al. reported the synthesis of Zn_xFe_1−xS₂ NCs with a broadened direct bandgap ranging from 1.219 eV to 1.32 eV compared to the direct bandgap of FeS₂ (as discussed in the iron disulfide section), which was close to the ideal bandgap for a single-junction PV device.²⁶⁷ The synthesis was carried out through a hot-injection method with iron(II) chloride and zinc stearate as the Fe and Zn precursors, respectively. The final ratio of Zn in the NCs was effectively tuned from 0% to 6.0 at% by properly adjusting the feeding precursor ratio of Zn/(Fe + Zn). The authors reported a promising photoconductive response in Zn_xFe_1−xS₂ photodetector devices, which resulted from a significant reduction of the dark current, which was attributed to the increased bandgap due to the zinc-alloying. PV devices based on Zn_xFe_1−xS₂ can be anticipated with the synthesis of high-quality NCs and an optimized device design and fabrication.

A few studies about composition-tuneable alloyed NCs based on SnS and SnSe (as discussed in the tin sulfide and tin selenide sections) have been reported. Wei et al. developed the synthesis of colloidal SnS_xSe_1−x NCs with a tuneable bandgap from about 0.92 eV to 1.24 eV in an intrinsic linear variation by changing the stoichiometric ratios of S and Se.²⁶⁸ The alloying of SnSe with the cations Ge and Sb to produce colloidal Sn_xGe_1−xSe and Sn_xSb_1−xSe NCs was performed by Buckley et al. and Hu et al., respectively.^269,270 In Buckley et al.'s study, the partial cation exchange of Ge²⁺ with the initially synthesized SnSe NCs was reported and led to the formation of colloidal Sn_xGe_1−xSe NCs. The composition was tuneable throughout the entire alloy range (0 ≤ x ≤ 1) by modulating the starting ratio of SnI₄ and GeI₄ precursors, resulting in an adjustable bandgap of Sn_xGe_1−xSe ranging from 0.87 eV to 1.13 eV. Meanwhile, Hu et al. prepared colloidal Sn_xSb_1−xSe nanorods by introducing tin(II) chloride dihydrate and antimony triacetate as Sn and Sb precursors, respectively. The direct and indirect bandgaps of the as-synthesized Sn_xSb_1−xSe NCs could be modulated from 1.39 eV to 1.53 eV and 0.93 eV to 1.28 eV, respectively, by increasing the Sb concentration from 0 to 0.2. The suitable bandgaps of alloyed NCs based on SnS and SnSe NCs imply the possibility of deploying these materials as light-absorbing candidates for PV applications.

The colloidal strategy of alloying Cu₂SnS₃ and Cu₂SnSe₃, both of which possess appropriate bandgaps, high absorption coefficients and high electron and hole mobilities for PV applications, to produce Cu₂Sn(S_3−xSe_x) NCs was explored by Liang et al.²⁷¹ The nanostructured Cu₂Sn(S_3−xSe_x), with x ranging from 0 to 1, was achieved by deploying elemental sulfur and selenium as S and Se precursors, respectively, and by tuning the feeding molar ratio of the S and Se precursors. The bandgap of Cu₂Sn(S_3−xSe_x) was tuned from 1.55 eV to 1.87 eV by increasing the concentration of sulfur in the NCs. The authors also studied the optoelectronic properties of the as-synthesized NCs by fabricating a PV device with the structure of ITO/Cu₂Sn(S_1.5Se_1.5)/Al. The device exhibited an enhancement in the current under light illumination compared to in the dark state. Further improvements in the NCs’ quality and optimization of the device assembly might lead to a more promising PV performance of Cu₂Sn(S_3−xSe_x).

Cu₂Ge(S_3−xSe_x) stands out as a particular semiconducting material for PV applications in view of its tuneable bandgap fitting in the ideal range for light absorption, where Cu₂GeS₃ has a p-type direct bandgap of 1.5 eV and Cu₂GeSe₃ holds a p-type direct bandgap of 0.77 eV.^272,273 To the best of our knowledge, there are only two reports on the synthesis of colloidal Cu₂Ge(S_3−xSe_x) NCs: one through a hot-injection approach and another through partial cation exchange.^274,275 The hot-injection route involved the injection of S–Se–oleylamine solution into a mixture of copper(II) acetylacetonate, germanium tetrachloride and oleylamine at a temperature of 160 °C.²⁷⁴ By tuning the molar ratio of the precursors, Cu₂Ge(S₂Se) NCs were prepared and chosen as the model material to investigate the PV properties of Cu₂Ge(S_3−xSe_x) because its bandgap levels are compatible with those of CdS and due to its ease in fabrication of smooth films by a solution approach. Pioneering exploration of PV devices with Cu₂Ge(S₂Se) NCs as the absorbing material led to a PCE of 0.20% under AM 1.5G illumination (100 mW cm⁻²).

Apart from ternary and quaternary alloyed semiconductors, quinary copper chalcogenide Cu₂(Ge_xSn_1−x)(S_ySe_1−y)₃ NCs also show appealing features for PV applications on account of the above-mentioned properties of its parent materials. This was demonstrated by Wu et al., where colloidal Cu₂(Ge_xSn_1−x)(S_ySe_3−y) NCs were synthesized with a tuneable composition in the ranges of 0 ≤ x ≤ 1 and 0 ≤ y ≤ 3.²⁷⁶ The bandgaps could be tuned from 1.45 eV to 2.57 eV for Cu₂(Ge_xSn_1−x)S (0 ≤ x ≤ 1) NCs and 1.35 eV to 1.95 eV for Cu₂(Ge_0.5Sn_0.5)(S_ySe_3−y) (0 ≤ y ≤ 3) NCs. The incorporation of as-synthesized Cu₂(Ge_0.5Sn_0.5)(S₂Se) NCs with a mean size of 12.6 nm as the light-absorbing material into PV devices resulted in a reasonably low PCE of 0.31%, largely owing to the long-chain organic ligands on the surface of the NCs, which hamper the efficient charge dissociation and transport. Further optimization of the device fabrication is necessary for Cu₂(Ge_0.5Sn_0.5)(S₂Se) NCs-based solar cells to maximize their performance.

Perspectives

Colloidal semiconducting NCs with environmentally friendly and earth-abundant elements are an exceptionally booming and prolific research field. Recent advancements in this area are exerting a dramatic impact on the progress of low-cost processing and high-efficiency solar cells. We have endeavoured with this review to present readers with an overview of recent efforts directed at utilizing colloidal NCs to produce environmentally friendly PV devices. Many chalcogenides, including FeS₂, Cu₂S, SnS, SnSe, Bi₂S₃, CZTS and CZTS relatives, have been developed towards the fabrication of colloidal NCs-based solar cells. The best PCE data for each chalcogenide NCs are: 1.1% for FeS₂, 1.6% for Cu₂S, 0.71% for SnS, 0.06% for SnSe, 4.87% for Bi₂S₃, 6.0% for CZTS, 9.3% for CZTSSe, 9.4% for CZTGeSSe, 7.1% for ACZTSe and 6.3% for CZTGeS.^{70,120,163,177,200,229,230,233,234,237} Among these, the device performances for FeS₂, Cu₂S, SnS and SnSe are comparatively low, which is mainly associated with the unsatisfactory quality of the NCs and the underdeveloped device fabrication process. For example, FeS₂ NCs showed a tendency to aggregate during characterization and device fabrication, which resulted in an increased surface roughness of the FeS₂ films.⁷⁰ In the case of Cu₂S, the photoactive layer was deposited with only the as-synthesized NCs annealed at 150 °C for 10 min to remove the excess solvents after spin-casting, and no further treatment was introduced to dispose of the insulating ligands on the surface of the NCs.¹²⁰ With regard to SnS, the ligands on the surface of the as-synthesized NCs could be easily stripped by alcohol during precipitation and the NCs aggregated, which led to the increased surface trap density of SnS films in PV devices.¹⁶² As for the fabrication of SnSe NCs-based PV devices, neither post-annealing nor ligand-exchange was deployed to remove the long-chain ligands on the surface of the NCs.¹⁷⁷ Apropos of CZTS and CZTS-related NCs, the annealing and selenization approaches appear as the mainstream for device construction. This high-temperature treatment constrains the choice of substrate material, which hinders the implementation of low-cost solution processes for PV applications. On the other hand, the ligand-exchange procedure, which performs well on replacing long-chain ligands on the surface of Pb-based NCs for solution-processed PV devices, still remains nascent for CZTS NCs-based solar cells.

As mentioned in the introduction, pinpointing the optimum colloidal synthetic route for environmentally friendly and earth-abundant NCs with a narrow size-distribution, well-controlled shape, uniform morphology and precisely-manipulated composition represents a challenging task. Many of the reported synthetic procedures for FeS₂, Cu₂S, SnS and SnSe have provided NCs with a broad size-distribution, unacceptably controlled shape and poorly passivated ligands on the surface as well as aggregation of the as-synthesized NCs in some cases in the worst scenario. The development of reliable high-quality oriented syntheses of the above-discussed NCs remains a challenge, and will require the use of persistent trial-and-error processes and a complete comprehension of the nucleation and growth mechanisms of NCs preparation. The exploration of diverse surfactants and novel precursors to fine-tune the reactivities of the precursors is of utmost importance to precisely control the size, shape and composition of the NCs. Regardless of the abundance of colloidal approaches developed for the production of CZTS NCs, the future research direction points to the colloidal approaches for CZTS NCs with one-dimensional geometries, such as nanorods and nanowires, and two-dimensional geometries, such as nanosheets. Compared to optimization of the NCs synthesis, the surface functionalization of NCs by ligand-exchange with small molecules (amines, thiols, acids, etc.) or by removal of the original ligands are far less studied, though these strategies have been proven successful for PbS and PbSe NCs-based PV devices.

In summary, environmentally friendly and earth-abundant colloidal NCs for PV applications have seen tremendous development in the past decade, with novel synthetic methods discovered and inventive device fabrication techniques contrived. The combination of non-toxicity, earth abundance, solution processing and high performance of this research system will ultimately become a leading technology for next-generation solar cells.

Conflicts of interest

The author declares no competing financial interests.

Notes and references

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