Recent advances, challenges and prospects in ternary organic solar cells

Congcong Zhao ^a, Jiuxing Wang *^a, Xuanyi Zhao ^a, Zhonglin Du *^a, Renqiang Yang *^b and Jianguo Tang *^a
aInstitute of Hybrid Materials, National Center of International Joint Research for Hybrid Materials Technology, National Base of International Sci. & Tech. Cooperation on Hybrid Materials, College of Materials Science and Engineering, Qingdao University, 308 Ningxia Road, Qingdao 266071, China. E-mail: jiuxingwang@qdu.edu.cn
^bKey Laboratory of Optoelectronic Chemical Materials and Devices (Ministry of Education), School of Chemical and Environmental Engineering, Jianghan University, Wuhan 430056, China. E-mail: yangrq@qibebt.ac.cn

First published on 23rd December 2020

---

---

Congcong Zhao

Congcong Zhao received his B.S. degree in Materials Science and Engineering from Liaocheng University in 2018. He is currently pursuing his M.S. degree at the National Center of International Joint Research for Hybrid Materials Technology of Qingdao University under the supervision of Dr Jiuxing Wang. His research interests are the design and synthesis of efficient organic and polymer materials in solar cells.

Jiuxing Wang

Jiuxing Wang received his B.S. degree in Polymer Materials and Engineering from Sichuan University in 2011 and Ph.D. degree in Chemical Engineering with Prof. Renqiang Yang from the University of Chinese Academy of Sciences in 2016. He has been an assistant professor at the National Center of International Joint Research for Hybrid Materials Technology of Qingdao University since 2016. His research interests focus on organic and polymer materials for photovoltaic applications.

Zhonglin Du

Zhonglin Du received his Ph.D. degree from East China University of Science and Technology (ECUST) in 2017 under the supervision of Professor Xinhua Zhong. He worked as a postdoctoral fellow with professor Dongling Ma at the Institut National de la Recherche Scientifique (INRS) of Canada from 2019 to 2021. Currently, he is an associate professor at the National Center of International Joint Research for Hybrid Materials Technology of Qingdao University. His main research interests include photoelectronic functional organic–inorganic hybrid materials and their application in solar energy conversion, especially photocatalysis and solar cells.

Renqiang Yang

Renqiang Yang received his Ph.D. degree from South China University of Technology in 2004 and then conducted his postdoctoral work at the University of California Santa Barbara with Prof. Guillermo C. Bazan. In 2009–2019, he worked as a full professor at Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences. Currently he is a full professor at the School of Chemical and Environmental Engineering, Jianghan University. His research interests include molecular design and synthesis of advanced organic opto-electronic materials and device applications thereof.

Jianguo Tang

Jianguo Tang received his Ph.D. degree from Shanghai Jiaotong University in 2000, after which he worked at the University of Bayreuth as a visiting professor (2001). Then he joined Prof. L. A. Belfiore's group at Colorado State University as a research fellow (2002–2003). Currently he is the director of the National Center of International Joint Research for Hybrid Materials at Qingdao University. His research interests include hybrid organic–inorganic luminescent materials and solar cells.

---

1. Introduction

Due to the increasing global consumption of fossil fuels and serious problems of energy shortage, exploiting renewable and environmentally friendly energy resources has become an urgent issue around the world. Solar energy is considered as an ideal alternative energy resource, due to its clean and rich reserve advantages. Silicon-based solar cells have already been commercialized, but complex manufacture and high cost limit their further applications. In the past decades, organic solar cells (OSCs) have become a hot-spot topic in photovoltaic fields owing to their unique advantages such as light weight, low cost, mechanical flexibility and solution processability.^1–4 The rapid development of various donor and acceptor materials greatly improves the PCEs of OSCs.^5–11

OSC device configurations include single-layer, bilayer, bulk-heterojunction (BHJ), tandem and ternary structures. The first single-layer organic photovoltaic cell (Fig. 1a) was reported by Kearns and Calvin in 1958.¹² The maximum power output was only 3 × 10⁻¹² W, due to low-efficiency charge separation. Thereafter, much research attention has been paid to this emerging field.^13–15 In 1986, Tang reported the first bilayer solar cell (Fig. 1b).¹⁶ However, limited interfacial area and insufficient charge transport restricted the PCE to 1%. In 1995, the concept of BHJ was introduced by Heeger and coworkers to overcome the difficulties of short exciton diffusion length and limited exciton lifetime.¹⁷ In a BHJ structure (Fig. 1c), both donor and acceptor materials are mixed together to form a bi-continuous interpenetrating network with large interfacial area for efficient exciton dissociation. To date, the BHJ structure has become the most widely used architecture in OSCs. With the rapid development of materials design and device optimization, the PCEs of single-junction OSCs have exceeded 17%, demonstrating great potential for large-area manufacture and commercialization in the future.¹⁸

Generally, the active layer in the OSC is very thin (∼100 nm), in order to achieve efficient charge collection. However, such a thin blend film leads to insufficient photon harvesting, which limits further enhancement in the performance of single-junction OSCs. To address this issue, several strategies focusing on how to absorb a broad range of the solar spectrum in solar cell devices have been developed. Constructing tandem (Fig. 1d) and ternary device structures (Fig. 1e) is an efficient strategy to extend light absorption and improve PCE. Tandem OSCs consist of two or more single-junction solar cells, which are linked either in series or in parallel.¹⁹ The individual sub-cells work independently to absorb photons in different ranges of the solar spectrum. To fully utilize sunlight, the wide- and low-bandgap sub-cells are usually used as front and rear cells, respectively. However, the complicated fabrication of tandem OSCs may increase the cost.²⁷

The active layer of binary OSCs is composed of one donor (D) and one acceptor (A). The working mechanism of binary OSCs can be summarized as follows. Firstly, the photoexcitation of photovoltaic materials in the active layer generates electron–hole pairs, known as excitons. Secondly, the excitons diffuse to the donor–acceptor interface. Thirdly, the excitons dissociate at the donor–acceptor interface to form free electrons and holes. Finally, free holes transport to the anode through the donor material and free electron transport to the cathode through the acceptor material. Ternary OSCs have a composition of either one donor/two acceptors (D:A₁:A₂) or two donors/one acceptor (D₁:D₂:A) in the active layer. In recent years, ternary OSCs have emerged as a competitive candidate to achieve high photovoltaic performances. As shown in Fig. 2, the ternary OSCs usually outperform their binary counterparts. Until now, various third components including polymers, fullerene derivatives and small molecules have been used in ternary OSCs. Compared with binary OSCs, ternary OSCs have distinct advantages. (i) The third component can enhance the absorption, which is favourable for increasing the short circuit current density (J_sc).²⁸ However, some studies indicated that it is not always the case unless the ternary blend has quite narrow absorption.²⁹ (ii) The third component can also increase exciton dissociation and charge mobility by building efficient charge transport pathways, resulting in high fill factors (FF) in OSCs.³⁰ (iii) The morphologies of active layers in ternary OSCs could be optimized by the addition of the third component.³¹ (iv) The V_oc in ternary OSCs varies as a function of the blend ratio, and it could be more easily and finely tuned than that in binary devices.^32,33 The strategies for improving the photovoltaic performance of binary OSCs, such as tuning energy levels, adjusting donor:acceptor blend ratios and controlling phase separation, can also be applied in ternary OSCs. However, the disadvantages of ternary OSCs should also be noted. For some ternary systems, the introduction of the third component might lead to a more complex morphology, which may give rise to difficulty in morphology control. The V_oc value of the ternary OSCs is in between those of the two original binary OSCs, lower than the higher one of the two binary OSCs. Despite these disadvantages, ternary OSCs show great potential for high-performance solar cells. The highest PCE of over 17% of ternary OSCs has been reported recently, showing great potential for constructing highly efficient OSCs.²¹

Typically, four working mechanisms could be involved in a ternary OSC: charge transfer, energy transfer, parallel-like model and alloy model. In the charge transfer mechanism (Fig. 3a), the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the third component are located between those of the host donor and acceptor, forming a cascade energy level alignment.³⁴ This cascade energy level can facilitate charge separation and charge transfer at the D:A interface because the third component can function as a charge relay for electron and hole transport.³⁵ As for the energy transfer mechanism (Fig. 3b), the third component does not directly generate free charges but transfers the energy to either a donor or an acceptor through Dexter or Förster resonance energy transfer (FRET). In some cases, the mechanisms of charge transfer and energy transfer can simultaneously exist in a ternary blend. In the parallel-like model (Fig. 3c), the third component forms its independent hole-transport channels.³⁶ In the alloy model (Fig. 3d), the third component electronically couples with the host donor to form a new charge transfer state.³⁷

According to the possible combinations of three components, all the ternary OSCs can be classified into four categories including polymer/small molecule/small molecule, polymer/polymer/small molecule, all-polymer and all-small-molecule types (Fig. 4). In this review, we have summarized the recent progress of ternary OSCs and analyzed the relationships among the molecular structure, active layer morphology and photovoltaic performance. Especially, we have emphasized the critical roles of the third component in ternary blends. Finally, the potential challenges and promising outlook for ternary OSCs are also discussed.

2. Recent advances in ternary organic solar cells

2.1 Polymer/small molecule/small-molecule OSCs

In this type of ternary OSC, a small molecule as a third component adding to the active layer can not only extend the absorption which is beneficial for harvesting more photons, but also improve the crystallinity of the polymer and tune active layer morphology through the effect of small molecules.^38–40 Two non-fullerene acceptors or mixed acceptors composed of a fullerene acceptor and an non-fullerene acceptor could be used in this type of ternary device, and thereby it can be classified into four categories, which are polymer donor/non-fullerene acceptor/fullerene acceptor, polymer donor/non-fullerene acceptor/non-fullerene acceptor, polymer donor/small-molecule donor/fullerene acceptor, and polymer donor/small-molecule donor/non-fullerene acceptor types.

2.2. Polymer/polymer/small-molecule OSCs

Due to the narrow absorption spectra of conjugated polymers, the use of two donor polymers can broaden the light absorption of the active layer and facilitate charge dissociation and transport via the cascade energy structure and optimized morphology.¹⁰⁴ Thus, much research work has been focused on the utilization of polymer donors as guest donors in fullerene-free ternary OSCs aiming at improving the performance of the binary blend system. The structures of donors and acceptors for polymer/polymer/small-molecule ternary OSCs are shown in Fig. 15, and the photovoltaic parameters are summarized in Table 5.

---

Zhang's group designed ternary OSCs by introducing a polymer donor J71 into a blend of PF2 and Y6.¹⁰⁵ They found that the E_losss of OSCs gradually decreased from 0.68 to 0.53 eV with increasing J71 content in donors, which may be an important factor for PCE improvement of ternary OSCs. The photoluminescence (PL) spectra (Fig. 16a) of the ternary blend showed that the PF2 emission intensity increased monotonically with the increase of J71 content, indicating the existence of energy transfer from J71 to PF2. The energy transfer can be further confirmed from the decreased 635 nm emission lifetime and increased 745 nm emission lifetime in blend films, as shown in time-resolved photoluminescence (TRPL) spectra (Fig. 16b). When the J71 content was up to 30% in donors, the J_sc increased from 23.17 to 24.97 mA cm⁻² and the FF increased from 62.35% to 64.70%. As a result, a PCE of 12.12% was obtained. The same group also introduced J71 as the second donor into the PM6:J71 binary blend to fabricate ternary OSCs.²⁴ PM6 and J71 may form an alloyed donor due to the good compatibility, which was beneficial for optimizing photon harvesting and phase separation of ternary active layers. When using TEM measurements to characterize the morphology of blend films, the fibrillar structures were found to be well maintained in ternary blend films with 20% J71 content, suggesting an ideal phase separation for sufficient charge transport. It was found that some excitons on J71 may transfer their energy onto PM6 and then are dissociated into free carriers at PM6/Br-ITIC interfaces. Consequently, a PCE of 14.13% was achieved in the optimized ternary OSCs with improved J_sc (19.39 mA cm⁻²) and FF (78.4%).

To study the morphological and mechanical stability of ternary OSCs, Ade et al. reported a highly efficient ternary OSC based on a non-fullerene acceptor IT-M and two polymer donors (FTAZ and PBDB-T).¹⁰⁶ Due to the rigid polymer chains of PBDB-T, the crystallization behavior of ITM was significantly suppressed in the FTAZ:PBDBT:IT-M (0.8:0.2:1) blend film even after thermal annealing at 180 °C. Consequently, the corresponding ternary device still retained 90% (Fig. 17) of the initial efficiency (13.2%) after a storage time of ∼1000 h in a glovebox. Another ternary OSC was fabricated by employing PTB7-Th as the donor, FOIC as the acceptor, and a PBDTm-T1 polymer donor as the third component.¹⁰⁷ Incorporating 20% PBDTm-T1 in the host PTB7-Th:FOIC blend led to more balanced charge mobilities (μ_h = 8.9 × 10⁻⁴ cm² V⁻¹ s⁻¹, μ_e = 6.6 × 10⁻⁴ cm² V⁻¹ s⁻¹). GIWAXS patterns showed that the incorporation of PBDTm-T1 had negligible influence on the π–π stacking and face-on orientation of the blends. As a result, an improved PCE of 13.8% with a decreased non-radiative recombination loss of 0.271 eV in ternary OSCs was achieved. Chen et al. designed a ternary blend based on two polymer donors of PTB7-Th and PBDB-T with comparable HOMO energy levels and a non-fullerene acceptor O-IDTBR, which exhibited a high V_oc of 1.02 V.¹⁰⁸ With an optimized ratio of 0.7:0.3:1.5 for a PTB7-Th:PBDB-T:O-IDTBR blend, the ternary device displayed a PCE of 11.58%. The ternary device retained good PCEs of 9.37% after 168 h thermal annealing at 85 °C, much higher than 6.27% and 5.33% for PTB7-Th:O-IDTBR and PBDB-T:O-IDTBR binary devices, respectively. In addition, the authors also studied the photovoltaic performances of larger area devices of 0.37, 0.57, and 0.91 cm². The corresponding devices exhibited PCEs of 10.12%, 9.51%, and 9.01%, respectively.

Ma et al. fabricated ternary OSCs based on PBDB-T:PTB7-Th:FOIC blends by blade coating in ambient environment.¹⁰⁹ Upon the addition of PTB7-Th into PBDB-T:FOIC blends, the crystallinity of FOIC decreased owing to the better miscibility of PTB7-Th and FOIC than that of PBDB-T and FOIC, which exhibited more efficient charge transport. Consequently, the optimal ternary device based on PBDBT:PTB7-Th:FOIC blends (0.5:0.5:1) provided the best PCE of 12.02% with a high FF of 65.78%. By incorporating 20% PTB7-Th into a PBDB-T:IEICO-4F binary blend, Tang et al. fabricated efficient ternary OSCs with PCEs as large as 11.62%.¹¹⁰ The combination of three materials can cover a large photon harvesting range from 300 to 1000 nm. The energy transfer between PBDB-T and PTB7-Th may enhance exciton utilization efficiency. Thus, the two factors resulted in a high J_sc of 24.14 mA cm⁻². Due to the similar HOMO energy levels of the two donors, the V_oc of 0.74 V for ternary OSCs can remain unchanged. Moreover, PTB7-Th can also act as a regulator to adjust the PBDB-T molecular arrangement for more efficient charge transport, and a high FF of 65.03% was obtained. Li et al. investigated ternary OSCs with PTB7-Th as the second donor in J51:ITIC blends.¹¹¹ When the blend ratio of J51:PTB7-Th:ITIC was 0.8:0.2:1, the ternary device achieved a PCE of 9.7% together with a J_sc of 17.75 mA cm⁻². The enhanced absorption in the wavelength region of ∼650–750 nm, reduced phase separation and energy transfer between J51 and PTB7-Th were the reasons for the improved J_sc and high efficiency. Peng et al. reported ternary OSCs based on PBDBT:PTB7-Th:SFBRCN blends.¹¹² Such ternary devices exhibited a broad composition tolerance over the whole D₁:D₂ weight ratios (from 1:0 to 0:1), due to the good miscibility and desirable nano-phase morphology of the two donor materials when blending with acceptors. To explore the device stability with different ternary compositions, the OSCs were treated thermally from room temperature to 140 °C. From the results, OSCs based on the ternary blend with a 0.5 ratio of PTB7-Th showed the best thermal stability (over 0.8 at 140 °C). At the optimized weight ratio of 0.3:0.7:0.8 for a PBDB-T:PTB7-Th:SFBRCN ternary blend, the resulting devices exhibited an improved V_oc of 0.93 V, a J_sc of 17.86 mA cm⁻² and an FF of 73.9%, delivering a high PCE of 12.27%.

Ameri et al. introduced PTB7-Th as the third component into a PDCBT:PC₇₁BM system.¹¹³ The high intermolecular affinity between PDCBT and PTB7-Th endowed the two polymer donors with a particular blend microstructure, which notably improved the charge transport mechanism. The resulting ternary device yielded a PCE of 10.12% with a high FF of 74%. Sun et al. fabricated semitransparent ternary OSCs using a wide bandgap polymer donor PBT1-S as the third component.¹¹⁴ PTB7-Th and PBT1-S showed complementary absorption, which contributed to extending the absorption of active layers. Moreover, the addition of 10% PBT1-S did not significantly change its average visible transmittance. The champion ternary devices showed a high PCE of 10.3%. A ternary system based on PTB7-Th:PffBT4T-2OD:PC₇₁BM was reported by Wang and coworkers.¹¹⁵ It was found that the energy transfer from PffBT4T-2OD to PTB7-Th enhanced light harvesting. After incorporating 15% PffBT4T-2OD, the blend film showed hierarchical phase separation within 25 nm and improved phase purity, which was beneficial for achieving sufficient exciton separation and charge transport. Therefore, an improved PCE of 10.72% and J_sc (19.02 mA cm⁻²) as well as FF (72.62%) were obtained in the ternary device. Fahlman et al. used two polymer donors PBDTTS-FTAZ and PTB7-Th to fabricate fullerene-based ternary OSCs.¹¹⁶ Due to the well-matched PL and absorption spectra of the two polymers, the energy transfer from PBDTTS-FTAZ to PTB7-Th was realized.

Moreover, adding PBDTTS-FTAZ into the ternary blend could promote the exciton dissociation of PC₇₁BM and the donors. The ternary device with 20% PBDTTS-FTAZ exhibited a PCE of 9.2%, a V_oc of 0.816 V, a J_sc of 16.4 mA cm⁻² and an FF of 67.3%. Kim et al. reported high-performance ternary OSCs by adding PPDT2CNBT into a PPDT2FBT:PC₇₁BM binary blend.¹¹⁷ The fluorine atoms on the benzothiadiazole moiety of PPDT2CNBT were replaced with nitrile groups, which overcame the poor solubility in processing solvents. The additional donor extended the light absorption of the active layer in a range of 700–850 nm and improved the V_oc because of its deep HOMO energy level. As a result, the best-performing devices showed a PCE of 9.46%, a J_sc of 18.7 mA cm⁻², a V_oc of 0.76 V, and an FF of 67%.

The use of two compatible polymer donors may suppress the crystallization behavior of acceptors, and the energy transfer between them will enhance the exciton utilization of the active layer. However, the introduction of the additional polymer donor usually leads to interrupted crystallization of the host donor. Consequently, controlling the interactions between two polymer donors should be further investigated.

2.3. All-polymer ternary OSCs

To overcome the limited sunlight absorption in most of the binary all-polymer solar cells, all-polymer ternary OSCs have been gradually developed in recent years. The introduction of the third component will lead to difficulty in controlling the morphology of ternary blend films. Therefore, careful selection of the ternary components is important for optimizing all the photovoltaic parameters. The structures of donors and acceptors for all-polymer ternary OSCs are shown in Fig. 18, and the photovoltaic parameters are summarized in Table 6.

---

Introducing a second polymer donor with appropriate absorption, energy levels and crystallinity into a binary all-polymer system is one of the promising strategies for achieving high-performance all-polymer ternary OSCs. In 2016, Li and coworkers incorporated a moderate bandgap polythiophene derivative PBDD-ff4T into a low bandgap PTB7-Th:N2200 blend to complement the absorption of the host blend.¹¹⁸ PBDD-ff4T also exhibited high crystallinity, which can promote charge transport and suppress geminate recombination in all-polymer ternary OSCs. The introduction of 10% PBDD-ff4T in the ternary blends significantly improved the μ_h from 5.47 × 10⁻⁴ to 1.23 × 10⁻³ cm² V⁻¹ s⁻¹, increasing the PCE from 5.9% to 7.2%. Another example based on PBDTTT-EF-T:N2200 and a wide bandgap polymer donor PCDTBT was reported by Ito et al.¹¹⁹ When the PCDTBT loading amount in the ternary blend was 10%, the EQEs at visible wavelengths were successfully increased up to 65–70%. In addition, the excitons of PCDTBT can be directly transported to PBDTTT-EF-T and N2200 via resonant Förster energy transfer, contributing to efficient charge generation. Therefore, the ternary device achieved an improved J_sc of 14.4 mA cm⁻² and a PCE of 6.65%. Wang and coworkers improved the photovoltaic performance of PTB7-Th:PNDI-T10-based all-polymer solar cells by introducing a high bandgap polymer PBDTTS-FTAZ as the second donor.¹²⁰ PBDTTS-FTAZ served as a sensitizer in the visible region of 450–650 nm, broadening the absorption of the ternary blend film. The PL spectrum of PBDTTS-FTAZ was found to overlap well with the absorption spectra of PTB7-Th and PNDI-T10, suggesting that a Förster energy transfer from PBDTTS-FTAZ to PTB7-Th and PNDI-T10 can occur at D/A interfaces and thereby improve charge generation in all-polymer ternary OSCs. These advantages enabled an outstanding PCE of 9.0% in the optimized 1:0.15:1 ternary device.

In 2018, Cao et al. performed a series of excellent research studies on all-polymer ternary OSCs. For example, they synthesized a wide bandgap polymer donor PBTA-Si to blend with a PTzBI-Si:N2200 system.¹²¹ PBTA-Si contained a difluorobenzotriazole building block with a siloxane-terminated side chain, which can be dissolved in green solvent 2-methyltetrahydrofuran (MeTHF) at room temperature. The good compatibility between PBTA-Si and PTzBI-Si could drive the molecular stacking of N2200 in the ternary blend, leading to reduced structure defects and enhanced charge transport. Therefore, the ternary blend with a ratio of 1:1:1 for PBTA-Si:PTzBI-Si:N2200 showed a slightly higher μ_e (1.02 × 10⁻³ cm² V⁻¹ s⁻¹) than the PBTA-Si:N2200 blend (μ_e = 8.76 × 10⁻⁴ cm² V⁻¹ s⁻¹) and the PTzBI-Si:N2200 blend (μ_e = 5.63 × 10⁻⁴ cm² V⁻¹ s⁻¹). The corresponding ternary devices exhibited a high PCE of 9.56% with an impressive FF of 75.7%. Further investigations showed that the all-polymer OSCs achieved a PCE of 9.17% with an active layer of 350 nm and maintained a PCE of 8.34% with a 420 nm thickness active layer. A wide bandgap copolymer PEG-2%, which had the benzodithiophene-alt-difluorobenzotriazole as the backbone and a polyethylene glycol (PEG) modified side chain, was designed by Cao et al. to fabricate all-polymer ternary OSCs.¹²² The flexible side chains made PEG-2% soluble in the green solvent of MeTHF. At the optimal weight ratio (1.4:0.6:1) of the PEG-2%:PTB7-Th:N2200 blend, the resulting device achieved the highest μ_h (9.51 × 10⁻⁴ cm² V⁻¹ s⁻¹) and μ_e (5.68 × 10⁻⁴ cm² V⁻¹ s⁻¹) with a balanced μ_h/μ_e of 1.67, contributing to its enhanced J_sc (16.89 mA cm⁻²) and favorable FF (66.13%). AFM measurements suggested that the incorporation of PTB7-Th increased the miscibility of PEG-2% and N2200, resulting in a reduced RMS roughness (0.90 nm) compared with the PEG-2%:N2200 blend film (RMS = 1.30 nm). As a result, the PEG-2%:N2200 binary device's PCE of 5.98% was increased to 9.27% in the PEG-2%:PTB7-Th:N2200 all-polymer ternary OSC. Cao's group used a narrow bandgap conjugated copolymer (PNTB) containing the naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) unit to blend with the PBTA-BO:N2200 film.¹²³ The increased weight ratio of PNTB gradually improved the EQE response in the 650–800 nm region, and the ternary blend film containing 30% PNTB exhibited the optimum EQE response, contributing to the highest J_sc of 15.77 mA cm⁻². Upon adding PNTB to a PBTA-BO:N2200 binary blend, a reduced RMS roughness of 1.4 nm was observed, demonstrating a smoother surface morphology than that of the binary blend film (RMS = 4.67 nm). A larger (100) crystal coherence length was also observed from ternary blends compared with two binary blends, indicating improved crystallinity. Such an optimized morphology enabled good charge transport of ternary blends, resulting in a high PCE of 10.09% and a desirable FF of 74.98%. Another high-performance all-polymer ternary OSC achieving a PCE of 10.12% and a high FF of 78% was reported by Cao et al.²⁵ The ternary systems comprised PTzBI:PBTA-BO donors and N2200 as an acceptor. AFM showed that the PTzBI:N2200 binary blend films exhibited granular aggregates with an RMS roughness of 1.37 nm, while the ternary blend films with 30% PBTA-BO formed smoother fibrous structures with a decreased RMS roughness of 1.29 nm. In addition, when the weight ratio of PBTA-BO increased from 0% to 30%, a strong face-on orientation was observed in GIWAXS patterns, which can help reduce the structural disorder and thereby enhance the charge transport. Both enhanced charge mobility and the desirable morphology accounted for the superior PCE and FF.

In 2019, Han and coworkers reported a high-performance all-polymer ternary OSC using PTB7-Th as an additional polymer donor to blend with a J51:N2200 system.¹²⁴ The EQE of the ternary blend exhibited a dramatic improvement in the range from 640 to 850 nm, suggesting that the addition of PTB7-Th contributed to the light absorption in the long-wavelength regions. The increased PTB7-Th content in the ternary blend may enhance the crystallinity of N2200, resulting in the improvement of electron transport. At a ratio of 1.4:0.6:1 for J51:PTB7-Th:N2200, the ternary blend film exhibited balanced electron–hole mobility (μ_h = 3.54 × 10⁻⁴ cm² V⁻¹ s⁻¹, μ_e = 3.17 × 10⁻⁴ cm² V⁻¹ s⁻¹), which was consistent with an enhanced J_sc of 17.17 mA cm⁻² and an FF of 66.3%, and hence the PCE reached 9.60%. More importantly, the authors used a green solvent cyclopentyl methyl ether (CPME) to fabricate the ternary devices, which was profitable for practical applications.

Han et al. chose PCDTBT as a sensitizer of PTB7-Th:N2200 binary blend to fabricate ternary OSCs.¹²⁵ The HOMO and LUMO energy levels of PCDTBT were located between those of PTB7-Th and N2200, resulting in an energy cascade level for efficient charge transfer. The addition of PCDTBT also broadened the absorption of PTB7-Th:N2200 in the 300–420 nm range, which enhanced light harvesting. More importantly, the interaction between PTB7-Th and PCDTBT promoted the aggregation of PTB7-Th, leading to decreased domain sizes and enlarged D/A interfaces. As a result, the ternary device with a 15% PCDTBT content gave a PCE of 5.11%, a V_oc of 0.78 V, a J_sc of 11.44 mA cm⁻², and an FF of 56%. Liu et al. developed a novel principle of dual FRET in a PTB7-Th:PF12TBT:P(NDI2OD-T2) system, in which the third component (PF12TBT) could transfer energy to both the donor and acceptor.¹²⁶ The photocurrent generation process for the PTB7-Th:P(NDI2OD-T2):PF12TBT blend system is illustrated in Fig. 19. The introduction of PF12TBT could provide a nucleus to induce more and smaller microcrystals in the blend film, leading to decreased phase separation for efficient photon absorption and exciton dissociation. In addition, 3-hexylthiophene was selected as a solvent additive to improve the dispersity of PF12TBT, enabling efficient dual FRET. The corresponding ternary devices yielded a PCE of 6.07%, a V_oc of 0.81 V, a J_sc of 12.28 mA cm⁻², and an FF of 61%.

He et al. reported an efficient all-polymer ternary OSC by introducing a chlorinated polymer donor PBClT as the third component into PTB7-Th:NDP-V-C7 blend films.¹²⁷ Compared with PTB7-Th, the LUMO and HOMO energy levels of PBClT decreased due to the strong electrophilic effect of the two chlorine atoms. The formed cascade energy levels were beneficial for charge transfer. Moreover, the introduction of PBClT could also enhance miscibility and suppress crystallization in ternary blends, resulting in a favorable film morphology. As a result, the best-performing device with 15% PBClT exhibited a PCE of 9.03%, a V_oc of 0.78 V, a J_sc of 16.77 mA cm⁻², and an FF of 68.07%.

To improve the performance of all-polymer ternary OSCs, wide bandgap polymer donors are often incorporated into the host blend to complement the absorption of narrow bandgap polymer acceptors. But all-polymer blend films often exhibit large phase separation, which may lead to decreased FFs of the resulting devices. Therefore, improving the miscibility of polymers is a crucial issue in all-polymer systems.

2.4. All-small-molecule ternary OSCs

Compared with all-polymer OSCs, all-small-molecule OSCs have attracted increasing attention and achieved significant improvement due to the unique features of small molecules, such as their well-defined structure and molecular weight, high purity, and less batch-to-batch variation.^128–133 The structures of donors and acceptors for all-small-molecule ternary OSCs are shown in Fig. 20, and the photovoltaic parameters are summarized in Table 7.

---

In 2016, Deng et al. fabricated a series of all-small-molecule ternary OSCs with DRCN5T and DR3TSBDT as donors and PC₇₁BM as the acceptor.¹³⁴ DRCN5T exhibited a wider absorption range than DR3TSBDT, showing a complementary absorption in the range from 300 nm to 800 nm. The two donors with good compatibility preferred to form an alloy state, which would provide efficient hole transport channels in ternary active layers. Moreover, the nano-scale phase separation and bi-continuous interpenetrating network were well maintained in the optimized ternary blend films, which were beneficial for efficient exciton dissociation and charge transport, resulting in improved J_sc and FF. By incorporating 30% DR3TSBDT in a DRCN5T:PC₇₁BM blend, a champion PCE of 10.16% was obtained with a J_sc of 16.5 mA cm⁻², an FF of 67.0% and a V_oc of 0.92 V. Another all-small-molecule ternary OSC consisting of DR3TBDTT and DR3TBDTT-E as donors and PC₇₁BM as the acceptor was fabricated by Wei and coworkers.¹³⁵ The energy levels of DR3TBDTT-E were lowered due to the introduction of ester functional side chains, and thus energy cascades were formed in the LUMO and HOMO energy levels of three compounds, which was beneficial for charge transfer. As shown in Fig. 21, after thermal and solvent vapor annealing (TSA), the alloy-like model transformed to a cascade model because DR3TBDTT-E shifted from the mixed region to D/A interfaces, which facilitated the charge separation process. At the optimized ratio of DR3TBDTT:DR3TBDTT-E:PC₇₁BM (0.9:0.1:0.8), the ternary solar cell exhibited a maximum PCE of 10.26%.

In 2019, the same group synthesized a small molecule DR3TBDTT-S-E as a donor to blend with DR3TBDTT:PC₇₁BM and DCAO3TBDTT:IDIC systems, respectively.¹³⁶ They found that two ternary devices presented a much higher PCE over 10%, but the related mechanisms are different. In a DR3TBDTT:DR3TBDTT-S-E:PC₇₁BM system, DR3TBDTT-S-E could serve as a charge relay to facilitate hole extraction from PC₇₁BM to DR3TBDTT, derived from the cascade energy levels of three materials, while in a DCAO3TBDTT:DR3TBDTT-S-E:IDIC system, the ternary blend film exhibited a mixed face-on and edge-on orientation by incorporating 5% DR3TBDTT-S-E, indicating that the formed 3D charge pathways could promote efficient charge transport. A small molecule DPP-2TTP containing porphyrin and diketopyrrolopyrrole units was designed by Zhu et al. to fabricate ternary OSCs.²⁶ As expected, the DPP-2TTP exhibited a very strong absorption in ranges of 400–550 nm and 700–900 nm, and the weak absorption in the range of 550–650 nm was compensated by a wide-bandgap small molecule DR3TBDTTF. The large overlap between the emission spectrum of DR3TBDTTF and the absorption spectrum of DPP-2TTP suggested efficient energy transfer from DR3TBDTTF to DPP-2TTP, which was helpful in achieving high photocurrents in ternary OSCs. Moreover, the addition of 20% DR3TBDTTF in a DPP-2TTP:DR3TBDTTF:PC₇₁BM blend film can slightly reduce the RMS from 2.35 to 2.08 nm; such a smoother surface was beneficial for more efficient exciton dissociation and charge transport. Thus the corresponding ternary device showed a remarkable PCE of 11.15% with a significant FF of 77.1%, which was much better than that the DPP-2TTP:PC₇₁BM binary device (PCE = 9.30%).

Cao's group reported a highly efficient ternary OSC using two porphyrin derivatives (DPPEZnP-TBO and DPPEZnP-BzTBO) as donors and PC₆₁BM as the acceptor.¹³⁷ By replacing the alky-thiophene with alky benzothiophene units at two of the porphyrin meso positions, DPPEZnP-BzTBO showed deeper HOMO and LUMO energy levels than DPPEZnP-TBO, forming a cascade structure to facilitate charge transportation. With 20% incorporation of DPPEZnP-TBO, the EQE values of the ternary devices improved significantly in the region of 630–810 nm and up to 65% in the 700–800 nm wavelength range. Compared with DPPEZnP-BzTBO:PC₆₁BM binary devices, the μ_h of the resulting ternary device was enhanced from 3.93 × 10⁻⁴ to 5.52 × 10⁻⁴ cm² V⁻¹ s⁻¹, and the μ_h was slightly enhanced from 2.38 × 10⁻⁴ cm² V⁻¹ s⁻¹ to 2.53 × 10⁻⁴ cm² V⁻¹ s⁻¹, contributing to the higher J_sc and FF. As a result, the ternary device achieved a PCE up to 10.17% with an improved J_sc of 18.60 mA cm² and an FF of 70.10%.

Liu et al. incorporated a small molecule DIB-SQ into a BTR:PC₇₁BM system.¹³⁸ The 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIB-SQ) was selected as the third component to enhance photon harvesting of the active layer due to its high extinction coefficient of 4.5 × 10⁵ M⁻¹ cm⁻¹ in the long wavelength range. The PL emission intensity was obviously decreased along with increasing contents of DIB-SQ, indicating efficient energy transfer from BTR to DIB-SQ. The incorporation of 6% DIB-SQ decreased the crystallinity of BTR in the ternary blend film, forming an optimized morphology with a fibrillar structure. Therefore, balanced charge transport (μ_h/μ_e = 1.07) can be achieved in the ternary active layer. The optimized ternary device with 6% DIB-SQ in the active layer yielded a champion PCE of 10.3% with a J_sc of 15.44 mA cm⁻², a V_oc of 0.90 V and an FF of 73.8%. Tang et al. selected a high-crystallinity small molecule BTR as the second donor and morphology regulator to prepare ternary OSCs.¹³⁹ The EQE values of ternary devices improved in the whole spectral range due to enhanced photon harvesting in the short wavelength range on mixing 1.5% BTR. The crystal BTR may induce the ordered packing of DRCN5T in a DRCN5T:BTR:PC₇₁BM ternary blend, leading to the uniform nano-fibrillar structure for efficient charge transport. Consequently, more balanced charge mobility with a μ_h/μ_e of 1.12 was obtained in the ternary active layer, resulting in an increased FF. The optimal ternary device achieved a PCE of 10.05% with a J_sc of 16.54 mA cm⁻², a V_oc of 0.90 V and an FF of 67.5%.

Zhu et al. improved the performance of all-small-molecule ternary OSCs through controlling the morphology of blend films.¹⁴⁰ In their work, a crystalline compound DPP(TBFu)₂ was selected as the third component to blend with a DPPEZnP-THE:PC₆₁BM system. The addition of DPP(TBFu)₂ could lead to phase separation with PC₆₁BM due to accelerated crystallization under thermal annealing, which enhanced the crystallinity and domain purity of the host donor. The ternary device with a 20% DPP(TBFu)₂ content showed the best PCE of 7.1% with a V_oc of 0.79 V, a J_sc of 14.5 mA cm⁻², and an FF of 62%.

Laquai et al. revealed the complex photophysics of DR3:ICC6:PC₇₁BM-based ternary OSCs for the first time.¹⁴¹ It was found that the excitation of donor DR3 or acceptor ICC6 resulted in fast charge transfer, while the excitation of PC₇₁BM led to fast singlet energy transfer to ICC6. The incorporation of PC₇₁BM increased the crystallinity of ICC6, resulting in an improved charge carrier mobility. This improvement enhanced the EQE across the entire absorption range of the ternary blend. The optimized ternary devices yielded an average PCE of 10.8%, with an FF of 72%, a V_oc of 0.87 V, and a J_sc of 16.3 mA cm⁻².

As discussed above, the introduction of high-crystallinity small molecules could induce the ordered molecular packing of the host donor, leading to the formation of a uniform fibrillar structure for efficient charge transport in the ternary blend film. However, small molecules would prefer to form large crystalline domains due to the strong molecular interactions, which is unfavorable for exciton dissociation and charge transport. To optimize the morphology of ternary blends, manipulating the complicated molecular interactions plays a key role for enhanced efficiencies.

3. Challenges and strategies

Although ternary OSCs have achieved remarkable PCEs, there are still some key issues and challenges that should be noted and addressed to further enhance the photovoltaic performance.

(i) Designing and synthesizing new donor materials are crucial to promote the performance of ternary OSCs. Narrow bandgap non-fullerene acceptors have strong absorption in the long wavelength region (700–950 nm) with high absorption coefficients. Devices based on non-fullerene acceptors have exhibited very impressive photovoltaic performance with PCEs over 17% in binary solar cells. Therefore, there is an urgent need to develop high-performance medium bandgap and wide bandgap donor materials having matched energy levels and complementary absorption with non-fullerene acceptors.

(ii) Increasing the miscibility of each component is required for optimizing the active layer morphology, which is vital to photovoltaic performance. Due to the high crystallinity of non-fullerene acceptors, the tendency to achieve severe phase separation may prevent efficient exciton dissociation and charge transport. Furthermore, a poor miscibility between polymers and small molecules would limit the composition tolerance of ternary systems. These issues could be addressed through the rational design of photovoltaic materials and the optimization of processing conditions.

(iii) Investigating the V_oc behavior of ternary devices is of equal importance. In a binary OSC, the V_oc is related to the difference between the HOMO energy level of the donor and the LUMO energy level of the acceptor. But it will become complicated when it comes to ternary solar cells. The V_oc of a ternary device is dependent on the ratios of used materials and is sensitive to the morphology of the ternary blend. More studies are needed to provide a deep understanding of the V_oc for ternary solar cells.

(iv) Compared with the numerous studies on improving the PCE of OSCs, the research on device stabilities has attracted much less attention. Achieving long-term stability is a crucial issue for realizing commercial applications. Although some studies have reported better stabilities of ternary OSCs than binary devices, there is still a long way to go to achieve long-term stabilities. To improve the device stabilities, the following aspects need to be paid attention. (a) Enhance the chemical stabilities of photovoltaic materials, such as the resistance to photodegradation and heat degradation.¹⁴² (b) Improve the morphological stability of the active layer, such as cross-linking to increase intermolecular interactions which may stabilize the active layer morphology.¹⁴³ (c) Make the encapsulation technology of OSC devices perfect to avoid exposure to oxygen and/or water vapor.¹⁴⁴

To realize commercial applications of OSCs, other challenges also need to be focused on. To date, manufacturing large-area OSC devices with high PCE has been a huge challenge.^145–147 The efficiency of large area devices is much lower than that of small area counterparts. Roll-to-roll compatible printing techniques are usually used to fabricate large area OSCs, such as inkjet printing,¹⁴⁸ screen printing,¹⁴⁹ pad printing,¹⁵⁰ gravure printing,¹⁵¹ and offset printing.¹⁵² The processing techniques are of vital importance to enhance the efficiency of large area OSCs, and they need to be further improved to realize competitive PCEs with small area counterparts. To achieve commercialization, it is also a big challenge to enhance the thickness tolerance of active layers transforming from the lab-scale to industry-scale.

4. Conclusions and prospects

Overall, the recent advances in ternary OSCs based on polymer/small molecule/small molecule, polymer/polymer/small molecule, all-polymer and all-small-molecule classifications were thoroughly studied. The characteristics of each of the four categories of ternary OSCs were systematically summarized and analyzed. The ternary strategy is an efficient approach to enhance the photovoltaic performance of binary OSCs. Incorporating the third component into a binary blend can broaden the absorption spectra, facilitate exciton dissociation and charge transport, as well as adjust the morphology of the blend. As a result, the device performance and stability can be simultaneously enhanced. The main advantages of utilizing the ternary strategy in OSCs are listed as follows.

Firstly, the available solar spectrum for OSCs usually ranges from ultraviolet to near-infrared, and a wider spectral coverage is beneficial for obtaining a higher J_sc. Compared with binary OSCs, the introduction of the third component into ternary OSCs could easily broaden the absorption and improve the light harvesting ability, which is beneficial for enhancing the J_sc. For example, introducing the third component with a wide bandgap may enhance absorption in the short-wavelength region, and introducing the third component with a narrow bandgap may enhance absorption in the long-wavelength region.

Secondly, cascade charge transfer is an important process in ternary OSCs. When the third component with suitable energy levels is introduced into a binary blend to form a cascade energy alignment, it is possible for the third component to transfer holes and electrons to the host donor and acceptor, respectively. As a result, efficient charge transfer and suppressed charge recombination could be realized.

Thirdly, morphology control has a significant impact on the V_oc, J_sc, and FF, which determine the photovoltaic performance of OSCs. The third component could act as a morphological modulator to improve miscibility and compatibility with the binary blend, resulting in better stability, optimized phase separation, and appropriate crystallinity of the ternary blend. Moreover, the crystallization, domain size, and purity can easily be tuned by controlling the weight ratio of the third component.

Several key issues toward further development were also pointed out, including rational materials design, optimization of the active layer morphology, and in-depth understanding of complicated mechanisms in ternary systems. To further improve the device performance, efforts should focus on developing high-performance donors and acceptors, increasing the miscibility of the third component with the binary blend, and controlling the favorable morphology with better stability by device engineering. Owing to the advantages of broad and strong absorption, cascade energy levels, and easily adjustable morphology, ternary OSCs have great potential in the field of organic electronics. The ternary strategy could provide bright prospects to boost the development of OSCs for commercial applications.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by (1) the National Natural Science Foundation of China (51703104, 51773220, 51802169, and 52073122), the Shandong Provincial Natural Science Foundation (ZR2017BEM035), the China Postdoctoral Science Foundation (2017M612198), and the Qingdao Innovation Program Applied Basic Research Fund (18-2-2-8-jch), (2) the State Key Project of International Cooperation Research (2017YFE0108300 and 2016YFE0110800), (3) the Shandong Double-Hundred Project (2018), (4) the National Plan for Introducing Talents of Discipline to Universities (“111” plan), and (5) the 1st Class Discipline Program of Shandong Province.

Notes and references

1. C. Lee, S. Lee, G. U. Kim, W. Lee and B. J. Kim, Chem. Rev., 2019, 119, 8028–8086 CrossRef CAS .
2. Z. Genene, W. Mammo, E. Wang and M. R. Andersson, Adv. Mater., 2019, 31, 1807275 CrossRef .
3. Y. Cui, H. Yao, J. Zhang, T. Zhang, Y. Wang, L. Hong, K. Xian, B. Xu, S. Zhang, J. Peng, Z. Wei, F. Gao and J. Hou, Nat. Commun., 2019, 10, 2515 CrossRef .
4. P. Cheng, G. Li, X. W. Zhan and Y. Yang, Nat. Photonics, 2018, 12, 131–142 CrossRef CAS .
5. Y. Cui, H. Yao, L. Hong, T. Zhang, Y. Xu, K. Xian, B. Gao, J. Qin, J. Zhang, Z. Wei and J. Hou, Adv. Mater., 2019, 31, 1808356 CrossRef .
6. Y. Dong, Y. Zou, J. Yuan, H. Yang, Y. Wu, C. Cui and Y. Li, Adv. Mater., 2019, 31, 1904601 CrossRef CAS .
7. C. Zhao, J. Wang, J. Jiao, L. Huang and J. Tang, J. Mater. Chem. C, 2020, 8, 28–43 RSC .
8. H. Li, J. Wang, Y. Wang, F. Bu, W. Shen, J. Liu, L. Huang, W. Wang, L. A. Belfiore and J. Tang, Sol. Energy Mater. Sol. Cells, 2019, 190, 83–97 CrossRef CAS .
9. C. Yan, S. Barlow, Z. Wang, H. Yan, A. K. Y. Jen, S. R. Marder and X. Zhan, Nat. Rev. Mater., 2018, 3, 18003 CrossRef CAS .
10. N. Gasparini, A. Salleo, I. McCulloch and D. Baran, Nat. Rev. Mater., 2019, 4, 229–242 CrossRef .
11. D. Ding, J. Wang, Z. Du, F. Li, W. Chen, F. Liu, H. Li, M. Sun and R. Yang, J. Mater. Chem. A, 2017, 5, 10430–10436 RSC .
12. D. Kearns and M. Calvin, J. Chem. Phys., 1958, 29, 950–951 CrossRef CAS .
13. J. Wang, X. Bao, D. Ding, M. Qiu, Z. Du, J. Wang, J. Liu, M. Sun and R. Yang, J. Mater. Chem. A, 2016, 4, 11729–11737 RSC .
14. P. Bi and X. Hao, Sol. RRL, 2019, 3, 1800263 CrossRef .
15. J. Wang, M. Xiao, W. Chen, M. Qiu, Z. Du, W. Zhu, S. Wen, N. Wang and R. Yang, Macromolecules, 2014, 47, 7823–7830 CrossRef CAS .
16. C. W. Tang, Appl. Phys. Lett., 1986, 48, 183–185 CrossRef CAS .
17. G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 1995, 270, 1789–1791 CrossRef CAS .
18. Q. Liu, Y. Jiang, K. Jin, J. Qin, J. Xu, W. Li, J. Xiong, J. Liu, Z. Xiao, K. Sun, S. Yang, X. Zhang and L. Ding, Sci. Bull., 2020, 65, 272–275 CrossRef CAS .
19. L. Meng, Y. Zhang, X. Wan, C. Li, X. Zhang, Y. Wang, X. Ke, Z. Xiao, L. Ding, R. Xia, H. L. Yip, Y. Cao and Y. Chen, Science, 2018, 361, 1094–1098 CrossRef CAS .
20. R. Yu, H. Yao, Y. Cui, L. Hong, C. He and J. Hou, Adv. Mater., 2019, 31, 1902302 CrossRef .
21. L. Zhan, S. Li, T.-K. Lau, Y. Cui, X. Lu, M. Shi, C.-Z. Li, H. Li, J. Hou and H. Chen, Energy Environ. Sci., 2020, 13, 635–645 RSC .
22. T. Yan, J. Ge, T. Lei, W. Zhang, W. Song, B. Fanady, D. Zhang, S. Chen, R. Peng and Z. Ge, J. Mater. Chem. A, 2019, 7, 25894–25899 RSC .
23. S. Dai, S. Chandrabose, J. Xin, T. Li, K. Chen, P. Xue, K. Liu, K. Zhou, W. Ma, J. M. Hodgkiss and X. Zhan, J. Mater. Chem. A, 2019, 7, 2268–2274 RSC .
24. Q. An, J. Wang and F. Zhang, Nano Energy, 2019, 60, 768–774 CrossRef CAS .
25. Z. Li, L. Ying, R. Xie, P. Zhu, N. Li, W. Zhong, F. Huang and Y. Cao, Nano Energy, 2018, 51, 434–441 CrossRef CAS .
26. V. Piradi, X. Xu, Z. Wang, J. Ali, Q. Peng, F. Liu and X. Zhu, ACS Appl. Mater. Interfaces, 2019, 11, 6283–6291 CrossRef CAS .
27. H. Huang, L. Yang and B. Sharma, J. Mater. Chem. A, 2017, 5, 11501–11517 RSC .
28. Z. Hu, J. Wang, Z. Wang, W. Gao, Q. An, M. Zhang, X. Ma, J. Wang, J. Miao, C. Yang and F. Zhang, Nano Energy, 2019, 55, 424–432 CrossRef CAS .
29. N. Felekidis, E. Wang and M. Kemerink, Energy Environ. Sci., 2016, 9, 257–266 RSC .
30. X. Guo, D. Li, Y. Zhang, M. Jan, J. Xu, Z. Wang, B. Li, S. Xiong, Y. Li, F. Liu, J. Tang, C. Duan, M. Fahlman and Q. Bao, Org. Electron., 2019, 71, 65–71 CrossRef CAS .
31. J. Lee, S. M. Lee, S. Chen, T. Kumari, S. H. Kang, Y. Cho and C. Yang, Adv. Mater., 2019, 31, 1804762 CrossRef .
32. Z. Hu, F. Zhang, Q. An, M. Zhang, X. Ma, J. Wang, J. Zhang and J. Wang, ACS Energy Lett., 2018, 3, 555–561 CrossRef CAS .
33. P. P. Khlyabich, B. Burkhart and B. C. Thompson, J. Am. Chem. Soc., 2011, 133, 14534–14537 CrossRef CAS .
34. Q. An, F. Zhang, L. Li, J. Wang, Q. Sun, J. Zhang, W. Tang and Z. Deng, ACS Appl. Mater. Interfaces, 2015, 7, 3691–3698 CrossRef CAS .
35. B. Fan, W. Zhong, X.-F. Jiang, Q. Yin, L. Ying, F. Huang and Y. Cao, Adv. Energy Mater., 2017, 7, 1602127 CrossRef .
36. T. Liu, X. Xue, L. Huo, X. Sun, Q. An, F. Zhang, T. P. Russell, F. Liu and Y. Sun, Chem. Mater., 2017, 29, 2914–2920 CrossRef CAS .
37. Y. Chen, P. Ye, Z. G. Zhu, X. Wang, L. Yang, X. Xu, X. Wu, T. Dong, H. Zhang, J. Hou, F. Liu and H. Huang, Adv. Mater., 2017, 29, 1603154 CrossRef .
38. D. Baran, T. Kirchartz, S. Wheeler, S. Dimitrov, M. Abdelsamie, J. Gorman, R. S. Ashraf, S. Holliday, A. Wadsworth, N. Gasparini, P. Kaienburg, H. Yan, A. Amassian, C. J. Brabec, J. R. Durrant and I. McCulloch, Energy Environ. Sci., 2016, 9, 3783–3793 RSC .
39. H. Li, K. Lu and Z. Wei, Adv. Energy Mater., 2017, 7, 1602540 CrossRef .
40. R. Yu, H. Yao and J. Hou, Adv. Energy Mater., 2018, 8, 1702814 CrossRef .
41. W. Li, Y. Yan, Y. Gong, J. Cai, F. Cai, R. S. Gurney, D. Liu, A. J. Pearson, D. G. Lidzey and T. Wang, Adv. Funct. Mater., 2018, 28, 1704212 CrossRef .
42. T. Zhang, X. Zhao, D. Yang, Y. Tian and X. Yang, Adv. Energy Mater., 2018, 8, 1701691 CrossRef .
43. S. V. Dayneko, A. D. Hendsbee, J. R. Cann, C. Cabanetos and G. C. Welch, New J. Chem., 2019, 43, 10442–10448 RSC .
44. N. Y. Doumon, F. V. Houard, J. Dong, P. Christodoulis, M. V. Dryzhov, G. Portale and L. J. A. Koster, J. Mater. Chem. C, 2019, 7, 5104–5111 RSC .
45. A. Tang, B. Xiao, Y. Wang, F. Gao, K. Tajima, H. Bin, Z.-G. Zhang, Y. Li, Z. Wei and E. Zhou, Adv. Funct. Mater., 2018, 28, 1704507 CrossRef .
46. Z. G. Zhang, Y. Yang, J. Yao, L. Xue, S. Chen, X. Li, W. Morrison, C. Yang and Y. Li, Angew. Chem., Int. Ed., 2017, 56, 13503–13507 CrossRef CAS .
47. M. Nam, M. Cha, H. H. Lee, K. Hur, K. T. Lee, J. Yoo, I. K. Han, S. J. Kwon and D. H. Ko, Nat. Commun., 2017, 8, 14068 CrossRef CAS .
48. Z. Wang, Z. Peng, Z. Xiao, D. Seyitliyev, K. Gundogdu, L. Ding and H. Ade, Adv. Mater., 2020, 2005386 CrossRef CAS .
49. J. Yuan, Y. Q. Zhang, L. Y. Zhou, G. C. Zhang, H. L. Yip, T. K. Lau, X. H. Lu, C. Zhu, H. J. Peng, P. A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. F. Li and Y. P. Zou, Joule, 2019, 3, 1140–1151 CrossRef CAS .
50. T. Yan, W. Song, J. Huang, R. Peng, L. Huang and Z. Ge, Adv. Mater., 2019, 31, 1902210 CrossRef .
51. M.-A. Pan, T.-K. Lau, Y. Tang, Y.-C. Wu, T. Liu, K. Li, M.-C. Chen, X. Lu, W. Ma and C. Zhan, J. Mater. Chem. A, 2019, 7, 20713–20722 RSC .
52. H. Fu, C. Li, P. Bi, X. Hao, F. Liu, Y. Li, Z. Wang and Y. Sun, Adv. Funct. Mater., 2019, 29, 1807006 CrossRef .
53. Y. Xie, F. Yang, Y. Li, M. A. Uddin, P. Bi, B. Fan, Y. Cai, X. Hao, H. Y. Woo, W. Li, F. Liu and Y. Sun, Adv. Mater., 2018, 30, 1803045 CrossRef .
54. H. H. Gao, Y. Sun, X. Wan, X. Ke, H. Feng, B. Kan, Y. Wang, Y. Zhang, C. Li and Y. Chen, Adv. Sci., 2018, 5, 1800307 CrossRef .
55. W. T. Hadmojo, F. T. A. Wibowo, W. Lee, H. K. Jang, Y. Kim, S. Sinaga, M. Park, S. Y. Ju, D. Y. Ryu, I. H. Jung and S. Y. Jang, Adv. Funct. Mater., 2019, 29, 1808731 CrossRef .
56. Z. Liang, J. Tong, H. Li, Y. Wang, N. Wang, J. Li, C. Yang and Y. Xia, J. Mater. Chem. A, 2019, 7, 15841–15850 RSC .
57. C. Xue, T. Zhang, K. Ma, P. Wan, L. Hong, B. Xu and C. An, Macromol. Rapid Commun., 2019, 40, 1900246 CrossRef CAS .
58. H. Shi, R. Xia, G. Zhang, H.-L. Yip and Y. Cao, Adv. Energy Mater., 2019, 9, 1803438 CrossRef .
59. D. Huang, F. Bian, D. Zhu, X. Bao, C. Hong, P. Zhou, Y. Huang and C. Yang, J. Phys. Chem. C, 2019, 123, 14976–14984 CrossRef CAS .
60. W. Zhao, S. Li, S. Zhang, X. Liu and J. Hou, Adv. Mater., 2017, 29, 1604059 CrossRef .
61. Z. Xiao, X. Jia and L. Ding, Sci. Bull., 2017, 62, 1562–1564 CrossRef CAS .
62. H. Lu, J. Zhang, J. Chen, Q. Liu, X. Gong, S. Feng, X. Xu, W. Ma and Z. Bo, Adv. Mater., 2016, 28, 9559–9566 CrossRef CAS .
63. X. Xu, Z. Bi, W. Ma, G. Zhang, H. Yan, Y. Li and Q. Peng, J. Mater. Chem. A, 2019, 7, 14199–14208 RSC .
64. K. Weng, C. Li, P. Bi, H. S. Ryu, Y. Guo, X. Hao, D. Zhao, W. Li, H. Y. Woo and Y. Sun, J. Mater. Chem. A, 2019, 7, 3552–3557 RSC .
65. B. Wang, Y. Fu, Q. Yang, J. Wu, H. Liu, H. Tang and Z. Xie, J. Mater. Chem. C, 2019, 7, 10498–10506 RSC .
66. X. Du, T. Heumueller, W. Gruber, A. Classen, T. Unruh, N. Li and C. J. Brabec, Joule, 2019, 3, 215–226 CrossRef CAS .
67. T. Liu, Y. Guo, Y. Yi, L. Huo, X. Xue, X. Sun, H. Fu, W. Xiong, D. Meng, Z. Wang, F. Liu, T. P. Russell and Y. Sun, Adv. Mater., 2016, 28, 10008–10015 CrossRef CAS .
68. X. Song, N. Gasparini, M. M. Nahid, S. H. K. Paleti, J.-L. Wang, H. Ade and D. Baran, Joule, 2019, 3, 846–857 CrossRef CAS .
69. M. Zhang, W. Gao, F. Zhang, Y. Mi, W. Wang, Q. An, J. Wang, X. Ma, J. Miao, Z. Hu, X. Liu, J. Zhang and C. Yang, Energy Environ. Sci., 2018, 11, 841–849 RSC .
70. R. Yu, S. Zhang, H. Yao, B. Guo, S. Li, H. Zhang, M. Zhang and J. Hou, Adv. Mater., 2017, 29, 1700437 CrossRef .
71. L. Yang, W. Gu, L. Hong, Y. Mi, F. Liu, M. Liu, Y. Yang, B. Sharma, X. Liu and H. Huang, ACS Appl. Mater. Interfaces, 2017, 9, 26928–26936 CrossRef CAS .
72. W. Su, Q. Fan, X. Guo, X. Meng, Z. Bi, W. Ma, M. Zhang and Y. Li, Nano Energy, 2017, 38, 510–517 CrossRef CAS .
73. T. Ameri, P. Khoram, J. Min and C. J. Brabec, Adv. Mater., 2013, 25, 4245–4266 CrossRef CAS .
74. L. Lu, M. A. Kelly, W. You and L. Yu, Nat. Photonics, 2015, 9, 491–500 CrossRef CAS .
75. Y. Ma, X. Zhou, D. Cai, Q. Tu, W. Ma and Q. Zheng, Mater. Horiz., 2020, 7, 117–124 RSC .
76. J. Song, C. Li, L. Zhu, J. Guo, J. Xu, X. Zhang, K. Weng, K. Zhang, J. Min, X. Hao, Y. Zhang, F. Liu and Y. Sun, Adv. Mater., 2019, 31, 1905645 CrossRef CAS .
77. Y. Chang, T.-K. Lau, M.-A. Pan, X. Lu, H. Yan and C. Zhan, Mater. Horiz., 2019, 6, 2094–2102 RSC .
78. M. Zhang, R. Ming, W. Gao, Q. An, X. Ma, Z. Hu, C. Yang and F. Zhang, Nano Energy, 2019, 59, 58–65 CrossRef CAS .
79. R. Ma, Y. Chen, T. Liu, Y. Xiao, Z. Luo, M. Zhang, S. Luo, X. Lu, G. Zhang, Y. Li, H. Yan and K. Chen, J. Mater. Chem. C, 2020, 8, 909–915 RSC .
80. J. Gao, R. Ming, Q. An, X. Ma, M. Zhang, J. Miao, J. Wang, C. Yang and F. Zhang, Nano Energy, 2019, 63, 103888 CrossRef CAS .
81. Y. Chang, X. Zhang, Y. Tang, M. Gupta, D. Su, J. Liang, D. Yan, K. Li, X. Guo, W. Ma, H. Yan and C. Zhan, Nano Energy, 2019, 64, 103934 CrossRef CAS .
82. L. Wu, L. Xie, H. Tian, R. Peng, J. Huang, B. Fanady, W. Song, S. Tan, W. Bi and Z. Ge, Sci. Bull., 2019, 64, 1087–1094 CrossRef CAS .
83. R. Lv, D. Chen, X. Liao, L. Chen and Y. Chen, Adv. Funct. Mater., 2019, 29, 1805872 CrossRef .
84. F. Pan, L. Zhang, H. Jiang, D. Yuan, Y. Nian, Y. Cao and J. Chen, J. Mater. Chem. A, 2019, 7, 9798–9806 RSC .
85. X. Ma, M. Luo, W. Gao, J. Yuan, Q. An, M. Zhang, Z. Hu, J. Gao, J. Wang, Y. Zou, C. Yang and F. Zhang, J. Mater. Chem. A, 2019, 7, 7843–7851 RSC .
86. Z. Liu and N. Wang, J. Mater. Chem. C, 2019, 7, 10039–10048 RSC .
87. H. Jiang, X. Li, J. Wang, S. Qiao, Y. Zhang, N. Zheng, W. Chen, Y. Li and R. Yang, Adv. Funct. Mater., 2019, 29, 1903596 CrossRef .
88. H. Jiang, X. Li, Z. Liang, G. Huang, W. Chen, N. Zheng and R. Yang, J. Mater. Chem. A, 2019, 7, 7760–7765 RSC .
89. H. Huang, X. Li, S. Chen, B. Qiu, J. Du, L. Meng, Z. Zhang, C. Yang and Y. Li, J. Mater. Chem. A, 2019, 7, 27423–27431 RSC .
90. L. Zhong, H. Bin, Y. Li, M. Zhang, J. Xu, X. Li, H. Huang, Q. Hu, Z.-Q. Jiang, J. Wang, C. Zhang, F. Liu, T. P. Russell, Z. Zhang and Y. Li, J. Mater. Chem. A, 2018, 6, 24814–24822 RSC .
91. M. Zhang, Z. Xiao, W. Gao, Q. Liu, K. Jin, W. Wang, Y. Mi, Q. An, X. Ma, X. Liu, C. Yang, L. Ding and F. Zhang, Adv. Energy Mater., 2018, 8, 1801968 CrossRef .
92. P. Xue, Y. Xiao, T. Li, S. Dai, B. Jia, K. Liu, J. Wang, X. Lu, R. P. S. Han and X. Zhan, J. Mater. Chem. A, 2018, 6, 24210–24215 RSC .
93. T. Liu, R. Ma, Z. Luo, Y. Guo, G. Zhang, Y. Xiao, T. Yang, Y. Chen, G. Li, Y. Yi, X. Lu, H. Yan and B. Tang, Energy Environ. Sci., 2020, 13, 2115–2123 RSC .
94. J. Zhang, Y. Zhang, J. Fang, K. Lu, Z. Wang, W. Ma and Z. Wei, J. Am. Chem. Soc., 2015, 137, 8176–8183 CrossRef CAS .
95. T. Kumari, S. M. Lee, S.-H. Kang, S. Chen and C. Yang, Energy Environ. Sci., 2017, 10, 258–265 RSC .
96. R. Fan, L. Gu, X. Li, G. Fu and S. Yang, Org. Electron., 2017, 41, 209–214 CrossRef CAS .
97. X. Ma, F. Zhang, Q. An, Q. Sun, M. Zhang, J. Miao, Z. Hu and J. Zhang, J. Mater. Chem. A, 2017, 5, 13145–13153 RSC .
98. G. Zhang, K. Zhang, Q. Yin, X. F. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang and Y. Cao, J. Am. Chem. Soc., 2017, 139, 2387–2395 CrossRef CAS .
99. Y. Zhang, D. Deng, K. Lu, J. Zhang, B. Xia, Y. Zhao, J. Fang and Z. Wei, Adv. Mater., 2015, 27, 1071–1076 CrossRef CAS .
100. J. Fang, D. Deng, J. Zhang, Y. Zhang, K. Lu and Z. Wei, Mater. Chem. Front., 2017, 1, 1223–1228 RSC .
101. B. Xia, L. Yuan, J. Zhang, Z. Wang, J. Fang, Y. Zhao, D. Deng, W. Ma, K. Lu and Z. Wei, J. Mater. Chem. A, 2017, 5, 9859–9866 RSC .
102. M. Zhang, F. Zhang, Q. An, Q. Sun, W. Wang, J. Zhang and W. Tang, Nano Energy, 2016, 22, 241–254 CrossRef CAS .
103. X. Du, J. Zhao, H. Zhang, X. Lu, L. Zhou, Z. Chen, H. Lin, C. Zheng and S. Tao, J. Mater. Chem. A, 2019, 7, 20139–20150 RSC .
104. X. Liu, Y. Yan, Y. Yao and Z. Liang, Adv. Funct. Mater., 2018, 28, 1802004 CrossRef .
105. X. Ma, Q. An, O. A. Ibraikulov, P. Lévêque, T. Heiser, N. Leclerc, X. Zhang and F. Zhang, J. Mater. Chem. A, 2020, 8, 1265–1272 RSC .
106. H. Hu, L. Ye, M. Ghasemi, N. Balar, J. J. Rech, S. J. Stuard, W. You, B. T. O'Connor and H. Ade, Adv. Mater., 2019, 31, 1808279 CrossRef .
107. Y. Xie, T. Li, J. Guo, P. Bi, X. Xue, H. S. Ryu, Y. Cai, J. Min, L. Huo, X. Hao, H. Y. Woo, X. Zhan and Y. Sun, ACS Energy Lett., 2019, 4, 1196–1203 CrossRef CAS .
108. Z. Wang, Y. Nian, H. Jiang, F. Pan, Z. Hu, L. Zhang, Y. Cao and J. Chen, Org. Electron., 2019, 69, 174–180 CrossRef .
109. L. Zhang, X. Xu, B. Lin, H. Zhao, T. Li, J. Xin, Z. Bi, G. Qiu, S. Guo, K. Zhou, X. Zhan and W. Ma, Adv. Mater., 2018, 30, 1805041 CrossRef .
110. X. Ma, Y. Mi, F. Zhang, Q. An, M. Zhang, Z. Hu, X. Liu, J. Zhang and W. Tang, Adv. Energy Mater., 2018, 8, 1702854 CrossRef .
111. L. Zhong, L. Gao, H. Bin, Q. Hu, Z.-G. Zhang, F. Liu, T. P. Russell, Z. Zhang and Y. Li, Adv. Energy Mater., 2017, 7, 1602215 CrossRef .
112. X. Xu, Z. Bi, W. Ma, Z. Wang, W. C. H. Choy, W. Wu, G. Zhang, Y. Li and Q. Peng, Adv. Mater., 2017, 29, 1704271 CrossRef .
113. N. Gasparini, S. Kahmann, M. Salvador, J. D. Perea, A. Sperlich, A. Baumann, N. Li, S. Rechberger, E. Spiecker, V. Dyakonov, G. Portale, M. A. Loi, C. J. Brabec and T. Ameri, Adv. Energy Mater., 2019, 9, 1803394 CrossRef .
114. Y. Xie, L. Huo, B. Fan, H. Fu, Y. Cai, L. Zhang, Z. Li, Y. Wang, W. Ma, Y. Chen and Y. Sun, Adv. Funct. Mater., 2018, 28, 1800627 CrossRef .
115. F. Zhao, Y. Li, Z. Wang, Y. Yang, Z. Wang, G. He, J. Zhang, L. Jiang, T. Wang, Z. Wei, W. Ma, B. Li, A. Xia, Y. Li and C. Wang, Adv. Energy Mater., 2017, 7, 1602552 CrossRef .
116. C. Wang, W. Zhang, X. Meng, J. Bergqvist, X. Liu, Z. Genene, X. Xu, A. Yartsev, O. Inganäs, W. Ma, E. Wang and M. Fahlman, Adv. Energy Mater., 2017, 7, 1700390 CrossRef .
117. T. H. Lee, M. A. Uddin, C. Zhong, S.-J. Ko, B. Walker, T. Kim, Y. J. Yoon, S. Y. Park, A. J. Heeger, H. Y. Woo and J. Y. Kim, Adv. Energy Mater., 2016, 6, 1600637 CrossRef .
118. W. Su, Q. Fan, X. Guo, B. Guo, W. Li, Y. Zhang, M. Zhang and Y. Li, J. Mater. Chem. A, 2016, 4, 14752–14760 RSC .
119. H. Benten, T. Nishida, D. Mori, H. Xu, H. Ohkita and S. Ito, Energy Environ. Sci., 2016, 9, 135–140 RSC .
120. Z. Li, X. Xu, W. Zhang, X. Meng, Z. Genene, W. Ma, W. Mammo, A. Yartsev, M. R. Andersson, R. A. J. Janssen and E. Wang, Energy Environ. Sci., 2017, 10, 2212–2221 RSC .
121. B. Fan, P. Zhu, J. Xin, N. Li, L. Ying, W. Zhong, Z. Li, W. Ma, F. Huang and Y. Cao, Adv. Energy Mater., 2018, 8, 1703085 CrossRef .
122. Z. Li, B. Fan, B. He, L. Ying, W. Zhong, F. Liu, F. Huang and Y. Cao, Sci. China: Chem., 2018, 61, 427–436 Search PubMed .
123. Z. Li, R. Xie, W. Zhong, B. Fan, J. Ali, L. Ying, F. Liu, N. Li, F. Huang and Y. Cao, Sol. RRL, 2018, 2, 1800196 CrossRef .
124. Q. Zhang, Z. Chen, W. Ma, Z. Xie, J. Liu, X. Yu and Y. Han, ACS Appl. Mater. Interfaces, 2019, 11, 32200–32208 CrossRef CAS .
125. R. Zhang, H. Yang, K. Zhou, J. Zhang, J. Liu, X. Yu, R. Xing and Y. Han, J. Polym. Sci., Part B: Polym. Phys., 2016, 54, 1811–1819 CrossRef CAS .
126. J. Liu, B. Tang, Q. Liang, Y. Han, Z. Xie and J. Liu, RSC Adv., 2017, 7, 13289–13298 RSC .
127. H. Chen, Y. Guo, P. Chao, L. Liu, W. Chen, D. Zhao and F. He, Sci. China: Chem., 2018, 62, 238–244 CrossRef .
128. Z. Zhou, S. Xu, J. Song, Y. Jin, Q. Yue, Y. Qian, F. Liu, F. Zhang and X. Zhu, Nat. Energy, 2018, 3, 952–959 CrossRef CAS .
129. R.-Z. Liang, Y. Zhang, V. Savikhin, M. Babics, Z. Kan, M. Wohlfahrt, N. Wehbe, S. Liu, T. Duan, M. F. Toney, F. Laquai and P. M. Beaujuge, Adv. Energy Mater., 2019, 9, 1802836 CrossRef .
130. N. D. Eastham, A. S. Dudnik, B. Harutyunyan, T. J. Aldrich, M. J. Leonardi, E. F. Manley, M. R. Butler, T. Harschneck, M. A. Ratner, L. X. Chen, M. J. Bedzyk, F. S. Melkonyan, A. Facchetti, R. P. H. Chang and T. J. Marks, ACS Energy Lett., 2017, 2, 1690–1697 CrossRef CAS .
131. L. Yang, S. Zhang, C. He, J. Zhang, H. Yao, Y. Yang, Y. Zhang, W. Zhao and J. Hou, J. Am. Chem. Soc., 2017, 139, 1958–1966 CrossRef CAS .
132. Q. An, F. Zhang, J. Zhang, W. Tang, Z. Deng and B. Hu, Energy Environ. Sci., 2016, 9, 281–322 RSC .
133. H. Wang, J. Cao, J. Yu, Z. Zhang, R. Geng, L. Yang and W. Tang, J. Mater. Chem. A, 2019, 7, 4313–4333 RSC .
134. Q. An, F. Zhang, X. Yin, Q. Sun, M. Zhang, J. Zhang, W. Tang and Z. Deng, Nano Energy, 2016, 30, 276–282 CrossRef CAS .
135. Z. Wang, X. Zhu, J. Zhang, K. Lu, J. Fang, Y. Zhang, Z. Wang, L. Zhu, W. Ma, Z. Shuai and Z. Wei, J. Am. Chem. Soc., 2018, 140, 1549–1556 CrossRef CAS .
136. Y. Chang, Y. Chang, X. Zhu, X. Zhou, C. Yang, J. Zhang, K. Lu, X. Sun and Z. Wei, Adv. Energy Mater., 2019, 9, 1900190 CrossRef .
137. L. Xiao, T. Liang, K. Gao, T. Lai, X. Chen, F. Liu, T. P. Russell, F. Huang, X. Peng and Y. Cao, ACS Appl. Mater. Interfaces, 2017, 9, 29917–29923 CrossRef CAS .
138. M. Zhang, J. Wang, F. Zhang, Y. Mi, Q. An, W. Wang, X. Ma, J. Zhang and X. Liu, Nano Energy, 2017, 39, 571–581 CrossRef CAS .
139. M. Zhang, F. Zhang, Q. An, Q. Sun, W. Wang, X. Ma, J. Zhang and W. Tang, J. Mater. Chem. A, 2017, 5, 3589–3598 RSC .
140. W.-Y. Tan, K. Gao, J. Zhang, L.-L. Chen, S.-P. Wu, X.-F. Jiang, X.-B. Peng, Q. Hu, F. Liu, H.-B. Wu, Y. Cao and X.-H. Zhu, Sol. RRL, 2017, 1, 1600003 CrossRef .
141. S. Karuthedath, Y. Firdaus, R. Z. Liang, J. Gorenflot, P. M. Beaujuge, T. D. Anthopoulos and F. Laquai, Adv. Energy Mater., 2019, 9, 1901443 CrossRef .
142. T. Kim, R. Younts, W. Lee, S. Lee, K. Gundogdu and B. J. Kim, J. Mater. Chem. A, 2017, 5, 22170–22179 RSC .
143. P. Cheng and X. Zhan, Chem. Soc. Rev., 2016, 45, 2544–2582 RSC .
144. N. Kim, W. J. Potscavage, A. Sundaramoothi, C. Henderson, B. Kippelen and S. Graham, Sol. Energy Mater. Sol. Cells, 2012, 101, 140–146 CrossRef CAS .
145. Y. Lin, S. Dong, Z. Li, W. Zheng, J. Yang, A. Liu, W. Cai, F. Liu, Y. Jiang, T. P. Russell, F. Huang, E. Wang and L. Hou, Nano Energy, 2018, 46, 428–435 CrossRef CAS .
146. Y. Lin, C. Cai, Y. Zhang, W. Zheng, J. Yang, E. Wang and L. Hou, J. Mater. Chem. A, 2017, 5, 4093–4102 RSC .
147. C. Cai, Y. Zhang, R. Song, Z. Peng, L. Xia, M. Wu, K. Xiong, B. Wang, Y. Lin, X. Xu, Q. Liang, H. Wu, E. Wang and L. Hou, Sol. Energy Mater. Sol. Cells, 2017, 161, 52–61 CrossRef CAS .
148. C. N. Hoth, S. A. Choulis, P. Schilinsky and C. J. Brabec, Adv. Mater., 2007, 19, 3973–3978 CrossRef CAS .
149. F. C. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T. D. Nielsen, J. Fyenbo, K. Larsen and J. Kristensen, Sol. Energy Mater. Sol. Cells, 2009, 93, 422–441 CrossRef CAS .
150. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 484–490 CrossRef CAS .
151. J. M. Ding, A. de la Fuente Vornbrock, C. Ting and V. Subramanian, Sol. Energy Mater. Sol. Cells, 2009, 93, 459–464 CrossRef CAS .
152. D. Zielke, A. C. Hübler, U. Hahn, N. Brandt, M. Bartzsch, U. Fügmann, T. Fischer, J. Veres and S. Ogier, Appl. Phys. Lett., 2005, 87, 123508 CrossRef .

This journal is © The Royal Society of Chemistry 2021