Acceptor aggregation induced hole mobility degradation in polymer solar cells

Abstract

Charge carrier transport plays a critical role in determining the stability of organic solar cells (OSCs). It is widely acknowledged that changes in the morphology of donor and acceptor structural domains influence the degradation of hole and electron mobility. However, there has been limited research conducted on the temporal evolution and correlation between electron and hole mobility in highly mixed bulk heterojunction (BHJ) films. This research gap hampers our understanding of the mechanism behind charge carrier decay and impedes further improvements in device stability for OSCs. Here, we observed that the molecular conformation of electron acceptors significantly influences the evolution of hole mobility under different conditions, even when the same donor material and weight fraction are used. Notably, we find that the decay trends of electron mobility exhibit a cooperative behavior with those of hole mobility in both systems. Additionally, we observed that the variation in hole transport properties becomes less sensitive in donor-rich cases (with a weight ratio of 99 : 1), further confirming that the distinct decay trends of hole mobilities in PM6-based films are caused by the presence of acceptor molecules. Through morphological characterization, we have concluded that changes in acceptor aggregation during the aging process impact the distribution of polymer donors, thereby influencing the degradation of hole carriers. This research not only provides direct evidence of an acceptor-induced mechanism for hole carrier decay but also offers perspectives on improving the device stability of OSCs.

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Hang Yin

Hang Yin is a Professor at the School of Physics, Shandong University. He is also a member of the State Key Laboratory of Crystal Materials. He obtained his BS degree from Qufu Normal University and MS and PhD degrees from Hong Kong Baptist University. He was a postdoctoral fellow in Hong Kong Baptist University and Hong Kong Polytechnic University. His major research interest is in the physics and chemistry of thin film materials, including charge carrier transport and defect analysis of organic semiconductors, fabrication of organic solar cells and thin film transistors, and device physics for energy applications.

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Introduction

Stability is a main issue preventing the commercialization process of OSCs.^1–3 Currently, people usually adopt either morphology or molecular structure evolution models to describe the device degradation process under various exposure conditions.^3–9 Excessive molecular aggregation and crystallization,^4,10–13 as well as structural changes in the mixing phase of BHJ films,^10–12 are significant factors contributing to the performance degradation of OSCs. Moreover, chemical reactions (including oxidation and photocatalytic reactions) can lead to device degradation by causing damage to organic molecules.^8,14–16 However, the understanding of the degradation process in highly disordered BHJ films is still incomplete,^17–19 as the donor and acceptor domains create independent pathways for hole and electron transport.²⁰ Due to the blend nature of BHJ films, modifying one material inevitably affects the other, causing unforeseen impacts on the carrier transport behaviors. Hence, elucidating the interaction between donors and acceptors in BHJ films is of paramount significance for advancing our understanding and enhancing the stability of organic photovoltaic (OPV) devices.

Charge carrier mobility is one of the most important parameters to evaluate the decay process of OSCs.^3,21–23 In order to address the limited stability of charge carrier mobility in organic systems, the concept of dimerized small molecule acceptors (DSMAs) has been introduced. DSMA design aims to achieve high crystallinity and collectivity by combining the high electron mobility of small molecule acceptors (SMAs) with the mechanical robustness of polymerized small-molecule-acceptors (PSMAs). This approach enables the development of highly stable OPVs.²⁴ Meanwhile, donor polymers with high crystallinity exhibit extensive intermolecular interactions and stable polymeric chains, which promote hole transport and mitigate mobility degradation. These characteristics significantly enhance the stability of OSCs.²⁵ However, the existing studies focusing solely on the impact of donor materials on hole transport stability or acceptor materials on electron transport stability are insufficient to fully understand the degradation mechanism of BHJ films with mixed-phase structures.²⁶ Therefore, it is of great significance to investigate the interactions between donor and acceptor materials on such stability issues.

Herein, we investigated the hole transport degradation of PM6:Y6 and PM6:PY-IT BHJ films, and observed that the selection of acceptor materials can significantly affect the hole transport properties in PM6-based OSCs. The hole mobility of the PM6:PY-IT case is more stable than that of the PM6:Y6 counterpart. However, there was no obvious difference in the hole mobility decay when 1 wt% of acceptors were blended into PM6, which further confirms that the hole mobility decay can be significantly contributed by acceptor materials. Grazing incidence wide angle X-ray scattering (GIWAXS) and photo-induced force microscopy (PiFM) were performed to evaluate the morphological differences of the blend films before and after the thermal aging process. The results indicate that small molecule acceptors exhibit significant aggregation phenomena and tend to generate extra vacancies for molecular migration during the thermal aging process,²³ which markedly disrupts the original distribution of polymer donors and adversely affects the stability of hole mobilities.²⁷ Conversely, the polymer acceptor system demonstrates stable molecular stacking, and the PM6:PY-IT device exhibits a stronger ability to retain a trap-free state and desirable thermal stability compared to the PM6:Y6 counterpart. This work not only provides direct evidence of the acceptor-induced hole carrier decay mechanism, but also supports the perspective of improving the device stability of OSCs.

Results and discussion

Fig. 1a and b illustrate the chemical structures and energy levels of organic materials selected in this work. Fig. 1c shows the film morphology evolution mechanism before and after thermal aging of PM6:Y6 and PM6:PY-IT systems, and a detailed interpretation is provided in the following section.

Fig. 1 (a) The chemical structures of PM6, Y6, and PY-IT. (b) The corresponding energy levels. (c) Morphological changing mechanism of PM6:Y6 and PM6:PY-IT films during thermal aging.

Hole-only and electron-only devices were fabricated to evaluate the charge carrier transport behaviors with the structures of ITO/PEDOT:PSS/active layer/spiro-TPD/Au for holes and ITO/Al/active layer/PDINN/Ag for electrons. Fig. 2b shows normalized hole mobilities of PM6:Y6 and PM6:PY-IT BHJ films as a function of time (absolute values shown in Fig. S1† and raw data shown in Fig. 2a). After a 300-hour thermal aging at 60 °C, the hole mobility of the PM6:Y6 system decreased from 1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ to 2.3 × 10⁻⁹ cm² V⁻¹ s⁻¹, while that of the PM6:PY-IT system decreased from 1.7 × 10⁻⁴ cm² V⁻¹ s⁻¹ to 7.0 × 10⁻⁷ cm² V⁻¹ s⁻¹. Additionally, the electron mobility of the PM6:PY-IT film under the same thermal conditions decreased from 4.8 × 10⁻⁵ cm² V⁻¹ s⁻¹ to 1.2 × 10⁻⁶ cm² V⁻¹ s⁻¹, which is more stable compared to that of the PM6:Y6 cell (from 1.3 × 10⁻⁴ cm² V⁻¹ s⁻¹ to 9.6 × 10⁻⁹ cm² V⁻¹ s⁻¹) (see Fig. S2†). To determine whether the difference in mobility degradation arises from the acceptor, we reduced the concentration of the acceptor in the blend system to 1 wt% and repeated the same measurement. The hole mobility of the PM6:1% Y6 system decreased from 3.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ to 6.6 × 10⁻⁷ cm² V⁻¹ s⁻¹ after 300 hours of thermal aging, while that of the PM6:PY-IT system decreased from 3.9 × 10⁻⁵ cm² V⁻¹ s⁻¹ to 1.8 × 10⁻⁶ cm² V⁻¹ s⁻¹ as shown in Fig. 2d (with original data shown in Fig. S3† and raw data shown in Fig. 2c). These two systems exhibited similar degradation in hole mobility, which suggests that the reactive force of acceptor degradation influences donor degradation in D : A blend films.

Fig. 2 (a) and (c) J × d in response to the applied electric field in response to the applied electric field for PM6:Y6 and PM6:PY-IT systems before and after thermal aging. (a) D : A = 1 : 1.2 and (c) D : A = 99 : 1. (b) and (d) Normalized hole mobility as a function of thermal aging time. (b) D : A = 1 : 1.2 and (d) D : A = 99 : 1.

We further analyzed the morphology of BHJ films based on Y6 and PY-IT in order to explore the heterogeneous interactions of active layers during thermal aging and identify the reasons for different hole transport behaviors. The thin film crystalline features were investigated using two-dimensional (2D) GIWAXS characterization.²⁸ The 2D GIWAXS patterns and 1D line-cut profiles are shown in Fig. 3a–h. Both systems exhibit prominent (010) peaks (q = 1.73 Å⁻¹, q = 1.67 Å⁻¹) in the out-of-plane (OOP) direction, along with (100) peaks (q = 0.29 Å⁻¹) in the in-plane (IP) direction, indicating predominantly face-on orientation. In the OOP direction (Fig. 3i), the significant reduction in crystalline coherence lengths (CCLs) in PM6:Y6 after thermal aging (24.97 to 21.44 Å) indicates weakened crystallinity and homogeneous interactions, which are responsible for the electron mobility degradation in SMAs.²⁹ In contrast, relatively stable molecular packing behaviors in the crystalline region were observed in the PM6:PY-IT system before and after thermal aging treatment (21.34 to 20.97 Å). In the IP direction (Fig. 3j), the packing distance (d-spacing) is nearly unchanged in both films. However, the crystallinity markedly decreased in PM6:Y6 (from 70.85 to 67.14 Å) while remaining consistent in PM6:PY-IT (from 78.02 to 76.89 Å), exhibiting the same crystalline degradation rate in OOP and IP directions. This dual degradation effect on crystallinity in both the donor and acceptor underscores their critical role in influencing the synchronous descending tendency in electron and hole mobility in different films.³⁰

Fig. 3 (a)–(f) 2D GIWAXS images of PM6:Y6 and PM6:PY-IT films before and after thermal aging. (g) and (h) GIWAXS intensity profiles of the corresponding films. (i) and (j) D-spacing and CCL in (i) out-of-plane and (j) in-plane directions for PM6:Y6 and PM6:PY-IT films before and after thermal aging.

To further investigate how the distinct heterogeneous interaction in polymer donor:small molecular acceptor/polymer acceptor drives dual crystallinity degradation, we employed PiFM (and AFM in Fig. S4†) to reveal the nanoscale chemical specificity in thin films.³¹ By scanning the samples at characterized frequencies of donors and acceptors (Fig. S5†), the aggregated distribution images of the donor and acceptor before and after thermal aging (Fig. 4a–h) were obtained. The poor domain interconnectivity due to significant aggregation observed in the Y6 film after thermal aging suggests that broken interpenetrating networks can dramatically decrease electron mobility. Furthermore, a clear fibrillar network appears in PM6:PY-IT films before and after aging, which enables desirable domain interconnectivity while retaining small domain widths. Correspondingly, Fig. 4e–h reveal that a more pronounced aggregation of PM6 is observed in the small molecule acceptor system. Therefore, the weak molecular self-interaction of SMAs suggests that the low activation energy of thermal diffusion induces dramatic phase segregation, leading to rapid loss of bipolar carrier mobility and damage to the percolation networks. The activation energy reveals that the diffusion of SMA is not dominated by existing free voids, but rather by the creation of additional voids to facilitate the movement of SMA molecules.²³ The rearrangement of SMA disrupts the molecular packing behavior of the donors, resulting in a synchronized decline in both crystallinity and charge carrier mobility (Fig. 1c).

Fig. 4 (a)–(d) PiFM images at the wavenumber of 1287 cm⁻¹ for PM6:Y6 films and 1696 cm⁻¹ for PM6:PY-IT films before and after thermal aging (at this wavenumber, only the acceptor signaled). (e)–(h) PiFM images at the wavenumber of 1649 cm⁻¹ for PM6:Y6 and PM6:PY-IT films before and after thermal aging (at this wavenumber, only PM6 signaled). The yellow color corresponds to the detected material signal.

We further determined the vertical distribution of the D : A ratio during the thermal aging process using film-depth-dependent light absorption spectroscopy. The evolution of the vertical phase distribution of donors and acceptors is shown in Fig. S6.† The PM6:PY-IT film exhibits a more stable vertical distribution in accordance with gentle crystallinity degradation. Conversely, the donor-rich sublayer is enriched in the middle of the PM6:Y6 films, indicating strong aggregation evolution after thermal aging.³²

Fig. S7 and S8† display the UV-vis absorption spectra and the photoluminescence (PL) spectra of the blend films before and after thermal aging, respectively. Notably, the PM6:PY-IT film exhibits negligible changes in absorption features after 100 hours of thermal aging. In contrast, the PM6:Y6 film shows a reduced 0–1/0–0 vibronic peak intensity ratio and a redshifted 0–0 transition peak (see Fig. S7b and c†). Both blends exhibit comparable diminished PL emission intensity after aging, indicative of increased radiative recombination losses and potential degradation of exciton dissociation efficiency.

Fig. 5a displays the current density–voltage (J–V) curves for PM6:Y6 and PM6:PY-IT OSCs before and after thermal aging. The different degradation behavior induced reduction in device performance is shown in Fig. 5b, c and S9.† Fig. S10† presents the light-intensity-dependent open-circuit voltage (V_OC) and short-circuit current (J_SC) curves. We observed a severe decline in the efficiency of the PM6:Y6 system, with performance falling to 62% of the initial value after 340 hours. Only 77% of the original fill factor (FF) value remained after the thermal aging test. In contrast, PM6:PY-IT films exhibit enhanced stability, with a relative power conversion efficiency (PCE) of 71% and an 86% FF value retained after aging. This enhanced stability is attributed to strong self-interaction, which suppressed the degradation of phase segregation and crystallinity. Consequently, a more gentle synchronous decrease in bipolar carrier mobility stabilizes the decline in device performance.

Fig. 5 (a) J–V curves for PM6:Y6 and PM6:PY-IT solar cells before and after thermal aging. (b) Normalized PCE and (c) normalized FF as a function of thermal aging time for PM6:Y6 and PM6:PY-IT solar cells.

Conclusion

We explored the acceptor-induced hole mobility degradation mechanism in BHJ systems. We elucidated the intricate interrelation between the molecular conformation of electron acceptors and the stability of bipolar carrier mobility. Our findings reveal that highly self-interacting systems with high diffusion activation energy can effectively suppress molecular aggregation, thereby retaining optimal percolation networks for bipolar carriers. The enhanced operational stability observed in the PM6:PY-IT system compared to the PM6:Y6 counterpart is attributed to the stabilized molecular packing behaviors that retain good domain interconnectivity and high FF. This work provides a different perspective to understand the degradation mechanism of OPV devices.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

Yiyun Li: data curation; formal analysis; investigation; methodology; visualization; writing – original draft. Dongcheng Jiang: data curation; investigation; methodology; writing – review & editing. Jiangkai Sun: data curation; investigation; writing – review & editing. Rui Shi: investigation. Yu Chen: investigation. Mingsheng Xu: investigation. Xiaoyan Du: resources. Guofu Xv: writing – review & editing. Hang Yin: conceptualization; funding acquisition; project administration; resources; supervision; writing – review & editing.

Conflicts of interest

The authors have no conflicts to disclose.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant no. 12204272). H. Y. acknowledges the Shandong Provincial Natural Science Foundation (no. ZR2021QF016/ZR202111090074) and the Qilu Young Scholar Program of Shandong University. A portion of this work is based on the data obtained at BSRF-1W1A. The authors gratefully acknowledge the cooperation of the beamline scientists at the BSRF-1W1A beamline. The authors thank Aimin Zhang from Shandong University (Testing and Manufacturing Center for Advanced Materials) for help with AFM-IR measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01481f

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