Localized transport channels through nanoscale phase separation for efficient inverted perovskite solar cells

Abstract

Interfacial recombination at the perovskite/PCBM interface is a critical factor for efficiency loss in inverted perovskite solar cells (PSCs). Different from the commonly used fully covered PCBM transport layer, we developed a localized electron transport channel to reduce the perovskite/PCBM direct contact area by introducing nanoscale separated PM6:PCBM. In this structure, electrons cannot be transported throughout PM6. However, these electrons can effectively circumvent PM6 and be extracted by PCBM due to the bi-continuous and nanoscale phase separation in PM6:PCBM, which will effectively reduce the contact area between perovskite and PCBM, thus inhibiting interfacial recombination. The resulting PSCs exhibited a high efficiency of 25.60% with good stability, retaining 93.1% of the initial efficiency after 65 °C aging for 2000 h.

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Introduction

Perovskite solar cells (PSCs) have attracted widespread interest due to their high efficiency, low cost, and enormous application potential, with certified efficiency exceeding 26%.^1–6 The properties of perovskite film and transport materials have been well optimized.^7–13 Interfacial energy loss has become the biggest obstacle for PSC performance. In inverted PSCs, the bottom contact interface can be well optimized by introducing various hole-transport materials, such as PTAA, P3CT, or SAMs.^14–16 However, it is necessary to stop the energy loss at the top contact interface because the dominant materials currently used in inverted PSCs are PCBM or C60. Previous reports have demonstrated that the direct contact between perovskite and C60 will induce large quasi-Fermi-level splitting loss in inverted PSCs.^17–19 Therefore, reducing the energy loss at the perovskite/C60 interface is the primary concern for further development of inverted PSCs.²⁰

Many previous works have aimed to reduce interfacial energy loss through surface passivation and 2D/3D perovskite heterojunctions.^21–26 Previously, we also reported an interfacial heterojunction with surface sulfidation and substrate-induced p–n transition approaches to inhibit such energy loss.^27,28 Despite great progress, these works cannot prevent the direct contact between perovskite and C60, and non-negligible interfacial energy loss still exists in inverted PSCs. Recently, Xu et al. reported a porous insulator with nanoscale openings by introducing Al₂O₃ nano-particles to reduce the contact area between perovskite and the indium–tin oxide/self-assembled monolayer (ITO/SAM) transport layer, which can also reduce the interfacial recombination in PSCs.^29,30 Thus, if PCBM or C60 is non-replaceable in inverted PSCs, reducing their contact area is also a feasible approach to reduce interfacial energy loss. The key difficulty lies in how to construct uniform and nanoscale openings in PSCs, because too small openings don't work, while too large openings will block electron extraction.

Motivated by the nanoscale phase separation in typical organic solar cells, we propose a localized transport channel to reduce the contact area between perovskite and PCBM by introducing a PM6:PCBM mixed transport layer (Fig. 1). PM6 is a typical donor material that easily forms a nanoscale but bi-continuous phase separation with PCBM (Fig. S1, ESI†), which will reduce the perovskite/PCBM contact area, but not interrupt electron transport through PCBM. Importantly, the wide bandgap nature of PM6 (>1.8 eV) will block electron transport from perovskite to PM6,^31–33 and thus it acts as an ‘insulator’ for perovskite, which is necessary for the construction of localized transport channels in PSCs. Additionally, PM6 contains thiophene, C=O, and –F groups, which are beneficial for defect passivation. As a result, inverted PSCs with PM6:PCBM show a high efficiency of 25.60% with much improved V_oc and FF. In addition, device stability is also greatly increased, retaining over 90% of the initial efficiency after 65 °C aging for 2000 h or 85 °C aging for 500 h.

Fig. 1 Schematic diagram of the localized transport channels in PSCs with PM6:PCBM.

Results and discussion

The energy levels of PM6 and perovskite are shown in Fig. 2a and b, respectively, which is obtained from ultraviolet photoelectron spectroscopy and UV-Vis absorbance (Fig. S2, ESI†). PM6 exhibits a shallower conduction band (−3.64 eV) as compared to perovskite (−3.91 eV), indicating blocked electron transport from perovskite to PM6. Therefore, in perovskite with PM6:PCBM, electrons will laterally transport and circumvent PM6, and then be extracted by PCBM, which will reduce the contact area between perovskite and PCBM, thus inhibiting interfacial recombination. The energy level structure of PSCs with PM6:PCBM is shown in Fig. 2c, and a cross-sectional SEM image of perovskite/PM6:PCBM is provided in Fig. S3 of the ESI.†

Fig. 2 UPS spectra of (a) PM6 and (b) perovskite films. (c) Energy level structure of PSCs. AFM phase images of (d) and (g) PCBM, (e) and (h) PM6, and (f) and (i) PM6:PCBM (weight ratio of 1 : 3) deposited on perovskite.

Atomic force microscopy (AFM) with phase imaging is conducted to investigate the phase separation of PM6:PCBM on perovskite films. If PM6 is over-aggregated, the electrons generated in perovskite beneath PM6 will not effectively circumvent PM6 to contact PCBM, thus affecting electron extraction and inducing efficiency loss in PSCs. As shown in Fig. 2d–i, PCBM is uniform and compact on perovskite films, which is consistent with the results presented in the AFM height images (Fig. S4, S5 and S6, ESI†). Pure PM6 tends to form fiber-like structures due to its aggregation. However, in PM6:PCBM, PCBM will fill the space among the PM6 fibers and form a two-phase morphology, which agrees with previous works on phase separation in organic solar cells. Such phase separation is bi-continuous, which will reduce the contact area between perovskite and PCBM, but will not interrupt electron extraction through PCBM.

Kelvin probe force microscopy (KPFM) measurements (Fig. S7, ESI†) show that the PCBM and PM6:PCBM samples exhibit uniform potential distribution. In the PM6:PCBM sample, the potential fluctuation seems slighter larger, which may be induced by the phase separation of PM6 and PCBM. This result further demonstrates that uniform phase separation is indeed generated in PM6:PCBM films.

Apart from suitable energy level, PM6 contains thiophene, C=O, and –F groups, facilitating defect passivation in perovskite. When PM6 is deposited on perovskite, the X-ray photoelectron spectroscopy (XPS) signal of S 2p_1/2 and S 2p_3/2 shifts from 164.5 eV and 163.3 eV (pure PM6) to 165.2 eV and 164.0 eV (Fig. 3a), respectively, which should be induced by the interaction between S element (thiophene) in PM6 and perovskite. Additionally, the Pb 4f_5/2 and Pb 4f_7/2 XPS signal also shifts from 143.1 eV and 138.3 eV for perovskite to 142.7 eV and 137.9 eV in perovskite/PM6 (Fig. 3b), respectively, further confirming the interaction between PM6 and perovskite. In addition, the XPS shift of F (Fig. S8, ESI†) and the FTIR test (Fig. S9, ESI†) also indicate the interaction between PM6 and perovskite. Such interaction will assist in passivating defects in perovskite, which will be beneficial for device performance.

Fig. 3 XPS of (a) S 2p and (b) Pb 4f in pure PM6, perovskite, and perovskite/PM6. (c) PL spectra of PM6, perovskite, perovskite/PCBM, perovskite/PM6, and perovskite/PM6:PCBM. (d) TRPL of ITO/perovskite/PCBM and ITO/perovskite/PM6:PCBM.

Photoluminescence (PL) test is conducted to study the carrier transfer between perovskite and PCBM or PM6. As shown in Fig. 3c, the PL of perovskite is effectively quenched in the perovskite/PCBM and perovskite/PM6:PCBM samples, indicating that PM6:PCBM will not affect the electron transfer between perovskite and PCBM. In perovskite/PM6, the PL is obviously increased, indicating that photo-generated electrons in perovskite will not be transferred to PM6 due to its shallower conduction band (the energy level in Fig. 2).

The PL of perovskite/PM6 shows a blueshift from 795 to 790 nm in comparison with pure perovskite, which should be induced by the defect passivation ability of PM6. In addition, the PL peak of PM6 at approximately 675 nm disappears in perovskite/PM6, which should be induced by electron transfer from PM6 to perovskite. This can also explain the greatly increased PL intensity of perovskite in perovskite/PM6 samples. Time-resolved photoluminescence (TRPL, Fig. 3d) of perovskite is fitted with a bi-exponential function. The first lifetime of τ₁ represents interfacial carrier transfer, while the second lifetime of τ₂ represents carrier recombination in bulk. Perovskite/PM6:PCBM exhibits a shorter τ₁ of 8.19 ns as compared to control perovskite/PCBM (14.09 ns), indicating that PM6 introduction will not inhibit the electron transfer between perovskite and PM6:PCBM, which agrees with the PL results. In addition, the τ₂ is also increased in perovskite/PM6:PCBM (123.16 ns) in comparison with that in the control sample (35.24 ns), which should be induced by the defect passivation of PM6.

Transient photocurrent (TPC) and transient photovoltage (TPV) measurements are employed to explore the charge extraction behavior in PSCs with PM6:PCBM. The TPC decay lifetime is greatly reduced from 2.32 μs (control PSCs with PCBM) to 1.54 μs in PSCs with PM6:PCBM (Fig. 4a), indicating faster carrier extraction, which agrees with the TRPL results. The TPV decay lifetime is greatly increased from 8.86 μs to 20.9 μs in PSCs with PM6:PCBM (Fig. 4b), indicating the inhibited carrier recombination due to the reduced contact area between PCBM and perovskite.

Fig. 4 (a) TPC and (b) TPV of PSCs with PCBM or PM6:PCBM. (c) Capacitance-frequency spectra of PSCs in the dark. (d) Gaussian fitting of tDOS distribution in the control and PM6:PCBM PSCs.

The frequency dependence of device capacitance is shown in Fig. 4c. The large capacitance at low frequency regions is ascribed to the electrode polarization and carrier accumulation at the perovskite interface.³⁴ PSCs with PM6:PCBM even exhibit slightly smaller low-frequency capacitance than devices with PCBM, indicating that no electron accumulation occurs at the perovskite/PM6:PCBM interface. In addition, with capacitance-frequency Gaussian fitting, the trap density of states (tDOS) is obtained, as shown in Fig. 4d. In PSCs with PM6:PCBM, the trap density is obviously reduced by 0.3–0.4 eV, consistent with the DLTS results (Fig. S11, ESI†), indicating the passivated defects in PSCs with PM6:PCBM.

Inverted PSCs are constructed with the configuration of glass-ITO/Meo-2PACz/perovskite/PM6:PCBM/C60/TPBi/Ag (Fig. 5a). In PSCs with PM6:PCBM (Tables S2 and S3, ESI†), the device FF (76.83% vs. 81.40%) and V_oc (1.176 V vs. 1.193 V) are clearly increased (Fig. 5b, J–V hysteresis in Fig. S12, ESI†) due to the reduced interfacial recombination. In addition, device J_sc is also increased from 25.77 to 25.71 mA cm⁻² (EQE spectra in Fig. S13, ESI†). Device efficiency of 25.60% is obtained in PSCs with PM6:PCBM, which is much higher than that of the control devices (24.31%). The stabilized power output reached 25.60%, as shown in the Fig. 5b inset. PSCs with different PM6:PCBM ratios are also fabricated, and the related device performance is shown in Table S3 (ESI†).

Fig. 5 (a) Device structure of inverted PSCs. (b) J–V curves and stable maximum power point output of PCBM and PM6:PCBM devices. (c) V_oc dependence on the light intensity of PCBM and PM6:PCBM devices. (d) Mott–Schottky characteristics of the devices measured at 10 kHz in the dark to determine the built-in potential. Statistics of (e) V_oc and (f) PCE of the PCBM and PM6:PCBM devices.

Electrochemical impedance spectroscopy (EIS) of PSCs indicates that PSCs with PM6:PCBM show smaller R_s (23 vs. 28 Ω) and larger Rrec (8050 vs. 2416 Ω) values than control PSCs with PCBM, which should be induced by the inhibited interfacial recombination and is beneficial to device FF. V_oc dependence on light intensity indicates that PSCs with PM6:PCBM show a lower ideality factor (n of 1.38) than control devices (n of 1.84), further demonstrating inhibited interfacial recombination (Fig. 5c; J–V curves under different light intensities in Fig. S14, ESI†), which should be induced by the effectively reduced contact area between PCBM and perovskite.

Mott–Schottky characterization shows a higher built-in potential (V_bi) of 1.173 V in PSCs with PM6:PCBM (Fig. 5d), agreeing with their high V_oc in the J–V curves. Fig. 5e and f show the V_oc and PCE distribution among 20 separated devices. The average V_oc (from 1.176 V to 1.192 V), and PCE (from 23.90% to 25.50%) are significantly improved in PM6-based devices. In addition to efficiency, thermal stability can also be increased in PSCs with PM6:PCBM, retaining 93.1% of the initial efficiency after 65 °C aging for 2000 h (Fig. 6a), while control PSCs retain 85.1% of their initial value under the same conditions. In addition, PSCs with PM6:PCBM also retained 91.9% of their initial efficiency after 85 °C aging for 500 h (Fig. 6b), which is much better as compared to the control devices.

Fig. 6 Thermal stability of PSCs heated at (a) 65 °C and (b) 85 °C in an N₂ environment.

Conclusions

Localized electron transport channel is developed to inhibit interfacial recombination through introducing nanoscale phase-separated PM6:PCBM as an electron transport layer in PSCs. Photo-generated electrons in perovskite cannot be transferred through PM6 due to its higher bandgap, but circumvent PM6 to reach and be extracted by PCBM, thus reducing the effective contact area of perovskite/PCBM and inhibiting interfacial recombination. Importantly, due to the nanoscale phase preparation and bi-continuous structure between PM6 and PCBM, electron extraction by the PCBM section in PM6:PCBM will not be disturbed. The resulting PSCs with PM6:PCBM exhibit a high efficiency of 25.60% with good thermal stability, retaining >90% of the initial efficiency after 65 °C aging for 2000 h.

Author contributions

X. Li and J. Fang conceived the idea. B. Feng designed and conducted the experiments. W. Li and Z. Cui improved the devices. Y. Li, Q. Weng, and J. Xu conducted device characterizations. Y. Mao and T. You conducted stability tests and analyzed the results. T. Shu and W. Zhang discussed and modified the manuscript.

Data availability

The data in the main text and ESI,† are available from the corresponding author on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (62274062, 62374058, 52173161, 62104070), the National Science Fund for Distinguished Young Scholars (T2325011), the Shanghai Science and Technology Innovation Action Plan (24DZ3001202), and the National Youth Top-notch Talent Support Program.

References

1. Y. Gao, Z. Song, Q. Fu, Y. Chen, L. Yang, Z. Hu, Y. Chen and Y. Liu, Adv. Mater., 2024, 36, 2405921.
2. S. Liu, J. Li, W. Xiao, R. Chen, Z. Sun, Y. Zhang, X. Lei, S. Hu, M. Kober-Czerny, J. Wang, F. Ren, Q. Zhou, H. Raza, Y. Gao, Y. Ji, S. Li, H. Li, L. Qiu, W. Huang, Y. Zhao, B. Xu, Z. Liu, H. J. Snaith, N.-G. Park and W. Chen, Nature, 2025, 632, 536–542.
3. M. Li, B. Jiao, Y. Peng, J. Zhou, L. Tan, N. Ren, Y. Ye, Y. Liu, Y. Yang, Y. Chen, L. Ding and C. Yi, Adv. Mater., 2024, 36, 2406532.
4. H. Chen, A. Maxwell, C. Li, S. Teale, B. Chen, T. Zhu, E. Ugur, G. Harrison, L. Grater, J. Wang, Z. Wang, L. Zeng, S. M. Park, L. Chen, P. Serles, R. A. Awni, B. Subedi, X. Zheng, C. Xiao, N. J. Podraza, T. Filleter, C. Liu, Y. Yang, J. M. Luther, S. De Wolf, M. G. Kanatzidis, Y. Yan and E. H. Sargent, Nature, 2023, 613, 676–681.
5. S. Li, Y. Jiang, J. Xu, D. Wang, Z. Ding, T. Zhu, B. Chen, Y. Yang, M. Wei, R. Guo, Y. Hou, Y. Chen, C. Sun, K. Wei, S. M. H. Qaid, H. Lu, H. Tan, D. Di, J. Chen, M. Gratzel, E. H. Sargent and M. Yuan, Nature, 2024, 635, 82–88.
6. S. Li, Y. Xiao, R. Su, W. Xu, D. Luo, P. Huang, L. Dai, P. Chen, P. Caprioglio, K. A. Elmestekawy, M. Dubajic, C. Chosy, J. Hu, I. Habib, A. Dasgupta, D. Guo, Y. Boeije, S. J. Zelewski, Z. Lu, T. Huang, Q. Li, J. Wang, H. Yan, H.-H. Chen, C. Li, B. A. I. Lewis, D. Wang, J. Wu, L. Zhao, B. Han, J. Wang, L. M. Herz, J. R. Durrant, K. S. Novoselov, Z.-H. Lu, Q. Gong, S. D. Stranks, H. J. Snaith and R. Zhu, Nature, 2024, 635, 874–881.
7. X. Li, H. Yang, A. Liu, C. Lu, H. Yuan, W. Zhang and J. Fang, Energy Environ. Sci., 2023, 16, 6071–6077.
8. H. Yang, X. Li, X. Guo, C. Lu, H. Yuan, A. Liu, W. Zhang and J. Fang, ACS Energy Lett., 2023, 8, 3793–3799.
9. X. Li, W. Zhang, Y.-C. Wang, W. Zhang, H.-Q. Wang and J. Fang, Nat. Commun., 2018, 9, 3806.
10. X. Li, S. Fu, W. Zhang, S. Ke, W. Song and J. Fang, Sci. Adv., 2020, 6, eabd1580.
11. X. Li, S. Fu, S. Liu, Y. Wu, W. Zhang, W. Song and J. Fang, Nano Energy, 2019, 64, 103962.
12. X. Li, S. Ke, X. Feng, X. Zhao, W. Zhang and J. Fang, J. Mater. Chem. A, 2021, 9, 12684–12689.
13. X. Guo, C. Lu, W. Zhang, H. Yuan, H. Yang, A. Liu, Z. Cui, W. Li, Y. Hu, X. Li and J. Fang, ACS Energy Lett., 2024, 9, 329–335.
14. H. Chen, C. Liu, J. Xu, A. Maxwell, W. Zhou, Y. Yang, Q. Zhou, A. S. R. Bati, H. Wan, Z. Wang, L. Zeng, J. Wang, P. Serles, Y. Liu, S. Teale, Y. Liu, M. I. Saidaminov, M. Li, N. Rolston, S. Hoogland, T. Filleter, M. G. Kanatzidis, B. Chen, Z. Ning and E. H. Sargent, Science, 2024, 384, 189–193.
15. X. Li, X. Liu, X. Wang, L. Zhao, T. Jiu and J. Fang, J. Mater. Chem. A, 2015, 3, 15024–15029.
16. X. Li, Y.-C. Wang, L. Zhu, W. Zhang, H.-Q. Wang and J. Fang, ACS Appl. Mater. Interfaces, 2017, 9, 31357–31361.
17. F. Ye, S. Zhang, J. Warby, J. Wu, E. Gutierrez-Partida, F. Lang, S. Shah, E. Saglamkaya, B. Sun, F. Zu, S. Shoaee, H. Wang, B. Stiller, D. Neher, W.-H. Zhu, M. Stolterfoht and Y. Wu, Nat. Commun., 2022, 13, 7454.
18. A. A. Said, E. Aydin, E. Ugur, Z. Xu, C. Deger, B. Vishal, A. Vlk, P. Dally, B. K. Yildirim, R. Azmi, J. Liu, E. A. Jackson, H. M. Johnson, M. Gui, H. Richter, A. R. Pininti, H. Bristow, M. Babics, A. Razzaq, T. G. Allen, M. Ledinsky, I. Yavuz, B. P. Rand and S. De Wolf, Nat. Commun., 2024, 15, 708.
19. Y. Wang, R. Lin, C. Liu, X. Wang, C. Chosy, Y. Haruta, A. D. Bui, M. Li, H. Sun, X. Zheng, H. Luo, P. Wu, H. Gao, W. Sun, Y. Nie, H. Zhu, K. Zhou, H. T. Nguyen, X. Luo, L. Li, C. Xiao, M. I. Saidaminov, S. D. Stranks, L. Zhang and H. Tan, Nature, 2024, 635, 867–873.
20. Z. Xing, F. Liu, S.-H. Li, X. Huang, A. Fan, Q. Huang and S. Yang, Angew. Chem., Int. Ed., 2023, 62, e202305357.
21. D. W. deQuilettes, J. J. Yoo, R. Brenes, F. U. Kosasih, M. Laitz, B. D. Dou, D. J. Graham, K. Ho, Y. Shi, S. S. Shin, C. Ducati, M. G. Bawendi and V. Bulovic, Nat. Energy, 2024, 9, 762.
22. H. Yang, Z. Cui, W. Li, X. Guo, C. Lu, H. Yuan, Y. Hu, W. Zhang, X. Li and J. Fang, Adv. Funct. Mater., 2024, 34, 2407828.
23. L. He, H. Zhang, D. Zhang, C. Gao, H. Su, D. Du, D. Ding, H. Liu and W. Shen, Adv. Funct. Mater., 2024, 34, 2403020.
24. G. Pica, L. Pancini, C. E. Petoukhoff, B. Vishal, F. Toniolo, C. Ding, Y.-K. Jung, M. Prato, N. Mrkyvkova, P. Siffalovic, S. De Wolf, C.-Q. Ma, F. Laquai, A. Walsh and G. Grancini, Nat. Commun., 2024, 15, 8753.
25. R. Azmi, D. S. Utomo, B. Vishal, S. Zhumagali, P. Dally, A. M. Risqi, A. Prasetio, E. Ugur, F. Cao, I. F. Imran, A. A. Said, A. R. Pininti, A. S. Subbiah, E. Aydin, C. Xiao, S. I. Seok and S. De Wolf, Nature, 2024, 628, 93–98.
26. X. Zhou, L. Zhang, J. Yu, D. Wang, C. Liu, S. Chen, Y. Li, Y. Li, M. Zhang, Y. Peng, Y. Tian, J. Huang, X. Wang, X. Guo and B. Xu, Adv. Mater., 2022, 34, 2205809.
27. X. Li, W. Zhang, X. Guo, C. Lu, J. Wei and J. Fang, Science, 2022, 375, 434–437.
28. Z. Cui, W. Li, B. Feng, Y. Li, X. Guo, H. Yuan, Q. Weng, T. You, W. Zhang, X. Li and J. Fang, Adv. Mater., 2024, 36, 2410273.
29. W. Peng, K. Mao, F. Cai, H. Meng, Z. Zhu, T. Li, S. Yuan, Z. Xu, X. Feng, J. Xu, M. D. McGehee and J. Xu, Science, 2023, 379, 683–690.
30. C. Das, M. Kot, T. Hellmann, C. Wittich, E. Mankel, I. Zimmermann, D. Schmeisser, M. K. Nazeeruddin and W. Jaegermann, Cell Rep. Phys. Sci., 2020, 1, 100112.
31. S. Wu, Z. Li, J. Zhang, X. Wu, X. Deng, Y. Liu, J. Zhou, C. Zhi, X. Yu, W. C. H. Choy, Z. Zhu and A. K. Y. Jen, Adv. Mater., 2021, 33, 2105539.
32. Z. Zhao, S. Chung, Y. Y. Kim, M. Jeong, X. Li, J. Zhao, C. Zhu, S. Karuthedath, Y. Zhong, K. Cho and Z. Kan, Energy Environ. Sci., 2024, 17, 5666–5678.
33. J. Lv, H. Tang, J. Huang, C. Yan, K. Liu, Q. Yang, D. Hu, R. Singh, J. Lee, S. Lu, G. Li and Z. Kan, Energy Environ. Sci., 2021, 14, 3044–3052.
34. H.-S. Pang, H. Xu, C. Tang, L.-K. Meng, Y. Ding, J. Xiao, R.-L. Liu, Z.-Q. Pang and W. Huang, Org. Electron., 2019, 65, 275–299.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01209k

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