Composition tuned hybrid perovskites: from interface and grain engineering to stability and performance improvement

Ph.D.Recently, wide bandgap (WB) organic-inorganic hybrid perovskites (OIHPs) have shown great potential to boost the efficiency of single-junction solar cells, by forming a tandem structure with narrow bandgap OIHPs, silicon (Si) or copper indium gallium diselenide (CIGS) through low-cost solution processing. However, such materials suffer from abnormal phase segregation under light illumination, which leads to a huge open circuit voltage (Vₒ) deficit and poor long-term stability in solar cells. Attempts have been made to solve this critical issue, however, an effective technique that can fully address the performance and stability limits are yet to be developed.This thesis aims to elucidate the mechanism of the photo-induced phase segregation and to develop defects elimination techniques to improve the photostability and the photovoltaic performance of WB-OIHPs. Firstly, the impact of bromine incorporation on the electronic properties of the OIHP solar cells has been studied. Through elemental analysis we found that bromine exhibits an increased concentration towards the interface between perovskite and TiO₂. Opto-electrical characterization results further suggest that the carrier lifetime in the perovskite and the charge extraction at the perovskite/TiO₂ interface are improved in the mixed halide perovskite samples. Accordingly, the optimal thickness of the perovskite active layer is increased from 330 nm for MAPbI₃ to 510 nm for MAPb(I₀.₈₈Br₀.₁₂)₃, resulting in an average efficiency increase of 1.6%.Next, the stability of the OIHPs under various application conditions, and in particular combined light and heat stress, has been well studied in Cs0.15FA0.85Pb(I0.73Br0.27)3 with an optical bandgap of ~1.72eV. We found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a post-deposition treatment with benzylamine (BA) molecules, the highly defective regions (e.g. film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA-modified perovskite solar cells also exhibited excellent photovoltaic performance, with the champion device reaching a stabilized power output efficiency of 17.1% and a Vₒ of 1.24 V.Finally, we demonstrate the photo-induced phase segregation and Vₒ loss can be addressed by combining grain crystallization and grain boundaries passivation, achieved simultaneously through tuning of perovskite precursor composition. Using Cs₀.₁₇FA₀.₃₈PbI₃₋ₓBrₓ (x=0.8, 1.2 1.5 and 1.8), with a varied bandgap from 1.72 eV to 1.93 eV, as the model system we illustrate how precursor additive Pb(SCN)₂ should be matched with a proper ratio of FAX (I and Br) to realize large grains with defect-healed grain boundaries. The optimized WB-OIHPs showed good photostability at both room-temperature and elevated temperature. Moreover, the corresponding solar cells exhibited excellent photovoltaic performances with the champion Vₒ/stabilized power output efficiency reaching 1.244 V/18.60%, 1.284 V/16.51%, 1.296 V/15.01% and 1.312 V/14.35% for WB-OIHPs with x=0.8, 1.2, 1.5 and 1.8, respectively.近期，寬带隙有機-無機混合的鈣鈦礦材料展示了通過簡單的溶液法，製備與窄帶隙鈣鈦礦，硅或銅-銦-鎵-硒疊層的多節太陽能電池的潛力，以期獲得突破單節太陽能電池肖克利-奎伊瑟極限的效率。然而，這類材料在光照下展現出反常的相分離现象，導致製備的太陽能電池的開路電壓出現巨大損失，長期工作穩定性也表现得非常差。儘管一些研究工作嘗試解決這個問題，簡單有效的方法仍然難以實現。本論文旨在獲取對光照相分離現象更深入的理解，併發展出能夠有效消除寬帶隙鈣鈦礦材料缺陷的技術，以期提高這類材料的光穩定性及光伏性能。首先，我們研究了在鈣鈦礦材料中引入溴離子對其電學性能的影響。通過元素分我們發現溴離子在鈣鈦礦/二氧化鈦介面上擁有更高濃度。光學-電學表徵發現溴離子的引入有利於提高載流子壽命，及電子在鈣鈦礦/二氧化鈦介面的抽取。相應的，太陽能電池中最佳的鈣鈦礦厚度從純碘鈣鈦礦的330 nm提高到碘溴混合鈣鈦礦的510 nm, 效率也獲得了1.6%的提高。隨後，鈣鈦礦在各種應用條件下的穩定性，尤其是光-熱條件下的穩定性，在帶隙為1.72 eV的碘溴混合鈣鈦礦中得到了較好的研究。我們發現光和熱協同作用能夠加速新鮮鈣鈦礦的相分離及分解。另一方面，通過對鈣鈦礦膜用苯甲胺分子做後處理，薄膜中的缺陷富集區域（晶粒表面）得到很好的鈍化，抑制了材料在光熱條件下的相分離和分解。除此之外，經過苯甲胺分子修飾的鈣鈦礦薄膜表現出了優異的光伏性能，製備的最好器件表現出17.1%穩態輸出效率及1.24 V的開路電壓。最後，我們展示了通過調節鈣鈦礦前驅體溶液的組分，可以同時提高帶隙在1.72 eV-1.93 eV範圍內碘溴混合鈣鈦礦的結晶性和晶粒尺寸，使得這些材料的光照穩定性和開路電壓同時得到提高。這些優化過的寬帶隙材料在室溫和更高溫度的光照穩定性都得到了提高。 除此之外，相應製備的太陽能電池表現出優異的光伏性能，最高的開路電壓/穩態輸出效率在1.72 eV, 1.79 eV, 1.86 eV和1.96 eV鈣鈦礦製備的太陽能電池中分別達到了1.244 V/18.60%, 1.284 V/16.51%, 1.296 V/15.01% 和1.312 V/14.35%。Zhou, Yang.Thesis Ph.D. Chinese University of Hong Kong 2018.Includes bibliographical references (leaves 111-139).Abstracts also in Chinese.Title from PDF title page (viewed on 4 June, 2021)