低压气相辅助溶液法制备非铅Bi基钙钛矿太阳电池及其光电性能研究

The low chemical stability in air and the toxicity of lead are the most important obstacle for further commercialization of perovskite solar cells. Nevertheless, the overall PCEs of the Bi-based perovskite solar cells (MBI) so far are still low, which is most likely caused by the poor film morphology, high carrier densities and large bandgap. To overcome this problem, it is of great importance to develop other effective strategies to promote the film. In the present study, MBI will be fabricated by the low-pressure vapor-assisted solution method. The introduction of HCl in the precursor solution will effectively increase the solubility of BiI3, avoiding the poor morphology caused by the rapid crystallization of BiI3 in DMF. The presence of defects and a high background carrier density within MBI results in predominantly first order recombination processes. In this study, the carrier concentration of perovskite film will be decreased by using BiF3 in precursor, which could reduce the concentration of Bi vacancies and hence the carrier mobility. Apart from the poor morphology and high carrier concentration, the relatively large indirect band gap of MBI is one of the performance limitations. Compared to the typical lead-based perovskite materials for perovskite solar cells, the device performance was mainly contributed by its high photocurrent density due to its extremely low band gap (~1.5 eV). Therefore, reducing the band gap by developing other bismuth based organic–inorganic materials may be a promising strategy to further improve its solar cell performance. In this Study, we will prepare the in situ sulfur doped MBI, via the thermal decomposition of Bi(xt)3 (xt=ethyl xanthate) or thiourea precursor. The doping of sulfur in X position will decrease the bandgap and broaden the absorption range of MA3Bi2I9. Moreover, the most prepare method to fabricate MA3Bi2I9 perovskites are a solution approach. Compared with the conventional solution processing, vapor deposition show several particular advantages. First, it avoids solvation and hydration processes as well as undesirable structural transitions that may occur during solution processing. Second, it also effectively reduces the too-fast reaction rate between BiI3 and CH3NH3I, thereby resulting in an optimized perovskite surface morphology. The effect of the composition of start precursor solution and other reaction condition on the morphology of the films will be carefully studied. The purpose of the study is to realize the controllable preparation of the MBI films by establishing the relationship between the films structure and the preparation conditions, and finding the formation mechanism of the films. The photoelectric properties of the as-prepared MBI films will be evaluated by fabricating the perovskite solar cells. The relationship between the films structure and the photoelectric properties will be studied. The carriers transform layer and the modification of the interface in the solar cells will be systematically studied.These researches will provide useful theory and practice experiences for the design and preparation lead-free perovskite solar cells with excellent photoelectric properties.

非铅Bi基钙钛矿电池因能有效避免传统钙钛矿太阳电池中Pb引起的污染和稳定性差等问题而备受关注，但目前该类电池效率很低，主要原因在于薄膜表面均匀性差、载流子浓度高以及带隙过宽，这严重制约了该电池的发展。基于此，本项目提出采用气相辅助溶液法制备非铅Bi基钙钛矿电池，通过在前驱体溶液中引入BiF3和HCl以降低Bi3+空位并有效提高Bi3+的溶解度，利用乙基黄原酸铋将S原位引入到X位上，从而降低材料的带隙。将溶液法制备的BiI3薄膜与MAI蒸气反应生成钙钛矿薄膜，相比液相反应，气相反应能有效地避免溶剂化和水解反应，减少由此产生的复生结构，通过调整气相反应条件可对反应速度进行控制，进而获得均匀的薄膜。将系统研究影响钙钛矿薄膜结构的关键因素，阐明钙钛矿薄膜生长机制，实现其结构可控。探明太阳电池的光电性能与薄膜结构之间的关系，揭示电池内部载流子传输影响机制，最终获得具有优良性能的非铅钙钛矿太阳电池。

本项目针对非铅Bi基钙钛薄膜表面均匀性差、载流子浓度高以及带隙过宽，进而影响电池光电性能的问题，采用低压气相辅助溶液法制备非铅Bi基钙钛矿太阳电池。相比液相法，气相法能有效地避免溶剂化和水解反应，进而获得了表面致密、均匀的MA3Bi2I9薄膜。采用黄原酸铋作为Bi源，通过加热使其分解释放出硫元素。S能够部分取代I的位置，进而降低了材料的带隙，最终获得带隙宽度为1.67eV的MA3Bi2I9-2xSx钙钛矿薄膜。项目利用DMF分子在BiI3不同晶面上的吸附能的差异，将旋涂后的BiI3薄膜在室温下直接放置于密闭容器中，保证了DMF缓慢的蒸发，促进了BiI3薄膜（113）和（300）晶面取向生长，同时伴随着（003）晶面的减弱。随后利用低压气相辅助溶液法将BiI3薄膜与MAI气体反应，BiI3的取向限制了MBI（006）晶面的生长，与保证了载流子快速传输。进而通过调节薄膜中MBI和BiI3的比例构成异质结，有效改善了器件的性能，最终实现了钙钛矿电池1.53%的PCE。该电池在室温下的氮气环境中60天后，仍然能够保持其原始光电转换效率的89.23%，呈现了优良的稳定性。项目引入硫脲诱导了Cs3Sb2I9薄膜（201）晶面择优取向，并有效调控了钙钛矿结晶，实现了Cs3Sb2I9电池2.22%的光电转换效率。使用戊基吡啶(2-py)代替tBP作为Spiro-OMeTAD中的添加剂制备了Ag2BiI5 钙钛矿太阳电池。研究结果表明，tBP的存在严重腐蚀了Ag2BiI5薄膜，只能获得0.61%的PCE。由于tBP的不稳定性，该器件显示较低的稳定性。通过在空穴传输层中引入2-py，我们极大提高了器件的稳定性。2-py的引入提高了LiTFSI的溶解度，促进了载流子的转移，光电转换效率从0.61%提高到1.32%，提高了2.16倍。使用Ca2+对Pb2+进行取代，有效提升了全无机钙钛矿太阳电池的环境稳定性，在低温60oC下获得了9.20%光电转换效率，明显地高于未掺杂Ca2+的电池5.54%的效率。随着Ca2+掺杂浓度的增加，电池的湿、热稳定性稳定性得到明显提升。成功制备了全无机Cs2PbI2Cl2/CsPbI2.5Br0.5 2D/3D混维异质结钙钛矿薄膜。2D/3D异质结有效提升了器件内的载流子传输。面积为1 cm2 电池实现了12.74％的光电转换效率。