Additive engineering on spiro-OMeTAD hole transport material for CsPbI3 all-inorganic perovskite solar cells with improved performance and stability

CsPbI3 all-inorganic perovskite has now drawn much attention due to its superior thermal stability compared with the organic-inorganic hybrid counterparts. Despite the great progress achieved recently in this field, CsPbI3 still suffers from low phase stability when exposed to moisture. We found the phase transition of CsPbI3 would be accelerated after depositing the conventional hole transport material (HTM) 2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]− 9,9′-spirobifluorene (spiro-OMeTAD). This is attributed to the negative effects of the additives in spiro-OMeTAD HTM: the hydrolysis of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), the corrosivity of 4-tert-butylpyridine (TBP), and the evaporation of TBP. By slightly modifying the additive content, full coordination between LiTFSI and TBP could be achieved and the negative effects mentioned above could be mitigated. Furthermore, tris(2-(1 H-pyrazol-1-yl)− 4-tert-butylpyridine)cobalt(III)-tris(bis(trifluoromethylsulfonyl)imide) (Co(III)TFSI) was added to promote oxidation of spiro-OMeTAD HTM in inert environment. With these approaches of additive engineering, HTMs with better interface contact and charge transport capability could be obtained. The device with optimized spiro-OMeTAD HTM achieved a champion power conversion efficiency (PCE) of 10.61% compared with 6.63% of the control one. Moreover, the optimal device maintained 81% of its initial PCE after storage for 30 days, exceeding that (68%) of the control one. Our results highlight the importance of spiro-OMeTAD HTM on the stability of CsPbI3 all-inorganic perovskite and provide a facile and feasible way to increase the stability and performance of corresponding solar cells.