New additive as Li+ source for charge transfer improvement at triple-cation perovskite/Spiro-OMeTAD interface

Abstract

One of the main obstacles to perovskite solar cells (PSCs) entering the market is the doping-induced degradation of expensive hole transport materials (HTMs), which results in poor device stability. Although lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (4-tBp) as additives are commonly used in HTMs, they often lead to low device stability and reproducibility due to their hygroscopic and volatile nature. In this study, we incorporated the dilithium phthalocyanine (Li 2 Pc) molecule into the Spiro-OMeTAD precursor solution with Li-TFSI and 4-tBp to produce triple-cation PSCs with an n–i–p configuration operating with high efficiency and stability. The addition of the Li 2 Pc molecule not only improves the perovskite/Spiro-OMeTAD interface but also increases the π–π stacking in the thin-film phase according to the absorption and Raman results. Based on space-charge-limited current, photoluminescence, time-resolved photoluminescence and lifetime analysis measurements, charge transfer is improved with the Li 2 Pc molecule and charge recombination losses are reduced. As a result, the control device showed a power conversion efficiency (PCE) of 20.0% with J SC = 24.6 mA/cm 2 and fill factor (FF) = 72.7%, whereas the Li 2 Pc-doped device exhibited an improved PCE of 20.6% with J SC = 25.0 mA/cm 2 and FF = 73.6%. The addition of Li 2 Pc resulted in more stable and high-efficiency solar cells that maintained approximately 90% of the initial efficiency after 40 days. • The addition of the Li 2 Pc molecule can increase the π–π stacking in the thin-film phase. • Li 2 Pc can effectively passivate the defects at the triple-cation perovskite/Spiro-OMeTAD interface. • The efficiency of triple-cation perovskite solar cell is increased from 20.0% to 20.6%. • The device with Li 2 Pc retained about 90% of the initial efficiency after 40 days compared to the control device.