All-Inorganic Perovskite Solar Cells Using Treated Carbon Materials for Effective Charge Extraction

Abstract

Perovskite solar cells (PSCs) have become the leader in the field of the third-generation solar cells due to their outstanding photovoltaic performance and low cost. However, the instability stemming from low tolerance to humidity and heat as well as residual solvents and ultraviolet illumination would be challenging for long-term stability. Recently, all inorganic perovskites, especially carbon-based hole-transporting material (HTM) CsPbBr3 perovskite, have attracted increasing attentions because of their enhanced moisture and thermal stability compared to hybrid counterparts.For the CsPbBr3 device, a challenging route to increase power conversion efficiency (PCE) is to take interfacial charge transfer into consideration and reduce interfacial energy-level differences to reduce space charge accumulation and enhance charge extraction. It is known that graphene-based derivatives are promising candidates for interfacial materials due to their excellent electronic properties. For example, graphene quantum dots (GQDs) between perovskite and TiO2 could significantly enhance the performance of PSCs due to dramatically enhanced electron extraction by the inserted GQDs; polyaniline/graphite composites were incorporated into carbon electrode to tailor work function and to improve hole-selectivity of back electrode for enhanced energy level alignment and interfacial hole extraction, leading to a remarkably reduced energy loss and charge recombination.Herein, we developed a strategy to use carbon dots (CDs) with tunable bandgaps and high absorption coefficients for electron extraction performance. The CDs were prepared by exfoliation method from oxidized graphite, and CDs in dimethylformamide solution were spin-coated onto FTO/c-TiO2/m-TiO2 substrate, then a multistep solution-processed method was used to prepare Cs-Pb-Br films with controllable CsPbBr3 phase. Specifically, CsBr/methanol solution (~15 mg mL-1) was spin-coated onto PbBr2 film and then annealed at 250 °C for 5 min, and the processes were repeated for 5 times. The obtained cesium lead bromide films were washed by 2-propanol. Finally, a carbon back-electrode with an average area of 0.09 cm2 was deposited onto the device. We fabricated all-inorganic PSCs with the FTO/c-TiO2/m-TiO2/CDs/CsPbBr3/carbon structure. Arising from the suppressed interfacial charge recombination, the optimal device yielded a PCE up to 10.67% under one sun illumination. These findings suggest that graphene-based derivatives can be used as interfacial modification materials toward high-performance carbon-based HTM-free PSCs.