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Charge extraction efficiency is one of the key factors to determine the performance of solar cells, which depends on the electronic properties of the electron and hole transporting layers. Sol–gel derived NiOx as the hole transporting layer (HTL) used in perovskite solar cells (PVSCs) promises both low-cost and high-stability. However, current NiOx based PVSCs have a relatively low fill factor (FF) and device efficiency. Here, we show that using an oxide composite approach by incorporating thermally reduced graphene oxide (rGO) into the NiOx layer (NiOx:rGO) can effectively improve the film conductivity without sacrificing its high work function. The improved charge extraction efficiency reduces the interfacial recombination loss and increases the carrier lifetime in NiOx:rGO based PVSCs, which are evidenced by transient photocurrent and transient photovoltage measurements. The NiOx:rGO based PVSC achieves a promising PCE of 19.1% with a significantly improved FF from 73% to 81% compared to the device using solely NiOx. Moreover, the NiOx:rGO based PVSC shows a decent photo-stability tracked at the maximum-power-point and a long shelf-lifetime with negligible degradation in device performance after 70 days. It is believed that the oxide composite transporting layer would be a promising alternative to further develop highly efficient and stable PVSCs. [ABSTRACT FROM AUTHOR]

Mixed-organic-cation FA1−xMAxPbI3 perovskites are currently attracting intensive attention for their high photovoltaic performance. However, FA1−xMAxPbI3 usually has an undesired non-perovskite phase leading to a compromised efficiency, and a facile strategy to achieve pure-phase FA1−xMAxPbI3 has been rarely reported. Here, we demonstrate a facile approach to form δ-phase free FA1−xMAxPbI3 using a non-stoichiometric precursor solution. It is found that a small amount of excess methylammonium iodide (MAI) added to the precursor solution has a profound effect on perovskite crystallization during the liquid-to-solid phase transformation. Using an in situ photoluminescence spectroscopy measurement, it is found that the excess MAI can promote the formation of FA1−xMAxPbI3 and facilitate the cation-exchange between organic FA+ and MA+ during the film formation. Based on the optimized pure-phase FA1−xMAxPbI3, the perovskite solar cells exhibit an encouraging power conversion efficiency (PCE) of 17.40%. This study demonstrates a facile method to achieve high performance FA dominating mixed-organic-cation perovskite devices while providing insight into the ion-exchange process during perovskite crystallization. [ABSTRACT FROM AUTHOR]

Developing facile strategies that enable high-performance perovskite solar cell (PSC) fabrication in ambient air is appealing yet challenging for low-cost and large-scale industrial deployment. However, attempts in this regard are still limited to methylammonium lead triiodide (CH3NH3PbI3), which lags significantly behind the rapid development of the state-of-the-art formamidinium (FA) based mixed perovskites. Here, a PbI2–(CsI)0.15–(FAI)x intermediate complex is proposed to unlock the processing limitation and efficiently facilitate the growth of high-quality mixed-cation Cs0.15FA0.85PbI3 perovskite films in ambient air with relative humidity (RH) as high as 70 ± 10%. It is found that the PbI2–(CsI)0.15–(FAI)x (x = 0.2 to 0.4) intermediate complex consisting of a mixture of composites is favorable for the conversion of PbI2 to high-quality Cs0.15FA0.85PbI3 perovskite. Eventually, it reduces the required FAI concentration deposited on the intermediate for the conversion of PbI2 to perovskite, which allows the strong hygroscopic FAI to be processed in ambient air. With the PbI2–(CsI)0.15–(FAI)0.3 intermediate complex, a low-temperature air-processed planar Cs0.15FA0.85PbI3 PSC achieves a champion power conversion efficiency (PCE) of 15.56% with superior thermal-stability and photo-durability, demonstrating its great potential for the large scale development of PSCs. [ABSTRACT FROM AUTHOR]

The origin of the impact of mobile ions in perovskite solar cells (PVSCs) has recently become a hot topic of debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm−3 to 1017 cm−3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in the literature, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible for the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated devices is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduces the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ions in the degradation mechanism of PVSCs. [ABSTRACT FROM AUTHOR]