The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells (PSCs) with both low cost and boosted high power conversion efficiency (PCE) over 22%. Despite the achievements, MAPbI3 suffers from inherent instability over ambient operation conditions due to the low formation energy of the material itself and the high hydrophilicity of the organic cations. Efforts such as developing novel device architectures as well as exploring novel materials have been tried to improve the device stabilities. Among them, the two-dimensional (2D) perovskites that are crafted using bulkier alkylammonium cations in place of methylammonium exhibit appealing environmental stability. However, the insulating alkylammonium spacer cations hinder charge transport and limit the efficiencies of the devices based on such perovskites. In this scenario, an exploration of alternative yet effective organic spacer cations that increase the charge transfer is imperative to enhance the efficiency.
Herein, we design such an alternative bi-functional conjugated cation AB. We report the first time the incorporation of AB in 2D/3D perovskites and its implementation on solar cells. The use of bi-functional conjugated cations enhances significantly the performance of the cells, reaching a highest power conversion efficiency of 15.6% with improved stability. The efficiency remained around 90% of the initial value after 100 h continuous illumination, much more stable than MAPbI3 perovskite. By comparing this cation with a mono-functional cation and a bi-functional non
conjugated cation with similar structure, we found that the bi-functional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. Our approach helps explore new rational designs of cations in perovskites.
The exceptional photovoltaic properties demonstrated for organic-inorganic hybrid lead halide perovskites have attracted tremendous attention around the world. The intriguing optoelectrical characteristics include strong absorption coefficient, high carrier mobility and long charge diffusion length. The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have now been boosted up to a certified 22.7% within a few years. Based on ambipolar type carrier transport properties of the perovskites, hole-conductor-free (HTMfree) PSCs have been developed, which simplifies the configuration of the devices. Previously, our group has developed a carbon based HTM-free printable mesoscopic PSC and achieved a certified PCE of 12.8 % with high stability. Herein, we reported a new hybrid carbon electrode based on ultrathin graphite/carbon black for HTM-free printable mesoscopic PSCs, and obtained a PCE of 13.83%.