Efficient planar heterojunction perovskite solar cells with weak hysteresis fabricated via bar coating

- A scalable bar-coating method is employed to prepare perovskite solar cells with maximum PCE of 13% with weak hysteresis.
- Similar device efficiency can be achieved by bar-coating the perovskite in air under a relatively high humidity around 50%, eliminating a low humidity requirement.
- Anti-solvent plays an importance role in determining quality and morphology of perovskite;
- Bar-coating can fabricate perovskite films with large grains with long aspect ratio.
- The grain boundaries in perovskite films have high charge recombination and therefore high PL intensity.

We report efficient, planar heterojunction perovskite solar cells prepared via bar coating that are compatible with large-scale fabrication on flexible substrates. Our MAPbI3 perovskite devices by bar coating method achieved a maximum power conversion efficiency (PCE) of 13.0% using a ITO/PEDOT: PSS/CH3NH3PbI3(MAPbI3)/PCBM/Ag structure, and there is very weak hysteresis in these devices. After formulation of the precursor solution and optimization of the coating process, planar perovskite thin films containing large-size grains with low surface roughness were obtained. We found that anti-solvent plays an important role in determining the uniformity, size of grains and width of grain boundaries in perovskite films. The addition of 5 vol% isopropanol into the precursor solution leads to the formation of large aspect ratio grains with a length ca. 200 µm and width of a few hundred nanometers, together with narrow grain boundaries. However, the absence or excessive of isopropanol result in perovskite films with high surface roughness and broad grain boundaries due to inefficient crystal growth or the formation of ridges between crystal grains. These grain boundaries have high PL intensity due to charge recombination and consequently deteriorate device efficiency. When bar-coating the perovskite films in air, best devices with PCE over 11% can be achieved with a relatively high humidity around 50%, eliminating the low humidity requirement during air processing.