High-performance carbon- based CsPbIBr<sub>2</sub> perovskite solar cells fabricated by precursor film preparation process

Abstract

All-inorganic perovskite has attracted extensive attention due to its photovoltaic properties and stability. Typically, the α-phase CsPbI<sub>3</sub> has an ideal bandgap of 1.73 eV suitable for the construction of high performance inorganic PSCs. But it suffers phase instability under ambient condition because of the unsatisfactory tolerance factor. By incorporating Br atoms into the perovskite structure, can greatly enhance the phase stability can be greatly enhanced. For example, CsPbBr<sub>3</sub> shows an excellent ambient stability and a wide bandgap of 2.3 eV that results in a limited light absorbtion. With the consideration from the unified perspective of the bandgap and the ambient phase stability, CsPbIBr<sub>2</sub> has a relatively appropriate bandgap (2.05 eV) and higher stability than CsPbI<sub>3</sub> and CsPbI<sub>2</sub>Br, which is made a good option for stable and efficient PSCs. However, there exist numerous defects on the CsPbIBr<sub>2</sub> film prepared by conventional one-step deposition method, which seriously affect the photoelectric conversion efficiency (PCE) of perovskite solar cells (PSCs). Considering the short dripping time and poor reproducibility of conventional anti-solvent technology, a precursor film preparation process is proposed to fabricate efficient and stable carbon-based CsPbIBr<sub>2</sub> perovskite solar cells. Using isopropyl alcohol (IPA) as the anti-solvent, the nucleation position of perovskite can be adjusted by regulating the evaporation rate of DMSO in the precursor film. In addition, guanidine thiocyanate (C<sub>2</sub>H<sub>4</sub>N<sub>4</sub>S) is added into IPA solution as a passivator to regulate the nucleation and crystallization process of perovskite. The carboxylic acid group of C<sub>2</sub>H<sub>4</sub>N<sub>4</sub>S can crosslink to Pb<sup>2+</sup> of CsPbIBr<sub>2</sub> via a chelating interaction, resulting in the easier decomposition of the CsI-DMSO-PbBr<sub>2</sub> intermediate phase in the spin-coating process of the precursor film. The amino group of C<sub>2</sub>H<sub>4</sub>N<sub>4</sub>S can also promote the crystallization and suppress the ion migration of the perovskite film through hydrogen bonds. The result shows that the compactness of the optimized CsPbIBr<sub>2</sub> film is significantly improved and the average grain size is about 800nm. The crystallinity and grain orientation are improved, and thus achieving better carrier separation and transport efficiency. The highest PCE of carbon-based CsPbIBr<sub>2</sub> PSC is obviously improved from 5.29% to 6.71%, i.e. increased by almost 21.16% compared with the control sample. Furthermore, the PSCs with precursor film preparation process possesses better long-term stability. The results obtained in this paper demonstrate that the new preparation technology can improve the quality of inorganic perovskite films in pure DMSO solvent system.