Silver-bismuth double perovskite semiconductors: variations on a theme

Abstract

Lead-halide perovskites (LHPs) have experienced tremendous development within 15 years from their first report as light harvesters in solar cells until today, in which their power conversion efficiency has been gradually improved from 3.8% to 26.1%. These high efficiencies in combination with their easy and low-cost processability as thin films make them a potential alternative to conventional silicon-based solar cells. However, compared to the latter they possess a low environmental stability and pose an environmental and human health hazard due to their lead content. While the stability has been gradually improved by extensive chemical engineering and encapsulating techniques, the toxicity aspect can only be tackled by substituting lead. Cs2AgBiBr6 is such a material – which this dissertation revolves around - in which lead is substituted by equal amounts of silver and bismuth, thus named double perovskite, which is characterized by high environmental stability and low toxicity. This dissertation can be divided into two main topics: The tuning of absorption and emissive processes in Ag-Bi double perovskites via structural engineering and the investigation and improvement of hole transport material (HTM)-free Cs2AgBiBr6 solar cells. The first topic covers two different approaches to influence the emission of Ag-Bi double perovskites. On the one hand, Cs2AgBiBr6 was doped with the lanthanides Eu3+ and Yb3+, resulting in two new emission features: A weak emission from dopant-introduced trap states (Eu3+) and strong emission due to an efficient energy transfer towards the dopant (Yb3+), respectively. On the other hand, Cs2AgBiBr6 has been dimensionally reduced to create monolayered 2D perovskites which enlarges the material’s bandgap and introduces new characteristic emission. Regarding the second topic, HTM-free Cs2AgBiBr6 solar cells have been prepared using ultrasonic spray-coating to deposit electrodes that consist of carbon black, won from upcycled industry waste. Thus, this approach represents an end-of-waste strategy to fabricate “green” solar cells. To further improve these devices, the surface of the Cs2AgBiBr6 thin films has been modified to create 2D/3D mixed phases. The 2D/3D modification improves the perovskite’s band alignment towards the back electrode and theoretical calculations further suggest that it reduces the defect concentration close to the perovskite/electrode interface. Accordingly, the application of the 2D/3D modification represents a valuable alternative to HTMs for Cs2AgBiBr6 solar cells that utilize carbon-based electrodes.