# The impact of tris(pentafluorophenyl)borane hole transport layer doping on interfacial charge extraction and recombination

## TL;DR

This paper studies how doping hole transport layers in perovskite solar cells affects charge extraction and recombination at interfaces.

## Contribution

The study introduces a new method using tris(pentafluorophenyl)borane doping to improve junction quality and reduce recombination in solar cells.

## Key findings

- Doping with tris(pentafluorophenyl)borane improves the junction quality in spiro-OMeTAD and PTAA-based devices.
- The enhanced performance is due to reduced recombination at the perovskite/hole transporter interface under illumination.

## Abstract

Selective charge transport layers have a strong influence on the overall efficiency and stability in perovskite solar cell devices. Specifically, the charge extraction and recombination occurring at the interfaces between the perovskite and these materials can be a limiting factor for performance. A lot of effort has been put into improving the conductivity of selective contacts, as well as the junction quality and energetic alignment with the absorber. On the hole extracting side, organic semiconductors have been extensively used due to their flexibility and favorable properties. Two of such compatible materials that have yielded high performing devices are the small molecule 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) and the polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). In this work, we investigate the impact of hole transport layer doping on the performance and potential distribution in solar cells based on these materials. To do so on operating solar cells, we created samples with exposed cross-sections and examined their potential profile distributions with Kelvin probe force microscopy (KPFM), implementing our comprehensive measurement protocol. Using the Lewis acid tris(pentafluorophenyl)borane (BCF), we enhanced the hole extracting material/perovskite junction quality in spiro-OMeTAD and in PTAA based devices. Measurements under illumination show that the improvement is caused by a reduced recombination rate at the perovskite/hole transporter interface.