Abstract text (incl. figure legends and references)

Introduction

Triple-halide perovskites¹ form a promising but complex material system that offers the possibility to vary band-gap energies via compositional changes for tandem solar cell applications. Perovskite-silicon tandem solar cells already exceeded the 31% of efficiency² and represent today's most promising tandem solar cell technology. Main obstacles to bring this technology to the market are the upscaling to industrial cell sizes and through-puts.

Objectives

The aim of the present study is to investigate triple-halide perovskite absorber layers for solar cells by correlative electron microscopy. These layers were deposited by slot-die coating plus N2 gas quenching, a deposition technique suitable for large-area and high-throughput production. Optimal process parameters for this technique, such as substrate temperatures, have not yet been well investigated. Therefore, we analyzed absorber layers produced at various substrate temperatures by several electron microscopy methods and correlated these results with photovoltaic parameters of cells made of absorbers with identical composition.

Materials & Methods

In the present study, triple-halide perovskite thin films with the composition (Cs_0.22FA_0.78)Pb(I_0.85Br_0.15)₃ + 5 mol% MAPbCl₃ were analyzed. These thin films were prepared by means of slot-die coating and subsequently annealed at temperatures of 125 °C, 150 °C, 160 °C and 170 °C. We conducted secondary electron (SE) imaging, electron backscatter diffractometry (EBSD), energy-dispersive X-ray spectroscopy (EDX), and cathodoluminescence (CL) in scanning electron microscopy on these thin films and determined solar cell parameters (short-circuit current J_SC, open-circuit voltage V_OC, fill factor FF and photo-conversion efficiency PCE) under one sun illumination in a solar simulator.

Results

The SE imaging indicated and EBSD measurements confirmed an increased average grain size with increasing annealing temperature, as it had been expected. By means of EBSD, EDX and CL, PbI₂ precipitates were detected and their area fraction on the perovskite-type thin films were quantified (Fig. 1). When varying the annealing temperature from 125 to 170 °C the efficiency maximum of almost 20% was found at 125 °C, and the efficiency decreases with increasing temperature (mainly owing to decreasing V_oc and FF values). The area fraction of PbI₂ increases from 5% to more than 20% within the studied temperature range, which enhances nonradiative recombination (thus, decreasing the V_oc) and increases significantly the series resistance of the solar cells (thus, decreasing the FF).

Figure 1: Triple-halide perovskite films analyzed by SE imaging, EBSD, CL and EDX. Green colors correspond to the triple-halide perovskite phase, and blue to PbI₂.

Conclusion

We found an increasing amount of PbI₂ precipitates with increasing annealing temperatures on the surface of triple-halide perovskite films by means of various electron microscopy techniques. It was possible to correlate the presence of the PbI₂ phase on the triple-halide perovskite thin-film surface to deteriorated solar cell performances. For the further improvement of these solar cells, it is mandatory to reduce the area fraction of the PbI₂ precipitates to a minimum.

References

¹Xu, J., et al. Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems. Science 367, 1097–1104 (2020).

²NREL efficiency chart (www.nrel.gov/pv/cell-efficiency.html)