Solution processing of perovskite thin films

Chinese researchers offer a simple route to bandgap engineering

Lead-halide perovskite solar cells are one of the hottest topics in materials science. And for good reason – in less than a decade, their conversion efficiency has grown from 3.8 % to over 20 %, bringing them into the range of more traditional solar photovoltaic materials. But their reliance on lead (Pb), and their chemical instability has meant that they haven’t yet made any significant commercial progress. Lead-free alternatives are under development, and amongst the most promising are heterovalent double-perovskites.

These are the focus of a new Materials Today paper [DOI: 10.1016/j.mattod.2019.04.023] from researchers from Tsinghua University, who have successfully synthesised double-perovskite thin films using a solution-based method, for the first time. The team looked specifically at Cs₂AgBiBr₆, a wide-bandgap material that had been previously only been fabricated via a complex, vacuum-sealed process. In addition they explored improving the solar conversion efficiency of the films though by tin (Sb) substitution, to narrow the bandgap.

They started by adding varying quantities of CsBr, AgBr, SbBr₃, and BiBr₃ into a DMSO solution. This was heated to 180°C – close to the solvent’s boiling point – until the precursors had fully dissolved. Quartz slides were dipped into the solution to coat them, and then heated to evaporate the DMSO, leaving the active ingredients behind. Samples were annealed to produce a series of Cs₂AgSb_xBi_1-xBr₆ (where x = 0, 0.25, 0.50, 0.75) thin films, which varied from yellow to red as the Sb-content increased.

X-ray diffraction indicated that all four native films were highly-crystalline, with SEM confirming an average grain size of approximately 10 µm. However, the bismuth-dominated samples were found to be of a higher quality than films that contained mostly Sb. All samples exhibited strong absorption in the visible light region, and the bandgap was found to decrease gradually from 2.22 to 1.97 eV as the proportion of Sb increased from 0 to 0.75.

The films were then used to create multilayer, double-perovskite solar cells, all with identical architecture. Their electrical performance was characterised under simulated sunlight, and the Cs₂AgSb_0.25Bi_0.75Br₆ solar cell was found to outperform their reference Cs₂AgBiBr₆ cell. The open-circuit voltage of the Sb-doped cell was 64 % higher, and its conversion efficiency 31 % higher than the undoped cell. However, once the proportion of Sb was further increased from to 0.50, the cell’s conversion efficiency sharply declined. The authors attribute this to pin-hole-like defects that were seen in the Sb-dominated films, and suggest that careful optimisation of the interface could solve this issue.

This paper offers a versatile, solution-based method for producing double-perovskite materials. While there is still work to be done on their integration, this fabrication route could help to accelerate their development, making them a possible option for practical photovoltaic devices.

---

Chenwei Li, Degang Jiang, Bingbing Huo, Meichun Ding, Congcong Huang,Dedong Jia, Haoxiang Li, Chen-Yang Liu, Jingquan Liu. “Bandgap-tunable double-perovskite thin films by solution processing” Materials Today, In Press (2019). DOI: 10.1016/j.mattod.2019.04.023