Perovskites on Ice: An Additive-Free Approach to Increase the Shelf-Life of Triple-Cation Perovskite Precursor Solutions

Abstract

The development of stable perovskite precursor solutions is critical if solution-processable perovskite solar cells (PSCs) are to be practically manufacturable. Ideally, such precursors should combine high solution stability without using chemical additives that might compromise PSC performance. Here, it was shown that the shelf-life of high-performing perovskite precursors could be greatly improved by storing solutions at low-temperature without the need to alter chemical composition. Devices fabricated from solutions stored for 31 days at 4 °C achieved a champion power conversion efficiency (PCE) of 18.6 % (97 % of original PCE). The choice of precursor solvent also impacted solution shelf-life, with DMSO-based solutions having enhanced solution stability compared to those including DMF. The compositions of aged precursors were explored using NMR spectroscopy, and films made from these solutions were analysed using X-ray diffraction. It was concluded that the improvement in precursor solution stability is directly linked to the suppression of an addition-elimination reaction and the preservation of higher amounts of methylammonium within solution.

Keywords: perovskites; photovoltaics; solar cells; solution chemistry; solution processing.

Scheme 1

(a) Hydrolysis of DMF to produce DMA. (b) i) Deprotonation of MAX component where X represents a halide ion. ii) Schematic of the addition‐elimination reaction producing MFA. iii) Schematic of the addition‐elimination reaction producing DMFA. For simplicity, the NH₃ reaction products are not shown. Reaction scheme (b) reproduced from [11].

Figure 1

(a) PCE of perovskite device with the stack ITO/SnO₂/perovskite/spiro‐OMeTAD/Au fabricated from RT‐aged TC‐mixed (DMF/DMSO) solutions. (b) J‐V curves and (c) stabilized power output for the best performing devices from RT‐aged solutions. (d) PCE of devices fabricated from low‐temperature‐aged solutions, with (e) J‐V curves and (f) stabilized power output for the best performing cells. Full J‐V sweep parameters for (b) and (e) and all device data for boxplots in (a) and (d) are given in Tables S2 and S3.

Figure 2

UV/Vis absorption spectra of TC‐mixed perovskite films prepared from solutions that have been aged at (a) RT and (b) at a low temperature of 4 °C. (c) Images of films prepared from a precursor solution aged for 115 days at RT and at 4 °C (LT) next to a film fabricated from a freshly prepared control ink (marked C). SEM images comparing films fabricated from (d) RT‐ and (e) LT‐aged (115 days) solutions to those fabricated from a (f) freshly prepared control solution.

Figure 3

XRD patterns for films made from RT‐aged TC‐mixed solutions after 3, 31, 60 and 115 days. Red stars mark cubic perovskite peaks and yellow circles mark emerging intermediate peaks.

Figure 4

XRD highlights for films made from TC‐mixed solutions that have been stored for 115 days at RT (yellow) and LT (blue) together with a freshly prepared control film (purple). Here, films from the LT‐aged ink still exhibit strong scattering from the perovskite phase, whereas in the RT‐aged solution a non‐perovskite phase is present (see Figure S12).

Figure 5

(a–c) NMR spectra of TC‐mixed solution dissolved in DMF/DMSO measured at several time points over a 1‐month period (full spectra in Figures S15–S18). (d) Absolute change in molarity for the various organic components over time, estimated compared to an internal standard for which the molarity remains constant. (e) Relative concentration of the organic cations over time determined as described in Supporting Information Note 3. Here we see a significant reduction in the relative concentration of both MA and FA, along with an increased concentration of MFA and DMFA.

Figure 6

Relative intensity of different cationic species in the TC precursor solutions after 6 and 216 days after storing at either RT or 4 °C (LT). Data is shown for solutions prepared from a DMF/DMSO solvent mix or DMSO‐only.