Influence of the Synthetic Procedures on the Structural and Optical Properties of Mixed-Halide (Br, I) Perovskite Films

Abstract

The influence of thermal treatments on the properties of mixed bromide-iodide organolead perovskites (MAPbI3−xBrx, MA=CH3NH3) is investigated in films prepared in air by single-step solution processes based on different precursor solutions. Initially, the bandgap energy (EG) dependence on composition is reconsidered on films obtained by mixtures of tri-halide solutions.An EG(x) relation is obtained that is expected to be independent of the film properties and can be used to assess perovskite composition. In these samples recombination centres are observed whose energy depth increases with x, likely involving the simultaneous presence of iodide and bromide, while the Urbach energy increases with the grain surface-to-volume ratio, which points out that the defects giving sub-bandgap absorption originate from grain boundaries. Tri-halide mixtures allow perovskite synthetic processes suitable for solar cell production, being fast and reproducible. A slight MABr excess in the solution made of MABr and PbI2 gives MAPbI2Br films free of PbI2 phases and with a high compositional stability, but non-radiative recombination channels can make the material not appropriate for high efficiency solar cells.Finally, the solution made of MAI and PbBr2 (3:1 molar ratio) is the less promising for solar cell production because its non-stoichiometric nature synthesis reproducibility an issue.

Effect of Halide Composition on the Photochemical Stability of Perovskite Photovoltaic Materials

“…Absorptiond egradation was found to correlate with increased full width at half maximum (FWHM) of the XRD peaks for mixed halide perovskites compared to MAPbI 3 and MAPbBr 3 (Figure 5a nd Ta ble 1). As imilar behavior of the FWHM of the XRD peaks was observedi n MAPb(I 1Àx Br x ) 3 films by Fedeli et al, [14] who also observed correlated changes in the Urbach energy,w hich may be related to structurald isorder.F urthermore, both Fedeli et al [14] and Sadhanala et al [10] found that changes in the FWHM of PL peaks were correlated with the Urbach energy variations, that is, the band-to-band PL peak becamen arrower when the Urbach energy was smaller (less structurald isorder) at low and high x values, and vice versa at intermediate x values. The decreasedg rain sizes in the mixed halide films (inc ompar- ison with those in MAPbI 3 and MAPbBr 3 samples) have been confirmed by scanning electron microscopy ( Figure S4).…”

“…[15,16] The E g values for MAPbI 3 and MAPbBr 3 samples are in agreement with previously reported ones (i.e., 1.58 [17] and 2.3 eV, [18 ] respectively). Sadhanala et al [10] and Fedeli et al [14] confirmed that the ratios of Br to Ii nt heir MAPb(I 1Àx Br x ) 3 films are the same as those in the solutionsw hen as toichiometric precursor compositioni n as ingle deposition step was used. We concludet hat the mixed halide films have asinglephase structure (solid solution)with Br contentslightly larger than that expected from the precursor materials, probablyo wing to the sequential, two-step deposition method (see the Experimental Sectionf or details).…”

“…The relationship between the halide compositionofthe precursor solution and the films can be strongly dependento nt he technology of the film preparation. Sadhanala et al [10] and Fedeli et al [14] confirmed that the ratios of Br to Ii nt heir MAPb(I 1Àx Br x ) 3 films are the same as those in the solutionsw hen as toichiometric precursor compositioni n as ingle deposition step was used. However,i no ther studies, [19] where an excessive amounto fM As alts was involved in the film preparation, adiscrepancy between actual halide compositions in solid perovskites and initial precursor solutionsw as reported.…”

“…The actual halide compositiono ft he films wasd etermined and comparedt ot hat in the precursors olutions. Recently,F edeli et al [14] demonstrated as mall negative deviation from Vegard's law for x ! [13] Noh et al [4] suggested the relevance of this rule for their MAPb(I 1Àx Br x ) 3 films at the entire range of x (0 x 1).…”

“…Applying Vegard's law to our XRD results showed as malld ifference in the Br content between the expected and actual composition (Table 1). Recently,F edeli et al [14] demonstrated as mall negative deviation from Vegard's law for x ! 0.19 in MAPb(I 1Àx Br x ) 3 films, which they referred to the shift from tetragonalt oc ubic [a] Ratio in the organic precursor solution.…”