The power conversion efficiency (PCE) of silicon-based solar cells is rapidly approaching its practical limit of 29%,[1] highlighting the need for alternative absorber materials. Emerging photovoltaic materials such as hybrid halide perovskites (HHPs) have shown to be a good alternative to silicon due to their high PCE (26.7%) as well as low-cost solution-based processing methods.[2], [3]

In order to understand the impact of the precursor solution in a device processed from solution, it is necessary to gain an insight into how the precursors affect the early stage of crystallisation. To have a better understanding of the role of the solvent in HHPs precursor solutions, we investigated MAPbI₃ precursor solution in γ-butyrolactone (GBL), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) and mixtures thereof using small angle X-ray scattering (SAXS).

SAXS is a non-destructive technique based on the scattering length density difference between the scattering objects and the matrix. SAXS allows us to determine the average distance between the scattering objects, their size and shape as well as the interaction with each other.[4] We performed SAXS experiments at HZB synchrotron radiation source BESSY II at the four-crystal monochromator beamline[5] of the Physikalisch-Technische Bundesanstalt using the ASAXS endstation.[6]

All the SAXS patterns obtained from the measurements show a clear maximum in the scattered intensity at q-values between 2.7 and 3.8 nm^-1 (values for GBL and DMSO respectively), a peak in the SAXS pattern is an indication of the presence of agglomerates in solution. The average distance between the scattering objects (d_exp) varies depending on the solvent used, showing that the solvent must be part of the scattering objects. In a previous study[7] we showed that the scattering objects in solution have a core-shell configuration, with a solvent shell surrounding a [PbI₆] core, which can be arranged as a single octahedron or as corner-sharing octahedra. In this study, we show that solvents with high donor number (DN), such as NMP or DMSO, present a lower d_exp compared to DMF and GBL. This indicates that solvents with high DN favour the single octahedron arrangement in the core. The results obtained from the analysed SAXS data (using SASfit[8]) indicate that solvents with high DN favour monodisperse solutions, i.e. all the scattering objects have the same size. DMF and GBL favour polydispersity, which follows a lognormal size distribution. This is in agreement with the proposed core-shell model since having only a single octahedron in the core can explain the monodispersity in high DN solvents.

We will discuss how the nature of the solvent influences the arrangement of the precursors in the precursor solution since it has the potential to impact the crystallisation process of the HP and, therefore, the performance of a device produced from solution processing.