Publication date: 8th January 2019
Optoelectronic devices based on hybrid perovskites have undergone a remarkable development in recent years mainly due to enormous efforts in optimizing their film properties. However, the relatively complex film formation dynamics as they occur, e.g. during solution processing, has not yet been addressed and understood in detail. Here we report measurements of absorption and photoluminescence (PL) during a solution based two-step synthesis of the model hybrid perovskite CH3NH3PbI3 in situ while spin coating. We observe changes in both, absorption and PL during the spin-coating process which allows us to identify different time regions. They are associated with specific spectral changes which are (i) a fast initial bathochromic shift of the PL, while absorption has hardly evolved yet, (ii) a continuously increasing absorption intensity while the PL remains unchanged, (iii) a narrowing of the PL from the higher energy edge with concomitant dynamic modulation of absorption spectrum, (iv) finally, a distinct bathochromic shift of both absorption and PL accompanied by an intense step-like increase of the absorption intensity. By changing the substrate temperature between 15 – 80°C we find that the spectral dynamics becomes faster with temperature yet the general sequence of spectral changes remains unchanged up to 60°C. Above 60°C, we observe significantly different spectral behaviors. Our results suggest that prior to film formation, a variety of intermediate states exist in the electronic structure to which the transition rates can be altered with temperature and which change fundamentally above 60°C.
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