Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
We present the optical constants determined from spectroscopic ellipsometry of single crystals of CH3NH3PbX3 (where X = I, Br, Cl). These results are interpreted by comparison with optical constants and band structure derived from the highest level of ab initio calculations using the relativistic the quasiparticle self-consistent GW approximation (QS-GW). The calculations reproduce the majority of observed absorption features. However the increasingly strong absorption peak observed at the band edge moving from X = I to Br to Cl attributed to excitonic absorption is not predicted by QS-GW since electron-hole interactions are not accounted for by the theory. Strikingly, the analysis shows that transitions from the highest valence band (VB1) to the lowest conduction band (CB1) at different momenta are responsible for all three of the main absorption peaks at (1.6, 2.5 and 3.1 eV) in CH3NH3PbI3 (only minor contributions from the second highest VB and to the second lowest CB are involved).
This observation of is particular relevance to the interpretation of transient absorption spectroscopy (TAS) measurements, the nature of which are the subject of considerable debate. We use our optical constants and energy band diagram of CH3NH3PbI3 to simulate its TAS and find good agreement with measurement. The results suggest that the bleaching states observed at 480 and 760 nm can be assigned to a reduction in the VB1 → CB1 transition at two different regions of momentum space. It is not necessary to invoke additional transitions from VB2 to explain the spectra.
Our calculations also indicate that the orientation of CH3NH3 cations can have a significant influence (up to ~0.14 eV) on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties.
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