3D hybrid perovskites are currently superstar materials for photovoltaics and attract increasing attention of the scientific community [1]. Recently, they also revealed attractive photoluminescence, as it is already well known for 2D layered hybrids. In the quest for a deeper understanding of such appealing properties, theoreticians have recently taken up the challenge. Our scientific approach starts from the identification of a multifaceted change of paradigm related to the recent advances in this field [1,2] and is based on concepts of solid-state physics originally developed for conventional semiconductors. We will show how the broad light-harvesting abilities and attractive transport properties of 3D metal-halide hybrid perovskites can be related to the multi-bandgap and multi-valley nature of their band structure [2]. Channels in reciprocal space facilitate carrier redistribution after optical excitation. Extensive analysis of older experimental data, as well as comparison to 2D layered hybrids and conventional semiconductor heterostructures, allow clarifying the nature of the photoexcited species. From dielectric responses over a wide frequency range we show that the Wannier-like exciton evidenced at low temperature becomes almost entirely screened at room temperature. Besides the screening by optical phonons, analogy with the reorientation dynamics of CN- in alkali-cyanides [3] and comparison to all-inorganic perovskites suggests further screening due to collective rotational motion of the organic cations [2]. This picture is consistent with the recent experimental findings.As a conclusion, the change of paradigm from DSSC to a new class of semiconductor solar cells is most probably associated at room temperature to free carriers and Bloch states of the inorganic lattice, rather than excitons. The collective rotations of the organic cations are proposed as a fundamental screening mechanism in 3D hybrids.

[1] Burschka, J. et al.Nature. 499, 316 (2013). Liu, M. et al. Nature. 501, 395 (2013). Stranks, S. D. et al. Science. 342, 341 (2013). Xing, G. et al. Science. 342, 344 (2013).

[2] Even, J. et al, Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites J. Phys. Chem. C. DOI: 10.1021/jp503337a (2014). Theoretical insights into multibandgap hybrid perovskites for photovoltaic applicationsProc. SPIE 9140, Photonics for Solar Energy Systems V, 91400Y (2014)

[3] Ortiz-Lopez, J. Phys. Rev. Lett. 50, 1289–1292 (1983). Lynden-Bell, R. M, Rev. Mod. Phys., 66, 721-761 (1994).