Abstract
The small exciton binding energy Eb in hybrid organic–inorganic perovskites (HOIPs) enables their extraordinary photovoltaic performance. The measured Eb in MAPbI3 (MA = CH3NH3), the most studied HOIP, is 50% smaller in the orthorhombic phase than estimated from the high-frequency dielectric constant and drops an additional 25% in the tetragonal phase. Here we show that these puzzling exciton behaviors can be quantitatively explained by incomplete screening arising from polar optical phonons (POPs) and randomly oriented MA ions. Emission/absorption of POPs introduces a virtual radius of electron (hole), within which the screening due to POPs is ineffective. Randomly oriented dipoles of MA ions in the tetragonal phase mediate an extra long-range coupling between the electron and hole. The exciton Hamiltonian is accurately solved by using variational hydrogenic wave functions as the basis set. Our results consistently account for the observed exciton properties and reveal the impact of polar coupling on excitons in HOIPs.
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