Ultralow Absorption Coefficient and Temperature Dependence of Radiative Recombination of CH₃NH₃PbI₃ Perovskite from Photoluminescence

Spectrally resolved photoluminescence is used to measure the band-to-band absorption coefficient α_BB(ℏω) of organic–inorganic hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10^–14 cm^–1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α_BB(ℏω) is calculated from the photoluminescence spectra of CH₃NH₃PbI₃ in the temperature range 80–360 K. Based on the temperature-dependent α_BB(ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, <i>B</i>(<i>T</i>) × <i>n</i>_<i>i</i>², is also obtained

Inverted Hysteresis in CH₃NH₃PbI₃ Solar Cells: Role of Stoichiometry and Band Alignment

J–V hysteresis in perovskite solar cells is known to be strongly dependent on many factors ranging from the cell structure to the preparation methods. Here we uncover one likely reason for such sensitivity by linking the stoichiometry in pure CH₃NH₃PbI₃ (MAPbI₃) perovskite cells with the character of their hysteresis behavior through the influence of internal band offsets. We present evidence indicating that in some cells the ion accumulation occurring at large forward biases causes a temporary and localized increase in recombination at the MAPbI₃/TiO₂ interface, leading to inverted hysteresis at fast scan rates. Numerical semiconductor models including ion accumulation are used to propose and analyze two possible origins for these localized recombination losses: one based on band bending and the other on an accumulation of ionic charge in the perovskite bulk

Supplement 1: Total absorption of visible light in ultrathin weakly absorbing semiconductor gratings

Supplemental document Originally published in Optica on 20 June 2016 (optica-3-6-556

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells

Perovskite material with a bandgap of 1.7–1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation