Photoconductivity measurements of CH₃NH₃PbI₃ deposited between two dielectric-protected Au electrodes show extremely slow response. The CH₃NH₃PbI₃, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH₃NH₃PbI₃ films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH₃NH₃PbI₃ transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH₃NH₃PbI₃ to take the photoinduced structural changes into consideration when measuring and interpreting the results.

With solar conversion efficiencies surpassing 20%, organometallic perovskites show tremendous promise for solar cell technology. Their high brightness has also led to demonstrations of lasing and power-efficient electroluminescence. Here we show that thin films of methylammonium lead iodide, prepared by solution processing at temperatures not exceeding 100 °C, exhibit a highly nonlinear intensity-dependent refractive index due to changes in the free-carrier concentration and for femtosecond excitation at higher intensities undergo saturation that can be attributed to the Pauli blocking effect. Nonlinear refractive index and nonlinear absorption coefficients were obtained by the Z-scan technique, performed simultaneously in open- and closed-aperture configurations. Both nanosecond- and femtosecond-pulsed lasers at multiple wavelengths were used in order to distinguish between the mechanisms inducing the nonlinearities. The magnitude and sign of the nonlinear refractive index n₂ were determined. For resonant excitation, free carrier generation is the dominant contribution to the nonlinear refractive index, with a large nonlinear refractive index of n₂ = 69 × 10^–12 cm²/W being observed for resonant femtosecond pumping and n₂ = 34.4 × 10^–9 cm²/W for resonant nanosecond pumping. For nonresonant femtosecond excitation, bound-charge-induced nonlinearity leads to n₂ = 36 × 10^–12 cm²/W. These values are equivalent to the best reported metrics for conventional semiconductors, suggesting that organometallic perovskites are promising materials for optical switching and bistability applications.

In this study we systematically explored the mixed cation perovskite Cs _x (CH ₃ NH ₃ ) _1−x PbI ₃ .