Anisotropic Electric Field Effect on the Photoluminescence of CH₃NH₃PbI₃ Perovskite Sandwiched between Conducting and Insulating Films

Photoluminescence (PL) of a nanocrystalline film of methylammonium lead iodide perovskite (MAPbI₃) sandwiched between an electrode of a fluorine-doped tin oxide (FTO) layer and an insulating film of poly­(methyl methacrylate) is found to increase and decrease significantly with the application of an external electric field (<i><b>F</b></i>_ext), depending on the direction of the applied field, based on the measurements of electrophotoluminescence (E-PL) spectra, i.e., field-induced change in PL spectra. The field-induced change in PL intensity is confirmed to originate from the field-induced change in the number of free carriers which induce radiative recombination, based on temporally resolved E-PL measurements. We propose that an internal field (<i><b>F</b></i>_int) exists even without application of <i><b>F</b></i>_ext. The anisotropic behavior of the effect of <i><b>F</b></i>_ext on PL is interpreted in terms of a synergy effect of <i><b>F</b></i>_int and <i><b>F</b></i>_ext; both fields are additive with the applied field direction from Ag to FTO electrode (positive direction) or subtractive with the opposite applied field direction (negative direction), where FTO is the positive electrode, resulting in an increased or decreased total electric field as well as quenching or enhancement of PL, respectively. The PL lifetime in the nanosecond region increased and decreased with an application of an electric field in the positive and negative directions, respectively, which is attributed to a field-induced change in the concentration of free carriers

Electroabsorption Studies of Multicolored Lead Halide Perovskite Nanocrystalline Solid Films

Electroabsorption (E-A) spectra of lead halide perovskites, that is, CH₃NH₃PbI_3–<i>x</i>Br_<i>x</i> (<i>x</i> = 0–3), which show large spectral shift depending on the ratio between iodine and bromide, have been measured. By analyzing E-A spectra with the integral method, spectral shape of the absorption spectra for the first exciton band and binding energy of exciton have been determined. Magnitudes of the change in electric dipole moment and molecular polarizability following excitation into the exciton state have been also evaluated in these perovskites. The binding energy of electron and hole in exciton of these materials as well as the magnitude of change in electric dipole moment following exciton absorption is roughly the same, suggesting that the difference of the photoenergy conversion in photovoltaic cells using these materials comes from the difference in light harvesting effect and difference in carrier mobility, not from the difference in carrier generation efficiency. The frequency-dependent third-order nonlinear susceptibility χ⁽³⁾ has also been calculated, based on the E-A spectra

Ag Doping of Organometal Lead Halide Perovskites: Morphology Modification and p‑Type Character

We report a simple synthetic approach to grow uniform CH₃NH₃PbI₃ perovskite (PSK) layers free of pinholes via varied portions of silver iodide (AgI) added to the precursor solution. XRD/EDS elemental mapping experiments demonstrated nearly uniform Ag distribution inside the perovskite film. When the 1% AgI-assisted perovskite films were fabricated into a p-i-n planar device, the photovoltaic performance was enhanced by ∼30% (PCE increased from 9.5% to 12.0%) relative to the standard cell without added AgI. Measurement of electronic properties using a hall setup indicated that perovskite films show p-type character after Ag doping, whereas the film is n-type without Ag. Transients of photoluminescence of perovskite films with and without AgI additive deposited on glass, p-type (PEDOT:PSS), and n-type (TiO₂) contact layers were recorded with a time-correlated single-photon counting (TCSPC) technique. The TCSPC results indicate that addition of AgI inside perovskite in contact with PEDOT:PSS accelerated the hole-extraction motion whereas that in contact with TiO₂ led to a decelerated electron extraction, in agreement with the trend observed from the photovoltaic results. The silver cationic dopant inside the perovskite films had hence an effect of controlling the morphology to improve photovoltaic performance for devices with p-i-n configuration