We report a systematic investigation on the role of excess PbI content in CHNHPbI perovskite film properties, solar cell parameters and device storage stability. We used the CHNHI vapor assisted method for the preparation of PbI-free CHNHPbI films under a N atmosphere. These pristine CHNHPbI films were annealed at 165 °C for different time intervals in a N atmosphere to generate additional PbI in these films. From XRD measurements, the excess of PbI was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CHNHPbI films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI contents on degradation of the CHNHPbI films. We find that the rate of degradation of CHNHPbI is accelerated due to the amount of PbI present in the film. A comparison of solar cell parameters of devices prepared using CHNHPbI samples having different PbI contents reveals a strong influence on the current density-voltage hysteresis as well as storage stability. We demonstrate that CHNHPbI devices do not require any residual PbI for a high performance. Moreover, a small amount of excess PbI, which improves the initial performance of the devices slightly, has undesirable effects on the CHNHPbI film stability as well as on device hysteresis and stability.

We discuss whether electron transfer from a photoexcited polymer donor to a fullerene acceptor in an organic solar cell is tractable in terms of Marcus theory, and whether the driving force ΔG 0 is crucial in this process. Considering that Marcus rates are presumed to be thermally activated, we measured the appearance time of the polaron (i.e., the radical-cation) signal between 12 and 295 K for the representative donor polymers PTB7, PCPDTBT, and Me-LPPP in a blend with PCBM as acceptor. In all cases, the dissociation process was completed within the temporal resolution of our experimental setup (220–400 fs), suggesting that the charge transfer is independent of ΔG 0. We find that for the PCPDTBT:PCBM (ΔG 0 ≈ −0.2 eV) and PTB7:PCBM (ΔG 0 ≈ −0.3 eV) the data is mathematically consistent with Marcus theory, yet the condition of thermal equilibrium is not satisfied. For MeLPPP:PCBM, for which electron transfer occurs in the inverted regime (ΔG 0 ≈ −1.1 eV), the dissociation rate is inconsistent with Marcus theory but formally tractable using the Marcus–Levich–Jortner tunneling formalism which also requires thermal equilibrium. This is inconsistent with the short transfer times we observed and implies that coherent effects need to be considered. Our results imply that any dependence of the total yield of the photogeneration process must be ascribed to the secondary escape of the initially generated charge transfer state from its Coulomb potential.

In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.

and with low trap-state densities. [ 3 ] This enabled the fabrication of perovskites solar cells that convinced the solar cell community with high performances such as power conversion effi ciencies of over 20%, [ 4,5 ] while offering the possibility for low cost production, e.g., by solution-processing. [ 6 ] Meanwhile, further device applications for organic-inorganic mixed halide perovskites have been discovered. For example, in 2014, low threshold levels for amplifi ed spontaneous emission showed that mixed halide perovskites can also be used for the facile fabrication of lasers with high quality factors. [7][8][9][10] Furthermore, it is known that most halide perovskite materials can exist in different crystal structures, depending on environmental conditions such as temperature. [11][12][13] Here, we show that a coexistence of tetragonal and orthorhombic phases within apparently the same crystalline grain can be optically induced into the halide perovskite CH 3 NH 3 PbI 3 at low temperatures, leading to amplifi ed spontaneous emission (ASE) simultaneously at two distinct wavelengths. The ASE feature associated with the (high temperature) tetragonal phase can be reproducibly written, read-out, and erased at 5 K by choosing appropriateThe photoluminescence in a lead halide perovskite is measured for different temperatures (5-300 K) and excitation fl uences (21-1615 µJ cm −2 ). It is found that amplifi ed spontaneous emission (ASE) is observed for an excitation density larger than about 1 × 10 18 cm −3 for both the tetragonal phase above 163 K and the orthorhombic phase below about 163 K. The fl uence that is required to obtain this excitation density depends on temperature and phase since the nonradiative decay of excitations is temperature activated with different activation energies of 85 20 ± and 24 5 meV ± for the tetragonal and orthorhombic phase, respectively. The ASE from the tetragonal phase-usually prevailing at temperatures above about 163 K-can also be observed at 5 K, in addition to the ASE from the orthorhombic phase, when the sample is previously exposed to a fl uence exceeding 630 µJ cm −2 at a photon energy of 3.68 eV. This additional ASE can be removed by mild heating to 35 K or optically, by exposing the sample by typically a few seconds with a fl uence around 630 µJ cm −2 . The physical mechanism underlying this optically induced phase transition process is discussed. It is demonstrated that this phase change can, in principle, be used for an all-optical "write-read-erase" memory device.