Perovskite-Polymer Blends Influencing Microstructures, Nonradiative Recombination Pathways, and Photovoltaic Performance of Perovskite Solar Cells.

Solar cells based on organic-inorganic halide perovskites are now leading the photovoltaic technologies because of their high power conversion efficiency. Recently, there have been debates on the microstructure-related defects in metal halide perovskites (grain size, grain boundaries, etc.) and a widespread view is that large grains are a prerequisite to suppress nonradiative recombination and improve photovoltaic performance, although opinions against it also exist. Herein, we employ blends of methylammonium lead iodide perovskites with an insulating polymer (polyvinylpyrrolidone) that offer the possibility to tune the grain size in order to obtain a fundamental understanding of the photoresponse at the microscopic level. We provide, for the first time, spatially resolved details of the microstructures in such blend systems via Raman mapping, light beam-induced current imaging, and conductive atomic force microscopy. Although the polymer blend systems systematically alter the morphology by creating small grains (more grain boundaries), they reduce nonradiative recombination within the film and enhance its spatial homogeneity of radiative recombination. We attribute this to a reduction in the density of bulk trap states, as evidenced by an order of magnitude higher photoluminescence intensity and a significantly higher open-circuit voltage when the polymer is incorporated into the perovskite films. The solar cells employing blend systems also show nearly hysteresis-free power conversion efficiency ∼17.5%, as well as a remarkable shelf-life stability over 100 days.

Thiophene-Functionalized Hybrid Perovskite Microrods and their Application in Photodetector Devices for Investigating Charge Transport Through Interfaces in Particle-Based Materials.

Particle-based semiconductor materials are promising constituents of future technologies. They are described by unique features resulting from the combination of discrete nanoparticle characteristics and the emergence of cooperative phenomena based on long-range interaction within their superstructure. (Nano)particles of outstanding quality with regards to size and shape can be prepared via colloidal synthesis using appropriate capping agents. The classical capping agents are electrically insulating, which impedes particle-particle electronic communication. Consequently, there exists a high demand for realizing charge transport through interfaces especially for semiconductors of relevance like hybrid perovskites (HYPEs), for example, CH3NH3PbI3 (MAPI) as one of the most prominent representatives. Of particular interest are crystals in the micrometer range, as they possess synergistic advantages of single crystalline bulk properties, shape control as well as the possibility of being functionalized. Here we provide a synthetic strategy toward thiophene-functionalized single crystalline MAPI microrods originating from the single source precursor CH3NH3PbI3TEG2 (TEG = triethylene glycol). In the dark, the microrods show enhanced charge transport characteristics of holes over 2 orders of magnitude compared to microscale cuboids with insulating alkyl surface modifiers and nonfunctionalized random sized particles. In large-area prototype photodetector devices (2.21 cm2), the thiophene functionalization improves the response times because of the interparticle charge transport (tON = 190 ms, tOFF = 430 ms) compared to alkyl-functionalized particles (tON = 1055 ms, tOFF = 60 ms), at similar responsivities of 0.65 and 0.71 mA W-1, respectively. Further, the surface functionalization and crystal grains on the micrometer scale improve the device stability. Therefore, this study provides clear evidence for the interplay and importance of crystal size, shape and surface modification of MAPI crystals, which is of major importance in every optoelectronic device.

H-aggregate analysis of P3HT thin films-Capability and limitation of photoluminescence and UV/Vis spectroscopy.

Polymer morphology and aggregation play an essential role for efficient charge carrier transport and charge separation in polymer-based electronic devices. It is a common method to apply the H-aggregate model to UV/Vis or photoluminescence spectra in order to analyze polymer aggregation. In this work we present strategies to obtain reliable and conclusive information on polymer aggregation and morphology based on the application of an H-aggregate analysis on UV/Vis and photoluminescence spectra. We demonstrate, with P3HT as model system, that thickness dependent reflection behavior can lead to misinterpretation of UV/Vis spectra within the H-aggregate model. Values for the exciton bandwidth can deviate by a factor of two for polymer thicknesses below 150 nm. In contrast, photoluminescence spectra are found to be a reliable basis for characterization of polymer aggregation due to their weaker dependence on the wavelength dependent refractive index of the polymer. We demonstrate this by studying the influence of surface characteristics on polymer aggregation for spin-coated thin-films that are commonly used in organic and hybrid solar cells.