Relaxor Solar Cell Absorbers

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Solar cell absorbers must fulfill a number of requirements for high performance: Appropriate band-gap, high charge carrier mobility and lifetime, appropriate work function to match charge carrier selective conductive layers, and an appropriate design for light capturing. In order to avoid charge carrier recombination, classical semiconductors necessitate exclusive crystal purification techniques removing point and extended defects to very low concentrations. Recently, we showed that the outstanding performance of the new halide perovskite absorbers is partly due to dielectric screening effects. The competition between classical phonon based polaron formation and dipolar dielectric screening permits charge carriers (electrons as well as holes) to propagate very long distances in the crystal without formation of small polarons (states deep in the band-gap) and negligible interaction with existing point defects (vacancies or foreign atoms). This proposal deals with the transfer of this result to those oxide perovskites, in which also two independent polar mechanisms arise, namely relaxor ferroelectrics. Polar nanoregions (PNRs) as well as the classical Fröhlich polaron screening provide independent screening mechanisms. We assume that a similar performance as in the halides can be achieved at much improved lifetime and robustness of the material. The project aims at tuning the band gap of existing relaxor systems towards the optimum for the solar spectrum (around 1.3 eV). The promising candidate relaxor system PbFe0.5Nb0.5O3 (PFN) possesses a band gap of around 1.0 eV. The chemical versatility of the perovskites will allow us to tailor the bandgap and the work function by cation doping. This facile material design will enable the development of a (graded) heterojunction solar cell containing a PFN layer and adjacent layers of doped PFN with properly aligned energy levels, thus obtaining cells with efficient photon capture as well as charge extraction. Efforts of thin film and cell engineering will be continually accompanied by investigation of band structures, structural properties, and charge carrier dynamics. Thus, on the one hand we will achieve new technical developments in the field of heterojunction solar cells and on the other hand we will gain and provide new insights into relaxor based photovoltaics of interest for a broad community in solid state physics.The teams have experience in investigation of the photovoltaic effect in perovskite materials and fabrication of solar cells. They also have a strong background in the study of ferroelectrics and relaxors. A close collaboration between the teams has been established in the last 5 years offering a multitude of complementary expertise necessary for the success of the project: nanoparticle synthesis, thin films preparation, solar cell development, structural characterization, measurements of functional properties on the macroscopic and local scales, dielectric and Raman spectroscopy.