AmericanChemicalSociety.com

The aim of the project has been to develop and perform an experiment that uses the optical and electrical detection of coherently manipulated spin-dependent transition rates in order to enhance the microscopic understanding of the wide range of charge carrier recombination processes in hydrogenated amorphous silicon (a-Si:H), a thin film semiconductor material that is used for solar cell applications. Figure 1 displays a sketch that illustrates the complexity of the a-Si:H band diagram and some of the many recombination processes. The experiment allows a categorization (“mapping”) of these mechanisms into a range of Lande-(g)-factors and charge carrier spin coupling regimes. The purpose of the mapping is to answer the long debated question of which of the recombination processes are geminate (without effect on conductivity) and which are non-geminate (with effect on conductivity), a question that is of great significance for the understanding of solar cell efficiency limitations.

Fig. 1: Band diagram of strongly disordered hydrogenated amorphous silicon. Localized band gap states lead to many qualitatively different recombination mechanisms which are detrimental to the device efficiency of solar cells.

The project was awarded about two years after the PIs relocation to the University of Utah, when most of the PIs laboratory had been set up and the preliminary experiments for the funded activities had been demonstrated. These preparation experiments included the demonstration of pulsed electrically and pulsed optically detected magnetic resonance spectroscopies (pEDMR and pODMR, respectively) performed on different charge carrier recombination and transport mechanisms in various organic and inorganic semiconductor materials. The project consisted of (i) a further development of these experiments for the application to a-Si:H thin film solar cell devices and (ii) to map g-factors and charge carrier spin coupling regimes using pEDMR and pODMR experiments that are measured under identical conditions in order to allow a determination of how an observed recombination path affects the optical and electrical materials properties.

Twenty month after the beginning of this project, the main goals of the funded activities have been achieved. A-Si:H thin fim solar cell devices suitable for pEDMR measurements have been development in collaboration with MV Systems, Inc., the industry partner of the project. The comparative mapping between pEDMR and pODMR on a-Si:H revealed unambiguously the different natures of the detected geminate and non-geminate recombination processes as shown in the plots of Fig. 2 and explained in the caption. In addition to these mile stones, the project also has led to a number of unanticipated synergies with other research activities that were previously deemed to be beyond the scope of this project. For instance, the development of the specific pEDMR probehead design for the a-Si:H thin film samples turned out to be very useful for experiments on other disordered thin film semiconductors (organic semiconductors) which are proposed for similar photovoltaic applications as the inorganic a-Si:H.

The remainder of the project duration will be used to apply this characterization work to the improvement of the a-Si:H material. This includes the investigation on how charge carrier recombination changes in a-Si:H when it is doped with small amounts of nitrogen (to create non-stoichiometric silicon nitride). It is anticipated that the doped materials may oppress recombination of geminate charge carrier pairs and as well as possible degradation of the materials (e.g. due to the Staebler-Wronski effect). The results of these activities are expected to determine future directions of the effort to optimize an important group of thin film semiconductor materials for photovoltaic applications.

Fig. 2: Plots of the optically (a) and electrically (b) detected recombination rate as a function of the magnetic resonance induced spin-Rabi nutation frequency W and the g-factor of the recombining charge carriers. The measurements were conducted on identical a-Si:H films and under identical conditions. The optically detected measurement shows a variety of different recombination channels while the electrically detected data reveals recombination for one g-factor and one nutation frequency only. Thus, most of the optically detected recombination does not influence the conductivity. It is geminate recombination.

The funded project enabled a talented and academically excelling physics graduate student, Mr. Sang-Yun Lee, to approach the completion of his PhD thesis. Mr Lee has made significant contributions to the understanding of charge carrier recombination in disordered semiconductors during the time when he was supported by the project. So far, the work accomplished through this project has contributed to four publications in Physical Review B, Physical Review Letters, Physica Status Solidi B and a book chapter. The paper in Physica Status Solidi B was highlighted on the Journal title page and this Journal title was later awarded the “Title of the Year” of the Physica Status Solidi journals. Currently, two additional manuscripts have been submitted for publication in Applied Physics Letters, and the Journal of the American Chemical Society and several further manuscripts are expected to be prepared and submitted for publication during the remainder of this project and after its completion.

The execution of this project has significantly contributed to the expansion of the coherent semiconductor spectroscopy capabilities and the range of investigated materials in the PI's lab. After the foundation of the PIs research group in 2006, this project was the first significant extramural award that the PI's research group had received. Much of the work that has been accomplished due to this support has led to further funding from other sources such as two National Science Foundation projects (including one CAREER Award) and one project supported by the U.S. Department of Energy. While these projects pursue different goals, they utilize in part the experimental capabilities that were developed and demonstrated in the course of this project for the application to amorphous silicon. Beyond its scientific impact, it is likely that the impact of this project has also significantly contributed to the early promotion of the PI as well as the early award of tenure to the PI by the University of Utah in July 2010.