Browsing by Author "Al Tarabsheh, Anas"

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Amorphous silicon based solar cells
(2007) Al Tarabsheh, Anas; Werner, Jürgen (Prof. Dr. rer. nat. habil.)
This thesis focuses on the deposition of hydrogenated amorphous silicon (a-Si:H) films bymeans of plasma enhanced chemical vapour deposition (PECVD). This technique allows the growth of device quality a-Si:H at relatively low deposition temperatures, below 140 °C and, therefore, enables the use of low-cost substrates, e.g. plastic foils. The maximum efficiencies of a-Si:H solar cells in this work are η= 6.8 % at a deposition temperature Tdep = 180 °C and η = 4.9 % at a deposition temperature Tdep = 135 °C. Decreasing the deposition temperature deteriorates the structural and electronic quality of a-Si:H films. Therefore, the deposition conditions are carefully optimized at low temperatures. The mismatch in the mechanical properties of the plastic foils and the inorganic semiconductor layers have less effect on the a-Si:H films at low deposition temperatures. As a result, the deposition temperatures should be decreased to minimize mechanical deterioration of the films but without losing too much of the electronic properties of the films. A novel analytical description of the current density/voltage (J/V) characteristics of p-i-n solar cells well represents experimental J/V curves of a-Si:H solar cells. The extended model solves the continuity and transport equations for electrons and holes, and fully accounts for the contributions of the drift and the diffusion currents. Many analytical models neglect the contribution of the diffusion current in describing the a-Si:H solar cells. Other existing models assume the diffusion lengths of electrons and holes to be equal, resulting in a symmetric distribution of carrier concentrations around the center of the intrinsic layer of the p-i-n solar cells. Both restrictions strongly limit the ability of these analytical models to accurately reproduce the J/V-characteristics of real solar cells. In contrast to existing analytical models, the new analytical description solves the continuity and transport equations of carriers at each location within the i-layer for the whole range of applied voltages. The peculiar extension of this model over previous ones enables a more realistic description of solar cells. My novel analytical model implements i) different values of the diffusion lengths, or mobility-lifetime products, of electrons and holes, and ii) realistic wavelength and depth dependencies of the photogeneration rate of charge carriers. The results of the model demonstrate that the location of the main recombination path of the photogenerated carriers inside the i-layer is voltage dependent, rather than being fixed at the middle of the i-layer as existing models assume. For a realistic description of the solar cell optics in calculating the J/V-characteristics, I fully account for the reflection of photons at the back contact. The model proves that the performance of a-Si:H solar cells which are illuminated through the p-layer is better than the one of cells illuminated through the n-layer. Testing corresponding J/V-characteristics from this model against experimental data of bifacial a-Si:H solar cells with transparent front and backside contacts, reveals that this extended analytical model well describes the output characteristics of real a-Si:H p-i-n solar cells. The model proves that the current collection of bifacial p-i-n solar cells is larger if the light enters through the p-layer because the mobility μn of electrons is larger than the mobility μp of holes. This thesis also investigates the dependence of the electrical and optical properties of a-Si:H films on the deposition conditions, and how those properties are enhanced by optimizing the deposition conditions. I apply the optimized layers to solar cells deposited on glass and on polyethylene terephtalate (PET) substrates. The incorporation of a buffer layer or a microcrystalline layer enhances the performance of the cells.