Nanocrystalline silicon and silicon oxide contact layers for silicon heterojunction solar cells

Crystalline-silicon solar cells are presently the main technology to supply the rapidly growing market for competitive photovoltaic electricity. To drive conversion efficiency of such cells further up, silicon heterojunction (SHJ) solar cells are promising. This thesis addresses strategies to improve the efficiency of such cells by the integration of nanocrystalline silicon (nc-Si:H) and nanocrystalline silicon oxide (nc-SiOx:H) contact layers grown by plasma enhanced chemical vapor deposition, replacing the commonly used amorphous silicon (a-Si:H) contact layers. The aim is to take advantage of both the improved charge carrier transport and contact resistance and a reduced parasitic absorption. The key challenge is to develop very thin, highly crystalline layers that require fast nucleation. As prerequisite, we first had to establish a reference process with p-doped a-Si:H with a high degree of reproducibility. With this we reached a conversion efficiency of 21.3%. We investigated the relation between deposition gas composition, optoelectronic material properties and structural features of (p)nc-Si:H films on cell stacks. The a-Si:H passivation layer growth regime (slight epitaxial or fully amorphous) is relevant for the nanocrystalline evolution and critically influences the fill factor (FF) and open circuit voltage of completed solar cells. Furthermore, nc-Si:H layers exhibited the ability to enhance surface passivation similarly to what was found with post-deposition hydrogen plasma treatment. A CO2 plasma treatment of the (i)a-Si:H surface prior to the emitter deposition was optimized as method to fast nucleate the thin nc-Si:H films without deteriorating the underlying passivation. The combination of optimized deposition parameters, plasma treatments and film thickness resulted in solar cells exhibiting an open circuit voltage of 727 mV, a short circuit current density of 38.9 mA/cm2, a FF of 74.6% and a conversion efficiency of 21.1%. The optical interaction between the incoming light and the multilayer stack placed on the illuminated side of the c-Si absorber was simulated varying the doped contact material (a-Si:H, nc-Si:H, nc-SiOx:H and doping type) in stack with (i)a-Si:H passivation and In2O3:Sn layers. We identified the optimal film thickness and refractive index as function of the substrate texture to maximize the generated current. The use of a tailored nc-SiOx:H layers allowed for the creation of a stack with refractive indexes that consecutively decreases from silicon to the ambient air resulting in a high short circuit current density of 40.4 mA/cm2 with a gain of 2 mA/cm2 as compared to using the (p)a-Si:H reference, but with limited FF of 72.9%. The causes of the FF limitations experimented were ascribed to both the poor contact to the front transparent conductive oxide and the initial stage of growth of the thin nc-Si:H emitter. Finally, an alternative procedure showed the potential to overcome both problems by passivating the silicon wafer surfaces with an ultra-thin amorphous silicon oxide film growth by PECVD. An exceptionally high crystalline volume fraction of 72% was measured in a thin layer on a cell stack. Preliminary cells exhibited a high FF and reduced current loss but a low open circuit voltage due to insufficient passivation. However, the approach opens up perspectives to further exploit the full potential of nc-Si:H contact layers.