The authors report on the fabrication of microcrystalline silicon p-i-n solar cells with efficiencies close to 10%, using glass coated with zinc oxide (ZnO) deposited by low pressure chemical vapor deposition (LPCVD).LPCVD front contacts were optimized for p-i-n microcrystalline silicon solar cells by decreasing the free carrier absorption of the layers and increasing the surface roughness. These modifications resulted in an increased current density of the solar cell but also in significantly reduced fillfactor (FF) and open-circuit voltage (Voc). In order to avoid these reductions, a new surface treatment of the ZnO is introduced. It transforms profoundly the surface morphology by turning the typical V-shaped valleys of the LPCVD ZnO into U-shaped valleys and by erasing from the surface small-sized pyramids and asperities. As a result, for fixed deposition parameters, the p-i-n microcrystalline silicon solar cell efficiency increased from 3.3% to 9.2%Further optimization of the microcrystalline silicon solar cell on this 'new' type of LPCVD ZnO front contact has led to an efficiency of 9.9%.
Perovskite/crystalline silicon tandem solar cells have the potential to reach efficiencies beyond those of silicon single-junction record devices. However, the high-temperature process of 500 °C needed for state-of-the-art mesoscopic perovskite cells has, so far, been limiting their implementation in monolithic tandem devices. Here, we demonstrate the applicability of zinc tin oxide as a recombination layer and show its electrical and optical stability at temperatures up to 500 °C. To prove the concept, we fabricate monolithic tandem cells with mesoscopic top cell with up to 16% efficiency. We then investigate the effect of zinc tin oxide layer thickness variation, showing a strong influence on the optical interference pattern within the tandem device. Finally, we discuss the perspective of mesoscopic perovskite cells for high-efficiency monolithic tandem solar cells.
We present a novel passivating contact structure based on a nanostructured siliconbased layer. Traditional poly-Si junctions feature excellent junction characteristics but their optical absorption induces current losses when applied to the solar cell front side. Targeting enhanced transparency, the poly-Si layer is replaced with a double-layer stack consisting of a nanostructured silicon oxide capped with a nanocrystalline silicon (nc-Si) layer. The nanostructured silicon oxide layer consists of an amorphous SiOx matrix with incorporated Si filaments connecting one side of the layer to the other, and is referred to as nanocrystalline silicon oxide (nc-SiOx) layer. We investigate passivation quality, measured as saturation current density, and nanostructural changes, characterized by Raman spectroscopy and transmission electron microscopy, carefully studying the influence of annealing dwell temperature. Excellent surface passivation on n-type and also p-type wafers is shown. An optimum annealing temperature of 950 °C is found, resulting in a saturation current density of 8.8 fA cm-2 and 11.0 fA cm-2 for n-type and p-type wafers, respectively. Efficient current extraction is presented with specific contact resistivities of 86 mΩ cm 2 on n-type wafer and 19 mΩ cm 2 on p-type wafers, respectively. Highresolution transmission electron microscopy reveals that the layer stack consists of intermixed SiOx and Si phases with the Si phases being partly crystalline already in the asdeposited state. Thermal annealing at temperatures ≥ 850 °C further promotes crystallization of the Si-rich regions. We show that the addition of the SiOx phase enhances the thermal stability of the contact and we expect it to allow to tune the refractive index and improve transparency while still providing efficient electrical transport thanks to the crystalline Si phase, which extends throughout almost the entire layer.
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