• Energy Research

This article investigates the optimal efficiency of a photovoltaic system based on a silicon thin film tandem cell using polymorphous and microcrystalline silicon for the top and bottom elementary cells, respectively. Two ways of connecting the cells are studied and compared: (1) a classical structure in which the two cells are electrically and optically coupled; and (2) a new structure for which the “current-matching” constraint is released by the electrical decoupling of the two cells. For that purpose, we used a computer simulation to perform geometrical optimization of the studied structures as well as their electrical performance evaluation. The simulation results show that the second structure is more interesting in terms of efficiency.

The influence of textured transparent conducting oxide (TCO) substrate and p-layer on the performance of single-junction hydrogenated polymorphous silicon (pm-Si:H) solar cells has been addressed. Comparative studies were performed using p-i-n devices with identical i/n-layers and back reflectors fabricated on textured Asahi U-type fluorine-doped SnO2, low-pressure chemical vapor deposited (LPCVD) boron-doped ZnO and sputtered/etched aluminum-doped ZnO substrates. The p-layers were hydrogenated amorphous silicon carbon and microcrystalline silicon oxide. As expected, the type of TCO and p-layer both have a great influence on the initial conversion efficiency of the solar cells. However they have no effect on the defect density of the pm-Si:H absorber layer.

We demonstrate a novel method to fabricate passivated interdigitated back contact (IBC) crystalline silicon solar cells incorporating a maskless, patterned plasma etching step. After deposition in a plasma-enhanced chemical vapor deposition (PECVD) chamber, the intrinsic and doped hydrogenated amorphous silicon and microcrystalline silicon layers (necessary for the passivated interdigitated contacts in such heterojunction technology (HJT) devices) are patterned via a single, maskless etching step, also performed in a PECVD chamber. The patterning step relies on selectively lighting an etching plasma within slits in the patterned powered electrode when it is placed very close to the substrate. The process flow is characterized and optimized at each step using spectroscopic ellipsometry, photoluminescence, and surface photovoltage mapping. It is shown that a critical step is the removal of a damaged layer formed on the surface after the patterned etching. Without this step, the IBC-HJT solar cells systematically exhibit S-shaped curves in their current-voltage (I–V) characteristics (giving fill factors below 25%). Once this critical step is included, the solar cells display I–V curves with fill factors above 65%, demonstrating the advantage of the maskless plasma patterning process.