Progress in Hot-Wire deposited nanocrystalline silicon solar cells

Abstract

The tendency towards cost reduction in the photovoltaic industry has led to the development of the so-called thin film silicon technology. Among the different possible morphologies, a material that presents very promising features is nanocrystalline silicon (nc Si:H), which consists in an aggregate of crystallites with sizes in the order of few tens of nanometers embedded in an amorphous matrix. The deposition technique used at Universitat de Barcelona to grow nc-Si:H solar cells is Hot-Wire Chemical Vapour Deposition (HWCVD), which features several advantages in comparison with Plasma Enhanced CVD, the most widely used one in industry at the moment (higher deposition rates, simpler and cheaper deposition geometry and easier to scale to large area deposition). In order to grow a completely Hot-Wire deposited nanocrystalline silicon solar cell with enhanced efficiency, different strategies have been followed. First of all, the effects of the different deposition parameters, especially the tantalum filament temperature, have been studied aiming to the obtaining of compact material. Several modifications of the set-up have also been performed with the same finality. Material with good structural, electrical and transport properties has been obtained at low filament temperature (1600ºC). Secondly, the effects of the substrate nature on the microstructure of the nc-Si:H layers has been considered in the three possible cases (intrinsic, n-type and p-type doped layers). Later on, the performance of different p-i-n solar cells has been studied. Firstly, the effects of different preliminary light trapping strategies has been considered: the use of a textured front contact and of a back reflector. Finally, the relation existent between the microsctructure of the material and the performance and stability of the devices has been investigated. Different degradation mechanisms that affected our devices were detected and their possible causes identified (contact degradation, effects outside the p-i-n nc Si:H structure, active layer degradation). A best efficiency of 5.2% in a completely Hot-Wire deposited p-i-n solar cell has been achieved at low filament (1600ºC) and substrate (200ºC) temperatures and by using preliminary light trapping strategies. Stability of this device was verified after more than 1000 hours of light soaking. The preliminary character of this device makes it a very promising starting point to further increase the conversion efficiency of Hot-Wire deposited nanocrystalline silicon solar cells.