Advances in solar cells made with hot wire chemical vapor deposition (HWCVD): superior films and devices at low equipment cost

Abstract

This paper reviews the history of hot wire chemical vapor deposition (HWCVD) and discusses the unique silicon thin film materials that can be obtained with this deposition method. The structure of the silicon films can be widely varied from fully amorphous to completely polycrystalline. Moreover, the hydrogen content can be controlled over a wide range. A characteristic feature of HWCVD is the high deposition rate for all types of films. A ‘HW’ solar cell toolbox is presented and the first efficient a-Si/poly-Si multibandgap tandem cells have been made using developed component cells. These cells consist of two stacked n–i–p-type solar cells on plain stainless steel using plasma deposited n- and p-type doped layers and hot wire deposited intrinsic (i) layers, where the i-layer is either amorphous (band gap 1.8 eV) or polycrystalline (band gap 1.1 eV). For the two absorber layers, we used individually optimized parameters such as gas pressure, hydrogen dilution ratio, substrate temperature, filament temperature and filament material. The solar cells do not comprise an enhanced back reflector, but feature a natural mechanism for light trapping, due to the texture of the (220) oriented poly-Si absorber layer and the fact that all subsequent layers are deposited conformally. The deposition rate for the throughput-limiting step, the poly-Si i-layer, is ≈5 Å/s. This layer also determines the highest substrate temperature required during the preparation of these tandem cells (<500 °C). The efficiency obtained for these tandem cells is 8.1%. The total thickness of the silicon nip/nip structure is only 1.1 μm.