• Energy Research

AbstractHot‐wire chemical vapor deposition was utilized to develop rapidly grown and high‐quality phosphorus‐doped amorphous silicon (a‐Si:H) thin films for poly‐crystalline silicon on tunnel oxide carrier‐selective passivating contacts. Deposition rates higher than 150 nm/min were obtained for the in situ phosphorus‐doped a‐Si:H layers. To optimize the passivating contact performance, material properties such as microstructures as well as hydrogen content were characterized and analyzed for these phosphorus‐doped a‐Si:H films. The results show that a certain microstructure of the films is crucial for the passivation quality and the conductance of passivating contacts. Porous silicon layers were severely oxidized during high‐temperature crystallization, giving rise to very low conductance. The insufficient effective doping concentration in these layers also yields inferior passivation quality due to lack of field‐effect passivation. On the other hand, dense silicon layers are insensitive to oxidation but very sensitive to blistering of the films during the subsequent high‐temperature process steps. By optimizing the deposition parameters, a firing‐stable‐implied open‐circuit voltage of 737 mV and a contact resistivity of 10 mΩ·cm2were achieved at a high deposition rate of 100 nm/min while 733 mV and 90 mΩ·cm2were achieved at an even higher deposition rate of 150 nm/min.

AbstractA highly transparent passivating contact (TPC) as front contact for crystalline silicon (c-Si) solar cells could in principle combine high conductivity, excellent surface passivation and high optical transparency. However, the simultaneous optimization of these features remains challenging. Here, we present a TPC consisting of a silicon-oxide tunnel layer followed by two layers of hydrogenated nanocrystalline silicon carbide (nc-SiC:H(n)) deposited at different temperatures and a sputtered indium tin oxide (ITO) layer (c-Si(n)/SiO2/nc-SiC:H(n)/ITO). While the wide band gap of nc-SiC:H(n) ensures high optical transparency, the double layer design enables good passivation and high conductivity translating into an improved short-circuit current density (40.87 mA cm−2), fill factor (80.9%) and efficiency of 23.99 ± 0.29% (certified). Additionally, this contact avoids the need for additional hydrogenation or high-temperature postdeposition annealing steps. We investigate the passivation mechanism and working principle of the TPC and provide a loss analysis based on numerical simulations outlining pathways towards conversion efficiencies of 26%.

A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.