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Present address: Arizona State University, School of Electrical, Computer and Energy Engineering, 551 E. Tyler Mall, Tempe, AZ 85287, USA.Reducing wafer thickness while increasing power conversion efficiency is the most effective way to reduce cost per Watt of a silicon photovoltaic module. Within the European project 20 percent efficiency on less than 100-mm-thick, industrially feasible crystalline silicon solar cells ("20plms"), we study the whole process chain for thin wafers, from wafering to module integration and life-cycle analysis. We investigate three different solar cell fabrication routes, categorized according to the temperature of the junction formation process and the wafer doping type: p-type silicon high temperature, n-type silicon high temperature and n-type silicon low temperature. For each route, an efficiency of 19.5% or greater is achieved on wafers less than 100 mm thick, with a maximum efficiency of 21.1% on an 80-mm-thick wafer. The n-type high temperature route is then transferred to a pilot production line, and a median solar cell efficiency of 20.0% is demonstrated on 100-mm-thick wafers.

Hydrogenated amorphous silicon oxynitride (a-SiO x N y :H) films, which are deposited by the plasma decomposition of silane and nitrous oxide at low temperatures (T dep o 300 1C), are investigated in order to evaluate the potential of these films for photovoltaic applications. In this work both, intrinsic and doped a-SiO x N y :H films are investigated in terms of their electrical, optical and structural properties using Fourier-transform infra-red (FTIR) and secondary-ion mass-spectroscopy (SIMS), as well as highresolution transmission electron microscopy (HRTEM), photo-conductance decay (PCD), spectral ellipsometry and temperature-dependent conductivity measurements. The plasma deposition parameters are optimized in terms of effective minority carrier lifetime, dark conductivity and low absorbance (i.e. high optical band gap). The optical band gap of the a-SiO x N y :H films can be widened up to 2.2 eV compared to a-Si:H due to the incorporation of oxygen and nitrogen into the amorphous network. Not only the optical band gap but also the passivation quality and the dark conductivity of the films are well correlated with the oxygen and nitrogen concentration, which are monitored by means of SIMS measurements. When applying an a-SiO x N y :H film with an optical band gap of 2.0 eV, a very high effective minority carrier lifetime of 2.5 ms is measured. In case of doped films, conductivities up to s dark ¼ 4.5 Â 10 À 3 S/cm for the n-type doping and s dark ¼ 3.9 Â 10 À 4 S/cm for the p-type doping are achieved. Combining the intrinsic and doped a-SiO x N y :H films to heteroemitter stacks on a crystalline Si base, a very high implied open circuit voltage of up to 733 mV is demonstrated. FTIR and HRTEM measurements reveal a homogenous distribution of Si-Si and Si-O-Si bonds in the a-SiO x N y :H films.

The effective surface recombination velocity of amorphous-silicon-coated crystalline silicon wafers is measured after illumination for various durations to investigate the stability of the surface passivation. We develop a defect model to determine the densities of dangling bond states at the a-Si:H/c-Si interface from fitting the experimental lifetime data. The surface recombination velocity of both p-type and n-type substrates is Seff=3±1 cm/s at τn=1015 cm−3 in the as-deposited state. Illumination induces an increase to Seff=16±5 cm/s due to an increase in the dangling bond density by one order of magnitude. This increase is reversible by annealing at 300 °C for 5 min.

Thin films of nanocrystalline SiO x N y are studied in view of their application in silicon heterojunction (SHJ) solar cells. In particular, the formation of the nanocrystals and their effects on the electrical and optical properties of the films are investigated. The role of the oxygen content on the properties of the layers is clarified as well. The obtained layers show very high conductivity (44 S/cm), low activation energy (1.85 meV) and high Tauc gap (2.5 eV), promising features for their application in photovoltaics.