We have analyzed the role of the bond densities of a-SiNx:H films on the passivation properties at the SiNx:H/Si interface. The films are deposited onto silicon wafers by plasma enhanced chemical vapor deposition using a 13.56 MHz direct plasma system and a SiH4/N2/H2 gas mixture. Fourier transform infrared spectroscopy measurements are performed in order to obtain the bonding concentration of Si–Si, Si–H, Si–N and N–H. The passivation properties are deduced by lifetime measurements using a microwave-detected photoconductance decay technique. Carrier lifetimes of the SiNx:H-passivated silicon wafers of up to 1200 μs correlate to surface recombination velocities, Seff, as low as 4–6 cm/s. This means that the films provide excellent passivation of silicon surfaces, which is necessary for high-efficiency solar cells. The Si–H bond density and the total bond density are considered as measures of the passivation quality. Models for the formation of K+ centers and for the passivation pathways during the plasma deposition are proposed. The addition of a further hydrogen source to the plasma gas (H2) leads to a better defect passivation of Si dangling bonds during the deposition.

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Articles you may be interested inEnhanced performance of solar cells with optimized surface recombination and efficient photon capturing via anisotropic-etching of black silicon Appl. Phys. Lett.Effect of plasma polymerization film on reducing damage of reactive ion etched silicon substrates with CHF3+O2 plasmas J.New concepts in silicon solar cell design require dry processing technologies. For this reason two reactive ion etching ͑RIE͒ processes have been developed: one for surface cleaning and one for the removal of phosphorous glass ͑PSG͒. However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF 6 RIE cleaning has been investigated by means of secondary ion mass spectroscopy, positron annihilation, and minority charge carrier lifetime measurements. Particles contained in the etch gas can be detected up to a depth of 50-80 nm in the silicon sample. A two layer model of vacancy distribution has been established: A layer of high vacancy concentration (10 19 cm Ϫ3 ) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 m with a vacancy concentration of 10 16 cm Ϫ3 . Effective minority charge carrier lifetimes decrease to about 10% of the lifetime of the wet etched control during RIE. If a heavily damaged layer of 20 nm is being removed by anodic oxidation, lifetimes return to the initial value. Under certain etching conditions it is possible to anneal plasma induced damage at 400°C. The influence of RIE induced damage on solar cells is quantified by open circuit voltage analysis: Long process times, addition of oxygen to the etch gas, and high rf power or self-induced dc bias result in a significant decrease in open circuit voltage. Nearly damage free RIE processes have been developed for surface cleaning as well as PSG removal. Dry processed solar cells thus show the same performance as wet etched cells.

The introduction of selective emitters underneath the front contacts of solar cells can considerably increase the cell efficiency. Thus, cost-effective fabrication methods for this process step would help to reduce the cost per W p of silicon solar cells. Laser Chemical Processing (LCP) is based on the waterjet-guided laser (LaserMicroJet®) developed and commercialized by Synova S.A., but uses a chemical jet. This technology is able to perform local diffusions at high speed and accuracy without the need of masking or any high-temperature step of the entire wafer.We present experimental investigations on simple device structures to choose optimal laser parameters for selective emitter formation. These parameters are used to fabricate high-efficiency oxide-passivated LFC solar cells that exceed 20% efficiency.