No abstract

No abstract

Higher open circuit voltages of the microcrystalline silicon bottom cell have a direct impact on the efficiency of the micromorph (μc-Si:H/a-Si:H) tandem cell. In this paper it is shown that open circuit voltages over 500 mV can be achieved leading to gc-Si:H cell efficiencies of 8.5 %. The behaviour of such cells is characterised both by the illuminated and the dark I-V characteristics in function of cell temperature. Microcrystalline cells with Voc-values higher than 500 mV and micromorph tandems possess in general a lower value of the temperature coefficient of the fill factor and thus of the efficiency, when compared to c-Si. Temperature-dependent dark I-V measurements suggest that the dominant recombination mechanism in lgc-Si:H cells is different from that prevailing in a-Si:H solar cells.

We performed plasma impedance measurements at room temperature for a hydrogen plasma using an impedance analyser. The plasma excitation frequency range spans from 40 to 70 MHz. Both the real and imaginary part of the impedance decrease monotonously with increasing frequency. These measurements are in agreement with prior experimental observations [1], that the required peak-to-peak voltage between the electrodes is reduced at higher excitation frequencies. Using a simple equivalent circuit for the plasma this effect can be mainly attributed to the increased sheath capacitance. Furthermore, by modelling the sheath with a simple parallel plate capacitor, its thickness could be estimated: it decreases from 2 nun at 40 MHz to about 1.4 mm at 70 MHz plasma excitation frequency. Finally, a possible link between the decreasing sheath thickness on the increase of deposition rate is discussed.

In the present work, several series with variation of deposition parameters such as hydrogen dilution ratio, VHF-power and plasma excitation frequency f have been extensively analyzed. Compared with`conventionala more-stable layers obtained at 200}2503C and high H dilution ratios of about 10, it was observed that electrical transport properties after light-induced degradation of layers deposited at`moderately higha temperatures (300}3503C) are equivalent but required lower H dilution ratios (between 2 and 4). As a consequence, the deposition rate of more stable layers obtained at moderately high temperatures is increased by a factor of 2. Moreover, optical gaps of a-Si:H deposited at 300}3503C are signi"cantly lower (by approx. 10 meV); furthermore, they decrease with f .