Hydrogenated amorphous silicon films grown by pulsed laser deposition

Abstract

Summary form only given. Amorphous hydrogenated Si (a-Si:H) thin-film solar cells offer power conversion efficiencies up to 12% and use the advantage of the existing infrastructure of Si industry, therefore they are of high technologic interest. Moreover, amorphous Si is cost-effective compared to crystalline Si and it is only needed in small quantities for thin-film solar cell production on rigid or flexible substrates. Pulsed laser deposition (PLD) is a simple, versatile, and cost-effective technique [1] for the deposition of a-Si:H layers.We employ pulsed laser deposition for the fabrication of a-Si:H solar cells in the p-i-n configuration for increased efficiency, as shown in Fig. 1, because the short minority-carrier lifetime in a-Si:H, especially in the doped varieties, requires electric field assistance for efficient collection of photogenerated carriers. The efficiency of the solar cell can be further improved by the incorporation of metallic nanoparticles for increased light absoprtion [2,3]. We irradiate silicon targets by a large number of laser pulses produced by an Nd:YAG laser system (10 ns pulse duration), operating at 355 nm in hydrogen atmosphere. In order to achieve doping for the n-type and p-type layers, we use highly doped silicon targets. Varying the PLD parameters, such as the laser fluence, number of pulses, substrate temperature, and hydrogen pressure, we optimize the morphology, electric conductivity, and optical properties of the a-Si:H layers for maximum efficiency.The conductivity of the a-Si:H films depends strongly on the laser fluence, while the optical absorption depends on the hydrogen pressure during deposition. The doping level of the a-Si:H layers can be further controlled by the simultaneous irradiation of the silicon target and a doping target by two synchronized lasers [4]. Post-deposition annealing also improves the properties of the deposited films.