Superior Phosphorous Doping in Nanocrystalline Silicon Thin Films and Their Application as Emitter Layers in Silicon Heterojunction Solar Cells

Abstract

Phosphorous doping in the nc-Si network induces gradually reduced crystallinity; however, preferential growth along <220>-oriented crystallites promotes the columnar-like growth morphology. At optimum doping, substitution by donor P + -atoms in the c-Si lattice contributes surplus free electrons and high carrier mobility, resulting in superior electrical conductivity in the n -nc-Si network. An elevated doping leads to incorporating elemental P 0 atoms in the interstitial position or forming P–Si–H clusters and generating voids between the crystalline columns. Segregation of defects contributes to decreasing carrier mobility and reducing conductivity after diminishing crystallinity and narrowing the optical band gap. By precisely controlling the growth at 250 °C and efficient electrically active doping by P + -atoms, n -nc-Si thin films with superior dark conductivity (∼10 1 S cm –1 ) are produced in which the percolation of charge carriers through the crystalline columns could facilitate stacked-layer devices. The optimum n -nc-Si thin films are used as the emitter layers in n -nc-Si/ p -c-Si heterojunction solar cells (HJSCs). Furthermore, an ultrathin a-Si:H buffer layer on the p -c-Si minimizes the junction carrier recombination loss, and subsequent postdeposition short-time H-plasma treatment (PSHPT) ensures seeds for superior nanocrystallization in the n -nc-Si emitter layer. The n -nc-Si/(PSHPT) i -nc-Si(buffer layer)/ p -c-Si HJSC delivers a PV conversion efficiency, η ∼12.35%, via a reasonable fill factor of ∼0.647 and sensible J SC of ∼32.75 mA cm –2 . Further improvement in the PV performance could be possible using suitably thinner p -c-Si wafers, harmonizing with the effective carrier diffusion length, and fabricating a high-quality passivation structure on the backside.

Topics & Concepts