Toward film-silicon solar cells on display glass

We describe recent progress in developing epitaxial film crystal silicon (c-Si) solar cells that can be grown at low temperature (<;760 °C) on seed-on-glass substrates. This low-cost approach is enabled by rapid epitaxy (up to 300 nm/min) of Si films with low dislocation density (<; 1×10⁵ cm^-2) at glass-compatible temperatures by hot-wire chemical vapor deposition (HWCVD). Epitaxial test cells on heavily-doped ´dead´ Si wafers provide insight into the quality of the Si absorber and the physics that limit device performance. Our best 2-3 μm thick, film silicon heterojunction (c-Si/a-Si) solar cells have reached ~6.7% efficiency (V_oc ~ 570 mV, J_sc ~18 mA/cm^-2) without rapid thermal anneal, defect passivation or light trapping. Unpassivated dislocations are strong recombination centers and limit effective minority carrier diffusion lengths to less than 15-20 μm (roughly half the distance between dislocations). We also report devices without light-trapping on layer-transfer Si seed layers bonded to display-glass; these seed layers template growth of high-quality HWCVD cSi. Our initial devices have V_oc = 460 mV, J_sc = 16.2 mA/cm², Eff. = 4.8 %, but will benefit from post-growth anneals, hydrogenation and new surface treatments before epitaxy. We discuss junction transport physics in the devices and explore the role of post-growth H-passivation and rapid thermal annealing treatments on device performance.