New generation transparent LPCVD ZnO electrodes for enhanced photocurrent in micromorph solar cells and modules

Summary form only given: Polycrystalline ZnO:B deposited by low-pressure chemical vapor deposition (LPCVD) was proven as an efficient electrode material for thin film silicon solar cells application, thanks to high transparency, good electrical conductivity and strong light scattering via self-textured surface. However, high doping used to lower resistivity of ZnO films, induces free carrier absorption (FCA), detrimental to current generation in the bottom microcrystalline cell of a micromorph device. Here we describe optimized 2 μm thick LPCVD ZnO:B bilayers, combining of a thin nucleation layer, plus a bulk layer, having different doping levels. This arrangement in one growth-step enables a separate control of electrical and optical properties of the films. It promotes the growth of strongly light diffusive structures with enhanced electron mobility (~45cm²/Vs) and low electron density (~2×10¹⁹ cm^-3). This results in low FCA and moderate sheet resistance, that should easily be lowered to <;20Ω_/sq. In an industrially scalable process, the bilayers approach provides highly transparent electrodes, well adapted for the development of micromorph solar cells. Indeed, a micromorph device generates less J_sc than its two separate junctions, for a higher voltage, allowing thus the use of a more resistive electrode. The potential of such bilayer front electrodes for power improvement and cost reduction of industrial micromorph modules is currently tested at Oerlikon Solar. First experiments already show a very promising gain of 3W_p/module, compared to modules on standard doped LPCVD ZnO.