Silicon epitaxy below 200°C: Towards thin crystalline solar cells

Low temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RFPECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175 °C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 μm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices.