High-Voc top-cells for multi-junction thin-film silicon photovoltaic devices

E. V. JOHNSON; F. VENTOSINOS; S. PARK; P. ROCA I CABARROCAS; D.Y. KIM; M. ZEMAN; R.A.C.M.M VAN SWAAIJ

Kyoto

Congreso; 6th World Conference on Photovoltaic Energy Conversion; 2014

The predicted but as-yet unfulfilled dominance of thin-film silicon photovoltaics (PV) has underlined an important lesson in mass-produced photovoltaics ? that cost-per-Watt is an insufficient measure of a PV technology's competitiveness. Due to balance of system costs (some of which scale by area), low efficiency technologies suffer an inherent disadvantage. For this reason, the most-promising way forward for glass-based thin-film silicon is higher efficiencies through the use of multi-junctions. The materials and device design palette for thin-film silicon extends far beyond the well-known micromorph configuration, to silicon oxides for high-bandgap cells, and germanium alloys for full-usage of the solar spectrum. Recent work has shown that the potential exists to go to 20% using triple and quadruple junctions [1], and all without deviating from large-area, low temperature plasma deposition techniques. Furthermore, through device design one can use the multijunction configuration to mitigate other drawbacks of thin-film technology, such as the well-known light-induced degradation of hydrogenated amorphous silicon (a-Si:H).In this work, we examine two cell combinations that could be used as the top two cells in a triple or quad- configuration. In both designs, a variant on a-Si:H ? polymorphous silicon ? is used as the absorber layer in the second cell in the stack. This material is known to be more stable than a-Si:H, but possesses a larger bandgap, which reduces the photocurrent it generates. However, as the already small amount of degradation in photocarrier collection observed by pm-Si:H occurs primarily for carriers generated by blue photons (i.e. at the front of the cell), this device configuration is expected to be more stable under light-induced degradation.In the first design, we investigate the use of a very wide-band-gap top cell with an a-SiOX:H absorber layer, (deposited at the TU Delft) followed by a pm-Si:H bottom cell (deposited at the LPICM). External quantum efficiency (EQE) curves for this design are present in Figure 1(a). This design, notably, gives elevated values of VOC (as high as 1.85V). Efficiencies for the initial structures are low due to poor current matching (6.5%).Secondly, we investigate a counter-intuitive cell stack design, where the stability issues of a-Si:H and polymorphous silicon (pm-Si:H) are offset by using a very thin a-Si:H top cell, followed a "blue-filtered" pm-Si:H bottom cell. Initial cell efficiencies approaching 8.5% are achieved in this case due to better current matching, and the use of a better optical elements (front electrode and back reflector), although the VOC's achieved are lower (1.74V). Although studies of the degradation of the cells remain to be completed, the two top-cell designs presented show great potential as high-gap absorbers in multi-junction cells. For the moment, their absolute values of efficiency are limited by external elements (sub-optimal optical components) and by current matching, which can be optimized by absorber layer thicknesses.