Microcrystalline silicon and micromorph tandem solar cells

Abstract

Abstract. "Micromorph" tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency.The justification of worldwide, intensified research on thinfilm solar cells is not based on a lack of success in the efficiency performance of wafer-based silicon or GaAs technologies. It is based on a lack of hope in the cost reduction potential of wafer-based technology. Indeed, in order to render photovoltaics a competitive energy source in future, it is imperative to adopt the approach of low-cost solar cells.

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

“…16,17 Thus, a stack consisting of a thin amorphous silicon top cell and a microcrystalline bottom cell in a so-called micromorph tandem structure offers the potential for fairly high efficiency at low cost in mass production. [18][19][20][21] A stabilized efficiency of 12.0% has recently been published for a tandem structure module 22 ͑for comparison, solar modules with sliced single crystalline and polycrystalline silicon have been reported to show efficiencies of up to 22.7% and 15.3%, respectively 23 ͒. Even though silicon thin-film modules still show rather low efficiencies compared to the other approaches, from the application point of view the annual energy production and especially the cost per unit of annual energy production is much more relevant.…”

Resonances and absorption enhancement in thin film silicon solar cells with periodic interface texture

“…Surprisingly, this is true for all types of silicon, regardless whether the devices are based on amorphous, 1,2 nano-, micro-, 3,4 or polycrystalline material; 5 it even applies to wafer-based multicrystalline or single-crystal devices. 6 In order to absorb sufficient amounts of light in such thin devices, light scattering at the interfaces has proven to be a highly successful concept; Redfield proposed to use surface corrugations in the range of tens of micrometers in order to refract light and ideally to trap it inside the absorber layer by total internal reflection; 7 this concept has been applied ever since to wafer-based cells with typical thicknesses between 170 and 250 mm, and it becomes increasingly important as the cell thickness is reduced in modern designs.…”

Exploring dark current voltage characteristics of micromorph silicon tandem cells with computer simulations

“…2 The micromorph cell does not incorporate any silicon alloys in the absorber layers; it has the optimal bandgap combination for terrestrial applications, and it is quite stable to light induced degradation. 3,4 Tandem solar cells are regularly characterized by their light current-voltage ͑J-V͒ characteristics under AM1.5 illumination and by the spectral responses ͑SRs͒ under blue bias light and under red bias light. [2][3][4] To our knowledge the dark J-V curves that have been widely used and studied in single solar cells have not been yet systematically explored in tandem solar cell structures to see if they can provide some useful information about the electrical quality of the device constituents.…”