The efficiency of single-junction silicon solar cells is approaching their theoretical upper limits, prompting researchers to pursue a multilayer design approach in the form of tandem solar cells. Technical University of Denmark researchers seek to surpass these limits by integrating two absorbers with different bandgaps into a single device and have designed the first monolithic selenium/silicon tandem solar cell.

Source: R. Nielsen/Technical University of Denmark

Selenium, long-established as an effective photovoltaic material, incurs lower processing costs relative to silicon, promising to keep a cap on technology economics. It forms the top layer of the device and serves as a wide bandgap photoabsorber to harness different wavelengths of light. Researchers first coated silicon solar cells with various conductive oxide layers, followed by application of thin films of crystallized selenium.

The fabricated cells described in PRX Energy 3 generated maximum voltages of 1.68 V when exposed to sunlight. Power conversion efficiencies of 2.7% were well below the 26.8% maximum efficiency documented for silicon but the measured voltage aligns with energy goals for multimaterial cells.

Device efficiency can be improved tenfold by reducing voltage losses from electrical resistance in the circuit and by increasing electron transport. Such an efficiency gain can be realized by fine-tuning the conductive materials used to connect the silicon with the selenium.