Nanoimprint diffraction gratings improve solar-cell efficiency

Because crystalline silicon (c-Si) solar-cell manufacturers are using thinner wafers to reduce costs, solar light-trapping technologies are becoming increasingly important. Researchers from the Fraunhofer Institute for Solar Energy Systems (ISE; Freiburg, Germany) and Universidad Politécnicat de Madrid (Madrid, Spain) are using rear-side diffraction gratings to capture photons—especially near the c-Si band edge (1.2 μm wavelengths)—that are lost as wafer thickness decreases. The gratings deflect the photons within the cell where they are contained through total internal reflection and have longer path lengths to increase absorption chances.

The researchers used two different dielectric buffer layer (DBL) processes and a low-cost aluminum backreflector in a nanoimprint-lithography process to fabricate the gratings. The DBLs were designed specifically to prevent absorption by the rear-reflector mirror—a problem that sometimes occurs with grating-based light-trapping designs. Binary gratings with a 50% duty cycle and 1 μm period were predicted to be optimum for photon absorption in 40-μm-thick c-Si substrates. Solar-cell efficiency was found to be enhanced between 0.8% and 1.3% using the rear-side gratings. Contact Alexander Mellor at [email protected].