UC San Diego

Solar cells are specially engineered semiconductor diodes that have the ability to convert solar energy, in the form of light, into electricity. Manufacturing high-efficiency low-cost photovoltaic devices has been the goal of researchers since it was discovered in 1954 that a voltage developed across a semiconductor pn junction when the lights in the room on. The costs associated with manufacturing solar modules can be greatly reduced if thinner semiconductor wafers are used. In order to maintain cell efficiency, novel light trapping methods that increase photon path lengths must be employed to ensure the usable portion of the solar spectrum is fully absorbed by thinner cells. Due to the planar symmetry of semiconductor wafers, any light transmitted into the cell from the top surface will be confined to the continuum of radiation modes only. Transmitted photons that reflect from the back of the cell will be incident onto the front of the cell at angles that reside within the exit cone of the semiconductor as mandated by reciprocity. Thus, a large portion of unabsorbed photons are transmitted out of the cell after a single round-trip through the material. To trap light in the cell, it is required to break the non -ergodic geometry of the planar material so that transmitted photons propagate at angles greater than what is required to totally internally reflect at the cell boundaries. This dissertation reports on absorption enhancing methods that employ the unique light scattering properties of metal and dielectric nanoparticles deposited onto the planar surfaces of a-Si and InP/InGaAsP quantum- well solar cells. The nanoparticles scatter incident light not only into radiation modes, but also into laterally propagating trapped modes. Due to increased path traveled by laterally propagating photons, enhanced absorption and increased energy conversion efficiency is observed. The non-ergodic geometry of planar cells treated in this manner is broken without modifying or damaging the surface of the semiconductor, thereby mitigating losses incurred by increased surface recombination of minority carriers. This method provides a new way to enhance solar energy conversion in solar cells that are too thin or too delicate to be textured using conventional approaches