Photovoltaic InGaN/GaN Solar Cells Based on Inverted Structures
Our structure utilizes a unique p-down Ga-polar structure that makes the intrinsic polarization fields in the structure achieve a better carrier sweep-out and leads to a higher current, therefore a higher efficiency. A GaN solar cell consists of p- and n-doped regions, with an undoped (intrinsic) region inserted in between. The photogenerated current is extracted to the front and rear contacts of the cell. Typically, the undoped region is a lower bandgap and low defect density region surrounded by higher bandgap material in the p- and n-regions. The intrinsic region is shorter than the slowest carriers (holes) diffusion length to increase the extraction efficiency. Solar cells separate the photogenerated carriers by using the built-in voltage created across the p-n junction to deliver the current to the outside circuit. The presence of a field-free region of comparable length to the diffusion length at the center of the intrinsic region would drastically reduce the collection efficiency and would make the internal quantum efficiency lower. Any photogenerated carriers that recombine non-radiatively due to material imperfections are lost from the process. Thus, the challenge of creating a high-efficiency solar cell is to remove all defects that can lead to non-radiative recombination. In order to get low defect density, one needs to start the growth of the structure with a low density substrate material. This requires controlling the threading dislocations density in the template material. The solar cells are processed in the nanofabrication facility of the University of Maryland.
The GaN technology will permit the realization of high energy conversion efficiency multi-junction devices, all fabricated in the same material technology. In future phases of development, multi-junction solar cells will be developed and full wafer-scale solar cells will be implemented. The University of Maryland has established a close collaboration with the Army Research Lab in Adelphi.