THE UPPER LIMIT TO THE THEORETICAL EFFICIENCY OF P-N HOMOJUNCTION AND INTERFACIAL LAYER HETEROJUNCTION SOLAR CELLS.

摘要

The physical mechanisms governing photovoltaic energy conversion in p-n homojunction and interfacial layer heterojunction (ILH) solar cells are examined. The usefulness of minority carrier mirrors (MCM) in such cells is studied by solving the minority carrier diffusion equation in the n- and p-type quasi-neutral regions of the cell, with boundary conditions representing MCM's at the ends of these regions. In this formalism, the MCM is considered to be an interfacial plane having zero surface recombination velocity. Non-zero values are also considered. It is shown that the MCM improves the open circuit voltage of the solar cell when it is located within a diffusion length of the junction between the n- and p-type regions. The effect of the MCM diminishes as the distance between it and the junction increases.;The analysis is also applied to cells made from silicon. A cell thickness of approximately 300 microns is necessary to absorb all the light owing to the indirect bandgap of silicon, yet the solar cell must be made thin in order to attain the maximum effect of the MCM's. A concept of internal light trapping is discussed; this trapping causes the light to undergo multiple reflections within the thin cell. By solving the minority carrier diffusion equation with appropriate generation function, it is shown that the upper limit to the efficiency is approximately 27%, for a cell of 15 microns in width.;The ILH solar cell is examined. A model describing current transport in the ILH cell is discussed and applied to the MIS solar cell. A new type of solar cell, the back surface MIS cell, is considered. The model is applied to this type of cell and the efficiency is calculated.;The ILH model is applied to the ITO-SiO(,2)-(p-type)Si SIS solar cell. The loss mechanisms in such cells are included in the model and the reduction in efficiency due to each mechanism is identified. The role of the SiO(,2)-Si interface states is examined.;The above analysis is applied to the direct gap materials CuInSe(,2) (E = 1.0eV) and GaAs (E = 1.43eV). It is shown that the theoretical upper limit to the conversion efficiency for devices employing MCM's on the front and back is approximately 26% for a CuInSe(,2) cell of width 2 microns.