A UNIFIED APPROACH FOR MODELING FUNDAMENTAL CHARACTERISTICS OF HIGHLY EFFICIENT SILICON BASED P-N JUNCTION AND HETEROJUNCTION SOLAR CELLS

摘要

We developed and applied physically transparent and efficient formalism of photoconversion in single junction solar cells. The photoconversion efficiency η of high-efficiency silicon based solar cells (SCs) is analyzed focusing on the role of combined surface recombinations at the illuminated and back surfaces with its combined velocity S_s. Both silicon based p-n junction SCs and α-Si:H-Si or α-SiC:H-Si heterojunctions (the so-called heterojunction with intrinsic thin layer (HIT) structures) based SCs are studied within the same general approach. It is shown that the open circuit voltage V_(oc) increase due to an additional contribution from the back surface is the common and essential feature of the photoconversion and high efficiency in the above SCs. Equations that comprehensively describe the photoconversion processes were derived and solved. This includes the most basic mechanisms of recombination: Shockley-Read-Hall (SRH) recombination, radiative recombination, surface recombination, recombination in the space charge region (SCR) and band-to-band Auger recombination. Calculated photoconversion efficiency η of the above systems were compared to the experimental values at AM1 .5 condition for highly efficient Si based SCs with η = 25% for p-n junction based systems, η = 24.7% for α-Si:H-Si HIT structures and η = 20.3% for a-Si_xC_(1-x):H-Si SCs. Comparison of theory and experiment allowed us to estimate the combined surface recombination velocity S_s, which was minimized in the above solar cells to achieve record high photoconversion. Our calculated maximum efficiency η_(max) for the silicon based SCs also fits very well the η_(max) calculated using more complex formalisms, available in the literature, thus confirming practicality of our theoretical results.