Simulation on Optimum Doping Levels in Si Solar Cells

Abstract

The two key variables of an Si solar cell, i.e., emitter (n-type window layer) and base (p-type substrate) doping levels or concentrations, are studied using Medici, a 2-dimensional semiconductor device simulation tool. The substrate is ptype and 150 μm thick, the pn junction is 2 μm from the front surface, and the cell is lit on the front surface. The doping concentration ranges from 1 × 1010 cm−3 to 1 × 1020 cm−3 for both emitter and base, resulting in a matrix of 11 by 11 or a total of 121 data points. With respect to increasing donor concentration (Nd) in the emitter, the open-circuit voltage (Voc) is little affected throughout, and the short-circuit current (Isc) is affected only at a very high levels of Nd, exceeding 1 × 1019 cm−3, dropping abruptly by about 12%, i.e., from Isc = 6.05 × 10−9 A·μm−1, at Nd = 1 × 1019 cm−3 to Isc = 5.35 × 10−9 A·μm−1 at Nd = 1 × 1020 cm−3, likely due to minority-carrier, or hole, recombination at the very high doping level. With respect to increasing acceptor concentration (Na) in the base, Isc is little affected throughout, but Voc increases steadily, i.e, from Voc = 0.29 V at Na = 1 × 1012 cm−3 to 0.69 V at Na = 1 × 1018 cm−3. On average, with an order increase in Na, Voc increases by about 0.07 V, likely due to narrowing of the depletion layer and lowering of the carrier recombination at the pn junction. At the maximum output power (Pmax), a peak value of 3.25 × 10−2 W· cm−2 or 32.5 mW· cm−2 is observed at the doping combination of Nd = 1 × 1019 cm−3, a level at which Si is degenerate (being metal-like), and Na = 1 × 1017 cm−3, and minimum values of near zero are observed at very low levels of Nd ≤ 1 × 1013 cm−3. This wide variation in Pmax, even within a given kind of solar cell, indicates that selecting an optimal combination of donor and acceptor doping concentrations is likely most important in solar cell engineering.