Korean Journal of Materials Research ISSN:1225-0562(Print) 2287-7258(Online) 한국재료학회지

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.