EFFICIENCY OF TANDEM SOLAR-CELL SYSTEMS AS A FUNCTION OF TEMPERATURE AND SOLAR-ENERGY CONCENTRATION RATIO

This paper presents the results of a comprehensive theoretical analysis of tandem photovoltaic solar cells as a function of temperature and solar concentration ratio. The I-V characteristics of the solar cells were assumed to be governed by the relation I = I0(eqV/AkT - 1) with I0 = Ke-E and A = B The overall efficiencies of tandem cell stacks consisting of as many as 24 cells having energy gaps in the 0.7 to 3.6 eV range were calculated for temperatures of 200, 300, 400 and 500 K and for illumination by an AMO solar spectrum having concentration ratios C = 1, 100, 500 and 1000 suns. For ideal diodes (A = B = 1), the calculations show that the optimized overall efficiency has a limiting value ηopt ≈ 70% for T = 200 K and C = 1000. For T = 300 K and C = 1000, this limiting efficiency approaches 60%. Most of the gain in efficiency occurs with the number of semiconductors in the tandem system between six and ten, e.g. for T = 300 K and C = 1000, an optimized six cell system has a theoretical limit efficiency of about 53%. Calculations were also conducted for the A = B = 2 case (non-ideal diode behavior); in this case the limiting value of η for a 24 cell system is about 65% at 200 K and 55% at 300 K. © 1979.