Quantum well solar cells for concentrator applications

The insertion of quantum wells into the i region of a bulk solar cell extends the absorption to longer wavelengths. This results in ef?ciency enhancement when the reduction in voltage is smaller than the increase in photocurrent. In addition to this advantage the strain balanced quantum well solar cell (SB QWSC) achieves longer wavelength absorption without the formation of dislocations [1,2] which should lead to longer device lifetimes. Further ef?ciency enhancements are possible by using wavelength selective mirrors such as Distributed Bragg Re?ectors (DBRs) to re?ect unabsorbed photons through the structure for a second pass thus increasing photocurrent. This paper examines the performance of GaAsP/InGaAs SB QWSCs for concentrator applications. At concentrator current levels the dark currents of the SB QWSC show ideal behaviour with ideality n ~ l. Modelling suggests that this is as a result of the dominance of radiative recombination [3,4]. The radiative dominance at concentrator current levels also means that in a QWSC with a DBR a significant proportion of the photons from the radiative recombination in the wells will be re?ected by the DBR and re absorbed in the well thus reducing the dark current and enhancing the open circuit voltage. The DBR enhances the efficiency from 25% to 26.4% in AMl.5D at around 3lO suns. Measurements of the temperature dependence of the dark currents confirm that the n=l component extracted from the fitted dark current data is the due to the radiative recombination and this holds from 25°C to lO5°C.