A superlattice solar cell with enhanced short circuit current and minimized drop in open circuit voltage

Abstract

Multiple quantum wells (MQWs) solar cell can increase the quantum efficiency of solar cell devices. Extra adsorption from quantum wells at longer wavelengths leads to a photovoltaic device with a high-density current output, which can solve the issue of current matching in tandem solar cells. However, enhancement in current density of a cell usually necessitates deeper wells in MQWs. As a result, open-circuit voltage would suffer from serious degradation due to a reduced quasi-Fermi-level splitting. Therefore, an optimization of a MQWs solar cell presents a trade-off between open-circuit voltage and short-circuit current. Our strategy is to avoid narrowing of the bandgap for the well material while maximizing the quantum efficiency in the sub-bandgap wavelength of the host material. This necessitates enhancement in carrier escape, which in turn requires thin enough barriers to allow tunneling of electrons and holes over an entire set of superlattice (SL). In this study, by the use of interface management technique for the crystal growth of MQWs, we have successfully implemented an ultra high efficiency SL solar cell by reducing the barrier thickness to 3.1 nm. For this cell, an increment of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> as large as 3.0 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was obtained as compared to a control GaAs p-i-n cell. A remarkable achievement was quite a small degradation in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> , 0.02V, in spite of substantial increment in I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> . An overall efficiency was 18%, which was a bit larger than that of the GaAs p-i-n control cell. Behind that remarkable achievement of almost no degradation in total conversion efficiency associated with the inclusion of quantum wells and enhanced output current, substantial contribution of tunneling transport has been confirmed for the SL cell by external quantum efficiency measurement at 77K.