Contents I. Introduction
The trade-off between the maximization of the current and the voltage of solar cells obtained through smaller and larger band-gap-materials respectively sets their fundamental efficiency limit to 31% [1]. Insertion of thin layers of smaller-bandgap material (the quantum well) in a host of larger-bandgap material (the quantum barrier), as shown in Fig. 1, can be a trick to break this fundamental limit [2], [3]. Known as quantum well solar cells (QWSC), this type of solar can harvest longer wavelength photons which are simply transmitted past the thin-film solar cells configured to absorb light with wavelengths approximately 900 nm, also evident from Fig. 1. Early-days QWSCs used p-n structure [2], which are later replaced by the p-i-n structure to provide uniform electric field across all the quantum wells inserted in the i-layer i.e. in the sandwiched intrinsic layer [Fig. 1]. The photo-generated carriers can escape from the wells through the thermal escape and tunneling [4] –[6]. However, the efficiency of QWSC is enhanced in comparison with the bulk cells (which has no quantum wells) when the loss in voltage is counter-balanced by the photo-current enhancement [7]. The work in [8] experimentally verified the voltage enhancement in QWSC; the prospect of enhanced efficiency has been discussed in [9], [10].
Energy band diagram of a quantum well solar cell [11].
