A Japanese research group proposed a photovoltaic (PV) cell that does not use a PN junction and potentially realizes a conversion efficiency of 70-80%.

The group includes Syuuichi Emura, a researcher at the Institute of Scientific and Industrial Research, Osaka University. The cell was proposed at PVJapan2014.

The proposal was to use the polarity inside crystal (the gradient of the internal electric field caused by spontaneous polarization) for the separation of excitors (pairs of an electron and a hole). Though silicon (Si), which is a common material for PV cells, does not have a polarity, many compound crystals used as materials have strong polarities.

When photons are absorbed and excitors are generated due to the gradients of the internal electric fields of such materials, electrons and holes are spontaneously separated in different directions, Emura said. Specifically, he is considering a PV cell having a device structure in which a 300nm-350nm-thick InGaN layer (bandgap: 0.92eV) is sandwiched between an InN layer and electrodes.

In the case of commonly-used PV cells, PN junction plays the role of separating excitors and extracting electrons and holes at different electrodes. There are several advantages in separating excitors by using only the gradient of the internal electric field, but the largest advantage is that it prevents the recombination of electrons and holes as well as thermal relaxation to some extent.

For example, the thickness of the photoactive layers of general Si-based PV cells is several tens of micrometers or more to improve optical absorptance. As a result, short-wavelength, high-energy photons become so-called "hot excitors" away from the PN junction, and they are lost before reaching the PN junction and being separated into electrons and holes due to recombination and thermal relaxation.

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