Intermediate Band Solar Cells based on ZnTeO.

Abstract

Low-cost, high efficiency solar cells are of tremendous interest for the realization of a renewable and clean energy source. The conversion efficiency of a single junction solar cell with an Intermediate Band in the bang gap is theoretically predicted to be as high as that of a triple junction solar cell by expanding the absorption spectrum without sacrificing the open-circuit voltage. In this work, an Intermediate Band Solar Cell based on the ZnTeO material system is investigated both experimentally and theoretically ZnTeO thin films synthesized by MBE and PLD are compared by characterizing their structural, chemical and electrical properties, which conclude that low oxygen partial pressure in MBE growth preferentially leads to subtitutional oxygen impurity in ZnTe. These states and related complexes, located at 0.4-0.7eV below the conduction band, with a radiative recombination coefficient of 1.2×10-10 cm3 sec-1 and an absorption coefficient of 104 cm-1 are excellent candidates to demonstrate the IBSC technique. The theoretical work reveals that both the open circuit voltage and fill factor degrade but the short circuit current increases in a ZnTe junction with oxygen states in comparison to those of a junction without oxygen states. When the optical cross-sections of oxygen states are larger than 10-14cm2, the conversion efficiency of ZnTeO based IBSC can be more than 20% with the optimal oxygen states density in range of 1020-1021 cm-3, nearly double of that of a ZnTe cell without oxygen states. Experimentally, ZnTeO based cells exhibit an expanded response to the solar spectrum in comparison to ZnTe due to photo-excitation below the bandgap energy, resulting in an approximate double of the short circuit current, while suffering an approximate 15% decrease in open circuit voltage. Sub-bandgap excitation experiments under 650nm and 1550 nm excitation demonstrate absorption of two photons in sequence, which is desired for intermediate band solar cells and serve as a basis for further efforts to develop ZnTeO for enhanced solar cell conversion efficiency. The diode structure was further optimized by n-ZnO/p-ZnTe hetero-junction with enhanced diode characteristics. A ZnSe buffer grown by MBE between the ZnO/ZnTe interface demonstrated a significant improvement of photovoltaic effect