ZnSiP2 Thin Film Growth for Si-Based Tandem Photovoltaics

ZnSiP2 is a ternary III - V analog with 0.5% lattice mismatch with Si and a 2.1 eV band gap, in the appropriate range for a top cell on a Si-based tandem device. We have previously shown that ZnSiP2 has many properties suitable for applications to Si-based tandem photovoltaics using bulk single crystals grown in a Zn flux. The favorable results obtained from characterization of bulk material encourage the development of ZnSiP 2as a photovoltaic absorber. To pursue this development, we have constructed a thin film growth reactor. This reactor employs a combination of chemical vapor deposition, using silane and phosphine as precursor gases, and physical vapor deposition, using an effusion cell to evaporate elemental Zn. We will present the results of ZnSiP2 film growth on (100) Si substrates. The composition, structure, and morphology of these films have been characterized by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron diffraction, and electron microscopy, respectively. These promising results represent significant advancement towards implementing ZnSiP2 as a top cell material on Si-based tandem photovoltaics.

I. Introduction

The detailed balance efficiency limit of intermediate band solar cells (IBSCs) (see Figure 1(a)) has been calculated at 63% at full concentration [1]; higher than the Shockley-Queisser limit of 41% [2] for conventional single-bandgap solar cells. Suggested implementations of IBSCs have been based on semiconductor nanostructures such as quantum dots [3], bulk semiconductor highly mismatched alloys [4] and bulk semiconductor materials containing a high density of deep-level impurities [5]. While the key IBSC operating principles have been demonstrated [6], [7] the Shockley-Queisser limit is still far from being exceeded in practice. In present implementations, the increased recombination via the IB levels is usually more significant than the improvement in photocurrent offered by the intermediate band absorption.