Nanopatterning and bandgap grading to reduce defects in CdTe solar cells

Abstract

We present simulation and experimental results proving the feasibility of a novel concept to increase efficiency of CdTe based solar cells. In order to achieve $0.50/W price in CdTe based modules, higher efficiencies need to be attained. The high defect density due to lattice-mismatch between CdS and CdTe reduces lifetime, voltage, and efficiency of the cells. We propose the use of a graded composition structure and a patterned substrate to reduce defects, increase lifetime, and efficiency of the cells. Innovative simulations using high-fidelity molecular dynamics predict that defect-free films are possible if the CdTe film is graded with Zn and is constructed as nano-islands with sizes below 90 nm. Both graded structure and nano-islands reduce the lattice-mismatch stresses. Also, the graded composition creates a back surface field and an enhanced ohmic contact. We have attempted to grow ZnTe and CdTe films on CdS substrates using a template of micro and nano-islands. Selective growths on patterned substrates have shown fewer grain boundaries when the island size decreases below 300 nm. Also, larger grain sizes were obtained using a CdTe/ZnTe stack when compared to a single layer CdTe. The simulation and experimental results demonstrate for the first time the ability to use nanopatterned substrates to enhance uniformity in thin film solar cells.

A prediction of dislocation‐free CdTe/CdS photovoltaic multilayers via nano‐patterning and composition grading

“…The approach demonstrated earlier enables us, for the first time, to use predictive simulations to accurately derive quantitative relations between misfit dislocation density and system dimensions and to identify defect‐free multilayers especially in a nanoscale regime where experimental studies require significant investment. As a motivation of our work, modern nano‐patterning technology currently allows nanoislands to be grown on a lattice mismatched substrate . An example of the concept is illustrated in Figure .…”

Molecular Dynamics Simulations of CdTe / CdS Heteroepitaxy - Effect of Substrate Orientation

“…Past efforts (Aguirre et al, 2014) (Cruz-Campa et al, 2012) (Zubia et al, 2007) have focused on reducing defects during the fabrication of the epilayers, although the fundamental mechanisms of defect formation are not well understood. Experimental studies have shown that careful preparation of the substrate surface termination (Myers et al, 1983) and the orientation of the substrate (Smith et al, 2000) (Sarney & Brill, 2004) (Terheggen et al, 2003) can greatly influence the microstructure and the quality of the heteroepitaxial film.…”

A molecular dynamics study on defect reduction in thin film Cd<inf>1&#x2212;x</inf>Zn<inf>x</inf>Te/CdS solar cells

“…For the CdTe/CdS multilayers commonly used for thin film solar cells, a very high misfit dislocation density is expected due to the large lattice mismatch (~10%) between the two materials [4]. Our group is currently exploring novel solar cell designs that use Cd 1-x Zn x Te nano islands to reduce defects [5,6]. The growth of the Cd 1-x Zn x Te allows graded composition where x continuously changes from x = 1 (i.e., ZnTe) to x = 0 (CdTe) in order to reduce the epilayer's strain when grown on top of CdS.…”