New ATK paper on interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells

Mar 09 2017

A new paper using ATK has been published in Applied Physics Letters, discussing interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells. It is written in collaboration between QuantumWise and a group of researchers as the Technical University of Denmark.

Cu2ZnSnS4 (CZTS) is a promising material for use in thin film photovoltaics. Since it is a direct bandgap semiconductor, devices based on CZTS should absorb light more effectively than conventional Silicon-based solar cell devices which suffer from the indirect bandgap of Silicon. 

Moreover CZTS is based purely on earth abundant elements making it very cost effective compared to already successful thin film solar cell technologies like CdTe. So far CZTS devices have suffered from inexplicably low open circuit voltages (OCV). Disadvantageous (cliff-like) band-alignment between CZTS and its CdS buffer-layer has been the most popular explanation for the deficit in OCV, even though measurement on the band-alignment in state-of-the-art CZTS device with over 7% efficiency all show favorable (spike-like/flat) band-alignments.

atkpaper-bandgapnarrowingLocal density of states of the CZTSe/CdS interface (left) and of the CZTS/CdS interface (central) resolved along the direction perpendicular to the interface plane. (right) Spatially-resolved DOS of the localized states at the CZTS/CdS interface.

In the recently published article, the researchers put forward an alternative explanation for the OCV-deficit. Using a method [1] based on tools available in ATK they studied in detail the electronic structure of the interface between CZTS and CdS, where they identified a shallow state localized at the interface. When included in device level simulations such a state leads to significant deterioration of the OCV, and as shown in the paper measurements on state-of-the-art CZTS solarcells can be reproduced quantitatively. This shows how parameters needed for device characterization can be extracted from atomistic device simulations and how the interplay between atomistic and device level simulations can be a powerful tool in characterizing the thin-film solarcell devices of the future.

You can find the paper here:

A. Crovetto, M. Palsgaard, T. Gunst, T. Markussen, K. Stokbro, M. Brandbyge and O. Hansen "Interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells"Appl. Phys. Lett. 110, 083903 (2017).

References:

[1] M. Palsgaard, A Crovetto, T. Gunst, T. Markussen, O. Hansen, K. Stokbro and M. Brandbyge, "Semiconductor band alignment from first principles: a new nonequilibrium Green's function method applied to the CZTSe/CdS interface for photovoltaics", 2016 International Conference on Simulation of Semiconductor Processes and Devices (sispad) - 2016, pp. 377-380.