Erika Robert

Quantum Efficiency Measurements and Modeling as Tools to Monitor Air Annealing of Cu2SnS 3 Solar Cells

Air annealing of chalcopyrite solar cells at either 200 °C or higher is often known to increase their power conversion efficiency. In this paper, we investigate the nature of this effect for Cu2SnS3 (CTS) solar cells by modeling the experimental external quantum efficiency. We find that the cell efficiency increase stems from increased diffusion length and depletion width and decreased interface recombination at the p-n junction. The increased diffusion length is also reproduced when only the absorber layer is air annealed. When solar cells are annealed in N2, no increase in diffusion length is measured. Hence, we attribute the increase in diffusion length to passivation of the grain boundaries in the bulk by oxygen. The larger depletion width on air and N2 annealing in the devices is independent of the CdS buffer layer thickness and occurs in its absence. We ascribe it to copper diffusion from the absorber layer to the n-type buffer and window layers. Interface recombination positively correlates with increasing buffer layer thickness. Based on our modeling, we conclude that the CTS absorber layer is still too highly doped to obtain large depletion widths and is highly recombination active at the p-n interface.

Crystallographic and optoelectronic properties of the novel thin film absorber Cu2GeS3

Thin films of Cu2GeS3 are grown by annealing copper layers in GeS and S gaseous atmosphere above 460°C. Below 500°C the cubic polymorph is formed, having inferior optoelectronic properties compared to the monoclinic phase, formed at higher temperature. The bandgap of the cubic phase lies below that of the monoclinic phase: they are determined from absorption measurements to be 1.23 and 1.55 eV respectively. Photoluminescence measurements are performed and only the monoclinic Cu2GeS3 shows a photoluminescence signal with a peak maximum at 1.57 eV. We attribute this difference between cubic and monoclinic to the higher quasi fermi level splitting of the monoclinic phase. Wavelength dependent photoelectrochemical measurements demonstrate the Cu2GeS3 to be p-type with an apparent quantum efficiency of less than 3 % above the band gap.

Chemical stability of the Cu 2 SnS 3 /Mo interface

Cu<inf>2</inf>SnS<inf>3</inf> is an earth abundant semiconductor researched for photovoltaic applications. Due to the small energy difference in the Sn^2+/4+ oxidation states and low free energy of MoS<inf>2</inf>, the Cu<inf>2</inf>SnS<inf>3</inf>/Mo interface is unstable and Cu<inf>2</inf>SnS<inf>3</inf> decomposes. The interface is stabilized by growing Cu<inf>2</inf>SnS<inf>3</inf> on a thin MoS<inf>2</inf> layer. Photoluminescence occurs only at the back of the Cu<inf>2</inf>SnS<inf>3</inf> layers when grown on MoS<inf>2</inf> and no quantifiable amounts of Cu and Sn are measured at the MoS<inf>2</inf> substrate. The quenching of emission of Cu<inf>2</inf>SnS<inf>3</inf> grown on Mo is due to binary sulfides formed in presence of Mo which are not formed when Cu<inf>2</inf>SnS<inf>3</inf> is grown on MoS<inf>2</inf>.