Back contact interface formation and extrinsic doping of solution-processed CZTSSe solar cells

Naylor, Matthew C. (2024) Back contact interface formation and extrinsic doping of solution-processed CZTSSe solar cells. Doctoral thesis, Northumbria University.

Text (Doctoral thesis) naylor.matthew_phd(16037906).pdf - Submitted Version Restricted to Repository staff only until 23 November 2024. Download (136MB) | Request a copy

Abstract

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) is a promising material for thin film photovoltaics as it possesses a direct energy bandgap, large absorption coefficient and high theoretical power conversion efficiency (PCE) according to the Shockley–Queisser limit. CZTSSe has attracts and retains scholarly interest due to the low-cost, low-toxicity and Earthabundant nature of the constituent elements. In contrast to the commercially available Cu(In, Ga)(S, Se)2 (CIGSSe), the performance of solution-processed CZTSSe devices has surpassed that of traditional physical vapour deposition (PVD) devices and thus, paving the way for low-cost manufacture at scale. This work uses an established solution-based CZTSSe method where a Cu2ZnSnS4 (CZTS) nanocrystal based ink is synthesised first, followed by a reactive anneal to promote recrystallisation. Comparable performance is observed in devices fabricated using small-scale ink deposition means, i.e. spin-coating as well as up-scaled ink deposition means, i.e. slot-die coating. As with the current state-ofthe- art CZTSSe research, this work adopts the substrate device architecture from CIGSSe technology where light enters through the top of the structure and not through the substrate. At the back contact interface (Mo/CZTSSe) an intermediate Mo(S, Se)2 layer is formed under the Se-rich environment and elevated temperatures imposed for CZTSSe processing. In an isolated experiment, it is found that by varying the deposition parameters of the Mo back contact the thickness of the somewhat resistive Mo(S, Se)2 layer can be reduced. X-ray diffraction measurements reveal that the two-dimensional Se-Mo-Se sheets which constitute the Mo(S, Se)2 layer exhibit multiple orientations. The orientation where the Se-Mo-Se sheet is positioned perpendicular to the substrate is found to be the most prominent. Varying the incident angle of the X ray beam reveals that towards the top of MoSe2 layer the orientation of Se-Mo-Se sheets tends towards the parallel plane. It is probable that parallel-to-tilted Se-Mo-Se orientation inhibit further Se diffusion and MoSe2 growth. A peak performance of 4.86% PCE is measured for a CZTSSe solar cell with a 185 nm thick Mo(S, Se)2 layer. Device performance with thinner Mo(S, Se)2 layers suffered from current shunting effects whilst the current collection in devices with thicker layers was inhibited by a large series resistance. The introduction of Ge in the CZTSSe absorber through ex-situ and in-situ means was investigated in an effort to manage Sn-related defects in the material. The in-situ route involved Ge-alloying at the point of CZTS synthesis and resulted in fundamental change to the crystallographic properties of the nanocrystalbased ink. Despite layering CZTS and Ge-alloyed CZTS inks, a Ge composition gradient was not achieved however, a peak PCE and open-circuit voltage of 5.00% and 355 mV was measured from devices fabricated with a Ge-alloyed capping layer. The ex-situ approach combined both PVD and solution based deposition modes to form an elemental Ge layer above (overlayer) and beneath (underlayer) the CZTS precursor. The Ge overlayer proved detrimental to the CZTSSe morphology, which was attributed to Ge-Se interplay during the CZTS recrystallisation step. Whereas the Ge underlayer benefited the morphology as it diffused up through the precursor and enhanced the recrystallisation, this resulted in reduced Sn(S, Se)2 on the CZTSSe surface.

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