Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings

Abstract

Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu 2 (Zn,Sn)(S,Se) 4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al 2 O 3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V O C ; +17%rel.), short-circuit current (J S C ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

CZTS solar cell device simulations with varying absorber thickness

“…In fact, back contact recombination is dominating for all thicknesses when ≥ 1500 nm. Further empirical studies in this subject are needed to evaluate the real influence of the back contact in CZTS devices [12]. …”

On the Importance of Joint Mitigation Strategies for Front, Bulk, and Rear Recombination in Ultrathin Cu(In,Ga)Se2 Solar Cells

“…The point contacts have 200 nm in diameter with a 2 μm pitch (center to center) in a quadratic array. A detailed process flow for the patterning approach can be found elsewhere. , Before the CIGS growth, 7 nm of NaF was evaporated on all substrates, ensuring Na supply, as the out-diffusion from the SLG might be blocked by the passivation layer. , The CIGS layer was grown using a three-stage co-evaporation step. The co-evaporation processes needed to be adjusted to account for the ultrathin CIGS layer.…”

“…A detailed process flow for the patterning approach can be found elsewhere. 36,43 Before the CIGS growth, 7 nm of NaF was evaporated on all substrates, ensuring Na supply, as the out-diffusion from the SLG might be blocked by the passivation layer. 44,45 The CIGS layer was grown using a three-stage coevaporation step.…”