The performance of kesterite-based photovoltaic devices is famously limited by significant open-circuit voltage (Voc) deficits that restrain their efficiency to levels lower than competing inorganic thin-film technologies such as Cu(In,Ga)Se2 (CIGS) and CdTe. The reason for this low Voc is widely attributed to excessive defect-assisted recombination and band tailing which originate from the off-stoichiometry typically encountered in the most efficient devices, the most successful composition so far being Cu-poor and Zn-rich. One strategy explored in the past decade to solve this Voc issue is to substitute Sn to Ge in a certain proportion x = Ge/(Ge+Sn), leading to Sn-Ge mixed kesterites denoted as Cu2Zn(Sn1-x,Gex)Se4 (CZTGSe). The benefit of Ge alloying is two-fold: it limits the quantity of Sn2+ species which are one of the main culprits for the electronic limitations mentioned above, and it allows bandgap tuning through the x ratio. In this work, we present the process development and opto-electrical characterization of Sn-Ge bandgap-graded CZTGSe thin-film absorbers and solar cells. The CZTGSe layers are deposited via a physical-based sequential process consisting in evaporation of layered metallic precursors then annealed in Se vapour. Polycrystalline kesterite thin-film absorbers are grown with acceptable morphology, Cu-poor & Zn-poor stoichiometry and a 1.04 eV PL bandgap. Their carrier lifetime is of the order of the nanosecond, as usually observed for kesterites as the consequence of high defect densities. ToF-SIMS profile indicates a Sn-Ge graded bandgap, likely due to the natural segregation of both elements during selenization, with a maximum bandgap around 1.4 eV at the back and a minimum bandgap around 1.1 eV at the front estimated from EDX composition. The obtained bandgap gradient towards the back interface should mitigate interface recombination, as observed in high-efficiency CIGS devices. Champion CZTGSe solar cell devices reach efficiency, Voc, Jsc and FF values of 7.1 %, 510 mV, 30.1 mA/cm² and 46.7 %, respectively, from I-V measurements whereas EQE spectrum reveals a slightly lower Jsc and an electrical bandgap of 1.23 eV in agreement with the estimated bandgap gradient. While Jsc attains an encouraging 70 to 80 % of its Shockley-Queisser (SQ) limit, the deficits in Voc and FF are particularly large and are the main culprits for poor performance in the studied devices. Notwithstanding the impact of non-ideal shunt and series resistances, EQE curve also highlights poor carrier collection in the absorber as well as high crystalline disorder, probably related to the point defects also limiting carrier lifetime. This tends to be confirmed by temperature-dependent IV and CV that identify the main Voc limiting factor to be relatively deep defect states likely close to the CZTGSe/CdS interface and 260 meV below the conduction band, combined with low hole mobility. These conclusions from experiments are supported by SCAPS 1-D simulations which suggest a highly defective surface layer between the CZTGSe absorber and CdS buffer. This study demonstrates the possibility the obtain Sn-Ge bandgap-graded kesterite solar cells but emphasizes the need to counteract near-CdS surface defect formation as the main limiting factor with regards to Voc.
Scaffidi, Romain ; et. al. Characterization and modelling of Sn-Ge bandgap-graded kesterite thin-film solar cells.13th European Kesterite Workshop (Barcelona, Spain, du 05/07/2023 au 07/07/2023).