Back Bandgap-Graded Kesterite Cu2Zn(Snx,Ge1-x)Se4 Thin Films for Solar Cell Applications

Scaffidi, Romain [UCL] et al.

Kesterite materials are promising alternatives to conventional Cu(In,Ga)Se2 and CdTe compounds for thin-film photovoltaics (PV) based on abundant and non-toxic elements. However, the efficiency of kesterite solar cells is still lagging due to large open-circuit voltage (Voc) losses that are mainly ascribed to electrostatic fluctuations and high density of point defects, related to their typical off-stoichiometric composition. Counteracting such losses can be achieved to a certain extent by substituting Ge to Sn in Cu2Zn(Sn1-x,Gex)Se4 (CZTGSe) compounds, while also allowing Sn-Ge bandgap tuning through the x=Ge/(Ge+Sn) ratio, which is the focus of this work. We implement a sequential process involving a physically evaporated Ge/Zn/Sn/Cu/Sn/Zn/Ge metallic precursor stack. This stack is pre-annealed in N2 to favour crystallization, and afterwards annealed with an optimized recipe in a Se-containing environment to activate and enhance grain growth. Polycrystalline CZTGSe absorbers with the expected composition, low surface roughness and close to micron-size crystalline grains are obtained, according to SEM-EDS and Raman data. TOF-SIMS measurements calibrated by EDX results demonstrate that a Sn-Ge back gradient is reached, with an average Ge/(Ge+Sn) ratio around 40%, corresponding to about 60% at the back and 20% at the front. This bandgap gradient likely has its minimum bandgap at the top surface, estimated to be 1.04eV from the PL spectrum. So as to attempt solving the TRPL-measured limited carrier lifetime, front surface sulfurization is investigated but still requires optimization. Using these CZTGSe absorbers, complete solar cells are processed and their IV characteristics are measured under AM1.5G illumination, leading to values of 487 mV and 28.1 mA/cm² for Voc and the short-circuit current density (Jsc), respectively. These encouraging values beyond 60% of the Shockley-Queisser limits are possibly the result of enhanced carrier collection by the Sn-Ge bandgap gradient, supporting the potential of Ge inclusion to boost the efficiency of kesterite solar cells. However, these devices are affected by a large fill factor deficit, which cannot be counteracted without a deeper understanding of the mainly responsible loss mechanisms.