Germanium Alloyed Kesterite Solar Cells with Low Voltage Deficits

It has previously been shown that substitution of germanium for tin in Cu2ZnSn(S,Se)(4) provides a means to change the band gap. Here, we show that Ge substitution can also be used to improve carrier collection and decrease the open-circuit voltage deficit that has hindered kesterites. Using a simple molecular ink, we spray coat continuous composition gradients of Cu2Zn(Sn,Ge)(S,Se)(4) spanning 0-90% Ge/(Ge + Sn). By mapping the absolute intensity photoluminescence and making devices from these gradients, we are able to clearly resolve changes in material properties and device performance with composition. We find that Ge can be used to increase the band gap as high as 1.3 eV (50% Ge/(Ge + Sn)) without any loss in optoelectronic material quality, but beyond 50% Ge/(Ge + Sn), the device efficiency decreases rapidly. We show evidence that the degradation results from both a deep defect located about 0.8 eV above the valence band and unfavorable band alignment. As the band gap increases, the defect moves toward midgap and becomes a stronger recombination site. After the completed devices were aged for 1 month in the ambient laboratory environment, the deep defect heals, and the V-oc improves by as much as 227 mV. We demonstrate an 11.0% efficient spray coated CZTGSSe device, without antireflective coating, that achieves 63% of the theoretical V-oc as compared to the 58% for the current record device.