Grain-Resolved Ultrafast Photophysics in Cu₂BaSnS_{4- x}Se _x Semiconductors Using Pump-Probe Diffuse Reflectance...

In this paper, we analyze fundamental photoexcitation processes and charge carrier kinetics in Cu₂BaSnS_{4- x}Se _x (CBTSSe), a recently introduced alternative to Cu(In,Ga)(S,Se)₂ and Cu₂ZnSnS_{4- x}Se _x (CZTSSe) photovoltaic/photoelectrochemical absorbers, using advanced laser spectroscopy and microscopy techniques. The broadband pump-probe diffuse reflectance spectroscopy technique facilitates monitoring the ultrafast processes in opaque CBTSSe films deposited on Mo-coated glass substrates, similar to the configuration found in functional devices. We spectrally resolve a sharp ground-state bleaching (GSB) peak for CBTSSe films, formed around the band edge transition, which is spectrally narrower than the GSB and stimulated emission in corresponding CZTSSe films. The presence of sharp electronic transitions is further deduced from the ensemble pump-probe spectroscopy and steady-state UV-vis diffuse reflectance spectra. Furthermore, using pump-probe diffuse reflectance scanning microscopy, we monitor the charge carrier formation and excited state pattern within the film grains at few hundred nanometer resolution and localize the kinetics of photogenerated carriers in each grain. The unique sensitivity of pump-probe microscopy and sharp electronic transitions allow for detection of small S/Se stoichiometry variations, Δ x ≤ 0.3, in CBTSSe grains-i.e., features that are largely unresolved for ensemble spectroscopy or luminescence measurements. By noting the sharp band edge transition, we show that the band tailing issue (prevalent for CZTSSe) is largely resolved for CBTSSe; however, other issues may remain, such as deep defects and fast carriers relaxations, which may still impact the photocurrent and open circuit voltage of the CBTSSe devices/films examined.