Visualization of Photoexcited Dynamics in Chalcogenide Materials Grains using State-Of-The-Art Pump-Probe Microscopy and Spectroscopy Techniques

IIn this paper, we integrate a combination of modern and complementary ultrafast transient absorption spectroscopy (TAS) and pump-probe transient absorption microscopy (TAM) techniques to discuss the ultrafast photophysics of efficient photovoltaic materials including Cu₂BaSnSSe (CBTSSe) chalcogenides^1,2 and Cu₂ZnSnS_4–xSe_x (CZTSSe). CBTSSe is a recently introduced alternative to Cu(In,Ga)(S,Se)₂ and Cu₂ZnSnS_4–xSe_x (CZTSSe). Pump-probe microscopic imaging enables to localize the photoexcitation patterns and early charge carrier kinetics within the grains of only a few hundreds of nanometers and localize the kinetics of photogenerated carriers in each grain with femtosecond time resolution.<br/> <br/>We compare the photoexcitation pattern in CBTS and CZTS films and discuss the excited state relaxation within film grains. For CBTS, we spectrally resolve a sharp ground-state bleaching (GSB) peak, formed around the band edge transition. The pump-probe microscopy images are heterogeneously composed of positive and negative regions identifying excited state absorption and ground sate bleaching in different grains. For CZTS samples, at identical condition, we observe that the pump-probe microscopy images are homogeneously composed of GSB. The broadband spectrum points to broad GSB absorption extended from visible to near infrared region for CZTSSe. Our pump-probe microscopy and spectroscopy results, all point to a reduction in shallow defects and band tailing in CBTSSe relative to CZTSSe films. The charge carrier relaxation resolved in CBTSSe single grains, or over an ensemble of grains, however, remains faster compared to CZTSSe, pointing to the need to perhaps better understand deep traps within this absorber family. Remarkably, we have shown that the unique sensitivity of pump-probe microscopy and sharp electronic transitions allow for detection of small compositional variations,— features that are largely unresolved for ensemble spectroscopy or luminescence measurements.<br/>References:<br/>1. Ghadiri, E.; Shin, D; Shafiee, A.; Warren, W.S.; Mitzi, D.B.; Grain-Resolved Ultrafast Photophysics in Cu₂BaSnS_4−xSe_x Semiconductors Using Pump−Probe Diffuse Reflectance Spectroscopy and Microscopy.<br/>2. Shin, D.; Zhu, T.; Huang, X.; Gunawan, O.; Blum, V.; Mitzi, D. B. Earth-abundant chalcogenide photovoltaic devices with over 5% efficiency based on a Cu₂BaSn(S,Se)₄absorber. <i>Adv. Mater</i>. <b>29</b>, 1606945 (2017).<br/>3. Ghadiri, E.; Zakeeruddin, S.M.; Hagfeldt, A.; Grätzel, M.; Moser, J.-E. Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy. <i>Sci. Rep.</i> <b>6</b>, 24465 (2016).<br/>4. Ghadiri, E.; Taghavinia, N.; Zakeeruddin, S. M.; Grätzel, M.; Moser, J.-E. Enhanced electron collection efficiency in dye-sensitized solar cells based on nanostructured TiO₂ hollow fibers. <i>Nano Lett</i>. <b>10</b>, 1632–1638 (2010).