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Sulfur/selenium grading is a widely used optimization strategy in kesterite thin-film solar cells to obtain a bandgap-graded absorber material and to optimize optical and electrical properties of the solar-cell device. In this work, we present a novel approach to introduce a [S]/([S] + [Se]) grading for Cu

This work reports a growth process for tuning the formation pathway and final composition of kesterite Cu2ZnSnSe4 (CZTSe) thin film absorbers. By adjusting the as-grown composition of the precursor, the kesterite formation can be started in a Cu-rich, stoichiometric, or Cu-poor environment. During the hightemperature stage of the annealing process, incorporation or loss of SnSe2–x vapor at the layer is used to achieve an in-process composition shift during further kesterite growth (e.g., Cu-rich to Cu-poor/Cu-poor toward Cu-rich). This approach partially decouples the composition of the as-grown precursor and the final absorber, which, thus, can be tuned independently. The film growth, specifically the composition shift during the process, is tracked by interruption experiments. The influence of the starting composition of the precursor and the resulting absorber composition on the optoelectronic properties and device performance of the final solar cells is discussed. The results show a refined composition region for reduced Urbach tailing and lower VOC deficit. The best solar cells show efficiencies > 11% and a VOC deficit < 320 mV (relative to the Shockley–Queisser limit).

Recently, organic solar cells have demonstrated high power conversion efficiencies, but extrinsic device stability remains to be a challenge. Often, delamination of the cathode upon water ingress leads to a massive active area loss and to a strong decrease in efficiency. We present highly efficient organic solar cells with an oligothiophene-based donor and two different cathodes. Devices are aged at four different climates under continuous illumination. Al and MoO3/Cr/Al cathodes are compared. Additional interlayers are introduced for enhanced cathode adhesion. Aging in different climates reveals a strong dependency of the degradation speed on absolute humidity for standard Al cathodes. However, using additional interlayers, degradation seems to be almost decoupled from the applied climate and device degradation is slowed immensely. Parallel WVTR measurements of the AlOx barrier allow for the assessment of absolute water quantities until devices are degraded to half of their initial efficiency: 24gm2 are found for standard Al cathodes, while devices with additional interlayers reach 58gm2. This difference is related to a completely different aging behavior, which trades strong short-circuit current density losses for moderate fill-factor losses.