The persistent double layer structure whereby two layers with different properties form at the front and rear of absorbers is a critical challenge in the field of kesterite thin-film solar cells, which imposes additional nonradiative recombination in the quasi-neutral region and potential limitation to the transport of hole carriers. Herein, an effective model for growing monolayer CZTSe thin-films based on metal precursors with large grains spanning the whole film is developed. Voids and fine grain layer are avoided successfully by suppressing the formation of a Sn-rich liquid metal phase near Mo back contact during alloying, while grain coarsening is greatly promoted by enhancing mass transfer during grain growth. The desired morphology exhibits several encouraging features, including significantly reduced recombination in the quasi-neutral region that contributes to the large increase of short-circuit current, and a quasi-Ohmic back contact which is a prerequisite for high fill factor. Though this growth mode may introduce more interfacial defects which require further modification, the strategies demonstrated remove a primary obstacle toward higher efficiency kesterite solar cells, and can be applicable to morphology control with other emerging chalcogenide thin films.
Keywords: CZTSe solar cells; back contact; bilayer structures; carrier collection; large-grain spanning monolayers.
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