The selenization of stacked elemental metallic layers (CuSn–Zn) is a commonly reported approach in kesterite Cu2ZnSnSe4 (CZTSe) processing. CZTSe formation via this approach usually involves a reaction route containing binary selenides, such as SnSe2−x. The high volatility of these phases at the necessary annealing temperatures (500–550 °C) makes this reaction pathway prone to Sn loss, which makes it challenging to control the composition and quality of the grown material. Herein, an approach based on stacked elemental and alloyed precursors is reported, and the benefits of using a Zn/CuSn/Zn configuration are discussed. The absence of nonalloyed elemental Sn helps in suppressing the formation and subsequent evaporation of SnSe2−x phases, preventing Sn loss from the film during selenization. This reaction pathway involves a process scheme which 1) starts with the growth of CZTSe in a “Cu‐rich” environment, 2) includes a shift of the composition by supply of SnSe2−x vapor, and 3) terminates in the “Cu‐poor” regime, leading to device efficiencies above 10%. This composition shift in the presented process appears similar to the final stage of the commonly known CIGSe three‐stage coevaporation.

A new diffractometer system was designed and built for the G2 experimental station at the Cornell High Energy Synchrotron Source (CHESS). A six-circle κ goniometer, which provides better access to reciprocal space compared to Eulerian cradles, was chosen primarily to perform large angle Bragg diffraction on samples with preferred crystallographic orientations, and can access both horizontal and vertical diffraction planes. A new atmosphere- and temperature-controlled sample stage was designed for thin film thermomechanical experiments. The stage can be operated in ultrahigh vacuum and uses a Be dome x-ray window to provide access to all scattering vectors above a sample’s horizon. A novel design minimizes sample displacements during thermal cycling to less than 160μm over 900°C and the stage is motorized for easy height adjustments, which can be used to compensate for displacements from thermal expansion. A new area detector was built and a new line detector was purchased. Both detectors cover a large region in reciprocal space, providing the ability to measure time-resolved phenomena. A detailed description of the design and technical characteristics is given. Some capabilities of the diffractometer system are illustrated by a strain analysis on a thin metal film and characterization of organic thin films with grazing incidence diffraction. The G2 experimental station, as part of CHESS, is a national user facility and is available to external users by application.

We report an approach to incorporate Ge into Cu2ZnSnSe4 using GeSe vapor during the selenization step of alloyed metallic precursors. The vapor incorporation slowly begins at T ≈ 480 °C and peaks at 530 °C, resulting in a Ge-based composition shift inside the previously formed kesterite layer. We initially observe the formation of a Ge-rich surface layer that merges into a homogeneous distribution of the incorporated element during the further dwelling stage of the annealing. This approach is very versatile and could be used in many similar fabrication processes for incorporating Ge into CZTSe-absorber layers. Because the vapor-based composition shift in the layer happens after the formation of the absorber film towards the end of the fabrication process, most process parameters and the precursor structure may not need any significant re-optimization. The careful integration of this step could help to reduce Sn-related deep defects and accompanying VOC losses. The best CZTGSe-power-conversion efficiency obtained in this series is 10.4 % (with EG = 1.22 eV, FF = 54%, JSC = 36 mA/cm2, VOC = 540 mV, VOCdef,SQ = 417 mV). These results demonstrate the potential of this approach for Ge incorporation into kesterite absorbers.

In this manuscript, we give an overview of the main insights into our growth procedure for kesterite solar cells and show the possibilities that are provided by this approach. The importance of using Cu–Sn alloy instead of elemental Sn and Cu in the precursor is shown. We discuss how the alloy approach stabilises the composition and helps guide the process along a preferred reaction pathway. A summary of our previously reported findings in the context of our latest results on kesterite solar cells prepared from Cu–Sn alloyed precursors is drawn. The positive impact of an alloy precursor configuration on the formation pathway, process control, and process resilience is demonstrated. Furthermore, a new optimisation strategy for kesterite, based on the reported pathway, is discussed, including a smooth phase transition from Cu-rich to Cu-poor kesterite. Finally, we demonstrate results on buffer optimisation and the application of a promising hybrid buffer configuration of CdS/Zn(O,S), which can reduce the optical losses in the solar cell structure.

This study show the influence of selenium amount during annealing of kesterite on the elemental composition of absorber and on the opto-electronic properties of solar cells. Enhanced carrier collection leads to device efficiencies approaching 12%.