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Alkali post deposition treatment (PDT) of the absorber material is an efficient strategy to boost the desired photovoltaic performance of kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, a simple and innovative solution-processed cesium PDT method was employed as an effective strategy for the CZTSSe absorber bulk and interface modification toward efficient CZTSSe-based photovoltaic devices. The material properties of the CZTSSe thin film and the photoelectric properties of the corresponding photovoltaic devices are investigated subsequently. After Cs treatment, the Cs element is mainly detected on the surface of the CZTSSe-based sample, while the phase structure itself is not affected. Systematic studies reveal that the presence of small amounts of cesium in the CZTSSe absorber bulk results in a low defect concentration, prevents the bandgap tail states, enlarges the depletion region width, and reduces the carrier recombination. Meanwhile, the majority of the Cs element detected on the surface inhibits the carrier recombination, and leads to improved carrier transport and extraction. Benefiting from the synergistic optimization effects brought by Cs treatment, a champion device with 12.83% efficiency is achieved using the 1.25 wt% CsCl hot solution with a soaking time of 30 s, which is ≈26% higher than that of its pristine counterpart (10.21%). Our work offers judicious direction to expedite design strategies for solving bottleneck problems regarding the bulk and interface recombination, showing a promising future in developing efficient kesterite-based CZTSSe photovoltaic devices. [ABSTRACT FROM AUTHOR]

In Cu2ZnSn(S,Se)4 photovoltaic devices, the exceptionally high density of 2CuZn + SnZn donor defects may induce bandgap fluctuations and thus limit Voc and PCE enhancement. We have previously reported that the partial substitution of Ag for Cu in Cu2ZnSn(S,Se)4 provides a promising way to effectively suppress CuZn defects and considerably enhance the cell performance for Cu2ZnSn(S,Se)4 photovoltaic devices. However, the bandgap fluctuations and detrimental band tailing fundamentally still limit cell performance. On the basis of a Ag-substituted CZTSSe system, in this paper, we demonstrate that partially substituting Zn with Mn could be a feasible tactic to pronouncedly decrease the bandgap fluctuations and prevent detrimental band tailing. Our in-depth investigation reveals that Mn substitution could increase depletion width, decrease the defect densities of the CuZn acceptor and 2CuZn + SnZn donor, and enable grain boundary (GB) inversion in a (Cu,Ag)2ZnSn(S,Se)4-based cell. As a result, an impressive PCE of 11.81% was achieved when the Mn substitution level was 10%, with a Voc of 478 mV, a Jsc of 37.89 mA cm−2, and an FF of 65.23%. This is so far the highest efficiency among Mn-substituted CZTSSe-based photovoltaic devices. By thoroughly understanding these photoelectric properties of the CZTSSe-based photovoltaic devices, we believe that the breakthrough in device efficiency will come soon through our continuous efforts. [ABSTRACT FROM AUTHOR]

Although low-temperature selenization technology offers several advantages in terms of production costs and beneficial composition profile controlling for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, excellent performance has not yet been achieved primarily due to the increased number of electronic disorders, smaller grain sizes, and more grain boundaries. In this study, a universal strategy for significantly increasing the power conversion efficiency (PCE) of low-temperature processed CZTSSe solar cells through directly dissolving the fluxing agent antimony into the as-prepared precursor solution is demonstrated. The key feature of this approach is that Sb atoms actually act without entering into the lattice and leave almost no residue within the absorber after taking effect. By appropriately adjusting the antimony incorporation concentration, the grain size, fine-grain layer thickness, local defect chemistry environment, and hole carrier concentration can be simultaneously improved, especially at low temperatures. More importantly, a larger band bending around the grain boundary (GB) is also observed by increasing the potential value from 40 mV to 100 mV after antimony incorporation, which can facilitate the GB-dominated carrier separation process. Due to the antimony-assisted morphology and enhancement in the nanoscale electrical properties, the CZTSSe device could maintain its performance at a lower temperature of 80 °C and directly decrease the selenization temperature down to 470 °C. This systematic study presents a new aspect for overcoming the challenges encountered at low temperatures, and our findings further support the great feasibility for the future application of low-temperature-processed CZTSSe photovoltaic technology to meet the industrial-scale production standards. [ABSTRACT FROM AUTHOR]

A non-hydrazine solution process has been considered as a promising way for the deposition of Cu(In,Ga)(S,Se)2(CIGSSe) thin films. However, from our previous work, the presence of a fine-grained layer in the absorber layer was shown to be an obstacle to further improve the solar cell efficiency. Here, an improved fabrication route is proposed to solve this problem. A mixture of Cu2S, In2Se3, Ga and Se was simultaneously dissolved in 1,2-ethanedithiol and 1,2-ethylenediamine, forming a homogeneous precursor solution to deposit CIGSSe nanocrystal films. By optimizing the selenization method by using rapid thermal processing (RTP) combined with an improved graphite box, large grains throughout the whole absorber layer were successfully obtained. Based on such a high-quality CIGSSe absorber layer, an encouraging power conversion efficiency of 11.8% has been achieved with a Jsc of 28.25 mA cm−2, a Voc of 596 mV and a FF of 69.79% and without an antireflection coating. [ABSTRACT FROM AUTHOR]