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A high-efficiency CZTSSe solar cell was achieved through the modification of the back interfacial contact with a MoO3 layer synthesized in situ using Na2S aqueous solution.

Cs treatment in a CZTSSe system could enhance device performance by affecting defect characteristics and carrier–transport properties.

A solution-processed ZnO:Li NP window layer is demonstrated to enhance charge collection and minimize interface recombination in kesterite solar cells.

The introduction of alkali metal (AM) post-deposition treatment (PDT) has led to the consecutive breakthrough of world record efficiencies for Cu(In,Ga)Se2 photovoltaics. However, for the derivative Cu2SnZn(S,Se)4 (CZTSSe) solar cells, the corresponding verdict seems partially unjustified due to the difference in defect chemical environment and phase structure, and the most effective incorporation means of AMs is still a mystery. In this work, for the first time, we developed a dual AM synergistic incorporation strategy combining light and heavy AMs by post-depositing CsF after NaF-PDT to improve the efficiency of CZTSSe solar cells. The relevant microscopic-electrical advantages with regard to the different roles of NaF and CsF PDT are discussed in more detail. It was found that the Na atoms are randomly dispersed both in the grain interior (GI) and grain boundaries (GBs), significantly increasing the acceptor concentration and passivating the deep level defects to enhance the Voc. In contrast, the Cs atoms are mainly segregated at the GBs, primarily increasing the potential of the GBs and improving the FF. The prominent electric benefits of Cs at the GBs and Na in the GI finally increase the device efficiency from 10.12% to 12.16%. Our findings solve the difficulty in obtaining the available Voc and FF for future high efficiency research and should serve as a basis for the in-depth understanding of AM-PDT in kesterite photovoltaics.

A major obstacle hindering further development of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the inherent recombination losses that occur both via bulk defects and charge extraction interfaces. Herein, we propose a generic Ag, Ge dual-gradient absorber architecture, involving a downshift of absorber VBM at the front contact and an upshift of absorber CBM at the back contact, to improve device performance. We experimentally show that the substitution of Cu by Ag allows for larger band bending at the p–n junction, and the substitution of Sn by Ge imposes an additional drift field within the quasi-neutral region. Additionally, the Ag and Ge ions diffusing into the bulk absorber passivate the deleterious CuZn and CuSn deep-level defects. In turn, accelerated charge extraction, transport and minimized recombination loss offered surprising gains in Voc and Jsc, and the dual-gradient device achieved a maximum efficiency of 12.26% with an improved Voc,deficit of 553 mV. This Ag, Ge graded-substitution approach offers an alternative absorber architecture to improve the Voc,deficit in future high-efficiency kesterite PV cells.

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have been receiving considerable attention in recent years, owing to their low cost and toxicity as well as a high abundance of their constituent elements. Solution-based (mainly precursor solution) methods have been widely used in the fabrication of CZTSSe-based thin film solar cells. Though most record solar cells since 2009 have been achieved using solution-based methods, carbonaceous residues are always formed during post deposition treatment (PDT). These carbonaceous residues are detrimental to grain growth of CZTSSe to form large grains. The presence of unwanted small grain layers will increase the series resistance and reduce the fill factor of the solar cell, thus consequently decreasing the efficiency of CZTSSe solar cells. Herein, we propose “green” supercritical carbon dioxide (SCCO2) treatment to overcome the challenge of carbonaceous residues. FT-IR, SEM, XRD and Raman characterization studies were used to confirm the reduction of carbonaceous residues in CZTSSe films. The solar cell fabricated from a selenized CZTSSe precursor film with SCCO2 treatment under optimal conditions showed the best conversion efficiency of 11.23% (VOC = 0.456 V, JSC = 36.6 mA cm−2, FF = 67.1%). To the best of our knowledge, this is the first report on boosting the efficiency of solution-based CZTSSe solar cells by using SCCO2 treatment. In addition, this unique SCCO2 treatment may also bring forth new ideas for other solution-based optoelectronic devices.

Although the substitution of Cu by Ag to suppress CuZn defects offers several advantages in overcoming the large open-circuit voltage (Voc) deficit for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, an excellent performance has not been achieved to date primarily due to the Fermi level pinning at the CdS/absorber interface and large recombination at the absorber/Mo interface. Herein, we developed a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the back and front surfaces of the (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) layer. The key advantages of this Ag-graded structure are as follows: the higher content towards the CdS/absorber interface can create weak n-type donor defects and retard Fermi level pinning, whereas the lower content at the interlayer maintains the conductivity and light absorption; moreover, the other higher content towards Mo back contact can effectively suppress the recombination and improve the utilization of long-wave incident light. By appropriately adjusting the Ag gradient, we demonstrated a significant increase in Voc, and an unexpected conversion efficiency of 11.2% was achieved. This is the highest efficiency achieved to date for Ag-substituted CZTSSe solar cells, and the result supports a new aspect that synthesis of a composition-graded CAZTSSe absorber has great potential for future research.