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The SnZn defect passivation and heterojunction interface energetics‐modification are crucial for further improvement of open‐circuit voltage (Voc) and efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, a simple but effective cation substitution strategy is reported to promote the performance of CZTSSe devices by simultaneously modifying the absorber and heterojunction interface. This is achieved by introducing the organic silicon salt C8H20O4Si into CZTSSe precursor solution as a source of silicon to partially substitute Sn with Si. Systematic studies reveal that there are two mechanisms synergistically contributing to the increase of efficiency: 1) This strategy can passivate the undesired SnZn defect in CZTSSe absorber more efficiently, leading to at least one order of magnitude lower SnZn defect density and beneficial carrier transport properties; 2) After Si incorporation, the conduction band minimum of the CZTSSe is upshifted, thus enlarging the bandgap, which ultimately optimizes the energy level structure at the CdS/CZTSSe interface and reduces interfacial energy loss by decreasing the carrier transport barrier. Consequently, the CZTSSe device with 7% Si substitution delivers a satisfactory efficiency of 13.02% owing to an increase in Voc. It is hoped the Si substitution strategy reported here can provide inspiration for developing advanced CZTSSe devices with broad application prospects. [ABSTRACT FROM AUTHOR]

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]

As a promising absorber material for thin film solar cells, the highest reported power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) is still far from the detailed balance limit of efficiency due to the large VOC deficit. Back contact engineering plays an important role in promoting the CZTSSe solar cell towards realizing high PCE. In this study, a MoO3-modified layer with the highest oxidation state 6+ and better thermal stability was in situ introduced by spin-coating Na2S aqueous solution on the Mo electrode. The results demonstrate that the Na2S layer at the rear contact provides more Na to diffuse into the absorber, thus improving the crystallinity of the film and the quality of the p–n heterojunction, and the carrier collection in the neutral region and at the front interface of the devices is enhanced. The formation of the MoO3 layer can also inhibit the decomposition reaction between Mo and CZTSSe, which minimizes the secondary phases and voids, and improves the quality of the back interface. Moreover, the band alignment and interfacial barrier at the back contact were modified by the MoO3 layer, resulting in better carrier collection and the reduction of photogenerated carrier recombination. By optimizing the Na2S (MoO3) layer, the average PCE of the CZTSSe solar cell increased to 12.48% from 10.22% with the large VOC increments of 60 mV and FF increments of 5%, and the best performance was achieved, with an active area efficiency of 12.74% without the anti-reflection coating layer. [ABSTRACT FROM AUTHOR]

Filter‐free narrowband photomultiplication‐type planar heterojunction (PHJ) organic photodetectors (PM‐PHOPDs) are first realized by employing a thick front donor layer and an ultrathin PC71BM layer. The thick front donor layer is employed as an optical field adjusting (OFA) layer. The sequentially coated PC71BM will diffuse slightly into OFA layer, which works as interfacial electron traps to capture photogenerated electrons for assisting hole tunneling injection. The P3HT/PC71BM‐based PM‐PHOPDs exhibit narrowband response with full‐width of half‐maximum of 32 nm and external quantum efficiency (EQE) of 1700% at 650 nm under −20 V bias. Due to the enhanced hole transport and reduced charge recombination in PHJ compared to those in bulk heterojunction (BHJ), the EQE of P3HT/PC71BM‐based narrowband PM‐PHOPDs is twice as P3HT:PC71BM BHJ‐based narrowband PM‐OPDs under the same bias. The response peak of PM‐PHOPDs is adjusted from 650 to 695 or 745 nm by incorporating SMPV1 or DRCN5T in OFA layers due to the red‐shifted absorption edge. The EQEs of 3600% at 695 nm and 870% at 745 nm are obtained for P3HT:SMPV1 and P3HT:DRCN5T‐based PM‐PHOPDs under −20 V bias, respectively. This work provides a smart strategy to achieve narrowband PM‐OPDs by designing different OFA layers. [ABSTRACT FROM AUTHOR]

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has emerged as a promising photovoltaic material not only because of its environmentally benign and earth‐abundant constituents, but also its outstanding photoelectronic properties. Unfortunately, the significant open‐circuit voltage (Voc) loss and inferior fill factor (FF) resulting from abundant nonradiative carrier recombination at depletion region has become a major obstacle for further improving device performance. Here, an effective strategy to passivate the deep trap and band‐tail states in the heterojunction is proposed, by modifying the CZTSSe absorber layer with GeSe2 post‐deposition treatment. The results reveal that the Ge4+ can migrate into the front surface of the absorber, which plays an active role in suppressing the CuSn deep defects and [2CuZn+SnZn] defect clusters, accordingly dramatically reducing severe interfacial nonradiative carrier recombination of CZTSSe photovoltaic device. Under optimal treatment conditions, the CZTSSe solar cell efficiency increases from 10.36% to 12.22%, mainly benefitting from the increasement of Voc and FF. [ABSTRACT FROM AUTHOR]

Abundant intrinsic defects and defect clusters in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells lead to severe nonradiative recombination and limited photoelectric performance. Therefore, developing effective method to suppress the detrimental defects is the key to achieve high‐efficiency solar cell. Herein, a convenient two‐step cooling strategy in selenization process is reported to suppress the CuZn and SnZn defects and defect clusters synergistically. The results show that rapid cooling during section from selenization temperature to turning temperature can inhibit the volatilization of Sn and restrain the corresponding Sn‐related defects, while slow cooling during the subsequent temperature section can reduce the degree of Cu‐Zn disorder. Benefitting from the synergistic effect of two‐step cooling, a significantly lowered concentration of SnZn and CuZn defect and their defect clusters [2CuZn+SnZn] in absorber is observed, meanwhile, a reduced band tailing effect and promoted carrier collection efficiency of the photovoltaic device is obtained. Finally, a device with improved open‐circuit voltage (Voc) of 505.5 mV and efficiency of 12.87% is achieved. This study demonstrates the impact of cooling process on defects controlling for the first time and provides a simple and effective new strategy for intrinsic defect control, which may be universal in other inorganic thin film solar cells. [ABSTRACT FROM AUTHOR]

The photovoltaic performance of the environmentally friendly Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is lower than its predecessor Cu(In,Ga)Se2 solar cells. Severe carrier recombination at the CZTSSe/CdS interface is one major reason that results in a large open‐circuit voltage loss. Doping zinc into CdS is a feasible strategy to modifying the CdS buffer layer film, but the present methods are not satisfactory. In this study, novel zinc incorporation strategy is developed to deposit a gradient composition ternary ZnxCd1–xS buffer layer for optimizing the heterojunction interface. The application of gradient composition ZnxCd1–xS buffer layer constructs a gradient conduction band energy configuration in the CZTSSe/buffer layer interface, which highly reduces the interface recombination. The suppressed interface recombination contributes to the enhanced open circuit voltage and device performance. Consequently, the CZTSSe solar cell based on gradient composition ZnxCd1–xS buffer layers achieves champion efficiency of 12.35% with VOC of 504.81 mV, JSC of 36.90 mA cm−2, and FF of 66.28%. It is worth noting that flammable and the toxic hydrazine solvent are replaced by the safe and low‐toxic 2‐methoxyethanol, making it more promising for the future commercialization of CZTSSe solar cells. [ABSTRACT FROM AUTHOR]