CZTSSe Solar Cells Research Articles

2D Ti3C2-MXene Serving as Intermediate Layer between Absorber and Back Contact for Efficient CZTSSe Solar Cells.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has been considered as the most promising absorber material for inorganic thin-film solar cells. Among the three main interfaces in CZTSSe-based solar cells, the CZTSSe/Mo back interface plays an essential role in hole extraction as well as device performance. During the selenization process, the reaction between CZTSSe and Mo is one of the main reasons that lead to a large open circuit voltage (VOC) deficit, low short circuit current (Jsc), and fill factor. In this study, 2D Ti3C2-MXene was introduced as an intermediate layer to optimize the interface between the CZTSSe absorber layer and Mo back contact. Benefiting from the 2D Ti3C2-MXene intermediate layer, the reaction between CZTSSe and Mo was effectually suppressed, thus, significantly reducing the thickness of the detrimental Mo(S,Se)2 layer as well as interface recombination at the CZTSSe/Mo back interface. As a result, the power conversion efficiency of the champion device fabricated with the 2D Ti3C2-MXene intermediate layer was improved from 10.89 to 13.14% (active-area efficiency). This study demonstrates the potential use of the 2D Ti3C2-MXene intermediate layer for efficient CZTSSe solar cells and promotes a deeper understanding of the back interface in CZTSSe solar cells.

Mechanism for Improving Kesterite Solar Cells Performance via Filed Passivation Effect Induced by V‐Doped MoSe2 Interface Layer at Back Interface

One of the key issues impeding the enhancement of power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the severe carrier recombination at CZTSSe/MoSe2 back interface, primarily arising from the reverse electric field formed between CZTSSe and n‐type MoSe2 produced after selenization. To inhibit recombination at back interface, herein, the MoSe2 layer is converted from n‐type to p‐type by V doping in site through reaction of V‐alloyed Mo (Mo:V) back electrode with Se during selenization, and CZTSSe solar cells with p+‐type V‐doped MoSe2 (MoSe2:V) interface layer are fabricated. It is found that the PCE of the device rises from 8.34% to 9.63% as back contact changes from soda lime glass (SLG)/Mo/n‐MoSe2 to SLG/Mo:V/p+‐MoSe2:V. The quantitative analysis demonstrates that the increased PCE predominantly originated from the decreased reverse saturated current density (J0), followed by the decreased series resistance (RS), and lastly by the increased photogenerated current density (JL). The influence mechanism of the SLG/Mo:V/MoSe2:V back contact on device performance is suggested by studying the properties of Mo:V and MoSe2:V films and CZTSSe/MoSe2:V heterojunction. This work emphasizes the vital significance of the back surface passivation field induced by p+‐MoSe2:V/p‐CZTSSe heterojunction, which is enlightening for optimizing the back contact in kesterite photovoltaics.

Investigating Ag2S quantum dot buffer layer in CZTSSe photovoltaics for enhanced performance

This study focuses on optimizing the performance of CZTSSe (Copper Zinc Tin Sulphide/Selenide) photovoltaic (PV) cells by incorporating an Ag2S quantum dot (QD) buffer layer. CZTSSe, with its significant Direct bandgap (1–1.5 eV) and the absorption coefficient (>104 cm−1), shows promise for efficient visible-range light absorption. Ag2S quantum dots, known in terms of their favourable attributes, such as high absorption, low solubility, and minimal surface recombination, are explored as a buffer layer material. The effects of the Ag2S quantum dot (QD) buffer layer on the optical and electrical characteristics of CZTSSe photovoltaic (PV) cells are comprehensively examined using simulation characterization. Key parameters, including Short-circuit (JSC), fill factor (FF), open-circuit voltage (VOC), and power conversion efficiency (PCE), are analyzed to validate device characteristics. The SCAPS-1d simulator is employed for performance assessment and enhancement through tuning device parameters such as energy bandgap, absorber layer thickness, buffer layer thickness, defect density, and acceptor concentrations of the absorber and hole transport layer (HTL), and donor concentrations of the buffer. Additionally, temperatures, as well as series-shunt resistance’s influence on device effectiveness, are explored. The study aims to maximize light absorption, enhance charge conduction, reduce carrier loss due to recombination, and upgrade CZTSSe PV cells’ overall performance. The CZTSSe solar unit achieves its highest PCE of 27.56% when employing an Ag2S buffer layer and Cu2O hole transport layer. The study provides valuable knowledge about the optimization of CZTSSe solar cells and the potential benefits of utilizing Ag2S QD in the role of buffer layer material. This research contributes to the understanding of enhancing CZTSSe PV cell performance through the incorporation of Ag2S QD buffer layers and presents pragmatic directives that can be employed to facilitate the progression of CZTSSe-based photovoltaic devices.

Improvement of Performance of Cu2ZnSn(S, Se)4 Solar Cells by Low‐Temperature Annealing of B‐Doped CdS

It is known that high carrier recombination at the p–n junction interface and low carrier separation ability (CSA) are two main problems resulting in low power conversion efficiency (PCE) of the Cu2ZnSn(S,Se)4 (CZTSSe) solar cell. To resolve these problems, one CZTSSe solar cell is prepared through substituting B‐doped CdS for CdS in a solar cell with the conventional structure of Ag/ITO/ZnO/CdS/CZTSSe/Mo/SLG and annealing B‐doped CdS/CZTSSe/Mo/SLG prior to deposition of ZnO, ITO, and Ag electrode. By optimizing the B doping content in the CdS and annealing temperature and time of the B‐doped CdS/CZTSSe, lattice mismatch between CdS and CZTSSe is decreased and width of depletion region is increased, leading to reduction in interfacial recombination, enhancement in CSA and intensity of incident light passing through the B‐doped CdS, and so increase in PCE from 7.89% of CZTSSe solar cell using CdS as buffer layer to 10.62%. A mechanism of the increment of the PCE induced by B doping and annealing is suggested. This work proposes a method of increasing PCE of CZTSSe solar cell and advances a deeper understanding of the mechanisms behind various parameters in CZTSSe solar cells through theoretical analysis and calculations.

In-doping collaboratively controlling back interface and bulk defects to achieve efficient flexible CZTSSe solar cells

Focusing on the low open circuit voltage (VOC) and fill factor (FF) in flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, indium (In) ions are introduced into the CZTSSe absorbers near Mo foils to modify the back interface and passivate deep level defects in CZTSSe bulk concurrently for improving the performance of flexible device. The results show that In doping effectively inhibits the formation of secondary phase (Cu(S,Se)2) and VSn defects. Further studies demonstrate that the barrier height at the back interface is decreased and the deep level defects (CuSn defects) in CZTSSe bulk are passivated. Moreover, the carrier concentration is increased and the VOC deficit (VOC,def) is decreased significantly due to In doping. Finally, the flexible CZTSSe solar cell with 10.01% power conversion efficiency (PCE) has been obtained. The synergistic strategy of interface modification and bulk defects passivation through In incorporation provides a new thought for the fabrication of efficient flexible kesterite-based solar cells.

Enhanced Carrier Collection in Cd/In-Based Dual Buffers in Kesterite Thin-Film Solar Cells from Nanoparticle Inks.

Increasing the power conversion efficiency (PCE) of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has remained challenging over the past decade, in part due to open-circuit voltage (VOC)-limiting defect states at the absorber/buffer interface. Previously, we found that substituting the conventional CdS buffer layer with In2S3 in CZTSSe devices fabricated from nanoparticle inks produced an increase in the apparent doping density of the CZTSSe film and a higher built-in voltage arising from a more favorable energy-band alignment at the absorber/buffer interface. However, any associated gain in VOC was negated by the introduction of photoactive defects at the interface. This present study incorporates a hybrid Cd/In dual buffer in CZTSSe devices that demonstrate an average relative increase of 11.5% in PCE compared to CZTSSe devices with a standard CdS buffer. Current density-voltage analysis using a double-diode model revealed the presence of (i) a large recombination current in the quasi-neutral region (QNR) of the CZTSSe absorber in the standard CdS-based device, (ii) a large recombination current in the space-charge region (SCR) of the hybrid buffer CZTSSe-In2S3-CdS device, and (iii) reduced recombination currents in both the QNR and SCR of the CZTSSe-CdS-In2S3 device. This accounts for a notable 9.0% average increase in the short-circuit current density (JSC) observed in CZTSSe-CdS-In2S3 in comparison to the CdS-only CZTSSe solar cells. Energy-dispersive X-ray, secondary-ion mass spectroscopy, and grazing-incidence X-ray diffraction compositional analysis of the CZTSSe layer in the three types of kesterite solar cells suggest that there is diffusion of elemental In and Cd into the absorbers with a hybrid buffer. Enhanced Cd diffusion concomitant with a double postdeposition heat treatment of the hybrid buffer layers in the CZTSSe-CdS-In2S3 device increases carrier collection and extraction and boosts JSC. This is evidenced by electron-beam-induced current measurements, where higher current generation and collection near to the p-n junction is observed, accounting for the increase in JSC in this device. It is expected that optimization of the heat treatment of the hybrid buffer layers will lead to further improvements in the device performance.

Tailoring band alignment of Cu2ZnSn(S,Se)4/CdS interface by Al2O3 drives solar cell efficiency

The p/n heterojunction with low surface recombination losses and good band alignment is one of the key factors for obtaining efficient photovoltaic (PV) devices. Here, ultrathin Al2O3 is introduced, and the effects of its thickness and growth temperature on the PV performance of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells are investigated. The result indicates that the increased depletion width and built-in voltage after appropriate Al2O3 treatment promote the separation and transport of photogenerated carriers. The absorber surface composition and defect are optimized, and the interface band alignment is modified by reducing the conduction band offset successfully, which drives the carrier collection and increases open-circuit voltage by 60 mV. The dual passivation mechanism of chemical passivation and field-effect passivation is realized by introducing Al2O3 into kesterite solar cells. This work not only reveals the intrinsic mechanism for performance enhancement CZTSSe solar cells with Al2O3, but also provides a significant guidance for the band adjustment at the interface.

Passivating Grain Boundaries via Graphene Additive for Efficient Kesterite Solar Cells.

Grain boundaries (GBs)-triggered severe non-radiative recombination is recently recognized as the main culprits for carrier loss in polycrystalline kesterite photovoltaic devices. Accordingly, further optimization of kesterite-based thin film solar cells critically depends on passivating the grain interfaces of polycrystalline Cu2 ZnSn(S,Se)4 (CZTSSe) thin films. Herein, 2D material of graphene is first chosen as a passivator to improve the detrimental GBs. By adding graphene dispersion to the CZTSSe precursor solution, single-layer graphene is successfully introduced into the GBs of CZTSSe absorber. Due to the high carrier mobility and electrical conductivity of graphene, GBs in the CZTSSe films are transforming into electrically benign and do not act as high recombination sites for carrier. Consequently, benefitting from the significant passivation effect of GBs, the use of 0.05wt% graphene additives increases the efficiency of CZTSSe solar cells from 10.40% to 12.90%, one of the highest for this type of cells. These results demonstrate a new route to further increase kesterite-based solar cell efficiency by additive engineering.

Energy Band Alignment at p–n Heterojunction Interface in CZTSSe Solar Cells with Novel ZnMgO Buffer Layer

By substituting Zn1−xMgxO (ZMO) for the conventional CdS buffer layer, the CZTSSe solar cells with the efficiency of 11.2% are obtained herein, which can be attributed to the conduction band regulation in the heterojunction. An appropriate conduction band offset (CBO) value can effectively reduce nonradiative recombination loss of charge carriers at the interface, thereby improving the open‐circuit voltage (Voc). Herein, ZMO buffer layer with the optical bandgap ranging from 3.34 to 3.90 eV is applied to enhance the CBO in the heterojunction from 0.14 to 0.72 eV. The ZMO‐buffered solar cells exhibit higher Voc and short‐current density (Jsc) compared to CdS‐buffered cells. The optimal efficiency of 11.2% is obtained in CZTSSe solar cell with the CBO of 0.27 eV, Jsc of 39.0 mA cm−2, fill factor of 69.6%, and Voc of 412 mV.

Heterojunction reconstruction via In doping towards high-efficiency CZTSSe solar cells

The Cu2ZnSn(S,Se)4 solar cells (CZTSSe) are emerging as a hot area of low-cost thin film photovoltaic technology owing to its non-toxicity and excellent photoelectric properties. However, the large open-circuit voltage deficit (Voc,deficit) caused by the inherently defective CdS/ CZTSSe heterojunction interface is still a huge challenge for improving power conversion efficiency. Herein, we reconstruct the heterojunction interface by inserting a CZTSSe:In into the CdS/CZTSSe heterojunction through an In2Se3 post-deposition treatment. The local occupation of In3+ at Zn2+ and Sn4+ sites decreases the disordered Zn- and Sn-related deep defects and generates benign InZn donors within CZITSSe transition layer simultaneously, which effectively enhances the surface p-to-n conversion. The combination of chemical and field-effect passivation regulates the charge extraction and passivates the charge recombination. As a result, the In element-buried absorber enables the Voc boost from 488 to 527 mV with an impressive active-area efficiency of 13.59 %. These results demonstrate the great potential of the In element-buried absorber in alleviating the Voc,deficit and enhancing the performance of CZTSSe solar cells.

An ITO-Free Kesterite Solar Cell.

Photovoltaic thin film solar cells based on kesterite Cu2 ZnSn(S, Se)4 (CZTSSe) have reached 13.8% sunlight-to-electricity conversion efficiency. However, this efficiency is still far from the Shockley-Queisser radiative limit and is hindered by the significant deficit in open circuit voltage (VOC ). The presence of high-density interface states between the absorber layer and buffer or window layer leads to the recombination of photogenerated carriers, thereby reducing effective carrier collection. To tackle this issue, a new window structure ZnO/AgNW/ZnO/AgNW (ZAZA) comprising layers of ZnO and silver nanowires (AgNWs) is proposed. This structure offers a simple and low-damage processing method, resulting in improved optoelectronic properties and junction quality. The ZAZA-based devices exhibit enhanced VOC due to the higher built-in voltage (Vbi ) and reduced interface recombination compared to the usual indium tin oxide (ITO) based structures. Additionally, improved carrier collection is demonstrated as a result of the shortened collection paths and the more uniform carrier lifetime distribution. These advances enable the fabrication of the first ITO-free CZTSSe solar cells with over 10% efficiency without an anti-reflectivecoating.

Sb Doping Strategy to Promote Growth and Suppress Defects in Solution-Processed CZTSSe Solar Cells for Improved Optoelectronic Performance

Sb Doping Strategy to Promote Growth and Suppress Defects in Solution-Processed CZTSSe Solar Cells for Improved Optoelectronic Performance

Visual Color Change During Spin-coating Guiding the Quality of Cu2 ZnSn(S,Se)4 Films.

Solution method provides a low-cost and environmentally friendly route for the fabrication of Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells. However, uncontrollable quality of the CZTSSe absorber layer will severely limit the device's performance. In this study, it is find that the thickness and the quality of the formed precursor is not stable because of the variation of the viscosity of the precursor solution. Combined by different characterization methods, the results disclose that such change is strongly related to the reflected color of the first coating layer during precursor growth. Further studies disclose that only by maintaining the appropriate reflected color can a well-crystallized CZTSSe film be prepared, thereby obtaining good solar cell efficiency. This semi-empirical pattern is confirmed by thin-film interference theory. Under the guidance of this method, CZTSSe absorbers with high quality are obtained easily, and the highly efficient CZTSSe solar cell can be fabricated easily. This study provides a feasible and effective strategy to obtain the optimal structure and composition of CZTSSe film toward the production of highly efficient kesterite solar cells, which can also be widely applied to the preparation of other films by solution-based method.

Boosting the open-circuit voltage of Cu2ZnSn(S,Se)4 solar cell by Cd/NH3 soaking

In this paper, the open-circuit voltages (V OC) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells were improved by soaking the CZTSSe films in Cd/NH3 solutions before the deposition of buffer layers. The effects of Cd/NH3 soaking on the properties of CZTSSe films and solar cells were investigated in depth. We found Cd/NH3 soaking can promote downward band bending of the CZTSSe layer at the surface region. In addition, the Cd/NH3 soaking can eliminate the deep level acceptor in CZTSSe and decrease the density of the defects at the absorber/buffer interface. These effects can significantly reduce the carrier recombination in the depletion region of CZTSSe solar cells. As a result, the V OC of CZTSSe solar cells were increased from ∼460 mV to ∼480 mV, the efficiency of the best CZTSSe solar cell was increased from 10.2% to 11.6% by the Cd/NH3 soaking process.

12.84% Efficiency Flexible Kesterite Solar Cells by Heterojunction Interface Regulation

AbstractDue to their environmental friendliness, low cost, and versatility in applications, flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells have garnered significant attention in recent years. However, the lack of alkali metal elements in the flexible substrate has posed a challenge to the advancement of flexible CZTSSe solar cells. In this study, a post‐treatment strategy is proposed involving Rb ions to simultaneously regulate the surface properties of the CZTSSe film and the reactions during the CdS chemical bath deposition (CBD) process. Material and chemical characterization results confirm that Rb ions effectively passivate the detrimental Se0 defect, leading to a more active surface for the CdS epitaxial growth. Additionally, the coordination between Rb ions and thiourea in the CBD solution improves the ion‐by‐ion deposition of the CdS layer. These beneficial effects significantly improve the heterojunction interface and consequently enable the flexible CZTSSe solar cell to achieve a record total‐area efficiency of 12.84% and excellent bending performance. Overall, the heterojunction interface regulation strategy employed in this work, along with the obtained remarkable results, offer promising prospects for the development of flexible CZTSSe solar cells.

Enhancement of power conversion efficiency of Ag-substituted Cu2ZnSn(S,Se)4 solar cells via tuning Cu2+/(Cu++Cu2+) percentage in precursor solution

Although trace silver (Ag) substitution for Cu in Cu2ZnSn(S, Se)4 (CZTSSe) offers benefits in improving open-circuit voltage (VOC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (JSC) owing to the increase of bandgap (Eg) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor JSC arising from the Ag substitution by tuning the Cu2+/(Cu++Cu2+) percentage in the (Cu, Ag)2ZnSnS4 (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu2+/(Cu++Cu2+) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu2+/(Cu++Cu2+) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (JL) and decrease in shunt resistance (Rsh). It is demonstrated that the increase in JL results from a widening in the depletion layer width (Wd) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu2+/(Cu++Cu2+) percentage. While the decrease in Rsh comes from the change of reaction pathways due to the change in Cu2+/(Cu++Cu2+) percentage.

Improving the efficiency of Cu2ZnSn(S,Se)4 solar cell by using localized K-doped precursor film

In this paper, we prepared Na–K co-doped Cu2ZnSn(S,Se)4 (CZTSSe) films by solution method. By optimizing the distribution of the K-doping (uniform or localized K-doping) in the precursor films, the efficiencies of CZTSSe solar cells were improved. The effect of the K-doping distribution on the properties of CZTSSe films and solar cells were investigated. Compared to the solar cells without K-doping, the uniform K-doping in the precursor film increased the solar cell efficiency from 10.4% to 11.7%. In comparison, the localized K-doping in the bottom and in the top of the precursor films yielded solar cells with efficiencies of 12.4% and 10.1%, respectively. It was determined that the K-doping distribution primarily had an influence on carrier recombination in the depletion regions of CZTSSe solar cells. Whereas k-doping at the top of the precursor film greatly enhances the carrier recombination in the depletion region, the bottom doping has the opposite effect. The reason could be that the variation of the K-doping distribution modified the crystallization of the CZTSSe films. As the result, the defect properties of the CZTSSe films were altered. Moreover, the effect of the bottom K-doping on the back contact properties of CZTSSe solar cells was investigated by impedance spectroscopy. Though the K-doping can reduce the back contact barrier of the solar cell, the back contact barrier still achieved the height of 228 meV. The results indicated that the back contact barrier would be an important limiting factor for the efficiency of CZTSSe solar cell.

Efficient and bending durable flexible Cu2ZnSn(S,Se)4 solar cells achieved by Ga doping stress regulation

Ga-doped CZTSSe is achieved by evaporating Ga layer in precursor combined with subsequent selenization to balance the efficiency and bending durability of flexible CZTSSe solar cells in this paper. Ga doping can simultaneously alleviate the residual stress and reinforce the microstructure of CZTSSe, retaining the original lattice structure of CZTSSe and suppressing Sn-related deep level defects. The gratifying residual stress reduction from -5.31 GPa to -3.82 GPa with decreased porosity from 6.24% to 3.24% of CZTSSe thin film spontaneously promotes the device performance after evaporating the 20 nm Ga layer. A desirable efficiency enhancement from 2.61% to 5.04% with preferable bending durability of flexible device is achieved, which strategy provides substantial assistance for the potential application of flexible solar cells.

A study on Cr incorporation to enhance performance of CZTSSe solar cells via optimizing selenization condition

Annealing Cu2ZnSnS4 (CZTS) precursor films in selenium atmosphere has been demonstrated to be an available means to enhance the quality of Cu2ZnSn(S,Se)4 (CZTSSe) absorber. Therefore, it is important to further explore the selenization conditions during annealing. Herein, Cr-doped CZTSSe absorber was synthesized using solution-processed, and the effects of different selenization conditions on the morphology, structure and photoelectrical performances of Cr-doped CZTSSe thin films were investigated firstly. Research shows that morphology of CCZTSSe films was greatly affected by different selenization conditions. By optimizing the selenization conditions, the crystallinity of CCZTSSe absorber was significantly optimized, and the compactness of the absorber was improved. It was further confirmed that the band gap of CCZTSSe absorber can be adjusted via changing the annealing conditions. The CCZTSSe under the optimum selenization conditions showed a better constituent composition, which further inhibited the harmful defects level of CuZn and [2CuZn + SnZn] in the absorber. Finally, the Cr-doped CZTSSe device at optimum selenidation condition of 520 ℃ for 15 min showed the best conversion efficiency of 5.86 %, in which VOC is 394 mV and FF is 54.07 %, respectively. A new strategy for high efficiency Cr-doped CZTSSe solar cells was proposed by optimizing selenization conditions.

Mechanism of improvement of efficiency of Cu2ZnSn(S,Se)4 solar cells by optimization of deposition temperature of CdS buffer layer

The physical properties of CdS buffer layers and the interface quality between CdS layer and Cu2ZnSn(S,Se)4 (CZTSSe) absorber have a significant impact on the performance of CZTSSe solar cells. In the present work, five types of CZTSSe solar cells with conventional structure (denoted as CdS-x-cells, x = 55, 60, 65, 70 and 75) were fabricated by using CdS prepared at deposition temperatures of 55, 60, 65, 70 and 75 ℃, respectively, as buffer layer, and influence mechanism of the deposition temperature of CdS layers on power conversion efficiency (PCE) of the CdS-x-cells was investigated. It is found that the PCE of the CdS-x-cells increases from 8.52% to 10.36% as the deposition temperature rises from 55 to 60 ℃ but falls from 10.36% to 7.14% from 60 to 75 ℃. The analysis on the photovoltaic and electrical parameters indicates that the increase in PCE is predominantly derived from the decreased reverse saturated current density (J0), followed by the decreased series resistance (RS) and increased shunt resistance (RSh). The decreased PCE with the further increasing deposition temperature is mainly contributed from the increased J0, followed by the decreased RSh and photogenerated current density (JL). The influence mechanism of deposition temperature of the CdS layer on PCE is suggested via quantitatively analysis of influence of electrical, optical properties and crystal quality of CdS prepared at various deposition temperatures on JL and electrical parameters of CdS-x-cells. This work is helpful for improvement of PCE of CZTSSe solar cells by modification of properties of buffer layer.