Kesterite Solar Cells Research Articles

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.

Controllable Double Gradient Bandgap Strategy Enables High Efficiency Solution‐Processed Kesterite Solar Cells

AbstractThe double gradient bandgap absorber has the potential to enhance carrier collection, improve light collection efficiency, and make the performance of solar cells more competitive. However, achieving the double gradient bandgap structure is challenging due to the comparable diffusion rates of cations during high‐temperature selenization in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) films. Here, it has successfully achieved a double gradient bandgap in the CZTSSe absorber by spin‐coating the K2S solution during the preparation process of the precursor film. The K2S insertion serves as an additional S source for the absorber, and the high‐affinity energy of K‐Se causes the position of the spin‐coated K2S solution locally Se‐rich and S‐poor. More importantly, the position of the bandgap minimum (notch) and the depth of the notch can be controlled by varying the concentration of K2S solution and its deposition stage, thereby avoiding the electronic potential barrier produced by an inadvertent notch position and depth. In addition, the K─Se liquid phase expedites the selenization process to the elimination of the fine grain layer. The champion CZTSSe device achieved an efficiency of 13.70%, indicating the potential of double gradient bandgap engineering for the future development of high‐efficiency kesterite solar cells.

High efficiency titanium nitride bowtie nanoparticle and upconverter layer incorporated kesterite tandem solar cell

Thin-film photovoltaic (PV) technology offers an economically viable and flexible way to harness solar energy. Using a material that absorbs more solar light than silicon significantly reduces material consumption and costs. Solar cells based on the kesterite mineral structure and its alloys stand out from other thin-film solar cells due to their earth-abundance and non-toxicity. Incorporating plasmonic nanoparticles in the kesterite absorber layer opens a promising path of enhancing light trapping efficiency. Again, the main losses in solar cells result from the incomplete utilization of the solar spectrum. The otherwise unused sub-bandgap photons can be utilized by adding an upconverting layer on a solar cell. This paper demonstrates a theoretical design and performance analysis of a high-efficiency kesterite solar cell by incorporating titanium nitride (TiN) plasmonic bowtie nanoparticles (NPs) and an upconverter layer. We compared the optical characteristics of various shapes and sizes of nanoparticles to find the optimum solar cell efficiency. The 100 nm thick bowtie-shaped NP had the highest power conversion efficiency, η of 24.80% and had the open circuit voltage Voc=0.59V, and short circuit current density, Jsc = 45.43 mA/cm2. The absorbed power for bowtie-shaped TiN NP incorporation enhanced the average absorption from ∼61% to ∼88% for the kesterite layer. For 100 nm thickness, the absorption enhancement, g, was greater than 1 for the whole spectral range, and the maximum value was ∼1.2. Incorporating a yttrium aluminum garnet upconverter layer increased the solar cell efficiency to ∼27.44.%.

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.

A Comprehensive Review on the Recent Strategy of Cation Substitution in CZTS(Se) Thin Films to Achieve Highly Efficient Kesterite Solar Cells

Kesterite Cu2ZnSnS4 (CZTS) and selenized CZTS (CZTSSe) are the most potential replacements of Cu(InxGa1−x)Se2 as absorber layers in thin‐film solar cells, aligning with modern green energy policies. In spite of a reported power conversion efficiency (η) of nearly 14.9%, their efficiency still lags much behind their predecessors like cadmium telluride and CIGS. Major obstacles hindering the performance of CZTS‐based thin‐film solar cells pertain to the formation of CuZn and ZnCu antisite defects, along with high density of 2CuZn + SnZn donor defects, which adversely affects open‐circuit voltage and η. To combat these challenges, researchers have explored cation substitution by incorporating alternative isovalent atoms, such as substituting Ag/Li for Cu, Mn/Cd for Zn, and Ge/Ti for Sn, as a potent strategy to inhibit severe potential fluctuations and undesirable tail states caused by Cu–Zn disorder and Sn‐related defects. Recently single‐cation substitution in CZTS (or CZTSSe) has garnered considerable attention, while dual‐cation incorporation has emerged as an intriguing avenue to enhance device performance, although in its nascent stage of research and development. This review discusses recent progress on cation substitution to suppress antisite defects in CZTS (or CZTSSe), leading to high‐efficiency thin‐film solar cells.

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.

Designing next-generation kesterite solar cells: A systematic numerical investigation of innovative structures and design variants for enhanced photovoltaic performance

This paper presents a thorough numerical investigation of copper zinc tin sulfide (CZTS)-based solar cells utilizing the SCAPS-1D simulation tool. In light of the scarcity of indium and gallium, CZTS is explored as a promising alternative absorber material. The study encompasses an extensive array of 280 device simulations, including a systematic optimization process targeting enhanced power conversion efficiency (PCE). Notably, this iterative optimization procedure results in a notable PCE increase from 11.01% to 14.24%. By delving into the realm of electron transport layer (ETL) materials, the integration of a CdO layer culminates in a significant PCE milestone of 15.71%, and a new structure was extrapolated from this step Back contact/CZTS/CdO/ZnO. Furthermore, an exploration involving 172 diverse configurations incorporating various hole transport layer (HTL) materials unveils a remarkable peak PCE of 21.14% (Back contact/SnS/CZTS/CdSe/ZnO). These findings provide valuable insights and directives for the advancement and optimization of CZTS-based solar cells.

Sulfur-graded kesterite structured film drives improvement of VOC.

Realizing the graded bandgap in absorber layer is very essential for high efficient thin film solar cells. However, such bandgap modification in kesterite-structured Cu2ZnSnSe4 is normally realized via high temperature sulfurization process (above 500°C), which is not only difficult to control the sulfurization depth, but also introduces additional deep defects because of the decomposition of absorber layer at such high temperature. In this study, a low-temperature sulfurization process (150°C) is developed. Such process not only inhibits the decomposition of Cu2ZnSnSe4 films and controls the elemental distribution very well, but also increase the surface bandgap of the absorber layer and form a gradient energy bandgap. Also, the density of deep-level defects in the Cu2ZnSnSe4 layer is reduced. As a consequence, the open circuit voltage of the solar cell is improved by 60 mV. This study paves the way towards the high efficient kesterite solar cell and other solar cells.

Performance assessment of earth-abundant Kesterite and lead-free chalcogenide-based perovskite solar cells using SCAPS-1D

One of the best-studied ways to produce renewable energy is efficiently converting photons into electrical energy using environmentally acceptable materials. In this theoretical investigation, BaZrS3, an inorganic, non-toxic chalcogenide perovskite, is used as the active layer in heterostructure perovskite photovoltaics, together with TiO2 as ETM and Kesterites as HTM. The SCAPS-1D simulation is adopted to conduct a thorough study and establish the highest PCE of the photovoltaic device. The performance was modified by carefully altering the bandgap, thickness, working temperature, series resistance, shunt resistance, dielectric permittivity, and mobility for the mentioned device. The reported study proved that BaZrS3-based devices are highly stable with temperature changes. Moreover, the observed CBO of 0.1 eV suppresses recombination at the absorber/ETM interface. An analysis of choosing the suitable dielectric constant for the absorber is also performed. Based on absorber energy gap optimization (1.6 eV), the PCEs for CZTSSe, CZTSe, and CZTS devices were 20.68%, 17.31%, and 21.24%, respectively. The present study shows that these materials could be employed as likely replacements for efficient active materials in solar utility with further exploration.

Controlling Selenization Equilibrium Enables High-Quality Kesterite Absorbers for Efficient Solar Cells

Kesterite Cu2ZnSn(S, Se)4 is considered one of the most competitive photovoltaic materials due to its earth-abundant and nontoxic constituent elements, environmental friendliness, and high stability. However, the preparation of high-quality Kesterite absorbers for photovoltaics is still challenging for the uncontrollability and complexity of selenization reactions between metal element precursors and selenium. In this study, we propose a solid-liquid/solid-gas (solid precursor and liquid/vapor Se) synergistic reaction strategy to precisely control the selenization process. By pre-depositing excess liquid selenium, we provide the high chemical potential of selenium to facilitate the direct and rapid formation of the Kesterite phase. The further optimization of selenium condensation and subsequent volatilization enables the efficient removal of organic compounds and thus improves charge transport in the absorber film. As a result, we achieve high-performance Kesterite solar cells with total-area efficiency of 13.6% (certified at 13.44%) and 1.09 cm2-area efficiency of 12.0% (certified at 12.1%).

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.

High Efficiency Kesterite Solar Cells Through a Dual Treatment Approach: Improving the Quality of both Absorber Bulk and Heterojunction Interface

AbstractCu2ZnSn(S,Se)4 (CZTSSe) solar cells, composed of earth‐abundant materials, face challenges in achieving high power conversion efficiency (PCE) due to non‐radiative recombination. The limitations primarily stem from the prevalence of CuZn‐related and SnZn‐related defects in the bulk of the absorber and at the heterojunction interface region. To overcome these challenges, a dual treatment approach is proposed that involves Ag‐alloying in the bulk and Al2O3 atomic layer deposition (ALD) process at the p–n interface. Comprehensive characterizations of the fabricated devices reveal that Ag‐alloying leads to a substantial reduction in deep defects within the bulk, while Al2O3 ALD process facilitates the formation of a well‐defined p–n interface region, along with the defect passivation. This synergetic effect enables PCE enhancement accompanied by the improvements of all device parameters. Notably, devices subjected to the additional Al2O3 ALD process exhibit a substantial improvement in fill factor, which is found to be consistent with a reduced element intermixing width at the p–n heterojunction region. Due to improvements in both the bulk absorber and the heterojunction, the highest PCE of 13.33% is achieved without an anti‐reflection coating.

Impact on generation and recombination rate in Cu2ZnSnS4 (CZTS) solar cell for Ag2S and In2Se3 buffer layers with CuSbS2 back surface field layer

AbstractFor photovoltaic (PV) applications, the earth‐abundant and non‐hazardous Kesterite Cu2ZnSnS4 (CZTS) is a possible substitute for chalcopyrite copper indium gallium selenide (CIGS). This research offers insight into the most innovative method for improving the performance of Kesterite solar cells (SCs) by using CuSbS2 back surface field (BSF) and Ag2S and In2Se3 as buffer layers, focuses on aligning energy bands, reducing non‐radiative recombination, and improving open‐circuit voltage (Voc). The proposed cells are Ni/CuSbS2/CZTS/In2Se3/ITO/Al and Ni/CuSbS2/CZTS/Ag2S/ITO/Al by adding interfaces. The optimized CZTS SCs with In2Se3 achieve a short‐circuit current density (Jsc) of 30.274 mA/cm2, fill factor (FF) of 89.15%, power conversion efficiency (PCE) of 31.67%, and Voc of 1.173 V. With the Ag2S buffer layer, PCE is 31.02%, FF is 88.61%, Jsc is 30.245 mA/cm2, and Voc is 1.157 V. These results depict the potential of CZTS‐based SCs with improved performance compared with conventional structures.

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.

Exploring the Potential of Pure Germanium Kesterite for a 2T Kesterite/Silicon Tandem Solar Cell: A Simulation Study.

Recently, the development of tandem devices has become one of the main strategies for further improving the efficiency of photovoltaic modules. In this regard, combining well-established Si technology with thin film technology is one of the most promising approaches. However, this imposes several limitations on such thin film technology, such as low prices, the absence of scarce or toxic elements, the possibility to tune optical properties and long lifetime stability. Therefore, to show the potential of kesterite/silicon tandems, in this work, a 2 terminal (2T) structure using pure germanium kesterite was simulated with combined SCAPS and transfer matrix methods. To explore the impact of individual modifications, a stepwise approach was adopted to improve the kesterite. For the bottom sub cell, a state-of-the-art silicon PERC cell was used with an efficiency of 24%. As a final result, 19.56% efficiency was obtained for the standalone top kesterite solar cell and 28.6% for the tandem device, exceeding standalone silicon efficiency by 4.6% and justifying a new method for improvement. The improvement observed could be attributed primarily to the enhanced effective lifetime, optimized base doping, and mitigated recombination at both the back and top layers of the CZGSSe absorber. Finally, colorimetric analysis showed that color purity for such tandem structure was low, and hues were limited to the predominant colors, which were reddish, yellowish, and purple in an anti-reflective coating (ARC) thickness range of 20-300 nm. The sensitivity of color variation for the whole ARC thickness range to electrical parameters was minimal: efficiency was obtained ranging from 28.05% to 28.63%.

Hyperactive Selenium Source Yields Kesterite Solar Cells with 12.86% Efficiency

AbstractOne of the main issues that limits the efficiency of kesterite solar cells is the low diffusion and chemical activity of selenium clusters. Here, this work proposes a simple and effective pre‐selenization strategy using Na2(Se2S) solution, which enables the direct introduction of hyperactive Se2, Se3, and Se4 into the precursors. The results demonstrate that Se2, Se3, and Se4 promote the formation of the Cu2‐xSe liquid phase and enhance the diffusion of elements from different micro‐regions. Consequently, the ratios of Cu/(Zn + Sn) and Sn/Zn in different micro‐regions of the absorber are controlled within the optimal range, exhibiting reduced fluctuations. The controlled environment suppresses the formation of CuZn and SnZn defects, as well as [2CuZn + SnZn] defect clusters. Finally, a power conversion efficiency (PCE) of 12.86% is achieved, which is the highest PCE for kesterite solar cells made under ambient pressure and with N,N‐dimethylformamide solvent.

EFFECT OF SERIES AND SHUNT RESISTANCE ON THE PERFORMANCE OF KESTERITE SOLAR CELLS

Kesterite devices are created from cheap, readily available, non-toxic basic components. Due to their p-type conductivity, direct band-gap shifts, comparatively high absorption coefficient, and adjustable band-gap, kesterite semiconductors have attracted a lot of attention from scientists for use in solar cells.The efficiency and overall performance of kesterite solar cells are influenced by various factors, including the incorporation of series and shunt resistances. In the present communication, we report the effect of series and shunt resistance on the photovoltaic performance of CZTSe kesterite solar cells. The relationship between series resistance and device parameters, such as fill factor, the open-circuit voltage, the short-circuit current density and the efficiency, is explored, providing valuable insights into device optimization. The efficiency was found to decrease with increase in the series resistance and vice-versa.

12.3% Efficient Low Voc Loss Pure Sulfide Kesterite Solar Cells from DMSO Solution via Cadmium Alloying

AbstractCd alloying has been theoretically proved to be an effective strategy to suppress Cu‐Zn antisite defects and related defect cluster for improving device performance of pure sulfide kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. However, the potential of Cd alloying has not been fully realized by solely doping without further post heat‐treatment. Here, Cd alloying CZTS (Cu2(Zn,Cd)SnS4, CZCTS) is reported through dimethyl sulfoxide (DMSO) solution and how alloying concentration affects reaction path, grain growth, and electronic properties of the CZCTS absorbers is investigated. This study found that Cd can be incorporated into CZTS through direct phase transformation grain growth, which sufficiently suppresses band tailing. High quality CZCTS absorber films and efficient solar cells are fabricated within a wide range of alloy concentration. A champion CZCTS device with a power conversion efficiency of 12.3% is achieved at 35% Cd concentration without any post heat treatment, improved by over 70% compared to 7.0% of CZTS. This device exhibits a high VOC gain to the Shockley–Queisser (Voc/VocSQ = 59.7%), the lowest VOC deficit achieved in pure sulfide kesterite solar cells. The results demonstrate the importance of the Cd alloying strategy for mitigating band tailing and achieving high efficiency pure sulfide kesterite solar cells.

Modeling of highly efficient CNGS based kesterite solar cell: A DFT study along with SCAPS-1D analysis

The electronic and optical properties of Cu2NiGeS4 (CNGS) are examined using the first-principle DFT calculations. A unique mBJ + U potential method is used for the band gap energy calculation of CNGS. With a remarkably high absorption coefficient (104 cm−1), CNGS has become a promising candidate for photovoltaic applications. SCAPS-1D tool is used to simulate a thin-film solar cell with a Mo/MoS2/Cu2NiGeS4 (CNGS)/CdS/ZnO/ZnO:Al structure. The impact of various factors, such as layer thickness, donor and acceptor concentrations, and defect density on the CNGS layer was explored. This study also explores the combination of suitable buffer layers (such as CdS, ZnS, and their alloy Cd1−xZnxS), along with different doping concentrations and thicknesses, to be used as suitable buffer layers in the CNGS solar cell. The simulation outcomes suggest that the optimal thickness for the absorption layer in CNGS solar cells is between 2000 and 2400 nm, while the ideal thickness for MoS2 is 100 nm. The buffer layer should be between 20 and 50 nm. Keeping the defect density of CNGS below 1014 cm−3 is crucial for high efficiency. The optimized results yield an efficiency conversion rate of 20.05%, a 66.77% fill factor, a short-circuit current of 29.67 mA/cm2 and an open-circuit voltage of 0.983 V.

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.