Band Tail Engineering in Kesterite Cu2ZnSn(S,Se)4 Thin-Film Solar Cells with 11.8% Efficiency

Element Diffusion Induced Carrier Transport Enhancement in High-Performance CZTSSe Self-Powered Photodetector

Cu2ZnSn(S,Se)4 (CZTSSe) has attracted great interest in thin-film solar cells due to its excellent photoelectric performance in past decades, and recently is gradually expanding to the field of photodetectors. Here, the CZTSSe self-powered photodetector is prepared by using traditional photovoltaic device structure. Under zero bias, it exhibits the excellent performance with a maximum responsivity of 0.77 A W-1, a high detectivity of 8.78 × 1012 Jones, and a wide linear dynamic range of 103 dB. Very fast response speed with the rise/decay times of 0.576/1.792 µs, and ultra-high switching ratio of 3.54 × 105 are obtained. Comprehensive electrical and microstructure characterizations confirm that element diffusion among ITO, CdS, and CZTSSe layers not only optimizes band alignment of CdS/CZTSSe, but also suppresses the formation of interface defects. Such a suppression of interface defects and spike-like band alignment significantly inhibit carrier nonradiative recombination at interface and promote carrier transport capability. The low trap density in CZTSSe and low back contact barrier of CZTSSe/Mo could be responsible for the very fast response time of photodetector. This work definitely provides guidance for designing a high performance self-powered photodetector with high photoresponse, high switching ratio, fast response speed, and broad linear dynamic range.

Improvement of Performance of CZTSSe Solar Cells by the Synergistic Effect of Back Contact Modification and Ag Doping

To improve the performance of Cu2ZnSn(S,Se)4 solar cells, a strategy is proposed to improve the quality of absorber and back interface simultaneously by substituting V-doped Mo (Mo:V) for a conventional Mo back electrode and incorporating Ag into the Cu2ZnSn(S,Se)4 (ACZTSSe) absorber in this work. Since p+-type V-doped MoSe2 (MoSe2:V) is formed in the site between the absorber and Mo:V during selenization, the conventional Mo/n-MoSe2 back contact is modified to Mo:V/p+-MoSe2:V, a back surface passivation field (BSPF) is established at the back interface, the band bending of MoSe2:V is downward and that of bottom of the absorber is upward. Further investigation reveals that the back contact modification and Ag doping have a synergistic effect on inhibiting carrier recombination, decreasing series resistance and increasing shunt resistance, thereby leading to the PCE of device without antireflection coating increased from 8.61 to 10.98%, which is larger than the sum of increase in PCE induced by Ag doping alone (8.61 to 9.66%) and back contact modification alone (8.61 to 9.63%). It is demonstrated that the synergistic effect stems mainly from the strengthened BSPF and the further reduced back contact barrier height. The former is due to the increased difference in work function (WF) between MoSe2:V and absorber induced by the reduced WF of the absorber after Ag doping and the raised WF of MoSe2:V after V doping. The latter is due to the downshifted valence band maximum of absorber after Ag doping. This work highlights the synergistic effect of back contact modification and Ag doping on improving the performance of CZTSSe solar cells and also provides an effective way to suppress carrier recombination.

Facile Approach for Metallic Precursor Engineering for Efficient Kesterite Thin-Film Solar Cells

Kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs) are a promising candidate for low-cost, clean energy production owing to their environmental friendliness and the earth-abundant nature of their constituents. However, the advancement of kesterite TFSCs has been impeded by abundant defects and poor microstructure, limiting their performance potential. In this study, we present efficient Ag-alloyed CZTSSe TFSCs enabled by a facile metallic precursor engineering approach. The positioning of the Ag nanolayer in the metallic stacked precursor proves crucial in expediting the formation of Cu-Sn metal alloys during the alloying process. Specifically, Ag-included metallic precursors promote the growth of larger grains and a denser microstructure in CZTSSe thin films compared to those without Ag. Moreover, the improved uniformity of Ag, facilitated by the evaporation deposition technique, significantly suppresses the formation of detrimental defects and related defect clusters. This suppression effectively reduces nonradiative recombination, resulting in enhanced performance in kesterite TFSCs. This study not only introduces a metallic precursor engineering strategy for efficient kesterite-based TFSCs but also accelerates the development of microstructure evolution from metallic stacked precursors to metal chalcogenide compounds.

Improvement of Efficiency in Kesterite Solar Cells by Intentionally Inserting a Thin MoS2 Layer into the Back Interface

A Mo(S,Se)2 interfacial layer is formed inevitably and uncontrollably between the Mo electrode and Cu2ZnSn(S,Se)4 (CZTSSe) absorber during the selenization process, which significantly influences the performance of CZTSSe solar cells. In this work, an ultrathin MoS2 layer is intentionally inserted into Mo/CZTSSe to reduce the recombination and thus optimize the interface quality. It is revealed that the absorber exhibits a continuous and compact morphology with bigger grains and remarkably without pinholes across the surface or cross-sectional regions after MoS2 modification. Benefitting from this, the shunt resistance (RSh) of the device increased evidently from ∼395 to ∼634 Ω·cm2, and simultaneously, the reverse saturation current density (J0) realized an effective depression. As a result, the power conversion efficiency (PCE) of the MoS2-modified device reaches 9.64% via the optimization of the thickness of the MoS2 layer, indicating performance improvements with respect to the MoS2-free case. Furthermore, the main contribution to the performance improvement is derived and analyzed in detail from the increased RSh, decreased J0, and diode ideality factor. Our results suggest that the Mo/CZTSSe interface quality and performance of CZTSSe solar cells can be modulated and improved by appropriately designing and optimizing the thickness of the inserted MoS2 layer.

Improving the Device Performance of CZTSSe Thin-Film Solar Cells via Indium Doping

Cation incorporation emerges as a promising approach for improving the performance of the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) device. Herein, we report indium (In) doping using the chemical bath deposition (CBD) technique to enhance the optoelectronic properties of CZTSSe thin-film solar cells (TFSCs). To incorporate a small amount of the In element into the CZTSSe absorber thin films, an ultrathin (<10 nm) layer of In2S3 is deposited on soft-annealed precursor (Zn-Sn-Cu) thin films prior to the sulfo-selenization process. The successful doping of In improved crystal growth and promoted the formation of larger grains. Furthermore, the CZTSSe TFSCs fabricated with In doping exhibited improved device performance. In particular, the In-CZTSSe-2-based device showed an improved power conversion efficiency (PCE) of 9.53%, open-circuit voltage (Voc) of 486 mV, and fill factor (FF) of 61% compared to the undoped device. Moreover, the small amount of In incorporated into the CZTSSe absorber demonstrated reduced nonradiative recombination, improved carrier separation, and enhanced carrier transport properties. This study suggests a simple and effective way to incorporate In to achieve high efficiency and low Voc loss.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu₂ ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30 nm). Interfacial engineering with LGO improved the V_oc of the CZTSSe TFSCs from 466 to 502 mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

Chalcogenide semiconductors offer excellent optoelectronic properties for their use in solar cells, exemplified by the commercialization of Cu(In,Ga)Se₂- and CdTe-based photovoltaic technologies. Recently, several other chalcogenides have emerged as promising photoabsorbers for energy harvesting through the conversion of solar energy to electricity and fuels. The goal of this review is to summarize the development of emerging binary (Sb₂X₃, GeX, SnX), ternary (Cu₂SnX₃, Cu₂GeX₃, CuSbX₂, AgBiX₂), and quaternary (Cu₂ZnSnX₄, Ag₂ZnSnX₄, Cu₂CdSnX₄, Cu₂ZnGeX₄, Cu₂BaSnX₄) chalcogenides (X denotes S/Se), focusing especially on the comparative analysis of their optoelectronic performance metrics, electronic band structure, and point defect characteristics. The performance limiting factors of these photoabsorbers are discussed, together with suggestions for further improvement. Several relatively unexplored classes of chalcogenide compounds (such as chalcogenide perovskites, bichalcogenides, etc.) are highlighted, based on promising early reports on their optoelectronic properties. Finally, pathways for practical applications of emerging chalcogenides in solar energy harvesting are discussed against the backdrop of a market dominated by Si-based solar cells.

Kesterite Solar Cells: Insights into Current Strategies and Challenges

Earth-abundant and environmentally benign kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative to its cousin chalcopyrite Cu(In,Ga)(S,Se)2 (CIGS) for photovoltaic applications. However, the power conversion efficiency of CZTSSe solar cells has been stagnant at 12.6% for years, still far lower than that of CIGS (23.35%). In this report, insights into the latest cutting-edge strategies for further advance in the performance of kesterite solar cells is provided, particularly focusing on the postdeposition thermal treatment (for bare absorber, heterojunction, and completed device), alkali doping, and bandgap grading by engineering graded cation and/or anion alloying. These strategies, which have led to the step-change improvements in the power conversion efficiency of the counterpart CIGS solar cells, are also the most promising ones to achieve further efficiency breakthroughs for kesterite solar cells. Herein, the recent advances in kesterite solar cells along these pathways are reviewed, and more importantly, a comprehensive understanding of the underlying mechanisms is provided, and promising directions for the ongoing development of kesterite solar cells are proposed.

Improving Ultraviolet Responses in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells Using Quantum Dot-Based Luminescent Down-Shifting Layer

Quantum dot (QD)-based luminescent down-shifting (LDS) layers were deposited on Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells via the drop-casting method. The LDS layers can easily widen the narrow absorption wavelength regions of single-junction solar cells and enable improvement of the short-circuit current. The optical properties of LDS layers deposited on glass and containing different QD contents were analyzed based on their transmittance, reflectance, and absorbance. The absorber films to be used in the CZTSSe solar cells were determined by X-ray diffraction measurements and Raman spectroscopy to determine their crystal structures and secondary phases, respectively. The completed CZTSSe solar cells with LDS layers showed increased ultraviolet responses of up to 25% because of wavelength conversion by the QDs. In addition, the impact of the capping layer, which was formed to protect the QDs from oxygen and moisture, on the solar cell performance was analyzed. Thus, a maximal conversion efficiency of 7.3% was achieved with the 1.0 mL QD condition; furthermore, to the best of our knowledge, this is the first time that LDS layers have been experimentally demonstrated for CZTSSe solar cells.

Influence of the Reaction Pathway on the Defect Formation in a Cu2ZnSnSe4 Thin Film

Point defect engineering in Cu₂ZnSnSe₄ (CZTSe) thin films is the main issue to improve its device performance. This study reveals the correlation between the reaction pathway and the point defects in the CZTSe film. The reaction pathway from a metallic precursor (Mo/Zn/Sn/Cu) to a kesterite CZTSe film is varied by changing the annealing process. The synthesized CZTSe films under different reaction pathways induce different device performances with different defect energy levels, although all CZTSe films have similar structural and optical properties (E_g ∼ 1.0 eV). The admittance spectroscopy demonstrates the correlations between point defect types (V_Zn, Zn_Sn, Zn_Cu, Cu_Zn, and V_Cu) and the reaction pathways for the formation of CZTSe films. The different growth rates of binary selenides, such as ZnSe and/or Sn-Se phases, during the annealing process are especially strongly related to the formation of point defects, leading to the different open-circuit voltages (396-451 mV) and fill factors (51-65%). The results of this study suggest that controlling the reaction pathway is an effective approach to adjust the formation of defects in the kesterite CZTSe film as well as to fabricate high-performance solar cell devices.

Anion exchange induced formation of kesterite copper zinc tin sulphide-copper zinc tin selenide nanoheterostructures

We report the colloidal synthesis of quaternary kesterite CZTS-CZTSe heterostructures via anion exchange reactions on a kesterite CZTS template. The crystal phase selectivity during the synthesis (kesterite vs. wurtzite) is due to the initial nucleation of cubic Cu9S5 seeds, followed by incorporation of Zn and Sn. Upon injection of Se-precursor, which triggered simultaneous anion exchange and overgrowth of the pristine CZTS template, sandwich CZTS-CZTSe (core-tip) nanoheterostructures were obtained. X-ray photoelectron spectroscopy (XPS) and optical band gap measurement results suggest a change of intrinsic electronic structure of CZTS by Se-treatment. Our study not only provides insight into mechanisms of formation of kesterite CZTS nanocrystals (NCs) and subsequent anion exchange reactions, but also opens doors to access novel CZTSSe nanostructures for potential applications.

A Facile Process for Partial Ag Substitution in Kesterite Cu2ZnSn(S,Se)4 Solar Cells Enabling a Device Efficiency of over 12

A cation substitution in Cu₂ZnSn(S,Se)₄ (CZTSSe) offers a viable strategy to reduce the open-circuit voltage (V_oc)-deficit by altering the characteristics of band-tail states, antisite defects, and related defect clusters. Herein, we report a facile single process, i.e., simply introducing a thin Ag layer on a metallic precursor, to effectively improve the device characteristics and performances in kesterite (Ag_x,Cu_1-x)₂ZnSn(S_y,Se_1-y)₄ (ACZTSSe) solar cells. Probing into the relationship between the external quantum efficiency derivative (dEQE/dλ) and device performances revealed the V_oc-deficit characteristics in the ACZTSSe solar cells as a function of Cu and Ag contents. The fabricated champion ACZTSSe solar cell device showed an efficiency of 12.07% and a record low V_oc-deficit of 561 mV. Thorough investigations into the mechanism underpinning the improved performance in the ACZTSSe device further revealed the improved band-tailing characteristic, effective minority carrier lifetime, and diode factors as well as reduced antisite defects and related defect clusters as compared to the CZTSSe device. This study demonstrates the feasibility of effectively suppressing antisite defects, related defect clusters, and band-tailing characteristics by simply introducing a thin Ag layer on a metallic precursor in the kesterite solar cells, which in turn effectively reduces the V_oc-deficit.

Density Functional Theory and Experimental Determination of Band Gaps and Lattice Parameters in Kesterite Cu2ZnSn(SxSe1-x)4

The structures and band gaps of copper-zinc-tin selenosulfides (CZTSSe) are investigated for a range of anion compositions through experimental analysis and complementary first-principles simulations. The band gap was found to be extremely sensitive to the Sn-anion bond length, with an almost linear correlation with the average Sn-anion bond length in the mixed anion phase Cu₂ZnSn(S_xSe_1-x)₄. Therefore, an accurate prediction of band gaps using first-principles methods requires the accurate reproduction of the experimental bond lengths. This is challenging for many widely used approaches that are suitable for large supercells. The HSE06 functional was found to predict the structure and band gap in good agreement with the experiment but is computationally expensive for large supercells. It was shown that a geometry optimization with the MS2 meta-GGA functional followed by a single point calculation of electronic properties using HSE06 is a reasonable compromise for modeling larger supercells that are often unavoidable in the study of point and extended defects.

Origin of Band-Tail and Deep-Donor States in Cu2ZnSnS4 Solar Cells and Their Suppression through Sn-Poor Composition

By comparing optical spectral results of both Sn-rich and Sn-poor Cu₂ZnSnS₄ (CZTS) with the previously calculated defect levels, we confirm that the band-tail states in CZTS originate from the high concentration of 2Cu_Zn + Sn_Zn defect clusters, whereas the deep-donor states originate from the high concentration of Sn_Zn. In Sn-rich CZTS, the absorption, reflectance, and photocurrent (PC) spectra show band-tail states that shrink the bandgap to only ∼1.34 eV, while photoluminescence (PL) and PC spectra consistently show that abundant Cu_Zn + Sn_Zn donor states produce a PL peak at ∼1.17 eV and abundant Sn_Zn deep-donor states produce a PL peak near 0.85 eV. In contrast, Sn-poor CZTS shows neither bandgap shrinking nor any deep-donor-defect induced PL and PC signals. These results highlight that a Sn-poor composition is critical for the reduction of band-tailing effects and deep-donor defects and thus the overcoming of the severe open-circuit voltage (V_oc) deficiency problem in CZTS solar cells.

8% Efficiency Cu2ZnSn(S,Se)4 (CZTSSe) Thin Film Solar Cells on Flexible and Lightweight Molybdenum Foil Substrates

The use of flexible and highly conducting molybdenum (Mo) foil as a substrate offers several advantages such as a high thermal stability, smooth surface, and chemical inertness for the fabrication of high-efficiency thin film solar cells (TFSCs) by lowering the manufacturing costs. Here, we report a record preliminary efficiency of ∼8% for sputtered-grown Cu₂ZnSn(S,Se)₄ (CZTSSe) TFSCs on flexible and lightweight Mo foils. Careful studies were focused on identifying the role of preparative parameters such as annealing temperature, absorber composition, and post-preparative optimization to bridge the obtained record efficiency of ∼8% to a previous record efficiency of 7.04% for Na-incorporated CZTSSe sputter-based TFSCs. Interestingly, the preliminary record efficiency of ∼8% for our CZTSSe device grown via a scalable sputtering method was achieved by optimizing the absorber quality and post-preparative device optimization. While our preliminary results with a record efficiency demonstrate the potential of sputtering method, there is much scope for further improvement in the device efficiency by thoroughly understanding alkali element doping in the absorber layer.