Voltage Deficit Research Articles

Concurrent Design of Alloy Compositions of CZTSSe and CdZnS Using SCAPS Simulation: Potential Routes to Overcoming VOC Deficit

Solar energy is the most used renewable energy source. CZTSSe uses earth-abundant elements and has promising optoelectronic properties, resulting in becoming a viable alternative to thin film PV. This work provides design guidelines for CZTSSe-based solar cells, where CZTSSe has a tunable affinity and energy gap. The analysis is based on incorporating a ternary compound material to serve as an electron transport material (ETM). In this regard, CdZnS is a potential candidate that can be utilized as an electron transport layer whose affinity and energy gap can be tuned to adjust the band alignment at the ETL/CZTSSe interface. In order to design a high-efficiency solar cell, one has to tune both the ETL and absorber layers to have a suitable conduction band offset (CBO), thereby minimizing the non-radiative recombination which, in turn, boosts the power conversion efficiency (PCE). Thus, in our presented simulation study, we provide a codesign of alloy compositions of both the CZTSSe photoactive layer and the CdZnS ETL using SCAPS-1D simulation. It is found that using the codesign of alloy compositions of the ternary compound ETL and the absorber enhances the PCE by about 2% and, more importantly, overcomes the main issue in CZTSSe which is its open-circuit voltage (VOC) deficit. Furthermore, upon optimizing the thickness and doping of both the ETL and absorber layer, as well as the bulk defect of the absorber layer, a PCE of 17.16% is attained in this study, while the calibrated PCE based on a previously published experimental work was 12.30%.

Voltage deficit in PV with suppressed recombination

The observed open circuit voltages in best performing solar cells are explained outside of the recombination paradigm, based on such factors as electrostatic screening, Meyer–Neldel effect, and lateral nonuniformities. The underlying concept of suppressed recombination presents a long neglected alternative pathway to efficient photovoltaic. The criteria of suppressed recombination and effective charge carrier extraction are consistent with the data for best performing solar cells.

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead-Free Perovskite Solar Cells.

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb-based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p-doping characteristics and possess abundant vacancy defects, which resulted in under-optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic "electron and defect compensation" strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol%) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p-type to weak p-type (i.e. up-shifting the Fermi Level by ~ 0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02%, which is ~46% higher than that of control device (9.56%). Notably, a record-high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb-based analogues (~ 0.30 V).

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead‐Free Perovskite Solar Cells

AbstractSn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb‐based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p‐doping characteristics and possess abundant vacancy defects, which result in under‐optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic “electron and defect compensation” strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p‐type to weak p‐type (i.e. up‐shifting the Fermi level by ~0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ~46 % higher than that of control device (9.56 %). Notably, a record‐high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb‐based analogues (~0.30 V).

Theoretical Insights of Degenerate ZrS2 as a New Buffer for Highly Efficient Emerging Thin‐Film Solar Cells

SnS, Sb2Se3, Cu2SnS3, CuSb(S,Se)2, and Cu2BaSn(S,Se)4 are emerging as promising light absorbers for thin‐film photovoltaics due to their extraordinary optoelectronic properties. However, improper band alignment with the buffer and large open‐circuit voltage (VOC) deficit limits their power conversion efficiencies (PCE). Therefore, finding a suitable buffer that overcomes these obstacles is crucial. Herein, ZrS2 as an alternative buffer for the aforementioned emerging thin‐film solar cells using SCAPS‐1D is proposed. The important ZrS2 parameters are optimized, including bandgap, thickness, carrier concentration, and defect density. Interestingly, ZrS2 behaves as a degenerate semiconductor at carrier concentrations >1E17 cm−3, improving the conductivity of the solar cells; it also demonstrates a high defect tolerance nature when the defect density lies between 1E12 and 1E18 cm−3. After ZrS2 parameters optimization, the built‐in potential of SnS, Sb2Se3, Cu2SnS3, CuSb(S,Se)2, and Cu2BaSn(S,Se)4 solar cells is enhanced by 0.2, 0.58, 0.05, 0.42, and 0.3 V, respectively, reducing recombination rate. Upon optimizing absorbers parameters, a PCE > 35% for SnS, Sb2Se3, and CuSb(S,Se)2 while >32% for Cu2SnS3 and Cu2BaSn(S,Se)4 solar cells is accomplished with low VOC loss (≈0.1 V). The absorbers must have high carrier concentration (1E20 cm−3) and low defect density (1E14 cm−3) to achieve these PCEs.

Interface Modification for Efficient and Stable Inverted Inorganic Perovskite Solar Cells.

Due to their excellent thermal stability and ideal bandgap, metal halide inorganic perovskite based solar cells (PSCs) with inverted structure are considered as an excellent choice for perovskite/silicon tandem solar cells. However, the power conversion efficiency (PCE) of inverted inorganic PSCs still lags far behind that of conventional n-i-p PSCs due to interfacial energy level mismatch and high nonradiative charge recombination. Herein, the performance of inverted PSCs was significantly improved by interfacial engineering of CsPbI3-x Brx films with 2-mercapto-1-methylimidazole (MMI). It is found that the mercapto group can preferably react with the undercoordinated Pb2+ from perovskite by forming Pb-S bonds, which appreciably reduces the surface trap density. Moreover, MMI modification results in a better energy level alignment with the electron transporting material, promoting carrier transfer and reducing voltage deficit. The above combination results in an open circuit voltage enhancement by 120mV, yielding a champion PCE of 20.6% for 0.09 cm2 area and 17.3% for 1 cm2 area. Furthermore, the ambient, operational and heat stabilities of inorganic PSCs with MMI modification are also greatly improved. Our work demonstrates a simple but effective approach for fabricating highly efficient and stable inverted inorganic PSCs. This article is protected by copyright. All rights reserved.

Sodium Incorporation for Performance Improvement of Solution-Processed Submicron CuIn(S,Se)2 Thin Film Solar Cells.

Low-cost solution-processed CuIn(S,Se)2 (CISSe) has great potential for large-scale production of photovoltaics (PV). However, low power conversion efficiency caused by poor crystallinity is one of the main drawbacks compared to vacuum-processed CISSe solar cells. In this work, three strategies for sodium (Na) incorporation into solution-processed CISSe by soaking in sodium chloride (NaCl) aqueous-ethanol solution [1molarity (M)for 10minutes (min)], either prior to absorber deposition (pre-deposition treatment, Pre-DT), before selenization (pre-selenization treatment, Pre-ST), or after selenization (post-selenization treatment, PST) are researched. The Pre-ST CISSe solar cells achieve a better PV performance than those from the other two strategies of Na incorporation. For optimization, soaking times (5, 10, and 15min) and NaCl concentrations (from 0.2 to 1.2m) of the Pre-ST are researched. The highest efficiency achieved is 9.6% with an open-circuit voltage (Voc ) of 464.5mV, a short-circuit current density (jsc ) of 33.4mAcm-2 , and a fill factor (FF) of 62.0%. Compared to the reference CISSe solar cell, Voc , jsc , FF, and efficiency of the champion Pre-ST CISSe device are improved absolutely by 61.0mV, 6.5mAcm-2 , 9%, and 3.8%, respectively. Simultaneously, the open-circuit voltage deficit, the back contact barrier, and the bulk recombination are found to be reduced for Pre-ST CISSe.

Thermomechanical Cleave of Polycrystalline CdTe Solar Cells and its Applications: A Review

One of the primary research challenges for cadmium telluride (CdTe) solar cells is addressing its open‐circuit voltage (VOC) deficit. While theoretical studies and single crystal work show VOC > 1 V is possible, devices remain stubbornly low at ≈800–900 mV. As absorber opto‐electronic properties (e.g., hole density, carrier lifetime) are improved, device modeling suggests that interfaces become limiting. Because CdTe‐based devices are typically grown in the superstrate configuration, the back interface is relatively accessible for manipulation and study, while the front interface (i.e., the heterojunction region) is buried under microns of material and inaccessible. NREL has developed a novel technique to thermomechanically cleave polycrystalline CdTe device stacks directly at the front interface, enabling characterization and controlled manipulation of this important region. Herein, recent work, primarily from NREL, will be reviewed, including considerations for achieving successful delamination; key scientific discoveries about the front interface that have been enabled by this technique; and practical applications, such as flexible, low‐cost solar with high power‐to‐weight ratio.

Suppressing the Photoinduced Halide Segregation in Wide‐Bandgap Perovskite Solar Cells by Strain Relaxation

AbstractEfficient and stable wide bandgap (WBG) perovskite solar cells (PSCs) are imperative for fabricating superior tandem devices. However, small crystal grains and light‐induced phase segregation of WBG perovskite result in large open‐circuit voltage (VOC) deficits, critically impeding the development of the related devices. Herein, the effective functional groups of Lewis‐base trans‐Ferulic acid (t‐FA) are employed to release the residual microstrain in the perovskite lattice. Larger perovskite crystals are formed by strengthening the interaction between the perovskite solute and solution. The lattice structure is stabilized to suppress light‐induced halide segregation. Finally, the power conversion efficiency (PCE) of the optimized device with a bandgap of ≈1.77 eV is increased from 17.33% to 19.31% with the enhancement of VOC. Moreover, replacing a mixture of MeO‐2PACZ and Me‐4PACZ as the hole transporting layer (HTL), the PCE further lifts to 19.9% and VOC is 1.32 V, one of the highest performances reported for WBG PSCs, especially for devices prepared entirely by solution spin‐coating. Therefore, this study provides a practicable alternative for realizing efficient WBG PSCs, which can contribute to the growth of perovskite‐based tandem devices.

Chloride-Based Additive Engineering for Efficient and Stable Wide-Bandgap Perovskite Solar Cells.

Metal halide perovskite based tandem solar cells are promising to achieve power conversion efficiency beyond the theoretical limit of their single-junction counterparts. However, overcoming the significant open-circuit voltage deficit present in wide-bandgap perovskite solar cells remains a major hurdle for realizing efficient and stable perovskite tandem cells. Here, a holistic approach to overcoming challenges in 1.8eV perovskite solar cells is reported by engineering the perovskite crystallization pathway by means of chloride additives. In conjunction with employing a self-assembled monolayer as the hole-transport layer, an open-circuit voltage of 1.25V and a power conversion efficiency of 17.0% are achieved. The key role of methylammonium chloride addition is elucidated in facilitating the growth of a chloride-rich intermediate phase that directs crystallization of the desired cubic perovskite phase and induces more effective halide homogenization. The as-formed 1.8eV perovskite demonstrates suppressed halide segregation and improved optoelectronic properties.

Novel Mg‐ and Ga‐doped ZnO/Li‐Doped Graphene Oxide Transparent Electrode/Electron‐Transporting Layer Combinations for High‐Performance Thin‐Film Solar Cells (Small 22/2023)

High-Performance Thin-Film Solar Cells In article number 2207966, Jung-Je Woo, Jin Hyeok Kim, Hyeong-Jin Kim, and co-workers demonstrate the Mg- and Ga-doped ZnO/Li-doped graphene oxide transparent electrode (TE)/electron-transporting layer (ETL) combination in thin-film solar cells. This newly designed TE/ETL combination not only increases the electron collection rate but also suppresses the open-circuit voltage deficit, thereby improving the conversion efficiency of the solar cells.

Gradient band gap CZTSSe prepared via sputtering from quaternary ceramic targets followed with annealing under different atmospheres

CZTSSe is a thin-film solar cell absorber material with long-time application potential, but its development is limited as its severe open-circuit voltage(Voc) deficit. To further improve the Voc, gradient band gap structure of absorber is expected to achieve this aim without large change of its overall band gap. In this work, a stack structure precursor was deposited by sputtering from quaternary ceramic targets. The top gradient band gap CZTSSe was achieved by annealing the precursor under pure Se or S/Se mix atmosphere. The effects of the atmosphere and the layers-stack precursor structure on the film properties and device performance were studied. Compared with selenization process, sulfo-selenization could effectively suppressed the severe reaction at the back contact interface. It would reduce Mo(S,Se)2 and secondary phases formation and improve solar cell efficiency. The Voc of the double-layers precursor solar cell successfully improved under both atmospheres as expected, representing this method could be a feasible direction to solve the large Voc,def problem. However, for higher efficiency sulfo-selenization cells, the Voc improvement in stack precursor samples did not bring obvious increase of efficiency due to the deterioration of fill factor, indicating that further optimization of device electrical properties should be applied in future study.

Halides-Enhanced Buried Interfaces for Stable and Extremely Low-Voltage-Deficit Perovskite Solar Cells.

The perovskite buried interfaces have demonstrated pivotal roles in determining both the efficiency and stability of perovskite solar cells (PSCs);however, challenges remain in understanding and managing the interfaces due to their non-exposed feature. Here, we proposed a versatile strategy of pre-grafted halides to strengthen the SnO2 -perovskite buried interface by precisely manipulating perovskite defects and carrier dynamics through alteration of halide electronegativity (χ), thereby resulting in both favorable perovskite crystallization and minimized interfacial carrier losses. Specifically, the implementation of fluoride with the highest χ induces the strongest binding affinity to uncoordinated SnO2 defects and perovskite cations, leading to retarded perovskite crystallization and high-quality perovskite films with reduced residual stress. These improved properties enable champion efficiencies of 24.2% (the control: 20.5%) and 22.1% (the control: 18.7%) in rigid and flexible devices with extremely low voltage deficit down to 386mV, all of which are among the highest reported values for PSCs with a similar device architecture. In addition, the resulting devices exhibit marked improvements in the device longevity under various stressors of humidity (>5000h), light (1000h), heat (180h), and bending test (10000times). This method provides an effective way to improve the quality of buried interfaces toward high-performance PSCs.

Overcoming Limitations in Water–Ethanol Sprayed Superstrate Solar Cells by Compositional Engineering of Cu2CdSn(S,Se)4

The increasing demand for solar energy requires materials from earth-abundant elements to ensure cost-effective production. One such light harvester Cu2CdSn(S,Se)4 fulfills this property. We report the development of functional solar cells based on Cu2CdSn(S,Se)4, which has been previously unreported. Furthermore, we deposited the thin films of Cu2CdSn(S,Se)4 by spray pyrolysis using environmentally benign solvents, in a superstrate architecture, reducing the potential cost of upscaling, the environmental hazards, and enabling its use in semitransparent or tandem solar cells. We analyze the Cu2CdSn(S,Se)4 and its optoelectronic characteristics with different sulfur and selenium ratios in the composition. We noted that Se is homogeneously distributed in the absorber and electron transport layer, forming a Cd(S,Se) phase that impacts the optoelectronic properties. The introduction of Se, up to 30%, is found to have a positive impact on the solar cell performance, largely improving the fill factor and absorption in the infrared region, while the voltage deficit is reduced. The device with a Cu2CdSn(S2.8Se1.2) composition had a 3.5% solar-to-electric conversion efficiency, which is on par with the reported values for chalcogenides and the first report using Cu2CdSn(S,Se)4. We identified the critical factors that limit the efficiency, revealing pathways to further reduce the losses and improve the performance. This work provides the first proof of concept of a novel material, paving the way for developing cost-efficient solar cells based on earth-abundant materials.

Low‐Dimensional 2‐thiopheneethylammonium Lead Halide Capping Layer Enables Efficient Single‐Junction Methylamine‐Free Wide‐Bandgap and Tandem Perovskite Solar Cells

AbstractWide‐bandgap (WBG) perovskite solar cells (PSCs) have garnered significant attention for their potential applications in tandem solar cells. However, their large open‐circuit voltage (VOC) deficit and serious photo‐induced halide segregation remain the main challenges that impede their applications. Herein, a post‐treatment strategy without thermal annealing is presented to form a 2D top layer of 2‐thiopheneethylammonium lead halide (n = 1) on WBG perovskites. This thermal annealing‐free post‐treatment method can more effectively passivate the defects of WBG methylamine (MA)‐free formamidinium/cesium lead iodide/bromide perovskite films and suppress photo‐induced perovskite phase segregation, as compared with the thermal annealing method that yields multi‐2D phases. The resulting opaque and semi‐transparent 1.66 eV‐bandgap perovskite solar cells deliver maximum power conversion efficiencies of 21.47% (a small VOC deficit of 0.43 V) and 19.11%, respectively, both of which are among the highest reports for inverted MA‐free WBG PSCs. Consequently, four‐terminal all‐perovskite tandem cells realize a remarkable efficiency of 26.64%, showing great promise for their applications in efficient multi‐junction tandem solar cells.

Surface Passivation of Organic-Inorganic Hybrid Perovskites with Methylhydrazine Iodide for Enhanced Photovoltaic Device Performance

Organic-inorganic hybrid perovskites are highly efficient in photovoltaic applications, making the commercialization of perovskite solar cells (PSCs) possible. However, the high density of defects on the surface significantly affects the performance of PSCs. To address this issue, we have demonstrated a facile post-treatment strategy utilizing methylhydrazine iodide (MHyI) to passivate the surface of the perovskite film. MHyI could co-ordinate with the dangling bonds on the surface of perovskite films, effectively passivating defects in the film and suppressing carrier non-radiative recombination. As a result, PSCs with MHyI modification exhibit a champion power conversion efficiency (PCE) of 23.19% and a high open-circuit voltage (VOC) of 1.14 V (0.43 V voltage deficit). Moreover, unencapsulated solar cells maintain their initial efficiency of 88% after 30 days of exposure to ambient air with 30% humidity, and the devices with encapsulation retained 57% of their initial efficiency after 200 h of maximum power point (MPP) loading under constant light irradiation in ambient air. Overall, our results provide a facile method for improving the performance and stability of PSCs.

Insights into the Efficiency Improvement for CZTSSe Solar Cells with over 12% Efficiency via Ga Incorporation

AbstractSuppressing the band tailing and nonradiative recombination caused by massive defects and defect clusters is crucial for mitigating open‐circuit voltage (Voc) deficit and improving the device performance of CZTSSe thin film solar cells. Cation substitution is one of the most commonly used strategies to address the above issues. The latest world record efficiency of 13.0% is obtained through this strategy (Ag substitution for Cu). Nevertheless, the importance of the approach to implementing metallic ion doping is easily overlooked by researchers. Here, different approaches are adopted to realize Ga doping and the differences in the efficacy and mechanism are thoroughly investigated. It is found that the secondary phase easily emerged when the physical‐based method is employed, and thus challenging to regulate the doping effect. In the case of the chemical‐based method, Ga doping can enlarge the depletion region widthand lower the defect activation energyand Urbach energy. Furthermore, GaSn defects located at grain boundaries can expand the energy band bending between GBs and grain interiors (GIs), thereby suppressing the deep defect states and nonradiative recombination. Consequently, power conversion efficiency as high as 12.12% with a Voc of 522 mV is achieved at 2% Ga doping.

Surface Defects Management by In Situ Etching with Methanol for Efficient Inverted Inorganic Perovskite Solar Cells

AbstractInorganic perovskite solar cells (IPSCs) have developed rapidly due to their good thermal stability and the bandgap suitable for perovskite/silicon tandem solar cells. However, the large open‐circuit voltage (VOC) deficit derived from the surface defects and the energy level structure mismatch impede the development of device performance, especially in the P‐I‐N structure IPSCs. Herein, an innovative in situ etching (ISE) treatment method is proposed to reduce surface defects through methanol without additional passivator. It is found that the perovskite films treated with methanol result in a slight excess of PbI2 on the surface and inserted into the grain boundaries. Therefore, the successful decrease of surface defects by methanol and the passivation of grain boundary defects by PbI2 greatly reduce the trap density of perovskite films. And the larger work function of PbI2 contributes to the energy band of perovskite surface bending downward and forms gradient energy level alignment at the I/N interface, which accelerates extraction of charge carriers. As a result, the efficiency of CsPbI2.85Br0.15 inverted IPSC is enhanced from 16.00% to 19.34%, which is one of the mostly efficient IPSCs. This work provides an original idea without additional passivator to manage the defects of inorganic perovskite.

Suppressed Voltage Deficit and Degradation of Perovskite Solar Cells by Regulating the Mineralization of Lead Iodide.

Both the uncoordinated Pb2+ and excess PbI2 in perovskite film will create defects and perturb carrier collection, thus leading to the open-circuit voltage (VOC ) loss and inducing rapid performance degradation of perovskite solar cells (PSCs). Herein, an additive of 3-aminothiophene-2-carboxamide (3-AzTca) that contains amide and amino and features a large molecular size is introduced to improve the quality of perovskite film. The interplay of size effect and adequate bonding strength between 3-AzTca and uncoordinated Pb2+ regulates the mineralization of PbI2 and generates low-dimensional PbI2 phase, thereby boosting the crystallization of perovskite. The decreased defect states result in suppressed nonradiative recombination and reduced VOC loss. The power conversion efficiency (PCE) of modified PSC is improved to 22.79% with a high VOC of 1.22V. Moreover, the decomposition of PbI2 and perovskite films is also retarded, yielding enhanced device stability. This study provides an effective method to minimize the concentration of uncoordinated Pb2+ and improve the PCE and stability of PSCs.

Identifying the relationships between subsurface absorber defects and the characteristics of kesterite solar cells

AbstractUnderstanding the defect characteristics that occur near the space‐charge regions (SCRs) of kesterite (CZTSSe) solar cells is important because the recombination loss at the CZTSSe/CdS interface is considered the main cause of their low efficiency. CZTSSe surfaces with different elemental compositions were formed without polishing (C00) and with polishing for 20 s (C20) and 60 s (C60). For C60, a specific region near the SCR was excessively Cu‐rich and Zn‐poor compared to C00 and C20. Various charged defects formed where the elemental variation was large. As the main deep acceptor defect energy level (Ea2) near the SCR increased, the efficiency, open‐circuit voltage deficit, and current density degraded, and this phenomenon was especially rapid for large Ea2 values. As the Ea2 near the SCR became deep, the carrier diffusion length decreased more for the CZTSSe solar cells with a low carrier mobility than for the CuInGaSe2 (CIGSe) solar cells. The large amplitude of the electrostatic potential fluctuation in the CZTSSe solar cells induced a high carrier recombination and a short carrier lifetime. Consequently, the properties of the CZTSSe solar cells were more strongly degraded by defects with deep energy levels near the SCR than those of the CIGSe solar cells.