Photovoltaic Absorber Research Articles

Constructing graded heterojunctions for rigid and flexible inverted photovoltaics under outdoor and indoor illumination

All inorganic perovskites (AIPs) have been widely used as an absorber for outdoor photovoltaics due to their high resistance toward thermal/light stress. Recently, CsPbI2Br with a wide bandgap of 1.93 eV has been considered optimal for indoor photovoltaics due to the higher efficiency limit than others. However, more often, inverted AIP devices suffer from serious charge recombination and inefficient carrier extraction, hence presenting unsatisfactory performance. In this work, solution-processable Bi2WO6 nanosheets (Nano-BWO) are prepared and deposited at the CsPbI2Br/PC61BM interface to form graded heterojunctions. A combined experimental and computational study reveals significant electron transfer from AIP to Nano-BWO and PC61BM due to their appropriate band structure offsets, leading to remarkably enhanced electron extraction in inverted devices. In addition, due to the defects passivation effect of Nano-BWO, the interfacial charge recombination is considerably reduced; meanwhile the electronic conductivity of the AIP absorber is remarkably enhanced. The resulting photovoltaics deliver a champion efficiency of 15.71% or 30.43% under one-sun (AM 1.5G) or white light emitting diode (WLED, 1000 lux) illumination. Furthermore, flexible devices with Nano-BWO achieve a high efficiency up to 13.74% (one-sun) or 24.50% (WLED). This work thus provides the first case of inverted AIP devices promising for indoor applications.

Protic Amine Carboxylic Acid Ionic Liquids Additives Regulate α-FAPbI3 Phase Transition for High Efficiency Perovskite Solar Cells.

The α-phase formamidinium lead tri-iodide (α-FAPbI3 ) has become the most promising photovoltaic absorber for perovskite solar cells (PSCs) due to its outstanding semiconductor properties and astonishing high efficiency. However, the incomplete crystallization and phase transition of α-FAPbI3 substantially undermine the performance and stability of PSCs. In this work, a series of the protic amine carboxylic acid ion liquids are introduced as the precursor additives to efficiently regulate the crystal growth and phase transition processes of α-FAPbI3 . The MA2 Pb3 I8 ·2DMSO phase is inhibited in annealing process, which remarkably optimizes the phase transition process of α-FAPbI3 . It is noted that the functional groups of carboxyl and ammonium passivate the undercoordinated lead ions, halide vacancies, and organic vacancies, eliminating the deleterious nonradiative recombination. Consequently, the small-area devices incorporated with 2% methylammonium butyrate (MAB) and 1.5% n-butylammonium formate (BAFa) in perovskite show champion efficiencies of 25.10% and 24.52%, respectively. Furthermore, the large-area modules (5cm × 5cm) achieve PCEs of 21.26% and 19.27% for MAB and BAFa additives, indicating the great potential for commercializing large-area PSCs.

Prospects for inorganic CsPbI3 perovskite solar cells with commercially viable lifetimes

Perovskite CsPbI3 is a promising photovoltaic absorber material, thanks to its ideal bandgap for Si-tandem solar cell applications and its excellent thermochemical stability compared with hybrid organic–inorganic perovskites. However, CsPbI3 has its own stability challenges as its photoactive β- and γ-polymorphs are thermodynamically unstable at room temperature compared with the yellow non-perovskite δ-phase. Stabilizing CsPbI3 has, thus, been the subject of considerable research in recent years. While some approaches, such as alloying with halides and reducing crystalline domain size, have proven effective in improving phase stability, these benefits have, thus far, come at the expense of photovoltaic efficiency compared with the state-of-the-art CsPbI3 solar cells. In this perspective, we discuss the progress and limitations of inorganic perovskite stabilization techniques and look forward at how to achieve inorganic perovskite solar cells with both commercially viable efficiencies and lifetimes.

Optical and morphological properties of Ge-incorporated polycrystalline Sb2Se3 thin-film for photovoltaic applications

Crystalline Ge-incorporated Sb2Se3 thin-films are the promising absorber material for third-generation solar cells due to its optical, electrical, and material properties with earth-abundant materials. In this contribution, we have grown polycrystalline Ge-incorporated Sb2Se3 thin-film absorbers and conducted morphological and optical analysis by Scanning Electron Microscopy and UV–Vis spectroscopy along with the chemical structure analysis with Raman Spectroscopy for photovoltaic absorber applications. Ge-incorporated Sb2Se3 thin-films were deposited with Vapor Transport Deposition techniques, using thermally evaporated Sb2Se3 and Ge from each Sb2Se3 and Ge nanopowder source as precursors. Evaporation temperatures of Sb2Se3 and Ge nanopowder sources were independently controlled in the different heating zones of the furnace. At the deposition temperature of 300 °C, grains of Ge-incorporated Sb2Se3 thin-film begin to grow due to the increased crystallization temperature by Ge incorporation. With temperature-dependent absorption spectrum and the indirect optical transition natures, the optical bandgap of Ge-incorporated Sb2Se3 absorbers is extracted as 1.15 eV and 1.23 eV for samples grown at 500 °C and 520 °C.

Photochemical Decomposition of Y‐Series Non‐Fullerene Acceptors Is Responsible for Degradation of High‐Efficiency Organic Solar Cells

AbstractOrganic photovoltaic cells that employ Y‐series non‐fullerene acceptors (NFAs) have recently achieved impressive power‐conversion efficiencies (>18%). To fulfill their commercial promise, it is important to quantify their operational lifetimes and understand their degradation mechanisms. In this work, the spectral‐dependent photostability of films and solar cells comprising several Y‐series acceptors and the donor polymer PM6 is investigated systematically. By applying longpass filters during aging, it is shown that UV/near‐UV photons are responsible for the photochemical decomposition of Y‐series acceptors; this degradation is the primary driver of early solar cell performance losses. Using mass spectrometry, the vinylene linkage between the core and electron‐accepting moieties of Y‐series acceptors is identified as the weak point susceptible to cleavage under UV‐illumination. Employing a series of device characterization, along with numerical simulations, the efficiency losses in organic photovoltaic cells are attributed to the formation of traps, which reduces charge extraction efficiency and facilitates non‐radiative recombination as the Y‐series acceptors degrade. This study provides new insights for molecular degradation of organic photovoltaic absorber materials and highlights the importance of future molecular design and strategies for improved solar cell stability.

Evidence of Polaron Formation in Halide Perovskites via Carrier Effective Mass Measurements

A combination of transient spectroscopy techniques provides an estimate of the time-resolved effective carrier mass and carrier-phonon coupling constant in a photovoltaic absorber, here applied to lead-halide perovskites.

Designing plant–transparent agrivoltaics

Covering greenhouses and agricultural fields with photovoltaics has the potential to create multipurpose agricultural systems that generate revenue through conventional crop production as well as sustainable electrical energy. In this work, we evaluate the effects of wavelength-selective cutoffs of visible and near-infrared (biologically active) radiation using transparent photovoltaic (TPV) absorbers on the growth of three diverse, representative, and economically important crops: petunia, basil, and tomato. Despite the differences in TPV harvester absorption spectra, photon transmission of photosynthetically active radiation (PAR; 400–700 nm) is the most dominant predictor of crop yield and quality. This indicates that different wavebands of blue, red, and green are essentially equally important to these plants. When the average photosynthetic daily light integral is > 12 mol m–2 d–1, basil and petunia yield and quality is acceptable for commercial production. However, even modest decreases in TPV transmission of PAR reduces tomato growth and fruit yield. These results identify crop-specific design requirements that exist for TPV harvester transmission and the necessity to maximize transmission of PAR to create the most broadly applicable TPV greenhouse harvesters for diverse crops and geographic locations. We determine that the deployment of 10% power conversion efficiency (PCE) plant-optimized TPVs over approximately 10% of total agricultural and pasture land in the U.S. would generate 7 TW, nearly double the entire energy demand of the U.S.

DFT + U study of electronic and optical properties of Cu3TMTe4: TM = V, Nb, Ta with incorporation of SOC

Density Functional Theory (DFT) calculations of structural, electronic and optical properties of copper sulvanite compounds (Cu3TMTe4; TM = V, Nb, Ta) was performed by incorporating spin orbit coupling (SOC) and Hubbard (U) correction for Cu-d orbitals. Analysis of the result shows that SOC effect slightly reduces the electronic band gap of each of the copper sulvanite compounds by 0.10–0.12 eV respectively. Furthermore SOC splitting of bands at R symmetry point of the valence band (VB) with split off energy Δso = 0.33, 0.34 and 0.34 eV for V, Nb and Ta family of compounds are respectively observed. Moreover the optical properties fulfill certain condition for promising solar photovoltaic (PV) absorber with minute observed SOC effect. Theoretical prediction provided with DFT + U calculation along with the incorporation of SOC may provide valuable data for development of nanodevices.

Ag Alloying in Cu2−yAgyBa(Ge,Sn)Se4Films and Photovoltaic Devices

Trigonal Cu2BaGe1−xSnxSe4(CBGTSe) has recently gained interest as a potential photovoltaic absorber to target mitigation of antisite defect formation in Cu2ZnSn(S,Se)4. This study examines partial substitution of Cu by Ag as a potential approach to tune the properties of Ag‐incorporated CBGTSe in the following aspects: 1) phase stability and crystal structure as a function of Ag content; 2) film morphology and grain structure; 3) charge carrier properties; 4) band positions; and 5) charge carrier kinetics and recombination. Up to 20% of Cu can be substituted by Ag in CBGTSe, while above 20% a phase mixture appears. Increasing Ag content induces larger average grain size and reduced hole carrier densities. In contrast, photoelectron spectroscopy and photoluminescence measurements reveal negligible impact of Ag substitution on ionization potential (≈5.4 eV) and electron affinity (≈3.7 eV). Also, Ag content offers negligible impact on carrier lifetimes (few ns). Consistent with these fundamental properties, solar cells based on two different Ag/(Ag + Cu) ratios (≈0% and ≈20%) show comparable power conversion efficiencies (≈2.7–2.8%). These results indicate that CBGTSe films and solar cells may be less sensitive to Ag substitution compared to Cu2ZnSn(S,Se)4, at least at the current level of absorber and device optimization.

Photocatalytic partial oxidation of methane to carbon monoxide and hydrogen over CIGS solar cell

Methane valorization is one of the main challenges in the modern chemical industry. However, existing processes require high reaction temperatures. The alternative photocatalytic routes for methane valorization at ambient conditions would be highly attractive. Today, photovoltaic (PV) generation of electricity is one of the main sources of renewable energy. PV absorbers could be excellent candidates for photochemical applications. Herein, we report selective methane photocatalytic oxidation at ambient conditions into CO and H2 by conventional Cu(In,Ga)Se2 (CIGS) absorbers used in solar cells. A thin film of CIGS coated over Mo exhibits exceptional performance in methane partial oxidation to CO and H2 with a stable CO productivity of 2.4 mmol per gram of CIGS and a selectivity to CO of over 80%. The reaction proceeds via the facile dissociation of methane into disordered carbon and hydrogen over CIGS surface with subsequent regeneration of the surface by partial oxidation of carbon into CO.

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals.

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.

Enhanced photoresponse of Cu2ZnSnS4 absorber thin films fabricated using multi-metallic stacked nanolayers.

Cu2ZnSnS4 (CZTS) thin films have attracted considerable attention as potential candidates for photovoltaic absorber materials. In a vacuum deposition technique, a sputtering stacked metallic layer followed by a thermal process for sulfur incorporation is used to obtain high-quality CZTS thin films. In this work, for fabricating CZTS thin films, we have done a 3LYS (3 layers), 6LYS, and 9LYS sequential deposition of Sn/ZnS/Cu metal stack (via. metallic stacked nanolayer precursors) onto Mo-coated corning glass substrate via. RF-sputtering. The prepared thin films were sulfurized in a tubular furnace at 550 °C in a gas mixture of 5% H2S + 95% Ar for 10 min. We further investigated the impact of the Sn/ZnS/Cu metal stacking layers on the quality of the thin film based on its response to light because metal inter-diffusion during sulfurization is unavoidable. The inter-diffusion of precursors is low in a 3-layer stack sample, limiting the fabricated film's performance. CZTS films with 6-layer and 9-layer stacks result in an improved photocurrent density of ∼38 μA cm-2 and ∼82 μA cm-2, respectively, compared to a 3-layer sample which has a photocurrent density of ∼19 μA cm-2. This enhancement can be attributed to the 9-layer approach's superior inter-diffusion of metallic precursors and compact, smooth CZTS microstructure evolution.

First-principles study of intrinsic and hydrogen point defects in the earth-abundant photovoltaic absorber Zn3P2

The shallow VZn acceptors are proposed as the source for p-type doping in the Zn3P2 solar absorber. Not only VZn but also deep-level defects PZn and Pi have increased concentrations in non-stoichiometric, P-rich Zn3P2.

Relativistic electronic structure and photovoltaic performance of K2CsSb

We identify K2CsSb as a potential photovoltaic absorber by considering it's optoelectronic properties and maximum theoretical power conversion efficiency.

Performance investigation of antimony chalcogenide based thin film solar cells via SCAPS simulation

Antimony chalcogenides thin films are promising photovoltaic absorber materials because of non-toxic and earth-abundant constituents. However, antimony chalcogenide based solar cells have yet to realize efficient performance compared with perovskite or CIGS based solar cells. In this work, a numerical simulation of Sb2(S1−xSex)3 thin films solar cells were investigated using one dimensional Solar Capacitance Simulator (SCAPS-1D). The solar cells parameters such as work temperature, defect density, electrode work function and band structure were investigated to determine the impact on device performance. The results show that compared with other device parameters, the main factors affecting device performance are band structure and defect concentration. By optimizing the parameters of the solar cell, the device efficiency is significantly improved from 9.08 % to 15.19 %. By preparing absorber layers with gradient band gap, reducing the defect density can significantly improve the device performance. These results contribute to the design and fabrication of efficient low-cost thin film solar cells and provide theoretical basis.

Effect of alkali metal adsorption over pristine Ga2STe janus monolayer in enhancing the visible region absorption

Density function theory calculations are used to examine the optical characteristics and electronic structure of Ga2STe Janus monolayer and alkali metal (Li, Na, K, Cs, Rb) adsorbed Ga2STe/Li, Ga2STe/Na, Ga2STe/K, Ga2STe/Cs, and Ga2STe/Rb structures. It is found from the results that pristine Ga2STe Janus monolayer has high absorption (∼13 × 104 cm −1) in the visible region range (∼630–780 nm). However, on adsorption of alkali metals, the absorption increases with wavelength and shifts toward the red region of the spectrum, which is also called redshift. Superior absorption is seen in the entire visible range (∼580–790 nm) for all alkali metal doped structures in comparison with pristine Ga2STe monolayer. Superior absorption in the visible range is required in optoelectronics applications and photovoltaic absorbers. Refractive index and dielectric function of alkali metal doped structures as a function of energy were also considered and were found absorption peaks are in consistent with dielectric function and refractive index values. Large absorption peaks were observed in the entire visible region for all metal adsorbed structures in comparison with pristine monolayer, because of which the proposed structures can be useful for optoelectronic applications.

Seed Layer Optimisation for Ultra-Thin Sb2Se3 Solar Cells on TiO2 by Vapour Transport Deposition.

Antimony selenide (Sb2Se3) material has drawn considerable attention as an Earth-abundant and non-toxic photovoltaic absorber. The power conversion efficiency of Sb2Se3-based solar cells increased from less than 2% to over 10% in a decade. Different deposition methods were implemented to synthesize Sb2Se3 thin films, and various device structures were tested. In search of a more environmentally friendly device composition, the common CdS buffer layer is being replaced with oxides. It was identified that on oxide substrates such as TiO2 using vacuum-based close-space deposition methods, an intermediate deposition step was required to produce high-quality thin films. However, little or no investigation was carried out using another very successful vacuum deposition approach in Sb2Se3 technology called vapour transport deposition (VTD). In this work, we present optimized VTD process conditions to achieve compact, pinhole-free, ultra-thin (<400 nm) Sb2Se3 absorber layers. Three process steps were designed to first deposit the seed layer, then anneal it and, at the final stage, deposit a complete Sb2Se3 absorber. Fabricated solar cells using absorbers as thin as 400 nm generated a short-circuit current density over 30 mA/cm2, which demonstrates both the very high absorption capabilities of Sb2Se3 material and the prospects for ultra-thin solar cell application.

Sputtering Al2O3 as an effective interface layer to improve open-circuit voltage and device performance of Sb2Se3 thin-film solar cells

Sb2Se3 as a promising photovoltaic absorber material is attracting increasing attention. However, high open-circuit voltage (VOC) loss, especially for superstrate structured Sb2Se3 solar cells, is one of the main bottlenecks for improving device efficiency. Here, sputtering aluminum oxide (Al2O3) as an interface layer between CdS and Sb2Se3 was applied in superstrate structured Sb2Se3 solar cells. We proved that the sputtered Al2O3 layer is not fully continuous on the rough CdS film even its thickness has reached 8.8 nm. Then we systematically investigated the influence of the Al2O3 layer on Sb2Se3 material properties and device performances. It is found that the Al2O3 layer can not only effectively inhibit [hk0]-oriented crystal growth and promote [hk1] growth orientation of Sb2Se3 films but also obviously weaken the disordered growth of prominent petal-like shape Sb2Se3 grains and reduce the surface roughness. Further, the Al2O3 layer can also availably passivate interfacial defects at the CdS/Sb2Se3 interface. Consequently, the average power conversion efficiency (PCE) of Sb2Se3 solar cells with an optimal Al2O3 layer thickness is 17.86% higher than that without the Al2O3 layer. Noteworthily, the VOC of Sb2Se3 solar cells is boosted to 0.462 V, which is the highest value in superstrate structured Sb2Se3 solar cells with absorber prepared by conventional physical methods. Finally, a champion efficiency of 6.25% has been achieved in a low-cost Sb2Se3 thin-film solar cell with FTO/CdS/Al2O3/Sb2Se3/Carbon device configuration.

Li/Ag Co‐Doping Synergistically Boosts the Efficiency of Kesterite Solar Cells Through Effective SnZn Defect Passivation

AbstractDespite the tremendous progress in the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic devices, open‐circuit voltage (Voc) deficit, mainly related to the SnZn‐related defects in the absorber, still remains one of the key issues that hinders the cell performance. To overcome this hurdle, here a facile synergistic post‐deposition treatment (PDT) strategy in CZTSSe family via AgF‐PDT after LiF‐PDT in the photovoltaic absorber to suppress SnZn defects, and thus, to promote the Voc and power conversion efficiency (PCE), is reported. Furthermore, a mechanism for improving device efficiency of CZTSSe cell is also proposed. The in‐depth investigation results demonstrate that Li/Ag co‐doping reduces the band tailing characteristic, enlarges the depletion region width, and enhances the carrier transport and collection, resulting in a lower bulk and interface recombination. More importantly, Li/Ag co‐doping strategy provides an effective method for SnZn defect passivation, thereby prolonging minority carrier lifetime in the ensuing devices and leading to enhancement in device performance. Correspondingly, the resulting Li/Ag co‐doped device delivers a champion efficiency of 12.58% with an impressive Voc of 507 mV. This work explores an effective strategy to kill the SnZn‐related defects, thereby overcoming the problems associated with the Voc deficit and lower cell efficiency in Kesterite‐based photovoltaic devices.

A facile Ion-Exchange assisted chemical bath deposition of CuSbS2 for thin film solar cells

Copper antimony sulfur (CuSbS2) possesses a band gap of about 1.5 eV, high absorption coefficient of > 104 cm−1 and abundant compositions, which is considered as one of the potential photovoltaic absorbers. In this work, we propose a facile ion-exchange assisted chemical bath deposition (IEACBD) method, which uses Cu+ to replace Sb3+ from the matrix of Sb2S3 and then combine into CuSbS2 alloy. Through systematic study, we found that this ion-exchange is a simultaneous interdiffusion process of Cu+ and Sb3+ ions, which enables a uniform mixture of those two elements. But it also inevitably produces a Sb2O3 by-product on the surface of the film because of the alkaline environment of this chemical bath. To further remove it, we introduce a mild etching process with BDCA solution for Sb2O3 and then obtain a high-quality CuSbS2. Finally, we assemble an inorganic CuSbS2 solar cell with the architecture of FTO/TiO2/CdS/CuSbS2/Carbon/Ag, and delivering a photoelectric conversion efficiency of 0.75 %. This enriches the preparation method of CuSbS2 thin film and also provides guidance for the synthesis of the similar multiple chalcogen compounds.