Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

Earth‐abundant Cu₂BaSnS₄ (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 10⁴ cm^?1, and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO₂ (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).     

Fe‐N‐C Oxygen Reduction Fuel Cell Catalyst Derived from Carbendazim: Synthesis,Structure, and Reactivity

New non‐PGM catalysts from the family of Fe‐N‐C pyrolyzed materials are reported. They are synthesized using a templating silica powder with iron nitrate and carbendazim (CBDZ) precursors (sacrificial support method). The synthesis involves high temperature pyrolysis, followed by etching of the sacrificial support (silica) and obtaining a “self‐supported” open frame morphology catalyst. Both the temperature of heat treatment and Fe to CBDZ ratio play a crucial role in the final catalytic activity in oxygen reduction reaction (ORR). Prepared materials have extremely high durability in RDE tests, ending up with more than 94% of initial activity (by E_1/2 value) after 10 000 cycles in an oxygen atmosphere, which is the result we report for the first time. Evaluation of these new M‐N‐C catalysts in a single membrane electrode assembly (MEA) has shown an exceptionally high open circuit voltage (OCV) of 1 V and the world's second best performance with no IR correction. MEA tests have shown high current density of 700 mA cm^‐2 at 0.6 V and 120 mA cm^‐2 at 0.8 V. In‐depth structure‐to‐property correlation presents an evidence that Fe‐N_x centers are the active sites playing a key role in oxygen reduction reaction.     

Achieving Both Enhanced Voltage and Current through Fine‐Tuning Molecular Backbone and Morphology Control in Organic Solar Cells

It is a great challenge to simultaneously improve the two tangled parameters, open circuit voltage (V_oc) and short circuit current density (J_sc) for organic solar cells (OSCs). Herein, such a challenge is addressed by a synergistic approach using fine‐tuning molecular backbone and morphology control simultaneously by a simple yet effective side chain modulation on the backbone of an acceptor–donor–acceptor (A–D–A)‐type acceptor. With this, two terthieno[3,2‐b]thiophene (3TT) based A–D–A‐type acceptors, 3TT‐OCIC with backbone modulation and 3TT‐CIC without such modification, are designed and synthesized. Compared with the controlled molecule 3TT‐CIC, 3TT‐OCIC shows power conversion efficiency (PCE) of 13.13% with improved V_oc of 0.69 V and J_sc of 27.58 mA cm^?2, corresponding to PCE of 12.15% with V_oc of 0.65 V and J_sc of 27.04 mA cm^?2 for 3TT‐CIC–based device. Furthermore, with effective near infrared absorption, 3TT‐OCIC is used as the rear subcell acceptor in a tandem device and gave an excellent PCE of 15.72%.     

Efficient Nanostructured TiO2/SnS Heterojunction Solar Cells

Tin sulfide (SnS) is one of the most promising solar cell materials, as it is abundant, environment friendly, available at low cost, and offers long‐term stability. However, the highest efficiency of the SnS solar cell reported so far remains at 4.36% even using the expensive atomic layer deposition process. This study reports on the fabrication of SnS solar cells by a solution process that employs rapid thermal treatment for few seconds under Ar gas flow after spin‐coating a precursor solution of SnCl₂ and thiourea dissolved in dimethylformamide onto a nanostructured thin TiO₂ electrode. The best‐performing cell exhibits power conversion efficiency (PCE) of 3.8% under 1 sun radiation conditions (AM1.5G). Moreover, secondary treatment using SnCl₂ results in a significant improvement of 4.8% in PCE, which is one of the highest efficiencies among SnS‐based solar cells, especially with TiO₂ electrodes. The thin film properties of SnS after SnCl₂ secondary treatment are analyzed using grazing‐incidence wide‐angle X‐ray scattering, and high‐resolution transmittance electron microscopy.     

Performance Enhancement of Polymer‐Free Carbon Nanotube Solar Cells via Transfer Matrix Modeling

Polymer‐free (6,5) single‐walled carbon nanotubes (SWCNTs) prepared using the gel permeation approach are integrated into SWCNT:C₆₀ solar cells. Evaporation‐driven self‐assembly is used to form large‐area SWCNT thin films from the surfactant‐stabilized aqueous suspensions. The thicknesses of various layers within the solar cell are optimized by theoretical modeling using transfer matrix calculations, where the distribution of the electric field within the stack is matched to light absorption by the SWCNTs through either their primary (S₁₁) or secondary (S₂₂) absorption peaks, or a combination thereof. The validity of the model is verified experimentally through a detailed parameter study and then used to develop SWCNT:C₆₀ solar cells with high open‐circuit voltage (0.44 V) as well as a cutting‐edge internal quantum efficiency of up to 86% through the nanotube S₁₁ transition, over an active area of 0.105 cm².     

Band Gap Control in Diketopyrrolopyrrole‐Based Polymer Solar Cells Using Electron Donating Side Chains

We compare the opto‐electronic and photovoltaic properties of two diketopyrrolopyrrole (DPP) based semiconducting polymers in which the DPP unit alternates along the chain with a conjugated bis(dithienyl)phenylene (4TP) unit. The two polymers differ only in the solubilizing substituents on the thiophene rings which are either alkyl (PDPP4TP) or alkoxy (PDPP4TOP) groups. We show that alkoxy groups lower the optical band gap and increase the ionization potential compared to the alkyl groups. As a result, PDDP4TOP provides a significantly higher charge generation efficiency and concomitant higher short‐circuit current, 18.0 mA cm^?2 vs. 12.4 mA cm^?2, compared to PDPP4TP in optimized devices with [6,6]phenyl‐C₇₁‐butyric acid methyl ester ([70]PCBM) as acceptor, but a simultaneous decrease in open circuit voltage, 0.51 vs. 0.67 V. The increased current arises from a higher external quantum efficiency and a wider spectral coverage. The net result is a small increase in power conversion efficiency from 5.8% for PDPP4TP to 6.0% for the PDPP4TOP in optimized devices. The optimized processing conditions and bulk heterojunction morphology are virtually identical for both photoactive layers. The study demonstrates that the side chains enable effective method for rationally designing new photoactive semiconducting polymers.     

The Introduction of Fluorine and Sulfur Atoms into Benzotriazole‐Based p‐Type Polymers to Match with a Benzotriazole‐Containing n‐Type Small Molecule: “The Same‐Acceptor‐Strategy” to Realize High Open‐Circuit Voltage

“The Same‐Acceptor‐Strategy” (SAS) adopts benzotriazole (BTA)‐based p‐type polymers paired with a new BTA based non‐fullerene acceptor BTA13 to minimize the trade‐off between the open‐circuit voltage (V_OC) and short circuit current (J_SC). The fluorination and sulfuration are introduced to lower the highest occupied molecular orbitals (HOMO) of the polymers. The fluorinated polymer of J52‐F shows the higher power conversion efficiency (PCE) of 8.36% than the analog polymer of J52, benefited from a good balance between an improved V_OC of 1.18 V and a J_SC of 11.55 mA cm^?2. Further adding alkylthio groups on J52‐F, the resulted polymer, J52‐FS, exhibits the highest V_OC of 1.24 V with a decreased energy loss of 0.48 eV, compared with 0.67 eV for J52 and 0.54 eV for J52‐F. However, J52‐FS shows an inferior PCE (3.84%) with a lower J_SC of 6.74 mA cm^?2, because the small ΔE_HOMO between J52‐FS and BTA13 (0.02 eV) gives rise to the inefficient hole transfer and high charge recombination, as well as low carrier mobilities. The results of this study clearly demonstrate that the introduction of different atoms in p‐type polymers is effective to improve the SAS and realize the high (V_OC) and PCE.     

A Bottom‐Up Approach toward All‐Solution‐Processed High‐Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting

The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S₂) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S₂ is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO₂ by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm^–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS₂ films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.     

Nanoscale Microstructure and Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells

Sulfurization with various atmosphere and postheat treatments has been reported for earth abundant kesterite Cu₂ZnSnS₄ (CZTS) preparation as cost‐effective material for next‐generation solar cells. A full understanding of the nanoscale microstructure and chemistry of CZTS/CdS interface obtained from these different fabrication routes is currently lacking, yet is critical to developing optimal processing routes for high‐performance kesterite solar cells. Here, the first detailed investigation of the interfacial microstructure and chemistry of CdS/Cu₂ZnSnS₄ heterojunctions is presented. For CZTS obtained from sulfurization in a sulfur‐only atmosphere where highly defective surfaces are present, air annealing followed by etching in the initial stage of chemical bath deposition (CBD) process can effectively eliminate interfacial defects and allow the epitaxial growth of CBD‐CdS, improving the minority lifetime, open circuit voltage (V_OC), and fill factor (FF) of the devices, while blocking Cd diffusion and deteriorating short circuit current (J_sc). For CZTS from sulfurization in a combined sulfur and SnS atmosphere where CBD‐CdS can directly epitaxially grow on CZTS and Cd‐diffusion is clearly observed, associated devices show the longest lifetime and the highest efficiency of 8.76%. Epitaxial growth of CdS and Cd diffusion into CZTS are found to be two crucial features minimizing interfacial recombination and achieving high‐efficiency devices. This will not only enhance the understanding of the device structure and physics of kesterite based solar cells, but also provide an effective way for designing other chalcogenide heterojunction solar cells.     

Engineering Electronic Structure of Stannous Sulfide by Amino‐Functionalized Carbon: Toward Efficient Electrocatalytic Reduction of CO2 to Formate

Engineering electronic structure to enhance the binding energies of reaction intermediates in order to achieve a high partial current density can lead to increased yield of target products. Herein, amino‐functionalized carbon is used to regulate the electronic structure of tin‐based catalysts to enhance activity of CO₂ electroreduction. The hollow nanotubes composed of SnS (stannous sulfide) nanosheets are modified with amino‐functionalized carbon layers, achieving a highest formate Faraday efficiency of 92.6% and a remarkable formate partial current density of 41.1 mA cm^?2 (a total current density of 52.1 mA cm^?2) at a moderate overpotential of 0.9 V versus reversible hydrogen electrode, as well as a good stability. Density functional theory calculations demonstrate that the superior activity is attributed to the synergistic effect among SnS and Aminated‐C in increasing the adsorption energies of the key intermediates and accelerating the charge transfer rate.     

Hybrid Perovskite‐Organic Flexible Tandem Solar Cell Enabling Highly Efficient Electrocatalysis Overall Water Splitting

Perovskite‐organic tandem solar cells are attracting more attention due to their potential for highly efficient and flexible photovoltaic device. In this work, efficient perovskite‐organic monolithic tandem solar cells integrating the wide bandgap perovskite (1.74 eV) and low bandgap organic active PBDB‐T:SN6IC‐4F (1.30 eV) layer, which serve as the top and bottom subcell, respectively, are developed. The resulting perovskite‐organic tandem solar cells with passivated wide‐bandgap perovskite show a remarkable power conversion efficiency (PCE) of 15.13%, with an open‐circuit voltage (V_oc) of 1.85 V, a short‐circuit photocurrent (J_sc) of 11.52 mA cm^?2, and a fill factor (FF) of 70.98%. Thanks to the advantages of low temperature fabrication processes and the flexibility properties of the device, a flexible tandem solar cell which obtain a PCE of 13.61%, with V_oc of 1.80 V, J_sc of 11.07 mA cm^?2, and FF of 68.31% is fabricated. Moreover, to demonstrate the achieved high V_oc in the tandem solar cells for potential applications, a photovoltaic (PV)‐driven electrolysis system combing the tandem solar cell and water splitting electrocatalysis is assembled. The integrated device demonstrates a solar‐to‐hydrogen efficiency of 12.30% and 11.21% for rigid, and flexible perovskite‐organic tandem solar cell based PV‐driven electrolysis systems, respectively.     

Efficient Perovskite Photovoltaic‐Thermoelectric Hybrid Device

An efficient perovskite photovoltaic‐thermoelectric hybrid device is demonstrated by integrating the hole‐conductor‐free perovskite solar cell based on TiO₂/ZrO₂/carbon structure and the thermoelectric generator. The whole solar spectrum of AM 1.5 G is fully utilized with the ≈1.55 eV band gap perovskite (5‐AVA)_x(MA)_1?xPbI₃ absorbing the visible light and the carbon back contact absorbing the infrared light. The added thermoelectric generator improves the device performance by converting the thermal energy into electricity via the Seebeck effect. An optimized hybrid device is obtained with a maximum point power output of 20.3% and open‐circuit voltage of 1.29 V under the irradiation of 100 mW cm^?2.     

High‐Performance Wide Bandgap Copolymers Using an EDOT Modified Benzodithiophene Donor Block with 10.11% Efficiency

Newly developed benzo[1,2‐b:4,5‐b′]dithiophene (BDT) block with 3,4‐ethylenedioxythiophene (EDOT) side chains is first employed to build efficient photovoltaic copolymers. The resulting copolymers, PBDTEDOT‐BT and PBDTEDOTFBT, have a large bandgap more than 1.80 eV, which is attributed to the increased steric hindrance between the BDT and EDOT skeletons. Both copolymers possess the satisfied absorptions, low‐lying highest occupied molecular orbital (HOMO) levels and high crystallinity. Using the fluorination strategy, PBDTEDOT‐FBT exhibits a wider and stronger absorption and a deeper HOMO level than those of PBDTEDOT‐BT. PBDTEDOT‐FBT:[6,6]‐Phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) blend also shows the higher hole mobility and better surface morphology compared with the PBDTEDOTBT:PC₇₁BM blend. Combination of above advantages, PBDTEDOT‐FBT devices exhibit much higher power conversion efficiency (PCE) of 10.11%, with an improved open circuit voltage (V_oc) of 0.86 V, short circuit current densities (J_sc) of 16.01 mA cm^?2, and fill factor (FF) of 72.6%. This work not only provides a newly efficient candidate of BDT donor block modified with EDOT conjugated side chains, but also achieves high‐performance large bandgap copolymers for polymer solar cells (PSCs) via the synergistic effect of fluorination and side chain engineering strategies.     

Efficient Vacuum‐Deposited Tandem Organic Solar Cells with Fill Factors Higher Than Single‐Junction Subcells

Efficient vacuum‐deposited tandem organic photovoltaic cells (TOPVs) composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2‐((2‐(5‐(4‐(diphenylamino)phenyl)thieno[3,2‐b]thiophen‐2‐yl)thiazol‐5‐yl)methylene)malononitrile for the visible absorption and 2‐((7‐(5‐(dip‐tolylamino)thiophen‐2‐yl)benzo[c]‐[1,2,5]thiadiazol‐4‐yl)methylene)malononitrile for the near‐infrared absorption, are reported. Two subcells are connected by the interconnection unit (ICU) composed of electron‐transporting layer/metal/p‐doped hole‐transporting layer. The p‐doped layer in the ICU enables increasing the short‐circuit current density (J _SC) of TOPVs by tuning the relative position of subcells in the tandem devices to have the maximum optical field distribution response, which is well matched with theoretical calculation. Moreover, the introduction of the doped layer benefits to the higher fill factor (FF) of the consisting subcells without losing open‐circuit voltage (V _OC) even with the thick active layers. As a result, power conversion efficiency of 9.2% is achieved with higher FF of 0.62 than that of single‐junction subcells (0.54, 0.57), J _SC of 8.7 mA cm^?2, and V _OC of 1.71 V using 80 nm thick active layers in both subcells.     

Additive‐Free Organic Solar Cells with Power Conversion Efficiency over 10%

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano‐morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo‐oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post‐treatments. The device displays a high open‐circuit voltage of 0.977 V, a large short‐circuit current density of 13.49 mA cm^‐2, and a high fill factor of 76.3%. These results represent an important step towards developing high‐efficiency additive‐free OSCs.     

Light‐Driven BiVO4–C Fuel Cell with Simultaneous Production of H2O2

Photoelectrochemical (PEC) systems have been researched for decades due to their great promise to convert sunlight to fuels. The majority of the research on PEC has been using light to split water to hydrogen and oxygen, and its performance is limited by the need of additional bias. Another research direction on PEC using light, is to decompose organic materials while producing electricity. In this work, the authors report a new type of unassisted PEC system that uses light, water and oxygen to simultaneously produce electricity and hydrogen peroxide (H₂O₂) on both the photoanode and cathode, which is essentially a light‐driven fuel cell with H₂O₂ as the main product at the two electrodes, meanwhile achieving a maximum power density of 0.194 mW cm^‐2, an open circuit voltage of 0.61 V, and a short circuit current density of 1.09 mA cm^‐2. The electricity output can be further used as a sign for cell function when accompanied by a detector such as a light‐emitting diode (LED) light or a multimeter. This is the first work that shows H₂O₂ two‐side generation with a strict key factors study of the system, with a clear demonstration of electricity output ability using low‐cost earth abundant materials on both sides, which represents an exciting new direction for PEC systems.     

Aqueous Solution‐Processable Small Molecular Metal‐Chelate Complex Electrolyte for Flexible All‐Solid State Energy Storage Devices

A small molecular metal‐chelate complex, tris(8‐hydroxyquinoline‐5‐sulfonic acid) aluminum (AlQSA₃), that has three sulfonic acid groups per molecule leading to an excellent solubility in water is reported as a liquid‐free perfect solid‐state electrolyte for flexible film‐type all‐solid‐state energy storage devices. The AlQSA₃ material is synthesized by one‐step reaction of aluminum triisopropoxide and 8‐hydroxyquinoline‐5‐sulfonic acid. The aqueous solutions of AlQSA₃ are applied to fabricate flexible film‐type all‐solid state electric double layer capacitors with indium‐tin oxide thin film electrodes. The ion conductivity of the AlQSA₃ film reaches 0.116 mS cm^?1, while a pronounced hysteresis are obtained in the cyclic voltammetry measurement. The AlQSA₃ film capacitors exhibit an output voltage of 1.5 V at 20 μA cm^?2, which is considerably stable by the repeated operation. In particular, the peak output voltage is well kept even after 180° bending for 500 times in the case of the flexible AlQSA₃ film capacitors.     

Understanding Film Formation Morphology and Orientation in High Member 2D Ruddlesden–Popper Perovskites for High‐Efficiency Solar Cells

2D Ruddlesden–Popper (RP) perovskites have recently emerged as promising candidates for hybrid perovskite photovoltaic cells, realizing power‐conversion efficiencies (PCEs) of over 10% with technologically relevant stability. To achieve solar cell performance comparable to the state‐of‐the‐art 3D perovskite cells, it is highly desirable to increase the conductivity and lower the optical bandgap for enhanced near‐IR region absorption by increasing the perovskite slab thickness. Here, the use of the 2D higher member (n = 5) RP perovskite (n‐butyl‐NH₃)₂(MeNH₃)₄Pb₅I₁₆ in depositing highly oriented thin films from dimethylformamide/dimethylsulfoxide mixtures using the hot‐casting method is reported. In addition, they exhibit superior environmental stability over thin films of their 3D counterpart. These films are assembled into high‐efficiency solar cells with an open‐circuit voltage of ≈1 V and PCE of up to 10%. This is achieved by fine‐tuning the solvent ratio, crystal growth orientation, and grain size in the thin films. The enhanced performance of the optimized devices is ascribed to the growth of micrometer‐sized grains as opposed to more typically obtained nanometer grain size and highly crystalline, densely packed microstructures with the majority of the inorganic slabs preferentially aligned out of plane to the substrate, as confirmed by X‐ray diffraction and grazing‐incidence wide‐angle X‐ray scattering mapping.     

Significant Efficiency Enhancement in Ultrathin CZTS Solar Cells by Combining Al Plasmonic Nanostructures Array and Antireflective Coatings

In the few past years, the economic and eco-friendly Cu₂ZnSnS₄ (CZTS) solar cells have caught lots of attentions. However, due to rather poor efficiency, identifying deficiencies and making improvements is necessary. In the present study, the performance improvement of ultrathin CZTS solar cells was achieved through (1) incorporation of anti-reflective coating (ARC) on the surface of cell and (2) embedding Al plasmonic nanostructures with different radius, periods, and vertical positions in the absorber layer. Various thicknesses of CZTS absorber layer were simulated optically and electrically using FDTD and DEVICE solver of Lumerical software. The reference solar cell consists of a 1.5-nm-thick CZTS absorber and exhibit an efficiency of up to 5.67%, short-circuit current density (Jsc) of 18.48 mA cm⁻² and open circuit voltage of 0.58 V. Result showed a remarkable performance enhancement of the solar cell in spite of a very thin absorber layer. For a 500-μm-thick CZTS solar cell with the assistance of ARC and embedding Al plasmonic nanostructures, the efficiency is increased to 7.45% due to an increase in Jsc to 22.62 mA cm⁻² with an open circuit voltage of 0.62 V.

     

Thin‐Film Solar Cells with InP Absorber Layers Directly Grown on Nonepitaxial Metal Substrates

The design and performance of solar cells based on InP grown by the nonepitaxial thin‐film vapor–liquid–solid (TF‐VLS) growth technique is investigated. The cell structure consists of a Mo back contact, p‐InP absorber layer, n‐TiO₂ electron selective contact, and indium tin oxide transparent top electrode. An ex situ p‐doping process for TF‐VLS grown InP is introduced. Properties of the cells such as optoelectronic uniformity and electrical behavior of grain boundaries are examined. The power conversion efficiency of first generation cells reaches 12.1% under simulated 1 sun illumination with open‐circuit voltage (V_OC) of 692 mV, short‐circuit current (J_SC) of 26.9 mA cm^?2, and fill factor (FF) of 65%. The FF of the cell is limited by the series resistances in the device, including the top contact, which can be mitigated in the future through device optimization. The highest measured V_OC under 1 sun is 692 mV, which approaches the optically implied V_OC of ≈795 mV extracted from the luminescence yield of p‐InP.