Establishing Stability in Organic Semiconductor Photocathodes for Solar Hydrogen Production.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm-2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS3, MoP, or RuO2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS3:MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm-2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO2 where initial photocurrent density (>8 mA cm-2) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.

Passivation Mechanism Exploiting Surface Dipoles Affords High-Performance Perovskite Solar Cells.

The employment of 2D perovskites is a promising approach to tackling the stability and voltage issues inherent in perovskite solar cells. It remains unclear, however, whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, the utilization of AzPbI3 leads to a new passivation mechanism due to the presence of surface dipoles resulting in a power conversion efficiency (PCE) of 22.4%. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a band gap of 1.55 eV.

Crown Ether Modulation Enables over 23% Efficient Formamidinium-Based Perovskite Solar Cells.

The use of molecular modulators to reduce the defect density at the surface and grain boundaries of perovskite materials has been demonstrated to be an effective approach to enhance the photovoltaic performance and device stability of perovskite solar cells. Herein, we employ crown ethers to modulate perovskite films, affording passivation of undercoordinated surface defects. This interaction has been elucidated by solid-state nuclear magnetic resonance and density functional theory calculations. The crown ether hosts induce the formation of host-guest complexes on the surface of the perovskite films, which reduces the concentration of surface electronic defects and suppresses nonradiative recombination by 40%, while minimizing moisture permeation. As a result, we achieved substantially improved photovoltaic performance with power conversion efficiencies exceeding 23%, accompanied by enhanced stability under ambient and operational conditions. This work opens a new avenue to improve the performance and stability of perovskite-based optoelectronic devices through supramolecular chemistry.

Lead Halide Perovskite Quantum Dots To Enhance the Power Conversion Efficiency of Organic Solar Cells.

The facile synthesis, solution-processability, and outstanding optoelectronic properties of emerging colloidal lead halide perovskite quantum dots (LHP QDs) makes them ideal candidates for scalable and inexpensive optoelectronic applications, including photovoltaic (PV) devices. The first demonstration of integrating CsPbI3 QDs into a conventional organic solar cell (OSC) involves embedding the LHP QDs in a donor-acceptor (PTB7-Th:PC71 BM) bulk heterojunction. Optimizing the loading amount at 3 wt %, we demonstrate a power conversion efficiency of 10.8 %, which is a 35 % increase over control devices, and is a record amongst hybrid ternary OSCs. Detailed investigation into the mechanisms behind the performance enhancement shows that increased light absorption is not a factor, but that increased exciton separation in the acceptor phase and reduced recombination are responsible.

Multidimensional Anodized Titanium Foam Photoelectrode for Efficient Utilization of Photons in Mesoscopic Solar Cells.

Mesoscopic solar cells based on nanostructured oxide semiconductors are considered as a promising candidates to replace conventional photovoltaics employing costly materials. However, their overall performances are below the sufficient level required for practical usages. Herein, this study proposes an anodized Ti foam (ATF) with multidimensional and hierarchical architecture as a highly efficient photoelectrode for the generation of a large photocurrent. ATF photoelectrodes prepared by electrochemical anodization of freeze-cast Ti foams have three favorable characteristics: (i) large surface area for enhanced light harvesting, (ii) 1D semiconductor structure for facilitated charge collection, and (iii) 3D highly conductive metallic current collector that enables exclusion of transparent conducting oxide substrate. Based on these advantages, when ATF is utilized in dye-sensitized solar cells, short-circuit photocurrent density up to 22.0 mA cm-2 is achieved in the conventional N719 dye-I3- /I- redox electrolyte system even with an intrinsically inferior quasi-solid electrolyte.

Iron Oxide Photoelectrode with Multidimensional Architecture for Highly Efficient Photoelectrochemical Water Splitting.

Nanostructured metal oxide semiconductors have shown outstanding performances in photoelectrochemical (PEC) water splitting, but limitations in light harvesting and charge collection have necessitated further advances in photoelectrode design. Herein, we propose anodized Fe foams (AFFs) with multidimensional nano/micro-architectures as a highly efficient photoelectrode for PEC water splitting. Fe foams fabricated by freeze-casting and sintering were electrochemically anodized and directly used as photoanodes. We verified the superiority of our design concept by achieving an unprecedented photocurrent density in PEC water splitting over 5 mA cm-2 before the dark current onset, which originated from the large surface area and low electrical resistance of the AFFs. A photocurrent of over 6.8 mA cm-2 and an accordingly high incident photon-to-current efficiency of over 50 % at 400 nm were achieved with incorporation of Co oxygen evolution catalysts. In addition, research opportunities for further advances by structual and compositional modifications are discussed, which can resolve the low fill factoring behavior and improve the overall performance.

Organic-inorganic halide perovskite/crystalline silicon four-terminal tandem solar cells.

Tandem solar cells constructed from a crystalline silicon (c-Si) bottom cell and a low-cost top cell offer a promising way to ensure long-term price reductions of photovoltaic modules. We present a four-terminal tandem solar cell consisting of a methyl ammonium lead triiodide (CH3NH3PbI3) top cell and a c-Si heterojunction bottom cell. The CH3NH3PbI3 top cell exhibits broad-band transparency owing to its design free of metallic components and yields a transmittance of >55% in the near-infrared spectral region. This allows the generation of a short-circuit current density of 13.7 mA cm(-2) in the bottom cell. The four-terminal tandem solar cell yields an efficiency of 13.4% (top cell: 6.2%, bottom cell: 7.2%), which is a gain of 1.8%abs with respect to the reference single-junction CH3NH3PbI3 solar cell with metal back contact. We employ the four-terminal tandem solar cell for a detailed investigation of the optical losses and to derive guidelines for further efficiency improvements. Based on a power loss analysis, we estimate that tandem efficiencies of ∼28% are attainable using an optically optimized system based on current technology, whereas a fully optimized, ultimate device with matched current could yield up to 31.6%.

Peripherally and axially carboxylic acid substituted subphthalocyanines for dye-sensitized solar cells.

A series of subphthalocyanines (SubPcs) bearing a carboxylic acid group either at the peripheral or axial position have been designed and synthesized to investigate the influence of the COOH group positions on the dye-sensitized solar cell (DSSC) performance. The DSSC devices based on SubPcs with axially substituted carboxylic acid groups showed low photovoltaic performance, whereas peripherally substituted one exhibited higher power conversion efficiency owing to improved injection from LUMO of SubPcs to the TiO2 conduction band.

Sterically hindered phthalocyanines for dye-sensitized solar cells: influence of the distance between the aromatic core and the anchoring group.

A new phthalocyanine (Pc) bearing bulky peripheral substituents and a carboxylic anchoring group directly attached to the macrocycle has been prepared and used as a sensitizer in DSSCs, reaching 5.57% power conversion efficiency. In addition, an enhanced performance for the TT40 dye, previously reported by us, was achieved in optimized devices, obtaining a new record efficiency with Pc-sensitized cells.

The role of π bridges in high-efficiency DSCs based on unsymmetrical squaraines.

A series of squaraine-based sensitizers with various π bridges and anchors were prepared and examined in dye-sensitized solar cells. The carboxylic anchor group was attached onto a squaraine dye through π bridges with and without an ethynyl spacer. DFT studies indicate that the LUMO is delocalized throughout the dyes, whilst the HOMO resides on the squaraine core. The dye that incorporates a 4,4-di-n-hexyl-cyclopentadithiophene group that is directly attached onto the π bridge, JD10, exhibits the highest power conversion efficiency in a DSC; this result is attributed, in part, to the deaggregative properties that are associated with the gem-di-n-hexyl substituents, which extend above and below the π-conjugated dye plane. Dye JD10 demonstrates a power-conversion efficiency of 7.3% for liquid-electrolyte dye-sensitized solar cells and 7.9% for cells that are co-sensitized by another metal-free dye, D35, which substantially exceed the performance of any previously tested squaraine sensitizer. A panchromatic incident-photon-to-current-conversion efficiency curve is realized for this dye with an excellent short-circuit current of 18.0 mA cm(-2). This current is higher than that seen for other squaraine dyes, partially owing to a high molar absorptivity of >5,000 M(-1) cm(-1) from 400 nm to the long-wavelength onset of 724 nm for dye JD10.

Highly soluble energy relay dyes for dye-sensitized solar cells.

High solubility is a requirement for energy relay dyes (ERDs) to absorb a large portion of incident light and significantly improve the efficiency of dye-sensitized solar cells (DSSCs). Two benzonitrile-soluble ERDs, BL302 and BL315, were synthesized, characterized, and resulted in a 65% increase in the efficiency of TT1-sensitized DSSCs. The high solubility (180 mM) of these ERDs allows for absorption of over 95% of incident light at their peak wavelength. The overall power conversion efficiency of DSSCs with BL302 and BL315 was found to be limited by their energy transfer efficiency of approximately 70%. Losses due to large pore size, dynamic collisional quenching of the ERD, energy transfer to desorbed sensitizing dyes and static quenching by complex formation were investigated and it was found that a majority of the losses are caused by the formation of statically quenched ERDs in solution.

Photoelectrochemical CO2 Reduction at a Direct CuInGaS2/Electrolyte Junction.

Photoelectrochemical (PEC) CO2 reduction has received considerable attention given the inherent sustainability and simplicity of directly converting solar energy into carbon-based chemical fuels. However, complex photocathode architectures with protecting layers and cocatalysts are typically needed for selective and stable operation. We report herein that bare CuIn0.3Ga0.7S2 photocathodes can drive the PEC CO2 reduction with a benchmarking 1 Sun photocurrent density of over 2 mA/cm2 (at -2 V vs Fc+/Fc) and a product selectivity of up to 87% for CO (CO/all products) production while also displaying long-term stability for syngas production (over 44 h). Importantly, spectroelectrochemical analysis using PEC impedance spectroscopy (PEIS) and intensity-modulated photocurrent spectroscopy (IMPS) complements PEC data to reveal that tailoring the proton donor ability of the electrolyte is crucial for enhancing the performance, selectivity, and durability of the photocathode. When a moderate amount of protons is present, the density of photogenerated charges accumulated at the interface drops significantly, suggesting a faster charge transfer process. However, with a high concentration of proton donors, the H2 evolution reaction is preferred.

Photogenerated charge transfer in Dion-Jacobson type layered perovskite based on naphthalene diimide.

Incorporating organic semiconducting spacer cations into layered lead halide perovskite structures provides a powerful approach to mitigate the typical strong dielectric and quantum confinement effects by inducing charge-transfer between the organic and inorganic layers. Herein we report the synthesis and characterization of thin films of novel DJ-phase organic-inorganic layered perovskite semiconductors using a naphthalene diimide (NDI) based divalent spacer cation, which is shown to accept photogenerated electrons from the inorganic layer. With alkyl chain lengths of 6 carbons, an NDI-based thin film exhibited electron mobility (based on space charge-limited current for quasi-layered 〈n〉 = 5 material) was found to be as high as 0.03 cm2 V-1 s-1 with no observable trap-filling region suggesting trap passivation by the NDI spacer cation.

Transparent Porous Conductive Substrates for Gas-Phase Photoelectrochemical Hydrogen Production.

Gas diffusion electrodes are essential components of common fuel and electrolysis cells but are typically made from graphitic carbon or metallic materials, which do not allow light transmittance and thus limit the development of gas-phase based photoelectrochemical devices. Herein, the simple and scalable preparation of F-doped SnO2 (FTO) coated SiO2 interconnected fiber felt substrates is reported. Using 2-5 µm diameter fibers at a loading of 4 mg cm-2 , the resulting substrates have porosity of 90%, roughness factor of 15.8, and Young's Modulus of 0.2 GPa. A 100 nm conformal coating of FTO via atmospheric chemical vapor deposition gives sheet resistivity of 20 ± 3 Ω sq-1 and loss of incident light of 41% at illumination wavelength of 550 nm. The coating of various semiconductors on the substrates is established including Fe2 O3 (chemical bath deposition), CuSCN and Cu2 O (electrodeposition), and conjugated polymers (dip coating), and liquid-phase photoelectrochemical performance commensurate with flat FTO substrates is confirmed. Finally, gas phase H2 production is demonstrated with a polymer semiconductor photocathode membrane assembly at 1-Sun photocurrent density on the order of 1 mA cm-2 and Faradaic efficiency of 40%.

Understanding and Mitigating the Degradation of Perovskite Solar Cells Based on a Nickel Oxide Hole Transport Material during Damp Heat Testing.

The development of stable materials, processable on a large area, is a prerequisite for perovskite industrialization. Beyond the perovskite absorber itself, this should also guide the development of all other layers in the solar cell. In this regard, the use of NiOx as a hole transport material (HTM) offers several advantages, as it can be deposited with high throughput on large areas and on flat or textured surfaces via sputtering, a well-established industrial method. However, NiOx may trigger the degradation of perovskite solar cells (PSCs) when exposed to environmental stressors. Already after 100 h of damp heat stressing, a strong fill factor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysis on cells and materials, completed by device simulation, the cause of the degradation is pinpointed and mitigation strategies are proposed. When NiOx is heated in an air-tight environment, its free charge carrier density drops, resulting in a band misalignment at the NiOx/perovskite interface and in the formation of a barrier impeding hole extraction. Adding an organic layer between the NiOx and the perovskite enables higher performances but not long-term thermal stability, for which reducing the NiOx thickness is necessary.

Cobalt electrolyte/dye interactions in dye-sensitized solar cells: a combined computational and experimental study.

We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO(2) surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)(3)](3+) species into contact with the TiO(2) surface. This translates into a high probability of intercepting TiO(2)-injected electrons by the oxidized [Co(bpy)(3)](3+) species, lying close to the N719-sensitized TiO(2) surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.

Convergent synthesis of near-infrared absorbing, "push-pull", bisthiophene-substituted, zinc(II) phthalocyanines and their application in dye-sensitized solar cells.

Zinc(II) phthalocyanine dyes that contain triarylamine-terminated bisthiophene and hexylbisthiophene groups have been synthesized by a convergent approach by using carboxytriiodo-ZnPc as a precursor. Further transformation of the iodo groups by a Pd-catalyzed reaction allowed easy preparation of further extended π-conjugated carboxy-ZnPcs. These dyes have been used as sensitizers in dye-sensitized solar cells, which exhibit a panchromatic response and moderate overall efficiencies.

Evaluation of a ruthenium oxyquinolate architecture for dye-sensitized solar cells.

Synthesis of the [Ru(dcbpy)(2)(OQN)](+) complex is reported in which dcbpy and OQN(-) are the bidentate 4,4'-dicarboxy-2,2'-bipyridyl and 8-oxyquinolate ligands, respectively. Spectroscopic, electrochemical, and theoretical analyses are indicative of extensive Ru(OQN) molecular orbital overlap due to degenerate Ru d(π) and OQN p(π) mixing. [Ru(dcbpy)(2)(OQN)](+) displays spectroscopic properties remarkably similar to those of the N3 dye, making it a promising candidate for application in dye-sensitized solar cell devices. However, its solar power conversion efficiency requires further optimization.

Successful demonstration of an efficient I(-)/(SeCN)2 redox mediator for dye-sensitized solar cells.

A new I(-)/(SeCN)(2) redox mediator has favorable properties for dye-sensitized solar cells (DSCs) such as less visible light absorption, higher ionic conductivity, and downward shift of redox potential than I(-)/I(3)(-). It was then applied for DSCs towards increasing energy conversion efficiency, giving a new potential for improving performance.

Graphene nanoplatelets outperforming platinum as the electrocatalyst in co-bipyridine-mediated dye-sensitized solar cells.

Graphene nanoplatelets (GNP) in the form of thin semitransparent films on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for the Co(bpy)3(3+/2+) redox couple in acetonitrile electrolyte solution. The GNP film is superior to the traditional electrocatalyst, that is, platinum, both in charge-transfer resistance (exchange current) and in electrochemical stability under prolonged potential cycling. The good electrochemical performance of GNP is readily applicable for dye-sensitized solar cells with Y123-sensitized TiO2 photoanodes and Co(bpy)3(3+/2+) as the redox shuttle. The dye-sensitized solar cell with GNP cathode is superior to that with the Pt-FTO cathode particularly in fill factor and in power conversion efficiency at higher illumination intensity.