Neutral water splitting catalysis with a high FF triple junction polymer cell

This document is the Accepted Manuscript version of a Published Work that appeared in final form in CS catalysis, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acscatal.6b01036We report a photovoltaics-electrochemical (PV-EC) assembly based on a compact and easily processable triple homojunction polymer cell with high fill factor (76%), optimized conversion efficiencies up to 8.7%, and enough potential for the energetically demanding water splitting reaction (V-oc = 2.1 V). A platinum-free cathode made of abundant materials is coupled to a ruthenium oxide on glassy carbon anode (GC-RuO2) to perform the reaction at optimum potential (Delta E = 1.70-1.78 V, overpotential = 470-550 mV). The GC-RuO2 anode contains a single monolayer of catalyst corresponding to a superficial concentration (Gamma) of 0.15 nmol cm(-2) and is highly active at pH 7. The PV-EC cell achieves solar to hydrogen conversion efficiencies (STH) ranging from 5.6 to 6.0%. As a result of the solar cell's high fill factor, the optimal photovoltaic response is found at 1.70 V, the minimum potential at which the electrodes used perform the water splitting reaction. This allows generating hydrogen at efficiencies that would be very similar (96%) to those obtained as if the system were to be operating at 1.23 V, the thermodynamic potential threshold for the water splitting reaction.Peer ReviewedPostprint (author's final draft

Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of largecations to obtain a perovskite with larger lattice parameter together with a large crystalsize to minimize interface nonradiative recombination. When applying the two-stepmethod for a better crystal control, it is rather challenging to fabricate perovskites withFA+cations, given the small penetration depth of such large ions into a compact PbI2film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl-anions achieving,via a solvent annealing-controlled dissolution-recrystallization, larger than 1µmperovskite crystals in a solar cell. This solar cell, with a largely increasedfluorescencequantum yield, exhibited an open-circuit voltage equivalent to 93% of thecorresponding radiative limit one. This, together with the low band gap achieved(1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.Peer ReviewedPostprint (author's final draft

Natural Random Nanotexturing of the Au Interface for Light Backscattering Enhanced Performance in Perovskite Solar Cells

As the efficiency of a solar cell approaches its limits, photonic considerations to further enhance its performance overtake electronic ones. It has been theoretically shown for GaAs solar cells that with the combined effects of a surface random texturing and a perfectly reflecting rear mirror, efficiencies close to the Shockley–Queisser limit can be reached, even when the absorber layer is very thin. In here, we demonstrate a method for taking advantage of surface random texturing to enhance the efficiency of planar perovskite solar cells. By naturally transferring the perovskite random nanotexturing to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, backscattering reduces light escape from the solar cell structure. This leads to a close to optimal light absorption that allows bringing the cell efficiency from 19.3% to 19.8%. Such a path we opened toward an ergodic behavior for maximum light absorption in perovskite cells may lead to the most efficient perovskite cells ever

4‑Terminal Tandem Photovoltaic Cell Using Two Layers of PTB7:PC₇₁BM for Optimal Light Absorption

A 4-terminal architecture is proposed in which two thin active layers (<100 nm) of PTB7:PC₇₁BM are deposited on a two-sided ITO covered glass substrate. By modeling the electric field distribution inside the multilayer structure and applying an inverse solving problem procedure, we designed an optimal device architecture tailored to extract the highest photocurrent possible. By adopting such a 4-terminal configuration, we numerically demonstrated that even when the two subcells use identical absorber materials, the performance of the 4-terminal device may overcome the performance of the best equivalent single-junction device. In an experimental implementation of such a 4-terminal device, we demonstrate the viability of the approach and find a very good match with the trend of the numerical predictions

UV-Induced Oxygen Removal for Photostable, High-Efficiency PTB7-Th:PC₇₁BM Photovoltaic Cells

Solution-processed ZnO sol–gel or nanoparticles are widely used as the electron-transporting layer (ETL) in optoelectronic devices. However, chemisorbed oxygen on the ZnO layer surface has been shown to be detrimental for the device performance as well as stability. Herein, we demonstrate that chemisorbed oxygen removal based on UV illumination of the ZnO surface layer under a nitrogen atmosphere can, simultaneously, improve the power conversion efficiency and photostability of PTB7-Th:PC71BM-based inverted polymer solar cells. By a systematic study of such a UV illumination procedure, we obtained optimal conditions where both the cell efficiency and stability were improved. We fabricated cells with a power conversion efficiency higher than 9.8% and with a T80 lifetime longer than 500 h, corresponding to about a 2.5-fold enhancement relative to non-UV-treated ZnO reference devices

Spatiotemporal Mapping Uncouples Exciton Diffusion from Singlet–Singlet Annihilation in the Electron Acceptor Y6

Understanding the spatial dynamics of nanoscale exciton transport beyond the temporal decay is essential for further improvements of nanostructured optoelectronic devices, such as solar cells. The diffusion coefficient (D) of the nonfullerene electron acceptor Y6 has so far only been determined indirectly, from singlet–singlet annihilation (SSA) experiments. Here, we present the full picture of the exciton dynamics, adding the spatial domain to the temporal one, by spatiotemporally resolved photoluminescence microscopy. In this way, we directly track diffusion and we are able to decouple the real spatial broadening from its overestimation given by SSA. We measured the diffusion coefficient, D = 0.017 ± 0.003 cm2/s, which gives a Y6 film diffusion length of L=Dτ≈35 nm. Thus, we provide an essential tool that enables a direct and free-of-artifacts determination of diffusion coefficients, which we expect to be pivotal for further studies on exciton dynamics in energy materials

Understanding the Internal Conversion Efficiency of BiVO₄/SnO₂ Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample’s thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott–Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential

Novel composites for nonlinear optics

A fully computerised Temperature-Gradient Zone-Melting (TGZM) apparatus was designed and built in order to produce novel and highly aligned composite films for Second Harmonic Generation (SHG). The TGZM apparatus consists of hot and cold aluminium blocks with glass-ceramic thermal insulator sandwiched between the two blocks. The composite films contain SHG-active guest crystals incorporated within a polymer matrix (host) forming a guest/host structure. These composites exhibit good optical performance in terms of SHG output (guest crystal), high mechanical strength, thermal and chemical stability (host polymer). These particular properties are of great importance especially for fibre-optical applications. 3-methyl-4-methoxy-4'-nitrostilbene (MMONS) is SHG-active guest material which was investigated by incorporating it in poly(methyl methacrylate) or PMMA(host polymer). Two PMMA average molecular weights (AMW) were used once at a time, in which the effect of that on the overall SHG intensity was clearly observed. It was found that a change of a polymer AMW does alter the output of the SHG signal. MMONS crystals were also embedded in another polymer host called Polystyrene (PS) in order to demonstrate the effect of using two different polymers on the SHG intensity of MMONS aligned films. The samples were cast on a glass slides and placed on the hot side of the TGZM apparatus (crystal growth from melt). Later they were drawn towards the cold side with a drawing rate closely matching the MMONS crystal growth rate and producing highly aligned composite films. A Nd:YAG laser beam (1064 nm) with 10 mJ fundamental energy was incident on the above samples (45 deg. from the optic axis z) using type II phase matching, resulting in a green second harmonic signal of 532 nm. The refractive index mismatch between MMONS and a polymer host such as poly(9-vinyle carbazole) or PVK (used in this project) could cause a major light scattering (i.e light loss) during SHG intensity measurements, leading to very much reduced harmonic signal. In order to overcome this problem, a certain amounts of dye (9-methylanthracene, 9-MA) was added in order to modify the overall refractive index of MMONS/PVK in a process called refractive index matching. Higher SHG output was observed than the case where no dye was involved. The refractive index measurements of MMONS/[PVK/9-MA] aligned films were performed using the waveguide prism coupler method to identify the point of refractive index matching at which the aligned film acts as a single medium. As a result of this process, the angular distribution of light transmission (using He-Ne laser beam at 633 nm) was observed to be at its highest level when the refractive index matching point is reached. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN031266 / BLDSC - British Library Document Supply CentreGBUnited Kingdo