Showing papers by "Martyn A. McLachlan published in 2020"

Light-intensity and thickness dependent efficiency of planar perovskite solar cells: charge recombination versus extraction

Abstract: Photoactive layer thickness is a key parameter for optimization of photovoltaic power conversion efficiency (PCE), yet its impact on charge extraction and recombination hasn’t been fully understood in perovskite solar cells (PSCs). Herein we find that in planar PSCs the perovskite thickness yielding maximal PCE is strongly light-intensity dependent. Whilst under 1 sun irradiation the PCE is relatively invariant for perovskite thicknesses between 250 to 750 nm, at lower light intensities (0.1–0.5 sun) the thickest devices yield strongly enhanced PCE, but at higher light intensities (>1 sun) the thinnest devices give optimal PCE. Our results unravel that increased perovskite thickness leads to enhanced light absorption, reduced interfacial recombination at open circuit but greater bimolecular recombination losses at short circuit thus is suitable for devices working under weak illumination, typical of many real-world applications. Reducing perovskite thickness, however, shows the contrast trend and is suitable for PSCs working under concentrated illumination.

Substitutional doping of hybrid organic–inorganic perovskite crystals for thermoelectrics

Abstract: Hybrid organic–inorganic perovskites have generated considerable research interest in the field of optoelectronic devices. However, there have been significantly fewer reports of their thermoelectric properties despite some promising early results. In this article, we investigate the thermoelectric properties of bismuth-doped CH3NH3PbBr3 (MAPbBr3) single crystals. The high-quality Bi-doped crystals were synthesized by inverse temperature crystallization and it was found that Bi substitutes onto the B-site of the ABX3 perovskite lattice of MAPbBr3 crystals with very little distortion of the crystal structure. Bi doping does not significantly alter the thermal conductivity but dramatically enhances the electrical conductivity of MAPbBr3, increasing the charge carrier density by more than three orders of magnitude. We obtained a negative Seebeck coefficient of −378 μV K−1 for 15% (x = 0.15) Bi-doped MAPb(1−x)BixBr3 confirming n-type doping and also measured the figure of merit, ZT. This work highlights routes towards controlled substitutional doping of halide perovskites to optimise them for thermoelectric applications.

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

Abstract: Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one-step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short-circuit current (Jsc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite-only devices. However, this increase in Jsc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high-performance devices.

Room Temperature Synthesis of Phosphine‐Capped Lead Bromide Perovskite Nanocrystals without Coordinating Solvents

Abstract: The room temperature synthesis of perovskite nanocrystals (NCs) is typically achieved by employing a ligand‐assisted reprecipitation (LARP) method, which can be handled in air, and its products are comparable to what is obtained using the traditional hot‐injection method. However, the LARP method typically requires the use of coordinating polar solvents such as dimethylformamide, which are not appropriate for large‐scale production due to toxicity concerns and can also degrade or form defective perovskite NCs. Herein, an amine and oleic‐acid‐free room temperature synthesis of lead bromide perovskite NCs is reported that uses a combination of trioctylphosphine oxide and diisooctylphosphinic acid ligands. This combination of ligands provides a stable platform for the polar‐solvent‐free synthesis in air of fully inorganic CsPbBr3 (fwhm ≈ 14 nm, emission = 519 nm) and hybrid organic‐inorganic FAPbBr3 (fwhm ≈ 19 nm) NCs with photoluminescence emission between 530 and 535 nm, which is in line with the Rec. 2020 color standards. In addition, it is shown that compared to a traditionally used ligand combination, phosphine ligands can be easily removed from the surface of the NCs, which is important for the future development of this technology in optoelectronic devices.

Enhancing the operational stability of unencapsulated perovskite solar cells through Cu–Ag bilayer electrode incorporation

Abstract: We identify a facile strategy that significantly reduces electrode corrosion and device degradation in unencapsulated perovskite solar cells (PSCs) operating in ambient air. By employing Cu–Ag bilayer top electrode PSCs, we show enhanced operational lifetime compared with devices prepared from single metal (Al, Ag and Cu) analogues. Time-of-flight secondary ion mass spectrometry depth profiles indicate that the insertion of the thin layer of Cu (10 nm) below the Ag (100 nm) electrode significantly reduces diffusion of species originating in the perovskite active layer into the electron transport layer and electrode. X-ray diffraction (XRD) analysis reveals the mutually beneficial relationship between the bilayer metals, whereby the thermally evaporated Ag inhibits Cu oxidation and the Cu prevents interfacial reactions between the perovskite and Ag. The results here not only demonstrate a simple approach to prevent the electrode and device degradation that enhance lifetime and stability but also provide insight into ageing related ion migration and structural reorganisation.

Quantum Confinement and Thickness‐Dependent Electron Transport in Solution‐Processed In 2 O 3 Transistors

Abstract: © 2020 Wiley-VCH GmbH The dependence of charge carrier mobility on semiconductor channel thickness in field-effect transistors is a universal phenomenon that has been studied extensively for various families of materials. Surprisingly, analogous studies involving metal oxide semiconductors are relatively scarce. Here, spray-deposited In_{2}O_{3} layers are employed as the model semiconductor system to study the impact of layer thickness on quantum confinement and electron transport along the transistor channel. The results reveal an exponential increase of the in-plane electron mobility (µe) with increasing In2O3 thickness up to ≈10 nm, beyond which it plateaus at a maximum value of ≈35 cm^{2} V^{−1} s^{−1}. Optical spectroscopy measurements performed on In_{2}O_{3} layers reveal the emergence of quantum confinement for thickness <10 nm, which coincides with the thickness that µe starts deteriorating. By combining two- and four-probe field-effect mobility measurements with high-resolution atomic force microscopy, it is shown that the reduction in µe is attributed primarily to surface scattering. The study provides important guidelines for the design of next generation metal oxide thin-film transistors.

Low Temperature Scalable Deposition of Copper(I) Thiocyanate Films via Aerosol-Assisted Chemical Vapor Deposition

Abstract: Copper(I) thiocyanate (CuSCN) is a stable, wide bandgap (>3.5 eV), low-cost p-type semiconductor widely used in a variety of optoelectronic applications, including thin film transistors, organic li...

Building on soft hybrid perovskites: highly oriented metal oxides as electron transport and moisture resistant layers

Abstract: Poor stability under humid environment is one of the key issues of inorganic–organic hybrid perovskite materials, which limits the transformation of perovskite optoelectronic devices from laboratory to industry. In this paper, a dense ZnO electron transport layer is fabricated directly on top of perovskite films by pulsed laser deposition (PLD) as the first line of defence against the penetration of H2O into perovskite, which shows excellent moisture resistance when compared with conventional organic and oxide nanoparticle counterparts. Smooth, dense and highly oriented crystalline ZnO films are obtained by deposition at near-room-temperature. And importantly, in the meantime, the structure and morphology of underlying perovskite layer are preserved.

Effect of processing temperature on film properties of ZnO prepared by the aqueous method and related organic photovoltaics and LEDs

Abstract: Here the influence of processing temperature on the properties of zinc oxide (ZnO) thin films fabricated using a carbon-free solution process is investigated. Our results show that the film processing temperature influences a wide range of structural and electro-optical properties. Films processed at 100 °C are shown to be formed of coalesced ZnO nanoparticles, whose dimensions increase with the processing temperature, accompanied by an increase in electron mobility. ZnO films processed at different temperatures were incorporated as electron transport layers (ETLs) in organic photovoltaic devices with PCDTBT:PC71BM as the active layer. We find that the ETLs processed at low temperature (100–200 °C) exhibit good device performance compared with those prepared at elevated temperatures, an effect we attribute to shifts in the work function and electrical conductivity. Interestingly a similar trend is observed when our ZnO is used as an electron injection layer in organic light emitting diodes, where the EILs processed at >200 °C show higher turn-on voltages and lower efficiencies than those annealed in the 100–200 °C range.