Bright Walker

Bio: Bright Walker is an academic researcher from Kyung Hee University. The author has contributed to research in topics: Polymer solar cell & Organic solar cell. The author has an hindex of 37, co-authored 98 publications receiving 7895 citations. Previous affiliations of Bright Walker include Ulsan National Institute of Science and Technology & University of California, Santa Barbara.

Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.

Abstract: Metal halide perovskites of the general formula ABX3—where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion—have shown great potential as light harvesters for thin-film photovoltaics1–5. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells6–9, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO−) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent), have long-term operational stability (450 hours) and show intense electroluminescence with external quantum efficiencies of more than 10 per cent. Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance. Incorporation of the pseudo-halide anion formate during the fabrication of α-FAPbI3 perovskite films eliminates deleterious iodide vacancies, yielding solar cell devices with a certified power conversion efficiency of 25.21 per cent and long-term operational stability.

Small Molecule Solution-Processed Bulk Heterojunction Solar Cells†

Abstract: Although most research in the field of organic bulk heterojunction solar cells has focused on combinations of a p-type conducting polymer as a donor and a fullerene-based acceptor, recent work has demonstrated the viability of solution-processed heterojunctions composed entirely of molecular solids. Molecular solids offer potential advantages over conjugated polymer systems in terms of easier purification, amenability to mass-scale production and better batch-to-batch reproducibility. This article reviews the major classes of molecular donors that have been reported in the literature in the past several years and highlights some of key considerations in molecular heterojunction design compared to polymer-based bulk heterojunctions.

Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells

Abstract: Research relating to organic solar cells based on solution-processed, bulk heterojunction (BHJ) films has been dominated by polymeric donor materials, as they typically have better film-forming characteristics and film morphology than their small-molecule counterparts. Despite these morphological advantages, semiconducting polymers suffer from synthetic reproducibility and difficult purification procedures, which hinder their commercial viability. Here, a non-polymeric, diketopyrrolopyrrole-based donor material that can be solution processed with a fullerene acceptor to produce good quality films is reported. Thermal annealing leads to suitable phase separation and material distribution so that highly effective BHJ morphologies are obtained. The frontier orbitals of the material are well aligned with those of the fullerene acceptor, allowing efficient electron transfer and suitable open-circuit voltages, leading to power conversion efficiencies of 4.4 ± 0.4% under AM1.5G illumination (100 mW cm−2). Small molecules can therefore be solution processed to form high-quality BHJ films, which may be used for low-cost, flexible organic solar cells.

Semi-crystalline photovoltaic polymers with efficiency exceeding 9% in a ∼300 nm thick conventional single-cell device

Abstract: We report a series of semi-crystalline, low band gap (LBG) polymers and demonstrate the fabrication of highly efficient polymer solar cells (PSCs) in a thick single-cell architecture. The devices achieve a power conversion efficiency (PCE) of over 7% without any post-treatment (annealing, solvent additive, etc.) and outstanding long-term thermal stability for 200 h at 130 °C. These excellent characteristics are closely related to the molecular structures where intra- and/or intermolecular noncovalent hydrogen bonds and dipole–dipole interactions assure strong interchain interactions without losing solution processability. The semi-crystalline polymers form a well-distributed nano-fibrillar networked morphology with PC70BM with balanced hole and electron mobilities (a h/e mobility ratio of 1–2) and tight interchain packing (a π–π stacking distance of 3.57–3.59 A) in the blend films. Furthermore, the device optimization with a processing additive and methanol treatment improves efficiencies up to 9.39% in a ∼300 nm thick conventional single-cell device structure. The thick active layer in the PPDT2FBT:PC70BM device attenuates incident light almost completely without damage in the fill factor (0.71–0.73), showing a high short-circuit current density of 15.7–16.3 mA cm−2. Notably, PPDT2FBT showed negligible changes in the carrier mobility even at ∼1 μm film thickness.

Endohedral fullerenes for organic photovoltaic devices

Abstract: A limiting factor of the power conversion efficiencies of organic photovoltaic devices is low voltage output Methano derivatives of the trimetallic endohedral fullerene Lu3N@C80 have now been synthesized and used as the acceptor in organic photovoltaics The open circuit voltage of the devices is significantly above those made using alternative fullerenes So far, one of the fundamental limitations of organic photovoltaic (OPV) device power conversion efficiencies (PCEs) has been the low voltage output caused by a molecular orbital mismatch between the donor polymer and acceptor molecules Here, we present a means of addressing the low voltage output by introducing novel trimetallic nitride endohedral fullerenes (TNEFs) as acceptor materials for use in photovoltaic devices TNEFs were discovered in 1999 by Stevenson et al 1; for the first time derivatives of the TNEF acceptor, Lu3N@C80, are synthesized and integrated into OPV devices The reduced energy offset of the molecular orbitals of Lu3N@C80 to the donor, poly(3-hexyl)thiophene (P3HT), reduces energy losses in the charge transfer process and increases the open circuit voltage (Voc) to 260 mV above reference devices made with [6,6]-phenyl-C61-butyric methyl ester (C60-PCBM) acceptor PCEs >4% have been observed using P3HT as the donor material This work clears a path towards higher PCEs in OPV devices by demonstrating that high-yield charge separation can occur with OPV systems that have a reduced donor/acceptor lowest unoccupied molecular orbital energy offset

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Polymer solar cells

Abstract: This Review summarizes recent progress in the development of polymer solar cells. It covers the scientific origins and basic properties of polymer solar cell technology, material requirements and device operation mechanisms, while also providing a synopsis of major achievements in the field over the past few years. Potential future developments and the applications of this technology are also briefly discussed.

Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency

Abstract: Today's best perovskite solar cells use a mixture of formamidinium and methylammonium as the monovalent cations. With the addition of inorganic cesium, the resulting triple cation perovskite compositions are thermally more stable, contain less phase impurities and are less sensitive to processing conditions. This enables more reproducible device performances to reach a stabilized power output of 21.1% and ∼18% after 250 hours under operational conditions. These properties are key for the industrialization of perovskite photovoltaics.

Polymer‐Fullerene Bulk‐Heterojunction Solar Cells

Abstract: Solution-processed bulk-heterojunction solar cells have gained serious attention during the last few years and are becoming established as one of the future photovoltaic technologies for low-cost power production. This article reviews the highlights of the last few years, and summarizes today's state-of-the-art performance. An outlook is given on relevant future materials and technologies that have the potential to guide this young photovoltaic technology towards the magic 10% regime. A cost model supplements the technical discussions, with practical aspects any photovoltaic technology needs to fulfil, and answers to the question as to whether low module costs can compensate lower lifetimes and performances.

Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

Abstract: All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb + ) can be embedded into a “cation cascade” to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.