Bright Walker

Endohedral fullerenes for organic photovoltaic devices

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. ; for the first time derivatives of the TNEF acceptor, Lu(3)N@C(80), are synthesized and integrated into OPV devices. The reduced energy offset of the molecular orbitals of Lu(3)N@C(80) 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-C(61)-butyric methyl ester (C(60)-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.

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

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.

A low band gap, solution processable oligothiophene with a dialkylated diketopyrrolopyrrole chromophore for use in bulk heterojunction solar cells

Bulk heterojunction solar cells are fabricated from blends of oligothiophene with a dialkylated diketopyrrolopyrrole chromophore:[6,6]-phenyl C71 butyric acid methyl ester. Absorption and photocurrent of the films extend to 800 nm. A power conversion efficiency (PCE) of 3.0% is obtained under simulated 100 mW/cm2 AM1.5 illumination with a 9.2 mA/cm2 short-circuit current density and an open-circuit voltage of 0.75 V. The hole and electron mobilities in the 50:50 blend are fairly balanced, 1.0×10−4 and 4.8×10−4 cm2/V s, respectively. This is the highest PCE reported to date for solar cells using solution processable small molecules.

Small‐Bandgap Polymer Solar Cells with Unprecedented Short‐Circuit Current Density and High Fill Factor

Small-bandgap polymer solar cells (PSCs) with a thick bulk heterojunction film of 340 nm exhibit high power conversion efficiencies of 9.40% resulting from high short-circuit current density (JSC ) of 20.07 mA cm(-2) and fill factor of 0.70. This remarkable efficiency is attributed to maximized light absorption by the thick active layer and minimized recombination by the optimized lateral and vertical morphology through the processing additive.

Quantification of Geminate and Non‐Geminate Recombination Losses within a Solution‐Processed Small‐Molecule Bulk Heterojunction Solar Cell

Direct measurements of the field-dependent efficiency with which electron-hole pairs are dissociated (1) can be combined with direct measurement of the carrier-density dependent rate at which they subsequently recombine (2) to determine the proportion of carriers which may be usefully extracted (3) for a class of solution-processed organic small-molecule bulk-heterojunction solar cells.

Improved Injection in n-Type Organic Transistors with Conjugated Polyelectrolytes

To improve injection in n-type organic thin film transistors (OTFTs), a thin conjugated polyelectrolyte (CPE) layer was interposed between electrodes and the semiconductor layer. OTFTs were fabricated with [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) and Au source and drain contacts. We demonstrate that the insertion of CPEs beneath top-contact Au source/drain electrodes can be a very effective strategy for improving the carrier injection and reducing turn-on threshold voltages of n-channel OTFTs. Ultraviolet photoemission spectroscopy (UPS) indicates that the decrease of the electron injection barrier is consistent with organized dipoles at the metal/organic interface.

Optimization of energy levels by molecular design: evaluation of bis-diketopyrrolopyrrole molecular donor materials for bulk heterojunction solar cells

We report a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, a material with a hydrolyzed dithieno(3,2-b;2′,3′-d)silole (SDT) core and 2-benzofuran (BFu) end capping groups leads to the most favorable properties for solar cells, capable of generating photocurrent up to 800 nm while producing an open-circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other bulk heterojunction systems. Device properties can be greatly improved through the use of solvent additives such as 2-chloronaphthalene and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.

Capillary Printing of Highly Aligned Silver Nanowire Transparent Electrodes for High-Performance Optoelectronic Devices

Percolation networks of silver nanowires (AgNWs) are commonly used as transparent conductive electrodes (TCEs) for a variety of optoelectronic applications, but there have been no attempts to precisely control the percolation networks of AgNWs that critically affect the performances of TCEs. Here, we introduce a capillary printing technique to precisely control the NW alignment and the percolation behavior of AgNW networks. Notably, partially aligned AgNW networks exhibit a greatly lower percolation threshold, which leads to the substantial improvement of optical transmittance (96.7%) at a similar sheet resistance (19.5 Ω sq(-1)) as compared to random AgNW networks (92.9%, 20 Ω sq(-1)). Polymer light-emitting diodes (PLEDs) using aligned AgNW electrodes show a 30% enhanced maximum luminance (33068 cd m(-2)) compared to that with random AgNWs and a high luminance efficiency (14.25 cd A(-1)), which is the highest value reported so far using indium-free transparent electrodes for fluorescent PLEDs. In addition, polymer solar cells (PSCs) using aligned AgNW electrodes exhibit a power conversion efficiency (PCE) of 8.57%, the highest value ever reported to date for PSCs using AgNW electrodes.

Self‐Assembly and Charge‐Transport Properties of a Polythiophene–Fullerene Triblock Copolymer

Adv. Mater. 2010, 22, 1835–1839 2010 WILEY-VCH Verlag G Organic photovoltaic devices based on blended films of a donor conjugated polymer and an acceptor fullerene have been the focus of studies for nearly 15 years. Most of this research has been centered around bulk heterojunction devices comprising poly(3hexylthiophene) (P3HT) blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). While extensive efforts to optimize processing conditions have resulted in power-conversion efficiencies of around 5%, the record for P3HT:PCBM devices has been stalled near this value for the past few years. The major factor limiting the efficiency of bulk heterojunction (BHJ) solar cells is the difficulty in controlling the nanoscale morphology of the two components. It is important that the donor and the acceptor domain sizes are approximately the same as the exciton diffusion length, usually on the order of 10 nm. Ideally, each of these domains would form a direct path from the cathode to the anode to facilitate efficient charge transport. Within each of these columnar domains, it is also necessary for themolecules to pack in an ordered fashion because highly crystalline domains result in higher charge mobility and more efficient devices. Recent studies have shown that P3HT:PCBM films can form three-dimensional networks with a domain size of 10 nm; however, the P3HTand PCBM networks do not provide a very direct path for electrons and holes to travel toward the appropriate electrodes. Isolated domains are often observed. Several processing methods have been employed to control the morphology of BHJ solar cells including thermal annealing, solvent-vapor annealing, additives, and slow evaporation. Although thermal annealing leads to more ordered networks, it is often responsible for large-scale phase segregation; thus, only low temperature annealing (<150 8C) and short annealing time are used (<30min). Annealing P3HT:PCBM films under temperature and time conditions that are optimal for forming P3HTcrystalline domains results in the formation of PCBM crystals on the micrometer length scale. Such large domains reduce the interfacial area available for charge separation and result in low-efficiency devices. Thus, it is highly desirable to find methods for controlling a self-assembled process to yield high carrier mobilities and efficient charge separation. Jenekhe and co-workers have shown that using preassembled P3HT nanowires, prepared via the solution method, in BHJ solar cells results in higher hole mobilities and higher power-conversion efficiencies than devices fabricated without P3HT nanowires. Another approach to form nanowires/nanostructures is to use block copolymers. Block copolymers have been shown to phase-separate into nanostructures upon film casting. Particularly, a class of block copolymers containing flexible and rigid blocks, called rod–coil diblock polymers, is known to form long fibers. When cast in films, the rod blocks of different polymer chains often stack along the surface to create the center of the fiber, while the coil groups hang to either side of the crystalline center. Using this approach, one can incorporate p-type and n-type materials into the two different parts of a copolymer that self-assembles into donor and acceptor domains. Based on the success of conjugatedpolymer:fullerene-BHJ systems, it would be desirable to incorporate a conjugated polymer block to serve as the donor and a fullerene-containing block to serve as the acceptor. Previous studies have investigated the self-assembling characteristics of rod–coil block copolymers using a poly(p-phenylenevinylene) (PPV) or poly(thiophene) rod block for hole transport and a flexible coil block that contained fullerene (C60) pendant groups for electron transport; however, little is known about thin-film or device characteristics such as morphology, nanoscale photocurrent properties, and charge mobilities. We have recently demonstrated the preparation of a welldefined mono trithiocarbonate P3HT RAFT (reversible additionfragmentation chain transfer) agent via a slightly modified version of the method previously developed by McCullough et al. We then used the RAFT agent in the development of a solution-processable donor–acceptor rod–coil triblock copolymer based on P3HT and C60. [50] The development of the synthetic methodology, synthesis, and characterization of the block copolymer with C60 has been described in detail in our previous study. We also prepared an imino PCBM, namely [5,6]-open azafulleroid, and showed that due to the perturbation of the fullerene’s p-electrons, the field-effect electron mobility is higher than that of highly optimized PCBM. This improvement in performance prompted us to investigate the synthesis of

Structure-function relationships of conjugated polyelectrolyte electron injection layers in polymer light emitting diodes

The characteristics of conjugated polymer light-emitting diodes containing poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) as the emissive layer and cationic or anionic conjugated polyelectrolytes (CPEs) as the electron injection layer are reported. Structure variations involving backbone, type of counterion, and charge were used to establish structure-function relationships. More efficient electron injection from Al and better device performance are attained with CPEs bearing negative charges. For cationic CPEs having the same counterion but different conjugated structures, one observes better device efficiency using the material with higher electron mobility. Thus, both charge and backbone are important for optimizing device performance.