Seth B. Darling

Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer: Fullerene Bulk Heterojunction Solar Cells

PTB7 semiconducting copolymer comprising thieno[3,4-b]thiophene and benzodithiophene alternating repeat units set a historic record of solar energy conversion efficiency (7.4%) in polymer/fullerene bulk heterojunction solar cells. To further improve solar cell performance, a thorough understanding of structure-property relationships associated with PTB7/fullerene and related organic photovoltaic (OPV) devices is crucial. Traditionally, OPV active layers are viewed as an interpenetrating network of pure polymers and fullerenes with discrete interfaces. Here we show that the active layer of PTB7/fullerene OPV devices in fact involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, indicating that the nanostructural characteristics at multiple length scales is one of the key factors determining the performance of PTB7 copolymer, and likely most polymer/fullerene systems, in OPV devices.

Morphology characterization in organic and hybrid solar cells

Organic and hybrid organic–inorganic photovoltaics are among the most promising options for low-cost and highly scalable renewable energy. In order to fully realize the potential of these technologies, power conversion efficiencies and stability will both have to be improved beyond the current state-of-the-art. The morphology of the active layer is of paramount importance in the photon to electron conversion process in organic and hybrid solar cells, with all length scales, from molecular ordering to intradevice composition variability, playing key roles. Given the central influence of morphology, characterizing the structure of these surprisingly complex material systems at multiple length scales is one of the grand challenges in the field. This review addresses the techniques, some of which have only recently been applied to organic and hybrid photovoltaics, available to scientists and engineers working to understand—and ultimately improve—the operation of these fascinating devices.

Vacuum-Deposited Small-Molecule Organic Solar Cells with High Power Conversion Efficiencies by Judicious Molecular Design and Device Optimization

Three new tailor-made molecules (DPDCTB, DPDCPB, and DTDCPB) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor-acceptor-acceptor molecular architecture, in which various electron-donating moieties are connected to an electron-withdrawing dicyanovinylene moiety through another electron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of DTDCPB and the previously reported DTDCTB were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure-property relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing DTDCPB as the donor and C(70) as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 ± 0.2% (the highest PCE of 6.8%), along with an open-circuit voltage (V(oc)) of 0.93 ± 0.02 V, a short-circuit current density (J(sc)) of 13.48 ± 0.27 mA/cm(2), and a fill factor (FF) of 0.53 ± 0.02, under 1 sun (100 mW/cm(2)) AM 1.5G simulated solar illumination.

Nanoscopic Patterned Materials with Tunable Dimensions via Atomic Layer Deposition on Block Copolymers

Selective self-limited interaction of metal precursors with self-assembled block copolymer substrates, combined with the unique molecular-level management of reactions enabled by the atomic layer deposition process, is presented as a promising controllable way to synthesize patterned nanomaterials (e.g., Al{sub 2}O{sub 3} see Figure, TiO{sub 2}, etc.) with uniform and tunable dimensions.

Block copolymers for photovoltaics

Photovoltaic energy conversion is arguably the most promising option for supplying renewable, carbon-neutral energy on a global scale. In order to reach grid parity, however, costs must be reduced substantially. Inexpensive materials generally exhibit efficiencies too low for practical application, but by controlling the morphology on the nanoscale there are opportunities to achieve significant improvements in this area. Block copolymers, which naturally self-assemble into periodic ordered nanostructures, can be utilized in diverse ways to control morphology, ranging from active layers to structure directors to a combination of these methodologies.

Assumptions and the levelized cost of energy for photovoltaics

Photovoltaic electricity is a rapidly growing renewable energy source and will ultimately assume a major role in global energy production. The cost of solar-generated electricity is typically compared to electricity produced by traditional sources with a levelized cost of energy (LCOE) calculation. Generally, LCOE is treated as a definite number and the assumptions lying beneath that result are rarely reported or even understood. Here we shed light on some of the key assumptions and offer a new approach to calculating LCOE for photovoltaics based on input parameter distributions feeding a Monte Carlo simulation. In this framework, the influence of assumptions and confidence intervals becomes clear.

Synthesis and Photovoltaic Effect in Dithieno[2,3‐d:2′,3′‐d′]Benzo[1,2‐b:4,5‐b′]dithiophene‐Based Conjugated Polymers

The recent surge of enthusiasm in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) is driven by their potential for fabricating fl exible and light-weight solar cells via facile, lowcost solution processing techniques. [ 1 ] New materials are crucial in order for OPVs to mature fully from research and development into cost effective products. The power conversion effi ciency (PCE) of large-area OPV solar cells is still inferior to the corresponding inorganic devices and should be continuously improved through major advances in new materials and enhancing our understanding of structure-property relationships. [ 2– 6 ]

A route to nanoscopic materials via sequential infiltration synthesis on block copolymer templates.

Sequential infiltration synthesis (SIS), combining stepwise molecular assembly reactions with self-assembled block copolymer (BCP) substrates, provides a new strategy to pattern nanoscopic materials in a controllable way. The selective reaction of a metal precursor with one of the pristine BCP domains is the key step in the SIS process. Here we present a straightforward strategy to selectively modify self-assembled polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) BCP thin films to enable the SIS of a variety of materials including SiO(2), ZnO, and W. The selective and controlled interaction of trimethyl aluminum with carbonyl groups in the PMMA polymer domains generates Al-CH(3)/Al-OH sites inside the BCP scaffold which can seed the subsequent growth of a diverse range of materials without requiring complex block copolymer design and synthesis.

Tetrathienoanthracene-based copolymers for efficient solar cells.

A series of semiconducting copolymers (PTAT-x) containing extended π-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/PC(61)BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC(61)BM.

Optoelectronics using block copolymers

Block copolymers, either as semiconductors themselves or as structure directors, are emerging as a promising class of materials for understanding and controlling processes associated with both photovoltaic energy conversion and light emitting devices. The increasing interest in block copolymers originates not only from their potential technological advantages but also from their ability to naturally self-assemble into periodic ordered nanostructures. In this article, we emphasize methods by which block copolymer self-assembly can be utilized to rationally design and control the shape and dimension of resulting nanostructures and therefore to develop idealized morphologies. Incorporating these self-organized materials into optoelectronic device fabrication processes or directly into devices will lead to new insights into structure-property relationships and perhaps, ultimately, increases in device efficiency.