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  1. Abstract

    Photodetectors that can sense not only light intensity but also light's polarization state add valuable information that is beneficial in a wide array of applications. Polymer semiconductors are an attractive material system to achieve intrinsic polarization sensitivity due to their anisotropic optoelectronic properties. In this report, the thermomechanical properties of the polymer semiconductors PBnDT‐FTAZ and P(NDI2OD‐T2) are leveraged to realize bulk heterojunction (BHJ) films with record in‐plane alignment. Two polymer blends with distinct weight average molar masses (Mw) are considered and either a strain‐ or rub‐alignment process is applied to align the polymer blend films. Optimized processing yields films with dichroic ratios (DR) of over 11 for the highMwsystem and nearly 17 for the lowMwsystem. Incorporating the aligned films into photodetectors results in a polarized photocurrent ratio of 15.25 with corresponding anisotropy ratio of 0.88 at a wavelength of 530 nm, representing the highest reported photocurrent ratio for photodiodes that can operate in a self‐powered regime. The demonstrated performance showcases the ability of polymer semiconductors to achieve BHJ films with exceptional in‐plane polymer alignment, enabling high performance polarization sensitive photodetectors for incorporation into novel device architectures.

     
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  2. Abstract

    Greenhouse vegetable production plays a vital role in providing year‐round fresh vegetables to global markets, achieving higher yields, and using less water than open‐field systems, but at the expense of increased energy demand. This study examines the life cycle environmental and economic impacts of integrating semitransparent organic photovoltaics (OPVs) into greenhouse designs. We employ life cycle assessment to analyze six environmental impacts associated with producing greenhouse‐grown tomatoes in a Solar PoweRed INtegrated Greenhouse (SPRING) compared to conventional greenhouses with and without an adjacent solar photovoltaic array, across three distinct locations. The SPRING design produces significant reductions in environmental impacts, particularly in regions with high solar insolation and electricity‐intensive energy demands. For example, in Arizona, global warming potential values for a conventional, adjacent PV and SPRING greenhouse are found to be 3.71, 2.38, and 2.36 kg CO2eq/kg tomato, respectively. Compared to a conventional greenhouse, the SPRING design may increase life cycle environmental burdens in colder regions because the shading effect of OPV increases heating demands. Our analysis shows that SPRING designs must maintain crop yields at levels similar to conventional greenhouses in order to be economically competitive. Assuming consistent crop yields, uncertainty analysis shows average net present cost of production across Arizona to be $3.43, $3.38, and $3.64 per kg of tomato for the conventional, adjacent PV and SPRING system, respectively.

     
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  3. Abstract

    Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.

     
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  4. Abstract

    Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature‐dependent aggregation, namely PffBT4T‐2OD [poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5‐diyl)], also known as PCE‐11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O‐IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O‐IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.

     
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  5. Abstract

    Organic solar cells (OSCs) are one of the most promising cost‐effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long‐term stability remains challenging, particularly if the material choice is restricted by roll‐to‐roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT‐M) that broke the 10% benchmark when blade‐coated in air, a second donor material (PBDB‐T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small‐molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs.

     
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  6. Abstract

    Casting of a donor:acceptor bulk‐heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco‐compatible sequential deposition approach is demonstrated for polymer/small‐molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X‐ray scattering indicate that applying (R)‐(+)‐limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single‐ink bulk‐heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.

     
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  7. Abstract

    Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

     
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  8. Abstract

    In this report, a high‐performance all‐polymer organic photodetector that is sensitive to linearly polarized light throughout the visible spectrum is demonstrated. The active layer is a bulk heterojunction composed of an electron donor polymer PBnDT‐FTAZ and acceptor polymer P(NDI2OD‐T2) that have complementary spectral absorption resulting in efficient detection from 350 to 800 nm. The blend film exhibits good ductility with the ability to accommodate large strains of over 60% without fracture. This allows the film to undergo large uniaxial strain resulting in in‐plane alignment of both polymers making the film optically anisotropic and intrinsically polarization sensitive. The films are characterized by UV–vis spectroscopy and grazing incidence wide‐angle X‐ray scattering showing that both polymers have similar in‐plane backbone alignment and maintain packing order after being strained. The films are integrated into devices and characterized under linear polarized light. The strain‐oriented detectors have maximum photocurrent anisotropies of 1.4 under transverse polarized light while maintaining peak responsivities of 0.21 A W−1and a 3 dB cutoff frequency of ≈1 kHz. The demonstrated performance is comparable to the current state of the art all‐polymer photodetectors with the added capability of polarization sensitivity enabling new application opportunities.

     
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  9. Abstract

    Intra‐ and intermolecular ordering greatly impacts the electronic and optoelectronic properties of semiconducting polymers. The interrelationship between ordering of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase in six representative semiconducting polymers, determined from distinct spectroscopic and diffraction signatures, is reported. The sidechain ordering exhibits unusually large coherence lengths (≥70 nm), induces torsional/twisting backbone disorder, and results in a vertically multilayered nanostructure with ordered sidechain layers alternating with disordered backbone layers. Calorimetry and in situ variable temperature scattering measurements in a model system poly{4‐(5‐(4,8‐bis(3‐butylnonyl)‐6‐methylbenzo[1,2‐b:4,5‐b′]dithiophen‐2‐yl)thiophen‐2‐yl)‐2‐(2‐butyloctyl)‐5,6‐difluoro‐7‐(5‐methylthiophen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole} (PBnDT‐FTAZ) clearly delineate this competition of ordering that prevents simultaneous long‐range order of both moieties. The long‐range sidechain ordering can be exploited as a transient state to fabricate PBnDT‐FTAZ films with an atypical edge‐on texture and 2.5× improved field‐effect transistor mobility. The observed influence of ordering between the moieties implies that improved molecular design can produce synergistic rather than destructive ordering effects. Given the large sidechain coherence lengths observed, such synergistic ordering should greatly improve the coherence length of backbone ordering and thereby improve electronic and optoelectronic properties such as charge transport and exciton diffusion lengths.

     
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  10. Abstract

    The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale‐up. Despite the rapid increase in power conversion efficiency (PCE) of spin‐coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high‐performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT‐M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin‐coated FTAZ:IT‐M devices using a single halogen‐free solvent. More importantly, chlorine‐free, blade coating of FTAZ:IT‐M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X‐ray scattering results reveal that large π–π coherence length, high degree of face‐on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low‐cost, low‐toxicity, and high‐efficiency OSCs by high‐throughput production.

     
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