• Energy Research

AbstractShortwave‐infrared (SWIR) photodetectors are vital for many scientific and industrial applications including surveillance, quality control and inspection. In recent decades, photodetectors based on organic semiconductors have emerged, demonstrating potential to add real value to broadband and narrowband imaging and sensing scenarios, where factors such as thermal budget sensitivity, large area aperture necessity, cost considerations, and lightweight and conformal flexibility demands are prioritized. It is now recognized that the performance of organic photodetectors (OPDs), notably their specific detectivity, is ultimately limited by trap states, universally present in disordered semiconductors. This work adopts an approach of utilizing these mid‐gap states to specifically create a SWIR photo‐response. To this end, this work introduces a somewhat counter‐intuitive approach of “trap‐doping” in bulk heterojunction (BHJs) photodiodes, where small quantities of a guest organic molecule are intentionally incorporated into a semiconducting donor:acceptor host system. Following this approach, this work demonstrates a proof‐of‐concept for a visible‐to‐SWIR broadband OPD, approaching (and, to some extent, even exceeding) state‐of‐the‐art performance across critical photodetector metrics. The trap‐doping approach is, even though only a proof‐of‐concept currently, broadly applicable to various spectral windows. It represents a new modality for engineering photodetection using the unconventional strategy of turning a limitation into a feature.

AbstractOrganic photovoltaics (OPV) have attracted great interest as a solar cell technology with appealing mechanical, aesthetical, and economies‐of‐scale features. To drive OPV toward economic viability, low‐cost, large‐scale module production has to be realized in combination with increased top‐quality material availability and minimal batch‐to‐batch variation. To this extent, continuous flow chemistry can serve as a powerful tool. In this contribution, a flow protocol is optimized for the high performance benzodithiophene–thienopyrroledione copolymer PBDTTPD and the material quality is probed through systematic solar‐cell evaluation. A stepwise approach is adopted to turn the batch process into a reproducible and scalable continuous flow procedure. Solar cell devices fabricated using the obtained polymer batches deliver an average power conversion efficiency of 7.2 %. Upon incorporation of an ionic polythiophene‐based cathodic interlayer, the photovoltaic performance could be enhanced to a maximum efficiency of 9.1 %.

We report the controlled preparation of water processable nanoparticles (NPs) employing the push-pull polymer PCDTBT and the fullerene acceptor PC71BM in order to enable solar cell processing using eco-friendly solvent (i.e. water). The presented method provides the possibility to separate the formation of the active layer blend and the deposition of the active layer into two different processes. For the first time, the benefits of aqueous processability for the high-potential class of push-pull polymers, generally requiring high boiling solvents, are made accessible. With our method we demonstrate excellent control over the blend stoichiometry and efficient mixing. Furthermore, we provide visualization of the nanomorphology of the different NPs to obtain structural information down to ~2 nm resolution using advanced analytical electron microscopy. The imaging directly reveals very small compositional demixing in the PCDTBT:PC71BM blend NPs, in the size range of about <5 nm, indicating fine mixing at the molecular level. The suitability of the proposed methodology and materials towards the aspects of eco-friendly processing of organic solar cells is demonstrated through a processing of lab scale NPs solar cell prototypes reaching a power conversion efficiency of 1.9%.

Abstract Document bibliographic coupling analysis was applied to a dataset of 23,953 journal papers in order to provide an overview of topics that were present in the academic literature on organic solar cells (OSCs) and perovskite solar cells (PvSCs) during the period of 2008–2017. Ten yearly networks were constructed by linking documents based on shared references, and clusters of closely connected papers were identified as the representations of topics in OSC and PvSC research. Next, 1,437 document titles were read in order to label the 80 most relevant clusters, together containing 11,352 papers. The resulting cluster labels show that research focused on bulk heterojunction polymer/fullerene solar cells in the beginning of the studied period, that OSC material development topics became more important in the middle of the decade, and that continued efforts to improve power conversion efficiencies spurred the rise of new concepts like PvSCs, fullerene-free OSCs, and ternary OSCs in the later years. While traditional literature reviews are valuable for in-depth reviews of specific topics, bibliometric reviews based on document bibliographic coupling are better suited for broad research synthesis. By leveraging the relationships present within the body of literature, they can objectively present the topic structure of large research domains.

This paper reviews the available life cycle analysis (LCA) literature on organic photovoltaics (OPVs). This branch of OPV research has focused on the environmental impact of single-junction bulk heterojunction polymer solar cells using a P3HT/PC60BM active layer blend processed on semi-industrial pilot lines in ambient surroundings. The environmental impact was found to be strongly decreasing through continuous innovation of the manufacturing procedures. The current top performing cell regarding environmental performance has a cumulative energy demand of 37.58 MJp m−2 and an energy payback time in the order of months for cells having 2% efficiency, thereby rendering OPV cells one of the best performing PV technologies from an environmental point of view. Nevertheless, we find that LCA literature is lagging behind on the main body of OPV literature due to the lack of readily available input data. Still, LCA research has led us to believe that in the quest for higher efficiencies, environmental sustainability is being disregarded on the materials' side. Hence, we advise the scientific community to take the progress made on environmental sustainability aspects of OPV preparations into account not only because standard procedures put a bigger strain on the environment, but also because these methods may not be transferrable to an industrial process. Consequently, we recommend policy makers to subsidize research that bridges the gaps between fundamental materials research, stability, and scalability given that these constraints have to be fulfilled simultaneously if OPVs are ever to be successful on the market. Additionally, environmental sustainability will have to keep on being monitored to steer future developments in the right direction.

Abstract P3HT:PCBM blends applied as active layers for bulk heterojunction organic solar cells generally show unstable morphologies upon prolonged thermal annealing, severely limiting the lifetime of the devices. As such, the thermodynamic instability of the blend is a limiting factor in the overall performance of organic photovoltaics, and a strong disadvantage in the fierce competition with other photovoltaic technologies. This paper shows whether different blend preparation conditions and intrinsic structural changes in the side chains of poly(3-alkylthiophene) (P3AT) derivatives can influence the thermal stability of the resulting solar cells. A combination of Bright Field Transmission Electron Microscopy (BFTEM) and the analysis of Selected Area Electron Diffraction (SAED) patterns revealed that the investigated preparation conditions do not really affect the thermal stability, whereas the introduction of a small ratio (10%) of specific functional moieties in the side chains of random P3AT copolymers does improve the thermal stability significantly. It was demonstrated that demixing of the blend components upon prolonged thermal annealing is strongly delayed in the functionalized P3AT:PCBM blends. The enhanced thermal stability was confirmed by in-situ monitoring of the short circuit current of organic solar cells based on the respective active layers. The introduction of functionalized side chains hence represents an attractive approach to increase the operational stability of organic photovoltaics based on the bulk heterojunction concept.

Conjugated polymers and small molecules based on alternating electron-donating (D) and electron-accepting (A) building blocks have led to state-of-the-art organic solar cell materials governing efficiencies beyond 10%. Unfortunately, the connection of D and A building blocks via cross-coupling reactions does not always proceed as planned, which can result in the generation of side products containing D-D or A-A homocoupling motifs. Previous studies have reported a reduced performance in polymer and small molecule solar cells when such defect structures are present. A general consensus on the impact of homocouplings on device performance is, however, still lacking as is a profound understanding of the underlying causes of the device deterioration. For differentiating the combined effect of molecular weight and homocouplings in polymer solar cells, a systematic study on a small molecule system (DTS(FBBTh2)2) is presented. The impact of homocouplings on nanomorphology, thermal, and electro-optical properties...

AbstractPhosphonium‐based polythiophene conjugated polyelectrolytes (CPEs) with three different counterions (dodecylsulfate, octylsulfate and perfluorooctane sulfonate) are synthesized to determine how the nature of the counterion affects the thermal properties, the self‐assembly in thin films and the performance as the cathode interfacial layer in polymer solar cells (PSCs). The counterion has a significant effect on the thermal properties of the CPEs, affecting both their glass transition and crystalline behaviour. Grazing‐incidence wide‐angle X‐ray scattering studies also indicate that changing the nature of the counterion influences the microstructural organization in thin films (face‐on versus edge‐on orientation). The affinity of the CPEs with the underlying photoactive layer in PSCs is highly correlated with the counterion species. Finally, in addition to an increase of the power conversion efficiency of ca 15% when using these CPEs as cathode interfacial layers in PSCs, a higher device stability is noted, compared to a reference device with a calcium interlayer. © 2020 Society of Industrial Chemistry