Non-fullerene Devices Research Articles

Contrastive manipulations on vertical stratifications by a fluorescent guest component in ternary nonfullerene and fullerene organic solar cells

Herein, a simple fluorescent material TPA2O (5,5′-((4′-(diphenylamino)-[1,1′-biphenyl]-2,6-diyl)bis(methaneylylidene))bis(1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione)) is designed and successively introduced into both polymer:nonfullerene and polymer:fullerene blends to construct ternary organic solar cells with an inverted structure of ITO/ZnO/active layer/MoO3/Ag. Remarkably improved power conversion efficiency by up to 25% (from 9.21% to 11.47%) and sufficiently enhanced photocurrent (from 21.01 to 25.25 mA cm−2) are realized in nonfullerene ternary devices by incorporating a small amount of TPA2O, while reduced PCE is found in ternary fullerene systems. Characterizations have proven that efficient Förster resonance energy transfer (FRET) exists between TPA2O and the host PTB7-Th donor. And the distinguishable device performance in different ternary systems is determined by the contrastive influences on lateral and longitudinal direction morphologies. Specifically, by incorporating TPA2O, the cross-section morphology is degenerated in ternary fullerene blends but stable in ternary nonfullerene blends. More interestingly, for vertical-section morphology, PTB7-Th is more enriched in the upper layers in ternary nonfullerene blends, which is close to hole-transport MoO3 and inducing more efficient hole transport. Whereas PTB7-Th is less concentrated within the top layers in ternary PC71BM-based blends. Moreover, remarkably enhanced thermal stability is observed in TPA2O-based nonfullerene devices, with 94% of the initial PCE after baking at 80 °C for 144 h, compared to 75% and 62% of IEICO-4F and PC71BM binary devices, respectively.

Reconciling models of interfacial state kinetics and device performance in organic solar cells: impact of the energy offsets on the power conversion efficiency.

Achieving the simultaneous increases in the open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) necessary to further increase the power conversion efficiency (PCE) of organic photovoltaics (OPV) requires a unified understanding of how molecular and device parameters affect all three characteristics. In this contribution, we introduce a framework that for the first time combines different models that have been used separately to describe the different steps of the charge generation and collection processes in OPV devices: a semi-classical rate model for charge recombination processes in OPV devices, zero-dimensional kinetic models for the photogeneration process and exciton dissociation and one-dimensional semiconductor device models. Using this unified multi-scale model in conjunction with experimental techniques (time-resolved absorption spectroscopy, steady-state and transient optoelectronic measurements) that probe the various steps involved in charge generation we can shed light on how the energy offsets in a series of polymer: non-fullerene devices affect the charge carrier generation, collection, and recombination properties of the devices. We find that changing the energy levels of the donor significantly affects not only the transition rates between local-exciton (LE) and charge-transfer (CT) states, but also significantly changes the transition rates between CT and charge-separated (CS) states, challenging the commonly accepted picture of charge generation and recombination. These results show that in order to obtain an accurate picture of charge generation in OPV devices, a variety of different experimental techniques under different conditions in conjunction with a comprehensive model of processes occurring at different time-scales are required.

Understanding the Interplay of Transport‐Morphology‐Performance in PBDB‐T‐Based Polymer Solar Cells

Polymer–polymer blends have been reported to exhibit exceptional thermal and ambient stability. However, power conversion efficiencies (PCEs) from devices using polymeric acceptors have been recorded to be significantly lower than those based on conjugated molecular acceptors. Herein, two organic nonfullerene bulk heterojunction (BHJ) blends ITIC:PBDB‐T and N2200:PBDB‐T, together with their fullerene counterpart, PCBM:PBDB‐T, are adopted to understand the effect of electron acceptors on device performance. Free charge carrier properties using time‐resolved microwave conductivity (TRMC) measurements are comprehensively investigated. The nonfullerene devices show an improved PCE of 10.06% and 6.65% in the ITIC‐ and N2200‐based cells, respectively. In comparison, the PCBM:PBDB‐T‐based devices yield a PCE of 5.88%. The optimal N2200:PBDB‐T produced the highest TRMC mobility, longest lifetime, and greatest free‐carrier diffusion length. It is found that such phenomena can be associated with the unfavorable morphology of the all‐polymer BHJ microstructure. In contrast, the solar cells using either the PCBM or ITIC acceptors display a more balanced donor and acceptor phase separation, leading to more efficient free‐carrier separation and transport in the operating device. By sacrificing efficiency for superior stability, it is shown that the improved structure in all‐polymer blend can deliver a more stable morphology under thermal stress.

Rational Design of 2D p–π Conjugated Polysquaraines for Both Fullerene and Nonfullerene Polymer Solar Cells

AbstractSo far, squaraine‐based polymer donors have been less explored for the bulk heterojunction (BHJ) polymer solar cells. In this work, two new p–π conjugated polysquaraines (PASQ‐BDT1 and PASQ‐BDT2) with different electron‐rich subunits on the squaraine skeleton are rationally developed as new polymer donors based on the 2D structure design concept. PASQ‐BDT2 with N,N‐diisobutylaniline subunits shows superior device performances in both fullerene and nonfullerene PSCs compared to PASQ‐BDT1 containing triphenylamine subunits, with power conversion efficiencies (PCEs) of 4.34% and 3.72%, respectively, owing to increased light‐harvesting ability and more favorable nanoscale morphology in the BHJ films. Moreover, its demonstrated that solvent effects can play an effective role in elevating the device performance. For the PASQ‐BDT2/PC71BM blend, the PCE is improved from 3.19% to 4.34% after solvent vapor annealing treatment, mainly attributed to the optimized film morphology and increased hole mobility. More interestingly, when the processing solvent for nonfullerene devices is changed from chlorobenzene to chloroform, a significant enhancement on PCE from 1.96% to 3.72% is yielded for the PASQ‐BDT2/ITIC blend, due to suppressed charge recombination and enhanced crystallinity in the chloroform‐processed BHJ films.

(Invited) Role of Domain Purity in Non-Fullerene Acceptor Based Organic Solar Cells

The field of non-fullerene organic solar cells (OSC) has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors with device efficiencies approaching 15%. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure–property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates a large pool of possible donor–acceptor combinations, which poses a daunting challenge for rationally screening material combinations. Resonant soft X-ray scattering (RSoXS) has proven to be a powerful technique to measure domain purity in OSC active layer blends. In polymer-fullerene systems, RSoXS has shown that higher average domain purity is generally correlated to device performance. A simple interpretation of RSoXS purity measurements is complicated by the consideration that real morphologies may be composed of three or more phases, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Whilst charges are understood to migrate to the aggregated domains due to a favorable electronic landscape, these charges might be effectively trapped in aggregates if there is insufficient number of percolating pathways in the amorphous mixed regions. Conversely, if the mixed regions have a composition much beyond the percolation threshold or if the volume faction of the mixed regions is large, then recombination is enhanced. Therefore, understanding the role of domain purity over different length scales and controlling the mixed amorphous phase for maximum charge creation and minimum recombination remains a principal challenge for non-fullerene OSCs. We will summarize several of our RSoXS studies that compare non-fullerene acceptor based organic solar cells having different primary chemical structures of the constituents and with significant processing variations. Our analysis suggests that mixed regions are often detrimental to good performance in these systems, typically having a pronounced negative impact on FF. Optimizing performance continues to be a delicate balance of possibly competing factors and the volume fraction of the ideal morphology remains to be determined. However, the miscibility of blend components, which might be probed using near-edge X-ray absorption spectroscopy as a screening tool, can guide expectations of achievable phase purity and ultimately the achievable performance of the active layer.

A Wide-Bandgap Conjugated Polymer Based on Quinoxalino[6,5-f ]quinoxaline for Fullerene and Non-Fullerene Polymer Solar Cells.

A wide-bandgap conjugated polymer, PNQx-2F2T, based on a ring-fused unit of quinoxalino[6,5-f ]quinoxaline (NQx), is synthesized for use as electron donor in polymer solar cells (PSCs). The polymer shows intense light absorption in the range from 300 to 740 nm and favorable energy levels of frontier molecular orbitals. The polymer has afforded decent device performance when blended with either fullerene-based acceptor [6,6]-phenyl-C71 -butylric acid methyl ester ([70]PCBM) or non-fullerene acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone-methyl))-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3-d: 2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (IT-M). The highest PCEs of 7.9% and 7.5% have been achieved for [70]PCBM or IT-M based PSCs, respectively. Moreover, the influence of molecular weight of PNQx-2F2T on solar cell performance has been investigated. It is found that fullerene-based devices prefer higher polymer molecular weight, while non-fullerene devices are not susceptible to the molecular weight of PNQx-2F2T. The device results are extensively explained by electrical and morphological characterizations. This work not only evidences the potential of NQx for constructing high-performance photovoltaic polymers but also demonstrates a useful structure-performance relationship for efficiency enhancement of non-fullerene PSCs via the development of new conjugated polymers.

Assessing the energy offset at the electron donor/acceptor interface in organic solar cells through radiative efficiency measurements

The radiative efficiency of non-fullerene devices is modulated by the energy offset, making electroluminescence a powerful tool for energy offset evaluation.

The effect of one- or two-dimensional conjugated benzodithiophene in polymeric donors on the device performance of non-fullerene organic solar cells

The device performance of bulk heterojunction organic solar cell significantly dependents on the molecular structure of donors or acceptors. Here we studied the differences in photovoltaic properties when employing polymeric donor consisting of one dimensional and two dimensional backbones blending with molecular acceptor with indacenodithiophene core. The necessity of polymeric donor with two dimensional backbone in non-fullerene devices based on indacenodithiophene-containing acceptors is demonstrated.

Side-Chain-Promoted Benzodithiophene-based Conjugated Polymers toward Striking Enhancement of Photovoltaic Properties for Polymer Solar Cells.

In this work, we have reported a highly efficient photovoltaic material, PBDTTz-SBP, by fine-tuning the side chains of the benzodithiophene (BDT) unit. With the replacement of alkoxy chains with alkylthio chains, a large increase in power conversion efficiency (PCE) was realized. Non-fullerene polymer solar cells (PSCs) without any post-treatment generate an optimal PCE of up to 12.09%, with a high VOC of 0.914 V, JSC of 18.52 mA cm-2, and fill factor of 71.43%. Notably, the efficiency of a PBDTTz-SBP-based solar cell was about 1.31-fold of the PCE (9.20%) of its counterpart based on the polymer, PBDTTz-BP, with alkoxy chains, indicating the striking modulation effect of side-chain engineering. Although VOC and JSC were lower than those of non-fullerene devices, the PSCs with PC71BM as the acceptor exhibited a fairly high fill factor of up to 76.69%, affording a moderate PCE. Our work reported a highly efficient polymer solar cell with a PCE of 12.09% and clearly demonstrated the great tuning effect of alkylthio chains on photovoltaic performance.

Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors

The field of non-fullerene organic solar cells has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure–property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates an extremely large pool of possible donor–acceptor combinations, which poses a daunting challenge for rational material screening and matching. This Review describes several important conceptual aspects of the emerging non-fullerene devices by highlighting key contributions that provided fundamental insights regarding rational material design, donor–acceptor pair matching, blend morphology control and the reduced voltage losses in non-fullerene organic solar cells. We also discuss the key challenges that need to be addressed to develop more-efficient non-fullerene organic solar cells.

Perylene Diimide-Based Zwitterion as the Cathode Interlayer for High-Performance Nonfullerene Polymer Solar Cells.

Nonfullerene polymer solar cells (PSCs) have earned widespread and intense interest on account of their properties such as tunable energy levels, potential for low-cost production processes, reduced energy losses, and strong light absorption coefficients. Here, a water-/alcohol-soluble zwitterion perylene diimide zwitterion (PDI-z) consisted of sulfobetaine ion as a terminal substituent and PDI as a conjugated core was synthesized. PDI-z was employed as an electron-transport layer (ETL) for nonfullerene PSC devices, obtaining an optimal power conversion efficiency (PCE) above 11.23%. Moreover, nonfullerene PSCs with the PDI-z cathode interlayer displayed an excellent performance on a large scale of interlayer thickness, which was compatible with printing fabrication techniques. Additionally, the PDI-z interlayer presented good ability of modifying high work function metals (for instance, Au, Cu, and Ag) in nonfullerene devices, and the Ag device displayed a PCE of 9.38%. This work provides a good alternative ETL for high-efficiency nonfullerene PSCs.

High-Performance Additive-/Post-Treatment-Free Nonfullerene Polymer Solar Cells via Tuning Molecular Weight of Conjugated Polymers.

In recent years, rapid advances are achieved in polymer solar cells (PSCs) using nonfullerene small molecular acceptors. However, no research disclosing the influence of molecular weight (Mn ) of conjugated polymer on the nonfullerene device performance is reported. In this work, a series of polymers with different Mn s are synthesized to systematically investigate the connection between Mn and performance of nonfullerene devices for the first time. It is found that the device performance improves substantially as the Mn increases from 12 to 38 kDa and a power conversion efficiency (PCE) as high as 10.5% is realized. It has to be noted this PCE is achieved without using any additives and post-treatments, which is among the top efficiencies of additive- and post-treatment-free PSCs. Most importantly, the variation trend of the optimal active layer thickness and morphology is significantly different from the device with fullerene as acceptor. The findings clarify the effect of Mn on the performance of nonfullerene PSCs, which would benefit further efficiency improvement of nonfullerene PSCs.

Influence of substrate temperature on the film morphology and photovoltaic performance of non-fullerene organic solar cells

A novel 1,8-naphthalimide-based small molecular acceptor (NI-T-C8) was designed and synthesized. The LUMO level of NI-T-C8 was high-lying, which would reduce the energy loss of WBG polymer PCDTBT-C12 based devices. When non-fullerene devices based on PCDTBT-C12: NI-T-C8 were spin-coated from substrates with a temperature of 30°C, the aggregation of NI-T-C8 would lead to an unoptimized film morphology and result in a low power conversion efficiency (PCE) of 2.68%. After elevating the substrates from 30 to 45°C, the domain size of blend films was decreased and a PCE of 3.61% with a high open circuit voltage (Voc) of 1.28V was acquired, but the aggregation of NI-T-C8 for blend films fabricated in 45°C based substrates was still influence the exciton dissociation in the active layers, lower Jsc values for NI-T-C8 based devices in comparison to PC71BM based ones were therefore acquired. This is the first single junction organic solar cell with a PCE higher than 3% could achieve a Voc higher than 1.2V. Our results also demonstrated that elevating the temperature of substrates could reduce the domain size of non-fullerene acceptors, which could provide a strategy to adjust the film morphology of active layer comprising easy-aggregating non-fullerene acceptor.