Acceptor-based Organic Solar Cells Research Articles

Dimerized Nonfused Electron Acceptor Based on a Thieno[3,4-c]pyrrole-4,6-dione Core for Organic Solar Cells.

In this work, the first dimerized nonfused electron acceptor (NFEA), based on thieno[3,4-c]pyrrole-4,6-dione as the core, has been designed and synthesized. The dimerized acceptor and its single counterpart exhibit similar energy levels but different absorption spectra due to their distinct aggregation behavior. The dimerized acceptor-based organic solar cells (OSCs) demonstrate a higher power conversion efficiency of 11.05%, accompanied by enhanced thermal stability. This improvement is attributed to the enhancement of the short-circuit current density and fill factor, along with an increase in the glass transition temperature. Characterizations of exciton dynamics and film morphology reveal that a dimerized acceptor-based device possesses an enhanced exciton dissociation efficiency and a well-established charge transport pathway, explaining its improved photovoltaic performance. All these results indicate that the dimerized NFEA as a promising candidate can achieve efficiency-stability-cost balance in OSCs.

2,5-dichloro-3,4-diiodothiophene as a versatile solid additive for high-performance organic solar cells

Volatile solid additive (SA) with diverse chemical structures has been proved to be an alternative to prepare high-efficient organic solar cells (OSCs). While, various organic photovoltaic materials raise challenges in selecting suitable SAs during the device optimization. Herein, based on the simple and commonly used thiophene unit, we proposed a perhalogenated 2,5-dichloro-3,4-diiodothiophene (SA-T5) as the SA for the device optimizations. As results, the capacity of SA-T5 in optimizing molecular packings of small-molecular acceptor L8-BO, dimeric acceptor DL8, and polymeric acceptor PY-DT based blend films has been demonstated. Their optimized morphologies assisted by SA-T5 contribute to the suppressed charge recombination and enhanced charge transportation, resulting in their improved short-circuit current density and fill factor, simultaneously. In comparsion with as-cast devices, SA-T5-treated devices showed significantly improved power conversion efficiencies (PCEs) of 18.8 % for PM6:L8-BO, 17.8 % for PM6:DL8, and 17.6 % for PM6:PY-DT, respectively. Furthermore, the potential of SA-T5 in obtaining high-performance ternary OSCs was also confirmed, for example, a 18.1 % efficiency was realized in PM6:DL8:L8-BO based device. To our knowledge, the performance of these SA-T5-treated devices reaches the best results of different acceptor-based OSCs. Importantly, this work provides a rare and successful case to develop versatile SAs for high-performance OSCs.

Crossbreeding Effect of Chalcogenation and Iodination on Benzene Additives Enables Optimized Morphology and 19.68% Efficiency of Organic Solar Cells.

Volatile solid additives have attracted increasing attention in optimizing the morphology and improving the performance of currently dominated non-fullerene acceptor-based organic solar cells (OSCs). However, the underlying principles governing the rational design of volatile solid additives remain elusive. Herein, a series of efficient volatile solid additives are successfully developed by the crossbreeding effect of chalcogenation and iodination for optimizing the morphology and improving the photovoltaic performances of OSCs. Five benzene derivatives of 1,4-dimethoxybenzene (DOB), 1-iodo-4-methoxybenzene (OIB), 1-iodo-4-methylthiobenzene (SIB), 1,4-dimethylthiobenzene (DSB) and 1,4-diiodobenzene (DIB) are systematically studied, where the widely used DIB is used as the reference. The effect of chalcogenation and iodination on the overall property is comprehensively investigated, which indicates that the versatile functional groups provided various types of noncovalent interactions with the host materials for modulating the morphology. Among them, SIB with the combination of sulphuration and iodination enabled more appropriate interactions with the host blend, giving rise to a highly ordered molecular packing and more favorable morphology. As a result, the binary OSCs based on PM6:L8-BO and PBTz-F:L8-BO as well as the ternary OSCs based on PBTz-F:PM6:L8-BO achieved impressive high PCEs of 18.87%, 18.81% and 19.68%, respectively, which are among the highest values for OSCs.

Building-up relations between intra- and intermolecular interactions, miscibility, and performance for low-cost, efficient fully non-fused acceptor-based organic solar cells

Although the molecular electronic forces (e.g., intra- and intermolecular interactions) within active layers largely govern the device performance of organic solar cells (OSCs), they are complicated and less understood. In this study, we have synthesized two low-cost isomeric non-fused acceptors (TT-Naph1 and TT-Naph2) with 1-naphthyl and 2-naphthyl aromatic chains, respectively and quantified the molecular interaction−photovoltaic performance relationship. Benefiting from the enhanced dipole moment, TT-Naph2 possesses a strong dipole–dipole intermolecular interaction, while the improved backbone planarity endows TT-Naph1 with a strengthened intramolecular charge-transfer effect, which can regulate the desired blend morphology with the D18 donor polymer as a result of its low miscibility with D18. Less miscible nanostructures are more pronounced in the layer-by-layer (LBL) systems than bulk heterojunction (BHJ) ones, increasing the power conversion efficiencies (PCEs) in the sequence of TT-Naph2 BHJ < TT-Naph1 BHJ < TT-Naph2 LBL < TT-Naph1 LBL. Notably, a ternary LBL OSC based on TT-Naph1 achieved remarkable PCE of 18.41%, one of the top values for LBL-type OSCs. Our findings provide insights into the controlling effect of intra- and intermolecular interactions on the active layers for efficient non-fused acceptor-based OSCs.

Charge Carrier Dynamics in Non-Fullerene Acceptor-Based Organic Solar Cells: Investigating the Influence of Processing Additives Using Transient Absorption Spectroscopy.

In this study, we present a comprehensive investigation into the charge generation mechanism in bulk-heterojunction organic solar cells employing non-fullerene acceptors (NFAs) both with and without the presence of processing additives. While photovoltaic devices based on Y6 or BTP-eC9 have shown remarkable power conversion efficiencies, the underlying charge generation mechanism in polymer:NFA blends remains poorly understood. To shed light on this, we employ transient absorption (TA) spectroscopy to elucidate the charge transfer pathway within a blend of the donor polymer PM6 and NFAs. Interestingly, the charge carrier lifetimes of neat Y6 and BTP-eC9 are comparable, both reaching up to 20 ns. However, the PM6:BTP-eC9 blend exhibits substantially higher charge carrier generation and a longer carrier lifetime compared to PM6:Y6 blend films, leading to superior performance. By comparing TA data obtained from PM6:Y6 or PM6:BTP-eC9 blend films with and without processing additives, we observe significantly enhanced charge carrier generation and prolonged charge carrier lifetimes in the presence of these additives. These findings underscore the potential of manipulating excited species as a promising avenue for further enhancing the performance of organic solar cells. Moreover, this understanding contributes to the advancement of NFA-based systems and the optimization of charge transfer processes in polymer:NFA blends.

Enhancement of Power Conversion Efficiency of Non-Fullerene Organic Solar Cells Using Green Synthesized Au-Ag Nanoparticles.

Organic-based photovoltaics are excellent candidates for renewable energy alternatives to fossil fuels due to their low weight, low manufacturing cost, and, in recent years, high efficiency, which is now above 18%. However, one cannot ignore the environmental price of the fabrication procedure due to the usage of toxic solvents and high-energy input equipment. In this work, we report on the enhancement of the power conversion efficiency non-fullerene organic solar cells by incorporating green synthesised Au-Ag nanoparticles, using onion bulb extract, into the hole transport layer poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS) of Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3 fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]: 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (PTB7-Th: ITIC) bulk-heterojunction organic solar cells. Red onion has been reported to contain quercetin, which serves as a capping agent that covers bare metal nanoparticles, thus reducing exciton quenching. We found that the optimized volume ratio of NPs to PEDOT: PSS is 0.06:1. At this ratio, a 24.7% enhancement in power conversion efficiency of the cell is observed, corresponding to a 9.11% power conversion efficiency (PCE). This enhancement is due to an increase in the generated photocurrent and a decrease in the serial resistance and recombination, as extracted from the fitting of the experimental data to a non-ideal single diode solar cell model. It is expected that the same procedure can be applied to other non-fullerene acceptor-based organic solar cells, leading to an even higher efficiency with minimal effect on the environment.

A follow-up investigation: Organic solar cells based on chalcogenophene-Thieno[3,4-c]pyrrole-4,6-dione-chalcogenophene containing random conjugated polymers

Herein, we conducted a comprehensive comparison between fullerene (FA) and non-fullerene (NFA) acceptor-based organic solar cells (OSCs). Four chalcogenophene-thieno[3,4-c]pyrrole-4,6-dione-chalcogenophene containing random conjugated polymers were used as donor materials in the active layer of OSCs. Our results showed that selenophene-containing polymer P1 performed better with ITIC (NFA), resulting in a PCE of 7.94%, whereas when blended with PC71BM (FA), it performed a PCE of 7.62%. One other selenophene containing polymer P2 achieved a PCE of 1.96% with ITIC and 6.30% with PC71BM. P3 and P4 (furan-containing polymers) exhibited 1.40% and 1.32% with ITIC, whereas when blended with PC71BM, 1.46% and 2.45%, respectively.

Designing of asymmetric non-fullerene based acceptor materials by re-modification of π-spacers with PCE > 20 % for organic solar cell

In this study, four new organic molecules with different compounds called the π-linkers, embedded between dithieno[3,2-b:2′,3′-d]pyrrole donor core and dicyanoindanone end-capped acceptor molecules were theoretically designed and analyzed to be proposed as the new asymmetric small molecule acceptors (SMAs) with the highest power conversion efficiency (PCE) in non-fullerene organic solar cells (OSCs). The charge transportability, stability, electronic, and optical properties were calculated based on the molecular structure analysis with the density functional theory (DFT) approach and the results were compared with dithieno[3,2-b:2′,3′-d]pyrrole based asymmetric SMAs named TPIC with dicyanoindanone end-capped groups. As a result of efficient π-π stacking, the energy differences decreased by 17.71 % and the charge transport increased by 49 %. The molecules showed broad absorption spectra ranging from 400 nm to 1200 nm with significant redshifted wavelength towards the near IR region coupled with low excitation energies. The LUMO energy level of the external donor PM7 is aligned with HOMO energy levels of the architecture acceptor molecules via bandgap tuning and the open-circuit voltage (VOC) increased from 1.002 V to 2.44 V for the highest combination PM7:C4. The efficacious charge transport properties, high excitonic generation with low excitation energies, facile separation, and increased VOC via band-gap tunning; all four designed molecules achieved a champion PCE ranging from 20 % to 34 % at short circuit current density 9, 12, and 15 mA cm−2. The molecules exhibited the highest PCE value for any asymmetric non-fullerene small molecule acceptor-based organic solar cells.

Role of the energy offset in the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

The quantum yield of long-range spatial dissociation of electron–hole pairs decreased with a decrease in the energy offset between the excited and charge-transfer states, leading to a threshold that can ensure high charge photogeneration efficiency.

Recent progress in non-fused ring electron acceptors for high performance organic solar cells

The recent progress in non-fused ring electron acceptor-based organic solar cells has been reviewed from the perspective of material design strategies.

Thermal annealing effect on non-fused ring acceptor based bulk heterojunction investigated by transient absorption spectroscopy

Organic solar cells based on non-fused ring fullerene acceptors have more potential to realize low-cost fabrication and commercialization. However, the power conversion efficiencies (PCEs) are much lower than those of the fused-ring non-fullerene acceptors. Thermal annealing treatment as a performance optimization way is performed on PBDB-T blend film based on fully non-fused ring acceptor, o-4TBC-2F, and the PCE is enhanced from 7.43% to 10.26%. Herein, transient absorption spectroscopy measurement is conducted to unveil the thermal annealing effect in the perspective of ultrafast dynamics. Two types of J-aggregations are present in the o-4TBC-2F film. Thermal annealing treatment induces more o-4TBC-2F molecules in the form of strong intermolecular coupling J-aggregation with long-range order. The aggregation is less heterogeneous, leading to a slow yet efficient hole transfer process, and the generated hole polarons are delocalized and long-lived. The dynamics results well explained the improved performance after thermal annealing treatment. The observations also show that the aggregation in blends can optimize the charge generation, which can be beneficial for designing highly efficient non-fused ring acceptor-based organic solar cells.

Synergy of backbone and end-group engineering for efficient non-fused-ring asymmetric electron acceptor-based organic solar cells

Two silole-containing non-fused-ring asymmetric small-molecule electron acceptors were designed and synthesized with an A–D1–D2–A configuration, featuring the assembly of dithienosilole and isooctoxy indenothiophene.

Novel wide bandgap benzodithiophene-based polymer donors with electron-withdrawing indolin-2-one side chains for efficient organic solar cells with high open circuit voltage

Two new π-conjugated polymers with donor backbone and π-conjugated indolin-2-one side chains, PBDTTI and PBDTTIF, are designed and synthesized as wide bandgap donors for non-fullerene acceptor-based organic solar cells (OSCs). The monomers containing electron accepting indolin-2-one side chains, (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-1-methylindolin-2-one (M1) and (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-5-fluoro-1-methylindolin-2-one (M2), can be easily synthesized via Knoevenagel condensation between 2,5-dibromothiophene-3-carbaldehyde and 2-oxindole or 5-fluoro-2-oxindole, respectively. Stille coupling polymerization of the electron donating benzodithiophene (BDT)-containing monomer 1,1′-[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl]bis [1,1,1-trimethylstannane] and M1 or M2 produced PBDTTI or PBDTTIF, respectively. The strong electron accepting π-conjugated 1-methylindolin-2-one and 5-fluoro-1-methylindolin-2-one side chains can achieve low-lying HOMO energy levels of −5.59 eV for PBDTTI and −5.60 eV for PBDTTIF, which is beneficial for realizing high open circuit voltage (VOC) of the resulting OSCs. On the other hand, since the electron acceptor units are on the side chains, the polymer backbone containing only electron donor units could maintain wide bandgaps of 1.91 eV and 1.89 eV for PBDTTI and PBDTTIF, respectively. When PBDTTI and PBDTTIF were used as donors and a small bandgap non-full acceptor ITIC as an acceptor, the resulting OSCs devices achieved VOC of 0.97 and 1.00 V, short circuit current densities (JSC) of 15.60 and 13.70 mAcm-2, and fill factors (FF) of 0.60 and 0.59, resulting in power conversion efficiencies of 8.00 and 7.70%, respectively.

Understanding what determines the organic solar cell stability

Understanding the fundamental origin of morphological degradation in non-fullerene acceptor-based organic solar cells is challenging. In the April 2021 issue of <i>Nature Materials</i>, Ghasemi et al. reveal that the most thermodynamically unstable and low-miscibility systems with high Flory-Huggins interaction parameter (χ) exhibit the most kinetically stable (low diffusion) morphology for superior device operation stability under thermal stress.

Understanding the low voltage losses in high-performance non-fullerene acceptor-based organic solar cells

A detailed voltage loss analysis of organic solar cells based on the record breaking material combination D18:Y6 and their fullerene-based counterparts (D18:PC70BM) cast light on the superior performance of state-of-the-art non-fullerene acceptors.

Correction: Indole-based A–DA′D–A type acceptor-based organic solar cells achieve efficiency over 15% with low energy loss

Correction for ‘Indole-based A–DA′D–A type acceptor-based organic solar cells achieve efficiency over 15% with low energy loss’ by Yu Chen et al., Sustainable Energy Fuels, 2020, 4, 6203–6211, DOI: 10.1039/D0SE01343A.

Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells

To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3–6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).

Comprehensive and Comparative Analysis of Photoinduced Charge Generation, Recombination Kinetics, and Energy Losses in Fullerene and Nonfullerene Acceptor-Based Organic Solar Cells.

In this work, a unique comprehensive and comparative analysis of photoinduced charge generation, recombination kinetics, and energy losses has been carried out to study the effect of different fullerene-based acceptors (FBAs) and nonfullerene acceptors (NFAs) on the performance of organic solar cells (OSCs). For this, different FBAs, specifically ICBA, PC60BM, and PC70BM, and NFAs, namely, ITIC, IT-4F, and IEICO-4F, were employed independently along with a particular donor polymer, PBDB-T, to fabricate bulk heterojunction OSCs and their performances have been compared. This donor molecule is known to give similar power conversion efficiency (PCE) with FBAs and NFAs and hence is ideal for comparative studies. The origin of the higher PCE of NFA-based OSCs vs FBA-based OSCs is analyzed in terms of spectral coverage, charge generation, recombination, and energy loss. It is found that the energy loss (ΔEloss) is ∼0.8 to 1 eV for FBA-based OSCs, while it is 0.5-0.7 eV for NFA-based OSCs. Interestingly, for the PBDB-T:IEICO-4F-based system, energy losses due to charge generation (ΔECT) are ∼0 eV and therefore this system has minimum ΔEloss among all of the studied devices. Providing a systematic, comprehensive, and comparative outlook, our study may further be extended to new upcoming NFA systems and beyond the donor system used in this work.

Alkylthio-substitution on wide bandgap conjugated polymers for non-fullerene acceptor-based organic solar cells

We investigated three conjugated polymers, PSBDB-T, P1SBDB-TS, and PSBDB-TS, having different numbers of alkylthio groups. The polymers showed a wide bandgap of 1.76–1.81 eV with broad absorption range from 300–700 nm and were used as donor polymers in organic photovoltaic (OPV) devices with the non-fullerene acceptor, IT-M. By increasing the number of alkythio groups in their chemical structures, the HOMO levels of the polymers down-shifted as −5.46 eV, −5.51 eV and −5.56 eV, and consequently, the VOC of the OPV cells gradually increased as 0.91 V, 0.92 V and 0.94 V for PSBDB-T, P1SBDB-TS, and PSBDB-TS, respectively. P1SBDB-TS having three alkylthio groups (two for benzo[1,2-b:4,5-b']dithiophene and one for benzo[1,2-c:4,5-c’]dithiophene-4,8-dione unit) showed the best power conversion efficiency of 6.88% among three polymers due to its high crystallinity and low bimolecular recombination in bulk heterojunction system.

Indole-based A–DA′D–A type acceptor-based organic solar cells achieve efficiency over 15 % with low energy loss

In order to increase the power conversion efficiency (PCE) of organic solar cells, developing high-performance non-fullerene small molecule acceptors is important for efficient charge generation and small energy loss.