Enhancing the performances of all-small-molecule ternary organic solar cells via achieving optimized morphology and 3D charge pathways

With the emergence of non-fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of all-small-molecule organic solar cells (ASM-OSCs) have been significantly improved. However, due to the strong crystallinities of small molecules, it is much more challenging to obtain the ideal phase separation morphology and efficient charge transport pathways for ASM-OSCs. Here, a high-efficiency ternary ASM-OSC has been successfully constructed based on H11/IDIC-4F system by introduction of IDIC with a similar backbone as IDIC-4F but weak crystallinity. Notably, the addition of IDIC has effectively suppressed large-scale phase aggregation and optimized the morphology of the blend film. More importantly, the molecular orientation has also been significantly adjusted, and a mixed face-on and edge-on orientation has formed, thus establishing a more favorable three-dimensional (3D) charge pathways in the active layer. With these improvements, the enhanced short-circuit current density (J_SC) and fill factor (FF) of the ternary system have been achieved. In addition, because of the high lowest unoccupied molecular orbital (LUMO) energy level of IDIC as well as the alloyed structure of the IDIC and IDIC-4F, the promoted open circuit voltage (V_OC) of the ternary system has also been realized.     

Diketopyrrolopyrrole-based conjugated polymers as additives to optimize morphology for polymer solar cells

Novel random copolymers for optimizing the morphology of the active layer for high performance organic photovoltaic devices have been demonstrated. Three ternary random copolymers PTBDTDPPSi CN(3/7), PTBDTDPPSi CN(5/5), PTBDTDPPSi CN(7/3) were prepared by polymerization of electron-donating thienyl-substituted benzodithiophene(TBDT) with 2,5-bis[8-(1,1,3,3,5,5,5-heptamethyltrisiloxane-3-yl)octly]-pyrrolo[3,4-c]pyrrole-1,4-dione(DPPSi) and 2,5-dio[5-(5-cyano-5,5-dimethyl-pentyl)]-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione(DPPCN) of different ratios. The DPPCN block can well-tune the light absorption and molecular packing, while the DPPSi block is in favor of enhancing the charge mobility. And the formation of organic Si―O―Si networks is beneficial to stabilize the morphology of the active layer. These new copolymers have narrow bandgaps and broaden visible light absorption from 500 nm to 1000 nm. Careful balance of the contents of the trimethoxysilyl group and the cyano group can well-tune the surface energy and morphology of the copolymers. Incorporation of these novel copolymers as additives into the blend of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C60-butyric acid methyl ester(PC_(61)BM) is found to effectively broaden the light absorption, improve the compatibility and morphology of the active layer. As a result, some devices with certain ratios of these copolymers as additives achieve the enhanced efficiency compared with the device based on pristine P3HT:PC_(61)BM.     

Additive-free non-fullerene organic solar cells with random copolymers as donors over 9% power conversion efficiency

The PBDB-TBT1:ITIC-based device obtains PCE of 9.09%, and is insensitive to additive and thermal annealing, and forms microstructural morphology.     

Ternary organic solar cells based on polymer donor,polymer acceptor and PCBM components

In this work,ternary organic solar cells(OSCs)combining a fullerene derivative PC71BM with a nonfullerene acceptor N2200-F blended with a polymer donor PM6 were reported.Compared with the binary systems,the highest power conversion efficiency(PCE)of 8.11%was achieved in ternary solar cells with 30 wt%N2200-F content,mainly due to the improved short-circuit current density(Jsc)and fill factor(FF).Further studies showed that the improved Jsc could attribute to the complementary abso rption of the two acceptors and the enhanced FF was originated from the higher hole mobility and the fine-tuned morphology in the ternary system.These results demonstrate that the combination of fullere ne and nonfullerene acceptors in ternary organic solar cells is a promising approach to achieve high-performance OSCs.     

Enhanced efficiency of ternary organic solar cells by doping a polymer material in P3HT:PC61BM

Doping a low‐bandgap polymer material (PDTBDT‐DTNT) as a complementary electron donor in poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyricacid methyl ester (PC₆₁BM) blend is experimented to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The PCE of OSCs was increased from 3.19% to 3.75% by doping 10 wt% PDTBDT‐DTNT, which was 17.55% higher than that of the OSCs based on binary blend of P3HT:PC₆₁BM (host cells). The short‐circuit current density (J_sc) was increased to 10.11 mA·cm⁻² compared with the host cells. Although the PCE improvement could partly be attributed to more photon harvest for complementary absorption of 2 donors by doping appropriate PDTBDT‐DTNT, the promotion of charge separation and transport as well as the suppression of charge recombination due to a matrix of cascade energy levels is also important. And the better morphology of the active layer films is beneficial to the optimized performance of ternary devices.     

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

The power conversion efficiency of an organic solar cell has now exceeded the 10% mark, which is a significant improvement in the last decade. This has been made possible due to the development of low-band-gap polymers with tunable electron affinity, ionization potential, solubility, and miscibility with the fullerene acceptor, and the improved understanding of the factors affecting the critical device parameters such as the V_OC and the J_SC. This review examines the latest strategies, results, and trends that have evolved in the design of solar cells with better efficiency and durability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Effect of additives on the photovoltaic properties of organic solar cells based on triphenylamine-containing amorphous molecules

Photovoltaic performance of the organic solar cells(OSCs)based on 2-((5′-(4-((4-((E)-2-(5′-(2,2-dicyanovinyl)-3′,4-dihexyl-2,2′-bithiophen-5-yl)vinyl)phenyl)(phenyl)amino)styryl)-4,4′-dihexyl-2,2′-bithiophen-5-yl)methylene)malononitrile(L(TPAbTV-DCN))as donor and PC70BM as acceptor was optimized using 0.25 vol%high boiling point solvent additive of1-chloronaphthalene(CN),1,6-hexanedithiol(HDT),or 1,8-diodooctane(DIO).The optimized OSC based on L(TPA-bTVDCN)–PC70BM(1:2,w/w)with 0.25 vol%CN exhibits an enhanced power conversion efficiency(PCE)of 2.61%,with Voc of0.87 V,Jsc of 6.95 mA/cm2,and FF of 43.2%,under the illumination of 100 mW/cm2 AM 1.5 G simulated solar light,whereas the PCE of the OSC based on the same active layer without additive is only 1.79%.The effect of the additive on absorption spectra and the atomic force microscopy images of L(TPA-bTV-DCN)–PC70BM blend films were further investigated.The improved efficiency of the device could be ascribed to the enhanced absorption and optimized domain size in the L(TPA-bTV-DCN)–PC70BM blend film.     

A near-infrared porphyrin-based electron acceptor for non-fullerene organic solar cells

A star-shaped electron acceptor with porphyrin as core and rhodanine-benzothiadiazole as end groups linked with ethynyl units was developed for non-fullerene solar cells, in which a PCE of 1.9% with broad photo response was achieved when combining with a diketopyrrolopyrrole-polymer as electron donor.     

Water soluble amino grafted silicon nanoparticles and their use in polymer solar cells

Stable aqueous amino-grafted silicon nanoparticles(SiNPs-NH2) were prepared via one-pot solution method. By grafting amino groups on the particle surface, the dispersion of SiNPs in water became very stable and clear aqueous solutions could be obtained. By incorporating SiNPs-NH2 into the hole transport layer of poly(3,4-ethylenedioxythiophene)/polystyrene sulfonic acid(PEDOT:PSS), the performance of polymer solar cells composed of poly[2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylene vinylene](MEH-PPV):[6,6]-phenyl-C61-butyric acid methyl ester(PCBM) as active layer can be improved. SiNPs-NH2 are dispersed uniformly in the PEDOT:PSS solution and help form morphologies with small-sized domains in the PEDOT:PSS film. SiNPs-NH2 serve as screens between conducting polymer PEDOT and ionomer PSS to improve the phase separation and charge transport of the hole transport layer. As a result, the sheet resistance of PEDOT:PSS thin films is decreased from(93 ± 5) × 105 to(13 ± 3) × 105 ?/□. The power conversion efficiency(PCE) of polymer solar cells was thus improved by 9.8% for devices fabricated with PEDOT:PSS containing 1 wt% of SiNPs-NH2, compared with the devices fabricated by original PEDOT:PSS.     

Ethynyl-linked perylene bisimide based electron acceptors for non-fullerene organic solar cells

Star-shaped electron acceptors based on perylene bisimide as end groups and spiro-aromatic core linked with ethynyl units were developed for nonfullerene solar cells. Ethynyl linkers are able to improve the planarity of conjugated backbone, resulting in enhanced electron mobility and power conversion efficiency in solar cells.     

Morphology and mechanical properties of poly(ether-imide)-modified polycyanurates

A high temperature thermosetting bisphenol A dicyanate (BADCy) was modified with a novel thermoplastic poly(ether-imide) (PEI) at various compositions. Fourier transform infrared spectroscopy was used to measure the conversion of BADCy. Curing kinetics of the BADCy/PEI blend were studied by the dynamic differential scanning calorimeter method. Morphologies of PEI-modified polycyanurates were investigated by scanning electron microscopy (SEM). It was found that the phase structures of the blends changed dramatically with the PEI content and molecular weight. The tensile results showed that the mechanical properties could be correlated with the morphologies. The blend with cocontinuous morphology had the highest tensile strength and elongation at break while the blend with ribbon-like morphology had the lowest one despite PEI molecular weight.     

Ternary organic solar cells:Improved optical and morphological properties allow an enhanced efficiency

The power co nversion efficiency(PCE) of OFQx-T:PC_(71)BM blend films reaches 7.59%.On this basis,ternary organic solar cells(OSCs) were fabricated with ITIC or PTB7-Th as the third component.The ternary OSCs with 50 wt% ITIC in acceptors exhibits an enhanced efficiency,from 7.59% to 8.17%.Also,the PCE of ternary OSCs with 50 wt% PTB7-Th in donors achieves 8.72%,which is 13% higher than that of binary OSCs.The PCE improvement of two ternary OSCs is mainly due to the increase of short-circuit current density(J_(SC)),which can be attributed to the complementary absorption spectra and improved film morphology.This work suggests that the selection of an appropriate third component plays a critical role in improving the PCE of ternary OSCs.     

Synthesis and characterization of benzobisthiazole based polymers as donor materials for organic solar cells

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Synthesis of new di-anchoring organic sensitizer based on quinoxaline acceptor for dye-sensitized solar cells

New quinoxaline-based organic sensitizer bearing di-anchoring group for dye-sensitized solar cells (DSSCs) was synthesized from diethyl 4,5-diaminophthaltate, in which was prepared under mild condition by using Takehito’s method. The synthesized sensitizer was compared with mono-anchoring sensitizer through absorption spectra, emission spectra, J-V curve, and IPCE spectra, indicating the di-anchoring group leads to a noticeable improvement of J_sc value owing to more efficient intramolecular charge transfer and channel number increment.     

Recent advances in intermixed phase of organic solar cells: Characterization,regulating strategies and device applications

Organic solar cells (OSCs) have unique advantages of low-cost solution processing, light weight, flexibility, and semitransparency, which is a promising photovoltaic technology. The intermixed phase plays a key role in determining the power conversion efficiencies (PCE) of OSCs. The intermixed phase is an amorphous region, where the donor and acceptor mix at the molecular level. Great efforts have been devoted to optimize the content and the composition of the intermixed phase. This perspective focuses on the functions of intermixed phase and elaborates the relationship between intermixed phase behavior and photophysical process, in particular, the exciton dissociation and charge transport. Then the characterization methods, including quantitative and qualitative characterizations, for the content and composition of intermixed phases are introduced. Meanwhile, this review also introduces the strategies to control the intermixed phase behavior, such as adjusting the miscibility between donor and acceptor, changing the ratio of donor to acceptor, regulating the crystallinity and so on. Moreover, representative examples are given and discussed to understand the key parameters on tuning the intermixed phase behavior. Finally, a future controlling and development of intermixed phase behavior is briefly outlooked, which may help to achieve high PCE of OSCs.     

Influence of different copolymer sequences in low band gap polymers on their performance in organic solar cells

The chemical design of a polymer can be tailored by a random or a block sequence of the comonomers in order to influence the properties of the final material. In this work, two sequences, PCPDTBT and F8BT (F8), were polymerized to form a block or a random copolymer. Differences between the various polymers were examined by exploring the surface topography and charge carrier mobility. A distinct surface texture and a higher charge carrier mobility was found for the block copolymer with respect to the other materials. Solar cells were prepared with polymer:PC₇₁BM blend active layers and the best performance of up to 2% was found for the block copolymer, which was a direct result of the fill factor. Overall, the sequences of different copolymers for solar cell applications were varied and a positive impact on efficiency was found when the block copolymer structure was utilized. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Novel small oligothiophene molecules with phenylene and naphthalene cores as promising absorber materials for organic solar cells

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