High-performance fullerene-free polymer solar cells with solution-processed conjugated polymers as anode interfacial layer

A series of conjugated polymers based on PFS derivatives with π-conjugated 5-(9H-fluoren-2-yl)-2,2′-bithiophene(fluorene-alt-bithiophene) backbones, namely PFS-3C, PFS-4C and PFS-6C, were synthesized for their use as the anode interfacial layers(AILs) in the efficient fullerene-free polymer solar cells(PSCs). Alkyl sulfonate pendants with different lengths of alkyl side chains were introduced in the three polymers in order to investigate the effect of the alkyl chain length on the anode modification. The obtained three polymers exhibited similar absorption bands and energy levels, indicating that changing the length of the alkyl side chains did not affect the optoelectronic properties of the conjugated polymers. Based on the PBDB-T:ITIC active layer, we fabricated the fullerene-free PSCs using the three polymers as the AILs. The superior performance of the fullerene-free PSC device was achieved when PFS-4C was used as the AIL, showing a power conversion efficiency(PCE) of 10.54%. The high performance of the PFS-4C-modified device could be ascribed to the high transmittance, suitable work-function(WF) and smooth surface of PFS-4C. To the best of our knowledge, the PCE obtained in the PFS-4C-modified device is among the highest PCE values in the fullerene-free PSCs at present. These results demonstrate that the PFS derivatives are promising candidates in serving as the AIL materials for high-performance fullerene-free PSCs.     

All-polymer solar cells with perylenediimide polymer acceptors

Four polymers based on perylenediimide co-polymerized with thiophene, bithiophene, selenophone and thieno[3,2-b]thiophene were investigated as the acceptor materials in all-polymer solar cells. Two different donor polymers, 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](PTB7-Th) and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-dodecyltetradecyl)-2,2′;5′,2″;5″,2′′′-quaterthiophen-5,5′′′-diyl)](Pff BT4T-2DT), with suitably complementary absorption spectra and energy levels were applied and examined. Among all different donor-acceptor pairs studied here, the combination of PTB7-Th:poly[N,N′-bis(1-hexylheptyl)-3,4,9,10-perylenediimide-1,6/1,7-diyl-alt-2,5-thiophene](PDI-Th) exhibited the best power conversion efficiency(PCE) of 5.13%, with open-circuit voltage(V_(oc)) = 0.79 V, short-circuit current density(J_(sc)) = 12.35 mA·cm~(-2) and fill-factor(FF) = 0.52. The polymer of PDI-Th acceptor used here had a regio-irregular backbone, conveniently prepared from a mixture of 1,6- and 1,7-dibromo-PDI. It is also noteworthy that neither additive nor posttreatment is required for obtaining such a cell performance.     

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.     

Self-assembled monolayers for interface engineering in polymer solar cells

Polymer solar cell (PSC) has been developed vastly in the past decade due to the advantages of low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, semitransparency, and solution processing. The performance and lifetime of PSCs are highly dependent on the properties of both active materials and their interfaces. The combination of the versatility of organic chemistry and the multitude of well-understood ligand–metal interactions allows self-assembled monolayers (SAMs) of organic molecules to direct control over the electronic and chemical properties at the inorganic–organic interfaces. Thus, SAMs are an attractive pathway to reconcile interfaces with tunable interface properties in PSCs. Hence, this review describes the application of SAMs in PSCs at different interfaces. First, SAMs as alternatives of traditional transporting materials to reduce the barrier at indium tin oxide (ITO)/active layer interface due to the ability of tuning work function of ITO electrode are discussed. Second, the modifications of metal oxide by SAMs to control the electrical contacts at transporting layer/active layer interface are described. Third, tailoring the properties of the donor/acceptor interface by SAMs to improve the performance of PSCs are summarized. Finally, perspectives and challenges are pointed out for developing highly stable and high-performance PSCs by applying SAMs.     

An acrylated fullerene derivative for efficient and thermally stable polymer solar cells

The continuous microstructure evolution occurring in active layers of polymer-fullerene solar cells is one of the main causes for their device instability. With aim to tackle it, this work developed a new polymerizable fullerene acceptor, [6,6]-phenyl-C₆₁-butyl acrylate (PC₆₁BA). It was found that PC₆₁BA has similar light-absorption properties and HOMO and LUMO energy levels as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and can be converted into insoluble oligomers upon heating at 150 °C. Polymer-fullerene solar cells using poly(3-hexylthiophene) (P3HT) as donor and PC₆₁BA as acceptor exhibited an optimized efficiency of 3.54%, the performance comparable to P3HT/PC₆₁BM cells (optimized efficiency: 3.70%). But, the former possess much better thermal stability than the latter owing to aggregation suppression by the polymerized PC₆₁BA. These results indicate that PC₆₁BA, unlike most previous reported, is a unique polymerizable fullerene derivative that can be used alone as acceptor to achieve both efficient and thermally stable polymer solar cells.     

Thienopyrazine or dithiadiazatrindene containing low band gap conjugated polymers for polymer solar cells

Four new low-band-gap alternating copolymers （P-1, P-2, P-3 and P-4） based on electron-rich benzodithiophene and newly developed electron-deficient units, thienopyrazine or dithiadiazatrindene derivatives, were synthesized by Stille polycondensation. All polymers exhibit good solubility in common organic solvents and a broad absorption band in the visible to near-infrared regions. The film optical band gaps of the polymers are in the range of 1.28-2.07 eV and the highest occupied molecular orbital （HOMO） energy levels are in the range of-4.99 eV to -5.28 eV. Bulk heterojunction polymer solar cells （PSCs） of the polymers were fabricated with phenyl-C61-butyric acid methyl ester （PC61BM） as acceptor material, and a power conversion efficiency of 0.80% was realized with P-1 as donor material.     

Rylene diimide and dithienocyanovinylene copolymers for polymer solar cells

Two polymers containing(E)-2,3-bis(thiophen-2-yl)acrylonitrile(CNTVT) as a donor unit, perylene diimide(PDI) or naphthalene diimide(NDI) as an acceptor unit, are synthesized by the Stille coupling copolymerization, and used as the electron acceptors in the solution-processed organic solar cells(OSCs). Both polymers exhibit broad absorption in the region of 300–850 nm. The LUMO energy levels of the resulted polymers are ca. –3.93 eV and the HOMO energy levels are –5.97 and –5.83 eV. In the binary blend OSCs with PTB7-Th as a donor, PDI polymer yields the power conversion efficiency(PCE) of up to 1.74%, while NDI polymer yields PCE of up to 3.80%.     

聚合物太阳能电池材料研究进展

本文介绍了几种常见的聚合物太阳电池材料。综述了聚合物太阳电池材料的合成、发展历史和现状,对其应用前景进行了展望。参考文献59篇。     

Effect of spacer insertion in a commonly used dithienosilole/benzothiadiazole-based low band gap copolymer for polymer solar cells

Dithienosilole-benzothiadiazole based low bandgap copolymers remain promising material for organic photovoltaics. A new copolymer, poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-{4,7-bis[2-(3-hexyl)thienyl]-2,1,3-benzothiadiazole-5,5′-diyl}] (PDTSDTBT) was designed by introducing a thiophene spacer bearing a hexyl chain at β-position in the main backbone and compared to its analog poly[(4,4′-dioctyldithieno[3,2-b:2′,3′-d]silole-2,6-diyl)-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PDTSBT). In PDTSDTBT, linear alkyl chains on silicon were chosen due to facile and cheap access and the inserted 3-hexylthiophene units were chosen to increase solubility and molar mass, a weak point with PDTSBT. The two parameters are important to optimize photovoltaic performances. To compare characteristics, PDTSDTBT of molar masses greater than, and equal to a sample of PDTSBT, were prepared. Pd-catalyzed Stille cross-coupling reactions in a micro-wave reactor to promote an efficient copolymerisations. A strong absorption ranging from 370 nm to 800 nm and a good thermal stability were observed. PDTSDTBT showed better solubility and higher degree of crystallinity. Facile synthesis of high molar masses meant that higher efficiencies, around 40% greater, could be obtained with PDTSDTBT. The polymer was demonstrated to be susceptible to improvement through the use of device-additives. For example, under initial optimisations using PDTSDTBT:PC₆₀BM blend at a ratio of 1:1 delivered a power conversion efficiency of 2.13% with J_SC = 7.73 (mA/cm²), under AM 1.5 (100 mW/cm²) illumination.     

Donor-acceptor (D-A) terpolymers based on alkyl-DPP and t-BocDPP moieties for polymer solar cells

A series of low band gap terpolymers based on 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b¢] dithiophene (BDTT) and diketopyrrolopyrrole (DPP) with varied solubilizing groups (i.e., tert-butoxycarbonyl, t-Boc and 2-octyldodecyl) are developed as electron donors for bulk heterojunction (BHJ) polymer solar cells (PSCs). The results reveal that the one with 50% t-Boc concentration (P3) performs better than the other terpolymers used in this study in conventional PSC devices with a power conversion efficiency of 2.92%.     

Rationally pairing photoactive materials for high-performance polymer solar cells with efficiency of 16.53%

The emergence of non-fullerene acceptors(NFA) offers a promising opportunity to develop high-performance donor/acceptor pairs with high power conversion efficiency,as NFAs offer tunable energy levels,broad absorption and suitable aggregation property.In order to enhance light-harvesting capability of active layers,we choose a wide bandgap polymer PTQ10 as the donor to blend with a narrow bandgap NFAY6 as the acceptor.In comparison with PTQ10:IDIC blend,~130 nm red-shifted absorption spectrum is observed in the PTQ10:Y6 blend,which potentially enhance the short-circuit current density(Jsc) for the PSCs.In addition,the optimal PTQ10:Y6 blend shows higher photoluminescence quenching efficiency and more efficient charge separation,higher charge mobilities,as well as weaker bimolecular recombination over the PTQ10:IDIC blend,which leads to an outstanding power conversion efficiency(PCE) of 16.53%,with a notable Jsc of 26.65 mA cm^-2 and fill factor(FF) of 0.751.     

Broad absorbing low-bandgap polythiophene derivatives incorporating separate and content-tunable benzothiadiazole and carbazole moieties for polymer solar cells

Three novel low-bandgap (LGB) conjugated polythiophenes (PThBTD_mCz_n) incorporating separate and content-tunable benzothiadiazole and carbazole moieties have been designed and synthesized for application in bulk heterojunction polymer solar cells (PSCs). The absorption spectral, thermal, electrochemical and photovoltaic properties of the random copolymers were investigated. Broad absorption from a single polymer covering the visible region from 300 to 800 nm was observed, which was ideal for highly efficient harvesting of the solar spectrum. DSC analysis showed that the polymers readily crystallized, indicating highly ordered intermolecular packing, which is beneficial for efficient charge-carrier transport. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open-circuit voltage while blending them with fullerene derivatives as electron acceptors. Polymer solar cells using 1:1 or 1:2 wt/wt polymer: PC₆₁BM (methanofullerene [6,6-phenyl C61-butyric acid methyl ester] blends as the photoactive layers were fabricated and characterized. The preliminary investigation on the photovoltaic device of the PThBTD_mCz_n polymers gave similar power conversion efficiency of 1.1-1.2% with V_oc of 0.64-0.68 V under simulated solar light AM 1.5 G (100 mW/cm²).     

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.     

Naphthodithiophene-based donor materials for solution processed organic solar cells

As an emerging donor building block, naphthodithiophene (NDT) is causing more concerns in the field of organic semiconductors. With the rigid and coplanar molecule structure, NDT will exhibit more application space relying on its own advantage for facilitating the charge carrier transport. In this review article, we have summarized the development progress on the NDT-based donor materials for solution processed organic solar cells. Discussions and comments on those representative NDT type materials about structure and property are also presented.     

The evolution of organic materials for efficient dye-sensitized solar cells

In the past three decades, dye-sensitized solar cells (DSSCs) have gained increased recognition as a potential substitute for inexpensive photovoltaic (PV) devices, and their maximum efficiency has grown from 7% to 14.3%. Recent developments in DSSCs have attracted a plethora of research activities geared at realizing their full potential. DSSCs have seen a revival as the finest technology for specific applications with unique features such as low-cost, non-toxic, colourful, transparent, ease of fabrication, flexibility, and efficient indoor light operation. Several organic materials are being explored and employed in DSSCs to enhance their performance, robustness, and lower production costs to be viable alternatives in the solar cell markets. This review provides a concise summary of the developments in the field over the past decade, with a special focus on the incorporation of organic materials into DSSCs. It covers all elements of the DSSC technology, including practical approaches and novel materials. Finally, the emerging applications of DSSCs, and their future promise are also discussed.     

A chlorinated non-fullerene acceptor for efficient polymer solar cells

After additive and thermal annealing treatment, the PM6:Y15 based device obtains a high power conversion efficiency of 14.13%.     

Solidifying liquid electrolytes with fluorine polymer and silica nanoparticles for quasi-solid dye-sensitized solar cells

Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) and silica nanoparticles were employed to solidify 3-methoxypropionitrile based liquid electrolytes containing an ionic liquid, 1-methyl-3-propylimidazolium iodide, as iodide resource. These new quasi-solid-state electrolytes were successfully used for regenerative phtoelectrochemical cells that yielded about 6.7% efficiency under simulated full sunlight (air mass 1.5, 100 mW cm⁻²) in combination with an amphiphilic ruthenium polypyridyl photosensitizer. The as-fabricated device showed a good thermostability at 80 °C for 30 days, maintaining higher than 90% of its initial performance.     

A chlorinated non-fullerene acceptor for efficient polymer solar cells

Improving the performance and reducing the manufacturing costs are the main directions for the development of organic solar cells in the future. Here, the strategy that uses chemical structure modification to optimize the photoelectric properties is reported. A new narrow bandgap (1.30 eV) chlorinated non-fullerene electron acceptor (Y15), based on benzo[d][1,2,3] triazole with two 3-undecyl-thieno[2′,3′:4,5] thieno[3,2-b] pyrrole fused -7-heterocyclic ring, with absorption edge extending to the near-infrared (NIR) region, namely A-DA'D-A type structure, is designed and synthesized. Its electrochemical and optoelectronic properties are systematically investigated. Benefitting from its NIR light harvesting, the fabricated photovoltaic devices based on Y15 deliver a high power conversion efficiency (PCE) of 14.13%, when blending with a wide bandgap polymer donor PM6. Our results show that the A-DA'D-A type molecular design and application of near-infrared electron acceptors have the potential to further improve the PCE of polymer solar cells (PSCs).     

Graphene-based materials for the electrochemical determination of hazardous ions

The use of graphene in the field of electrochemical sensors is increasing due to two main properties that make graphene and derivatives appealing for this purpose: their conductivity and high surface area. In addition, graphene materials can be easily functionalized with nanoparticles (Au, Pt, etc.) or organic molecules (DNA, polymers, etc.) producing synergies that allow higher sensitivity, lower limit of detection as well as increased selectivity. The present review focuses on the most important works published related to graphene-based electrochemical sensors for the determination of hazardous ions (such as As(III), Cd²⁺, Pb²⁺, Hg²⁺, Cr(VI), Cu²⁺, Ag⁺, etc.). The review presents examples of the use of graphene-based electrodes for this purpose as well as important parameters of the sensors such as: limit of detection, linear range, sensitivity, main interferences, stability, and reproducibility. The application of these graphene-based electrodes in real samples (water or food matrices) is indicated, as well. There is room for improvement of these type of sensors and more effort should be devoted to the use of doped graphene (doped for instance with N, B, S, Se, etc.) since electrochemically active sites originated by doping facilitate charge transfer, adsorption and activation of analytes, and fixation of functional moieties/molecules. This will allow the sensitivity and the selectivity of the electrodes to be increased when combined with other materials (nanoparticles/organic molecules).     

Effect of degradation on electronic properties of polymer solar cells

We present here the effect of degradation on electronic properties of polymer solar cells. Investigations were performed on two types of solar cells based on the bulk‐heterojunction network of poly(3‐hexylthiophene) and phenyl [6,6] C₆₁ butyric acid methyl ester, one with slow degradation whereas other with faster degradation. Samples were prepared in identical conditions with controlled atmosphere, but for faster degradation, one of the samples was exposed to ambient air (rich in O₂ and H₂O molecules) before deposition of top metal electrode. The sample with slow degradation showed linear degradation in short circuit current density (J_sc), whereas the sample with faster degradation exhibited exponential degradation in J_sc. Linear degradation happens due to degradation in the active layer only whereas the exponential degradation is because of through degradation of the solar cell. The effect of degradation is investigated on different diode parameters. Because of different degradation processes in the two samples, the variations in diode parameters with time are different. Copyright © 2013 John Wiley & Sons, Ltd.