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%.     

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).     

Reducing energy loss via tuning energy levels of polymer acceptors for efficient all-polymer solar cells

Science China Chemistry - The open-circuit voltage (Voc) of all-polymer solar cells (all-PSCs) is typically lower than 0.9 V even for the most efficient ones. Large energy loss is the main reason...     

Recent advances in rylene diimide polymer acceptors for all-polymer solar cells

In recent years, a large library of n-type polymers have been developed and widely used as acceptor materials to replace fullerene derivatives in polymer solar cells(PSCs), stimulating the rapid expansion of research on so-called all-polymer solar cells(a PSCs). In particular, rylene diimide-based n-type polymer acceptors have attracted broad research interest due to their high electron mobility, suitable energy levels, and strong light-harvesting ability in the visible region. Among various polymer acceptors, rylene diimide-based polymers presented best performances when served as the acceptor materials in a PSCs. Typically, a record power conversion efficiency(PCE) of 7.7% was very recently achieved from an a PSC with a rylene diimide polymer derivative as the acceptor component. In this review, we highlight recent progress of n-type polymers originated from two significant classes of rylene diimide units, namely naphthalene diimide(NDI) and perylene diimide(PDI), as well as their derivatives for a PSC applications.     

Novel polymer acceptors achieving 10.18% efficiency for all-polymer solar cells

Polymer acceptors based on extended fused ring p skeleton has been proven to be promising candidates for all-polymer solar cells(all-PSCs), due to their remarkable improved light absorption than the traditional imide-based polymer acceptors. To expand structural diversity of the polymer acceptors, herein,two polymer acceptors PSF-IDIC and PSi-IDIC with extended fused ring p skeleton are developed by copolymerization of 2,20-((2 Z,20 Z)-((4,4,9,9-tetrahexadecyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b']dithio phene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1 H-indene-2,1-diylidene))dimalononitrile(IDIC-C16) block with sulfur(S) and fluorine(F) functionalized benzodithiophene(BDT) unit and silicon(Si) atom functionalized BDT unit, respectively. Both polymer acceptors exhibit strong light absorption.The PSF-IDIC exhibits similar energy levels and slightly higher absorption coefficient relative to the PSi-IDIC. After blended with the donor polymer PM6, the functional atoms on the polymer acceptors show quite different effect on the device performance. Both of the acceptors deliver a notably high open circuit voltage(V_OC) of the devices, but PSi-IDIC achieves higher V OCthan PSF-IDIC. All-PSC based on PM6:PSi-IDIC attains a power conversion efficiency(PCE) of 8.29%, while PM6:PSF-IDIC-based device achieves a much higher PCE of 10.18%, which is one of the highest values for the all-PSCs reported so far. The superior device performance of PM6:PSF-IDIC is attributed to its higher exciton dissociation and charge transport, decreased charge recombination, and optimized morphology than PM6:PSi-IDIC counterpart. These results suggest that optimizing the functional atoms of the side chain provide an effective strategy to develop high performance polymer acceptors for all-PSCs.     

15.4% Efficiency all-polymer solar cells

We report all-polymer solar cells(All-PSCs) with record-high power conversion efficiency(PCE) through tuning the molecular weights of the polymer donor(PBDB-T) to form optimal active layer morphology. By combining the polymer donors with a newly reported polymer acceptor(PJ1), an unprecedented high PCE of 15.4% and fill factor over 75% were achieved for the AllPSCs with the medium molecular weight polymer donor(PBDB-T_(MW)), which is the highest value for All-PSCs reported so far.Detailed morphology investigation revealed that the proper phase separation in the PBDB-T_(MW):PJ1 blend should account for the superior device performance as PBDB-T_(MW) exhibits appropriate miscibility with the polymer acceptor PJ1. These results demonstrated that the device performance of All-PSCs could be fully comparable to that of small molecular acceptor-based PSCs. The formation of optimized morphology via precise control of molecular weights of polymer donors and acceptors is crucial to achieve this goal.     

8.0% Efficient all-polymer solar cells based on novel starburst polymer acceptors

The polymer N2200, with its π-conjugated backbone composed of alternating naphthalene diimide(NDI) and bithiophene(DT)units, has been widely used as an acceptor for all-polymer solar cells(all-PSCs) owing to its high electron mobility and suitable ionization potential and electron affinity. Here, we developed two naphthalene diimide derivatives by modifying the molecular geometry of N2200 through the incorporation of a truxene unit as the core and NDI-DTas the branches. These starburst polymers exhibited absorption spectra and molecular orbital energy levels that were comparable to N2200. These copolymers were paired with the wide-bandgap polymer donor PTz BI-O to fabricate all-polymer solar cells(all-PSCs), which displayed impressive power conversion efficiencies up to 8.00%. The improved photovoltaic performances of all-PSCs based on these newly developed starburst acceptors can be ascribed to the combination of increased charge carrier mobilities, reduced bimolecular recombination, and formation of more favorable film morphology. These findings demonstrate that the construction of starburst polymer acceptors is a feasible strategy for the fabrication of high-performance all-PSCs.     

High-performance all-polymer solar cells enabled by a novel low bandgap non-fully conjugated polymer acceptor

The non-fully conjugated polymer as a new class of acceptor materials has shown some advantages over its small molecular counterpart when used in photoactive layers for all-polymer solar cells(all-PSCs), despite a low power conversion efficiency(PCE)caused by its narrow absorption spectra. Herein, a novel non-fully conjugated polymer acceptor PFY-2 TS with a low bandgap of~1.40 eV was developed, via polymerizing a large π-fused small molecule acceptor(SMA) building block(namely YBO) with a nonconjugated thioalkyl linkage. Compared with its precursor YBO, PFY-2 TS retains a similar low bandgap but a higher LUMO level.Moreover, compared with the structural analog of YBO-based fully conjugated polymer acceptor PFY-DTC, PFY-2 TS shows a similar absorption spectrum and electron mobility, but significantly different molecular crystallinity and aggregation properties,which results in optimal blend morphology with a polymer donor PBDB-T and physical processes of the device in all-PSCs. As a result, PFY-2 TS-based all-PSCs achieved a PCE of 12.31% with a small energy loss of 0.56 eV enabled by the reduced non-radiative energy loss(0.24 eV), which is better than that of 11.08% for the PFY-DTC-based ones. Our work clearly demonstrated that non-fully conjugated polymers as a new class of acceptor materials are very promising for the development of high-performance all-PSCs.     

Printable and stable all-polymer solar cells based on non-conjugated polymer acceptors with excellent mechanical robustness

All-polymer solar cells(all-PSCs)trigger enormous commercial applications,and great progress has been made in recent years.However,from small-area devices to large-area modules,the poor adaption of the materials for printing methods and the large efficiency loss are still great challenges.Herein,three novel non-conjugated polymer acceptors(PTH-Y,PTClm-Yand PTClo-Y)are developed for all-PSCs.It can be found that non-conjugated polymer acceptors can effectively minimize the technique and efficiency gaps between small-area spin-coating and large-area blade-printing method,which can facilitate the preparation of large-area flexible device.By directly inheriting the spin-coating condition,the blade-coating processed device based on PTCloY achieves an impressive power conversion efficiency(PCE)of 12.42%,comparable to the spin-coating processed one(12.74%).Such a non-conjugated polymer system also can well tolerate large-scale preparation and flexible substrate.Notable PCE of 11.94%for large-area rigid device and 11.56%for large-area flexible device are obtained,which is the highest value for large-area flexible all-PSCs fabricated by blade-coating.In addition,the non-conjugated PTClo-Y-based devices show excellent thermal stability and mechanical robustness.These results demonstrate that the non-conjugated polymer acceptors are potential candidates for the fabrication of highly-efficient,large-area and robust flexible all-PSCs by printing methods.     

Synthesis and application of poly(fluorene-alt-naphthalene diimide) as an n-type polymer for all-polymer solar cells

A new alternating copolymer of fluorene and naphthalene diimide, PF-NDI, was synthesized and characterized. The highest power conversion efficiency of all-polymer solar cells based on P3HT:PF-NDI reached 1.63% with a relatively high fill factor of 0.66 by using 1,8-diiodooctane as a solvent additive to optimize the mixing morphology.     

Non-equivalent D-A copolymerization strategy towards highly efficient polymer donor for polymer solar cells

D-A copolymerization is a broadly utilized molecular design strategy to construct high efficiency photovoltaic materials for polymer solar cells (PSCs),and all the D-A copolymer donors reported till now are the alternate D-A copolymers with equal D-and A-units.Here,we first propose a non-equivalent D-A copolymerization strategy with unequal D-and A-units,and develop three novel non-equivalent D-A copolymer donors (PM6-D1,PM6-D2 and PM6-D3 with D/A unit ratio of 1.1:0.9,1.2:0.8 and1.3:0.7,respectively) by inserting more D units into the alternate D-A copolymer PM6 backbone to finely tune the physicochemical and photovoltaic properties of the polymers.The three non-equivalent D-A copolymers show the down-shifted highest occupied molecular orbital (HOMO) energy levels,higher hole mobility,higher degree of molecular self-assembly and higher molecular crystallinity with the increase of D-unit ratio in comparison with the alternate D-A copolymer PM6.As a result,all the three non-equivalent D-A copolymer-based PSCs with Y6 as acceptor achieve improved power conversion efficiency (PCE)with higher V_(oc),larger J_(sc)and higher FF simultaneously.Particularly,the PM6-D1:Y6 based PSC achieved a high PCE of17.71%,which is significantly higher than that (15.82%) of the PM6:Y6 based PSC and is one of the highest performances in the binary PSCs.     

Rhodanine flanked indacenodithiophene as non-fullerene acceptor for efficient polymer solar cells

A fused-ring electron acceptor IDT-2BR1 based on indacenodithiophene core with hexyl side-chains flanked by benzothiadiazole rhodanine was designed and synthesized.In comparison with its counterpart with hexylphenyl side-chains(IDT-2BR),IDT-2BR1exhibits higher highest occupied molecular orbital(HOMO)energy but similar lowest unoccupied molecular orbital(LUMO)energy(IDT-2BR1:HOMO=-5.37eV,LUMO=-3.67eV;IDT-2BR:HOMO=-5.52eV,LUMO=-3.69eV),red-shifted absorption and narrower bandgap.IDT-2BR1 has higher electron mobility(2.2×10~(-3)cm~2 V~(-1)s~(-1))than IDT-2BR(3.4×10~(-4)cm~2 V~(-1)s~(-1))due to the reduced steric hindrance and ordered molecular packing.Fullerene-free organic solar cells based on PTB7-Th:IDT-2BRl yield power conversion efficiencies up to 8.7%,higher than that of PTB7-Th:IDT-2BR(7.7%),with a high open circuit voltage of0.95 V and good device stability.     

An isoindigo-based low band gap polymer for efficient polymer solar cells with high photo-voltage

A new low band gap polymer (E(g) = 1.6 eV) with alternating thiophene and isoindigo units was synthesized and characterized. A PCE of 3.0% and high open-circuit voltage of 0.89 V were realized in polymer solar cells, which demonstrated the promise of isoindigo as an electron deficient unit in the design of donor-acceptor conjugated polymers for polymer solar cells.     

A crosslinked polymer as dopant-free hole-transport material for efficient n-i-p type perovskite solar cells

A new crosslinked polymer,called P65,with appropriate photo-electrochemical,opto-electronic,and thermal properties,has been designed and synthesized as an efficient,dopant-free,hole-transport material(HTM)for n-i-p type planar perovskite solar cells(PSCs).P65 is obtained from a low-cost and easily synthesized spiro[fluorene-9,90-xanthene]-30,60-diol(SFX-OH)-based monomer X65 through a freeradical polymerization reaction.The combination of a three-dimensional(3 D)SFX core unit,holetransport methoxydiphenylamine group,and crosslinked polyvinyl network provides P65 with good solubility and excellent film-forming properties.By employing P65 as a dopant-free hole-transport layer in conventional n-i-p type PSCs,a power conversion efficiency(PCE)of up to 17.7%is achieved.To the best of our knowledge,this is the first time a 3 D,crosslinked,polymeric dopant-free HTM has been reported for use in conventional n-i-p type PSCs.This study provides a new strategy for the future development of a 3 D crosslinked polymeric dopant-free HTM with a simple synthetic route and low-cost for commercial,large-scale applications in future PSCs.     

Conjugation expansion strategy enables highly stable all-polymer solar cells

The stability issue is one of the key factors hindering the commercial application of organic solar cells. All-polymer organic solar cell is one of the effective ways to solve the stability problem. In this work, we designed and synthesized two polymer donor materials PBDT and PDTBDT with different conjugation ranges, and demonstrated for the first time that extending the conjugation range of donor materials in all polymer solar cells can significantly improve device efficiency and stability. The experimental results of materials and devices show that PDTBDT with a larger conjugation range has stronger crystallinity and a more planar structure, which endows the active layer in its corresponding device with higher exciton dissociation probability, lower carrier recombination probability, more balanced charge transport properties and more favorable film morphology. As a result, the PDTBDT:PYF-T-o devices display an outstanding PCE of 13.38%, which is much higher than PBDT with smaller conjugation range based devices. Moreover, the PDTBDT:PYF-T-o device retains 0.86 of the initial PCE after over 500 h in the air atmosphere, exhibiting significantly improved stability. The improved stability is attributed to the enhanced moisture and air tolerance of active layer film thanks to the strong crystallinity of the donor material. These results demonstrate that the conjugation expansion strategy is one of the effective ways to obtain efficient and stable all-polymer organic solar cells.     

Side-chain modification of polyethylene glycol on conjugated polymers for ternary blend all-polymer solar cells with efficiency up to 9.27%

With the rapid progress achieved by all-polymer solar cells (all-PSCs), wide-bandgap copolymers have attracted intensive attention for their unique advantage of constructing complementary absorption profiles with conventional narrow-bandgap copolymers. In this work, we designed and synthesized a wide bandgap ternary copolymer PEG-2% which has the benzodithiophene-alt-difluorobenzotriazole as the backbone and the polyethylene glycol (PEG) modified side chain. The PBTA-PEG-2%:N2200 can be processed with a non-chlorinated solvent of 2-methyl-tetrahydrofuran (MeTHF) for the binary all-PSC, which exhibits a moderate photovoltaic performance. In particular, the ternary all-PSCs that consisting an additional narrow bandgap polymer donor PTB7-Th can also be processed with MeTHF, resulting in an unprecedented power conversion efficiency (PCE) of 9.27%, and a high PCE of 8.05% can be achieved with active layer thickness of 240 nm, both of which are the highest values so far reported from all-PSCs. Detailed investigations revealed that the dramatically improved device performances are attributable to the well-extended absorption band in the photoactive layer. Hence, developing novel copolymers with tailored side chains, and introducing additional polymeric components, can broaden the horizon for high-performance all-PSCs.     

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.     

11.2% Efficiency all-polymer solar cells with high open-circuit voltage

Herein,we fabricated all-polymer solar cells(all-PSCs)based on a fluorinated wide-bandgap p-type conjugated polymer PM6 as the donor,and a narrow bandgap n-type conjugated polymer PZ1 as the acceptor.In addition to the complementary absorption and matching energy levels,the optimized blend films possess high cystallinity,predominantly face-on stacking,and a suitable phase separated morphology.With this active layer,the devices exhibited a high V_(oc)of 0.96 V,a superior J_(sc)of 17.1 mA cm~(-2),a fine fill factor(FF)of 68.2%,and thus an excellent power conversion efficiency(PCE)of 11.2%,which is the highest value reported to date for single-junction all-PSCs.Furthermore,the devices showed good storage stability.After 80 d of storage in the N_2-filled glovebox,the PCE still remained over 90%of the original value.Large-area devices(1.1 cm~2)also demonstrated an outstanding performance with a PCE of 9.2%,among the highest values for the reported large-area all-PSCs.These results indicate that the PM6:PZ1 blend is a promising candidate for scale-up production of large area high-performance all-PSCs.     

Chlorinated polymer solar cells simultaneously enhanced by fullerene and non-fullerene ternary strategies

To achieve efficient polymer solar cells(PSCs)with full utilization of the whole spectrum,the multicomponent devices are of great importance to be deeply explored,especially for their capability of one-step fabrication.However,the research about one same binary system simultaneously derivated various multi-component PSC is still very limited.Herein,we achieved the whole constructions from one binary host to different ternary systems and even the quaternary one.The ternary strategies with fullerene acceptor,PC₇₁BM,and non-fullerene acceptor,BT₆IC-BO-4Cl,as the third component,both boosted the device efficiencies of PBT4Cl-Bz:IT-4F binary system from about 9% to comparatively beyond 11%.Despite the comparable improvement of performance,there existed other similarities and differences in two ternary strategies.In detail,the isotropic carrier transport of PC₇₁BM which largely elevated the fill factor(FF)in the corresponding devices,while the strong absorption of BT₆IC-BO-4Cl enhanced the short current density(J_SC)most.More interestingly,quaternary devices based on PBT4Cl-Bz:IT-4F:PC71 BM:BT₆IC-BO-4Cl could combine both advantages of fullerene and non-fullerene ternary strategies,further pumped the J_SC from 16.44 to the highest level of 19.66 mA cm^-2 among all devices,eventually resulted in an optimized efficiency of 11.69%.It reveals that both fullerene and non-fullerene ternary strategies have their unique feature to elevate the device performance either by efficient isotropic carrier transport or better coverage of whole sunlight spectrum and easy tunable energy levels from organic materials.The key is how to integrate the two pathways in one system and provide a more competitive solution facing high-quality PSCs.