Gradually modulating the three parts of D-π-A type polymers for high-performance organic solar cells

Organic solar cells(OSCs)have received great attention for the prominent advantage of low-cost,light-weight and potential for fabricating flexible and semi-transparent device via roll-to-roll printing toward making better use of inexhaustible renewable clean energy during the past years[1-4].

Gradual chlorination at different positions of D-π-A copolymers based on benzodithiophene and isoindigo for organic solar cells

Isoindigo(IID)has been widely used as strong acceptor unit(A)to construct narrow bandgap polymers in organic field effect transistors(OFETs)and organic solar cells(OSCs).Combing with IID,we chose benzodithiophene(BDT)as the donor unit(D)and thieno[3,2-b]thiophene(TT)as theπbridge to construct a new type of D-π-A polymer PE70.Based on PE70,we adopt the chlorination strategy to fine-tune photoelectric characteristics and film morphology,and then developed PE74 and PE75.By blending with non-fullerene acceptor(NFA)Y6,device based on PE74 with chloride substitution on the BDT unit showed increasing photovoltaic performance.In addition,further chlorine substitution on the IID(PE75)would greatly reduce the non-radiative voltage loss(ΔV3),and the distorted molecular conformation also took responsible for the excessive recombination.As results,PE74:Y6-based device achieves a power conversion efficiency(PCE)of 11.06%with open-circuit voltage(VOC)of 0.76 V,which are higher than those of PE70:Y6(PCE of 10.40%and VOC of 0.72 V)and PE75:Y6-based device(PCE of 6.24%and VOC of 0.84 V).This work demonstrates the regularity of the photovoltaic performance caused by chlorination strategy in polymer in the non-fullerene OSC devices,which provide important insights into highperformance photovoltaic materials.

聚3-己基噻吩:非富勒烯太阳能电池中的量子效率损失和电压损失

聚3-己基噻吩(P3HT)以其合成工艺简单、成本低廉的优势,成为有机光伏领域中最具吸引力的电子给体材料之一。然而,目前P3HT:非富勒烯太阳能电池的光伏性能仍然较差。在本工作中,我们证明了与P3HT:富勒烯太阳能电池相比,较快的电荷转移态的非辐射衰减速率(K_(nr))是导致P3HT:非富勒烯太阳能电池中较低的量子效率和较高的电压损失的原因。然后,我们研究了基于非富勒烯受体ZY-4Cl的太阳能电池的工作机理。研究结果表明与P3HT:非富勒烯体系相比,P3HT:ZY-4Cl中K_(nr)的降低改善了器件的量子效率,同时降低了电压损失。K_(nr)降低的原因可以部分归因于电荷转移态能量的增加。此外,给体分子和受体分子之间的距离(DA间距)的增大也是K_(nr)减少的重要原因。因此,我们得出结论:为了提高P3HT太阳能电池的性能,需进一步降低器件的K_(nr),这可通过增加活性层中的DA间距来实现。

Ternary blend strategy in benzotriazole-based organic photovoltaics for indoor application

Organic photovoltaics(OPVs)suitable for application in indoor lighting environments can power a wide range of internet of things(Io T)related electronic devices.The ternary structure has huge advantages in improving the photovoltaic performance of OPVs,including broadening the light absorption,improving the charge transport,manipulating the energy loss(E_(loss))and so on.Herein,we use wide-bandgap photo-active materials,including the benzotriazole-based polymer donor(J52-F),chlorinated polymer donor(PM7)and A_(2)-A_1-D-A_1-A_(2)-structured acceptor(BTA3),to construct ternary OPVs for indoor light applications.Benefitting from the introduction of PM7 as the third component in J52-F:BTA3-based blend,a gratifying PCE of 20.04%with a high V_(OC)of 1.00 V can be obtained under the test conditions with an illumination of 300 lx from an LED lighting source with a color temperature of 3000 K.The excellent device performance is inseparable from the matched spectrum,enhanced light absorption and the reduced E_(loss),while the improved charge transport capability and suppression of carrier recombination also play an indelible role.Our work shows a potential material system to meet the requirement of devices applied under indoor light.Moreover,these findings demonstrate that designing multi-component OPVs is indeed a feasible way to further improve the performances of the photovoltaic energy conversion system for indoor applications.

Tuning the optoelectronic properties of vinylene linked perylenediimide dimer by ring annulation at the inside or outside bay positions for fullerene-free organic solar cells

Among various perylenediimide(PDI)-based small molecular non-fullerene acceptors(NFAs),PDI dimer can effectively avoid the excessive aggregation of single PDI and improve the photovoltaic performance.However,the twist of perylene core in PDI dimer will destroy the effective conjugation.Thus,ring annulation of PDI dimer is a feasible method to balance the film quality and electron transport,but the systematic study has attracted few attentions.Herein,we choose a simple vinylene linked PDI dimer,V-PDI2,and then conduct further studies on the structure-property-performance relationship of four kinds of derived fused-PDI dimers,namely V-TDI2,V-FDI2,V-PDIS2 and V-PDISe2 respectively.The former two are incorporated thianaphthene and benzofuran at the inside bay positions,and the latter two are fused thiophene and selenophene at the outside bay positions,respectively.Theoretical calculations reveal the inside-and outside-fused structures largely affect the skeleton configuration,the former two tend to be planar structure and the latter two maintain the distorted backbone.The photovoltaic characterizations show that the inside-fused PDI dimers offer high open circuit voltage(VOC),while the outside-fused PDI dimers afford large short-circuit current density(JSC).This variation tendency results from the reasonably tunable energy levels,light absorption,molecular crystallinity and film morphology.As a result,PBDB-T:V-PDISe2 device exhibits the highest power conversion efficiency(PCE)of 6.51%,and PBDB-T:VFDI2 device realizes the highest VOC of 1.00 V.This contribution indicates that annulation of PDI dimers in outside or inside bay regions is a feasible method to modulate the properties of PDI-based non-fullerene acceptors.

-1.2 V open-circuit voltage from organic solar cells

Organic solar cells(OSCs)have achieved rapid advance due to the continuous development of high-performance key materials.Recently,the power conversion efficiencies(PCEs)of OSCs under 1 Sun condition(AM 1.5 G,100 mW/cm2)are striving toward 19%[1−5].The PCE improvement benefits from the largely enhanced short-circuit current density(Jsc)and fill factor(FF).However,these cells show relatively low open-circuit voltage(Voc)around 0.8-0.9 V.

高开路电压与低能量损失有机太阳能电池研究进展

有机太阳能电池(organic solar cells,OSCs)是光伏发电和能源功能材料的重要组成部分,因其制作简单、材料来源广泛、轻量化、柔性好等突出优势成为电池材料领域研究的热点.然而,与无机/钙钛矿太阳能电池相比,有机太阳能电池存在较大的电压损失(即能量损失),限制了效率的进一步提升.目前实验室报道最高效率远低于肖克利-奎伊瑟(Shockley-Queisser)理论所定义的极限效率.因此,最大化降低有机太阳能电池的电压损失,是进一步提升电池器件效率的关键.针对上述问题,国家纳米科学中心的相关科研人员做出了很多努力,在高开路电压、低能量损失的有机太阳能电池分子设计、理论计算以及新型器件结构等方面做了非常出色的工作.本文综述了近年来的相关研究进展,希望为有机太阳能电池相关领域的研究者提供借鉴并对有机太阳能电池的发展起到促进作用.

Selective fluorination on donor and acceptor for management of efficiency and energy loss in non-fullerene organic photovoltaics

Although fluorination has been proved effective to modulate optoelectronic properties and film morphology,knowledge of managing power conversion efficiency(PCE)and energy loss(Eloss)of organic photovoltaics(OPVs)by selective fluorination on the donor and/or acceptor is lacking.Herein we designed and synthesized three 1,2,3-benzotriazole(BTA)-based linear polymers(PE45,PE46 and PE47)with different numbers of fluorine atom substitution on the conjugated phenyl side chain.Two classic non-fullerene acceptors(NFAs)Y5(without fluorination)and Y6(with fluorination)were utilized to manage the device efficiency and energy loss.The results revealed that increasing fluorine substitutions on polymer donor improved the OPV efficiencies when the fluorinated Y6 was used as the acceptor,whereas decreased the PCEs when the non-fluorinated Y5 was used as the acceptor.The energy loss declined with the growing fluorine substitutions on polymer donor,and Y5 systems gave the lower values in comparison with the corresponding Y6 cases.It turns out that PE47:Y6 achieved the highest PCE of 15.58%with an open-circuit voltage(VOC)of 0.84 V(Eloss=0.56 e V)due to the highest and balanced hole/electron mobilities,suppressed bimolecular recombination and fibril network morphology,which is the highest value in the BTA-based polymers.Furthermore,PE47:Y5 attained an ultralow non-radiative energy loss of 0.15 e V,which is one of the lowest values among the reported OPVs.Our work could not only give a direct path on how to manage the efficiency and energy loss by selective fluorination on donor and acceptor,but also show a deep understanding on charge generation,transport and collection induced by selective fluorination.

The development of A-DA’D-A type nonfullerene acceptors containing non-halogenated end groups

Compared with perovskite solar cells and silicon solar cells,the excessive voltage loss(Vloss)becomes a stubborn stone that seriously hinders the further improvement of organic photovoltaic(OPV).Thus,many researchers focus on finding an effective material system to achieve high-performance OPVs with low Vloss.In recent 5 years,acceptor-donor-acceptor’-donor-acceptor(A-DA’D-A)type non-fullerene acceptors(NFAs)have attracted great attention because of their promising photovoltaic performance.Among them,A-DA’D-A type NFAs containing non-halogenated end group(NHEG)exhibit the large potential to achieve high open-circuit voltage(VOC)for the state-of-the-art OPVs,because of high-lying molecular energy levels and decreasing Vloss.In this review,we systematically summarize the recent development of A-DA’D-A type NHEG-NFAs and the impact of different NHEGs on the optoelectronic properties as well as the photovoltaic performance.In addition,we especially analyze the Vloss of NHEG-NFAs in the binary and ternary OPV devices.At last,we provide perspectives on the further molecular design and future challenges for this kind of materials as well as suggested solutions.

Tuning the intermolecular interaction of A_(2)-A_(1)-D-A_(1)-A_(2) type non-fullerene acceptors by substituent engineering for organic solar cells with ultrahigh V_(OC) of ~1.2 V

For non-fullerene acceptors(NFAs)with linear A_(2)-A_(1)-D-A_(1)-A_(2) backbone,there are three kinds of possible intermolecular interaction,A_(1)-A_(1),A_(1)-A_(2) and A_(2)-A_(2) stacking.Hence,it is a huge challenge to control this interaction and investigate the effect of intermolecular stacking model on the photovoltaic performance.Here,we adopt a feasible strategy,by utilizing different substituent groups on terminal A2 unit of dicyanomethylene rhodanine(RCN),to modulate this stacking model.According to theoretical calculation results,the molecule BTA3 with ethyl substituent packs via heterogeneous interaction between A_(2) and A_(1) unit in neighboring molecules.Surprisingly,the benzyl group can effectively transform the aggregation of BTA5 into homogeneous packing of A_(2)-A_(2) model,which might be driven by the strong interaction between benzyl and A1(benzotriazole)unit.However,different with benzyl,phenyl end group impedes the intermolecular interaction of BTA4 due to the large steric hindrance.When using a BTA-based D-π-A polymer J52-F as donor according to“Same-A-Strategy”,BTA3-5 could achieve ultrahigh open-circuit voltage(VOC)of 1.17–1.21 V.Finally,BTA5 with benzyl groups realized an improved power conversion efficiency(PCE)of 11.27%,obviously higher than that of BTA3(PCE=9.04%)and BTA4(PCE=5.61%).It is also worth noting that the same trend can be found when using other four classic p-type polymers of P3HT,PTB7,PTB7-Th and PBDB-T.This work not only investigates the intermolecular interaction of A_(2)-A_(1)-D-A_(1)-A_(2) type NFAs for the first time,but also provides a straightforward and universal method to change the interaction model and improve the photovoltaic performance.

Wide Band Gap Non-Fullerene Small Molecular Acceptors Containing Spirobifluorene and Benzotriazole with Three Different End-Capped Groups for P3HT-Based Organic Solar Cells

Spirofluorene （SF） and benzo[d][1,2,3]triazole （BTA） have been considered as promising building blocks to construct n-type photovoltaic materials. Herein, three new small molecule acceptors （SMAs） named BTA21, BTA23 and BTA27 with the structure of A2=A1-D-AI^A2 have been designed, in which SF and BTA were used as a central unit of D and bridged acceptor unit of A1, respectively. In addition, 3-ethylrhodanine, 2-（3-ethyl-4-oxothiazolidin-2-ylidene）malononitrile and malononitrile were chosen as terminal acceptor units to modulate the properties of the final SMAs. Three SMAs show wide optical band gaps （Eg） of 2.19, 2.15 and 2.22 eV, respectively, with gradually down-shift of the lowest unoccupied molecular orbital {LUMO） levels in the order of BTAZl, BTA23 and BTA27 depending on the electron-withdrawing capability of terminal acceptor units. BTA21 shows great advantages with respect to donor poly（3-hexylthiophene） （P3HT） over BTA23 and BTA27, such as well energy-level matching, complementary absorption and proper morpholgy, Concequently, P3HT：BTA21 shows the best power conversion efficiency （PCE） value of 3.28% with an open-circuit voltage （Voc） of 1.02 V, a short-circuit current （Jsc） of 5.45 mA.cm-2 and a fill factor （FF） of 0.59. These results indicate that the terminal acceptor group end-capped in SMAs plays a significant role in controlling their optical, electronic, and photovoltaic properties.

PTB7-Th based organic solar cell with a high V_(oc) of 1.05 V by modulating the LUMO energy level of benzotriazole-containing non-fullerene acceptor

The open-circuit voltage(V_(oc))of classical photovoltaic polymers,such as P3HT and PTB7-Th,are always restricted when combining with fullerene derivatives,due to the difficulty of modulating the energy levels of fullerene derivatives.Thus,design of new non-fullerene small molecule acceptor(NFSMA)is very significant to match with these mature polymer donors and improve the V_(oc)and power conversion efficiency(PCE).Here,a new benzotriazole(BTA)-based NFSMA,BTA7 was synthesized by adopting A_2@A_1ADAA_1@A_2type molecular backbone.By using a strong electron-accepting unit of malononitrile(M)as terminal segment A_2,BTA7 demonstrates strong crystallinity,red-shifted absorption spectrum and down-shifted lowest unoccupied molecular orbital(LUMO)energy levels in comparison with BTA_1and BTA_2.Organic solar cells(OSCs)based on PTB7-Th:BTA7 realized a high V_(oc)of 1.05 V with a moderate PCE of 4.60%.The energy loss(E_(loss)=E_gàe V_(oc))of 0.53 e V is lower than the experiential minimum value of0.6 e V,which indicates PTB7-Th still has large potential to improve the V_(oc)and photovoltaic performance after the development of novel electron acceptors.

A small molecular electron acceptor based on asymmetric hexacyclic core of thieno[1,2-b]indaceno[5,6-b']thienothiophene for efficient fullerene-free polymer solar cells

A novel A-D-A(acceptor-donor-acceptor)type non-fullerene small molecule,A201,consisting of an asymmetric thieno[1,2-b]indaceno[5,6-b^0]thienothiophene(TITT)unit as middle D part and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile(IC)groups as end-capped A parts was designed and synthesized.The asymmetric TITT building block showed a higher dipole moment of 0.85 Debye(1 Debye=3.33564?10^(à30)cm)compared with the symmetric analogues of indacenodithiophene(IDT)and indacenodithieno[3,2-b]thiophene(IDTT)of 0.098 and 0.13 Debye,respectively.The solution-processed bulk heterojunction solar cells using a benzotriazole(BTA)-based polymer of J71 as donor and A201 as acceptor,showed a power conversion efficiency(PCE)of 9.36%with an open-circuit voltage(V_(oc))of0.88 V,a short-circuit current(J_(sc))of 13.15 m A cm^(à2),and a fill factor(FF)of 0.67,under the illumination of AM 1.5G at 100 m W cm^(à2).The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility(l_h)and electron mobility(l_e)of 9.56?10^(à4)and 5.17?10^(à4)cm^2V^(à1)s^(à1),respectively.These results indicate that the asymmetric electron-donating segments are promising to construct A-D-A type small molecular acceptors,which could largely enhance the diversity of building blocks to design photovoltaic materials.

The first application of isoindigo-based polymers in non-fullerene organic solar cells

Although isoindigo(IID)-based polymers can realize high charge mobility,these materials are currently confined to fullerenebased organic solar cells(OSCs).Herein,we designed a pair of alternative D-π-A type copolymers,PE71 and PE72,to expand the application in non-fullerene OSCs,where benzo[1,2-b:4,5-b′]thiophene(BDT),thieno[3,2-b]thiophene(TT)and IID units were used as D,A andπ-bridge,respectively.The aim of optimizing the length of alkyl chains on TT bridge is to ensure polymer solubility,crystallinity as well as miscibility with acceptor molecules.We find that PE71 and PE72 exhibit similar optical and electronic properties,but PE71 with shorter hexyl chain tends to aggregate into fiber-like structure.In the end,Y6 is selected as the electron acceptor because of suitable energy levels and complementary absorption spectrum.Finally,PE71:Y6 device realizes a power conversion efficiency(PCE)of 12.03%,which is obviously higher than that of PE72:Y6 device(9.74%)and is also the highest value for IID-based photovoltaic polymers.The charge transport,molecular aggregation,film morphology and energy loss analysis were systematically investigated.The improved photovoltaic performance of PE71:Y6 mainly originates from the better interpenetrating network structure toward facilitating exciton seperation and free charge carrier transportation.Our results indicate that IID-based D-π-A polymers can also be utilized in non-fullerene OSCs and the alkyl chains on the thieno[3,2-b]thiopheneπ-bridge have a vital effect on the photovoltaic performance.

Effects of BTA2 as the third component on the charge carrier generation and recombination behavior of PTB7:PC71BM photovoltaic system

Effects of a benzotriazole(BTA)-based small molecule,BTA2,as the third component on the charge carrier generation and recombination behavior of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7):[6,6]-phenyl-C71-butyric acid methyl ester(PC71BM)organic solar cells(OSCs)were investigated by optical simulation of a transfer matrix model(TMM),photo-induced charge extraction by linearly increasing voltage(photo-CELIV)technique,atomic force microscope(AFM),and the Onsager–Braun model analysis.BTA2 is an A_(2)-A_(1)-D-A_(1)-A_(2)-type non-fullerene small molecule with thiazolidine-2,4-dione,BTA,and indacenodithiophene as the terminal acceptor(A_(2)),bridge acceptor(A_(1)),and central donor(D),respectively.The short-circuit current density of the OSCs with BTA2 can be enhanced significantly owing to a complementary absorption spectrum.The optical simulation of TMM shows that the ternary OSCs exhibit higher internal absorption than the traditional binary OSCs without BTA2,resulting in more photogenerated excitons in the ternary OSCs.The photo-CELIV investigation indicates that the ternary OSCs suffer higher charge trap-limited bimolecular recombination than the binary OSCs.AFM images show that BTA2 aggravates the phase separation between the donor and the acceptor,which is disadvantageous to charge carrier transport.The Onsager-Braun model analysis confirms that despite the charge collection efficiency of the ternary OSCs being lower than that of the binary OSCs,the optimized photon absorption and exciton generation processes of the ternary OSCs achieve an increase in photogenerated current and thus improve power conversion efficiency.

The effect of alkyl chain branching positions on the electron mobility and photovoltaic performance of naphthodithiophene diimide(NDTI)-based polymers

Conjugated polymers are widely used in organic optoelectronic devices due to their solution processability,thermal stability and structural diversity.Generally,alkyl side chains must be utilized to increase the solubility of final polymers in the processing solvent.However,the effects of different type alkyl chains on the properties of type photovoltaic polymers have rarely been investigated.In this article,we synthesized three naphthodithiophene diimide(NDTI)based polymers containing bulky alkyl chains with different branching position,named as NDTI-1,NDTI-2 and NDTI-3,respectively.We systematically investigated the effect of different branching point on the molecular packing,charge transport and photovoltaic performance.When moving the branching point away from the backbone,the intermolecular interaction became stronger,which could be proved by 2D grazing incidence wide angle X-ray scattering(GIWAXS)measurement.Therefore,the electron mobilities in organic field-effect transistors gradually increased from 2.11×10^3 cm^2 V^-1 s^-1 for NDTI-1 to 4.70×10^-2 cm^2 V^-1 s^-1 for NDTI-2 and 9.27×10^-2 cm^2 V^-1 s^-1,for NDTI-3,which are quite high values for polymers with face-on orientation.In addition,the NDTI-2 and NDTI-3 thin films exhibited redshifted absorption spectra compared with NDTI-1.When blending with three classic donor polymers PBDB-T,PTB7-Th and PE61,NDTI-2 based devices always showed the higher power conversion efficiencies(PCEs)than the other two polymers(beside the comparable result of PTB7-Th:NDTI-3 combination)as a result of the high photocurrent response and high fill factor.Our results indicate that bulky alkyl chain with branching point at 2-position should be a good and safe choice for the design of naphthodithiophene diimide-based and even naphthalene diimide-based n-type photovoltaic polymers.

Side chain engineering of quinoxaline-based small molecular nonfullerene acceptors for high-performance poly(3-hexylthiophene)-based organic solar cells

Poly(3-hexylthiophene)(P3HT)is one of the most used semiconducting polymers for organic photovoltaics because it has potential for commercialization due to its easy synthesis and stability.Although the rapid development of the small molecular non-fullerene acceptors(NFAs)have largely improved the power conversion efficiency(PCE)of organic solar cells(OSCs)based on other complicated p-type polymers,the PCE of P3HT-based OSCs is still low.In addition,the design principle and structure-properties correlation for the NFAs matching well with P3HTare still unclear and need to be investigated in depth.Here we designed a series of NFAs comprised of acceptor(A)and donor(D)units with an A2-A1-D-A1-A2 configuration.These NFAs are abbreviated as Qx3,Qx3 b and Qx3c,where indaceno[1,2-b:5,6-b′]dithiophene(IDT),quinoxaline(Qx)and 2-(1,1-dicyanomethylene)rhodanine serve as the middle D,bridged A1 and the end group A2,respectively.By subtracting the phenyl side groups appended on both IDT and Qx skeletons,the absorption spectra,energy levels and crystallinity could be regularly modulated.When paired with P3 HT,three NFAs show totally different photovoltaic performance with PCEs of 3.37%(Qx3),6.37%(Qx3b)and 0.03%(Qx3 c),respectively.From Qx3 to Qx3b,the removing of phenyl side chain in the middle IDT unit results in the increase of crystallinity and electron mobility.However,after subtracting all the grafted phenyl side groups on both IDT and Qx units,the final molecule Qx3 c exhibits the lowest PCE of only 0.03%,which is mainly attributed to the serious phase-separation of the blend film.These results demonstrate that optimizing the substituted position of phenyl side groups for A2-A1-D-A1-A2 type NFAs is vital to regulate the optoelectronic property of molecule and morphological property of active layer for high performance P3HT-based OSCs.