•ARTICLES• April 2020 Vol.63 No.4: 490–496https://doi.org/10.1007/s11426-019-9668-8

17.1%-Efficiency organic photovoltaic cell enabled with twohigher-LUMO-level acceptor guests as the quaternary strategy

Kun Li1,3, Yishi Wu1, Xuemei Li2*, Hongbing Fu1* & Chuanlang Zhan3*

1Department of Chemistry, Capital Normal University, Beijing 100048, China;2School of Chemistry & Chemical Engineering, Linyi University, Linyi 276000, China;

3CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

Received December 9, 2019; accepted December 20, 2019; published online March 11, 2020

Quaternary blended organic solar cells utilize four blended material components (one donor plus three acceptors, two donors andtwo acceptors, or three donors plus one acceptor) as the active layer materials. The use of four material components allows us tohave more material selections and more mechanism choices to improve the photon-to-electron conversion efficiency. In thiscontribution, we present a new case of quaternary material system, that shows 17.1% efficiency obtained by adding IDIC andPC71BM as the guest acceptors of the host binary of PM6:Y6. The lowest unoccupied molecular orbital (LUMO) levels of IDICand PC71BM are both higher than that of Y6, which is one reason to obtain increased open-circuit voltage (Voc) in the quaternarydevice. Upon introduction of IDIC and PC71BM as the acceptor guests, the hole and electron mobilities are both increased, whichcontributes to the increased short-circuit current-density (Jsc). Effects of the weight ratios of the three acceptor components areinvestigated, which demonstrates that the increased hole and electron mobilities, the accelerated hole-transfer, and the reducedmonomolecular recombination are the factors contributing to the increased Jsc and fill-factor. This case of quaternary devicedemonstrates the applicability of the quaternary strategy in increasing the device functions and hence the efficiencies in the fieldof organic photovoltaic cells.

quaternary solar cell, organic photovoltaic, small-molecule acceptor, nonfullerene, fullerene

Citation: Li K, Wu Y, Li X, Fu H, Zhan C. 17.1%-Efficiency organic photovoltaic cell enabled with two higher-LUMO-level acceptor guests as the quaternarystrategy. Sci China Chem, 2020, 63: 490–496, https://doi.org/10.1007/s11426-019-9668-8

1 Introduction

The invention of Y6 (2,2′-((2Z,2′Z)-(12,13-bis(2-ethylhex-yl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylyli-dene))-bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)dimalononitrile) [1] based electron acceptor ma-terials pushes a great advance in the research of organicphotovoltaic cells. The power conversion efficiencies(PCEs) have reached over 16% via selection of the pairing

donor polymer [2–4], modifications on the molecular struc-ture of Y6 [5,6], or using a ternary material strategy [7–12].In comparison to the donor:acceptor binary blend, the use

of an additional donor or acceptor component, calling theternary approach, enables to improve the device functions[13–16]. For example, introduction of a smaller bandgapdonor or acceptor can help to achieve a wider coverage of thenear infrared solar emission spectrum so that the short-circuitcurrent-density (Jsc) can be increased [17–24], while byloading an electron donor material with its highest occupiedmolecular orbital (HOMO) level deeper than the HOMOlevel of the host donor [25] or introducing an electron ac-ceptor material with its lowest unoccupied molecular orbital(LUMO) energy higher than the LUMO energy of the host

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*Corresponding authors (email: (xuemei_li@yeah.net; hbfu@cnu.edu.cn;clzhan@iccas.ac.cn)

acceptor [26–29], the open-circuit voltage (Voc) can be in-creased. Again, the fill factor (FF) can be improved if theloaded third component can form alloy-like phase with theimprovement of the film-morphology [30–34].We and others have recently presented the quaternary

strategy in which four components, either one donor plusthree acceptors, two donors and two acceptors, or three do-nors plus one acceptor, are blended to act as the photoactivelayer. The inclusion of four components in the quaternarystrategy can allow us to have more material selections andhence more mechanism choices to enhance the photon-to-electron conversion. We observed that the addition of mixedPCBM and ICBA as the acceptor guest of PBDB-T:ITICenabled the reduction of the ITIC optical bandgap, con-tributing to increased Jsc. Like that the PCBM and ICBAperformed with synergetic effects in their ternary blends, themixing of PCBM and ICBA as the acceptor guests also showsynergetic effects with the benefits of concurrently increasedVoc and FF [35]. The addition of one more fullerene as theforth component enabled to increase the phase crystallinity[36] and push more nonfullerene molecules for formingcompacted π-π-stacks [37]. He et al. [38] observed that thequaternary strategy can help to improve charge transporta-tion with a broad tolerance of acceptor composites. Ma et al.[39] observed that the quaternary material system can bedesigned by introducing an absorption-complementary ma-terial as the forth component. Through the quaternary strat-egy, PCEs of 12.0%–13.3% have been achieved [40].In this work, we report a quaternary organic photovoltaic

cell with 17.1% efficiency. This new quaternary materialsystem is designed by selecting the crystalline IDIC andPC71BM as the guest acceptors of the host binary of PM6:Y6.Figure 1 shows the molecular structures of the four materials.The reasons for the selections are as follows. We have ob-served that the addition of IDIC as the acceptor guest of thebinary host of PM6:Y6 enabled to increase the phase crys-tallinity with the hole and electron mobilities both increased[7]. And we and others observed that the addition of PCBMas the acceptor guest of PM6:Y6 enabled to increase thephase purity [12] and reduce the nonradiative energy loss

[10]. On basis of these observations, we speculated that theloading of mixed IDIC and PCBM as the quaternary strategywould benefit IDIC and PCBM synergized with the obtain-ing of an even larger Jsc and FF, in addition to the higher Vocas a result of the higher LUMO levels and reduced Voc loss,which have been demonstrated by the experimental resultspresented herein.

2 Results and discussion

Figure 2(a) shows the absorption spectra of the polymer andthe host (Y6) and guest (IDIC and PC71BM) acceptors. Theabsorption spectra of the four materials are complementaryto each other, which is beneficial for the more full coverageof the solar emission spectrum from 300 to 950 nm when thefour materials are combined to form a quaternary film. Forthe energy levels (Figure 2(b)), the LUMO levels of IDICand PC71BM are both about −3.90 eV that is higher than theLUMO energy (−4.11 eV) of Y6. The HOMO level(−5.60 eV) of IDIC is nearly parallel to the HOMO energy(−5.62 eV) of Y6. The HOMO of PC71BM is much deeperwith a value of −5.96 eV. The matched energy levels of the

Figure 1 Molecular structures of the donor polymer and the host andguest acceptors (color online).

Figure 2 Film absorption spectra (a) and energy levels (b) of donor polymer and the host (Y6) and guest (IDIC and PC71BM) acceptors. The energy levelsof PM6 and the host and guest acceptors were measured under the same experimental conditions [7,12] (color online).

491Li et al. Sci China Chem April (2020) Vol.63 No.4

host and guest acceptors demonstrate that they can be used asthe acceptor guests of PM6:Y6 in a quaternary strategy.The quaternary devices were fabricated with a device

structure ITO/PEDOT:PSS/active layer/PDINO/Al. On basisof the optimized weight ratio of the ternary blend of PM6:Y6:IDIC, which is 1:1:0.2 [7], we added PC71BM as the forthcomponent. The best quaternary device was obtained as 0.1weight ratio amount of PC71BM was blended (Table S1,Supporting Information online). The optimal conditionswere the use of 0.75% CN and an annealing temperature of90 °C (Tables S2, S3). Figure 3(a, b) are the current-density-voltage (J-V) curve and external quantum efficiency (EQE)spectrum of the optimized quaternary solar cell. The photo-voltaic data are shown in Table 1. The best quaternary devicehas a PCE of 17.07% with a Voc of 0.866 V, a Jsc of26.19 mA/cm2, and an FF of 75.29%. The integrated Jscvalue is 25.28 mA/cm2 that agrees well with the Jsc valueobtained from the J-V curve. The highest EQE is 83.7%,occurring at 625 nm. As the control, the photovoltaic para-meters of the PM6:Y6 binary device are as follows: Voc=0.838 V, Jsc=25.45 mA/cm

2, and FF=72.33%.The effects of the guest-to-guest and the host-to-guest

weight ratios on the device functions of the quaternary solarcell are investigated. The J-V curves and the solar cellparameters are shown in Figure 4(a, b), respectively. As

shown in Table 1, above 16% efficiencies are readily ob-tained and all of the devices exhibit a good FF of about 75%as the IDIC-to-PC71BM ratio is changed from the optimal0.2:0.1 to 0.1:0.2 (with the keeping of the ratio of PM6:Y6 as1:1) and the host-to-guest ratio is changed from 1.0:0.3 to1.2:0.3 (with the maintenance of the IDIC-to-PC71BM ratioas 0.2:0.1), demonstrating a relatively broad ranging toler-ance of the high efficiency of the PM6:Y6:IDIC:PC71BMbased quaternary solar cell. Compared to the device fabri-cated with PM6:Y6:PC71BM=1:1:0.3, the introduction ofIDIC supplies a relatively larger Voc than the addition ofPC71BM (0.866–0.869 V vs. 0.851 V).The trend of the Jsc value is consistent with that of the hole

and electron mobilities (Figure 4(c)). The hole and electronmobilities were measured using the space-charge-limited-current (SCLC) method with the dark J-V data shown inFigure S1 (Supporting Information online). The largest holeand electron mobilities and again the most balanced hole-to-electron mobilities are observed at the optimized weight ratioof PM6:Y6:IDIC:PC71BM= 1.0:1.0:0.2:0.1. Compared to the1.0:1.0:0.1:0.2 quaternary blend, the 1.0:1.0:0.2:0.1 blendexhibits a larger hole and again a larger electron mobilityvalue, which is due to that the use of IDIC as the guest showsa relatively larger hole and electron mobilities than the use ofPC71BM as the acceptor guest (Figure 4(c)): for the IDIC

Figure 3 The J-V curve (a) and EQE spectrum (b) of the optimized quaternary device. The insert in (a) is the histogram of the PCEs of the quaternary solarcells (color online).

Table 1 The photovoltaic data with different electron-acceptor materials. The electron-donor material is PM6 for all PSCs. All data were obtained underillumination of AM 1.5G (100 mW/cm2) light source

PM6:Y6:IDIC:PC71BM Voca) (V) Jsc

a) (mA/cm2) FF a) PCEavea) (%)

1:1.0:0:0 0.838 25.45 72.33 15.46

1:1.0:0:0.3 0.851 (0.850±0.001) 25.28 (25.01±0.31) 75.15 (74.86±0.35) 16.16 (16.01±0.18)

1:1.0:0.1:0.2 0.866 (0.865±0.001) 25.61 (25.18±0.57) 74.72 (74.31±0.45) 16.57 (16.41±0.22)

1:1.0:0.2:0.1 0.866 (0.865±0.001) 26.19 (25.91±0.31) 75.29 (74.99±0.31) 17.07 (16.98±0.11)

1:1.1:0.2:0.1 0.867 (0.866±0.001) 25.88 (25.67±0.28) 74.15 (73.82±0.38) 16.64 (16.49±0.19)

1:1.2:0.2:0.1 0.868 (0.867±0.001) 25.12 (24.96±0.23) 75.02 (74.76±0.34) 16.34 (16.21±0.15)

1:1.0:0.3:0 0.869 (0.867±0.002) 24.55 (24.31±0.26) 75.59 (75.11±0.52) 16.13 (16.04±0.14)

1:0:1.0:0b) 0.924 10.81 71.6 7.13

1:0:0:1.0c) 0.911 13.4 65.1 8.00

a) Average values from 20 devices shown in parentheses. b) Data from ref. [7]. c) Data from ref. [12].

492 Li et al. Sci China Chem April (2020) Vol.63 No.4

ternary blend (in a weight ratio PM6:Y6:IDIC=1:1:0.3), thehole and electron mobilities (μh/μe) are 5.45/8.62×10−4 cm2 V−1 s−1 and for the PC71BM ternary blend(also in a weight ratio PM6:Y6:IDIC=1:1:0.3), the values are5.09/7.86×10−4 cm2 V−1 s−1.Under illumination, the Jsc value is mainly contributed by

the photogenerated current-density (Jph) that is the differencebetween the current-density under illumination and underdark, as shown in Figure S2. The trending to a saturatedvalue of Jph implies that the photogenerated hole and electroncarriers can be efficiently collected by the right electrode at alarge effective internal field regime. The Jph values at theshort-circuit point (Jph,sc) and at the high applied voltagepoint (here, 1.5 V) (Jph,sat) are plotted in Figure 4(d). BothJph,sc and Jph,sat change in a similar trend to the Jsc value,consistent with the trend of the charge mobilities, suggestingthat the change of the charge mobilities is the factor de-termining the scale of the Jsc value in these quaternary de-vices when the acceptor ratios are varied accordingly. Therelatively larger collection efficiency as demonstrated by theJph,sc/Jph,sat (Figure 4(d)) is observed in a similar trend to the

Jsc value, being another contribution to scale of Jsc.The recombination mechanisms at short-circuit and open-

circuit are studied by following the light dependent mea-surements of J-V measurements. Figure S3 shows the plotsof the Jsc and Voc as a function of the incident light intensity.The linear fitting gives a slope value from which the α andnkT/q value is obtained, respectively. Here, k, T, and q are theBoltzmann constant, temperature in Kelvin, and the ele-mentary charge, respectively. The α and n values are plottedin Figure 4(e). For all the quaternary solar cells, close αvalues (all between 0.99–1.0) and close n values (all about1.15) are obtained, meaning that the charge recombinationdoes not change upon the change of the acceptor ratios,which is consistent with the maintenance of the device FF. Atthe optimal weight ratio of PM6:Y6:IDIC:PC71BM=1.0:1.0:0.2:0.1, the series resistance (Rs) and the shunt re-sistance (Rsh) are 3.9 and 1.9 kΩ cm2, respectively. With thechange of the ratios of the acceptors, the Rs and Rsh values arenearly maintained (Figure 4(e)). Again, all the quaternarydevices show low leakage current at the low applied voltageregion (Figure 4(f)).

Figure 4 (a) The illuminated J-V curves of the quaternary solar cells fabricated under different acceptor ratios. (b–e) The plots of the photovoltaicparameters (b), of the hole and electron mobilities (c), of the Jph,sc and Jph,sat (d), and of the values α, n, Rs and Rsh (e) versus the compositions of the threeacceptor materials. (f) The dark J-V curves of the quaternary solar cells fabricated under different acceptor ratios (color online).

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Photo-fluorescence (PL) spectra shown in Figure 5(a)demonstrated that the fluorescent intensity of PM6 and theacceptor is largely reduced when they are blended to formbinary blend, due to the photo-induced charge transfer be-tween polymer and the electron acceptor materials. Com-pared to the fluorescence intensity of the PM6:Y6 binaryblend, the addition of 0.3 weight ratio amount of PC71BM orIDIC as the guest acceptor slightly reduced the fluorescenceintensity of Y6, indicating the hole transfer from Y6 to PM6could be slightly enhanced. For the quaternary blend, the useof IDIC and PC71BM in a weight ratio of 0.2:0.1 as theacceptor guests further reduces the intensity of Y6 and PM6,providing evidence of the enhancement of the hole transferfrom Y6 to PM6 and again the electron transfer from PM6 tothe acceptor.Femtosecond transient absorption (fs-TA) spectroscopy

was performed to study the hole transfer dynamics of thebinary, ternary and quaternary blends. A pump wavelength of832 nm was used to selectively excite Y6, at that wavelength

no signal of PM6 was populated in PM6-only film. The fs-TA spectra of neat Y6 film, the IDIC and PC71BM ternaryand the optimal quaternary blends were shown as the ex-amples in Figure 6(a, b) and Figure S4(a, b), respectively. TAspectra of the neat Y6 film (Figure 6(a)) show two groundstate bleaching (GSB) bands at 854 and 730 nm, as well astwo excited state absorption (ESA) bands peaked at 573 and921 nm, respectively. The 854-nm GSB signal decays multi-exponentially with lifetimes of 0.34, 2.5, 84.4 and 4.7 ns(Table S4). When the binary, ternary, and quaternary systemsare excited, a new negative band appears at 630 nm (Figure 5(b) and S4). This signal can be assigned to GSB of PM6. Theobservation of PM6 GSB indicates the hole transfer takesplace from Y6 to PM6. Figure 6(c, d) gives the comparisonsof the charge carrier dynamics shown in the kinetic curves at630 nm for the blended systems. The rate for the holetransfer to generate charge carrier can be determined fromthe initial rising time stage of 630-nm curve, resulting in arising lifetime of 0.19–0.34 ps for the binary, ternary and

Figure 5 Photo-fluorescence (PL) spectra of the pure donor polymer and the host and guest acceptor films (a) and binary, ternary and quaternary blendedfilms (b) (color online).

Figure 6 (a, b) fs-TA spectra of neat Y6 and PM6:Y6:IDIC:PC71BM (1:1.0:0.2:0.1) quaternary blends obtained under excitation with 832 nm wavelengthlight. (c, d) Comparison of the the kinetic and fitting curves at 600 nm for the binary, ternary and quaternary blends (color online).

494 Li et al. Sci China Chem April (2020) Vol.63 No.4

quaternary systems (τ1 in Table S4). The addition of IDIC asthe acceptor guest slightly accelerates the hole transfer(0.21 ps) compared to the use of 0.3 weight ratio amount ofPC71BM as the guest (0.25 ps), and among the quaternarysystems, the most accelerated hole transfer (0.20 ps) is ob-served in the optimal quaternary system with PM6:Y6:IDIC:PC71BM=1:1.0:0.2:0.1. The accelerated hole transfer ac-companying with the largest charge mobilities (Figure 4(c))and the most reduced monomolecular recombination (Figure4(e)) contributes to the largest Jsc and FF among the fourquaternary systems.The film-morphology of the quaternary solar cell blend

films was investigated with the transmission electron mi-croscopy (TEM) and atomic force microscopy (AFM) heightand phase techniques (Figure 7). TEM images (a–f) indicatethe formation of nanoscale film-morphology for all of thequaternary blends. The maintenance of the nanoscale mor-phology can be explained by the structural similarity be-tween Y6 and IDIC and the good miscibility betweenPC71BM and the nonfullerene acceptor, which benefits theformation of the alloy-like acceptor phase. Nevertheless,quite different morphology can be seen, demonstrating thefine-tuning of the film-morphology as the result of thechange of the acceptors weight ratios. Compared to the na-noscaled particle-like morphology of the white domains seenon the 1:1:0.3 blended PM6:Y6:PC71BM film, thinner fibrilsplus particles are more frequently seen on the 1:1:0.3 blen-ded PM6:Y6:IDIC film. Upon replacement of the PC71BMcontent (here, 0.3) with 0.1 or 0.2 weight ratio amount ofIDIC, affording the 1:1:0.1:0.2 (Figure 7(b)) or 1:1:0.2:0.1(Figure 7(c)) blended quaternary blend, smaller and lessparticle-like white and dark domains are seen and the thinner

fibrils (white) become more frequently seen, which helps tomake a finer interpenetrating 3-dimentional (3-D) networks,which could be the contribution to the increased hole andelectron mobilities. Upon increasing the Y6 content from the1:1:0.2:0.1 blend to 1:1.1:0.2:0.1 (Figure 7(d)) and then1:1.2:0.2:0.1 (Figure 7(e)), the white fibers become thickerand the dark particles seems to become larger, which agreeswith the decreased charge mobilities.The AFM height (Figure 7(g–l)) and phase (Figure 7(m–r))

images also demonstrate the differences of the surfacemorphology of the quaternary blends. The fibrils with lightcolor seen on the AFM height image are corresponding to thedonor polymer rich phases. On the AFM phase images, thefibrils are clearly seen as indicated with the light-color. Re-latively, the brown regions having less fibril feature corre-spond to the acceptor-rich domains. As indicated on theheight images, the loading of IDIC tends to help the donorpolymer to be connected to form a 2-D surface morphologyand the acceptor domains are surrounded separately. Thefinest and smoothest 2-D surface morphology is seen on theoptimal quaternary blend with PM6:Y6:IDIC:PC71BM=1:1:0.2:0.1, which is consistent with the highest chargecollection observed in this device.

3 Conclusions

In the summary, we present a new quaternary material sys-tem that shows a PCE of 17.1%, the record efficiency re-ported to date from the organic photovoltaic cells. Thismaterial system is achieved by introducing the crystallineIDIC and PC71BM into the recently emerging Y6 material

Figure 7 The TEM (a–f) and AFM height (g–l) and phase (m–r) images of the quaternary solar cell blend films fabricated under different acceptor ratios:PM6:Y6:IDIC:PC71BM=1:1:0:0.3 (a, g, m), 1:1:0.1:0.2 (b, h, n), 1:1:0.2:0.1 (c, i, o), 1:1.1:0.2:0.1 (d, j, p), 1:1.2:0.2:0.1 (e, k, q), and 1:1:0.3:0 (f, l, r) (coloronline).

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system. The higher LUMO levels of IDIC and PC71BM arethe reason of the obtaining of the increased Voc. The in-creased hole and electron mobilities upon the addition of themixed IDIC and PC71BM as the third and fourth componentcontribute to the increased Jsc value. Over 16% efficienciesare readily obtained with the weight ratios of the three ac-ceptors varied, demonstrating the high efficiency of thisquaternary material system.

Acknowledgements This work was supported by the National NaturalScience Foundation of China (91433202, 21773262, 21327805) and TaishanScholars Program of Shandong Province (tsqn201812101).

Conflict of interest The authors declare that they have no conflict ofinterest.

Supporting information The supporting information is available online athttp://chem.scichina.com and http://link.springer.com/journal/11426. Thesupporting materials are published as submitted, without typesetting orediting. The responsibility for scientific accuracy and content remains en-tirely with the authors.

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