Enabling Efficient Tandem OrganicPhotovoltaics with High Fill Factor viaReduced Charge RecombinationYuqiang Liu,†,‡,# Pei Cheng,†,# Jun Yuan,§ Tianyi Huang,† Rui Wang,† Dong Meng,† Bineh Ndefru,†

Yingping Zou,§ Baoquan Sun,*,‡ and Yang Yang*,†

†Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States‡Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials(FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University,Suzhou 215123, China§College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

*S Supporting Information

ABSTRACT: Compared with inorganic photovoltaics, organic photovoltaics(OPVs) still have a low fill factor (FF) that restricts their power conversionefficiency (PCE). The FF is determined by the resistances that are influencedby carriers recombination. In this work, the tandem structure isdemonstrated to be a feasible solution to improve the FF of OPVs. Therecombination losses of subcells in the tandem structure are reduced andconsequently improves the FF. When the two subcells are stacked in atandem structure, the highest value of the FF reaches 76.6%, much higherthan that of 66.1% achieved by its single junction device. Finally, a PCE of14.1% with a FF of 73.6% is achieved in a tandem OPV after optimizing thethickness of each active layer. This work not only demonstrates the inherentmechanism for a higher FF in tandem OPVs but also presents a feasiblesolution for high-performance photovoltaic devices.

Organic photovoltaics (OPVs) are promising candi-dates for cost-effective photovoltaic devices.1−9 Todate, several approaches, including tandem struc-

ture,10−12 ternary system,13−16 new donors,17−19 and non-fullerene acceptors,20−22 have been presented to improve theperformance of OPVs, and very recently, a certified powerconversion efficiency (PCE) over 15% has been achieved.23

However, there are still some fundamental challenges that havenot been fully addressed.The diffusion length of excitons in organic semiconductor

materials is usually much shorter than that in inorganic ones.Hence, the excitons would experience recombination beforethey dissociate into free carriers at donor/acceptor interfaces inOPVs. To overcome this problem, the concept of a bulkheterojunction has been presented, where organic acceptorsand donors are blended to compose the active layer.24,25 Thethickness of these active layers could reach greater than 100nm to meet the requirements for light absorption. However,thicker active layer causes the fill factor (FF) of OPVs becomespoorer than that of inorganic ones. The FF is determined bythe shunt resistance (RSH) and the series resistance (RS). Ahigh RSH and a low RS will result in an enlarged FF. The RSH isinfluenced by the monomolecular recombination of carriers

through the cell, and increases in monomolecular recombina-tion cause a lower RSH. The RS arises from the bimolecularrecombination caused by the flow resistance of charge carriers,and increases in bimolecular recombination lead to a largerRS.

26−29 Especially, the recombination is more significant at ahigh carrier density. Hence, minimizing the recombination isessential to achieve a satisfactory PCE in OPVs.30,31

The tandem structure has proven to be a promisingapproach to improve the PCE of OPVs, and an enlightenedopen-circuit voltage (VOC) over 2.1 V has been achieved.32,33

Herein, we present that the tandem structure is a feasiblesolution to resolve the problem of recombination in OPVswithout consideration of interface recombination. Afterstacking into a tandem structure, both the monomolecularrecombination and the bimolecular recombination are reduceddue to the decrease in light intensity and the active layerthickness in subcells. The reduced recombination improves theFF to a champion value of ∼77% in a tandem OPV. With anoptimized thickness of active layers, a PCE of ∼14% with a FF

Received: April 26, 2019Accepted: June 6, 2019Published: June 7, 2019

Letterhttp://pubs.acs.org/journal/aelccpCite This: ACS Energy Lett. 2019, 4, 1535−1540

© 2019 American Chemical Society 1535 DOI: 10.1021/acsenergylett.9b00896ACS Energy Lett. 2019, 4, 1535−1540

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of 74% is achieved. This work demonstrates a feasibleapproach for high-performance OPVs and other photovoltaicdevices.Figure 1a is the schematic diagram of a tandem OPV. A

blend film of a donor (PBDB-T, Figure 1d) and a nonfullerene

acceptor (Y1, Figure 1e) is used as the active layer and theabsorption curve of the blend film is shown in Figure S1. Apoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE-DOT:PSS) film serves as an interconnecting electrode. Zincoxide (ZnO) and molybdenum trioxide (MoO3) are used asinterface layers for charge carrier collection. Indium tin oxide(ITO) and silver (Ag) are electrodes. The tandem OPV couldbe separated from the PEDOT:PSS layer into two subcells, i.e.,a rear cell (Figure 1b) and a front cell (Figure 1c). The frontcell is equivalent to a semitransparent OPV. When the sunlightilluminates onto the front cell, part of the sunlight willpenetrate the device, followed by absorption by the rear cell.Hence, the rear cell is equivalent to an OPV illuminated byfiltered sunlight, where the front cell is regarded as the sunlightfilter.The interface recombination losses are not considered. The

schematic diagram of a rear cell sheltered by a sunlight filter isshown in Figure 2a. Since the sunlight is mainly absorbed bythe active layer of PBDB-T:Y1, different thicknesses of PBDB-T:Y1 films serve as the filter that covered onto the rear cell.The structure of the rear cell is ITO/ZnO/PBDB-T:Y1/MoO3/Ag, where the thickness of the active layer is ∼110 nm.Figure 2b is the current density−voltage (I−V) curves of a rearcell with or without a filter. The electrical performances of rearcells are summarized in Table 1 and Table S1. The devicewithout a filter yields a VOC of 0.86 V, a short-circuit currentdensity (JSC) of 22.6 mA/cm2, a FF of 66.1%, and a PCE of12.9%. The FF is improved from 66.1% to 69.3% after coveringwith a 50 nm filter, and further improvement of the FF isachieved with thicker filters. As shown in Figure 2d, the values

of FF are 69.7%, 70.8%, and 72.3%, which correspond to filterthicknesses of 70, 90, and 110 nm, respectively. When thethickness of the filter is 110 nm, an improvement of ∼10% isachieved compared with that of the device without a filter. Toprove the reproducibility of improving FF with a filter, ahistogram is summarized in Figure S2 that demonstratesalmost all devices have an improved FF. Furthermore, a rearcell with a 90 nm active layer is fabricated and the I−V curvesare shown in Figure S3 that exhibits a similar trend aftercovering with filters.In an equivalent circuit diagram of photovoltaic devices, the

resistance could be simplified to the RSH and the RS. And theFF is determined by the two resistances.26,27 Figure 2edemonstrates the RSH of rear cells covered with differentthickness filters, where RSH improves with increasing thethickness of filters. Taking a rear cell with a 110 nm active layeras an example, the RSH of a rear cell without the filter is 682 Ω.After being covered with 50, 70, 90, and 110 nm thicknessfilters, the values of RSH are improved to 910, 1126, 2267, and3003 Ω, respectively.The value of RSH is influenced by the monomolecular

recombination that is a recombination of photogeneratedexcitons before separating into free electrons and holes.28 Themonomolecular recombination varies with the change ofconcentration of photogenerated excitons, where a higherconcentration causes a larger level of monomolecularrecombination. In other words, the monomolecular recombi-nation will be dependent on the sunlight intensity, since theconcentration of photogenerated excitons is dependent on thesunlight intensity. After penetrating a filter, the intensity ofsunlight will decrease, thus lowering the monomolecularrecombination, which improves RSH and consequently elevatesFF.The value of RS also increased after covering with a filter,

which is detrimental to FF. To consider the influence of bothRSH and RS, a calculated FF (FF*) is determined from thefollowing equation:

ikjjjjj

y{zzzzzr

rFF FF (1 ) 1

10 s

sh* = − −

where FF0 is the ideal FF and can be estimated from VOCthrough an empirical equation in the Supporting Informa-tion.34,35 The values of rs and rsh are the influence factors of RSand RSH, respectively, given by the equation of rs = RS/RCH andrsh = RSH/RCH, where RCH is the characteristic resistancecalculated by RCH = VOC/ISC. The values of FF0 are ∼85%,meaning the FF values of all devices have a similar maximum.However, after combining the influence of RSH and RS, anobvious change is demonstrated in FF*, as shown in Figure 2d.The improvement of FF* from 73% to 77% after covering witha 110 nm filter.When the device is covered with a filter, JSC decreases

obviously, owing to the filter absorbing a large amount ofsunlight. The external quantum efficiencies (EQE) spectra inFigure 2c also display a similar decreasing trend with increasingthe thickness of filters. The VOC value of the rear cell with a110 nm filter is 0.03 V lower than that of rear cell without afilter. However, the change in VOC has a negligible influence onFF0 according to Figure 2d.To further prove the aforementioned analyses, the I−V

curves of a rear cell are measured under different sunlightintensities from 10 to 100 mW/cm2, as shown in Figure S4.

Figure 1. Schematic diagrams of (a) a tandem OPV, (b) a separaterear cell, and (c) a separate front cell. Chemical structure of (d)PBDB-T and (e) Y1.

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Due to the reduced monomolecular recombination withdecreasing the sunlight intensity, an improved RSH is achievedresulting in an enlarged FF (Figure 2f). When subcells arestacked into a tandem structure, the sunlight intensity of eachsubcell is lower than their single junction structure. Hence,both the rear cell and the front cell will have a higher FF in atandem structure.The front cell is based on a semitransparent OPV with a

structure of ITO/ZnO/PBDB-T:Y1/MoO3/Ag (15 nm), asshown in Figure 3a. The transmittance curves of front cells are

shown in Figure S5. The I−V curves of the front cell withdifferent thickness of active layers are presented in Figure 3band the electrical performances are summarized in Table S2.The change in VOC of front cells is negligible. With decreasingthe thickness of active layers, the values of JSC display anobvious reduction, which is consistent with the EQE spectra inFigure 3c. The lower JSC is due to the fact that a fraction of thesunlight could penetrate the front cells. The values of FF are68.9%, 68.0%, 65.2%, and 63.4%, which correspond to 50, 70,90, and 110 nm thickness active layers, respectively. Animproved FF is achieved by decreasing the thickness of activelayers (Figure 3d). The thinner active layers decrease the flowresistance of carriers, leading to a reduced bimolecularrecombination occurring between the free electrons andholes.36,37 The reduced bimolecular recombination decreasesRS and thus improves FF. The PCE of front cells is 9.85% and9.93% when 110 and 90 nm PBDB-T:Y1 films serve as activelayers, respectively. An obvious decrease in PCE isdemonstrated after the thickness reduces to 70 and 50 nm.The main reason for the decrease in PCE is owing to the

Figure 2. (a) Schematic diagram of a rear cell under a filter (a PBDB-T:Y1 film is used as the filter). (b) I−V curves, (c) EQE spectra, (d) FFand (e) RSH of devices under different thickness filters, and (f) RSH and FF of a rear cell under different sunlight intensities. FF: fill factor.FF*: calculated fill factor. FF0: ideal fill factor.

Table 1. Electrical Performances of Rear Cells underDifferent Thickness Filters

thicknessVOC(V)

RSH(Ω)

RS(Ω)

FF(%)

FF0(%)

FF*(%)

without filter 0.86 682 3.65 66.1 85.6 73.150 nm filter 0.86 910 4.85 69.3 85.6 73.570 nm filter 0.84 1126 5.46 69.7 85.4 73.890 nm filter 0.84 2267 6.99 70.8 85.4 75.7110 nm filter 0.83 3003 7.09 72.3 85.3 77.1

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thinner active layer, which causes a reduced JSC. However, amildly thick active layer is enough for a front cell in a tandemOPV, since the light harvesting property is enhanced in thetandem structure. Hence, the front cell could have a thinneractive layer to raise the value of FF.The structure of the tandem OPV is ITO/ZnO/PBDB-

T:Y1/MoO3/PEDOT:PSS/ZnO/PBDB-T:Y1/MoO3/Ag(Figure 1a), where MoO3/PEDOT:PSS/ZnO compose theinterconnecting layer to connect the two sequential subcellsinto a tandem structure. Meanwhile, the interconnecting layeris used as a charge recombination zone to align the quasi-Fermilevel of the tandem structure, as shown in Figure 4a of theenergy alignment. Holes from the front cell and electrons fromthe rear cell would be recombined in this zone, and electronsfrom the front cell and holes from the rear cell would becollected at electrodes (Figure S6).The I−V curves of tandem OPVs are demonstrated in

Figure 4b, and detailed parameters are summarized in Table 2

and Table S3. The FF varies with changing the thickness ofactive layers of front cells, and a champion FF of 76.6% isachieved in a tandem OPV. However, the PCE is 10.7% due tothe unbalanced JSC. The improvement is ∼16% compared withthe value of 66.1% in the single junction OPV, which is due tothe reduced the monomolecular and bimolecular recombina-tion of the carriers. The VOC value is ∼1.7 V without obviousvariation with changing the thickness. To achieve the highestPCE of a tandem OPV, the JSC of each subcell must be well-

Figure 3. (a) Schematic diagram of a semitransparent front cell. (b) I−V curves, (c) EQE spectra, and (d) FF of devices based on differentthickness active layers.

Figure 4. (a) Band energy diagram and (b) I−V curves of tandem OPVs.

Table 2. Electrical Performances of Tandem Devices

thickness (nm)

front cell/rear cell VOC (V) JSC (mA/cm2) FF (%) PCE (%)

110/110 1.70 8.2 76.6 10.790/110 1.71 10.3 74.8 13.170/110 1.70 11.2 73.6 14.150/110 1.71 10.9 71.5 13.2

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matched. That is, the thickness of the active layers of eachsubcell should be matched. The JSC of tandem devices is tunedfrom 8.2 to 11.2 mA/cm2 by changing the active layerthickness of front cells. The optimized thickness of the frontcell and the rear cell are 70 and 110 nm, respectively. Thetandem OPV achieves a champion PCE of 14.1% with a FFvalue of 73.6%.In summary, a tandem structure to improve the FF and PCE

of OPVs is presented. The concentration of photogeneratedexcitons is reduced due to the decreased sunlight intensity insubcells, which reduces the monomolecular recombination.Meanwhile, a thinner active layer has a reduced bimolecularrecombination due to decreased flow resistance of carriers.These changes of recombination lead to the larger RSH andlower RS and consequently improve the FF to ∼77%. Finally, aPCE of ∼14% is achieved in tandem OPVs. This workdemonstrates a feasible solution to further improve theperformance of photovoltaic devices.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.9b00896.

Experimental section, empirical equation to calculateFF0 from VOC, absorbance curve of a PBDB-T:Y1 film,summary of the FF with or without a filter, I−V curvesof a rear cell, I−V curves of a rear cell under differentsunlight intensities, transmittance curves of semi-transparent front cells, energy level diagram of tandemOPVs during working conditions, photovoltaic andelectrical performances of solar cells, and resistances oftandem devices (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*Email: bqsun@suda.edu.cn (B.S.).*Email: yangy@ucla.edu (Y.Y.).ORCIDYuqiang Liu: 0000-0003-3494-6390Dong Meng: 0000-0001-6776-0707Yingping Zou: 0000-0003-1901-7243Baoquan Sun: 0000-0002-4507-4578Yang Yang: 0000-0001-8833-7641Author Contributions#Y. Liu. and P. Cheng contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSY.Y. acknowledges the Air Force Office of Scientific Research(AFOSR) (FA2386-18-1-4094), Office of Naval Research(ONR) (N00014-17-1-2,484), and UC-Solar Program (MRPI328368) for their financial support. Y.L. and B.S. acknowledgethe National Key Research and Development Program ofChina (2016YFA0202402), the National Natural ScienceFoundation of China (61674108, 91833303), the 111 Programand Collaborative Innovation Center of Suzhou Nano Scienceand Technology, and the Chinese Scholars Council(201706920073) for their financial support.

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