materials Article
Efficient Inverted Organic Solar Cells Based on a Fullerene Derivative-Modified Transparent Cathode Yifan Wang 1,2 , Hailin Cong 1,2, * 1 2 3 4
*
ID
, Bing Yu 1,2 , Zhiguo Zhang 3 and Xiaowei Zhan 4
ID
Institute of Biomedical Materials and Engineering, College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, China; [email protected] (Y.W.); [email protected] (B.Y.) Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China; [email protected] Correspondence: [email protected]; Tel.: +86-532-8595-3995; Fax: +86-532-8595-5529
Received: 18 July 2017; Accepted: 7 September 2017; Published: 11 September 2017
Abstract: Indium tin oxide (ITO) is a transparent conductive material which is extensively used in organic solar cells (OSCs) as electrodes. In inverted OSCs, ITO is usually employed as a cathode, which should be modified by cathode buffer layers (CBLs) to achieve better contact with the active layers. In this paper, an amine group functionalized fullerene derivative (DMAPA-C60 ) is used as a CBL to modify the transparent cathode ITO in inverted OSCs based on PTB7 as a donor and PC71 BM as an acceptor. Compared with traditional ZnO CBL, DMAPA-C60 exhibited comparable transmittance. OSCs based on DMAPA-C60 show much better device performance compared with their ZnO counterparts (power conversion efficiencies (PCEs) improved from 6.24 to 7.43%). This is mainly because a better contact between the DMAPA-C60 modified ITO and the active layer is formed, which leads to better electron transport and collection. Nanoscale morphologies also demonstrate that the surface of DMAPA-C60 -modified ITO is plainer than the ZnO counterparts, which also leads to the better device performance. Keywords: organic solar cell (OSC); cathode buffer layer (CBL); transparent conducting material; fullerene derivative; ZnO
1. Introduction Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been regarded as one of the most advanced photovoltaic technologies because of their advantages over traditional silicon solar cells, which include low cost, light weight, and flexibility [1–4]. Nowadays, OSCs based on fullerene as acceptors or non-fullerene organic materials as acceptors have achieved power conversion efficiencies (PCEs) of over 11% [5–9]. Inverted OSCs, which are mostly based on conductive indium tin oxides (ITO) as cathodes and silver (Ag) as anodes, possess major advantages of enhanced device performance as well as increased stability compared with conventional OSCs [10,11]. In recent years, inverted OSCs have developed rapidly, mostly benefiting from the development of novel donor and acceptor materials and interfacial engineering [12]. Electrode-semiconducting active layer contacts in organic electronics are plagued by interfacial barriers for charge transfer due to poor alignment between the electrode work function and the Fermi level of the semiconductor [13]. As a result, the cathode buffer layer (CBL) between the active layer and conductive ITO cathode plays an important role in the performance of inverted OSCs because it can decorate the ITO cathode and promote the electron transfer and extraction process [14,15]. Materials 2017, 10, 1064; doi:10.3390/ma10091064
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CBLs can be grouped into two categories: inorganic and organic materials. Inorganic materials 2 of 8 Materials 2017, 10, 1064 are commonly transition-metal oxides, such as CsCO3 [16,17], TiOx [18,19], and ZnO [20–22]. Different from inorganic CBLs, organic CBLs exhibit the advantage of structural tunability, which could can offerbe more opportunities of realizing better interfaceand modification and high-performance fully CBLs grouped into two categories: inorganic organic materials. Inorganic materials solution-processed OSCs [23,24]. In view various types of organic CBLs have drawn much are commonly transition-metal oxides, such of as this, CsCO [16,17], TiO [18,19], and ZnO [20–22]. Different x 3 attention and have been recently developed. For example, water (alcohol)-soluble conjugated from inorganic CBLs, organic CBLs exhibit the advantage of structural tunability, which could offer more polymers such as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9opportunities of realizing better interface modification and high-performance fully solution-processed dioctylfluorene)] (PFN) and its derivatives [25,26] have been successfully utilized as CBLs in inverted OSCs [23,24]. In view of this, various types of organic CBLs have drawn much attention and OSCs and have been proved to be comparable to or even better than the conventional ZnO CBL. have Other been organic recentlyCBLs developed. For example, water (alcohol)-soluble conjugated polymers such as such as polyethylenimine ethoxylated (PEIE) [27] and titanium (diisopropoxide) 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and its poly[(9,9-bis(3 bis(2,4-pentanedionate) (TIPD) [28] have also been proved to be potential CBLs in inverted OSCs with derivatives [25,26] better have than beenZnO successfully utilizedfullerene-based as CBLs in inverted beenasproved performances CBLs. Recently, materialsOSCs have and beenhave designed a to benew comparable to Fullerene-based or even bettermaterials than the ZnO Otherfor organic CBLs such type of CBL. areconventional considered to be idealCBL. candidates CBLs, because their structures are similar to the conventionally used fullerene acceptors, which could smoothly as polyethylenimine ethoxylated (PEIE) [27] and titanium (diisopropoxide) bis(2,4-pentanedionate) bridge electrons transporting from the fullerene acceptor to the cathode [29–32]. For example,better Alex than (TIPD) [28] have also been proved to be potential CBLs in inverted OSCs with performances and co-workers reported that, with a new kind of fullerene derivative Bis-OMe fulleropyrrolidinium ZnO CBLs. Recently, fullerene-based materials have been designed as a new type of CBL. Fullerene-based iodide (FPI) blended PEIE CBL in inverted OSCs, a much better performance is achieved compared materials are considered to be ideal candidates for CBLs, because their structures are similar to the with using only PEIE as the CBL [33]. An amine group functionalized fullerene complex (DMAPAconventionally used fullerene acceptors, which could smoothly bridge electrons transporting from the C60, Figure 1a) was successfully synthesized as an ideal candidate for CBLs to modify the Al cathode fullerene acceptor toOSCs the cathode [29–32]. amino For example, Alexpotentially and co-workers that, with a new in conventional [34]. Its terminal group could providereported a dipole moment, just kind of iodide (FPI) blended PEIE CBL(LUMO) in inverted as fullerene the aminoderivative group doesBis-OMe in PFN. fulleropyrrolidinium Furthermore, the lowest unoccupied molecular orbital OSCs,energy a much better performance achieved with using only PEIE thehighest CBL [33]. An amine level is estimated to be is 3.58 eV, closecompared to that of PC 71BM (about 3.85 eV).asThe occupied groupmolecular functionalized (DMAPA-C , Figure 1a) was successfully synthesized as an ideal orbitalfullerene (HOMO) complex energy level is estimated to be 5.52 eV, which is low enough to efficiently 60 block holes from various donors. candidate for CBLs to modify the Al cathode in conventional OSCs [34]. Its terminal amino group could this paper, we demonstrate 60 cangroup be independently used as a CBL to modify potentiallyInprovide a dipole moment,that justDMAPA-C as the amino does in PFN. Furthermore, the lowest the transparent conductive cathode ITO in inverted OSCs based on poly{4,8-bis[(2unoccupied molecular orbital (LUMO) energy level is estimated to be 3.58 eV, close to that of PC71 BM ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno (about 3.85 eV). The highest occupied molecular orbital (HOMO) energy level is estimated to be 5.52 eV, [3,4-b]thiophene-4,6-diyl} (PTB7) as a donor and PC71BM as an acceptor. Compared with the which is low enough to efficiently block holes from various donors. traditional ZnO CBL, a much better device performance was realized (PCE improved from 6.24 to 7.43%).
(a)
(b)
Figure 1. (a) Chemical structures of PTB7, PC71 BM, and DMAPA-C60 ; (b) Device structure of the inverted OSC.
In this paper, we demonstrate that DMAPA-C60 can be independently used as a CBL to modify the transparent conductive cathode ITO in inverted OSCs based on poly{4,8-bis[(2-ethylhexyl)oxy]benzo dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4,[3,4-b]thiophene-4,6-diyl} (PTB7)
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Figure (a) Chemical structures of PTB7, PC71BM, and DMAPA-C60; (b) Device structure of the as a donor and1.PC 71 BM as an acceptor. Compared with the traditional ZnO CBL, a much better device inverted OSC. performance was realized (PCE improved from 6.24 to 7.43%).
Results and Discussion 2. 2. Results and Discussion 2.1. Absorption and Transmittance 2.1. Absorption and Transmittance ToTo modify ITO cathodes in in inverted OPV devices, thethe buffer layers should bebe asas transparent asas modify ITO cathodes inverted OPV devices, buffer layers should transparent possible, which can guarantee the more sunlight will penetrate the cathode and bebe absorbed byby thethe possible, which can guarantee the more sunlight will penetrate the cathode and absorbed active layers. The normalized spectra ofof optical absorption of of thethe ZnO buffer layer and DMAPA-C active layers. The normalized spectra optical absorption ZnO buffer layer and DMAPA-C 60 60 buffer layer areare shown in in Figure 2a.2a. Both ZnO and DMAPA-C buffer weak buffer layer shown Figure Both ZnO and DMAPA-C bufferlayers layersexhibit exhibitvery very weak 60 60 absorption transmittancespectra spectraofofthe theITO ITOlayer, layer, ZnO-modified ITO layer, absorptionatat400~800 400~800nm. nm. The transmittance ZnO-modified ITO layer, and and DMAPA-C ITOlayer layerare areshown shownin in Figure Figure 1b. They of of DMAPA-C 60-modified ITO They all allshow showexcellent excellenttransmittance transmittance 60 -modified over 85%, demonstrating the excellent light transmissivity the buffer layers, which suitable over 85%, demonstrating the excellent light transmissivity ofof the buffer layers, which is is suitable forfor modifying ITO cathode. modifying thethe ITO cathode.
Figure UV-Vis absorption spectra ZnO and DMAPA-C 60 films; Transmittance spectra Figure 2. 2. (a)(a) UV-Vis absorption spectra of of ZnO and DMAPA-C (b)(b) Transmittance spectra of of 60 films; 60and , and ITO/ZnO films. ITO, ITO/DMAPA-C ITO, ITO/DMAPA-C , ITO/ZnO films. 60
2.2. Photovoltaic Performances 2.2. Photovoltaic Performances fabricated BHJ OSCs with a structure of ITO/CBL/PTB7:PC 71BM/MoO 3/Ag using classical WeWe fabricated BHJ OSCs with a structure of ITO/CBL/PTB7:PC using classical 71 BM/MoO 3 /Ag 71 BM as an acceptor material. Table 1 shows low-bandgap polymer PTB7 as a donor material and PC low-bandgap polymer PTB7 as a donor material and PC71 BM as an acceptor material. Table 1 shows the the device parameters of DMAPA-C 60-based OSCs with different DMAPA-C 60 concentrations, device parameters of DMAPA-C -based OSCs with different DMAPA-C concentrations, indicating 60 60 indicating thickness ofdependence of these After OSCs optimization, OSCs with 2 mg/mL the thicknessthe dependence these devices. After devices. optimization, with 2 mg/mL DMAPA-C 60 DMAPA-C 60 shows the best performance with a PCE of 7.43%. Figure 3a shows the current shows the best performance with a PCE of 7.43%. Figure 3a shows the current density-voltagedensity(J-V) voltage curves of the based on different CBLs (under theofillumination of mW AM1.5G, curves of (J-V) the OSCs based onOSCs different CBLs (under the illumination AM1.5G, 100 ·cm−2100 ). −2). The corresponding device parameters are summarized in Table 2. The devices with bare mW·cm The corresponding device parameters are summarized in Table 2. The devices with bare ITO as a −2 mA·cm ITO asshows a cathode shows the PCEwith of 4.93%, a very small SC of·cm 12.0 and an fill cathode the lowest PCElowest of 4.93%, a verywith small JSC of 12.0 JmA and an−2fill factor factor (FF) 58.5%. of only 58.5%. This be ascribed the direct contact of ITO with active layer, leading (FF) of only This can becan ascribed to thetodirect contact of ITO with thethe active layer, leading a large series resistance S) and a small parallel resistance (R). SH). Whenwe weutilize utilize ZnO CBL toto a large series resistance (RS(R ) and a small parallel resistance (RSH When ZnO CBL toto modify ITO cathode, device performance enhances significantly, with VOC improving from modify thethe ITO cathode, thethe device performance enhances significantly, with V OC improving from −2 −2 −2−2, ,FF 0.69 0.71 J SC from 12 mA·cm to 12.8 mA·cm FF from 58.5 to 66.9%, and PCE from 4.93 0.69 VV to to 0.71 V, JV, from 12 mA · cm to 12.8 mA · cm from 58.5 to 66.9%, and PCE from 4.93 to to SC − 6.24%. Simultaneously, it can be seen that with ZnO as the CBL, the R S decreases from 7.03 Ω·cm 6.24%. Simultaneously, it can be seen that with ZnO as the CBL, the RS decreases from 7.03 Ω·cm −22 to that better contact is formed between thethe cathode andand thethe active layer, and to6.22 6.22Ω·cm Ω·cm−2−, 2indicating , indicating that better contact is formed between cathode active layer, carrier trapping and recombination inside the device is reduced. With DMAPA-C 60 as the CBL, and carrier trapping and recombination inside the device is reduced. With DMAPA-C60 as the CBL,an the enhancement enhancement of ofJJSC SC from from 12.8 12.8 mA mA·cm aneven evenhigher higherPCE PCEof of 7.43% 7.43% is is achieved, mostly due to the ·cm−−22 to −2 −2 − 2 − 2 is is consistent with thethe fact that thethe RS R value (3.50 Ω·cm ) of the 60 device to15.7 15.7mA·cm mA·cm . This . This consistent with fact that (3.50 Ω·cm ) ofDMAPA-C the DMAPA-C S value 60 −2). The − 2 is much smaller than that of the ZnO device (6.22 Ω·cm external quantum efficiency (EQE) device is much smaller than that of the ZnO device (6.22 Ω·cm ). The external quantum efficiency curvecurve is shown in Figure 2b. The Jsc values integrated fromfrom the the EQEEQE spectra are are 11.9, 15.1, 12.3 (EQE) is shown in Figure 2b. The Jsc values integrated spectra 11.9, 15.1, −2 for −2 for mA·cm thetheno-CBL, 60-based, and ZnO-based respectively, which 12.3 mA·cm no-CBL,DMAPA-C DMAPA-C -based, and ZnO-based devices, respectively, which is is 60 consistent with the values shown in Table 2. consistent with the values shown in Table 2.
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Table 1. Device performances of OSCs based on DMAPA-C60 with different DMAPA-C60 Materials 2017, 10, 1064 concentrations.
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DMAPA-C60
VOC
JSC
FF
PCE (%)
Table 1. Device(mg/mL) performances of OSCs on DMAPA-C with different −2) a 60 60 concentrations. (V) based (%) (mA·cm BestDMAPA-C Average
1 1.5 DMAPA-C60 (mg/mL) 2 1 2.5 1.5 3 2 2.5 3
0.69 0.69 V OC (V) 0.70 0.69 0.70 0.69 0.69
a
14.2 57.3 5.61 14.3 63.4 6.26 − 2 FF (%) JSC (mA·cm ) 15.7 65.7 7.43 15.414.2 61.4 57.36.62 14.814.3 62.9 63.46.42
0.70 15.7 65.7 The average PCE was obtained from 25 devices. 0.70 15.4 61.4 0.69 14.8 62.9
5.48 PCE (%) 6.1 Best 7.27 Average a 5.61 6.45 5.48 6.26 6.29 6.1 7.43 6.62 6.42
7.27 6.45 6.29
Figure 3c shows the dark aJ-V curves of these three kinds of devices. It can be seen that under the The average PCE was obtained from 25 devices. reverse voltage, the device with the DMAPA-C60-modified ITO cathode shows the lowest dark current, which suggests after adding the DMAPA-C60 CBL, the bimolecular recombination is reduced Figure 3c shows the dark J-V curves of these three kinds of devices. It can be seen that under remarkably, leading to the enhancement of the light current. In addition, Voc can be calculated based the reverse voltage, the device with the DMAPA-C60 -modified ITO cathode shows the lowest dark on the well-known equation: current, which suggests after adding the DMAPA-C60 CBL, the bimolecular recombination is reduced remarkably, leading to the enhancement of the light current. In addition, Voc can be calculated based JSC nkT on the well-known equation: (1) VOC ≈ ln , nkT J0SC q VOC ≈ ln , (1) q J0 where q is the elemental charge, k is the Boltzmann coefficient, T is the temperature, and J00 is the reverse saturation toto Equation (1),(1), thethe smaller the the J0, the the Vthe OC. As a result, saturationcurrent. current.According According Equation smaller J0 ,larger the larger V OC . As a the VOCthe of the 60 device60 slightly comparedcompared with the other twoother devices. from result, V OCDMAPA-C of the DMAPA-C deviceincreases slightly increases with the twoSo, devices. the weabove, can draw a conclusion that DMAPA-C 60 is a promising to modify cathodes in So, above, from the we can draw a conclusion that DMAPA-C promising CBLITO to modify ITO 60 is a CBL inverted cathodesOSCs. in inverted OSCs.
Figure 3. (a) J-V J-V curves, curves, (b) (b) EQE EQE curves curves and, and, (c) (c) Dark Dark J-V J-V curves curves of of OSCs OSCs based based on on different different CBLs. CBLs. Table CBLs. Table 2. 2. Device Device performances performances of of OSCs OSCs based based on on different different CBLs. Structure Structure ITO ITO/DMAPAC60 ITO ITO/DMAPA-C ITO/ZnO60 ITO/ZnO
PCE (%) FF RSH JSC VOC RS PCE (%) a −2)S −2SH FF V OC(V) JSC cm−2) (%) ) (mA (Ω cmR (Ω cmR Best Average −2 ) −2 −2 ) a (%) (Ω · cm (Ω · (V)0.69 (mA· cm Best Average 12.0 58.5 4.93 4.59 7.03 427.3cm ) 15.7 65.7 7.43 7.27 3.507.03 580.2 0.690.70 12.0 58.5 4.93 4.59 427.3 0.700.71 15.7 65.7 7.43 7.27 580.2 12.8 66.9 6.24 6.01 6.223.50 685.4 0.71 a The average 12.8 PCE was 66.9 6.01 6.22 685.4 obtained6.24 from 25 devices. a
The average PCE was obtained from 25 devices.
2.3. Morphology Atomic force microscopy (AFM) was employed to investigate the nanoscale morphology of ITO 2.3. Morphology films modified by different CBLs. The height images are shown in Figure 4. It can be observed that Atomic microscopy (AFM) was employed investigate the nanoscale morphology of ITO the bare ITO force film exhibits a relatively rough surface to with the root mean square (RMS) roughness of films modified different CBLs. The height images are shown in Figure 4. It can observed that 10.10 nm. Afterbydecorating with ZnO, the RMS decreases to 7.25 nm. With the be modification of the bare ITO film exhibits a relatively rough surface with the root mean square (RMS) roughness DMAPA-C 60, a much smoother surface is obtained with an RMS of only 4.57 nm. Thus, devices with of 10.10 nm. After decorating with ZnO, thebetween RMS decreases to 7.25 nm.and With the modification of DMAPA-C60 as CBLs possess better contact the active layers cathodes, suppressing DMAPA-C60 , a much smoother surface is obtained with an RMS of only 4.57 nm. Thus, devices with DMAPA-C60 as CBLs possess better contact between the active layers and cathodes, suppressing
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charge chargerecombination recombination recombinationin ininthe the theinterface. interface. interface.This This Thiscould could couldbe bebeanother another anotherreason reason reasonto totoexplain explain explainthe the the improvement improvementof ofof charge improvement PCE PCE of of the the DMAPA-C DMAPA-C 6060 device. device. PCE of the DMAPA-C60 device.
2) 2) Figure Figure4. 4.4.AFM AFMheight heightimages images(3(3 of2of (a)ITO, ITO, (b) (b)DMAPA-C DMAPA-C 60-decorated 60-decorated ITO, ITO,and and(c)(c)ZnOZnOFigure AFM height images (3× ×3×3μm 3μm µm )(a) of (a) ITO, (b) DMAPA-C 60 -decorated ITO, and (c) decorated decoratedITO. ITO. ZnO-decorated ITO.
2.4. 2.4.Stability Stability Stability 2.4. After Afterheating heating heatingat atat130 130 130◦°C for for120 120 120min, min, min,the the thePCE PCE PCEof ofofZnO-based ZnO-based ZnO-baseddevices devices devicesdecays decays decaysfrom from from6.24 6.24 6.24to toto4.61%, 4.61%, 4.61%, After C°Cfor preserving preserving 73.9% 73.9% of of its its original original value. value. Replacing Replacing ZnO ZnO with with DMAPA-C DMAPA-C 60,60the , the PCE PCE decays decays from from 7.43 7.43 toto preserving 73.9% of its original value. Replacing ZnO with DMAPA-C60 , the PCE decays from 7.43 5.2%, 5.2%, showing showing 70% 70% itsoriginal originalvalue, value,which whichindicates indicatesthat that thatDMAPA-C DMAPA-C DMAPA-C 60-based 60-baseddevices devicesexhibit exhibit to 5.2%, showing 70%ofof ofits its original value, which indicates 60 -based devices exhibit similar similar good good thermal thermal stabilities stabilities as as ZnO-based ZnO-based devices devices (as (as shown shown in in Figure Figure 5). 5). similar good thermal stabilities as ZnO-based devices (as shown in Figure 5).
◦ C for Figure 5.5.5. Stability curves of ZnO and DMAPACC60C devices under continuous heating at 130 Figure Figure Stability Stability curves curves ofof ZnO ZnO and and DMAPADMAPA60 60 devices devices under under continuous continuous heating heating atat 130 130°C °Cfor for 120 min. 120 120 min. min.
3. 3.3.Materials Materials Materialsand and andMethods Methods Methods 3.1. 3.1. 3.1.Materials Materials Materials Unless Unless Unlessstated stated statedotherwise, otherwise, otherwise,solvents solvents solventsand and andchemicals chemicals chemicalswere were wereobtained obtained obtainedcommercially commercially commerciallyand and andwere were wereused used used without further purification. DMAPA-C was synthesized according to Reference [34]. 60 without withoutfurther furtherpurification. purification.DMAPA-C DMAPA-C 6060 was wassynthesized synthesizedaccording accordingtotoReference Reference[34]. [34]. 3.2. Preparation of ZnO 3.2. 3.2.Preparation PreparationofofZnO ZnO Zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O] was first dissolved in anhydrous 2-methothyethanol Zinc Zincacetate acetatedihydrate dihydrate[Zn(CH [Zn(CH 3COO) 3COO) 2·2H 2·2H 2O] 2O] was wasfirst firstdissolved dissolvedininanhydrous anhydrous2-methothyethanol 2-methothyethanol (0.5 M concentration) and then ethanolamine was added to the solution as sol stabilizer (0.5 M (0.5 (0.5MMconcentration) concentration)and andthen thenethanolamine ethanolaminewas wasadded addedtotothe thesolution solutionasassol solstabilizer stabilizer(0.5 (0.5MM concentration) followed by thorough a mixing process with a magnetic stirrer for 12 h. concentration) concentration)followed followedby bythorough thorougha amixing mixingprocess processwith witha amagnetic magneticstirrer stirrerfor for1212h.h.
3.3. 3.3.Fabrication Fabricationofofthe theOSCs OSCs Organic Organicsolar solarcells cellswere werefabricated fabricatedwith withthe thefollowing followingstructures: structures:ITO/PTB7:PC ITO/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag, 3/Ag, ITO/ZnO/PTB7:PC ITO/ZnO/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag, 3/Ag,ITO/DMAPA-C ITO/DMAPA-C 60/PTB7:PC 60/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag. 3/Ag.Patterned PatternedITO ITOglass glass
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3.3. Fabrication of the OSCs Organic solar cells were fabricated with the following structures: ITO/PTB7:PC71 BM/MoO3 /Ag, ITO/ZnO/PTB7:PC71 BM/MoO3 /Ag, ITO/DMAPA-C60 /PTB7:PC71 BM/MoO3 /Ag. Patterned ITO glass (sheet resistance = 15 Ω·−1 ) was precleaned in an ultrasonic bath with deionized water, acetone, and isopropanol (each for 15 min). Then different CBLs were spin-coated on ITO. For ZnO as the CBL, the prepared ZnO sol-gel was spin-coated on the ITO substrate with 3000 rpm and then annealed at 200 ◦ C for 1 h in the air. For DMAPA-C60 as the CBL, methanol solution of DMAPA-C60 was deposited atop the ITO substrate at 3000 rpm for 30 s. Then the active layer was formed by spin-coating from o-dichlorobenzene (o-DCB) solution containing 10 mg·mL−1 PTB7 and 15 mg·mL−1 PC71 BM at 1000 rpm for 1 min. 1,8-Diiodooctane with a 3% volume ratio was added to the o-DCB solutions and stirred before use. A MoO3 (ca. 5 nm) and silver layer (ca. 80 nm) was then evaporated onto the surface of the active layer under vacuum (ca. 10−5 Pa) to form the positive electrode. The active area of the device was 4 mm2 . 3.4. Characterizations Current density-voltage characteristics were measured inside a N2 -filled glove box by using a source meter (2400, Keithley Instruments, Cleveland, OH, USA) controlled by a LabVIEW program (National Instruments, Austin, TX, USA) in the dark and under white light illumination (100 mW·cm−2 ). The light intensity was calibrated using a silicon photodiode with a KG5 filter (S1133, Hamamatsu Photonics, Hamamatsu, Japan). The EQE spectra were recorded using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd., Kaohsiung, Taiwan). The light intensity at each wavelength was calibrated by using a standard single crystal Si photovoltaic cell. Thin-film UV-vis absorption and transmission curves were recorded on a JASCO V-570 spectrophotometer. The nanoscale morphology of the blended films was observed using a Veeco Nanoscope V atomic force microscope in the tapping mode. The stability test of solar cells was carried out under continuous heating at 130 ◦ C and the J-V was measured using a B2912A source meter (Angilent Technologies, Palo Alto, CA, USA) every 15 min for a total of 120 min under AM 1.5 G illumination. These tests were carried out in the glove box. 4. Conclusions In conclusion, an easy-accessible fullerene derivative, DMAPA-C60 , was developed as a CBL to modify the transparent conducting cathode ITO in inverted OSCs based on a PTB7:PC71 BM system. Compared with traditional ZnO CBLs, DMAPA-C60 exhibited comparable transmittance. Meanwhile, OSCs based on DMAPA-C60 showed much better device performance compared with their ZnO counterparts (PCE improved from 6.24 to 7.43%). This is mainly caused by the much lower Rs, that is to say, a better contact between the DMAPA-C60 -modified ITO and the active layer is formed and a better electron collection at the cathode is realized. Nanoscale morphologies also demonstrated the plainer surface of DMAPA-C60 -modified ITO compared with the ZnO counterparts, which leads to the lower Rs and higher PCE. The success of DMAPA-C60 as a CBL layer indicates that the amine groups functionalized fullerene derivatives could be very promising in replacing conventional ZnO CBLs for low-cost, high-performance inverted OSCs. Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (21574072, 21375069, 21675091, 21404065), the Natural Science Foundation for Distinguished Young Scientists of Shandong Province (JQ201403), the Taishan Young Scholar Program of Shandong Province (tsqn20161027), the Key Research and Development Project of Shandong Province (2016GGX102028, 2016GGX102039), the Project of Shandong Province Higher Educational Science and Technology Program (J15LC20), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (20111568), the People’s Livelihood Science and Technology Project of Qingdao (166257nsh, 173378nsh), the Innovation Leader Project of Qingdao (168325zhc), the China Postdoctoral Science Foundation (2017M612199), the Postdoctoral Scientific Research Foundation of Qingdao, and the First Class Discipline Project of Shandong Province.
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Author Contributions: Y.W., X.Z., and Z.Z. conceived and designed the experiments. Y.W. and Z.Z. conducted the experiments. H.C., B.Y., and Y.W. discussed the results. Y.W., H.C., and B.Y. wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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Efficient Inverted Organic Solar Cells Based on a Fullerene Derivative-Modified Transparent Cathode Yifan Wang 1,2 , Hailin Cong 1,2, * 1 2 3 4
*
ID
, Bing Yu 1,2 , Zhiguo Zhang 3 and Xiaowei Zhan 4
ID
Institute of Biomedical Materials and Engineering, College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, China; [email protected] (Y.W.); [email protected] (B.Y.) Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China; [email protected] Correspondence: [email protected]; Tel.: +86-532-8595-3995; Fax: +86-532-8595-5529
Received: 18 July 2017; Accepted: 7 September 2017; Published: 11 September 2017
Abstract: Indium tin oxide (ITO) is a transparent conductive material which is extensively used in organic solar cells (OSCs) as electrodes. In inverted OSCs, ITO is usually employed as a cathode, which should be modified by cathode buffer layers (CBLs) to achieve better contact with the active layers. In this paper, an amine group functionalized fullerene derivative (DMAPA-C60 ) is used as a CBL to modify the transparent cathode ITO in inverted OSCs based on PTB7 as a donor and PC71 BM as an acceptor. Compared with traditional ZnO CBL, DMAPA-C60 exhibited comparable transmittance. OSCs based on DMAPA-C60 show much better device performance compared with their ZnO counterparts (power conversion efficiencies (PCEs) improved from 6.24 to 7.43%). This is mainly because a better contact between the DMAPA-C60 modified ITO and the active layer is formed, which leads to better electron transport and collection. Nanoscale morphologies also demonstrate that the surface of DMAPA-C60 -modified ITO is plainer than the ZnO counterparts, which also leads to the better device performance. Keywords: organic solar cell (OSC); cathode buffer layer (CBL); transparent conducting material; fullerene derivative; ZnO
1. Introduction Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been regarded as one of the most advanced photovoltaic technologies because of their advantages over traditional silicon solar cells, which include low cost, light weight, and flexibility [1–4]. Nowadays, OSCs based on fullerene as acceptors or non-fullerene organic materials as acceptors have achieved power conversion efficiencies (PCEs) of over 11% [5–9]. Inverted OSCs, which are mostly based on conductive indium tin oxides (ITO) as cathodes and silver (Ag) as anodes, possess major advantages of enhanced device performance as well as increased stability compared with conventional OSCs [10,11]. In recent years, inverted OSCs have developed rapidly, mostly benefiting from the development of novel donor and acceptor materials and interfacial engineering [12]. Electrode-semiconducting active layer contacts in organic electronics are plagued by interfacial barriers for charge transfer due to poor alignment between the electrode work function and the Fermi level of the semiconductor [13]. As a result, the cathode buffer layer (CBL) between the active layer and conductive ITO cathode plays an important role in the performance of inverted OSCs because it can decorate the ITO cathode and promote the electron transfer and extraction process [14,15]. Materials 2017, 10, 1064; doi:10.3390/ma10091064
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CBLs can be grouped into two categories: inorganic and organic materials. Inorganic materials 2 of 8 Materials 2017, 10, 1064 are commonly transition-metal oxides, such as CsCO3 [16,17], TiOx [18,19], and ZnO [20–22]. Different from inorganic CBLs, organic CBLs exhibit the advantage of structural tunability, which could can offerbe more opportunities of realizing better interfaceand modification and high-performance fully CBLs grouped into two categories: inorganic organic materials. Inorganic materials solution-processed OSCs [23,24]. In view various types of organic CBLs have drawn much are commonly transition-metal oxides, such of as this, CsCO [16,17], TiO [18,19], and ZnO [20–22]. Different x 3 attention and have been recently developed. For example, water (alcohol)-soluble conjugated from inorganic CBLs, organic CBLs exhibit the advantage of structural tunability, which could offer more polymers such as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9opportunities of realizing better interface modification and high-performance fully solution-processed dioctylfluorene)] (PFN) and its derivatives [25,26] have been successfully utilized as CBLs in inverted OSCs [23,24]. In view of this, various types of organic CBLs have drawn much attention and OSCs and have been proved to be comparable to or even better than the conventional ZnO CBL. have Other been organic recentlyCBLs developed. For example, water (alcohol)-soluble conjugated polymers such as such as polyethylenimine ethoxylated (PEIE) [27] and titanium (diisopropoxide) 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and its poly[(9,9-bis(3 bis(2,4-pentanedionate) (TIPD) [28] have also been proved to be potential CBLs in inverted OSCs with derivatives [25,26] better have than beenZnO successfully utilizedfullerene-based as CBLs in inverted beenasproved performances CBLs. Recently, materialsOSCs have and beenhave designed a to benew comparable to Fullerene-based or even bettermaterials than the ZnO Otherfor organic CBLs such type of CBL. areconventional considered to be idealCBL. candidates CBLs, because their structures are similar to the conventionally used fullerene acceptors, which could smoothly as polyethylenimine ethoxylated (PEIE) [27] and titanium (diisopropoxide) bis(2,4-pentanedionate) bridge electrons transporting from the fullerene acceptor to the cathode [29–32]. For example,better Alex than (TIPD) [28] have also been proved to be potential CBLs in inverted OSCs with performances and co-workers reported that, with a new kind of fullerene derivative Bis-OMe fulleropyrrolidinium ZnO CBLs. Recently, fullerene-based materials have been designed as a new type of CBL. Fullerene-based iodide (FPI) blended PEIE CBL in inverted OSCs, a much better performance is achieved compared materials are considered to be ideal candidates for CBLs, because their structures are similar to the with using only PEIE as the CBL [33]. An amine group functionalized fullerene complex (DMAPAconventionally used fullerene acceptors, which could smoothly bridge electrons transporting from the C60, Figure 1a) was successfully synthesized as an ideal candidate for CBLs to modify the Al cathode fullerene acceptor toOSCs the cathode [29–32]. amino For example, Alexpotentially and co-workers that, with a new in conventional [34]. Its terminal group could providereported a dipole moment, just kind of iodide (FPI) blended PEIE CBL(LUMO) in inverted as fullerene the aminoderivative group doesBis-OMe in PFN. fulleropyrrolidinium Furthermore, the lowest unoccupied molecular orbital OSCs,energy a much better performance achieved with using only PEIE thehighest CBL [33]. An amine level is estimated to be is 3.58 eV, closecompared to that of PC 71BM (about 3.85 eV).asThe occupied groupmolecular functionalized (DMAPA-C , Figure 1a) was successfully synthesized as an ideal orbitalfullerene (HOMO) complex energy level is estimated to be 5.52 eV, which is low enough to efficiently 60 block holes from various donors. candidate for CBLs to modify the Al cathode in conventional OSCs [34]. Its terminal amino group could this paper, we demonstrate 60 cangroup be independently used as a CBL to modify potentiallyInprovide a dipole moment,that justDMAPA-C as the amino does in PFN. Furthermore, the lowest the transparent conductive cathode ITO in inverted OSCs based on poly{4,8-bis[(2unoccupied molecular orbital (LUMO) energy level is estimated to be 3.58 eV, close to that of PC71 BM ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno (about 3.85 eV). The highest occupied molecular orbital (HOMO) energy level is estimated to be 5.52 eV, [3,4-b]thiophene-4,6-diyl} (PTB7) as a donor and PC71BM as an acceptor. Compared with the which is low enough to efficiently block holes from various donors. traditional ZnO CBL, a much better device performance was realized (PCE improved from 6.24 to 7.43%).
(a)
(b)
Figure 1. (a) Chemical structures of PTB7, PC71 BM, and DMAPA-C60 ; (b) Device structure of the inverted OSC.
In this paper, we demonstrate that DMAPA-C60 can be independently used as a CBL to modify the transparent conductive cathode ITO in inverted OSCs based on poly{4,8-bis[(2-ethylhexyl)oxy]benzo dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4,[3,4-b]thiophene-4,6-diyl} (PTB7)
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Figure (a) Chemical structures of PTB7, PC71BM, and DMAPA-C60; (b) Device structure of the as a donor and1.PC 71 BM as an acceptor. Compared with the traditional ZnO CBL, a much better device inverted OSC. performance was realized (PCE improved from 6.24 to 7.43%).
Results and Discussion 2. 2. Results and Discussion 2.1. Absorption and Transmittance 2.1. Absorption and Transmittance ToTo modify ITO cathodes in in inverted OPV devices, thethe buffer layers should bebe asas transparent asas modify ITO cathodes inverted OPV devices, buffer layers should transparent possible, which can guarantee the more sunlight will penetrate the cathode and bebe absorbed byby thethe possible, which can guarantee the more sunlight will penetrate the cathode and absorbed active layers. The normalized spectra ofof optical absorption of of thethe ZnO buffer layer and DMAPA-C active layers. The normalized spectra optical absorption ZnO buffer layer and DMAPA-C 60 60 buffer layer areare shown in in Figure 2a.2a. Both ZnO and DMAPA-C buffer weak buffer layer shown Figure Both ZnO and DMAPA-C bufferlayers layersexhibit exhibitvery very weak 60 60 absorption transmittancespectra spectraofofthe theITO ITOlayer, layer, ZnO-modified ITO layer, absorptionatat400~800 400~800nm. nm. The transmittance ZnO-modified ITO layer, and and DMAPA-C ITOlayer layerare areshown shownin in Figure Figure 1b. They of of DMAPA-C 60-modified ITO They all allshow showexcellent excellenttransmittance transmittance 60 -modified over 85%, demonstrating the excellent light transmissivity the buffer layers, which suitable over 85%, demonstrating the excellent light transmissivity ofof the buffer layers, which is is suitable forfor modifying ITO cathode. modifying thethe ITO cathode.
Figure UV-Vis absorption spectra ZnO and DMAPA-C 60 films; Transmittance spectra Figure 2. 2. (a)(a) UV-Vis absorption spectra of of ZnO and DMAPA-C (b)(b) Transmittance spectra of of 60 films; 60and , and ITO/ZnO films. ITO, ITO/DMAPA-C ITO, ITO/DMAPA-C , ITO/ZnO films. 60
2.2. Photovoltaic Performances 2.2. Photovoltaic Performances fabricated BHJ OSCs with a structure of ITO/CBL/PTB7:PC 71BM/MoO 3/Ag using classical WeWe fabricated BHJ OSCs with a structure of ITO/CBL/PTB7:PC using classical 71 BM/MoO 3 /Ag 71 BM as an acceptor material. Table 1 shows low-bandgap polymer PTB7 as a donor material and PC low-bandgap polymer PTB7 as a donor material and PC71 BM as an acceptor material. Table 1 shows the the device parameters of DMAPA-C 60-based OSCs with different DMAPA-C 60 concentrations, device parameters of DMAPA-C -based OSCs with different DMAPA-C concentrations, indicating 60 60 indicating thickness ofdependence of these After OSCs optimization, OSCs with 2 mg/mL the thicknessthe dependence these devices. After devices. optimization, with 2 mg/mL DMAPA-C 60 DMAPA-C 60 shows the best performance with a PCE of 7.43%. Figure 3a shows the current shows the best performance with a PCE of 7.43%. Figure 3a shows the current density-voltagedensity(J-V) voltage curves of the based on different CBLs (under theofillumination of mW AM1.5G, curves of (J-V) the OSCs based onOSCs different CBLs (under the illumination AM1.5G, 100 ·cm−2100 ). −2). The corresponding device parameters are summarized in Table 2. The devices with bare mW·cm The corresponding device parameters are summarized in Table 2. The devices with bare ITO as a −2 mA·cm ITO asshows a cathode shows the PCEwith of 4.93%, a very small SC of·cm 12.0 and an fill cathode the lowest PCElowest of 4.93%, a verywith small JSC of 12.0 JmA and an−2fill factor factor (FF) 58.5%. of only 58.5%. This be ascribed the direct contact of ITO with active layer, leading (FF) of only This can becan ascribed to thetodirect contact of ITO with thethe active layer, leading a large series resistance S) and a small parallel resistance (R). SH). Whenwe weutilize utilize ZnO CBL toto a large series resistance (RS(R ) and a small parallel resistance (RSH When ZnO CBL toto modify ITO cathode, device performance enhances significantly, with VOC improving from modify thethe ITO cathode, thethe device performance enhances significantly, with V OC improving from −2 −2 −2−2, ,FF 0.69 0.71 J SC from 12 mA·cm to 12.8 mA·cm FF from 58.5 to 66.9%, and PCE from 4.93 0.69 VV to to 0.71 V, JV, from 12 mA · cm to 12.8 mA · cm from 58.5 to 66.9%, and PCE from 4.93 to to SC − 6.24%. Simultaneously, it can be seen that with ZnO as the CBL, the R S decreases from 7.03 Ω·cm 6.24%. Simultaneously, it can be seen that with ZnO as the CBL, the RS decreases from 7.03 Ω·cm −22 to that better contact is formed between thethe cathode andand thethe active layer, and to6.22 6.22Ω·cm Ω·cm−2−, 2indicating , indicating that better contact is formed between cathode active layer, carrier trapping and recombination inside the device is reduced. With DMAPA-C 60 as the CBL, and carrier trapping and recombination inside the device is reduced. With DMAPA-C60 as the CBL,an the enhancement enhancement of ofJJSC SC from from 12.8 12.8 mA mA·cm aneven evenhigher higherPCE PCEof of 7.43% 7.43% is is achieved, mostly due to the ·cm−−22 to −2 −2 − 2 − 2 is is consistent with thethe fact that thethe RS R value (3.50 Ω·cm ) of the 60 device to15.7 15.7mA·cm mA·cm . This . This consistent with fact that (3.50 Ω·cm ) ofDMAPA-C the DMAPA-C S value 60 −2). The − 2 is much smaller than that of the ZnO device (6.22 Ω·cm external quantum efficiency (EQE) device is much smaller than that of the ZnO device (6.22 Ω·cm ). The external quantum efficiency curvecurve is shown in Figure 2b. The Jsc values integrated fromfrom the the EQEEQE spectra are are 11.9, 15.1, 12.3 (EQE) is shown in Figure 2b. The Jsc values integrated spectra 11.9, 15.1, −2 for −2 for mA·cm thetheno-CBL, 60-based, and ZnO-based respectively, which 12.3 mA·cm no-CBL,DMAPA-C DMAPA-C -based, and ZnO-based devices, respectively, which is is 60 consistent with the values shown in Table 2. consistent with the values shown in Table 2.
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Table 1. Device performances of OSCs based on DMAPA-C60 with different DMAPA-C60 Materials 2017, 10, 1064 concentrations.
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DMAPA-C60
VOC
JSC
FF
PCE (%)
Table 1. Device(mg/mL) performances of OSCs on DMAPA-C with different −2) a 60 60 concentrations. (V) based (%) (mA·cm BestDMAPA-C Average
1 1.5 DMAPA-C60 (mg/mL) 2 1 2.5 1.5 3 2 2.5 3
0.69 0.69 V OC (V) 0.70 0.69 0.70 0.69 0.69
a
14.2 57.3 5.61 14.3 63.4 6.26 − 2 FF (%) JSC (mA·cm ) 15.7 65.7 7.43 15.414.2 61.4 57.36.62 14.814.3 62.9 63.46.42
0.70 15.7 65.7 The average PCE was obtained from 25 devices. 0.70 15.4 61.4 0.69 14.8 62.9
5.48 PCE (%) 6.1 Best 7.27 Average a 5.61 6.45 5.48 6.26 6.29 6.1 7.43 6.62 6.42
7.27 6.45 6.29
Figure 3c shows the dark aJ-V curves of these three kinds of devices. It can be seen that under the The average PCE was obtained from 25 devices. reverse voltage, the device with the DMAPA-C60-modified ITO cathode shows the lowest dark current, which suggests after adding the DMAPA-C60 CBL, the bimolecular recombination is reduced Figure 3c shows the dark J-V curves of these three kinds of devices. It can be seen that under remarkably, leading to the enhancement of the light current. In addition, Voc can be calculated based the reverse voltage, the device with the DMAPA-C60 -modified ITO cathode shows the lowest dark on the well-known equation: current, which suggests after adding the DMAPA-C60 CBL, the bimolecular recombination is reduced remarkably, leading to the enhancement of the light current. In addition, Voc can be calculated based JSC nkT on the well-known equation: (1) VOC ≈ ln , nkT J0SC q VOC ≈ ln , (1) q J0 where q is the elemental charge, k is the Boltzmann coefficient, T is the temperature, and J00 is the reverse saturation toto Equation (1),(1), thethe smaller the the J0, the the Vthe OC. As a result, saturationcurrent. current.According According Equation smaller J0 ,larger the larger V OC . As a the VOCthe of the 60 device60 slightly comparedcompared with the other twoother devices. from result, V OCDMAPA-C of the DMAPA-C deviceincreases slightly increases with the twoSo, devices. the weabove, can draw a conclusion that DMAPA-C 60 is a promising to modify cathodes in So, above, from the we can draw a conclusion that DMAPA-C promising CBLITO to modify ITO 60 is a CBL inverted cathodesOSCs. in inverted OSCs.
Figure 3. (a) J-V J-V curves, curves, (b) (b) EQE EQE curves curves and, and, (c) (c) Dark Dark J-V J-V curves curves of of OSCs OSCs based based on on different different CBLs. CBLs. Table CBLs. Table 2. 2. Device Device performances performances of of OSCs OSCs based based on on different different CBLs. Structure Structure ITO ITO/DMAPAC60 ITO ITO/DMAPA-C ITO/ZnO60 ITO/ZnO
PCE (%) FF RSH JSC VOC RS PCE (%) a −2)S −2SH FF V OC(V) JSC cm−2) (%) ) (mA (Ω cmR (Ω cmR Best Average −2 ) −2 −2 ) a (%) (Ω · cm (Ω · (V)0.69 (mA· cm Best Average 12.0 58.5 4.93 4.59 7.03 427.3cm ) 15.7 65.7 7.43 7.27 3.507.03 580.2 0.690.70 12.0 58.5 4.93 4.59 427.3 0.700.71 15.7 65.7 7.43 7.27 580.2 12.8 66.9 6.24 6.01 6.223.50 685.4 0.71 a The average 12.8 PCE was 66.9 6.01 6.22 685.4 obtained6.24 from 25 devices. a
The average PCE was obtained from 25 devices.
2.3. Morphology Atomic force microscopy (AFM) was employed to investigate the nanoscale morphology of ITO 2.3. Morphology films modified by different CBLs. The height images are shown in Figure 4. It can be observed that Atomic microscopy (AFM) was employed investigate the nanoscale morphology of ITO the bare ITO force film exhibits a relatively rough surface to with the root mean square (RMS) roughness of films modified different CBLs. The height images are shown in Figure 4. It can observed that 10.10 nm. Afterbydecorating with ZnO, the RMS decreases to 7.25 nm. With the be modification of the bare ITO film exhibits a relatively rough surface with the root mean square (RMS) roughness DMAPA-C 60, a much smoother surface is obtained with an RMS of only 4.57 nm. Thus, devices with of 10.10 nm. After decorating with ZnO, thebetween RMS decreases to 7.25 nm.and With the modification of DMAPA-C60 as CBLs possess better contact the active layers cathodes, suppressing DMAPA-C60 , a much smoother surface is obtained with an RMS of only 4.57 nm. Thus, devices with DMAPA-C60 as CBLs possess better contact between the active layers and cathodes, suppressing
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charge chargerecombination recombination recombinationin ininthe the theinterface. interface. interface.This This Thiscould could couldbe bebeanother another anotherreason reason reasonto totoexplain explain explainthe the the improvement improvementof ofof charge improvement PCE PCE of of the the DMAPA-C DMAPA-C 6060 device. device. PCE of the DMAPA-C60 device.
2) 2) Figure Figure4. 4.4.AFM AFMheight heightimages images(3(3 of2of (a)ITO, ITO, (b) (b)DMAPA-C DMAPA-C 60-decorated 60-decorated ITO, ITO,and and(c)(c)ZnOZnOFigure AFM height images (3× ×3×3μm 3μm µm )(a) of (a) ITO, (b) DMAPA-C 60 -decorated ITO, and (c) decorated decoratedITO. ITO. ZnO-decorated ITO.
2.4. 2.4.Stability Stability Stability 2.4. After Afterheating heating heatingat atat130 130 130◦°C for for120 120 120min, min, min,the the thePCE PCE PCEof ofofZnO-based ZnO-based ZnO-baseddevices devices devicesdecays decays decaysfrom from from6.24 6.24 6.24to toto4.61%, 4.61%, 4.61%, After C°Cfor preserving preserving 73.9% 73.9% of of its its original original value. value. Replacing Replacing ZnO ZnO with with DMAPA-C DMAPA-C 60,60the , the PCE PCE decays decays from from 7.43 7.43 toto preserving 73.9% of its original value. Replacing ZnO with DMAPA-C60 , the PCE decays from 7.43 5.2%, 5.2%, showing showing 70% 70% itsoriginal originalvalue, value,which whichindicates indicatesthat that thatDMAPA-C DMAPA-C DMAPA-C 60-based 60-baseddevices devicesexhibit exhibit to 5.2%, showing 70%ofof ofits its original value, which indicates 60 -based devices exhibit similar similar good good thermal thermal stabilities stabilities as as ZnO-based ZnO-based devices devices (as (as shown shown in in Figure Figure 5). 5). similar good thermal stabilities as ZnO-based devices (as shown in Figure 5).
◦ C for Figure 5.5.5. Stability curves of ZnO and DMAPACC60C devices under continuous heating at 130 Figure Figure Stability Stability curves curves ofof ZnO ZnO and and DMAPADMAPA60 60 devices devices under under continuous continuous heating heating atat 130 130°C °Cfor for 120 min. 120 120 min. min.
3. 3.3.Materials Materials Materialsand and andMethods Methods Methods 3.1. 3.1. 3.1.Materials Materials Materials Unless Unless Unlessstated stated statedotherwise, otherwise, otherwise,solvents solvents solventsand and andchemicals chemicals chemicalswere were wereobtained obtained obtainedcommercially commercially commerciallyand and andwere were wereused used used without further purification. DMAPA-C was synthesized according to Reference [34]. 60 without withoutfurther furtherpurification. purification.DMAPA-C DMAPA-C 6060 was wassynthesized synthesizedaccording accordingtotoReference Reference[34]. [34]. 3.2. Preparation of ZnO 3.2. 3.2.Preparation PreparationofofZnO ZnO Zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O] was first dissolved in anhydrous 2-methothyethanol Zinc Zincacetate acetatedihydrate dihydrate[Zn(CH [Zn(CH 3COO) 3COO) 2·2H 2·2H 2O] 2O] was wasfirst firstdissolved dissolvedininanhydrous anhydrous2-methothyethanol 2-methothyethanol (0.5 M concentration) and then ethanolamine was added to the solution as sol stabilizer (0.5 M (0.5 (0.5MMconcentration) concentration)and andthen thenethanolamine ethanolaminewas wasadded addedtotothe thesolution solutionasassol solstabilizer stabilizer(0.5 (0.5MM concentration) followed by thorough a mixing process with a magnetic stirrer for 12 h. concentration) concentration)followed followedby bythorough thorougha amixing mixingprocess processwith witha amagnetic magneticstirrer stirrerfor for1212h.h.
3.3. 3.3.Fabrication Fabricationofofthe theOSCs OSCs Organic Organicsolar solarcells cellswere werefabricated fabricatedwith withthe thefollowing followingstructures: structures:ITO/PTB7:PC ITO/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag, 3/Ag, ITO/ZnO/PTB7:PC ITO/ZnO/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag, 3/Ag,ITO/DMAPA-C ITO/DMAPA-C 60/PTB7:PC 60/PTB7:PC 71BM/MoO 71BM/MoO 3/Ag. 3/Ag.Patterned PatternedITO ITOglass glass
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3.3. Fabrication of the OSCs Organic solar cells were fabricated with the following structures: ITO/PTB7:PC71 BM/MoO3 /Ag, ITO/ZnO/PTB7:PC71 BM/MoO3 /Ag, ITO/DMAPA-C60 /PTB7:PC71 BM/MoO3 /Ag. Patterned ITO glass (sheet resistance = 15 Ω·−1 ) was precleaned in an ultrasonic bath with deionized water, acetone, and isopropanol (each for 15 min). Then different CBLs were spin-coated on ITO. For ZnO as the CBL, the prepared ZnO sol-gel was spin-coated on the ITO substrate with 3000 rpm and then annealed at 200 ◦ C for 1 h in the air. For DMAPA-C60 as the CBL, methanol solution of DMAPA-C60 was deposited atop the ITO substrate at 3000 rpm for 30 s. Then the active layer was formed by spin-coating from o-dichlorobenzene (o-DCB) solution containing 10 mg·mL−1 PTB7 and 15 mg·mL−1 PC71 BM at 1000 rpm for 1 min. 1,8-Diiodooctane with a 3% volume ratio was added to the o-DCB solutions and stirred before use. A MoO3 (ca. 5 nm) and silver layer (ca. 80 nm) was then evaporated onto the surface of the active layer under vacuum (ca. 10−5 Pa) to form the positive electrode. The active area of the device was 4 mm2 . 3.4. Characterizations Current density-voltage characteristics were measured inside a N2 -filled glove box by using a source meter (2400, Keithley Instruments, Cleveland, OH, USA) controlled by a LabVIEW program (National Instruments, Austin, TX, USA) in the dark and under white light illumination (100 mW·cm−2 ). The light intensity was calibrated using a silicon photodiode with a KG5 filter (S1133, Hamamatsu Photonics, Hamamatsu, Japan). The EQE spectra were recorded using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd., Kaohsiung, Taiwan). The light intensity at each wavelength was calibrated by using a standard single crystal Si photovoltaic cell. Thin-film UV-vis absorption and transmission curves were recorded on a JASCO V-570 spectrophotometer. The nanoscale morphology of the blended films was observed using a Veeco Nanoscope V atomic force microscope in the tapping mode. The stability test of solar cells was carried out under continuous heating at 130 ◦ C and the J-V was measured using a B2912A source meter (Angilent Technologies, Palo Alto, CA, USA) every 15 min for a total of 120 min under AM 1.5 G illumination. These tests were carried out in the glove box. 4. Conclusions In conclusion, an easy-accessible fullerene derivative, DMAPA-C60 , was developed as a CBL to modify the transparent conducting cathode ITO in inverted OSCs based on a PTB7:PC71 BM system. Compared with traditional ZnO CBLs, DMAPA-C60 exhibited comparable transmittance. Meanwhile, OSCs based on DMAPA-C60 showed much better device performance compared with their ZnO counterparts (PCE improved from 6.24 to 7.43%). This is mainly caused by the much lower Rs, that is to say, a better contact between the DMAPA-C60 -modified ITO and the active layer is formed and a better electron collection at the cathode is realized. Nanoscale morphologies also demonstrated the plainer surface of DMAPA-C60 -modified ITO compared with the ZnO counterparts, which leads to the lower Rs and higher PCE. The success of DMAPA-C60 as a CBL layer indicates that the amine groups functionalized fullerene derivatives could be very promising in replacing conventional ZnO CBLs for low-cost, high-performance inverted OSCs. Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (21574072, 21375069, 21675091, 21404065), the Natural Science Foundation for Distinguished Young Scientists of Shandong Province (JQ201403), the Taishan Young Scholar Program of Shandong Province (tsqn20161027), the Key Research and Development Project of Shandong Province (2016GGX102028, 2016GGX102039), the Project of Shandong Province Higher Educational Science and Technology Program (J15LC20), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (20111568), the People’s Livelihood Science and Technology Project of Qingdao (166257nsh, 173378nsh), the Innovation Leader Project of Qingdao (168325zhc), the China Postdoctoral Science Foundation (2017M612199), the Postdoctoral Scientific Research Foundation of Qingdao, and the First Class Discipline Project of Shandong Province.
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Author Contributions: Y.W., X.Z., and Z.Z. conceived and designed the experiments. Y.W. and Z.Z. conducted the experiments. H.C., B.Y., and Y.W. discussed the results. Y.W., H.C., and B.Y. wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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