energies Article
Low Temperature Aqueous Solution-Processed ZnO and Polyethylenimine Ethoxylated Cathode Buffer Bilayer for High Performance Flexible Inverted Organic Solar Cells Hailong You 1 , Junchi Zhang 1 , Zeyulin Zhang 2 , Chunfu Zhang 1, *, Zhenhua Lin 1, *, Jingjing Chang 1 , Genquan Han 1 , Jincheng Zhang 1 , Gang Lu 3 and Yue Hao 1 1
2 3
*
Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, School of Microelectronics, Xidian University, Xi’an 710071, China;
[email protected] (H.Y.);
[email protected] (J.Z.);
[email protected] (J.C.);
[email protected] (G.H.);
[email protected] (J.Z.);
[email protected] (Y.H.) School of Textiles and Materials, Xi’an Polytechnic University, Xi’an 710048, China;
[email protected] Huanghe Hydropower Solar Industry Technology Co. Ltd., 369 South Yanta Road, Xi’an 710061, China;
[email protected] Correspondence:
[email protected] (C.Z.);
[email protected] (Z.L.); Tel.: +86-29-882-017-59 (C.Z.); +86-29-882-048-89 (Z.L.)
Academic Editor: Claudia Barolo Received: 19 February 2017; Accepted: 31 March 2017; Published: 6 April 2017
Abstract: High performance flexible inverted organic solar cells (OSCs) employing the low temperature cathode buffer bilayer combining the aqueous solution-processed ZnO and polyethylenimine ethoxylated (PEIE) are investigated based on Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 -butryric acid methyl ester (P3HT:PC61 BM) and Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene2,6-diyl}{3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71 -butyric acid methyl ester (PTB-7:PC71 BM) material systems. It is found that, compared with pure ZnO or PEIE cathode buffer layer (CBL), the proper combination of low-temperature processed ZnO and PEIE as the CBL enhanced the short circuit current density (JSC ), resulting in better device performance. The increased JSC results from the enhanced electron collection ability from the active layer to the cathode. By using the ZnO/PEIE CBL, a power conversion efficiency (PCE) as high as 4.04% for the P3HT:PC61 BM flexible device and a PCE as high as 8.12% for the PTB-7:PC71 BM flexible device are achieved, which are higher than the control devices with the pure ZnO CBL or pure PEIE CBL. The flexible inverted OSC also shows a superior mechanical property and it can keep 92.9% of its initial performance after 1000 bending cycles with a radius of 0.8 cm. These results suggest that the combination of the low temperature aqueous solution processed ZnO and PEIE can be a promising cathode buffer bilayer for flexible inverted OSCs. Keywords: organic solar cells (OSCs); cathode buffer layer (CBL); ZnO; polyethylenimine ethoxylated (PEIE)
1. Introduction Due to the potential of low-cost, flexibility, light weight and compatibility with roll-to-roll fabrication, organic solar cells (OSCs) have attracted much research attention [1–4]. After continuous efforts in recent years, OSCs with power conversion efficiency (PCE) above 10%–12% have been achieved, based on the widely used bulk heterojunction (BHJ) of donor-accepter blend [5–7]. Generally, the structure of OSCs could be classified into two categories, namely conventional structure and
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inverted structure. The conventional structure usually uses a poly(3,4-ethylenedioxithiophene): poly(styrenesulfonate) (PEDOT:PSS) hole transport layer on indium-tin-oxide (ITO) to collect holes and a low-work-function metal [aluminum (Al) or calcium (Ca)] top cathode to collect electrons. However, the acidic PEDOT:PSS layer can corrode the ITO electrode, leading to the indium diffusion into the active layer and resulting in interface instability [8–11]. And the top low-work-function metal can also be easily oxidized in air, resulting in poor stability [12–14]. Compared with the conventional structure, the inverted structure have the opposite electrode polarities, where the modified ITO acts as the cathode and a high-work-function metal such as silver (Ag) or gold (Au) acts as the top anode, so that both the commonly used acidic PEDOT:PSS and low-work-function metal top cathode can be avoided and morphology of the active layer become more stable. This structure has been considered as an efficient approach for improving the cell stability [15–19]. In inverted OSCs, an electron-selective layer between the ITO cathode and active layer is indispensable so that an ohmic contact for the decrease or elimination the electron-extraction barrier could be formed. N-type metal oxides such as zinc oxide (ZnO), titanium oxide (TiOx ), and tin oxide (SnO2 ) have been introduced as a cathode buffer layer (CBL) to modify ITO as an effective electron-collecting electrode in inverted OSCs [1,2,15,20]. In particular, ZnO is more attractive because of its beneficial properties such as high electron mobility, high optical transparency, low-cost and simple solution process. Besides the metal oxides, polyelectrolyte such as polyethylenimine ethoxylated (PEIE) has also been widely used as CBL, which could also be processed by the simple solution processing method. PEIE contains simple aliphatic amine groups, which can produce surface dipoles and reduce the work function of the ITO electrode. Thus, the energy mismatch between the electrode and the active layer could be lowered so that the carriers can be efficiently collected by the electrode. The PCE of inverted OSCs with PEIE as CBL was comparable to inverted PSCs using ZnO as CBL. Recent reports [21,22] have shown that by combining ZnO and PEIE as the cathode buffer bilayer, the performance of OSCs could be further improved. However, in the reported cathode buffer bilayer, the deposition of ZnO is almost on the sol-gel method. This method usually requires a high process temperature (usually ≥300 ◦ C), which is not compatible with flexible substrates such as polyethylene terephthalate (PET). Colloid-processed Nc-ZnO could be processed at a low temperature, but it possesses environmentally sensitive electrical performance in ambient atmosphere. In order to fabricate the flexible devices, the CBL combining a stable low temperature ZnO and PEIE should be developed. Recent results [21,23–25] have shown that the aqueous solution of ammine-zinc complex is a promising technique to afford low temperature conversion to dense ZnO thin films. ZnO deposited using this method was first adopted to fabricate thin film transistor and later applied in organic light-emitting diodes and inverted OSCs. In particular, inverted OSCs based on ZnO CBL deposited by this method have showed efficient performance [20,23,24]. Ka et al. [25] and we [20,24] have shown that ZnO deposited by this aqueous solution could be processed at a low temperature so that most flexible substrates can withstand. In previously work [24], we have demonstrated that the PCE of flexible OSCs could reach 7.6% by using the aqueous solution processed ZnO as CBL. Besides, PEIE could be deposited at a low temperature (100 ◦ C). This makes PEIE also very attractive for using in flexible devices. However, there is still no report about the cathode buffer bilayer combining the low temperature aqueous solution processed ZnO and PEIE used in flexible inverted OSCs. In this work, we employed the low temperature cathode buffer bilayer combining the aqueous solution-processed ZnO and PEIE in flexible inverted OSCs based on Poly(3-hexylthiophene2,5-diyl):[6,6]-phenyl-C61 -butryric acid methyl ester (P3HT:PC61 BM) and Poly({4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl}): [6,6]-phenyl-C71 -butyric acid methyl ester (PTB-7:PC71 BM). It is found that with proper combination of ZnO and PEIE as the CBL, the short circuit current density (JSC ) is obviously improved, resulting in better device performance compared with pure ZnO or PEIE CBL. The increased JSC results from the enhanced electron collection ability from the active layer to the cathode. At the same time, the flexible
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inverted OSCs showed a superior mechanical property. These results suggest that the combination of the low temperature aqueous solution processed ZnO and PEIE can be a promising cathode buffer Energies 2017, 10, 494 3 of 11 bilayer for flexible inverted OSCs.
2. Results Results and and Discussion Discussion 2. Theschematic schematicdevice devicestructure structure flexible inverted OSCs the energy diagram the The of of flexible inverted OSCs andand the energy bandband diagram of theof used used materials are illustrated in Figure 1a,b. The inverted OSCs were fabricated on flexible PET materials are illustrated in Figure 1a,b. The inverted OSCs were fabricated on flexible PET substrates substrates with the structure of ITO/CBLs/P3HT:PC 61BM or PTB7:PC71BM/MoO3/Ag. ZnO has and with theand structure of ITO/CBLs/P3HT:PC 61 BM or PTB7:PC71 BM/MoO3 /Ag. ZnO has conduction conduction energy of around −4.4 eV band and valence band energy ofeV, around eV, which band energyband of around −4.4 eV and valence energy of around −7.8 which−7.8 suggests that suggests that electrons from the active layer can be transported into ZnO, while holes from thecan active electrons from the active layer can be transported into ZnO, while holes from the active layer be layer canAtbethe blocked. At the same time, simple PEIE contains amine groups andsurface it can blocked. same time, PEIE contains aliphaticsimple amine aliphatic groups and it can produce producebetween surface the dipoles between the cathode active layer.layer A very thinreduce PEIE layer could reduce dipoles cathode and active layer. Aand very thin PEIE could the work function the work function of cathode and help the electron extraction. There are four different combinations of cathode and help the electron extraction. There are four different combinations of ZnO and PEIE for of ZnOpure andZnO, PEIE pure for CBLs: ZnO, pure ZnO/PEIE and PEIE/ZnO. Inverted based on CBLs: PEIE,pure ZnO/PEIE andPEIE, PEIE/ZnO. Inverted OSCs based on theOSCs four different the four different CBLs are fabricated. CBLs are fabricated. -2.3eV -3eV
Ag Anode
MoO3 P3HT:PC BM/ PTB7:PC BM 61
Cathode
PEIE -3.4eV -3.7eV -4.3eV -4.4eV + MoO3 P3HT -4.8eV PTB7 PC 71 B + PC61BM M ZnO -5eV -5.2eV +
71
ITO CBL
-
-6.1eV -6.0eV +
-7.4eV
-5.3eV
-4.3eV
Ag
-
ITO PET
Figure 1. (a) The layer structure and (b) the corresponding energy band diagram of materials of the flexible Figure 1. (a) The layer structure and (b) the corresponding energy band diagram of materials of the flexible organic solar cells cells (OSCs). (OSCs). P3HT:PC61BM: organic solar P3HT:PC61BM: Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butryric Poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butryric acid methyl ester; PTB7:PC71BM: Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiopheneacid methyl ester; PTB7:PC71BM: Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl} 71-butyric acid 2,6-diyl}{3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C {3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C 71 -butyric acid methyl ester; methyl ester; CBL: cathode buffer layer; ITO: indium-tin-oxide; PET: polyethylene terephthalate; and CBL: cathode buffer layer; ITO: indium-tin-oxide; PET: polyethylene terephthalate; and PEIE: PEIE: polyethylenimine ethoxylated. polyethylenimine ethoxylated.
The current density versus voltage (J-V) characteristics of the P3HT:PC61BM devices with The current density versus voltage (J-V) characteristics of the P3HT:PC61 BM devices with different different CBLs are shown in Figure 2a, and the extracted device parameters are summarized in CBLs are shown in Figure 2a, and the extracted device parameters are summarized in Table 1. Table 1. The parameters are extracted according to the Shockley equation: The parameters are extracted according to the Shockley equation: q ( V Rs J ) V Rs J J J 0 ( exp ( Jp ) 1) (1) q(V −nkRBsTJ ) V −RRshs J J = J0 (exp( ) − 1) + − Jp (1) nkB T Rsh where J0 is the saturation current, Jp the photocurrent, Rs the series resistance, Rsh the shunt where J0 is the current, photocurrent, Rs the series Rshconstant, the shunt and resistance, resistance, n saturation the ideality factor,Jp qthethe electron charge, kB theresistance, Boltzmann T the ntemperature. the ideality factor, q the electron charge, k the Boltzmann constant, and T the temperature. By using By using Equation (1) with our proposed explicit analytic expression method [26], the B Equation (1) with proposed explicit expressioncould method [26], the experimental were experimental dataour were extracted and analytic these parameters rebuild I-V curves ofdata the OSCs extracted and CBLs these as parameters could rebuild theconfirmed I-V curves the OSCs with different CBLs as with different shown in Figure 1a, which theofvalidity of the extracted parameters. 2, an shown Figure 1a,pure which confirmed the of the extracted parameters. For device witha For thein device with ZnO CBL, a JSC ofvalidity 9.16 mA/cm open circuit voltage (V OC)the of 0.65 V, and 2 pure ZnO(FF) CBL,ofa63.61% JSC of 9.16 mA/cm which , an open circuit voltage (V OC )The of 0.65 V, and fill factor (FF) fill factor are achieved result in a PCE of 3.84%. device withapure PEIE CBL 2 of 63.61% are achieved which result in a PCE of 3.84%. The device with pure PEIE CBL shows shows a similar device performance with a PCE of 3.79%, a VOC of 0.65 V, a JSC of 9.19 mA/cm , andaa 2 , and a FF of similar device These performance with a PCE of 3.79%, a Vbetter 0.65 V, a JSC mA/cm FF of 62.87%. results are comparable or even to of the9.19 reported OSCs with pure OC ofcompared 62.87%. are comparable or we evenuse better compared to the OSCs pure ZnO or ZnO orThese PEIEresults CBL [24,27,28]. When PEIE/ZnO CBL in reported the device, a with decreased device 2 PEIE CBL [24,27,28]. When we use PEIE/ZnO CBL in the device, a decreased device performance is performance is obtained with a PCE of 3.12%, a VOC of 0.64 V, a JSC of 8.21 mA/cm , and a FF of 58.98%. 2 obtained with a PCE 3.12%, a V OCthe of device 0.64 V, awith JSC of 8.21 mA/cm and a FFthe of best 58.98%. Comparing Comparing with theofabove OSCs, ZnO/PEIE CBL ,exhibits performance as shown in Figure 2c,d, which obtains a PCE as high as 4.04%, with a JSC of 9.81 mA/cm2, a VOC of 0.65 V, and a FF of 63.83%. The obviously increased JSC should account for the performance improvement. To understand the improvement in the photovoltaic efficiency of the device with ZnO/PEIE CBL, the collected photocurrent (Jph) as a function of effective voltage (Veff), which reflects the internal field in the device, is plotted. Jph is obtained by subtracting the current density in the dark from the current
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with the above OSCs, the device with ZnO/PEIE CBL exhibits the best performance as shown in Figure 2c,d, which obtains a PCE as high as 4.04%, with a JSC of 9.81 mA/cm2 , a V OC of 0.65 V, and a FF of 63.83%. The obviously increased JSC should account for the performance improvement. To understand the improvement in the photovoltaic efficiency of the device with ZnO/PEIE CBL, the collected photocurrent (Jph ) as a function of effective voltage (V eff ), which reflects the internal4field Energies 2017, 10, 494 of 11 in the device, is plotted. Jph is obtained by subtracting the current density in the dark from the current density illumination. The TheVV defined V a , where V ocompensation is the compensation density under illumination. effeff is is defined as as VeffV=effVo=−VVoa, − where Vo is the voltage voltage as the where voltageJphwhere JphVa=is0the andapplied V a is the applied bias.inAs shown Jph defined defined as the voltage = 0 and bias. As shown Figure 2b,inJphFigure in the 2b, device in thethe device with the ZnO/PEIE in other from open-circuit to with ZnO/PEIE CBL is higherCBL thanisinhigher other than devices fromdevices open-circuit condition to condition short-circuit short-circuit condition. These results indicate that higher charge collection ability is achieved by the condition. These results indicate that higher charge collection ability is achieved by the device with device with theCBL, ZnO/PEIE which isfor responsible itsThe higher PCE. The low series the ZnO/PEIE which isCBL, responsible its higher for PCE. relatively lowrelatively series resistance for resistance thethe device with the CBLthat also confirms thatbilayer the ZnO/PEIE bilayer could the device for with ZnO/PEIE CBLZnO/PEIE also confirms the ZnO/PEIE could further lower the further lower the energythe barrier between and the ITO electrode and thethen active layer, transport and then from electron energy barrier between ITO electrode the active layer, and electron the transport from the active layer toThis ITO is is the facilitated. This is reason active layer to ITO is facilitated. main reason forthe themain higher JSC. for the higher JSC . (a)
(b) 12
12 PEIE/ZnO ZnO PEIE ZnO/PEIE Fitting curve
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Figure 2. 2. (a) (a) The The current current density density versus versus voltage voltage (J-V) (J-V) characteristics; characteristics; (b) (b) the the collected collected photocurrent photocurrent as as Figure ph -V eff ) characteristics; (c) statistical chart of fill factor (FF) and open a function of effective voltage (J a function of effective voltage (Jph eff ) characteristics; (c) statistical chart of open OC); and short short circuit circuit circuit voltage voltage (V (VOC circuit ); and and (d) (d) statistical statistical chart chart of of power power conversion efficiency (PCE) and SC)) of current density density (J(JSC current of the the OSCs based on P3HT:PC61 BM with with different different CBLs. CBLs. 61BM Table 1.1. Photovoltaic parameters for for flexible flexible inverted inverted P3HT:PC P3HT:PC6161BM-based BM-based and and Table Photovoltaic performance performance parameters PTB7:PC 71 BM-based OSCs on PET. PTB7:PC71 Active layer
Active Layer
CBLs
CBLs
ZnO
JSC (mA/cm2) V VOC (V) FF (%) FF (%) OC (V)
JSC (mA/cm2 )
9.16
0.65
63.61
ZnO PEIE PEIE P3HT:PC 61BM P3HT:PC61 BM ZnO/PEIE ZnO/PEIE PEIE/ZnO PEIE/ZnO
9.16 9.19 9.19 9.81 9.81 8.21 8.21
0.65 0.65 0.65 0.65 0.65 0.64 0.64
63.61 62.87 62.87 63.83 63.83 58.98 58.98
ZnO ZnO PEIE PEIE ZnO/PEIE ZnO/PEIE PEIE/ZnO
15.39 15.39 14.74 14.74 16.48 16.48 13.48
0.75 0.75 0.75 0.75 0.75 0.75 0.75
65.91 65.91 65.32 65.32 66.01 66.01 62.14
PTB-7:PC 71 BM 71BM PTB-7:PC
PEIE/ZnO
13.48
0.75
62.14
PCE (%) PCE (%) Rs (Ω/cm2R) s Rsh (Ω/cm Rsh 2) Best Ave (Ω/cm2 ) (Ω/cm2 ) Best Ave 3.84 3.74 2.1 501.8 3.84 501.8 3.79 3.68 3.74 3.1 2.1 473.9 3.79 3.68 3.1 473.9 4.04 479.7 4.04 3.93 3.93 2.7 2.7 479.7 3.12 2.95 3.2 449.8 3.12 2.95 3.2 449.8 7.63 610.5 7.63 7.59 7.59 4.9 4.9 610.5 7.28 7.09 7.09 5.6 5.6 594.3 7.28 594.3 8.12 8.00 4.1 645.1 8.12 8.00 4.1 645.1 6.31 5.99 6.4 757.9 6.31 5.99 6.4 757.9
The four different CBLs were also applied to PTB-7:PC71BM-based flexible inverted OSCs to test its feasibility in other material systems. Figure 3a shows the J-V curves and Table 1 shows the parameter summary of the flexible inverted PTB-7:PC71BM OSCs. These devices show the same variation tendency of performance as that in the devices based on the P3HT:PC61BM material
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The four different CBLs were also applied to PTB-7:PC71 BM-based flexible inverted OSCs to test its feasibility in other material systems. Figure 3a shows the J-V curves and Table 1 shows the parameter summary of the flexible inverted PTB-7:PC71 BM OSCs. These devices show the same variation tendency of performance as that in the devices based on the P3HT:PC61 BM material system. The device with pure ZnO CBL achieves a PCE of 7.63% and with a V OC of 0.75 V, a JSC of 15.39 mA/cm2 and a FF of 10, 65.91%. And the device with pure PEIE CBL achieves a PCE of 7.28% with a V OC of 0.75 V, Energies 2017, 494 5 of 11 a JSC of 14.74 mA/cm2 and a FF of 65.32%. Both of them have a comparable performance with the performance with reported ZnO or the PEIE CBL device [27–29].performance And the lowest device reported OSCs withthe pure ZnO orOSCs PEIEwith CBL pure [27–29]. And lowest is obtained performance is obtained the PEIE/ZnO CBL is introduced. It onlywith shows a OC PCE 6.31% when the PEIE/ZnO CBLwhen is introduced. It only shows a PCE of 6.31% aV ofof 0.75 V, awith JSC ofa VOC ofmA/cm 0.75 V,2 a, and JSC of 13.48 mA/cm2When , and awe FFuse of ZnO/PEIE 62.14%. When wethe use ZnO/PEIE CBL, the highest 13.48 a FF of 62.14%. CBL, highest device performance is 2, device performance is obtained and aisPCE as high as 8.12% a VJOC a higher obtained and a PCE as high as 8.12% obtained with a V OCisofobtained 0.75 V, awith higher of 0.75 16.48V,mA/cm SC of 2 JSC ofa 16.48 , and a as higher FFFigure of 66.01%, shown Figure 3b,c. These results demonstrate that and highermA/cm FF of 66.01%, shown 3b,c. as These results demonstrate that the ZnO/PEIE bilayer the ZnO/PEIE bilayer could effectively enhance the performance of PTB-7:PC 71 BM inverted OSCs could effectively enhance the performance of PTB-7:PC BM inverted OSCs and confirm its feasibility 71 and confirmmaterial its feasibility in different material systems. in different systems.
2
JSC(mA/cm )
(a)
5
PEIE PEIE/ZnO ZnO ZnO/PEIE Fitting curve
0 -5 -10 -15 -20 -0.2
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Figure 3. 3. (a) (a) J-V J-V characteristics; characteristics; (b) (b) statistical statistical chart chart of of FF FF and and VVOC,, and (c) statistical chart of PCE and Figure OC and (c) statistical chart of PCE and the OSCs based on PTB-7:PC 71BM with different CBLs. JJ SC of SC of the OSCs based on PTB-7:PC71 BM with different CBLs.
Figure 4 shows the statistical PCE of OSCs with different CBLs based on P3HT:PC61BM and Figure 4 shows the statistical PCE of OSCs with different CBLs based on P3HT:PC61 BM and PTB-7:PC71BM systems and the results show that the devices based on the ZnO/PEIE bilayer PTB-7:PC71 BM systems and the results show that the devices based on the ZnO/PEIE bilayer achieve a better performance, which confirms the validity of our above discussion. The incident achieve a better performance, which confirms the validity of our above discussion. The incident photon-to-electron conversion efficiency (IPCE) spectra (SCS 100 IPCE system, Zolix instrument Co. photon-to-electron conversion efficiency (IPCE) spectra (SCS 100 IPCE system, Zolix instrument Ltd., Beijing, China) of the OSCs devices with ZnO/PEIE CBL based on P3HT:PC61BM and Co. Ltd., Beijing, China) of the OSCs devices with ZnO/PEIE CBL based on P3HT:PC61 BM and PTB7:PC71BM are shown in Figure 5. The IPCE value approaches 68% around 550 nm for the device PTB7:PC71 BM are shown in Figure 5. The IPCE value approaches 68% around 550 nm for the device based on P3HT:PC61BM. The JSC calculated from integration of the IPCE spectrum from 300 nm to based on P3HT:PC61 BM. The JSC calculated from integration of the IPCE spectrum from 300 nm to 800 nm is 9.64 mA/cm2. For the device based on PTB-7:PC71BM, the IPCE value approaches 77% 800 nm is 9.64 mA/cm2 . For the device based on PTB-7:PC71 BM, the IPCE value approaches 77% around 600 nm. The JSC calculated from integration of the IPCE spectrum from 300 nm to 800 nm is around 600 nm. The JSC calculated from integration of the IPCE spectrum from 300 nm to 800 nm is 16.14 mA/cm22. The calculated JSC is near the values obtained from the I-V measurements. 16.14 mA/cm . The calculated JSC is near the values obtained from the I-V measurements. 8.5
4.5
8.0
3.5 3.0 2.5
7.5
PCE(%)
PCE(%)
4.0
7.0 6.5 6.0 5.5
Ltd., Beijing, China) of the OSCs devices with ZnO/PEIE CBL based on P3HT:PC61BM and PTB7:PC71BM are shown in Figure 5. The IPCE value approaches 68% around 550 nm for the device based on P3HT:PC61BM. The JSC calculated from integration of the IPCE spectrum from 300 nm to 800 nm is 9.64 mA/cm2. For the device based on PTB-7:PC71BM, the IPCE value approaches 77% is around2017, 60010, nm. Energies 494The JSC calculated from integration of the IPCE spectrum from 300 nm to 800 nm 6 of 12 16.14 mA/cm2. The calculated JSC is near the values obtained from the I-V measurements. 8.5
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Figure 4. 4. Performance Performance statistical statistical results results of of OSCs OSCs with with different different CBLs CBLs based based on on (a) (a) P3HT:PC P3HT:PC61BM BM and and Figure 61 (b) PTB-7:PC PTB-7:PC71BM. (b) Energies 2017, 10, 49471 BM. 6 of 11 0.8
IPCE
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Figure 5. Incident Figure 5. Incident photon-to-electron photon-to-electron conversion conversion efficiency efficiency (IPCE) (IPCE) spectra spectra of of the the OSCs OSCs devices devices with with BM. ZnO/PEIE ZnO/PEIECBL CBLbased basedon onP3HT:PC P3HT:PC6161BM BMand andPTB-7:PC PTB-7:PC7171 BM.
It can be seen from Figures 2 and 3, the lowest device performance is obtained for the OSC with It can be seen from Figures 2 and 3, the lowest device performance is obtained for the OSC with the the PEIE/ZnO CBL. The combination of ZnO and PEIE in this configuration leads to the negative PEIE/ZnO CBL. The combination of ZnO and PEIE in this configuration leads to the negative effects on effects on the JSC and FF. In fact, the obvious transmittance difference between the PEIE/ZnO sample the JSC and FF. In fact, the obvious transmittance difference between the PEIE/ZnO sample and other and other samples can be distinguished by eyes in the experiments. To investigate the optical samples can be distinguished by eyes in the experiments. To investigate the optical property differences property differences of the four different CBL films, the ultra violet visible (UV-Vis) wavelength of the four different CBL films, the ultra violet visible (UV-Vis) wavelength optical transmittance optical transmittance spectra (LAMBDA-950, PERKIN-ELMER, Waltham, MA, USA) of the films spectra (LAMBDA-950, PERKIN-ELMER, Waltham, MA, USA) of the films with pure ZnO, pure PEIE, with pure ZnO, pure PEIE, ZnO/PEIE, and PEIE/ZnO CBLs were measured and shown in Figure 6. ZnO/PEIE, and PEIE/ZnO CBLs were measured and shown in Figure 6. As can be seen, all the As can be seen, all the samples with the configurations of pure ZnO, pure PEIE, and ZnO/PEIE have samples with the configurations of pure ZnO, pure PEIE, and ZnO/PEIE have good transmittances good transmittances in the visible wavelength range. These results indicate that the ZnO interlayer, in the visible wavelength range. These results indicate that the ZnO interlayer, PEIE interlayer and PEIE interlayer and ZnO/PEIE interlayer have minimal effect on the light absorption of active layer. ZnO/PEIE interlayer have minimal effect on the light absorption of active layer. However, there is However, there is obviously lower light transmittance for the sample with the configuration of obviously lower light transmittance for the sample with the configuration of PEIE/ZnO at wavelength PEIE/ZnO at wavelength from 400 nm to 700 nm. The lower light transmittance of PEIE/ZnO film in from 400 nm to 700 nm. The lower light transmittance of PEIE/ZnO film in the visible wavelength the visible wavelength range would result in a lower JSC in the corresponding OSC. This must be one range would result in a lower JSC in the corresponding OSC. This must be one important reason important reason for the inferior performance of device with PEIE/ZnO CBL. One possible for the inferior performance of device with PEIE/ZnO CBL. One possible explanation is that PEIE explanation is that PEIE could be dissolved in water and the used ZnO is deposited by an aqueous could be dissolved in water and the used ZnO is deposited by an aqueous solution method. During solution method. During the deposition of the ZnO precursor, the aqueous solution of ammine-zinc the deposition of the ZnO precursor, the aqueous solution of ammine-zinc complex may destroy complex may destroy the underlined PEIE film and then results in the lower light transmittance. the underlined PEIE film and then results in the lower light transmittance. In order to verify our In order to verify our hypothesis, we take optical microscopy images of the four CBLs. The optical hypothesis, we take optical microscopy images of the four CBLs. The optical microscopy images of microscopy images of ITO/ZnO, ITO/ZnO/PEIE, ITO/PEIE/ZnO, and ITO/ZnO are presented in ITO/ZnO, ITO/ZnO/PEIE, ITO/PEIE/ZnO, and ITO/ZnO are presented in Figure 7. It can be seen in Figure 7. It can be seen in the optical microscopy image of ITO/PEIE/ZnO that there are many the optical microscopy image of ITO/PEIE/ZnO that there are many winkles or cracks in this sample, winkles or cracks in this sample, which is obviously different from other samples where the surface which is obviously different from other samples where the surface is rather smooth. This confirms our is rather smooth. This confirms our hypothesis that the deposition of ZnO destroys the underlying hypothesis that the deposition of ZnO destroys the underlying PEIE film. PEIE film. 100
ttance(%)
80 60
PET
In order to verify our hypothesis, we take optical microscopy images of the four CBLs. The optical microscopy images of ITO/ZnO, ITO/ZnO/PEIE, ITO/PEIE/ZnO, and ITO/ZnO are presented in Figure 7. It can be seen in the optical microscopy image of ITO/PEIE/ZnO that there are many winkles or cracks in this sample, which is obviously different from other samples where the surface is rather smooth. This confirms our hypothesis that the deposition of ZnO destroys the underlying Energies 2017, 10, 494 7 of 12 PEIE film. 100
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40 20 0
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Figure 6. Transmittance spectra of the films with ZnO, ZnO/PEIE, PEIE, PEIE/ZnO CBL.
Energies 2017,Figure 10, 4946. Transmittance spectra of the films with ZnO, ZnO/PEIE, PEIE, PEIE/ZnO CBL.
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Figure microscopyimages imagesofof(a)(a) PET/ITO/ZnO;(b) (b)PET/ITO/PEIE; PET/ITO/PEIE;(c) (c)PET/ITO/ZnO/PEIE; PET/ITO/ZnO/PEIE; Figure 7. 7. The The optical optical microscopy PET/ITO/ZnO; and (d) PET/ITO/PEIE/ZnO. The particles in (a–c) are used for focus to make the and (d) PET/ITO/PEIE/ZnO. The particles in (a–c) are used for focus to make the optical optical microscopy microscopy images clear. images clear.
To further investigate the surface characteristic of CBLs, an atomic force microscope (AFM) To further investigate the surface characteristic of CBLs, an atomic force microscope (AFM) (Agilent 5500, Agilent Technologies, Palo Alto, CA, USA) was adopted to measure the surface (Agilent 5500, Agilent Technologies, Palo Alto, CA, USA) was adopted to measure the surface morphology of ITO/ZnO, ITO/ZnO/PEIE, ITO/PEIE/ZnO, and ITO/ZnO. The measurement results morphology of ITO/ZnO, ITO/ZnO/PEIE, ITO/PEIE/ZnO, and ITO/ZnO. The measurement results are illustrated in Figure 8. The root mean square (RMS) roughness was 1.3 nm, 1.5 nm, 2.1 nm, and are illustrated in Figure 8. The root mean square (RMS) roughness was 1.3 nm, 1.5 nm, 2.1 nm, 10.5 nm for the samples with the configuration of PEIE, ZnO/PEIE, ZnO, and PEIE/ZnO, and 10.5 nm for the samples with the configuration of PEIE, ZnO/PEIE, ZnO, and PEIE/ZnO, respectively. The RMS value (10.5 nm) of PEIE/ZnO is far larger than other three RMS values of ZnO respectively. The RMS value (10.5 nm) of PEIE/ZnO is far larger than other three RMS values (2.1 nm), ZnO/PEIE (1.5 nm), and PEIE (1.3 nm). The rough surface of PEIE/ZnO suggests that the of ZnO (2.1 nm), ZnO/PEIE (1.5 nm), and PEIE (1.3 nm). The rough surface of PEIE/ZnO suggests that PEIE film is destroyed by the following ZnO deposition, which is consistent with the transmittance the PEIE film is destroyed by the following ZnO deposition, which is consistent with the transmittance and optical microscopy measurements. On the other hand, by capping PEIE on ZnO, a smaller RMS and optical microscopy measurements. On the other hand, by capping PEIE on ZnO, a smaller RMS value (1.5 nm) of ZnO/PEIE is achieved compared with that of ZnO (2.1 nm). This means that there is value (1.5 nm) of ZnO/PEIE is achieved compared with that of ZnO (2.1 nm). This means that there a smoother surface for the sample of ZnO/PEIE compared to the pure ZnO and the smoother is a smoother surface for the sample of ZnO/PEIE compared to the pure ZnO and the smoother morphology supplies excellent contact between the active layer and CBL, which is consistent with morphology supplies excellent contact between the active layer and CBL, which is consistent with the the better device performance. better device performance.
PEIE film is destroyed by the following ZnO deposition, which is consistent with the transmittance and optical microscopy measurements. On the other hand, by capping PEIE on ZnO, a smaller RMS value (1.5 nm) of ZnO/PEIE is achieved compared with that of ZnO (2.1 nm). This means that there is a smoother surface for the sample of ZnO/PEIE compared to the pure ZnO and the smoother morphology excellent contact between the active layer and CBL, which is consistent 8with Energies 2017, 10,supplies 494 of 12 the better device performance.
Figure 8. Surface morphology of (a) ZnO; (b) PEIE; (c) ZnO/PEIE; and (d) PEIE/ZnO. Figure 8. Surface morphology of (a) ZnO; (b) PEIE; (c) ZnO/PEIE; and (d) PEIE/ZnO.
Another possible reason for the different JSC values is that the effects of the different surface Another possible reason for the different JSC values is that the effects of the different surface energy of CBLs on the active layer formation. To study the influence of different CBLs on the active energy of CBLs on the active layer formation. To study the influence of different CBLs on the active layer, we carried out a contact angle measurement of the CBL films. The consequence of water contact layer, we carried out a contact angle measurement of the CBL films. The consequence of water angle measurement is displayed in Figure 9. As illustrated in Figure 9, due to application of UV Energies 2017, 10, 494 8 of 11 contact angle measurement is displayed in Figure 9. As illustrated in Figure 9, due to application of◦ ozone treatment, all of the four different CBLs show good hydrophilicity. The contact angles of 16.8 UV ozone treatment, all of the four different CBLs show good hydrophilicity. The contact angles of ◦ (PEIE/ZnO) (ZnO), 18.1◦ (PEIE), 17.7◦ (ZnO/PEIE), and 21.2and were very close. Theclose. delicate 16.8° (ZnO), 18.1° (PEIE), 17.7° (ZnO/PEIE), 21.2° (PEIE/ZnO) were very Thedifference delicate would not cause on filmimpact formation active layer. This indicated the influence causedthe by difference wouldmuch not impact cause much on of film formation of active layer. This indicated hydrophilicity of by CBL on thickness could influence caused hydrophilicity of CBLbeonneglected. thickness could be neglected.
Figure CBL; (b) (b) PEIE PEIE CBL; CBL; (c) (c) PEIE/ZnO PEIE/ZnO CBL; Figure 9. 9. The The water water contact contact angle angle of of OSC OSC devices devices with with (a) (a) ZnO ZnO CBL; CBL; and (d) ZnO/PEIE CBL. and (d) ZnO/PEIE CBL.
The flexibility property of OSCs with different CBLs (pure ZnO, pure PEIE, ZnO/PEIE, and The flexibility property of OSCs with different CBLs (pure ZnO, pure PEIE, ZnO/PEIE, PEIE/ZnO) based on PTB-7:PC71BM material system was also studied. The consequence of bending and PEIE/ZnO) based on PTB-7:PC BM material system was also studied. The consequence of test is illustrated in Figure 10. It can 71 be seen that all of OSCs with pure ZnO CBL, pure PEIE CBL, bending test is illustrated in Figure 10. It can be seen that all of OSCs with pure ZnO CBL, pure PEIE and ZnO/PEIE CBL show similar flexibility property. After 1000 bending cycles with a radius of CBL, and ZnO/PEIE CBL show similar flexibility property. After 1000 bending cycles with a radius 0.8 cm, OSCs with pure ZnO CBL, pure PEIE CBL, and ZnO/PEIE CBL could keep 90.5%, 93.0%, and of 0.8 cm, OSCs with pure ZnO CBL, pure PEIE CBL, and ZnO/PEIE CBL could keep 90.5%, 93.0%, 92.9% of their initial performance, respectively. However, the PCE of OSC with PEIE/ZnO CBL, and 92.9% of their initial performance, respectively. However, the PCE of OSC with PEIE/ZnO CBL, which is also the device with lowest PCE, decreases quickly in the bending test and only keeps by which is also the device with lowest PCE, decreases quickly in the bending test and only keeps by 80% 80% of its original value after 1000 bending cycles. The inferior characteristic of PEIE/ZnO film of its original value after 1000 bending cycles. The inferior characteristic of PEIE/ZnO film affects not affects not only the device performance but also the flexibility property of OSCs. only the device performance but also the flexibility property of OSCs.
Normalized PCE
1.0
0.9
0.8
PEIE/ZnO ZnO PEIE
and ZnO/PEIE CBL show similar flexibility property. After 1000 bending cycles with a radius of 0.8 cm, OSCs with pure ZnO CBL, pure PEIE CBL, and ZnO/PEIE CBL could keep 90.5%, 93.0%, and 92.9% of their initial performance, respectively. However, the PCE of OSC with PEIE/ZnO CBL, which is also the device with lowest PCE, decreases quickly in the bending test and only keeps by 80% of2017, its 10, original value after 1000 bending cycles. The inferior characteristic of PEIE/ZnO9 film Energies 494 of 12 affects not only the device performance but also the flexibility property of OSCs.
Normalized PCE
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PEIE/ZnO ZnO PEIE ZnO/PEIE
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Figure Normalizeddevice devicePCE PCEas asaafunction function of of of 0.8.0.8. Figure 10.10. Normalized of the the number numberof ofbending bendingcycles cycleswith witha radius a radius
3. Materials and Methods 3. Materials and Methods 3.1. 3.1. Preparation Preparation of of Materials Materials For For the the ZnO ZnO precursor precursor solution, solution, ZnO ZnO aqueous aqueous solution solution was was prepared prepared by by dissolving dissolving 10 10 mg mg ZnO ZnO 2+ solution. Then the solution was ultrasonically powder in 1 mL ammonia to form 0.125 M Zn(NH 3)42+ powder in 1 mL ammonia to form 0.125 M Zn(NH3 )4 solution. Then the solution was ultrasonically processed for 5–10 5–10 min min and and stored stored in in refrigerator refrigerator at at 0–10 0–10 ◦°C than 12 12 h h before before use. use. For processed for C for for more more than For PEIE PEIE solution, PEIE was dissolved in 2-methoxy ethanol to form 0.2 wt% PEIE solutions. The solutions solution, PEIE was dissolved in 2-methoxy ethanol to form 0.2 wt% PEIE solutions. The solutions were were then stirred h at room temperature then stirred for 12for h at12room temperature beforebefore use. use. For P3HT:PC 61BM, a mixture of P3HT and PC61BM at a weight ratio of 1:0.8 was dissolved in For P3HT:PC61 BM, a mixture of P3HT and PC61 BM at a weight ratio of 1:0.8 was dissolved 1,2-dichlorobenzene(1,2-DCB) andand thenthen put put on the heating stagestage stirred for more than than 12 h.12For in 1,2-dichlorobenzene(1,2-DCB) on the heating stirred for more h. PTB-7:PC 71BM, a mixture of PTB-7:PC71BM at a weight ratio of 1:1.5 was dissolved in dichlorobenzene For PTB-7:PC BM, a mixture of PTB-7:PC BM at a weight ratio of 1:1.5 was dissolved in 71
71
dichlorobenzene (CB). Then, 1,8-diiodooctane (DIO) was added to the solution at a concentration of 3 vol%. Finally, just like the preparation process of P3HT:PC61 BM, the solution was also stirred for more than 12 h. The ITO-coated PET substrates were supplied by Zhuhai Kaivo (Zhuhai, China). PCBM and DIO were purchased from Nano-C (Westwood, MA, USA) and Alfa (Heysham, UK), respectively. PTB7 and P3HT were provided by 1-materials (Dorval, QC, Canada) and Rieke Metals (Lincoin, NE, USA,). ZnO, CB, 1,2-DCB, and MoO3 were supplied by Sigma Aldrich (St. Louis, MO, USA). 3.2. Fabrication and Measurement of Flexible Organic Solar Cells The devices were fabricated with a structure of ITO/CBLs/active layer/MoO3 /Ag as in Figure 1a, in which CBLs are pure ZnO, pure PEIE, ZnO/PEIE, or PEIE/ZnO, and the active layer are P3HT:PC61 BM or PTB-7:PC71 BM films. Figure 1 shows the layer structure of the device. The fabrication processes were as follows: ITO-coated PET substrates were sequentially cleaned with detergent, deionized water, and ethanol in an ultrasonic bath for 20 min, and then were blow-dried with a nitrogen gun. Then, UV ozone was applied to the ITO surface for 30 min. The ZnO aqueous solution was spin-cast on the cleaned ITO-PET substrate at 3000 rpm for 40 s, and then annealed in the oven at 150 ◦ C for 30 min. The PEIE layer was deposited via spin-coating method at 5000 RPM for 60 s and followed by annealing at 100 ◦ C in the oven too. The polymer solution was then spun-casted on the CBL at 1000 RPM for 60 s for PTB7:PC71 BM or at 800 RPM for 120 s for P3HT:PC61 BM in glove box, respectively. After the films were slow dried in glove box for 3 h, the P3HT-based devices should be extra pre-annealed on a heating stage at 150 ◦ C for 10 min. Then 10 nm MoO3 and 80 nm Ag were thermally evaporated onto the active layer through a metal shadow mask. The devices’ area was about 8 mm2 The J-V characteristics of the devices were measured by a Xenon lamp (XEC-300M2, SANEI ELECTRIC, Shizuoka, Japan) with an air mass (AM) 1.5 G filteratan intensity of 100 mW/cm2 and a Keithley 2400 source-measure unit [30]. The Xenon lamp is verified through a standard
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Si solar cell calibrated by the National Renewable Energy Laboratory (NREL). The transmittance spectra of the PET/ITO/ZnO, PET/ITO/PEIE, PET/ITO/ZnO/PEIE, and PET/ITO/PEIE/ZnO were measured by a UV/Vis/NIR spectrophotometer (LAMBDA-950, PERKIN-ELMER, Waltham, MA, USA). The surface optical morphology of PET/ITO/ZnO, PET/ITO/PEIE, PET/ITO/ZnO/PEIE, and PET/ITO/PEIE/ZnO was investigated by the Leica DM4000M optical microscopy (Wetzlar, Germany). Tapping mode AFM tests were performed to test the thin films morphology by an Agilent 5500 scanning probe system (Agilent 5500, Agilent Technologies, Palo Alto, CA, USA). The water contact angle measurement was carried out by a contact angle meter (JC2000DM, Beijing Zhongyikexin Science and Technology Co. Ltd., Beijing, China). 4. Conclusions In summary, we have fabricated and investigated high performance flexible inverted OSCs by employing low temperature aqueous solution processed ZnO and PEIE bilayer film with high transparency and good electron transporting properties as the CBL. By using the ZnO/PEIE CBL, the highest PCE of 4.04%, based on P3HT:PC61 BM, and 8.12%, based on PTB-7:PC71 BM, are achieved, which are higher than the control device with the pure ZnO CBL or pure PEIE CBL. Meanwhile, the inverted OSC also shows superior flexibility and it can keep 92.9% of its initial performance after 1000 bending cycles with a radius of 0.8 cm. This study indicates that the low-temperature solution-processed ZnO/PEIE bilayer significantly enhanced charge collection efficiency which is beneficial for high performance inverted OSCs and suitable for flexible devices. Acknowledgments: This study was partly financially supported by National Natural Science Foundation of China (61334002, 61106063, 61534004, 61604119) and the Fundamental Research Funds for the Central Universities (JB151406, JB161101, JB161102). Author Contributions: Chunfu Zhang and Zhenhua Lin conceived the idea and guided the experiment, Hailong You and Junchi Zhang conducted most of device fabrication, data collection. Chunfu Zhang wrote the manuscript, Zhenhua Lin and Jingjing Chang revised the manuscript, Zeyulin Zhang, Genquan Han, Jincheng Zhang, Gang Lu helped the device measurement, and Yue Hao supervised the team. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.
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