Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells

Organic Electronics 37 (2016) 35e41

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Amphiphilic fullerene derivative as effective interfacial layer for inverted polymer solar cells Ting Hu a, 1, Ping Jiang a, Lie Chen a, b, 1, Kai Yuan a, b, Hanjun Yang a, Yiwang Chen a, b, * a b

College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 April 2016 Received in revised form 13 June 2016 Accepted 13 June 2016

Amphiphilic fullerene derivative with poly(ethylene glycol) chain (C60-PEG) was applied as effective interfacial layer to improve the performance of inverted polymer solar cells. C60-PEG could not only be used as cathode buffer layer alone by replacing ZnO, but also be used as a self-assembled monolayer to modify ZnO. C60-PEG can tune energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, due to the strong interaction between ZnO nanoparticles and PEG chain, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. Consequently, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]- phenyl C71-butyric acid methyl ester (PC71BM) present a PCE of 6.6% by incorporating C60-PEG into as cathode buffer layer. Moreover, an improved PCE of 7.4% with good long-term stability in air were further achieved by using C60-PEG/ZnO interlayer. In this work, C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in largescale printed polymer solar cells. © 2016 Elsevier B.V. All rights reserved.

Keywords: Polymer solar cells Amphiphilic Compatibility Modification

1. Introduction The polymer solar cells (PSCs) have been researched as a promising alternative to conventional silicon-based solar cells in the past decade owing to their mechanical flexibility and large area roll-to-roll manufacturing at low temperatures [1e4]. Nowadays, the highest power conversion efficiency (PCE) of PSCs exceeds 11% [5]. It is believable that PSCs could be applied in our lives widely. In order to further improve the performance of PSCs, various efforts were adopted, such as designing efficient active materials [6e8], developing effective processing technique [5,9,10], and using suitable device structure [11,12]. Meanwhile, interface engineering is crucially important in improving the performance of PSCs. Interfacial layers possess multiple functions, for example, tuning the energy level alignment [13,14], adjusting the light absorbing [15,16] and improving the interfacial stability [17]. Generally, interfacial materials contain conducting polymer [4,18], metal oxides [19e21], conjugated polymer electrolytes [12,22], carbon nanomaterials

* Corresponding author. College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China. E-mail address: [email protected] (Y. Chen). 1 T. Hu and L. Chen contributed equally to this work. http://dx.doi.org/10.1016/j.orgel.2016.06.018 1566-1199/© 2016 Elsevier B.V. All rights reserved.

[23e25] and crosslinkable materials [26]. Among them, fullerene-based interfacial materials were used widely as good cathode buffer layer (CBL) due to their special chemical compatibility to active layer. Some fullerene derivatives are blended with active layer, then forming a buffer layer on the top of active layer spontaneously, like fullerene derivative with a fluorocarbon chain [27], fullerene with poly(ethylene glycol) chain [28] and amine-based fullerene [29]. However, the drawback of the fullerene-based CBL is the relatively low inherent conductivity. Therefore most of fullerene-based interfacial materials are applied as self-assembled molecular layer (SAM) to modify the zinc oxide (ZnO) but used alone, such as phosphonic acid-anchored C60 [30], carboxylic acid-anchored C60 [25], phenyl-C61-butyric acid$(PCBA) [31] and so on. Although ZnO is a potential CBL with relatively high electron mobility, the compatibility between organic polymer and inorganic metal oxide should not be ignored. In this case, a good solubility is particularly important for fullerene interfacial materials to be used as SAM. Herein, amphiphilic fullerene derivative C60-PEG was synthesized by modifying PC61BM with hydrophilic poly(ethylene glycol) side chain. C60-PEG could not only be used as CBL alone to show a comparable performance to ZnO, but also act as a layer of SAM to modify ZnO with a better performance. C60-PEG can tune energy

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level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, C60-PEG can passivate the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. As a result, inverted polymer solar cells based on thieno[3,4-b]thiophene/benzodithiophene (PTB7) [6,6]:- phenyl C71-butyric acid methyl ester (PC71BM) with C60-PEG as CBL presented a comparable PCE of 6.6% to ZnO. Meanwhile, a further enhanced PCE of 7.4% with good long-term stability in air was achieved when C60-PEG was employed as modification layer. In addition, C60-PEG could be prepared by spin coating at room temperature without annealing, which provides the possibility for large-area printed PSCs. 2. Experimental 2.1. Synthesis of C60-PEG C60-PEG was synthesize by Steglich esterification according to the literature [32]. At first, PCBM was hydrolyzed to get [6,6]phenyl-C61-butyric acid (PCBA) [33]. Then, a blend solution was obtained by dissolving PCBA (0.1 g, 0.112 mmol) and PEG (0.456 g, 0.076 mmol) in the mixed solvent of 1,2-dichlorobenzene and methylbenzene (V:V ¼ 1:1), and the solution was sonicated for 1 h to dissolve PCBA completely. Triphenylphosphine (0.08 g, 0.290 mmol) and the diethyl azodicarboxylate (0.055 g, 0.290 mmol) was added dropwise to the solution. The reaction was held at room temperature for two days. At last, the products were purified by column chromatography with ethyl acetate and methyl alcohol. 2.2. Device fabrication ITO conductive glass (35 U cm2) were cleaned by alcohol, detergent, deionized water and isopropyl, then dried by N2 flow in sequence. Before the spin-coating of cathode buffer layers, ITO substrates were treated by plasma 15 min. In this work, ZnO was synthesized by the hydrolysis of precursor at 200  C for 40 min [34]. C60-PEG was dissolved in CH3OCH2CH2OH (10 mg/ml), and was spin-coated on top of ITO (2000 rpm) or ZnO (6000 rpm) without annealing. After that, the blended solution P3HT‫׃‬PC61BM or PTB7‫׃‬PC71BM was spin coated on top. Finally, anode buffer layer MoO3 (7 nm) and anode Ag (90 nm) was prepared by thermal evaporation. Current-voltage (J-V) characteristics were tested using Keithley 2400 Source Meter in the dark and under simulated AM 1.5 G (100 mW cm2) irradiation (Abet Solar Simulator Sun2000). Incident photon-to-current efficiency (IPCE) were measured under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the incident light was performed with a monocrystalline silicon diode. 2.3. Characterizations PerkinElmer Lambda 750 spectrophotometer was performed to research the ultravioletevisible (UVevis) absorption spectra. Hitachi F-7000 spectrofluorophotometer was carried out to study photoluminescence (PL). The morphology images of ZnO, C60-PEG and ZnO/C60-PEG were obtained from atomic force microscope (AFM) (Digital Instrument Nanoscope 31) test. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements were realized by employing AXIS-ULTRA DLD spectrometer (Kratos Analytical Ltd.) under monochromatic light source of He (I) (21.2 eV). The thicknesses of all the layers were measured by surface profilometry (Alpha-Step-IQ). JC2000A contact angle instrument was used to measure the water contact angle.

3. Results and discussion The chemical structures of active materials and C60-PEG employed for device preparation and the inverted device architecture with the schematic diagram of cathode buffer layer are presented in Fig. 1. The synthetic details of C60-PEG are described in experimental section. As we reported previously [35], C60-PEG has a good air stability and a good solubility in alcohols and organic solvents. Due to the hydrophilic PEG side chain, the water contact angle of ZnO/C60-PEG decreased in contrast with that of bare ZnO (Fig. S1). In this paper, ZnO modified by C60-PEG is noted as ZnO/ C60-PEG for easily describing. UV/Vis absorption profiles of the samples are shown in Fig. S2. C60-PEG CBL has an extremely low absorption in the Vis-NIR wavelengths. The same absorption around 378 nm for ZnO and ZnO/C60-PEG is assigned to the usual band-edge emission in the UV. At the same time, the transmittance of C60-PEG and ZnO/C60PEG CBLs are comparable to that of ITO (Fig. S3). It is proved that C60-PEG has a suitable optical property to be used as CBL for inverted PSCs. Atomic force microscopy (AFM) was used to look into the morphology of different CBL and the height images are displayed in Fig. 2. Compared to ZnO (Fig. 2a), bare C60-PEG shows a different morphology with an increased root-mean-square (RMS) roughness value of 3.8 nm (Fig. 2b). When the surface of ZnO was modified by C60-PEG, the morphology changed to homogeneous network structure (Fig. 2c). There is a strong interaction between ZnO nanoparticles and PEG molecular chain [36], which drives the hydrophilic and oxygen-rich PEG side chain to the ZnO surface. In this sense, C60-PEG molecular moved and formed the homogeneous network film spontaneously. Further spin coating the active layer on the top of C60-PEG or ZnO/C60-PEG, the P3HT:PC61BM represented a smooth film. To research the origin of the different morphology for C60-PEG on ZnO and ITO, X-ray photoelectron spectroscopy (XPS) was carried out on three CBLs. The O 1s XPS spectrum of bare ZnO exhibits two typical peaks, located at about 530.0 eV and 531.4 eV (Fig. 3a). The peak at lower binding energy corresponds to the ZneO bonds [37]. The other peak at higher binding energy is ascribed to oxygen atoms from hydroxyl oxygen [37,38]. Besides the two peaks from ZnO, an additional peak (~532.5 eV) from C60-PEG appears for O 1s XPS spectrum of the ZnO modified by C60-PEG, which associated with O-C-O. Due to the long PEG side chain, the main peak for O 1s XPS spectrum of C60-PEG is originated from O-C-O. The O 1s peak from carboxyl groups (COO-) [39] may be included in the wide peak of O-C-O, so it cannot be distinguished in the O 1s spectrum. Due to the C60-PEG on top, the peak at 530.0 eV for Zn-O of ZnO/C60-PEG is weaker than that of pristine ZnO. Fig. 3b presents the C 1s XPS spectra of ZnO, C60-PEG and ZnO/C60-PEG. It is found that all the samples have two peaks ~285 eV (CeOR, R ¼ C2H5) and ~288.5 eV (COOR), which is assigned to the C atoms of carbonyl groups [40]. Compared to ZnO, the different peak at ~286.2 eV of C60-PEG and ZnO/C60-PEG for O-C-O is attributed to aromatic carbons from C60PEG [40,41]. The atomic concentrations of carbon, oxygen and zinc in all the samples based on the C 1s, Zn 2p and O 1s XPS spectra are summarized in Fig. 3c. C60-PEG possesses the highest atomic concentrations of carbon, oxygen because of its chemical structure. The atomic concentrations of carbon for ZnO/C60-PEG is higher than that for bare ZnO, but the atomic concentrations of oxygen is lower. When C60-PEG was spin coated on the top of ZnO, the hydrophilic and oxygen-rich PEG side chain tend to the ZnO surface, leaving the hydrophobic and carbon-rich fullerene cage facing toward the opposite direction. In that case, more carbon and less oxygen were detected. Meanwhile, the photoluminescence (PL) was performed to study the function of C60-PEG on the surface

T. Hu et al. / Organic Electronics 37 (2016) 35e41

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Fig. 1. Chemical structures of the active materials and C60-PEG used for device fabrication and the structure of the inverted device employed.

Fig. 2. AFM height images (5 mm  5 mm) of (a) ZnO, (b) C60-PEG, (c) ZnO/C60-PEG, (d) ZnO/P3HT:PCBM, (e) C60-PEG/P3HT:PCBM, and (f) ZnO/C60-PEG/P3HT:PCBM. The inset of each picture is the roughness value of root-mean-square (RMS).

defects of ZnO. It is observed that the PL spectra of ZnO and ZnO/ C60-PEG reveal two emission peaks at the same wavelength (Fig. S4). The emission peak at 371 nm origins from the usual bandedge emission and the band edge emission peak in blue region is attributed to the transitions involving Zn interstitial defect states as reported [42]. By comparison, the intensity of the peak in blue region for ZnO in the presence of C60-PEG is lower, indicating the C60-PEG passivated the shallow trap sites of ZnO. The passivation of defects could reduce the recombination of charge carriers. As shown in Fig. 4a and Figure S5, ultraviolet photo-electron spectroscopy (UPS) was employed to clarify the energy level of the ZnO, C60-PEG and ZnO/C60-PEG. The highest occupied

molecular orbital (HOMO) level energies are defined according to the equation [43].

  HOMO EHOMO ¼ hy  Eonset  Ecutoff where hn is the incident photon energy of 21.2 eV, and Ecutoff is gained from the high binding energy cutoff of a spectrum [43,44], HOMO is delivered from the right panel. As a result, the HOMO and Eonset energies for ZnO, C60-PEG and ZnO/C60-PEG are 7.65 eV, 6.92 eV and 7.42 eV, respectively. Based on these HOMO energies and the optical band gap obtained from the UVevis absorption spectra (Fig. S2) [45], the LUMO energy levels were

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Fig. 3. (a) O 1s, (b) C 1s spectra of ZnO, C60-PEG, and ZnO/C60-PEG, and (c) Atomic concentrations of carbon, oxygen and zinc based on the corresponding XPS spectra.

Table 1 Energy levels of ZnO, C60-PEG and ZnO/C60-PEG. Buffer layer

Eg

HOMO (UPS)

LUMO (Eg)

ZnO C60-PEG ZnO/C60-PEG

3.33 2.68 3.30

7.65 6.92 7.42

4.32 4.24 4.12

estimated as 4.32 eV for ZnO, 4.24 eV for C60-PEG and 4.12 eV for ZnO/C60-PEG, which are summarized in Table 1. Here, the band gap for C60-PEG is obtained from cyclic voltammetry curve (Fig. S6). At the same time, Kelvin probe was also applied to further demonstrate the result. Work function images from matrix and error graph of the bare ITO, ITO/ZnO, ITO/C60-PEG and ITO/ZnO/ C60-PEG are displayed in Fig. S7. The values of the work function obtained from Kelvin probe measurement is consistent with that from UPS, indicating C60-PEG has a suitable energy level to be CBL (Fig. 4b). Due to the passivation of ZnO surface traps by C60-PEG

and the N-type property of C60-PEG, the work function of ZnO modified by C60-PEG decreased. As a consequence, a better energy alignment was built in the device for improving the charge extraction and collection, and Voc as well [46]. In order to directly characterize the electron transport of CBLs in the vertical direction, the space-charge-limited-current (SCLC) was measured by adopting electron-only devices (inset of Fig. 5). The fitted curves using the SCLC model of ZnO, C60-PEG and ZnO/C60PEG are presented in Fig. 5, and the corresponding electron mobility calculated according to the MotteGurney SCLC equation [47,48] are listed in Table 2. The electron mobility of the device with C60-PEG is a little higher than that of ZnO, and the device with ZnO/ C60-PEG owns the highest electron mobility. Thanks to the special chemical compatibility to active layer [28,49]. fullerene cage facing toward the active layer and hydrophilic PEG spontaneously facing down to ITO or ZnO. For ZnO/C60-PEG, this phenomenon should be more obvious because of the strong interaction between ZnO nanoparticles and PEG molecular chain. Therefore, the active layer

Fig. 4. (a) UPS spectra of ZnO, C60-PEG and ZnO/C60-PEG; (b) the corresponding energy level diagram of the components of the devices.

T. Hu et al. / Organic Electronics 37 (2016) 35e41

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Fig. 5. Log J vs. log V plots for MotteGurney SCLC fitting of the electron-only device. Inset shows the configuration of the electron-only device.

Table 2 Electron mobility of the device with ZnO, C60-PEG and ZnO/C60-PEG. Devices

me (cm V 2

ZnO 1

1

s

)

C60-PEG 4

8.76  10

9.23  10

ZnO/C60-PEG 4

1.21  103

could have an ideal vertical distribution of acceptor in the active layer with acceptor near to CBL, which is good for the electron transport. Furthermore, this function also make a contribution to the light absorption of the active layer (Fig. S8). The inverted device was fabricated with the structure ITO/CBL/ PTB7‫׃‬PC71BM/MoO3/Ag by incorporating ZnO, C60-PEG and ZnO/ C60-PEG. The current densityvoltage (J-V) curves under illumination are shown in Fig. 6a. As summarized in Table 3, the device with C60-PEG delivers a comparable PCE of 6.6%, in contrast to 6.7% for the device with bare ZnO. Notably, the device with ZnO/C60PEG shows the best PCE of 7.4% with the highest parameters, a short circuit current density (Jsc) of 15.37 mA cm2, an open circuit voltage (Voc) of 0.734 V, a fill factor (FF) of 65.4%. The increased Jsc and FF is attributed to a good compatibility produced by C60-PEG between inorganic ZnO and organic active layer. The enhanced Voc is led by the better energy alignment. J-V characteristics of PSCs measured without illumination are illustrated in Fig. 6b. It is reported that the high shunt resistance (RSH) means less leakage current in the devices [50]. The less leakage current of the device with ZnO/C60-PEG implies that the recombination of charge was decreased [46,51]. Besides, the IPCE spectra in Fig. 6c are in good agreement with the Jsc values. To further verify the effect of C60PEG on the performance enhancement, light absorption, and the exciton dissociation process of the solar cell, we determined the saturation current density Jsat and charge dissociation probabilities P(E,T) of the device based on ZnO, C60-PEG and ZnO/C60-PEG. Fig. 7a displays the photocurrent density (Jph) versus effective voltage (Veff) curves for the solar cells under illumination at 100 mW/cm2. Jph is calculated according to Jph ¼ JL  JD (JL and JD are the photocurrent densities under illumination and in the dark) [52]. Veff is defined as Veff ¼ V0  Va (V0 is the voltage when Jph equals zero and Va is the applied bias voltage) [53]. Jph linearly increases with the increasing Veff and saturates at high Vint. Then, the maximum photoinduced carrier generation rate per unit volume (Gmax) can be gained by using the equation Jsat ¼ q L Gmax (q is the electronic charge and L is the thickness of the active layer) [54,55].

Fig. 6. Performance of devices under simulated AM 1.5 G (100 mW$cm-2) illumination. (a) Illuminated J-V characteristics, (b) J-V curve under dark and (c) IPCE spectra of the devices ITO/cathode buffer layer/PTB7‫׃‬PC71BM/MoO3/Ag.

The estimate values of Gmax are 5.98  1027 m3 s1, 6.31  1027 m3 s1 and 6.75  1027 m3 s1 for the devices with ZnO, C60-PEG and ZnO/C60-PEG, respectively. It is inferred that absorption of light and the exciton generation are improved in the device with C60-PEG and ZnO/C60-PEG. P(E,T) is defined by normalizing Jph with Jsat (Jph/Jsat) [56] and the result plots are in Fig. 7b. Compared to that of the device with ZnO, the values increased from 77.6% to 83.4% for the device with C60-PEG and to

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Table 3 Photovoltaic parameters of the devices with ITO/cathode buffer layer/PTB7‫׃‬PC71BM/MoO3/Ag structure. Devices

Jsc [mA cm2]

Voc [V]

FF [%]

Rs [U cm2]

Rsh [U cm2]

PCE [%]

ZnO C60-PEG ZnO/C60-PEG

14.89 15.24 15.37

0.725 0.733 0.734

61.8 59.3 65.4

5.34 6.41 2.63

955.9 402.2 1327.9

6.7 6.6 7.4

Fig. 7. (a) Plots of photocurrent density as a function of the effective bias for devices prepared with ZnO, C60-PEG and ZnO/C60-PEG. (b) Exciton dissociation probability [P(E,T)] plotted with respect to Veff for the device prepared ZnO, C60-PEG and ZnO/C60-PEG.

87.2% for the device with ZnO/C60-PEG. It is demonstrated that the C60-PEG could facilitate charge dissociation. Furthermore, ZnO, C60-PEG and ZnO/C60-PEG were also tested in the P3HT:PC61BM system (Fig. S9 and Table S1). Meanwhile, the devices with ZnO/ C60-PEG shows the best stability than the devices with pristine ZnO and C60-PEG (Fig. S10). The long term stability of the device with ZnO/C60-PEG results from the passivation of defects. 4. Conclusions In summary, the amphiphilic fullerene derivative C60-PEG was successfully employed in the inverted device as cathode buffer layer and modification layer of ZnO with improved performance. C60-PEG tuned energy level alignment and improve the interfacial compatibility between active layer and ITO or ZnO. Moreover, C60PEG passivated the surface defects and traps of ZnO, and facilitate the charge selective and dissociation. With C60-PEG as cathode buffer layer, the inverted polymer solar cells present a PCE of 6.6%, which is comparable to that of PSCs with ZnO. For the device with ZnO/C60-PEG, an enhanced PCE of 7.4% with a relatively long-term stability were observed. It’s worth noting that C60-PEG could be prepared by solution process at room temperature without additional annealing, which shows the potential in large-scale printed PSCs. Acknowledgments This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51273088, 51263016 and 51473075), and National Basic Research Program of China (973 Program 2014CB260409). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2016.06.018.

Notes Competing financial interests. competing financial interest.

The

authors

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