Fluorinated Reduced Graphene Oxide as an Efficient Hole-Transport Layer for Efficient and Stable Polymer Solar Cells

In this work, we have rationally designed and successfully synthesized a reduced graphene oxide (GO) functionalized with fluorine atoms (F-rGO) as a hole-transport layer (HTL) for polymer solar cells (PSCs). The resultant F-rGO has an excellent dispersibility in dimethylformamide without any surfactants, leading to a good film-forming property of F-rGO for structuring a stable interface. The recovery of conjugated C=C bonds in GO oxide after reduction increases the conductivity of F-rGO, which enhances the short-circuit current density of photovoltaic devices from 15.65 to 16.89 mA/cm2. A higher work function (WF) (5.1 eV) of F-rGO than that of GO (4.9 eV) is attributed to the fluorine group with a high electronegativity. Naturally, the better-matched WF with the highest occupied molecular orbital level of the PTB7-Th (5.22 eV) donor induces an improved energy alignment in devices, resulting in a superior open-circuit voltage of the device (0.776 vs 0.786 V). Consequently, the device with F-rGO as the HTL achieves a higher power conversion efficiency (8.6%) with long-term stability than that of the devices with GO HTLs and even higher than that of the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) control device. These results clearly verify that the F-rGO is a promising hole-transport material and an ideal replacement for conventional PEDOT/PSS, further promoting the realization of low-cost, solution-processed, high-performance, and high-stability PSCs.


■ INTRODUCTION
Polymer solar cells (PSCs) based on bulk-heterojunction structures hold great potential owing to their advantageous properties such as low-cost fabrication, light weight, flexibility, and use of nontoxic elements. 1 There has been significant progress in the power conversion efficiency (PCE) of PSCs because of the tremendous efforts in the development of new photoactive materials and fabrication techniques over the past few years. 2−4 To date, the PCE of single cells has reached over 11%. 5−9 Because the interfaces between the electrodes and the photoactive layer play an important role in improving the overall photovoltaic performance of PSCs, the charge-transport layers between the photoactive layer and electrodes are notably indispensable. The functions of charge-transport layers involved reducing energy barriers for charge carrier transport, forming a selective contact for the corresponding charge carrier at two electrodes, and optimizing the morphology and concentration distribution of the photoactive layer for enhanced device performance. 10,11 So far, poly(3,4-ethylenedioxythiophene)/poly-(styrenesulfonate) (PEDOT:PSS), a commercial conducting polymer, has been most commonly used as a hole-transport layer (HTL) to increase hole contact and to improve anode collection in PSCs. However, the properties of high acidity and hygroscopicity of PEDOT:PSS limit the further application of devices. 12,13 Inorganic materials including NiO, V 2 O 5 , and MoO 3 , as other currently predominant HTLs, are introduced because they could offer a high work function (WF) with good transparency and exhibit favorable durability for long-term device operation. However, it cannot be ignored that the requirement of a thermal treatment and the costly vacuum process have vastly limited their broad applications in PSCs. 14,15 As a promising alternative to PEDOT:PSS, graphene oxide (GO) possesses numerous advantages, such as tunable electronic structures, low manufacturing cost, and compatibility with roll-to-roll processing. GO is generally prepared from the oxidation of graphite by a strong oxidant concentrated sulfuric acid and potassium permanganate, which was first introduced by Li et al. 16 Herein, it is observed that the graphene sheet in graphite oxide is decorated with oxygen functionalities at the edges, including epoxy, hydroxyl, carboxyl, and carbonyl groups. Consequently, the optoelectronic properties of GO, like the van der Waals interactions between the graphene sheets in graphite oxide, could be tuned. In addition, GO (rGO) with oxygen-containing functional groups is endowed with excellent solubility in water or polar organic solvents. Thus, it is a particularly promising solution-processable interfacial material for application at a large scale. 17 −19 In spite of the aforementioned superiority of GO, it is undisputable that GO

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Article has insulating properties against the π−π structure of graphite being seriously damaged. Yet, the performance of PSCs with GO interlayers is sensitive to the thickness of GO. Thus, with the increasing thickness of GO over one monolayer, the internal resistance of the photovoltaic device could get increased, further the fill factor (FF) could get lowered, and eventually the performance of PSCs could be affected. 20 Therefore, some efforts have been made to improve the conductivity of GO. 17−19 Thus, the devices' FF and performance have been significantly improved by adding a small amount of single-walled carbon nanotubes in the GO layer because of the improvement of conductivity. 20 Moreover, the conductivity could be elevated by partially reducing GO. However, there exist some problems; one point is that the WF of partially reduced GO generally decreases as a result of the removal of electronegative oxygen groups. Consequently, the reduced GO cannot match the highest occupied molecular orbital (HOMO) of the donor material well for the lower WF, which induces the interface barrier, especially for the narrowbandgap polymer donor. 21 Another one is that the reduced GO with an insufficient number of oxygen groups on the basal plane tends to aggregate in solution, which is unbeneficial for fabricating thin uniform films by spin-coating. 22,23 Herein, we developed a fluorine-functionalized, reduced GO (F-rGO) using 2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenylhydrazine as a HTL for PSCs. In detail, the developed reductant had seven fluorine atoms, which is less toxic in contrast to the intensively toxic and explosive hydrazine. 24,25 The resultant F-rGO presents an excellent dispersibility (about 5 mg/mL in dimethylformamide (DMF) without any surfactants), helpful in forming a film to develop a stable interface. More importantly, F-rGO has both a higher conductivity and a satisfactory WF (5.1 eV) compared to those of GO. Finally, when applied to PSCs, the desirable PCE (8.6%) could be achieved with F-rGO as the HTL. The good device performance contributed to the enhancement of the short-circuit current density (16.89 mA/cm 2 ), open-circuit voltage (0.79 V), and FF (64.8%), meaning that F-rGO is appropriate as a HTL for PSCs.

■ RESULTS AND DISCUSSION
The hole-transport material from reduced GO functionalized with fluorine atoms (F-rGO) was prepared by reduction and simultaneous functionalization of GO using a reductant, 2,3,5,6tetrafluoro-4-(trifluoromethyl)phenyl-hydrazine, which possesses seven fluorine atoms, as illustrated in Figure 1a. To confirm that GO has been succeeded in reduction, X-ray photoelectron spectroscopy (XPS) measurements were conducted and the change in the composition and bonding type of GO and F-rGO was explored. The fitting results of the C1s curves of GO and F-rGO are shown in Figure 1b,c. Figure S1 presents the XPS C1s curve. It could be observed that the C1s spectrum of the GO film consists of several components, assigned to C−C (284.7 eV), C−OH (286.5 eV), C−O−C (286.9 eV), and COOH (288.8 eV). However, for F-rGO, the peak intensity of the four components evidently differs from that of the GO, especially the enhanced intensity of C−C. This is due to the restoration of the conjugated CC bonds of the GO sheets after reduction, meaning the successful reduction of GO. Moreover, there occur another two compositions corresponding to C−N (285.8 eV) and C−F 3 (293.1 eV), revealing that the reductant is successfully attached to the basal plane and edges of the GO sheets. The clear chemical structure change is confirmed by Raman spectroscopy measurements. Figure 1d shows the Raman spectra of GO and F-rGO. The peak intensity ratio of the D and G bands is believed to be closely related to the degree of disorder and is inversely proportional to the size of the sp 2 C domains. The observed result shows that the ratio of I D /I G of the F-rGO increases to 1.12 (compared to 0.95 for GO) after reduction, demonstrating that numerous small-size sp 2 C domains were formed after reduction, 26,27 further verifying that the GO was reduced and functionalized with fluorine atoms successfully, consistent with the aforementioned result of XPS. Figure 1e illustrates the UV− visible (UV−vis) spectra of GO, which show a strong absorbance peak at 228 nm. However, the absorbance peak of GO is red-shifted to 265 nm and the absorbance in the whole spectral region is enhanced, indicating that the conjugated CC bonds of GO are restored by reduction. 28,29 More importantly, the resultant F-rGO shows an excellent dispersibility ( Figure S2) of 5 mg/mL in DMF without any surfactants. This is because the CF 3 groups from the bulky unit can effectively reduce the van der Waals interaction and thereby prevent the graphene sheets from aggregating in solution. 30 The enhanced dispersibility contributes to the solution processability of F-rGO. To evaluate the conductivity of the GO and F-rGO layers, we fabricated the diodes with a structure of ITO/GO(F-rGO)/Al, as shown in Figure 2a. Note that the different slopes of the current density−voltage (J−V) curves imply that GO(F-rGO) thin films have different electrical conductivities. Normally, the greater the slope, the better the conductivity of thin film. It could be seen from Figure 2a that F-rGO shows a distinct increase in the conductivity compared to that of the pristine GO, which is attributed to the partial restoration of conjugated CC bonds. Atomic force microscopy (AFM) measurements were also conducted to analyze the morphology of GO and F-rGO (presented in Figure 2b). It could be found that the rootmean-square roughness values of F-rGO increase slightly (from 1.49 to 1.98 nm) after the reduction, probably ascribed to the bulky phenyl functional groups being grafted onto the graphene sheet with disorder. 19 However, from AFM topography images of photoactive layers on different substrates ( Figure S3), it could be observed that there are no obvious differences in films' morphologies when the photoactive layers are deposited on different substrates. Furthermore, it is seen from the transmittance of ITO with different HTLs (presented in Figure S4) that the F-rGO has transmittance (>80%) comparable to that of GO and PEDOT after reduction. More importantly, the contact angle measurements depict that the F-rGO/ITO shows a larger contact angle of 71.0°than that of GO/ITO (45.5°) and PEDOT/ITO (13.5°) (given in Figure S5), owing to the decrease of the oxygen-containing groups during reduction. The result indicates that the enhanced hydrophobic surface of the F-rGO-modified ITO is more compatible with the upper active layer in comparison to that of GO-modified ITO. The above results show that the F-rGO is suitable for anode interlayer material.
The ultraviolet photoelectron spectroscopy (UPS) measurements were carried out to explore the WFs of GO and F-rGO. The following equation is used to calculate the WFs of GO and F-rGO deposited onto the ITO substrate where hν is the excitation energy of He(I), equals to 21.2 eV, E F is the Fermi level, and E cutoff is the high-binding-energy cutoff. 30 It is obvious that, compared to the WFs of bare ITO (4.5 eV), PEDOT (4.9 eV), and GO (4.9 eV), the WF of F-rGO is significantly higher, with the value of 5.1 eV, as shown in Figure 3a. The upshifted WF of F-rGO can be majorly due to the fluorine atoms with a high electronegativity, inducing p-type self-doping of the graphene. 31,32 Eventually, the higher WF of F-rGO could match the HOMO of the donor polymer (P7B7-Th, 5.22 eV) better (Figure 3b), leading to a better energy level alignment. Simultaneously, the built-in potential could improve and the interface resistance reduces, thereby enhancing the performance of PSCs.
To assess the viability of F-rGO as a HTL in PSCs, we fabricated the conventional device structure (ITO/HTL/ PTB7-Th:PC 71 BM/PFN/Al, Figure 4a) based on PTB7-Th:PC 71 BM. Reference devices were also prepared without any modification, with PEDOT:PSS and GO as HTLs. Figure  4a shows the current density−voltage (J−V) characteristics of the devices, and the related device parameters are summarized in Table 1. The device without the HTL shows a poor V OC of 0.650 V, FF of 52.3%, J SC of 15.02 mA/cm 2 , and PCE of 3.3%, whereas the device with GO as the HTL demonstrates a PCE (7.8%) with a V OC of 0.776 V, FF of 63.9%, and J SC of 15.65 mA/cm 2 . It is noteworthy that the device with F-rGO as the HTL displays an obviously improved PCE of 8.6%, with enhanced device parameters including V OC of 0.786 V, J SC of 16.89 mA/cm 2 , and FF of 64.8%. The unexceptionable J SC could be attributed to the higher conductivity of F-rGO after reduction, leading to the better carrier transport property, whereas the slightly increased V OC is ascribed to the more matched WF (5.1 eV), promoting efficient hole extraction with sufficient built-in potential. In addition, the smaller R s (Table 1) of the device with the F-rGO interlayer just confirms the favorable hole-transport property. Figure 4b Figure 4b, which are in good accordance with the J SC values (within 3% mismatch) from the J−V characteristics under light conditions. Furthermore, the dark current densities of the PSCs with PEDOT:PSS, GO, and F-rGO as HTLs (as shown in Figure S6) show that F-rGO is

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Article more efficient in blocking electrons and collecting holes than PEDOT and GO. We also measured the hole mobility of different HTLs ( Figure S7) with space-charge-limited current. 15 The hole mobilities of F-rGO, GO, and PEDOT are exhibited in Table S1. Obviously, F-rGO has the highest hole mobility among these three HTLs, contributing to the J SC enhancement.
To expand the potential of F-rGO as an outstanding HTL, the device stability of the PSCs with F-rGO was investigated with a function of time of exposure under an ambient environment without encapsulation. As a comparison, the device stability with GO/ITO, PEDOT:PSS/ITO, and bare ITO as anodes was also evaluated. As shown in Figure 4c, a rapid degradation rate could be observed for PCEs with bare ITO. After 4600 min, the device with bare ITO continues to show a very poor PCE due to the poor surface property of ITO. Simultaneously, the device with PEDOT:PSS retains just about 23% of its primal PCE, which originates from the hygroscopic PEDOT:PSS layer by absorbing water from the atmosphere, thus leading to the degradation of hole extraction. In contrast, the devices fabricated with GO and F-rGO HTLs show a greatly enhanced device stability with the PCEs retaining 67 and 63% of their initial PCEs after 4600 min, respectively, which is because of the GO layer, blocking water and oxygen. The result suggests that the conventional PSCs with F-rGO as the HTL could act as an encapsulation at both bottom to obtain a device with high performance and long-term air stability.

■ CONCLUSIONS
In conclusion, we synthesized a fluorine-functionalized, reduced GO with 2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl-hydrazine as a reductant. The resultant F-rGO has an excellent dispersibility, making the subsequent solution processing feasible. Compared with GO, F-rGO has a higher conductivity and higher WF, resulting from the recovery of conjugated C C bonds in GO after reduction and the grafting of fluorine atoms with a high electronegativity, respectively. Consequently, the F-rGO device acquires a higher efficiency than that of GO device, with the increased short-circuit current density and open-circuit voltage. It is worth noting that the F-rGO device even obtains an ultimately better PCE (8.6%) in comparison to that of the device with GO. In addition, the F-rGO-based photovoltaic device shows a stability superior to that of the GO-based and PEDOT-based devices. These results clearly demonstrate that the F-rGO is a promising hole-transport material and an ideal replacement for conventional PEDOT, further promoting the realization of low-cost, solutionprocessed, high-performance, and high-stability PSCs.

■ EXPERIMENTAL PROCEDURES
Preparation of GO. The GO was prepared by a modified Hummer's method. Briefly, graphite (2.0 g) was added to

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Article concentrated sulfuric acid (50 mL, 0°C) under stirring, then sodium nitrate (1.0 g) was added, and the reaction system was stirred for about 0.5 h at 0°C. Under violent agitation, potassium permanganate (8.0 g) was added slowly to keep the temperature of the mixture lower than 20°C. Then, the suspension was transferred into a 40°C water bath and vigorously stirred for about 5.0 h. Subsequently, water (100 mL) was added and the solution was stirred for 20 min at 90°C . An additional 360 mL of water was added. Then, H 2 O 2 (30%, 0.5 mL) was added slowly, turning the color of the mixture from dark brown to yellow. The suspension was filtered and washed with 1:10 hydrochloric acid aqueous solution (200 mL) to remove metal ions. The precipitate was dispersed into deionized water to form a uniformly dispersed suspension. The suspension was centrifuged (at a low speed) to remove the incompletely oxidized product. The remaining supernatant was centrifuged (at a high speed) to obtain the precipitate, which was freeze-dried for next use.
Preparation of F-rGO. First, 0.8 g of the prepared GO was dispersed in 200 mL of deionized water using an ultrasonic batch cleaner. Then, 5.1 g of 2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl-hydrazine was added to the suspension and stirred at 60°C for 6 h. Finally, the suspension was filtered and washed with methyl alcohol. The filtered product was dried in a vacuum oven at 50°C for 24 h.
Characterization of Films. XPS measurements for GO and F-rGO were performed using a Thermo-VG Scientific ESCALAB 250 system. The morphology of GO, F-rGO, and active layers was investigated using AFM (Agilent 5500). The WFs of the samples were obtained using UPS (Kratos Analytical Ltd.). The Raman spectra of GO and F-rGO were acquired from LabRam-1B. The transmittance spectra and UV−vis spectrum were analyzed by PerkinElmer Lambda 750. The current−voltage characteristics of the devices under illumination (the light intensity was 100 mW/cm 2 ) and in the dark were tested by a Keithley 2400 Source Meter.
Device Fabrication.  [3,4-b]thiophenediyl]] (PTB7-Th) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM), were purchased from 1-material Inc. and Solarmer Energy, Inc., respectively. The electron transport material, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)alt-2,7-(9,9-dioctylfluorene)] (PFN), was provided by Solarmer Energy, Inc. The device structure is ITO/HTL/PTB7-Th:PC 71 BM/PFN/Al. The ITO glasses were cleaned by an ultrasonic batch cleaner in acetone, soap water, deionized water, and isopropanol, successively. Then, the ITO glasses were dried with nitrogen and treated with O 3 plasma for 10 min. The interface layers of the devices were deposited on the ITO glasses using the following spin-cast conditions. The PEDOT layer was spin-cast from the solution at 5000 rpm for 60 s, followed by baking for 20 min at 120°C. The GO layer was spin-cast from its solution in water (1 mg/mL) using a gradient spin, from 2000 to 5000 rpm for 60 s, followed by baking for 10 min at 150°C. The F-rGO layer was spin-cast from its solution in DMF (2.0 mg/mL) using a gradient spin, from 2000 to 5000 rpm for 60 s, followed by baking for 10 min at 150°C. As photoactive layers, PTB7-Th and PC 71 BM were spin-cast from a mixed solvent of chlorobenzene/1,8-diiodoctane (97:3 v/v) solution (with a total concentration of 25 mg/mL, stirred for at least 12 h at 70°C) at 1000 rpm for 2 min. The resulting photoactive layer was dried in a glove box with nitrogen before the deposition of the PFN layer. The PFN layer was spin-cast from its solution in methanol (0.4 mg/mL) in the presence of a small amount of acetic acid at 5000 rpm for 60 s. Then, the devices were transferred into a vacuum chamber for thermal deposition of Al (100 nm) at a pressure of 10 −7 Torr. The area of each device was 4 mm 2 , as defined by the overlap of ITO and the evaporated Al.

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.