Self-Organized Hole Transport Layers Based on Polythiophene

Article pubs.acs.org/cm

Self-Organized Hole Transport Layers Based on Polythiophene Diblock Copolymers for Inverted Organic Solar Cells with High Efficiency Kai Yao,† Lie Chen,†,‡ Xun Chen,† and Yiwang Chen*,†,‡ †

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

‡

S Supporting Information *

ABSTRACT: Novel fluoroalkyl side-chain diblock copolymers, poly(3-hexylthiophene)-block-poly[3-(4(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxy)phenyl)decyloxy)thiophene] (P3HT-b-P3FAT), were successfully synthesized by Grignard metathesis (GRIM) polymerization. Driven by the low surface energy of fluoroalkyl side chains, the fluorinated polymers can spontaneously segregate on the surface of poly-(3-hexylthiophene) (P3HT) during spincoating processes. As the P3HT block increases in the copolymer, higher concentrations of fluoropolymers are required to form the self-assembled monolayer on the surface. The fluorinated part forms an interfacial dipole that shifts the work function of the anode metal, while the P3HT block can interact with the P3HT donor for hole transport. With this selfassembly hole transport layer to align the energy levels, P3HT:PCBM photovoltaic devices are easily fabricated to achieve improved performance. Overall, devices prepared with 1.5 mg mL−1 copolymer PFT-3HT with a 3:1 ratio of P3HT to P3FAT block in the active layer solution displayed PCE values of up to 4.6% (50% PCE increase over a PEDOT:PSS control device) and showed a significant long-term stability in excess of 300 h in air. KEYWORDS: block copolymers, self-assembly, fluoropolymers, organic photovoltaics

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INTRODUCTION

Since interfacial layers can be used to tune the work functions of both bottom and top electrodes, inclusion of appropriate interfacial layers can alter charge extraction efficiency and selectivity by matching the Fermi level to either the quasi-Fermi levels of holes (Ef,h) of the donor or the quasiFermi levels of electrons (Ef,e) of the acceptor in the BHJ for hole or electron collection, respectively.8 Therefore, OPVs can be fabricated based on two types of device structures including the conventional structure and the inverted structure in which the electrode polarity is reversed. Operating a polymer BHJ OPV in an ‘‘inverted mode,’’ where electrons are extracted to the transparent electrode and holes transport to the reflective electrode, especially requires tailoring of the electrode work functions using interfacial modifiers. This is often advantageous with respect to performance stability, design flexibility, and compatibility with stacked architectures.9 In polymer solar cells, poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) is extensively applied as a hole transporting layer (HTL) to improve hole collection at the anode. Yet, a major drawback of PEDOT:PSS is associated with stability issues. It is known that the hygroscopicity and acidity of PEDOT:PSS can

Solution-processed organic solar cells offer the potential to provide solar energy at a lower cost than conventional photovoltaics due to scalable printing manufacturing and low materials costs.1 The most studied active-layer structure is that of the bulk heterojunction (BHJ), which ideally consists of an interpenetrating network of electron-donor and electronacceptor materials, usually conjugated polymer donor and fullerene acceptor, and can be fabricated by easily solution casting processing.2 Polymer:fullerene bulk heterojunction (BHJ) solar cells have recently achieved power conversion efficiencies over 9% in single junction devices.3 Several methods for improving the power conversion efficiency (PCE) of BHJ solar cells are known to be effective, including the design and synthesis of new low band gap materials,4 stable device structures,5 and efficient device processing to improve the nanoscale morphology.6 In addition, interface engineering also plays a critical role in determining the performance of OPV (organic photovoltaic) devices.7 The development of new interfacial materials with desired charge selectivity and compatibility for all-solution-processed multilayer devices, along with proper integration of the interfacial layer with new active materials, is important to further improve the efficiency and stability of OPVs. © 2013 American Chemical Society

Received: November 27, 2012 Revised: February 22, 2013 Published: February 22, 2013 897

dx.doi.org/10.1021/cm400297p | Chem. Mater. 2013, 25, 897−904

Chemistry of Materials

Article

Scheme 1. Synthetic Routes to the Block Copolymers P3HT-b-P3FAT (PFT-3HT and PFT-6HT) and Homopolymer PFAT

cause degradation of the active layer and the indium tin oxide (ITO) electrode, resulting in a decreased lifetime of the solar cell.10 Moreover, on the anode side, many inverted BHJ solar cells have been reported with an interlayer of wet processed PEDOT:PSS.11 Over the past few years, much effort has been expended in producing a material system that can act as an efficient hole extraction layer for OPVs as a substitute for PEDOT:PSS.12 The development of new solution-processed hole transport interfacial layers that can replace PEDOT:PSS remains to be a challenging area for OPVs since several criteria must be taken into consideration for an efficient anodic buffer layer. The interfacial layer should first and foremost ensure proper energy level alignment between the ionization energy of the donor and the work function of the electrode. Second, they should possess reasonable hole mobility to minimize resistance across the interfacial layer. Finally, they need to have good orthogonal solvent-processability and film forming properties to avoid eroding into the bottom BHJ layer. Hashimoto et al. showed that fullerene derivatives with fluoroalkyl chains (FCn) spontaneously form a monolayer on the surface of a nonfluorinated fullerene derivative (PCBM) film during the coating process and named it surface segregated monolayer (SSM).13 It is well-known that materials having a low surface energy such as fluorinated compounds prefer to migrate to the air/liquid interface during coating. This is called surface segregation, which is driven by the total energy minimization of the system. Recently, they also reported the application of the modified surface of the donor (P3HT) film with a surface-segregated monolayer (SSM) of fluorinated polythiophene.14 Besides, other types of solution-processable polythiophenes were used as hole transport materials.15

However, the performance is still limited, with potential improvement on the hole transport and collection, particularly at the anode interface. In our own pursuit of “PEDOT:PSSfree” anodes, block polythiophenes with a fluorinated side chain have been applied because of a number of unique features. The fluoroalkyl chain block can spontaneously form a monolayer on the surface of the active layer film during the coating process, which acts as a buffer layer with dipole moments at the active layer interface. Moreover, the P3HT block can interact with the donor materials in the active layer to enhance the compatibility for hole collection and transport.

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RESULTS AND DISCUSSION Synthetic routes for the target copolymers are shown in Scheme 1. The P3HT block was first prepared by Grignard metathesis (GRIM) polymerization (using Ni(dppp)Cl2) to give P3HT with a living chain-end, which was chain-extended into the diblock structure using the bromo-alkyl-substituted thiophene monomer with different feed ratios.16 In the medium copolymers poly(3-hexylthiophene)-b-poly-(3-(10bromodecyl)thiophene) (P3HT-b-P3BrDT), the bromide groups in P3HT-b-P3BrDT were substituted by the Williamson ether reaction with fluoroalkyl chains (FA), to yield the resulting copolymers P3HT-b-P3FAT. All copolymers were purified by sequential Soxhlet extraction using methanol, hexane, and chloroform in succession. The complete fluorinated substitution of the copolymers was evidenced by the shift of the 1H NMR resonance of the methylene group (δ =3.40 ppm) in α to the terminal bromine atom (Figure S1). Two diblock copolymers were studied in detail, having different targeted P3FAT:P3HT molar ratios of 1:3 and 1:6 denoted PFT-3HT and PFT-6HT, respectively. The relative block ratios 898



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Figure 1. (A) F 1s and C1s regions of XPS profiles of the P3HT:PFT-3HT film prepared with different PFT-3HT concentrations. (B) F/C atomic ratio on the surfaces of P3HT:PFT-3HT thin films measured by XPS and plotted as a function of PFT-3HT concentration of the spin-coating solutions and F/C ratios calculated from the polymer compositions in the solutions.

(molar monomer equivalent) were calculated by 1H NMR spectroscopy, comparing the signal intensity of the methylene group (adjacent to the thiophene ring) at 2.80 ppm to the methylene group (adjacent to the oxygen atom) at 4.0 ppm. The actual compositions were very close to the feed ratios (25:75 and 15:85) of the monomers. Moreover, the homopolythiophene (PFAT) with the same fluoroalkyl side chains was prepared as a reference. The homopolymer PFAT can be regarded as a block copolymer without P3HT block; therefore, the P3FAT:P3HT ratios of the PFAT, PFT-3HT, and PFT-6HT are 1:0, 1:3, and 1:6, respectively. The absorption spectra of the polymer films spin-coated from the o-dichlorobenzene (ODCB) solution are shown in Figure S2. The normalized solid-state absorption spectra of these block copolymers are thereby essentially similar to that seen in P3HT homopolymer. Moreover, both results from the X-ray scattering (XRD) and small-angle X-ray scattering (SAXS) show that increasing the block ratio of P3FAT can disturb the conjugated structures of ordered P3HT crystals (Figure S3, see the Supporting Information). Nevertheless, these data also demonstrate that, by controlling the content of fluoralkyl part and the rod−rod interactions in poly(3-alkythiophene)

derivatives, well-ordered lamellar structures of polythiophene can be maintained in the PFT-3HT and PFT-6HT.17 To investigate the surface segregation of fluorinated polythiophenes during spin coating, X-ray photoelectron spectroscopy (XPS) was carried out. The films were prepared on ITO-coated glass substrates by spin coating the solutions with various concentrations of fluorinated polymers and a fixed concentration of P3HT at 20 mg mL−1. We first carried out the pure P3HT film (20 mg mL−1) and P3HT:PFT-3HT with different PFT-3HT concentrations. The peak of F 1s can be clearly observed on the film surface with fluorinated compounds (Figure 1A), and the F/C atomic ratios of the surface are calculated from the peak intensities (F 1s/C 1s) and plotted in Figure 1B as a function of the fluoroalkyl polythiophenes concentration in the solutions.14b The F/C atomic ratios calculated from the polymer compositions of the mixed solutions, which represent the expectations for homogeneously mixed films, are also shown in Figure 1B for comparison. Interestingly, the surface F/C ratios of the films were much higher than the calculated ratio of the solution for all the concentrations, which indicates the segregation of PFT3HT to the film surface during the spin-coating process. The F/ 899



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Figure 2. Atomic force microscopy (AFM) height images (5 μm × 5 μm) of annealed P3HT:PCBM:PFT-3HT films with PFT-3HT concentrations of 1.0, 1.2, 1.5, 1.8, and 2.0 mg mL−1. The pure active layer film P3HT:PCBM (20:20 mg mL−1) is given as a reference.

ration concentrations, excess PFT-3HT might remain in the bulk of the film, as observed in an enlarged view of the F 1s region in the XPS depth profile. For films with PFT-3HT concentrations of 2.0 mg mL−1, a low concentration of PFT3HT remained in the film after etch for 60 s in the magnified picture in the Figure S4B. Since the morphology of the bicontinuous phases of donor and acceptor components in the active layer is desirable for BHJ PSCs, and the two blocks: fluoroalkyl chains and alkyl chains in P3HT-P3FAT block copolymers can also form two phases, we use atomic force microscopy in tapping mode to measure surface morphology of P3HT:PCBM (20:20 mg mL−1) blending with addition of block copolymer PFT-3HT, as shown in Figure 2. The surface topographical image of pure P3HT:PCBM shows a fine phase separation and bicontinuous network morphology (Figure 2F). When the PFT-3HT is added into P3HT:PCBM, the surfaces of the films are flat and uniform for PFT-3HT concentrations of 1.0, 1.2, and 1.5 mg mL−1. At PFT-3HT concentrations below 1.5 mg mL−1, the maximum height differences of the films are less than 2 nm, and the arithmetic roughness (Ra) is less than 0.36 nm. When the concentration is 1.8 mg mL−1, the maximum height differences increase to 4.22 nm. Further increasing of the concentration, large aggregations gradually appear on the surface, possibly consisting of the aggregation of the fluoroalkyl polymers. The result supports the formation of densely packed fluoroalkyl on the surface, similarly to the XPS analysis. To determine the energetic of the dipole moment formation above P3HT:PCBM layers, we performed ultraviolet photoe m i s s i o n s p e c t ro s c o p y ( U P S ) m e a s u r e m e n t s o n P3HT:PCBM:PFT-3HT thin films on ITO-coated glass substrates.20 The UPS profiles of a pristine P3HT:PCBM film and films with various concentrations of PFT-3HT layers on the surface are shown in Figure 3A. Figure 3A shows the photoemission onset, while the inset figure presents the magnified spectra of the low binding energy region. There is

C atomic ratio increases almost linearly with the concentration of block copolymers in the lower concentration region. At a PFT-3HT concentration of around 1.5 mg mL−1, the F/C atomic ratio increased up to approximately 0.21 and began to show signs of saturation. This saturation behavior was also observed in previous work of Hashimoto et al. on the fluoroalkyl-based self-assembled monolayers.18 According to the model (in the Supporting Information)19,14b with a densely packed fluorinated monolayer, the calculated F/C atomic ratio value (0.22) on the surface at the saturated concentration is close to the observed one (0.21). This result suggests that at high concentrations above the saturated point, the surface is almost completely covered by fluoroalkyl chains of PFT-3HT. The saturated concentration for PFAT and PFT-6HT films are around 0.8 and 2.5 mg mL−1, respectively. It indicates that more fluoroalkyl polymers are needed to form the selfassembled monolayers on the P3HT surface as the P3HT block increased in the copolymer. However, the F/C atomic ratio of PFAT, PFT-3HT, and PFT-6HT saturate at approximately 34%, 21%, and 15%, respectively. The variation is attributed to the fact that the maximum surface density of the fluoroalkyl chain can be depressed with the increasing of the large P3HT block in the polymer chains. XPS depth profiles (Figure S4) are used to further confirm the surface segregation of the block copolymer PFT-3HT. The surface of the film is etched with an argon ion beam. After 60 s of etching from the surface of the film with 1.5 mg mL−1 PFT-3HT, the F 1s peak disappears completely. There are no F 1s peaks observed inside the film, indicating that all PFT-3HT segregates at the surface (Figure S4A). The thickness of the film determined from AFM analysis was 100 nm with an etching duration of approximately 1500 s for the entire film. Therefore, the thickness of the layer containing fluorine can be estimated as less than 4 nm. This indicates that PFT-3HT segregated on the surface is a very thin layer, nearing monolayer thickness. However, for the P3HT:PFT-3HT films blended with PFT-3HT above satu900



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Table 1. Summary of XPS and UPS Results for the Films with or without Self-Assembly Layer selfassembly layer no PFAT PFT3HT PFT6HT

saturated point (mg mL−1)a

F/C atom ratio on surfacea,b

active layer ionization potential (eV)b,c

Ag work function (eV)c,d

0 0.8 1.5

0 0.34 0.21

4.6 5.9 5.5

4.2 5.1 4.9

2.5

0.15

5.4

4.7

a

Measured by XPS. bAll the blending ratios are above the saturated concentration. cMeasured by UPS. dThin Ag films deposited on various fluorinated polymer films (see Figure 4).

film was higher than that of the P3HT:PCBM:PFT-3HT film. The shift tendency is also proved by the copolymer PFT-6HT with lower IE values. Since the direct interface between Ag and the fluorinated polymer is not easily accessible with solution based materials, the molecular level alignment was measured by UPS on the surface of a 5 nm thin Ag film deposited onto the polymer surface (the details are given in the Supporting Information) and not from the very interface. Yet, the change of the various metal/organic interfaces can impact the values of work functions observed in the measurement.22 Under these conditions, the work function of Ag was found to be 4.2 eV and shifted by 0.91, 0.74, and 0.54 eV for the Ag/PFAT, Ag/ PFT-3HT, and Ag/PFT-6HT samples, respectively. Although we cannot fully clarify the mechanisms of this rise in the effective work function of Ag, it is likely that this large shift in work function arises from an interfacial dipole effect, provided by the fluorinated alkyl moieties. The dipole moment of these moieties will contribute to the work function variation and further allow holes to be collected by the Ag anode more easily. In addition, work function shifts observed in metal electrodes (Ag, Au) modified with a fluoroalkyl thiol SAMs have been reported.23 Besides, the work function differences among the three polymers indicate that the increasing of fluoroalkyl side chains contributes to the enhanced interfacial dipole and raised work function, and the small variation dipole moment of the interfacial layer may result in different device performance. Moreover, the device based on P3HT:PCBM active layer need preannealing to achieve the best performance. To clarify this point, the thermal effect on the interfacial film has been examined, and it is found that annealing the fluoropolymer films before Ag deposition leads to no obvious change in the energy levels. Photovoltaic devices containing fluorinated polymers blends in the inverted configuration ITO/ZnO/ P3HT:PC61BM:fluorinated polythiophene/Ag (Supporting Information) are fabricated.24 The fluoroalkyl polythiophene layer as the buffer layer used in organic solar cells spontaneously formed by spin-coating the P3HT:PC61BM: fluoroalkyl polymer ternary blends. Additionally, the overall ratio of polymer to fullerenes is maintained at 1:1, as is the thermal annealing temperature (140 °C). Details of the methods for device fabrication and characterization are provided in the Supporting Information. Table 2 lists the average values of VOC, JSC, FF, and η as the varied concentrations of fluorinated polymers and device structures. In a comparative study, two reference devices, including a standard ITO/ZnO/P3HT:PC61BM/PEDOT:PSS/Ag solar

Figure 3. (A) High binding energy cutoff of the active layer (P3HT:PCBM) as-cast films UPS profiles with various concentrations of PFT-3HT. The inset show the blow-up of the low binding energy region with marks at the edge of HOMO levels. (B) Ionization energy of the P3HT:PCBM:PFT-3HT films plotted as a function of the concentration of PFT-3HT in the solution.

a large shift of the high binding energy cutoff of the UPS spectra, but the valence band edge (HOMO level) position appears nearly unaffected by the dipole formation. When the concentrations of the PFT-3HT increase to 1.5 mg mL−1, the secondary edge shifts to 16.5 eV and saturated above that. The corresponding ionization energies (IE) are determined from the width of the UPS profiles (from the edges of HOMO level to the high binding energy cutoff)20a,21 and are plotted against the concentration of PFT-3HT (Figure 3B). The surfacesegregated PFT-3HT layers increased the IE proportional to the concentration of PFT-3HT up to about 1.5 mg mL−1, close to results of XPS analysis. The IE values calculated from the spectral width are summarized in Table 1. From these results, we can clearly see the shift of IE when the PFAT or P3HT-bP3FAT copolymers are mixed in the P3HT:PCBM solutions. The maximum IE value of the P3HT:PCBM:PFAT film reaches approximately 5.9 eV. This shift compared with the case of the PFT-3HT film can be attributed to the higher density of the dipole moments, because the fluoroalkyl side chains of homopolymer PFAT can be packed more densely than those of block copolymers PFT-3HT and PFT-6HT, as deduced from the molecular structures. This is supported by the fact that the F/C atom ratio on the surface of the P3HT:PCBM:PFAT 901



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Figure 4. The measured work functions of thin Ag film coated on the surface of different polymer layers with glass as substrate, based on UPS results.

Table 2. Device Performance Parameters of Inverted P3HT:PCBM Solar Cells Incorporating Fluoroalkyl Polythiophene (PFT-3HT, PFT-6HT, and PFAT) with Different Concentrations devicea,b ITO/ZnO/P3HT:PCBM/ PEDOT:PSS/Ag ITO/ZnO/P3HT:PCBM/Ag ITO/ZnO/P3HT:PCBM:PFT3HT/Ag (1.0)d ITO/ZnO/P3HT:PCBM:PFT3HT/Ag (1.5) ITO/ZnO/P3HT:PCBM:PFT3HT/Ag (2.0) ITO/ZnO/P3HT:PCBM:PFT3HT/PEDOT:PSS/Ag (1.5) ITO/ZnO/P3HT:PCBM:PFT6HT/Ag (1.5) ITO/ZnO/P3HT:PCBM:PFT6HT/Ag (2.0) ITO/ZnO/P3HT:PCBM:PFT6HT/Ag (2.5) ITO/ZnO/ P3HT:PCBM:PFAT/Ag (0.8) ITO/ZnO/ P3HT:PCBM:PFAT/Ag (1.0)

JSC (mA cm‑2)

VOC (V)

FF (%)

η (%)

8.8 (9.0)c

0.59

55.7

2.9 ± 0.3

6.9 9.9

0.57 0.59

42.3 56.2

1.7 ± 0.3 3.3 ± 0.2

11.3 (11.1)

0.60

64.9

10.6

0.60

61.0

4.4 ± 0.2 (4.6)e 3.9 ± 0.2

10.2

0.60

61.3

3.7 ± 0.2

9.0

0.59

57.6

3.1 ± 0.2

9.6

0.59

60.2

3.4 ± 0.2

10.4

0.59

66.0

9.3

0.56

56.0

4.1 ± 0.2 (4.3) 2.9 ± 0.2

7.8

0.54

51.3

2.2 ± 0.3

Figure 5. Current−voltage characteristics of P3HT:PCBM based device with different self-organized buffer layer (PFAT, PFT-3HT, and PFT-6HT) and various concentrations (from 0.8 mg mL−1 to 2.5 mg mL −1 ). Two reference devices: a standard cell ITO/ZnO/ P3HT:PC61BM/PEDOT:PSS/Ag (control one) and a device without buffer layer were also fabricated. The values in the parentheses present the fluoropolymer concentrations (mg mL−1) in the active layer solutions.

conversion efficiency (PCE) of 2.9%. When a small amount of PFT-3HT is mixed in the spin-coated solution (P3HT:PCBM:PFT-3HT 1:0.8:0.1 w/w/w) to replace the PEDOT:PSS layer, a considerable improvement of the PCE is observed. Compared with the control device without PEDOT:PSS or any buffer layer, the efficiency enhancement from 1.7% to 3.3% proves the function of surface energy modification induced by fluoroalkylated polythiophene layer. Moreover, the performance is improved up to 4.4% with values for JSC of 11.3 mA cm−2, VOC of 0.60 V, and FF of 0.65 when the concentration of PFT-3HT is increased to 1.5 mg mL−1. The self-assembled hole transport layer also give a lower leakage current in the dark under the reverse bias compared to PEDOT:PSS (Figure S6). However, the device containing 2.0 mg mL−1 PFT-3HT shows a decrease in both JSC and FF values. Above the saturation concentration, excess block copolymer can remain in the bulk of the film to affect the device performance, as observed in XPS depth profiles. However, for low concentrations of PFT-3HT in the solution, the buffer layer cannot cover the whole active layer and would allow current leakage at the interface. These observations are well correlated with the loading dependent surface roughness, wherein high PFT-3HT loadings lead to decreased FF and JSC values and increased series resistances.

a All values represent averages from six 0.04 cm2 devices on a single chip. bAll the devices were annealed at 140 °C for 10 min before the anode (Ag) evaporation. cThe calculated short-circuit current density from the integration of the EQE spectra (JSC, calc.) of the test cells were shown. The error between the JSC, calc., and the JSC is less than 3% for all the tested cells. dThe values in the parentheses present the concentrations (mg mL−1) in the active layer solutions. eThe values show the best performance of all the testing devices.

cell (control sample) and a device without PEDOT:PSS layer are fabricated. Figure 5A shows the current density−voltage (JV) curves under illumination for solar cells fabricated with and without P3HT-b-P3FAT in the active layer solution. We first characterize the effects of block copolymer PFT-3HT concentrations on the device performance in the absence of PEDOT:PSS. The control P3HT:PCBM bulk heterojunction device (with PEDOT:PSS) shows a short-circuit current density (JSC) of 8.8 mA cm−2, an open-circuit voltage (VOC) of 0.59 V, and a fill factor (FF) of 0.56, resulting in a power 902



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In order to further investigate the effects of block ratios variation in the copolymer on the device performance, homopolymer PFAT and block copolymer PFT-6HT are added to form the buffer layer (Figure 5B). The homopolymer PFAT can be regarded as a block copolymer without P3HT block; therefore, the P3FAT:P3HT ratios in the PFAT, PFT3HT, and PFT-6HT are 1:0, 1:3, and 1:6, respectively. For PFT-6HT, the best photovoltaic performance is observed in the device with a PFT-6HT concentration of 2.5 mg mL−1, displaying PCE values of up to 4.1%. These observations are consistent with the PFT block surface segregation summarized in Table 1. As mentioned above, higher PFT-6HT loadings are needed to form a uniform interfacial layer, while the requirement concentration of PFAT is lower, about 0.8 mg mL−1. Moreover, the lower VOC, JSC, and FF of devices based on the PFAT results in a poor efficiency, compared with that of PFT-6HT and PFT-3HT. Indeed, the device performance of the PFAT is similar with that of PEDOT:PSS. This can be explained by the fact that both the homopolymer and PEDOT:PSS have little interaction with an active layer. However, for the block copolymers of PFT-3HT and PFT6HT with the large P3HT block, the fluoroalkyl block can be accumulated on the surface of active layer and can interact with the P3HT for hole collection and transport. Therefore, we believe that both the surface dipole moment induced by the fluoroalkyl block layer and the interaction between the active layer and buffer layer caused by the alkyl block are the origin of the improvement in the device performance. Figure 6A shows the external quantum efficiencies (EQE) of the inverted devices, with the PEDOT:PSS layer or PFT-3HT (1.5 mg mL−1), respectively. It can be seen from Figure S7 that the absorption of the blend layer does not change with the introduction of the PFT-3HT layer. However, the EQE values with the PFT-3HT blending device from 450 to 550 nm are over 70%, much higher than that of the PEDOT:PSS prepared device. The increasing Jsc values calculated from the EQE curves under the standard AM 1.5G conditions match well with those obtained from the J−V measurements (Table 2). This indicates that the PFT-3HT device has better hole transport and collection efficiency than the normal type. Among the challenges facing current-generation PSCs, performance degradation when exposed to ambient air limits implementation on a large scale. The inverted cell architecture minimizes environmental oxidation of low work function cathodes.25 Moreover, the use of the PFT-3HT monolayer as a hole transport layer prevents the degradation of PEDOT:PSS in the normal inverted cell, therefore enhancing PSC durability.10a The stability of the inverted P3HT:PCBM solar cells (with selfassembly PFT-3HT layer as the hole transport layer) is shown in Figure 6B. The solar cells are exposed continuously to air at room temperature (without any encapsulation barrier). The PCEs remain above 70% of the original value even after storage in air for more than two weeks. This value is almost equal with function of metal oxides hole transport. Nevertheless, for P3HT:PCBM cells prepared with PEDOT:PSS, the PCE decreases significantly when exposed to air, and the PCE is reduced by a factor of 2 after air exposure for 7 days.

Figure 6. (A) EQE spectra of inverted solar cells with PEDOT:PSS or self-assembled PFT-3HT layer as hole transport materials. (B) Normalized PCEs for inverted P3HT:PCBM solar cells with different buffer layers as a function of storage time in air under ambient conditions.

is presented. Control of the block ratios enables continuous control of the work function of anode (Ag) and the ionization potentials of the films depending on the concentration of fluorinated polymers in the solution. This shift difference between copolymers and homopolymer can be attributed to the variation density of the dipole moments, because the fluoroalkyl side chains of homopolymer PFAT can be packed more densely than those of block copolymers PFT-3HT and PFT-6HT. Therefore, work function of Ag is shifted by 0.91, 0.74, and 0.54 eV for Ag/PFAT, Ag/PFT-3HT, and Ag/PFT-6HT, respectively. However, for the block copolymers of PFT-3HT and PFT-6HT with large P3HT block, the P3FAT block can be accumulated on the surface, while the P3HT block can interact with the donor polymer for hole collection and transport. This leads to an improvement of the PCE from 1.7% to 4.4% for the device based on P3HT:PCBM with PFT-3HT as the selfassembled buffer layer; the device with homopolymer PFAT that without P3HT block gives a PCE of 2.9%, similar to PEDOT:PSS performance. Furthermore, the copolymer interfacial layers offer excellent long-term air stability. All of these features indicate that surface-segregated polythiophene block copolymers interfacial layer is a promising alternative to PEDOT:PSS as the hole transport layer for polymer solar cells.

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CONCLUSIONS We present a novel hole transport block copolymer P3HT-bP3FAT composed of polythiophene with two different side chains. The spontaneous formation of the surface segregated layer driven by the low surface energy of fluoroalkyl side chains 903



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ASSOCIATED CONTENT

S Supporting Information *

Text giving the experimental details, instrumentation, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 791 83969562. Fax: +86 791 83969561. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51273088, and 51263016).

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REFERENCES

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