Hole Extraction Enhancement for Efficient Polymer Solar Cells with

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Hole Extraction Enhancement for Efficient Polymer Solar Cells with Boronic Acid Functionalized Carbon Nanotubes doped Hole Transport Layers Yang Dang, Si Shen, Yunhe Wang, Xiangwei Qu, Shuai Huang, Qingfeng Dong, S. Ravi P. P Silva, and Bonan Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04791 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Sustainable Chemistry & Engineering

Hole Extraction Enhancement for Efficient Polymer Solar Cells with Boronic Acid Functionalized Carbon Nanotubes doped Hole Transport Layers Yang Dang†, Si Shen†, Yunhe Wang†, Xiangwei Qu†, Shuai Huang†, Qingfeng Dong‡, S. Ravi P. Silva§ and Bonan Kang†,* †

State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and

Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China. E-mail: [email protected] ‡

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, 2699 Qianjin Street, Changchun 130012, China. §

Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford,

Surrey GU2 7XH, UK KEYWORDS: polymer solar cells, bf-MWCNTs, hole transport layer, hole extraction, mobility, conductivity

ABSTRACT: Boronic acid functionalized multi-walled carbon nanotubes (bf-MWCNTs) were synthesized via a facile low temperature process and introduced in PEDOT:PSS as the composite

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hole transport layer (HTL), which improved the power conversion efficiency (PCE) of polymer solar cells (PSCs). The devices utilized PCDTBT:PC71BM active layers had achieved an optimal PCE of 6.953%, leading to 28% enhancement comparing to the device based on pristine PEDOT:PSS HTL. The PEDOT:PSS:bf-MWCNTs composite HTLs exhibited remarkable enhancement on hole mobility and electrical conductivity, which were beneficial to the hole extraction and transport on interface. Meanwhile, the work function (WF) of HTLs had an increase after bf-MWCNTs doping, which was matched with the highest occupied molecular orbital (HOMO) of the donor material, further improving the hole transport. Therefore, the incorporation of bf-MWCNTs efficiently improved the hole extraction and transport from active layer to the electrode.

INTRODUCTION Polymer solar cells (PSCs) have been widely studied due to their advantages of flexibility light weight and ease of fabrication.1-6 The most common architecture is consistsed of conjugated polymer donor and fullerene acceptor. The power conversion efficiency (PCE) of 11% has been achieved in BHJ PSCs.7-11 Generally speaking, there are two strategies to improve the PCE of PSCs. On the one hand is to develop device engineering, such as fabricating technology and device structures.12-15 On the other is to synthesis new active and interface materials with excellent properties.9, 16-17 However, the performance improvement is further needed to meet the requirement of practical application. Among all the methods mentioned above, improving contact and interfaces could make further progress in efficiency of PSCs. The charge transport and extraction of the BHJ PSCs are generally determined by interfaces between photoactive layer and electrodes.18-20 In order to effectively collecting and extracting carriers, hole transport layers (HTLs) and electron transport layers (ETLs) are utilized which can


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reduce the contact barrier between active layers and electrodes.21-22 Recently, HTLs of PSCs have been widely studied and lots of achievements have been obtained. Many HTLs materials have

been

reported

including

poly(3,4-ethylenedioxythiophene):

poly(styrenesulfonate)

(PEDOT:PSS),23-24 semiconduction metal oxides,25-27 small molecule organic materials28-30 and graphene oxides.31-33 Among the materials mention above, PEDOT:PSS is the most commonly used HTL material in traditional PSCs, which exhibits high optical transparency and smoothens surface of ITO electrode via solution processing. However, PEDOT:PSS has many defects, for example, the low conductivity and poor hole selectivity. Moreover, the existence of the barrier between the highest occupied molecular orbital (HOMO) of the donor and the work function of PEDOT:PSS makes it difficult to achieve Ohmic contact. A straightforward way to address these problems in PSCs is to introduce nanosheets or nanotubes into HTLs materials.34-36 In this respect, carbon nanotubes (CNTs) show not only excellent carrier mobility, but also promote exciton dissociation.37-38 Moreover, CNTs can be easily functionalized with functional groups, and thus tune the optical and electrical properties.3940

Nevertheless, PSCs with CNTs HTLs have the defect of lacking charge selectivity, and the

CNTs are difficult to achieve a full coverage coating on electrodes at a time. Therefore, developing functional CNTs with excellent electrical conductivity charge selectivity and establishing a rational application in PSCs have been challenging issues. Herein, we demonstrated a facile method to synthesis boronic acid functionalized multi-walled CNTs (bf-MWCNTs) and PEDOT:PSS:bf-MWCNTs composite layers were used as HTLs for high efficiency PSCs. The PEDOT:PSS:bf-MWCNTs composite films as the HTLs improved the hole transport capacity of the electrode. Compared to pristine PEDOT:PSS HTLs, the composite HTLs not only enhanced the electrical conductivity but also improved the charge transport due to


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the introduction of the bf-MWCNTs. Moreover, the work function (WF) of HTLs increased by incorporating the bf-MWCNTs, and thus the energy barrier was reduced which facilitated the hole extraction from active layer to ITO. Therefore, a PCE of 6.953% for the PSCs based on PCDTBT and PC71BM was obtained under an AM 1.5 illumination of 100 W m-2. This study provided an efficient way to improve HTLs for PSCs, which was helpful in exploiting the practical application of functionalized CNTs in organic photovoltaic devices in the future. EXPERIMENTAL SECTION Preparation of Carboxylated MWCNTs (c-MWCNTs). For carboxyl modification of multiwalled CNTs (MWCNTs), MWCNTs were added into a mixture of H2SO4 and HNO3 (volume ratio of 3:1) and ultrasonicated for 10 h. Then the solution was filtered by a polytetrafluroethylene membrane (0.45 µm) and washed for three times. Finally, the black solid was dried at 60°C and the c-MWCNTs were obtained. Preparation of bf-MWCNTs. Firstly, c-MWCNT (3 mg) was dispersed in 20 mL phosphate buffered solution (PBS, pH=7.4). Then, 3-aminobenzeneboronic acid (APBA, 24 mg) and 1ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC, 32 mg) were added into the solution and stirred for 3 h at room temperature. The solution was further dialyzed for 72 h to obtain bfMWCNTs. Device Fabrication. Photoactive layer materials PCDTBT and PC71BM (1-material Chemscitech) were used as received. PCDTBT (7 mg) and PC71BM (28 mg) were dissolved into 1,2-dichlorobenzene (DCB, 1 mL) and the solution was stirred at 60°C overnight before used. The indium tin oxide (ITO) substrates were ultrasonicated with acetone, isopropanol, and deionized water for 15 min, respectively. Then, the substrates were treated with O2 plasma for 60 s. The bf-MWCNTs were dispersed in PEDOT:PSS aqueous solution with different weight ratio


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(0.1-0.5 wt%) and sonicated for 1 h. Then the above solution was spun coated on the prepared ITO for 1 min at 3000rpm. Subsequently, the substrates were annealed at 120°C for 10 min. The photoactive layer was deposited onto the HTLs by spun coating at 1500 rpm for 30 s and then thermal treated at 70°C for 30 min in a N2 filled glovebox. Finally, 0.8 nm of LiF buffer layer and 100 nm of Al cathode were deposited on photoactive layer by thermal evaporation. The fabricated device structure was ITO/PEDOT:PSS:bf-MWCNTs/PCDTBT:PC71BM/LiF/Al. The schematic diagram and chemical formula of HTLs were shown in Figure 1.

Figure 1. The schematic diagram of PSCs and chemical formula of HTLs Device Characterization. The morphology as-prepared bf-MWCNTs were examined by using a scanning electron microscopy (SEM, JEOL, JSM 6700F) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 s-twin). The current density-voltage (J-V) characteristics measurements of all devices were measured by Keithley 2400 Source Meter at room temperature, using Newport 9225 1A-1000 solar simulator using an AM 1.5 G filter (100 mW cm-2) in a N2 glovebox. Incident photo-to-electron conversion efficiency (IPCE) were measured by Pharos Technology QEM1000 under short circuit conditions with respect to a calibrated silicon diode. The morphology of HTLs were tested by atomic force microscope (AFM) in tapping mode using a Veeco multimode with a nanoscope III controller. The impedance spectra were recorded by HP Hewlett Packard 4248A at 1 V in the frequency range from 20 Hz to 1


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MHz (CHI 760). The absorption spectra of devices were measured by ultraviolet/visible spectrometer (UV 1700, Shimadzu). Result and Discussion

Scheme 1. Synthesis route of bf-MWCNTs In this work, a facile method was employed to synthesis bf-MWCNTs. The c-MWCNT were surface functionalized by carboxylic groups after acid treatment and the oxygen-rich functional groups were active and ease of modification. Then the c-MWCNT were modified by APBA functional groups through a solution method under room temperature. The synthesis route is shown in Scheme 1.

Figure 2 (a) SEM and (b) TEM images of bf-MWCNTs The SEM and TEM of bf-MWCNTs were examined and shown in Figure 2. The SEM image of bf-MWCNTs (Figure 2a) exhibited a smooth surface and a uniform size and the average length was 318 nm (The length distribution of bf-MWCNTs was shown in Figure S1). Figure 2b showed a TEM image of bf-MWCNTs, it was shown that the functionalized MWCNTs were well-dispersed in aqueous solution. In this study, the bf-MWCNTs were doped into PEDOT:PSS HTLs. Therefore, The dispersity of the bf-MWCNTs in PEDOT:PSS solution was also estimated (Supporting Information Figure S2) and the result showed that bf-MWCNTs were well dispersed


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in the PEDOT:PSS solution. The c-MWCNT and bf-MWCNTs were confirmed by FTIR and Raman spectra. As shown in Figure 3a, the FTIR spectra of c-MWCNTs displayed a peak at 1654 cm-1, which assigned to the characteristic peak of -COOH. After successful boronic acid functionalized the c-MWCNT, as shown in Figure 3a, the peak assigned to the carboxyl decreased or disappeared and the peak for amide groups (-CONH, 1578 cm-1) was observed. Moreover, the vibrations of aromatic boronic acids located at 1331 and 1424 cm-1, which were assigned to B-O bending modes. The Figure 3a inset presented the FTIR spectra in the range of 1200-2000 cm-1. Additionally, the bf-MWCNTs were further confirmed by XPS spectra (Supporting Information Figure S3).

Figure 3 (a) FTIR of c-MWCNT and bf-MWCNTs. (b) Raman spectra of pristine MWNCTs, cMWCNT and bf-MWCNTs Figure 3b showed the Raman spectra of the pristine MWCNTs, c-MWCNT and bf-MWCNTs. The Raman spectra of MWCNTs exhibited two remarkable peaks at 1340 cm-1 and 1571 cm-1, corresponding to the disorder mode (D-band) and tangential modes (G-band), respectively. The G band was assigned to the E2g vibration mode of sp2 carbon domains, which reflected the degree of graphitization of the MWCNTs. The D band was related to the structural defects and the disorder structure. The extent of the defects in MWCNTs can be quantified by the intensity ratio of the D to G band (ID/IG).41 Comparing with the pristine MWCNTs, the ID/IG ratio of cMWCNT was increased from 0.95 to 1.03 after carboxylated functionalization, indicating acid


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treatment caused structural damage of MWCNTs. It is notable that the bf-MWCNTs exhibited a smaller ID/IG ratio of 0.99 comparing with the c-MWCNT. This was mainly because parts of APBA were absorbed on MWCNTs via π-π stacking and covered the surface defects. According to this result, the bf-MWCNTs should retain better electrical conductivity than the c-MWCNTs, which was beneficial to the electrical properties and should be an excellent HTL materials in PSCs.

Figure 4. J-V curves of PSCs with pristine PEDOT:PSS, PEDOT:PSS:MWCNTs and PEDOT:PSS:bf-MWCNTs HTLs at different doping concentrations (a) under illumination and (b) in dark. (c) IPCE spectra of PSCs with pristine PEDOT:PSS HTLs and with PEDOT:PSS:bfMWCNTs HTLs at different doping concentrations. (d) the absorption spectra of devices with pristine PEDOT:PSS HTL and with 0.4 wt% bf-MWCNTs doped HTL. Table 1. The performance parameters of the PSCs with pristine MWCNTs doped PEDOT:PSS HTLs and with different concentrations of bf-MWCNTs doped PEDOT:PSS HTLs. Dopant

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

MWCNTs

0.865±0.02

11.726±0.08

62.38±0.02

6.327±0.13

7.81

364.63

bf-MWCNT

0.839±0.02

10.852±0.12

59.52±0.03

5.421±0.14

9.02

115.08


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0 wt% bf-MWCNT 0.1 wt%

0.870±0.01

11.238±0.08

60.87±0.01

5.952±0.12

8.22

278.60

bf-MWCNT 0.2 wt%

0.869±0.01

11.581±0.13

60.93±0.01

6.132±0.09

7.85

347.21

bf-MWCNT 0.3 wt%

0.881±0.03

11.902±0.09

61.59±0.03

6.458±0.12

7.30

447.59

bf-MWCNT 0.4 wt%

0.880±0.02

12.509±0.12

63.14±0.02

6.953±0.13

6.26

621.03

bf-MWCNT 0.5 wt%

0.882±0.01

11.792±0.08

62.18±0.02

6.469±0.11

6.42

530.15

PSCs devices with PEDOT:PSS:bf-MWCNTs and pristine PEDOT:PSS HTLs were fabricated to evaluate their effect on devices performance. The J-V curves of PCDTBT:PC71BM solar cells were tested under AM 1.5 G illumination and shown in Figure 4a. All the parameters of PSCs were average values of 50 devices and summarized in Table 1. For pristine PEDOT:PSS HTLs devices, it showed an open-circuit voltage (Voc) of 0.839 V, a short circuit current (Jsc) of 10.852 mA cm-2, a FF of 59.52%, and yielding a PCE value of 5.421%. For PEDOT:PSS:bf-MWCNTs HTLs device, the optimal PCE value of 6.953% was achieved. The improvement of PCE could be mainly attributed to the Jsc increasing to 12.509 mA cm-2. Additionally, the Voc was increased to 0.880 V, and the FF was increased to 63.14%. The J-V curves in darkness were also measured. Figure 4b presents the dark current of devices with PEDOT:PSS:bf-MWCNTs and pristine PEDOT:PSS HTLs. The PSCs with PEDOT:PSS:bf-MWCNTs composite HTLs showed lower leakage currents at negative voltage, indicating a a smaller series resistance (Rs) and a larger shunt resistance (Rsh) , which could suppress charge recombination and increase the Jsc and FF.


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In addition, the dark current rose rapidly in the positive voltage, demonstrating an enhancement in charge transport. To further understanding the enhancement of Jsc, IPCE spectra of devices were measured and the results were shown in Figure 4c. IPCE of device with pristine PEDOT:PSS HTLs showed a maximum of 63.9% (480 nm) and devices with PEDOT:PSS:bf-MWCNTs composite HTLs showed a maximum of 68.7% (475 nm), and the calculated Jsc were 10.433 mA cm-2 and 12.055 mA cm-2, respectively, which were consistent with the Jsc of PSCs. As shown in the IPCE spectra, The IPCE of devices with PEDOT:PSS:bf-MWCNTs HTLs exhibited an decrease in the wavelength range from 330 to 390 nm. In order to explain this phenomenon, the absorption spectra of PEDOT:PSS and PEDOT:PSS:bf-MWCNTs HTLs were measured. The absorption of PEDOT:PSS:bf-MWCNTs (Supporting Information Figure S4) was larger than that of pristine PEDOT:PSS in the range of 330-390 nm, indicating that absorption of active layer was decreased in the same range due to the bf-MWCNTs absorbed a part of incident light, which directly reduced the IPCE. The enhancement in photocurrent of the devices with PEDOT:PSS:bf-MWCNTs HTLs was primarily in the ranges of 370-480 nm and 520-630 nm. However, comparing with the PEDOT:PSS HTLs devices, the UV-Vis spectra of devices with 0.4 wt% bf-MWCNTs doped HTLs (Figure 4d) showed barely change in the corresponding wavelength. Therefore, the increase of photocurrent cannot be attributed to the light absorption of devices. There must be other factors that improve the photocurrent such as the enhancement of hole extraction from the photoactive layer to the PEDOT:PSS:bf-MWCNTs HTLs.


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Figure

5.

Photoluminescence

(PL)

spectra

of

PCDTBT/PEDOT:PSS

film

and

PCDTBT/PEDOT:PSS with different concentration bf-MWCNTs doping. Firstly, we investigated the impact of the bf-MWCNTs on charge transfer behaviour, PL spectra of PEDOT:PSS/PCDTBT and PEDOT:PSS:bf-MWCNTs/PCDTBT were measured. As can be seen in Figure 5, the fluorescence emission of PCDTBT was observed at 700 nm. It is worth noting that the PL intensity of the PEDOT:PSS:bf-MWCNTs/PCDTBT was reduced with the increase of the bf-MWCNTs doping concentration. These experimental data indicated that the free holes generated in PCDTBT upon illumination were extracted by the bf-MWCNTs in the composite HTLs. As a result, incorporation of the bf-MWCNTs in PEDOT:PSS films enhanced the charge transfer from active layer to HTLs.

Figure 6 (a) J-V characteristics in the dark and (b) J0.5-V characteristics of the hole-only devices based on PEDOT:PSS:MWCNTs HTLs, PEDOT:PSS HTLs and PEDOT:PSS:bf-MWCNTs HTLs at different doping concentrations.


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For further exploring the role of bf-MWCNTs in promoting the Jsc, J-V characteristics of holeonly

devices

were

measured

(Figure

6).

The

hole-only

devices

ITO/PEDOT:PSS:bf-MWCNTs/PCDTBT:PC71BM/MoO3/Al

structures

were and

ITO/PEDOT:PSS/PCDTBT:PC71BM/MoO3/Al. The results were fitted with the Mott-Gurney space-charge-limited-current (SCLC) model.42 ଽ

௏మ

‫ܬ‬ୗେ୐େ = ߝ଴ ߝ௥ ߤ య ଼ ௅ (1)

where J is the current density, L is the thickness of active layer, V is the applied voltage, εr and ε0 are the dielectric permittivity, and the µ is the carrier mobility. The hole mobility for device with PEDOT:PSS HTL was 1.127×10-5 cm2 V-1 s-1. 0.4 wt% bf-MWCNTs doped device showed an optimal hole mobility of 5.836×10-5 cm2 V-1 s-1. The detail hole mobility data of all devices were summarized in Table S1. These results indicated that the hole extraction capacity was largely enhanced by introducing bf-MWCNTs into PEDOT:PSS films, thus improving charge transport and reducing the exciton recombination, leading to a significant increase in Jsc. In order to examine the function of boronic acid groups, PSCs devices with pristine MWCNTs doped PEDOT:PSS HTLs were fabricated. The J-V curves of PSCs based on pristine MWCNTs doped PEDOT:PSS HTLs at different doping concentrations under illumination were measured (Supporting Information Figure S5). The performance parameters of the devices were summarized in Table S2. The devices with pristine MWCNTs doped HTLs showed the optimal Jsc of 11.726 mA cm-2 and PCE of 6.327%, which were lower than that of devices with 0.4 wt% bf-MWCNTs doped HTLs (Jsc=12.509 mA cm-2, PCE=6.953%). We further fabricated the holeonly devices with pristine MWCNTs doped HTLs and measured the hole mobility of the devices. The J-V curve in dark of the hole-only device with pristine MWCNTs doped HTL was shown in


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Figure 6 and the result was fitted with the SCLC model. Devices with MWCNTs doped HTLs exhibited a hole mobility of 2.0422×10-5 cm2 V-1 s-1, much lower than that of bf-MWCNTs doped HTLs (5.836×10-5 cm2 V-1 s-1). This indicated that the boronic acid functional group played an important role in improving the hole mobility. The bf-MWCNTs exhibited more excellent hole selectivity and contributed to the improvement of devices performance.

Figure 7. (a) J-V curves of PEDOT:PSS and PEDOT:PSS:bf-MWCNTs composite HTLs obtained under dark condition. (b) Impedance spectra of devices with PEDOT:PSS HTL and with PEDOT:PSS:bf-MWCNTs HTLs at the doping concentration of 0.1-0.5 wt%. (c) The fitting model of Equivalent circuit. Through the above characterization, we knew that the bf-MWCNTs had great influence on electrical properties of HTLs. Subsequently, we measured the conductivities and impedance spectra of PEDOT:PSS films with and without bf-MWCNTs doping and the results were showed in Figure 7. The J-V characteristics of the diodes with the ITO/PEDOT:PSS/Al and ITO/PEDOT:PSS:bf-MWCNTs/Al structures were measured. The slopes of the J-V curves demonstrated the changes of conductivities of devices. In Figure 7a, slopes of J-V curves for the devices with bf-MWCNTs doped films were larger than that of undoped devices. Among them,


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the PEDOT:PSS film with 0.4 wt% bf-MWCNTs doping exhibited the largest slope, leading to the highest conductivity. However, with the increasing of doping concentration, the conductivity showed an obvious decrease, which in consistent with the trend of Rs. The higher conductivity could improve the charge transport, resulting in a lower internal resistance. Therefore, the increasing conductivity also led to the enhancement of the Jsc. Figure 7b presented the impedance spectra for devices with PEDOT:PSS HTL without and with different concentrations of bf-MWCNTs doped under dark condition. It can not only obtain the kinetics information and energetic processes of devices,43 but also observe electrical properties that cannot be observed in the direct current regime.44 The impedance spectra were measured with an alternating current signal in the frequency range from 20 Hz to 1 MHz and the results were all semicircle. The impedance spectra fitted with an equivalent circuit (Figure 7c). As we can see in the circuit, R0 is the resistance of anode interface, buffer layer and wire for devices. R1 is transport resistance and C1 represents dielectric and geometrical capacitance. Rrec is the recombination resistance and Cµ is usually called as chemical capacitance which is the capacitance of bulk heterojunction layers and relates to the excess of carrier storage of active layers. The diameters of the impedance spectra were affected by R0, R1 and Rrec. As shown in Figure 7b, with the increase of bfMWCNTs doping concentration, the diameter of impedance spectra became smaller. This indicated that the PSCs devices resistance was decreased by incorporating bf-MWCNTs doped HTLs, which was in accordance with the trend of Jsc. On the other hand, the series resistance Rs was defined as sum of R0 and R1. As listed in Table 1, Rs was decreased because of the incorporation of bf-MWCNTs into HTLs, which was beneficial to interfacial charge transport.


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Figure 8. (a) The Jph and (b) Pc characteristics versus Veff of devices with PEDOT:PSS HTL and with PEDOT:PSS:bf-MWCNTs HTLs at different doping concentrations. To understand the influence of the composite HTLs on exciton generation and dissociation, we determined the maximum photoinduced carriers generation rate (Gmax) of devices. Figure 8a depicted the influence of bf-MWCNTs doped HTLs on photocurrent density (Jph) versus effective voltage (Veff). Jph could be given by JL-JD, where JL and JD are the current density under illumination and in dark, respectively. Veff is given by V0-Va, where V0 is the compensation voltage at Jph=0 and Va is the applied bias voltage. The Jph increased at low Veff and reached saturation at large Veff region (defined as Jsat). Ideally, all the photogenerated excitons dissociated and converted to current in the large Veff region. However, the Jsat was limited only by the Gmax in the saturation region. The Gmax value of devices without and with bf-MWCNTs doped HTLs could be estimated from Jsat=qGmaxL, where q is the electron charge (1.6×10-19) and L is the thickness of active layer (100 nm). Gmax of the control devices was 8.43×1027 m-3 s-1 (Jsat=134.90 A m-2), while the devices with 0.4 wt% bf-MWCNTs doped HTLs exhibited a significant increased Gmax of 1.11×1028 m-3 s-1 (Jsat=177.45 A m-2). The improved hole extraction and transport capacity of the composite HTLs were mainly contributed to the increase of Gmax. However, not all the photogenerated excitons could dissociate into free carriers. Only a part of photogenerated excitons can dissociate into free charge carriers, and thus, contributing to the photocurrent. The charge carrier collection efficiency (Pc) versus Veff was shown in Figure 8b, the Pc was an important parameter reflecting the probability of excitons dissociation and carriers


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collection, which increased from 77.80% for the control device to 86.80% for the device with 0.4 wt% bf-MWCNTs doped HTL. These results revealed that incorporation of bf-MWCNTs into HTLs played an positive role in hole extraction, achieving a better photocurrent. The detail data of Jsat, Gmax and Pc of all devices are summarized in Table S3.

Figure 9. Energy level alignment of PSCs. The WF of the PEDOT:PSS and bf-MWCNTs composite layer was tested using Kelvin probe measurements (Figure 9). The WF of the PEDOT:PSS HTL was 5.02 eV. After 0.4 wt% bfMWCNTs incorporating into PEDOT:PSS film, the WF exposed an significant increase (5.39 eV), which could reduce the energy barrier and accelerated the hole collection from the donor material to the ITO. On the other hand, the enhancement of Voc was in accordance with these the observations. Generally speaking, Voc is influenced by the the difference between the HOMO of donor and the LUMO of acceptor, and It is also related to the WF of interfacial layer and electrode materials. The electrodes and active layer materials did not changed in our study. Therefore, the increased Voc was mainly because of the increase of WF of hole transport layer. The images of WF measured by Kelvin probe and detail WF data of all HTLs are summarized in Figure S6 and Table S4, respectively.


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Figure 10. AFM height images of (a) pristine PEDOT:PSS film and PEDOT:PSS films with (b) 0.1 wt%, (c) 0.2 wt%, (d) 0.3 wt%, (e) 0.4 wt% and (f) 0.5 wt% bf-MWCNTs doping. AFM was utilized to investigate the influence of bf-MWCNTs on the morphology of the HTLs. Figure 10 displayed the AFM topographic images (3 µm×3 µm) of pristine PEDOT:PSS film and bf-MWCNTs doped PEDOT:PSS films (0.1-0.5 wt%). For the undoped PEDOT:PSS film, it showed a room-mean-square (RMS) value of 1.50 nm. For the bf-MWCNTs doped films, the RMS values were 1.47, 1.47, 1.49, 1.50 and 1.50 nm at each doping concentration (0.1-0.5 wt%), respectively, which were almost same as the pristine PEDOT:PSS film. The corresponding phase images were exhibited in Figure S7 (Supporting Information). The bfMWCNTs domains were observed. We assigned the bright phase to the bf-MWCNTs for the reason that the bright phase emerges with the addition of bf-MWCNTs. It could be seen that the doping of the bf-MWCNTs had little effect on the roughness of films. Conclusions In conclusion, boronic acid functionalized MWCNTs with enhanced hole extraction capability and high conductivity had been synthesized and applied as PEDOT:PSS:bf-MWCNTs composite HTLs for high efficiency PSCs. The device with 0.4 wt% bf-MWCNTs doped HTLs showed a maximal PCE of 6.953%, increasing by 28% than that of the control device. The enhancement of the PCE was mainly contributed from the increase of the Jsc and Voc. With the addition of 0.4


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wt% bf-MWCNTs into the PEDOT:PSS, the HTLs presented a significant improvement on hole mobility and conductivity, which were the main reasons for the Jsc enhancement. Furthermore, with the WF of HTLs increasing, it matched well with the HOMO level of PCDTBT, resulting in higher Voc. This work showed that the low temperature, solution synthesized bf-MWCNTs could be an excellent hole transport material for organic optoelectronic devices. ASSOCIATED CONTENT Supporting Information. Length distribution of bf-MWCNTs (Figure S1). SEM images of a PEDOT:PSS:bf-MWCNTs film under different magnification (Figure S2). XPS spectra of the bf-MWCNTs (Figure S3). Hole mobility of devices with PEDOT:PSS and PEDOT:PSS:bf-MWCNTs HTLs (Table S1). The absorption spectra of pristine PEDOT:PSS HTL and with 0.4 wt% bf-MWCNTs doped HTL (Figure S4). The J-V curves of PSCs with pristine MWCNTs doped PEDOT:PSS HTLs at different doping concentrations under illumination (Figure S5). The performance parameters of the devices with different concentrations of pristine MWCNTs doped PEDOT:PSS HTLs (Table S2). Jsat, Gmax and Pc of devices with PEDOT:PSS HTL and with PEDOT:PSS:bf-MWCNTs HTLs at different doping concentrations (Table S3). Work function of PEDOT:PSS film and PEDOT:PSS:bf-MWCNTs composite films with different bf-MWCNTs doping concentrations measured by Kelvin probe (Figure S6) and the calculated data (Table S4). AFM phase images of pristine PEDOT:PSS film and PEDOT:PSS films with various concentration of bf-MWCNTs doping (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was supported by the National Key Basic Research and Development Program of China (Grant No. 2016YFB0401001) Founded by MOST. The authors are grateful to the State Key Laboratory on Integrated Optoelectronics and State Key Laboratory of Supramolecular Structure and Materials. SRPS acknowledges support from the Royal Society International programme. REFERENCES (1) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y., Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591-595. (2) Yang, D.; Fu, P.; Zhang, F.; Wang, N.; Zhang, J.; Li, C., High efficiency inverted polymer solar cells with room-temperature titanium oxide/polyethylenimine films as electron transport layers. Journal of Materials Chemistry A 2014, 2, 17281-17285. (3) Li, Z.; Zhang, X.; Liu, C.; Zhang, Z.; He, Y.; Li, J.; Shen, L.; Guo, W.; Ruan, S., The Performance Enhancement of Polymer Solar Cells by Introducing Cadmium-Free Quantum Dots. Journal of Physical Chemistry C 2015, 119, 26747-26752.


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For Table of Contents Use Only. The device structure of PSCs with bf-MWCNTs doped HTLs. The composite HTLs exhibited enhancement on hole selectivity which beneficial to the performance of PSCs.


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Hole Extraction Enhancement for Efficient Polymer Solar Cells with

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