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The effect of methanol treatment on the performance of polymer solar cells

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 484003 (http://iopscience.iop.org/0957-4484/24/48/484003) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 24 (2013) 484003 (7pp)

doi:10.1088/0957-4484/24/48/484003

The effect of methanol treatment on the performance of polymer solar cells Kai Zhang, Zhicheng Hu, Chunhui Duan, Lei Ying, Fei Huang and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China E-mail: [email protected] and [email protected]

Received 3 June 2013, in final form 7 August 2013 Published 6 November 2013 Online at stacks.iop.org/Nano/24/484003 Abstract Significant performance enhancement was observed for the bulk-heterojunction polymer solar cell of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ poly[N-900 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzodithiazole)]: [6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PC71 BM)/Al when the as-cast active layer was rinsed with methanol before the deposition of the metal electrode. Comparison of independent anode interfacial layers of PEDOT:PSS and MoO3 indicated that the effects of methanol treatment on the improvement of device performance are more pronounced for PEDOT:PSS-based devices. No discernible changes can be observed in film thickness, surface topography and UV–vis absorption profiles of the photoactive layer, indicating the absence of film reconstruction and the improvement of device performance are hence attributed to the modification of the interface between the PEDOT:PSS and the fresh active layer. Further examination of the devices containing a cathode interlayer of poly[(9,9-bis(30 -(N,Ndimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) also demonstrates the potential modification of the interface between the PEDOT:PSS and the active layer by methanol in addition to the widely observed PFN functionality. (Some figures may appear in colour only in the online journal)

1. Introduction

To achieve high power conversion efficiency (PCE), significant efforts have been dedicated to the development of new materials [3–6], the innovation of device architectures [7–9], the optimization of processing methods [10–12], and interface engineering [13–15]. A variety of materials have been utilized for interface modification, such as alkali metal salts [16, 17], metal chelates [18], self-assembled monolayers [19, 20] and water/alcohol soluble conjugated polymers [13, 21, 22]. It was reported very recently that the PCEs of PSCs can be considerably enhanced by rinsing the active layer with polar solvents before deposition of metal electrodes [23–25]. The explanations for these enhancements include the optimization of the phase separation in the active layer, and a possible influence of the interface between the active layer and the PEDOT:PSS layer underneath [24], the increase of

Polymer solar cells (PSCs) have emerged as a promising photovoltaic technology due to the unique advantages of light weight, compatibility with low-cost solution processing, and great potential for flexible devices [1, 2]. Of particular interest is the bulk-heterojunction (BHJ) architecture, which consists of a nanoscale phase separated interpenetrating network of a π -conjugated polymer electron donor and a fullerene electron acceptor. A conventional BHJ PSC applies the sandwich configuration, where the organic photoactive layer is sandwiched between an electron collecting electrode of a low work function metal and a hole collecting electrode of indium tin oxide (ITO) coated with a hole transport layer (HTL) poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). 0957-4484/13/484003+07$33.00

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surface potential, the increase of built-in potential across the device due to passivation of surface traps or increased surface charge density after methanol treatment [25]. Similar effects of solvent treatment are found to be favorable for the modification of the crystallinity of P3HT-based solar cells [26]. A solvent effect was also found in polymer light emitting diodes [27]. However, these interesting events remain poorly understood. In this contribution, we demonstrate that the interface modification by simple methanol treatment is dependent on both device fabrication procedures and the interface between the HTL and active layer underneath. It was noted that the positive effects of device performances for the fresh active layer treated straightforwardly with methanol are more pronounced than that observed for the active layer stored under vacuum for 12 h before methanol treatment and cathode evaporation. The improvement of PCE in terms of open circuit voltage (Voc ) and short circuit current (Jsc ) is more effective for devices with PEDOT:PSS as the HTL layer than the molybdenum oxide (MoO3 ). Further investigation by depositing a water/alcohol soluble polymer poly[(9,9-bis(30 -(N,N-dimethylamino)propyl)2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) on top of an active layer reveals that the improvement in device performance may be due to a combination of the presence of a thin PFN layer that can facilitate electron extraction and methanol treatment, while PFN plays the dominant role in the improvement of the resulting PSCs’ performance.

Scheme 1. Chemical structure of PCDTBT and PC71 BM, and solar cell device configuration used in this study.

baked at 80 ◦ C in an oven for 12 h. To fabricate photovoltaic devices, a HTL (ca. 40 nm) of PEDOT:PSS (Baytron PVP AI 4083, filtered at 0.45 µm) was spin-coated on the plasma treated ITO-coated glass substrates at 3000 rpm and baked at 200 ◦ C for 15 min under ambient conditions, while the HTL MoO3 was thermally evaporated onto pre-cleaned ITO substrate under a high vacuum (3 × 10−4 Pa) with a thickness of 10 nm. The substrates were then transferred into an argon-filled glove-box. Subsequently, the PCDTBT:PC71 BM (1:4) active layer (ca. 90 nm) was spin-coated on the top of the PEDOT:PSS or MoO3 layer from its chlorobenzene:1,2-dichlorobenzene (1:3, V:V) solution. Then, methanol or PFN solution (0.5 mg l−1 in methanol) was dropped onto the active layer and spin-coated at 2000 r min−1 for 1 min to complete the methanol treatment or PFN deposition. After that, the substrates were transferred into a chamber and pumped down to a high vacuum (2×10−6 mbar), and aluminum (100 nm) was thermally evaporated through shadow masks. The effective device area was measured to be 0.16 cm2 . The current density–voltage (J–V) curves were measured on a computer-controlled Keithley 2400 sourcemeter under a 1 sun, AM 1.5G spectrum from a class solar simulator (Japan, SAN-EI, XES-40S1); the light intensity was 100 mW cm−2 as calibrated by a NREL certified reference photodiode (Hamamatsu).

2. Experimental details 2.1. Materials Poly[N-900 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzodithiazole)] (PCDTBT) was purchased from 1-material Chemscitech Inc. (St-Laurent, Quebec, Canada). [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) was purchased from Nano C (MA, USA) with a purity >99.5%. Methanol was purchased from Guangzhou reagent factory with a purity >99.9%. All of these materials are used as received without further purification. Poly[(9,9-bis(30 -(N,N-dimethylamino) propyl)-2,7-fluorene)alt-2,7-(9,9-dioctylfluorene)] (PFN) was synthesized according to the reported literature [28]. 2.2. Measurement and characterization UV–vis absorption spectra were measured on an HP 8453E UV–visible spectrophotometer. Atomic force microscopy (AFM) measurement was carried out on a DINS4 AFM (Veeco). The thickness of the films was measured by a Dektak 150 surface profiler.

3. Results and discussion Scheme 1 shows the configuration of a solar cell device with a conventional architecture of ITO/HTL/PCDTBT:PC71 BM/Al, where an optimized weight ratio of 1:4 was applied for the active layer of PCDTBT:PC71 BM. Initial efforts focused on the devices with PEDOT:PSS as the HTL.

2.3. Solar cell device fabrication and characterization ITO-coated glass substrates were cleaned by sonication in acetone, detergent, deionized water and isopropanol and 2

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Table 1. Photovoltaic performances of solar cells measured at AM 1.5G irradiation with the intensity of 100 mW cm−2 under various fabrication conditions with PCDTBT:PC71 BM (1:4 wt:wt) as the active layer. (Note: the device performance data are generated from ten devices, and the effective device area is 0.16 cm2 .) Device

HTL

Conditions of MeOH

Jsc (mA cm−2 )

Voc (V)

FF (%)

PCE (%)

D1 D2 D3 D4 D5 D6 D7 D8 D9

PEDOT:PSS PEDOT:PSS PEDOT:PSS PEDOT:PSS MoO3 MoO3 MoO3 PEDOT:PSS PEDOT:PSS

w/o Type Aa Type Bb Type Cc w/o Type Aa Type Bb Type Dd Type Ee

9.91 ± 0.2 10.49 ± 0.3 9.99 ± 0.4 10.24 ± 0.3 8.57 ± 0.2 8.89 ± 0.2 8.55 ± 0.2 10.38 ± 0.2 10.30 ± 0.3

0.80 ± 0.01 0.92 ± 0.01 0.82 ± 0.02 0.92 ± 0.01 0.83 ± 0.01 0.85 ± 0.01 0.83 ± 0.02 0.93 ± 0.01 0.90 ± 0.01

53.6 ± 0.6 57.3 ± 0.7 52.4 ± 1.2 57.5 ± 0.8 52.0 ± 1.3 52.6 ± 1.2 51.6 ± 1.2 62.8 ± 0.5 63.1 ± 0.7

4.3 ± 0.1 5.5 ± 0.2 4.3 ± 0.3 5.4 ± 0.2 3.7 ± 0.2 4.0 ± 0.1 3.7 ± 0.2 6.1 ± 0.2 5.9 ± 0.3

a

The active layer was treated with methanol, followed by cathode evaporation (Type A). The active layer was stored under vacuum for 12 h, followed by methanol treatment and cathode evaporation (Type B). c The active layer was treated with methanol and stored under vacuum for 12, followed by cathode evaporation (Type C). d The active layer was deposited by PFN solution and followed by cathode evaporation (Type D). e The active layer was stored under vacuum for 12 h, followed by PFN solution deposition and cathode evaporation (Type E). b

12 h to remove the solvent residue of chlorobenzene/1,2dichlorobenzene and methanol, which was followed by cathode evaporation. For comparison, a control device based on an active layer without methanol treatment was also prepared. The details of device fabrication are shown in section 2. The current density–voltage (J–V) characteristics are shown in figure 1 and the corresponding photovoltaic parameters are summarized in table 1. A moderate device performance was obtained for the control device (D1), which exhibited a PCE of 4.3% with a Jsc of 9.91 mA cm−2 , a Voc of 0.80 V and a fill factor (FF) of 53.6%. It was found that the D2, based on Type A, exhibited distinctly improved PCE, particularly in terms of Jsc and Voc . Device D2 exhibited a Jsc of 10.49 mA cm−2 , a Voc of 0.92 V and a FF of 57.3%, and a relatively high PCE of 5.5%. This phenomenon has also been observed in devices based on PTB7:PC71 BM as the active layer [25]. It is speculated that the residual methanol on the active layer may lead to dipole formation, which can reduce the work function of metal cathodes [27]. Hence, devices Type B and Type C were fabricated to verify this hypothesis. Device D4 that based on Type C exhibited nearly identical performance to device D2, which means that the active layer has little change after being stored in vacuum for 12 h. In the meantime, device D3 that based on Type B unexpectedly exhibited nearly identical performance to the control device D1, which means that the residual methanol has no or an insignificant effect on device performance enhancement if the residual chlorobenzene/1,2-dichlorobenzene in the active layer was removed before methanol rinsing. Thus, the performance enhancement after methanol treatment may originate from the potential modification of the under layer instead of the interface between the active layer and the cathode, as the thickness of ca. 90 nm may allow for the diffusion of methanol molecules across the whole film in the presence of residual chlorobenzene/1,2-dichlorobenzene.

Figure 1. J–V characteristics of PSC devices based on different processing conditions with PEDOT:PSS as the HTL. a The active layer was treated with methanol and followed by cathode evaporation (Type A); b the active layer was stored under vacuum for 12 h, then followed by methanol treatment and cathode evaporation (Type B); c the active layer was treated with methanol and stored under vacuum for 12 h, then followed by cathode evaporation (Type C).

The influence of the deposition and subsequent removal of methanol atop the pre-fabricated active layer on device performances was studied on the basis of three types of device fabrication procedures. Device Type A: the fresh active layer was straightforwardly treated with methanol and followed by cathode evaporation; device Type B: the active layer was stored under vacuum for 12 h to remove the chlorobenzene/1,2-dichlorobenzene solvent residue, then followed by methanol treatment and cathode evaporation; device Type C: the fresh active layer was treated with methanol at first and then stored under vacuum for 3

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Figure 2. UV–vis absorption of (a) PCDTBT:PC71 BM blend films and (b) the variation of active layer thickness with storage time under vacuum. a Active layer was treated with methanol; b active layer was stored under vacuum for 12 h, then treated with methanol.

Figure 3. Surface topographic AFM images of a PCDTBT:PC71 BM (1:4, wt:wt) film atop a PEDOT:PSS layer: (a) pristine film; (b) methanol treated film; (c) vacuum 12 h, then treated with methanol; and a PCDTBT:PC71 BM (1:4, wt:wt) film atop a MoO3 layer: (d) pristine film; (e) methanol treated film; (f) vacuum 12 h, then treated with methanol. The size of all images is 5 µm × 5 µm.

To further probe the influence of methanol treatment on HTL, an alternative device architecture utilizing MoO3 instead of PEDOT:PSS as the HTL was also fabricated. The corresponding photovoltaic devices are denoted as D5 (control device without methanol treatment), D6 (follow device fabrication procedures Type A) and D7 (follow device fabrication procedures Type B), respectively. The J–V characteristics are shown in figure 4 and photovoltaic parameters are summarized in table 1. The performances of the methanol treated devices D6 and D7 are very close to that of the control device D5 with a Voc of 0.83 V, a Jsc of 8.57 mA cm−2 , a FF of 52%, and a PCE of 3.7%, indicating that the influence of methanol treatment on the performance of MoO3 -based devices was much less pronounced than that of PEDOT:PSS-based devices. Therefore, the modification of the

The possible changes of the active layer itself after methanol treatment were probed at first. No obvious changes were observed in the absorption spectra of PCDTBT:PC71 BM (1:4 in wt:wt) after methanol treatment (figure 2(a)). The active layer thickness has little change regardless of the durations under vacuum (figure 2(b)), indicating the absence of obvious reconstruction of the internal donor structures. The surface morphologies of the blend films (figures 3(a)–(c)) were examined by atomic force microscopy (AFM). All BHJ films have quite similar surface topographic morphology with a root-mean-square (rms) value of ca. 1.0 nm on the top of both PEDOT:PSS and MoO3 substrates, indicating that methanol treatment has little influence on the morphology of the active layer. 4

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The electron mobilities of devices based on a pristine active layer and a methanol treated active layer with conditions of Type A and Type B are nearly identical (6.7 × 10−3 cm2 V−1 s−1 ), which means that methanol treatment has no effect on the electron mobility of devices. In contrast, hole-only devices based on PEDOT:PSS as the HTL demonstrated a close hole mobility of 8.9×10−4 cm2 V−1 s−1 for the pristine active layer and methanol treated active layer with condition Type B, while an improved hole mobility of 1.5 × 10−3 cm2 V−1 s−1 for the methanol treated active layer with condition Type A. It is worth noting that the improved hole mobility for methanol treated films with condition Type A will lead to more balanced charge carrier transport in the BHJ layer, which can lead to improved FF in the resulting device (D2) by restricting the build-up of space charges and reducing charge recombination subsequently [25], and thus, improved device performance. On the other hand, no obvious variation was observed for hole-only devices using MoO3 as the HTL, which exhibited a hole mobility of 2.7 × 10−3 cm2 V−1 s−1 . It was noted that even though MoO3 -based devices have a slightly higher hole mobility than those of PEDOT:PSS-based devices, the performances of devices based on MoO3 are actually lower than that of the latter. This is understandable since the PCE relied not singly on the charge carrier transport properties; a range of other factors can also influence the device performances. For instance, the adhesion between the bulk-heterojunction layer and the underlying PEDOT:PSS or MoO3 , the surface morphology of the bulk-heterojunction layer, the contact resistance of the devices and so forth. Nevertheless, these results indicated that the improvement of hole mobility should correlate to the interface layer between the PEDOT:PSS and the active layer instead of the intrinsic charge carrier mobility of the BHJ film. Further investigation of the interfacial modification was carried out to distinguish the effect of methanol and water/alcohol soluble conjugated copolymer PFN, which has been extensively used as an efficient cathode interlayer. Solar cell devices with an architecture of

Figure 4. J–V characteristics of PSC devices with MoO3 as the HTL. a Active layer was treated with methanol, followed by cathode evaporation (Type A); b active layer was stored under vacuum for 12 h, followed by methanol treatment and cathode evaporation (Type B).

interface between the PEDOT:PSS and active layer may play an important role in the device performance improvement. The charge carrier mobility of the active layer with different methanol treatment conditions was investigated using single-carrier diodes and a space-charge limited current (SCLC) model. Electron- and hole-only devices were fabricated with device structures of ITO/Al/PCDTBT:PC71 BM/Ca/Al and ITO/HTL/PCDTBT:PC71 BM/MoO3 /Al, respectively, where the HTL is PEDOT:PSS or MoO3 . The BHJ film of PCDTBT:PC71 BM was treated with methanol with conditions of both Type A and Type B. The mobility of the devices was determined by fitting the dark current to the single-carrier space-charge limited current (SCLC) model. The electron and hole mobility was calculated from the slope of the J 1/2 –V characteristics as shown in figure 5.

Figure 5. J 1/2 –V characteristics of (a) an electron-only device with a configuration of ITO/Al/PCDTBT:PC71 BM/Ca/Al; (b) a hole-only device with a configuration of ITO/PEDOT:PSS/PCDTBT:PC71 BM/MoO3 /Al; and (c) hole-only device with configuration of ITO/MoO3 /PCDTBT:PC71 BM/MoO3 /Al. a Active layer was treated with methanol, followed by cathode evaporation (Type A); b active layer was stored under vacuum for 12 h, followed by methanol treatment and cathode evaporation (Type B). 5

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4. Conclusion In conclusion, the effects of post-deposition treatment of PCDTBT:PC71 BM film with methanol on the efficiency of polymer solar cells were found to be more effective for the devices using PEDOT:PSS as the HTL than those using MoO3 as the HTL. The underlying mechanism of the ‘methanol effect’ can be attributed to the modification of the interface between the PEDOT:PSS and the active layer rather than the interface between the active layer and the metal cathode. Further investigation by depositing water/alcohol soluble polymer PFN atop the active layer reveals that the improvement in the PCE may originate from the combination of the potential modification of the interface between the PEDOT:PSS and the active layer by methanol and the interface modification function of PFN, but the latter plays a dominant role. Figure 6. J–V characteristics of PSC devices with or without PFN as the cathode interlayer. d Active layer was deposited by PFN solution and followed by cathode evaporation (Type D); e active layer was stored under vacuum for 12 h, followed by PFN solution deposition and cathode evaporation (Type E).

Acknowledgments This work was financially supported by the Natural Science Foundation of China (Nos 21125419, 50990065, 51010003 and 51073058), the Ministry of Science and Technology, China (MOST) National Research Project (Nos 2009CB623601 and 2009CB930604), Guangdong Natural Science Foundation (Grant No. S2012030006232) and Research Fund for the Doctoral Program of Higher Education of China (20120172140001).

ITO/PEDOT:PSS/PCDTBT:PC71 BM/PFN/Al were fabricated, where PFN was deposited from its methanol solution with a concentration of 0.5 mg ml−1 in the presence of a trace amount of acetic acid. Compared to the control device of D1, it was found that all photovoltaic parameters of Voc , Jsc and FF were simultaneously improved for device D8 (following device fabrication procedures Type D: active layer was treated with PFN solution and followed by cathode evaporation) containing a PFN interlayer, which exhibited a PCE of 6.1% with a Voc of 0.93 V, a Jsc of 10.38 mA cm−2 and a FF of 62.8%. This observation agrees with the previously reported results [22, 29]. In order to exclude the positive influence of methanol on device performance, a device with the active layer stored in vacuum for 12 h then treated with PFN solution before cathode evaporation was fabricated. This device was denoted as D9 (following device fabrication procedures Type E), which showed a slightly decreased PCE of 5.9% with a Voc of 0.90 V, a Jsc of 10.30 mA cm−2 and a FF of 63.1%, but still much higher than the control device without the PFN interlayer. As the effects of methanol treatment on the performance of both devices D9 and D3 were negligible, the significantly improved PCE of D9 relative to D3 could be mainly contributed by the incorporated PFN interlayer. The J–V characteristics of the relevant devices are presented in figure 6. We suppose that the PFN interlayer can facilitate the electron selective property of the electrode, which will in turn lead to reduced electrical leakage current, and enhanced built-in potential and thus slightly increased Voc [22]. In addition, the amino groups in the PFN side chain are hole trapping centers that can act as an effective hole blocking layer [30], and can optimize the rectification ratio and thus lead to increased fill factors in the devices.

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The effect of methanol treatment on the performance of polymer solar cells.

Significant performance enhancement was observed for the bulk-heterojunction polymer solar cell of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesu...
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