Improvement in polymer solar cell performance and eliminating light soaking effect via UV-light treatment on conjugated polyelectrolyte interlayer

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Improvement in polymer solar cell performance and eliminating light soaking effect via UV-light treatment on conjugated polyelectrolyte interlayer Xianqiang Li a,b, Jie Liu c, Xiaohong Tang b, Shifeng Guo a, Jun Li a, Hong Wang b, Bin Liu a,c, Wei Lin Leong a,⇑ a

Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A⁄STAR), 3 Research Link, Singapore 117,602, Singapore School of Electrical and Electronics Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore c Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore b

a r t i c l e

i n f o

Article history: Received 7 April 2015 Received in revised form 5 June 2015 Accepted 5 June 2015 Available online xxxx Keywords: Inverted polymer solar cell Conjugated polyelectrolyte Light-soaking effect Electron-transporting layer

a b s t r a c t A new conjugated polyelectrolyte material, namely, poly [9,9-bis((60 -N,N,N-trimethylamino)hexyl)fluorene-alt-co-benzoxadiazole dibromide] (PFBD) is reported as electron transport layer (ETL) in polymer solar cells. We observed a light-soaking effect and described how a pre UV-light treatment on PFBD ETL is essential for attaining higher efficiencies (>7%) and negate the light-soaking problem. The pre UV-light treatment on PFBD layer is found to directly influence its molecular structure and result in reduction of the work function and increased electron mobility in PFBD which corroborates well with the observed low series resistances obtained from dark current analysis and impedance spectra, and therefore enhancement in open-circuit voltage and fill factor. Moreover, after the pre-UV light treatment, the maximal efficiency of the solar cells retains at a nearly similar level for at least 26 days. Ó 2015 Published by Elsevier B.V.

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1. Introduction

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Polymer solar cells (PSCs), a blend of conjugated polymers and fullerene derivatives, are promising candidate for clean and large-area power sources. To date, polymer solar cells have achieved power conversion efficiencies (PCE) over 10% [1–4]. The advancement of this thin-film photovoltaic technology is mainly driven by developing new light harvesting materials and improving the absorption properties of the materials. The development of innovative interfacial materials that can be incorporated between the electrodes and the photoactive layer is also important as they can improve the charge collection efficiency by reducing the contact resistance and interfacial recombination loss, thereby resulting in higher short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) [5–11]. Conjugated polyelectrolytes (CPEs) are one of the promising candidates for interfacial modification materials. In general, CPEs are polymers containing hydrophobic p-conjugated backbones with hydrophilic surfactant-like ionic pendant groups. CPEs can be easily processed from environmentally friendly polar solvents

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⇑ Corresponding author. E-mail addresses: [email protected] (X. Tang), [email protected] (B. Liu), [email protected] (W.L. Leong).

(such as alcohol or water) due to the ionic nature of the side chains, which allow the fabrication of multilayer polymer-based devices with orthogonal solubility [12–15]. Their highly polar pendant groups enable excellent interface modification functions by (1) creating strong interface and/or molecular dipoles at the metal/ organic interface that lead to a substantial reduction of the electrode work function, or (2) allowing the ion motion to redistribute the electric fields within a device. These changes are able to minimize interfacial resistances and facilitate electron transport, making them promising interfacial layer materials in organic electronic devices [16–19]. In addition, the use of CPEs can avoid high temperature processing, which is usually required in other interfacial materials, such as metal oxides [20,21] and hence reduces manufacturing costs [22]. The use of CPEs as electron transport layer (ETL) in solar cells has been widely reported to efficiently enhance PCE of PSCs [13,19,23–26]. For instance, cationic polythiophene, [13] and polyfluorene derivatives containing amine salts and alkoxy groups [23,24] have been reported to be effective ETLs in solar cells. The neutral precursors of the organic electrolytes (without ionic parts), such as, poly[(9,9-bis(30 -(N,N-dimethylamino) propyl)-2,7-fluore ne)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was also incorporated successfully as electron transport layer in a thieno[3,4-b]-thiophe ne/benzodithiophene copolymer based PSC and a high PCE of

http://dx.doi.org/10.1016/j.orgel.2015.06.012 1566-1199/Ó 2015 Published by Elsevier B.V.

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9.2% was achieved. [16] Despite the successful application of CPEs or their neutral precursors as ETLs, there have also been reports on the need for light-soaking effects [27,28] wherein the fill factor (FF) and open circuit voltage (Voc) increase with light illumination until they reach saturated values. Zilberberg et al. observed light-soaking phenomenon when using poly(3-[6-(N-methylimida zolium)hexyl]thiophene) bromide (P3ImHT) as ETL [27]. The light-soaking effect was attributed to a reduction of work function of ITO after UV illumination in combination with the dipole effect of CPE on lowering the work function of ITO. A recent report studied the light-soaking effect for inverted PSCs using PFN as ETL and explained the phenomenon through the influences of oxygen on ITO originated from the ambient air device processing [28]. In this paper, we report a new CPE material, namely, poly [9,9-bis((60 -N,N,N-trimethylamino)hexyl)-fluorene-alt-cobenzoxadiazole dibromide] (PFBD), as electron transport layer in thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]phenyl C71-butyric acid methyl ester (PC71BM) based solar cells. We also observed a light-soaking effect and described how a pre UV-light treatment on PFBD layer is essential for attaining higher efficiencies (>7%) and negate the light-soaking problem. Impedance analysis reveals that the pre-UV light treatment on PFBD interlayer leads to reduced series and bulk resistance and increased flat-band potential (VFB) of the inverted solar cell, thereby improving the fill factor (FF) and open-circuit voltage (Voc). Moreover, after the pre-UV light treatment, the maximal efficiency of the solar cells retains at a nearly similar level for at least 26 days.

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2. Experiment section

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All materials were used as received without purifications. The inverted PSCs have a structure of ITO/PFBD/PTB7:PC71BM/ MoO3/Al. PTB7:PC71BM blends were prepared from chlorobenzene at a weight ratio of 1:1.5 and total concentration of 25 mg/mL. A 3% v/v 1,8-diiodoctane (DIO) [29] solvent was also added into the final solution. The blend solution was stirred overnight at 70 °C in a glove box before spin-casting. PFBD was synthesized according to previous report [30]. ITO substrates were cleaned using soap water and sonicating sequentially with de-ionized water, acetone and isopropanol for 15 min each. The ITO substrates were dried at 100 °C in oven for several hours, followed by exposure to ultraviolet light and ozone for 15 min to produce a hydrophilic surface. The cleaned ITO substrates were then transferred into a nitrogen-filled glove box. The PFBD interlayers were spin-coated on the surface of ITO substrate from a methanol solution (0.2 mg/mL) followed by drying of the films at 100 °C for 10 min. PTB7:PC71BM blend film was spin-coated at 1000 rpm for 2 min and annealed at 70 °C for 10 min to form an active layer with thickness of about 90 nm. A 10 nm MoO3 layer and a 100 nm Al layers were deposited by thermal evaporation at a base pressure of 1.0  106 mbar to form the hole-collecting electrode. The device area was defined to be 4.5 mm2. Inverted devices using PFN as electron transporting layer were also fabricated as a reference with a similar structure of ITO/PFN/PTB7:PC71BM (1:1.5 by weight)/MoO3/Al. PFN solution with concentration of 1 mg/mL was prepared by dissolving PFN into methanol mixed with 2 lL/mL acetic acid [16]. The current density–voltage (J–V) characteristics of devices were measured by a Keithley 2400 parameter analyzer under one sun solar spectrum illumination (AM 1.5G) from a solar simulator with intensity of 100 mW/cm2 calibrated via a silicon reference cell. Incident photon-to-current efficiency (IPCE) measurements were conducted on devices under short-circuit conditions through a lock-in amplifier (SR510, Stanford Research System) at a

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chopping frequency of 233 Hz during illumination with a monochromatic light from a Xenon arc lamp. The J–V and IPCE measurements were carried out in a nitrogen-filled glove box. The thickness of PTB7:PC71BM active layer was measured by a KLA Tencor P-16+ Surface Profiler while the thickness of PFBD layer was obtained from AFM measurements. The absorption spectrum of PFBD layers coated on quartz substrates was characterized by a Shimadzu UV-3101PC UV–VIS-NIR Spectrophotometer. The NanoWizard III instrument (JPK Instruments AG, Berlin, Germany) equipped with the NanoWizard head and controller was used in the AFM measurement. The morphology of the model surfaces was visualized by tapping mode AFM imaging under ambient conditions using standard silicon probes (k  40 N/m, Tap 300AL-G, Budget sensors). All the images were processed using the JPK data processing software (Version 4.2). The work functions of the ITO and ITO/PFBD were measured with Kelvin probe apparatus (KP Technology, UK). The thickness, AFM, absorption and work function measurements were characterized in ambient air. The impedance measurement was carried out through an automated potentiostat (Solartron-analytical, 1470E) coupled with a frequency response analyzer (Solartron-analytical, 1255B) in a three-electrode electrochemical system. The devices were encapsulated for impedance testing in air with a UV-curable epoxy and covered with a glass slide. The AC impedance measurement was done in Z–h mode with a fixed AC drive bias of 25 mV with the frequency varying from 500 Hz to 1 MHz under one sun spectrum illumination. A constant DC bias which equals to the open-circuit voltage was applied and superimposed on the AC bias. The capacitance–voltage measurement was conducted in dark condition at the room temperature. The capacitance was measured at different applied bias voltage with AC excitation amplitude of 10 mV at frequency of 10 kHz.

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3. Results and discussion

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Fig. 1 shows the molecular structures of the component materials and solar cell architecture. The transparent ITO functions as the electron-collecting electrode while the hole-collecting electrode consists of Al. The bulk heterojunction polymer/fullerene photovoltaic devices consists of blended films of thieno[3,4-b]thiophe ne/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Electron transporting layers (ETLs), which are inserted between the ITO and photoactive layer, are either PFBD or PFN. The thickness of PFBD interlayer was optimized (Table S1) and the photovoltaic parameters of the best inverted PTB7:PC71BM devices using different ETLs and without ETL (ITO-only) are shown in Table 1. The average PCE values were obtained from at least three devices. For the device using PFBD as ETL (optimized thickness of PFBD is 2 nm), a high PCE of 7.21% is exhibited. The measured Jsc is 16.44 mA/cm2, which is in good agreement with the calculated Jsc from incident photon-to-current collection efficiency (IPCE) spectra (Fig. S1 of Supporting information). The performance is also comparable to that of a conventional solar cell structure (ITO/PEDOT:PSS/PTB7:PC71BM/Al) (not shown) as well as the inverted device using PFN as ETL (Table 1). We also performed the dark current analysis (Fig. S1(c) and Table 1). The low Rs value of 1.9 X cm2 in PFBD based device suggests good contacts between active layers and ITO electrode, which facilitate free charge carrier extraction while the high Rsh of 1.4  104 X cm2 indicates the ability in electron selectivity and low charge recombination losses at the interface between the PFBD and photoactive layer. The result indicates that the PFBD acts as an effective ETL in inverted solar cells.

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Fig. 1. Chemical structures of the materials used in this work and schematic structure of the inverted PSCs.

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Fig. 2(a) presents the typical current–voltage (J–V) characteristics of the PTB7:PC71BM inverted organic solar cells with PFBD as electron transport layer upon illumination, exhibiting an S-shape characteristics, with an initial Voc of 0.62 V, Jsc of 16.33 mA/cm2 and FF of 44%. However, as the illumination time increases, we observed that all the device parameters improved, the S-shape characteristics disappeared and ultimately attained a high efficiency of 6.9% after 15 min of continuous illumination. This phenomenon was not observed in inverted PSCs based on PFN interlayer which were fabricated in the same batch (Fig. S2(a)). The control device without ETL (ITO only) also displayed negligible difference after continuous illumination (Fig. S2(b)), ruling out the possibility that the Al/MoO3 contact, adsorbed oxygen species on ITO electrode [27,31] or photoactive layer is responsible for the soaking effect. The ITO/PFBD interface and/or the PFBD layer itself are most likely to be responsible for the light soaking behavior. To investigate the origin of this light soaking effect, the J–V characteristics of the solar cells under illumination were first measured with and without the UV spectral components. The UV spectral components were excluded through the application of an UV long-pass filter with a cutoff at 430 nm. It can be clearly seen from Fig. 2(b) that light soaking effect under UV spectral components considerably improved the device performance. In particular, we note that the improved J–V characteristics after light-soaking (with UV spectral components) is stable and do not return to its initial S-shape state over time, providing the first indication that a permanent photochemical modification of PFBD has occurred upon light illumination. To further ascertain the hypothesis that the UV spectral components is the main factor for the improvement in device performance, a pre-UV light treatment was performed on the PFBD coated ITO substrate, followed by completion of the device

fabrication. The behavior of detailed photovoltaic parameters of inverted PSCs with and without pre-UV treatment is plotted in Fig. 3. Compared to the characteristics exhibited by devices without pre-UV treatment of PFBD (as prepared), the initial Voc and FF of devices with 30 min of pre-UV treatment are substantially higher. The efficiency of devices with 30 min pre-UV treatment quickly saturates from 6.7% to 7.2%, and the devices display negligible light soaking effect. The device with pre-UV treatment also displayed good stability. Its J–V characteristic remains almost the same after 26 days (Fig. S3(a)), hence the pre-UV treatment method improved the performance of device to a stable level. This is in contrast to the inverted PSC using PFN as ETL which degrades faster over time (Fig. S3(b)). It is clear that light soaking problem is from the influence of UV spectral components on the PFBD layer, which was successfully solved after introduction of the pre-UV treatment method. Kelvin Probe measurements were also conducted on ITO and ITO/PFBD electrodes with and without UV-treatment. The work functions of bare ITO and as-prepared ITO/PFBD are 5.2 and 4.9 eV respectively while the work function of ITO/PFBD is further reduced to 4.6 eV after 30 min of UV-treatment. The lowered work function of ITO/PFBD matches well with the LUMO level of PC71BM (4.3 eV), [16] enabling a more efficient charge collection and hence higher efficiency. To further understand the influence of UV-treatment on device performance, the series and bulk resistance of inverted solar cells were measured by impedance analysis. The Nyquist plots of the measured impedance of inverted solar cells based on as-prepared and 30 min pre-UV light treated PFBD interlayers are shown in Fig. 4(a). There are two characteristic changes upon UV irradiation: (1) The series resistance Rs, which can be obtained by extrapolating the impedance curve to x-axis at high frequency (left), of device

Table 1 Photovoltaic parameters of inverted PTB7:PC71BM devices using PFBD or PFN as ETL. Characteristics of an ITO-only device without PFBD and PFN interlayers are shown as reference. ETL

Jsc (mA/cm2)

Voc (V)

FF (%)

PFBD PFN ITO only

16.44 15.24 13.15

0.69 0.72 0.28

63.2 66.7 34.8

PCE (%) Average

Best

7.10 ± 0.19 7.15 ± 0.13 0.95 ± 0.28

7.21 7.27 1.27

Rsh (X cm2)

Rs (X cm2)

14,000 75,000 80

1.9 2.6 10.9

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Fig. 2. (a) J–V characteristics of inverted solar cells based on PFBD ETL with different illumination time. (b) Power conversion efficiency as a function of illumination time with and without the UV filter.

Fig. 3. Photovoltaic parameters, PCE (a), FF (b), Voc (c) and Jsc (d) as a function of different illumination time, of inverted PSCs based on PFBD interlayer with and without UV treatment on PFBD layer.

Fig. 4. (a) Nyquist plots of the impedance of inverted PTB7:PC71BM solar cells based on as-prepared and 30 min pre-UV light treated PFBD interlayers. (b) Mott–Schottky characteristics of inverted devices based on as-prepared and 30 min pre-UV light treated PFBD interlayers.

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based on pre-UV treated PFBD layer is slightly reduced to 2.2 X cm2, as compared to the Rs for the as-prepared device (4.9 X cm2). (2) The charge transfer resistance of device based on pre-UV treated PFBD layer is also significantly reduced (reduced radius of semicircle in Nyquist plot). The charge transfer resistance originates from interfaces through which charge transfer occurs, and there are three such interfaces in the device: PFBD–PC71BM; PTB7–PC71BM; PTB7–MoO3. Since the UV treatment is not affecting the latter two, it is most likely resulting from the change in charge transfer efficiency through PFBD–PC71BM interface. This is consistent with the observed reduction in work function of ITO/PFBD from Kelvin probe measurements (and associated improvement in FF and reduction in Rs identified from dark I–V curves) (i.e., reduced barrier against electron collection from PC71BM to ITO electrode). The corresponding Mott–Schottky characteristics of inverted solar cells based on as-prepared and 30 min UV treated PFBD interlayers are shown in Fig. 4(b). The flat-band potential (VFB) is obtained by extrapolating the curve to x-axis in the linear region. [33] The VFB of as-prepared device is 0.53 V while the device with UV-treated PFBD has a much higher VFB (0.68 V). The enhancement of VFB correlates well with the higher Voc value. From the impedance analysis results, it can be concluded that UV-treatment improves the device performance by reducing the bulk and series resistance and increasing the VFB. Since the PTB7:PC71BM morphology can be potentially affected by the surface energy of the underlying PFBD layer after UV treatment, AFM measurement was conducted on as-prepared and

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30 min pre-UV treated PFBD layers as well on the corresponding PTB7:PC71BM active layers (see Fig. 5). The PFBD layers were found to uniformly cover the surface of ITO electrode (see Fig. S4). The as-prepared PFBD interlayer displayed a smoother morphology with root-mean-square (rms) roughness of 0.54 nm. Similar rms roughness of the UV treated PFBD layer (0.57 nm) is observed, indicating that the UV treatment does not influence the surface topography of PFBD layer. The PTB7:PC71BM blend film on top of as-prepared PFBD interlayer is also very uniform and smooth with rms of 1.24 nm. No obvious change was observed in PTB7:PC71BM blend film on the top of UV treated PFBD interlayer (rms  1.18 nm). From the AFM results, it can be concluded that UV illumination does not change the surface morphology of PFBD layer and the surface morphology of PTB7:PC71BM blend film is independent on the pre-UV light treatment of the underlying PFBD layer. We have also carried out optical measurements to analyze the UV light treated PFBD films. However, no obvious peak was found in Fourier transform infrared (FT-IR) spectra due to thin thickness of PFBD film. We next measured the absorption spectra of PFBD film prepared on quartz substrate (Fig. 6). The absorption maximum of as-prepared PFBD films (black line) occurs at 325 and 480 nm, corresponding to the localized p–p⁄ transitions of the conjugated backbones and the intramolecular charge transfer (ICT) from the donor of fluorene to the acceptor of benzoxadiazole (BD) units, respectively. After UV-treatment for various times, ranging from 30 min to 2 h, the absorbance of the PFBD films at longer wavelength (480 nm) decreased accompanied with gradual

Fig. 5. AFM topographic images of as-prepared (a) and 30 min UV treated (b) PFBD interlayer on ITO and PTB7:PC71BM blend film on the top of as-prepared (c) and 30 min UV treated (d) PFBD interlayer.

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Fig. 6. UV–vis absorption spectra of PFBD films at various UV illumination durations.

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blue-shift, indicating that the ICT effect has decreased. The absorbance at 325 nm remains almost the same, implying that the fluorene units are not affected by the UV-treatment. The highly localized electron densities in the LUMO on electron-deficient BD units would be expected to decrease the electron mobility in PFBD [34]. Hence, one possible explanation for the role of UV treatment is that some of the BD units are broken down by UV irradiation and result in increased electron mobility in PFBD which corroborates well with the observed low series resistances obtained from dark current analysis and impedance spectra.

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4. Conclusions

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In summary, we have demonstrated inverted polymer solar cells using thin conjugated polyelectrolyte, PFBD, as ETL and they can be as efficient (>7%) as inverted solar cells using the commonly used PFN. We have also shown that the light soaking issue can be remedied by employment of UV light treatment on the PFBD layer. The UV illumination treatment of PFBD leads to reduced work function and increased electron mobility, thus enabling a more efficient electron collection. Moreover, the maximal efficiency of these solar cells retains at a similar level for at least 26 days, which demonstrates that the process occurred during the UV-light treatment on the PFBD layer improved the device performance to a stable level. Our studies suggested that proper selection of the conjugated main chains is critical for the development of CPEs as interfacial materials and the interfacial property can be well-modified by employment of UV treatment. More detailed study on the relationship of the structures of the CPEs with UV treatment is underway.

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5. Uncited reference

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[32].

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Acknowledgements

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X. Li is grateful to NTU for NTU Research Scholarship. W.L. Leong thanks A⁄STAR IMRE Molecular Materials Project (IMRE/13-1P0909) for funding support.

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Appendix A. Supplementary data

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.orgel.2015.06. 012.

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