Towards high performance inverted polymer solar cells

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Towards high performance inverted polymer solar cells Chao Yi and Xiong Gong Attempts to develop inverted polymer solar cells (PSCs) as required for achieving high efficiency and good stability have, however, met with limited success. Here, we report that a high efficiency of 8.4% was achieved for inverted PSCs. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, leading to facilitate electron transport and suppress bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that the inverted PSCs are comparable with the conventional PSCs. Address Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325, United States Corresponding author: Gong, Xiong ([email protected])

Current Opinion in Chemical Engineering 2013, 2:125–131 This review comes from a themed issue on Materials engineering Edited by Thein Kyu For a complete overview see the Issue and the Editorial Available online 12th October 2012 2211-3398/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.coche.2012.09.001

Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on polymer–fullerene composites are of growing interest as a potential source of renewable and clean energy owing to their attractive advantages of low cost fabrication in large areas and light weight on flexible substrates [1,2,3,4,5]. Despite over 8% power conversion efficiency (PCE) reported for BHJ PSCs with a conventional device configuration of ITO/PEDOT:PSS/ BHJ composite/Ba(Al) [6,7,8,9]. Here the anode is indium tin oxide (ITO); PEDOT:PSS (poly(ethylenedioxythiophene):polystyrene sulphonate) is used to smooth the surface of ITO, Barium (Ba) coated Aluminum (Al) is used as the cathode. It was found out that PEDOT:PSS on ITO are unstable with indium contaminating the polymer and thus degrading the performance of PSCs [10,11]; meanwhile, Ba/Al is inherently flawed in lower long-term stability [12,13]. By contrast, the device architecture, inverted PSCs was developed. In the inverted PSCs, ITO is used as the cathode and Ag or Au is used as the anode. The inverted PSCs possess better stability owing to the introduction of air-stable high-work-function metals, typically, Ag or Au, www.sciencedirect.com

and the elimination of the PEDOT:PSS layer [14,15,16,17,18,19]. Moreover, in the inverted PSCs, the anode, such as Ag or Au, which can be formed coating or printing technology from Ag (or Au) pastes, to simplify and lower the cost of manufacturing [20]. In the inverted PSCs, the challenge to reverse the layer sequence of PSCs is to prepare a selective contact bottom cathode [21]. n-type metal oxide such as titanium oxide (TiOx), zinc oxide (ZnO), cesium carbonate (Cs2CO3) and others were used to modify cathode electrode and to change the polarity of the device by depositing these materials on top of ITO. For example, titanium oxide (TiOx), as an electron-transporting, a hole blocking [22] and an optical spacer, was also used for modification of the interface between organic layer and ITO cathode [23,24,25]. All these studies indicated that the performance of inverted PSCs is sensitive to the interface between ITO cathode and the active layer. Owing to the organic/ITO interface modified by the introduction of buffer layer, inverted PSCs with PCEs ranging from 3% to 7% were reported [26–28,29,30,31]. However, the PCEs observed from PSCs with inverted device structure are still not comparable with those obtained from PSCs with the conventional device configuration [32,33]. Therefore, the need to improve the PCEs requires the implementation of new materials and exploration of new device structures. Recently, ZnO has drawn much attention because it has a good transparency in whole visible region and a good electron mobility [34–36]. Moreover, ZnO thin film can be easily deposited from corresponding solution via various solution processes followed by thermal annealing treatment. All these features make ZnO an ideal material as a buffer layer for PSCs with an inverted device structure [16,37]. Here we report high performance inverted PSCs. By casting a thin conjugated polyelectrolyte layer onto the solution-processed ZnO thin film, the interface between ZnO electron extraction layer and BHJ active layer is re-engineered, as a result, a record PCE of 8.4% under AM (Air Mass) 1.5 G irradiation is achieved for BHJ PSCs with the inverted structure. All these results demonstrate that electron extraction layer plays an important role in enhancing the performance of inverted PSCs.

Preparation of ZnO and ZnO/PFN-Br thin films ZnO thin film: ZnO thin films coated on top of ITO glass were obtained from solution by sol–gel method. Before coating ZnO thin films, the ITO glass substrates were cleaned by ultrasonic treatment in acetone, detergent, Current Opinion in Chemical Engineering 2013, 2:125–131

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deionized water and isopropyl alcohol sequentially. ZnO precursor was prepared by dissolving zinc acetate (ZnAc: Sigma–Aldrich) and ethanolamine (Guangzhou Chemical Reagent Factory) in the solution of 2-methoxyethanol with a concentration of 0.5 mol L1. Approximately 40 nm ZnO thin films were obtained by spin-coating above solution followed by being annealed at 200 8C in air for 1 hour. Note that the ZnO precursor was not heated gradually from room temperature to the temperature being treated [35]. ZnO thin films were then ultrasonicated in acetone and isopropyl alcohol, and subsequently dried in an oven for the fabrication of PSCs. ZnO/PFN-Br thin film: ZnO/PFN-Br thin film was prepared by depositing PFN-Br from PFN-Br water–alcohol (1:1 by volume) solution with a thickness of 5 nm on the top of ZnO thin film. Here PFN-Br is poly[(9,9bis(30 ((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7,fluorene)-alt-2,7-(9,9-dioctylfluorene)].

Absorption spectra of ZnO and ZnO/PFN-Br thin films The absorption spectra of solid thin films ZnO and ZnO/ PFN-Br casted on quartz glass were measured by a HP 8453 UV–vis spectrophotometer. Figure 1 shows the absorption spectrum of ZnO and ZnO/PFN-Br thin films. The ZnO/PFN-Br thin film only absorbs UV light and is transparent from 450 nm to 900 nm, which is similar to the absorption spectrum of ZnO thin film. This indicated that the visible light is allowed to pass through ZnO/PFN-Br thin layer into BHJ composite active layer. Therefore, the photodegradation of polymer active layer is reduced; as a Figure 1

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Absorption spectra of ZnO and Zno/PFN-Br thin films. Current Opinion in Chemical Engineering 2013, 2:125–131

result, a good photochemical stability from inverted PSCs is anticipated [17].

Electronic conductivity of ZnO and ZnO/PFNBr thin films The electronic conductivities of ZnO and ZnO/PFN-Br thin films were measured on Bruker Dimension Icon system with Peak Force Tapping Tunneling AFM (Atomic force microscopy) (PFTUNA) module. The probe was the (PFTUNA) probe with spring constant of 0.5 N/m with 20 nm Pt/Ir coating on both front and back side. The spring was constantly measured using thermal tune method. The peak currents were measured with bias voltage (1 V) applied to the sample. The ramp rate of 0.4 Hz and the force setpoint of 60 nN were for both thin films. PFTUNA module was used to measure the peak currents with bias voltage applied to the ZnO and ZnO/PFN-Br thin layers [14]. We found that the maximum peak current observed from pristine ZnO thin layer is about 5 pA within a 7 mm zoom, however, the maximum peak current observed from ZnO/PFN-Br thin layer is about 25 pA within a 4 mm zoom. These qualitative studies indicated that the electronic conductivity of ZnO/PFN-Br thin layer is much higher than that of ZnO thin layer. This high electronic conductivity will facilitate electron transport from BHJ composite to corresponding electrode [33,38].

BHJ composite The BHJ materials under this study are a D–A conjugated polymer, {4,8-bis(4,5-didecyl-2-thienyl)-benzo[1,2-b:4,5b0 ]dithiophene-alt-[4,9-bis(4-hexyl-2-thienyl)naphtho[1,2-c:5,6-c0 ]bis[1,2,5]thiadiazole-5,5-diyl]} (PBDT-DTNT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The molecular structures, synthesis and characterization of PBDT-DTNT and PFN-Br in details were reported elsewhere [39,40]. The PC71BM was purchased from One-Material Inc. and used without further purification. Figure 2 shows the absorption spectra of PBDT-DTNT, PC71BM, BHJ composite of PBDT-DTNT:PC71BM and ZnO/PFN-Br thin films. The bandgap of PBDTDTNT is 1.48 eV, which is close to the ideal bandgap for PSCs donor materials [41]. PBDT-DTNT has absorption from 400 nm to 800 nm. PC71BM has absorption from 400 nm to 700 nm. The absorption spectrum of PBDT-DTNT:PC71BM BHJ composite is the superposition of PBDT-DTNT and PC71BM, which implies that both PBDT-DTNT and PC71BM will contribute photocurrent for PSCs fabricated by PBDTDTNT:PC71BM.

Fabrication of inverted PSCs Photovoltaic characteristics were investigated for the inverted PSCs with the device architectures of ITO/ ZnO/PBDT-DTNT:PC71BM/MoO3/Ag and ITO/ZnO/ www.sciencedirect.com

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Absorption spectra of PBDT-DTNT, PC71BM and PBDT-DTNT:PC71BM.

PFN-Br/PBDT-DTNT:PC71BM/MoO3/Ag. For the inverted PSCs with ZnO thin layer as an electron extraction layer, an approximately 40 nm ZnO thin film was deposited on the top of ITO glass. For the inverted PSCs with ZnO/PFN-Br as an electron extraction layer, an approximately 5 nm thick PFN-Br layer was spin cast on the top of 40 nm thick ZnO layer which was coated on the ITO glass. Photoactive layer PBDTDTNT:PC71BM BHJ composite with a thickness of approximately 250 nm was then spin coated on either ZnO or ZnO/PFN-Br layer from 1,2-dichlorobenzene solution composed of PBDT-DTNT and PC71BM (PBDT-DTNT:PC71BM = 1:1.5, by weight). The photoactive layer was then thermally annealed at 110 8C for 10 min. After that, about 8 nm molybdenum oxide (MoO3) was thermally deposited on the top of PBDTDTNT:PC71BM layer with an evaporation rate of 0.1 A˚ s1 under the vacuum of 3  104 Pa. Ultimately, about 60 nm Ag film was deposited on the top of MoO3 layer through a shade mask. The device area was 0.16 cm2. All the devices were encapsulated with resin and exposed to ultraviolet light for 5 min in the glove-box with nitrogen atmosphere before testing in air.

Performance of inverted PSCs J–V characteristics were shown in Figure 3. As shown in Figure 3a, without PFN-Br interfacial layer, inverted PSCs showed a short-circuit current (Jsc) of 15.2 mA cm2, an open-circuit voltage (Voc) of 0.69 V, a fill factor (FF) of 55% and a corresponding PCE of 6.1%. www.sciencedirect.com

After depositing a thin conjugated polyelectrolyte layer, PFN-Br, onto ZnO thin film to reengineer the interface between ZnO electron extraction layer and PBDTDTNT:PC71BM, inverted PSCs showed a Jsc of 17.4 mA cm2, a Voc of 0.75 V, a FF of 61% and a corresponding PCE of 8.4%. Over 37% enhanced PCEs were observed. Device performance optimization involved over 500 identical devices fabricated from over 100 independently PBDT-DTNT:PC71BM, and ZnO/PFN-Br thin films; PCEs between 8.2% and 8.6% were obtained. The average device yields PCE of 8.4%, with Jsc of 17.4 mA cm2 (the calculated Jsc value from IPCE spectrum was 17.2 mA cm2). This high efficiency is the highest value reported from inverted BHJ PSCs so far. The Voc of 0.75 V from the inverted PSCs with PFN-Br interfacial layer was larger than Voc of 0.69 V from the inverted PSCs without PFN-Br interfacial layer. This difference was owing to the different band alignments in the inverted PSCs with and without PFN-Br interfacial layer. Figure 3b shows the J–V characteristics of inverted PSCs with and without PFN-Br interfacial layer measured in dark. It was found that the dark current densities (Jdark) under the reverse bias from the inverted PSCs with PFNBr interfacial layer are approximately 10 times smaller than those from the inverted PSCs without PFN-Br interfacial layer. The low dark current densities observed from the inverted PSCs with PFN-Br interfacial layer Current Opinion in Chemical Engineering 2013, 2:125–131

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Figure 3

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J versus V characteristics of inverted PSCs with the device architecture of ITO/ZnO/active layer/Ag and ITO/ZnO/PFN-Br/active layer/Ag (a) under illumination and (b) in dark.

More importantly, the low dark current densities observed from the inverted PSCs with PFN-Br interfacial layer implied that the inverted PSCs with PFN-Br interfacial layer possess larger Voc than those without PFN-Br interfacial layer [38]. Towards the end, we have modeled the J–V characteristics of PSCs with an equivalent circuit, consisting of single diode with series resistance (Rs) and shunt resistance (Rsh). The addition of a parallel photocurrent source, Jph, leads to the well-known equation [38] J ¼ J 0 ½expððqðV  Rs AJÞ=nkT Þ  1 þ ðV  Rs AJÞ=ðRsh AÞ  J ph ðV Þ

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The enhanced Jsc and FF in the inverted PSCs with PFN-Br interfacial layer implied that Rs in the inverted PSCs with PFN-Br interfacial layer was smaller than that without PFN-Br interfacial layer, and Rsh in the inverted PSCs with PFN-Br interfacial layer is larger than that Figure 4

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where J0 is reverse dark current density. According to Eq. (2), lower J0, higher Voc. Therefore, inverted PSCs with PFN-Br interfacial layer possess a higher Voc than that PSCs without PFN-Br interfacial layer. Current Opinion in Chemical Engineering 2013, 2:125–131

IPCE (%) versus wavelength for inverted PSCs with the device structures of ITO/ZnO/PBDT-DTNT:PC71BM/MoO3/Ag and ITO/ZnO/PFN-Br/ PBDT-DTNT:PC71BM/MoO3/Ag. www.sciencedirect.com

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without PFN-Br interfacial layer. In order to confirm this hypothesis, we have calculated Rs and Rsh based on the J– V curves shown in Figure 3b. The Rs for the inverted PSCs with and without PFN-Br interfacial layer are 6.1 V cm2 and 10.3 V cm2, respectively. The Rsh for the inverted PSCs with and without PFN-Br interfacial layer are 757.6 V cm2 and 490.2 V cm2, respectively. It is indeed that the Rs in the inverted PSCs with PFN-Br interfacial layer is smaller than that without PFN-Br interfacial layer, and the Rsh in the inverted PSCs with PFN-Br interfacial layer is larger than that without PFNBr interfacial layer. Figure 4 compared the incident photon-to-current efficiency (IPCE) spectra of the inverted PSCs with and without PFN-Br interfacial layer. Inverted PSCs with PFN-Br interfacial layer exhibited a substantial enhanced response from 400 nm to 800 nm as compared with that without PFN-Br interfacial layer. The inverted PSCs with PFN-Br interfacial layer showed a high IPCE response over 64% from 400 nm to 750 nm, with a maximum value of 76% at 550 nm. Compared with the inverted PSCs without PFN-Br interfacial layer, the

inverted PSCs with PFN-Br interfacial layer also exhibited a small difference in the spectrum from 490 nm to 720 nm. This difference is probably originated from different internal optical electric field distributions owing to complex index of refraction and layer thickness of ZnO and ZnO/PFN-Br [43,44]. As shown in Figure 3b, a large forward current was observed in the inverted PSCs with PFN-Br interfacial layer rather than that without PFN-Br interfacial layer [44,45]. This indicated the contact between PFN-Br interfacial layer and BHJ composite active layer is better than that between ZnO layer and BHJ composite active layer. Therefore, an enhanced IPCE response was observed from the inverted PSCs with PFN-Br interfacial layer. AFM was carried out to further investigation of the influence of PFN-Br interfacial layer on the device performance. Figure 5a and b compared the surface morphologies of pristine ZnO and ZnO/PFN-Br thin films. With scale of 10 mm  10 mm, the root mean square (RMS) for ZnO/PFN-Br is 2.335 nm, and RMS for ZnO is 1.375 nm. The rougher interface offers greater surface area as compared to smoother interface and thus provides

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Tapping-mode AFM images: (a) the surface of pristine ZnO thin film, RMS: 1.375 nm (10 mm  10 mm), (b) the surface of ZnO/PFN-Br thin film, RMS: 2.335 (10 mm  10 mm), (c) PBDT-DTNT:PC71BM on the top of pristine ZnO thin film, RMS: 3.353 nm (20 mm  20 mm), (d) PBDT-DTNT:PC71BM on the top of ZnO/PFN-Br thin film, RMS: 5.858 nm (20 mm  20 mm). www.sciencedirect.com

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larger contact area between PBDT-DTNT:PC71BM BHJ composite layer and ZnO electron extraction layer to interact and thus the interfacial adhesion is expected to be better [3,46]. Figure 5c and d displayed the surface morphologies of PBDT-DTNT:PC71BM on the pristine ZnO thin film and on the ZnO/PFN-Br thin film, respectively. The RMS for PBDT-DTNT:PC71BM on the pristine ZnO thin film and on the PFN-Br/ZnO thin film are 3.353 nm and 5.858 nm, respectively. Different roughness surfaces implied that the interfacial adhesion between MoO3 hole extraction layer and PBDTDTNT:PC71BM layer are different. The rougher surface, the better interfacial adhesion is prospected [3,46]. Consequently, a better charge transport is expected from a good interfacial adhesion between MoO3 hole extraction layer and PBDT-DTNT:PC71BM BHJ layer.

Conclusions Through a conventional active-layer processing methodology, efficient BHJ PSCs with the inverted device structure based on PBDT-DTNT:PC71BM composites have been fabricated. Power conversion efficiency of 8.4% under AM 1.5 G illumination was achieved from the inverted PSCs by using a PFN-Br interfacial layer to reengineer ZnO electron extraction layer. Without PFNBr interfacial layer, the inverted PSCs only showed an efficiency of 6.1% under the same condition. The overall enhanced short-circuit current density, open-circuit voltage, fill factor and corresponding high efficiency in the inverted PSCs with a thin conjugated polyelectrolyte interfacial layer, were attributed to the good contact between ZnO electron extraction layer and PBDTDTNT:PC71BM BHJ active layer, good interfacial adhesion between the electron extraction layer and PBDT-DTNT:PC71BM BHJ active layer, and enhanced charge transport via suppressed bimolecular recombination. These results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

Acknowledgements The author thanks the University of Akron and 3M Company for financial support.

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Current Opinion in Chemical Engineering 2013, 2:125–131