MoO3 Anode Bilayer Buffer Layers for Improved Performance in

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PTFE/MoO3 Anode bi-layer Buffer Layers for Improved Performance in PCDTBT:PC71BM Blend Organic Solar Cells Panpan Zhang, Xu Xu, Yang Dang, Shuai Huang, Xin Chen, Bonan Kang, and S. Ravi P Silva ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01252 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016

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ACS Sustainable Chemistry & Engineering

PTFE/MoO3 Anode bi-layer Buffer Layers for Improved Performance in PCDTBT:PC71BM Blend Organic Solar Cells Panpan Zhang†, Xu Xu†, Yang Dang†, Shuai Huang†, Xin Chen†, Bonan Kang*†, S. R. P. Silva‡ †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China [email protected]

‡

Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom

KEYWORDS: organic solar cells, bi-layer anode buffer layer, bulk heterojunction solar cells, work function, charge transport

ABSTRACT: This paper discusses the effects of an anode bi-layer buffer layer on poly[N-9’’hepta-decanyl-2,7-carbazolealt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-ben-zothiadiazole)]:[6,6]-phenylC71-butyric acid methyl ester (PCDTBT:PC71BM) based organic solar cells (OSCs) by thermally evaporating poly(tetrafluoroethylene) (PTFE) and molybdenum trioxide (MoO3) successively between indium-tin-oxide (ITO) and the active layer. The PTFE/MoO3 bi-layer forms an interfacial dipole that increases the surface work function of the ITO anode, which

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contributes to the extraction of holes and the suppression of carrier recombination at the interface.

The

replacement

of

the

conventional

ethylenedioxythiophene):poly-(styrenesulfonate)

hygroscopic

(PEDOT:PSS)

and with

acidic

poly(3,4-

PTFE/MoO3

is

advantageous for efficient charge transport and reduction of the fabrication costs. Significant improvement in terms of the fill factor and power conversion efficiency (PCE) is obtained compared to the reference devices. The improvement of device performance in PTFE/MoO3 bilayer structure results from its combined effects of tuning work function ability and electronblocking capacity of PTFE and excellent hole-extracting capability of MoO3.

INTRODUCTON In recent years, Bulk heterojunction (BHJ) organic solar cells have attracted attention due to their low-cost, light-weight, mechanical flexibility, solution processability etc.1-10 Although the reported efficiency of OSCs has been steadily increasing to over 10% in the last few years11-19, further effort is needed to increase the PCE and device stability in order to enable viable commercialization that needs to consider scale up and long term stability too13,20-23. An important element in determining the performance of OSCs is the efficient charge transport in the active layer, which depends on good interfacial properties between the active layer and electrodes.24,25 However, direct contact between the active layer and the deposited electrodes have led to quenching of excitons.26 In order to obtain efficient charge transport, anode buffer layers (ABL) that are either high work function metal oxides, such as MoO3 or the conducting polymer PEDOT:PSS is normally used in OSCs.27-32 Although PEDOT:PSS exhibits many advantages in thermal stability, film formability etc.,33-36 the hygroscopic and acidic nature of PEDOT:PSS also raises some issues in terms of corrosion of the OSCs anodes (e.g., indium tin oxide (ITO)) and thus lower overall stability37-39. As a result, the device performance is degraded. Promising


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alternatives and currently heavily studied substitutes for PEDOT:PSS derive from transition metal oxides, such as MoO3.28 For anode buffer layers, there is another important characteristic that blocking electrons from the acceptor material in order to avoid undesirable recombination at the interface. However, a bare MoO3 layer has been demonstrated not to be an efficient electron blocking layer.40 Therefore the introduction of an electron blocking layer between ITO and MoO3 seems to be a practical way. Based on the above, in this study, a bi-layer anode buffer layer consisting of PTFE and MoO3 is presented to substitute for the PEDOT:PSS and the bare MoO3. Our goal is to study the effect of bi-layer structure on device performance and to prepare efficient OSCs. Fluorocarbon based materials such as CFx and PTFE has been demonstrated to be effective for improvement of organic light-emitting diodes (OLEDs) and OSCs.24,41-44 Here, we use PTFE as one of the materials of the bi-layer anode. PTFE has a number of advantageous features, such as chemical and thermal stability, light weight, moisture resistance, flexible, low cost and so on.24,45,46 Furthermore, as described in our previous papers, the fluorine component will align on top of the ITO surface, which creates a dipole surface with the dipole moment directed inward the ITO.24 As a result, the energy barrier between ITO and MoO3 is reduced. Meanwhile, MoO3 has been provided with excellent hole extracting capabilities.27,47-49 One of the issues addressed in this work is therefore whether a very thin layer of PTFE can improve the MoO3 contact by preventing electrons recombining from anode and adjusting the work function of the anode without providing a detrimental pathway for electron recombination. EXPERIMENTAL SECTION Materials and solutions. The poly[N-9’’-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2thienyl-2’,1’,3’-ben-zothiadiazole)] (PCDTBT) was purchased from 1-Material(Canada), while



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the fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was supplied from American Dye Source. PCDTBT was dissolved in 1,2-dichlorobenzene to make 8 mg/ml solution, followed by blending with PC71BM in a ratio of 1:4 w/w. The blend then was stirred for about 14 h in a glovebox while heating at 60 °C. The tris(8-hydroxy quinoline) aluminum (Alq3) with the ability of transferring electron was used as cathode buffer layer. Device Fabrication. In our study, devices were fabricated with the structure of ITO/PTFE/MoO3/PCDTBT:PC71BM/Alq3/Al. The ITO coated glass substrates were cleaned prior to device fabrication by ultrasonic bath in acetone, distilled water, and isopropyl alcohol. Before device fabrication, the ITO substrates were treated with an oxygen plasma for 5 min. Then, the PTFE and MoO3 films were thermally evaporated on the top of ITO surfaces successively at a base pressure of 7.0×10-4 Pa. The thickness of the PTFE layer varied between 0.3, 0.8, 1.0, 1.5 and 2 nm at a deposition rate of about 0.05 Å/s. MoO3 films were deposited to a thickness of 5 nm at a rate of about 0.2 Å/s. The PEDOT:PSS layer of about 25 nm thickness was obtained by spin coating an aqueous solution onto ITO glass, followed by baking at 120 °C for 10 min in an ambient atmosphere. The PCDTBT:PC71BM blend was spin-cast on top of the anode buffer layers at 1500 rpm for 40 s in a glovebox, followed by annealing at 70 °C for 30 min. Finally, the devices were completed by the evaporation of the Alq3 (~1 nm) and Al (~120 nm) under a base vacuum of 7.0×10-4 Pa. The active area of the device, defined by shadow mask, was 0.05 cm2. Measurements. The thickness of PEDOT:PSS was measured using a Veeco Dektak 150 alpha step surface profiler. The layer thickness and the deposition rate of the deposited materials were controlled by calibrated thin film thickness monitor (TDM-200) during vapor deposition. The contact potential differences (CPD) were measured by the Kelvin Probe Force Microscopy


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(KPFM, SKP5050, KP Technologies), which was based on atomic force microscopy (AFM). Potential was obtained by detecting a cantilever deflection caused by an electrostatic force between a tip and a sample. This potential meant CPD between a tip and a sample, namely the work function difference between a tip and a sample. Before measurement, the equipment was calibrated using a standard Au sample. The electrode tip scanned across the sample surface in a certain area and we tested 100 points per sample. The current density-voltage (J-V) characteristics were measured by a Keithley 2400 source meter under standard 1 sun, AM 1.5 G test conditions using a solar simulator. The incident photon-to-current efficiency (IPCE) spectra were carried out by Crowntech QTest Station 1000 AD. The visible absorption and transmittance spectra were obtained by means of ultraviolet/visible spectrometer (UV1700, Shimadzu). All the measurements were carried out at room temperature in an ambient atmosphere. RESULTS AND DISCUSSION The schematic structure and energy level diagrams of the completed devices are shown in Figure 1. As shown, the fluorine component aligning on top of the ITO surface creates a dipole surface with the dipole moment directed inward towards the ITO24, which results in the reduced energy barrier between the ITO and MoO3 interface. To demonstrate the effect of the evaporated PTFE on the work function of ITO, KPFM has been conducted in the dark, which displays the information of topography and local work function at the nanometer scale.50 The KPFM gives the CPD between the sample materials and the tip. According to the calculating formula of

(1), we obtained the work function of the samples from the

measured CPD, as shown in Figure 2, all of which were checked for accuracy. Here, WSample is the work function of the sample, △WAu is the error value calibrated by standard Au sample and △WSample is the measured CPD of the sample. The work function of the ITO layer was estimated



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to be 4.7 eV. In contrast to the bare ITO, the ITO/PTFE layer showed increased work function of about 5.15 eV. Moreover, the work function of PTFE/ MoO3 bi-layer (5.45 eV) seemed to be higher than that of MoO3 (5.3 eV), which was better matched to the highest occupied molecular orbital (HOMO) of PCDTBT. This leads to a more conducive extraction of holes and the suppression of electron carrier recombination at the interface. As a consequence, the overall device obtained had a significantly improved performance.

Figure 1. (a) The schematic structure and (b) theoretical energy level diagrams of different materials involved in the polymer solar cells. To confirm the effect of the thickness dependence of the PTFE on the performance of the BHJ solar cells, six thicknesses of PTFE (0 nm, 0.3 nm, 0.8 nm, 1.0 nm, 1.5 nm, 2.0 nm) are used. As shown in Figure 3(a), when the thickness of PTFE is 0 nm, the device exhibits a PCE of 6.05% (VOC=0.86 V, JSC=12.16 mA/cm2, FF=57.74%). When we introduce the PTFE layer, the device performance begins to improve slightly. The devices with a combined PTFE/MoO3 ABL exhibited the best average PCE of 7.31% (VOC=0.90 V, JSC=12.93 mA/cm2, FF=63.12%) when the thickness was increased to 1.5 nm, while the devices with thicknesses of 2 nm or more showed a gradually decreasing performance. Thus, the thickness of 1.5 nm is chosen as the


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optimum thickness. This may be because after inserting the PTFE layer, a strong dipole layer is created by the negatively charged fluorine rich PTFE layer that has modified the surface and increased the effective work function of the ITO. In addition, by inserting the PTFE layer, efficient hole extraction can reduce the hole accumulation at the ITO/organic interface and as a result, the JSC is improved. Since PTFE is an insulating material, with an extremely high resistivity of 1018 Ω/cm and a large ionization potential of 9.8 eV, it is reasonable to expect that the shortcircuit current densities of OSCs decrease with increasing thickness of the PTFE layer.24 Therefore, there is an optimum thickness of PTFE. At this optimum thickness of the ABL we notice there is an improvement in both the VOC and the JSC, which we believe can be replicated in any generic device with the correctly designed anodes. The higher VOC is expected due to the effective down-shifting of the combined PTFE/MoO3/PCDTBT HOMO level, which will effectively adjust the open circuit voltage between this and the PCBM LUMO level. The higher JSC current we believe could have arisen due to less recombination of holes generated via exciton dissociation with electrons from the ITO. The optimization of the balance of charge transport to excitonic dissociation can also be examined by studying the effects of bi-layer devices versus BHJ OSC.51



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Figure 2. The work function of ITO, ITO/PTFE, ITO/MoO3, ITO/PEDOT:PSS and ITO/PTFE/MoO3 calculated according to the data measured by Kelvin Probe Force Microscopy in the dark. The J-V characteristics of fabricated devices under illumination without ABL and with ABLs of bare MoO3, PEDOT:PSS, PTFE and PTFE/MoO3 bi-layer are shown in Figure 3(b), and the photovoltaic parameters are summarized in Table 1. The device with PTFE/MoO3 is shown to have the best average PCE of 7.31% as noted above. The reference device without ABL shows the lowest PCE of 3.21% with VOC=0.63 V, JSC=10.77 mA/cm2, FF=47.50% while the reference devices with only MoO3 or PEDOT:PSS as the hole transport layer show respectively VOC=0.86 V, JSC=12.16 mA/cm2, FF=57.74%, PCE=6.05% and VOC=0.88 V, JSC=11.31 mA/cm2, FF=58.00%, PCE=5.79%. The device with only PTFE is also included for comparison. As we


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can see, although its VOC is much higher, increasing from 0.63 V to 0.86 V, its JSC is similar to that of device without ABL. As we know, gaining a high VOC not only requires a deep HOMO level of donor but also needs a high work function of anode. Compared to the device without ABL, the VOC of devices with ABLs are much higher, especially the device with bi-layer ABL. That maybe because the high work function of bi-layer ABL reduces the hole injection barrier, leading to an optimum band matching at the interface of anode/donor. As a result, both the JSC and the VOC of device with bi-layer structure are improved significantly, resulting in a remarkable improvement of FF and PCE.



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Figure 3. Current density versus voltage characteristics under illumination for devices (a) with different thickness of PTFE in the system of PTFE/MoO3 bi-layer and (b) without ABL and with different ABLs. (c) Dark current density versus voltage. Figure 3(c) shows the dark current versus voltage characteristics of devices without ABL and with different ABLs. All the devices with ABLs show smaller dark current at reverse bias, which indicates a lager shunt resistances (Rsh) that helps to suppress the leakage current. In addition, a better rectifying effect is achieved for devices with bi-layer structure compared with reference devices. The dark current in the bi-layer structure is about one order of magnitude lower than that in the device with bare MoO3, proving that the bi-layer assists in increasing charge transport and blocking electrons from anode.

Table 1. Device characteristics of PCDTBT:PC71BM BHJ without ABL, and with ABLs of PEDOT:PSS, PTFE, MoO3 and PTFE/MoO3.

Different

JSC

VOC

FF

PCE

Rs

Rsh

ABL

(mA/cm2)

(V)

(%)

(%)

(Ω·cm2)

(Ω·cm2)

w/o

10.77±0.02

0.63±0.01

47.50±0.03

3.21±0.04

5.99

330.02

PEDOT:PSS

11.31±0.04

0.88±0.01

58.00±0.03

5.79±0.02

9.23

505.74

PTFE

10.93±0.03

0.86±0.01

57.84±0.02

5.42±0.02

6.33

573.17

MoO3

12.16±0.02

0.86±0.01

57.74±0.01

6.05±0.01

9.42

935.85

PTFE/MoO3

12.93±0.02

0.90±0.01

63.12±0.02

7.31±0.03

6.96

1016.83

Series resistances (Rs) and Rsh of the devices without ABL and with ABLs of bare MoO3, PEDOT:PSS, PTFE and PTFE/MoO3 bi-layer are also listed in Table 1. We can find that Rs of



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the devices with PTFE/MoO3 decreases when compared to devices with bare MoO3 and PEDOT:PSS, which contributes to the improvement of JSC. At the same time, Rsh of devices with ABLs are significantly increased, especially Rsh of devices with PTFE/MoO3 rising to 1016.8 Ω ·cm2, which is consistent with the dark current in Figure 3(c). The decrease of RS and the increase of Rsh are all advantageous to the improvement of FF in spite of a small degree.23,52,53 Figure 4 shows the IPCE spectra of all device architectures. The maximum IPCE for the devices with PTFE/MoO3 reached 68.5%, while the IPCE is more than 60% in the range of 350600 nm, indicating an efficient photon-to-electron conversion, which is in agreement with the higher JSC obtained above. In addition, the IPCE spectra for the devices with PTFE/MoO3 bilayer have a blue shift compared with the devices with only the PEDOT:PSS or without the HTL. This is expected with the higher HOMO-LUMO gap we have created in our devices with the shifting of the ‘effective’ HOMO level for carriers with the ABL. We also find that the IPCE for the devices with PEDOT:PSS is higher after 430 nm. However, its current doesn’t increase accordingly. That may be due to the negative influence on the anode. All of these are consistent with the results of J-V characteristics in Figure 3(b). The interfacial charge recombination seems to be restrained in the devices with PTFE/MoO3 buffer layer since the high IPCE and FF have been achieved.


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Figure 4. IPCE (in %) of devices based on PCDTBT:PC71BM without ABL and with bare MoO3, PEDOT:PSS or PTFE/MoO3 bi-layer as ABL. To examine the surface morphologies of the PTFE layer and PTFE/MoO3 bi-layer, the atomic force microscopy (AFM) and transmission electron microscopy (TEM) were carried out, as shown in Figure 5. The root mean square (RMS) roughness of PTFE layers and PTFE/MoO3 bilayers were all 1.34 nm, which indicates that the PTFE interfacial layer did not increase the roughness and allows for a flat active layer. As we also note, when MoO3 was evaporated, it formed uniform and continuous particles attached to the PTFE layer. This makes it possible to obtain a good phase separation, which is critical for the efficient exciton separation and charge transport within the active layers.



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Figure 5. (a) and (b) display the AFM images and (c) and (d) show the TEM images of the PTFE layer and the PTFE/MoO3 bi-layer. Figure 6(a) is the visible absorption spectra of PCDTBT:PC71BM film without the ABL and with PEDOT:PSS, MoO3 or PTFE/MoO3 bi-layer as ABL. It shows that the absorption of active layer films with PTFE/MoO3 bi-layer has slightly increased during the range 300 to 450 nm. This indicates that a thin layer of PTFE will not interfere with the light absorption because of its good light transparency which ensures the light passing through PTFE before reaching the active layer. It leads to a similar generation of photogenerated carrier in prepared samples with various ABLs. Therefore, the improvement of device performance depends mainly on efficient charge transport and suppression of carrier recombination at the interface benefiting from PTFE/MoO3 bi-layer


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ABL. Figure 6(b) is the transmittance spectra which is consistent with the data shown in Figure 6(a), and the analysis that followed.

Figure 6. The absorption spectra (a) and the transmittance spectra (b) of PCDTBT:PC71BM without ABL and with different ABLs. CONCLUSION In summary, we successfully fabricated high-performance PCDTBT:PC71BM BHJ solar cells by inserting a PTFE/MoO3 anode buffer bi-layer which plays an important role in enhancing the PCE (7.31%, 127.8% increase) with the increased JSC, VOC, and FF compared with solar cells without the anode buffer layer. Furthermore, the device with PTFE/MoO3 ABL also exhibits increased overall performance compared with devices with only MoO3 or PEDOT:PSS ABL. The improvement due to PTFE/MoO3 results from its multiple functionalities of enhanced extraction of holes, effective electron blocking and suppression of carrier recombination at the interface, and formation of an interfacial dipole with stable chemical and physical properties. The BHJ solar cell with PTFE/MoO3 bi-layer as ABL is an encouraging architecture for achieving high-performance devices, and can be used as a generic route for high performance solar cells.



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AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +86-137-5655-3011 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the Key Projects of Science and Technology Development Plan of Jilin province (20110412, 20130206019GX). SRSP acknowledges support from the Royal Society International programme. REFERENCES (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltiac Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789-1790. (2) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid Nanorod-Polymer Solar Cells. Science 2002, 295, 2425-2427. (3) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617-1622.


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For Table of Contents Use Only:

Table of Contents Graphic

PTFE/MoO3 Anode bi-layer Buffer Layers for Improved Performance in PCDTBT:PC71BM Blend Organic Solar Cells Panpan Zhang, Xu Xu, Yang Dang, Shuai Huang, Xin Chen, Bonan Kang, S. R. P. Silva Synopsis: A novel and promising device structure for improving the work function of anode has been provided, which can significantly enhance the performance in organic solar cells.


MoO3 Anode Bilayer Buffer Layers for Improved Performance in

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