Effect of Zinc Oxide Electron Transport Layers on Performance and

Jun 21, 2011 - Bulk heterojunction organic photovoltaic devices with a low work function cathode often suffer from rapidly degraded performance when e...
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Effect of Zinc Oxide Electron Transport Layers on Performance and Shelf Life of Organic Bulk Heterojunction Devices Summer R. Ferreira,* Ping Lu, Yun-Ju Lee,† Robert J. Davis,‡ and Julia W. P. Hsu*,† Sandia National Laboratories, PO Box 5800, MS-1415, Albuquerque, New Mexico 87185, United States

bS Supporting Information ABSTRACT: Bulk heterojunction organic photovoltaic devices with a low work function cathode often suffer from rapidly degraded performance when exposed to ambient environment. Here, we report improved performance in blended PCBM:P3HT photovoltaic devices with the addition of a combination of zinc oxide nanoparticles infiltrated with a sol gel zinc oxide thin film as the electron transport layer. Furthermore, the zinc oxide electron transport layer dramatically improves device shelf life for unencapsulated devices tested and stored under ambient conditions. Transmission electron microscopy results help shed light on the origin behind the improved device stability.

’ INTRODUCTION Bulk heterojunction (BHJ) organic photovoltaic (OPV) devices are of interest for their relative high efficiency, up to 7.4%,1 and their low cost of materials and fabrication. Conventional architecture BHJ OPV devices use low work function metals as the electron-collecting electrode and can not withstand exposure to ambient environment. They are typically made in controlled atmosphere and encapsulated to prevent rapid degradation. For OPV to be a viable technology, efforts must be made to improve the performance and stability of such devices. In these organic solar cells, the photoexcited electrons and holes generated in the BHJ active layer need to be transported to their respective electrodes. It has been shown that the performance of OPV devices is improved by adding an electron transport layer (ETL) of a wide band semiconductor such as TiOx,2 ZnO,3 and CrOx4 that facilitates electron transport from the active layer to the cathode while blocking holes. These wide band gap ETLs have also been shown to improve performance by acting as optical spacers5,6 and oxygen barriers.7 TiOx2,7 and CrOx4 ETLs have also demonstrated significant improvement in device stability under testing, albeit under relatively short test durations of 5 12 days. However, previous work has not investigated the impact of ZnO ETLs on device stability. Here, we compare the performance and shelf life in ambient environment over 2 months between control BHJ devices of blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with aluminum (Al) electrodes and ones which use zinc oxide (ZnO) as an ETL between the blended active layer and the Al cathode. By examining the degradation of the interfaces in the device under electron beam exposure in transmission electron microscopy, we find evidence of significant increase in interface r 2011 American Chemical Society

strength with the addition of the ZnO ETL, which may explain the dramatic improvement in shelf life of these devices.

’ EXPERIMENTAL METHODS Devices are prepared on patterned indium tin oxide (ITO) substrates (R e 15 Ω/0) (Thin Film Devices, Anaheim, CA) that are sonicated in soapy water and isopropyl alcohol (ACS grade, Fisher Chemical) for 10 min each and then dried under flowing nitrogen, followed by a 20 min UV ozone treatment. An as-received Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (H.C. Starck, Clevios) layer was deposited on the ITO by spinning at 2000 rpm for 1 min followed by heating at 120 °C for 10 min in air to remove water. The control PCBM:P3HT blend device architecture, without an ETL, is shown schematically in Figure 1A in which an active layer is deposited on the PEDOT:PSS. The active layer solution is prepared with 24 mg/mL each of P3HT (Rieke Metals, electronics grade regioregular P3HT 4002-E) and PCBM (Solenne BV, C60PCBM >99%) in o-dichlorobenzene by stirring on a hot plate at 60 °C in a nitrogen (N2) glovebox overnight. The solution is allowed to cool to room temperature and filtered through a 0.2 um PTFE filter prior to spin coating. All spin coating was carried out in a nitrogen box. P3HT:PCBM films are deposited by spin coating ∼400 μL of the active layer solution at 600 rpm. The substrates are immediately placed into a covered Petri dish and covered with Al foil. Samples are left in the N2 glovebox overnight to dry and annealed in N2 for 10 min at 120 °C. Received: April 15, 2011 Revised: June 2, 2011 Published: June 21, 2011 13471

dx.doi.org/10.1021/jp203539k | J. Phys. Chem. C 2011, 115, 13471–13475

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Table 1. Device Open Circuit Voltage (Voc), Current Density (Jsc), Fill Factor (FF), Efficiency (η), and Series Resistance (Rs) ETL

Voc (mV) Jsc (mA/cm2) FF (%)

η (%)

Rs (Ω cm2)

no ETL

362 ( 3

5.86 ( 0.59 42.1 ( 1.2 0.89 ( 0.08 22.9 ( 1.5

ZnO NP

534 ( 3

6.97 ( 0.22 28.1 ( 1.0 1.05 ( 0.03 27.3 ( 1.3

ZnO NP:SG 522 ( 5

7.56 ( 0.25 40.4 ( 1.1 1.59 ( 0.02 26.7 ( 2.3

Figure 1. Schematic illustration of the device architectures for bulk heterojunction devices with (A) no electron transport layer (ETL) (control devices), (B) a ZnO nanoparticle ETL (ZnO NP ETL devices), and (C) both a ZnO nanoparticle and ZnO sol gel ETL (ZnO NP:SG ETL devices).

Devices containing either a ZnO nanoparticle (NP) ETL or a ZnO NP layer infiltrated with a ZnO sol gel thin film ETL are illustrated schematically in Figure 1, B and C, respectively, in which the ZnO is deposited on the active layer. ZnO nanoparticles (NPs) were synthesized in methanol (Fisher Chemical, ACS grade) following a published procedure8 and rinsed three times in methanol to a final concentration of 350 mg/mL. The NPs were then diluted to a final concentration of 7 mg/mL in butanol (ACS certified, Fisher Chemical). The ZnO NP ETL (Figure 1B, C) is prepared by spinning four layers of ZnO NP from butanol solution at 1000 rpm in the N2 box. The sol gel zinc acetate infiltrating layer (Figure 1C) is prepared by spin coating a 17.5 mM zinc acetate solution in ethanol on top of the ZnO NP layer at 1000 rpm and annealing on a hot plate at 150 °C for 10 min in N2 to form the ZnO sol gel. The devices are allowed to cool before they are removed to air. Finally, 100 nm aluminum is thermally evaporated on top of the devices to form the electron-collecting electrodes. Device performance was measured under simulated one-sun illumination, using a KG5 filtered tungsten halogen lamp adjusted with an NREL calibrated Si photodiode to 100 mW/ cm2. A UV filter was not used. Each of the six individual devices per sample was tested upon initial exposure to light, and the results were averaged to get the initial current voltage characteristics. Subsequently, the devices were stored in the dark and tested periodically for 78 days. Devices were stored in the dark in air and tested under ambient conditions. Temperature and humidity were not controlled during storage or testing, but remained between approximately 20 and 25 °C and 10 and 40% humidity. The device cross sections were prepared by focused ion beam (FIB) and were imaged using a Philips CM30 transmission electron microscope (TEM) at an operating voltage of 300 kV.

’ RESULTS AND DISCUSSION The device performance is compared between the three types of devices: the control devices without an ETL, and those with a ZnO NP ETL and with a ZnO NPs infiltrated with a sol gel solution ETL (ZnO NP:SG ETL). Devices with only a sol gel ETL are not presented because the sol gel solution, with ethanol as the solvent, does not form a uniform film on the active layer when directly deposited. Without an ETL, the control device has an efficiency of 0.89%. Upon the addition of a ZnO NP ETL, the open circuit voltage (Voc) is increased by almost 50%, from 362 mV in the control to 534 mV. A similar increase in Voc is observed in the device with a ZnO NP:SG ETL to 522 mV (see Table 1). The current density (Jsc) in the devices

Figure 2. Current density voltage response of each of the three device architectures measured under a KG5 filtered tungsten halogen lamp at 100 mW/cm2. Inset: current density voltage taken in the dark for each device. The legend describes the electron transport layer (ETL) used in the device: “No ETL” indicating the control device (black), “ZnO NP” for a ZnO nanoparticle ETL (blue), and “ZnO NP:SG” for devices with a ZnO nanoparticle layer infiltrated with sol gel ZnO (red).

also increased with the inclusion of ZnO ETLs. With the ZnO NP layer the Jsc is increased by over 18% and by almost 30% in the NP:SG ETL device. Because the ZnO NP ETL device exhibits the characteristic “S-shape” current voltage characteristics, whose origin could be due to defects at interfaces9 or in the ZnO NPs,10 it suffers from poor fill factor (FF) (Table 1 and Figure 2), resulting in a relatively small increase in efficiency of 18% between the control device and the ZnO NP ETL device, to 1.05% overall efficiency. On the other hand, the devices with the ZnO NP:SG ETLs do not suffer from poor fill factor, and therefore the improved Voc and Jsc lead to an increase in efficiency of over 75% from the control to 1.59%. The series resistances (Rs) of all three devices are similar; however, the Rs increases slightly with the addition of ZnO ETLs, from 23 to 27 Ω cm2 because neither the nanoparticles nor the sol gel layer is highly conducting. Note that all our devices have efficiencies significantly below that of the best reported P3HT:PCBM BHJ devices. Without an ETL layer, efficiencies of above 4% have been reported.11 The main difference lies, we believe, in that those high-efficiency devices were prepared and tested in an oxygen- and humiditycontrolled glovebox, and use a Ca/Al anode to optimize performance. By contrast, in this work the active layer was spun in an enclosed box with flowing nitrogen, containing moderate oxygen and humidity. Second, it has been shown that annealing after the deposition of the Al electrodes significantly improves performance,12,13 which was not done in this study. Our device performance is comparable to those without post-annealing in 13472

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Figure 3. Shelf-life study: normalized device performance as a function of aging in air for 78 days. Device open circuit voltage (Voc) (upper left), current density (Jsc) (upper right), fill factor (lower left), and device efficiency (lower right), with error bars determined from the standard deviation of six devices.

refs 12 and 13. Nonetheless, since all our devices were fabricated and tested under the same conditions, the conclusions drawn on the effect of the ZnO ETL are valid. The improvement in device performance with the use of a ZnO layer between the active layer and electrode may be attributed to the ZnO acting to both collect electrons and block holes. In addition to the improved device efficiency provided by ZnO ETLs, there is also a significant improvement in the device stability with the addition of the ETLs. We tested these unencapsulated devices over 78 days with testing and storage carried out in air under ambient conditions. Devices were stored in the dark in between measurements. The shelf life results are plotted in Figure 3. We observe that the control device without ETL suffers very rapid degradation as reported in previous publications.2,4,7,14,15 Within 1 day the control device has degraded to 0% efficiency with a complete loss of Jsc. In contrast, the device with a ZnO NP ETL has a Voc and fill factor that are high over the full time tested, remaining at 70% and 67%, respectively, of the initial values after 78 days of aging in air. However, the Jsc drops rather significantly within the first several days to 65% of its initial value, and further degrades steadily with time to 11% of the initial current density at day 78. Although the ZnO NP layer results in greatly extended device stability over the control devices, the efficiency eventually drops to less than 10% of the initial efficiency by the end of the 78 days of testing. There is a marked further improvement in device stability with the use of a combination of ZnO NPs and a solution-based zinc acetate layer spun on top of the NPs, which is then annealed. The Voc is slightly above the initial value, while the Jsc remains over 75% of the initial value after 78 days. The fill factor degrades most of the three parameters to 67% of the initial fill factor, similar to the ZnO NP device. As a result of the improved stability in the Voc and Jsc over the ETL with ZnO NPs alone, even after 78 days the performance remains at nearly 60% of the initial efficiency for

the ZnO NP:SG ETL device. Five different sets of six devices with and without ETLs that we tested all exhibit qualitatively similar behavior: the efficiency of the devices without an ETL drop to zero in less than 2 days while devices with ZnO ETLs show photovoltaic responses for weeks, and the efficiency remains most stable in devices with a combined ZnO NP: SG ETL. In order to understand the improved device stability observed in the devices with ZnO ETLs over the control device, we image a cross section of each type of devices using TEM. The cross sections were prepared by FIB and were imaged using a Philips CM30 TEM at an operating voltage of 300 kV. The devices were exposed to air for 55 days before being made as TEM specimens. Figure 4 shows representative cross sections of three types of devices. The upper row shows images taken immediately after being loaded in the TEM, and the lower row shows the same sample after given amounts of time under electron beam exposure in the TEM. In the initial image of the control device, one can see some small voids between the active layer and the Al electrode as identified by arrows in Figure 4A. It is well documented that Al readily reacts with organics.16,17 Two separate groups have reported the presence of a ∼4 nm thick amorphous layer containing Al, O, and C at the interface of P3HT:PCBM and the Al metal.12,13 A similar interfacial layer, i.e., with darker contrast and comparable thickness, is observed in all our TEM images when Al is in direct contact with the P3HT: PCBM layer. Since our TEM samples have been exposed to air for over 55 days, the voids are probably formed during this period. From Figure 4A, these voids appear to locate at the interfacial layer. After 30 min of electron beam exposure, the voids grow in number and size, again denoted by arrows in Figure 4B, which shows an overlapping region (see caption). While these voids are the result of electron beam damage, they indicate the relative bonding strength of different interfaces in 13473

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Figure 4. Transmission electron microscope images of device cross sections for each of the three devices taken initially, and after a given electron beam exposure time. Arrows point to voids present in the device. Overlapping regions in a control device (A) before and (B) after 30 min of electron beam exposure, where the far right void of (A) is the far left void shown in (B). Overlapping regions in a device with a ZnO NP layer (C) before and (D) after 15 min of beam exposure, where the left void denoted in (C) is the rightmost arrow denoted in (D). Finally a device with a ZnO NP:SG layer is shown in (E) before and (F) after 2 h of electron beam exposure.

the device. Since the entire specimen was exposed to the same dosage of the electron beam but only the Al-BHJ interface develops these voids, they indicate that Al-BHJ interface is the most weakly bonded interface in the control device. In addition to the weak interface, Al has been shown to allow water and oxygen to transport though the Al grains and does not provide a good diffusion barrier.14 Similar to the control, small voids are present in the initial image for the device with a 10 20 nm thick ZnO NP ETL. However, these voids are smaller in number and size, compared to the control device, and occur between the ZnO ETL and the active layer (Figure 4C). In addition to facilitating electron transport, the ZnO NP ETL separates the active layer from the Al and prevents reaction between the Al and polymer, similar to the role of LiF.18 As with the control device, the number and size of voids increases with 15 min of electron beam exposure (Figure 4D). Again, these voids suggest that the ZnO BHJ active layer may form a weakly bonded interface. The ZnO NP ETL may provide some protection of the active layer from oxygen compared to the control devices. However, because of the weak interface and the porous nature of the NP layer, this

ZnO NP ETL may not provide a good oxygen barrier between the environment and the active layer over long times. By contrast, the device with the ZnO NP:SG ETL does not contain any observable voids (Figure 4E), nor do such voids develop (Figure 4F) even with 2 h of electron beam exposure, which is over 4 times as long as those explored for the control and ZnO NP ETL devices. It is also observed that the combined NP: SG layer is approximately twice as thick as the ZnO NP ETL, ranging between 25 and 50 nm thick. This provides evidence that the combination of the ZnO NP, infiltrated with the ZnO SG does provide a stronger interface between the ETL and the active layer, leading to the lack of voids caused by electron beam exposure. The thicker ZnO layer may act as a better oxygen barrier for the polymer layer than the thinner ZnO NP layer. In addition, the sol gel solutions most likely fill the gaps between the ZnO NP and may result in a denser ZnO layer, which is more effective in protecting the active layer and preventing the thermally deposited Al from contacting the polymer. While both increased thickness and reduced porosity can contribute to the observed improved performance and shelf life, we believe the latter is more important since the interfacial integrity under 13474

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The Journal of Physical Chemistry C electron beam exposure is qualitatively different for the two kinds of ZnO ETLs. These TEM results suggest that the high Voc and Jsc and the enhanced shelf life of the device with the ZnO NP:SG ETL are related to the more strongly bonded interfaces and thicker ZnO ETL.

’ CONCLUSIONS We have demonstrated that the addition of a ZnO ETL between the active blend layer and the Al electrode results in improved device performance and shelf life in PCBM:P3HT BHJ organic solar cells. The best results were obtained for ETL that contains additional infiltration of a ZnO sol gel solution to a ZnO NP layer. TEM results reveal a correlation between device shelf life and the existence of compromised interfaces in the structure. In the control devices, which demonstrate very rapid degradation in air, Al reacts with the active organic layer and degradation occurs at the Al BHJ interface. With the ZnO NP ETL, Al reaction with the polymer is minimized, although the adhesion between ZnO and organic is weak. In contrast, the ZnO NP:SG ETL device, which retains nearly 60% of the initial efficiency after 78 days in air, does not show any evidence of a weak interface. Ongoing efforts are being made to optimize the ZnO ETL for further improvements in device performance and stability.

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’ ASSOCIATED CONTENT

bS

Supporting Information. A detailed description of experimental methods is available. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.R.F.), [email protected] (J.W.P.H.). Present Addresses †

Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080. ‡ General Electric Global Research, Niskayuna, NY 12309.

’ ACKNOWLEDGMENT We thank Michael Rye for the preparation of TEM samples by FIB. This work was supported by the AOP PV Program through the Energy Efficiency and Renewable Energy within the Department of Energy and by Sandia’s Laboratory Directed Research and Development program. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin company, for the United States Department of Energy under contract DE-AC0494AL85000. ’ REFERENCES (1) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135. (2) Hayakawa, A.; Yoshikawa, O.; Fujieda, T.; Uehara, K.; Yoshikawa, S. Appl. Phys. Lett. 2009, 90, 163517. (3) Yip, H. L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K. Y. Adv. Mater. 2008, 20, 2376. (4) Wang, M. D.; Tang, Q.; An, J.; Xie, F. Y.; Chen, J. A.; Zheng, S. Z.; Wong, K. Y.; Miao, Q. A.; Xu, J. B. ACS Appl. Mater. 2010, 2, 2699. 13475

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