Stamping Transfer of a Quantum Dot Interlayer for Organic

Article pubs.acs.org/Langmuir

Stamping Transfer of a Quantum Dot Interlayer for Organic Photovoltaic Cells Ji Hye Jeon,† Dong Hwan Wang,†,∥ Hyunmin Park,⊥ Jong Hyeok Park,*,‡ and O Ok Park*,†,§ †

Department of Chemical and Biomolecular Engineering (BK21 Graduate Program), Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ School of Chemical Engineering and SAINT, Sungkyunkwan University, Suwon 440-746, Republic of Korea § Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 50-1, Sang-ri, Hyeonpung-myeon, Dalseong-gun, Daegu 711-873, Republic of Korea S Supporting Information *

ABSTRACT: An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode.

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INTRODUCTION Organic solar cells have been attracting much interest as a nextgeneration energy conversion system due to their efficiency, ease of fabrication, and flexibility.1−3 Recently, researchers attempted to fabricate organic solar cells with easy and simple processes such as printing techniques.4−6 Moreover, one of the most important research aspects of organic solar cells is to develop new processing methods and environmentally friendly materials while the cells’ high efficiency is maintained.7−9 Conjugated polymers and fullerene derivatives have thus far been adopted as electron donor and acceptor materials, respectively, to make bulk heterojunction (BHJ) type organic solar cells, where an active layer can be prepared from a mixture of these two substances.10−12 The resultant solar energy conversion efficiency of these BHJ devices has reached as high as 8%.13,14 The overall power conversion efficiency of organic solar cells depends on a combination of the following steps: (1) exciton generation after absorbing incident solar light, (2) dissociation of the electron−hole pairs at the p−n interface, (3) transport of electrons and holes to both electrodes, and (4) collection of charges at the electrodes.15 In efforts to optimize each step, vigorous studies on synthesizing low band gap substances, modifying electrodes, and applying heat treatments to devices have been conducted.16−18 Also, many studies have reported improved efficiency and lifetime of organic solar cells through the introduction of a solution processable interlayer between an active layer and a cathode, especially to optimize step 4.15,19 © 2012 American Chemical Society

Several solution processable interlayer materials for efficient electron extraction in organic solar cells have been reported.20−26 For instance, Heeger’s group fabricated air stable polymer solar cells in 2007 by introducing a TiOx layer onto an active layer.25 Cell performances and stability have been greatly improved owing to the introduction of unique interlayers in organic solar cells. However, in most cases, metal oxide interlayers have been coated directly on the active layers, and the solvent used (water or alcohol) can damage the active layer. On the other hand, some researchers have studied thin interlayers made from alkaline fluorides (LiF, NaF, KF) and calcium.17 The insertion of thin interlayers into solar cells enhances the cells’ efficiency and stability.27 However, these layers are thermally deposited onto the active layer in a high vacuum system, a process that is not only difficult to control but also costly. Here, we have introduced a cadmium selenide (CdSe) quantum dot (QD) interlayer prepared by a stamping transfer process instead of a direct spin-coating process on the active layer for bulk heterojunction solar cells. There have been numerous studies on polymer/QD hybrid solar cells with the application of different QD shapes, e.g., particles or rods.28 However, there have been few reports on the effects of the QD interlayer on organic solar cells. Generally, it is very difficult to prepare a bilayer structure with an active layer and a QD Received: April 11, 2012 Revised: May 18, 2012 Published: May 26, 2012 9893

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at 150 °C for 30 min. The defined active area was 9 mm2, as determined using a digital microscope. Measurement. The current−voltage characteristics (J−V) of photovoltaic cells under simulated AM 1.5 white light illumination were measured with a Hewlett-Packard 4155A semiconductor parameter analyzer (Yokohama Hewlett-Packard, Tokyo, Japan). The voltage scan rate was 20 mV/s from positive potential to negative potential. The AM 1.5 white light was produced with a solar simulator based on a filtered Xe lamp (Oriel, 91193), and its intensity was adjusted with a Si reference cell (Fraunhofer ISE, certificate no. CISE269) for 1 sunlight intensity of 100 mW/cm2. All the photovoltaic properties were evaluated in ambient air at constant temperature (25 °C). TEM images were collected on a 200 kV field emission transmission electron microscope (FE-TEM, JEOL Ltd.). Fluorescence images were observed by a confocal microscope (Carl Zeiss). The work function of the CdSe quantum dot interlayer was measured with an ultraviolet photoelectron spectroscope produced by Thermo VG Scientific (sigma probe).

interlayer through a solution process, because most solvents for QDs can dissolve and destroy the bottom layer. To solve these drawbacks, Bulovic et al. studied QD-light emitting diodes and solar cells with a QD layer transferred from a polydimethylsiloxane (PDMS) mold as an active layer.29,30 In this study, by using a novel ultraviolet (UV) curable resin-coated polycarbonate (PC) film instead of PDMS material in the printing process, we successfully spin-coated a QD layer on a UVcurable film directly without any pretreatment (such as intermediate solvent treatment or plasma treatment) rather than transfer the layer onto the active layer. By this process, we could introduce a CdSe QD interlayer onto an active layer without damaging the active layer.

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EXPERIMENTAL SECTION

QD Transfer and Device Fabrication. In this experiment, bulk heterojunction photovoltaic cells with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene:poly(styrene sulfonate) (PEDOT:PSS)/P3HT:PCBM/Al were fabricated. To prepare the devices, ITO glass was cleaned and then exposed to oxygen plasma for 10 min prior to use. A buffer layer of PEDOT:PSS (AI4083) was then spincoated to a thickness of ∼40 nm and baked at 115 °C for 15 min. An active layer composed of P3HT (Rieke Met. Inc. P200, regioregular >98%) and PCBM (Nano-C) (1:0.6 in weight ratio) was spin-coated from chlorobenzene to produce 90 nm ± 5 nm thin films. Thermal annealing of the samples was performed at 150 °C for 30 min. CdSe QDs with a diameter of ∼3 nm were purchased from QD Solution Inc. (NanodotTM-C01-550). To prepare the QD interlayer on the active layer, a UV-curable PUA resin (NOA63, Norland Product Inc.) was dropped onto a SAMs-treated Si wafer. A polycarbonate (PC) film was then attached to the resin and then rubbed by a roller under uniform pressure. The curing process was conducted with a UV-lamp (400 W) for 3 min and the film was subsequently separated from the Si wafer.31 A 30 μL portion of the CdSe QD solution with various concentrations (50, 25, 12.5, and 6.25 mg/mL) was then dropped onto the PUA/PC film (2.5 cm × 2.5 cm) and spin-casted at 2500 rpm for 30 s. The QD layer was thermally treated using a hot plate at 50 °C for 1 min to remove the residual solvent. After fabricating the CdSe layer onto the PUA/PC film, the film was placed on an active layer and pressed with a cotton swab on the hot plate while the ambient temperature was maintained at 50 °C to transfer the film onto the active layer. (Figure 1 shows a schematic diagram of the process.) Note that there were no significant changes in the thickness of the active layer before and after the transfer, as determined from α-step measurements. The metal electrode is comprised of Al (Aldrich 99.999%) and was thermally deposited using a thermal evaporator to a thickness of 100 nm. Both processes were performed under a vacuum of 3 × 10−6 Torr. After the deposition, thermal annealing of the samples was performed

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RESULTS AND DISCUSSION Figure 1a shows a schematic diagram of the device structure with the transferred QD interlayer between the active layer and metal cathode. A schematic diagram of the transfer process of the QD interlayer is shown in Figure 1b. In this study, a PUAcoated PC film with good solvent resistance was chosen as a mother substrate to transfer the QD interlayer onto the active layer.31 In general, QDs with uniform size are synthesized with the help of a surfactant, where the QD surface has organophilic characteristics. This means that the synthesized QDs should be dispersed in organic solvents (such as toluene or chloroform). Hence, this is one of the obstacles to directly applying the QD interlayer on an active layer, as the organic solvent for dispersing QDs can dissolve and destroy the active layer. A method to transfer the QD layer onto the active layer without damaging the latter was thus designed. Although the organic layer transfer has been implemented using a PDMS mold, the PDMS can also be damaged by organic solvent during the spincoating process, thereby impeding the formation of uniform film thickness.32 The QD layer was therefore transferred by using PUA in order to produce a QD interlayer in the form of a monolayer on top of the active layer. The TEM technique was used to verify the film formation of the QD interlayer onto the active layer. First, a P3HT:PCBM film was coated onto a NaCl substrate instead of using ITO glass, followed by transfer of the QD interlayer onto the active layer, as described in the Experimental Section. The NaCl substrate with the active layer/QD interlayer was then immersed in water to dissolve the NaCl substrate. The floating film was only taken for TEM observation. Figure 2 shows TEM images of the QD/active layer as a function of the QD thickness. Because the QD interlayer is thinner than the particle size of the QDs, we denote the thickness according to the concentration of the used QD solution. Figure 2a−d represents the QD morphologies prepared from the solutions with concentrations of 6.25, 12.5, 25, and 50 mg/mL, respectively. (We also fabricated devices prepared from a higher concentration of CdSe than 50 mg/mL. See more information in Figure S1 of the Supporting Information.) According to the order of the concentrations of QD solution, the coverable area by QDs on the active layer increased gradually and the QD layers from the 50 mg/mL solution show compactly covered QDs. These experimental conditions are comparable to previously reported information, where spin-coating was

Figure 1. (a) Device structure of organic solar cells with CdSe quantum dot interlayer. (b) Schematic diagram of the CdSe QD layer transfer technique. 9894

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Figure 4, Figure S1 (Supporting Information), and Table 1 show photocurrent−voltage (J−V) curves and cell performance

Figure 2. TEM images of the transferred CdSe quantum dots on P3HT:PCBM films with different concentrations: (a) 6.25 mg/mL, (b) 12.5 mg/mL, (c) 25 mg/mL, and (d) 50 mg/mL.

Figure 4. Illumination J−V curves of the photovoltaic cells with transferred CdSe QD interlayer.

utilized to prepare a monolayer of QDs with a concentration of 20 mg/mL.33 As can be seen in Figure 2, the layer was wellcovered without any clefts in the QDs based on TEM images, especially in cases c and d. Generally, TEM images can only represent a very small area. To verify whether the coverage of QDs on the top of the active layer is uniform, confocal microscopy, as presented in Figure 3, was also carried out. Although individual QD images could not be obtained due to the amplification of light, it was verified that the QDs were well-covered, ranging from hundreds of μm2 to 1 mm2, using fluorescence images. In this study, the CdSe QDs have ∼3 nm diameter, and thereby the maximum emission and absorption were observed at 550 nm and 535 nm, respectively. According to the TEM observations, with increasing concentration of the QD solutions, brighter emission from the QDs was observed, reflecting complete coverage of the QDs onto the active layer.

Table 1. Cell Performances of Organic Solar Cells with Different Concentrations of QD Interlayers reference CdSe, 50 mg/mL CdSe, 25 mg/mL CdSe, 12.5 mg/mL CdSe, 6.25 mg/mL

Voc

Jsc

FF

efficiency

0.57 0.63 0.62 0.61 0.59

10.4 9.9 9.9 9.9 9.9

0.40 0.49 0.47 0.45 0.44

2.39 3.08 2.88 2.72 2.66

parameters of the organic solar cells that incorporate QD interlayers with different thickness, respectively. To confirm the effectiveness of the QD interlayer, a reference device without an interlayer was also fabricated. In the case of the reference device, open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF) were determined as 0.57 V, 10.4 mA/cm2, and

Figure 3. Laser scanning confocal analysis of the CdSe QD interlayer on the P3HT:PCBM films. Images magnified by a factor of 100 with different concentrations: (a) 50 mg/mL, (b) 25 mg/mL, (c) 12.5 mg/mL, and (d) 6.25 mg/mL. Images magnified by a factor of 700 with different concentrations: (e) 50 mg/mL, (f) 25 mg/mL, (g) 12.5 mg/mL, and (h) 6.25 mg/mL. 9895

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Figure 5. (a) UPS spectra of a CdSe QD layer. (b) Energy diagram of organic solar cells using CdSe quantum dots as an interlayer with a transfer technique.

HOMO level were determined according to the following equations:38

0.40, respectively, and the power conversion efficiency was 2.39%. In the case of the devices with the transferred QD interlayer prepared from the lowest QD concentration of 6.25 mg/mL, Voc, Jsc, and FF were 0.59 V, 9.9 mA/cm2, and 0.44, respectively, and the power conversion efficiency was 2.66%. Increasing the concentration further, FF and Voc gradually improved and reached 0.49 and 0.63, respectively. In the case of the device with a QD interlayer prepared from a solution of 50 mg/mL concentration, Voc, Jsc, and FF were 0.63 V, 9.9 mA/ cm2, and 0.49, respectively, and the power conversion efficiency was 3.08%. A trend of increased Voc and FF in organic solar cells after application of a rubbing process has been reported in previous studies.34,35 The incident pressure during the rubbing process improves the contact interface between ITO glass and PEDOT:PSS and/or PEDOT:PSS and the active layer, which might increase Voc and FF. The factors influencing Voc are still in dispute. Generally, it is believed that the difference in potential between the lowest unoccupied molecular orbital (LUMO) of PCBM and the highest occupied molecular orbital (HOMO) of P3HT affects Voc when the contact is ohmic. However, when the contact is nonohmic, the work function of two electrodes is believed to affect Voc. The improved contact between ITO and the active layer from the rubbing process may provide the contact with a more ohmic condition. That is one of possible reasons why all samples with the CdSe interlayer showed an enhanced Voc. However, it is notable that Voc increases linearly with the thickness of CdSe interlayer, as shown in Table 1, up to 0.63 V for a solution concentration of 50 mg/mL. This behavior is coincident with the results from other interlayers.22,36,37 The FF of the cells was also enhanced as the thickness of the CdSe interlayer was increased up to 50 mg/mL CdSe. The FF is generally determined by the series and shunt resistance of the cells. The CdSe interlayer between the active layer and cathode can affect the series resistance of the cells. Interestingly, the cells with CdSe interlayers showed lower Jsc values compared with the reference cell. This is presumably due to a decrease of the root-mean-square (rms) value of the active layer after the rubbing process, which would lead to a decrease of the contact area between the active layer and metal cathode.31 Finally, the HOMO and LUMO energy levels of the CdSe interlayer were measured using ultraviolet photoelectron spectroscopy (UPS) and are shown in Figure 5a. From the UPS spectra, the CdSe QD interlayer work function and

φ = hν − |Ecutoff − Ef | ,

E LUMO = E HOMO − Eg

Where φ is the work function, hν is the incident photon energy, Ecutoff is the secondary electron cutoff position, Ef is the Fermi level, ELUMO is the onset energy of the LUMO level, EHOMO is the energy of the HOMO level, and Eg is the band gap.39,40 The light source is a He|discharge of hν = 21.2 eV. If Ef = 0, the work function (WF) was determined from the secondary electron cutoff. Therefore, the WF and HOMO and LUMO levels of the CdSe QD used in this study were expected to be −4.7, −6.7, and −4.4 eV, respectively. Figure 5b shows an energy diagram of the solar cells containing the QD interlayer. From the energy diagram, it is anticipated that the CdSe interlayer acts as an effective hole-blocking layer as well as an electron-transporting layer. This interlayer can also prevent direct contact between Al and P3HT in the active layer, which may decrease the possibility of charge recombination at the active layer/cathode interface.41

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CONCLUSIONS

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ASSOCIATED CONTENT

A quantum dot interlayer was successfully fabricated on top of an active layer using a stamping transfer method, which does not damage the active layer. The amount of QDs in the interlayer was effectively controlled by adjusting the concentration of the CdSe solution. It was verified that the power conversion efficiency increased as the concentration of QDs was increased. This is attributed to more uniform coverage of QDs onto the active layer. Because of the roles of QDs as an electron-transporting layer and hole-blocking layer, substantially improved cell performance was observed. This study provides new directions for using hydrophobic materials that should be processed by an organic solvent as an effective interlayer in conventional organic solar cells.

S Supporting Information *

Supplementary figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org. 9896

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (O.O.P.), [email protected] (J.H.P.). Present Addresses ∥

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 931065090. ⊥ Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 50−1, Sang-ri, Hyeonpung-myeon, Dalseong-gun, Daegu 711−873, Republic of Korea. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) [No. 20120000821, 20110023215, 20110027677, NCRC program (2011-0006268)] and a WCU grant from MEST (R32-2008-000-10142-0).

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