Effect of Metal-Reflection and Surface-Roughness Properties on

Dec 10, 2009 - We investigated the effect of the reflectivity of various metal cathodes on the power-conversion efficiency (PCE) of organic photovolta...
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J. Phys. Chem. C 2009, 113, 21915–21920

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Effect of Metal-Reflection and Surface-Roughness Properties on Power-Conversion Efficiency for Polymer Photovoltaic Cells Su-Hwan Lee, Dal-Ho Kim, Ji-Heon Kim, Gon-Sub Lee, and Jea-Gun Park* Nano-SOI Process Laboratory, Hanyang UniVersity, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, Republic of Korea ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: NoVember 16, 2009

We investigated the effect of the reflectivity of various metal cathodes on the power-conversion efficiency (PCE) of organic photovoltaic (OPV) cells fabricated with a P3HT:PCBM blended layer. The reflectivity of the cathodes correlated well with the short-circuit current (Jsc) influenced by absorption of visible light reflected from the cathodes; that is, a higher reflectivity of the cathodes led to a higher Jsc. In addition, we analyzed how the surface roughness of a metal cathode influences the performance on the polymer PV cells. As a result, a higher reflectivity and a lower surface roughness of the metal cathodes gave a higher PCE. In addition, it was confirmed that the interface property between the metal cathodes and organic hole/exciton blocking layer strongly influences the PCE of OPV cells. Introduction Recently, organic photovoltaic (OPV) cells have attracted attention as a renewable, sustainable source of electricity, because they are a clean energy source, have a low fabrication cost, and can be processed on a flexible substrate.1-7 In particular, many researchers have sought to improve the photovoltaic (PV) cell performances such as the power conversion efficiency (PCE).8-19 The PCE of an OPV cell is generally determined by a multitude of factors, including the light absorption in the electron-donor material, light absorption in the electron-acceptor material, transport of light-generated excitons from both layers to the donor-acceptor interface, efficiency of the hole/exciton separation into electron-hole pairs at the interface, transport of holes across the donor layer to the anode, transport of electrons across the acceptor layer to the cathode, quality of the contact between the donor layer and the anode, and quality of the contact between the acceptor layer and the cathode. Thus, many researchers have made efforts to improve the PCE of OPV cells by modifying the donor, acceptor, blocking material, and PV cell structure.16-25 However, most of researchers have not investigated the effect of the metal optical property on the PCE. Our study investigates how the reflectivity, resistance, and surface roughness of a metal cathode influence the performance on the polymer PV cells such as shortcircuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE. Experimental Methods The poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) were dissolved in chlorobenzene at a weight ratio of 2:1 and stirred on a hot plate at 50 °C for more than 72 h in a nitrogen glovebox before spincasting to form the blend layer. Here, P3HT was selected as the electron-donor material, and PCBM was used as the electronacceptor material. To fabricate the PV cells, the ITO glass was treated by a 354 nm UV lamp continuously for 60 s and was then placed for 30 s under a 200 W oxygen plasma flow. * E-mail: [email protected].

Generally, many researchers have used OPV cell structure without isolations between the active areas of light absorption, as shown in Figure 1a. In this case, the active areas of four OPV cells were fully covered by PEDOT:PSS and P3HT:PCBM without isolation of the active areas, and ITO electrode’s edge has a damage after ITO patterning process; see the crosssectional view of the OPV cells in the bottom figure of Figure 1a. Thus, we have generally obtained the PCE of ∼3.99%, shown in Figure 2a. To overcome these issues (isolation and edge damage), we recently developed a new OPV cell structure, as shown in Figure 1b. Four active areas were isolated from each other by a positive photoresistor (PR), and the PR protected the active layer from damage of ITO electrode’s edge by using lithography after ITO patterning; see the cross-sectional view of the OPV cells in the Figure 1b. Thus, we generally obtained the PCE of ∼6.53%; see Figure 2b. In addition, the inset of Figure 2b shows the external quantum efficiency (EQE) of a PV cell fabricated with a new substrate and an Al top electrode. Note that the active areas were 1.5 mm × 1.5 mm. Particularly, the PR used is photosensitive to I-line (wavelength of 365 nm), which is not influenced in the visible-light range (wavelength of 380-800 nm). A thin layer of poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS, Baytron PVPAI 4083) was spin-coated onto the ITO glass at a speed of ∼2,000 rpm for 60 s after being filtered with a 5 µm pore size PTFE syringe filter (Whatman) and then baked at 140 °C for 10 min in a nitrogen glovebox. The P3HT:PCBM blended solution was filtered by using a 5 µm pore size PTFE syringe filter. The P3HT:PCBM blended layer was then spin-cast at ∼1,000 rpm for 60 s on the PEDOT:PSS layer and was then baked at 150 °C for 10 min in a nitrogen glovebox. The 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layer was continuously deposited by thermal evaporation as a hole/exciton blocking layer on the P3HT:PCBM blended layer under a pressure of ∼5 × 10-7 Torr and evaporated until it was ∼12 nm thick. Finally, metal cathodes (Al, Ag, Au, Ni, Fe, and Ti) that were ∼80 nm thick were deposited by thermal evaporation at a pressure of ∼5 × 10-7 Torr. The evaporation rates of the BCP layer and the metal electrode were ∼0.5 Å/s and ∼5 Å/s, respectively. Figure 1c shows a schematic of the cross-sectional

10.1021/jp9072813  2009 American Chemical Society Published on Web 12/10/2009

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Figure 1. (a) Schematic of the structure of a conventional PV cell. (b) Schematic of the structure of a PV cell fabricated with various metal electrodes. (c) Energy-level diagram showing HOMO and LUMO energy levels of each component material.

Figure 2. Photocurrent-density vs voltage (J-V) characteristics of PV cells fabricated with Al cathode. (a) Conventional OPV cell without isolating active areas. (b) OPV cell with isolating active areas. Inset is EQE of polymer PV cell fabricated with a new substrate and an Al top electrode.

structure and the energy-band diagram of the OPV cell. After being fabricated, the polymer OPV cell was transferred to a glovebox without vacuum break. In the glovebox, we dropped epoxy resin outside the OPV cell and attached the cover glass including CaO patch as a desiccant. Then, the OPV cell was exposed by UV light (354 nm) with 50 W power to passivate the OPV cell from water contamination. All electrical measurements were performed under nitrogenglovebox conditions at room temperature. The photocurrentdensity-versus-voltage (J-V) characteristics was analyzed by using a HP 4155C source-measure unit. The photocurrent was measured under AM 1.5G solar illumination at 100 mW/cm2 (1 sun) supplied by using a Newport 150 W solar simulator. The light intensity was monitored with a calibrated silicon photodiode for the AM 1.5G spectrum. The light-absorption

spectrum was measured with a UV-visible (Varian, Cary 5000, USA) spectrophotometer. Results and Discussion Figure 3a shows the light-absorption spectrum of the P3HT: PCBM blended layer in the wavelength range of visible light. To measure the light-absorption spectrum, a specific sample was fabricated: P3HT:PCBM blended layer on PEDOT:PSS on ITO glass. The light absorption of the P3HT:PCBM blended layer was weak at wavelengths of above 650 nm but had a strong band at wavelengths in the range 400-650 nm. In other words, for the P3HT:PCBM blended layer, the light absorption in the wavelength range 400-650 nm had a significant influence on the performance of the PV cell. In particular, Jsc should be influenced more because light absorption is mainly related to

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Figure 3. (a) UV-visible-light absorption spectrum of P3HT:PCBM bulk heterojunction composite layer in the visible range. (b) Resistances of various metal electrodes analyzed by using the same length (∼20 mm) and thickness (∼80 nm).

Figure 4. Reflectivity of various metal electrodes (Al, Ag, Au, Ni, Fe, and Ti) in the visible range.

Jsc. Thus, for the PV cell with the same structure, the increase of the light absorption in the wavelength range 400-650 nm will enhance the PCE of the PV cell. In the wavelength range 400-650 nm, the light absorption can be improved by using a metal cathode with a higher reflectivity of visible light. However, the resistance of the metal cathode should be considered even though a higher-reflectivity cathode would supply a greater absorption of visible light, because the resistance of the metal cathode determines the final photocurrent density of the PV cell. First, the resistances of various metal electrodes were estimated by using the same area (∼3 mm in width × ∼20 mm in length) and thickness (∼80 nm). The resistances of aluminum (Al), silver (Ag), gold (Au), nickel (Ni), iron (Fe), and titanium (Ti) were 7.5, 2.7, 2.2, 7.8, 20.3, and 195.2 ohm, respectively, as shown in Figure 3b. Second, the reflectivity of various metal cathodes, which were evaporated on the glass substrate, as shown in Figure 4, were estimated with a metal layer of ∼80 nm in thickness, 50 mm in width, and 50 mm in length. For Al and Ag layers, visible light almost always reflected in the wavelength range 400-650 nm, but the reflectivity of Ag was larger than that of Al (i.e., 94.4% for Al and 96.2% for Ag at a wavelength of 518 nm). For the Ni cathode, visible light reflected approximately ∼70% in the wavelength range 400-650 nm (i.e., 70.1% at a wavelength of 518 nm). For the Au layer, the visible light reflected from 400 to 484 nm with a reflectivity of ∼40% and then root-squarely increased with the wavelength. The reflectivity of the Au layer was 64.8% at a wavelength of 518 nm. For Fe and Ti layers,

visible light was almost always transmitted in the wavelength range 400-650 nm, but the reflectivity of the Fe layer was larger than that of Ti (i.e., 39.3% for Fe and 26.2% for Ti at a wavelength of 518 nm). From the reflectivity of various metal layers in Figure 4, it is likely that the sequence of higher absorption in the P3HT:PCBM blended layer, produced by the visible light reflected from metal cathodes, is followed by Al, Ag, Ni, Au, Fe, and Ti. As a result, it is also expected that the sequence of higher Jsc is followed by Al, Ag, Ni, Au, Fe, and Ti. Third, we tested the dependency of the PCE on the BCP layer thickness for the OPV cells which were fabricated with the same OPV cell structure as that in Figure 1b, as shown in Figure 5. Figure 5a shows the dependence of the photocurrent-density versus voltage (J-V) characteristics of the P3HT:PCBM blended PV cells depending on the BCP layer thickness. PCE is obtained by multiplying Jsc with Voc and FF. Voc, FF, and Jsc varied with the BCP layer thickness. The dependence of Voc and FF on the BCP layer thickness is shown in detail in Figure 5b. Voc abruptly increased at BCP layer thicknesses up to ∼1.0 nm and then slightly increased at thicknesses up to ∼12.0 nm. Furthermore, it saturated above ∼12.0 nm as the BCP layer thickness continued to increase. The FF also rapidly increased at BCP layer thicknesses up to ∼1.0 nm and then very slightly increased at thicknesses up to ∼8 nm. Furthermore, it slightly decreased when the BCP layer thickness was above ∼8 nm. The variation of Voc and FF were ∼179% (from 0.37 to 0.66 V) and ∼165% (from 0.41 to 0.68), respectively. In addition, the detailed dependence of Jsc and PCE on the BCP layer thickness is shown in Figure 5c. Jsc rapidly increased at BCP layer thicknesses up to ∼1.0 nm and very slightly increased at thicknesses up to ∼12.0 nm. Furthermore, it decreased when the BCP layer thickness was above ∼12.0 nm. The variation of Jsc was ∼138.0% (from 11.08 to 15.26 mA/cm2). The PCE of the polymer PV cell without inserting the hole/exciton blocking layer (BCP) showed a variation of 2.02% which was much lower than that of the PV cell with the same cell structure reported in ref 26 (∼5.0%). However, the PCE rapidly increased at BCP layer thicknesses up to ∼1.0 nm and then slightly increased at thicknesses up to ∼12.0 nm. Furthermore, it slightly decreased when the BCP layer thickness was above ∼12.0 nm. The maximum PCE of 6.65% was obtained at a specific BCP layer thickness of ∼12.0 nm, improving PCE by about 327% (from 2.02 to 6.65%). This happens when the exciton is separated into the hole and the electron is at the interface of donor (P3HT) and acceptor (PCBM) materials. When the hole moves to the bottom electrode, the electron will move to the top electrode. But some separated holes and nonseparated exciton transport

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Figure 5. Dependence of polymer PV cell performance on the BCP layer thickness. (a) J-V characteristics. (b) Voc and FF. (c) Jsc and PCE.

Figure 6. (a) Photocurrent-density vs voltage (J-V) characteristics of PV cells fabricated with various metal electrodes. (b) Photocurrent-density vs voltage (J-V) characteristics of PV cells fabricated with LiF layer by using Au electrode. PV cell fabricated with P3HT:PCBM blended layer: ITO/PEDOT:PSS (∼2000 rpm)/P3HT:PCBM (∼1000 rpm)/BCP (∼12 nm)/LiF (0, 0.5 nm)/Au electrode (∼80 nm).

to the top electrode instead of the bottom electrode, and then, recombination occurs at the interface of the top electrode. This phenomenon decreases PCE of PV cell. To prevent this recombination, the BCP layer is deposited as shown in figure 1c. The BCP layer forms an energy barrier of about 1.1 eV between the P3HT: PCBM blended layer and the top electrode. Therefore, the energy barrier formed by the BCP prevents the exciton and the hole from moving toward the top electrode. Consequently, the BCP layer enhances the PCE of the polymer PV cell thaks to the hole/exciton blocking layer that prevents recombination. Therefore, the deposition of the hole/exciton blocking layer (BCP) on the P3HT blended with the PCBM remarkably improved the PCE of polymer PV cells as a result of the blocking effect. Finally, the PV performance for cells with various metal cathodes was estimated, as shown in Figure 6a. The photocurrent density versus voltage strongly depended on the metal-cathode property. For the PV cell with the Ti cathode, the cell did not behave typically for a PV cell under visible-light illumination. This behavior is probably related to the transparent characteristic of visible light (see Figure 4). The detailed PV performances such Jsc, Voc, FF, and PCE are described in Table 1. Jsc of PV cells for Al, Ag, Ni, Au, Fe, and Ti is 15.36, 14.66, 12.83, 12.04, 8.56, and 1.17 mA/cm2, respectively. Voc of PV cells for Al, Ag, Ni, Au, Fe, and Ti is 0.645, 0.635, 0.625, 0.235, 0.305, and 0.055 V, respectively. FF of PV cells for Al, Ag, Ni, Au, Fe, and Ti is 65.9, 63.0, 63.7, 44.5, 35.5, and 25.6%, respectively. PCE of PV cells for Al, Ag, Ni, Au, Fe, and Ti is 6.53, 5.87, 5.11, 1.26, 0.93, and 0.02%, respectively. The sequence of higher Jsc and PCE for OPV cells for various metal cathodes is well correlated welwith the sequence of higher reflectivity when one compares Figure 4 with Table 1. As the results show, there is no clear correlation between the PV performance and the resistance and work function of the metal

TABLE 1: Summary of Metal Work Function and Properties of PV Cells Fabricated with Various Metal Electrodes under 100 mW/cm2 AM 1.5G Illumination metal work electrode function (eV) Voc (V) Jsc (mA/cm2) FF (%) PCE (%) Al Ag Au Ni Fe Ti

4.28 4.26 5.10 5.15 4.70 4.33

0.645 0.635 0.235 0.625 0.305 0.055

15.36 14.66 12.04 12.83 8.56 1.17

65.9 63.0 44.5 63.7 35.5 25.6

6.53 5.87 1.26 5.11 0.93 0.02

cathode when one compares Figure 3b with Table 1. Therefore, the reflectivity of the metal cathodes obviously affects the PCE of PV cells principally rather than the resistance of the metal cathodes. However, the interface property between the metal cathode and small-molecule hole/exiton blocking-layer (BCP) in PV cells should be carefully considered although the visiblelight absorption reflected from the metal cathodes in PV cells primarily influences the PCE of PV cells. This is because the PCE of the Au cathode (1.26%) was lower than expected, even though the Jsc of Au cathode (12.04 mA/cm2) was slightly lower than that of the Ni cathode (12.83 mA/cm2). In addition, note that the Ni cathode showed a very high PCE although its reflectivity was ∼70%; compare the PCE and reflectivity between Ni and Al cathode in Table 1 and Figure 4. This result is probably related to the interface property between the exiton/ hole blocking layer and metal cathode or work function difference for Ni and Al cathodes. We need further study to clarify this issue. To find the cause for PCE difference among PV cells with the electrode of Al, Ag, Ni, and Au, the reflectivity and the surface roughness at the interface of metal electrodes were analyzed by using an optical interferometer (white-light interferometry, NanoView E-1000, Nanosystem, Korea). Figure 7 shows the images of the surface roughness. In addition, Figure

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Figure 7. Surface-roughness images obtained by optical interferometer (a) Al, (b) Ag, (c) Ni, (d) Au, (e) Fe, and (f) Ti. The samples were fabricated with a 80 nm thickness on glass substrate.

Figure 8. Rms values of various metal electrodes.

8 shows the root-mean-square (rms) values of Al, Ag, Ni, Au, Fe, and Ti electrodes (1.08, 1.20, 0.89, 1.13, 0.86, and 0.58 nm, respectively.) First, we compared the reflectivity and surface

roughness of Al and Ag electrodes to determine which one is the more dominant factor to impact on PCE. Although the reflectivity of Al electrodes is lower than that of Ag electrode, polymer PV cell fabricated with Al electrode showed higher PCE than Ag electrode. Therefore, for Al and Ag electrodes, PCE of PV cell is affected dominantly by the surface roughness rather than by the reflectivity of metal electrode. Second, the rms value of the Ni electrode (0.89 nm) shows better interfacial properties than Au (1.13 nm) and Ag electrodes (1.20 nm). However, the reflectivity of the Ni electrode (70.1%) is 27.1% lower than that of the Ag electrode (96.2%); therefore, the PCE of PV cell fabricated with the Ni electrode is ∼13% lower than that of the Ag electrode, illustrating that the reflectivity of the metal electrode affects more dominantly than the surface roughness. Meanwhile, the PCE of PV cell fabricated with the Ni electrode is ∼306% higher than that of the Au electrode. This big PCE difference is probably caused by the lower surface roughness value of Ni than that of Au electrode but not by the

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difference of reflectivities. Thus, it is notable that the surface roughness affects more dominantly than the reflectivity to PCE unless the difference of reflectivities is higher than 2%. In addition, to confirm the interface effect, the LiF of 0.5 nm was inserted into the Au cathode and small-molecule hole/exiton blocking layer (BCP). Voc of the PV cell was improved from 0.235 to 0.385 V, as shown in Figure 6b. Other than this, Jsc of the PV cell did not change nearly as much as expected. As a result, the PCE of the PV cell with the LiF interface layer changes by ∼201% (from 1.26 to 2.53%). Note that the Voc of PV cells is mainly determined by the energy-level difference between the HOMO of donor organic material and the LOMO of acceptor organic material and the interface property between the metal cathode and small-molecule hole/exiton blocking-layer (BCP) visible-light absorption, thereby determining the final PCE of PV cells.27-29 Conclusions In summary, we investigated how the reflectivity of various metal-electrode materials (Al, Ag, Au, Ni, Fe, and Ti) influences the OPV cell performance. The light absorption strongly depended on the reflectivity, much more than on the resistance and work function of the metal electrode. The light absorption was improved by using a metal electrode with a better reflectivity of the visible light. Therefore, more excitons were generated, and the photocurrent was enhanced in proportion to the increased light absorption when using a metal electrode which had a good reflectivity. In addition, the PCE of PV cell was affected by the surface roughness more dominantly than by the reflectivity to PCE, unless the difference of reflectivities was higher than 2%. Consequently, the PV cell fabricated with a good reflection electrode and surface roughness results in improved performance of the PV cell. Finally, we observed PCE values of 6.53, 5.87, 5.11, 1.26, 0.93, and 0.02% for PV cells fabricated with an Al, Ag, Ni, Au, Fe, and Ti electrodes, respectively. Acknowledgment. This project was supported by The National Research Program for Tera-bit-level Non-volatile Memory Development, sponsored by the Korean Ministry of Knowledge Economy. References and Notes (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15.

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