J. Phys. Chem. C 2009, 113, 16807–16810
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Polymer-Electrode Interfacial Effect on Photovoltaic Performances in Poly(3-hexylthiophene):Phenyl-C61-butyric Acid Methyl Ester Based Solar Cells Hui Jin,* Markus Tuomikoski, Jussi Hiltunen, Pa¨lvi Kopola, Arto Maaninen, and Flavio Pino VTT Technical Research Centre of Finland, P.O. Box 1100, FI-02044 VTT, Oulu, Finland ReceiVed: July 3, 2009; ReVised Manuscript ReceiVed: August 7, 2009
The effect of the polymer-electrode interface on the photovoltaic performance of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester based solar cells was investigated. Four forms of cathodes, Ca/Ag, Ca/Al, LiF/Al, and Al, were deposited on photoactive films. The Ca/Al cathode showed the best FF of 0.69, while Al produced the worst one of 0.55. The efficiency-limiting effect, a concavity in the fourth quadrant of the current-voltage characteristics, can be caused by a thermally degraded poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) anode or an oxidized Ca cathode. An inorganic material interlayer of CdSe between the photoactive layer and the cathode can, to some extent, inhibit the negative effect of polymer-metal interface. Introduction As the demand for renewable energy sources is growing, polymer solar cell has been showing a powerful potential due to its promise of low-cost manufacturing on flexible substrates. The power conversion efficiencies of polymer-based bulk heterojunction (BHJ) solar cells, poly(3-hexylthiophene) (P3HT) as a donor and phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor, have reached up to 4-5% under 1.5 AM (100 mW/cm2) illumination.1,2 To achieve a satisfying power conversion efficiency, a desirable FF is required, which is directly related to a balanced charge transport in the photoactive layer and a good charge collection at two electrodes. Furthermore, charge transport and charge collection largely rely on the morphology of photoactive films and the polymer-metal interfacial effect. Actually, the morphology in organic BHJ solar cells has a bigger effect on FF than the effect on the other photovoltaic (PV) parameters like open-circuit voltage (Voc) and short-circuit current (Isc). Fibrillar morphology may result in an inhomogeneous organic layer-metal interface, which can lead to a lowered FF.3 Also, the deposition conditions of a metal cathode can cause a difference in the FF. Especially in the case of Al, a wellcontrolled growth rate and other conditions like a controlled evaporation temperature can mean a higher FF.4 In the present paper, the polymer-electrode interfacial effect on PV parameters, especially on FF, was investigated in P3HT: PCBM based solar cells. It was found that the different cathodes led to different FF values. Besides, a drop in FF caused by a concavity in the fourth quadrant of current-voltage characteristics was observed in the devices fabricated with a degraded polyethylenedioxythiophene:poly(styrene sulfonate) (PEDOT: PSS) film or an oxidized Ca cathode. Although some groups reported a similar feature,5-7 an unambiguous understanding still needs to be worked out. Finally, it was found that an inorganic interlayer of CdSe between polymer and cathode can effectively inhibit the long-term corrosion of metal on a photoactive layer. Experimental Section The devices were fabricated by spin-coating a blend of P3HT (Rieke Metals)/PCBM (Nano-C). The blends were weighted * To whom correspondence should be addressed. Phone: 358-40-486 5373. Fax: 358-40-722 2320. E-mail:
[email protected].
with the ratio of 1:1 in the solid state, and then dissolved in the distilled 1,2-dichlorobenzene with a concentration of 40 mg/ mL. The ITO-coated glass substrates were cleaned by ultrasonic treatment in deionized water, acetone, and isopropyl alcohol sequentially. Shortly after the UV-ozone treatment, a layer (about 40 nm) of PEDOT:PSS (Clevios P VP A1 4083) was spin-coated. The normal PEDOT:PSS layers were fabricated by heating on a hot plate for 20 min at 150 °C, and the degraded PEDOT layers were prepared by heating for 60 h at 120-150 °C. The surface topography and the surface roughness of the PEDOT layers were observed and recorded by a Veeco Dektak 150 surface profiler. P3HT:PCBM solution was spin-coated in the air to form an active layer on the PEDOT:PSS film. The final thickness of a photoactive film was also measured by Dektak 150. Four different cathodes, which consisted of Ca (25 nm)/Ag (80 nm), Ca (20 nm)/Al (100 nm), LiF (1 nm)/Al (100 nm), or Al (100 nm), were evaporated. If there is no special explanation, a Ca/Ag cathode was used to complete the devices. To determine the effect of the different cathodes, the photoactive films were prepared in identical processes, which guaranteed similar film morphology. Before cathode deposition, the blend films were thermally annealed at 110 °C for 5 min in a glovebox. The oxidized Ca cathode was prepared by exposing a device with a Ca layer to the low-vacuum environment before evaporating Ag. An interlayer of CdSe between the photoactive layer and the metal cathode was deposited by E-beam depositor. The shadow mask of ∼10 mm2 was utilized during cathode deposition to define the active area of the devices. All devices were encapsulated by the UV-cured epoxy (DELO 681) in the glovebox full of dry nitrogen atmosphere. All the characterization was done in ambient air at room temperature. Steady state illumination tests were carried out under 1.5 AM irradiation by using a 300 W Cermax lamp-based solar simulator. Current density-voltage (J-V) curves were measured with a Keithley 2400 source measurement unit. The UV-vis absorption spectra were recorded on a Varian Cary 5000 spectrophotometer. A yttrium aluminum garner laser with a pulse width of 6 ns and a wavelength of 532 nm was used for testing the transient photocurrent of the thin-film devices. The devices were illuminated from the ITO side and the transients were detected by a Tektronix TDS 744A digital storage oscilloscope.
10.1021/jp906277k CCC: $40.75 2009 American Chemical Society Published on Web 08/21/2009
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Figure 1. J-V characteristics of P3HT:PCBM solar cells with the cathode of Ca (25 nm)/Ag (80 nm) (square), Ca (20 nm)/Al (100 nm) (circle), LiF (1 nm)/Al (100 nm) (triangle), or Al (100 nm) (star), respectively, in the dark (the inset) and under AM1.5 (100 mW/cm2) illumination.
Figure 2. J-V characteristics of the P3HT:PCBM samples fabricated by using a normal prepared and a thermally degraded PEDOT:PSS layer as a hole transport layer, under AM1.5 illumination (100 mW/ cm2) and in the dark (the inset).
TABLE 1: Summary of the P3HT:PCBM Device Parameters with Different Cathodes Presented in Figure 1a parameters
Ca/Ag
Ca/Al
LiF/Al
Al
Voc [V] Jsc [mA/cm2] FF η [%] R Rs [Ω · cm2] Rsh [MΩ · cm2]
0.56 6.52 0.57 2.07 54.3 2.09 0.001
0.58 7.27 0.69 2.90 1.02 × 105 2.96 1.51 × 103
0.52 7.34 0.65 2.52 4.46 × 103 2.93 71.0
0.35 7.68 0.55 1.49 3.82 × 104 5.73 2.36 × 102
a The illumination intensity (AM 1.5) is 100 mW/cm2. Voc: open-circuit voltage; Jsc: short-circuit current density; FF: fill factor; η: power conversion efficiency; R: rectification; Rs: serial resistance; Rsh: shunt resistance.
Results and Discussion Figure 1 depicts the J-V characteristics of P3HT:PCBM solar cells with a cathode of Ca/Ag, Ca/Al, LiF/Al, or Al, respectively, in the dark (the inset) and under 1.5 AM (100 mW/cm2) illumination. The device with the Ca/Al cathode showed a quite good FF of 0.69, while the Al cathode led to a value of only 0.55 (Table 1). Since the photoactive layers were fabricated in the same processes, the variation of FF can be more likely attributed to the interfacial effect between the polymer and the metal, which crucially has an impact on charge transfer and charge collection at the contacts. A LiF layer has been considered to cause a significant vacuum level offset due to the strong dipole moment of LiF,8 resulting in a higher FF of 0.65. The addition of a thin-film Ca between Al and the organic layer has been believed to form ohmic contact and then result in an increase in FF.9 However, it was found that a similar Ca interlayer between Ag and the photoactive layer did not cause the same increase in FF. That means the Ag or the Al layer not only played a role in protecting the Ca film from the corrosion of moisture and oxygen, but also forms an alloy layer with Ca, which has an effect on the resulting performances. According to the J-V characteristic in the dark (the inset in Figure 1), the rectification (R) value at (1 V can be calculated, and also, the serial resistance (Rs) and the shunt resistance (Rsh) can be estimated from the slope close to 1 and 0 V, respectively (Table 1).10 As one can see, a quite small Rsh of 0.001 MΩ · cm2 was obtained in the device with the Ca/Ag cathode. A low Rsh means the occurrence of charge recombination and leakage current, which can be the main reason for the lower FF and the smaller Jsc.11 The quite low R at (1 V provided evidence for a large reverse leakage current in the devices with the Ca/Ag cathode. The Rs for the device with the Al cathode was almost twice
Figure 3. The surface topographies of the fresh (a) and the degraded (b) PEDOT layers recorded by the video camera (×166) of Dektak 150.
those for the other three cathodes, while a relatively high Rsh implied that the Al cathode can form fewer pinholes or defects at the interface between Al and the photoactive layer. As a result, the changes in both Rs and Rsh led to a decline only in FF but not in Jsc for the device with the Al cathode. Figure 2 compares the J-V characteristics of the samples fabricated by using a fresh and a thermally degraded PEDOT: PSS layer, respectively. Under illumination, a detrimental concavity occurred in the fourth quadrant of the J-V curve for the sample with the degraded PEDOT layer. Panels a and b of Figure 3 show the surface topographies of the fresh and the degraded PEDOT layer, respectively. The surface of the fresh PEDOT layer was quite smooth, while on the degraded PEDOT layer, some areas became quite different (Figure 3b). The roughness of the surface area shown in panels a and b of Figure 3 was 0.4 and 3.49 nm, respectively. The roughness ranges for all the layers were 0.3-1.5 and 1.6-3.5 nm for the fresh and the degraded ones, respectively. These surface changes on PEDOT layers have been attributed to thermal degradation, which destroyed the morphology and thus decreased the conductivity of PEDOT:PSS films.12,13 The degraded PEDOT film will directly cause a poor contact between the photoactive layer and the PEDOT:PSS film. According to the J-V characteristics in the dark (the inset in Figure 2), the estimated Rs for the devices with the fresh and the degraded PEDOT layer were 7.2 and 80.0 Ω · cm2, respectively. The increased Rs and the decreased conductivity led to the obvious drop in both of Jsc and FF in the device with the degraded PEDOT layer, and even a concavity occurred in the fourth quadrant of the J-V curve. Some previous reports paid more attention to the effect of the metal-polymer interface on the formation of this concavity, and considered that it can be explained by a counter diode effect or charge accumulation at the metal-polymer interface.6,7,9 In the present paper, it was observed that a similar effect also can occur at the anode side, and a poor PEDOT-polymer contact and a low conductivity also can be reasons for a sharp decline in FF. Therefore, a suitable preparation process for the PEDOT:
Effect of the Cathodes on P3HT:PCBM Solar Cells
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16809 TABLE 2: Summary of the P3HT:PCBM Devices Parameters with a CdSe Interlayer Shown in the Inset of Figure 5a Voc [V] Jsc [mA/cm2] FF η [%] Rs [Ω · cm2] Rsh [KΩ · cm2] a
as-produced
5 nm
8 nm
20 nm
0.59 5.72 0.65 2.45 4.16 20.40
0.58 5.81 0.60 2.23 4.55 20.36
0.45 5.81 0.50 1.52 6.71 1.35
0.32 1.18 0.31 0.13 25.6 1.24
The illumination intensity (AM 1.5) is 90 mW/cm2.
Figure 4. J-V characteristics of the P3HT:PCBM samples fabricated by using the Ag-protected Ca and the oxidized Ca as the cathode under AM1.5 illumination (100 mW/cm2) and in the dark (the inset).
Figure 5. A concavity in J-V characteristics under AM1.5 illumination (90 mW/cm2) caused by poor deposited Ca (square) and the disappearance of this concavity by using a 5 nm CdSe interlayer (circle). The inset shows the thickness dependence of a CdSe interlayer on PV performances.
PSS layer is one way to resolve the contact problem at the interface between PEDOT and the photoactive layer. At the cathode side, some chemical degradation, or a quite unstable or high growth rate of metal may result in a concavity in J-V characteristics under illumination. Figure 4 compares the J-V characteristics of the samples fabricated by using a Ag-protected Ca cathode and an oxidized Ca cathode under illumination and in the dark (the inset). The oxidized Ca cathode caused a large decline in PV performances and also an S-shape J-V curve was shown, which has been attributed to the chemical degradation from oxygen on the Ca-polymer interface.3 The dark current in logarithmic scale (the inset in Figure 4) for the device with the oxidized Ca cathode was almost symmetric, the R was thus quite low, and a much high Rs of 560 Ω · cm2 was obtained. The overly high Rs value explained the decline in PV performances. Figure 5 shows the J-V curve of an asproduced device with a poorly deposited Ca cathode (not oxidized) under illumination (square). The J-V concavity in this situation has been explained by the inefficient charge transfer at the polymer-metal interface.14 One way for resolving the metal-polymer interface problem is to insert an interlayer between the photoactive layer and metal, which can reduce the direct impact of the metal on the photoactive layer. In our experiments, as a typical inorganic semiconductor material, CdSe was selected due to its low cost and good electron conductivity property. A CdSe interlayer of 5 nm between the P3HT:PCBM layer and the Ca/Ag cathode removed the Sshaped curve (circle), which was caused by the poor deposition (square) (Figure 5). The inset in Figure 5 shows the thickness
Figure 6. Shelf lifetime of the encapsulated P3HT:PCBM composite devices without (a) and with (b) an interlayer of CdSe (5 nm) during a 130-day test. The illumination intensity is 90 ( 2 mW/cm2. The inset in panel a compares the initial absorption spectra of the encapsulated P3HT:PCBM film with that after 2 months.
dependence of the CdSe interlayer. When the thickness of the CdSe layer was up to 8 nm, a decline in the Voc and the FF limited the efficiency of the devices. According to the estimated Rs and Rsh from the J-V curves in the dark condition (Table 2), a thicker CdSe interlayer caused a higher Rs and a lower Rsh. Therefore, a CdSe interlayer can help to avoid the detrimental effect at the metal-polymer interface, but the suitable thickness of CdSe should be about 5 nm. Otherwise, the increased Rs and the decreased Rsh will cause a large drop in Voc and FF. In fact, it was also found that a long-term corrosion on the polymer-metal contact can cause a concavity in J-V curves. A CdSe interlayer not only prevented the occurrence of this concavity, but also largely enhanced the shelf lifetime. Panels a and b of Figure 6 present J-V characteristics of the encapsulated P3HT:PCBM composite devices with and without an interlayer of CdSe (5 nm) during 130 days for shelf lifetime test. The Voc, Jsc, FF, and η for the as-produced P3HT:PCBM device initially were 0.59 V, 5.73 mA/cm2, 0.65, and 2.41%,
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Jin et al. shortened the tail of the transient. Therefore, the transient photocurrent of the 130-days-degraded sample falls faster than that of the fresh sample. Conclusions
Figure 7. Short-circuit transient photocurrent responses of a fresh P3HT:PCBM sample without a concavity in J-V characteristics (circle) and the 130-days-degraded sample that was characterized in Figure 6a (square).
respectively. By adding the interlayer of CdSe, the PV parameters for the initial J-V test were 0.56 V, 5.72 mA/cm2, 0.60, and 2.14%, respectively. During 130 days, the as-produced device gradually degraded and showed an S shape in the J-V characteristics, while the device with the CdSe interlayer presented a much slower degradation. After 130 days, the Voc, Jsc, FF, and η for the as-produced sample decreased to 0.53 V, 5.24 mA/cm2, 0.29, and 0.92%; and for the sample with the CdSe interlayer, the parameters came to 0.53 V, 5.86 mA/cm2, 0.47, and 1.71%. The η decreased by 62% and 20%, respectively. As can be seen, the decline in FF is a most crucial factor that causes the degradation of the device. According to the absorption spectra (the inset in Figure 6a) of the encapsulated P3HT:PCBM film, no big difference was observed after 2 months. Therefore, the significant degradation of the sample without the CdSe interlayer should not come from the photoactive layer itself, but most likely from the contact on the cathode side. One possibility was proposed that the permeability of Ca into the photoactive layer caused the degradation of solar cells. The transient photocurrent responses of the thin-film devices shed some light on this issue. Figure 7 shows the short-circuit transient photocurrent responses of a fresh P3HT:PCBM sample without a concavity in the J-V curve and the 130-days-degraded sample that was characterized in Figure 6a. No obvious difference in rise time was observed, while the fall times were quite different. The sharp rise of photocurrent was attributed to charge carriers near the two electrodes. Under the extraction power of the built-in electric field, free charge carriers drifted toward the electrodes. On the off-edge of the light pulse, recombination and dissociation rate under the gradually faded build-in field determined the tail of transients. When calcium permeated into a P3HT:PCBM layer, similarly with organoaluminum compounds, organocalcium compounds can be formed by adding calcium to organic compounds, and anion radicals can be created on the polymer by single-electron transfer.15-17 Positive charge carriers which diffuse near the cathode can be annihilated on anion radicals, and thus the recombination rate increases. The increased recombination rate
In conclusion, the effect of the cathodes on P3HT:PCBM solar cells has been observed. The outer metals of Ag and Al not only protected the inner metal Ca from the corrosion of moisture and oxygen, but also produced the alloy that caused quite different Rs and Rsh values in the resulting devices. Besides, the polymer-electrode interfaces, both on the anode and the cathode side, had a big effect on the PV parameters, especially FF. Low conductivity and high resistance caused by the polymer-electrode interfacial effect can be one reason for the S-shaped J-V curve of the degraded sample. The long-term corrosion from moisture and oxygen in the air caused an increase in recombination rate at the polymer-metal interface. This interfacial degradation led to an obvious drop in FF, while an inorganic interlayer of CdSe can weaken the effect of the polymer-metal interface and prolong the shelf lifetime. Acknowledgment. This work is supported by VTT’s Printed Intelligence Strategic Program. References and Notes (1) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (2) Kim, J. Y; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. AdV. Mater. 2006, 18, 572. (3) Djara, V.; Berne`de, J. C. Thin Solid Films 2005, 493, 273. (4) Riede, M. K.; Sylvester-Hvid, K. O.; Glatthaar, M.; Keegan, N.; Ziegler, T.; Zimmermann, B.; Niggemann, M.; Liehr, A. W.; Willeke, G.; Gombert, A. Prog. PhotoVoltaics 2008, 16, 561. (5) Sullivana, P.; Jones, T. S. Org. Electron. 2008, 9, 656. (6) Ferenczi, T. A. M.; Nelson, J.; Belton, C.; Ballantyne, A. M.; Quiles, M. C.; Braun, F. M.; Bradley, D. D. C. J. Phys.: Condens. Matter 2008, 20, 475203. (7) Oey, C. C.; Djurisic, A. B.; Wang, H.; Man, K. K. Y.; Chan, W. K.; Xie, M. H.; Leung, Y. H.; Pandey, A.; Nunzi, J.-M.; Chui, P. C. Nanotechnology 2006, 17, 706. (8) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Appl. Phys. Lett. 2002, 80, 1288. (9) Gupta, D.; Bag, M.; Narayan, K. S. Appl. Phys. Lett. 2008, 92, 093301. (10) Shirland, F. A. AdV. Energy ConVers. 1966, 6, 201. (11) Kim, M.-S.; Kim, B.-G.; Kim, J. Appl. Mater. Interfaces 2009, 1, 1264. (12) Vitoratos, E.; Sakkopoulos, S.; Dalas, E.; Paliatsas, N.; Karageorgopoulos, D.; Petraki, F.; Kennou, S.; Choulis, S. A. Org. Electron. 2009, 10, 61. (13) Huang, J.; Miller, P. F.; Wilson, J. S.; Mello, A. J.; de; Mello, J. C. de; Bradley, D. D. C. AdV. Funct. Mater. 2005, 15, 290. (14) Glatthaar, M.; Riede, M.; Keegan, N.; Sylvester-Hvid, K. O.; Zimmermann, B.; Niggemann, M.; Hinsch, A.; Gombert, A. Sol. Energy Mater. Sol. Cells 2007, 91, 390. (15) Cros, S.; Firon, M.; Lenfant, S.; Trouslard, P.; Beck, L. Nucl. Instrum. Methods Phys. Res., Sect. B 2006, 251, 257. (16) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92, 686. (17) Lo¨gdlund, M.; Bre´das, J. L. J. Chem. Phys. 1994, 101, 4357.
JP906277K
