- Email: [email protected]
Origin of the open-circuit voltage of organic thin-film solar cells based on conjugated polymers
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 759–761
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Origin of the open-circuit voltage of organic thin-film solar cells based on conjugated polymers Toshihiro Yamanari a,, Tetsuya Taima a, Jun Sakai b, Kazuhiro Saito a a Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Advanced Technologies Development Laboratory, Matsushita Electric Works, Ltd., 1048, Kadoma, Osaka 571-8686, Japan
a r t i c l e in fo
abstract
Article history: Received 22 December 2007 Accepted 16 September 2008 Available online 5 November 2008
Recently, efficiency of organic solar cells (OSCs) with conjugated polymers and fullerene derivatives has been increased, in most cases by improving their short-circuit current density. Regarding the other photovoltaic parameters, the open-circuit voltage (VOC) remains controversial. In this study, we investigated the relation between the VOC and the energy difference (DE) between the highest occupied molecular orbital of the donor material and the lowest unoccupied molecular orbital of the acceptor. The simple linear relation between VOC and DE was not observed in the polymer-based OSCs. There must be some factor that decreased the VOC of the cells. & 2008 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Conjugated polymers Open-circuit voltage Material energy levels
1. Introduction Organic solar cells (OSCs) have potential advantages like the low manufacturing cost, light-weight and mechanical flexibility [1]. Recently, polymer-based bulk heterojunction-type OSCs with conjugated polymers and fullerene derivatives have become one of the most attractive solar cells, because of their convenient fabrication methods and the relatively high efficiency compared to small-molecular-weight OSCs. Especially, the cells containing regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) have been focused and increase in their efficiencies has been obtained, in most cases by improving their short-circuit current density (JSC) [2–7]. Consequently, their efficiencies have been reached at around 5% [2–4]. The incident photon-to-electron conversion efficiency of the high-performance OSCs showed very high (about 70–80%) covering over the visible region [4–6]. Therefore, it might be difficult to get further improvement of JSC. However, the efficiency is not yet sufficient for commercial implementation. Regarding the other photovoltaic parameters, the origin of open-circuit voltage (VOC) remains controversial and there have been few strategies to successfully improve VOC [8–11]. For polymer-based OSCs, Scharber et al. [10] proposed that VOC of a conjugated polymer:PCBM OSCs could be estimated by VOC ¼ DE0.3, where DE is the energy difference between the highest occupied molecular orbital (HOMO) level of the donor polymer and the lowest unoccupied molecular orbital Corresponding author. Tel.: +81 29 861 3474; fax: +81 29 861 3475.
E-mail address: [email protected] (T. Yamanari). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.09.022
(LUMO) level of the acceptor PCBM. On the other hand, as for small-molecular-weight OSCs, Rand et al. [11] have found a very simple relation (VOC ¼ DE) with no offset. We believe understanding of the origin of VOC will be an effective guideline to accelerate the development of highperformance OSCs. In this study, we investigated the relation between energy levels of the donor/acceptor materials and VOC of OSCs based on five different conjugated polymers. In addition, incident light intensity dependencies of VOC for three different polymer-based OSCs were also examined.
2. Experimental methods 2.1. Sample preparation The device structure is shown in Fig. 1. PCBM was used as the acceptor material. Donor polymers tested in this study were P3HT, poly[tris(2,5-bis(hexyloxy)-1,4-phenylenevinylene)alt-(1,3-phenylenevinylene)] (PTDPV), poly[2-methoxy-5-(30 ,70 dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2). To prepare polymer:PCBM solutions, a polymer and PCBM were dissolved in chlorobenzene. Glass substrates with an indium-tin oxide (ITO) electrode were used for sample preparation. The substrates were exposed to ozone for 30 min. Poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited by spin coating at 3000 rpm (after passing a 0.45 mm filter)
ARTICLE IN PRESS 760
T. Yamanari et al. / Solar Energy Materials & Solar Cells 93 (2009) 759–761
10
A
Polymer : PCBM
V
PEDOT : PSS ITO Glass substrate
Current density / mA cm-2
Al
VOC = 0.33 V
5
0
-5 0.59
0.84
Fig. 1. Schematic device structure of organic solar cells fabricated in this study.
-10 -1.0
and then dried at 135 1C for 10 min. Subsequently, the polymer:PCBM solutions were spin coated onto the substrates. Al electrodes of approximately 60-nm thickness were deposited by vacuum evaporation. The active area of the cell was 0.04 cm2. Once the electrodes were deposited, the devices were stocked in a N2-filled glove box. As for P3HT:PCBM cells, thermal annealing treatment was carried out at 140 1C for 10 min under nitrogen atmosphere after the cell fabrication.
Fig. 2. J–V curves of the PTDPV:PCBM cells with different active layer thickness and polymer content. The illumination light was AM1.5G (100 mW/cm2).
2.2. Measurements
Photovoltaic layer
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
P3HT:PCBM PTDPV:PCBM MEH-PPV:PCBM MDMO-PPV:PCBM F8T2:PCBM
8.7 (0.2) 2.93 (0.11) 3.6 (0.2) 3.1 (0.4) 2.9 (0.3)
0.62 (0.02) 0.843 (0.009) 0.75 (0.04) 0.80 (0.04) 0.84 (0.07)
0.611 (0.007) 0.46 (0.02) 0.44 (0.02) 0.50 (0.04) 0.53 (0.02)
3.28 (0.06) 1.13 (0.02) 1.16 (0.09) 1.3 (0.2) 1.26 (0.08)
The power conversion efficiency (PCE) was calculated from the current density–voltage (J–V) characteristics under Air Mass 1.5 global (AM1.5G) solar simulated light irradiation of 100 mW/cm2. J–V characteristics were measured by the semiconductor characterization system (KEITHLEY 4200, Keithley Co. Ltd.) at room temperature in N2 atmosphere. The ionization potential of donor/ acceptor materials in air was determined using a photo-electron spectrometer (AC-2, Riken Keiki Co. Ltd.). UV–vis spectrum of each homogeneous donor/acceptor-material film was measured by a spectrophotometer (IMUC-7000, Otsuka Electronics Co. Ltd.). HOMO and LUMO levels of donor/acceptor materials were estimated by ionization potentials and absorption edges of UV–vis spectra. For measurement of the incident light intensity dependencies of the cell parameters for polymer:PCBM cells, the illumination intensity was varied controlling an iris of the illumination equipment and using a collective lens.
3. Results and discussion At the beginning, it is worth noting that low-performance cells showed wide variation of VOC. For example, VOC of PTDPV:PCBM cells varied from 0.3 to 0.9 V (see Fig. 2). This result would be brought about by insufficiency of optimization of the device fabrication condition, such as the active layer thickness and the polymer/PCBM ratio. Thinner film thickness causes a larger leakage current. It is known that the large leakage current brings low fill factor (FF) and low VOC from the equivalent circuit study [9]. In addition, van Durren et al. [12] reported that high polymer content cells show high VOC but very low PCE. Therefore, we had to optimize their active layer thickness and the ratio of polymer to PCBM for each polymer:PCBM cell. VOC of the cells with PCE o1% and FF o0.4 were omitted to remove the ambiguity. The photovoltaic parameters of each cell that meets the criteria are presented in Table 1. Standard deviations of VOC for every polymer:PCBM cell were under 0.1 V, indicating that the criteria were enough to obtain reliable VOC. Fig. 3(a) shows VOC of five different polymer-based solar cells plotted versus the HOMO level of the polymers and the energy
-0.5
0.0 Voltage / V
0.5
1.0
Table 1 Photovoltaic performances of five different polymer-based organic solar cells
The measurements were carried out under AM1.5G (100 mW/cm2) irradiation. Each photovoltaic parameter was averaged from 8–12 samples. Values presented in parentheses were the standard deviations.
difference (DE). DE is the energy difference between the HOMO level of the donor polymer and the LUMO level of the acceptor PCBM, as shown in Fig. 3(b). While Scharber et al. [10] reported a liner relation between VOC and DE with offset (VOC ¼ DE0.3; broken line in Fig. 2) for polymer:PCBM OSCs, our results obviously deviate from the relation. Recently, Rand et al. [11] have investigated the incident light intensity and temperature dependencies and have proposed a greatly simple liner relation between maximum VOC, which was not always observed under a typical condition (100 mW/cm2 intensity, 25 1C), and DE without offset (V Max OC ¼ DE; solid line in Fig. 2) for small-molecular-weight OSCs fabricated by vacuum evaporation technique. We have also observed the relation VOC ¼ DE for the small-molecular-weight OSCs by optimizing device structures as for each donor/acceptor materials to obtain best performance under AM1.5G at 100 mW/cm2 intensity (which will be published elsewhere [13]). Taking account of the results in small-molecular-weight OSCs, there must be some factor that decreases the VOC in polymer-based OSCs. To obtain further insight, we studied the incident light intensity dependence of VOC. Fig. 4 shows VOC of the three different polymer:PCBM cells in the intensity range 20–1000 mW/cm2. Although the maximum VOC (V Max OC ) of each cell was slightly higher than VOC under the standard intensity (100 mW/cm2), the simple linear relation has not been observed between VOC and DE. The light intensities that showed the maximum VOC (IV max ) were significantly different from each polymer:PCBM cells. IV max of the P3HT:PCBM, MDMO-PPV:PCBM and F8T2:PCBM cells were 750, 300 and 78 mW/cm2, respectively. In the intensity region lower than IV max , VOC was increased by elevating the light intensity.
ARTICLE IN PRESS T. Yamanari et al. / Solar Energy Materials & Solar Cells 93 (2009) 759–761
recombination of generated photo carriers. Insufficient charge distribution may result in reduced VOC. To achieve higher VOC, it will be necessary to control the internal morphology and remove the impurities in the bulk heterojunction layer of the cells. Regarding the control of the internal morphology, polymer-based OSCs that are fabricated by solution chemistry seem to have a disadvantage when compared to small-molecular-weight OSCs. Vacuum evaporation technique used for fabrication of small-molecular-weight OSCs has more flexibility and that enables making optimized structures, such as complex multilayered device structures [14]. This might be a reason why the relation between VOC and DE has no offset. Conversely, there are possibilities for further improvement of VOC in polymer-based OSCs.
Δ E (HOMOPolymer−LUMOPCBM) / eV 0.8
0.6
1.0
1.2
1.4
1.6
1.8
1.0 0.9 VOC / V
761
0.8 0.7 0.6 0.5 0.4 4.8
4.6
5.0
5.2
5.4
5.6
4. Conclusions
HOMOPolymer Level / eV
2.5
LUMO
2.7
2.8 3.1
3.9
4.8 eV ITO
5.0 PEDOT: PSS
4.8 P3HT
4.3 Al
ΔE 5.0 PTDPV
5.2 MDMO-PPV MEH-PPV
Acknowledgments
5.6 6.0
F8T2
PCBM
HOMO
Fig. 3. (a) VOC of different polymer-based solar cells plotted versus HOMO level of the donor polymers and the energy difference (DE). Solid and broken lines represent the relations VOC ¼ DE and VOC ¼ DE0.3, respectively. (b) Schematic energy diagram of materials used in this study.
1.0
VOC / V
0.9
0.8
0.7
0.6
0.5 0
200
400
600
Light intensity / mW
The simple linear relation between VOC and DE was not observed in the polymer-based OSCs. However, the relation V Max OC pDE seemed to hold. From this point of view, our result does not contradict previous reports [10,11]. There must be some factor that decreases VOC in polymer-based OSCs. Elucidation of the factors and introducing more sophisticated technique to control the internal structure of the bulk heterojunction layer will bring further improvement in the performance of polymer-based OSCs.
800
1000
cm-2
Fig. 4. Incident light intensity dependence of VOC for three different polymerbased solar cells: a P3HT:PCBM cell (square), a MDMO-PPV:PCBM cell (triangle) and an F8T2:PCBM cell (circle).
This result might be accounted for the imperfection of the internal morphology and the presence of impurities in the bulk heterojunction layer. It is possible that defects of internal morphology and impurities trap the charge carriers. The trapped charges form space charge which disturbs the charge distribution within the bulk heterojunction layer. The defects and traps also bring
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade, and Industry (METI). References [1] S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Organic-based photovoltaics: toward low-cost power generation, MRS Bull. 30 (2005) 10–19. [2] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622. [3] M. Reyes-Reyes, K. Kim, D.L. Carroll, High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methocycarbonyl)propyl-1-phenyl-(6,6)C61 blends, Appl. Phys. Lett. 87 (2005) 083506. [4] J.Y. Kim, S.H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger, New architecture for high-efficiency polymer photovoltaic cells using solutionbased titanium oxide as an optical spacer, Adv. Mater. 18 (2006) 572–576. [5] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.-S. Ha, M. Ree, A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells, Nat. Mater. 5 (2006) 197–203. [6] S. Yoo, W.J. Potscavage, B. Domercq, J. Kim, J. Holt, B. Kippelen, Integrated organic photovoltaic modules with a scalable voltage output, Appl. Phys. Lett. 89 (2006) 233516. [7] T. Yamanari, T. Taima, J. Sakai, K. Saito. Highly efficient organic thin-film solar cells based on poly(3-hexylthiophene) and soluble C70 fullerene derivative,, Jpn. J. Appl. Phys. 47 (2008) 1230–1233. [8] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, J.C. Hummelen, Origin of the open circuit voltage of plastic solar cells, Adv. Funct. Mater. 11 (2001) 374–380. [9] A. Moliton, J.-M. Nunzi, How to model the behavior of organic photovoltaic cells, Polym. Int. 55 (2006) 583–600. [10] M.C. Scharber, D. Mu¨hlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Design rules for donors in bulk-heterojunction solar cells— towards 10% energy-conversion efficiency, Adv. Mater. 189 (2006) 789–794. [11] B.P. Rand, D.P. Burk, S.R. Forrest, Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells, Phys. Rev. B 75 (2007) 115327. [12] J.K.J. van Durren, X. Yang, J. Loos, C.W.T. Bulle-Lieuwma, A.B. Sieval, J.C. Hummelen, R.A.J. Janssen, Relating the morphology of poly(p-phenylene vinylene)/methanofullerene blends to solar-cell performance, Adv. Funct. Mater. 14 (2004) 425–434. [13] T. Taima, J. Sakai, T. Yamanari, K. Saito, in preparation. [14] S.R. Forrest, The limits to organic photovoltaic cell efficiency, MRS Bull. 30 (2005) 28–32.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Origin of the open-circuit voltage of organic thin-film solar cells based on conjugated polymers Toshihiro Yamanari a,, Tetsuya Taima a, Jun Sakai b, Kazuhiro Saito a a Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Advanced Technologies Development Laboratory, Matsushita Electric Works, Ltd., 1048, Kadoma, Osaka 571-8686, Japan
a r t i c l e in fo
abstract
Article history: Received 22 December 2007 Accepted 16 September 2008 Available online 5 November 2008
Recently, efficiency of organic solar cells (OSCs) with conjugated polymers and fullerene derivatives has been increased, in most cases by improving their short-circuit current density. Regarding the other photovoltaic parameters, the open-circuit voltage (VOC) remains controversial. In this study, we investigated the relation between the VOC and the energy difference (DE) between the highest occupied molecular orbital of the donor material and the lowest unoccupied molecular orbital of the acceptor. The simple linear relation between VOC and DE was not observed in the polymer-based OSCs. There must be some factor that decreased the VOC of the cells. & 2008 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Conjugated polymers Open-circuit voltage Material energy levels
1. Introduction Organic solar cells (OSCs) have potential advantages like the low manufacturing cost, light-weight and mechanical flexibility [1]. Recently, polymer-based bulk heterojunction-type OSCs with conjugated polymers and fullerene derivatives have become one of the most attractive solar cells, because of their convenient fabrication methods and the relatively high efficiency compared to small-molecular-weight OSCs. Especially, the cells containing regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) have been focused and increase in their efficiencies has been obtained, in most cases by improving their short-circuit current density (JSC) [2–7]. Consequently, their efficiencies have been reached at around 5% [2–4]. The incident photon-to-electron conversion efficiency of the high-performance OSCs showed very high (about 70–80%) covering over the visible region [4–6]. Therefore, it might be difficult to get further improvement of JSC. However, the efficiency is not yet sufficient for commercial implementation. Regarding the other photovoltaic parameters, the origin of open-circuit voltage (VOC) remains controversial and there have been few strategies to successfully improve VOC [8–11]. For polymer-based OSCs, Scharber et al. [10] proposed that VOC of a conjugated polymer:PCBM OSCs could be estimated by VOC ¼ DE0.3, where DE is the energy difference between the highest occupied molecular orbital (HOMO) level of the donor polymer and the lowest unoccupied molecular orbital Corresponding author. Tel.: +81 29 861 3474; fax: +81 29 861 3475.
E-mail address: [email protected] (T. Yamanari). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.09.022
(LUMO) level of the acceptor PCBM. On the other hand, as for small-molecular-weight OSCs, Rand et al. [11] have found a very simple relation (VOC ¼ DE) with no offset. We believe understanding of the origin of VOC will be an effective guideline to accelerate the development of highperformance OSCs. In this study, we investigated the relation between energy levels of the donor/acceptor materials and VOC of OSCs based on five different conjugated polymers. In addition, incident light intensity dependencies of VOC for three different polymer-based OSCs were also examined.
2. Experimental methods 2.1. Sample preparation The device structure is shown in Fig. 1. PCBM was used as the acceptor material. Donor polymers tested in this study were P3HT, poly[tris(2,5-bis(hexyloxy)-1,4-phenylenevinylene)alt-(1,3-phenylenevinylene)] (PTDPV), poly[2-methoxy-5-(30 ,70 dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2). To prepare polymer:PCBM solutions, a polymer and PCBM were dissolved in chlorobenzene. Glass substrates with an indium-tin oxide (ITO) electrode were used for sample preparation. The substrates were exposed to ozone for 30 min. Poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was deposited by spin coating at 3000 rpm (after passing a 0.45 mm filter)
ARTICLE IN PRESS 760
T. Yamanari et al. / Solar Energy Materials & Solar Cells 93 (2009) 759–761
10
A
Polymer : PCBM
V
PEDOT : PSS ITO Glass substrate
Current density / mA cm-2
Al
VOC = 0.33 V
5
0
-5 0.59
0.84
Fig. 1. Schematic device structure of organic solar cells fabricated in this study.
-10 -1.0
and then dried at 135 1C for 10 min. Subsequently, the polymer:PCBM solutions were spin coated onto the substrates. Al electrodes of approximately 60-nm thickness were deposited by vacuum evaporation. The active area of the cell was 0.04 cm2. Once the electrodes were deposited, the devices were stocked in a N2-filled glove box. As for P3HT:PCBM cells, thermal annealing treatment was carried out at 140 1C for 10 min under nitrogen atmosphere after the cell fabrication.
Fig. 2. J–V curves of the PTDPV:PCBM cells with different active layer thickness and polymer content. The illumination light was AM1.5G (100 mW/cm2).
2.2. Measurements
Photovoltaic layer
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
P3HT:PCBM PTDPV:PCBM MEH-PPV:PCBM MDMO-PPV:PCBM F8T2:PCBM
8.7 (0.2) 2.93 (0.11) 3.6 (0.2) 3.1 (0.4) 2.9 (0.3)
0.62 (0.02) 0.843 (0.009) 0.75 (0.04) 0.80 (0.04) 0.84 (0.07)
0.611 (0.007) 0.46 (0.02) 0.44 (0.02) 0.50 (0.04) 0.53 (0.02)
3.28 (0.06) 1.13 (0.02) 1.16 (0.09) 1.3 (0.2) 1.26 (0.08)
The power conversion efficiency (PCE) was calculated from the current density–voltage (J–V) characteristics under Air Mass 1.5 global (AM1.5G) solar simulated light irradiation of 100 mW/cm2. J–V characteristics were measured by the semiconductor characterization system (KEITHLEY 4200, Keithley Co. Ltd.) at room temperature in N2 atmosphere. The ionization potential of donor/ acceptor materials in air was determined using a photo-electron spectrometer (AC-2, Riken Keiki Co. Ltd.). UV–vis spectrum of each homogeneous donor/acceptor-material film was measured by a spectrophotometer (IMUC-7000, Otsuka Electronics Co. Ltd.). HOMO and LUMO levels of donor/acceptor materials were estimated by ionization potentials and absorption edges of UV–vis spectra. For measurement of the incident light intensity dependencies of the cell parameters for polymer:PCBM cells, the illumination intensity was varied controlling an iris of the illumination equipment and using a collective lens.
3. Results and discussion At the beginning, it is worth noting that low-performance cells showed wide variation of VOC. For example, VOC of PTDPV:PCBM cells varied from 0.3 to 0.9 V (see Fig. 2). This result would be brought about by insufficiency of optimization of the device fabrication condition, such as the active layer thickness and the polymer/PCBM ratio. Thinner film thickness causes a larger leakage current. It is known that the large leakage current brings low fill factor (FF) and low VOC from the equivalent circuit study [9]. In addition, van Durren et al. [12] reported that high polymer content cells show high VOC but very low PCE. Therefore, we had to optimize their active layer thickness and the ratio of polymer to PCBM for each polymer:PCBM cell. VOC of the cells with PCE o1% and FF o0.4 were omitted to remove the ambiguity. The photovoltaic parameters of each cell that meets the criteria are presented in Table 1. Standard deviations of VOC for every polymer:PCBM cell were under 0.1 V, indicating that the criteria were enough to obtain reliable VOC. Fig. 3(a) shows VOC of five different polymer-based solar cells plotted versus the HOMO level of the polymers and the energy
-0.5
0.0 Voltage / V
0.5
1.0
Table 1 Photovoltaic performances of five different polymer-based organic solar cells
The measurements were carried out under AM1.5G (100 mW/cm2) irradiation. Each photovoltaic parameter was averaged from 8–12 samples. Values presented in parentheses were the standard deviations.
difference (DE). DE is the energy difference between the HOMO level of the donor polymer and the LUMO level of the acceptor PCBM, as shown in Fig. 3(b). While Scharber et al. [10] reported a liner relation between VOC and DE with offset (VOC ¼ DE0.3; broken line in Fig. 2) for polymer:PCBM OSCs, our results obviously deviate from the relation. Recently, Rand et al. [11] have investigated the incident light intensity and temperature dependencies and have proposed a greatly simple liner relation between maximum VOC, which was not always observed under a typical condition (100 mW/cm2 intensity, 25 1C), and DE without offset (V Max OC ¼ DE; solid line in Fig. 2) for small-molecular-weight OSCs fabricated by vacuum evaporation technique. We have also observed the relation VOC ¼ DE for the small-molecular-weight OSCs by optimizing device structures as for each donor/acceptor materials to obtain best performance under AM1.5G at 100 mW/cm2 intensity (which will be published elsewhere [13]). Taking account of the results in small-molecular-weight OSCs, there must be some factor that decreases the VOC in polymer-based OSCs. To obtain further insight, we studied the incident light intensity dependence of VOC. Fig. 4 shows VOC of the three different polymer:PCBM cells in the intensity range 20–1000 mW/cm2. Although the maximum VOC (V Max OC ) of each cell was slightly higher than VOC under the standard intensity (100 mW/cm2), the simple linear relation has not been observed between VOC and DE. The light intensities that showed the maximum VOC (IV max ) were significantly different from each polymer:PCBM cells. IV max of the P3HT:PCBM, MDMO-PPV:PCBM and F8T2:PCBM cells were 750, 300 and 78 mW/cm2, respectively. In the intensity region lower than IV max , VOC was increased by elevating the light intensity.
ARTICLE IN PRESS T. Yamanari et al. / Solar Energy Materials & Solar Cells 93 (2009) 759–761
recombination of generated photo carriers. Insufficient charge distribution may result in reduced VOC. To achieve higher VOC, it will be necessary to control the internal morphology and remove the impurities in the bulk heterojunction layer of the cells. Regarding the control of the internal morphology, polymer-based OSCs that are fabricated by solution chemistry seem to have a disadvantage when compared to small-molecular-weight OSCs. Vacuum evaporation technique used for fabrication of small-molecular-weight OSCs has more flexibility and that enables making optimized structures, such as complex multilayered device structures [14]. This might be a reason why the relation between VOC and DE has no offset. Conversely, there are possibilities for further improvement of VOC in polymer-based OSCs.
Δ E (HOMOPolymer−LUMOPCBM) / eV 0.8
0.6
1.0
1.2
1.4
1.6
1.8
1.0 0.9 VOC / V
761
0.8 0.7 0.6 0.5 0.4 4.8
4.6
5.0
5.2
5.4
5.6
4. Conclusions
HOMOPolymer Level / eV
2.5
LUMO
2.7
2.8 3.1
3.9
4.8 eV ITO
5.0 PEDOT: PSS
4.8 P3HT
4.3 Al
ΔE 5.0 PTDPV
5.2 MDMO-PPV MEH-PPV
Acknowledgments
5.6 6.0
F8T2
PCBM
HOMO
Fig. 3. (a) VOC of different polymer-based solar cells plotted versus HOMO level of the donor polymers and the energy difference (DE). Solid and broken lines represent the relations VOC ¼ DE and VOC ¼ DE0.3, respectively. (b) Schematic energy diagram of materials used in this study.
1.0
VOC / V
0.9
0.8
0.7
0.6
0.5 0
200
400
600
Light intensity / mW
The simple linear relation between VOC and DE was not observed in the polymer-based OSCs. However, the relation V Max OC pDE seemed to hold. From this point of view, our result does not contradict previous reports [10,11]. There must be some factor that decreases VOC in polymer-based OSCs. Elucidation of the factors and introducing more sophisticated technique to control the internal structure of the bulk heterojunction layer will bring further improvement in the performance of polymer-based OSCs.
800
1000
cm-2
Fig. 4. Incident light intensity dependence of VOC for three different polymerbased solar cells: a P3HT:PCBM cell (square), a MDMO-PPV:PCBM cell (triangle) and an F8T2:PCBM cell (circle).
This result might be accounted for the imperfection of the internal morphology and the presence of impurities in the bulk heterojunction layer. It is possible that defects of internal morphology and impurities trap the charge carriers. The trapped charges form space charge which disturbs the charge distribution within the bulk heterojunction layer. The defects and traps also bring
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade, and Industry (METI). References [1] S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Organic-based photovoltaics: toward low-cost power generation, MRS Bull. 30 (2005) 10–19. [2] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622. [3] M. Reyes-Reyes, K. Kim, D.L. Carroll, High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methocycarbonyl)propyl-1-phenyl-(6,6)C61 blends, Appl. Phys. Lett. 87 (2005) 083506. [4] J.Y. Kim, S.H. Kim, H.-H. Lee, K. Lee, W. Ma, X. Gong, A.J. Heeger, New architecture for high-efficiency polymer photovoltaic cells using solutionbased titanium oxide as an optical spacer, Adv. Mater. 18 (2006) 572–576. [5] Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. McCulloch, C.-S. Ha, M. Ree, A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells, Nat. Mater. 5 (2006) 197–203. [6] S. Yoo, W.J. Potscavage, B. Domercq, J. Kim, J. Holt, B. Kippelen, Integrated organic photovoltaic modules with a scalable voltage output, Appl. Phys. Lett. 89 (2006) 233516. [7] T. Yamanari, T. Taima, J. Sakai, K. Saito. Highly efficient organic thin-film solar cells based on poly(3-hexylthiophene) and soluble C70 fullerene derivative,, Jpn. J. Appl. Phys. 47 (2008) 1230–1233. [8] C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, J.C. Hummelen, Origin of the open circuit voltage of plastic solar cells, Adv. Funct. Mater. 11 (2001) 374–380. [9] A. Moliton, J.-M. Nunzi, How to model the behavior of organic photovoltaic cells, Polym. Int. 55 (2006) 583–600. [10] M.C. Scharber, D. Mu¨hlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Design rules for donors in bulk-heterojunction solar cells— towards 10% energy-conversion efficiency, Adv. Mater. 189 (2006) 789–794. [11] B.P. Rand, D.P. Burk, S.R. Forrest, Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells, Phys. Rev. B 75 (2007) 115327. [12] J.K.J. van Durren, X. Yang, J. Loos, C.W.T. Bulle-Lieuwma, A.B. Sieval, J.C. Hummelen, R.A.J. Janssen, Relating the morphology of poly(p-phenylene vinylene)/methanofullerene blends to solar-cell performance, Adv. Funct. Mater. 14 (2004) 425–434. [13] T. Taima, J. Sakai, T. Yamanari, K. Saito, in preparation. [14] S.R. Forrest, The limits to organic photovoltaic cell efficiency, MRS Bull. 30 (2005) 28–32.