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The synthesis of MDMO-PPV capped PbS nanorods and their application in solar cells
Current Applied Physics 9 (2009) 1175–1179
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
The synthesis of MDMO-PPV capped PbS nanorods and their application in solar cells Zhijie Wang a, Shengchun Qu a,*, Xiangbo Zeng a, Junpeng Liu b, Changsha Zhang a, Mingji Shi a, Furui Tan a, Zhanguo Wang a a b
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 13 November 2008 Received in revised form 24 December 2008 Accepted 19 January 2009 Available online 23 February 2009 PACS: 73.61.Ph 73.61.Tm 84.60.Jt
a b s t r a c t Poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) capped PbS nanorods about 100 nm in diameter and 400 nm in length were synthesized via a hydrothermal route in toluene and dimethylsulfoxide solution. By blending the PbS nanorods with the MDMO-PPV as the active layer, bulk heterojunction solar cells with an indium tin oxide (ITO)/polyethylenedioxythiophene/polystyrenesulphonate (PEDOT: PSS)/MDMO-PPV: PbS nanorods/Al structure were fabricated in a N2 filled glove box. Current density–voltage characterization of the devices showed that the solar cells with PbS nanorods hybrid with MDMO-PPV as active layer were better in performance than the devices with the polymer only. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanorods Solar cells
1. Introduction PbS is a semiconductor with a narrow bandgap (0.41 eV), a large exciton Bohr radius (18 nm) and a strong quantum-size effect in nanocrystalline form. This makes it be used in many fields such as solar cells [1–6] and IR detectors [1,7]. Furthermore, an efficient multiple exciton generation has been detected in PbS quantum dot, thus rendering it a promising candidate for highly efficient photovoltaic conversion devices [8,9]. So far, PbS nanocrystals have been prepared by many methods, and many different PbS morphologies were attained, such as quantum dots, nanorods, nanostars, nanotubes, and nanomultipods [10– 18]. However, the most successful PbS nanocrystals used in solar cells and photodetectors are oleic acid capped PbS quantum dots or their post-synthetic ligand exchange products. Zhang et al. [2] reported bulk heterojunction solar cells made with MEH-PPV and octylamine-capped PbS quantum dots exchanged from oleic acid capped PbS quantum dots, the power conversion efficiency was 0.15%. Klem et al. [6] reported all inorganic bulk heterojunction infrared photovoltaic cells made with ITO and butylamine capped PbS quantum dots exchanged from oleic acid capped ones. They ob* Corresponding author. Tel.: +86 10 82304240; fax: +86 10 82305052. E-mail address: [email protected] (S. Qu). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.01.008
tained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaic devices. Although the postsynthetic ligand exchange is used to replace the oleic acid with shorter ligands to improve charge transport in the device, this makes the process complex. Furthermore, the shorter ligands still impede charge transport from the one PbS quantum dot to another or other material more or less. Therefore, a few methods were proposed to prepare PbS quantum dots in polymer directly without using any insulator surfactant. Watt et al. [12] and Wang et al. [19] reported the direct synthesis of PbS quantum dots in MEHPPV and MDMO-PPV solution, respectively. However, when the quantum dots are blended with the polymer as the active layer of the devices, the present effective conduction paths for charge transport remain a challenge, because the electrons generated by the dissociation of excitons in the polymer transport between the quantum dots by hopping. In order to reduce the hopping events, it is necessary to elongate the quantum dots to nanorods. In this paper, we adopted a new route to prepare PbS nanorods directly in MDMO-PPV matrix. Blending the MDMO-PPV with the MDMO-PPV capped PbS nanorods as the active layer, bulk heterojunction solar cells were fabricated. The current density–voltage (J–V) measurement shows that the solar cells with blend of the PbS nanorods and MDMO-PPV as the active layer have higher
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photovoltaic performance than the ones with the polymer only, even higher than the device reported by Mcdonald et al. [1]. 2. Experimental
controlled Keithley 2400 Source Meter under 100 mW/cm2 illumination from a solar simulator. A 500 W xenon lamp served as the light source and the light intensity was calibrated using a standard silicon solar cell. In the J–V measurement process, the fill factor (FF) was calculated using
2.1. Synthesis of the MDMO-PPV capped PbS nanorods
FF ¼ ðJVÞmax =J sc V oc ;
Fifteen milli gram of MDMO-PPV (purchased from Aldrich) was dissolved in 30 mL of toluene, forming an orange semi-transparent solution. After 10 min stirring, upon the addition of 6 mL dimethylsulfoxide (DMSO), the solution became transparent immediately, then 50 mg lead acetate (Pb(OAc)2) was added subsequently. Once the Pb(OAc)2 was dissolved in the solution completely, 3 mL of DMSO solution containing 8 mg thioacetamide was added slowly, under vigorous stirring. After 8 min, the colour of the solution became dark red from orange. Then the solution was transferred to a Teflon-lined stainless steel autoclave, heated to 160 °C and maintained for more than 24 h then allowed to cool to room temperature. The resulting dark red solution was transferred to a 250 mL beaker and precipitated with excess ethanol. The precipitate was centrifuged at 6000 rpm for 6 min to remove the unwanted molecular byproducts and washed with ethanol several times.
where (JV)max is the maximum product of J and V that can be calculated from the J–V curve, Jsc is the short circuit current density and the Voc is the open circuit voltage. Defined as the ratio of the electric power output of the cell at the maximum power point to the incident optical power (Plight), the power conversion efficiency (EFF) can be calculated using the following equation,
2.2. Fabrication of the solar cells
EFF ¼ FF J sc V oc =P light :
ð1Þ
ð2Þ
3. Results and discussion 3.1. Characterization of the MDMO-PPV capped PbS nanorods Fig. 2 shows the TEM and HRTEM images of the product. Nanorods with diameter about 100 nm and length about 400 nm are clearly observed. From the HRTEM image, the clearly discernible
The photovoltaic devices were fabricated by spin-coating a blend of the PbS nanorods and MDMO-PPV in an appropriate ratio, sandwiched between a transparent anode and a cathode. In a typical procedure, ITO-coated glass substrate as the anode was cleaned by ultrasonic treatment in detergent, deionized water, acetone and isopropyl alcohol sequentially. Then a thin layer of PEDOT: PSS (Baytron P) was spin-coated to modify the ITO surface. After annealed at 140 °C for 10 min, the substrate was transferred to a nitrogen-filled glove box. To prepare the active layer, MDMOPPV was first dissolved in chlorobenzene to make a 6 mg/mL solution, followed by blending it with the PbS nanorods. The blend was ultrasonicated for 1 h and stirred for 3 h, then transferred to the nitrogen-filled glove box. The active layer was obtained by spincoating the blend at 1500 rpm for 60 s. After the active layer was annealed in the glove box at 110 °C for 22 h, aluminium electrode was deposited under a high vacuum as the cathode. Fig. 1 demonstrates the structure of the solar cells. 2.3. Characterization High-resolution transmission electron microscopy (HRTEM) and TEM for MDMO-PPV capped PbS nanorods were performed using a TECNAI F20 TEM equipped with a field emission gun operating at 200 kV accelerating voltage. Ultraviolet–visible–near infrared (UV–vis–NIR) absorption spectrum was carried out by an UV3100 UV–vis–NIR recording spectrophotometer (Shimadzu). J–V measurements were performed in forward bias with a computer-
Fig. 1. Structure of the bulk heterojunction solar cells.
Fig. 2. (a) TEM image of the MDMO-PPV capped PbS nanorods. (b) HRTEM image of the MDMO-PPV capped PbS nanorods.
Z. Wang et al. / Current Applied Physics 9 (2009) 1175–1179
crystalline lattice demonstrates that the nanorods have a high degree of crystallinity. To confirm the existence of PbS, energy dispersive spectroscopy (EDS) was measured on the nanorods. Fig. 3 demonstrates that the main elements of the sample are Pb and S. The peaks of C, Cu, and O are from the copper grid substrate. UV–vis–NIR absorption spectroscopy was recorded to investigate the absorption properties of the PbS nanorods. Fig. 4 exhibits the absorption spectra of the MDMO-PPV capped PbS nanorods and MDMO-PPV. Compared with the MDMO-PPV absorption spectrum, the MDMO-PPV capped PbS nanorods have a wide absorption range from UV to NIR. The peak from 400 to 550 nm is from the MDMO-PPV capping ligand, which indicates the existence of MDMO-PPV on the surface of the PbS. Since the size of the obtained PbS nanorods is 100 nm in diameter and 400 nm in length, larger than the 18 nm Bohr radius, quantum confinement effect is not expected. The absorption onset should be expected at 3020 nm, the same with the bulk PbS. As a result, we can not find any PbS absorption peak in the range from 300 to 1000 nm. However, the absorption outside the absorption range of MDMO-PPV can be improved when the PbS nanorods exist in the MDMO-PPV matrix. Beyond 600 nm, where MDMO-PPV has no absorption at all, the wide absorption range of the MDMO-PPV capped PbS nanorods can only results from the PbS. The absorption peak of the MDMO-PPV capped PbS nanorods is somewhat blue-shifted as compared with that of the MDMO-PPV, which may also be explained by the presence of PbS that has a good absorption property in the UV range. 3.2. Performance of the solar cells 3.2.1. Effect of ambience in fabrication process In PPV, the oxygen reacts with a vinylene group and forms a carbonyl group that has a strong electron affinity which makes it favorable for the electron of an exciton to transfer into the carbonyl group thereby dissociating the exciton [20,21]. The hole can be free to move along the polymer as a free charge carrier, while the electron is trapped at the site of the carbonyl group [22]. However, this process is not considered to be used as a charge separation mechanism, because it can reduce the transport properties of electrons and holes in the device by building up space charges [22]. To prevent oxidation, devices are usually fabricated and handled in an oxygen-free environment or encapsulated immediately after production. To confirm the analysis of Martin Drees above, we have fabricated solar cells with a 3:1 PbS nanorods to MDMO-PPV weight ratio in air and N2 respectively, and the J–V curves are shown in
Fig. 3. EDS spectrum of the MDMO-PPV capped PbS nanorods.
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Fig. 4. Absorption spectra of the MDMO-PPV and MDMO-PPV capped PbS nanorods. The curves were normalized at the peak of MDMO-PPV.
Fig. 5. The solar cells did not undergo any annealing process. Although the open circuit voltage (Voc) of the device made in N2 is lower than that made in air, the short circuit current density (Jsc) and FF of this solar cell are both higher. As a result, the energy conversion efficiency of the device made in N2 is higher. This can be explained by the oxygen induced space charges. Once the space charges form, the transport properties of electrons and holes are reduced and the charge recombination is increased. For the performance of solar cells, the low Jsc and FF are induced. On the other hand, the photo-oxidation can be interpreted as p-doping the polymer by increasing the number of free holes. When the MDMO-PPV forms p–n junction with PbS nanorods, the oxidation induced pdoping polymer results in a higher work function difference between the polymer and the nanorods as compared with the polymer without oxidation process. Thus, the Voc of the sample made in air is higher. 3.2.2. Effect of annealing For the bulk heterojunction solar cells, the film morphology that reflects crystallinity and aggregation of the acceptor and donor in the film is the key factor that influences the efficiency of solar cells. An effective method to control film morphology is annealing, especially at an annealing temperature above the glass transition temperature of the polymer. Annealing near or above the glass transition temperature of the polymer could allow morphology changes in the film that might yield more favorable electrical
Fig. 5. J–V curves of the solar cells fabricated in air and N2, respectively.
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transport properties in both polymer and nanocrystal phases. Thus, the power conversion efficiency can be greatly improved. Zhang et al. [2], has reported that for MEH-PPV: PbS quantum dots solar cells, annealing at 220 °C for 1 h can improve the efficiency by 600 times. In our system, annealing at 110 °C for 22 h before Al deposition can improve the Voc by nearly 3 times, while annealing at 110 °C for 22 h after Al deposition improves the Voc by about 2 times, though the Jsc of the two devices experienced annealing is a little lower than the device without annealing (Fig. 6). Thermal annealing can help in removing the residue solvent, reducing the free volume and improving the interface with the electrode. As a result, the Voc of the solar cells is improved dramatically. Furthermore, FF is also improved by annealing due to the improved charge transport. Compared with the solar cells that were annealed before Al electrode deposition, the samples annealed after Al electrode deposition have worse Voc and Jsc performance. Thus, the power conversion efficiency is reduced dramatically. This may result from the diffusion of the Al atoms to the active layer during the annealing process, which inhibits charge separation and transport. 3.2.3. Effect of PbS concentration on the performance In order to optimize the performance of the solar cells, influence of the different PbS nanorods contents in the active layer was considered. Fig. 7 displays the performance of solar cells with different PbS contents in weight. All the devices here were fabricated in N2 and annealed at 110 °C for 22 h before Al deposition. As the content increases to 75 wt%, the Jsc, Voc and FF increase correspondingly, thus the energy conversion efficiency is improved obviously. In polymer, the excitons generated by the absorption of photons can not dissociate into free charges due to the high binding energy. Only when the excitons travel to the interface between the polymer and a second material, such as PbS, can they dissociate into free charges due to the difference of Fermi-level. Furthermore, the diffusion length of the excitons in polymer is usually smaller than 10 nm. As s result, when the excitons travel longer than 10 nm, most of them decay. This is the main loss process in the polymer based solar cells. In order to reduce the travel length below the diffusion length and improve the excitons dissociation efficiency, PbS nanorods were added to the MDMO-PPV. Because of the low bandgap of the PbS nanorods, the photon that can not be absorbed by the MDMO-PPV can be absorbed by the PbS nanorods, thus the exciton generation efficiency can also be improved simultaneously. When the excitons dissociate into free charges at the interface of the MDMO-PPV and PbS nanorods, the electrons trans-
Fig. 7. J–V curves of the solar cells with different PbS nanorods contents in weight.
port along the PbS nanorods and then are collected by the Al cathode. The holes transport in the polymer and are collected by the ITO anode. While the content reaches 80 wt%, the conversion efficiency decreases due to the reduction of Voc and FF. Too many nanorods in the polymer can damage the morphology of the polymer and inhibit the hole transport. Therefore, the solar cell with 75 wt% PbS nanorods content fabricated in N2 and annealed at 110 °C for 22 h before Al deposition has the best performance, the best performance is about 0.01%, higher than the performance reported by Mcdonald et al. [1]. 4. Conclusion MDMO-PPV capped PbS nanorods were prepared and applied as acceptor in the active layer of bulk heterojunction solar cells first. Absorption spectra showed that adding the nanorods to the polymer can improve the absorption outside the MDMO-PPV absorption range. To optimize the performance, the influences of ambience, annealing process, and PbS nanorods contents were considered. We found that the solar cell with 75 wt% PbS content fabricated in N2 annealed at 110 °C before Al deposition had the best performance. Acknowledgments This work is supported by the National Natural Science Foundation of China (Contract Nos. 60736034, 60576036) and the National Basic Research Programme of China with Contract Nos 2006CB202604 and 2006CB604904, the National High Technology Research and Development Program of China with Contract Nos 2006AA03Z408. References
Fig. 6. J–V curves of the solar cells undergone different annealing processes. The samples of ‘‘NonA”, ‘‘PreA” and ‘‘PostA” show the solar cells without an annealing process, annealed at 110 °C for 22 h before Al deposition and annealed at 110 °C for 22 h after Al deposition, respectively.
[1] S.A. Mcdonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J.D. Klem, L. Levina, E.H. Sargent, Nature Materials 4 (2005) 138. [2] S. Zhang, P.W. Cyr, S.A. McDonald, G. Konstantatos, E.H. Sargent, Appl. Phys. Lett. 87 (2005) 233101. [3] S. Günes, K.P. Fritz, H. Neugebauer, N.S. Sariciftci, S. Kumar, G.D. Scholes, Sol. Energ. Mater. Sol. Cells 91 (2007) 420. [4] P. Wang, L. Wang, B. Ma, B. Li, Y. Qiu, J. Phys. Chem. B 110 (2006) 14406. [5] R. Bayon, R. Musembi, A. Belaidi, M. Bär, T. Guminskaya, M.-Ch. Lux-Steiner, Th. Dittrich, Sol. Energ. Mater. Sol. Cells 89 (2005) 13. [6] E.J.D. Klem, D.D. MacNeil, P.W. Cyr, L. Levina, E.H. Sargent, Appl. Phys. Lett. 90 (2007) 183113. [7] P. Gadenne, Y. Yagil, G. Deutscher, J. Appl. Phys. 66 (1989) 3019. [8] R.D. Schaller, M. Sykora, J.M. Pietryga, V.I. Klimov, Nano Lett. 6 (2006) 424. [9] R.J. Ellingson, M.C. Beard, J.C. Johnson, P. Yu, O.I. Micic, A.J. Nozik, A. Shabaev, A.L. Efros, Nano Lett. 5 (2005) 865. [10] Y. Zhou, H. Itoh, T. Uemura, K. Naka, Y. Chujo, Langmuir 18 (2002) 5287.
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X. Lu, Y. Zhao, C. Wang, Adv. Mater. 17 (2005) 2485. A.A.R. Watt, H. Rubinsztein-dunlop, P. Meredityh, Synth. Met. 154 (2005) 57. S. Wang, F. Gu, M. Lü, Langmuir 22 (2006) 398. A. Qin, Y. Fang, W. Zhao, H. Liu, C. Su, J. Cryst. Growth 283 (2005) 230–241. S. Wu, H. Zeng, Z.A. Schelly, Langmuir 21 (2005) 686. D. Kuang, A. Xu, Y. Fang, H. Liu, C. Frommem, D. Fenske, Adv. Mater. 15 (2003) 1747. [17] K.R. Choudhury, Y. Sahoo, S. Jang, P.N. Prasad, Adv. Funct. Mater. 15 (2005) 751. [18] J. Ge, J. Wang, H. Zhang, X. Wang, Q. Peng, Y. Li, Chem. Eur. J. 11 (2005) 1889.
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[19] Z.J. Wang, S.C. Qu, X.B. Zeng, C.S. Zhang, M.J. Shi, F.R. Tan, Z.G. Wang, J.P. Liu, Y.B. Hou, F. Teng, Z.H. Feng, Polymer 49 (2008) 4647. [20] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (5) (1994) 744. [21] L.J. Rothberg, M. Yan, S. Son, M.E. Galvin, E.W. Kwock, T.M. Miller, H.E. Katz, R.C. Haddon, F. Papadimitrakopoulos, Synth. Met. 78 (1996) 231. [22] M. Drees, Polymer/fullerene photovoltaic devices – nanoscale control of the interface by thermally-controlled interdiffusion, Doctor Thesis, Virginia Polytechnic Institute and State University, 2003, p. 14.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
The synthesis of MDMO-PPV capped PbS nanorods and their application in solar cells Zhijie Wang a, Shengchun Qu a,*, Xiangbo Zeng a, Junpeng Liu b, Changsha Zhang a, Mingji Shi a, Furui Tan a, Zhanguo Wang a a b
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
a r t i c l e
i n f o
Article history: Received 13 November 2008 Received in revised form 24 December 2008 Accepted 19 January 2009 Available online 23 February 2009 PACS: 73.61.Ph 73.61.Tm 84.60.Jt
a b s t r a c t Poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) capped PbS nanorods about 100 nm in diameter and 400 nm in length were synthesized via a hydrothermal route in toluene and dimethylsulfoxide solution. By blending the PbS nanorods with the MDMO-PPV as the active layer, bulk heterojunction solar cells with an indium tin oxide (ITO)/polyethylenedioxythiophene/polystyrenesulphonate (PEDOT: PSS)/MDMO-PPV: PbS nanorods/Al structure were fabricated in a N2 filled glove box. Current density–voltage characterization of the devices showed that the solar cells with PbS nanorods hybrid with MDMO-PPV as active layer were better in performance than the devices with the polymer only. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanorods Solar cells
1. Introduction PbS is a semiconductor with a narrow bandgap (0.41 eV), a large exciton Bohr radius (18 nm) and a strong quantum-size effect in nanocrystalline form. This makes it be used in many fields such as solar cells [1–6] and IR detectors [1,7]. Furthermore, an efficient multiple exciton generation has been detected in PbS quantum dot, thus rendering it a promising candidate for highly efficient photovoltaic conversion devices [8,9]. So far, PbS nanocrystals have been prepared by many methods, and many different PbS morphologies were attained, such as quantum dots, nanorods, nanostars, nanotubes, and nanomultipods [10– 18]. However, the most successful PbS nanocrystals used in solar cells and photodetectors are oleic acid capped PbS quantum dots or their post-synthetic ligand exchange products. Zhang et al. [2] reported bulk heterojunction solar cells made with MEH-PPV and octylamine-capped PbS quantum dots exchanged from oleic acid capped PbS quantum dots, the power conversion efficiency was 0.15%. Klem et al. [6] reported all inorganic bulk heterojunction infrared photovoltaic cells made with ITO and butylamine capped PbS quantum dots exchanged from oleic acid capped ones. They ob* Corresponding author. Tel.: +86 10 82304240; fax: +86 10 82305052. E-mail address: [email protected] (S. Qu). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.01.008
tained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaic devices. Although the postsynthetic ligand exchange is used to replace the oleic acid with shorter ligands to improve charge transport in the device, this makes the process complex. Furthermore, the shorter ligands still impede charge transport from the one PbS quantum dot to another or other material more or less. Therefore, a few methods were proposed to prepare PbS quantum dots in polymer directly without using any insulator surfactant. Watt et al. [12] and Wang et al. [19] reported the direct synthesis of PbS quantum dots in MEHPPV and MDMO-PPV solution, respectively. However, when the quantum dots are blended with the polymer as the active layer of the devices, the present effective conduction paths for charge transport remain a challenge, because the electrons generated by the dissociation of excitons in the polymer transport between the quantum dots by hopping. In order to reduce the hopping events, it is necessary to elongate the quantum dots to nanorods. In this paper, we adopted a new route to prepare PbS nanorods directly in MDMO-PPV matrix. Blending the MDMO-PPV with the MDMO-PPV capped PbS nanorods as the active layer, bulk heterojunction solar cells were fabricated. The current density–voltage (J–V) measurement shows that the solar cells with blend of the PbS nanorods and MDMO-PPV as the active layer have higher
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photovoltaic performance than the ones with the polymer only, even higher than the device reported by Mcdonald et al. [1]. 2. Experimental
controlled Keithley 2400 Source Meter under 100 mW/cm2 illumination from a solar simulator. A 500 W xenon lamp served as the light source and the light intensity was calibrated using a standard silicon solar cell. In the J–V measurement process, the fill factor (FF) was calculated using
2.1. Synthesis of the MDMO-PPV capped PbS nanorods
FF ¼ ðJVÞmax =J sc V oc ;
Fifteen milli gram of MDMO-PPV (purchased from Aldrich) was dissolved in 30 mL of toluene, forming an orange semi-transparent solution. After 10 min stirring, upon the addition of 6 mL dimethylsulfoxide (DMSO), the solution became transparent immediately, then 50 mg lead acetate (Pb(OAc)2) was added subsequently. Once the Pb(OAc)2 was dissolved in the solution completely, 3 mL of DMSO solution containing 8 mg thioacetamide was added slowly, under vigorous stirring. After 8 min, the colour of the solution became dark red from orange. Then the solution was transferred to a Teflon-lined stainless steel autoclave, heated to 160 °C and maintained for more than 24 h then allowed to cool to room temperature. The resulting dark red solution was transferred to a 250 mL beaker and precipitated with excess ethanol. The precipitate was centrifuged at 6000 rpm for 6 min to remove the unwanted molecular byproducts and washed with ethanol several times.
where (JV)max is the maximum product of J and V that can be calculated from the J–V curve, Jsc is the short circuit current density and the Voc is the open circuit voltage. Defined as the ratio of the electric power output of the cell at the maximum power point to the incident optical power (Plight), the power conversion efficiency (EFF) can be calculated using the following equation,
2.2. Fabrication of the solar cells
EFF ¼ FF J sc V oc =P light :
ð1Þ
ð2Þ
3. Results and discussion 3.1. Characterization of the MDMO-PPV capped PbS nanorods Fig. 2 shows the TEM and HRTEM images of the product. Nanorods with diameter about 100 nm and length about 400 nm are clearly observed. From the HRTEM image, the clearly discernible
The photovoltaic devices were fabricated by spin-coating a blend of the PbS nanorods and MDMO-PPV in an appropriate ratio, sandwiched between a transparent anode and a cathode. In a typical procedure, ITO-coated glass substrate as the anode was cleaned by ultrasonic treatment in detergent, deionized water, acetone and isopropyl alcohol sequentially. Then a thin layer of PEDOT: PSS (Baytron P) was spin-coated to modify the ITO surface. After annealed at 140 °C for 10 min, the substrate was transferred to a nitrogen-filled glove box. To prepare the active layer, MDMOPPV was first dissolved in chlorobenzene to make a 6 mg/mL solution, followed by blending it with the PbS nanorods. The blend was ultrasonicated for 1 h and stirred for 3 h, then transferred to the nitrogen-filled glove box. The active layer was obtained by spincoating the blend at 1500 rpm for 60 s. After the active layer was annealed in the glove box at 110 °C for 22 h, aluminium electrode was deposited under a high vacuum as the cathode. Fig. 1 demonstrates the structure of the solar cells. 2.3. Characterization High-resolution transmission electron microscopy (HRTEM) and TEM for MDMO-PPV capped PbS nanorods were performed using a TECNAI F20 TEM equipped with a field emission gun operating at 200 kV accelerating voltage. Ultraviolet–visible–near infrared (UV–vis–NIR) absorption spectrum was carried out by an UV3100 UV–vis–NIR recording spectrophotometer (Shimadzu). J–V measurements were performed in forward bias with a computer-
Fig. 1. Structure of the bulk heterojunction solar cells.
Fig. 2. (a) TEM image of the MDMO-PPV capped PbS nanorods. (b) HRTEM image of the MDMO-PPV capped PbS nanorods.
Z. Wang et al. / Current Applied Physics 9 (2009) 1175–1179
crystalline lattice demonstrates that the nanorods have a high degree of crystallinity. To confirm the existence of PbS, energy dispersive spectroscopy (EDS) was measured on the nanorods. Fig. 3 demonstrates that the main elements of the sample are Pb and S. The peaks of C, Cu, and O are from the copper grid substrate. UV–vis–NIR absorption spectroscopy was recorded to investigate the absorption properties of the PbS nanorods. Fig. 4 exhibits the absorption spectra of the MDMO-PPV capped PbS nanorods and MDMO-PPV. Compared with the MDMO-PPV absorption spectrum, the MDMO-PPV capped PbS nanorods have a wide absorption range from UV to NIR. The peak from 400 to 550 nm is from the MDMO-PPV capping ligand, which indicates the existence of MDMO-PPV on the surface of the PbS. Since the size of the obtained PbS nanorods is 100 nm in diameter and 400 nm in length, larger than the 18 nm Bohr radius, quantum confinement effect is not expected. The absorption onset should be expected at 3020 nm, the same with the bulk PbS. As a result, we can not find any PbS absorption peak in the range from 300 to 1000 nm. However, the absorption outside the absorption range of MDMO-PPV can be improved when the PbS nanorods exist in the MDMO-PPV matrix. Beyond 600 nm, where MDMO-PPV has no absorption at all, the wide absorption range of the MDMO-PPV capped PbS nanorods can only results from the PbS. The absorption peak of the MDMO-PPV capped PbS nanorods is somewhat blue-shifted as compared with that of the MDMO-PPV, which may also be explained by the presence of PbS that has a good absorption property in the UV range. 3.2. Performance of the solar cells 3.2.1. Effect of ambience in fabrication process In PPV, the oxygen reacts with a vinylene group and forms a carbonyl group that has a strong electron affinity which makes it favorable for the electron of an exciton to transfer into the carbonyl group thereby dissociating the exciton [20,21]. The hole can be free to move along the polymer as a free charge carrier, while the electron is trapped at the site of the carbonyl group [22]. However, this process is not considered to be used as a charge separation mechanism, because it can reduce the transport properties of electrons and holes in the device by building up space charges [22]. To prevent oxidation, devices are usually fabricated and handled in an oxygen-free environment or encapsulated immediately after production. To confirm the analysis of Martin Drees above, we have fabricated solar cells with a 3:1 PbS nanorods to MDMO-PPV weight ratio in air and N2 respectively, and the J–V curves are shown in
Fig. 3. EDS spectrum of the MDMO-PPV capped PbS nanorods.
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Fig. 4. Absorption spectra of the MDMO-PPV and MDMO-PPV capped PbS nanorods. The curves were normalized at the peak of MDMO-PPV.
Fig. 5. The solar cells did not undergo any annealing process. Although the open circuit voltage (Voc) of the device made in N2 is lower than that made in air, the short circuit current density (Jsc) and FF of this solar cell are both higher. As a result, the energy conversion efficiency of the device made in N2 is higher. This can be explained by the oxygen induced space charges. Once the space charges form, the transport properties of electrons and holes are reduced and the charge recombination is increased. For the performance of solar cells, the low Jsc and FF are induced. On the other hand, the photo-oxidation can be interpreted as p-doping the polymer by increasing the number of free holes. When the MDMO-PPV forms p–n junction with PbS nanorods, the oxidation induced pdoping polymer results in a higher work function difference between the polymer and the nanorods as compared with the polymer without oxidation process. Thus, the Voc of the sample made in air is higher. 3.2.2. Effect of annealing For the bulk heterojunction solar cells, the film morphology that reflects crystallinity and aggregation of the acceptor and donor in the film is the key factor that influences the efficiency of solar cells. An effective method to control film morphology is annealing, especially at an annealing temperature above the glass transition temperature of the polymer. Annealing near or above the glass transition temperature of the polymer could allow morphology changes in the film that might yield more favorable electrical
Fig. 5. J–V curves of the solar cells fabricated in air and N2, respectively.
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transport properties in both polymer and nanocrystal phases. Thus, the power conversion efficiency can be greatly improved. Zhang et al. [2], has reported that for MEH-PPV: PbS quantum dots solar cells, annealing at 220 °C for 1 h can improve the efficiency by 600 times. In our system, annealing at 110 °C for 22 h before Al deposition can improve the Voc by nearly 3 times, while annealing at 110 °C for 22 h after Al deposition improves the Voc by about 2 times, though the Jsc of the two devices experienced annealing is a little lower than the device without annealing (Fig. 6). Thermal annealing can help in removing the residue solvent, reducing the free volume and improving the interface with the electrode. As a result, the Voc of the solar cells is improved dramatically. Furthermore, FF is also improved by annealing due to the improved charge transport. Compared with the solar cells that were annealed before Al electrode deposition, the samples annealed after Al electrode deposition have worse Voc and Jsc performance. Thus, the power conversion efficiency is reduced dramatically. This may result from the diffusion of the Al atoms to the active layer during the annealing process, which inhibits charge separation and transport. 3.2.3. Effect of PbS concentration on the performance In order to optimize the performance of the solar cells, influence of the different PbS nanorods contents in the active layer was considered. Fig. 7 displays the performance of solar cells with different PbS contents in weight. All the devices here were fabricated in N2 and annealed at 110 °C for 22 h before Al deposition. As the content increases to 75 wt%, the Jsc, Voc and FF increase correspondingly, thus the energy conversion efficiency is improved obviously. In polymer, the excitons generated by the absorption of photons can not dissociate into free charges due to the high binding energy. Only when the excitons travel to the interface between the polymer and a second material, such as PbS, can they dissociate into free charges due to the difference of Fermi-level. Furthermore, the diffusion length of the excitons in polymer is usually smaller than 10 nm. As s result, when the excitons travel longer than 10 nm, most of them decay. This is the main loss process in the polymer based solar cells. In order to reduce the travel length below the diffusion length and improve the excitons dissociation efficiency, PbS nanorods were added to the MDMO-PPV. Because of the low bandgap of the PbS nanorods, the photon that can not be absorbed by the MDMO-PPV can be absorbed by the PbS nanorods, thus the exciton generation efficiency can also be improved simultaneously. When the excitons dissociate into free charges at the interface of the MDMO-PPV and PbS nanorods, the electrons trans-
Fig. 7. J–V curves of the solar cells with different PbS nanorods contents in weight.
port along the PbS nanorods and then are collected by the Al cathode. The holes transport in the polymer and are collected by the ITO anode. While the content reaches 80 wt%, the conversion efficiency decreases due to the reduction of Voc and FF. Too many nanorods in the polymer can damage the morphology of the polymer and inhibit the hole transport. Therefore, the solar cell with 75 wt% PbS nanorods content fabricated in N2 and annealed at 110 °C for 22 h before Al deposition has the best performance, the best performance is about 0.01%, higher than the performance reported by Mcdonald et al. [1]. 4. Conclusion MDMO-PPV capped PbS nanorods were prepared and applied as acceptor in the active layer of bulk heterojunction solar cells first. Absorption spectra showed that adding the nanorods to the polymer can improve the absorption outside the MDMO-PPV absorption range. To optimize the performance, the influences of ambience, annealing process, and PbS nanorods contents were considered. We found that the solar cell with 75 wt% PbS content fabricated in N2 annealed at 110 °C before Al deposition had the best performance. Acknowledgments This work is supported by the National Natural Science Foundation of China (Contract Nos. 60736034, 60576036) and the National Basic Research Programme of China with Contract Nos 2006CB202604 and 2006CB604904, the National High Technology Research and Development Program of China with Contract Nos 2006AA03Z408. References
Fig. 6. J–V curves of the solar cells undergone different annealing processes. The samples of ‘‘NonA”, ‘‘PreA” and ‘‘PostA” show the solar cells without an annealing process, annealed at 110 °C for 22 h before Al deposition and annealed at 110 °C for 22 h after Al deposition, respectively.
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