J. Phys. Chem. C 2010, 114, 9161–9166
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High-Efficiency Hybrid Polymer Solar Cells with Inorganic P- and N-Type Semiconductor Nanocrystals to Collect Photogenerated Charges Shuyan Shao,†,‡ Fengmin Liu,† Zhiyuan Xie,*,† and Lixiang Wang† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: February 11, 2010; ReVised Manuscript ReceiVed: March 30, 2010
Efficient charge collection is one of the most important factors to achieve high power conversion efficiency for the bulk-heterojunction solar cells based on polymer blend films. We report enhanced hybrid polymer solar cells based on blends of a semiconducting polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as electron donor and crystalline n-type ZnO nanoparticle as electron acceptor by using p-type semiconducting Cu2O nanocrystal as an anode buffer layer between the indium tin oxide (ITO) anode and the MEH-PPV:ZnO blend layer. The p-type Cu2O nanocrystals of 30-40 nm diameter dispersed on ITO anode surface percolate into the MEH-PPV:ZnO blend layer serving as hole collection antenna to efficiently collect holes. The results show that the charge recombination in the MEH-PPV:ZnO blend layer resulting from low charge mobility of MEH-PPV is effectively restrained via hole-transport bypass of Cu2O nanocrystals. Compared to the control device with poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a flat anode buffer layer, the short-circuit current increases 45% for the device with Cu2O nanocrystal as an anode buffer layer. Moreover, the lower ionization potential of Cu2O nanocrystal renders the device a high open-circuit voltage compared to the device with PEDOT:PSS as an anode buffer layer. The optimal device with Cu2O nanocrystal as an anode buffer layer shows a power conversion efficiency of 2.0%, a 132% increase compared to the device with PEDOT:PSS as an anode buffer layer. I. Introduction Solar cells have been studied extensively in recent years as clean, sustainable, and renewable energy sources due to the increasing global energy crisis. Polymer photovoltaic (PV) cells have evolved as promising alternatives to silicon-based solar cells due to their potential to be low-cost, lightweight, and flexible. Significant progress with power conversion efficiency (PCE) approaching 7% has been made for polymer-fullerene derivative blends by finely manipulating the active-layer morphology and developing new PV polymer materials.1-7 Although the donor/acceptor bulk heterojunction structure overcomes well the limitation of short exciton diffusion length of semiconducting polymers (∼10 nm) by providing abundant donor/acceptor interfaces for charge separation, low charge mobility of the semiconducting polymers is still a drawback for polymer PV technology, which influences the charge collection efficiency and the final PCE. Limited by low charge mobility of the semiconducting polymers, the active layer comprised of polymer-fullerene derivative blend should be thin enough to match the short hole drift length and restrain current loss from charge recombination.8,9 However, the thin active layer would reduce the light-harvesting efficiency and hence hinder the further enhancement of PCE. Among alternative electron acceptors are inorganic semiconductor nanocrystals in view of the complicated synthetic route of fullerene derivatives and the tendency toward phase segregation for the polymer-fullerene blend over time. Actually, much * To whom correspondence should be addressed. E-mail: xiezy_n@ ciac.jl.cn.. † Changchun Institute of Applied Chemistry. ‡ Graduate School of Chinese Academy of Sciences.
attention has been focused on polymer-inorganic hybrid solar cells with the expectation of integrating the advantages of the two materials: solution processability and high absorption of semiconducting polymers, high electron mobility, and good chemical stability of inorganic semiconductor nanocrystals. Up to now, several n-type inorganic semiconductor materials, including CdSe nanodots, nanorods and tetrapods,10-12 CdS nanorods,13 TiO2,14-19 and ZnO nanoparticles,20-22 have been used as electron acceptors in polymer hybrid solar cells. In principle, using high electron-mobility n-type inorganic semiconductor nanocrystals as electron acceptors does not favor resolving the aforementioned less efficient charge-collection caused by low hole mobilities of semiconducting polymers. Herein, we demonstrate a polymer-inorganic hybrid PV cell with high-mobility inorganic p-type and n-type semiconductor nanocrystals to efficiently collect holes and electrons, respectively, and a PCE of 2.0% of the resulting PV cell is achieved. A p-type cuprous oxide (Cu2O) nanocrystal is used as a hole collector on the indium-tin oxide (ITO) anode surface to replace the commonly used poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) anode buffer layer. The blend comprising poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) plus ZnO nanoparticles with a weight ratio of 1:3.3 is used as the active layer in PV cells. The p-type Cu2O nanocrystals of 30-40 nm diameter dispersed on ITO anode surface percolate into the MEH-PPV:ZnO blend layer to serve as a hole collection antenna for efficiently collecting holes. The PV cell with Cu2O as an anode buffer layer demonstrates a short-circuit current (JSC) of 3.92 mA cm-2 and a PCE of 2.0%, about a 45% and 132% increase, respectively, as compared to the control device with PEDOT:PSS as an anode buffer layer. Moreover, the open-circuit voltage (VOC) and fill
10.1021/jp1013169 2010 American Chemical Society Published on Web 04/22/2010
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factor (FF) are also improved considerably. This is the best result reported so far for PPV-based hybrid PV cells and the underlying mechanism for the enhancement of PV performance is discussed. II. Experimental Section Materials. MEH-PPV was purchased from Canton OLEDKING Optoelectronic Materials Co. Ltd. and used as received. ZnO nanoparticles with a diameter of about 5 nm were prepared following the procedure reported by Beek et al.23 A 2.95 g sample of zinc acetate dihydrate (>99%) was dissolved in 120 mL of methanol at 60 °C under vigorous stirring. Then 1.48 g of KOH (>85%) was dissolved in 60 mL of methanol. The KOH solution was dropped into the zinc acetate dihydrate solution in 10 min under vigorous stirring. The solution temperature was held at 60 °C and stirred for 4 h, and then the heating and stirring were removed to allow particles to precipitate for an additional 12 h. The precipitate was cleaned by centrifugation of the dispersion and washed twice with 50 mL of methanol. The washed nanoparticles were dissolved in 5 mL of chlorobenzene (100-120 mg mL-1). Cu2O nanoparticles were synthesized by modifying the method reported by Zhu et al.24 The reaction was carried out in a closed three-necked bottle. Next 1.2 g of Cu(NO3)2 (>99.5%) was dissolved in 50 mL of glycol and the mixture was heated to 140 °C under vigorous stirring. After an hour, the color of the mixture turned deep green from light blue and a reddish brown gas was accumulated over the glycol; the bottle cork was oened to let the reddish brown NO2 gas out and then closed. The mixture was heated for another 1 h and the color turned a shallow yellow. Then the heater was removed and the stirring stopped. PV Cell Fabrication and Measurement. The ITO-coated glass substrate with a sheet resistance of 10 Ohm/square was cleaned with detergent and deionized water in sequence and then dried at 120 °C for 30 min. Cu2O nanoparticles were deposited onto the ITO substrate via spin-coating from glycol solution (7 mg mL-1) and dried in a vacuum chamber for 1 h, then the substrate was moved into a nitrogen-filled glovebox for spin-coating the active layer. In the case of the device with PEDOT:PSS as an anode buffer layer, a 40 nm thick PEDOT: PSS (Baytron P4083) layer was spin-coated onto the ITO substrate and baked at 120 °C for 30 min in ambient. The chlorobenzene solution comprised of MEH-PPV (3-3.5 mg mL-1) plus ZnO (9.9-11.55 mg mL-1) was then spin-coated on top of the PEDOT:PSS or Cu2O layer to produce the active layer. Finally, the device was pumped down under vacuum ( 0.1 V, higher photocurrent and smaller saturation voltage are observed for the device with Cu2O nanocrystal as the anode buffer layer, indicating that this device possesses high charge-collection efficiency. The PV cells with ITO-only and ITO/PEDOT:PSS structures show similar photocurrent variation with the effective applied voltages. The photocurrent of the two devices decreases rapidly with lowering the effective applied
voltages in the region of V0 - V < 1 V, and shows a squareroot dependence on the effective applied voltages in the region of V0 - V > 0.2 V. Such a variation of photocurrent on the effective applied voltages indicates that recombination of charge carriers becomes considerable possibly due to short hole drift length in the MEH-PPV phase and the space-charge-limited effect caused by the much lower hole mobility of MEH-PPV compared to the electron mobility of ZnO in the MEH-PPV: ZnO blend film (see Figure S6 in the Supporting Information),33 while this effect is not obvious for the device with a Cu2O nanocrystal as the anode buffer layer, indicating that the recombination of charge carriers is restrained to some extent due to efficient collection of holes by p-type Cu2O nanocrystals. The light-intensity (Plight) dependence of the photocurrent has also been investigated for the MEH-PPV:ZnO (140 nm) devices with ITO-only, ITO/PEDOT:PSS, and ITO/Cu2O structures and the dependence of Jph on Plight on a double-logarithmic scale is shown in Figure 7. The Plight was varied from 1000 W m-2 down to 3 W m-2, using a set of neutral density filters. It was demonstrated that the intensity dependence of photocurrent can be described by Jph ∝ PSlight, where the exponent S ranges from 0.75 in the case of space-charge-limited photocurrent to 1.0 for the space-charge-free limit.37 The fitted slope S of the Jph-Plight curves of the devices with ITO-only and ITO/PEDOT:PSS structures deviates from unity at low effective applied voltage demonstrating space-charge effect, while the Jph-Plight characteristics of the device with ITO/Cu2O structure do not show obvious space-charge effect with the fitted slope close to unity at all three effective applied voltages. It was reported that the buildup of space-charges due to unbalanced transport of electrons and holes in the polymer PV cell would dramatically reduce the device performance, since it causes fundamental limitation on FF and JSC.33 For our devices with ITO-only and ITO/PEDOT:PSS structures, the slope of 0.84-0.86 at an effective voltage of 0.7 V indicates that there exists considerable space-charges resulting in severe charge recombination. The
High-Efficiency Hybrid Polymer Solar Cells
J. Phys. Chem. C, Vol. 114, No. 19, 2010 9165 IV. Conclusions Limited by low hole mobilities and short hole drift length of the semiconducting polymer, the active layer thickness of polymer bulk-heterojunction photovoltaic cells based on MEHPPV:ZnO blend should be thin to avoid charge recombination and increase of series resistance, but this would sacrifice efficient photon absorption. Herein, the hole collection in the MEH-PPV: ZnO blend film is successfully enhanced by using p-type Cu2O nanocrystal as the anode buffer layer since the Cu2O nanocrystal percolates into the active layer to serve as a hole collector. The optimal thickness of the active layer is increased compared to that of the device with the commonly used PEDOT:PSS anode buffer layer, which allows efficient photon harvesting and results in a large JSC. More interestingly, the VOC is increased by using a p-type Cu2O nanocrystal as the anode buffer layer due to its deep HOMO level. This approach may provide a way to enhance the power conversion efficiency of bulk-heterojunction PV cells based on low-mobility semiconducting polymers. Acknowledgment. The authors acknowledge financial support from the National Natural Science Foundation of China (nos. 50873100, 20834005, and 20921061), 973 Project of Ministry of Science and Technology of China (2009CB623602 and 2009CB930603), and the Chinese Academy of Sciences (KJCX2-YW-M11). Supporting Information Available: X-ray diffraction pattern of Cu2O power, EQE of the PV cells, and the charge transport characteristics of the MEH-PPV:ZnO blend film are elucidated in detail. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 7. Intensity dependence of photocurrent for different PV cells at different effective applied voltages. The slope (S), determined from the linear fit to the experimental data, is shown by solid lines.
resulting devices with ITO-only and ITO/PEDOT:PSS structures show JSC values of 2.6 and 2.7 mA cm-2, and FFs of 0.36 and 0.38, respectively (shown in Figure 5). The device with ITO/ Cu2O structure shows a slope of 0.93, indicating that the spacecharge effect is prevented to some extent due to efficient hole collection by the Cu2O nanocrystal and the recombination of electrons and holes is restrained.33 The device with ITO/Cu2O structure shows a JSC of 3.92 mA cm-2 and an FF of 0.51, respectively, much larger than those of the devices with ITOonly and ITO/PEDOT:PSS structures. These concomitant results highlight that the Cu2O nanocrystal as an anode buffer layer effectively reduces the recombination between electrons and holes and enhances the charge-collection efficiency, and finally the PCE of the PV cells.
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