High-Efficiency Organic Solar Cells Based on End-Functional-Group-Modified Poly (3-hexylthiophene)

High-Efficiency Organic Solar Cells Based on End-Functional-Group-Modified Poly...

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Figure 3. Surface composition of the end-functional-group-modified P3HT/PCBM-blend films measured by X-ray photoelectron spectroscopy (XPS). (a) as-prepared, (b) annealed at 1008C for 15min, (c) representative XPS spectra with changes of the S2p peak of both sides of the

PEDOT:PSS and air/film interface in P3HT CF3 and OH/PCBM-blend film.

4 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–6 Final page numbers not assigned w.MaterialsViews.com w.advmat.de

is not fully developed at this stage, thus, in case of nonannealed devices, device properties of modified P3HTs show no big differences compared with annealed devices (Fig. 4 inset, efficiency h¼0.6 1.4%). However, by thermal treatment, the morphology is fully developed and the effect of the P3HT end group becomes clearer, which results in a big difference in the device performance, as shown in Figure 4. The annealing process is one of the most promising post-treatments to enhance the device properties. The annealing process is known to cause an increase of the crystallization of the donor polymer, PCBM clustering for percolation through the device, and the pathway keeping of the carrier transport. In our study, enhanced device properties can be achieved through the annealing process in any kind of end-functional-group modification. However, especially in the case of the P3HT CF3 and CH3, a dramatic increase of the efficiency caused by the optimal morphology and phase separation was obtained compared with other groups. For annealed P3HT CF3/PCBM films the F is 0.69 and the efficiency is 4.5% (average values, summarized in Table 2). The larger values for Jsc (short-circuit current density) and F imply a lower series resistance and a higher carrier mobility (the series resistance, Rs, decreases from the P3HT OH/PCBM value of

Rs¼20.10V cm2 to Rs¼1.40V cm2 for P3HT CF3/PCBMafter annealing). Thus, the increased efficiency as a function of the end functional group of P3HTresults from morphology improvement achieved by a better mixing of two components in the BHJ film.

In conclusion, we have demonstrated the preparation of polymer solar cells with a power-conversion efficiencies of 4.5% by altering the end functional group of P3HTand by introducing thermal annealing post-fabrication. The higher efficiencies may be attributed to thermally induced morphology improvements, resulting from the matched surface energies of the two components, which improve transport across the interface between the BHJ materials. The phase separation of the active layer, in both the vertical and horizontal directions throughout the films, can be controlled by surface-energy matching of the donor/ acceptor materials. Improved nanoscale morphology results in more-efficient charge generation, and this reduces series resistance and raises F, both of which contribute to higher solar cell efficiency.

Experimental

The structures of BHJ solar cells, using end-functional-group-modified P3HT as the electron donor and PCBM as the acceptor, were fabricated as: ITO/PEDOT:PSS/P3HT:PCBM/LiF/Alandeachdevicehadanactiveareaof 0.0515cm2. The synthesis and characterization of the endfunctional-group-modified P3HT are described in the Supporting Information. GPC (Waters) was performed using THF as the eluent, and the molecular weight was calibrated based on PS standards. MALDI-TOF (Bruker) MS experiment was performed using a dithranol matrix without any salt.

For a blend solution with PCBM, the synthesized P3HT was stirred for 1 day with PCBM (purchased from Nano-C) in chlorobenzene (1wt% P3HT/PCBM blend 1:1). The ITO glass was cleaned by sequential 30min sonication in solutions of detergent (Mucasol), acetone, and isopropyl alcohol and then rinsed with deionized (DI) water. The ITO substrate was treated with UV–ozone to improve the wettability of PEDOT:PSS. The P3HT/PCBM-blend solution was spin-coated at 700rpm for 2min onto the PEDOT/PSS-coated ITO glass (PEDOT/PSS; Baytron P TP AI 4083, Bayer AG; spin-coating conditions of 90 s at 2000rpm, producing a film thickness of 30nm) and a thin film of the P3HT/PCBM blend was obtained. Subsequently, the devices were thermally annealed at 1008C for 15min. LiF (7A) and Al electrodes (each 150nm thick) were sequentially thermal evaporated. The performance of solar cells on ITO substrates was determined by measuring the J–V characteristics in the dark and under AM 1.5 solar illumination (with respect to a reference cell PVM 132, calibrated with NREL) at an intensity of 100mW cm 2 with an Oriel 1kW solar simulator, using a programmable Keithley mode 4200 power source. All electrical measurements and active layer fabrication were executed in a nitrogen gas-purged glove box.

XRD and TEM were used for structural analysis and morphology. XRD experiments were performed at the 3C2, 10C1, 8C1 beamline (1.54A˚ wavelength) at the Pohang Accelerator Laboratory (PAL). The measurements were obtained in a scanning interval of 2u between 38 and 258.T o

Figure 4. a,b) Schematic diagram of the representative annealed films of

P3HT CF3/PCBM-and P3HT OH/PCBM-blend films. c) J–V characteristics of the photovoltaic devices for various end-functional-group-modified

P3HT/PCBM-blend films after annealing ( Br (filled circle), OH (open circle), CH3 (filled square), CF3 (open square)). Inset: device performance of the as-prepared film.

Table 2. Summary of the photovoltaic device performance for various end-functional-group-modified P3HT/PCBM blends.

End Group Voc [V] [a] Jsc [mA/cm2]F F h [%] Rs [V cm2]

[a] Voc is the open circuit voltage.

w.advmat.de w.MaterialsViews.com prepare a sample for TEM (HITACHI-7600 operating at 100kV) and high resolution TEM (HR-TEM; Jeol-1200 EX I operating at 200kV), P3HT/ PCBM-blend filmswere floatedonwaterand placedontoa coppergrid.The optical properties of the PV cells were characterized by UVabsorbance and photoluminescence. UV absorbance was measured using an Agilent 8453 spectrometer (HP) in transmission geometry and PL measurements were performed on a Fluorolog-3 spectrofluorometer (Jobin Yvon Horiba). The surface energy was calculated from contact-angle measurements performed with a FACE contact-angle meter (Kyowa Kaimenkagabu). Film thickness was determined with an ellipsometer (M-200V, H. A. Woollam), and a surface profiler (Alpha-step 500). The vertical phase separation of the films was analyzed by XPS, recorded on a VG ESCALAB 220i spectrometer on the Mg Ka X-ray line (1253.6eV) operated at 15kV and 20mA. Because the depth profile process for Arþ sputtering causes the films to collapse, we only attempted XPS analysis at two points on the PEDOT:PSS and air/film interfaces. Therefore, to obtain XPS data of the PEDOT:PSS side, the film was floated on water and attached to an ITO substrate.

Acknowledgements

J. S. K. and Y. L. contributed equally to this work. This work was supported by the National Research Foundation of Korea Grant (KRF-2008-313- D00254), National Creative Research Initiative Program (R-16-2004-003- 01001-0), and the Pohang Accelerator Laboratory for providing the 3C2, 8C1 and 10C1 beamlines used in this study. Supporting Information is available online from Wiley InterScience or from the author.

Revised: September 19, 2009 Published online:

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