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

By Jong Soo Kim, Youngmin Lee, Ji Hwang Lee, Jong Hwan Park, Jin Kon Kim,* and Kilwon Cho*

Organic photovoltaic devices have drawn much attention in research and industry due to their low cost, easy fabrication, mechanical flexibility, and the general applicability of organic materials. Recently, power conversion efficiencies (PCEs) in excess of 5% have been reported for the poly(3-hexylthiophene)/

[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) solar cell.[1–4] The P3HT/PCBM pair successfully enhances the PCE of organic photovoltaics (OPVs) because these constituents mix homogeneously[5] in blend solutions. Phase separation, which can affect the efficiency of charge separation, is controllable with post-treatments such as thermal or solvent annealing.[6–8] Phase separation between P3HT and PCBM is a critical issue for device efficiency because the exciton diffusion length of P3HT is extremely short, on length scales of 10nm. Charge separation can only occur at the donor/ acceptor interface. Additionally, a percolation pathway to each electrode must be well established to reduce loss due to the recombination of separated charges. A nanometer-scale bicontinuous network that is suitable for enhanced charge transport[9–12] is, therefore, essential for OPV device function.

The morphology[13–15] of the active layer depends on the choice of layer components.[16,17] In particular, the physical interaction[18] between donor and acceptor is the primary determinant of phase separation in the active layer, which affects the device performance.[19–21] The miscibility of constituents as well as crystallinity control can also significantly affect the device performance. However, an optimized morphology from blend films is difficult to obtain. PCBM diffusion, which is the determining factor for phase separation, and P3HTcrystallization occur simultaneously and interfere with attempts to modify the blend by altering the solvent type,[2] concentrations of the blend components, deposition technique, solvent evaporation rate, or thermal/solvent annealing.

Recently, Kim et al. reported that an end functional group modified with H Ho rH Br of P3HT can affect the performance of the photovoltaic device,[23] especially with respect to charge trapping and device hysteresis. They focused on the bromine end group, which is a typical end group for P3HT and showed that the different sizes and electrostatic interactions of the H and Br atoms, which determine chain packing and charge trapping, can affect the device performances. However, their study focused on the electrical properties without examining the active-layer morphology, which is the most important factor affecting device efficiency. Moreover, they used only two types of end-functional P3HTs. Therefore, systematic tests of a variety of end-group-modified P3HT/PCBM blends will establish the correlation between functional-group modification and both active-layer morphology and the performance of the photovoltaic devices.

In the present work, we synthesized several end-groupmodified P3HTs and fabricated a device from the modified P3HT/PCBM blend. We controlled the phase separation of the P3HT/PCBM blend by changing the surface energy of P3HT via end-group modification. From the optimized P3HT/PCBM-blend morphology, we obtained enhanced PCE of 4.5% with a fill factor (F) of 0.69.

Bromo end-capped P3HT (P3HT Br) was synthesized according to the organozinc coupling method, which features nickel-catalyzed cross coupling of thiophene derivative monomers that are prepared by reactions of 3-hexylthiophene with lithium diisopropylamine and zinc chloride. P3HT-Br was then converted to hydroxyl, ethyl, or perfluoro end-functional P3HT

(P3HT OH, P3HT CH3, P3HT CF3, Fig. 1a). The synthesized materials were characterized by NMR,

Fourier-transform infrared (FT-IR) spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), and gel permeation chromatography (GPC) (Supporting Information). The number-average molecular weight

To study the effect of the end groups, a relatively low molecular weight of P3HTwas used. In the view point of molecular weight, that is not an optimal condition. However, a rather high PCE was obtained by optimizing blend morphology using endgroup-modified P3HT. The composition of end groups was analyzed by MALDI-TOF MS. After synthesis of P3HT (P3HT Br), the end group of the polymer was always a mixture of hydrogen and bromine due to the termination mechanism of the polymerization. The amount of the bromo end group was w.MaterialsViews.com w.advmat.de

[*] Prof. K. Cho, Prof. J. K. Kim, Y. Lee, J. H. Park

Department of Chemical Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea) E-mail: kwcho@postech.ac.kr jkkim@postech.ac.kr

Dr. J. S. Kim, Dr. J. H. Lee School of Environmental Science and Engineering Polymer Research Institute Pohang University of Science and Engineering Pohang, 790-784 (Korea)

DOI: 10.1002/adma.200902803

w.advmat.de w.MaterialsViews.com

calculated to 50% in P3HT Br. After synthesis of a tetrahydropyranyl (THP)-protected intermediate, the composition of the end functional group was confirmed again by comparing molecular weight from MALDI-TOF MS with the integration ratio of related peaks in the NMR spectrum. Although the end functional moiety was not ideal, it was enough to observe the effect of the end functional group.

We used cyclic voltammetry (CV) to verify that the endfunctional-group modification does not significantly change the molecular band gap, an important factor in solar cells. We found that the highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) levels of the modified P3HTs are similar to those of the pristine P3HT, 5.2 and 2.8eV, respectively. Blend solutions (P3HT/PCBM¼1:1 by weight) were prepared with chlorobenzene and these solutions were spin-coated onto poly- (3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)- coated indium tin oxide (ITO) glass. The devices were annealed at 1008C for 15min.

The morphology of the active layer was verified by transmission electron microscopy (TEM). Figures 1b–d show the

P3HT CF3, Br, and OH films, respectively, blended with

PCBM as described. Relatively dark regions in the TEM image indicate PCBM-rich regions or aggregates.[24] For the

P3HT CF3/PCBM blend, a fully mixed (the PCBM aggregation regions are very tiny) morphology was obtained before annealing.

However, a significantly different morphology for P3HT OH/ PCBM was observed despite the lack of thermal treatment. It is clear that the initial film morphology of the blend films depends on the end functional group of P3HT. After annealing, the morphology becomes very different.

P3HT CF3/PCBMfilmshavesmaller(20nm)aggregatedPCBM regions than the 400–500-nm-sized P3HT OH/PCBM-blend films. The addition of the CF3 end group to P3HT suppresses the overgrowth of PCBM aggregates. Control of the nanoscale morphologyoftheP3HT/PCBMblendresultsinoptimizedphase separation for efficient charge separation and transport and, thereby, leads to high-efficiency solar cells. In addition to the phase separation mechanism of the active layer correlated with the P3HT crystallization and nanocrystal formation of PCBM,[24,25] we can speculate that the phase separation can be controlled by the end functional group of the donor polymer. The surface energy of the P3HT changes with end-group

Figure 1. a) The molecular structures of materials used in this work. Various end functional groups were modified for P3HT. TEM images of the end- functional-group-modified P3HT/PCBM film morphology. As-prepared films of P3HT CF3/PCBM (b), P3HT Br/PCBM (c), and P3HT OH/PCBM (d). Annealed (at 1008C) film of P3HT-CF3/PCBM (e), P3HT Br /PCBM (f), and P3HT-OH /PCBM (g).

2 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 modification. Table 1 shows the surface energies of the various end-functional-group-modified P3HT and PCBM compounds. Among the four end-functional-group-modified P3HTs, the surface energy (34.3mJ m 2) of P3HT CF3 is similar to that of PCBM (34.2mJ m 2). Therefore, similar surface energies of these individual compounds result in homogeneous blends and well-mixed films, resulting from optimal phase separation of the PCBM domains after thermal annealing.

Figure 2a shows the UV absorption of thin films of the end-group-modified derivatives of P3HT. In blend films, the absorption regions of PCBMat 340nm and P3HTat 500nmwere separated. After annealing, the peaks were red-shifted and showed a slight increase in intensity (Fig. 2a, inset). Photoluminescence (PL) spectra, as shown in Figure 2b, indicated dramatic PL quenching in P3HT CF3 and P3HT CH3 compared to P3HT OH, P3HT Br. Significantly,

P3HT CF3 emission was quenched by 91% relative to P3HT but P3HT OHwasquenchedbyonly57%.Thisdramaticincreasein

PL quenching indicates that efficient charge separation occurred at the P3HT/PCBM interface in P3HT CF3, limiting the PL emission from the active layer. For organic PV devices, the separation of excitons into separated electrons and holes only occurs at the donor/acceptor interface.[26,27] Therefore, the limited interfacial surface area of the abrupt and flat heterojunction for the case of P3HT OH and PCBM limits its efficiency because excitons generated far from the interface recombine prior to dissociation. If the distance between conjugated polymer and PCBM is larger than the exciton diffusion length, some excitons cannot reach a domain of PCBM and subsequently recombine to give a PL signal. In contrast, a well-mixed heterointerface, especially for the case of the P3HT CF3/ PCBM blend ensures more-efficient exciton diffusion and charge separation. As the miscibility of the donor and acceptor increases, the size of the phase-separated domains decreases and the interfacial surface area between the two components is larger. The excited-state energy can, therefore, contribute to the photocurrent of the device instead of being emitted as a PL signal. Separated PCBM clusters may be the cause of PL quenching and charge separation between P3HT CF3/PCBM occurs more effectively. Furthermore, to investigate the composition of end- functional-group-modified P3HT/PCBM-blend films as a function of the film depth, X-ray photoelectron spectroscopy (XPS) experiments were performed. Because only the P3HT molecule contains sulfur atoms, sulfur composition in the blend films is an indicator of the amount of modified P3HT in the blend films. Figures 3a and 3b show the relative ratios of sulfur atoms as a function of depth for the as-prepared films and films annealed

at 1008C for P3HT Br, OH, CH3, and CF3. The XPS depth-profile experiments allowed comparison of the sulfur- component ratios of both sides of the film, i.e., the PEDOT:PSS side and the air/film interface. First, we measured the percentage of sulfur in the homogeneous P3HT film. As shown in Figures 3a and 3b, the sulfur content is uniformly 10% throughout the film for homogeneous P3HT. Next, the sulfur content of the active layers for various end-functionalized P3HT/PCBM blends was measured and normalized to the sulfur content of the homogeneous P3HT. As can be seen in Figure 3, the composition of the P3HT Br and OH spin-coated films or annealed films are similar. In spin-coated films, both P3HT Br and OH are rich at the PEDOT:PSS substrate. Enrichment might be caused by the surface-energy difference between P3HT Br or OH and PCBM. Because PCBM is more hydrophobic than P3HT, there is a relatively stronger affinity between P3HT and PEDOT:PSS-coated ITO glass. The P3HT component of the blends is preferentially adsorbed onto the PEDOT:PSS-coated ITO substrate, which results in the P3HT-rich region at the

PEDOT:PSS surface. Although the P3HT CF3/PCBM film has similar tendencies to P3HT OH, the slope of the relationship between composition change and thickness for P3HT CF3 is

Table 1. Surface energies of various end-functional-group-modified P3HTs and PCBM.

Figure 2. a) UV–vis absorption of homogeneous, end-functional-group- modified P3HT; inset: UV absorption of a P3HT CF3/PCBM-blend film (prepared as described, film annealed at 1008C for 15min) b) PL spectra of the homogeneous P3HT and end-functional-group-modified P3HT/ PCBM blend.

w.advmat.de w.MaterialsViews.com flatter than that of P3HT OH, implying that

P3HT CF3 and PCBM are well mixed throughout the film.

When the blend films are thermally annealed, the compositional uniformity of the P3HT CH3 and CF3/PCBM-blend film remains. In the P3HT CH3 and CF3/ PCBM-blend film the surface energies between P3HT and PCBM are well matched and P3HT chains interrupt the PCBM diffusion into the air/film interface (X-ray diffraction (XRD) results also reflect the correlation between the P3HT orientation and PCBM diffusion as a function of the end functional group in Fig. S6 of the Supporting Information). Films with a relatively uniform composition gradient are obtained. In contrast, the composition of the P3HT OH and

Br-blend film changes more abruptly, because PCBM molecules with relatively low surface energy move toward the air/film interface. The diffusion of PCBM molecules to the air/film interface enlarges the size of the PCBM cluster and severe vertical phase separation occurs. The composition gradient, i.e., P3HT-rich at the PEDOT:PSS layer and PCBM-rich at air/film interface, seems to be advantageous for device properties. However, phase separation between P3HT OH, Br, and PCBM results in domains that are too large (200–400nm, which is much larger than the diffusion length of the exciton) and hence produces major defects in the device.

Therefore, although P3HT CH3 and CF3/ PCBM films have less vertical phase separation and composition gradients in the entire film, their smaller PCBM domains and better miscibility help exciton diffusion more effectively and reduce the series resistance of the active layers, leading to an increase in device efficiency.

Figure 4 shows a schematic diagram focusing on the vertical and horizontal phase separations between P3HT/PCBM. When the surface energies of a donor and an acceptor match well (P3HT CF3 and CH3/PCBM, Figure 4a), smaller PCBM domains exist throughout the entire film. On the other hand, when the surface energies do not match, severe PCBM phase separation occurs. In this case, the exciton generated from the bottom side of the films cannot contribute to charge transport (Figure 4b) because it cannot reach the P3HT/PCBM interface. This is reflected in the current density–voltage (J–V) characteristics of devices (Figure 4c). The morphology of the as-cast active layer is not the equilibrium morphology and the phase separation is not complete. The effect of the end group of P3HT

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