J. Phys. Chem. C 2009, 113, 7863–7869
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Photoconductance of Bulk Heterojunctions with Tunable Nanomorphology Consisting of P3HT and Naphthalene Diimide Siloxane Oligomers Wojciech J. Grzegorczyk,†,# Palaniswamy Ganesan,‡,|,# Tom J. Savenije,† Svetlana van Bavel,§,| Joachim Loos,§,| Ernst J. R. Sudho¨lter,⊥ Laurens D. A. Siebbeles,† and Han Zuilhof*,‡ Optoelectronic Materials Section, Department DelftChemTech, Delft UniVersity of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands, Laboratory of Organic Chemistry, Wageningen UniVersity and Research Center, Dreijenplein 8, 6703 HB Wageningen, The Netherlands, Laboratory of Materials and Interface Chemistry (SMG), EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX EindhoVen, The Netherlands, and Laboratory of Nano-organic Chemistry, Faculty of Applied Sciences, Department DelftChemTech, Delft UniVersity of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands ReceiVed: NoVember 1, 2008; ReVised Manuscript ReceiVed: March 2, 2009
The relation between the morphology, optical, and photoconductive properties of thin-film bulk heterojunctions of poly(3-hexylthiophene) (P3HT) with a series of electron-accepting siloxanes with a different number (x ) 2, 4, 5) of pendant naphthalene diimide (NDIS) moieties is reported. All NDIS siloxanes show good electronaccepting properties, when blended with P3HT. Interestingly, the film (nano)morphology can be controlled by relatively small changes in the molecular structure of the NDIS siloxanes. Spin-coating from orthodichlorobenzene (ODCB) yields complex film (nano)morphologies, being correlated with the weight ratios of P3HT and NDIS siloxanes, and the molecular structure of the latter. On the other hand, all blends spincoated from chloroform (CHCl3) show good mixing of the components at the molecular level. It is inferred that the nanomorphology of the blends can greatly influence their photoconductive properties: samples spincoated from ODCB invariably display a higher photoconductance than corresponding samples spin-coated from CHCl3. This is explained in terms of a higher mobility of holes in samples spin-coated from ODCB, as measured by time-resolved microwave conductivity measurements. These data are useful to delineate the conditions for the research in strives for efficient organic photovoltaics. I. Introduction Solar cells based on organic materials are potential low-cost alternatives for conventional cells made of (multi)crystalline silicon. Important advantages of the use of organic materials, such as conjugated polymers, are the ease of processability and low production costs. In silicon solar cells, light absorption directly leads to generation of free charge carriers. By contrast, in conjugated polymers, absorption of light results in the formation of bound electron-hole pairs in the form of neutral excited states, also known as excitons. These excitons may diffuse through the polymer to reach an interface with an electron acceptor, where the excitons can dissociate into charge carriers.1,2 However, according to recent studies, photoexcitation of regioregular poly(3-hexylthiophene) (P3HT) also leads to direct formation of a substantial amount of charge carriers in addition to excitons.3-7 The photogenerated electrons can diffuse toward an electron acceptor, which reduces the rate of recombination with holes in the polymer.5,6 The highest power conversion efficiency for bulk heterojunction (BHJ) solar cells realized until now exceeds 5% and has been realized for cells based on P3HT with the electron acceptor * Corresponding author. Phone: +31-317-482367. Fax: +31-317-484914. E-mail:
[email protected]. † Department of Chemical Engineering, Delft University of Technology. ‡ Wageningen University and Research Center. § Eindhoven University of Technology. | Dutch Polymer Institute. ⊥ Department DelftChemTech, Delft University of Technology. # These authors contributed equally to this work.
[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM).8-13 The morphology of the BHJ is one of the key parameters, which determines the efficiency.1,9,10,14,15 The morphology must be such that an intimate contact between the polymer and the electron acceptor exists, because the diffusion length of excitons or the electrons is only a few nanometers in polythiophene derivatives.16-18 In addition, continuous networks of the conjugated polymer and the electron acceptor must be present within the BHJ to provide percolation pathways to the charge-collecting electrodes. For efficient solar cells based on P3HT and PCBM, almost each absorbed photon leads to formation of an electron in the external circuit under short circuit conditions.11 The morphology of a blend layer is very sensitive to the preparation procedure (e.g., spin-coating and annealing conditions) and to the material composition of the BHJ.1,2,10,11,19 Despite extensive studies on the effects of the spin-coating solvent,9,13 of postproduction processes such as annealing,11,20 or of solvent vapor treatment,21 it is still difficult to predict and to control the resulting morphology. Therefore, it is of interest to investigate to which extent the BHJ morphology can be tuned via the molecular structure of the material components. In most studies, PCBM is used as electron-accepting component.1,2,13,22 Derivatives of perylene or naphthalene diimides are alternative electron acceptors with a reduction potential similar to that of PCBM.8,23,24 These materials are of particular interest, because the molecules tend to form stacks with strong π-π overlap, which facilitates efficient charge transport.25,26 A charge carrier mobility as high as 0.1 cm2/V · s has been obtained from
10.1021/jp809665k CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
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J. Phys. Chem. C, Vol. 113, No. 18, 2009
Grzegorczyk et al. and at 1800 RPM for the CHCl3-derived samples. This yielded layers of 60-70 nm in thickness. Transmission (IT) and reflection (IR) spectra were measured using a Perkin-Elmer Lambda 900 UV/vis/NIR spectrophotometer equipped with an integrating sphere (“Labsphere”).16 The optical density spectra were calculated using
OD ) -log10
( ) IT I0 - I R
(1)
in which I0 is the incident light intensity. The attenuation spectrum (FA), denoting the fraction of incident photons, which was actually absorbed by the sample, was calculated using
FA ) 1 -
Figure 1. Chemical structures of naphthalene diimide siloxane (NDIS) dimer (2NDIS), tetramer (4NDIS), and pentamer (5NDIS).
measurements on a field-effect transistor based on a naphthalene diimide (NDI) derivative with excellent processability.27 Previously, an n-type material based on a tetrahedral core with pendant NDI moieties has been reported, which yielded homogeneous layers in a blend with a poly(p-phenylene vinylene) derivative. Luminescence of these blend layers was completely quenched, which has been attributed to efficient charge separation.23,28 However, despite these promising observations, these tetrahedrally substituted materials yielded BHJs with poor photovoltaic properties. This was attributed to the rigidity of the tetrahedral core, which limited the possibility of continuous π-π stacking of molecules over a long range. To realize long-range π-π stacking, another class of more flexible n-type linear and cyclic oligosiloxanes has been synthesized with a varying number (x) of pending naphthalene diimide moieties (xNDIS, with x ) 2, 4, 5); see Figure 1.29 The present study aims to provide insight into the effects of the molecular structure of the xNDIS compounds on the morphology and on the photoconductance of blend films with P3HT. Particular attention is paid to the effects of the relative amount of P3HT and NDIS in the blend and the spin-coating solvents used to prepare thin films. The film morphology was characterized with UV/vis spectroscopy, transmission electron microscopy (TEM), and electron diffraction (ED). The photogeneration and decay of free charge carriers were studied using the time-resolved microwave conductivity (TRMC) technique.16 With this electrodeless AC conductivity technique, the effects of charge transport from the active organic layer to electrodes are avoided.
IT + IR I0
(2)
Photoluminescence (PL) spectra of the samples were recorded using a Photon Technology International fluorescence meter on photoexcitation of the samples at λ ) 350 and 430 nm. Bright-field transmission electron microscopy (BF-TEM) imaging and acquisition of electron diffraction (ED) patterns were performed on a TECNAI G2 20 transmission electron microscope (FEI Co., The Netherlands) operated at 200 kV. A detailed description of the TRMC setup can be found elsewhere.16 Briefly, the thin films on quartz substrates were placed in an X-band (8.4 GHz) microwave cavity and photoexcited with short (3 ns) linearly polarized laser light pulses, with wavelength at the absorption maximum of the polymer (500 nm). The pulses were generated by pumping an optical parametric oscillator (OPO) with the third harmonic of a Q-switched Nd:YAG laser (Infinity, Coherent). Photogeneration of mobile charge carriers in the sample leads to an increase of the time-dependent conductance, ∆G(t), and consequently to absorption of microwave power by the sample. The photoconductance, ∆G(t), was obtained from the normalized change in reflected microwave power (∆P(t)/P) from the cavity.30
∆P(t) ) -K∆G(t) P
(3)
The sensitivity factor K is determined by the microwave frequency and geometrical and dielectric properties of the media in the microwave cavity. The photoconductivity initially increases with time due to charge generation during the 3 ns laser pulse and reaches a maximum at a somewhat later time due to the 18 ns response time of the microwave cavity. Subsequently, the photoconductance decreases due to recombination or trapping of charges. From the maximum in the time-dependent change of the photoconductance (∆GMAX), the product of the quantum yield for photoinduced charge separation (φ) and the sum of mobilities (Σµ) of electrons and holes is derived, according to
II. Experimental Section The synthesis of xNDIS siloxane oligomers (x ) 2, 4, 5) has been described elsewhere.29 Regioregular P3HT (Rieke Metals Inc., USA) was used as received. The solutions were prepared by dissolving the compounds at various weight ratios, that is, 1:0, 1:0.25, 1:1, and 1:4 (P3HT:NDIS) in ortho-dichlorobenzene (ODCB) or chloroform (CHCl3). The solutions were stirred overnight and contained always 8 mg/mL P3HT. Subsequently, the films were spin-coated onto 1 mm thick 12 × 25 mm2 quartz substrates (ESCO Products) in a nitrogen-filled glovebox, with an oxygen concentration
