Jul 24, 2013 - National Institute of Advanced Industrial Science and Technology (AIST), ..... Mohebbi, A. R.; Seo, J.-H.; Leclerc, M.; Heeger, A. J. Breaking.
Article pubs.acs.org/JPCC
Charge Separation and Recombination of Charge-Transfer Excitons in Donor−Acceptor Polymer Solar Cells Jun’ya Tsutsumi,*,† Hiroyuki Matsuzaki,† Naoyuki Kanai,‡ Toshikazu Yamada,† and Tatsuo Hasegawa*,† †
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan Fuji Electric Co., Ltd., 1 Fujimachi, Hino 191-8502, Japan
‡
ABSTRACT: The generation and recombination of photocarriers in a series of donor−acceptor polymers were examined with a focus on the two unique absorption bands that originate from low-energy charge-transfer (CT) excitation and highenergy main-chain (MC) excitation, respectively. The activation energies for the generation of photocurrents were different for CT and MC excitations in pristine polymer films, whereas they were identical to one another in blended films containing a fullerene derivative. The results show that the photocarriers are generated directly by MC excitation, whereas they are generated indirectly by CT excitation through the formation of bound electron−hole pairs, the binding energies of which are correlated with their recombination lifetimes.
1. INTRODUCTION Considerable research attention has been recently focused on organic photovoltaic cells (OPCs) as one of the most important devices for producing renewable energy because of their high production processability as well as the lightweight, thin, and impact resistant characteristics.1−5 A recent major development in OPCs involves the use of donor−acceptor (DA) polymers that consist of alternating donor units and acceptor units along the polymer backbone; such polymers permit the realization of conversion efficiencies in excess of 9%.6−12 The unique nature of DA polymers is manifested in the emergence of two distinct absorption bands in the visible region and the near-infrared region, respectively. On the basis of molecular-orbital calculations, it has been proposed that the near-infrared absorption band corresponds to charge-transfer (CT) excitation between the donor and the acceptor units in the polymer backbone, whereas the visible absorption band corresponds to spatially extended main-chain (MC) excitation, as observed in conventional homopolymers such as poly(3hexylthiophene-2,5-diyl) (P3HT).13,14 This feature is closely analogous to that observed in donor−acceptor CT complexes of small molecules.15,16 It is believed that an extended range of active photon energies arising from CT excitation might be crucial for high efficiencies in DA polymer OPCs.3 Also, it has recently been pointed out that the efficiency of photocarrier generation is lower for CT excitation than for MC excitation and that this is associated with the spatially localized nature of the CT excited state.13,14,17 However, experimental evidence has not yet been established to discriminate between the individual photocarrier generation mechanisms associated with CT and MC excitations, respectively, in DA polymers. We systematically investigated photocarrier generation and recombination characteristics in response to CT and MC © 2013 American Chemical Society
excitations by measuring the intrinsic photocurrent yields and photoluminescence lifetimes for a series of DA polymers: poly{[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl} (PCDTBT),7,8 poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)oxycarbonyl]thieno[3,4-b]thiophenediyl]} (PTB7),10 and poly{2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]} (PCPDTBT)9 as well as for the homopolymer P3HT as a reference. The temperature-dependent photocurrent yields of the appropriate absorption bands were measured for pristine polymer films as well as bulk heterojunction (BHJ)-containing films of the polymers blended with the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We found that the two absorption bands of the pristine DA polymer films required different activation energies to produce photocurrent, whereas these were identical in the case of the blended films. On the basis of these results, we demonstrate that the activation energy by CT excitation in the pristine films should correspond to the binding energy of the relaxed CT excitons. We also show that both the binding energy and the photoluminescence lifetime vary systematically with changes in the CT gap energy in the DA polymers.
2. EXPERIMENT All films of the pristine polymers and the polymer−PCBM blends that were used as semiconductor layers were spin-coated and then annealed at 423 K for 30 min. All fabrication Received: April 25, 2013 Revised: July 23, 2013 Published: July 24, 2013 16769
dx.doi.org/10.1021/jp404094e | J. Phys. Chem. C 2013, 117, 16769−16773
The Journal of Physical Chemistry C
Article
Figure 1. Absorption spectra, photocurrent spectra, and activation energy of the photocurrent, Ea, measured for (a) PCDTBT, (b) PTB7, (c) PCPDTBT, and (d) P3HT. Spectra measured for a pristine polymer film and a blend film containing PCBM are shown in each case. The red solid line and the red broken line correspond to photocurrent spectra measured for stack-type and lateral-type cells, respectively. Note that the photocurrent spectra for the lateral-type cells are multiplied to permit comparison with those for the stack-type cells.
profiles of photoluminescence in the pristine DA polymer films were measured at various temperatures by using the output from the optical parametric amplifier pumped by a Ti:Al2O3 regenerative amplifier as the excitation light source (duration: 180 fs; repetition rate: 100 kHz; output power: 18 pJ mm−2 pulse−1). A streak camera (Hamamatsu Photonics, StreakScope C4334) equipped with a monochromator was employed to detect the photoluminescence and their time-decay profiles. The molecular orbital calculation was conducted with density functional theory (DFT) and time-dependent density functional theory (TDDFT)18 as coded in the software Gaussian 09.19 The calculations were carried out with the 6-31G(d) basis set at the CAM-B3LYP level20,21 for the smallest repeating unit of the DA polymer with its terminals at either end replaced by hydrogen atoms.
processes and measurements were performed under an inert atmosphere of nitrogen. In the photocurrent measurements, we used lateral-type cells consisting of a pair of Au electrodes with a gap distance of ∼150 μm on top of the semiconductor layer. Monochromated light from a halogen lamp was used as the excitation light source. To observe intrinsic photocarrier conduction without interference from the effects of charge separation at the metal−semiconductor interfaces, we restricted the area of illumination to the region within the semiconductor part inside the electrode gap that was subjected to an applied electric field of
