3014
J. Phys. Chem. C 2009, 113, 3014–3020
Photocurrent Spectra and Fast Kinetic Studies of P3HT/PCBM Mixed with a Dye for Photoconversion in the Near-IR Region Erik M. J. Johansson,*,† Arkady Yartsev,† Håkan Rensmo,‡ and Villy Sundstro¨m† Department of Chemical Physics, Lund UniVersity, Box 124, SE-22100 Lund, Sweden, and Department of Physics, Uppsala UniVersity, Box 530, SE-751 21 Uppsala, Sweden ReceiVed: September 29, 2008; ReVised Manuscript ReceiVed: December 19, 2008
Photoconversion properties are demonstrated for a device based on a small dye molecule, absorbing light in the near-IR region, mixed with two organic charge transport materials and together forming a dye-sensitized organic bulk heterojunction. The organic dye molecule, phthalocyanine (1,4,8,11,15,18,22,25-octabutoxy29H,31H-phthalocyanine), mixed with a blend of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM), shows a photoconversion spectrum extended more than 150 nm toward longer wavelengths, as compared to a device without such dye sensitization. In the dye-sensitized region of the photoconversion spectrum the maximum internal quantum efficiency was estimated to 40%. With higher dye concentrations the internal quantum efficiency decreases. Transient laser spectroscopy measurements show that after excitation of the dye there is an electron transfer from the dye to PCBM and a subsequent hole transfer from the dye to P3HT, which results in a long-lived (P3HT+/dye/PCBM-) charge-separated state. Introduction The interest in organic solar cells has grown in recent years, and the efficiency of these types of solar cells has been improved at a high rate.1-23 A major breakthrough was made with blending a polymer and a fullerene derivative to produce an interpenetrating network (a bulk heterojunction).2,3 Poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) blend solar cells based on this idea have recently shown high energy conversion efficiencies.5-12 In this mixture, P3HT has the highest absorbance of the two materials in the visible spectral region and dominates the light absorption at these wavelengths. Following light absorption the charges are separated in a process with P3HT as electron donor and PCBM as acceptor.8 The electrons and holes are then transported in the PCBM and P3HT phases, respectively, to the different contacts. The high regioregularity of the P3HT polymer promotes a crystalline character and high conductivity in the solar cells, and many experiments have been made to adjust the crystalline character by, for example, heating.6-12 However, the P3HT polymer only absorbs light with shorter wavelengths than 650 nm, and to improve the efficiency of organic solar cells a larger part of the solar spectrum should be utilized.13,14 Therefore, other polymers that absorb light at longer wavelengths have been synthezised.13-23 Quantum dots or dyes have been mixed with a polymer, and the resulting devices show light harvesting at longer wavelengths.24-27 Polymers that incorporate dyes have also been synthetisized.28-31 A different approach may be to use two organic materials for the charge transport (for example, a polymer and C60) and use a small dye molecule to absorb light in the longer wavelength region, in a similar way as in the dye-sensitized solar cells where TiO2 and an electrolyte transport the charges from a dye molecule.32 Solar * Corresponding author. E-mail:
[email protected]. Phone: + 46 70 22 33146. Fax: + 46 46 22 24119. † Lund University. ‡ Uppsala University.
cells based on dye-sensitized TiO2 and a solid-state holeconducting material such as triarylamine-based molecules33-38 or polythiophenes39-43 have been shown to be effective. Dyesensitized devices with an organic electron-conducting material have also been made,44-46 and recently an MEH-PPV/porphyrin-dye/PCBM device was demonstrated, which extends the absorption into the red region of the solar spectrum.46 In this paper we demonstrate the possibility to use an organic dye molecule in a P3HT/PCBM blend and that the dye can contribute efficiently to the photocurrent at long wavelengths where the polymer lacks absorption. Transient absorption measurements were performed to monitor the processes after light absorption in the dye molecule in the dye-sensitized bulk heterojunction, and the processes are discussed in view of the observed device performance. Experimental Section PCBM was purchased from Solenne; P3HT (regioregular; MW, 17 500) and the dye 1,4,8,11,15,18,22,25-octabutoxy29H,31H-phthalocyanine were purchased from Sigma-Aldrich. For the absorbance and transient absorption measurements, blends of P3HT/dye/PCBM (20:x:20 mg/mL, x ) 2 and 20 mg) in chlorobenzene and only the dye in chlorobenzene were spincoated (2000 rpm) on conducting glass substrates (F:SnO2 on glass). Thereafter the blend was heated to 140 °C for 5 min and sealed with surilyn and a coverglass in argon atmosphere. For the measurements in solution, the dye was dissolved in chlorobenzene (0.05 mg/mL). The absorbance measurements were obtained with a HP 8453 UV-vis spectrometer, and the background measurements were obtained from the conducting glass (for the blend) and the cuvette (for the dye solution). The experimental setup for the pump-probe measurements is based on a Clark MXR CPA-2001 regeneratively amplified oscillator system that produces light pulses at 775 nm with pulse durations of 150 fs at 1 kHz repetition rate. A part of the fundamental output was used as a pump. For the probe, two
10.1021/jp808610f CCC: $40.75 2009 American Chemical Society Published on Web 01/22/2009
Dye-Sensitized Organic Bulk Heterojunction noncollinear amplifiers (NOPA, Clark MXR) were used. With one of them, the differential spectra over two spectral ranges covering together the range of 530-920 nm were recorded using amplified supercontinuum as a probe and a reference light pulses. After passing through the sample both pulses were spectrally dispersed and then detected by two diode arrays. The chirp of the probe light was corrected numerically during data processing. Pump-probe kinetics were recorded using another NOPA as a source of tunable spectrally selected and temporally compressed pulses. This experimental setup allows for detection of photoinduced transient absorption signals with a probe noise level lower than 10-5 per ∼300 pairs of probe pulses with and without excitation, respectively. The pump-probe delay was varied up to 9 ns. The relative pump-probe polarization was kept at magic angle in all time-resolved experiments. The photon density of the pump pulse in the transient absorption measurements of the blends was about 5 × 1013 photons/cm2/pulse. For intensities higher than about 1014 photons/cm2/pulse the transient absorption measurements of the blends showed nonlinear behavior. The transient spectra were normalized to the number of absorbed photons at the excitation wavelength (775 nm), except for the P3HT/PCBM blend (Figure 6 inset) that show negligible absorption at 775 nm. For the external quantum efficiency (EQE) measurements, devices were prepared in the following manner. A PEDOT-PSS layer was spin-coated (2000 rpm) on conducting glass. The PEDOT-PSS layer was heated to 100 °C for 10 min in argon atmosphere. Thereafter, a blend of P3HT/dye/PCBM (20:x:20 mg/mL, x ) 0, 2, 4, 10, and 20 mg/mL) in chlorobenzene was spin-coated on the PEDOT-PSS yielding mixed P3HT/dye/ PCBM blend (1:x:1, x ) 0, 0.1, 0.2, 0.5, 1) layers with a thickness of about 100 nm measured by a Dektek3 surface profile meter. The sample was then heated to 140 °C for 5 min in argon atmosphere. An Al contact (9 mm2) was deposited onto the sample by thermal evaporation in a vacuum chamber with a base pressure of
