J. Phys. Chem. C 2009, 113, 2547–2552
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Ultrafast Studies of Charge Generation in PCBM:P3HT Blend Films following Excitation of the Fullerene PCBM Steffan Cook,* Ruyzi Katoh, and Akihiro Furube National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: June 10, 2008; ReVised Manuscript ReceiVed: NoVember 4, 2008
In order to better understand the limits of [6,6]phenyl C61-butyric acid methyl ester/poly(3-hexylthiophene) (PCBM:P3HT) organic solar cell device performance, ultrafast transient absorption spectroscopy has been used to probe the dynamics of charge creation in films of PCBM:P3HT following excitation of the fullerene. The time scale for splitting of the PCBM singlet excited state and “hole” transfer across the polymer-fullerene donor-acceptor interface is found to be faster than the system resolution of 250 fs and is calculated to occur with near unity efficiency. Ironically, this very quick electron transfer is discussed as being inefficient with regard to organic solar cell device power conversion efficiency as an excess amount of energy is used up in the electron transfer step. Introduction Organic solar cells (OSCs) are of increasing interest as they offer many advantages over their inorganic counterparts through lower production costs.1-6 The current challenge facing organic photovoltaics however is their relatively low efficiency, with most of the best OSC devices (excluding dye-sensitized solar cells) having between 2 and 4% power conversion efficiency (PCE)7-12 and only the very best such as copper pthalocyanine/ fullerene C6013,14 or [6,6]phenyl C61-butyric acid methyl ester/ poly(3-hexylthiophene) (PCBM/P3HT)15,16 having reported efficiencies as high as 5% or more (see also refs 17 and 18). This efficiency is low compared even to amorphous silicon solar cells which have a proven efficiency of greater than 12% in the field.19 OSC device efficiency therefore is currently an obstacle to the real world application of these devices, and studies on the limits of device efficiency are necessary. One of the primary processes which govern the performance of organic solar cells is the efficiency of the initial charge generation step. In these solar cells also termed “excitonic” solar cells,20 instead of the free charges formed in materials such as silicon, light absorption by the organic materials leads to a bound electron-hole pair called an exciton, which must be split at an interface with a second more electronegative material before free charges are formed. For solar devices to be efficient this “exciton” splitting step must also be as efficient as possible. Keeping the exciton splitting high however in practice means constraints on both the choice of materials (sufficient offset in electronegativity) and blend film morphology. For instance, it is in order to maximize this initial exciton splitting that a bulkheterojunction architecture with a large interface between donor and acceptor is commonly used for organic solar cell devices21-24 despite disadvantages this morphology may have for charge mobilities, another important factor in device efficiency. Despite the importance of this initial step, few studies of charge generation in OSCs have been published25-28 and none has yet to study the charge generation reaction from the perspective of the fullerene rather than the polymer. While absorption by the * Corresponding author,
[email protected]. Telephone: +81 29-861-5080, ext 55516. Fax: +81 29-861-5301.
fullerene molecule may be low, in films with a large component of PCBM compared to polymer18,29,30 (as is typically necessary where low hole mobility polymers are used31-33) PCBM contribution to light absorption in the film can become significant and therefore the efficiency of the charge generation proceeding excitation of the fullerene is of practical concern. Furthermore, fullerenes that show greater absorption in the visible region such as C70-PCBM9,34-36 and the so-called “emerald-green” fullerenes37,38 are beginning to be used which may lead to the fraction of light absorption by the fullerene component in the photoactive layers rising even further. Understanding the efficiency of “exciton splitting” and hence charge generation following light absorption by the fullerene is therefore relevant and will likely become more so in the future. In this report we describe the mechanism and efficiency of charge generation following fullerene excitation in one of the most common and efficient polymer/fullerene organic solar cell blend films,39-62 namely, poly(3-hexylthiophene) and C60PCBM. In order to study the charge transfer reaction, which typically occurs on ultrafast time scales,25-28 femtosecond transient absorption spectroscopy has been employed. This technique measures the nature and concentration of a photoinduced transient species formed in the sample following excitation, by changes in transmittance of a probe beam passing through the sample both as a function of time and wavelength. The time resolution of this pump-probe technique is limited by the width of the pump and probe laser pulses which can be as low as a few tens of femtoseconds, depending on the type of laser used, and has sufficient resolution therefore to observe ultrafast phenomena. Note in this report the time resolution of the pump-probe system is ∼250 fs. The pump-probe technique is particularly useful in that the intensity of probe absorption is directly related to the number of transient species created in the film, and therefore this technique can be used to directly quantify the initial concentration as well as changes to the concentration of species as a function of time. In this report the transient absorption spectra in films of PCBM and a blend film of P3HT with PCBM will be characterized and contrasted and
10.1021/jp8050774 CCC: $40.75 2009 American Chemical Society Published on Web 01/22/2009
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the transient absorption decay kinetics analyzed for evidence of charge formation. Experimental Section P3HT and PCBM were purchased from Aldrich and Solenne BV, respectively. The polythiophene was >98.5% regioregular (RR) with a Mw of 87000. The pristine PCBM films were dropcast from 2% (wt) chlorobenzene solution onto quartz substrates unless otherwise stated while the blend films were made from a common chlorobenzene solution at a 2:1 (PCBM: P3HT) weight ratio before spin coating at 3000 rpm for 60 s onto cleaned quartz substrates. A large weight ratio of PCBM to P3HT was used in the blend film as this reflects a common proportion of PCBM to polymer used in organic solar cells. Before testing, samples were placed under low-pressure vacuum to remove residual solvent. Film absorption spectra were taken using a Shimadzu UV-3100PC spectrometer, and steady-state photoluminescence was recorded using a Hitachi 850 fluorescence spectrometer. Femtosecond transient absorption was conducted using a regenerative amplifier system consisting of a “Hurricane” Ti:sapphire laser (800 nm wavelength, 160 fs fwhm pulse width, 1.0 mJ/pulse intensity, 1 kHz repetition rate) from Spectra Physics. The 800 nm laser light was split into two equal halves, and the first was directed into an optical parametric amplifier (Quantronix, Topas) tuned to give an output at 330 nm used for the pump pulse. The pump pulse was purified of unwanted fundamental laser light and light of other wavelengths by a “cold” NIR cut filter and a short pass filter. Intensity of the pump pulse was 6 × 1014 photons/cm2 unless otherwise specified in the text. The second half of the fundamental laser light was focused onto a sapphire plate (2 mm thick) to generate a white light continuum for the probe beam. To account for variance in the flux of the probe beam, part of the probe beam was split using a partially transparent mirror and the intensity monitored by a photodiode. The probe beam was focused at the center of the pump beam on the sample and the transmitted probe beam then detected by either a Si or InGaAs detector after passing through a monochromator (Acton Research, SpectraPro-150). A time difference between the arrival of pump and probe beams was achieved using a movable delay stage and every other pulse of the pump beam mechanically chopped so that a probe signal with and without pump excitation could be measured. The time resolution of the experiments was approximately 250 fs. Transient absorption spectra were not corrected for chirp in the white light spectrum. For reference however the chirp effect of this setup is expected to be negligible for probe wavelengths greater than 900 nm, and around 1 ps between 600 and 900 nm. Samples were tested at 22 °C in air but scanned on a moving platform during experiments to avoid degradation during testing. All results were reproducible over the duration of the experiment and between different samples, suggesting no disturbance to the real transient absorption signal due to photodegradation product buildup. For the time-resolved emission measurements, a streak camera (Hamamatsu, STREAK SCOPE C4334) was used to measure emission from the sample following excitation with 532 nm light at 1 kHz from the same laser system as above. Pump light was purified by a 532 nm bandpass filter and IR cutoff filters. The emission collected from the sample was at 90° to the excitation beam sample; however, an additional high pass filter was placed before the collection slit of the streak camera to cut out any stray laser or room light. System resolution of this setup was 30-50 ps.
Figure 1. Absorptance spectra (1-transmission) for a pristine PCBM film, pristine P3HT film, and a blend film of PCBM:P3HT (1:2 w/w).
Results Figure 1 shows a typical absorptance spectrum for pristine PCBM and P3HT films as well as a blend PCBM/P3HT film. Note that the y-axis units are absorptance which is equivalent to 1-transmission (reflection losses are ignored). The PCBM HOMO-LUMO gap is 1.8 eV, but due to absorption to the first neutral singlet state being symmetry forbidden, PCBM shows strongest absorption in the UV region corresponding to the formation of higher singlet excited states.63 Polythiophene absorption is well-known to be sensitive to the level of ordering present in the P3HT film.43 For pristine films of P3HT spin coated from a low vapor pressure solvent like chlorobenzene as was used here, the relatively slow film formation process allows the P3HT chains to be well aligned53,62,64 typically giving rise to a highly featured absorption spectra with an onset around ∼620 nm, shoulder at ∼550 nm, and peak at ∼520 nm. In the blend film, P3HT shows a less structured and blue-shifted absorption profile due to the presence of PCBM which hinders chain alignment. That the P3HT absorption maximum is shifted by over 80 nm in the blend compared to the pristine P3HT film suggests a fine level of mixing between the two components. Such a short distance is exemplified by the near unity (a factor of ∼26, data not shown here) quenching of the P3HT singlet emission compared to the pristine P3HT film. Emission from pristine PCBM films was observed;65 however a quantitative comparison of the degree of PCBM emission quenching by P3HT in the blend film was impossible due to, first, the small signal size and, second, the presence of remnants of P3HT emission which overlap with the PCBM emission spectra. From the absorption spectra (300-3000 nm) there is no evidence for any additional low energy absorption not present in the individual components and suggests no interaction in the ground state between P3HT and PCBM. In this work laser light of 330 nm was used to excite PCBM. When the blend film is pumped at this wavelength, almost exclusive light absorption56 by PCBM is possible, which simplifies analysis of the resultant signals. Figure 2 shows the transient absorption (TA) spectra for a PCBM film (a) at 0.3, 4.7, and 100 ps after excitation at 330 nm. The TA spectrum is broad and rises slowly from 600 to 1200 nm. Note the slight dip at 800 nm is an artifact of the equipment. Up to 100 ps the TA spectra do not significantly change shape with time suggesting that only one species is contributing to the excited-state spectra. Figure 3 shows the decay kinetics of the signal probed at 1200 nm, the peak of the film PCBM TA signal. Identical decays were seen at other probe
Charge Generation in Organic Solar Cell Devices
Figure 2. Transient absorption spectra for a pristine PCBM film. Excitation wavelength was 330 nm (6 × 1014 photons/cm2). Note the dip at 800 nm is an artifact of the equipment.
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Figure 5. Transient decay kinetics for a blend PCBM:P3HT film following excitation at 330 nm. Excitation intensity was 6 × 1014 photons/cm2. Transient decay kinetics (positive signal) were probed at 850 nm while the bleach signal (negative signal) was probed at 520 nm. A smooth red line to the positive transient decay is given as a guide to the eye. Inset shows the transient decay up to 100 ps.
for the transient species probed at 850 nm (black line). The signal rise is system limited with the signal decaying to roughly half its original value over the first 100 ps. The green line in Figure 5 shows the negative TA signal due to bleaching of the blend P3HT ground state absorbance probing at 520 nm. The bleach signal is small but is present and remains flat up to 100 ps in time. Figure 3. Transient decay kinetics for a pristine PCBM film. Excitation wavelength was 330 nm (6 × 1014 photons/cm2). Decay kinetics were probed at 1200 nm. The smooth red line is a guide to the eye. Inset shows the transient decay to longer times of 200 ps.
Figure 4. Transient absorption spectra for a blend PCBM:P3HT film following excitation at 330 nm. Excitation intensity was 6 × 1014 photons/cm2.
wavelengths such as 800 and 1000 nm. The decay of the transient absorption signal for the pristine film is multiexponential and loses approximately three-quarters of its intensity over the first 200 ps. The smooth red line in Figure 3 is a guide to the eye. Figure 4 shows the TA spectra and decay kinetics for the PCBM:P3HT blend film following excitation of the PCBM at 330 nm. The blend film TA signal is broad and semicircular with a center peak at around 850 nm. Immediately a contrast in the TA spectra with that of the pristine PCBM film (Figure 2) is clear. Once again there is no significant change to the shape of the transient absorption with time (0.7-100 ps) suggesting only one species is contributing to the TA signal; however, the level of noise on the data especially in the near-IR region may be masking any subtle changes. Figure 5 shows the decay profile
Discussion PCBM Film. Following excitation of the pristine PCBM film, the excited state present is probably that of the first neutral singlet excited state of solid film PCBM. Although we excite with an energy much above the band gap of PCBM, fast relaxation from higher singlet states to the lowest singlet state is expected before the resolution of our system. Potentially absorption by a charge transfer type transition in fullerene solid films is also possible;63 however, as the wavelength chosen for excitation is centered on an absorption peak to a known neutral singlet state transition,66 we believe this is not the case. For comparison we also measured the transient absorption spectra of the PCBM singlet excited state in solution (see Figure S1 in Supporting Information); however, singlet absorption in solution PCBM much like that for C60,67,68 has a distinct peak 1 eV) from PCBM to P3HT. The same can be said for the electron transfer from the P3HT neutral excited state to PCBM. Typically for charge transfer reactions which operate in the “normal” region of Marcus electron transfer, a large change is beneficial to charge generation as it ensures very quick and therefore almost complete “exciton” splitting. For device efficiency however, a large change in energy (∆G 0) between the neutral excited states and the charge separated state (CSS) is wasteful, as a large fraction of the photon energy is used up in the charge generation process. Of photons of the band gap energy, around 1.8 eV for
Charge Generation in Organic Solar Cell Devices
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PCBM, more than 1 eV of that energy is dissipated in the charge transfer reaction in the form of heat. The hole/electron transfer step therefore is a big, if not biggest, contributor to wasted efficiency in this system and therefore gains in efficiency are available if the energy involved was minimized. Calculating the minimum energy change needed to ensure efficient charge generation at the D-A interface, so that material energy levels can be tailored accordingly, is difficult however. Sometimes a ∆G 0 value of 0.3-0.5 eV is quoted in the literature as being the minimum needed,81,83 based on values of the polymer exciton binding energy;84 however, this is in our opinion a misconception. There is in our eyes no direct link between the polymer exciton binding energy and the energy needed to ensure charge transfer at the D-A interface. The first considers the energy needed to separate charges to an infinite distance in a single material whereas the second is the energy required to move an electron or hole a short distance across to another molecule in a binary blend system. There is no relation between these two concepts although this has come to be accepted. The actual minimum ∆G 0 needed to ensure efficient charge transfer across the D-A interface is not a concrete value. A high yield of charge separation requires that the rate of charge transfer (κet) be much faster than other competing decay processes for the singlet state (see eq 1). The rate of charge transfer itself can be determined by the Marcus equation85 and depends on many factors including the reorganization energy of the components involved, the product of two components which can be described as the strength and frequency of electronic coupling between the materials, as well as the ∆G0 for the reaction (see eq 2).
κet )
(
1 -(λ + ∆G0)2 2π HAB2 exp p 4λκbT √4πλκbT
)
(2)
Equation 2 is the Marcus-Hush theory for electron transfer where H2AB stands for the strength of electronic coupling, λ is the reorganization energy, and ∆G 0 is the Gibbs free energy change. Additionally in practice for a blend system some consideration must be given for the effect of exciton migration times to a D-A interface on charge transfer rates; however, in wellblended D-A systems this concern may be negligible. All these independent factors mean that the minimum ∆G 0 needed for efficient charge generation will vary from system to system, and in practice the minimum ∆G 0 is not a fixed value. In order to get realistic values for the minimum ∆G required, a more detailed and quantitative discussion than can be given here is needed; however to summarize, it is highly likely that a ∆G 0 greater than 0.3 eV will be needed to ensure efficient charge transfer at a D-A interface,86 not for energetic reasons but for kinetic ones. To close, we stress that while there is potential room for improvement in efficiency over P3HT/PCBM by developing new materials with similar attributes but optimal energy levels (for instance with higher polymer IP/HOMO levels), care must be taken to ensure that the efficiency of charge transfer should be the first priority in every design. Already in the literature there are several reports of the use of high HOMO energy polymers for OSC’s where incomplete charge transfer due to a high CSS energy has proved a problem for device efficiency.65,86-91 In the most extreme example, charge generation in blends of F8BT (IP ∼5.9 eV) and PCBM was found to be stopped completely65 due to a faster energy than electron transfer process from F8BT to PCBM.
Conclusions In blend films of P3HT:PCBM, hole transfer from the PCBM neutral excited state to P3HT is found to occur on a quicker time scale than the equipment resolution of
