Transient Absorption Studies of Bimolecular Recombination Dynamics

Nov 17, 2009 - be in excellent agreement with a bimolecular recombination model in the presence of an exponential distribution of localized (trap) sta...
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Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/ Fullerene Blend Films Tracey M. Clarke, Fiona C. Jamieson, and James R. Durrant* Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, England ReceiVed: June 29, 2009

Bimolecular recombination, an important loss mechanism in organic solar cells, has been investigated using transient absorption spectroscopy for poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films and analogues of these components. Data are analyzed as a function of blend composition, postdeposition thermal annealing, and excitation density. Comparison of transient spectra for P3HT:PCBM films with analogous films employing P3HS and PC70BM allows the assignment of the photoinduced absorption features. These decay dynamics are analyzed on the nanosecond to millisecond time scales and are shown to be in excellent agreement with a bimolecular recombination model in the presence of an exponential distribution of localized (trap) states. Thermal annealing results in an acceleration of these decay dynamics, which is assigned to a reduction in the depth of the trap states and correlated with an increase in film crystallinity. The decay dynamics are analyzed to obtain an effective recombination coefficient that is charge density dependent at low polaron densities, but becomes independent of charge density at high charge carrier densities (>1018 cm-3). This transition is assigned to trap filling, with the recombination coefficient measured at high charge density (k ) 3 × 10-12 cm3 s-1) corresponding to the trap-free limit. From transient spectroscopic behavior we estimate the density of intraband trap states in unannealed P3HT:PCBM blend films to be ∼7 × 1017 cm-3. Introduction Solar cells constructed from conjugated polymers have been steadily increasing in efficiency in recent years.1 In particular, cells with a poly(3-hexylthiophene):6,6-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction active layer have approached efficiencies of 5%.2,3 However, while the bulk heterojunction approach enables efficient exciton dissociation, it also increases the interfacial area available for bimolecular recombination of photogenerated charge carriers. Consistent with this limitation, we have recently reported that for such solar cells, bimolecular recombination is an important loss mechanism that limits device open circuit voltage.4 A range of techniques have been employed to analyze bimolecular dynamics in polymer:fullerene blends and devices.4-13 Of particular relevance, we have used transient absorption spectroscopy (TAS) to analyze such dynamics in MDMO-PPV:PCBM blend films,14,15 where a model describing an exponential distribution of trap states limiting recombination was applied.16 TAS of blend films provides an excellent method for investigating recombination dynamics in the active layer of photovoltaic devices: these films exhibit virtually the same recombination kinetics that complete devices do,4 but without added complications such as the macroscopic electric fields induced by electrodes in complete devices. In this paper we extend these studies to a detailed analysis of the nanosecond-millisecond recombination dynamics in P3HT:PCBM blends including systematic studies as a function of blend composition, annealing, and excitation intensity. We also employ these data to determine the charge density dependent recombination rate coefficient and demonstrate a transition from trap-limited to trap-free recombination as the excitation density is increased. * To whom correspondence should be addressed. E-mail: j.durrant@ imperial.ac.uk.

The photovoltaic performance of organic solar cells is highly dependent upon fabrication conditions employed;17-20 solvent, PCBM concentration, and spin-coating conditions have all been shown to contribute to the variations observed in solar cell performance. Thermal annealing, for instance, is known to improve the efficiency of P3HT:PCBM solar cells by up to an order of magnitude.21 For P3HT:PCBM blends, these factors have all been shown to alter the crystallinity of the polymer and it is this parameter that appears to have one of the greatest impacts on solar cell performance. This study therefore examines the effect of altering the blend films’ crystallinity through a variety of methods such as thermal annealing and changing the PCBM concentration. Three aspects of photovoltaic device function that primarily influence the performance are the efficiencies of light absorption, charge collection, and charge pair generation.1,22 The latter has been reported to be a significant contributor to the increase in P3HT:PCBM device efficiency upon thermal annealing: the charge generation yield approximately doubled after annealing.23 These charge generation (polaron) yields were measured by using TAS, which directly monitors the optical absorption, and therefore yield, of photogenerated charges on the microsecond time scale.14,15 In addition, TAS also provides information on the recombination dynamics of these photogenerated charges. Both geminate recombination (from Coulombically bound radical ion pairs before full charge separation into free charge carriers) and bimolecular recombination (from the fully charge separated species) represent significant loss pathways in photovoltaic cells; as such, it is important to investigate their contributions to a particular system. It has been previously established that the rates of geminate and bimolecular recombination depend upon the crystallinity of the blend system.24,25 For instance, a highly crystalline P3HT:PCBM film has en-

10.1021/jp909442s  2009 American Chemical Society Published on Web 11/17/2009

Polythiophene/Fullerene Blend Films hanced phase segregation and consequently large domain sizes for the two components; this potentially reduces the efficiency of geminate recombination by increasing the effective separation distance between the electron and hole in each bound radical pair. The TAS technique has been extensively applied to MDMOPPV:PCBM blends14,15 and in-depth analyses have been done. To identify the origin of the recombination dynamics, a laser excitation-dependent TAS study was performed. The MDMOPPV:PCBM blends (1:2) revealed a long-lived power law decay (∆OD ∝ t-R) extending into the millisecond time regime. The model applied to this system16 indicates that the power law results from bimolecular recombination of dissociated polarons in the presence of an exponential distribution of localized (trapped) states. The value of R (the gradient of the power law, which is linear on a log-log scale) provides an indication of the energetic distribution of polaron trap states. Thermal activation of the polarons out of these trap states is required before recombination can occur and this is the rate-limiting step. Prior to this slow power law phase, an excitation density-dependent fast phase was observed. This fast phase was assigned to recombination of free charge carriers generated when the density of photogenerated polarons exceeds the density of localized states. At lower laser powers, the fast phase was not observed; rather the decay dynamics were dominated by relatively slow, trap-limited recombination. The aim of this paper is to extend our previous studies of MDMO-PPV:PCBM films to P3HT:PCBM blend films. A prerequisite for such analyses is unambiguous assignments of the transient absorption features observed. For this purpose, data for P3HT:PC60BM films were compared with those obtained with the selenium analogue of P3HT, P3HS, and the C70 analogue of PC60BM, PC70BM. P3HS has a smaller optical gap (1.6 eV) than regioregular P3HT (1.9 eV);26 both polymers are estimated to have the same HOMO level (4.8 eV), therefore this decrease in band gap is due to a lower P3HS LUMO level. The extended absorption range (up to 760 nm) allows a greater spectral overlap with the solar emission spectrum, thus it is possible that P3HS might be a good candidate for polymer solar cells. After completing our assignments, we examine the effect parameters that influence the morphology and crystallinity of blend films have on the charge carrier decay dynamics. These decay dynamics are analyzed to yield the charge density dependence of the bimolecular recombination coefficient and are discussed in terms of trap-limited and trap-free recombination. Experimental Methods P3HT was obtained from Sigma-Aldrich, P3HS from Merck Chemicals Ltd., the PC60BM supplied by API Services Inc., and the PC70BM from Solenne. All pristine polymer and polymer: acceptor blend (1:1 by weight) solutions were prepared with chlorobenzene (BDH Laboratory Supplies) and stirred at 60 °C to fully dissolve the polymer. All films were spin-coated in a clean room with a filtered air environment. Annealing of films was accomplished at 140 and 150 °C for P3HT:PCBM and P3HS:PCBM films, respectively, both for 20 min. Steady state absorption spectra of the films were measured with a UV/vis spectrophotometer (Shimadzu, UV-1601) at room temperature. Transient absorption decays were measured by exciting the sample film, under a nitrogen atmosphere, with a dye laser (Photon Technology International Inc., GL-301) pumped by a nitrogen laser (Photon Technology International Inc., GL-3300). The excitation wavelength used was 500 nm for P3HT:PCBM and 520 nm for P3HS:PCBM, with pump intensities ranging

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Figure 1. The ground state absorption spectra of a pristine P3HT film and P3HT in a 1:1 blend film with PC60BM before and after annealing at 140 °C for 20 min.

between 0.7 and 75 µJ · cm-2 and a repetition frequency of 4 Hz. A 100 W quartz halogen lamp (Bentham, IL 1) with a stabilized power supply (Bentham, 605) was used as a probe light source, with a typical probe wavelength of 980 nm. The probe light passing through the sample film was detected with a silicon photodiode (Hamamatsu Photonics, S1722-01). The signal from the photodiode was preamplified and sent to the main amplification system with an electronic band-pass filter (Costronics Electronics). The amplified signal was collected with a digital oscilloscope (Tektronics, TDS220), which was synchronized with a trigger signal of the pump laser pulse from a photodiode (Thorlabs Inc., DET210). To reduce stray light, scattered light, and sample emission, two monochromators and appropriate optical cutoff filters were placed before and after the sample. To extend the TAS into the nanosecond regime, a photodiode with a quicker time response was used in conjunction with a 980 nm laser diode probe beam. Results Ground State Absorption Spectra. The ground state absorption spectrum of a pristine film of P3HT shows a maximum absorbance at 525 nm and a lowest energy band at 600 nm, as displayed in Figure 1. The addition of PCBM to create a 1:1 blend film causes significant changes to the spectrum, inducing a large blue-shift of the absorption maximum from 525 to ∼485 nm. The magnitude of the blue-shift is dependent upon the film fabrication conditions and hence the crystallinity of the resulting film. PCBM is known to disrupt the crystallinity of P3HT in a blend film: the PCBM molecules are dispersed between P3HT chains and consequently prevent the formation of polymer crystallite domains.24 Thus a greater blue-shift in the absorption spectrum is indicative of decreasing P3HT crystallinity. Indeed, the absorption spectra of the least crystalline blend films are quite similar to that in solution, indicating that the PCBM has an appreciable detrimental effect on the P3HT crystallinity, as reported previously.27 Interestingly, however, P3HT’s lowest energy transition at 600 nm does not shift upon addition of PCBM and its wavelength is independent of the film’s crystallinity. This peak has previously been assigned to an interchain transition.28 The absorption spectra of pristine P3HS and its 1:1 blend film with PCBM (Figure 2) follow similar trends to that observed for P3HT. The pristine film has an absorption maximum at 580 nm, which blue-shifts to ∼515 nm in the blend as a result of the reduced polymer crystallinity. The lowest

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Figure 2. The ground state absorption spectra of a pristine P3HS film and P3HS in a 1:1 blend film with PC60BM before and after annealing at 150 °C for 20 min.

Figure 3. The normalized transient absorption spectra of P3HT in annealed (140 °C for 20 min) 1:1 blend films with PC60BM and PC70BM. The inset shows the normalized transient absorption spectra of P3HT:PC70BM (1:1) blend films before and after annealing. All transient spectra were obtained 10 µs after 70 µJ · cm-2 excitation at 500 nm.

energy transition at 685 nm does not shift when the crystallinity is altered through addition of PCBM. The red-shift of the P3HS absorption bands relative to P3HT is a result of its lower LUMO, reducing the band gap to extend the absorption range and allow a greater light harvesting ability. The crystallinity of the blend film, with either P3HT or P3HS, can be enhanced by thermal annealing: this process increases the phase segregation between the polymer and PCBM, resulting in a stronger interchain interaction through the formation of crystalline polymer domains.20,29-32 This is manifested in the absorption spectrum as an increase in the oscillator strength and a red-shift of the absorption maximum, the magnitude of which depends on the crystallinity of the unannealed film (Figures 1 and 2). The lowest energy (interchain) excitation at 600 and 685 nm for P3HT and P3HS respectively does not shift after annealing. The same trends upon annealing are observed when the electron acceptor PC70BM is utilized (see the Supporting Information, Figure 1S). Transient Absorption Spectra. The transient absorption spectrum of a P3HT:PC60BM film shows a single broad band centered at ∼980 nm, shown in Figure 3. This band can be assigned to localized P3HT positive polarons (P3HT•+) due to its similarity with other results in the literature.33-35 The increasing absorbance from 900 nm toward 750 nm can possibly

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Figure 4. The normalized transient absorption spectra of a P3HS: PC60BM (1:1) blend film before and after annealing (150 °C for 20 min) obtained 10 µs after 70 µJ · cm-2 excitation at 515 nm.

be attributed to the presence of two-dimensional delocalized polymer polarons, the maximum absorbance of which has been reported at 690 nm.33,35 Altering the electron acceptor from PC60BM to PC70BM has no effect on the wavelength of the 980 nm peak, consistent with its assignment as a polymer transient species rather than PCBM: previous studies have shown that the C60 anion absorbs at 1070 nm,36,37 while the C70 anion absorbs at 1370 nm.38 The absence of the PCBM anion in the P3HT:PCBM transient spectrum can be attributed to the higher extinction coefficient of P3HT•+ (20 000 L · mol · cm-1, vide infra) compared to PCBM•- (6 000 L · mol · cm-1),39 thus the P3HT polaron absorption dominates the spectrum. Thermal annealing results in minor changes to the P3HT:PC60BM transient spectra in terms of absorbance maximum or band shape; only the magnitude of the signal size increases substantially. P3HS:PC60BM has a transient absorption maximum at 1150 nm (Figure 4): this is appreciably red-shifted from that of analogous P3HT blend’s maximum, consistent with the redshift measured in the ground state absorption spectra. Altering the electron acceptor has no significant effect on the P3HS blend spectra, as was also observed in the P3HT transient absorption spectra. The 1150 nm transient peak can therefore be assigned to the P3HS positive polaron. Thermal annealing does not result in a shift of the 1150 nm transition, but does result in the appearance of a new band at ∼820 nm, most probably due to the enhanced crystallinity after annealing, which increases the concentration of delocalized P3HS polarons and hence the absorbance of the associated absorption band (an analogous, but much weaker increase in the transient absorbance at 750 nm is observed for P3HT). The decay dynamics for P3HS:PC60BM are shown in Figure 2S in the Supporting Information; these are very similar to P3HT:PC60BM, as reported previously.26 Control data on the pristine polymers, P3HT or P3HS, give a very small TAS signal; typically a ∆OD of 10-5 was observed at the highest excitation density. This is significantly smaller than that of the blend films, which possess TAS signals over an order of magnitude greater under the same conditions, consistent with the high charge photogeneration efficiency expected in the presence of PCBM. Transient Recombination Dynamics. The annealed P3HT: PCBM (1:1) films’ recombination decays exhibit a slow phase from 3 µs to 1 ms that can be fitted to a power law with R ) 0.38 and an excitation density-dependent fast phase on time scales before ∼3 µs, as shown in Figure 5a. As such, this dynamics behavior is very similar to that observed for MDMO-

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Figure 5. The transient absorption decay kinetics of a P3HT:PC60BM (1:1) blend film (a) and a P3HT:PC70BM (1:1) blend film (b), both annealed at 140 °C for 20 min, obtained as a function of laser excitation density with a pump wavelength of 500 nm and a probe wavelength of 980 nm.

Figure 6. Transient absorption decays of an unannealed P3HT:PCBM (1:1) film obtained as a function of laser excitation density with a 532 nm pump wavelength and a 980 nm probe wavelength (a) and the normalized signal size measured at 50 ns and 3 µs as a function of laser excitation density, and where the ∆OD at 50 ns has had the contribution from the slow phase subtracted from it (b).

PPV blend films.14,15 The magnitude of the power law’s exponent R is similar to that measured for MDMO-PPV:PCBM blend films (R ) 0.4), indicating that the two systems have similar trap distribution energies. This gradient does not change as the laser excitation density is increased (only the signal amplitude changes). It should be noted that the decay dynamics of P3HT:PCBM presented in Figure 5a is a representative example: the value of R is very sensitive to film crystallinity and values of R between 0.25 and 0.65 have been observed for analogous films, depending upon film fabrication conditions and P3HT regioregularity.4,26 The decay dynamics of P3HT:PCBM differ from MDMO-PPV:PCBM in terms of the time range over which the fast phase exists: it extends into the microsecond time regime for P3HT but not for MDMO-PPV. In addition, the amplitude of the MDMO-PPV:PCBM power law decay reaches saturation when the laser excitation density is increased above 1 µJ · cm-2; this saturation does not occur as quickly for P3HT (indeed, full saturation has not yet been reached by 75 µJ · cm-2). Since the possibility of triplet states was discounted by using oxygen quenching studies, the fast decay phase of P3HT:PCBM can be attributed to either geminate or bimolecular recombination. The excitation density-dependent fast phase on the early microsecond time scale was therefore investigated further by extending the time resolution into the nanosecond regime. These data, shown in Figure 6a, clearly show the pronounced excitation density dependence of the fast decay phase. This point is further illustrated in Figure 6b, which compares the excitation density dependences of the fast and slow decay phases. It is apparent that there is a threshold excitation density of ∼5 µJ · cm-2; above

this threshold the amplitude of the slow phase (3 µs) starts to saturate, correlated with the appearance of the fast phase (50 ns). This significant excitation density dependence of the fast phase is consistent with bimolecular rather than geminate recombination. If geminate recombination was the primary contributor to this phase, then the fast phase would still be apparent at low excitation densities as a monomolecular decay;this is clearly not the case (Figures 5 and 6). We note that some contribution of geminate recombination dynamics to the fast phase cannot be ruled out; however, the pronounced excitation density dependence data shown in Figure 6 clearly indicate that this contribution, if present, will be relatively minor. As such, the observation of polaron decay dynamics correlated with excitation density, and hence the number of charges in the film, is consistent with second order (or higher) kinetics; bimolecular recombination. The presence of two bimolecular recombination phases is in good qualitative agreement with what we have reported previously for MDMO-PPV:PCBM blends14,15 and is thus consistent with the trap model proposed to account for the recombination behavior observed in this system.16 This comparison therefore strongly supports our assignment of these fast and slow phases of P3HT:PCBM blends to trap-free and trap-limited bimolecular recombination, respectively. More quantitatively, the amplitude of the slow decay phase shows a much less pronounced saturation with increasing excitation density than we observed previously for MDMO-PPV:PCBM blend films,14,15 an observation we discuss in more detail below. The previous work on the MDMO-PPV:PCBM system utilized continuous white light illumination15 in order to test

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Figure 7. The transient absorption decay kinetics of P3HT blend films with 5 wt % and 50 wt % PC60BM (the latter has been annealed at 140 °C for 20 min) obtained with 75 µJ · cm-2 excitation with a pump wavelength of 500 nm and a probe wavelength of 980 nm.

Figure 8. Transient absorption decay kinetics of a P3HT:PC60BM (1: 1) blend film obtained with 75 µJ · cm-2 excitation at 500 nm and a probe wavelength of 980 nm before and after annealing at 140 °C for 20 min.

the recombination model, that is, the presence of an exponential tail of trap states. Under constant illumination, a steady state carrier density is created in the blend film and these charge carriers fill the deepest trap states. The laser excitation generates additional charges that must therefore occupy the shallower trap states, enhancing the rate of their recombination. Indeed, this was observed for MDMO-PPV:PCBM, and also for P3HT: PCBM (Figure 3S, Supporting Information), where the gradient of the power law decay increased to R ) 0.88. This indicates a significant increase in the rate of bimolecular recombination, as predicted from the trap-filling model. The charge generation yield and recombination kinetics of polymer:PCBM blend films, as monitored by TAS, are dependent upon a number of factors but their crystallinities and the overall morphology are primary concerns. The crystallinity of the film’s constituents can be altered in a variety of ways, such as thermal annealing, variation in PCBM concentration, and other experimental parameters (solvent, film thickness); this change in crystallinity is reflected in the transient absorption data. Transient absorption decays measured for annealed P3HT blends with 5 wt % and 50 wt % PCBM, for example, reveal that both the charge generation yield and recombination rate change as a result of altering the PCBM concentration (Figure 7). Note that an accurate evaluation of the polaron yield requires a correction for the ground state absorbance oscillator strength of the film at the pump wavelength and knowledge of the polaron extinction coefficient (vide infra). The charge generation yield increases considerably as the PCBM concentration is increased (by at least a factor of 2) over the full time range studied, as we discuss in more detail elsewhere. This is consistent with higher efficiency devices fabricated with 50 wt % PCBM blends. In addition to this, the TAS decays show that the gradient of the power law decay (as defined by R) decreases from 0.65 to 0.38 as the PCBM concentration is increased from 5 to 50 wt %. Since R is indicative of the energetic distribution of the polaron trap states, it can be surmised that a higher PCBM concentration causes the presence of deeper polaron traps (this is often linked with a reduced crystallinity), slowing the rate of bimolecular recombination by decreasing the number of polarons thermally activated out of the traps. A previous study40 showed small increases in R with higher regioregularity and larger increases with annealing of P3HT:PCBM films, clearly showing the correlation between R and crystallinity. In addition, it has been previously reported that high concentrations of PCBM disrupt P3HT’s interchain interactions and decrease the overall

crystallinity41 (consistent with the decreased R);but also enhance the number of continuous percolation pathways required for efficient electron transport. Thus the highest efficiency P3HT: PCBM devices are those produced with an optimum quantity of PCBM: a balance is required between efficient charge transport and crystallinity. Another method to influence the crystallinity of a P3HT blend film is to utilize thermal annealing. A comparison of the decay kinetics between P3HT:PC60BM before and after annealing, shown in Figure 8, reveals the striking observation that the signal amplitude increases considerably. This has been discussed in detail elsewhere23 and is attributed to an increase in charge generation yield upon annealing due to a reduction in geminate recombination, which contributes significantly to the enhanced short circuit current and device efficiency. We note that, for the films studied herein, the higher initial ∆OD upon annealing cannot be attributed to differences in bimolecular recombination losses prior to 1 µs. The amplitude increase with annealing was observed for all excitation densities studied, and actually increases at lower excitation densities; under such lower excitation conditions submicrosecond bimolecular recombination losses can be expected to be relatively minor (see, for example, Figure 4S in the Supporting Information). There are other differences that thermal annealing induces in the recombination dynamics of P3HT:PC60BM. For instance, the annealed films generally show a faster power law decay (larger R) than the unannealed films;40 in the example shown in Figure 8, R increases from 0.25 to 0.38 upon annealing. This has been attributed to the improvement in crystallinity and suggests a structural/conformational origin of the polaron traps since they are affected by thermal treatment. In addition, the fast phase is most clearly resolved prior to annealing (with a steeper gradient), possibly indicative of a more complete saturation of the available trap density. Given that we observe more efficient charge photogeneration in annealed films, our comparison of the difference in decay dynamics data before and after annealing suggests that annealing results not only in shallower traps compared to the unannealed films, but potentially also a higher density of traps. We note, however, that such changes with annealing are difficult to quantify due to the variation in crystallinity between unannealed samples that occurs during film fabrication. In this work, the effect of altering the electron acceptor has also been investigated. P3HT:PC70BM (1:1) films showed similar initial charge generation yields to P3HT:PC60BM and

Polythiophene/Fullerene Blend Films similar recombination dynamics as a function of excitation density (Figure 5b) and annealing (Figure 5S, Supporting Information). However, the power law decay gradients of both the unannealed and annealed films increased when PC70BM is used instead of PC60BM (using the same batch of P3HT and identical film preparation conditions). The annealed film’s gradient, for example, increased from R ) 0.38 to 0.57 when PC60BM is replaced with PC70BM. The other major difference between P3HT:PC60BM and P3HT:PC70BM lies in the faster saturation of the power law amplitude with increasing excitation density for P3HT:PC70BM (as shown in Figure 5b). Discussion The annealed P3HT:PCBM (1:1) films’ transient absorption recombination decays (Figure 5) are very similar to that observed for MDMO-PPV (1:2) blend films. Both exhibit a slow power law phase during the microsecond to millisecond time range and an excitation density-dependent fast phase on earlier time scales. It is therefore probable that the existing model to explain MDMO-PPV blends’ recombination kinetics,16 involving bimolecular recombination in the presence of an exponential tail of trap states, can be applied to P3HT:PCBM and its analogues. As such, the slow decay phase arises from bimolecular recombination of dissociated polarons in the presence of an exponential distribution of localized (trapped) states, resulting in a power law decay. An excitation density-dependent fast phase exists prior to this, assigned to recombination of free charge carriers generated when the density of photogenerated polarons exceeds the density of localized states. Furthermore, the gradient of the fast phase approaches a value of 1 as the excitation density is increased, as expected for pure bimolecular recombination that is no longer influenced by the presence of trap states. Indeed, the similarities observed between the two blend systems suggest that this model could be generic for polymer:PCBM blend systems. However, there are a couple of important differences observed between the recombination kinetics of P3HT:PCBM and MDMO-PPV:PCBM blend films. The first major difference is the time range over which the fast phase exists, extending into the µs time regime for P3HT but not for MDMO-PPV. This may imply slower recombination kinetics of the P3HT free charge carriers and is consistent with time-of-flight and CELIV measurements of these two systems showing that crystalline (regioregular or annealed) P3HT:PCBM devices have smaller bimolecular recombination coefficients and longer charge carrier lifetimes than MDMO-PPV devices.5 The slower recombination kinetics measured for crystalline P3HT:PCBM is also consistent with the greater degree of phase segregation compared to MDMO-PPV:PCBM, which requires a significant fraction of PCBM to induce phase segregation.42,43 This results in higher device efficiencies for P3HT:PCBM. The second major difference between the recombination dynamics of P3HT:PCBM and MDMO-PPV blend films is the degree of signal saturation for the power law decay, where the amplitude of the MDMO-PPV:PCBM power law decay reaches saturation much more quickly with excitation density than P3HT:PCBM. For P3HT:PCBM blend films, the amplitude of the power law decay phase continues to increase with excitation density even in the presence of the fast phase. This strongly indicates that under low excitation densities a significant fraction of shallowly trapped P3HT polarons (corresponding to the nanosecond to microsecond decay dynamics) exists, even under conditions when the deep trap states (millisecond decay dynamics) are not fully occupied. This observation suggests an

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20939 incomplete thermalization of the P3HT polarons on the time scale of bimolecular recombination;such that under the pulsed excited conditions employed herein, the photogenerated polarons do not fully achieve a thermalized Fermi-Dirac occupancy of the available density of states prior to recombination. This is consistent with CELIV measurements that report the observation of time-dependent charge mobilities and recombination dynamics at early time scales, attributed to relaxation dynamics of the charge carriers into the tail (trap) states of the density of states distribution.9,10 A slow thermalization of the charge carriers in P3HT:PCBM may be correlated with the presence of crystalline polymer domains; this could lead to partial isolation of the polarons in each domain, thereby preventing them from accessing remote traps outside their local domain and thus achieving full thermalization. The observation that the P3HT:PC70BM film has enhanced signal saturation compared to P3HT:PC60BM suggests that the PCBM has a role in thermalization of polymer polarons into the trap states: PC70BM (or its influence on the morphology) has a different effect on the filling of the deepest traps compared to PC60BM. We now turn to quantification of the bimolecular recombination rate constant, k (or rate coefficient). Note that due to the thermalization effects apparent in the transient decays of P3HT: PCBM, a high excitation density was utilized in order to achieve a complete Fermi-Dirac occupancy of the trap states (which does not occur at lower excitation densities). We employ the methodology we have reported previously,4 where we analyze the transient absorption decay dynamics in terms of the polaron density dependence of this rate coefficient:

dn ) -k(n)n2 dt d∆OD dFεM 1000 k(n) ) dt ∆OD2 NA

(1) (2)

where t is time, n is the charge carrier density (assuming charge neutrality, n ) p) determined from the ∆OD (the TAS signal amplitude), dF is the film thickness, εM is the molar extinction coefficient for photogenerated polarons, and NA is Avogadro’s constant. The bimolecular recombination coefficient can therefore be established directly from the TAS results, where the measured ∆OD can be related to the charge carrier density n via the Beer-Lambert law. In this analysis, εM has been estimated at 20 000 L · mol · cm-1, using the initial ∆OD measured by TAS and an integrated short-circuit photocurrent transient for a device (to estimate the concentration of charge carriers), with both techniques under the same low excitation density conditions. The differential of ∆OD as a function of time was calculated from the differential of a double power law fit to the transient absorption data of an annealed P3HT:PCBM film at high excitation density (∆OD ) (4 × 10-11)t-1 + (5.3 × 10-7)t-0.36), consistent with our assignments of the fast and slow phases to trap-free and trap-limited bimolecular recombination, respectively. Determination of the differential of ∆OD directly from the transient data yielded the same dependence of k(n) upon n, but with a lower signal-to-noise. The results of this analysis for an annealed P3HT:PCBM blend film are shown in Figure 9. It is apparent that the bimolecular recombination coefficient, k, is strongly carrier density dependent up to n ≈ 1018 cm-3, corresponding to the charge carrier concentration range applicable to photovoltaic device operation (n ) 1015-1017 cm-3). This observation is in good agreement with our previous transient absorption and transient photovoltage analyses of P3HT:PCBM devices.4 This charge carrier dependent region corresponds to the slow phase

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Figure 9. Plot of bimolecular recombination coefficient, k, as a function of charge carrier density, n, calculated from the decay kinetics of an annealed P3HT:PCBM (1:1) film with an excitation density of 75 µJ · cm-2 (from Figure 5a). The line is a guide for the eye.

kinetics where trap-limited recombination dominates. This region of the k versus n plot can be fitted to the equation k(n) ) k0nβ, where β ≈ 1.8 for the data shown herein. We note that β is related to the power law exponent R determined by directly fitting the transient decays (by β ) 1/R - 1).4 As discussed above, this exponent R is strongly influenced by the crystallinity of the film, ranging in value between 0.25 for relatively amorphous films up to 0.65 for more crystalline films, corresponding to values for β in the range 3 to 0.5. Substituting k(n) ) k0nβ into eq 1 gives:

dn ) -k0nβ+2 dt

(3)

This is indicative, for β ) 1.8, of an overall reaction order of approximately 3.8. This behavior is consistent with our previous observation of approximately third-order decay dynamics reported previously for annealed P3HT:PCBM devices,4 with the quantitative reaction order depending strongly upon film processing and therefore crystallinity. The important conclusion, therefore, is that the decay dynamics during the slow phase are higher than the second order kinetics expected for pure bimolecular recombination. This corresponds to a bimolecular recombination process in which the bimolecular recombination coefficient, k, is carrier dependent, k(n). At higher charge carrier densities, the dependence of k upon charge density saturates until k is virtually independent of n (above 1018 cm-3). This is consistent with all traps being filled; the traps no longer limit recombination and trap-free recombination is the dominant process (the fast phase). This value therefore provides an estimation of the polaron trap density in this film. A schematic illustrating the two different recombination regimes (trap-limited and trap-free) is displayed in Figure 10. Furthermore, the concentration of P3HT polaron traps in the unannealed P3HT:PCBM film can be estimated from the nanosecond TAS data as the threshold charge density at which the fast phase appears and the slow phase shows clear saturation. This threshold can be observed in Figure 6b to correspond to an excitation density of approximately 5 µJ · cm-2. Assuming a unity quantum efficiency for charge photogeneration, this excitation density corresponds to a density of photogenerated charges of ∼7 × 1017 cm-3. This trap density, obtained from the excitation intensity studies, is similar to the charge density at which k(n) saturates in Figure 9 (∼1018 cm-3), as determined from kinetic analysis of a single transient decay for annealed P3HT:PCBM. We thus conclude that the charge density

Figure 10. The density of states for the two regimes observed in the TAS results and in Figure 10: (a) depicts trap-limited recombination, where thermal activation of holes from the exponential tail of localized state is required prior to bimolecular recombination (this is the region where k is charge carrier dependent), and (b) shows trap-free recombination, where all the trap states have been filled and an excess of charge carriers is present in the conduction band (this is the region where k no longer depends on charge carrier concentration).

threshold at which P3HT polaron transport moves from traplimited to trap-free regimes is of the order of 1018 cm-3. Since the charge carrier density for an organic solar cell under normal operation (approximately 1015-1017 cm-3) is lower than this value, this suggests that the majority of charges will be influenced by the presence of trap states and trap-limited recombination will dominate the kinetics. In contrast, field-effect transistors operate with greater charge carrier densities (>1018 cm-3, higher than the density of trap states) and charge carrier dynamics (e.g., mobility) are therefore likely to be dominated by trap-free kinetics. It is also important to note that the trap density estimated for P3HT:PCBM is an order of magnitude larger than that of MDMO-PPV.14,15 This implies that the higher charge carrier mobility measured for P3HT is not due to fewer trap states, but instead could be attributed to a higher mobility of the untrapped charges. This comparison also raises the question of the physical origin of the polaron trap states, which is currently not well understood. It is unlikely that they are due to chemical defects since polymer crystallinity (and regioregularity40) strongly affects the energetic depth of the trap states, implying a conformational influence. We note that thermal annealing appears to lower the ionization potential of P3HT,23 consistent with a distribution of HOMO energy levels as a function of polymer crystallinity and domain size. It seems likely that this energetic distribution is a potential origin of the distribution of “trap” state energies discussed herein. Conclusions The existing model to describe the decay kinetics of MDMOPPV blend films involves a power law resulting from bimolecular recombination in the presence of an exponential distribution of localized states and an excitation density-dependent fast phase assigned to recombination of free charge carriers that are generated when the density of photogenerated polarons exceeds the density of localized states. This model can now be extended to include P3HT:PCBM blends due to the similarities of the recombination kinetics between the two systems. The observation of a carrier density dependent recombination coefficient that becomes independent at high carrier densities provides strong experimental support for this model. As the charge carrier concentration is increased, the bimolecular recombination therefore progresses from trap-limited to trapfree kinetics. This occurs at approximately 1018 cm-3, implying that bimolecular recombination in P3HT:PCBM devices under normal operating conditions is dominated by trap-limited kinetics.

Polythiophene/Fullerene Blend Films The effect of crystallinity on the recombination dynamics of P3HT:PCBM was investigated by thermal annealing and altering the PCBM concentration. It was observed that enhancing the crystallinity of the blend film increased the rate of bimolecular recombination, as indicated by R, the slope of the power law decay. Acknowledgment. This work was supported by the EPSRC Nanotechnology Grand Challenge and Supergen Programme (S/P2/00476/00/00) with further support from BP Solar Ltd. We also thank Chris Shuttle and Hideo Ohkita for helpful discussions. Supporting Information Available: Ground state absorption spectra of P3HT with different acceptors, transient absorption decay kinetics of P3HS:PC60BM as a function of excitation density, P3HT:PC60BM under the influence of white light, P3HT:PC60BM under low excitation density conditions before and after annealing, and P3HT:PC70BM before and after annealing. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924–1945. (2) Ma, W. W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (3) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (4) Shuttle, C. G.; Regan, B. O.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Phys. ReV. B 2008, 78, 113201. ¨ sterbacka, R. Prog. (5) Pivrikas, A.; Sariciftci, N. S.; Juscaronka, G.; O PhotoVoltaics 2007, 15, 677–696. (6) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88, 052104. (7) Westerling, M.; Vijila, C.; Osterbacka, R.; Stubb, H. Chem. Phys. 2003, 286, 315–320. (8) Pivrikas, A.; Jusˇka, G.; Mozer, A. J.; Scharber, M.; Arlauskas, K.; ¨ sterbacka, R. Phys. ReV. Lett. 2005, 94, Sariciftci, N. S.; Stubb, H.; O 176806–176809. ¨ sterbacka, R.; Juska, G.; Arlauskas, K.; Stubb, H. (9) Pivrikas, A.; O Synth. Met. 2005, 155, 242–245. (10) Mozer, A. J.; Dennler, G.; Sariciftci, N. S.; Westerling, M.; Pivrikas, ¨ sterbacka, R.; Juaka, G. Phys. ReV. B 2005, 72, 035217. A.; O ¨ sterbacka, (11) Sliauzys, G.; Juska, G.; Arlauskas, K.; Pivrikas, A.; O R.; Scharber, M.; Mozer, A.; Sariciftci, N. S. Thin Solid Films 2006, 511512, 224–227. ¨ sterbacka, R.; (12) Dennler, G.; Mozer, A. J.; Juska, G.; Pivrikas, A.; O Fuchsbauer, A.; Sariciftci, N. S. Org. Electron. 2006, 7, 229–234. (13) Shuttle, C. G.; Maurano, A.; Hamilton, R.; Regan, B. O.; Mello, J. C. d.; Durrant, J. R. Appl. Phys. Lett. 2008, 93, 183501. (14) Montanari, I.; Nogueira, A. F.; Nelson, J.; Durrant, J. R.; Winder, C.; Loi, M. A.; Sariciftci, N. S.; Brabec, C. Appl. Phys. Lett. 2002, 81, 3001–3003. (15) Nogueira, A. F.; Montanari, I.; Nelson, J.; Durrant, J. R.; Winder, C.; Sariciftci, N. S.; Brabec, C. J. Phys. Chem. B 2003, 107, 1567–1573. (16) Nelson, J. Phys. ReV. B 2003, 67, 155209.

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