Time-Resolved EPR of Photoinduced Excited States in a

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Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend Lorenzo Franco,*,† Antonio Toffoletti,† Marco Ruzzi,† Luciano Montanari,‡ Claudio Carati,‡ Lucia Bonoldi,‡ and Riccardo Po’§ †

Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova, Italy Refining and Marketing Division Research Center, ENI S.p.A, via Maritano 26, 20097 San Donato Milanese, Italy § Research Center for non Conventional Energies Istituto Eni Donegani, ENI S.p.A, Via Fauser 4, 28100 Novara, Italy ‡

ABSTRACT: We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl1-phenyl-(6,6)C61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using timeresolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet−singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a more complete description of the photophysical processes taking place in this photoactive blend.

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INTRODUCTION Low-cost, large-area, and flexible solar cells have been the subject of a large and ever-increasing number of research works in the past years. A class of the most promising materials for flexible photovoltaic devices is based on organic semiconducting polymers mixed with fullerene derivatives, in so-called bulk heterojunction cells. Impressive increase of the photovoltaic efficiency has been achieved so far, by virtue of a tailored development of new polymeric materials, whose optimal characteristics depend on several factors. The HOMO and LUMO energies are extremely relevant parameters to be modified, since they strongly influence the photophysical processes following light absorption in presence of a fullerene derivative. The polymers in the photoactive blends usually play the role of electron donors (p-type materials), whereas fullerene derivatives are used as the electron acceptors (ntype materials). The fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM, Figure 1) is considered the best performing n-type material so far and is used as a reference material for performance evaluation. A large number of experimental data have been collected on polymer/PCBM blends by means of spectroscopic methods, electrochemical measurements, microscopic investigations, and other techniques that afforded a better insight into the mechanisms and the parameters affecting the photoinduced generation of charge carriers in blends of semiconducting polymers with electron acceptors. © XXXX American Chemical Society

Figure 1. Polymer (F8BT) and the fullerene derivative (PCBM) investigated in this work. The HOMO and LUMO levels from literature data (see text) are also indicated.

One of the most investigated class of polymers so far is the polythiophenes, among which the poly(3-hexylthiophene) Received: June 26, 2012 Revised: November 13, 2012

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PCBM. On the other hand, nonstationary time-resolved EPR techniques are frequently used to investigate the dynamics of formation and decay of short-lived photoexcited paramagnetic states such as triplet states and transient radicals species (spincorrelated and/or separated radical ion pairs)14−17 with a submicrosecond time resolution. Triplet-state formation in conjugated polymer is usually considered a performance-decreasing factor because it competes with the generation of separated charges from bound charge transfer states (polaron pairs). Moreover, in the presence of molecular oxygen the triplet state can efficiently lead to singlet oxygen formation which is a strong oxidant, responsible for increased material degradation. From the point of view of charge carrier lifetime, the recombination of photogenerated charge carriers to triplet excitons represents an additional decay pathway and therefore a loss in photovoltaic performances. It is clear that the identification of all the different possible processes that can take place in a complex materials such as conjugated polymer/fullerene blends is necessary to establish the pathways for energy degradation after the initial light absorption. In this work we applied light-induced EPR (LEPR) and timeresolved EPR (TREPR) to investigate the nature and dynamics of the paramagnetic states produced in blends F8BT/PCBM (Figure 1) after photoexcitation in the UV or visible range. Although this blend does not represent an efficient photovoltaic material for the reasons mentioned above, it was chosen here because of the extended spectroscopic investigations already reported on it and the complexity of the photophysical processes that have been identified in this material. In the following we will demonstrate that EPR experiments can provide information that complement the description of the photophysics obtained by optical spectroscopies. We observed the production, after photoexcitation, of a variety of paramagnetic states such as charge carriers and excited triplet states, and by means of TREPR, we were able to identify the photophysical pathways leading to the formation and decay of the different short-lived species. To the best of our knowledge, this is the first report on the identification of such processes in polymer/fullerene blends using time-resolved EPR spectroscopy.

(P3HT) is considered as a benchmark material. In recent years, however, many new polymers have been synthesized whose molecular structure is designed aiming at an extended solar light absorption and a suitable HOMO and LUMO energy level alignment with respect to the electron acceptor (PCBM). A good performance of the photovoltaic devices based on polymer/fullerene blends is expected whenever extended solar light harvesting and efficient photoinduced charge generation is achieved. Alternating fluorene copolymers represent promising photovoltaic materials because of their high hole mobility and efficient photoinduced charge transfer in the presence of PCBM.1−4 Simple polyfluorenes, however, show a poor absorption in the visible part of the solar spectrum: because of this limitation, it was proposed several years ago that alternating donor−acceptor structures in the polymer backbone could effectively decrease the band gap, allowing an improved absorption of the solar light.5 The polymer F8BT (poly[(9,9bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8diyl)], Figure 1) represents one of the simplest structures based on the alternating donor−acceptor sequence, the 9,9dialkylfluorene group being the electron-rich (donor) unit and the benzothiadiazole being the electron-poor (acceptor) unit. Unfortunately in many cases, including F8BT/PCBM blends, the measured photovoltaic efficiencies do not reach high performances with respect to P3HT/PCBM films. The basic reason for the low efficiencies is usually due to the poor absorption of solar light and/or to the energy mismatch of the HOMO and LUMO levels in the polymer−fullerene pairs. More detailed information on the causes of the low performances showed by polymer/PCBM films has been obtained in previous works by photophysical investigations that demonstrated the presence of a variety of processes competing with charge generation and collection in the photovoltaic device. Among the competitive processes, we may mention charge recombinations, excited triplet states and exciplexes generation,6−8 excited states quenching by impurities,9 and others, all of which can play a role in the overall photovoltaic efficiency.10 Most of the photophysical investigations on photoactive materials have been carried out by optical spectroscopies, with either steady-state or time-resolved methodologies. The signatures of the different photogenerated species are however sometimes difficult to recognize because of weak emission properties or partially overlapping absorption bands. By means of absorption, emission, and time-resolved emission spectra, Cook et al.10 showed that in F8BT/PCBM blends the main pathway for energy dissipation after light absorption proceeds via singlet−singlet energy transfer from the polymer to PCBM, followed by intersystem crossing (ISC) to the PCBM triplet state, without the formation of chargeseparated states. As we will demonstrate in this work, a more complete and precise description of the photoinduced processes in F8BT/PCBM blend can be obtained using electron paramagnetic resonance (EPR) spectroscopy. The EPR technique has been used since early investigations of photoinduced electron transfer from conjugated polymers to fullerene because it helped to uniquely identify the presence of charge-separated states or radical ions pairs, generated by photoexcitation of the donor−acceptor blends.11−13 The conventional steady-state EPR method, called light-induced EPR (LEPR), is able to reveal the existence of long-lived (from milliseconds to hours) radical ions of the polymer and of the

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MATERIALS AND METHODS The fullerene derivative PCBM, the polymer F8BT (poly[(9,9bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8diyl)], M n = 10 000−20 000, M w /M n < 3), and 1,2dichlorobenzene (ODCB) have been purchased from Aldrich and used without further purification. Solutions of PCBM or F8BT in ODCB (1 mg/mL) have been put into EPR quartz tubes (4 mm o.d.), thoroughly degassed by at least three cycles of freeze−pump−thaw, and sealed under vacuum (10−3 Torr). Films of polymer/PCBM blends have been obtained from mixed solutions of polymer and fullerene in ODCB (1:1 by weight) put into EPR quartz tubes and then letting the solvent to evaporate under vacuum. The tubes are finally flame-sealed to avoid any presence of oxygen. The EPR spectra were recorded with a Bruker ER200D spectrometer equipped with a nitrogen flow cryostat for sample temperature control (110−400 K). LEPR spectra have been obtained by subtracting the spectrum recorded before the lightirradiation from the spectrum recorded under illumination of the sample (indicated as “light on minus dark” in the B

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following). As the source of illumination we used white light from a 300 W xenon lamp, IR filtered and focused onto a quartz optical fiber, which delivered about 100 mW/cm2 of light irradiance to the sample inside the EPR resonant cavity. Typical LEPR experimental parameters were modulation amplitude 1 G, microwave power 0.2 mW, and scan time 60 s for 10 accumulated scans. LEPR intensity determination was obtained by the double integral of the simulated spectra, which provides the independent determination of all the paramagnetic species considered in the simulation. TREPR experiments were performed using a modified Bruker ER200D spectrometer with an extended detection bandwidth (6 MHz), disabling magnetic field modulation and using pulsed sample photoexcitation from the second harmonic of a Nd:YAG pulsed laser (Quantel Brilliant, λ = 532 nm, pulse length = 5 ns; E/pulse ≅ 5 mJ, 20 Hz repetition rate). The EPR direct-detected signal was recorded with a LeCroy LT344 digital oscilloscope, triggered by the laser pulse. The overall response time of the instrument was about 150 ns. Typical TREPR experimental parameters were: microwave power 2 mW, 256 field positions, and 200 transient signals averaged at each field position. From the data set obtained, the transient EPR spectrum at different time delays after the laser pulse or the EPR signal time evolution at selected magnetic field position can be extracted and processed. Spectral simulations were carried out using the Matlab toolbox Easyspin.18

Figure 2. Black line: experimental LEPR spectrum at T = 120 K of the F8BT/PCBM film (light on minus dark). Red line: calculated spectrum.

narrow asymmetric line centered at g = 1.9998 ± 0.0005 and a broader line at lower field (g = 2.0022 ± 0.0005). Similar EPR lines have been observed in several other polymer/PCBM blends and have been attributed to the PCBM anion (the high field line) and to the polymer polaron (the low field line).12,21 The identification of the presence of both the F8BT cation and the PCBM anion clearly demonstrates that a photoinduced charge separation is occurring in the blend and long-lived radical ions of the two components are produced. The longlived radicals can be identified with trapped charge carriers, produced after separation of charge-transfer states or geminate polaron pairs. However, the LEPR spectrum seems rather weak in comparison with LEPR spectra recorded in similar conditions in P3HT/PCBM blends (not shown). There are two possible explanations for the low intensity of the F8BT/ PCBM spectrum: either the charge generation process proceeds with low efficiency or the geminate charge recombination is a fast process which produces a low steady-state concentration of free radical ions. Even though the former case was demonstrated previously, the occurrence of efficient recombination processes is also supported by the TREPR results described in the following of the paper. It has also to be noted that the EPR line of photogenerated polymer cations (F8BT•+) shows a smaller intensity compared to that of the PCBM anions (PCBM•−). This result keeps on to be true even if the spectrum is recorded at low microwave power to prevent saturation effects. We attribute the different intensities of the LEPR lines to different decay rates for the two photogenerated species (F8BT•+, PCBM•−), the F8BT cation being less stable than the PCBM anion at 120 K. Indeed, for species with a very short lifetime the steady-state concentration may be too low to be detected by EPR. The different decay rate of the two photogenerated charge carriers implies that recombination between PCBM anions and F8BT cations is not the only possible decay pathway. Additional independent decay processes could be present, such as the interaction with metal catalyst impurities which can interact selectively with one or the other diffusing charges. In general, it is conceivable that the positive polarons (polymer cations) are distributed in a density of states different with respect to density of states for the anions, and the shorter mean lifetime of the positive polarons suggests that for the latter species there is a lower number of states at deep energy levels. TREPR Spectra. The TREPR spectrum recorded 0.5 μs after the laser pulse in a F8BT/PCBM blend at 120 K, using the photoexcitation wavelength of 532 nm, is shown in Figure 3. The spectrum is partially in enhanced absorption (Abs) and

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RESULTS The absorption spectrum of pristine F8BT films extends partially into the visible, with the onset of absorption at about 500 nm and an optical band gap of about 2.4 eV.19,20 On the other hand, the PCBM weakly absorbs over all the visible range, with a very weak tail from forbidden transitions up to 800 nm. The absorption spectra of F8BT/PCBM blends correspond to the superposition of the spectra of the two components, thus demonstrating the absence of a significant ground state interaction between the two materials.10 In particular, both F8BT and PCBM components in the blend are capable of absorbing light at 532 nm, with low efficiency. Therefore, at 532 nm the light absorption produces singlet excited states either in the polymer or in the PCBM. LEPR Spectra. LEPR spectra (light on minus dark) of neat F8BT films recorded at T = 120 K, using Xe light illumination, show an extremely weak EPR absorption at approximately g = 2.0030 ± 0.0005, with a line width of about 4 G (data not shown). The weak line can be attributed to polarons formed by intrachain and/or interchain photoinduced electron transfer. The weakness of the EPR signal is due to the very low yield of photoinduced charge-separated states in the neat polymer because of the absence of a suitable electron donor or acceptor. A stronger LEPR response is observed when the polymer F8BT is mixed with PCBM in a 1:1 weight ratio. Before illuminating the blend, there is almost no stable EPR spectrum, but a clear EPR spectrum is recorded under illumination of the sample. After switching off the light, the EPR intensity quickly decays within a few seconds to a negligible intensity. This fact indicates that the lifetimes of the majority the charge carriers at T = 120 K are less than a few seconds. At temperatures higher than 200 K, the LEPR spectrum vanishes completely, indicating probably that the decay times of the charged species become too short to produce a steady-state concentration of polarons sufficiently high to be detected by EPR. The LEPR spectrum recorded at T = 120 K (Figure 2) shows the presence of a C

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coincident to those found in other fullerene derivatives similar to PCBM.24,25 The line shapes used in the simulated spectra are mixed Lorentzian and Gaussian, indicating a partial inhomogeneous broadening.18 As mentioned before, the TREPR spectrum of the F8BT/ PCBM blend shows a similarity to the PCBM spectrum, but the shape is markedly different. The TREPR spectrum of F8BT recorded in an inert matrix of frozen ODCB is hardly visible by TREPR due to a low quantum yield. However, it is clear that the F8BT spectrum extends over a much wider range (about 900 G) and show a completely different polarization pattern, with the low field part in emission and the high field part in absorption (spectrum not shown). The TREPR spectrum of F8BT/PCBM blend can be properly fit as the superposition of two spectra: (a) the spectrum of the PCBM triplet state populated by normal ISC (i.e., with the triplet sublevel populations Px:Py:Pz = 0.3:1.0:0.0) with a slightly decreased ZFS parameter (D = 96 G) and (b) the spectrum of an additional PCBM triplet state with a different pattern of sublevel populations, namely an excess population in the spin sublevels labeled with the ms = +1 and ms = −1 quantum numbers (indicated as T+1 and T−1 states). The two calculated spectra and their sum are shown in Figure 4.

partially in emission (Em) and extends over a range of about 200 G.

Figure 3. TREPR spectra of F8BT/PCBM film (solid line) and of PCBM in frozen solution of ODCB (dashed line). All spectra are recorded 0.5 μs after the laser pulse at T = 120 K (Abs = absorption, Em = emission).

In Figure 3 the TREPR spectrum of PCBM in frozen ODCB solution is shown. The spectra of the blend and of the PCBM extend over a similar range and show a pattern partially in enhanced absorption (at low field) and partially in emission (at high field). The TREPR spectrum of PCBM in ODCB is easily assigned to the fullerene excited triplet state, which is produced in high quantum yield after photoexcitation of fullerene or fullerene derivatives.22 A common feature in photoexcited triplet states is the nonequilibrium population of spin sublevels produced by spin-selective intersystem crossing (ISC) from the excited singlet state. The nonequilibrium populations of triplet spin sublevels are responsible for the absorptive (Abs) and emissive (Em) lines in TREPR spectra. The EPR spectrum of a triplet state (S = 1) can be calculated on the basis of the eigenvalues and eigenvectors obtained from diagonalization of the spin Hamiltonian that includes the anisotropic Zeeman and the zero field splitting terms expressed by the g and D tensors, respectively: H = β B·g·S + S·D·S

Figure 4. Experimental (blue line) and calculated (red line) TREPR spectrum of F8BT/PCBM blend. The simulated spectrum is obtained as the sum of a calculated triplet spectrum of PCBM populated by ISC (dotted line) and a triplet spectrum of PCBM with an excess population in T+1 and T−1 states (dashed line).

(1)

where B is the magnetic field vector, S is the spin operator, and β is the Bohr magneton. It is common to evaluate the D tensor in terms of its eigenvalues (X, Y, Z) or in terms of the zero field splitting parameters D and E defined as D = −3/2Z, E = 1/2(Y − X).23 In general, there are two allowed EPR transition between the three triplet spin sublevels. Both terms in the Hamiltonian depend on the relative orientation of the molecule with regard to the magnetic field B (axis of quantization), and in a polycrystalline or disordered material the resulting spectrum is the sum of all the lines corresponding to the different orientation of the molecules (excited to their triplet states) in the sample. The best fit of the experimental TREPR spectrum of PCBM is obtained using the following zero field splitting parameters D and E and the g-tensor principal values: D = 104 G, E = 7 G, gxx ≈ gyy ≈ gzz ≈ 2.001. Moreover, in order to correctly simulate the experimental spectrum, the relative values of the three zerofield triplet spin sublevels population (Px, Py, Pz) must be considered, from which the population at high field are computed using the same mixing coefficients that transform the zero field states into the high field ones. The best fit values for the sublevel populations are Px:Py:Pz = 0.3:1.0:0.0. All the values for ZFS parameters, g-values, and populations are

The component of the triplet simulated by a ISC population indicates that some of PCBM molecules in the blend undergo a normal singlet to triplet conversion from the excited singlet state, either directly photoexcited or populated by energy transfer from the F8BT excited singlet state. The smaller D value for the PCBM triplet in the blend may be attributed to a distortion of the PCBM triplet wave function by specific interactions with the polymer. In a previous work singlet− singlet energy transfer in the same blend have been demonstrated to be an efficient process.10 The excess population on T+1 and T−1 states can be explained by considering the triplet state generation via a different pathway, namely via the recombination of the charge carriers (radical ion pair recombination). In the frame of the spin-correlated radical pair (SCRP) theory, 26 the pair composed of the interacting polymer cation and PCBM anion can exist in four spin states, which in general are linear combination of one spin singlet and three spin triplet states. The normal recombination pathway of charge carriers starts from the radical ion pair in singlet state, which decays to the singlet ground state by a spin-allowed back-electron transfer D

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from fullerene anion to the polymer cation. The singlet-state recombination eliminates all the pairs which possess ms = 0 spin components. According SCRP theory, the singlet state of the pair is partially admixed with the triplet state of the pair bearing ms = 0 (T0). Thus, also the population of the triplet T0 state is depleted by the singlet recombination process leaving an excess of population on ms = +1 and ms = −1 states (T+1 and T−1 states of the pair). Charge recombination can also happen from the radical ion pairs in triplet state, populating a lower energy neutral triplet state. The pairs in triplet state can therefore either dissociate or recombine to give a neutral triplet exciton, transferring in the latter case the excess population of T+1 and T−1 states of the pair onto the spin states of the lowest energy neutral triplet state, which results in a population pattern different from the pattern expected from ISC processes. A schematic diagram of the radical ion pair recombination to both the singlet ground state and the lowest energy triplet state of the system, showing the nonequilibrium population transfer, is depicted in Figure 5.

In the F8BT/PCBM blend it was reported that excitation of the polymer leads to a fast energy transfer to the excited singlet state of PCBM, followed by ISC to the triplet state and no or negligible generation of charge-separated states.10 According to our results the PCBM excited triplet state is certainly the final destination of the excess energy carried by the absorbed light, but charge transfer states and charge separated states (free charge carriers) are also partially generated, as demonstrated by LEPR results, and the radical ions are eliminated not only by recombination to the ground singlet state but also to the PCBM excited triplet state. The recombination to a triplet state from a pair of PCBM and polymer radical ions, sometimes described as an hole transfer, in a simplified view requires that an electron is transferred from the HOMO of the PCBM to the HOMO of the polymer, as schematically shown in Figure 6.

Figure 5. Possible recombination processes of a radical ion pair state to the ground singlet state or to an excited triplet state. Because of the mixing between the singlet and T0 states of the pair, the singlet state recombination depletes also the T0 state. If the triplet recombination occurs, the excess of populations in the T+1 and T−1 states of the pair is transferred to the neutral excited triplet state, resulting in a population pattern different from the pattern due to ISC mechanism.

Figure 6. Schematic diagram of the charge recombination to a triplet PCBM excited state (hole transfer) starting from a radical ion pair in the triplet state. The HOMO and LUMO energies are reported in the correct relative order, according to literature data. Apparently, the HOMO energies do not allow the electron transfer from the HOMO of PCBM to the HOMO of F8BT (dashed curved arrow), but TREPR results show that the process actually occurs.

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The hole transfer, under this simplified model, is therefore feasible only if the energetic requirement EHOMO‑polymer < EHOMO‑PCBM is satisfied. The HOMO and LUMO levels of PCBM are reported as −6.1 and −3.8 eV, respectively,27 while for F8BT they are reported as −5.9 and −3.3 eV (Figure 1).28 Thus, the HOMO level energies in principle would not allow the charge recombination to the PCBM triplet state. Our experimental results can be explained if the actual energies of the PCBM triplet state (not of the single HOMO orbitals) and the energy of CT states are considered. The requirement for an efficient radical ion pair recombination to the PCBM triplet state is more precisely defined as the existence of an excited triplet state at an energy lower than that of CT states. The PCBM triplet state energy is 1.55 eV above the ground singlet state.29 The energy of CT state is difficult to determine and has been measured only in very few cases where emission or absorption of the CT state could be observed or by using electroluminescence methods.30 Instead, approximate calculations of the CT state energy have been proposed by Veldman et al.,31 by using HOMO or LUMO energies and bandgaps obtained from electrochemical and optical data, and applying empirical corrections as described in eqs 2 and 3:

DISCUSSION Several types of paramagnetic species can be produced upon photoexcitation of a conjugated polymer/fullerene blend. After the primary absorption process, a singlet excited state (S = 0) is produced in the polymer and/or fullerene derivative. In isolated molecules embedded in an inert matrix, the main decay pathways of the excited state are the decay to the ground state (via radiative emission or solvent deactivation processes) or the conversion to the excited triplet state (S = 1) via ISC promoted by spin−orbit interaction. In the presence of a suitable electron donor or acceptor, the excited state (either singlet or triplet) can undergo an electron transfer, giving rise to a pair of bound ion radicals (CT states). The radical ion pair can immediately recombine (geminate recombination) and decay to the singlet ground state or may dissociate to produce free charge carriers in the material (CS states). Another possible evolution of CT states is the singlet to triplet interconversion, which is expected to proceed within a few tens of nanoseconds (the reciprocal of the two radicals resonant frequency difference). As a consequence, the CT states in triplet states may recombine to a lower-lying triplet excited state (triplet exciton). E

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The Journal of Physical Chemistry C ′ ′ ECT = |E HOMO (F8BT) − E LUMO (PCBM)| + Δ

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(2)

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AUTHOR INFORMATION

Corresponding Author

′ E HOMO (F8BT) = −eEoxF 8BT + PCBM ′ E LUMO (PCBM) = −eEred

*Tel +39 049 8275107, fax +39 049 8275248, e-mail lorenzo. [email protected].

1 F8BT F8BT (eEox − eEred − Eg ) 2 1 PCBM PCBM + (eEox − eEred 2

Notes

The authors declare no competing financial interest.

− Eg ) (3)

ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by ENI S.p.A. (contract no. 3500020406), by the University of Padova (PRAT CPDA085989/08), by Regione del Veneto (SMUPR no. 4148), and by MIUR (PRIN 20085M27SS).

where the E′ values represent the corrected electrochemical energies of the HOMO and LUMO orbitals, Eg are the optical bandgaps, and Δ is the electrostatic energy needed to separate the two charges (approximately set at 0.29 eV31). Using energy values reported in previous works, E′LUMO(PCBM) = −4.38 eV31 and E′HOMO(F8BT) = −5.8 eV,19 we can calculate ECT = 1.71 eV. The conclusion from this calculation is that the energy of the CT is expected to be slightly higher than the PCBM triplet state. This is in agreement with our experimental results, even though the energies predicted by eqs 2 and 3 are based on severe approximations and suffer from all the uncertainties on the level energies considered. Our experimental evidence is a direct and more reliable way to determine the relative order of the energies. We therefore conclude that the energy level of charge transfer states in F8BT/PCBM blends is higher than the PCBM triplet excited state, i.e., higher than 1.5 eV with respect to the ground state. Charge separation after photoexcitation is however partially possible, as revealed by LEPR, but the lifetime of CT states is shortened by the presence of a recombination pathway leading to PCBM excited triplet state.

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CONCLUSIONS

Our investigation of a F8BT/PCBM blend identified a variety of photophysical processes that can take place upon visible light excitation. By light-induced EPR we detected the photoinduced formation of charge-separated states, identified as the PCBM radical anion and polymer radical cation. The different LEPR line intensities of the two species suggest a different trap densities of states for the positive and negative charge carriers. By means of time-resolved EPR with microsecond time resolution, we detected the photoinduced formation of PCBM excited triplet state. The EPR spectral shape is correctly reproduced only assuming two pathways for the triplet generation: a normal intersystem crossing from PCBM excited singlet state and a charge recombination process from radical ion pairs to PCBM triplet state. The identification of this recombination pathway, allowed only if the radical ion pair energy is higher than the PCBM triplet state, permits the assessment of a lower limit for the CT state energies in this blend. The identification of an additional charge recombination pathway, observed by TREPR but hardly revealed by other spectroscopic techniques, gives additional information for the complete explanation of the low photovoltaic efficiency of this blend. The investigation reported here demonstrate that TREPR can be a useful method for the identification of photophysical pathways involving paramagnetic states. Currently, an extended TREPR analysis of a series of polymer/ fullerene blends is underway, whose results will be presented elsewhere. F

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