Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy

Chemical Physics Letters 542 (2012) 70–73

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Sub-bandgap absorption in polymer-fullerene solar cells studied by temperature-dependent external quantum efficiency and absorption spectroscopy Martin Presselt ⇑, Felix Herrmann, Sviatoslav Shokhovets, Harald Hoppe, Erich Runge, Gerhard Gobsch Institute of Physics and Institute of Micro- and Nanotechnologies, Ilmenau University of Technology, 98693 Ilmenau, Germany

a r t i c l e

i n f o

Article history: Received 13 January 2012 In final form 28 May 2012 Available online 4 June 2012

a b s t r a c t We study the sub-bandgap (SBG) absorption in solar cells made of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenylC61-butyric-acid-methyl-ester by photothermal deflection absorption spectroscopy and measurement of temperature-dependent external-quantum-efficiency (EQE) spectra. Several models for SBG absorption are critically reviewed in view of the EQE results. The latter suggest polaron-related transitions as origin of the Gaussian SBG peak near 1.6 eV. Intermolecular charge transfer (CT) excitations as an explanation cannot completely be ruled out. However, the assumption of CT excitons with large binding energies is difficult to reconcile with the rapid loss of weight of the Gaussian SBG-peak seen in EQE above room temperature. Ó 2012 Elsevier B.V. All rights reserved.

One of the most intensely studied material systems used for bulk heterojunction (BHJ) solar cells is a blend of poly(3-hexylthiophene-2,5-diyl) as electron donor and [6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) as electron acceptor, because rather good solar cells with energy-conversion efficiency above 5% can be produced [1–5]. Nevertheless, many question regarding fundamental processes particularly at the donor–acceptor interface remain open even for the corresponding bilayer system. Even more challenging are BHJs, where the complexity of the nanomorphology and the abundance of interfaces dramatically modify exciton and charge transport and increase the importance of interface-related phenomena [6–8]. Weak features observed in optical spectra of BHJs at energies below the bandgaps of the individual blended materials are assumed to be directly related to the electron donor–acceptor interface, see Refs. [9–11] and references therein. These weak sub-bandgap (SBG) features are best seen in photoluminescence measurements [12–14], which naturally are particulary sensitive to rather immobile states at low energy. In contrast, the present Letter studies temperature-dependent external quantum efficiency (EQE) spectra, which qua definition are sensitive to those states which contribute dominantly to charge transport and, thus, to power conversion. Various mechanisms for the origin of SBG transitions in P3HT:PCBM BHJs and their dependence on the nanostructure of the blend have been suggested in the literature. They are briefly categorized and summarized in the following and are illustrated in Figure 1. ⇑ Corresponding author. E-mail address: [email protected] (M. Presselt). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.05.063

(A) Disorder-induced absorption tails and trap-state absorption have been studied mostly for inorganic materials for more than half a century following the seminal work of Urbach [15]. Recently, they were evidenced for organic electron donor acceptor systems by photothermal deflection spectroscopy (PDS) [16], and their impact on fundamental processes in organic solar cells was investigated [17–19]. However, it is still debated controversially whether tail states contribute to sub-bandgap EQE spectra [9,20]. The trap state density is assumed to increase when approaching the P3HT-PCBM interface. (B) Molecularly dispersed PCBM in the P3HT-rich phase with inhomogeneously broadened transition energies would most likely lead to Gaussian-shaped peaks [9]. Again, it seems reasonable to assume that the density of molecularly dispersed PCBM is increased near the P3HT-PCBM interface. (C) Intermolecular charge transfer (CT) excitations from the highest occupied molecular orbital of the donor to the lowest unoccupied molecular orbital of the acceptor are often referred to as charge transfer exciton if their energy is sufficiently below that of the intramolecular transitions [9,20,21]. However, it is discussed controversially whether CT states are involved as essential intermediate steps of charge separation in P3HT:PCBM BHJs. Photoluminescence [13,14], electroluminescence [22], and transient absorption spectroscopy [23–25] were applied to answer this question. Stable CT states were identified for other organic donor–acceptor systems [26–28], and possess binding energies between 130 and 200 meV [29,11]. Naturally, CT states are connected to the P3HT-PCBM interface, but are assumed to yield rather trapped than free polarons [10,30].

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Figure 1. Scheme of possible sub-bandgap transitions (black arrows). Transitions between highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals are given for comparison (grey arrows). (A) Disorder-induced absorption tails resulting from local variations of HOMO and LUMO energies; (B) Molecularly dispersed PCBM in a P3HT-rich phase with energy levels deviating from bulkPCBM; (C) Intramolecular charge transfer transition between P3HT-HOMO and PCBM-LUMO; (D) Possible transitions at a hole–polaron: The Coulomb attrraction reduces locally the HOMO–LUMO gap (P2 transition), modifies other transitions into the LUMO (P3), and induces new intra-band transitions (P1).

Figure 2. Absorption spectra of P3HT, PCBM and corresponding 2:3 and 3:2 blended films detected via photothermal deflection spectroscopy at room temperature. Dashed graphs indicate simulated spectra obtained by linear combinations (LC) of the absorption spectra of the pristine films weighted according to the blending ratios. All features of the experimental blend absorption spectra can be reproduced, except for a non-additive shoulder between 1.4 and 1.7 eV (shown enlarged as inset), which is discussed in detail in the body of the text. The weak peak around 1.27 eV is assigned to a P2–hole–polaron transition at P3HT, while the weak peak at 1.76 eV is assigned to a PCBM transition.

(D) Charged polaron-related transitions [31–35] are absorption events in organic semiconductors happening near a (charged) polaron. The presence of a charge carrier modifies locally the energy levels of the single-particle diagram Figure 1D. Analogously to the fact that the total energies of both the H ion and the Hþ 2 molecule are lower than that of the neutral atom, optical transitions leading to a complex of two electrons and one hole or one electron and two holes occur at frequencies below the neutral exciton transition. Such three particle complexes, so-called trions, have been studied extensively in inorganic semiconductors [36]. Charged three particle complexes exist in organic materials as well, see e.g. Ref. [37] for recent experiments in a PPV-related polymer (PPV: polyphenylene vinylene). However, they are less often referred to as trions. The optical transition may or may not occur on the same molecule where the extra charge, i.e. the polaron, resides. The latter case includes charged charge-transfer excitations. The polaron itself can be localized or delocalized. Recently, we have demonstrated that the SBG EQE spectrum is constituted of two contributions at least: an approximately exponential absorption tail and a Gaussian-shaped absorption peak [9]. While the exponential contribution could be attributed to disorder in agreement with literature [16], the Gaussian function has not yet been assigned unambiguously. The present Letter extends our earlier work by a systematic study of the temperature-dependent EQE and by absorption spectroscopy, i.e., the combination of ellipsometry with photothermal deflection spectroscopy [38]. In order to resolve SBG features, both experiments were performed with an outstanding intensity resolution. We compare results for P3HT and PCMB solar cells with two blends of composition P3HT:PCBM = 2:3 and 3:2. Solar cells made from blends of the latter composition show for suitable morphology an optimized maximal power conversion efficiency of the order of 5%. The EQE was measured for solar cells composed of

PEDOT:PSS (poly (3,4-ethylene dioxythiophene:poly (styrene sulfonate)) and P3HT:PCBM layers, successively spin-cast on ITO (indium tin oxide) covered glass substrates. Aluminum contacts were deposited on top by physical vapor deposition. All solar cells and films were annealed for 5 min at 150 C [39]. The EQE-measurements were performed under short-circuit conditions and weak monochromatic illumination intensities. Temperature equilibrations were assured by a waiting period of 30 min for each temperature setting. The PDS samples of the active layer were spin-coated on 20  5 mm Heraeus SuprasilÒ substrates with a typical film thickness of 100 nm. They were produced and annealed with the same parameters as the solar cells used in the EQE experiments. PDS data were recorded using an experimental setup consisting of a 250 mm monochromator with halogen lamp as excitation source. The Suprasil cell was filled with perfluorohexane. The deflection was probed by a helium neon laser. A pristine PCBM film (Figure 2) shows a weak SBG absorption peak at 1.76 eV and an absorption tail at lower energies. The absorption tail of the P3HT film is stronger and a P2–polaron transition [34] (cf. Figure 1D) causes a weak absorption peak at 1.27 eV. The absorption spectra of the blends can be approximated quantitatively as linear combination of the spectra of the pristine films weighted with the blending ratios, except for a broad shoulder between 1.4 and 1.7 eV (see Figure 2), which is constituted of the absorption tail (Figure 1A) and the Gaussian SBG peak to be assigned in the following. If molecularly dispersed PCBM molecules (model B) would be the origin of the SBG absorption in the P3HT:PCBM BHJs, the PCBM peak at 1.76 eV should decrease with increasing SBG absorption, rather than being reproduced by the linear combination of the P3HT and PCBM spectra (dashed lines in Figure 2). Furthermore, one would not expect the disorder-broadened Gaussian to be

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centered up to 200 meV below rather than at the 1.76 eV PCBM peak. Hence, transitions of molecularly dispersed PCBM appear to be an improbable explanation for SBG absorption. This leaves CT (model C) and polaron transitions (model D) as likely origins of the SBG absorption peak near 1.6 eV. The fact that the latter occurs in blends only, can be explained in either model: A CT transition is simply not possible in the pristine materials, while the interpretation of polaron transitions in the blend around 1.6 eV is more complex. For this transition energy, two different models are discussed in the literature: It could be a P3 transition from HOMO-1 to LUMO, which by the interaction with the extra charge is lowered below the HOMO–LUMO transition, i.e., the gap energy, see Figure 1D. It is symmetry forbidden for an isolated P3HT chain, but its optical weight is expected to increase with increasing intermolecular interactions and upon blending [32]. Alternatively, it can be a so-called DP2 (’’delocalized polaron’’) transition. It resembles the P2 transition, but involves a more mobile hole–polaron which is delocalized over several molecules [31]. Thus, it gains less energy from lattice deformation and the corresponding excitonic transition occurs at an energy considerably above the P2 absorption feature. In contrast to the P3 transition but analogously to the P2 transition at 1.27 eV [34], the DP2 absorption in the P3HT-rich phase is expected to change little upon blending. Therefore, within the polaron model the peak around 1.6 eV would be assigned to a P3 absorption. In order to discriminate between model C (charge transfer excitons) and D (polaron-related transitions), we studied the temperature-dependencies of external quantum efficiencies as they have been shown to be sensitive to exciton hopping [39] and the generation of a photocurrent in model D includes the additional step of (charged) exciton diffusion towards the interface. Figure 3 presents EQE spectra recorded at several temperatures ranging from 80 to 390 K. We will compare the temperature dependence of the weak EQE signal (0.01–0.5%) at 1.6 eV as representative SBG energy, with the temperature dependence of the EQE

EQE [%]

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T [K] 80 100 125 160 200 250 295 347 390

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P3HT:PCBM=2:3

EQE [%]

EQE [%]

10

at energies which are either typical for P3HT (2.1, 2.5 eV) or for PCBM (1.8, 3.5 eV) [39]. As can be seen in Figure 4A and C, P3HT-related EQE(T) values pass through a typical maximum around room temperature for both BHJ solar cells. It is caused by the interplay between thermal activation and phonon scattering [40,35,41,39]. This is also true for PCBM-related EQE values for the P3HT-rich solar cell, but they are still increasing above room temperature for the PCBM-rich solar cell. The difference between both cells results most likely from changes in the nano-morphology: At the larger PCBM weight fraction, the PCBM clusters are larger and more crystalline, thus phonon scattering is reduced and the activation energy is decreased by about 10 meV. In contrast, electron-donating polymer clusters are rather changing in number than in size upon varying their weight fraction in a BHJ [42,43]. The temperature-dependence of EQE spectra is notoriously difficult to interpret quantitatively, because they are influenced by exciton, electron–polaron and hole– polaron transport, the mechanism of exciton dissociation as well as the temperature dependence of the two contacts. However, differences between the two solar cells in the non-SBG region will most likely be dominated by differences in the diffusive exciton transport. The temperature-dependent EQE in the non-SBG region can be modeled by the form EQE ðTÞ  T m expðDE=kB TÞ, [39] which has been suggested for the combined influence of thermal activation and phonon scattering [35]. Note that the low-temperature regime 80–200 K is important for reliable quantitative fits [39]. The SBG EQE(T) curve recorded in the subband-gap region at h  x ¼ 1:6 eV follows qualitatively the EQE(T) spectra at higher energies. The temperature dependence of the latter is most likely dominated by the interplay of thermal activation and phonon scattering of excitons and polarons, yielding a drop of EQE at high temperatures [39]. For the SBG EQE(T) curve, one can expect different temperature dependence for the disorder-related exponential contribution to SBG absorption (model A) and for the Gaussian peak

0.0 400

Photon energy [eV] Figure 3. External quantum effciency (EQE) spectra at temperatures between 80 and 390 K of solar cells with P3HT:PCBM blending ratios of 2:3 and 3:2 (upper (A) and lower (B) part) are shown on the right side on a linear scale. On the left, the EQE spectra are plotted between 1.2 and 1.9 eV on a semi-logarithmic scale to visualize the weak sub-bandgap features. Vertical dotted, dashed and dash-dotted lines indicate energies typical for P3HT, PCBM and sub-bandgap absorption, respectively, at which the EQE spectra are evaluated.

Figure 4. On the left, external quantum effciencies (EQE) evaluated at energies typical for the blended materials and for sub-bandgap (SBG) absorption (cf. vertical lines in Figure 3) are shown for solar cells with P3HT:PCBM blending ratios of 2:3 and 3:2 (panels (A) and (C)). On the right (panels (B) and (D)), temperature dependencies of the exponential and the Gaussian contribution to the SBG EQE spectra are shown. These contributions were determined by fitting the SBG spectra and integrating the area of the resulting Gaussians and exponential functions (0– 1.65 eV) for each investigated temperature.

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(models C and D). Therefore, we separate those via line fits which also include the contributions which are present in the pristine films as well: the PCBM-related Gaussian near 1.8 eV and the tails of the fundamental P3HT absorption starting between 1.9 and 2.0 eV. The resulting weights are shown in Figure 4B and D. For both investigated solar cells, the disorder-related exponential SBG EQE tails increase with increasing temperature, while the Gaussian SBG EQE contributions show pronounced maxima between 250 and 300 K. The decrease towards higher temperatures is considerably stronger than that of the above-bandgap-EQE spectra. Assuming charge-transfer excitons as origin of the SBG peak (model C), exciton diffusion would be absent and the impact of phonon scattering on the EQE temperature dependence should be weak. The observed pronounced EQE decrease above room temperature could possibly result from thermal dissociation of CT excitons. However, quantum chemical calculations [30] and electricfield-dependent photoluminescence experiments suggest binding energies in the range of 130 to 200 meV for BHJs made of PPV and PCBM [11,29], which is much larger than thermal energies. As we are not aware of equally reliable corresponding data for P3HT:PCBM, we shall assume that charge-transfer-exciton binding energies are not dramatically different for both polymeric BHJ systems. On this condition of thermally rather stable CT-excitons, the rapid temperature loss of the SBG EQE Gaussian is difficult to reconcile with a direct CT excitation as origin of this Gaussian. In contrast, it seems natural to assume that charged polaron-related transition (’’trions’’) are subject to rather strong interactions with phonons and, therefore, strongly temperature-dependent. Thus, the strong temperature dependence at high temperatures provides a strong indication in favor of a polaronic origin (model D). In summary, four models possibly explaining sub-bandgap absorption in BHJ solar cells were evaluated: (A) Disorder-induced absorption tails are identified in the absorption spectra of the pristine P3HT and PCBM films and in the blends. Related EQE tails are increasing with temperature since disorder is related to molecular vibrations. (B) Absorption of molecularly dispersed PCBM in the P3HT-rich phase appears to be an improbable explanation of the considered SBG Gaussian. (C) Thermally stable intermolecular charge transfer (CT) excitations can very well explain absorption data. However, this explanation is difficult to reconcile with the pronounced temperature dependence of the EQE, i.e. the rapid loss of weight of the Gaussian SBG EQE-peak around 1.6 eV above room temperature seen in Figure 4B and D. (D) Charged polaron-related transitions provide a consistent interpretation of SBG absorption and SBG EQE(T) spectra of P3HT:PCBM BHJs because (i) the estimated SBG Gaussian energy agrees with the values reported in the literature for P3-type polaron transitions [34,32]; (ii) the SBG Gaussian is more pronounced in the absorption spectra of the BHJs than in the spectra of the pristine P3HT films since the P3 transition is assumed to be strengthened due to intermolecular interactions; and (iii) the temperature dependence of the SBG EQE Gaussian indicates a strong impact of phonon scattering on exciton diffusion as expected for excitons which are inherently linked to polarons, i.e., the P3 exciton. In conclusion, SBG absorption and SBG EQE features are caused by disorder-related absorption tails and Gaussian-shaped absorp-

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