J. Phys. Chem. C 2010, 114, 19535–19539
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Substitutional Impurities in PPV Crystals: An Intrinsic Donor-Acceptor System for High VOC Photovoltaic Devices Arrigo Calzolari,*,† Alice Ruini,‡ Carlo Cavazzoni,§ and Marı´lia J. Caldas| CNR-IOM Istituto Officina dei Materiali, Trieste, Italy, S3 National Center, CNR-NANO Istituto Nanoscienze and Physics Department, UniVersity of Modena e Reggio Emilia, Modena, Italy, CINECA, Casalecchio di Reno, Bologna, Italy, and UniVersidade de Sa˜o Paulo, Sa˜o Paulo, Brazil ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: September 21, 2010
Defects are usually present in organic polymer films and are commonly invoked to explain the low efficiency obtained in organic-based optoelectronic devices. We propose that controlled insertion of substitutional impurities may, on the contrary, tune the optoelectronic properties of the underivatized organic material and, in the case studied here, maximize the efficiency of a solar cell. We investigate a specific oxygen-impurity substitution, the keto-defect -(CH2-CdO)- in underivatized crystalline poly(p-phenylenevinylene) (PPV), and its impact on the electronic structure of the bulk film, through a combined classical (force-field) and quantum mechanical (DFT) approach. We find defect states which suggest a spontaneous electron-hole separation typical of a donor-acceptor interface, optimal for photovoltaic devices. Furthermore, the inclusion of oxygen impurities does not introduce defect states in the gap and thus, contrary to standard donor-acceptor systems, should preserve the intrinsic high open circuit voltage (VOC) that may be extracted from PPV-based devices. Introduction Organic conjugated polymers offer the interesting possibility of combining the convenient optoelectronic properties of conventional semiconductors with the flexibility and lightweight of plastic materials, which entails a wide potential for device purposes,1 in particular for photovoltaic (PV) cells.2 Contrary to standard devices (such as diodes) that generate current under the application of an external bias, i.e., absorbing power, photovoltaic devices are designed to generate power providing both load current and potential drop. The maximum values for the photocurrent and the produced voltage are the so-called short-circuit current (jsc) and open-circuit voltage (VOC), which in principle determine the maximum energy that can be obtained from the cell.2,3 The schematic diagram in Figure 1a shows the working principle of a standard organic PV (OPV) device, which involves the promotion of one electron from the valence band top (VBT) to the conduction band minimum (CBM) of the active layer (conjugated polymer) under illumination. Ideally, the produced exciton should easily dissociate, yielding carriers, both electrons and holes, free to flow through the semiconductor and be collected by two external electrodes, with low and high work function energies (LWFE and HWFE in Figure 1), respectively. The difference between the electrode work functions is the resulting VOC. The efficient operation of the cell is strictly connected to an appropriate alignment of the energy levels of the electrodes and the active layer (Figure 1a). However, since excitons have a sizable binding energy in most conjugated polymers, dissociation occurs mainly at the * To whom correspondence should be addressed. E-mail: arrigo.calzolari@ democritos.it. † Theory@Elettra Group, Democritos Simulation Center, CNR-IOM Istituto Officina dei Materiali. ‡ S3 National Center, CNR-NANO Istituto Nanoscienze and Physics Department, University of Modena e Reggio Emilia. § CINECA. | Instituto de Fı´sica, Universidade de Sa˜o Paulo.
organic/LWFE interface. Due to the limited exciton diffusion length, efficient photoconversion occurs only for absorption close to the interface, which can cause hole accumulation on the organic side.4 One step toward a more efficient dissociation is to create a heterojunction within the active region: two materials with different electron affinities and ionization potentials (e.g., conjugated polymer and fullerene), forming a donor-acceptor interface as represented in Figure 1b. One common solution is the creation of a so-called “bulk heterojunction” architecture, where an acceptorswhich can be a molecule such as fullerene, a nanotube, or a semiconductor nanoparticlesis dispersed directly in the donor polymer film. Upon exciton dissociation, the hole flows through the polymer film, and the electron is transferred to the acceptor and then to the electrode. Besides the problematic issues of miscibility and morphology control,5 one of the most challenging drawbacks of donor-acceptor systems6-8 is the inherent reduction of VOC (Figure 1b): the lowering of the effective CBM of the composite forces the choice of an electrode with a higher work function (LWFE(1) in Figure 1), in order to fulfill the correct energy level alignment, degrading the overall efficiency of the OPV cell. The best strategy would be to design, for each polymer system, a way to dissociate the exciton without affecting too much the effective energy gap, so as not to lower the VOC. In this direction, we propose the alternative (Figure 1c) of devising intrinsic donor-acceptor behavior through introduction of a suitable defect in the bulk polymer system. We find that for poly(p-phenylenevinylene) (PPV) the proper dilute substitution of one of the vinylene blocks by a keto-defect [-(CH2-CdO)-] should indeed enhance photoconductivity, leaving virtually untouched the polymer band gap. Choice of System While defect engineering is a well-established field of research in the case of inorganic semiconductors, this is not the case for
10.1021/jp105765d 2010 American Chemical Society Published on Web 10/21/2010
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Figure 1. Principle of operation of PV devices. (a) Behavior of single-component organic device. (b) Two-component donor-acceptor system, bilayer or bulk heterojunction. (c) Intrinsic (single-component) donor-acceptor system.
semiconducting polymers, and defects, related to the presence of impurities or conjugation breaking, have been usually invoked as the main cause for low efficiencies or device degradation.9,10 In recent years, however, it has been accepted that there is still lack of fundamental understanding,11 and the importance of investigating in depth the role of morphology and defects11-14 on device performance has gained new attention. PPV is one of the most investigated conjugated polymers for organic electronics and is generally very sensitive to oxidation,15,16 like other organic polymers. Oxygen can be unintentionally introduced in the film, due to the wet-chemistry processing or by ambient contamination, and in some cases the presence of carbonyl (CdO) groups has been detected, together with luminescence quenching.17 The hypothesis of chain cleavage (i.e., photolysis) as a consequence of the oxygen-driven formation of aromatic aldehydes in the vinyl segments9 was usually assumed to explain the observed quenching;16 however, the same presence of carbonyl groups was associated with photoconductivity enhancement,18 which motivates further study. As such, related on-chain keto-defects have been already studied19 from first-principles for the isolated PPV chains: a disruptive effect on carrier transport was demonstrated, with interesting electronhole decoupling, which offers a possible alternative explanation for the experimentally observed photoluminescence bleaching and photoconductivity enhancement. Still the impact of the same substitution on bulk systems was not clear, since the effect could be lost, and this is our focus here. When discussing defect engineering, it is extremely important to have a well-established knowledge base for the electronic and optical properties of the host system. For conjugated polymers this is even more critical, since the actual morphology of the film is very rarely known in depth, and usually different materials are grouped under the same denomination. As a clear example here, the luminescence quenching has been detected in the references quoted before9,10,16,17 in very different materials: underivatized PPV,17 poly(bis(cholestanoxy)phenylenevinylene) (BCHA-PPV),9 and poly(methoxy(dimethyloctyloxy)phenylenevinylene) (MDMO-PPV),10 while most other studies20 are for poly(methoxy(ethnylhexyloxy)-p-phenylenevinylene) (MEHPPV). Now, films composed of these polymers show completely different morphological properties; while the side chains in most polymers facilitate casting, because they dissolve more easily in a proper wet-chemistry processing, the same side chains also dictate the very disordered amorphous final configuration, with each conjugated segment either electronically isolated from the neighbors or at most with π -stacked segments20 as in MEHPPV. On the other hand, pristine PPV aggregates in very compact crystallites in herringbone (HB) configuration,21,22 which are immersed in amorphous regions. It is to be noted that the phenomenon of PC enhancement has been detected in
underivatized PPV samples18 and not, to our knowledge, in derivatized PPV films. Indeed, another effect from different morphological configurations regards the optical properties, an object of active debate for PPV films.23-25 Some points can be extracted in this case from previous theoretical studies carried out with ab initio methodologies: results for the optical properties of single-chain PPV26 indicate a well-defined excitonic optical structure, with large binding energy for the first exciton, which has also a large extension (∼ 50 Å) in real space. Solid state effects on the electronic transport and optical properties of conjugated polymers have been already studied, also by means of theoretical ab initio techniques.23,27-30 The specific 3D chain packing, π-stack or HB, was demonstrated to crucially affect both transport and optical properties. In particular, the exciton binding energy is found to decrease27 for HB-PPV compared to π-stack or isolated chains; one more positive point for HB-PPV is that the lowest exciton is dark23 (optically inactive): this should enhance exciton migration prior to radiative recombination. Finally, charge-transfer excitons, with electron and hole on different neighboring chains, originate as a consequence of interchain interaction, favoring also transversal exciton migration. While on-chain processes possibly dominate in the amorphous regions, where interchain interactions are weaker, inside the crystalline grains the three-dimensional properties prevail and lead us to proceed with the study presented here. Methodology In this paper, we investigate the solid state effects on the electronic structure of bulk HB-PPV in the presence of oxygen keto-defects. We performed ab initio electronic structure calculations based on the density-functional theory (DFT) as implemented in the Quantum-Espresso suite of codes.31 We adopted PBE32 generalized gradient correction to the exchange correlation functional and ultrasoft pseudopotential of the Vanderbilt type.33 The electronic wave functions were expanded in plane waves up to an energy cutoff of 30 Ry, while the expansion for the density was taken to 300 Ry. For the density calculation, the Brillouin zone of the system (Figure 2) was sampled34 with 16 k points in the one-dimensional case of the single chain (see below), and with the Γ point only in the case of the large bulk supercell. For the bulk cell, we also extracted values for the Z point. In the crystalline HB configuration there are two inequivalent chains35 per unit cell: in order to study diluted keto-defects in the bulk crystal, we considered a large supercell corresponding to (3 × 3) HB lateral periodicity, i.e., 18 parallel chains, each containing 7 phenylenevinylene units (3 × 3 × 7). The resulting triclinic cell for the clean PPV bulk has dimensions a1 ) 2.42 nm, a2 ) 4.69 nm, a3 )1.81 nm (R ) β ) 90°, γ ) 57°) and
Substitutional Impurities in PPV Crystals
Figure 2. Crystal structure: top (a) and side (b) view of the typical herringbone chain arrangement for bulk PPV in the presence of the keto-defect on the central chain. Labels 1-3 identify three inner chains in the cell, taken as a reference in the text. Chain 2 includes the defect. Inset of panel (b): zoom on the (CdO) keto-termination in vinyl group. (c) Brillouin zone corresponding to the real-space monoclinic cell of panel (a).
includes 1764 atoms and 4788 electrons. The substituted system is obtained by adding a single oxygen atom and rearranging a central vinylene group to the ketone configuration. The simulated cell is shown in Figure 2a, b, while panel c displays the corresponding Brillouin zone. It is important to take into account dispersion forces,36 e.g., van der Waals and London interactions, that rule the assembly of polymeric chains into the solid state phase. Given the huge dimension of the system, which is well beyond the standard even for bare DFT calculations, we adopted a combined approach19,37 based on both force-field and first-principles techniques. We optimize the atomic structure of the overall system by means of a classical relaxation procedure, based on the empirical COMPASS force field,38 which properly includes van der Waals contributions. Then, keeping the atoms fixed, we calculate the ground state electronic structure of the resulting geometry. The simulations for the single-chain limit were obtained extracting a single chain from the force-field relaxed bulk.19,37 Results and Discussion In Figure 3 we show the results in terms of electronic properties: the band structure for the single chain, in both the keto-defect (dashed line) and clean (solid line) configuration, is displayed in the left panel, while the right panel exhibits the density of states (DOS) of the keto-defect in the bulk (Figure 2) compared to the clean HB bulk. The electronic structure of the PPV single chain (left panel) is dominated in the region near the gap by π-like dispersive bands. The presence of the oxygen defect in the single chain breaks the conjugation and localizes the states at the band edges, as seen by the flattening of the energy bands, in agreement with previous single-chain results.19 In Figure 3 the DOS for the bulk systems should be interpreted taking into account that we have just two k points, and as such we do not clearly distinguish the usual van-Hove-type dispersion even for the clean bulk.28,29 We observe that in both the clean and the keto-defect bulk samples, the effect of the crystalline environment results in the formation of gathered manifolds of states, which correspond to the broad peaks in the density of states. Each manifold contains a number (or multiplet) of levels equal to the number of PPV chains included in the simulation cell, i.e., 18 in the present case. Thus, compared to the picture of the primitive cell,28 we also explicitly see the broadening of the levels due to interchain interaction, summed to the effect
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Figure 3. Effect of the keto-defect on the electronic properties of PPV, single-chain, and bulk system: we show in the left panel the electronic band structure of the isolated chain and in the right panel the density of states of the bulk; solid black lines refer to the clean systems, dashed red lines to the systems in the presence of the defect. Γ-Z direction in the Brillouin zone corresponds to the polymer axis in the real space. Results for the bulk are the convolution for Γ and Z points. Labels follow those in Figure 2c.
of band-folding in the longitudinal direction. The orbitals in a manifold have identical symmetries with some degree of hybridization between neighboring chains. We can see anyhow that HB packing produces a slight gap reduction with respect to the single-chain case. In the crystalline structures, the effect of the oxygen impurity is on the whole much less destructive than for the single chain: The formation of the keto-defect locally destroys the electronic conjugation only in the substituted chain, leaving the neighbors electronically unchanged. The defect states in the valence band localized on the oxygen atom are found to accumulate at about -2 eV, i.e., well below the frontier orbital energies, and are fully occupied, that is, inert. Indeed the electronic states belonging to the perturbed chain remain immersed in the crystalline manifolds, without insertion of defect states in the gap: in other words, the interesting electron-hole separating states detected for the single chain are still seen here, as band edge and not as gap states. The absence of gap states may have relevant consequences in the search for a high VOC in PPVbased solar cells. In order to gain further insight, we calculate within DFT the electrical gap as the difference between the ionization potential (IP) and the electron affinity (EA), that are in turn defined as the difference in total energies between neutral and charged states: IP ) E(N - 1) - E(N) and EA ) E(N) E(N + 1), where N is the number of electrons. Charged systems are simulated by adding/removing an electron in the whole periodic cell. We find that the electrical band gap Eg is basically unaffected by the presence of the defect (Eg ) 1.37 for the clean case, Eg ) 1.34 with the defect). It should be noted that the quantitatiVe values we obtain are bound to be affected by not taking into account the beyond-DFT many-body corrections.39,40 However, at this time, a full calculation within the many-body perturbation theory scheme is not accessible for a system of this size. Moreover, due to the very similar (delocalized) character of both the substituted and clean states in the gap region, we do not expect large errors in the comparatiVe values we now present; in fact, calculations of many-body effects on the electronic structure of several systems (see, e.g., refs 41-43) agree in that the quasiparticle corrections to the DFT energies are pretty similar for states having the same real-space localization. We now consider the possible effects induced by the ketodefect on exciton migration and photocurrent generation. We
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Figure 4. Convoluted band edge states for bulk PPV in presence of the keto-defect (top view on the left and side view of the central chains on the right). The exciton-splitting effect is evident from the side view, where the real-space separation of c-HOMO and c-LUMO orbitals at the substitution site is clearly rendered. Chains at the lateral positions (grayish orbitals in right panels) are not shown on the left for clarity. Labels 1-3 follow those of Figure 2.
focus on the convolution of the HOMO and LUMO peaks (labeled c-HOMO and c-LUMO, respectively) of the substituted structure, shown in Figure 4. For the sake of simplicity in visualization, in Figure 4 a zoom on three central chains in the cell is also reported, two of them clean (labeled 1 and 3) and the one including the keto-defect (labeled 2). The remaining chains of the cell behave as the unperturbed chains 1-3, so we may expect that a photon absorbed in the vicinity ofsbut not atsthe defect will have the same characteristics as those found in the underivatized crystal. Then, Figure 4 shows that, while the convoluted frontier orbitals are uniformly delocalized over the entire crystal, the HOMO and LUMO states on the substituted chain are delocalized over different semiaxes, with the same origin on the defect itself; i.e., they remain perfectly conjugated up to the defect but do not overlap. We may then also expect that, if the exciton migrates by attraction of the electron-hole pair to the substitutional impurity, the pair will split at the defect. Therefore, our calculations for the bulk crystal confirm the behavior already reported for the isolated chain: for HB-PPV crystallites the on-chain keto-defect is expected to help the exciton dissociation in a free electron-hole pair, acting as an intrinsic donor-acceptor system (Figure 1c). This would configure an optimal realization for solar cell production: PPV intrinsically absorbs light in the visible range and can provide a high nominal VOC potential. The inclusion of keto-impurities leads to the formation of an intrinsic donoracceptor system that favors exciton splitting and thus photocurrent generation. At the same time, the substituted system does not imply in any lowering of the open-circuit voltage VOC, as in typical heterojunction-based systems. Summary and Conclusions We investigated the effect of a dilute substitutional defect in a bulk semiconducting polymer crystal, namely, -(CH2-CdO)- (keto-defect) substituting for one vinylene group in poly(p-phenylenevinylene). We employed for that scope ab initio density-functional theory applied to a large supercell, in the standard simulation scheme adopted for defects in conventional semiconductors. We find that the isolated impurity, in the compact herringbone structure of bulk underivatized PPV, produces a sharp spatial splitting in both conduction and valence band edge states of the substituted chain, but does not introduce active gap levels, and furthermore leaves mostly undisturbed all neighbor chains. The combination of
these properties with well-established results for the excitonic properties of bulk HB-PPV can be used to understand puzzling experimental data regarding the optical activity induced by the carbonyl group in PPV films. These results should be viewed as a new concept, or a change of paradigm for organic devices: even considering the advantages of ambient or wet-chemistry processing, there is real need to probe the fundamentals of electronic properties, through realistic simulations tools coupled to model experimental setups, in order to fully explore the advantages of any given system. In the case studied here, the existence of an intriguing effectsluminescence quenching coupled to photoconductivity enhancementsin a particular system triggered our investigation. The possibilities for using this specific defect system depend on control of morphology, to make possible the use of underivatized HB-PPV, and on controlling, in the synthesis, the defect concentration, which should be matched to the exciton diffusion length and not degrade carrier mobility. Acknowledgment. Computational resources were provided at CINECA by CNR-INFM through “Commissione Calcolo Parallelo”. M.J.C. acknowledges support from FAPESP and CNPq, Brasil and CNR-NANO S3, Italy. References and Notes (1) Friend, R.; Gymer, R.; Holmes, A.; Burroughes, J.; Marks, R.; Taliani, C.; Bradley, D.; Santos, D. D.; Bre´das, J.; Lo¨gdlung, M.; Salaneck, W. Nature (London) 1999, 397, 121. (2) Spanggaard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125. (3) Dennler, G.; Saricifti, N. S.; Brabec, C. J. Conjugated PolymerBased Organic Solar Cells. In Semiconducting Polymers: Chemistry, Physics and Engineering; Hadziioannou, G., Malliaras, G., Eds.; Wiley-VCH: Weinhem, Germany, 2000. (4) Antoniadis, H.; Hsieh, B.; Abkowitz, M.; Jenekhe, S.; Stolka, M. Synth. Met. 1994, 62, 265. (5) Quist, P. A. C.; Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J.; Savenije, T. J.; Siebbeles, L. D. A. J. Phys. Chem. B 2006, 110, 10315. (6) Olson, D. C.; Shaheen, S. E.; White, M. S.; Mitchell, W. J.; van Hest, M. F. A. M.; Collins, R. T.; Ginley, D. S. AdV. Funct. Mater. 2007, 17, 264. (7) Servaites, J. D.; Ratner, M. A.; Marks, T. J. Appl. Phys. Lett. 2009, 95, 163302. (8) Xin, H.; Reid, O. G.; Ren, G.; Kim, F. S.; Ginger, D. S.; Jenekhe, S. A. ACS Nano 2010, 4, 1861. (9) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough, R. L. J. Am. Chem. Soc. 1995, 117, 10194. (10) Chambon, S.; Rivaton, A.; Gardette, J.-L.; Firon, M.; Lutsen, L. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 317. (11) Gregg, B. A. Soft Matt. 2009, 5, 2985.
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