Role of Molecular Architecture in Organic Photovoltaic Cells - The

PERSPECTIVE pubs.acs.org/JPCL

Role of Molecular Architecture in Organic Photovoltaic Cells D. Venkataraman,* Serkan Yurt, B. Harihara Venkatraman, and Nagarjuna Gavvalapalli Department of Chemistry, 710 North Pleasant Street, University of Massachusetts Amherst, Amherst, Massachusetts 01003

ABSTRACT What are the stumbling blocks for achieving high-efficiency organic photovoltaic devices? This question is examined from a molecular architecture and molecular packing perspective. The intermolecular interaction between the electron donor and electron acceptor influences the charge separation. The packing of electron donors and acceptors influences the charge transport. Therefore, there is a need to strike a balance between the chargetransfer interactions and packing interactions and obtain nanoscale segregated morphologies for efficient charge separation and charge transport. Molecular architecture is key toward striking this balance and, therefore, its impact on charge-transfer interactions and packing interactions; thus, the active-layer morphology and photovoltaic metrics are examined. A variety of molecular architectures for the packing of π-conjugated organic molecules to structures relevant for photovoltaic devices is also discussed.

T

here is widespread interest in developing organicbased photovoltaic (OPV) devices for solar energy conversion.1-13 Using π-conjugated organic materials will allow solution-based fabrication methods that may, in turn, lead to low-cost, lightweight, flexible solar cells for applications such as solar roof shingles, solar curtains, or solar backpack chargers. Thus far, the highest overall power conversion efficiency achieved in OPVs is ∼7%.14-16 In comparison, commercial Si-based solar cells have overall power conversion efficiencies of ∼20%. The research in the field of OPVs is focused on achieving power conversion efficiencies greater than 10%.17 What are the stumbling blocks for achieving this efficiency target? In all organic photovoltaic cells, the active material consists of π-conjugated electron donors and electron acceptors. The nature of the intermolecular interaction between electron donors and electron acceptors influences the charge transfer. The packing of electron donors and the packing of electron acceptors influences the charge transport. The active-layer morphology should strike a balance between the intermolecular interaction between electron donors and electron acceptors and the intermolecular interactions within the electron donors and electron acceptors. Molecular architecture plays a key role in achieving this balance. This Perspective examines how the molecular architecture influences the active-layer morphology and photovoltaic metrics. In a photovoltaic device, photons are absorbed by an active medium, and this process creates excited electron-hole pairs. These electron-hole pairs, the excitons, can either recombine to emit light or heat or, in a more useful process, dissociate and move to their respective electrodes to result in a photovoltaic current. In OPV devices, excitons are tightly

r 2010 American Chemical Society

bound and have short diffusion distances, typically less than 10 nm.4,11,18 Therefore, in order to efficiently harvest the charge carriers, a heterojunction should exist within the exciton diffusion distance to dissociate excitons, and a continuous conducting pathway should exist for the charge carriers to reach the electrodes. Photoefficiency also depends on the ability of the photoactive materials, the active medium, to capture photons; typical photon capture distances in conjugated organic molecules/polymers are ∼0.2 μm.19 In essence, in order to achieve higher efficiency in solar cells, the absorption spectrum of the active material(s) should overlap with the solar emission spectrum (AM 1.5), and the following requirements have to be met: (a) hole conductors and

Using π-conjugated organic materials will allow solution-based fabrication methods that may, in turn, lead to low-cost, lightweight, flexible solar cells for applications such as solar roof shingles, solar curtains, or solar backpack chargers. Received Date: January 20, 2010 Accepted Date: February 16, 2010 Published on Web Date: February 25, 2010

947

DOI: 10.1021/jz1000819 |J. Phys. Chem. Lett. 2010, 1, 947–958


Figure 1. Illustration of (a) bulk heterojunction morphology, (b) a bilayer morphology in organic photovoltaic devices, and (c) a typical current density-voltage curve under dark (red) and under illumination (green), showing the photovoltaic metrics used for calculating overall power conversion efficiency.

define efficiency. The fill factor is the ratio of a photovoltaic cell's actual maximum power density output (MPP) to its theoretical power density output if both the current density (Jsc) and voltage (Voc) are at their maxima (see Figure 1c). The short-circuit current density largely depends on the number of photons captured by the active material, the efficiency of charge separation, and the efficiency of charge collection. The FF depends on the parasitic resistances of the device, whereas Voc is related to the energy difference between the LUMO of the electron acceptor and the HOMO of the electron donor. The bulk heterojunction and bilayer are two common morphologies for OPVs with respectable power conversion efficiencies (Figure 1a and b). We first consider the bulk heterojunction morphology, which is characterized by the presence of segregated, bicontinuous nanoscale domains of the charge conductors in the entire active material and is commonly observed in conjugated polymer-based OPV devices.11,22,23 In devices that have high efficiency, a thiophenebased conjugated polymer is used as the hole conductor, and a fullerene derivative (PC61BM or PC71BM) is used as the electron conductor (Figure 2). Thin films of the conjugated polymer-PCBM blend are solvent or thermally annealed for specific times to obtain bulk heterojunction structures. Although protocols have been reported to obtain these morphologies,24 there is an element of irreproducibility in the annealing process, and two samples processed under the same annealing conditions may not have the same activelayer morphology. Moreover, continued annealing of the samples leads to macrophase segregation of the conjugated polymer and PCBM, which degrades the device performance. Thus far, the OPVs with the highest reported efficiencies have this morphology,14-16 and current research is focused on designing new conjugated polymers that have absorption spectrum with better overlap with solar emission spectrum.9

electron conductors should be assembled into structures such that a heterojunction should exist at lengths scales of a few nanometers for efficient charge separation, (b) continuous phases of charge-carrier conductors should exist for efficient charge transport, and (c) the active medium (i.e., the thickness of the OPV device) should have a characteristic length scale of micrometers to match the typical photon capture distance.

The active-layer morphology should strike a balance between the intermolecular interaction between electron donors and electron acceptors and the intermolecular interactions within the electron donors and electron acceptors. We will define the terms that describe device efficiency before we venture into the discussion on morphologies that satisfy the aforementioned requirements. The power conversion efficiency (PCE or η) in photovoltaic cells is defined as the amount of power density produced by a solar cell relative to the power density available in the incident solar radiation (Pin). The equation to calculate PCE is shown in Figure 1c, where Jsc is the short-circuit current density, Voc is the opencircuit voltage, and FF is the fill factor.18,20,21 Pin is the sum over all wavelengths and is generally fixed at 100 W/cm2 when solar simulators are used and is the most general way to


948



Figure 2. (top) A schematic representation of the stacking of rrP3HT lamella in annealed films and the packing of the fullerene. (bottom) A schematic representation of the morphology evolution upon annealing in rrP3HT-PC61BM blends showing the formation of PCBM domains and rrP3HT domains.

In a seminal paper, Bradley, Campoy-Quiles, Nelson, and co-workers investigated the morphology evolution of the regioregular poly(3-hexylthiophene) (rrP3HT)-PC61BM blends.25 Using variable-angle spectroscopic ellipsometry (VASE) data, they modeled the changes in the distribution of PC61BM in the active layer upon annealing. The VASE models were also correlated to the changes in the oscillator strength of rrP3HTand PC61BM upon annealing. It was found that upon annealing, rrP3HT rapidly crystallizes, and PC61BM migrates after the rrP3HT crystallization into areas where the polymer is of low density. These sites act as nucleation sites for PC61BM. It is also well-known that upon prolonged annealing, the PC61BM crystallites grow larger in size and eventually macrophase segregate. At this stage, the efficiency of the device drops precipitously. Why do the conjugated polymer and PC61BM form bulk heterojunctions and why do they macrophases segregate upon continued annealing? The answer lies in the molecular architecture, the intermolecular interaction, and the packing propensities of the π-conjugated moieties. Specifically, it is the interplay between rrP3HT-PC61BM, rrP3HT-rrP3HT, and PC61BM-PC61BM interactions. The first factor that comes into play is the rrP3HT-PC61BM interaction. rrP3HT is electron-rich, and PC61BM is electron-poor, and there is a natural tendency for the electron-rich π-conjugated compounds to favorably interact with electron-poor π-conjugated compounds.26,27 The rrP3HT-PC61BM interaction in the blends has been observed as a subgap charge-transfer absorption band in longer wavelengths and weak, red-shifted emission signals.28 This interaction is key for charge dissociation, and the nature of this interaction impacts the open-circuit voltage, Voc.28 It is predicted that stronger charge-transfer interactions will lower the open-circuit voltage.28,29 We will return to the relationship between Voc and charge transfer later in the section on bilayered systems.


A second factor that comes into play is packing. rrP3HT has a high degree of planarity, and in annealed thin films of rrP3HT, the hexyl chains on the thiophene rings interdigitate to form lamellar structures; the lamella stack through face-toface π-π stacking interactions at a typical distance, d100, of ∼3.5 Å.30,31 PC61BM on the other hand is nonlinear and packs with a center-to-center distance of ∼10.5 Å.32 The size of PC61BM (∼7 Å) is twice that of typical π-π stacking distances (∼3.5 Å). Thus, the packing propensities for rrP3HT and PC61BM are different (Figure 2, top). How do these factors come into play in terms of morphology? The crystallinity of unannealed films of the blend is low. At this stage, PC61BM is dispersed in the rrP3HT matrix possibly stabilized through the charge-transfer interactions between rrP3HTand PC61BM. Upon annealing, as mentioned before, rrP3HT crystallizes first,25 and the crystalline rrP3HT domain cannot accommodate PCBM because of the differences in the packing propensities of rrP3HTand PC61BM. This is consistent with the observation that PCBM starts to diffuse out of the rrP3HTcrystalline domains and into domains where the rrP3HT is poorly crystalline or amorphous (Figure 2, bottom);25 the packing of rrP3HT wins over charge-transfer interactions. If the annealing is stopped at this stage, we obtain a bulk heterojunction structure with crystalline domains of rrP3HT, poorly crystalline domains of PC61BM, and interfacial regions between the rrP3HTand PC61BM domains. If the annealing proceeds further, the PC61BM crystallites will continue to grow, eventually leading to macrophase segregation. It is important to note that during annealing, rrP3HT crystallizes rapidly but PC61BM does not. In sum, the macroscopic phase segregation of rrP3HT-PC61BM results from the inability of the rrP3HT crystallites to accommodate PC61BM. Therefore, the bulk heterojunction results from the inability of the rrP3HTcrystallites to accommodate PC61BM and the large differences in the rates of crystallization of the rrP3HT and PC61BM. Thus, the bulk heterojunction is a result of striking

949



Figure 3. A schematic illustration of the packing of conjugated polymers with fullerene-based molecules as pendant groups. For efficient electron transport, the fullerene molecules should pack with a center-to-center distance of 10.5 Å. For efficient hole transport, the conjugated polymer should pack with a stacking distance of 3.5 Å.

the balance between the packing forces and the intermolecular interactions between rrP3HT and PC61BM.33 With this insight, we look at two other approaches that have been reported to stabilize bulk heterojunction structures. The first approach involves covalently attaching fullerenebased molecules as pendant groups to conjugated polymers to prevent the macrophase segregation.34-36 This molecular architecture has the potential to obtain nanoscale-segregated morphologies for OPVapplications. Moreover, such molecules can also act as bipolar conductors.37 Interestingly, the overall power conversion efficiencies reported for these systems are around 0.6% or less, far below the efficiency of the blends.34 It has been shown that in these systems, the photoinduced charge transfer from the conjugate polymer to the fullerene leads to long-lived charge-separated states. The charge recombination has been shown to be a nongeminate bimolecular recombination, indicating that the charges can migrate within the molecule. It has been argued that the low efficiency in covalently attached systems may be due to competition of the energy-transfer processes with the electron-transfer process.38 In oligophenylenevinylene-C60 dyads, it has been shown that only a few percent of the light that is absorbed by oligophenylenevinylene results in electron transfer.38 Another reason for the low efficiency in these systems may be related to the mobility of the charges between the molecules. As mentioned before, the plane-to-plane stacking distance of the conjugated polymer is 3.5 Å, whereas the center-to-center distance of the fullerene is 10.5 Å. Since the packing propensities of the conjugated polymer and fullerene are different, for this molecular architecture, it will be difficult to simultaneously satisfy the packing requirements for the conjugate polymer and fullerene for effective charge transport (Figure 3).


Figure 4. Schematic illustration of the packing in the copolymer-PCBM blend. The reduced regioregularity of the copolymer creates regions with poor packing, and PCBM migrate to these domains.

The second approach, reported by Frech et and co-workers, involves reducing the regioregularity of rrP3HT using a 3,4-dialkylthiophene unit in rrP3HT (Figure 4).39 The decrease in regioregularity is expected to lead to regions with improper polymer packing; the PCBM can then diffuse into these regions. Frech et and co-workers have shown that reducing the regioregularity does increase the stability of the bulk heterojunction morphology to the annealing conditions. However, it has been shown by Kim, Nelson, and Bradley that regioregularity of the conjugated polymer influences the mobility and thus device performance; the higher the regioregularity, the better the efficiency.30 Therefore, in

950



Figure 5. Illustration of the diblock copolymer architecture, morphologies as a function of volume fraction of the constituent blocks, and the device architectures that can be obtained using these morphologies.

Several research groups have reported interesting morphologies with semiconducting diblock polymers, but the efficiencies remain low.47-53 At this point, it is unclear whether the limitation is charge separation or charge transport or both and requires further studies. Nonetheless, the block copolymer architecture provides a powerful pathway to assemble charge carrier conductors into nanoscale segregated structures.

this approach, the regioregularity of the conjugated polymer needs to be optimized for device metrics and for morphological stability. Recently, Campoy-Quiles reported the mixing of irregioregular P3HT to the rrP3HT-PCBM blend to create amorphous regions for accommodating PCBM domains.40 Replacing fullerene-based molecules with planar electron acceptors such as perylene diimide in polymer blend devices has been attempted.41,42 The important difference is that the plane-to-plane packing of planar electron acceptors is commensurate with the plane-to-plane stacking distances of conjugated polymers. Therefore, molecules can intimately mix with the conjugated polymer through charge-transfer interactions or macrophase segregation due to packing interactions. The maximum reported efficiency for planar electron acceptors is ∼0.3%, using a compatibilizer.43 The challenge in these systems is to balance the charge-transfer interactions and packing interactions to achieve nanoscale segregated morphologies. The block copolymer is a promising molecular architecture that is being pursued to control the morphology of the active layer independent of the nature of the electron acceptor or donor.44 Block copolymers, two chemically different polymers joined together at one end, microphase separate into arrays of domains comparable to the dimension of the polymer chain.45,46 The size and separation of these nanoscopic domains depend upon the molecular weight of the copolymer, while the type of morphology (body-centered arrays of spheres, hexagonally close-packed cylinders, gyroids, and alternating lamellae, as shown in Figure 5) depends on the volume fraction of the components. Thus, in principle, through this approach, one can assemble OPV devices with different morphologies and study the impact of the morphology on the photovoltaic metrics. This approach also allows us to use electron acceptors other than fullerene-based molecules and obtain heterojunctions that are ordered in bulk.


The block copolymer architecture provides a powerful pathway to assemble charge carrier conductors into nanoscale segregated structures. A bilayered structure is characterized by the presence of a layer of the hole conductor and a layer of the electron conductor with an interface between the layers (Figure 1b). The exciton created in either of these layers needs to move to the interface for charge splitting. For efficient charge splitting, the thickness of the layers must be on the order of the diffusion distance, ∼10 nm. However, for efficient photon absorption, the thickness of the layers should be around 0.2 μm. In bilayered structures, it is difficult to reconcile the requirements for efficient charge splitting with efficient photon absorption. Nonetheless, a recent study by Schwartz, Tolbert, and co-workers shows that power conversion efficiencies of 3.5% can be obtained from a P3HTPC61BM bilayered device.54 Bilayered structures also serve as convenient architectures for answering some of fundamental questions in OPV devices. Often π-conjugated organic molecules are used to create OPVs with bilayered structures through controlled multistep

951



Figure 6. Illustration of the bilayer morphology obtained through self-assembly of substituted hexabenzocoronene and N,N0 -bis(1-ethylpropyl)-3,4,9,10-perylenebisdicarboximide.

0.55 V, whereas rubrene had a Voc of 0.92 V. The HOMO energy of tetracene was 5.1 eV, and that of rubrene was 5.3 eV. The measured dark current density was 2.5 10-3 μA/cm2 for rubrene and 7.7 10-2 μA/cm2 for tetracene. In OPV, the dark current density is dominated by the thermal excitations in the charge-transfer complexes formed at the heterojunction. Therefore, it was argued that when compared with the planar tetracene, rubrene, with four pendant orthogonal phenyl rings, may interact poorly with the electron acceptor, C60. Thus, the hypothesis that is advanced through this work is that weaker donoracceptor interactions result in a low dark current density ( Js) and thus high Voc. Nonetheless, the maximum attainable Voc is determined by the energy differences between the HOMO of the donor and the LUMO of the acceptor. Similar observations have also been made by Vandewal and co-workers in conjugated polymer-based photovoltaic cells.28 Although the donor molecules can be designed to have weak intermolecular interactions with acceptor molecules to minimize the dark current density, it is also imperative to take into account the impact of the molecular design on packing of the donor molecules; the packing impacts charge mobility. The work by Thompson, Forrest, and Vandewal provides the impetus to study the relationship between the molecular architectures, molecular packing, charge mobility, charge dissociation, and dark current density. A key advantage of the bulk heterojunction structure over the bilayer structure is the presence of the donor-acceptor interface in the entire active layer. Thus, the bulk heterojuction structure satisfies the length scale requirements for efficient charge dissociation and efficient photon absorption. In general, one of two types of packing, a mixed stack motif or the segregated stack motif, is observed when electronrich conjugated molecules are mixed with electron-poor conjugated molecules (Figure 7). Electron conductors are

layer deposition of the semiconductors by vacuum processing techniques. Since most organic π-conjugated materials have low vapor pressure, they are not easy to vacuum process. In 2001, Friend, M€ ullen, and co-workers showed that solution processing of N,N0 -bis(1-ethylpropyl)-3,4,9,10-perylenebisdicarboximide and hexa-substituted hexabenzocoronene results in a bilayered structure through self-assembly with a large interfacial area between the perylenediimide, the electron conductor, and hexabenzocoronene, the hole conductor (Figure 6).55 The maximum power conversion efficiency reported for this system was 1.95% at 495 nm. The driving force for formation of the segregated structure in this work was attributed to differences in the solubility of perylenediimide and hexabenzocoronene molecules. This work paves the way for designing molecules to obtain bilayered structures through self-assembly by tuning the nature of the side chains. Recently, Schwartz, Tolbert, and co-workers reported the formation of bilayered devices using sequential spin-casting.54 In 2009, Thompson, Forrest, and co-workers reported their studies on the role of molecular architecture on the photovoltaic metrics in bilayered structures using C60 as the electron acceptor and various π-conjugated molecules as the electron donor.29 Specifically, they probed the influence of the molecular architecture on the dark current density (Js) and thus on Voc. The relationship between Voc, Js, and the short-circuit current density, Jsc, is given in eq 1, where ΔEDA is the energy difference between the LUMO of the acceptor and the HOMO of the donor, Jso is a pre-exponential factor that depends on the molecules, and n is the diode ideality factor.20 nkt Jsc ln where Js ¼ Jso eð -ΔEDA =2nkTÞ Voc ð1Þ q Js For two electron donors with similar HOMO energies, it was found that the Voc was different. For example, it was found that when tetracene was used as the electron donor, the Voc was


952



Figure 7. Illustration of mixed packing and segregated arrangements of electron-rich conjugated molecules (blue, hole conductor, donor) and electron-poor conjugated molecules (orange, electron conductor, acceptor). Although the illustration shows planar electron-poor compounds, the mixing also happens with nonplanar, fullerene-based compounds.

electron-deficient, and hole conductors are electron-rich; there is a natural tendency for electron-deficient compounds to mix with electron-rich compounds through aromatic stacking interactions or donor-acceptor interactions.26,27 The mixed stack is a common motif, whereas the segregated stack is a rare motif in condensed-phase structures. How can we overcome charge-transfer interactions and assemble small-molecule organic semiconductors into segregated structures? To answer this question, we look into some of the excellent work done in the area of organic superconductors.56-59 It has been shown that segregated structures can indeed be achieved if there is a ground-state electron transfer between the donor and the acceptor molecules to form ionic salts.56-59 Most of these structures show metallic behavior and thus are not considered in the context of photovoltaic devices. In the 1980s, Bernstein, Bittner, and co-workers reported a molecular design in which two donor moieties (phenyl or naphthyl rings) were attached to a tetracyanoquinodimethane (TCNQ) molecule, resulting in a donor-acceptor-donor triad.60,61 In crystal structures of these triads, it has been shown that this design does result in a one-dimensional segregated stack of TCNQ and naphthalene molecules without the ground-state electron transfer (see Figure 8). However, donor-acceptor interaction between TCNQ and naphthalene dominates the second dimension. In 2002, a similar triad architecture was reported by W€ urthner, Meijer, and co-workers using peryelene bisimide and oligo-PPV.62-64 The triads were assembled through hydrogen bonds and through covalent bonds.62-64 Although the exact structure of this assembly is not known, it has been proposed that OPV-PERY-OPV systems (see Figure 9) form chiral stacks through π-π interactions similar to the stacking shown in Figure 8. For both triads, it was found that the forward electron transfer was fast, with a rate constant >1012 s-1. Interestingly, it was found that in the aggregated state, the rate of the charge recombination in the covalent triad was slower (k = 2 109 s-1) than that in the hydrogen-bonded triad (k = 6.3 109 s-1). Also, there were only small differences in the


Figure 8. Illustration of the 1-D stacks of donors and acceptors along the c-axis in the donor-acceptor-donor triad architecture. However, there are charge-transfer interactions along the a-axis.

charge recombination rates between the aggregated and nonaggregated states for the covalent triad. The lack of difference in charge recombination rates may indicate that the charge is delocalized on a single triad but does not move between the stacks. Further studies on mobility may shed light on the nature of the interaction between the π-conjugated units in these assemblies. However, for the hydrogen bond triad, charge recombination was found to be slower in the disassembled state. This may indicate that the assembly may be dominated by donor-acceptor interactions. The OPV-PERY-OPV triads have been reported to have poor diode characteristics, which has been ascribed to orientation of the triad assembly with respect to the electrode for charge transport. Nonetheless, the triad architecture provides a pathway to self-assemble donors and acceptors into structures relevant for OPV devices. Side chains are often appended to π-conjugated organic molecules to increase their solubility and processability. In the condensed phase, side chains impact the packing of π-conjugated organic molecules and thus impact electronic properties.65,66 We reasoned that the side chains can be judiciously chosen to direct the packing of organic semiconductors into segregated structures in molecular dyads and thus provide a general strategy for the creation of bulk heterojunction structures (Figure 10).67 Therefore, we devised a molecular architecture in which electron-rich conjugated molecules will bear side chains that have incompatible packing with the side chains that are attached to electron-poor conjugated molecules; the side chains will be mutually phobic. The interplay between side-chain interactions and π-π stacking interactions will help stabilize the segregated stack and destabilize the mixed stack of hole-conducting and electron-conducting molecules.

953



Figure 9. Chemical representations of the hydrogen-bonded and covalently bonded OPV-PERY-OPV triad molecules studied by Meijer, W€ urthner, and co-workers.

The interplay between side-chain interactions and π-π stacking interactions will help stabilize the segregated stack and destabilize the mixed stack of hole-conducting and electron-conducting molecules. On the basis of the chemical nature and packing propensities, it is possible to identify various pairs that are immiscible. The archetypical example of such a pair is where one moiety is hydrophobic and another moiety is hydrophilic. Similarly, it is well-known that an aliphatic fluorocarbon will not form a cocrystal with an aliphatic hydrocarbon. Consequently, if a molecule has hydrocarbon and fluorocarbon sections, then in the solid-state packing, these sections phase separate. Using dyads that have naphthalene diimide and naphthyl ether units such as molecule 1 (see Figure 11), we showed using single-crystal X-ray diffraction that indeed segregated assemblies can be obtained in bridged electron-rich and electron-poor π-conjugated moieties using the mutual phobicity of aliphatic hydrocarbon and fluorocarbon chains. Noteworthy is the fact that both naphthalene diimide and naphthyl ether pack through π-π stacking interactions.67 If the side

Figure 10. Illustration of the use of side chains with incompatible packing to guide the assembly of donors and acceptors into segregated structures and the examples of side-chain pairs with incompatible packing.


954



chains were similar, then the packing would be dominated by naphthalene diimide-naphthyl ether interactions. The efficiency of charge separation, charge mobility, and photovoltaic metrics in these molecules is currently being studied. Subsequent to our work, in 2008, Aida and co-workers reported the use of lipophilic and hydrophilic side chains to control the packing of sexithiophene-fullerene dyads into “bicontinuous electron donor and acceptor arrays” (Figure 12).68 Using small-angle X-ray scattering and UV absorption data, they proposed a model wherein the amphiphilic dyad packs in a lamellar segregated arrangement.

However, in lipophilic dyads, absorption attributable to charge-transfer interactions between the sexithiophene and fullerene units was observed. The charge-separated stated was long-lived in the amphiphilic dyad, indicating the absence of any charge-transfer interactions. The examples from our group and Aida's group indicate that the side chains in π-conjugated systems can be exploited to direct the packing of electron donors and acceptors. They also indicate the important role that side chains play in molecular packing and hence in charge separation and charge transport. From materials purity to device fabrication, there are several factors that come into play in the power conversion efficiency of a photovoltaic device. The examples in this Perspective provide a glimpse of the role of molecular architecture in controlling the morphology of the active layer, charge dissociation, and charge mobility. For designing efficient organic photovoltaic cells, it is imperative that we understand the correlation between photovoltaic metrics, the molecular architecture, packing, and intermolecular interactions. A key barrier to develop this understanding is the lack of a method to reliably and reproducibly organize electron- and hole-conducting semiconductors into nanoscale structures that provide efficient charge separation and charge mobility. A general method to organize semiconductors will allow us to address some of the fundamental questions in OPV devices in the future. These questions include the following. (1) What molecular architectures provide stable segregated structures of electron-rich and electron-poor semiconductors through self-assembly? (2) Within the segregated stacks, how does the packing geometry affect charge mobility? (3) Between the segregated stacks, how do charge separation, charge recombination, and dark current density depend on the interaction geometry? (4) How do the photovoltaic metrics correlate to molecular packing and arrangement? (5) What is the optimal structure for an efficient PV device?

Figure 11. Illustration of the packing of the naphthalimide and naphthyl ether stacks in the single-crystal structure of molecule 1, showing the segregation of electron-rich and electron-poor aromatic compounds.

Figure 12. Molecules used by Aida and co-workers for the self-assembly of donor and acceptor molecules in dyads into segregated structures.


955



(6)

For designing efficient organic photovoltaic cells, it is imperative that we understand the correlation between photovoltaic metrics, the molecular architecture, packing, and intermolecular interactions.

(7)

(8)

(9)

(10)

AUTHOR INFORMATION (11)

Corresponding Author: *To whom correspondence should be addressed. E-mail: dv@ chem.umass.edu.

(12)

Biographies

(13)

D. Venkataraman, aka D.V., is an associate professor of chemistry at the University of Massachusetts Amherst. His current research focuses on the directed assembly of molecules in the condensed state for efficient charge transport. For further information, see http://people.chem.umass.edu/dv Serkan Yurt is a graduate student in the D.V. Group at the University of Massachusetts Amherst and works on using diblock conjugated polymers for directing the active-layer morphologies in OPV devices. B. Harihara Venkatraman is a graduate student in the D.V. Group at the University of Massachusetts Amherst and is exploring novel supramolecular strategies for assembling electron donors and acceptors into bulk heterojunction structures. Nagarjuna Gavvalapalli is a graduate student in the D.V. Group at the University of Massachusetts Amherst. His research focuses on the elucidating the relation between side chains, packing, and charge transport in π-conjugated systems.

(14)

(15)

(16)

(17)

(18)

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the American Chemical Society Petroleum Research Fund, the National Science Foundation MRSEC on Polymers, the U. S. Army Research Office (W911NF-08-1-0412), and the National Science Foundation Center for Fueling the Future (CHE-0739227).

(19) (20) (21)

REFERENCES (1)

(2) (3)

(4)

(5)

Goetzberger, A.; Hebling, C.; Schock, H. W. Photovoltaic Materials, History, Status and Outlook. Mater. Sci. Eng., R 2003, 40, 1–46. Hoppe, H.; Sariciftci, N. S. Organic Solar Cells: An Overview. J. Mater. Res. 2004, 19, 1924–1945. Nogueira, A. F.; Longo, C.; De Paoli, M. A. Polymers in Dye Sensitized Solar Cells: Overview and Perspectives. Coord. Chem. Rev. 2004, 248, 1455–1468. Spanggaard, H.; Krebs, F. C. A Brief History of the Development of Organic and Polymeric Photovoltaics. Sol. Energy Mater. Sol. Cells 2004, 83, 125–146. Li, B.; Wang, L. D.; Kang, B. N.; Wang, P.; Qiu, Y. Review of Recent Progress in Solid-State Dye-Sensitized Solar Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 549–573.


(22)

(23)

(24)

(25)

956

Riede, M.; Mueller, T.; Tress, W.; Schueppel, R.; Leo, K. SmallMolecule Solar Cells - Status and Perspectives. Nanotechnology 2008, 19, 424001. Yum, J. H.; Chen, P.; Gratzel, M.; Nazeeruddin, M. K. Recent Developments in Solid-State Dye-Sensitized Solar Cells. ChemSusChem 2008, 1, 699–707. Bredas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691–1699. Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868–5923. Heremans, P.; Cheyns, D.; Rand, B. P. Strategies for Increasing the Efficiency of Heterojunction Organic Solar Cells: Material Selection and Device Architecture. Acc. Chem. Res. 2009, 42, 1740–1747. Thompson, B. C.; Frechet, J. M. J. Polymer-Fullerene Composite Solar Cells. Angew. Chem. Int. Ed. 2008, 47, 58–77. Helgesen, M.; Sondergaard, R.; Krebs, F. C. Advanced Materials and Processes for Polymer Solar Cell Devices. J. Mater. Chem. 2010, 20, 36–60. Kaake, L. G.; Barbara, P. F.; Zhu, X.-Y. Intrinsic Charge Trapping in Organic and Polymeric Semiconductors: A Physical Chemistry Perspective. J. Phys. Chem. Lett. 2010, 1, 628–635. Cho, S.; Coates, N.; Moon, J. S.; Park, S. H.; Roy, A.; Beaupre, S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photon. 2009, 3, 297–303. Chen, H.-Y.; Hour, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photon. 2009, 3, 649–653. Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future;Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, DOI: 10.1002/adma.200903528. Scharber, M. C.; M€ uhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, a. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells;Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789–794. Benanti, T. L.; Venkataraman, D. Organic Solar Cells: An Overview Focusing on Active Layer Morphology. Photosynth. Res. 2006, 87, 73–81. Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533–4542. Nelson, J. The Physics of Solar Cells; Imperial College Press: London, 2003. Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551–1566. Janssen, R. A. J.; Hummelen, J. C.; Saricifti, N. S. PolymerFullerene Bulk Heterojunction Solar Cells. MRS Bull. 2005, 30, 33–36. Yang, X.; Loos, J. Toward High-Performance Polymer Solar Cells: The Importance of Morphology Control. Macromolecules 2007, 40, 1353–1362. Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617–1622. Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology Evolution Via Self-Organization and Lateral and Vertical Diffusion in Polymer:Fullerene Solar Cell Blends. Nat. Mater. 2008, 7, 150–164.



(26) (27) (28)

(29)

(30)

(31)

(32)

(33)

(34)

(35) (36)

(37) (38)

(39)

(40)

(41)

(42)

(43)

Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. Aromatic Interactions. J. Chem. Soc., Perkin Trans. 2 2001, 651–669. Hunter, C. A.; Sanders, J. K. M. The Nature of π-π Interactions. J. Am. Chem. Soc. 1990, 112, 5525–5534. Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Ingan, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904–909. Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Molecular and Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices. J. Am. Chem. Soc. 2009, 131, 9281–9286. Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. A Strong Regioregularity Effect in SelfOrganizing Conjugated Polymer Films and High-Efficiency Polythiophene:Fullerene Solar Cells. Nat. Mater. 2006, 5, 197–203. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, HighMobility Conjugated Polymers. Nature 1999, 401, 685–688. Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Influence of the Solvent on the Crystal Structure of PCBM and the Efficiency of MDMO-PPV: PCBM 'Plastic' Solar Cells. Chem. Commun. 2003, 2116–2118. M€ uller, C.; Ferenczi, T. a. M.; Campoy-Quiles, M.; Frost, J. M.; Bradley, D. D. C.; Smith, P.; Stingelin-Stutzmann, N.; Nelson, J. Binary Organic Photovoltaic Blends: A Simple Rationale for Optimum Compositions. Adv. Mater. 2008, 20, 3510–3515. Cravino, A.; Sariciftci, N. S. Double-Cable Polymers for Fullerene Based Organic Optoelectronic Applications. J. Mater. Chem. 2002, 12, 1931–1943. Mozer, A.; Sariciftci, N. Conjugated Polymer Photovoltaic Devices and Materials. CR Chim. 2006, 9, 568–577. Cravino, A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson, M. R.; Neugebauer, H.; Sariciftci, N. S. A Novel Polythiophene with Pendant Fullerenes: Toward Donor/Acceptor Double-Cable Polymers. Chem. Commun. 2000, 2487– 2488. Cravino, A.; Sariciftci, N. Molecules as Bipolar Conductors. Nat. Mater. 2003, 2, 360–361. Eckert, J. F.; Nicoud, J. F.; Nierengarten, J. F.; Liu, S. G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. Fullerene-Oligophenylenevinylene Hybrids: Synthesis, Electronic Properties, and Incorporation in Photovoltaic Devices. J. Am. Chem. Soc. 2000, 122, 7467–7479. Sivula, K.; Luscombe, C. K.; Thompson, B. C.; Fr echet, J. M. J. Enhancing the Thermal Stability of Polythiophene:Fullerene Solar Cells by Decreasing Effective Polymer Regioregularity. J. Am. Chem. Soc. 2006, 128, 13988–13989. Campoy-Quiles, M.; Kanai, Y.; El-Basaty, A.; Sakai, H.; Murata, H. Ternary Mixing: A Simple Method to Tailor the Morphology of Organic Solar Cells. Org. Electron. 2009, 10, 1120–1132. Dittmer, J. J.; Petritsch, K.; Marseglia, E. A.; Friend, R. H.; Rost, H.; Holmes, A. B. Photovoltaic Properties of MEH-PPV/PPEI Blend Devices. Synth. Met. 1999, 102, 879–880. Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Electron Trapping in Dye/Polymer Blend Photovoltaic Cells. Adv. Mater. 2000, 12, 1270–1274. Rajaram, S.; Armstrong, P. B.; Kim, B. J.; Fr echet, J. M. J. Effect of Addition of a Diblock Copolymer on Blend Morphology and


(44)

(45) (46)

(47)

(48)

(49) (50)

(51)

(52)

(53)

(54)

(55)

(56)

(57) (58)

(59)

(60)

(61)

957

Performance of Poly(3-Hexylthiophene):Perylene Diimide Solar Cells. Chem. Mater. 2009, 21, 1775–1777. Segalman, R. A.; McCulloch, B.; Kirmayer, S.; Urban, J. J. Block Copolymers for Organic Optoelectronics. Macromolecules 2009, 42, 9205–9216. Bates, F. S.; Fredrickson, G. H. Block Copolymers - Designer Soft Materials. Phys. Today 1999, 52, 32–38. Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics - Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525–557. Lindner, S. M.; Thelakkat, M. Nanostructures of n-Type Organic Semiconductor in a p-Type Matrix Via Self-Assembly of Block Copolymers. Macromolecules 2004, 37, 8832–8835. Sommer, M.; Lindner, S. M.; Thelakkat, M. Microphase-Separated Donor-Acceptor Diblock Copolymers: Influence of Homo Energy Levels and Morphology on Polymer Solar Cells. Adv. Funct. Mater. 2007, 17, 1493–1500. Sun, S. S. Design of a Block Copolymer Solar Cell. Sol. Energy Mater. Sol. Cells 2003, 79, 257–264. Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. Semiconducting Diblock Copolymers Synthesized by Means of Controlled Radical Polymerization Techniques. J. Am. Chem. Soc. 2000, 122, 5464–5472. Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; L evque, P.; Audinot, J. N.; Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. A New Supramolecular Route for Using Rod-Coil Block Copolymers in Photovoltaic Applications. Adv. Mater. 2009, 21, 1–6. Zhang, Q. L.; Cirpan, A.; Russell, T. P.; Emrick, T. Donor-Acceptor Poly(Thiophene-block-Perylene Diimide) Copolymers: Synthesis and Solar Cell Fabrication. Macromolecules 2009, 42, 1079–1082. Lee, J. U.; Cirpan, A.; Emrick, T.; Russell, T. P.; Jo, W. H. Synthesis and Photophysical Property of Well-Defined Donor-Acceptor Diblock Copolymer Based on Regioregular Poly(3-Hexylthiophene) and Fullerene. J. Mater. Chem. 2009, 19, 1483–1489. Ayzner, A. L.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J. Reappraising the Need for Bulk Heterojunctions in Polymer-Fullerene Photovoltaics: The Role of Carrier Transport in All-Solution-Processed P3HT/PCBM Bilayer Solar Cells. J. Phys. Chem. C 2009, 113, 20050–20060. Schmidt-Mende, L.; Fechtenk€ otter, A.; M€ ullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics. Science 2001, 293, 1119–1122. Williams, J. M.; Schultz, A. J.; Geiser, U.; Carlson, K. D.; Kini, A. M.; Wang, H. H.; Kwok, W. K.; Whangbo, M. H.; Schirber, J. E. Organic Superconductors ; New Benchmarks. Science 1991, 252, 1501–1508. Jerome, D.; Schulz, H. J. Organic Conductors and Superconductors. Adv. Phys. 1982, 31, 299–490. Wudl, F. From Organic Metals to Superconductors ; Managing Conduction Electrons in Organic-Solids. Acc. Chem. Res. 1984, 17, 227–232. Williams, J. M.; Beno, M. A.; Wang, H. H.; Leung, P. C. W.; Emge, T. J.; Geiser, U.; Carlson, K. D. Organic Superconductors - Structural Aspects and Design of New Materials. Acc. Chem. Res. 1985, 18, 261–267. Becker, J. Y.; Bernstein, J.; Bittner, S.; Levi, N.; Shaik, S. S. Strategic Design of Organic Conductors ; Structure of a Prototypical Molecule. J. Am. Chem. Soc. 1983, 105, 4468–4469. Becker, J. Y.; Bernstein, J.; Bittner, S.; Levi, N.; Shaik, S. S.; Zerzion, N. Synthesis and Properties of Archetypical Do-



(62)

(63)

(64)

(65)

(66)

(67)

(68)

nor-Acceptor Donor (D2A) Molecules. J. Org. Chem. 1988, 53, 1689–1694. Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer, E. W. Photoinduced Electron Transfer in a Mesogenic Donor-Acceptor-Donor System. Chem.;Eur. J. 2002, 8, 4470–4474. Wurthner, F.; Chen, Z. J.; Hoeben, F. J. M.; Osswald, P.; You, C. C.; Jonkheijm, P.; von Herrikhuyzen, J.; Schenning, A.; van der Schoot, P.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J. Supramolecular p-n-Heterojunctions by CoSelf-Organization of Oligo(p-Phenylene Vinylene) and Perylene Bisimide Dyes. J. Am. Chem. Soc. 2004, 126, 10611– 10618. Schenning, A.; von Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.; Wurthner, F.; Meijer, E. W. Photoinduced Electron Transfer in Hydrogen-Bonded Oligo(p-Phenylene Vinylene)-Perylene Bisimide Chiral Assemblies. J. Am. Chem. Soc. 2002, 124, 10252–10253. Shukla, D.; Nelson, S. F.; Freeman, D. C.; Rajeswaran, M.; Ahearn, W. G.; Meyer, D. M.; Carey, J. T. Thin-Film Morphology Control in Naphthalene-Diimide-Based Semiconductors: High Mobility n-Type Semiconductor for Organic Thin-Film Transistors. Chem. Mater. 2008, 20, 7486–7491. Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y. Y.; Dodabalapur, A. A Soluble and Air-Stable Organic Semiconductor with High Electron Mobility. Nature 2000, 404, 478–481. Benanti, T. L.; Saejueng, P.; Venkataraman, D. Segregated Assemblies in Bridged Electron-Rich and Electron-Poor πConjugated Moieties. Chem. Commun. 2007, 692–694. Li, W.-S.; Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.; Tagawa, S.; Masunaga, H.; Sasaki, S.; Takata, M.; Aida, T. Amphiphilic Molecular Design as a Rational Strategy for Tailoring Bicontinuous Electron Donor and Acceptor Arrays: Photoconductive Liquid Crystalline Oligothiophene-C60 Dyads. J. Am. Chem. Soc. 2008, 130, 8886–8887.


958


Role of Molecular Architecture in Organic Photovoltaic Cells - The

Recommend Documents