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Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices Rahul Bhowmik, Rajiv J Berry, Vikas Varshney, Michael F. Durstock, and Benjamin J. Leever J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09765 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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The Journal of Physical Chemistry

Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices Rahul Bhowmik‡ ˩, Rajiv J. Berry‡, Vikas Varshney‡†, Michael F. Durstock‡, Benjamin J. Leever‡* ‡

Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA ˩

†

UES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432, USA

Universal Technology Corporation, 1270 N Fairfield Road, Dayton, OH 45432, USA

ABSTRACT

Molecular modeling methods are used to understand the interfacial properties between the holetransport and active layers in organic photovoltaic (OPV) devices. The hole-transport layer (HTL) consists of a blend of poly(styrene-sulfonate) and poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), whereas the active layer (AL) consists of a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Simulation results on the HTL confirm the interpenetrating lamellar structure with alternating PSS and PEDOT domains as observed in experiments. In addition, interfacial results show high PCBM interactions with the HTL, which results in PCBM migration to the HTL surface. The observed PCBM concentration profile is

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discussed from the perspective of attractive interactions, and it is shown that these interactions are governed by the side chain of PCBM. Calculations also suggest that OPV device performance could be improved by, for example, increasing the number of benzene rings and backbone -CH2- groups in the PCBM side chain, which would be expected to reduce PCBM concentration at the HTL surface. The results yield important insights into molecular interactions associated with the HTL and AL interfaces that contribute to final device morphology and thus provide guidelines toward materials design approaches for optimized device performance.

INTRODUCTION Flexible organic electronics, such as organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and organic thin-film transistors (OTFTs), have gained much attention in recent years because of their wide area of applications including energy harvesting, lighting, flexible displays, radio-frequency identification (RFID), and chemical sensors.1-6 In particular, OPV devices have shown great promise due to low fabrication costs, mechanical stability, and operational versatility.7-12 Researchers have demonstrated impressive increases in OPV device performance over the past 10 years, with power conversion efficiency increasing from 2.5 % to ~11 %,13-17 primarily as a result of the synthesis of novel low band gap polymers combined with an improved understanding of the role of active layer (AL) morphology in these devices. Specifically, donor-acceptor domain sizes and their interconnectivity have been shown to govern the transport of charge carriers through the film and eventually the generation of photocurrent.1821

In the case of the ubiquitous poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) donor-acceptor system, the choice of solvent and annealing conditions has been


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shown to dramatically impact active layer morphology and therefore device performance.22-27 OPV devices also commonly include hole transport layers (HTLs) such as poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Deposited on the ITO anode in conventional OPV devices, the PEDOT:PSS layer serves a number of purposes including electron-blocking (to prevent recombination at the anode), modification of work function and surface energy, and screening “hot-spots” at the ITO surface.28 Since holes generated in the active layer must pass through the HTL en route to the anode; and because numerous reports have shown that the HTL can actually influence active layer morphology;29-30 a careful understanding of the role of the HTL in OPV devices is essential to improve device performance. To understand the origin of the vertical concentration profile observed within the active layer, researchers have employed several techniques, including electron tomography,31 near edge X-ray absorption fine structure (NEXAFS),32-34 X-ray photoelectron spectroscopy (XPS),35-36 neutron reflectivity,37 spectroscopic ellipsometry,38-39 and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS)40.

Despite significant experimental efforts, the influence of

PEDOT:PSS on the active layer morphology and composition profile remains unclear. For example, several researchers have detected a P3HT-rich region at the active layer/HTL interface,31-33 while others report a high concentration of PCBM at this interface,34-37 and some reports show a 50:50 P3HT:PCBM blend at the interface.38-40 The differences in observed results could be attributed to several factors such as processing conditions, solvents used, or weight ratios of starting materials. Although additional experimental analysis or improved analytical techniques may eventually lead to greater consensus within the community, these approaches will not provide insight into the fundamental origin of vertical concentration gradients in OPV


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devices. Incorporating theoretical/simulation methods at atomic/molecular scales is therefore critical for obtaining an in-depth understanding of interfaces in these devices. Recently, several computational studies have been performed to model the morphological behavior of the active layer using all-atom41-44 and coarse-grained (CG) simulations.45-49 Allatom MD simulations have been used to show that the ordered configuration of π-π stacking between thiophene rings41 and the effective conjugation length42 of oligothiophene decrease in the presence of a fullerene crystal surface. Similarly, different blend compositions of C60-MEHPPV were simulated and showed that phase separation is controlled by blend ratio and chain connectivity.43

In addition, simulations have shown that PCBM and C60 are both likely to

aggregate on amorphous and (100) crystal surface P3HT, with less favorable interactions with the (010) crystal surface of P3HT.44 At larger length scales, CG simulations were performed to study the morphology of the active layer consisting of various types of polymer blends.45-49 The study of interfaces between P3HT and PCBM reveal that face-on orientation is the most energetically favorable orientation of the P3HT with the PCBM surface45. Huang et al. have identified the local morphologies of P3HT:C60 systems.46-47 Similarly, Lee et al. have developed a multi-scale approach to understand the morphological behavior of P3HT:PCBM system.48 It is also shown that the morphology of the active layer depends on side-chain spacing and backbone orientation of conjugated polymeric systems.49 Although these studies are important to understand the morphology of the active layer under various conditions, none of the modeling work focuses on the interfaces between the HTL and the active layer, which is believed to influence the overall device performance.29-30 For the PEDOT:PSS HTL, even fewer theoretical and simulation studies have been carried out, primarily due to the complexity in the structural arrangement. The structure consists of long PSS


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chains, each associated with many short chains (oligomers) of PEDOT. The structural complexity arises during synthesis of PEDOT by oxidizing (doping) EDOT in a polyelectrolyte solution of PSS,50-52 making both PSS and PEDOT highly charged species, which may induce polarity in their structure. Due to the large complex structure of PEDOT:PSS, quantum mechanics calculations are not feasible, even with current state of the art computational resources. Nevertheless, local regions of PEDOT:PSS can be studied with ab-initio calculations. Recently, the interactions between charged PEDOT, with eight monomers of EDOT, and ptoluenesulfonic acid (a monomeric unit of PSS) have been studied using molecular mechanics coupled with ab-initio calculations. The results have shown that the perpendicular arrangement of the sulfonic group of p-toluenesulfonic acid with the thiopehene ring of PEDOT has the most favorable interactions. Also, the molecular mechanics calculations on multiple molecules showed PEDOT oligomers arranged in a parallel configuration, allowing π-π stacking with ptoluenesulfonic acid located either perpendicular to the plane or lateral to polymer chains.53-54 Furthermore, studies have been performed to understand the impact of doping levels on density, structure factors, and solubility parameters for various polymers using molecular modeling methods,55-56 which could be useful to get detailed understanding of the evolution of the structure of PEDOT:PSS. Although these studies are important to determine the local arrangements, they do not provide any information regarding the nature of interactions with respect to how PEDOT:PSS interacts with active layer components. In the present work, molecular dynamics (MD) simulations have been conducted to analyze the P3HT:PCBM/PEDOT:PSS interface and examine how interfacial interactions drive the development of morphology and vertical phase segregation in the active layer. To the best of the authors’ knowledge, this is the first investigation to model the evolution and dynamics of the


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HTL/active layer interface with full atomistic detail. The results not only illuminate how molecular interactions contribute to vertical phase segregation in the active layer, but also suggest an approach for tailoring this phase segregation to optimize device performance. COMPUTATIONAL DETAILS Atomic charge calculations Several structures were constructed in order to obtain the atomic charges. Oligomer conformations of PSS (six structures), PEDOT (three structures) and P3HT (five structures) as well as three conformations of PCBM were utilized. The charges were computed using the restricted electrostatic potential (RESP) approach available within the R.E.D (Restricted ESPcharge Derive) program.57 The RESP scheme is typically used during General Amber Force Field (GAFF)58 atomic partial charge calculations,59-61 where it utilizes the Gaussian 09 software62 to optimize the structures at the HF/6-31G* level, followed by molecular electrostatic potential (MEP) calculations to obtain atomic point charges using the Merz-Singh-Kollman method.63 The initial and optimized structures are listed in Table S1 in Supplement A of the supporting information. The average charges from different conformations with standard deviations are also included on Tables S2 to S7 of Supplement A. The structures for the optimized geometries of oligomers and PCBM are included in Supplement B of the supporting information.

Simulation Details All simulations were carried out using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)64 package developed at Sandia National Laboratory. GAFF was utilized for the simulations, and its functional form is shown in the supporting information. GAFF is non-


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polarizable Force Field (FF) and thus does not capture the polarizability of the simulated system. To the best of authors’ knowledge, the polarizable or polarized Force Field for the studied molecular systems have not been developed. However, in the present study, the focus is on the semi-quantitative understanding of the evolution of the interface between the HTL and active layers, which can be well characterized by GAFF. Here, the GAFF parameters were assigned using the Antechamber package.65 Visual Molecular Dynamics (VMD)66 was used to visualize all models, and the simulations were conducted under periodic boundary conditions. Other simulation conditions employed were the PPPM67 method for long-range electrostatics, the L-J 12-6 potential for van der Waals interactions within a 12 Å cutoff, the Nose-Hoover thermostat68 for temperature control, and the Verlet algorithm69 with time step of 1 fs to integrate the equations of motion. All models were initially equilibrated at 300 K and 1 bar pressure using NPT dynamics for ~30 ns. Next, the models were gradually warmed and held at 773 K for 1 ns and then slowly cooled back down to 300 K. The gradual heating and cooling were each performed over 100 ps. The models are then simulated for another 5 ns at 300 K. The annealed models were further equilibrated under NVT conditions for 1 ns and used for the surface and interface simulations. The individual surfaces containing P3HT:PCBM and PEDOT:PSS were constructed by placing an ~300 Å vacuum layer above the annealed models. The surfaces thus created were 2-D periodic in the x-y plane and sandwiched between vacuum layers in the z-direction. These surfaces were equilibrated further at 300 K for 1 ns under NVT conditions. The distribution of the molecular components in the equilibrated surfaces was assessed via concentration profile calculations. The equilibrated surfaces were brought together to construct composite models of the interface. The combined systems were equilibrated at 300 K for 1 ns, followed by thermal


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cycling at 773 K for 1 ns. Finally, the models were cooled back down to 300 K and simulated for 5 ns to produce trajectories for subsequent analysis. Two different models of the P3HT:PCBM structure were built using the Amorphous cell module of the Materials Studio software (Version 6.1) from Accelrys, Inc. (San Diego, CA). The number of P3HT chains was fixed at 16, with each chain containing 55 repeat units. The 1:1 and 5:3 models contained 14080 (160 molecules of PCBM) and 8448 (96 PCBMs) atoms, respectively. The starting models built with the Amorphous Cell module are illustrated in Figure S1 of the supporting information. In addition, the LAMMPS data files with force field parameters of the equilibrated PEDOT:PSS/P3HT:PCBM systems are included in the supporting information.

Interaction energy per unit cross-section area of interface The interaction energy calculations between different species are based on a combination of van der Waals and electrostatic energies. The interaction energy represents the behavior of interactions between different entities, with negative values representing attractive interactions and vice-versa. The values are calculated from GAFF’s electrostatics and van der Waals terms as the average value during the last 50 ps of simulations for equilibrated models. The normalized values of interaction energies were used to analyze the interaction trends in three different models with distinct cross-sectional area. The normalization is attained by mathematical division of the interaction energy with the cross-sectional area of the interfaces.

Concentration profile


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Concentration profiles are calculated for the last 50 ps of equilibrated structures. First, the whole structure was divided in several regions along the z-axis (or along the thickness of a model), each 0.1 nm thick. The number of atoms belonging to a molecular species (PSS, PEDOT, P3HT, or PCBM) is then evaluated in that region. The local concentration profile was then calculated by mathematical division of the number of atoms with the volume of a 0.1 nm thick region. Finally, this process is repeated for all the 0.1 nm thick regions along the z-axis to obtain the concentration profile of the whole structure.

Interface point and interface region The interface point is defined as the point along the thickness of model when the PSS or PEDOT contacts either P3HT or PCBM, whichever comes first in concentration profile plots. The interface region is then defined as 5 nm region with 2.5 nm thickness on either direction along the z-axis from the interface point.

RESULTS AND DISCUSSION In conventional OPVs, the primary source of photocurrent is a result of visible light absorption and exciton generation in the P3HT phase. Upon exciton dissociation, the holes and electrons are transported to their respective electrodes via donor (P3HT) and acceptor (PCBM) materials as depicted in Figure 1. As shown, the electrons are transferred directly to the cathode (Al) from PCBM molecules. However, before reaching the anode (ITO) the holes must pass through the HTL, in which they are transported via the PEDOT phase of the PEDOT:PSS structure. Therefore, the interface between the HTL and the active layer plays an important role in influencing hole transport and eventually the overall photocurrent of the device. In the present


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study, the molecular concentrations at the interfaces and individual active and hole transport layers were studied by modeling concentration profiles and interaction energies of the OPV constituents. As described previously, the PEDOT:PSS is a highly conductive layer that is comprised of long chains of negatively charged polystyrene sulfonate (PSS) electrostatically bound to short, conjugated PEDOT chains that are oxidized (p-doped) and hence electrically conductive.70 Generally, PEDOT:PSS is coated on a substrate by spin casting, followed by thermal annealing. This process takes several minutes, during which the morphology of the PEDOT:PSS evolves to its final structure as the solvent evaporates. Due to the time-scales involved, MD simulations cannot model the morphological evolution of the experimentally observed structure. As the objective of this modeling study is to investigate the morphology at the active layer/HTL interface (and not the morphological evolution of PEDOT:PSS by itself), the PEDOT:PSS model development was focused on efficiently generating structures consistent with what has been observed experimentally. In order to understand the final PEDOT:PSS system, it is necessary to analyze the structure at atomic, molecular, and nanoscale levels. At atomic/molecular length scales, each PEDOT chain is oxidized and has a +2 charge distributed over six to eight EDOT repeat units, with each PEDOT oligomer varying from 6 to 18 repeat units in length.71-73 The positive charge of PEDOT is balanced by the ionized component of the sulfonate moieties (SO3-1) in PSS. In the present study, each PEDOT chain consists of eight monomer units. The PSS typically utilized in experiments is made up of ~2174 repeat units.74 However, due to size and time constraints, PSS containing 192 repeat units was used to model the local behavior of the long chain. In molecular simulations, it is well established that such representations featuring short chains are capable of


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reproducing many experimental results from much longer chains. In particular, molecular simulations of polymers with low molecular weight are regularly used for identifying many morphological properties that are consistent with experimental results.75-77 In the constructed PSS model, the ionized SO3-1 groups are separated by three neutral -SO3H groups (Figure 2). This ionization scheme assumes that the charges on PSS are balanced by the charges of PEDOT, as it was observed that the size of eight units of PEDOT was approximately equal to the eight units of PSS. At the molecular/nanoscale level, the long chains of PSS (with electrostatically bound oligomers of PEDOT) are known to form spherical particles when suspended in water.70,

78-80

When these particles are spun-cast on a substrate and thermally annealed, the water molecules evaporate and the spherical particles form elongated spaghetti-like structures. The process is depicted in Figure S3 in Supplement A of the supporting information. This results in an insulating PSS region at the top surface. When this surface is treated with chlorobenzene, the surface conductivity has been shown to increase, which has been attributed to an increase in the relative concentration of PEDOT at the HTL surface.81 Additionally, in some regions, PEDOT may be buried a few nanometers below a PSS-rich surface. Due to the differing concentration profiles experimentally observed at the PEDOT:PSS surface, we have considered three different models: model 1 (M1), model 2 (M2), and model 3 (M3), to represent different regions of the PEDOT:PSS surface. These regions are shown in Figure 2 and further detailed in Figure S4 of Supplement A of the supporting information. M1 consists of only PSS molecules with SO3H and SO3-1 groups. Here, there are three SO3H groups in between two SO3-1 groups, which are neutralized by H+ ions. SO3H has bonds between sulfur and hydrogen atoms, whereas SO3-1H+ has only non-bonded interactions. M2 consists of both


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PSS and PEDOT with no H+ ions, where all the SO3-1 groups are neutralized by net positive charges of PEDOT molecules. Finally, M3 contains PEDOT in the mid-region of the model, where it is neutralized by PSS molecules However, the top and bottom layers consist of PSS only, where the SO3-1 groups are neutralized by H+ ions similar to M1. The H+ ions in M1 and M3 are strongly bound to the nearest SO3-1 via non-bonded interactions. Therefore, H+ ions are not mobile in the polymeric matrix. The number of atoms in M1, M2, and M3 were 46104, 63380, and 61080, respectively. The numbers of PSS chains in these models were 12, 10, and 12, respectively, while the PEDOT chains numbered 0, 288 and 144, respectively. The surface layers of starting and equilibrated models M1, M2, and M3 are shown in Figure S5 in Supplement A of the supporting information, which shows that the PSS has a folding (or winding) type structure. Here PSS is charge neutralized by either H+ ions (M1 and M3) or PEDOT (M2). The concentration profiles of the PEDOT:PSS models under different conditions are shown in Figure 3. As represented in Figure 3a, 3d, and 3g, the starting models were ~45 nm thick with maximum concentrations of PSS and PEDOT of ~40 atoms/nm3 and ~30 atoms/nm3 respectively. After equilibration, all bulk models resulted in a thickness of 17-20 nm as shown in Figure 3b, 3e, and 3h. This shows that the models had decreased by over 50% from original thickness due to equilibration. The initial and equilibrated final structural dimensions and volume of all the models are shown in Table S8 in Supplement A of the supporting information. In addition, the maximum concentrations of PSS in M1, M2, and M3 were calculated as 99 atoms/nm3, 87 atoms/nm3, 81 atoms/nm3, respectively. Similarly, the maximum concentration of PEDOT in models 2 and 3 were estimated to be 69 atoms/nm3 and 58 atoms/nm3, respectively. This consolidation occurred due to the thermal motion during equilibration as favorably interacting


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atoms came closer to each other. More importantly, it is observed in M2 and M3 that PEDOT and PSS form continuous domains throughout the layer thickness. The equilibrated bulk models were subsequently used to build surface models of the three PEDOT:PSS regions. In the surface models, pseudo 2D periodic boundary conditions were used as compared to 3D boundary conditions used in bulk models. Here, the concentration profiles are very similar to the bulk models with continuous domains of PEDOT and PSS for M2 and M3 models. This type of continuous structure is essential for hole transport along PEDOT from the active layer to the anode. The maximum thickness computed for these lamellae was ~2.5 nm in M2, and ~5.0 nm in M3, which is consistent with AFM phase images of the cleaved PEDOT:PSS layer82-84 showing similar lamella structures with observed thicknesses of ~2-6 nm. It should be noted that the focus of this work is the relationship between the composition of the PEDOT:PSS surface and the concentration profile of the P3HT:PCBM film. Although M2 and M3 would be expected to exhibit the best charge transfer characteristics based on the proximity of PEDOT and P3HT, quantum calculations would be needed to verify this assumption. Following PEDOT:PSS analysis, the equilibrated active layer and HTL were combined to generate a model of their interface, which was subsequently equilibrated. Active layers with 1:1 and 5:3 P3HT:PCBM weight ratios were selected to mimic typical experimental conditions35. Again, concentration profiles were calculated for the combined systems and are shown in Figure 4. Figure 4a, 4c, and 4e show the concentration profiles of M1, M2, and M3 with the 1:1 P3HT:PCBM system. For M1, at the interface, the concentration of PSS decreases from ~17 nm and becomes zero at ~19 nm, while both the P3HT and PCBM concentrations start to increase from ~17 nm, demonstrating that both P3HT and PCBM molecules penetrated the HTL about 2 nm. For M2, like M1, both P3HT and PCBM penetrate the surface by ~2 nm. In M3,


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interestingly, both the P3HT and PCBM showed greater degree of permeation inside the HTL surface with PCBM permeating more than P3HT (5.6 nm vs. 3.7 nm). To understand the impact of the HTL on the 5:3 P3HT:PCBM system, the concentration profiles were calculated and shown in Figure 4b, 4d, and 4f. The permeation of P3HT and PCBM inside the HTL is similar to the penetration observed in the 1:1 P3HT:PCBM system, i.e. the PCBM and P3HT were penetrated by about 2 nm in M1 and M2 whereas they were penetrated by about 5.6 nm and 3.7 nm in M3. Due to a higher weight percentage of P3HT in 5:3 P3HT:PCBM, P3HT shows a higher concentration (about 2 times) as compared to PCBM at the interface., It is also observed for all the studied cases that both active layer components (P3HT and PCBM) diffused inside the surface of PEDOT:PSS. For more detailed investigation of P3HT and PCBM concentrations at the interface, the fractional atomic concentrations of the components were calculated at the interface region and are summarized in Table 1. The fractional concentrations are the fraction of a specific molecular species at the interface region, which is 2.5 nm on either side of interface point. The table shows that the fraction of PSS and PEDOT are very similar for 1:1 and 5:3 P3HT:PCBM blends. However, the concentration of active layer components in this region was significantly altered as compared to the bulk. While the PCBM fraction was higher at the interface region for 1:1 P3HT:PCBM, the opposite was observed for 5:3 P3HT:PCBM, for which the P3HT concentration is higher in the bulk. Although a significant concentration of P3HT at the HTL interface would seem to be required for optimal hole transport to the anode, the simulation results have shown that PCBM deposits preferentially on the HTL for the 1:1 P3HT:PCBM active layer composition. This result is consistent with our recent experimental study, where it has been shown that the HTL interface has a higher concentration of PCBM.35


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In order to better understand the propensity of PCBM to preferentially migrate toward the HTL surface, the HTL/active layer interface was further examined through interaction energy analysis. As described in the Computational Details section, the interaction energy between different species is based on a combination of van der Waals and electrostatic energies, with negative values representing attractive interactions and vice-versa. It has been shown during several studies that the diffusion process is strongly influenced by molecular interactions,85-87 and the most direct way to analyze the molecular interactions is to calculate the interaction energy between the molecules.88 The computed interaction energies between the HTL and active layer components (P3HT and PCBM) are depicted in Figure 5a and 5b. For 1:1 P3HT:PCBM, both the P3HT and PCBM show attractive interaction energy with the HTL. The favorability of P3HT interactions with the HTL decreased in the order of M3>M1>M2. PCBM shows the same decreasing order. However, the PCBM interaction energies were much stronger as compared to P3HT, which is thought to be the primary reason for the high concentration of PCBM at the HTL interface. For 5:3 P3HT:PCBM, again both the P3HT and PCBM show attractive interaction energy. Here the interactions of PCBM were stronger for M1 and M3 and weaker for M2. Importantly, though, the difference in interaction energies for P3HT and PCBM in the 5:3 case were much smaller than for the 1:1 system.

The higher concentration of P3HT at the HTL

interface for the 5:3 system is therefore attributed to the combination of similar P3HT and PCBM interaction energies and the higher bulk concentration of P3HT. It should be noted, however, that in the case of M1 and M3, for which PCBM has a slightly higher interaction energy than P3HT, the PCBM concentration at the surface could potentially increase if the simulation time were extended beyond 5 ns.


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To more fundamentally understand the origin of these interaction energies, the favorable interaction of PCBM with the HTL can be further decomposed into energy components arising from the C60 and side chain as shown in Figure 5c and 5d. The C60 portion of PCBM showed large, positive (i.e., unfavorable) interaction energies with the HTL for all the models for the 1:1 P3HT:PCBM system. Similar though somewhat less unfavorable values were calculated for the interaction energy of C60 with the HTL in the 5:3 P3HT:PCBM system. The data further show that C60 has higher repulsive interactions in the order of M1, M3, and M2. Conversely, the side chain had large, negative (i.e., attractive) interaction energies with the HTL for M1, M2, and M3 in both the 1:1 and 5:3 P3HT:PCBM systems. Larger attractive and repulsive interactions were observed for C60 and the side chain in 1:1 P3HT:PCBM as compared to 5:3 P3HT:PCBM system and could be attributed to the higher concentration of PCBM at the interface of HTL and 1:1 P3HT:PCBM. Comparing the interaction energies in Figure 5c and 5d, it is clear that attractive interactions between the HTL and the side chain dominate over the repulsive interactions of C60 with the HTL, leading to a net attractive interaction of PCBM with the HTL. This result provides a more fundamental basis for the PCBM interaction with PEDOT:PSS and equally importantly provides an opportunity to modify the side chain in order to engineer a more favorable active layer concentration profile. In order to understand the role of individual PCBM side chain atoms in the interaction energy between the HTL and the active layer, an energy breakdown analysis was performed for each atom in the PCBM side chain. The results are presented in Table S10. A subset of this data, the interaction energy of side chain functional groups which had high repulsive interactions, was calculated and is shown in Figure 6. The analysis shows that the methyl acrylate group shows attractive behavior with interaction energy in the range of -671 - -2477 mJ/m2, and suggests that


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increasing their numbers would therefore be expected to enhance PCBM migration toward the interface. Conversely, the benzene ring has overall repulsive interactions with the HTL in the range of 20 - 150 mJ/m2. Backbone –CH2- groups involving atoms C73 and C76 also show repulsive behavior with interaction energy in the range of 72-717 mJ/m2. Finally, the end methyl group shows repulsive interactions in the range of 215 - 1257 mJ/m2. Therefore, it is clear that if the number of benzene rings and backbone –CH2- groups in the side chain were increased the repulsive behavior of the side chain would be enhanced. Therefore, side-chain modification would likely decrease the preferential diffusion of PCBM to the HTL interface and thereby improve hole charge collection through an increased number of P3HT domains at this interface. Although chemical modification of PCBM seems the most likely approach for modifying the active layer composition profile, a similar energy breakdown analysis could be conducted to identify similar opportunities to modify P3HT or the PEDOT:PSS blend.

CONCLUSIONS Three different models of PEDOT:PSS have been constructed to represent different regions of the HTL surface. After equilibrating for a sufficiently long time, concentration profiles of different models were calculated. The concentration profiles showed lamellar structures with alternating PSS and PEDOT domains and are consistent with numerous experimental reports. Two different models of the active layer were subsequently constructed based on the weight percentages of P3HT and PCBM commonly used in OPV devices. These models were simulated in bulk and surface conditions. The surfaces of the active layer models were then brought close to different models of the HTL surfaces for interface simulations to evaluate the concentration gradient. Here, it was observed that PCBM preferentially migration on the HTL layer surface.


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The increased migration was more pronounced for the higher weight percentage of PCBM and was attributed to greater attractive interactions between PCBM and the HTL surface. Furthermore, analysis of interaction energy between the HTL and PCBM suggested that the PCBM side chain exhibits significant interactions with the HTL surface, explaining the favorable migration of PCBM at the surface. Finally it was shown that device performance could potentially be enhanced by modifying PCBM side chain composition to increase the number of benzene and -CH2- backbone groups, which would be expected to lead to a higher P3HT concentration at the HTL interface.

ASSOCIATED CONTENT Supporting Information Content of Supplement A: (1)Details about the evolution of PEDOT:PSS structure, (2) the starting and equilibrated structures of PEDOT:PSS and P3HT:PCBM, (3) GAFF functional form, (4) structures used for partial atomic charge calculations, (5) average charges with standard deviation and, (6) interaction energy between PCBM’s side-chain atoms with PEDOT:PSS layer. This supplement includes six figures and ten tables. Content of Supplement B: (1) structures of the optimized geometries of oligomers and PCBM after ab-initio calculations, and (2) LAMMPS data files of final equilibrated structures of models M1, M2, and M3 interacting separately with 1:1 and 5:3 P3HT:PCBM. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENTS


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This work was funded by the Air Force Research Laboratory Materials & Manufacturing Directorate. We acknowledge the use of the Department of Defense’s supercomputing resources and the Consolidated Customer Assistance Center. AUTHOR INFORMATION Corresponding Author * Tel.: +01 937 255 9141; Fax: +01 937 656 6327; Email address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Figure 1. Conventional OPV device structure with various molecular structures.


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Figure 2. Three different starting models of PEDOT:PSS as used in simulations.


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START

BULK

SURFACE

M1

M2

M3

Thickness (nm)

Figure 3. PEDOT:PSS concentration profiles of M1 (a, b, and c), M2 (d, e, and f), and M3 (g, h, and i).


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1:1 P3HT:PCBM

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5:3 P3HT:PCBM

M1

M2

M3

Thickness (nm)

Figure 4. PEDOT:PSS/P3HT:PCBM concentration profiles of M1 (a and b), M2 (c and d), and M3 (e and f) with 1:1 (a, c, and e) and 5:3 (b, d, and f) P3HT:PCBM weight ratios.


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Figure 5. The interaction energy between HTL and AL components for 1:1 and 5:3 P3HT:PCBM (a and b), and the interaction energy between HTL and PCBM constituents (C60 and Side Chain) (c and d)


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Figure 6. Interaction energy of the HTL with various side chain groups of PCBM. Here the side chain groups are Benzene Ring, -CH2- Group1, -CH2- Group2, and Methyl Group. (Benzene Ring: C62+C63+C65+C67+C69+C71+H64+H66+H68+H70+H72, -CH2- Group1: C73+H74+H75, -CH2-Group2: C76+H77+H78, Methyl Group: C85+H86+H87+H88. The numbering of side-chain atoms is shown in the inset diagram.)


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Table 1. Molecular fractions of PSS, PEDOT, P3HT, and PCBM at the interface region as defined in the text.

M1

1:1 P3HT:PCBM

5:3 P3HT:PCBM

fPSS

0.454

0.494

fP3HT

0.253

0.366

fPCBM

0.293

0.140

fPSS

0.272

0.268

fPEDOT

0.211

0.205

fP3HT

0.172

0.360

fPCBM

0.345

0.167

fPSS

0.386

0.429

fPEDOT

0.000

0.002

fP3HT

0.288

0.426

fPCBM

0.326

0.143

M2

M3


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Molecular Modeling of Interfaces between Hole ... - ACS Publications

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