The Morphology of a Bulk Heterojunction Photovoltaic Cell with Low

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The Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration Thomas Lee, Audrey Sanzogni, Ningxin Zhangzhou, Paul L. Burn, and Alan Edward Mark ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10321 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

The Morphology of a Bulk Heterojunction Photovoltaic Cell with Low Donor Concentration Thomas Lee,† Audrey Sanzogni,† Ningxin Zhangzhou,† Paul L. Burn,∗,†,‡ and Alan E. Mark∗,† †School of Chemistry & Molecular Biosciences, The University of Queensland, St Lucia Campus, Brisbane 4072, Australia ‡Centre for Organic Photonics & Electronics, The University of Queensland, St Lucia Campus, Brisbane 4072, Australia E-mail: [email protected]; [email protected] Phone: +61 (0)7 3365 4180. Fax: +61 (0)7 3365 3872 Abstract Atomistic non-equilibrium molecular dynamics simulations have been used to model the morphology of small-molecule bulk heterojunctions formed by vapor deposition as used in organic photovoltaics. Films comprising C60 and 1, 5, 10, and 50 wt % of 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC) were compared to films of neat C60 . The simulations suggest that if holes can hop between donor molecules separated by as little as 1.2–1.5 nm, then a TAPC concentration of 5 wt % is sufficient to form a percolating donor network and facilitate charge extraction. The results provide an explanation for why low donor content organic photovoltaics can still have high efficiencies. In addition, the roughness, porosity, and crystallinity of the films was found to decrease with increasing TAPC content. Keywords: organic photovoltaics, thin films, molecular simulation, vapor deposition, bulk heterojunctions

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Introduction

The most efficient organic photovoltaic (OPV) devices developed to date are comprised of a bulk heterojunction photoactive film. A bulk heterojunction film contains one or more donor and acceptor materials with the ratio of the components optimized for light absorption, exciton separation into free charges, and extraction of the charge carriers. For efficient exciton separation and charge extraction, the photoactive blend film morphology should be such that there is an interface between a donor and acceptor chromophore within the exciton diffusion length as well as a percolation pathway through the material via which the charges can reach the electrodes. 1 Generally, the optimal weight ratio of the donor and acceptor material is in the range of 1:4 to 1:1 regardless of the type of donor and acceptor materials (polymeric or non-polymeric). However, there have been a number of recent reports of so-called low donor content solar cells that also give good efficiencies. Such cells all contain a fullerene derivative as the acceptor. Donors have included nonpolymeric, 2–9 dendrimeric, 10 and polymeric materials. 11 In all cases, a blend containing a few weight percent of donor can give rise to OPV devices that have a large open-circuit voltage (Voc ≈ 1.0 V) and an efficiency of 3–5%. The question that naturally arises is how OPV devices with such low concentrations of donor in the acceptor matrix can have relatively high efficiencies. Given the small amount of donor in the film, the fullerene acceptor is expected to be the main absorber. This is especially true for donors like 1,1-bis[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC, Figure 1(a)) that absorb in the deep blue. Hence, in TAPC:C60 blends charge generation must occur via photoinduced hole transfer. 12 In the first published report of high-efficiency low donor content fullerene solar cells, Zhang et al. described the cells as bulk heterojunction Schottky diodes with the organic film–MoOx interface giving rise to the large open-circuit voltage. 2 The efficiencies of the cells containing TAPC volume fractions of 1.2%, 5.0%, 12.5% and 25% were 1.1, 2.8, 2.5, and 1.6%, respectively, where the volume fractions were calculated based on the relative deposition rates of the two components. These volume fractions correspond to 0.8, 3.4, 8.8, and 18 wt % TAPC, based on the densities of the neat materials. The increase in performance of 2


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films containing up to 3-9 wt % TAPC was attributed to an increase in the availability of hole percolation pathways through the TAPC component. Similarly, the decrease in performance at 18 wt % TAPC and above was attributed to a loss of electron percolation pathways through the fullerene component. Implicit in their analysis was that even at low TAPC concentrations there were percolation pathways through the film for holes via the hole transporting material. It should be noted that fullerenes are reported to be essentially unipolar for electrons. 13 However, Vandewal et al. proposed an alternative explanation for the high open-circuit voltage. 14 They concluded from external quantum efficiency measurements that the donor molecules were “well-dispersed” in the fullerene matrix and that the open-circuit voltage was dependent on the interfacial area available for charge carrier recombination. However, if the donor molecules are fully dispersed in the fullerene matrix then it is not clear whether percolation pathways can exist through which holes can be extracted from the device. To truly understand the origin of the high efficiency of low donor content organic photovoltaics, it is necessary to understand the spatial distribution of the donor molecules in the acceptor phase. Experimentally, it is not possible to unambiguously identify individual guest molecules in a host matrix, particularly when the guest is at low concentration. It is, however, possible to model these distributions computationally. We recently reported the use of atomistic non-equilibrium molecular dynamics simulations to mimic the vapor deposition of blends of materials that are used in the emissive layer of organic light-emitting diodes (OLEDs). 15,16 Such blends typically have around 6–10 wt % of the emissive compound in a host matrix and are analogous to low donor content solar cells. In the case of the OLED materials, it was found that even at a low weight percent there were significant interactions between the emissive molecules, leading to percolation pathways for charge carriers. In this work, we apply the same approach to study the distribution of TAPC (the donor) within films containing a C60 host (the acceptor) (Figure 1(a-b)).

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a

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N

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Figure 1: Chemical structures of (a) 1,1-bis[4-bis(4-methylphenyl)aminophenyl] cyclohexane (TAPC) and (b) buckminsterfullerene (C60 ). Space-filling representations of the united-atom models used in the simulations are illustrated. (c) Snapshots (side-on and oblique) from the simulations during (left) and immediately following (right) the deposition of a film containing 5 wt % TAPC in C60 . Carbon atoms in C60 are colored green; carbon atoms and united CH2 and CH3 groups in TAPC are colored red; nitrogen and hydrogen atoms are colored blue and white, respectively; carbon atoms in graphene are colored gray.

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Results and Discussion

In order to determine the detailed morphology of low donor content organic solar cells, atomistic non-equilibrium molecular dynamics simulations have been used to mimic the vapor-deposition of films of neat C60 and bulk heterojunction films composed of C60 and 1, 5, 10, and 50 wt % TAPC. The molecules were randomly deposited onto a 17.0 nm by 16.7 nm graphene substrate as illustrated in Figure 1(c). The deposition protocol was similar to that previously reported by Tonnelé et al. 15 The films generated by these simulations have been characterized in terms of their density, roughness, porosity, and spatial distribution of donor molecules. By comparing the morphology of the blended films with that of neat C60 films, it was found that the addition of the dopant reduces roughness, porosity, and ordering. By considering the pathways formed by donor molecules through which holes can move through the film, it is possible to rationalize the large difference in efficiency between films containing 1 and 5 wt % TAPC. We find that in films containing 1 wt % TAPC there are no percolating 4


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Figure 2: (a) Snapshot of a C60 film immediately after deposition. Atoms are represented as in Figure 1. (b) The height of the film shown in (a) as a function of position over the substrate. (c) The density of three independently deposited C60 films as a function of distance from the substrate (blue, red, orange) and the average density (black). The dashed blue line shows the change in the blue density profile after annealing.

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donor networks whereas at 5 wt % TAPC we observe a high degree of donor connectivity. An example of a typical morphology of a neat C60 film generated by simulating the process of vacuum deposition is shown in Figure 2(a). During the early stages of the deposition a significant proportion of the C60 molecules bounced off the graphene substrate (92% of the first 200 molecules deposited). This is a consequence of the high rigidity of the C60 molecules and occurred despite the use of a “soft” graphene substrate (see experimental section). The likelihood of an incoming C60 molecule sticking was much greater if it landed on an already deposited molecule rather than the substrate, as landing on a molecule allowed additional energy to be dissipated during the collision. Overall, out of 12,000 attempted depositions across three replicates, 15% of the C60 molecules bounced and were removed. The density profiles of three replicate C60 films with respect to the distance from the substrate (z) are shown in Figure 2(c) (blue, red, and orange lines) along with the average density (black line). The well defined peaks in the density profile, particularly close to the substrate, correspond to layers of C60 in the films. The average density in the central region of the film, calculated by dividing the mass of material in the region 4 ≤ z < 8 nm by the volume of that region, was 1.27 g cm−3 . As a cross-check, the density was also calculated by dividing the total mass of C60 deposited by the volume of the region defined by 0 < z < h, where h is the mean height of the film, resulting in a value of 1.33 g cm−3 . For comparison, Xiang et al. 17 has reported a value of 1.46 ± 0.02 g cm−3 for vacuum deposited thin films of C60 that were 100–400 nm thick. The radial distribution of the C60 molecules was also calculated and is provided as supporting information (Figure S1). Sharp peaks were observed in the radial distribution at distances within 1% of the those expected within a face-centered cubic lattice with the experimentally-determined lattice parameter of C60 (1.146 nm). 18 As illustrated in Figure 2(a), the neat C60 films show a high degree of surface roughness. Figure 2(b) shows a height-map generated by placing a grid over the surface with a spacing of 0.5 nm and taking the z- coordinate of the atom furthest from the substrate as the height. Averaged over the three films, it was found that the mean film height was

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h = 10.8 nm (range: 10.7–10.9 nm), and the root mean square (RMS) roughness was hRMS = 2 nm (range: 1.5–2.5 nm). In principle, the film roughness observed in the simulations can be directly compared with atomic force microscopy (AFM) measurements. However, in this case the comparison is complicated by the fact that the radii of probes typically used in AFM are on the order of 10 nm, comparable to the size of the domain simulated. Thus AFM generally probes features larger than those observed in these simulations. Nevertheless, the roughness observed in the simulations is of the same order as experimental values reported for C60 films on mica (1.2–1.4 nm) 19 and glass (13 nm). 20 In addition to the surface roughness, large voids, or pores, were observed within the neat C60 films, with diameters multiple times larger than the size of a C60 molecule. In some cases these extended through the film from the film-air interface to the substrate. The porosity (φ) was estimated from the free volume of the domain simulated, Vfree , the mean height of the film, h, and the RMS roughness, hRMS :

φ=

Vfree − (Lz − htop )Lx Ly htop Lx Ly

(1)

where Lx , Ly , Lz are the dimensions of the domain simulated and htop = h + hRMS is an estimate of the top of the film. The free volume Vfree was estimated by attempting to randomly insert a spherical probe into the system. Vfree was estimated as the fraction of non-overlapping insertions multiplied by the total volume of the domain simulated. The probe radius was 0.35 Å, which is comparable to the radius of a C60 molecule. Using this method, the average porosity was 17% (range: 13–22%). The porosity of the film can also be estimated by comparing its density with experimental measurements of the density of crystalline C60 (1.56–1.65 g cm−3 ). 21,22 This comparison results in a porosity of 15–23%, consistent with that calculated using the probe method. It should be noted that the precise value of the porosity is likely to be influenced by the nature of the interactions of the deposited molecules with the substrate. The use of periodic boundary conditions could also affect the nature of the pores. To determine whether the morphology obtained was stable, one of the neat C60 films was annealed at 750 K for 100 ns. An increase in the crystallinity of the film was observed, although changes in the density, height, roughness, 7



and porosity were negligible. A snapshot of the film before and after annealing is provided as supporting information (Figure S2). 5 wt %

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0 0

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Figure 3: (a) Snapshots of films containing 5 and 50 wt % TAPC and (b) the corresponding height-maps. To examine the formation of a bulk heterojunction, TAPC was deposited together with C60 . The deposition of films containing C60 and 1, 5, 10, and 50 wt % TAPC was simulated. Examples of the blended films containing 5 and 50 wt % TAPC are shown in Figure 3(a) alongside the corresponding height-maps illustrating the surface roughness (Figure 3(b)). For films containing 1, 5, and 10 wt % TAPC, the mean RMS roughness of individual films was 1.56, 1.71, and 1.62 nm, respectively, slightly less than that of neat C60 (2.0 nm). The RMS roughness of the 50 wt % TAPC film was lower again, with a mean of 1.3 nm (range: 1.1–1.5 nm). The porosity of the films was also observed to decrease with increasing TAPC concentration, falling to 5% at 50 wt % TAPC (see Figure S3 in the supporting information). The decrease in roughness and porosity with increasing TAPC content is consistent with experimental observations of other smallmolecule:C60 vacuum-deposited blends. 23 To determine the degree of anisotropy in the films, the distribution of the orientation of the triarylamine chromophores of the TAPC molecules was analysed. The orientation was characterized in terms of the angle between the substrate and the vector connecting 8


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the central aliphatic carbon atom of the TAPC molecule to the central nitrogen of the triarylamine moiety. The distribution was close to the isotropic case with a slight preference towards parallel alignment of the vector with the substrate in the films containing 20 and 50 wt % TAPC. A figure showing the distribution of the orientation in the blended films is provided in the supporting information (Figure S4). The degree of layering, as indicated by the amplitude of the oscillations in the density profile, also decreased with increasing TAPC content. This suggests that the inclusion of the TAPC disrupts the packing of the C60 molecules and leads to a more amorphous material. Although throughout most of the film the TAPC content was within 0.1 wt % of the target, an excess of TAPC at the interface with the graphene was observed. One reason for this excess was that unlike the more rigid C60 molecules, the TAPC molecules flexed on impact and rarely bounced. In addition, the TAPC molecules are more flexible than C60 and able to interact closely with the substrate as well as occupy the voids between the spherical C60 molecules. The average density of the blended films as a function of distance from the substrate, as well as the ratio of the TAPC density to the target TAPC density, are provided in the supporting information (Figure S5(a) and (b), respectively). The reduced roughness and porosity may in part be due to the ability of the TAPC molecules to mediate the interactions between the C60 molecules. For example, the presence of TAPC reduces the probability of C60 molecules bouncing on impact. When the blended films were annealed at 750 K for 100 ns, the porosity decreased in all cases, essentially falling to zero at 10 wt % and above. As discussed above, annealing led to a significant increase in the ordering in neat C60 films. However, only a small increase in the ordering was apparent in the density profile of the 1 wt % TAPC film after annealing and little to no change in the density profiles of the 5 wt % and 10 wt % TAPC films was observed. In the film containing 50 wt % TAPC, annealing did result in a significant change in the distribution of the components. The density of TAPC increased at the vapor interface, leaving a C60 -rich region between 2–5 nm from the top surface of the film. Such a vertical phase separation might also occur at lower concentrations of TAPC (1–10 wt %) on longer timescales. The porosity and the density profiles of the

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pre- and post-annealed films are provided in the supporting information (Figure S3 and Figure S6, respectively).

a

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Figure 4: (a, b) The fraction of TAPC molecules in the film offering a path for hole migration to the substrate/anode as a function of the allowed hole hopping distance used to determine the connectivity of the TAPC phase. Either (a) the nitrogen atoms or (b) the nitrogen atoms and the para aryl carbon atoms were treated as points of connectivity. (c) Illustrations of the networks formed by anode-connected TAPC molecules using the connectivity criteria in (b) with an allowed hopping distance of 1.3 nm. The periodic simulation domain is repeated twice in the x and y axes. Images were produced using VMD. 24 To provide insight into hole mobility within low donor content bulk heterojunction films, the connectivity of the donor phase within the films was investigated. For a low donor content solar cell to function, an exciton generated on a C60 molecule by photon absorption must diffuse to an interface with a TAPC molecule for separation into charges. The exciton diffusion length within films of C60 has been reported to be 30–35 nm, 25 while the typical distance to a donor:acceptor interface would be less than 3 nm in a 10


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1 wt % film. Photoluminescence measurements have shown that more than 90% of C60 excitations form charge transfer states in TAPC:C60 films containing 1 wt % TAPC. 14 Therefore, charge extraction, not exciton diffusion, is expected to limit the efficiency of the solar cells. Given that the fullerenes show predominantly electron transport in the diode configuration, 13 hole transport within the donor TAPC phase will be a limiting factor in charge extraction. Following a charge separation event, a hole is generated on a TAPC molecule. To be extracted from the bulk heterojunction, the hole must migrate to the anode by “hopping” between nearby TAPC molecules. If no pathway exists through which a hole can reach the anode, the hole can be considered trapped and will eventually be lost to recombination. To estimate the proportion of TAPC molecules with a “connection” to the anode through which holes can migrate out of the device, two different criteria for connectivity were investigated. In one, two TAPC molecules were considered connected if the nitrogen atoms lay within a given “hopping distance”. Note, the holes are expected to be localized on the triarylamine moieties. Alternatively, two TAPC molecules were considered connected if any of the nitrogen atoms or the aryl carbon atoms in the para position relative to the nitrogen atoms lay within the hopping distance. The second criteria better accounts for the shape of the delocalized π-system of the triarylamine moiety. For films containing 1, 5, and 10 wt % TAPC, the fraction of TAPC molecules for which there is pathway allowing the migration of holes to the anode as a function of the allowed hole hopping distance under these two connectivity criteria are shown in Figure 4(a) and (b), respectively. Both criteria show the same qualitative trend. The fraction of TAPC molecules connected to the anode increases as the allowed hole hopping distance is increased. When hole movement was restricted to small hopping distances (e.g. 0.5 nm) only TAPC molecules lying directly on the substrate are connected. As the hopping distance is increased, the fraction of TAPC molecules connected to the anode rises sigmoidally before plateauing at 100%. For a given hopping distance, increasing the effective size of the triarylamine chromophore by including the aryl carbon atoms in addition to the central nitrogen results in greater connectivity. As the TAPC content

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was increased, the allowable hole hopping distance at the inflection point and at complete connectivity both decreased. At 50 wt % TAPC (not shown), a continuous donor phase is formed independent of the allowed hopping distance such that hole transport is not expected to limit efficiency. Also, annealing the blended films for 100 ns at 750 K did not significantly affect the connectivity of the TAPC phase. Experimentally, the optimal TAPC concentration is approximately 5 wt % (based on the volume fractions reported by Zhang et al. 2 ). Allowing a hopping distance of 1.2– 1.5 nm, depending whether the aryl carbon atoms are included in the connectivity analysis in addition to the nitrogen atoms would imply that over 50% of the TAPC molecules would be connected to the anode in a film containing 5 wt % TAPC. This hopping distance corresponds approximately to the diameter of a C60 molecule. Examples of the anode-connected donor networks assuming a hopping distance of 1.3 nm are illustrated in Figure 4(c). The red surfaces in the figure are isosurfaces based on a volumetric map ρ(~r) that is a sum a Gaussian function centered at each connected atom: ρi (~r) = exp −|~rr2−~ri |

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hop

where rhop = 1.3 nm is the hopping distance and r~i is the position of the ith atom. Figure 4(c) shows the isosurface corresponding to ρ(~r) = 1. TAPC molecules that were part of a “dead-end” network, lacking a connection to the anode, were excluded. At 1 wt % it is clear that only those TAPC molecules in direct contact with the anode (39%) can contribute to charge extraction. The remaining 61% of TAPC molecules in this film (not shown) act as traps. At 5 wt %, a large majority of TAPC molecules (73%) have a connection to the substrate. This network spans the film from one interface to the other. At 10 wt % TAPC, nearly all (99%) of the TAPC molecules belong to an interconnected network percolating throughout the film. By providing a model of the structure of bulk heterojunctions at the atomic level, these simulations provide unique insight into the connection between morphology and device efficiency. For example, it has been reported that high crystallinity in TAPC:C70 (5 vol. % TAPC) solar cells improved hole mobility. 26 In the simulations, increasing TAPC content led to a reduction in crystallinity, which could contribute to the experimentally observed decrease in efficiency at high TAPC concentration.

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Another consistent finding in this work was that the porosity and roughness decrease with increasing TAPC content, especially at high TAPC concentrations after annealing. This could have implications for the interpretation of the origin of device efficiency. For example, in previous studies attempts have been made to characterize small molecule bulk heterojunctions in terms of the volume fraction of the donor and to relate this to device efficiency. 2,14 However, how this can be calculated without proper allowance being made for the presence of voids and the effect of TAPC content on these voids is uncertain. Certainly, methods that rely on the relative deposition rates of the donor and acceptor could yield inappropriate values. The fact that the morphology changes on annealing at high TAPC concentrations also has potential implications for the performance of the devices. At 50 wt % TAPC, the donor was found to accumulate at the film-air interface. This resulted in a depletion of the acceptor at the interface and of the donor in the region a few nanometers below the interface. It is expected that, given sufficient time, such a phase separation may also occur at lower donor concentrations. The depletion of the acceptor at the cathode would lead to poor electron extraction from the bulk heterojunction, and thus reduced device efficiency. The phase separation would also lead to a loss of hole percolation pathways in the upper region of the film.

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Conclusions

The simulations suggest that the donor phase in low donor content bulk heterojunctions has substantially greater connectivity at 5 wt % than at 1 wt %. This lends weight to the hypothesis that an improvement in hole mobility is the primary driver of the improvement in solar cell efficiency over this TAPC concentration range. While the exact hole hopping distance in TAPC:C60 blends is uncertain, the simulations suggest that hops on the order of 1.2–1.5 nm would be sufficient for extensive percolation pathways to exist at 5 wt % TAPC. It is important to note, however, that in this work the blended films were 10-13 nm thick, while low donor content solar cells generally contain bulk heterojunction layers up to 50 nm thick. In thicker films, the proportion of TAPC molecules connected

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to the anode via a hole percolation pathway would decrease for a given hopping distance. The porosity of the material is also likely to impact exciton and charge mobility within the bulk heterojunction layer. The tendency of TAPC to reduce the roughness, porosity, and crystallinity of the films potentially adds an additional factor to consider during the optimization of these blends. Knowledge of the microscopic structure of a bulk heterojunction is key to obtaining a detailed picture of the local environment in which both the donor and acceptor are found, allowing the more accurate prediction of optoelectronic properties of these materials. Together with our earlier investigations, the work again demonstrates the utility of atomistic molecular simulations in understanding the morphology and properties of vacuum-deposited organic thin films.

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Methods

All deposition simulations were performed using the GROMACS simulation package version 4.6. 27 During the deposition process, new molecules were inserted 2 nm from the top of the film every 10 ps with a random orientation and an initial velocity towards the surface to ensure they reach the growing film. The net velocity was randomly selected from a normal distribution with a mean of 0.05 nm ps−1 and a standard deviation of q

kB T /m where m is the mass of the molecule. Molecules that were more than 1 nm

above the top of the film and moving away from the substrate (after having bounced on impact or sublimed from the surface) were removed prior to the insertion of the next molecule. Films 10–13 nm thick were formed by attempting to deposit 4000 molecules, of which 100–900 bounced. For mixed films, the species deposited was randomly selected, with a target TAPC:C60 ratio of 29:2497 (1 wt %), 2:33 (5 wt %), 23:180 (10 wt %), or 23:20 (50 wt %). Three independent deposition simulations were performed for each composition. For each composition, one of the obtained films was annealed at 750 K for 100 ns. These annealing simulations were performed using GROMACS version 5.1 with GPU acceleration. Charges, van der Waals interactions, and bonded interactions for C60 and TAPC

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were assigned using the Automated Topology Builder (ATB) 28 (molid 28246 and 130088, respectively). The CH2 and CH3 groups of TAPC were represented as united atoms. The carbon atoms in the graphene substrate were assigned to the atom type ‘C’ as defined in the GROMOS 54A7 forcefield. 29 Interactions between neighboring graphene carbon atoms were represented using a quartic potential with equilibrium distance 1.42 Å and spring constant 8.66 × 106 kJ mol−1 nm−4 and the z-coordinate of each graphene carbon atom was restrained using a harmonic potential with spring constant 1000 kJ mol−1 nm−2 .

Acknowledgement The authors acknowledge funding from the Australian Research Council (ARC) grant DP150101097. A.E.M. is a University of Queensland Vice-Chancellor’s Research Focused Fellow. P.L.B. is an ARC Laureate Fellow (FL160100067). This work was supported by computational resources provided by the Australian Government through the National Computational Infrastructure under the National Computational Merit Allocation Scheme. This work falls under the activities of the Australian Centre for Advanced Photovoltaics supported by the Australian Government through the Australian Renewable Energy Agency. Responsibility for views, information or advice expressed herein is not accepted by the Australian Government.

Supporting Information Available Snapshots of the neat C60 films before and after annealing and plots of the porosity, the radial distribution of C60 molecules, the distribution of chromophore orientation, and the vertical density profiles of the films are supplied as supporting information. The GROMACS topology and force field files used for the simulations and tables listing the compositition, height, roughness, porosity, and density of the films are also provided.

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Graphical TOC Entry C60

Neat C60

TAPC

5 wt % TAPC

20

50 wt % TAPC


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The Morphology of a Bulk Heterojunction Photovoltaic Cell with Low

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