Non-Conjugated Polymers for Organic Photovoltaics - ACS Publications

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Non-Conjugated Polymers for Organic Photovoltaics: Physical and Optoelectronic Properties of Poly(perylene diimides) Michael Woodhouse, Craig L. Perkins, Matthew T. Rawls, Russell A. Cormier, Ziqi Liang, Alexandre M. Nardes, and Brian A. Gregg* The National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401 ReceiVed: NoVember 11, 2009

Most polymers employed in organic photovoltaic cells have π-conjugated backbones. Here we investigate the physical and optoelectronic properties of three nonconjugated polymers based on perylene diimides. The optical properties, exciton diffusion length, band edge positions, conductivity, and carrier mobility of these polymers are described. They are electron-conducting and highly photostable. The film structure and optoelectronic properties vary with deposition conditions and annealing procedures. One of the polymers has an unusually long exciton diffusion length of 22 nm. The free electron density resulting from n-type charged defects is ∼3 × 1015 cm-3 in one polymer and is even lower in the others. Simple bilayer solar cells are limited by their series resistance suggesting that these semiconductor films would benefit from doping. Introduction The development of renewable energy is essential to sustain our ecosystem. The photoconversion of sunlight directly into electricity and/or fuels seems to be the only viable long-term strategy for a sustainable energy future, although wind power will also play a role.1–4 Organic semiconductors, OSCs, show promise for photoconversion through their synthetic variability, their low-temperature “plastic” processing, and the possibility of producing lightweight, flexible, easily manufactured, and inexpensive solar cells.5–13 OSCs may eventually replace inorganic semiconductors, ISCs, in photovoltaic cells as they have been slowly replacing ISCs in other applications that require a combination of low cost, large area, and flexibility without requiring rapid switching speeds. Examples are photocopiers, laser printers, light-emitting diodes, and white light panels for room lighting.14,15 The widespread implementation of renewable energy requires competitive efficiency, stability, and cost. Organic photovoltaic, OPV, systems6,16–19 have a potential advantage over inorganic PV in both module costs and balance-of-systems costs. But efficiency and especially stability6,20,21 remain too low. There are useful niche applications, such as the portable charging of electronic devices, in which the power needs are minimal and the lifetime of the solar cell need only match that of the device. Any eventual large-scale deployment, however, for grid-tied industrial and residential applications will require 10-30 year lifetimes. Although encapsulation can mitigate the instability problem of OSCs, rigorous encapsulation is expensive. Flexible modules, which are probably required for truly low-cost manufacturing, may be even more difficult to encapsulate adequately than those on a rigid glass substrate. With such challenges in mind, we pursue new photostable polymers for OPV that should require only minimal encapsulation. As in ISCs, the observed electrical properties of OSCs may be influenced, or even controlled, by the uncharged and charged defects (or dopants) present in the material. Charged defects are thought to be a major source of photochemical instability * To whom correspondence should be addressed. E-mail: brian.gregg@ nrel.gov.

in regioregular poly(3-hexylthiophene), P3HT, one of the most common OPV materials.22,23 Furthermore, charged defects are known to quench excitons24,25 and decrease carrier mobilities.26,27 Some of these defects, which may occur at distorted sp2hybridized carbons in the π-conjugated backbone, can be removed by chemical treatments with subsequent improvement in photostability, carrier mobility, and exciton diffusion length.22,23 Arguably, however, π-conjugated polymers may always have a high density of electronic defects because the inevitable kinks and twists in the sp2 carbon polymer backbone will generate electronic states in the bandgap.28 Despite their mostly deleterious influence, charged defects can also be beneficial if they act as dopants. Doping an OSC can mitigate resistance limitations in devices and generate interfacial electric fields that promote carrier separation.28–31 Less defective (lower doped) materials may suffer from increased recombination because the field is not strong enough at the interface to separate the geminate carriers.28,32 Nevertheless, we believe that low-defect semiconductors when properly doped will ultimately prove superior to the highly defective materials currently in use that are doped in an uncontrolled fashion by random defects. In an attempt to create polymers for OPV having a lower charged defect density, we investigate here three new nonconjugated photoactive polymers consisting of the typical small molecule OSC, perylene diimide. The molecular units are linked together through a nonelectroactive sp3-hybridized backbone that also provides solubility. One reason to utilize polymeric forms of molecular semiconductors is to improve both the film forming ability of the material and also its adhesion to other OPV cell layers. The ability to produce smooth, pinhole free films is necessary for preventing electrical shorts in devices, especially when using an evaporated metal contact. Adhesion is essential, as eventual crystallization of materials and delamination of interfaces is a common degradation path for OPV cells, quite apart from chemical instabilities.6,20,21 The film-forming properties of polymers are often superior to those of small molecules; however, their electrical properties are often poorer.27,28 There is commonly a trade-off between these two attributes. We hoped to combine some of the best properties of both polymers and

10.1021/jp910738a  2010 American Chemical Society Published on Web 03/24/2010

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Figure 1. Molecular structures and nomenclature of the polymers.

small molecules in this work. We report the optoelectronic characterization of three new perylene diimide based polymers along with stability studies and preliminary OPV cell preparation. Experimental Methods Polymers. The polymers were synthesized as described elsewhere;33 the molecular weights are in the range 8 000-10 000. The molecular structures are shown in Figure 1. The degree of polymerization was small (n ) 10-12 in Figure 1) possibly owing to the poor solubility of the product during synthesis. Solubilities were determined by preparing saturated solutions in a solvent or solvent mixture, heating to 50 °C for approximately 1 h, and then stirring overnight at room temperature before passing through a 0.45-µm Teflon syringe filter. From the absorbance of diluted solutions at 525 nm and an extinction coefficient of 92 000 M-1 cm-1,34 the saturated concentration was calculated. Films. Films were prepared on microscope slides cleaned by sequential sonication in acetone and IPA followed by oxygen plasma cleaning. Polymer films were prepared by spin-casting or drop-casting followed by annealing at 125 °C for 15 min in air. The highest quality polymer films were obtained on microscope slides coated with a silane layer:35 plasma-cleaned substrates were dipped in 100 mL of ethanol containing 250 µL of acetic acid and 2 mL of diphenyl dimethoxysilane (Gelest) for ∼2 min. They were then removed, rinsed with ethanol, and baked in air at 110 °C for 10 min. Measurements. Current-voltage curves were measured with a Keithley Model 236 Source Measure Unit. The dark conductivity was obtained from the current near zero field in a polymer film on platinum interdigated electrodes with 10 µm spacing and 5.8 × 10-4 cm2 area (Abtech Scientific). Absorption and emission spectra were measured on an HP 8453 UV-vis spectrometer and a PTI LPS-220B flourimeter, respectively. Film thickness was measured with a Dektak 8 profilometer system. The exciton diffusion length, Lex, was estimated as before22,23,36 by applying a quencher to a series of films and measuring the ratio of the quenched to unquenched fluorescence intensities as a function of film thickness. Excitation was at 470 nm and passed through a 470 nm notch filter; peak emission

was at 670 nm and passed through a 550 nm long pass filter. The exciton quencher was a ∼20-nm-thick film of evaporated copper(II) phthalocyanine, CuPc. The optical field intensity distribution in the polymer/CuPc bilayer films on glass was modeled with use of a custom simulation program. The field was largely unaffected by optical interference from reflections at the interfaces.37 SEM images were obtained on an FEI 630 NOVA NanoSEM Field Emission Microscope operating at 2-kV accelerating voltage. AFM images were obtained in tapping mode with a Veeco DI3100 AFM. To eliminate the possibility that ions contribute to the measured conductivity, the most conductive of the polymers, R300, was subjected to a number of deionizing procedures. Within experimental error (a factor of ∼2), none of these procedures changed the conductivity. R300 was (1) recrystallized from chloroform/methanol, (2) dissolved in chloroform, washed copiously with deionized water, and precipitated by addition of petroleum ether (this procedure was repeated six times), (3) run through a neutral alumina column eluted with 0.5 to 5% methanol-chloroform, (4) run through a column containing a mixture of proton (Dowex 50X8-100) and hydroxide (Dowex Marathon MSA) exchange beads, and (5) run in chloroform through a column containing Phenomenex Strata ABW desalting sorbent (formulated for use in organic solvents). Photoelectron Spectroscopy. X-ray and ultraviolet photoelectron spectra (XPS, UPS) were collected with a home-built surface analysis cluster tool described previously.38 An ultrahigh vacuum transport system allows films to be transferred among the various electron spectrometers without air contamination. Transfers between glove boxes employed a sealed KF nipple filled with dry nitrogen or argon. UPS experiments were conducted with a -3.5 V bias applied to the sample to ensure that secondary electron cut-offs were due to the sample rather than the spectrometer. The binding energy scale of UPS experiments was determined by using the Fermi edge of a clean piece of molybdenum foil. Satellites in UPS spectra from the nonmonochromatic He I radiation were numerically subtracted assuming contributions of 2% at 23.087 eV and 0.5% at 23.742 eV to the principal resonance line at 21.218 eV, which was taken to be 100%.39 To aid in the identification of the polymer HOMO levels, data were obtained on evaporated thin films of analogous perylene-based small molecules and analyzed by using the assumption that the HOMO levels of the polymeric materials are derived primarily from their perylene diimide cores. Initial work demonstrated that the materials studied were susceptible to damage by X-rays. Damage manifested itself in the form of increasing intensity in the UPS spectra near the Fermi energy. Because of this and because our sample alignment is done with X-ray excitation, the X-ray source was turned on just long enough for alignment, the sample was then moved to an undamaged area, and UPS spectra were taken on a fresh portion of each film. X-ray Diffraction. Films of greater than 100 nm thickness were prepared for XRD by drop casting on low-noise quartz substrates (Gem Dugout). The substrate was then placed in a

TABLE 1: Solubility of the Polymers in 95:5 Chloroform:Chlorobenzenea polymer

solubility (mg/mL)

Rmax (µm-1)

σ0 (S/cm)

µe (cm2/(V s))

nf (cm-3)

R300 R400 B400

10 6 6

4.3 4.0

2 × 10-7 1 × 10-8 3 × 10-11

5 × 10-4

3 × 1015 (1 × 1014) (2 × 1011)

Absorption coefficient, R, at 472 nm; zero-field dark conductivity, σ0; electron mobility, µe; and free electron density, nf (values in parentheses assume µe ) 5 × 10-4 cm2/(V s)). a

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Figure 2. (a) Absorption spectra of R300 and R400 films compared to R400 in chloroform solution. (b) Bleaching rate in air under 10 mW/cm2 white light illumination measured at peak absorbance of R300, B400, and P3HT. The inset shows the change in the absorption spectra of P3HT after 7 days of illumination; there was no change in absorption of the perylene diimide polymers over this time.

glass Petri dish with approximately 1 mL of pure solvent around the edges to ensure slow solvent evaporation. XRD measurements were performed with a Scintag PTS goniometer (Bragg-Brentano geometry) using Cu KR radiation of wavelength 0.154 nm detected with a liquid nitrogen-cooled Ge detector. Fluorescence Decay. Fluorescence decays were collected by using the time- correlated single photon counting apparatus described previously.40 Briefly, the excitation source was a pulsed diode laser (Horiba Jobin Yvon NanLED-460) with a 460 nm, 200 ps pulse width. The instrument response function, IRF, was ∼220 ps. Upon deconvolution of the IRF, the temporal resolution was ∼44 ps. OPV Cells. Photovoltaic devices were prepared on photolithographically patterned indium tin oxide, ITO, substrates (Thin Film Devices) with an active area of 0.11 cm2. The substrates were cleaned with a Sonicare toothbrush in aqueous detergent, rinsed with deionized water, sonicated first in acetone then in IPA, and finally cleaned with an oxygen plasma for 5 min. A 30 nm thick layer of PEDOT:PSS (Baytron P VP AI 4083, passed through a 0.45-µm filter) was prepared by spincoating two layers (each at 6000 rpm for 60 s) onto the substrate and annealing at 150 °C for 10 min in air.41 In addition to enhancing rectification, the PEDOT:PSS layer was found necessary to prevent electrical shorts, presumably by improving the surface wetting of the polymer and/or by providing a more planar surface to prevent protrusions of ITO from shunting to the evaporated metal contacts. CuPc was evaporated onto the PEDOT:PSS layer at a rate of 1-2 Å/s. Polymer solutions were spin-coated onto the CuPc layer with a KW-4A spin-coater (Chemat Technology) at 850 rpm. Devices were annealed on a hot plate at 125 °C for 15 min in air before evaporating 10 nm bathocuproine (BCP) at a rate of 1-2 Å /s. Back electrodes of 75 nm Ag were evaporated through shadow masks in a glovebox-integrated deposition chamber (Angstrom Engineering) at a rate of 0.5-1 Å /s. Photocurrent-voltage curves were measured with a tungsten halogen bulb (ELH) providing up to 100 mW/cm2 as measured with two Hamamatsu S1787-04 Si reference diodes. Results and Discussion Solubility. Solution processing requires relatively high material solubility, a nontrivial challenge when working with OSC’s. Adding polyoxyethylene chains (R300 and R400) or polyoxypropylene chains (B400) to the semiconductor improves

Figure 3. Time-of-flight mobilities measured in a 1:1 mixture of R300 (electron conductor) and P3HT (hole conductor).

solubility but increases the average distance between chromophores and thus compromises the transport properties in solid films. The chain lengths employed here represent the minimum necessary for sufficient solubility. The best solvent for the three polymers is chloroform, and they are somewhat less soluble in chlorobenzene or pyridine. Spin-coating from chloroform solution often yielded poor quality films because the solvent evaporated too quickly. Better results are obtained with mixed chloroform-pyridine (75:25) or chloroform-chlorobenzene (95: 5) solutions. Spin coating saturated solutions produces films in a thickness range of 25-75 nm depending on the solvent mixture and the substrate surface properties. The solubility of each polymer in the latter solvent mixture is listed in Table 1. The solubility of R300 in 75:25 chloroform:pyridine (which was employed to make more structurally ordered films) is ∼5 mg/ mL. Absorbance and Photobleaching. The absorption coefficients for two of the polymer films are shown in Figure 2, as is a solution-phase spectrum for R400. The film absorption is shifted to slightly higher energy compared to that of the solution spectrum. The black phase of the perylene diimide,28,42,43 which is expected to be more photoactive than the red phase obtained here, was not observed. Despite the similarity in side chains between the new polymers and the liquid crystal perylene diimides studied earlier,29,44 the polymer films apparently do not have the ability to self-organize into the lower energy black phase. As expected for a polymer with a higher density of chromophores, the absorption coefficient of R300 is greater than that of R400.

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Figure 4. XRD patterns for two morphologies of R300 obtained from (a) 75:25 chloroform:pyridine mixture and (b) 95:5 chloroform:chlorobenzene. The insets show AFM images of the films and the image on the right is the height profile. The rms (root-mean-square) roughness is 1.33 (left) and 0.653 nm (right).

gives the zero field conductivity, σ0. The conductivity is related to the free electron density, nf, and the electron mobility, µe, by

σ ) qnfµe

Figure 5. UPS spectra of drop-cast films of two perylene diimide polymers and of thermally evaporated thin films of PTCDA and PPEI. The zero of the binding energy scale is equal to the Fermi energy. The intensity scale of the inset is expanded by a factor of 100 and shows features near EF.

As an initial test for chemical stability (and later photovoltaic device stability), the photobleaching rates in air at room temperature under ∼10 mW/cm2 illumination were monitored and compared to those of the common π-conjugated polymer, P3HT (Figure 2b). The perylene diimide polymer films are unaffected by this treatment for 3 weeks while the P3HT film is almost completely bleached (inset to Figure 2b) in this time. Electrical Properties. The conductivity was obtained from polymer films spin-cast onto Pt interdigitated electrodes, IDE’s, having a 10-µm spacing. This large spacing ensures that bulk properties, not electrode effects, are measured. Better film quality was often obtained on IDEs than on planar substrates. Current density-voltage, JV, curves were measured in the dark from +5 to -5 V. The slope as the curve passes through 0 V

(1)

where q is the electronic charge. Values of σ (Table 1) were surprisingly high for both reported polymers with poly(oxyethylene) chains (R300 and R400) as well as for a similar third polymer that is not yet fully characterized. Rigorous purification and deionizing procedures (see Experimental Methods) had no effect on σ, suggesting that it is not caused by mobile ions trapped in the films. Exposure to air (containing O2 and H2O) decreases σ slightly (by 2-3×) consistent with the reversible compensation of n-type defects by O2. The reported poly(oxypropylene) polymer, B400, as well as a similar polymer that is not yet fully characterized, showed a conductivity in the expected range of ∼10-11 S/cm. The polymer film quality on ITO substrates was often poor, which led to electrical shorting when the back contacts of a device were deposited. Thus, no reliable mobility measurements for a pure film were obtained. However, in a film made from a 1:1 bulk heterojunction mixture of R300 and P3HT it was possible to measure mobilities by the time-of-flight, TOF, method.22 In this configuration, the electrons move through the R300 while the holes move through the P3HT. Figure 3 shows both electron and hole mobilities over a small field range. The low-field hole mobility in P3HT is 3 × 10-4 cm2/(V s) (Figure 3), similar to literature values.22,45 The electron mobility in R300 was slightly higher at ∼5 × 10-4 cm2/(V s). The free electron density, nf, in the R300 films (Table 1) is calculated from eq 1. For the other two polymers it was estimated by assuming the same mobility as for R300. Although the free carrier density in R300 is an order of magnitude less than that in P3HT,23 it is still surprisingly high for a molecular material. In materials with no purposely added dopants, free carriers must derive from charged defects of some kind. Since

TABLE 2: Summary of Data Obtained from the UPS Experiments As Described in the Text material

VBM - EF (eV)

Φ (eV)

IP (eV)

IP (lit.)

R300/Si R400/Si PPEI/Si PTDCA/Si R300/CuPc/PEDOT:PSS CuPc/PEDOT:PSS

2.27 ( 0.06 2.06 ( 0.07 2.12 ( 0.02 2.34 ( 0.07 1.77 ( 0.04 0.67 ( 0.06

3.84 ( 0.04 3.63 ( 0.07 4.20 ( 0.04 4.32 ( 0.07 4.35 ( 0.05 4.40 ( 0.06

6.11 ( 0.07 5.69 ( 0.10 6.32 ( 0.04 6.66 ( 0.10 6.12 ( 0.06 5.07 ( 0.08

5.648 6.7046,47 5.0449

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Figure 6. UPS spectra of ITO/PEDOT:PSS/CuPc and of ITO/PEDOT: PSS/CuPc/R300.

Figure 9. Fluorescence decays of perylene diimide polymer films fit to a triexponential decay.

Figure 7. Schematic of the band edge positions for the R300 polymer and the underlayers used in device fabrication. The HOMO-LUMO gap of the polymer was taken as the sum of the optical gap (2.0 eV) and a typical value for an exciton binding energy of 0.3 eV. The CuPc transport gap was taken from a literature value.51

most carriers are not free in a low dielectric medium, the actual charged defect density is estimated to be ∼102-103-fold greater than nf.27,28,44 The carrier density in the poly(oxyethylene)-based polymers, R300 and R400, is several orders of magnitude higher than that in the poly(oxypropylene)-based B400. Since we have excluded the possibility that the conductivity comes from mobile ions, we must conclude that the poly(oxyethylene) chains somehow induce a substantial density of n-type charged defects in the perylene diimide polymers, whereas this occurs at a much lower level in the poly(oxypropylene) polymer. Possibly the poly(oxyethylene) chains reorient to generate dipoles under an applied field. Dipoles are known to create charges in OSC’s.44

Film Order. Obtaining crystalline polymer films could potentially be of great benefit for photovoltaic applications, if the film quality is sufficient. Our polymer films were initially examined in a polarizing optical microscope. Several types of films were selected for further study by AFM and XRD. In Figure 4, AFM images of R300 films prepared from 75:25 chloroform:pyridine and from 95:5 chloroform:chlorobenzene are shown as insets to their respective XRD spectra. The films prepared from chloroform:pyridine appear more highly ordered: the intensity of the XRD reflection at 2θ ) 2.80° is narrower and ∼3× larger than that from the chloroform:chlorobenzene sample (2θ ) 3.09°), and a second order peak appears implying a greater degree of organization. The smaller d-spacing (29.1 Å) in the chloroform:pyridine sample compared to that in the chloroform:chlorobenzene sample (d ) 31.5 Å) indicates tighter chromophore packing in the direction normal to the substrate. The AFM images (Figure 4 insets) indicate that both film surfaces are quite rough with sharp features on the 100-200 nm scale. Films of R400 and B400 were less uniform than films of R300. Electronic Structure. UPS spectra are shown in Figure 5 for drop-cast films of R300 and R400 as well as evaporated films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and perylene diphenethylimide (PPEI). The films were prepared on H-terminated Si (100) wafers. The PTCDA and PPEI films were used to identify the HOMO levels of the

Figure 8. (a) Fluoresence spectra of the quenched and unquenched parts of a 28 nm thick R300 film. CuPc was the quencher; the geometry of the experiment is shown in the inset. (b) Fluorescence quenching ratio versus film thickness. Data are fit to eq 2; values of the exciton diffusion lengths are shown in parentheses.

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Figure 10. Current-voltage curves of an ITO/PEDOT:PSS/CuPc/R300/BCP/Ag cell in the dark, under 1.1 suns, and under 0.031 sun multiplied by 35. The cell schematic is shown in the inset.

TABLE 3: Device Performance Characteristicsa intensity (suns)

VOC (mV)

JSC (mA cm-2)

FF

efficiency (%)

Rs (Ω cm2)

Rsh (Ω cm2)

1.1 0.031

588 406

1.74 0.083

31 36

0.30 0.39

150 1900

480 9400

a

The series resistance, Rs, is calculated as the inverse of the slope at Voc and the shunt resistance, Rsh, is calculated as the inverse of the slope at Jsc.

polymer, which, due to low intensity signals resulting from a relatively low density of states near the Fermi energy, EF, were not immediately obvious. The inset of Figure 5 shows that the feature in the polymer spectra centered on ∼2.9 eV is close in energy to the features commonly taken to be the HOMO of PTCDA and of PPEI,45–48 and for this reason, the 2.9 eV feature was taken as the polymer HOMO. On some samples a lowintensity feature was observed extending ∼1 eV below what we have taken as the polymer HOMO (see, e.g., the R400 UPS spectrum between 1 and 2 eV). We attribute this feature, which varied somewhat from sample to sample, to the disorder and impurities that are known to contribute to the density of states in organic semiconductors.48 Our band position calculations ignore this feature. Extrapolating the linear portion of the low binding energy side of the proper HOMO peaks to the energy axis as recommended by Krause et al.45 yields values for the valence band maximum with respect to EF. These values were used in conjunction with the work functions obtained from the secondary electron cutoffs to calculate the ionization potentials of the various materials. These and some literature values for PTCDA and CuPc are given in Table 2 and agree quite well with the exception of PPEI for which we arrive at an IP that is ∼0.7 eV lower than in ref 48. To determine the band offset between the CuPc donor and the perylene diimide acceptors, we first prepared CuPc films on ITO/PEDOT:PSS substrates by evaporation and examined them by UPS. A solution of R300 polymer was then spun onto this CuPc layer and probed by UPS. Figure 6 shows that the work function of the CuPc/polymer structure is essentially that of the CuPc underlayer, and thus different from the work function of the same polymer on H-terminated Si (100). The hole injection barrier, as determined by the onset of the polymer HOMO with respect to the Fermi energy, is shifted by an amount identical to the work function shift. Thus, within experimental error, the ionization potential of the polymer is the same (6.12

eV) on both substrates. Energy levels are summarized in the band diagram of Figure 7. Given the results for the CuPc/R300 system, we can conclude that the two materials follow the vacuum level alignment rule (Schottky-Mott limit), which is expected because the CuPc work function of 4.40 eV is significantly smaller than the polymer ionization potential.49–51 The UPS results are summarized in Table 2. Exciton Diffusion Length, Lex. The data in Figure 7 show that excitons created within the perylene diimide should inject holes into CuPc. If we assume that this fluorescence quenching process is infinitely fast, we can use it to measure Lex. If quenching is slower, the measured Lex will be a lower limit to the actual value. The ratio of steady-state fluorescence intensity with (Flq) and without (Fluq) an evaporated film of CuPc for a number of different film thicknesses (d) is shown in Figure 8b, as is data for P3HT reported previously.22 Figure 8a shows an example of the emission spectra from which the data are collected. Data are fit to an approximate model for optically thin films:22,37,52

( )

Flq Lex d )1tanh Fluq d Lex

(2)

In two of the polymers Lex was quite short (7 nm in R400 and 8 nm in B400) similar to the 7 nm measured previously in P3HT.22 In R300, however, Lex ) 15 nm when prepared from chloroform:chlorobenzene (data not shown) and Lex ) 22 nm when prepared from chloroform:pyridine. The fact that R300 formed better quality, more highly ordered films than the other two polymers may explain its longer values of Lex. The fluorescence decay kinetics of R300 and B400 films are shown in Figure 9. Films were excited at 460 nm. Decays were fit to a sum of three exponentials. The weighted average lifetime, τavg, of R300 was 1066 ps. For comparison, in P3HT τavg ≈ 300 ps.53 In B400 τavg ≈ 854 ps. The shorter lifetime of B400,

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which is possibly caused by its poorer film quality, may also contribute to its shorter exciton diffusion length. OPV Cells. The best OPV cells were prepared from R300 in chloroform:chlorobenzene, although more highly ordered films were obtained from chloroform:pyridine (Figure 4). Devices prepared from the latter solvent mixture suffered from poor film quality and were often shorted. Photovoltaic devices were fabricated in the following configuration: ITO/PEDOT: PSS, 30 nm/CuPc, 25 nm/R300, 20 nm/BCP, 10 nm/Ag, 100 nm. Around 1 sun illumination intensity, short-circuit current densities were typically ∼2 mA/cm2 and open-circuit photovoltages were 500-600 mV (Figure 10). Device parameters for the cell shown in Figure 10 are collected in Table 3. The low fill factor results partly from the high series resistance, Rs, which in turn is expected from the low conductivity of the undoped R300 film (Table 1). The other two polymers are even more resistive. A low shunt resistance, Rsh, also contributes to the poor fill factor. One common cause of low Rsh is the penetration of evaporated back contact metal in some spots into the soft organic films. With decreasing light intensity, the fill factor and power conversion efficiency increased (Figure 10), suggesting again that the cells are limited by a high series resistance. Doping these materials is expected to improve their photovoltaic behavior. Conclusions Three nonconjugated polymers based on perylene diimides are synthesized and characterized. They are electron conducting and far more stable against photo-oxidation than many π-conjugated polymers. Their low molecular weight and limited solubility, however, contribute to rather poor film-forming properties. The exciton diffusion length in one polymer reaches 22 nm. The conductivity in three polyoxyethylene-based polymers (∼10-7 S/cm) is much higher than that in two polyoxypropylene-based polymers (∼10-11 S/cm). Rigorous deionizing has no effect on the conductivity. Carrier mobility is moderately high at 5 × 10-4 cm2/(V s). The free electron density, derived from the n-type charged defect density, was rather low in R300 (3 × 1015 cm-3) and apparently much lower in B400 (∼2-1011 cm-3). OPV cells were resistance limited, consistent with the low carrier density, suggesting that doping should be employed in future studies. Acknowledgment. We thank Andrew Fergeson, Matthew Reese, and Anthony Morfa for helpful discussions and Bobby To for the SEM and AFM images. The optical field intensity simulation was carried out by Jao van de Lagemaat. This work was funded by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences and by the Solar Energy Technology Program of the U.S. Department of Energy through Contract No. DE-AC36-08GO28308 to NREL. References and Notes (1) Basic Research Needs For Solar Energy Utilization; BES Workshop on Solar Energy Utilization, 2005. (2) Directing Matter and Energy: Five Challenges for Science and the Imagination; Report from the BES Advisory Committee, 2007. (3) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (4) Smalley, R. E. MRS Bull. 2005, 30, 412. (5) Organic PhotoVoltaics: Mechanisms, Materials, and DeVices; Sun, S.-S., Sariciftci, N. S., Eds.; Taylor and Francis: Boca Raton, FL, 2005. (6) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 94, 3693. (7) Organic-Based Photovoltaics MRS Bull. 2005, 30, 1.

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