High Open-Circuit Voltage Photovoltaic Cells with a Low Bandgap

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High Open-Circuit Voltage Photovoltaic Cells with a Low Bandgap Copolymer of Isothianaphthene, Thiophene, and Benzothiadiazole Units Jung Yong Kim,† Yang Qin,‡ Derek M. Stevens,† Vivek Kalihari,† Marc A. Hillmyer,*,‡ and C. Daniel Frisbie*,† Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue Southeast, Minneapolis, Minnesota 55455, and Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455 ReceiVed: August 18, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009

A novel conjugated copolymer (PITN-co-ThBTD) composed of alternating isothianaphthene, thiophene, and benzothiadiazole units was synthesized and characterized. The polymer has a low bandgap of 1.55 eV as a result of the intrachain coupling between electron-donating/withdrawing units. Thermal analysis and wideangle X-ray scattering (WAXS) reveal that the polymer has a largely amorphous structure. Blends of PITNco-ThBTD with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were studied as a function of increasing PCBM content by WAXS, atomic force microscopy, charge transport, and photovoltaic measurements. The PCBM solubility limit, i.e., the phase-separation point, was estimated to be 30 wt % PCBM, beyond which charge carrier transport switches from hole only to ambipolar (both electron and hole) in a field-effect transistor testbed. Bulk heterojunction solar cells were constructed from PITNco-ThBTD films blended with varying weight fractions of PCBM. The best performance was observed at high PCBM compositions (∼70-80% PCBM) rather than at the phase separation point. The power conversion efficiency of 0.9% with short circuit current, Jsc ) 3.4 mA/cm2, open circuit voltage, Voc ) 0.83 V, and fill factor, FF ) 32%, was measured under AM 1.5, 100 mW/cm2 illumination. The high Voc is a promising result for low bandgap polymer-based photovoltaics, while the low FF is a performance-limiting factor originating from the disordered structure of the polymer and the thickness of the film (100 nm). Introduction Low bandgap conjugated polymer/fullerene bulk-heterojunction (BHJ) solar cells have received growing attention because of their potential to extend the absorption of the solar spectrum to the near-infrared region.1-15 The resultant increase in short circuit current (Jsc), in principle, makes it possible to achieve 10% power conversion efficiencies (PCE).16,17 Thus small bandgap (Eg < 1.8 eV) polymers are of crucial importance for realizing the commercialization of low-cost polymer photovoltaics (PVs).18-38 The bandgap (Eg) of conjugated polymers is known to be determined by several contributions, i.e., bondlength alternation, coplanarity, aromatic resonance, electrondonating or -withdrawing groups, and intermolecular or intrachain coupling.2-9 By properly considering these factors, the design of novel conjugated polymers can be tailored to reduce the bandgap. However, the alignment of energy levels between the polymer and an electron acceptor such as a fullerene derivative must also be considered.16,17 Excitons dissociate when the donor/acceptor (D/A) offset energy is larger than the exciton binding energy. Furthermore, the open circuit voltage (Voc) depends on the energetic difference between donor highestoccupied molecular orbital (HOMO) and acceptor lowestunoccupied molecular orbital (LUMO).39-43 To date, several low bandgap polymer solar cells have been reported with relatively high Voc values (> 0.8 V).44-46 * To whom correspondence should be addressed. E-mail: hillmyer@ umn.edu (M.A.H.); [email protected] (C.D.F.). † Department of Chemical Engineering and Materials Science, University of Minnesota. ‡ Department of Chemistry, University of Minnesota.

The synthesis and characterization of low bandgap polymers dates back to the mid 1980s, and there are several known synthesis strategies for making these materials.2-9 Poly(isothianaphthene) (PITN) has a low Eg ≈ 1 eV resulting from the aromaticity of the thiophene and benzene fused rings.3,47,48 Poly(thienylene vinylene) (PTV) exhibits Eg ≈ 1.65 eV deriving from improved π-delocalization and coplanarity in the polymer backbone due to the presence of a vinylene group between the thiophene rings.27,48-56 In addition, several low bandgap copolymers having alternating D/A units in the polymer chain also have been reported.5-9,11 To make these materials, thiophene and pyrrole are commonly used as electron donors, and benzothiadiazole, thienopyranozine, and structural units with cyano (-CN) or nitro (-NO2) groups are used as electron acceptors.5-9,11 Among this class of low bandgap polymers, benzothiadiazole copolymer-based solar cells have been most successful in PVs, showing PCE ≈ 6%.10,12,14,15,30,33,46 Here we also have employed the same intramolecular D/A method in the synthesis of a new copolymer (PITN-co-ThBTD) composed of isothianaphthene (ITN), thiophene (Th), and benzothiadiazole (BTD) units. In this report, we have characterized the phase behavior and device performance in blends of PITN-co-ThBTD with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The solubility limit of PCBM in the polymer was identified by characterizing blend films using a combination of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), and charge transport measurements in a field effect transistor testbed. Both ambipolar charge transport and phase separation of PCBM from the amorphous PITN-co-ThBTD were observed at 30 wt % PCBM. BHJ solar

10.1021/jp9079926  2009 American Chemical Society Published on Web 12/09/2009

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SCHEME 1: Synthesis of PITN-co-ThBTD

cells were fabricated and studied as a function of composition, light intensity, thickness, and solvent evaporation rate. The optimum solar cell performance was observed at ∼70-80 wt % PCBM, far from the point of phase separation, a trend that is commonly observed in solar cells from other amorphous polymers, such as poly(phenylene vinylene) (PPV) derivatives.41,57,58 Experimental Section Materials and General Methods. Commercially available solvents and compounds were purchased from Aldrich and used as received. PCBM was purchased from American Dye Source. Poly(3,4-ethylene dioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) (Baytron P VP Al 4083) was purchased from H. C. Starck. Indium-tin oxide (ITO) glass slides (sheet resistance, 8-12 Ω/sq) were obtained from Delta Technologies. Heavily doped p-type Si wafers were obtained from Silicon Valley Microelectronics. Toluene and tetrahydrofuran (THF) were purified by first bubbling N2 through for 30 min and then passing through a homemade column system.59 Air- and moisturesensitive reactions were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inert-atmosphere glovebox (MBraun). 1,3-Bis(trimethylstannyl)isothianaphthene32 and 2,5-bis[5-(2-iodo-3-decyl)thienyl]benzothiadiazole60 were synthesized according to previously reported procedures. The 499.867-MHz 1H and 125.692-MHz 13C NMR spectra were recorded on a Varian INOVA 500 MHz spectrometer. All solution 1H and 13C NMR spectra were referenced internally to residual protio solvent peaks. Sizeexclusion chromatography (SEC) analyses were performed in CHCl3 (1 mL/min) using a Hewlett-Packard (HP) 1100 system equipped with a HP 1100 autosampler, a HP 1100 HPLC pump, a HP 1047A refractive index (RI) detector, and an Agilent 1200 UV-vis detector. Three styragel columns (Polymer Laboratories; 5 µm Mix-C), which were maintained in a column heater at 35 °C, were used for separation. The columns were calibrated with polystyrene standards (Polymer Laboratories). Ultravioletvisible (UV-vis) absorption spectra of polymer solutions and thin films were taken on a Spectronic Genesys 5 spectrometer over a wavelength range of 250-1100 nm. Solution measurements were performed in 1-cm quartz cuvettes, and polymer films were obtained by drop-casting toluene solutions (ca. 5 mg/ mL) onto glass substrates. Cyclic voltammetry was carried out on a 100B analyzer from BAS. The three-electrode system consisted of a glassy carbon disk as working electrode, a Ag wire as auxiliary electrode, and Ag/AgCl (calibrated with ferrocene +1/0 couple, 0.40 V vs SCE in 0.1 M [NBu4][PF6] acetonitrile solution)61 as the reference electrode. Polymer films were obtained by drop-casting toluene solutions (ca. 5 mg/mL) onto the glassy carbon electrode, and the cyclic voltammograms were recorded in CH3CN containing [Bu4N][PF6] (0.1 M) as the supporting electrolyte. The redox potentials are reported relative to the saturated calomel electrode (SCE). Differential scanning calorimetry (DSC) measurements were acquired using

a TA Q1000 calorimeter with 10-15 mg of the polymer at a scan rate of 10 °C/min. The results reported are from the second heating cycle. An indium standard was used to calibrate the instrument and nitrogen was used as the purge gas. Synthesis of PITN-co-ThBTD. 1,3-Bis(trimethylstannyl)isothianaphthene (0.656 g, 1.43 mmol), 2,5-bis[5-(2-iodo-3decyl)thienyl]benzothiadiazole (1.19 g, 1.43 mmol), and bis(tritert-butylphosphine)palladium (0) (22 mg, 0.043 mmol) were charged into a Schlenk tube and dissolved in 20 mL of THF under N2. The mixture was heated at 60 °C for 72 h, and PITNco-ThBTD was isolated by precipitation into acetone. The polymer was further purified by Soxhlet extraction with acetone and chloroform and isolated by precipitation of the chloroform fraction into acetone to give a black powder (0.90 g, 90%). 1H NMR (499.867 MHz, CDCl3) δ ) 8.1, 7.9-7.8, 7.2 (aromatic Hs), 2.8, 1.7, 1.4-1.1, 0.8 (decyl Hs); 13C NMR (75.477 MHz, CDCl3) δ ) 152.8, 143.4, 139.4, 137.2, 130.4, 130.1, 126.5, 125.7, 125.5, 124.7, 121.9 (aromatic Cs), 32.1, 31.2, 29.9, 29.7, 29.6, 22.9, 14.4 (decyl Cs). SEC (CHCl3, 1 mL/min, RI) Mn ) 5.7 kg/mol, Mw ) 10.4 kg/mol, PDI ) 1.8. UV-vis (1.82 × 10-5 M repeat units in CHCl3): λmax ) 547 nm, εsolution ) 2.31 × 104 L · mol-1 · cm-1; UV-vis (film) λmax ) 610 nm, λedge ) 800 nm. Transistor Fabrication. The SiO2 side of the heavily doped p-type Si substrate was exposed to saturated hexamethyldisilazane (HMDS) vapor for several minutes.32,56,62 On top of this surface, a few drops of ca. 5 mg/mL polymer:fullerene solution (PITN-co-ThBTD:PCBM in o-dichlorobenzene (DCB)) were deposited and spin-coated at 2000 rpm for 60 s under ambient laboratory conditions. The spun polymer:fullerene films were ∼20 nm thick. The blend films were annealed at 150 °C for 24 h. Gold top contact source and drain electrodes were formed by vacuum evaporation of 45 nm Au through a Si stencil mask. The finished transistor had a channel width of 4000 µm and a length of 40 µm.

Figure 1. UV-vis absorption spectra of PITN-co-ThBTD in toluene solution (solid line) and film (dashed line).

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Figure 2. Room-temperature WAXS pattern for the drop-cast PITN-co-ThBTD:PCBM film as a function of PCBM wt %. (a) Out-of-plane WAXS pattern for the untreated films, (b) GIXS pattern for the untreated films, (c) out-of-plane WAXS pattern for the annealed films (150 °C, 24 h), and (d) GIXS pattern for the annealed films (150 °C, 24 h).

PV Device Fabrication. PV devices were fabricated on precleaned patterned ITO glass substrates. PEDOT/PSS was spin-coated on the ITO glass slides (2.54 × 2.54 cm2) at a speed of 4000 rpm for 45 s and annealed at ∼120 °C for 5 min.32,56 The PEDOT/PSS film was ∼30 nm thick. On top of the PEDOT/ PSS, a few drops of PITN-co-ThBTD:PCBM (conc. ∼25-80 mg/mL) in DCB were deposited and then spin-coated at a speed of ∼600-3000 rpm for 60 s inside a glovebox. The spun PITNco-ThBTD:PCBM films were ∼100 nm ((10 nm) thick (for composition dependence of solar cell performance) or ∼100-170 nm thick (for spin-rate dependence) or ∼50-160 nm thick (for active layer thickness dependence), which was measured using a KLA Tencor P-10 profilometer. After drying overnight by slow solvent evaporation under N2, the spun film was mounted inside the bell jar of a Denton Vacuum Bench Top Turbo III thermal evaporator. Aluminum was evaporated to deposit a 100 nm thick electrode on top of the PITN-co-ThBTD:PCBM films. The device active area was 0.1 cm2. Characterization. Thermal behavior was measured using DSC (TA Instruments) and thermogravimetric analysis (TGA, Perkin-Elmer TGA 7). Glass transition temperatures were taken as the midpoint of the transition between two baselines using the Universal Analysis 2000 software.56,62 WAXS (Bruker-AXS Microdiffractometer with 2.2-kW sealed Cu KR X-ray source) was performed on drop-cast PITN-co-ThBTD:PCBM films with thicknesses of ca. 4 µm. AFM images were acquired using a

Veeco Metrology Nanoscope IIIa microscope operating in tapping mode. Electrical characterization of transistors was performed in the dark using a Desert Cryogenics probe station with a base pressure of 5 × 10-7 Torr. Mobility (µ) was calculated in the linear regime using the equation, µ ) |∂ID/∂VG|VD ) const/(CiVDW/L): where ID, VG, VD, and Ci are drain current, gate voltage, drain voltage, and gate insulator SiO2 capacitance, respectively. For PV measurements, the illumination source was a 150-W Xe-arc lamp (Oriel) with an AM 1.5G filter. Several attenuators with constant optical density were used to obtain the range of light intensities. Current-voltage characteristics of PV cells were measured with an Agilent 4155C Semiconductor Parameter Analyzer. Photocurrent action spectra were obtained using a monochromator (Cornerstone 130 1/8 m) equipped with commercially available gratings and filters (Newport Corp.) in conjunction with a Keithley 2400 source meter controlled by customized LabView code.32,56 Results and Discussion 1. PITN-co-ThBTD Synthesis and Characterization. PITNco-ThBTD was obtained through a Stille-type coupling reaction of monomers 1 and 2, as shown in Scheme 1. Monomer 1 has been shown to be a highly versatile building block for the construction of various conjugated polymers with different dihalogenated coupling partners, the nature of which determine the physical and electronic properties of the resulting polymers.32



Figure 4. Linear mobility (µh at VD ) -15 V, µe at VD ) 15 V) of the PITN-co-ThBTD:PCBM blends as a function of increasing PCBM wt %. Annealing process was done at 150 °C, 24 h. Inset: gold bottomgate and top-contact PITN-co-ThBTD:PCBM transistor.

Figure 3. The AFM tapping mode height (left column) and simultaneously acquired phase images (right column) of the PITN-co-ThBTD: PCBM blend films of various compositions. All the images are 1 × 1 µm2, and the various compositions shown in the figures correspond to PCBM wt % in PITN-co-ThBTD:PCBM blend. Image height range, maximum peak-to-valley is 40 nm (0-50%) and 8 nm (80%).

Benzothiadiazole (BTD) and its derivatives have been widely used as electron deficient components in low bandgap conjugated polymers with alternating donor-acceptor structures.18,63-65 The reaction was conducted in THF at 60 °C using the highly active catalyst, Pd[PtBu3]2,66 and PITN-co-ThBTD was isolated by precipitation into acetone and Soxhlet extraction as a black solid in 90% yield. The polymer was characterized by both 1H and 13C NMR spectroscopy (see Figures S1 and S2 in Supporting Information

Figure 5. Composition dependence of the ∼100 nm thick PITN-coThBTD:PCBM solar cells: (a) Jsc and Voc, under simulated AM 1.5 spectrum. Inset: device structure. (b) FF and PCE under simulated AM 1.5 spectrum. Inset: a picture of the PITN-co-ThBTD solar cell.

for corresponding spectra). As seen from the 1H NMR spectrum, the relatively sharp signals and well-resolved splitting pattern of the resonance at 7.8 ppm from the isothianaphthene protons indicate a regioregular structure of the polymer, as expected. The estimated number average molecular weight of this polymer, by SEC against polystyrene standards, is Mn ) 5.7 kg/mol (corresponding to degree of polymerization, n ≈ 8) with a polydispersity index of PDI ) 1.8. UV-vis spectra for PITNco-ThBTD thin films and solutions, shown in Figure 1, show a broad absorbance across the visible range, with an onset of

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Figure 6. Light intensity dependence of the ∼100 nm thick PITN-co-ThBTD:PCBM ) 30:70 solar cells: (a) JV characteristics under white light with various intensities, (b) semilog JV curve under white light with various intensities, (c) Jsc and Voc, and (d) FF and PCE.

800 nm, corresponding to an optical band gap of 1.55 eV. Furthermore, from the oxidation onset of Eox ) 0.9 V in the cyclic voltammetry measurements (supplied in Figure S3 in Supporting Information), we estimate the HOMO level of this polymer to be -5.3 eV,67,68 and the LUMO level is thus estimated from the optical bandgap to be -3.7 eV. We determined the extinction coefficient (ε ≈ 4.2 × 104 cm-1 at λmax ) 610 nm) of spin coated PITN-co-ThBTD films from a plot of absorption vs polymer film thickness (see Figure S4 in Supporting Information). This extinction coefficient is very similar to the value of poly(3-hexyl-2,5-thienylene vinylene) (P3HTV) (ε ≈ 4.1 × 104 cm-1 at λmax ) 574 nm).56 In addition, the extinction coefficient is approximately 50% of the value of poly(3-hexylthiophene) (P3HT), which is ε ≈ 7.9 × 104 cm-1 at λmax ) 520 nm. Thus in order to absorb 99% of light at 610 nm, a ∼240 nm thick PITN-co-ThBTD film is needed without accounting for the reflection at the contacts in a practical device. We also studied the thermal behavior of the PITN-co-ThBTD polymer, (Figure S5 of Supporting Information). The DSC curve shows no melting behavior and a glass transition temperature (Tg) at 65 °C consistent with a disordered amorphous structure. In addition, a TGA thermogram revealed that PITN-co-ThBTD was stable at least up to 300 °C under N2 while ∼10% of the material was lost at 400 °C. 2. Structure and Morphology of PITN-co-ThBTD:PCBM Films. WAXS was carried out on drop-cast films of PITN-coThBTD:PCBM blends to investigate the phase behavior including the PCBM solubility limit. Out-of-plane WAXS and inplane grazing-incidence X-ray scattering (GIXS) patterns for the as-deposited films are shown in parts a and b of Figure 2. For pure PITN-co-ThBTD films (i.e., 0 wt % PCBM), we obtained a weak diffraction peak at qz ) 0.35 Å-1 corresponding to a d spacing of 2 nm. This d spacing likely reflects the length of the alkyl side chains plus the ring system of the polymer

and indicates there is some ordering of the polymer in layers running parallel to the substrate. There is also a very broad amorphous halo around qx,z ) 1.5 Å-1 in both qx and qz directions that corresponds to an average van der Waals contact between aromatic rings. Overall, the quality of the diffraction spectrum for pure PITN-co-ThBTD supports the earlier conclusion from the DSC data that the polymer is largely amorphous but with some weak ordering perpendicular to the substrate. For pure PCBM films, three broad diffraction peaks are observed at qx,z ) 0.7 Å-1, qx,z ) 1.4 Å-1, and qx,z ) 2.0 Å-1, as is typically observed.62 For unannealed blend films, we cannot clearly determine the PCBM phase separation point due to the broad nature of the PCBM diffraction. Thus, to observe the PCBM solubility limit clearly,62 we annealed the PITN-co-ThBTD:PCBM films at 150 °C (higher than Tg of PITN-co-ThBTD and lower than Tm of PCBM) for 24 h under an argon environment and repeated the WAXS studies in parts c and d of Figures 2. Through this annealing, we can expect the crystallization of PCBM for phase separated composites. As shown in Figure 2c, the diffraction peaks at qz ) 1.4 Å-1 are sharpened for 30-100 wt % PCBM, which we ascribe to the phase-separated PCBM crystals. However, the qz ) 1.4 Å-1 diffraction peak from the 30 wt % PCBM composition is rather weak. We have also examined this peak from the GIXS data for the annealed blend, Figure 2d. Here we can observe not only the diffraction peak at qx ) 1.4 Å-1 but also a very broad signal originating from the PCBM peak, qx ≈ 2.0 Å-1. However, this latter peak is not observed at 20 wt % PCBM. Thus based on the WAXS data, we estimate that the phase separation point of PITN-co-ThBTD:PCBM blends is ∼30 wt % PCBM. Figure 3 shows AFM height (left column) and corresponding phase (right) images for different PITN-co-ThBTD:PCBM blends. In the case of the pure PITN-co-ThBTD film, i.e., 0


Figure 7. (a) Log Jsc-IL curve with various PCBM compositions. (b) Variation of R as a function of PCBM composition.

wt % PCBM, the phase image shows no contrast; consistent with the amorphous nature of pure PITN-co-ThBTD films. A drastic change takes place in the phase images when the PCBM content in the blend is increased from 20 to 30 wt %. The 20 wt % phase images look similar to the 0 wt % phase image, but a considerable phase contrast is seen for the 30 wt % composite. The phase contrast is consistent with phase separation inferred from the WAXS data. Fourier transform analysis of the same image shows that the domain size is ∼14 nm (see Figure S6 in Supporting Information). The 50 wt % phase image also shows the phase contrast, while the higher contents of PCBM (80%) display no apparent contrast in the phase images. 3. Transport Behavior in a Transistor Testbed. We have examined electrical transport in PITN-co-ThBTD:PCBM transistors to determine the relation between phase behavior and charge carrier mobility, Figure 4. The field effect hole mobility of the pure PITN-co-ThBTD film is 3 × 10-6 cm2/(V · s), Figure S7 of Supporting Information. This low mobility is ascribed to the disorganized amorphous structure of the polymer with a relatively small molecular weight. For unannealed films, the hole mobilities are almost constant between 0 and 30 wt % PCBM (µh ) ∼2-3 × 10-6 cm2/(V · s)), followed by a gradual decrease up to 50 wt % (3 × 10-7 cm2/(V · s)). Beyond 50 wt % there is no hole conduction. In contrast, for the same as-cast films, no electron mobility was observed until the PCBM concentration exceeded 40 wt %. This composition is somewhat higher than the estimated PCBM solubility limit (30 wt %) determined by

J. Phys. Chem. C, Vol. 113, No. 52, 2009 21933 WAXS and AFM. However, electron conduction will not occur until the PCBM domains are interconnected and it is reasonable that this will happen for PCBM concentrations exceeding the solubility limit. However, after annealing, a measurable electron mobility was observed at 30 wt %, corresponding to the estimated phase separation composition. The transfer and output characteristics of the transistors show comparable electron (µe ) 4 × 10-7 cm2/(V · s)) and hole (µh) 1 × 10-7 cm2/(V · s)) mobilities at a PITN-co-ThBTD:PCBM composition of 70:30, parts c and d of Figure S7 of Supporting Information. Thus as shown in our previous studies,56,62 ambipolar (electron and hole) transport was observed only after phase separation, in which a percolating pathway for n-channel conduction is formed. 4. Solar Cell Composition Dependence. PV devices were fabricated with the ITO/PEDOT:PSS(30 nm)/PITN-co-ThBTD: PCBM (∼100 nm)/Al configuration, as seen in the inset to Figure 5a. Pure PITN-co-ThBTD films are navy blue in appearance, as seen in the inset of Figure 5b. PITN-co-ThBTD: PCBM films were spin-coated at a relatively low speed, 600 rpm, as commonly employed for P3HT:PCBM solar cells to obtain an optimized morphology for enhanced performance.69 Figure 5 shows the PITN-co-ThBTD:PCBM solar cell performance as a function of increasing PCBM composition under 100 mW/cm2 simulated AM 1.5 irradiation. Jsc, Voc, and FF all increased with increasing PCBM wt %, from 27 µA/cm2, 0.54 V, and 27% (pure PITN-co-ThBTD) to 2.3 mA/cm2, 0.69 V, and 32% (70 wt % PCBM), respectively. The devices with 70 wt % PCBM exhibited the highest efficiencies (PCE ) 0.5%). Solar cells based on amorphous polymers usually have shown the best performance at high PCBM compositions, e.g., ∼80 wt % PCBM.32,41,57,58 5. Solar Cell Light Intensity Dependence. Using a series of neutral density of filters, we have studied the PITN-coThBTD:PCBM (30:70) solar cell performance as a function of increasing light intensity, IL, ranging from 3 to 290 mW/cm2.56 The JV characteristics and intensity dependence of Jsc, Voc, FF, and PCE are shown in Figure 6. For 70 wt % PCBM, Jsc increases approximately linearly with the relation, Jsc ≈ ILR (R ) 0.96), and Voc follows a logarithmic function, Voc ) Vtln (Jsc/Js + 1), as expected, where Vt ) kT/e is the thermal voltage (k, Boltzmann’s constant; T, temperature; e, electronic charge) and Js is the reverse-bias saturation current.70 In contrast, FF decreases for IL > 10 mW/cm2 owing to the effect of series resistance (low carrier mobility) or carrier recombination. The overall PCE curve is governed by the trend in the fill factor in spite of the increase of Jsc and Voc with increasing light intensity; PCE peaks at 10 mW/cm2 illumination. We also have checked the light intensity dependencies for different PCBM compositions. Figure 7a shows a double log plot for Jsc vs IL with varying PCBM compositions, with the slope of each plot (R) shown in Figure 7b. Below phase separation, R is 0.78 (10 wt %) and 0.81 (20 wt %), but above the phase separation, it increases to about ∼0.92-0.96 for 40-80 wt %. The increased R value after phase separation indicates a lower rate of charge recombination in spite of the intrinsic transport limitations of this polymer. 6. Spin Coating Process Dependence. We have further optimized the PITN-co-ThBTD:PCBM (30:70) solar cells by controlling the thickness of the photoactive layer. For identical spin-coating conditions (600 rpm), PCE decreases from 0.4-0.5% for a 100 nm thick film to 0.2-0.3% for a 170 nm thick film. Devices with active layers less than 100 nm were also inferior

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Figure 8. Photoactive layer spin-rate dependence of the PITN-co-ThBTD:PCBM ) 20:80 solar cells. Thickness of photoactive layer: ∼170 nm (600 rpm), ∼140 nm (1000 rpm), and ∼100 nm (1500-3000 rpm). (a) Jsc and Voc under simulated AM 1.5 spectrum, (b) FF and PCE under simulated AM 1.5 spectrum, (c) JV characteristics under simulated AM 1.5 spectrum and in dark, and (d) photocurrent action spectra of the solar cells under illumination with monochromatic light. Inset: a picture of the PITN-co-ThBTD:PCBM ) 20:80 solar cell.

due to a reduced Voc, presumably caused by the formation of microscopic shorts, or pinholes, in the film. We have also examined the impact of spin-coating speed on device performance. Spin-coating is a nonequilibrium process, and the BHJ morphologies that develop often rely significantly on the evaporation rate of solvent from the drying film. Hence, we have also fabricated a series of devices from the same solution, varying the spin speed from 600 to 3000 rpm. Figures 8a and b show Jsc, Voc, FF, and PCE as a function of spin speed for the PITN-co-ThBTD:PCBM) 20:80 solar cell.71 Jsc, FF, and PCE all increase drastically with increasing spin-rate before leveling off beyond 2000 rpm. This performance increase is similar to the measured thickness change of the photoactive layer, which decreases from 170 nm (600 rpm) to ∼100 nm (g1500 rpm). As a result, we measured the highest PCE of 0.9% with Jsc ) 3.4 mA/cm2, Voc ) 0.83 V, and FF ) 32% at 2000 rpm, Figure 8c. Similarly, the PITN-co-ThBTD:PCBM) 30:70 solar cell showed a PCE ≈ 0.8%; see Figure S8 of Supporting Information. By comparison of the results in Figure 8 with those of Figure 5, we see that, for a constant film thickness of 100 nm, increasing the spin speed from 600 to 2000 rpm improves PCE by nearly a factor of 2, from 0.4-0.5% to 0.8-0.9%. While the AFM images of the film surfaces are nearly identical, we suspect the increased rate of solvent evaporation in the 2000 rpm films yields a favorable bulk morphology by limiting the size of phase separated domains. Thus, the important role of spin speed (e2000 rpm) is to change both film thickness and morphology simultaneously. Figure 8d shows the photocurrent action spectra of the 80 wt % PCBM devices, and the inset shows a picture of this device, light brown in color due to the high PCBM fraction. The device spin coated at 2000 rpm shows ∼22% peak external

quantum efficiency or incident photon to current efficiency (IPCE). The IPCE graph follows the absorption spectrum of PITN-co-ThBTD:PCBM (20:80), which is significantly blueshifted (i.e., optical band gap difference, ∆Eg ≈ 0.22 eV) from that of a pure PITN-co-ThBTD film, Figure S9 of Supporting Information. This blue shift is simply due to the reduced intermolecular interactions brought about by blending with PCBM (see also Figure 1 for UV-vis absorption data for the extreme case: solution vs film). With a band tail reaching to 760 nm, the blend harvests energy across most of the visible light range. 7. Solar Cell Thickness Dependence. Thickness is an important design parameter because it determines the optimum balance between light absorption and charge extraction.56 Thus, we have studied the impact of active layer thickness on device performance at a fixed spin rate of 2000 rpm (as optimized in the previous section). Current-voltage characteristics under light and in dark are seen in parts a and b of Figure 9. Table 1 gives the dark currents in terms of shunt resistance (RpA), series resistance (RsA), and rectification ratio (RR). RpA and RsA were determined from the slope of dark current-voltage curve at 0 and 2 V, respectively. RR was calculated from the ratio of absolute dark current values at ( 2 V. RpA increases from ∼104 Ω · cm2 for 50-70 nm thick films to ∼105-106 Ω · cm2 for 85-160 nm thick films, presumably due to the elimination of pinholes and other defects prone to thinner films. RsA shows a minimum (∼2 Ω · cm2) at ∼70-120 nm thick film. Similarly, RR was improved from 101 to 105 by increasing film thickness from 50 to 160 nm. The values of RpA, RsA, and RR in the dark should be directly related with the photovoltaic diode performance under light illumination.



Figure 9. Active layer thickness dependence of the PITN-co-ThBTD:PCBM ) 20:80 solar cells: (a) JV characteristics under simulated AM 1.5 spectrum, (b) JV characteristics in dark, (c) Jsc and Voc under simulated AM 1.5 spectrum, and (d) FF and PCE under simulated AM 1.5 spectrum.

TABLE 1: RpA (Ω · cm2 at 2 V), RsA (Ω · cm2 at 0 V), and RR ((2 V) of PITN-co-ThBTD:PCBM ) 20:80 Solar Cells as a Function of Photo-Active Layer Thickness active layer thickness (nm) 50 R pA R sA RR

55

1.40 × 10 3.83 × 100 1.21 × 101 4

60

3.20 × 10 3.89 × 100 1.08 × 103 4

70

1.22 × 10 3.31 × 100 1.83 × 103 4

85

2.00 × 10 2.35 × 100 7.23 × 103 4

The solar cell parameters are summarized in parts c and d of Figure 9. Jsc increases with thickness up to ∼100 nm and then decreases, owing to increased series resistance and recombination loss. Voc increases up to ∼85 nm and levels off at higher thickness. The lower Voc of the thin films (50-70 nm) can also be attributed to parasitic leakage current caused by film defects, such as pinholes. FF decreases monotonically with increasing active layer thickness owing to the low hole mobilities of amorphous PITN-co-ThBTD as do R values from light intensity measurements (see Figure S10 of Supporting Information). Thus, overall PCE peaks at ∼0.8% for a ∼100 nm thick film. Conclusions An amorphous PITN-co-ThBTD has been used as an electron donor material in PITN-co-ThBTD:PCBM BHJ solar cells. The photoactive PITN-co-ThBTD blends were studied as a function of increasing PCBM compositions using WAXS and AFM. The PCBM solubility limit is ∼30 wt % PCBM. The relation between electrical transport and phase behavior was examined in detail. For the annealed transistors, below the PCBM solubility limit (∼30 wt %), only hole transport was observed, and above the solubility limit, ambipolar (electron and hole) transport was evident. A series of PITN-co-ThBTD:PCBM solar cells were fabricated with an optimum performance of

100

6.32 × 10 2.60 × 100 2.14 × 104 5

120

8.04 × 10 2.07 × 100 9.56 × 104 5

140

7.65 × 10 2.15 × 100 9.93 × 104 5

160

7.53 × 10 4.40 × 100 6.89 × 104 5

3.50 × 106 4.52 × 100 2.55 × 105

∼0.8-0.9% PCE for ∼70-80 wt % PCBM blends. These cells exhibit high Voc values on the order of 0.8 V. The principal performance limiting bottleneck is the low hole and electron mobility values. Further work is aimed at developing low bandgap polymers with better transport characteristics. Acknowledgment. This work was funded by the Initiative for Renewable Energy and the Environment at the University of Minnesota (UMN) and the Xcel Energy Renewable Development Fund. It was also supported by the UMN Materials Research Science and Engineering Center (MRSEC), funded by the NSF (DMR-0819885), and also partly through DMR0706011. C.D.F. thanks Prof. Russell Holmes for making his solar cell testing equipment available. Supporting Information Available: 1H and 13C NMR spectra, cyclic voltammogram, thermal behavior of PITN-coThBTD, UV-vis absorption spectra for determination of extinction coefficient, power spectra density of the AFM phase image of 30 wt % film, current-voltage characteristics for PITN-co-ThBTD based transistors, the performance of PITNco-ThBTD:PCBM ) 30:70 solar cells, and variation of R as a function of active layer thickness are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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