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Keywords:

  • conducting materials;
  • dendrimers;
  • molecular devices;
  • molecular electronics;
  • photophysics

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

A series of gradient π-conjugated dendrimers and their corresponding models based on 5,5,10,10,15,15-hexahexyltruxene moieties as nodes and oligo(thienylene vinylene) (OTVs) units with different lengths as branching arms are synthesized in good yields through Wittig–Horner reactions. All new compounds are fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and MALDI-TOF MS or ESI-MS. Investigation of their photophysical properties reveals that the gradient dendritic scaffold not only results in a higher molar absorption coefficient and broader absorption region than those of their corresponding model compounds, but also improves the PL quantum yields relative to the corresponding OTVs. The suitable HOMO and LUMO levels as well as excellent film forming properties make these molecules potential candidates for organic solar cells. Solution-processed bulk heterojunction solar cells using these dendrimers as donor and [6,6]-phenyl-C61 butyric acid methyl ester as acceptor are prepared and tested. The power conversion efficiency of the devices based on G0-4-2 is 0.40 % under illumination of air mass 1.5 and 100 mW cm−2. This is the highest record value for OTV-based materials to date. Although the absorption band of dendrimer G0-4-2 is much narrower than that of poly(3-hexylthienylene vinylene) (P3HTV), the efficiency of its solar cell device is almost twice that of the device based on P3HTV. This result shows clearly the advantage of gradient dendritic structures as active materials for photovoltaic cells.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

π-Conjugated dendrimers have attracted considerable interest because of their well-defined molecular structure, high purity, and high absorption intensity.13 Light-absorbing chromophores can be incorporated into the branching units to achieve high molar extinction coefficients over a broad absorption region, which is an important requirement for organic solar cell materials.4 Moreover, the stepwise synthetic methodology of such dendrimers offers the possibility to engineer their electronic properties in each dendritic branch.5 To date, the power conversion efficiencies of photovoltaic devices based on small organic molecules are still much lower than those of π-conjugated polymers because of their narrower light absorption range and lower absorption coefficients in the red and near-infrared region.6, 7 Therefore, extensive research was conducted to improve the overlap between the absorption of the active layer materials and the solar spectrum.8

Oligo- and poly(thienylene vinylene)s (OTVs), although having small energy band gaps and good hole mobility (both of which are attractive features for organic solar cells9), exhibit low power conversion efficiency in solar cell devices10 because of the inherent nonluminescent properties of longer OTVs in solution and thin films.11, 12 We envisage that there is plenty of room to further develop OTV materials for various applications if we can, by structural modification, improve their photophysical properties.

In our previous work, we developed a family of novel π-conjugated thienylethynylene dendrimers G0, G1, and G2 for light-harvesting purposes.13 We wished to explore whether desirable photophysical properties could be realized in lower generation dendrimers, which are more practical for applications.2i Herein, we present two gradient π-conjugated dendrimers G0-2-1 and G0-4-2,

  • magnified image

in which 5,5,10,10,15,15-hexahexyltruxene units are employed as the nodes and OTVs with different lengths are employed as light-harvesting branching arms. Our intention was to study the relationship between several key features of molecular design and device performance, while keeping the basic dendrimer backbone and generation constant. The introduction of OTV units as the branching arms results in high molar extinction coefficients over a broad absorption region and good hole-carrier properties at the zeroth generation. On the other hand, the introduction of alkylated truxene units dramatically improves the dendrimer’s fluorescence quantum yields relative to corresponding OTVs. To improve their solubility12 and processability, we introduced multiple alkyl groups at the nodes and at the terminal groups. According to molecular modeling results,14 G0-4-2 has a diameter of about 8.5 nm and a molecular weight up to 6250 Da. Finally, we investigated their performances in solution-processed bulk heterojunction solar cells and show that, despite their similar chemical structure, these molecules exhibit drastically different photovoltaic behavior.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Synthesis and Characterization

Scheme 1 illustrates the synthetic approach to dendrimer G0-2-1. Reduction of 115 with NaBH4 afforded compound 2 in 94 % yield. Previously, we observed that the target bromide obtained by reaction of alcohol 2 with PBr3 decomposed readily.16 Therefore, a one-pot reaction of 2 with triethylphosphate, iodine, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at 125 °C was employed to afford 3 in 85 % yield.17 Dendrimer G0-2-1 was obtained in 64 % yield through a threefold Wittig–Horner reaction between trialdehyde 415 and phosphonate 3.18

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Scheme 1. Synthesis of dendrimer G0-2-1 and the core of dendrimer G0-4-2.

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Reduction of (E)-5-(2-(thiophen-2-yl)vinyl)thiophene-2-carbaldehyde with NaBH4 gave 5 in 97 % yield. As shown in Scheme 1, a one-pot reaction of 5 with triethylphosphate, iodine, and DBU afforded 6 in 52 % yield. The threefold Wittig–Horner reaction between 4 and phosphonate 6 gave 7 in 80 % yield.18 Under typical Vilsmeier reaction conditions,19 7 formed an overformylated by-product, which was difficult to purify. Therefore, 7 was treated with nBuLi, followed by addition of anhydrous DMF, to give trialdehyde 8 in 86 % yield.20

Scheme 2 illustrates the synthetic approach to G0-4-2. The key component, the “masked” AB2 building block 10, bears one unprotected thiophene group for further modification and two thiophene groups with hexyl chains. The introduction of hexyl groups in the terminal thiophene ring greatly improved the solubility of the final product. The Wittig–Horner reaction between 915 and diethyl(5-hexylthiophen-2-yl)methylphosphonate21 afforded 10 in 83 % yield. Compound 10 was converted into monoaldehyde 11 in 83 % yield by treatment with nBuLi and anhydrous DMF. Reduction of 11 with NaBH4 gave alcohol 12 in 95 % yield. Horner reagent phosphonate 13 was then prepared in 63 % yield by a procedure similar to that for 3. G0-4-2 with all-E stereochemistry of the double bonds was obtained through threefold Wittig–Horner coupling reaction between trialdehyde 8 and monophosphonate 13 in 62 % yield.

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Scheme 2. Synthesis of G0-4-2.

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To better understand the photophysical properties of the newly synthesized gradient π-conjugated dendrimers, compounds Tr(TV)2Tr15 and Tr(TV)4Tr (Tr=truxene; TV=thienylene vinylene) were prepared as the linear models of dendrimers G0-2-1 and G0-4-2. Tr(TV)4Tr was synthesized by Wittig–Homer reaction between (E)-5,5′-(ethene-1,2-diyl)dithiophene-2-carbaldehyde and 3 in 71 % yield.

All new compounds are readily soluble in common organic solvents, such as chlorobenzene, THF, and CHCl3. The structure and the purity of all new compounds were fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and MALDI-TOF MS or ESI-MS. The 1H NMR spectrum of G0-4-2 possesses the correct integral ratio of the number of aliphatic CH2 protons (adjacent to the terminal thiophene ring) to the sum of all aromatic protons. Figure 1 shows the MALDI-TOF MS spectrum of G0-4-2, in which the sharp molecular ion peak at m/z=6246 Da (calcd: m/z=6250 Da) and another signal corresponding to dehexylated fragment [M−C6H13]+ (calcd: m/z=6165 Da, found: m/z=6161 Da) was clearly observed, which was similar to those of other truxene derivates.22 All these results clearly indicated the molecular identity and purity of G0-4-2. The thermal properties of our dendrimers were analyzed by thermal gravimetric analysis (TGA). All compounds have high thermal stability and decomposition temperatures higher than 390 °C in a nitrogen atmosphere. The glass transition temperature of G0-4-2 was 115 °C, measured by differential scanning calorimetry (DSC). Other compounds exhibit no apparent glass transition during heating.

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Figure 1. MALDI-TOF mass spectrum of G0-4-2.

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Photophysical and Electrochemical Properties

Figure 2 shows the absorption and photoluminescence (PL) spectra of dilute solutions of the dendrimers and model compounds in THF. The photophysical data are summarized in Table 1. The absorption spectra of the dendrimers and model compounds show two distinct absorption bands arising from different OTV chromophores. G0-2-1, Tr(TV)2Tr, and Tr(TV)4Tr exhibit a peak at about 341 nm owing to the same peripheral moiety, namely, the monothiophene-functionalized truxene chromophore.15 For G0-4-2, the absorption in the short wavelength range peaked at about 414 nm, which was assigned to the (TV)2-functionalized truxene unit. The dendrimers exhibited another absorption peak at 446 nm for G0-2-1, and at 501 nm for G0-4-2. The absorbance in this region was assigned to the longest branch of dendrimers, since Tr(TV)2Tr and Tr(TV)4Tr have similar absorption peaks in this region (443 nm and 499 nm, respectively). The effective conjugation length was extended and the molar extinction coefficient of the absorption peak of dendrimer G0-2-1 (2.9×105M−1 cm−1) to G0-4-2 (4.5×105M−1 cm−1) increased by about three times relative to those of their corresponding models (Tr(TV)2Tr and Tr(TV)4Tr). Meanwhile, both dendrimers have higher molar extinction coefficients in the longer wavelength range than those of the model compounds. All absorption spectra show well-defined vibronic structure, reflecting the planar, rigid geometry of the OTV units. Comparison of G0-2-1 and G0-4-2 (λmax values of 360 nm and 465 nm, respectively) with the corresponding unsubstituted OTVs21 revealed that the terminal truxene moiety participated in the conjugated system and thus induced a red shift in the λmax value of 86 nm for G0-2-1 and 36 nm for G0-4-2.23 Meanwhile, compared with G013 and G113 with oligothienylethynylene as the branch, dendrimers G0-2-1 and G0-4-2 exhibited a much broader absorption region, especially for G0-4-2, because of the better conjugation through the double bond than the triple bond. As mentioned earlier, such a broad and strong absorption spectrum is of great importance for organic solar cell materials with high energy-conversion efficiency.

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Figure 2. Absorption and photoluminescence spectra of dendrimers and model compounds in THF (10−6M) at room temperature. Photoluminescence spectra were recorded under excitation at the absorption maximum (λmax).

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Table 1. Photophysical properties of dendrimers and model compounds in solution and in thin films.
CompdAbs λmax[a] [nm (log ε)]Abs λmax[b] [nm]Emis λmax[a] [nm]Emis λmax[b] [nm]ΦPL[c]
  1. [a] In THF (10−6M); [b] in thin films; [c] in THF with Coumarin 152 (ΦPL=0.21 in ethanol) as the standard.

G0-2-1341 (5.41), 446 (5.46)343, 44749651654 %
Tr(TV)2Tr341 (5.30), 442 (5.04)343, 44649551156 %
G0-4-2414 (5.74), 501 (5.65)420, 5135670.5 %
Tr(TV)4Tr341 (5.31), 499 (5.21)343, 5075650.6 %

PL spectra of G0-2-1 and Tr(TV)2Tr in THF showed almost identical features (a maximum peak at 496 nm and a shoulder), similar to those of linear (TV)2. G0-4-2 and Tr(TV)4Tr showed a weak emission maximum at about 567 nm and exhibited red-shifts of 71 nm, relative to that of G0-2-1, owing to the increase of effective conjugation length in the branching arms. In comparison, shorter OTV oligomers ((TV)2, (TV)3, and (TV)4) have very low fluorescence quantum yields. Longer OTV oligomers did not show any detectable emission owing to the optical forbidden transition from the lowest energy excited state (2Ag) to the ground state (1Ag).24 The fluorescence quantum yields (ΦPL) of dilute solutions of these molecules in THF were measured to be 0.54 for G0-2-1, 0.56 for Tr(TV)2Tr, 0.005 for G0-4-2, and 0.006 for Tr(TV)4Tr.25 Although quite low, the fluorescence quantum yields of G0-4-2 and Tr(TV)4Tr were dramatically improved relative to that of (TV)4 (0.0009).26 This result shows that the highly fluorescent truxene moieties in the dendritic structure effectively influence the optical properties of the OTV segments.22 The improved fluorescence quantum yields are beneficial to photoinduced electron transfer efficiency in organic solar cells, in which the donor material was mixed with fullerene derivatives.27 Thus, our design strategy might provide a general strategy to improve the performance of organic solar cells based on OTVs and other nonluminescent molecules.

Thin films used for UV/Vis and PL measurements were obtained by spin-coating a solution of corresponding compounds in toluene (ca. 10 mg mL−1) onto quartz plates at 1000 rpm. All dendrimers and models exhibited excellent film-forming properties, rendering our dendrimers suitable for future applications in organic solar cells. Figure 3 shows their absorption and PL spectra in thin films. All absorption spectra were principally composed of two distinct absorption bands. The absorption maxima were observed at 447 nm (343 nm) for G0-2-1 and at 446 nm (343 nm) for Tr(TV)2Tr, which are very similar to those of the dilute solutions. Meanwhile, absorption maxima were observed at 513 nm (420 nm) for G0-4-2 and 507 nm (343 nm) for Tr(TV)4Tr in the longer wavelength region, red-shifted only by 12 nm and 8 nm, respectively, relative to those of the dilute solutions. Such small shifts implied that intermolecular aggregation (usually observed for OTV derivatives in thin films) was, to a large extent, suppressed by the truxene moieties with hexyl substituents and the overall dendritic structure.13, 28 The band gaps of dendrimers were estimated from the onset of absorption spectra in thin films. G0-2-1 has a band gap of 2.43 eV and G0-4-2 has a band gap of 2.00 eV; these values are slightly larger than that of PTV (Eg=1.74 eV).27 Such a decrease in band gap is in agreement with an increase of the effective conjugation length from G0-2-1 to G0-4-2. The PL spectra of G0-2-1 and Tr(TV)2Tr in thin films became very broad relative to those of the dilute solutions. Meanwhile, their maxima were red-shifted 20 nm and 16 nm, respectively, indicating the formation of excimers in excited states. Moreover, G0-4-2 and Tr(TV)4Tr were nonluminescent in thin films, owing to further reduction in the fluorescence quantum yields in the solid state.29 The broad and strong optical absorption of gradient dendrimer G0-4-2 in a thin film suggested its potential for photovoltaic applications.30

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Figure 3. Absorption and photoluminescence spectra of dendrimers and model compounds in thin films. Photoluminescence spectra were recorded under excitation at the absorption maximum (λmax).

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The electrochemical properties of the active compounds are important for organic solar cells.31 Therefore, the HOMO/LUMO energy level and the corresponding band gap of these gradient dendrimers and model compounds were investigated by cyclic voltammetry on a Pt electrode. Figure 4 shows the cyclic voltammetric response of these new compounds in thin films. The cyclic voltammograms of G0-2-1 and Tr(TV)2Tr show irreversible oxidation waves at 1.81 and 1.75 V vs. Ag/AgCl. The other compounds exhibit reversible oxidation waves peaking at 1.16 V for G0-4-2 and 1.08 V for Tr(TV)4Tr (vs Ag/AgCl). The HOMO levels of the thin films were determined from the onset positions of the oxidation peaks using Equation (1).32

  • equation image((1))
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Figure 4. Cyclic voltammograms of dendrimer and model compound films on a Pt electrode in 0.1 M Bu4NPF6 in CH3CN measured at a scan rate of 100 mV s−1.

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However, no reduction potentials were detected for these dendrimers. Therefore, the LUMO levels can only be inferred from the oxidation potential and the optical energy band gap calculated previously. The thus-obtained HOMO and LUMO levels were −5.54/−3.11 eV for G0-2-1, −5.55/−3.09 eV for Tr(TV)2Tr, −5.29/−3.29 eV for G0-4-2, and −5.32/−3.24 eV for Tr(TV)4Tr. Table 2 summarizes the electrochemical data of our dendrimers and model compounds. These results also revealed a significant change in the HOMO and LUMO energy levels as the length of the conjugated OTV branching arm increases while the dendritic generation was kept the same. These results are consistent with UV/Vis absorption spectra mentioned above. Our molecules show a higher stability against oxygen because of the lower HOMO levels relative to PTV (−4.80 eV).24 G0-4-2 has the highest HOMO level and the lowest LUMO level among our compounds. Comparison with the HOMO levels of −6.1 eV and the LUMO levels of −4.3 eV for PCBM33 (PCBM=[6,6]-phenyl-C61 butyric acid methyl ester) suggests possible photoinduced intermolecular charge transfer between our molecule and PCBM.34

Table 2. Electrochemical properties and absorption properties of dendrimers and model compounds in thin films.[a]
CompdEox (onset) [V]Epa [V]Abs λonset [nm]EHOMO [eV]ELUMO [eV]Band gap [eV]
  1. [a] Reference electrode: Ag/AgCl.

G0-2-11.141.81510−5.54−3.112.43
Tr(TV)2Tr1.151.75505−5.55−3.092.46
G0-4-20.891.16620−5.29−3.292.00
Tr(TV)4Tr0.921.08595−5.32−3.242.08

Photovoltaic Properties

The motivation of the design and synthesis of the gradient π-conjugated dendrimers G0-2-1 and G0-4-2 was to search for novel OTVs for photovoltaic cells. Hence, we used them as electron donor materials to fabricate bulk heterojuction solar cells with the structure ITO/PEDOT:PSS/dendrimer:PCBM (1:4 w/w)/Al (ITO=indium tin oxide; PEDOT=poly(3,4-ethylenedioxythiophene); PSS=poly(styrene sulfonate)), where PCBM was used as the electron acceptor. For comparison, devices based on G0-4-2, Tr(TV)4Tr, and G0-2-1 were fabricated with the same structure. The thickness of the photoactive layer was optimized for each donor compound/PCBM mixture. In general, all devices showed good diode-like behavior in the dark and a photovoltaic effect under illumination. Figure 5 shows the JV curves of the solar cells based on our molecules under the illumination of air mass (AM) 1.5, 100 mW cm−2, and Table 3 lists the photovoltaic properties obtained from the JV curves. The highest power conversion efficiency (PCE) is 0.40 % for G0-4-2, which is one of the highest values reported so far for solution-processable bulk heterojunction solar cells based on dendritic structures.

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Figure 5. J–V curves of single layer bulk heterojuction solar cells with dendrimers and PCBM as photoactive layers under illumination of AM 1.5 at 100 mW cm−2.

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Table 3. Device performance of single layer bulk heterojunction solar cells with dendrimers and PCBM as photoactive layers (1:4 w/w) under the illumination of AM 1.5 at 100 mW cm−2.
CompdVoc [V]Jsc [mA cm−2]FFPCE [%]
G0-4-20.821.3336.30.40
G0-2-10.850.4230.80.11
Tr(TV)4Tr0.850.6027.50.14

The Voc values of our compounds were about 0.82–0.85 V, which is remarkably around 0.3 V higher than that of P3HTV/PCBM devices (P3HTV=poly(3-hexylthienylenevinylene).35 These Voc values were among the highest values for devices using OTVs as electron donor, resulting from the lower HOMO energy level of our compounds than that of P3HTV derivatives.27 We note that the values of Jsc increased up to 1.33 mA cm−2 with increasing length of OTV branching arm in our gradient dendrimers from G0-2-1 to G0-4-2. This observation could be explained by the introduction of a longer OTV branching arm, which shifted the absorption spectrum to the longer wavelengths where the solar photon flux increased. Meanwhile, the Jsc value of G0-4-2 increased significantly relative to that of the corresponding model compound Tr(TV)4Tr because of its enhanced absorption intensity. This result clearly shows the advantage of using a dendritic structure in the photoactive layer. From the J–V curves, FF (fill factor) values were deduced to be between 0.27 and 0.36. The FF value was also enhanced with increasing size of the molecules. The PCE of the device based on G0-4-2 ranks among the highest recorded for OTV derivatives. Moreover, the efficiency of the device based on dendrimer G0-4-2 was also almost two times that of devices based on P3HTV (0.19 %),27 while its absorption band was much narrower than that of P3HTV. The higher efficiency of the device based on our dendrimers could be ascribed to the gradient dendritic structure and their improved photophysical properties such as PL quantum yield.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

In conclusion, we have developed a family of gradient shape-persistent π-conjugated dendrimers G0-2-1 and G0-4-2 based on OTV units as the branch and truxene moieties as the node for photovoltaic cells. By varying the conjugated length of the OTV units between two truxene moieties while keeping the generation unchanged, we achieved higher molar absorption coefficients and much broader absorption spectra, both of which help to improve the light-harvesting ability of the materials. Such a synthetic strategy obviates the difficulty in the preparation of higher generation dendrimers, which is more practical for real applications. Moreover, the introduction of truxene units and gradient dendritic structure was shown to improve the photophysical properties. Studies of single-layer bulk heterojunction solar cells based on these gradient π-conjugated dendrimers as donor materials and PCBM as acceptor showed an increase in PCE (up to 0.40 % for G0-4-2) while keeping the generation unchanged. Moreover, the device results based on these conjugated small molecules are even better than those of polymeric systems based on OTVs. This result suggests that the structure modification of OTVs with detectable emission and broader absorption in thin films is an effective strategy to explore new conjugated small molecules for photovoltaic applications. The relative lower PCE can be explained in part by the modest matching of the absorption spectra of our dendrimers with the solar spectrum. Gradient π-conjugated dendrimers with longer OTV segments in the branch might have better Jsc and PCE values, and studies to this end are in progress in our laboratory. We are also currently exploring nonlinear optical applications of these novel gradient dendrimers.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

General Methods

Chemicals were purchased and used as received. 1H and 13C NMR spectra were recorded on a 300 (300 MHz), ARX400 (400 MHz) NMR instrument using CDCl3 as solvent unless otherwise noted. Chemical shifts were reported in parts per million (ppm) relative to internal tetramethylsilane (δ=0 ppm) or residual CHCl3 (δ=7.26 ppm). UV/Vis spectra were recorded on a Perkin–Elmer Lambda 35 UV/Vis spectrometer. PL spectra were measured on a Perkin–Elmer LS55 luminescence spectrometer. MALDI-TOF MS spectra were recorded on a time-of-flight (TOF) mass spectrometer using a 337 nm nitrogen laser and dithranol as matrix. Elemental analyses were carried out on an Elementar Vario EL instrument (Germany). Differential scanning calorimetry analyses were performed on a METTLER TOLEDO instrument DSC822e calorimeter. Cyclic voltammetry was performed using BASi EC epsilon workstation; scan rate: 100 mV s−1; working electrode: Pt disk; auxiliary electrode: Pt wire; reference electrode: Ag/AgCl; supporting electrolyte Bu4NPF6 (0.1 M, CH3CN).

Fabrication and Measurements of Photovoltiac Devices

The single layer bulk heterojuction organic solar cells were fabricated in the configuration of the traditional sandwich structure with an ITO glass anode and a metal cathode. A thin layer (40 nm) of Baytron PEDOT:PSS was spin-coated on the ITO glass in a preheating treatment at 50 °C. The photosensitive active layer was then prepared by spin-coating a blend of solution of our dendrimers in chloroform or CH2Cl2 and PCBM (1:4 w/w) on the top of the PEDOT:PSS layer. Samples were dried in petri dishes under N2 atmosphere without any heating treatment. Al metal contacts were then thermally deposited onto the dried organic layer at a pressure around 4∼6×10−4 Pa. The active area of a device was 0.15 cm2. The current–voltage (JV) measurements of the devices were conducted on a computer-controlled Keithley 236 source measuring unit. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as light source, and the optical power at the sample was 100 mW cm−2.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

This work was supported by the Major State Basic Research Development Program from the Ministry of Science and Technology (Nos. 2006CB921602, 2007CB808000, and 2009CB623601) and the National Natural Science Foundation of China.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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