New Molecular Donors with Dithienopyrrole as the Electron-Donating

Article pubs.acs.org/cm

New Molecular Donors with Dithienopyrrole as the ElectronDonating Group for Efficient Small-Molecule Organic Solar Cells Hung-I Lu,† Chih-Wei Lu,‡ Ying-Chi Lee,† Hao-Wu Lin,*,‡ Li-Yen Lin,† Francis Lin,† Jung-Hung Chang,§ Chih-I Wu,§ and Ken-Tsung Wong*,† †

Department of Chemistry, and §Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan ‡ Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Six new molecules with donor−acceptor−acceptor (D−A−A) configuration bearing coplanar electron-donating dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or aryl-substituted DTP as the D unit and the electron-withdrawing pyrimidine−dicyanovinylene or benzothiadiazole−dicyanovinylene as the A−A block are synthesized. The introduction of aryl groups (p-tolyl or p-anisolyl) onto the α position of DTP is found to greatly benefit the chemical stability and extend the molecular conjugation of the DTP-based D−A−A molecules. The observed broad absorption spectra and anisotropic molecular orientation in the thin films allow for these new D−A−A molecules to perform good capability of light harvesting. These new D−A−A donors are subject to the fabrication of vacuum-processed small-molecule organic solar cells (SMOSCs). The results showed that the structural modulation on the central A block (pyrimidine versus benzothiadiazole) as well as the terminal substitution (p-tolyl versus p-anisolyl) give an evident trade-off between the open circuit voltage (Voc) and short circuit current density (Jsc) values. Among them, SMOSC-adopted D−A−A molecule TDPM composed of a p-tolyl terminal group and a D−A−A configuration of DTP− pyrimidine−dicyanovinylene as the electron donor combined with C70 as the electron acceptor shows a Jsc of 11.34 mA/cm2, a Voc of 0.94 V, and a fill factor (FF) of 0.52, giving a best power conversion efficiency (PCE) as high as 5.6%.

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SMOSC.26 The promising achievement in SMOSCs has opened a new space for the innovative ideas of designing new small molecules with novel structure and designated functions. Reported small donor molecules featured with central symmetric structures, such as phathalocyanines,27 subphthalocyanines,28−30 and squaraines,31−33 typically performed good efficiencies in SMOSCs. Particularly interesting, vacuum-processed SMOSCs adopted with oligothiophene end capped with dicyanovinylene groups as the donor have been reported to deliver a PCE up to 6.9%.34−36 The low molecular dipole and high crystallinity account for the good performance of these central symmetric molecules. In contrast, lower PCEs were observed for small molecules configured with an unsymmetrical donor−π−acceptor (D−π−A) architecture,37−40 despite their high extinction coefficient and long wavelength absorption features. The high molecular dipole resulting from the strong push−pull electronic interaction in D−π−A-type donors is suspected to retard the charge carrier transport propensity.41−43 Interestingly, the high molecular dipole of a D−π−A-type molecule can be diminished by the formation of a dimer with antiparallel crystal packing.44 In this

INTRODUCTION Organic solar cells or organic photovolatics (OPVs) are emerging as one of the promising technologies for renewable energy sources because of their potential low-cost fabrication, color-tunable feature, and mechanical flexibility.1−8 Many research activities have endeavored to develop new organic materials and device configurations for improving the efficiency and practical durability of OPVs. Among them, the developments of new conjugated polymers possessing enhanced lightharvesting ability and/or better charge mobility have drawn most of our attention. In this regard, solution-processed bulk heterojunction (BHJ) OPVs comprised of a polymer as the donor in conjunction with a fullerene derivative as the acceptor have been achieved with great successes.9−16 On the other hand, the progress of OPVs adopting a small molecule as the donor lags behind the polymer-based counterpart. Until recently, solution-processed BHJ OPVs with elegantly tailored small donor molecules have successfully realized outstanding power conversion efficiency (PCE) of over 8%,17−25 indicating the great potential of small-molecule organic solar cells (SMOSCs). Besides the solution process, SMOSCs can also be fabricated by the vacuum deposition with layer-by-layer configurations, giving an extra opportunity for engineering the device structures. To date, an impressive PCE of 12% has been achieved for a vacuum-processed triple-junction tandem © 2014 American Chemical Society

Received: March 11, 2014 Revised: July 22, 2014 Published: July 23, 2014 4361

dx.doi.org/10.1021/cm5018147 | Chem. Mater. 2014, 26, 4361−4367

Chemistry of Materials

Article

Scheme 1. Synthetic Pathways of DTP-Based New D−A−A Donor Molecules

introduced as a coplanar π spacer of sensitizers for achieving highly efficient dye-sensitized solar cells (DSSCs).57−63 However, only limited works related to DTP-containing small molecules were reported to serve as the donor in SMOSCs,64−66 in which a PCE of 1.3% was achieved.67 Very recently, a solution-processed SMOSC based on a smallmolecule donor with a N-ethylhexyl DTP core together with an electron acceptor [phenyl-C61-butyric acid methyl ester (PC61BM)] has been reported to deliver a PCE of 4.8%,68 indicating the potential of DTP as the electron-donating block in small-molecule donors. In comparison to coplanar cores, such as dithieno[3,2-b:2′,3′-d]silole, 4H-cyclopenta[2,1-b:3,4b′]bithiophene, and dithieno[3,2-b:2′,3′-d]thiophene, the Nalkyl DTP can provide better electron-donating ability69,70 to induce a sufficient electron coupling with the A−A component, rendering the new D−A−A-type molecules to have satisfactory long wavelength absorption and high extinction coefficient. In addition, the aromatic character of the central pyrrole ring in DTP can effectively reduce the conformational variations on the N center, leading to better intermolecular interactions. For the six new D−A−A-type donors reported in this work, the electron-donating N-ethylhexyl DTP was introduced to hybrid with the previously established A−A components, such as benzothiadiazole−dicyanovinylene or pyrimidine−dicyanovinylene. In addition, the aryl group (p-tolyl or p-anisolyl) was introduced as the end-capping groups of the new D−A−A chromophore. The terminal aryl substitutions are greatly benefical to block the high chemical reactivity of unprotected DTP and extend the molecular conjugation for giving more red-shifted absorption and, thus, better light-harvesting

regard, merocyanine was an excellent example of molecular donors using a D−π−A configuration to give SMOSC with a PCE over 6.1%.45 We have expanded this strategy to develop small donor molecules configured with a D−A−A architecture,46,47 where electron-deficient heteroarenes (benzothiadiazole or pyrimidine) were introduced to serve as the central A component bridging the typical triarylamine donor and dicyanovinylene acceptor. Such molecular design has led the donor molecule to exhibit a smaller excitation energy and a lower highest occupied molecular orbital (HOMO). Consequently, the D−A−A-type donor molecule showed capability to concurrently enhance the short-circuit current density (Jsc) and open-circuit voltage (Voc) in SMOSCs, where the best device has achieved a PCE up to 6.8%.48 It is worth mentioning that a D−A−A-type molecule (YF25), where dithieno[3,2b:2′,3′-d]silole was adopted as the donor moiety to couple with benzothiadiazole−dicyanovinylene (A−A), showed a potential alternative of the fullerene derivative as the electron acceptor blending with a typical donor P3HT.49 Inspiration by the crystal packing observed in our D−A−A-type donor molecules, where the terminal aryl groups attaching to the tetrahedral N center hinder the interactions between neighboring antiparallel dimers, provides the possibility to further refine on the D−A−A structure by modifying the donor moiety. In this work, we replaced the triarylamino group with a rigid and coplanar dithieno[3,2-b:2′,3′-d]pyrrole (DTP) as the electron-donating component to make new D−A−A-type donors for efficient SMOSCs. DTP has been widely used as the electron-donating moiety in alternating D−A polymers for OPVs50−54 and fieldeffect transistors.54−56 In addition, DTP has also been 4362



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RESULTS AND DISCUSSION Synthesis. The syntheses of new DTP-based D−A−A donors are shown in Scheme 1. The DTP core was prepared according to the literature procedures developed by Rasmussen and co-workers.71 Following the reported monostannylation method,57 the treatment of DTP with n-BuLi gave a monolithiated intermediate that was quenched with Bu3SnCl. The resulting solution was directly subject to the chemoselective Stille coupling reaction of 2-iodo-5-bromopyrimidine in the presence of a Pd(II) catalyst to afford the bromo compound 1 in good yield. The treatment of compound 1 with n-BuLi at −100 °C gave the lithiated intermediate, which was subsequently quenched with ethyl formate to afford the aldehyde 2 with a moderate yield. The first target D−A−A donor DPM was obtained in a high yield by the Knoevenagel condensation of compound 2 with malononitrile under a modified L-alanine-catalyzed procedure.35 Then, the bromination of compound 2 with N-bromosuccinimide (NBS) smoothly afforded the bromo derivative 3, which was then treated with p-tolyl boronic acid or p-anisolyl boronic acid under a Suzuki coupling condition to give the desired intermediates 4 and 5, respectively. The compounds 4 and 5 were then condensed with malononitrile to afford aryl-groupcapped TDPM and ADPM, respectively. Another series of DTP-based new D−A−A donors (DBT, TDBT, and ADBT) adopting benzothiadiazole−dicyanovinylene as the A−A component was synthesized basically following the similar synthetic scheme of the DTP−pyrimidine−dicyanovinylene donors. However, the available key intermediate of 7bromobenzothiadiazole-4-carbaldehyde (6) renders the syntheses of DBT, TDBT, and ADBT more efficiently. Properties. Figure 1 depicts the electronic absorption spectra of new DTP-based D−A−A donors in solution

capability. On the basis of this systematic study, a clear structure−property−performance relationship has been successfully established. Among these six new D−A−A-type donors, the donor TDPM composed of a p-tolyl terminal group and a D−A−A configuration of DTP−pyrimidine− dicyanovinylene in conjunction with C70 as the acceptor gives an efficient SMOSC with impressive performance: Voc of 0.94 V, Jsc of 11.34 mA/cm2, fill factor (FF) of 0.52, and PCE up to 5.6%.

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Article

EXPERIMETAL SECTION

Synthesis. The detail synthetic procedures and characterizations of new compounds are provided in the Supporting Information. Energy Level Measurements. Photoemission experiments were carried out by ultraviolet (UV) photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) simultaneously in an ultra high vacuum (UHV) chamber with a base pressure of 10−10 Torr. The ionization potential (IP) was measured by UPS (He I, 21.2 eV) with an experiment resolution of 0.15 eV, and the kinetic energy of photoelectrons was measured by a cylindrical mirror analyzer (CMA). The electron affinity (EA) was measured by IPES in the isochromat mode with a resolution of around 0.45 eV. The Fermi level reference in UPS and IPES was established by a clean Au surface. All of the materials were prepared in a deposition chamber and transferred in situ to the analysis chamber without breaking the vacuum to study the electric structure by UPS and IPES measurements. The UPS and IPES spectra of new D−A−A donor molecules are provided in the Supporting Information. Solar Cell Fabrication and Measurements. Fullerene C70 and 4,7-diphenyl-1,10-phenanthroline (Bphen) were purified by temperature-gradient sublimation before use in this study. A filtered dispersion of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spun on precleaned indium tin oxide (ITO) at 8000 rpm and then annealing at 135 °C for 30 min. The metal oxide and organic thin film as well as metal electrodes were orderly deposited on the PEDOT:PSS layer in a high-vacuum chamber with a pressure of ∼2 × 10−6 Torr. The sheet resistance of ITO was ∼15 Ω/square. The deposition rates of all organic compounds are ∼0.3−1.5 Å/s. The active area of the solar cells had an average size of 5 mm2 and were carefully measured device by device using a calibrated optical microscope. Devices were encapsulated using an UV-cured sealant (Everwide Chemical Co., Epowide EX) and a cover glass under an anhydrous nitrogen atmosphere after fabrication and were measured in air. Organic films for ellipsometry measurements were vacuum-deposited on fused silica substrates. The J−V characteristics were measured with a SourceMeterKeithley2636A under AM1.5G simulated solar illumination at an intensity of 100 mW/cm2 [calibrated with a National Renewable Energy Laboratory (NREL)-traceable KG5 filtered silicon reference cell]. The current densities were all corrected by spectra mismatch factors. The deviation values were obtained from device−device variations of 4−8 devices. The external quantum efficiency (EQE) spectra were taken by illuminating chopped monochromatic light with a continuous-wave bias white light (from halogen lamp) on the solar cells. The photocurrent signals were extracted with a lock-in technique using a current preamplifier (Stanford Research System), followed by a lock-in amplifier (AMETEK). The EQE measurement is fully computer-controlled, and the intensity of monochromatic light is carefully calibrated with a National Institute of Standards and Technology (NIST)-traceable optical power meter (Ophir Optronics). Ellipsometry measurements were carried out with a J. A. Woollam, Inc. V-VASE variable-angle spectroscopic ellipsometer. The anisotropic optical constants of a sample were determined by the combination of reflection and transmission ellipsometry, which, in principle, is simpler and eliminates the risk of sample−sample variation.

Figure 1. Absorption spectra of DTP-based D−A−A donors in CH2Cl2.

(CH2Cl2). The data are summarized in Table 1. The parent donor DBT showed a red-shifted absorption λmax (605 nm) compared to that of DPM (515 nm), which can be ascribed to the better quinoidal character of benzothiadiazole compared to that of pyrimidine because it shows better aromatic behavior.72 However, the λmax values of DPM and DBT were relatively blue-shifted in comparison to those of previously reported D− A−A donors with the same A−A components, such as DTDCTP 47 and DTDCTB, 46 where the di(p-tolyl)aminothienyl group was the D moiety. This result indicates that the electron-donating ability of DTP is inferior to that of the di(p-tolyl)aminothienyl group because of the lack of direct 4363



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Table 1. Physical Properties of New DTP-Containing Dyes a film λsoln max/λmax (nm)

DPM TDPM ADPM DBT TDBT ADBT

515/480 554/529 562/537 605/586 649/645 660/660

ε (M−1 cm−1)b 55100 67300 68500 44500 80500 68600

c Eopt g (eV)

Eox (V)d

2.15 1.97 1.93 1.80 1.66 1.63

h

0.77 0.50 0.45 0.65h 0.43 0.35

Ered (V)d

IP (eV)e

EA (eV)f

Td (°C)

anisotropicity (S)g

−1.41 −1.45i −1.49i −1.13 −1.09 −1.07

−5.6 −5.4 −5.1 −5.4 −5.2 −5.0

−3.2 −3.2 −2.9 −3.4 −3.4 −3.4

295 335 347 302 328 328

−0.195 −0.255j −0.269k 0.024l −0.192 −0.276

i

a From dichloromethane solution and from vacuum-deposited film. bIn dichloromethane solution. cCalculated from the onset of absorption spectra in dichloromethane solution. dDetermined from the E1/2 of the voltammogram. eIP determined by UPS. fEA determined by IPES. gS = (ke − ko)/(ke + 2ko), where ke and ko are from the peak wavelength. hDetermined from the Epa of the voltammogram. iDetermined from the Epc of the voltammogram. jCalculated from 530 nm. kCalculated from 535 nm. lCalculated from 585 nm.

stabilize the radical anion than pyrimidine. It is worth noting that the reduction potentials of both DPM and DBT series are rather inert to the nature of terminal groups. For oxidation, the unprotected parent donors, DPM and DBT, showed irreversible oxidation waves, which were ascribed to the existence of the remaining α position on DTP. The electrochemical oxidations become quasi-reversible as the aryl substitutions were introduced to block the α position, where the more electron-donating p-anisolyl-substituted molecules (ADPM and ADBT) exhibited lower oxidation potentials compared to those of p-tolyl-substituted molecules (TDPM and TDBT). The IP and EA of these new donor films were determined by UPS and IPES, respectively. The data are summarized in Table 1. As indicated, the EAs of these new donors are sufficient to ensure an efficient electron transfer upon photoexcitation to those of C60 and C70. On the other hand, the IPs of these donors are relatively low, suggesting for high Voc to be obtained because they are applied as a donor in photovoltaic devices. From the structural feature point of view, the donors with benzothiadiazole moiety have lower EAs and higher IPs compared to those of their pyrimidine-based counterparts. These results agree with the better quinoidal character of benzothiadiazole that can effectively reduce the energy gap. Furthermore, the observed red-shifted absorption maxima upon introducing aryl terminal groups can be rationalized by raising up the IP levels in both DPM- and DBT-based series, where the stronger electron-donating panisolyl-group-implanted molecules have higher IP levels compared to those of p-tolyl-group-substituted counterparts. Optical constants (refractive index, n, and extinction coefficient, k) of vacuum-deposited thin films of DPM and DBT series molecules were determined by the combination of reflection and transmission ellipsometry. Figure 3 depicts the extinction coefficient (k) of vacuum-deposited thin films of DPM- and DBT-based donor molecules. As shown, all of the thin films show uniaxial anisotropic characteristics with the optical axis along the surface normal. The larger ordinary extinction coefficient (ko) in comparison to extraordinary extinction coefficient (ke) for these new DTP-based D−A−A donors (except DBT) gave the negative orientation parameter S74,75 (Table 1), which is defined as (ke − ko)/(ke + 2ko) at the peak wavelength, indicating that the new DTP-based D−A−A molecules preferred the orientations parallel to the surface plane upon vacuum deposition onto substrates. This uniaxial anisotropy usually contributes to more intensive absorption for normally incident light and, therefore, may advance higher short-circuit current density (Jsc).39,48 The thermal stability is essential for organic donors because the photovoltaic device is fabricated by thermal evaporation in

contribution of the N substitution in DTP to the main conjugated backbone. However, in comparison to those of DTDCTP and DTDCTB, the molecules DPM and DBT showed larger extinction coefficients that will compensate for the loss of light harvesting because of the short absorption λmax values. Upon end capping the DTP cores of DPM and DBT with the p-tolyl or p-anisolyl group, further bathochromic shifts with larger extinction coefficients were observed, indicating the effective extension of conjugation by the introduction of aryl substitution as the terminal group was successful. The electronic character of the terminal aryl substitutions (p-tolyl versus p-anisolyl) provides an extra effect on shifting the λmax values to further long wavelength (∼10 nm). The electronic absorption spectra of the thin films were broadened with a slightly hypsochromic shift in λmax values (see Figure S1 of the Supporting Information) compared to those observed in solution, which is likely due to the conformational changes of the molecular backbone and/or intermolecular interactions that occurred in the solid states.73 To further elucidate the relationship between energy levels and chemical structures, the electrochemical characteristics of new DTP-based D−A−A molecules were studied by cyclic voltammetry with a scan rate of 100 mV/s, where tetrabutylammonium perchlorate (TBAP) as an electrolyte in tetrahydrofuran (THF) for the reduction and tetrabutylammonium hexafluorophosphate (TBAPF6) as an electrolyte in CH2Cl2 for the oxidation were used. Figure 2 shows the cyclic

Figure 2. Cyclic voltammograms of DTP-based donor molecules.

voltammograms of new DTP-based D−A−A compounds. The DPM series with pyrimidine−dicyanovinylene as the A−A component showed irreversible reductions, whereas the DBT series with benzothiadiazole−dicyanovinylene as the A−A component showed promising quasi-reversible reductions at lower potentials. Apparently, beznothiadiazole can better 4364



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Table 2. Photovoltaic Parameters of Planar Mixed Heterojunction Solar Cells under AM1.5G Simulated Solar Illumination at an Intensity of 100 mW/cm2 dye DPM TDPM TDPMa ADPM DBT TDBT ADBT a

Jsc (mA/cm2) 7.97 10.05 11.39 10.43 11.56 11.63 9.72

± ± ± ± ± ± ±

0.03 0.24 0.29 0.42 0.21 0.12 0.22

Voc (V) 0.98 0.93 0.94 0.85 0.89 0.76 0.66

± ± ± ± ± ± ±

0.02 0.01 0.01 0.01 0.02 0.01 0.01

FF 0.33 0.49 0.50 0.41 0.37 0.46 0.37

± ± ± ± ± ± ±

PCE (%) 0.01 0.01 0.02 0.01 0.01 0.01 0.01

2.5 4.6 5.5 3.8 3.9 4.1 2.6

± ± ± ± ± ± ±

0.1 0.2 0.1 0.1 0.1 0.1 0.1

The active layer thickness is 45 nm, and the others are 40 nm.

Figure 3. Ordinary (o, in-plane) and extraordinary (e, out-of-plane) extinction coefficients (k) of (a) DPM-based donor molecules and (b) DBT-based donor molecules.

vacuum. The N-alkyl group and dicyanovinylene group of new DTP-based D−A−A molecules are suspected to be thermally labile. Therefore, thermogravimetric analysis (TGA) was used to probe the thermal stability of these D−A−A donors. As indicated in Table 1, all of these new D−A−A donors exhibit relatively high decomposition temperatures (Td) corresponding to 5% weight loss, ranging from 295 to 347 °C. Interestingly, the aryl-substitution-capped molecules performed higher Td values than those of the parent molecules DPM and DBT. This result fortifies the advantage of introducing an aryl group as the terminal group. We envision that these new D−A−A donors with satisfactory thermal stability are robust enough toward the fabrication of thin-film devices using the vacuum process. Device Characteristics. Planar-mixed heterojunction (PMHJ) solar cells were fabricated using these newly synthesized compounds as the electron donors and C70 as the electron acceptor with a general device architecture of ITO/PEDOT:PSS (30 nm)/MoO3 (5 nm)/donor (7 nm)/ donor:C70 (1:2 by volume, 40 nm)/C70 (7 nm)/Bphen, (6 nm)/Ca (1 nm)/Ag. Device performances were measured under ambient atmosphere using a calibrated solar simulator, and the device parameters are summarized in Table 2. Current density to voltage (J−V) characteristics of these devices are presented in Figure 4a. As one might expect, the trend of the obtained open circuit voltage (Voc) values is consistent with the magnitudes of the relative IPs of the donor thin films. For example, the DPM-based device showed a maximum Voc of 1.00 V, reflecting the fact that DPM possesses the lowest IP level. Furthermore, it is noteworthy that Voc decreases progressively along the order of uncapped dyes > dyes capped

Figure 4. (a) Spectral mismatch-corrected J−V characteristics (under 1 sun, AM1.5G illumination) and (b) EQE spectra of (□) DPM:C70, (○) TDPM:C70, (△) ADPM:C70, (▽) DBT:C70, (◇) TDBT:C70, and (⬡) ADPM:C70 solar cells.

with p-tolyl > dyes capped with p-anisolyl in both DPM and DBT series, indicating that the introduction of the p-tolyl or panisolyl group at the 2 position of DTP would raise the IPs and consequently reduce the Voc of the devices. The EQE spectra of the solar cells are shown in Figure 4b. It is obvious that the devices of using DBT dyes exhibit more red-shifted EQE spectra in comparison to their DPM counterparts, agreeing with the observed absorption spectra (see Figure S1 of the Supporting Information) as well as the k spectra (Figure 3). Despite the narrow spectral coverage, the DPM-based devices showed higher EQEs compared to those of DBT-based devices. Apparently, the structural modulation on the central A block as well as the terminal substitution gave an evident trade-off between the Voc and Jsc values. Among these devices, the ADBT-based cells was observed with the unexpected low Jsc value. This could be attributed to the energy levels misalignment between ADBT and PEDOT:PSS. The IP of ADBT 4365



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giving higher Jsc but lower Voc compared to those of pyrimidineembedded counterparts. Therefore, the structural modification on the central A block as well as the terminal substitution gave an evident trade-off between the Voc and Jsc values. Among these six new donors, the vacuum-processed device using TDPM composed of a p-tolyl terminal group and a D−A−A configuration of DTP−pyrimidine−dicyanovinylene as the electron donor combined with C70 as the electron acceptor delivered the best efficiency with a Jsc of 11.34 mA/cm2, a Voc of 0.94 V, and a FF of 0.52, and a PCE as high as 5.6%. We believe that the established structure−property−performance relationship based on DTP-based D−A−A donors can trigger new ideas for designing more promising small-molecule donors for achieving highly efficient SMOSCs.

(−5.0 eV), which is higher than that of PEDOT:PSS (−5.2 eV), could be regarded as a barrier for hole transporting. Consequently, regardless of the fact that ADBT possesses panchromatic absorption spectra across the UV−vis to nearinfrared (NIR) wavelength range, the devices show relatively lower performance, leaving room for the further improvement by structural modifications. On the other hand, it is noteworthy to mention that the FFs of TDPM- and TDBT-based devices are the highest within their individual groups, indicating that the p-tolyl group is the appropriate end-capping substitution in these molecular families. Therefore, the best performance was obtained from the solar cell using TDPM as an electron donor, exhibiting a balance between the photovoltage and photocurrent, as well as high FF, thus resulting in a PCE as high as 4.8% with a Voc of 0.94 V, Jsc of 10.29 mA/cm2, and FF of 0.50. Further tuning the thickness of the D/A mixed layer has been employed to pursue better performance of solar cells using TDPM as an electron donor. As the thickness of the D/A blended layer increased from 40 to 45 nm, the device parameters extracted from the J−V curves (Figure 5) are listed

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic methods, absorption spectra of all donor molecules in solution and thin film, UPS and IPES spectra, and copies of 1H and 13C nuclear magnetic resonance (NMR) spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Council of Taiwan (NSC 101-2113-M-002-009-MY3, NSC-102-2221-E-007-125MY3, and NSC-101-2112-M-007-017-MY3) and the Low Carbon Energy Research Center, National Tsing-Hua University, for financial support.

Figure 5. Mismatch-corrected J−V characteristics (under 1 sun, AM1.5G illumination) and EQE spectrum (inset) of a TDPM-based optimized device.

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in Table 2. The corresponding EQE spectrum shown in the inset of Figure 5 exhibits high plateaus of nearly or higher than 60% throughout the 400−600 nm wavelength range. With an optimized TDPM:C70 layer thickness (45 nm), the device delivers an impressive performance, with a Voc of 0.94 V, Jsc of 11.34 mA/cm2, FF of 0.52, and PCE up to 5.6%.

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CONCLUSION The coplanar DTP was adopted as the electron-donating (D) group to combine with the electron-withdrawing dicyanovinylsubstituted pyrimidine or benzothiadiazole to give new D−A− A-type donor molecules. The electrochemical and thermal stabilities as well as the light-absorption capability of DTPimplanted D−A−A molecules were improved upon introducing an aryl group (p-tolyl or p-anisolyl) onto the electroactive site of DTP. The broad absorption spectra and anisotropic molecular orientation observed in the thin films allow for these new D−A−A molecules to perform good capability of light harvesting. Besides the terminal aryl substitution, the central A unit (pyrimidine versus benzothiadiazole) was found to play a more crucial role for governing the properties and performance of DTP-based D−A−A donor molecules. Because of the quinoidal character, benzothiadiazole is beneficial for giving D−A−A molecules with red-shifted absorption, thus 4366



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