1,1,4,4-Tetracyanobuta-1,3-diene Substituted Diketopyrrolopyrroles

Article pubs.acs.org/JPCC

1,1,4,4-Tetracyanobuta-1,3-diene Substituted Diketopyrrolopyrroles: An Acceptor for Solution Processable Organic Bulk Heterojunction Solar Cells Yuvraj Patil,† Rajneesh Misra,*,† M. L. Keshtov,‡ and Ganesh D. Sharma*,§ †

Department of Chemistry, Indian Institute of Technology, Indore, Madhya Pradesh 452020, India A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Street, 28, Moscow 119991, Russia § Molecular Electronics and Optoelectronics Device Research Laboratory Department of Physics, The LNM Institute of Information Technology, Jamdoli, Rajasthan 302031, India ‡

S Supporting Information *

ABSTRACT: Two small molecules composed of 1,1,4,4-tetracyanobuta1,3-diene substituted diketopyrrolopyrroles (DPPs) denoted as DPP5 and DPP6 were synthesized and their photophysical and electrochemical properties were investigated. The frontier molecular orbitals based on empirical relation between cyclic voltammetry redox potentials, experimental IP, and EA energies indicate that these two small molecules can be used as an electron acceptor for the polymer bulk heterojunction solar cells. The BHJ solar cells combined with a low band gap D−A copolymer P as an electron donor exhibits promising power conversion efficiency of 3.90% and 4.95%, with DPP5 and DPP6, respectively, after the optimization of active layers, indicating that these small molecules based on DPPs can be the alternative as an electron acceptor to replace fullerene, leading to the low-cost solution-processed polymer solar cells.

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which may enhance the Voc and Jsc of BHJ OSCs.19−27 When compared to the fullerene and its derivatives, nonfullerene small-molecule acceptors have wider spectral absorption that allows absorbing more sunlight to generate electricity. Recently, Yao et al. reported a power conversion efficiency (PCE) of 6.1% for solution processed BHJ organic solar cells using perylene diimide acceptor and conjugated polymer.28 More recently, Zhan et al. have reported a PCE of 6.8% for a device consisting of an electron acceptor based on a fused-ring core end-capped with 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (INCN) units and conjugated polymer without any treatment.29 Zhao et al. have reported PCE of 6.3% for the device with nonfullerene acceptor and low band gap copolymer.30 Many other promising nonfullerene acceptors have been used for solution processed organic solar cells with overall PCE in the range 4−8%.31−38 A variety of nonfullerene acceptors have been designed that have shown improved device efficiency. The dicyano (−CN) acceptors have been extensively employed in optoelectronic devices,15,39−43 which offer an alternative for electron acceptor.15,44−56 The −CN substituent can increase the electron affinity and promote the formation of crystallite architecture by

INTRODUCTION The solution processable derivatives of fullerene such as phenyl-C60/70 butyric acid methyl ester (PC60/70BM) have been widely used as an electron acceptor in bulk heterojunction (BHJ) solar cell due to their excellent electron acceptor properties,1−8 such as (i) high n-channel mobility, (ii) lowest unoccupied molecular orbital (LUMO), which is delocalized over the entire molecule and facilitates better electron transport, (iii) exhibiting reversible electrochemical reduction that results in stable reduced charge species, and (iv) formation of domains from solution processed film that are on the appropriate length scale for exciton dissociation with mixed phase with electron donor created. Despite these excellent properties, fullerene acceptors exhibit some drawbacks, such as weak absorption with poor tunability over the intense regions of solar spectrum, which limits the contribution to the photocurrent, morphological instability in thin film blends over time, leading to macroscopic crystallite formation and device degradation, and high synthetic costs.9 To address these problems, new acceptor materials from simple synthetic steps, high yield, and inexpensive synthetic processes for BHJ active layer used in organic solar cells (OSCs) are needed.10−12 Recently, substantial progress has been made in the design and synthesis of solution processed nonfullerene electron acceptors.13−18 The optical and electrochemical properties of nonfullerene acceptors can be easily tuned and tailored, © XXXX American Chemical Society

Received: December 16, 2015 Revised: March 3, 2016

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DOI: 10.1021/acs.jpcc.5b12307 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Chemical structure of DPP5, DPP6, and copolymer P.

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RESULTS AND DISCUSSION Synthesis and Characterization. The symmetrical and unsymmetrical TPA based 4 and 3 were synthesized by the reaction of (4-ethynylphenyl) diphenylamine with monobromo 1 and dibromo 2 under Pd-catalyzed Sonogashira crosscoupling reaction. The reaction of TPA based 3 and 4 under [2 + 2] cycloaddition−retroelectrocyclization reaction with tetracyanoethylene resulted in TCBD derivatives DPP5 and DPP6, respectively (Scheme S2, Supporting Information). Their photophysical, thermal, and electrochemical properties were explored. The push−pull character of these DPP small molecules can induce intramolecular charge transfer and extend the absorption spectra toward the near-infrared region of the solar spectrum. The [2 + 2] cycloaddition-retroelectrocyclization reaction of 3 and 4 with one and two equivalents of tetracyanoethylene (TCNE) at room temperature resulted D−A−D−A′−D′ and D′−A′−D−A−D−A′−D′ type TCBD derivatives DPP5 and DPP6 in 85% and 88% yield, respectively (Scheme S2, Supporting Information). The precursors monobromo 1 and dibromo 2 were synthesized by the reported procedure (Scheme S1, Supporting Information).76 The DPP5 and DPP6 were purified by silica-gel column chromatography and recrystallization techniques. The DPP5 and DPP6 are readily soluble in common organic solvents like dichloromethane, chloroform, toluene, tetrahydrofuran, and well characterized by 1 H NMR, 13C NMR, and HRMS techniques (Supporting Information). The compound with one TCBD unit is thermally more stable compared to a two TCBD-containing compound.77 The decomposition temperatures for DPPs 3, 4, 5, and 6 were found to be 402 °C, 398 °C, 357 °C, and 338 °C at 10% weight loss respectively (Figure S1, Supporting Information). The trend in the decomposition temperature follows the order 3 > 4 > 5 > 6, which reveals that the incorporation of TCBD lowers the thermal stability. The lower thermal stability of DPP6 compared to DPP5 is related to the additional TCBD unit. The thermal properties of DPP5 and DPP6 were also investigated by differential scanning calorimetry (DSC) at a heating rate of 10 °C min−1 under a nitrogen atmosphere (Figure S1, Supporting Information). In first round of heating,

secondary interaction and/or local dipole alignments that favor efficient charge transport.57 Recently, Park et al. reported dicyanodistyrylbenzene-based acceptor along with P3HT as the electron donor for solution-processed BHJ solar cells and achieved a moderate efficiency of 2.71%.58 The diketopyrrolopyrrole (DPP) can be easily functionalized at the amide−nitrogen atom, show strong light absorption, high thermal stability, and have been used as electron acceptors for organic solar cells.19,59−63 The DPP core is commonly substituted at 3 and 6 position with thienyl moiety (DPPTh), resulting in greater planarity of the molecular backbone and increased π−π interaction.12 The 1,1,4,4-tetracyanobuta-1,3diene (TCBD) unit exhibits a strong electron withdrawing character and imparts good solubility in all organic solvents because of the nonplanarity of the molecule,59 which leads to efficient solubility and prevents the formation of aggregates.60 The electron-accepting ability of the DPP core has been improved by incorporating the electron-withdrawing groups.61,62 The use of two strong withdrawing units in the small molecule makes it possible to obtain a n-type organic semiconductors, which can be utilized as nonfullerene electron acceptors for BHJ organic solar cells.19,63−73 The materials based on multiple acceptors possess excellent photophysical behaviors such as strong light absorption, coplanar structure, and good photochemical stability.74 Recently, Jo et al. have reported a PCE of 5% for the solution processed organic bulk heterojunction solar cell using an electron acceptor, 4,7-dithien2-yl-2,1,3-benzothiadiazole (DTBT), as a core unit and DPP as a flanking unit.75 In this article, we report the design, synthesis, and solar cell properties of unsymmetrical DPP5 and symmetrical DPP6 triphenylamine based TCBD substituted DPPs. The DPP5 and DPP6 exhibit low band gap with excellent absorption ability from 500−800 and 500−900 nm for DPP5 and DPP6, respectively. The copolymer that possess the complementary absorption (stronger absorption where DPPs have weak absorption) to these DPPs has been used as the donor to harvest the photon covering from visible to near-infrared region of solar spectrum. The devices based on optimized active layers of DPP5 and DPP6 as electron acceptor and conjugated D−A as electron donor showed over all PCE of 3.90% and 4.95%, respectively. B


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Figure 2. Normalized optical absorption spectra of DPP5 and DPP6 in solution and thin films.

Figure 3. CV (red line) and DPV (black line) plots of DPP5 and DPP6.

melting temperature was observed at 143 and 113 °C for DPP5 and DPP6, respectively. However, no apparent transitions were observed in the first round of cooling, which may be due to the low crystalline nature of compounds. Optical and Electrochemical Properties. The DPP5 and DPP6 were used as electron acceptors and copolymer P77 as the electron donor (chemical structures are shown in Figure 1) for the present investigation. The normalized optical absorption spectra of DPP5 and DPP6 in dilute tetrahydrofuran (THF) solution and thin film cast from THF solvent are shown in Figure 2. The absorption bands of DPP5 (λmax = 476 nm with molar extinction coefficient = 1.5 × 104 M−1 cm−1) and DPP6 (λmax = 480 nm with molar extinction coefficient of 2.6 × 104 M−1 cm−1) in the shorter wavelength region are attributed to the π−π* transition and are quite similar, whereas the absorption bands in the longer wavelength region for DPP5 (λmax = 683 nm with molar extinction coefficient of 1.7 × 104 M−1 cm−1) and DPP6 (λmax = 738 nm with molar extinction coefficient of 1.9 × 104 M−1 cm−1) are attributed to the intramolecular charge transfer (ICT) between donor and acceptor present in the molecules. The red shift in the ICT band for DPP6 compared to DPP5 indicates that the stronger

D−A interaction due to the presence of two TCBD units may be due to the larger dipole moment of DPP6 for intramolecular charge transfer. The thin film absorption spectra of DPP5 and DPP6 are broader and red-shifted compared to absorption spectra in solution indicating effective aggregation in the solid state due to strong intermolecular π−π interaction derived from their highly conjugated backbone. The optical band gaps of these DPP5 and DPP6 were estimated from the onset edge of absorption spectra in thin film and are 1.58 and 1.46 eV, respectively. The absorption spectrum of D−A copolymer P in solution shows two absorption bands;78 the band located in the shorter wavelength region (λmax = 396 nm) can be attributed to the π−π* transition of conjugated polymer main chains and the longer wavelength absorption band located at 500−700 nm (λmax = 528 nm) is caused by the ICT interaction between electron-rich benzo[1,2-b:4,5-b′]dithiophene (BDT) and electron-withdrawing PTQD units. In solid state film, the absorption spectrum of P(PTQD-BDT) shows a redshift, as compared with its absorption spectrum in solution. The main absorption peak is red-shifted to 556 nm and absorption edge extended to 712 nm as a result of more planar polymer chains C


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The Journal of Physical Chemistry C structure and/or the degree of interchain π-stacking in solid film. The absorption spectra of DPPs and P are complementary to each other and we expect that both P and DPPs are contributing the photogeneration process. The electrochemical properties of DPPs were explored by cyclic voltammetry and differential pulse voltammetry (CV and DPV) techniques in dichloromethane solvent using 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The CV and DPV plots of DPPs are shown in Figure 3 and the corresponding data are listed in Table 1. The TCBD-substituted unsymmetrical DPP5 shows

LUMO energy levels were calculated using following expressions ⎛ Fc ⎞ onset − E1/2⎜ + ⎟ + 4.8] E HOMO = −[Eox ⎝ Fc ⎠ ⎛ Fc ⎞ onset − E1/2⎜ + ⎟ + 4.8] E LUMO = −[Ered ⎝ Fc ⎠

where E1/2(Fc/Fc+) is the half wave potential of Fc/Fc+ couple (the oxidation potential of which is assumed to at 4.8 eV against the Ag/Ag+ electrode. The HOMO and LUMO energy levels were estimated from the onset oxidation and reduction potentials of the first oxidation and reduction peak, respectively.73−77 The HOMO/LUMO energy levels are −5.54/−4.22 eV and −5.64/−4.36 eV for DPP5 and DPP6, respectively. We have used a conjugated D−A copolymer denoted as P based on weak electron-donating BDT moiety and strong 9-(2-octyldodecyl)28H-pyrrolo[3,4-b]bisthieno[2,3f:3′,2′-h]quinoxa-line-8,10(9H)-dione (PTQD) unit as the electron donor. The copolymer P exhibits the HOMO and LUMO energy levels of −5.34 and −3.52 eV, respectively.78 The acceptors DPP5 and DPP6 exhibit large LUMO offset with P (0.70 and 0.84 eV for DPP5 and DPP6, respectively). Because the exciton binding energy in most of the organic semiconductors is 0.3−0.5 eV,79 the LUMO offset between DPP5 or DPP6 and P provides sufficient driving force for efficient exciton dissociation and effective electron transfer from donor to acceptor in the active layer. Additionally, the gaps between the LUMO of DPPs and the HOMO of P are 1.12 and 0.94 eV for DPP5 and DPP6, respectively, which could result in high open circuit voltage (Voc) of the resulting devices. The blending of P with either DPP5 or DPP6 results in significant fluorescence quenching of P (as shown in Figure 4), indicating effective photoinduced charge transfer occurred between P and DPPs in the blend film and most of the excitons generated is the active layer after the absorption of photons are dissociated.72 This effective photoinduced charge transfer is beneficial for producing short circuit current (Jsc) in OSCs. We have performed time-dependent density functional theory (TD-DFT) calculation and the results show that the ICT band arises from triphenylamine to the TCBD and DPP

Table 1. Electrochemical Properties of DPP5 and DPP6

DPP5 DPP6

E4 reda

E3 reda

E2 reda

E1 reda

E1 oxida

E2 oxida

E3 oxida

−0.91 −0.86

−0.53 −0.64

−0.30 −0.19

1.13 1.30

1.29 1.81

1.82

−1.50

a

The electrochemical analysis was performed in a 0.1 M solution of Bu4NPF6 in dichloromethane at 100 mV s−1 scan rate versus Ag/Ag+ at 25 °C.

three oxidation waves whereas symmetrical DPP6 shows only two oxidation waves. The first oxidation potential value follows the order DPP6 > DPP5, which indicates introduction of TCBD hardens the oxidation. Both the unsymmetrical and symmetrical TCBD derivatives (DPP5 and DPP6) show four reduction waves with two corresponding to the TCBD unit and two corresponding to the DPP unit. The incorporation of electron withdrawing TCNE in ethyne-bridged DPPs (3 and 4) show additional reduction waves in DPP5 and DPP6 corresponds to TCBD unit in CV and DPV. The reduction of TCBD derivatives (DPP5 and DPP6) is easier than ethynebridged DPPs (3 and 4) (as shown in Figure S2, Supporting Information) due to the presence of the TCBD unit. The reduction of DPP moiety is hardened by incorporation of TCBD. The potentials were measured versus Ag/Ag+ as a quasi reference electrode. After each experiment, the potential of the Ag/Ag+ electrode was calibrated against the Fc/Fc+ redox couple. The highest occupied molecular orbital (HOMO) and

Figure 4. Fluorescence spectra of pristine P and P:DPP5 and P:DPP6 thin films. D


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0.92 V and FF = 0.52) and 4.95% (10.28 mA/cm2, Voc = 0.86 and FF = 0.56) for P:DPP5 and P:DPP6, respectively (Table 3). The DPP5 based device showed higher Voc than that for DPP6, because the LUMO level of DPP5 (−4.22 eV) is higher than that of DPP6 (−4.36 eV). The value of Jsc for DPP6 based devices is higher than that for DPP5 based devices. To get information about the higher values of Jsc for DPP6, the incident photon-to-current efficiency (IPCE) spectra of these devices were measured. The IPCE spectra (Figure 6b) show that the values of IPCE of the DPP6 based devices is larger than that of DPP5 and response is broader for DPP6 as compared to DPP5. The IPCE spectrum of the devices closely resembles with the absorption spectra of the active layers of corresponding devices, indicating that both copolymer and DPPs are contributing the photocurrent generation. In the longer wavelength region, the photocurrent generation is due to the absorption of photon by DPPs whereas in the wavelength region between 350−600 nm, both P and DPPs contributes to the photocurrent generation. The Jsc values estimated from the integration of IPCE spectra are complied in Table 2, consistent with the values observed in J−V characteristics under illumination. In a BHJ organic solar cell, dissociation of photogenerated excitons at the donor/acceptor interface present in the active layer depends mainly on the free energy for charge transfer (CT) process, which is directly related to the energetic offset between excitonic and charge separated (CS) states82−84 and therefore, the free energy for photoinduced electron transfer (ΔGCT) between the electron donor (P) and electron acceptors DPP5 or DPP6 can be estimated by the expression, ΔGCT = ECT − Eg, where ECT is the charge transfer energy and Eg is the optical band gap (1.74 eV) of donor copolymer P.76 We have used the same empirical formula between ECT and Voc as used for polymer/fullerene systems85,86 that is, ECT = qVoc + 0.47 to estimate the ECT. The calculated ECT value for the device based on DPP5 (1.39 eV) is slightly higher than that for DPP6 (1.33 eV) based device and accordingly ΔGCT for DPP6 (−0.41 eV) is slightly higher for DPP5 (−0.35 eV). Therefore, only a slight difference between the values of ΔGCT between two devices cannot explain fully the difference between the photogeneration between the devices based on these two acceptors. But both DPP5 and DPP6 exhibit a low photon energy loss, defined as Eg − qVoc of 0.86 eV for DPP5 and 0.92 eV for DPP6 are higher than the optimal energy loss for exciton dissociation, indicating that these acceptors can have strong potential to replace the fullerene.87,88 The increase in Jsc may be related with the light harvesting ability of the active layer. To get information about the change in the absorption profile, we have measured the blended layer processed with SVA treatment and compared with counter ascast films. After SVA treatment, the absorption bands corresponds to either donor (P) or acceptors (DPPs) significantly stronger than as-cast blended films, attributed to the self-aggregation of donor and acceptor molecules because of the better phase separation, leading the enhancement in the light-harvesting ability of active layer. To get information about the charge carrier transport in the organic solar cells, we have measured the hole and electron mobilities in the blended thin films processed with and without SVA treatment and measured employing SCLC method. For hole only devices, structure of ITO/PEDOT:PSS/P:DPP5 or DPP6/Au was used. The large energy barrier (0.78−0.64 eV) between work function of Au (−5.0 eV) and LUMO energy

unit. The main transitions corresponding to CT occur from HOMO−1 to LUMO in DPP5, and HOMO−2 and HOMO to LUMO in DPP6 (Table 2), which support our Table 2. Calculated Electronic Transitions of DPP5 and DPP6 in the Gas Phase DPP5 DPP6 a

wavelength (nm)

composition

fa

664 462 732

HOMO−1→LUMO (−0.21) HOMO−1→LUMO+1 (0.69) HOMO−2→LUMO (0.50) HOMO→LUMO (0.49)

0.72 0.55 1.23

f = oscillator strength.

interpretation. In order to understand the geometry and the electronic structure of DPP5 and DPP6, DFT calculations were carried out using the Gaussian 09W program.80 The geometry optimization was carried out in gas phase and the frontier molecular orbitals (FMOs) are displayed in Figure 5. The TDDFT calculations were performed at the B3LYP/6-31G level.81 The main electronic transitions with composition, oscillator strengths, and assignments for DPP5 and DPP6 are as shown in Table 2. The LUMO of unsymmetrical DPP5 is localized on the DPP and TCBD unit and HOMO−1 is localized on the triphenylamine unit. The results of the TD-DFT calculation indicates that DPP5 and DPP6 show two main transitions in the visible region. In unsymmetrical DPP5, the ICT from triphenylamine to TCBD and DPP units corresponds to HOMO−1 to LUMO transition. The other π−π* transition in triphenylamine and TCBD units in the short wavelength region corresponds to HOMO−1 to LUMO+1 and HOMO−2 to LUMO transition. In symmetrical DPP6 HOMO is distributed over whole molecule whereas HOMO−2 is localized on two triphenylamine units. The charge transfer transitions in DPP6 at the long wavelength region from donor TPA to TCBD and DPP acceptors units corresponds to HOMO−2 → LUMO and HOMO → LUMO transition. The localization of LUMO on DPP and TCBD core in DPP5 and DPP6 indicates the acceptor nature of DPP and TCBD. The theoretical absorption wavelengths of DPP5 and DPP6 were found to be lower than that of experimental values. Photovoltaic Properties. We have used P as donor and DPP5 or DPP6 as an acceptor to fabricate BHJ polymer solar cells with structure ITO/PEDOT:PSS/P:DPP5 or DPP6/Al, where ITO is indium tin oxide and PEDOT:PSS is poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate). The concentration of donor to acceptor in the blended active layer is critical for overall PCE of organic solar cell, because there should be a balance between absorption and the charge transport within the active layer. We have fabricated the devices with different weight ratios of acceptor (DPP5 or DPP6) and donor (P) and found the optimized weight ratio of 1:2 with total concentration of 20 mg/mL, showed the best performance. The current−voltage (J−V) characteristics of the devices under illumination are shown in Figure 6a and corresponding photovoltaic parameters are summarized in Table 2. When THF was used as solvent for the preparation of the active layer thin films, the devices fabricated from optimized P:DPP5 and P:DPP6 showed overall PCE of 1.16% and 1.57%, respectively. To enhance the PCE of the devices further, solvent vapor annealing of THF cast films was adopted. The PCE was significantly improved up to 3.90% (Jsc = 8.15 mA/cm2, Voc = E


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Figure 5. FMOs of DPP5 and DPP6 estimated by DFT calculation at B3LYP level.

level of DPPs (−4.22 eV and −4.36 eV for DPP5 and DPP6, respectively) would suppress the electron injection from Au electrode and thus provide hole only device. However, for electron only devices the structure ITO/Al/P:DPP5 or DPP6/ Al was used. In this case, the electron injection barrier is negligible, while the large energy barrier (1.14 eV) between the work function of Al (−4.2 eV) and HOMO of P (−5.34 eV) would suppress the hole injection and thus behave as electron only device. The hole and electron mobilities of as-cast and

SVA treated P:DPP5 and P:DPP6 were estimated from the J− V characteristics as shown in Figure 7a (hole only) and 7b (electron only). Both the cast P:DPP5 and P:DPP6 exhibit similar hole mobilities of 2.68 × 10−5 cm2/(V s) and 2.56 × 10−5 cm2/(V s), respectively, while P:DPP6 thin film showed higher electron mobility of 7.84 × 10−6 cm2/(V s) than that for P:DPP5 (4.53 × 10−6 cm2/(V s)), indicating the symmetrical nature of DPP increases electron transport. The hole mobility (7.58 × 10−5 cm2 /(V s) and 6.86 × 10−5 cm2/(V s) for F


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Figure 6. (a) J−V curves and (b) IPCE spectra of BHJ organic solar cells fabricated using optimized P:DPP5 and P:DPP6 active layers.

Table 3. Photovoltaic Parameters of Devices Based on DPP5 and DPP6 Acceptors and Copolymer P as Electron Donor and the Optimized Weight Ratio is 1:2 active layer a

P:DPP5 P:DPP5b P:DPP6a P:DPP6b a

Jsc (mA/cm2)

Jsc (mA/cm2)c

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

3.88 8.15 5.82 10.28

3.76 8.23 5.91 10.21

0.88 0.92 0.82 0.86

0.34 0.52 0.38 0.56

1.16 3.90 1.57 4.95

54 32 46 22

253 386 292 412

Cast from THF. bSVA-treated active layer. cEstimated from the IPCE spectra.

Further studies were carried out by plotting the photocurrent density (Jph, Jph = JL − JD, where JL and JD are current densities under illumination and in dark, respectively) as a function of effective voltage (Veff, defined as Vo − Vappl,, where Vo is the voltage at which Jph is zero and Vappl is applied voltage)89−91 and shown in Figure 8. The calculated charge dissociation probabilities are 0.54, 0.58, 0.68, and 0.76 for P:DPP5 (as-cast), P:DPP6 (as-cast) P:DPP5 (SVA), and P:DPP6 (SVA) based devices, respectively, which could be the another factor of lower

DPP6:P and DPP5:P, respectively) and electron mobilities (1.45 × 10−5 cm2/(V s) and 4.34 × 10−5 cm2/(V s) for DPP5:P and DPP6:P, respectively) in the active layers were increased upon the SVA treatment leading to the better charge transport, which is beneficial to the enhancement in Jsc of the devices, therefore, the devices with SVA-treated active layers showed higher Jsc and FF compared to as-cast counterpart devices. G


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based devices. From J−V characteristics of the devices, we have estimated the series resistance (Rs) of OSC and found that Rs is smaller for the devices based on SVA treated active layers as compared to as-cast counterpart (Table 2). The adsorbed solvent may render relaxation and self-organization of molecules, which promotes intermolecular interaction and ordered packing94 and reduces the defects and Rs, resulting improvement in Voc.95,96 Figure 9 shows the TEM images of the as-cast and SVA processed P:DPP6 (1:2) thin films. When the film is cast from THF solvent, the active layer shows poor phase separation between acceptor and donor. However, when the as-cast thin film was treated with SVA process it showed nanoscale phase separation and both illumination and dark domains with the sizes of 25−30 nm. This is consistent with the increase in hole and electron mobilities and reduction of the recombination loss and increase the Jsc and FF. In order to get information effect of SVA treatment on the crystalline nature of the blended thin films, we have measured the X-ray diffraction (XRD) pattern of these blended thin films with and without SVA treatment (shown in Figure 10 only for P:DPP6). The as-cast blend film contains the diffraction peaks from both DPP6 (2θ = 5.83°) and P (2θ = 4.53°), respectively, suggesting that donor P and acceptor DPP6 are mixed without disturbing the crystalline organization of each domain. After the SVA treatment of blend film, the diffraction peak corresponding to P as well as for DPP6 increases. The above results demonstrate that SVA treatment promotes the crystallization and stacking of both P and DPP6 in the blend film resulting in the much better light harvesting and charge transport capability within the active layer.

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CONCLUSIONS We have synthesized two solution processable small molecules DPP5 and DPP6 as electron acceptors for organic bulk heterojunction solar cells. These small molecules exhibit excellent thermal stability, broad and strong absorption in NIR region complementary to conjugated polymer P, appropriate energy levels (estimated from the cyclic voltammetry and theoretical TD-DFT simulation) matching with P and moderate electron mobility. The organic solar cells based on solvent vapor annealed P:DPP5 and P:DPP6 showed PCE of 3.90% and 4.95%, respectively, which are higher than that for as-cast active layers. The broader IPCE spectrum of the devices, which closely resembles the absorption spectra of both DPPs and P, indicates that the both donor and acceptor are contributing to the photocurrent generation. These conclusions are based on an empirical relation between CV and experimental IP and EA. The results obtained in this study will be useful for design of nonfullerene acceptors for BHJ− organic solar cells.

Figure 7. J−V characteristics for (a) hole only and (b) electron only devices. The solid lines showed the SCLC fitting.

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EXPERIMENTAL SECTION The synthesis and characterization of DPP5 and DPP6 were described in Supporting Information. The ITO-coated glass substrates were cleaned by sonication in detergent, deionized water, acetone, and isopropyl alcohol and then dried in ambient atmosphere. Then, a thin layer (∼40 nm) of PEDOT:PSS was spin-coated on the precleaned ITO-coated glass substrates at 4000 rpm and baked at 110 °C for 15 min under ambient conditions. The active layer of blend of donor (copolymer P) and acceptor (DPP5 or DPP6) in different weight ratio in THF

Figure 8. Variation of photocurrent (Jph) with effective voltage (Veff) for the devices based in P:DPP5 and P:DPP6 active layers cast from THF and SVA treated.

values of IPCE for devices with SVA treated active layers. It is reported in literature that the lower charge dissociation probability is related to stronger germinate recombination at the donor−acceptor interface in the blended active layer.92,93 It can be seen from the table that the devices with SVA active layers exhibit higher Voc compared to that as-cast active layer H


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Figure 9. TEM images of P:DPP6 (1:2).

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AUTHOR INFORMATION

Corresponding Authors

*(R.M.) E-mail: [email protected]. *(G.D.S.) E-mail: [email protected]. Phone: 91-1415191741. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M. thanks to DST and CSIR, Government of India. Y.P. thanks to MHRD. G.D.S. is thankful to DST, Government of India (DST-RFBR Joint Research Project).

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Figure 10. X-ray diffraction pattern of as-cast and SVA treated P:DPP6 (1:2) films.

solution (15 mg/mL) with an approximate thickness of 85 ± 5 nm was spin coated on the top of PEDOT:PSS film. SVA was carried out by dropping 100 μL of THF around the as-cast active layer and covering with a Petri dish in a way that the film is exposed to the THF atmosphere for 60 s. The cathode aluminum (Al) electrodes were deposited in a vacuum chamber with base pressure of 1 × 10−6 Torr. The effective active area of the devices is about 20 mm2. The current−voltage (J−V) characteristics of photovoltaic devices were measured on a computer-controlled Keithley 238 source meter under 1 sun, AM 1.5 G spectrum from a solar simulator, consist of xenon lamp and optical filter as light source. The IPCE of the devices was measured at illuminating the device through the light source and monochromator and the resulting current was measured using a Keithley electrometer under short circuit condition.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12307. Text, figures, and tables giving TGA, DSC data, general experimental methods, 1H, 13C, and HRMS spectra of all new compounds. (PDF) I


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1,1,4,4-Tetracyanobuta-1,3-diene Substituted Diketopyrrolopyrroles

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