Donor–acceptor–π–acceptor based charge transfer chromophore as electron donors for solution processed small molecule organic bulk heterojunction solar cells

Organic Electronics 19 (2015) 76–82

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Donor–acceptor–p–acceptor based charge transfer chromophore as electron donors for solution processed small molecule organic bulk heterojunction solar cells P. Gautam a, R. Misra a,⇑, S.A. Siddiqui b, G.D. Sharma c,⇑ a

Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India Department of Electrical Engineering, Vivekanand Institute of Technology (VIT), NRI Road, Jagatpura, Jaipur, Rajasthan 303012, India c R & D Center for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College, Kukas, Jaipur, Rajasthan 302028, India b

a r t i c l e

i n f o

Article history: Received 1 December 2014 Received in revised form 13 January 2015 Accepted 23 January 2015 Available online 31 January 2015 Keywords: Small molecules Bulk heterojunction solar cells Power conversion efficiency Solvent additives

a b s t r a c t Two benzothiazole (BT) based donor–acceptor–p–acceptor (D–A–p–A) molecular system denoted as BT3 and BT4 have been designed, synthesized and their optical and electrochemical properties were investigated. The BT4 show wider absorption profile and lower bandgap as compared to BT3 due to the strong electron withdrawing ability of dicyanoquinodimethane (DCNQ) as compared to tetracyanobutadiene (TCBD). The solution processed bulk heterojunction solar cells were fabricated using BT3 and BT4 as electron donor and PC71BM as electron acceptor. The organic solar cells optimized dichloromethane (DCM) processed BT3:PC71BM (1:1) and BT4:PC71BM (1:1) showed PCE of 2.56% and 3.68%, respectively. The higher PCE of BT4:PC71BM is related to the wider absorption of the blend and better ordered domain sizes in the blend as compared to BT3:PC71BM. The devices processed with 1,8-diiodoctane (DIO) additives showed PCE of 3.77% and 5.27%, for BT3:PC71BM and BT4:PC71BM blends, respectively. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Bulk heterojunction, comprised of a blend of electron donor (polymers or small molecules) and electron acceptor (fullerene derivatives) have attracted considerable attention due to their potential for fabrication of low cost devices with high power conversion efficiency (PCE) [1]. The power conversion efficiencies (PCE) of polymer/fullerene BHJ solar cells have increased over the past two decades and approached 9.2% in single junction cells [2], and more than 10% for tandem solar cells [3]. In spite of the high PCE of these organic solar cells, polymer donor materials always suffer from batch to batch variations, difficulty ⇑ Corresponding authors. E-mail addresses: [email protected] (R. Misra), sharmagd_in @yahoo.com, [email protected] (G.D. Sharma). http://dx.doi.org/10.1016/j.orgel.2015.01.032 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

of purification, and polydispersity [4]. Compared to polymers, the small molecules have advantages including a well defined molecular structure, definite molecular weight and high purity without batch to batch variations [5], and have garnered lots of attention for solution processed organic BHJ solar cells [6]. Solution processed BHJ organic solar cells based on small donor molecules have shown outstanding PCE of over 8% in the recent years [7]. These results indicate that small molecule based BHJ organic solar cells have a great potential. At present, an impressive PCE of 12% has been achieved for a vacuum processed triple junction tandem small molecule organic solar cell (SMOSC) [8]. In order to obtain high PCE in organic solar cells based on small molecules, the synthesis of low bandgap small molecules containing electron donating (D) and electron accepting (A) groups have become a widely used strategy

P. Gautam et al. / Organic Electronics 19 (2015) 76–82

over the past few years [9]. In the case of D–A based organic material, the intramolecular charge transfer (ICT) between the D and A unit in the material effectively extends the light absorption to the near infrared (NIR) region of the solar spectrum. However, it decreases the absorption of the material in the visible region resulting in the reduction in short circuit current density (Jsc) [10]. This issue can be overcome by incorporating an additional electron rich (1A/2D system) or electron deficient unit (2A/D system) as a third unit in the material, which can extend the range of light absorption through the appearance of new p ? p⁄ or ICT peak, respectively. Moreover, the Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be controlled by tuning the strength of the three different units because the degree of electron donating and accepting ability affects the HOMO and LUMO energy levels, respectively, of the materials. Two small molecules BT3 and BT4 having A1–p–A2–D molecular structure with the same A1 (benzothiazole), p-linker (phenyl) and D triphenylamine (TPA) but different A2, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BT3 and BT4 were synthesized, and their optical, electrochemical and theoretical properties were investigated. These small molecules were applied as donor along with PC71BM as acceptor for the fabrication of solution processed bulk heterojunction organic solar cells and showed PCE of 2.56% and 3.68% for BT3:PC71BM and BT4:PC71BM blends cast from DCM solvent. Through the incorporation of 1,8-diiodoctane (DIO) additive during the spin coating of active layer the PCE of organic solar cells has been enhanced up to 3.77% and 5.27%, for BT3:PC71BM and BT4:PC71BM blends, respectively.

2. Experimental details 2.1. Device fabrication and characterization The BHJ organic solar cells were prepared using indium tin oxide (ITO) coated glass substrate as anode, Al as cathode and a blended film of BT3 or BT4:PC71BM between the two electrodes as photoactive layer as follows. Firstly, ITO-coated glass substrates were cleaned with detergent, ultrasonicated in acetone and isopropyl alcohol, and subsequently dried in an oven for 12 h. An aqueous solution of PEDOT:PSS (Heraeus, Clevios P VP,Al 4083) was spin cast on the ITO substrates obtaining a film of about 40 nm thick. The PEDOT:PSS film was then dried for 10 min at a temperature of 120 °C in ambient conditions. Then, 10 mg/mL solutions of BT3 or BT4/PC71BM blends in different solvents were prepared with different weight ratios spun cast on the top of the PEDOT:PSS layer, and dried at 80 °C for 10 min. The solvents include dichloromethane (DCM) and DCM containing 1%, 2%, 3% and 4% (v%) DIO. The thickness of the photoactive layer was about 100 ± 10 nm. Finally 90 nm thick Al electrodes were deposited on the top of BHJ film under reduced pressure (<10 6 Torr). All the devices were fabricated and tested in ambient atmosphere without encapsulation. The active area of the devices is about 0.20 cm2.

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The current–voltage characteristics of the devices were measured using a computer controlled Keithley 238 source meter in the dark as well as under an illumination intensity of 100 mW/cm2. A xenon light source coupled with AM1.5 optical filter was used as light source to illuminate the surface of the devices. The incident photon to current efficiency (IPCE) of the devices were measured illuminating the device through the light source and monochromator and resulting current was measured using Keithley electrometer under short circuit condition. 3. Results and discussions 3.1. Synthesis and characterization of BT3 and BT4 The synthesis of donor–acceptor–p–acceptor (D–A–p–A) benzothiazoles (BTs) BT3 and BT4 are shown in Scheme 1. The condensation of 2-aminothiophenol with 4-bromobenzaldehyde in DMSO at 190 °C for 1 h resulted in bromo BT1 with 52% yield [9e]. The donor triphenylamine unit was incorporated via Pd-catalyzed Sonogashira crosscoupling reaction of BT1 with 4-ethynyl-N-N-diphenyaniline which resulted in compound BT2 with 70% yield. The [2 + 2] cycloaddition–retroelectrocyclization reaction of acetylene linked compound BT2 with TCNE and TCNQ yielded BT3 and BT4, respectively (Scheme 1) [9f]. The details of synthesis and characterization can be found in supporting information. 3.2. Optical and electrochemical properties The normalized absorption spectra of dilute solutions of BT3 and BT4 in dichloromethane (DCM) and of thin films deposited on quartz glass are shown in Fig. 1 and the summary of optical data, i.e. absorption peak wavelengths   (kmax) and optical bandgap Eopt are complied in Table 1. g The solution of BT3 exhibits two absorption bands with absorption peaks at 400 nm and 480 nm having molar extinction coefficients of 3.38  104 M 1 cm 1 and 2.82  104 M 1 cm 1, respectively, whereas BT4 exhibits two absorption bands having absorption peaks at 342 nm and 663 nm with molar extinction coefficients of 2.31  104 M 1 cm 1 and 2.05  104 M 1 cm 1 respectively, and an additional shoulder at 450 nm. Compared to BT3, BT4 exhibits a wider and red-shifted absorption band in the longer wavelength region, attributed to the strong electron withdrawing capability of DCNQ. The absorption spectra of these small molecules in thin films showed broadening and red-shifting of the optical absorption in the low energy band as compared to the absorption spectra in solution. This can be attributed to greater p-electron delocalization and enhanced inter-chromophore interactions in solid state [11]. The optical bandgaps were estimated from the onset of absorption spectra in long wavelength region and are 1.95 eV and 1.45 eV, for BT3 and BT4, respectively. The lower value of optical bandgap may be attributed to the strong electron withdrawing nature of DCNQ unit in BT4. The redox properties of BT3 and BT4 have been investigated in dichloromethane and the results are summarized

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Scheme 1. Synthesis of BT3 and BT4.

in Table 2. The relatively lower HOMO–LUMO gap for BT4 as compared to BT3 is ascribed to the lower value of LUMO of BT4. The trends of the HOMO–LUMO level is consistent with the trend observed in the optical gap. Moreover, the HOMO energy level of BT3 ( 5.44 eV) is deeper than that for BT4 ( 5.35 eV), which may be beneficial for high Voc in organic solar cells. Moreover, the energy difference in LUMO energy levels, i.e., BT3 ( 3.45 eV) or BT4 ( 3.67 eV) and PC71BM ( 4.0 eV) is higher than the exciton binding energy (0.3 eV), indicating there is sufficient driving force for exciton dissociation and the small molecules can be used as donor for the fabrication of solution processed BHJ organic solar cells. The electrochemical bandgaps for both the small molecules are higher than the corresponding optical bandgaps, which is commonly observed in the organic semiconductors. 3.3. Theoretical calculations

Fig. 1. UV–visible absorption spectra of (a) BT3 and (b) BT4 in DCM solution and thin film cast from DCM solvent.

In order to understand the electronic structure of donor–acceptor–p–acceptor (D–A–p–A) benzothiazoles BT3 and BT4, density functional theory (DFT) calculations were performed at the B3LYP/6-31G⁄⁄ level. The minimum-energy conformation and frontier orbitals are shown in Fig. 2. The largest coefficients in the HOMO orbital of BT3 and BT4 are delocalized on the electron donating TPA unit, and the electron density distribution of the LUMO orbital is located on the BT, TCBD and DCNQ acceptor units. The HOMO energies of BT3 and BT4 are 5.53 eV and 5.54 eV whereas the LUMO energies are 2.30 eV and 3.13 eV respectively. The results indicate that incorporation of TCBD and DCNQ unit results in similar HOMO energy level whereas the LUMO level is significantly lowered in BT4 due to the strong electron withdrawing DCNQ

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P. Gautam et al. / Organic Electronics 19 (2015) 76–82 Table 1 Optical data of the BT3 and BT4.

a b

1

cm

1 a

kabs (nm)/(absorption coefficient  104 cm

Compound

kabs (nm)/e (M

)

BT3 BT4

480 (28,212), 400 (33,815) 663 (20,483), 450 (shoulder), 342 (23,132)

1 b

)

405 (1.5), 508 (0.85) 346 (1.08), 457 (0.62) (shoulder), 695 (1.02)

Eopt (eV) g 1.95 1.45

In solution. Thin film.

Table 2 Electrochemical data of BT3 and BT4.

a b

Compound

Eox (V vs NHE)a

BT3 BT4

1.03 0.94

EHOMO (eV)a 5.44 5.35

ELUMO (eV) = EHOMO 3.45 3.67

Eelec

Eelec (eV)a 1.99 1.68

EHOMO (eV)b 5.53 5.54

ELUMO (eV)b 2.30 3.13

Eelec (eV)b 3.23b 2.41

Data from cyclic voltammetry. Computational data (frontier molecular orbitals of benzothiazoles at the B3LYP/6-31G** level for C, N, S, and H).

Fig. 2. HOMO and LUMO energy level of BT3 and BT4 calculated from B3LYP/6-31G⁄(d) level for C, H, N and S.

unit. The incorporation of strong acceptor DCNQ in BT4 results in greater stabilization of the LUMO level leading to low HOMO–LUMO gap. 3.4. Photovoltaic properties Based on the optical and electrochemical properties of BT3 and BT4, we focused our efforts on the incorporation of these small molecules as donor for the fabrication of

solution processed BHJ organic solar cells and subsequent optimization. The HOMO/LUMO energy levels of BT3 and BT4 make them excellent candidates for use with PC71BM as acceptor component [12]. We varied the weight ratio of these small molecules and PC71BM from 1:0.5, 1:1 and 1:1.5, and found that the optimized ratio is 1:1. Therefore we have limited our results only to this ratio. The absorption spectra of BT3:PC71BM and BT4:PC71BM blend films cast from DCM are shown in Fig. 3 which reveal that

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Fig. 3. UV–visible absorption spectra of BT3:PC71BM and BT4:PC71BM blended films cast from DCM.

BT3:PC71BM and BT4:PC71BM exhibit broad absorption spectra from 350 to 700 nm and 350 to 900 nm, respectively. The BT4:PC71BM film showed absorption over the entire UV visible spectrum that extends into the NIR, which is an attractive feature. The BHJ organic solar cells using BT3 and BT4 as electron donor materials and PC71BM as the electron acceptor material with a device structure of ITO/PEDOT:PSS/ donor:acceptor/Al were fabricated. The best result was observed for a donor:acceptor weight ratio of 1:1. The current–voltage (J–V) characteristics of the devices are shown in Fig. 4a and photovoltaic parameters, i.e. short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) and PCE are summarized in Table 3. The optimized device based on BT3:PC71BM exhibits a Jsc of 6.36 mA/cm2, Voc of 0.96 V, and FF of 0.42, which yield a PCE of 2.56% whereas the BT4:PC71BM based device exhibits a high PCE of 3.68% with a Jsc of 9.12 mA/cm2, Voc of 0.84 V and FF of 0.48. The incident photon to current efficiency (IPCE) curves of the BHJ devices based on these small molecules under monochromatic light is shown in Fig. 4b. The IPCE spectra of the devices closely resemble the absorption spectra of the corresponding blends, indicating that both small molecules and PC71BM contribute to the photogeneration processes [13]. The Jsc calculated from the integral of IPCE curves based on BT3 and BT4 were 6.24 mA/cm2 and 8.98 mA/cm2, respectively, consistent with the Jsc values obtained from J–V characteristics under illumination. The higher value of Jsc for the BT4 based device is attributed to the broader absorption spectra of the BT4:PC71BM blend as compared to BT3:PC71BM. The higher value of Voc for the BHJ organic solar cells based on BT3 is attributed to the deeper HOMO level of BT3 as compared to BT4, since the Voc in a BHJ device is directly related to the energy difference in LUMO of acceptor and HOMO of donor. The hole mobility of BT3 and BT4 in blends with PC71BM was measured using a space charge limited current model in the dark employing a hole-only diode (ITO/PEDOT:PSS/BT3 or BT4:PC71BM/Au) [14] (as shown in Fig. 5). The BT3 and BT4 exhibited hole mobilities

Fig. 4. (a) Current–voltage characteristics under illumination, and (b) IPCE spectra of BHJ organic solar cells based on BT3:PC71BM and BT4:PC71BM processed with DCM and DIO/DCM.

Table 3 Photovoltaic parameters for small molecule BHJ solar cells based on BT3 and BT4 as donor and PC71BM as acceptor with different processing solvents. Blend a

BT3:PC71BM BT4:PC71BMa BT3:PC71BMb BT4:PC71BMb a b

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

6.36 9.12 8.20 10.82

0.96 0.84 0.92 0.84

0.42 0.48 0.50 0.58

2.56 3.68 3.77 5.27

DCM cast film. DIO/DCM cast film.

6.7  10 6 and 9.4  10 6 cm2/V s, respectively. The difference in hole mobilities for BT3 and BT4 in the blends may also be attributed to the difference in the values of Jsc and PCE. We have also estimated the electron mobility using electron-only (ITO/Al/BT3 or BT4:PC71BM/Al) devices to be about 2.34  10 4 cm2/V s. To get more information about the crystallinity of BT3 and BT4, X-ray diffraction patterns of pristine BT3 and BT4 films as well as their blends with PC71BM were

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Fig. 7. XRD patterns of BT3:PC71BM and BT4:PC71BM thin films cast from DCM and DIO/DCM solvent. Fig. 5. Current–voltage characteristics of the devices processed with and without DIO additives for hole only devices, solid lines are SCLC fitting.

measured. XRD patterns of the pristine BT3 and BT4 are shown in Fig. 6. XRD patterns of BT3 and BT4 illustrate that both BT3 and BT4 have significant (1 0 0) diffraction peaks at 2h = 4.36° and 4.85°, respectively, resulting in corresponding d-spacings 19.4 Å and 16.8 Å, respectively. The smaller value of BT4 confirmed the tighter packing to BT4 as compared to BT3, may be due to the addition phenyl unit. The XRD patterns of blend films are shown in Fig. 7. It can be seen from this figure that the d-spacings of the blend molecules were identical to those of the pristine films, which suggest a more compact packing of BT4 in the blend. Moreover, the width of the half peak of the (1 0 0) of BT4 is narrower than that of BT3, indicating that BT4 has better ordered crystalline domains in the blends than BT3. In the case of BT3, due to the loosely stacked molecules, PC71BM can easily be inserted between BT3 molecules in the blend film and results in higher miscibility of BT3 and PC71BM, which has an adverse effect on the purity of crystalline domains [13] and thus leads to the broadening of the half peak of the (1 0 0). The purer crystalline domains of the donor material promote the escape of charges from the intermixed phase, which leads to less bimolecular recombination, helps in absorbing more

Fig. 6. XRD patterns of BT3 and BT4 thin films cast from DCM solvent.

photons, yielding a higher Jsc [13]. Therefore, the high value of Jsc for the device based on BT4 is mainly ascribed to the broader absorption profile and the denser and purer crystallinity of BT4 in the blend. Since the PCE of organic solar cells based on these small molecules is low as compared to the polymer solar cells, we have added 1,8-diiodoctane (DIO) [6a,7f] to the blend solution prior to spin coating. The J–V characteristics of the devices processed with 0.3% DIO (v/v)/DCM solvent under illumination are shown in Fig. 4a and photovoltaic parameters are complied in Table 3. The performance of organic solar cells based on BT3:PC71BM and BT4:PC71BM improved after addition of 0.3% DIO (v/v). The organic solar cell based on BT3:PC71BM showed a PCE of 3.77% with a Jsc of 8.20 mA/cm2, Voc of 0.92 V and FF of 0.50. The device fabricated from BT4:PC71BM blend processed with 0.3% DIO/ DCM showed a PCE of 5.27% with a Jsc of 10.82 mA/cm2, Voc of 0.84 V and FF of 0.58. The IPCE spectra of the devices are also shown in Fig. 4b. It can be seen from the IPCE spectra, the addition of DIO enhanced the IPCE peaks in both BT3:PC71BM and BT4:PC71BM, which is consistent with the increased values of Jsc and PCE. To get information about the effect of additive on the device performance, we measured an XRD pattern to characterize the crystalline nature of the BT4:PC71BM blend, shown in Fig. 7. It can be seen from XRD pattern that the film processed with DIO showed a more intense and narrower (1 0 0) peak than the film without DIO. This indicates that the additive improves the crystallization degree and purity of the crystalline materials [15]. The degree of crystallization influences the phase purity and interfacial area in the blend film, which influences charge recombination and charge separation, respectively. The addition of DIO enhanced the formation an interpenetrating network in the blend that may facilitate the exciton diffusion and charge separation, thereby producing high Jsc. We have also measured the hole mobility of BT3 and BT4 in the blend processed with DIO/DCM additive employing the J–V characteristics in dark for hole only devices (as shown in Fig. 5) and space charge limited current (SCLC) model [14]. We have used thickness and relative dielectric constant of the active layer as 90 nm and 3.5, respectively, for the calculation of hole mobility. The

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estimated values of hole mobility are 3.24  10 5 and 9.34  10 5 cm2/V s for BT3 and BT4, respectively. The increased value of hole mobility may be attributed to the enhancement in the crystallinity induced by the solvent additive. The increased values of hole mobilities are favorable for more balanced charge transport in the devices, leading to an enhancement in Jsc and PCE. 4. Conclusions As a continuation of our research work on the development of small molecules for use as electron donors in solution processed small molecule organic solar cells, we have synthesized two D–A–p–A small molecules BT3 and BT4 having A1–p–A2–D molecular structures with same A1 (benzothiazole), p-linker (phenyl) and D (TPA) but different A2, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BT3 and BT4, respectively, and investigated their optical, electrochemical and computational properties. BT4 has broader absorption profile and lower bandgap as compared to BT3, because of the electron withdrawing ability of DCNQ is stronger than TCBD. These small molecules were used as electron donor along with PC71BM for the fabrication of solution processed organic solar cells. The solar cell based on BT4 has higher Jsc and PCE as compared to BT3, resulting from the wider solar light absorption of BT4 than BT3. The PCE has been further improved up to 3.77% and 5.27% for the BT3:PC71BM and BT4:PC71BM blends, respectively, processed with DIO additive. The enhancement in PCE is mainly due to the increased value of Jsc and FF, attributed to the increase in the hole mobility induced by the crystallization effect, leading to more balanced charge transport.

[4] [5] [6]

[7]

[8]

[9]

Acknowledgments We are thankful to Department of Physics, LNMIIT, Jaipur and Material Science Research Laboratory, MNIT, Jaipur for providing the facilities for device fabrication and characterization. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2015.01.032.

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