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Dicyanoquinodimethane-substituted benzothiadiazole for efficient small-molecule solar cells† Prabhat Gautam,a Rajneesh Misra*a and Ganesh D. Sharma*b Two unsymmetrical donor–acceptor–acceptor–p–acceptor type benzothiadiazoles (BTD3 and BTD4) functionalized with tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) modules, showing strong absorption in the visible region are reported. The bulk heterojunction solar cells based on BTD4:PC71BM
Received 13th January 2016, Accepted 9th February 2016 DOI: 10.1039/c6cp00243a
and BTD3:PC71BM based active layers processed with chloroform (CF), thermal annealing and subsequent solvent vapor annealing, i.e. two step annealing (TSA), exhibited PCEs of up to 6.02% and 5.36%, respectively, which is significantly higher than those of the corresponding devices based on the as-cast blend active layer. This enhancement is related to the improvement in exciton dissociation efficiency and more balanced charge
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transport in the devices based on the active layer processed with TSA treatment.
1. Introduction Organic solar cells (OSCs) based on a bulk heterojunction active layer are marvelous approach to convert solar energy into electrical energy and have gained considerable attention in last two decades, because of their prominent advantages of low cost, light weight and flexibility.1–3 A record power conversion efficiency (PCE) exceeding 10% has been achieved by single BHJ solar cells with soluble conjugated polymers4,5 via developing new polymer donors and morphology engineering, and 11.8% has been achieved for triple junction solar cells.6 However, the polydispersity of polymers affects the reproducibility of synthesis, purification, and the optical and electrochemical properties of final materials. On the other hand, conjugated small molecules (SMs) benefit from their well-defined molecular structure and definite molecular weight and have attracted the scientific community working in the field of OSCs.7–10 The PCEs of solution processed BHJ OSCs has reached up to 9–10%.11,12 One of the strategies to increase the PCE is the design of new organic SMs having low optical band gaps that can absorb solar energy in a broader wavelength region.13 In this regard, the current research on organic solar cells focuses on the a
Department of Chemistry, Indian Institute of Technology, Indore, MP, 452017, India. E-mail: [email protected] b Molecular Electronics and Optoelectronics Research Laboratory, Department of Physics, The LNM Institute of Information Technology (Deemed University), Jamdoli, Jaipur, Rajasthan, India. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Characterization data for BTD3 and BTD4, 1H, 13C NMR and HRMS spectra of new compounds. The DFT calculation data and the electrochemical data. See DOI: 10.1039/c6cp00243a
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design of new SMs built by connecting the electron donating (D) and accepting (A) units through a p-conjugated spacer. The strength of the D–A interaction is determined by the donor and acceptor moieties as well as connecting p-bridge. The photovoltaic parameters of the OSC using these SMs can be easily controlled by proper combination of donor and acceptor units and p-linkers.14 Our group has explored an unsymmetrical D–A–A–p–D type of benzothiadiazole based SM and used it as a donor for a solution processed BHJ, achieving a PCE of 4.61% after the optimization of solvent additive concentration.15,16 In search of new organic semiconductors, herein we report the synthesis of two SMs, BTD3 and BTD4, having a D–A1–A2–p–A3 molecular structure with the same D (TPA), A2 (benzothiadiazole), p-linker (ethynyl) and A3 (pyridine) but a different A1, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BTD3 and BDT4, respectively, and their optical and electrochemical properties were investigated. These SMs were applied as donors along with PC71BM as an acceptor for the fabrication of solution processed bulk heterojunction solar cells. The devices, based on optimized BTD3:PC71BM and BTD4:PC71BM processed with TSA treatment, showed PCEs of 5.36% and 6.02%, respectively, which are higher than those of the corresponding devices based on as-cast blends. The enhancement in the PCE has been mainly attributed to the increase in Jsc and FF, which are related with the improvement in exciton dissociation efficiency and more balanced charge transport.
2. Experimental details Synthesis of BTD3 and BTD4 The synthesis of the small molecules is described in the ESI.†
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Device fabrication and characterization The solution processed BHJ organic solar cells, using BTD3 and BTD4 as donors and PC71BM as electron acceptor, were fabricated with the device structure ITO/PEDOT : PSS/BTD3 or BTD4 : PC71BM (different weight ratios)/Al. The indium tin oxide (ITO) coated glass substrates were cleaned sequentially by ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol for 20 min. A layer of PEDOT:PSS was spin coated (3000 rpm, 40 nm thick) onto the ITO glass substrate and baked at 120 1C for 20 min. The active layer was spin cast from different weight ratio 1 : 05, 1 : 1, 1 : 1.5, 1 : 2 and 1 : 2.5 blends of BTD3 or BTD4 and PC71BM in chloroform (CF) solution. The total concentration of the mixture was 14 mg mL 1 in all blends and the films were spin cast at spinning speed of 2500 rpm. For the two step annealing (TSA) treatment of the active layer, first the active layer was placed on the hot plate at 120 1C for 5 min, for thermal annealing (TA) and then cooled down to room temperature; after that the active layer was kept in petri dish containing 100 mL of THF for 1 min. The average thickness of the active layers was about 85 5 nm. Finally, the 60 nm thick aluminum (Al) layer was deposited onto the active layer under high vacuum by a shadow mask to define the effective active area of 20 mm2. All the devices were fabricated and their characterizations were performed in ambient atmosphere without any encapsulation. The current–voltage ( J–V) characteristics of the BHJ organic solar cells were measured using
Chart 1
Fig. 1
a computer controlled Keithley 238 source meter in dark as well as under simulated AM1.5G illumination of 100 mW cm 2. A xenon light source coupled with an optical filter was used to give the stimulated irradiance at the surface of the devices. The incident photon to current efficiency (IPCE) of the devices was measured illuminating the device through the light source and monochromator and the resulting current was measured using a Keithley electrometer under short circuit condition.
3. Results and discussion Optical and electrochemical properties The chemical structures of BTD3 and BDT4 are shown in Chart 1. The detailed procedure for the synthesis of the SMs are described in Scheme S1 (see ESI†). The optical absorption spectra of BTD3 and BTD4 in dilute THF solution and thin films are shown in Fig. 1. In solution BTD3 shows two absorption bands with absorption peaks at 283 nm and 446 nm with molar extinction coefficients of 5.78 104 M 1 cm 1 and 6.16 104 M 1 cm 1, respectively, whereas BTD4 exhibits three absorption bands at 298 nm, 397 nm and 693 nm with molar extinction coefficients of 7.1 104 M 1 cm 1, 6.70 104 M 1 cm 1 and 3.71 104 M 1 cm 1, respectively. The absorption band in the shorter wavelength region results from localized p–p* transitions whereas the absorption band in the longer wavelength region is attributed
Chemical structure of BTD3 and BTD4.
Normalized optical absorption spectra of BTD3 and BTD4 in chloroform solution and thin film cast from chloroform solvent.
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to the intramolecular charge transfer (ICT) between donor and acceptor moieties present in the SMs. After replacing TCBD (BTD3) with a DCNQ (BTD4) unit, the ICT absorption band was red shifted to 693 nm with a molar extinction coefficient 3.71 104 M 1 cm 1. This strong redshift is attributed to the extension of the p-conjugation by replacing TCBD with DCNQ. The change of acceptor moiety from TCBD to DCNQ leads to a significant bathochromic shift due to the increase in the electron withdrawing strength associated with a reduced molar extinction coefficient. Compared to the solution spectra, the ICT absorption bands of both BTD3 and BTD4 in thin films were red-shifted with absorption bands at 474 nm and 722 nm for BTD3 and BTD4, respectively, and significantly extended toward lower energies, which is important for the light harvesting efficiency of the active layer used in OSCs. Additionally, a vibronic shoulder peak around 562 nm and 778 nm for BTD3 and BTD4 respectively was also observed which indicates that both SMs have stronger intermolecular p–p packing interactions with the molecular backbone in the solid state. The optical bandgaps of the molecules estimated from the absorption band edge in thin film decreased from 1.64 eV (BTD3) to 1.38 eV (BTD4), as a result of elongation of the p-system. On the basis of the broad absorption profile of BTD4 as compared to BTD3, this should be beneficial for achieving higher Jsc. The redox properties of BTD3 and BTD4 have been investigated by cyclic voltammetry (Fig. 2 and Table 1). The HOMO and LUMO energy levels of these SMs were calculated from the onset of the first oxidation and reduction wave, respectively, using EHOMO/ELUMO = (Eonset + 4.4 eV). The HOMO energy level for both SMs is almost same ( 5.56 eV and 5.50 eV for BTD3 and BTD4, respectively). The different LUMO levels ( 3.64 eV and 3.78 eV for BTD3 and BTD4, respectively) may be attributed to the different A1 (TCBD and DCNQ for BTD3 and BTD4, respectively). In D–A type organic materials, the LUMO is determined by the strength of the acceptor unit used. The DCNQ is a stronger acceptor than TCBD, and leads to a deeper LUMO energy level of BTD4 as compared to the BTD3. The LUMO offset between these SMs and PC71BM ( 4.1 eV) is higher than the exciton
Fig. 2
Table 1
Optical and electrochemical properties of BTD3 and BTD4
c Eoxd Eredd Small lmax (nm) lmaxb E opt g molecule e (M 1 cm 1)a (nm) (eV) (V) (V)
d E ele EHOMOe ELUMOe g (eV) (eV) (eV)
BTD3
283 (57 868) 446 (61 638)
285 474
1.64 1.28
0.35 1.69 0.73
5.45
3.2
BTD4
298 (71 092) 397 (67 048) 693 (37 102)
293 413 722
1.34 1.11
0.28 1.38 0.34
5.44
3.3
a
Absorption spectra were recorded in THF at 1 10 5 M concentration. b Absorption spectra of a thin film. c Calculated from the onset absorption edge of absorption spectra in a thin film. d Recorded by cyclic voltammetry, in 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in DCM at 100 mV s 1 scan rate versus SCE electrode, and for the irreversible redox process, the peak potential is quoted. e Estimated from DFT calculations.
binding energy, guaranteeing a sufficient driving force for efficient exciton dissociation at the D–A interface and photoinduced charge transfer. In order to investigate the electronic structure of BTD3 and BTD4, density functional theory (DFT) calculation was performed at the B3LYP/6-31G** level. The contours of the HOMO and LUMO of BTD3 and BTD4 are shown in Fig. 3. The theoretical results indicate the following: (a) the HOMO orbitals in BTD3 and BTD4 are localized over the triphenylamine donor, whereas the LUMOs are delocalized over the benzothiadiazole, the TCBD/DCNQ and the pyridine unit. (b) The calculated HOMO and LUMO energies of the ground state optimized geometry of the BTD3 were 5.45 and 3.20 eV, whereas for BTD4 they were 5.44 and 3.30 eV, respectively. (c) The optimized geometries of BTD3 and BTD4 show that the ethynyl linked pyridine acceptor unit adopts a planar orientation with the BTD core. Photovoltaic properties The organic BHJ solar cells were fabricated using the BTD3 or BTD4 as the donor along with PC71BM as the electron acceptor with a conventional solar cell structure of ITO/PEDOT:PSS/ BTD3 or BTD4:PC71BM/Al. It is well known that the relative
Cyclic voltammograms of BTD3 and BTD4 at 0.01 M concentration in 0.1 M TBAPF6 in dichloromethane, recorded at a scan rate of 100 mV s 1.
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Fig. 3
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Frontier molecular orbitals of BTD3 (top) and BTD4 (bottom).
concentration of donor and acceptor used in the BHJ active layer strongly influences the over all PCE of a BHJ solar cell and there should be a balance between the absorbance of solar photons and the subsequent charge transporting processes in the active layer. When the concentration of acceptor is low, the electron transporting ability will be limited, whereas with a higher amount of acceptor concentration in the blend, the absorbance of the active layer will be inadequate, since the donor component is mainly responsible for the absorption of light. In addition to absorbance the hole transporting ability in the active layer also decreased. The performances of the devices were optimized by varying the weight ratios of the SM and PC71BM and it was found that the optimized weight ratio was 1 : 2. We have further employed a two step annealing process (TSA), i.e. thermal annealing followed by the solvent vapor annealing. The normalized absorption spectra of BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) blends are shown in Fig. S1 (see ESI†), showing a combination of both donor (BTD3 or BTD4) and acceptor absorption features. The J–V characteristics under illumination (AG1.5, 100 mW cm 2) and incident photon to current efficiency (IPCE) spectra of the optimized BHJ organic solar cells are shown in Fig. 4 (upper part) and the photovoltaic parameters are summarized in Table 2. The device based on the optimized BTD3:PC71BM and BTD4:PC71BM active layers processed with CF (chloroform) showed PCEs of 2.24% ( Jsc = 6.34 mA cm 2, Voc = 0.98 V and FF = 0.36) and 3.40% ( Jsc = 8.86 mA cm 2, Voc = 0.96 V and FF = 0.40), respectively. The IPCE spectra of the devices closely resemble the absorption spectra of the corresponding BHJ active layers (Fig. S1, ESI†) which indicates that both PC71BM and donor (BTD3 and BTD4) are contributing to the photocurrent generation. The higher value of the PCE for BTD4:PC71BM has been attributed to the higher values of both Jsc and FF. The higher value of Jsc is attributed to the lower value of the optical band gap of BTD4, due to the improved solar spectrum coverage of BTD4:PC71BM, as evidenced by its absorption spectrum. Moreover, in the IPCE spectra, the IPCE spectrum is broader for BTD4:PC71BM than
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that for BTD3:PC71BM which also favors the larger value of Jsc for the device based on BTD4:PC71BM active layer. The fact that the values of Voc for the devices based on BTD3 and BTD4 as the donor are almost same, may be attributed to the same HOMO energy level for both the small molecules, since the Voc for BHJ organic solar cells is directly related to the energy difference between the donor and acceptor components employed in the active layer. The reason for the slightly higher value of Voc for the BTD3 based device (0.98 V) than the BTD4 one (0.96 V) may be the deeper HOMO energy level of BTD3. The difference in Voc of two device is only 0.02 V, where as the difference in the HOMO levels of BTD3 and BTD4 is 0.06 eV; the reduced Voc difference may be due to the voltage losses at the interfaces between the active layer and the cathode and anode. In order to improve the overall PCE of our devices based on the optimized active layers, we have employed a two step annealing (TSA) treatment of the active layer before the deposition of the final Al electrode as reported in the literature for SM solar cells.17,18 Firstly, we have tested the photovoltaic performance for the devices when the active layer was only thermally annealed. The devices based on BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) show PCEs of 3.85% ( Jsc = 7.88 mA cm 2, Voc = 0.94 V and FF = 0.52) and 4.88% ( Jsc = 9.84 mA cm 2, Voc = 0.92 V and FF = 0.54), respectively. The J–V characteristics of the devices processed by TSA treatment are shown in Fig. 4a and the photovoltaic parameters are compiled in Table 2. After the TSA treatment of the active layer, the devices showed overall PCEs of 5.36% and 6.02% based on BTD3:PC71BM and BTD4:PC71BM based active layers, respectively. The improvement in the PCE is mainly due to the higher values of both Jsc and FF. The improvement in Jsc is consistent with the IPCE spectra (Fig. 4, lower part) of the corresponding devices based on the active layers processed with TSA treatment. The Jsc of the BHJ organic solar cell is directly related to the light harvesting capability of the active layer, exciton generation and their subsequent dissociation into electron and hole, and the transportation of holes and electrons through the active
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Fig. 4 J–V characteristics under illumination (upper part) and IPCE spectra (lower part) of the optimized devices based on as cast from chloroform and TSA treated active layers.
Table 2 Photovoltaic parameter of BHJ organic solar cells based on BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) active layers processed under different conditions
Active layer
Jsc Voc (mA cm 2) (V)
BTD3:PC71BMa 6.34 BTD3:PC71BMb 9.84 BTD4:PC71BMa 8.86 BTD4:PC71BMb 11.28 a
As cast from chloroform.
0.98 0.94 0.96 0.92 b
FF
PCE (%)
0.36 0.58 0.40 0.60
2.24 5.36 3.40 6.02
(2.13)c (5.28)c (3.32)c (5.94)c
Rs Rsh (O cm2) (O cm2) 68.23 24.5 54.35 18.46
266 435 316 639
TSA. c Average of five devices.
layer towards anode and cathode, respectively.19 The light harvesting capability of the device mainly depends upon the absorption profile of the active layer employed. In order to investigate the effect of TSA on the device performance, we have investigated the UV-visible spectra of the active layers and the IPCE spectra of the corresponding devices. The optical absorption spectra of the BTD3:PC71BM and BTD4:PC71BM blends processed
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under different conditions are shown in Fig. S3 (see ESI†). In comparison with the absorption spectrum of the as cast film, the absorption spectrum of the film with TSA treatment shows a redshift and also exhibits a vibronic shoulder around 625 nm and 784 nm for the BTD3:PC71BM and BTD4:PC71BM blends, respectively, which is related to enhanced p–p stacking. The TSA treated blend films showed increased absorption intensity thus resulting in higher Jsc for the corresponding devices. Moreover, as shown in the IPCE curves, a uniform increase of the spectral response across the whole wavelength region of measurement was observed for either of the devices with TSA treatment. This is consistent with the trend observed in the optical absorption spectra. The integrated values of Jsc from the integration of the IPCE spectra (9.76 mA cm 2 and 11.18 mA cm 2 for BTD3 and BTD4 based devices, respectively) are consistent with the values estimated from J–V characteristics. The charge transport properties of the active layer can also provide important information about the photovoltaic performance
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Fig. 5 J–V characteristics of the hole only devices based on as cast and TSA treated BTD4:PC71BM active layers.
of the BHJ organic solar cells, including FF which depends upon the carrier sweep-out rate relative to the bimolecular charge recombination rate.20 The hole and electron mobilities of the BHJ active layer were measured using space charge limited current method employing a hole only device (ITO/PEDOT:PSS/active layer/Au) and an electron only device ITO/Al/active layer/Al, respectively. The J–V measured curves for the hole only devices using optimized BTD4:PC71BM processed with as cast and TSA treated films are shown in Fig. 5. Similar curves were also observed for the BTD3:PC71BM based devices. The hole and electron mobilities for the active layers are compiled in Table S1 (see ESI†). The hole mobility of active layer processed with TSA treatment is higher than that for the as cast active layer. However, the electron mobility is not influenced by the processing treatment of the active layer. The enhancement in the hole mobility and almost the same electron mobility, leads to more balanced charge transport within the active layer resulting in improvement in both FF and Jsc. To understand the increase in the Jsc of the devices based on active layers processed with TSA treatment, we analyzed the differences in generation, transportation and recombination of free charge carriers. In order to get information about the difference in the Jsc values in the devices with and without processing treatment of the active layers, we have studied the charge photogeneration in the devices. Fig. 6 shows the variation of photocurrent density ( Jph) with internal effective voltage (Veff) for the devices based on BTD4:PC71BM active layers as cast and with TSA treatment. Similar trends in the device based on BTD3:PC71BM were also observed. The measured photocurrent is given by Jph = JL JD, where JL is current density under illumination and JD is current density under darkness. The Veff is defined as Veff = Vo Vapp20 and corresponds to the strength of the internal electric field within the device to extract the free charge carriers. It can be seen from this figure that the Jph linearly increases with Veff at low voltage and Jph starts to saturate for large reverse bias (Veff 4 0.7 V) and almost all the
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Fig. 6 Variation of Jph with effective voltage (Veff) for the devices based on the as cast and TSA treated BTD4:PC71BM active layer.
free charge carriers were extracted by the applied field and collected by the electrodes. In the case of the device based on the active layer processed without any treatment, i.e. as cast, the Jph shows a stronger field dependence across the large bias range and has not fully saturated, even at Veff B 3.0 V, suggesting a significant germination and/or bimolecular recombination and less efficient charge collection at the electrodes, thus a lower FF.21 However, for the device processed with TSA treatment, Jph value starts saturation at 0.53 V, reaching a saturation value at 3.0 V. The saturated values for Jph for the as cast, and TSA processed films are 12.97 mA cm 2 and 15.23 mA cm 2, respectively, suggesting that the charge generation in the devices are influenced by the TSA treatment. We have also estimated the maximum exciton generation rate (Gmax) and exciton dissociation probability Pc for these devices, using Jphsat = qGmaxL and Pc = Jphsat/Jsc, respectively. The values of Gmax estimated from the above expression are 9.0 1027 m3 s 1 and 1.07 1028 m3 s 1, for the as cast and TSA treated active layers, respectively. This difference is attributed to the change in the absorption profile of the active layer as evidenced by the absorption spectra of the blend (Fig. 3). The estimated values of Pc for the devices are 0.68 and 0.86, for the as cast and TSA active layers, respectively. The enhanced value of Pc may be attributed to the balanced charge transport as indicated in the higher value of the hole mobility induced by the combined effect of thermal and solvent annealing treatment. The variation of Jsc with illumination intensity for the as cast and TSA treated blend based devices was measured to understand the charge recombination in the devices and is shown in Fig. S2 (ESI†). In general Jsc follows a power dependence on the illumination intensity Plight as Jsc p Palight. If the value of a is unity then weak charge carrier losses occur due to bimolecular recombination. The observed power law exponent (a) was 0.82 and 0.91 for as cast, TA and TSA based devices, respectively. The increased value of a for TSA as compared to the as cast processed film means that there is a significant reduction in the recombination loss in the TSA processed device. This can
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also explain the lower FF of the device based on the as cast blend with a high series resistance (Rs) and low shunt resistance (Rsh) as compared to other devices (as shown in Table 2).
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4. Conclusions In conclusion, we have designed two molecular structures of the type D–A1–A2–p–A3 with the same D (TPA), A2 (benzothiadiazole), p-linker (ethynyl) and A3 (pyridine) but different A1, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BTD3 and BDT4, respectively, and investigated their optical and electrochemical properties. BTD4 showed a broader absorption profile and a lower optical bandgap than those of BTD3, due to the stronger electron withdrawing ability of DCNQ than TCBD. These SMs were used as electron donors along with PC71BM as an electron acceptor for the fabrication of solution processed BHJ organic solar cells. The solar cell based on BTD4 showed a higher PCE (3.40%) than that of BTD3 (2.24%), attributed to its broader absorption profile, leading to a greater light harvesting ability of the BTD4:PC71BM active layer as compared to BTD3:PC71BM. The PCEs of the devices improved to 5.36 and 6.02% for the TSA treated BTD3:PC71BM and BTD4:PC71BM active layers, respectively. The enhancement in PCE is mainly due to the increased values of Jsc and FF, attributed to the increase in hole mobility leading to more balanced charge transport.
Acknowledgements RM thanks CSIR, and DST, New Delhi for financial support. PG thanks CSIR for SRF.
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Dicyanoquinodimethane-substituted benzothiadiazole for efficient small-molecule solar cells† Prabhat Gautam,a Rajneesh Misra*a and Ganesh D. Sharma*b Two unsymmetrical donor–acceptor–acceptor–p–acceptor type benzothiadiazoles (BTD3 and BTD4) functionalized with tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) modules, showing strong absorption in the visible region are reported. The bulk heterojunction solar cells based on BTD4:PC71BM
Received 13th January 2016, Accepted 9th February 2016 DOI: 10.1039/c6cp00243a
and BTD3:PC71BM based active layers processed with chloroform (CF), thermal annealing and subsequent solvent vapor annealing, i.e. two step annealing (TSA), exhibited PCEs of up to 6.02% and 5.36%, respectively, which is significantly higher than those of the corresponding devices based on the as-cast blend active layer. This enhancement is related to the improvement in exciton dissociation efficiency and more balanced charge
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transport in the devices based on the active layer processed with TSA treatment.
1. Introduction Organic solar cells (OSCs) based on a bulk heterojunction active layer are marvelous approach to convert solar energy into electrical energy and have gained considerable attention in last two decades, because of their prominent advantages of low cost, light weight and flexibility.1–3 A record power conversion efficiency (PCE) exceeding 10% has been achieved by single BHJ solar cells with soluble conjugated polymers4,5 via developing new polymer donors and morphology engineering, and 11.8% has been achieved for triple junction solar cells.6 However, the polydispersity of polymers affects the reproducibility of synthesis, purification, and the optical and electrochemical properties of final materials. On the other hand, conjugated small molecules (SMs) benefit from their well-defined molecular structure and definite molecular weight and have attracted the scientific community working in the field of OSCs.7–10 The PCEs of solution processed BHJ OSCs has reached up to 9–10%.11,12 One of the strategies to increase the PCE is the design of new organic SMs having low optical band gaps that can absorb solar energy in a broader wavelength region.13 In this regard, the current research on organic solar cells focuses on the a
Department of Chemistry, Indian Institute of Technology, Indore, MP, 452017, India. E-mail: [email protected] b Molecular Electronics and Optoelectronics Research Laboratory, Department of Physics, The LNM Institute of Information Technology (Deemed University), Jamdoli, Jaipur, Rajasthan, India. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Characterization data for BTD3 and BTD4, 1H, 13C NMR and HRMS spectra of new compounds. The DFT calculation data and the electrochemical data. See DOI: 10.1039/c6cp00243a
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design of new SMs built by connecting the electron donating (D) and accepting (A) units through a p-conjugated spacer. The strength of the D–A interaction is determined by the donor and acceptor moieties as well as connecting p-bridge. The photovoltaic parameters of the OSC using these SMs can be easily controlled by proper combination of donor and acceptor units and p-linkers.14 Our group has explored an unsymmetrical D–A–A–p–D type of benzothiadiazole based SM and used it as a donor for a solution processed BHJ, achieving a PCE of 4.61% after the optimization of solvent additive concentration.15,16 In search of new organic semiconductors, herein we report the synthesis of two SMs, BTD3 and BTD4, having a D–A1–A2–p–A3 molecular structure with the same D (TPA), A2 (benzothiadiazole), p-linker (ethynyl) and A3 (pyridine) but a different A1, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BTD3 and BDT4, respectively, and their optical and electrochemical properties were investigated. These SMs were applied as donors along with PC71BM as an acceptor for the fabrication of solution processed bulk heterojunction solar cells. The devices, based on optimized BTD3:PC71BM and BTD4:PC71BM processed with TSA treatment, showed PCEs of 5.36% and 6.02%, respectively, which are higher than those of the corresponding devices based on as-cast blends. The enhancement in the PCE has been mainly attributed to the increase in Jsc and FF, which are related with the improvement in exciton dissociation efficiency and more balanced charge transport.
2. Experimental details Synthesis of BTD3 and BTD4 The synthesis of the small molecules is described in the ESI.†
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Device fabrication and characterization The solution processed BHJ organic solar cells, using BTD3 and BTD4 as donors and PC71BM as electron acceptor, were fabricated with the device structure ITO/PEDOT : PSS/BTD3 or BTD4 : PC71BM (different weight ratios)/Al. The indium tin oxide (ITO) coated glass substrates were cleaned sequentially by ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol for 20 min. A layer of PEDOT:PSS was spin coated (3000 rpm, 40 nm thick) onto the ITO glass substrate and baked at 120 1C for 20 min. The active layer was spin cast from different weight ratio 1 : 05, 1 : 1, 1 : 1.5, 1 : 2 and 1 : 2.5 blends of BTD3 or BTD4 and PC71BM in chloroform (CF) solution. The total concentration of the mixture was 14 mg mL 1 in all blends and the films were spin cast at spinning speed of 2500 rpm. For the two step annealing (TSA) treatment of the active layer, first the active layer was placed on the hot plate at 120 1C for 5 min, for thermal annealing (TA) and then cooled down to room temperature; after that the active layer was kept in petri dish containing 100 mL of THF for 1 min. The average thickness of the active layers was about 85 5 nm. Finally, the 60 nm thick aluminum (Al) layer was deposited onto the active layer under high vacuum by a shadow mask to define the effective active area of 20 mm2. All the devices were fabricated and their characterizations were performed in ambient atmosphere without any encapsulation. The current–voltage ( J–V) characteristics of the BHJ organic solar cells were measured using
Chart 1
Fig. 1
a computer controlled Keithley 238 source meter in dark as well as under simulated AM1.5G illumination of 100 mW cm 2. A xenon light source coupled with an optical filter was used to give the stimulated irradiance at the surface of the devices. The incident photon to current efficiency (IPCE) of the devices was measured illuminating the device through the light source and monochromator and the resulting current was measured using a Keithley electrometer under short circuit condition.
3. Results and discussion Optical and electrochemical properties The chemical structures of BTD3 and BDT4 are shown in Chart 1. The detailed procedure for the synthesis of the SMs are described in Scheme S1 (see ESI†). The optical absorption spectra of BTD3 and BTD4 in dilute THF solution and thin films are shown in Fig. 1. In solution BTD3 shows two absorption bands with absorption peaks at 283 nm and 446 nm with molar extinction coefficients of 5.78 104 M 1 cm 1 and 6.16 104 M 1 cm 1, respectively, whereas BTD4 exhibits three absorption bands at 298 nm, 397 nm and 693 nm with molar extinction coefficients of 7.1 104 M 1 cm 1, 6.70 104 M 1 cm 1 and 3.71 104 M 1 cm 1, respectively. The absorption band in the shorter wavelength region results from localized p–p* transitions whereas the absorption band in the longer wavelength region is attributed
Chemical structure of BTD3 and BTD4.
Normalized optical absorption spectra of BTD3 and BTD4 in chloroform solution and thin film cast from chloroform solvent.
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to the intramolecular charge transfer (ICT) between donor and acceptor moieties present in the SMs. After replacing TCBD (BTD3) with a DCNQ (BTD4) unit, the ICT absorption band was red shifted to 693 nm with a molar extinction coefficient 3.71 104 M 1 cm 1. This strong redshift is attributed to the extension of the p-conjugation by replacing TCBD with DCNQ. The change of acceptor moiety from TCBD to DCNQ leads to a significant bathochromic shift due to the increase in the electron withdrawing strength associated with a reduced molar extinction coefficient. Compared to the solution spectra, the ICT absorption bands of both BTD3 and BTD4 in thin films were red-shifted with absorption bands at 474 nm and 722 nm for BTD3 and BTD4, respectively, and significantly extended toward lower energies, which is important for the light harvesting efficiency of the active layer used in OSCs. Additionally, a vibronic shoulder peak around 562 nm and 778 nm for BTD3 and BTD4 respectively was also observed which indicates that both SMs have stronger intermolecular p–p packing interactions with the molecular backbone in the solid state. The optical bandgaps of the molecules estimated from the absorption band edge in thin film decreased from 1.64 eV (BTD3) to 1.38 eV (BTD4), as a result of elongation of the p-system. On the basis of the broad absorption profile of BTD4 as compared to BTD3, this should be beneficial for achieving higher Jsc. The redox properties of BTD3 and BTD4 have been investigated by cyclic voltammetry (Fig. 2 and Table 1). The HOMO and LUMO energy levels of these SMs were calculated from the onset of the first oxidation and reduction wave, respectively, using EHOMO/ELUMO = (Eonset + 4.4 eV). The HOMO energy level for both SMs is almost same ( 5.56 eV and 5.50 eV for BTD3 and BTD4, respectively). The different LUMO levels ( 3.64 eV and 3.78 eV for BTD3 and BTD4, respectively) may be attributed to the different A1 (TCBD and DCNQ for BTD3 and BTD4, respectively). In D–A type organic materials, the LUMO is determined by the strength of the acceptor unit used. The DCNQ is a stronger acceptor than TCBD, and leads to a deeper LUMO energy level of BTD4 as compared to the BTD3. The LUMO offset between these SMs and PC71BM ( 4.1 eV) is higher than the exciton
Fig. 2
Table 1
Optical and electrochemical properties of BTD3 and BTD4
c Eoxd Eredd Small lmax (nm) lmaxb E opt g molecule e (M 1 cm 1)a (nm) (eV) (V) (V)
d E ele EHOMOe ELUMOe g (eV) (eV) (eV)
BTD3
283 (57 868) 446 (61 638)
285 474
1.64 1.28
0.35 1.69 0.73
5.45
3.2
BTD4
298 (71 092) 397 (67 048) 693 (37 102)
293 413 722
1.34 1.11
0.28 1.38 0.34
5.44
3.3
a
Absorption spectra were recorded in THF at 1 10 5 M concentration. b Absorption spectra of a thin film. c Calculated from the onset absorption edge of absorption spectra in a thin film. d Recorded by cyclic voltammetry, in 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in DCM at 100 mV s 1 scan rate versus SCE electrode, and for the irreversible redox process, the peak potential is quoted. e Estimated from DFT calculations.
binding energy, guaranteeing a sufficient driving force for efficient exciton dissociation at the D–A interface and photoinduced charge transfer. In order to investigate the electronic structure of BTD3 and BTD4, density functional theory (DFT) calculation was performed at the B3LYP/6-31G** level. The contours of the HOMO and LUMO of BTD3 and BTD4 are shown in Fig. 3. The theoretical results indicate the following: (a) the HOMO orbitals in BTD3 and BTD4 are localized over the triphenylamine donor, whereas the LUMOs are delocalized over the benzothiadiazole, the TCBD/DCNQ and the pyridine unit. (b) The calculated HOMO and LUMO energies of the ground state optimized geometry of the BTD3 were 5.45 and 3.20 eV, whereas for BTD4 they were 5.44 and 3.30 eV, respectively. (c) The optimized geometries of BTD3 and BTD4 show that the ethynyl linked pyridine acceptor unit adopts a planar orientation with the BTD core. Photovoltaic properties The organic BHJ solar cells were fabricated using the BTD3 or BTD4 as the donor along with PC71BM as the electron acceptor with a conventional solar cell structure of ITO/PEDOT:PSS/ BTD3 or BTD4:PC71BM/Al. It is well known that the relative
Cyclic voltammograms of BTD3 and BTD4 at 0.01 M concentration in 0.1 M TBAPF6 in dichloromethane, recorded at a scan rate of 100 mV s 1.
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Fig. 3
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Frontier molecular orbitals of BTD3 (top) and BTD4 (bottom).
concentration of donor and acceptor used in the BHJ active layer strongly influences the over all PCE of a BHJ solar cell and there should be a balance between the absorbance of solar photons and the subsequent charge transporting processes in the active layer. When the concentration of acceptor is low, the electron transporting ability will be limited, whereas with a higher amount of acceptor concentration in the blend, the absorbance of the active layer will be inadequate, since the donor component is mainly responsible for the absorption of light. In addition to absorbance the hole transporting ability in the active layer also decreased. The performances of the devices were optimized by varying the weight ratios of the SM and PC71BM and it was found that the optimized weight ratio was 1 : 2. We have further employed a two step annealing process (TSA), i.e. thermal annealing followed by the solvent vapor annealing. The normalized absorption spectra of BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) blends are shown in Fig. S1 (see ESI†), showing a combination of both donor (BTD3 or BTD4) and acceptor absorption features. The J–V characteristics under illumination (AG1.5, 100 mW cm 2) and incident photon to current efficiency (IPCE) spectra of the optimized BHJ organic solar cells are shown in Fig. 4 (upper part) and the photovoltaic parameters are summarized in Table 2. The device based on the optimized BTD3:PC71BM and BTD4:PC71BM active layers processed with CF (chloroform) showed PCEs of 2.24% ( Jsc = 6.34 mA cm 2, Voc = 0.98 V and FF = 0.36) and 3.40% ( Jsc = 8.86 mA cm 2, Voc = 0.96 V and FF = 0.40), respectively. The IPCE spectra of the devices closely resemble the absorption spectra of the corresponding BHJ active layers (Fig. S1, ESI†) which indicates that both PC71BM and donor (BTD3 and BTD4) are contributing to the photocurrent generation. The higher value of the PCE for BTD4:PC71BM has been attributed to the higher values of both Jsc and FF. The higher value of Jsc is attributed to the lower value of the optical band gap of BTD4, due to the improved solar spectrum coverage of BTD4:PC71BM, as evidenced by its absorption spectrum. Moreover, in the IPCE spectra, the IPCE spectrum is broader for BTD4:PC71BM than
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that for BTD3:PC71BM which also favors the larger value of Jsc for the device based on BTD4:PC71BM active layer. The fact that the values of Voc for the devices based on BTD3 and BTD4 as the donor are almost same, may be attributed to the same HOMO energy level for both the small molecules, since the Voc for BHJ organic solar cells is directly related to the energy difference between the donor and acceptor components employed in the active layer. The reason for the slightly higher value of Voc for the BTD3 based device (0.98 V) than the BTD4 one (0.96 V) may be the deeper HOMO energy level of BTD3. The difference in Voc of two device is only 0.02 V, where as the difference in the HOMO levels of BTD3 and BTD4 is 0.06 eV; the reduced Voc difference may be due to the voltage losses at the interfaces between the active layer and the cathode and anode. In order to improve the overall PCE of our devices based on the optimized active layers, we have employed a two step annealing (TSA) treatment of the active layer before the deposition of the final Al electrode as reported in the literature for SM solar cells.17,18 Firstly, we have tested the photovoltaic performance for the devices when the active layer was only thermally annealed. The devices based on BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) show PCEs of 3.85% ( Jsc = 7.88 mA cm 2, Voc = 0.94 V and FF = 0.52) and 4.88% ( Jsc = 9.84 mA cm 2, Voc = 0.92 V and FF = 0.54), respectively. The J–V characteristics of the devices processed by TSA treatment are shown in Fig. 4a and the photovoltaic parameters are compiled in Table 2. After the TSA treatment of the active layer, the devices showed overall PCEs of 5.36% and 6.02% based on BTD3:PC71BM and BTD4:PC71BM based active layers, respectively. The improvement in the PCE is mainly due to the higher values of both Jsc and FF. The improvement in Jsc is consistent with the IPCE spectra (Fig. 4, lower part) of the corresponding devices based on the active layers processed with TSA treatment. The Jsc of the BHJ organic solar cell is directly related to the light harvesting capability of the active layer, exciton generation and their subsequent dissociation into electron and hole, and the transportation of holes and electrons through the active
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Fig. 4 J–V characteristics under illumination (upper part) and IPCE spectra (lower part) of the optimized devices based on as cast from chloroform and TSA treated active layers.
Table 2 Photovoltaic parameter of BHJ organic solar cells based on BTD3 : PC71BM (1 : 2) and BTD4 : PC71BM (1 : 2) active layers processed under different conditions
Active layer
Jsc Voc (mA cm 2) (V)
BTD3:PC71BMa 6.34 BTD3:PC71BMb 9.84 BTD4:PC71BMa 8.86 BTD4:PC71BMb 11.28 a
As cast from chloroform.
0.98 0.94 0.96 0.92 b
FF
PCE (%)
0.36 0.58 0.40 0.60
2.24 5.36 3.40 6.02
(2.13)c (5.28)c (3.32)c (5.94)c
Rs Rsh (O cm2) (O cm2) 68.23 24.5 54.35 18.46
266 435 316 639
TSA. c Average of five devices.
layer towards anode and cathode, respectively.19 The light harvesting capability of the device mainly depends upon the absorption profile of the active layer employed. In order to investigate the effect of TSA on the device performance, we have investigated the UV-visible spectra of the active layers and the IPCE spectra of the corresponding devices. The optical absorption spectra of the BTD3:PC71BM and BTD4:PC71BM blends processed
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under different conditions are shown in Fig. S3 (see ESI†). In comparison with the absorption spectrum of the as cast film, the absorption spectrum of the film with TSA treatment shows a redshift and also exhibits a vibronic shoulder around 625 nm and 784 nm for the BTD3:PC71BM and BTD4:PC71BM blends, respectively, which is related to enhanced p–p stacking. The TSA treated blend films showed increased absorption intensity thus resulting in higher Jsc for the corresponding devices. Moreover, as shown in the IPCE curves, a uniform increase of the spectral response across the whole wavelength region of measurement was observed for either of the devices with TSA treatment. This is consistent with the trend observed in the optical absorption spectra. The integrated values of Jsc from the integration of the IPCE spectra (9.76 mA cm 2 and 11.18 mA cm 2 for BTD3 and BTD4 based devices, respectively) are consistent with the values estimated from J–V characteristics. The charge transport properties of the active layer can also provide important information about the photovoltaic performance
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Fig. 5 J–V characteristics of the hole only devices based on as cast and TSA treated BTD4:PC71BM active layers.
of the BHJ organic solar cells, including FF which depends upon the carrier sweep-out rate relative to the bimolecular charge recombination rate.20 The hole and electron mobilities of the BHJ active layer were measured using space charge limited current method employing a hole only device (ITO/PEDOT:PSS/active layer/Au) and an electron only device ITO/Al/active layer/Al, respectively. The J–V measured curves for the hole only devices using optimized BTD4:PC71BM processed with as cast and TSA treated films are shown in Fig. 5. Similar curves were also observed for the BTD3:PC71BM based devices. The hole and electron mobilities for the active layers are compiled in Table S1 (see ESI†). The hole mobility of active layer processed with TSA treatment is higher than that for the as cast active layer. However, the electron mobility is not influenced by the processing treatment of the active layer. The enhancement in the hole mobility and almost the same electron mobility, leads to more balanced charge transport within the active layer resulting in improvement in both FF and Jsc. To understand the increase in the Jsc of the devices based on active layers processed with TSA treatment, we analyzed the differences in generation, transportation and recombination of free charge carriers. In order to get information about the difference in the Jsc values in the devices with and without processing treatment of the active layers, we have studied the charge photogeneration in the devices. Fig. 6 shows the variation of photocurrent density ( Jph) with internal effective voltage (Veff) for the devices based on BTD4:PC71BM active layers as cast and with TSA treatment. Similar trends in the device based on BTD3:PC71BM were also observed. The measured photocurrent is given by Jph = JL JD, where JL is current density under illumination and JD is current density under darkness. The Veff is defined as Veff = Vo Vapp20 and corresponds to the strength of the internal electric field within the device to extract the free charge carriers. It can be seen from this figure that the Jph linearly increases with Veff at low voltage and Jph starts to saturate for large reverse bias (Veff 4 0.7 V) and almost all the
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Fig. 6 Variation of Jph with effective voltage (Veff) for the devices based on the as cast and TSA treated BTD4:PC71BM active layer.
free charge carriers were extracted by the applied field and collected by the electrodes. In the case of the device based on the active layer processed without any treatment, i.e. as cast, the Jph shows a stronger field dependence across the large bias range and has not fully saturated, even at Veff B 3.0 V, suggesting a significant germination and/or bimolecular recombination and less efficient charge collection at the electrodes, thus a lower FF.21 However, for the device processed with TSA treatment, Jph value starts saturation at 0.53 V, reaching a saturation value at 3.0 V. The saturated values for Jph for the as cast, and TSA processed films are 12.97 mA cm 2 and 15.23 mA cm 2, respectively, suggesting that the charge generation in the devices are influenced by the TSA treatment. We have also estimated the maximum exciton generation rate (Gmax) and exciton dissociation probability Pc for these devices, using Jphsat = qGmaxL and Pc = Jphsat/Jsc, respectively. The values of Gmax estimated from the above expression are 9.0 1027 m3 s 1 and 1.07 1028 m3 s 1, for the as cast and TSA treated active layers, respectively. This difference is attributed to the change in the absorption profile of the active layer as evidenced by the absorption spectra of the blend (Fig. 3). The estimated values of Pc for the devices are 0.68 and 0.86, for the as cast and TSA active layers, respectively. The enhanced value of Pc may be attributed to the balanced charge transport as indicated in the higher value of the hole mobility induced by the combined effect of thermal and solvent annealing treatment. The variation of Jsc with illumination intensity for the as cast and TSA treated blend based devices was measured to understand the charge recombination in the devices and is shown in Fig. S2 (ESI†). In general Jsc follows a power dependence on the illumination intensity Plight as Jsc p Palight. If the value of a is unity then weak charge carrier losses occur due to bimolecular recombination. The observed power law exponent (a) was 0.82 and 0.91 for as cast, TA and TSA based devices, respectively. The increased value of a for TSA as compared to the as cast processed film means that there is a significant reduction in the recombination loss in the TSA processed device. This can
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also explain the lower FF of the device based on the as cast blend with a high series resistance (Rs) and low shunt resistance (Rsh) as compared to other devices (as shown in Table 2).
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4. Conclusions In conclusion, we have designed two molecular structures of the type D–A1–A2–p–A3 with the same D (TPA), A2 (benzothiadiazole), p-linker (ethynyl) and A3 (pyridine) but different A1, i.e. tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) for BTD3 and BDT4, respectively, and investigated their optical and electrochemical properties. BTD4 showed a broader absorption profile and a lower optical bandgap than those of BTD3, due to the stronger electron withdrawing ability of DCNQ than TCBD. These SMs were used as electron donors along with PC71BM as an electron acceptor for the fabrication of solution processed BHJ organic solar cells. The solar cell based on BTD4 showed a higher PCE (3.40%) than that of BTD3 (2.24%), attributed to its broader absorption profile, leading to a greater light harvesting ability of the BTD4:PC71BM active layer as compared to BTD3:PC71BM. The PCEs of the devices improved to 5.36 and 6.02% for the TSA treated BTD3:PC71BM and BTD4:PC71BM active layers, respectively. The enhancement in PCE is mainly due to the increased values of Jsc and FF, attributed to the increase in hole mobility leading to more balanced charge transport.
Acknowledgements RM thanks CSIR, and DST, New Delhi for financial support. PG thanks CSIR for SRF.
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