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Low Energy Gap Triphenylamine-Heteropentacene-Dicyanovinyl Triad for Solution-Processed Bulk-Heterojunction Solar Cells Amaresh Mishra, Christoph Wetzel, Rahul Singhal, Peter Bäuerle, and Ganesh D. Sharma J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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The Journal of Physical Chemistry

Low Energy Gap Triphenylamine-HeteropentaceneDicyanovinyl Triad for Solution-Processed BulkHeterojunction Solar Cells Amaresh Mishra,*,†,‡,# Christoph Wetzel,† Rahul Singhal,∥ Peter Bäuerle,† Ganesh D. Sharma*§ †

Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-EinsteinAllee 11, 89081 Ulm, Germany ‡

School of Chemistry, Sambalpur University, Jyoti Vihar-768019, Sambalpur, India. Email: [email protected] #

Nano Research Centre, Sambalpur University, Jyoti Vihar-768019, Sambalpur, India.

∥Department

of Physics, Malaviya National Institute of Technology, Jaipur-302017, Rajasthan, India

§

Department of Physics, The LNM Institute of Information Technology (Deemed University), Rupa ki Nagal, Jamdoli, Jaipur, Rajasthan-302031, India. Email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT A low bandgap D-D-A type conjugated molecular triad TPA-SN5-DCV has been implemented in solution-processed organic solar cells using PC71BM as acceptor. The molecule showed a narrow optical energy gap of 1.59 eV, and a broad absorption spectrum ranging from 350 to 750 nm. Cyclic voltammetry measurements were used to estimate the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels and are -5.10 eV and -3.53 eV, respectively. After the optimization of the donor and acceptor (PC71BM) weight ratio, the device based on as cast TPA-SN5-DCV:PC71BM (1:2) active layer exhibited overall PCE of 3.34% which was improved to 6.31% using 0.3 vol% 1,8-diiodooctane (DIO) as solvent additive (SA). Further optimization of the DIO-treated active layer via thermal annealing (TA) resulted in a markedly improved PCE of 7.26%. The enhancement in PCE after either SA or SA+TA treatment is largely accredited to the increase in both JSC and FF which is associated to the superior nanoscale morphology, broadening of absorption and balanced charge transport. Introduction Organic solar cells (OSCs) containing π-conjugated oligomers and polymers are emerging as a promising technology for low cost photovoltaic systems due to the tunable material properties, lowering of production cost using solution-processing and potential for the fabrication of lightweight and flexible devices.1-3 The advancement of this technology is largely due to the fundamental scientific interest for designing new materials and improvements in device optimization conditions which successfully minimize the limitations of unbalanced charge transport and poor film forming property of oligomer-based OSCs. During the last two decades, a number of studies has been devoted to improve the power conversion efficiency (PCE) and as a result, PCEs now approach to 13% in laboratory scale bulk-heterojunction solar cells (BHJSC) 2 ACS Paragon Plus Environment

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by integrating new organic semiconductors as donor and/or acceptor,4-6 morphological control of the photoactive layer,7-9 and interfacial engineering.10-17 The growing interest in oligomer-based OSCs is also encouraged by numerous potential advantages over conjugated polymers, like more reproducible synthesis and purification,18 while maintaining the prospective for the control of active layer morphology through use of additives,8,19 annealing,20,21 and thus avoiding batch-tobatch variations. A large variety of high performance oligomers based on A-D-A, D-A-D, D-A (D = donor; A = acceptor) scaffolds have been designed in last few years which often contain oligothiophenes, bridged-thiophene derivatives, diketopyrrolopyrroles, or fused heteroacenes as core motifs.14,22-30 Among the donor materials, the strong electron donating triphenylamines (TPA), due to their good hole transporting abilities, have been used as versatile building blocks to develop a series of linear and star-shaped molecular materials.31-36 The push-pull (D-A) molecular structures facilitate efficient intramolecular charge transfer. It has been reported that the TPA group substantially stabilizes separated holes after exciton dissociation and thus improve the hole transportation in the devices.31,37 Star-shaped molecules in combination with oligothiophene spacer and dicyanovinyl or benzothiadiazole acceptor have been used in BHJ solar cells generating PCEs up to 4.8%.38-40 Ko and co-workers have prepared some star-shaped molecules containing TPA and fused TPA core.41,42 The fused TPA unit led to the extension of πconjugation and increased the intermolecular interactions, thus exhibiting red-shifted absorption and facilitated charge transport showing PCEs up to 5.8%. Acceptor-functionalized S,N-heteroacenes due to their rigid molecular backbone exhibit close π–π stacking in thin films, strong absorption in the visible region and good charge transport properties. These heteroacenes combine the higher stability of oligothiophenes and the planar



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and extended π-conjugated scaffold of oligoacenes normally exhibiting high charge-carrier mobilities.44 In BHJ solar cells, prepared by both solution- and vacuum-deposition techniques, these heteroacenes have shown excellent performance with PCEs as high as 7%.27,28,43,44 Recently, we have developed a push-pull type D-D-A triad TPA-SN5-DCV by combining the good hole transporter TPA with the planar heteropentacene (SN5) as combined donor unit, which was terminated by an electron-accepting dicyanovinylene (DCV) group. The triad allowed us to obtain a low energy gap material with intense absorption in the visible region and suitable energies of the frontier orbitals for solar cell applications. Recently, this D-D-A triad was used as a hole transport material in perovskite solar cells generating high PCEs of 17.7% and good stability.45 In this work, we now describe the use of our D-D-A triad as strong absorbing donor in solution-processed BHJOSCs showing the versatility of the molecular systems in various types of solar cell technologies. Excellent PCEs of 7.26% have been obtained for TPA-SN5DCV:PC71BM devices by using diiodooctane (DIO) as solvent additive followed by thermal annealing. The nanoscale morphology and charge transport properties across the interface were investigated in order to investigate the influence of processing conditions and their impact on the device performance.

Results and discussion Optical and electrochemical properties The synthesis of the D-D-A triad TPA-SN5-DCV 1 is reported elsewhere45 and the chemical structure is shown in Figure 1a. The absorption spectra of TPA-SN5-DCV in dichloromethane solution and thin films deposited from THF solution is shown in Figure 1b. In solution the triad exhibited an intense intramolecular charge transfer (ICT) absorption band peaking at 591 nm (ε = 4 ACS Paragon Plus Environment

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93,700 M-1 cm-1). In thin-films, the ICT band is broadened and red-shifted to 654 nm exhibiting an onset of absorption at 780 nm. This relates to an optical energy gap of approximately 1.59 eV which is 0.26 eV lower than that in solution and a typical value for D-A-based molecular systems. The red-shifted shoulder peak in the film spectrum arising from the self-organization of the molecules in the solid state. Cyclic voltammetry (Figure 1c) was used to calculate the energy level of the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO/LUMO = -5.10 eV/-3.53 eV), from which we can estimate the electrochemical energy gap of 1.57 eV. The LUMO energy level of the D-D-A triad is sufficiently elevated than the LUMO energy level of PC71BM (-4.1 eV) which important for exciton dissociation, demonstrating that this triad can be used as a suitable donor along with PC71BM acceptor in BHJOSCs (Figure 1d).



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Figure 1. a) Chemical structure of TPA-SN5-DCV donor; b) Optical absorption spectra of TPASN5-DCV in dilute THF solution and film cast from THF; c) Cyclic voltammograms of the TPA-SN5-DCV measured in dichloromethane at 25 °C, supporting electrolyte TBAHPF (0.1 M), scan rate 100 mV s−1. The HOMO/LUMO energy levels were estimated from the Eonset potential values of the first oxidation and reduction wave using Fc/Fc+ energy level as –5.1 eV vs. vacuum; d) Schematic energy levels of the solar cell components.

The structure of TPA-SN5-DCV 1 was optimized by density functional theory (DFT) using the B3LYP functional/TZVP basis set (Figure 2). The electron density of the HOMO of TPA-SN5DCV 1 is mainly located on the terminal TPA moiety and the heteropentacene donor bridge. Where, the LUMO is mainly located on the terminal electron accepting DCV group with substantial contributions from the heteropentacene bridge. This electron distribution results in a push-pull type chromophores providing sufficient overlap between the terminal DCV acceptor and the TPA donor via the heteropentacene bridge assuring fast charge transfer upon excitation. The calculation revealed that a TPA unit can play a significant role to partially stabilize the holes separated from excitons which may improve the hole transport properties in corresponding devices.


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Figure 2. Isodensity surface plots of TPA-SN5-DCV (hexyl chains were replaced by methyl substituents) calculated by the density functional theory (DFT) using the B3LYP/TZVP.

Photovoltaic properties BHJOSCs using TPA-SN5-DCV 1 as donor were fabricated using the device assembly of ITO/PEDOT:PSS/TPA-SN5-DCV:PC71BM/PFN/Al. The conjugated polymer, poly[(9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)]

(PFN),

which

is

soluble in alcohol, was used as a cathode interfacial layer.46 After varying the D:A weight ratio, the optimal weight ratio was found to be 1:2. The as cast active layer is denoted as device A. The current–voltage (J-V) features of the optimized device A is presented in Figure 3a and corresponding photovoltaic data are compiled in Table 1. The TPA-SN5-DCV:PC71BM devices without any post-treatment showed rather a moderate PCE of 3.34% with a short circuit current density (JSC) of 9.74 mA cm-2, an open circuit voltage (VOC) of 0.78 V and fill factor (FF) of 0.44. The moderate performance may be assigned to a poor nanoscale morphology of the BHJ layer. The nanoscale morphology of the BHJ layer should comprise a bi-continuous network of Dand A-phases representing the best compromise between two contradictory requirements: (i) the D- or A-rich regions should be small enough so that the excitons can reach the D-A interface before they are lost due to recombination by radiative or non-radiative processes, and (ii) they should be large enough in order to avoid recombination of charge carriers at the D:A interface. The recombination rate is sensitive to interface to bulk ratio, as rationalized by Monte Carlo simulation.47 Therefore numerous constraints must be managed to obtain the optimized nanoscale morphology of a BHJ layer for efficient dissociation of exciton into free charges,



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transport, and extraction at the electrode.48,49 Firstly, the donor and acceptor phase separation in the nanoscale, impact excitons on their way to reach critical D:A interfaces.50,51 Secondly, internal order within these phases affects the charge carrier transport.52 Finally, the formation of interpenetrating networks of donor and acceptor phases changes the paths of photogenerated holes and electrons that are collected at anode and cathode, respectively.20,53 For an efficient OSC, all these requirements are to be fulfilled during the film formation of the BHJ active layer.54 An ideal morphology can be obtained during the formation of BHJ film due to specific materials properties, such as solubility and miscibility, and can be improved by external treatments.55 Various strategies, such as solvent optimization for active layer, solvent additives, solvent vapour annealing, thermal annealing and combination of these, have been employed to manage the active layer morphology in BHJSCs. In order to exploit the potential of our D-D-A triad, we further employed solvent additive (SA) and post-treatment by thermal annealing (TA) in order to control the morphology of active layer and improve the device performance.2,9,56 We have also tested only thermal annealed active layer without solvent additive but the PCE was merely improved up to 4.71% (JSC =11.48 mA cm-2, VOC = 0.72 V and FF = 0.57). Figure 3a presents the J-V characteristics of devices using SA (device B) and SA+TA (device C) and the data are depicted in Table 1. The PCE of solar cells cast from chloroform (CF) containing 0.3 vol% 1,8-diiodooctane (DIO) as additive exhibited a simultaneous increase in JSC (13.54 mA cm-2) and FF (0.63) resulting in an overall enhancement of the PCE to 6.31%. When the SA-containing active layer was additionally thermally annealed, the PCE was further increased to 7.24%, benefited from the enhanced JSC (14.22 mA cm-2) and FF (0.69). The external quantum efficiency (EQE) spectra of the devices are displayed in Figure 3b. The EQE of the devices were closely related to the corresponding absorption spectra of the


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blended films (Figure 4), demonstrating that both PC71BM and TPA-SN5-DCV contributes to the photocurrent generation. In comparison to the EQE spectra of the as-cast device A, the SA (device B) and SA+TA (device C) treatment not only increase the EQE in the entire absorption range but also red-shifted the onset on the low energy side of the EQE spectra with a shoulder at around 724 nm. The calculated JSC values obtained by integration of the EQE spectra are about 9.65 mA cm-2, 13.43 mA cm-2 and 14.15 mA cm-2 for devices A, B, and C, respectively, are comparable with the JSC values measured from J-V curves.

Figure 3. (a) Current–voltage (J-V) characteristics under illumination (AM1.5, 100 mW cm-2) and (b) EQE spectra of devices A, B, and C. Table 1. Photovoltaic parameters for the BHJSCs based on TPA-SN5-DCV:PC71BM processed and post-treated under different conditions. Device

JSC

VOC

(mA cm-2)

(V)

FF

PCE

µh

µe

(%)

(cm2 V-1s-1)

(cm2 V-1s-1)

µe/µh

Device A 9.74 (as cast)

0.78

0.44

3.34 (3.25)a

6.54 x10-5

2.72 x10-4

4.16

Device B 13.54 (SA)

0.74

0.63

6.31 (6.24)a

1.12 x10-4

2.81 x10-4

2.51

Device C 14.22 (SA+TA)

0.74

0.69

7.26 (7.18)a

2.11 x10-4

2.88 x10-4

1.36



a

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Average PCE values obtained from eight identical devices are shown in parentheses.

The difference in the performance of devices A (untreated), B and C (solvent additive and post-annealing) may be related to the chemical structure, blend morphology optimization and charge transport kinetics. Therefore, we have examined the effect of the solvent additive and TA as well on the optical absorption spectra in order to obtain information regarding the source of the enhancement in JSC. As shown in Figure 4, the both the SA- and SA+TA-processed TPASN5-DCV:PC71BM blend film demonstrate an obvious increase in the absorption intensity and spectral broadening in the 500-750 nm region in comparison to as-cast film (λmax = 642 nm). Moreover, a vibronic shoulder peak at 692 nm and 702 nm was observed for the SA and SA+TA treated films, respectively. These results indicated that both the SA and TA treatment, can lead to enhanced D/A interaction and crystalline ordering in the blended film.57 Therefore, the absorption intensity of the SA and the SA+TA blended films enhanced and broadened as compared to as cast counterpart. These features are consistent with the EQE response of the devices (Figure 3b).

Figure 4. Normalized absorption spectra of TPA-SN5-DCV:PC71BM blended thin films prepared under different treatments.


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The enhancement of the JSC and FF for the devices B and C in comparison to device A were also confirmed by the evaluation of hole and electron mobilities for hole-only (ITO/PEDOT:PSS/ active layer/Au) and electron-only (ITO/Al/active layer/Al devices using the space charge limited current (SCLC) method.58 Table 1 summaries the hole and electron mobilities estimated from the SCLC model and corresponding J-V characteristics are shown in Figure S1. The hole and electron mobilities for as-cast TPA-SN5-DCV:PC71BM are 6.54 × 10-5 cm2 V-1s-1 and 2.72 × 10-4 cm2 V-1s-1, respectively. After the SA and TA treated devices, the hole mobilities were improved up to 1.12 × 10-4 cm2 V-1s-1 and 2.11 × 10-4 cm2 V-1s-1, respectively. However, the electron mobilities were slightly increased upon either by SA (2.81 × 10-4 cm2 V-1s-1) or by SA+TA (2.88 × 10-4 cm2 V-1s-1) treatment. The electron/hole mobility ratio (µe/µh) for as-cast, SA and TA-treated active layers were 4.16, 2.51 and 1.36, respectively. Therefore the unbalanced charge transport in the device A may be one of the causes for the lower FF, JSC and PCE for this device. The decrease in the µe/µh ratio with either SA or SA+TA-treatment demonstrates more balanced charge transport in devices B and C. Because of the better charge transport properties, the majority of the photogenerated charge carriers can be more efficiently collected at the electrodes, which is supported by the high FF for the OSCs based on SA and SA+TA-processed active layers. Therefore, we rationalize that the further balanced charge transport and fewer charge recombination losses in devices B and C are the reason for a significant improvement of FF using SA and/or TA.59,60 The photocurrent dependency Jph (Jph = JL-JD, where JL and JD represents the current densities under light and in the dark, respectively) on the effective voltage Veff (Veff = Vo-Vapp, where Vapp is the applied voltage and Vo is when Jph = 0) was used to explore the kinetics associated to the exciton dissociation and charge transport and their extraction routes in the devices and presented



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in Figure 5. As noticed from the figure, Jph exhibited a linear dependence on Veff at low voltage region and subsequently reaches saturation at value of Veff. In the case of device A, a stronger field dependence is observed for Jph across the wider bias voltage regime, and is not completely saturated even at Veff = 2.4 V, signifying that device A undergoes major bimolecular recombination and the internal electric field is not enough to pull out all charge carriers. After the use of SA, the Veff, at which the Jph is saturated (Jphsat), shifted to lower value and even further lower for device C. These results prove the improvement of exciton dissociation and charge collection efficiency for device B and C. Information about the overall exciton dissociation and charge collection efficiency is acquire from the ratio Jph(sc)/Jphsat at short circuit condition.61,62 The values of Jph/Jphsat for devices A, B and C are 0.76, 0.88 and 0.91, respectively, and confirms that the exciton dissociation and charge collection efficiency is improved for device B and even more for C, reflected with higher JSC and FF values. We have also determined the maximum of free charge carriers (Gmax) generation rate using Jphsat = qLGmax, (where, L is the thickness of the active layer and q is the electronic charge). The calculated values of Gmax for the devices A, B and C are 9.67 × 1027 m3 s-1, 1.14 × 1028 m3 s-1 and 1.18 × 1028 m3 s-1, respectively. The observed trends in the Gmax for the three devices are in agreement with the increase in JSC and broadening of absorption spectra of corresponding active layers (Figure 4), which confirms the efficient generation of excitons and their dissociation into free charge carriers in the BHJ layer prepared using SA or SA+TA treatment. The enhancement of Gmax and Jph(sc)/Jphsat for the devices B and C compared to the untreated device A is credited to better phase separation of the BHJ layer, enhanced hole mobility, reduced bimolecular recombination and superior balanced charge transport in the devices.


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Figure 5. Variation in photocurrent density (Jph) with the effective voltage (Veff) for devices based on active layers prepared under different treatments. The morphology of the photoactive layer also plays a vital role in the photocurrent enhancement, as this controls the efficiency of the exciton dissociation, charge separation and charge collection. Figure 6 demonstrates the X-ray diffraction patterns of the TPA-SN5DCV:PC71BM thin films processed under different conditions. It is clear from these patterns that weak reflections corresponding to (010) and (100) planes are obtained in the as-casted film, but the crystalline nature of the film was found to be superior upon SA and SA+TA dual treatment, which is evident by the improved scattering peak and intensities in the XRD pattern for these films. For the as-cast TPA-SN5-DCV:PC71BM blended film, the (100) diffraction peak is observed at 2θ = 5.08°, which gives a d-spacing distance of ~1.74 nm in the molecular system. In addition to it, another weak (010) diffraction peak is observed at 2θ = 22.52°, which corresponds to π-π stacking distance of ~0.39 nm and crystal coherence length (CCL) of ~19.23 nm. Such a short π-π stacking distance between the small molecule backbones suggests a strong intermolecular interactions and endorses an efficient charge transport with elevated FF. The enhancement in the intensity of the diffraction peaks for (100) and (010) reflections in case of SA-treated TPA-SN5-DCV:PC71BM blend indicates the improvement in crystalline ordering in 13 ACS Paragon Plus Environment


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the blended films. Moreover, the crystal size and CCL are also enhanced for SA-containing film. These results indicate that the SA treatment raised the charge carrier mobility of donor molecule by improving the crystallinity due to nucleation growth. The TA treatment further improved the crystallinity and diffraction intensities in both (100) and π-π stacking directions. The results therefore demonstrated that collective SA+TA treatment is more efficient in improving crystalline ordering in BHJ blend leading to enhanced charge transport and charge collection efficiencies. These findings accounted for higher FF and markedly improved PCE of the BHJSCs.

Figure 6. X-ray diffraction patterns of TPA-SN5-DCV:PC71BM blend films. The morphological changes of the BHJ layers processed under different conditions (SA and SA+TA) were investigated using transmission electron microscopy (TEM) and the results are shown in Figure 7. It is clear from the bright field TEM image of as-cast blended film that there is almost negligible phase separation between the donor and acceptor phases (Figure 7a) which suggests

an

intermixed

TPA-SN5-DCV:PC71BM

micro-structure

without

significant

crystallization. In contrast, SA (Figure 7b) and SA+TA (Figure 7c) treated films show a clear phase separation between the PC71BM acceptor and the TPA-SN5-DCV donor within the range 14 ACS Paragon Plus Environment

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of approximately 30-40 nm resulting in a bi-continuous interpenetrating network, which is beneficial for both, exciton dissociation and efficient charge transport. It is therefore clear that SA and SA+TA treatment improve both JSC and FF and ultimately boost the overall PCE.

Figure 7. TEM images of blended films (a) as cast, (b) SA and (c) SA+TA treatment. The scale bar is 200 nm.

Conclusions In this article, we report the BHJ solar cell performance of a D-D-A triad TPA-SN5-DCV 1 comprising triphenylamine and heteropentacene as donor and a dicyanovinyl acceptor. In solution-processed BHJSCs the as-cast TPA-SN5-DCV:PC71BM devices exhibited PCEs of 3.34% which was significantly improved to 7.26% by using diiodooctane as solvent additive followed by thermal annealing of the active layer. The PCE reported here is the highest value obtained so far for unsymmetrical push-pull molecular systems. The enhancement of the PCE is due to the broad and strong absorption of the active layer covering the region from 300-750 nm. The solvent additive and post-treatment resulted in a significant enhancement of the fill factor from 0.44 to 0.69, which could be attributable to the improvement in the nanoscale morphology and charge transport properties. Based on the combination of the strong electron-donating TPA and heteroacene system for the rational design of donor materials for OSCs or of hole-transport



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materials for perovskite solar cells,45 the current results offer an encouragement for further analyses of structure-device performance relationships for this type of D-A molecules.

Experimental Methods The BHJ solar cells were prepared in an ITO/TPA-SN5-DCV:PC71BM/PFN/Al configuration. Firstly, ITO-coated glass substrates were ultrasonically cleaned for 10 min each with deionized water, acetone, ethanol and iso-propanol, successively and then dried. The PEDOT:PSS solution was spin-coated onto the pre-clean ITO substrates at 2000 rpm followed by baking at 120 °C for 15 min in air.63 The TPA-SN5-DCV donor and PC71BM with different weight ratios (1:1, 1:1.5, 1:2 and 1:2.5 ) with same concentration of 16 mg/mL, were dissolved in chloroform (CF) to make the blend for active layer. Then in order to get the thin active layer above prepared solution was deposited by spin-coating on top of the PEDOT:PSS layer at 2500 rpm for 60 s and then dried at 40 °C for 10 min. For the solvent additives, 0.3 vol% of DIO was mixed in chloroform solvent.64 The BHJ layer was then thermally annealed at 110 °C for 30 s. The methanolic solution of polyelectrolyte interlayer poly[9,9-bis(3'-(N,N-dimethylamino)-propyl-2,7-fluorene)alt-2,7-(9,9-dioctylfluorene)] (PFN) (1.5 mg/mL) in the presence of a trace amount of acetic acid (~2 µl mL-1) was spin-coated on top of the BHJ layer at 3000 rpm for 30 s. Finally, aluminium top electrode was deposited by thermal evaporation at 2.0x10-5 Pa. The active area of the devices was about 16 mm2. All devices were prepared and tested in an ambient atmosphere without encapsulation.

The

hole-only

(ITO/PEDOT:PSS/active

layer/Au

and

electron-only

(ITO/Al/active layer/Al) devices were prepared to measure the hole and electron mobility, respectively. The current–voltage (J–V) characteristics of the BHJOSCs were measured with a computer controlled Keithley 2400 source meter in the dark and under a simulated AM 1.5G


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illumination of 100 mW cm-2. A xenon light source attached with the optical filter was used to give the stimulated irradiance. The external quantum efficiency (EQE) of the devices was measured by illuminating the device through the light source and the monochromator and the resultant current was recorded using a Keithley electrometer under short circuit conditions.

Supporting Information. The following file is available free of charge on the ACS Publications website. Dark current– voltage characteristics of hole-only devices and electron only devices.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] ACKNOWLEDGMENT The authors thank the German Federal Ministry of Education and Research (BMBF, program LOTsE 03EK3505G), and Department of Science and Technology (DST), New Delhi (DST/TMD/SERI/D05) for financial support. REFERENCES (1)

Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of

Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397–7457.



(2)

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Mishra, A.; Bäuerle, P. Small Molecule Organic Semiconductors on the Move: Promises

for Future Solar Energy Technology. Angew. Chem. Int. Ed 2012, 51, 2020–2067. (4)

Zhang, S.; Ye, L.; Hou, J. Breaking the 10% Efficiency Barrier in Organic Photovoltaics:

Morphology and Device Optimization of Well-Known PBDTTT Polymers. Adv. Energy Mater. 2016, 6, 1502529. (5)

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TOC Graphic


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Figure 1. a) Chemical structure of TPA-SN5-DCV donor; b) Optical absorption spectra of TPA-SN5-DCV in dilute THF solution and film cast from THF; c) Cyclic voltammograms of the TPA-SN5-DCV measured in dichloro¬methane at 25 °C, supporting electrolyte TBAHPF (0.1 M), scan rate 100 mV s−1. The HOMO/LUMO energy levels were estimated from the Eonset potential values of the first oxidation and reduction wave using Fc/Fc+ energy level as –5.1 eV vs. vacuum; d) Schematic energy levels of the solar cell components. 1416x970mm (110 x 110 DPI)

ACS Paragon Plus Environment


Figure 2. Isodensity surface plots of TPA-SN5-DCV (hexyl chains were replaced by methyl substituents) calculated by the density functional theory (DFT) using the B3LYP/TZVP. 150x137mm (120 x 120 DPI)


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Figure 3. (a) Current–voltage (J-V) characteristics under illumination (AM1.5, 100 mW cm 2) and (b) EQE spectra of devices A, B, and C. 177x124mm (300 x 300 DPI)



Figure 3b 177x124mm (300 x 300 DPI)


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Figure 4. Normalized absorption spectra of TPA-SN5-DCV:PC71BM blended thin films prepared under different treatments. 177x124mm (300 x 300 DPI)



Figure 5. Variation in photocurrent density (Jph) with the effective voltage (Veff) for devices based on active layers prepared under different treatments. 177x124mm (300 x 300 DPI)


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Figure 6. X-ray diffraction patterns of TPA-SN5-DCV:PC71BM blend films. 177x124mm (300 x 300 DPI)



Figure 7. TEM images of blended films (a) as cast, (b) SA and (c) SA+TA treatment. The scale bar is 200 nm. 1387x400mm (110 x 110 DPI)


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TOC Graphic 205x144mm (150 x 150 DPI)


Low Energy Gap Triphenylamine ... - ACS Publications

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