Symmetrical and unsymmetrical triphenylamine based diketopyrrolopyrroles and their use as donors for solution processed bulk heterojunction organic solar cells

Abstract

Two small molecules DPP3 (D–π–A) and DPP4 (D–π–A–π–D) with triphenylamine (TPA) donors and diketopyrrolopyrrole (DPP) acceptors linked with ethyne linkers were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photonic, electronic, thermal and computational properties were investigated. The red shift in the electronic absorption spectra of DPP4 as compared to DPP3 is related to extended conjugation and increased donor–acceptor interaction. We have used DPP3 and DPP4 as electron donors along with PC₇₁BM as an electron acceptor for solution processed bulk heterojunction organic solar cells. The solar cells prepared from DPP3:PC₇₁BM and DPP4:PC₇₁BM (1 : 2) processed from chloroform (CF) exhibit a power conversion efficiency (PCE) of 2.23% (J_sc = 6.74 mA cm⁻², V_oc = 0.92 V and FF = 0.36) and 3.05% (J_sc = 8.26 mA cm⁻², V_oc = 0.88 V and FF = 0.42), respectively. The higher PCE of the device with DPP4 compared to DPP3 was demonstrated as to the higher hole mobility and broader IPCE spectra. The devices based on DPP3:PC₇₁BM and DPP4:PC₇₁BM processed with solvent additive (1 v% DIO, 1,8-diiodooctane) showed PCE values of 4.06% and 5.31%, respectively. The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive.

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Introduction

Bulk heterojunction (BHJ) solar cells from a blend of electron donor and electron acceptor organic semiconducting materials are a potential source of renewable energy sources due to their being inexpensive and lightweight with flexibility.¹ Research on organic solar cell (OSC) materials has been focused on the design and synthesis of low bandgap conjugated polymers that are capable of both light absorption and charge transport. After the optimization of bandgap and electrochemical energy levels of conjugated polymer donors, control of the nanomorphology of the BHJ active layer, and interfacial layers, the power conversion efficiency (PCE) of OSCs based on these materials has exceeded 10%.² Despite this advancement, these materials suffer from several drawbacks, such as batch to batch variation, side length polydispersity, structural defects and difficult synthesis and purification.³ To overcome these limitations the research is diverted to “small molecule” BHJ (SM-BHJ) solar cells, where polymer donors are replaced by conjugated molecular systems which can be synthesized with well defined molecular architecture with desired frontier molecular orbitals to match the light harvesting of the best performing polymeric materials.⁴ The PCE of SM-BHJs has been reached in the range of 9–10%.⁵ The PCE of SM-BHJ solar cells could be further improved by exploring the design of new SMs that can used as donor in BHJ active layer.

The frequently used strategy to design high performance organic small molecule is to use the push–pull chromophore structure in which electron donating (D) and electron accepting (A) units are coupled together, designing semiconducting organic materials with highly conjugated backbone structure.⁶ This type of small molecules exhibit low band gap, intense absorption in visible region, strong intra- and intermolecular interaction, and efficient charge transport, which is beneficial for high PCE of organic solar cells. Among the various SMs, designed for OSCs, the SMs containing diketopyrrolopyrrole (DPP), with two fused electron deficient lactam rings have been attracted lot of attention because of its unique properties such as high absorption coefficient in visible region, strong electron withdrawing property and high coplanarity facilitate intermolecular packing. DPP derivatives possess high thermal stability and used as brilliant colorants.⁷ The DPP is reported as potential electron acceptor and functionalization with electron donor will results in donor–acceptor molecular system. Moreover, strong donor–acceptor (D–A) interaction between DPP core and adjacent thiophenes generate strongly hybridized frontier molecular orbitals, low lying LUMO and high lying HOMO orbitals of DPP based SMs. Therefore, most of the SMs with DPP possesses low band gap and well ordered structure in thin film and thus exhibit high J_sc in OSCs.^8,11 The state of art with the PCE of 7% has been reported for BHJ solar cells based on DPP SMs⁹ and in the range of 6–9% for DPP based D–A copolymers.¹⁰ Our group is involved in the design and synthesis of donor–acceptor molecular systems with low HOMO–LUMO gap for optoelectronic applications.¹¹ Ziessel et al. have reported the symmetrical and unsymmetrical triphenylamine functionalized diketopyrrolopyrroles.¹²

In this article we wish to report the synthesis and solar cell properties of DPP3 and DPP4 as electron donor along with PC₇₁BM as electron acceptor in solution processed BHJ organic solar cells. The acetylene linker between DPP and TPA is used to afford the low HOMO–LUMO gap through extended conjugation and also increase the ionization potential of resulting molecule due to relatively larger electron-withdrawing characteristics of sp hybridization over sp² hybridization.¹³ After the optimization, we have achieved the PCE of 4.06% and 5.31% for active layer DPP3:PC₇₁BM and DPP4:PC₇₁BM, respectively.

Experimental part

Synthesis of DPP3

In 100 ml round bottom flask monobromodiketopyrrolopyrrole 1 (0.100 g, 0.15 mmol) and (4-ethynylphenyl)diphenylamine (0.041 g, 0.15 mmol) were dissolved in dry THF (10 ml) and triethylamine (6 ml). The reaction mixture was degassed with argon for 10 minutes and Pd(PPh₃)₄ (0.0.018 g, 0.015 mmol) and CuI (0.0028 g, 0.015 mmol) were added. The reaction mixture was stirred at 70 °C for overnight. After completion of reaction, the reaction mixture was allowed to cool down to room temperature. The solvents were removed under vacuo and the product was purified by silica-column chromatography with hexane : dichloromethane (3 : 1) as an eluent in 75% yield.

¹H NMR (400 MHz, CDCl₃, δ in ppm): 8.93 (2H, s), 7.64 (1H, s), 7.36 (3H, m), 7.28 (4H, m), 7.12 (7H, m), 7.00 (2H, d, J = 8 Hz), 4.05 (4H, s), 2.35 (1H, s), 1.74 (4H, s), 1.41 (4H, s), 1.25 (23H, s), 0.86 (6H, s); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): 161.3, 161.2, 148.7, 146.9, 139.9, 139.1, 135.5, 135.4, 132.6, 132.5, 130.8, 129.9, 129.8, 129.5, 129.2, 128.7, 125.3, 124.0, 121.6, 114.5, 108.3, 107.9, 98.5, 81.7, 42.3, 31.9, 30.1, 30.0, 29.5, 29.31, 29.26, 26.9, 22.7, 14.1; HRMS (ESI) m/z calcd for C₅₄H₆₁N₃O₂S₂ + Na: 870.4097 [M + Na]⁺, found 870.4094 [M + Na]⁺.

Synthesis of DPP4

In 100 ml round bottom flask dibromodiketopyrrolopyrrole 2 (0.100 g, 0.14 mmol) and (4-ethynylphenyl)diphenylamine (0.073 g, 0.28 mmol) were dissolved in dry THF (10 ml) and triethylamine (6 ml). The reaction mixture was degassed with argon for 10 minutes and Pd(PPh₃)₄ (0.015 g, 0.014 mmol) and CuI (0.0025 g, 0.014 mmol) were then added. The reaction mixture was stirred at 70 °C for overnight. After completion, the reaction mixture was allowed to cool down to room temperature. The solvents were removed under vacuo and colored product was purified by silica-column chromatography with hexane : dichloromethane (3 : 1) as an eluent in 80% yield.

¹H NMR (400 MHz, CDCl₃, δ in ppm): 8.93 (2H, s), 7.36 (6H, m), 7.29 (9H, m), 7.10 (11H, m), 7.00 (4H, d, J = 8 Hz), 4.07 (4H, m), 3.13 (2H, m), 1.75 (4H, m), 1.45 (6H, m), 1.25 (21H, s), 0.85 (6H, m); ¹³C NMR (100 MHz, CDCl₃, δ in ppm): 161.2, 148.7, 147.0, 139.0, 135.7, 132.6, 130.0, 129.7, 129.5, 129.4, 125.4, 124.0, 121.7, 114.5, 108.5, 98.7, 81.8, 46.1, 42.4, 31.9, 30.1, 29.7, 29.6, 29.31, 29.26, 26.9, 22.7, 14.2, 8.7; HRMS (ESI) m/z calcd for C₇₄H₇₄N₄O₂S₂ + Na: 1137.5145 [M + Na]⁺, found 1137.5127 [M + Na]⁺.

Results and discussion

The small molecules DPP3 and DPP4 were synthesized by Pd-catalyzed Sonogashira cross-coupling reaction of mono-bromo DPP 1 and di-bromo DPP 2 with one and two equivalents of (4-ethynylphenyl)diphenylamine in 75% and 80% yield respectively (Scheme 1). The precursors mono-bromo DPP 1 and di-bromo DPP 2 were synthesized by following reported procedure.¹⁴ The DPP3 and DPP4 were purified by repeated silica-gel column chromatography and recrystallization techniques. Both the DPPs are readily soluble in common organic solvents like dichloromethane, chloroform, toluene, tetrahydrofuran and were well characterized by ¹H NMR, ¹³C NMR, and HRMS techniques.

Scheme 1 Synthesis of triphenylamine based DPP3 and DPP4.

Photophysical and thermal properties

The electronic absorption spectra of DPP3 and DPP4 in dilute chloroform (CF) solution as well thin film cast from CF, are shown in Fig. 1 and data are listed in Table 1. Both the DPPs show absorption bands in UV-visible region from 300 nm to 750 nm. The absorption bands at shorter (below 500 nm) and longer wavelength (510–750 nm) correspond to the π–π* transition and intramolecular charge transfer (ICT) from TPA to DPP respectively. The red shift of CT band in DPP4 as compared to DPP3 is related to the extended conjugation and increased donor–acceptor interaction. The more intense absorption for DPP4 may be due to the presence of two TPA fragments is likely to be attributed to an internal charge transfer induced by the strongly electron donating TPA unit and the π-accepting DPP unit. Moreover, both materials have shown vibrational absorption peaks in the higher wavelength region. In contrast, the absorption spectra of these DPP SMs in thin films are red shifted compared to those in solution. Such feature is attributed to a more planar conjugated backbone, further ordered structure in solid state and a higher π-electron delocalization though the molecular backbone, which could be beneficial to greater hole mobility. The optical bandgaps of these DPP3 and DPP4 were estimated from the onset edge of absorption spectra in thin film and are 1.84 and 1.72 eV, respectively.

Fig. 1 Electronic absorption spectra of DPP3 and DPP4 in CF solution (10⁻⁴ M) and thin films cast from CF.

Table 1 Photophysical, electrochemical, thermal and computational properties of DPP3 and DPP4

	λmax^a (nm) ε/10^4 (M^−1 cm^−1)	λmax^b (nm)	E^optg^c (eV)	E^elect.g^d (eV)	E^DFTg^e (eV)	Te^f (°C)
a In dilute CF solution.b Thin film cast from CF solution.c E^optg = 1240/λonset, λonset is onset absorption edge is absorption spectra in thin film.d Estimated from electrochemical analysis.e Estimated from DFT calculation.f Estimated from TGA.
DPP3	583 (2.3), 551 (2.8)	618	1.84	1.43	2.24	402
DPP4	627 (3.3), 587 (2.3)	658	1.72	1.30	2.07	398

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The thermal properties of DPP3 and DPP4 were investigated by thermogravimetric analysis (TGA) under nitrogen atmosphere and curves shown in Fig. S1.† The decomposition temperatures for 10% weight loss in DPP3 and DPP4 were 402 °C and 398 °C, respectively (Table 1), indicating that both DPPs are thermally stable.

Electrochemical and computational properties

The electrochemical properties of DPPs were explored by cyclic voltammetry and differential pulse voltammetry (CV and DPV) techniques in dichloromethane solvent using 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as supporting electrolyte. The CV and DPV curves are shown in Fig. 2 and the corresponding data are listed in Table 2. Both the DPPs exhibit four oxidation waves. The first oxidation potential of DPP3 and DPP4 are almost same. The DPP3 and DPP4 show two reduction waves in CV and DPV corresponding to formation of mono and dianion. The potentials were measured vs. Ag/Ag⁺ as quasi reference electrode. After each experiment, the potential of the Ag/Ag⁺ electrode was calibrated against the Fc/Fc⁺ redox couple.

Fig. 2 Cyclic voltammetry (CV) (red line) and differential pulse voltammetry (DPV) (black line) plots of (a) DPP3 (b) and DPP4.

Table 2 Electrochemical properties of DPP3 and DPP4^a

	E^2 red	E^1 red	E^1 oxid	E^2 oxid	E^3 oxid	E^4 oxid
a The electrochemical analysis was performed in a 0.1 M solution of Bu4NPF6 in dichloromethane at 100 mV s^−1 scan rate, versus Ag/Ag^+ at 25 °C.
DPP3	−1.51	−1.10	0.86	1.03	1.41	1.79
DPP4	−1.51	−0.89	0.91	1.22	1.47	1.82

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The HOMO and LUMO energy levels were estimated from the onset oxidation and reduction potentials respectively. The HOMO/LUMO energy levels are −5.12/−3.69 eV and −5.07/−3.77 eV for DPP3 and DPP4, respectively. Because the exciton binding energy in an organic semiconductor is 0.3–0.5 eV, the LUMO offset between DPP3 or DPP4 and PC₇₁BM afford sufficient driving force for efficient exciton dissociation and effective electron transfer in the active layer. The deeper HOMO energy level is desirable to achieve the high V_oc, since V_oc is determined by the energy difference between the HOMO level of donor and LUMO energy of acceptor used in the active layer of BHJ organic solar cell.

The density functional theory (DFT) calculations were carried to understand the geometry and electronic structure of DPP3 and DPP4 using the Gaussian 09W program at the B3LYP/6-31+G** level. The geometry optimizations were carried out in the gas phase and frontier molecular orbitals (FMOs) are displayed in Fig. 3. The HOMO of both the DPPs are distributed on the whole molecule whereas LUMOs are localized mainly on the DPP core. The localization of LUMO on DPP core indicates the acceptor nature of DPP. This shows the typical donor–acceptor (D–A) interaction and charge transfer from TPA to DPP.¹⁵ The theoretical band gap values obtained from DFT calculations were found to be in consistent with the electrochemical band gap values calculated from DPV and the optical band gap values from the UV-vis absorption (Table 1).

Fig. 3 The frontier molecular orbitals of DPP3 and DPP4 estimated by DFT calculations.

Photovoltaic properties

The solution processed BHJ solar cells based on DPP3 and DPP4 as donor along with PC₇₁BM as acceptor were made with a device structure ITO/PEDOT:PSS/DPP3 or DPP4:PC₇₁BM/Al. The DPP3 or DPP4:PC₇₁BM active layer was formed by spin coating of a constant concentration of 14 mg ml⁻¹, compromising a mixture of DPP3 or DPP4 and PC₇₁BM in CF. The photovoltaic performance of the BHJ organic solar cells are strongly influenced by the concentration of donor and acceptor component used in the active layer, since there should be a balance between the absorption profile of active layer and charge transport within the active layer towards the final collecting electrodes. The weight ratio between DPPs and PC₇₁BM were varied from 1 : 1, 1 : 1.5, 1 : 2 and 1 : 2.5 in order to optimize the ratio of donor and acceptor in active layer. The best performance was achieved for the 1 : 2 weight ratio for both the DPPs as donor. We have observed that the higher concentration of donor reduced the electron mobility leading the reduction of PCE, may be attributed to the poor interpenetrating pathways between the donor and acceptor. Furthermore, the concentration of DIO solvent additive was varied from 0 to 0.5%, 1% and 1.5% in order to improve the morphology of BHJ active layer. It was observed that the optimized weight ratio is 1 : 2 and solvent concentration is 1% of DIO in CF solution. The current–voltage (J–V) characteristics of the optimized devices based on DPP3:PC₇₁BM and DPP4:PC₇₁BM active layers cast from CF solution are shown in Fig. 4a and c, respectively and photovoltaic parameters are summarized in Table 3. Under optimized DPP3:PC₇₁BM (1 : 2) cast from CF based device showed relatively high V_oc of 0.92 V, but low PCE of 2.23% with a J_sc of 6.74 mA cm⁻² and FF of 0.36. However, under the similar condition, the device based on DPP4:PC₇₁BM exhibited a PCE of 3.05% with J_sc of 8.26 mA cm⁻², V_oc of 0.88 V and FF of 0.42. The increase in PCE for the DPP4 based device is mainly due to the higher values of J_sc and FF. The relatively higher V_oc for DPP3 based device is proportional to its slightly deeper HOMO energy level compared to DPP4. The higher value of J_sc for DPP4 is also confirmed from the IPCE spectra of the devices (Fig. 4b and d). The IPCE spectra of devices based on DPP3 and DPP4 exhibited broad response, consistent with absorption spectra of their corresponding active layers (Fig. 4a and b). It can be seen from the figure that absorption spectra of the DPPs:PC₇₁BM blends is the combination of both PC₇₁BM and DPPs (the absorption band corresponds around 390 nm, attributed to PC₇₁BM whereas the absorption band in longer wavelength region corresponds to DPPs), indicating both PC₇₁BM and DPP are contributing to the exciton generation and resulting photocurrent in the device. Comparing the Fig. 4b and d, it can be seen that DPP3 based device showed response from 350–660 nm with maximum IPCE value of 31% at 610 nm, whereas DPP4 based device showed IPCE response from 350 to 690 nm with maximum value of 40% at 624 nm. The J_sc values of DPP3 and DPP4 based devices calculated from the integration of IPCE spectra are 6.58 mA cm⁻² and 8.14 mA cm⁻², respectively, which are consistent with the measured values from J–V characteristics under illumination.

Fig. 4 Current–voltage (J–V) characteristics under illumination (AM15.G, 100 mW cm⁻²) (a and c) and IPCE spectra (b and d) for the BHJ organic solar cells based on optimized DPP3:PC₇₁BM and DPP4:PC₇₁BM active layers cast from CF and DIO (3 v%)/CF solutions.

Table 3 Photovoltaic parameters for the BHJ organic solar cells using optimized DPP3:PC₇₁BM and DPP4:PC₇₁BM active layer under different processing conditions

Active layer	Jsc (mA cm^−2)	Voc (V)	FF	PCE (%)	μh (cm^2 V^−1 s^−1)	μe/μh
a CF cast.b DIO/CF.c Average to 8 devices.
DPP3:PC71BM^a	6.74	0.92	0.36	2.23 (2.18)^c	5.34 × 10^−6	44
DPP4:PC71BM^a	8.26	0.88	0.42	3.05 (2.98)^c	7.82 × 10^−6	31
DPP3:PC71BM^b	8.88	0.88	0.52	4.06 (3.97)^c	4.26 × 10^−5	5.68
DPP4:PC71BM^b	11.16	0.85	0.56	5.31 (5.24)^c	9.18 × 10^−5	2.71

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We measured the J–V characteristics of hole only devices based on DPP3:PC₇₁BM and DPP4:PC₇₁BM (weight ratio 1 : 2) active layers (as shown in Fig. 5a and b), and applied space charge limited current (SCLC) model to extract hole mobility (μ_h) using hole only device configuration ITO/PEDOT:PSS/active layer/Au. The estimated values of hole mobility for DPP3 and DPP4 are 5.34 × 10⁻⁶ and 7.82 × 10⁻⁶ cm² V⁻¹ s⁻¹, respectively (Table 3). Moreover, we have also estimated the electron mobilities (μ_e) in the active layers using similar manner employing electron only device (ITO/Al/active layer/Al), are 2.34 × 10⁻⁴ cm² V⁻¹ s⁻¹ and 2.42 × 10⁻⁴ cm² for DPP3 and DPP4 based active layer. This indicates that the electron to hole mobility ratio for DPP3 and DPP4 based devices is 44 and 31, respectively. Although in both devices, the electron and hole transport in unbalanced resulting lower PCE, but relatively more balanced charge transport for DPP4 based device, leading to relatively higher PCE for this device as compared to DPP3 based device.

Fig. 5 Current–voltage (J–V) characteristics of the hole only devices based on (a) DPP3:PC₇₁BM and (b) DPP4:PC₇₁BM active layers processed with CF and DIO (3 v%)/CF solutions. The solid lines represent the SCLC fitting.

The PCE of the devices processed with CF solution is moderate. Although the V_oc is quite respectable but the low value of PCE is mainly due to the low J_sc and FF. It is well known that the high J_sc of the BHJ organic solar cell is related to the exciton generation and dissociation at D–A interface and their efficient charge transport with any recombination losses during their transportation towards the electrodes and directly linked with the nanophase morphology of the BHJ active layer. The nanomorphology of the BHJ active layer can be strongly affected by the processing conditions.¹⁶ A small amount of DIO was employed as solvent additive to improve the nanophase morphology of the active layer to improve the exciton diffusion and dissociation. The J–V characteristics of the devices based on the active layers processed with optimized DIO/CF are shown in Fig. 4a and c (red color) and corresponding photovoltaic parameters are summarized in Table 3. Comparing to the devices based on the active layer processed with CF, the devices processed with DIO (1 v%)/CF showed higher PCE i.e. 4.06% (J_sc = 8.88 mA cm⁻², V_oc = 0.88 V, and FF = 0.52) and 5.31% (J_sc = 11.16 mA cm⁻², V_oc = 0.85 V, and FF = 0.56) for DPP3 and DPP4, respectively. The enhancement in the PCE is attributed to the increase in both J_sc and FF. The enhancement in J_sc with DIO additive is consistent with the IPCE spectra as shown in Fig. 4b and d (red color), where the IPCE value has been increased throughout the entire wavelength region of measurement. The calculated J_sc values of devices from the IPCE spectra are 8.76 mA cm⁻² and 11.08 mA cm⁻², for DPP3 and DPP4 based devices respectively which are in good agreement with measured values.

The origin of increase in the J_sc for the devices processed with DIO/CF was investigated by absorption spectra of active layers. Fig. 6a and b show UV-visible optical absorption spectra of active layers processed with and without DIO additive. As compared to CF cast active layer, the DIO additive films showed a red shift, corresponding to the DPPs. This red shift is an indication of the improved ordered structure of active layer. Moreover, the absorption intensity, particularly the absorption band corresponds to DPP also increased. The broader absorption profile and increase in the absorption intensity, results more exciton generation in the active layer and may be one of the reasons for the increase in J_sc.

Fig. 6 Normalized absorption spectra of (a) DPP3:PC₇₁BM and (b) DPP4:PC₇₁BM thin films processed with and without DIO solvent additives.

It is well known that the nanoscale morphology of BHJ active layer can be strongly affected by the solvent processing.¹⁷ The X-ray diffraction pattern gives information about the molecular packing, crystallinity and D–A phase separation.¹⁸ To get information about the effects of DIO additive on above parameter, XRD measurements were carried out on the pristine films of DPP3:PC₇₁BM and DPP4:PC₇₁BM, spin cast from CF and DIO (1 v%)/CF and shown in Fig. 7a and b. The blend film cast from CF showed weak broad scatting at 2θ = 5.53° and 5.98°, for DPP3 and DPP4, respectively, corresponding to the d-spacing of 1.34 nm and 1.18 nm, which is originated from very small crystalline domains of DPP3 and DPP4 respectively.¹⁹ It can be seen that the diffraction peak for DPP4 is stronger than that for DPP3, indicating more ordered stacking existed in DPP4, due to the additional TPA donor unit, consequential improved crystallinity, which is in agreement with the absorption spectra of DPP4. It was observed that with the addition of additive DIO in CF, the DPPs:PC₇₁BM blended films shows the diffraction peaks with increased diffraction intensity, indicates that the processing additive significantly enhance the crystallinity of DPP in the active layer and thus increase the degree of order. This can be interpreted that the processing additive allows the DPPs to crystallize more completely by providing increased drying time during the film formation.²⁰ Moreover, reduction in the d-spacing is related to the more closed packing, which is beneficial for efficient charge transport within the active layer.²¹

Fig. 7 XRD patterns of (a) DPP3:PC₇₁BM and (b) DPP4:PC₇₁BM thin films cast from CF and DIO/CF solutions.

The transmission electron microscopy (TEM) of blended film of DPP4 and PC₇₁BM are shown in Fig. 8. The bright regions can be attributed to DPP4 domains, whereas the dark regions can be attributed to PC₇₁BM domains due to its high electron scattering density.²² Without DIO additive, DPP4:PC₇₁BM films showed large phase separation of the order of 35–40 nm, which indicates a limited interface between DPP4 and PC₇₁BM. However, when 1% DIO was added, the blend film morphology for DIO/CF processed film slightly changed, revealing the small phase separation in the range of 25–30 nm, leading the increase in the interfacing area for exciton dissociation. Similar change in the morphology was observed in DPP3:PC₇₁BM active layer. In general, the PC₇₁BM component in the active layer is selectively soluble in less volatile solvent such as DIO, therefore, addition of DIO results aggregation of the donor material during the film drying process leading to formation of more highly ordered D–A phases, leading efficient charge transport.²³

Fig. 8 TEM images of DPP4:PC₇₁BM (1 : 2) films processed with CF and DIO/CF solution. The scale bar is 100 nm.

We further investigated the effect of solvent additive on the charge transporting properties of active layers, measuring the J–V characteristics of hole only devices employing the active layers processed with DIO (1 v%/CF) (Fig. 5a and b red color) as described earlier. The hole mobilities are compiled in Table 3. The electron mobility of the solvent additive active layers is almost same as for CF cast active layer. It was observed that the blended films cast from CF exhibited very low hole mobilities and relatively high electron mobilities. The low hole mobilities and the unbalanced electron/hole transport are the main reasons for poor performance of the devices based on the active layers without solvent additives. Solvent additive significantly enhance the hole mobility to 4.26 × 10⁻⁵ and 9.18 × 10⁻⁵ cm² V⁻¹ s⁻¹ for DPP3 and DPP4, respectively. The addition of DIO, greatly increases the hole mobility for both blended films, indicates that the enhancement in the crystallinity induced by DIO can promote the vertical percolate pathways for blended films. In addition, the electron mobility significantly improved for the active layer processed with solvent additive, and more balanced electron/hole transport (5.68 and 2.71 for DPP3 and DPP4, respectively) was achieved. The relative more balanced charge transport is in good agreement with increased FF of the devices.

The improved PCE of the devices with solvent additive processed active layers are supported by the relation of photocurrent density (J_ph) with effective voltage (V_eff). J_ph = J_L − J_D, where J_L and J_D are the current densities under illumination and in dark, respectively. V_eff = V₀ − V_app, V₀ is the voltage at which J_ph = 0 and V_app is applied voltage.²⁴ The variation of J_ph with V_eff for the devices based on DPP4:PC₇₁BM processed with CF and DIO (1 v%)/CF are shown in Fig. 9. Similar plots have been observed for DPP3:PC₇₁BM based devices. In the case of device based on active layer processed with CF solvent, J_ph shows stronger field dependence across the large bias range and has not fully saturate even V_eff = 3.0 V, suggesting a significant germinate and/or bimolecular recombination and less efficient charge collection at the electrodes, thus a lower FF. However, the device processed with DIO/CF, J_ph has a linearly dependence on voltage at the low value of V_eff, and J_ph reaches saturation, when the effective voltage reaches to 2.4 V and 2.0 V, for DPP3:PC₇₁BM and DPP4:PC₇₁BM active layer, respectively. This suggest that the photogenerated excitons are dissociated into free charge carriers and charge carriers are collected at the electrodes efficiently with reduced bimolecular recombination for the devices based on blends processed with DIO/CF. We have estimated the maximum generation rate of free charge carriers G_max, according to J_phsat = qG_maxL, where q is the elementary charge and L is the active layer thickness. The G_max is influenced by the solvent additive with the values 0.96 × 10²⁷ m³ s⁻¹, 2.1 × 10²⁷ m³ s⁻¹, 1.08 × 10²⁷ m³ s⁻¹ and 3.4 × 10²⁷ m³ s⁻¹ for DPP3:PC₇₁BM (CF cast), DPP3:PC₇₁BM (DIO/CF cast), DPP4:PC₇₁BM (CF cast) and DPP4:PC₇₁BM (DIOCF cast), respectively. The trend observed for G_max with different treatments is consistent with the enhancement of J_sc as well as the UV-visible absorption coefficient, indicating more efficient exciton generation and separation in the devices with DIO/CF cast. The ratio of J_ph/J_sat under short circuit conditions gives information about the overall exciton dissociation and charge collection efficiency. Under short circuit conditions, the ratios are 0.56, 0.72, 0.63 and 0.78 for DPP3:PC₇₁BM (CF cast), DPP3:PC₇₁BM (DIO/CF cast), DPP4:PC₇₁BM (CF cast) and DPP4:PC₇₁BM (DIO/CF cast), respectively. The greater value of P_C with DIO/CF processed devices is attributed to the better phase separation in the active layer, increased hole mobility, improved balance in charge transport and enhancement in the light absorption ability.

Fig. 9 Variation of photocurrent density (J_ph) with effective voltage (V_eff) for devices based on DPP4:PC₇₁BM processed with CF and DIO/CF solvents under constant incident light intensity.

Conclusions

In conclusion, we have synthesized unsymmetrical and symmetrical triphenylamine based diketopyrrolopyrroles (DPP3 and DPP4) by Sonogashira cross-coupling reaction and investigated their optical, thermal, electrochemical and computational properties. The red shift in the absorption spectra of DPP4 compared to DPP3 is related to extended conjugation and increased of donor–acceptor interaction. The HOMO and LUMO energy levels suggested that these DPPs can be used as electron donor along with PC₇₁BM as electron acceptor for the fabrication of solution processed small molecule bulk heterojunction solar cells. The solution processed BHJ small molecule processed with CF solution showed PCE of 2.23% and 3.05% for DPP3:PC₇₁BM and DPP4:PC₇₁BM active layers, respectively. The higher PCE of device based on the later active layer than that of former is attributed to the broader absorption profile of DPP4 and larger hole mobility. In an attempt to improve the PCE, we have the adopted solvent additive (SA), i.e. 1% v DIO/CF technique and achieved PCE of 4.06% and 5.31%, for DPP3:PC₇₁BM and DPP4:PC₇₁BM active layer, respectively. The enhancement in the PCE with solvent additive based device is attributed to the balanced charge transport, higher light harvesting ability, and better nanoscale morphology for exciton dissociation and charge transport and collection, supported by the, XRD, TEM and mobility results. The results presented here shows that symmetrical DPP4 shows higher PCE than unsymmetrical DPP3 and employing additional ethyne bridged triphenylamine unit results in increased efficiency (PCE) of device from of 4.06% to 5.31%.

Acknowledgements

Y. P. thanks Ministry of Human Resource Development (MHRD), R. M. thanks Department of Science and Technology (Project No. EMR/2014/001257) and the Council of Scientific and Industrial Research (Project No. 01/(2795)/14/emr-II), New Delhi for the financial support. GDS is thankful to DST, Government of India (DST-RFBR Joint Research Project). F. C. Chen thanks the support by the Ministry of Science and Technology of Taiwan (grant number: MOST 103-2923-E-009-001-MY3). We are also thankful to Faculty of Physics, Lomonosov Moscow State University, 1-2 Leninskiye Gory, Moscow, 119991, Russian Federation.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, TGA curves, ¹H NMR, ¹³C NMR spectra and HRMS data of compounds. See DOI: 10.1039/c6ra10442h

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