Furan-bridged thiazolo [5,4-d]thiazole based D–π–A–π–D type linear chromophore for solution-processed bulk-heterojunction organic solar cells

Abstract

Novel furan-bridged thiazolo[5,4-d]thiazole based π-conjugated organic chromophore (RFTzR) was formulated and utilized for small molecule organic solar cells (SMOSCs). The presence of furan spacer along with two terminal alkyl units significantly improved its absorption and solubility in the common organic solvents. RFTzR exhibited the reasonable HOMO and LUMO energy levels of −5.36 eV and −3.14 eV, respectively. The fabricated SMOSCs with RFTzR (donor) and PC₆₀BM (acceptor) as photoactive materials presented relatively high power conversion efficiency of ∼2.72% (RFTzR : PC₆₀BM, 2 : 1, w/w) along with good open-circuit voltage of ∼0.756 V and high photocurrent density of ∼10.13 mA cm⁻², which might attribute to its improved absorption, electrochemical properties and the presence of strong electron-withdrawing furan moieties.

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1. Introduction

Solution-processed small molecule organic solar cells (SMOSCs) have recently grown as an alternative photovoltaic device to the conventional photovoltaics due to various advantages over their polymer counterparts, such as well-defined molecular structures, definite molecular weight, easy synthetic processes and also easy purification techniques.^1–7 Small π-conjugated organic chromophores are considered as a good alternative for low band-gap organic conjugated polymers in solution-processed bulk-heterojunction (BHJ) small molecule organic solar cells (SMOSCs).^8–11 Zhou et al. recently synthesized a linear chromophore of A–D–A type (DR₃TBDT) for the SMOSCs and demonstrated the high power conversion efficiency (PCE) of >7%.¹² The performance of SMOSCs is usually influenced by various fabrication parameters, such as composition ratio, film-thickness, homogeneity of materials and annealing temperatures.^13–16 In particular, the size of the donor (D) and acceptor (A) unit in the organic chromophore is a crucial factor for the exciton-diffusion towards the D–A interface and then charge-separation for the effective charge-transport to the electrodes.^17–19

Small organic molecules with a thiazolo[5,4-d]thiazole unit present a rigid and coplanar structure with good electron-accepting tendency owing to the presence of imine (C=N–) groups and fused-heterocyclic rings.^20–23 It is known that the electronegativity of the oxygen atom is stronger than that of the sulfur atom, which might help to reduce the highest occupied molecular orbital (HOMO) level of the chromophore by introducing oxygen based heterocyclic moieties. In BHJ organic solar cells, the high open circuit voltage (V_OC) is generally the difference between the HOMO of the electron donor and lowest unoccupied molecular orbital (LUMO) of the electron acceptor.^24,25 Furan, a heterocyclic unit, has been proved a good alternative for the thiophene unit because of its good electron withdrawing ability and better planarity, and it tunes the HOMO–LUMO level appropriately in organic conjugated chromophores and dyes for solar cells.^26–29 In furan-based organic materials, the smaller size of the oxygen atom compared with the sulfur atom leads to less steric demand in oligofurans than in oligothiophenes, which might also contribute to the significant difference in the rigidity.^30,31 Importantly, furan-based materials are biodegradable and could be obtained directly from the natural sources. Furan based materials are significantly more electron-rich, show higher fluorescence, better molecular-packing, and greater rigidity with better processability (due to their greater solubility) than their thiophene-counterparts. Recently, organic solar cells fabricated with furan-containing polymers and PC₇₁BM showed quite high power conversion efficiencies.^32,33 Hitherto, various D–A–D or A–D–A type thiazolo[5,4-d]thiazole small organic molecules have aroused a lot of interest for the efficient solution-processed organic photovoltaic materials.^34–37 In this manuscript, a new and novel furan-bridged thiazolo[5,4-d]thiazole based D–π–A–π–D type linear chromophore, 2,5-bis (5-(5-(5-hexylthiophen-2-yl)thiophen-2-yl) furan-2-yl) thiazolo[5,4-d]thiazole (RFTzR), has been synthesized, characterized and applied for the fabrication of solution processed SMOSCs. The terminal alkyl units at both ends of the chromophore have considerably improved its solubility in common organic solvents, which is an essential criterion for solution-processed fabrication devices to facilitate the charge conduction on the donor–acceptor interface.

2. Experimental

2.1. Materials and equipments

All the chemicals and reagents were purchased from commercial sources and used without further purification. The flash column chromatography was performed on a column packed with silica gel (300–400 mesh). The thin layer chromatography (TLC) plates of aluminum silica gel 60 F254 (Merck) were used to monitor the reaction progress.

2.2. Synthesis of chromophore

The synthetic route of π-conjugated linear organic chromophore, RFTzR, is shown in Scheme 1. The monomeric precursors of the thiazolo[5,4-d]thiazole-based chromophores were synthesized by previously reported procedures.³⁸ Compound, 5-di(furan-2-yl)thiazolo[5,4-d]thiazole (2) was synthesized by a ring-closing reaction of furfural (1) and dithiooxamide, which on bromination with N-bromosuccinimide gives 2,5-bis(5-bromofuran-2-yl)thiazolo[5,4-d]thiazole (3). Suzuki cross-coupling reactions of 3 and 4 yielded the furan-based linear organic chromophore in good yield under inert atmosphere.

Scheme 1 Synthetic route of the furan-bridged organic chromophore.

2.2.1. Synthesis of 2,5-di(furan-2-yl)thiazolo[5,4-d]thiazole, 2. A solution of furfural (1) (1.2 g, 12 mmol) and dithiooxamide (0.60 g, 5 mmol) in nitrobenzene (∼20 ml) was heated to reflux at 130 °C for 24 h under inert atmosphere. The reaction changes were monitored by TLC. After completion of the reaction, the reaction mixture was cooled to room temperature and diethyl ether was added (50 ml) to obtain the precipitates. The obtained precipitates were washed by ether, ethanol and hexane several times. The residue was purified by column chromatography on silica gel with hexane : DCM (1 : 1, v/v) as eluents. The final product was dissolved in chloroform and heated for 15 min. The obtained solution was filtered and allowed to cool. The desired product was finally obtained by recrystallization in diethyl ether as greenish needle-like crystals with a bitter almond smell (0.98 g, 72% yield). ¹H NMR (600 MHz, CDCl₃, ppm) δ: 6.45 (d, 2H), 7.00 (d, 2H), 7.10 (d, 2H). FT-IR (KBr, cm⁻¹): 3144, 3106, 3069, 1648, 1596, 1500, 1455, 1384, 1313, 1241, 1214, 1192, 1147, 1032, 1014, 925, 880, 845, 815, 755, 660, 595, 560.

2.2.2. Synthesis of 2,5-bis(5-bromofuran-2-yl)thiazolo[5,4-d]thiazole, 3. 2,5-Di(furan-2-yl)thiazolo[5,4-d]thiazole, 2 (0.48 g, 1.8 mmol) was dissolved in anhydrous dimethylformamide (∼20 ml). A solution of N-bromosuccinimide (0.82 g, 4.5 mmol) in anhydrous dimethylformamide (∼8 ml) was added, and the reaction mixture was refluxed for 2 h under inert atmosphere. The precipitates were collected and dissolved in dichloromethane (DCM). After solvent removal by the reduced pressure, the residue was washed several times with diethyl ether, hexane and ethanol. The desired product was obtained by recrystallization from hexane as yellow crystals (0.65 g, 83.3% yield). ¹H NMR (600 MHz, CDCl₃, ppm) δ: 6.52 (d, 2H), 7.03 (d, 2H) FT-IR (KBr, cm⁻¹): 3118, 3106, 3097, 2985, 2927, 2853, 1732, 1584, 1497, 1466, 1312, 1228, 1198, 1080, 1017, 941, 918, 845, 797, 700, 641.

2.2.3. Synthesis of 2,5-bis(5-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)furan-2-yl) thiazolo[5,4-d]thiazole (RFTzR). 2,5-Bis(5-bromofuran-2-yl)thiazolo[5,4-d]thiazole (3) (0.20 g, 0.43 mmol) and n-hexyl-bithiophene boronic acid pinacol ester (4) (2.1 eq.) (0.25 g, 0.95 mmol) were dissolved in ∼15 ml anhydrous toluene into a two-neck round bottom flask. The reaction mixture was flushed with N₂ for 20 min, and then Pd(PPh₃)₄ (15 mg, 5 mol%) was added. The reaction mixture was flushed again with N₂ for another 10 min followed by slow addition of degassed aqueous K₂CO₃ solution (2 M) (4 ml) by syringe, and the reaction mixture was stirred at 110 °C for 24 h under inert atmosphere. The reaction mixture was cooled to room temperature and extracted with dichloromethane (DCM). The organic layer was collected and washed with water and brine and then dried over anhydrous MgSO₄. After solvent removal by reduced pressure, the residue was purified by column chromatography on silica gel with hexane : DCM (2 : 1, v/v) and then recrystallized from dichloromethane : methanol (2 : 1) to obtain the target product, RFTzR (0.14 g, 71% yield) as red crystals. ¹H NMR (600 MHz, CDCl₃, ppm) δ: 7.31–7.33 (d, 2H), 7.16–7.18 (d, 2H), 7.08–7.10 (d, 2H), 7.05–7.07 (d, 2H), 6.72–6.75 (d, 2H), 6.67–6.69 (d, 2H), 2.80–2.84 (t, 4H), 1.69–1.73 (m, 4H), 1.29–1.40 (m, 12H), 0.86–0.92 (t, 6H). ¹³C NMR (400 MHz, CDCl₃, ppm) δ: 152.61, 152.40, 151.20, 150.98, 147.71, 147.57, 145.19, 145.07, 139.40, 139.18, 138.61, 138.45, 137.10, 136.91, 135.46, 135.27, 128.14, 127.92, 127.11, 126.90, 125.66, 125.46, 104.32, 104.22, 43.30, 43.19, 32.37, 31.13, 30.92, 29.31, 29.18, 22.88, 22.95, 13.98, 13.90. FT-IR (KBr, cm⁻¹): 3113, 3068, 2954, 2917, 2848, 1836, 1741, 1643, 1541, 1464, 1398, 1325, 1203, 1161, 1129, 1047, 990, 863, 802, 786, 717, 667, 575. MS (ESI): m/z calc. for C₄₀H₃₈N₂F₂S₆⁺: 770.23; found: 770.81 [M⁺].

2.3. Device fabrication

For SMOSCs fabrication, the synthesized organic chromophore was dissolved in chlorobenzene and mixed in different amounts with PC₆₀BM. The structure of solar cell devices used in the present study is ITO/PEDOT:PSS/RFTzR:PC₆₀BM/Ag. The ITO substrates were first cleaned with detergent, deionized water, acetone and isopropyl alcohol using ultrasonication, and subsequently dried overnight in a vacuum oven. Poly (3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a buffer layer (thickness ∼80 nm) was deposited by solution spin-coating on ITO substrates with the scan rate of ∼4000 rpm for 60 s. This deposited layer was annealed at 130 °C for 10 min in a vacuum oven. The photoactive layers (thickness ∼60 nm) of RFTzR : PC₆₀BM (1 : 1, 2 : 1 and 3 : 1, w/w) were deposited on PEDOT:PSS/ITO thin films by spin-coating, and finally a top Ag layer of ∼100 nm thickness was deposited by thermal evaporation. The active area of the solar cell devices was 1.5 cm².

2.4. Characterizations

The nuclear magnetic resonance (NMR) spectra were obtained by using JEOL FT-NMR spectrophotometer in CDCl₃ as reference solvent (¹H at 600 MHz and ¹³C at 100 MHz). The chemical shift values were measured as δ values in parts per million (ppm) compared to the tetramethylsilane (TMS) as internal standard. The Fourier-transform-infrared (FT-IR) spectroscopy was performed by FT/IR-4100 (JASCO) spectrometer as solid pellets mixed with KBr. The ultraviolet-visible (UV-vis) absorption spectra were measured by V-670 (JASCO) spectrophotometer in chloroform solvent, and photoluminescence spectra (PL) were also obtained by the FP-6500 (JASCO) fluorometer in dilute chloroform solvent with excitation source of ∼350 nm. The cyclic voltammetry (CV) measurements were performed using WPG 100 potentiostat/galvanostat (WonA Tech) at a scan rate of 100 mV s⁻¹ with a three-electrode cell containing a glassy carbon working electrode, a saturated calomel reference electrode (SCE) and a platinum wire counter electrode. Herein, RFTzR was dissolved in chloroform solvent, and thin film was deposited on the glassy carbon working electrode by drop casting and dried at 60 °C for 4 h under nitrogen. The CV measurement was performed in 0.1 M of tetrabutylammonium hexafluorophosphate (TBAPF₆) in acetonitrile as the supporting electrolyte. Thermo-gravimetric analysis (TGA) was carried out with a TA instruments Q-50 thermogravimetry analyzer at a scan rate of 10 °C min⁻¹ under inert atmosphere. Differential scanning calorimetry (DSC) was characterized by TA instrument DSC-2910 at a heating and cooling rate of 10 °C min⁻¹ under nitrogen atmosphere. The photovoltaic properties of the cells were measured under simulated AM 1.5 radiation at 100 mW cm⁻² using 1000 W metal halide lamp (Phillips), which served as a simulated sun light source, and its light intensity (or radiant power) was adjusted with a Si photo detector fitted with a KG-5 filter (Schott) as a reference calibrated at NREL (USA). The power conversion efficiency (η) is calculated by the following equation:

η = J_SC × V_OC × FF/P_in

where J_SC is the short-circuit photocurrent density, V_OC is the open-circuit voltage, FF is the fill factor and P_in is the incident radiation power. The incident photon-to-current conversion efficiency (IPCE) spectra were measured by an IPCE measurement system (SM-250E, Bunkoukeiki). Before the sample measurements, a standard silicon photodiode was used as the reference for the calibration of the system. The blend RFTzR:PC₆₀BM thin film was analyzed in terms of morphology by performing atomic force microscopy (AFM) measurement using AFM Nanoscope IV, Digital Instruments, Santa Barbara, USA.

3. Results and discussion

The thermal behaviour of RFTzR is analysed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under N₂ atmosphere. The synthesized chromophore, RFTzR, shows a good thermal stability with ∼5% weight loss at ∼394 °C, as shown in the TGA plot (Fig. 1). The DSC analysis (Fig. 1 inset) reveals that the synthesized chromophore exhibits a strong melting peak (T_m) at ∼205 °C and a weak melting peak at ∼225 °C, suggesting the crystalline property of the chromophore. This chromophore also displays a weak isotropic-to-crystalline peak at ∼222 °C with a strong peak at ∼187 °C during the cooling cycle. The alkyl chains of organic molecules induce the solubility and the liquid-crystalline (LC) nature of the chromophore via self-assembly behavior. Generally, self-assembly behavior is the electrostatic interactions, which might be due to the result of π–π staking and the hydrogen bonding ability of the organic molecules. The present chromophore has shown the possibility of hydrogen bonding due to oxygen (furan spacer) as well as strong π–π staking ability.^39,40

Fig. 1 TGA and DSC (inset) thermograms of furan-based linear RFTzR chromophore.

The UV-vis absorption spectra of RFTzR in dilute (1 × 10⁻⁴ M) chloroform (Fig. 2a) shows a strong absorption band at λ_max ≈ 473 nm due to the π–π* transition, which is red shifted to ∼484 nm in thin film state (Table 1), and hence RFTzR obtains a higher absorption edge and lower optical band gap than that of previously reported RTzR.²⁰ The organic chromophore shows a reasonably high molar absorption coefficient (ε) ∼2.0 × 10⁴ M⁻¹ cm⁻¹ in solution at the absorption maxima (λ_max ≈ 473 nm), suggesting a better intramolecular charge transfer (ICT) transition between thiazolo [5,4-d]thiazole-core and thiophene rings due to the rigid molecular geometry in the furan-bridged linear chromophore.^20,41 The red shifting occurs due to its strong intermolecular ordered packing through a long planar molecular structure in solid state thin film on glass.^42,43 Moreover, the replacement of thiophene by furan spacer unit in the RFTzR molecule might also cause the higher shifting in the absorption band because furan has strong electro-negativity and a small sized oxygen atom compared to the thiophene sulphur atom. In this case, the optimized RFTzR film thickness is estimated as ∼60 nm, which delivers the uniform and homogenous thin film on glass substrate. Therefore, the molecular geometry of the RFTzR molecule permits the good electronic interaction between acceptor and donor moieties, which might result in the improved internal charge transfer. It is noticed that the chromophore, RFTzR, exhibits a relatively small optical band gap of ∼2.18 eV, which is calculated using the wavelength of the absorption edge of the thin film of the small organic molecule. Hence, this value could be considered as a reasonable optical band gap for small organic molecules.²⁰

Fig. 2 (a) UV-vis spectra of RFTzR in chloroform solution and RFTzR thin film deposited on glass substrate by spin coating; (b) photoluminescence spectra of RFTzR thin film on ITO substrate and RFTzR : PC₆₀BM (2 : 1 w/w) blend layer thin film deposited by solution spin coating.

Table 1 Optical and electrochemical properties of the furan-bridged chromophore, RFTzR

Chromophore	λmax^a (nm)	λmax^b (nm)	HOMO^c (eV)	LUMO^d (eV)	Eg^e (eV)
a Absorption in chloroform solution.b Absorption of thin film on ITO.c Estimated from the onset of oxidation potential from the formula: HOMO = −(4.4 + E^onsetox) (eV).d Estimated from the formula: LUMO = E^optg + HOMO.e Optical band gap calculated from the onset of the UV-vis spectra of the film from the formula: E^optg = 1240/(λonset)film.
RFTzR	473	484	−5.36	−3.14	2.18

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To examine the donor–acceptor charge transfer process, the photoluminescence spectroscopy (Fig. 2b) of the synthesized chromophore has been analyzed in thin film on glass substrate and blend with the PC₆₀BM layer. The RFTzR thin film displays a strong emission band at ∼625 nm, which might reduce the possibility of charge recombination in film state, and hence induces exciton-diffusion towards the donor–acceptor interface, which is ultimately beneficial for exciton-dissociation.^44,45 Moreover, the RFTzR : PC₆₀BM blend active layer thin film (2 : 1, w/w) displays the complete quenching in PL emission, suggesting a better blend formation and charge transport for the organic chromophore, which might enhance the photovoltaic properties of the conjugated chromophore during solution-processed fabrication.⁴⁶

The cyclic voltammetry (CV) studies of RFTzR thin film in 0.1 M acetonitrile solution of tetrabutylammonium hexafluorophosphate [ⁿBu₄N]⁺[PF₆]⁻ at a potential scan rate of 100 mV s⁻¹ has been carried out, as shown in Fig. 3. The furan-bridged chromophore exhibits relatively low electrochemical stability because the oxidation potential peak is not completely reversible, which might affect the photovoltaic properties of the organic chromophore. The oxidation potential peak is situated at the onset values of E^onset_ox = +0.96 ± 0.02 eV. From the CV observations, HOMO value of −5.36 eV is obtained using the equation: HOMO = −(4.4 + E^onset_ox) (eV). The LUMO energy level of −3.14 eV is estimated using the formula: LUMO = E^opt_g + HOMO.^47–49 In general, the proper electron transfer from donor to the acceptor molecule requires a higher LUMO level of donor by at least ∼0.3 eV to the LUMO energy level of the acceptor molecule.⁴⁸ The LUMO energy level of PC₆₀BM has values that range between −4.0 and −4.3 eV.⁴⁹ In support, a LUMO–LUMO offset of 0.3–0.4 eV is necessary for an exciton-dissociation and the efficient electron transfer from donor to PC₆₀BM.^50–52 In our case, the LUMO–LUMO offset between RFTzR and PC₆₀BM is larger than ∼0.3 eV; therefore, it could be expected that the exciton might easily dissociate at the donor–acceptor interface. The substitution of thiophene from the furan unit and terminal alkyl units at both ends in RFTzR might improve the intermolecular charge transfer (ICT) transition and hence, induces higher absorption and self-assembly behavior via liquid crystal properties of RFTzR and therefore, displays better efficiency.^53,54

Fig. 3 Cyclic voltammetry of the furan-bridged organic chromophore with 0.1 M TBAPF₆ as supporting electrolyte in anhydrous acetonitrile solution at a scan rate of 100 mV s⁻¹.

The photovoltaic parameters of the fabricated SMOSCs are evaluated by the current density (J)–voltage (V) measurements (Fig. 4a) under the 1 sun light (100 mW cm⁻², 1.5 AM). Among the fabricated SMOSCs, a high power conversion efficiency (PCE) of ∼2.72% is achieved by SMOSC fabricated with RFTzR : PC₆₀BM (2 : 1, w/w), whereas other fabricated SMOSCs exhibit inferior PECs of ∼1.72% for RFTzR : PC₆₀BM (1 : 1, w/w) and ∼1.94% for RFTzR : PC₆₀BM (3 : 1, w/w). SMOSC fabricated RFTzR : PC₆₀BM (2 : 1, w/w) presents the high short-circuit current density (J_SC) of ∼10.13 mA cm⁻² due to higher wavelength and scattering behavior along with high open circuit voltage (V_OC) of ∼0.756. The lowering in the V_OC value at low and high concentrations of RFTzR in the blend layer might relate to their morphological features. As shown in AFM analysis, RFTzR : PC₆₀BM (1 : 1, w/w) and RFTzR : PC₆₀BM (1 : 1, w/w) blend thin films exhibit high roughness, indicating less homogeneous film between RFTzR and PC₆₀BM molecules, which might result in the low V_OC. It is believed that the weight ratio 2 : 1 (RFTzR : PC₆₀BM) might optimize the sample containing the highly homogeneous thin film of RFTzR and PC₆₀BM. The improved J_SC and V_OC might be explained by the ultrafast and complete intermolecular charge transfer (ICT) between RFTzR and PC₆₀BM due to the introduction of the furan unit with the thiazolo[5,4-d]thiazole backbone. Furthermore, the introduction of the furan unit as spacer in RFTzR chromophore might be an effective supplier of holes owing to its smaller resonance energy (16 kcal mol⁻¹) than the thiophene unit as spacer (29 kcal mol⁻¹).^55,56 Compared to the thiophene unit, the small size of the oxygen atom in the furan unit might induce the planarity of the molecule and improve the morphology of blend thin film, which significantly enhances the photocurrent density and performances in solar cell devices.⁵⁷ In support, the presence of two terminal alkyl chains in the molecules might also induce aggregation and hence, facilitate the charge transfer rate and increase the photocurrent density of the devices. At large amounts of RFTzR, the low V_OC and fill factor is related to the fast electron transfer and the recombination rate at the interface of TCO and the active organic layer of the device.^58–60 The incident photon-to-current efficiency (IPCE) measurements have been conducted to examine the contribution of incident photons to the photocurrent for the fabricated SMOSCs. Fig. 4b shows the IPCE plots of the fabricated SMOSCs with different active layer ratios. SMOSC with RFTzR : PC₆₀BM (2 : 1, w/w) records the maximum IPCE of 37% in the broad absorption wavelengths between 450 and 650 nm, whereas SMOSC with RFTzR : PC₆₀BM (3 : 1, w/w) shows low IPCE of 25% in the same absorption range. The obtained IPCE results are in good accordance with the photovoltaic parameters, especially J_SC values (Table 2).

Fig. 4 (a) J–V curves of fabricated SMOSCs with the active layer of RFTzR : PC₆₀BM at various ratios of 1 : 1, 2 : 1 and 3 : 1 (w/w). (b) IPCE spectra of the fabricated SMOSCs with the active layer of RFTzR : PC₆₀BM at various ratios of 2 : 1 and 3 : 1 (w/w).

Table 2 Photovoltaic parameters of the furan-bridged organic chromophore fabricated with PC₆₀BM in SMOSCs

RFTzR : PC60BM	Photovoltaic parameters
	JSC (mA cm^−2)	VOC (V)	FF	PCE (%)
1 : 1, w/w	6.38	0.667	0.40	1.72
2 : 1, w/w	10.13	0.756	0.34	2.72
3 : 1, w/w	8.29	0.723	0.32	1.94

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The morphological analysis is investigated to explain the homogeneity of the blend RFTzR : PC₆₀BM active layer (1 : 1, 2 : 1, 3 : 1, w/w) using the atomic force microscopy (AFM), as depicted in Fig. 5. The AFM samples were prepared via spin-coating of the active layer on ITO/PEDOT:PSS coated ITO substrate for a better understanding and comparison of film morphology, as presented elsewhere.⁶¹ From Fig. 5(c and d), The blended thin film of RFTzR : PC₆₀BM (2 : 1, w/w) exhibits homogeneous and smooth morphology of low surface roughness (R_rms = 1.94 nm) with nanoscale phase separation, suggesting a good mixing between RFTzR and PC₆₀BM in chlorobenzene solvent, whereas other blended thin films of RFTzR : PC₆₀BM (1 : 1 and 3 : 1 w/w) record the high surface roughness of R_rms = 2.73 nm and R_rms = 2.02 nm, respectively. These results show that the active layer of RFTzR : PCBM (2 : 1, w/w) is at an optimized ratio for achieving the best performance of organic solar cell devices. It is known that the smooth morphology and homogeneous blend with the nanoscale phase separation are responsible for the large donor–acceptor interface area needed for exciton dissociation.⁶² Herein, the morphological analysis reveals that RFTzR : PCBM (2 : 1, w/w) blend thin film depicts the low surface roughness value (1.94 nm) compared to other RFTzR : PC₆₀BM ratios (1 : 1 and 3 : 1, w/w), which suggests the homogeneity of RFTzR and PC₆₀BM molecules in the blend layer and provides enough surface area for exciton-dissociation. This significantly improves the J_SC and power conversion efficiency of SMOSCs. The fill factor value is low for all SMOSCs devices due to a number of reasons, such as domain size, film morphology, misalignment of energy levels and series resistance. The series resistance is composed of the resistance of the different semiconductor layers of the cell in addition to the metal semiconductor contacts. The low FF is related to the increase in the series resistance of RFTzR:PC₆₀BM/ITO, resulting in the high recombination rate.^63,64 The high FF in RFTzR : PCBM (1 : 1, w/w) based SMOSC is related to its low series resistance of 6.16 Ω cm² as compared to other SMOSCs with high series resistances of ∼9.18 Ω cm² (RFTzR : PC₆₀BM (2 : 1, w/w)) and ∼11.42 Ω cm² (RFTzR : PC₆₀BM (3 : 1, w/w)). The AFM images of the active layer RFTZR : PC₆₀BM (2 : 1, w/w) suggest that RFTzR has a good miscibility with PC₆₀BM in the blended films, and hence a spontaneous phase-segregation process in the blend layers could form a bicontinuous network structure, which acts as percolation channels for the efficient carrier collection within the active layer of BHJ solar cells.⁶⁵ Therefore, the presence of furan spacer in linear chromophore with the homogeneous blended thin film of RFTzR : PC₆₀BM (2 : 1, w/w) substantially improves the interface area for exciton dissociation, resulting in the high photocurrent density and the power conversion efficiency of the devices.

Fig. 5 Topographical and 3D AFM images of different active layer blend thin films of (a and b) RFTzR : PC₆₀BM (1 : 1, w/w), (c and d) RFTzR : PC₆₀BM (2 : 1, w/w), and (e and f) RFTzR : PC₆₀BM (3 : 1, w/w).

4. Conclusions

A new and novel furan-bridged thiazolo[5,4-d]thiazole based π-conjugated organic (RFTzR) chromophore is synthesized and applied as an active material for the fabrication of SMOSCs. The synthesized chromophore RFTzR is highly soluble in common organic solvents due to the presence of two terminal alkyl units at both ends of the molecule. The RFTzR chromophore substantiates the reasonable HOMO and LUMO energy levels of −5.36 eV and −3.14 eV, respectively. The fabricated SMOSCs with RFTzR : PC₆₀BM (2 : 1, w/w) exhibits relatively high power conversion efficiency of ∼2.72% with high photocurrent density of ∼10.13 mA cm⁻² and high V_OC of 0.756 V. The improvement in the efficiency for the furan-bridged chromophore might attribute to the better solubility, good miscibility with PC₆₀BM and uniform film morphology of the devices, which serves an ultrafast and complete intermolecular charge transfer (ICT) between RFTzR and PC₆₀BM. The introduction of the furan unit in place of thiophene in the thiazolo[5,4-d]thiazole core organic chromophore significantly increases absorption, solubility and better thin film morphology of solar cell devices and hence, this study has shown a promising way for the furan based organic chromophores in future.

Acknowledgements

Dr Sadia Ameen acknowledges the Research Funds of Chonbuk National University in 2012. This work is fully supported by NRF Project no. 2014R1A2A2A01006525. This work is also supported by “Leaders in Industry-University Cooperation Project, (2014)” supported by the Ministry of Education, Science and Technology (MEST) and the National Research Foundation of Korea (NRF). We acknowledge the Korea Basic Science Institute, Jeonju branch, for utilizing the research supported facilities.

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