Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells

Article pubs.acs.org/JPCC

Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency Based on a Benzodithiophene Donor Core and Dicyano Acceptor Units

Downloaded by UNIV OF CAMBRIDGE on September 2, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.jpcc.5b07130

Challuri Vijay Kumar,† Lydia Cabau,† Aurelien Viterisi,† Subhayan. Biswas,‡ Ganesh D. Sharma,*,§ and Emilio Palomares*,†,∥Instituteof Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Avda. Països Catalans 16, 43007 Tarragona, Spain Departmentof Physics, The LNM Institute of InformationTechnology (LNMIIT), Jaipur-302031, IndiaR&DCenter for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College, Kukas, Jaipur-303101, IndiaCatalanInstitution for Research and Advance Studies (ICREA), Avda. Lluis Companys 23, 08010 Barcelona, Spain S Supporting Information *

ABSTRACT: A novel semiconductor organic molecule, denoted as VC89, having A-D-D1-D-A structure, was synthesized and all relevant physical and chemical features for its application in solar cells were investigated. The structure comprises 2ethylhexoxy substituted BDT (donor D1 unit) as a core and a dicyano acceptor unit (DCV) as the terminal acceptor group (A) linked through cyclopentadithiophene (CDT) (donor D) moiety. The BHJ OSC VC89:PC71BM (1:2), processed with chloroform (CF) as solvent, showed an overall power conversion efficiency (PCE) of 4.63% with short circuit current JSC = 9.28 mA/cm2, open circuit voltage VOC = 0.96 V, and fill factor (FF) = 0.52. When the active layer was processed using DIO as a solvent additive (3% v/v in CF), the corresponding solar cell showed a PCE of 6.05% with JSC = 10.96 mA/cm2, VOC = 0.92, and FF = 0.60. The PCE was further improved to 6.66% with JSC = 11.68 mA/cm2, VOC = 0.92, and FF = 0.62, when the DIO/CF (3% v/v)-processed active layer was treated with THF vapors (solvent vapor annealing, SVA). The increase in PCE was due to the enhancement in both the JSC and FF due to the use of the dicyano groups as electron acceptor units. On one hand, JSC is determined by the enhancement of the film light absorbance, which is reflected in a better IPCE and better charge collection. On the other hand, we show herein that the use of solvent annealing after treatment with chemical additives also leads to better nanomorphologies that substantially improve the solar cell efficiency.

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INTRODUCTION Organic solar cells (OSCs), a complementary solar-to-energy conversion technology, to the well-established silicon-based solar cells for converting solar energy into electrical energy have gained a lot of attention in the past two decades, because of their attractive features such as light weightlessness, solution processability at low temperature, low cost of fabrication, and the potential for large scale production through roll to roll printing.1−10 Among the different architectures employed in solution processed OSCs, the bulk heterojunction (BHJ) is among the most commonly used.11−13 It is based on solution© XXXX American Chemical Society

processed active layers of blends of an organic electron donor and an electron acceptor materials, phase-separated at the nanometer scale. The development of device architectures using BHJ active layers has led to record power conversion efficiencies (PCEs) of over 10% using soluble π-conjugated polymers as donors in single junction BHJ solar cells.14−17 Yet, the higher degree of polymer structures (for example, differ Received: July 23, 2015 Revised: August 20, 2015

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DOI: 10.1021/acs.jpcc.5b07130 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthetic Route of VC89a


a

Reaction conditions: (i) malononitrile, β-alanine dichloroethane/ethanol mixture, 12 h, reflux.

central donor unit and 3-ethylrhodamine (RH) as electron withdrawing end groups linked through a cyclopentadithiophene (CDT) (D) π-spacer and used it for solution processed OSCs as a donor and achieved a PCE of 6%.48 Herein, we report the synthesis and photophysical and photovoltaic properties of a new D−A organic small molecule, denoted VC89, comprising a dialkoxy-substituted BDT unit as the central donor unit moiety and DCV as the electron withdrawing moieties. The replacement of the RH group with DCV in VC89 induces a downward shift of the LUMO energy level, due to the higher electron affinity of the DCV moiety,39,40 and therefore lowers the band gap of VC89 with respect to VC90. Moreover, the weak electron donating properties of the BDT core provide for a deep HOMO energy level (−5.38 eV). VC89 was used in conjunction with PC71BM as the acceptor in solution processed BHJ organic solar cells. The devices based on the VC89:PC71BM blend in optimized weight ratio of 1:2 and cast from CF solvent showed an overall PCE of 4.63% with JSC = 9.28 mA/cm2, VOC = 0.96 V, and FF = 0.52. We have employed a solvent additive (SA) method, i.e., DIO (3 vol%) in CF to improve the PCE of the devices and achieved an overall PCE of 6.05% with JSC = 10.96 mA/cm2, VOC = 0.92 V, and FF = 0.60. Moreover, when the DIO/CF processed active layer was subjected to solvent vapor annealing (SVA), the PCE was further improved to 6.66% with JSC = 11.68 mA/cm2, VOC = 0.92 V, and FF = 0.62. This improvement was ascribed to an increase in hole mobility, more balance charge transport, and higher charge collection efficiency. The SA-processing and SVA treatment have both been shown to have an impact on the crystallinity and phase separation at the nanoscale of the active layer, which accounted for the improvement of the devices’ characteristics.

chain length) affects the reproducibility of synthesis, purification, and their properties of the targeted materials. In recent years, small molecules have emerged as an alternative to polymers, owing to their well-defined chemical structure, reproducibility of synthesis, ease of purification, and fine-tuning of optical and electronic properties.18−22 Recently, solutionprocessed bulk heterojunction solar cells based on small molecule donors with PCEs have shown similarity to polymer based solar cells.23−27 These high performances are the result of an interdisciplinary research effort in device optimization, control of interfacial morphology using additives, solvent annealing techniques, and the synthesis of new donor small molecules.28,29 The optical and electronic properties of photoactive layers employed in OSCs are among the most determining factors to achieve high PCEs.30−32 The ideal donor material should fulfill some fundamental requirements such as (i) a broad absorption profile with high molar extinction coefficient, (ii) high charge carrier mobility with low carrier recombination kinetics, (iii) optimal energetic alignment of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels with those of the acceptor material, and (iv) the ability to form a nanostructured active layer when blended with the acceptor. The above requirements can be fulfilled by designing the electron donor to comprise both electron donating (D) and electron accepting (A) moieties into their backbone. One can simultaneously tune the HOMO level and the bandgap of the donor by making the proper choice of D and A segments. Among the various small molecules with alternating D−A moieties reported for solution processed BHJ solar cells, molecules incorporating the benzo-[1,2-b:4,5-b′]dithiophene (BDT) as the donor building block have shown high performance. Indeed, BDT is an attractive building block due to its high structural symmetry. The rigidity of the fused aromatic system has been shown to enhance electron delocalization and promote cofacial π−π stacking in the solid state.33−36 Moreover, the relatively weak donor ability of BDT induces a deeper HOMO in the small molecule it is integrated too.37,38 Many promising end group acceptors, in particular, dicyanovinyl (DCV),39−41 alkyl cyanoacetate,42−45 and 3ethylrhodanine,46,47 can be found in the organic solar cell literature. Recently, we have designed and synthesized a small molecule donor (VC90) containing a dialkoxy-substituted BDT as the

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EXPERIMENTAL SECTION Materials and Methods. Unless stated otherwise, all reagents were purchased from commercial sources and used without further purification. Pd(PPh3)4 and malononitrile were purchased from Sigma-Aldrich. The THF and toluene solvent were further purified as described.48 Experimental Details. The absorption profiles in solution (UV−visible spectra) were carried out in a 1 cm path-length quartz cell using a Shimadzu model 3600 spectrophotometer. The solution fluorescence spectra were recorded using a Spex model Fluoromax-3 spectrofluorometer using a 1 cm quartz B


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The Journal of Physical Chemistry C cell. All 1H and 13C NMR spectra were recorded on Bruker AV 300 and AV 500 instruments, respectively, at a constant temperature of 300 K, unless otherwise stated. 1H spectra were referenced to tertramethylsilane. ESI mass spectra were recorded on a Water Quattro micro (Water Inc., USA). Cyclic voltammetric experiments were carried out with a PCcontrolled CH instruments model CHI620C electrochemical analyzer. Synthesis and Characterization. The synthesis of VC89 is depicted in Scheme 1. The intermediate BDT-CPDTCHO(1) was synthesized according to reported procedures.13 The target molecule,VC89, was prepared by Knoevenagel condensation of BDT-CPDT-CHO(1) with malononitrile in the presence of β-alanine in a dichloroethane/ethanol mixture. The important intermediates and the targeted molecules were well characterized by 1H NMR, 13CNMR, and MALDI mass spectroscopy. The characterization data are given in the Supporting Information. The novel small molecule VC89 demonstrates reasonable solubility in chloroform, chlorobenzene, and THF. Synthesis of BDT-CPDT-CHO (1). A solution of 6-bromo4,4-dihexyl-4H-cyclopenta [2,1-b:3,4-b′] dithiophene-2-carbaldehyde (i) (300 mg, 0.387 mmol) and 2,6-bis(trimethyltin)4,8-bis(2-ethylhexoxy)benzo-[1,2-b:4,5-b′]dithiophene (2) (380 mg, 0.852 mmol) in dry toluene (20 mL) was degassed twice with argon followed by the addition of Pd(PPh3)4 (44 mg, 0.04 mmol). After being stirred at 120 °C for 48 h, the reaction mixture was added into water and extracted with CH2Cl2. The organic layer was washed with water and then dried over MgSO4. The solvent was removed under reduced pressure and the crude product was purified by column chromatography on silica gel using a mixture of CH2Cl2 and hexane (1:1) as eluent to afford compound BDT-CPDT-CHO (1) (290 mg, 39%). 1H NMR (300 MHz, CDCl3) δH: 9.86 (S, 2H), 7.59 (S, 2H), 7.56 (S, 2H), 7.19 (S, 2H), 4.23 (d, J = 5.4 Hz), 1.92 (m, 2H), 1.69 (m, 8H), 1.46 (m, 4H), 1.26 (m, 44H), 0.81−0.90 (m, 24H). 13C NMR (100 MHz, CDCl3) δ: 182.23; 162.73; 157.92; 149.69; 147.37; 146.81; 143.87; 143.64; 141.64; 136.53; 135.46; 133.58; 132.26; 129.78; 128.85; 119.29; 115.92; 54.05; 40.47; 38.49; 33.75; 31.34; 30.24; 29.47; 29.39; 29.02; 28.06; 24.39; 24.23; 23.53; 22.96; 22.72; 22.37; 14.00; 11.15. MALDI: m/z calcd for C70H94O4S6 1190.55, found 1190.7. Synthesis of VC89. BDT-CPDT-CHO (1) (200 mg, 0.167 mmol), malononitrile (67 mg, 1.00 mmol), and β-alanine (1.78 mg, 0.02 mmol) were dissolved in a dichloroethane/ethanol mixture (20 mL, 1:1 v/v), and the resulting solution was refluxed and stirred for 12 h under argon. The reaction mixture was then extracted with CH2Cl2, washed with water, and dried over MgSO4. The solvent was removed under reduced pressure and the crude product was purified by column chromatography on silica gel using CH2Cl2 and hexane (1:1) as eluent and subsequent size exclusion chromatography using THF as eluent. The solid was purified again as described herein: The solid was dissolved in 15 mL of CHCl3, and poured by syringe through a 0.2 μm PTFE filter in a 250 mL graduated cylinder. A buffer layer (10 mL) consisting of a 1:1 mixture of CHCl3:Hexane was carefully poured over the solution (filtered through 0.2 μm PTFE filter). Finally, pure hexane was carefully poured over the buffer layer (filtered through 0.2 μm PTFE filter) up to the top of the graduated cylinder and the solution was left in the dark undisturbed. VC89 precipitated as dark blue shiny crystals over

a period of 2 to 3 weeks approximately, after which they were recovered by filtration and washed with plenty of hexane (200 mL) (110 mg, 58%). 1 H NMR (500 MHz, CDCl3) (Figure S1, Supporting Information) δH:7.76 (S, 2H), 7.60 (S, 2H), 7.56 (S, 2H), 7.23 (S, 2H), 4.19 (m, 4H), 1.95 (m, 2H), 1.71 (m, 8H), 1.46 (m, 4H), 1.20 (m, 44H), 0.81−0.90 (m, 24H). 13CNMR (100 MHz, CDCl3) (Figure S2, Supporting Information) δ: 164.96; 159.21; 149.69; 150.27; 144.52; 144.27; 136.64; 136.22; 135.24; 135.26; 132.78; 129.36; 119.70; 117.92; 115.02; 114.50; 54.58; 40.68; 37.63; 31.48; 30.59; 29.64; 29.20; 24.67; 23.91; 23.23; 22.51; 14.22; 13.95; 11.35. MALDI: m/z calcd for C76H94N4O2S6 1286.5701, found 1286.5776. MALDITOF (spectra of compound VC89 shown in Figure S3, Supporting Information). Anal. Calcd for C, 70.87; H, 7.36; N, 4.35; S, 14.94. Found C,70.45; H,7.51; N,4.27;S,14.03. Device Fabrication and Characterization. The solar cells were fabricated using the glass/ITO/PEDOT:PSS/ VC89:PC71BM/Al device architecture. The indium tin oxide (ITO) patterned substrates were cleaned by ultrasonic treatment in aqueous detergent, deionized water, isopropyl alcohol, and acetone, sequentially, and finally dried in air. The anode was modified with a PEDOT:PSS layer (60 nm) by spin coating and heated for 10 min at 100 °C. Blends of VC89 with PC71BM with weight ratios of 1:1, 1:1.5, and 1:2 and 1:2.5 in chloroform (CF) were prepared as reported before.48 For the VC89:PC71BM blend processed from 1%, 2%, 3%, 4%, and 5% v/v of DIO in CF solvent mixture only the 1:2 weight ratio mixture was used. The total concentration of the solution was 10 mg/mL for each active layer. The approximate thickness of the active layers was 90 nm. Finally, the aluminum (Al) top electrode was thermally evaporated on the active layer at a pressure of 10−5 Torr through a shadow mask of area of 20 mm2. All devices were fabricated and tested in air without encapsulation. The hole-only and electron-only devices with ITO/PEDOT:PSS/VC89:PC 7 1 BM/Au and ITO/Al/ VC89:PC71BM/Al architectures were fabricated in an analogous way. The current−voltage (J−V) characteristics of the BHJ organic solar cells were measured using a computer controlled Keithley 238 source meter in the dark and under simulated AM 1.5G illumination of 100 mW/cm2. A xenon light source coupled with optical filters 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 by illuminating the device through the light source and a monochromator. The resulting current was measured using a Keithley electrometer under short circuit condition. XRD measurements were recorded on a Bruker D8 Advanced model diffractometer with Cu Kα radiation (λ = 1.542 Å) at a generator voltage of 40 kV.

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RESULTS AND DISCUSSION Optical and Electrochemical Properties. Figure 1 displays the absorption spectra of VC89 in solution in CF and thin film cast from CF. The UV−visible spectra of VC89 exhibits the dual band characteristics of D−A type conjugated chromophores:49 the less intense high energy band with a maximum absorption in the 350−380 nm region is assigned to characteristic π−π* transition of the conjugated backbone and the band at longer wavelength is attributed to intramolecular charge transfer (ICT) between the electron rich BDT and electron deficient DCV segments. In solution, VC89 exhibits its highest absorbace at 590 nm with ε = 2.61 × 105 M−1 cm−1. C


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The Journal of Physical Chemistry C


Figure 1. Normalized absorption spectra of VC89 in chloroform solution and cast from chloroform solution.

Figure 2. Normalized absorption spectra of VC89:PC71BM (1:2) cast under different conditions.

VC89 showed a significant increase in molar extinction coefficient, compared to VC90, (7.8 × 104 M−1 cm−1 vs 2.61 × 105 M−1 cm−1, respectively) illustrating the higher delocalization of the π-conjugation in VC89 due to the stronger electron withdrawing effect of DCV. The presence of strong electron withdrawing CN groups at both ends of the BDT core linked through CDT lead to a bathochromic shift of 8 nm in VC89 with respect to the RH-based small molecule, VC90. The absorption spectrum of VC89 in thin film is redshifted to 622 nm with a shoulder at 670 nm, which is attributed to stronger intermolecular interactions in the solid state. The shoulder observed in the thin film absorption spectra may be the result of the formation of J aggregates that, as known, increase the absorbance toward the IR region of sun spectra. In solution, the “shoulder” cannot be seen because the sample concentration is very low to allow the measurement without any saturation of the UV−visible detector system. We estimated the optical bandgap for VC89 from the onset of the absorption of thin films to be 1.68 eV. Cyclic voltammetry (CV) was used to estimate the HOMO of VC89 (Figure S4). The detail regarding the estimation of HOMO and LUMO level of VC89 is described in Supporting Information. The HOMO energy level is at −5.38 eV. The calculated LUMO energy level is −3.42 eV in comparison with the PC71BM LUMO energy level (−4.1 eV) ensures good charge transfer. VC89 exhibits similar HOMO level to VC90, but different LUMO energy level, attributed to the different acceptor end groups. Figure 2 shows the UV−visible absorption spectra of optimized VC89:PC71BM (1:1) active layers cast from CF. The absorption spectra show two distinct absorption bands, i.e., an absorption band centered at 388 nm corresponding to the PC71BM fraction and an absorption band centered at 608 nm corresponding to the VC89 fraction. It can be seen from Figure 2 that the absorption band corresponding to VC89 in the VC89:PC71BM blend film is blue-shifted relative to the absorption band observed in films of pristine VC89, indicating that the PC71BM may disrupt intermolecular π−π packing interactions in the VC89 fraction when blended with VC89. The room temperature luminescence emission spectrum of the film recorded to investigate on the exciton separation efficiency. The spectra of VC89 and VC89:PC71BM are shown in Figure 3. As illustrated in this figure the film of pristine VC89 showed a strong emission peak at 736 nm, while when VC89 is blended with PC71BM, this emission peak is significantly

Figure 3. Photoluminescence (PL) spectra of VC89, VC89:PC71BM (CF cast), VC89:PC71BM (DIO/CF), and VC89:PC71BM (SVADIO/CF) blend thin films.

quenched.50 The magnitude of the quenching becomes gradually higher when the active layer is processed with a solvent additive (SA) and when it is additionally treated via SVA.51 This increase in PL quenching suggests that the excitons generated by the absorbed photons would be dissociated to free charge carriers (electrons and holes) more effectively.52 Photovoltaic Properties. Solution processed solar cells were made using VC89 as electron donor and PC71BM as electron acceptor with a device structure as this one: (ITO)/ PEDOT:PSS/VC89:PC71BM/Al. The device optimization was carried out by varying the weight ratio of donor vs acceptor and the active layer was deposited from CF. The best photovoltaic characteristics were achieved with a 1:2 ratio with a donor/ acceptor concentration of 10 mg/mL. The active layer thickness was 85−90 nm. The J−V characteristics of the optimized device are shown in Figure 4a and the corresponding photovoltaic parameters are summarized in Table 1. The device has an overall PCE of 4.63% with JSC = 9.28 mA/cm2, VOC = 0.96 V, and FF = 0.52. This PCE is higher than the previously reported devices based on VC90,13 and the relative increase in JSC suggests that it could be attributed to the broader absorption of VC89 compared to VC90 and higher yield of exciton generation. However, VC89 induces a similar VOC than VC90, due to the similarity in their HOMO energy levels. The overall PCE of the solution processed BHJ organic solar cell based on VC89:PC71BM (CF cast) is inferior to that of the D


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values of the device processed with SA were higher than those of the device without SA in the entire visible region of the spectrum, and further increased for SVA-treated devices. The calculated values of JSC from the IPCE spectra were in good agreement, within experimental error, with the values observed in the J−V characteristics of devices under illumination (Table 2). In order to investigate the impact of SA and SVA on the devices’ performance, we recorded UV−visible absorption spectra of the blend films processed under different conditions (Figure 2, red and blue lines for SA and SA-SVA processed films, respectively). As shown in Figure 2, in comparison with the spectrum of active layers cast from pure CF, the spectra of the DIO/CF-cast active layer showed a redshift and a shoulder peak at 670 nm, which is related to the enhanced π−π stacking.63,64 For active layer films processed with SA and subsequently SVA-treated, the absorption increased consistently with the increase in JSC observed experimentally. As mentioned above, a suitable nanomorphology is key to balancing the different physical processes involving the collection of carriers (electrons and holes) after the excition dissociation at the donor−acceptor organic interface. The VC89:PC71BM morphology was analyzed by transmission electron microscopy (TEM). Figure 5 shows the TEM images VC89:PC71BM films cast from CF and DIO/CF and with SVA postdeposition treatment. As seen from these figures, the SAprocessed film exhibits a greater degree of phase separation than that of the active layer cast from pure CF, which is further increased with the additional SVA treatment. Additionally, a better bicontinuous interpenetrating network of donor and acceptor was observed, which may contribute to high exciton dissociation and charge transport.65 The crystallinity of the active layer is another factor that is paramount in the charge transport and collection, thereby affecting the overall performance of the BHJ organic solar cell. To get information about the change in crystallinity of the VC89:PC71BM we recorded the X-ray diffraction patterns (outof-plane; Bragg−Brentano configuration) of the blend films processed with CF and DIO/CF, and with postdeposition SVA treatment (Figure 6). The film cast from CF shows a relatively broad diffraction peak at 2θ = 5.72° corresponding to crystalline domains of VC89. However, in the SA-processed films, this diffraction peak becomes narrower, as seen by the fwhm value, and more intense, advocating for the growth of a greater number and also larger crystallites of VC89, compared to the film cast from CF. This is probably due to the difference in boiling point between CF and DIO, which presumably slows down the drying rate of the active layer after spin coating, assisting in the formation of numerous crystallites of VC89 and promoting their growth to greater sizes. The intensity of the diffraction peak is seen to further increase with the SVA treatment, while the FHWM is seen to remain roughly constant. This interesting trend shows that SVA does not

Figure 4. (a) Current−voltage (J−V) characteristics under illumination and (b) IPCE spectra of the BHJ organic solar cells based on differently processed active layers of VC89:PC71BM.

organic solar cells reported in the literature, mainly attributed to the low values of both Jsc and FF. This is due to the poor morphology of the active layer, which is crucial for charge separation, carrier transport, and efficient carrier collection. Recently, different processing techniques such as thermal annealing,53 solvent additive (SA) processing,54−56 interface engineering,23,57,58 and solvent vapor annealing (SVA)59−64 have been applied successfully to improve the PCE of the BHJ small molecule solar cells. As such, to improve the PCE of our devices, we added small quantities of DIO in the blend solution, and submitted the active layers to a subsequent SVA step (THF). The results are summarized in Figure 4a (J−V characteristics under illumination) and Table 2. The PCE increased to 6.05% (JSC = 10.96 mA/cm2, VOC = 0.92 V, and FF = 0.60) upon DIO addition, and to 6.66% (JSC = 11.68 mA/ cm2, VOC = 0.92, and FF = 0.62) when the active layer was subsequently SVA-treated. The enhancement in the JSC observed in the latter two cases is consistent with the corresponding IPCE spectra. As seen in Figure 4b, the IPCE Table 1. Optical and Electrochemical Properties of VC89 small molecule

λabsa(nm) (solution)

λabsb(nm) (film)

λema (nm)

E0−0c (eV)

f Eopt g (eV)

Eox (V vs Fc/Fc+)

EHOMOd (eV)

ELUMOe (eV)

VC89

590 (261000)

622

732

1.97

1.68

0.50

−5.38

−3.42

Measured in chloroform, in parentheses molar extinction efficient at λabs (in M−1 cm−1). bMeasured in thin film cast from chloroform solution. E0−0 was determined from the intersection of absorption and emission spectra in dilute solutions. dEHOMO was calculated using EHOMO (vs vacuum) = −4.88 − Eox (vs Fc/Fc+). eELUMO was calculated using ELUMO = EHOMO + E0−0. fEstimated from the onset absorption edge in absorption spectra of VC89 in thin film (Eopt g = 1240/λonset). a c

E


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Table 2. Photovoltaic Parameters of BHJ Organic Solar Cells Based on Optimized VC89:PC71BM (1:1 Weight Ratio) Blend Processed under Different Conditions processing condition With any treatment SA (3 v% DIO/CF) SVA-SA


a

JSC (mA/cm2) 9.28 10.96 11.68

JSC calc.a 9.16 10.98 11.56

Voc (V) 0.96 0.92 0.92

FF 0.52 0.60 0.62

PCE (%) b

4.63 (4.54) 6.05 (5.98)b 6.66 (6.56)b

Rs (Ω cm2)

Rsh (Ω cm2)

38.5 24.04 18.16

227 350 434

Calculated from the integration of IPCE spectra. bAverage of 5 devices.

Figure 5. TEM images of the VC89:PC71BM blends processed under different conditions, scale bar is 100 nm.

yield or slower germinate recombination kinetics. The PL quenching revealed that the active layer processed with either DIO/CF or SVA-DIO/CF exhibits more favorable features for higher JSC value. To gather evidence on the influence of the different treatments on the charge transport properties, charge carrier mobility was measured using space charge limited current (SCLC) method.68 We have used hole-only and electron-only devices with the structures ITO/PEDOT:PSS/VC89:PC71BM/ Au and ITO/Al/VC89:PC71BM/Al, to estimate the hole (μh) and electron mobility (μe), respectively. The J−V characteristics of hole-only devices processed under different conditions in the dark are shown in Figure 7. The hole and electron mobility in CF-cast blends is found to be 3.45 × 10−5 cm2/(V s) and 2.34 × 10−4 cm2/(V s), respectively. The resulting ratio between

Figure 6. XRD pattern VC89:PC71BM (1:2 w/w) thin film processed under different conditions.

enhance the effect of the DIO additive in terms of crystallite growth; however, the solvent vapors promote the formation of a higher number of crystallites accounting for the higher overall crystalline volume.66 The observed increase in crystallites’ size between active layers processed from pure CF and those with additive is consistent with the significant increase in FF as previously demonstrated,67 while the increase in FF is moderate for SVA treated active layers with respect to those deposited from SA. The overall increase in order in the molecular packing improves the film photon-to-current conversion efficiency and is likely to be responsible for the increase in JSC between the SA-processed device and the SVA-treated device.. It can be seen from Figure 3 that the degree of PL quenching is more with for DIO/CF cast film (blue color) and further increases with subsequent SVA (SVA-DIO/CF) (green color). The more effective PL quenching suggests that the exciton dissociation was improved with SA and further enhanced with subsequent SVA as compared to CF cast blend. The increase in PL quenching may be related to either the exciton dissociation

Figure 7. Dark current−voltage (J−V) characteristics of hole only devices based on the VC89:PC71BM (1:2) active layer processed under different conditions. The solid lines represent the SCLC fitting. F


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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. The series resistance (Rs) and shunt resistance (Rsh) were estimated from the slope of the J−V characteristics of the devices, under illumination, around the V OC and JSC , respectively, and complied in Table 2. As illustrated in Table 2, the value of Rs was decreased with DIO/CF and further decreased with SVA-DIO/CF and can attributed to the balance charge transport and hence improved the FF of the devices72,73 due to the presence of the cyano groups in the structure of the small organic molecule. The Rsh is correlated with the leakage current74,75 and thus slight decrease in VOC is observed for the solar cells made with either DIO/CF and SVA-DIO/CF.


electron and hole mobility (μe/μh) is 14.74. However, when processed with SA, the μh and μe were found to be 7.8 × 10−5 and 2.28 × 10−4 cm2/(V s), resulting in a more balanced charge transport (μe/μh = 2.92). The ratio is further improved (μe/μh = 2.27) upon SVA with values of μh and μe of 9.6 × 10−5 cm2/ (V s) and 2.23 × 10−4 cm2/(V s), respectively. Interestingly, the mobility ratio is seen to decrease in the SA-processed and SVA treated active layers. This is mostly due to a significant increase in the hole mobility, in line with the increase in the overall crystallinity of the active layers. The mobility ratio follows the increase in FF showing a link between the two features as previously demonstrated.69 To investigate the effect of SA and subsequent SVA treatment on the charge generation in the VC89:PC71BM BHJ organic solar cells, we measured the photocurrent density (Jph) as a function of effective voltage (Veff)70 as shown in Figure 8. The measured photocurrent density is defined as Jph =

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CONCLUSIONS In this paper, we have reported the synthesis of a novel A−D− D1−D−A small molecule, VC89, with a low bandgap (1.68 eV) and investigated its properties and potential use in organic solar cells. The use of cyano groups, instead of other suitable electron acceptor units (for example, rhodanine48), leads to a better response on the near IR spectrum and, thus, am improved photocurrent. Moreover, we have shown that the combination of room temperature solvent annealing process, using THF as a solvent, together with the use of chemical additives as DIO leads to a PCE of 6.66%, which is among the best values for small molecule solution processed solar cells. We demonstrated, using a set of different characterization techniques, that processing the active layers with a solvent additive and subsequently submitting them to a solvent annealing increases both the overall crystalline volume of the active layer and the size of the crystallites of donor. This induces an improvement of the phase segregation impacting various photophysical parameters. The increase in crystallinity was shown to have a positive impact on the charge transport, while the improvement of the phase segregation was shown to have an impact on the light absorption of the active layer, on the charge generation, and on charge collection efficiency. The improvement of these morphological features was linked to the J−V characteristics of the devices, and shown to account for the enhancement in JSC and FF observed experimentally. Further work in the direction of the use of cathode buffer is in progress to increase the PCE of the organic solar cell using VC89 as donor along with PC71BM as acceptor.

Figure 8. Variation of photocurrent density (Jph) with the effective voltage (Veff) for devices with active layers processed under different conditions.

JL − Jd, where JL and Jd are the current density under illumination at 100 mW/cm2 and in the dark, respectively. Veff is given by Vo − V, where Vo is the compensation voltage defined as Jph (Vo) = 0, and V is the applied voltage. The J−V plots indicate that at low Veff, the Jph increases linearly with Veff and then saturates for Veff > 1.5 V. Therefore, we assume that at full saturation, all the excitons generated after photon absorption in the active layer are dissociated into free electrons and holes and subsequently collected by the cathode and the anode, respectively. This allows us to estimate the maximum generation rate of free charge carriers Gmax according to Jphsat = qGmaxL,71 where q is the elementary charge and L is the active layer thickness. Gmax is significantly influenced by SA and SVA with values of 9.9 × 1021 cm−3 s−1, 1.04 × 1022 cm3 s−1, and 1.12 × 1022 cm3 s−1, extracted for active layer processed from pure CF, DIO/CF, and SVA-treated, respectively. The trend observed for Gmax with different treatments is consistent with the enhancement of JSC as well as the UV−visible absorption coefficient, indicating more efficient exciton generation and separation in the devices with either DIO/CF cast or SVADIO/CF active layer. The charge collection probability (PC; PC = JSC/Jphsat) for the devices made using CF, DIO/CF, and SVADIO/CF was found to be 0.65, 0.73, and 0.75, respectively. The greater value of PC either with DIO/CF and SVA-treated

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07130. NMR and MALDI-TOF spectra, cyclic voltammagram, absorption and emission spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells

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