An effective molecular engineering approach for employing a halogen

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Organic Electronic Devices

An effective molecular engineering approach for employing a halogen-free solvent for the fabrication of solution-processed small molecule solar cells Zaheer Abbas, Jawon Shin, Raju Alta, Shafket Rasool, Chang Eun Song, Hang Ken Lee, Sang Kyu Lee, Won Suk Shin, Won-Wook So, Soon-Ki Kwon, Yun-Hi Kim, and Jong Cheol Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14888 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Applied Materials & Interfaces

An effective molecular engineering approach for employing a halogen-free solvent for the fabrication of solution-processed small molecule solar cells Zaheer Abbas,a,b Jawon Shin,a Raju Alta,c Shafket Rasool,a,b Chang Eun Song,*,b,d Hang Ken Lee,d Sang Kyu Lee,a,b Won Suk Shin,a,b Won-Wook So,a Soon-Ki Kwon,e Yun-Hi Kim,*,c and Jong-Cheol Lee*,a,b aAdvanced

Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141

Gajeongro, Yuseong, Daejeon 34114, Republic of Korea. E-mail: [email protected] bAdvanced

Materials and Chemical Engineering, University of Science and Technology (UST),

217 Gajeongro, Yuseong, Daejeon 34113, Republic of Korea. cDepartment

of Chemistry and RIGET, Gyeongsang National University, Jinju 660-701,

Republic of Korea. E-mail: [email protected] dEnergy

Materials Research Center, Korea Research Institute of Chemical Technology (KRICT),

141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea. E-mail: [email protected] eDepartment

of Materials Engineering and Convergence Technology and ERI, Gyeongsang

National University, Jinju 660-701, Republic of Korea.

ACS Paragon Plus Environment

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Keywords: dithienobenzodithiophene (DTBDT), halogen-free processing, small molecule-based organic solar cells (SMOSCs), molecular engineering, pinhole-free SMOSCs

Abstract

To utilize the potential of small-molecule-based organic solar cells (SMOSCs), proper designs of the photoactive materials which result in reasonable performance in a halogen-free solvent system and thicknesses tolerance over a range are required. One of the best approaches to achieve these requirements is via the molecular engineering of the small-molecule electron donor. Here, we have modified a previously reported dithienobenzodithiophene (DTBDT) based small molecule (SM1) via the dimerization approach, i.e., the insertion of an additional DTBDT into the main backbone of the small molecule (SM2). SM1-based photoactive film showed severe pinhole formation throughout the film when processed with a halogen-free o-xylene solvent. On the other hand, the modified small-molecule SM2 formed an excellent pinhole-free film when processed with the oxylene solvent. Due to the dimerization of the DTBDT in the SM2 core, highly crystalline films with compact lamellae and an enhanced donor/acceptor interdigitation were formed, and all of these factors led to high-efficiency of 8.64% with the CF and 8.37% with the o-xylene solvent systems. To the best of our knowledge, this study represents one of the best results with SM donor and fullerene derivative acceptor materials that have shown the device performance with halogenfree solvents.


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1. Introduction Organic solar cells (OSCs) have attracted considerable attention over the past two decades owing to their distinct advantages, such as the ability to undergo roll-to-roll (R2R) coating processes, their light weight, good flexibility and the potential for low-cost fabrication and environmental friendliness.1-5 The basic structure of OSCs includes a photoactive layer containing the heterojunctions of the electron donor (D) and electron acceptor (A) materials spread throughout the bulk and sandwiched between two electrodes to extract photo-generated charge carriers.6-8 In OSCs, polymers and small molecules (SMs) are most commonly employed as electron donors, while [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) serves as an electron acceptor. Molecular engineering of the electron donor materials can be used to tune their optical and electrical properties; therefore, this class of OSCs has distinct advantages over other types of solar cells. Recently, innovations in the design of these materials have led to increases in the power conversion efficiency (PCE) rates which exceed 10%, thereby provided better recognition of OSCs relative to that of market-dominated inorganic solar cells.9-11 The performances of OSCs can be improved further by optimizing certain device processing conditions, for example, by changing the processing solvents, using solvent additives, applying thermal annealing (TA), conducting solvent vapour annealing processes (SVA) or employing the hot conditions of the photoactive solution and substrate.12-18 All of these approaches are useful under specific conditions depending on the photoactive materials. Most importantly, the choice of the processing solvent is critical when these OSCs may be subjected to R2R coating techniques for real applications, however, most state-of-the-art OSCs are processed with halogenated solvents. Unfortunately, these halogenated solvents are harmful to the human health and environment and required to produce relatively costly synthetic steps. Therefore, the


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halogen-free solvent system is preferable for the commercialization of these OSCs to fabricate large-area modules via the R2R method.19-21 Among polymer-based OSCs and small-moleculebased OSCs (SMOSCs), nearly all high-performance SMOSCs are prepared using halogenated solvents owing to the high solubility of SMs in solvents such as chloroform (CF), chlorobenzene (CB) along with their good morphological control and film-forming properties.9-13,22-25 Ternary component-based photoactive layers in OSCs have recently been proved as an efficient strategy to improve device performances.26-29 However, the energy level alignment and miscibility of the three components in the photoactive layer are still a challenge. In addition to the choice of the processing solvent and ternary component selection, another critical factor related to the upscaling of SMOSCs is the thickness tolerance, as precise control of the thickness during the R2R process is somewhat difficult. Most reported high-efficiency SMOSCs are sensitive to the thickness, and the best PCEs are achieved with photoactive layers nearly 100 nm thickness.30-32 Therefore, thickness tolerance is an important requisite in the R2R printing process. Molecular engineering in the SMOSCs have been developed in this area to tune the optoelectronic and physical, photochemical properties of SMs, and there are number of methods which can be employed to achieve required properties, such as donor-acceptor unit modification, side-chain engineering, functional-group modulation and increasing the conjugated chains lengths within the SM.33-36 SMs typically have three constituting structural components: a conjugated backbone, side chains and substituents. Among all of these, the conjugated backbone plays a vital role in dictating the intrinsic optoelectronic properties and defines the energy levels, film morphologies, energy band-gaps, and charge carrier mobilities. Kwon et al. synthesized a donor material, termed SM1 here, based on a DTBDT monomer linked with terminal rhodanine acceptor units via alkyl terthiophene and applied this SM as an electron


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donor to SMOSCs. The large heteroacene unit in DTBDT imparted a narrow band-gap and higher charge carrier mobilities. As a result, the PCE exceeded 7% when processed with PC71BM using CF as a processing solvent.35 Previously, our group reported a series of BDTbased small molecules and inserted different numbers of BDT units into the backbone of the SM.36 We found that the dimer BDT in the backbone of the SM exhibited better performance compared to the monomeric BDT containing a counterpart due to the improved morphology, enhanced intermolecular interactions and interconnected structures. In this study, our newly synthesized SM2 was processed with PC71BM using CF as the main processing solvent, it showed a PCE rate of 8.64% with a fill factor (FF) as high as 75%. More importantly, when photoactive materials were processed in o-xylene, SM2-based SMOSCs showed PCE rates as high as 8.37% with a FF of 75%, whereas reference SM1-based SMOSCs did not show any efficiency at all due to the poor film formation. This approach can be used as an important guideline in cases where replacing the toxic halogenated solvents commonly used in the fabrication of high-performance SMOSCs is required. 2. Results and Discussion 2.1. Synthesis and characterization of SM2 The chemical structures of SM1 and SM2 are shown in Figure 1(a), and the synthetic route for newly synthesized SM2 is shown in Scheme S1. The target molecule (SM2) was synthesized via the Knoevenagel condensation reaction of the dialdehyde intermediate with 3-ethylrhodanine at a reasonable yield (Supporting Information). The final target oligomer SM2 structure was characterized by 1H-NMR, 13C-NMR and MALDI-TOF spectroscopy, and the purification of SM2 was performed by column chromatography (see the Supporting Information). The SM2


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shows sufficient solubility in various common organic solvents, such as CF, CB, orthodichlorobenzene (o-DCB), and tetrahydrofuran (THF) as well as in the eco-friendly solvent toluene and o-xylene. The thermal stability of SM2 was measured by means of a thermogravimetric analysis (TGA), demonstrating excellent thermal stability with a 5% weight loss at 397 °C (SM1 undergoes a 5% weight loss at 380°C), as shown in Figure S1a. Differential scanning calorimeter (DSC), employed to measure the melting temperature (Tm) and the recrystallization temperature (Tc), found these to be 243 oC and 228 °C, respectively (Figure S1b). 2.2. Optical and electrochemical properties Figure 1(b) displays the normalized UV-vis absorption spectra of the pristine SM1 and SM2 samples in solution and film states using chloroform as a solvent. The detailed absorption data for the solution and film are summarized in Table 1. In the solution state, both SMs exhibit similar absorption spectra throughout the visible range, with the absorption maxima (λmax) for both found at 510 nm. When films of the SMs were prepared from the chloroform solutions, the absorption edge became red-shifted by more than 100 nm; it was found to be 703 nm for SM1 and 712 nm for SM2, while λmax was found to be 567 nm for SM1 and 569 nm for SM2. Specifically, SM2 showed broader absorption as compared to SM1 in the range of 580 nm to over 700 nm, and a weak shoulder peak appeared at around 640 nm. The broader absorption and the shoulder peak appeared due to the stronger interaction of the backbones of the SM2.37 The better interaction of the backbone imparts higher crystallinity and more effective π–π stacking in the molecular backbone in the solid state.


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Increased the conjugation length by increasing the DTBDT in the backbone of the SM has a direct effect on the energy levels. We used cyclic voltammetry (CV) to measure the shift in the highest energy occupied molecular orbital (HOMO) levels in SMs (Figure S2). From the value of the onset oxidation potential, the HOMO energy levels were determined to be -5.36 eV and -5.45 eV for SM1 and SM2, respectively (Table 1). The HOMO level was shifted downward by 0.09 eV, which is beneficial when attempting to obtain a higher open-circuit voltage (VOC) in an actual working device. The lowest energy unoccupied molecular orbital (LUMO) levels of SM1 and SM2 were calculated to be -3.60 eV and -3.71 eV, respectively, as estimated from the optical band-gaps (Egopt) and HOMO energy levels (Table 1). SM2 with the DTBDT dimer core unit exhibits deeper HOMO energy levels with lower band-gaps than SM1 due to the greatly πextended ring and strong interaction of the backbones of the SM2. 2.3. Photovoltaic performance Conventional devices were fabricated with the device architecture ITO/PEDOT:PSS(30nm)/photoactive layer(100 nm)/Ca(2nm)/Al(100nm), while all of the photoactive thicknesses were ensured to be very close to 100 nm, unless specified otherwise. An energy level diagram of the devices is shown in Figure 1c. To optimize the processing conditions for the photoactive films, the SVA and TA conditions were initially optimized. Because the performance capabilities of SM-based SMOSCs can be tuned via SVA and TA,37-39 it was found in the current SMOSCs that the SVA has a more pronounced effect on the PCE as compared to the TA (Tables S1 and S2 and Figure S3 and S4). In these SMOSCs, the PCE was improved from 3.95% to more than 7.00% for SM1 within 15 seconds SVA when CF and CH2Cl2 (DCM) were used. The PCE in SM2:PC71BM devices was even higher than 8.00% within 15 seconds SVA when CF, DCM and THF solvents were applied for SVA. The SVA time was then


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optimized by scanning from zero to 90 seconds with both SM:PC71BM systems, and a 15 sec annealing time was found to be best in both cases (Figure S5 and S6 and Tables S3 and S4). The combined effect of SVA and TA on the device performance outcomes was also studied and the SMOSCs prepared with SM1 and SM2 were initially subjected to 15 seconds of SVA followed by thermal annealing for 10 minutes at 90 oC and 120 oC; the photovoltaic parameters in these cases are shown in Table S5 (J-V curves are shown in Figure S7). There were no improvements in the performance outcomes of SMOSCs. In contrast, the PCE dropped with higher TA conditions, as shown in Figure S7 and Table S5. A weight ratio scan of the SM2:PC71BM system was also carried out, and it was found that the best device performance could be achieved with a donor-to-acceptor (D/A) ratio of 1:1 (Table S6 and Figure S8). Therefore, a D/A ratio of 1:1, a SVA time of 15 seconds and DCM as a solvent for SVA were utilized in the remaining experiments. SMOSCs were prepared from PC71BM blends with SM1 and SM2 under optimized conditions using CF as the main processing solvents (J-V curves are shown in Figure 2b and detailed photovoltaic parameters are summarized in Table 2). Photographic images of as-cast and SVA neat SM films as well as their blend films with PC71BM are shown in Figure 2a. All of the photovoltaic parameters were improved in newly synthesized SM2-based devices as compared to SM1-based SMOSCs. Due to the down-shifted HOMO energy level, SM1-based SMOSCs showed a higher VOC (0.83 eV) as compared to that of the SM1 (0.80 eV) based SMOSCs. The broader absorption in the range of 580nm to 700nm in SM2 and the appearance of a shoulder peak around 640nm (Figure 1b) resulted in a higher short-circuit current density (JSC) in SM2, i.e., 13.86 mAcm-2 as opposed to the JSC in SM1 of 12.93 mAcm-2 (Table 2), which was further corroborated when their external quantum efficiency (EQE) was measured (Figure 2c). The calculated current density as measured from the EQE spectra for SM2 is higher (13.55 mAcm-2)


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in comparison to that of the SM1-based SCs (12.13 mAcm-2). To investigate the overall enhancement in the performance outcomes of the SM2:PC71BM devices, further characterizations were carried out. These results are discussed in the upcoming sections. When SM2 was synthesized, it was postulated that the dimer-type SM would be more planar as compared to the monomeric SM1 due to the higher interdigitation of the dimer units in the backbone of the SM. This increased interdigitation will occur due to the interlocking of the sidechains of the SM2, resulting in higher crystallinity, while this feature is absent in SM1. This planarity may lead to enhanced crystallinity in the SM2 as compared to SM1, and it may also promote π-π as well as lamellae stacking in the SMs. These features are highly desired in SMOSCs because they have an effect on the incident light absorption, exciton dissociations at the donor acceptor (D/A) interface, and charge transport properties within the photoactive layer.40-43 To confirm this, pristine SMs and their blended films with PC71BM were characterized with the 2D grazing incidence wide-angle scattering (2D-GIWAXS) technique. GIWAXS can be used to gain many types of information about the photoactive layer, i.e., the orientation of the photoactive materials along the in-plane (IP) or out-of-plane (OOP) direction with respect to the incident light, the characteristics of the π- π stacking, and the size of the lamellae in the films.44-46 No noticeable difference in the crystallinity or orientation of the SMs was found between pristine films processed from SM1 and SM2, as shown in images in Figure S9 (line profiles are shown in Figure S10a and S10b). When these SMs were blended with PC71BM, 2D-GIWAXS characteristics are shown in Figure 3 and the d-spacing parameters are summarized in Table 3. The prominent halo ring around 1.35 Å along the IP and OOP direction comes from the PC71BM molecules,47-48 and the intensity of this peak is reduced after SVA (Figure 3a-3d), indicating that


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their blending with the SMs increased. The preferential edge-on orientation of the SMs was found in both blended films, as shown in Figure 3. A long-range (h00) lamellae order in the IP direction and (010) π-π stacking peak were found in blended films with SVA (Figure 3b, d) as compared to photoactive films without SVA (Figure 3a, c), which are strong evidence of the enhanced crystallinity of these blended films with SVA process. These enhancements in the crystallinity are also revealed in a colour change of the blend films from brown to purple in photography images (Figure 2a).49-50 Most importantly, the (h00) peak position was shifted towards a higher q value, as indicated in the OOP and IP line profile of the SM2:PC71BM film (Figure 3e, f), which is an indication of enhanced interaction between the lamellae. As a result, the size of the lamella in the SM2:PC71BM films was found to be smaller than that in the SM1:PC71BM film along the OOP and IP direction (Table 3). Further morphological characterizations were carried out using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The surface roughness levels were increased after the SVA process (Figure S11), possibly resulting in enhanced interaction and packing between the donor SMs (AFM phase images are shown in Figure S12). Additionally, when the bulk morphology was characterized using TEM, it was clearly noted that the SVA induces a fibril type of interpenetrating network with strong crystallinity within the bulk of the blended films (Figure S13). After SVA, both blended films showed similar morphological changes, but the domains in the SM2 were smaller as compared to those in the SM1-based films (Figure S13). All of these abovementioned morphological factors, for example, the reduction in the lamellae sizes and the formation of interdigitated structure have profound effects on the photovoltaic performances of the SMOSCs. Therefore, when the solar cells based on SM1 and SM2 were fabricated, the SM2-based SMOSCs outperformed the SM1-based solar cells. Owing to the


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enhanced crystallinity induced by the interdigitation of the side chains of SM2 and the formation of compact lamellae, all of the photovoltaic parameters were improved, especially JSC and FF (Figure 2, Table 2). With the photoactive film thicknesses being kept at 100 nm, the neat SM2 and SM2:PC71BM blended photoactive films demonstrated higher and wider absorption coefficients as compared to those of the SM1 based films (Figure S14a and 14b), which may have factored into the higher photocurrents in SMOSCs. And the formation of a fibril network in the SMOSCs (TEM images, Figure S13) positively affected all of the photovoltaic parameters.5152

To deeply understand FF value as high as 75% in the SM2-based SMOSCs, hole and electron

mobilities (µh and µe) were measured using the space-charge limited current (SCLC) method. It was found that the neat SM2 and SM2:PC71BM blended films have higher values of µh and µe compared to those of the SM1 based films (Figure S15 and Table 2). Along with the charge mobilities, the types (bimolecular or monomolecular) and the extent of recombinations were also measured with the incident light intensity (Figure S16) by means of a previously reported method.40-42 The J-V curves with respect to the incident light intensity are shown in Figure S17. When VOC is plotted against the incident light intensity (LI), the slope of the SM2:PC71BM blended film is found to be lower (1.46) than that of the SM1:PC71BM blended films (1.57), indicating that the SM2-based SMOSCs have fewer monomolecular or trap-assisted recombinations as compared to the SM1-based SMOSCs.53-55 Moreover, the SM2:PC71BMbased SMOSCs have high photocurrents (Jph) as compared to the SM1:PC71BM-based SMOSCs (Figure S18). When the photoluminescence (PL) was measured, the PL quenching was higher in the SM2 blended film as compared to the SM1 blended films (Figure S19). Although similar interpenetrating fibril networks in the SM:PC71BM films were found in the TEM images (Figure S13), the fibrils and size of the lamellae of the SM2 were smaller. Therefore the SM2 blended


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films had a higher PL quenching rate, indicating that the photo-generated excitons reaching the D/A interface were more efficiently dissociated into free carriers. The tolerance of the photoactive thickness is a highly desirable factor when using R2R printing techniques in SMOSCs, the SM1- and SM2-based blends were screened with thicknesses ranging from ~70 nm to ~350nm (J-V curves are shown in Figure S20 and Figure S21 and photovoltaic parameters are tabulated in Tables S7 and S8). The SM2-based SMOSCs indicated much higher PCEs as compared to those of the SM1-based SMOSCs over a wide range of thicknesses. As shown in Figure S22a, the EQE in the SM2:PC71BM-based SMOSCs is very high in a ~280-nm-thick photoactive layer in comparison to that in the SM1:PC71BM-based SCs. This may have been caused by the higher absorption coefficient and higher charge carrier mobilities in the SM2:PC71BM photoactive layer than the SM1-based photoactive layer. Moreover, the difference in the photovoltaic parameters over a wide range of the photoactive thickness is less in the SM2:PC71BM-based SMOSs than in the SM1:PC71BM-based devices (Figure S22b, c). Additionally, when inverted SMOCSs were fabricated using ZnO NPs as an electron transport layer (J-V and EQE shown in Figure S23), the SM2:PC71BM devices showed higher PCEs as compared to SM1:PC71BM-based solar cells (Table S9). Owing to the higher mobilities, fewer charge recombinations, higher photocurrents and higher PL quenching levels, the SM2:PC71BM-based SMOSCs outperformed the SM1:PC71BM-based SMOCs and showed remarkably higher FF (75%) as compared to the SM1:PC71BM-based SMOCs (72%). 2.4 Halogen-free SMOSCs Halogenated solvents such as CF, CB, and oDCB do not exist in nature and require additional processing steps to synthesize them. The polymer-based OSCs and SMOSCs processed with


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non-halogen solvent systems have already been reported with fullerene and non-fullerenes electron acceptors.13, 56-58 However, the different device performance of the OSCs processed with non-halogenated solvents depending on the molecular chemical structures has been rarely studied. To translate the lab-scale performances to an industrial R2R method, the crucial step is to use an environmentally friendly solvent with the suitable boiling point such as the o-xylene. Therefore, the halogen-free processing of photoactive layer in SMOSCs was carried out using oxylene as the main processing solvent. Bulk heterojunction SM:PC71BM based SMOSCs were fabricated with device structure of ITO/PEDOT:PSS/photoactive layer/Ca/Al. The J-V characteristics and EQE spectra of the best SMOSCs for each SM donor are shown in Figure 4a and b. Contact angle measurements of the photoactive solutions were performed to examine the interfacial contact properties between photoactive films and substrate (Figure 4c, d). No big difference could be observed between samples based on SM1 and SM2 photoactive solution, however, the forming of the SM1:PC71BM blended film could not be done; moreover, pinholes were found throughout the film, as shown in Figure 4e. In contrast, our newly synthesized SM2 blended film led to a pinhole-free film, as shown in Figure 4f. This behaviour is explained by the hypothesis that simultaneously increasing the degree of interactions and interdigitations between SM2 donors in photoactive films compared to the SM1 based films could induced continuous and pinhole-free over the substrate. As a result, 8.37% PCE was achieved with o-xylene processed SM2:PC71BM-based SMOSCs. These results demonstrate that the newly synthesized SM2 is more versatile and therefore processable from non-halogenated solvents. 3. Conclusions In conclusion, a novel small molecule (SM2) was synthesized via a dimerization approach to enhance the interdigitation of the side chains of the SM. Extended conjugation due to


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dimerization rendered a reduced band-gap and deeper the HOMO and LUMO levels in SM2. SM2:PC71BM blended photoactive layer exhibited not only higher PCE and thickness tolerance levels due to the higher crystallinity and fibril nature of the films, but it also performed very well in non-halogenated solvent systems, showing similar performance outcomes compared to the halogenated solvent. This molecular engineering approach is an effective methodology with which to tune the electronic properties and thereby design such molecules which can perform halogen-free solvent systems for real photovoltaic applications. 4. Experimental Section Device fabrication and characterization Conventional devices with the architecture Glass/ITO/PEDOT:PSS/photoactive Layer/Ca/Al were fabricated. Firstly, ITO patterned glass substrates were cleaned with cleaning detergent in deionized water followed by acetone and IPA, three times each. These ultrasonically cleaned glass substrates were put inside an oven at 140 oC for 8 hours. Photoactive solutions were prepared by dissolving the SMs and PC71BM into CF or o-xylene, and put onto hot plate for thorough mixing at 50 oC for four hours, no processing is used in the experiments. ITO coated glass substrates were treated with ozone for 15 minutes prior to the spin coating of the PEDOT:PSS as a hole transport layer. And then, spin coating of PEDOT:PSS (AI4083, HERAEUS) at 4000rpm for 30 seconds was carried out and a uniform thickness of nearly 30 nm was achieved. Solutions of photoactive materials were filtered with 0.45 µm filter and then, solutions and substrates were transferred to glove box where photoactive layers were spin coated at 3000 rpm for 30 seconds in the case of CF solvent and at 1000 rpm in the case of o-xylene solvent. SVA with different solvents was carried out by putting the dry substrates into a Petri


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dish and about 0.15 mL of solvent was placed along the walls of the Petri dish and was covered with the lid. After required SVA time, the devices were transferred to metal deposition chamber where 2 nm of Ca and 100 nm of Al were thermally evaporated and deposited over the substrates using a shadow mask. The vacuum inside the deposition chamber during the metal depositions was around 10-7 torr. Power conversion efficiencies were measured under 100 mW/cm2 AM 1.5G irradiation from a xenon arc lamp with an AM 1.5 global filter using a computer-controlled Keithley 236 source measure unit (Keithley Instruments), after calibration with NREL certified silicon diode with an dintegrated KG5 optical filter. EQE was measured by K3100 measurement system (McScience, South Korea). All device measurements were carried out in air at room temperature. Synchroton X-ray diffraction analysis Grazing-incidence wide angle X-ray Scattering (GIWAXS) measurements were conducted at PLS-II 3C beamline of Pohang Accelerator Laboratory (PAL) in Korea. The X-rays coming from the in-vacuum undulator (IVU) are monochromated using Si(111) double crystals and focused at the detector position using K-B type mirrors. GIWAXS patterns were recorded with a 2D CCD detector (Rayonix SX165) and X-ray irradiation time was 2 ~ 120 seconds dependent on the saturation level of detector. The incidence angle (0.1 ~ 0.14o) was carefully selected to allow for complete penetration of the X-rays into the film. SCLC devices fabrication and testing Mobility measurements of small molecule:PC71BM (w/w, 1.0:1.0) blends were carried out using space-charge limited current (SCLC) method with the following diode structures:


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ITO/PEDOT:PSS/active layer/Au for hole-only devices and ITO/ZnO NPs/PEIE/photoactive layer/Ca/Al for electron-only devices by taking current-voltage measurements and fitting the results to a space-charge limited form. The charge carrier mobilities were calculated using the SCLC model, where the SCLC is described by: J = 9ε0εrµV2/8L3, where J is the current density, L is the film thickness of the photoactive layer, μ is the hole or electron mobility, εr is the relative dielectric constant of the transport medium (which is usually kept at 3 for polymers and small molecules), ε0 is the permittivity of free space (8.85 × 10–12 F m–1), V is the internal voltage in the device, and V = Vappl – Vr – Vbi, where Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance between the electrodes, and Vbi is the built-in voltage due to the relative work function difference of the two electrodes.


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Figure 1. (a) Chemical structures of SM1 and SM2, (b) UV-Vis absorbance of SM1 and SM2 in solution and film states, and (c) energy level diagram of the SMOSCs with SM and PC71BM.

Table 1. Thermal, optical and chemical properties of SM1 and SM2. Small Molecule

Td [°C]a

λmax [nm] solution

λmax [nm] film

λonset [nm] film

Eg opt [eV]b

HOMO [eV]

LUMO [eV]c

SM 1

383d

511

567

703

1.76

-5.36

-3.60

SM 2

397

510

569

712

1.74

-5.45

-3.71

aDecomposition bEstimated cCalculated dThe

temperature (Td) was determined by TGA (with 5% weight loss).

values from the UV-vis absorption edge of the thin films (Egopt = 1240/ λonset eV). from HOMO energy levels and optical band gaps.

value is taken from reference 35.


17


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Figure 2. (a) Neat and blend films of SM1 and SM2 without and with CH2Cl2 SVA. (b) J-V curves of SMOSCs processed with CF and their (c) external quantum efficiencies.


18

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Table 2. Photovoltaic performances of the SM1:PC71BM and SM2:PC71BM based SMOSCs processed with the CF solvent. Small Molecule

PCE [%]

µh × 10-5 [cm2/V s]c

µe × 10-5 [cm2/V s]d

Annealing

VOC [V]

JSC [mA/cm2]

FF [%]

W/O

0.88 0.88±0.01

9.72 (9.55)a 9.65±0.08

46 45±1

3.95 (3.72±0.22)b

3.13 (3.04±0.08)b

0.62 (0.51±0.11)b

SVA

0.80 0.79±0.02

12.93 (12.13)a 12.34±0.61

72 71±2

7.48 (7.16±0.33)b

6.94 (6.82±0.13)b

8.92 (8.79±0.10)b

W/O

0.90 0.90±0.01

11.59 (11.16)a 11.33±0.27

56 56±1

5.77 (5.59±0.18)b

5.16 (5.02±0.13)b

2.08 (1.95±0.12)b

SVA

0.83 0.83±0.01

13.86 (13.55)a 13.65±0.22

75 74±1

8.64 (8.51±0.15)b

18.30 (18.11±0.21)b

15.00 (14.89±0.12)b

SM 1

SM 2

aThe

value is calculated from EQE data.

bThe

average value in the brackets is obtained from over 20 independent devices.

cHole-only

device is ITO/PEDOT:PSS/photoactive Layer/Au.

dElectron-only

device is ITO/ZnO NPs/PEIE/photoactive Layer/Ca/Al.


19


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Figure 3. 2D-GIWAXS images of the blends of (a, b) SM1:PC71BM and (c, d) SM2:PC71BM without and with SVA, respectively. Line profiles of the blend films in the (e) OOP and (f) IP directions.

Table 3. Summary of the d-spacing parameters. Film

Annealing

(100)a [Å]

(100)b [Å]

(010)b [Å]

W/O

22.97

23.91

-

SVA

21.41

24.10

3.75

W/O

20.88

21.26

-

SVA

20.51

22.09

3.74

SM 1:PC71BM

SM 2:PC71BM

aCalculation

from the out-of-plane direction.

bCalculation

from the in-plane direction.


20

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Figure 4. (a) J-V characteristics of the SM:PC71BM SMOSCs processed with o-xylene (b) relevant EQE spectra of the SM2:PC71BM (c, d) contact images of the photoactive solution on the substrate processed from o-xylene and (e, f) optical images of the photoactive films with (c, e) SM1:PC71BM, (d, f) SM2:PC71BM, respectively.


21


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic methods, additional figures as mentioned in the main text, 1H and 13C NMR spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.E.S.) *E-mail: [email protected] (Y.-H.K.) *E-mail: [email protected] (J.-C.L.) Author Contributions Z.A. and T.T.T.B. synthesized small molecules. R.A., S.R. and C.E.S. have contributed to device fabrication and data analysis. Z.A. and C.E.S. wrote the manuscript. H.K.L. and S.K.L. measured GIWAXS and analyzed the data. W.S.S., W.-W.S. and S.-K.K. suggested experimental advice. All authors discussed and commented on the paper. C.E.S., Y.-H.K. and J.-C.L. supervised the experiment and manuscript. Notes The authors declare no competing of financial interest Acknowledgements This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT &


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TOC


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An effective molecular engineering approach for employing a halogen

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