Impact of the Crystalline Packing Structures on Charge Transport and

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Impact of the crystalline packing structures on charge transport and recombination via alkyl chain tunability of Dppbased small molecules in bulk heterojunction solar cells Chang Eun Song, Yu Jin Kim, Sanjaykumar R Suranagi, Gururaj P. Kini , Sangheon Park, Sang Kyu Lee, Won Suk Shin, Sang-Jin Moon, In-Nam Kang, Chan Eon Park, and Jong-Cheol Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01576 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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ACS Applied Materials & Interfaces

Impact of the crystalline packing structures on charge transport and recombination via alkyl chain tunability of DPP-based small molecules in bulk heterojunction solar cells

Chang Eun Song,‡ab Yu Jin Kim,‡c Sanjaykumar R. Suranagi,‡ab Gururaj P. Kini,ab Sangheon Park,ad Sang Kyu Lee,ab Won Suk Shin,ab Sang-Jin Moon,ab In-Nam Kang,e Chan Eon Park*c and Jong-Cheol Lee*ab

a

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

Daejeon, 305-600, Republic of Korea. b

Chemical Convergence Materials Major, University of Science and Technology (UST),

Daejeon, 305-350, Republic of Korea. c

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Poha

ng University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea d

Department of Physics, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si,

Gyeong Gi-do, Republic of Korea. e

Department of Chemistry, The Catholic University of Korea, Bucheon, Gyeong Gi-do, 420-

743, Republic of Korea.

ACS Paragon Plus Environment


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ABSTRACT A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2ethylhexyl (EH), 2-butyloctyl (BO) and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing and, hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.

KEYWORDS Alkyl chain length, Benzodithiophene, Bulk heterojunction solar cell, Diketopyrrolopyrrole, Small molecule solar cell


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INTRODUCTION Solar energy has attracted considerable attention as a green energy resource. Bulk heterojunction (BHJ) organic photovoltaics (OPVs) consisting of a blend mixture of conjugated donor materials and fullerene-derivative electron acceptors constitute a promising component of inexpensive, lightweight, solution-processable, large-area flexible devices for use in energy-generating applications.1-4 Among these applications, solution-processed smallmolecule OPVs have emerged as attractive alternatives to their more widely studied polymeric counterparts. They display advantages such as significantly defined molecular structures particularly for compounds of intermediate dimension and molecular weights, relatively high-level purity and good reproducibility from one batch to the next.5-7 Consequently, by combining efforts at small-compound design, active-morphology engineering, structural modification and device structure controlling, BHJ OPVs based on small-molecule/fullerene blend mixtures have reached significant milestones with over 9% power conversion efficiencies (PCEs).8-9 Among the strategies being explored, the creative choice of donor (D) and acceptor (A) moieties in the molecular design has shown considerable potential for the design of small molecule materials for use in high-performance photovoltaics. Some studies have focused on understanding the effect of alkyl side chains on the device performance of OPVs based on alternating D–A small molecules. Recent research has demonstrated that the grafting of different alkyl side chains onto small molecules can dramatically improve the opto-electronic properties, intermolecular packing, nano-scale morphology of blends and, consequently, photovoltaic performance.10-14 We have taken into account the above considerations to design and synthesize three new small molecules based on triisopropylsilylethynyl (TIPS)-substituted benzo[1,2-b:4,5b']dithiophene (BDT) units and diketopyrrolopyrrole (DPP) groups to determine the effect of



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alkyl side-chain length on the photovoltaic performance. We expected the TIPS motif to enhance π-conjugation and rigidify the structure, as well as to produce a more electron-rich BDT core that would allow effective intramolecular charge transfer (ICT) and self-packed molecular structures.15-16 Most of all, small compounds having TIPS-substituted BDT units have rarely been reported in the OPV field. Furthermore, the DPP unit, with two fused electron-withdrawing lactam units, and is hence a strong acceptor building block with a high molar absorption coefficient. Thus, it has been widely introduced in solution processed BHJ solar cells with high photocurrent density. 17-19 For the three small-molecule semiconductors, we chose 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD) as side chains on the DPP unit, and obtained SM1, SM2 and SM3 compounds. We systematically investigated the effects of side-chain size (EH vs. BO vs. HD) on the sun-light absorption, charge transport and recombination characteristics of the resulting small molecules as well as on the their conformations and photovoltaic properties. The research reported herein is a detailed study of the impact of the respective alkyl chain lengths on DPP-based small-molecule OPVs.

RESULTS AND DISCUSSION Synthesis, characterization and thermal properties The synthesis of the three small molecules is depicted in Scheme 1. Starting from the reaction of 4,8-dihydrobenzo[1,2-b:4,5-b’]dithiophene-4,8-dione with TIPS-acetylide, the 2,7-bis(trimethylstannyl)TIPS–BDT 2 was prepared in two steps following procedures described in the literature.20 DPP-functionalization was done by reaction of DPP 3 (3,6dithiophene-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) with three different alkyl chains using K2CO3 as a base in dimethylformamide (DMF) as demonstrated in the already reported-literature, followed by the subsequent mono-bromination with N-bromosuccinimide


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(NBS) to obtain 5a, 5b, and 5c, respectively.21-22 The small molecules (SMs) were obtained by a Pd(PPh3)4- assisted Stille coupling reaction between TIPS–BDT 2 and DPP 5a, 5b, and 5c, respectively. The result materials were distinctly characterized by using 1H NMR and 13C NMR spectroscopies and mass spectrometry (Supporting Information). The three obtained small molecules were observed to be quite soluble in commonly used organic solvents such as tetrahydrofuran, chloroform (CF), chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) without gel formation. It is very important that the small molecules be thermally stable if they are to be used effectively in optoelectronic devices. The thermal properties of the synthesized compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements. The three synthesized DPP-based small molecules were found to be quite thermally stable: they displayed onset decomposition temperatures for 5% weight loss (Td) above 360°C, as shown in Figure 1a and Table 1. Obviously, the thermal stabilities of SM1, SM2 and SM3 is sufficiently adequate for use in OPVs and other opto-electronic devices.23 The thermal behaviors of the target molecules were further studied by DSC (Figure 1b). The DSC data showed the same trend for melting transition temperature (Tm) and crystallization temperature (Tc) in the order: SM1 (Tm = 257°C and Tc = 240°C) > SM2 (Tm = 193°C and Tc = 174°C) > SM3 (Tm = 170°C and Tc = 152°C) (Table 1). A higher transition temperature indicates a more ordered microstructure, and thus SM1 with its relatively short EH chains had a higher crystallinity than the other two compounds.24

Photophysical and electrochemical properties The photo-physical (optical) characteristics of the SM1, SM2 and SM3 small molecules were studied by ultraviolet-visible (UV-vis) absorption spectroscopy in dilute CF solutions



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and as solid-state thin films (Figure 2a, b, Table 1). In the diluted solution state, all of the BDT-DPP derivatives yielded quite similar absorption bands between 300–700 nm. That at 550–650 nm was ascribed to ICT band between the main-site donor (BDT) and side-site acceptor DPP moieties.25 The other absorption bands (300–450 nm) were assigned to localized π–π* transitions and the conjugation framework.26 Compared to their corresponding solution state spectra, the solid-state thin film absorption bands of all three small molecules were broadened/extended with their maxima (λmax) redshifted (Figure 2b). These absorption features resulted from increased interchain interactions in the solid films, which may have been due to the greater extent of π-π* interaction stacking of the backbones or enhanced polarizability of the film or both.27-28 The shorter alkyl side chain is from HD to BO to EH, the more systematic red-shifting of the absorption band is. This was because more planar structures caused by stronger intermolecular interactions were manifested in the sequence of SM3 < SM2 < SM1;28 this trend is in agreement with the trends in crystallinity obtained from the DSC profiles. The optical bandgaps (Eg opt) of SM1, SM2 and SM3 were 1.66, 1.68 and 1.69 eV, respectively, calculated from the λonset of their solid-state films (Table 1). Cyclic voltammetry (CV) experiments were carried out on the thin films of each of these three compounds to help understand how their chemical structures influenced their electrochemical properties (Figure 2c, Table 1). The highest occupied molecular orbital (HOMO) energy and lowest unoccupied molecular orbital (LUMO) energy were determined from the onset oxidation potentials and onset reduction potentials. The HOMO level systematically increased as the side chain lengthened from EH to BO to HD: –5.85 for SM1, –5.80 for SM2 and –5.71 eV for SM3. The LUMO levels of the three compounds systematically decreased: –3.44 for SM1, –3.39 for SM2 and –3.37 eV for SM3. Evidently, the different alkyl side-chains on the DPP units affected both the HOMO and LUMO energy


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levels of the resulting small molecules. However, the similarity in the values indicated that the different alkyl side-chain lengths incorporated into the molecules had only a minor effect on the electro-chemical behavior of the compounds. Furthermore, the CV results demonstrated that the three molecules had proper frontier energy levels that were wellaligned with those of PC71BM (Figure 2d).29

Charge carrier transport characteristics Hole carrier mobility is an important parameter to consider when assessing the effectiveness of photovoltaic donor materials. Here, we constructed organic field-effect transistors (OFETs) to investigate the hole mobilities of the three small compounds. To measure these carrier mobilities, OFETs having a bottom-gate/top-contact architecture were fabricated using a heavily n-doped conductive silicon wafer with a 300-nm-thick layer of SiO2. The hole carrier mobility was derived from the source–drain current (IDS) vs gate voltage (VG) transfer curve in a clear saturation regime (Vsd = –80 V) (Figure 3a). The SM1 thin film had the highest hole mobility of 6.3 × 10–4 cm2 V–1 s–1, while the SM2 sample exhibited a medium hole mobility of 2.4 × 10–4 cm2 V–1 s–1 and the SM3 device had the lowest hole mobility of 6.8 × 10–5 cm2 V–1 s–1. These OFET mobilities are consistent with their UV–vis absorption data. Our small molecules having the shorter alkyl chains attached to the DPP moiety showed higher charge carrier mobility, which was because of more ordered packing structures resulting from lower steric hindrance.30 Higher mobilities of small molecule materials boost the efficiency of organic solar cells.31 The hole mobility was also evaluated using the space charge limited current (SCLC) model. Hole-only diodes with the structure of ITO/PEDOT:PSS/Active blend/Au were fabricated and characterized to demonstrate their reliabilities (The details of how SCLC devices were fabricated and how the measurements were taken are described in the Experimental section).



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Figure 3b shows the dark J-V curves for the hole-only devices. The SCLC mobilities followed the order 2.4 × 10-4, 9.8 × 10-5 and 2.1 × 10-5 cm2 V-1 s-1 for the SM1, SM2 and SM3 cases, respectively. The trend in the mobilities was the same for the SCLC and OFET data and indicated that higher hole mobilities derived from stronger intermolecular interactions enabled higher photocurrent densities.32 Moreover, a higher donor material carrier mobility can allow holes to be collected more quickly, thereby reducing the rate of competing second-order recombination and boosting the JSC.33

Small-molecule photovoltaic properties Bulk heterojunction solar cell devices with a traditional ITO/PEDOT:PSS/small molecule:PC71BM/Ca/Al configuration were fabricated to determine the effects of varying the alkyl side-chains on the photovoltaic properties of the DPP-based small molecules. The best result was obtained from a solution blend of a small molecule donor/fullerene acceptor compositional ratio of 1:1 at a concentration of 20 mg mL–1 in CB. The performances were further improved by processing in CF as an alternative solvent. Figure 4a and b shows the current density-voltage (J–V) characteristics the corresponding data is summarized in Table 2. First, all of the three small-molecule-based devices showed substantially higher device performances (ca. two-fold higher values) when they were processed in CF rather than in CB; similar results have been previously reported for other small molecule OPVs.34-36 For the three devices spin-coated from CF, the maximum PCEs of 4.53% (open circuit voltage (Voc) = 0.92 V, short circuit current (Jsc) = 10.3 mA cm–2, fill factor (FF) = 54%), 2.67% (Voc = 0.91 V, Jsc = 5.26 mA cm–2, FF = 46%) and 0.88% (Voc = 0.86 V, Jsc = 2.75 mA cm–2, FF = 37%) were achieved for the SM1-, SM2- and SM3-based devices. Although the backbone structures of the compounds were identical, differences in the alkyl side chains on the DPP moiety had a marked effect on their photovoltaic device performances in solution-processed


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BHJ solar cells. The radical difference in PCE values was mainly attributed to the significant variations in photocurrent (JSC) and FF. The boosted photocurrent density with increasing fill factor may have resulted from enhanced charge carrier movement, elevated carrier collection efficiency, or formation of an optimal morphology. This is supported by the results of the hole carrier transport data; the active-morphology studies and charge recombination analyses are discussed below. Next, to further enhance the device efficiency of single-jucntion solar cells made from SM1, we investigated the effect of the high-boiling additives 1,8-diiodooctane (DIO), 1chloronaphthalene (CN) and diphenyl ether (DPE) (3%, by volume) on the photovoltaic properties (Figure 4c and Table 3). The PCEs were substantially improved for the devices processed with CN and DPE additives but not with the DIO additive. Specifically, when 3 vol% CN was added to the SM1-based cell, the Jsc and FF improved to 11.90 mA cm–2 and 60%, respectively, and thus elevated the PCE to 5.54%. This is a much higher value than those reported for current DPP-based small-molecule solar cells. Figure 4d displays the incident photon vs. conversed current efficiency (IPCE) curves of the photovoltaic devices under monochromatic light illumination. The SM1 OPVs processed with the different additive solvents exhibited relatively broad IPCE responses that ranged from 300–800 nm. The CN-treated device exhibited the highest IPCE value of over 53% in the 300–550 nm range, which provided the highest JSC value for CN-treated devices.37

Nano-scale morphological characteristics The nano-structural active blend morphology is one of the main factors that determine photovoltaic device performance. Atomic force microscopy (AFM) was used to explore the effects of various alkyl side-chains on the morphology and device performance of DPP-based small molecule:PC71BM films. The surface morphologies of the blend films processed from



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CF were studied; these blend films were prepared according to the device procedure. Figure 5 shows that the active layer of SM2 (b) displayed a somewhat more aggregated surface and slightly larger domains with a higher root-mean-square (RMS) roughness of 0.56 nm compared with the SM3:PC71BM blend films (RMS = 0.25 nm) (c). After using the active layer with SM1 (a), larger domains and aggregates were apparent in the height image, and the RMS roughness increased to 1.22 nm. These aggregates of the larger segregated domain phase most likely arose from the more extensive inter-molecular interactions of their materials,38 which agrees with the hole carrier mobility results (Figure 3). Therefore, a relatively rough surface and more ordered structure benefited charge transport and resulted in increases of JSC and FF as well as improved device efficiency.39 While AFM can be used to probe the blend morphology of the top surface of a film, this technique can’t provide bulk state morphological information. Two-dimensional grazingincidence wide-angle X-ray scattering (2D–GIWAXS) was used to investigate the bulk morphology of the blend films. Figure 5d–f shows 2D–GIWAXS patterns of small molecule:PC71BM mixture films deposited from CF solutions. Reflection peaks corresponding to lamellar packing structures were observed in the out-of-plane axis for all of the small molecule:PC71BM films; lamellar stacked-peaks (h00) were shown up to (300) scattering peaks, with a Bragg distance of 22.93 Å (the corresponding qz,(100) = 0.274 Å–1) arising from the donor compounds. The small molecules strongly self-stacked networking in an edge-on population when spin-casted onto the PEDOT:PSS-substrates.40 Furthermore, a broad diffraction peak was seen with halo scattering, which derived from aggregated PC71BM domains.41 Interestingly, the peak location was the same for all of the blend films, but the peak intensity varied considerably. Stronger scattering intensity exhibited as a larger peak size appeared in the 2D–GIWAXS images in the sequence: SM1 > SM2 > SM3 blend films. Notably, the crystallinity and order of the structures of the compounds decreased in the order:


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SM1 > SM2 > SM3.42 These results are also consistent with the film morphology as observed using AFM (Figure 5a–c).

Relatively recombination Analysis for the dependence of the J-V characteristics on the intensity of the irradiated light The dependence of the J-V characteristics on the intensity of the irradiated light was investigated to further determine how the device performance was influenced by charge transport and blend morphology, which themselves depended on the nature of the alkyl side chain. Figure 6 represents the J–V plots of the illuminated SM1:PC71BM, SM2:PC71BM and SM3:PC71BM devices obtained under different light intensities ranging from 3.2–100 mW cm–2. These data were used to reveal the dependence of Voc on the light intensity for the three device systems (Figure 6d). Recently, the Voc vs. light intensity has been used to better understand the influence of trap-assisted recombination or second-order recombination in the devices.43 The light intensity (I) and Voc are given by according to the following expression: Voc ≈

(1 − ) ln

This formula shows the dependence of the Voc on varied light intensity because the exciton (hole-electron pairs) generation rate (G) is a direct term proportioning to illumination intensity, I.43-44 In general, the curve slope of Voc vs. I (logarithm) is equal to k(Boltzmann constant)T(temperature)/q(probability for exciton separation) for second-order (Langevin) recombination. However, a stronger dependence of Voc as a function of I has been observed in a system in which obvious trap-assisted recombination occurs, with the slope in such a case equal to 2kT/q.44 In our cases, the SM1:PC71BM device had the lowest slope of 1.08 kT/q while SM2:PC71BM and SM3:PC71BM devices had higher slopes of 1.11 and 1.37 kT/q, respectively. These results implied that the ideal packing morphology of the SM1:PC71BM



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blends permitted fewer trapping defects and yielded reduced trap densities existing between interfacial surface and the active layer in the stacked-device, and in this way suppressed trapassisted recombination and allow to an increased photocurrent density.45 Additional insight into the recombination mechanism was obtained by investigating Jsc as a function of light intensity (Figure 6e). A power law dependence of JSC upon the light intensity can be expressed in below equation, sc = γ(ℎ ! "#)$

where α is the exponential constant factor and γ is a constant.46 Under the ideal condition,

the second-order recombination should be minimized (α ≈ 1); any deviation of α from 1 implies that second-order recombination has taken place.47 Figure 6e shows that the extracted α values were 0.96, 0.91 and 0.85 for the SM1, SM2 and SM3-based devices, respectively.

The proximity to unity (α ≈ 1) followed the order SM1 > SM2 > SM3, which illustrated that the carrier sweep-out was more efficient and fully suppressed second-order recombination in the devices in the sequence SM1:PC71BM, SM2:PC71BM and SM3:PC71BM.47-48 Finally, the variation of FF as a function of illuminated light intensity was examined to establish the reason for the higher FF in the devices processed with small molecules having relatively short alkyl chains with second-order recombination factors (Figure 6f). All of the devices showed decreasing FF values with decreasing light intensity. However, the FF declined in the order SM3:PC71BM, SM2:PC71BM and SM1:PC71BM, which showed a larger variation with various light intensity, I, explaining that the recombination losses dominated at high current densities.49

CONCLUSION In conclusion, a series of solution-processed small compounds based on TIPS-substituted


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BDT units and DPP moieties were designed and synthesized for BHJ OPVs. These new small molecules had identical backbone structures, but had different alkyl side chains, i.e., ethylhexyl (EH) for SM1, butyloctyl (BO) for SM2 and hexyldecyl (HD) for SM3. The different types and shapes of the side chains strongly influenced the light absorption, charge transport properties and charge recombination factors of the small molecules as well as the morphological characteristics and photovoltaic properties of the small-molecule-based solar cells. Those small molecules with shorter alkyl side chains had more ordered structures with higher crystallinities in the thin films, which improved charge free-carrier movement and reduced recombination. The performance of the solar cell devices declined in the manner SM1 > SM2 > SM3 with PCEs of 4.53, 2.67 and 0.88%, respectively. Furthermore, the effect of processing small amount additive solvents on the enhancement of photovoltaic performance was demonstrated; the JSC and FF of the SM1:PC71BM device were particularly noteworthy. After optimization, the highest PCE of 5.54% was found for SM1:PC71BM cells treated with CN solvent. This work provides insight into the advantages of using side-chain engineering to efficiently construct photovoltaic small molecules.

EXPERIMENTAL SECTION Materials All chemicals were purchased/obtained from commercial suppliers and used as received to final product. All solvents were purified by distillation prior to use. Reactions were conducted under an atmosphere of nitrogen (N2) and monitored by thin layer chromatography (TLC) on silica gel 60 F254 (Merck). Flash chromatography was performed using Merck silica gel 60 (particle size 230–400 mesh).

Synthesis



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As noted above, 4,8-dihydrobenzo[1,2-b:4,5-b']dithiophen-4,8-dione with TIPS–acetylide, TIPS–BDT 1 and 2,7-bis(trimethylstannyl)TIPS–BDT 2 were prepared by using established synthetic methods.18 Compounds 3 and 4 were also prepared according to literature procedures.19-20

General procedure for the synthesis of mono-brominated DPP compounds 5a, 5b and 5c. N-bromosuccinimide (1.0 mmol) was added to a solution of the DPP derivative compound 4 (1.0 mmol) in CF (40 mL) under a dry N2 atmosphere. The resulting mixture was continuously stirred for 12 h and then poured into water (50 mL). The non-water based organic layer was clearly separated; the aqueous region was extracted with CHCl3 (3 × 40 mL) and the combined organic regions were dried over anhydrous MgSO4 and filtered, and the solvent was removed under reduced pressure. The residue was chromatographically purified on silica gel by eluting with CH2Cl2/hexane (1/1, v/v) to afford the products (5a, 5b and 5c) as solids.

5a. 3-(5-Bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione: dark brown solid (283 mg, 47%). 1H NMR (300 MHz, CDCl3) (δ ppm); 8.90 (d, J = 3.9, 1H), 8.61 (d, J = 4.2 Hz, 1H), 7.64 (d, J = 5.0, 1H), 7.29–7.19 (m, 2H), 4.05 (m, 2H), 3.90 (m, 2H), 1.84 (br s, 2H), 1.42–1.21 (m, 16H), 1.02–0.85 (m, 12H). MALDI– TOF, m/z: Calcd., 603.6; found, 603.8 (M+). 5b. 3-(5-Bromothiophen-2-yl)-2,5-bis(2-butyloctyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione: dark brown solid (315 mg, 44%). 1H NMR (300 MHz, CDCl3) (δ ppm); 8.89 (d, J = 3.9, 1H), 8.60 (d, J = 4.2 Hz, 1H), 7.66 (d, J = 5.0, 1H), 7.27–7.18 (m, 2H), 4.10 (d, J = 7.7 Hz, 2H), 3.85 (d, J = 7.7 Hz, 2H), 1.90 (br s, 2H), 1.40–1.15 (m, 32H), 0.85 (m, 12H). MALDI–TOF, m/z: Calcd., 715.8; found, 716.0 (M+).


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5c. 3-(5-Bromothiophen-2-yl)-2,5-bis(2-hexyldecyl)-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione: brown solid (372 mg, 45%). 1H NMR (300 MHz, CDCl3) (δ ppm); 8.83 (d, J = 3.9, 1H), 8.55 (d, J = 4.2 Hz, 1H), 7.63 (d, J = 5.0, 1H), 7.30–7.19 (m, 2H), 3.94 (d, J = 7.7 Hz, 2H), 3.89 (d, J = 7.7 Hz, 2H), 1.82 (br s, 2H), 1.40–1.05 (m, 48H), 0.85 (m, 12H). MALDI–TOF, m/z: Calcd., 828.1; found, 828.4 (M+).

General procedure used for the synthesizing the SMs Pd(PPh3)4 (5.0 mol%) in toluene (8 mL) was added and poured to a solution of 3-(5bromothiophen-2-yl)-2,5-dialkyl-6-(thiophen-2-yl)-2,5-dihydropyrrolo

[3,4-c]pyrrole-1,4-

dione (5a/5b/5c, 0.160 mmol) and 2,6-bis(trimethylstannyl)benzo[1,2-b:4,5-b']dithiophene4,8-diyl)bis(ethyne-2,1-diyl))bis(triisopropylsilane) (TIPS-BDT 2, 0.078 mmol) under a dry N2 atmosphere. The resulting mixture was heated to reflux at 120°C for 24 h. After cooling to room temperature and precipitating by adding MeOH (20 mL), the precipitate was filtered and purified by column chromatography on silica gel by eluting with CH2Cl2/hexane (1/1, v/v) to afford the products (SM1, SM2 and SM3) as solids.

SM1: dark brown solid (104 mg, 84%), 1H NMR (300 MHz, CDCl3) (δ ppm); 9.04 (d, J = 4.02 Hz, 2H), 8.94 (d, J = 3.75 Hz, 2H), 7.75 (s, 2H), 7.64 (d, J = 4.98 Hz, 2H), 7.50 (d, J = 4.14 Hz, 2H), 7.29 (m, 2H), 4.08 (m, 8H), 1.98 (m, 4H), 1.43–1.11 (m, 74H), 0.97–0.83 (m, 24H);

13

C NMR (125.75 MHz, CDCl3) (δ ppm); 161.59, 141.83, 140.76, 140.42, 139.44,

137.71, 136.90, 135.55, 130.74, 129.79, 128.47, 126.72, 120.17, 111.69, 108.65, 108.08, 103.21, 101.78, 45.97, 45.88, 39.09, 39.06, 30.15, 30.02, 28.29, 23.55, 23.46, 23.07, 23.01, 18.80, 14.06, 14.04, 11.31, 10.43. MALDI–TOF, m/z: Calcd., 1594.73; found: 1595.55. Anal. Calcd. for C92H122N4O4S6Si2: C, 69.21; H, 7.70; N, 3.51; S, 12.05. Found: C, 69.1; H, 8.1; N, 3.5; S, 11.9%.



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SM2: dark brown solid (113 mg, 80%), 1H NMR (300 MHz, CDCl3) (δ ppm); 8.97 (d, J = 4.29 Hz, 2H), 8.90 (d, J = 0.92 Hz, 2H), 7.77 (s, 2H), 7.65 (d, 2H, J = 0.93 Hz, 2H), 7.51 (d, 2H, J = 4.05 Hz, 2H), 7.29 (m, 2H), 4.07 (m, 8H), 2.01 (m, 4H), 1.45–1.16 (m, 106H), 0.950.76 (m, 24H);

13

C NMR (125.75 MHz, CDCl3) (δ ppm); 161.67, 141.82, 140.73, 140.57,

139.51, 139.45, 137.72, 136.66, 135.45, 130.76, 129.88, 129.79,

128.47, 126.76, 120.33,

111.74, 108.78, 108.13, 103.20, 101.79, 46.38, 46.23, 37.67, 31.78, 31.11,

30.93, 30.83,

30.68, 29.67, 28.36, 28.28, 26.14, 26.03, 23.04, 22.63, 18.80, 14.09, 14.04, 11.30, 10.43. MALDI–TOF, m/z: Calcd., 1818.98; found: 1819.79. Anal. Calcd. for C108H154 N4O4S6Si2: C, 71.24; H, 8.52; N, 3.08; S, 10.56. Found: C, 71.0; H, 9.0; N, 3.0; S, 10.1%.

SM3: dark brown solid (127 mg, 80%), 1H NMR (300 MHz, CDCl3) (δ ppm); 8.98 (d, J = 4.17 Hz, 2H), 8.91 (m, 2H), 7.77 (s, 2H), 7.65 (d, J = 0.93 Hz, 2H), 7.50 (d, J = 4.11 Hz, 2H), 7.29 (m, 2H), 4.05 (m, 8H), 2.11 (m, 4H), 1.41–1.15 (m, 138H), 0.98–0.80 (m, 24H);

13

C

NMR (125.75 MHz, CDCl3) (δ ppm); 161.65, 141.81, 140.72, 140.57, 139.50, 139.44, 137.71, 136.68, 135.49, 130.76, 129.87, 129.79,

128.47, 126.75, 120.32, 111.73, 108.78, 108.12,

103.18, 101.80, 46.40, 46.24, 37.73, 37.65, 31.87, 31.85, 31.78, 31.76, 31.13, 30.94, 30.03, 30.00, 29.69, 29.67, 29.56, 29.50, 29.34, 29.30, 26.15, 26.06, 26.02, 22.63, 18.80, 14.12, 14.09, 14.06, 11.30; MALDI–TOF, m/z: Calcd., 2043.23; found: 2044.00. Anal. Calcd. for C124H186N4O4S6Si2: C, 72.82; H, 9.17; N, 2.74; S, 9.40. Found: C, 72.5; H, 9.7; N, 2.7; S, 9.1%.

General information A Bruker DPX300 instrument operating at 300 MHz and a Bruker AVANCE500 instrument operating at 125 MHz were used to acquire 1H and

13

C nuclear magnetic resonance (NMR)


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spectra, respectively. Mass spectra (MALDI–TOF/TOF) were measured using a Bruker Autoflex Speed TOF/TOF mass spectrometer. The elemental analysis was carried using Thermo Scientific FLASH EA-2000 Organic Elemental Analyzer (EA) equipment to determine C, H, N, S. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were carried out under a N2 atmosphere at a 10°C min-1 heating rate with a TA Instruments Q5000 V3.17 (build 265) analyzer and a TA Instruments Q1000 V9.9 (build 303) analyzer, respectively. UV-Vis absorption experiments were carried out with a Shimadzu UV–2550 spectrophotometer (double-beam system) over a wavelength range of 300–900 nm. To search the electrochemical properties of the small molecules, cyclic voltammetry (CV) experiments were performed, using an IviumStat instrument at a scan rate of 50 mV s-1 at 25°C under argon with 0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile as the electrolyte. The working electrode was made out of platinum coated with a thin small-molecule film. A Pt wire and an Ag/AgCl electrode served as the counter and reference electrodes, respectively. The height AFM images (1.5 × 1.5 µm2) were obtained using a Multimode IIIa instruments in a tapping mode. 2D–GIWAXS techniques were performed at the PLS–IIA U–SAXS (9A) beam line at the Pohang Accelerator Laboratory in Korea. The diffraction X-ray beam coming from the in vacuo undulator (Ek = 11.24 keV, wavelength 1.103 Å) was focused horizontally and vertically at the sample position. The incidence angle of the X-ray beam was adjusted to 0.11–0.13° and the irradiation time was 5– 10 s. 2D–GIWAXS pattern images were detected using a 2D charge-coupled device (CCD) detector and a sample-to-detector distance of approximately 232 nm. For AFM and 2D– GIWAXS analyses, the samples were prepared prior to 24 h by spin-coating layers of the small molecule:PC71BM on a Si-wafer substrate.

Fabrication and characterization of the organic field-effect transistors



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Solution-processed OFET devices using SM1, SM2 and SM3 were fabricated with a topcontact/bottom-gate architecture. Silicon (Si) wafer having dielectric layer of 300 nm silicon oxide was selected to gate-contact substrate, which was firstly cleaned with piranha solution (a mixture of sulfuric acid and hydrogen peroxide). And then, residue solution on Sisubstrates was dried to nitrogen blowing gas, and sequentially they was surface-treated with octyltrichlorosilane (OTS-8) solution (10 mM in toluene) at room condition for 2 h. The pure small molecule solutions (0.5 wt% in chloroform) was prepared before spin-coating to layer for 1 h. After that the small molecule semiconductor films were spin-coated onto OTS-8trated substrate at 1500 rpm for 50 s with a ca. 50 nm thickness. Finally, gold electrodes (source and drain) were deposited in high-vacuum chamber with ca. 100 nm. The electrical characteristics of fabricated OFETs (which has channel length of 12 µm and width of 120 µm) were measured using both Keithley 2400 and 236 source/measurement units.

Fabrication and characterization of the small molecule solar cells The small molecule photovoltaic devices were fabricated with a conventional geometry of ITO/PEDOT:PSS/Active layer/Ca/Al, where poly(3,4-ethylenedioxythiophene):poly-(styrene sulfonate) (PEDOT:PSS) was used to interlayer for effective hole extraction and calcium (Ca) was used for efficient hole blocking/electron extraction purposes. First, ITO-coated glass subtrates were ultrasonically cleaned in detergent, deionized water, high purity (99%) acetone and (99.5%) isopropyl alcohol for each of 20 min. The ITO-coated glass was then subjected to an ultraviolet ozone (UVO) treatment for 15 min. A hole-transporting PEDOT:PSS (Clevios P purchased from H. C. Starck) layer was formed onto a UVO-pretreated ITO anode to spin-coating method at 5,500 rpm for 60 s and baked for 15 min at 140°C in the oven. The active blend solution (20 mg mL–1) was then spin-coated onto the PEDOT:PSS layer, which was prepared by dissolving the small molecules and PC71BM in a 1:1 blend of CB and CF


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solvents. Solutions (CF) containing three different additives (DIO, CN and DPE) were also prepared for the SM1:PC71BM blends. All of the solutions were stirred overnight at 30°C on a hot-plate in a N2-filled glove box. Finally, a 2nm-thick layer of Ca and 100nm-thick layer of aluminum were thermally evaporated and deposited on the top of a defined 0.09 cm2 area of the active layer at a pressure of 3 × 10–6 Torr. The current density–voltage (J–V) plots were obtained with shadow masks under 100 mW cm–2 AM 1.5 G irradiation conditions from a xenon-arc lamp with an AM 1.5 global filter. The IPCE spectra were measured under short circuit condition using a Oriel monochromator; the photocurrent and absolute photon flux were detected using a lock-in amplifier and calibrated silicon photodiode, respectively. All measurements made using the devices were carried out in air at room temperature.

Hole mobility by SCLC measurements To determine charge hole mobility measurements with the SCLC method, a hole-only device was fabricated with the method used to fabricate the photovoltaic device, except for the

cathode

electrode.

Therefore,

the

structure

of

the

hole-only

diode

was

ITO/PEDOT:PSS/active layer (small molecule:PC71BM)/Au, with a gold electrode (Au, 100 nm) vacuum-deposited on the active layer as the cathode. The mobilities were determined using the Mott–Gurney relationship to fit the current–voltage curves in the range from 0–3 V under dark conditions using the following equation: =

(,-.. − ,/ )2 ,-.. − ,/ 9 (0 (* +ℎ exp (0.8917 ) 3 8 1 1

where J represents the current density, L represents the film thickness of the active layer, µh represents the hole mobility, ε0εr represents the dielectric permittivity of the active layer, Vappl is the applied voltage, Vbi is the built-in voltage and γ is the field activation factor.



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ASSOCIATED CONTENT Supporting Information Detailed 1H,

13

C NMR and mass spectra. The Supporting Information is available free of

charge on the ACS Publications website at DOI: –.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Contact author) *E-mail: [email protected]

Author Contributions ‡

The authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by a New and Renewable Energy grant from the Korean Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Korean Government Ministry of Knowledge Economy (No. 20123010010140).

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(49) Bartesaghi, D.; Perez, I. C.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J. A. Competition between Recombination and Extraction of Free Charges Determines the Fill Factor of Organic Solar Cells. Nat. Commun. 2015, 6, 7083.



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Scheme 1. Synthesis and chemical structures of the diketopyrrolopyrrole (DPP)-based small molecules SM1, SM2 and SM3.


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110

10

(a)

100

5

Heat flow (W/g)

80 70 60 50

SM1 SM2 SM3

40 30

(b)

152 oC

90

Weight (%)

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


0

100

200

300

400

o

Temperature ( C)

500

240 174 oC

0

193 oC

-5 -10 -15 100

oC

SM1 SM2 SM3

170 oC

150

257 oC

200

250

300

o

Temperature ( C)

Figure 1. TGA traces (a) and differential scanning calorimetry (DSC) profiles (b) for the SM1, SM2 and SM3 small molecules.



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

Figure 2. Absorption spectra of the DPP-based compounds in (a) chloroform (CF) solution and (b) thin films. (c) Cyclic voltammograms for the three target small molecules. (d) Energy level diagram of the small-molecule solar cells investigated in this work.


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-6

(a) 10

SM1

1.0x10

-3

8.0x10

-4

6.0x10

-4

4.0x10

-4

2.0x10

-4

-7

10

-8

-9

10

10

-10

10

-11

10

-12

1 0.1

-60 -40 -20 0 Gate Voltage,VG(V)

-3

-7

-4

8.0x10

-8

( ID )1/2 (A)1/2

10

-9

10

-10

10

-11

10

-12

100

1.0x10

0.1

1

10

Vappl - Vbi (V)

SM2

10

10

0.01 0.01

0.0 20

-4

6.0x10

-4

4.0x10

-4

Dark J (mA/cm 2)

-80 -6

ID (A)

10

SM1

10

10 1 0.1

2.0x10

SM2

-100

-80

-6

10


-4

-8

(ID ) 1/2 (A) 1/2

-9

10

-10

10

-11

10

-12

1

10

100

1.0x10 8.0x10

10

0.1

Vappl - Vbi (V) -3

SM3

-7

10

0.01 0.01

0.0 20

10

ID (A)

(b) 100

-4

6.0x10

-4

4.0x10

-4

Dark J (mA/cm 2)

ID (A)

10

(ID ) 1/2 (A)1/2

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


Dark J (mA/cm 2)

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10 1 0.1

2.0x10

SM3

-100

-80


0.0 20

0.01 0.01

0.1

1

10

Vappl - Vbi (V)

Figure 3. (a) IDS and IDS1/2 vs. gate voltage for the DPP-containing compound-based thin-film transistors. (b) Current–voltage characteristics of a hole-only diode for the DPP-based small compounds (space charge limited current [SCLC] method).



Current Density (mA/cm2)

) 2

Current Density (mA/cm

2

(a) 0 -2 -4 SM1 SM2 SM3

-6 -8

0.0

0.2

0.4

0.6

0.8

0

(b)

-2 -4 -6 SM1 SM2 SM3

-8 -10

1.0

0.0

0.2

0.4

0.6

0.8

(d)

60 50

DIO CN DPE

40

IPCE (%)

SM1:PC71BM

-3 -6 -9

30 20 10

(c)

-12 0.0

0.2

0.4

0.6

1.0

Voltage (V)

0

2

)

Voltage (V)


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

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0.8

1.0

Voltage (V)

SM1:PC71BM

DIO

CN

DPE

0 400

500

600

700

800

Wavelength (nm)

Figure 4. Characteristic current density vs. voltage curves of the SM1, SM2 and SM3 devices processed with chlorobenzene (CB) (a) and CF (b). (c) J–V curves for the SM1:PC71BM devices processed in the presence of 1,8-diiodooctane (DIO), 1chloronaphthalene (CN) and diphenyl ether (DPE) additives and the corresponding incident photon vs. current efficiency (IPCE) spectra (d).


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Figure 5. Atomic force microscopy (AFM) height images (1.5 × 1.5 µm2) (upper panel) and two-dimensional grazing-incidence wide-angle X-ray scattering (2D–GIWAXS) patterns (lower panel) of (a and d) SM1:PC71BM, (b and e) SM2:PC71BM and (c and f) SM3:PC71BM blend films.



-2 -4

100.0 91.6 79.4 63.1 50.1 39.8 31.6 10.0 3.2

-6 -8 -10 -12

0.0

0.2

0.4

0.6

0.8

2

)

(c)

0

-2

100.0 91.6 79.4 63.1 50.1 39.8 31.6 10.0 3.2

-4

-6

1.0

0.0

0.2

Voltage (V)

0.4

0.6

0.8

(e)10

SM1 SM2 SM3

-1

-3 0.0

0.6 slope = 1.08 slope = 1.11 slope = 1.37

10

100 2

Light Intensity (mW/cm )

0.2

0.4

0.6

0.8

1.0

Voltage (V)

(f) 0.55

SM1 SM2 SM3

0.50 0.45

FF (%)

0.7

100.0 91.6 79.4 63.1 50.1 39.8 31.6 10.0 3.2

-2

1.0

2

0.8

0.5

0

Voltage (V)

JSC (mA/cm )

(d) 0.9



2

0

2


(b) )

)

(a)

V OC (V)

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

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1 α = 0.96 α = 0.91 α = 0.85

0.1

10

100

0.40 0.35 0.30

SM1 SM2 SM3

0.25 0.20

2


10

100 2


Figure 6. Illuminated J–V characteristics of the devices based on SM1:PC71BM (a), SM2:PC71BM (b) and SM3:PC71BM (c) under various light intensities ranging from 3.2–100 mW cm–2. The light intensity dependence is shown for the open circuit voltage (d), photocurrent density (e) and fill factor (f).


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Table 1. Thermal, photophysical and electrochemical properties of the SM1, SM2 and SM3 compounds. Small

Thermal properties

molecule

Td (°C)

Tm (°C)a

SM1

360

SM2 SM3

Tc

Electrochemical properties

Optical properties Eg

opt

(°C)b

λmax (nm) solution

λmax (nm) film

λonset (nm) film

(eV)

257

240

621

630, 685

747

362

193

174

621

629, 681

371

170

152

621

628, 680

HOMO (eV)

LUMO (eV)

1.66

–5.85

–3.44

740

1.68

–5.80

–3.39

735

1.69

–5.71

–3.37

a

c

Melting temperature evaluated by DSC. Crystallization temperature determined by DSC. c Estimated values from the UV-vis absorption edge of the thin films (Eg opt = 1240/ λonset eV). HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital b



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Table 2. Device performance of the three different solar cell devices SM1:PC71BM, SM2:PC71BM and SM3:PC71BM processed with CB and CF.

Active layer

Condition

VOC

JSC

FF

PCE

(solution)

(V)

(mA cm–2)

(%)

(%)

CB

0.91

6.81

48

2.96

CF

0.92

10.3

54

4.53

CB

0.91

3.04

42

1.16

CF

0.91

5.26

46

2.67

CB

0.85

1.98

40

0.68

CF

0.86

2.75

37

0.88

SM1:PC71BM

SM2:PC71BM

SM3:PC71BM

FF, fill factor; PCE, power conversion efficiencies; CB, chlorobenzene; CF, chloroform


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Table 3. Photovoltaic performance of the SM1:PC71BM device.

Active layer

SM1:PC71BM

VOC (V)

JSC (mA cm–2)

FF (%)

PCE (%)

DIO

0.77

10.77

45

3.72

CN

0.79

11.90

60

5.54

DPE

0.79

11.86

51

4.80

Condition (solution)

CF

FF, fill factor; PCE, power conversion efficiencies; CF, chloroform; DIO, 1,8-diiodooctane; CN, chloronaphthalene; DPE, diphenyl ether



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Table of Contents (TOC)


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Impact of the Crystalline Packing Structures on Charge Transport and

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