Structural and morphological tuning of dithienobenzodithiophene-core small molecules for efficient solution processed organic solar cells

Dyes and Pigments 115 (2015) 23e34

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Structural and morphological tuning of dithienobenzodithiophenecore small molecules for efficient solution processed organic solar cells Minwoo Jung a, Dongkyun Seo b, Kyungwon Kwak b, Ajeong Kim c, Wonsuk Cha c, Hyunjung Kim c, Youngwoon Yoon a, Min Jae Ko a, Doh-Kwon Lee a, Jin Young Kim a, Hae Jung Son a, d, *, BongSoo Kim a, d, e, * a

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea c Department of Physics, Sogang University, Seoul 121-742, Republic of Korea d Nanomaterials Science and Engineering, Korea University of Science & Technology, Daejeon 305-350, Republic of Korea e Green School, Korea University, 1, Anam-dong 5-ga, Seongbuk-gu, Seoul 136-701, Republic of Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 October 2014 Received in revised form 3 December 2014 Accepted 5 December 2014 Available online 13 December 2014

The fused dithieno[2,3-d:20 ,30 -d0 ]benzo[1,2-b:4,5-b0 ]dithiophene (DTBDT) structure was coupled with diketopyrrolopyrrole (DPP) moieties to generate highly planar bis(2,5-bis(2-ethylhexyl)-3,6-di(thiophen2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione)dithieno[2,3-d:20 ,30 -d0 ]benzo[1,2-b:4,5-b0 ]dithiophene (DTBDTDPP-EH) and bis(2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole1,4-dione)dithieno[2,3-d:20 ,30 -d0 ]benzo[1,2-b:4,5-b0 ]dithiophene (DTBDTDPP-BO) molecules, where the EH and BO stands for 2-ethylhexyl and 2-butyloctyl groups respectively. The morphology of the DTBDTDPP-EH alone or DTBDTDPP-EH:[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film was controlled using post-thermal annealing at 130  C or addition of 1,8-diiodooctane (DIO) additives. The DIO-additive treatment was more effective than thermal annealing at increasing crystallinity; the DIOadditives promoted the formation of nanoscopically well-connected molecular crystalline domains in the blend films. This observation well explained the ordering of the photovoltaic properties of DTBDTDPP-EH:PCBM devices: from worst to best, as-cast, thermally treated, and DIO-treated photoactive films. The DTBDTDPP-BO:PCBM device followed the similar trend with lower performances due to the presence of irregularly overgrown domains. Overall, we demonstrate that it is critical to optimize nanoscale film morphologies by engineering alkyl chains and selecting an appropriate processing method. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Organic solar cells Power conversion efficiency Organic semiconductors Conjugated small molecules Crystallinity Nanoscale phase separation

1. Introduction Organic photovoltaic devices (OPVs) that may be fabricated using simple processing techniques are the subject of extensive investigations in academia and industries. Such devices may potentially be mass produced at low cost [1e10]. Over the past decade, polymer-based photovoltaic cells (polymer-OPVs) have attracted significant attention and have yielded dramatic

* Corresponding authors. Current address: Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5516; fax: þ82 2 958 6649. E-mail addresses: [email protected] (H.J. Son), [email protected] (B. Kim). http://dx.doi.org/10.1016/j.dyepig.2014.12.003 0143-7208/© 2014 Elsevier Ltd. All rights reserved.

improvements in device power conversion efficiencies (PCEs), which have reached levels exceeding 9% [4e6,11]. Recently, solution-processable small molecule-based BHJ OPVs (SM-OPVs) have also attracted attention for OPVs because they present several advantages: (i) the synthesis and purification of small molecules is relatively simple and easy; (ii) the molecular energy levels of the molecules may be controlled and predicted using theoretical approaches; (iii) batch-to-batch variations may be minimized, which can increase the reproducibility of the electrical properties; and (iv) promising high PCEs approaching 10% have been obtained [12e14]. SM-OPVs have emerged as an alternative future energy source. The PCEs of SM-OPVs could be further enhanced by exploring the development of new small molecules that can act as electron donors in the photoactive layer.

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M. Jung et al. / Dyes and Pigments 115 (2015) 23e34

The characteristics of ideal electron donor molecules include broad and intense absorption, efficient exciton separation at the interface between the electron donors and electron acceptors, fast charge transport and carrier extraction, and nanoscale phase separation from the electron acceptor molecules such as [6,6]-phenylC61-butyric acid methyl ester (PCBM) [1,2,15]. These characteristics may be obtained through the careful design of the electron donor chemical structures. Successful approaches to the design of molecules involve combinations of electron-rich (D) moieties and electron-deficient (A) moieties in AeDeA or DeAeD systems [1,2]. Benzo[1,2-b;4,5-b0 ]dithiophene (BDT) and dithienosilole (SI), which contain alkyl chains, are representative planar conjugated electronrich moieties that can enhance intermolecular pep stacking and charge carrier mobilities [5,12,13,16]. The exemplary electrondeficient moieties of benzothiadiazole [3,17e20], thiadiazolopyridine [17,21], and diketopyrrolopyrroles (DPPs) [16,22e31], have been reported. Among these the DPPs are most effective at broadening the absorption spectra of small molecules because they have strong electron-accepting properties and their planar structures facilitate intermolecular packing. Moreover, the DPPs are photochemically stable and have a high absorption cross-section [30,32]. In addition to obtaining suitable molecular backbone by using D and A moieties, the molecular blend morphologies in photoactive films must be optimized to achieve high photovoltaic properties. In this regard, solubilizing alkyl chain groups must be carefully selected and appended to molecular backbones to obtain high performance SM-OPV [1,2,33], and polymer-OPVs [34e37]. Moreover, thermal annealing and the use of the 1,8-diiodooctaine (DIO) solvent additive were very effective in enhancing the photovoltaic performances of polymer-OPVs. These methods have been employed to achieve high PCEs in SM-OPV devices [19,28e31,38e43]. However, the effects of these treatments on small molecule-based photoactive films are not yet fully understood, and additional studies are needed to articulate general guidelines for optimizing the conditions for fabricating new solar cell device materials.

In this work, we explored the use of the highly conjugated electron-rich dithieno[2,3-d:20 ,30 -d0 ]benzo[1,2-b:4,5-b0 ]dithiophene (DTBDT) moiety for use in AeDeA structured molecules. The DTBDT structure is a highly planar fused ring system that can facilitate intermolecular packing and retain a low molecular HOMO level [44,45]. Combining DTBDT units with DPP moieties yielded two AeDeA type small molecules, DTBDTDPP-EH and DTBDTDPP-BO. Both molecules displayed narrow bandgaps of 1.7 eV and high extinction coefficients of ~1  105 M1 cm1 at the light absorption maxima, and they maintained low HOMO levels (5.34 eV). The DTBDTDPP molecules were used to fabricate BHJ OPV devices whose photoactive films were treated according to one of two methods: (i) post-thermal annealing at 130  C or (ii) photoactive film formation using a 1,8-diiodooctaine (DIO) solvent additive, followed by pre-annealing for 10 min at 70  C. DTBDTDPP-EH:PCBM devices were prepared using DIO additives, which proved to be more effective than thermal annealing in forming nanoscopic (~10 nm) bicontinuous interpenetrating DTBDTDPP-EH molecular domains with a high crystallinity. The DTBDTDPP-EH:PCBM blend devices exhibited a PCE of 4.35%. The DTBDTDPP-BO molecules, which were prepared with longer alkyl chains and incorporated into DTBDTDPP-BO:PCBM devices, tended to form more aggregates and larger domains that reduced the device PCE (<3%). This work demonstrated the interplay between the delicate management of molecular structures and device fabrication methods as a means for controlling the photovoltaic performances. 2. Synthesis and characterization Scheme 1 shows the synthetic routes of DTBDTDPP-EH and DTBDTDPP-BO. Molecules of 1 and 3 were synthesized according to previously reported procedures [44,46], and 2 was purchased from Lumtec. The detailed syntheses of 1, 3, DTBDTDPP-EH, and DTBDTDPP-BO are provided in the Supporting information. The

Scheme 1. Synthetic routes of DTBDTDPP-EH and DTBDTDPP-BO.

M. Jung et al. / Dyes and Pigments 115 (2015) 23e34

final products were synthesized via a Stille cross-coupling reaction and were confirmed using nuclear magnetic resonance spectrometry (NMR), elemental analysis (EA), and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDITOF). The DTBDTDPP-EH and DTBDTDPP-BO molecules were soluble in common organic solvents, including dichloromethane, chloroform, toluene, and chlorobenzene. 3. Thermal properties The thermal properties of the DTBDTDPP-EH and DTBDTDPP-BO molecules were measured using differential scanning calorimetry (DSC). DSC thermograms measured from the molecules are shown in Fig. S1, and the crystallization temperatures (Tc) and melting temperatures (Tm) are listed in Table 1. The alkyl chain lengths dramatically influenced the thermal properties. DTBDTDPP-EH had a higher melting temperature (Tm: 328.5  C) and a higher crystallization temperature (Tc: 267.5  C) than DTBDTDPP-BO (Tm: 260.4  C, Tc: 231.5  C). These results indicated that the shorter branched alkyl chains on the DPP units more effectively promoted cohesive interactions than the longer branched alkyl chains, which were associated with the packing of conjugated aromatic backbones, as revealed by the X-ray diffraction results (see below). The DSC data indicated that the substituted alkyl chains were key to controlling the intermolecular interactions. 4. Optical and electrochemical properties The optical properties of the DTBDTDPP-EH and DTBDTDPP-BO molecules were measured using UVevisible absorption spectroscopy. Fig. 1 shows the UVevisible absorption spectra of the molecules in the chloroform solution and in the as-cast thin film state. Table 1 summarizes the optical properties, electrochemical

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properties, and bandgaps. A strong UVevisible absorption band at 610 nm was observed in both the solution and solid states. In the solution state, the UVevisible absorption features were nearly independent of the identity of the substituted alkyl chains on the DPP units. This is presumably because the molecular backbone structures of the DTBDT and DPP units are quite flat and the branched alkyl chains do not make steric hindrance with aromatic rings on the backbone, which can be visualized in optimized molecular geometries obtained from quantum mechanical calculations ((Fig. 2, S2). The extinction coefficients at the absorption maxima were quite high (ε ¼ ~1  105 M1 cm1). In the solid state, the absorption maxima of both of the as-cast molecular films displayed high extinction coefficients (a ¼ ~8.3  104 cm1) as well. The absorption edge was red-shifted by ~70 nm, and a new peak appeared at ~670 nm, consistent with the signature for intermolecular stacking interactions among conjugated backbones as a result of solidification [47,48]. The optical bandgaps of both the molecular films were estimated to be 1.73 eV (~720 nm) based on the absorption edges. Interestingly, annealing at 130  C weakened the peak at 670 nm, and a relatively large increase was observed in the absorption band at 570 nm. The DIO-use films displayed a more dramatic change, especially in the DTBDTDPP-EH film. This change was attributed to partial switching from the J-aggregate to Haggregate molecular packing motifs [24,47e51], which was accompanied by a higher molecular crystallinity, as shown in the grazing-incidence X-ray diffraction (GIXD) experiments. The DTBDTDPP-BO film exhibited the similar behavior with the smaller changes. Deeper insights into the electronic structure, molecular geometries, and electronic transitions were gained by conducting quantum mechanical calculations using density functional theory (DFT) and time-dependent DFT (TD-DFT). The energy levels of the frontier orbitals and the geometries of the DTBDTDPP-EH

Table 1 Thermal, optical, and electrochemical properties of the DTBDTDPP molecules. Molecules

DSC Tm (oC)

DTBDTDPPeEH DTBDTDPPeBO a b c d e

328.5 260.4

UVevisible absorption Tc (oC)

267.5 231.5

Solution

Cyclic voltammetry Film as-cast (130  C-annealed)

As-cast film

lmax (nm)

lonset (nm)

lmax (nm)

lonset (nm)

Egopt (eV)

ox Eonset (V)a

HOMO (eV)b

red (V)c Eonset

LUMO (eV)d

EgEC (eV)e

618 618

667 667

612 (612) 607 (607)

727 (727) 715 (717)

1.71 (1.71) 1.73 (1.72)

0.54 0.53

5.34 5.33

1.35 1.33

3.45 3.47

1.89 1.86

The electrochemical oxidation onset potential in films with respect to the ferrocene/ferroceneþ. (Eox ¼ 4.8 eV). HOMO is calculated by e(Eox onset þ 4.8 eV). The electrochemical reduction onset potential in films with respect to the ferrocene/ferrocenium. HOMO is calculated by e(Eox onset þ 4.8 eV). Electrochemical bandgap, i.e. (LUMOeHOMO).

Fig. 1. UVevisible absorption spectra of (a) DTBDTDPP-EH and (b) DTBDTDPP-BO in chloroform solutions and thin films. Molecular films were annealed at RT (as-cast) and 130  C for 10 min. The arrows point corresponding y-axis.

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M. Jung et al. / Dyes and Pigments 115 (2015) 23e34

Fig. 2. (a) Energy levels and frontier molecular orbitals and (b) energy minimum molecular geometry of DTBDTDPP-EH.

molecules were very similar to those of the DTBDTDPP-BO molecules, except for the alkyl chains, as shown in Fig. 2, S2. Surface plots of the HOMO and LUMO orbitals of the DTBDTDPP molecules displayed highly delocalized orbitals along the molecular backbone. Mixing among the molecular orbitals on the DTBDT and DPP units produced relatively low HOMO and high LUMO levels, as confirmed by cyclic voltammetry. Interestingly, the electron-rich fused system comprising the DTBDT moiety contributed less to the HOMO-1 and LUMO þ 1 orbitals than the side DPP moieties (Fig. 2a, S2a). The optimized molecular geometries revealed that all the dihedral angles between the conjugated planes along the backbone were less than 12.2 (Fig. 2b, S2b). These observations suggested that DTBDT is an excellent electron donor moiety that provides effective overlap between the p-orbitals and keeps HOMO level deep in addition to generate the planar backbone, which can improve the properties

of high-performance OPVs [52]. The alkyl side chains did not contribute to the HOMO or LUMO levels, as expected; however, they filled the spaces out of the molecular backbone plane and influenced the intermolecular packing and interactions, as shown in Fig. 2b. A more detailed analysis is presented in the context of the GIXD data, below. The TD-DFT calculations reproduced well the overall features of the UVevisible spectra of the DTBDTDPP molecules in solution state (Fig. 3a, S3a). Our calculation results overestimated the transition energy by only 0.15 eV, a much smaller value than the typical error associated with TD-DFT calculations [53]. The transitions underlying the UVevisible absorption bands were further understood by performing natural transition orbitals (NTOs) analysis based on the calculated transition density matrices after the TDDFT calculations. The NTO calculation provided a compact orbital

Fig. 3. (a) Experimental and theoretical UVevisible absorption spectra of DTBDTDPP-EH in solution phase and (b) NTOs based on TD-DFT calculation of DTBDTDPP-EH are plotted for transitions at 416 and 573 nm. Hole orbitals and electron orbitals are shown in the left and right, respectively.

M. Jung et al. / Dyes and Pigments 115 (2015) 23e34

representation of the transition density between the ground and excited states with respect to an expansion into single-particle transitions (hole and electron states for each given excitation) [54e56]. It should be noted that the NTOs are not the same as unoccupied and occupied canonical MO pairs that form the ground state geometry optimization calculations. From the NTO calculation, we obtained an NTO pair comprising a hole- and an electronorbital for an excitation. The NTOs that made the most dominant contributions are plotted for each transition in Fig. 3b. The transition at 416 nm mainly possesses the contribution from the central DTBDT unit. The transition at longer wavelengths around 573 nm included partial charge transfer from the central DTBDT unit to the peripheral DPP units, as shown in the bottom of Fig. 3b. These NTO orbitals also show a change from aromatic form in the hole-orbital to quinoidal structure in the electron-orbital. This observation agreed well with our molecular design concept, i.e. the absorption band was extended by internal charge transfer from the donor moiety to the acceptor moieties. We note that this excited state structure is beneficial in solar cell applications in that it promotes electron transfer from the electron donor molecules to the PCBM molecules. The structure can additionally prevent bimolecular recombination at the electron donor/PCBM interfaces. The electrochemical properties of the molecular films were examined using cyclic voltammetry (CV). Cyclic voltammograms of the DTBDTDPP molecules are shown in Fig. S4. The energy levels of the frontier orbitals were determined from the onsets of the oxidation and reduction curves to be 5.33 eV for the HOMO and 3.45 eV for the LUMO levels. The electrochemical bandgaps ðEEC g Þ were ~1.88 eV, slightly higher than the optical bandgaps (~1.73 eV). The low-lying HOMO levels implies that the DTBDTDPP molecules may yield high Voc values, and the high-lying LUMO levels would be favorable for the efficient exciton splitting at the

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interface between the DTBDTDPPs and PCBMs in the photoactive layer [57,58]. These optical and electrochemical properties suggested that the DTBDTDPP molecules are appropriate for SM-OPV applications.

5. Grazing incidence X-ray diffraction The molecular packing structures and molecular orientations in the DTBDTDPP molecular films were examined using GIXD experiments. Fig. 4 shows GIXD images and line-cut profiles, respectively, of the as-cast, 130  C-annealed, and DIO-use DTBDTDPP molecular films. The as-cast film of linear planar DTBDTDPP-EH molecules displayed ordered crystalline features with high-order peaks, although the molecules were radially distributed with the mainly “edge-on” orientation (Fig. 4a, g and h). A strong (100) peak appeared at qz ¼ 0.45 Å1, corresponding to a d100 spacing of 14.0 Å, and a pep stacking (010) peak appeared at qy ¼ 1.58 Å1, corresponding to a d010 spacing of 3.98 Å. The 130  C-annealed films showed slightly improved crystallinity (Fig. 4b, g and h). The small crystallinity changes presumably arose from the high thermal stability of the DTBDTDPP-EH molecules, as indicated by the DSC measurements that revealed a high Tm with no additional phase transitions. On the other hand, the 0.6 vol.% DIO-use molecular film showed a highly crystalline feature with a slightly reduced qz ¼ 0.42 Å1, corresponding to a d100 spacing of 15.0 Å. The (010) peak was more pronounced at qy ¼ 1.59 Å1, corresponding to a d010 spacing of 3.95 Å (Fig. 4c, g and h). Moreover, the comparison of the (100) peak intensities in the out-of-plane direction and in the in-plane direction indicated that the edge-on orientations were further developed. The larger lamellar spacing and increased crystallinity reflected that the DTBDTDPP-EH molecules became more ordered and edge-on oriented. Thus, we concluded that the

Fig. 4. GIXD images of (a) as-cast, (b) 130  C-annealed, and (c) 0.6 vol.% DIO-used DTBDTDPP-EH films; GIXD images of (d) as-cast, (e) 130  C-annealed, and (f) 0.4 vol.% DIO-used DTBDTDPP-BO films. Line-cut profiles of DTBDTDPP-EH films in (g) out-of-plane (qz) and (h) in-plane (qy) directions; line-cut profiles of DTBDTDPP-BO films in (i) out-of-plane (qz) and (j) in-plane (qy) directions.

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use of a small amount of DIO provided a very effective means to increase the intermolecular interactions in the thin film. The as-cast film of DTBDTDPP-BO molecules were crystalline with a greater degree of edge-on orientations compared to the DTBDTDPP-EH molecules (Fig. 4d, i and j), presumably because they contained slightly longer side chains. The (100) peak was observed at qz ¼ 0.41 Å1, corresponding to a d100 spacing of 15.3 Å, but the pep stacking (010) peak was indistinct. Annealing of the DTBDTDPP-BO films at 130  C significantly increased the crystallinity compared to the DTBDTDPP-EH films (Fig. 4e, i and j). This is associated with their higher thermo-responsivity, i.e. a lower melting point for the DTBDTDPP-BO molecules and a higher degree of intermolecular packing due to the more flexible longer alkyl chains [37]. The increase in crystallinity was accompanied by a slight reduction in qz ¼ 0.38 Å1 (d100 ¼ 16.5 Å). The 0.4 vol.% DIOuse DTBDTDPP-BO film indicated the most crystalline feature among the three film preparation conditions, as in the DTBDTDPPEH film (Fig. 4f, i and j). The (100) peak became more pronounced as qz ¼ 0.36 Å1 was further reduced (d100 ¼ 17.5 Å), and the distinct (010) peak was observed at qy ¼ 1.58 Å1 (d010 ¼ 4.0 Å). The larger intensity difference between the (100) peak intensity in the out-ofplane direction and that in the in-plane direction suggested that the DTBDTDPP-BO molecules tended to assume a more “edge-on” molecular orientation than the DTBDTDPP-EH molecules. The larger d100 spacing in the DTBDTDPP-BO molecules than that in the DTBDTDPP-EH can be explained by the fact that they had longer alkyl side chains. The comparison of each calculated molecular width along the direction of the fully stretched-out alkyl chains in the energyminimum geometries of the DTBDTDPP-BO and the DTBDTDPPEH molecules (23.52 Å and 18.30 Å, respectively) and their measured d100 spacings (17.5 Å and 15.0 Å, respectively) revealed that the DTBDTDPP-BO molecule had a larger difference (D ¼ 6.02 Å) than DTBDTDPP-EH (D ¼ 3.30 Å). These results

reflected the greater degree of interdigitation among the alkyl side chains in the DTBDTDPP-BO film as a result of intermolecular stacking. The broader (010) peak was obtained from the DTBDTDPP-BO film in the high q range of 1.4e1.6 Å1 compared to that obtained from the DTBDTDPP-EH film presumably because the longer and bulkier branched alkyl chains could generate various cofacial interactions among the aromatic rings in the molecular film. 6. Photovoltaic properties OPV devices based on the DTBDTDPP molecules were fabricated in a structure of ITO/PEDOT:PSS/DTBDTDPP:PCBM (6:4, wt/wt)/ TiO2/Al and the photovoltaic properties of these devices were measured under AM 1.5 G, 1 sun illumination. Fig. 5a and b shows the photocurrent density (J) versus voltage (V) curves. Table 2 summarizes the photovoltaic properties of the best devices. The as-cast DTBDTDPP-EH:PCBM and DTBDTDPP-BO:PCBM blend devices exhibited similar PCEs of 1.1%. These devices yielded high open-circuit voltages (Voc) of 0.95 V, as expected from the deep HOMO energy levels. The devices generated low short-circuit currents (Jscs) and fill factors (FFs) as a result of the weak crystalline nature of the molecular domains and the concomitant presence of well-mixed phases and aggregated phases in the DTBDTDPP:PCBM blend films (discussed below). Fig. 6c and d shows the external quantum efficiency (EQE) spectra of both devices, revealing that 300e750 nm photons participated in photocurrent generation. Maximum EQE values of 20% were obtained from both DTBDTDPP devices prepared from the as-cast blend films. The photovoltaic properties were improved using two methods: (i) post-thermal annealing of the OPV devices at 130  C (denoted “130  C-annealing”) and (ii) film formation by spincoating blend solutions containing a small amount of the DIO additive solvent and then pre-annealing of the resulting films at 70  C for 10 min

Fig. 5. JeV characteristics of (a) DTBDTDPP-EH:PCBM and (b) DTBDTDPP-BO:PCBM devices fabricated in the three conditions: as-cast, 130  C-annealed, and DIO-used. Photovoltaic properties were measured under AM 1.5G, 100 mW/cm2. The corresponding EQE spectra of (c) DTBDTDPP-EH:PCBM and (d) DTBDTDPP-BO:PCBM devices.

M. Jung et al. / Dyes and Pigments 115 (2015) 23e34

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Table 2 Summary of photovoltaic parameters and hole mobilities of DTBDTDPP:PCBM devices. Device

Treatment

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

mh (cm2 V1 s1)

DTBDTDPPeEH:PCBM (6:4, wt/wt)

As-cast 130  C-annealed 0.6 vol.% DIO As-cast 130  C-annealed 0.4 vol.% DIO

0.95 0.77 0.85 0.94 0.71 0.82

4.0 9.1 9.9 3.9 6.1 7.3

29.0 45.9 51.3 29.2 37.1 45.2

1.10 3.19 4.35 1.08 1.61 2.69

7.0 3.5 3.4 3.0 7.0 1.7

DTBDTDPPeBO:PCBM (6:4, wt/wt)

(denoted “DIO-use”). The best performance results from each condition were presented and discussed below, and all device performance properties under various conditions are summarized in the Supporting Information (Tables S1 and S2 and Figs. S5 and S6). C Post-thermal annealing at 130 improved the DTBDTDPP:PCBM device performance significantly. The PCE of the DTBDTDPP-EH:PCBM devices increased significantly from 1.1 to 3.2%, mainly due to the dramatic improvements in both the shortcircuit current density (Jsc), 9.1 mAcm2, and the fill factor (FF), 0.46. Accordingly, the maximum EQE values improved significantly to 53%. The Voc values, however, decreased from 0.95 to 0.77 V, respectively. The dependence of photovoltaic characteristics on the annealing temperature is provided in greater detail in Table S1 and Fig. S5. The improvement in Jsc and accompanied Voc loss has been commonly observed among thermally annealed SM-OPVs [29e31]. The Voc drop can be attributed to an increase in recombination at the interface between the blend film and TiO2/Al. For instance, the 110  C-annealed DTBDTDPP-EH:PCBM device without the TiO2 layer yielded an even smaller Voc of 0.69. Moreover, it is supported by Leong et al.’s report that the reduction in Voc is mainly due to unfavorable vertical phase separation, which may aggravate the

     

106 106 105 106 106 105

interfacial recombination [29]. Furthermore, it is less likely that the surface roughness might have affected to the Voc loss because the roughness trend of the blend surfaces is not correlated with the Voc values in the devices prepared by the as-cast, 130  C-annealing, and DIO-use methods (see below for more discussion). In the same manner, the bimolecular recombination inside the photoactive layer is not responsible for the Voc drop because the nanoscale phase separation was improved in the 130  C-annealing, and DIOuse films (see below results of morphology study). On the other hand, the DTBDTDPP-BO:PCBM devices showed a relatively small improvement with an increase in PCE from 1.1 to 1.6% when compared with the DTBDTDPP-BO:PCBM devices. Although both the devices showed a similar drop in Voc (from 0.94 to 0.71 V), the short-circuit current densities were much less increased. The reduced photovoltaic properties of the DTBDTDPP-BO:PCBM device compared to the DTBDTDPP-EH:PCBM are discussed below. The DIO-use method enhanced the DTBDTDPP:PCBM device performances more significantly. The DTBDTDPP-EH:PCBM devices submitted to 0.6 vol.% DIO-use yielded the best PCE of 4.35%, with Jsc ¼ 9.9 mAcm2, Voc ¼ 0.85 V, and FF ¼ 0.51. Similarly, the 0.4 vol.% DIO use yielded the best PCE of 2.69%, for the DTBDTDPP-BO:PCBM devices, with Jsc ¼ 7.3 mAcm2, Voc ¼ 0.82 V, and FF ¼ 0.45. These

Fig. 6. GIXD images of DTBDTDPP-EH:PCBM blend films prepared in the three different conditions: (a) as-cast, (b) 130  C-annealed, and (c) 0.6 vol.% DIO-used. GIXD images of DTBDTDPP-BO:PCBM blend films prepared in the three different conditions: (d) as-cast, (e) 130  C-annealed, and (f) 0.4 vol.% DIO-used. Line-cut profiles of DTBDTDPP-EH:PCBM films in (g) out-of-plane (qz) and (h) in-plane (qy) directions; line-cut profiles of DTBDTDPP-BO:PCBM films in (i) out-of-plane (qz) and (j) in-plane (qy) directions.

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improvements were strongly associated with the formation of optimal nanoscopic crystalline domains and the achievement of a high hole mobility, as discussed below. The dependence of the photovoltaic properties on the device preparation conditions was investigated by characterizing the ascast, 130  C-annealed, and DIO-use DTBDTDPP:PCBM blend films using GIXD, tunneling electron microscopy (TEM), atomic force microscopy (AFM), and hole mobility measurement techniques. The GIXD experiments revealed the crystalline features of the DTBDTDPP:PCBM blend films. Fig. 6 shows GIXD images and linecut profiles of the as-cast, 130  C-annealed, and DIO-use DTBDTDPP:PCBM blend films. The crystallinities of both DTBDTDPP molecules decreased upon the incorporation of PCBM molecules, compared to the DTBDTDPP molecular films [59]. The DTBDTDPP-EH:PCBM molecular domains were weakly crystalline in the as-cast films. A weak (100) peak appeared at qz ¼ 0.45 Å1 and corresponded to a d100 spacing of 14.0 Å. A broad peak observed at qy ¼ 1.39 Å1 originated mainly from the stacked PCBM [59,60], and partly from the third peak in the crystalline DTBDTDPP-EH film (see Fig. 4a and g). Thermal annealing at 130  C significantly increased the crystallinity of the DTBDTDPP-EH:PCBM film, as indicated by the appearance of the (100) and (200) peaks. The DIOuse DTBDTDPP-EH:PCBM film exhibited even stronger (100) and (200) peaks in the qz direction. The (100) peak appeared at qz ¼ 0.43 Å1 and corresponded to a slight increase in the d100 spacing to 14.6 Å. The PCBM stacking peak was slightly narrowed and shifted to a position at qz ¼ 1.35 Å1. A (010) peak was observed at qz ¼ 1.57 Å1, corresponding to a d010 spacing of 4.0 Å. The crystallinity and molecular orientations of the DTBDTDPPBO:PCBM blend films followed a feature similar to that displayed by the DTBDTDPP-EH:PCBM blend films under each film preparation condition. The as-cast film was less crystalline, whereas both the post-thermally annealed and the DIO-use films displayed significant increases in the crystallinity. The DTBDTDPP-BO:PCBM blend films showed a large increase in the d100 spacing along the qz direction, from 16.1 Å in the as-cast film to 17.0 Å in both in the 130 C-annealed film and in the DIO-use film. These results indicated that DTBDTDPP-BO containing the longer thermo-responsive alkyl chains transitioned to a more ordered packing that resembled the configuration in the molecule-only film. Likewise, the DIO molecules interacted with the longer alkyl chains in the DTBDTDPP-BO molecules and helped form long-range molecular ordering, as observed in the molecule-only films. The significant change in UVeVisible absorption spectra was also observed as a result of the increased crystallinity under the conditions of 130  C thermal annealing or the DIO-use. Fig. 7 plots

the UVevisible absorption spectra of the DTBDTDPP-EH:PCBM and DTBDTDPP-BO:PCBM blend films. The as-cast DTBDTDPP-EH:PCBM and DTBDTDPP-BO:PCBM blend films showed similar UVevisible absorption features, with a strong peak around 670 nm that arose mainly from the J-aggregate DTBDTDPP molecular packing structure, that resembled the structure found in molecule-only films. The blend films submitted to 130  C-annealing and DIO-use conditions yielded spectral shapes that were nearly indistinguishable from those obtained in the molecule-only films (see Fig. 1). Collectively, the GIXD and UVevisible absorption characteristics indicated that both the 130  C-annealing and DIO-use conditions provided an effective means for improving the crystallinity and molecular ordering, both of which properties correlated well with high solar cell performance properties. TEM and AFM imaging revealed the morphologies of the DTBDTDPP-EH:PCBM and DTBDTDPP-BO:PCBM blend films. Figs. 8 and 9 show the TEM and AFM images, respectively, of the as-cast, 130  C-annealed, and DIO-use blend films. The TEM images of the as-cast blend films revealed that the DTBDTDPP molecules were well mixed with the PCBM molecules (Fig. 8a and d). Consistent with these results, the AFM images revealed that the surfaces of the as-cast films were very smooth, with root-mean-square (rms) roughness values of 0.30 and 0.31 nm for the as-cast DTBDTDPPEH:PCBM and DTBDTDPP-BO:PCBM films, respectively (Fig. 9a and d). Given the low crystallinity of the blend films, this morphology suggested that few bicontinuous interpenetrating networks had formed in the as-cast films. Therefore, the as-cast film devices of both molecules showed low photovoltaic properties, with the low Jsc and the low FF values. Thermal annealing of the DTBDTDPP-EH:PCBM films at 130  C for 10 min altered the film morphology and improved the photovoltaic properties. The 130  C-annealed DTBDTDPP-EH:PCBM film displayed very fine nanoscale domains with a width of ~7.5 nm (Fig. 8b). The observation of molecular domains was consistent with the GIXD and UVevisible absorption spectral results obtained from the blend films. The narrow molecular domains were beneficial for increasing the contact area between the DTBDTDPP-EHs and the PCBMs, which resulted in the high Jsc (9.06 mAcm2) in the DTBDTDPP-EH devices, more than a two-fold improvement over the devices prepared from the as-cast films. On the other hand, very fine domains with widths less than 10 nm yielded a limited increase in the FF due to bimolecular recombination [61]. Note that the 130  C-annealed film appeared to have optimal blend morphology as compared to the films that had been thermally annealed at other temperatures. The TEM images of these films are shown in Fig. S7. On the other hand, the DTBDTDPP-BO:PCBM films

Fig. 7. UVevisible absorption spectra of (a) DTBDTDPP-EH:PCBM (b) DTBDTDPP-BO:PCBM blend films prepared from the three different conditions: as-cast, 130  C-annealing, and DIO-use.

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Fig. 8. TEM images of DTBDTDPP-EH:PCBM blend films: (a) as-cast, (b) 130  C-annealed, and (c) 0.6 vol.% DIO-used. TEM images of DTBDTDPP-BO:PCBM blend films: (d) as-cast (e) 130  C-annealed, and (f) 0.4 vol.% DIO-used.

Fig. 9. Topographic images of DTBDTDPP-EH:PCBM blend films: (a) as-cast, (b) 130  C-annealed, and (c) 0.6 vol.% DIO-used. Topographic images of DTBDTDPP-BO:PCBM blend films: (d) as-cast (e) 130  C-annealed, and (f) 0.4 vol.% DIO-used.

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annealed at 130  C yielded aggregated DTBDTDPP-BO domains with poor connectivity and irregular domain sizes (7e20 nm) (Fig. 8e). Thus, the DTBDTDPP-BO devices showed a limited increase in the Jsc, FF, and PCE values. The AFM images revealed that the surface roughness values increased slightly to 0.43 and 0.77 nm, respectively, for the DTBDTDPP-EH:PCBM and DTBDTDPPBO:PCBM films, presumably due to the growth of molecular domains (Fig. 9b and e). Under the DIO-use conditions, the film morphology was even more favorable for OPV applications. The DTBDTDPP-EH domains in the DTBDTDPP-EH:PCBM blend were slightly more developed, formed extensive fibrous connections among domains, and displayed domain widths of ~12 nm (Fig. 8c). These observations were consistent with the observation that the DIO-use DTBDTDPP-EH devices displayed the highest Jsc (9.93 mAcm2) and FF (0.51) values and yielded the best PCE of 4.35%. Similarly, the DIO-use condition generated better connections among the DTBDTDPP-BO molecules in the DTBDTDPP-BO:PCBM films by forming ~19 nm thick fibrous domains (Fig. 8f), consistent with the GIXD data. The better connections increased the PCE to 2.69%, although the PCE remained limited by the relatively large degree of phase separation in the DTBDTDPP-BO:PCBM films. We note that achieving an optimal morphology depends critically on the use of an appropriate amount of DIO. Too much DIO induced the formation of poor film morphology and reduced the photovoltaic performances of the devices, as discussed further in the Supporting information (Table S2 and Figs. S6 and S8). AFM imaging revealed that the surface roughness values increased significantly to 3.72 and 3.44 nm, respectively, for the DTBDTDPP-EH:PCBM and DTBDTDPPBO:PCBM films (Fig. 9c and f), as a result of the more extensive phase separation between the DTBDTDPP and PCBM molecules. The TEM and AFM studies confirmed that both the post-thermal annealing and DIO-use methods significantly affected film formation, and the DIO-use conditions provided a more effective method for controlling the molecular domains and forming an optimal film morphology. This morphology, with a domain size of 10 nm and good connections, was critical for promoting exciton dissociation, charge transport, and, thus, a high PCE with excellent Jsc and FF values. The shorter alkyl chains in the DTBDTDPP-EH molecules formed uniform nanoscale domains, whereas the longer alkyl chains in the DTBDTDPP-BO molecules promoted the formation of larger and more irregular aggregates. These film properties resulted from the fact that the longer alkyl chains in the DTBDTDPP-BO molecules were more sensitive to the thermal and DIO treatments and tended to increase the degree of intermolecular stacking. The hole mobilities of the DTBDTDPP-EH:PCBM and DTBDTDPPBO:PCBM blend films were measured under dark conditions using the space-charge-limited-current (SCLC) method. Hole-only devices were fabricated with the structure ITO/PEDOT:PSS/ DTBDTDPP:PCBM (6:4, wt/wt)/Au, and their currentevoltage characteristics were measured under dark conditions. Fig. S9 displays the current densityevoltage characteristics, and Table 2 summarizes the hole mobilities of the blend films prepared from the as-cast, 130  C-annealed, and DIO-use conditions. The 130  Cannealed blend films displayed hole mobilities of 106 cm2 V1 s1, similar to those obtained from the as-cast films. These results indicated that thermal annealing increased the molecular crystallinity and induced the formation of slightly larger domains among the DTBDTDPP molecules; however, the interconnections among the molecular domains appeared to be limited. The hole mobility of the DIO-use DTBDTDPP-EH:PCBM blend film increased significantly to 3.4  105 cm2 V1 s1. In the same manner, the DIO-use DTBDTDPP-BO:PCBM blend film showed a significant increase in the hole mobility, from 3.0  106 to 1.7  105 cm2 V1 s1. The approximately 5-fold increase in the hole mobility was attributed

to the improved crystallinity and interconnectivity among the phase-separated DTBDTDPP domains in the DTBDTDPP:PCBM blend films, as revealed by the GIXD and TEM studies. Therefore, the use of DIO to form photoactive films in the SM-OPVs provides an effective way to create continuous carrier transport pathways. This feature contributed to the reduced bimolecular recombination inside photoactive layers, which increased the values of Voc, Jsc, and FF in the DIO-use molecular devices relative to the corresponding values obtained from the thermally annealed devices [62]. Several important findings were discussed here. First, the planar well-conjugated DTBDT core in the DTBDTDPP molecular structures maintains the overall molecular planarity and keeps molecular HOMO level low. This suggests that the DTBDT moiety may be potentially useful in the development of other SM-OPV materials. The DPP moieties in the side arms effectively lowered the bandgaps and induced an effective internal charge transfer. The DPPcontaining molecules that formed linear planar structures, as discussed in this and in previous studies [31,33,51], tended to form Haggregates, in addition to J-aggregates, in the well-packed solid state. Second, the selection of an appropriate alky chain length was critical for fine-tuning the intermolecular packing structure and nano-separated morphology in the electron donor molecule:PCBM blend films. Importantly, our morphological studies indicated that both the thermal annealing and the DIO-use processes assisted the development of appropriately crystalline structures and domains in the final film state, while the DIO-use treatment was more effective in optimizing the properties of both the DTBDTDPP-only and DTBDTDPP:PCBM blend films. The DTBDTDPP-EH and DTBDTDPPBO molecules in the DTBDTDPP:PCBM blend films self-organized to form distinct domains, and the PCBM molecules selfsegregated from the DTBDTDPP molecules with the assistance of the DIO molecules. The improved crystallinity of the DIO-use DTBDTDPP films was attributed to the fact that the high boiling point DIO solvent provided a longer drying time for both the DTBDTDPP and PCBM domains, which facilitated the formation of phase-separated nanoscale domains with percolated continuous network production, as reported in the literature [39,40,63,64]. This hypothesis was verified by comparing TEM images of regularly processed and fast-dried DIO-use DTBDTDPP-EH:PCBM films. The formation of molecular domains and the extent of interconnectedness were quite poor in the 0.6 vol.% DIO-use DTBDTDPPEH:PCBM blend film that was fast-dried under vacuum immediately after spin-coating (Fig. S10). We also note that the role played by the DIO during the film morphology development process in the small molecule:PCBM blend film differed from the role played by DIO in the morphological development of the polymer:PCBM film. It is well-known that in the case of the polymer:PCBM blend films, the DIO additive mainly affected the morphology of the PCBM domains because the DIO has little solvating power for polymers [39,40,62]. By contrast, in the former case, the presence of very small amounts of DIO in the molecular solution significantly altered the crystallinity and molecular packing structures of both the electron donor molecules and the PCBM molecules simultaneously during film drying, as demonstrated in this work. These effects may explain why the optimal DIO concentration for the preparation of small electron donor molecule:PCBM films tended to be around 0.4 vol.%, whereas it was commonly a few vol.% in the preparation of polymer:PCBM films. Therefore, the DIO additive concentration should be carefully optimized especially for each SM-OPV device composition. 7. Conclusions To development high performance electron donor molecules for high PCE and proper morphology, we synthesized DTBDTDPP

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molecules employing the DTBDT and DPP moieties bearing 2ethylhexyl (EH) or 2-butyloctyl (BO) alkyl side chains on the DPP units. DTBDTDPP-EH and DTBDTDPP-BO were highly thermo-stable and provided broad strong absorption bands with narrow bandgaps, and low-lying HOMO and high-lying LUMO energy levels that matched the corresponding energy levels of the PCBMs, thereby providing high Vocs and good exciton separation. The as-cast DTBDTDPP-EH:PCBM and DTBDTDPP-BO:PCBM blend films were weakly crystalline, and the DTBDTDPP molecules were not wellconnected. These devices afforded a low PCE of 1.1%. The PCE of the DTBDTDPP-EH:PCBM device was increased to 2.39% by postthermal annealing at 130  C of the devices and its PCE was more significantly enhanced to 4.35% by the use of DIO to form the blend film. The DTBDTDPP-BO:PCBM blend films followed a similar trend and displayed low PCEs. A full characterization of the blend films revealed that the DIO additive promoted effectively molecular packing, carrier transport, and the formation of extensive interconnections among the molecular domains, although thermal annealing at 130  C also improved the crystallinity of the DTBDTDPP molecules to some degree. In regard to the effect of the length of branched alkyl side chains, the DTBDTDPP-EH molecules bearing shorter alkyl chains formed better nanoscale bicontinuous electron donor channels to achieve high carrier generation and carrier transport than the DTBDTDPP-BO molecules. As a result, the DTBDTDPP-EH:PCBM blend films displayed the higher PCE. Overall, this work highlights that the use of an appropriate concentration of the DIO processing additive provides a method for controlling the blend film morphology in electron donor and electron acceptor materials, and also it is necessary to fine-tune chemical structures of electron-donors by introducing proper solubilizing groups, such that we can utilize the full beneficial characteristics of the planar conjugated backbones. Acknowledgments This work was supported by New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry & Energy (MTIE) (20133030000130, 20113030010060), and by Korea Institute of Science and Technology (KIST) Internal Project, and by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (2013, University-Institute corporation program). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2014.12.003. References [1] Coughlin JE, Henson ZB, Welch GC, Bazan GC. Design and synthesis of molecular donors for solution-processed high-efficiency organic solar cells. Acc Chem Res 2014;47(1):257e70. [2] Chen Y, Wan X, Long G. High performance photovoltaic applications using solution-processed small molecules. Acc Chem Res 2013;46(11):2645e55. [3] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446. [4] He Z, Zhong C, Su S, Xu M, Wu H, Cao Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photonics 2012;6(9):591e5. [5] Zhang M, Guo X, Zhang S, Hou J. Synergistic effect of fluorination on molecular energy level modulation in highly efficient photovoltaic polymers. Adv Mater 2014;26:1118e23. [6] Liu S, Zhang K, Lu J, Zhang J, Yip H-L, Huang F, et al. High-efficiency polymer solar cells via the incorporation of an amino-functionalized conjugated metallopolymer as a cathode interlayer. J Am Chem Soc 2013;135(41):15326e9.

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