Controlling the Morphology of BDTT-DPP-Based Small Molecules via

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Controlling the Morphology of BDTT-DPP-based Small Molecules via End-group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells Ji-Hoon Kim, Jong Baek Park, Hoichang Yang, In Hwan Jung, Sung Cheol Yoon, Dongwook Kim, and Do-Hoon Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b05248 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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ACS Applied Materials & Interfaces

Controlling the Morphology of BDTT-DPP-based Small Molecules via End-group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells Ji-Hoon Kim,† Jong Baek Park,† Hoichang Yang,|| In Hwan Jung,‡ Sung Cheol Yoon,‡ Dongwook Kim, § and Do-Hoon Hwang*,† †

Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea

‡

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Republic of Korea ||

Department of Applied Organic Materials Engineering, Optoelectronic Hybrids Research Center, Inha University, Incheon 402-751, Republic of Korea

§

Department of Chemistry, Kyonggi University, San 94-6, Iui-dong, Yeongtong-gu, Suwon 443-760, Republic of Korea

KEYWORDS: organic photovoltaic device; small molecules; benzodithiophene; diketopyrrolopyrrole; inverted tandem solar cells.

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ABSTRACT: A series of narrow-band-gap, π-conjugated small molecules based on diketopyrrolopyrrole (DPP) electron acceptor units coupled with alkylthienyl-substitutedbenzodithiophene (BDTT) electron donors were designed and synthesized for use as donor materials in solution-processed organic photovoltaic cells. In particular, by end-group functionalization of the small molecules with fluorine derivatives, the nanoscale morphologies of the photoactive layers of the photovoltaic cells were successfully controlled. The influences of different fluorine-based end-groups on the optoelectronic and morphological properties, carrier mobilities, and the photovoltaic performances of these materials were investigated. A high power conversion efficiency (PCE) of 6.00% under simulated solar light (AM 1.5G) illumination has been achieved for organic photovoltaic cells based on a small-molecule bulk heterojunction system consisting of a trifluoromethylbenzene (CF3) end-group-containing oligomer (BDTT-(DPP)2-CF3) as the donor and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) as the acceptor. As a result, the introduction of CF3 endgroups has been found to enhance both the short circuit current density (JSC) and fill factor (FF). A tandem photovoltaic device comprising an inverted BDTT-(DPP)2-CF3:PC71BM cell and a poly(3-hexylthiophene) (P3HT): indene-C60-bisadduct (IC60BA)-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.30%. These results provide valuable guidelines for the rational design of conjugated small molecules for applications in high-performance organic photovoltaic cells. Furthermore, to the best of our knowledge, this is the first report on the design of fluorine-functionalized BDTT-DPP-based small molecules, which have been shown to be a viable candidate for use in inverted tandem cells.


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1. INTRODUCTION Recently, organic photovoltaic cells (OPVs) based on solution-processable organic small molecules (SMs) have been extensively studied. This interest is due to their unique advantages such as high purity, well-defined molecular weights, excellent reproducibility without batch-to-batch variations, and relatively simple purification procedures compared to polymer-based OPVs.1-3 The state-of-the-art SM-based OPVs exhibit over 9% power conversion efficiency (PCE) in single layer cells4 and over 10% in tandem cells.5 The most successful OPV architecture is the bulk heterojunction (BHJ) structure, which is prepared by mixing electron donors and acceptors with that show nanoscale phase separation. The BHJ structure offers a high density of heterojunction interfaces in the active layers.6,7 Therefore, the choice of method for controlling the morphology of the photoactive layer is extremely important to achieve a highly efficient device. The most well-known methods for controlling the morphologies of blended films are thermal annealing and treatment with additives such as 1,8-diiodooctane (DIO)8,9 and 1-chloronaphthalene (1-CN). 10,11

To further enhance the intermolecular interactions and the fibrous structure of the

polymer in the photoactive layer, end-group functionalization of the donor polymers with fluorine atoms has been reported.12-15 Yang et al. synthesized end-fluorinated poly-(3hexylthiophene) (P3HT) and controlled the miscibilities of P3HT and PCBM to obtain a bicontinuous interpenetrating network of donor/acceptor components, thus, optimizing the morphology of the photoactive layer.16 Cho et al. reported that solar cell performance was greatly improved (a PCE of over 6.02%) on the introduction of trifluoromethylbenzene as an end-capping group for the donor polymer.17 However, in the case of SMs, although end-group functionalization with various fluorine derivatives is likely to be a powerful tool, it has not been studied in depth.



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In this study, we have introduced a fluorine end-group to the SM system. We report first the strategic end-group engineering of low-band-gap SMs based on the alkylthienyl substituted-benzodithiophene (BDTT)-diketopyrrolopyrrole (DPP) system with the aim of achieving highly efficient OPVs. DPP chromophores have been extensively explored for organic optoelectronic applications, owing to their unique π-conjugated systems, high optical densities, and exceptional stabilities. In particular, DPP-based oligomers or SMs are widely used for OPVs,18–20 organic thin film transistors (OTFTs),21,22 dye-sensitized solar cells (DSSC),23 and chemical sensors,24 etc. A series of low-band-gap SMs containing benzo[1,2b:4,5-b‘]dithiophene (BDT) and DPP units have been synthesized by several research groups. For example, Yao et al. reported a BDT-DPP-based SM (optical band gap (Egopt) ≈ 1.64 eV) that had a PCE of 5.29%.25 Subsequently, Adachi et al. demonstrated a device with a PCE of 5.8%, obtained by optimizing the π-conjugation in the BDT-DPP backbone.26 As shown in Figure 1, we investigated the effect of molecular tuning of the endgroups on the key parameters determining the PCEs by incorporating a series of functionalized end-groups such as benzene, fluorobenzene, and trifluoromethylbenzene in the SMs. BHJ OPVs fabricated with SMs containing trifluoromethylbenzene as the end-group had PCEs, at maximum, of 6.0% in single junction devices, which is the highest value observed among devices using BDTT-DPP-based SMs. We finely tuned the morphology of the SMs by varying the architecture of the end-groups, in order to promote better π–π stacking and chain ordering in the film state. Thus, we demonstrated that subtle changes in the end-groups can have a significant influence on the photovoltaic parameters. Also, we showed that fluorine end-group functionalization in the SM system is one of the best methods to enhance the performance of solar cells. However, improving the PCEs of single junction OPVs is challenging and, generally, the PCE value is limited to 9% in OPVs with a BHJ device structure.27–29 In this type of cell, 4 ACS Paragon Plus Environment

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the narrow absorption band of the electron donor in the active layer cannot absorb the abundant solar energy. Therefore, the concept of tandem solar cells was developed to reduce these absorption losses. Tandem solar cells contain double-junction or multijunction sub-cells with different active layers, each absorbing different parts of the solar spectrum.30,31 Thus, to enhance the overall performance of the OPVs, tandem OPVs have also been fabricated using the best performing low-band-gap SMs BDTT-(DPP)2-CF3 and P3HT:indene-C60-bisadduct (IC60BA) as the top and bottom cell components, respectively. Combining these components in

tandem solar cell architecture and using end-group functionalization, we report a much

improved PCE of 8.30%.

2. RESULTS AND DISCUSSION 2.1. Synthesis The synthetic routes for the three SMs with acceptor-donor-acceptor structures, namely BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3, are outlined in Scheme 1 and Scheme S1 (Supporting Information). The three SMs were synthesized using the Stille and Suzuki cross-coupling reactions. The structures of the final molecules were confirmed by 1H-, 13

C-, and

19

F-NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight

(MALDI-TOF) mass spectrometry, and elemental analysis. The synthetic procedures and characterization data are described in detail in the experimental section and supporting information. All oligomers considered in the study are soluble in common organic solvents such as chloroform, toluene, and chlorobenzene at room temperature, owing to the presence of multiple solubilizing alkyl chains. Thermogravimetric analyses (TGA) indicate that BDTT(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 possess good thermal stability with 5% weight-loss temperatures (Td) of 397, 406, and 414 °C, respectively, measured under N2 atmosphere (Figure S1). Clear melting temperatures (Tm) at 281, 283, and 287 oC, and 5 ACS Paragon Plus Environment


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recrystallization points (Tc) at 256, 258, and 261 oC were measured for BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3, respectively. (Figure S1, inset)

2.2. Molecular Energy Level Measurement The electronic structures of the SM donor molecules were investigated by conducting cyclic voltammetry (CV) experiments (Figure 3(a)), ultraviolet photoelectron spectroscopy (UPS) measurements (Figure 3(b, c)), and density functional theory (DFT) calculations (Figure 3 (d) and Table 1). CV measurements for BDTT-DPP-B, BDTT-DPP-F and BDTT-DPP-CF3 were used to calculate their respective HOMOs. These were found to be approximately -5.15, -5.15, and -5.18 eV, while, from UPS measurements, their ionization potentials (IPs) were found to be approximately 5.29, 5.30, and 5.52 eV, respectively. Theoretical results are also in fair agreement with these experimental results. IPs were calculated to be approximately 5.37, 5.41 and 5.52 eV for BDTT-DPP-B, BDTT-DPP-F, and BDTT-DPP-CF3, respectively. Theoretical electron affinities (EAs) were also calculated. Although these tend to be underestimates of the true values, the trends they reveal are expected to be reliable.32 The EAs were found to be approximately 2.05, 2.10, and 2.23 eV for these molecules. These results suggest that if the end phenyl groups contain fluoromethyl substituents, the HOMO and LUMO are moderately stabilized; in contrast, fluoride substitutions have only marginal impact on the frontier orbitals. The HOMO–LUMO gaps33 for these molecules were calculated to be approximately 3.32, 3.32, and 3.29 eV, respectively, and thus remain intact. Notably, given that that all three measurements were conducted in different phases (solution phase for CV, solid phase for UPS, and gas phase for DFT calculations), we consider that the effect of the environment on their molecular electronic structures is insignificant. 2.3. Optical Properties


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Figure 2(a) and (b) show the absorption spectra of the SMs dissolved in chloroform and spun-cast on quartz substrates, respectively. The optical data, including the absorption peak wavelengths (λabs), absorption edge wavelengths (λedge), and optical band gaps (Egopt), are summarized in Table 1. All the three SMs show a strong and broad absorption peak at longer wavelengths originating from a HOMO-to-LUMO intramolecular charge transfer between the donor and acceptor units in the SMs. Also, all three show a weak absorption peak at shorter wavelengths, which is attributed to π-π transitions.34 For absorption spectra acquired in the thin film state, the maximum absorption wavelengths (λmax) as well as λedge are red-shifted relative to the spectra obtained in the solution state. Interestingly, the UV absorption peaks of the BDTT-(DPP)2-F and BDTT-(DPP)2-CF3 films are red-shifted compared to those of the BDTT-(DPP)2-B film and include a wide shoulder region (at 700–800 nm), implying that intermolecular interactions in the BDTT-(DPP)2-F and BDTT-(DPP)2-CF3 chains may be stronger than those in the BDTT-(DPP)2-B chains.17 The optical band gap energies (Egopt) of the BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 films, which are estimated from the wavelength corresponding to the onset of the UV-visible absorption in the SM films, are 1.57, 1.56, and 1.55 eV, respectively. Again, the results from the theoretical calculations are similar. As expected from the rather similar fundamental gaps, the transition energies for BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 are computed to be approximately 1.80, 1.80, and 1.78 eV, respectively. In addition, the theoretical results indicate that the LUMOs tend to be primarily localized on the DPP units, whereas the HOMOs are delocalized along the length of the molecules, as shown in Figure 3. Because they correspond to HOMOto-LUMO transitions, the S1 states of all three SMs are the products of intramolecular charge transfer. 2.4. Properties of Single Photovoltaic Cells



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To investigate the photovoltaic properties of the SMs, BHJ OPVs were fabricated with the SMs as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor in a device with an ITO/PEDOT:PSS/photoactive layer/Ca/Al structure. The active layer was spun-cast from a chloroform solution of the donor and acceptor molecules. 1-CN (2.5 vol%) was used as a processing additive to optimize the morphology of the active layer. For OPV devices based on BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT(DPP)2-CF3, the ratio of the SM to PC71BM was adjusted from 2:1 to 1:2 (w/w); the optimized weight ratio between the SM and PC71BM was 1:1. (for details, please see the supporting information) Figures 4(a) and (b) show the current–voltage (J−V) characteristics of the OPVs based on SM/PC71BM (1:1 w/w), both with and without 1-CN, under AM 1.5G illumination at 100 mW cm-2. The photovoltaic parameters of the devices are summarized in Table 2. In the absence of the additive, the devices containing BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT(DPP)2-CF3 had low PCEs of 1.27, 1.57, and 1.6%, respectively. The open-circuit voltage (VOC) is affected not only by the frontier orbital energy levels of the donor and acceptor molecules but, also, other factors such as the film quality and the morphology of the active layer. For these reasons, the OPVs fabricated using the three SMs can have similar VOC values despite small changes in their HOMO energy levels by the UPS. The measured JSC values of the devices fabricated using BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 were 3.88, 3.82 and 5.50 mA cm-2, respectively. The performances of the OPVs fabricated using BDTT-(DPP)2-B:PC71BM, BDTT-(DPP)2-F:PC71BM, and BDTT-(DPP)2-CF3:PC71BM (1:1 w/w) films were greatly enhanced by using 1-CN, as previously stated. Adding 2.5 vol% of 1CN to the CF solutions increased the PCEs dramatically from 1.27 to 3.65% for the BDTT(DPP)2-B:PC71BM device, 1.57 to 4.96% for the BDTT-(DPP)2-F:PC71BM device, and 1.69 to 6.00% for the BDTT-(DPP)2-CF3:PC71BM device. The VOC values of the 1-CN-containing 8 ACS Paragon Plus Environment

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devices remained constant or were slightly reduced. This significant improvement in the PCE was likely a result of the significant increase in the JSC values and FF. The JSC and FF of an OPV is affected by many factors including the absorbance of the active layer, film morphology, and charge carrier mobility.35,36 Interestingly, the JSC values of the devices fabricated using the three SMs were significantly altered when 1-CN was used as a processing additive. The measured JSC values of the devices fabricated using BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 were 10.21, 12.48, and 13.60 mA cm-2, respectively. Two-dimensional (2D) grazing-incidence X-ray diffraction (GIXD) was performed to investigate the crystalline structures of the active layers. All the samples were spun-cast onto PEDOT:PSS-coated Si substrates from SMs and their blend solutions. Figure 5 shows the 2D-GIXD patterns of the pure SMs and SM:PC71BM (1:1 w/w) blends. For all the pure SM films, the order of crystallinity, based on the peak intensities of the 1D GIXD patterns (see 1D out-of-plane X-ray profiles shown in Figure S7(a)), is as follows: BDTT-(DPP)2-CF3 > BDTT-(DPP)2-F ≥ BDTT-(DPP)2-B. This implies that the fluorinated SM molecules packed better than the non-fluorinated molecules, forming an improved film structure during the fast solvent evaporation that occurs in spin-casting. The BDTT-(DPP)2-B and BDTT-(DPP)2-F films had mixed crystal structures that included chains oriented both edge-on and face-on chains with respect to the substrate. In contrast, the BDTT-(DPP)2-CF3 film had a crystalline phase with the chains mostly oriented edge-on, yielding strong X-ray reflections from the (h00) crystal planes along the Qz axis. In the BDTT-(DPP)2-B film, the average interval between (h00) crystal reflections along the Qz axis is 0.2982 Å-1, which corresponds to the (100) crystal planes and a d(100) spacing of 21.07 Å. Additionally, a weak X-ray reflection at Qz = 1.612 Å-1 corresponds to the layer spacing of the (010) crystal planes, d(010) = 3.90 Å; this is known to be the intermolecular π-stacking distance. Using the same X-ray analysis, d(100) and d(010) values of BDTT-(DPP)2-F were found to be 22.26 and 3.83 Å, respectively. 9 ACS Paragon Plus Environment


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Interestingly, the crystalline BDTT-(DPP)2-CF3 film contains the crystallites with the closest edge-on chain interactions of the SMs: d(100) = 22.49 Å and d(010) = 3.71. Å. Unlike the pure SM systems, the corresponding blend films with PC71BM (1:1 w/w) showed discernible changes in the crystal orientations of the SMs. The proportion of face-on chains in the BDTT(DPP)2-B: and BDTT-(DPP)2-F:PC71BM blends reduced, becoming a minor component; in contrast, the face-on conformation of BDTT-(DPP)2-CF3 molecules in the blend film increased, becoming dominant over the edge-on orientation (see Figures 5(d−f)). Overall, the crystallinity of the SMs in these blends decreased in comparison with the corresponding pure systems due to the existence of the amorphous PC71BM phase, which is identified by open circles at Q = 1.34 Å-1 in the GIXD patterns. However, the unit-cell parameters of each πconjugated SM were maintained in the blend films. Particularly, the order of crystallinity of the SMs in the blend films is the same as that of the pure SM films. Based on the 2D GIXD results, it was found that π-conjugated distances between fluorinated SMs along the nearest chains, given by the d(010) values, were much smaller than that of BDTT-(DPP)2-B. That is, 3.83, 3.71, and 3.90 Å for BDTT-(DPP)2-F, BDTT-(DPP)2-CF3, and BDTT-(DPP)2-B, respectively. Particularly, we believe that the vertical π-conjugation that occurs with the faceon orientation of the BDTT-(DPP)2-CF3 molecules in the blend film (1:1 w/w) is responsible for the high JSC values observed in OPVs containing BDTT-(DPP)2-CF3 in comparison to those of the BDTT-(DPP)2-B and BDTT-(DPP)2-F based OPVs. In OPV devices, hole and electron mobility is an important parameter for donor materials to ensure efficient charge-carrier transport to the electrodes and to suppress photocurrent loss by competing charge recombination. Using the steady-state space-chargelimited current (SCLC) technique, hole mobilities (µh) and electron mobilities (µe) of the three SMs were evaluated in vertical hole and electron-only devices. The hole-only and electrononly devices were fabricated with ITO/PEDOT:PSS/pure SMs (neat state) and SMs:PC71BM 10 ACS Paragon Plus Environment

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(1:1 w/w, 2.5 vol% 1-CN) (blend state)/Au and ITO/ZnO/SMs:PC71BM (1:1 w/w, 2.5 vol% 1CN) (blend state)/LiF/Al structures, respectively; the mobilities were calculated using the Mott-Gurney equation.37 The measured hole mobilities of the BDTT-(DPP)2-B, BDTT(DPP)2-F, and BDTT-(DPP)2-CF3 films were 4.21 × 10−4, 1.40 × 10−3, and 2.07 × 10−3 cm2 V−1 s−1, respectively, as shown in Figure 6. On the other hand, the measured hole mobilities of the BDTT-(DPP)2-B:PC71BM (1:1, w/w), BDTT-(DPP)2-F:PC71BM (1:1, w/w), and BDTT(DPP)2-CF3:PC71BM (1:1, w/w) blends were 1.55 × 10−5, 3.11 × 10−5, and 4.57 × 10−4 cm2 V−1 s−1, respectively. This could be due to the higher crystallinity of BDTT-(DPP)2-CF3 film compared to BDTT-(DPP)2-B and BDTT-(DPP)2-F and, also, to the face-on orientation of the BDTT-(DPP)2-CF3 chains, which have the shortest π-π stacking distance in the BDTT(DPP)2-CF3:PC71BM blend film, as shown from the 2D-GIXD analysis. In addition, the balance between the hole (µh, blend) and electron (µe, blend) mobilities in the active layer is important for improving the FFs of OPVs. The measured electron to hole mobility µe/µh ratios of the BDTT-(DPP)2-B:PC71BM, BDTT-(DPP)2-F:PC71BM, and BDTT-(DPP)2-CF3:PC71BM blend films were 6.9, 4.0, and 1.9, respectively. The higher hole mobility and better charge balance of the BDTT-(DPP)2-CF3:PC71BM blend are consistent with the higher JSC and FF values than those of both BDTT-(DPP)2-B and BDTT-(DPP)2-F. The mobilities extracted from the SCLC are summarized in Table 3. The morphologies of the SM:PC71BM active layers were investigated using atomic force microscopy (AFM) and transmission electron microscopy (TEM). For the films prepared without 1-CN, the TEM images reveal severe phase-segregation for all three SMs (Figure 7). In contrast, much more homogeneous morphologies were observed for BDTT(DPP)2-B:PC71BM,

BDTT-(DPP)2-F:PC71BM,

and

BDTT-(DPP)2-CF3:PC71BM

films

processed with 1-CN, as shown in Figures 7(d)–(f). Interestingly, the BDTT-(DPP)2CF3/PCBM blend contains nanofiber structures (Figure 7(f)). In general, the dark regions in 11 ACS Paragon Plus Environment


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the TEM images arise due to the PCBM clusters or aggregates, which have greater electron densities compared to the regions containing the electron donors.38 Therefore, the nanofibers in the bright regions in the TEM image may be BDTT-(DPP)2-CF3. The BDTT-(DPP)2-CF3 fibrillar structures arise as a result of the fluorine atoms in the end-group of the molecules. Fluorine can form numerous secondary interactions with other molecules. In particular, strong dipole-dipole interactions induced by the electronegative fluorine atom are likely to exert a significant influence on the molecular structure, packing, and morphology of the thin films.17, 39-41

Figures 4(c) and (d) show the external quantum efficiency (EQE) curves of OPVs fabricated under the same optimized conditions as those used for the J–V measurements. It is evident that the EQEs of the devices containing SMs prepared with 1-CN are much higher than those without 1-CN. The 1-CN-containing devices fabricated with BDTT-(DPP)2-CF3 had higher EQEs than those containing either BDTT-(DPP)2-B or BDTT-(DPP)2-F. All of the devices exhibited efficient PCEs in the wavelength range of 350–800 nm, with EQE values of 45–55%. The JSC values of all the devices, determined by integration from the EQE curves, were consistent (within 5% error) with the values obtained from the J–V measurements.

2.5. Inverted Tandem Photovoltaic Properties Tandem photovoltaic cells with inverted configurations were fabricated using the newly designed low-band-gap, high-efficiency BDTT-(DPP)2-CF3 films. A tandem photovoltaic cell enables the effective harvesting of a broader portion of the solar spectrum and makes more efficient use of the photonic energy compared to a single junction structure.38–40 Tandem OPVs with inverted configurations were fabricated using BDTT(DPP)2-CF3:PC71BM as the top cell component. P3HT and IC60BA were used as the bottom 12 ACS Paragon Plus Environment

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cell donor and acceptor materials, respectively, because P3HT has a complementary absorption to the top cell SMs and IC60BA can produce a high Voc in combination with P3HT. The structure of the fabricated tandem cell was ITO/PEIE/P3HT:IC60BA (150 nm)/PEDOT:PSS/polyethyleneamine ethoxylated (PEIE)/BDTT-(DPP)2-CF3:PC71BM (100 nm)/MoO3/Ag. The tandem OPVs fabricated for this study have an inverted architecture and contain a conducting SM layer of PEDOT:PSS coated with PEIE as the charge recombination layer.42,43 Figure 8 shows the device structure and the UV-visible absorption spectra of P3HT and BDTT-(DPP)2-CF3 in the solid state. The J–V characteristics of the tandem cells and the performance parameters are shown in Figure 9(a). The maximum PCE of the BDTT-(DPP)2-CF3-based tandem cells was 8.30% and a VOC of 1.53 V, which is equal to the sum of the VOC’s of the single bottom and top cells. In addition, a JSC of 8 mA cm-2 and FF of greater than 68% were achieved. The photovoltaic performance of the inverted tandem devices is summarized in Table 4. To further confirm the JSC values, the EQEs of the two sub-cells in the tandem device were measured

using

a

previously

method.44–46 The

reported

EQE

curves

of

the

P3HT:IC60BA/BDTT-(DPP)2-CF3:PC71BM tandem device are shown in Figure 9(b). The bottom cell containing a P3HT:IC60BA-based tandem OPV had measureable quantum efficiency from 300 to 650 nm, with a maximum EQE of 70% at about 520 nm. In addition, the integrated Jsc was calculated to be 8.63 mA cm-2. On the other hand, the top cell based on BDTT-(DPP)2-CF3 had a broad photoresponse from 300 to 800 nm. The maximum EQE was over 58% from 600 to 700 nm. Also, from the EQE curve, the integrated JSC was determined to be 7.57 mA cm-2. The current of a tandem cell is typically determined by the sub-cell with the lowest JSC. Therefore, the measured JSC of 8.00 mA cm-2 was close to the integrated JSC of 7.57 mA cm-2 from the top cell.



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3. CONCLUSIONS We have successfully synthesized three new π-conjugated SMs, BDTT-(DPP)2-B, BDTT(DPP)2-F, and BDTT-(DPP)2-CF3, and investigated the effect of the fluorine end-groups on controlling the nanoscale morphologies of the photoactive layers of the photovoltaic cells. All three SMs had low optical band gaps (~1.55 eV) and strong π–π stacking shoulder peaks in the UV-visible absorption spectra. Fluorine end-group functionalization increased the fibrous structure of the SMs in the photoactive layer. Additional UV/visible spectroscopy and 2DGIXD results suggest that the fibrous structures with enhanced π-conjugated stacking and ordering increased the hole mobility. As a result, the PCE of BDTT-(DPP)2-CF3, which contains CF3 functional groups, increased to 6.0%, higher in comparison with BDTT-(DPP)2B, which does not contain a fluorinated group (PCE: 3.65%). Moreover, the tandem OPVs fabricated using BDTT-(DPP)2-CF3:PC71BM as the top cell and P3HT:IC60BA as the bottom cell exhibited maximum PCEs of 8.30%. We believe that the fluorine end-group functionalization, particularly in SM systems, is one of the most efficient methods for the enhancement of the photovoltaic performance of OPVs. Furthermore, this approach has the potential to open up new research avenues into the chemically mediated control of nanoscale blend morphologies to fabricate highly efficient OPVs.


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Synthesis of BDTT-(DPP)2-B: To a mixture of BDTT (0.50 g, 0.55 mmol) and 3-(5bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)-6-(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione (DPP-B) (0.86 g, 1.27 mmol) in dry toluene (20 mL), Pd(PPh3)4 (0.05 g, 0.04 mmol) was added and the mixture was stirred for 24 h at 100 °C. After cooling to room temperature, the reaction mixture was poured into water and then extracted with chloroform. The combined organic layers were washed with water and dried over anhydrous MgSO4. After filtration and evaporation, the product was purified by silica gel column chromatography (eluent: chloroform), recrystallized from chloroform/methanol, and dried under vacuum to afford BDTT-(DPP)2-B as a dark green solid, which was characterized by HPLC. (yield = 0.70 g, 81%, purity > 99.9%). 1H NMR (300 MHz, CDCl3) δ: 9.03-9.02 (d, 2H), 8.99-8.97 (d, 2H), 7.55 (s, 2H), 7.52-7.50 (d, 4H), 7.40-7.38 (d, 2H), 7.33-7.32 (d, 2H), 7.29-7.25 (m, 4H), 7.21-7.18 (d, 4H), 6.99-6.97 (d, 2H), 3.95-3.90 (m, 8H), 2.98-2.96 (d, 4H), 1.86-1.82 (m, 6H), 1.59-1.24 (m, 48H), 1.10-0.84 (m, 36H).

13

C NMR (75 MHz, CDCl3) δ:

161.5, 161.2, 150.8, 149.7, 146.3, 144.0, 143.8, 141.8, 139.9, 138.8, 138.5, 137.3, 136.7, 136.4, 136.1, 131.9, 130.8, 129.0, 128.5, 128.3, 126.2, 122,2, 122.0, 121.9, 120.3, 108.5, 107.9, 49.5, 41.8, 37.9, 35.2, 32.2, 32.1, 31.1, 30.4, 29.6, 29.3, 26.1, 25.3, 23.0, 15.3, 11.9, 11.6. Anal. Calcd. for C106H126N4S8: C, 71.66; H, 7.15; N, 3.15; S, 14.44. Found: C, 71.60; H, 7.18; N, 3.09; S, 14.39. MS (MALDI-TOF) m/z: [M]+, 1774.75; Found, 1774.521. Tm: 281 oC.

Synthesis of BDTT-(DPP)2-F: This compound was prepared by a method similar to BDTT(DPP)2-B, using BDTT (0.5 g, 0.55 mmol), 3-(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)6-(5-(4-fluorophenyl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-F) (0.88 g, 1.27mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol). The product was obtained as a dark green solid and characterized using HPLC. (yield = 0.68 g, 82%, purity > 99.9%). 1H NMR (300 MHz, CDCl3) δ: 9.03-9.01 (d, 2H), 8.95-8.93 (d, 2H), 7.53 (s, 2H), 7.46 (d, 4H), 7.41 (s, 2H), 15 ACS Paragon Plus Environment


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

7.22 (d, 2H), 7.16 (d, 2H), 7.00 (d, 2H), 6.93 (d, 2H), 3.90 (m, 8H), 2.99 (d, 4H), 1.85-1.83 (m, 6H), 1.59-1.24 (m, 48H), 1.10-0.84 (m, 36H). 13C NMR (75 MHz, CDCl3) δ: 162.3, 162.1, 152.1, 150.6, 148.3, 145.1, 144.7, 143.5, 140.1, 139.5, 138.9, 138.0, 137.6, 136.7, 136.4, 132.3, 131.7, 130.1, 129.4, 128.1, 127.5, 123,1, 122.7, 122.0, 121.8, 109.1, 108.3, 49.3, 42.0, 38.0, 36.1, 32.8, 32.5, 31.4, 30.7, 29.9, 29.5, 26.2, 25.1, 23.3, 14.2, 12.1, 11.9. 19F NMR (600 MHz, CDCl3, δ): −160.1 (s, 2F). Anal. Calcd. for C106H124N4S8: C, 70.24; H, 6.90; N, 3.09; S, 14.15. Found: C, 70.13; H, 6.99; N, 3.05; S, 14.09. MS (MALDI-TOF) m/z: [M]+, 1810.74; Found, 1810.50. Tm: 283 oC.

Synthesis of BDTT-(DPP)2-CF3: This compound was also prepared by a method similar to BDTT-(DPP)2-B, using BDTT (0.5 g, 0.55 mmol), 3-(5-bromothiophen-2-yl)-2,5-bis(2ethylhexyl)-6-(5-(4-(trifluoromethyl)phenyl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (DPP-CF3) (0.95 g, 1.27mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol). The product was obtained as a dark green solid and characterized using HPLC. (yield = 0.80 g, 88%, purity > 99.9%). 1H NMR (300 MHz, CDCl3) δ: 9.02 (d, 2H), 8.91 (d, 2H), 7.55 (s, 2H), 7.49-7.43 (m, 8H), 7.34 (d, 2H), 7.12 (d, 2H), 7.02 (d, 2H), 3.90 (m, 8H), 3.00 (d, 4H), 1.85-1.82 (m, 6H), 1.58-1.22 (m, 48H), 1.09-0.89 (m, 36H). 13C NMR (75 MHz, CDCl3) δ: 163.1, 162.9, 160.3, 152.4, 150.3, 148.2, 145.1, 144.4, 142.1, 140.3, 139.2, 138.4, 137.2, 136.1, 136.2, 132.6, 132.1, 131.2, 130.2, 128.3, 127.2, 123,5, 122.6, 122.6, 122.1, 110.1, 109.2, 49.2, 42.4, 38.1, 36.3, 33.1, 32.8, 31.5, 30.6, 29.7, 29.1, 26.5, 25.3, 23.2, 13.6, 12.2, 12.0. 19F NMR (600 MHz, CDCl3, δ): −62.3. (s, 6F). Anal. Calcd. for C108H124N4S8: C, 67.82; H, 6.53; N, 2.93; S, 13.41. Found: C, 67.50; H, 6.49; N, 2.90; S, 13.38. MS (MALDI-TOF) m/z: [M]+, 1910.73; Found, 1910.47. Tm: 287 oC.


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■ ACKNOWLEDGMENT This work was supported by a grant (NRF-2014M3A6A5060936) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, and National Research Foundation (NRF) grants, funded by the Korean government (NRF-2014R1A2A2A01007318 and No. 2011-0030013 through GCRC SOP). ■ASSOCIATED CONTENT Supporting Information The experimental details of synthesis of monomers, 1H,

19

F-NMR spectra, TGA, CV

measurements of SMs, photovoltaic properties, XRD 1D-plot, and the additional data. This material is available free of charge via the Internet at http://pubs.acs.org ■ AUTHOR INFORMATION Corresponding Author *Prof. D.-H. Hwang ([email protected])



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Table 1. Optical and electrochemical properties of the synthesized SMs.

SMs

λmax, abs

λmax

λedge

(nm)

(nm)

(nm)

Solution[a]

Film[b]

Film[b]

Egopt

Expt.

Calc.

(eV)[c]

(eV)[d]

HOMO

LUMO

(eV)

(eV)[g]

-5.15[e] BDTT-(DPP)2-B

652

642, 701

788

1.57

1.80

-5.29[f]

-2.05

[g]

-5.37

-5.15[e] BDTT-(DPP)2-F

651

654, 710

795

1.56

1.80

-5.30[f]

-2.10

[g]

-5.41

-5.18[e] BDTT-(DPP)2-CF3

653

655, 715

800

1.55

1.78

-5.52[f]

-2.23

-5.52[g] a)

Solution UV-visible spectra measured in chloroform solutions at room temperature;

chloroformsolution onto glass slides;

c)

b)

Thin film (ca. 100nm) spin-coated from

Calculated from the absorption band edge of the thin films, Egopt = 1240/λedge; d)Results from TD-

DFT calculation using B3LYP/6-31G(d)

e)

HOMO = -e(Eoxonset + 4.70) (eV); f)Determined by UPS for each spin-coated thin film;

HOMO[LUMO] corresponds to adiabatic –IPs[EAs] calculated at B3LYP/6-31G(d): IP = E(cation)-E(neutral) and EA = E(anion)-E(neutral).

25

ACS Paragon Plus Environment

g)


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Table 2. Photovoltaic properties of the OPVs containing the SMs and PC71BM (1:1 w/w) as the donors and acceptors, respectively, under AM 1.5G (100 mW cm-2) illumination.

SMs

BDTT-(DPP)2-B

BDTT-(DPP)2-F

BDTT-(DPP)2-CF3 [a]

Ratio

1-CN

[w/w]

[2.5 vol%]

1:1

[a]

[a]

VOC [V]

JSC [mA/cm2]

No

0.77±0.01

3.88±0.5

1:1

Yes

0.70±0.02

1:1

No

0.77±0.01

1:1

Yes

0.69±0.01

1:1

No

0.77±0.01

1:1

Yes

0.69±0.01

10.21±0.2 [9.94]

FF[a]

[a]

PCE [%]

[%]

[b]

3.82±0.3 12.48±0.4 [11.97]

[b]

4.47±0.3 13.60±0.2 [12.66]

[b]

43±0.3

1.09±0.18(1.27)

51±0.2

3.30±0.35(3.65)

53±0.2

1.31±0.26(1.57)

58±0.4

4.64±0.32(4.96)

49±0.3

1.42±0.27(1.69)

64±0.3

5.87±0.13(6.00)

The device architecture is ITO/PEDOT:PSS/photoactive layer/Ca/Al configuration. Only the optimized recipes were considered for the

estimation of the average PCE; data have been averaged 10 devices; the performance of the best device is given in parentheses. [b]

Calculated from the external quantum efficiency (EQE) spectra.

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Table 3. Calculated hole mobilities of the SMs and SMs/PC71BM (1:1 w/w) system under optimized conditions using the SCLC method Active layer

Solvent

Donor: PC71BM

Thickness

Neat State

Blend State

Blend State

(nm)

µh[a]

µh[a]

µe[b]

µh/µe

[1:1 wt ratio] BDTT-(DPP)2-B

CF/1-CN

100

4.21×10−4

1.55×10−5

2.23×10-6

6.9

BDTT-(DPP)2-F

CF/1-CN

100

1.40×10−3

3.11×10−5

7.76×10-6

4.0

BDTT-(DPP)2-CF3

CF/1-CN

100

2.07×10−3

4.57×10−4

2.37×10-4

1.9

[a]

The hole-only devices with structure of ITO/PEDOT:PSS (30 nm)/pure SMs and

SMs:PC71BM (1:1 w/w)/Au.

[b]

The electron-only devices with structure of ITO/ZnO (30

nm)/SMs:PC71BM (1:1 w/w)/LiF/Al.



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Table 4. Characteristics of the BDTT-(DPP)2-CF3-based inverted tandem OPVs (inverted single and inverted tandem) under AM 1.5G (100 mW cm-2) illumination. Cell

Active Layer

Inverted

Weight ratio

Voc

Jsc

FF

PCE

[w/w]

[V]

[mA/cm2]

[%]

[%]

[a]

Tandem

Bottoma)

P3HT:IC60BA

1:1

0.83

9.21

67

5.11

Cells

Topb)

BDTT-(DPP)2-CF3:PC71BM

1:1

0.70

13.56

64

6.12

Tandemc)

-

-

1.53±0.01

8.00±0.56

68±0.1

8.11±0.19 (8.30)

a)

ITO/PEIE/P3HT:IC60BA/MoO3/Ag configuration. Only the optimized recipes were considered for the estimation of the average PCE; data

have been averaged 10 devices; the performance of the best device is given in parentheses. b)

ITO/PEIE/ BDTT-(DPP)2-CF3:PC71BM/MoO3/Ag configuration.

c)

ITO/PEIE/P3HT:IC60BA/PEDOT:PSS/PEIE/BDTT-(DPP)2-CF3:PC71BM/MoO3/Ag configuration.

28


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Figure 1. Chemical structures of BDTT-(DPP)2-X small molecules.

Scheme 1. Synthetic route for BDTT-(DPP)2-X small molecules.



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Figure 2. UV-visible absorption spectra of BDTT-(DPP)2-B (black), BDTT-(DPP)2-F (red), and BDTT-(DPP)2-CF3 (blue) in (a) in chloroform solution and (b) as-spun solid thin films.


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Figure 3. (a) Cyclic voltammograms of BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3. UPS spectra of (b) the onset and (c) the secondary edge region of BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3. (d) Energy level diagram of the synthesized SMs. a) Energy-minimized structure (B3LYP/6-31G(d)) of the HOMO and LUMO of the dimer model compounds at the bottom and top of the image, respectively, b) HOMO energy levels measured by CV, UPS, DFT, and c) LUMO corresponds to adiabatic –IPs[EAs] calculated at B3LYP/631G(d): EA = E(anion)-E(neutral).

31



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Figure 4. J–V curves of the OPVs based on SMs:PC71BM (1:1, w/w) prepared (a) without and (b) with 1-CN, measured under AM 1.5G, 100 mW cm-2 illumination. EQE curves of the OPVs based on SMs:PC71BM (1:1, w/w) prepared (c) without and (d) with 1-CN.


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Figure 5. 2D-GIXD patterns of (a)–(c) pure BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT(DPP)2-CF3 films and (d)–(f) BDTT-(DPP)2-B, BDTT-(DPP)2-F, and BDTT-(DPP)2-CF3 films blended with PC71BM (1:1 w/w).



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Figure 6. J–V curve of the hole only (a) pure SMs and (b) blend state, and electron only (c) blend state devices of pure SMs and SM:PC71BM blend films calculated from the hole/electron-only devices by fitting the J–V curves in the SCLC regime.


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Figure 7. TEM images of SMs:PC71BM blends (1:1 w/w) cast from chloroform: (a) BDTT(DPP)2-B:PC71BM (1:1) without 1-CN, (b) BDTT-(DPP)2-F:PC71BM (1:1) without 1-CN, (c) BDTT-(DPP)2-CF3:PC71BM (1:1) without 1-CN, (d) BDTT-(DPP)2-B:PC71BM (1:1) with 1CN, (e) BDTT-(DPP)2-F:PC71BM (1:1) with 1-CN and (f) BDTT-(DPP)2-CF3:PC71BM (1:1) with 1-CN. (Inset) AFM tapping-mode height images of the blend films.



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Figure 8. Inverted tandem photovoltaic device. (a) Device structure of the inverted tandem photovoltaic device, (b) UV-visible spectra of the P3HT of bottom cell, the BDTT-(DPP)2CF3 of top cell, and the solar spectrum, (c) chemical structures of the bottom cell materials (P3HT and IC60BA) and top cell materials (BDTT-(DPP)2-CF3 and PC71BM).


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Figure 9. (a) J−V characteristics of a single junction bottom cell, single junction top cell, and inverted tandem cell under AM 1.5G illumination (100 mW cm-2), (b) EQE of the P3HT:IC60BA- based bottom cell and BDTT-(DPP)2-CF3:PC71BM-based top cell in a typical tandem device.



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Table of Contents


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Controlling the Morphology of BDTT-DPP-Based Small Molecules via

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