Toward the Realization of A Practical Diketopyrrolopyrrole-Based Small Molecule for Improved...

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Toward the Realization of A Practical Diketopyrrolopyrrole-Based Small Molecule for Improved Effi ciency in Ternary BHJ Solar Cells

Junghoon Lee, Myoung Hee Yun, Jonggi Kim, Jin Young Kim, Changduk Yang*

An easily accessible DPP-based small molecule ( DMPA-DTDPP ) has been synthesized by a simple and effi cient route. The resulting molecule, when incorporated into a P3HT:PCBM-based BHJ solar cell, is found to signifi cantly improve the effi ciency. The utility of DMPA-DTDPP as an additive yields an increase in the short circuit current den-sity ( J sc ) because DMPA-DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion effi ciency, compared to the P3HT:PCBM control device. Considering the trouble-free and cost effective synthesis of DMPA-DTDPP , it may prove very useful in high-performance solar cells.

1. Introduction

Since the discovery of effi cient charge transfer through the bi-continuous pathways between a conjugated polymer and fullerene, [ 1 ] bulk-heterojunction (BHJ) devices containing regioregular poly(3-hexylthiophene) (rr-P3HT) as a donor and (6,6)-phenyl C 61 -butyric acid methyl ester (PCBM) as an acceptor have been widely investigated and represent the most effi cient polymeric solar cell (PSC) struc-ture, approaching power conversion effi ciencies (PCEs) of 4–5%. [ 2–5 ] One of the saliently critical features in BHJ solar cells based on P3HT:PCBM is their sensitivity to the pertur-bation in morphology induced by various factors such as processing additives, [ 6 ] solvent annealing, [ 3 , 7 ] and thermal annealing. [ 2 , 8 ] These choices affect the complex correla-tions between various parameters that govern phase

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J. Lee , M. H. Yun , J. Kim , Prof. J. Y. Kim , Prof. C. Yang Interdisciplinary School of Green Energy and KIER-UNIST Advanced Center for Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea. E-mail: [email protected]

separation. [ 9–11 ] Recently, we have demonstrated that the use of a diblock copolymer as an additive can play a critical role in tuning the phase segregation. [ 12 ] As another novel approach, Chu and co-workers [ 13 ] have applied a small molecule to form a ternary cascade structure in the BHJ solar cells, resulting in about a 15% increase in PCE, when compared to a pristine P3HT:PCBM device. However, tedious synthetic methodologies towards both the diblock copolymer and the TQTFA have restricted the realization for their usage in BHJ devices.

In this regard, we paid our attention to 3,6-dithien-2-yl-2,5-dialkylpyrrolo[3,4- c ]pyrrole-1,4-dione (DTDPP)-based pigments as additives within the BHJ active layer of which the fundamental strategies are based on the following: i) DTDPP derivatives show outstanding per-formance in both organic photovoltaic devices (OPVs) and organic fi eld effect transistors (OFETs). [ 14–21 ] Of par-ticular interest is their ambipolar transport with nearly balanced electron and hole mobilities, [ 15 , 16 , 21 ] which makes it possible not only to inject both holes and elec-trons effi ciently at the interfaces in BHJ binary blends but also to avoid the charge trapping in themselves. ii) The DTDPP motif is synthesized in a single step

library.com DOI: 10.1002/marc.201100643

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from commercially inexpensive compounds and easily modifi ed to tailor its optoelectronic properties. This is important since the barrier for preparation of mate-rials in terms of a cost effectiveness must be overcome to realize the commercial potential of solar cells. In this communication, we have designed and synthesized a DTDPP-cored small molecule comprising electron-rich dimethylphenlyamine (DMPA) moieties, namely 3,6-di(5-(4-(dimethylamino)phenyl)thien-2-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4- c ]pyrrole-1,4-dione (DMPA-DTDPP) to combine its pseudo three-dimensionality with the bulky side chains in the main backbone to benefi t from the amorphous structure and suppress aggregation in moleucles containing it. Furthermore, based on the donor (D)–acceptor (A) architecture, its energy levels can meet the energetic requirement of the cascaded energy levels of P3HT and PCBM. We demonstrate how the use of this molecule in P3HT:PCBM blends can lead to a sub-stantial improvement in effi ciency in ternary BHJ solar cells.

2. Experimental Section

2.1. General

All chemicals were purchased from Aldrich chemical company and used without further purifi cation. Anhydrous solvents were used for synthesis. Regioregular P3HT (55 kDa) was purchased from Rieke Metal Inc. and PCBM ( > 99.5%) was obtained from Nano-C. THF was purifi ed and freshly distilled over sodium and benzophenone. 1 H and 13 C NMR spectra were recorded on a Varian VNMRS 600 (600 MHz) using TMS as an internal standard. MALDI-TOF spectra were measured with a Bruker Ultrafl ex III and elemental analyses of carbon, hydrogen, sulfur, and nitrogen were carried out with a Flash 2000 elemental analyzer. Absorp-tion spectra were recorded on a Varian Cary 5000 spectropho-tometer at room temperature. The electrochemical data were obtained using a VersaSTAT 3 (Princeton Applied Research). The electrochemical measurements were investigated by cyclic vol-tammetry (CV) at room temperature at a scan rate 50 mV s − 1 under argon in o - dichlorobenzene (DCB) solution. The experi-ments were performed using tetra- n -butylammonium hexafl uor-ophosphate ( n -Bu 4 NPF 6 ) (0.1 M ) as the supporting electrolyte, a platinum working electrode, a platinum-wire auxiliary electrode, a Ag wire pseudo-reference electrode, and ferrocene (Fc)/Fc + as the external standard, respectively.

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Table 1. Photovoltaic parameters of P3HT:PCBM (1:0.8, w/w) solar ce

Composite DMPA-DTDPP a)

P3HT:PCBM (1:0.8 w/w) 0%

5%

10%

a ) The device performance was consistent and reproducible (see Expe

2.2. Device Fabrication and Characterization

The BHJ fi lms were prepared under optimized conditions according to the following procedure: An indium tin oxide (ITO)-coated glass substrate was fi rst cleaned with distilled water, ultrasonicated in distilled water, acetone, and isopropyl alcohol, and subsequently dried overnight in an oven. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) PEDOT:PSS in aqueous solution was spin-cast to form a fi lm with a thickness of approxi-mately 40 nm. The substrate was dried for 10 min at 140 ° C in air, and then transferred into a glove-box to spin-cast the photoac-tive layer. A solution containing a mixture of 2 wt% P3HT:PCBM (1:0.8, w/w) in DCB with varying weight concentration of addi-tives from 0 to 10% was then spin-cast on top of the PEDOT:PSS layer. Herein, the ratios of P3HT:PCBM:DMPA-DTDPP used were 1:0.8: 0, 1:0.8:0.05 and 1:0.8:0.1 respectively, which are expressed as 0, 5, and 10%. Three samples with different ratios of DMPA-DTDPP were prepared for measurement. The device was then pumped down to lower than 10 − 7 torr and an ∼ 100 nm thick Al electrode was deposited on top. Thermal annealing was carried out by directly placing the completed devices on a hot plate at 150 ° C in a glove box fi lled with nitrogen gas. After annealing, the devices were put on a metal plate and cooled to room tem-perature before the measurements were carried out. Measure-ments were carried out with the solar cells inside a dry box by using a high quality optical fi ber to guide the light from the solar simulator equipped with a Keithley 2635A source measurement unit. The solar cell devices were illuminated at an intensity of 100 mW cm − 1 . For a more accurate comparison of relative sur-faces, atomic force microsocpy (AFM) images were obtained of a 5 μ m × 5 μ m area (veecoAFM) in tapping mode. UV-vis (Varian Cary 5000) spectra were measured for fi lms of P3HT:PCBM with monomer and without composites. To probe the reproducibility and the trend of the devices with DMPA-DTDPP, all the data were confi rmed by the optimal 10 devices and presented in Table 1 .

2.3. Synthesis of 3,6-Di(5-(4-(dimethylamino)phenyl)thien-2-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4- c ]pyrrole-1,4-dione ( DMPA-DTDPP ):

N,N - Dimethyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenylamine (160 mg, 0.65mmol), 3,6-di(5-bromothien-2-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4- c ]pyrrole-1,4-dione (300 mg, 0.26mmol), [ 16 , 25 ] Aliquat336 (15 mg, 15 mol%), 2.0 M aqueous K 2 CO 3 (10 mL), and toluene (20 mL) were taken together in a 250 mL Schlenk fl ask and purged with argon for 10 min. To this solu-tion, tetrakis(triphenylphosphine)palladium (14 mg, 13.0 μ mol) was added and the reaction mixture was heated at 95 ° C under

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lls with various DMPA-DTDPP concentrations (0, 5, and 10 wt%) .

J sc [mA cm − 1 ]

V oc [V]

FF b) PCE [%]

8.27 0.60 0.61 3.02

9.84 0.60 0.58 3.37

9.17 0.60 0.57 3.13

rimental Section); b) Fill factor.

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Scheme 1 . Synthetic route of DMPA-DTDPP a .

vigorous stirring for 24 h. The compound was extracted in CHCl 3 , washed with brine, and dried over MgSO 4 . The solvent was evap-orated under reduced pressure. The crude product was purifi ed by chromatography on silica with 0–50% dichloromethane in hexane as eluent. Isolated yield = 0.15 g (52%) as a thick viscous purplish oil. 1 H NMR (600 MHz, CDCl 3 , δ ): 8.95 (m, 2H), 7.57 (dd, J = 2.09 and 8.79 Hz, 4H), 7.31 (m, 2H), 4.08 (d, J = 6.80 Hz, 4H), 3.03 (s, 12H), 2.02 (s, 2H), 1.34–1.21 (m, 80H), 0.88–0.86 (m, 12H). Anal. calcd for C 68 H 105 BrN 4 O 2 S 3 : C 68.82, H 8.92, N 4.72; found: C 68.61, H 9.09, N 4.61. MALDI-TOF MS m / z : 1211.79 (M + ).

3. Results and Discussion

The synthetic route to DMPA-DTDPP is outlined in Scheme 1 . Compounds 1 , 2 , 3 , and 4 were prepared in moderate to good isolated yields according to established methods. [ 22 , 23 ] A Suzuki coupling reaction of 3,6-di(5-bromothien-2-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4- c ]pyrrole-1,4-dione ( 3 ) and N,N -dimethyl-4-(4,4,5,5-tetrame-thyl-1,3,2-dioxaborolan-2-yl)-phenylamine ( 4 ) in the pres-ence of a catalytic amount of Pd(PPh 3 ) 4 , in toluene at 95 ° C for 24 h afforded DMPA-DTDPP (52%).

Figure 1 a illustrates the UV-vis absorption spectra of P3HT, PCBM, and DMPA-DTDPP in thin fi lms. DMPA-DTDPP dis-plays two wide-ranging bands at 300–485 and 525–735 nm. The optical HOMO–LUMO energy gap ( E g ) is 1.67 eV, esti-mated from the onset for the DMPA-DTDPP thin fi lm. The shorter wavelength absorbance probably stems from DMPA and thiophene units in the arms, while the lower energy bands with well-resolved vibronic structure are attributed to the charge transfer band from electron-rich DMPA and thiophene units to the electron-defi cient DPP core. From the absorption feature covering practically the whole visible region, we speculated that the DMPA-DTDPP molecule can effectively harvest the longer sunlight wave-lengths that P3HT and PCBM can not absorb.

CV was employed to investigate the redox behavior (Figure 1 b). The onset oxidation occurs at 0.7 eV versus Ag/Ag + for DMPA-DTDPP. According to the empirical equation

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( E HOMO/LUMO = [( E ox/red – E ferrocene ) + 4.8] eV), the HOMO energy level is located at –5.25 eV and the LUMO level cal-culated based on the optical bandgap is at –3.58 eV, which are positioned in an ideal range that lies between those of P3HT and PCBM. The energy diagrams of P3HT, PCBM, and DMPA-DTDPP are shown in Figure 1 c. Such cascaded energy levels of P3HT, PCBM, and DMPA-DTDPP prompted us to utilize DMPA-DTDPP as an additive for enhanced photocurrent density in a ternary blend system relative to that of the binary blend of P3HT:PCBM.

The optical absorption spectra for BHJ fi lms comprised of P3HT:PCBM (1:0.8, w/w) with addition of various DMPA-DTDPP concentrations (0, 5, and 10 wt%, respectively) are shown in Figure 1 d. To precisely access the changes of absorption profi les and intensities upon addition of DMPA-DTDPP, the almost similar thickness ( ∼ 100 nm) of the blend fi lms are monitored. There is a broad absorption of P3HT:PCBM at around 390–550 nm. As the amount of DMPA-DTDPP is increased in the blend, a detectable absorption band at around 675–700 nm, arising from DMPA-DTDPP absorption, is observed. Notably, the vibrational band at around 600 nm, which is indicative of the crystallization of P3HT, [ 24 ] is clearly observed even in the presence of DMPA-DTDPP, implying that the crystallization of P3HT is not disturbed by the addition of DMPA-DTDPP. Interestingly, by adding DMPA-DTDPP into the blend mixture, the π – π ∗ absorption band position no longer changes but only an increase in absorb-ance intensity is observed. However, with the addition of 10% DMPA-DTDPP, the intensity apparently decreases. This suggests that the P3HT:PCBM fi lm processed using 5% DMPA-DTDPP is optimal.

The BHJ solar cells were fabricated with a layer-by-layer structure of glass/ITO/PEDOT:PSS/P3HT:PCBM (1:0.8, w/w):DMPA-DTDPP/Al (100 nm). Current density–voltage ( J - V ) characteristics and the related parameters of P3HT:PCBM blends with different ratios of DMPA-DTDPP are shown under AM 1.5G illumination (100 mW cm − 2 ) in Figure 2 a and summarized in Table 1 .

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Figure 1 . Absorption spectra of P3HT, PCBM, and DMPA-DTDPP in fi lm (a). Cyclic voltammograms of DMPA-DTDPP on a Pt electrode in 0.1 M n -Bu 4 NPF 6 /DCB solution (b). Energy level diagrams for P3HT, PCBM, and DMPA-DTDPP (c). Absorption spectra of P3HT:PCBM (1:0.8, w/w) blend fi lms wtih various DMPA-DTDPP concentrations (0, 5, and 10 wt%) (d).

The P3HT:PCBM device as a standard reference has a short circuit current density ( J sc ) of 8.27 mA cm − 2 , an open circuit voltage ( V oc ) of 0.60 V, and a fi ll factor (FF) of 61%, yielding a PCE of 3.02%. The device made using DMPA-DTDPP exhibits enhanced J sc compared to the fi lm without DMPA-DTDPP additive. The best performance is observed in the P3HT/PCBM (1:0.8, w/w) cell containing 5% DMPA-DTDPP, which shows a substantially improved J sc of 9.84 mA cm − 2 , whereas the V oc of 0.60 V and the FF of 58% remain similar to those of that obtained for the device lacking DMPA-DTDPP. The corresponding PCE is 3.37%, approximately 12% higher relative to that of the pristine P3HT:PCBM. Such an increase in the J sc with DMPA-DTDPP inclusion is a result of increased absorp-tion breadth leading to a rise in the number of photogen-erated carriers.

To address the origin of the increase in J sc upon the addition of DMPA-DTDPP, we have measured the incident photon-to-current efficiency (IPCE) spectra

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of P3HT:PCBM and P3HT:PCBM:DMPA-DTDPP devices, respectively. In the IPCE of the ternary blend solar cells, as shown in Figure 2 b, the photocurrent peak is not only observed in the wavelength range (400–600 nm) of the P3HT absorption but also a negligible photocurrent signal centered at 700 nm of the DMPA-DTDPP absorp-tion is unveiled. This indicates that the photocurrent in this region is mainly a result of the generation of excitons from the absorption of photons by P3HT and their subsequent dissociation into free charge car-riers at P3HT:PCBM interfaces present in the blend. By adding DMPA-DTDPP, the IPCE corresponding to the P3HT absorption ( ∼ 60% at 450 –550 nm) is higher than that for the binary blend control device without DMPA-DTDPP ( ∼ 50%). This suggests that the DMPA-DTDPP molecules are not directly involved in the photoabsorp-tion but nonetheless promote charge generation from P3HT excitons indirectly. Therefore, one can conclude that the increase in J sc is mainly attributable to the

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Figure 2 . J – V characteristics (a) and incident photon-to-current effi ciency (IPCE) spectra (b) of P3HT:PCBM (1:0.8, w/w) blend fi lms wtih various DMPA-DTDPP concentrations (0, 5, and 10 wt%). Inset is a device confi guration of the tenary blend solar cell.

indirect contribution of DMPA-DTDPP to the improve-ment in the charge generation from P3HT excitons rather than the direct photoabsorption of DMPA-DTDPP chromophores.

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Figure 3 . AFM images of P3HT:PCBM (1:0.8, w/w) fi lms incorporating D

In order to further assess the nature of the improved device performance upon addition of DMPA-DTDPP, we evaluated the surface topography by AFM.

In Figure 3 , although all AFM images of the active layer fi lms show a smooth surface, the root mean square (rms) roughness differences are evaluated as 25.7, 4.79, and 7.38 nm for the P3HT:PCBM fi lm, P3HT:PCBM:5% DMPA-DTDPP fi lm, and the P3HT:PCBM:10% DMPA-DTDPP fi lm, respectively. Despite the fact that the fi lm with 10 wt% of DMPA-DTDPP shows a coarser morphology than the pris-tine P3HT:PCBM, it exhibits a slightly higher J sc . This can be explained by the fact that, because of the large amount of DMPA-DTDPP (10%), the enlarged interface between P3HT and PCBM leads to more opportunities for charge separation and transport, which increases the J sc . From these observations, one can conclude that the addition of DMPA-DTDPP into P3HT:PCBM allows tailoring of the mis-cibility between P3HT and PCBM leading to more homo-geneous fi lms with smoother surfaces than the pristine fi lm itself, as well as optimizing the device morphology. In the case of 5% addition of DMPA-DTDPP, the morpholog-ical requirement of the active layer for high performance OPVs is thus optimized nanometer-scale phase separa-tion, which could provide a large interface area for charge dissociation as well as a continuous percolating path for hole and electron transport to respective electrodes.

4. Conclusion

In summary, we have synthesized DMPA-DTDPP in one step from easily accessible compounds. Considering the cascaded energy levels of P3HT, PCBM, and DMPA-DTDPP, we have applied DMPA-DTDPP into P3HT:PCBM-based solar cells as an additive for improving the properties of the active layer. With various amounts of DMPA-DTDPP added, the enhanced PCE can be related to the allocation of DMPA-DTDPP molecules at P3HT:PCBM interfaces without the formation of DMPA-DTDPP aggregates, which contributes

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MPA-DTDPP at 0 (a), 5 (b), and 10 wt% (c).


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to photocurrent through the effective dissociation of the generated excitons in the P3HT phase at the absorp-tion region. The best device obtained by 5% DMPA-DTDPP shows an ∼ 12% increase in PCE to 3.37%, when compared to the controlled reference cell without the additive. On the basis of the simple, cost effective synthesis of DMPA-DTDPP, this protocol can be extended to various material combinations as well as directly to existing industrial proc-esses, hence boosting the photocurrent generation even for the best PSCs.

Acknowledgements : This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0002494) and the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (2010-0019408), (2010-0026916), (2010-0026913), and (NRF-2009-C1AAA001-2009-0093020).

Received: September 30, 2011 ; Revised: October 19, 2011 ; Published online: December 2, 2011; DOI: 10.1002/marc.201100643

Keywords: bulk-heterojunction solar cells; dimethylpheny-lamine; diketopyrrolopyrrole; donor–acceptor materials; ternary systems

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