Vacuum processable donor material based on dithieno[3,2-b:2′,3′-d]thiophene and pyrene for efficient organic solar cells

Abstract

We have developed a new donor material, 2,6-di(pyren-1-yl) dithieno[3,2-b:2′,3′-d]thiophene (DDT), consisting of dithieno[3,2-b:2′,3′-d]thiophene (DTT) and pyrene units. The bulk heterojunction (BHJ) device based on DDT : C₇₀ = 1 : 4 exhibited an efficient power conversion efficiency (PCE) of 3.60%, under simulated AM 1.5 solar irradiation at 100 mW cm⁻².

---

Organic solar cells (OSCs) are emerging as a clean and renewable energy resource due to their unique features including low-cost manufacturing, easy synthesis of active layer materials, possibility of flexible panels, easy device fabrication, and other potential applications.¹

Since the first report of OSCs by Tang,¹ researchers have focused on improving the power conversion efficiency (PCE) of the devices using new organic small molecules. In recent years, the PCE of OSCs has been steadily improved by the use of various organic small molecules.^2–4 Further, the PCEs of 8–9% have been achieved for polymer and small molecule based OSCs with the most promising bulk heterojunction (BHJ) architecture.⁵

Pyrene-based organic materials have been utilized as active materials of organic thin-film transistors (OTFTs), and blue light-emitting and electron-transporting materials in organic light-emitting diodes (OLEDs), and donor materials for OSCs.^6–10 Oligothiophene-based organic materials have been widely investigated for optoelectronic applications such as OSCs, OTFTs, and OLEDs, owing to their light-emitting and absorbing, hole-transporting, and charge carrying transport properties.¹¹ In particular, linearly fused oligothiophene-based compounds derived from dithieno[3,2-b:2′,3′-d]thiophene (DTT) have been utilized in a variety of organic optoelectronic applications, because the DDT unit has a relatively high hole mobility due to the fused thiophene core.^12–15 Therefore, the synthesis of DDT derivatives has received much attention.¹⁶ However, to the best of our knowledge, DTT-based small molecules have not been successfully utilized for high performance OSCs.¹⁷

In this paper, we report on the development of a thermally stable and efficient donor material, 2,6-di(pyren-1-yl)dithieno[3,2-b:2′,3′-d]thiophene (DDT), consisting of DTT and pyrene units, for OSCs. The device performance was optimized at DDT : C₇₀ = 1 : 4; the maximum open circuit voltage (V_oc), short circuit current (J_sc), and PCE value of the device were 0.98 V, 9.24 mA cm⁻², and 3.60%, respectively, under simulated AM 1.5 solar irradiation at 100 mW cm⁻².

Compound DDT was synthesized by the palladium catalyzed Suzuki cross coupling reaction between 2,6-dibromodithieno[3,2-b:2′,3′-d]thiophene¹⁸ and 1-pyreneboronic acid (Scheme 1). Compound DDT was identified by HR-mass and EA analyses.

Scheme 1 Synthesis of DDT.

Fig. 1a shows the absorption (UV) and emission (PL) spectra of DDT in CH₂Cl₂ and in solid film. The UV spectrum of DDT in CH₂Cl₂ shows a main absorption band with λ_max at 387 nm, which originates from the DTT and pyrene unit. The UV spectrum of DDT in solid film shows an absorption maximum band at 412 nm, which is 25 nm red-shifted when compared to the absorption in CH₂Cl₂. The PL spectrum of DDT shows maximum bands at 499 nm in CH₂Cl₂ and at 569 nm in film, which is 70 nm red-shifted when compared to the absorption in CH₂Cl₂. The UV and PL spectra of the solid film of DDT were both red-shifted relative to the CH₂Cl₂ solution spectra, presumably owing to the more planar structure of DDT in the film than in CH₂Cl₂. Fig. 1b and S1† show the PL spectra of DDT in neat film and the blended film of DDT : C₇₀ = 1 : 1, 1 : 4. The PL spectra of DDT in DDT : C₇₀ = 1 : 1, 1 : 4 blended film were significantly quenched by C₇₀. That means that an efficient intermolecular photoinduced charge transfer (PICT) process from the excited state of DDT to C₇₀ occurred. This PICT process of DDT is similar to the case of conducting conjugated polymer and fullerene blend system.^3g

Fig. 1 UV spectra (a) and PL spectra (a and b) of DDT at 400 nm excitation in 0.02 mM CH₂Cl₂ and in solid film.

The thermal stability of DDT was investigated by thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). DDT shows a high thermal stability of T_d = 432 °C, as revealed by the TGA thermogram (Fig. 2), suggesting that DDT is stable during the vacuum thermal sublimation process used in OSC fabrication. When the sample of DDT was heated, a melting peak was observed at around 358 °C (Fig. S2†).

Fig. 2 TGA data of DDT.

As shown in Fig. S3†, the cyclic voltammograms (CVs) of DDT shows irreversible oxidation process in 1,2-dichlorobenzene (ODCB). The oxidation potential energy of DDT is observed at E_onset = 1.00 V, calibrated against the ferrocene/ferrocenium (Fc/Fc⁺) redox couple. The highest occupied molecular orbital (HOMO) energy level of DDT (−5.66 eV) is calculated from E_onset after correction against the Fc/Fc⁺ redox couple. The lowest unoccupied molecular orbital (LUMO) energy level of DDT (−2.95 eV) is calculated from the CV and cross sectional wavelength between the UV and PL spectra (Table 1).¹⁹

Table 1 Optical, electrochemical and thermal properties of DDT

	UV (nm)		PL (nm)		HOMO (eV)	LUMO (eV)	Td (°C)	Tm (°C)
	sol	film	sol	film
DDT	387	412	499	569	5.66	2.95	432	358

---

DDT-deposited films exhibit no diffraction peaks present in the X-ray diffraction (XRD) patterns, indicating that the DDT molecule has poor crystalline morphology in the solid films (Fig. S4, ESI†).

To investigate the correlation between the molecular structure and the photophysical properties, we performed molecular orbital calculations using density functional theory (DFT). The ground state geometries of DDT were optimized in vacuum using the B3LYP/6-31 G* level in the Gaussian03 package. The optimized structures and the frontier molecular orbitals are shown in Fig. 3. The HOMO level of DDT is located on both the DTT and pyrene units. The LUMO level of DDT is mainly located on the pyrene unit.

Fig. 3 Electron density distribution of HOMO and LUMO orbital of DDT.

To investigate the photovoltaic properties of DDT, we fabricated planar heterojunction (PHJ) devices with a configuration of ITO/MoO₃ (5 nm)/DDT (10, 20, 30 nm)/C₇₀ (40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (8 nm)/Al (100 nm). Fig. S5, ESI† shows the current density (J) vs. voltage (V) characteristic curves of the PHJ solar cells. All the PHJ devices exhibit the poor photovoltaic behavior. The optimized 20 nm DDT-based device exhibited a J_sc of 3.15 mA cm⁻², a V_oc of 0.76 V, a fill factor (FF) of 0.22; thus a PCE of 0.52% was obtained. In contrast, the device with a 30 nm DDT based device had a reduced J_sc of 0.71 mA cm⁻², a V_oc of 0.88 V, and a FF of 0.17; thus, a reduced PCE of 0.11%. The low-lying HOMO energy levels of DDT results in a large energy difference relative to the LUMO of fullerenes, leading to the relatively large V_oc values. The V_oc value increases from 0.60 V to 0.88 V with increasing the thickness of DDT film from 10 nm to 30 nm.²⁰ Overall, the PCE of DDT-based PHJ devices were not high compared with other small-molecule-based OSCs due to the short absorption range and poor crystalline property of DDT (Fig. S4, ESI†). Fig. S5, ESI† shows the incident photon to current conversion efficiency (IPCE) spectra of PHJ solar cells. PHJ solar cells show IPCE values of 30% at 526 nm for 20 nm DDT based device and 25% at 538 nm for 30 nm DDT based device (Fig. S5, ESI†).

To improve the PCE, we fabricated bulk heterojunction devices with a configuration of ITO/MoO₃ (5 nm)/DDT : C₇₀ = 1 : 1, 1 : 4 (50 nm)/BCP (8 nm)/Al (100 nm). Fig. 4a shows the J–V characteristic curves of the BHJ solar cells. The DDT : C₇₀ = 1 : 1 based device exhibited a J_sc of 2.27 mA cm⁻², a V_oc of 0.91 V, and a PCE of 0.38%. On the other hand, the DDT : C₇₀ = 1 : 4 based device exhibited a significantly increased efficiency: a J_sc of 9.24 mA cm⁻² a V_oc of 0.98 V, and a PCE of 3.60% (Fig. 4a). The higher PCE of the DDT : C₇₀ = 1 : 4 based device can be explained by the increased dissociation of excitons (Fig. S1, ESI†) and increased carrier mobility at a low donor concentration.²¹

Fig. 4 (a) J–V characteristic curves and (b) IPCE spectra of ITO/MoO₃ (5 nm)/DDT : C₇₀ = 1 : 1,1 : 4 (50 nm)/BCP(8 nm)/Al (100 nm) device under simulated AM 1.5 solar irradiation at 100 mW cm⁻².

The dependence of the photocurrent (J_ph) on the light intensity (I), taken from J–V characteristic curves of the device with DDT : C₇₀ = 1 : 1 and DDT : C₇₀ = 1 : 4 (Fig. S6, ESI†), reveals which recombination process is dominant (Fig. 5). The light intensity-dependent photocurrent shows a power law behavior and α is taken from the slope in the log-scale. The devices with DDT : C₇₀ = 1 : 1 and DDT : C₇₀ = 1 : 4 show values of α = 0.54 and α = 0.82, respectively. The square-root dependence of the photocurrent on the light intensity in the device with DDT : C₇₀ = 1 : 1 suggests that the bimolecular recombination process is dominant.²² In other words, it can be explained that the path for the carrier transport is more or less prevented because of the mixing of the donor and the acceptor molecules. On the contrary, the device with DDT : C₇₀ = 1 : 4 shows the three-fourth power law dependence of the photocurrent on the light intensity, which indicates that the space-charge limited recombination process is dominant. This means that the path for the electron and the hole transport is relatively well built in the active layer.²² It is noteworthy that the PCE of 3.60%, to the best of our knowledge, is the highest value among DTT-based OSC devices due to the efficient exciton dissociation and space charge limited recombination process. Fig. 4b shows the IPCE spectra of BHJ solar cells utilizing DDT and C₇₀. The IPCE spectra of the BHJ devices based on DDT and C₇₀ show a broad response extending from 300 to 700 nm. BHJ devices based on DDT : C₇₀ show IPCE values with a maximum of 41% at 394 nm for DDT : C₇₀ = 1 : 1 based device and 60% at 512 nm for DDT : C₇₀ = 1 : 4 based device (Fig. 4b). The higher PCE and J_sc for the device based on DDT : C₇₀ = 1 : 4 compared to the device based on DDT : C₇₀ = 1 : 1 is reflected in higher IPCE values in the long wavelength region. The overall device performance data of DDT are summarized in Table 2.

Fig. 5 Incident light intensity dependence of the J_ph at the short-circuit condition taken from the J–V characteristic curves of the device with DDT : C₇₀ = 1 : 1 and DDT : C₇₀ = 1 : 4 are displayed in black square and red circle, respectively.

Table 2 OSC data based on DDT and C₇₀ under simulated AM 1.5 solar irradiation at 100 mW cm⁻²

Material	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)
DDT, 10 nm	0.60	1.02	0.16	0.10
DDT, 20 nm	0.76	3.15	0.22	0.52
DDT, 30 nm	0.88	0.71	0.17	0.11
DDT : C70 = 1 : 1	0.91	2.27	0.18	0.38
DDT : C70 = 1 : 4	0.98	9.24	0.40	3.60

---

In order to explain the device performance difference when using different ratios of DDT and C₇₀, we investigated the morphology of these DDT : C₇₀ blended films by atomic force microscopy (AFM).²³ The AFM images of the blended film of DDT : C₇₀ are shown in Fig. 6. The morphology of DDT film exhibits the formation of an aggregated large domain (Fig. S7, ESI†). The morphology of DDT : C₇₀ = 1 : 1 blended film exhibits the formation of an aggregated large domain (Fig. 6a). These aggregated domains of DDT and DDT : C₇₀ = 1 : 1 blended films might hinder the efficient exciton dissociation and carrier transport, resulting in low J_sc and PCE. On the other hand, the morphology of DDT : C₇₀ = 1 : 4 blended film exhibits the formation of a relatively small domain (Fig. 6b), which presumably leads to a higher J_sc and a high PCE of 3.60%.

Fig. 6 1 × 1 μm AFM images of DDT : C₇₀ = 1 : 1 (a) and DDT : C₇₀ = 1 : 4 (b) deposited thin films.

Conclusions

We have demonstrated thermally stable and efficient donor material DDT, consisting of DDT and pyrene units. The maximum V_oc, J_sc and PCE values of the BHJ device based on DDT : C₇₀ = 1 : 4 were 0.98 V, 9.24 mA cm⁻², and 3.60%, respectively, under simulated AM 1.5 solar irradiation at 100 mW cm⁻². A PCE of 3.60%, to the best of our knowledge, is the highest efficiency among OSC devices fabricated using DTT derivatives.

Acknowledgements

This study was supported by a grant from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (no. 2008-0061903), and by the New & Renewable Energy Technology Development Program of the KETEP grant (no. 20113020010070) funded by the Ministry of Knowledge Economy.

References

1. C. W. Tang, App. Phys. Lett., 1986, 48, 183.
2. A. Mishra and P. Bäuerle, Angew. Chem., Int. Ed., 2012, 51, 2020.
3. (a) B. C. Thompson and J. M. J. Fréchet, Angew. Chem., Int. Ed., 2008, 47, 58; (b) Y. Shirota, J. Mater. Chem., 2000, 10, 1; (c) Y. Shirota, J. Mater. Chem., 2005, 15, 75; (d) J. Roncali, Acc. Chem. Res., 2009, 42, 1719; (e) D. Demeter, T. Rousseau and J. Roncali, RSC Adv., 2013, 3, 704; (f) V. Jeux, D. Demeter, P. Leriche and J. Roncali, RSC Adv., 2013, 3, 5811; (g) N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science, 1992, 258, 1474.
4. (a) J. Kwon, W. Lee, J.-y. Kim, S. Noh, C. Lee and J.-I. Hong, New J. Chem., 2010, 34, 744; (b) J. Kwon, M. K. Kim, J.-P. Hong, W. Lee, S. Noh, C. Lee, S. Lee and J.-I. Hong, Org. Electron., 2010, 11, 1288; (c) J. Kwon, J.-P. Hong, W. Lee, S. Noh, C. Lee, S. Lee and J.-I. Hong, Org. Electron., 2010, 11, 1103; (d) J. Kwon, M. K. Kim, J.-P. Hong, W. Lee, S. Lee and J.-I. Hong, Bull. Korean Chem. Soc., 2013, 34, 1355.
5. (a) Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 593; (b) J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem. Soc., 2013, 135, 8484.
6. F. Moggia, C. Videlot-Ackermann, J. Ackermann, P. Raynal, H. Brisset and F. Fages, J. Mater. Chem., 2006, 16, 2380.
7. (a) K.-C. Wu, P.-J. Ku, C.-S. Lin, H.-T. Shih, F.-I. Wu, M.-J. Huang, J.-J. Lin, I.-C. Chen and C.-H. Cheng, Adv. Funct. Mater., 2008, 18, 67; (b) J. Kwon, J.-P. Hong, S. Lee and J.-I. Hong, New J. Chem., 2013, 37, 2881.
8. H.-Y. Oh, C. Lee and S. Lee, Org. Electron., 2009, 10, 163.
9. T. M. Figueira-Duarte and K. Müllen, Chem. Rev., 2011, 111, 7260.
10. (a) J. Kwon, J.-P. Hong, S. Noh, T.-M. Kim, J.-J. Kim, C. Lee, S. Lee and J.-I. Hong, New J. Chem., 2012, 36, 1813; (b) J.-W. Mun, I. Cho, D. Lee, W. S. Yoon, O. K. Kwon, C. Lee and S. Y. Park, Org. Electron., 2013, 14, 2341; (c) O. P. Lee, A. T. Yiu, P. M. Beaujuge, C. H. Woo, T. W. Holcombe, J. E. Millstone, J. D. Douglas, M. S. Chen and J. M. J. Fréchet, Adv. Funct. Mater., 2011, 23, 5359.
11. (a) J. E. Anthony, Chem. Rev., 2006, 106, 5028; (b) J. E. Anthony, Angew. Chem., Int. Ed., 2008, 47, 452.
12. X. C. Li, H. Sirringhaus, F. Garnier, A. B. Holmes, S. C. Moratti, N. Feeder, W. Clegg, S. J. Teat and R. H. Friend, J. Am. Chem. Soc., 1998, 120, 2206.
13. L. Wang, Q. Chen, G. B. Pan, L. J. Wan, S. M. Zhang, X. W. Zhan, B. H. Northrop and P. J. Stang, J. Am. Chem. Soc., 2008, 130, 13433.
14. L. Zhang, L. Tan, Z. H. Wang, W. P. Hu and D. B. Zhu, Chem. Mater., 2009, 21, 1993.
15. X. W. Zhan, Z. A. Tan, E. J. Zhou, Y. F. Li, R. Misra, A. Grant, B. Domercq, X. H. Zhang, Z. S. An, X. Zhang, S. Barlow, B. Kippelen and S. R. Marder, J. Mater. Chem., 2009, 19, 5794.
16. J. Frey, A. D. Bond and A. B. Holmes, Chem. Commun., 2002, 2424.
17. J. K. Lee, S. Lee and S. J. Yun, Bull. Korean Chem. Soc., 2013, 34, 2148.
18. T.-H. Kwon, V. Armel, A. Nattestad, D. R. MacFarlane, U. Bach, S. J. Lind, K. C. Gordon, W. Tang, D. J. Jones and A. B. Holmes, J. Org. Chem., 2011, 76, 4088.
19. (a) J. Kwon, T.-H. Kwon, H. S. Cho, M. K. Kim, I.-S. Shin, D.-Y. Shin, S. J. Park and J.-I. Hong, New J. Chem., 2008, 32, 1368; (b) M. K. Kim, J. Kwon, T.-H. Kwon and J.-I. Hong, New J. Chem., 2010, 34, 1317.
20. M. D. Perez, C. Borek, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2009, 131, 9281.
21. (a) M. Zhang, H. Wang, H. Tian, Y. Geng and C. W. Tang, Adv. Mater., 2011, 23, 4960; (b) Y.-q. Zheng, W. J. Potscavage, Jr, T. Komino, H. Hirade, J. Adachi and C. Adachi, Appl. Phys. Lett., 2013, 102, 143304.
22. (a) I. Riedel, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande and J. C. Hummelen, Adv. Funct. Mater., 2004, 14, 38; (b) L. J. A. Koster, V. D. Mihailetchi, H. Xie and P. W. M. Blom, Appl. Phys. Lett., 2005, 87, 203502; (c) V. D. Mihailetchi, J. Wildeman and P. W. M. Blom, Phys. Rev. Lett., 2005, 94, 126602.
23. J.-Y. Pan, L.-J. Zuo, X.-L. Hu, W.-F. Fu, M.-R. Chen, L. Fu, X. Gu, H.-Q. Shi, M.-M. Shi, H.-Y. Li and H.-Z. Chen, ACS Appl. Mater. Interfaces, 2013, 5, 972.

---

Footnotes

† Electronic supplementary information available: Detailed experimental procedures, photophysical data, thermal data, electrochemical data, XRD data, device data, and AFM data, additional supporting data. See DOI: 10.1039/c4ra02895c

‡ These authors contributed equally to this work.

---