Application of Small Molecule Donor Materials Based on

Mar 27, 2014 - State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Centre on. Molecul...
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Application of Small Molecule Donor Materials Based on Phenothiazine Core Unit in Bulk Heterojunction Solar Cells Qin Tan,† Xichuan Yang,*,† Ming Cheng,† Haoxin Wang,† Xiuna Wang,† and Licheng Sun*,†,‡ †

State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Centre on Molecular Devices, Dalian University of Technology (DUT), 2 Linggong Road, 116024 Dalian, China ‡ School of Chemical Science and Engineering, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 30, 10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: A D-π-A type small molecule PTZ1 and an A-π-D-π-A type small molecule PTZ2 with phenothiazine as the central building block and dicyanovinyl as the electronwithdrawing end-group have been designed and synthesized. Compared with D-π-A type donor material PTZ1, the donor material PTZ2 with A-π-D-π-A configuration shows much wider response to solar light. The donor material PTZ1 possesses more positive highest occupied molecular orbital level, and higher Voc was obtained for devices with PTZ1/PC71BM blend as the active layer. An improved efficiency of 3.25% was obtained for the PTZ2/PC71BM based solar cells.

1. INTRODUCTION In the circumstance of lack of energy, as a new generation of solar cell, organic solar cells (OSCs) are attracting more and more attention worldwide due to their potential to be costeffective and flexible solar energy conversion devices.1−4 In the past few years, many research efforts have been focused on polymer solar cells and impressive results were achieved.5−10 To date, power conversion efficiencies (PCE) approaching 10% have been obtained for both solution-processed single-junction polymer solar cells and the tandem devices.2 Compared with polymer solar cells, though the efficiency is a little lower, smallmolecule OSCs are still of great interest to the research community owing to the distinct advantages of small-molecule semiconductors, such as well-defined structures, easier synthesis, and batch-to-batch stability.3 With consistent effort, the PCE of small-molecule OSCs had been improved from 0.03% to 9.02% in the past few years.11−24 However, there is still much work needed to be done to further improve the performance of small-molecule OSCs, including designing and exploiting better matched materials and device structure engineering. During the processing of small-molecule OSCs, the active layer is generally made into a bulk heterojunction (BHJ). Two components, a p-type small molecule as a donor and another n-type material such as the soluble fullerene derivative as an acceptor, are cast synchronously from solution and upon drying, and then form a BHJ composite film. The interpenetrating networks of donor and acceptor phases make photogenerated excitons well separated on the donor−acceptor interface. Studies prove that the structure and properties of the BHJ film have great influence on the device performance.25 So © XXXX American Chemical Society

the active materials, especially the donor materials are the key factor for high PCE of small-molecule BHJ OSCs. In this context, we focused our attention on the design and synthesis of new small molecule donor materials. From our previous studies of dye-sensitized solar cells (DSSCs), we learned that organic dyes with a donor-π bridge-acceptor (D-π-A) molecular configuration possess effectively intramolecular charge transfer once excited.26,27 Phenothiazine is a heterocyclic compound with electron-rich sulfur and nitrogen atoms, and the phenothiazine ring has a nonplanar geometry. While, to our best knowledge, there were only some reports on the application of dyes containing a phenothiazine moiety in DSSCs.28−38 Herein, we recently reported one D-π-A-type small molecule donor material PTZ1, and one A-π-D-π-A-type small molecule donor material PTZ2, in which the phenothiazine moiety was employed as block unit. The detailed structures of these two small molecules are shown in Figure 1. The detailed synthetic routes and characterization of donor materials PTZ1 and PTZ2 are shown in Supporting Information.

2. RESULTS AND DISCUSSION 2.1. Optical and Electronic Properties. The UV−vis absorption spectra of these two small molecule donor materials Special Issue: Michael Grätzel Festschrift Received: January 13, 2014 Revised: March 21, 2014

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Figure 1. The structures of small molecule PTZ1 and PTZ2.

Figure 2. Absorption spectra of PTZ1and PTZ2 (a) in chloroform solution and (b) thin films fabricated by spin coating.

in chloroform are depicted in Figure 2. Both of these molecules showed an intense absorption band in the visible light region. PTZ1 exhibits two well-defined peaks at 381 and 496 nm, with molar extinction coefficients (ε) of 16400 M−1·cm−2 and 25700 M−1·cm−2, respectively. Compared to PTZ1, a significant bathochromic shift (25 nm) together with an increase in ε of the absorption band was found for PTZ2 mainly due to the increased conjugation length of the molecular frame. The absorption bands of these two molecules when the material was cast as a thin film were broadened and red-shifted in comparison to those observed for the material in solution. This is likely due to the intermolecular π−π stacking of the molecules in the solid state. The optical band gaps (E0−0opt) in the PTZ1 and PTZ2 films were 1.98 and 1.82 eV, respectively. Energy levels of donor materials, such as the highest occupied orbital (HOMO) level and the lowest unoccupied orbital (LUMO) level are crucial to exciton separation in OSCs operations. Cyclic voltammetry (CV) was employed on PTZ1 and PTZ2 in chloroform to roughly estimate their energy levels (Figure 3). The test results are listed in Table 1. The HOMO and LUMO energies determined from the first oxidation and reduction potentials are −5.54 and −3.56 eV versus vacuum level for donor material PTZ1. While for symmetric structured donor material PTZ2, the HOMO and LUMO levels are −5.51 and −3.69 eV, respectively. The HOMO levels of these two molecules are very close, so a similar open-circuit voltage (Voc) can be obtained when applied to OSCs. The LUMO energy levels of PTZ1 and PTZ2 lie above the LUMO of PC71BM, thus ensuring a sufficient driving force for exciton separation (see Figure 4). From CV test results, we can see that PTZ1 and PTZ2 show energy gaps (E0−0CV) of 2.09 and 1.81 eV, respectively, which are in good accordance with E0−0opt. To give insight into the electronic structure of PTZ1 and PTZ2, we optimized the configuration of these two compounds

Figure 3. Cyclic voltammogram of PTZ1 and PTZ2 in chloroform solution.

by using Gaussian 09 software with density functional theory (DFT) at the B3LYP/6-31G level. The calculated results are shown in Figure 5. Both of these compounds have a good plane conjugation system. Their HOMO orbitals mainly localize on the phenothiazine ring while the LUMO orbitals localize on the dicyanovinyl. The orbital data match well with experiment data got by CV. 2.2. Photovoltaic Performance of BHJ Solar Cells. The devices with an ITO/ZnO (30 nm)/active layer (100 nm)/ MoO3 (5 nm)/Ag (50 nm) device configuration were fabricated. The detailed fabrication process is shown in the Supporting Information. In these devices, the MoO3 thin film acts as the hole-transporting layer and the optimal thicknesses of the active layers are dependent on the optical field distribution as well as exciton diffusion lengths and carrier recombination rates in the thin films. The optimized thicknesses of the active layers in PTZ1- and PTZ2-based devices were found to be around 100 nm. The devices B

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Table 1. Optical and Electrochemical Data of PTZ1 and PTZ2 donor materials

λmaxsolution (nm)

ε at λmax (M−1·cm−1)

λmax film (nm)

E0−0opt (eV)

E0−0CV (eV)

HOMO (eV)

LUMO (eV)

PTZ1 PTZ2

496 521

25 700 36 900

513 537

1.98 1.82

2.09 1.81

−5.54 −5.51

−3.56 −3.69

Figure 4. (a) Device configuration, (b) proposed energy levels diagram of the PTZ1- or PTZ2-based BHJ OSCs.

Figure 6. J−V curves of solar cells with an active layer composed of PTZ1/PC71BM (1:1, w/w) and PTZ2/PC71BM (1:1, w/w).

Table 2. Device Performance Parameters for BHJ OSCs Based on PTZ1 and PTZ2

Figure 5. HOMO and LUMO orbitals after optimization with Gaussian 09.

fabricated with a donor material/PC71BM weight ratio of 1:1 in the active layer gave the best results. Device performance measurements were carried out under AM 1.5G (100 mW· cm−2) simulated solar illumination. The current−voltage (J−V) curves are shown in Figure 6. The corresponding parameters are collected in Table 2. A PCE of 1.59% was achieved for the PTZ1/PC71BM (1:1) based device, with a Voc of 0.993 V, a short-circuit current (Jsc) of 5.32 mA·cm−2 and a fill factor (FF) of 30.1%. Under the same conditions by using the PTZ2 as donor material instead, much higher PCE (3.25%) was obtained. The improvement of PCE can be mainly attributed to the doubled Jsc (10.3 mA· cm−2), considering the similar FF (32.5%) and a little lower Voc (0.968 V). The Voc values obtained in these devices are consistent with the magnitudes of the HOMO levels of the donor materials. On the other hand, the Jsc values generally followed the trend in donor band gap. To address the origin of the enhanced device performances, we have measured the external quantum efficiency (EQE) spectra for the PTZ1- or PTZ2-based BHJ devices (Figure 7). The EQE spectra confirmed the higher photocurrent

donor material (P)

P/N(PC71BM)

Jsc (mA· cm−2)

Voc (V)

FF (%)

η (%)

PTZ1 PTZ2

1:1 1:1

5.32 10.3

0.993 0.968

30.1 32.5

1.59 3.25

Figure 7. EQE spectra and the integrated current of solar cells with an active layer composed of PTZ1/PC71BM (1:1, w/w) and PTZ2/ PC71BM (1:1, w/w).

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OSCs can be achieved through rational molecule design, and the A-π-D-π-A configuration seems more promising.

generation effciencies for the device made with PTZ2 relative to PTZ1. When PTZ1 was used as donor material in active blend, the maximum EQE value was just about 38.5%, while, for the PTZ2 based device, a pronounced enhancement in photoresponse ranging from 400 to 700 nm was observed and the maximum EQE value was improved to 65.9%. The current densities obtained from EQE measurements reached about 4.93 mA·cm−2 and 9.62 mA·cm−2, which were approximately the same as those from the J−V curves. 2.3. Film Morphologies. The atomic force microscopy (AFM) image of the PTZ1/PC71BM blend film exhibits a coarse morphology, with a root-mean-square (RMS) roughness of 10.4 nm (Figure 8), whereas the PTZ2/PC71BM blend film

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ASSOCIATED CONTENT

S Supporting Information *

The detailed synthetic routes and characterization of donor materials PTZ1 and PTZ2 and the detailed fabrication process of devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 411 84986247. Fax: +86 411 84986250. *E-mail: [email protected]. Fax: +46-8-791-2333. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of this work from China Natural Science Foundation (Grant Nos. 21120102036, 51372028, 91233201, 21276044), the National Basic Research Program of China (Grant No. 2014CB239402), the Swedish Energy Agency, K&A Wallenberg Foundation, and the State Key Laboratory of Fine Chemicals (KF0805).

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Figure 8. Tapping-mode AFM images of active layer (a) height image of PTZ1/PC71BM (1:1, w/w), (b) phase image of PTZ1/PC71BM (1:1, w/w), (c) height image of PTZ2/PC71BM (1:1, w/w), (d) phase image of PTZ2/PC71BM (1:1, w/w). Scan size: 5 μm × 5 μm.

has a smooth surface (RMS = 7.5 nm). This coarse surface structure of the blend films is ascribed to the low entropy of mixing between PTZ1 and PC71BM during the spin-coating process. The blend film with a lower value of RMS may form a better interpenetrating network which benefits to exciton separation and leads to an increased Jsc for the corresponding solar cell.

3. CONCLUSION We designed and synthesized two novel PTZ-based small molecule donor materials and applied them into BHJ solar cells. Through detailed study, we found that the A-π-D-π-A-type small molecule PTZ2 has a narrower band gap and shows much wider response to the solar spectrum. Also, blend films with more smooth surface structure and a better interpenetrating network can be fabricated with a PTZ2/ PC71BM blend. A much higher PCE of 3.25% was achieved for PTZ2/PC71BM-based devices as applied to BHJ solar cells. The improvement of PCE can be mainly attributed to the double Jsc, considering the similar FF (32.5%) and a little decreased Voc. The PCE of the PTZ2-based device is not very high, but the results indicate that higher efficiency for SMD

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