All polymer solar cells with diketopyrrolopyrrole-polymers as electron donor and a naphthalenediimide-polymer as electron acceptor

Abstract

Four typical diketopyrrolopyrrole (DPP)-based conjugated polymers were used as electron donors in all-polymer solar cells (PSCs) with a naphthalenediimide-based polymer N2200 as the electron acceptor. The four DPP polymers have near-infrared absorption spectra up to 1000 nm and suitable energy levels for charge separation from donor to acceptor. DPP polymer : N2200 cells were found to have high open circuit voltages in comparison to fullerene-based solar cells but with low short circuit current densities and fill factors, so that the power conversion efficiencies of these cells were relatively low (0.45–1.7%). These blends relatively had balanced but low hole and electron mobilities from space charge limit current measurements, small surface roughness, and highly quenched photoluminescence (PL) from steady-state PL. These studies show that the low photocurrent and performance arise from the miscibility of the DPP and N2200 polymers, which enhances the charge recombination. The finding was further confirmed by grazing incidence X-ray diffraction and resonant soft X-ray scattering. All the PSCs based on DPP polymers were investigated, opening further studies based on these systems due to the broad absorption, high carrier mobilities and good crystalline properties of DPP polymers.

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Introduction

All-polymer solar cells (PSCs) that use conjugated polymers both as electron donors and electron acceptors have attracted increasing attention in recent years.^1,2 A number of semiconducting polymers have been successfully developed as electron acceptors, showing distinct absorption spectra, energy levels and charge carrier mobilities.^3–9 Hence, the power conversion efficiencies (PCEs) of all-PSCs have approached 8%,^9,10 close to the PCEs of 10% for conventional solar cells based on fullerene derivatives as electron acceptors.^11–13

Conjugated polymers as electron donors are central to all-PSCs. Initially, these polymer donors were developed for high performance fullerene-based solar cells, in which the so-called “push–pull” polymers were the most successful donors.¹⁴ Conjugated polymers containing different electron-deficient units, such as benzothiadiazole (BT)^15,16 thienopyrroledione (TPD),^17,18 thieno[3,4-b]thiophene (TT)^19,20 and diketopyrrolopyrrole (DPP),²¹ have shown PCEs >9% as electron donors with fullerene derivatives as the electron acceptor. Conjugated polymers consisting of BT, TPD and TT units show wide or medium band gaps with absorption onsets below 800 nm, which were also reported as electron donor in all-PSCs with PCE >6%.^7,9,22 However, small band gap conjugated polymers with DPP units have been less studied for all-PSCs with PCEs below 3%.^6,23

The DPP unit is a strong electron-withdrawer, which enhances near-infrared absorption up to 1000 nm.²⁴ In addition, DPP polymers have high carrier mobility,^25,26 good crystalline properties²⁷ and are relatively easy to synthesize.²⁸ These attributes have enabled DPP-based polymers to show high performance in fullerene-based solar cells and field-effect transistors. Therefore, these polymers are of interest for PSCs for potential high performance with a broad photo-response.

In this work, four typical DPP polymers (Fig. 1a) are selected as electron donors with a typical naphthalenediimide-based electron acceptor, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (Polyera ActivInk N2200, Fig. 1b),²⁹ to construct all-polymer PSCs. These DPP polymers absorbed in the near infrared and had varied frontier energy levels, and had PCEs of 0.45–1.7% in all-polymer PSCs. The low PCEs were further analyzed by space charge limited current (SCLC), atomic force microscopy (AFM), steady-state photoluminescence (PL), grazing incidence X-ray diffraction (GIXD) and resonant soft X-ray scattering (RSoXS) measurement, indicating that DPP polymers and N2200 are highly miscible which increases charge recombination. Our findings indicate that, by enhancing the phase separation between DPP polymers and N2200, provides a route to improve the device performance.

Fig. 1 Polymer structures of (a) the electron donor polymers PDPPTPT, PDPP5T, PDPP2TBDT and PDPP2TDTP; (b) electron acceptor polymers N2200. (c) Energy level of the polymers in this work. The values of HOMO and LUMO levels were also included.

Experimental

Materials and measurements

The DPP polymers, PDPPTPT,³⁰ PDPP5T,³¹ PDPP2TBDT³² and PDPP2TDTP²⁴ were synthesized according to the literature procedures. The polymer N2200 was obtained from Polyera Corporation and used as received. Optical spectra were recorded on a JASCO V-570 spectrometer. Cyclic voltammetry was conducted with a scan rate of 0.1 V s⁻¹ under an inert atmosphere with 1 M tetrabutylammonium hexafluorophosphate in o-DCB as the electrolyte. The working electrode was a platinum disk, the counter electrode was a silver electrode, and an Ag/AgCl quasi-reference electrode was used. The concentration of the sample in the electrolyte was approximately 1 mM, based on monomers. Fc/Fc⁺ was used as an internal standard. AFM images were recorded using a Digital Instruments Nanoscope IIIa multimode atomic force microscope in tapping mode. Steady state fluorescence spectra were recorded at room temperature using an Edinburgh Instruments FLS980 double-monochromator luminescence spectrometer equipped with a nitrogen-cooled near-IR sensitive photomultiplier (Hamamatsu).

Grazing incidence X-ray diffraction (GIXD) was performed at beamline 7.3.3, Advanced Light Source (ALS), Lawrence Berkeley National Lab (LBNL). The sample was put inside a helium chamber, and Pilatus 2M detector was used to collect the signal. GIXD results were analyzed using Nika software package and peak information was accessed by a Gaussian fitting. RSoXS was performed at beamline 11.0.1.2 ALS, LBNL. Thin films were floated and transferred onto Si₃N₄ substrate and experiments were done in the transition mode.

Photovoltaic devices with an inverted configuration were made by spin coating a ZnO sol–gel at 4000 rpm for 60 s onto pre-cleaned, patterned ITO substrates. The photoactive layer was deposited by spin coating a chloroform solution of the DPP polymer and N2200, and processing additive such as diiodooctane (DIO), 1-chloronaphthalene (1-CN), or o-DCB in air. MoO₃ (10 nm) and Ag (100 nm) were deposited by vacuum evaporation at ca. 4 × 10⁻⁵ Pa as the back electrode.

The active areas of the cells were 0.04 cm². The J–V characteristics were measured by a Keithley 2400 source meter unit under AM1.5G spectrum from a solar simulator (Enlitech model SS-F5-3A). Solar simulator illumination intensity was determined at 100 mW cm⁻² using a monocrystalline silicon reference cell with a KG5 filter. Short circuit currents (J_sc) under AM1.5G conditions were estimated from the spectral response and convolution with the solar spectrum. The external quantum efficiency was measured by a Solar Cell Spectral Response Measurement System QE-R3011 (Enli Technology Co., Ltd.). The thickness of the active layers in the photovoltaic devices were measured with a Veeco Dektak XT profilometer.

Results and discussion

Optical and electrochemical properties

The DPP polymers in this work consist of a DPP core, thienyl bridges and electron-rich aromatic units (Fig. 1a). By introducing electron-donating conjugated segments from benzene to terthiophene, benzodithiophene with alkylthiophene as side groups and dithienopyrrole, the polymers PDPPTPT, PDPP5T, PDPP2TBDT and PDPP2TDTP had optical band gaps (E_g) from 1.53 eV to 1.23 eV (Fig. 2a and Table 1). The four polymers have exhibited high short circuit current densities (J_sc), up to 20 mA cm⁻², and PCEs up to 7% in fullerene-based solar cells.^24,30 In addition, these polymers also have high molecular weights of ∼100 kg mol⁻¹.

Fig. 2 Optical absorption spectra of (a) the pure DPP polymers and N2200 in solid state films and (b) blended thin films of DPP polymer : N2200 (1 : 1) spin coated from CHCl₃ solution.

Table 1 Optical and energy level of the DPP polymers and N2200

Polymer	E^filmg (eV)	EHOMO^a (eV)	ELUMO (eV)	ΔELUMO^b (eV)
a Determined as ELUMO − E^filmg.b ΔELUMO = q(ELUMO(polymer) − ELUMO(N2200)).c Ref. 33.d Ref. 6.e Ref. 24.
PDPPTPT	1.53	−5.19	−3.66^c	0.36
PDPP5T	1.45	−5.08	−3.63^d	0.39
PDPP2TBDT	1.44	−5.28	−3.84	0.18
PDPP2TDTP	1.23	−4.91	−3.68^e	0.34
N2200	1.49	−5.51	−4.02	—

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The absorption spectra of pure polymers and blends are shown in Fig. 2. The polymer N2200 has an optical band gap of 1.49 eV that is slightly lower than that of PDPPTPT, but higher than that of other polymers. When DPP polymers are blended with N2200 at a ratio of 1 : 1, the absorption spectra is similar to that of the pure polymers. For PDPP2TDTP : N2200 an absorption shoulder at ∼700 nm is observed, but the intensity is weak compared to the absorption peak at ∼900 nm of PDPP2TDTP. These results indicate that DPP polymers have a high absorption coefficient in thin films in comparison to N2200.

The energy levels of DPP polymers and N2200 determined from cyclic voltammetry are summarized in Fig. 1c and Table 1. N2200 has the lowest unoccupied molecular orbital (LUMO) level of −4.02 eV, which is slightly higher than that of fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with a LUMO level of −4.16 eV under the same measurement conditions.⁶ The high-lying LUMO level of N2200 will help improve the open circuit voltage (V_oc) in solar cells. Meanwhile, DPP polymers show deep LUMO levels of −3.68 eV to −3.84 eV, which makes the LUMO offset between the DPP polymers and N2200 approach 0.30 eV. Especially, PDPP2TBDT : N2200 shows the lowest LUMO offset of 0.18 eV. Recently, some studies revealed that the LUMO offset between donor and acceptor close to 0.1 eV could still provide enough energy for exciton dissociation into free charges.^34–36 Therefore, we can speculate that the DPP polymer : N2200 cells in this work have efficient charge separation, which will be further confirmed by PL measurements.

Polymer–polymer solar cells performance

The DPP polymers as electron donors and N2200 as the electron acceptor were used in all-polymer PSCs with inverted device configuration, where ITO/ZnO and MoO₃/Ag were used as the electrodes. Solutions with different contents of the additive, the ratio of donor to acceptor, and the thickness of active layers varied to optimize the performance of the solar cells. All the fabrication conditions and corresponding results are summarized in Tables S1–S3 and Fig. S1–S3.† The optimized results of these cells are summarized in Table 2.

Table 2 Solar cell parameters of optimized inverted solar cells of DPP polymers : N2200

Polymer^a	Jsc^b (mA cm^−2)	Voc [V]	FF	PCE [%]	Eloss (eV)
a Ratio of donor to acceptor is 1 : 1. Optimized spin coating solvent for active layer is CHCl3 with 2.5% DIO as additive.b Jsc as calculated by integrating the EQE spectrum with the AM1.5G spectrum. The thickness of active layers is around 70–90 nm.
PDPPTPT	1.4	0.88	0.38	0.45	0.65
PDPP5T	5.2	0.68	0.48	1.7	0.77
PDPP2TBDT	4.0	0.80	0.48	1.5	0.64
PDPP2TDTP	5.9	0.50	0.38	1.1	0.73

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In general, all the cells based on these DPP polymers provide the best PCEs when the active layers have the ratio of donor to acceptor of 1 : 1 and are solution-processed from chloroform with 2.5% DIO (Table 1). All the cells show high V_oc compared to their fullerene-based solar cells, which is due to the high-lying LUMO level of N2200. However, the PCEs of these all-polymer PSCs are much lower than those of fullerene-based solar cells.^22,30–32 The polymer PDPPTPT copolymerized with weak donating phenyl units show the lowest PCE of 0.45% with short circuit current density (J_sc) of 1.4 mA cm⁻² and FF of 0.38. PDPP5T with five thienyl units on the conjugated backbone has the best PCE of 1.7% among these cells, with J_sc of 5.2 mA cm⁻² and FF of 0.48. The polymer PDPP2TBDT containing two-dimensional benzodithiophene units provides PCE of 1.5% with J_sc of 4.0 mA cm⁻². PDPP2TDTP has the smallest band gap of 1.23 eV and broad absorption spectra, but the PCE is only 1.1% with J_sc of 5.9 mA cm⁻². The low PCEs in these cells are generally attributed to their low J_scs, which is also evidenced by the external quantum efficiencies (EQEs) of these cells (Fig. 3b). Although the photon response of these cells can extend to near-infrared region (up to 1000 nm), the maximum EQE is below 0.25.

Fig. 3 (a) J–V characteristics in dark (dashed lines) and under white light illumination (solid lines) of optimized solar cells of DPP polymers blended with N2200 (1 : 1) fabricated from CHCl₃ with 2.5% DIO as additive. (b) EQE of the same devices.

Morphology investigation

The efficiency of photon conversion into free charges relates to three physical processes: exciton diffusion into the interface of donor and acceptor, charge dissociation into free electron and hole, and charge transport into the electrode. Charge dissociation is generally related to the LUMO offset of the donor and the acceptor, which can further relate to energy loss (E_loss) that is defined as the difference between E_g and eV_oc. It is widely accepted that E_loss >0.6 eV can provide sufficient driving force for exciton dissociation into free charges.³⁷ In these DPP polymer : N2200 cells, E_loss is between 0.64 eV and 0.77 eV (Table 2), indicating efficient charge separation. Therefore, the low PCEs in these cells may arise from other physical processes: exciton diffusion to the interface between donor and acceptor and charge (hole and electron) transport to the electrode, both of which strongly relate to the morphology of mixed donor and acceptor phase.

The hole and electron mobilities in bulk-heterojunction (BHJ) blending films are crucial for charge transportation process, where the unbalanced hole and electron mobilities will induce charge accumulation at the electrodes and reduce the charge transport. In this work, we use space charge limit current (SCLC) to calculate the hole and electron mobilities, where the device configuration of ITO/PEDOT:PSS/active layer/Au was used for hole-only devices and ITO/ZnO/active layer/LiF/Al was used for electron-only devices. The results are summarized at Table 3 and Fig. 4. These cells have hole and electron mobility around 10⁻⁴ cm² V⁻¹ s⁻¹ with relatively balanced ratio as seen from μ_h/μ_e (Table 3), indicating similar transportation for hole and electron. However, it is difficult to get conclusion about morphology from this single measurement.

Table 3 Hole and electron mobility in all-PSCs by SCLC measurement^a

Polymer	μh (cm^2 V^−1 s^−1)	μe (cm^2 V^−1 s^−1)	μh/μe
a Hole mobility was realized by the device configuration of ITO/PEDOT:PSS/active layer/Au. Electron mobility was realized by the device configuration of ITO/ZnO/active layer/LiF/Al.
PDPPTPT	1.20 × 10^−4	5.53 × 10^−5	2.17
PDPP5T	1.37 × 10^−4	1.69 × 10^−4	0.81
PDPP2TBDT	1.42 × 10^−4	6.79 × 10^−5	2.09
PDPP2TDTP	1.92 × 10^−4	1.33 × 10^−4	1.44

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Fig. 4 J–V characteristics under dark for (a) hole-only devices and (b) electron-only devices.

AFM images of the DPP polymer : N2200 cells are shown in Fig. 5. PDPPTPT and PDPP5T based cells have small surface roughnesses of 2.62 nm and 2.69 nm, respectively, while PDPP2TBDT and PDPP2TDTP based cells have relatively high roughnesses of 3.86 nm and 4.76 nm, respectively. The chemical nature of DPP polymers and N2200 is similar, both containing conjugated backbones and long alkyl substituted side chains. The lack of surface topography suggests that the polymers are miscible, though care must be exercised in over interpreting the AFM results which are characteristic only of the surface.

Fig. 5 AFM height images (3 μm × 3 μm) of optimized DPP polymer : N2200 (1 : 1) active layers spin coated from chloroform containing 2.5% DIO. (a) PDPPTPT, (b) PDPP5T, (c) PDPP2TBDT and (d) PDPP2TDTP. The root mean square (RMS) roughness values are 2.62 nm, 2.69 nm, 3.86 nm and 4.76 nm.

We further use steady-state PL spectra to study the charge generation of DPP polymer : N2200 films (Fig. 6). The pure DPP polymers and N2200 exhibit reasonable PL spectra between 800 and 1100 nm, except for PDPP2TDTP with negligible emission. When the DPP polymers were blended with N2200, PL of DPP polymers were quenched, confirming the efficient charge transfer from donor to acceptor. It has been reported that large phase separation in binary systems would not effectively quench the donor PL spectra,³⁸ and thus the observation here indicated the trend of good mixing of the blends. It is notable that, PDPP2TBDT has relatively low PL intensity, which induces less PL quenching in the blend.

Fig. 6 Photoluminescence spectra of the DPP polymers, N2200 and blends of DPP polymer : N2200 (1 : 1) fabricated from CHCl₃ with 2.5% DIO. The thin films were excited at 760 nm for measurement, except for PDPP2TDTP and PDPP2TDTP : N2200 with excitation wavelength of 875 nm.

The structure order of DPP polymers and N2200 and their BHJ blends were investigated using grazing incidence X-ray diffraction (GIXD). The diffractograms and line-cut profiles are summarized in Fig. 7. As shown in PDPPTPT, a (100) peak is seen at 0.35 A⁻¹ (corresponding to a distance of 1.80 nm) in the out-of-plane direction, along with high ordered reflections. The π–π stacking shows a distinctive peak at 1.68 A⁻¹ (corresponding to a distance of 0.37 nm) with large azimuthal angle spread. PDPP5T showed a similar (100) reflection with less pronounced peak intensity. The π–π stacking peak is clearly evident in the out-of-plane direction at 1.73 A⁻¹ (corresponding to a distance of 0.36 nm). PDPPBDT showed a predominant “face-on” crystal orientation, as evidenced by a strong π–π stacking reflection in the out-of-plane direction (1.67 A⁻¹, corresponding a distance of 0.38 nm). The corresponding (100) peak located in the in-plane direction has a distance of 1.9 nm. PDPPDTP shows similar behaviour to that of PDPPBDT, with a π–π stacking reflection in the out-of-plane direction (1.72 A⁻¹, corresponding to a distance of 0.36 nm). The favourable π–π stacking for these polymers facilitates charge hopping across the devices, and serves as good channels to transfer hole carriers. N2200 shows a characteristic “face-on” crystal orientation. The π–π stacking located at 1.71 A⁻¹, corresponds to a distance of 0.37 nm. A well-pronounced (100) peak is seen in the in-plane direction, with a distance of 2.4 nm. Characteristics of BHJ thin films are shown in Fig. 7k. The overlapping of π–π stacking from DPP polymers and N2200 makes it hard to distinguish details of each polymer. However, from the disappearance of high ordered reflections from PDPPTPT in the PDPPTPT : N2200 blends, it is evident that N2200 disrupts the ordering of the PDPPTPT due, more than like to favorable interactions between the two polymers. This disruption of the order and the mixing results in a reduction in the device performance.

Fig. 7 GIXD of pure polymers and bulk heterojunction blends. (a) PDPPTPT, (b) PDPP5T, (c) PDPP2TBDT, (d) PDPP2TDTP, (e) N2200; (f) PDPPTPT : N2200, (g) PDPP5T : N2200, (h) PDPP2TBDT : N2200, (i) PDPP2TDTP : N2200. (j) and (k) are line-cut profiles of pure polymer and bulk heterojunction blends (solid line: out-of-plane line cuts; dotted lined: in-plane line cuts; the profiles are labeled using color text).

The phase separation of BHJ thin films is studied using resonant soft X-ray scattering (RSoXS) method.³⁹ Shown in Fig. 8 are RSoXS curves at a 287 eV photon energy, which gives the best contrast around carbon K-edge for the systems investigated. As can be seen in Fig. 8, PDPPTPT : N2200 blends shows a monotonic decay without clear features, thus no clear morphological feature can be observed. A more detailed Guinier and Debye–Bueche analysis did not yield useful results in radius of gyration or cord length (Fig. S4†). And thus a poor device performance is obtained. PDPP5T and PDPPBDT based blends show similar scattering characteristics without much feature to be analyzed. PDPPDTP : N2200 blends show a hump at around 0.006 A⁻¹, corresponding to a distance of 105 nm, yet the intensity is quite weak. Thus a poor mesh network is envisioned. RSoXS characterization reveals the weak phase separation for DPP polymers and N2200 blends. This can be an important reason for the low performance of their solar cell devices.

Fig. 8 RSoXS of bulk heterojunction blended thin films at 287 eV photon energy.

Conclusions

In conclusion, we investigated the typical DPP polymers as electron donor materials to pair with N2200 as an electron acceptor to fabricate all-polymer solar cells. The strong absorption, good energy level alignment, and high crystallinity of DPP polymers argue that these can be valuable systems with high potential to achieve high device performance. The mixing of the polymers, however, led to suppression of polymer crystallization, creating mixed domains that are less efficient in generating useful charge carriers and, therefore, relative low J_scs and PCEs in devices. It would be necessary to develop new DPP polymers that can phase separate with acceptor polymers to create a more favorable bicontinuous BHJ morphology to facilitate charge collection in all polymer solar cells, which is in due course in our lab.

Acknowledgements

This work was supported by the Recruitment Program of Global Youth Experts of China. The work was further supported by the National Natural Science Foundation of China (21574138, Y5A1141501) and the Strategic Priority Research Program (XDB12030200) of the Chinese Academy of Sciences. FL and TPR were supported by the U.S. Office of Naval Research under contract N00014-15-1-2244. Portions of this research were carried out at beamline 7.3.3 at the Advanced Light Source and Molecular Foundry, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

Notes and references

1. A. Facchetti, Mater. Today, 2013, 16, 123–132.
2. Y. Lin and X. Zhan, Mater. Horiz., 2014, 1, 470–488.
3. X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen and S. R. Marder, J. Am. Chem. Soc., 2007, 129, 7246–7247.
4. D. Mori, H. Benten, H. Ohkita, S. Ito and K. Miyake, ACS Appl. Mater. Interfaces, 2012, 4, 3325–3329.
5. T. Earmme, Y.-J. Hwang, N. M. Murari, S. Subramaniyan and S. A. Jenekhe, J. Am. Chem. Soc., 2013, 135, 14960–14963.
6. W. Li, W. S. C. Roelofs, M. Turbiez, M. M. Wienk and R. A. J. Janssen, Adv. Mater., 2014, 26, 3304–3309.
7. J. W. Jung, J. W. Jo, C.-C. Chueh, F. Liu, W. H. Jo, T. P. Russell and A. K. Y. Jen, Adv. Mater., 2015, 27, 3310–3317.
8. Y. Zhou, T. Kurosawa, W. Ma, Y. Guo, L. Fang, K. Vandewal, Y. Diao, C. Wang, Q. Yan, J. Reinspach, J. Mei, A. L. Appleton, G. I. Koleilat, Y. Gao, S. C. B. Mannsfeld, A. Salleo, H. Ade, D. Zhao and Z. Bao, Adv. Mater., 2014, 26, 3767–3772.
9. Y.-J. Hwang, B. A. E. Courtright, A. S. Ferreira, S. H. Tolbert and S. A. Jenekhe, Adv. Mater., 2015, 27, 4578–4584.
10. L. Gao, Z.-G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei and Y. Li, Adv. Mater., 2016, 28, 1884–1890.
11. Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade and H. Yan, Nat. Commun., 2014, 5, 5293.
12. H. Hu, K. Jiang, G. Yang, J. Liu, Z. Li, H. Lin, Y. Liu, J. Zhao, J. Zhang, F. Huang, Y. Qu, W. Ma and H. Yan, J. Am. Chem. Soc., 2015, 137, 14149–14157.
13. J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li and J.-X. Tang, Adv. Mater., 2015, 27, 1035–1041.
14. Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868–5923.
15. S. Beaupre and M. Leclerc, J. Mater. Chem. A, 2013, 1, 11097–11105.
16. C. Gao, L. Wang, X. Li and H. Wang, Polym. Chem., 2014, 5, 5200–5210.
17. Y. P. Zou, A. Najari, P. Berrouard, S. Beaupre, B. R. Aich, Y. Tao and M. Leclerc, J. Am. Chem. Soc., 2010, 132, 5330–5331.
18. N. Zhou, X. Guo, R. P. Ortiz, T. Harschneck, E. F. Manley, S. J. Lou, P. E. Hartnett, X. Yu, N. E. Horwitz, P. M. Burrezo, T. J. Aldrich, J. T. López Navarrete, M. R. Wasielewski, L. X. Chen, R. P. H. Chang, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2015, 137, 12565–12579.
19. Y. Liang and L. Yu, Acc. Chem. Res., 2010, 43, 1227–1236.
20. Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T. P. Russell and Y. Cao, Nat. Photonics, 2015, 9, 174–179.
21. H. Choi, S.-J. Ko, T. Kim, P.-O. Morin, B. Walker, B. H. Lee, M. Leclerc, J. Y. Kim and A. J. Heeger, Adv. Mater., 2015, 27, 3318–3324.
22. T. Kim, J.-H. Kim, T. E. Kang, C. Lee, H. Kang, M. Shin, C. Wang, B. Ma, U. Jeong, T.-S. Kim and B. J. Kim, Nat. Commun., 2015, 6, 8547.
23. A. Zhang, C. Xiao, D. Meng, Q. Wang, X. Zhang, W. Hu, X. Zhan, Z. Wang, R. A. J. Janssen and W. Li, J. Mater. Chem. C, 2015, 3, 8255–8261.
24. K. H. Hendriks, W. Li, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2014, 136, 12130–12136.
25. C. B. Nielsen, M. Turbiez and I. McCulloch, Adv. Mater., 2013, 25, 1859–1880.
26. Y. Ji, C. Xiao, Q. Wang, J. Zhang, C. Li, Y. Wu, Z. Wei, X. Zhan, W. Hu, Z. Wang, R. A. J. Janssen and W. Li, Adv. Mater., 2015, 28, 943–950.
27. W. Li, K. H. Hendriks, A. Furlan, W. S. C. Roelofs, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2013, 135, 18942–18948.
28. R. Po, G. Bianchi, C. Carbonera and A. Pellegrino, Macromolecules, 2015, 48, 453–461.
29. H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz, M. Kastler and A. Facchetti, Nature, 2009, 457, 679–686.
30. K. H. Hendriks, G. H. L. Heintges, V. S. Gevaerts, M. M. Wienk and R. A. J. Janssen, Angew. Chem., Int. Ed., 2013, 52, 8341–8344.
31. W. Li, W. S. C. Roelofs, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2012, 134, 13787–13795.
32. L. T. Dou, J. B. You, J. Yang, C. C. Chen, Y. J. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180–185.
33. K. H. Hendriks, G. H. L. Heintges, V. S. Gevaerts, M. M. Wienk and R. A. J. Janssen, Angew. Chem., Int. Ed., 2013, 52, 8341–8344.
34. W. Li, K. H. Hendriks, A. Furlan, M. M. Wienk and R. A. J. Janssen, J. Am. Chem. Soc., 2015, 137, 2231–2234.
35. K. Kawashima, Y. Tamai, H. Ohkita, I. Osaka and K. Takimiya, Nat. Commun., 2015, 6, 10085.
36. N. A. Ran, J. A. Love, C. J. Takacs, A. Sadhanala, J. K. Beavers, S. D. Collins, Y. Huang, M. Wang, R. H. Friend, G. C. Bazan and T.-Q. Nguyen, Adv. Mater., 2016, 28, 1482–1488.
37. D. Veldman, S. C. J. Meskers and R. A. J. Janssen, Adv. Funct. Mater., 2009, 19, 1939–1948.
38. W. Li, Y. Zhou, B. V. Andersson, L. M. Andersson, Y. Thomann, C. Veit, K. Tvingstedt, R. P. Qin, Z. S. Bo, O. Inganäs, U. Wurfel and F. L. Zhang, Org. Electron., 2011, 12, 1544–1551.
39. E. Gann, A. T. Young, B. A. Collins, H. Yan, J. Nasiatka, H. A. Padmore, H. Ade, A. Hexemer and C. Wang, Rev. Sci. Instrum., 2012, 83, 045110.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03681c

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