Enhancing the light-harvesting and charge transport properties of polymer solar cells by embedding NaLuF₄:Yb,Tm nanorods

Abstract

In this contribution, we study the effects of NaLuF₄:Yb,Tm nanorods (NRs) dopant on the performance of inverted polymer bulk heterojunction (BHJ) solar cells. The study allows us to investigate the impact of NRs on the light-harvesting and photocurrent in BHJ solar cells fabricated with PCDTBT blended with [6,6]-phenyl-C71-butyric acid methylester (PC₇₁BM) acceptor. By controlling the doping concentrations, we demonstrate that the power conversion efficiency (PCE) of PCDTBT:PC₇₁BM solar cells increases from 6.02% to 7.18% mainly due to improving light absorption and photocurrent. The single carrier devices reveal that the electron and hole mobilities both rise in doped devices, contributing to the enhancement of photoconductivity and consequent PCEs.

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1. Introduction

As a result of the use of low-cost materials, ease of processing, and rapid increase in power conversion efficiencies (PCE), solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) have attracted considerable attention as a potentially practical approach to solar energy harvesting.^1–3 PSCs efficiency has reached over 10%, which has been a promising alternative to conventional inorganic photovoltaic devices.^4–8 Compared with their inorganic counterparts, the PSCs efficiency is still lower, and further improvements are necessary to make PSCs commercially viable. The efficiency of PSCs is limited by a few factors, such as low light-trapping, unsatisfactory excition migration and dissociation, charge transport, and charge recombination, which are significantly influenced by the active layer films. Typically, the active layer of PSCs involves a phase-separated blend of an electron-donating conjugated polymer and an electron-accepting fullerene derivative.^9–11 Tremendous efforts have been made to optimize the active layer formation and the device configuration,^12,13 such as different solvents doping, thermal annealing of the active layer, optical spacer adding, and tandem structure, which have obtained effective improvement in PSCs performance.^14–22

As we know, about 52% of total solar energy flux is in the infrared (IR) and near-infrared (NIR) range, so the utmost using of the NIR part of the solar radiation is a practical method to further enhance the performance of solar cells. Highly efficient solar cells require absorbing photons over the possibly broad range of the solar spectrum followed by effective generation and collection of charge carriers. One widely adopted strategy to enhance the number of absorbed photons and resultant photocurrent is by decreasing the optical band gap of the active layer to achieve a better absorption overlap with the solar spectrum. Up-conversion (UC) is a luminescence of transforming two or more low-energy photons into one high-energy photon.^23–29 UC is a smart concept to exploit the IR and NIR photons, which may enhance the response of solar cells in the IR region. A great deal of work has been done to utilize inorganic nanoparticles (NPs) including UC effect to enlarge the absorption coefficient of active materials in polymer solar cells.^30,31 In addition, incorporation of NPs with UC function into the solar cells can not only increase light trapping of active layer but also apparently improve charge transport properties.^32–35 Meanwhile, the NPs can play a role as light-scattering centers and tune the morphology of active layer. Lanthanide rare-earth ions, such as NaLuF₄ and NaYF₄ possess superior UC efficiency, have been used in optoelectronic devices to enhance light capture in NIR region and improve charge transport and charge injection. In this paper, the cubic phase NaLuF₄:Yb,Tm nanorods (NRs) were prepared by a facile solvent thermal approach and introduced into active layer of PSCs. The NRs have an average size of ∼50 nm and can be well dispersed in the blend solution. The performances of PSCs fabricated with different doping concentrations are investigated and compared.

2. Experimental

2.1 Material preparation

LnCl₃ (Ln = Lu, Yb, and Tm, 99.999%) compounds were purchased from Shanghai Shabo Chemical Technology Company. Oleic acid (OA) and 1-octadecene (ODE) were obtained from Alfa Aesar. Oleylamine (OM) and sodium oleate (NaOA) were obtained from Aladdin. Ammonium fluoride (NH₄F) with A.R. purity was bought from Beijing Fine Chemical Company, China. All other chemicals were analytical grade and used without further purification.

A typical procedure for the synthesis of β-NaLuF₄:20%Yb,0.7%Tm is as follows: 1 mmol LnCl₃ (Ln = Lu, Yb, and Tm) was added to a 100 mL three-necked flask containing a certain amount of OA, OM and ODE. The solution was stirred and heated at 110 °C for 1 h under vacuum. The solution was then cooled to room temperature using a gentle flow of argon gas. Solid NaOA and anhydrous NH₄F were added to the flask, and the mixed solution was then heated at 320 °C for 1 h. When the solution had cooled to room temperature, solid state products were precipitated with 50 mL of ethanol, then collected after centrifugation, washed with ethanol twice, and finally redispersed in methylbenzene for further experiments. X-Ray diffraction (XRD) analysis of NaLuF₄:Yb,Tm powder was carried out with a powder diffractometer (Model Rigaku RU-200b), using Ni-filtered Cu Kα radiation (λ = 1.5406 Å) with 200 mA current and 50 kV voltage across the tube to generate powerful X-rays. The XRD measurement was performed at a scan rate of 18° min⁻¹ and step size of 0.02°. Fig. 1(a) exhibits that NaLuF₄:Yb,Tm is hexagonal phase. Fig. 1(b) shows the transmission electron microscope (TEM) images of synthesized cubic phase NaLuF₄:Yb,Tm crystals, in which we can see that NaLuF₄:Yb,Tm NRs are 50 nm in length and 15 nm in diameter on average. Fig. 1(c) is the up-conversion emission spectrum NaLuF₄:Yb,Tm NRs, and the emission peaks are located in 360 nm and 470 nm. In Yb³⁺–Tm³⁺ codoped systems, Yb³⁺ ions absorb pumping photons and successively transfer energy to Tm³⁺ to populate their ³H₅, ³F_3,2, and ¹G₄ levels in turn. The ¹D₂ state is populated through the cross relaxation ³F₃ → ³H₆: ³F₃ → ¹D₂ (Tm³⁺) due to the large energy mismatch (∼3500 cm⁻¹) between ¹G₄ and ¹D₂, which cannot be directly populated by the ET ²F_5/2 → ²F_7/2 (Yb³⁺): ¹G₄ → ¹D₂ (Tm³⁺). The ³P₂ level of Tm³⁺ is populated by the ET ²F_5/2 → ²F_7/2 (Yb³⁺): ¹D₂ → ³P₂ (Tm³⁺) and then relaxes rapidly to the ¹I₆ level.^36–40

Fig. 1 (a) XRD image, (b) TEM image, and (c) up-conversion emission spectrum of NaLuF₄:Yb,Tm crystals.

2.2 Device fabrication and characterization

β-NaLuF₄:20%Yb,0.7%Tm NRs was dissolved in toluene solution and concentration is 1 mg mL⁻¹. The device structure is shown in Fig. 2. The invert OSC devices have a configuration of indium tin oxide (ITO)/titanium dioxide (TiO₂) (25 nm)/active layer (100 nm)/molybdenum oxide (MoO₃) (10 nm)/sliver (Ag) (100 nm). The TiO₂ film covered on the ITO acts as an electron-selective layer, while MoO₃ high work function (−5.9 eV) at the photoactive layer/Ag interface will enhance hole collection.^41–44 Patterned ITO-coated glass substrates were sonicated consecutively with acetone, isopropyl alcohol, and deionized water for 10 min and subsequently dried with nitrogen. Anatase TiO₂ layer was prepared onto ITO substrate and the thin film was then thermally annealed at 450 °C for 2 h in the muffle furnace. For the active layer, the 1,2-dichlorobenzene (DCB) solution composed of PCDTBT (7 mg mL⁻¹) and PC₇₁BM (28 mg mL⁻¹) were doped with 0, 10 μL, 20 μL, 30 μL, 40 μL NaLuF₄:20%Yb,0.7%Tm NRs solution, respectively. The corresponding weight ratio (wt) of NRs and PCDTBT:PC₇₁BM are 0, 0.029 wt%, 0.058 wt%, 0.087 wt%, 0.116 wt%. Accordingly, different doping devices are named as device A, device B, device C, device D, and device E. Poly-[N-9′′-hepta-decanyl-2,7-carbazolealt-5,5-(40,70-di-2-thienyl-20,10,30-ben-zothia-diazole)] (PCDTBT) is electron donor and fullerene derivative [6,6]-phenyl-C₇₁-butyric acid methylester (PC₇₁BM) is acceptor. The blend solution was spin-cast at 2000 rpm on top of the TiO₂ layer in air, and the thickness of active layer is about 100 nm. The samples with PCDTBT:PC₇₁BM were baked in the atmosphere of argon in the glove box at 70 °C for 20 min. Finally, the devices were completed by thermal evaporation of MoO₃ and Ag electrode under a base vacuum of 5.0 × 10⁻⁴ Pa. The active area of the inverted devices is about 0.064 cm².

Fig. 2 The device structure of the inverted polymer solar cells based on NaLuF₄:Yb,Tm NRs doping.

Current density–voltage (J–V) characteristics were measured with a computer-programmed Keithley 2400 source/meter under AM1.5 G solar illuminations with an Oriel 300 W solar simulator intensity of 100 mW cm⁻² (about one sun) in air without encapsulation. The light intensity was measured with a photometer (Internationallight, IL1400) corrected by a standard silicon solar cell. The incident photon-to-current efficiency (IPCE) was measured with Crowntech QTest Station 1000 AD. The absorption and transmittance spectra were measured by means of ultraviolet/visible spectrometer (UV 1700, Shimadzu).

3. Result and discussion

The J–V characteristics of all finished devices are shown in Fig. 3. The device A without NRs doping exhibits a short circuit current density (J_sc) of 13.946 mA cm⁻², open-circuit voltage (V_oc) of 0.867 V and fill factor (FF) of 49.8%, leading to a PCE of 6.022%. As expected, the performance of devices C has been apparently improved, the PCE of device C made with an optimal doping amount of 0.058 wt% is 7.175%, with a J_sc of 15.585 mA cm⁻², V_oc of 0.865 V and FF of 53.24%. J_sc and FF are both improved, J_sc rises from 13.946 mA cm⁻² to 15.585 mA cm⁻² and FF rises from 49.8% to 53.24%, which directly brings an 19% enhancement of PCE than device A.

Fig. 3 J–V characteristics of the devices with different amount of NRs under AM 1.5 G illumination with an intensity of 100 mW cm⁻² in ambient air, inset is J–V characteristics of devices with different doping amount of NRs in dark.

Meanwhile, the device B and D were also conducted, and they demonstrate relatively higher PCE than contrast device. The detailed results are summarized in Table 1, and the values are typical average of 50 devices. Noticeably, V_oc remained the same level as that of the reference device, which suggests that the NaLuF₄:20%Yb,0.7%Tm NRs have no large influence on the energy level distribution of the devices. It is widely believed that the V_oc mainly originates from the energy difference between the highest occupied molecular orbital (HOMO) of donor and the lowest unoccupied molecular orbital (LUMO) of acceptor.^45,46 In addition, device E with doping amount of 0.116 wt% NRs shows a poor performance especially in FF compared with other devices. The excess NRs doping is adverse to the quality of the film of active layer, which might result in a poor phase separation and then decrease exciton dissociation and charge mobility. The inset of Fig. 3 shows the dark J–V curves of control and doped devices in the absence of illumination. The doped devices show smaller leakage current at negative voltages. Meanwhile, higher current in the space charge limited current dominated regime was achieved for doped devices, indicating an increase of charge transfer speed and a decrease of series or contact resistance, which will produce a well rectifying effect compared with control devices.⁴⁷

Table 1 Device performance, including open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF), power conversion efficiency (PCE), series resistance (R_s) and shunt resistance (R_sh) of devices in our study

Device	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)	Rs (ohm)	Rsh (ohm)
A	0.867	13.946	49.80	6.022	322.646	4348.32
B	0.867	14.199	52.62	6.478	179.967	4093.80
C	0.865	15.585	53.24	7.175	180.211	4416.98
D	0.869	14.316	50.33	6.236	220.150	3462.04
E	0.874	13.949	46.75	5.699	344.285	3839.60

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In order to further understand the effect of NaLuF₄:Yb,Tm NRs, another contrast experiment was performed by simply adding NaLuF₄ (0.058 wt%) NRs into PCDTBT:PC₇₁BM solution without Yb and Tm ions (Fig. 4). The result shows that the J_sc and PCE of the device with NaLuF₄ NRs are both higher than that of the undoped devices, lower than that of NaLuF₄:Yb,Tm doping cells. This indicates that doped NaLuF₄ and NaLuF₄:Yb,Tm into active layer of PSCs can both improve the performance of cells, but the device performance of doped NaLuF₄:Yb,Tm are much higher, which is attributed to the UC of Yb and Tm ions. Why the properties of cells doped NaLuF₄ NRs was also higher than that of control devices. The following possible factors may illustrate the effect of NaLuF₄ NRs. Firstly, the optical path length in the active layer can be increased because of scattering of NaLuF₄ NRs. Secondly, the near field will extend into the active layer at a large extent, and the possibility of interaction coupling between the near field and the incident light will be enhanced, improving the photo-generated excitons of active layer.^48–50

Fig. 4 J–V characteristics comparison of cells without doping, doped with 0.058 wt% NaLuF₄ NRs, and doped with 0.058 wt% NaLuF₄:Yb,Tm NRs.

The incident photon-to-current efficiency (IPCE) spectra of all devices are shown in Fig. 5. The IPCE of the device A shows a maximum of 62% at 525 nm. When incorporating 0.058 wt% NRs, the IPCE of device C shows a maximum of 69.5% at 475 nm, a little higher than that of device A. This is consistent with the data from J–V curves. IPCE of devices doped with NRs are apparently improved from 350 nm to 630 nm compared with device A. As shown in Fig. 1(b), the emission peaks of NaLuF₄:Yb,Tm are located in 360 nm and 470 nm, which probably contributes to the enhancement of IPCE. Fig. 6(a) is light-harvesting spectrum of active layer film with and without NRs. It can be seen that light harvesting of doped film got modestly increase. Fig. 6(b) is the light transmittance spectra of active layers with different ratios of NRs, which is corresponding with Fig. 6(a). The enhanced light trapping in the NRs doped devices can be attributed to the increase of light path from the scattering of cubic phase NaLuF₄ NRs, and UC effect deriving from Yb³⁺ and Tm³⁺ ions also makes a contribution to improve light absorption.

Fig. 5 The IPCE spectra of all finished solar cells.

Fig. 6 (a) The absorption spectra of the active layers with different doping amount of NRs, (b) the transmittance spectra of the devices with different doping amount of NRs.

In order to investigate the role of NRs on charge transport properties, we fabricated two kinds of single carrier devices. The hole-only device configuration is ITO/MoO₃/active layer/MoO₃/Ag, the MoO₃ connected with ITO is electron blocking layer. The 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is used as hole blocking layer, and the electron-only device configuration is ITO/TiO₂/active layer/BCP/Ag. The J–V characteristics in dark of these two kinds of devices were shown in Fig. 7. We observe that J_sc of doped devices has been mildly enhanced respective of doping amount in both the hole-only devices and the electron-only devices. According to the data of Fig. 7, charge carrier mobilities were calculated from Mott–Gurney law, including field dependence. It will apply a realistic evaluation on the apparent charge carrier mobility in the active layer according to J–V characteristics of single charge carrier devices. At a typical applied voltage of 5.0 V, corresponding to an electric field of 5 × 10⁵ V cm⁻¹ across the bulk of a 100 nm device, apparent hole mobilities of 7.52 × 10⁻⁵ cm² V⁻¹ s⁻¹, 8.78 × 10⁻⁵ cm² V⁻¹ s⁻¹, 1.19 × 10⁻⁴ cm² V⁻¹ s⁻¹, 8.67 × 10⁻⁵ cm² V⁻¹ s⁻¹, and 8.53 × 10⁻⁵ cm² V⁻¹ s⁻¹, and apparent electron mobilities of 3.22 × 10⁻⁴ cm² V⁻¹ s⁻¹, 5.67 × 10⁻⁴ cm² V⁻¹ s⁻¹, 7.85 × 10⁻⁴ cm² V⁻¹ s⁻¹, 5.96 × 10⁻⁴ cm² V⁻¹ s⁻¹, and 4.17 × 10⁻⁴ cm² V⁻¹ s⁻¹ have been determined for the device A and device B, C, D and E, respectively. Both the hole and electron mobilities were increased and a more balanced charge transport in doped devices can be achieved. This indicates incorporating NRs into active layer is benefit to increase exciton dissociation and enhance carriers transport properties thus increasing the J_sc of devices. Simultaneously, we obtain the largest J_sc with 0.058 wt% NRs doping, which is consistent with the tendency of the photocurrent in Fig. 3, suggesting that the charge transport properties are substantially improved. To investigate the impact of NRs on the interface resistance of devices, the impedance spectra of all devices were measured with frequency of 20 to 1 MHz, and the results are shown in Fig. 8. The semicircle's diameter in the diagram stands for the impedance. In other words, the larger diameter means larger impedance. It indicates that the semicircle's diameter of devices doped with NRs are smaller than control devices. Thus, the additive of NRs brings significant decline to the devices impedance, which explains the increase of the J_sc.

Fig. 7 J–V characteristics of single carrier device in dark (a) electron-only device and (b) hole-only device.

Fig. 8 The complex impedance characteristics of all devices.

The surface morphology of active layer is very important to the photovoltaic performance of polymer BHJ solar cells. Fig. 9 shows the surface morphology of active layers of device A, device B, device C, and device D in AFM (atom force microscopy) images. Their surfaces are quite smooth and the root-mean-square (RMS) roughness of the four active layers is 0.36 nm, 0.48 nm, 0.54 nm, and 0.66 nm, respectively. The doped and undoped films have similar surface morphologies, while doped films have a higher RMS. According to the BHJ solar cell model, the V_oc is determined by the energy difference between HOMO level of the donor and the LUMO level of the acceptor and little influenced by the film morphology, so V_oc is almost same for all devices. It has been well acknowledged that continuous interpenetrating networks with proper domain size are essential for the efficient exciton separation and charge transport, which are key factors to enhance the J_sc and FF. Comparison of the AFM images of all active layers indicates that the higher RMS roughness of doped films leads to higher J_sc and FF, which is consistent with the J–V characteristics shown in Fig. 3.

Fig. 9 AFM images of PCDTBT:PC₇₁BM film without and with different ratios NRs.

4. Conclusion

In conclusion, we have investigated the role of NaLuF₄:Yb,Tm NRs dispersed into the active layer on the performance of inverted BHJ OSCs based on PCDTBT:PC₇₁BM blending. Compared with the reference device without NRs, the maximum 19% enhancement in power conversion efficiency (from 6.022% to 7.175%) was achieved. And increasing J_sc, FF and unchangeable V_oc were observed in the devices containing the NRs with optimized doping amount of 0.058 wt%. The evaluations of device photovoltaic characteristics demonstrated that the generation rate and the dissociation probability of excitons were improved after introducing NRs, resulting in improved J_sc and FF. The introduction of NaLuF₄ NRs can not only increase the optical path length as scattering center, but also can convert IR and NIR photons into visible photons and thus increases the light harvesting of active layer, leading to a higher PCE.

Acknowledgements

Supported by National Natural Science Foundation (Grant nos 61275035, 61274068, 61077046), Chinese National Programs for High Technology Research and Development (Grant no. 2013AA030902), Doctoral Found of Ministry of Education of China (Grant nos 20100061120045 and 20110061130004), Project of Science and Technology Development Plan of Jilin Province (Grant nos 20130206075SF, 20110314, 20120324), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (no. IOSKL2012KF03).

Notes and references

1. J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante and A. J. Heeger, Science, 2007, 317, 222.
2. F. C. Krebs, H. Spanggard, T. Kjær, M. Biancardo and J. Alstrup, Mater. Sci. Eng., B, 2007, 138, 106.
3. L. Blankenburg, K. Schultheis, H. Schache, S. Sensfuss and M. Schrödner, Sol. Energy Mater. Sol. Cells, 2009, 93, 476.
4. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 1636.
5. F. C. Krebs, Org. Electron., 2009, 10, 761.
6. F. C. Krebs, M. Jorgensen, K. Norrman, O. Hagemann, J. Alstrup, T. D. Nielsen, J. Fyenbo, K. Larsen and J. Kristensen, Sol. Energy Mater. Sol. Cells, 2009, 93, 422.
7. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 465.
8. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 394.
9. C. Edwards, A. Arbabi, G. Popescu and L. L. Goddard, Light: Sci. Appl., 2012, 1, e30.
10. S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz and J. C. Hummelen, Appl. Phys. Lett., 2001, 78, 841.
11. M. T. Rispens, A. Meetsma, R. Rittberger, C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Chem. Commun., 2003, 3116.
12. M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang and T. J. Marks, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 2783.
13. R. Steim, S. A. Choulis, P. Schilinsky and C. J. Brabec, Appl. Phys. Lett., 2008, 92, 093303.
14. S. E. Shaheen, C. J. Brabec and N. S. Sariciftci, Appl. Phys. Lett., 2001, 78, 841.
15. F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv. Funct. Mater., 2003, 13, 85.
16. R. A. Taylor, T. Otanicar and G. Rosengarten, Light: Sci. Appl., 2012, 1, e34.
17. C. Xiang, W. Koo, F. So, H. Sasabe and J. Kido, Light: Sci. Appl., 2013, 2, e74.
18. W. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617.
19. J. Y. Kim, K. Lee, N. E. Coates, D. Mose, T. Q. Nguyen, M. Dante and A. J. Heeger, Science, 2007, 317, 222.
20. S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297.
21. J. Perez-Juste, I. Pastoriza-Santos and L. M. Liz-Marzan, Coord. Chem. Rev., 2005, 249, 1870.
22. D. H. Wang, D. Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. Park and A. J. Heeger, Angew. Chem., 2011, 50, 5633.
23. C. F. Guo, T. S. Sun, F. Cao, Q. Liu and Z. F. Ren, Light: Sci. Appl., 2014, 3, e161.
24. L. L. Huang, X. Z. Chen, B. F. Bai, Q. F. Tan, G. F. Jin, T. Zentgraf and S. Zhang, Light: Sci. Appl., 2013, 2, e70.
25. X. Chen, B. H. Jia, Y. A. Zhang and M. Gu, Light: Sci. Appl., 2013, 2, e92.
26. Z. Holman, S. Wolf and C. Ballif, Light: Sci. Appl., 2013, 2, e106.
27. J. de Wild, J. K. Rath, A. Meijerink, W. G. J. H. M. Van Sark and R. E. I. Schropp, Sol. Energy Mater. Sol. Cells, 2010, 94, 2395.
28. T. Trupke, A. Shalav, B. S. Richards, P. Wurfel and M. A. Green, Sol. Energy Mater. Sol. Cells, 2006, 90, 3327.
29. T. Trupke, M. A. Green and P. Wurfel, J. Appl. Phys., 2002, 92, 4117.
30. A. Shalav, B. S. Richards and T. Trupke, Appl. Phys. Lett., 2005, 86, 013505.
31. B. S. Richards, Sol. Energy Mater. Sol. Cells, 2006, 90, 1189.
32. C. W. Lin, D. Y. Wang, Y. T. Wang, C. C. Chen, Y. J. Yang and Y. F. Chen, Sol. Energy Mater. Sol. Cells, 2011, 95, 1107.
33. H. C. Liao, C. S. Tsao, T. H. Lin, M. H. Jao, C. M. Chuang, S. Y. Chang, Y. C. Huang, Y. T. Shao, C. Y. Chen, C. J. Su, U. S. Jeng, Y. F. Chen and W. F. Su, ACS Nano, 2012, 6, 1657.
34. B. Paci, G. D. Spyropoulos, A. Generosi, D. Bailo, V. R. Albertini, E. Stratakis and E. Kymakis, Adv. Funct. Mater., 2011, 21, 3573.
35. D. H. Wang, D. Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park and A. J. Heeger, Angew. Chem., Int. Ed., 2011, 50, 5519.
36. R. S. Khnayzer, J. Blumhoff, J. A. Harrington, A. Haefele, F. Deng and F. N. Castellano, Chem. Commun., 2012, 48, 209.
37. M. Zhang, Y. Lin, T. J. Mullen, W. Lin, L. D. Sun, C. H. Yan, T. E. Pattern, D. Wang and G. Liu, J. Phys. Chem. Lett., 2012, 3, 3188.
38. J. Méndez-Ramos, P. Acosta-Mora, J. C. Ruiz-Morales, T. Hernandez, M. E. Borges and P. Esparza, RSC Adv., 2013, 3, 23028.
39. S. Obregón and G. Colón, Chem. Commun., 2012, 48, 7865.
40. G. F. Wang, W. P. Qin, L. L. Wang, G. D. Wei, P. F. Zhu and R. J. Kim, Opt. Express, 2008, 16, 11907.
41. J. Meyer, S. Hamwi, M. Kroger, W. Kowalshy, T. Riedl and A. Kahn, Adv. Mater., 2012, 24, 5408.
42. M. Kroger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky and A. Kahn, Org. Electron., 2009, 10, 932.
43. Y. H. Su, Y. F. Ke, S. L. Cai and Q. Y. Yao, Light: Sci. Appl., 2012, 1, e14.
44. D. Lepage, A. Jimenez, J. Beauvais and J. J. Dubowski, Light: Sci. Appl., 2012, 1, e28.
45. G. Dennier, M. C. Scharber, T. Ameri, P. Denk, K. Forberich, C. Waldauf and C. J. Brabec, Adv. Mater., 2008, 20, 579.
46. M. C. Scharber, D. Wuhlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789.
47. C. Tao, S. P. Ruan, G. H. Xie, X. Z. Kong, L. Shen, F. X. Meng, C. X. Liu, X. D. Zhang, W. Dong and W. Y. Chen, Appl. Phys. Lett., 2009, 94, 043311.
48. J. L. Wu, F. C. Chen, Y. S. Hsiao, F. C. Chien, P. Chen, C. H. Kuo, M. H. Huang and C. S. Hsu, ACS Nano, 2011, 5, 959.
49. M. Heo, H. Cho, J. W. Jung, J. R. Jeong, S. Park and J. Y. Kim, Adv. Mater., 2011, 23, 5689.
50. I. Kim, T. S. Lee, D. S. Jeong, W. S. Lee and K. S. Lee, J. Phys. D: Appl. Phys., 2012, 45, 065101.

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