Versatility and robustness of ZnO:Cs electron transporting layer for printable organic solar cells

Abstract

In this work, a Cs doped sol–gel ZnO film (ZnO:Cs) as an efficient and robust electron transporting layer (ETL) in versatile systems of organic solar cells (OSCs) is developed, which can be simply formulated by blending the Cs₂CO₃ with the Zn(Ac)₂ precursor in solutions. The Cs doping significantly increased the ZnO film conductivity and lowered its work function, as unveiled by the conductive atomic force microscope and the ultra-violet photoelectron spectroscopy. Decent device performance enhancements of OSCs with versatile photovoltaic materials, e.g. P3HT : PC₆₁BM, P3HT:ICBA, and PTB7 : PC₇₁BM, were observed with the doped ZnO:Cs as the ETL compared with the pristine ZnO ETL. The enhanced device performance was also found in the tandem solar cells. Moreover, the device performance shows little drop with the thickness of the doped ZnO:Cs ETL ranging from 40 to 520 nm, indicating the less thickness dependence for the doped ZnO:Cs ETL. The current work verifies the potential of the Cs doped ZnO as a high performance ETL material for printable OSCs.

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Introduction

Simple Metal/Semiconductor/Metal (MSM) structured photovoltaic devices^1–3 are emerging as one promising technique for solar energy conversion, e.g. organic solar cells (OSC).^4,5 The inherent merits of OSC include low cost, flexibility, light-weight, and compatibility with the Roll to Roll (R2R) processing,^6,7 which endow it the strong competitiveness for future commercialization.^8,9 In this structure, the active layer functions as both light absorbing and charge transporting layer.¹⁰ Due to the bipolar charge transporting property of these sandwiched active layers, the device polarity is determined mainly by electrodes, e.g. the electronic structure or the energy level alignment at the electrode interfaces. To realize high device performance, efficient and selective transporting of only one charge species is required to reduce the charge recombination.^11–16 As a result, the cathode work function comparable with the Lowest Unoccupied Molecular Orbit (LUMO) is desired to form Ohmic contact for the electron extraction, and vice verse for the anode with work function comparable to the Highest Occupied Molecular Orbit (HOMO) for the hole extraction. Since the intrinsic work function of the electrode is limited by its own nature, modification of the electrode is essentially important to achieve the optimal interface contact and the high device performance.^17–21

The commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a versatile anode modification layer due to the doping induced high conductivity and high work function.²² However, the cathode interfacial modification materials or the electron transporting layer (ETL) with versatility are lacked. A general rule for designing the optimal ETL is to lower the work function and increase the conductivity of ETL, as unveiled previously.¹⁵ Recently, a variety of candidates for cathode modification were reported, e.g. bathocuproine (BCP),^23,24 aluminum 8-hydroxyquinolinate (AlQ₃),²⁵ LiF,²⁶ ZnO,²⁷ TiO₂,²⁸ poly electrolytes,⁸ etc. Most of them could lower the electrode work function to efficiently extract electrons while suppress the holes, with the mechanisms of doping effect,^29,30 intrinsic low work function^31–33 or the formation of favorable dipole moments.^34,35 Although these ETLs could promise the OSC with enhanced device performance, the film thickness was critically limited within tens of nanometers. Generally, thicker films would block the charge extraction and ruin the device performance due to the low conductivity of these ETLs. Notably, the thin films (several tens of nm) could hardly make a full coverage on the rough electrode surfaces, which might lead to punctures and cause charge recombination. And the ultra-thin and homogeneous film on large area substrates is especially difficult to form on the rough R2R processing.³⁶ Therefore, it is urging to develop new ETLs with high efficiency, versatility, and processing robustness for R2R integration.^36,37 One possible strategy to resolve this problem is by doping, with which the film conductivity would increase appreciably and the work function could also be tuned to an appropriate energy level.^38–42

In this work, we demonstrated a Cs doped sol–gel ZnO film (ZnO:Cs for short) as the efficient and robust ETL for printable OSCs. Compared with the pristine ZnO ETL, the device performance with the Cs doped ZnO as ETL was enhanced in OSCs with a variety of photovoltaic materials (chemical structures shown in Fig. 1a), e.g. poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM), P3HT:indene-C60 bisadduct (ICBA), and poly(benzodithiophene-alt-thienothiophene) (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), in both single junction (Fig. 1b) and tandem (Fig. 1c) device architectures. Less thickness dependence was also found for the doped ZnO:Cs ETL, indicating its versatility and robustness as a potential ETL material for printable OSCs.

Fig. 1 (a) Chemical structures of the materials used in this work, (b) schematic device structure of the inverted single junction organic solar cells, (c) schematic device structure of the inverted tandem organic solar cells.

Experimental section

Materials and equipments

All of the chemicals are purchased from Sigma Aldrich if not specified. The P3HT and PTB7 are purchased from 1-materials Corp and used as received. The IC₆₀BA and PC₆₁BM are purchased from Lumi. Tech. (Taiwan, China), and used as received. The UPS measurement was carried out on an integrated ultrahigh vacuum system equipped with multi-technique surface analysis system (Thermo ESCALAB 250Xi) with the He(I) (21.2 eV) UV excitation source. The atomic force microscopy (AFM) was recorded by the Nanscope IIIa multi-mode scanning probe microscope. The conducting AFM (c-AFM) was measured at contact mode with a Pt coated tip. A plastic tip holder was used for the measurement.

Device fabrication and measurement

Single junction organic photovoltaic devices. The patterned ITO/glass substrates were cleaned sequentially by detergent, acetone, isopropanol in ultrasonic bath. After blew dry by N₂ gas, the substrates were further cleaned by UV-Ozone machine. ZnO precursor solution (0.5 M Zn(Ac)₂·2H₂O in 2-methoxyethanol, with 0.5 M monoethanolamine, with or without 0.005 M Cs₂CO₃) was spun on the ITO substrate at 3000 rpm and annealed at 160 °C for 30 min to form 30–40 nm ZnO or the doped ZnO:Cs ETL. The substrates were transferred into the glove-box to deposit the BHJ layer. For P3HT : PC₆₁BM layer, 30 mg ml⁻¹ (P3HT : PC₆₁BM = 1 : 1 wt%) o-dichlorobenzene solution is spun on the substrate to form a 120 nm film. To control the morphology of the active layer, slow growth method was adopted. The dried P3HT : PC₆₁BM film was further annealed at 120 °C for 5 min to improve the crystalline properties. Finally, 10 nm MoO₃ and 100 nm Ag layers were deposited sequentially in vacuum to complete the device fabrication process. The deposition of P3HT:ICBA is the same to that of P3HT : PC₆₁BM, except that the modified PEDOT:PSS (m-PEDOT:PSS) instead of MoO₃ is used as HTL. The deposition of the m-PEDOT:PSS is described elsewhere.⁴⁴ For the PTB7 : PC₇₁BM solar cell, 25 mg PTB7 : PC₇₁BM composite (PTB7 : PC₇₁BM = 1 : 1.5, wt%) was dissolved in 1 ml chlorobenzene mixed with 3% diiodooctane by volume, and stirred over night at 70 °C for complete dissolution. Then, the solution was spin-casted onto ITO/ZnO substrates to form a film of 80 nm thickness. After that, 10 nm MoO₃ and 100 nm Ag layers were deposited sequentially.

Tandem solar cells. The fabrication of glass/ITO/ZnO or ZnO:Cs/P3HT:ICBA/m-PEDOT:PSS substrates is the same as that for the single junction device mentioned above. On the prepared substrates, the ZnO precursor solution, consisting of 20 mg ml⁻¹ zinc acetylacetonate hydrate in anhydrous methanol was spin-coated onto cleaned ITO-coated glass, followed by thermal annealing in air at 90 °C for 3 min (ca. 20 nm). Subsequently, PFN in methanol solution was deposited on ZnO layer using a spin-coating process. The same procedure for the active layer in the single junction device was used for the PTB7 : PC₇₁BM deposition. Finally, the substrates were transferred into the vacuum for deposition of 10 nm MoO₃ and 100 nm Ag layers to complete the tandem device.

Device measurement. The I–V characteristic curve of PSC was recorded on the Keithley source unit under AM 1.5 G 1 sun intensity illumination by a solar simulator from Abet Corp. The intensity of the solar simulator is calibrated by a standard silicon diode, which was certified by National Renewable Energy Lab.

Results and discussion

Electrical properties of ZnO:Cs ETL

The work function of cathode or ETL is critical for device performance of OSCs. The lowered work functions are preferred to reduce the interfacial barriers for the electron extraction. UPS measurement is adopted to examine the work function variations of ZnO films with/without Cs₂CO₃ doping. As shown in Fig. 2a, the pristine ZnO film shows a work function of 4.17 eV. However, with Cs₂CO₃ doping, the work function of ZnO:Cs film is lowered to 3.78 eV. Fig. 2b shows the energy level structures of all the layers in sequence of the device stacking. The work function 4.17 eV of the pristine ZnO film is higher than the LUMO of fullerene derivatives or the acceptors (3.9 eV for PC₆₁BM or PC₇₁BM, and 3.74 eV for ICBA, Fig. 2b), which indicates the possible formation of electron extraction barrier at the ITO cathode. However, the doped ZnO:Cs film shows a much lowered work function of 3.78 eV, which is comparable or even lower than the acceptor LUMO levels. As a result, the efficient electron injection from the ZnO:Cs layer to the acceptor is expected, which would induce electron Ohmic contact. In this regard, the ZnO:Cs film shows the promise for the further improving the device performance of OSCs.

Fig. 2 (a) Ultra-violet photoelectron spectroscopy of the pristine sol–gel ZnO and ZnO:Cs films. (b) Energy level structure of each layer in the single junction organic solar cells.

The conductivity of the ZnO layer, especially along the vertical direction, is critical for efficient charge extraction and high device performance. Here, we investigated the vertical conductivity of ZnO:Cs films by the conducting mode AFM measurement. This technique allows to mapping the conductivity of each pixel on a small area and vertical to the films. The measurement of film conductivity is performed at contact mode, and a bias voltage of minus 2 V is applied. Fig. 3 shows the C-AFM images of 120 nm ZnO (Fig. 3a) and ZnO:Cs films (Fig. 3b) on the glass/ITO substrate. The current intensity of ZnO:Cs film is significantly stronger compared with that of the pristine ZnO film. This result clearly shows that the Cs₂CO₃ doping of sol–gel ZnO would increase the conductivity.

Fig. 3 Conducting atomic force microscope images of (a) the pristine ZnO (120 nm) and (b) the doped ZnO:Cs (120 nm) films.

PV devices

To testify the efficacy of the doped Zn:Cs layer as ETL for photovoltaic devices, we fabricated solar cells based on a variety of photovoltaic materials systems, e.g. P3HT : PC₆₁BM, P3HT:ICBA, and PTB7 : PC₇₁BM, in both single junction and tandem device architectures.

Organic single junction solar cells

The inverted single junction OSC device is used to evaluate the efficacy of the doped ZnO:Cs film as ELT. The device structure of ITO/ZnO or ZnO:Cs/active layer/MoO₃/Ag is presented in Fig. 1b. The intensively studied organic photovoltaic materials, e.g. P3HT : PC₆₁BM, P3HT:ICBA, and PTB7 : PC₇₁BM, were selected as the active layers to testify the versatility of ZnO:Cs as ETL. Fig. 4a shows the I–V characteristics of these OSC devices, and the detailed device parameters are summarized in Table 1. As shown, the P3HT : PC₆₁BM based single junction device with the pristine ZnO as ETL shows a J_SC of 8.61 mA cm⁻², a V_OC of 0.60 V, a FF of 0.628, and a power conversion efficiency (PCE) of 3.25%. With the doped ZnO:Cs layer as ETL, all the device parameters get increased, with a J_SC of 9.53 mA cm⁻², a V_OC of 0.62 V, a FF of 0.649, and a PCE of 3.84%. This constitutes a 18% enhancement in PCE compared with the control device. For the P3HT:ICBA based organic solar cell, the doping of sol–gel ZnO boosts the device PCE from 5.23% to 6.42%, J_SC from 10.32 to 11.41 mA cm⁻², and FF from 0.626 to 0.683. In the PTB7 : PC₇₁BM device, the device FF increases from 0.657 to 0.726, and device PCE increases from 7.01% to 7.89%. The enhancement in the J_SC of all of the single junction devices with the doped ZnO:Cs film could be confirmed by the EQE spectra shown in Fig. 4b and c. As shown, for the P3HT based OSCs, the EQE spectra are enhanced in the whole absorption region. This indicates that the electron extraction at the cathode interface becomes more efficient, which could be attributed to the improved Ohmic contact at the cathode interface. However, for the PTB7 : PC₇₁BM system, the EQE is only slightly enhanced in the PC₇₁BM absorption region, consistent to the slight increase of J_SC (from 14.42 to 14.88 mA cm⁻²). This could be attributed to the suppression exciton quenching in PCBM phase, which is possibly resulted from the faster charge transfer at the interface of ZnO:Cs/PC₇₁BM.

Fig. 4 (a) I–V characteristics of P3HT : PC₆₁BM, PTB7 : PC₇₁BM, and P3HT:ICBA based OSCs with ZnO and ZnO:Cs as ETL. (b) EQE spectra of P3HT based solar cells (P3HT : PC₆₁BM and P3HT:ICBA) with ZnO and ZnO:Cs as ETL, (c) EQE spectra of PTB7 : PC₇₁BM based solar cells with ZnO and ZnO:Cs as ETL.

Table 1 Device parameters of P3HT : PC₆₁BM, PTB7 : PC₇₁BM, and P3HT:ICBA based single junction and tandem photovoltaic devices with ZnO and ZnO:Cs as ETLs

Active layer	ETL	JSC (mA cm^−2)	VOC (V)	FF (/)	Efficiency (%)		Rs (Ω cm^−2)
					Best	Ave.
P3HT : PC61BM	ZnO	8.61	0.60	0.63	3.25	3.14 ± 0.03	5.2
	ZnO:Cs	9.53	0.62	0.65	3.84	3.59 ± 0.06	3.4
P3HT:IC60BA	ZnO	10.32	0.81	0.63	5.23	5.02 ± 0.08	7.2
	ZnO:Cs	11.41	0.81	0.68	6.42	5.89 ± 0.18	4.1
PTB7 : PC71BM	ZnO	14.42	0.73	0.67	7.01	6.84 ± 0.05	1.4
	ZnO:Cs	14.88	0.73	0.73	7.89	7.59 ± 0.11	1.2
Tandem	ZnO	8.14	1.53	0.64	8.03	7.71 ± 0.11	7.7
	ZnO:Cs	8.16	1.52	0.69	8.56	8.02 ± 0.21	4.3

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The general enhancement of device performance based on a variety of popular device system with the doped ZnO:Cs suggests the efficacy and versatility of the ZnO:Cs as an efficient ETL in OSC. We attribute the decent device performance enhancement to the lowered work function and the increased charge transporting with Cs doping, which could be derived from the decreased series resistance (R_s) of the OSC devices based on all the three materials systems upon doping (Table 1).

ZnO:Cs thickness dependent device performance

As mentioned, the interfacial layer design with OSC device performance insensitive to the layer thickness is required to be robust for device manufacturing. In this regard, interfacial layer with high conductivity is favored. One of the most prominent features with the doped ZnO:Cs film is the decent increase in conductivity, as revealed by the C-AFM measurement. Here, we fabricated OSC devices with different thicknesses of ETL to examine the efficacy of ZnO:Cs as the robust ETL. The PTB7 : PC₇₁BM blend is used as the active layer in the devices. Fig. 5 shows the I–V characteristics of the devices with different thicknesses of ZnO:Cs as ELT. For comparison, the OSC devices with different thicknesses of the pristine sol–gel ZnO layer as ETL are also fabricated, with their I–V characteristics shown in Fig. 5a. Due to the optical spacer effect, the J_SC of organic solar cells would vary with the thickness of ZnO layer.²⁸ Also, the thicker ZnO or ZnO:Cs film would induce the severe edge effect due to the increased conductance,⁴³ which resulting in the overestimation of J_SC. Therefore, the origin of J_SC variation with different thicknesses of ZnO or ZnO:Cs film is confusing. It is very difficult to distinguish the doping effect, the optical spacer or the edge effect counting for the variation of J_SC. Besides of the J_SC, the FF is also influenced by and even more sensitive to the interfacial contact at the cathode. Here, we normalized the J_SC for each I–V curve and observed the FF variation with the thickness of ZnO or ZnO:Cs ETL. The variations of FF with the ETL thickness of ZnO or ZnO:Cs are shown in Fig. 5c. For OSCs with the pristine ZnO as ETL, the FF drops dramatically with the ZnO thickness beyond 80 nm, which is corresponding to the concave of I–V curve with increasing the ZnO thickness. Accordingly, the R_s of OSC increases from 1.41 to 4.97 Ω cm⁻², and R_sh dramatically decreases from 1.21 × 10⁵ to 1.79 × 10³ Ω cm⁻² with increasing the ZnO thickness. In the case of the doped ZnO:Cs, the FF increases (0.74) with the thickness coming to 80 nm. Further increase in the ZnO:Cs thickness shows little influence on the FF of the device, where the FF of the device remains 0.67 even the thickness of the ZnO:Cs comes to 520 nm. Correspondingly, the R_s varies from 1.21 to 2.56 Ω cm⁻² and R_sh from 8.72 × 10⁵ to 9.37 × 10⁴ Ω cm⁻² with the thickness of ZnO:Cs increasing from 40 to 520 nm. Fig. 5d shows the I–V characteristics of OSC with 320 nm ZnO or ZnO:Cs as ETL at dark, which directly shows the leakage current is significantly suppressed and forward current enhanced with the doped ZnO:Cs ETL. The insensitivity of the device FF with the thickness of ZnO:Cs indicates the robustness of ZnO:Cs as ETL for application in OSC. These results verify the potential of the doped Zn:Cs film as an efficient and robust ETL for developing highly efficient printable OSCs (Table 2).

Fig. 5 I–V characteristics of PTB7 : PC₇₁BM solar cells with different thicknesses of the pristine ZnO (a) and the doped ZnO:Cs (b) as ETL. (c) Variation of FF with the thickness of the pristine ZnO and the doped ZnO:Cs ETLs. (d) Dark I–V characteristics of PTB7 : PC₇₁BM solar cells with 320 nm thickness of the pristine ZnO or the doped ZnO:Cs as ETL.

Table 2 Device parameters of PTB7 : PC₇₁BM based single junction photovoltaic devices with different thicknesses of the pristine ZnO and the doped ZnO:Cs as ETLs

ETL	Thickness nm	FF (/)	Rs Ω cm^−2	Rsh Ω cm^−2
ZnO	40	0.67	1.41	1.21 × 10^5
	80	0.72	1.11	7.09 × 10^4
	200	0.63	3.52	1.26 × 10^4
	320	0.56	4.97	1.79 × 10^3
ZnO:Cs	40	0.73	1.21	8.72 × 10^5
	80	0.75	0.97	1.12 × 10^6
	200	0.70	1.98	2.58 × 10^5
	320	0.69	2.32	3.46 × 10^5
	520	0.68	2.56	9.37 × 10^4

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Organic tandem solar cells

Series connected tandem architecture has the potential to efficiently utilize the solar irradiance for OSCs, in which the ETL is also very important for high device performance. Here, series connected tandem device architecture (Fig. 1c) is designed to test the efficacy of the ZnO:Cs as ETL. We used the P3HT:ICBA blend as the front cell active layer due to the large band gap (∼2.0 eV) of P3HT and the high V_OC for the thermalization loss reduction, and PTB7 : PC₇₁BM blend as the back cell active layer due to the low band gap (∼1.7 eV) and the potential for transmittance loss alleviation. The UV-visible absorption spectra of P3HT:ICBA and PTB7 : PC₇₁BM films are shown in Fig. 6a. The complementary absorption spectra indicates the suitability of the two systems to compose a series tandem device, where the current density of sub-cells should be matched. The interconnecting layer composed of the modified PEDOT:PSS and ZnO/poly[(9,9-bis(3-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylflorene)] (PFN) multi-layers for the sub-cells connection. The doped ZnO:Cs and MoO₃ were used as ETL and HTL, respectively. The I–V curves of the tandem devices with the pristine ZnO and the doped ZnO:Cs as ETLs are shown in Fig. 6b. After optimization, the best device performance of the tandem solar cells with the doped ZnO:Cs as ETL shows a PCE of 8.56%, a V_OC of 1.53 V, an FF of 0.69, and a J_SC of 8.16 mA cm⁻², which exhibits appreciable enhancement compared to that with the pristine ZnO as ETL. With regard to each individual cell, the PCE of the tandem solar cell is also improved (see Table 1). The V_OC of the tandem solar cell is nearly the summed value of the two sub-cells. Especially, the FF of the tandem device is approaching 70%. These results indicate the efficacy of the doped ZnO:Cs layer as ETL in series connected tandem organic solar cells.

Fig. 6 (a) UV-visible absorption spectra of P3HT:IC₆₀BA and PTB7 : PC₇₁BM films. (b) I–V characteristics of organic tandem solar cells with the pristine ZnO or the doped ZnO:Cs as ETL. The tandem device structure: glass/ITO/ZnO or ZnO:Cs/P3HT:ICBA/PEDOT:PSS/ZnO/PFN/PTB7 : PC₇₁BM/MoO₃/Ag.

Conclusion

In summary, we demonstrate the Cs₂CO₃ doped sol–gel ZnO is an efficient and robust ETL with versatile application for a wide range of organic solar cells, e.g. P3HT : PC₆₁BM, P3HT:ICBA, and PTB7 : PC₇₁BM in both single junction and tandem device structures. With the Cs₂CO₃ doping, the work function of the sol–gel ZnO is lowered, and the conductivity is increased significantly, which reduces the extraction barrier. Remarkably, the device performance shows marginal drop with the thickness of the doped ZnO:Cs ETL ranging from 40 to 520 nm, indicating its robustness for roll to roll integration. We stress the importance of chemical doping method for developing highly efficient interfacial layer, and the potential of the doped ZnO:Cs film as efficient and robust ETL for device optimization and future commercialization.

Acknowledgements

This work was supported by the Major State Basic Research Development Program (2014CB643503), the National Natural Science Foundation of China (Grants 91233114 and 51261130582), the Postdoctoral Science Foundation of China (2015M570505), and the program for Innovative Research Team in University of Ministry of Education of China (IRT13R54).

References

1. J. Peet, M. L. Senatore, A. J. Heeger and G. C. Bazan, Adv. Mater., 2009, 21, 1521–1527.
2. B. C. Thompson and J. M. J. Fréchet, Angew. Chem., Int. Ed., 2008, 47, 58–77.
3. M. Gratzel, Nat. Mater., 2014, 13, 838–842.
4. T. D. Nielsen, C. Cruickshank, S. Foged, J. Thorsen and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2010, 94, 1553–1571.
5. H. Spanggaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2004, 83, 125–146.
6. F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2009, 93, 465–475.
7. F. C. Krebs, T. Tromholt and M. Jorgensen, Nanoscale, 2010, 2, 873–886.
8. Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591–595.
9. J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun., 2013, 4, 1446.
10. L.-J. Zuo, X.-L. Hu, T. Ye, T. R. Andersen, H.-Y. Li, M.-M. Shi, M. Xu, J. Ling, Q. Zheng, J.-T. Xu, E. Bundgaard, F. C. Krebs and H.-Z. Chen, J. Phys. Chem. C, 2012, 116, 16893–16900.
11. Y. Hirose, A. Kahn, V. Aristov, P. Soukiassian, V. Bulovic and S. R. Forrest, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 13748–13758.
12. S. R. Cowan, P. Schulz, A. J. Giordano, A. Garcia, B. A. MacLeod, S. R. Marder, A. Kahn, D. S. Ginley, E. L. Ratcliff and D. C. Olson, Adv. Funct. Mater., 2014, 24, 4671–4680.
13. R. A. Street, M. Schoendorf, A. Roy and J. H. Lee, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 205307.
14. Z.-K. Tan, K. Johnson, Y. Vaynzof, A. A. Bakulin, L.-L. Chua, P. K. H. Ho and R. H. Friend, Adv. Mater., 2013, 25, 4131–4138.
15. L. Zuo, J. Yao, H. Li and H. Chen, Sol. Energy Mater. Sol. Cells, 2014, 122, 88–93.
16. W. Tress, K. Leo and M. Riede, Adv. Funct. Mater., 2011, 21, 2140–2149.
17. R. Po, C. Carbonera, A. Bernardi and N. Camaioni, Energy Environ. Sci., 2011, 4, 285–310.
18. R. Steim, F. R. Kogler and C. J. Brabec, J. Mater. Chem., 2010, 20, 2499–2512.
19. E. D. Gomez and Y.-L. Loo, J. Mater. Chem., 2010, 20, 6604–6611.
20. L.-M. Chen, Z. Xu, Z. Hong and Y. Yang, J. Mater. Chem., 2010, 20, 2575–2598.
21. E. L. Ratcliff, B. Zacher and N. R. Armstrong, J. Phys. Chem. Lett., 2011, 2, 1337–1350.
22. J. Huang, P. F. Miller, J. S. Wilson, A. J. de Mello, J. C. de Mello and D. D. C. Bradley, Adv. Funct. Mater., 2005, 15, 290–296.
23. H. Gommans, B. Verreet, B. P. Rand, R. Muller, J. Poortmans, P. Heremans and J. Genoe, Adv. Funct. Mater., 2008, 18, 3686–3691.
24. P. Peumans and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 126–128.
25. M. F. Lo, T. W. Ng, S. L. Lai, F. L. Wong, M. K. Fung, S. T. Lee and C. S. Lee, Appl. Phys. Lett., 2010, 97, 143304.
26. C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci and P. Denk, Appl. Phys. Lett., 2002, 80, 1288–1290.
27. Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter and A. J. Heeger, Adv. Mater., 2011, 23, 1679–1683.
28. 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, 297–302.
29. Y. Gao, H.-L. Yip, K.-S. Chen, K. M. O'Malley, O. Acton, Y. Sun, G. Ting, H. Chen and A. K. Y. Jen, Adv. Mater., 2011, 23, 1903–1908.
30. G. Parthasarathy, C. Shen, A. Kahn and S. R. Forrest, J. Appl. Phys., 2001, 89, 4986–4992.
31. M. Rusu, S. Wiesner, I. Lauermann, C.-H. Fischer, K. Fostiropoulos, J. N. Audinot, Y. Fleming and M. C. Lux-Steiner, Appl. Phys. Lett., 2010, 97, 073504.
32. L. Motiei, Y. Yao, J. Choudhury, H. Yan, T. J. Marks, M. E. v. d. Boom and A. Facchetti, J. Am. Chem. Soc., 2010, 132, 12528–12530.
33. L. Zuo, X. Jiang, M. Xu, L. Yang, Y. Nan, Q. Yan and H. Chen, Sol. Energy Mater. Sol. Cells, 2011, 95, 2664–2669.
34. Y. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A. J. Giordano, H. Li, P. Winget, T. Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W. Haske, E. Najafabadi, T. M. Khan, H. Sojoudi, S. Barlow, S. Graham, J.-L. Brédas, S. R. Marder, A. Kahn and B. Kippelen, Science, 2012, 336, 327–332.
35. H. Wang, E. D. Gomez, Z. Guan, C. Jaye, M. F. Toney, D. A. Fischer, A. Kahn and Y.-L. Loo, J. Phys. Chem. C, 2013, 117, 20474–20484.
36. R. Søndergaard, M. Hösel, D. Angmo, T. T. Larsen-Olsen and F. C. Krebs, Mater. Today, 2012, 15, 36–49.
37. F. C. Krebs, R. Søndergaard and M. Jørgensen, Sol. Energy Mater. Sol. Cells, 2011, 95, 1348–1353.
38. D. Gao, M. G. Helander, Z.-B. Wang, D. P. Puzzo, M. T. Greiner and Z.-H. Lu, Adv. Mater., 2010, 22, 5404–5408.
39. Z. Lu, J. Zhou, A. Wang, N. Wang and X. Yang, J. Mater. Chem., 2011, 21, 4161–4167.
40. S. Kwon, K.-G. Lim, M. Shim, H. C. Moon, J. Park, G. Jeon, J. Shin, K. Cho, T.-W. Lee and J. K. Kim, J. Mater. Chem. A, 2013, 1, 11802–11808.
41. Z. Zulkifli, M. Subramanian, T. Tsuchiya, M. S. Rosmi, P. Ghosh, G. Kalita and M. Tanemura, RSC Adv., 2014, 4, 64763–64770.
42. Z. Zulkifli, G. Kalita and M. Tanemura, Phys. Status Solidi RRL, 2015, 9, 145–148.
43. D. Gupta, M. Bag and K. S. Narayan, Appl. Phys. Lett., 2008, 93, 163301.
44. L. Zuo, C.-C. Chueh, Y.-X. Xu, K.-S. Chen, Y. Zang, C.-Z. Li, H. Chen and A. K. Y. Jen, Adv. Mater., 2014, 26, 6778–6784.

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