Dual Roles of the Fullerene Interlayer on Light Harvesting and

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Dual Roles of Fullerene Interlayer on Light Harvesting and Electron Transfer for Highly Efficient Polymer Solar Cells Zhiqi Li, Chunyu Liu, Zhihui Zhang, Xinyuan Zhang, Wenbin Guo, Shengping Ruan, Liu Zhang, and Yongbing Long J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00412 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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The Journal of Physical Chemistry

Dual Roles of Fullerene Interlayer on Light Harvesting and Electron Transfer for Highly Efficient Polymer Solar Cells Zhiqi Li,1 Chunyu Liu,1 Zhihui Zhang,1 Xinyuan Zhang,1 Wenbin Guo,*1 Shengping Ruan1, Liu Zhang,*2, and Yongbing Long3 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering , Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2

College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Street, Changchun 130061, People’s Republic of China

3

School of Electronic Engineering, South China Agricultural University, Guangzhou, 510642, China

Abstract There are two critical strategies to achieve the high photoelectric conversion efficiency of polymer photovoltaic cells: broadening the light absorption spectra of the active layer and efficiently splitting photo-generated excitons with low probability of recombination. Herein, we demonstrate the improved light trapping and reduced photo-generated excitons recombination probability of the inverted heterojunction solar cells by incorporating C60 fullerene modification. This approach ameliorates traditional multi-blend/layer systems, and allows multiple acceptor materials to synergistically work. After fullerene modification layer was incorporated, the optimal device presents a 25.5% improvement of power conversion efficiency (PCE) up to 9.458 % with an open circuit voltage (Voc) of 0.8 V, a short-circuit current (Jsc) of 18.575 mA/cm2, and a fill factor (FF) of 63.4%. This study provides a novel inspiration for the structure development of high-efficiency photovoltaic devices.

1. Introduction The bulk heterojunction (BHJ) polymer solar cells (PSCs) based upon conjugated polymer and fullerene derivative have been widely investigated as a potential unit for portable, cost-efficient, and large-area of mechanization energy conversion devices.1-6 Among them, the inverted structure of PSCs, which compose of a BHJ photoactive layer between indium tin oxide (ITO) as bottom cathode and high work-function metal (Ag, Al or Au) as a top anode, is a promising strategy to achieve printability and long-term device stability.7-9 This construction creates a larger functional interface for sufficient charge separation between donor and acceptor phases, and efficient excitons dissociation occurs at this interface due to an abrupt potential change (strong interfacial electric fields) caused by different excited-state energy levels of two materials.10,11 Extraordinarily, inverted PSCs can invent vertical phase separation and concentration gradient between different photoactive phases, which allows high extraction efficiency.12-14 To increase the photoelectric conversion efficiency of inverted PSCs, many efforts have been expended on some mainly fundamental issues for broadening light harvesting and increasing the quantum yields of incident photons such as development of high-performance materials,15,16 optimization of morphology by advanced processing methods or addition,17-21 molecular 1

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or quantum dot doping,22-24 and optimization of electrical contact at different interfaces by selfassembled monolayer.25,26 Still, considering the viable practical commercialization of this burgeoning photovoltaic technology, further improvements of the efficiency up to the threshold for commercial applications are urgently required. A common way towards high efficiency is obtaining strong absorption of sunlight as well as efficient charge separation and transport without recombination in the photoactive interpenetrating networks.27-31 Thus, many groups attempted to explore ternary-blend PSCs (Low band gap material or small molecule doping) which utilize a third material to either extend the solar spectrum or control the phase separation to decrease charge carrier recombination.22-34 Unfortunately, the third additives into the photoactive layer, such as porphyrin-based dyes,35 even have a negative impact on device performance duo to accessional defects, trap and/or detrimental effects on blend morphology, which results in a large quantity of recombination centers in the photoactive layer. Additionally, an ideally bicontinuous interpenetration network in ternary-blend is difficult to control, which also causes a high energy loss because of charge recombination. Moreover, the large energy barrier between the conduction bands of the organic layer and TiO2/ZnO results in unbalanced charge injection and transport and inefficient electron-hole recombination. Poor electrical contact between the hydrophobic organic layer and the hydrophilic metal oxide also leads to low fill factor (FF) and power conversion efficiency (PCE) of the devices.36-38 Therefore, the design of functional charge transport layers to improve electrical contact between electrode and polymer active layers and decrease inefficient electron-hole recombination is highly essential and desirable for high-performance PSCs. In this study, we describe an efficient interface modification for BHJ PSCs by incorporating fullerene (C60) interlayer between poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6diyl]

[3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7):[6,6]-phenyl-C71-

butyric acid (PC71BM) active layer and TiO2 buffer layer to improve the photon absorption and decrease charge carrier recombination. Unlike common multi-blend systems, PTB7:PC71BM/C60-based systems enable the effective utilization of multiple acceptors without additional detrimental effect on blend layer, thereby leading to the improvements of light absorption and energy conversion. It is also found that the C60 can act as sticky agent between photoactive layer and buffer layer, which could efficiently increase interfacial contact of different films and boosted the formation of perpendicular phase separation of binary mixed polymer on a surface immersing in solvents. In addition, the photoluminescence spectra on these blends shows decreased carrier recombination occurred among whole photoactive film due to the improved morphology and matched energy levels alignment between the PTB7 emission and PC71BM/C60 absorption. Both processes enhanced the short-circuit current density (Jsc) and fill factor (FF) for PSCs, resulting in an improved PCE from 7.535 % up to 9.458 %.

2. Results and Discussion In this section, we present the results of improved performance for PSCs with C60 interlayer including photovoltaic performance, nanomorphology analysis, optical characterizations, and photophysics mechanism. The device configuration is Glass/ITO/TiO2/C60/active-layer/MoO3/Ag, which is shown in Figure 1a. Also, the control devices with the structure of Glass/ITO/TiO2/activelayer/MoO3/Ag were fabricated for comparison. All the details about the device fabrication and characterization are described in the experimental section. 2


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Figure 1 (a) the device structure of PSCs, (b) the current density–voltage (J–V) characteristics of PSCs with different thickness of C60 interlayer.

2.1 Photovoltaic Performance Figure 1b indicates the current density–voltage (J–V) characteristics of reference and optimized devices under one sun illumination simulated AM 1.5G irradiation at 100 mW/cm2. It can be seen that the control devices based on structure of ITO/TiO2/PTB7:PC71BM/MoO3/Ag have a typical PCE of 7.535 %, accompanying a short-circuit current density (Jsc) of 15.824 mA/cm2, an open-circuit voltage (Voc) of 0.80 V, and a fill factor (FF) of 59.33 %. Compared to the control device, the incorporated C60 layer between TiO2 and PTB7:PC71BM resulted in the enhancement of Jsc and FF values. At a typical thickness of C60 layer (3 nm), the modified devices owned better performance due to the enhanced Jsc and FF, but Jsc began to decrease at a bigger thickness (more than 15 nm). In particular, the optimum devices with C60 thickness of 15 nm yielded a very promising PCE of 9.458 % including a Jsc of 18.575 mA/cm2, a Voc of 0.8 V, and a FF of 63.40 %. A summary of the average device performance parameters extracted from over 32 devices are listed in Table 1, and the corresponding thickness of C60 films for optimized devices are 3, 10, 15, and 20 nm, respectively. Table 1. Stabilized device parameters of control and optimized PSCs with various thickness of C60 layer, which are average values of 32 individual devices.

C60 thickness

PCEbest (PCEave ± error)

Voc (V)

Jsc (mA/cm2)

FF (%)

Rs (ohm/mm2)

0

7.535 (7.527±0.008) %

0.80

15.824

59.33

21.65

3 nm

8.383 (8.377±0.006) %

0.80

16.982

61.42

15.57

10 nm

9.048 (9.041±0.005) %

0.80

17.982

62.59

15.22

15 nm

9.458 (9.451±0.004) %

0.80

18.575

63.40

13.55

20 nm

9.050 (9.042±0.006) %

0.80

18.088

62.54

17.05

2.2 Study of Film Morphology

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Figure 2 AFM images of TiO2 (a and b) and C60 coated TiO2 (c and d with the thickness of 15 nm, e and f with the thickness 20 nm) films. In order to investigate the role of C60 interlayer on the improved device performance, we measured the atomic force microscopy (AFM) high images of TiO2 film (Figure 2a and b) and C60 coated TiO2 (Figure 2c and d with 15 nm; Figure 2e and f with 20 nm) films by tapping-mode. It is noteworthy that the slightly increased surface roughness from 1.21 nm (Figure 2a) to 1.56 nm (Figure 2c) after coating the C60 layer onto TiO2 was observed. In addition, as shown in the AFM 3D images (Figure 2b and 2d), it can be found an obvious difference of the surface morphology without and with coated C60 layer on the TiO2 film. Remarkably, the TiO2 film show unshaped lattice grain morphology, while the TiO2/ C60 films shows hilly lattice grain morphology. Meanwhile, the composite TiO2/C60 film exhibits less-aligned grains on the surface compared to pristine TiO2 layer. The sizes of these aggregates are very small, which is assigned to small aggregated fullerene crystallites, and the layer is therefore rugged. The improved roughness of interlayer and smaller particles might improve the physical contact between the TiO2 (C60) layer and the active layer, leading to the decreased series resistance (Rs).39,40 However, the surface roughness increased to be 1.92 nm (Figure 2e) compared to 4


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previous TiO2 film when the C60 thickness is more than 15 nm. As shown in Figure 2e (C60 thickness in 20 nm), the morphology difference becomes less prominent between the TiO2 surface and the C60 thick film coated TiO2 surface because of the large macromolecules stacking structure of C60 crystallite grains compared to the smoother surface of the thin interlayer. The exaggerated surface roughness of C60 gave a cross-linking of polymerisation, which caused a bad interfacial contact with active layer, leading to an increased interfacial charge recombination probability. Meanwhile, the rough layer increases the interface defects, which could also cause charge carrier recombination of the interface of the C60, hence a bad performance of device is obtained.41

Figure 3 Water contact angles of (a) TiO2, (b) UV-treated TiO2, (c) ITO, and (d) C60-coated TiO2 surface. Interestingly, it was found that the spreading property of the PC71BM chlorobenzene solutions on C60 surface was better than bare TiO2 and UV treated TiO2 films, which reflects different wetting or coating character for the active layers (blend of PTB7:PC71BM) on the C60-coating TiO2 layers. To prove this conjecture, the contact angles of the nano-films with de-ionized water (Figure 3) were measured. As presented in Figure 3, the C60 surface is more hydrophobic than the pristine TiO2 surface, since the contact angle changes from 14 ° for TiO2 to 87 ° for C60. Furthermore, the surfaces of TiO2 (14 °) and UV-treated TiO2 (11 °) are not conductive to the spin of hydrophobic PC71BM compared to ITO (31.5o), which is adverse of the formation of perpendicular phase separation of binary mixed polymer on a surface immersing in solvents. Incorporation of C60 generates hilly lattice grain morphology, which enhanced surface affinity due to increased roughness.42 All of these account for the different micro-morphology of different thin films.

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Figure 4 Tapping-mode AFM phase image of PTB7:PC71BM (a) without and (b) with C60 layer, the mechanism of phase separation and gelation process (c) without and (d) with C60 layer. We also prepared sample of PTB7:PC71BM solution on TiO2 and C60 coating TiO2 to perform AFM morphology measurements, and the film preparation condition for AFM test was the same as that for device fabrication so as to guarantee the reliability of comparison. As shown in Figure 4, the PTB7: PC71BM blend on C60 (Figure 4b) seems to be strongly affected and indication of a well-proportioned phase separation is visible except for some increased roughness. Taking into consideration that slightly higher contact angles were observed for the C60-coating TiO2 and ITO layers, it can be inferred that the surface of C60 film is easily covered with the fullerene derivative. Therefore, these surface examinations confirm that the presence of the C60 layer have an energetic impact on the active layer coating because there is some mutual induction. Figure 4c and 4d present schematic diagram of the phase separation of photoactive layer on C60-coating TiO2 and TiO2 films in ternary BHJ blends. Hydrophobic PC71BM attached more easily on C60 sample than on primary ones, which tends to settle onto the surface of the coatings and there are lots of micro-cracks in them.43 Hence, the employment of C60 makes is conductive to form fullerene-rich functional group on tegmental area, which facilitated the formation of perpendicular phase separation of binary mixed photoactive polymer on the surface immersing of solvents. Therefore, the highly ordered and homogeneous interpenetrating crystalline domains are obvious in the blend film of PTB7: PC71BM on the top of C60 (Figure 4d), which could not be found in the film without C60 (Figure 4c), resulting in the enhanced phase separation in ternary BHJ blends.44-46 Additionally, the achieved

interpenetrating network would create a larger functional

interface between donor and acceptor phases for sufficient charge separation, thus efficient exciton dissociation occurs at this interface, which would effectively promote the excitons separation and 6


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transfer. Extraordinarily, this symmetrical interpenetrating photoactive layer could reduce the defects and traps introduced by particles aggregating, leading to an efficient charge carrier transmission. 47

2.3 Optical and Photophysical Characteristics of PSCs

Figure 5 (a) The photoluminescence spectra of PTB7-only, PTB7/C60, and PTB7:PC71BM films, (b) PTB7:PC71BM photoluminescence spectra on different thickness of C60 films. To deeply explore the hypothesis of decreased carrier recombination occurring among whole photoactive film, we tested the photoluminescence spectra of pristine PTB7, PTB7/C60, PTB7: PC71BM, PTB7: PC71BM /C60 films excited at 329 nm (Figure 5a). A excitation of PTB7 film can be seen, resulting from the maximized excitons recombination within itself. On the contrary, a very weak emission is detected for PTB7: PC71BM film as the direct excitons recombination is not occurring and photo-excited electron transfers from the PTB7 to PC71BM through the symmetrical interpenetrating network. Moreover, the decreased excitation spectra of composite PTB7: PC71BM /C60 film confirmed that incorporation of C60 decreased molecule recombination occurred inside active films, which contributed to the improved photocurrent.48 Figure 5b displays the photoluminescence spectra of PTB7:PC71BM active layer on the C60 films with different thickness. The thick PTB7/C60 film shows a high excitation. When the C60 was coated on the TiO2, the emission from the composite films decreased, resulting from the decreased recombination. After the C60 thickness increases to 15 nm, the emission from the composite films gives an optimal decrease, because hilly lattice grain morphology leads to an excellent phase separation of active layer. However, the emission intensity continuously rises at around 850 nm when the C60 thickness is more than 15 nm. Figure A shows that C60 gives the emission peaks at around 700 nm and 850 nm, while PTB7 gives a peak at 750 nm. Hence, it is reasonable to think that this decrease is resulting from the bad current transport and recombination occurring inside too thick C60 films, which is consistent with the decreased device performance.

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Figure 6 (a) The absorption spectrum of PSCs with different thickness of C60 films, (b) the comparison of calculated and experimental light absorption spectrum, (c) IPCE of control and optimized device with different thickness of C60 layers, (d) the charge transfer mechanism between active layer and C60 layer. The absorption spectrum of control and optimized devices with various thickness of C60 layer is shown in Figure 6a, the device with a C60 modification shows a holistic absorption enhancement in the region of 350-750 nm with two peaks at 640 nm and 710 nm. The visible light harvesting of the devices was getting higher with the increase of film thickness. To further understand the reason of improved optical absorption, we measured the absorption of PTB7: PC71BM, bare C60 (15 nm), and PTB7: PC71BM /C60 (15 nm) films. Also, the calculated optical absorption of PTB7: PC71BM /C60 (15 nm) film (imaginary line) is exhibited in Figure 6b. It is worth noting that optical absorption of experimental measurement is higher than the theory calculation, which may originate from two respects: (1) light absorption of the C60 film itself; (2) optical electric field distribution regulation of device induced by incorporated C60 film. In particular, the experimental absorption indicates a high degree of ordered enhancement from 550 nm to 750 nm compared to theoretical data, which corresponds to distinct enhancements from 550 nm to 750 nm in the incident photon-to-electron conversion efficiency (IPCE) spectrum (Figure 6c). We calculated the photocurrents using integrals of IPCE spectrum to quantify the improvement in IPCE. The photocurrent in the 300–780 nm regions increased from 14.95 mA /cm2 at 0 nm to 15.96 mA/cm2 at 3 nm, 17.13 mA/cm2 at 10 nm, 17.87 mA/cm2 at 15 nm, and then gradually decreased at 17.19 mA/cm2 at 20 nm. The IPCE spectra is consistent with Jsc values of all the devices. The reason for the enhancement of IPCE can be ascribed into three parts: (1) improved absorption of photoactive layer, which is calculated by the difference 8


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between experimental current and theoretical values in the absorption spectra, revealing efficient harvesting of the photons by optical electric field distribution control; (2) decreased recombination inside photoactive layer due to the improvement of phase separation; (3) intramolecular energy transfer between PTB7 and C60.

Figure 7 (a) The emission spectrum of PTB7 overlapped the absorption of C60, (b) charge transport process inside PSCs, (c) Energy levels alignment of all materials used in this study. As shown in Figure 7a, the emission spectrum of PTB7 overlapped the absorption of C60 in the range of 300-700 nm. In this case, C60 works as an electron acceptor and efficient exciton dissociation occurs at C60/PTB7 interface (shown in Figure 6d and 7b). For the reference device, the photocurrent is generated by excitons in PTB7 and then dissociated at the PTB7:PC71BM interfaces. Hence, the increased IPCE in the range of 350-500 nm is attributed to the additional excitons dissociated into free charges in PTB7/C60 interface; (4) improved charge transfer channel, the lowest unoccupied molecular orbital (LUMO) of C60 is -4.1 eV, which is slightly lower than LUMO of PC71BM.49 Considering the perpendicular phase separation of binary mixed polymer for inverted PSCs, PC71BM was thought to directly contact with TiO2, so we measured the work-function of TiO2 and PC71BM. Figure 7c shows the energy level of TiO2 and PC71BM without and with C60. There is a little difference between the work-function of TiO2 and PC71BM, hence, when pristine TiO2 layer contacts with PC71BM, unified Fermi level will be established and the energy bands of PC71BM will be bended, leading to an apparent electron barrier occurring in the side of PC71BM (middle of Figure 7c), which blocks the electron transport from PC71BM to TiO2. While the TiO2/C60 contacts with PC71BM (right of Figure 7c), the interface electron barrier in PC71BM will decrease, removing the restriction on the outward transfer of electrons, which promotes electron transport from PC71BM to TiO2. In other words, when electrons jump from the ground-state to the excited-state, they tend to shift to the conduction band of TiO2 through the LUMO of C60 because an electron could trend to jump into one of the states with a small barrier height. Therefore, it is possible that C60 serves as electron transfer medium and the photo9



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excited electron would be injected into cathode and collected by the external circuit (Figure 7). Meanwhile, the highest occupied molecular orbital (HOMO) of C60 is -6.4 eV, which makes it play a role on hole blocking. To further probe the mechanism of boosted charge transfer, the electron-only devices with the structure of ITO/TiO2/C60/PTB7:PC71BM/BCP/Ag were fabricated and J−V characteristics were examined in a bias voltage range of 0−8 V (Figure 8a) for control and C60-coating (15 nm) devices. Upon applying a positive bias Ag anode, electrons are injected into PTB7 and the space charge limited current (SCLC) occurs, and the mobility can be extracted from the Mott−Gurney equation:

8d 3 µ= 9ε 0ε r

2

 J    V  Here µ is the carrier mobility, d is the photoactive layer thickness, and ε0  a  ,

(8.85×10−12 F/m) and εr (equal to 3 for organic materials) are the permittivity of vacuum and relative permittivity, respectively. device is 5.35×10

−5

50

According to the above equation, the electron mobility of the control

2

cm /(Vs). After incorporating C60 interlayer, the electron mobility is gradually

increased to 8.14×10−5 cm2/(Vs), which indicates that the electron transfer capacity was improved. Figure 8b shows the dark current versus bias voltage characteristics of all PSCs in this paper. For the optimized devices, the dark current at reverse bias and leakage current at zero bias are obviously reduced, which improves the diode rectifying ratios. The decreased dark current unravels a smaller series resistance (Rs) whcih could facilitate charge transport and then increase FF. However, the dark current in the forward bias region rapidly grows, which indicates that C60 modification does facilitate electron transport and lead to a decrease of detrimental charge recombination.

Figure 8 (a) J−V characteristics of electron-only device in a bias voltage range of 0−8 V, (b) the dark current versus bias voltage characteristics of the devices.

3. Conclusion In summary, we have successfully investigated high-efficiency BHJ PSCs comprising PTB7 and PC71BM by incorporating a C60 fullerene modification between cathode buffer layer and photoactive layer. Absorption spectroscopic study of devices reveals that C60 interlayer could efficiently adjust the optical electric field distribution regulation of whole devices and improve the optical trapping of photoactive film. Surface morphology analysis and contact angles measurement of TiO2/C60 film 10


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prove that C60 interlayer improves the coverage of active layer onto TiO2 buffer layer, which is beneficial for the interfacial contact between active layer and Ag cathode, and consequently the electron transfer from active layer to Ag cathode is boosted. Additionally, C60-coating layer could also prohibit electron-hole recombination of PTB7:PC71BM interpenetrating network and the interface between the active layer and TiO2 buffer layer, resulting in dramatically increased electron extraction efficiency.

4. Experimental Section 4.1 Device Fabrication The indium tin oxide (ITO) glass substrate with a sheet resistance of 12 Ω/sq was purchased from Dongguan Xiangcheng Group, China. Electron donor material of PTB7 was obtained from 1-Material Inc. and used as received. Electron acceptor material of PC71BM was purchased from Lumtek Corp. without purification. Electron transport layer material of TiO2 was synthesized by the sol-gel method in our lab following the procedure reported in ref.51-54 The inverted device structure was ITO/TiO2/C60(0 nm, 3 nm, 15nm, 20 nm)/PTB7: PC71BM/MoO3/Ag as described in our previous report. Fistly, The ITO-coated glass substrates were ultrasonicated in degegent, deionized water, acetone, and isopropanol for 15 min every time, and subsequently dried in an oven for 4 h. They were cleaned in a UV ozone oven for 15 min prior to use. Then, a 40 nm TiO2 thin layer was coated on top of the precleaned ITO substrate by spin-casting at 3,000 rotations per min (rpm) for 20 s. Before deposition of the photoactive layer, a x nm (x= 3 nm, 10 nm, 15 nm, 20 nm) C60 layer were evaporated on TiO2 film. Thirdly, the PTB7:PC71BM blend active layer solution (a total concentration of 25 mg mL−1) with a nominal thickness of ∼100 nm was prepared by spin-coating a mixed solvent of chlorobenzene/1,8diiodoctane (97%:3% by volume) solution at 1,000 rpm for 2 min. Finally, the device was transferred into a vacuum chamber ( ～ 10-5 Torr), and a 4 nm MoO3 layer and a 100 nm Ag layer were subsequently evaporated through a shadow mask to define the active area of the devices (6.4 mm2) and form the top anode. The complete devices were transferred into air for all the tests.

4.2 Measurements and Characterization After fabrication, the PCE was extracted from J–V curve measurements (using a Keithley 2,400 sourcemeter) under a 1 sun, AM 1.5G spectrum from a solar simulator (Oriel model 91,192; 1,000 W m−2). The illumination intensity was determined by a NREL-calibrated silicon solar cell with KG-5 color filter. All electrical measurements were carried out in air at room temperature. The active area of the device irradiated by the light was defined as 6.4 mm2 by using a photomask (2 mm×3.4 mm), so no extra current outside the defined area was collected. Current density–voltage (J–V) curves were measured with a Keithley 2601 source measurement unit. The incident photon-to-current conversion efficiency (IPCE) measurements were performed in air using a PV Measurements QEX7 system. The morphologies of the TiO2 and C60 films were characterized by atomic force microscopy (AFM) in tapping mode (Dimension Icon Scanasyst). The thicknesses of the active layer and evaporated layers were recorded with a Woollam M2000UI Spectroscopic Ellipsometer and verified by AFM. Water contact angles of different substrates were measured using Contact Angle Tester. 11



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Author Information *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

Acknowledgements The authors are grateful to National Natural Science Foundation of China (61370046, 11574110), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10), Project of Graduate Innovation Fund of Jilin University (2016090), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No.2014A030306005), Foundation for High-level Talents in Higher Education of Guangdong Province, China (Yue Cai-Jiao [2013]246, Jiang Cai-Jiao[2014]10) for the support to the work.

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Table of Contents

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Dual Roles of the Fullerene Interlayer on Light Harvesting and

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