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Strong Enhancement of Photoelectric Conversion Efficiency of Co-hybridized Polymer Solar Cell by Silver Nanoplates and Core-shell Nanoparticles Wenfei Shen, Jianguo Tang, Yao Wang, Jixian Liu, Linjun Huang, Weichao Chen, Lanlan Yang, Wei Wang, Yanxin Wang, Renqiang Yang, Jungheum Yun, and Laurence A. Belfiore ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13671 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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ACS Applied Materials & Interfaces

Strong Enhancement of Photoelectric Conversion Efficiency of Co-hybridized Polymer Solar Cell by Silver Nanoplates and Core-shell Nanoparticles Wenfei Shen,† Jianguo Tang,*† Yao Wang,† Jixian Liu,† Linjun Huang,† Weichao Chen,‡ Lanlan Yang,† Wei Wang, † Yanxin Wang, † Renqiang Yang,*‡ Jungheum Yun,∥ Laurence A. Belfiore*†§ †

Academy of Hybrid Materials, National Base of International Sci. & Tech. Cooperation on Hybrid Materials, Qingdao University, 308 Ningxia Road, Qingdao 266071, P. R. China

‡

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Acadamy of Sciences, 189 Songling Road, Qingdao, 266101, P.R.China

§

Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523, USA

∥

Surface Technology Division, Korea Institute of Materials Science, Changwon, Gyeongnam,

641-831, Republic of Korea. * Corresponding author: Jianguo Tang ([email protected]); Tel. : +86 532 85951519

Fax: +86 532 85951519

Renqiang Yang ([email protected]) Tel.: +86 532 80662700

Fax: +86 532 80662778

Laurence A. Belfiore ([email protected]) Tel.: +01 970 4915395 Fax: +01 970 4915395

ABSTRACT A new way was meticulously designed to utilize the localized surface plasmon resonance (LSPR) effect and the light scattering effect of silver nanoplate (Ag-nPl) and core-shell Ag@SiO2 nanoparticles (Ag@SiO2-NPs) to enhance the photovoltaic performances of polymer solar cells (PSCs). To prevent direct contact between silver

ACS Paragon Plus Environment


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nanoparticles (Ag-NPs) and photo-active materials which will cause electrons quenching, bare Ag-nPl were spin-coated on indium tin oxide and silica capsulated Ag-NPs were incorporated to PBDTTT-C-T:PC71BM active layer. As a result, the devices incorporated with Ag-nPl and Ag@SiO2-NPs showed great enhancements. With the dual effects of Ag-nPl and Ag@SiO2-NPs in devices, all wavelength sensitization in visible range was realized, therefore, the power conversion efficiency (PCE) of PSCs showed a great enhancement of 14.0% to 8.46%, with an increased short circuit current density of 17.23 mA/cm2. The improved photovoltaic performances of devices were ascribed the LSPR effect and the light scattering effect of metallic nanoparticles. Apart from optical effects, the charge collection efficiency of PSCs was improved after the incorporation of Ag-nPl.

Key Words: efficient polymer solar cells, metallic nanoparticles, LSPR effect, light scattering effect, avoid excitons quenching.

INTRODUCTION Bulk hetero-junction (BHJ) polymer solar cells (PSCs) have gained tremendous developments in the last decade for their virtues, including light-weight, flexibility, low-cost and easy fabrication processes, compared to inorganic solar cells1-5. To date, with the utilizing of novel low band gap materials and advanced processing technologies, the highest power conversion efficiency (PCE) of PSCs have been over 10%6-10, which is still far lower than the value of inorganic solar cells11 and commercialization requirements. One factor that restricts the photovoltaic


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performance of PSC is the weak light absorption1-2, 10, 12. Although the light absorption can be improved by increasing the thickness of the active layer11, 13, the thick active layer will decrease the extraction efficiencies of electrons and holes11, 14 and thus restricts the PCE of PSC. To dissolve this issue, tandem solar cells6-7, 15 and ternary solar cells16-19 utilizing complementary light absorption donor materials were becoming effective strategies to broaden the light absorption range of the device. However, the fabrication processes of the tandem solar cell are very complicate, and for the ternary devices the selecting of two donors needs to be very rigorous, because a large portion of ternary blends cause excitons quenching. On the contrary, a more simple and universal strategy can be utilized to enhance the light absorption of the PSCs, which is using the metallic nanoparticles20-22 (MNPs). The MNPs can act as the light traps to harvest light energy through localized surface plasmon resonance (LSPR) effect23-25, and will transfer the energy to active layer materials in their nearest vicinity. With this LSPR effect, the electromagnetic field of light is amplified near the surface of MNPs

14, 21

. Very

importantly, the LSPR effect of MNPs can be effectively tuned by sizes, shapes and crystalline structures, as well as the ambient medium where the metallic nanoparticles exist25. In addition, the MNPs can act as the light scattering center, which will enhance the optical path, thus enhance the light absorption of the PSCs. Therefore, MNPs are expected to be an nano-additive in the PSCs to improve the light absorption and to improve the photovoltaic performance23, 26-29.



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Currently, the published research works concerned mostly about single shape of MNPs and added into different layers of PSC devices, such as hole transport layer20, 30-34

, electron transport layer21, 35 or active layer13-14, 22, 36. In this contribution, we

creatively improved the single shape method into dual-shape hybrid strategy, to realize the adjustment of PSC efficiency through shape effect of metallic nano-additives. The combination of the light absorbance with different shape nano-metallic additives is much effective to enhance the light harvesting and photovoltaic performances of PSCs. As we have demonstrated in our previous works37, avoiding directly contact between metallic nanoparticle surface and donor/acceptor materials is an effective way to improve the photovoltaic performances of PSCs. Therefore, we incorporated the bare Ag nanoplates on ITO glass, and core shell nanostructure Ag@SiO2 nanoparticles (Ag@SiO2-NPs) to active layer. The results show that the photovoltaic performance of PSCs was improved by 14.0%. In conclude, this work systematically illustrated the enhancements with the comparisons among virgin reference polymer, single shape additions, and dual-shape combinations, and to give confirmation to show the highest photovoltaic efficiency of double-shape combined hybrid PSCs.

EXPERIMENT DETAILS Preparations of Ag nanoplate and Ag@SiO2 nanoparticles

Ag nanoplate (Ag-nPl) was prepared according the one-step method firstly reported by Zhiqiang Gao38. The mixted solution of deionized water (39.2 mL), silver nitrate


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(0.1 M, 40µL), H2O2 (30 wt%, 112 µL), and trisodium citrate (0.1 M, 600 µL) were vigorously stirred at room temperature. Sodium borohydride (NaBH4, 100 mM, 400 µL) was rapidly injected to the mixed solution to initiate the reaction, the solution color transferred gradually from colorless to yellow, red, and blue in 5 minutes. Ag nanoplate was condensed by centrifugation at 8000 rpm for 10min, and the precipitate was re-dispersed in ethanol. Ag@SiO2-NPs were synthesized according to our previous published methods. To improve the compatibility of Ag@SiO2-NPs with active

layer

materials,

the

Ag@SiO2-NPs

were

treated

with

Gamma-glycidoxypropyltrimethoxysilane (KH560 50 V%) aqueous solution. 1 ml KH560 50 V% was added to 10 ml Ag@SiO2-NPs ethanol solution, the pH of the solution was altered to 8 with dilute ammonia solution and kept stirring for for 12 h at room temperature. The treated Ag@SiO2-NPs were purified for 3 times by centrifugations, then the Ag@SiO2-NPs were dried in vacuum oven at a low temperature. The fabrication of PSCs

The device structure of PSCs in this study is: Glass/ITO/Ag-nPl/PEDOT:PSS/Active layer with Ag@SiO2-NPs /Ca/Al, and Figure 1 shows the schematic diagram of the PSCs structure. ITO conductive glasses with conductivity of 15 Ω·sq-1 were ultrasonic cleaned 15 mins by cleaning agent, deionized water, acetone, de-ionized water, and isopropyl alcohol, respectively. The ITO glasses substrates were cleaned in a Plasma cleaning chamber for 6 min prior to the deposition of Ag-nPl and PEDOT:PSS films. For devices with Ag-nPl, Ag-nPl solution was spin-coated at 2000 rpm for 40 s. The



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PEDOT:PSS solution was spin-coated at 4000 rpm for 20 s to form a hole transport layer and thermal annealed at 160 ℃ for 30 min. After thermal annealing, the ITO substrates with hole transport layers were transferred to nitrogen-filled glove box. In this work, low band gap donor material Poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl) -benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thi ophen-4,6-diyl]}(PBDTTT-C-T)

and

fullerene

derivatives

acceptor

material

C71-butyric acid methyl ester (PC71BM) were chosen as photo-active materials, and their chemical structures were shown in Figure 1. The mass ratio of PBDTTT-C-T and PC71BM in 1,2-dichlorobenzene solution was 1:1.5, and the polymer concentration was 10 mg ml-1. Besides, 3v% 1,8-diiodooctane (DIO) was added to the photo-active materials blend solution to forming idea interpenetrating network with appropriate donor and acceptor domain sizes in active layer. The PBDTTT-C-T/PC71BM blend solution was spin-coated on the hole transport layer at 800 rpm for 60 s to form a ~100 nm active layer. In devices with Ag@SiO2-NPs, Ag@SiO2-NPs were prior incorporated to the PBDTTT-C-T/PC71BM blend solution and the blend solution was subjected to 10 min ultrasonic treatment before spin-coating. At last, 10 nm Ca and 100 nm Al electrodes were thermal evaporated onto the active layers in a vacuum coating equipments one after another to form cathodes at a vacuum ≤ 5*10-4 pa 14, 23.


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Figure 1. Schematic of the normal structure PSCs with metallic nanoparticles on ITO and

in

active

layer,

Glass/ITO/Ag-nPl/PEDOT:PSS/PBDTTT-C-T:PC71BM+

Ag@SiO2-NPs /Ca/Al, and the chemical structures of PBDTTT-C-T and PC71BM. CHARACTERIZATIONS The transmission electron microscopy (TEM) images of Ag@SiO2-NPs and Ag-nPl were characterized by a JEM-2000 Ex. The absorbance spectra of nanoparticles and PBDTTT-C-T:PC71BM films were measured by a Varian Cary 50 UV-Vis spectrometer. The atomic force microscopy (AFM) measurements for Ag-nPl thickness were obtained by an Agilent 5400 AFM at ambient temperature. The scanning electron microscopy images (SEM) were scanned by a FE-SEM Hitachi S-4800. The current density-voltage (J-V) characteristics of devices were obtained under AM 1.5 solar illuminations with the intensity of 100 mW/cm2 on Newport solar simulator by a Keithley 2420 source measurement in glove box. The external quantum efficiencies (EQE) of PSCs were characterized by a certified Newport incident

photon

conversion

efficiency

(IPCE)

measurement

system.

The

photoluminescence spectra of PBDTTT-C-T films were measured by using a Horiba Jobin Yvon Fluoro Max 4.



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FDTD solution parameters: The LSPR effects of Ag-nPl and Ag@SiO2-NPs were simulated by a finite-difference time-domain (FDTD) method developed by OptiWave®. According to the TEM results, the simulation sizes of Ag-nPl and Ag@SiO2-NPs were 50 nm and 60 nm, and silica shell thickness of Ag@SiO2-NPs was set as 15 nm. The Coated-Sphere Groups object structure was selected to simulate the LSPR effect of Ag@SiO2-NPs, the refractive index of silica was 1.46, and the other simulation parameters were set based on that of SiO2(Glass)-Palik in Material Database. For all the FDTD simulations of this work, the incident source was plane wave, and the incident wavelength was set from 350 nm to 900 nm, the simulation material were Ag (Silver)-Johnson and Chtisty, the simulation region was 100 nm*100 nm, and the maximum mesh step was 1 nm.

RESULTS AND DISCUSSIONS TEM images of Ag-nPl and Ag@SiO2-NPs were shown in Figure 2. By one-step method, the size of the Ag-nPl is widely distributed, which will broaden the absorption (in Figure 3). From Figure 2(a, b), we can see that the size of Ag-nPl is 30-80 nm. From the absorption spectrum of Ag-nPl (in Figure 3), the absorption range of the Ag-nPl is 400-1000 nm, and the absorption peaks of Ag-nPl are at 750 nm and 331 nm. The strong absorption peak of Ag-nPl at 750 nm can be attributed to the in plane dipole resonance absorption of Ag nanoplates, and the characteristic absorption peak of 331 nm is the out of plane dipole and quadrupole resonance peak of Ag nanoplate 39. And the emerged peak at 331 nm is the characteristic absorption peak of


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lamellar-structure nanosilver. In order to characterize the thickness of the lamellar Ag-nPl, we took the AFM measurement of Ag-nPl on Si substrate. As shown in Figure 4, the thickness of Ag-nPl is about 4 nm, which is much thinner than the PEDOT:PSS layer (~30 nm). For Ag@SiO2-NPs, the core shell structure can be clearly seen in Figure 2(c, d), the Ag core is 30-50 nm, and the homogeneous silica shell is about 15 nm. To date, the strongest enhancements have been reported for molecular located within 10-20 nm from the metallic surface, so the 15 nm silica thickness is appropriate for better enhancements. The absorption peak of Ag@SiO2 is 425 nm (in Figure 3), and the absorption peak of Ag@SiO2-NPs is relative narrow, which demonstrates that the sizes of Ag@SiO2-NPs are more homogeneous. From Figure 3, we can see that combining the absorption of Ag@SiO2-NPs and Ag-nPl will realize full wavelength coverage in the visible light range, which demonstrated that all wavelength in the visible light range will be sensitized the incorporated metallic nanoparticles. To clearly characterize the LSPR effect of Ag-nPl and Ag@SiO2-NPs, we simulated the magnified electromagnetic field by FDTD solutions, which calculated according the Maxwell equations. As seen from simulation results in Figure 2e and f, the electromagnetic field is magnified up to 5 times near the surface of nanoparticles, and the magnified electromagnetic field gradually decreases with increasing the distance from the surface. Therefore, the magnified electromagnetic field can be absorbed by surrounding materials thus increasing the light absorption of surrounding photo-active materials. Although the metallic nanoparticles can act as the light scattering dots to increase optical path, strong light scattering usually occurs



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when the size of nanostructures are comparable to the wavelengths of incident light, which means Mie scattering40. So the relatively small size of metallic nanoparticles utilized in this work may contribute less. Based on this point, it is reasonable to believe that LSPR effect plays a relatively important role to enhance the photovoltaic performances of polymer solar cells.

Besides, it is worth to note that bare

nanometals in the active layer may transfer excited electrons to active polymer. Thus, the encapsulation of metallic nanoparticles by SiO2 will effectively avoid this phenomenon13. Hence, in this work, we incorporated silica coated Ag NPs to active layer and un-capsulated Ag-nPl on ITO glass. By this way, the LSPR effect of Ag@SiO2-NPs and Ag-nPl can be effectively utilized to enhance the photovoltaic performances of PSCs.

Figure 2 TEM images of Ag-nPl (a, b), and Ag@SiO2-NPs (d, e). The FDTD simulation results of Ag-nPl (c), and Ag@SiO2-NPs (f).


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Figure 3 Absorption spectra of Ag@SiO2-NPs, Ag-nPl in aqueous solutions, and the absorption spectrum of PBDTTT-C-T solid film.

Figure 4 AFM characterization results of lamellar Ag-nPl on Si substrate. To investigate the effects of Ag-nPl and Ag@SiO2-NPs on enhancing the photovoltaic performance of PSCs, devices with Ag@SiO2-NPs in active layer, devices with Ag-nPl on ITO glasses, devices with Ag@SiO2-NPs in active layer and Ag-nPl on ITO glasses, and reference devices without metallic nanoparticles were fabricated. The incorporating concentrations of nanoparticles were prior optimized, as the results shown in Table S1, S2 and Figure S1, S2 (Supporting Information), the optimized



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Ag@SiO2-NPs weight percent to PBDTTT-C-T:PC71BM in o-DCB solution was 1.5 wt%, and Ag-nPl concentration in ethanol was 2 mg/ml.

The detailed photovoltaic

parameters of optimized devices with metallic nanoparticles and the reference device are summarized in Table 1, and the corresponding current density-voltage (J-V) characteristics under the illumination of AM 1.5 g (100 mW/cm2) are shown in Figure 5. The PBDTTT-C-T:PC71BM devices without metallic nanoparticles showed an average PCE of 7.34% and a maximal PCE of 7.42% with an open-circuit voltage (Voc) of 0.76 V, a short-circuit current density (Jsc) of 16.21 mA/cm2， and a fill factor (FF) of 0.606. With the sensitization effect of metallic nanoparticles, devices with only Ag@SiO2-NPs, Ag-nPl and both of them showed various degree of enhancement on photovoltaic performances. Devices with Ag@SiO2-NPs in PBDTTT-C-T:PC71BM active layer showed a maximal PCE of 7.93%, with an enhanced Jsc of 16.94 mA/cm2. Devices with Ag-nPl on ITO glasses showed a maximal PCE of 7.89%, with an enhanced Jsc of 16.42 mA/cm2 and an enhanced FF of 0.633. With the dual effect of Ag@SiO2-NPs and Ag-nPl, devices showed a best photovoltaic performance of 8.46%, with a greatly enhanced Jsc of 17.32 mA/cm2 and an enhanced FF of 0.652. The Voc of all devices showed no discrepancy, which indicates that the incorporation of metallic nanoparticles will not change the electric properties. The enhanced Jsc can be ascribed to the improved light absorption of PSCs, which are the dual effects of LSPR effect and the scattering effect of nanoparticles. FF values are affected by many factors such as chare carriers transport, recombination and extraction. Since the incorporation of the dielectric silica


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capsulated Ag nanoparticle will not cause the changes of electric properties, so the enhanced FF value in devices with Ag-nPl can be ascribed to the enhanced hole extraction properties after incorporating Ag-nPl on ITO, this results are in accordance with the previous published papers30, 41. To verify the effectiveness of the device structure incorporated by Ag@SiO2-NPs and Ag-nPl, the devices based on PTB7-Th/PC71BM active materials were fabricated, and the corresponding photovoltaic parameters were listed in Table S3. The results indicate that the PCE of the device incorporated by both Ag@SiO2-NPs and Ag-nPl was improved to 9.93% from the value 8.87% of the reference device. In conclusion, the device structures incorporated by Ag@SiO2-NPs in active layer and by Ag-nPl on ITO glass were also effective to PTB7-Th/PC71BM systems. Table 1 Photovoltaic parameters of the devices incorporated by different nanometals. Voc (V)

Jsc (mA/c m2)

FF

Average PCE (%) a

Maximal PCE (%)

Rs (Ωcm2 )b

Reference device

0.76± 0.01

16.21±0.23 0.606±0.013

7.34± 0.08

7.42

4.53

Devices with Ag@SiO2-NPs

0.76± 0.01

16.94±0.15

0.616± 0.023

7.77± 0.16

7.93

3.42

Devices with Ag-nPl

0.76± 0.01

16.42±0.09

0.633± 0.013

7.71± 0.17

7.89

2.88

Devices with Ag-nPl + Ag@SiO2

0.75± 0.01

17.23±0.14

0.652± 0.008

8.32± 0.14

8.46

3.12

a

The average PCE value was obtain from 15 devices, b Series resistance (Rs) obtained

at 1.5 V in dark condition.



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Figure 5 J-V curves of the devices incorporated with different metallic nanoparticles. The external quantum efficiencies (EQEs) of PSCs with Ag@SiO2-NPs and Ag-nPl are showing in Figure 6, besides, the enhancement of the EQE values in devices were also illustrated in Figure 6. With the EQE cures, the integrated photocurrent densities were calculated. As results, the integrated photocurrent densities of Ag@SiO2-NPs devices, Ag-nPl devices, Ag@SiO2-NPs + Ag-nPl devices and reference device are 16.79 mA/cm2, 16.35 mA/cm2, 16.82 mA/cm2 and 16.13 mA/cm2, respectively8. There are about 5% differences between integrated values and measured values, which is the results of the light spectra mismatches between sun and xenon lamp42. In additions, the EQE enhancement curves of Ag@SiO2-NPs devices and Ag-nPl devices are in accordance with light absorption curves, hence, we can believe that the enhanced Jsc values are caused by the LSPR effect of metallic nanoparticles.


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Figure 6 EQE curves and the EQE enhancements curves of devices incorporated with different metallic nanoparticles. To investigate the discrepancies of devices on photocurrent generation process and charge collection efficiencies, photocurrent density (Jph)-effective voltage (Veff) curves of reference device and device with Ag-nPl and Ag@SiO2-NPs were shown in Figure 7. The Jph-Veff analysis method was firstly published by Blom43. Jph is defined by formula: Jph=JL-JD, for JL and JD are the current densities under illuminations and dark conditions, respectively. Veff is difined by formula: Veff =V0 – Va, V0 is the voltage at the condition of Jph=0, and Va is the applied voltage. As results in Figure 6, the photocurrent densities in device with Ag-nPl and Ag@SiO2-NPs saturates at a certain Veff value (≈4 V), this indicates that all the light generated charges are completely swept out to electrodes over this bias voltage value44. However, much smaller and continuous increased photocurrent curves without saturation is shown in reference device, which indicates that the charge collecting efficiency in reference device is



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lower. The enhanced saturated photocurrent density (Jsat) demonstrates that incorporating Ag-nPl and Ag@SiO2-NPs can enhance the photocurrent of PSCs. And the enhanced Jsat is caused by the LSPR effect and the scattering effect of metallic nanoparticles. The charge collection probabilities (Pc) are defined by Jph/Jsat, at the short-circuit conditions for applied voltage=0, Veff =V0 = Voc. The Pc of reference device and device with Ag-nPl and Ag@SiO2-NPs are 76.4% and 86.8%, respectively. Such a high Pc value combined with the high photocurrent density seems to be responsible

for

the

high

PCE

of

8.46%

in

device

with

Ag-nPl

and

Ag@SiO2-NPs45.

Figure 7 Jph-Veff curves of reference device and device with Ag-nPl and Ag@SiO2-NPs. As has been illustrated, Ag-nPl were spin-coated on ITO glasses, then the hole transport layers were fabricated. Whereas, Ag@SiO2-NPs were incorporated to the PBDTTT-C-T:PC71BM active layer. To investigate the morphologies of Ag-nPl on


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ITO and Ag@SiO2-NPs in active layer, SEM and film TEM characterizations were taken. In Figure 8a, b, we can see that Ag-nPl disperse well and keep their original sizes. In Figure 8c, d, we can see that Ag@SiO2-NPs homogeneously dispersed in the active layer. The well dispersed Ag@SiO2-NPs keep their nano-characteristics and functioned as a light trap to enlarge the electro-magnetic field, and as a result enlarged the light absorption of the active layer. Figure 8e shows the TEM image of PBDTTT-C-T:PC71BM active layer with Ag@SiO2-NPs. From the TEM image, we can clearly see the interpenetrating network of white phase (polymer) and black phase (PC71BM). In the enlarged image (Figure 8f), core-shell structure of Ag@SiO2-NPs can be clearly seen, and no obvious interface observed between Ag@SiO2-NPs and photo-active materials. All the results demonstrated that the compatibility of Ag@SiO2-NPs PBDTTT-C-T:PC71BM active layer is good, thus it is understandable to believe that the incorporation of Ag@SiO2-NPs will not destroy the interpenetrating network of active layer. Carefully comparing the SEM images and TEM images of Ag@SiO2-NPs in Figure 8, we can find that the size of Ag@SiO2-NPs characterized by the SEM results is little bigger than the sizes demonstrated by the TEM images. This may be caused by the good encapsulation of polymer on the surface of Ag@SiO2-NPs, which once again indicates the good compatibility of Ag@SiO2-NPs and PBDTTT-C-T:PC71BM active layer.



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Figure 8 SEM images of Ag-nPl on ITO glasses (a, b); SEM images of PBDTTT-C-T:PC71BM film with Ag@SiO2-NPs (c, d); TEM image of PBDTTT-C-T:PC71BM film with Ag@SiO2-NPs (e), the magnified image of Ag@SiO2-NPs in PBDTTT-C-T:PC71BM film (f). In order to check the optical discrepancies of the PBDTTT-C-T:PC71BM films in different devices, absorption measurements were taken, and the results are shown in Figure 9.The absorption curves of all the PBDTTT-C-T:PC71BM films with the same thickness

show

consistent

shapes,

but

the

intensities

show

differences.

PBDTTT-C-T:PC71BM film with Ag@SiO2-NPs shows intensity enhancement in 400-500 nm compared with that of pure PBDTTT-C-T:PC71BM film, which is in accordance

with

the

LSPR

wavelength

of

Ag@SiO2-NPs.


Similarly,

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PBDTTT-C-T:PC71BM films with Ag-nPl show intensity enhancement in 600-750 nm, which is also in accordance with the LSPR wavelength of Ag-nPl. All of these results indicate that the incorporation of metallic nanoparticles will improve the light absorption of active layer. Therefore, the enhanced light absorption promises a better photovoltaic performance of a PSC.

Figure 9 Absorption spectra of active layers that with metallic nanoparticles. To further verify the incorporation of Ag-nPl and Ag@SiO2-NPs will enhance the light absorption of active layer, photoluminescence measurements were taken. PBDTTT-C-T material without PC71BM was selected to take photoluminescence measurement, because the absence of PC71BM will ensure most of absorbed energy transfer to radiation energy other than electric energy37. Figure 10 shows the photoluminescence results, and the excitation wavelength of PBDTTT-C-T film was 652 nm. The PBDTTT-C-T film with both Ag-nPl and Ag@SiO2-NPs shows the maximal photoluminescence intensity, PBDTTT-C-T films with Ag-nPl or Ag@SiO2-NPs show various intensity enhancements, and the pure PBDTTT-C-T



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obtained a minimum intensity. This result is consistent with the absorption spectra of active layer and the photovoltaic performances of corresponding devices. And this result can be explained by the enhanced light absorption. For instance, the incorporation of metallic nanoparticles will enhance the light absorption of PBDTTT-C-T films, thus leads to higher emission intensity of the PBDTTT-C-T film.

Figure 10 Photoluminescence spectra of PBDTTT-C-T films that with metallic nanoparticles, excitation wavelength 652 nm.

CONCLUSIONS This work reported the enhancement of photovoltaic performance of hybrid PSCs by dual-shapes of metallic nano-additives of Ag-nPl and Ag@SiO2-NPs. Ag-nPl and Ag@SiO2-NPs

were

spin-coated

on

ITO

glass

and

incorporated

to

PBDTTT-C-T:PC71BM active layer, respectively, to utilizing the LSPR effects to enhancing the light absorption of the active layer in polymer solar cells. With the full visible light sensitization by the incorporations of both Ag-nPl and Ag@SiO2-NPs, the polymer solar cell obtained the optimal photovoltaic performance with maximum


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PCE of 8.46%, and Jsc of 17.23 mA/cm2. According to the systematic characterizing, the enhanced Jsc was ascribed to the improved light absorption of active layer by the incorporation of metallic nanoparticles. Importantly, the methodology of multiple shape combination of metallic nano-additives improves the photovoltaic performance of PSCs much effectively comparing single shape method, which will be the new way to increase the efficiency of PSCs.

ACKNOWLEDGEMENT This work was supported by: (1) The National One-Thousand Foreign Expert Program (WQ20123700111); (2) The Program for Introducing Talents of Discipline to Universities (“111” plan); (3) Natural Scientific Foundation of China, Grant #51273096, #51373081, #51473082, #51603109, #51503112; (4) Shandong Provincial Natural Science Foundation, Grant ZR2016EMB03. Thanks to Prof. Penghui Wu at Zhejiang University to perform FDTD simulations for this work.

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