Prominent Efficiency Enhancement in Perovskite Solar Cells

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Prominent Efficiency Enhancement in Perovskite Solar Cells Employing Silica-Coated Gold Nanorods Runsheng Wu, Bingchu Yang, Chujun Zhang, Yulan Huang, Yanxia Cui, Peng Liu, Conghua Zhou, Yuying Hao, Yongli Gao, and Junliang Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00309 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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The Journal of Physical Chemistry

Prominent Efficiency Enhancement in Perovskite Solar Cells Employing Silica-coated Gold Nanorods Runsheng Wu, † Bingchu Yang, *, † Chujun Zhang, † Yulan Huang, † Yanxia Cui, *, ‡ Peng Liu, † Conghua Zhou, † Yuying Hao, ‡ Yongli Gao,† and Junliang Yang *, † †

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South University, Changsha 410083, China

‡

Key Laboratory of Advanced Transducers and Intelligent Control System (Ministry of

Education), Taiyuan University of Technology, Taiyuan 030024, China

*Corresponding author email: [email protected] (B. C. Yang), Telephone: +86-731-88879525 [email protected] (Y. X. Cui), Telephone: +86-351-6018862 [email protected] (J. L. Yang), Telephone: +86-731-88660256

Complete list of affiliations: Runsheng Wu, †, § Bingchu Yang, *, † Chujun Zhang, † Yulan Huang, † Yanxia Cui, *, ‡ Peng Liu, † Conghua Zhou, † Yuying Hao, ‡ Yongli Gao,†, // and Junliang Yang *, † †

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South University, Changsha 410083, China

‡

Key Laboratory of Advanced Transducers and Intelligent Control System (Ministry of Education),

Taiyuan University of Technology, Taiyuan 030024, China §

School of New Energy Science and Engineering, Xinyu University, Xinyu 338004, China

//

Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA

1

ACS Paragon Plus Environment


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Abstract Highly efficient planar heterojunction (PHJ) perovskite solar cells (PSCs) with a structure

of

ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al

were

fabricated

by

low-temperature, solution process. As employed silica-coated gold (Au@SiO2) nanorods at the interface between the hole transport layer PEDOT:PSS and the active layer CH3NH3PbI3, the average power conversion efficiency (PCE) showed over 40% enhancement, of which the average PCE was improved from 10.9 % for PHJ-PSCs without Au@SiO2 to 15.6 % for PHJ-PSCs with Au@SiO2, and the champion one up to 17.6 % was achieved. Both experiment and simulation results proved that prominent efficiency enhancement come from the localized surface plasmon resonance of Au@SiO2 nanorods which could improve the incident light trapping as well as improve the transport and collection of charge carrier, resulting in the enhancement in device parameters. The results suggest that metal nanorods, e.g., Au@SiO2, could be employed to fabricate high-efficiency and low-cost PHJ-PSCs.

2


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I. Introduction The halide perovskite (CH3NH3PbX3, X= Cl, Br and I) solar cells (PSCs) have attracted considerable attention in photovoltaic community during the past several years due to the advantages of high efficiency, low cost, and the compatibility with roll-to-roll (R2R) fabrication.1-5 The power conversion efficiencies (PCEs) underwent a rapid development from firstly reported value of 3.8 % to a certified value of over 20 %, which is comparable to that of commercial silicon-based solar cell.

6-13

It will

be a very promising candidate in new energy realm, solar plants, building integrated photovoltaics (BIPV) as well as energy supply for portable electric devices and so on. Although multidisciplinary efforts have led to the rapid increase of the PCE to the level of silicon solar cells, the further improvement of PSCs performance is still necessary for accelerating the commercialization.12 In order to maximize the short circuit current density (Jsc) and then enhance the PCE, one of effective and simple strategies is to increase the light absorption in the active layer using adequate thick perovskite films. However, the thickness of active layer is limited due to the long transport distance inevitably intensifying adverse recombination of electrons and holes. Normally, the optimized perovskite film thickness in PSCs is in the range of 280 to 350 nm,14, 15 which is inadequate for the light absorption. In order to improve the light absorption at the optimized thickness, incorporating metal nanoparticles (NPs) into PSCs is probably a compelling choice. Metal NPs have been successfully used to moderately improve the performance of organic solar cells (OSCs) and dye sensitized solar cells (DSSCs).16-25 It is well known that the size, shape, component and dielectric environment of metal NPs play very important roles in affecting photovoltaic performance.16, 17, 20-23 Metal NPs can excite localized surface plasmon (SP) resonance, leading to an improvement in light absorption, and then an increase in exciton generation.17, 20, 21 Normally, metal NPs enwrapped by insulated dielectric material can induce better photovoltaic performance than bare metal NPs because the insulator layer can avoid the direct contact between metal NPs and the active layer to eliminate charge recombination and 3



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exciton quenching loss at the metal surface.21, 26 Moreover, a series of studies suggest that doping metal NPs into solar cells can improve the electrical properties, e.g., the carrier transport and collection.17, 18, 21-22 Especially, metal nanorods greatly exceed spherical-shaped metal NPs in enhancing local electromagnetic field and hence exhibit superior photovoltaic performance.17 Snaith et al. reported the performance improvement in meso-superstructured PSCs via incorporating spherical-shaped metal NPs into Al2O3 matrix, of which the average PCE was enhanced from 8.5 % to 9.5%.18 Park et al. reported that Au NPs were introduced into Spiro-OMeTAD layer and could improve the performance of meso-superstructured PSCs.27 Jeng and Liao et al. reported that the plasmonic effect of silver NPs or nanoplates introduced into the hole transport layer on the photovoltaic performance of PSC devices, resulting in the PCE enhancement of about 10% by controlling the size, shape and concentration of silver NPs or nanoplates.28,

29

They attributed the enhancement in photovoltaic

performance of PSCs to either the reduced exciton binding energy or the electrical properties. 18, 27-29 Herein, we prepared planar heterojunction (PHJ) PSC devices with the structure of

ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al

containing

silica-coated

gold

(Au@SiO2) core-shell nanorods at the interface between the hole transport layer PEDOT:PSS and the active layer CH3NH3PbI3 via low-temperature solution process. As a result, the average PCE of 15.6 % and the champion PCE of 17.6 % were obtained in the PHJ-PSC devices with Au@SiO2 nanorods under one sun in AM 1.5G. Compared with PHJ-PSC devices without Au@SiO2 nanorods (of which the average PCE is 10.9 %), an impressive improvement over 40 % in average PCE was achieved. The results offer a possible method to dramatically enhance the performance of PHJ-PSC devices by employing metal nanoparticles, e.g., Au@SiO2 nanorods.

2. Experiments The PHJ-PSC devices with or without Au@SiO2 nanorods were fabricated as shown in Fig. 1a-b. The ITO-coated substrate was ultrasonically cleaned in acetone, detergents, distilled water, and isopropyl alcohol for 15 min, respectively. After the 4


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cleaning and drying, the substrate was treated using UV-ozone for 15 min to reform the ITO substrate surface. The PEDOT: PSS (Baytron, PVP AI 4083) was spin-coated on the ITO substrate and annealed on hot plate at 150 °C for 15 min, resulting in a thickness of about 50 nm. For PHJ-PSC devices with Au@SiO2 nanorods, Au@SiO2 nanorods dispersed in ethanol with the concentration of 0.032 pM, 0.047 pM, and 0.095 pM were spin-coated on top of the PEDOT: PSS layer at a speed of 3000 rpm for 30 s and then annealed on hot plate at 120 °C for 5 min. The Au nanorods, as core of the metal NPs, were synthesized by the seed-mediated growth method,30 and the SiO2 shell was surfaced surrounding the core according to a previously published procedure.31 It is noted that the solution was ultrasonically agitated for 10 min to fully disperse Au@SiO2 nanorods. In order to obtained high-quality perovskite film, the solvent-induced-fast-crystallization deposition (SIFCD) method was used to fabricate the perovskite layer in a nitrogen-filled glove box (both H2O and O2 < 1.0 ppm).13, 15 On top of the PEDOT:PSS layer with or without Au@SiO2 nanorods, the mixture of methylammonium iodide (CH3NH3I, jingge, Wuhan) and lead iodide (PbI2, Zhengpin, shanghai) dissolved in anhydrous N,N-dimethylformamide (DMF, J&K Seal) (550mg/ml) was spin-coated at a speed of 4000 rpm for 30 s, resulting in a thickness of about 290 nm. Subsequently, the sample was annealed on a hot plate at 100 °C for 10 min in glove box. Then a 15 mg/ml fullerene derivative PCBM (Dye Source, American) dissolved in anhydrous chlorobenzene (CB, J&K Seal) was spin-coated at a speed of 3000 rpm on top of CH3NH3PbI3 layer, resulting in a thickness of about 30 nm. Finally, Al electrode (~100 nm) was evaporated through a shadow mask under a vacuum of ~ 4×10–6 mbar to complete the fabrication of PHJ-PSC device with an active area of 0.09 cm2. The absorption spectra of CH3NH3PbI3 films and Au@SiO2 nanorods in ethanol solution were measured by ultraviolet-visible spectrophotometer (UV-vis, Persee, T9, China). Transmission electron microscope (TEM) and atomic force microscope (AFM, Agilent Technologies 5500 AFM/SPM System, USA) was employed to characterize Au@SiO2 nanorods or PSC devices. To characterize the crystallinity and morphology of CH3NH3PbI3 films with and without Au@SiO2 nanorods, X-ray diffraction (XRD, 5



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Rigaku D, Max 2500, Japan) and scanning electron microscope (SEM) were employed. PHJ-PSC devices were tested by digital source meter (Keithley, model 2420) with a solar simulator (91160s, Newport, AM 1.5G) from -1.5 V to +1.5 V at scanning speed of 0.3 V/s if without specified note. The standard silicon solar cell was used to calibrate the incident light power as 100 mW/cm2. The incident photon to current conversion efficiency (IPCE) spectra of PHJ-PSC devices were measured and analyzed by quantum efficiency measurement system (Saifan, Beijing). The electrochemical impedance spectroscopy (EIS, CHI606D, Chenhua, Shanghai) spectra were recorded under 1-sun AM 1.5G illumination with different bias voltage.

(a)

(b)

Al electrode

Al electrode PCBM

PCBM

CH3NH3PbI3 CH3NH3PbI3

Au@SiO2 nanorods

PEDOT:PSS

PEDOT:PSS ITO

ITO

Fig. 1. (a-b) The schematic cross-sectional views of PHJ-PSC devices without and with Au@SiO2 nanorods, respectively.

3. Results and Discussion The morphology of Au@SiO2 core/shell nanorods characterized by TEM is shown in Fig. 2a-b. Fig. 2b clearly suggests that Au nanorods are embedded in dielectric matrix SiO2 forming a core/shell structure. The size distribution of Au@SiO2 core/shell nanorods is shown in Fig. 2c. The average diameter and length of Au nanorod as well as the thickness of SiO2 shell are 16.8 nm, 34.7 nm and 9.5nm, respectively. The SiO2 shell completely coats on Au nanorod core. The normalized absorption spectrum of Au@SiO2 nanorods in ethanol is presented in Fig. 2d. It can be found that Au@SiO2 nanorods exhibit a transverse localized SP resonance peak at about 522 nm and a longitudinal localized SP resonant peak at about 700 nm. It is the 6


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dual resonant peaks of Au@SiO2 nanorods that make Au@SiO2 nanorods exhibits better improvement on the performance of photovoltaic devices than that of Au@SiO2 spherical-shaped NPs.17 The AFM and cross-section SEM images of Au@SiO2 nanorods deposited on the PEDOT:PSS layer as well as cross-section SEM images for PSC devices with and without Au@SiO2 nanorods are shown in Fig S1 and S2 in Supporting Information. It is found that Au@SiO2 nanorod clusters are distributed like the branches on the PEDOT:PSS layer, as shown in Fig. S1a. Fig. S1b is cross-section SEM image which obviously presents Au@SiO2 clusters. Both kinds of PSC devices with and without Au@SiO2 exhibit well-defined layer-by-layer configuration (Fig. S2a-b). The perovskite layer in both PSC devices is homogeneous and well crystalline, proved by XRD results as well below. Fig. S2b clearly indicates that Au@SiO2 nanorod clusters are embedded at the interface between the PEDOT:PSS layer and the perovskite layer.

(b)

(a)

80 Diamerer Length

40 20

Absorbance (a.u.)

1.2

(c) 60

SiO2

5 nm

200 nm

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1.0 0.8 0.6 0.4 0.2

10 20 30 40 50 Size distribution of Au@SiO2 nanorods (nm)

(d)

400

500 600 700 Wavelength (nm)

800

Fig.2. (a-b) TEM morphology of Au@SiO2 nanorods, and inset in (b) is the fourier transform

pattern. (c) The size distribution of Au@SiO2 nanorods. Au@SiO2 nanorods dispersed in ethanol 7


(d) Absorption pattern of


Fig. 3a exhibits the XRD patterns of CH3NH3PbI3 thin films deposited on the PEDOT:PSSS layer with and without Au@SiO2. The diffraction peaks at the 2 theta angles of 14.4o, 28.7o, 32.1o, 40.9o and 43.4o can be assigned to (110), (220), (310), (224) and (314) crystal faces, respectively.32, 33 The two kinds of CH3NH3PbI3 thin films show very similar diffraction peaks as well as the peak intensity, suggesting the two CH3NH3PbI3 thin films have similar crystallinity. The surface morphology of both perovskite thin films is similar as well, which is continuous and compact (Fig. 3b and 3c). 10k (110)

Without Au@SiO2

(404)

4k

(224) (314)

6k

With Au@SiO2

(220) (310) (312)

8k

(112) (211) (202)

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2k

10

20

30

40

50

60

2-Theta (degree)

(b)

With Au@SiO2

(c)

Without Au@SiO2

500nm

500nm

Fig. 3. (a) XRD patterns of perovskite thin films deposited on the PEDOT:PSSS layer. (b-c) SEM images of perovskite thin films deposited on the PEDOT:PSSS layer with and without Au@SiO2 nanorods, respectively.

8


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The typical J-V curves of PHJ-PSC devices with and without Au@SiO2 nanorods are shown in Fig. 4a. For the reference device without Au@SiO2 nanorods, the Voc, Jsc and FF are 1.01 V, 18.0 mA/cm2, 65.9 %, respectively, resulting in a PCE of 12.1 %. For maximizing the advantage of Au@SiO2 nanorods for improving photovoltaic performance, J-V characteristics of PHJ-PSC devices incorporated Au@SiO2 nanorods with the different concentrations of 0.032 pM, 0.047 pM and 0.095 pM dispersed in ethanol were investigated, respectively. For PHJ-PSC devices with Au@SiO2 nanorods at a concentration of 0.032 pM, the Voc, Jsc and FF are 0.97 V, 19.7 mA/cm2 and 64.9 %, respectively, resulting in a PCE of 12.5 %. Increasing the concentration to 0.047 pM, PHJ-PSC devices show a PCE of 16.1 % with Voc of 1.03 V, Jsc of 22.0 mA/cm2 and FF of 70.6 %, respectively. However, the device parameters decrease contrarily as further increasing the concentration to 0.095 pM, and the Voc, Jsc and FF are 1.01 V, 20.6 mA/cm2, and 67.9%, respectively, resulting in a PCE of 14.3 %. The above results suggest that Au@SiO2 nanorods inserted between the PEDOT:PSS layer and the CH3NH3PbI3 layer could dramatically improve the performance of PHJ-PSCs, and the optimized concentration is 0.047 pM. Although the Au@SiO2 nanorods can efficiently trap the incidence light and enhance local electric field in the active layer, benefitting the improvement of photovoltaic performance16, 17, 22 they would have the risk of increasing the interface transport resistance, as shown in Fig. S4 in Supporting Information. The details will be discussed below in the EIS section. Hence, the concentration of Au@SiO2 nanorods is also an important factor to efficiently improve the photovoltaic performance of PHJ-PSC devices.

9


(a) -5

Without Au@SiO2 With Au@SiO2 at 0.032 pM

-10

With Au@SiO2 at 0.047 pM With Au@SiO2 at 0.095 pM

-15 -20

-0.5

0.0 0.5 Voltage (V) 0

1.0

1.5

(b)

Current Density (mA/cm

2

)

0 2

Page 10 of 24

0

-25 -1.0

Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current Density (mA/cm2)


From -1.5 V to +1.5 V Jsc = 21.1 mA/cm2 Voc = 1.07 V FF = 77.3 % PCE = 17. 6 %

-5 -10

From +1.5 V to -1.5 V Jsc = 21.2 mA/cm2 Voc = 1.06 V FF = 77.7 % PCE = 17. 7 %

-15 -20 -1.0

-0.5

0.0 0.5 Voltage (V)

1.0

1.5

(c) -5 -10 -15

500 mV/s 300 mV/s 150 mV/s 100 mV/s

PCE = 17.6 % PCE = 17.7 % PCE = 17.4 % PCE = 17.2 %

-20 -1.0

-0.5

0.0 0.5 Voltage (V)

1.0

Fig. 4. (a) The typical J-V curves of PHJ-PSC devices with and without Au@SiO2 nanorods at the different concentrations. (b-c) The J-V curves of the champion PHJ-PSC device with 0.047 pM Au@SiO2 nanorods under different scanning directions and scanning speeds, respectively.

Furthermore, based on the optimized concentration at 0.047 pM, a champion PHJ-PSC device with the PCE up to 17.6 % was achieved under forward scanning direction from -1.5 V to +1.5 V, of which the Voc, Jsc and FF are 1.07 V, 21.1 mA/cm2, 77.3 %, respectively, as shown in Fig. 4b. More attractively, the PHJ-PSC device with Au@SiO2 nanorods didn’t show obvious hysteresis. Under reverse scanning direction, a PCE of 17.7% was obtained as well. If the scanning speed was changed, the PCEs of PHJ-PSC devices exhibit negligible variation, e.g. the PCEs of 17.6%, 17.7%, 17.4%, and 17.2% were obtained at the scan speeds of 500 mV/s, 300 mV/s, 150 mV/s, and 100 mV/s, respectively (Fig. 4c). The results suggest that almost no 10


1.5

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hysteresis was observed and the J-V curves are almost the same regardless of the scanning direction and scanning speed during the test for PHJ-PSC device with Au@SiO2 nanorods. To objectively evaluate the photovoltaic performance, more experiments were carried out and the statistic performance parameters are shown in Fig. 5a-d and Table 1. The results indicate that the Voc is kept at about 1 V, regardless of containing Au@SiO2 nanorods in PHJ-PSCs or not. The average parameters of Jsc, FF and PCE are 18.3 mA/cm2, 59.5 % and 10.9 %, respectively, for the reference devices without Au@SiO2 nanorods. The average PCEs are sharply increased to be 11.2 % and 15.6 % when Au@SiO2 nanorods with the concentration of 0.032 pM and 0.047 pM were introduced at the interface between the PEDOT:PSS layer and the CH3NH3PbI3 layer. The former one shows the average Jsc of 18.9 mA/cm2 and FF of 61.3%, while the latter one shows the average Jsc of 20.7 mA/cm2 and FF of 72.0 %. If further increasing the concentration of Au@SiO2 nanorods to 0.095 pM, PHJ-PSCs, in contrary, show a little decrease in performance parameters, of which the average PCE is 14.5 % with the average Jsc of 20.3 mA/cm2 and the average FF of 70.1 %, respectively. With Au@SiO2 nanorods inserted in PHJ-PSC devices, the enhancements in the average Jsc are about 3.7 %, 13.5 % and 11.1 %, as well as the enhancements in the average FF are about 3.0 %, 21.0 % and 17.8% for the concentrations of Au@SiO2 nanorods of 0.032 pM, 0.047 pM, and 0.095 pM, respectively, resulting in the impressive improvement in the average PCEs by 2.9 %, 43.7 % and 33.4 %, respectively. One should note that the performance parameters for PHJ-PSC devices with Au@SiO2 nanorods show well distributions with small standard deviations (Fig. 5a-d and Table 1), implying the good reliability and reproducibility. The improvement of PCE for PHJ-PSCs with Au@SiO2 nanorods originates from the increase in Jsc and FF, whereas Voc remains almost the same. The reasons for the increase in Jsc and FF will be explained as follows using absorbance, IPCE and EIS analysis.

11



80

Without Au@SiO2 With Au@SiO2 at 0.032 pM

60

(a) Percentage (%)

Percentage (%)

100

With Au@SiO2 at 0.047 pM

60

With Au@SiO2 at 0.095 pM

40 20 0

60

(b)

30 20

Without Au@SiO2 With Au@SiO2 at 0.032 pM With Au@SiO2 at 0.047pM With Au@SiO2 at 0.095pM

60

65

70

100

(c)

20

55

16

17

18

19

20

21

22

23

Short Circult Current Density (mA/cm2)

40

0 50

40

0

0.92 0.96 1.00 1.04 1.08 1.12

Percentage (%)

80

Without Au@SiO2 With Au@SiO2 at 0.032 pM With Au@SiO2 at 0.047 pM With Au@SiO2 at 0.095 pM

50

10

Open circult Voltage (V)

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

75

80 60

(d)

40 20 0

80

Without Au@SiO2 With Au@SiO2 at 0.032 pM With Au@SiO2 at 0.047 pM With Au@SiO2 at 0.095 pM

9 10 11 12 13 14 15 16 17 18

Power Conversion Efficiency (%)

Fill Factor (%)

Fig. 5. The statistic performance parameters for PHJ-PSC devices without and with Au@SiO2 nanorods at the concentration of 0.032 pM, 0.047 pM, and 0.095 pM, respectively.

Table 1. The average performance parameters obtained from J-V curves for PHJ-PSC devices with and without Au@SiO2 nanorods. The best photovoltaic parameters are shown in the bracket. Concentration of

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0

0.99±0.05 (1.04)

18.3±1.2 (20.3)

59.5±4.9 (66.6)

10.9±1.2 (13.5)

0.032

0.95±0.03 (0.99)

18.9±1.7 (21.3)

61.3±2.8 (64.9)

11.2±1.3 (12.8)

0.047

1.04±0.03 (1.08)

20.7±1.0 (22.2)

72.0±3.4 (77.3)

15.6±0.9 (17.6)

0.095

1.01±0.02 (1.04)

20.3±1.0 (22.1)

70.1±1.1 (71.6)

14.5±0.8 (16.0)

Au@ SiO2 nanorods (pM)

12


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The IPCE spectra of PHJ-PSCs with and without Au@SiO2 nanorods are measured as shown in Fig. 6a, which indicates that significant enhancement in IPCE takes place over almost the whole wavelength range. It is found that the integrated Jsc derived from the IPCE spectra for both types of PHJ-PSCs degrade a bit since the measurement was carried out in air, but the air degradation does not affect the increase in the integrated Jsc (~15%), which agrees well with that obtained from J-V curves. Furthermore, the absorption spectra of CH3NH3PbI3 films with and without Au@SiO2 nanorods are also measured as shown in Fig. 6b. It is obvious that the multilayer film with Au@SiO2 nanorods at the interface between the PEDOT:PSS layer and the CH3NH3PbI3 layer exhibits higher absorption over a broad wavelength range than the control film. Then, we calculated the wavelength-dependent enhancement factors of the IPCE (∆IPCE) and absorption (∆Abs) for the structures with Au@SiO2 nanorods with respect to the corresponding control and plot them in Fig. 6c. It is seen that the two enhancement factor curves are roughly similar to each other in shape with the intensities moderate over the whole wavelength range except for that longer than 750 nm. Besides, it is also noticed that the enhancement in IPCE is higher than that of absorption. Consequently, the increase in Jsc could partly attribute to the improved absorption with the presence of Au@SiO2 nanorods. It is believed that Au@SiO2 nanorods could improve the electrical properties in PHJ-PSCs, contributing to the improvement in IPCE as well (see the subsequent EIS discussion).

13



1.0 Absorption (a.u.)

IPCE

0.8 0.6 0.4

Without Au@SiO2 With Au@SiO2

0.2 0.0 300

0.6

∆IPCE

5

(a)

400


0.6

0.4

0.4

0.2

0.2

400

500

600

700

(b) Without Au@SiO2 With Au@SiO2

3 2 1

(d)

(c)

0.0 300

4

0 300

800

∆Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400


λ = 720 nm

E

λ = 530 nm

0.0 800

Page 14 of 24

E

800

10

3 λ = 760 nm 0

E

0 6

3 λ = 760 nm 0

E

0

Wavelength (nm) Fig. 6. (a, b) The IPCE spectra of PHJ-PSCs and absorption spectra of CH3NH3PbI3 films with and without Au@SiO2 nanorods, respectively. (c) The calculated wavelength-dependent enhancement factors of the IPCE (∆IPCE) and absorption (∆Abs). (d) The calculated electric field distributions for Au@SiO2 nanorods (single or pair) at the denoted wavelengths and polarization.

The detailed explanation of the improved absorption after incorporating Au@SiO2 nanorods are presented as follows. As known, the longitudinal resonance of Au@SiO2 nonorods is much stronger than their transverse resonance. As indicated in Fig. 2d, the transverse and longitudinal resonances of Au@SiO2 nonorods in ethanol solution locate at ~520 and ~700 nm, respectively, which should red-shift to longer wavelengths because the perovskite film has much larger refractive indices

34

than

ethanol in the investigated wavelength range. It is deduced that the sharp rise in ∆IPCE and ∆Abs at the wavelength range longer than 750 nm arises from the longitudinal resonance of Au@SiO2 nanorods and the neighbored low peak ~720 nm is produced by the transverse resonance of Au@SiO2 nanorods. Because the edge of 14


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the absorption band of perovskite film is around 770 nm, the peak of ∆Abs corresponding to the longitudinal resonance of Au@SiO2 nanorods cannot be identified. Here, in Fig. 6d, the top left and top right panels display the electric field distributions at ~720 nm (with the transverse resonance excited) and ~760 nm (with the longitudinal resonance excited, though at off-resonant condition), respectively, with the polarization of the incident electric field indicated in the plots. The simulation of electric field distribution is carried out by the finite element method and the detail is exhibited in the Supporting Information. The distribution maps clearly suggest that the electric field around Au@SiO2 nanorods is indeed significantly enhanced triggered by the excitation of SP resonance. It is the Au@SiO2 nanorods functioned as subwavelength “antennas” to couple light into the perovskite film, thereby improving the absorption of light in perovskite.22 At the wavelength band between 430 nm and 630 nm, although the Au@SiO2 nanorods are off-resonant, ∆Abs presents an apparent peak at 530 nm. The bottom left panel in Fig. 6d shows the electric field distribution around Au@SiO2 nanorod at 530 nm. One sees that at 530 nm, the field outside of the nanorods is similar to that at 720 nm in profile but a lower intensity, while the field inside of the nanorods turns into much brighter. At this situation, the absorption of light in the metallic nanorods becomes significant, yielding a non-negligible resistive heating, which may benefit the exciton generation in perovskite. Besides, at the wavelength band shorter than 430 nm, a peak of ∆Abs is also identified, which might benefit from inter-band transitions between the electronic bands in the bulk Au. Normally, Au@SiO2 nanorods tend to form the clusters on the PEDOT:PSS layer (Fig. 2a, Fig. S1a-b and Fig. S2b), thereby the electric field distribution of two neighbored nanorods at 760 nm was calculated as well (bottom right, Fig. 6d). It is clearly seen that light can be trapped at the gap between the nanorods, which could be another factor affecting the absorption in the pervoskite film. To summarize this part, the enhancement of electric field resulted from the excitation of different types of SP modes by the incorporated Au@SiO2 nanorods is responsible for the increase in absorption in the perovskite film, thereby contributing to the improvement in PCE and then a higher Jsc. 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In order to probe the physical mechanism of FF and Jsc increase for the PHJ-PSC devices with and without Au@SiO2 nanorods, the EIS technology was employed. EIS is a powerful and effective technique for characterizing the interfacial charge-transfer properties and recombination dynamics in solar cells.35-41 Normally, FF is directly related to the series and recombination resistances, which can reflect the charge transport and recombination processes inside of solar cells.38-40 The EIS of PHJ-PSC devices with and without Au@SiO2 nanorods are exhibited in Fig. S3 in the Supporting Information. The tests were carried out at different bias voltages under AM 1.5G solar illumination, where the high frequency is ascribed to the series resistance Rs and low frequency is attributed to the recombination resistance Rrec.35-36, 40

Fig. 7a and 7b present the series and recombination resistances for the PHJ-PSC

devices with Au@SiO2 nanorods (at the optimized concentration) and without Au@SiO2 nanorods under different bias voltages, respectively. It is found that the series resistance of PHJ-PSC devices with Au@SiO2 nanorods is smaller than that of PHJ-PSC devices without Au@SiO2 nanorods. The result is in excellent agreement with the change trend of series resistances extracted from J-V curves, where the average series resistances for PHJ-PSC devices with and without Au nanorods are about 164 Ω and 59 Ω, respectively. It is clearly seen from Fig. 7b that the recombination resistance in both systems decreases exponentially with increasing the bias voltage, which is attributed to the fact that the built-in electric field in device was removed by increasing the bias voltage, preventing the directional movement of photo-generated charge carriers. It also indicates that the recombination resistance of PHJ-PSC devices with Au@SiO2 nanorods is larger than that of PHJ-PSC devices without Au@SiO2 nanorods. Again, it is in agreement with the rule extracted from the J-V curves, which reveals the average shunt resistance of the device with Au@SiO2 nanorods (~2.8 ×10 4 Ω) is larger than that of the control (~1.3×10 4Ω). The increase in recombination and shunt resistance for PSC devices with Au@SiO2 nanorods can be attributed to the localized SP resonance effect of Au@SiO2 nanorods. The excitation of the localized SP resonance can benefit the dissociation and restrain the recombination of charge carriers.17,

21, 22

To sum up, the improvement of charge

16


Page 16 of 24

Page 17 of 24

transport and reduction of charge recombination could be responsible for the enhancement of FF and Jsc for PHJ-PSC devices with Au@SiO2 nanorods, resulting in the increase of PCE.

(a)

With Au@SiO2

10000

Without Au@SiO2

(b)

With Au@SiO2 Without Au@SiO2

1000 Rrec ( Ω⋅⋅cm2 )

10

Rs ( Ω⋅⋅cm2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

100 10 1 0.1

0.0

0.2

0.4

0.6

0.01

0.8

0.0

0.2

0.4

0.6

0.8

1.0

Vapp (V)

Vapp (V)

Fig. 7. The series and recombination resistances for PHJ-PSC devices with and without Au@SiO2 nanorods.

4. Conclusions In conclusion, highly efficient PHJ-PSC devices with a structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Al were fabricated by introducing Au@SiO2 nanorods at the interface between the hole buffer layer PEDOT:PSS and the active layer CH3NH3PbI3, of which the PCEs up to 17.6 % was achieved. Under optimized condition, PHJ-PSCs with Au@SiO2 nanorods presented remarkable enhancement in FF and Jsc, resulting in over 40 % enhancement in PCEs. The experiment and simulation results confirmed that Au@SiO2 nanorods not only increase the incident light trapping in the active layer by triggering localized SP resonance, but also improve the transport and collection of charge carrier in PHJ-PSC devices. The in-depth analysis of the absorption, IPCE and EIS proved that the enhancement of Jsc and FF results from the lower serial resistance, higher recombination resistance, longer recombination lifetime, and larger exciton generation rate. The metal NPs play an important role in the enhancement of photovoltaic performance and pave a boulevard for fabricating high-efficiency and low-cost PHJ-PSCs. 17



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Page 18 of 24

Supporting Information AFM and cross-section SEM morphology for Au@SiO2 nanorods on the PEDOT:PSS layer; Cross-section SEM images of PSC devices with and without Au@SiO2 nanorod; the EIS curves for PSC devices with and without Au@SiO2 nanorod; the transport resistance for PSC devices with and without Au@SiO2 nanorod. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This work was supported by the National Natural Science Foundation of China (11334014, 61475109, and 61274056), the Program for New Century Excellent Talents in University (NCET-13-0598), Hunan Provincial Natural Science Foundation of China (2015JJ1015), and the Project of Innovation-driven Plan in Central South University (2015CXS036). Y. Gao thanks the National Science Foundation (Grant No. CBET-1437656). R. Wu thanks the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC 201518) and Dr. Innovation fund of Central South University (2014ZZTS012).

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Modulated Photovoltage/Photocurrent Spectroscopy. J. Phys. Chem. C 2015, 119, 3456-3465.

23



Table of Contents Graphic Al PCBM CH3 NH3PbI3 Au@SiO2

PEDOT:PSS ITO

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

0 -5 -10 -15

From -1.5 V to +1.5 V PCE=17.6% From +1.5 V to -1.5 V PCE=17.7%

-20 -1.0

-0.5

0.0

0.5

Voltage (V)

24


1.0

Prominent Efficiency Enhancement in Perovskite Solar Cells

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