Unique Gold Nanorods Embedded Active Layer Enabling Strong

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The Unique Gold Nanorods Embedded Active Layer Enabling Strong Plasmonic Effect to Improve the Performance of Polymer Photovoltaic Devices Chunyu Liu, Chaoyang Zhao, Xulin Zhang, Wenbin Guo, Kun Liu, and Shengping Ruan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00459 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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The Journal of Physical Chemistry

The Unique Gold Nanorods Embedded Active Layer Enabling Strong Plasmonic Effect to Improve the Performance of Polymer Photovoltaic Devices Chunyu Liu, a Chaoyang Zhao, b Xulin Zhang, a Wenbin Guo, *,a Kun Liu, *,b Shengping Ruan a a

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

b

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,2699 Qianjin Street, Changchun 130012, P. R. China

ABSTRACT It is been widely reported that plasmonic effects of metallic nanomaterials can enhance light-harvesting in polymer soar cells (PSCs). However, the improved light trapping degree is closely related to the shape of the nanoparticles (NPs), which inevitably limits the efficiency enhancement for PSCs. In this paper, we demonstrated that the incorporation of Au arrow-head nanorods (AHNRs) into the active layer of inverted PSCs can dramatically lead a 28.7% effieiciency enhancement as compared to pre-optimize control PSCs. Both theoretical and experimental results show that the origin of the improved power conversion efficiency (PCE) can be attributed to not only the optical absorption enhancement but also charge transport capacity improvement. The metal tip of AHNRs can lead to a significant enhancement of local field and long-range scattering. In addition, a wide-band absorption improvement was observed and charge carriers mobilities increased by an order of magnitude. These results offer an effective approach to enhance the efficiency for PSCs. *

Corresponding author:[email protected](W.B. Guo),[email protected](K. Liu)

Author Contributions: Chunyu Liu and Chaoyang Zhao contributed equally to this work.

1. INTRODUCTION Polymer solar cells (PSCs) have been the subject of much research interest in the past few decades.1,2 Remarkable attraction of PSCs are the advantages of low cost, lightweight, and large-area fabrication on flexible substrates compared with inorganic solar cells.3-8 To date, the power conversion efficiency (PCE) of single PSCs based on low bandgap polymer as electron donor has reached to 10.5%.9 Moreover, higher PCEs of 11.3% for the cascade PSCs and 11.83% for the triple-junction have been achieved.10,11 However, this maximum efficiency of

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PSCs is not high enough to satisfy the commercial application, which are limited by insufficient light absorption and low charge carrier mobility within the active thin film.12 Hence, there is a need to increase the absorption by the photoactive film without increasing the film thickness, so to avoid the increase of the charge recombination.13,14 Light-harvesting enhancement can be realized through plasmonic and scattering effects within the photoactive film,15,16 buffer layers,17 or both.18,19 Noble metal nanoparticles (NPs) exhibiting localized surface plasmon resonance (LSPR)20-22 have been reported as additive to increase light absorption,23-25 such as nanosphere,14,26 nanorod,27 nanowall,28,29 nanodisk,30 and nano-core-shell structure.31 The shapes of NPs strongly affect the light trapping degree thus limiting the efficiency enhancement for PSCs. Among these, the metallic tip can create a strong field enhancement and scattering effect localizing at the apex.32-34 Meawhile, the influence of NPs on charge separation and transport inside high efficiency bulk heterojunction (BHJ) solar cells have been explored and studied in depth. Therefore, the employment of metal NPs could efficiently solve the problems of light absorption and charge transport property.35,36 In this communication, we report high-performance PSCs by incorporation of Au nanorods (NRs) with two arrow-head

ends

into

the

poly

[N-9´´-hepta-decanyl-2,7-carbazole-alt-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-

benzothiadiazole) (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) layer. Embedding Au arrowhead NRs (AHNRs) into the active layer and revealing advantages of its unique tip shape have not been reported yet. The incident light can induce field enhancement from a high surface charge density at the tip. Moreover, Au AHNRs with a relatively large size can strongly reflect and scatter light, acting as a high angle scattering center. These Au AHNRs can produce both near field and far field scattering effects. Also, direct mixing of Au AHNRs in an active layer could reduce the cell resistance and improve charge transport.

2. RESULTS AND DISCUSSION To demonstrate the role of plasmon enhancement due to the AHNRs, we choose the well-studied PSC system of PCDTBT: PC71BM. The molecule structure of donor and acceptor materials are exhibited in Figure1a, and device structure and energy levels are shown in Figure 1b. The Au AHNRs were synthesized by over-growth method (Supporting Information), which has an average length of about 100 nm. Figure 2 is the representative transmission electron microscope (TEM) image of the Au AHNRs, indicating that Au AHNRs possess well-defined shape and uniform size. The histogram of Au AHNRs length and diameter are shown in Figure S1 (Supporting Information). To embed the hydrophilic AHNRs into the hydrophobic active blend layer of PCDTBT: PC71BM, the surface ligand


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of AHNRs, cetyltrimethylammonium bromide, was replaced with hydrophobic thiol-teminated polystyrene, which prevents the aggregation of AHNRs in the active layer. Meanwhile, the absorbance spectrum of Au AHNRs solution shows two characteristic LSPR bands at 535 and 830 nm, assigned to transverse and longitudinal band, respectively (Figure S2, Supporting Information). As an initial study, we examined the influence of the Au AHNRs dopants on the morphological property of the active layers. The topographic images of PCDTBT:PC71BM films (5 µm × 5 µm) with various ratio (0, 0.5, 1.0, 1.5wt%) of Au AHNRs additive are showed in Figure S3 (Supporting Information) measured by atomic force microscope (AFM). There was ignorable difference of the surface roughness observed between undoped film and films doped with Au AHNRs, which indicates that majority of the Au AHNRs were embedded within the active layer and did not affect their morphologies. We studied the impact of Au AHNRs doping on the photovoltaic property. Figure 3a shows the typical current density-voltage (J-V) characteristics of fabricated devices. Detailed device parameters such as short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and PCE of the devices doped with different amounts of Au AHNRs were summarized in Table 1. All devices have a similar active layer thickness of ca. 120 nm, as determined by a profilometer. For the control device, a PCE of 5.75% was obtained with a Jsc of 13.06 mA/cm2, a Voc of 0.87 V, and a FF of 50.61%. After the incorporation of Au AHNRs in active layer, Jsc, FF, and PCE exhibited considerable enhancements, while the Voc of the doped devices remained the same. In particular, the doped device with 1.0wt% Au AHNRs exhibited the highest Jsc value at 15.54 mA/cm2, with a Voc at 0.87 V and a FF at 55.37%, resulting in a 28.7% enhancement for PCE. Besides, the series resistance (Rs) of the optimized device reduced from 15.1 to 8.2 Ω cm2, which contributed to the increase of FF. To further understand the reason for the increase of Jsc, the incident photon-to-current conversion efficiencies (IPCE) of all fabricated device were measured, and the results are illustrated in Figure 3b as a function of illumination wavelength. The control device showed the maximum IPCE of 65% at 490 nm, while the value of optimal doped device was 76% at the same wavelength. Meanwhile, the cells demonstrated a consistent increase of IPCE spectra within the wavelength range from 350 to 600 nm with Au AHNRs doping. This wavelength region coincides with the increased absorption range of the active layer with the Au AHNRs incorporation (Figure S4, Supporting Information), indicating that Au AHNRs did indeed improve the photocurrent.


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Since charge carrier mobility plays an important role on charge extraction by electrodes, hole-only and electron-only devices were fabricated and the charge carrier mobilities were also studied (or measured). The configuration of hole-only devices was ITO/MoO3/PCDTBT:PC71BM:Au AHNRs/ MoO3/Ag, where the MoO3 connected with ITO was used as an electron-blocking layer. The structure of electron-only devices was ITO/TiO2/PCDTBT:PC71BM:Au AHNRs/BCP/Ag, where the BCP was used as a hole-blocking layer. The dark J-V characteristics of single charge carrier devices were measured and fitted using the space-charge limited current (SCLC) model and the Mott–Gurney law that included field-dependent mobility. Figure 4a shows the J-V characteristics of hole-only device measured at driving voltage range of 0-8 V. The results indicated that Jsc of all doped devices were larger than that of the control device, and it was well agreement with the photocurrent in Figure 3a. Therefore, the doped devices possessed better capacity for hole transport due to the addition of Au AHNRs.37-39 Meanwhile, it can be seen from Figure 4b that Au AHNRs also conduced to the improvement of electron transport property. In order to make a realistic evaluation on the enhancement of charge transfer, charge carrier mobilities were calculated. At a typical applied voltage of 1.2 V, corresponding to an electric field of 105 Vcm-1 across the bulk of a 120 nm active layer, the apparent hole mobilities raise from 2.86×10-5 to 2.46×10-4 cm2V-1s-1 and the apparent electron mobilities increased from 6.65×10-6 to 1.07×10-4 cm2V-1s-1. After the introducing of Au AHNRs, the hole and electron mobilities both raised, and a more balanced charge transport was achieved.40-43 Higher carrier mobilities mean that charges are transported to the electrodes more quickly, which reduces current losses via recombination.4446

The presence of Au AHNRs in the PCDTBT:PC71BM blend layer provides an interface for charge transfer, which

enables the formation of percolation pathways for electron transport, thus reducing the effects of space charge formation and mitigating the charge recombination.40,44 The charge transport capacity is largely improved by incorporating the additive, leading to a significant improvement for Jsc and FF. To further investigate the role of Au AHNRs on optical absorption, we estimated the maximum photoinduced carrier generation rate (Gmax) in the devices with and without Au AHNRs. Figure 5a shows the dependence of the photocurrent density (Jph) on the effective voltage (Veff) for control and doped devices. Here Jph=JL-JD, where JL and JD are the current density under illumination and in the dark, respectively, and Veff= Vo-Va, where Vo is the voltage when Jph equals zero (i.e. JL=JD) and Va is the applied voltage.47,48 Apparently, Jph increases linearly at low Veff range and saturates at a high Veff. Therefore, the values of the saturation photocurrent density (Jsat) can be calculated from Figure 5a. Then Gmax was got from Jph= qGmaxL, where q is the electron charge and L is the thickness of active layer.


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The values of Gmax for the control and doped devices were 7.60 × 1027 m-3s-1 (Jsat=146 Am-2) and 8.59 × 1027 m-3s-1 (Jsat=165 Am-2), respectively. Thus, a considerable improvement in Gmax occurred after incorporating the Au AHNRs into the active layer. As the value of Gmax is related to the maximum absorption of incident photons,49,50 the increase of Gmax suggests an enhanced light absorption in the active layer of the doped device, which is consistent with the result in Figure S4 (Supporting Infornation). Meanwhile, due to the addition of Au AHNRs, schottky barrier could be formed around the area of the contact between Au AHNRs and semiconductor materials in the active layer. Under the light conditions, depletionregion will generate some excitons, and then they would be separated quickly due to the effect of build-in electric field. Excitons are dissociated into free carriers and transfer to other parts of the active layer, leading to an increase of photoinduced carrier generation rate. Furthermore, the exciton dissociation probabilities [P(E,T)] (or charge collection probability (PC)) were compared, which are related to the electric field (E) and temperature (T) for solar cells. Only a portion of photogenerated excitons can dissociate into free carriers for PSCs. Therefore, Jph can be expressed using the equation, Jph=qGmaxP(E, T)L. The value of P(E,T) at any bias can be obtained from the plot of the normalized photocurrent density (Jph/Jsat) with respect to Veff.51 Figure 5b reveals that the value of P(E,T) under the short-circuit conditions (Va=0 V) increased from 89.3% (for the control device) to 94.0% (for the optimal device), indicating that the excitation of LSPR originating from Au AHNRs facilitated excitons to dissociate into free carriers. Thus, the excitation of the LSPR increased both the exciton generation rate and the dissociation probability, thereby enhancing the Jsc for doped PSCs. The steady state photoluminescence (PL) spectra by using wavelength excitation source (λexc=470 nm) were employed to further examine the exciton dissociation in the active films. Figure 5c shows the PL spectra of PCDTBT:PC71BM films without and with various weight ratio of Au AHNRs annealed at 70 °C for 20 min. PL quenching was observed for the active film with Au AHNRs doping comparing to the one without doping, which contribute to the enhancement of the exciton dissociation due to the increased separation interfaces and reduced electron/hole recombination. Moreover, the PL spectra also indicate that there was an ultra-fast photoinduced charge transfer in the active layer. Both high charge separation and acceleration of the charge transfer were beneficial to the device performance. In order to explore the role of Au AHNRs on the resistance of the devices, the impedance spectra of PCDTBT:PC71BM based on cells with and without Au AHNRs were measured at frequency range of 20 Hz to 1 MHz as shown in Figure 5d. It can be seen that the shapes of impedance spectra are commendable semicircles that are beneficial to inquiry the resistance for PSCs. The diameters of semicircles are related with the


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series resistance. It is worth noting that semicircles diameters of all semicircles are very different, that is the more Au AHNRs was doped, and the smaller diameter was observed. It reveals that the Au AHNRs embedded in active layer can dramatically reduce the series resistance of the device. Some comparisons between Au AHNRs and Au spherocylindrical NRs are also expected. We did some experiments to confirm that arrow-heads play a considerable role on the performance improvement. As shown in Figure S5 (Supporting Information), the TEM images show that Au NRs possess the similar size with Au AHNRs. The parameters and J-V characteristics for optimized process of devices doped with Au NRs are shown in Table S1 and Figure S6 (Supporting Information). J-V characteristic of devices fabricated with optimal concentration of Au NRs is displayed in Figure 6, as well as the control device and optimal devices doped with Au AHNRs. Figure 6 shows that the PCE of the optimal device with Au NRs increases to 6.84% (Jsc of 14.29 mA/cm2, Voc of 0.87 V, and FF of 55.02%), however, it lags behind the optimized device with Au AHNRs. It can be confirmed that arrow-heads play an indispensable role in the enhanced performance of cells. The most important application of metal NPs as addition into active layer is the effect of LSPR. We deduce that the special tip shape of Au AHNRs could demonstrate better results than the spherocylindrical Au NRs. So the theoretical analog calculations of electric field profiles in spherocylindrical NRs and AHNRs doped into active layers were carried out by the finite-difference time-domain (FDTD) method. Figure 7 shows the distribution of field intensity around the AHNRs and spherocylindrical NRs in the wavelength of 500 nm which corresponds to the peaks in IPCE enhancements in Figure 3b, including end view (Figure 7a and b) and side view (Figure 7c and d). The magnitude of enhanced electric field was shown by the color scale. It was found that LSPR effects of AHNRs and spherocylindrical NRs in active layers were both excited. Moreover, rigorous calculation for Au AHNRs has shown an intensity enhancement factor of roughly 8 over the incident light. The field enhancement arises from a high surface charge density at the tip that is induced by the incident light polarized along the tip axis.32-34 The strong near field caused by Au AHNRs contributed to light trapping enhancement.

3. CONCLUSION In summary, we have investigated the effects of Au AHNRs doped into the active layer on the performance of inverted PSCs based on PCDTBT:PC71BM blending. Increasing Jsc and FF were observed in the cells including Au AHNRs with optimal doping ratio. And the maximum enhancement of 28.7% in PCE (from 5.75% to 7.40%) was achieved compared with the control device without Au AHNRs. LSPR induced local field enhancement not only


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leads to increased absorption of active layers but also benefits charge separation and transport, resulting in increased charge carrier density and mobility. In addition, the better results original from Au AHNRs than Au

spherocylindrical NRs were demonstrated due to their special arrow-heads. We believe that the results of our study offer an effective approach to enhance the efficiency of PSCs.

4. EXPERIMENTAL METHODS Materials and Preparation: PCDTBT and PC71BM were purchased from Lumtec corp. Au AHNRs were synthesized by ourselves using anover-mediated growth reported previously. 52,53 TiO2 solution was also prepared by ourselves according to some reported reference.54,55 MoO3 and Ag were also purchased from Lumtec Corp.

Device Fabrication: The device structure is indium tin oxide (ITO)/nano-crystal titanium dioxide (ncTiO2)/PCDTBT:PC71BM: Au AHNRs/molybdenum oxide (MoO3)/silver (Ag). Detailed process of devices preparation just likes the description in our previous papers.56,57 Au AHNRs solution was blended into active layer solution, and the weight ratio of Au AHNRs and donor/acceptor materials are 0.5wt%, 1.0wt%, 1.5wt%, and 2.0wt% respectively.TiO2 was spin-cast on top of the precleaned ITO substrate at 3000 RPM for 18 s then annealed at 450 °C for 2 h in the muffle furnace and cooled by nature. The PCDTBT: PC71BM (1:4 by weight) active blend layers doping with various ratio Au AHNRs were prepared by spin-coating the 1,2-dichlorobenzene (DCB) solution at 2000 RPM. Subsequently, the devices were completed by thermal evaporation of MoO3 and Ag electrode with the thickness of 10 nm and 100 nm, respectively.

Device Characterizations: Transmission electron microscopy (TEM) was performed on a Hitachi H800 operating at an acceleration voltage of 200KV. The light absorption and transmittance spectra were measured by means of ultraviolet/visible spectrometer (UV 1700, Shimadzu). The surface morphology was analyzed by Bruker Dimension Icon Atomic Force Microscope (AFM).Current density-voltage (J-V) characteristics were measured by a Keithley 2601 source/meter under AM 1.5G solar illuminations with an Oriel 300 W solar simulator intensity of ~100 mWcm-2. The incident photon-to-current efficiency (IPCE) was measured with Crowntech QTest Station 1000 AD. The impedance was analyzed by a Precision Impedance Analyzer 6500B Serious of Wayne Kerr Electronics.


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ASSOCIATED CONTENT Supporting Information Available The synthesis of Au arrow-head nanorods, the histogram of Au AHNRs length and Au AHNRs diameter, absorption spectrum of Au AHNRs solution, AFM topography of active films doping with different amounts Au AHNRs, absorption spectra of active layer doping with different amounts of Au AHNR.TEM image of Au NRs, J-V characteristics of devices doping with different amounts of Au NRs, device performance parameters of control and with different amounts of Au NRs. The Supporting Information is available free of charge on the ACS Publications website or the authors.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61275035, 61370046, 11574110, 21474040, 21534004), Project of Science and Technology Development Plan of Jilin Province (20130206075SF), Scientific Frontier and Interdiscipline Innovative Projects of Jilin University (2013ZY18), Key Technology Research and Development Program of Changchun (13KG66), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10), Project of Graduate Innovation Fund of Jilin University (2015098) for the support to the work.

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Table I. Device performance, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE), dependent on the different Au AHNRs. All parameters are typically average of 35 devices for each doping amount. Doping amount (wt%)

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0

0.87±0.01

13.06±0.12

50.61±0.19

5.75±0.16

0.5

0.87±0.01

13.40±0.10

54.73±0.10

6.38±0.11

1.0

0.87±0.01

15.54±0.15

55.37±0.23

7.40±0.20

1.5

0.87±0.01

14.32±0.13

54.66±0.22

6.81±0.18

2.0

0.87±0.01

13.29±0.20

50.87±0.13

5.88±0.14


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Figures:

Figure 1. (a) Molecule structure of PCDTBT and PC71BM, (b) structure and energy level of inverted PSCs.

Figure 2. TEM image of Au AHNRs.


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Figure 3. (a) J-V characteristics and (b) IPCE spectra of PSCs devices doping with different amounts of Au AHNRs.

Figure 4. J-V characteristics of single carrier device in dark (a) hole-only device and (b) electron-only device.


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Figure 5. (a) Photocurrent density (Jph) as a function of the effective voltage (Veff) for control and doped devices under constant incident light intensity, (b) exciton dissociation probability [P(E,T)] plotted with respect to effective bias (Veff) for these PSCs devices, (c) PL spectra of active films with different concentration of Au AHNRs, and (d) the impedance graph of PSCs devices doping with various amounts Au AHNRs in dark.


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Figure 6. J-V characteristics of undoped devices, optimal devices doped with Au NRs and Au AHNRs


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Figure 7. Electric field profiles of (a) Au AHNRs in end view, (b) Au NRs in end view, (c) Au AHNRs in side view, (d) Au NRs in side view.


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Table of contents:


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Unique Gold Nanorods Embedded Active Layer Enabling Strong

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