1 Plasmonic Metal Nanoparticles with Core-Bishell Structure for High

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Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells Kai Yao, Hongjie Zhong, Zhiliang Liu, Min Xiong, Shifeng Leng, Jie Zhang, Yun-Xiang Xu, Wenyan Wang, Lang Zhou, Haitao Huang, and Alex K.-Y. Jen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00135 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells

Kai Yao,*,†,‡ Hongjie Zhong,† Zhiliang Liu,† Min Xiong,† Shifeng Leng, † Jie Zhang,§ Yun-xiang Xu, Wenyan Wang,*, Lang Zhou, † Haitao Huang*,‡ and Alex K.-Y. Jen*,§,# †Institute

of Photovoltaics/Department of Materials Science and Engineering, Nanchang

University, Nanchang 330031, China ‡Department

of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon,

Hong Kong, China §Department

of Materials Science & Engineering, City University of Hong Kong, Kowloon,

Hong Kong, China College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China #Department

of Materials Science and Engineering, University of Washington, Seattle, WA

98195, USA *Email: [email protected]; [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT: To maximize light coupling into the active layer, plasmonic nanostructures have been incorporated into both activelayers of organic solar cells (OSCs) and perovskite solar cells (PSCs) with the aim of increasing light absorption, but reports have shown controversial results in electrical characteristics. In this work, we introduce a core-bishell concept to build plasmonic nanoparticles (NPs) with metal-inorganic semiconductor-organic semiconductor nanostructure. Specifically, Ag NPs were decorated with titania/benzoic-acid-fullerene bishell (Ag@TiO2@Pa), which enables the NPs possess nature compatibility with fullerene acceptors or perovskite absorber. Moreover, coating the Ag@TiO2 NP with a fullerene shell can activate efficient plasmon-exciton coupling and eliminate the charge accumulation, thus facilitating exciton dissociation and reducing the monomolecular recombination. The improved light absorption and enhanced carrier extraction of devices with Ag@TiO2@Pa nanoparticles are responsible for the improved short-circuit current and fill factor, respectively. On the basis of the synergistic effects (optical and electrical), a series of plasmonic OSCs exhibited enhancement of 12.3-20.7% with a maximum PCE of 13.0%, while the performance of plasmonic PSCs also showed an enhancement by 10.2% from 18.4% to 20.2%. This core-bishell design concept of plasmonic nanostructures demonstrates a general approach to improving the photovoltaic performance with both optical and electrical contribution.

KEYWORDS: plasmonic nanostructures, light absorption, organic solar cells, perovskite solar cell, charge recombination


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Organic solar cells (OSCs) comprising of bi-continuous networks of semiconducting donor/acceptor derivatives have attracted great attention due to their attractive properties such as cost effectiveness, light weight, large scale solution processing, mechanical E = .

! and etc.1-3

At present, the widely used materials in organic photovoltaics are still dominated by fullerene acceptors in combination with donor polymers.4 Although significant progress has been made to enhance their performance through the continuous development of high-performance materials (non-fullerene acceptor)5 and device engineering,6 their performance still lags behind that of other inorganic photovoltaic technologies. One of the fundamental reasons for this is the low charge mobility of organic materials, which has put OSCs into a dilemma. Consequently, a tradeoff between light absorption and charge extraction has to made, resulting in failure to absorb all of the incoming light. Therefore, it is appealing to consider optical engineering strategies that can enhance light absorption in certain families of bulk heterojunction (BHJ) OSCs. Among various available strategies, such as using folded device architecture (tandem cells),7 aperiodic dielectric stack,8 diffraction grating,9 and plasmon resonant metallic nanostructure,10-12 utilizing plasmonic metallic nanostructures as light concentrators offer a highly attractive solution to this predicament. The utilization of the localized surface plasmon resonance (LSPR) from metal nanoparticles is promising because of the ability to produce dense layers of solution-processed particles with strong light-matter interactions.13 Three widely proposed mechanisms for plasmon-enhanced solar cells, including far-field scattering, near-field enhancement, and plasmon-induced charge transfer, have been extensively investigated to improve the performance of solar cells.14,15 Deeper understanding on each of these mechanisms shows that cooperative plasmonic effects from both optical and electrical aspects will always occur. It is worth noting


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that the strongest confinement field of LSPRs and enhanced light scattering can be expected when embedding metallic NPs within the active layer.16 However, the effectiveness of this approach on increasing the efficiencies of OSCs is convoluted by a complex interplay of competing effects. In spite of a few successful reports on direct mixing of metal NPs in the active layer, there is considerable evidence to suggest that bare metal nanoparticles may promote charge recombination and exciton quenching losses at the metal surface due to dipole-dipole interaction and charge- accumulating mechanisms.17,18 In addition, the plasmon-electrical effects arise from morphology changes, redistribution of exciton generation and hot carrier injection, which are difficult to precisely control.19,20 Thus, competition between positive optical effects and negative electrical effects determines the final device performance, which could explain the contradictory results presented in previous work. A potential way to address this issue is to coat the noble metal nanoparticles with a thin dielectric layer (mostly SiO2), inhibiting the charge transfer yet still allowing them to retain their attractive optical properties.21-23 Moreover, plasmonic metal NPs with core-shell nanostructure, such as Au@SiO2 and Ag@SiO2, are also incorporated into the perovskite solar cells (PSCs). Apart from light trapping, some studies have also proposed the benefits of plasmonic material in perovskite device characteristics such as exciton dissociation, carrier mobility and electron transfer.24 However, by using an insulating shell, part of the photogenerated carriers from the most absorption-enhanced absorber located around the surface of metal NPs as well as the plasmonically generated hot carriers would be lost due to the inhibition of charge injection through SiO2 shell.25 Instead, oxide semiconductors (such as TiO2, ZnO and CuOx) should be chosen as the shell material rather than insulating components because the carriers can be easily transferred to surrounding absorber through Schottky metal/semiconductor interface.26 A


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conductive TiO2 layer enables charge accumulation and transfer of carriers to the active layer through plasmon-induced resonance energy transfer and hot electron injection processes,27 while silica only allow hot electrons to transfer over the Schottky barrier when the thickness of the SiO2 layer is less than its tunneling barrier height (~2 nm).28 Furthermore, the choice of the outer shells also can affect the charge transport and morphology of the OSC. As we know, metal NPs should be uniformly dispersed within the embedding layer, eliminating aggregation of the NPs and disruption of the OSC morphology. However, bare metal NPs are difficult to fulfill the uniform dispersion within the active layer while maintaining good control of the activelayer morphology.29 For example, it was found that the morphology of the active layer could be disrupted when Au NPs were blended into the active layer. And the aggregated Au NPs result in more quenching energy states and recombination centers inside the active layer, accompanied by a high leakage current in the device.30 On the other side, capping the metallic NPs with suitable organic layer may cancel the influence on morphological evolution of the BHJ active layer during the spin-coating process. It has been reported that the use of P3HT-terminated Au NPs in the P3HT:PCBM system appears to be an efficient way to suppress deterioration of the active layer morphology.31 However, poorly capped NPs with only organic ligands may still act as charge trapping sites and result in losses via charge/energy transfer between organic materials and the metallic nanostructure. We think that a combination of an oxide semiconductors inner-shell with an additional organic out-shell may be able to eliminate such problems. In this study, we proposed a bishell concept through a combination of an inorganic semiconductor outer shell (for charge-transport) with an additional organic capping shell (for morphology) to eliminate both negative effects induced by metallic NP incorporation. As a proof


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of concept, Ag@TiO2@Pa NPs, by coating fullerenes out-shell (Pa) onto TiO2 inner-shell nanoparticles through carboxylic linkage, has been designed and prepared. On one hand, the presence of fullerene outer-shell has good compatibility with fullerene acceptors and provides a powerful approach for achieving good dispersity with a large set of polymer–fullerene BHJ material systems, comprising binary and ternary blending. On the other hand, the combination of TiO2-Pa bishell with suitable energy alignment is beneficial to the charge transport and collection in the plasmonic device due to the participation of plasmon-exciton coupling. Consequently, both the optical effect and electrical effect induced by the LSPRs were properly regulated to improve the photocurrent and FF simultaneously. On the basis of the synergistic effects, we have achieved a critical advance over core-shell structured metal nanoparticles for plasmonic OSCs and PSCs, providing a versatile and effective route for performance enhancement with physical understanding. RESULTS AND DISCUSSION The Design Role of Core-bishell Nanostructure. To take advantage of plasmonic-electrical effects on the device performance, a number of conditions need to be simultaneously satisfied: (i) location in the active layer to achieve the optimal near-field coupling (Figure 1a) (ii) the orbital coupling between metal NPs and adsorbates on transfer of hot-electrons with spectral matching, (iii) a static build-in field to separate plasmonically created electron-hole pairs before they thermalize, and (iv) a suitable interface to reduce recombination and control the junction between metal NPs and activelayer. Previous work demonstrated that inorganic shell is critical for preventing the charge recombination and organic shell is decisive for the morphological control of the BHJ active layer embedded with metal NPs. Therefore, we expect that the corebishell nanostructure (Figure 1b) can promote LSPR-induced electron transfer between noble


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In our initial experiments, we explored the electron transfer of nano-plasmonic solar cell by mimicking the core-bishell nanostructure with similar energy alignment. Here we chose fullerene as outer-shell, owing to their excellent mobility and good compatibility with fullerenebased BHJ activelayer. The schematic of the nano-plasmonic structures is shown in Figure 1c, while the fabrication process is presented in Scheme S1. Before the deposition of C60, thin TiO2 or SiO2 films are deposited on top of nanostructured Ag rear electrode as spacer and the final device consists of a Glass/Ag/TiO2/C60/ITO or Glass/Ag/SiO2/C60/ITO architecture. Under illumination, the injection of excited electrons from Ag nanostructured arrays into C60 semiconductors is expected. However, the electron transfer will meet three competitive processes and be lost via intra-band thermalization, inter-band transitions, or surface recombination,32 where the recombination can be mitigated by interface passivation (Figure 1d and 1e). With the presence of TiO2 interface, electron transferred through the TiO2 rapidly thermalize in the conduction band of C60, giving rise to open circuit voltage determined by the built-in electric field of the junction. However, only the excited electrons with enough energy can either tunnel through to the C60 layer or undergo Schottky emission over the barrier when the interface is passivated with SiO2 layer. Besides, the increase of the barrier thickness would significantly reduce the efficiency of the carrier injection.28 Current-voltage (J-V) characteristics (Figure 1f and 1g) of the nano-plasmonic device demonstrate the ability of these devices on carrier extraction. The TiO2 device showed very high photocurrent but with a small open-circuit voltage (VOC). In contrast, the insertion of the SiO2 layer leads to the formation of Schottky barrier @OSB) with reduced short-circuit current (JSC) but high Voc (0.6 V). The change of short-circuit photocurrent was further evaluated from the external quantum efficiency (EQE), as shown in Figure S1. Therefore, it is evident that the Ag-TiO2-C60 (metal-semiconductor-semiconductor,


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MSS)33 stacked structure enables efficient transfer of the excited electrons to the fullerene. Stimulated by the results of the macroscopic Ag/TiO2/C60 films, we therefore further explore a similar design on the metal NPs with TiO2/fullerene bishell nanostructure. Preparation and Characteristics of Plasmonic Metal Nanoparticles. Ag NPs were prepared following a two-step chemical reaction, that is, synthesizing Ag NPs at 120 oC and then forming TiO2 shells at room temperature.34 Although the oxide shell reduces the recombination and back reaction of electrons on the surface of metal NPs by providing an energy barrier between metal and absorber, the dispersity of metallic NPs in the BHJ layer still bring limitations on the performance. Furthermore, Ag@TiO2 NPs were modified with fullerene derivates, PCBMsubstituted benzoic acid (Pa). The detailed synthetic procedures of plasmonic nanostructures are described in experimental section. Figure 2a shows the high-resolution TEM (HRTEM) images of Ag (average diameter ~ 25 nm) with the lattice fringes of Ag crystalline structure. Figures 2b and 2c show the Ag@TiO2 NPs with an amorphous TiO2 shell about 2~3 nm in thickness and fullerene acid treated Ag@TiO2, termed Ag@TiO2@Pa, with assembled monolayer anchoring to the surface of TiO2 shell-layer, respectively. The formation of Ag@TiO2 and Ag@TiO2@Pa nanostructures was also confirmed by optical absorption spectra (Figure 2d). The absorption peak from the surface plasmon resonance shifts from 403 nm (bare Ag) to a longer wavelength of 421 nm (Ag@TiO2) and 424 nm (Ag@TiO2@Pa), due to the high dielectric constant amorphous TiO2 and amorphous Pa. With the loading of fullerene, the absorption of Ag@TiO2 below 400 nm can be complemented. The Fourier transform infrared spectrometry (FTIR, Figure 2e) of fullerene acid absorbed on Ag@TiO2 show two bands at 1711 and 1677 cmR$, which are attributed to splitting of two similar carbonyl (C=O) groups. These bands are assigned to either bidentate chelating or bidentate bridging ring structure, as shown in the inset.35 Figure


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Extended from our previous works, high-efficiency BHJ consisting of ladder-type donor polymer, poly(indaceno-dithieno[3,2-b]thiophene difluorobenzothiadiazole) (PIDTT-DFBT),37 and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were chosen as photoactive layer. As shown in the TEM images (Figure 3a) of PIDTT-DFBT:PC71BM films with NPs, the Ag@TiO2 with organic ligand attachment showed severe aggregation, while the Ag@TiO2@Pa dispersed well in the activelayer. It indicated that the morphology of activelayer may be disturbed by the incorporation of Ag@TiO2 NPs, while coating the NP with fullerene shell is found to give compatible blends with pristine activelayer. In contrast, the presence of fullerene outer-shell in the Ag@TiO2@Pa NPs can eliminate the negative effect. In order to investigate this further, atomic force microscopy (AFM) was conducted to study the morphology of activelayer film with various metal NPs (Figure S4). The Ag@TiO2@Pa devices show similar height and phase images as the reference devices, indicating a relatively unchanged morphology in the bulk heterojunction. On the other hand, the activelayer films with Ag@TiO2 loading show significantly increased roughness as well as some abrupt bumps on the surface, due to the aggregations of the Ag@TiO2 NPs. We further extracted the absorption (A) from diffuse reflection (R) and transmission (T), using 1 – R – T (Figure 3b). The absorber film doped Ag@TiO2@Pa with exhibits greater light absorption than reference film in the range of 400-800 nm, showing a noticeable peak around 500 nm. In addition, these results also show that Ag@TiO2-doped films have slightly improved absorbance and much redshifted extinction spectra when compared to that of Ag@TiO2@Pa. These results also suggest that the optical enhancement originate from the plasmonic optical effect of LSPRs, which typically shows a wavelength dependency.


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spectrum when compared with that of well-dispersed Ag@TiO2@Pa. This aggregation-induced red-shift was also evidenced when we used other simple packing modes (Figure S6). Meanwhile, we found that the coupled surface plasmon resonances in aggregated nanostructures would cause an electric field enhancement, especially at the nanogap. Besides, we also investigated the effect of core-shell plasmonic NPs on the long-lived charge carrier yields of the BHJ film. We performed photoinduced absorption (PIA) spectroscopy to monitor the absorption of free charge carriers (polarons).38 As shown in Figure 3e, both the PIA spectra of PIDTT-DFBT:PC71BM films with Ag@TiO2 and Ag@TiO2@Pa incorporation show an increased enhancement in the magnitude of the polaron absorption (1.35 eV). The increased intensity also confirms that the enhanced plasmonic fields raise the exciton generation rate. It is worth noting that PIA spectra based on Ag@TiO2 also shows a bleach around 1.8 eV, which may attributed to the excess trapped electrons that survive within the aggregated Ag@TiO2. Device Performance. Based on the optical effect of Ag@TiO2 or Ag@TiO2@Pa within active layer, we first fabricated OSCs to evaluate how the dispersity of NPs in BHJ layer influences device performance. Here, we selected PIDTT-DFBT:PC71BM and PTB7-Th:PC71BM as representative disorder dominated polymer system and semi-crystalline polymer system, respectively. We fabricated three groups of devices in an inverted configuration (Figure 4a). Except the devices with the incorporation of Ag@TiO2 or Ag@TiO2@Pa into the activelayers, reference devices without NPs were also prepared. Before the experimental study, we firstly performed a preliminary calculation on the integrated absorption distributions in the xoy plane of the different position along z axis in the PIDTT-DFBT based devices. Figure 4b exhibits the absorption distribution in active layer of the reference OSC, which is manifested as the uniform over the intrinsic absorption band of the blend material PIDTT-DFBT:PC71BM. Compared to the


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Next, we directly investigated the effect of plasmonic NPs embedded in a activelayer on the device performance. The densities of NPs were optimized (Figure S7) and the representative J–V characteristics of the devices based on PIDTT-DFBT are shown in Figure 4c and Table 1. Surprisingly, approximately 2 wt% addition of the Ag@TiO2@Pa nanoparticles in activelayer films can lead to the best result in each group. The champion PIDTT-DFBT device shows VOC of 0.98 V, JSC of 14.24 mA cm-2, FF of 0.66, and PCE of 9.20%, which is 20.1% higher than that of the control device. However, the incorporation of Ag@TiO2 NPs resulted in inferior performance than desired with only slightly increased JSC. It is important to note that the PTB7Th based system achieved similar results. The PTB7-Th reference devices without NPs gave a VOC of 0.80 V, JSC of 16.38 mA cm-2, FF of 0.72 and PCE of 9.40%. After the introduction of 1.0 wt% of Ag@TiO2@Pa NPs in PTB7-Th:PC71BM, the optimal device shows an obvious increase of the PCE to 11.05% with the VOC, JSC, and FF of 0.82 V, 18.14 mA cm-2 and 0.74, respectively. After the introduction of Ag@TiO2 with the same concentration in the activelayer, performance enhancement was eliminated and the PCE values dramatically decreased with a significantly low FF of 0.67. The drop in FF in the devices doped with Ag@TiO2 is consistent with the lower shunt resistance (RSH) and higher leakage currents, as shown in the dark J–V characteristics on a log-linear scale (Figure S8). In contrast, the control device based on Ag@SiO2@Pa only exhibited obvious photocurrent enhancement (Figure S9), which is consistent with the extracted absorption (Figure S10). We believe that insulating SiO2 coatings have eliminated electronic effects and performance enhancement are dominated by plasmonic optical effects,12 while both optical and electrical characteristics are affected by using semiconducting TiO2 shell. To confirm the contribution from plasmonic effect, another comparative experiment was conducted by introducing TiO2@Pa NPs (Figure S11) without metal Ag core into the activelayer. However,


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incorporating TiO2@Pa (metal-free) brought a negligible change on device performance and we could not verify the existence of LSPR effect from the extracted absorption spectra. Table 1. Characteristics of plasmonic solar cell incorporating metal nanostructures at optimal concentration in various activelayers under AM 1.5G illumination (100 mW cm-2). Activelayer Nanostructure PIDTTDFBT :PC71BM

PTB7-Th :PC71BM

JSC (mA cm-2)

VOC (V)

FF

b) PCE ( (%)a) W@DA

Reference

12.66 ± 0.12 (12.74) 0.96 ± 0.00 (0.96) 0.62 ± 0.01 (0.63)

7.54 (7.66)

Y

Ag@TiO2

13.24 ± 0.19 (13.33) 0.95 ± 0.01 (0.96) 0.60 ± 0.02 (0.61)

7.56 (7.81)

+2.0

Ag@TiO2@Pa 14.13 ± 0.16 (14.24) 0.98 ± 0.01 (0.98) 0.65 ± 0.01 (0.66)

9.03 (9.20)

Reference

16.32 ± 0.13 (16.38) 0.80 ± 0.00 (0.80) 0.71 ± 0.01 (0.72)

9.28 (9.40)

+20.1 Y

Ag@TiO2

16.82 ± 0.21 (16.91) 0.79 ± 0.01 (0.80) 0.65 ± 0.02 (0.67)

8.65 (9.01)

-4.1

Ag@TiO2@Pa 18.05 ± 0.18 (18.14) 0.81 ± 0.01 (0.82) 0.73 ± 0.01 (0.74) 10.72 (11.05) +17.6 P3HT :IC60BA

10.56 ± 0.11 (10.63) 0.83 ± 0.00 (0.83) 0.67 ± 0.01 (0.68)

5.89 (6.03)

Y

Ag@TiO2@Pa 11.96 ± 0.15 (12.06) 0.85 ± 0.01 (0.85) 0.70 ± 0.02 (0.71)

7.10 (7.28)

+20.7

Reference

Y Reference 20.70 ± 0.12 (20.76) 0.77 ± 0.01 (0.78) 0.71 ± 0.01 (0.72) 11.36 (11.58) PTB7-Th: BTR:PC71BM Ag@TiO @Pa 21.90 ± 0.18 (21.99) 0.78 ± 0.01 (0.79) 0.73 ± 0.02 (0.75) 12.51 (13.01) +12.3 2 MAPbI3 a) Best

Reference

20.32 ± 0.42 (20.71) 1.10 ± 0.00 (1.12) 0.78 ± 0.02 (0.79) 17.58 (18.37)

Y

Ag@TiO2@Pa 21.27 ± 0.46 (21.69) 1.11 ± 0.01 (1.13) 0.81 ± 0.02 (0.83) 19.23 (20.24) +10.2

values in the brackets; b) The change factors @W % compared to the best performance of reference devices.

%

ref)

of plasmonic devices

The EQE spectra were recorded in Figure 4d (PIDTT-DFBT based) and 4e (PTB7-Th based) for better evaluation of the photocurrent change. It can be observed that EQE is enhanced in a broad range (350 to 800 nm) upon the incorporation of Ag@TiO2@Pa, while it maximizes at 510 nm. This enhancement region was consistent with the absorption enhancement factor from reflectance spectra. On the contrary, although improved and redshift absorption was obtained in the Ag@TiO2 blending cells, EQE enhancement factor decreases as compared with that of Ag@TiO2@Pa cell. In spite of the enhanced absorption and plasmonic field, we postulate that the deterioration of the activelayer morphology, as a result of the aggregated Ag@TiO2 NPs, would bring recombination pathways. Furthermore, we calculated the IQE (IQE = EQE/A) by


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measuring the total absorption spectrum (A) and EQE of the PIDTT-DFBT based OSCs (Figure S12) to determine the plasmonic electrical effect of core-bishell nanoparticles on charge collection. Note that the variation of the IQE of plasmonic devices with different nanostructures is in an opposite trend. This trade-off in morphology deterioration would cancel the net benefit for the plasmonic device. With the addition of different nanoparticles, the evolution of BHJ morphology, including crystallinity and donor/acceptor heterostructure interface, is certainly a critical factor affecting the overall device performance. It is important to note that PTB7-Th based system with strong molecular packing is more sensitive to the blend morphology. The origin of the Performance Enhancement. For Ag@TiO2@Pa system, both the JSC and FF are dominant factors for device performance enhancement. We therefore can infer that the incorporation of metal NPs will affect the charge carrier generation and transport. In order to understand the plasmonic effects on the improvement of JSC, we further explore the effects of NPs on exciton generation and dissociation. Figure S13 reveals the positive effect of the incorporation of various types of NPs on saturation photocurrent density (Jsat) and the corresponding exciton generation rate Gmax.39 We assume that all generated electron–hole pairs are dissociated and collected at high effective voltage, and the saturation current (Jsat) is limited just by the total amount of absorbed incident photons (Gmax). Such an increase suggests enhanced light absorption in the active layer of the plasmonic device, which is consistent with the increased photocurrent for both devices. The value of exciton dissociation probabilities, P(E,T) (Jph/Jsat) increased from 84.3% for the PTB7-Th reference device to 89.2% for the Ag@TiO2@Pa device. However, it decreased to 80.9% for the Ag@TiO2 device. This opposite change of exciton dissociation probability is consistent with the IQE result. It indicates that LSPR effect induced by the dispersed bishell Ag NPs would facilitate excitons to dissociate into


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free carriers, while the strong aggregation of plasmonic NPs is harmful to efficient dissociation, thereby enhancing recombination rates. Further proof of the strong dependence of recombination on the aggregation comes from the light intensity dependence of the J-V characteristics of the devices under illumination, which is a valuable indicator to probe the dominant recombination mechanism. As can be observed from the PTB7-Th system (Figure S14), the nearly linear dependence of JSC and VOC on the incident light intensity with exponential factors @[A close to 0.94 and power law of (s) 1.1, respectively, for both reference and Ag@TiO2@Pa cells. Therefore, it can be assumed that the recombination mechanism in both devices is dominated by bimolecular Langevin recombination. In parallel, the Ag@TiO2-based solar cell shows a significantly steeper slope of VOC (1.48 kT/q), implying the presence of additional trap-assisted recombination pathways, as a result of disrupted morphology or/and NP-induced chargetrapping. The light-dependence results suggest that the aggregation of Ag@TiO2 NPs would act as charge trapping sites. Actually, a strong decrease in FF with the incorporation of Ag@TiO2 is strongly suggestive of inefficient charge collection. To verify, we then analyzed the transport behavior of plasmonic devices. We firstly investigated variations in the charge mobility of the BHJ layers as a function of shell structures of Ag NPs by space charge-limited current (SCLC) method. Figure S15 compares the nanoparticle dependence of carrier mobilities for PIDTTDFBT:PC71BM and PTB7-Th:PC71BM blends. For both activelayers, the incorporation of Ag@TiO2 nanoparticles has essentially no obvious positive effect on the electron/hole mobility. In contrast, incorporation of Ag@TiO2@Pa NPs favors improved carrier mobility. Particularly, the electron mobility is affected strongly by the shell structure of nanoparticles due to their excellent electron affinity. For example, the electron mobilities of the reference PTB7-


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Th:PC71BM device as well as those blended with Ag@TiO2 and Ag@TiO2@Pa as calculated from the currents in the square law region are (7.66 ± 0.41)×10-4, (7.02 ± 0.46)×10-4 and (1.01 ± 0.048)×10-3 cm2 V-1 s-1, respectively. Compared with bishell Ag NPs, the decreased electron mobility for the aggregated Ag@TiO2 cells is not surprising and can be attributed to the unfavorable morphology. Therefore, this result suggests that the metal/semiconductor interface constructed by the core-bishell structure may facilitate charge collection. However, the unbalanced carrier mobilities can hardly be the primary factor for the increasing of JSC and FF in the Ag@TiO2@Pa case. To acquire insight into the dynamics of the photogenerated excitons within the photoactive blend, we performed steady state PL and time-resolved PL (TPRL) measurements. The PL intensity of PIDTT-DFBT:PC71BM activelayer films was enhanced in the presence of Ag@TiO2 or Ag@TiO2@Pa when they were excited with 485 nm light, confirming the increase in density of photogenerated excitons upon the excitation. The results are consistent with enhanced polaron signal in the PIA spectra and the greater value of Gmax in the plasmonic device, since excitation of the plasmonic NPs resulted in local enhancement of the electromagnetic field in the vicinity of the Ag NPs induced by LSPRs. Further, the PL decay profiles of PIDTT-DFBT:PC71BM films in the presence and absence of Ag NPs are displayed in Figure 5a, in which the PL decay data were adequately fitted using a double exponential function (Table S1). The corresponding effective exciton lifetimes, .exc, were calculated to be 263, 272 and 108 ps for the reference, Ag@TiO2 and Ag@TiO2@Pa-based blends, respectively. Interestingly, incorporation of Ag@TiO2 into the blend film leads to the longest decay time, especially for the fast decay process (quenching of free carriers). This TPRL results suggest that excitons in this blend need to diffuse for a longer time to reach a donor/acceptor interface compared to the pristine activelayer.


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To further trace the effect of plasmonic NPs on charge collection, we performed transient photovoltage (TPV) measurements to compare the lifetime of charges (.) in blends as a function of carrier density (n).40 The small perturbation lifetime, .Wn, is extracted from the singleexponential decays in dependence of the respective open circuit voltage at various illumination levels. The total charge lifetime, . (n), can be determined through, ( )=

( + 1)

(1)

And the slope of n–.Wn gave the order of recombination, d = d

0

+1

(2)

where 0+1 (R) is the so-called recombination order. Figure 5b depicts . versus n for the cells doped with Ag NPs as well as the reference cells. Taking PTB7-Th:PC71BM activelayer as an example, the TPV signal of reference device showed a power law dependence with slope (R = 2.19) and a reasonable carrier lifetime (. = 2.06 µs) biased at 1 Sun light intensity, which agrees with the results from reported values. Under 1 Sun illumination, the increased charge carrier density values also agrees well with the higher photocurrent obtained for plasmonic based devices, in which n continuously increases from n = 2.10 × 1016 cm-3 of control PTB7Th:PC71BM cell to 2.20 × 1016 cm-3 of Ag@TiO2-based cell and finally to 2.45 × 1016 cm-3 of Ag@TiO2@Pa-based cell. Further, the addition 1.5 wt% of Ag@TiO2@Pa NPs enhances the carrier lifetime to . = 2.80 µs, while the incorporation of Ag@TiO2 NPs decreases the . to 0.68 µs but increases the R to 2.85. As we know, a recombination order R equal to 2 indicates perfect bimolecular recombination at open circuit voltage conditions. Therefore, the obtained R = 2.09 for the Ag@TiO2@Pa case, confirms the aforementioned ideal bimolecular recombination for this system, which proves our hypothesis that the presence of plasmon-exciton coupling is


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beneficial to carrier extraction and justifies the significantly improved FF value. In contrast, the recombination order close to 3 suggests trap-assisted recombination dominating at Ag@TiO2 system, which should be correlated to the effect of NP-induced traps as well as morphological traps within the active layer of organic solar cell. Considering the formation of an aggregated Ag@TiO2 subnetwork inside the active layer, accumulation of electrons and inhomogeneous charge carrier distribution can be expected, consistent with the decreased FF and inferior performance. To gain a complete picture of the loss mechanisms in the Ag@TiO2-based devices, we also explored dark current characteristics that are highly dependent on the transport and recombination processes.17,41 The common diode equation with the presence of a series resistance RS and a parallel shunt resistance RSH is expressed as,

=

[ (

0

) ]

exp

1 +

(3) !

where ndark is the dark ideality factor, J0 is (reverse bias) saturation current density, while k, T and q are the Boltzmann constant, temperature and elementary charge, respectively. The ndark determines the slope of exponential regime of the dark J-V characteristics on a semi-logarithmic plot and can be expressed as,

dark

=

(

&'( &

)

1

(4)

Figure S16 depicts the dark J-V characteristics with fitted slope under forward bias. For the sensitive PTB7-Th:PC71BM system, ndark increases from 1.34 in the control cells and 1.31 in the Ag@TiO2@Pa-based cell to 1.92 in the cells doped with Ag@TiO2—a trend consistent with light-dependent measurements. Since the ideality factors under dark forward bias is dependent


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on the deeply trapped carriers in the BHJ activelayer, we interpret the larger ideality factor arises from an increase in the energetic disorder and a loss in the charge collection stemming from the presence of aggregated Ag@TiO2-induced traps. All these results indicate that embedding the Ag@TiO2 NPs within the charge transport layers could also act as charge trapping sites, other than the induced morphology changes. Since we chose TiO2 rather than insulating SiO2 as the shell material, the excitons could be quenched by either energy transfer to the metal nanoparticles (Ag core), a speeding up of the radiative or non-radiative decay rate, or via induced charge separation. However, the improved charge transport is a surprise since we would not expect the fullerene shell (Pa) to be able to sustain rapid electron conduction. In order to better understand the advantages of TiO2 and fullerene combination, the linear current-voltage (J-V) measurement was conducted. The conductivity of the fullerene film doped with Ag@TiO2@Pa in the dark state is found to be slightly higher than that of Ag@TiO2 (Figure 5c). Moreover, it is found that the conductivity of the PC71BM:Ag@TiO2@Pa shows a great improvement after illumination. The obvious light-induced enhancement in the conductivity of PC71BM:Ag@TiO2@Pa indicates the presence of surface plasmon enhanced photo-conductance which may be ascribed to charge transfer process of plasmonically excited electrons from metal NPs to bishell. Judging by the conduction band (CB) of amorphous TiO2 (4.0 eV) and the lowest unoccupied molecular orbital (LUMO) level of PC71BM (4.2 eV), we can infer that the energy barrier predominantly blocks forward electron transfer into the TiO2, rather than fast charge transport. In the Ag@TiO2-based devices, the accumulation of free charge carriers at the interface is consistent with increased trap-assisted recombination. By contrast, coating the Ag@TiO2 NP with a fullerene shell can facilitate efficient device action and eliminate the undesired recombination even without plasmonic excitation. In addition, the electrons on the Pa


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are blocked from transferring onto the TiO2 because of the poor electronic coupling across the B_B

>

benzoic acid group binding the two.42 Therefore, charges now are being

“stored” in these fullerene molecules, which will benefit the electron extraction in the plasmonic device. Overall, we proposed a mechanism in Figure 5d and 5e, in which electrons transfer from both the donor polymer and plasmonic excitation to the Pa shell, upon which the electrons are then collected through fullerene channel.43 The mechanism is consistent with the significantly improved FF and slightly increased voltages in Ag@TiO2@Pa device. The inhibition of electron injection into the TiO2 reduces the chemical capacitance in the device and push up the built-in potential (Figure S17), which is consistent with slightly increased VOC. Generality Investigation. Lastly, in a bid to prove the generality of our method, we employed the bishell Ag@TiO2@Pa into other typical absorber materials, namely P3HT:IC60BA and PTB7-Th:BTR:PC71BM44 (0.75:0.25:1.5 wt/wt/wt) systems, in which the BTR named benzodithiophene terthiophene rhodanine. P3HT has stronger intermolecular packing with crystalline aggregation than PTB7-Th, while the ternary blend exhibited a state-of-the-art performance among fullerene OSCs with a PCE over 11%. Figure 6a and 6b depict the J–V curves of plasmonic devices based on various activelayers, and Table 1 presents the photovoltaic parameters of the cells. Upon the introduction of plasmonic Ag@TiO2@Pa NPs, both P3HT:IC60BA and the ternary devices show similar behaviors to that of PTB7-Th:PC71BM. Benefited from the LSPR effect induced by Ag@TiO2@Pa, the PCE of P3HT:IC60BA device is substantially improved from 6.03% to 7.28% (20.7% enhancement), and the addition of bishell Ag NPs into ternary system was found to improve the maximum PCE from 11.58% to 13.01% (12.3% enhancement). The photocurrent enhancements are also demonstrated by the EQE spectra (Figure S18). Taking ternary system as an example, the derived absorption spectra (A= 1


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– R – T) of Ag@TiO2@Pa-doped film only showed slight increase in comparison with the respective reference (Figure S19), because the addition of BTR into binary PTB7-Th:PC71BM have already induced the formation of more ordered polymer domains that improve the photon absorption range and facilitate charge transport. However, in line with the PTB7-Th-based results, both the photocurrent and FF of the plasmonic device are obviously enhanced in comparison with the reference device, suggesting that the incorporation of Ag@TiO2@Pa provides a general approach to improved performance from both charge generation (optical properties) and charge transport (electronic properties) simultaneously. To confirm this, we further calculated the internal quantum efficiency (IQE) by measuring the total absorption spectrum and EQE of the binary (PTB7:PC71BM) and ternary OSCs. As expected, the IQE value of Ag@TiO2@Pa-doped PSCs stays above 93% throughout the entire absorption spectrum (380– 770 nm), whereas the IQE of the device without Ag nanoparticles is below 90%. Since the IQE is the ratio of the number of charge carriers collected by the solar cell to the number of absorbed photons, the higher IQE of plasmonic device confirmed suppressed recombination and higher charge collection efficiency due to the plasmonic nanoparticles. Moreover, it is noteworthy that the performance based on core-bishell NPs over 13% is one of the highest among the PCE values for plasmonic OSCs (see Table S2).


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no harm to the growth of perovskite films composed of densely packed large grains. J–V curves and EQE spectra of the optimized reference and plasmonic devices are demonstrated in Figure 6c, 6d and Figure S21, respectively. The best PCE of the bare MAPbI3 PSCs was significantly enhanced by 10.2% from 18.37% to 20.24% with the addition of 0.2 wt% Ag@TiO2@Pa. Furthermore, it is notable that the high-efficiency inverted plasmonic perovskite cell achieves a very high fill factor of 0.83, which is one of the highest values of perovskite solar cells reported in the literature. The films embedded with Ag@TiO2@Pa showed more uniform PL emission distribution than pure MAPbI3 with reduced quantity of the prominent dark regions (details in Figure S22a), suggesting that plasmonic NPs could remedy non-radiative defects in the perovskite film. The reduced trap-assisted recombination promotes efficient charge separation of photogenerated excitons in the PSCs, which is consistent with the TRPL decay results (Figure S22b). To confirm these superior carrier dynamics characteristics within the device, we further conducted the transient photovoltage (TPV, Figure S22c) measurement and the electrochemical impedance spectroscopy (EIS, Figure S22d). In distinct contrast, incorporation of Ag@TiO2@Pa into perovskite film exhibit a long charge lifetime and a low charge transfer resistance, guaranteeing effective charge collection efficiency. Due to the subtle interactions of excitonplasmon coupling (Figure S23), the presence of the plasmonic nanoparticles can not only enhance generation of free charge carriers, but also tune the carrier-transport properties. Although there is significant difference in the loss mechanisms between OSC and PSC and there are additional improvements beyond the intrinsic ones described in the organic material, we have credibly confirmed the excellent generality of bishell design of plasmonic metallic NPs in all fullerene-based organic and perovskite solar cells.


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CONCLUSIONS In conclusion, a design concept of plasmonic nanostructures by decorating Ag nanoparticle with titania/benzoic-acid-fullerene bishell (Ag@TiO2@Pa) is proposed in order to minimize loss mechanisms which occurred when bare Ag nanoparticles were dispersed within activelayer. We attribute these optimizations to the skillful combination of inorganic inner-shell and organic outer-shell, in which the dense TiO2 remove the exciton quenching and the fullerene coating enable uniform dispersion within the embedding layer without morphology disruption. Elimination of metal NP-induced traps provides a platform to probe the charge dynamics and enhancement mechanisms in plasmonic solar cell unambiguously. Optically, Ag NPs improved light absorption with increase of exciton generation by strongly confined field of the LSPR and more efficient light scattering within the active layers. Electronically, the TiO2-fullerene bishell enables the generated carriers to be transferred through NPs and then be collected through fullerene channel, hence increasing the carrier collection efficiency, as validated by extensive electrical measurements. By incorporating Ag@TiO2@Pa into activelayer, improved performance in plasmonic OSCs and PSCs via strong plasmon-coupling effects is widely demonstrated with efficiency reaching 13.0% and 20.2%, respectively, which are the one of the highest values reported to date for plasmonic OSCs and PSCs using metal nanoparticles. The insights into the plasmonic technology could shed light on the existing controversy and provide guidelines for nanostructure design and device fabrication.

MATERIALS AND METHODS Materials. Silver nitrate (AgNO3, 99.5%), titanium isopropoxide (TPO, 97%) and polyvinylpyrrolidone with an average molecular weight of 10000 (PVP-10) were purchased from


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Sigma-Aldrich; ethanol (99.5%), acetone (99.5%), nitric acid (70%), ammonia (28 30 wt% NH3 in water) and ethylene glycol (99.9%) were purchased from Alfa Aesar. All chemicals were used as received and water was deionized (18.2

b milli-Q pore).

Synthesis of NPs. Ag and Ag@TiO2 NPs. Small size Ag NPs with a diameter of 20–30 nm were synthesized by a modified polyol process: typically, 0.1 mmol of silver nitrate and 0.5 g of PVP-10 were added into 25 mL of ethylene glycol solution, and the mixture was kept stirring at room temperature until completely dissolved. Then, the solution was slowly heated up to 120 oC and kept at that temperature for 1 h with constant stirring. At the end of reaction, the NPs were separated from ethylene glycol by addition of acetone (200 mL) and subsequent centrifugation at 5000 rpm to remove the supernatant. Later, the Ag NPs were washed with ethanol, centrifuged at 5000 rpm, and redispersed in 18 mL of ethanol and 2 mL of 4% ammonia in ethanol. This solution was stirred and sonicated more than 30 min, which were directly used for coating the TiO2 shell by adding TPO solution in ethanol while stirring the solution vigorously. Typically, 20 c- of TPO in 1 mL of ethanol was added into the Ag NP solution (20 ml), yielding a shell of TiO2 around 2–3 nm thick. Ag@TiO2@Pa NPs. The Ag@TiO2 NPs capped with fullerene acid (Pa) was prepared according to Figure 2. The NPs were dispersed in tetrahydrofuran (THF) and sonicated more than 30 min, before adding into the tetrahydrofuran solution of Pa (50 mL, 30 mg/L). The solution was refluxed at 70 oC for 8 h with purging N2. Subsequently, the solid was collected by filtration, and washed with tetrahydrofuran to remove the excess fullerene-COOH. Finally, the product was dried at 60 * under vacuum for 4 h. Device Fabrication. For nano-plasmonic device, our fabrication process begins with the nanostructured Ag rear electrode (as shown in Scheme S1). Firstly, a 10 nm thin Ag film was


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deposited onto cleaned glass substrates using thermal evaporation. Then, thin Ag film was heated at 300 °C for 30 minutes in glove box to generate isolated Ag nanostructured arrays on the glass substrate (Scheme S1). An additional 100 nm thick Ag film was deposited on the hemispherical Ag nanostructured arrays on the glass surface to achieve an opaque rear electrode. In the next, spacer interface and C60 were progressively built for the completed devices. TiO2 and SiO2 were deposited

by

atomic

layer

deposition

using

titanium

isopropoxide

and

3-

aminopropyltriethoxysilane as precursor, respectively. Finally, a 200 nm thick ITO film was deposited as anode by DC magnetron sputtering. For plasmonic OSCs fabrication, BHJ reference solutions consisted of PIDTT–DFBT:PC71BM (20 mg/mL, 1:3, w/w) in a mixture of 1,2-dicholobenzene (o-DCB) and 1-chloronaphthalene (v/v, 97:3) solution, PTB7-Th:PC71BM (25 mg/mL, 1:1.5, w/w) in a mixture of chlorobenzene (CB) and 1,8-diiodoctane (v/v, 97:3) solution, P3HT:IC60BA (1:1 w/w, 30 mg/mL) in CB/1,8-diiodoctane (v/v, 97.5:2.5) solution and PTB7-Th:BTR:PC71BM (30 mg/mL, 0.75:0.25:1.5, w/w/w) in CB/1,8-diiodoctane (v/v, 99:1) solution were prepared. The plasmonic PIDTT–DFBT:PC71BM solar cell device solution was prepared by adding a solution of the Ag@TiO2@Pa core-bishell NPs into the BHJ solution so that the final concentration of the NPs was 0.40 mg/mL. Similarly, the final concentration of the NPs in the PTB7-Th:PC71BM, P3HT:IC60BA and PTB7-Th:BTR:PC71BM were 0.25 mg/mL, 0.45 mg/mL and 0.3 mg/mL, respectively. Taking PIDTT–DFBT:PC71BM devices for example, ITO-coated (15 b /sq) glass substrates were cleaned sequentially with a detergent, DI-water, acetone, and isopropanol. Prior to the fabrication of the organic layers, the ITO surface was treated with plasma for 20 s. As for the devices with an inverted structure, the prepared ZnO sol-gel was spin-coated on the precleaned ITO-coated glass substrate at 4000 rpm. The ZnO films were annealed at 200 °C for 1 h


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in the air. Afterwards, PIDTT–DFBT:PC71BM solutions with or without NPs incorporation were spin-coated on the ZnO layer at 1000 rpm inside a nitrogen glove box (O2 and H2O concentration < 1 ppm), and subsequently dried in a vacuum chamber under nitrogen atmosphere to obtain a film thickness of approximately 90 nm. Finally, 8 nm MoO3 and 100 nm Ag were deposited sequentially under high vacuum by thermal evaporation onto the active layer through shadow masks to define a device area of 0.108 cm2. Device Characterization. All the J-V curves in this study were recorded using a Keithley 2400 source meter unit. The device photocurrent was measured in the dark and under AM1.5 illumination condition at intensity of 100 mW cm-2. The illumination intensity of the light source was accurately calibrated employing a standard Si photodiode detector equipped with a KG-5 filter, which can be traced back to the standard cell of the National Renewable Energy Laboratory (NREL). The EQE spectra performed here were obtained from an IPCE setup consisting of a Xenon lamp (Oriel, 450 W) as the light source, a monochromator, a chopper with a frequency of 100Hz, a lock-in amplifier (SR830, Stanford Research Corp), and a Si-based diode (J115711-1-Si detector) for calibration. All masked tests gave consistent results with relative errors within 1%. Calculated JSC values obtained by integrating the EQE spectrum under the AM 1.5G illumination condition agreed well with the measured value (within 3%). Other characterizations are presented in the supporting information.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Materials information and the preparation method of core-shell,


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nanoparticles theoretical calculated absorption enhancement and morphology investigation of activelayer, detailed device characteristics.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] *Email: [email protected] *Email: [email protected] Author Contributions K. Yao, H. Huang, L. Zhou and A. Jen supervised the work and co-wrote the manuscript. K. Yao conceived the idea and proposed the experimental design. K. Yao, H. Zhong, M. Xiong and Y. Xu prepared the NPs and analyzed film properties. K. Yao, Z. Liu and S. Leng performed the device fabrication and testing. J. Zhang performed morphology measurements. W. Wang. performed the theory simulation for the BHJ film. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 51863013 and 61874052) and the Natural Science Foundation of Jiangxi Province, China (Grant 20161ACB21004). K.Y. thanks the the financial support of Hong Kong Scholars program (XJ2016048). This work was supported by the Hong Kong Polytechnic University (G-YZ98). A.K.-Y.J. thanks the Boeing-Johnson Foundation for their fnancial support. REFERENCES (1) Bredas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: the Challenges. Acc. Chem. Res. 2009, 42, 1691-1699.


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(2) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (3) Carlé, J. E.; Helgesen, M.; Hagemann, O.; Hösel, M.; Heckler, I. M.; Bundgaard, E.; Gevorgyan, S. A.; Søndergaard, R. R.; Jørgensen, M.; García-Valverde, R.; Chaouki-Almagro, S.; Villarejo, J. A.; Krebs, F. C. Overcoming the Scaling Lag for Polymer Solar Cells. Joule 2017, 1, 274-289. (4) Ganesamoorthy, R.; Sathiyan, G.; Sakthivel, P. Fullerene based Acceptors for Efficient Bulk Heterojunction Organic Solar Cell Applications. Sol. Energy Mater. Sol. Cells 2017, 161, 102148. (5) Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K.-Y.; Marder, S. R.; Zhan, X. NonFullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003. (6) Yip, H.-L.; Jen, A. K.-Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994-6011. (7) Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X.; Wang, Y.; Ke, X.; Xiao, Z.; Ding, L.; Xia, R.; Yip, H. L.; Cao, Y.; Chen, Y. Organic and Solution-Processed Tandem Solar Cells with 17.3% Efficiency. Science 2018, 361, 1094-1098. (8) Betancur, R.; Romero-Gomez, P.; Martinez-Otero, A.; Elias, X.; Maymó, M.; Martorell, J. Transparent Polymer Solar Cells Employing a Layered Light-Trapping Architecture. Nat. Photonics 2013, 7, 995-1000. (9) Chen, J.-D.; Cui, C.; Li, Y.-Q.; Zhou, L.; Ou, Q.-D.; Li, C.; Li, Y.; Tang, J.-X. SingleJunction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035-1041. (10) Polman, A.; Atwater, H. A. Photonic Design Principles for Ultrahigh-Efficiency


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Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells

A design concept of plasmonic nanostructures by decorating Ag nanoparticle with titania/fullerene bishell is developed in order to minimize loss channels from morphology disruption and the charge trapping which occurs when bare Ag nanoparticles are dispersed within activelayer. Herein, a general approach to improving the photovoltaic performance with synergistic contributions from both optical and electrical effects has been demonstrated.


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1 Plasmonic Metal Nanoparticles with Core-Bishell Structure for High

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