Improved Efficiency of Inverted Perovskite Solar Cells

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Apr 23, 2018 - Improved Efficiency of Inverted Perovskite Solar Cells. Via Surface Plasmon Resonance Effect of [email protected] Core–Shell Tetrahedra ...

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Improved Efficiency of Inverted Perovskite Solar Cells Via Surface Plasmon Resonance Effect of [email protected] Core–Shell Tetrahedra Nanoparticles Hao Hao, Liang Wang, Xiaoqian Ma, Kun Cao, Hongtao Yu, Minghao Wang, Wenwen Gu, Rui Zhu, Muhammad Sabieh Anwar, Shufen Chen,* and Wei Huang* halide ions of Cl, Br, and I) have Sufficient absorption to incident solar illumination, long exciton diffusion attracted a great deal of attention in photovoltaic fields due to their large length, and efficient dissociation are extremely critical factors to guarantee an absorption coefficient, weakly bounded acquirement of high power conversion efficiency (PCE) in solar cells, including excitons and long charge diffusion perovskite solar cells (PSCs). In this work, [email protected](4-styrenesulfonate) (PSS) length.[1–6] The power conversion efficiencore–shell tetrahedra nanostructures, synthesized by seed mediated growth cies (PCEs) based on above-mentioned method, are incorporated into PSCs for the first time to improve light perovskites have been boosted up to more than 22% in recent researches.[7–9] Among absorption of methylammonium lead iodide via surface plasmon resonance the various organometal halide perovskite effect. Both the use of Au tetrahedra core and the introduction of ultrathin PSS materials, methylammonium lead iodide shell are beneficial for generating a strong local field and preventing from (MAPbI3) has been extensively used as an exciton quenching at the surface of nanoparticles (NPs). With an optimal excellent light harvester in perovskite solar concentration of [email protected] tetrahedra NPs, the PCE achieves 16.53%, cells (PSCs) due to proper bandgap of about showing a significant improvement factor of 18.83% compared to the 1.55 eV and large absorption coefficient of 105 cm1.[10–15] Even so, the MAPbI3 reference device without NPs. Analyses indicate that in addition to perovskite active layer shows a relative promotion of light absorption of active layer over the broad wavelength weak absorption over the wavelength range range, the [email protected] tetrahedra NPs also increase exciton dissociation and of 750 nm, thus it is necessary charge transfer efficiency by enhancing the recombination resistance to enhance light harvesting over the weak inside PSCs and reducing photoluminescence intensity and exciton/carrier absorption region of perovskite so as to potentially improve the PCE of PSCs. lifetime of perovskite films. Surface plasmon resonance (SPR) effect has been found to be a fascinating solution to improve light absorption in solar cells 1. Introduction by utilizing strong local field and light scattering generated by metallic nanoparticles (NPs) or periodic metal nanostrucIn the past few years, inorganic–organic metal halide perovskites tures.[16–19] To date, metallic NPs have been widely utilized in with chemical formula of ABX3 (A cation is CH3NH3þ (MA), HC-(NH2)2þ (FA) or Csþ, B is Pb2þ or Sn2þ, and X is one or two almost all types of solar cells, for example, dye-sensitized and

H. Hao, L. Wang, X. Ma, Dr. K. Cao, H. Yu, M. Wang, W. Gu, Prof. S. Chen, Prof. W. Huang Key Laboratory for Organic Electronics and Information Displays Jiangsu Key Laboratory for Biosensors Institute of Advanced Materials (IAM) Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM) Nanjing University of Posts & Telecommunications (NUPT) 9 Wenyuan Road, Nanjing 210023, China E-mail: [email protected]; [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.201800061.

DOI: 10.1002/solr.201800061

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Dr. R. Zhu State Key Laboratory for Artificial Microstructure and Mesoscopic Physics Department of Physics Peking University 5 Yiheyuan Road, Beijing 100871, China Prof. M. Anwar Department of Physics Syed Babar Ali School of Science and Engineering Lahore University of Management Sciences (LUMS) Opposite Sector U, D.H.A. Lahore 54792, Pakistan Prof. S. Chen, Prof. W. Huang Institute of Flexible Electronics (SIFE) Northwestern Polytechnical University (NPU) 127 West Youyi Road, Xi’an 710072, Shaanxi, China

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organic solar cells (OSCs),[20–24] due to advantages of welldispersion of metal NPs in a variety of aqueous/organic solutions and the easy control over NPs’ shape, size, and density.[25–30] Metal NPs with different shapes of sphere, star, triangle, rod, octahedron and cube have been successfully synthesized by Choy, Yang, Lee, Wong, Heeger, Wang, and Chen’s groups.[31–43] By doping one or dual plasmonic nanostructures into different functional layers to improve light capture inside solar cells or broaden absorption region of photoactive layer, 10–30% increases in PCE have been observed in succession.[44–48] More recently, these metal NPs have also been gradually employed into PSCs. Al NPs were doped into PEDOT:PSS by Kakavelakis et al., which promoted the efficiencies of PSCs to exceed 13.5% and extended device’s stability upon prolonged illumination under ambient conditions.[49] The 40 nm Au-Ag alloy nanospheres were inserted into two TiO2 layers by Cheng et al., with a significant PCE improvement from 12.6 to 14.8%.[50] What’s more, it has been demonstrated that locating metal NPs into the photoactive layer is beneficial to fully employ the local field generated by metal NPs, but these PSCs also suffer a largely increased exciton quenching due to the existence of charge carrier recombination at the surface of the bare metal NPs.[51] Coating insulated shells onto these metal NPs can sufficiently solve this problem by substantially preventing from the charge recombination on the surface of NPs.[51–52] Following this idea, the [email protected] nanospheres and [email protected] nanospheres were doped into the Al2O3 layer and active layer, respectively, with PCE enhancement factors of 11.4 and 16.3% being observed.[53–54] The [email protected] nanospheres were used as a modifier of the mp-TiO2 layer by Ye et al., realizing a 20.3% increase in PCE of PSCs with tri-cation and dual-anion mixed perovskites.[55] Furthermore, [email protected] nanorods were incorporated by Zhang et al. into the interface of perovskite/2,20 ,7,70 -tetrakis-(N,N-di-p-methoxyphenylamine)9,90 -spiro-bifluorene (spiro-OMeTAD), and enhanced the PCE of PSCs from 14.39 to 17.39%, which represents a pretty high improvement factor among reported literatures. The significant performance improvements were ascribed to the enhanced charge separation, light scattering and absorption processes.[56] In this work, we develop our research work by considering the following two factors: First, we choose a nanostructure with a tetrahedra shape because they can generate an extremely strong local field based on our calculation result with a finite differential time domain (FDTD) method, whose field intensity is 102 higher than the common sphere and rod shapes.[57] In addition, we use an insulated polymer poly(sodium 4-styrenseulfonate) (PSS) to assemble the Au tetrahedra NPs, which is electronegative and easily combines with the positive Au tetrahedra NPs, forming an ultrathin shell of 2 nm onto the NPs. Note that in our previous work we have demonstrated that metallic NPs with a relatively thin shell is particularly helpful to sufficiently use the high local field generated by metal NPs.[57] Overall, this method has advantages of easy control of shell’s thickness and simple synthesis process. Applying the [email protected] tetrahedra core-shell NPs into the inverted PSCs using the MAPbI3 active layer, we observe the maximum short-circuit current density (Jsc) and PCE of 23.34 mA cm2 and 16.53% with enhancement factors of 15.4 and 18.8%, respectively. The large increase in device performance is significantly ascribed to the strong localized SPR

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property induced by [email protected] tetrahedra NPs, which promotes the light absorption of the active layer and finally enhances the light harvesting efficiency of devices with enhanced Jsc. Moreover, the presence of [email protected] tetrahedra NPs significantly depresses exciton/carrier recombination and rapidly transfers holes from MAPbI3 into the poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS), which efficiently enhances exciton dissociation and charge transfer efficiency with eventual fill factor (FF) and PCE of PSCs improved.

2. Results and Discussion [email protected] tetrahedra NPs were synthesized with a seed mediated growth method with detailed synthesis process described in experimental section. They were characterized with transmission electron microscope (TEM) and ultravioletvisible (UV–vis) absorption spectra. Figure 1a and b depict the TEM images of Au tetrahedra NPs without and with 20.5 nm PSS shell, respectively. As the statistics shown in Figure S1, Supporting Information, the average edge length of Au tetrahedra NPs is 54 nm. From Figure 1c, the absorption peak of the Au tetrahedra NPs in ethanol is observed at 540 nm and shows a negligible shift with the coating of the PSS shell, possibly due to extremely thin shells coated on Au NPs and the relative low refractive index of PSS. To evaluate the feasibility of [email protected] tetrahedra NPs in the planar inverted PSCs, a series of devices with architecture of ITO/PEDOT:PSS/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag were fabricated, as shown in Figure 2a. Here, different concentrations of [email protected] tetrahedra NPs in ethanol were coated onto the PEDOT:PSS hole transport layer (HTL) to optimize the distribution density of Au NPs. At the same time, a control device without Au NPs was manufactured for comparison. Considering the dispersion of [email protected] tetrahedra NPs into ethanol and potential effect of ethanol solvent on device performances, another control device was also fabricated with its PEDOT:PSS HTL treated with ethanol. The electrical performances for all the devices measured under AM 1.5G simulated solar illumination and dark conditions were shown in Figure 2b and Figure S2, Supporting Information. From the current density–voltage (J–V) curves in Figure 2b and the results summarized in Table 1, obvious increases in Jsc and FF were observed in PSCs doping with the [email protected] tetrahedra NPs, although their open-circuit voltage (Voc) showed a mild decline of 30 mV. The Jsc and PCE increased from 20.23 mA cm2 and 13.91% without [email protected] tetrahedra NPs to 23.34 mA cm2 and 16.53% when the doping concentration of Au NPs reached 4 vol.%, showing significant enhancement factors of 15.4 and 18.8%, respectively. With a further deviation from the optimal concentration, both Jsc and PCE began to decrease, and the decline of these parameters was especially serious when the concentration of NPs exceeded 6 vol.%. In addition, devices were scanned under reverse and forward directions to observe the hysteresis phenomenon. And the hysteresis index was calculated with the equation of (JRS(0.8Voc)  JFS(0.8Voc))/JRS(0.8Voc), with results shown in Table S1, Supporting Information. Note that JRS(0.8VOC) and

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Figure 1. The TEM images for Au tetrahedra coated (a) without and (b) with 2 nm PSS. (c) The absorption spectra of the Au tetrahedra without and with PSS shell. The Au tetrahedral NPs were dissolved in ethanol.

JFS(0.8VOC) represented the current density at 80% VOC under reverse and forward scanning conditions, respectively. On the whole, all devices exhibited extremely weak hysteresis with low hysteresis indexes of 0.03, indicating good crystallinity and phase purity of devices despite of Au NPs. In order to explore the potential influence of NPs on devices, the distributions of [email protected] tetrahedra NPs on the PEDOT:PSS layer were measured with scanning electron microscopy (SEM). As displayed in Figure 3a–d, the distribution density of NPs increased significantly with its increasing concentration. When the doping concentration exceeded 4 vol.%, some NPs began to aggregate on the PEDOT:PSS layer, and this situation seemed to be more serious with the concentration of over 8 vol.%, which might be the reason why Jsc dropped quickly with the concentration of [email protected] tetrahedra NPs exceeding 4 vol.%. The statistic average distance for two Au NPs and the surface coverage of different concentrations Au NPs were shown in Figure S3, Supporting Information. Notably, at the optimal concentration of 4 vol.%, the statistic average distance for two Au NPs was 877.9 nm, which was used to set a periodic structure model in FDTD software to calculate the local field distribution around [email protected] tetrahedra NPs and thus observe the influence of NPs on the internal field intensity of PSCs. Here, the NPs-induced alterations of the electric field intensities were compared with those without [email protected]

tetrahedra NPs in the control device, with the simulation results of some typical wavelengths of 620, 700, and 800 nm extracted into Figure 4 (XZ plane at Y ¼ 0), Figure S4, Supporting Information (YZ plane at X ¼ 0) and Figure S5, Supporting Information (XY plane at Z ¼ 0). The detailed threedimensional structure of our NPs in a rectangular coordinate system was illustrated in the inset of Figure 5a. With the use of the [email protected] tetrahedra NPs, an extremely strong local field was generated on XZ plane with intensity exceeding 4200 in the near infrared range (800 nm, Figure 4f), three orders of magnitude higher than that (1.5, Figure 4c) without tetrahedra NPs. The as-generated local field by NPs decreased with the shift of wavelength toward shorter wavelength, for example, it dropped to an intensity of 1000 (Figure 4d) when the wavelength blue-shifted to 620 nm, showing an enhancement factor of 1000. Above analysis on the simulated electric fields indicated that the SPR of the Au tetrahedra NPs induced a strong local field over a wide wavelength range, which was very beneficial to promoting absorption of the perovskite active layer. Similar enhancements of field intensity were also observed on YZ and XY planes (Figures S4 and S5, Supporting Information), but their amplitudes were obviously smaller than those on XZ plane (Figure 4), which was mainly due to a supposed polarization direction of incident solar illumination along with the X axis.

Figure 2. a) The structural diagram of our perovskite solar cells. b) The J–V characteristics for all PSCs with different concentrations of [email protected] tetrahedra NPs.

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Table 1. Photovoltaic characteristics for PSCs with different concentrations of [email protected] tetrahedra NPs. The provided data are based on five groups of devices (active area ¼ 0.1 cm2), measured with a voltage scan rate of 50 mV s1 under AM 1.5 G solar illumination of 100 mW cm2. Voc [V]

Jsc [mA cm2]

FF

PCE [%]

Untreated

1.040  0.028

19.36  1.05

65.2  2.1

13.14  0.77

Ethanol-treated

1.033  0.019

17.74  0.97

66.8  3.9

12.31  0.67

2 vol.%

1.036  0.017

19.74  1.19

65.9  3.4

13.56  0.52

4 vol.%

1.036  0.020

21.88  1.46

66.9  3.6

15.19  1.34

6 vol.%

1.029  0.011

20.74  1.31

65.9  3.5

14.08  1.11

8 vol.%

1.029  0.023

18.30  1.14

62.9  2.8

11.89  0.84

Device

To better understand the Au NPs’ influence on active layer and observe if the local field would be still magnified inside the perovskite layer, electric field intensity along x axis satisfying Y¼0 nm and Z¼2 nm was extracted, with cross-section of Au tetrahedra on XZ plane (at Y¼0) illustrated in Figure 5a and simulated results shown in Figure 5b–d. Figure 5b–d corresponded to the electric field intensity along x axis at 620, 700, and 800 nm. Inset of Figure 5b–d only showed the local field in the perovskite layer given the NP’s boundaries of (16.5, 2.0) and (33.0, 2.0) at Z ¼ 2 nm, from which obviously enhanced electric field intensity was observed at all wavelengths and it was especially remarkable at 620 nm with an enhancement factor of 277. Above analysis demonstrated that although the local field intensity around NPs went down exponentially with distance, it still remained a high value in the active layer due to use of an ultrathin PSS shell, which was helpful to improve the absorption of MAPbI3, thus resulting in increases in Jsc and PCE. The as-measured UV–vis absorption of the

MAPbI3 with [email protected] tetrahedra NPs further demonstrated this point (Figure 6a). With the use of [email protected] tetrahedra NPs, the absorption intensity of the perovskite layer was obviously higher than that of the control sample without Au NPs over a wide wavelength range of 400–780 nm, which was in good agreement with previously reported results.[47,58–60] Moreover, the absorption intensity continuously increased with the increasing NPs concentration and it was more clearly observed with calculated ΔAbsorption (Figure S6a, Supporting Information). However, the measured Jsc in Figure 2b showed a different trend with above absorption curves, that is, the Jsc for the PSC with 4 vol.% NPs concentration was higher than the ones with 6 and 8 vol.% NPs although they owned higher absorption intensities. The abnormal result was probably due to the aggregation of high NPs concentrations, which increased the likelihood of electron–hole recombination and eventually led to a lower current, as the SEM images exhibited in Figure 3. The photocurrent density (Jph) versus effective voltage (Veff ) characteristic curves were illustrated in Figure 7 to investigate photo-generated carriers. Jph was calculated according to the equation of Jph ¼ qGMAX L, where q and L are the electronic charge and the thickness of the MAPbI3 layer (280 nm), and GMAX is the maximum carrier generation rate. Assuming all the photogenerated excitons were dissociated into free charge carriers and collected by electrodes at a high Veff region, the as-calculated GMAX values for 0, 2, 4, 6, and 8 vol.% NPs were 4.18  1027, 4.32  1027, 4.51  1027, 4.40  102, and 4.34  1027 m3 s1, respectively. The enhanced values suggested that incorporating [email protected] tetrahedra NPs into PEDOT:PSS/MAPbI3 interface increased the degree of light harvesting in MAPbI3 layer since the value of GMAX was only limited by total amount of absorbed incident photons,[61,62] which was further confirmed by the enhanced absorption

Figure 3. The SEM images of (a) 2 vol.%, (b) 4 vol.%, (c) 6 vol.%, and (d) 8 vol.% concentrations of [email protected] tetrahedra NPs on the PEDOT:PSS layer.

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Figure 4. The local field distributions on XZ plane at Y ¼ 0 nm. Here, incident light propagation and polarized directions of this incident light beam are along Z and X axes, respectively. (a–c) correspond to the field intensities at 620, 700, and 800 nm without [email protected] tetrahedra NPs and (d–f) correspond to the field intensities at 620, 700, and 800 nm with [email protected] tetrahedra NPs.

spectra (Figure 6a and Figure S6a, Supporting Information) for the samples with different concentrations of [email protected] tetrahedra NPs. In this part, X-ray diffraction (XRD) spectra of a series of perovskite samples grown on PEDOT:PSS, ethanol-treated PEDOT:PSS and NPs-covered PEDOT:PSS were measured to investigate the Au NPs’ effect on the crystallization of MAPbI3, as shown in Figure 8. The peaks at 14.1 , 28.4 , and 42.1 represented the characteristic peaks of the (110), (220), and (330) planes of the MAPbI3 perovskite crystalline structure.[63,64] The almost identical XRD patterns among the different samples indicated that the perovskite absorber was not affected by the presence of the [email protected] tetrahedra NPs. Both the strong perovskite crystalline peaks and the extremely weak PbI2 peak at 12.8 demonstrated the complete transformation of PbI2 into MAPbI3, suggesting high crystalline quality of the perovskite, which eventually contributed to high PCEs with large Jscs and FFs. The SEM images in Figure 9 exhibited the surface morphologies of above-mentioned perovskite films. All of the perovskite films showed continuous pinholefree morphologies and good crystallinities, with the grain sizes counted in Figure S7, Supporting Information. The average grain size of the untreated sample was 372 nm, while the ethanol-treated and NPs-inserted ones were among 383–390 nm. The little change of the grain size with the introduction of Au tetrahedra NPs implied that the incorporation of NPs scarcely impacted the formation and crystallization of the perovskite active film. To deeply investigate potential reason why FF increases in those plasmonic PSCs (Table 1), electrochemical impedance

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spectroscopy (EIS) of the PSCs with different concentrations of Au NPs was tested with the Nyquist plots shown in Figure 10a. The recombination resistance (Rrec ) in the lowfrequency region of the Nyquist plots could be used to characterize the recombination process occurring at PEDOT: PSS/perovskite/PC 61BM heterojunction. Since the recombination rate was inversely proportional to Rrec , a high R rec represented lower recombination of carriers/excitons at these interfaces. Calculated results indicated that the Au NPs doped PSC showed a larger Rrec (421 Ω cm 2) than the untreated one (338 Ω cm 2), demonstrating the successful depression of recombination and increased exciton dissociation efficiency at the interface of PEDOT:PSS/MAPbI3 with Au NPs doped, all of which resulted in eventual increase in FF. The steady-state photoluminescence (PL) spectra and time-resolved PL decay curves were measured to explore the effect of the SPR on the carriers/excitons lifetime, as shown in Figure 10b,c. Samples employed structures of PEDOT:PSS/ perovskite and PEDOT:PSS/[email protected] tetrahedra NPs/perovskite. Compared to the sample without NPs, the perovskite film with the insertion of [email protected] tetrahedra NPs clearly showed a significant PL quenching (Figure 10b), indicating that the [email protected] tetrahedra NPs accelerated excitons ionization and charge separation from the perovskite to the PEDOT:PSS HTL.[65] The time-resolved PL measurements also verified this point. The sample without NPs had a carrier/exciton lifetime of 43.7 ns, while it reduced to 31.6 ns after the [email protected] tetrahedra NPs were inserted into the interface of perovskite/PEDOT:PSS. The shorter lifetime for

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Figure 5. a) The cross-section schematic of [email protected] tetrahedra with partial cell structure (XZ plane at Y ¼ 0 nm). Its three-dimensional structure in a rectangular coordinate system is illustrated in the inset. (b–d) correspond to the electric field intensity at 620, 700, and 800 nm when Z ¼ 2 nm. Inset figures are the field distributions in the perovskite layer.

the Au NPs-modified perovskite film suggested that the [email protected] tetrahedra NPs were beneficial to exciton dissociation and charge transfer. Finally, incident photon-to-electron conversion efficiency (IPCE) curves of PSCs with different concentrations of

[email protected] tetrahedra NPs were compared, as shown in Figure 6b. As the NPs were added into the PSCs, the IPCE values enhanced apparently over a wide wavelength range of 350–750 nm, of which the 4 vol.% concentration-based device reached a peak value of 92% in the visible region. The

Figure 6. a) The UV–vis absorption spectra of perovskite on PEDOT:PSS and different concentrations of [email protected] tetrahedra NPs. b) IPCE curves for the devices without and with different concentrations of [email protected] tetrahedra NPs.

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3. Conclusion

Figure 7. Photocurrent density vs effective voltage for the devices with different concentrations of [email protected] tetrahedra NPs.

calculated Jsc values from IPCE curves coincided well with the experimental Jsc ones, as shown in Figure S8, Supporting Information. Note that the error between the theoretical and experimental data was within 6%. The variation on IPCE (ΔIPCE) for plasmonic PSCs was also calculated, with results displayed in Figure S6b, Supporting Information. Similar to ΔAbsorption in Figure S6a, Supporting Information, the IPCE values showed broad improvements over 350–750 nm. Given the IPCE was decided by the product of the light harvesting efficiency (ηA), exciton diffusion efficiency (ηED), exciton dissociation efficiency (ηET), free charge transfer efficiency (ηCT) and charge collection efficiency (ηCC), the improved IPCEs in our work mainly originated from the enhanced ηA, ηET, and ηCT due to the promotion of light absorption and exciton dissociation in active layer and accelerated hole transfer at the interface of PEDOT:PSS/MAPbI3 with the [email protected] tetrahedra NPs doped.

Figure 8. The XRD patterns of the MAPbI3 films on the PEDOT:PSS, ethanol-treated PEDOT:PSS and [email protected] tetrahedra NPs-covered PEDOT:PSS with different NPs concentrations of 2–8 vol.%.

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In summary, a novel type of core-shell tetrahedra nanostructure, [email protected] tetrahedra NPs, was synthesized with seed mediated growth method and then incorporated into the MAPbI3-based inverted PSCs. With the optimal doping concentration of 4 vol.%, the best performing cell achieved a Jsc of 23.34 mA cm2 and a PCE of 16.53%, showing enhancement factors of 15.4 and 18.8% compared with those in the standard device without Au NPs. The XRD and SEM characterizations indicated that it showed a negligible influence on the crystallization and formation of the active film when doping the [email protected] tetrahedra NPs into PSCs. Simulated and measured results on film’s optical characteristics demonstrated that the performance improvement on PSCs was on one hand originated from the absorption enhancement of the active layer induced by strong SPR effect and on the other hand owing to increased exciton dissociation and accelerated hole transfer caused by metallic NPs’ large recombination resistance and rapid carrier transfer ability. This study is anticipated to provide an efficient method of light manipulation for further improving the cell performance. And the understanding gained from this study is expected to allow for the construction of highperformance optoelectronic devices that could find new applications in the fields of organic solar cells, perovskite/ organic light-emitting diodes, and potentially many other devices employing metallic nanoparticles.

4. Experimental Section Chemicals: Hexadecyltrimethylammonium bromide (CTAB, 99%), cetyltrimethylammonium chloride (CTAC, 25 wt.% in water), tetrachloroaurate trihydrate (HAuCl4  3H2O), sodium borohydride (NaBH4, 99%), L-ascorbic acid (AA, 99%), and sodium chloride (NaCl, 99%) were purchased from J&K Chemical. Poly(sodium 4-styrenseulfonate) (PSS, 30 wt.% in water) was bought from Sigma Aldrich. Perovskite precursors such as lead iodide (PbI2) and methylammonium iodide (MAI) were purchased from Xi’an p-OLED Corp., while PEDOT:PSS (AI 4083) and PC61BM were purchased from Heraeus Materials Technology Co. Ltd. and Nano-C, respectively. All solvents used were obtained from Sigma-Aldrich. All the chemicals were used as received without any purification. Synthesis of Au Tetrahedra NPs: Firstly, CTAB-capped Au clusters were synthesized. A total of 10 mL of mixed aqueous solution containing 0.25 mM HAuCl4 and 100 mM CTAB was prepared. A total of 0.6 mL of 10 mM fresh NaBH4 aqueous solution was quickly injected into above Au(III)-CTAB mixed solution using a pipette. The solution color immediately changed from yellow to brown upon the addition of NaBH4. The mixture was stirred at a speed of 300 rpm for 5 min and then left undisturbed at 27  C for 3 h to ensure complete decomposition of NaBH4 in the mixed solution. Then Au spherical seeds were prepared. A total of 0.1 mL of CTAB-capped Au clusters was added into the mixed aqueous solution containing 2 mL of 0.5 mM HAuCl4, 2 mL of 200 mM CTAC, and 1.5 mL of 100 mM AA. The reaction was allowed to proceed for 10 min at 27  C. The product was treated with a centrifugation of 14 500 rpm for 30 min, washed with deionized water once, and then dispersed into 1 mL of 20 mM CTAC aqueous solution. Finally, Au tetrahedra NPs were synthesized. 0.5 mL of 100 mM CTAB, 0.75 mL of 200 mM CTAC, 1.0 mL of 100 mM AA and 20 mL of above-synthesized Au spherical seeds were mixed with 0.75 mL of deionized water, followed by dropwise addition of 1.0 mL of 0.5 mM HAuCl4 aqueous solution using a syringe pump at an injection rate of 0.25 mL h1. The reaction was allowed to continue at 27  C for 10 min after the injection had been completed. The reaction

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Figure 9. The SEM images of the perovskite films on the (a) PEDOT:PSS, (b) ethanol-treated PEDOT:PSS, and [email protected] tetrahedra NPs-covered PEDOT: PSS with different NPs concentrations of (c) 2 vol.%, (d) 4 vol.%, (e) 6 vol.%, and (f) 8 vol.%.

product was further treated with a centrifugation of 13 200 rpm for 10 min, washed with deionized water twice, and finally dispersed into 0.5 mL deionized water. Synthesis of Au [email protected] NPs: To coat the Au tetrahedra with PSS, 0.5 mL of Au tetrahedra NPs solution, 0.4 mL of 210 mg mL1 PSS aqueous solution, and 0.2 mL of 45 mM NaCl solution were added into 0.75 mL deionized water. And then the mixed solution was stirred vigorously at a speed of 800 rpm for 3 min. The mixture was kept undisturbed at 27  C for 3 h and followed by centrifugation of the mixture at 12 000 rpm for 10 min. The Au [email protected] [email protected] nanostructures were obtained and washed with deionized water two times, which was finally dispersed into 0.5 mL ethanol. Simulation work: The [email protected] tetrahedra NPs were supposed to be periodically distributed onto the PEDOT:PSS layer. The electronic field distributions were calculated with periodic boundary conditions for X and Y axes and perfectly matched layer for Z axis (with a distribution period of 880 nm for both X and Y axes and a nonperiodic distribution for Z axis, referred to the statistical average distance of 4 vol.% NPs distribution in Figure S2, Supporting Information). The plane wave light source was used in our simulation model. The override mesh sizes setting was 1, 1, and 1 nm for X, Y, and Z axes, respectively, which is small enough for getting an accurate result.

Device Fabrication: Firstly, ITO glasses were washed with acetone, ethanol and deionized water in sequence, followed by ultrasonic cleaning with deionized water, acetone, and ethanol for 15 min. After drying with high purity N2 and vacuum drying in an oven, the ITO substrates were treated with UV-ozone for 15 min. Then, PEDOT:PSS was spin-coated onto the patterned ITO glass substrates at 2000 rpm for 45 s and dried at 110  C for 20 min, forming a 50 nm thick film. The Au [email protected] NPs-doped ethanol solutions with concentrations of 0, 2, 4, 6, and 4 vol.% were respectively spin-coated onto the PEDOT:PSS layer at 2000 rpm for 45 s, and then dried at 110  C for 20 min. For the ethanol-treated sample, it followed a same way with those using Au [email protected] NPs. After the samples were fully dried, they were transferred into a glovebox to form a 280 nm MAPbI3 film via a two-step deposition method. Note that PbI2 was dissolved into DMF/DMSO solvent at a ratio of 4:1 (v/v) with a concentration of 1.4 mol L1 and then the PbI2 solution was stirred at 70  C overnight. MAI was dissolved in isopropanol with a concentration of 55 mg mL1. For the fabrication of perovskite layer, PbI2 layer was first spin-coated onto the 60  C-preheated substrates at 6000 rpm for 20 s with an acceleration time of 2 s, followed by a thermal annealing at 100  C for 30 s. After cooled down to room temperature, the MAI solution was spin-coated on top of the PbI2 layer at 4500 rpm for 30 s with an acceleration time of 1.5 s, followed by a thermal annealing at 100  C for

Figure 10. a) EIS spectra of the untreated PSC and 4 vol.% [email protected] tetrahedra NPs doped PSC under 0 V. (b) Steady-state PL spectra and (c) timeresolved PL decay curves of standard PEDOT:PSS/perovskite and PEDOT:PSS/4 vol.% [email protected] tetrahedra NPs/perovskite films. The steady-state PL spectra were excited with a wavelength of 445 nm and a peak wavelength of 771 nm was observed.

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40 min. During the annealing process, the samples obviously changed from pale yellow to dark brown. After this, 20 mg mL1 PC61BM in chlorobenzene was spin-coated onto the perovskite layer at 3000 rpm for 60 s and then dried at room-temperature for 10 min. Finally, all the samples were transferred into a vacuum chamber for BCP and silver electrode deposition under a pressure of 2  104 Pa. Herein, a 6 nm thick BCP and a 100 nm thick Ag were sequentially deposited onto the perovskite layer. Characterization: The shape and size for the [email protected] tetrahedra NPs were measured with a transmission electron microscope (TEM) (Hitachi, HT7700). The surface morphology images of the Au NPs-coated PEDOT:PSS films and the perovskite layer were carried out by using a Hitachi S-4800 field emission SEM. The crystal structures of the MAPbI3 thin films were characterized using the Bruker D8 ADVANCE XRD equipment. The thicknesses of the PEDOT:PSS, the perovskite, and the PC61BM films were determined with a Bruker DektakXT Stylus Profiler. The current density–voltage characteristic curves of the devices were operated with a Keithley 2400 source meter under a simulated AM 1.5G illumination (100 mW cm2, Oriel Sol3A Class Solar Simulator (94023A)) at a scan rate of 200 mV s1. The UV–vis absorption spectra of the perovskite films were obtained using a PerkinElmer Lambda 650S spectrophotometer. The photoluminescence measurements were carried out with a Horiba Fluoromax 4 spectrometer.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements H.H. and L.W. contributed equally to this work. The authors acknowledge financial support from the National Foundation for Science and Technology Development of China (973 project, Grant No. 2015CB932203), the National Key Research and Development Program of China (Grant No. 2017YFB0404501), the National Natural Science Foundation of China (Grant Nos. 61274065, 61704091, 61377025, 91433203, and 11121091), the Young 1000 Talents Global Recruitment Program of China, the National Research Program for Universities of Pakistan administered by the Higher Education Commission (Grant No. 2028), the Pakistan Science Foundation (Grant No. PSF/Res/P-LUMS/Phys (159)), the Science Fund for Distinguished Young Scholars of Jiangsu Province of China (Grant No. BK20160039), the Natural Science Foundation of Jiangsu Province (Grant Nos. BM2012010 and BK20170899), the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. YX030002), the Jiangsu National Synergetic Innovation Center for Advanced Materials, the Synergetic Innovation Center for Organic Electronics and Information Displays, and the National Postdoctoral Program for Innovative Talents (Grant No. BX201700122).

Conflict of Interest The authors declare no conflict of interest.

Keywords Au tetrahedra, carrier transfer, local field, perovskite solar cells, surface plasmon resonance

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Received: February 27, 2018 Revised: April 23, 2018 Published online:

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