Working from Both Sides: Composite Metallic Semitransparent Top

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Working from Both Sides: Composite Metallic Semitransparent Top Electrode for High Performance Perovskite Solar Cells Xuezeng Dai, Ye Zhang, Heping Shen, Qiang Luo, Xingyue Zhao, Jianbao Li, and Hong Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10830 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 7, 2016

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ACS Applied Materials & Interfaces

Working from Both Sides: Composite Metallic Semitransparent Top Electrode for High Performance Perovskite Solar Cells Xuezeng Dai†, Ye Zhang†, Heping Shen†, Qiang Luo†, Xingyue Zhao†, Jianbao Li†,‡, Hong Lin*, † †

State Key Laboratory of New Ceramics & Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

‡

Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, Materials and Chemical Engineering Institute, Hainan University, Haikou 570228, China

KEYWORDS: perovskite solar cells, silver nanowires, composite metallic, top electrode, semitransparent

ABSTRACT: We report herein perovskite solar cells using solution-processed silver nanowires (AgNWs) as transparent top electrode with markedly enhanced device performance as well as stability by evaporating an ultra-thin transparent Au (UTA) layer beneath the spin-coated AgNWs forming a composite transparent metallic electrode. The interlayer serves as a physical separation sandwiched in between the perovskite/hole transporting material (HTM) active layer

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and the halide-reactive AgNWs top-electrode to prevent undesired electrode degradation and simultaneously functions to significantly promote ohmic contact. The as-fabricated semitransparent PSCs feature a Voc of 0.96 V, a Jsc of 20.47 mA cm-2, with an overall PCE of over 11% when measured with front illumination and a Voc of 0.92 V, a Jsc of 14.29 mA cm-2 and an overall PCE of 7.53% with back illumination, corresponding to approximately 70% of the value under normal illumination conditions. The devices also demonstrate exceptional fabrication repeatability and air stability.

INTRODUCTION Organic-inorganic lead halide perovskites offer intriguing optoelectric characteristics including high carrier mobility, long charge diffusion length and particularly cost-effectiveness and have thus been a central research topic in photovoltaic applications1-3. With rapidly advancing research, perovskite solar cells (PSCs) have attained certified power conversion efficiency (PCE) exceeding 20% (albeit marked as unstablized)4,5 since the initial discovery derived from a dyesensitized solar cell prototype. The versatile carrier transporting properties allow for different device

designs

to

be

furnished,

i.e.

TiO2

compact

layer/perovskite/HTM6-8

or

PEDOT:PSS/perovskite/PCBM9-12. Due to the robust absorption coefficients of organometal halide perovskites, films with much smaller thickness are entailed to ensure sufficient light absorption in comparison with conventional thin film photovoltaic technologies13. In this regard, PSCs have also demonstrated unparalleled potential to be integrated as semitransparent building window components as opposed to rigid silicon-based devices14. Eperon et al fabricated devices taking advantage of spontaneous dewetting to create perovskite islands with a proper length-


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scale to appear visibly continuous yet to enable unattenuated transmission of light14. Gaspera et al showed efficient PSCs with color to neutrality utilizing dielectric-metal-dielectric multilayered top electrode with high transparency and conductivity15. For the completion of semitransparent photovoltaics, to deposit a conductive yet transparent electrode is of critical importance. In contrast to their conventional opaque counterparts, it is essential for transparent electrodes to simultaneously have inherent transmittance and electrical conductivity. In addition, the fabrication process has to be compatible with that of PSCs since perovskite would readily decompose in the presence of a polar solvent, such as water or ethanol, which excludes many a procedure. Among very few candidates, silver nanowires (AgNWs) afford benchmarking optoelectric properties comparable to that of commercial transparent conducting oxides (e.g. FTO) and have been applied as top electrodes of organic solar cells deposited using either a spray-coating16,17, spin-coating18 or transfer process19,20. Despite the targeted application as transparent electrode from which side light is supposed to be shone, most existing reports fall short of showing photovoltaic performance based on such measuring conditions where the AgNWs function merely as an electrical contact similar to their opaque counterparts rather than a bifunctional conductive transparent electrode21,22. Yet the major obstacle for stable device operation lies in the reactive nature of halides and silver for prolonged working durations. It has been well known that depending on specific device configurations, elements like iodine would migrate onto the surface of the charge selective layer/electrode interface and form irreversibly stable silver halide compounds, which is a thermodynamically favorable reaction. It has been reported that direct deposition of AgNWs on the underlying halide-containing perovskite layer would cause darkened electrodes after ambient storage for merely several days with much worsened photovoltaic performance, indicating the


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formation of undesired material phases, possibly AgI23. The AgNWs-based devices are also reported to have undergone rather rapid degradation24 even if they are stored in a N2 atmosphere, which is also the case for thick opaque evaporated Ag layers25-27. In this sense, it would be highly desirable if a conductive yet transparent layer can be introduced beneath the AgNWs to physically separate the direct contact of AgNWs from the halide-containing active layer. Guo et al21 introduced a thin layer ZnO between the PCBM and AgNWs layers which serves dual functionalities such as to ensure ohmic contact of the electrode interfaces and to protect the underlying perovskite active layer. Despite the efforts, the stability issue persists, probably because of the semiconducting nature of ZnO not capable of fully screening the mobile halide ions. In this work, by inserting an ultrathin Au interlayer we managed to avoid the direct engagement of the reactive AgNWs from the halide-containing active layer and the stability issue is successfully resolved. By engineering the Au layer thickness an optimized device performance was obtained. The electrodes showed no observable morphological change after storage while the ones fabricated following conventional route displayed considerable degradation. EXPERIMENTAL SECTION Materials: Unless specified otherwise, all materials were purchased from either Alfa Aesar or Sigma-Aldrich.

2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenylamine)9,9′-spirobifluorene

(spiro-

OMeTAD) was purchased from Borun Chemical Co., Ltd. (Ningbo, Zhejiang, China). Silver nanowires solution was purchased from Shangke New Materials Technology Co., Ltd. (Jinan, Shandong, China). Methylammonium iodide was synthesized according to previous report28.


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Device fabrication: Devices were fabricated on FTO-coated glass (Nippon Sheet Glass Co., Ltd.). Initially, FTO was removed from regions under the anode contact by etching the FTO with 2 M HCl and zinc powder. Substrates were then cleaned sequentially in soap, deionized water, ethanol, acetone, isopropanol and oxygen plasma. A compact layer of TiO2 was subsequently deposited by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 2000 rpm for 60 s, and annealed at 500 °C for 30 minutes. After cooling to room temperature, samples were transferred into a nitrogen-filled glovebox. To deposit perovskite films, a 45 wt.% N,Ndimethylformamide solution of methylammonium iodide and PbI2 (1:1 molar ratio) was first dropped onto the TiO2 coated FTO substrate. The substrate was then spun at 5000 rpm and after 4 seconds chlorobenzene was quickly dropped onto the center of the substrate. This instantly changed the color of the substrate from transparent to light brown. Then placed the bowl cover over the bowl immediately. The spun was stopped when the substrate change to transparent again. The obtained films were then dried at 100 °C for 10 min. The hole-transport layer was deposited by spin-coating the spiro-OMeTAD in chlorobenzene solution with added tertbutylpyridine (tBP) and lithium bis (trifluoromethanesulfonyl) imide (Li-TFSI). To fabricate transparent top electrode, a defined thickness of Au film was firstly deposited by thermal evaporation under vacuum of ~ 10-4 Pa. The silver nanowires dispersion in isopropanol (2 mg mL-1) was spin-coated on substrates at 2500 rpm by 10 drops to form AgNWs conducting networks. The device active area was 0.06 cm-2 defined by a mask during the measurement, though the ultrathin gold layer and the AgNWs layer are distributing completely on the whole substrate which is 2 cm × 2 cm.


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Characterization: Current-voltage characteristics were recorded under AM 1.5 100 mW cm-2 simulated sunlight (Newport Oriel 92192) with a Keithley 2400, previously calibrated with a calibrated Si solar cell. The surface morphologies were investigated using a Nanonavi SPA400 AFM unit in non-contact mode and a field emission SEM (Zeiss LEO-1530) operating at 15 kV. The transmission spectra of gold and AgNWs deposited on glass substrates were measured using a Perkin-Elmer Lambda 950 spectrophotometer. The energy dispersive X-ray spectroscopy analysis and mapping of single silver nanowire were detected using a transmission electron microscope (FEI Technai G20) equipped with an energy dispersive X-ray spectroscopy detector. The chemical compositions at the surface and across the thin depth were determined and confirmed by electron probe microanalyzer (JEOL JXA-8230). The thickness of evaporated gold films were determined by X-ray reflectivity (Bruker D8 Discover) and Nanonavi SPA400 AFM unit. RESULTS AND DISCUSSION Typical current density-voltage (J-V) characteristics of PSCs with conventional opaque Au electrodes, AgNWs and ultrathin Au layers + AgNWs were recorded under simulated AM 1.5 G with a light intensity of 100 mW cm-2 (Figure 1). The control device with opaque Au electrode shows a fill factor (FF) of 0.63 (Figure S6). Whereas in the absence of the ultrathin Au layer, straightforward deposition of AgNWs onto the perovskite/HTM active layer surface would result in poor photovoltaic performance, with a PCE of less than 2.5% and a FF of 0.23 even when illuminated from the FTO side, indicating high series resistance resulted from non-ideal contact of AgNWs and perovskite/HTM interface, as evidenced from much higher calculated series resistance (Table 1). The Rs (series resistance) was extracted from J-V curves as reported from previous literature15. Notably, it has been reported that placing AgNWs in direct contact with


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perovskite active layer would not yield working devices due to the non-optimal contact for charge extraction21. After the intermediate Au layer is inserted, a significantly augmented FF (over 0.5) was obtained regardless of the illumination directions. The appreciable improvement can be attributed to the morphological as well as electrical features belonging to the deposited transparent Au (UTA) layer as will be further discussed below. Note that due to the nonconductive nature of the UTA layer, without further depositing AgNWs electrodes, device would not yield measurable photoresponses.

Figure 1. (a) J-V curves of the PSCs fabricated with opaque Au electrode, AgNWs, ultrathin Au layer + AgNWs illuminated from FTO side and illuminated from transparent electrode (TE) side. (b) Schematic illustration of devices with UTA Au layer + AgNWs composite metallic electrode (c) SEM cross-sectional image of complete fabricated device. The colored area is marked to indicate different components. The inset shows the image of the full fabricated cell taken in front of background. The electrical properties of spin casted AgNWs are reported to be sensitive to the surface configurations of the underlying layer. Previous works have shown that by post-treatment


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solvent engineering, one is able to obtain ultrasmooth perovskite layers, which paved the way for preparing solution-based transparent electrodes29-31. By employing such methods we obtained much more consistent data in respect of batch-to-batch device PCE variances and observable perovskite material phase differences. Notably, the precise timing of solvent casting is essential for the fabrication of highly reproducible perovskite films. Traditional perovskite deposition techniques for planar device fabrications would commonly give rise to uneven surface morphologies, resulting in conglomeration and non-optimal contact of the interconnected AgNWs network. It can be evidenced from the widespread photovoltaic performances even with evaporated thick metallic top electrodes (Figure 2). AgNWs feature an averaged sheet resistance of 153 Ω sq-1 when casted on chlorobenzene (CB)-treated active layer coated with a spiroOMeTAD film. Toluene treatment of the perovskite layer gives similar film surface geometry but somewhat inferior performances30. For comparison conventional approaches such as sequential deposition and one-step deposition with a mixed halide precursor, the film sheet resistances show significant inconsistency with an averaged measured value of 279 kΩ sq-1 and 32 MΩ sq-1, respectively. To probe the morphology and functionalities of the intermediate Au layer, we varied the Au thickness by modulating the deposition parameters in the thermal evaporation process monitored by a crystal oscillator and subsequently confirmed by atomic force microscope (AFM) and X-ray reflectivity (XRR) measurements (Table S1). Cross-sectional SEM images fall short in giving accurate thickness information due to the ultrathin nature of the deposited films. It can be observed that the evaporated Au particles nucleated on the perovskite/HTM surface rather sparsely and formed particle-wise isolated metallic islands32-34 (Figure 3). It could also be found that the ultrathin Au layer itself is essentially non-conductive, with the electrical resistance below


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the detection limit of either multimeter or four-point probe at all thicknesses of 3 nm, 6 nm, 9 nm and 15 nm. The non-conductive nature could be explained by the physical discontinuity of the UTA thin films.

Figure 2. AFM and SEM images of perovskite films fabricated by one step method using CH3NH3PbI3-xClx precursor (a)(d)(g), sequential deposition method (b)(e)(h) , and solvent engineering with CB (c)(f)(i). And SEM images of AgNWs coated on above-mentioned perovskite films respectively (j)(k)(l)


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Figure 3. SEM images of Au layer of different thickness (a) 3 nm (b) 6 nm (c) 9 nm (d) 15nm and corresponding solution-process AgNWs top electrode (e) 3 nm (f) 6 nm (g) 9 nm (h) 15 nm.


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We also note that due to the centrifugal force during the spinning process, the AgNWs tend to align along a particular direction, rather than forming a crosslinking network when casted directly on bare perovskite surface or perovskite/HTM films with no Au loading (Fig 2l). The anisotropic “orientation” behavior would presumably cause non-optimal electrical conduction since the lateral charge transfer would be retarded. This phenomenon is much improved when the UTA layer reaches a thickness of 15 nm, forming a self-intersecting AgNWs network (Figure S3). The apparent morphological alternation could be the result of different surface energies and subsequent interaction of perovskite/HTM layer and the UTA layer when brought into contact with AgNWs solution during spin coating. In general, metals always have higher energy than organic materials due to the much stronger forces of metallic bonds than hydrogen bonding. Meanwhile, high energy substrates are more easily wet than low energy one35. Therefore, AgNWs solution will spread more quickly on the substrate with UTA. Correspondingly, more AgNWs would fast land on the substrate before they started to be orientated by the centrifugation of isopropanol solution during spin coating. We subsequently fabricated PSCs with ultrathin Au middle layers of different thicknesses. Their photovoltaic performances with illumination from both FTO and AgNWs sides are shown in Figure 4 and Table 1. In general, when illuminated from the FTO side, devices tend to yield superior performances comparing to when illuminated from the AgNWs side.


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Figure 4. J-V curves of PSCs with different intermediate Au layer thickness (a) illuminated from the FTO side and (b) illuminated from the AgNWs side. Table 1. Photovoltaic performance of PSCs employing ultrathin Au middle layer and 3 layers of AgNWs illuminated from both FTO and AgNWs sides.

Au thickness (nm)

Voc (mV)

Jsc (mA cm-2)

FF

η (%) Rs (Ω)

FTO AgNWs

FTO AgNWs FTO AgNWs

FTO AgNWs

0

978

--

10.90

--

0.23

--

2.33

--

63.6~66.0

3

953

920

15.79

14.29

0.56

0.57

8.38

7.53

17.3~21.3

6

956

932

17.46

9.38

0.54

0.54

9.04

6.43

17.2~20.6

9

979

930

16.15

9.07

0.57

0.59

8.97

4.94

16.8~20.8

15

956

914

17.73

8.35

0.53

0.58

8.93

4.42

17.6~21.0

*“FTO” indicates illumination from FTO side, “AgNWs” indicates illumination from AgNWs side. Despite the apparent discontinuous nature of the UTA film, a pronounced PCE enhancement from 2.33% (0 nm) to 8.38% (3 nm) can be observed with FTO side illumination, probably owing to the interface contact and conductivity improvement. It can be observed form the SEM


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images that although there are holes from underneath AgNWs, the charge collection is not adversely affected, since the effective contact of the middle UTA layer/spiro-OMeTAD and UTA layer/AgNWs interface. The increase of Jsc and FF value with Au loading may also be resulted from Au island filling in the pinholes of the perovskite/HTM surface as can be clearly observed from Figure 3 (c) and (d) 36. As the UTA layer thickness continues to rise, the PCE with illumination from the FTO side reaches its maximum at 6 nm of 9.04% followed by a plateau. The PCEs however, showed a negative trend when illuminated from the AgNWs side. The Jsc dropped from 14.3 mA cm-2 for device with a 3 nm UTA layer to 8.3 mA cm-2 once the UTA thickness is increased to 15 nm, while the PCEs drop markedly to 4.42%, which may be associated with the reduction of transmittance when increasing film thickness. The Voc and FF of different devices, however, remained almost constant. It can be concluded that the isolating nature of the sparsely distributed Au islands does not necessarily indicate inferior charge collection, but appears to allow adequate light penetration to the underlying active layer. With increasing UTA coverage the light transmittance is adversely affected, hence the reduction of the Jsc. A critical property of the UTA layer/AgNWs hybrid electrode would be its optical characteristics. We have thus prepared samples on glass slides since the devices are opaque when completed under operating conditions. It can be observed that the deposition of 3 layers of AgNWs result in a higher transmittance than that of FTO owing to light scattering effects37, which could greatly enhance solar cell performance (Figure 5, 0 nm). In particular, the AgNWs electrode features an average visible transmittance (AVT) of 87% at a wavelength of 550 nm, proving comparable performance to conventional transparent conducting oxides (typically AVT = 83% for solar cells). To deposit a UTA layer underneath the AgNWs would give rise to an


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appreciable AVT decline at 550 nm (Figure 5). Notably, 6 nm UTA + 3 layers AgNWs show an acceptable AVT of 70% at 550 nm.

Figure 5. Transmittance spectra of films with different Au thickness. Table 2. Photovoltaic performance of PSCs employing 6 nm UTA middle layer and different layers of AgNWs illuminated from both FTO and AgNWs sides.

AgNWs layers

Voc (mV)

Jsc (mA cm-2)

FF

η (%) Rs (Ω)

FTO

AgNWs

FTO

AgNWs

FTO

AgNWs

FTO

AgNWs

1

870

847

1.57

1.42

0.20

0.22

0.31

0.29

412~426

2

967

897

14.19

9.22

0.47

0.43

6.42

3.54

28.0~31.7

3

968

913

20.47

11.00

0.56

0.61

11.07

6.10

13.7~16.7

4

892

875

15.18

8.32

0.50

0.53

6.71

3.89

21.1~24.8

To investigate the effect of different AgNWs layers on device performance, we fabricated PSCs with 1-4 AgNWs layers on top of the evaporated UTA layer. As can be observed from


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Figure 6, depositing 1 layer of AgNWs would cause rather sparse distribution, revealing incomplete coverage of the underlying Au layer. As the layer numbers mount, more dense and cross-stacked nanowires can be readily seen. We further measured the sheet resistance (Rsh) of the perovskite/HTM samples with different AgNWs layers. It was found that with the fixed thickness of UTA (6 nm), 1 AgNWs layer generally yields sheet resistance of over 100 kΩ sq-1, whereas film 2 AgNWs layers shows dramatically decreased sheet resistance of merely 132.4 Ω sq-1. The considerably enhanced conductivity is possibly due to the much improved contact of overlapping AgNWs (Table 3). When the deposition reaches 3 layers, the film sheet resistance is further lowered to 34 Ω sq-1, which is optimal for electrical conducting purpose. The photovoltaic performances of devices with different layers of AgNWs have been shown in Table 2. The cells with 3 layers AgNWs and 6 nm UTA yield a highest efficiency of 11.07%, and an average up to 9.80% for 105 samples contributing to our study (Figure S5). Yet 4 layers would result in a metallic tint visible to bare eyes of the film and decrease the film transmittance. Meanwhile, more times of isopropanol infiltration will affect the properties of organic layers underneath, which will influence the AgNWs consequently. To investigate this, the cell with 3 layers AgNWs has been dip coated in isopropanol repeatedly. The sheet resistance tabulated in Table S2 shows an obvious increase after serveral times of dip coating, confirming that isopropanol was deleterious for the perovskite device by increasing Rsh. Table 3. Sheet resistance and transmittance of the composite electrodes with different layers of AgNWs on top of UTA layer. 6 nm Au+

1 layer

2 layers

3 layers

4 layers

Average Rsh (Ω sq-1)

192.6k

132.4

34.0

38.7

Transmittance (%)

76.2

73.9

70.0

65.2


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Figure 6. SEM images of AgNWs of (a) 1 layer (b) 2 layers (c) 3 layers and (d) 4 layers. As mentioned earlier in the text, the prevalent stability issue brought by the directly contact of reactive Ag with perovskite/HTM active layer poses great challenge towards fabricating high performance and durable devices. Also ambient moisture, oxygen and ultraviolet are widely known to cause a synergic and irreversible structural damage to perovskite material. The degradation mechanism of the perovskite-based device may be expressed as follows38: CH3NH3PbI3(s) ⇌ PbI2(s) + CH3NH3I(aq)

（1）

CH3NH3I(aq) ⇌ CH3NH2(aq)+HI(aq)

（2）

4HI(aq) +O2(g) ⇌ 2I2(s)+2H2O(l)

（3）

2HI(aq) ⇌ H2(g)+I2(s)

（4）

In order to investigate the device stability when incorporating an UTA middle layer, we have stored devices with AgNWs as top electrode in a desiccator for one week. Not surprisingly, those


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with AgNWs directly in contact with perovskite/HTM proved to be almost non-operational with an electrode sheet resistance of over 100 kΩ sq-1, whereas devices with an intermediate UTA layer show a performance of 9.12%, 82.4% of its original PCE with sheet resistance of 153 Ω sq1

. The AgNWs were found to be covered with nodules grown on each nanowires in the absence

of a UTA layer, which may be responsible for the markedly decreased sheet resistance hence device performance (Figure 7a). The ones that have an UTA layer underneath showed much improved morphological integrity, almost identical to that before storage (Figure 7b). Subsequent Transmission Electron Microscope (TEM) images confirmed the generation of the nodules on the outer surface of the nanowires, corresponding to a large volume expansion (Figure 8a). The elemental probing of Ag, I and O on an individual Ag nanowire showed an even distribution, corroborating the reactive nature of Ag when directly exposed to perovskite surface, the mechanism of which was illustrated in Figure 8b. Silver halides are known to be photosensitive and commonly would decompose into silver impurities, hence a gray coloration. To accurately identify the species proved to be difficult due to the nanoscale size and negligible loading of the material comparing to the matrix during electrode degradation.

Figure 7. SEM images of AgNWs films stored in ambient environment for one week (a) directly on top of perovskite/HTM active layer and (b) on top of intermediate UTA layer.


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Figure 8. (a) TEM image (b) schematic illustration of the degrading mechanism of Ag and elemental mapping of (c) Ag-K (d) Ag-L (e) I-K (f) I-L and (g) O-K. To further probe the elemental and electrode material evolution during storage, Electron Probe Microanalysis (EPMA) was carried out for film surface mapping (Figure 9). Given the metal halide perovskite is fully covered by organic HTM spiro-OMeTAD, the surface iodine (I) content is assigned to those that have migrated onto the active layer/electrode interface. It could be clearly evidenced that more I and O contents were significantly increased, suggesting Ag2O and AgI formation.


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Figure 9. EPMA elemental mapping: (a) (e) Au; (b) (f) Ag; (c) (g) I; (d) (h) O. The upper row belongs to the sample without the UTA layer, the lower row to that with the UTA layer.


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The iodine-containing compound originated from the perovskite could migrate through the spiro-OMeTAD film and then react with Ag as reported by Kato et al36, thus leading to poor stability for the perovskite solar cell based on Ag contact Moreover, pinholes usually exist in the spiro-OMeTAD layer, which enabled the moistures in the air to diffuse through themselves and decompose the perovskite layer. Here, by employing an ultrathin Au layer, the abovementioned iodine migration and also the moisture diffusion could be effectively blocked, which have been illustrated in Figure 10.

Figure 10. The mechanism of halide migration (a) without UTA layer (b) with UTA layer In light of all the evidences we can conclude that the insertion of an ultrathin Au middle layer serves to 1) ensure optimal ohmic contact of AgNWs and the active layer, 2) facilitate charge collection by lowering the orientation of AgNWs during spinning, and 3) enhance the stability of the fabricated device by providing a physical separation of the AgNWs from possible halide migration and reaction. CONCLUSIONS In summary, by depositing an ultrathin layer of Au in between perovskite/HTM active layer and AgNWs transparent top electrode, device stability as well as photovoltaic performance could be much enhanced. It was revealed that the introduction of an intermediate Au layer functions to improve the orientation of spin coated AgNWs, ensure ohmic contact of the active layer and


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AgNWs top electrode and prevent the formation of undesirable silver compounds in the absence of a physical separator. By optimizing the fabrication process, an overall PCE of 11.1% could be obtained from front illumination, and approximately 70% of the original PCE could be maintained when illuminated from the transparent electrode side. This approach opens up possibilities in potential used as transparent electrodes in flexible devices and tandem structures with much enhanced performance and stability. ASSOCIATED CONTENT Supporting Information. UTA thickness data probed by XRR and AFM; XRD pattern of the fabricated perovskite; photovoltaic performances of devices with semitransparent electrode illuminated from double-side. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Authors received funding from the Ministry of Science & Technology, Israel and the Ministry of Science & Technology, P.R. China: the China-Israel Cooperative Scientific Research Fund


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(2015DFG52690) and the Projects of International Cooperation and Exchanges NSFC (51561145007). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors express their gratitude for the support provided by the Ministry of Science & Technology, Israel and the Ministry of Science & Technology, P.R. China: the China-Israel Cooperative Scientific Research Fund (2015DFG52690) and the Projects of International Cooperation and Exchanges NSFC (51561145007). REFERENCES (1) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (2) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (4) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476-480.


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Table of Contents/Abstract Graphics


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Working from Both Sides: Composite Metallic Semitransparent Top

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