Robust and recyclable substrate template with ultrathin nanoporous

Subscriber access provided by READING UNIV

Article

Robust and recyclable substrate template with ultrathin nanoporous counter electrode for organic-hole-conductor-free monolithic perovskite solar cells Ming-Hsien Li, Yu-Syuan Yang, Kuo-Chin Wang, Yu-Hsien Chiang, Po-Shen Shen, Wei-Chih Lai, Tzung-Fang Guo, and Peter Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12367 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

ACS Applied Materials & Interfaces

Robust and recyclable substrate template with ultrathin nanoporous counter electrode for organichole-conductor-free monolithic perovskite solar cells Ming-Hsien Li1, Yu-Syuan Yang1, Kuo-Chin Wang1, Yu-Hsien Chiang1, Po-Shen Shen1, Wei-Chih Lai1,2,3, Tzung-Fang Guo1,2,3, Peter Chen1,2,3,* 1

2

Department of Photonics, National Cheng Kung University, Tainan, 701, Taiwan Center for Micro/Nano Science and Technology (CMNST), National Cheng Kung University,

Tainan, 701, Taiwan 3

Advanced Optoelectronics Technology Center (AOCT), National Cheng Kung University,

Tainan, 701, Taiwan *E-mail: [email protected]

KEYWORDS: perovskite solar cell; n-i-p heterojunction; organic-hole-conductor free; nanoporous electrode; all-inorganic solar cell template; multi-component perovskite

ACS Paragon Plus Environment

1


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

Page 2 of 29

ABSTRACT:

A robust and recyclable monolithic substrate applying all-inorganic metal-oxide selective contact with nanoporous (np) Au:NiOx counter electrode is successfully demonstrated for efficient perovskite solar cells, of which the perovskite active layer is deposited in the final step for device fabrication. Through annealing of the Ni/Au bilayer, the nanoporous NiO/Au electrode is formed in virtue of interconnected Au network embedded in oxidized Ni. By optimizing the annealing parameters and tuning the mesoscopic layer thickness (mp-TiO2 and mp-Al2O3), a decent PCE of 10.25% is delivered. With the mp-TiO2/mp-Al2O3/np-Au:NiOx as a template, the original perovskite solar cell with 8.52% PCE can be rejuvenated by rinsing off perovskite material with DMF and refilling with newly deposited perovskite. Renewed device using the recycled substrate once and twice respectively achieved PCE of 8.17% and 7.72% that are comparable to original performance. This demonstrates that the novel device architecture is possible to recycle the expensive transparent conducting glass substrates together with all the electrode constituents. Deposition of stable multi-component perovskite materials in the template also achieves an efficiency of 8.54% which shows its versatility for various perovskite materials. The application of such novel NiO/Au nanoporous electrode has promising potential for commercializing costeffective, large scale, and robust perovskite solar cells.

INTRODUCTION:

The emerging perovskite solar cells (PSCs) have attracted tremendous attention due to their superior photonic properties, high power conversion efficiency and low-cost fabrication process.1-4 As evidenced from many characterizations such as UV-Vis spectrum, time-resolved photoluminescence, electron beam induced current profile, and time-resolved terahertz


2

Page 3 of 29

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


spectroscopy, the hybrid organic-inorganic perovskite possesses strong absorption over the visible range,1 promising ambipolar transport property with long carrier diffusion length,5-6 and high carrier mobility.7-8 Moreover, the facile solution-processed fabrication of perovskite makes it possible for the aim of cost-effective solar cells. These advantages make perovskite solar cell one of the most promising alternatives to compete with the current existing photovoltaic technologies. Despite the extremely high device efficiency achieved by solution-processed approach, fabrication of large-area PSCs remains challenging using spin-coating process due to the demand on uniformity. Currently, several issues have been addressed in the field if this technology is going to make real impact for commercialization. First, the stability has to be significantly improved either by introducing new absorber materials or novel architectures that can survive light soaking. Secondly, the fabrication cost shall be affordable and comparable to the market existing technologies. As inspired from the monolithic dye-sensitized solar cells (DSCs),9 mesoporous counter electrode have recently been employed for perovskite solar cells, where the perovskite precursor is ultimately infiltrated in the final step for device fabrication.10-25 Since perovskite is vulnerable to high temperature process and polar solvent, low-temperature processed soft chemistry is in general applied on top of perovskite. For example, spin-coating of organic hole transport materials (HTMs) or electron accepter such as PCBM are conducted after perovskite to avoid the degradation of the underneath perovskite film in conventional layer-bylayer n-i-p or p-i-n sandwiched structure. Thus, deposition of perovskite in the last step for device fabrication offers several advantages. First, it allows the use of high temperature or harsh chemical process for making selective electrodes and provides us more selections of charge transport materials as well as their fabrication process and perovskite formation.24,

26

Some

investigation pointed out that the use of organic HTM with its corresponding additive and


3


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

Page 4 of 29

solvent can react with perovskite.27 Thus perovskite deposition as the last step in device fabrication prevents the unnecessary reaction. Secondly, the demand for large area smooth morphology is alleviated as the contact is a mesoscopic junction. Thirdly, the most expensive constituent and most energy-demanding process in device fabrication can be saved if the selective contact integrated substrate can be recycled. Transparent conducting glass is considered the most expensive part in the whole device28 and sintering process for preparation of mesoporous layer is energy-consuming. If the substrates can be recycled by removing the degraded perovskite and reloading the low-cost perovskite material to revive the device, such architecture would lower the cost for commercial application and reduce the energy budget for device fabrication. Some reports have demonstrated the revitalization of perovskite solar cell by immersing the fabricated perovskite solar cells in polar aprotic solvent to recycle the transparent conductive glass substrate.29-32 Although the dissolved gold electrode and HTM can be further reused, recycling process for these materials is necessary. More efforts shall be conducted to achieve recycled perovskite solar cell in virtue of re-deposition of perovskite light absorber, Spiro-OMeTAD and gold electrode, sequentially, onto the recycled substrate. In this investigation, we develop a superior device configuration, which all the constituents of the perovskite cell except the perovskite absorber can be recycled. In such architecture, the perovskite layer can be deposited in the final step for device fabrication with the merits of nonorganic carrier transport layer, robust and recyclable substrate. Moreover, combining with scalable and low-temperature process for the fabrication of mesoporous layer, a large area of fully inorganic template with energy-saving budget is favorable for the future commercial products. For example, Müller-Buschbaum et al. deposited mesoporous TiO2 layer using spraydeposition incorporated with block-copolymer-assisted sol-gel synthesis.33 The amount of


4

Page 5 of 29

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


presynthesized TiO2 nanoparticles determines the pore size and specific surface area of mesoporous TiO2 layer. Jen et al. applied low-temperature combustion process to synthesize copper-doped NiOx with superior crystallinity and electrical conductivity than the pristine NiOx. A promising efficiency of 17.74% was demonstrated with a p-i-n inverted architecture.34 Hou et al. conducted low-temperature processed NiO nanocrystal ink to serve as HTM for planar inverted device and obtained impressive VOC of 1.11 V due to the reduction of non-radiative loss.35 Zhao and Wong et al. exchanged the solvent of the commercial NiO suspension by 1butanol to directly spin coat onto the surface of perovskite. With insertion of CuSCN interfacial layer to form NiO/CuSCN hybrid HTM, the normal n-i-p heterojunction PSCs with all-inorganic carrier transport layer exhibited remarkable long-term stability for ~140 days.36 Zhou et al. presynthesized NiO nanocrystals by using noninjection thermolysis method with ligand protection to prepare NiO nanocrystals solution. The low-temperature solution-processed NiO nanocrystals were further employed as HTM in the inverted planar and normal mesoscopic architectures to deliver 15.9% and 9.11% efficiency, respectively.37 Wang et al. further employed self-assembled monolayer to passivate the surface defects of the low-temperature solution-processed NiO nanocrystals. The PSCs using interfacial modified NiO effectively reduced the trap-assisted recombination to deliver an outstanding efficiency of 18.4%.38 Furthermore, room-temperature solution-processed NiOx nanoparticles are prepared by controlling the PH value of the NiO-based precursor for well dispersion in the deionized water. The spin-coated NiOx nanoparticles produce a compact and pinhole-less thin film as p-type contact for efficient perovskite solar cells39-43 and photodetectors44. These facile preparations for NiOx with low-temperature process can eliminate the conventional high-temperature sintering process, and reduce the energy budget for fabrication.


5


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

Page 6 of 29

Han’s group firstly introduced the mesoporous (mp) carbon electrode for monolithic device and infiltrated the perovskite precursor into the mesoporous structure by sequential deposition45 upon the mp-carbon electrode.10 Due to the excellent ambipolar transport feature combined with long carrier diffusion length, the perovskite simultaneously functions as the light absorber and hole transport material in the devices.10-18,

23

The HTM-free architecture composed of mp-

TiO2/mp-ZrO2/mp-carbon was demonstrated to display an efficiency of 12.8%.10 In addition, the mp-carbon-based mesoscopic device employing hetero p-n junction of mp-TiO2/mp-NiO facilely separates electron and hole flow in opposite direction, leading to an efficiency of 11.4%.19 By further inserting a non-injection semiconductor of mp-ZrO220-21 or mp-Al2O322 to separate the mp-TiO2 and the mp-NiO as a n-i-p heterojuntion, the TiO2/NiO interfacial charge recombination is significantly suppressed. Such design separates selective contact electrode and improves the PCE to ~15%.20-22 A thickness over 10 um of mesoporous carbon electrode is usually applied to improve electrical conductivity; however, a fragile over-thick carbon hinders the future application. The potential candidate of metal mesoporous electrode, such as Au and Ni, are further employed to replace mp-carbon electrode. Yu et al. de-alloyed the Ag/Au alloy leaf to form the nanoporous Au electrode and transferred it onto the mesoporous substrate composed of mp-TiO2/mp-Al2O3.46 Fan et al. employed mp-Ni electrode to substitute the mpcarbon by reducing the mp-NiO into mp-Ni with an annealing process in presence of the hydrogen.47 These conversions of metal into mesoporous metal electrode, such as Ag elimination or NiO reduction into Ni, suffer from some redundant or unnecessary processes which increase the cost and energy required for fabrication. More recently, we employed Ni/Au bilayer in the inverted planar perovskite solar cells to replace the transparent conducting oxide by oxidizing the e-beam-evaporated nickel/gold film in oxygen atmosphere with an annealing process to form a


6

Page 7 of 29

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


Au network embedded in NiOx.48 The advantages of thermal and chemical stability, suitable work function, effective hole transport, capability of to block electrons and diversity of preparation make NiOx as an effective HTM for effective perovskite solar cells.49 For the porous metal electrode, Au is a promising candidate because of its high electrical conductivity, chemical stability, favorable work function and agglomeration to form worm-like interconnected porous feature. The Au:NiOx is thus formed to simultaneously function as HTM and conductive electrode. This bifunctional p-type electrode requires a balance between optical transparency and electrical conductivity, thus the thickness of Au layer is limited to 7 nm. Inspired by this previous work, we further adopt such oxide/metal network for perovskite solar cells as counter electrode, where the demand of optical transparency is absent. To achieve a stable, thin and robust nanoporous-electrode with high conductivity, Ni and Au are sequentially deposited onto the mesoporous layer and directly converted into an nanoporous (np) Au:NiOx electrode by annealing process under ambient atmosphere. The bifunctional np-Au:NiOx is applied for the n-i-p heterojunction perovskite solar cell. Compared to the mp-carbon electrode based device, a much thinner nanoporous Au:NiOx electrode less than 100 nm exhibits compatible conductivity to facilitate the charge extraction. With the mpTiO2/mp-Al2O3/np-Au:NiOx as a template, the original perovskite solar cell with 8.52% PCE can be rejuvenated by rinsing perovskite material with DMF and refilling the newly deposited perovskite. A two-time recycled device was further demonstrated to reuse the TCO-contained substrate, providing a cost-effective strategy for commercial application. The thickness of the mesoporous layer is optimized to deliver an efficiency of 10.25%. To demonstrate that the novel device architecture is versatile to various perovskite materials, multi-component perovskite of Csx(MAyFA1-y)1-xPb(BrzI1-z)3 is further loaded as light absorber to deliver an efficiency of 8.54%.


7


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

Page 8 of 29

A promising perovskite solar cell configuration is thus demonstrated with the merits of simplicity, robustness and recyclability.

RESULTS and DISCUSSION:

The fabrication process is illustrated in Scheme 1. The mesoporous layers composed of the TiO2 working electrode and Al2O3 scaffold are sequentially spin-coated and sintered as the base, followed by thermal deposition of the nickel and gold thin film. Annealing the FTO/compact TiO2/mp-TiO2/mp-Al2O3/Ni/Au substrate converts the Ni/Au film into nanoporous NiO/Au thin film on top of the based template. The porosity of the nanoporous electrode in terms of pore size and interconnectivity are crucial factors that would affect the infiltration of perovskite precursor and the conductivity of np-electrode. The annealing temperature and period are conducted to control the porosity and morphology of the nanoporous counter electrode.

Scheme 1. The illustration of fabrication process for the monolithic perovskite solar cell composed of FTO/compact (cp) layer/mesoporous (mp) layer/Ni/Au. The device fabrication process is: (I) etching FTO substrate; (II) spray deposition of compact TiO2 layer; (III) spin-


8

Page 9 of 29

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


coating of mesoporous layer; (IV) thermal deposition of Ni/Au bilayer; (V) annealing of Ni/Au bilayer to form nanoporous (np) electrode; (VI) deposition of perovskite light absorber.

From the real-time monitoring of sputtered Au on the silicon wafer50 or polymer film51, the change of surface morphology depends on the Au thickness and undergoes an evolution of nucleation, island growth, island coalescence, domain coarsening and percolation, and finally layer growth to form a film. When the Au thickness exceeds the threshold thickness for percolation (6~8 nm), the predominant growth process is the layer growth. Therefore, 60 nmthick gold film is supposed to be continuous metal films. The surface morphology SEM image of as-deposited gold film is firstly examined in Fig. S1(a) and exhibits a polycrystalline morphology composed of interconnected grains. The top-view SEM image of the underneath mp-TiO2/mp-Al2O3 substrate is presented in Fig. S1(b) for comparison, indicating the full coverage of Ni/Au film on the top of mp-TiO2/mp-Al2O3 layer. The polycrystalline morphology of as-deposited Au film might be resulted from the surface roughness of underneath mesoporous layer. Figure 1 shows the SEM images of Ni (10 nm)/Au (60 nm) bilayer top-surface morphology by changing the annealing temperature from 300°C (Fig. 1(a)), 400°C (Fig. 1(b)) to 500°C (Fig. 1(c)) with a constant period of 15 min under ambient atmosphere. By annealing the Ni/Au film at 300°C, dark spots apparently appear to form pores on the surface due to the agglomeration of Au. A marginal increase of porosity is observed with increasing the annealing temperature to 500°C. To avert the melt of glass substrate for higher annealing temperature, we further fix the annealing temperature of 500°C with variant period from 30 min (Fig. 1(d)), 45 min (Fig. 1(e)) to 60 min (Fig. 1(f)) to observe the surface morphology. When the annealing period prolongs to 30 min, enlarged pores is formed with the continuation of agglomeration that increases the porosity. A well-interconnected worm-like Au network is achieved by annealing


9


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

Page 10 of 29

the Ni/Au film under annealing temperature of 500°C for 45 min that exhibits a porosity of ~44% with a promising sheet resistivity of 5.8 Ω/square, as revealed in Figure 1(e). Such morphology represents a very important feature that the requirements for perovskite filling and descent conductivity can be both satisfied. Although the porosity of Ni/Au bilayer increases to ~51% for the annealing period extending to 60 min, a fractured Au network significantly raises the sheet resistivity over 30 kΩ/square which is unfavorable for the carrier transport through the nanoporous electrode, as shown in Figure 1(f).

Figure 1. SEM images with a scale bar of 500 nm for oxidized Ni (10 nm)/Au (60 nm) bilayer deposited on mesoporous layer under annealing condition of (a) 300°C for 15 min, (b) 400°C for 15 min, (c) 500°C for 15 min, (d) 500°C for 30 min, (e) 500°C for 45 min, and (f) 500°C for 60 min. Their corresponding sheet resistance is inserted for comparison. The device architecture shown in Figure 2(a) depicts the n-i-p heterojunction FTO/cpTiO2/mp-TiO2/mp-Al2O3/np-Au:NiOx/CH3NH3PbI3 perovskite configuration, in which the perovskite is ultimately infiltrated into the mesoporous layer by two-step sequential deposition.45 The PbI2 precursor is dropped and spin-coated onto the nanoporous Au:NiOx electrode, followed by immersing in the CH3NH3I solution to convert PbI2 into CH3NH3PbI3 perovskite. As


10

Page 11 of 29

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


mentioned above, the porosity of np-Au:NiOx plays the key role for the device performance. Figure S2 presents the current density-voltage (J-V) curves for the devices applying Au:NiOx npelectrode prepared by thermally oxidizing the Ni (10 nm)/Au (60 nm) bilayer at 500°C for 15 min, 30 min, and 45 min, namely Ni/Au 15 m, Ni/Au 30 m, and Ni/Au 45 m, respectively. The statistic photovoltaic parameters of devices with variant annealing period are summarized in Table S1. The Ni/Au 15 m cell exhibits an open-circuit voltage (VOC) of 0.88±0.02 V, a shotcircuit photocurrent (JSC) of 9.99±1.09 mA/cm2, and a fill factor (FF) of 0.51±0.01, delivering a PCE of 4.44±0.51% under AM 1.5G simulated solar irradiation. With increasing the porosity by rising the annealing period, Ni/Au 45 m cell exhibits similar VOC but increased JSC as compared to the Ni/Au 30 m counterpart. The enhanced porosity benefits the infiltration of perovskite to improve the loading of perovskite light harvester. However, all these devices deliver a lower FF of ~0.5. This is probably due to the thick (~700 nm) mp-Al2O3 scaffold we initially used to separate TiO2 and counter electrode. Figure 2(b) shows the cross-sectional SEM image of the Ni/Au 45 m cell, in which the constituent layers of mp-TiO2 (~700 nm)/mp-Al2O3 (~700 nm)/npAu:NiOx with a capping overlayer of cuboid perovskite are clearly observed. It is obvious that the surface morphology on top of the counter electrode is not a concern anymore for making good electronic contact.


11


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

Page 12 of 29

Figure 2. (a) Device configuration of the nanoporous electrode based perovskite solar cells with a FTO/cp-TiO2/mp-TiO2/mp-Al2O3/np-Au:NiOx template. Sequential deposition of perovskite is thus formed by infiltration from the np-Au:NiOx electrode, above which a overlayer of cuboid perovskite is capped. (b) The cross-sectional SEM image of the as-fabricated device using oxidized Ni/Au bilayer formed at 500°C for 45 min. In our previous report, annealing the Ni/Au metal would induce Ni diffusion and NiO formation on the Au surface.48 The verification of NiO resulted from the oxidized Ni(10 nm)/Au(60 nm) bilayer covered on the mp-TiO2/mp-Al2O3 is examined via the X-ray diffraction (XRD). Initially, the samples are prepared by thermal deposition Ni thin film on the top of mesoporous layer. By annealing the Ni single layer on the mesoporous layer at 500°C under ambient atmosphere, the conversion from Ni (labeled by ○) to NiO (labeled by *) is observed from the XRD patterns, in which the characteristic diffraction peaks at 37.2°, 43.2° and 62.8° are respectively assigned to the (111), (200) and (220) crystalline facets, as shown in Figure 3(a). The peaks labeled by # correspond to the underneath mp-Al2O3 layer. The conversion is also confirmed from the UV-Vis spectra when the opaque Ni (green line in Figure 3(b)) transfers to the transparent NiO (yellow line in Figure 3(b)) with an ultra-thin thickness. The Ni/Au bilayer is thus deposited on the mesoporous layer and oxidized under the same annealing condition. After the formation of Au:NiO, its characteristic peaks are coincidence with that of NiO film, among which the diffraction peak at 37.2° slightly increased due to the overlap of NiO and Au signals. The blue line in Figure 3(b) presents the transmittance spectra of oxidized Ni(10 nm)/Au(60 nm). Compared to the red line of prime Ni(10 nm)/Au(60 nm), a relatively increased transmittance is achieved when Ni transfers to NiO.


12

Page 13 of 29

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


Figure 3. (a) XRD patterns for the as-deposited Ni film (green), annealed Ni film (yellow), asdeposited Ni/Au film (red) and annealed Ni/Au film (blue) to form Au:NiOx film. The metal layers are deposited on Al2O3 mesoporous layer. (b) The corresponding transmittance spectra of as-deposited Ni film, annealed Ni film, as-deposited Ni/Au film, and Au:NiOx film. To scrutinize the formation of Au:NiO, Figure 4(a) shows the secondary ion mass spectrometry (SIMS) depth profile of the as-deposited Ni(10 nm)/Au(60 nm) bilayer on the top of mp-Al2O3. The Au/Ni and Ni/Al interfaces are clear identified and the Al profile is consistent with the oxygen profile, revealing a layer-stacked structure of Au/Ni/Al2O3 as schematically illustrated in Figure 4(c). When the Au/Ni bilayer is annealed at 500°C under ambient


13


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

Page 14 of 29

atmosphere for 45 min, the conversion of NiO is confirmed as the oxygen profile overlapped well with the Ni profile (Figure 4(b)). For the annealed Au/Ni bilayer, the oxidized Ni diffuses around the Au surface to form NiO layer as observed in our previous results.48 Thus we expect the gold, nickel and oxygen signal to be detected on the surface once such diffusion phenomena occurs. Figure 4(d) schematically presents the structure of Au:NiOx porous thin film, in which the resultant NiO layer encloses the Au network.

Figure 4. Normalized SIMS profiles of relevant elements including Al, O, Ni, and Au as probed from the (a) Ni/Au bilayer, and (b) sample in (a) after annealing at 500°C for 45 minutes to nanoporous electrode (np-Au:NiOx) on the mp-Al2O3. The scheme of (c) Ni/Au bilayer and (d) oxidized Ni/Au bilayer covered on the mesoporous layer. The preliminary experiment for solar cells utilizing variant mp-TiO2/mp-Al2O3 thickness with an overall thickness of ~1400 nm is conducted to realize the effect of mp-TiO2/mp-Al2O3


14

Page 15 of 29

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


thickness on the photovoltaic parameters as summarized in Table S2. It is found that the film thickness plays a vital role on the devices performance, in conjunction with the porosity of Au:NiOx electrode. Notably, the perovskite solar cells using 700 nm thick TiO2 combined with 700 nm thick Al2O3 exhibit the best efficiency, mainly due to improvement of JSC. With thicker non-injection mp-Al2O3 scaffold thickness over 700 nm, the photo-generated carriers would need to transport over 1 µm to reach the contact electrode that is exceeding the carrier diffusion length for CH3NH3PbI3 perovskite; thus, higher recombination loss could be the main reason for the reduced JSC. On the other hand, a slightly lower JSC is also observed when increasing the thickness of mp-TiO2 over 700 nm. When the electrons inject into the mp-TiO2, the low electron mobility of mp-TiO2 dominates the transport of electrons;52 hence, a thicker mp-TiO2 would hinder the electrons to reach the FTO. These preliminary tests imply that it is critical to optimize the layer thickness for the balance between light harvesting and carrier transport. To facilitate the carriers transport to the electrode, the thickness of mp-TiO2 and mp-Al2O3 are remarkably reduced to be 70 nm and 350 nm, respectively, which is based on the highly efficient perovskite solar cells using 70 nm thick mp-TiO2 covered with a perovskite capping layer of ~400 nm.53 The resultant cell (denoted as 70/350) exhibits a decent VOC of 0.91 V, a modest FF of 0.66, and a low Jsc of 4.7 mA/cm2, delivering a low PCE of 2.8%, whose J-V curve is displayed in Figure 5(a). Despite reduction of the mp-Al2O3 thickness by half, the carriers generated from perovskite still transit over 500 nm to cause recombination and diminish the photocurrent. For the purpose of equivalent perovskite absorption, the thickness of mp-TiO2 and mp-Al2O3 are modified to be 300 nm and 150 nm with a similar overall thickness for the devices (named 300/150). A significantly enhanced Jsc of 15.4 mA/cm2 with an improved PCE of 8.5%, as shown in Figure 5(a) and Table 1, is achieved when the thickness of mp-Al2O3 reduces from 350 nm to 150 nm


15


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

Page 16 of 29

and that of mp-TiO2 increases from 70 nm to 300 nm. A thinner mp-Al2O3 could lower carriers recombination before carriers reach mp-TiO2 or nanoporous Au:NiOx electrode, while a relatively thick mp-TiO2 layer could effectively extract the electrons excited from the perovskite located within the mp-Al2O3 layer. Thus, we conclude that the thickness of mp-Al2O3 less than 300 nm facilitates the electron transport and extraction to improve the Jsc along with the device performance. Figure S3 reveals the distribution of perovskite cross the device by applying the SIMS measurement to detect the constituent elements, C, N, Pb and I, of the CH3NH3PbI3 perovskite, in which each layer of the template is labeled with color bar from the synchronous SIMS depth-profile by tracing the metal elements of Au, Ni, Al, Ti and Sn. It is clear that C, N, Pb and I signals distribute from the top of np-Au:NiO layer to the bottom layer of mp-TiO2, proving the infiltration of perovskite cross the template. Figures S4 and S5 depict the statistic distribution of efficiencies resulted from 23 devices and the J-V curves of 300/150 device with different scan direction (same scan rate of 0.2 mV/s), respectively. A negligible hysteresis is observed under forward scan (-0.2 V to 1.0 V) and reverse scan (1.0 V to -0.2 V) direction, and the corresponding photovoltaic parameters are presented in Table S3. A negligible hysteresis might be attributed to the balanced charge transport behavior, since the comparable carrier mobility for TiO2 and NiO.52 To further demonstrate the reuse capability of np-electrode based PSC, we remove the perovskite light absorber by washing with solvent (DMF). Subsequently, we reload the perovskite washed-off substrate again with the newly deposited perovskite material. Two-time recycled process for the all-inorganic template is evidenced from the photovoltaic performance as depicted in Figure 5(a) along with photovoltaic characteristics shown in Table 1. The recycled device presents comparable efficiency with the pristine 300/150 cell, indicating that the robust template is promising for recycled application. The inferior efficiency obtained from


16

Page 17 of 29

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


the recycled cells is mainly due to reduced photocurrent and voltage. The incident photon to electron conversion efficiency (IPCE) spectra of the pristine 300/150 cell and recycled once are displayed in Figure 5(b). The peak IPCE of pristine 300/150 cell reaches over 60% at approximately 350~600 nm and the photocurrent received from the IPCE spectrum integrating with the AM 1.5G solar spectrum provides a reasonable current density of 13.96 mA/cm2. The decreased performance of recycled devices is presumably attributed to the passivated mp-TiO2 surface that might be resulted from the binding of residual lead iodine onto the mp-TiO2 surface after rinsing the MAPbI3 perovskite active layer.32 After replacing with newly perovskite, the bound lead iodine could not convert into perovskite and form a passivation channel to hinder the carrier transport at the TiO2/perovskite interface. The lead iodine bound mp-TiO2 surface could also reduce the loading amount of perovskite due to its reduced specific surface area. With increasing the recycle times, such effect has impact on the device performance in terms of reduced JSC and VOC, simultaneously. However, the compatible efficiency of recycled devices indicates that the robust template is promising for recycle application. A comparative experiment is conducted to demonstrate the recyclability of our robust architecture. A carbon-based device with a monolithic structure composed of FTO/cp-TiO2/mp-TiO2/mp-ZrO2/Carbon is fabricated for the recycle test as referred to Figure S6(a). The perovskite light absorber filled in carbonbased device via sequential deposition is shown in Fig. S6(b). After rinsing perovskite material with DMF, partial carbon electrode is removed due to poor adhesion as indicated by white arrows in Fig. S6(c). From the recycle demonstration of supplementary video, carbon debris is peeling-off from the carbon counter electrode. On the contrary, the nanoporous electrode of Au:NiOx remains intact and presents well-conductivity after removing the perovskite with DMF as demonstrated in supplementary video. Compared to the pristine np-Au:NiOx-based template,


17


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

Page 18 of 29

the np-Au:NiOx electrode is still untouched for the perovskite-filled device rinsed by DMF (Fig. S6(e)), recycled once device rinsed by DMF (Fig. S6(f)), and recycled twice device rinsed by DMF (Fig. S6(g)).

Figure 5. (a) J-V curves for mesoscopic perovskite solar cells with a FTO/cp-TiO2/mp-TiO2/mpAl2O3/np-Au:NiOx/CH3NH3PbI3 perovskite configuration, in which the thickness of mpTiO2/mp-Al2O3 is modulated to be 70 nm/350 nm (namely 70/350) and 300 nm/150 nm (namely 300/150). A reused device for mp-TiO2/mp-Al2O3 of 300 nm/150 nm once (namely Recycled cell (1st)) and twice (namely Recycled cell (2nd)) are presented for comparison. (b) Incident photon to electron conversion efficiency (IPCE) response of pristine 300/150 cell and recycled 300/150 cell once.

Table 1. Photovoltaic parameters for perovskite solar cells employing np-Au:NiOx electrode with thickness of mp-TiO2/mp-Al2O3 of 70 nm/350 nm and 300 nm/150 nm. Photovoltaic parameters of recycled devices once and twice from the 300/150 cell are also shown for comparison. Cell

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

70/350

0.91

4.7

0.66

2.82


18

Page 19 of 29

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


300/150

0.89

15.4

0.62

8.52

Recycled cell (1st)

0.83

15.0

0.66

8.17

Recycled cell (2nd)

0.80

14.4

0.67

7.72

Eventually, a champion cell with a PCE of 10.25% is achieved by fine tuning the thickness of mp-TiO2 (~450 nm) and mp-Al2O3 (~200 nm), as displayed in Figure 6, which is comparable with the carbon-based counterpart.12,

54-55

With increasing the whole device thickness, the

loading amount of perovskite increases to ameliorate the light absorption capability and the photovoltaic parameters. Based on the same thickness of mesoporous layer, multi-component perovskite of Csx(MAyFA1-y)1-xPb(BrzI1-z)356 is utilized to serve as light absorber, achieving a PCE of 8.54% with its photovoltaic parameters summarized in Table. 2. The inferior performance might result from the un-optimized perovskite crystallinity within the mesoporous structure due to the solvent effect of perovskite precursor.26, 54 It is worth noting that the state of the art perovskite solar cells employed porous carbon counter electrode reaches a PCE of~15%, whose configuration consists of mp-TiO2/mp-ZrO2/mp-NiO/carbon mesoporous architecture.20, 22 A lack of mp-NiO layer in the present work could reduce the perovskite loading along with charge (hole) collection capability. The perovskite crystallization within the template is another key role for improving efficiency. The solvent effect26,

54

, constitution14 and treatment24 of

perovskite may significantly improve the device performance. The further optimization on device architecture and perovskite crystallinity within the mesoporous layer would be attempted to improve the device performance.


19


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

Page 20 of 29

Figure 6. J-V curves of champion devices using a thickness of mp-TiO2/mp-Al2O3 being 450 nm/200 nm incorporated with MAPbI3 and multi-component perovskite. Table 2. Photovoltaic parameters for np-Au:NiOx electrode based perovskite solar cells employing MAPbI3 and multi-component MAPbI3 perovskite. Perovskite

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

MAPbI3

0.91

17.04

0.66

10.25

Multi-component perovskite

0.93

13.91

0.66

8.54

CONCLUSION:

The conventional all-inorganic template with carbon porous electrode has been demonstrated to deliver high efficiency and excellent stability. However, carbon porous electrode is easy to peel off from the template when the perovskite-filled device is rinsed by DMF for the aim of reuse. Due to the fragility of carbon porous electrode, there is no report to demonstrate the recyclability for the carbon-based device. The published works for recyclable cells mainly focus on the


20

Page 21 of 29

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


sandwiched structure and recycle the FTO substrate only. Therefore, a robust template for recyclability is highly demanded to provide a cost-effective and energy-efficient device in the long run. An organic hole conductor free perovskite solar cells applying recycled template composed of all-inorganic metal-oxide selective contacts with nanoporous Au:NiOx counter electrode is successfully fabricated. Without any organic carrier transport layer, the all-inorganic metal-oxide template is durable and stable. The perovskite active layer can be deposited in the final step for device fabrication. With the optimization of annealing parameter and modification of the mesoscopic layer thickness (mp-TiO2 and mp-Al2O3), a decent PCE of 10.25% is delivered. We further demonstrate that the process of reloading perovskite material can rejuvenate the device and make this novel device architecture possible to recycle the expensive transparent conducting substrates containing all essential constituents of electrode materials. After rinsing off perovskite material with DMF, the nanoporous Au:NiOx counter electrode remains intact, indicating the novel device architecture is robust for reuse. The mesoscopic template employing the novel NiO/Au nanoporous electrode presents the merits of cost-effective, robustness and recyclability. Filling stable perovskite materials such as multi-component or 2D/3D hybrid perovskite57 in the template manifests the potential for stable perovskite solar cells. Since the perovskite is infiltrated in the final step, the preparation of all-inorganic template has more freedom on the process window such as sintering temperature and use of solvent and chemicals.

EXPERIMENTAL SECTION

SOLAR CELL FABRICATION:


21


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

Page 22 of 29

The fluorine-doped tin oxide (FTO) glass was sequentially cleaned with detergent, deionized water, ethanol and acetone. TiO2 compact layer (cp-TiO2) was deposited onto cleaned FTO substrates by spray pyrolysis using titanium diisopropoxide bis(acetylacetonate) precursor solution diluted in ethanol (20 mM). During spray pyrolysis, FTO substrate was keep at 500˚C on a hot plate, and each spray was pumped out using an atomizer (Glaskeller) with carrier gas of oxygen. 10 mL of diluted solution was spray-coated onto the substrates of 10×20 cm2. After the spray deposition was completed, the substrate was sintered at 500˚C for 30 min. The mesoporous TiO2 layer (mp-TiO2) was formed by spin coating diluted TiO2 paste (30NRT from Dyesol, 1:4 paste to ethanol volume ratio) solution at 4000 rpm for 30 s and sintered at 500°C for 30 min. The mesoporous Al2O3 layer (mp-Al2O3) was covered on the mp-TiO2 by spin coating diluted Al2O3 paste solution at 4000 rpm for 30 s and sintered at 500°C for 30 min. Ni and Au were sequentially deposited onto the top of mp-Al2O3 via thermal evaporation with a deposition rate of 0.1 Å/s and 0.5 Å/s, respectively, under a working pressure

Robust and recyclable substrate template with ultrathin nanoporous

Recommend Documents