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Apr 10, 2018 - Room-Temperature and Aqueous Solution-Processed Two-. Dimensional TiS2 as an Electron Transport Layer for Highly Efficient and Stable ...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Room-Temperature and Aqueous Solution-Processed TwoDimensional TiS2 as an Electron Transport Layer for Highly Efficient and Stable Planar n−i−p Perovskite Solar Cells Peng Huang,† Ligang Yuan,† Kaicheng Zhang,† Qiaoyun Chen,† Yi Zhou,*,† Bo Song,*,† and Yongfang Li†,‡ †

Laboratory of Advanced Optoelectronic Materials, College of Chemistry Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ‡ CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: In this study, a room-temperature and aqueous solution-processed two-dimensional (2D) transition-metal dichalcogenide TiS2 was applied as an electron transport layer (ETL) in planar n−i−p perovskite solar cells (Pero-SCs). Upon insertion of the 2D TiS2 ETL with UV-ozone (UVO) treatment, the power conversion efficiency (PCE) of the planar Pero-SCs was optimized to 18.79%. To the best of our knowledge, this value should be the highest efficiency to date among those PCEs of the n−i−p Pero-SCs with room-temperature-processed metal compound ETLs. More importantly, the n−i−p Pero-SCs with the UVOtreated 2D TiS2 as an ETL also show extremely high stability, where the average PCE remained over 95% of its initial value after 816 h storage without encapsulation. KEYWORDS: planar n−i−p perovskite solar cells, UVO-treated 2D TiS2, electron transport layer, room-temperature and aqueous solution-processed

1. INTRODUCTION

Considerable efforts have been made on exploring ETLs prepared at low (or room) temperature, but we are still not satisfactory on the presently developed materials/methods. For example, Li et al. achieved a PCE as high as 17.6% with the application of an Al-doped ZnO layer prepared by radio frequency sputtering as an ETL; however, this technique relies on an expensive and complicated equipment that can be a limit for massive production.23 ZnO was presented as a good roomtemperature-processed ETL and has drawn wide attention; however, up to date, the moderate PCEs (450 °C)-processed ETLs (e.g., TiO2),16−18 and low-temperature (∼150 °C)-processed ETLs (e.g., ZnO19,20 and SnO221,22) have been developed. Seeking for room-temperature and aqueous solution-processed ETLs is not only for saving the cost by simplifying the processing procedure but also for bringing a chance to achieve highly stable photovoltaic devices. © XXXX American Chemical Society

Received: February 24, 2018 Accepted: April 10, 2018 Published: April 10, 2018 A

DOI: 10.1021/acsami.8b03225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

probable thickness of the nanosheets determined by depth analysis was approximately 3.9 nm (corresponding to three layers).41 It is worth to note that 2D TiS2 is quite transparent for the visible light, which can be evidenced by the UV−vis spectra (Figure S1 in the Supporting Information) of ITO substrates before and after modification of approximately 10 nm of 2D TiS2. In the following, the work function (WF) of ITO with the 2D TiS2 film was determined by a Kelvin probe force microscope (KPFM) using highly oriented pyrolytic graphite (HOPG) as a reference and the measurement method referred in the literature.42 As shown in Figure 2, the WF of pristine

and aqueous solution-processed 2D MoS2 and WS2 as HTLs in Pero-SCs.33 Along this research clue, we attempt to seek good 2D TMDs that can be employed as ETLs in n−i−p Pero-SCs. In the present study, room-temperature and aqueous solutionprocessed 2D TiS2 was prepared by exfoliation and used as an ETL because of its unique electronic and chemical properties.34−36 We found that the 2D TiS2 ETL with the treatment of UV-ozone (UVO) led to a great improvement of the device performance, and a PCE as high as 18.79% was achieved. To the best of our knowledge, this should be the highest PCE using room-temperature-processed metal compound ETLs. More importantly, the stability of the Pero-SCs became extremely high, and 95% of the average PCE was retained after 816 h storage without encapsulation. We believe that the room-temperature and aqueous solution-processed UVOtreated 2D TiS2 possesses a great potential as easily processible and environment friendly ETLs for highly efficient and stable Pero-SCs.

2. RESULTS AND DISCUSSION The 2D TiS2 was prepared by an ultrasonication-enhanced lithium interaction method.37 In brief, the TiS2 powder was reacted with n-butyllithium under vigorous ultrasonication and thus exfoliated to nanosheets with a few atomic layers. X-ray diffraction (XRD) was employed to characterize TiS2 before and after exfoliation. As shown in Figure 1a, the diffraction

Figure 2. WFs of ITO with the modification of 2D TiS2 and UVOtreated 2D TiS2.

ITO was 4.75 eV, which is consistent with that reported in the literature.43 Upon modification with 2D TiS2, the WF increased to 4.79 eV. Such a WF does not match the conduction band (approximately 4.4 eV) of perovskite. Interestingly, a 15 min UVO treatment can effectively decrease the WF of the 2D TiS2modified ITO to 4.64 eV. After UVO treatment, the energy level difference between ITO and conduction band of perovskite is