Development of Inverted Organic Solar Cells with TiO 2 Interface Layer by Using Low-Temperature Atomic Layer Deposition Zhenhua Lin, †,‡ Changyun Jiang,* ,† Chunxiang Zhu,* ,‡ and Jie Zhang † † Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore 117602 ‡ Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 * S Supporting Information ABSTRACT: Organic solar cells (OSCs) with inverted structure have attracted much attention in recent years because of their improved device air stability due to the use of stable materials for electrodes and interface layers. In this work, TiO 2 films, fabricated using low temperature (e.g., 130-170 °C) atomic layer deposition (ALD) on ITO substrates, are used as electron selective interface layers to investigate inverted OSCs. It is found that though the as-deposited TiO 2 films are high resistive due to the presence of oxygen defects, the defects can be significantly reduced by light soaking. PV cells with 15-nm-thick amorphous-TiO 2 layers fabricated at low temperature show better performance than those with poly crystal TiO 2 with same thickness deposited at 250 °C. The low temperature ALD-grown TiO 2 films are dense, stable and robust with capability of conformal coating on nanostructural surfaces, showing a promising interface layer for achieving air-stable plastic OSCs with roll-to-roll mass production potential. KEYWORDS: low-temperature fabrication, TiO 2 thin film, atomic layer deposition, inverted structure, organic solar cells, light soaking 1. INTRODUCTION Organic solar cells (OSCs) have attracted much attention in recent years due to their unique advantages: capability of low- cost and large area fabrication process, and lightweight and mechanical flexibility using plastic substrates. 1-4 Bulk hetero- junction of donor-acceptor has been proven a successful route toward solution processable OSCs with power conversion efficiency (PCE) up to ∼8%. 5 In most conventional OSCs, an acidic poly(3,4-ethyl-enedioxythiophene): poly- (styrenesulfonate) (PEDOT:PSS) hole transport layer and low work function metal electrode such as aluminum or calcium are generally used. However, the strong acidic property of the PEDOT:PSS layer is detrimental to the indium-tin-oxide (ITO) electrode, and the low-work-function metal can be easily oxidized in air, both degrading devices’ performance easily in air and leading to poor stability of the cells. 6-8 One attempt to improve the air stability of OSCs is to use inverted structure, 9-12 where an n-type metal oxide film is used as the electron selective layer on ITO substrate (cathode) and a high work function metal such as silver or gold is used as the top anode. In the inverted structure, both the low-work- function metal and the interface of ITO/PEDOT:PSS can be avoided and thus enhances the air stability. One key challenge in making efficient inverted OSCs lies in the electron selective buffer layer: it should work as a high conductive path for efficient electron extraction while exhibit good hole blocking capability. A number of works have been carried out in inverted OSCs using TiO 2 film as electron selective layer, owing to its superior air stability as compared to other semiconductor oxides. The synthesis of TiO 2 has been investigated a lot, 13-19 and there are several methods commonly used to fabricate a compact TiO 2 thin film, including sol-gel (or suspension) solution processing, 17,18 and spray pyrolysis. 19 Spray pyrolysis usually employs a high-temperature (>250 °C) process for decomposition of titanium oxide precursors, which may be not suitable for low-cost plastic substrates. The sol-gel solution processing methods have demonstrated promising potential for low temperature and roll-to-roll fabrication of TiO x thin film for OSCs. However, it has its limitations in precisely controlling the thickness and uniformity of the ultrathin TiO x film over large area and its poor conformal coating capability on nanostructured surfaces. 20-22 Low-temperature atomic layer deposition (ALD) seems more suitable for fabrication of compact TiO 2 film due to its capability in achieving uniform, dense and highly conformal films on 2D and 3D surfaces, and on plastic substrates. 23 However, there are very few reports on inverted OSCs using ALD-processed TiO 2 film as the electron selective layer. Kang et al. ever reported an efficient inverted OSC fabricated by using nanocrystalline TiO 2 film deposited by ALD at 220 °C. 24 However, this deposition temperature is too high for commonly used plastic substrates such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). In a recent study of using TiO 2 /ZnO selection layer for inverted Received: October 8, 2012 Accepted: January 21, 2013 Published: January 21, 2013 Research Article www.acsami.org © 2013 American Chemical Society 713 dx.doi.org/10.1021/am302252p | ACS Appl. Mater. Interfaces 2013, 5, 713-718