Letter Embedded organic hetero-junction and negative-differential-resistance photocurrent based on bias-assisted natural-drying of organic drops Xian Ning Xie a, * , Yuzhan Wang b , Xingyu Gao b , Kian Keat Lee a , Chorng Haur Sow a,b , Kian Ping Loh a,c , Andrew Thye Shen Wee a,b a NUS Nanoscience and Nanotechnology Initiative (NUSNNI), National University of Singapore, Singapore b Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore c Department of Chemistry, National University of Singapore, Singapore article info Article history: Received 22 April 2010 Received in revised form 25 May 2010 Accepted 20 June 2010 Available online 6 July 2010 Keywords: Photo current negative differential resistance Organic hetero-junction Bias-assissted drop casting abstract This work reports a bias-assisted natural-drying method in film drop-casting for the forma- tion of an interfacial layer inside the bulk organic film. The natural-drying facilitates phase segregation in the polymer blend, while the simultaneous substrate bias application induces host electronic states in the interfacial layer. Consequently, an embedded organic hetero-junction is formed between the interfacial layer and bulk film. The hetero-junction not only enhances the separation of photoexcited electron–hole (e p  h p ) pairs, but also allows for the selective storage and extraction of photocarriers through the bias-induced host states. Unusual photocurrent with negative-differential-resistance (NDR) is thus obtained for the first time for organic optoelectronic devices. The typical peak-to-valley ratio of the NDR peak is P500, and the NDR behavior is stable for prolonged photocurrent measurements. The bias-assisted natural-drying method is simple and cost-effective, and can be used as a new approach to the design and control of organic interfaces. Ó 2010 Elsevier B.V. All rights reserved. Interface plays a key role in organic-based optoelectronic devices, yet the formation of interface is very complex. When two materials are in contact, their energy levels and surface charges will rearrange to reach an equilibrium state. This process is often complicated by additional factors such as chemical bonding, surface dipole, and defect state forma- tion at the interface [1]. In photovoltaics, the interface or hetero-junction not only affects the dissociation and life- time of photoexcited electron–hole (e p  h p ) pairs, but also determines the transport energetics of photocarriers throughout the device [2–4]. For inorganic Si-based photo- voltaics [5], the interface formation is achieved by contact- ing p-type Si with its n-type counterpart. The depletion layer associated with the formation of p–n junction gener- ates a build-in field E in , which provide the driving force for the separation of the e p  h p pairs. In organic photovoltaics, the analogous hetero-junction is obtained by interfacing electron-donating organics with electron-accepting organ- ics [6–12]. Compared to inorganic p–n junctions, the control of organic interfaces in solution processes is much more dif- ficult as their formation is very sensitive to the specific wet process such as the solvent used, the rate of drying, and the post-drying heat treatments [3,9,10]. Intrinsic current with negative-differential-resistance (NDR) has been widely observed for conducting organic films and composites [13–17]. In general, the NDR mani- fests as a current peak in the current–voltage (IV) curve, and it originates from charge injection, storage and release through host electronic states in the films. The NDR is an interface phenomenon, and is useful in novel applications such as resistive switching and memory devices [13–17]. To date, there is no report on photocurrent NDR in organic optoelectronics. This is mainly due to the absence of host states at the organic interfaces for the accommodation and storage of photocarriers. 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.06.014 * Corresponding author. E-mail address: nnixxn@nus.edu.sg (X.N. Xie). Organic Electronics 11 (2010) 1543–1548 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel