Solution processed polymer tandem cell utilizing organic layer coated nano-crystalline TiO 2 as interlayer Won-Suk Chung a,c , Hyunjung Lee a , Wonmok Lee b,* , Min Jae Ko a , Nam-Gyu Park a , Byeong-Kwon Ju c , Kyungkon Kim a, * a Materials Science and Technology Division Solar Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea b Department of Chemistry, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea c Display and Nanosystem Lab, College of Engineering, Korea University, Seoul 136-701, Republic of Korea article info Article history: Received 14 June 2009 Received in revised form 8 November 2009 Accepted 5 December 2009 Available online 11 December 2009 Keywords: Solar cells Organic photovoltaics Polymer tandem solar cells Polymer solar cells Titanium dioxide nanoparticles abstract A solution processed polymer tandem cell has been fabricated by utilizing organic layer coated TiO 2 nanoparticle (OL-TiO 2 ) as an interlayer. The crystalline phase of the OL-TiO 2 was anatase. The dispersed solution of the OL-TiO 2 showed high optical transparency and excellent film forming property. The top and bottom cell were clearly separated by the OL-TiO 2 interlayer without interlayer mixing, which was not observed for the tandem cell utilizing commercially available TiO 2 nanoparticle (N-TiO 2 ) as an interlayer. The con- version efficiency of a polymer tandem cell was enhanced from 1.43% to 3.44% by replacing the interlayer from N-TiO 2 to OL-TiO 2 . The tandem cell performance was further enhanced by adjusting the thicknesses of the active layers in the subcells and adjusting the conduc- tivity of the PEDOT:PSS layer in the bottom cell. The highest conversion efficiency of 3.66% was obtained from the tandem cell having the structure of ITO/Baytron P VP AI 4083/ P3HT:PCBM (100 nm)/OL-TiO 2 /Baytron PH 500/P3HT:PCBM (100 nm)/Al. In addition that, it was found that the OL-TiO 2 interlayer enhanced the stability of the tandem cell compar- ing to that of the single junction cell by the reduction of the oxygen diffusion to the bottom layer by the interlayer. It is expected that the performance of the tandem cell can be further enhanced by adopting efficient low band gap materials. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The single junction solar cell has limitations to over- come Shockley–Queisser limitation because excess energy of the photons, absorbed with energies greater than the semiconductor band gap, are lost as heat and photons with energies less than the semiconductor band gap are not ab- sorbed [1]. The tandem solar cell technology could be one way to overcome the limits of the single junction solar cell. However, it is difficult to fabricate polymer tandem solar cells because the previously deposited layer could be de- stroyed by the following solution process. Therefore, it is important to use a proper separation layer (or interlayer), which isolates two subcells. The interlayer protects the bot- tom cell as well as recombining the charge carriers from the subcells. There are several reports on the formation of the interlayer and middle electrode by the vacuum deposition method [2–7]. However, it is desirable to fabricate polymer tandem cells with all solution processes in terms of large area fabrication, the production cost and the high through- put. It is essential to develop solution processed interlayer materials to fabricate the polymer solar cell with solution process. Semiconducting metal oxides are thought to be promising candidates for the interlayer due to their optical transparency, electron mobility and chemical stability. Gi- lot et al., used ZnO nanoparticles/PEDOT interlayer for the 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.12.007 * Corresponding authors. E-mail addresses: wonmoklee@sejong.ac.kr (W. Lee), kimkk@kist.re.kr (K. Kim). Organic Electronics 11 (2010) 521–528 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel