1 3 Microsyst Technol DOI 10.1007/s00542-017-3389-6 TECHNICAL PAPER ZnO/TiO 2 core–shell photoelectrodes for dye-sensitized solar cells by screen printing and room temperature ALD He Sun 1 · Hayato Kurotaki 1 · Kensaku Kanomata 2 · Fumihiko Hirose 2 · Matthew Schuette White 3 · Tsukasa Yoshida 1 Received: 9 March 2017 / Accepted: 24 March 2017 © Springer-Verlag Berlin Heidelberg 2017 TiO 2 is amorphous and can only be converted to crystal- line anatase TiO 2 by annealing at 450 °C. The crystalliza- tion recovered current but lost the advantage of the voltage increase. EIS has indicated a high trap density introduced by the amorphous ALD TiO 2 , which could be reduced by crystallization, but accompanied with an increased rate of recombination. 1 Introduction Dye-sensitized solar cells (DSSCs) employing zinc oxide (ZnO) as the photoelectrode material could offer poten- tial advantages to the traditionally used titanium dioxide (TiO 2 ), in that high temperature sintering process might be avoided to permit use of flexible plastic substrates. Strongly adhering porous ZnO layers can be fabricated by low tem- perature methods such as electrodeposition (Goux et al. 2006) and screen printing of nano-particles with subse- quent warm water treatment (Yane and Yoshida 2012). This is a welcome feature not only for cost and weight reduction of the device, but also for versatility in its use, especially as indoor energy harvesting devices to substitute batteries which are seen as one of the most promising applications of organic solar cells (Lungenschmieda et al. 2007). While high efficiencies above 10% have been reported with TiO 2 , efficiency of DSSCs with ZnO remained low, up to about 6% (Saito and Fujihara 2008). One of the reasons is the lack of a broad absorbing dye matched to ZnO. Broadly absorbing Ru(II) polypyridine dyes are well suited to TiO 2 and achieve a short circuit current (J SC ) over 20 mA cm -2 for the best examples (Chen et al. 2009). On the other hand, that of ZnO sensitized with a narrow absorbing organic dye is somewhere around 13 mA cm -2 , despite the high incident photon to current efficiency Abstract Mesoporous photoelectrodes for dye-sensitized solar cells (DSSCs) in a ZnO/TiO 2 core–shell structure have been fabricated at low temperature by combination of screen printing of ZnO nanoparticles and room tempera- ture atomic layer deposition (ALD) of TiO 2 . ALD nicely achieved a uniform coating of the individual ZnO particles with a thin skin of TiO 2 of uniform thickness. The overall mesoporous structure was unchanged, so that a high sur- face area for dye adsorption and pores for penetration of the electrolyte solution were preserved. DSSCs employ- ing the core–shell electrode sensitized with D149 indoline dye achieved a higher open circuit voltage (V OC ) than that with ZnO, but accompanied with a decrease of short circuit photocurrent density (J SC ) due to decreased efficiency of electron injection. Increase of the recombination resistance to extend the electron lifetime by the TiO 2 shell has been confirmed by electrochemical impedance spectroscopy (EIS). X-ray diffraction analysis has revealed that the ALD Electronic supplementary material The online version of this article (doi:10.1007/s00542-017-3389-6) contains supplementary material, which is available to authorized users. * Tsukasa Yoshida yoshidat@yz.yamagata-u.ac.jp He Sun sunhepeter@hotmail.com 1 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan 2 Department of Electrical Engineering, Faculty of Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan 3 Department of Physics, Materials Science Program, University of Vermont, Burlington, VT 05405, USA