Integrated polymer solar cells in serial architecture with patterned charge-transporting MoO x for miniature high-voltage sources Seong-Min Cho 1 , Chang-Min Keum 1 , Hea-Lim Park 1 , Min-Hoi Kim 2 , Jin-Hyuk Bae 3 , and Sin-Doo Lee 1 * 1 School of Electrical Engineering, Seoul National University, Seoul 151-600, Korea 2 School of Global Convergence, Hanbat National University, Daejon 305-719, Korea 3 School of Electronics Engineering, Kyungpook National University, Daegu 702-701, Korea E-mail: sidlee@plaza.snu.ac.kr Received September 26, 2013; accepted January 31, 2014; published online March 13, 2014 We develop miniature high-voltage sources from polymer solar cells (PSCs) with charge-transporting molybdenum oxide (MoO x ) integrated in a serial architecture through sacrificial layer (SL)-assisted patterning. The MoO x layer, being patterned by the lift-off process of the SL of a hydrophobic fluorinated-polymer, as a hole transporting layer plays a critical role on the reduction of the dark current and the increase of a high open circuit voltage of an integrated PSC array. The underlying mechanism lies primarily on the elimination of the lateral charge pathways in the MoO x layer in the presence of the electrode interconnection. Two miniature voltage sources consisting of 20 PSCs and 50 PSCs are demonstrated in the operation of a liquid crystal display and an organic field-effect transistor, respectively. Our SL-assisted integration approach will be directly applicable for implementing the self-power sources made of the PSCs into a wide range of the electronic and optoelectronic devices. © 2014 The Japan Society of Applied Physics 1. Introduction Polymer solar cells (PSCs) have been paid much attention for the electrical power generation from the solar energy in a wide range of applications. 1–4) In the past decade, much effort has been made toward increasing the power conversion effi- ciency (PCE) of the PSC. Particularly, a hole transport layer (HTL) plays a significant role on the enhancement of the PCE through the charge selective contacts between a photoactive layer (PL) and the anode. The HTL of poly(3,4-ethylenedi- oxylenethiophene)–poly(styrene sulfonic acid) (PEDOT:PSS) has been widely used as an anode interfacial layer 5,6) but it often disrupts the device stability because of the hygro- scopic and acidic nature. 7,8) As an alternative, molybdenum oxide (MoO x ) was proven to substantially improve the PCE as well as the device stability through selective charge extracting and refractive index matching between the MoO x layer and the PL. 7–9) Besides the enhancement of the PCE of a single PSC, it is very important to construct miniature high-voltage sources by the integration of the individual PSCs for use as self-contained power supplies in various electronic circuits and autonomous micro-electro-mechanical systems requiring typically tens of volts. 10,11) For developing miniature high- voltage sources from the PSCs, there are two major issues; one concerns the realization of a high open circuit voltage by reducing the dark current paths inherent to the interconnec- tion of the individual PSCs, which is evident from the relationship between the open circuit voltage V oc and the dark current J s for a single photovoltaic cell, V oc ³ (Gk B T/q) ln(J ph /J s ) with the electronic charge q, the ideality factor G which is about one to two, the Boltzmann constant k B , and temperature T for J ph /J s º 1, where J ph denotes the photocurrent. 12,13) The other is how to integrate a large number of the PSCs in small volume, meaning that a chemically compatible and durable, high-resolution pattern- ing technique is inevitably required for integrating the PSCs into an array. In previous studies, a conventional photoli- thography was employed for patterning electrodes in planar interconnection of nanomodules. 14) In this case, however, it suffers from the complexity of processing, low PCE of 0.008%, and a low fill factor (FF) of 25%. The use of a patterned conductive polymer by mechanical scribing as well as selective coating on a self-assembling monolayer is rather simple but limits feature resolutions. 15) The orthogonal solvent method, which is a more advanced photolithographic variant for fabricating organic electronic devices, involves the deterioration of underlying organic layers by the exposure of ultraviolet light and plasma etching. 16) In this work, we develop miniature high-voltage sources from the PSCs having the patterned HTL of MoO x integrated in a serial architecture through sacrificial layer (SL)-assisted patterning. The elimination of lateral charge pathways in the MoO x layer plays a primary role in the reduction of the dark current and the increase of a high open circuit voltage of an integrated PSC array. In other words, patterning of MoO x is critical for suppressing the lateral charge flow in the PSC array. 2. Experimental procedure The patterning processes of MoO x in sequence are schemati- cally shown in Fig. 1; (i) a glass substrate with pre-patterned indium–tin-oxide (ITO), being as the anode, in Fig. 1(a) was cleaned sequentially with acetone, isopropyl alcohol, and methyl alcohol in an ultrasonicator for 10 min each. At each cleaning step, the substrate was rinsed with deionized water for 5 min in an ultrasonicator and purged with nitrogen gas. The cleaned substrate was then dried at 90 °C in a vacuum oven for 10 min to remove any residual water. (ii) The SL patterns complementary to desired MoO x patterns were transfer-printed onto the substrate using an elastomeric stamp of poly(dimethylsiloxane) (PDMS) as shown in Fig. 1(b). The SL patterns, dip-coated on the PDMS stamp, were well transferred without high pressure and/or heat since the adhesion of the SL material to the PDMS at the interface is weaker than that to the underlying substrate. 17) In our study, the SL material was a fluorinated-polymer (3M Novec TM EGC-1700) dissolved in a highly fluorous solvent (3M Novec TM HFE-7100). Note that the fluorous solvent is chemically inert to most of organic and inorganic materials. Depending on the resolution of the PDMS stamp, the SL patterns can be down to a few micrometers. 17–19) The Japanese Journal of Applied Physics 53, 042301 (2014) http://dx.doi.org/10.7567/JJAP.53.042301 REGULAR PAPER 042301-1 © 2014 The Japan Society of Applied Physics