Conjugated polyelectrolyte and zinc oxide stacked structure as an interlayer in highly efficient and stable organic photovoltaic cells† Yi-Ming Chang * and Chi-Yi Leu A highly efficient and stable inverted organic photovoltaic cell has been demonstrated by incorporating a zinc oxide (ZnO)/conjugated polyelectrolyte (CPE) stacked structure as an electron-transporting layer. The CPE not only improved the electron collection efficiency in inverted devices but also smoothened the ZnO surfaces, thereby reducing the leakage current and improving the device reliability. In this work, two CPEs, poly[(9,9-bis(3 0 -(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) and poly[3-(6-trimethylammoniumhexyl)thiophene] (PTMAHT), are used, and three different interlayers, ZnO, ZnO/PFN, and ZnO/PTMAHT, are systematically studied. By coating PFN onto the ZnO surface, a high power conversion efficiency of 8.54% (with a fill factor of 73%) and 17% efficiency improvement are achieved in thieno[3,4-b]-thiophene/benzodithiophene copolymer, PTB7, and [6,6]-phenyl C 71 butyric acid methyl ester bulk heterojunction solar cells, along with excellent stability. Introduction The organic photovoltaics (OPVs) have attracted broad research attention because organic semiconductors have many advan- tages compared to conventional inorganic materials, including mechanical exibility, lightness, and ease of large-area fabri- cation. 1–3 Recently, a power conversion efficiency (PCE) of more than 10% has been demonstrated in OPVs. 4 These results also indicate the signicant future potential of organic solar energy cells. Considering various breakthroughs during the develop- ment of OPVs, it is noted that improvements that result in high performance always rely on both material innovation and state- of-the-art device architecture. Among these breakthroughs, interface modication is commonly used to simultaneously improve the device efficiency and stability. 5 According to our knowledge, OPV devices fabricated with a regular architecture have limited air stability; further, rapid oxidation of the low-work-function cathodes in OPV cells is observed, 6 and indium tin oxide (ITO) is gradually etched by the acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) charge-transport layer. 7 As a result, the device performance strongly degrades, leading to poor lifetimes. 8 Accordingly, to improve the stability of OPVs, an inverted archi- tecture has been designed, where the positions of the cathode and the anode are reversed and the low-work-function electrode is replaced by an air-stable electron-transporting and hole-blocking interlayer, such as zinc oxide (ZnO). 9–12 ZnO has been widely used as the interlayer in the fabrication of inverted OPV cells because it is a safe, environmentally stable, and cheap material. Moreover, ZnO can be prepared by various synthesis routes and converted into different morphologies. 13–15 Careful adjustment of the interface characteristics between different materials may further improve the PCE. Recently, inverted OPVs with a high PCE of 7.3% and a ll factor (FF) of 69% were demonstrated in a device that incorporated a bulk heterojunction (BHJ) layer composed of thieno[3,4-b]-thiophene/benzodithiophene copolymer (PTB7) (Fig. 1a) and [6,6]-phenyl C 71 butyric acid methyl ester (PC 70 BM) on a ZnO interlayer. 9 The PCE of the thus-prepared inverted devices is comparable to that of regular devices reported in the literature; 16 further, the device stability in air is improved. Moreover, Small et al. demonstrated an inverted poly- dithienogermole–thienopyrrolodione:PC 70 BM solar cell using an ultraviolet-ozone-treated ZnO–poly(vinyl pyrrolisone) composite sol–gel lm as the interlayer. 17 The thus-prepared device showed a PCE of as high as 8.5% with a FF of 67%. By incorporating a cross-linked fullerene-based electron- transporting interlayer, [6,6]-phenyl-C 61 -butyric styryl dendron ester, the interface characteristics of ZnO in the inverted architecture can be further modulated. The thus-prepared device based on poly(3-hexylthiophene) (P3HT):indene-C 60 bis- adduct BHJ exhibited an improved photocurrent and FF, yielding a high PCE of 6.2%. 18 Recently, conjugated poly- electrolytes (CPEs) have emerged as a novel class of electron- transporting interlayers. The CPE layer is believed to form dipoles at the organic–metal interface and facilitate efficient Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan. E-mail: ymc@itri.org.tw; Fax: +886-3- 5820093; Tel: +886-3-5915021 † Electronic supplementary information (ESI) available: Contact angle measurement, UV-visible absorption spectra of CPE lms, stability test of encapsulated devices, Best J–V curves of the OPVs. See DOI: 10.1039/c3ta10537g Cite this: J. Mater. Chem. A, 2013, 1, 6446 Received 4th February 2013 Accepted 27th March 2013 DOI: 10.1039/c3ta10537g www.rsc.org/MaterialsA 6446 | J. Mater. Chem. A, 2013, 1, 6446–6451 This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry A PAPER