Thin Metal Electrodes for Semitransparent Organic Photovoltaics

Abstract

We demonstrate semitransparent organic photovoltaics (OPVs) based on thin metal electrodes and polymer photoactive layers consisting of poly(3-hexylthiophene) and [6,6]-phenyl C 61 butyric acid methyl ester. The power conversion efficiency of a semitransparent OPV device comprising a 15-nm silver (Ag) rear electrode is 1.98% under AM 1.5-G illumination through the indium-tinoxide side of the front anode at 100 mW/cm 2 with 15.6% average transmittance of the entire cell in the visible wavelength range. As its thickness increases, a thin Ag electrode mainly influences the enhancement of the short circuit current density and fill factor. Its relatively low absorption intensity makes a Ag thin film a viable option for semitransparent electrodes compatible with organic layers.Keywords: Thin metal electrodes, semitransparent, organic photovoltaics. Manuscript received Dec. 13, 2012; revised June 11, 2013; accepted June 19, 2013 I. IntroductionOrganic photovoltaic (OPV) cells are promising candidates for various power generating applications, and their recent progress shows ascending power conversion efficiency (PCE) of up to 10.6% based on a tandem structure [1]. It is thought that the manufacturing processes of OPV cells are more costeffective than those of crystalline silicon solar cells and inorganic thin film solar cells by minimizing the vacuum deposition process [1]- [3]. The flexibility of substrates using plastic films, stainless steel foil, and even paper is also considered a unique advantage of OPV devices [3], [4]. The ultra-thinness and absorption band tunability of OPV photoactive layers enable the development of transparent and colorful PV products for power-generating windows of building-integrated photovoltaics and automatic vehicles [5]- [9].To enhance the transparency of OPV electrodes for various applications, different approaches have been reported. Transparent conducting oxides, conducting polymers, carbon nanotubes, metal nanowires, graphene, thin metals, and combinations of these materials have been introduced in OPV devices with improving efficiencies [4], [7]-[13]. Conducting polymers, metal nanowires, and thin metals have shown desirable compatibility with organic photoactive layers. However, consecutive solution-processed electrodes on polymer layers might result in the dissolution of bottom photoactive layers or provide differences in wettability between polymers and electrode materials [10]. In this study, thin metal electrodes as top cathodes are investigated to achieve higher PCE of semitransparent OPV devices without solvent damage or a wettability issue of the bottom organic layers. The current density-voltage (J-V) characteristics are obtained using a source measure unit under AM 1.5-G illumination through the ITO side of the front anode at 100 mW/cm 2 . The device fabrication steps and J-V measurements of the OPV devices are processed in nitrogen glove boxes. Their external quantum efficiencies (EQEs) and internal quantum efficiencies (IQEs) are evaluated in air at wavelengths of 35...

Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

“…Figure a compares the optical transmittance of 15 nm Al, Au, and Ag films with an 80 nm thick Ag layer . The average transmittance of the 15 nm thick films is relatively low and, thus, these type of coatings are suitable only for bottom electrical contacts of semitransparent solar cells .…”

“…The average transmittance of the 15 nm thick films is relatively low and, thus, these type of coatings are suitable only for bottom electrical contacts of semitransparent solar cells . Further reduction of the thickness of metal electrodes will not help since the increase of the sheet resistance will significantly reduce the fill factor FF that will result in the drop of the photoelectrical performance …”

“…a) Optical transmittance of different metal thin films. Reproduced with permission . Copyright 2013, Wiley.…”

Distinction of [220] and [204] textures of Cu(In,Ga)Se2 film and their growth behaviors depending on substrate nature and Na incorporation

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