BCP/Ag/MoO3 Transparent Cathodes for Organic Photovoltaics

IO N Flexible solar cells have attracted attention due to their advantages in low cost roll-to-roll fabrication and potential application in fl exible electronic devices. [ 1–6 ] Typical fl exible solar cells on a transparent substrate compose of bottom anode, thin-fi lm semiconductor, and a top cathode. [ 4 ] When the plastic fi lm was used as the substrate, the direction of incoming light is from substrate, a front illumination. [ 7 ] However, there is a distinct disadvantage in the plastic substrate, a high water vapor permeation rate (10 1 –10 − 1 g/m 2 /day at 25 ° C). [ 8 ] Such problem could be solved by employing a thin metal foil and stainless steel substrate. However, those substrates are opaque. Thus the device structure of fl exible solar cells on the opaque substrate composed of a refl ective bottom anode, a thin-fi lm semiconductor, and a transparent top cathode, [ 4 ] leading to a back illumination structure. In this confi guration of fl exible solar cells, the light incomes from top transparent cathode and makes photoexcited electrons in the semiconductor materials. [ 9 ] Thus, high power conversion effi ciency (PCE) of solar cells could be achieved by improving the optical transmittance of cathode in the devices. However, because the transparent top cathode composed by thin-fi lm of metal layer such as Ag/ Al and Ag/Au has low transmittance ( < 60%), more than 40% of the incident light is refl ected at the surface of top electrode, resulting in low PCE (1.1%) of the organic photovoltaic (OPV) solar cells. [ 10 , 11 ] This low transmittance of the top cathode has become one of the main obstacles to achieve high effi ciency solar cells. Hence, several other techniques such as alkali metal/Ag double layer and transparent conducting oxide (TCO) cathode have been studied to improve the optical transmittance of top electrode. [ 12–17 ] When an alkali metal surface such as Ca, Li, or Ba was deposited prior to the Ag or Al thin fi lm, one could obtain a high transmittance value ( > 70%) and low sheet resistance ( ∼ 10 ohm/sq), [ 12 , 13 ] leading to an increased PCE (1.7%) of OPV cells. But, such alkali metals caused the degradation of OPVs during the operation because of their susceptibility of moisture which is one of the most important factors on OPV device stability. [ 18 , 19 ] TCO materials such as ZnO, IZO and ITO were another candidate for transparent cathode. [ 20–23 ] Even though the TCO electrode has a good conductivity and high transmittance to obtain a high effi ciency (1.5%) in OPV devices, it was suffering from