Organic Tandem Solar Cell Using a Semi-transparent Top Electrode for Both-side Light Absorption

We fabricated organic tandem solar cells having all transparent electrodes, which allowed light absorption from both sides. The transparent bottom, middle, and top electrodes are indium tin oxide (ITO), LiF (0.5 nm)/Al (3 nm)/MoO(3) (10 nm) and LiF (0.5 nm)/Al (3 nm)/Au (15 nm), respectively. The active layers of the bottom and the top cells are a bulk heterojunction of poly(3-hexylthiopene) (P3HT):phenyl-C61-butyric acid methylester (PCBM) and a bilayer of zinc phthalocyanine (ZnPC, 15 nm)/fullerene (C(60), 40 nm), respectively. The top LiF/Al/Au electrode shows a transmittance (T) of more than 40% over the spectral range of 400 - 700 nm. The solar cell performances are similar under the solar simulated illumination of AM 1.5G 100 mW/cm(2) to both sides: short-circuit current (J(SC)) = 1.21 mA/cm(2), open-circuit voltage (V(OC)) = 1.07 V, fill factor (FF) = 52.84%, and power conversion efficiency (PCE) = 0.68% for illumination to the ITO side; and J(SC) = 1.25 mA/cm(2), V(OC) = 1.09 V, FF = 44.42 %, and PCE = 0.61% for illumination to the LIF/Al/Au side. Since the V(OC) is approximately the sum of the top and the bottom cells for illumination to both electrodes, the middle electrode of LiF (0.5 nm)/Al (3 nm)/MoO(3) (10 nm) performs properly to connect both unit cells.