Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells
Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.