Inverted polymer solar cells integrated with small molecular electron collection layer

An efficient inverted polymer solar cell (PSC) is reported by integrating a small molecular electron collection layer (ECL) between indium tin oxide (ITO) cathode and the photoactive layer of blended poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM). The ECL is composed of a cesium carbonate-doped tris(8-hydroxyquinolinato) aluminum (Cs ₂CO₃:Alq₃) layer. As determined by photoelectron spectroscopy and electrical measurements, the Cs ₂CO₃ doping induces suitable energy level alignment at the ITO/Cs₂CO₃:Alq₃/PCBM interface and the increase in bulk conductivity of organic ECL, which are favorable to electron extraction through Cs₂CO₃:Alq₃ to ITO cathode. In addition, optical simulation indicates that the Cs₂CO ₃:Alq₃ layer can act as an optical spacer to modulate the region of highest incident light intensity within the photoactive layer, where absorption and charge dissociation are efficient. The inverted PSC with an optimized Cs₂CO₃:Alq₃ ECL exhibits a power conversion efficiency of 4.83%. The method reported here provides a facile approach to achieve high-performance inverted PSCs at low processing temperature.