Degradation of small-molecule organic solar cells

Small-molecule organic solar cells with a structure of indium tin oxide (ITO)-8-hydroxy-quinolinato aluminum (Alq3) (2nm) (C60) (40nm) phthalocyanine (CuPc) (32nm) (40nm) were fabricated. The shelf lifetime of unencapsulated devices was over 1500h, and the power conversion efficiency reached 0.76% under AM1.5G (air mass 1.5 global) 75mW∕cm2. The long lifetime was attributed to the inverted structure compared to the conventional ITO60 structure since the former could effectively protect C60 from the diffusion of oxygen and modify interfacial electrical properties. The introduction of a 2nm Alq3 layer into the cells enhanced the power conversion efficiency by more than 20 times. The presence of the thin Alq3 film on the ITO substrate lowered the substrate work function and hence increased the electric field in the organic layers, which was beneficial to the collection of free carriers. The reasons for the degradation of such kind of organic solar cells are analyzed in detail.