Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells

Chloro-aluminum phthalocyanine (ClAlPc) film growth on 1H,1H,2H,2H- perfluorodecyltrichlorosilane (FDTS) and MoO₃ is studied and correlated to ClAlPc/C₆₀ solar cell performance for both planar and bulk heterojunction (HJ) architectures. On top of unheated substrates, ClAlPc films grow amorphous independent of the substrate surface. When heated to 105 °C, ClAlPc grows with a face-on orientation on MoO₃, with a crystalline phase I-like absorption profile. On FDTS, the film is optically characterized as phase II, and adopts an edge-on orientation. Implemented in planar HJ cells, the latter films show a substantially higher current compared to the other growth conditions, leading to 3% efficient cells. This current increase is investigated with spectral response and reflectivity measurements, and is found to be related to a more efficient exciton dissociation. Next, ClAlPc and C₆₀ are co-evaporated on FDTS and MoO₃ modified ITO substrates to fabricate bulk HJ devices. Notably, we find that when a thin pure "templating" layer of ClAlPc is grown first, the subsequently grown ClAlPc:C₆₀ bulk HJ propagates the templating effect, and films show a higher crystallinity than without this templating layer, with higher fill factors as a result. On MoO₃, this approach yields efficiencies above 4%.