Enhanced efficiency in plastic planar heterojunction solar cells by AuNPs positioned at donor-acceptor interfaces

A new class of copolymers containing oligothiophene moieties with different length and fullerene units have been designed and prepared by an easy and inexpensive one-step synthetic approach. The incorporation of small quantities of these copolymers into bulk heterojunction (BHJ) solar cells with donor regioregular polythiophene (P3HT) and acceptor fullerene derivate (PCBM) results in a good control of the phase separation process without further affecting the BHJ optoelectronic properties. Indeed, these copolymers allow modulating under thermal annealing the growth of domains whose size depends on the length of the copolymer repetitive units. Domain size on the same length scale of the P3HT exciton diffusion length with a good continuity between the electrodes gives an efficient exciton dissociation and charge mobility. Thus by employing copolymers containing oligothiophenic chains with size of about 8 nm, the power conversion efficiency (PCE) (4.46 %) and short current density (JSC) (16.15 mA cm-2) values results in the highest reported so far for P3HT:PCBM solar cells on plastic substrates.