Synthesis and characterization of copolymers based on benzotriazoles and different atom-bridged dithiophenes for efficient solar cells

Abstract

A series of donor (D)–acceptor (A) alternating copolymers based on a benzotriazole (BTZ) acceptor and different dithiophene donors, such as dithieno[3,2-b:2′,3′-d]pyrrole (NT), dithieno[3,2-b:2′,3′-d]silole (ST) and cyclopenta[2,1-b:3,4-b′]dithiophene (CT), have been successfully synthesized to study the effect of bridging atoms on the properties for application in polymer solar cells (PSCs). The resulting copolymers exhibit good solubility in organic solvents, good thermal stability and low bandgaps. It is important to note that their absorption spectra, electronic energy levels as well as photovoltaic properties can be facilely modulated by changing the bridging atom of donor segments from N, Si to C. The UV-vis and cyclic voltammetry results show that PNTBTZ containing NT block has the best light-harvesting ability and the narrowest bandgap because of the strongest electron-donating property of NT unit compared to ST and CT. However, PSTBTZ exhibits the lowest lying HOMO energy level, which is beneficial to achieve the high V_oc of the related polymer solar cells. The basic electronic structures of D–A model compounds of these copolymers were also studied by DFT calculations at the B3LYP/6-31G* level. The PSCs using these copolymers were fabricated and evaluated with a typical structure of ITO/PEDOT:PSS/copolymer:PC₇₁BM/Ca/Al under the illumination of AM 1.5G, 100 mW cm⁻². As expected, the PNTBTZ device has the highest V_oc value among these PSCs for the deepest HOMO energy level. However, the device based on PCTBTZ with a C-bridged dithiophene (CT) donor segment exhibits the highest J_sc of 9.24 mA cm⁻², FF of 0.64 and PCE of 4.02% due to the broadest and strongest spectral response as well as ideal morphology compared to PNTBTZ and PSTBTZ.