Improved Performance for Inverted Organic Photovoltaics via Spacer between Benzodithiophene and Benzothiazole in Polymers

In this work, the effects of the spacer between benzo[1,2-b;3,4-b′]dithiophene (BDT) and dialkoxybenzothiadiazole (ROBT) in polymers were investigated for applications in organic solar cells. Polymer PBDT-2T-ROBT has a bithiophene (2T) spacer between the BDT and ROBT units, whereas PBDT-ROBT is a direct copolymer of BDT and ROBT. The polymer/PC₇₀BM solar cells using both polymers were fabricated and optimized via polymer/fullerene ratio, solvent, and solvent additives. The spacer has significantly improved solar cell performance from 1.28% (V_oc =0.77 V, J_sc = 3.13 mA/cm², FF = 53.11%) to 5.11% (V_oc = 0.66 V, J_sc = 13.33 mA/cm², FF = 58.12%). The X-ray diffraction (XRD) spectra show the PBDT-2T-ROBT/PC₇₀BM blended film is semicrystalline, whereas the PBDT-ROBT/PC₇₀BM film is amorphous. This indicates that the spacer facilitates polymer organization for higher carrier mobility in the film. Atomic force microscopy (AFM) topography and phase images show that PBDT-2T-ROBT/PC₇₀BM films form fibrillar networks, whereas PBDT-ROBT/PC₇₀BM films exhibit larger granular morphology. The photoinduced charge extraction by linearly increasing voltage (Photo-CELIV) measurements show that PBDT-2T-ROBT/PC₇₀BM has a mobility of 9.59 × 10^-5 cm²/(V·s), which is higher than the mobility of 8.64 × 10^-5 cm²/(V·s) for PBDT-ROBT/PC₇₀BM film.