Efficiency improvement using bis(trifluoromethane) sulfonamide lithium salt as a chemical additive in porphyrin based organic solar cells

Two new conjugated acceptor–π-donor–π-acceptor (A–π-D–π-A) porphyrins have been synthesised using 3-ethylrhodanine (1a) or dicyanovinylene (1b) groups as acceptor units. Their optical and electrochemical properties made these materials excellent electron donors along with PC₇₁BM as the acceptor for solution-processed bulk heterojunction organic solar cells. The devices based on 1a:PC₇₁BM (1:2) and 1b:PC₇₁BM (1:2) processed with CB showed low power conversion efficiencies (PCE) of 2.30% and 2.80%, respectively. Nonetheless, after processing the active layer using a mixture of 3 vol% of a pyridine additive in THF, the PCE was enhanced up to 5.14% and 6.06% for 1a:PC₇₁BM and 1b:PC₇₁BM, respectively. Moreover, when we used LiTFSI as the chemical additive in pyridine/CB-processed 1b:PC₇₁BM an excellent PCE of 7.63% was recorded. The effects over the film morphology and the device characteristics (J_sc, V_oc and FF) due to the introduction of LiTFSI are discussed.