Two unsymmetrical donor-acceptor-acceptor-π-acceptor type benzothiadiazoles (BTD3 and BTD4) functionalized with tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) modules, showing strong absorption in the visible region are reported. The bulk heterojunction solar cells based on BTD4:PC71BM and BTD3:PC71BM based active layers processed with chloroform (CF), thermal annealing and subsequent solvent vapor annealing, i.e. two step annealing (TSA), exhibited PCEs of up to 6.02% and 5.36%, respectively, which is significantly higher than those of the corresponding devices based on the as-cast blend active layer. This enhancement is related to the improvement in exciton dissociation efficiency and more balanced charge transport in the devices based on the active layer processed with TSA treatment.