Poly(3-hexylthiophene) (P3HT) is one of the most-studied conjugated polymers due to its appropriate band gap energy (~1.9 eV) for front cell of tandem organic photovoltaics (OPVs), relatively high short-circuit current density of over 10 mA/㎠ and high external quantum efficiency of ~70%. In this presentation, we will report on the improved performance of P3HT-based OPV by incorporating a small amount of additive in active layer even without post-thermal annealing. The non-annealed OPV with P3HT:PCBM:Additive blend film showd an increase in short-circuit current and open-circuit voltage, resulting in an enhancement in power conversion efficiency, compared to that of reference cell without additive.