Background: Polymeric solar cells (PSC) are advantageous over silicon-based photovoltaic devices in many ways, such as mechanical flexibility and low cost of production, butthe low power conversion efficiency (PCE) of PSCs and their relatively small lifetimes due to polymer degradation when exposed to UV radiation constitute considerable disadvantages. The aim of this paper is to enhance the PCE of the produced TiO2 nanosponge/spin-coated P3HT solar cells by doping of the PEDOT:PSS buffer layer with the coumarin known as umbelliferone. Methods: Devices were produced by spin-coating a P3HT layer over an electrochemically grown TiO2 nanostructured thin film. The PEDOT:PSS buffer layer having umbelliferone as an additive was deposited over the P3HT layer using air spray technique. Finally, the samples were thermally treated by annealing at 180°C. Result: The buffer layer, where the PEDOT:PSS layer was doped by umbelliferone, was characterized by NMR, MEV and conductivity measurements. It was observed that the conversion efficiency is enhanced for devices containing umbelliferone in the buffer layer, and the best result indicated a conversion efficiency of around 1.65%, with open circuit voltages and short circuit current as high as 320mV and 0.180mA, respectively, for devices with effective area of around 1mm2. Conclusion: It was observed that the conversion efficiencies of the umbelliferone additivate devices were almost twice as large as the non-additivated devices. The physical reason for the PCE enhancement is associated with chemical bond between umbelliferone molecules and PSS ionomers, producing a conductivity increase and morphological effects on the buffer layer.