NOVELTY - An improved bulk heterojunction solar cells (100) comprises an anode with thickness of 500-1500 Angstrom composed of a glass substrate (110) coated with indium tin oxide (ITO) (120) and a film of a hole transport layer (130) Poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS), an active layer (140) with thickness 500-2000 Angstrom containing an electron donor and an electron acceptor in weight ratio of 1:1 to 1:2, and aluminum cathode (150) with thickness of 1000-1500 Angstrom .    USE - As bulk heterojunction solar cells.    ADVANTAGE - High power conversion efficiency (PCE) in polymer solar cell can be achieved with PC60BM acceptor in combination with a blend of two low band gap conjugated polymers. The polymer solar cell developed on above concept/combination is cost effective and efficient.    DETAILED DESCRIPTION - An improved bulk heterojunction solar cells (100) based on modified poly(4,8-bis((2-ethylhexyl)oxy)benzo(1,2-b:4,5-b')dithiophene-2,6-diyl)(3-fluoro-2-((2-ethylhexyl)carbonyl)thieno(3,4-b)thiophenediyl) (PTB7)-(6,6)-phenyl-C61-butyric acid methyl ester (PC60BM) comprises an anode with thickness of 500-1500 Angstrom composed of a glass substrate (110) coated with indium tin oxide (ITO) (120) and a film of a hole transport layer Poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS) (130), an active layer (140) with thickness 500-2000 Angstrom containing an electron donor and an electron acceptor in weight ratio of 1:1 to 1:2, and aluminum cathode (150) with thickness of 1000-1500 Angstrom . The modification of PTB7 is done by blending it with poly(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl) (PCDTBT), in a solvent preferably chlorobenzene. An INDEPENDENT CLAIM is included for preparation of the improved bulk heterojunction solar cell involving cleaning the ITO coated glass, ultrasonicating and UV/ozone treating followed by spin coating thin film of PEDOT:PSS at 4000-5000 rpm for 40-60 seconds, modifying PTB7 by blending it with PCDTBT in a solvent, preferably chlorobenzene by stirring 10-14 hours at room temperature to obtain a composite electron donor; blending the electron donor composite with an electron acceptor PC60BM to obtain the active layer; spin casting the active layer on to the anode; vacuum depositing Al electrode (cathode) on the top of active layer.    DESCRIPTION OF DRAWING(S) - The figure shows schematic layout of a polymer solar cell.    Solar cell (100)    Glass substrate (110)    Indium tin oxide (120)    Hole transport layer (130)    Active layer (140)    Cathode (150)