Synthesis and photovoltaic properties of new medium band gap polymers based on phthalimide units

Three novel medium band gap polymers based on phthalimide (PHI) units, namely, poly{[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b′]dithiophene]-alt-[3,6-(N-(2-ethylhexyl)phthalimide)]} (P1), poly{[2,6-(N-(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]pyrrole]-alt-[3,6-(N-(2-ethylhexyl)phthalimide)]} (P2), and poly{[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl)]-alt-[3,6-(N-(2-ethylhexyl)phthalimide)]} (P3) have been synthesized using Stille coupling reaction. The polymers were characterized by ¹H NMR spectroscopy, gel permeation chromatography, UV–Vis absorption spectroscopy, and cyclic voltammetry. Photovoltaic study of solar cells with these polymers as donor and [6,6]-phenyl-C₆₁-butyric acid methylester (PCBM) as acceptor showed power conversion efficiency (PCE) in the range of 0.36–2.24 %. The highest PCE of 2.24 % was achieved for P3/PCBM blend without any interface engineering or post-treatment. This study offers a useful and important insight for designing PHI-based medium band gap polymers used for efficient poly mer solar cells.