We designed and synthesized a family of three structurally related conjugated alternating copolymers, namely poly [2,6-(4,4-dioctyl-4H-cyclopenta [2,1-b:3,4-b] dithiophene)-alt-2,7-(4,5-dioctylbenzo[2,1-b:3,4-b']dithiophene)] (PBDT), poly [2,6- (4,4-dioctyl-4H-cyclopenta[2,1-b: 3,4-b'] dithiophene)-alt-2,9-(5,6-dioctylnaphtho[2,1-b:3,4-b']dithiophene)](PNDT ), and poly [2,6-(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-6,9-(2,3-b is((S)-2,6-dimethylheptyl)dithieno[3,2-f:2',3'-h]quinoxaline] (PQDT). The repeating units of these three copolymers consist of two modified bithiophene units with one of them planarized by bridging benzo, naphtho, and quinoxalino segments, respectively. The 2,6-(4,4-dioetyl-4H-cyclopenta)[2,1-b;3,4-b']dithiophene moieties were introduced as the other bithiophene unit to improve the solubility of resultant copolymers, facilitating the polymers characterization and photovoltaic devices fabrications. All polymers were fully characterized, and their applications as donor materials in conjunction with PCBM as acceptor in bulk heterojunction (BHJ) photovoltaic devices were tested. The increased conjugation from naphtha and quinoxalino segments over benzo unit led to the decreased band gap of PNDT and PQDT compared to that of PBDT. A lower HOMO level was observed from PQDT due to the presence of two nitrogen atoms in the planarized pi system, which rendered the conjugated molecule electron-deficient. BHJ photovoltaic devices made from PQDT:PCBM film (1:1.6, 100 nm) exhibited a V-oc value of 0.53 V, a J(sc) value of 4.56 mA/cm(2), and a FF of 0.47, offering an overall power conversion efficiency of 1.14% and a peak quantum efficiency of 37% from IPCE measurement. The best performance of PQDT among three polymer investigated was correlated with its lower optical band gap (1.96 eV), lower HOMO level (-5.15 eV), and higher hole mobility (5.15 x 10(-5) cm(2) V-1 s(-1)). We concluded that the intrinsic electronic properties of these planarized bithiophene moieties offered moderate flexibility in fine-tuning electronic properties of the corresponding copolymers.