Two low-bandgap (LGB) conjugated polymers (P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 x 10(-5) cm(-1) (lambda approximate to 520 nm) and 1.4 x 10(5) cm(-1) 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 x 10(-3) cm(2)/VS for P2) allowing efficient charge extraction and a good fill-factor for PSC application. High-performance power conversion efficiency (PCE) of 4.4% is obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from PSC device with an active layer containing 25 wt% P2 and 75 wt% [6,6]-phenyl-C71 -butyric acid methyl ester (PC71BM), which is superior to that of the analogous P3HT cell (3.9%) under the same experimental condition.