A series of novel polythiophene derivatives comprised of alternating structures of N-alkylated thieno[3,4-c]pyrrole-4,6-dione (nTPD, n = number of carbon atoms from 11 to 18 in alkyl chains) and pristine thiophene were synthesized by Stille coupling reaction between 2,5-dibromo- or 2,5-diiodo-nTPD and 2,5-bis(tributylstannyl)thiophene. The effect of alkyl side-chain structures at the N-atom of TPD on the optical, electrochemical, and photovoltaic properties has been investigated systematically. In addition, these properties were compared with a widely used polythiophene derivative, i.e., poly(3-hexylthiophene) (P3HT). Optical bandgaps of the obtained polymers PnTPDTs estimated from absorption and fluorescence spectra were 1.8–2.1 eV, which were comparable or smaller than that of P3HT (2.0 eV). HOMO and LUMO energy levels of PnTPDTs were determined by electrochemical and optical measurements. The HOMO levels are −5.4 to −5.7 eV, which are lower than that of P3HT (−5.2 eV). The photovoltaic properties of the devices consisting of PnTPDT (n = 11, 12, 13, 18) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were investigated. The short-circuit currents (JSC) in the PnTPDT:PCBM devices were relatively low and varied significantly with alkyl side-chain structures (0.81–2.29 mA cm−2), whereas the open-circuit voltage (VOC) values were higher by 0.1–0.2 V than that in the P3HT:PCBM device. The P12TPDT:PCBM device exhibited the photocurrent generation exceeding wavelength of 750 nm, but the power conversion efficiency was found to be moderate (0.75%).