In order to prevent the inter-molecular aggregation and retard electron recombination, blocking side substituents, i.e. alkyl and alkylphenyl, were incorporated into the bithiophene bridge and two new carbazole-based sensitizers (CzD2 and CzD3) were synthesized. A reference dye (CzD1) comprising bithiophene spacer was also synthesized for comparative purposes. Their photophysical, electrochemical, structural and electronic properties were extensively investigated for highlighting the effects of side substituents. Moreover, their photovoltaic properties were evaluated using cobalt-containing electrolyte. The CzD3 with the bulkiest side group yielded the highest power conversion efficiency (PCE) of 6.20% due to the largest open circuit voltage (Voc) and short circuit photocurrent (Jsc). Furthermore, incident photon-to-current efficiency (IPCE) spectra and electrochemical impedance spectroscopy (EIS) analysis were performed to reveal the impact of side substituents on the performance of devices. Therefore, this work demonstrates that incorporating blocking side groups into the ?-bridge is an effective way to obtain high PCEs of sensitized cells with cobalt redox.