We report two series of D-A-pi-A metal-free organic sensitizers for dye-sensitized solar cells (DSSCs) based on triphenylamine and N-hexyl-carbazole as electronic donors, respectively. Through varying auxiliary acceptors and pi-spacers, several significant consequences on cell efficiency were identified: (i) a broadened UV-Vis absorption spectrum and low-lying LUMO level with [1,2,5]selenadiazolo[3,4-c] pyridine as auxiliary acceptor; (ii) compensation for the absorption valley around 400 nm in the UV-vis spectra by the introduction of a thiophene unit into the pi-bridge; (iii) effective improvement of the power conversion efficiency (PCE) by means of cosensitization, leading to dye OKT-1, 3.10% PCE, increased to 4.19% with squaraine dye SQ2 as co-sensitizer. The design criteria identified have opened the door for further optimization of this new dye family. (C) 2017 Elsevier Ltd. All rights reserved.