In this study, diethyl oxalate (DEOX) is applied as an effective inexpensive additive based iodide/triiodide electrolyte in the dye-sensitized solar cells (DSSCs). Addition the suitable amount of DEOX as 1 M into the electrolyte shows dramatically improvement in the short circuit current (J(sc)) and consequently, in the total conversion efficiency (eta). The fabricated devices based on N719 and 2-cyano-3-(4-(diphenylamino) phenyl) acrylic acid (TPA) sensitizers with modified electrolyte show the efficiency of 7.33% and 2.63% at an irradiation of AM1.5, and an 37% and 22% energy conversion efficiency increments, respectively. The boost in the photocurrent density mainly is due to the molecular complex formation between DEOX and redox species in the electrolyte solution that promotes the electrochemical properties of electrolyte. Also, electrochemical impedance measurements indicate adsorbing of DEOX on the semiconductor surface leads to an incensement in the lifetime (tau) and the electron density (n(s)) in the conduction band (CB) of TiO2 that shifts the Fermi level (Et) which leads to small enhancement in the V-oc. Adsorbing of DEOX on the titania surface retards the interfacial charge recombination that has a beneficial effect on the V-oc and J(sc). Furthermore, we compares the effect mechanisms of DEOX and 4-tert-butylpyridine (TBP) additives on the cell performance by applying different electrolytes containing the additives. These results show that TBP increases V-oc while DEOX additive has effect on the J(sc) and using of combination of additives leads to a remarkable improvement in eta. As a result, DEOX is a new promising co-additive which can be used for high efficient and low cost DSSCs. (C) 2015 Elsevier Ltd. All rights reserved.