Efficient solvent-assisted external treatment for planar heterojunction small-molecule organic solar cells

Abstract

We developed a novel solvent-assisted treatment (SAT) technique to modify the nanomorphology of the planar heterojunction (PHJ) bilayer active layers (ZnPc/C₆₀) of organic photovoltaics (OPVs). The SAT technique uses organic solvent vapors under reduced pressures, which partially dissolves one component (the donor molecule, ZnPc, in this study) of PHJ layers prepared by vacuum deposition. Because of the partial mixing of the two layers, the PHJ layers develop a bulk heterojunction (BHJ)-like intermixed morphology. The performance of the resulting OPVs is considerably improved because of (i) the increased interfacial area of ZnPc/C₆₀, (ii) the healing of the intrinsic micropores within the active layers, which originate from the deposition process, and (iii) enhanced light absorption due to the rearrangement of ZnPc molecules. After the SAT, the power conversion efficiency (PCE) of OPVs improved more than three-fold (2.58%), with an open-circuit voltage (V_OC) of 0.61 V, a short-circuit current (J_SC) of 7.50 mA cm⁻², and a fill factor (FF) of 0.56, as compared to that of the as-prepared PHJ-OPVs (PCE = 0.83%, with V_OC = 0.38 V, J_SC = 5.3 mA cm⁻², and FF = 0.42). Our unique SAT technique provides an alternative route for controlling the nanomorphology of organic thin films by vacuum deposition, which may be very difficult to achieve using more conventional methods.