An all-inclusive investigation of the effect of postprocessing thermal treatment on all vacuum-deposited small-molecule organic solar cells (SM-OSC) is presented. Herein, DTDCTB is chosen as the donor (D) molecule, and three fullerene-derivative acceptor (A) molecules, namely, ICBA, C70, and C60, are chosen for the study, and the devices were optimized for PV. As the first step, OSC cells were fabricated and characterized for photophysical, morphology, as well as various photovoltaic parameters, such as VOC, JSC, FF, and η, and external quantum efficiency for different processing parameters, such as active layer concentration ratios and annealing temperatures. The devices based on ICBA were found to have outperformed the devices using C70 and C60. OSC devices using ICBA as an acceptor are chosen for further characterization to establish the role of thermal treatment on their device performance. To this, 1-diode Shockley equation modeling is employed, and a qualitative relationship between diode saturation current and thermal annealing is obtained. Additionally, the charge recombination dynamics of the binary bulk heterojunction systems were investigated using the light intensity-dependent J-V characteristics, and the role of annealing in the reduction of trap-assistance recombination was established that corroborates well with the obtained annealing-dependent morphology information from AFM measurement. © 2011-2012 IEEE.