Effect of substrate temperature on ultrasonic spray deposited film morphology and coffee stain effect

In this work, polymer PTB7, small-molecule PC 71 BM and also their blend films have been deposited by ultrasonic spray method and studied the effect of substrate temperature on the resulting film morphology, coffee stain effect and the resultant solar cell performance. A coffee stain is a dark-coloured edge that remains on the surface when the solvent evaporates from a drop containing particles. We observed that at a lower temperature, the spray droplets coalesce in the resulting film and therefore large structures were seen in the pristine films. In addition, the coffee stain effect was more prominent for lower temperatures. For PTB7 and PC 71 BM droplets, the values are 13 and 135 μm at 40°C, and 3 and 7 μm at 60°C. In blend film, the coffee stain effect follows the same trend as pristine films but to a lesser extent. We also find that the coffee ring width was more (7 μm at 60°C) in droplets with PC 71 BM having a lower molecular weight compared to that of PTB7 (3 μm at 60°C). The solar cell performance however shows that at lower substrate temperatures the device performs better, indicating that the disadvantages of the coffee stain effect are compensated by active-layer properties coated at lower temperatures. The device performance at 42°C substrate temperature was best due to low R s = 93 Ωcm 2 and highest V oc = 0.64 V and J sc = 3.53 mA cm –2 values. As the substrate temperature was increased to 48°C, the R s value increased and was equal to R sh (500 Ωcm 2 ), which led to low J sc = 1.70 mA cm –2 and V oc = 0.50 V and eventually the lowest PCE. However, when coated at too low substrate temperatures the devices were shorted. Variations of J sc and V oc with irradiation intensity show enhanced recombination mechanisms in higher substrate temperatures.