Influence of annealing treatments on solution-processed ZnO film deposited on ITO substrate as electron transport layer for inverted polymer solar cells

• We realized a solution-processed ZnO thin film as electron transport layer for PSCs.
• We studied the effect of the annealing on the properties of ZnO and ITO layers.
• The best PCE (7%) was achieved using a ZnO film annealed at 150 °C for only 5 min.
• The ITO sheet resistance increases upon annealing worsening the device performances.
• A correlation between the PSC series resistances the ITO sheet resistance was shown.

In this work we studied the influence of the annealing treatments of a sol–gel derived ZnO electron transport layer deposited on ITO substrate, on the performances of inverted bulk heterojunction polymer solar cells using a blend of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] and [6,6]-phenyl C71 butyric acid methyl ester. Since the annealing treatments needed to complete the formation of the solution-processed ZnO film can modify the underlying ITO electrode, we analyzed the performance of the fabricated cells in terms of the properties of ITO and ZnO films. We found a linear relationship between the sheet resistance of the ITO layer and the series resistance of the corresponding device, which strongly influences the fill factor. The best power conversion efficiency (7%) under simulated AM 1.5G illumination of 100 mW/cm2 was achieved for the polymer solar cell fabricated using a ZnO film annealed at 150 °C for only 5 min. Higher annealing temperatures and times increase the sheet resistance of the ITO worsening the device performances.