Optimization of Active Layer and Anode Electrode for High-Performance Inverted Bulk-Heterojunction Solar Cells

Inverted ZnO–NPs/C₆₀-self-assembled monolayer (SAM)/poly(3-hexyl-thiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester solar cell devices were systemically optimized by varying the weight blend ratio of donor and acceptor from 1:0 to 1:1, the active layer thickness, the annealing temperature, the annealing time, and the top anode electrode. These inverted cells using C₆₀-SAM modification show a transition to a more bulkheterojunction device at blend ratios of 1:0.3 to 1:0.4, thus leading to large increase in device efficiencies from 1.6% to 3.5%. Further increase in the ratio shows eventual saturation in device efficiency to 4.5%, thus indicating an optimum blend composition. A strong dependence of the annealing temperature and time on the fill factor of the device is observed, which is correlated to changes in the morphology of the active layer. The anode metal electrode work function was varied using Ca/Al, Al, Ag, Cu, Au, and Pd as the contacts. Devices incorporating a layer of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) between the active layer and anode electrode exhibit similar V_oc regardless of electrode work function, whereas devices without the PEDOT:PSS layer exhibited behavior more similar to the MIM model. Optimized devices show efficiencies of 4.5% using a blend ratio of 1:0.7, an active layer thickness of ∼200 nm, and a thermal annealing condition of 160^◦C for 10 min using Ag as the top-metal-anode contact. Inverted polymer-based solar cells using cheaper metals like Cu showed similar device efficiency.