Elimination of Burn-in Open-Circuit Voltage Degradation by ZnO Surface Modification in Organic Solar Cells

Photodegradation of inverted organic solar cells based on ZnO as an electron transport layer (ETL) was studied over short time scales of 5 min and 8 h. Devices with ZnO as ETL reproducibly exhibited a steep loss of open-circuit voltage, <i>V</i>_OC, and shunt resistance, <i>R</i>_SH, in a matter of minutes upon illumination. Removing the UV-content of illumination minimized <i>V</i>_OC loss and impact on the device’s shunting behavior, indicating its role in the loss. Application of an ultrathin layer of Al on ZnO led to almost negligible photoinduced <i>V</i>_OC loss up to 8 h of exposure. By applying the fundamental Shockley diode equation, we approximated the <i>V</i>_OC loss to be caused by dramatic increases in reverse saturation current <i>I</i>₀. We attribute the increased rate of recombination to diminished carrier selectivity at the ZnO/organic interface. Devices with Al modified ZnO ETL demonstrated remarkable <i>R</i>_SH (1.4 kΩ cm² at 1 sun), rectification ratio (10⁶) and reverse saturation current density (2.1 × 10^–7 mA/cm²)