Area dependent behavior of bathocuproine (BCP) as cathode interfacial layers in organic photovoltaic cells

Abstract

Standard and inverted configuration small molecule OPV cells incorporating bathocuproine (BCP) as electron transport and exciton blocking layer is investigated, demonstrating that 2 mm2 standard and inverted cells display a maximum performance for BCP thicknesses of 10 nm and 1.5 nm, respectively. The reason for the different optimum BCP thicknesses for the two device configurations is the BCP-metal complex formed between the Ag electrode and the BCP layer in the standard configuration OPV devices. Interestingly, at optimum BCP thicknesses, the inverted OPV cells outperform the standard devices. Upon up-scaling of the device area of the cells from 2 mm2 to 10 and 100 mm2, device failure becomes prominent for the inverted OPV cells, due to aggregation of the evaporated BCP layer on the ITO surface. This demonstrates that although BCP can be adopted for efficient ETL in inverted configuration OPV devices on small scale, it is not suitable for device up-scaling due to severely decreasing device yields. In this work, a possible solution where an ultrathin layer of C70 is evaporated between the ITO and BCP layer is proposed. It is demonstrated that the proposed solution holds a strong potential to minimize the device failures of the BCP based inverted OPV cells to a significant extent, while maintaining good device performances.

Degradation pathways in standard and inverted DBP-C70 based organic solar cells

“…Although both device types have identical active layers, the devices based on the standard structure show an overall better performance, reaching 130 mV higher V OC and higher PCE. Recent work has demonstrated that this trend, however, may depend on device area 41 . We note that the reported performances here are similar to those reported for bilayer DBP/C 70 cells in the literature 32,42 .…”

Inverted organic solar cells with non-clustering bathocuproine (BCP) cathode interlayers obtained by fullerene doping

“…Figure 2a shows an atomic force microscopy (AFM) image of 3 nm pure BCP deposited on top of an ITO coated glass substrate. The clustering of BCP occurs due to a large interface energy between ITO and BCP 33 , and may take place immediately after BCP deposition even at room temperature 34 . Such clustering can be explained by Ostwald ripening, in which some aggregates grow at the expense of others by adsorbing molecules from the surrounding surface area 45 .…”

“…Such clustering can be explained by Ostwald ripening, in which some aggregates grow at the expense of others by adsorbing molecules from the surrounding surface area 45 . At larger surface area, the probability of forming clusters that cause device shunts are larger 33 , making device upscaling more challenging in inverted OPV architectures. One possible solution to overcome BCP aggregation is to conduct co-evaporation with another organic small molecule in order to obtain smoother films 34 .…”

“…Several studies have focused on the function of BCP as ETL and EBL in OPV devices with standard device architecture 24,31,32 , highlighting the aforementioned properies. In our previous study, an area-dependent behavior of BCP used as ETL and EBL in inverted OPV devices was reported 33 . It was observed that while scaling up the OPV device area, the performance and device yield of the inverted OPV devices decrease significantly compared to standard configuration cells, which was demonstrated to be due to the clustering of BCP on ITO surfaces 33,34 .…”

“…In our previous study, an area-dependent behavior of BCP used as ETL and EBL in inverted OPV devices was reported 33 . It was observed that while scaling up the OPV device area, the performance and device yield of the inverted OPV devices decrease significantly compared to standard configuration cells, which was demonstrated to be due to the clustering of BCP on ITO surfaces 33,34 . While BCP on small device areas works well as both EBL and ETL, the probability of BCP clusters penetrating the active layer (approx.…”