Investigation of the Selectivity of Carrier Transport Layers in Wide-Bandgap Perovskite Solar Cells

Excellent contact passivation and selectivity are prerequisites to realize the full potential of high-material-quality perovskite solar cells, first to maximize the internal voltage (or quasi-Fermi-level separation) iV within the absorber, then to translate this high internal voltage into a high external voltage V. Experimental quantification of contact passivation and selectivity is, thus, key to improving device performance. Here, open-circuit measurements of iV(oc) and V-oc, combined with surface photovoltage measurements, are used to systematically quantify the passivation-using iV(oc) as a metric-and the selectivity-defined as S-oc = V-oc/iV(oc)-of a range of common carrier transport layers to wide-bandgap (1.67 eV) perovskite absorbers. The resulting solar cells suffer from large voltage deficits, particularly when NiOx is used as the hole transport layer, even though it provides better passivation than its polymer-based counterparts (PTAA and PTAA/PFN). This indicates a poor selectivity of NiOx (S-oc < 0.81 for NiOx-based devices), whereas devices using polymer-based hole transport layers exhibit high selectivity (S-oc = 0.94-0.95). In agreement with recent reports, this low selectivity is attributed to the formation of an interlayer of non-perovskite material with high resistance to holes at the perovskite/NiOx interface. These measurements also imply that the selectivity of the C60-based electron transport layers is relatively good.

Automated summary

Excellent contact passivation and selectivity are crucial for high-quality perovskite solar cells. Experimental quantification of these parameters can help improve device performance. Devices using NiOx as the hole transport layer exhibit large voltage deficits and poor selectivity, while those using polymer-based transport layers show higher selectivity.