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 NiO_x 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 NiO_x (S_oc < 0.81 for NiO_x-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/NiO_x interface. These measurements also imply that the selectivity of the C60-based electron transport layers is relatively good.