Reduced hysteresis in perovskite solar cells using metal oxide/organic hybrid hole transport layer with generated interfacial dipoles

Herein, we report a facile method to reduce the J–V hysteresis using nickel oxide(NiO_x)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) double hole transport layer (DHTL) fabricated p-i-n perovskite solar cells. The device using the NiO_x/PEDOT:PSS DHTL shows a reduced hysteresis with an average hysteresis index of 0.020. This is smaller than a single NiO_x device (0.142) or single PEDOT:PSS (0.093). First-principles calculations reveal that O and S in PEDOT:PSS couple with the Ni in NiO_x, and PSS forms hydrogen bonds with O on NiO_x. This coupling generates an interfacial dipole moment directed toward the PEDOT: PSS layer, evidenced by dark current-voltage plots. Electrochemical impedance spectroscopy (EIS) indicates that these dipoles lead to kinetically-favorable charge extraction and contribute to the suppressed hysteresis. Kelvin probe force microscopy indicates that the generated dipoles dispel the accumulated iodine ion vacancies at the hole transport layers side that enhanced charge collection and thereby decreased the hysteresis. Intensity modulated photocurrent spectroscopy demonstrates that the device with the NiO_x/PEDOT:PSS DHTL shows faster charge transport rate k_trans than that of device with single PEDOT:PSS or NiO_x, and further demonstrates that a more efficient outward charge flow in PSCs with the NiO_x/PEDOT:PSS DHTL reduces hysteresis.