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

Highlights

• The solution-processed p-i-n perovskite solar cells with the NiO_x/PEDOT:PSS double hole transport layer show negligible hysteresis.

• The dipole moment size of the generated interfacial dipoles from the PEDOT:PSS to NiO_x is 2.977 Debye.

• The dipoles dispel accumulated iodine ion vacancies at HTL side which improves the charge collection and lowers hysteresis.

Abstract

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.

Introduction

With the rapid development of novel perovskite solar cells (PSCs), the characteristics of current density–voltage (J–V) hysteresis have garnered deep analysis [1,2]. The origin of hysteresis has been an aspect of debate, mainly concerning the charge carrier trapping/detrapping processes [3], ion migration affected by charge accumulation [4], the capacitive effects [5], ionic displacement [6], and ferroelectric effects [7]. It is generally believed that the halide ions and their vacancies critically affect the J–V curve [[8], [9], [10], [11], [12]]. These mobile ions respond slowly to the scanning bias, and that affects the built-in field and changes the energy landscape of the interface [13]. Accumulation of these mobile ions at the interface temporarily affect the charge transfer and collection in the device. Driven by the inner electric field, mobile ionic charge accumulates at the interface of electron and hole transport layer interface. As a result, the inner electric field is shielded by the ionic charge at the interface, and cannot drive the extraction of photogenerated charge without overpotential and leads to hysteresis [14].

The reduction or elimination of the hysteresis effect is necessary to achieve stable output and reliability of PSCs [27]. Earlier work attempted to reduce hysteresis through passivation of perovskite film and charge transport layer [[15], [16], [17]]. These methods tried to improve the absorber and the interface of the absorber/transporting material to reduce charge accumulation and enhance charge flow. The presence of defects provides a pathway for the migration of ions by reduced steric hindrance. Huang et al. showed that the trap density can be reduced through the deposition of fullerenes on perovskite films and achieved a nearly eliminated hysteresis [18]. Park and colleagues reported that the hysteresis may be controlled by placing Frenkel defects of iodide ions into the O-gap position or by doping with Kalium Iodidum [19]. While these methods are successful, they are cumbersome and require expensive materials.

At present, both PEDOT:PSS and NiO_x are widely used as hole transport layer (HTL) candidates for p-i-n structured perovskite solar cells due to their visible light transparency, easy processing, and suitable energy levels for hole transfer in PSCs [[20], [21], [22], [23], [24], [25]]. Earlier reported with PEDOT:PSS or NiO_x HTLs still have various hysteresis issues [[20], [21], [22], [23], [24], [25]]. This work reports on a facile method to reduce the hysteresis by using nickel oxide (NiO_x)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) double hole transport layer (DHTL) for PSCs. The work demonstrates that the solution processed p-i-n PSC with the NiO_x/PEDOT:PSS DHTL layers shows negligible hysteresis. Moreover, the underlying mechanism of the inorganic/organic DHTL reduced hysteresis was studied by First-principles calculations and operational characterization.