Passivation in perovskite solar cells: A review

Highlights

• We have reviewed the research progress of passivation in different interfaces of PSCs.

• Passivation effects on devices hysteresis and stability were discussed.

• We look into the prospects and challenges in the passivation of PSCs.

Abstract

Photovoltaic device based on inorganic–organic hybrid perovskite structured materials have been one of the brightest spotlights in the energy-conversion research field in recent years. However, due to their inherent properties and the architecture of the fabricated device, many defects trap states or carrier transport barriers are present at the interfaces between each functional layer and at the grain boundaries of the perovskite. These defects cause undesirable phenomena such as hysteresis and instability in the perovskite solar cells, which has slowed their commercialization. To address these issues, intensive research effort has been devoted recently to the development of passivation materials and approaches that can reduce the amount of interface and surface defect states in perovskite solar cells. Here, we have reviewed the state of the research progress in the development of passivation of different interfaces in the perovskite solar cell, including the interface (a) between transparent conductive oxide and electron transport material; (b) between the electron transport material and perovskite; (c) between the perovskite grains (grain boundaries); (d) between the perovskite and hole transport layer; (e) between the hole transport layer and electrode, and (f) between the electrode material and atmospheric environment. We also look into the prospects and challenges in the passivation of hybrid perovskite solar cells.

Introduction

In the five years since the first fabrication of the all solid-state perovskite solar cell (PSC) by Nam Gyu Park et al. in 2012 [1], the photoelectric conversion efficiencies (PCEs) of PSCs have experienced an explosive growth. Certified PCEs of 22.7% and 19.7% for small area PSCs with areas of 0.095 cm² and 1.0 cm², respectively [2], and of 12.1% for large area of 36.1 cm² [3] have been achieved. This remarkable achievement has resulted in PSCs being considered the most promising class of third generation photovoltaic devices to replace the currently widely used silicon solar cells.

In order to commercialize PSCs, three major barriers remain to be overcome [4], [5]: (a) The environmental toxicity caused by the use of Pb in PSCs, (b) their unsatisfactory stability against temperature, humidity and light exposure, and (c) the dependence of their differential J–V curves on the scan directions (i.e., from open circuit voltage to short circuit current or vice versa), which is the so-called “hysteresis” phenomenon [6].

The searching for Lead-free perovskite with high photovoltaic performance still has a long way to go. Theoretical investigation has implied that a promising perovskite absorber should exhibit high electronic dimensionality, a criterion that presently fulfilled only by Pb-based three dimensioned (3D) structured perovskite. Some reported double perovskites, such as the Ag- and Bi-based halide double perovskites are structurally 3D but electronically 0 dimensioned (0D), making it quite difficult to find promising candidates to replace the current Pb-based perovskite [7].

Fortunately, research into eliminating hysteresis and improving stability has resulted in many substantial achievements recently. PSCs with low or even no hysteresis [8], [9], [10], [11], [12], [13], and with outstanding stability towards temperature [14], humidity [15], [16], [17], [18], [19], and light exposure [20], [21], [22], [23], [24], [25] have been successfully fabricated.

Interface passivation is one of the most commonly used and efficient strategies to improve the photovoltaic performance of PSCs.

According to International Union of Pure and Applied Chemistry (IUPAC), passivation, in physical chemistry and engineering, refers to a material becoming “passive,” that is, less affected or corroded by the environment in which it will be used. Passivation involves the application of an outer layer of a shielding material as a micro-coating, created by chemical reaction with the base material [26]. The transition process from the “active state” to the “passive state” by the formation of a passivating film [27]. For perovskite solar cells, passivation generally refers to either chemical passivation, which reduces the defects trap states in order to optimize the charge transfer between various interfaces [9], [28], [29], [30], [31], or physical passivation, which isolates certain functional layers from the external environment to avoid degradation of the device.

Typical PSC devices contain six main interface, including (a) the interface between the transparent conductive oxide and electron transport layer (ETL); (b) the interface between the electron transport material and perovskite; (c) the interface between the perovskite grains (grain boundaries); (d) the interface between the perovskite and hole transport layer (HTL); (e) the interface between the hole transport layer and electrode, and (f) the interface between the electrode material and atmospheric environment [32].

In this review, we summarize the research advances of the past several years, and focus on interface passivation in perovskite solar cells, organized according to the interface classifications listed above. A brief prospective on the challenges and opportunities in passivation technology for enhancing the performance and stability of perovskites solar cells is also provided. It should be noted that in addition to perovskite solar cells, passivation strategies have also been applied to perovskite nanocrystals/quantum dots [33], [34], [35] and light emitting devices [36], [37], [38], [39]; however, these will not be discussed here.