Charge selective contacts, mobile ions and anomalous hysteresis in organic–inorganic perovskite solar cells

Abstract

High-efficiency perovskite solar cells typically employ an organic–inorganic metal halide perovskite material as light absorber and charge transporter, sandwiched between a p-type electron-blocking organic hole-transporting layer and an n-type hole-blocking electron collection titania compact layer. Some device configurations also include a thin mesoporous layer of TiO2 or Al2O3 which is infiltrated and capped with the perovskite absorber. Herein, we demonstrate that it is possible to fabricate planar and mesoporous perovskite solar cells devoid of an electron selective hole-blocking titania compact layer, which momentarily exhibit power conversion efficiencies (PCEs) of over 13%. This performance is however not sustained and is related to the previously observed anomalous hysteresis in perovskite solar cells. The “compact layer-free” meso-superstructured perovskite devices yield a stabilised PCE of only 2.7% while the compact layer-free planar heterojunction devices display no measurable steady state power output when devoid of an electron selective contact. In contrast, devices including the titania compact layer exhibit stabilised efficiency close to that derived from the current voltage measurements. We propose that under forward bias the perovskite diode becomes polarised, providing a beneficial field, allowing accumulation of positive and negative space charge near the contacts, which enables more efficient charge extraction. This provides the required built-in potential and selective charge extraction at each contact to temporarily enable efficient operation of the perovskite solar cells even in the absence of charge selective n- and p-type contact layers. The polarisation of the material is consistent with long range migration and accumulation of ionic species within the perovskite to the regions near the contacts. When the external field is reduced under working conditions, the ions can slowly diffuse away from the contacts redistributing throughout the film, reducing the field asymmetry and the effectiveness of the operation of the solar cells. We note that in light of recent publications showing high efficiency in devices devoid of charge selective contacts, this work reaffirms the absolute necessity to measure and report the stabilised power output under load when characterizing perovskite solar cells

Perovskite Photovoltaics: Rare Functions of Organo Lead Halide in Solar Cells and Optoelectronic Devices

“…Recently, transient p/n doping of intrinsic perovskite semiconductor as a result of ionic migration has been proposed by Snaith et al as a most probable model to explain slow photocurrent response and hysteresis. 69 However, there was no direct solid evidence that supports these proposed mechanisms and the origin of hysteresis remains in need of in-depth study. We have found that external bias voltage applied to the CH 3 NH 3 PbI 3 cell before obtaining the JV curve can lead to transient hysteresis that is significantly enhanced in planar-type cell structures.…”

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…As a result, the effective charge-transfer process through these interfaces can be changed by the distribution of ions, which are driven by the external electric field. [189] Li et al [115] also mentioned the often-observed kinks illustrated in Figure 6 in the J-V curve measurement. Several groups have observed a similar behavior, [15,117,128,190,191] and this phenomenon especially prevails when using fast scan rates.…”

Advances in Perovskite Solar Cells

“…Some "ETL-free" cells exhibited very low stabilized power output even though decent PCEs were obtained from J-V measurements. [ 55 ] Therefore, the working mechanism for these cells needs further investigation. We note that ITO and FTO behave as ETL, thus the term "ETL-free" should be taken with a grain of salt.…”