It Takes Two to Tango—Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis

Abstract

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO and SnO, the organic fullerene derivatives C, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

CdS as Electron Transport Layer for Low‐Hysteresis Perovskite Solar Cells

“…According to the literature, hysteresis occurs especially in planar n–i–p perovskite devices, as the contact area between TiO x and perovskite is reduced compared to mesoporous devices . Hence, TiO x with its low electron extraction properties is not well‐suited as ETL for planar devices.…”

Gradient Energy Alignment Engineering for Planar Perovskite Solar Cells with Efficiency Over 23%

“…[6] Recently, Kim et al investigated the effects of methylammonium chloride (MACl) on the performance of perovskite films and fabricated a device that achieved a certified PCE of 23.5%. [9] Among them, TiO 2 , [10] SnO 2 , [11] and [6]-phenyl-C 61 -butyric acid methyl ester (PCBM) [12] are the most commonly used ones.The lower efficiency of devices based on planar structures is closely related to insufficient charge extraction [13] and imperfect interfacial band alignment. Developing low-temperature-processed electron-transport layer (ETL) [8] for planar structure devices is of great importance.…”

“…The lower efficiency of devices based on planar structures is closely related to insufficient charge extraction [13] and imperfect interfacial band alignment. [14] The insufficient charge extraction between perovskite and ETL can result in charge accumulation, which negatively affects device performance.…”

A PCBM-Modified TiO2 Blocking Layer towards Efficient Perovskite Solar Cells

“…Xing et al [24] showed that there are high density hole-deficient traps located at the TiO 2 surface, which leads to an electron transfer potential barrier between CH 3 NH 3 PbI 3 and TiO 2 . This potential barrier is absent between the perovskite layer and the PCBM layer, and then the PCBM layer has a faster charge extraction compared to TiO 2 [24,25]. In order to ensure the function of PCBM, the PCBM layer should be avoided from the damage when the perovskite precursor is deposited.…”