Cobalt Chloride Hexahydrate Suppresses Energy Loss in Perovskite Solar Cells

Original Article:
Cobalt Chloride Hexahydrate Assisted in Reducing Energy Loss in Perovskite Solar Cells with Record Open-Circuit Voltage of 1.20 V

Pengyang Wang, et al.

ACS Energy Lett. 2021, 6, 6, 2121-2128.

10.1021/acsenergylett.1c00443

SnO₂ thin films are one of the most efficient electron transport layers in perovskite solar cells (PSCs). However, SnO₂ thin films usually suffer from harmful defects and energy-level alignment mismatch, which limits the open-circuit voltage (V_OC). In this work, the authors introduced cobalt chloride hexahydrate (CoCl₂ ·6H₂O) into SnO₂ thin films to improve the problems of defects and band structure.

A New Passivation Method of Cobalt Chloride Hexahydrate

Cobalt Chloride Hexahydrate Suppresses Energy Loss in Perovskite Solar Cells

Although the SnO₂ electron transport layer (ETL) is widely used in PSCs, its high electron mobility leads to reduced charge accumulation at the interface between the ETL and the perovskite, resulting in hysteresis suppression.

On the one hand, the energy-level alignment between perovskite and ETL is crucial for device voltage deficits. According to previous reports, there are many hydroxyl groups on the surface of metal oxides, which lead to non-radiative recombination at the interface between perovskite and ETL.

On the other hand, defect management of SnO₂ thin films is also very important for improving device performance. It has been shown that Cl has a good passivation effect on perovskites and enhances V_OC and power conversion efficiency (PCE).

Based on the above considerations, this work aims to develop a new passivation method to fully exploit the potential of PSCs by introducing cobalt chloride hexahydrate into the SnO₂ precursor solution.

How does cobalt chloride hexahydrate work?

According to previous reports, there are hydroxyl groups on the surface of SnO₂, which can bind to metal sites. There are two main active sites on the surface of SnO₂ crystals, which can interact with water to form hydroxyl groups, and after UV-ozone treatment, the surface has a higher degree of hydroxylation. The Co²⁺ in CoCl₂ can interact with the hydroxyl groups on the surface of the SnO₂ film because the hydroxyl groups are basic and tend to dissociate into -OH.
Cobalt chloride hexahydrate can passivate the defects in the SnO₂ film and the bottom of the perovskite film, thereby inhibiting the interfacial charge recombination. Furthermore, although the addition of cobalt chloride hexahydrate did not change the crystal structure and crystallinity, the SnO₂-CoCl₂ ETL exhibited enhanced transmittance in the visible region and minimized optical loss.
The presence of cobalt chloride hexahydrate also tunes the energy-level alignment of the perovskite/ETL interface, which facilitates electron extraction.

Performance of Cobalt Chloride Hexahydrate Modified SnO₂ ETL

The SnO₂-CoCl₂ -based PSC exhibits a record-breaking voltage of 1.20 V (minimum energy loss of 0.34 V) and a champion PCE of 23.82%.
The SnO₂-CoCl₂ -based PSC showed enhanced stability, maintaining an initial efficiency of 83.5% after 200 h under continuous irradiation.
The SnO₂-CoCl₂ -based PSC retained 79.6% of their initial efficiency after 100 h at 60 °C in ambient air (relative humidity >50).

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