Behaviour of Perovskite Solar Cells Under Reverse Bias and Interface Engineering for Efficient and Stable Solar Cells

Abstract

Perovskites have drawn tremendous attention due to their excellent optoelectronic properties, low-cost and versatile solution-processibility. The power conversion efficiency of single-junction perovskite solar cells (PSCs) has reached 26.1%, showing promising prospects for commercialization. Herein, we mainly focus on the behaviour of PSCs under reverse bias and the interface engineering for efficient and stable PSCs. The results are summarized as follows:

(1) An in-situ temperature and current measurement technique was developed. Intriguingly, some hot spots were observed and then quickly disappeared in the reverse biased PSCs, along with the simultaneous increase in the current and local temperature. Also, the potential mechanism has been revealed and analysed. An abnormal bulge in the perovskite film was found at the hot spot. Accordingly, the appearance and disappearance of hot spots were perfectly explained by band bending and tunnelling current caused by ion accumulation. Additionally, statistical analysis suggested that sparkling hot spots were related to reverse voltage and efficiencies of PSCs.

(2) It is found that the device performance can be improved by manipulating the migration of iodine ions via reverse-biasing, for example, at -0.4 V for 3 min in dark. Characterizations suggest that reverse bias can increase the charge recombination resistance, improve carrier transport, and enhance built-in electric field. Iodine ions including iodine interstitials in perovskites are confirmed to migrate and accumulate at the tin dioxide (SnO2)/perovskite interface under reverse-biasing, which fill iodine vacancies at the interface and interact with SnO2. First-principles calculations suggest that the SnO2/perovskite interface with less iodine vacancies has a stronger interaction and higher charge transfer, leading to larger built-in electric field and improved charge transport. Iodine ions that may pass through the SnO2/perovskite interface are also confirmed to be able to interact with Sn4+ and passivate oxygen vacancies on the surface of SnO2. Consequently, an efficiency of 23.48% with the open-circuit voltage of 1.16 V is achieved for PSCs with reverse-biasing.

(3) Due to the limited interface contact and weak interfacial interaction, planar heterojunction PSCs have space for further improvement. Herein, a structural and chemical crosslinking interface is proposed and constructed by introducing an extra layer, which blends SnO2 nanoparticles with chloride salts. Since the incorporated materials can be dissolved during the fabrication of perovskite, the quality of perovskite films was improved, leading to larger grain size and reduced trap-state density. Also, more chloride ions at the SnO2/perovskite interface were observed and the interaction between Cl- and Sn4+ was confirmed. It results in more pronounced n-type SnO2 with better conductivity and deeper conduction bands, leading to preferable energy level alignment between SnO2 and perovskite. Consequently, the open-circuit voltage and fill factor of the devices increased, and target cells presented better stability, retaining 98% of initial efficiencies after more than 10000 h storage in dry air (5% relative humidity) and maintaining 85.50% of the initial efficiency after 1000 h of operation under light. This strategy enabled the achievement of 25.28% efficiency with a low bandgap (1.53 eV) perovskite composition, and it has been confirmed to be universal when other related materials are utilized.