Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions

Abstract

Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH3NH3PbI3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand, solar cells with noncompact morphologies (open GBs, high trap density) are sensitive to the sign of the traps and hence to the cell preparation methods. Even in the presence of traps at GBs, trap-assisted recombination at interfaces (between the transport layers and the perovskite) is the dominant loss mechanism. We find a direct correlation between the density of traps, the density of mobile ionic defects, and the degree of hysteresis observed in the current–voltage (J–V) characteristics. The presence of defect states or mobile ions not only limits the device performance but also plays a role in the J–V hysteresis.

Origin of Open‐Circuit Voltage Enhancements in Planar Perovskite Solar Cells Induced by Addition of Bulky Organic Cations

“…As we discuss above, NMA is primarily located at grain surface. There is extensive evidence for trap location at grain surface in MAPbI 3 . We thus conclude that NMA is able to passivate grain surface trap states, resulting in the observed increase in device voltage with NMA concentration.…”

Vapor transport deposition of antimony selenide thin film solar cells with 7.6% efficiency

“…These trapped photo-generated carriers would most probably contribute to recombination loss. Furthermore, the energy levels of H2 and E1 are located near to the midgap, which significantly increase the recombination possibility of photo-generated carriers 46 . Therefore, the H2 and E1 are the dominant defects that influence the shift of quasi-Fermi levels and trap-assisted recombination, and then the V OC and J SC of the solar cells.…”

How Methylammonium Cations and Chlorine Dopants Heal Defects in Lead Iodide Perovskites

“…For the most stable arrangements ( Figure S13, Supporting Information) of the iodine vacancies in such electron-rich regions, a self-localization of the electronic excitation ensues upon geometric relaxation (Figure 2a) and a discrete trap level emerges below the conduction band edge (Figure 2b). Notably, deep states have been identified experimentally as electron traps, [21,32,36] Since the conduction band is predominantly composed of p orbitals of lead ions, the deep electron traps could possibly originate from the unsaturated lead atoms. The resulting localized electronic excitation in Figure 2a is thus prompted by the formation of a lead-centered defect with two excess holes.…”