Sakshi Sharma

(Advisor: Dr. Juan Pablo Correa-Baena)

will defend her master’s thesis entitled,

SURFACE PASSIVATION FOR ENHANCED STABILITY AND PERFORMANCE IN PEROVSKITE SOLAR CELLS

On

Wednesday, December 6th at 02:00 p.m.

MRDC 3515

Committee

• Prof. Juan Pablo Correa-Baena – School of Materials Science and Engineering (Advisor)
• Prof. Faisal Alamgir– School of Materials Science and Engineering
• Prof. Antonio Facchetti– School of Materials Science and Engineering 

Abstract

Lead halide perovskite solar cells (PSC) have emerged as promising next generation photovoltaics. Their unique ABX3 stoichiometry- where ‘A’ is a monovalent cation, ‘B’ is a divalent metal cation and ‘X’ is a halogen- provides tremendous potential for composition and bandgap engineering to obtain desired optoelectronic properties, enabling high power conversion efficiencies exceeding 25%. 

Despite their growing appeal, commercialization of PSC technology faces challenges due to device instabilities in ambient conditions. Particularly, device interfaces between the active perovskite layer and adjacent charge transport layers are vulnerable to defects which can accelerate perovskite degradation under environmental stressors such as heat, moisture, or oxygen, limiting their long-term viability. Interfaces also significantly impact charge transport, collection and recombination mechanisms in devices and thus require optimization. To address these challenges, research has concentrated on interface modification to passivate surface defects, protect the bulk of perovskite from external environment, and tune the charge transfer properties at the surface. 

Conjugated organic ammonium salts have been used at interfaces to introduce hydrophobicity on the perovskite film and promote charge delocalization brought on by conjugation. However, most surface treatment strategies relying on organic molecules introduce an electrically insulating spacer layer under thermal stress.  Heat induced diffusion of molecules can reconstruct the interface into lower dimensional phases, which impedes charge extraction and affects photo-conversion efficiency (PCE) of devices. This brings a tradeoff between the benefits of passivation and charge extraction.

For proper interface design, it is essential to study the thermal behavior of these passivation layers and establish their relationship with the optoelectronic properties of solar cells. This work explores the thermal behavior of passivation agents, specifically employing long-chain thiophene-functionalized π-conjugated molecules (2TI and 4TmI, with two and four thiophene rings, respectively) on interfacial structural stability and charge extraction. Tailoring the steric hindrance of the bulky cations used to treat perovskite surfaces presents an opportunity to control cation mobility, and consequently any phase changes resulting at elevated temperatures.

Structural studies reveal that the length of the cation backbone regulates the rate of interfacial perovskite structure reconstruction on prolonged heating. Consequently, faster phase conversion is observed in 2TI compared to larger 4TmI, with the formation of a n=1 A’PbI4 two- dimensional phase which consists of inorganic PbI6 octahedra monolayers separated by an organic spacer layer, A’ being either 2T or 4Tm. The oligothiophene tail in these molecules further contributes to spacer layer conductivity, prompting distinct charge extraction and recombination behaviors in 2TI versus 4TmI passivated devices, confirmed by synchrotron-based X-ray measurements. Results show that despite the observed phase changes, 2TI treated devices can tune the surface potential to promote efficient hole extraction to the overlying hole transport layer and reduce carrier recombination. This interfacial steric engineering translates to high performing passivated solar cells, with 2TI/CsFAPbI3 devices exhibiting efficiency exceeding 20%, an open-circuit voltage of 1.07 V and minimal changes under continuous thermal exposure.

By identifying the nature and impact of heat induced dynamical structural changes at passivated perovskite interfaces, this work highlights the key to surface functionalization so that solar cell performances can be maintained at high operating temperatures.