Hybrid organic-inorganic perovskites (HOIPs) have emerged as a promising photovoltaic material over past few years, as their unique properties arising from a combination of both organic and inorganic components make them ideal for a plethora of applications. The power conversion efficiency of HOIP-based solar cells skyrocketed from a mere 3% to >25% in just under a decade. This remarkable improvement is unprecedented in the field of photovoltaics. However, there is a major roadblock preventing further progress in the field. HOIPs are unstable under sunlight and humidity – the very conditions they would need to endure to realize their potential in real-world devices. Without increased stability, HOIP-based devices will remain as laboratory curiosity instead of a technological breakthrough. I am a part of D. Venkataraman’s ALIEN group (Advanced Laboratory for Iontronic, Electronic, and Nanomaterials), where we look into the fundamental science behind the instability of HOIPs to generate new knowledge and rationally design new compositions and interfaces with enhanced stability and superior device performance. You can read the full list of scientific publications here

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1. Ion Transport in HOIPs

 

Emerging evidence suggests that transport of ions through HOIPs may be a cause of both the instability of these materials and many of its unique properties. However, there is no consensus on which ions are moving, which conditions cause them to move, or how they move through the material. Resolution of this matter has been complicated by the fact that HOIPs are mixed conductors.

I am working on determining the nature of mobile species and what activates their movement. I mainly use electrochemical impedance spectroscopy (EIS) to separate movement of charged species in a material by time-scale, which makes it a great tool to study mixed conductors like HOIPs. We fabricate various compositions of HOIPs and modify the interfaces to understand their impact on the ionic movement. Our ultimate goal is to use this knowledge for improving the stability of HOIPs.

 

2. Composition Engineering of HOIPs

 

The stability and efficiency of a HOIP-based solar cell primarily depends on the composition of HOIP. It has been shown that the replacement of A-site methylammonium ion with a larger formamidinium ion not only improves the stability of HOIPs but also boosts the power conversion efficiency. The exact mechanism by which formamidinium improves the stability is still debatable however, it is associated with the large ionic radius, smaller dipole moment and multiple hydrogen bonding sites as opposed to the single amine group present on methylammonium.  We want to explore the flexibility of the 3D structure by incorporating large cations at the A-site with different shape, dipole and strength of hydrogen bonding. Eventually, I incorporate these large cation-based HOIPs in solar cells  to test their stability and performance.

 

3. Interface Engineering

 

There is a growing awareness about the crucial role of interfaces in perovskite solar cells (PSCs) which has lead to the designing of efficient contact layers, additives for interfacial modification and strategies for defect passivation in HOIPs. Yet, we lack in our fundamental knowledge about the interfacial processes occurring in PSCs. For instance, a complete understanding of the charge transport, collection and recombination still remain a challenge. Our experiments include modification of the interfacial layers, integration into devices and characterization through microscopic and standard electronic measurements. We believe that the understanding of interfacial characteristics in HOIP-based PSCs is key to enhance the efficiency of these devices.