Faculty Advisor: Prashant Kamat

Revealing the Role of Iodine Expulsion in Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged as one of the exciting candidates in solar energy conversion devices. The progress made regarding the cell architecture to achieve power conversion efficiency compatible with Si photovoltaics has drawn the attention towards commercialization. However, in recent years, the increase in efficiency has only been incremental, mostly achieved by decreasing cell area. Increasing the cell area without significant losses and with improved stability remains a challenge. To overcome this challenge, it is crucial to understand the processes that prevent achieving the desired long-term stability.

In the literature it has been shown that under photoirradiation iodine gets expelled from perovskites to non-polar solvents (like dichloromethane, toluene) due to hole accumulation and subsequent oxidation of iodide ions. Recently, it has been shown that iodine may get expelled to the organic hole transport layer in devices as well. This project will study the effect of iodine expulsion into one of the most commonly used organic hole transport layers in perovskite solar cell devices, spiro-MeOTAD. Specifically, I will induce iodine expulsion in fully operating perovskite solar cells by light soaking, and I will periodically evaluate the photovoltaic performance and probe the absorption changes on the same devices.

Research Objectives

The primary goal of this project is to elucidate the effect of iodine expulsion on the photoconversion efficiency of perovskite solar cells.

(1) The project will begin with fabricating methylammonium lead iodide and spiro-MeOTAD films with thicknesses used in solar cell devices. Spiro-MeOTAD films will be prepared with and without the addition of dopants. The films will be characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) to establish the properties of the pristine films.

(2) Next, the effect of light soaking will be studied by physically pairing the perovskite and hole-transport layers. This approach will allow me to see the changes in each film individually with the same techniques proposed in the first step. This I will be able to determine the effect of expulsion on spectral features, crystallographic changes and morphology changes on both the perovskite and hole transport layer.

(3) After establishing the effects mentioned above, I will fabricate perovskite solar cell devices using MAPbI3 and spiro-MeOTAD as the absorber and hole transport layer, respectively. He devices will be designed in a way that will allow me to probe the UV-Vis absorption changes while monitoring the changes in the photovoltaic performance. Given that, this part of the project will start with evaluating the solar cell performance and UV-Vis absorption before any light soaking. Next, solar cells will be continuously photoirradiated in inert conditions to exclude reactions due to moisture. The photovoltaic performance and UV-Vis absorption will be periodically probed to study the kinetics of the process.

(4) Finally, I will study the reversibility of iodine expulsion. It has been shown in the literature that organic transport layers – depending on their HOMO levels – return to their original oxidation state over time. I will study the reversibility of this process in the case of spiro-MeOTAD via applying heat or vacuum. The effect of this reversibility on the solar cells performance will also be studied.