Establishing perovskite solar cells' efficiencies from first-principles simulations

Despite their relatively recent introduction, organic-inorganic lead halide perovskite solar cells have achieved efficiencies typical of cells based on mono-crystalline silicon. Perovskite solar cells still present several unresolved issues, such as their rapid degradation and the presence of toxic lead, which still prevent their viability on the market. Fortunately, perovskite crystalline structure allows for great compositional variety, for example a great effort is being made to replace lead with tin. Perovskite are therefore a very promising candidate for future developments in solar cells technologies. As a consequence of perovskite great compositional variety, the fundamental physical parameters of these materials take on different values. Most of these parameters can be predicted through firstprinciple calculations based on density functional theory. It is therefore, at least in principle, possible to theoretically determine the maximum efficiency achievable by a certain device. Consequently, the development of new solar cells can be guided by theoretical predictions. In this thesis we will determine the power conversion efficiency of simple models of perovskite solar cells with parameters calculated by first principles methods by our group of found present in literature. Specifically, we will consider a pin thin film solar cell made of Titanium dioxide, as electron transport material, Methylammonium Lead Iodide (MAPbI3) perovskite, as optical absorber, and Spiro-meOTAD, as hole transport material. We will simulate the cells with SCAPS, a one dimensional solar cell simulation program, and only consider radiative recombination. Finally, we will explore how the cell’s fundamental parameters affect efficiency. The perovskite fundamental parameters considered are band gap, affinity, carriers mobility, density of states, and dielectric constant. This thesis is composed of three chapters. In the first chapter we introduce the models and basic equations of solar cells simulations. In the second chapter we describe perovskites cells and explain the first principle methods used to determine its properties. In the third chapter we review our solar cell simulation and explore how parameters affect the cell’s efficiency.

Anche se introdotte da meno di dieci anni, le celle solari basate su perovskiti miste organiche inorganiche hanno raggiunto efficienze tipiche delle celle basati su silicio mono-cristallino. Le celle a perovskite presentano ancora diversi problemi irrisolti, come il loro rapido degrado e la presenza di piombo tossico, che tutt’ora impediscono l’accesso al mercato. Fortunatamente, la stessa struttura cristallina permette una grande varietà composizionale. Ne consegue che parametri fisici fondamentali (come le band gap o le mobilità elettroniche) di tali materiali assumono valori diversi. La gran parte di tali parametri può essere predetta tramite calcoli a principi-primi basati sulla teoria del funzionale della densità (DFT). Questo rende in principio possibile la determinazione teorica della massima efficienza raggiungibile da un certo dispositivo. Di conseguenza lo sviluppo di nuove celle solare può essere guidato da predizioni teoriche. In questa tesi determineremo l’efficienza di semplici modelli di celle solari a perovskite a partire da parametri per le loro componenti fondamentali calcolati a principi-primi dal nostro gruppo o presenti in letteratura. Simuleremo le celle con SCAPS, un programma di simulazione delle celle solari unidimensionale, e a tal fine considereremo solo la ricombinazione radiativa. Infine, esploreremo come i parametri fondamentali della cella influiscono sull’efficienza.