The rapid evolution of highly efficient perovskite solar cells

Perovskite solar cells (PSCs) have attracted much attention because of their rapid rise to 22% efficiencies. Here, we review the rapid evolution of PSCs as they enter a new phase that could revolutionize the photovoltaic industry. In particular, we describe the properties that make perovskites so remarkable, and the current understanding of the PSC device physics, including the operation of state-of-the-art solar cells with efficiencies above 20%. The extraordinary progress of long-term stability is discussed and we provide an outlook on what the future of PSCs might soon bring the photovoltaic community. Some challenges remain in terms of reducing non-radiative recombination and increasing conductivity of the different device layers, and these will be discussed in depth in this review.

Energy Yield and Electricity Management of Thin-Film and Crystalline Silicon Solar Cells

Yannick Samuel Riesen

In the case of high photovoltaic (PV) penetration into the electricity grid, the energy produced by a PV system that is effectively used (useful energy) depends on the energy yield and on how this energy is managed to avoid detrimental effects occurring at high PV injection, e.g. during the midday peak. The overall goal of this thesis is to provide guidelines for maximizing the useful energy of a PV system by quantifying losses incurred during operation at both the solar cell device and the system levels. Solar cells are usually optimized for the standard test conditions (STC). However, the conditions are generally different during operation. This work assesses how solar cell materials and designs can be optimized to maximize the energy yield for specific operating condition. We mainly focus on thin-film silicon solar cells because of their challenging metastable behavior. The temperature dependence of the performance of thin-film amorphous silicon (a-Si:H) and microcrystalline silicon solar cells is thus measured for different deposition parameters and cell designs. We observe that, by tuning the intrinsic layer thickness of a-Si:H cells, the cells with the best (STC) efficiency do not necessarily provide the highest energy output. We also explain the presence of a maximum in the value of the fill factor as a function of temperature. The temperature dependence study is then extended to thin-film silicon multi-junction, crystalline silicon heterojunction (SHJ) and other crystalline silicon solar cells. For thin-film silicon solar cells, spectral effects and degradation or recovery effects due to the metastable character of a-Si:H (due to the Staebler-Wronski effect) significantly impact the energy yield. Based on indoor and outdoor degradation/recovery experiments, we show that it is challenging to describe this metastability with a diode model. However, such a model with a current loss term and an additional temperature dependence for the saturation current and ideality factors accurately reproduces the current-voltage characteristics of a-Si:H solar cells over a wide range of irradiance levels and operating temperatures. On the system level, we model a PV system with local storage to evaluate several strategies to reduce the detrimental midday injection peaks. The impact of such measures on the useful energy is also investigated. We develop a simple control algorithm that minimizes the losses due to a feed-in limit and maximizes self-consumption without the need of a production forecast. We show that heat storage using a boiler or a heat pump performs as well as battery storage. In general, a feed-in limit reduces significantly peak injection but only a relatively small storage capacity is needed to reduce losses (due to this limit). Changes in tilt and orientation of modules also reduce losses resulting from feed-in limits and shrink the winter/summer production ratio by more than a factor of two. We also develop a statistical method that estimates - from loads measured every 15 min - when different electrical appliances in a household are commonly used. This model indicates that about 8% of the total load could be shifted easily to the midday period, thereby reducing the midday injection peak. Finally, we combine device and system aspects to show that varying cell technology (e.g. with different temperature response) has a limited but not negligible impact on system output.

EPFL2016

Fundamentals of perovskite formation for photovoltaics

Amita Ummadisingu

Perovskite solar cells have become strong contenders in the arena of photovoltaics due to the stunning rise in their efficiency from 3 % to over 20 % in just seven years. In this time, numerous device architectures and thin film deposition methods have been explored. The sequential deposition and anti-solvent methods are among the most widely used for preparing perovskite solar cells. Optimizing these perovskite deposition processes to tune the perovskite film morphology plays a key role in the race for the highest efficiencies as homogeneity of the film correlates with superior photovoltaic performance. To date, the factors controlling perovskite film formation in various deposition methods and the precise mechanism are little understood. In light of this, the aim of the thesis is to unravel the salient aspects of perovskite film formation. I first study the formation of methylammonium lead iodide in sequential deposition using confocal laser scanning fluorescence microscopy (CLSM) and scanning electron microscopy (SEM). I discover that illumination during film formation is a major factor in the reaction as it greatly accelerates perovskite formation and tunes the final film morphology by controlling nucleation of lead iodide. The nucleation density increases logarithmically with the illumination intensity. I uncover the mechanism behind this effect and demonstrate that it is a quantum phenomenon and not merely a heating effect. I posit that absorbed light lowers the surface tension, thereby facilitating nucleation. Not only is this effect present in the main deposition methods â sequential deposition and the anti-solvent methods, but also in perovskites of more complex compositions that are employed in state-of-the-art solar cells. Second, I scrutinize the individual stages in the formation of methylammonium lead iodide perovskite in sequential deposition. SEM-cathodoluminescence imaging identifies the presence of mixed crystalline aggregates composed of methylammonium lead iodide perovskite and lead iodide in forming perovskite films. Using cross-sectional CLSM for the first time, I identify the directionality of perovskite formation. Kinetic monitoring and model fitting bring forward the Avrami models as the most suitable to quantify and represent the formation of perovskite under different conditions of temperature, film thickness and illumination. Finally, I identify a maturation effect in the perovskite film in solar cells stored in the dark and investigate the dynamic spontaneous coalescence of crystals in the film. During maturation, small crystals merge to form larger ones and this reduces the number of grain boundaries and the associated trap states, supressing non-radiative recombination. The photovoltaic performance of high efficiency solar cells thus increases and it is accompanied by a reduction in the hysteresis. Overall, apart from providing insights into the dynamic nature of deposited films, I present a fundamental understanding of the perovskite formation process in different deposition methods and for various compositions. Such a comprehensive picture aids in the achievement of high photovoltaic performance.

EPFL2018

Formation and Stabilization of Inorganic Halide Perovskites for Photovoltaics

Wei Guo, Ulf Anders Hagfeldt, Zaiwei Wang, Wanchun Xiang, Jiahuan Zhang

In the last decade, halide perovskites have emerged as new semiconductors that revolutionize the photovoltaic field. Among this family of materials, all-inorganic halide perovskites are becoming a research paradigm because of their outstanding photophysical properties combined with the potential of long-term stabilities against moisture and thermal stresses in practical applications. By forming high-quality inorganic halide perovskite thin films and stabilizing the three-dimensional perovskite phases, in the past few years, the state-of-the-art inorganic perovskite solar cells have achieved a power conversion efficiency of 20.37% and a promising long-term operation stability. In this review, based on the latest research progress of inorganic halide perovskites for photovoltaics, we summarized (1) the crystal structural, optical, and photophysical properties of materials; (2) the film formation methods; and (3) the stabilization strategies of inorganic perovskite phase, to provide useful guideline for fabricating devices with further enhanced performance and stability, and promoting commercial applications.