Energy Harvesting and Storage

This chapter presents a detailed discussion of the evolution of c-Si solar cells and state-of-the-art Si solar cell technologies. The salient features of the high-efficiency c-Si photovoltaic structures, their characteristics, and efficiency enhancements are presented, including the PERC family, TOPCon, IBC, and HIT solar cells. The importance of passivation layers, strategies to obtain better passivation, carrier selectivity and low-cost fabrication techniques are also presented in general for the readers. The industrial status and prospects of c-Si solar cell technology are briefly elucidated. The fundamentals of thin film solar cells and sensitized solar cell technologies are expounded in the latter part. This chapter serves as a prelude to the following next three chapters in the book.

This chapter redefines silicon-based solar cells by introducing the concept of charge-carrier selective contacts. In this sense, heterojunction solar cells use crystalline silicon as a high-quality light absorber. Then, two complementary electron- and hole-transport-layers must select the photogenerated charge-carriers at their corresponding electrodes. The solar cell electrical characteristics can be explained considering the electrochemical-potential as the driving-force for electrons flow. It is shown that doped p–n junctions are not essential for effective photovoltaic energy conversion, though they are indeed a good and very mature solution. Alternatively, non-conventional materials can be used as selective contacts for silicon heterojunction solar cells with important technological advantages. Transition-metal-oxides have particularly excelled as the hole-selective-contacts of silicon heterojunction solar cells. Regarding the electron-selective-contact, titanium oxide is probably the most studied alternative. Full dopant-free silicon heterojunction solar cells with efficiencies above 20% have been already demonstrated. These devices can be fabricated at low-temperature by simple deposition techniques and using safe and abundant materials. On the other hand, the metallic electrode is also very important to design good charge-carrier selective contacts. It is shown that dipolar interface layers can apparently shift the work function of the electrode and reinforce its charge-carrier selectivity. The most novel approaches based on the use of spin-coated conjugated polyelectrolytes is explained by the end of this chapter.

Hybrid organic–inorganic perovskites are materials that recently gained huge scientific attention for their superior characteristics. High optical absorption, ambipolar conductivity, high carrier mobility, bandgap tunability, and easy processing at low cost are properties that make this material unique. The hybrid nature of perovskite material containing both the properties of organic and inorganic ions makes it superior to other direct bandgap semiconductors in the field of photovoltaics. The chapter offers information on the structural, optical, and electrical properties of organic–inorganic halide perovskite materials. It introduces other varieties of perovskites that are obtainable by incorporating different mixtures of cations and anions. The evolution of perovskite-based solar cells is explained in detail, including their fabrication process and working principles. Perovskite-based solar cell efficiency has rapidly progressed from 3.81% in 2009 to the current status where single-junction perovskite solar cell efficiency reached 25.2%. The chapter illustrates advanced perovskite solar cell structures like back contact cells and various tandem cells based on perovskites. It also discusses qualitatively different methods to fabricate large-area perovskite solar cells. Commercializing perovskite-based solar cells faces challenges such as their instability in ambient conditions and the toxicity of lead. Researches are in progress to tackle these drawbacks and those attempts are also presented. The chapter concludes with a discussion on the current status of perovskite-based solar cells.

It is shown, that conventional CH₃NH₃PbI₃ based perovskite solar cells, in the reported case with an initial efficiency of about 12%, show almost no degradation, when irradiated with 68 MeV protons for doses of up to 10¹² protons cm⁻². The 15% degradation, observed for a dose of 10¹³ protons cm⁻², mainly due to a decrease of the short circuit current, is to a large amount only caused by the glass substrate coloring. Other solar cell parameters, like the fill factor and the open circuit voltage are not at all degrading even after this high radiation dose exposure. All solar cell parameters have also been monitored continuously in-situ during the exposure of the devices to proton irradiation and the results have been compared to conventional silicon photodiode degradation, confirming the much better stability under irradiation of the perovskite solar cells. Furthermore two other research activities, regarding perovskite related research at Salerno University are highlighted: By the characterization of perovskite solar cells with a new technique, based on the measurement of the temperature dependence of the low-frequency electrical noise spectra, the electronic defect structure and the solar cell efficiency could be clearly correlated. In another research, utilizing the long-term reproducibility of the hysteretic behavior of conventional perovskite solar cells, the realization of a Resistor Random Access Memory (ReRAM) for data storage has been demonstrated and a possible underlying physical mechanism has been proposed.

Even though renewable energy sources are abundant, their intermittent nature demands inevitability of storage devices for the generated energy so that energy required can be supplied upon demand. Among rechargeable batteries lithium ion batteries (LIBs) have drawn extreme attention as promising devices for energy storage applications, owing to their advantageous aspects of high energy density, low self-discharge and long cycle life. In particular, high energy density LIBs are considered as ideal power sources for electric vehicles (EV) and hybrid electric vehicles (HEV). Silicon and transition metal oxides are widely used anode materials for Li-ion cells due to high theoretical capacity, environmental friendliness and suitable and safe voltage profile, they are endowed with. However, large volume expansion up to 300% on lithium insertion, poor electrical conductivity and formation of unstable solid electrolyte interface (SEI) layers are some of the serious drawbacks to tackle, when they are used as anode materials. This chapter is devoted for a detailed discussion on different strategies that can be effectively implemented to address the challenges associated with high—capacity anode materials and arrive at suitable solutions to identify ideal anode materials for developing high energy density Li-ion cells for applications in next generation energy storage systems.

Lithium-ion (Li-ion) cell technology is one of the most booming electrochemical energy storage technologies owing to its high volumetric and gravimetric energy densities, long cycle life, high efficiency and low self-discharge. The energy density of Li-ion cells depends to a greater extent on the type of cathode material used. A plethora of cathode materials have been developed and deployed in Li-ion cells viz. LiCoO₂, LiNi_0.8Co_0.15Al_0.05O₂, LiMn_0.33Ni_0.33Co_0.33O₂, LiFePO₄, LiMn₂O₄, etc. The requirements for Li-ion cells with improved energy density for various applications push researchers to develop newer cathode materials with superior specific capacities and high working voltage. This chapter discusses the state-of-the-art research activities related to high capacity cathode materials, including their structural and electrochemical features, challenges and strategies that have been adopted to improve their performance.

Among the various configurations available for lithium-ion cells, the pouch type has been grabbing attention because of its high energy density, design flexibility, low cost and lightweight. Such a pouch pack enables further reduction in the size and weight of portable electronic devices where they power and for the same reason the pouch cells are not only suitable for terrestrial applications but also attractive for space applications too. Lithium-ion pouch cells have been successfully used in many applications including space. However, this design has certain limitations. The poor rigidity of the pouch case makes them more susceptible to external mechanical damage and swelling under elevated temperature and overcharging. Therefore, the manufacturers are constantly striving to improve the performance of pouch cells. This chapter provides a brief overview about the different aspects of lithium ion pouch cells and the various strategies introduced in upgrading the performance of this thin design.

The electrolytes in batteries are one of the vital components that can decide the overall performance of any secondary storage device. The conventionally used liquid electrolytes possess many safety hazards while operating at extreme conditions of temperature and climate. Polymer based solid electrolytes are identified as one of the key candidates that can offer safety, good battery performance, flexible and compact battery design. This chapter gives an overview of the various polymer based electrolytes that are currently under research and development for use in LIBs. Prior to explaining the polymer based electrolytes, the general properties of electrolytes are outlined with some of the conventionally used liquid electrolyte system and their characteristics. Special emphasis is given to solid polymer electrolytes, Gel polymer electrolytes, the ion transport mechanism in these systems and their characteristics are highlighted with examples from literature.

Supercapacitors are emerging energy storage devices for future energy technology in respect of high power density and longer cycle life. Various parameters have been introduced for the analysis of the electrochemical performance of supercapacitor devices, such as specific capacitance, cyclic stability, and internal resistance of the device. Several materials have been developed for supercapacitor electrode applications. Carbon-based composite materials have attracted supercapacitor electrode applications due to their high surface area and electrochemical conductivity. Many researchers have developed carbon-based composite supercapacitors with excellent cyclic stability and high specific capacitance. Several attempts were made in the fabrication of electrode materials to enhance the specific capacitance. In this chapter, the focus is on the different types of supercapacitors such as electric double-layer capacitor EDLC, pseudo-capacitor, and hybrid capacitors, their analytical techniques, and potential nanostructured electrode materials such as carbon nanomaterials and carbon-based composite materials for high-performance supercapacitors.

The increasing trend of wearable smart electronics and health monitoring systems demands efficient and flexible energy storage devices for powering them. The flexible and wearable supercapacitors have recently gained much attention due to their inherent ability of fast charging, long cycle life, and wearer safety attributes. To make supercapacitors flexible, it is required to design the system with all flexible components. This can be achieved only by employing flexible substrates, thin-film electrodes, and polymer gel electrolytes. This chapter focuses on the various flexible substrates used in supercapacitors from one-dimensional to three-dimensional and different electrode deposition processes to make flexible and thin electrode layers. At the end of the chapter, a brief discussion on the assembling and packaging of the supercapacitor for various wearable applications is also given.