Table of contents

With the increasing demand for Internet of Things (IoT) with integrated wireless sensor networks (WSNs), sustainable power supply and management have become important issues to be addressed. Thermal energy in forms of waste heat or metabolic heat is a promising source for reliably supplying power to electronic devices; for instance, thermoelectric power generators are widely being researched as they are able to convert thermal energy into electricity. This paper specifically looks over the application of thermoelectricity as a sustainable power source for IoT including WSNs. Also, we discuss a few thermoelectric systems capable of operating electronic skin (e-skin) sensors despite their low output power from body heat. For a more accurate analysis on body heat harvesting, models of the human thermoregulatory system have been investigated. In addition, some clever designs of heat sinks that can be integrated with thermoelectric systems have also been introduced. For their power management, the integration with a DC–DC converter is addressed to boost its low output voltage to a more usable level.

Solid oxide fuel cells (SOFCs) are promising candidates for next-generation energy conversion devices, and much effort has been made to lower their operating temperature for wider applicability. Recently, atomic layer deposition (ALD), a novel variant of chemical vapor deposition, has demonstrated interesting research opportunities for SOFCs due to its unique features such as conformality and precise thickness/doping controllability. Individual components of SOFCs, namely the electrolyte, electrolyte–electrode interface, and electrode, can be effectively engineered by ALD nanostructures to yield higher performance and better stability. While the particulate or porous structures may benefit the electrode performance by maximizing the surface area, the dense film effectively blocks the chemical or physical shorting even at nanoscale thickness when applied to the electrolyte, which helps to increase the performance at low operating temperature. In this article, recent examples of the application of ALD-processed nanostructures to SOFCs are reviewed, and the quantitative relationship between ALD process, ALD nanostructure and the performance and stability of SOFCs is elucidated.

This paper provides an overview of the physical vapor technologies used to synthesize Cu₂ZnSn(S,Se)₄ thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. A final section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading.

Kesterite semiconductors, derived from the mineral Cu₂(Zn,Fe)SnS₄, adopt superstructures of the zincblende archetype. This family of semiconductors is chemically flexible with the possibility to tune the physical properties over a large range by modifying the chemical composition, while preserving the same structural backbone. In the simplest case, three metals (e.g. Cu, Zn and Sn) occupy the cation sublattice, which gives rise to a range of competing orderings (polymorphs) and the possibility for order–disorder transitions. The rich physics of the sulphide, selenide, and mixed-anion materials make them attractive for computer simulations in order to provide deeper insights and to direct experiments to the most promising material combinations and processing regimes. This topical review assesses the status of first-principles electronic structure calculations, optical modelling, and photovoltaic device simulations of kesterite semiconductors. Recent progress is discussed, and immediate challenges are outlined, in particular towards overcoming the voltage deficit in Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ solar cells.

An important challenge when attempting to identify the role of microstructure on the properties of doped energy materials is to distinguish the behaviour of each grain boundary. In this paper we describe our recent work using atomistic simulations to investigate the structure, composition and oxygen transport of gadolinium doped cerium dioxide tilt grain boundaries. We find that energy minimisation can be systematically employed to screen grain boundary structures and dopant segregation. When dopants are distributed equally across grains, molecular dynamics simulations reveal oxygen vacancies reside near dopants, resulting in higher oxygen diffusivity. Once the dopants accumulate at the grain boundaries these grain boundaries become saturated with oxygen vacancies. We see fast oxygen diffusion within the grain boundary plane, although the depletion layer, as shown via the electrostatic potential appears to block transport across the grain boundary. However, this is highly dependent on the grain boundary structure as we find striking differences of the electrostatic potential and the segregation behaviour between each of interface studied.

Organometallic lead-halide solar cells exhibited immense potential over the past years and reached the transition point from lab to industry-scale fabrication. However, bridging this gap and establishing perovskites as a viable competitor to conventional Si-based photovoltaics, hinges on the success of cost-effective upscaling process. The key factor impeding this transition is operational stability of solar cells under realistic photoconversion conditions. To this extent, reducing the dimensionality of cell constituents appears as a promising and very attractive approach to tackle this issue. The beneficial influence of such materials on device stability, which is explicitly tied to the engineered interface quality with underlying layers, comes as a result of complex interplay between energy alignment, strain-induced interactions and barrier-like properties of 2D components. The aim of this perspective is to briefly outline key challenges regarding the exploitation of 2D materials within the framework of perovskite photovoltaics, as well as to suggest further development directions.

Kesterite Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) semiconductor materials have been extensively studied over the past decade, however despite significant efforts, the open circuit voltage remains below 60% of the theoretical maximum. Understanding the optical and electrical properties is critical to explaining and solving the voltage deficit. This review aims to summarize the present knowledge of optical and electrical properties of kesterites and specifically focuses on experimental data of intrinsic defects, charge carrier density and transport, and minority carrier lifetime and related rate-limiting recombination mechanisms. It concludes with suggestions for further investigation of the electrical and optical properties of kesterite materials.

The formation of the solid electrolyte interphase during the formation and conditioning steps, is a very time consuming and expensive process. We present an active formation method in LiNi_1/3Mn_1/3Co_1/3O₂ (NMC-111) versus graphite lithium-ion batteries, which maintains the cycling performance of the cells. Ten different active formation protocols were evaluated, which consisted of cycling between an upper (V_u) and lower (V_l) voltages. The cells were evaluated using electrochemical impedance spectroscopy (EIS) and cycling. X-ray photoelectron spectroscopy was used to analyse the surface of the electrodes after cycling. Cycling performance and resistance measurements from the EIS results confirm the different effect of formation protocols in the lifetime and performance of the cells. We show that during the formation protocol the interface composition is optimised through the transport of lithium ions through the initial organic decomposition layer on the graphite at higher cell voltages (>3.65 V). These higher voltage cycling formation protocols giving an interface with greater stability and enhanced cycling are observed in the cells.

Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu₂ZnSn(S,Se)₄ matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se)₂ films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se)₂ can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd_1−xZn_xS or Zn_1−xSn_xO_y, result in higher efficiencies than devices with CdS buffers when sulfur-rich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells.

The effect of (NH₄)₂S and CS₂ chemical etches on surface chemistry and contacting in Sb₂Se₃ solar cells was investigated via a combination of x-ray photoemission spectroscopy (XPS) and photovoltaic device analysis. Thin film solar cells were produced in superstrate configuration with an absorber layer deposited by close space sublimation. Devices of up to 5.7% efficiency were compared via current–voltage measurements (J–V) and temperature-dependent current–voltage (J–V–T) analysis. XPS analysis demonstrated that both etching processes were successful in removing Sb₂O₃ contamination, while there was no decrease in free elemental selenium content by either etch, in contrast to prior work. Using J–V–T analysis the removal of Sb₂O₃ at the back surface in etched samples was found to improve contacting by reducing the potential barrier at the back contact from 0.43 eV to 0.26 eV and lowering the series resistance. However, J–V data showed that due to the decrease in shunt resistance and short-circuit current as a result of etching, the devices show a lower efficiency following both etches, despite a lowering of the series resistance. Further optimisation of the etching process yielded an improved efficiency of 6.6%. This work elucidates the role of surface treatments in Sb₂Se₃ devices and resolves inconsistencies in previously published works.