Polycrystalline thin-film solar cells have reached a levelized cost of energy that is competitive with all other sources of electricity. The technology has significantly improved in recent years, with laboratory cell efficiencies for cadmium telluride (CdTe), perovskites, and copper indium gallium diselenide (CIGS) each exceeding 22 percent. Both CdTe and CIGS solar panels are now produced at the gigawatt scale. However, there are ongoing challenges, including the continued need to improve performance and stability while reducing cost. Advancing polycrystalline solar cell technology demands an in-depth understanding of efficiency, scaling, and degradation mechanisms, which requires sophisticated characterization methods. These methods will enable reseachers and manufacturers to improve future solar modules and systems. This work provides researchers with a concise overview of the status of thin-film solar cell technology and characterization. Chapters describe material systems and their properties and then provide an in-depth look at relevant characterization methods and the learning facilitated by each of these. Following an introductory chapter, the book provides systematic and thorough coverage of the following topics: trends to improve CdTe solar cell performance; Cu(In,Ga)Se2 and related materials; perovskite solar cells; photovoltaic device modelling; luminescence and thermal imaging of thin-film photovoltaic materials, devices, and modules; application of spatially resolved spectroscopy characterization techniques on Cu2ZnSnSe4 solar cells; time-resolved photoluminescence characterization of polycrystalline thin-film solar cells; fundamentals of electrical material and device spectroscopies applied to thin-film polycrystalline chalcogenide solar cells; nanometer-scale characterization of thin-film solar cells by atomic force microscopy-based electrical probes; scanning transmission electron microscopy characterization of solar cells; photoelectron spectroscopy methods in solar cell research; time-of-flight secondary-ion mass spectrometry and atom probe tomography; and solid-state nuclear magnetic resonance characterization for photovoltaic applications. The final chapter provides an overview and describes future prospects.
Inspec keywords: semiconductor thin films; photoelectron spectroscopy; scanning-transmission electron microscopy; copper compounds; NMR spectrometers; photoluminescence; solar cells
Other keywords: polycrystalline thin-film solar cells motivation; time-of-flight secondary-ion mass spectrometry; thermal imaging; Cu2ZnSnSe4 solar cells; photovoltaic device modeling; device spectroscopies; Luminescence; spatially resolved spectroscopy characterization techniques application; CdTe solar cell performance control; thin-film polycrystalline chalcogenide solar cells; Solid-state NMR characterization; electrical material fundamentals; Nanometer-scale characterization; atom probe tomography; PV applications; multiphysics approach; time-resolved photoluminescence characterization; photoelectron spectroscopy methods; perovskite solar cells; thin-film photovoltaic materials; Cu(In,Ga)Se2; Cu2ZnSnSe4; STEM characterization
Subjects: Physics literature and publications; Photoelectric conversion; solar cells and arrays; Thin film growth, structure, and epitaxy; General electrical engineering topics; Solar cells and arrays
Thin-film solar cell development may be conceptualized as improving processes to lower costs and improve performance, with characterization as a guide along the way. Yet, pushing the boundaries of characterization is as much at the core of thin-film solar cell development as material synthesis. As PV continues to mature, there is an endless drive to improve performance to compete. Hence, PV technology is striving for increasing levels of perfection, requiring a corresponding sophistication in characterization and understanding. Consequently, the topic of this book -advanced characterization-is timely and critical to develop the potential of thin-film solar technology.
Thin-film polycrystalline CdTe solar photovoltaic (PV) cells are the most successful thin-film PV technology in history and currently represent the largest single challenger to the mass-produced wafer silicon products that dominate the market. In 2016 combined sales of single- and multi -crystalline wafer silicon PV were estimated as -300 GW p , while those of CdTe were -3 GW p . Set against the ongoing expansion of silicon PV module manufacture and tumbling production costs, the challenge for thin-film CdTe is to maintain its competitiveness and market share. To this end, the manufacturers and the research community are faced with a continual challenge of decreasing the costs of producing thin-film modules and increasing their photon conversion efficiency (PCE).
"CIGS" is the common term used to describe a broad class of chalcopyrite materials consisting of a group I element cation, a group III cation, and a group V anion. The materials are alloys with compositions such as (Ag,Cu)(In,Ga)(Se,S) 2 -in other words, they are mixtures of two elements from groups I, III, and V and are a subset of a larger set of I, III, VI 2 compounds. We will use "CIGS" as short form for them. The materials are attractive for the semiconductor optical absorber layer in photovoltaics because they have very high optical absorption coefficients and span a wide range of energy gaps.
In the following sections of this chapter, we first discuss the structural and optoelectronic properties as well as the defect tolerance of halide perovskites for solar cell applications. We then compare the various common device architectures for PSCs and discuss several perovskite fabrication strategies, including solution deposition and vacuum processes, that are key to preparing high -quality perovskite thin films for high-performance PSCs. Many research and development (R&D) challenges exist that must be addressed to ready PSCs for practical applications. We review key issues on stability (moisture, thermal, light, and chemical compatibility), material toxicity (Pb and Pb-free PSCs), and scaling up (material, device architecture, and coating approach selection). A key focus area for further PSC development is characterization. Since perovskite behaves differently than conventional semiconductors, specific characterization protocols must be established to reliably evaluate the progress among different research groups. Stability characterization needs special attention given the complexity of perovskite in response to various external stress factors. Finally, we provide an outlook on the research trends in PSC development toward commercialization.
This chapter provides the essential features of advanced photovoltaic (PV) device modeling and specific examples for cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and silicon (Si)-based technologies. It is instructive to compare polycrystalline thin-film and crystalline devices.
In this chapter, we describe luminescence and thermal imaging in the context of thin-film PV devices. First, we discuss both traditional and developing experimental designs, including dark LIT (DLIT), illuminated LIT, PL using different illumination configurations, traditional EL performed under forward -bias current injection, and contactless EL induced with sub -cell illumination patterns. Next, we provide some brief theoretical background to highlight relationships between fundamental materials properties and the resulting luminescence and thermal images. Finally, we give examples of thin-film PV PL, EL, and LIT images to demonstrate the utility of these imaging methods ranging from understanding the microstructure of active layers to managing the quality and reliability of full PV modules.
Raman spectroscopy is named after C.V. Raman who observed the Raman effect for the first time in 1928. Today, Raman spectroscopy is widely used in various fields -from fundamental research to applied solutions. This spectroscopic technique takes advantage of light interaction with vibrational and rotational states of materials.
In this paper some photoluminescence (PL) approaches to characterize thin film solar cells with emphasis on time-resolved methods. Spectral PL analysis is complementary and was recently reviewed. Since TRPL is not very commonly used in PV characterization. consider interface and bulk recombination in the test structures and in devices . Briefly describe charge -carrier transport and recombination microscopy. Finally summarize and compare some CdTe, CdSeTe, CIGS, kesterite, and perovskite EO characteristics.
This chapter examines the fundamentals of characterization methods using AC modulated electrical excitation and measurement of the complex-valued response as applied to chalcogenide polycrystalline thin-film solar cells. Capacitance voltage (CV), drive-level capacitance profiling (DLCP), admittance spectroscopy (AS), deep-level transient spectroscopy (DLTS) and its variants, so-called transient photocurrent (TPI) and transient photocapacitance (TPC), and deep-level optical spectroscopy (DLOS) are the members of this class sometimes referred to as capacitance-based techniques. The primary desirable trait of capacitance measurements compared to others is their ability, in ideal cases, to accurately quantify charge and thus densities of states as functions of space and energy. However, other material and device features may produce similar signals; and only in very restricted cases (such as an epitaxial pn homojunction with two Ohmic contacts) are traps or defects in the depletion width the only possible origin of a defect-like signature. The applicability of assumptions of capacitance spectroscopies and interpretations developed for single-crystalline and epitaxial materials and devices are examined in the context of today's chalcogenide thin-film photovoltaic (PV) technologies, which present both material and band-diagram nonidealities in this regard.
KPFM and SSRM, which relate to the following characterization reviews, will be introduced briefly in Section 10.2. Some recent nm-scale characterizations of perovskite, CIGS, and CdTe thin-film solar cell materials and devices will be reviewed in Sections 10.3-10.5 and closing remarks will be given in Section 10.6.
In this chapter, the author will summarize some of the more recent results of atomic resolution transmission electron microscopy of defects and GBs in CdTe. Aberration-corrected STEM has developed into a powerful tool to characterize the local atomic and electronic structures of materials. Due to the incoherent nature of the imaging process, the atomic-resolution high-angle annular dark-field (HAADF) images can be used directly to determine the atomic configurations at GBs or dislocation cores. Moreover, recent developments in spectrometer technology now enable atomic-resolution chemical quantification and measurements of the local density of unoccupied states above the Fermi level. When combined with first principles modeling, electron-beam-induced current (EBIC) measurements, or device-level modeling, STEM characterization can provide powerful information about the role that GBs and individual defect complexes play on Voc, FF, and thus, the overall cell conversion efficiency.
Photoelectron spectroscopy (PES), also referred to as photoemission spectroscopy, is a direct experimental method for assessing the chemical and electronic properties of materials. The technique is becoming increasingly important in the research of photovoltaic (PV) devices -where, more specifically, X-ray photoelectron spectroscopy (XPS) is used primarily to measure the chemical properties such as composition and contamination of solar cell materials, whereas ultraviolet photoelectron spectroscopy (UPS) reveals key electronic properties such as work function and electronic energy -level positions. PES is a surface -sensitive technique ideally suited for the analysis of thin films and interfaces, either completed ones or during their formation process. Because the new generation of PV devices comprise a multitude of complex interfaces -each of which plays a critical role for performance and functionality-PES analysis of functional cell components has gained even more relevance.
Time-of -flight secondary-ion mass spectrometry (TOF-SIMS) and atom probe tomography (APT) have many similarities. Both detect the chemical makeup of a solid material at ppm or better sensitivity, while retaining the spatial location information from the signal.
The purpose of this chapter is to show the capabilities of ssNMR in the context of PV materials, to give enough background to understand research that uses the technique, and to help the reader evaluate whether ssNMR would be helpful to their research endeavors.
In the various chapters of this book, numerous characterization techniques are presented that can be applied to thin-film solar cells to determine (micro)structural, compositional, electrical, and optoelectronic properties. What has yet to be more fully elucidated is to what extent these characterization techniques can be combined, in a correlative way, to enhance the information gathered on materials and devices. Indeed, it is valuable to consider combining techniques to verify the relevance of the measured materials properties or to obtain them on different length scales-to compare surface with bulk properties, or to correlate structure and composition of materials with electrical and optoelectronic properties.