Solar cell performance can be further optimized by adjusting the stoichiometry of Zn(O,S), and by tuning the electrical properties of Zn(O,S) through various in situ or post-annealing treatments. Zn(O,S) can be post-annealed in oxygen atmosphere or doped with nitrogen, by ammonium hydroxide or ammonia gas, during the ALD growth to reduce the carrier concentration, which can be critical for reducing interface recombination at the p-n junction. High carrier concentration buffer layers can be critical for reducing contact resistance with the ITO layer. Zn(O,S) can also be incorporated with aluminum by trimethylaluminum (TMA) doses to either increase or decrease the carrier concentration based on the stoichiometry of Zn(O,S).

The fundamental concept of the book is to explain how to make thin film solar cells from the abundant solar energy materials by low cost. The proper and optimized growth conditions are very essential while sandwiching thin films to make solar cell otherwise secondary phases play a role to undermine the working function of solar cells. The book illustrates growth and characterization of Cu2ZnSn(S1-xSex)4 thin film absorbers and their solar cells. The fabrication process of absorber layers by either vacuum or non-vacuum process is readily elaborated in the book, which helps for further development of cells. The characterization analyses such as XPS, XRD, SEM, AFM etc., lead to tailor the physical properties of the absorber layers to fit well for the solar cells. The role of secondary phases such as ZnS, Cu2-xS,SnS etc., which are determined by XPS, XRD or Raman, in the absorber layers is promptly discussed. The optical spectroscopy analysis, which finds band gap, optical constants of the films, is mentioned in the book. The electrical properties of the absorbers deal the influence of substrates, growth temperature, impurities, secondary phases etc. The low temperature I-V and C-V measurements of Cu2ZnSn(S1-xSex)4 thin film solar cells are clearly described. The solar cell parameters such as efficiency, fill factor, series resistance, parallel resistance provide handful information to understand the mechanism of physics of thin film solar cells in the book. The band structure, which supports to adjust interface states at the p-n junction of the solar cells is given. On the other hand the role of window layers with the solar cells is discussed. The simulation of theoretical efficiency of Cu2ZnSn(S1-xSex)4 thin film solar cells explains how much efficiency can be experimentally extracted from the cells. - One of the first books exploring how to conduct research on thin film solar cells, including reducing costs - Detailed instructions on conducting research

To sustain future civilization, the development of alternative clean-energy technologies to replace fossil fuels has become one of the most crucial and challenging problems of the last few decades. The thin film solar cell is one of the major photovoltaic technologies that is promising for renewable energy. The current commercial thin film PV technologies are based on Cu(In, Ga)Se2 and CdTe. Despite their success in reducing the module cost below $1/Wp, these absorber materials face limitations due to their use of scarce (In and Te) and toxic (Cd) elements. One promising candidate for an alternative absorber material is tin monosulfide (SnS). Composed of cheap, non-toxic and earth-abundant elemental constituents, SnS can potentially provide inexpensive PV modules to reach the global energy demand in TW levels.

"You, 0 Sun, are the eye of the world You are the soul of all embodied beings You are the source of all creatures You are the discipline of all engaged in work" - Translated from Mahabharata 3rd Century BC Today, energy is the lifeline and status symbol of "civilized" societies. All nations have therefore embarked upon Research and Development pro grams of varying magnitudes to explore and effectively utilize renewable sources of energy. Albeit a low-grade energy with large temporal and spatial variations, solar energy is abundant, cheap, clean, and renewable, and thus presents a very attractive alternative source. The direct conver sion of solar energy to electricity (photovoltaic effect) via devices called solar cells has already become an established frontier area of science and technology. Born out of necessity for remote area applications, the first commercially manufactured solar cells - single-crystal silicon and thin film CdS/Cu2S - were available well over 20 years ago. Indeed, all space vehicles today are powered by silicon solar cells. But large-scale terrestrial applications of solar cells still await major breakthroughs in terms of discovering new and radical concepts in solar cell device structures, utilizing relatively more abundant, cheap, and even exotic materials, and inventing simpler and less energy intensive fabrication processes. No doubt, this extraordinary challenge in R/D has led to a virtual explosion of activities in the field of photovoltaics in the last several years.

This book presents a comparison of solar cell materials, including both new materials based on organics, nanostructures and novel inorganics and developments in more traditional photovoltaic materials. It surveys the materials and materials trends in the field including third generation solar cells (multiple energy level cells, thermal approaches and the modification of the solar spectrum) with an eye firmly on low costs, energy efficiency and the use of abundant non-toxic materials.

Thin-film kesterites have been explored as promising absorbers in future photovoltaic devices due to their earth-abundant and non-toxic constituents, which do not impose any future production limitations. However, the current record conversion efficiency of polycrystalline kesterite devices is 12.6%--i.e., at least 2.4% short of the efficiency threshold needed to make this material competitive with chalcogenide-based thin film technologies. This shortage in conversion efficiency has been in part ascribed to the large extent of carrier recombination by defects at the grain boundaries and contact/absorber interfaces. In this work, methods nanoscale compositional and electrical characterization of grain boundaries and contact/absorber interfaces in kesterite solar cells have been developed, using a unique combination of advanced nano-characterization tools including Auger Nanoprobe Microscopy (NanoAuger), Kelvin Probe Force Microscopy (KPFM) and Cryogenic Focused Ion Beam (Cryo-FIB). NanoAuger and KPFM measurements on high-performance CZTSSe thin film PV devices revealed that the presence of SnOx at the grain boundaries is essential to the high VOC. This passivation layer needs to be formed by an air anneal process performed after the film deposition. In contrast to the oxide at the grain boundary, oxide layer on the top surfaces of the grains has been found to be (Sn,Zn),O. A new cross-sectioning method via grazing angle of incidence Cryo-FIB milling, has been developed where smooth cross-sections with at least 10x scale expansion have been prepared. These surfaces were characterized for CIGSe monitor films confirming the presence of MoSe2 interlayer acting as a proper hole contact on the back surface.

Beginning with an overview and historical background of Copper Zinc Tin Sulphide (CZTS) technology, subsequent chapters cover properties of CZTS thin films, different preparation methods of CZTS thin films, a comparative study of CZTS and CIGS solar cell, computational approach, and future applications of CZTS thin film solar modules to both ground-mount and rooftop installation. The semiconducting compound (CZTS) is made up earth-abundant, low-cost and non-toxic elements, which make it an ideal candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe solar cells which face material scarcity and toxicity issues. The device performance of CZTS-based thin film solar cells has been steadily improving over the past 20 years, and they have now reached near commercial efficiency levels (10%). These achievements prove that CZTS-based solar cells have the potential to be used for large-scale deployment of photovoltaics. With contributions from leading researchers from academia and industry, many of these authors have contributed to the improvement of its efficiency, and have rich experience in preparing a variety of semiconducting thin films for solar cells.

Tin(II) sulfide (SnS) is an earth-abundant, inexpensive, and non-toxic absorber material for thin film solar cells. SnS films are deposited by atomic layer deposition (ALD) through the reaction of a tin precursor, bis(N,N'-diisopropylacetamidinato)tin(II), and hydrogen sulfide. The SnS films demonstrate excellent surface morphology, crystal structure, phase purity, stoichiometry, elemental purity, and optical and electrical properties.

This book examines the electronic structure of earth-abundant and environmentally friendly materials for use as absorber layers within photovoltaic cells. The corroboration between high-quality photoemission measurements and density of states calculations yields valuable insights into why these materials have demonstrated poor device efficiencies in the vast literature cited. The book shows how the materials’ underlying electronic structures affect their properties, and how the band positions make them unsuitable for use with established solar cell technologies. After explaining these poor efficiencies, the book offers alternative window layer materials to improve the use of these absorbers. The power of photoemission and interpretation of the data in terms of factors generally overlooked in the literature, such as the materials’ oxidation and phase impurity, is demonstrated. Representing a unique reference guide, the book will be of considerable interest and value to members of the photoemission community engaged in solar cell research, and to a wider materials science audience as well.

In recent years, kesterite Cu2ZnSnS4 (CZTS) has become an interesting alternative to copper indium gallium (di)selenide (CIGS) due to its non-toxic and earth abundant constituents. A variety of methods is being used to fabricate kesterite thin films, such as coevaporation, sputtering, electrodeposition, spray pyrolysis and others. Most of them include an annealing step to stimulate elemental mixing and interdiffusion. Although conversion efficiencies of kesterite solar cells have increased among different research groups, the record value of 12.6% set by IBM in 2014 has not been broken yet. Therefore, experimental and theoretical studies are needed to predict the effect of the secondary phases and detrimental defects on the electronical properties of the CZTS based solar devices. The work presented here studies non-destructive techniques for in situ process control and monitoring. With the aim to detect phases and phase transitions to optimize crucial processing steps such as pre-annealing of metal precursors, high temperature annealing and vacuum deposition of Cu-Sn-Zn-S based thin films. The research consists of three parts in which Raman spectroscopy, X-ray diffraction (XRD) and reflectometry are used to explore this objective. In the first part Raman spectroscopy is investigated as an in situ monitoring technique during high temperature annealing of thin films. It investigates whether the occurrence of CZTS can be monitored when it is created from annealing a Mo/CTS/ZnS layered thin film. CuS, SnS, ZnS and CTS (Cu-Sn-S) films are prepared by physical vapor deposition. The Raman scattering intensity was compared to investigate whether their specific vibrational modes can be distinguished from each other at room temperature. Then, the CTS film is annealed between 50 and 550 °C in order to investigate whether CTS vibrational modes can be identified at elevated temperatures and to see which transitions take place within the thin film. Also, a CZTS reference film is annealed between 50 and 550 °C for reference purposes. The temperature dependence of the main CZTS modes is examined to investigate whether it can be used for in situ temperature control. Finally, a ZnS layer is deposited on the unannealed CTS film to obtain a Mo/CTS/ZnS layered film. This film is used to study the conversion of CTS/ZnS into CZTS at elevated temperatures. It was found that Raman spectroscopy can successfully be used to monitor formation of CZTS by identifying its main vibrational mode during the annealing process. The intensity of the CTS modes reduces at elevated temperatures. At 450 °C, the main CZTS mode at 338 cm-1 can be clearly identified. The second part also focuses on high temperature annealing. However, in this part the focus lies on annealing of the metal precursor films. It is explored whether specific alloys benefit or hinder the formation of secondary phases during formation of the CZTS absorber films. Also, to what extent this influences solar cell performance. In situ XRD was investigated for in situ monitoring of the pre-annealing process. Cu-poor metal precursor films are prepared by sputtering deposition. The precursors are annealed at 150 °C, 200 °C, 300 °C and 450 °C in a three zone tube furnace. The effect on the structural properties is analysed by XRD to study the formation mechanism of alloys. The precursor films are then sulfurized in a three zone tube furnace. The structural properties of the absorber are analysed and correlated with structures in the precursor. It is found that formation of SnS2 in the absorber is proportional to the remaining Sn in the pre-annealed precursor. Also, electron micrographs showed that pre-annealing temperature influences grain growth and surface precipitation of Sn-S and Zn-S. Pre-annealed absorbers at 450 °C did not exhibit these phases on the surface. Solar devices are fabricated from the absorber films and best performing devices were obtained from pre-annealed absorbers at 450 °C. They showed absence of Sn and SnS2 in, respectively, the precursor and absorber. It could be concluded that SnS2 phases are detrimental to device efficiency and that SnS2 XRD peak intensity follows an inverse proportionality with device efficiency. The third part explores reflectometry as a method to monitor a growing film during thermal evaporation in a physical vapor deposition (PVD) system. A set of six CZTS absorbers is examined by ex situ Raman spectroscopy and reflectometry to study the influence of secondary phases CuS and ZnS on reflection spectra. Composition strongly influences reflection spectra and CuS leaves a characteristic dip in the reflection spectrum at about 600 nm. An integration method was used to analyze this phenomenon quantitatively. Subsequently, a reflectometry setup is designed, developed and integrated in the PVD system. Four different CZTS co-evaporated and multi-layered films are deposited. Structural, morphological and vibrational properties are investigated. The reflection spectra are monitored during deposition and time-dependent reflection spectra are analyzed for characteristic aspects related to properties such as thickness, band gap and phase formation. CuS could not be detected in the films by the integration method due to the superposition of the CuS dip with developing interference fringes during film growth. However, in multilayered CTS/ZnS film it is found that the onset of ZnS deposition can be detected by increased reflection intensity due to reduced surface roughness. Additionally, the shifting onset of the interference fringes to lower photon energies can be used as a characteristic fingerprint during the deposition process. In conclusion, this work showed that Raman spectroscopy, XRD and reflectometry could be successfully implemented for in situ process control and monitoring of high temperature annealing and vacuum deposition of Cu-Sn-Zn-S based precursors and absorbers. The application of these in situ techniques can lead to the optimization of thin film material properties and solar cells. As such, this study has paved the way for further improvement of Cu-Sn-Zn-S based precursors and thin film absorbers. Innerhalb der letzten Jahre hat sich Kesterit Cu2ZnSnS4 (CZTS) aufgrund seiner ungiftigen Bestandteile und deren hoher Verfügbarkeit zu einer interessanten Alternative zu Kupfer Indium Gallium (di-)Selenid (CIGS) entwickelt. Zur Herstellung von Kesterit Dünnschichten wird eine Vielzahl von Methoden verwendet wie Ko-Verdampfung, Sputtern, Elektrodeposition, Spray Pyrolyse und andere. Die meisten davon beinhalten einen Temper-Schritt um die Durchmischung und Interdiffusion der Elemente zu stimulieren. Obwohl der Wirkungsgrad der Kersterit Solarzellen von verschiedenen Forschungsgruppen erhöht wurde, ist der Rekordwert von IBM von 12,6 % noch nicht gebrochen worden. Daher werden experimentelle und theoretische Studien benötigt, die den Einfluss von Fremdphasen und schädlichen Defekten auf die elektronischen Eigenschaften der CZTS Solarzellen vorhersagen. Die vorliegende Arbeit untersucht zerstörungsfreie Methoden für die in situ Prozesskontrolle und -überwachung. Dabei ist das Ziel, entscheidende Prozessschritte wie das Vortempern der Metall-Vorläufer sowie das Hochtemperatur-Tempern und die Vakuum-Abscheidung von Cu-Sn-Zn-S basierten Schichten zu optimieren. Die Untersuchung besteht aus drei Teilen, in denen Raman-Spektroskopie, Röntgendiffraktion (XRD) und Reflektometrie benutzt werden um dieses Ziel zu erreichen. Im ersten Teil wird die Ramanspektroskopie als in situ Methode zur Überwachung des Hochtemperatur-Temperns von Dünnschichten betrachtet. Es wird untersucht, ob das Entstehen von CZTS beim Tempern von gestapelten Mo/CTS/ZnS Dünnschichten beobachtet werden kann. CuS, SnS, ZnS und CTS (Cu-Sn-S) Schichten werden durch physikalische Gasabscheidung hergestellt. Die Intensität der Raman Streuung wurde vergleichen um zu untersuchen, ob die spezifischen Vibrations-Moden bei Raumtemperatur voneinander unterschieden werden können. Dann werden die CTS Schichten zwischen 50 °C und 550 °C getempert um zu untersuchen, ob die CTS Vibrations-Moden bei höheren Temperaturen identifiziert werden können und um festzustellen, welche Übergänge innerhalb der Schicht auftreten. Außerdem wurde eine CZTS Referenzschicht zwischen 50 °C und 550 °C für Referenzzwecke getempert worden. Die Temperaturabhängigkeit der CZTS Haupt-Moden werden betrachtet, um zu untersuche, ob sie für die in situ Temperaturüberwachung verwendet werden können. Abschließend wurde eine ZnS Schicht auf einem nicht getemperten CTS Film abgeschieden, um eine gestapelte Mo/CTS/ZnS Schicht zu erhalten. Diese Schicht wird verwendet, um die Umwandlung von CTS/ZnS zu CZTS bei erhöhten Temperaturen zu untersuchen. Es wurde festgestellt, dass Raman Spektroskopie erfolgreich verwendet werden kann, um die Bildung von CZTS zu überwachen, indem die Haupt-Vibrations-Moden während des Temperns identifiziert werden. Die Intensität der CTS Moden verringert sich bei höheren Temperaturen. Bei 450 °C kann die CZTS Hauptmode bei 338 cm-1 klar identifiziert werden. Der zweite Teil konzentriert sich ebenfalls auf das Hochtemperatur-Tempern. In diesem Teil liegt der Fokus allerdings auf dem Tempern der Metal-Vorläufer-Schichten. Es wird erforscht, ob bestimmte Legierungen die Entstehung von Fremdphasen während der Entstehung der CZTS Absorberschichten begünstigen oder hemmen und welchen Einfluss dies auf die Leistung der Solarzelle hat. In situ XRD wird verwendet, um die Prozesse des Vortemperns zu überwachen. Kupfer arme Metall-Vorläufer-Schichten werden durch Sputtern aufgetragen. Die Vorläufer werden bei 150 °C, 200 °C, 300 °C und 450 °C in einem Drei-Zonen-Röhren-Ofen getempert. Die Auswirkungen auf die strukturellen Eigenschaften werden mit XRD analysiert, um den Entstehungsmechanismus der Legierungen zu untersuchen. Die Vorläuferschichten werden dann in einem Drei-Zonen-Röhren-Ofen sulfurisiert. Die strukturellen Eigenschaften des Absorbers werden analysiert und mit der Struktur der Vorläufer korreliert. Es wurde festgestellt, dass die Entstehung von SnS2 im Absorber proportional zum verbleibenden Sn im vorgetemperten Vorläufer ist. Außerdem zeigen Bilder des Rasterelektronenmikroskops, dass die Temperatur des Vortemperns das Kornwachstum und das Abschieden von Sn-S und Zn-S an der Oberfläche beeinflusst. Bei 450 °C vorgetemperte Absorber weisen keine dieser Phasen an der Oberfläche auf. Solarzellen werden aus diesen Absorber-Schichten hergestellt und die besten Zellen entstanden aus den bei 450 °C vorgetemperten Absorbern. Bei diesen traten Sn und SnS2 weder im Vorläufer noch im Absorber auf. Es konnte geschlussfolgert werden, dass SnS2 Phasen schädlich für den Wirkungsgrad der Zellen sind und dass die Intensität der SnS2 XRD Peaks invers proportional zum Wirkungsgrad der Zellen ist. Der dritte Teil erforscht die Reflektometrie als Methode zur Überwachung des Schichtwachstums während des thermischen Verdampfens in einer Anlage zur physikalischen Gasabscheidung (PVD). Ein Satz aus sechs CZTS Absorbern wird mittels ex situ Raman-Spektroskopie und Reflektometrie vermessen, um den Einfluss der Fremdphasen CuS und ZnS auf die Reflexionsspektren zu untersuchen. Die Zusammensetzung beeinflusst die Reflexionsspektren stark und CuS hinterlässt eine charakteristische Senkung bei 600 nm im Reflexionsspektrum. Eine Integrationsmethode wurde verwendet um dieses Phänomen quantitativ zu analysieren. Anschließend wurde ein Reflektometrieaufbau entworfen, entwickelt und in die PVD-Anlage integriert. Vier verschiedene CZTS koverdampfte und Mehrschicht-Filme wurden abgeschieden. Strukturelle, morphologische und Vibrationseigenschaften werden untersucht. Die Reflexionsspektren werden während des Abscheidens aufgenommen und zeitabhängige Reflexionsspektren werden auf charakteristische Aspekte im Zusammenhang mit Eigenschaften wie Dicke, Bandlücke und Entstehung von Phasen untersucht. CuS konnte in den Schichten mit der Integrations-Methode wegen der Überlagerung der CuS Senkung mit dem entstehenden Interferenzmuster nicht detektiert werden. Allerdings wurde in gestapelten CTS/ZnS Schichten beobachtet werden, dass der Beginn der ZnS Abscheidung durch eine ansteigende Intensität der Reflektion aufgrund der verringerten Oberflächenrauigkeit detektiert werden kann. Zusätzlich kann die Verschiebung des Startpunkts der Interferenzen zu niedrigeren Photonenenergien als charakteristischer Fingerabdruck während des Abscheidungsprozesses verwendet werden. Zusammenfassend zeigt diese Arbeit, dass Raman-Spektroskopie, XRD und Reflektrometrie erfolgreich als in situ Prozesskontrolle und –überwachung bei Hochtemperatur-Tempern und Vakuum-Abscheidung von Cu-Sn-Zn-S basierten Vorläufern und Absorbern realisiert werden konnten. Die Anwendung dieser in situ Techniken kann zu einer Optimierung der Eigenschaften von Dünnschicht-Materialien und von Solarzellen führen. Als solche hat diese Untersuchung den Weg für weitere Verbesserung von Cu-Sn-Zn-S basierte Vorläufer und Dünnschicht-Absorber geebnet.