Secondary phases are likely to occur in the Cu2ZnSnS4 (CZTS) films since the CZTS is thermodynamically stable in only a narrow region of the phase diagram. The CZTS solar cell performance can be influenced by the existence and precipitated position of secondary phases. Therefore, locally investigate the distribution of secondary phases is important to further improve CZTS solar cell efficiency. In this study, two different kinds of transmission electron microscopy imaging techniques, bright field scanning TEM image (BF-STEM) and High-angle annular dark-field (HAADF) image, are applied to analyze the distribution of secondary phases. Due to the atomic number differences between CZTS and secondary phases, secondary phases are evident in the HAADF images. Therefore, HAADF image is a more powerful and convenient method to analyze the secondary phases than the BF-STEM image.

In this study, we have investigated various approaches to improve CIGS solar cells after thin film deposition. CIGS devices have been fabricated by a hydrazine solution based process. Post-deposition treatments by sulfurization were studied with focuses on the change of material structures and physical properties. Sulfurization has shown to increase grain size and band gap of the absorber layers at higher temperatures. This property change has shown a direct impact on open circuit voltage of the solar cell devices. Through these post-deposition processes, improved quality of CIGS materials can be obtained and the associated solar cell devices show better performance.

We describe appropriate wafer cleaning procedure and surface passivation characteristics of various passivants used for making measurement of minority carrier lifetime (τB ) of very high quality Si wafers. These passivants include: iodine ethanol (I-E), quinhydrone methanol (QH-M), SiO2, and Al2O3. The issues related to the passivation stability and the spatial uniformity for mapping τB are also discussed.

We report a stable CdS/Sb2S3/SnSe heterojunction thin film solar cell deposited on SnO2:F (FTO) – coated glass substrates. Thermal evaporation at 10-5 Torr with substrate temperature of 400 °C was used to deposit Sb2S3 and SnSe thin films of 450 nm and 160 nm, respectively. Thin film Sb2S3 has an optical band gap (Eg) of 1.48 eV and photoconductivity (σp) of 4x10-7 Ω-1 cm-1 and thin film SnSe has an Eg of 1.28 eV and σp of 2 Ω-1 cm-1. The chemically deposited CdS thin film heated at 400 °C shows an Eg of 2.34 eV and σp of 0.1 Ω-1 cm-1. Stabilized solar cell structures with these thin films, FTO/CdS/Sb2S3/SnSe/C-Ag, showed open circuit voltage (Voc) of 0.60 V, short circuit current density (Jsc) of 5.51 mA/cm2 and power conversion efficiency (η) of 0.96% with a fill factor FF of 0.29. In the absence of the SnSe layer, Jsc decreases to 4.77 mA/cm2.

The highest efficiency CuIn1-xGaxSe2 (CIGS) based solar cells have been produced from films with x∼0.3 which gives a value of Eg around 1.1-1.2eV. Increasing the Ga content of the CIGS absorber provides an increase in Voc, allows tuning of the band gap that can enhance performance under actual operating conditions, and potentially makes it possible to use CIGS films in multi-junction devices. However, champion cells have not yet been produced for values of x significantly greater than 0.3. This work focuses on how increased Ga content in CIGS films affects the recombination behavior of grain boundaries. Cathodoluminescence spectral imaging (CLSI) measurements on fully processed devices allow us to compare device properties with recombination behavior and optical properties of grain boundaries in films with different Ga content. Our data suggests that grain boundaries in high efficiency films with x∼0.3 exhibit a significant red shift in the CL spectra whereas grain boundaries in films with higher Ga content typically show either a small shift or none at all. This shift indicates band bending near the boundaries which could enhance charge separation and subsequent collection of carriers generated near grain boundaries. This is investigated statistically to identify trends in different regions of the films.

Grain boundaries (GBs) in polycrystalline silicon (poly-Si) thin film solar cells are frequently found to be detrimental for device performance. Biaxiallytextured silicon with grains that are well-aligned in-plane and out-of-plane can possess fewer GB defects. In this work, we use TCAD Sentaurus device simulator and known experimental work to investigate and quantify the potential performance gains of biaxially-textured silicon. Simulation shows there can be performance gain from well-aligned grains when GB defects dominate carrier recombination or when grains are small. On the other hand, when intra-grain defects dominate recombination and grains are large, well-aligned grains do not lead to much performance gain. Another important result from our simulation is when intra-grain and GB defects are few, Jsc is almost independent of grain size while Voc drops with decreasing grain size.

III-V compound multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency III-V compound multi-junction solar cells, understanding and controlling imperfections (defects) are very important. This paper reviews fundamentals of defects and defect management for III-V compound materials, single-junction, multi-junction, space and concentrator solar cells.

Chemically deposited thin film stack of SnSe-ZnSe-Cu2-xSe was heated in nitrogen with Se vapor at 350-400 oC to produce Cu2ZnSnSe4 (CZTSe) thin films. For this, a thin film of SnSe with 180 nm thickness was deposited at 26 °C from a chemical bath containing tin(II) chloride, triethanolamine, sodium hydroxide, sodium selenosulfate, and a small quantity of polyvinylpyrrolidone. Thin films of ZnSe and Cu2-xSe were subsequently deposited on this SnSe film, also from chemical bath. The CZTSe thin film produced this way shows X-ray diffraction pattern matching that of Cu2ZnSnSe4 (kesterite/stannite) and have a Zn-rich composition. The film has an optical band gap of 0.9-1.0 eV and p-type electrical conductivity, 0.2-0.06 Ω-1 cm-1.

Thin films of AgSbS2 (150 nm) are prepared (75 min at 40 °C) via chemical deposition using a solution mixture containing SbCl3, Na2S2O3 and AgNO3. As-deposited films are amorphous. When they are heated in nitrogen at 180-320 °C, crystalline cubic-AgSbS2 films are formed. They show an optical band gap 1.89 eV and photoconductivity 1.8x10-5 Ω-1cm-1. Silver antimony sulfide-selenide film, AgSb(SxSe1-x)2, is produced from the initial amorphous film when it is heated in presence of Se-vapor. XRD analysis confirms the formation of solid solution AgSbS1.25Se0.75 or AgSbSe2 depending on the extent of Se-vapor available during heating. SnO2:F/CdS/AgSbS2/C solar cell shows Voc 610 mV, Jsc 0.88 mA/cm2,FF 0.53 and η 0.28%. In SnO2:F/CdS/Sb2S3/AgSb(SxSe1-x)2/C solar cell, Voc is 582 mV, Jsc 0.99 mA/cm2, FF 0.51 and η 0.29%.

Cu2ZnSnS4 (henceforth CZTS) absorber layers are successfully synthesized by a sulfurization technique of physical vapor deposited precursors. In our previous report, we have clarified that the off-stoichiometry composition of Cu-poor and Zn-rich is desirable to achieve high conversion efficiency. By using CZTS compound target that provide such active composition, we could conduct a simple single sputtering method to prepare CZTS absorber. In our laboratory, a two-stage process of precursor preparation followed by sulfurization is a major fabrication method from the start of this study. We think that this method is suitable for a mass production. An optimization of the sulfurization process is a quite important issue because the active composition was already revealed. In this paper, TG/DTA system available in the H2S atmosphere is introduced to optimize the sulfurization condition. As a result, bump-free CZTS films were prepared successfully and the fluctuation of J-V properties in one substrate was drastically suppressed.

Thin-film absorber layers for photovoltaics have attracted much attention for their potential for low cost per unit power generation, due both to reduced material consumption and to higher tolerance for defects such as grain boundaries. Cu2ZnGeSe4 (CZGSe) comprises one such material system which has a near-optimal direct band gap of 1.6 eV for absorption of the solar spectrum, and is made primarily from earth-abundant elements.

CZGSe metallic precursor films were sputtered from Cu, Zn, and Ge onto Mo-coated soda lime glass substrates. These were then selenized in a two-zone close-space sublimation furnace using elemental Se as the source, with temperatures in the range of 400 to 500 C, and at a variety of background pressures. Films approximately 1-1.5 µm thick were obtained with the expected stannite crystal structure.

Next, Cu2ZnSnSe4 (CZTSe), which has a direct band gap of 1.0 eV, was prepared in a similar manner and combined with CZGSe as either compositionally homogeneous or layered absorbers. The compositional uniformity of selenide absorbers made by selenizing compositionally homogeneous Cu-Zn-Ge-Sn precursor layers was determined and the band gap as a function of composition was investigated in order to demonstrate that the band gap is tuneable for a range of compositions. For layered Cu-Zn-Ge/Cu-Zn-Sn precursor films, the composition profile was measured before and after selenization to assess the stability of the layered structure, and its applicability for forming a band-gap-graded device for improved current collection.

An excellent candidate for an earth abundant absorber material is WSe2 which can be directly grown as a p-type semiconductor with a band gap near 1.4 eV. In this work we present the structural, optical, and electrical properties of thin film WSe2 grown via the selenization of sputter deposited tungsten films. We will show that highly textured films with an optical band gap in range of 1.4 eV, and absorption coefficients greater than 105/cm across the visible spectrum can be easily achieved. In addition we will present Hall Effect and carrier density measurements as well, where will show densities in the 1017cm-3 range and p-type Hall mobilities greater than 10 cm2/V-s range can be obtained. We employ these results to numerically simulate solar cells based on this material, where we will show efficiencies greater than 20% are possible.

Using first-principles full-field electromagnetic simulations, we demonstrate that near-perfect above-band-gap solar absorption can be achieved in nanostructured, ultra-thin-film iron oxide photoanodes for photoelectrochemical (PEC) water splitting. In our designed core-shell nanocone structures, all regions of hematite (α-iron oxide) are away from the interface between hematite and water by a minimum distance of less than the hole diffusion length in hematite, which is assumed to be no greater than 20nm. The optical absorption in our structure corresponds to a photocurrent density of 12.5mA/cm2 if one assumes an air mass 1.5 solar spectrum and a unity absorbed photon-to-current efficiency (APCE) for all wavelengths in that spectrum. Our photon management strategy eliminates the trade-off between optical absorption and carrier collection as commonly found in conventional designs of PEC cells, and variants of the strategy are generally applicable to other material systems.

The effect of interfacial phases on the electrical properties of Au/Ti/SiO2/InSb metal-insulator (oxide)-semiconductor (MIS or MOS) structures was investigated by capacitance-voltage (C-V) measurements. With increasing the deposition temperature of silicon oxide from 100 to 350°C using PECVD, the change in the interfacial phases between SiO2 and InSb were analyzed by resonant Raman spectroscopy to verify the relation between the breakdown of C-V characteristics and the change of interfacial phases. The shape of C-V characteristics was dramatically changed when the deposition temperature was above 300°C. The C-V measurements and Raman spectra represented that elemental Sb accumulation resulted from the chemical reaction of Sb oxide with InSb substrate was responsible for the failure in the C-V characteristics of MIS structure.

Fe2SiS4 and Fe2GeS4 crystalline materials posses direct bandgaps of ∼1.55 and ∼1.4 eV respectively and an absorption coefficient larger than 105 cm–1; their theoretical potential as solar photovoltaic absorbers has been demonstrated. However, no solar devices that employ either Fe2SiS4 or Fe2GeS4 have been reported to date. In the presented work, nanoprecursors to Fe2SiS4 and Fe2GeS4 have been fabricated and employed to build ultra-thin-film layers via spray coating and rod coating methods. Temperature-dependent X-Ray diffraction analyses of nanoprecursors coatings show an unprecedented low temperature for forming crystalline Fe2SiS4 and Fe2GeS4. Fabricating of ultra-thin-film photovoltaic devices utilizing Fe2SiS4 and Fe2GeS4 as solar absorber material is presented.

We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally resolved luminescence images of solar cell absorbers. The system is calibrated to yield the luminescence flux in absolute values. This system enables to quantitatively image physical parameters such as the photovoltage with an uncertainty of less than 30mV. The wide field illumination, low power excitation and fast acquisition brings new insights compared to classical setups such as confocal microscope. Several types of absorbers have been analyzed. For instance, we can investigate spatial fluctuations of the Quasi Fermi Levels splitting in CIGS polycristalline absorbers and link those fluctuations to transport properties. The method is general to the point that third generation PV cells absorbers can also be evaluated. We illustrate the great potential of our setup by imaging carrier temperature in Hot Carriers Solar cells absorbers and quasi Fermi levels splitting in Intermediate Band Solar cells.