Doping thin films used for photovoltaic absorbers is both critical to maximize device voltage and challenging due to complex interactions between point defects in these materials. Such interactions can result in compensation of the intended dopant species, meaning that the active charge-carrier concentration is lower than the concentration incorporated dopants. Charge-carrier compensation is directly related to the open-circuit voltage (VOC) deficit, or magnitude of VOC relative to the theoretical limit. Understanding how the carrier concentration varies within thin-films is necessary to design material processing schedules to minimize this VOC deficit and produce more efficient devices. Unfortunately, measurements of the free carrier concentration are generally relevant at the device level and cannot resolve local differences. Resolving local doping differences in thin-films such as Cd(Se,Te), CZTS, and CIGSe requires techniques with micron or sub-micron spatial resolution due to the polycrystalline structure as well as intended and unintended composition variations in these materials. In this contribution, we show how simultaneous measurement of cathodoluminescence (CL) and electron-beam-induced current (EBIC) can be used to expose doping variations in Cd(Se,Te) thin-films. Simultaneous collection of these signals reveals unexpected differences in the electric field strength through the device thickness due to spatial variation in the carrier concentration.
KEYWORDS: Copper indium gallium selenide, Photovoltaics, Gallium, Transmission electron microscopy, Gallium arsenide, Solar energy, Scanning electron microscopy, Optoelectronics, Thin films
Over the last several decades, champion photovoltaic (PV) devices using CuInGaSe2 (CIGS) as the absorber material have been achieved using polycrystalline films exclusively. This has led to the assumption that polycrystalline CIGS generally outperform single-crystal CIGS in PV devices. However, recently, very high-quality epitaxial CIGS has been grown on GaAs substrates producing PV device efficiencies of 20.0%. These results have revived the debate on what effects grain boundaries have on PV device efficiencies. In this contribution, we compare the optoelectronic properties of polycrystalline CIGS films with those of high-efficiency epitaxial CIGS films. This comparison reveals that grain boundaries are associated with properties that negatively impact PV device efficiency. Additionally, we find that the grain interiors in polycrystalline films exhibit properties that are similar to the high-performance epitaxial films. Our results suggest that it may be possible to achieve higher device efficiencies with epitaxial CIGS than with polycrystalline films.
KEYWORDS: 3D modeling, Thin film solar cells, Monte Carlo methods, Electrons, Diffusion, Numerical simulations, MATLAB, Renewable energy, Electron holes, Thin films
Cathodoluminescence (CL) measurements can be applied to assess grain-boundary (GB) and grain-interior (GI) recombination in thin-film solar cell materials and made quantitative if we can develop CL models that account for material and measurement complexities. Recently, we developed a three-dimensional numerical CL model, based in MATLAB, that simulates the GI CL intensity as a function of four parameters: grain size, GI lifetime, and GB and surface recombination velocities. The model assumes that GB electrostatic potentials are screened by the high excesscarrier densities used in the CL measurement such that transport is governed by ambipolar diffusion. Here, we develop models to address directly GB potentials and their effects on these measurements. First, we transfer the MATLAB-based model to COMSOL software, and then introduce shallow donors to the GBs to produce potentials. We also develop a two-dimensional model in COMSOL to simulate CL GB contrast with GB potentials. Simulations indicate that GB potentials can increase or decrease CL intensities relative to the zero-potential case. However, the high electron-beam currents typically applied in CL measurements minimize the impact of GB potentials.
Alkali post-deposition treatments significantly improve the performance of CuInGaSe2 (CIGS) devices, but there is still room for improvement. Here, we investigate the effects of potassium fluoride alkali post-deposition treatment on the defect chemistry and recombination at grain boundaries and grain interiors using temperature- and injection-dependent cathodoluminescence (CL) spectrum imaging. We study CIGS thin films grown on alkali-free sapphire substrates to isolate the effects of alkali treatment from alkali metals that can diffuse from standard soda-lime glass substrates. We find that alkali treatment affects the energy and temperature dependence of the luminescence peaks, as well as the defect activation energies. CL spectrum images reveal that the luminescence transitions at grain boundaries have a distinct power dependence after alkali treatment and substantially different defect chemistry. This work shows that temperatureand injection-dependence CL spectrum images can provide unique insight into the defect chemistry and recombination behavior of CIGS thin films.
In this work, we report on the synthesis and characterization of Cu2ZnSnS4 (CZTS) thin films prepared by annealing of co-sputtered metal precursors in sulfur atmosphere. Radio-frequency magnetron sputtering was applied to deposit the metal layers from single metal targets on Mo-coated soda-lime glass substrates. The chemical composition of the precursors was controlled by varying the sputtering working power, resulting in films with various compositions. X-ray fluorescence was used to determine the elemental concentration of these metal films. The metal precursors were then converted into CZTS in a tube furnace using different sulfurization conditions to investigate the effect of the annealing process on the properties of the final CZTS films. Film structural characterization and phase identification results were supported by X-ray diffraction (XRD) and Raman spectroscopy. Surface and cross-sectional film morphology was carried out by scanning electron microscopy (SEM). For the sulfurized films, significant Sn loss was noticed. However, the loss of Sn was successfully controlled by depositing precursors with an excess of Sn. After optimizing the composition of the metal precursor, XRD and Raman scattering results revealed single-phase CZTS films without clear signs of secondary phases. SEM showed improved morphology in the form of dense structures and smooth surfaces for the films sulfurized at 600°C. Our first solar cell, based on a CZTS film originating from a precursor sulfurized at 550°C for 60 min, showed an open-circuit voltage of 471 mV, a short-circuit current density of 9.92 mA/cm-2, a fill factor of 36.9%, and an efficiency of 1.72%.
Hematite is a potential candidate for hydrogen production by photoelectrochemical (PEC)
decomposition of water due to its good bad gap and excellent chemical stability. However, its poor
conductivity limits its PEC performance. Titanium has been predicted to be a good choice of dopant for
improving the conductivity. Most of the Ti-doped hematite films are produced by solution based
method. However, such procedure may introduce impurities. RF sputtering is a clean vacuum deposition
technique, which is perfect for the synthesis of metal oxide. In this paper, we report our synthesis of Tidoped
hematite thin films by RF magnetron co-sputtering of iron oxide and titanium targets at various
conditions. Our work shows that the structure and morphology of iron oxide can be modified by
controlling the doping concentration of titanium. Moreover, we confirmed that the PEC performance of
Ti-doped iron oxide film is significantly better than the undoped one.
Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain
electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough
understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among
the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in
aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies
have indicated that the group IIIA delafossite family (CuMO2, M = Al, Ga, In) do not exhibit direct band gaps. Their
fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB
delafossite family (CuMO2, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, band gaps are
above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate
alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types
of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also,
the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed
that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also
discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of
the photo-electrons.
KEYWORDS: Silicon, Transmission electron microscopy, Glasses, Thin films, Crystals, Photovoltaics, Scanning electron microscopy, Solar cells, Silicon films, Thin film solar cells
The material-quality limiting factors of evaporated solid-phase crystallized (SPC) poly-Si thin films fabricated on
planar glass for photovoltaic applications are investigated by a study combining scanning electron microscopy and
transmission electron microscopy. The grains in the investigated thin films are found to be randomly oriented, with an
average grain size of ~2.1 μm. In general, the grains are found to have a high defect density, although some grains are
more defective than others. We also observe a high level of impurity incorporation, in particular, oxygen, into the film.
The optical activity of the Si films is dominated by deep band tail states. We conclude that the high intragrain defect
densities and the high impurity levels are two major limiting factors for obtaining high-quality evaporated SPC poly-Si
thin films for photovoltaics.
We combine first-principles density functional theory, material synthesis and characterization, and photoelectrochemical
(PEC) measurements to explore methods to effectively reduce the band gap of ZnO for the application of PEC water
splitting. We find that the band gap reduction of ZnO can be achieved by N and Cu incorporation into ZnO. We have
successfully synthesized ZnO:N thin films with various reduced band gaps by reactive RF magnetron sputtering. We
further demonstrate that heavy Cu-incorporation lead to both p-type doping and band gap significantly reduced ZnO thin
films. The p-type conductivity in our ZnO:Cu films is clearly revealed by Mott-Schottky plots. The band gap reduction
and photoresponse with visible light for N- and Cu-incorporated ZnO thin films are demonstrated.
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