Browsing by Author "Sites, James R., advisor"
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Item Open Access Analysis of impact of non-uniformities on thin-film solar cells and modules with 2-D simulations(Colorado State University. Libraries, 2010) Koishiyev, Galymzhan Temirkhanovich, author; Sites, James R., advisor; Lear, Kevin L., committee member; Leisure, Robert Glenn, 1938-, committee member; Harton, John L., committee memberClean and environmentally friendly photovoltaic (PV) technologies are now generally recognized as an alternative solution to many global-scale problems such as energy demand, pollution, and environment safety. The cost ($/kWh) is the primary challenge for all PV technologies. In that respect, thin-film polycrystalline PV technology (CdTe, Cu(In,Ga)Se2, etc), due to its fast production line, large area panels and low material usage, is one of the most promising low-cost technologies. Due to their granular structure, thin-film solar cells are inherently non-uniform. Also, inevitable fluctuations during the multistep deposition process of large area thin-film solar panels and specific manufacturing procedures such as scribing result in non-uniformities. Furthermore, non-uniformities can occur, become more severe, or increase in size during the solar-panel's life cycle due to various environmental conditions (i.e. temperature variation, shading, hail impact, etc). Non-uniformities generally reduce the overall efficiency of solar cells and modules, and their effects therefore need to be well understood. This thesis focuses on the analysis of the effect of non-uniformities on small size solar cells and modules with the help of numerical simulations. Even though the 2-D model developed here can analyze the effect of non-uniformities of any nature, only two specific types of microscopic non-uniformities were addressed here: shunts and weak-diodes. One type of macroscopic non-uniformity, partial shading, was also addressed. The circuit model developed here is a network of diodes, current-sources, and transparent-conductive-oxide (TCO) resistors. An analytic relation between the TCO-resistor, which is the primary model parameter, and TCO sheet resistance ρS, which is the corresponding physical parameter, was derived. Based on the model several useful general results regarding a uniform cell were deduced. In particular, a global parameter δ which determines the performance of a uniform solar cell depending on sheet resistance ρS, cell length L, and other basic parameters, was found. The expression for the lumped series resistance in terms of physical parameters was also derived. Primary power loss mechanisms in the uniform case and their dependence on ρS, L, and light generated current JL were determined. Similarly, power losses in a small-area solar cell with either a shunt or a weak-microdiode were identified and their dependence on ρS, JL, and location of the non-uniformity with respect to the current collecting contact was studied. The impact of multiple identical non-uniformities (shunts or weak-diodes) on the performance of a module was analyzed and estimates of efficiency loss were presented. It was found that the efficiency of the module strongly depends not only on the severity and fractional area of non-uniformities but also on their distribution pattern. A numerical parameter characterizing distribution pattern of non-uniformities was introduced. The most and least favorable distribution patterns of shunts and weak-diodes over the module area were determined. Experimentally, non-uniformities may be detected with the help of spatially resolved measurements such as electroluminescence (EL). The 2-D circuit model was also used to develop the general framework to extract useful information from experimental EL data. In particular, a protocol that can help distinguish a shunt from a weak-diode and estimate the severity of the non-uniformity based on the EL data was developed. Parts of these simulation results were verified with experimental EL data obtained by other authors. The thesis also discusses the effect of partial shading (a macroscopic non-uniformity) on the operation and safety of thin-film solar panels. A detailed analysis of the current-voltage characteristics of partially shaded module was performed. Conditions that result in a shaded cell experiencing high reverse voltage were shown. A mathematical formalism was developed to distinguish two extremes: when reverse-bias shunting or breakdown dominates. It was shown that in the shunt-dominated case in extreme situations the voltage across the shaded cell can be quite large (~ 20V). High voltage across the shaded cell results in both high power dissipation and elevated temperature. Depending on the light generated current, the temperature above ambient of the shaded cell can be as high as ~100-300°C, implying potential safety issues. The analysis covered all basic rectangular shade configurations.Item Open Access Design strategies for high-efficiency CdTe solar cells(Colorado State University. Libraries, 2017) Song, Tao, author; Sites, James R., advisor; Kanevce, Ana, committee member; Gelfand, Martin, committee member; Wu, Mingzhong, committee member; Sampath, W. S., committee memberWith continuous technology advances over the past years, CdTe solar cells have surged to be a leading contributor in thin-film photovoltaic (PV) field. While empirical material and device optimization has led to considerable progress, further device optimization requires accurate device models that are able to provide an in-depth understanding of CdTe device physics. Consequently, this thesis is intended to develop a comprehensive model system for high-efficiency CdTe devices through applying basic design principles of solar cells with numerical modeling and comparing results with experimental CdTe devices. Four key topics about high-efficiency CdTe cells are covered in this dissertation: (a) material optimization of CdTe absorber, (b) roles of emitter/absorber interface on carrier transport, (c) substrate choices for monocrystalline CdTe cells, and (d) back contact configurations for thin-film polycrystalline CdTe cells. Finally, comparisons between simulation and experiment are carried out to identify both beneficial and detrimental mechanisms for CdTe cell performance and to guide future cell optimization. The CdTe absorber is central to cell performance. Numerical simulation has shown the feasibility of high energy-conversion efficiency (open-circuit voltage VOC > 1000 mV, efficiency η > 25%), which requires both high carrier density (p >1016 cm-3) and long minority carrier lifetime (τn > 100 ns). As the minority carrier lifetime increases (τn > 10 ns), the carrier recombination at the back surface becomes a limitation for cell performance with absorber thickness < 3 µm. Hence, either a thicker absorber or an appropriate back-surface-field layer is a requisite for reducing the back-surface recombination. When integrating layers into devices, more careful design of interfaces is needed. One consideration is the emitter/absorber interface. It is shown that a positive conduction-band offset ΔEC ("spike") at the interface is beneficial to cell performance, since it can induce a large valence-band bending which suppresses the hole injection near the interface for the electron-hole recombination, but too large a spike is detrimental to photocurrent transport. In a heterojunction device with many defects at the emitter/absorber interface (high SIF), a thin and highly-doped emitter can induce strong absorber inversion and hence help maintain good cell performance. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. In terms of specific emitter materials, the calculations suggest that the (Mg,Zn)O alloy with 20% Mg, or a similar type-I heterojunction partner with moderate ΔEC (e.g., Cd(S,O) or (Cd,Mg)Te with appropriate oxygen or magnesium ratios) should yield higher voltages and would therefore be better candidates for the CdTe-cell emitter. The CdTe/substrate interface is also of great importance, particularly in the growth of epitaxial monocrystalline CdTe cells. Several substrate materials (CdTe, Si, GaAs, and InSb) have been discussed and all have challenges. These have generally been addressed through the addition of intermediate layers between the substrate and CdTe absorber. InSb is an attractive substrate choice for CdTe devices, because it has a close lattice match with CdTe, it has low resistivity, and it is easy to contact. However, the valence-band alignment between InSb and p-type CdTe, which can both impede hole current and enhance forward electron current, is not favorable. Three strategies to address the band-offset problem are investigated by numerical simulation: (a) heavy doping of the back part of the CdTe layer, (b) incorporation of an intermediate CdMgTe or CdZnTe layer, and (c) formation of an InSb tunnel junction. Each of these strategies is predicted to be helpful for higher cell performance, but a combination of them should be most effective. In addition, the CdTe/back contact interface plays a significant role in carrier transport for conventional polycrystalline thin-film CdTe devices. A significant back-contact barrier φb caused by metallic contact with low work function can block hole transport and enhance the forward current and thus result in a reduced VOC, particularly with fully-depleted CdTe devices. A buffer contact layer between CdTe absorber and metallic contact is strongly needed to mitigate this detrimental impact. The simulation has shown that a thin tellurium (Te) buffer as well as a highly doped p-type CdTe layer can assume such a role by reducing the downward valence-band bending caused by large φb and hence enhancing the extraction of the charge carriers. Finally, experimental CdTe cells are discussed in parallel with the simulation results to identify limiting mechanisms and give guidance for future efficiency improvement. For the monocrystalline CdTe cells made at NREL, it is found that the sputter damage causing large numbers of defect states near the Cd(S,O)/CdTe interface plays an important role in limiting cell performance, particularly for cells with low oxygen Cd(S,O) (with a "cliff" band offset). Other effects, such as the large series resistance and reflection, also reduce the cell performance. A lattice-matched material with less deposition damage and with a type-I interface is suggested to introduce less interfacial recombination in future emitter growth on epitaxial CdTe absorbers. For polycrystalline CdTe solar cells made at CSU, it is demonstrated that an MZO emitter forms a spike at the MZO/CdTe interface and a Te buffer layer mitigates large back-contact barrier φb. Both play very important roles in achieving good cell performance (VOC ~ 860 mV, η ~ 18.3%). The simulation has also shown that the electron reflector would be an effective approach to further increase VOC even with a relative low CdTe carrier concentration (~1014 cm-3).Item Open Access Distortions to current-voltage curves of CIGS cells with sputtered Zn(O,S) buffer layers(Colorado State University. Libraries, 2013) Song, Tao, author; Sites, James R., advisor; Wu, Mingzhong, committee member; Sampath, Walajabad, committee memberSputtered-deposited Zn(O,S) is an attractive alternative to CdS for Cu(In,Ga)Se2 (CIGS) thin-film solar cells' buffer layer. It has a higher band gap and thus allows greater blue photon collection to achieve higher photon current. The primary goal of the thesis is to investigate the effects of the secondary barrier at the buffer-absorber interface on the distortions to current-voltage (J-V) curves of sputtered-Zn(O,S)/CIGS solar cells. A straightforward photodiode model is employed in the numerical simulation to explain the physical mechanisms of the experimental J-V distortions including J-V crossover and red kink. It is shown that the secondary barrier is influenced by both the internal material properties, such as the conduction-band offset (CBO) and the doping density of Zn(O,S), and the external conditions, such as the light intensity and operating temperature. A key parameter for the sputter deposition of Zn(O,S) has been the oxygen fraction in the argon beam. It is found that the CBO varies with the oxygen fraction in the argon beam at a fixed temperature. With a greater CBO (∆EC > 0.3 eV), the resulting energy barrier limits the electron current flowing across the interface and thus leads to the J-V distortion. Two different ZnS targets, non-indium and indium-doped one, were used to deposit the Zn(O,S) buffer layer. At the same oxygen fraction in argon beam, a non-In-doped Zn(O,S) buffer with a smaller amount of doping forms a greater secondary barrier to limit the electron current due to the compensation of the Zn(O,S) buffer layer. In addition, the temperature-dependent J-V crossover can be explained by the temperature-dependent impact of the secondary barrier - at lower temperature in the dark, the maximum distortion-free barrier is reduced and results in a more serious current limitation, indicating a greater J-V crossover. It is also found that, under low-intensity illumination, there is a lower doping density of Zn(O,S) due to a smaller amount of photons with hν > Eg(Zn(O,S)) which can excite the buffer layer to release the trapped electrons from the deep-level defect state. The result is a greater secondary barrier to limit the electron current through the interface and shift the light J-V curve right towards the dark J-V curve at high bias (V > VOC) which reduces the J-V crossover. Finally, the quantitative comparison of J-V distortion between simulation and experiment is employed to examine the credibility of the secondary barrier theory.Item Open Access Effects of contact-based non-uniformities in cadmium sulfide/cadmium telluride thin-film solar cells(Colorado State University. Libraries, 2008) Davies, Alan R., author; Sites, James R., advisorTo strongly contribute to the near-term electricity supply, CdTe-based photovoltaic devices must continue to improve in performance under the constraint of simple and cost efficient fabrication methods. This dissertation focuses on characterization and modeling of devices with non-uniform performance induced by the cell contacts. Devices were obtained from a commercially viable pilot-scale fabrication system at Colorado State University. Current versus voltage (J-V), quantum efficiency (QE) and laser-beam-induced current (LBIC) were the main characterization techniques applied in this work. The p-type CdTe semiconductor has a large work-function and thus tends to form a Schottky barrier when the back-electrode is formed. A common strategy of mitigating the performance-limiting contact barrier is to prepare the CdTe surface with a chemical etch, and include Cu to reduce the effective barrier. Non-uniformity of the etch or Cu inclusion, or insufficient application of Cu can result in a non-uniform contact, with regions of high- and low-energy Schottky barriers participating in the cell performance. Barrier non-uniformities in devices with little or no Cu were identified with the LBIC measurement and a model for their influence was developed and tested using PSpice circuit modeling software. Because of their superstrate configuration, CdTe cells feature front contacts made from transparent-conducting oxides (TCOs). Fluorine-doped tin oxide (F:SnO2) is a common choice because of its availability and acceptable optical and electrical properties. When the n-CdS layer of the CdS/CdTe structure is thinned to encourage greater current generation, non-uniformities of the solar cell junction arise, as CdTe comes into sporadic contact with the TCO layer. Device simulations suggest that the SnO2/CdTe junction is weaker than CdS/CdTe because of a large conduction-band offset induced by the differing electron affinities in the heterojunction. LBIC was used to verify increasing non-uniformity in devices with thin CdS and whole-cell performance followed the trends predicted by simulations. An empirical relationship between CdS thickness and relative influence the weaker junction was developed. The practical limit of CdS thickness was determined to be about 120 nm for CSU devices.Item Open Access Electron-reflector strategy for CdTe thin-film solar cells(Colorado State University. Libraries, 2010) Hsiao, Kuo-Jui, author; Sites, James R., advisor; Sampath, W. S. (Walajabad S.), committee member; Gelfand, Martin Paul, committee member; Leisure, Robert Glenn, 1938-, committee memberThe CdTe thin-film solar cell has a large absorption coefficient and high theoretical efficiency. Moreover, large-area photovoltaic panels can be economically fabricated. These features potentially make the CdTe thin-film solar cell the leading alternative energy source. However, the record CdTe efficiency (16.5%) is much less than its theoretical maximum efficiency (29%), primarily because the open-circuit voltage (0.845 V) is well below what is expected for its band gap (1.5 eV). The incorporation of an electron reflector is a strategy to improve the open-circuit voltage of solar cells, and thus a strong possibility to improve the efficiency of CdTe thin-film solar cells. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface. Different methods to create an electron reflector are explained in the thesis: (1) expanded band gap, either an expanded-band-gap layer or a bulk-band-gap reduction, and (2) alteration to the band bending through a reversed back barrier or a heavily-doped back surface. Investigation shows that the expanded-band-gap layer is the most efficient and practical mechanism for an electron reflector, and the combination of any two mechanisms does not yield additional improvement. To have the optimal effect from the electron-reflector strategy, reasonable CdTe lifetime (1 ns or above) and full depletion of the CdTe layer are required to ensure high carrier collection. Furthermore, a good-quality reflector interface between the p-type CdTe layer and the electron-reflector layer is essential. Preliminary experimental evidence has shown that CdTe cells with a ZnTe back layer do have a slightly higher open-circuit voltage. An electron reflector should be particularly beneficial for thin (less than 2 μm) CdTe cells which have a fully-depleted CdTe absorber layer. Thin CdTe cells can also benefit from the optical reflection at the back surface. To investigate the possibility of still higher efficiency, both electron and optical reflection were numerically applied to the CdTe record-cell baseline model. However, there is little improvement for CdTe thicknesses greater than 2 μm. To have the optimal effect from combined electron and optical reflection, cells approximately one micron thick are required. Even without the improvement to the current quality of CdTe, cell efficiency above 19% should be achievable with a 0.2-eV electron reflector. Moreover, efficiency above 20% should be possible if one can also achieve large optical back reflection. At the same time, competitive CdTe cell performance at a thickness as thin as 0.4 um should be possible. This thesis gives a comprehensive numerical investigation of the electron-reflector strategy for CdTe thin-film solar cells.Item Open Access Imaging as characterization techniques for thin-film cadmium telluride photovoltaics(Colorado State University. Libraries, 2014) Zaunbrecher, Katherine, author; Sites, James R., advisor; Gelfand, Martin, committee member; Buchanan, Kristen, committee member; Sampath, W. S., committee memberThe goal of increasing the efficiency of solar cell devices is a universal one. Increased photovoltaic (PV) performance means an increase in competition with other energy technologies. One way to improve PV technologies is to develop rapid, accurate characterization tools for quality control. Imaging techniques developed over the past decade are beginning to fill that role. Electroluminescence (EL), photoluminescence (PL), and lock-in thermography are three types of imaging implemented in this study to provide a multifaceted approach to studying imaging as applied to thin-film CdTe solar cells. Images provide spatial information about cell operation, which in turn can be used to identify defects that limit performance. This study began with developing EL, PL, and dark lock-in thermography (DLIT) for CdTe. Once imaging data were acquired, luminescence and thermography signatures of non-uniformities that disrupt the generation and collection of carriers were identified and cataloged. Additional data acquisition and analysis were used to determine luminescence response to varying operating conditions. This includes acquiring spectral data, varying excitation conditions, and correlating luminescence to device performance. EL measurements show variations in a cell's local voltage, which include inhomogeneities in the transparent-conductive oxide (TCO) front contact, CdS window layer, and CdTe absorber layer. EL signatures include large gradients, local reduction of luminescence, and local increases in luminescence on the interior of the device as well as bright spots located on the cell edges. The voltage bias and spectral response were analyzed to determine the response of these non-uniformities and surrounding areas. PL images of CdTe have not shown the same level of detail and features compared to their EL counterparts. Many of the signatures arise from reflections and severe inhomogeneities, but the technique is limited by the external illumination source used to excite carriers. Measurements on unfinished CdS and CdTe films reveal changes in signal after post-deposition processing treatments. DLIT images contained heat signatures arising from defect-related current crowding. Forward- and reverse-bias measurements revealed hot spots related to shunt and weak-diode defects. Modeling and previous studies done on Cu(In,Ga)Se2 thin-film solar cells aided in identifying the physical causes of these thermographic and luminescence signatures. Imaging data were also coupled with other characterization techniques to provide a more comprehensive examination of nonuniform features and their origins and effects on device performance. These techniques included light-beam-induced-current (LBIC) measurements, which provide spatial quantum efficiency maps of the cell at varying resolutions, as well as time-resolved photoluminescence and spectral PL mapping. Local drops in quantum efficiency seen in LBIC typically corresponded with reductions in EL signal while minority-carrier lifetime values acquired by time-resolved PL measurements correlate with PL intensity.Item Open Access Luminescence measurements inform a strategy for unlocking the full potential of CdTe-based photovoltaics(Colorado State University. Libraries, 2023) Jundt, Pascal M., author; Sites, James R., advisor; Sampath, Walajabad S., committee member; Yost, Dylan C., committee member; Gelfand, Martin P., committee member; Kuciauskas, Darius, committee memberCadmium telluride (CdTe) photovoltaics are characterized by simplicity and speed of fabrication with low usage of materials, all of which translate into low cost. These significant advantages have earned CdTe the second-highest adoption rate of all photovoltaic technologies. However, conversion efficiencies, while functional, are significantly lower than the theoretical limit for this material. This discrepancy is almost entirely a discrepancy in voltage, and the so-called "voltage deficit" of CdTe has stubbornly persisted for decades. While many strategies are being pursued to attempt to reduce the voltage deficit, this issue is fundamentally one of excessive nonradiative recombination due to defects within the absorber material, as will be demonstrated in this dissertation. Recombination is evaluated primarily by luminescence measurements, and as such this class of measurements is particularly relevant to the challenges faced by CdTe research today. The rate of recombination is parameterized by the carrier lifetime, and time-resolved photoluminescence (TRPL) is the most common method of determining this parameter in CdTe. Historically, accurate determination of bulk lifetime was as simple as extracting the time constant of the slowest component of a TRPL decay. However, significant gains in material passivation and doping over the last few years have both decreased the relative influence of trap-assisted recombination and increased the influence of p-n junction fields on TRPL measurements. Consequently, when measurements are performed on complete cells, extracting the tail time constant from a TRPL decay no longer necessarily gives an accurate representation of the bulk material lifetime, and the result is distorted by field effect contributions. This fact is not necessarily well-known by the CdTe community, and extraction of the tail time constant is still the most common way to report lifetimes, even in measurements on complete state-of-the-art cells. This dissertation demonstrates the skewing effects of junction fields, and identifies under which conditions they manifest and how. To probe field effects, external electrical bias was incorporated during TRPL measurements, which allows fairly precise manipulation of fields. Biased TRPL measurements were performed on a variety of samples, and a model was developed to substantiate and better explain the results. It was found that the same characteristics which enable good performance (high lifetime, doping, and mobility) are the same which add complexity to TRPL interpretation. It was also found that field effects can be effectively suppressed by significant forward bias, leading to far more accurate determination of bulk lifetime. TRPL and external radiative efficiency (ERE) luminescence measurement results have indicated very low rates of nonradiative recombination and associated very high lifetime for some CdTe-based materials deposited at Colorado State University, particularly the cadmium selenium telluride (CdSeTe) alloy. While these attributes should allow voltages approaching 1 V and efficiencies on the order of 25%, when incorporated into "traditional" cell architectures these materials typically achieve middling performance at best, and often no performance at all. To unlock the great potential indicated by luminescence measurements, a different cell architecture is proposed which aims to accommodate these materials and take advantage of their characteristics. In an n-i-p configuration, an intrinsic absorber material is sandwiched between two carrier-selective contacts, at least one of which must be transparent. This design eliminates the requirement that the absorber be doped, which penalizes lifetime. Based on the findings of modeling reported here, undoped CdSeTe appears to be an ideal intrinsic layer material. The currently-utilized SnO2:F/MgZnO front contact appears to be excellent as the n-type electron-selective layer. The one missing component is the p-type hole-selective layer; modeling in this dissertation describes in detail what attributes are required of this material. Most important is band alignment with CdSeTe, which should produce a valence band offset as close to zero as possible, and a conduction band offset which forms a sufficiently high electron barrier. Sufficient p-type doping is also quite important. Based on these criteria, ZnTe was identified as a suitable candidate material, and several cells were fabricated with this architecture. While preliminary cells achieved relatively poor performance compared with traditional designs, J-V curves were surprisingly well-behaved, and the almost immediate achievement of functioning cells using an entirely new approach is promising. Luminescence characterization of these structures identified several areas for improvement, namely the use of a p-type dopant other than copper and the replacement of ZnTe with another material with similar band structure but more compatible lattice constant.Item Open Access Processing and characterization of thin cadmium telluride solar cells(Colorado State University. Libraries, 2017) Wojtowicz, Anna, author; Sites, James R., advisor; Sampath, W. S., committee member; de la Venta, José, committee memberCadmium telluride (CdTe) has the highest theoretical limit to conversion efficiency of single-junction photovoltaic (PV) technologies today. However, despite a maximum theoretical open-circuit voltage of 1.20 V, record devices have historically had voltages pinned around only 900 mV. Voltage losses due to high recombination rates remains to be the most complex hurdle to CdTe technology today, and the subject of on-going research in the physics PV group at Colorado State University. In this work, an ultrathin CdTe device architecture is proposed in an effort to reduce bulk recombination and boost voltages. By thinning the CdTe layer, a device's internal electric field extends fully towards the back contact. This quickly separates electrons-hole pairs throughout the bulk of the device and reduces overall recombination. Despite this advantage, very thin CdTe layers also present a unique set of optical and electrical challenges which result in performance losses not as prevalent in thicker devices. When fabricating CdTe solar cells, post-deposition treatments applied to the absorber layer are a critical step for achieving high efficiency devices. Exposure of the polycrystalline CdTe film to a chlorine species encourages the passivation of dangling bonds and larger grain formation, while copper-doping improves device uniformity and voltages. This work focuses on experiments conducted via close-space sublimation to optimize CdCl2 and CuCl treatments for thin CdTe solar cells. Sweeps of both exposure and anneal time were performed for both post-deposition treatments on CdTe devices with 1.0 μm absorber layers. The results demonstrate that thin CdTe devices require substantially less post-deposition processing than standard thicker devices as expected. Additionally, the effects of CdTe growth temperature on thin devices is briefly investigated. The results suggest that higher growth temperatures lead to both electrical and stoichiometric changes in CdTe closely associated with lower carrier lifetimes and poorer overall performance.