Thin-film PV Technologies Research Articles

Cation and anion mutations in kesterites: A way to flexible band gap energy tuning

• Solar cells based on Kesterite-type absorber layers are the only critical raw material (CRM) free thin film PV technology. • Quaternary chalcogenide semiconductors, e.g. Cu 2 ZnGeS 4 /Se 4 or Cu 2 ZnSiSe 4 show band gap energies between 1.5 eV and 2.2 eV. • Flexible bandgap energy tuning in compound semiconductors is possible by cation and anion mutation (alloying). • Different alloying schemes (e. g. anion alloying or cation alloying) show various special features (e. g. miscibility gap).

Comparison of different simplistic prediction models for forecasting PV power output: Assessment with experimental measurements

This paper tested the energy outputs of different types of PV modules and evaluated the accuracies of different simplistic PV module power prediction models. A test rig was developed in Hong Kong to assess the PV power outputs of ten PV modules. The solar radiation, ambient temperature and power generation are recorded. The evaluated models include simple efficiency model, temperature correction model and one-diode model. The results show that the mono-Si PV module is the highest in terms of the annual energy outputs per unit area, and a-Si PV modules is the lowest. The simple efficiency model overestimates the power output of all types of PV modules for more than 10% except for a CdTe PV module. The one-diode model demonstrated the highest accuracy for mono-Si and poly-Si PV modules. The accuracies of the evaluated models are low for thin-film PV technologies. Although the mean bias error of the one-diode model is larger than 10% for the thin-film PV modules (expect for one CdTe PV module), the one-diode model has the highest accuracy among the three models. Further studies should be conducted to investigate the energy performance of thin-film PV modules and then improve their prediction accuracy.

A Detailed Analysis of Daily, Seasonal and Yearly Performance Values of Photovoltaic Modules Using by a Simplified Method

The performance of panels for PV system design is determined according to the performance values in standard test conditions specified in the panel catalog, without knowing the performance under actual operating conditions. However, the operating performance of the PV panel depends on meteorological characteristics of place where the PV system is installed. Especially, if outside temperature values are above test conditions, the efficiency of the PV panel decreases and generation losses are observed. In this study, performance parameters of photovoltaic panel were calculated for four different PV panel technologies only by using their catalogue values like NOCT temperature, power-temperature coefficient etc. For this purpose, real working conditions were simulated using 3-year climate data for the meteorological conditions of Sanliurfa, Turkey. In the end, PV panel efficiency, electricity generation values and performance ratios were calculated in accordance with the temperature. According to the results obtained, the PV panels’ performance ratios decreased up to 0.75 during the summer months. The highest unit energy generation was achieved with thin film PV technology.

Quantitative Analysis of the Separate Influences of Material Composition and Local Defects on theVocof PV Devices: An Exemplary Study on CIGS

A lot of effort is required in order to close the efficiency gap between laboratory record Cu(In,Ga)Se2 (CIGS) solar cells and commercially produced modules. An obstacle easily overlooked is the fact that reduced module performance may have multiple root causes. Here, we show how to isolate two main factors that influence the open circuit voltage ( V oc) of photovoltaic devices, the material composition and the local defects, here shunts in particular, i.e., regions of significantly lowered resistivity. In order to characterize the material composition, we used electroluminescence spectroscopy. Additionally, we investigated the effect of local defects on the V oc, by the use of thermography. Which of both factors dominates the resulting V oc depends on the insolation intensity under which the device is operated. We established a model in order to estimate the V oc as a function of the luminescence peak wavelength, the quantification of the thermal hot spots caused by local defects, and the insolation intensity. Subsequently, we investigated how these factors affect the electrical conversion efficiency of the device. The combination of these two measurement techniques results in a more conclusive evaluation of the electrical parameters, thereby enabling an improved quality assessment to allow a successive process optimization. This approach is also applicable to other thin film PV technologies such as perovskite and organic solar cells.

Electrospun ZnO nanofiber interlayers for enhanced performance of organic photovoltaic devices

Organic photovoltaics (OPV) has improved significantly in recent years and power conversion efficiencies are getting close to those of hybrid and inorganic thin film PV technologies. In this work, we demonstrate high performance inverted organic photovoltaic cells from electrospun nanofiber based electron transport layers, which outperform their thin-film counterpart. FTO/ZnOfilm/ZnOnanofibers/PTB7:PC70BM/MoO3/Ag devices were designed with different ZnO nanofiber based electron transporting layers (ETL) varying the nanofiber width and thickness, using a conventional electrospinning technique for the nanofiber ETL fabrication. We demonstrate that 35 nm thick films of electrospun ZnO nanofibers results in an improvement of the short-circuit current density (Jsc) and fill factor (FF) in the cells, leading to power conversion efficiency of up to 7.6%. The improvement is assigned to an improved charge collection efficiency, due to realization of direct and continuous pathway for electron extraction inside the solar cell structure. The results highlight the application of one-dimensional nanofiber based charge extraction layers as a promising route for development of high performance OPV devices in the future.

Materials, energy, water, and emissions nexus impacts on the future contribution of PV solar technologies to global energy scenarios

PV technologies are increasingly making significant contribution to global energy generation (GEG), attributed to their high potential of increasing efficiency, cost reduction, and improving energy security. These technologies however rely on metals that are identified as critical due to risks associated with their supply, and other materials that require energy and water for their production. In this paper, a comprehensive assessment of required materials for PV technologies, an analysis of their materials inflows, outflows, and stocks, an estimate of their maximum contribution to global energy scenarios (GES), and an estimate of energy and water required for their material production and associated CO2 emissions under the nexus approach, have been carried out using a dynamic material flow-stock model. A total of 100 energy-material nexus scenarios, which combines 10 GES and 10 materials scenarios, have been analysed. Results indicate that although most GES are difficult to be realized under current PV technologies market share and condition; these technologies could make significant contribution to GEG in future. The three commercial thin-film PV technologies could produce between 3% and 22% of electricity generation in IEA-450 scenario. Energy required for PV materials production is expected to reach between 5.9% and 11.8% of electricity generated (EG) by PV solar and between 0.76% and 1.52% of total EG in IEA-450 scenario by 2050. CO2 emissions associated with material production are expected to be between 0.94% and 2.2% of total CO2 emissions in IEA-450 scenario by 2050.

A study on the degradation rates and the linearity of the performance decline of various thin film PV technologies

In most available analyses of the annual degradation rate (RD, in %/year) of thin film PV (TFPV), light-induced degradation is not separated from long-term degradation and degradation linearity is virtually not explored.A study on the performance loss of various TFPV technologies in Jaén (Spain) is presented with a double goal: (a) providing more reliable values of RD by avoiding the approaches above and (b) assessing the suitability of some performance metrics and statistical techniques intended to estimate RD, depending on PV degradation linearity. Thus, monthly time series were constructed with PR DC, temperature-corrected PR DC and experimental peak power for two grid-connected TFPV technologies -a-Si and a-Si/µc-Si- and four TFPV modules -a-Si, a-Si/µc-Si, CdTe and CIGS- tested outdoors over a five-and-a-half-year and a six-and-a-half-year period, respectively. Classical seasonal decomposition (CSD) and Year-on-Year (YOY) statistical techniques were subsequently employed. The confidence intervals of RD found for the a-Si and a-Si/µc-Si PV fields overlap for CSD and YOY over the five-and-a-half-year measurement period and its sub-periods analysed, with RD ∼ 1.3%/year. This holds for the three performance metrics under scrutiny, a fact that is attributable to the closely linear degradation experienced by both PV fields. Mainly due to nonlinearities, CSD applied to any of the three performance metrics for the four TFPV modules fails to provide reliable values of RD. However, the values of RD obtained by applying YOY to such performance metrics for the aforementioned four TFPV modules over the five-and-a-half-year measurement period and its sub-periods studied are within the uncertainty range.

Modelling an optimised thin film solar cell

A phenomenological model has been developed to simulate the efficiency of thin film solar cells. The model uses key equations for p-n heterojunctions and includes radiative recombination, Auger recombination, Shockley–Read–Hall recombination and surface recombination losses. This framework is appropriate for final year undergraduate and Master’s students. Key solar cell phenomena are related to an equivalent circuit enabling the maximum conversion efficiency to be determined under standard AM1.5 solar illumination. The underlying physical basis of the model is presented together with algorithms to allow numerical simulation of a solar cell under the full range of operating conditions. The simulation accounts for optical losses within the device and uses a shunt resistance to account for recombination losses. Solar cells can be optimised for efficiency, or other operating characteristics, by adjusting layer thicknesses and doping levels. The model is used to investigate an emerging solar technology: thin film p-n heterojunction Cu2ZnSnS4/CdS solar cells. An optimised solar cell is found to have an overall PV conversion efficiency of (10 ± 1)%, however significant uncertainties on the values of some Cu2ZnSnS4 material properties mean that the trends predicted by the model are a more useful output from the simulation as these can be related to underlying physical phenomena in a solar cell. A region of maximum efficiency is found for absorber layer thicknesses of the order of microns. The range of CdS thicknesses for which this region is maximised is found when the n-type doping concentration of the CdS is maximised. An abrupt drop in efficiency is found when the CdS doping concentration is less than the doping in the Cu2ZnSnS4. Varying device and material parameters provides physical insights into the operation of solar cell devices. Strategies for managing optical losses and carrier losses can be tested leading to the identification of designs optimised for high efficiency thin film solar cells. The model can be used for other thin film PV technologies by inputting material properties.

Evaluation and comparison of crystalline silicon and thin-film photovoltaic solar cells technologies using data envelopment analysis

In the recent years, electricity production from renewable energy sources especially solar energy has progressed a great deal. Solar energy is a locally available renewable resource which is the most plentiful, unfailing and clean of all the renewable energy resources till date. A solar cell also called photovoltaic cell or PV is the technology used to convert energy from sunlight directly into electricity. As the PV market grows, it is becoming increasingly important to understand the energy performance of photovoltaic solar cells technologies. The purpose of this paper is to evaluate and compare the performance of crystalline silicon and thin film PV technologies with the DEA (data envelopment analysis) method in four states (considering payback period factor, total factors, technical and economic factors). The results indicate that in the three states where in “total factors”, “technical factors” and “economic factors” have been considered as performance factors, the mean of efficiency scores for crystalline silicon is higher than the mean of efficiency scores for thin-film. But in the state where in “payback period factor” has been considered as performance factor, the mean of efficiency score for Thin-film is higher than the mean of efficiency score for crystalline Silicon.

Current status and future prospects of thin-film PV technologies

Current status and future prospects of thin-film PV technologies

Design, modeling and performance monitoring of a photovoltaic–thermal (PVT) water collector

Over the years, many research efforts have been spent on the development of hybrid photovoltaic–thermal (PVT) technology, which is able to produce electricity and thermal energy at the same time with better overall performances of the two separated solar technologies. The aim of this work is the design of an innovative glazed PVT component with water as heat transfer fluid and the construction of a detailed mathematical model for the estimation of its electrical and thermal production. The paper presents the design of a covered PVT collector, made with thin film PV technology and a roll-bond flat plate absorber, and a simulation model, developed through the elaboration of several mathematical equations, to evaluate the performance of covered PVT water collectors. Finally, considerations about daily and annual yield of the proposed PVT collector, compared to a standard photovoltaic module, are discussed.

Rapid Directed Sintering of CdTe/CdS Thin Film Solar Cells Using Intense Pulsed Light

Traditional printing of low cost thin film materials for solar energy applications would transform the power generation landscape. Thin film PVs have been touted as a low cost alternative to the dominant crystalline silicon (c-Si) technology. Goodrich, et al. estimated the costs of c-Si modules in 2010 to be $2.25 per watt (/W) compared to $0.76/W for Cadmium Telluride (CdTe) thin film technology. In the past two years, the costs of c-Si modules have dropped well below $1/W due to surplus production and razor thin and often negative margins from manufacturers. These cost pressures have driven a number of thin film manufacturers from the market. Although the materials costs of thin film technologies are low, the high heat, high vacuum, and specialty gas techniques are energy and capital intensive processes that drive up costs. A big challenge for thin film PV technologies is to drastically reduce the manufacturing and capital expenditures as well as installed costs to aggressively compete with the falling costs of c-Si. Therefore, printing thin films at near-atmospheric conditions has a high likelihood of driving down production costs. This will require innovative low temperature techniques that can be adapted into current production schemes.In this work we have made nanoparticle inks of CdS and CdTe that are deposited using traditional printing techniques. The inks are then exposed to extremely short pulses (~ 1 ms) of high energy light, which is absorbed by the semiconducting nanoparticles and released almost instantaneously as heat. The temperature of the films can exceed several hundred degrees celcius for very brief moments precipitating changes in the morphology of the films. This Intense Pulsed Light (IPL) process can be accomplished over an area of nearly 60 cm2 and uses a very broad spectrum of light, making this process simpler and less expensive to scale than a laser source. The deposition and processing of the thin films is accomplished at atmospheric conditions reducing manufacturing, handling and substrate costs.The IPL process has been utilized quite extensively in the sintering of metals specifically in the area of printed electronics.[1] To date very little work has been done on the processing of semiconductors such as CdS and CdTe. In this work we have studied the IPL process on thin films of CdS and CdTe nanoparticle thin films. This includes an in-depth look at the behavior of the band gap, crystal structure, grain size and defects during the process. Specific analysis included UV-Vis, SRD, SEM, TEM and PL and we also incorporated a finite element model to plot the history of the process.We observed a stark change in the band gap of CdS with increased power per pulse and determined that lower total power was required to affect a significant change in the crystallinity of the films at higher pulse powers (Fig 1).[2] We also observed the growth of CdTe grains during IPL processing that resulted in a pin-hole free film, which also exhibited fewer point defects (Fig 2).[3] Finally a CdTe/CdS solar cell was manufactured and measured. Jha, M.; Dharmadasa, R., Amos, D.; Druffel, T., Low Temperature Production of Conductive Copper Using Intense Pulsed Light Sintering, ACS Applied Materials and Interfaces, 2014, 5(24): p. 13227-13234.Dharmadasa, R.; Druffel, T., Intense Pulsed Light Sintering of CdS Thin Films, Advanced Engineering Materials, 2014, DOI: 10.1002/adem.201400008.Dharmadasa, R.; Lavery, B. W.; Dharmadasa, I. M.; Druffel, T., Light Induced Sintering of Cadmium Telluride Nanoparticle based Thin Films, ACS: Applied Materials and Interfaces, 2014, DOI:10.1021/am500124t.

Uncertainty in PV Module Measurement---Part II: Verification of Rated Power and Stability Problems

Standard testing condition power of PV modules, especially thin-film modules, is not a constant value. Exposure to irradiance and temperature, as well as electrical bias and dark storage, cause changes in STC power, which complicate the interpretation of quality assurance measurements and tests. This paper presents a literature review and summarizes current knowledge of metastability and stability problems of PV modules with focus on their influence on quality assurance, especially the verification of rated module power. Three groups of thin-film PV technologies are addressed: amorphous silicon, cadmium telluride (CdTe), and chalcopyrite technologies (CIGS). Amorphous silicon is affected mostly by light-induced degradation and seasonal annealing, whereas sensitivity to dark storage is the most relevant effect for CdTe and CIGS PV modules. In conclusion, it is not possible to quantify the impact of specific (meta)stability effects on measured STC power for all modules of one technology in general, as there is a strong sensitivity to the exact composure and processing of the modules. With regard to the total uncertainty inherent in the verification of rated power of new modules, this paper introduces the concept of differentiating between measurement-related and module-stability-related uncertainty and suggests a procedure for its determination.

Cu(Sb,Bi)(S,Se)2 as Indium-free Absorber Material with High Optical Efficiency

Ternary Cu(Sb,Bi)(S,Se)2 compounds such as CuSbS2, CuSbSe2, CuBiS2, and CuBiSe2, are potential photovoltaic materials that contain earth abundant and low-cost elements. Employing an atomistic first-principles modeling within the density function theory we calculate the electronic and optical properties of Cu(Sb,Bi)(S,Se)2 compounds. We find that these four materials have indirect fundamental band gaps in the range of 1.1–1.7eV. The indirect gap nature is different to the previous experimental measurements (direct gap). Calculations reveal that these materials have strong optical absorption compared to chalcopyrite CuIn(S,Se)2 and kesterite Cu2ZnSn(S,Se)4. The strong absorption of these compounds is explained by the localized p-states of cation Sb/Bi in the lower region of conduction band. Thereby, ternary Cu(Sb,Bi)(S,Se)2 compounds have the potential to be used as absorber material in thin-film PV technologies.

Dynamic carbon mitigation analysis: the role of thin-film photovoltaics

We assess the net carbon emissions during a transition to photovoltaic power generation, comparing established and emerging thin-film PV technologies.

Study of optical losses in mechanically stacked dye-sensitized/CdTe tandem solar cells

ABSTRACTIn a constant effort to capture effectively more of the spectral range from the sun, multi-junction cells are being investigated. In this context, the marriage of thin film and dye-sensitized solar cells (DSC) PV technologies may be able to offer greater efficiency whilst maintaining the benefits of each individual technology. DSC devices offer advantages in the nature of both the metal oxide photo-electrode and dye absorption bands, which can be tuned to vary the optical performance of this part of a tandem device, while CdTe cells absorb the majority of light above their band-gap in only a few microns of thickness. The key challenge is to assess the optical losses with the goal of reaching a net gain in photocurrent and consequently increased conversion efficiency. This study reports on the influence of optical losses from various parts of the stacked tandem structure using UV-VIS spectrometry and EQE measurements. A net gain in photocurrent was achieved from a model developed for the DSC/CdTe mechanically stacked tandem structure.

Change of the Characterization Techniques as Progress of CuInSe<sub>2</sub>-Based Thin-Film PV Technology

In the CuInSe2(CIS)-based thin-film PV technology, various characterization techniques have been applied to measure the composition, crystal structure, depth profile and defect chemistry and so on, since Boeing Aerospace, for the first time, has come to the 10 % milestone in a thin-film form in 1980 by fabricating a very small single cell with top grids. More advanced and comprehensive characterization techniques are being applied after over 18 % total-area efficiency was consistently achieved employing the “three stage method”, which was developed by National Renewable Energy Laboratory (NREL). Comparing to the CIS-based absorber, there are not so many researches to investigate the absorber/buffer interface because the buffer is too thin to analyse separately and precisely and there are quite limited information on reaction pathways and composition of the buffer layer. However, in order to achieve the aperture-area efficiency of over 18 % on over 800cm2-sized large-area integrated circuits, it is remarkably important how to enhance the quality of absorber/buffer interface. Therefore, analytical works to understand how to improve the FF should tend to be more and more important.

Anodic bonding of activated tin solder alloys in the liquid state: A novel large-area hermetic glass sealing method

A new variant of the anodic bonding technique (ALTSAB: Activated Liquid Tin Solder Anodic Bonding) is described, which offers a number of advantages for direct glass to metal sealing over conventional methods. It employs a tin solder alloy containing an activating metal which is anodically bonded to alkali rich glass substrates in the liquid state. Two different test specimens were fabricated and characterized under vacuum at a pressure of 5×10 −4 mbar and a bonding temperature of 300 °C and compared with standard Al/glass anodic bonds. The ability to fabricate dual interface bonds of good quality and high shear strength >16 MPa by the ALTSAB method was demonstrated. With the sealant metal in the liquid state, this methodology promises superior surface adaptation and therefore joint performance when compared with the standard anodic bonding. More importantly, there is no need for premetallization or surface pretreatment of the glass substrates, and unlike in conventional anodic bonding, there is no need for mechanically loading the joining assembly. Wetting of the float glass substrates by the activated solder alloy SnAl0.6 after successful anodic bonding was confirmed by optical microscopy. The enrichment of the activating element Al at the bonding interface was confirmed by SEM and EDS element mapping. The methodology presented here shows great promise for various large-area sealing applications, particularly vacuum glazing, thin film and organic PV and OLED technology.

Interface control to enhance the fill factor over 0.70 in a large-area CIS-based thin-film PV technology

To improve the fill factor (FF) over 0.7, the following research has been performed, such as 1) our sulfurization after selenization (SAS) process condition for the Cu(InGa)(SeS) 2 (CIS)-based absorber formation is fixed to attain the open-circuit voltage ( V oc) of over 0.630 V/cell, 2) thickness of a double buffer structure with both Zn(O,S,OH) x deposited by a chemical-bath deposition (CBD) technique and intrinsic ZnO deposited by a metal–organic chemical vapor deposition (MOCVD) technique is adjusted by optimizing the series resistance ( R s) and the shunt resistance ( R sh), 3) post-deposition annealing in vacuum after CBD is applied to reduce the hydroxide (OH) concentration, and 4) narrower pattern-2 and -3 scribe lines contribute to make the interconnect width less than 200 μm and to keep the short-circuit current density ( J sc) over 34 mA/cm 2. As the result, for the first time a FF of over 0.7 has been achieved in our CIS research and development (R&D) since FY1993. This is the biggest contributor to achieve our FY2007 milestone of 15% efficiency with aperture area larger than 800 cm 2. We have achieved this milestone in the form of a laminated module.

Nanocrystalline dye‐sensitized solar cells having maximum performance

AbstractThis paper presents an overview of the research carried out by a European consortium with the aim to develop and test new and improved ways to realise dye‐sensitized solar cells (DSC) with enhanced efficiencies and stabilities. Several new areas have been explored in the field of new concepts and materials, fabrication protocols for TiO2 and scatterlayers, metal oxide blocking layers, strategies for co‐sensitization and low temperature processes of platinum deposition. Fundamental understanding of the working principles has been gained by means of electrical and optical modelling and advanced characterization techniques. Cost analyses have been made to demonstrate the potential of DSC as a low cost thin film PV technology. The combined efforts have led to maximum non‐certified power conversion efficiencies under full sunlight of 11% for areas <0ċ2 cm2 and 10ċ1% for a cell with an active area of 1ċ3 cm2. Lifetime studies revealed negligible device degradation after 1000 hrs of accelerated tests under thermal stress at 80°C in the dark and visible light soaking at 60°C. An outlook summarizing future directions in the research and large‐scale production of DSC is presented. Copyright © 2006 John Wiley & Sons, Ltd.