Physics-based electrical modelling of CIGS thin-film photovoltaic modules for system-level energy yield simulations
Copper indium gallium selenide (CIGS) is a commercialized, high-efficiency thin-film photovoltaic (PV) technology. The state-of-the-art energy yield models for this technology have a significant normalized root mean square error (nRMSE) on power estimation: De Soto model—26.7%; PVsyst model—12%. In this work, we propose a physics-based electrical model for CIGS technology which can be used for system-level energy yield simulations by people across the PV value chain. The model was developed by considering models of significant electrical current pathways from literature and adapting it for the system-level simulation. We improved it further by incorporating temperature and irradiance dependence of parameters through characterisation at various operating conditions. We also devised a module level, non-destructive characterization strategy based on readily available measurement equipment to obtain the model parameters. The model was validated using the measurements from multiple commercial modules and has a significantly lower power estimation nRMSE of 1.2%.
Introduction
Photovoltaics has evolved from an additional power source in small calculators to a clean energy source in mainstream energy production. The growing concerns over climate change have fuelled further evolution towards innovative concepts like vehicle-integrated PV, net-zero energy buildings and net-zero energy districts. In many of these applications, thin-film PV technologies are more suitable than crystalline silicon (c-Si) solar cells. The thin-film technologies have existed for decades, but c-Si PV has dominated the PV industry with its high efficiency, stability, and mature manufacturing processes 1. The thin-film PV technologies are known for their aesthetic appeal and the possibility to fabricate them on flexible substrates. With such characteristics, thin-film PV technologies may gain market share in the domain of integrated photovoltaics (IPV). A report by Becquerel Institute 2 states that, in 2020, the European IPV market was valued at almost 600 million euros and projected to triple reaching 1800 million euros by 2023. The case study in the report highlights the cost-effectiveness of thin-film technologies. The report also identifies digitization as one of the major factors needed to boost the economy around IPV. A modelling infrastructure for reliable energy yield prediction is a step in that direction as it assists in developing and optimizing thin-film PV systems for maximum energy yield. In this work, we FOCUS on creating an energy yield model for system-level simulation for the CIGS technology.
CIGS solar cells have additional current mechanisms that make the current–voltage (IV) characteristics unique compared to conventional solar cells. In c-Si solar cells, the current–voltage (IV) characteristics can be constructed by super positioning the dark behaviour and the photogenerated current at short circuit. This superposition fails in CIGS solar cells because of the voltage-dependent photogeneration 3,4,5. This can be seen in the measured dark and illuminated IV curves shown in Fig. 1. The current mechanisms that contribute to the photocurrent in CIGS solar cells are drift current across the depletion region to the heterojunction interface, thermionic emission across the junction, bulk recombination, and diffusion to the back contact. The voltage dependence is caused by the drift and the thermionic emission losses that are linked to the change in the electric field and increased back contact recombination 6,7,8. The electric field and the depletion layer width, in turn, are voltage dependent. The parasitic currents in CIGS solar cells are junction recombination current, ohmic shunt current, space charge limited current (SCLC), and tunnelling currents 9. The recombination current in CIGS solar cells varies with illumination as some defects are activated upon illumination 10,11. Shunt current is related to the presence of pinholes in the absorber. The absence of buffer and window layers results in a local metal-semiconductor-metal contact which leads to SCLC 12,13,14,15. The tunnelling currents are caused by mid-gap defects that are significant at temperatures below 250 K 11 and can be ignored for the energy yield estimation.
One of the commonly used models for PV module performance is the Sandia PV Array Performance Model (SAPM) 16. This model uses IV characteristics to estimate fitting coefficients to represent the relation of the IV parameters at different operating conditions. This model was first developed for c-Si modules but has also been used for thin-film modules based on CIGS and cadmium telluride (CdTe) technologies 17 ,18. A database of the fitting coefficients for different modules is publicly available and integrated into open-source software packages such as System Advisory Model (SAM) 19 and pvlib 20. and commercial software like PVsyst 21. The Loss Factor Model (LFM) 22 like SAPM, uses the outdoor measurement data to obtain a set of coefficients to correct temperature and spectral mismatch. LFM and SAPM are both empirical models. These models require a significant amount of onsite measurement data to calibrate the parameters for realistic energy yield estimation.
Physics-based models consider the variation of the physical parameters with the external operating conditions to simulate the performance of solar cells. The availability of such models is minimal. The De Soto model 23 (aka the five-parameter model) is a physics-based model based on the superposition of the dark and the illuminated IV curve of a solar cell. It has been widely used by the PV industry to represent the performance of the c-Si solar cells. PVsyst 21 uses a modified version of the De Soto model where it considers an exponential relation between shunt and illumination intensity instead of a linear relation. The efficacy of De Soto and PVsyst models are tested and reported in the section “Statistical significance”. The analytical model described by Sun et al. 6 is a physics-based model developed specifically for CIGS. It addresses the superposition failure by modelling the voltage dependence of photogeneration. However, this model does not consider the variation of saturation current and shunt resistance with illumination intensity, which are significant at low irradiance. The model was only validated at intensities above 400 W m −2 with laboratory-scale cells. The temperature and illumination dependence validated at the cell level may not be valid at the module level.
The reference parameters are parameters that represent the module characteristics at the Standard Test Conditions (STC). They are inputs to a model to estimate module performance at other operating conditions. Multiple techniques are available for estimating the value of such parameters. The nonlinear least-squares method is the most common technique to fit a model to data and obtain the parameters. It iteratively minimises the least square error. The requirement of initial values and bounded solution space for the parameters become a constraint for this method. Wrong initial estimates may lead to non-convergence or convergence to local minima. As the number of parameters increases, uncertainty in parameter estimation increases. This limits its use in large models. Another method is to reduce the number of model parameters by representing the possible dependent variables as a function of independent variables and iteratively solving the model to correctly estimate the power ( \(P_\mathrm\) ) at the maximum power point (MPP), the open-circuit voltage ( \(V_\mathrm\) ) and the short-circuit current ( \(I_\mathrm\) ) 24. Both the above techniques are suitable for a simple model like the De Soto model.
The model described by Sun et al. 6 has many parameters and they simplify the problem by individually fitting the current curves with their respective model i.e., fitting the reverse-biased dark current to their shunt current model, forward bias dark current to their diode current model. The photocurrent curve is obtained by separating the diode current and shunt current from the illuminated IV curve. Then, the photocurrent model is fitted to the photocurrent curve. For certain parameters, estimates were obtained through the numerical simulation software ADEPT, which requires material-specific parameters (e.g., layer thickness, mobility, doping densities, and defects). This methodology by Sun et al. may be an interesting approach as it reduces uncertainty by splitting the model into three smaller parts. However, this strategy cannot be applied to parameterise the model for a commercial module. It is neither possible to measure the reverse-bias IV curve of commercial PV modules because of the presence of bypass diodes nor to have material-specific data like doping concentration, mobility, etc.
To set a benchmark, the accuracy of the existing state-of-the-art models to estimate the IV characteristics of CIGS modules were analysed. Although the CIGS-specific model with a parameter extraction methodology described by Sun et al. 6 should be considered as the state of the art, it cannot be used at the module level. In this work, the variations of both De Soto and PVsyst models are considered as the state of the art for module-level simulations. The De Soto model is used along with the parameter extraction technique described by Laudani et al. 24. The performance estimation data of the PVsyst model was obtained from the module database within the PVsyst software. The IV curves for different modules were estimated using both models and compared with the measured curves to calculate their accuracy.
Figure 2a compares the Power–Voltage (P–V) curves estimated by the De Soto and PVsyst models with the measured IV curves for different illumination intensities at 25 °C. The De Soto model overestimates \(V_\mathrm\) and \(P_\mathrm\) at low intensities and PVsyst overestimates them at high intensities. Figure 2b compares the model estimates with measured curves for different cell temperatures at 1000 W m −2. At high temperatures, the De Soto model underestimates \(V_\mathrm\) and PVsyst overestimates \(P_\mathrm\). The nRMSE in estimating the maximum power point using the De Soto and PVsyst model are 26.7% and 12%, respectively (The graph representing this data is discussed later in the text).
In this work, we develop an electrical model for energy yield estimation adapting the models of significant current pathways from literature and incorporating the temperature and irradiance dependence of various parameters. These relations were determined by characterization. We also develop a step-by-step characterization strategy to obtain the parameters for using the model. The proposed characterization methods are non-destructive and can be easily done with basic PV lab instrumentation. Apart from model parameters, module temperature and incident irradiance are the only external parameters required for energy yield prediction. Thus, the proposed model can be easily combined with a thermal model and irradiance model in any of the existing energy yield prediction infrastructure. With modular and simplistic approach proposed, we ensure that the model can be used across the PV value chain. Table 1 gives the summary of the state-of-the-art models available for simulating the performance of CIGS devices and compares it with the developments made in this work.
Shunt resistance
Shunt resistance ( \(R_\mathrm\) ) can be determined as the inverse slope of the IV curve at \(I_\mathrm\) ( \(V\) = 0) 27. Assuming same shunt resistance for all cells, single cell shunt resistance can be obtained by multiplying the \(\frac
\) to the module shunt resistance. The measurement of shunt resistance at different incidence irradiance shows that it varies non-linearly with irradiance. We propose to use Eq. (7) to represent the shunt resistance variation for CIGS technology. \(R_\mathrm\_\mathrm\) is the shunt resistance at reference irradiance \((G_\mathrm)\) of 1000 W m −2 and \(G\) is the incident irradiance:
Figure 4a shows the irradiance dependence of shunt resistance for different CIGS modules. The power relation (7) was used to fit the data for each module to obtain the empirical parameter \(u\) that defines the relation. The values extracted from the data are given in inset table of Fig. 4a.
Ideality factor and saturation current pre-factor
The ideality factor (n) and dark saturation current pre-factor \((I_)\) can be estimated from the \(I_\mathrmV_\mathrm\) curve obtained by plotting \(I_\mathrmV_\mathrm\) (cell parameters) at different irradiance intensities. This method is used with an assumption that at \(V_\mathrm\). diode current is equal to photocurrent and shunt current is negligible. However, our model considers voltage-dependent photogeneration, illumination-dependent dark saturation current, and non-negligible shunt current. The value of n and \(I_\) cannot be directly used as the reference parameters. Equation (8) was used to fit the \(I_\mathrmV_\mathrm\) to get an initial estimate for \(n\) and \(I_\).
Curve fitting
At this point, the reference values \(R_\mathrm,\mathrm\). \(E_\mathrm\). and \(R_\mathrm,\mathrm\) (cell parameters) have been estimated. Now, the IV curve at STC can be fitted to the Eq. (5) using python SciPy curve fit function 28 to obtain the remaining parameters \(\alpha _\mathrm\). \(V_\mathrm,\mathrm\). \(n\). \(I_00,\mathrm\). and \(\gamma _\mathrm\). Equations (9) and (10), adapted from 6 give the temperature relation of the parameters \(\alpha\) and \(V_\mathrm\). respectively. Here \(\Delta E_\mathrm\) refers to the conduction Band offset between the buffer and absorber. Value of \(\Delta E_\mathrm\) = 0.1 eV fits well for the CIGS technology. For some modules, γ exhibited slight temperature dependence given by Eq. (11), where m is either 0 or 1:
Analyzing a Fast-Growing Solar Cell Technology
Developments in research and manufacturing have pushed copper gallium indium selenide (CIGS) solar panel to the forefront in the adoption of photovoltaic technology for energy generation. CIGS is a highly stable, high performance, and mature thin film PV technology. While the composition of these PV material has not been substantially altered since 1986, its popularity is due to improvements in manufacturing to lower costs and expand installation opportunities such built-environments, portability and into diffuse lighting and high-heat installations.
Silicon crystalline solar panels
Over the past few decades monocrystalline and polycrystalline silicon solar panels have experienced the majority PV installations. Silicon is an abundant material and has proven to be an easy and relatively inexpensive product to produce and sell. Monocrystalline panels offer the greatest energy conversion efficiency, but are more expensive than polycrystalline panels, which are composed of melted fragments of silicon joined to form the wafers for the panel. Owning to manufacturing processes, monocrystalline panels are the most expensive material on the market. Improvements in polycrystalline technologies have made them more widely adopted, but due to lower efficiency more panels are needed to cover a given area.
Thin film solar panels
Thin film cells exhibit lower efficiencies than crystalline panels and require the most space for the same amount of power. Since they are becoming the cheapest panels to produce because of the low material costs for thin film they are quickly becoming the more economically efficient panel types. Their adaptability to structures such as building siding or rooftop shingles and improved aesthetic appearance compensates for space requirements. Thin film panels are generally flexible and lighter weight, adding to their versatility in installations.
Amorphous solar panels
Like conventional solar panels, amorphous solar panels are made from silicon, but they are constructed by depositing non-crystalline silicon on a substrate like glass, plastic, or metal. Unlike many other thin-film panel options, amorphous silicon panels use very little toxic materials. When compared mono- or poly-crystalline solar panels, amorphous panels use much less silicon. However, this technology is the least efficient.
Cadmium telluride (CdTe) panels
Another common photovoltaic technology is cadmium telluride (CdTe) panels. CdTe thin film panels are made from several thin layers: one main energy producing layer made from the compound cadmium telluride, and surrounding layers for electricity conduction and collection. Slightly higher in efficiency than amorphous solar panels, the biggest drawback to CdTe is that cadmium, while abundant, is one of the most toxic materials known, making manufacturing and disposal an environmental concern.
Copper gallium indium selenide (CIGS) solar panels
Over the past years, Copper gallium indium selenide (CIGS) has become the fastest growing thin film PV technology. Sandwiched between conductive layers and deposited on substrates such as glass, plastic, steel, and aluminum, CIGS layers are thin enough to be allowed full-panel flexibility. While also using the toxic chemical cadmium, its implementation is at a lower percentage than CdTe panels.
Due to its promise as the highest efficiency of thin film PV technologies, considerable public and private research has gone into making its production cheaper and more efficient compared to other solar technologies.
Some of the technological advantages of CIGS panels are:
- Low temperature coefficient: CIGS efficiency does not decrease as quickly as silicon panels when in high temperatures – making it ideal for installation in the fast-growing solar markets. Coupled with superior performance in diffuse light conditions, CIGS PV remains a high yield technology even in less than ideal environments.
- High absorption: This direct-bandgap material can absorb a significant portion of the solar spectrum, enabling it to achieve the highest efficiency of any thin-film technology.
- Tandem design: A tunable bandgap allows the possibility of tandem CIGS devices.
- Protective buffer layer: The grain boundaries form an inherent buffer layer, preventing surface recombination and allowing for films with grain sizes of less than 1 micrometer to be used in device fabrication.
Japan’s Solar Frontier is currently the largest CIGS producer, with 1 GW of production capacity and 5 GW of modules deployed globally. New, large-scale investments in CIGS manufacturing from major energy and industrial players is currently underway, primarily in China. Around 600 MW of CIGS production capacity was added in 2018 with expansion plans for multiple gigawatts of production. CIGS research institutes and endeavors in countries including Germany, France, Switzerland, the Netherlands, Sweden, and Spain make Europe the leading international center for CIGS technology development. When this fundamental expertise is combined with the established network of advanced production equipment suppliers, Europe provides a promising ecosystem for CIGS technology development – with laboratory developments readily transferable into scale production machinery and solutions.
Analyzing a CIGS solar cell
In a recently published application note, we used energy-dispersive X-ray fluorescence (EDXRF) to examine a glass substrate coated with a Mo layer on which CIGS is deposited and sealed with layers of CdS, ZnO and Al doped ZnO. EDXRF as a truly nondestructive technique is ideal for the characterization of CIGS solar cells, to determine both layer composition and thickness. Total measurement times of 1-minute live time provide excellent repeatability values, both for the layer composition and thickness. The fundamental parameters-based thin film software allows measurement of thickness, mass and composition of up to 6 layers containing any number of elements.
Sale of CIGS Solar Cell Panels Expected to Reach 1 Billion by 2013
Over 2.3 Billion has been invested by Venture Capital Firms in 35 different CIGS Solar Cell Technology companies. Of this amount, the top 5 firms, shown below have received 1.8 Billion.
Note: CIGS is an abbreviation for solar cells made from Copper Indium Gallium Selenide (CIGS).
With the phenomenal market success of First Solar, evidence of higher efficiency rates in the laboratory and using a material that does not contain toxic cadmium, one can imagine the investment appeal of CIGS solar cell technology.
Unfortunately for the investors, while laboratory results have been very promising, mass producing panels using CIGS solar cell technology has proven to be extremely difficult. Its’ manufacture and behavior has not yet been mastered as evidenced by the very few CIGS firms in true commercial production. Almost every CIGS firm has experienced schedule delays, personnel shake-ups, or massive re-working of processes and technological approaches, and none have been able to reach the high efficiency levels first proved possible in the laboratory.
A few firms such as Wuerth Solar and Global Solar are currently shipping commercial CIGS product. Others such as Solyndra and Nanosolar boast of winning large purchase orders but true large-scale production at promised costs and efficiencies is not yet proven, and already some firms seem to be dropping out of the race.
However, because of continuing interest from developers and solar installers, CIGS is still a viable solar cell technology to watch, and some of the companies, especially those backed by larger companies such as Q-Cells’ subsidiary Solibro and Global Solar, which Solon has a stake in, will likely survive. In fact, at least one research companies are predicting that the sale of solar panels using CIGS solar cell technology will grow from 321 million in revenue in 2009 to around 1 billion by 2013, despite the fact that many of today’s CIGS companies won’t be there to see the turn around.
GBI Research even came out with a report in June 2010 that predicts that CIGS solar cell technology will emerge as the major solar cell technology by 2020. The report provides key data, information and analysis on the current status and future outlook of Global Thin film industry.
Manufacturing
99% of the light shining on a CIGS solar cell will be absorbed in the first micrometer of the material. Cells made from CIGS are usually heterojunction structures—structures in which the junction is formed between semiconductors having different bandgaps. The most common material for the top or window layer in CIS devices is cadmium sulfide (CdS), although zinc is sometimes added to improve transparency.
Adding small amounts of gallium to the lower absorbing CIS layer boosts its bandgap from its normal 1.0 electron-volts (eV), which improves the voltage and therefore the efficiency of the device. This particular variation is commonly called a copper indium gallium diselenide or CIGS solar cell.
Basically, a CIGS solar cell consists of the following parts:
The CIGS solar cell companies will all have similar products. The difference between them lay in how they produce those cells.
While there are many companies trying to make CIGS solar cheaper than the incumbent technology of silicon solar cells, one of the gaping structural gaps in a CIGS builder’s business plan is the lack of standard manufacturing tools. This means that every CIGS player has to be an equipment builder as well as a PV module and panel vendor. That’s technically risky and a highly inefficient use of VC investor capital.
Currently, the following methods are used to manufacture CIGS solar cells:
The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenide vapor to form the final CIGS structure. An alternative is to directly co-evaporate copper, gallium, indium and selenium onto a heated substrate.Most CIGS solar cells are produced using a co-evaporation technique that involves vacuums and can be costly and time-consuming. The elements are heated and deposited on a surface in a vacuum
NanoSolar uses a non-vacuum-based alternative process that mixes the materials into a liquid then deposits nano-particles of the precursor materials on the substrate and then sinters them in situ.
Electroplating is another low cost alternative to apply the CIGS layer. SoloPower uses this method.
A UCLA team has created its copper-indium-diselenide solar cell without going through the vacuum evaporation process. Instead, they dissolve their material into a liquid, apply it to a surface and bake it. In solution form, their solar absorber layer.- the part made from the copper-indium-diselenide or CIGS materials and critical to the performance of the cell.- can be easily painted or coated onto a surface.
In our method, [an] advantage is our solution technology has the potential to be fabricated in a continuous roll-to-roll process, Hou said, which is an important cost breakthrough.
AQT uses a dry reactive sputtering process and is targeting a 65 per Watt capital cost and 1.06 per Watt cell cost.
Efficiency Levels
With record laboratory CIGS solar cell efficiency at just below 20% for several years, the new trend of CIGS research has shifted to investigation on lower-cost deposition methods that could be an alternative to expensive vacuum processes.
Applied Quantum Technology (AQT) which raised only 4.75 million from undisclosed sources produced CIGS materials (in less than a year) with an NREL tested efficiency of greater than 10%. It has taken most of the other players several years to reach that efficiency milestone.
SoloPower earlier this year said that it had achieved 11 percent efficiency for its panels, which is relatively good compared with other CIGS makers.Currently, non-vacuum efficiencies of 10%-15% have been achieved by many parties, such as ISET, Nanosolar and IBM – which is slightly higher than the efficiency rating of Cadmium Telluride panels produced by First Solar.
Advantages of CIGS Technologies
So far the promise of CIGS solar cell technology has been greater than the reality, but certain advantages of this technology are beginning to emerge, namely:
The active layer (CIGS) can be deposited in a polycrystalline form directly onto molybdenum coated glass sheets or steel bands. This uses less energy than growing large crystals, which is a necessary step in the manufacture of crystalline silicon solar cells. Also unlike crystalline silicon, these substrates can be flexible.
One environmental advantage of CIGS solar cell technologies have over Cadmium Telluride solar cell panels is that it uses a much lower level of cadmium, in the form of cadmium sulfide. In some designs, sometimes zinc is used instead of cadmium sulfide all together.
Like Cadmium Telluride panels, CIGS solar cell panels show a better resistance to heat than silicon based solar panels.
Disadvantages of CIGS Solar Cell Panels
Like all thin film solar panels, CIGS panels are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%. They are however, the most efficient of the thin film technologies.
So far being able to produce solar panels at that can compete with polycrystalline or cadmium telluride panels has not been possible. There is growing concern by some parties, that the cost of fabricating the product makes it difficult to be competitive with current grid prices.
It may take several more years to solve the manufacturing problems and bring the production costs in line with the other leading producers of solar panels.
For Updated Industry News
Some industry observers believe 2010 was the coming out year for CIGS companies as a number of companies started selling substantial numbers of CIGS modules. To read the full article on our blog site click here.
NREL verifies MiaSole’s 15.7% CIGS Thin-Film Module Conversion Efficiencies. To read the full article click here.
Click on the appropriate link to return to the top of this page about CIGS solar cell technology or to return to the previous section about Thin Film Technologies.
Sale of CIGS Solar Cell Panels Expected to Reach 1 Billion by 2013
Over 2.3 Billion has been invested by Venture Capital Firms in 35 different CIGS Solar Cell Technology companies. Of this amount, the top 5 firms, shown below have received 1.8 Billion.
Note: CIGS is an abbreviation for solar cells made from Copper Indium Gallium Selenide (CIGS).
With the phenomenal market success of First Solar, evidence of higher efficiency rates in the laboratory and using a material that does not contain toxic cadmium, one can imagine the investment appeal of CIGS solar cell technology.
Unfortunately for the investors, while laboratory results have been very promising, mass producing panels using CIGS solar cell technology has proven to be extremely difficult. Its’ manufacture and behavior has not yet been mastered as evidenced by the very few CIGS firms in true commercial production. Almost every CIGS firm has experienced schedule delays, personnel shake-ups, or massive re-working of processes and technological approaches, and none have been able to reach the high efficiency levels first proved possible in the laboratory.
A few firms such as Wuerth Solar and Global Solar are currently shipping commercial CIGS product. Others such as Solyndra and Nanosolar boast of winning large purchase orders but true large-scale production at promised costs and efficiencies is not yet proven, and already some firms seem to be dropping out of the race.
However, because of continuing interest from developers and solar installers, CIGS is still a viable solar cell technology to watch, and some of the companies, especially those backed by larger companies such as Q-Cells’ subsidiary Solibro and Global Solar, which Solon has a stake in, will likely survive. In fact, at least one research companies are predicting that the sale of solar panels using CIGS solar cell technology will grow from 321 million in revenue in 2009 to around 1 billion by 2013, despite the fact that many of today’s CIGS companies won’t be there to see the turn around.
GBI Research even came out with a report in June 2010 that predicts that CIGS solar cell technology will emerge as the major solar cell technology by 2020. The report provides key data, information and analysis on the current status and future outlook of Global Thin film industry.
Manufacturing
99% of the light shining on a CIGS solar cell will be absorbed in the first micrometer of the material. Cells made from CIGS are usually heterojunction structures—structures in which the junction is formed between semiconductors having different bandgaps. The most common material for the top or window layer in CIS devices is cadmium sulfide (CdS), although zinc is sometimes added to improve transparency.
Adding small amounts of gallium to the lower absorbing CIS layer boosts its bandgap from its normal 1.0 electron-volts (eV), which improves the voltage and therefore the efficiency of the device. This particular variation is commonly called a copper indium gallium diselenide or CIGS solar cell.
Basically, a CIGS solar cell consists of the following parts:
The CIGS solar cell companies will all have similar products. The difference between them lay in how they produce those cells.
While there are many companies trying to make CIGS solar cheaper than the incumbent technology of silicon solar cells, one of the gaping structural gaps in a CIGS builder’s business plan is the lack of standard manufacturing tools. This means that every CIGS player has to be an equipment builder as well as a PV module and panel vendor. That’s technically risky and a highly inefficient use of VC investor capital.
Currently, the following methods are used to manufacture CIGS solar cells:
The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenide vapor to form the final CIGS structure. An alternative is to directly co-evaporate copper, gallium, indium and selenium onto a heated substrate.Most CIGS solar cells are produced using a co-evaporation technique that involves vacuums and can be costly and time-consuming. The elements are heated and deposited on a surface in a vacuum
NanoSolar uses a non-vacuum-based alternative process that mixes the materials into a liquid then deposits nano-particles of the precursor materials on the substrate and then sinters them in situ.
Electroplating is another low cost alternative to apply the CIGS layer. SoloPower uses this method.
A UCLA team has created its copper-indium-diselenide solar cell without going through the vacuum evaporation process. Instead, they dissolve their material into a liquid, apply it to a surface and bake it. In solution form, their solar absorber layer.- the part made from the copper-indium-diselenide or CIGS materials and critical to the performance of the cell.- can be easily painted or coated onto a surface.
In our method, [an] advantage is our solution technology has the potential to be fabricated in a continuous roll-to-roll process, Hou said, which is an important cost breakthrough.
AQT uses a dry reactive sputtering process and is targeting a 65 per Watt capital cost and 1.06 per Watt cell cost.
Efficiency Levels
With record laboratory CIGS solar cell efficiency at just below 20% for several years, the new trend of CIGS research has shifted to investigation on lower-cost deposition methods that could be an alternative to expensive vacuum processes.
Applied Quantum Technology (AQT) which raised only 4.75 million from undisclosed sources produced CIGS materials (in less than a year) with an NREL tested efficiency of greater than 10%. It has taken most of the other players several years to reach that efficiency milestone.
SoloPower earlier this year said that it had achieved 11 percent efficiency for its panels, which is relatively good compared with other CIGS makers.Currently, non-vacuum efficiencies of 10%-15% have been achieved by many parties, such as ISET, Nanosolar and IBM – which is slightly higher than the efficiency rating of Cadmium Telluride panels produced by First Solar.
Advantages of CIGS Technologies
So far the promise of CIGS solar cell technology has been greater than the reality, but certain advantages of this technology are beginning to emerge, namely:
The active layer (CIGS) can be deposited in a polycrystalline form directly onto molybdenum coated glass sheets or steel bands. This uses less energy than growing large crystals, which is a necessary step in the manufacture of crystalline silicon solar cells. Also unlike crystalline silicon, these substrates can be flexible.
One environmental advantage of CIGS solar cell technologies have over Cadmium Telluride solar cell panels is that it uses a much lower level of cadmium, in the form of cadmium sulfide. In some designs, sometimes zinc is used instead of cadmium sulfide all together.
Like Cadmium Telluride panels, CIGS solar cell panels show a better resistance to heat than silicon based solar panels.
Disadvantages of CIGS Solar Cell Panels
Like all thin film solar panels, CIGS panels are not as efficient as crystalline silicon solar cells, for which the record efficiency lies at 24.7%. They are however, the most efficient of the thin film technologies.
So far being able to produce solar panels at that can compete with polycrystalline or cadmium telluride panels has not been possible. There is growing concern by some parties, that the cost of fabricating the product makes it difficult to be competitive with current grid prices.
It may take several more years to solve the manufacturing problems and bring the production costs in line with the other leading producers of solar panels.
For Updated Industry News
Some industry observers believe 2010 was the coming out year for CIGS companies as a number of companies started selling substantial numbers of CIGS modules. To read the full article on our blog site click here.
NREL verifies MiaSole’s 15.7% CIGS Thin-Film Module Conversion Efficiencies. To read the full article click here.
Click on the appropriate link to return to the top of this page about CIGS solar cell technology or to return to the previous section about Thin Film Technologies.