Copper Indium Gallium Selenide Solar Cells: Wikis

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Copper-Indium-Gallium-Selenium (CuIn1-xGaxSe2 or CIGS) is a direct bandgap semiconductor. The bandgap varies from 1.0 eV (CIS) to 1.65 eV (CGS) depending on the Ga/(In+Ga) ratio. CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons.[1] CIGS's absorption coefficient is higher than any other semiconductor used for solar modules. Devices made with CIGS belong to the thin-film category of photovoltaics (PV). The market for thin-film PV grew at a 60% annual rate from 2002-2007 and is still growing rapidly (Figure 1) .[2] Therefore, a strong incentive exists to develop and improve deposition methods for these films that will allow lower cost and increased throughput.

Figure 1: The projected growth of the thin film PV industry
Figure 2: CIGS device structure

The most common device structure is for CIGS solar cells is shown in Figure 2. Glass is commonly used as a substrate, however, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils.[3] A molybdenum layer is deposited (commonly by sputtering) which serves as the back contact and to reflect most unabsorbed light back into the absorber. Following Mo deposition a p-type CIGS absorber layer is grown by one of several unique methods. A thin n-type buffer layer is added on top of the absorber. The buffer is typically CdS deposited via chemical bath deposition. The buffer is overlaid with a thin, intrinsic ZnO layer which is capped by a thicker, Al doped ZnO layer. Despite increasing the series resistance, the intrinsic ZnO layer is beneficial to cell performance. The precise mechanism for the improvement is still being debated.[4] The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible.

Contents

General properties of high performance CIGS absorbers

All high performance CIGS absorbers in solar cells have several similarities independent of the growth technique used. First, they are polycrystalline α-phase which has the chalcopyrite crystal structure shown in Figure 3. The second property is an overall Cu deficiency.[5] Cu deficiency increases the majority carrier (hole) concentration by increasing the number of Cu vacancies. These vacancies act as electron acceptors. Also, when CIGS films are In rich (Cu deficient) the surface layer of the film forms an ordered defect compound (ODC) with a stoichiometry of Cu(In,Ga)3Se5. The ODC is n-type, forming a p-n homojunction in the film at the interface between the α phase and the ODC. Recombination velocity at the CIGS/CdS interface is decreased by presence of the homojunction. The drop in interface recombination attributable to ODC formation is demonstrated by experiments which have shown recombination in the bulk of the film is the main loss mechanism in Cu deficient films, while in Cu rich films the main loss is at the CIGS/CdS interface.[4][5]

Figure 3: CIGS unit cell. Red = Cu, Yellow = Se, Blue = In/Ga

Na incorporation is also necessary for optimal performance. Ideal Na concentration is considered to be approximately 0.1 at%. Na is commonly supplied by the soda-lime glass used as the substrate, but in processes that do not use this substrate the Na must be deliberately added. Beneficial effects of Na include increases in p-type conductivity, texture, and average grain size. Furthermore, Na incorporation allows for performance to be maintained over larger stoichiometric deviations.[1] Simulations have predicted that Na on an In site creates a shallow acceptor level and that Na serves to remove In on Cu defects (donors), but reasons for these benefits are still being debated. Na is also credited with catalyzing oxygen absorption. Oxygen passivates Se vacancies that act as compensating donors and recombination centers.

Alloying CIS (CuInSe2) with CGS (CuGaSe2) increases in the bandgap. To reach the ideal bandgap for a single junction solar cell, 1.5 eV, a Ga/(In+Ga) ratio of roughly 0.7 would be optimal. However, at ratios above ~0.3 device performance drops off. Industry currently targets the 0.3 Ga/(In+Ga) ratio, resulting in bandgaps between 1.1 and 1.2 eV. The decreasing performance has been postulated to be a result of CGS not forming the ODC, which is necessary for a good interface with CdS.[5]

The highest efficiency devices show a high degree of texturing, or preferred crystallographic orientation. Until recently record efficiency devices displayed a (112) texture, but now a (204) surface orientation is observed in the best quality devices.[1] A smooth absorber surface is preferred to maximize the ratio of the illuminated area to the area of the interface. The area of the interface increases with roughness while illuminated area remains constant, decreasing open circuit voltage (VOC). Studies have also linked an increase in defect density to decreased VOC. recombination in CIGS has been suggested to be dominated by non-radiative processes. Theoretically, recombination can be controlled by engineering of the film, as opposed to being intrinsic to the material.[6]

Precursor deposition and post processing

Perhaps the most common method used to create CIGS films for commercial use is deposition of precursor materials – always including Cu, In, and Ga, and sometimes including Se – onto a substrate and processing these films at high temperatures under a proper atmosphere. The following sections outline the various techniques for precursor deposition processing, including sputtering of metallic layers at low temperatures, printing of inks containing nanoparticles, electrodeposition, and a technique inspired by wafer-bonding.

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General selenization concerns

The Se supply and selenization environment is extremely important in determining the properties and quality of the film produced from precursor layers. When Se is supplied in the gas phase (for example as H2Se or elemental Se) at high temperatures the Se will become incorporated into the film by absorption and subsequent diffusion. During this step, called chalcogenization, complex interactions occur to form a chalcogenide. These interactions include formation of Cu-In-Ga intermetallic alloys, formation of intermediate metal-selenide binary compounds, and phase separation of various stoichiometric CIGS compounds. Because of the variety and complexity of the reactions taking place, the properties of the CIGS film are difficult to control.[1]

Differences exist between films formed using different Se sources. Using H2Se yields the fastest Se incorporation into the absorber. 50 at% Se can be achieved in CIGS films at temperatures as low as 400 °C. By comparison, elemental Se only achieves full incorporation with reaction temperatures of 500 °C and above. Below 500 °C films formed from elemental Se were not only Se deficient, but also had multiple phases including metal selenides and various alloys. Use of H2Se also provides the best compositional uniformity and the largest grain sizes. However, H2Se is highly toxic and is classified as hazardous to the environment.

Sputtering of metallic layers followed by selenization

In this method of forming CIGS absorbers, a metal film of Cu, In, and Ga is sputtered at or near room temperature and reacted in a Se atmosphere at high temperature. This process has higher throughput than coevaporation and compositional uniformity can be more easily achieved.

Sputtering a stacked multilayer of metal – for example a Cu/In/Ga/Cu/In/Ga... structure – produces a smoother surface and better crystallinity in the absorber, when compared to a simple bilayer (Cu-Ga alloy/In) or trilayer (Cu/In/Ga) sputtering. These attributes result in higher efficiency devices, but forming the multilayer is a more complicated deposition process and is likely not worth the cost of extra equipment or the added process complexity.[5] Additionally, the reaction rates of Cu/Ga and Cu/In layers with Se are different. If the reaction temperature is not high enough, or not held long enough, CIS and CGS form as separate phases. The same considerations outlined in the previous section apply to Se incorporation.

Companies currently using similar processes include Showa Shell, Shell Solar, Miasolé, Avancis, and Energy Photovoltaics (EPV).[7] Showa Shell sputters a Cu-Ga alloy layer and an In layer, followed by selenization in H2Se and sulfurization in H2S. The sulfurization step appears to passivate the surface in a way similar to CdS in most other cells. Thus, the buffer layer used is Cd-free which eliminates the worries related to the toxicity and environmental impact of Cd. Showa Shell has reported a maximum module efficiency of 13.6% with an average of 11.3% for 3600 cm2 substrates.[3] Shell Solar uses the same technique as Showa Shell to create the absorber; however, they use a CdS layer deposited by chemical vapor deposition. Modules sold by Shell Solar have a specification of 9.4% module efficiency.

Miasole has had great success in procuring venture capital funds for its process and scale up. However, little is known about their sputtering/selenization process beyond their stated efficiency of 9 to 10% for modules.

EPV uses a hybrid between coevaporation and sputtering in which In and Ga are evaporated in a Se atmosphere. This is followed by Cu sputtering and a selenization step. Finally, In and Ga are again evaporated in the presence of Se. Based on Hall measurements, these films have a low carrier concentration and high mobility compared to other devices. EPV films have also been shown to have a low defect concentration.

Chalcogenization of particulate precursor layers

In this method, metal or metal-oxide nanoparticles are used as the precursors for CIGS growth. These nanoparticles are generally suspended in a water based solution and then applied to large areas by various methods, with printing the most common. The film is then dehydrated and, if the precursors are metal-oxides, reduced in a H2/N2 atmosphere. Following dehydration, the remaining porous film is sintered and selenized at temperatures greater than 400 °C.[5][6][8]

Nanosolar and International Solar Electric Technology (ISET) are attempting to scale up this process.[3] ISET uses oxide particles while Nanosolar is extremely secretive about their ink. The ink's composition is unknown but there is some implication that Se is also incorporated into Nanosolar's ink. The advantages of this process include uniformity over large areas, non-vacuum or low-vacuum equipment, and adaptability to roll-to-roll manufacturing. When compared to laminar metal precursor layers, the selenization of sintered nanoparticles is more rapid. The increased rate is a result of the greater surface area associated with porosity. Decreasing high temperature selenization reduces the thermal budget. Unfortunately, the drawback of porosity is a tendency towards rougher absorber surfaces. Use of particulate precursors allows for printing on a large variety of substrates with high materials utilization, around 90% or more. A disadvantage is that little research and development exists in this area of deposition. In Nanosolar's manufacturing the printed rolls are cut into cells and must be binned and integrated in a fashion similar to how Si devices are made today. The binning process is different from the monolithic integration that many CIGS companies are using. Monolithic integration is far more adaptable to inline production.

Nanosolar has reported a cell (not module) efficiency of 14%, however this has not been verified by any national laboratory testing, nor are they allowing any onsite inspections of their facilities to verify this and other claims made in the past. In independent testing[6] ISET's absorber had the 2nd lowest efficiency at 8.6%. However, all the modules that beat ISET's module were coevaporated, a process which has manufacturing disadvantages and higher costs. ISET's sample suffered most from low Voc and low fill factor, indicative of a rough surface and/or a high number of defects aiding recombination. Related to these issues, the film had poor transport properties including a low Hall mobility and short carrier lifetime.

Electrodeposition followed by selenization

Precursors can also be deposited by electrodeposition. Two different methodologies exist: deposition of elemental layered structures, and simultaneous deposition of all elements (including Se). Both methods require thermal treatment in a Se atmosphere to make device quality films. Because electrodeposition requires conductive electrodes, metal foils are a logical substrate. Electrodeposition of elemental layers is similar to the sputtering of elemental layers. Currently no company is scaling up this process.

Simultaneous deposition is performed using a working electrode (cathode), a counter electrode (anode), and a reference electrode as in Figure 4. A metal foil substrate is used as the working electrode in industrial processes. An inert material is used for the counter electrode, and the reference electrode exists to measure and control the potential difference between the anode and cathode. The reference electrode allows the process to be performed potentiostatically, meaning the potential of the substrate can be controlled.[5]

Figure 4: CIGS electrodeposition apparatus

Electrodeposition of all elements simultaneously is a difficult processing problem for a variety of reasons. First, the standard reduction potentials of the elements are not the same, causing preferential deposition of a single element. This problem is commonly alleviated by adding different counter ions into solution for each ion to be deposited (Cu2+, Se4+, In3+, and Ga3+), thus changing the reduction potential for that ion. Second, the Cu-Se system has a complicated behavior and the composition of the film depends on the Se4+/Cu2+ ion flux ratio which can vary over the film surface. Because of this behavior the deposition conditions, specifically precursor concentrations and deposition potential, need to be optimized. Even with optimization, reproducibility is low over large areas due to composition variations and potential drops along the substrate.

The resulting films have small grains, are Cu-rich, and generally contain Cu2-xSex phases along with impurities from the solution. Annealing is required to improve crystallinity. In order to achieve efficiencies higher than 7%, a stoichiometry correction is also required. The correction is done via high temperature physical vapor deposition which is not practical in industry.

Having solved the optimization issues, Solopower is currently producing cells with >13.7% conversion efficiency as per NREL. SoloPower is currently attempting to scale up the process, but few details have been released. SoloPower is relying on the advantages of roll-to-roll manufacturing and flexible metal foil substrates.

Precursor combination by wafer-bonding inspired technique

Figure 5: Schematic illustration of wafer-bonding inspired technique

In this process, two different precursor films are deposited separately on a substrate and a superstrate. The films are pressed together and heated to release the film from the superstrate leaving a CIGS absorber on the substrate. This technique allows the superstrate to be reused (Figure 5). Heliovolt patented this procedure and has named it the FASST process. Therefore, Heliovolt is the only company currently scaling up the technique. In principle, the precursors can be deposited at low temperature using low cost deposition techniques, lowering the final cost of the module. However, the first one or two generations of product will still use higher temperature PVD methods and not achieve full cost cutting potential. Flexible substrates could eventually be used in this process.

Typical film characteristics are not known outside of the company as no research has been conducted by independently funded laboratories. However, Heliovolt has claimed a top cell efficiency of 12.2%.

Coevaporation

Coevaporation, or codeposition, is the most prevalent CIGS fabrication technique in the laboratory and an important method in industry. The Boeing coevaporation process deposits bilayers of CIGS with different stoichiometries onto a heated substrate and allows them to intermix. The National Renewable Energy Laboratory (NREL) has developed another process which involves three deposition steps and produced the current CIGS efficiency record holder at 19.9%. The first step in NREL's method is codeposition of In, Ga, and Se. This is followed by Cu and Se deposited at a higher temperature to allow for diffusion and intermixing of the elements. In the final stage In, Ga, and Se are again deposited to make the overall composition Cu deficient.[5]

Würth Solar has been producing CIGS cells using an inline coevaporation system since 2005 with module efficiencies between 11% and 12% by the end of that year. They have subsequently opened another production facility and continued to improve efficiency and yield. Other companies scaling up coevaporation processes include Global Solar and Ascent Solar.[7] Global Solar also uses an inline three stage deposition process. In all of the steps Se is supplied in excess in the vapor phase. In and Ga are first evaporated followed by Cu and then by In and Ga to make the film Cu deficient. These films performed quite favorably in relation not only to other manufacturers but also to absorbers grown at NREL and the Institute for Energy Conversion (IEC).[6] However, fully fabricated modules of Global Solar’s films did not perform as well. The property in which the module most obviously under-performed was a low Voc, which is characteristic of high defect density and high recombination velocities. Interestingly, Global Solar’s absorber layer outperformed the NREL absorber in carrier lifetime and hall mobility. However, as completed cells the NREL sample performed better. This is evidence of a poor CIGS/CdS interface, possibly due to the lack of an ODC surface layer on the Global Solar film.

As most of the CIGS research at national laboratories and universities covers coevaporation, companies using this technique stand to gain the most from the scientific community. However, they also face significant disadvantages including uniformity issues over large areas and the related difficulty of coevaporating elements in an inline system. Another disadvantage is high growth temperatures which raise the thermal budget and cost. Additionally, coevaporation is plagued by low material utilization (deposition on chamber walls instead of the substrate) and expensive vacuum equipment.[3][8]

Chemical vapor deposition

CVD has been implemented in multiple ways for the deposition of CIGS. Processes include atmosphere pressure metal organic CVD (AP-MOCVD), plasma enhanced CVD (PECVD), low-pressure MOCVD (LP-MOCVD), and aerosol assisted MOCVD (AA-MOCVD). Current work is focused on changing the typical dual-source precursors to single-source precursors.[5] Multiple source precursors must be homogeneously mixed and the flow rates of the precursors have to be kept at the proper stoichiometry. Single-source precursor methods do not suffer from these drawbacks and should enable better control of film composition compared to multiple source precursors.

CVD is not yet being used by any companies for CIGS synthesis. Currently, CVD produced films have low efficiency and a low Voc, partially a result of a high defect concentration. Additionally, the surfaces of the films are generally quite rough which serves to further decrease the Voc. However, the requisite Cu deficiency has been achieved using AA-MOCVD along with a (112) orientation.

However, if the film quality produced by CVD can be improved, any company using this technique could benefit from knowledge gained in other industries using large area CVD deposition, such as glass coating manufacturers. CVD deposition temperatures are lower than those used for other processes such as co-evaporation and selenization of metallic precursors. Therefore, CVD has a lower thermal budget, reducing the cost. Potential manufacturing problems include difficulties converting CVD to an inline process as well as the expense of handling volatile precursors.

References

  1. ^ a b c d B. J. Stanbery, Critical Reviews in Solid State & Materials Science 27, 73 (2002).
  2. ^ http://www.sustainableenergyworld.eu/energy/thin-film-wins-pv-market-share-three-new-plants-in-germany-total-almost-50-mw-289.html. Retrieved 3-21-09.
  3. ^ a b c d N. G. Dhere, Solar Energy Materials and Solar Cells 91, 1376 (2007).
  4. ^ a b A. Ihlal et al., Thin Solid Films 515, 5852 (2007).
  5. ^ a b c d e f g h M. Kemell et al., Critical Reviews in Solid State and Material Sciences 30, 1 (2005).
  6. ^ a b c d B. J. S. D. L. Y. S. S. L. W. K. M. C. L. P. W. N. S. M. E. I. L. Repins, Progress in Photovoltaics: Research and Applications 14, 25 (2006).
  7. ^ a b H. S. Ulal, and B. von Roedern, Solid State Technology 51, 52 (2008).
  8. ^ a b K. Derbyshire, Solid State Technology 51, 32 (2008).

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