The Institute of Energy Conversion’s research predominantly features three solar energy technologies: Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS), and Silicon.
CdTe is an attractive contender for terrestrial solar power generation with its high conversion efficiency and low production cost. The IEC’s focus for CdTe is on device fabrication and analysis, high throughput processing methods, incorporation of sub-micron CdTe absorber layers, and the development of flexible solar cells, as it continues to push the envelope toward low-cost CdTe modules with 15% efficiency.
With their wider bandgap, CIGS also have great promise for broad application, providing the highest efficiencies of any thin-film solar cells, as well as the ability to be manufactured on low cost glass or flexible substrates. At IEC, research centers on alloy deposition either by thermal co-evaporation or selenization. A baseline process for complete cell fabrication is maintained, and cells with efficiencies as high as 18% have been produced.
The production of silicon solar cells continues to grow globally at rates of greater than 30% per year, and forms the basis of a multi-billion dollar industry. IEC continues to develop new device structures which combine the high efficiency and large manufacturing capacity of crystalline silicon photovoltaics with the low cost of thin film solar cells. Silicon heterojunction technology, in particular, provides considerable flexibility that has allowed IEC to achieve record efficiencies.
DOE Foundational Program to Advance Cell Efficiency (FPACE)
“Low Cost Back Contact Heterojunction Solar Cells on Thin c-Si Wafers: Integrating Laser and Thin Film Processing for Improved Manufacturability”
PI: Steve Hegedus
The objective of this research is to achieve low cost high efficiency c-Si solar cells on thin wafers < 50 microns in an interdigitated back contact (IBC) structure with passivated heterojunction (HJ) emitters and contacts. The IBC design offers a unique advantage of high short circuit current since the front surface has no grid shading and is optimized for maximum optical generation. Additionally, low temperature plasma chemical vapor deposition (CVD) deposited a-Si / c-Si HJ emitters and contacts provide high open circuit voltage due to excellent chemical and field-assisted passivation. The potential of this device architecture has been confirmed by both Panasonic and Sharp having produced champion Si IBC-HJ solar cells having efficiency > 25%. The challenge is to simultaneously achieve a low cost, high performance, high yield and manufacturable IBC-HJ solar cell process. We are focusing on new laser contacting and patterning approaches with our partners at MIT (Prof. Buonassisi) and IPG (Dr. Mendes).
“Understanding the Effect of Na in Improving the Performance of CuInSe2 Based Photovoltaics”
PI: Kevin Dobson
The objective of this project is to develop an understanding of how Na impacts the properties of Cu(InGa)Se2 (CIGS) thin films, irrespective of processing method, through identifying pathways which Na improves CIGS devices. The Na is incorporated in the CIGS film from either the soda-lime glass substrate or by using NaF. The presence of Na greatly improves the performance of CIGS devices, enhances grain size, reduces compensating donor density and passivates grain boundaries (GBs), however, there is a lack of fundamental understanding of mechanisms. Aspects of this project, conducted with our partner Univ. of Illinois (Prof. Rockett), have included understanding diffusion of Na and the role of oxygen in Mo contact layers, diffusion and chemistry of Na in CIS-based absorber thin films, and the role of Na in device operation. For comparison, in-grain Na diffusion was studied using CIS single crystals.
“Advanced Precursor Reaction Processing for Cu(InGa)(SeS)2 Solar Cells”
PI: Bill Shafarman
This project seeks to provide pathways to improve module efficiency and manufacturability of Cu(InGa)(SeS)2 films using the precursor reaction approach which is being developed for commercial manufacturing of Cu(InGa)(SeS)2 modules. Using IEC’s unique hydride gas reactor facility to react Cu-Ga-In precursors in H2Se and H2S, we have been addressing critical issues with this process including control of through-film composition and bandgap to enable high open circuit voltage solar cells; and poor quality of the back contact which can result in poor adhesion and typically contains relatively large voids that may affect device performance or stability.
“Reduced Cu(In,Ga)Se2 Thickness in Solar Cells Using a Superstrate Configuration”
PI: Bill Shafarman
The objective of this program, is to develop approaches to reduce the cost of Cu(In,Ga)Se2 manufacturing by reducing the thickness of the Cu(In,Ga)Se2 absorber layer to ~ 0.5 µm compared to typical thicknesses 1.5 - 2.5 µm. Previous results have shown that the most critical issue for sub-micron absorber layers is that short circuit current is significantly reduced due to incomplete absorption in the Cu(InGa)Se2 and poor reflection at the Mo back contact. Our approach is to implement light scattering for increased optical path length and an improved back reflector using a superstrate cell configuration where light is incident through the glass unlike the conventional Cu(In,Ga)Se2 configuration where the light is incident directly on the semiconductor layers. Implementing a thin absorber layer with the superstrate structure requires the development of a transparent back contact and incorporation of advanced optical enhancement methods.
“Fabrication of High Efficiency Thin-Film ACIGS Solar Cells”
PI: Bob Birkmire
Subcontract with NREL under their DOE FPACE program “Enabling the CIGS Thin-Film PV Technology to Meet the DOE Goal of $0.50/W Module Price”
The overall objectives of this work effort are to increase thin film Ag1-yCuyIn1-xGaxSe2 (ACIGS) solar cell conversion efficiency and reduce cost barriers to enable copper indium diselenide-based module prices to be on the trajectory toward $0.50/W by the end of this decade. IEC has the unique role to advance the deposition and device technology of the Ag-alloy ACIGS system where the lower melting point could lead to reduced defects and improved efficiency at higher bandgaps. The specific objectives of this work effort are to: 1) Increase the laboratory cell efficiency toward 22%; 2) Reduce the gap between laboratory efficiency (20.3%) and average commercially available module efficiency of 12-13%; and 3) Show continuous improvement in deposition processes and alternative window layers that can potentially reduce cost ($/m2).
DOE Foundational Program to Advance Cell Efficiency 2 (FPACE2)
PI: Brian McCandless
“Growth and Characterization of Cu2ZnSn(S,Se)4 Single Crystals”
IEC is a team member on the DOE FPACE2 project led by IBM. The project goal is reduction of the VOC deficit in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. The VOC deficit is the difference between the absorber band gap and the measured VOC, presently ~600 mV, and is targeted for reduction to <475 mV. The project is a collaborative effort between IBM, IEC, Harvard, and University of California at San Diego. The investigation is being carried out using two model systems - epitaxial and bulk single-crystal CZTSSe - in conjunction with theoretical modeling of bulk CZTSSe properties and development of state-of-the-art polycrystalline devices at IBM, to provide a fundamental understanding of recombination and VOC -limiting mechanisms in this promising PV system. IEC has developed bulk single crystal growth methods and is assessing electronic response of crystals and single-crystal solar cells to variations in composition, bulk defects, and surface passivation. Single crystals with up to 5 mm facets have been made without the use of flux agents. Conductivity spanning 3 orders of magnitude has been demonstrated by variation of the Cu content in the kesterite phase. Solar cells with VOC deficit <540 mV have been fabricated.
DOE Next-Generation Photovoltaics II
“Novel Low Symmetry Light Grating Layers”
PI: Bob Birkmire
This is a collaborative effort with UD’s Department of Materials Science and Engineering. The objective of this project is to develop novel wavelength-scale low-symmetry gratings fabricated in a high-refractive-index glass using low-cost nanoimprint/molding techniques as light trapping elements. The novel inherent low symmetry of both the proposed structural unit and the Bravais lattice ensures that all guided modes, irrespective of their symmetry, are excited and contribute to light trapping enhancement to exceed the 4n2 Lambertian limit. The ultimate goal of the program is to apply these light trapping structures to thin c-Si wafers <50 μm. While thin wafers significantly reduce material costs and may result in improvements in device efficiency by minimizing bulk recombination, they require effective light trapping designs due to the decreased optical absorption. Approaches employed in this study have included simulation of 1-D gratings to determine an optimized structure, development of stamping and nano-imprint methods to process As-Se glass-based gratings, development of bi-facial heterojunction solar cells on Si wafers < 50 µm and implementation of optimized grating layers onto these thin c-Si devices to demonstrate improved cell performance.
DOE Bay Area Photovoltaic Consortium (BAPVC)
“Advanced Evaporation Source Design”
PI: Greg Hanket
Thin film photovoltaic technologies have the potential to offer improved economics over silicon-based technologies. The highest laboratory thin film cell efficiency is 21.7%, fabricated by the elemental vacuum evaporation of Cu(InGa)Se2. To date, however, production module efficiencies using this technique only achieve 14-15% efficiency, in spite of laboratory cells achieving these efficiencies 20 year ago. While some of the disparity can be attributed to module losses due to cell interconnection and encapsulation optical losses, it is also the case that the manufacturing technology for reproducing laboratory-scale deposition conditions over 1 sq meter areas simply does not exist. The objective of this project is develop advanced source designs to meet the high rates required for economically-viable manufacture of Cu(InGa)Se2 modules by elemental evaporation.
NSF/DOE Engineering Research Center (ERC): Quantum Energy and Sustainable Solar Technologies (QESST): Arizona State University
“Cu(InGa)Se2 Tandem Cell Development”
PI: Bill Shafarman & Bob Birkmire
In the QESST program we are working to demonstrate the feasibility of thin film tandem cells based on Cu(InGa)Se2 alloy materials to increase module efficiency and reduce costs. One task is working to characterize the deposition process and material growth mechanisms using a three-stage evaporation process for wide bandgap (AgCu)(InGa)Se2 films, viewed as the critical enabling technology for tandem cell development. In this work, we have incorporated Ag-alloying to give significant grain size enhancement, greater intermixing during growth, and improved solar cell performance. A second research task is to address manufacturing issues for Cu(InGa)Se2-based thin films by developing models for high rate / continuous processing and to validate reactor design based on these models. In addition, we have developed a stochastic growth model to predict through-film composition as a function of process variables and precursor composition.
NSF Integrative Graduate Education and Research Traineeship (IGERT): The Solar Economy (SEIGERT): Purdue University
PI: Bob Birkmire
I. Electro- & Photo- luminescence Imaging and Characterization
Electroluminescence (EL) is an electro-optical characterization technique used to image and characterize spatial uniformity of solar cells and modules. This project is focused on developing a robust EL system, capable of supporting multiple users and research programs. We have developed a suit of analysis tools, and characterized the capabilities of our system, measuring a wide range of photovoltaic device structures and technologies, including a-Si/c-Si heterojunction devices, CdTe, Cu(In,Ga)Se2, and CZTS based technologies. Additionally, we are adding the capability to perform photo-luminescent (PL) measurements on films and completed devices to compliment the EL.
II. Approaches to Increasing Voc in CdTe Solar Cells
This project is focused on developing CdTe solar cells with enhanced voltage by: 1) evaluating alternative back contacts consisting of C61-butyric acid methyl ester, PCBM, grown using different chemistries as the primary contact followed by a current carrying metal; and 2) reducing the thickness of the CdTe film to less than an absorption length to minimize recombination in the space charge region.
Accelerated Testing for Increasing Module Lifetimes > 25 Years
PI: Steve Hegedus
Solar modules must be designed and manufactured to produce energy for over 20 years of continuous exposure to combinations of conditions or ‘stressors’: sun, rain, hot and cold temperatures, hail, UV radiation, and a range of electrical bias conditions. However, it is not practical to verify the impact of every new material, device design or process on the module lifetime with 20+ years of outdoor exposure. Therefore, accelerated exposure is commonly used to force the appearance of material or manufacturing defects in a period of months. Such tests do not predict the long term performance but rather the appearance of failure mechanisms. This is accomplished by exposing cells or modules to combinations of stressors at higher values than would normally be experienced. Common examples include higher temperature and humidity (85°C at 85% relative humidity) or freeze-thaw cycles (10 cycles of -10°C to +40°C). The IEC has well developed facilities to study both cause and effect: creating failures by applying accelerated stressors, and analyzing the effects using a suite of device characterization tools.