Copper Indium Gallium Diselenide Solar Cells

The National Center for Photovoltaics (NCPV) at NREL has significant capabilities in copper indium gallium diselenide (CIGS) thin-film photovoltaic research and device development. CIGS-based thin-film solar cell modules represent the highest-efficiency alternative for large-scale, commercial thin-film solar cells. Record small-area single-junction efficiency now tops 22% and several companies have confirmed module efficiencies exceeding 16%.

Most of these companies are building upon work performed by NCPV CIGS scientists during the past 20-plus years of research. Central to CIGS advances was our development of the "three-stage process." This process enables the formation of a CIGS thin-film layer that is of the proper composition and structure to allow the photo-generated charge carriers to exist long enough in the CIGS layer of the device so that they can be separated and collected at the front and back contacts. This separation and collection is critical for demonstrating high conversion efficiency.

Schematic illustration of a typical CIGS substrate thin-film photovoltaic device. In this design, the layers of the device are deposited onto a glass, metal, or polymer substrate. Sunlight enters through the top layer of the device (the transparent conducting oxide) and produces electrical current and voltage in the lower layers.

Current Research Areas

The primary research areas that are currently our focus are the following:

• Understanding effects of material and process choices. NCPV scientists have been key in helping industry understand how cost-effective, industrially relevant process choices can impact the ultimate performance and reliability of CIGS modules.
• Cell reliability for advanced device designs. Although the present CIGS designs can meet the required reliability goals for rigid modules, new CIGS products and markets are envisioned (e.g., flexible products) that may require a higher level of reliability to meet deployment requirements. The NCPV is pursuing new materials for inclusion into the cell structure that will meet these requirements.

Tools and Capabilities

We use the following in our CIGS thin-film cell research and development:

• Two 3"x3" co-evaporators with electron-impact ionization spectrometer (EIES) rate control for CIGS deposition
• Co-evaporator tool that is integrated with appropriate sputtering and analysis capabilities to study interface formation and industrially relevant processes
• 12"x12" system for ZnO and ZnO:Al deposition
• 1.5"x1.5" and 3"x3" sputter system for Cd₂SnO₄, Zn₂SnO₄, and CdS deposition.
• Suite of cell testing techniques, including current-voltage and quantum efficiency testing of thin-film devices
• Extensive collaboration with NCPV's groups involved in measurements and characterization as well as materials theory to study functionality of existing materials and devices, and develop ideas for new product avenues.
• CIGS cluster tool.

Projects

In the Manufacturing and Reliability Science for CIGS Photovoltaics project, our goal is to overcome the largest challenges to investor confidence and long product lifetime in CIGS: metastability, potential-induced degradation, and shading-induced hot spots. We need to maintain or improve efficiencies while eliminating damage due to shading-induced hot spots and minimizing metastability and potential-induced degradation (PID) to the low levels observed in successful silicon modules.

Using research-scale samples, we are documenting the magnitude of the effect, explaining the underlying physical phenomena, and providing solutions demonstrated to be effective on both NCPV and industrial partner samples.

Performance changes with light and voltage history. (Illustration from Solar Frontier)

Metastability. We are validating a stabilization procedure designed to minimize variations in repeated power measurements at standard test conditions caused by transient light-induced metastabilities in CIGS modules. Such metastable effects frustrate the repeatable and accurate measurement of a module's performance in the electrical state to which it stabilizes under normal operation outdoors.

For example, if a module is measured after a long period of storage in the dark, the power measured may yield different results from one made immediately after prolonged exposure to light. Short-term transients can be induced by perturbing the voltage bias or temperature. Another associated challenge is that elevated temperatures used for accelerated reliability testing of modules can excite transient changes that obscure permanent degradations.

Potential-Induced Degradation. PID is degradation that manifests in some photovoltaic modules primarily from the effects of system voltage stress and interactions with environmental factors such as humidity, water, soiling, light, and temperature. Mechanisms at the root of PID may include ion migration, especially sodium from the glass to or from the cell, leading to corrosion and charge redistribution; this may cause shunting and recombination of photo-generated carriers at the surfaces, defect complexes, or within the absorber layer.

Schematic cross-section for different types of CIGS module packages. We are characterizing them and trying to mitigate PID.

Our work surveys several thin-film module types in an environmental chamber and an outdoor test field for PID mechanisms. We are comparing observed degradation mechanisms, rates of degradation for stressing in-chamber, and Coulomb transfer rates for the various designs. We are collecting evidence of whether tracking of leakage current and Coulombs transferred can be reliably used for projecting power degradation for lifetime prediction or test-method standardization.

Shading-Induced Damage. Photovoltaic cells can be damaged by reverse-bias stress, which can occur in operation when a module is partially shaded. Partial shade in monolithic thin-film modules can cause reverse-bias operation, which leads to stress in temperature, current density, and voltage. The magnitude of this stress is sensitive to the transmissivity and extent of the mask causing the shade. Masks that leave portions of the masked cells illuminated lead to higher temperature and current density because of the preferential flow of current through the illuminated portions.

Visual (left) and electroluminescence (right) images of CIGS module damaged by partial shading.

The reverse breakdown of CIGS depends on illumination. The reverse-breakdown voltage of some CIGS cells is much smaller under illumination than in the dark. The physical origin of this light-enhanced reverse-breakdown (LERB) effect, which is different from illumination-dependent shunt current, is an active area of research. We have incorporated this LERB effect for the first time in a module-scale model, which we used to predict thermal and electrical stress due to partial shade.

Comparative Cell Analysis. High-efficiency CIGS-based devices can be made by many different manufacturing techniques. In fact, recent world-record CIGS solar cells have been made by different processes. To study this further, a team that included the NCPV, universities, and industrial partners has characterized devices from multiple solar cell fabricators to gain insight into their similarities and differences. The goal was to identify the strengths of each method and to connect process details to the observed characteristics. Losses were quantified to show how each device's performance departed from an ideal device. Through this investigation, we sought to 1) learn from comparisons between device structures and absorber processes and 2) approach tangible solutions for closing efficiency gaps between lab cells and large-scale manufacturing.

Band-Edge Effects. CIGS device efficiency is affected by band-edge effects such as grading, electrostatic fluctuations, bandgap fluctuations, and band tails. Studying a variety of NCPV absorbers, researchers have examined aspects of the relationship between band-edge phenomena and device performance. Recent increases in diffusion length of CIGS devices imply changes to the optimum bandgap profile, and CIGS shows evidence of electrostatic and bandgap fluctuations, within grains and at grain boundaries. The NCPV advances in CIGS device performance use changes in the bandgap grading and manipulation of potential fluctuations via post-deposition treatment. Controlling these band-edge effects in new materials is needed to surpass the 20%-plus efficiency level.

Working with Us

Visit Working with Us to learn more about NCPV's PV partnership opportunities. Contact us for specific information on the NCPV's R&D in the area of CIGS materials and devices.