Understanding the Influence of Belt Furnaces and Firing Parameters on the Efficiency of Thin Film CIGS Solar Cells
Thin film photovoltaic¡¯s is a promising path for achieving higher efficiencies at lower costs. With advancements in materials and processing technologies, thin film photovoltaic¡¯s has been gaining significant importance in solar cell technology. Betwixt the various thin film technologies, CIGS solar cells have been reported as being the most efficient. Currently, extensive efforts are underway for improving the efficiency of CIGS solar cells while considerably increasing its production volume. This exertion will explain the processing of CIGS solar cells and the importance of the firing process. In addition, the study will shine light on an ideal belt furnace for selenization of CIGS solar cells.
The ever increasing demand and growing cost of silicon has made thin film photovoltaic¡¯s a competing substitute to silicon solar. Thin film photovoltaic (PV) modules are seen as a realistic alternative for a cost effective generation of electricity from sunlight. They are often referred to as the second generation of photovoltaic technology. Some promising materials for thin film solar cells include amorphous silicon, cadmium telluride, CuInSe 2 and its alloys. While all of these materials are low band gap, they are all polycrystalline as well leading to a loss in efficiency due to grain boundary recombination. Amid the polycrystalline thin film solar, CIGS solar cells have been documented in various pieces of literature as being unrivalled in efficiency.
A typical fabrication of a CIGS solar cell encompasses the sputtered deposition of molybdenum back contact material on a substrate. The substrate material can be a rigid soda lime glass or a flexible polyimide. The sputtering of the back contact is followed by the deposition of the CIGS absorber layer, CDS buffer layer and the final Zno contact layer. Molybdenum is primarily used as a back contact due to its ability to form an ohmic contact, as well as, inert behavior to corrosive gasses. Cadmium sulphide is used with CIGS material to form an n-type semiconductor material along with a p-type CIGS absorber material. Some of the widely used methods for depositing CdS include chemical bath deposition (CBD), sputtering and closed space sublimation (CSS). Zno is used as the front contact material due to its superior electrical and optical properties. Figure 1 illustrates a material stack up of the CIGS solar cell.
Figure 1: Material Stack up of CIGS solar Cell
Processing CIGS in a Belt Furnace
The fabrication of CIGS solar cells within a production environment contains the deposition of copper, indium and gallium on the selected substrate material while annealing them at elevated temperatures in controlled atmospheres. First, the substrate material is cleaned and then heated to an elevated temperature. Afterwards, copper, indium, gallium and selenium are deposited through a sputtering process and then the deposited parts are selenized in an elevated temperature profile. The selenization process involves ramping up from room temperature to 450C in about 4 minute¡¯s. The samples are soaked at this temperature for 7 minutes and then elevated to 550C for approximately 4 minutes. The samples are then held at 550C for another 7 minutes and then cooled down. Finally, while held at an elevated temperature, hydrogen sulphide gas is introduced to sulphurization. A detailed profile is illustrated in Figure 2.
Figure 2: Selenization profile of CIGS solar Cell
Effect of Selenization Profile on the Efficiency of Solar Cell
The temperature and soaking times play two very important roles in determining the final efficiency of the CIGS solar cell. Many researches have revealed significant evidences for how the different sintering temperatures and soaking times have a direct impact on the efficiency outcome. The results of the experiment by Kadam et al demonstrate that a sintering temperature above 500 oC and a soaking period between 30 and 60 minutes can improve the efficiency of the cell significantly.
In his experiments, when the samples were selenized at 400 C for 10 minutes with a temperature ramp of 6C per minute, it resulted in smaller, non-faceted grains. This is an indication that either the temperature or the soaking time is inadequate. When the soaking time was increased from 10 to 15 minutes, no significant improvements were noted in the results. Therefore, the ramp rates can be construed as having a larger impact on efficiency. In addition, Kadam et al studied the impact of various heating rates. Changes in the parameters were made: heating rate was increased from 6 C per minute to 20 C per minute and the temperature was increased to 425 C with a soak period of 30 minutes. The resulting solar cell film appeared to be more homogeneous. Furthermore, the temperature was increased to see the effects on the grain size and cell efficiency, which indicated that at 475 C, the grain size ranged from 0.5 um to 1.5 um with an efficiency of 5.56%.
|Condition||Grain size||Film thickness||Efficiency||Comments|
|Temp, Soak time|
|400 C, 10 min||Varies from <1-2 um||5 um||<1%||Inadequate parameters|
|475 C, 30 min||0.5 ¨C 1.5 um||3.5-3.6 um||5.56%||Better efficiency obtained|
|500 C, 30 min||2 um||2.7-2.8 um, rough surface||4.80%||Non-uniform grain sizes|
|500 C, 60 min||>1 um, well faceted||2.6 um||9%||High efficiency|
|515 C, 50 min||>1 um, highly faceted||2.6-2.7 um||8%||High efficiency|
At the same time, they detected that increasing the peak temperatures above 500 C could also help with improving efficiency. However, at higher temperatures the grains become well faceted and less thick. It is fair to conclude that high quality CIGS solar cells should be prepared at temperatures above 500 C with an ideal soaking time between 30 and 60 minutes. Understanding that the heat rate, temperature, and soaking times are important factors in determining the ultimate cell efficiency, a belt furnace with a wide range of firing temperatures and a fast heat rate design are desired for constructing highly efficient solar cells.
Belt Furnace for CIGS Processing
A furnace that processes CIGS solar cells should be capable of operating to 650C or higher. The aim is for a wider belt with superior cross belt uniformity so that larger substrates can be contained in the future. More over, a muffle is required to ensure a cleaner operating environment because various processes in CIGS processing, such as Sulpharization, involves the introduction of different gasses at different time periods. The muffle needs to have the capability to control the gas type and gas flow in each zone. With all of this in mind, Torrey Hills Technologies has designed a furnace that is well suited to match the requirements for thin film solar applications. Figure 4 illustrates a firing furnace that is capable of processing CIGS solar cells and Table 3 lists the technical specifications for the ideal CIGS furnace.
The designed HSA furnace uses ceramic heater boards in order to achieve elevated temperatures. Aside from the standard belt size of 350 and 650mm, wider belts have been designed as well to accommodate wider glass substrates. While a 700mm wide belt has been successfully engineered, efforts are underway to build a belt as wide as 1000mm. As a standard feature, this furnace is equipped with a steel brush for cleaning the conveyor belt, however, Ultrasonic belt cleaning is available as an extra option.
A microprocessor based PID controller is what controls the furnace. Type K thermo-couples are used in determining the zone temperatures and the controls are located on the right hand side of the furnace which can be viewed from the entrance. The central processing unit (CPU) is mounted under the exit table and the CPU is primed with a Windows operating system for easy computing. The computer system is pre-installed with a program for controlling the furnace parameters, including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved as well for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly into the microprocessor. This feature allows for the monitoring and recording of actual temperatures experienced by the part. Software is also included with the computer to capture, display, printout and store the furnace profile. Additionally, the furnace is equipped with a redundant overheat safety protection system which incorporates an additional type K thermocouple in the center of each controlled zone and the multi-loop alarm. The specification of a HSA 7503 belt furnace is listed in Table 2.
Figure 3: CIGS Sintering Furnace Requirement
Figure 4: HSA Series Belt Furnace
|Rate Temperature||900 deg. C max|
|Normal operating||850 deg C|
|Belt Width||30 inch ± (750mm)|
|Effective Above Belt Clearance||1 inch ± (25mm)|
|Conveyor Speed||2-8 IPM (40-200mm/min)|
|Loading Table||59 inch ± (1500mm)|
|Unloading Table||59 inch ± (1500mm)|
|Belt||Balanced V Weave, SUS314|
|Heating Elements||FEC Heater Board|
|Insulation||High quality ceramic fiber|
|Temperature Controller||Intelligent PID Shimaden Controller|
|Alarm||Thermocouple, Over Temp, Belt Stop. Audio and Visual Alarm|
|Atmosphere||6 pipes of dry clean air or N 2 . 2-6 m 3 /h, 1.1-3.3 CFM|
|Cooling||Forced Air cooling|
|Across Belt Temperature Uniformity||+/- 4 deg C|
|Overall System Width||60.5" (1540mm)|
|Overall System Length||343 inch ± (8705mm)|
|Overall System Height||54 inch ± (1,350mm)|
|Power||Three-phase, 480VAC, 60Hz, 38 KVA Max|
|Normal operating power draw is about 15 KVA|
Table 2: Specification of HSA7503 belt furnace for CIGS Processing
The selenization process has a significant impact on the efficiency of CIGS solar cells. The proposed HSA furnace has the capabilities to elevate at a much higher temperature, maintaining cross belt uniformity and achieving the required profile. At the same time, the muffle design allows the introduction of specific gasses in specific firing zones per requirement. In short, the HSA series is ideal for the selenization process in thin film photovoltaics.
Edwards, B, M., ¡°Screen and Stencil Print Technologies for Industrial N-Type Silicon Solar Cells¡±, PhD Thesis, University of New South Wales, Australia.
Kadam, A. A., ¡°PREPARATION OF EFFICIENT CuIn1-xGaxSe2-ySy/CDS THIN-FILM SOLAR CELLS BY OPTIMIZING THE MOLYBDENUM BACK CONTACT AND USING DIETHYLSELENIDE AS SELENIUM PRECURSOR¡±. University of Central Florida, 2006, pp. 80-120.
Kapur, V. K., Bansal, A., Le, P., ¡°NON-VACUUM PROCESSING OF CIGS SOLAR CELLS ON FLEXIBLE POLYMERIC SUBSTRATES.¡± International Solar Electric Technology, Inc. (ISET), Inglewood, California, 2003
Annual Technical Report, ¡°LAB TO LARGE SCALE TRANSITION FOR ON-VACUUM THIN FILM CIGS SOLAR CELLS.¡± International Solar Electric Technology, Inc. (ISET), Inglewood, California. NREL Subcontract No. XCQ-230630-30.
Dwyer, D., Repins, I., Efstathiadis, H., Haldar, P., ¡°DEPOSITION OF CuInAlSe2 FILMS USING CO-SPUTTERED PRECURSORS AND SELENIZATION¡±, College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany.