Polycrystalline CdTe-based Photovoltaics

The major emphasis in our group is on the fabrication of thin-film CdTe-based solar cells on glass substrates. In this effort we fabricate complete solar cell structures starting with soda-lime glass substrates coated with a transparent conducting oxide usually SnO2:F. We often use commercial "Low-E" glass from Pilkington as the starting substrate. Our deposition process then uses radio-frequency sputtering (13.56 MHz) to grow a CdS layer about 150 nm thick and a CdTe layer about 2000 nm thick. The cell is completed by evaporation of a thin metal contact layer on the CdTe surface.


The solar cell structure is shown in the sketch. Notice that the cell has a superstrate configuration so that in operation the cell is placed (upside down) with the thin films protected under the glass and sunlight passing first through the glass.

 



On the commercial Low-E glass, we have fabricated a cell that was tested at the National Renewable Energy Lab (NREL) at 12.6% under one-sun illumination.  The second figure shows the test results.  The left curve is the current-voltage relationship which indicates an open-circuit voltage of  Voc = 0.8204 V and a short-circuit current density of Jsc = 20.7 milliamps/sq.cm.  The maximum power is Pmax = 1.8833 milliwatts which yields a “fill factor” of 73.96%.  (Fill factor is defined as the product of Voc x Jsc divided by Pmax.)

Recently we have fabricated a cell on aluminosilicate glass (Corning 1737) using our own sputtered ZnO:Al.  One of these cells has recently been tested at NREL with an overall efficiency of 14.0% at one-sun illmination.  The structure of this cell is shown in the figure.
  The official NREL cell tests are shown below.


 



The second emphasis is on the optimization of laser scribing for the fabrication of monolithic cell interconnections. Scribing, either with a mechanical stylus or a laser is used for fabricating series interconnections allowing a large, thin-film photovoltaic panel to be made with, for example, 100 cells monolithically interconnected. The series interconnection allows the same electrical power to be produced from the panel with a voltage 100 times that of an individual cell and the current equal to that of one cell. For CdTe thin films, the individual cell voltage is about 0.8 V. The figure shows the typical structure of a series interconnection—in this case it is for cells made on a polymer substrate (rather than a superstrate such as glass). Notice that the interconnection process here utilizes three scribes--one in the molybdenum back contact, one through the CdTe and CdS, and one in the top, transparent contact layer (ZnO:Al). In recent work we have studied and compared the results of several different lasers for laser scribing.  These lasers are: three different types of Nd:YAG lasers, a copper vapor laser, an XeCl excimer laser, and a KrF excimer laser.   

 


Personnel:

Faculty members: Alvin Compaan, Randy Bohn, Victor Karpov, Dean Giolando
Research Assistant Professor: Akhlesh Gupta

Postdoctoral fellows: Diana Shvydka, Sunghyun Lee, Sanli Wang
Research Lab technician: Robert Burmeister
Graduate students:  Jennifer Drayton, Xiangxin Liu, Karthik Allada, Viral Parikh, Anthony Vasko

Undergraduate students: Levi Gorrell, Melissa Haugen, Chris Verzella