CdTe Thin Film Solar Cell Characteristics as a Function

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					                                                         CdTe Thin Film Solar Cell Characteristics
                                                              as a Function of Temperature                                                                                                                                                                                                                                                                          REU 2006
                                                 Andrew Quecan, Sara Harrison, Vijay Padma, and Dr. Chris Ferekides                                                                                                                                                                                                                                                Poster#: EE.5

      As energy costs continue to rise and sources of energy still have a profound impact on international politics, alternative energy development is an important, if not essential, research discipline. Studied by The Lewis
Group at The California Institute of Technology, on a global scale, solar energy has the potential to practically produce over six hundred terawatts of power. Of course, individual steps to this end goal are being researched
each day. To develop more cost-efficient CdTe (cadmium telluride) solar cells, the characteristics of those cells are needed to enhance overall understanding. To acquire a range of CdTe characteristics such as IV curves and
fill factors, a vacuum-chambered cylindrical device was developed for testing a range of temperatures (100K-400K). The device uses a liquid nitrogen pump and flexible heaters to vary the temperature of the solar cell while
conducting four-point measurements to obtain the needed data for IV curves, fill factors, and other useful characteristics. As expected, the open-circuit voltage of CdTe cells increased as temperature decreased. As more
cells with are tested with varying light intensities, more characteristics will be attained to lead to a more efficient solar cell.

CdTe SOLAR CELLS                                                                                                 METHODOLOGY
    Cadmium telluride (CdTe) solar cells are heterojunction semiconductors, heavily doped, so that it            1. A single solar cell is placed within the vacuum chamber device and connected to a four-point instrument.
behaves as a one-sided pn junction. Since Na >> Nd , for a p-type semiconductor like CdTe, the total space       2. The chamber is sealed and a rough vacuum with a rotary vane pump is created to remove any moisture
charge width is approximately equal to the space charge width in the low-doped (n-region) space charge             within the device.
width. Using these assumptions, junction capacitance reduces to:                                                 3. Testing begins after flexible heaters raise the cell’s temperature to approximately 400K. A liquid nitrogen

                                  (1 / C )  2(V  V ) / e N
                                         2                                                                         pump, connected to the chamber, gradually reduces the temperature of the cell. Temperature is reduced by
                                                   bi     R       s d                                              varying the speed of the liquid nitrogen pump. Temperature is increased by varying the voltage of the
Therefore, determination of capacitance will lead to doping concentration.                                         flexible heaters.
    Solar cell performance characteristics and efficiency are given by VOC, ISC, and fill factor (FF). Open      4. The J-V data is collected using a four-probe measurement technique and a computer controlled
circuit voltage (VOC) occurs when I=0. Conversely, short circuit current (ISC) occurs when V=0. In addition,       electrometer/power supply. Measurements are taken in intervals of 10K. A DC voltage sweep is applied at
optimal performance is when power is maximum PM=IMVM. Fill factor relates the rectangular areas formed             each interval for voltages from -1.5V to 1.5V.
as a ratio:
                                     FF  I M VM / I SCVOC                                                       5. To obtain light J-V characteristics of the solar cells, a light source is introduced after initial voltage is
                                                                                                                   applied. The light source is held constant during the DC voltage sweep.
    As shown in Fig. 1 below, a CdTe solar cell is fabricated on a glass substrate. The thin-film (SnO2:F)       6. To obtain C-V characteristics of the solar cells, a frequency of 3KHz is applied during a voltage sweep from
increases photoconductivity and reduces reflection. Cadmium sulfide (CdS) is part of the heterojunction.           -2.5V to .5V. Same process described above is used to obtain light C-V characteristics.                 1.00E+15
                      Fig. 1                                                                                                                                                                                                                                                                                                                                                    9.00E+14

                                                                                                                                          Saturated Current [Amps]

                                                                                                                                                                                                                                                                                                 Doping Concentration [cm-3]
                                                                                                                                                                                 0.0025   0.0027     0.0029    0.0031   0.0033   0.0035       0.0037   0.0039
                                                                                                                                                                                                                                                                                                                                                                                8.00E+14       50
                      Configura-   ISC                                                                                                                                   1.00E-08                                                                                                                                                                                                              40
                                                              PM                                                                                                                                                                                                                                                                                                                7.00E+14
                      tion of a                                                                                                                                          1.00E-09                                                                                                                                                                                               6.00E+14
                      thin-film                                                                                                                                          1.00E-10                                                                                                                                                                                               5.00E+14       0

                      CdTe solar                                                                                                                                         1.00E-11                                                                                                                                                                                               4.00E+14
                      cell                                                                                                                                               1.00E-12
                                                                                                                                                                                                                                                                                                                                                                                3.00E+14       -30
                                                                                                                                                                                                    Temperature [K ]                                                                                                                                                            2.00E+14       -40

                                                                                                                                                                                                                                                                104                                                                                                                            -50
                      Fig. 2                                                                                                                 0.1
                                                                                                                                                                     0          0.2           0.4             0.6          0.8            1              1.2    58
                                                                                                                                                                                                                                                                                                                                                                                0.00E+00       -70
                      IV and FF                                    VOC                                                                    0.01
                                                                                                                                                                                                                                                                                                                                   -2          -1.5           -1         -0.5              0   -75

                                                                                                                                                                                                                                                                          Temperature (ºC)
                                                                                                                                                                                                                                                                                                                                                       Voltage [Volts]                         Room
                                                                                                                                                                                                                                                                20                                                   1.4

                                                                                                                     Current [Amps]
                                                                                                                                                                                                                                                                10                                                   1.2

DEVICE DEVELOPMENT                                                                                                                      0.0001                                y = 5E-09e22.583x

                                                                                                                                                                                                                                                                                             VOC [Volts]
     Initial design considerations included: a clean environment created by a low pressure vacuum                                      0.00001
 environment, cooling and heating system, variability of the intensity of light, and four-point measurements                                                                                                                                                    -31
 for J-V data. A T-shaped vacuum chamber was selected to alternate the three main variables on each                                   0.000001
                                                                                                                                                                                                                                                                -50                                                  0.2

 flange: voltage, temperature, and light intensity. Each flange was adjusted to provide for these variables:                      0.0000001
                                                                                                                                                                                                                                                                                                                                   150   200          250          300   350        400

 a glass flange was made for light, while the other two flanges were drilled and epoxied for the copper                                                                                             Voltage [Volts]                                             Expon.
                                                                                                                                                                                                                                                                (104_2)                                                                               Temperature [K]

 tubes and wires. A brass cylinder was selected primarily for its thermal conductivity and designed with a
 testing platform for the solar cells and probes. Copper tubing was installed within the cylinder and is used     SUMMARY
 as the cooling system while the liquid nitrogen flows through the tubing and pumping system. Flexible                  In order to obtain a better understanding of cadmium telluride solar cells, a cylindrical vacuum-
 heaters were wrapped around the cylinder to provide a heating mechanism. The cylinder is placed inside             chambered device was developed to measure a range of CdTe characteristics. Experimental results
 the vacuum chamber, which is pressurized by a rotary vane pump. A four-point measurement system was                support the theory that the open-circuit voltage of CdTe cells will increase as temperature decreases.
 developed after designing probes that would be sufficient for solar cells.                                         Further testing is needed on CdTe cells in order to acquire their complete range of characteristics. A
                                                                                                                    better understanding of CdTe cells will lead to the development of a more cost-efficient solar cell.
                                           Presented at the 4th Annual USF College of Engineering Undergraduate Research Symposium
                                                                              Tampa, FL April 6, 2006
                                                       Department of Electrical Engineering, Clean Energy Research Center
                                                                   University of South Florida, Tampa, FL 33620