College Papers

1.1.1. proper calibration. The LED dosimeter will have

1.1.1.      (C-V) Characteristics

         From (C-V) measurements, at different gamma exposure levels, it is clearly seen that (Fig. 5), the diode diffusion capacitance decreases linearly with the increase in the gamma dose. Such behavior of the LEDs indicates that these devices can be successfully employed to measure low radiation doses after proper calibration. The LED dosimeter will have the following advantages:

        i.            it can be used in routine dosimetry due to its cheapness;

      ii.            it has a remarkably high sensitivity to gamma doses over a wide dose range;

    iii.            it is easy to handle and small in size; the smaller the dosimeter the higher the accuracy of dose measurements;

    iv.            it is commercially available in the local market.

Fig. (5): Gamma-dose dependence of green LED capacitance.

1.1.1.1.      Annealing of LED Devices

          The term annealing is usually used for thermal defect modification and the changes in the damage with time 9. Following irradiation, the damaged samples were subjected to annealing procedures; the group of green and red samples shown in Fig. (6), irradiated up to 2 MGy, were kept at room temperature (20-30 °C). The output characteristics of the devices were plotted daily up to 40 days, and then followed weekly up to total storage time of 240 days. It is observed clearly that the room temperature recovery in the value of the emitted light intensity is small. After 240 days the green and red samples recover around 19.2% and 23.5% of their original values, respectively.

0

50

100

150

200

250

0

10

20

30

40

50

Storage Temperature = 20 – 30

  O

C

Intensity Recovery, %

Storage Time, Days

 Green LED

 Red LED

Fig. (6): Shelf annealing of green – and red – LEDs – irradiated up to 2.0 MGy.

1.2.            Photovoltaic Cells           

1.2.1.      Photo Voltage Decay Technique

         A plot of the photo voltage decay versus time for a device in the decay mode will have up to three distinct regions, ideally, as shown in Fig. (7).The first region (I) corresponds to a condition of high-level injection, where the excess minority carrier concentration exceeds the equilibrium majority carrier concentration in the base region of the cell. When this condition is met, the decay curve is linear, and the minority carrier lifetime (?) may be derived using the following expression 10-12:

                                                      (2)

Where, Voc : open circuit voltage and t : time.

         The second region (II) of the decay curve corresponds to a condition of intermediate injection, where the excess minority carrier concentration in the base is greater than the thermal – equilibrium minority carrier concentration but less than the thermal equilibrium majority carrier concentration. Under these conditions, the decay curve is again linear, and the lifetime can be computed from the following expression.

                                                        (3)

           Finally, in the third region (III) of the decay curve, a low injection condition exists, where the excess minority carrier concentration is less than the equilibrium minority carrier concentration. Here, as Voc becomes much less than kT/q, the Voc decay approaches exponential time dependence:

                                         (4)

Where, V(o) is the open circuit voltage at termination of excitation.

Fig. (7): Theoretical photo voltage decay curve.

 

This analysis is based on the assumption of a one-sided junction so that the contribution to the photo voltage from the heavily doped emitter is negligible. It also requires that the amount of excess charge stored in the space-charge region be negligible compared to that stored in the base; the decay is then characterized by the base minority carrier lifetime. These conditions are met by most silicon photovoltaic cells of interest.

1.2.2.      Photovoltaic Samples

Fabricated photovoltaic samples. Silicon photovoltaic cells with different base resistivity doping levels were fabricated 13, where the starting materials were single-crystal, p-type silicon wafers of 10 cm2 area, with orientation <111>, and with an acceptor concentration of 5 x 1015 cm-3.

Commercial photovoltaic cells. In addition to the locally fabricated photovoltaic cells, the study was also extended to include a set of commercially available cells. The cells were collected from the following different manufacturers:

·              Optical Coating Laboratories, Inc., OCLI,

·              Solarex Corporation,

·              Sargent-Welch, and

·              Semicon, Inc.

1.2.3.      Measuring Technique.

            A schematic diagram of the experimental setup used for minority carrier lifetime measurement is shown in Fig. (8), where, the light source is an electronic stroboscope. The strobe flash effectively meets the requirement of being an abruptly terminated excitation source since its fall time is very short relative to the Voc decay times observed in this work. The strobe intensity at a distance of 10.0 cm is equivalent to approximately ten times full solar intensity level (= 1350 klux).

     Photo voltage decay curve was monitored with a Tektronix oscilloscope utilizing a high gain differential amplifier plug-in unit. To ensure that the photovoltaic cell under test was not being loaded down by the measuring circuit, high input impedance, low output impedance buffer (operational amplifier in the unity gain configuration) was used.

Fig. (8): Photo-induced voltage decay measuring system.

1.2.4.      Initial Characteristics

            The lifetime of the minority carries was calculated by application of the photo voltage decay technique. Fig. (9) gives an indication of a typical oscilloscope tracing of the induced photo voltage decay for an ”n/p” silicon photovoltaic cell. On the other hand, Fig. (10) shows the photo-induced voltage decay for the silicon photovoltaic cells made with different base resistivity values of 10, 5.0, 1.0 and 0.30 (ohm.cm). Region (I) is observed only in the cells with high base resistivity, where the source intensity is sufficient to produce the high injection condition. Region (II) can be masked by a slow rate of discharge of the stored charge in the junction depletion region. Its discharge rate is inversely proportional to the effective ”RC” time constant of the cell, where C and R are the junction capacitance and resistance, respectively. Because the cell resistance increases by an order of magnitude as the base minority carrier density approaches its dark thermal equilibrium level, this discharge of junction capacitance can slow down the last stage of the Voc decay. The behavior in region (III) is generally observed in silicon photovoltaic cells because the source intensity, which is insufficient to reach the high injection condition, is usually enough to excite the cells to an injection level which is substantially higher than that at which the discharge of the junction capacitance limits the Voc decay rate. The decay rate is determined from the slope of the linear or semi-linear parts of the Voc decay curves, as shown in the dashed lines in each of the pulses. Finally, Table (1) summarizes the room temperature (kT/q=0.025 eV) VOC decay rates and the calculated base minority carrier lifetime for the investigated samples.

Fig. (9): Oscilloscope tracing of the photo-induced voltage decay for  silicon photovoltaic cell.

 

Fig. (10):  Photo-induced voltage decay of silicon photovoltaic cells

fabricated with different base resistivity values.

 

Table (1): Photo-induced open circuit voltage decay rates and calculated lifetime values for silicon photovoltaic cells.

Cell Type

Base
Resistance,
ohm.cm

dVoc/dt,
V/sec

Minority carrier
Lifetime, µsec

Solarex

4040

6.00

OCLI

1340

19.00

Sargent-Welch

2352

20.00

Semicon, Inc.

1260

21.00

Locally Fabricated

 

 

 

 

10.0

1000

25.10

 

5.0

1360

19.00

 

1.0

2500

10.00

 

0.3

3650

7.00

 

1.2.5.      Radiation Effects

         The effect of different radiation types, fluence and energy on the minority carriers lifetime were studied, as shown in Fig. (11). It was found that the permanent radiation damage on silicon photovoltaic cells is attributed mainly to the change in the lifetime of minority carriers contained in the base region and consequently the mean of their diffusion length. The serious reduction in the lifetime (?), from its initial value down to a certain level, which depends on the radiation type, fluence and energy, obeys the relation 14:

                                                 (5) 

. Using the relation:

                                                                          (6)

then, Equation 4 will be:

                                                      (7)

                                                                             (8)

 

Where, KL (=Kt/D) is the diffusion length radiation damage constant and L, is the diffusion length before irradiation.

Fig. (11): Effect of electron fluence and energy on the minority carrier lifetime.

         The lifetime of the minority carries was calculated by application of the open circuit voltage decay technique. Noting that, there are several other effects which may influence the observed open circuit voltage decay behavior. If the cell has a low internal shunt resistance, it may not be in the open circuit condition even though a high impedance buffer amplifier is employed. This loading of the photovoltaic cell increases the observed decay rate and an underestimate of the lifetime is obtained. If the minority carrier lifetime depends upon the injection level and changes during the decay, then the simple decay behavior predicted in Fig. (7) will not be observed. Fig. (12) shows the open circuit voltage decay curves of different commercial photovoltaic cells, Sargent-Welch, OCLI,  Solarex and Semicon.

 

          Electrical noise from the stroboscope circuits may create initial transients in the oscilloscope trace that can sometimes be reduced through electrical shielding. Other rapid initial transients in the decay curve may occur, which are the manifestations of a high surface or interface recombination velocity at the illuminated surface, in the case of the vertically illuminated cells, and at the high-low junction in the case of the horizontal junction cells. Surface or interface recombination may also influence the decay behavior after the rapid initial transient and will affect the determination of lifetime. Also, non­uniform carrier generation in horizontal junction photovoltaic cells of relatively large area (>1.0 cm2) can cause an initially low VOC decay rate. During this initial period lateral currents flow in the base, the emitter region, and in the front contact grid to establish uniform injection conditions in the cell. After these initial fluctuations, the open circuit voltage decay settles down to at least an approximately linear decay that is governed by recombination of minority carriers in the base, subject to the other considerations mentioned above.

Fig. (12): Photo voltage decay curves of different commercial photovoltaic cells,

Sargent-Welch, OCLI,  Solarex and Semicon.

       Finally, Figs. (13 through 15) show the following:

–          Electron fluence on short circuit current (Isc), open circuit voltage (Voc) and conversion efficiency of silicon photovoltaic cells,

–          Dependence of diffusion length, lifetime, and output power and percentage reduction in short circuit current of silicon photovoltaic cells on neutron fluence.

–          Dependence of efficiency and short circuit current of silicon photovoltaic cells on neutron energy.

 

Fig. (13): Effects of electron fluence on short circuit current (Isc), open circuit voltage (Voc) and conversion efficiency of silicon photovoltaic cells.         

Fig. (14): Dependence of diffusion length, lifetime, and output power and percentage reduction in short circuit current of silicon photovoltaic cells on neutron fluence.

Fig. (15): Dependence of efficiency and short circuit current of silicon photovoltaic cells

on neutron energy.

Conclusions

          From the study, analysis and experimental and theoretical results, the following conclusions can be deduced:

–          Cobalt-60 irradiation of LED’s result in an increase followed by a decrease in the output light intensity, and brightness of the devices. The increase was due to low doses of a few kGy while doses above 100 Gy causes degradation of the photoemission. The changes are probably due to the introduction of defect levels into the diffusion region of the PN-junction.

–          Shelf annealing recovery was noticed to be after 240 days the devices recovered19.2% and 23.5%, for green and red ones, of their initial characteristics.

–          LEDs are sensitive to gamma radiation over a wide dose range and can substitute for a chemical dosimeter in the low-dose range. This can be considered a new possibility in the field of solid state dosimetry.

–          (C-V) measurements of gamma irradiated Varactors of the type BA102 showed that both the depletion – and the diffusion – capacitances are dose dependent for very high doses up to 1.30 MGy. Thus Varactor type BA102 could be used for high dose dosimetry. Under reverse bias, the Varactor maximum to minimum capacitance ratio increased from 2.7 at zero dose to 45 at 50 kGy which might be used to improve the Varactor performance in a tuning circuit.

–          The photo-induced open circuit voltage decay technique is a reliable and useful method for determining base minority carrier lifetime in photovoltaic cells.

–          The base resistivity of the photovoltaic cells controls to a large extent their characteristics.

–          The effect of radiation on photovoltaic cells performance is mainly due to the changes in lifetime of minority carriers contained in the base region.