3 Description of laboratory research facility and methodology of measurements

Figure 28 – photocells’ CVC at different light intensities.

Figure 29 – Scheme of work 6-10 research facility

Scheme of the research facility represented on a Fig. 29. As the source of light we have same LED as in work 6-9 with known dependence of light emission power P_LIGHT from supply current I_LED. By varying resistance on resistors bank R₃ we change current I_LED and, consequently, emission power P_LIGHT. Task for this work is to measure light CVCs of studying photodiode for two light intensities of LED (two values of I_LED), determine short-circuit current I_SC and open-circuit voltage V_OC, calculate generated power and determine optimal load(maximum efficiency mode) for given photocell. For this purpose in a photodiode circuit we have switched voltmeter and shunt resistance bank R_sh. As the voltmeter has its input resistance R_V the common load resistance:

.

By varying R_sh (at fixed I_LED) we change photocurrent I_PD and measure photovoltage U. Generated power can be calculated by formula

,

and efficiency

,

where P_LIGHT must be taken from work 6-9. Maximum of η(R_LOAD) dependency gives as optimal load resistance R_OPT.

4 Work execution order and experimental data analysis

1. Mount scheme Fig. 29.

2. Turn on power, and, by varying resistance R₃ set the value I_LED = 10 mA.

3. On the shunt resistance bank R_sh set first value given in the table of measurements (see sample protocol below) and measure photovoltage U.

4. Do p.3 for all values of R_sh given in the table for I_LED = 10 mA.

5. Set I_LED = 30 mA.

6. Do p.3 for all values of R_sh given in the table for I_LED = 30 mA.

7. Calculate load resistance.

8. Calculate photocurrent I_PD and generated power P.

9. From work 6-9 take values of P_LIGHT corresponding to I_LED = 10 mA and I_LED = 30 mA and calculate efficiency η of the photocell.

10. Plot I_PD(U) dependence for each value of I_LED and determine shot-circuit current I_SC and open-circuit voltage V_OC.

11. Plot η(R_LOAD) dependence for each value of I_LED and determine optimal load for each case.

12. Make conclusion about which mode of operation of the photocell is more efficient.

5 Control questions

1. How the photoeffect occurs in a p-n junction?

2. What critical modes of operation of a photocell are possible?

3. How the short-circuit photocurrent depends on light intensity?

4. How the open-circuit photovoltage depends on light intensity?

5. What is photocell’s optimal load?

6. What is efficiency of the photocell?

7. What is photoeffect red border?

8. Describe the light and the dark CVCs of p-n junction.

7 Content of the report

Laboratory work № 6-10 ι. Homework.

(answer on control questions from p. )

…

Ιι. Laboratory work № 6-10 implementation protocol.

1) Topic:

Studying of a photoeffect in a p-n junction

2) Goal: Studying the photoeffect in a p-n junction and measuring of dark and light current-voltage characteristics.

3) Scheme of laboratory research facility:

B - supply battery mA – miliamperemeter; R3 – 1 – 9999 Ω resistors bank; LED – light emitting diode; PD – photodiode; Rsh – shunt resistance bank; V – voltmeter.

4) Table of measuring instruments:

№	Name	Type	Serial №	Grid limit	Grid unit	Accuracy class
1.	Milliamperemeter	М906		25 mА	1 mА	1 %
2.	Voltmeter(multimeter)	DT-830B		200 mV	0.1 mV	0.5 %
				2000 mV	1 mV
3.	Resistors bank R3	Р33	-	9999 Ω	1 Ω	0,2 %
4.	Shunt resistor Rsh (resistors bank R3)	Р33	-	9990 Ω	10 Ω	0,2 %

Accesories: the LED, the photodiode.