Task 2　Application Technology of Solar Cell's Current-Voltage Curve

Ⅰ.Basic knowledge of solar cell's current-voltage curve

Silicon solar cells are semiconductor devices using semiconductor photovoltage effects. Light radiated on the battery presents a variety of situations. In order to maximize energy conversion efficiency，solar cells must be designed to maximize light absorption and that after reflection. When sunlight irradiates on the battery，voltage and current（voltampere characteristic）of the battery can be simply represented by characteristic curve shown in Fig.1-2-1，where U_OC is the open circuit voltage；I_SC being short circuit current；U_mp being the best working voltage；and I_mp being the best operating current.

The optimum operating point corresponds to maximum output of the battery，which is P_m. And its maximum is obtained by multiplying the optimum operating voltage and the optimum operating current. When actually used，battery is influenced by load and sunshine conditions. The operating point will deviate from the best working point.

（1）Open-circuit voltage U_OC

Disconnect load of PV battery circuit and measure voltage of two ends，that is，I=0. The voltage at this time is called open-circuit voltage.

（2）Short-circuit current I_SC

The current at both ends of photovoltaic cell is short-circuited，that is，U=0. Current at the moment is short-circuit current.

When the voltage of solar cell（component）rises，such as the resistance of load increases or the voltage of battery（component）increases from 0（short-circuit condition），the output power of battery（component）will grow from 0.

When voltage reaches a certain value and the power reaches it's maximum，if resistance continues to increase，the power will jump over the maximum point and gradually reduced to 0，that is，the voltage reaches the open-circuit voltage U_OC. And battery（component）output power reaches it's maximum point，which is called maximum power point；the corresponding voltage is called maximum power point voltage U_mp，also known as maximum operating voltage；the corresponding current is called maximum power point current，also known as maximum operating current I_mp；power of the point is called maximum power P_m.

Ⅱ.Calculation of solar cell load current

As shown in Fig.1-2-2，circuit and equivalent circuit of the solar cell.R_L is external load resistance of the battery. When R_L =0，measured current is the short-circuit current of battery. The so-called short-circuit current I_SC，is the current through solar cells when placed under standard light irradiation and the output end is short-circuited. The method of measuring shortcircuit current is to connect ammeter whose internal resistance is less than 1Ω to the ends of solar cell. The wider solar cells' area is，the larger the I_SC data are. In general，I_SC value of 1cm²silicon solar cell is about 16～30mA.

Under the irradiation of 100mW/cm²spectral radiation，open-circuit voltage of silicon solar cell is 450～600mV and the maximum is 690mV. When incident spectral irradiance changes，the opencircuit voltage of solar cell is proportional to the logarithm of incident spectral irradiance. When environment temperature rises，the open-circuit voltage value will decrease. For every 10℃ rise，U_OC value will decrease by 2～3mV.

I_D（diode current）is the total diffusion current across p-n junction and have an opposite direction from I_SC.R_S is a series resistor，which is mainly composed of battery's body resistance，surface resistance，electrode conductor resistance and contact resistance between electrode and silicon surface.

In the same solar cell，the I_SC is proportional to the irradiance of incident light. When environment temperature rises，I_SC value rises slightly. For every 10℃ rise，the I_SC value rises about 78μA. When R_L → ∞，the measured voltage is open-circuit voltage of the battery. The so-called open-circuit voltage U_OC is solar cell's output voltage when it is placed under 100mW/cm²light source illumination and both ends are open. Open-circuit voltage of the battery can be measured by DC millivolt meter with a high-resistance.

Open-circuit voltage of solar cell is directly related to spectral irradiance and irrelevant with the size of battery area.

R_sh is bleeder resistance which is caused by unclean wafer edge or defected body. An ideal solar cell has a small series resistance R_S and a very large parallel resistance R_sh Since R_S and R_sh are respectively connected in series and parallel in circuit，they are negligible when performing ideal circuit calculations. At this point，the current flowing through load is I_L：

I_L =I_SC -I_D

The ideal p-n junction characteristic curve equation is：

I_D ——saturation current of solar cells in the absence of light，A；

　q——electron charge，C；

　e——natural logarithmic bottom；

　K——Boltzmann constant；

　T——thermodynamics temperature，K；

　A——constant factor（When positive bias voltage is large A is 1，and when small A is 2）.

When I_L =0，voltage U is U_OC.

According to the above two equations，we can get the current-voltage curve of solar cells，as is shown in Fig.1-2-3. This curve can be named as I-U curve or volt-ampere curve for short.

In Fig.1-2-4，curve 1 is the I-U curve when the battery is not exposed to light；curve 2 is the I-U curve when the battery is exposed to light which can be obtained by moving curve 1 by I_SC value to the fourth quadrant. After coordinatesexchange，commonly used current-voltage characteristics curve of solar cells is formed，as shown in Fig.1-2-4.

The current-voltage characteristic curve of solar cell shows the relationship between current I_m and voltage U_m which transmits through solar cell（components）at a specific solar irradiance.

If the solar cell（components）circuit is short-circuited，that is U=0，then current is short-circuit current I_SC；if the circuit is open，that is I=0，voltage at this time is open circuit voltage U_OC. The output power of solar cell（components）is equal to the product of current and voltage flo wing through the battery（components），that is P=UI. Voltampere characteristics of solar panels are affected by solar irradiance，solar spectrum distribution and operating temperature of solar cells（components）. The standards are 1000W/m²spectral irradiance，AM1.5 spectrum；25℃ battery temperature. For condition limit，the volt-ampere characteristic curve tested by training equipment may differ from the standard.

Ⅲ.Solar panel's Vol-t ampere curve and test training of maximum power output point

①Switch on main control panel's air switch and start the control screen.

②Turn on the three simulating light source lamps by switching on “SD1，SD2，SD3” on the “Simulating light source” of main control panel and turn on switches of“SF1、SF2、SF3”to start all the cooling fans before that. Fix the battery plate at a certain posi tion（fixed irradiance）.

③Disconnect K1，K2 and connect training lines according to Fig.1-2-5.

④Measure data of voltage of the solar panel and its change with load resistance. When load resistance is zero，measured short-circuit current is I_{S C} and when load resistance is infinite，measured open circuit voltage is U_OC.

Specifically，by increasing load resistance or voltage of the battery（component），voltage will gradually increase from 0（short-circuit condition：R_L =0），which is increasing resistance value. As load resistance increases，current will gradually decrease，and terminal voltage of solar panel gradually increases. Current and voltage begin to show a linear relation. Record A₁ and V₁ values at different R_L values and draw volt-ampere characteristics curve.

【Exercise】

（1）Draw a volt-ampere characteristics curve according to training principles.

（2）Look up relevant information and find out what are differences between volt-ampere characteristics curve and standard volt-ampere characteristic curve? What are the reasons for these differences?

（3）Why the maximum of ideal factor can not reach 1?