Characteristic Resistance

Source: FasterCapital

The Characteristic Resistance of a Solar Cell

The characteristic resistance of a solar cell is a crucial parameter that determines the cell’s output resistance at its maximum power point. When the load resistance matches the characteristic resistance of the solar cell, the maximum power is efficiently transferred to the load, allowing the solar cell to operate at its peak performance level.

Understanding Characteristic Resistance

The characteristic resistance, denoted as R_CH, is the inverse of the slope of the line representing the voltage at the maximum power point (V_MP) divided by the current at the maximum power point (I_MP). In most cases, R_CH can be approximated by the open-circuit voltage (V_OC) divided by the short-circuit current (I_SC):

R_CH = V_MP / I_MP ≈ V_OC / I_SC

The units of R_CH are typically in ohms (Ω) when using I_MP or I_SC, but when current density (J_MP or J_SC) is used, the units of R_CH are in ohm cm².

Application in Solar Cell Analysis

The characteristic resistance is valuable for analyzing solar cells as it provides insights into series and shunt resistance. For instance, commercial silicon solar cells are known for being high current and low voltage devices. A standard 156 mm square solar cell typically exhibits a current of 9 to 10 amps and a maximum power point voltage of 0.6 volts, resulting in a characteristic resistance of 0.067 Ω. In contrast, a 72-cell module made from these cells may have a higher R_CH of 4 to 5 ohms.

Series Resistance and Power Loss

The characteristic resistance also facilitates the conversion between fractional power loss and series resistance in ohms or ohm cm²:

R_series = f × R_CH

f = R_series / R_CH

Here, f represents the fraction of power loss ranging from 0 to 1. Both R_series and R_CH share the same units, either ohms or ohm cm².

Shunt Resistance and Power Loss

Similarly, the shunt resistance can be linked to power loss through the equation:

R_shunt = f × R_CH

f = R_CH / R_shunt

Where f indicates the fraction of power loss from 0 to 1. The units of R_shunt align with those of R_CH, either ohms or ohm cm².

Source: Electrical Academia