cell reference temperature in kelvin, TRef = 298 oK (25 oC), G
is the solar insolation in W/m2 and Gr represents the reference
solar radiation W/m2, Gr = 1kW/m2. Short-circuit current is
always measured under standard test conditions of reference
temperature 25oC and solar radiation of 1kW/m2 [4] and [5-7]