Short-circuit current, maximum power, and open circuit voltage during a single flash are determined by varying intensity, voltage, and current. An apparatus determines the substrate doping and the series resistance of the solar cell. The series resistance of the cell is determined from a voltage step from the maximum power voltage operating point to the open-circuit condition. Methods are described for determining the substrate doping from stepping or sweeping the voltage. The first uses a voltage step and finds the change in charge that results. This determines a unique doping if the series resistance is known. The second uses data for a case of varying current, voltage, and light intensity, and compares this data to the case of varying voltage and intensity with no current. By transposing both cases into the steady state, agreement between the two data sets is found for unique doping and series resistance values.