The basic function of a photovoltaic cell is to convert input — sunlight energy expressed in irradiance (W/m2) — into output — useable electrical power — with as little loss as possible. To quantify the ability of the system to accomplish this conversion, one can simply compare the output to the input by forming a ratio of the two. This ratio, expressed in percentages, is known as the power conversion efficiency (PCE) of the device and it is a key parameter of electrical performance. Since the PCE is used to compare the performance of photovoltaic devices, it is critical that accurate estimates be made for the PCE. The estimate is dependent on knowing, with a high degree of accuracy, the actual conditions, including irradiance and cell temperature, under which the parameter is measured.
Irradiance is normally measured in the field with an irradiance sensor, or in a simulated environment using a solar reference cell. These devices are calibrated using the American Society for Testing and Materials (ASTM) and/or International Electrotechnical Commission standards. Calibration services are offered by a number of labs, including Newport Corporation’s Technology and Applications Center (TAC) PV Lab; the National Renewable Energy Lab oratory’s (NREL) Device Performance Group; Sandia’s Photo voltaic System Evaluation Laboratory (PSEL); the Fraunhofer Institute for Solar Energy (ISE) in Freiburg, Germany; and others.
Standard conditions for calibration are 25°C and 1000 W/m2 of sunlight or simulated sunlight. The Newport TACPV Lab is ISO-17025 certified and A2LA accredited. We use simulated sunlight produced by a solar simulator, and the source of light in the solar simulator is a Xenon arc bulb. One of the challenges to making accurate calibrations is reproducing the standard conditions for a variety of photovoltaic materials in a variety of sizes and shapes.
A typical solar reference cell is a 2cm × 2cm solar cell packaged in a metal housing and protected under a glass or fused silica window (Figure 1). Terminals for interfacing with a digital multi-meter are built into the package, and a temperature sensor is required for measuring the temperature of the cell. One of the important electrical performance parameters for a solar reference cell is its short-circuit current. The packaged cell can be used as a reference cell when its short-circuit current is known within some degree of accuracy at the standard conditions (25°C and 1000 W/m2 of sunlight). Since the reference solar cell is a linear device, it can then be used to measure the irradiance under other combinations of temperature and irradiance, if the temperature coefficients are known.