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.
A significant source of uncertainty in PV testing is the lack of knowledge of the temperature, Tcell, at the space charge region of the cell. The package that houses a cell in a solar reference cell, for example, acts as a heat sink that cools the back of the cell (by conduction) faster than the top surface of the cell (by convection).This makes the bottom surface, generally speaking, a few degrees C cooler than the top surface of the cell. The built-in thermocouple in a solar reference cell measures the temperature at the back of the cell which is different from Tcell. A lack of knowledge of the temperature difference translates into uncertainty in the performance parameter, which is proportional to the temperature coefficient.
The difference in temperatures is apparent when the parameter (e.g. open-circuit voltage) is plotted against temperature (at the back of the cell) when the cell is heated or cooled (Figure 2). As the best estimate of the open circuit voltage, the two lines in Figure 2 can be extrapolated to 25°C using the temperature coefficient, and the midpoint of the two Voc intercepts at 25°C can be calculated. For more accurate measurement, we let the cell equilibrate at room temperature and measure the parameters using only momentary illumination (~ 1 sec). The measurement then is extrapolated to 25°C using their respective temperature coefficients.
Additional uncertainty (i.e. “error” ) arises from a mismatch of light source spectra and spectral response of the cell. This error source is known as the spectral mismatch error and is expressed in terms of the absolute deviation of the spectral mismatch factor M from unity. The mismatch refers to the relative differences in spectral distributions of the light used to measure the reference and the device under test, respectively, and the relative differences in the spectral responses of the two devices. Matching the spectra and responses as closely as possible minimizes this error. A value of M = 1 indicates a perfect match. Deviations from unity can be as much as 50%1.
The Newport TAC — PV Lab uses a solar reference cell with a KG1 filter window (instead of the standard fused silica) to test organic devices. Doing so keeps the mismatch below 10%. Of course, there is error in the correction factor M itself due to inaccurate knowledge of the spectral distribution of sunlight and spectral response of either device. To meet this challenge, Newport’s Oriel division has developed an instrument, the IQE-200, which can accurately measure the spectral response of most PV devices.
Another error encountered in PV measurements employing simulated sunlight is due to the spatial non-uniformity in the solar simulator beam (Figure 3).
Solar simulator light is typically more concentrated in the center (around the optical axis) than at the edge of the illuminated area and maps into a domed surface, the height of which can be used as a metric for spatial non-uniformity. Spatial non-uniformity is minimized (under 2%) in Class AAA solar simulators like the Sol3A from Newport Oriel (Figure 4). The residual amount of spatial non-uniformity causes irradiance error that is proportional to the relative areas of the solar reference cell and device under test, and on the relative locations of the two cells within the working plane of the solar simulator. Alternatively, a factor (analogous to the spectral mismatch factor) can be calculated and applied to correct for this error2.
Unlike packaged solar reference cells, experimental or prototype cells sometimes arrive at the Newport TAC-PV Lab unpackaged. In many cases, they degrade with exposure to air, light, heat and humidity. These are delicate structures as small as 0.04 cm2 that may simply be sandwiched between two microscope slides. Light exposure during testing must be short to be non-invasive, but no shorter than the response time of the cell. There is often no convenient way to control or directly measure the temperature of these devices. The TAC-PV Lab tests experimental cells under short exposure to light (~ 1 sec) as produced by the Newport Oriel Sol3A solar simulator with a built-in shutter with 300 ms switching time. This technique perturbs the cell only slightly from being in equilibrium with room temperature. Variations in Voc during the exposure to light can be used as a measure of deviation from equilibrium3. Repeating these short exposures at different bias voltages generates an I-V curve from which all the electrical performance parameters can be calculated.
- C. R. Osterwald, “Translation of device performance measurements to reference conditions.” Solar Cells, 18, pp. 269-279 (1988).
- Klaus Heidler, Heike Fischer, and Siegfried Kunzelmann, “New approaches to reduce uncertainty in solar cell efficiency measurements introduced by nonuniformity or irradiance and porr FFdetermination.” Proceedings of the 9th EC Photovoltaic solar energy conference, Freiburg, Germany, pp. 791-4 (1989).
- K. Emery and T. Moriarty, “Accurate measurement of organic solar cell efficiency,” Proc. SPIE Optics + Photonics, session 7052-12, San Diego, CA, Aug. 10-14, (2008).