efficiency

Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight. The energy conversion efficiency ( ) of a solar cell is the percentage of the solar energy to which the cell is exposed that is converted into electrical energy. This is calculated by dividing a cell’s power output (in watts) at its maximum power point (Pm) by the input light (E, in W/m2) and the surface area of the solar cell (Ac in m2). : By convention, solar cell efficiencies are measured under standard test conditions (STC) unless stated otherwise. STC specifies a temperature of 25 C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These conditions correspond to a clear day with sunlight incident upon a sun-facing 37 -tilted surface with the sun at an angle of 41.81 above the horizon.ASTM G 173-03, “Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 Tilted Surface,” ASTM International, 2003. This represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. Under these test conditions a solar cell of 20% efficiency with a 100 cm2 ( (10 cm)2 ) surface area would produce 2.0 W. The efficiency of the solar cells used in a photovoltaic system, in combination with latitude and climate, determines the annual energy output of the system. For example, a solar panel with 20% efficiency and an area of 1 m will produce 200 W at STC, but it can produce more when the sun is high in the sky and will produce less in cloudy conditions and when the sun is low in the sky. In central Colorado, which receives annual insolation of 2200 kWh/m , such a panel can be expected to produce 440 kWh of energy per year. However, in Michigan, which receives only 1400 kWh/m /yr, annual energy yield will drop to 280 kWh for the same panel. At more northerly European latitudes, yields are significantly lower: 175 kWh annual energy yield in southern England. Several factors affect a cell’s conversion efficiency value, including its reflectance efficiency, thermodynamic efficiency, charge carrier separation efficiency, and conduction efficiency values. Because these parameters can be difficult to measure directly, other parameters are measured instead, including quantum efficiency, VOC ratio, and fill factor. Reflectance losses are accounted for by the quantum efficiency value, as they affect “external quantum efficiency.” Recombination losses are accounted for by the quantum efficiency, VOC ratio, and fill factor values. Resistive losses are predominantly accounted for by the fill factor value, but also contribute to the quantum efficiency and VOC ratio values. As of September 2013, the highest efficiencies have been achieved by using multiple junction cells at high solar concentrations (44.7% by the Fraunhofer ISE, Soitec, CEA-Leti and the Helmholtz-Zentrum Berlin).