Characterization of Organic Solar Cells

Abstract

This chapter guides you through electrical and physical characterization of photovoltaic devices. There is a special focus on the practical aspects of calibrating sun simulators and accurately determining power conversion efficiencies for devices under simulated and real sunlight conditions. The sun is a reliable source of luminous energy with an essentially constant power output. From this point of view, it is an excellent light source when testing the power conversion efficiency of a device and it is of course also the real source of light energy against which one has to measure the efficiency of a given device when it comes to energy production using a photovoltaic in a real application. As practice would have it, the incident intensity of sunlight at the surface of the earth is, in many regions, highly variable and depends on the weather (cloud cover, rain, snow, etc.), the time of the day (day, night), and the time of the year (summer, winter). For this reason it is practical to have a light source that can be used inside the laboratory such that measurements can be carried out repeatedly at any point in time and also the source can be switched off. As discussed earlier, the spectral distribution of the sunlight follows blackbody radiation from a source with the surface temperature of the sun (5800 K). In space the distribution is continuous and near perfect, while at the surface of the earth various atmospheric gasses absorb part of the spectrum giving it a particular spectrum depending on where you are on the globe. The light sources that are available in the lab are exclusively electric arc lamps (while chemical light sources are possible by burning gasses with a high flame temperature). For this reason the spectrum of simulated sunlight cannot exactly reproduce the spectrum of the sun. Generally there is a tendency to have too much ultraviolet and visible light intensity and too little infrared light intensity. Furthermore, the spectrum often has discrete emission lines from the elements present in the arc lamp.