Air Mass AM

Based on the Sun s direction at different point of time, AM differs and can be expressed as  

Here 6 refers to angle between sunrays falling normal to earth surface and actual angle of simrays at different time during the day. For 0 equal to 48.2, AM is 1.5, which is typically used for characterizing solar cells. A standard test condition for solar cells is AM 1.5 with an incident power density of 100 mWcm at a temperature of 25 C [32]. 

As the performance of solar cell is critically dependent on the various aforementioned device parameters, their detailed understanding is necessary for development of efficient and stable OPVs via designing of materials and manipulation of device architecture. 

Open-circuit voltage is the value of voltage at zero current, and in real sense it represents the maximum possible photo-voltage generated across the device. In case of OPVs, it is determined by the difference between the HOMO level of donor (D) and LUMO of acceptor (A). It has been shown that [33] the theoretical V can be approximated as 

The can be increased by either moving the donors HOMO farther away from vacuum level or pushing the acceptors LUMO closer to vacuum level. However, while doing so, energy offset of larger than exciton binding energy 0.3eV) should be maintained between the LUMOs of donor and acceptor. 

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Specific solar radiation conditions are defined by the air mass (AM) value. The spectral distribution and total flux of radiation outside the Earth s atmosphere, similar to the radiation of a black body of 5,900 K, has been defined as AM-0. The AM-1 and AM-1.5 are defined as the path length of the solar light relative to a vertical position of the Sun above the terrestrial absorber, which is at the equator when the incidence of sunlight is vertical (90°) and 41.8°, respectively. The AM-1.5 conditions are achieved when the solar flux is 982 Wm2. However, for convenience purpose the flux of the standardized AM-1.5 spectrum has been corrected to 1,000 Wm2. 

As an approximation to the solar distribution at the earth s surface, we will take Boer s (T/S) distribution for AM 1.2 which is for a bright sunny summer day near noon ( ). (AM stands for air mass. AM 0 would correspond to incident solar power outside the earth s atmosphere. AM 1 would correspond to incident solar power at sea level with the sun at the zenith.) If we make the ideal assimption that a(X) = 1 for X Xg and a(X) = 0 for X then we can calculate an ideal value of rig as a function of This is shown as curve E in Fig. 2. ri has a maximiam value at 1110 nm for AM 1.2 but the maximum is very broad in that rig is > 45% between 800 nm and 1300 nm. It is a common misconception among photochemists that r g represents the fraction of solar power that can be converted to electricity or chemical energy. The next section will attairpt to show the fallacy of this view. 

Figure 5.2 Pictorial definitions of (air mass (AM) units of absorption by the atmosphere...

The most efficient silicon cells produced are based on p — n homojunctions and convert 23.1% of the energy in incident light set to simulate the global air mass (AM) 1.5 spectrum, an artificial reference spectrum used to standardize measurement of PV power, with an intensity of 1000 W/m2... 

AM (0) sunlight The solar irradiance in space just above the earth atmosphere (air mass, AM, zero). Also called extraterrestrial global irradiance. 

ASTM global irradiance standard solar spectrum (air mass (AM) 1.5) up to wavelength of 1000 nm, incident on a plane tilted 38° facing south, normalized to 1000 W m" for the whole spectrum. 

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