For amorphous modelling, I have performed a research project for determining the better way to model this technology. Comparing model results with long-term module measurements at sun, in any irradiance and temperature conditions, I have found that we can use the standard one-diode model, with 3 corrections:
- The Rshunt
resistance (inverse of the slope around Isc) has an exponential-like behavior
according to Irradiance. This partly explains the better yields of amorphous at low irradiance.
- One should add a specific term for taking the recombination loss
in the -i- layer into account. This acts mainly on the Voltage (especially Voc), but has a significant implication on the thermal behaviour of Pmpp.
If this is not sufficient for retrieving the manufacturer's temperature specification on Pmpp, sometimes we have still to adjust the gamma thermal behaviour for obtaining the required muPmpp (but is it really correct ?).
- PVsyst also applies a spectral correction
as a function of Air Mass and Clearness Index Kt which has been proposed by the University of Loughborough.
See the publications Mermoud 2010
and Mermoud 2005
(in french) on our web site.
NB: To my opinion (and according to this model), the improvement of performance at low irradiances is not related to diffuse spectral effects (as stated by many manufacturers) , but essentially to electrical effects like exponential behaviour of the Rshunt as function of irradiance, and high Rserie values which induces losses going with the square of the current).