The "Low-light efficiency" is mainly dependent on the Rserie and Rshunt (exponential behaviour) parameters (as well as di²MuTau for amorphous technology).
The loss goes with the square of the current (Rs * I²), therefore increasing quadratically
Take now a module designed for an irradiance under 100 W/m2: if the Rserie is high (bad), the losses are higher at high irradiances (orange curve).
Rserie_EfficComp_100W.png [ 6.96 KiB | Viewed 13224 times ]
Now the characteristics of the module you buy is specified at STC.
Therefore if you want to design a module of the same Power at STC as your "good" module, this new module with high Rseries should be globally
of better quality at low irradiances:
Rserie_EfficComp_STC.png [ 8.1 KiB | Viewed 13224 times ]
Therefore a module of same STC performance, but with bad Rseries will give better low-light performances !
In other words: the module is globally of much better quality, but the high Rserie value penalizes the STC performances.
The shunt resistance at STC has a very low impact on the Low-light efficiency.
However the exponential behavior
as function of the irradiance will decrease the Rshunt loss, and therefore enhance the efficiencies under 400 W/m².
The picture shows the case of a crystalline module
, with a dynamics Rsh(0) / Rsh(STC) of 4.
Rshunt_EfficComp.png [ 8.24 KiB | Viewed 13224 times ]
However the amorphous modules
have a much lower Rshunt at STC, and a much higher dynamic Rsh(0) / Rsh(STC) of around 12.
The figure shows that the efficiency difference would be significant between different Rshunt if they were constant. But the recovery of the exponential behavior leads to very similar curves.
Rshunt_EfficComp_Amorphous.png [ 8.76 KiB | Viewed 13224 times ]
Therefore the recovery
of low-light efficiency is better with low Rshunts and high exponential dynamics.