In a multi-junction solar cell, individual cells are stacked upon each other. But while the (theoretical) limiting conversion efficiency scales with the number of series-connected sub-cells, this might not be the case when the sub-cells are all connected in parallel (= device with two terminals).
Consequently, if the overall voltage is limited by the bandgap of the bottom cell, the limiting efficiency of multiple parallel-connected junctions remains the SQ-limit.
What am I missing here?
If you put one top and one bottom cell in parallel I think it's actually worse. Imagine a top cell with 1.7eV bandgap and bottom cell with 1.1eV, so typical Vmpp for each cell alone might be 1.0V and 0.6V. If you put them in parallel there is a difference of 0.4V, that is the top cell is applying a bias to the bottom cell. The voltages have to match, so presumably the top cell will run below its mpp, and some of its power will be used to pump the bottom cell to the same voltage.
I'd need to run a circuit simulation e.g. LTSpice to tell you for sure - which you can do, the software is free - but basically putting a single bottom cell and single top cell in parallel sounds like a terrible idea.
However, what has been done is to put 2 bottom cells in parallel with one top cell. If their Vmpp differs roughly 2x it works. Similarly for any other ratio. Look for "voltage matched tandem", or see below.
https://patents.google.com/patent/US20150340528A1/en
https://ieeexplore.ieee.org/abstract/document/8366407
Dear, Manuel Schnabel,
Actually, When connecting multiple solar panels in a 12-48 volt off-grid system, you have a few options: parallel, series, or a combination of the two. In this article, we’ll give you the basics on wiring solar panels in parallel and in series. Let’s start off with a quick comparison of parallel circuits and series circuits.
Parallel circuits have multiple paths for the current to move along. If an item in the circuit is broken, current will continue to move along the other paths, while ignoring the broken one. This type of circuit is used for most household electrical wiring. For example: when you turn off your TV, it doesn’t also turn off your lights.
When wiring solar panels in parallel, the amperage (current) is additive, but the voltage remains the same. eg. If you had 4 solar panels in parallel and each was rated at 12 volts and 5 amps, the entire array would be 12 volts and 20 amps.
Series circuits have only one path for current to travel along. Therefore, all the current in the circuit must flow through all the loads. A series circuit is a continuous, closed loop - breaking the circuit at any point stops the entire series from operating. An example of a series circuit is a string of old Christmas lights - if one bulb breaks, the whole string turns off.
When wiring solar panels in a series, the voltage is additive, but the amperage remains the same. eg. If you had 4 solar panels in a series and each was rated at 12 volts and 5 amps, the entire array would be 48 volts and 5 amps.
Remember: just like batteries, solar panels have a negative terminal ( - ) and a positive terminal ( + ). Current flows from the negative terminal through a load (current consumed by a piece of equipment) to the positive terminal.
And The Shockley–Queisser limit for the efficiency of a solar cell, without concentration of solar radiation. The curve is wiggly because of absorption bands in the atmosphere. The solar spectrum was approximated by a smooth curve, the 6000K blackbody spectrum. As a result, the efficiency graph was smooth and the values were slightly different.
In physics, the Shockley–Queisser limit (also known as the detailed balance limit, Shockley Queisser Efficiency Limit or SQ Limit, or in physical terms the radiative efficiency limit) refers to the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell.
Hope you are satisfy with answer, if still face any problem related to simulation and designing kindly tell me i will help you with my best efforts.
Thanks
Thank you for your reply.
You're right in all what you say. But my question does not refer to the wiring of solar panels but to the physical, actual architecture of a solar cell.
While all solar panels see the same incident solar spectrum, each sub-cell of a multi-junction solar cell will see a different spectrum. For example, as the top cell(s) "filter" the higher energy photons out, a bottom cell might only see NIR photons. Consequently, multi-junction solar cells can exceed the SQ-limit of single junction solar cells, because thermal losses are greatly reduced.
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