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I am currently working on improving a solar cell, which is partly transparent and therefore utilizes two transparent electrodes (TCO on top and PEDOT:PSS on the bottom). As PEDOT:PSS alone is not conductive enough, a silver electrode patterned as a honeycomb structure forms the bottom end.

Rough design of the solar cell stack.

As I investigated which thickness of PEDOT:PSS would be beneficial, I came across an interesting phenomena, for which I might have an explanation but am not 100% satisfied yet. As I don't want to bias anybody, I would like to keep my explanation and hope for alternative inputs. Here is the problem:

At first, PEDOT:PSS was deposited a few micrometres thick. Investigating the cell with the electroluminescence (EL) setup, the following image came up:

EL-image of the cell with onefold thickness.

You can see the honeycomb pattern of the silver grid (diameter of one comb is roughly around 1 mm). It is obvious that charge carriers entering the cell through the electrodes mainly recombine above the grid.

When the thickness of PEDOT:PSS is increased to twice the thickness by depositing once, waiting for the layer to dry and depositing twice, this is what shows up:

EL-image of the cell with twofold thickness.

Quite remarkable, the situation is reversed. Either radiative recombination is suppressed above the grid, leaving only some kind of background between it, or recombination is strongly enhanced between, in contrast to above the grid.

I am very curiously looking forward to your appreciated explanations. Thanks a lot in advance!

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