In agreement with Markus, the instability of the organic solar cells is observed due to structural changes with environmental stresses as temperature. One of the papers which investigates such phenomena is given in the link:http://www.elp.uji.es/recursos/paper275.pdf

wish you success

Dear Dr. Hösel and Dr. Zekry,

Thank you for your quick reply!

Here, I am talking about the successive measurements. The measurements were taken in order of one after the other, i.e. during the same session.

Also, I could not open the link of the paper mentioned above.

I will get back to you, regarding the difficulty that I am facing with more details, as soon possible.

Kind regards,

K. Usman

Any experimental values always come with some extent of variation. In fabricating OPV devices, process in every step, the fabrication environment, instrument condition, etc. are never the same. I recommend plotting the efficiency data and determined the dispersion of the data (like standard deviation) to see if your different values are significant enough.

There is a review paper about degradation mechanisms in short time that can help you:

Stability/degradation of polymer solar cells Solar Energy Materials and Solar Cells, 92 (2008) 686-714

http://web.mit.edu/3.042/team1_10/NanoSol/References_files/PPCefficiency.pdf

Dear Khuram,

As stated by the early contributors, degradation phenomenon would be one of the most important argument here. This can be due

1. UV- during light exposure,

2. heat but also 

3. Oxygen and water vapor within the layers themselves and the measurement environment

As an experiment, I assume that you are able to minimize/avoid the third point on contribution of oxygen and water vapor in the measurement environment  but if care is not taken during device fabrication the first part of the 3rd point can already start at the interface of the active and the metal electrodes which would get accelerated due to light exposure.

Bear in mind that there is a already huge drop in PCE even within interval of a minute to hours as already explained (Burn-in effect) before device stabilization.  As an example in my own case my samples may lose up to 10-15% of their PCE within the first 2 to 5 minutes. and this mostly due to light exposure.

I hope this helps. By the way, I could download the paper provided by Abdelhalim Zekry.

Cheers

Thanks to the versatility of Dye sensitized solar cells, if made well even in lab room conditions, improve instead of degradation and stability measurements can be made for weeks and months....

Dear All,

I appreciate your time to answer my question.

Sorry for my very late reply.

Kind regards,