4.2 Overview of the Light Soaking Results
4.2.1 Light Soaking of CdS Buffer Layer Cells
Four cells with a CdS buffer layer have been investigated. The cell CdS 4, CdS 8 and CdS 32 have increasing VOC and PM P P with high FF. Cell CdS 28 has one of the lowest VOC and PM P P of the CdS buffer layer sample but one of the highest ISC. CdS 32
The CdS buffer layer cell with the best performance shows very interesting effects under light soaking as can be seen in figure 4.3. Under white light with low irradiance, all
4.2 Overview of the Light Soaking Results
the parameters except the ISC decrease logarithmically over time after ten seconds, indicated by almost linear declining lines in the diagrams with logarithmic time axis.
The FF and the VOC decline by about 7%, the PM P P by 14%. The same effect occurs under blue light but with the relative decrease is less (4% in VOC, 6% in FF and 10%
in PM P P).
In green light the FF develops quite similar, decreasing by 7%. The VOC however initially increases during the first few minutes and then remains leveled until it de-creases again after half an hour. The plateau is at around 4% and the final value of VOC remains 2% above the initial value. The PM P P is stable for the first six minutes and then decreases at the same rate as under blue light.
Under yellow light the FF still shows similar behavior and also a decrease by around 7%. The VOC initially increases as under green light but with a higher plateau of 14%.
Again, after half an hour the VOC decreases again with a final value 11% higher than the initial one. As a result of the VOC development, the PM P P also increases until reaching a plateau after one minute at 11% and decreases back to plus 5% during the last one and a half hours.
4.2 Overview of the Light Soaking Results
CdS 4 8 32
white
blue
green
yellow
red
infrared
Figure 4.3: Results of the Light Soaking experiments of CIGS solar cells with CdS buffer layer. The x-axes show the logarithmic time of the measurement in seconds after starting the light soaking and the y-axes show the relative change of the parameters. The lines are for the guidance of the eye.
4.2 Overview of the Light Soaking Results
Under red light the FF no longer decreases but instead remains quite stable with changes of less than 2%. The VOC increases even further than under yellow light. It’s increase has a steeper phase for the first fifteen minutes and a flatter phase after that, but never reaches a plateau until it’s final value of plus 23%. The PM P P increases slightly steeper than the VOC and reaches a maximum at plus 23% after one hour.
It then slightly decreases due to a decrease in FF. The extremely high increase of VOC might be caused by a decrease of the cell’s series resistance, as will be discussed in section 4.3.
Under infrared light the FF develops as under blue light, decreasing by 7%. The VOC initially decreases but this might also be due to a measurement error in the first measurement. It then increases steadily in a logarithmic way over time and finally reaches plus 5%. The PM P P initially decreases due to the drop of VOC but remains rather stable within ±2%. This means that the decrease in FF and the increase in VOC compensate each other.
As the white light includes all other colors but solely shows the effect of the blue light, it is quite probable, that the blue light effect is dominant for this cell.
While the FF development is the same for every light with the exception of red, the development of VOC drastically changes with the color of incident light. It “bends upwards” with decreasing photon energy. The fact that it increases before increasing indicates that two different physical effects are working in parallel, serial, both having different time dependencies. This “bending” effect collapses under infrared light, prob-ably because the photons don’t have enough energy anymore to excite certain states of the energy band structure.
CdS 8
The CdS buffer layer cell with the medium performance also has a stable ISC during any light soaking treatment. Under white light the FF slightly decreases by 1%. The VOC and the PM P P initially increase by 1%. The VOC then decreases to a final value of minus 2% and the PM P Pto a final minus of 3%. Under blue light the development for all four parameters is very similar, with the main differences being the PM P P not rising significantly but rather staying constant during the first minute and the final VOC only being 1% lower than the initial value.
Under green light the initial increase of VOC and PM P P is larger than under blue light, just like it was for the cell CdS 32. The VOC increases by 2% and has a plateau starting after 30 seconds and lasting for around five minutes. then it decreases again until it reaches the initial value after two hours of light soaking. The FF shows a very small decrease at the beginning and then slowly returns to the initial value as well.
The PM P P initially increases a little slower than the VOC (due to the initial decrease in FF). After 6 minutes the PM P P reaches its maximum of +2%.
Under yellow light the ISC abruptly increases by 4% after from the first to the second measurement. Having in mind that the ISC stays stable in almost all other experiments, it is rather likely that this jump is a measurement error and the development should be compared to the second measurement. From there, the FF steadily increases by 5%.
The VOC increases by 3% until it reaches a plateau after around 8 minutes and then decreases again by 1%, leading to an overall increase of 2%. The PM P P shows a very logarithmic increase for the first 9 minutes reaching a plateau at plus 7%.
4.2 Overview of the Light Soaking Results
CdS 8 28
white
blue
green
yellow
red
infrared
Figure 4.4: Results of the Light Soaking experiments of a CIGS solar cells with CdS buffer layer and relatively high ISC but low VOC. The CdS 8 cell from before is shown as comparison. The x-axes show the logarithmic time of the measurement in seconds after starting the light soaking and the y-axes show the relative change of the parameters. The lines are for the guidance of the eye.
4.2 Overview of the Light Soaking Results
Under red light the FF initially drops to minus 5% and then starts to steeply increase logarithmically to a total increase of 10% after 1 minute. The VOC shows a similar development but turns around at minus 1%. The PM P P initially decreases to minus 6% and after 1 minute increases logarithmically until it reaches plus 20%.
The parameter’s development under infrared light is very similar to the one under red light. The minimum of FF and PM P P are only 2% and the VOC rather stalls initially than decreases before increasing. The final relative increase of VOC and PM P P are the same as under red light, the final relative increase of FF is only 7%.
As for the cell CdS 32, the cell CdS 8 shows the same development under blue light as under white light, meaning that the blue light effect again is dominant. It also shows the “bending” of the VOC, it already starts under blue light consisting of photons with approximately 2.8 eV. Overall, the increase of VOC stays smaller than in the CdS 32 cell, but the development of the FF of the CdS 8 cell also “bends upwards” with decreasing photon energy, leading to similarly high increases in PM P P. Furthermore these effects remain under infrared light.
CdS 4
The cell CdS 4 with the poor performance shows very different behavior under white and blue light. There is no more total decrease in VOC andPM P P but instead the initial increase from CdS 8 continues logarithmically until the VOC reaches a final increase of plus 5% and the PM P P a final increase of 3%. The FF decreases by slightly more than 2%, under blue light quicker than under red light, leading to an initial decrease of PM P P by 1% under blue light. All developments are logarithmic over time.
Under green light the FF of the CdS 4 cell slightly decreases by 1% before return-ing to its initial value again. The VOC remains stable for around one minute before increasing by a total of 4%. The PM P P slightly decreases at first and finally reaches a total increase of 4%.
Under yellow light the ISC increases by around 1%, again probably due to measure-ment errors rather than representing a real physical effect. The FF also increases by 1%.
The VOC increases by 5%, this time without an initial plateau. The PM P P increases by slightly more than 6%.
Under red light the FF and theVOC show a slight initial decrease of around 1%
and then increase to final values of plus 3% (FF) and 7% (VOC). As a result, the PM P P initially decreases by 2% and reaches a final overall increase of 12%.
Under infrared light the CdS 4 cell shows a decrease in FF and VOC by 2%. The FF then increases to its initial value and the VOC increases to 2%. The PM P P initially decreases by 2% and then increases to 2%.
CdS 28
The parameters of cell CdS 28 under STC are very different from the other CdS cells.
CdS 28 has high ISC and the lowest VOC, FF and PM P P. It also shows very different behavior under light soaking.
Under low irradiation white light the VOC initially decreases rapidly, reaching a minimum of -4% after 6 minutes, whereas the biggest VOC decrease amongst the other CdS cells is 4% after two hours. After 45 minutes the VOC of the CdS 28 cell increases
4.2 Overview of the Light Soaking Results
to reach a final overall increase of 2%. The FF increases to plus 2% after a decrease of around 1%. The PM P P initially decreases by 5% and then increases with the VOC to finally reach +4%.
Under blue light the FF increases by 1%. The VOC and the PM P P initially decrease by 4% and then increase to a final total decrease of 2%.
Under green light there is a jump in the measurement indicated by the sudden change of ISC to the second measurement. From there the FF increases by 7%, while the VOC increases by 2%. The PM P P increases by 10% after the second measurement.
The developments under green light are logarithmic with time.
Under yellow light the FF increases logarithmically by 2% and the VOC decreases logarithmically by 2%. The PM P P stays constant.
The red light experiment shows a relatively big increase in VOC (9%), FF (22%) and PM P P (33%) after nine minutes.
Under infrared light the FF remains constant. The VOC and the PM P P decrease logarithmically by 6% within the first thirty minutes before increasing to a total of minus 5%.