As you observe leakage current in the dark, there could be difficulties with conduction. 

Low JSC is due to low conductivity and high resistance. The problem with resistance is: It can be external or internal type.

I would try the following:

1) try to etch the electrode surface, remove oxide layers and do measurement.

2) Do stability measurement (at zero applied potential measure JSC with time. If your JSC improves with time, this indicates removal of oxide layer an enhancement of JSC.

3) If value of JSC does not get better, you may have other troubles. Try to anneal your electrode to enhance carrier mobility between particles/grains.

4) Also think of the contact between the semiconductor layer and the conducting back contact electrode.

These are tips to take care of.

Hi Prof. Hilal, Thank you for your kind suggestions.

I agree with you the low Jsc might be related to charge transport. But I do not think my electrode or buffer layers can cause high resistance, because the same device configuration and electrodes work quite well in organic solar cells. I guess it is more related to the active layer per se, e.g. due to traps rooted from various defects. But on the other hand, I could not understand why charge recombination due to these traps did not affect Voc and FF. 

And to minimize the traps, I tried to anneal the perovskite layer for longer duration or change the ratio between PbI2 and MAI, it seemed these methods did not solve the problem.

Hello Sun

Carrier mobility between particles could also be a problem. This causes high resistance. My question to you is:

Did your annealing enhance inter-particle connections? If not this could be a still existing reason for low JSC.

However, as your FF and VOC are still good. The reason for low JSC could be due to low absorption of light by the electrode.  You may check this simply by studying effect of light intensity on measured JSC value (change light intensity and measure JSC). I appreciate if you could update me on your results, as I am anxious to know now. 

I still believe that you need to study value of JSC vas time at constant  I appreciate if you do that and update me. We have seen up-rise in value of JSC with time due to removal of dirts from the electrode surface.

Please update me of your findings. Best wishes

To Prof. Hilal,

I tried a few experiments you had suggested. Thermal annealing enhances Jsc. My perception is thermal annealing improves crystal domain size and crystallinity. But even after thermal annealing, the Jsc is still quite low.

We can rule out the possibility that the low Jsc is related to low light absorption, as the film thickness and UV-vis absorption profile suggest the film can absorb enough light. Unfortunately I can not perform the light intensity dependent Jsc study due to the experimental setup in the lab. 

The Jsc can increase a little bit under prolonged light illumination, after reaching a peak value, the Jsc starts to fall. I guess this is related to the "light soaking" effect of this type of solar cells. 

To Emilio,

I used reverse scan, from 1.2 to -0.2 V with 500 mV/s. The forward scan reduces the efficiency a bit. May I know the reason behind the fast scan rate (1000 mV/s)?

You are right. We adopted the "organic" stack cell architecture. Do you have any suggestion on enhancing Jsc for such a cell architecture? How do you normally treat the perovskite layer to facilitate the reaction between PbI2 and MAI without causing pinholes or rough surface?

Hi Nikhil,

Thanks a lot for the details of fabrication procedures. It looks like improving crystallinity and contact are the key to high performance.

Cheers,

Kuan

Dear Kuan,

the best test for the reasons of your low jsc would be quantum efficiency measurements, do you have the opportunity to measure this? How do the IV-measurements in the dark compare to the one under illumination (are they just shifted by the photocurrent or do they have a different shape)? Without these informations it is difficult to guess, but the interfaces in these multilayers often play the crucial role and can for example efficiently block the photocurrent if there are large band offsets etc. I hope I could point you some directions of further improvements...

All the best.

Dear Paul,

Thanks for your new insight. The photocurrent and dark current followed the same shape. However, I noted some cells exhibited a tail in both photocurrent and dark current at the beginning of the sweep, i.e. the current increased first, passing a hump, then decreased, as the voltage reduced from 1.2 V. Is this a sign for too many trap sites?

Dear Emilio,

After I reduced the thickness for the PCBM layer, the current increased to 12-13. But still much lower than reported 18-20. The pvk thickness is between 200 and 250 nm. Maybe switching to MAPbI3-xClx will work if the current limiting factor is the free charge diffusion length.

Dear Kuan,

I would follow Paul's suggestion. Please check EQE of your cells, make sure your cells have correct light absorption range. Optical band gap might be different to other perovskite material.

best regards,

Chang-Qi

You should check both EQE and absorption spectra. What's the film thickness for perovskite you are using?

Hello Sun

Value of Jsc depends on many factors:

- Effective radiation intensity and absorptivity of the film

- Quality of the film: surface states lower Jsc. Internal resistance lowers Jsc. Poor connection to back contact lowers Jsc. Poor mobility lowers Jsc. Lack of dopant and majority carriers lower Jsc. Bad contact between grains lowers Jsc. Excessive imperfections lower Jsc. You may have one or more of these factors. 

The first thing I would do is to anneal the film at suitable temperature. Check radiation intensity, etc

However, before making a conclusion, you need to calculate efficiency (because you still have soundly good FF ~70%). If you get for example soundly high efficiency then your Jsc value is acceptable, and you need not worry at all. Please check that and inform me before proceeding with solutions.

Best wishes

Hey guys,

Thank all of you for suggestions and comments. Problem was solved. Here are the three important changes I made.

1. Dope the perovskite with Cl ion.

2. Expose the perovskite thin film to air before evaporation

3. Insert rhodamine 101/LiF double interlayer between the PCBM and Ag electrode (see attachment).

Based on these changes, I guess the low Jsc was related to charge transport. But I still yet to understand the role of air exposure.

Cheers,

Kuan

Article Efficiency enhancement of planar perovskite solar cells by a...

Hello,

I greatly appreciate the information put out here. I would like to know the theoretical limits on Jsc values. How can we explain a very Jsc, lke 30mA/cm2.?