Let's imagine a water pump. If it had leakage of the water, the water pressure decreases. If the water outlet chocked, the current decreases. The pump corresponds to a solar cell, and the water pressure and the current correspond to the voltage and the electric current respectively. In your cell, there is a leakage which corresponds to carrier recombination due to the poor crystal quality or harmful impurities.

If the band gap was narrow than that you intended, the voc also goes down. But, the low F.F. indicate poor crystal quality or existence impurities. In general, it is very difficult to obtain a good compound semiconductor crystal. The crystal defects which affect the cell performance are not visible in SEM. Precision XRD is better to evaluate your cells.        

 Normally  its depends also for the temperature.I don't know at   which thermal conditions do  you recover theses result ?Because as other when temperature increase jsc  increase and Voc  fall  very fast than Jsc.May be more details about your experimental test condition will be more helpful to  try to understand   origin of results obtained now .

Dear Muhammed,

what is the shape of your I-V curve? can you display one?

The energy mismatch between the the hole and electron transport layer may cause lowering of the open circuit voltage voltage. Also, their high resistance can cause the reduction of the fill factor. As far as i know the organic materials suffer from limited conduction capability. Interface recombination may cause the declining of the collected photocurrent with cell loading.

Any how it is advisable to investigate the dark current and the photo current and the illuminated i-v curves. If you desire to discuss the matter please display some of your results.

Best wishes

Dear Muhamad,

experimentally, this happens when your contact was unsuitable for your solar cell active layer, replace Al with Ag or Au and see the result. 

Dear Muhamad

It is known that an intrinsic layer with low energy bandgap in pin solar cells has higher solar photocurrent  than that with high energy bandgap.

Dear Muhamad

In this structure, achieving high Voc and Jsc is possible with both Al and Ag contact. But, a

Let's imagine a water pump. If it had leakage of the water, the water pressure decreases. If the water outlet chocked, the current decreases. The pump corresponds to a solar cell, and the water pressure and the current correspond to the voltage and the electric current respectively. In your cell, there is a leakage which corresponds to carrier recombination due to the poor crystal quality or harmful impurities.

If the band gap was narrow than that you intended, the voc also goes down. But, the low F.F. indicate poor crystal quality or existence impurities. In general, it is very difficult to obtain a good compound semiconductor crystal. The crystal defects which affect the cell performance are not visible in SEM. Precision XRD is better to evaluate your cells.        

Dear P. D. Dongo, I am certain that the cell temperature was controlled to be max at 35 degree C. So its not the problem.

Prof AbdelHalim, I am sorry that the data was in the simulation's computer and i don't have the data with me now. However I do have plotted the tafel curve for the dark IV curve. The plot can show me how the cell behave in general (Rsc Roc, ideal diode behaviour etc that give rise to recombination) but it certainly cannot tell me the origin of those problem and the main cause (that is so many things to rule out!). I guess the detailed scan of Xray crystallography might help me to understand more. 

Mehrdad, Benslimane, and Farzaneh, I tried Ag as well as the back contact. It got a little bit improvement in Voc but still low. I will try to use thin BCP, MoO3 or Ca next time to match the energy level.

 Yoshiki Fukada, Thats exactly what I am thinking! I guess maybe there are so much crystal defects present in those samples.

Anyway, thank you all for your valuable comments and suggestion!

Agree with Abdelhalim Zekry. It may well because the leakage current introduced in your cell fabrication, as Voc will decrease with the increase of J0 in Mott-Schottky equation.

Good luck

Dear Muhamad,

The first step in diagnosing the solar cell is investigate its I-V curves. For example the presence of the S-shape in the 4 th quadrant points out the nonohmic behavior of the metal contacts to etl and or to the hole transport layer.  This reduces much the fill factor.

The presence of high ohmic resistance points out that one of the transport layers may have high resistivity. As a consequence the fill factor will be low and the shape of the i-v in the fourth quadrant will be triangular.

The presence of many defects in the material can lower the carrier lifetime and increase the dark current leading to a decrease in the open circuit voltage. In order to get more evidence on the recombination you can measure directly the carrier lifetime by the open circuit voltage decay. 

The presence of the  shunt resistance points out that the junction is partially shorted by pin holes and metal penetration from the top to the bottom. This can be further investigated by a microscope.

As the electrical and optoelectronic properties depend directly on the physcial and technological parameters, it is expected that these parameters affects the shape of dark and illuminated performance of the solar cells.

In fact, if one measures the different layer characteristics before their assembly to a device one may get the expected results provided well functioning interfaces.

Therefore i told you to display the characteristics of the solar cells as inyour question you did not speak from some important other parameters that can be used to diagnose the solar cells.

Best wishes

Thank you Prof. Abdelhalim,

I did the fabrication again, and this time the curve characteristics i.e Voc FF, are getting improved. This time I got 0.73V of Voc, but not as high as expected (about 1.1V) for the MAPbIxBr3-x. The band gap for the active layer is 1.73eV.  I attached the IV curve here for you to diagnose. The bias was done in forward direction from -0.2 to 1V. I cant see the S-shape in the 4th quadrant. Thanks!

Dear Muhamad,

Congratulations, an appreciable proceeding!

The phenomenon that the illuminated i-v curve cuts the dark curve is normally observed in the solar cells in which the collected photocurrent decreases with forward biasing of the solar cells. Because of the low mobility and the relatively low lifetime, together with low effective electric field with increasing forward bias, the collected photogenerated current decreases in comparison to the the short circuit photcurrent as the the collection electric field will be at its maximum value. It is so, that the electric field at short circuit= the contact difference of potential phi/ the thickness d of the i-region which is the same time the perovskite layer. At any forward bias the electric field will become= (phi- V)/ d.

It is o that the photogenerated electrons and holes will be collected by the electric field. If they are not collected at a time smaller than the lifetime they will recombine and get lost.

Consequently in such cells there are three currents:

- the shortcircuit photocurrent, Ips

- lost recombination current from the photogenerated electron hole pairs we call the recombined photcurent, Ipr

- The dark current Id 

In the conventional solar cells Ipr is vanishing.

So, it seems that:

The lifetime of carriers are relatively small, you have to increase it by better interfaces and bulk of the pervoskite. This will improve also the mobility of the charge carriers.

As a test you can decrease the active layer thickness, the crossing may disappear, however , the shortcircuit may be reduced.

Best wishes

Dear Muhamad, 

In case of of using PEDOT:PSS as growing substrate of perovskite, it has disadvantage due to more hydrophilic properties of PEDOT:PSS. Thus make perovskite formation will be poor. Unfortunately your perovskite thickness is thick enough (~600 nm). I suggested you to read this paper from Prof. Gratzel. Please check it. (High efficiency stable inverted perovskite solar cells without current hysteresis, Energy Environ. Sci., 2015,8, 2725-2733)

You may also read and follow this high inverted structure with PCE over 20%.

Good luck.

Article The synergistic effect of H2O and DMF towards stable and 20%...

The solar cell devices exhibit low efficiency due to the possibility of the recombination loss of the photogenerated charge carriers attributed to surface defects. The recombination of electrons-holes pair reduces the photovoltaic energy conversion efficiency of the planar planar P-I-N junction perovskite solar cell device. However, the large short circuit current density (Jsc) in your device structure is attributed to a large number of electron–hole pairs are generated within a short time due to the small band gap of perovskite layer. On the other hand, the small value of open circuit voltage (Voc) of your device can be ascribed to the decrease of electrons lifetime in Perovskite layer. This phenomenon is an indicator of the presence of internal recombination loss in the planar P-I-N perovskite solar cell based device.

Dear Colleagues,

I would like to pay your attention about that i newly published an analytical models of the organic and perovskite solar cells based on physical insight and i gave the cause of the intersection of the illuminated current with the dark current.

Please follow the link: Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells

Best wishes