This I-V curves for SnS photodetector with silver electrodes. It can be noted that the both dark and light curves do not intersect with origin point. Although the voltage in the range of (0-0.02 V) is positive value, but the value of dark current is negative.
Usually, I-V curve is plotted and analyzed for values more than 0.02 V. About the curve, i think this behavior may arise from the very low amount of electron diffusion from electrodes to semiconductor. These electrodes can pass the Schottky barriers, so the negative current is reported.
From the I-V can estimate the differential resistance ~3X10E7 Ohm (dark) and ~3X10E6 Ohm (light), to analyses the features require more information about the sample structure and I-V measurement results (larger bias ranges, change some conditions, etc.).
Thanks a lot for all colleagues. However, in all my previous works (when platinum was used as electrodes material), the curves of I-V characteristics in the dark and light conditions intersected with origin point. The new thing here is the negative value of dark current with positive voltage value. Furthermore, this result also occurred with current - time curve at bias voltage 0 V. How can I explain producing current under illumination at 0 V bias and the I-V curve behaviour is ohmic?
Dear Dr. Mohammad Soroosh
I believe that the electron diffusion from electrodes to semiconductor (the current caused by the electrons) did not mean the current has a negative value when the voltage has a positive value. Thanks again for your participation.
Platinum contact (Pt/S) has better Ohmic contact feature than Sn/S contact. The Sn/S I-V in dark shows ~0.022 V built-in voltage (when I=0) and resistance Rd~3X10E7 Ohm, and in light shows abouit -0.005 V built-in voltage and resistance Rd~3X10E6 Ohm. Light might increase conductivity carrier concentration and change the total built-in voltage (from positive to negative). Good Ohmic contact shows good linear I-V, total built-in voltage related to contact interfaces, material and light distribution, etc.
At dark, a small current at zero bias is likely caused by stray light. The device behaves like a solar cell (not a good one), because the two electrodes are not perfectly symmetric.
Or simply your measurement system as an offset. You cannot get current without providing some energy :), either fromm light or bias.
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