Current transport in copper indium gallium diselenide solar cells comparing mesa diodes to the full cell

Mesa diodes were formed on CdS/CIGS/stainless steel solar cells to investigate current transport when edge leakage and spot defects are avoided. Current conduction mechanisms in the device were determined from current–voltage (I–V) and current–voltage–temperature (I–V–T) characteristics. Space charge limited (SCL) current in the mobility regime with an exponential distribution of traps was found in the voltage range of V>0.6 V based on I∝Vm where m>2. In the voltage region of 0.2 V<V<0.6 V, recombination was the dominant mechanism based on the ideality factor, n, in the equation I=Ae(qV/nkT), close to 2. For −0.2 V<V<0.2 V, a combination of tunneling and SCL current in the ballistic regime was suggested because of the weak temperature dependency and approximation to I∝V1.5. For the reverse bias region where V<−0.2 V, the device exhibited either SCL current in the velocity saturation regime or tunneling based on the unity I–V relation and the weak temperature dependency. A previous report on full size CIGS cells indicated a higher degree of tunneling for V<0.2 V. Thus, the mesa diodes show some difference in mechanism compared to “good” full cells and much difference compared to “poor” full cells.