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 > 0.6V based on I ∝ V^m 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 ∝ V^1.5. 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.