**7. I-V Characterization of solar cell**

Current voltage measurement of the solar cells was carried out using Agilent 4155c (Agilent, Santa Clara, CA) semiconductor parameter analyzer equipped with solar cell simulator in SDSU. Fig. 10 shows the experimental set up used for measuring I-V response of the solar cells, where 2 SMUs (source measurement unit) were used. The SMU s could operate as a voltage source (constant sweep voltage) or a current source and it could measure voltage and current at the same time. The SMUs could measure from 10-12 A to 1 A and -10 V to 10 V. SMU 1 was set as voltage sweep mode from -1 V to 1 V with steps of 0.01 V, and SMU 2 was set to measure current of the solar cell during IV measurement. IV responses were measured under both dark and illuminated condition. During illumination, the intensity of the simulated light was 100 mW·cm-2 and calibrated using the NREL calibrated standard cell.

Fig. 10. Experimental set up for measuring IV response of a solar cell.

Table 3 shows the current voltage characteristics of p-n junction solar cells with structures AlSb/TiO2, AlSb/ZnO. The active cell area was 0.16 cm2 and fabricated on ITO coated glass surface.

The *V*oc of the best cell with ZnO as an n type layer was found out to be 120 mV and *I*sc to be 76 uA. The *FF* of the cell was calculated to be 0.24 and the efficiency was 0.009%. The cell with TiO2 as an n layer has even lower *V*OC and *I*sc. TiO2 is less suitable n-type layer for making junction with AlSb than ZnO because it is far more conductive than TiO2. A number of reasons may be attributed for this low efficiency. First, is due to small electric field at the junction between AlSb and the n type material (ZnO or TiO2). This severely limits the charge


separation at the junction and decreases VOC of the device. A better material needs to be explored to dope AlSb n type to increase the built in field. The field could also be extended using the p-*i*-n structure to design the solar cells.

Table 3. Current-voltage characteristics of p-n junction solar cells

Interesting results were obtained with a p-*i*-n junction, CuSCN/AlSb/ZnO. The used cell has an active cell area of this cell was 0.36 cm2 and fabricated on Mo coated glass surface. Charge was collected from the silver epoxy fingers casted on top of ITO surface and Mo back contact. The cell showed a *V*OC of ~ 500 mV and a *J*SC of 1.5 mA/cm2. With a FF value of 0.5, the efficiency of this cell was calculated to be 0.32%. This observation may be attributed to the more efficient charge separation than that in the p-n junction devices due to a strong build-in field. However, the efficiency of the p-*i*-n junction device is very low in comparison to other available thin film solar cells devices. There are still many unknown factors including the interfaces in the junction. Such a low efficiency could be attributed to the defects along the AlSb interface with both the p- and n-type of layers. Interfaces between AlSb and other layers needed to be optimized for a better performance.
