Bifacial amorphous Si quintuple‐junction solar cells for IoT devices with high open‐circuit voltage of 3.5V under low illuminance

Hydrogenated amorphous Si(a‐Si:H) quintuple‐junction solar cells, which consist of a‐SiO x :H/a‐SiO x :H/a‐Si:H/a‐SiO x :H/a‐SiO x :H, were fabricated by plasma CVD method. The total thickness was 0.6‐0.8 μm. Irradiation intensity ( Pin ) dependence of the open circuit voltage ( V oc ) of quintuple‐junction solar cells was measured. The decreasing amount ΔV oc (1/10) of the open‐circuit voltage when the irradiation intensity became 1/10 was 62mV/cell. Voc drops rapidly from around the irradiation intensity of 1mW/cm 2 (approximately 1,000 lux). This large Voc reduction is due to leakage current. Then, we discussed the origin of the leakage current, and, finally, by improving the leakage current, a very high open‐circuit voltage V oc of 3.5 V was demonstrated under LED light illumination. Furthermore, we theoretically analyzed Voc as a function of the irradiation intensity, including effects of the leakage current and the film quality of i‐a‐Si(O):H. It was found from the simulation results that it is necessary to increase the shunt resistance Rsh and to lower the defect density of i‐a‐Si(O):H in order to obtain a sufficient Voc ‐ Pin characteristics for IoT devices application under low illuminance.