Planar multijunction high voltage solar cell chip

Integrated circuit technology has been successfully developed to design and fabricate a new type of small area planar multijunction high voltage solar cell chip for concentrated sunlight applications. Each of these 1×1 cm solar cell chips was a monolithic device consisting of six internally series interconnected unit cells, fabricated on 75 μm thick p-type single crystal silicon substrate. Each chip consisted of 1.42×9.63 mm n+/p collecting junctions on the back of the wafer. The illuminated front surface area was divided into 0.3 μm deep n+ regions which form front surface field regions corresponding to the n+/p unit cells positioned beneath. A five photomask level photolithographic process together with a standard microelectronics batch process technique was employed to construct the PMJ solar cell chip. The open circuit voltage of the solar cell chip increased rapidly with illumination up to about 4 AMl suns, and then began to saturate at the sum of the individual unit cell voltages of 3.5 V above 4 AMl suns. A short circuit current density per unit cell of 300 mA/cm2 at 20 AMl suns was observed. This low value is attributed to a low minority carrier diffusion length in the base region of the solar cell chip. The suggested process modifications should significantly increase the device efficiency.