Early history of vertical p-n-junctions and advantages of vertical structures

The first silicon solar cell was made entirely by accident in early 1940 by Russell Ohl [1,2]. When he used a flashlight for illuminating a piece of silicon he was studying, the voltmeter, which was connected across, showed a surprisingly large reading. The samples of cells, cut from recrystallized material, allowed to investigate both types of p — n junction geometry: with the junction, which was perpendicular and parallel to the illuminated surface. It is interesting to see (Fig.1 [1,2]) that the first solar cells were made with vertical p-n-junctions.

The first practical solar cell was developed in 1954 after considerable theoretical and experimental work. This solar cell was made from monocrystalline silicon and had the planar junction. It was the forerunner of today’s solar cell.

Later vertical p-n junctions were analysed, current-voltage characteristic with the assumption of infinite velocity of recombination and without build-in field was calculated
and conclusion that SC with vertical p-n junctions (SCVJ) are not suitable for conversion of solar energy was made.

At the same time the main problem: to decrease series resistance for conversion of concentrated light, is much easier to solve for vertical design.

SCVJ have a number of advantages in comparison to conventional SC:

1. low series resistance leads to the absence of mutual contradictions between sheet resistance of emitter, spectral sensitivity, surface of electrical contact grid and so on;

2. high tolerance to damage radiation (it is easier to optimize the radiation resistance in space solar cells);

3. lower equilibrium temperature (no need of metallization on the front and back surfaces, that is why SCVJ are transparent to IR behind the main absorption band);

4. in comparison to planar double-sided (bifacial) SC, SCVJ have ideal symmetrical frontal and back sensitive surfaces, that allows to use a both side illumination;

5. can be used as component part of tandem SC (because they are transparent behind the main absorption band;

6. high output voltage (series connection of cells) and small current under the same power lead to more efficient battery (because of decreasing losses, which are arising at the high current cells).