Toward 30% Efficiency Silicon Cells

In 1985, Gray and Schwartz presented a paper entitled ‘Why don’t we have a 30% efficient silicon solar cell?’ [18]. At that time, the highest reported efficiency for silicon solar cell was 22% under concentrated sunlight. In that paper, the authors concluded that 30% efficiency would be attainable with a silicon solar cell if: [11]

• a novel heterojunction or heteroface contact with small contact resistance and low emitter saturation current was developed.

A few years later, the first three steps to attain the 30% efficiency target had been addressed and resolved: the PC solar cell with a thin substrate and long carrier lifetime, with a textured and passivated front surface, proved to be the best design for high efficiency concentrator solar cell [19, 20]. In 1989, the highest reported efficiency for silicon PC solar cell was 28.3% [21]. The same year, Swanson responded to Gray’s paper in a publication entitled “Why we will have a 30% efficient silicon solar cell” [22] where he demonstrates that all the pieces are now in place to fabricate a 30% efficient silicon solar cell. The last milestone to reach the 30% breakthrough, namely the development of polysilicon emitters with a low contact resistance, and low emitter saturation currents had just been demonstrated at Stanford [23, 24]. Swanson announced: Within one year, cells will be reported with efficiency in excess of 30% [22].

We believe that 30% efficient silicon solar cells are possible in a manufacturing environment. In order to achieve this goal, polysilicon or heterojunction emitter technology needs to be implemented, along with several other improvements to the existing technology (thinner cells, improved light trapping, and reduced dimension geometries).