A team of researchers at Japanese chemical firm Kaneka Corporation have developed solar cells with 26.3% efficiency, which breaks the previous record of 25.6% and is far ahead of the 20-ish% efficiency of high-end commercial cells. The researchers submitted their findings to Nature Energy, after which they further optimized their design to reach an efficiency of 26.6%. The team used “industry-compatible processes”, so their improvements might actually be coming to a roof near you.
One of the techniques used to improve solar cell efficiency was high-quality thin-film heterojunction. This is a well-known technique which involves layering silicon very carefully to minimize the “band gap”: the energy range in the cell where no electron states can exist. When the band gap is low, it takes relatively little energy to convert a valence electron (bound to an atom) to a conduction electron (which can jump between atoms).
This cell has a passivation layer at the front of the cell (which light shines onto) and low-resistance electrodes in the back. Previous high efficiency cell designs would place passivation layers on the back and the front, or place electrodes on both the back and front. Having electrodes at the front of the cell means losing some photons that might otherwise reach the cell. Thus, according to the paper, the entire front of the cell “can be optimized focusing on optical and passivation properties”.
The theoretical limit of solar cell efficiency is about 29%. Kaneka researchers analyzed the sources of efficiency losses for their cell: 0.5% was due to resistive loss, 1% was in light collection, and 1.2% was due to electrons being attracted to positively charged holes in the cell rather than moving into current collection components. The researchers claim that these losses can be reduced by reducing the resistance in the rear of the panel and depositing an insulating reflection material to the “separation region” of the cells.
Increasing solar cell efficiency means that the cells can more quickly recapture their installation cost, making them more attractive for consumers and paving the way for a solar-powered planet.