Hybrids and related materials - Page 9

Researchers from Helmholtz-Zentrum Berlin (HZB) and the university École polytechnique fédérale de Lausanne(EPFL) in Switzerland have combined a silicon heterojunction solar cell with a perovskite solar cell monolithically into a tandem device and reached a record efficiency of 18%, stating it has the potential to reach 30% after additional modifications.

Designing such silicon-perovskite tandem cells can be challenging, as perovskite cells tend to require coating onto titanium dioxide layers, which must first be sintered at around 500 °C. The amorphous silicon layers that cover the crystalline silicon wafer in silicon heterojunction degrade at this temperature. Now the team from EPFL and HZB has managed to overcome hurdles and manufacture this kind of monolithic tandem cell, by depositing a layer of tin dioxide at low temperatures instead of using titanium dioxide. A thin layer of perovskite could then be spin-coated onto this intermediate layer and covered with hole-conductor material. The team also used a transparent protective layer to avoid the metal oxides sputtering and consequently destroying the perovskite layer and hole-conductor material.

Researchers at the National Renewable Energy Laboratory (NREL) announced that they have figured out a pathway for dealing with the heat loss problem by deploying hot-carrier technology in perovskite solar cells. Hot carrier solar cells offer simplicity of design, low cost, and high efficiency, but are a long way from being commercialized, as one big challenge is revving up the kinetic energy transfer in order to prevent energy loss.

This recent study provides a pathway for pushing perovskite levels upwards, possibly as high as 66%. It determines that charge carriers created by absorbing sunlight by the perovskite cells encounter a bottleneck where phonons (heat carrying particles) that are emitted while the charge carriers cool cannot decay quickly enough. Instead, the phonons re-heat the charge carriers, thereby drastically slowing the cooling process and allowing the carriers to retain much more of their initial energy for much longer periods of time. This potentially allows this extra energy to be tapped off in a hot-carrier solar cell.

Researchers at the Helmholtz-Zentrum Berlin (HZB) developed a process for coating perovskite layers with graphene for the first time, so that the graphene acts as a front contact.

A traditional silicon absorber converts the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions and a particularly effective complement to conventional silicon is perovskite. However, it is normally very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Researchers at The Hong Kong Polytechnic University (PolyU) have developed efficient and low-cost semitransparent perovskite solar cells with graphene electrodes. The power conversion efficiencies (PCEs) of this novel invention are around 12% when they are illuminated from Fluorine-doped Tin Oxide bottom electrodes (FTO) or the graphene top electrodes, compared with 7% of conventional semitransparent solar cells.

PolyU researchers used graphene as an electrode material, after creating simple processing techniques for enhancing the conductivity of graphene to meet the requirement of its applications in solar cells. To begin with, the conductivity of graphene was dramatically improved by coating a thin layer of conductive polymer that was also used as an adhesion layer to the perovskite active layer during the lamination process. Im addition, the performance of this novel invention is further optimized by improving the contact between the top graphene electrodes and the hole transport layer on the perovskite films.