Power/Performance Bits: Jan. 20

Boosting silicon solar cells
According to Stanford researchers, stacking crystalline perovskites onto a conventional silicon solar cell dramatically improves the overall efficiency of the cell.

The researchers reminded that silicon solar cells dominate the world market, but the power conversion efficiency of silicon photovoltaics has been stuck at 25 percent for 15 years. One inexpensive way to improve efficiency is to build a tandem device made of silicon and another inexpensive photovoltaic material.

This approach is desirable because one solar cell is simply put on top of the other, which produce efficiency than either could do by itself. From a commercial standpoint, it makes a lot of sense to use silicon for the bottom cell but until recently there wasn’t a good material for the top cell, then perovskites came along.

Perovskite is a crystalline material that is inexpensive and easy to produce in the lab. In 2009, scientists showed that perovskites made of lead, iodide and methylammonium could convert sunlight into electricity with an efficiency of 3.8 percent. Since then, researchers have achieved perovskite efficiencies above 20 percent, rivaling commercially available silicon solar cells and spawning widespread interest among silicon manufacturers.

The goal is to leverage the silicon factories that already exist around the world, and with tandem solar cells, a billion-dollar capital expenditure to build a new factory is not necessary. Instead, a layer of perovskite can be added to an existing silicon module at relatively low cost, the researchers noted.

Laser-induced graphene
Advancing their recent development of laser-induced graphene (LIG) by producing and testing stacked 3D supercapacitors, Rice University scientists detailed their energy-storage devices that are important for portable, flexible electronics.

A team from the Rice lab of chemist James Tour discovered last year that firing a laser at an inexpensive polymer burned off other elements and left a film of porous graphene, which the researchers viewed as the perfect electrode for supercapacitors or electronic circuits.

To prove it, team members have since extended their work to make vertically aligned supercapacitors with laser-induced graphene on both sides of a polymer sheet. The sections are then stacked with solid electrolytes in between for a multilayer sandwich with multiple microsupercapacitors.

A schematic shows the process developed by Rice University scientists to make vertical microsupercapacitors with laser-induced graphene. The flexible devices show potential for use in wearable and next-generation electronics. (Source: Rice University)

The flexible stacks show excellent energy-storage capacity and power potential and can be scaled up for commercial applications. LIG can be made in air at ambient temperature, perhaps in industrial quantities through roll-to-roll processes.