JAIST researchers have developed three types of silicon ink:  aB-doped p-type, an i-type (intrinsic) and a P-doped type.
Credit T. Shimoda; JAIST.

Typically, photovoltaic units composed of thin-film silicon materials do not involve amorphous or single-crystalline silicon, and making thin-films with poly-silicon is still a frontier field (see previous post). But researchers at the Japan Advanced Institute of Science and Technology may have developed the world’s first thin-film amorphous silicon photovoltaic cell made by using liquid silicon “inks.”

The group says its units have an energy conversion efficiency of 1.79 percent, according to Tatsuya Shimoda, professor at the JAIST School of Materials Science who is leading the team.

The energy conversion level is not a revelation, but the printing process may be a significant innovation.

(WARNING: Some of the details of what Shimoda’s group is doing are a little sketchy because the translations of their work from Japanese to English range from pretty bad to really, really bad. So, be forewarned that some of the following details may be faulty.)

The cells that are created by the group are pin-type (sometimes noted as p-i-n-type), where the p-, i- and n- layers are added to a glass substrate using an innovative “printing” technique.

It looks like the group uses a method that starts with cyclopentasilane. The CPS is polymerize to make polysilanes. The materials were developed a few years ago when the researchers were learning how to make the polysilane from the CPS (the polymer molecules are made by bonding SiH2 like a chain). At that point, JAIST  researchers were able to form amorphous silicon-thin film transistors using a chemical vapor deposition procedure.

In time, the group was able to make pin-type cells, however the group found it difficult to create a uniform polysilane film with all layers being formed through the CVP method. Thus, they shifted their focus to a printing-based process.

The researchers say that by printing the layers, they are able to increase the conversion efficiency from the previous (all CVP) method of 0.51% to 1.79%. They feel confident they can improve conversion efficiency, but they have a long way to go before they catch up the leading thin-film silicon cells, such as United Solar’s 12% efficiency level recently confirmed by NREL.

Perhaps it will be more import that they have been able to come up with a process to mass produce amorphous silicon PV cells using roll-to-roll manufacturing.