Dye-sensitized cell redesign

Adv. Mater. doi: 10.1002/adma.201001006 (2010)

Credit: WILEY VCH

Researchers in Australia have now developed a new electrode configuration for dye-sensitized solar cells (DSCs) in an attempt to improve their cost and efficiency. DSCs typically sandwich a dye-sensitized titanium dioxide layer between two fluorine-doped tin oxide electrodes. However, this design requires a compromise between the cell's electronic and optical properties to allow incident light to pass through the upper electrode and into the cell.

In contrast, in the back-contact DSCs developed by scientists at Monash University and the firm MiniFAB in Australia, both electrodes are on the same side of the cell. The team fabricated their cells by producing interdigitated electrodes on a single fluorine-doped tin oxide sheet using mask-based laser ablation. The resulting cells delivered an incident photon-to-electron conversion efficiency of 54% when illuminated from behind, compared with 75% for a cell with a conventional electrode design. However, the back-contact cell achieved an efficiency of 56% when illuminated from the front, compared with just 39% in the conventional configuration. “Very little optimization has been done on this first generation of back-contact devices, but already their photovoltaic performance is comparable to conventional DSCs,” explained Udo Bach from Monash University.

3D photonic-crystal solar cell

Appl. Phys. Lett. 96, 242102 (2010)

Researchers from Japan and Canada have produced the first ever solar cell containing an integrated 3D silicon photonic crystal. The cell is based on an intrinsic silicon layer with an inverse opal structure — a 3D photonic crystal with an omnidirectional photonic bandgap — sandwiched between n- and p-doped crystalline silicon layers. The cell traps light and prolongs the lifetime of charge carriers, thus reducing leakage current and improving the cell's fill factor.

Although the initial, unoptimized cell only attained a modest conversion efficiency of 0.32%, Geoffrey Ozin, the lead researcher from the University of Toronto, Canada, believes that this marks an important step towards amplified photon collection and photoelectron generation in solar cells. “Cleverly designed silicon inverse opal components could reduce electron–hole radiative recombination and enhance absorption processes,” he explained.

As well as cells based on intrinsic material, the researchers also developed a doping method for creating n- and p-type inverse silicon opals, following which the electrical conductivity of these structures was evaluated.

“What is most amazing is being able to show that the charge-transport properties of these silicon opals are on par with semiconductor-quality silicon,” Ozin said. “This was not anticipated, and it is a very important observation with respect to future advances in enhanced-efficiency silicon photonic-crystal solar cells.”

Embedded scatterers

Opt. Express 18, A139–A146 (2010)

Implanting nanoscale spheres of SiO2 into thin-film silicon solar cells can potentially increase cell efficiency by increasing light absorption. That's the conclusion of a modelling study performed by Michael Scarpulla and James Nagel of the University of Utah in the USA.

“Conventional silicon thin-film photovoltaic cells are typically a few micrometres thick,” explained Scarpulla. “Silicon has poor light absorption at red and infrared wavelengths, meaning that not all of the light will be absorbed.”

Scattering the light using nanoparticles gives it a longer path length through the silicon cell, thus increasing the chance of absorption. Previous studies exploited this idea by placing nanoparticles on top of the cell, but this is difficult to integrate with an antireflective coating, which is a key component of any solar cell. Embedding SiO2 nanoparticles directly into the cell material therefore avoids this problem.

The researchers showed that this approach could increase the total light absorbed in the top 1 μm of an infinitely thick silicon layer by about 10%. However, Scarpulla pointed out that there are obstacles to putting this approach into practice. “The real question is how to implement this concept without completely ruining the electronic behaviour of the device,” he explained.

Best of all solar worlds

Nano. Lett. 10, 2609–2612 (2010)

Uniting inorganic and organic optoelectronics may be the answer to improving the efficiency and reliability of DSCs, according to researchers in Korea and Switzerland. The team produced inorganic–organic heterojunction solar cells by depositing stibnite and poly(3-hexylthiophene) (P3HT) on the surface of a mesoporous titanium dioxide layer.

“Conventional DSCs have acceptable efficiencies, given their low fabrication costs,” explained Sang Il Seok from the Korea Research Institute of Chemical Technology. “However, they have issues sealing in their liquid hole conductors.” His team therefore used solid organic P3HT as both a hole conductor and light absorber. In addition, stibnite was chosen because its bandgap and absorption coefficient make it well-suited for use as a sensitizer, and also because it is cheaper and more stable than the dyes used in conventional DSCs.

The resulting cells delivered an incident photon-to-electron conversion efficiency of 80% and a power conversion efficiency of 5.15% under full-sun conditions, which compares favourably with DSC and organic solar-cell record efficiencies of 11.5% and 6.77%, respectively. “The conversion efficiency was amazing,” Seok said. “This indicates that we can easily increase it by tuning the individual key components.”

CdTe films get thinner

Appl. Phys. Lett. 96, 242103 (2010)

Researchers from the University of California at Santa Cruz in the USA have produced ultrathin cadmium telluride (CdTe) solar cells that use only around 10% of the material needed to fabricate conventional CdTe cells. To cut the amount of semiconductor material required, the team spun-cast colloidal solutions of CdTe nanorods onto substrates coated with indium tin oxide. Coating and sintering the resulting film in cadmium chloride and adding aluminium contacts produced a cell that was able to deliver a power conversion efficiency of 5%, even at a thickness of only 360 nm. “This result proves that using solution-deposited colloidal quantum dots as the solar-cell material can provide charge-transport efficiencies that are competitive with bulk thin films,” said Sue Carter, leader of the research team. “This research demonstrates the potential for achieving power efficiencies suitable for commercial photovoltaic applications from printed ultrathin films of CdTe nanoparticles.”