Solar cells: Pillars of light

One of the major challenges in the world today is the energy crisis. The high demand and low supply of fossil fuel are driving up oil and food prices. Silicon-based solar cells are one of the most promising technologies for generating clean and renewable energy.

Using these devices to convert just a fraction of the sunlight that hits the earth each day into electricity could drastically cut society’s dependence on fossil fuels. Unfortunately, however, high-grade silicon crystals demand great care during the manufacturing process, making the resulting high production cost one of the main obstacles in the road to commercialization.

One way to bring down the production cost of these solar cells is to deposit layers of silicon onto cheaper substrates such as plastic or glass. However, this approach has one drawback: silicon thin films have lower power conversion efficiencies than bulk silicon crystals because they absorb less light and contain more defects. Patrick Lo at the A*STAR Institute of Microelectronics and co-workers[1] have now discovered an approach for increasing the power conversion efficiency of silicon thin films deposited on cheap substrates.

Low-grade silicon thin films suffer from one inherent problem: they cannot absorb photons whose wavelengths are larger than their film thickness. For instance, a standard, 800-nm-thick thin film may capture short-wavelength blue light, but will completely miss longer-wavelength red light. “To keep material costs low and improve light efficiency, the trick is to trap more photons, including those with medium wavelengths,” says Lo.

One way to trap more photons in the silicon thin film is to carve tiny silicon pillars—hundreds of nanometers in size—in the silicon surface (see image). Lo explains that the silicon nanopillars are like a forest of trees, in which light enters and cannot easily get out. “When light strikes the surface, it bounces a few more times along or inside the pillars before penetrating the bottom flat surface,” he says. “Each bouncing event increases the chances of photon absorption.”

Lo and co-workers used computer simulations to determine the best configuration for extracting electrical charges from the defect-ridden silicon films. They found that the upper portion of each pillar can be made extremely conductive by introducing large amounts of dopants. Lo and co-workers are now using these practical guidelines to engineer a prototype of this unique concept. “Working with nanostructures is a wonderful way to open paths that could overcome the limits set by conventional physics,” he notes.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics

Journal information

[1] Wong, S. M. et al. Design high-efficiency Si nanopillar-array-textured thin-film solar cell. IEEE Electron Device Letters 31, 335–337 (2010).