Researchers Develop Groundbreaking Flexible Solar Cell Technique

  • Published
  • By Dr. Benjamin Leever
  • Materials and Manufacturing
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  A groundbreaking research technique demonstrated by the Air Force Research Laboratory detects performance variability at a microscopic scale during operation of flexible solar cells. This technique will be essential in evaluating new materials and architectures for organic solar cells, and improving their efficiency for Air Force applications.

The Air Force needs power, and flexible solar cells have the potential to affect a wide range of applications, including those used for air vehicles, personal power, and even airbases. Integrating solar cells onto the wings of unmanned aerial vehicles will dramatically increase their range, and the logistical burden of powering a deployed airbase could be significantly reduced by building solar cells directly onto structures such as tents.

A large class of flexible solar cells is based on plastics, which offer many advantages including possible manufacture by low-cost techniques such as spraying, casting, and printing. The solar cells work via photovoltaic properties of the plastic, which convert a portion of the light that hits the solar cells to electricity. Because these materials are very effective at absorbing light, the plastic can be less than one ten-thousandth of an inch thick, allowing flexibility and semi-transparency.

In addition to the plastic layer, these devices include a transparent electrode, called indium-tin-oxide (ITO), which is also used in flat-panel TVs and computer monitors. Although it was well known that this transparent electrode was not electrically uniform, the impact of this variability on the performance of solar cells was not understood. Working with Northwestern University, AFRL researchers developed a new technique to determine how the electrical variability in the transparent electrode layer impacts the solar cell performance.

The technique is called Atomic Force Photovoltaic Microscopy (AFPM), and it works by scanning a nano-scale stylus across an array of microscopic solar cells. These small solar cells are illuminated with simulated solar light so that they actually function during the experiment, a significant improvement over previous methods. The small size of the solar cells enables the identification of variations in the performance of the plastic at a very small scale. The technique allows full characterization of each microscopic solar cell, and it also enables the generation of a "map" of the plastic film showing locations of improved and degraded performance.

AFPM results establish a clear link between the variability of the transparent conductor surface and the performance of the photovoltaic plastic layer. This new insight suggests that tailoring the surface properties of the transparent conductor layer could be crucial in improving solar cell performance.