A new device that pairs a solar cell with heat harvesters captures more of the sun’s energy and converts it into electricity than the solar cell alone (ACS Nano 2015, DOI: 10.1021/acsnano.5b04042). If the hybrid device can be scaled up to larger sizes, it could be used to make energy-saving windows, among other applications, the researchers say.
“Commercial solar panels only harvest part of the radiation they’re exposed to,” says Eunkyoung Kim, who develops organic polymers at Yonsei University, in Seoul. The rest of the solar spectrum is lost, much of it as heat. To capture that heat, researchers have tried combining solar cells with pyroelectric or thermoelectric devices that convert heat into electricity.
But bringing all the components together is challenging, Kim says. The right materials have to be selected so that light and heat can transfer through one part of the device into the others. Previous hybrid devices didn’t have enough power output, says Kim. She decided to engineer clear conductive polymers to make the device more efficient and with sufficient voltage to power electrochromic windows that darken in response to electricity.
Kim and her colleagues built a multilayer device that combines different types of energy harvesters to capture more of the available visible light and heat. The top layer is a dye-sensitized solar cell, which has a low efficiency, but has the benefit of being transparent. Solar radiation that’s not captured and converted into electricity passes through to the layers below. The second layer is the key to the device: a stack of polymer electrodes made of transparent, pyroelectric polymer coated on both sides with conductive, transparent PEDOT polymer. This part of the device converts uncaptured visible light into heat, and some of the heat into electricity. Finally this photothermal/pyroelectric layer is glued with conductive paste to a thermoelectric layer that converts remaining heat into electricity.
Adding the heat conversion devices to the solar cell boosted the overall efficiency by 20% compared to the solar cell alone, Kim says. The hybrid device also gets an important boost in its voltage output—enough to power a light-emitting diode and an electrochromic display, which the solar cell alone can’t do. Kim wants to make larger versions of the device to power windows that would darken in response to sun and heat, keeping a building cooler. Under concentrated light, the hybrid device has a power conversion efficiency of 41.3%.
Rachel A. Segalman, a chemical engineer at the University of California, Santa Barbara, who works on thermoelectric materials, says combining different kinds of heat-converting devices with a solar cell, instead of just using either a pyroelectric or a thermoelectric device, is an “intriguing” idea. It’s analogous to tandem solar cells, she says, which use multiple layers of materials to capture a broader spectrum of visible and infrared light.
Still, the resulting power conversion is low, Segalman says. Even if each component has high efficiency, much can be lost in the connections between them, reducing the overall efficiency of the device.
Kim says she’s working on improving the output of the hybrid system, first by optimizing the performance of each component, for example by using nanostructured materials. She is also making devices with a larger area, which would be needed to make energy-saving windows.