Light absorption in thin film photovoltaic technologies, such as those based on organic materials, quantum dots or perovskites, is well below the theoretical maximum. Indeed, more than 20% of photons are thought to be scattered out of these devices or absorbed outside of the active light-absorbing layer. Over the last decades, researchers have been trying to remedy this problem by developing nanostructures that better absorb light. Some examples include incorporating hexagonal arrays of nanocolumns in solar cells, or embossing nanoholes in the active layer. They have also tried integrating nanoimprinted electrodes capable of both diffracting light and collecting photogenerated charge carriers (electrons and holes), 1D photonic crystals or Bragg reflectors for transparent cells, nanospheres to couple light in “whispering gallery” modes and metallic nanoparticles to increase light absorption.

A team at ICFO-The Institute of Photonic Sciences led by Jordi Martorell, also a professor at the Universitat Politecnica de Catalunya, has now developed a new resonating optical cavity for thin film solar cells that absorbs light at two different “non-harmonic” frequencies. “When integrated with the polymer PCE-10 (or PTB7-Th as it also known), we measured a PCE of 11.1%, which is the maximum efficiency ever measured for this polymer,” explains Martorell. “This cavity could also be used for other kinds of thin films, and in particular for those made from the novel perovskites.”

Combining two optical cavities

“The new cavity is formed by combining two optical cavities resonating at different non-harmonic frequencies,” he adds. “The resulting cavity is non-resonant at either one of the two frequencies, but at the same time light propagation within it has a reminiscence of both resonances,” he tells nanotechweb.org.

“This unusual effect can also be achieved in two-handed guitar tapping when a given string is tapped, first with one hand, and then with the other hand while the string is still vibrating from the first tap. In the transition from one note to another, the string is said to vibrate inharmonically, producing a sound that has the ‘character’ of both notes. This is why we call our cavity a ‘two-resonance tapping cavity’ – an effect we found quite unexpectedly when we started to study different ways to optimize light harvesting in thin film photovoltaic materials.”

The cavity absorbs light over a broad range of frequencies. “It is made by combining a thin (high refractive index) dielectric layer and a thin metallic layer (ideally made of silver) on one side of the cavity,” explains Martorell. “On the other side, we have a high reflector, such as thick metal layer. The only complicated issue is to find the optimal combination of layer thicknesses but we solved this problem by implementing an inverse problem solving computer simulation, taking into the account the Sun’s spectrum and the absorption characteristics of the photovoltaic materials we employed.

Optimal light harvesting

“Light harvesting in the cavity is optimal, which means that we confine light in the active material in the most effective way for 1D structures, he adds. “For the polymer we used, we observed close to a 20% improvement in the short circuit current – which is an extremely large increase for such a parameter when considering photonics enhancements in solar cells. What is more, the optical cavity does not degrade other photovoltaic parameters (like the fill factor or the open circuit voltage) either.

“The cavity might be used to improve thin film solar cells, which are better than those made of silicon in many ways, except that they have a reduced light harvesting capacity. Our TRTC solves this issue almost completely.”

The next step for the researchers is to show the TRTC effect in perovskite-based solar cells. “So far, we have proven it theoretically, but there are still some fabrication issues that need to be resolved,” adds Quan Liu, first author of the research, which is detailed in Advanced Energy Materials DOI: 10.1002/aenm.201700356.