Semiconducting Polymer Donor Materials Boost Photon Harvesting in Organic Solar Cells

Organic solar cells are constantly improving with the development of new materials for the active layer, especially when materials are stacked in a bulk heterojunction design that leverages various combined absorption windows to utilize photons at more parts of the spectrum.

Grazing-incidence wide-angle X-ray scattering was used to profile the molecular packing and orientation of the binary system’s material with PBDB-T-SF and IT-4F added in different amounts. Image Credit: Data obtained at Beamline I07 at the Diamond Light Source (UK)

Non-fullerene materials are particularly favorable in binary organic solar cells, enabling optical and energy properties to be adjusted. However, regardless of their benefits, these materials possess narrow absorption windows. Efforts to integrate non-fullerene acceptors into organic solar cells include the addition of a third component to boost photon harvesting.

It is vital to carefully choose the third component material so it does not have an impact on the molecular form and structure in ways that lower efficiency but ensure charge and energy transfer in the right direction.

A study reported recently in Applied Physics Reviews, from AIP Publishing, offers a practical guide for choosing materials for ternary organic solar cells. The authors used component engineering to prolong the light absorption and effectiveness of solar cells in a simple, physical way, rather than using the complex process of synthesizing novel semiconductors.

They began with an exclusive non-fullerene electron acceptor known as COi8DFIC, which has high-power conversion efficiency because of its high bandgap and the ability to change its molecular orientation from lamella orientations to H-and J-type aggregations through hot substrate casting.

During the research, they combined a PTB7-Th:COi8DFIC binary system with the polymer electron donor PBDB-T-SF and the small molecular electron acceptor IT-4F to establish the suitability of each material for ternary devices.

They learned that either a donor or acceptor material can be used positively in ternary devices: it was found that IT-4F and PBDB-T-SF are effective when incorporated into the binary PTB7-Th:COi8DFIC system in quantities of 15% and 10%, respectively.

The materials improved photon-harvesting, enhanced the spectral response, and had an impact on the molecular order of the host materials to optimize π-π stacking. Stacking the molecular planes parallel to the device electrode directly adds to power conversion efficiency, charge mobility, and preserving fine phase separation.

The coexistence of H-and J-type aggregations means the device has a broader absorption spectrum and will absorb more photons in both short and long wavelength ranges and convert them into charges, resulting in higher efficiency.

Tao Wang, Study Author

Going forward, the authors hope to investigate physical techniques to better regulate the formation of the material, to prevent H-type and boost J-type aggregation, which prolongs the light absorption toward near-infrared, making semi-transparent organic solar cells viable.

Source: http://www.aip.org

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