Introduction to Next-Generation Renewable Energy:

Organic Photovoltaics

By Assistant Prof. Yu-Wei Su of Molecular Science and Engineering

The current development trend of photovoltaic (also known as solar cells) can be categorized based on the types of materials, which includes traditional inorganic photovoltaics, such as silicon (Si)-based, cadmium telluride (CdTe), and copper indium gallium selenide (Cu(In_xGa_1-x)Se₂) thin-film cells. Due to the high-temperature manufacturing processes for forming high-crystallinity inorganic material of inorganic photovoltaics, it is a challenge to modify the materials themselves to achieve desired component characteristics. On the other hand, photovoltaics based on organic materials include organic photovoltaics (OPVs) and dye-sensitized solar cells. Due to the advancements of organic photovoltaics on its low manufacturing costs and suitable for flexible substrates in lightweight applications, modifying interface layers and electrode thickness have enabled the development of semi-transparent photovoltaic devices, particularly in the field of agrivoltaics in the recent years.

In organic materials, particularly in conjugated polymers, the dielectric constant typically lies between 3 and 4. The excitons generated in these polymers possess binding energies ranging from 0.5 to 1 eV, which is markedly higher than the thermal energy at room temperature (0.025 eV). Therefore, when conjugated polymers are employed as electron donors in OPVs, they combined with electron acceptor materials to establish an internal electric field at the donor-acceptor interface, enabling the dissociation of excitons into free electrons and holes. Over the past two decades, research in OPVs has advanced rapidly, driven by four primary factors: (1) an efficient photon-to-electron conversion mechanism; (2) the development of novel non-fullerene small molecules that absorb infrared light and allow energy level tuning via molecular design; (3) the formation of optimized microstructures in the active layer with donor-acceptor conjugated polymers; and (4) the introduction of new device architectures, especially those with innovative interface layers. The main focus of OPV research has been bulk heterojunction structures, with power conversion efficiencies surpassing 19%. As depicted in Figure 1, to further enhance efficiency, we utilize a layer-by-layer spin-coating technique to construct an active layer with a p-i-n structure. This approach allows precise control over the donor/acceptor interface in the vertical dimension, promoting exciton dissociation and charge transport through the p-i-n structure. Additionally, we employ grazing incidence wide-angle and small-angle X-ray scattering (GIWAXS/GISAXS) to assess the crystallographic orientation of the conjugated polymers and the size distribution of polymer/small molecule aggregates within the active layer. Furthermore, we employed density functional theory (DFT) calculations to investigate the impact of side-chain substituents (-H, -F, -OCH₃) on electron transfer in the HOMO and LUMO levels of conjugated polymers (Figure 2). This computational method offers a practical tool for efficiently screening potential molecular structures prior to the design of future conjugated polymer molecules.

[1] Y.-W. Su, S.-C. Lan, K.-H. Wei, Mater. Today 2012, 15 (12), 554-562.

[2] Y.-W. Su, C.-E. Tsai, T.-C Liao, K.-H. Wei, Solar RRL 2024, 8 (5), 2300927.

[3] Y.-W. Su, Y.-C. Lin, K.-H. Wei, J. Mater. Chem. A 2017, 5 (46), 24051-24075.

[4] P.-T. Chen, Y.-W. Su, Chem. Phys. Lett. 2024, 850, 141447.