The Influence of the Central Metal Ion on the Electronic and Photovoltaic Properties of Metalloporphyrins [M= Zn(II), Ni(II), Cu(II), Au(III)] Systems
Authors: Cuesta Virginia , Vartanian Maida , de la Cruz Pilar , Sharma Ganesh D. , Langa Fernando 
Published: September 2023

Although the pioneering use of porphyrins in organic solar cells (OSCs) was disappointing as reported efficiencies were very low, the situation has changed over the last five years as Zn-porphyrins with ABAB structures linked to acceptor units through triple bonds have been applied as donors resulting in efficiencies of up to 12% in binary OSCs and more than 15% in ternary OSCs.

The optical and electrochemical properties of porphyrins can be adjusted by molecular design and functionalization on the b or meso positions of the porphyrin ring as well as by introduction of different central metal ions. The choice of the centrally bounded metal cation within the porphyrin core plays an important role in the electronic properties allowing the modulation of the frontier orbital levels and thereby use the porphyrin-based molecule as donor or acceptor component of the device.

Due to the high HOMO level of Zn-Porphyrin derivatives, the Voc of OSCs using this family of compounds are lower than 0.9 V limiting the PCE of devices.

We have described that Ni-porphyrins derivatives allow the design of OSCs with voltage higher than 1V. These systems present slightly higher optical bandgaps but the deeper highest occupied molecular orbital (HOMO) energy level make available to reach good PCE values of the solar devices.

While porphyrins and their derivatives have been extensively used as donor component in the BHJ active layer of OSCs, the investigation of porphyrin-based materials as electron acceptor is very limited. Recently, we designed a new A2-D-A1-D-A2 non fullerene small molecule acceptor (NFSMA) with broad absorption up to 920 nm and a great absorption coefficient where A1 core is Au(III) porphyrin. This molecule, when used in OSC with a polymer donor yielded a PCE=9.24%. Nevertheless, the use of other central metals such as Cu(II) in the porphyrin core has been scarcely used in OSCs, the studied devices presented low efficiencies (≤ 3%). These outcomes demonstrated that metalloporphyrins, which is different from Zn-porphyrins, also have great potential for the design of NFSMAs for efficient OSCs. Also, we have studied A-π-D-π-A small molecules based on Cu(II) Porphyrin, scarcely used in OSCs, presenting an ambipolar behaviour, as donor and as acceptor. Here, I´ll present our recent work in design, synthesis, and application of porphyrin-based small molecules with different central metal for highly efficient OSCs.

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