Publication date: 17th February 2025
In the past 13 years, perovskite solar cells (PSCs) have emerged as the most promising third generation solar cell technology for large-scale energy production. Although performance-wise they are already capable of rivaling with silicon solar cells in lab-scale devices, their commercialisation is still impeded by the poor long-term stability of the perovskite light absorber. The nature of the hole transporting layer is one of the main factors for this instability: organic small molecules, which are responsible for the very high performances of PSCs, are also responsible for their quick degradation, as they require hygroscopic additives to reach the desired conductivity (the perovskite layer is very sensitive to moisture), and they can migrate into the light absorbing layer, disrupting it [1]. To solve these issues, the use of polymeric materials is a viable solution, as they are intrinsically more conductive than small molecules, thus often avoiding the use of dopants [2]. Metal complexes are a viable alternative to organic molecules, as their energy levels can be fine-tuned by tweaking the ligand environment or changing the metal centre [3]. They can be intrinsically doped by adding two different oxidation states of the metal centre to the hole transporting layer, being able to overcome state-of-the-art organic small molecules both in efficiency and stability [4].
In this work, we developed covalent polymers with a rigid backbone based on porphyrin monomers to use as hole transporters in perovskite solar cells. These materials take advantage of the higher conductivity of metal complexes compared to organic compounds and couple them with the greater stability of polymers compared to small molecules. Careful design of the ligand environment allows the formation of robust macromolecules that can be dissolved in perovskite-compatible solvents and that present a good degree of hydrophobicity, to help with the preservation of the light absorbing layer over time. Preliminary cyclic voltammetry measurements show that monomers with different metal centres have HOMO levels ranging from −5.65 to −5.40 eV and that, once polymerised, these values further decrease of about 0.25 eV, producing compounds compatible with a broad range of high-performing perovskite light absorbers.
The authors acknowledge support from Project CH4.0 under the MUR program “Dipartimenti di Eccellenza 2023–2027” (CUP: D13C22003520001), and from the PNA4Energy project, funded under the MUR program “PNNR M4C2 Initiative 1.2: Young Researcher - Seal of Excellence” (CUP: D18H22001950007).
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