Challenges in the design and synthesis of self-assembling molecules as selective contacts in perovskite solar cells

Abstract

Self-assembling molecules (SAMs), as selective contacts, play an important role in perovskite solar cells (PSCs), determining the performance and stability of these photovoltaic devices. These materials offer many advantages over other traditional materials used as hole-selective contacts, as they can be easily deposited on a large area of metal oxides, can modify the work function of these substrates, and reduce optical and electric losses with low material consumption. However, the most interesting thing about SAMs is that by modifying the chemical structure of the small molecules used, the energy levels, molecular dipoles, and surface properties of this assembled monolayer can be modulated to fine-tune the desired interactions between the substrate and the active layer. Due to the important role of organic chemistry in the field of photovoltaics, in this review, we will cover the current challenges for the design and synthesis of SAMs PSCs. Discussing, the structural features that define a SAM, (ii) disclosing how commercial molecules inspired the synthesis of new SAMs; and (iii) detailing the pros- and cons- of the reported synthetic protocols that have been employed for the synthesis of molecules for SAMs, helping synthetic chemists to develop novel structures and promoting the fast industrialization of PSCs.

Fig. 1. Common architectures of PSCs (regular and inverted) and their working mechanism. h: holes, e: electrons.

Fig. 2. Organic small molecules as HTMs in different organizations: (a) random or (b) as SAMs.

Fig. 3. Structural components of the SAM and self-assembly process to form the monolayer.

Fig. 4. Commercial molecules are used for the formation of SAMs in PSCs, indicating the highest PCE obtained in each case.

Scheme 1. Synthesis of C₆₀-SAM 44 through Prato reaction. (Red) Anchoring group; (Blue) spacer group; (Brown) functional group.

Scheme 2. Click chemistry approach for the functionalization of fullerene core and synthesis of fullerene diol 49.

Scheme 3. Prato reaction for the synthesis of fulleropyrrolidine 52 substituted with the thiophene moiety.

Scheme 4. Synthesis of V1036 58, the first carbazole-based SAM in PSCs.

Scheme 5. Synthesis of 2PACz 61 and its derivatives under the reactions sequence: halogenation/N-alkylation/Arbuzov reaction.

Scheme 6. Synthesis of 2PACz-based molecules 75a–e with different spacer group lengths, n = 2, 4 and 6.

Scheme 7. Synthesis of MeO-2PACz analog 80 substituted in positions 2 and 7.

Scheme 8. Introduction of aromatic rings in the spacer group of molecules 83 and 84.

Scheme 9. Synthesis of molecules 89 and 91 with conjugated spacer groups and cyanoacetic moiety as anchoring group.

Scheme 10. Synthesis of π-expanded carbazoles 98 and 99 as an approach for the formation of stable and efficient SAMs.

Scheme 11. Synthesis of the first molecule based on the TPA fragment for SAM in PSCs.

Scheme 12. Synthesis of derivative 109 introducing the donor dimethoxyphenyl fragment on the TPA moiety.

Scheme 13. Synthesis of molecules 113a–c prepared to study the role of the substitution pattern in TPA-based SAMs in PSCs.

Scheme 14. Synthesis of TPA- TPA-benzothiadiazole HTMs for the fabrication of SAMs with different anchoring groups.

Scheme 15. Synthesis of electron-rich TPA derivatives 131 and 132 with conjugated spacer groups.

Scheme 16. Synthesis of phenothiazine 135 with TPA groups functionalized at positions 3 and 7.

Scheme 17. Synthesis of phenothiazines with different anchoring groups: –H 136, –COOH 137, –SO₃H 138, and –PO(OH)₂139.

Scheme 18. Synthesis of phenothiazine 144 substituted with electron-withdrawing groups at positions 3 and 7.

Scheme 19. Phenothiazines derivatives incorporating sulfur 144, oxygen 144a and selenium 144b on the head group.

Scheme 20. First examples of bithiophene derivatives 147 and 148a–d prepared to form SAMs in PSCs.

Scheme 21. Synthesis of bithiophene derivatives 150a–c having the TPA and carbazole moieties as functional groups.

Scheme 22. Synthesis of the thiophene derivative 152 with two anchoring groups for the formation of bidentate SAM.

Scheme 23. Examples of molecules with one thiophene ring are a spacer group, a similar anchoring group, and different functional groups showing different dipole moments.

Scheme 24. Synthesis of naphthalimide derivatives 160, 163, and 165 for the formation of SAMs in n-i-p perovskite solar cells.

Scheme 25. Preparation of functionalized ionic liquid 169 for the deposition of SAMs on the FTO surface.

Scheme 26. Synthesis of the carboxylated Zn-phthalocyanine derivative 171.

Scheme 27. Synthesis of molecules 177a and b bearing a methylene group to induce flexibility on the layer of SAMs formed on the ZnO surface.

Scheme 28. Synthesis of the triazatruxene derivatives 185–188 for the formation of multipodal SAMs in PSCs.

Scheme 29. Preparation of quinoxaline 192 containing two MeOTAP fragments as functional groups via condensation reaction.

Scheme 30. Synthetic route for the construction of the spacer group around the quinoxaline core in derivatives 194 and 196.

Scheme 31. Synthesis of the X-shape derivatives 197–200 with the MeOTAP donor group and nitrile functionalities as anchoring groups.

Scheme 32. Synthesis of the spiro OMeTAD derivative 211.

Carlos E. Puerto Galvis

Dora A. González Ruiz

Eugenia Martínez-Ferrero

Emilio Palomares