Polymeric Dopant-Free Hole Transporting Materials for Perovskite Solar Cells: Structures and Concepts towards Better Performances

Abstract

Perovskite solar cells are a hot topic of photovoltaic research, reaching, in few years, an impressive efficiency (25.5%), but their long-term stability still needs to be addressed for industrial production. One of the most sizeable reasons for instability is the doping of the Hole Transporting Material (HTM), being the salt commonly employed as a vector bringing moisture in contact with perovskite film and destroying it. With this respect, the research focused on new and stable "dopant-free" HTMs, which are inherently conductive, being able to effectively work without any addition of dopants. Notwithstanding, they show impressive efficiency and stability results. The dopant-free polymers, often made of alternated donor and acceptor cores, have properties, namely the filming ability, the molecular weight tunability, the stacking and packing peculiarities, and high hole mobility in absence of any dopant, that make them very attractive and a real innovation in the field. In this review, we tried our best to collect all the dopant-free polymeric HTMs known so far in the perovskite solar cells field, providing a brief historical introduction, followed by the classification and analysis of the polymeric structures, based on their building blocks, trying to find structure-activity relationships whenever possible. The research is still increasing and a very simple polymer (PFDT-2F-COOH) approaches PCE = 22% while some more complex ones overcome 22%, up to 22.41% (PPY2).

Keywords: dopant-free polymers; hole transporting materials; organic materials; perovskite solar cells.

Figure 1

Different visualization of the perovskite structure: (a) Crystal structure of organometal trihalide with chemical structure of ABX₃, where A is the organic cation (green), B the metal cation (blue), and X the halide anion (red); (b) perovskite structure showing the B cation assembled around X anions to form BX6 octahedra. (c) Tilting of BX₆ octahedra occurring from nonideal size effects and other factors. Reprinted with permission from (J. Phys. Chem. Lett. 2016, 7, 851–866, ref. [35]) Copyright (2016) American Chemical Society.

Figure 2

Different perovskite deposition methods (a) solution-based one-step method (b) co-evaporation; (c) solution-based two-step method; (d) sequential evaporation; (e) vapor-assisted solution process (VASP). Reprinted from Progr. Quantum Electron. 2017, 53, 1–37, Djurišić, A. et al., Perovskite solar cells–An overview of critical issues. [46] Copyright (2017), with permission from Elsevier.

Figure 3

Schematic representation of different PSC device architectures with the (a) mesoporous n–i–p configuration, (b) planar n–i–p configuration, and (c) planar-inverted (p–i–n) device stacks. Reprinted with permission from ref. [55] Copyright 2015 John Wiley and Sons.

Scheme 1

Main dopants/additive used in the doping process of organic Hole Transporting Materials (HTMs).

Scheme 2

The HTM reference small molecules, spiro-OMeTAD and first dopant-free HTM, TTF-1.

Scheme 3

Classical homopolymeric HTMs and some dopant-free polymeric HTMs (or their monomers).

Scheme 4

Polymers containing the 2,5-Dihydropyrrolo[3,4]Pyrrole-1,4-Dione (DDP) group.

Scheme 5

HTMs containing the BDT group.

Scheme 5

HTMs containing the BDT group.

Scheme 6

HTMs containing the benzothiadiazole (BTZ) and or the Carbazole (Cbz) group.

Figure 4

Possibilities of Quinoid States for Three Polycarbazoles and Their Main Charge Transport Processes. Reprinted with permission from (Xie, Y.et al. Macromolecules 2019, 52 (12), 4757–4764) [158]. Copyright (2019) American Chemical Society.

Scheme 7

Miscellaneous of dopant-free HTM polymers.

Scheme 7

Miscellaneous of dopant-free HTM polymers.

Figure 5

Different packing of polymers: edge-on (left); face-on (right). Reprinted with permission from ref. [171] Copyright 2015 John Wiley and Sons.