π-Conjugated Materials as the Hole-Transporting Layer in Perovskite Solar Cells

Abstract

Hybrid organometal halide perovskites have attracted much attention these past four years as the new active layer for photovoltaic applications. Researches are now intensively focused on the stability issues of these solar cells, the process of fabrication and the design of innovative materials to produce efficient perovskite devices. In this review, we highlight the recent progress demonstrated in 2015 in the design of new π-conjugated organic materials used as hole transporters in such solar cells. Indeed, several of these "synthetic metals" have been proposed to play this role during the last few years, in an attempt to replace the conventional 2,2 1 ,7,7 1 -tetrakis-(N,N-di-4-methoxyphenylamino)-9,9 1 -spirobifluorene (Spiro-OMeTAD) reference. Organic compounds have the benefits of low production costs and the abundance of raw materials, but they are also crucial components in order to address some of the stability issues usually encountered by this type of technology. We especially point out the main design rules to reach high efficiencies.

Toward Highly Efficient Inkjet‐Printed Perovskite Solar Cells Fully Processed Under Ambient Conditions and at Low Temperature

“…Several interfacial layers between ITO and the perovskite active layer can be used for perovskite devices . The most popular interfacial layers are TiO x and WO x .…”

Hole‐Transporting Materials for Perovskite‐Sensitized Solar Cells

“…TTF also hasa strong electron‐donating ability and tuning of the energy levels is easy due to facile substitution at the 2‐,3‐,6‐, and 7‐positions. TTF also aids in the facile construction of p‐type semiconductors with high hole mobilities and tunable energy levels . Although TTF was an efficient hole conductor, it was less soluble in common solvents, which created the requirement for expensive vacuum sublimation methods for deposition.…”

Quinoidal 2,2′,6,6′‐Tetraphenyl‐Dipyranylidene as a Dopant‐Free Hole‐Transport Material for Stable and Cost‐Effective Perovskite Solar Cells

“…In the last few years the development of perovskite solar cells (PSCs) has been unprecedented, with power conversion efficiencies (PCEs) exceeding 20 % . In current PSC devices spiro‐OMeTAD is the most commonly used hole‐transport material (HTM) . However, its high cost due to a nontrivial synthesis combined with the need to mix it with various dopant materials is a major hindrance towards cost‐effective PSCs.…”

“…[5][6][7][8] In current PSC devices spiro-OMeTAD is the most commonly used hole-transportm aterial (HTM). [9] However, its high cost due to an ontrivials ynthesisc ombined with the need to mix it with variousd opant materials is am ajor hindrance towards cost-effective PSCs.T herefore, it is mandatory to developa l-ternativeH TMs with good transportp roperties that are easy to synthetize.Anumber of conjugated polymers,o rganic small molecules,a nd inorganic HTMs were reported. [10][11][12] Organic-small-molecule HTMs are preferred because they offer some uniquea dvantages such as the possibility to tune their electronic properties by chemical design, few-steps, and low-costs ynthetic pathways;s olution processability;g ood stability;a nd its often environmental friendliness.M ost of the reported HTMs are inferior to spiro-OMeTAD,b ut no HTM has yet clearlye merged in terms of cost, efficiency, and stability.V arious classes of HTMs basedo nt hiophene, [13] carbazole, [14] triazatruxene, [15] star-shaped molecules, [16] fluorinated indolo [3,2-b]indole, [17] or triptycene, [18] were recently investigated in PSCs.H owever, ita ppearst hat an umber of these HTMs possess an on-planar molecular conformation that induces large intermolecular distances in the solid state; at ypicale xample are HTMs based on triarylamines.…”