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).

Tailored Polymeric Hole‐Transporting Materials Inducing High‐Quality Crystallization of Perovskite for Efficient Inverted Photovoltaic Devices

“…[11] Many efforts have been devoted to develop low-cost and dopant-free polymers as alternatives to PTAA. [12][13][14][15][16] Various polymeric HTMs have been developed with good efficiencies, such as donor-π-acceptor type conducting polymers, [17][18][19][20] crossconjugated polymers, [21] and nonconjugated polymers. [22] However, only few dopant-free materials can achieve the efficiency comparable to PTAA, and the synthesis steps of these materials are complicated.…”

“…However, until now, researches on developing polymeric HTMs have mainly focused on designing different π‐conjugated backbones in order to control the hole extracting and transporting properties, and there is rarely research on this issue. [ 12,14 ]…”

“…However, until now, researches on developing polymeric HTMs have mainly focused on designing different π-conjugated backbones in order to control the hole extracting and transporting properties, and there is rarely research on this issue. [12,14] Herein, we designed and synthesized three co-polymeric HTMs consisting of alternating 4-methoxy-N,N-diphenylaniline (TA) and para-(or meta-, or ortho-) phenylenevinylene (PV) repeating units in their main backbone structures, which named PTA-pPV, PTA-mPV, and PTA-oPV (Figure 1a). These three polymers could be easily polymerized by Wittig-Honor reaction without using any noble-metal catalysts.…”

Organic Solar Cells Parameters Extraction and Characterization Techniques

“…A voltage ramp in the reverse direction is used to collect the charges leading to a ramp in the displacement current density j(0). The time t max needed by the transient current to reach its peak is used to estimate the mobility in the device through the simple analytical formula given by (15) [107][108][109].…”

“…For example, polymers like polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are used as a substrate due to their high flexibility, and transparency [ 12 ]. An example of polymers as active layer materials are PVK-MEHPPV:PCBM [ 13 ], and P3HT:PCBM, which has been under massive research in the past decade [ 14 ], additive or dopant material for active and hole transport layers morphology modifications in different types of solar cells [ 15 ]. Polymers are used as an interfacial layer also.…”