Hole transport layers in organic solar cells: A review

Abstract

Thanks to huge research efforts, organic solar cells have become serious candidates in the field of renewable energy sources, with reported power conversion efficiencies above 19% and operating lifetime surpassing decades. In the thin film stack composing the organic solar cell, the transport layers at interfaces play a key role, as important as the photoactive material itself. Both electron (ETL) and hole (HTL) transport layers are indeed directly involved in the efficiency and stability of the devices, due to the very specific properties required for these interfaces. Focusing on the HTL interface, a large number of materials has been used in organic solar cells, such as 2D materials, conductive polymers or transition metal oxides. In this review, we present the evolution and recent advances in HTL materials that have been employed in manufacturing organic solar cells, by describing their properties and deposition processes, and also relating their use with the fullerene or the new non-fullerene acceptors in the active layer.

Scaling-Up of Solution-Processable Tungsten Trioxide (WO3) Nanoparticles as a Hole Transport Layer in Inverted Organic Photovoltaics

“…Thermally evaporated air-stable metals such as gold (Au) or silver (Ag) were also used as top electrodes due to their high work function. This allows for an improved charge extraction and reduced recombination losses and eliminates oxidation problems [29,30]. The transparent electron-conducting layer can consist of metal oxides such as zinc oxide (ZnO) or titanium oxide (TiO x ), which helps resolve the issue of the acidic PEDOT:PSS on ITO or FTO [30,31] used in direct architecture.…”

“…This allows for an improved charge extraction and reduced recombination losses and eliminates oxidation problems [29,30]. The transparent electron-conducting layer can consist of metal oxides such as zinc oxide (ZnO) or titanium oxide (TiO x ), which helps resolve the issue of the acidic PEDOT:PSS on ITO or FTO [30,31] used in direct architecture. This leads to organic photovoltaic cells (OPVs) with inverted device structures that have better compatibility with solution and roll-to-roll processing.…”

“…This leads to organic photovoltaic cells (OPVs) with inverted device structures that have better compatibility with solution and roll-to-roll processing. Additionally, these structures offer superior stability in ambient conditions since metal anodes can be deposited from commercially available colloidal solutions [30]. In contrast, the direct architecture typically employs low work-function metals like LiF/Al or Ca/Al, which can be easily oxidized in air [30,32].…”

Application of Buffer Layer to Improve the Efficiency of Organic Solar Cells

“…[29,47] Therefore, the ABL material needs a reasonable energy level to selectively extract holes and block electrons and at the same time has a high conductivity and light transmission. [31] At present, the anode of OSCs generally uses indium tin oxide transparent conductive film with excellent conductivity, light transmission, and high hardness. The hole transport materials used in OSCs are divided into inorganic hole transport materials and organic hole transport materials, it mainly includes organic polymers and transition metal oxides.…”

“…However, the energy difference between the valence band level and the positive Fermi level and between the conduction band level and the negative Fermi level of the photoactive layer is generally large, which is not conducive to the efficient injection of holes and electrons into the positive and negative terminals respectively. [ 31 ] In addition, the electrons in the conduction band level of the photoactive layer and the holes in the valence band level can be injected into the positive electrode and the negative electrode respectively to generate leakage current, which greatly restrict the improvement of the PCE of OSCs. [ 32–34 ] Electrode buffer layer is introduced into the structure of OSCs, which can effectively solve the above problems.…”