Recent progress in organic hole transport materials for energy applications

Highlights

• Organic hole transport materials are promising components for environmental sustainability and efficient optoelectronics.

• The synthesis and optoelectronics of more than 500 HTMs-based chemical structures are covered.

• The first part classifies the newly synthesized HTMs and their optoelectronic characteristics.

• The second part covers their applications in different fields, such as OLED, DSCs, OFET and PSCs.

• The HOMO-LUMO lying energy levels are crucial for the suitability of HTMs in energy applications.

Abstract

The ever-increasing demand for energy that necessitates fabrication of optoelectronic devices for efficient solar harvesting systems has driven scientists towards the development of the device components. One of the main components is the organic hole transport materials (HTMs) as a p-type semiconductor that has the advantages over inorganic materials of being biodegradable, low cost, and easily processed. Also, dopant free HTMs fulfilling easy film formation, thermal and light stability as well as good hole mobility are presented.

In this review, more than 500 HTMs, small molecules and some oligomers together with their synthetic approaches and how the chemical structure affects their properties as HTMs are discussed. The vast majority of HTMs were synthesized through metal-catalyzed cross-coupling reactions such as Ullmann reaction, Suzuki reaction, Buchwald-Hartwig reaction and Stille reaction. Therefore, the scope of this review comprises the importance of organic HTMs in energy applications, the synthetic approaches, and their classification according to the chemical structure. In addition, the review will present the structural dependent HOMO-LUMO energies, as these energy levels are of vital importance for the suitability of HTMs in different energy applications.

Introduction

Continuous interest has been paid to the development of organic HTMs by modulating their physical and chemical properties resulting in low-cost optoelectronics devices like organic field effect transistors [1,2], chemical and biological sensors [3], solar cells [[4], [5], [6]], light-emitting devices [[7], [8], [9]], vapor sensors [10], and photosensing devices [11,12].

Organic HTMs have many advantages, over their inorganic counterparts, such as good thin film-forming characteristics, low-cost, good solubility, infinite variety, environmentally friendliness, solution-processability, mechanical flexibility, the tunability of electronic properties and easy fabrication. Even though scientists in the materials area must deeply understand the architecture of HTMs as well as their microscopic organization in the active layer as this morphology will affect their performance. According to their role in optoelectronic devices, HTMs should have an amorphous character, and thus grain frames could be avoided in the amorphous film [13,14]. The glass transition temperature (T_g) is an essential character of HTMs. The T_g is the reversible transition in amorphous materials which allow rapid molecular motion under heating. Thus, the thin film included HTMs are allowed to be transited to a crystalline state upon heating above their T_g values. A significant relationship between stability comes from high T_g, and amorphous state was stated by Naito and Miura who stated that high stability could be achieved in large molecular weight compounds having asymmetric globular construction and weak intermolecular coherence [15,16].

In this review, more than 500 HTMs, small molecules, and some oligomers are presented. HTMs reviews have been of increasing attention in the last decade [5,[17], [18], [19]]. To the best of our knowledge, only a little attention has been paid towards the synthesis, classification and applications of organic HTMs. Therefore, the classification of organic HTMs and their synthetic pathways are presented and discussed. In addition, the applications of these HTMs are briefly highlighted in the last section of this review.