Abstract
Global energy demand is increasing; thus, emerging renewable energy sources, such as organic solar cells (OSCs), are fundamental to mitigate the negative effects of fuel consumption. Within OSC’s advancements, the development of efficient and stable interface materials is essential to achieve high performance, long-term stability, low costs, and broader applicability. Inorganic and nanocarbon-based materials show a suitable work function, tunable optical/electronic properties, stability to the presence of moisture, and facile solution processing, while organic conducting polymers and small molecules have some advantages such as fast and low-cost production, solution process, low energy payback time, light weight, and less adverse environmental impact, making them attractive as hole transporting layers (HTLs) for OSCs. This review looked at the recent progress in metal oxides, metal sulfides, nanocarbon materials, conducting polymers, and small organic molecules as HTLs in OSCs over the past five years. The endeavors in research and technology have optimized the preparation and deposition methods of HTLs. Strategies of doping, composite/hybrid formation, and modifications have also tuned the optical/electrical properties of these materials as HTLs to obtain efficient and stable OSCs. We highlighted the impact of structure, composition, and processing conditions of inorganic and organic materials as HTLs in conventional and inverted OSCs.
Highlights
Solar energy has enough power capacity to satisfy the whole world’s demand [1,2].According to Luqman et al [3], the amount of solar energy irradiated at the Earth’s atmosphere ranges from 200 to 250 Wm−2 per day, of which ca. 70% is available for conversion into power generation [4,5]
To achieve high-performance organic solar cells (OSCs), the materials used for hole transporting layers (HTLs) need to show (i) high work function (WF) that matches with the highest occupied molecular orbital (HOMO) energy level of the donor material and the anode energy level, (ii) transparency to increase the light absorption by the active layer, (iii) high hole mobility to lower the charge accumulation and recombination, (iv) a large band gap to block electron carriers, and (v) chemical resistance to external factors [38,71,72]
Modification in the particles’ size or addition of metal NPs results in the localized surface-plasmon resonance (LSPR) effect, which increases the light absorption. Inorganic materials such as Mo and Ni were doped with V and Cu-doped NiOx (Cu) to tune the WF and increase conductivity and transparency, resulting in a high Voc, fill factor (FF), and Jsc
Summary
Solar energy has enough power capacity to satisfy the whole world’s demand [1,2]. According to Luqman et al [3], the amount of solar energy irradiated at the Earth’s atmosphere ranges from 200 to 250 Wm−2 per day, of which ca. 70% is available for conversion into power generation [4,5]. Since Kearns and Calvin’s pioneering work on OSCs in 1958, one significant breakthrough in solar energy technology has been the efficient electron transfer between a conjugated polymer and fullerene derivative [15,16] It encouraged the interest in the light-harvesting of OSCs from the structure device into the materials used for their construction [16,17,18,19]. We presented an extensive state-of-the-art review about the advances in HTLs that show great potential for enhancing the efficiency (e.g., PCE) and stability of OSCs. The progress made on improving HTL properties of inorganic (metal oxides and sulfides), nanocarbon materials, conjugated polymers, and small organic molecules as HTLs in OSCs was discussed, focusing on solution-processing conditions, deposition methods, doping, composite/hybrid formation, and chemical modifications. Summary tables of the photovoltaic device architecture and their performance are presented at the end of Sections 4.3. and 4.5
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