A new carbazole-based hole-transporting material with low dopant content for perovskite solar cells

A low-cost asymmetric carbazole-based hole-transporting material for efficient perovskite solar cells

Perovskite solar cells (PSCs) have reached their highest efficiency with the state-of-the-art hole-transporting material (HTM) spiro-OMeTAD.

Simple 3,6-disubstituted Carbazoles as Potential Hole Transport Materials: Photophysical, Electrochemical and Theoretical Studies

Developing effective and low-cost organic hole-transporting materials (HTMs) is crucial for the construction of high-performance perovskite solar cells (PSCs) and to promote their production in commercial ventures. In this context, we herein report the molecular design, synthesis and characterization of two novel D-A-D-A-D architectured 9-(2-ethylhexyl)-9H-carbazoles, connecting the mono/dimethoxyphenyl substituted cyanovinylene sidearms symmetrically at 3^rd and 6^th positions of the carbazole heterocycle (CZ_1-2 ), as potential hole-transporting materials (HTMs). The current work highlights their structural, photophysical, thermal, electrochemical and theoretical investigations, including their structure-property correlation studies. Evidently, the optical studies showcased their excellent fluorescence ability due to their push-pull natured structure with extended π-conjugation. Further, in-depth solvatochromic studies demonstrated their intramolecular charge-transfer (ICT)-dominated optoelectronic behavior, supported by various correlation studies. Also, the optical results revealed that CZ₁  and CZ₂ display λ_abs  and λ_emi  in the order of 410-430 nm and 530-560 nm, respectively, with a bandgap in the range of 2.5-2.6 eV. Finally, their quantum chemical simulations have provided an insight into the predictions of their structural, molecular, electronic and optical parameters. Conclusively, the study furnishes a deeper understanding of the intricacies involved in the structural modification of carbazole-based HTMs for achieving better performance.

New Helicene-Type Hole-Transporting Molecules for High-Performance and Durable Perovskite Solar Cells

Three azahelicene derivatives with electron-rich bis(4-methoxyphenyl)amino or bis( p-methoxyphenyl)aminophenyl groups at the terminals were deliberately designed, synthesized, and characterized as hole-transporting materials (HTMs) for perovskite solar cells (PSCs). Optical and thermal properties, energy level alignments, film morphologies, hole extraction ability, and hole mobility were studied in detail. PSCs using the newly synthesized molecules as HTMs were fabricated. A maximum power conversion efficiency (PCE) of 17.34% was observed for the bis( p-methoxyphenyl)amino-substituted derivative (SY1) and 16.10% for the bis( p-methoxyphenyl)aminophenyl-substituted derivative (SY2). Longer-chain substituent such as hexyloxy group greatly diminishes the efficiency. In addition, the dopant-free devices fabricated with SY1 as the HTM shows an average PCE of 12.13%, which is significantly higher than that of spiro-OMeTAD (7.61%). The ambient long-term stability test revealed that after 500 h, the devices prepared from SY1 and SY2 retained more than 96% of its initial performance, which is much improved than the reference device with standard spiro-OMeTAD as the HTM under the same conditions. Detailed material cost analysis reveals that the material cost for SY1 is less than 8% of that for spiro-OMeTAD. These results provide a useful direction for designing a new class of HTMs to prepare highly efficient and more durable PSCs.

Facilely Synthesized spiro[fluorene-9,9'-phenanthren-10'-one] in Donor-Acceptor-Donor Hole-Transporting Materials for Perovskite Solar Cells

We have demonstrated two novel donor-acceptor-donor (D-A-D) hole-transport material (HTM) with spiro[fluorene-9,9'-phenanthren-10'-one] as the core structure, which can be synthesized through a low-cost process in high yield. Compared to the incorporation of the conventional HTM of commonly used 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD), the synthesis process is greatly simplified for the presented D-A-D materials, including a minimum number of purification processes. This results in an increased production yield (>55 %) and suppressed production cost (<30 $ g^-1 ), in addition to high power conversion efficiency (PCE) in perovskite solar cells (PSCs). The PCE of a PSC using our D-A-D HTM reaches 16.06 %, similar to that of Spiro-OMeTAD (16.08 %), which is attributed to comparable hole mobility and charge-transfer efficiency. D-A-D HTMs also provide better moisture resistivity to prolong the lifetime of PSCs under ambient conditions relative to their Spiro-OMeTAD counterparts. The proposed new type of D-A-D HTM has shown promising performance as an alternative HTM for PSCs and can be synthesized with high production throughput.

Stability of organometal halide perovskite solar cells and role of HTMs: recent developments and future directions

Perovskite solar cells (PSCs) have recently emerged as one of the most exciting fields of research of our time, and the World Economic Forum in 2016 recognized them as one of the top 10 technologies in 2016. With 22.7% power conversion efficiency, PSCs are poised to revolutionize the way power is produced, stored and consumed. However, the widespread use of PSCs requires addressing the stability issue. Therefore, it is now time to focus on the critical step i.e. stability under the operating conditions for the development of a sustainable and durable PV technology based on PSCs. In order to improve the stability of PSCs, hole transport materials (HTMs) have been considered as the paramount components. This is due to the fact that most of the organic HTMs possess a hygroscopic and acidic nature that leads to poor stability of the PSCs. This article reviews briefly but comprehensively the environmental stability issues of PSCs, fundamentals, strategies for improvement, the role of HTMs towards stability and various types of HTMs. Also the environmental parameters affecting the performance of perovskite solar cells including temperature, moisture and light soaking environment have been considered.

Hole-Transporting Materials for Printable Perovskite Solar Cells

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,2',7,7'-tetrakis-(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs.

Low-Cost Carbazole-Based Hole-Transport Material for Highly Efficient Perovskite Solar Cells

A low-cost carbazole-based small-molecule material, 1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethylcarbazole, was designed and synthesized through a facile three-step synthetic route. The material was characterized and applied as a hole-transport material (HTM) for low-temperature-processed planar perovskite solar cells (PSCs). Devices based on this new HTM exhibit a high power-conversion efficiency of 17.8 % that is comparable to that of PSCs based on the costly 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) (18.6 %) .

Novel efficient hole-transporting materials based on a 1,1′-bi-2-naphthol core for perovskite solar cells

New hole-transporting materials based on 1,1′-bi-2-naphthol and carbazole.