A Bifunctional Saddle‐Shaped Small Molecule as a Dopant‐Free Hole Transporting Material and Interfacial Layer for Efficient and Stable Perovskite Solar Cells

Abstract

Herein, a new bifunctional saddle-shaped organic small molecule named 2,2 0 ,7,7 0 -tetrakis(N, N-di-p-methoxyphenyl-aniline)-α, β-cycloocta[1,2-b:4,3-b 0 :5,6b 0 :8,7-b 000 ]tetrathiophenyl (α, β-COTh-Ph-OMeTAD) is synthesized. When compared with spiro-OMeTAD, a star hole transporting material (HTM) for highly efficient perovskite solar cells, the new material has a deeper highest occupied molecular orbital (HOMO) energy level of À5.30 eV, and a higher hole mobility of 2.88 Â 10 À4 cm 2 V À1 s À1 . With dopant-free α, β-COTh-Ph-OMeTAD as a HTM and an interfacial layer combinined with chlorobenzene (CB) as the anti-solvent, mesoporous perovskite solar cells (PSCs) are fabricated, which exhibit a power conversion efficiency (PCE) of 17.22% under AM 1.5 conditions, which is a little higher than that of devices based-on doped spiro-OMeTAD under the same conditions, which is 16.83%. Notably, the PSCs devices with dopant-free α, β-COTh-Ph-OMeTAD as both the HTM and interfacial layer show better stability, and after being stored in dark and dry air without encapsulation for nearly 800 h, the PCE can still be maintained at 86% of the maximum. This opens a new avenue for efficient and stable PSCs by exploring new dopant-free materials as alternatives to spiro-OMeTAD.

Dopant‐Free and Green‐Solvent‐Processable Hole‐Transporting Materials for Highly Efficient Inverted Planar Perovskite Solar Cells

“…The compounds of MF‐1 and MF‐2 (Scheme S1, Supporting Information) are synthesized by Suzuki cross‐coupling reaction, and details of synthesis of MF‐2 are shown in Supporting Information, whereas that of MF‐1 can be found in previous report. [ 27 ] The structure of MF‐2 is characterized by nuclear magnetic resonance (NMR), MALDI‐TOF MS, and the Fourier transform infrared spectroscopy (FT‐IR) spectra (shown in Figure S1–4, Supporting Information). The thermal behaviors of HTMs are determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figure S5a,b, Supporting Information).…”

“…In summary, we synthesized two saddle‐shaped materials, MF‐2 and MF‐1, [ 27 ] and applied them in inverted PSCs as dopant‐free HTMs. As a result, the MF‐2‐based device exhibits an impressive PCE of 18.53%, which is higher than that of MF‐1‐based device (17.59%) and significantly exceed 12.34% rendered by the device based on well‐known PEDOT:PSS.…”

“…In 2019, our group reported the first new HTM based on COTh, named α, β‐COTh‐Ph‐OMeTAD. [ 27 ] The devices of PSC based on α, β‐COTh‐Ph‐OMeTAD exhibits higher PCEs and better stabilities than those based on Spiro‐OMeTAD in regular mesoporous structure. Later, Nazeeruddin and co‐workers [ 28 ] and Yang and co‐workers [ 29 ] also independently reported similar results with PCE of 16.3% and 17.7%, respectively.…”

A Review on Solution‐Processable Dopant‐Free Small Molecules as Hole‐Transporting Materials for Efficient Perovskite Solar Cells

“…Up to now, the reported polycyclic cores can be grouped into four types according to their fused degree, namely tetracyclic, [121][122][123] pentacyclic, [124][125][126][127][128][129][130][131][132] hexacyclic, [133] and heptacyclic cores (Figure 9). [134][135][136] There are two kinds of structures for tetracyclic cores, fluoranthene and benzotrithiophene (BTT).…”

“…In contrast to tetracyclic cores, the pentacyclic ones had a much bigger family of efficient SM-HTMs in hand. [124][125][126][127][128][129][130][131][132] The first efficient HTMs is from Jang et al [124] in 2018, where di(1-benzothieno)[3,2-b:2′,3′-d]pyrrole (DBTP) was chosen as the core and meta-methoxy revised DPA as the arms to form the molecule mDPA-DBTP. The molecule adopted a face-on orientation in the solid state, and together with the high planarity of the fused DBTP core that endowed the molecule with a strengthened intermolecular π-π stacking, a correspondingly very high hole mobility of 6.34 × 10 −4 cm 2 V −1 s −1 was obtained, superior to the value of doped spiro-OMeTAD The TPV spectra and intensity-modulated photocurrent/photovoltage (IMPS/IMVS) analyses confirmed that mDPA-DBTP presented a shorter charge extraction time and a longer charge recombination time than doped spiro-OMeTAD, suggesting better hole extraction and recombination suppression abilities.…”