Indeno[1,2‐b]carbazole as Methoxy‐Free Donor Group: Constructing Efficient and Stable Hole‐Transporting Materials for Perovskite Solar Cells

Abstract

With perovskite-based solar cells (PSCs) now reaching efficiencies of greater than 20 %, the stability of PSC devices has become ac ritical challenge for commercialization. However,m ost efficient hole-transporting materials (HTMs) thus far still rely on the state-of-the-art methoxy triphenylamine (MOTPA) donor unit in whichm ethoxy groups usually reduce the device stability.H erein, ac arbazole-fluorene hybrid has been employed as am ethoxy-free donor to construct organic HTMs.The indeno[1,2-b]carbazole group not only inherits the characteristics of carbazole and fluorene,b ut also exhibits additional advantages arising from the bulky planar structure.Consequently,M129, endowed with indeno[1,2-b]carbazole simultaneously exhibits ap romising efficiency of over 20 %a nd superior long-term stability.T he hybrid strategy toward the methoxy-free donor opens an ew avenue for developing efficient and stable HTMs. Figure 1. Schematic illustration of the molecular design of M127-M129.

Understanding the Effects of Fluorine Substitution in Lithium Salt on Photovoltaic Properties and Stability of Perovskite Solar Cells

“…Significant progress has been made in the field of lead halide perovskite solar cells (PSCs) over the past decade owing to a combination of the excellent optoelectronic properties of perovskite active materials, including strong and tunable optical absorption, ambipolar charge transport, long charge-carrier diffusion length, small exciton binding energy, and cost-effective solution processability. − Adopting the device structure of dye-sensitized solar cells, the first PSCs with the utilization of CH 3 NH 3 PbI 3 as the light absorber reported by Miyasaka et al in 2009 was found to be unstable, because of the quick fade upon dissolution of the perovskite active materials in the triiodide/iodine liquid-state electrolyte . In order to address this shortcoming, solid-state hole-transporting material (HTM) 2,2′,7,7′-tetrakis­( N , N -di- p -methoxyphenyl-amine)-9,9′-spirobifluorene (Spiro-OMeTAD) was employed as an alternative to liquid electrolyte to successfully construct the first all solid-state mesoscopic heterojunction PSCs featuring a FTO/c-TiO 2 /m-TiO 2 /perovskite/HTM/Au cell configuration that was stable for 500 h, developed by Park and co-workers in 2012, in which the HTM is the key component for extracting/transporting the photogenerated hole from the perovskite to the back contact metal electrode, effectively minimizing charge-recombination loss at the perovskite/HTM interface and thus achieving good device performance in PSCs. − As reported so far, the commercially available Spiro-OMeTAD has been proven to be the most efficient and widely used HTM in most of the highly performing PSCs yielding the record power conversion efficiency (PCE) of PSCs over 25% . However, Spiro-OMeTAD shows low intrinsic hole mobility of about ∼10 –6 cm 2 ·V –1 ·s –1 in its pristine form, resulting in poor photovoltaic performances in devices. − Thus, it is extremely important to improve its hole-transfer ability by introducing some chemical additives such as Li-bis­(trifluoromethanesulfonyl)­imide (Li-TFSI) into the Spiro-OMeTAD-based film.…”

Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

“…Meanwhile, hysteresis free small scaled devices based on XY1 showed a competitive PCE of 18.78%. Further, several novel chemical structures of dopant‐free HTMs with moderate device data have been reported (summarized in Table 1 and 2) yet not discussed in detail here …”

Dopant-free small-molecule hole-transport material for low-cost and stable perovskite solar cells

“…68,69 Five-membered heterocyclic imidazoles have two different nitrogen atoms, one with electron-poor character, as in pyridine, and one with electron-rich character, as in pyrrole. 70,71 A pyridine-like nitrogen atom as a Lewis base passivates perovskite defects. Furthermore, incorporation of the phenanthrene moiety improves the planarity of the imidazole-derived motif, thus promoting intermolecular p-p stacking.…”