Randi Azmi

Diphenyl-2-pyridylamine-Substituted Porphyrins as Hole-Transporting Materials for Perovskite Solar Cells

The susceptibility of porphyrin derivatives to light-harvesting and charge-transport operations have enabled these materials to be employed in solar cell applications. The potential of porphyrin derivatives as hole-transporting materials (HTMs) for perovskite solar cells (PSCs) has recently been demonstrated, but knowledge of the relationships between the porphyrin structure and device performance remains insufficient. In this work, a series of novel zinc porphyrin (PZn) derivatives has been developed and employed as HTMs for low-temperature processed PSCs. Key to the design strategy is the incorporation of an electron-deficient pyridine moiety to down-shift the HOMO levels of porphyrin HTMs. The porphyrin HTMs incorporating diphenyl-2-pyridylamine (DPPA) have HOMO levels that are in good agreement with the perovskite active layers, thus facilitating hole transfers from the perovskite to the HTMs. The DPPA-containing zinc porphyrin-based PSCs gave the best performance, with efficiency levels comparable to those of PSCs using spiro-OMeTAD, a current state-of-the-art HTM. In particular, PZn-DPPA-based PSCs show superior air stability, in both doped and undoped forms, to spiro-OMeTAD based devices.

Performance Improvement in Low-Temperature-Processed Perovskite Solar Cells by Molecular Engineering of Porphyrin-Based Hole Transport Materials

Porphyrin derivatives have recently emerged as hole transport layers (HTLs) because of their electron-rich characteristics. Although several successes with porphyrin-based HTLs have been recently reported, achieving excellent solar cell performance, the chances to improve this further by molecular engineering are still open. In this work, Zn porphyrin (PZn)-based HTLs were developed by conjugating fluorinated triphenylamine (FTPA) wings at the perimeter of the PZn core for low-temperature perovskite solar cells (L-PSCs). The fluorinated PZn-HTLs (PZn-2FTPA and PZn-3FTPA) exhibited superior HTL properties compared to the nonfluorinated one (PZn-TPA). Moreover, their deeper highest occupied molecular orbital energy levels were beneficial for boosting open-circuit voltages, and their enhanced face-on stacking improved the hole transport properties. The L-PSC using PZn-2FTPA achieved the highest performance of 18.85%. Thus far, this result is one of the highest reported power conversion efficiencies among the PSCs using porphyrin-based HTLs.

High-Efficiency Air-Stable Colloidal Quantum Dot Solar Cells Based on a Potassium-Doped ZnO Electron-Accepting Layer

High-efficiency colloidal quantum dot (CQD) solar cells (CQDSCs) with improved air stability were developed by employing potassium-modified ZnO as an electron-accepting layer (EAL). The effective potassium modification was achievable by a simple treatment with a KOH solution of pristine ZnO films prepared by a low-temperature solution process. The resulting K-doped ZnO (ZnO-K) exhibited EAL properties superior to those of a pristine ZnO-EAL. The Fermi energy level of ZnO was upshifted, which increased the internal electric field and amplified the depletion region (i.e., charge drift) of the devices. The surface defects of ZnO were effectively passivated by K modification, which considerably suppressed interfacial charge recombination. The CQDSC based on ZnO-K achieved improved power conversion efficiency (PCE) of ≈10.75% ( VOC of 0.67 V, JSC of 23.89 mA cm-2, and fill factor of 0.68), whereas the CQDSC based on pristine ZnO showed PCE of 9.97%. Moreover, the suppressed surface defects of ZnO-K substantially improved long-term stability under air. The device using ZnO-K exhibited superior long-term air storage stability (96% retention after 90 days) compared to that using pristine ZnO (88% retention after 90 days). The ZnO-K-based device also exhibited improved photostability under air. Under continuous light illumination for 600 min, the ZnO-K-based device retained 96% of its initial PCE, whereas the pristine ZnO-based device retained only 67%.