3D hole-transporting materials based on coplanar quinolizino acridine for highly efficient perovskite solar cells

Abstract

Over the past five years, perovskite solar cells (PSCs) have gained intense worldwide attention in the photovoltaic community due to their low cost and high power conversion efficiencies (PCEs). One of the most significant issues in achieving high PCEs of PSCs is the development of suitable low-cost hole-transporting materials (HTMs). Here, we put forward a new concept of HTMs for PSCs: a 3D structure with a core of coplanar quinolizino acridine, derived from the conventional concept of 2D triphenylamine HTMs. A cheaper Ag nanolayer was utilized to replace Au as the counter electrodes, and the title HTM TDT-OMeTAD was synthesized via an easy four-step synthesis (total yield: 61%) to achieve the low cost and convenient manufacture of PSCs. Compared with the conventional 2D triphenylamine HTM, TTPA-OMeTPA, PSC devices based on the 3D HTM TDT-OMeTPA showed a significant improvement in PCE from 10.8% to 16.4%, even outperforming Spiro-OMeTAD (14.8%). TDT-OMeTAD's highest PCE mainly results from it having the highest open-circuit voltage (V_oc) of 1.01 V in this work, which is proven to be due to the higher hole mobility, matching energy levels, higher hydrophobicity and the smaller dark current. Moreover, an incident photon-current conversion efficiency (IPCE) test and time-resolved photoluminescence (TRPL) have been carried out to observe the better hole injecting efficiency and photoelectric conversion capability of TDT-OMeTPA based PSCs than Spiro-OMeTAD. The TDT-OMeTPA based PSCs exhibited >75% reproducibility (PCE > 15%) and retained 93.2% of the initial PCE after >500 hours.

Fig. 1. Molecular structures of (a) TDT-OMeTPA and (b) TTPA-OMeTPA. The (c) HOMO, (d) LUMO and (e) optimized molecular conformation of TDT-OMeTPA obtained from DFT calculations.

Fig. 2. (a) UV-vis absorbance of TDT-OMeTPA, TTPA-OMeTPA and Spiro-OMeTAD. SEM images of the (b) TDT-OMeTPA and (c) TTPA-OMeTPA layers accumulated on perovskite layers. Water contact angles on (d) Spiro-OMeTAD, (e) TDT-OMeTPA and (f) TTPA-OMeTPA.

Fig. 3. (a–c) AFM amplitude images (scale bar = 100 nm) of the (a) TDT-OMeTPA, (b) TTPA-OMeTPA and (c) Spiro-OMeTAD HTM layers. (d–f) AFM height images (scale bar = 100 nm) of the (d) TDT-OMeTPA, (e) TTPA-OMeTPA and (f) Spiro-OMeTAD HTM layers. (g) Corresponding line profiles of the HTM layers.

Fig. 4. (a) Energy level diagram for each layer of the PSC device based on the TDT-OMeTPA HTM. (b) Schematic of the planar PSC device. (c) J–V curves under simulated 1.0 M sunlight, (d) IPCE curves, and (e) J–V curves in the dark of PSC devices based on TDT-OMeTPA, TTPA-OMeTPA and Spiro-OMeTAD. (f) Histogram of 30 device PCEs based on the TDT-OMeTPA HTM.

Fig. 5. Time-course changes in the photovoltaic performance parameters of the PSC devices with TDT-OMeTPA, TTPA-OMeTPA and Spiro-OMeTAD as the HTMs, respectively, under simulated solar illumination (AM 1.5G 100 mW cm^–2).

Fig. 6. (a) The time-integrated photoluminescence spectra of the perovskite films with/without the different HTMs. (b) The time-resolved photoluminescence spectra measured at a wavelength near the band gap that yields the maximum photoluminescence signal upon exciting the perovskite films containing the different HTMs at 420 nm.