Passivation via atomic layer deposition Al2O3 for the performance enhancement of quantum dot photovoltaics
Introduction
Lead sulfide (PbS) colloidal quantum dot solar cells (CQDSCs) have gained considerable attention as the third-generation solar cell technology due to their size-dependent band gap, capability of multiple exciton generation, and solution processability for low-cost, large-area manufacturing [[1], [2], [3], [4]]. In the past decade, PbS CQDSCs have made significant progresses mainly by surface chemical modification of colloidal quantum dots (CQDs) [5,6] and optimization of device structures [[7], [8], [9]]. However, the large trap density of CQDs, resulted from their large surface-to-volume ratio, still restricted the carrier collection and increased the recombination loss of solar cells. Ligand exchange was an efficient strategy to passivate the surface traps of CQDs, which has been successfully performed in the solid-state and liquid-phase processes [10,11]. Especially, the combination of ligand exchange and a matrix engineering efficiently passivated PbS CQDs, and generated a power conversion efficiency (PCE) of 12% [12].
Except for passivating traps, the ligand exchange was reported to efficiently control the energy levels of PbS CQDs layer through ligand-induced surface dipoles. Bawendi and Bulović etc. reported that the energy levels of PbS CQDs could be shifted by up to 0.9 eV through different ligand treatments [13]. Nowadays, a CQDs bilayer, composed of a light-harvesting layer formed by iodide-passivated CQDs and a p-type hole-extraction layer formed by ethanedithiol-passivated PbS CQDs (PbS-EDT), were widely used in the state-of-art PbS CQDSCs [[14], [15], [16]]. The PbS-EDT layer possessed an up-shifted band edges compared with iodide-passivated CQDs, and thus played an important role in promoting the carrier collection due to the engineered band alignment. Nevertheless, the surface traps of PbS-EDT could not be well passivated by usual ligand exchange processes. V. Wood et al. have reported that the trap density of PbS-EDT layer was up to 3.0 × 1016 cm−3 [17]. The large density of traps resulted in more trap-assisted carrier recombination loss risk [[18], [19], [20]]. In addition, the large surface trap density of PbS-EDT caused the Fermi level pinning. This induced an inapposite work function alignment at the PbS-EDT/Au and led to a reverse Schottky barrier at this interface, which would block the hole extraction from PbS-EDT to Au electrode [[21], [22], [23]]. Several recent investigations of CQDSCs have also verified the existence of this reverse Schottky barrier at the PbS-EDT/Au interface and its negative effect on the hole extraction [24,25].
This negative influence of PbS-EDT became more serious in the PbS CQDSCs with the ordered bulky heterojunction (OBH) structure. As shown in Fig. 1a, OBH PbS CQDSCs usually utilized well-aligned ZnO nanowire (NW) array as the electron extraction layer. The interpenetration of ZnO nanowires (NWs) and PbS CQD light-harvesting materials realized the direction orthogonalization of light harvesting and electron extraction, and this could dramatically enhance electron (minority carrier) extraction of PbS CQDSCs. Therefore the thickness of PbS CQD light-harvesting layer could be raised to over 600 nm for the high-efficiency application of solar light, especially that in near-infrared region. However, this OBH structure enlarged the distance of hole extraction, and thus the influence of PbS-EDT became more apparent in the OBH PbS CQDSCs compared with that in the solar cells with planar structure. Thus, a new strategy for the passivation of PbS-EDT, keeping the band alignment inside the PbS bilayer, was of importance for the potential improvement of the ZnO NWs/PbS solar cell.
In this work, we sought to passivate the traps of PbS-EDT film of ZnO NWs/PbS solar cell, by post-depositing an aluminum oxide (Al2O3) layer using atomic layer deposition (ALD) technology. The energy dispersive X-ray spectroscopy analysis demonstrated that ALD Al2O3 not only spread over the surface of PbS-EDT film, but also slightly permeated into interior due to the diffusion of gaseous precursors in ALD method. The Al2O3 layer was proved to directly reduce traps of PbS-EDT film without breaking the band alignment by photoluminescence (PL) and Kelvin probe force microscopy (KPFM) measurements, leading to a significant decrease in the reverse Schottky barrier at PbS-EDT/Au contact and an enhanced carrier extraction. The ALD treatment generated a PCE increase from 5.28% to 7.07% in performance of ZnO NWs/PbS device under AM1.5G (100 mW/cm2) illumination. These findings suggested that the ALD Al2O3 ultrathin layer was an efficient strategy for the post-treatment of quantum dot optic and electronic device including solar cells, light-emitting diodes and photodetectors.
Section snippets
Results and discussion
The structure of the bulk-ordered heterjunction PbS CQDSCs was depicted in Fig. 1a. A well-aligned ZnO NW array (length: ~500 nm) was synthesized by the hydrothermal method on the FTO substrate, and used as the electron extraction layer of solar cells. PbS with tetrabutylammonium iodide ligand (PbS-TBAI) as the light harvesting layer were deposited on the ZnO NW array by the layer-by-layer spin-coating method. In this process, PbS-TBAI not only filled in the space between neighbouring ZnO NWs,
Conclusions
ALD Al2O3 was post-deposited to passivate the surface traps of PbS-EDT film for the performance improvement of PbS CQDSCs. The structural analysis indicated that ALD Al2O3 could overcoat and infill the PbS-EDT film at the same time. After the ALD Al2O3 treatment, the surface traps of PbS-EDT was efficiently passivated, and the proper band alignment at PbS-TBAI/PbS-EDT interface was successfully maintained for the fast hole extraction of CQDSCs. This method resulted in a CQDSCs with an improved
CRediT authorship contribution statement
Jinhuan Li: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Data curation. Yinglin Wang: Validation, Writing - review & editing, Visualization, Data curation. Fangxu Wan: Methodology. Meiqi An: Validation. Meiying Li: Validation. Lei Wang: Methodology. Xintong Zhang: Resources, Visualization, Project administration, Funding acquisition. Yichun Liu: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by the Natural Science Foundation of China [91833303, 51872044 and 51602047], the 111 project [B13013] and Jilin Scientific and Technological Development Program [20180520007JH].
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