Abstract
Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core–shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core–shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.
Highlights
Owing to its abundancy, non-toxicity, and relative ease to be grown as nanostructures by low-cost, low-temperature and implemented chemical deposition techniques [1,2], ZnO nanowires (NWs) have emerged as an important building block in nanostructured solar cells [3]
The ZnO/TiO2 core–shell NW heterostructures covered with a Sb2 S3 shell grown by chemical spray pyrolysis (CSP), using 30, 50, 70, and 90 cycles, were filled by P3HT, using immersion and thereafter covered with a thin layer of Au by thermal evaporation to form the complete extremely thin absorber (ETA) solar cell structure of indium tin oxide (ITO)/ZnO/TiO2 /Sb2 S3 /P3HT/Au
These findings show that the dimensional optimization of all the components in the ZnO/TiO2 /Sb2 S3 core–shell NW heterostructures is crucial to further improve the photovoltaic performance of the related ETA solar cells, while the issue of the hole-transporting material (HTM) is capital to properly collect the charge carriers, the holes
Summary
Non-toxicity, and relative ease to be grown as nanostructures by low-cost, low-temperature and implemented chemical deposition techniques [1,2], ZnO nanowires (NWs) have emerged as an important building block in nanostructured solar cells [3]. In the core–shell configuration used in ETA solar cells, the n-type ZnO NWs are basically coated with a thin shell as the optical absorber in the visible part of the electromagnetic spectrum (hereinafter shell) [3]. 2.0 eV and a type II band alignment with ZnO NWs [3]. These core–shell heterostructures benefit from a large number of assets, including (i) efficient light absorption phenomena through radiated and guided optical modes and (ii) efficient charge carrier management through charge carrier separation and collection [10,11,12,13]. The first efficient ETA solar cell integrating ZnO NWs was reported in 2005 with the use of a p-type CdSe shell and of
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