Zinc Oxide Nanorods (ZnO NRs) for photovoltaic applications

Ishwein, Zahra (2019) Zinc Oxide Nanorods (ZnO NRs) for photovoltaic applications. Doctoral thesis, Northumbria University.

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Abstract

Zinc oxide (ZnO) is a material that has highly attractive cost-effective features and can be grown using different methods leading to a wide range of nanostructured morphologies. In this work, zinc oxide nanorods (ZnO NRs) with various surface distribution density were sensitised using aqueous solutions based on zinc salt at 40°C85°C. Systematic investigations were carried out on the influences of zinc salt, different ZnO seed layer thickness and growth temperature (Tgrowth). The grown ZnO NRs were used in two different aspects. The first aspect was to grow the NRs on a releasable layer and here Omnicaot was used as a sacrificial layer and SU-8 photoresist as a support structural layer to reach the desired aim. Also, ZnO NRs were grown on polydimethylsiloxane (PDMS). This was used to improve the performance of different types of solar cells by mounting the full structure on top with help of optical gel. The results obtained showed that wet lift-off showed an increasing ɳ from 1.56% to 2.05% when a GaAs solar cell was used, whereas the same solar cell showed an efficiency ɳ increasing to about 2.03% when using dry peel-off nanostructures. CZTSSe (Cu2ZnSn(S,Se)4) solar cells showed an increase in ɳ from 1.3% to (1.79%, 1.65% and 2.15%) for ZnO NRs/SU-8, ZnO NRs/PDMS and flower-like/diluted PDMS, respectively. The second aspect was to fabricate extremely thin absorber solar cells. For lift-off process, the work presented herein provides a cost-effective, simple novel combination of lift-off processing with hydrothermally grown ZnO NRs on SU-8 or PDMS, in a low temperature range Tgrowth from 40°C- 85°C. This study addresses a controllable release/transfer of ZnO NRs when high growth temperature represents a barrier to carrying out an immediate growth on flexible substrates for example wearable electronics applications. The evolution of ZnO NRs with Tgrowth were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-Vis spectroscopy. The ZnO NRs/SU-8 and ZnO NRs/PDMS structures were successfully detached from the glass. The SEM images confirmed that, the ZnO NRs formed different diameters and lengths as Tgrowth increased. Transmitted/scattered light characteristics showed different trends depending on the Tgrowth and structure stack used. The findings of this study offer an easy method of lift-off ZnO NRs (and subsequent stack of layers) using the low-cost facilities and at low temperature. Current-voltage (I-V) and external quantum efficiency (EQE) measurements showed an affective influence of mounting the released ZnO NRs on CZTS and GaAs solar cells. Moreover, a study on ETA cell fabrication using CZTS is presented. The fabricated ETA cell structure was as the following: glass/ITO/ZnO seed/ZnO NRs/CdS/CZTS/P3HT/gold and the aim is for the CdS/CZTS to conformally coat the ZnO NRs, while P3HT acts as a hole transport layer which in turn helps avoiding shunting. ETA cell showed about 0.02% of efficiency (ɳ), 0.05V (Voc), 0.15mA/cm2 (Jsc), 27.46% (FF) and about 20% EQE in the 300nm-600nm spectra region. These results are clear indication of promising sight for ETA solar cells based on CZTS nanoparticles which can be more improved.

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