Investigating Zinc Oxide Nanoparticle Interlayer Materials and Applications for Stable Photovoltaic Devices


Student thesis: Doctoral Thesis

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Award date1 Sep 2020


Owing to the high optical transparency, low cost and excellent electron-transporting properties, zinc oxide (ZnO) has a wide range of applications, including optical waveguides, gas sensors, high-power transistors, light-emitting diodes, and thin film solar cells. Thus far, various approaches have been demonstrated to deposit ZnO thin films. Among them, solution processed ZnO nanoparticles (ZnO NPs) have been widely used as an electron transport layer (ETL) in developing organic and perovskite photovoltaic devices. However, the ZnO NPs are known to be sensitive to oxygen and moisture in the air with strong trap emission in the visible (400-700 nm) region in the photoluminescence (PL) measurement. Consequently, most reported high-efficiency solar cells with ZnO NP ETL are fabricated in an inert environment, which limits the large-scale industrial deployment of low-cost and long-term stable photovoltaic cells.

This thesis aims to investigate the intrinsic properties of ZnO NPs derived interlayer and explore strategies to enhance the stability of photovoltaic devices. Accordingly, the research works are divided into three main areas: (1) Investigation on the origin of defect emission and stability of solution derived ZnO NP thin films; (2) Development of air processable ZnO NP thin films for stable polymer solar cells; (3) Development of ZnO precursor solution formulation to resolve the commonly observed decomposition issue at ZnO/perovskite interface in photovoltaic application.

Understanding the intrinsic properties of ZnO NPs is crucial for achieving stable photovoltaic devices. The quality of solution processed ZnO NPs is often correlated with their PL spectral characteristics. Hence, this thesis firstly demonstrates the correlation between defects properties and PL emission of ZnO NPs. The results show that both the PL spectral line-shape and intensity are extremely sensitive to nitrogen (N2) and oxygen (O2) gas molecules. By conducting time-dependent PL and photothermal deflection spectroscopy (PDS) measurements under vacuum and different gases environments, it is found that both inert (N2) and reactive (O2) molecules can be absorbed on the ZnO NP surface and induce charge transfer (CT) with ZnO NPs. The CT states induced by N2 are non-radiative which significantly reduces the band emission. Whereas the CT states induced by O2 are radiative which results in the visible emission. The results support that the surface effect of the ZnO NPs is the major source of the widely observed visible emission rather than structural defects in ZnO NPs.

With a better understanding on the origin of defect emission of solution processed ZnO NPs, we sought to improve the efficiency and stability of polymer solar cells by pretreating the ZnO NP interlayer. The results show that the ultraviolet (UV) irradiation environment and treatment time have a pivotal effect on the ZnO defects density and the resulting photovoltaic performance. In case of nitrogen, the device performance is irrespective of the treatment time. Interestingly, when the UV irradiation is carried out in air, the high device performance can be achieved by treating the ZnO for a few seconds (<10 s), and prolonged UV irradiation results in inferior device performance and strong degradation. Based on our previous finding that O2 molecules are chemisorbed on the ZnO NP surface facilitated by electron transfer from photoexcited ZnO NPs to O2 molecules. It is proposed that there is a complete process of repairing and rebuilding the CT states in ZnO during the UV irradiation in air. As a result, devices using PCE10: PC71BM bulk-heterojunction with air-processed ZnO ETL achieve an encouraging PCE of 8.10% and retain 80% of the initial performance after two months of exposure in the air without encapsulation. This work not only demonstrates a facile approach to realize the ambient compatible fabrication of organic photovoltaic devices but also to identify the correlation between the surface effect of ZnO and the corresponding device performance.

Besides organic photovoltaic devices, solution processed ZnO NPs also have been widely used as electron transport layers (ETL) in perovskite photovoltaic devices. However, the CH3NH3PbI3 perovskite on the surface of ZnO interlayer decomposes to PbI2 upon thermal annealing at 100°C. From the X-ray photoelectron spectroscopy (XPS) measurement results, hydroxyl groups and superoxide attached on the ZnO surface are responsible for the decomposition of perovskite. On the other hand, the perovskite layer on the surface of aluminum-doped zinc oxide (AZO) nanoparticles shows improved thermal stability. Using time-dependent PL measurement, it is found that the doping of aluminum changes the NP surface properties. Therefore, the reactivity of metal oxide surface plays a key role in the thermal stability of perovskite. However, the morphology of the perovskite on AZO is in poor quality with significantly reduced grain size, which limits both the device efficiency and stability.

To further understand the stability issue in perovskite, organic polymers are incorporated in the precursor solution to hinder the ion dissociation in perovskite thin films. A co-polymer PCE10 with strong dipole moment and a mono-polymer polystyrene (PS) with negligible dipole moment are used as the additives. Surprisingly, our results show no obvious passivation effect from the polymers in perovskite as has been commonly ascribed in the community. Moreover, the optimized photovoltaic devices with and without polymer additives have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous illumination. Interestingly, it is found that although the initial mobile ion concentrations are comparable in both devices with and without polymer additives, the additive can strongly suppress the ion dissociation during the device operation. The suppression of ion dissociation is further confirmed by the enhanced electrical-stress tolerance. The devices with polymer additives can operate up to -2 V reverse bias, which is much larger than the breakdown voltage of -0.5 V that has been commonly observed in perovskite solar cells. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.