Elsevier

Solar Energy

Volume 167, June 2018, Pages 18-23
Solar Energy

Ferroelectric photovoltaic characteristics of pulsed laser deposited 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3/ZnO heterostructures

https://doi.org/10.1016/j.solener.2018.03.072Get rights and content

Highlights

  • 0.5BZT-0.5BCT/ZnO heterostructures exhibited enhanced photovoltaic (PV) properties.

  • The enhanced PV effect is attributed to the perfect alignment of the internal fields.

  • The influence of polarization interfacial charge coupling on the band structure is highlighted.

  • No diffusion at the 0.5BZT-0.5BCT/ZnO interfaces is observed as evidenced EELS.

Abstract

In this work, we investigate the photovoltaic response of Pt/0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3(0.5BZT-0.5BCT)/ITO structures through the insertion of a semiconductor ZnO layer at different positions. The values of short-circuit photocurrent density (Jsc) of the Pt/ZnO/0.5BZT-0.5BCT/ITO, Pt/0.5BZT-0.5BCT/ZnO/ITO and Pt/ZnO/0.5BZT-0.5BCT/ZnO/ITO capacitors are around 5.31, 0.0034 and 0.052 mA/cm2, respectively. The enhanced photovoltaic (PV) effect is observed when ZnO layer is inserted between Pt and the 0.5BZT-0.5BCT layer. The built-in field developed at the ZnO/ferroelectric interface in the same direction of the depolarizing field, provides a favorable electric potential for the efficient separation and transportation of photo generated e-h pairs. Furthermore, the polarization-dependent interfacial coupling effect enhances PV effect, which is confirmed by investigating the role of polarization flipping on switchable photo response. This work provides an efficient pathway in tuning the PV response in ferroelectric-based solar cells.

Introduction

The harvesting of solar energy through the photovoltaic (PV) effect is one the most important resources for the renewable energy. PV devices have traditionally been developed by optimizing the three key steps, which are photogeneration of electron-hole (e-h) pairs, separation of e-h pairs and their transport (Liu et al., 2014). Organic and other semiconductor based solar cells essentially require a p-n junction, where the internal electric field near the junction interface is used for e-h pair separation. The main drawback of these structures is that they cannot produce the open circuit voltage (Voc) above the band gap of materials and consequently causes low efficiency. Furthermore, these structures limit the material choices and also device fabrication to form a workable junction due to issues such as lattice mismatch, doping, and band alignment (Liu et al., 2014). Moreover, inevitable heavy doping processes in traditional solar cells can lead to significant levels of Auger recombination (Liu et al., 2014, Richtera et al., 2012).

Alternatively, ferroelectric materials are considered as promising candidates for photovoltaic applications due to their unique natural properties (Katiyar et al., 2015, Agarwal et al., 2015, Chen et al., 2011, Zhang et al., 2015). Due to its internal electric field, they do not need any p-n junction to produce photocurrent. The ferroelectric PV effect exhibits outstanding advantages over the conventional p-n junction based PV devices, such as high Voc beyond the band gap and polarization controlled PV response (Sharma et al., 2015). However, in these materials, the short-circuit current density (Jsc) is low due to inefficient radiation absorption and e-h generation (Fan et al., 2014). For these reasons, PV devices based on semiconductor and ferroelectric materials alone have poor photovoltaic properties due to low Voc and Jsc, respectively.

Therefore, the integration of semiconductor and ferroelectric materials (so called heterostructures) takes the advantage of both the semiconductor and the ferroelectric properties and may lead to high-efficiency photovoltaic effect (Fan et al., 2014). In these heterostructures, e-h pairs are created inside both the semiconductor and ferroelectric layers. The depolarizing field and the interface charge coupling between the ferroelectric and semiconductor are crucial for the separation and transport of photo generated e-h through the heterostructures (Fan et al., 2014, Cao et al., 2012). There are only few reports on the photovoltaic effect of heterostructures based on semiconductor-ferroelectrics (Fan et al., 2014, Cao et al., 2012, Pan et al., 2016, Chakrabartty et al., 2016). Fan et al. (2014) investigated the BiFeO3 thickness dependent photovoltaic effect in an In2O3-SnO2/ZnO/BiFeO3/Pt heterostructure in which highest efficiency of 0.33% was achieved at a thickness of 300 nm. Cao et al. (2012) have shown that the insertion of an n-type Cu2O cathode buffer layer into an ITO/Pb(Zr,Ti)O3/Pt solar cell leads to the 120 times enhancement in short-circuit photocurrent. On the other hand, Pan et al. (2016) investigated the thickness (tZnO) dependence of the ZnO layer, with an intrinsic spontaneous polarization (Ps), in a ITO/Pb(Zr,Ti)O3/ZnO/Au heterostructure. They revealed that the optimal thickness is 60 nm of ZnO in order to exhibit enhanced PV characteristics in the heterostructure due to attainment of maximum internal field in ZnO. However, very few reports have focused on interfacial coupling influence on the PV effect (Pan et al., 2016, Chakrabartty et al., 2016). Besides photovoltaic applications, interfacial charge coupling have also been investigated for other applications like resistive random access memories (RRAMs) (Silva et al., 2017b, Silva et al., 2017c).

In this work, we investigate the PV response of Pt/0.5BZT-0.5BCT/ITO structures through the insertion of a semiconductor ZnO layer at different positions. Moreover, we have further investigated the PV mechanism in a metal/semiconductor/ferroelectric/metal (MSFM) heterostructure, particularly the origin of the photocurrent enhancement and the role played by the polarization-dependent interfacial coupling effect, through systematic measurements of photovoltaic current-voltage curves at various polarization states and detailed analysis of energy band diagrams.

Section snippets

Experimental methods

Pulsed laser deposition (PLD) technique was used to deposit 0.5BZT-0.5BCT/ZnO thin film heterostructures on Pt/TiO2/SiO2/Si substrate. The PLD chamber is provided with a multi-target carousel system that allows the deposition of different layers without breaking the vacuum. The 0.5BZT-0.5BCT target prepared by conventional solid state reaction as described in Silva et al. (2015) and the commercially available ZnO (99.99% purity from Kurt Lesker) target were used as a source for the

Results and discussion

Fig. 1 shows the XRD patterns of different heterostructures, where the Bragg peaks corresponding to the 0.5BZT-0.5BCT and ZnO thin layers were indexed, according to standard powder diffraction data (JCPDS Card Nos. 27-0530 (0.5BZT-0.5BCT) and 36-1451 (ZnO)).

The presence of strong ZnO (0 0 2) peak in all heterostructures suggests that ZnO grows with a preferred c-axis orientation (Sekhar et al., 2013), regardless of the ZnO layer location within the multilayer sandwich. Usually, ZnO grows along

Conclusions

This work highlights the effect of the ZnO layer position on the photovoltaic response of pulsed laser deposited 0.5BZT-0.5BCT/ZnO heterostructures. XRD patterns revealed the formation of pure ZnO and 0.5BZT-0.5BCT phases only, while EELS showed that the there is no diffusion at the 0.5BZT-0.5BCT/ZnO interfaces. The enhanced photovoltaic effect in Pt/ZnO/0.5BZT-0.5BCT/ITO heterostructures was attributed to the perfect alignment of the internal fields and to the polarization-dependent

Acknowledgements

This work was supported by: (i) Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013 and (ii) Project Norte-070124-FEDER-000070 Nanomateriais Multifuncionais. Part of this work was supported by the COST Action MP1406 “Multiscale in modelling and validation for solar photovoltaics (MultiscaleSolar)”. The authors acknowledge the CERIC-ERIC Consortium for access to experimental facilities and financial support under proposal 20157018.

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