Effect of polarization on photoexcited carrier dynamics in ferroelectric thin films
Introduction
Ferroelectric materials have a unique depolarization electric field originating from the spontaneous polarization, the anomalous photovoltaic (PV) effect has potential application value in the photovoltaic area, which has attracted dramatic attention recently [[1], [2], [3], [4], [5], [6]]. Both the bulk photovoltaic and the anomalous photovoltaic effect are the origins of the ferroelectric photovoltaic effect. The shift current model is a leading theory in the bulk photovoltaic effect. It is a nonlinear optical effect, that is, the coherent evolution of electron and hole wavefunctions under external illumination [[7], [8], [9]]. However, the mechanism of the anomalous PV effect based on spontaneous polarization is quite different from the shift current in the bulk photovoltaic effect. The depolarization field generated with spontaneous polarization in ferroelectric materials is one of the factors affecting anomalous photovoltaics [2]. The main characteristic of this anomalous PV effect is to generate higher than band gap voltages, and the photovoltaic response is not limited by any energy barrier [[10], [11], [12]]. Unlike the internal electric field in conventional semiconductor p-n junction and heterojunction materials that only exist in the space-charge region, the depolarization field in ferroelectric materials spreads throughout the bulk region [[13], [14], [15]]. The depolarizing field has been proved to be obviously beneficial to photocurrent and photovoltage, both of which are important parameters of photovoltaic devices, which is predicted to be a critical factor that may assist the ferroelectric materials to achieve a high efficiency beyond the limit of conventional semiconductor materials [16,17]. It is reported that the photocurrent depends on the lifetime of the photogenerated non-equilibrium charges [2]. Although ultrafast dynamics provides an opportunity to study the carrier motion process of ferroelectric materials on a time scale [18,19], attention has been focused on soft mode theory in ferroelectric materials by ultrafast dynamics at present [[20], [21], [22]]. And there is a lack of discussion on the microscopic mechanism of photovoltaic effect. Pal et al. use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of soft-mode nonlinearities in ferroelectric SrTiO3 thin films. They found that soft mode is a hybrid mode of lattice motions and electronic interband transitions and provides the microscopic processes of soft mode change near ferroelectric phase transition [20]. Li et al. reported that excitation by terahertz electric field can dynamically induce transition to hidden ferroelectric phase in quantum paraelectric SrTiO3. The results show that the soft phonon mode plays a role in the structural transformation [21]. Kozina et al. observed the lattice response of the ferroelectric perovskite SrTiO3 by using femtosecond X-ray pulses. They demonstrated a mechanism to transfer energy into higher energy modes by exploiting the nonlinearity of coupled phonons [22]. However, there are almost no reports that directly study the spontaneous polarization can increase charge carrier lifetimes, limiting signifcantly the application of ferroelectric materials in photovoltaic area.
The photocurrent/photovoltage in conventional semiconductor solar cells is intrinsically determined by photocarrier dynamics. For instance, photocarriers with a long lifetime or a long diffusion length may lead to a high photocurrent [2,23]. Thus, to have an in-depth understanding of the relationship between the depolarization field and the photovoltaic parameters, we monitored photocarrier dynamics in several ferroelectric materials PbTi1-xNixO3 (PTNO, with x = 0.00, 0.01, 0.02, 0.05, and 0.1) with different depolarization field strength using ultrafast time-resolved reflectance (TRR) measurement. We found that in the PTNO films with stronger depolarization fields, the photocurrents are higher, and the photocarriers recombined slower compared with those in the weaker-field ones. We deduce that in films with larger polarizations, the depolarization fields are stronger, and the photogenerated electrons and holes are separated more effectively. Thus, the recombination of photocarriers is retarded and the photocurrent is improved.
Section snippets
Materials and methods
Doped PbTi1-xNixO3 (PTNO) thin films with x = 0.00, 0.01, 0.02, 0.05, and 0.1 were prepared on lanthanum aluminate (LAO) (001) and Nb-doped strontium titanate (Nb-STO) (001) substrates using Sol-gel method. Lead acetate trihydrate Pb(CH3COO)2·3H2O (≥99.5 %), nickel acetate tetrahydrate Ni(CH3COO)2·4H2O (≥99 %), and titanium (IV) isopropoxide (≥97 %) were used as precursors, which were all purchased from Sigma-Aldrich Incorporation. Acetic acid (CH3COOH) (≥99.5 %) and deionized water with volume
Results and discussion
The X-ray diffraction patterns of PTNO thin films at room temperature are shown in Fig. 1(a) and (b). The XRD profiles reveal that all the films are highly (00l) oriented and splitting of (00l) the peaks. The a and c domains coexist in the PbTiO3 film and the nanodomains in the PbTi0.9Ni0.1O3 film are observed by TEM images (TEM image of PbTi0.95Ni0.05O3 in Fig. S2 in the supplementary material), as shown in Fig. 1(c) and (d) [25,26]. The domain structure leads to the splitting of the peaks [27,
Conclusions
In summary, in thin films with larger ferroelectric polarization, the depolarization field is stronger, and the photogenerated electrons and holes are separated more effectively, thereby improving the anomalous photovoltaic response. The photocarrier dynamics are monitored of the PbTi1-xNixO3 thin films of ferroelectric materials with different polarization intensities using femtosecond time-resolved reflectance measurement. It is found that the depolarization field is stronger due to larger
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
This work is supported by the National Key Research and Development Program of China (2017YFA0303403), the National Natural Science Foundation of China (61674058, 61574058) and the Foundation of National Key Laboratory of Shock Wave and Detonation Physics under Grant (6142A03182007).
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