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by straining the ZnO MW component (Fig. 4).11 This is the case of p-n junction modulation described in Fig. 2(b). In this configuration, Ppz-induced interfacial charge redistribution forms a potential dip at the ZnO–GaN interface, which traps electrons or holes and facilitates their recombination. Similar to LEDs, in the case of a GaN–InGaN quantum well laser, the quantum well profile can be rectified by Ppz at the well’s interface.12 Whether there is a negative or positive effect on the quantum efficiency results depends on the polarity of this potential. A similar effect also exists in PV devices, whose performance relies on effective electron–hole separation at a semiconductor heterojunction. The built-in field at the heterojunction provides a critical driving force for charge separation and, thus, dictates opencircuit voltage and short-circuit current. When a PV structure involves a piezoelectric semiconductor, the presence of Ppz may lead to a considerable change of free carrier distributions in the piezoelectric material and its adjacent semiconductor or metal contacts and, thus, influences the device performance. One example has been demonstrated based on a n-ZnO–p-PbS quantum dot (QD) heterostructure, where the builtin field in the depleted PbS layer (φbi, PbS) is essential to charge extraction from the QD layer.13 Additional driving force could be expected when interfacial charge redistribution is induced by Ppz from strained ZnO. Figure 5(a) shows that, when positive Ppz appears at the ZnO–PbS interface, conduction and valence bands of PbS are bent further downward, producing a sharper, extended built-in field, which is preferable for sweeping excitons apart. In this scenario, the driving force for extracting electrons from the PbS QD assemblage is augmented to (φbi,PbS + Δφpz,PbS), and the width of the depletion region in PbS under zero external bias expands accordingly. This enlarged depletion region in the PbS QD layer is necessary for enhanced charge extraction. Meanwhile, the positive Ppz also may yield a shorter depletion region (δpz,ZnO) and shallower band bending on the ZnO side. Figure 5(b) schematically illustrates the overall change of depletion region at the ZnO–PbS interface. Therefore, positive Ppz at the ZnO–PbS interface is a favorable condition for charge extraction. The Ppz-engineered PV performance was tested on flexible QD solar cells (QDSCs) fabricated using p-type PbS QDs and an n-type textured, (0001) orientation ZnO thin film. Appreciable change in current density (Jph) occurred Figure 4. (a.) Optical images showing the strain-dependent emission intensity from a n-ZnO/p-GaN LED. (b) Electroluminescence spectra of the LED subject to various strains at a bias of 9 V. under various strains (Fig. 5(c)). A linear relationship was identified from the plot of Jsc versus strain (blue squares in Fig. 5(d)), where Jsc exhibited a 0.02 μA/cm2 (or 1.1%) increase per 0.01% strain drop. Under zero strain, the efficiency of the QDSC was ~3.1% (red circles in Fig. 5(d)). Approximately 4.0% efficiency was obtained at a compressive strain of –0.25%, corresponding to a ~30% improvement. The efficiency also exhibited an approximately linear relationship with strain within the testing range (-0.25% – 0.15%), where a 1.2% efficiency enhancement per 0.01% strain drop was identified. More significant band shifting in the piezoelectric material would be observed if the external contact material, for example, a polymer, had a very low carrier density. Depending on its electrical permittivity, the screening length of a polymer can be fairly large and the Ppz-induced electric field can be sensed far away from the interface in the polymer. This situation was first demonstrated in a polymer solar cell, where ZnO MWs served as electron conductors and poly(3-hexylthiophene) (P3HT) was the photon absorber.14 By straining the ZnO MW under photoillumination, the cell’s open-circuit voltage increased when positive Ppz appeared at the ZnO–P3HT interface and lowered the conduction band of ZnO. A similar effect also has been observed from a ferroelectric poly(vinylidene fluoride) (PVDF)– P3HT heterostructure, where the permanent polarization from PVDF enhances the PV performance.15 So far, all the experimental evidence indicates that the piezotronic effect holds great promise for improving the performance of PV devices by enhancing the effectiveness of charge extraction and modulating the open-circuit voltage. Piezocatalysis—Straining to split water In addition to modulating regular semiconductor functions, coupling piezopotential with electrochemical processes creates a new effect, denoted as piezocatalysis. Because the strain state and electronic state of these materials are strongly coupled, piezocatalysis could be prominent in piezoelectric materials. Piezocatalysis is the product of an intimate interaction between the native electronic state of the piezoelectric material, the chemistry of the surrounding (Adapted with permission from Ref 11. Copyright 2011, American Chemical Society.) medium, and a strain-induced piezoelectric potential. Mechanically deforming a piezoelectric material induces a perfuse electric field that augments the energetics of free and bound charges throughout the material. The thermodynamic feasibility and kinetics of electrochemical processes occurring at the surface of the piezoelectric material is sensitive to the electrochemical potential difference between charges on the piezoelectric’s surface and in the surrounding medium. Thus, piezoelectric potential, which can affect dra- American Ceramic Society Bulletin, Vol. 92, No. 6 | www.ceramics.org 21


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