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Abstract
The carrier transport performances play key roles in the photoelectric conversion efficiency for photovoltaic effect. Hence, the low carrier mobility and high photogenerated carrier recombination in ferroelectric materials depress the separation of carriers. This work designs a ferroelectric polarization-interface-free PN junction composed with P-type semiconductor BiFeO3 (BFO) derived from the variable valence of Fe and N-type semiconductor BiFe0.98Ti0.02O3 (BFTO) through Ti donor doping. The integration of the ferroelectricity decides the PN junction without polarization coupling like the traditional heterojunctions but only existing carrier distribution differential at the interface. The carrier recombination in PN junction is significantly reduced due to the driving force of the built-in electric field and the existence of depletion layer, thereby enhancing the switching current 3 times higher than that of the single ferroelectric films. Meanwhile, the carrier separation at the interface is significantly engineered by the polarization, with open circuit voltage and short circuit current of photovoltaic effect increased obviously. This work provides an alternative strategy to regulate bulk ferroelectric photovoltaic effects by carrier transport engineering in the polarization-interface-free ferroelectric PN junction.
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1. Introduction
Compared with traditional semiconductor PN junctions, ferroelectric materials with the bulk photovoltaic effect are considered as a promising candidate for photovoltaic applications because their open circuit voltage is not constrained by the bandgap but decided by the depolarization field throughout the materials that can effectively separate photogenerated carriers [1,2]. Even so, the low short circuit current makes it impossible to obtain the ideal photoelectric conversion performances due to the inherently high bandgap and unsatisfactory carrier transport characteristics for ferroelectric materials [3,4]. Therefore, suitable bandgap and excellent carrier separation transport characteristics become the key to improve the ferroelectric photovoltaic effect.
As a known room-temperature ferroelectric material with a small band gap (Eg ∼ 2.7 eV), BiFeO3 (BFO) provides a basic guarantee for photon absorption in the near visible region [5]. However, the photogenerated current is still very limited since the absorbed photons cannot be efficiently converted within the material due to the low carrier mobility and the high photogenerated carrier recombination [6,7]. For ferroelectric photovoltaic effect, the separation of photogenerated carriers is mainly dominated by the depolarization field and the Schottky barrier at the electrode interface, while the carrier transport is closely related to the internal conduction channel in materials. Based on this, as a way to reduce carrier recombination and improve carrier transport performances, the construction of ferroelectric-semiconductor heterojunction stands out among many methods to enhance photovoltaic performances [8,9]. Nevertheless, the interface design of these composite structure is based on the heterogeneous structure, in which the photoelectric performance is improved at the expense of ferroelectricity. Therefore, how to reduce carrier recombination to improve transport without weakening the separation effect derived from ferroelectric polarization is expected for the ferroelectric photovoltaics.
This work constructs a polarization-interface-free ferroelectric PN junction, where there are only differences in carrier distribution, and no coupling and aggregation effects of polarization at the interface, which is completely different from the traditional heterojunction with interface polarization. Specifically speaking, the pure BFO is selected as a P-type (P-BFO) ferroelectric semiconductor due to the valence fluctuations of Fe, and N-type (N-BFTO) ferroelectric semiconductor is obtained by doping the high-valence ion Ti in the BFO, and eventually the two contact to form polarization-interface-free PN junctions. On the one hand, the bulk ferroelectricity provides the driving force to effectively separate photogenerated carriers by ferroelectric depolarization field. On the other hand, the built-in electric field formed at the PN junction interface not only provides an additional driving force to promote carrier separation, but also effectively reduces the recombination of the photogenerated electron-hole pairs. This carrier transport engineering by the construction of the ferroelectric PN junction improves the photocurrent switching response by more than 3 times. Moreover, the carrier separation and transport performances at the interface can also be modulated by the polarization, where the modulation rate of open circuit voltage and short circuit current reaches 30% and 40% respectively at the polarization voltage of 50 V, demonstrating excellent photovoltaic response. The detailed mechanism of carrier transport engineering through the construction of polarization-interface-free PN junctions is revealed.
2. Experiments
All samples were prepared using the sol-gel method and the raw materials used were common materials commonly used in this method. Bi(NO3)3·5H2O with 99% purity, Fe(NO3)3·9H2O with 98% purity and [CH3(CH2)3O]4Ti with 97% purity purchased from Alfa Aesar were dissolved into ethylene glycol methyl-ether to form precursor solutions with a concentration of 0.04 mol/L. The corresponding precursor solutions were spun on Pt/Ti/SiO2/Si (100) substrate and heat-treated at 500°C with oxygen atmosphere to obtain P-BFO, N-BFTO and PN junction films. X-ray diffraction (XRD) and Raman spectrometry were used to characterize the crystal structure. Ferroelectric and leakage characteristics were collected by a ferroelectric analyzer. The J-V curves and the photocurrent dynamic switching response were obtained by a Keithley 2400 source meter and the solar simulator.
3. Results and discussion
Figure 1(a) shows the XRD patterns of all the films. The clear and sharp diffraction peaks of the P-BFO film are well matched to the standard card (JCPDS No: 72-2035) and correspond to the R3c space group rhombohedral polycrystalline structure. A clear Gaussian fit (110/1ī0) planes splitting peak is clearly observed at about 32° for the P-BFO film, as shown in the enlarged image of Fig. 1(b). Whereas, the diffraction double peaks tend to merge into single peaks in the N-BFTO film, indicating that Ti doping induces the structural transition from rhomboid to orthotropic phases [10]. For PN junction films, the morphology of peaks is between P-BFO and N-BFTO films, showing the physical superposition of the two peaks. That is to say, the formation of PN junction does not result in structural mutation at the interface. Figure 1(c) shows the Raman shift with the rhombohedral R3c primitive cell summarized by irreducible representation Γ = 4A1 + 9E [11]. Obviously, N-BFTO films show a significant reduction in the strength of A1 longitudinal optical mode compared with P-BFO due to the fluctuation of components caused by the Ti doping, resulting in a decrease in disorder and stereochemical activity [12]. The transverse optical E4 located at approximately 611 cm−1 is attributed to the Fe-O2 bond, where O2 represents the equatorial ion and is associated with the rotation of the octahedron. As shown in the enlarged figure in Fig. 1(d), the shift peaks of N-BFTO films show a slight blue shift and intensity enhancement compared with P-BFO, indicating the transformation of phase structure and the change in polarizability due to the lattice mismatch caused by Ti introduction [13]. Correspondingly, the peak changes in frequency for the PN junction show the physical superposition of the first two, which together with the XRD results further proves that the construction of PN junction has little effect on the original structure at the interface.
Fig. 1. Structural characterization of all films. (a) XRD patterns; (b) enlarged view of XRD pattern; (c) Raman shift; (d) enlarged Raman shift.
Figure 2(a) shows the polarization-electric field (P-E) loops of the P-BFO, N-BFTO and PN junction films, and all the films exhibit excellent hysteresis loops. Among them, P-BFO film exhibits the best ferroelectric properties with the maximum polarization and residual polarization reaching 127 and 92 µC/cm2, respectively, which is related to the large relative displacement of the inherent positive and negative ions. The reduced polarization of N-BFTO film is ascribed to the transformation of the phase structure caused by Ti doping, which enhances the symmetry of the structure. To reconfirm the change in symmetry, the tolerance factors (τ) for P-BFO and N-BFTO films were calculated to be 0.9543 and 0.9547, respectively [14]. The increase of τ in BFTO films represents a higher symmetry and indicates the decrease in polarization. Interestingly, PN junction film exhibits polarization characteristics between the first two, demonstrating that there is no polarization mutation caused by structure at the contact interface of P-type and N-type, but only the physical superposition of the two. More importantly, all films exhibit asymmetric P-E loops, indicating the presence of build-in electric fields inside the films. For P-BFO or N-BFTO films, the asymmetric coercive field is derived from the asymmetric barrier in contact with the electrodes on both sides. While for PN junction film, the asymmetric coercive field is the unification of the PN junction barrier in the depletion layer and the asymmetric barrier at the electrode interface. The voltage applied during the test is from the bottom electrode upwards, so it exhibits a positive bias coercive field. The built-in electric field is defined as Ei = (Ec++Ec-)/2, where Ec+ and Ec- are coercive fields in positive and negative directions respectively, and the Ei values corresponding to the three films are shown in Fig. 2(b) [15]. It can be seen that the PN junction film shows much higher Ei value than the single P-BFO and N-BFTO films, which is originated from the carrier diffusion at the interface, further proving the formation of polarization-interface-free PN junction and the existence of the built-in electric field brought by it. Figure 2(c) shows a mechanism sketch of the asymmetric coercive field caused by the Schottky barrier. For P-BFO or N-BFTO film, electrons flow through the material to the electrode at the contact interface and holes flow from the electrode to the material due to the difference in Fermi energy levels between the electrode and the material, so that the internal electric fields Ebi-1 and Ebi-2 directed from the material to the electrode are generated in the opposite direction at the electrode contact interface on both sides from the material to the electrode. Here, Ebi-1 is greater than Ebi-2 since the work function of Pt is greater than that of Au, and the two cancel out to obtain the total built-in electric field (Ei) in the sample towards the bottom electrode. For PN junction film, except for the internal electrode-related fields mentioned above, the internal electric field (Ein) from the bottom electrode upwards is also generated in the depletion layer due to the difference in carrier concentration at the contact surface of P-type and N-type. Therefore, the increase of the total built-in electric field in the PN junction results in the offset of the coercive field.
Fig. 2. Electrical and optical properties of films. (a) P-E loops at room temperature; (b) the variation of Ei in different samples indicates the presence of built-in electric field in PN junction, illustration of enlarged P-E loops; (c) band diagram of P/N type and PN junction films; (d) the ultraviolet–visible reflectance spectra, illustration of the estimated bandgap; (e) the leakage of all films; (f) log(J) vs log(E) at positive and negative voltages to reveal the leakage mechanism.
The U-V reflection spectra are exhibited in Fig. 2(d) and all films show sharp reflecting edges concentrated in the visible region. Compared with the P-BFO film, the reflected edge of the N-BFTO film shows obvious redshift, indicating the enhanced absorption of photons. This is because the introduction of Ti reduces the electron transition barrier, so that electrons in the valence band can transition more efficiently under photon excitation. Nevertheless, the reflective edge of PN junction films is between P-BFO and N-BFTO films, showing moderate light absorption capacity. Bandgap values of all films calculated according to Kubelka-Munk relationship are shown in the illustration in Fig. 2(d) [16]. The bandgap of BFO is 2.65 eV, which is consistent with what has been reported by other researchers [17]. However, that of N-BFTO film decreases to 2.3 eV because the introduction of Ti changes the ligand field of the central atom, making the splitting energy level of the d orbital closer to the valence band and reducing the bandgap [18]. In particular, the PN junction presents a moderate bandgap of 2.4 eV due to the physical superposition of the first two, indicating that the formation of the PN junction has not changed the electronic configuration of the original material.
The leakage current density (J) vs. electric field (E) curves are shown in Fig. 2(e), arising from carriers generated through thermal excitation, defects and other unfavorable factors, which are used to measure the quality of the films. All films exhibit asymmetric leakage curves due to differences in the contact barrier between the material and the different electrodes. The phenomenon of forward conduction and reverse cut-off is also consistent with the fact that the barrier height of Pt side is higher than that of Au side. As can be seen, the P-BFO film presents the worst leakage property because of the volatilization of Bi and the valence change of Fe. The slight improvement of leakage for the N-BFTO film is ascribed to valence compensation by Ti doping. In particular, the leakage of PN junction film is improved by more than 3 orders of magnitude compared with the P-BFO film, which provides a guarantee for obtaining the intrinsic photocurrent. This excellent leakage performances are beneficial from the depletion layer in PN junction, where there are almost no carriers in such a high resistance region. Figure 2(f) plots the log(J)-log (E) curves to describe the leakage mechanism. For the asymmetric leakage curves, the log(J)-log(E) curves under positive and negative bias are fitted respectively. Under forward bias, the curve is divided into two regions according to the slope. The slopes of P-BFO, N-BFTO and PN junction films are 0.95, 1.07 and 1.11 at low electric field respectively, which are basically close to the ohmic mechanism (S∼1). At high fields, the slopes of P-BFO, N-BFTO and PN junction films suddenly change 3.091, 5.59 and 4.159 respectively, which is in line with the Schottky emission mechanism of field-activated electrons passing through the barrier formed by the difference in Fermi levels [19]. While for the curves at negative bias at low electric field, the slopes of P-BFO, N-BFTO and PN junction films are 1.25, 1.67 and 1.15, respectively, which match the ohmic mechanism accompanied by the space-charge-limited conduction (SCLC) mechanism, indicating that both thermally excited free electrons and charge carriers injected by electrodes are present in the films [20]. With the further increase of electric field, the slopes of P-BFO, N-BFTO and PN junction films sharply increases to 10.5, 7.8 and 5.9 respectively, which is consistent with the interfacial constrained Schottky emission mechanism. That is to say, regardless of the direction of the electric field, the conduction mechanism is the ohmic mechanism and SCLC mechanism of bulk conduction effect at low electric field, and changes into interfacial conduction Schottky emission mechanism at high electric field. Interestingly, the transition voltages under positive bias are significantly lower than that under negative bias, which comes from the contribution of the asymmetric Schottky barrier at the interface. Specifically, the decrease of the overall barrier height makes it easier for the carrier to conduct the transition under positive bias, while the increase of the barrier height makes the carrier transition less efficient under negative bias, so a higher electric field is required to induce a change in conduction behavior.
Figure 3(a) shows the photocurrent density vs. voltage (J-V) characteristics under illumination, which represents the absorption and conversion ability of the material to light. The nonlinear J-V curves indicate Schottky contact on both sides between the material and electrode, rather than ohmic contact. It is worthy noting that the PN junction film exhibits a more pronounced asymmetric J-V curve than the others, demonstrating the formation of the depletion layer and thereby induced rectification effect. The J-V curves applying positive and negative test voltages are displayed in Fig. 3(b), where Voc and Jsc exhibit a reversal direction through inverting electric poling, in relation to the switching of polarization. In particular, for the same film, Voc and Jsc tested by the inverted electric poling show asymmetry due to the together contribution from both the Schottky barrier and the PN junction barrier. Further, the change of the Voc and Jsc under opposite voltage is shown in Fig. 3(c). The Voc under the positive voltage is greater than that under the negative, and the Jsc shows the opposite raw. These results provide sufficient proofs for the different contribution Schottky barrier the and depletion layer on the photocurrent and photovoltage in the PN junction.
Fig. 3. Photovoltaic properties of all films. (a) The J-V curve shows the presence of Schottky contacts at all interfaces; (b) the J-V curves are applied under the opposite voltage; (c) the changes in Voc and Jsc under opposite test voltages; (d) PL spectra; (e) the dark and light J–V curves; (f) the switching response curves of the current density (Js) over time.
PL spectra in Fig. 3(d) are used to characterize the recombination degree of photogenerated carriers. It is generally believed that the lower the luminous intensity, the lower the recombination ability of photogenerated carriers [21]. The single BFO or BFTO film exhibits a strong PL intensity, corresponding to higher carrier recombination due to rapid electron transition from conduction to valence band. However, PL spectra intensity of PN junction decreases significantly, indicating lower carrier recombination. The efficient carrier dissociation in PN junction comes from two aspects: On the one hand, the internal electric field in PN junction acts as the driving force of carrier separation together with the depolarization field. On the other hand, unlike the simultaneous carrier separation and recombination in the ferroelectric film, there is no recombination of carriers in the depletion layer region, which greatly improves the carrier dissociation efficiency.
Figure 3(e) shows the J-V properties for the films without polarization under light and dark conditions. All the films present obvious photovoltaic response. Voc and Jsc of the PN junction film reach 0.51 V and 4.88 µA/cm2 under light conditions, which are significantly higher than those of the P-BFO and N-BFTO films. Figure 3(f) shows the dynamic photocurrent response under the alternating state of the optical switch without external bias over a long period of time. It can be seen that all the films exhibit repeatable and sustainable response to solar light. The switch response of the PN junction film is higher 3 times than that of the P-BFO and N-BFTO films. Unlike most of the ferroelectric photovoltaic effects that rely on optimizing polarization and band gap, the excellent photoelectric conversion ability of the PN junction film mainly depends on the additional built-in electric field in the depletion layer to separate photogenerated carriers and lower photogenerated carrier recombination.
In order to further study the dependence of photovoltaic effect on polarization, the J-V curves under different polarization states were tested by applying polarization voltages on different directions. The modulation rate is defined as [9]:
Fig. 4. Photovoltaic characteristics under polarization voltage modulation. The modulation rate of (a) Voc, (b) Jsc, (c) switching current Js varies with different polarization voltage.
The modulation mechanism of polarization on photovoltaic performances in PN junction is represent by the band structure under different polarization states in Fig. 5. At the contact interface between P-BFO and N-BFTO films, depletion layer is formed and band bending is associated with the potential barrier, as shown in Fig. 5(a). In the circumstances, the built-in electric field in the PN junction makes the carriers quickly be separated and gather at the interface boundary, thereby reducing the recombination of photogenerated carriers and forming electron and hole channels to obtain good carrier transport performances. When the positive voltage is applied from bottom to top electrodes, it is defined as the upward polarization state. Figure 5(b) shows the band structure in the upward polarization state, with the polarization direction from P-type to N-type. The positive polarized charge gathered on the side near the interface in the P-BFO layer attracts a large number of negative majority carriers in the N-BFTO to migrate to the interface, while the negative polarized charge gathered on another side near the interface attracts a large number of positive majority carriers in the P-BFO layer to migrate to the interface. As a result, the barrier height decreases and the built-in electric field at the interface becomes larger due to a rapid thinning of the depletion layer compared to the original state. In this case, the increase of the built-in electric field and the decrease of the barrier height are beneficial for the separation and transport of photogenerated carriers to obtain excellent photovoltaic performances.
Fig. 5. Schematic diagram of band structure for PN junction film under different polarization states. (a) Without polarization; (b) under forward polarization; (c) under downward polarization.
On the contrary, the polarized charge accumulates at the interface in the downward polarization state, as shown in Fig. 5(c). As a result, a large number of positive and negative majority carriers in P-BFO and N-BFTO are repulsed by the positive and negative polarized charges, respectively, resulting in the further widening of the depletion layer and thereby decrease of the build-in electric field. Thus, the bending of the band structure induces a very high barrier at the interface due to incomplete shielding of the polarized charge. In this case, the carrier migration is depressed, making the collection of photogenerated carriers weaker than that in the upward polarization state. However, carrier recombination is decreased due to the widening of the depletion layer, which makes the photovoltaic performance better than that of the original un-polarized state. In a word, polarization state engineered carrier transport performances in PN junction can be attributed to the enhancement of polarization field and the adjustment of depletion layer width and barrier height by charge depletion and accumulation by polarization inversion.
4. Conclusions
In summary, a polarization-interface-free ferroelectric PN junction is constructed by the contact between P-BFO and N-BFTO, where ferroelectric photovoltaic effect is engineered by the recombination and the transport performances of carriers. Meanwhile, the transport characteristics of the carriers at the interface are regulated by polarization state, demonstrating excellent photovoltaic response. This work confirms an effective way to improve carrier transport performances by designing polarization-interface-free PN junctions for ferroelectric photovoltaic effect.
Funding
Natural Science Foundation of Inner Mongolia (2022ZD06); National Natural Science Foundation of China (12074204, 12374258).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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