The recent advances of the BPVE in ferroelectric perovskite oxides exceeding the S—Q limit have spurred further exploration for electronic systems with optimal device efficiency performance However, while the intriguing physics in the two-dimensional 2D limit, such as magnetism 24 , 25 , 26 , hyperferroelectricity 27 , 28 , 29 , 30 , and exotic topological phases 31 , 32 , 33 have been widely explored, the direct experimental study on BPVE in 2D materials remains unimplemented.
The shift current is directly measured in the family of 2D materials. The enhanced BPVE is associated to the room-temperature ferroelectricity of CuInP 2 S 6 as evidenced by the electric polarization, temperature, and thickness dependent behavior. The V oc and I sc can be controlled and even reversed by an external electric field. Based on this approach, the maximum V oc is obtained to be about 1. Notably, above T c , negligible photovoltage and photocurrent are observed in all devices in the paraelectric phase with restored high symmetry We also demonstrate a crossover from 2D to 3D photovoltaics when the thickness of the CIPS exceeds the free path length l 0 of photocarriers, beyond which the photocurrent reduces to the dark condition level even with inversion asymmetry.
These results highlight the potential of developing next-generation solar cells with ultrathin 2D materials. In previous investigations of the BPVE on bulk materials, a capacitor-like structure is utilized see Fig.
When it is irradiated with light, a non-zero I sc could be observed even without external bias. This zero-bias photocurrent is regarded as the characteristic feature of BPVE. The BPVE has been mostly demonstrated in perovskite oxides in the name of photoferroics 3. In a 1D nanotube with a large free path length, the photocurrent can be furtherly enhanced. Therefore, ultrathin vdW ferroelectrics with controllable thickness are ideal candidates for enhanced photocurrent generation via BPVE see Fig.
Inset shows the characteristic I — V curve of the bulk photovoltaic effect in 3D ferroelectrics. The black and red lines represent the I — V curves at the dark conditions and bright conditions, respectively. The region of top graphene, ultrathin CIPS, and bottom graphene are marked with orange, blue, and red dashed lines, respectively.
The off-center shift of the Cu cations leads to stable out-of-plane ferroelectricity 17 , Figure 1b presents the optical image of the 2D photovoltaic device with a sandwiched structure for device 1.
S1 and their ferroelectricity was confirmed by the piezoresponse force microscopy PFM measurement at ambient condition see Fig. In the device, bilayer and few-layer graphene were used as crossed bottom and top electrodes, respectively.
Benefited from the high optical transmission of graphene in the visible range 45 , 46 , our device is allowed to carry out the direct measurements on photo-generated carriers with the synchronous electric control of the polarization. From the I sc mapping, we find that the photo-generated current exists only in the overlapping area of the top and bottom graphene electrodes as evidenced by the spatial correspondence see Fig.
To check the spatial dependence of I sc in detail, we extracted one line profile from the I sc mapping see Fig. This spatial confinement feature is a direct manifestation of finite free path length of the hot photocarriers. Moreover, the sign and magnitude of the photocurrent are non-uniformly distributed, showing independence on sample topography see Figs. Since the device is not electrically polarized in advance, the spatially randomized photocurrent is the result of the spontaneous electric polarization in CIPS see Fig.
Next, we studied the photocurrent generation in CIPS by homogeneous light illumination on the whole device. In the open-circuit condition, we characterize the relation between photocurrent and voltage using The photocurrent density is related to the BPVE via the third-rank piezoelectric tensor as. Thus, the larger the photovoltaic field is, the stronger the photovoltaic voltage will be.
Figure 2a shows the characteristic output I —V curve from one typical device device 2. With light illumination, a large and positive I sc was observed. In contrast, when the laser is switched off dark condition , the current density at 0. Compared with previously reported bulk ferroelectric photovoltaic cells 35 , 36 , 37 , this result yields much larger I sc and V oc at the same excitation density These data yield photoconductivity at the order of 8. The dotted color lines represent the linear fit on the output I — V curves measured at bright conditions with different laser power.
The color represents the magnitude of the generated photocurrent density. The arrows indicate the measurement sequence. In ferroelectrics, the electric polarization is nonvolatile and electrically switchable. To verify the relation between the enhanced photocurrent and the ferroelectricity, we study the photocurrent as a function of poling voltage.
By scanning the polarizing voltage, the sign of the photocurrent can be controlled from negative to positive gradually. The asymmetry of the photocurrent between backward and forward scanning arises from the ferroelectric hysteresis of CIPS.
Specifically, the switchable photocurrent is summarized in Fig. To show the hysteresis behavior more clearly, we extract the J sc at each poling voltage.
As shown in Fig. Similarly, the V oc as the function of the poling voltage is shown in Fig. Upon heating the device to be above T c , the ferroelectric CIPS will transform to the paraelectric phase with restored inversion symmetry, in which the BPVE is forbidden strictly.
We also investigate the photocurrent dependence on light illumination power P in. The monotonical increase of generated J sc with P in is consistent with the prediction of Eq. By comparing the photoconductivity of CIPS with that from previous literatures 31 , 32 , 33 , 34 , 35 , we find comparable results in our devices. Insets display the crystal structures of CIPS in ferroelectric and paraelectric phases. In the ferroelectric phase, the off-center shift of the Cu ions induces a built-in electric field E bi perpendicular to the 2D plane.
Therefore, with an out-of-plane electric polarization, the vacuum levels at the two sides of the CIPS are different as illustrated in Fig. Furthermore, the sign of the asymmetric energy potential barrier between the top and bottom surfaces can be controlled by the direction of electric polarization, leading to the direction switchable photocurrent Fig.
There is no observable photocurrent from BPVE. The top and bottom graphene electrodes are represented with slabs of atoms in gray on the left and right sides of CIPS. The cartoon on the top depicts the corresponding energy shift of the conduction band and valence band. By considering the linear dependence of the photocurrent on light power density from Eq. For 3D materials, most of the ferroelectrics are oxide insulators with perovskite structure, in which the large gap width fundamentally limits the photocurrent density unless via subtle domain engineering refs.
In 1D WS 2 nanotube, the ultrahigh photocurrent density is benefited from the nanoscale diameter size of the tube in nature and the enhanced shift current The BPVE-induced photocurrent density is three orders of magnitude larger than that observed in 3D bulk photovoltaics.
However, as confined by the tube structure, the direction of the photocurrent could not be switched by the external electric field. In contrast, when the film thickness is larger than the above free path length, the photocurrent decreases to the noise level quickly. This is a clear demonstration of the crossover from 2D to 3D BPVE materials, which has not yet been observed in experiments.
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