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Nature Communications volume 16, Article number: 9839 (2025) Cite this article
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Bulk photovoltaic (BPV) effect primarily stems from shift currents in symmetry-breaking materials, providing the potential to smash the Shockley-Queisser limit that constrains the performance of conventional p-n junctions-based solar cells. However, limited open circuit voltages (V_oc) or short circuit current densities (J_sc) from BPV devices still cause a low photoelectric conversion efficiency. Here, combining theoretical analysis and experimental evidence, we identify a range of BPV materials where both V_oc and J_sc can be co-optimized, and greatly boost the efficiency through ferroelectric engineered shift current. We select ferroelectric NbOBr₂ as an example and construct a two-dimensional in-plane device with a giant shift current-dominated BPV effect. In spontaneous polarization state, the devices demonstrate a record-high J_sc among all ferroelectric materials. Moreover, the electrically aligned NbOBr₂ polarization enables the significant co-enhancement of both V_oc and J_sc, leading to a colossal improvement of photoelectric conversion efficiency up to four orders of magnitude (1.25%), which is approximately four times greater than that of state-of-the-art BPV devices. Our work provides a promising solution for screening and creating higher efficient BPV cells.
Solar cell technology can convert sunlight, the most abundant energy source, into electricity, and its quest for higher photoelectric conversion efficiency (PCE = J_scV_ocFF/P, where FF is the fill factor and P is the power density of incident light) is critical to address the increasing energy crisis^1,2. Conventional technology are p-n junctions-based photovoltaic cells, which rely on built-in fields at the interface to separate photo-generated electron-hole pairs. This mechanism fundamentally determines the low PCE that cannot exceed the Shockley-Queisser limit derived from thermodynamic principles^2,3,4. Alternatively, the bulk photovoltaic (BPV) effect, as an emerging technology for harvesting solar energy, can offer a viable pathway for breaking the Shockley-Queisser limit. The BPV effect primarily originates from shift currents in symmetry-breaking materials caused by the displacement of electron clouds with light-induced interband excitation. Furthermore, the BPV technology eliminates the need for p-n heterojunction interfaces and can generate stable photovoltages above the bandgap of excitation light^5,6,7.
Currently, various strategies have been used to create the BPV effect in high-symmetry low-dimensional materials (i.e., centrosymmetric materials), enabling the significant improvement of short-circuit current density. The approaches include interfacial design (e.g., twisted angular heterojunctions)^8,9, geometric engineering (e.g., one-dimensional nanotubes or nanoribbons)^10,11, and strain engineering (e.g., flexo-photovoltaic effect)^{12,13,14,15,16}. By lowering the crystal symmetry, these pioneering attempts have extended the BPV effect from symmetry-breaking materials to a wide range of centrosymmetric high-symmetry materials. However, albeit the realization of large J_sc in these studies, the V_oc remains at an exceptionally low level due to high electrical conductivity. Such an imbalance results in a low PCE that still falls below the Shockley-Queisser limit^2,17,18, which underscores the critical role of conductivity and bandgap screening in BPV performance. To enhance the V_oc of BPV effect, ferroelectric materials with electric field poling are applicable, which have received considerable attention so far^{1,2,7,16,19,20,21}. However, reported ferroelectric materials possess large band gaps and low conductivity, unfortunately leading to a small J_sc. In this context, seeking new ferroelectrics with feasible band gaps and simultaneously increasing the V_oc and J_sc is exceptionally vital to achieve higher PCE of BPV effect.
As shown in Fig. 1a, we propose two processes to augment the PCE of BPV devices. The first process involves screening suitable ferroelectric BPV materials by analyzing the trends in PCE-dependent bandgap and conductivity (left panel of Fig. 1a; for more details, see the Supplementary Note 2). The screening should not only rely on the range of the bandgap (1.9–2.3 eV) but also ensure appropriate conductivity (10⁻⁴− 1 S/cm). Such a screening is different from that of p-n junctions-based solar cells, in which high-conductivity materials are typically employed to enhance their PCE. However, in BPV devices, it is theoretically challenging to achieve both a high carrier concentration and a large open-circuit voltage simultaneously². The second process is to improve both V_oc and J_sc via ferroelectric increased shift current (right panel of Fig. 1a), which can be theoretically rationalized below. For the shift current BPV effect, the current density J_sc can be expressed below, given that the linear polarized light (E(omega )) is confined in the poling direction²²:
where (sigma) is the susceptibility tensor of shift current. The (sigma) can be considered an integral of the shift vector ({ebar{R}}_{{nm}}^{{ab}}) weighted by the rate of optical transition over the Brillouin zone. Because the shift vector indicates the change of charge center in real space during excitation, ({ebar{R}}_{{nm}}^{{ab}}) mainly determines the total shift current generated in 2D materials. Therefore, the current density J_sc can be simplified as:
where f_nm is the difference of Fermi distribution functions of band n and m. Moreover, the integral of the shift vector over the Brillouin zone is proportional to the inter-band polarization difference²³.
in which P_n and P_m denote the polarization of band n and m, respectively. Hence, we can infer that manipulating ferroelectric polarization presents a promising avenue for increasing the magnitude of shift current (right panel of Fig. 1a) and resulting J_sc. However, due to V_oc= dJ_sc/({{{rm{sigma }}}}) (d and ({{{rm{sigma }}}}) are the electrode distance and material conductivity, respectively)², a high V_oc cannot be achieved from narrow-bandgap semiconductor materials with high conductivity (see the Supplementary note 2). In summary, to increase the PCE of BPV devices, we initially screen suitable bandgap ferroelectrics and then co-optimize V_oc and J_sc via polarization-increased shift current.
a Two procedures are proposed to improve the PCE. Firstly, as shown in the left panel, we screen ferroelectric BPV materials with the potential for a high PCE (The data are provided in Supplementary Table 1). Yellow bar: 2D TMDs (< 1.9 eV, 1 ~ 100 S/cm; high J_sc and low V_oc). Gray bar: narrow-bandgap ferroelectrics (< 1.7 eV, 10⁻⁶ ~ 10⁻⁴S/cm; low J_sc and low V_oc). Blue bar: wide-bandgap ferroelectrics (> 2.3 eV, 10⁻¹² ~ 10⁻⁴S/cm; low J_sc and high V_oc). Red area: potential high-PCE materials. Then, as shown in right panel, we increase the efficiency of shift current-ferroelectric BPV by polarization engineering. The SC and P dictate shift current and polarization, respectively. Theoretically, the shift vector serves as an intermediate variable to establish a correlation between the shift current and polarization state. b Out-of-plane (top) and in-plane (bottom) schematic diagrams of the crystal structure of NbOBr₂ (space group C2, with a = 13.83 Å, b = 3.91 Å and c = 7.02 Å). The polarization direction (P) along the b-axis is marked by a green arrow. c Ferroelectric switching of a 16 nm thick NbOBr₂ flake. The upper panels show PFM butterfly loops with notable polarization switching. The bottom panels present topography, PFM in-plane amplitude and phase images, revealing spontaneous ferroelectric polarization. Scale bar, 500 nm. d Polar plots of SHG responses, indicating remarkable noncentrosymmetry along b-axis. e Schematic of the NbOBr₂-based lateral BPV device.
To this end, we adopt a 2D van der Waals ferroelectric NbOBr₂ crystal with extraordinary nonlinear optical effects as a model. NbOBr₂ has demonstrated compelling potential for photonic applications²⁴. Compared to traditional ferroelectric materials, this crystal exhibits a suitable band gap (~ 2.1 eV), strong exciton effect and moderate conductivity^25,26,27,28, which all lies in the ranges that we propose in Fig. 1a. Meanwhile, distinct from other shift-current BPV materials^11,13, NbOBr₂ allows for the modulation of intrinsic photoelectric properties via electric polarization, which is anticipated to warrant high PCE. Here, we designed a lateral BPV device with ferroelectric polarization oriented along b axis. By investigating the dependence of J_sc on the angle of polarized light, establishing a nonlinear numerical relationship between J_sc and optical power, and analyzing photocurrent spatial distribution^29,30, we determine that shift current predominantly governs the bulk photovoltaic effect of NbOBr₂^8,9,31,32. In spontaneous polarization state of NbOBr₂, the J_sc is significantly higher than that of other ferroelectric or non-centrosymmetric 2D materials. Furthermore, through the engineering of ferroelectric polarization, V_oc and J_sc are concurrently enhanced, resulting in a remarkable increase in PCE by 2-4 orders of magnitude compared to the spontaneous polarization state. This represents the first endeavor to optimize the performance of shift current BPV through polarization manipulation. Density functional theory analysis reveals that the NbOBr₂’s collective effects of large joint densities of states (JDOS) and absorption strengths contributes to the extraordinary shift current.
Single van der Waals (vdW) layers of NbOBr₂ crystal are ABC stacked along the a-axis direction and crystallized into a monoclinic structure with C2 space group, as shown in Fig. 1b. Within each vdW layer, the atoms in NbOBr₂ are arranged in the form of an [NbO₂Br₄] octahedron, similar to a typical perovskite structure³³. The Nb atom dimerizes along the c-axis (d1 > d2), resulting in noticeable first-order Peierls distortion. The eccentric displacement of Nb atoms caused by the second-order Perls distortion along the b-axis renders the separation of positive and negative charge centers, constituting in-plane spontaneous polarization (P_s)^24,34,35. Through lateral piezoelectric force microscopy (PFM), we characterize the in-plane spontaneous domains of NbOBr₂ (Fig. 1c). The ferroelectric switching is demonstrated by electrically switchable phase hysteresis and butterfly-shaped amplitude loops³⁶. Notably, the low crystal symmetry of NbOBr₂ is expected to produce significant BPV effect.
As shown in Fig. 1d, the second-harmonic generation (SHG) response in NbOBr₂ demonstrates in-plane noncentrosymmetric behavior essential for ferroelectric presence. We fitted the nonlinear second-order SHG response, revealing that the maximum signal is aligned along the b axis. This is consistent with crystal symmetry analysis. Significantly, the fracture direction is determined by the external tear orientation and the anisotropic nature of NbOBr₂’s fracture toughness. This synergy induces preferential crystal breaking along a specific direction during the exfoliation process, resulting in anisotropic shapes such as rectangles. Upon cleaving flakes from bulk crystal, the unique structure allows for easy differentiation between polarized and non-polarized directions in NbOBr₂ nanosheets. For example, the typically rectangular nanosheets feature the wide edge aligned with the polarization axis and the long edge aligned with the non-polarization axis. Based on these considerations, we fabricated lateral 2D NbOBr₂ BPV devices oriented along the polarization axis, as shown in Fig. 1d.
Most ferroelectric photovoltaic devices primarily utilize out-of-plane polarization to enable vertical BPV cells, with the BPV mechanism being closely associated with domain walls^7,15, depolarization fields¹⁹, and polarization interfaces³. Nevertheless, as a 2D room-temperature in-plane ferroelectric material, NbOBr₂ has been theoretically calculated to produce shift current-dominated BPV³⁵, and here we attempt to show relevant experimental evidence. Initially, under stable 405-nm laser illumination (Fig. 2a), a photovoltaic current of 10^-11A was measured without a bias in the spontaneous polarization state of NbOBr₂. However, due to its small J_sc and V_oc, which can be easily mistaken for dark current or device noise, we subsequently obtained the time-dependent response curve of I_sc with periodic illumination (Fig. 2b). The results confirm that the observed change in current is attributed to the photovoltaic response of NbOBr₂, in which a fast response time of 16.6 μs can be acquired (Supplementary Fig. S10).
a I–V characteristics of the NbOBr₂ device under illumination and dark conditions. b Dynamic photocurrent responses upon repeatedly turning on or off the incident laser. c Incident power density dependence of I_sc. d I_sc spatial distribution mapping of a device in the inset of (e). The photocurrent was collected from the white dashed box in the insert of (e), with the channel region between the two black solid lines. e Position dependence of I_sc taken along the white dashed lines in (d). Inset: Optical micrograph of the device, and the White scale bar represents 5 μm. f Polar plot of I_sc (blue dots) as a function of the linear polarization angle along the b axis for the NbOBr₂ device. A 405 nm laser with a power density of about 7500 W cm⁻² was used in (a, b, and f).
The numerical relationship between I_sc and incident optical power is further investigated, revealing a transition from linear to square-root dependence (see Fig. 2c and Supplementary Figs. S8, S9). This behavior distinguishes itself from the photovoltaic mechanism based on the Schottky barrier, and agrees well with the dependence of shift current on optical power^10,14,37. In addition, the spatial distribution of I_sc in the NbOBr₂ device was also collected, as depicted in Fig. 2d, e. We see that unidirectional photocurrent is predominantly localized within the channel region, implying a distinct mechanism from that generated by the Schottky junction region, where the photocurrent is primarily localized at two electrode-semiconductor interfaces with opposite directions of photocurrent^9,31. This phenomenon has been confirmed across multiple devices (Supplementary Fig. S5), especially those with longer channels (Supplementary Fig. S6). It is noteworthy that the consistency between theoretical calculation of shift current^29,30 and our experimental findings (spatial distribution of photocurrent in long channels) can rule out the significant contribution of ballistic current. Therefore, in conjunction with evidence of changes in I_sc related to variations in the angle of linearly polarized light (Fig. 2f), we conclude that shift current dominates the BPVE effect of NbOBr₂. It is noted that the angular dependence of linear polarization of the shift current is also attributed to the in-plane structural anisotropy of the NbOBr₂.
To quantify the BPV effect and evaluate its performance, we compare the J_sc and BPV coefficients from various ferroelectric photovoltaic devices. Firstly, Fig. 3a illustrates the reported J_sc magnitudes for different materials upon varying illumination power densities. At an excitation wavelength of 375 nm, the NbOBr₂ device exhibits a record-high J_sc in spontaneous polarization state. However, at a wavelength of 405 nm, the J_sc of NbOBr₂ falls slightly behind a few 2D ferroelectric materials, indicating that the performance of BPV effect depends on a few factors such as material band structure, device structure (vertical or lateral), and BPV mechanism. We note that the light wavelength-dependent properties of NbOBr₂ shift current will be discussed later to elucidate the underlying difference.
a Plots of short-circuit current density J_sc versus incident power density. Data for other ferroelectic materials is taken from the references (Bi-Mn-O³⁹, KBFO⁶², BFO⁶³, BFO:La³⁸, KBNNO⁶⁴, BTO⁶⁵, BFO:Mn⁶⁶, BTO⁶⁷, PZT⁶⁷, Sn₂P₂S₆⁴⁰, CIPS⁵, In₂Se₃¹⁹, SnS³¹). The underlined materials denote data obtained in the spontaneous polarization state, whereas the non-underlined materials represent those after poling. b Comparison of the BPV coefficient in various ferroelectric materials. Data for other materials is adopted from the references (PTO⁶⁸, LNO:Fe⁶⁹, KNO:Fe⁶⁸, BTO⁶⁵, PZT⁷⁰, BFO⁶⁶, CIPS⁵, In₂Se₃¹⁹, SnS³¹, 3 R MoS₂¹⁴).
The BPV coefficient is a third-order tensor that characterizes the strength of BPV effect¹³. We used the equation J_i = βEE^*P_in to estimate the BPV coefficient of NbOBr₂ along the polarization axis⁸, where J_i represents the current density of BPV, β denotes the BPV coefficient, E and E^* represent unit vectors for light polarization, and P_in is the optical power. We acquire an average BPV coefficient of 0.029 V⁻¹ within a linear response interval at 375 nm, while at 405 nm the coefficient is calculated to be 1.5 × 10⁻⁴V⁻¹. Notably, under a single ultraviolet band, NbOBr₂ exhibits a giant BPV coefficient among all ferroelectric materials. In summary, compared to the reported ferroelectric photovoltaic materials, the J_sc and BPV coefficients of NbOBr₂ devices demonstrate highly competitive performance, particularly at 375 nm.
As previously reported^1,38,39, ferroelectric photovoltaic materials can be electrically switched between positive and negative responses. However, their limitations involve small J_sc, greatly reducing the PCE of BPV effect. In the case of NbOBr₂ BPV devices, we not only realize this switching phenomenon, but also acquire an abnormally enhanced photovoltaic current after electric field poling. We present the output curves obtained under 375 nm illumination in three different states: initial state, after positive-voltage poling, and after negative-voltage poling (see Fig. 4a). It can be observed that − 5 V poling enables J_sc to increase from 0.001 A/cm² to 0.044 A/cm²; similarly, + 5 V poling endows J_sc with –0.011 A/cm². The rearrangement of ferroelectric domains can be intuitively employed to understand the raised photovoltaic performance of NbOBr₂ (see Fig. 4b). In the initial state, the spontaneous polarization domains are not well ordered, resulting in a negligible net photocurrent. However, upon applying an electric field to align all domains into the same direction, there is a dramatic increase or reversal in the net photocurrent^40,41.
a I_sc–V curves recorded under illumination before and after poling. The laser used in (a) is 375 nm with a power density of 0.46 W cm⁻² and a spot of 35.6 μm. b Schematic diagram of ferroelectric domains arrangement after electrical field modulation. c Statistics of J_sc, V_oc, and η enhancement, extracted from the I–V curves after poling of ten separate devices. d Comparing the photovoltaic efficiency of reported BPVE devices. Data for other materials is taken from references (BFO:La³⁸, KBNNO⁶⁴, BTO⁶⁵, BFO⁷, Sn₂P₂S₆⁴⁰, MAPbI₃⁴¹, MoS₂/BP⁸, 3R-MoS₂¹⁴, WS₂ NT¹⁰, In₂Se₃¹⁹).
Subsequently, we have conducted multiple experiments and observed that, even after the removal of the electric field, the enhancement effect on photocurrent can persist. This retention is closely associated with the switched nonvolatile ferroelectric polarization⁴². Finally, we obtained statistic J_sc and V_oc enhancement data from 10 devices, as illustrated in Fig. 4c. The J_sc enhancement typically ranges from one to two orders of magnitude, while the increase in V_oc is approximately one order of magnitude, resulting in a gigantic improvement of PCE by two to four orders of magnitude compared to the initial polarization state. To visually demonstrate the remarkable PCE of NbOBr₂ BPV devices, we compared them with currently reported ferroelectric and shift current-dominated BPV materials (see Fig. 4d and Supplementary Table 2 for detailed data). Surprisingly, the PCE of NbOBr₂ under spontaneous polarization is even comparable to that of Sn₂P₂S₆, BFO:La, and KBNNO with electric field poling. Remarkably, the NbOBr₂ PCE under polarization engineering can reach 1.25 %, which is approximately four times higher than that of shift current BPV, such as WS₂ nanotubes and MoS₂/BP devices. It is noteworthy that to strengthen the rigor of the PCE comparison, the J_sc values are extracted from the linear region of the photovoltaic response under low light intensity, with incident light power maintained as close as possible to the same order of magnitude. In addition, we not only extracted photovoltaic efficiency data from the references, but also fabricated lateral BPV devices with identical structures for the other three materials. Under standardized photovoltaic testing conditions, their best photovoltaic performance was obtained (see Supplementary Table S3). The best BPV efficiencies of the fabricated 3R-MoS₂, MoS₂ nanoscrolls, and β‘-In₂Se₃ devices are 0.32 %, 0.28 % and 0.09 %, respectively. The aforementioned results are consistent with the order of magnitude reported in the references^10,14,19. Their BPV efficiency at 375 nm exceeds that observed within the visible light spectrum. Nevertheless, the BPV efficiency of NbOBr₂ after poling remains the highest among all samples.
Compared with junction-type photovoltaic, the shift current-dominated BPV is not only affected by the energy band structure of photovoltaic materials, but also depends on Bloch wave functions and polarization states theoretically^43,44,45,46. We employed density functional theory to investigate the origin of the shift current of the NbOBr₂ family. Figure 5a shows the calculated conductivities of ({sigma }_{{sc}}^{{yyy}}left(omega right)) for NbOX₂ (X = Br, Cl, and I), which determine the strengths of the (y)-direction (or (c)-axis) shift current stimulated by (y)-direction linear polarized light. For ({sigma }_{{sc}}^{{yyy}}left(omega right)) in NbOBr₂, a steep increment of about (7,mu {{{rm{A}}}}/{{{{rm{V}}}}}^{2}) from Peak A (around 3.2 eV, 387 nm) to Peak B (around 3.74 eV, 331 nm) can be discerned, while ({sigma }_{{sc}}^{{yxx}}left(omega right)), representing shift current excited by (x)-direction linear polarized light, oscillates by only ~(1,mu {{{rm{A}}}}/{{{{rm{V}}}}}^{2}) (Supplementary Fig. 17a). The superiority of NbOBr₂ over the responses of other NbOX₂ (X = Cl, I) crystals in this specific wavelength region (3.2–3.74 eV, 331–387 nm) coincides well with the experimental observations at 375 nm. The microscopic origin of this steep increment originates from the collective effects of both joint densities of states (JDOS) and absorption strengths. Although Fig. 5b shows that the magnitudes of JDOS are on the same level for both Peaks A and B, the absorption coefficients in Supplementary Fig. 17b indicate that the difference between Peak A and B mainly originated from absorption strengths, ({|{r}_{{nm}}^{y}|}^{2}). The inter-band resonant absorption contributions of conductivities ({sigma }_{{sc}}^{{yyy}}left(omega right)) are parsed (see Supplementary Note 2), and the major band-resolved contributions at Peaks B and A are respectively presented in Supplementary Figs. 17c, d. Among the major inter-band contributions, the absorptions between first conduction (CB) and deep valence (VB + 9) bands and the transitions between CB and shallow valence (VB + 2) bands are representative⁴⁷, and orbital projections for these three electronic bands are presented in Fig. 5c. The (k_y, k_z)- distributions of absorption strength at ({k}_{x}=0) for both types of transitions shown in Fig.5d further illustrate that the transitions mainly arises from overlaps between ({d}_{{yz}})-orbitals of Nb atom and (p)-orbitals of Br atom at the region near (Gamma to {{{rm{Z}}}}) k-path. An order of magnitude difference shown in Fig. 5d indicates that the symmetry-allowed transitions from deep valence bands should be more pronounced than that from shallow valence bands in NbOBr₂, leading to noticeable differences between Peak B and Peak A.
a Conductivities of shift current, ({sigma }_{{sc}}^{{yyy}}left(omega right)), and (b) The joint densities of states. The results for the NbOX₂ (X = Br, Cl, I) family are respectively denoted by red, blue, and green lines. The peaks at (omega=3.2,{{{rm{eV}}}}) and (3.74,{{{rm{eV}}}}) are labeled by “A” and “B”, respectively. c Electronic band structure for bulk NbOBr₂. Red and blue lines denote Nb d- and Br O p- orbitals weights to the bands. In the first conduction band (CB), 3rd (VB + 2) and 10th (VB + 9) valence bands, the weights of Br p-, Nb d_yz-, Nb d_xy-, and the rest d orbitals of Nb (drest) are indicated by purple, red, blue, and green solid circles, respectively. d (left({k}_{y},{k}_{z}right)) Distributions (({k}_{x}=0)) of the absorption strengths. There ({vert {r}^{y}_{nm}vert}^{2}) are representative (CB, VB +2) and (CB, VB + 9) bands.
The discussion mainly focuses on the shift current-dominated BPV, whose efficiency can be effectively switched and enhanced through polarization engineering. Although previous theories have established a correlation between electric polarization and shift vector integrals^23,48,49, the experimental possibility of increasing both shift current and V_oc via the alignment of polarization remains unknown. In principle, the shift vector is the relative displacement of the center of electrons and holes during an optical transition. After the poling of an external electric field, the polarization vector is aligned in the same direction, subsequently amplifying the shift vector and leading to an increase in shift current. If selecting suitable bandgap ferroelectrics, such an increase can also boost the magnitude of V_oc theoretically and experimentally. Moreover, the direction of the shift vector will also vary as a function of the polarization direction (see Fig. 1a). We note that the dominance of shift current in NbOBr₂ BPV can be unambiguously demonstrated. However, as shown by the results in Fig. 4a, the other influencing factors, such as polarization charge redistribution at the metal-ferroelectric interface and Schottky barrier change, cannot be entirely disregarded as they may also involve the enhancement of V_oc^1,2,50,51.
Besides the DFT calculation, photovoltaic experiments on NbOCl₂ and NbOI₂ (see Fig. 5a, Supplementary Figs. S15 and S16) have also been conducted. Among the existing findings, the optimal response wavelength of the NbOX₂ family (X = Cl, Br, I, ranging from Cl to I) exhibits a gradual redshift. Consequently, under identical thickness and a 405 nm laser, the magnitude of photocurrent density from highest to lowest is as follows: NbOI₂ > NbOBr₂ > NbOCl₂ (see Supplementary Fig. S14). However, the iodine in NbOI₂ tends to precipitate easily, rendering the device particularly susceptible to failure. Overall, the NbOX₂ family exhibits exceptional performance in high PCE photovoltaic applications and holds particular promise for addressing the low PCE limitation associated with shift current BPV devices. If optimization can be achieved for both the ferroelectric-metal interface and the stability of NbOI₂ material, further efficiency enhancement could be expected.
In summary, by screening ferroelectric materials and leveraging in-plane ferroelectric NbOBr₂ BPV as a model, we demonstrate that ferroelectric polarization engineering can theoretically and experimentally allow for the co-optimization of both shift current and V_oc. We thus advance the low PCE in the 2D bulk photovoltaic field to a new record of 1.25 %, which surpasses the state-of-the-art value by fourfold at the light wavelength of 375 nm. Moving forward, we propose two possible approaches based on the mechanistic understanding of this work to further enhance the photovoltaic efficiency. Firstly, optimizing the Schottky interface is crucial because a poor interface can compromise photovoltaic performance after polarization alignment. Secondly, the 3D tier-by-tier integration of 2D photovoltaic materials remains largely unexplored. The number of stackable layers for 2D photovoltaic cells could far exceed that of perovskite p-n junction cells. Through precise design and stacking, the light absorption efficiency and photoelectric conversion efficiency of 2D BPV cells are expected to increase substantially.
Bulk NbOBr₂ crystals were synthesized by chemical vapor transport³³. Initially, chemical reactants (Nb₂O₅, 99.99%, from Aladdin; Br₂, 99.7%, from Aladdin; Nb, 99.95%, from Aladdin) in stoichiometric proportions were meticulously mixed and enclosed in an evacuated quartz tube with a length of ~ 20 cm under a vacuum of 10⁻⁶  torr and liquid nitrogen temperature. Subsequently, the sealed tube underwent heating in a two-zone furnace, maintaining the reaction zone at 650 °C and the growth zone at 600 °C for a duration of 5 days for the formation of NbOBr₂ crystals. The 10–80 nm nanosheets obtained by mechanical exfoliation were transferred to SiO₂/Si label substrate by the dry method, electrodes were designed in the b-axis direction by laser direct writing equipment (HEIDELBERG DWL66 + ), and Cr/Au electrodes (5 nm/35 nm) were deposited by thermal evaporation.
The crystal structure of the exfoliated flakes was studied by FEI Titan G2 80–200 ChemiSTEM. SHG measurement was performed with an MStarter 100 SHG Microspectral Scanning Test System, in which the excitation wavelength was 1064 nm and the pulse width was 6 ps (Nanjing Metatest Optoelectronics Co., Ltd, China). The Raman spectra were collected by a confocal Raman spectrometer (Horiba LabRAM Odyssey) equipped with a 532 nm laser. The spectral response of heterojunctions from 400 to 1500 was studied by a variable-wavelength photocurrent instrument (MStarter ABS DUV-NIR Microscopic Absorption Spectroscopy System, Nanjing Metatest Optoelectronics Co., Ltd.). Electrical measurements were performed in a Keithley 4200 Parameter Analyzer, while PFM were carried out using an Asylum MFP-3D with a 2 N/m probe. Scanning photocurrent mapping measurements were conducted by a photocurrent imaging instrument (Mstarter 200 High Precision Photocurrent Scanning Test Microscope) (supplied by Nanjing Metatest Optoelectronic Co., Ltd.) including 375 nm LED and 405 nm multimode laser light source, Keithley 6482, and Keithley 2614b. Response time was measured by using an LNPC (Nanjing Metatest Corporation).
The Kohn-Sham electronic structures of NbOX₂ were obtained by performing the density functional theory (DFT) calculations, implementing the Vienna Ab initio Simulation Package (VASP) code^52,53,54,55. The optimized crystal structures are obtained from references. The exchange-correlation energy of valence electrons was treated by the generalized gradient approximated (GGA) functionals of Perdew-Burke-Ernzerhof (PBE) type parameterization. An energy cutoff for the plane-wave basis was set to 400 eV, with a 3 × 9 × 5 Monkhorst-Pack k-point sampling. Convergence was reached when the energy difference between sequential steps of electronic self-consistent field calculation was less than 10⁻⁶eV. For the efficient evaluation of shift current conductivities, the Hamiltonian was obtained using the Wannier tight-binding method implemented in the Wannier 90 code^{56,57,58,59,60,61}. In the Wannier tight-binding calculations, a dense k-point mesh of 30 × 90 × 50 for converged integrals of the Brillouin zone and an energy width of η = 0.025 eV for smearing Dirac δ-functions were adopted. Scissors shifts, based on the band gaps of hybrid HSE calculations, are used to prohibit the underestimations of the frequencies of shift current responses.
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The data that support the findings of this study are presented in the paper and the Supplementary Information. Source data are provided in this paper.
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This research was supported by the National Science Foundation of China (grant no. 62404197 to P. F.; No. 62304202 to F. X.).
These authors contributed equally: Pu Feng, Zhihao Gong.
Center for Quantum Matter, School of Physics, Zhejiang University, Hangzhou, China
Pu Feng, Baoyu Wang, Zhongyi Wang, Haoran Xu, Lingrui Zou, Hua Wang, Fei Xue & Kai Chang
ZJU-Hangzhou Global Scientific and Technological Innovation Center, College of Integrated Circuits, Zhejiang University, Hangzhou, China
Pu Feng, Baoyu Wang, Zhongyi Wang, Haoran Xu, Lingrui Zou, Xun Han, Bin Yu & Fei Xue
Academy of Interdisciplinary Studies on Intelligent Molecules, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, College of Chemistry, Tianjin Normal University, Tianjin, China
Zhihao Gong
Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
Chen Liu & Xixiang Zhang
State Key Laboratory of Metastable Materials Science & Technology and Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao, China
Yingchun Cheng
Department of Materials Science and Engineering, National University of Singapore, Singapore, Singapore
Lain-Jong Li
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F.X. and P.F. conceived and guided the research. F.P. fabricated devices and tested their photoelectrical performance. Z.-Y.W. and H.-R.X. carried out the PFM measurements. B.-Y.W. and C.L. performed the STEM characterization. L.-R.Z. performed the Raman characterization. F.X., P.F., L.-J.L., and H.W. analyzed the data. P. F., Z.-H.G., and F.X. wrote the paper. Z.-H.G. performed DFT simulations under the guidance of H.W. Y.-C.C. provided bulk crystal materials. X.H., B.Y., X.-X.Z., and K.C. provided experimental resources.
Correspondence to Xun Han, Hua Wang or Fei Xue.
The authors declare no competing interests.
Nature Communications thanks Bing Huang, Ya Yang, and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.
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Feng, P., Gong, Z., Wang, B. et al. High-efficiency bulk photovoltaic effect with ferroelectric-increased shift current. Nat Commun 16, 9839 (2025). https://doi.org/10.1038/s41467-025-64807-y
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Received: 12 February 2025
Accepted: 26 September 2025
Published: 07 November 2025
Version of record: 07 November 2025
DOI: https://doi.org/10.1038/s41467-025-64807-y
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