Haoyun Wang received his B.S. degree in functional materials from Huazhong University of Science and Technology (HUST) in Wuhan, China in 2020. Currently, he is pursuing a doctor’s degree in material physics and chemistry from HUST. His current research interest is focused on 2D mateirals based electronic and optoelectronic devices.
Xingyu Song received her B.S. degree in materials science and engineering from Northeastern University in Shenyang, China in 2021. Currently, she is pursuing a M.S. degree in materials science from Huazhong University of Science and Technology in Wuhan. Her current research interest is focused on 2D materials based tunneling transistors.
Xing Zhou is a Professor of School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). He received his B.S degree in inorganic nonmetallic materials from Wuhan University of Science and Technology (WUST) in 2012, and received his Ph.D. from HUST in 2017. His current research interests focus on the controllable synthesis of 2D materials and their heterostructures for optoelectronics.
Tianyou Zhai received his B.S. degree in chemistry from Zhengzhou University in 2003, and then received Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2008. Afterward, he joined the National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow, and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor of School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their applications in optoelectronics.1. Introduction
Photovoltaic devices play an important part in solar-energy conversion, image sensing, communication, and other extensive fields[1−9]. Although photovoltaic devices based on conventional bulk semiconductors have been widely applied, the limitations such as low absorption coefficient, lattice mismatch, brittleness and surface dangling bonds impede the further development in high-performance and wearable photovoltaic devices.
In recent years, two-dimensional (2D) materials have attracted increasing interest due to the unique optoelectronic properties, including dangling-bond-free surface[10−16], strong light−matter interaction[17−28], broadband absorption[29−41] and flexibility[42−48]. Compared with conventional semiconductors, 2D materials exhibit great advantages in photovoltaic devices. Table 1 shows the properties of 2D materials and its significance for photovoltaic devices. Firstly, the dangling-bond-free surface allows different 2D materials to be integrated by van der Waals (vdW) force without the lattice mismatch. Therefore, the carrier transportation can be modulated by band engineering, and broadband light absorption can be achieved by stacking 2D materials with different bandgaps[49−54]. Moreover, such dangling-bond-free surface is self-passivated, avoiding the complex surface passivation process[55−57]. Secondly, the ultrathin body of 2D materials enables efficient modulation of electrical properties by external field and chemical surroundings, thus the photoelectric performances can be effectively modulated, providing new pathways for improving the photovoltaic performances[58−64]. Thirdly, 2D materials such as transition metal dichalcogenides (TMDs) exhibit strong light−matter interaction, showing larger light absorption in comparison with bulk semiconductors in nanoscale thickness, and thus ultra-thin photovoltaic devices can be achieved through 2D materials[65−67]. Last but not least, the mechanical flexibility of 2D materials makes them applicable in flexible and wearable photovoltaic devices. Therefore, 2D materials provide great opportunities for next-generation photovoltaic devices. In recent years, substantial progress of 2D photovoltaic devices has been made. A comprehensive summary and prediction for the further development of 2D photovoltaic devices are in urgent need.
| Properties of 2D materials | Significance for photovoltaic devices |
| Dangling-bond-free surface | Van der Waals integration for band engineering self-passivated surface |
| Ultrathin body | Strong modulation of electrical properties |
| Strong light−matter interaction | Ultra-thin devices with high performances |
| Mechanical flexibility | Flexible devices |
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Here, we demonstrate the recent advances in 2D photovoltaic devices. We first present the general classification of 2D photovoltaic devices, including 2D-material-based Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and bulk photovoltaic effect (BPVE) devices. Subsequently, recent representative researches on each structure are presented, and the strategies for improving the photovoltaic performances are demonstrated in detail. Finally, conclusions and outlooks are delivered, providing a guideline for the further development of 2D photovoltaic devices.
2. 2D photovoltaic devices
General photovoltaic device requires a built-in electric field, where the photo-generated carriers are separated and the photocurrent is generated. The built-in field can be generated by fabricating heterojunctions, homojunctions and Schottky junctions. The self-passivated surface of 2D materials provide a platform for fabricating devices through vdW interaction, and the ultrathin nature makes it highly tunable by external fields and chemical surroundings. Moreover, novel properties such as BPVE have been discovered in 2D materials. Thus, 2D materials provide great opportunity for novel photovoltaic devices. Fig. 1 shows the classification of 2D photovoltaic devices, including Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and BPVE devices.
2.1 2D Schottky junction devices
Schottky junctions are widely used in photovoltaic devices. Due to the difference in work functions between metal and semiconductors, charge transfer occurs when metal and semiconductor contact, and thus a built-in field is generated at the interface of the semiconductor, which separates photo-generated electron−hole pairs under illumination, resulting in photovoltaic effect.
Schottky junction photovoltaic devices based on 2D materials have been widely researched[68−72]. However, the ultrathin body makes 2D materials highly sensitive to the contact quality[73−76]. Conventional evaporation method to make metal−semiconductor contact usually introduces interface defects due to the impact of high-energy metal atoms, leading to severe Fermi-level pinning effect and unpredictable Schottky barrier. Moreover, the interface defects act as recombination center, reducing the carrier collection efficiency[77−81]. These factors become the limits of high-performance 2D Schottky photovoltaic devices.
In recent years, vdW metal contact method has been developed. Due to the self-passivated surface of 2D materials, vdW metal contact can be realized by transferring metal to the 2D materials. In 2018, Liu et al. reported the creation of vdW metal−semiconductor junction, as shown in Fig. 2(a)[82]. Compared with conventional evaporation method, the non-destructive method creates an interface that is free from lattice destruction and defects, eliminating the Fermi-level pinning effect. As a result, the value of Schottky barrier approaches the Schottky−Mott limit, indicating an ideal contact. Thus, Schottky junction can be designed by selecting metals and semiconductors with large difference in work functions. The authors demonstrated an Ag/MoS2/Pt asymmetric-contact Schottky device (Fig. 2(b)), and an excellent ideality factor of 1.09 is obtained, indicating an ideal Schottky diode. A pronounced photovoltaic effect under illumination is observed, as shown in Fig. 2(c), with the open-circuit voltage (VOC) reaching 1.02 V in one-layer device and 0.76 V in seven-layer device. In 2021, Zhang et al. reported a MoS2/1T’-MoTe2 vdW Schottky junction, and also showed that the vdW integration of the 2D semiconductor and 2D metal can effectively inhibit the Fermi-level pinning effect caused by lattice destruction and defects, resulting in ideal Schottky barrier height[83]. Furthermore, this work introduced the acid-induced sulfur vacancy self-healing (SVSH) effect to reduce the sulfur vacancies in the MoS2, which significantly decreases the intrinsic doping level of MoS2. As a result, the Schottky barrier width is evidently enlarged. The ideal Schottky junction exhibits a good ideality factor of ~1.6, and improved photovoltaic performances with high external quantum efficiency (EQE) of 20%.
To achieve high efficiency of carrier collection and realize scalable photoactive area, vertical contact device shows more advantages in photovoltaic devices. In 2019, Went et al. developed a transfer process for fabricating Schottky junction photovoltaic device with vertical vdW contact, as shown in Fig. 2(d)[84]. WS2 is used as absorber layer, and Au and Ag electrodes are used as asymmetric contacts. Due to the high-quality vdW interface and high carrier collection efficiency in the vertical channel, the device exhibits high photovoltaic performances. As shown in Fig. 2(e), high photovoltaic performances with VOC of 256 mV, short circuit current (ISC) of 4.10 mA/cm2, EQE of ~40%, and power conversion efficiency (PCE) of 0.46% are achieved, while similar device made with evaporated metal contact shows negligible photovoltaic effect. Moreover, the developed direct transfer process eliminates the lithography process, providing a feasible way for batch fabrication.
However, the EQE of most reported 2D photovoltaic devices are below 50%, which is much lower than the theoretical limit[85]. The main challenge lies in the photo-generated carrier recombination loss in channel. In 2022, Wang et al. demonstrated a 2D photovoltaic device with minimized defects and carrier recombination in channel, as shown in Fig. 2(f)[86]. Graphene/WS2/Pt vertical Schottky device is fabricated, and the vdW metal contact is applied to realize an interface without defects and Fermi-level pinning. Thereby, a large Schottky barrier can be realized by adopting metal with large work functions such as Pt. Furthermore, by modulating the thickness of WS2, the trade-off between carrier collection efficiency and light absorption is addressed. As a result, under illumination, the carriers can be swiftly separated in the depleted channel and collected by electrodes without recombination loss. As shown in Fig. 2(g), efficient carrier transportation without loss is realized. As shown in Figs. 2(h) and 2(i), a high EQE of 92% is achieved, which approaches the theoretical limit. Moreover, the proposed strategy of fabricating high-performance photovoltaic devices can be extended to other 2D materials such as MoSe2 and WSe2.
Electron-selective contact has also been demonstrated effective to improve the photovoltaic performances. In 2022, Kim et al. reported a Pt/WSe2/WOx/Gr photovoltaic device[87]. A WOx layer is introduced by plasma treatment of the WSe2 surface and acts as electron-selective contact, which promotes the extraction of electrons and inhibits the extraction of holes to graphene, reducing the recombination loss at the contact interfaces. As a result, a PCE up to 5.44% is realized. Furthermore, Nazif et al. demonstrated a high-specific-power flexible solar cell based on 2D Schottky device[88]. Vertical Au/WSe2/Gr vdW contact Schottky junction is fabricated on flexible and lightweight polyimide substrate. MoOx capping is used for doping as well as anti-reflection. High PCE of 5.1% under AM 1.5G is achieved. Meanwhile, the bending test suggests consistent performance levels under bending, and the use of ultrathin polyimide substrate achieves a high specific power of 4.4 W·g−1.
2.2 2D homojunction devices
P−n junction is another building block of photovoltaics. When a p−n junction is fabricated, the holes transfer from p-type material to n-type material, and electrons transfer from n−type material to p-type material, thus forming a built-in field at the junction interface, which separates photo-generated electron−hole pairs under illumination, resulting in photovoltaic effect. P−n homojunctions have been widely used in photovoltaic devices. Extensive researches have been focused on the p−n homojunctions based on 2D materials, and multiple strategies for realizing 2D homojunctions have been developed, such as doping and external field modulation[89−97].
Conventional doping method for bulk semiconductors could introduce severe defect states in ultrathin 2D materials, which seriously degrade the performances[98, 99]. Hence, it is vital to develop novel doping methods for 2D photovoltaic devices. In 2022, Li et al. developed a universal p-type doping method for TMDs[100]. As shown in Fig. 3(a), the SnCl4 solution is used to treat the 2D materials, where the Sn4+ ion exchanging process can effectively reduce the electron concentration in TMDs, and the Kelvin probe force microscopy (KPFM) image shows an evident increase in the work function of doped PdSe2. Moreover, by changing the concentration of SnCl4 solution, the doping level can be effectively modulated. By this means, an in-plane homojunction is fabricated (Fig. 3(b)), and pronounced photovoltaic effect is observed (Fig. 3(c)). 2D p−n homojunction photovoltaic devices based on other doping methods have also been reported, such as aluminum-doped black phosphorus (BP) homojunction[101], boron-doped BP homojunction[102], WSe2 homojunction metal contact doping[103], etc. P−i−n homojunctions can also be realized by doping of 2D materials. The introduction of intrinsic region extends the depletion region, where the carriers can be efficiently separated without recombination, thus higher carrier collection efficiency can be realized. In 2021, Zhang et al. fabricated a WSe2 p−i−n diode by surface plasma treatment doping[104]. O2 plasma and Ar plasma are used to treat the contact region and obtain p-type and n-type doping. The photoactive volume is increased by adding an intrinsic layer to extend the depletion region, which can effectively separate photo-generated carriers, resulting in enhanced photovoltaic performances, including ISC of 0.57 μA and VOC of 0.34 V. Lee et al. reported a MoTe2 p−i−n homojunction realized by selective H-doping and split-gate modulation[105]. The ISC of the p−i−n junction is 70 pA, showing large improvement compared with 15 pA of the p−n junction. The improvement is attributed to the greater depletion region with higher efficiency of carrier separation.
However, the chemical doping methods for fabricating p−n homojunction still introduce impurity dopants, which could degrade the electrical performances of 2D materials, and the doping is irreversible. In recent years, external field modulation has been developed for fabricating 2D p−n homojunctions. In 2021, Chen et al. fabricated split-gate WSe2 devices (Fig. 3(d)) to modulate the electron and hole doping in WSe2, and investigated the exciton behavior under illumination[106]. VdW metal contact is used to obtain an interface with minimum Shockley−Read−Hall recombination, and thus the intrinsic photophysics of the device can be realized. A p−n homojunction is obtained by electrostatic field modulation in the split-gate geometry. Under illumination, photovoltaic effect is observed, and the ISC under the modulation of gate voltage is investigated. In vdW contact device, a peak of ISC appears at low charge density by the gate modulation (Fig. 3(e)). The peak ISC is attributed to the suppressed exciton-charge Auger recombination at low charge density. However, in evaporated contact device, the peak ISC disappears due to the influence of interface defects. The photocurrent mapping shows much longer exciton diffusion length at low charge density region (Fig. 3(f)), further confirming the suppressed exciton−charge Auger recombination. The photovoltaic performances are greatly enhanced due to the reduced carrier recombination, EQE of 83.6% and VOC of 0.75 V are achieved in a three-layer device. In addition, ferroelectric materials have also been utilized for modulating 2D materials due to the strong electrostatic field and nonvolatile characteristic. In 2020, Wu et al. demonstrated ferroelectric polarization enabled MoTe2 p−n junctions[107]. Ferroelectric P(VDF−TrFE) is applied as dielectric and split-gate electrodes are used to modulate the channel. The device can be reconfigured as n−n, p−p, p−n, and n−p junctions by changing the polarization directions. High rectification ratio of 5 × 105 is realized in the p−n junction, and high photovoltaic performances including EQE of 40% and PCE of 2.5% are achieved.
Novel doping strategies have been explored in recent years. In 2020, Lee et al. reported a reversible solid-state doping of 2D materials by introducing superionic silver iodide layer in WSe2 device, as shown in Fig. 3(g)[58]. By applying an electric field to the device at a temperature above the superionic phase transition temperature, the Ag+ can be selectively accumulated or depleted at doping locations, thus non-volatile p-type or n-type doping can be reversibly realized in WSe2 channel, as shown in Fig. 3(h). The p−n homojunction exhibits pronounced photovoltaic effect with a high VOC of ~0.52 V (Fig. 3(i)). In addition, p−n homojunctions can also be realized by the intrinsic ion migration in 2D materials. In 2023, Zhu et al. presented a p−n homojunction based on 2D CuInP2S6[108]. The Cu+ ions migrate under the in-plane electric field, leading to p-type and n-type doped area, forming a lateral p−n homojunction. The p−n junction exhibits distinct rectification and photovoltaic performances. Moreover, the rectification and photovoltaic behavior can be effectively tuned and reversed by changing the poling voltage. As a result, good photovoltaic performances including VOC of 0.3 V and JSC of 20 mA/cm2 are achieved.
2.3 2D−2D p−n heterojunction devices
Owing to the self-passivated surface, different 2D materials can be assembled through vdW interaction despite lattice mismatch, thus p−n heterojunctions can be easily fabricated. VdW integration allows band engineering for charge transfer modulation and complementary bandgaps. Therefore, 2D p−n heterojunctions provide great potential for high-performance photovoltaic devices. 2D p−n heterojunction have been widely studied in recent years[109−115].
In 2021, Zou et al. reported a GaSe/MoS2 p−n heterojunction. Due to the high-quality heterojunction interface and the type-Ⅱ band alignment, a large VOC of 0.61 V is achieved[116]. Furthermore, heterojunctions provide the advantage of combining the characteristic of different materials. For example, Zhao et al. demonstrated a BP/InSe heterojunction in 2018, in which the BP provides broadband light absorption and polarization sensitivity, and the InSe provides high electron mobility. As a result, a broadband, polarization-sensitive, and fast response photovoltaic device is obtained[117]. Broadband photovoltaic devices have been realized by involving narrow-bandgap materials in p−n heterojunctions, such as WSe2/Bi2Te3[118], b-As0.4P0.6/MoTe2[119], AsP/InSe[120], etc.
Various 2D p−n heterojunction photovoltaic devices have been explored. However, the EQE of most reported devices are below 55%. One of the main challenges is the interface carrier recombination. In 2019, Wu et al. proposed a MoS2/AsP p−n junction, using AsP as a carrier selective contact and forming a unilaterally depleted type-Ⅰ heterojunction, as shown in Fig. 4(a)[121]. A large depletion region is formed in the MoS2 due to the large difference in work functions between MoS2 and AsP. Under illumination, photo-generated carriers in MoS2 are effectively separated by the built-in field, and only photo-generated electrons can transfer through the interface and are collected by the AsP via the rapid recombination with the opposite carriers in the accumulation region. Meanwhile, the holes in MoS2 side transfer to the source electrode (Fig. 4(b)). Thus, the interface recombination of the carriers can be effectively suppressed, significantly enhancing the carrier collection efficiency. As a result, the device exhibits high photovoltaic performances with an ISC of 1.3 μA, a VOC up to 0.61 V (Fig. 4(c)) and a high EQE of 71%. Furthermore, the unilaterally depleted heterojunction is applicable to other 2D materials, such as MoS2/BP heterojunction.
The photovoltaic performances of p−n heterojunctions are limited by the built-in field, impeding the further development. On this issue, Jin et al. demonstrated a 2D CsPbBr3/CdS p−n junction with excellent excitonic photovoltaic effect to address the limitation of built-in field in 2020 (Fig. 4(d))[122]. 2D CsPbBr3 possess large exciton binding energy, and the photo-generated excitons diffuse to the interface and dissociate under illumination. As a result, a large chemical potential energy gradient is formed to enhance the carrier separation (Fig. 4(e)). The photo-generated carriers are effectively separated under effect of both built-in field and the chemical potential energy, leading to improved carrier separation efficiency and subsequently the photovoltaic performance. As shown in Fig. 4(f), large VOC of ~0.76 V and PCE of ~17.5% are achieved.
The low light absorption efficiency is another challenge for 2D p−n heterojunction photovoltaic devices. Decorating 2D p−n heterojunctions using materials with large light absorption is an effective strategy to improve the light absorption. In 2022, Zeng et al. demonstrated a WSe2/MoS2 p−n junction decorated by PbS quantum dots (QDs) (Fig. 4(g))[123]. The PbS QDs act as the light-sensitive layer, and the electrons produced in the QDs layer can be effectively collected by the MoS2 under the built-in field. Then, the light absorption is significantly enhanced. As shown in Fig. 4(h), the device with PbS QDs layer exhibits greatly enhanced photoresponse and achieves a maximum EQE of 100% (Fig. 4(i)). 2D perovskites with strong light absorption also provide great potential for overcoming the challenge of low-absorption efficiency[124−126]. Fang et al. reported a Cs2AgBiBr6/WS2 p−n heterojunction[127]. The 2D perovskite Cs2AgBiBr6 serves as a light-sensitive layer, which enhances the sensitivity of the WS2, and prominent charge transfer at the junction interface is observed. Moreover, a vertical-stacked graphene layer is used to further enhance the carrier collection efficiency. As a result, pronounced photovoltaic effect with a large VOC of ~0.75 V is achieved. Wang et al. reported a perovskite/BP/MoS2 heterojunction[128]. The MAPbI3 is used as a light-sensitive layer to improve the performance of BP/MoS2 p−n junction, and the photo-generated carriers in MAPbI3 can be effectively transferred to the BP layer. As a result, a high EQE of ~80% is achieved.
Contact engineering has also been proved as an effective method to boost the performances of 2D p−n heterojunctions. In 2020, Yang et al. reported a WSe2/MoS2 p−n heterojunction with atomically thin WOx on the surface of WSe2, which serves as a charge transport layer[129]. A strong built-in electric field is formed at the WSe2/WOx junction, promoting the extraction of holes to graphene while blocking the transport of the dissociated electrons. In this manner, the rapid recombination of the separated charges at heterojunction is inhibited, resulting in significantly enhanced photovoltaic performance. The PCE increases from 0.7% to 5.0% after adding the WOx layer.
The carrier collection efficiency is also a key factor in photovoltaic performances. In a p−n junction, a depletion region is generated at the interface of heterojunction. The photo-generated carriers can be swiftly separated and swept out of the depletion region by the built-in field without recombination. However, during the diffusion process in the undepleted channel, carrier recombination occurs, resulting in decreased carrier collection efficiency and limited EQE[105, 130]. Therefore, the carrier transportation in the undepleted channel should be minimized. Fabricating vertical-channel p−n junctions with ultrathin 2D materials can minimize the carrier diffusion in the undepleted channel, meanwhile, large photoactive area can be obtained. In 2017, Wong et al. reported a WSe2/MoS2 p−n heterojunction with graphene and Au as top and bottom electrodes, as shown in Fig. 5(a)[85]. The quantum efficiency of device with graphene top electrode shows great improvement compared with that of the device without graphene top electrode (Fig. 5(b)), confirming the significant improvement of carrier collection efficiency in the vertical channel device. As a result, high EQE up to 50% is achieved (Fig. 5(c)). In 2020, Zhang et al. reported carbon nanotube (CNT)/WSe2/MoS2/CNT vertical point p−n heterojunctions, as shown in Fig. 5(d)[131]. A vertical channel is formed by the vertically cross-stacked CNTs. The photocurrent mapping shows that the photocurrent is mainly produced at the cross point of the CNTs (Fig. 5(e)), confirming that the vertical channel exhibits higher carrier collection efficiency. As a result, high EQE of ~30% is achieved (Fig. 5(f)).
2.4 2D−3D heterojunction devices
Although strong light−matter interaction has been reported, the light absorption of 2D materials is still limited by the ultrathin characteristic, indirect bandgap and strong exciton binding energy[132, 133], leading to unsatisfying photovoltaic performances. The self-passivated surface allows 2D materials to integrate with 3D bulk semiconductors through vdW interaction as well, which provides a solution for the limited optical absorption. Moreover, the mature CMOS technics is conducive to the on-chip integration of the devices. Thus, 2D−3D heterojunctions have attracted much attention on photovoltaics[134−136].
2D materials/Si heterojunctions have been widely researched due to the high absorbance of Si and the compatibility with CMOS technics. In 2018, Xie et al. fabricated a multilayered PtSe2/Si vertical hybrid heterojunction by selenylation of Pt films pre-deposited on Si substrates[137]. High photovoltaic performances with PCE of 8% and EQE of ~80% are achieved. The heterojunction also exhibits a good photoresponse over a wide spectrum ranging from 200 to 1550 nm, owing to the narrow bandgap of PtSe2. Moreover, the device is highly suitable for weak optical signal detection. In 2019, Wu et al. demonstrated a large-area 2D WS2/Si heterostructure[138]. Due to the facilitation of the photocarrier separation process by the type-Ⅱ band alignment, the fabricated device exhibits a photovoltaic effect with ISC of 5 × 10-6 A. Remarkably, the heterostructure exhibits broadband photovoltaic response up to 3043 nm.
In 2018, Xiao et al. demonstrated a novel 2D/3D heterojunction of reduced graphene oxide (RGO) −MoS2/pyramid Si via solution-processing method (Fig. 6(a))[139]. As shown in Fig. 6(b), carriers are produced in the junction under illumination, and the RGO serves as a high-conductivity path for electrons that greatly facilitates the carrier transportation. Thus, the carrier collection efficiency is greatly enhanced. The I−V characteristics of the device exhibits significant photovoltaic effect, where the VOC and ISC reach 300 mV and 4 × 102 μA, respectively (Fig. 6(c)). More importantly, the device also exhibits a broad-spectrum photovoltaic response, ranging from 350 nm to 4.3 μm, which can be attributed to the decreased bandgap of MoS2 caused by S vacancies.
Apart from Si, narrow-bandgap bulk semiconductors can be used for broadband photovoltaic devices, such as germanium (Ge) and HgCdTe (MCT). In 2020, a MoTe2/Ge heterojunction has been demonstrated by Chen et al., showing prominent photovoltaic effect under 915-nm illumination[140]. As a narrow-bandgap material with excellent absorption, MCT is highly desired for the progress of photovoltaic devices. However, traditional MCT photovoltaic detectors exhibited low quantum efficiency due to the numerous interface defects caused by ion implantation[141]. To address this issue, Wang et al. fabricated a graphene/MCT heterojunction for uncooled mid-wave infrared (MWIR) photodetector in 2022[142]. By transferring high-mobility multilayer graphene onto MCT, a clean and defect-free interface is obtained (Fig. 6(d)). The device exhibits a pronounced photovoltaic effect in a wide spectrum region ranging from 520 to 4225 nm (Fig. 6(e)). It is worth noting that the blackbody radiation was investigated on vdW-on-MCT heterostructures. Fig. 6(f) shows a high quantum efficiency (85%) under blackbody radiation, which is comparable to those of commercial MWIR photodetectors in 3.8−4.8 μm region.
In 2022, Jiao et al. also demonstrated a remarkable photovoltaic effect in the MCT/BP heterojunction (Fig. 6(g))[143]. Concretely, a broken-gap (type-Ⅲ) band structure is obtained, forming a strong built-in electric field at the junction interface. A large photocurrent is obtained due to the photo-induced direct band-to-band tunneling (BTBT) mechanism (Fig. 6(h)). Owing to the high absorption in broadband, the heterojunction exhibits a good photovoltaic performance in a broad spectrum ranging from 520 to 4330 nm (Fig. 6(i)). Moreover, broadband polarization sensitivity without external optical elements is realized by the optical anisotropy of BP.
2.5 BPVE devices
The S−Q limit becomes a constraint on photovoltaic efficiency once a Schottky junction or a p−n junction is formed, where a built-in field is necessary[19, 144−147]. However, BPVE means no need of built-in field to generate photocurrent at zero bias. Thus, BPVE device has been considered as a promising candidate for surpassing the Shockley−Queisser (S−Q) limit[148−150]. In the past few years, various 3D ferroelectric materials have boosted studies on BPVE. Most of these ferroelectric materials are insulators, such as barium titanate (BaTiO3) and bismuth iron oxide (BiFeO3), where the photocurrent density is fundamentally limited by the large bandgap (2.7−5 eV)[151, 152]. BPVE has also been discovered in 2D materials with broken inversion symmetry[153]. Recent studies have shown that BPVE in 2D materials can achieve higher photocurrent conversion efficiency than 3D crystal structures do[145, 146, 151−154]. Moreover, the unique properties such as dangling-bond-free surface and flexibility allow 2D materials to realize BPVE by symmetry engineering, such as fabricating vdW interface and nanotubes.
BPVE in ferroelectric 2D materials, such as In2Se3 and CuInP2S6 (CIPS), has been predicted and experimentally demonstrated[154]. In 2021, Li et al. fabricated CIPS-based photovoltaic device, where the ultra-thin CIPS is sandwiched by the graphene electrodes (Fig. 7(a))[155]. The device exhibits a prominent photovoltaic behavior, with a short-circuit current density (JSC) up to 10 mA·cm-2, which is significantly higher than conventional bulk ferroelectric materials. In addition, the BPVE in CIPS-based devices can be modulated by varying the external electric field, which can be explained by the tunable band alignment in the heterostructure device (Fig. 7(b)). The off-center shift of the Cu ions in CIPS results in out-of-plane ferroelectricity, and the generation of an internal electric field (Ebi) lead to different band alignment of graphene at different electrode interfaces (i.e., asymmetric energy potential barrier height ΔΦ). Under illumination, the carriers are separated by Ebi and drifted under ΔΦ. Notably, the direction of the photocurrent is switchable due to the controllable electric polarization. Asymmetric photocurrent is observed due to the ferroelectric hysteresis of CIPS, confirmed by the extracted JSC at each poling voltage, where the JSC follows an obvious hysteresis loop (Fig. 7(c)).
In addition to photoferroic devices, the intentional disruption of in-plane inversion symmetry in vdW heterojunctions can also lead to BPVE. In 2021, Akamatsu et al. designed a WSe2/BP heterojunction by stacking crystals with different rotational symmetries at specific angle to reduce the interfacial symmetry, showing prominent in-plane electronic polarization[156]. Owing to the threefold rotation symmetry of WSe2 being mismatched with the twofold rotational symmetry of BP, the interface of BP and WSe2 exhibits no rotational symmetry (Fig. 7(d)). And the mirror symmetry remains when the mirror planes of BP and WSe2 are parallel, resulting in electronic polarization along the parallel direction of the mirror plane, and the photocurrent appears along the polarization direction. The stripe moiré pattern is observed due to the lattice mismatch between WSe2 and BP, reflecting the in-plane polarity at the WSe2/BP junction interface (Fig. 7(e)). As shown in Fig. 7(f), a large ISC is observed under illumination, indicating a significant BPVE characteristic. Furthermore, by measuring the direction dependent photocurrent, it is confirmed that the direction of photocurrent generation and polarization are both parallel to the mirror direction. The results provide a strategy for symmetry engineering of various vdW interfaces.
The BPVE can also be observed in 2D TMDs when the inherent inversion symmetry is broken. When the mirror symmetries and threefold rotational of the 2D materials are broken due to bending or strain, BPVE can be realized, facilitating the research on 1D-nanotubes and flexo-photovoltaic-based devices[157−159]. In 2019, Zhang et al. fabricated WS2 devices based on non-centrosymmetric polar nanotubes with a hollow core (Fig. 7(g))[19]. As shown in Fig. 7(h), the ISC and VOC both monotonically vary with laser power, confirming the occurrence of BPVE. It is found that the reduction of crystal symmetry by moving from a 2D monolayer WS2 to a polar tubular WS2 can greatly enhance the BPVE. The ISC of WS2 nanotube devices (Fig. 7(i)) is significantly larger than that of the monolayer WS2 devices. Thus, reducing the crystal symmetry is essential for BPVE. In addition, the flexo-photovoltaic effect (FPVE) is a strain-gradient-induced BPVE which represents the photocurrent response caused by the symmetry breaking of asymmetric crystals. Jiang et al. proposed a strain-gradient engineering approach based on structural heterogeneity and phase transition of a MoS2 and VO2 hybrid system, demonstrating the FPVE in the MoS2[147]. Prominent ISC is observed under illumination, and the bulk photovoltaic coefficient of the MoS2 is significantly larger than that in most non-centrosymmetric materials. The FPVE discovered in 2D materials leads to a new guidance in potential photovoltaic applications.
3. Conclusion and outlook
2D materials have shown great potential in photovoltaic devices owing to the unique optoelectronic properties, including broadband light absorption, strong light−matter interaction, and flexibility. Moreover, the self-passivated surface allows the fabrication of devices through vdW interaction without the lattice mismatch, such as vdW metal contact and vdW p−n heterojunction. On this basis, band engineering can be realized and the photovoltaic performances such as EQE, PCE and VOC can be significantly enhanced. In this review, we have presented the recent advances in photovoltaic devices based on 2D materials, including Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and BPVE devices.
2D Schottky junctions based on vdW metal contact have shown great advantages in photovoltaic devices. The vdW metal contact leads to defect-free interface without the Fermi-level pinning, so that large built-in field and wide depletion width can be obtained, and the defect-induced recombination loss is also impeded. Therefore, the photovoltaic performance can be greatly enhanced. Moreover, vertical contact device reduces the carrier recombination loss during the transportation, further improving the photovoltaic performances. 2D homojunctions can be realized by doping and external field modulation. Doping provides a feasible way for fabricating p−n homojunctions, and multiple strategies such as plasma doping, ion exchanging doping and solid-state doping have been successfully applied in photovoltaic devices. The external field enables effective and reconfigurable modulation of carrier transportation in 2D materials to realize the p−n homojunctions, and the photovoltaic performances of the p−n homojunctions can be improved by the modulation. Novel strategies such as reversible solid-state doping have also been developed in recent years. P−n heterojunctions with different 2D materials can be easily fabricated through vdW interaction, which provide a new pathway for band engineering for high-performance photovoltaics. Strategies for fabricating heterojunctions such as unilateral depletion, excitonic photovoltaic effect, combination with high-absorption materials, contact engineering, and vertical carrier transportation have been demonstrated to realize high-performance photovoltaic devices. 2D−3D p−n heterojunctions utilize the advantage of conventional bulk semiconductors, such as large optical absorption and broadband absorption, which effectively overcome the limited absorption in 2D materials. The p−n heterojunctions using Si and Ge exhibit large light absorption. Moreover, the p−n heterojunctions using narrow-bandgap materials such as HgCdTe exhibit broadband absorption. BPVE has been discovered in 2D materials and demonstrated to be promising for overcoming the S−Q limit. BPVE can be realized in non-centrosymmetric 2D materials, and by symmetry engineering such as fabricating vdW interface and fabricating nanotubes. Table 2 shows the performance comparison of representative works in recent years.
| Devices | PCE (%) | EQE (%) | VOC (V) | FF | References |
| Pt/MoS2/Ag | 0.2 | 1.74 | 1.02 | 0.26 | [82] |
| 1T’-MoTe2/MoS2/Cr | − | 20 | 0.19 | − | [83] |
| WS2/Au | 0.46 | 40 | 0.256 | 0.44 | [84] |
| Gr/WS2/Pt | 5 | 92 | 0.4 | 0.39 | [86] |
| Gr/WOx/WSe2/Pt | 5.44 | − | 0.47 | 0.59 | [87] |
| MoOx/Gr/WSe2/Au | 5.1 | − | 0.476 | 0.617 | [88] |
| MoTe2 homojunction | − | 40 | 0.3 | 0.5 | [107] |
| WSe2 homojunction | − | 83.6 | 0.75 | − | [106] |
| CsPbBr3/CdS | 17.5 | − | 0.76 | 0.5 | [122] |
| Cs2AgBiBr6/WS2/Gr | − | 14.7 | − | − | [127] |
| Gr/WSe2/MoS2 | 3.4 | 50 | 0.38 | − | [85] |
| PbS/MoS2/WSe2 | 7.65 | 100 | 0.42 | − | [123] |
| Perovskite/BP/MoS2 | − | 80 | 0.32 | − | [128] |
| MoS2/AsP | 9 | 71 | 0.61 | 0.5 | [121] |
| Gr/WOx/WSe2//MoS2 | 5 | − | 0.46 | 0.45 | [129] |
| CNT/WSe2/MoS2/CNT | − | 42.7 | 0.35 | − | [131] |
| PtSe2/Si | − | 80 | 0.3 | − | [137] |
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In spite of the great progress achieved, the photovoltaic devices based on 2D materials are still facing some challenges. Fig. 8 shows the outlooks on the future development of 2D photovoltaic devices.
On the one hand, the performances of 2D photovoltaic devices need further improvement. The PCEs of most currently reported 2D photovoltaic devices are below 5%, showing large gap from the theoretical limit[160]. Extensive researches have been focused on improving the EQE and ISC. However, VOC is also a key parameter for realizing high PCE. Currently the most reported VOC values of 2D photovoltaic devices are below 0.5 V, while the bandgaps of most 2D semiconductors are in the range of 1−2 eV[133, 161−163], indicating that there is still significant room to improve the VOC. The key to address the low VOC lies in modulation of the built-in field. Doping can effectively modulate the built-in field by modulating the doping concentration. However, realizing controllable doping while maintaining good stability and high carrier mobility remains a great challenge. Novel doping methods need further development, such as solid-state doping and remote modulation doping[58, 164]. Contact engineering can modulate the carrier transportation at the interface. By reducing the carrier recombination and extra Schottky barrier, the VOC and PCE can be improved[83, 87, 129]. However, the currently developed methods exhibit limited performances. Novel strategies such as semi-metal contact with strong van der Waals interaction have pushed the carrier transportation to the theoretical limit, which can be further employed in photovoltaics[165]. The low-absorption efficiency is also a challenge limiting the VOC and PCE. The most 2D materials exhibit indirect bandgap and large exciton binding energy, which limit the optical absorption. The hybridization of 2D materials with large-optical-absorption materials has been demonstrated effective for improving the photovoltaic performance, such as the hybridization with bulk semiconductors and quantum dots.
On the other hand, the fabrication of large-scale 2D photovoltaic devices is a key challenge for the practical application. First, high-quality and controllable wafer-scale synthesis of 2D materials and vdW heterojunctions still faces great challenges. Currently, only a few wafer-scale 2D materials have been reported[112, 166−168]. Yet more 2D materials are needed for high-performance photovoltaic devices, such as WSe2 and black phosphorus. And the quality of wafer-scale single crystal also needs improvement. Second, wafer-scale transfer is vital for fabricating photovoltaic devices with p−n junctions or Schottky junctions. Wafer-scale transfer have been demonstrated in graphene and metals recently[169, 170], and could be further extended to other 2D semiconductors to fabricate large-scale photovoltaic devices. Last but not least, the flexibility of 2D materials provides great opportunity for flexible and wearable devices. And the research on the integration of large-scale 2D photovoltaic devices on flexible substrates is in desperate need for the practical application.
In conclusion, 2D materials are promising for next-generation high-performance photovoltaic devices, and great achievements have been made in recent years. To further push the practical application of 2D photovoltaic devices, novel strategies for improving the performances and technologies for large-scale fabrication need further development.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52322210, 52172144, 22375069, 21825103, and U21A2069), National Key R&D Program of China (2021YFA1200501), Shenzhen Science and Technology Program (JCYJ20220818102215033, JCYJ20200109105422876) and the Innovation Project of Optics Valley Laboratory (OVL2023PY007).
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