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Hao Ou, Sota Tsukamoto, Tenta Kitamura, Motoki Matsuno, Koshi Oi, Togo Takahashi, Takahiko Endo, [Yasumitsu Miyata](https://orcid.org/0000-0002-9733-5119), [Jiang Pu](https://orcid.org/0000-0002-1676-2072), [Taishi Takenobu](https://orcid.org/0000-0001-7313-6396)

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[Photovoltaic Device Based on Monolayer Compositionally Graded Transition Metal Dichalcogenide Alloy](https://mdr.nims.go.jp/datasets/475b3655-3251-402a-a847-5aea70769ca0)

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Photovoltaic Device Based on Monolayer Compositionally Graded Transition Metal Dichalcogenide AlloySmall Methods www.small-methods.comRESEARCH ARTICLEPhotovoltaic Device Based on Monolayer Compositionally Graded Transition Metal Dichalcogenide Alloy Hao Ou1 Sota Tsukamoto2 Tenta Kitamura1 Motoki Matsuno2 Koshi Oi2 Togo Takahashi2 Takahiko Endo3 Yasumitsu Miyata3 Jiang Pu1 Taishi Takenobu2 1 Department of Physics, Institute of Science Tokyo, Meguro, Tokyo, Japan 2 Department of Applied Physics, Nagoya University, Nagoya, Japan 3 Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan Correspondence: Hao Ou ( ou@phys.sci.isct.ac.jp) Jiang Pu ( pu@phys.sci.isct.ac.jp) Taishi Takenobu ( takenobu@nagoya-u.jp) Received: 15 October 2025 Revised: 12 January 2026 Accepted: 2 February 2026 Keywords: monolayer alloy | photoresponse | photovoltaic device | p-n junction | transition metal dichalcogenide ABSTRACT Monolayer transition metal dichalcogenides (TMDCs) have been widely studied for the fabrication of photovoltaic devices with high energy conversion efficiencies for future ultrathin optoelectronic devices. To create efficient photovoltaic devices, in-plane heterostructures, whose composition can be artificially tailored by chemical vapor deposition, are a promising approach to form p-n junctions spontaneously. Although sharp in-plane heterostructures are typically employed, their narrow heterointerfaces are prone to defect sensitivity and thermal losses, which can significantly reduce device performance. In this study, we demonstrated that the spontaneous p-n junction devices fabricated based on chemically synthesized compositionally graded monolayer WS2 x Se2(1- x ) alloys exhibited enhanced photoresponse performance. By conducting photocurrent and photoluminescence mappings, we revealed the correlation between the photocurrent generation behavior and local composition gradient. Moreover, the monolayer alloy device exhibited an open-circuit voltage as high as 0.66 V, highlighting the potential of a compositionally graded p-n junction for high-efficiency photovoltaic devices. Our study presents a new approach for the development of efficient TMDC-based optoelectronic devices.                    1 Introduction Photovoltaic devices, such as solar cells that convert photons intoelectrical signals, are essential for the future energy conversionindustry. As the demand for advanced solar cell technologiesincreases over the years, it becomes increasingly important todevelop more efficient and functionalized photovoltaic devices.In recent years, atomically thin transition metal dichalcogenides(TMDCs) have attracted significant attention due to their low-dimensional nature and strong light–matter interactions [ 1, 2 ].Typical semiconducting TMDCs exhibit band gaps ranging from1 to 2 eV, corresponding to photon energies ranging fromnearly red to infrared, making them promising candidates forhigh-performance solar cell components [ 3–6 ]. For instance,This is an open access article under the terms of the Creative Commons Attribution License, which permcited. © 2026 The Author(s). Small Methods published by Wiley-VCH GmbH Small Methods , 2026; 10:e01997 https://doi.org/10.1002/smtd.202501997monolayer TMDC-based photovoltaic devices have been widely explored, showing pronounced photovoltaic effects, with an open-circuit voltage ( Voc ) exceeding 0.8 V and photoconversionefficiency (PCE) greater than 50%, highlighting their potential for use in transparent and flexible solar cells [ 4, 7–10 ]. How-ever, existing methods used to fabricate p-n junction devicesoften require relatively complex structures, such as split gatestructures [ 5 ]. In addition, chemical methods require surfacetreatment or functionalization, which can compromise device performance and reproducibility [ 11, 12 ]. Although van der Waalsstacking can spontaneously form p-n junctions via appropriate selection of materials, it inevitably lacks practical scalabil- ity and reliability because of the manual fabrication process[ 13–16 ]. its use, distribution and reproduction in any medium, provided the original work is properly 1 of 8http://www.small-methods.comhttps://doi.org/10.1002/smtd.202501997https://orcid.org/0000-0002-1676-2072https://orcid.org/0000-0001-7313-6396mailto:ou@phys.sci.isct.ac.jpmailto:pu@phys.sci.isct.ac.jpmailto:takenobu@nagoya-u.jphttp://creativecommons.org/licenses/by/4.0/https://doi.org/10.1002/smtd.202501997http://crossmark.crossref.org/dialog/?doi=10.1002%2Fsmtd.202501997&domain=pdf&date_stamp=2026-02-12                                                                                     v             23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative CTo address these limitations, the direct growth of monolayerlateral heterostructures is regarded as a promising strategy fordesigning efficient photovoltaic devices. Chemically synthesizedlateral heterostructures possess naturally formed p-n junctions,enabling device fabrication by simply depositing electrodes insuitable locations [ 17–20 ]. For example, Duan et al. demonstratedthe epitaxial growth of a WS2 -WSe2 lateral heterostructure witha sharp interface separating the two material regions [ 17 ]. Anobvious photocurrent response was observed near the interface,confirming efficient electron–hole separation. However, becausethe width of the interface is typically ∼ 1 nm [ 18 ], the deviceperformance is highly sensitive to defects within this narrowregion. These defects can disrupt the band alignment and localelectric field, which degrades carrier generation and separationefficiency. Furthermore, the heterostructure only provides twodiscrete bandgap values, and therefore, the absorption of pho-tons with a continuous energy distribution inevitably leads tothermalization losses, which reduce the efficiency of the device[ 21, 22 ]. To overcome these limitations, we focused on the recentlyproposed compositionally graded monolayer TMDC alloys [ 23–25 ]. Unlike the formation of a sharp atomic interface in typicallateral heterostructures, the atomic registry in graded alloysshows a gradual variation over several micrometers ( µm). Con-sequently, the composition, band structure, and optical prop-erties continuously vary inside the alloy regions. Indeed, wehad demonstrated color-tunable light-emitting devices basedon monolayer WS2 x Se2(1- x ) alloys [ 25 ]. However, there is alack of detailed investigation of the photocurrent generationmechanism and demonstration of the photovoltaic device usingsuch compositionally graded alloys. Hence, in this study,we examined the photoresponse of a two-terminal mono-layer WS2 x Se2(1- x ) device. Correlated photoluminescence andphotocurrent mapping revealed the photovoltaic effect insidethe alloy region, in which the p-n junction formed sponta-neously, resulting from the spatially varying atomic compo-sition. We demonstrated the photovoltaic effect in a simpledevice structure with a high Voc ( ∼ 0.66 V), which exceededthe typical values of monolayer or heterostructure-based pho-tovoltaic devices. Their simple structure and notable perfor-mance indicate the great potential of compositionally gradedTMDC alloys for the fabrication of efficient photovoltaicdevices. 2 Results and Discussion 2.1 Spontaneous Formation of p-n Junction in Monolayer WS2x Se2(1-x) The compositionally graded monolayer WS2 x Se2(1- x ) was syn-thesized by chemical vapor deposition (CVD), as reported inour previous study [ 25 ]. Although the synthesis process wasessentially the same as that in our previous study, in thisstudy, we primarily exploited the spontaneous band structureof the compositionally graded alloy and demonstrated photo-voltaic carrier generation and separation under illumination. Inaddition, we performed atomic force microscopy (AFM) andspatial optical measurements to confirm the monolayer natureand composition-dependent optical property variation of the2 of 8synthesized sample (Figure S1 ). Based on optical measurementsand first-principle calculations, when the composition gradu- ally changed from WS2 to WSe2 , the band structure changedaccordingly [ 25 ]. Given that WS2 is typically n -type whereas WSe2 is p -type [ 26, 27 ], a gradual p-n junction is expected to formspontaneously within the alloy region (Figure 1a ). To verify this,we measured the photoluminescence (PL) spectra and surface potential variation across the WS2 x Se2(1- x ) alloy region, as shown in Figure 1b . The surface potential variation was characterizedby the contact potential difference (CPD) using Kelvin probeforce microscopy (KPFM). The alloy region was identified bythe PL peak energy map (Figure 1c ), in which the peak energygradually shifted from ∼ 2.0 to ∼ 1.65 eV from the upper partto the lower part, indicating a composition gradient from theWS2 -rich region to the WSe2 -rich region [ 28 ]. The CPD mapis presented in Figure 1d , which reveals a clear variation inthe surface potential. Notably, the surface potential contrastapproximately followed the PL peak energy shift, suggestinga composition-correlated surface potential landscape in the measured region. We obtained a surface potential difference of∼ 110 meV between the WSe2 -rich and WS2 -rich regions (insetof Figure 1d ). It shall be noted that even though multilayerislands were present in the sample (see Figure 1b ), they did notspatially correlate with the PL (Figure 1c ) and CPD (Figure 1d )variations because both figures exhibited gradual changes along the same direction, which was distinct from the multilayerislands. In particular, in the CPD map, the multilayer islandstypically exhibited lower values, and thus, shadows could beobserved (Figure 1d ). The CPD signals in these islands appearedas abrupt changes, and one could identify them from thebackground of the gradient. Therefore, the observed variation originated from the composition gradient. In addition, we didnot observe any phase separation in our samples [ 25 ], as con-firmed by the spatial optical measurements shown in FigureS1 . Furthermore, prior experimental studies and thermodynamic simulations have indicated the feasibility of CVD conditions forgrowing a miscible monolayer WS2 x Se2(1- x ) solid–solution alloy [ 29, 30 ]. Based on the results obtained from PL mapping, KPFM mapping,and previous theoretical calculations, we deduced a continually varying band structure across the alloy region, as schematicallyillustrated in Figure 2a . More importantly, the gradient of thesurface potential indicated the presence of a built-in electricfield. When the alloy was illuminated, photogenerated electron–hole pairs were separated by this built-in field, forming freecarriers swept toward the WS2 -rich region (electrons) or WSe2 -rich region (holes), resulting in a photocurrent. To furtherinvestigate this behavior, we measured the two-terminal current–oltage ( I –V ) relationship of the sample (labeled as device #1),where the channel region was aligned approximately parallel tothe composition gradient. As shown in Figure 2b , the devicesexhibited rectifying behavior in the absence of illumination (blackline), strongly supporting the existence of a p-n junction insidethe sample. However, the I –V curve differed from that of a typicalp-n junction, as there appeared to be a linear increase in thecurrent when the applied voltage was between 0 and + 2 V. Thismay be due to the presence of intrinsic (undoped) regions ofWS2 and WSe2 inside the channel, which introduce additional resistive components into the current. We then measured theI –V curve by illuminating the entire device with a 532-nmSmall Methods, 2026ommons LicenseFIGURE 1 Characterization of monolayer compositionally graded WS2 x Se2(1- x ) alloy. (a) Schematic of the composition variation in the monolayer alloy. (b) Optical image of a CVD-grown monolayer alloy. The scale bar is 10 µm. The green dot denotes the laser spot. The red and green rectangles indicate the regions on which (c) PL peak energy mapping and (d) surface potential mapping were carried out, respectively. The inset of (d) shows the CPD profile along the dashed line. The scale bars are 5 µm. FIGURE 2 Gradual p-n junction formation. (a) Schematic of the band diagram and photocurrent generation mechanism of the monolayer alloy. (b) Current–voltage ( I –V ) curves in the linear scale of the two-terminal device (labeled as device #1) under dark and illuminated conditions. The inset of (b) shows the curves in the logarithmic scale.            mechanism.  23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creativelaser (red line in Figure 2b ). We observed an increase in thereverse current compared with the dark current. Simultaneously,the current increased rapidly under a forward bias, which waspossibly due to the doping of intrinsic regions and reducedcontact resistance under illumination. The illuminated curvecan be explained by the modified Schockley diode equation inwhich the response of the current to the applied bias is stillSmall Methods, 2026exponential, whereas the reverse saturation current is modified (see Section S2 ) [ 29 ]. The change in the I —V characteristicsunder illumination strongly indicated that the p-n junction insidethe alloy region exhibited an observable optoelectronic response,with a clear zero-bias photocurrent (inset of Figure 2b ), whichdeserved a detailed investigation of the photocurrent generation 3 of 8 Commons LicenseFIGURE 3 Photocurrent generation characterization of the monolayer alloy device. (a) Microscopic image of the channel region of device #2. The scale bar is 5 µm. (b) Corresponding PL map. (c) Corresponding photocurrent map. (d) Peak energy, peak energy gradient, and photocurrent profiles along the dashed lines in (b) and (c).                                                23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative2.2 Spatial Photocurrent Mapping We then investigated the location-dependent photocurrent gen-eration of device #2. We defined the channel in the alloy region,which was free from any multilayer islands, to exclude theirpossible influence (Figure 3a ). It shall be noted that we useda 100 × lens to limit the diameter of the illuminating laserspot to ∼ 2 µm for spatial photocurrent mapping, which wassignificantly smaller than that of a recently reported photore-sponse study on alloy samples [ 23 ]. PL mapping was performedsimultaneously with short-circuit current mapping ( Isc , wherewe did not apply bias during the measurement), as describedin Methods. Therefore, we were able to correlate the localoptical properties with the photocurrent generation behavior.Based on the results of PL mapping (Figure 3a ), we observed apeak energy shift within the channel, indicating the existenceof a composition gradient. More importantly, the correspondingphotocurrent map revealed that when the alloy region wasilluminated by the laser, a photocurrent as high as 2.2 nA wasobserved (Figure 3b ). In contrast, we did not observe significantphotocurrent in regions where the PL peak energy was nearlyconstant. Considering the existence of the p-n junction insidethis region, the collected photocurrent provides solid evidenceof the photovoltaic response from the alloy region. It is worthnoting that although the location of the generated photocurrentwas close to the electrode, the Schottky junction between the4 of 8electrode and sample could not be regarded as the origin because(1) the photocurrent could still be generated when the centerof the focused laser spot was away from the electrode and themaximum photocurrent value was clearly not at the edge of theelectrodes (Figure 3b ), and (2) no photocurrent was observedwhen the laser spot was within proximity of the other electrode.These properties differed from those of typical Schottky junction-based devices. We also excluded the possibility of the device beinga phototransistor because it showed rectified transport behavior (Figure 2b ), and no bias or gate voltage was applied during thismeasurement. The photocurrent map of device #1 is shown in Figure S3 ,which had a longer channel length, and the alloy region waslocated near its center. The photocurrent map revealed a peakwithin the channel, located away from the electrodes. Thus,we confirmed again that a photocurrent was generated fromthe alloy region through the photovoltaic effect. The collectedcurrent value was only on the order of picoamperes (pA), whichwill be discussed later. The spontaneously formed, gradual p-njunction enabled the sample to be fabricated into a photovoltaicdevice with a highly simple structure. Moreover, as a previousstudy unveiled [ 23 ], owing to the spatially varying band structure,the device showed photoresponse within a spectral range fromnearly red to infrared illumination, highlighting its potential forhigh-performance broadband optoelectronic applications. Small Methods, 2026 Commons License                                                  FIGURE 4 Calculated distance and laser position dependence of the photocurrent. The top panel indicates the built-in electric field profile, and the right panel shows the photocurrent profile along the corresponding gray dashed lines in the main panel. The two dashed lines represent different locations of electrodes (see main text). It shall be highlighted that the laser center moves parallel to the channel and the built-in electric field. The negative current density indicates that the direction of current is reversed.                              23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative C2.3 Photocurrent Generation in the Gradual p-n Junction To examine the photocurrent generation behavior in detail, inFigure 3c , we plotted the spatial profiles of the PL peak energy, thegradient of the PL peak energy shift, and the corresponding pho-tocurrent along the black dashed line in the PL map (Figure 3a )and the white dashed line in the photocurrent map (Figure 3b ).The same analysis for device #1 is summarized in Figure S3 . Here,we used the gradient of the PL peak energy shift to approximatethe local built-in field, as previous band structure calculationsrevealed that both the conduction band edge and valence bandedge varied almost linearly with the composition variation [ 25 ].Based on the profiles shown in Figure 3c , both the peak energygradient and photocurrent exhibited peak behaviors, indicatingthe nonlinearity of the composition variation and the resultingspatial dependence of photocurrent generation. However, wefound that the maxima of these two profiles did not necessarilycoincide. This was particularly evident when the alloy regionwas relatively far from the electrodes (Figure S3 ). In an ideal p-n junction, the built-in field causes a drift of the photo-excitedexcess carriers, leading to a drift current. Therefore, for a givenlaser power, it is reasonable to relate the maximum PL energyshift gradient to the maximum photocurrent value because theformer indicates the local built-in field magnitude and the latter isrelated to the short-circuit current. However, this correspondencewas not evident in our devices. The peak mismatch was moreprominent in the device with a longer channel, in which the alloyregion was located far away from the electrodes (Figure S3 ). To address the above observations, we focused on the phe-nomenon in which photogenerated excess carriers moved underboth drift and diffusion before reaching the electrodes. Thecarriers might recombine with their counterparts during migra-tion, and hence, the recombined carriers did not contribute tothe photocurrent. To illustrate this, we adopted the continuityequation of carrier density n , considering the drift term due tothe built-in potential, which is expressed as [ 30, 31 ] 𝑑𝑛 𝑑𝑡 = 𝐺 + 1 𝑞 𝑑(𝐽𝑑 𝑟 𝑖 𝑓 𝑡 + 𝐽𝑑𝑖 𝑓 𝑓 𝑢𝑠𝑖 𝑜𝑛 ) 𝑑𝑥 − 𝑛 𝜏(1)Here, G is the generation rate of the excess carriers. The currentgeneration, due to carrier movement, consists of the drift current( Jdrift ) and diffusion current ( Jdiffusion ) densities. The last term onthe right-hand side of Equation ( 1 ) indicates the recombinationprocess of the carrier, where τ denotes the carrier lifetime. Wenumerically calculated the electron current distribution. Forconvenience, we considered a 1D condition and assumed that theelectric field satisfied a Gaussian distribution according to theshape of the peak shift gradient (Figure 3c ). The current densitywas calculated as a function of both laser spot positions, whichshifted from the left side of the center of the built-in field to theright side, and the channel length (represented by the locationof the electrode). The details of the calculation are provided inSection S4 . The resulting photocurrent density map is shown in the mainpanel of Figure 4 . The horizontal axis corresponds to the channeldirection, and the vertical axis corresponds to the laser centerposition, which also shifts along the channel direction. TheSmall Methods, 2026top panel shows the assumed Gaussian-shaped built-in electric field induced by composition variation in the alloy region. Thedashed vertical lines mark different electrode positions, allowing a direct comparison of the photocurrent profiles for differentchannel lengths. For instance, the maxima of the PL gradient andphotocurrent are consistent if the electrode is positioned at thecenter of the built-in electric field (the origin of the horizontalaxis of the main panel). To evaluate this effect, we collected the current values at twoselected positions (indicated by the two dashed lines (pos-1 andpos-2) in the main panel of Figure 4 ) as a function of the lasercenter position. The resulting line profile of the photocurrentagainst different electrode positions is plotted in the right panelof Figure 4 . At both positions, the maximum photocurrent didnot occur when the laser center overlapped with the strongestelectric field. In other words, when the electrodes were separatedfrom the center of the built-in field, the maximum photocurrentwas not equivalent to the largest value of the PL peak energyshift, which agreed with the PL and photocurrent mappingmeasurements (Figure 3c ). Instead, the current increased whenthe laser center began to move toward the electrodes (positivelaser center location from the center of the built-in field). Thiscan be explained by the recombination process of electrons beforethey reach the electrodes. Thus, the photocurrent peak was notconsistently located at the same location as that of the electricfield. Furthermore, because pos-1 was closer to the electric fieldcenter than pos-2, the maximum current value was significantlyhigher, indicating that a shorter channel length corresponded toa higher photocurrent value. It shall be highlighted that we didnot apply a gate voltage to the sample during the measurements,and thus, the electron lifetime of the undoped WSe2 or WS2 regions outside the alloy region was very short because of the5 of 8ommons LicenseFIGURE 5 Performance of the monolayer alloy photodiode. (a) Current–voltage ( I –V ) curves of the device under different back-gate voltages. The extracted ISC and VOC are presented in (b) as a function of the back-gate voltage VBG .                                                              23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Clarge defect density in typical CVD-grown samples [ 32, 33 ]. Indevice #1, where the electrodes were far from the alloy region,most of the photogenerated excess electrons recombined beforebeing collected by the electrodes, yielding a photocurrent onthe order of pA (Figure S3 ). In contrast, the shorter channellength of device #2 greatly prevented recombination loss, yieldinga photocurrent on the order of nanoamperes (nA) (Figure 3c ).The photocurrent peak was closer to the electrodes comparedwith that to the electric field, which supports the idea of carriertransport and recombination processes (see Figure S4 ). Thisbehavior also implies a compromise between the excess carriergeneration and recombination processes. It is worth noting thatonly qualitative agreement was obtained from the calculations,since the realistic parameters, such as the electric field distribu-tion, electron lifetime, and mobility, might vary inside the alloyregion. Further investigations are necessary to fully exploit thepotential of this material for optoelectronic devices. 2.4 Performance of Monolayer WS2x Se2(1-x) Photodiode Here, we discuss the device performance based on our under-standing of the photocurrent generation process. We illuminatedthe entire channel region of device #2 and measured the I –V curves. Simultaneously, we applied electron doping throughthe SiO2 back-gate structure. Under illumination, we obtaineda modified I –V relationship compared with that under darkconditions (Figure 5a ). By applying a gate voltage, both ISC andVOC increased (Figure 5b ), similar to those of previously reportedphotovoltaic devices [ 34 ]. Several mechanisms may contribute tothis behavior. For example, increased electron doping fills thedefect sites, which may prolong the electron lifetime and enhancethe mobility, which increases both ISC and VOC . The band bendingcaused by the gate voltage may also be important in this case. It is worth noting that the observed Voc values of the alloy device(0.41–0.66 V) were generally higher than those reported for pureTMDCs or heterostructure devices without chemical treatments,which were usually below 0.5 V. The comparison is summarizedin Section S5 . This suggests that a higher energy maximum can6 of 8be extracted, thereby enabling a higher conversion efficiency [ 7,21 ]. This can be attributed to the significantly wider built-in fieldregion in the compositionally graded alloy compared with thatin lateral heterostructures. At the same photon density, morefree carriers can be generated by the field, leading to enhanceddevice performance. Moreover, owing to the gradually changing composition, the potential barrier height in the alloy region isexpected to be significantly lower than that in lateral heterojunc-tions. In addition, the steep interfaces of lateral heterostructuresare more sensitive to defects, which can lead to unexpected bandalignment and/or electric field distribution, reducing the device performance. In this regard, the continuous p-n junction of thealloy may prevent such detrimental effects. For abrupt lateralheterostructures, the depletion/high-field region is confined to only a few nanometers (nm), and therefore, only a small fractionof photocarriers is generated and efficiently collected at thejunction, making the photocurrent highly sensitive to interfacial defects. In compositionally graded alloys, a micrometer-wide junction can increase the number of photocarriers generated within the junction region. However, for a fixed built-in potentialdrop, the reduced average field may reduce the carrier sepa-ration/collection efficiency, implying that an optimal junction width is required to attain a balanced carrier generation andfield strength. In contrast, the fill factor (FF) of our device wasbetween 22% and 24%, which was lower than the highest reportedvalues for in-plane heterostructures [ 18 ]. This was consistent withthe nearly linear I –V curves (Figure 5a ). The relatively low FFmight have resulted from the high series resistance, as a moderategate voltage application significantly increased the Voc value. The photovoltaic figures of merit are summarized in Section S6 .The external power conversion efficiency (PCE) and external quantum efficiency (EQE) were lower than those reported forstate-of-the-art lateral heterostructure photovoltaic devices [ 17–19, 35 ], which was possibly due to carrier collection limitations inthe lateral geometry and nonideal transport/contacts. However, a comparatively high VOC indicates a large built-in potentialacross the compositionally graded alloy, highlighting its potential for photovoltaic operation upon device optimization. Thus, to further improve device efficiency, it is imperative to optimizethe device structure, reduce the contact resistance, and devisesynthesis routes that will produce a less defective alloy material.Small Methods, 2026ommons License                                                          23669608, 2026, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202501997 by National Institute For, Wiley Online Library on [08/06/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Nevertheless, the 2D nature, spontaneous p-n junctions, andbroad bandgap range make monolayer TMDC alloys promis-ing candidates for simple-structured photovoltaic applications,including wearable or transparent self-powered devices andultrathin solar cells. 3 Conclusion In this study, we demonstrated the photocurrent generationin two-terminal monolayer WS2 x Se2(1- x ) alloy devices. KPFMmeasurements confirmed the presence of a gradual p-n junctionwithin the compositionally graded alloy region. SimultaneousPL and photocurrent mapping revealed that the observed pho-tocurrent originated from the photovoltaic effect. Furthermore,we demonstrated that the photocurrent generation was uniqueto the local composition gradient, as supported by the numericalcalculations. More importantly, the device based on the mono-layer alloy exhibited an open-circuit voltage as high as 0.66 V.The simple device structure, broadband absorption nature, andhigh open-circuit voltage highlight the potential of these devicesas efficient optoelectronic devices. 4 Methods 4.1 PL Mapping Measurements The PL peak energy mapping measurements were conductedusing a laser Raman spectrometer (JASCO NRS-5100 andHORIBA XploRA PLUS). The laser wavelength and powerwere 532 nm and 24 µW, respectively. The sample stage wassystematically motorized to perform spatial mapping. 4.2 KPFM Measurements KPFM measurements of the surface potential profiles were per-formed using a Bruker Multimode 8-HR atomic force microscope.The cantilever employed in the KPFM measurements was SCM-PIT-V2 with a Pt/Ir-coated Si tip. The force constant and resonantfrequency were ∼ 3 N/m and ∼ 75 kHz, respectively. The data wereacquired in amplitude-modulated KPFM (AM-KPFM) mode. 4.3 Photocurrent Mapping Measurements To measure the photocurrent generation, photolithography wasused to define the electrode pattern, and Au/Ni (50/3 nm)electrodes were deposited. Photocurrent mapping was performedusing a Keithley 6514 electrometer. Simultaneously, PL mappingwas conducted using a Raman spectrometer with a laser powerof 169 µW. For the I –V measurements, a bias was applied using aKeithley 2612 source meter, and the corresponding photocurrentvariation was collected using an electrometer. Acknowledgements This work was financially supported by JSPS KAKENHI (GrantNumbers: JP20H05867, JP20H05664, JP21H05232, JP21H05234,Small Methods, 2026CJP21H05236, JP22H00280, JP22H01899, JP22H04957, JP22K19059, JP22H01899, JP24H00044, and JP25K17908), JST CREST (Grant Numbers: JPMJCR23A4 and JPMJCR23O3), and JST FOREST (GrantNumbers: JPMJFR223Z and JPMJFR213X), Japan. Y.M. acknowledges the support from the World Premier International Research CenterInitiative (WPI), MEXT, Japan. This work was also supported by theIketani Science and Technology Foundation. Conflicts of Interest The authors declare no conflict of interest. Data Availability Statement The data that support the findings of this study are available from thecorresponding author upon reasonable request. References 1 . A. Jäger-Waldau, M. C. Lux-Steiner, and E. Bucher, “MoS2 , MoSe2 , WS2 and WSe2 Thin Films for Photovoltaics,” Solid State Phenomena 37 (1994):479–484, https://doi.org/10.4028/www.scientific.net/SSP.37-38.479 . 2 . K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically ThinMoS2 : A New Direct-Gap Semiconductor,” Physical Review Letters 105 (2010): 136805, https://doi.org/10.1103/PhysRevLett.105.136805 . 3 . Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. 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Supporting File : smtd70544-sup-0001-SuppMat.docx Small Methods, 2026 Commons Licensehttps://doi.org/10.1021/nl501962chttps://doi.org/10.1038/s41699-020-00179-9https://doi.org/10.1109/JSTQE.2016.2582318https://doi.org/10.1038/nnano.2014.222https://doi.org/10.1002/adma.201701168https://doi.org/10.1016/j.nanoen.2018.06.049https://doi.org/10.1002/smll.202002263https://doi.org/10.1002/0470068329https://doi.org/10.1038/nmat3263https://doi.org/10.1021/acsami.9b03608https://doi.org/10.1021/acsami.1c09176https://doi.org/10.1002/adma.202203250https://doi.org/10.1021/nl504256yhttps://doi.org/10.1002/adma.201503872https://doi.org/10.1021/acs.nanolett.5b03662https://doi.org/10.3390/mi15060778https://doi.org/10.1063/1.5063263https://doi.org/10.35848/1882-0786/acae1ahttps://doi.org/10.1038/nmat3633https://doi.org/10.1103/PhysRevMaterials.6.064005https://doi.org/10.1038/s41467-023-37174-9https://doi.org/10.1126/science.aab4097 Photovoltaic Device Based on Monolayer Compositionally Graded Transition Metal Dichalcogenide Alloy 1 | Introduction 2 | Results and Discussion 2.1 | Spontaneous Formation of p-n Junction in Monolayer WS2xSe2(1-x) 2.2 | Spatial Photocurrent Mapping 2.3 | Photocurrent Generation in the Gradual p-n Junction 2.4 | Performance of Monolayer WS2xSe2(1-x) Photodiode 3 | Conclusion 4 | Methods 4.1 | PL Mapping Measurements 4.2 | KPFM Measurements 4.3 | Photocurrent Mapping Measurements Acknowledgements Conflicts of Interest Data Availability Statement References Supporting Information