Tandem-structured, hot electron based photovoltaic cell with double Schottky barriers

We demonstrate a tandem-structured, hot electron based photovoltaic cell with double Schottky barriers. The tandem-structured, hot electron based photovoltaic cell is composed of two metal/semiconductor interfaces. Two types of tandem cells were fabricated using TiO2/Au/Si and TiO2/Au/TiO2, and photocurrent enhancement was detected. The double Schottky barriers lead to an additional pathway for harvesting hot electrons, which is enhanced through multiple reflections between the two barriers with different energy ranges. In addition, light absorption is improved by the band-to-band excitation of both semiconductors with different band gaps. Short-circuit current and energy conversion efficiency of the tandem-structured TiO2/Au/Si increased by 86% and 70%, respectively, compared with Au/Si metal/semiconductor nanodiodes, showing an overall solar energy conversion efficiency of 5.3%.

T here has been substantial research focused on converting solar energy into electricity since the development of the pn-junction solar cell. Recently, various strategies have been proposed to enhance the maximum possible efficiency of a solar cell with a band gap of 1.1 eV, 30% of which is called the single-junction Shockley-Queisser limit 1 . Semonin et al. demonstrated photocurrent enhancement assisted by multiple exciton generation on semiconductor quantum dots, which led to a quantum efficiency exceeding 100% in the quantum dot solar cell 2 . The plasmonic solar cell could be a viable thin solar cell capable of efficiently absorbing broad light wavelengths. As the thickness of the photoactive materials increases, light harvesting is improved, whereas carrier recombination easily occurs, leading to a trade-off between light absorption and carrier collection. For excellent optical absorption without considerable loss of charge carriers in a thin solar cell, the surface plasmon based solar cell has been studied. Dang et al. observed improved broadband light harvesting when using multiple core-shell structured oxide/metal/oxide plasmonic nanoparticles in a dye-sensitized solar cell (DSSC), resulting in an increased photovoltaic cell efficiency (PCE) from 8.3% to 10.8% 3 . Atwater et al. and Reineck et al. have suggested possible mechanisms for enhancement of solar cell performance based on plasmonic materials 4,5 . There are possibilities for implementation of metal nanoparticles in solar cells that have local near-field enhancement, a high scattering cross section, and an actual light harvesting element that induces charge separation. In addition, tunable surface geometry that increases the surface area for higher dye loading and light harvesting and reduced charge recombination in DSSC were reported by Ko et al. who fabricated a nanoforest composed of high-density, long-branched, multi-generation hierarchical ZnO nanowires 6 . However, an intrinsic challenge in solar cells is limited light absorption on the photoactive materials. A stacking structure generally consisting of a low-bandgap material and a high-bandgap material-called a tandem cell-was introduced to maximize absorption across the solar spectrum 7 . Recently, application of tandem cells has expanded from an inorganic solar cell to an organic solar cell that includes a polymer solar cell to overcome low carrier mobility (i.e., a limiting factor for power conversion efficiency) [8][9][10] . Dou et al. designed a tandem polymer solar cell that consists of a low-bandgap conjugated polymer spectrally matched with the front cell, showing an increased PCE (8.62%), compared with a single-layer polymer solar cell (6%) 11 .
The initial effort to develop a solar energy conversion device can be traced back to the metal/semiconductor (MS) junction with a thin metal film. The MS diode has two mechanisms for generation of excited electrons as photocurrent: band-to-band excitation on the semiconductor and internal photoemission from the thin metal film (i.e., hot electrons). It is known that hot electrons, which have high kinetic energy (1-3 eV), can be generated in metals when external energy is deposited on the surface, such as in the absorption of light 12 . Energetic charge carriers excited during photon illumination have been detected on thin metal/semiconductor contacts or metal/ insulator/metal tunneling junctions [13][14][15] . It was reported that surface modification of a thin metal film, such as a connected island structure or deposited nanowires, enhanced the yield of energetic charge carrier generation. Knight et al. showed the possibility for photodetection using energetic charge carriers injected over a Schottky energy barrier at the interface between a gold nanostructure (the antenna) and n-type silicon 16 . The gold nanostructure was coupled with incident photons and exhibited resonant plasmons that excited energetic charge carriers and resulted in a detectable photocurrent 17,18 . Wang et al. observed excitation of hot carriers via plasmon absorption on Au/Al 2 O 3 /Au junctions, suggesting that a simple planar metal/insulator/metal diode can play a role as a light harvesting device based on hot electron extraction 19 . Among approaches to increase the PCE in MS solar cells based on internal photoemission and band-to-band excitation, parallel energy harvesting with tandem cells could be ideal.
In this work, we demonstrate solar energy conversion using a tandem-structured, hot electron based photovoltaic cell with double Schottky barriers for multiple reflections of hot electrons and two semiconductor layers for enhancement of band-to-band excitation, as illustrated in Fig. 1a. We show a tandem-structured solar cell composed of multilayers with different absorption characteristics, which is a promising strategy to increase energy conversion efficiency.

Results and discussion
In the tandem-structured, hot electron based photovoltaic cell, the double Schottky barriers were built on the two interfaces of a thin gold film that makes contact with the TiO 2 and silicon. Fig. 1b shows a cross section of TiO 2 deposited on a Au film on silicon, where the Au layer plays a role as a common electrode that has Schottky contacts to two semiconductors. The corrugated Au layer (periodic pattern) refers to the cross-section view of the Au island structures. The Au layer is also an optical absorption layer for internal photoemission. The transport process of electrons on a metal/semiconductor contact is illustrated in the energy diagram in Fig. 1c. The proposed tandem-structured, hot electron based photovoltaic cell harvests the photo-excited charge carriers via two channels: band-to-band excitation on two semiconductors and internal photoemission (or hot carrier generation) based on absorption on the metal layer and transport across the double Schottky barriers.
Two pathways for internal photoemission and band-to-band excitations are shown as the red and blue lines in Fig. 1c, respectively. As the incident photon energy (hn) is greater than the band gap (Eg) of the semiconductors, electron-hole pairs are produced in the semiconductors while hot electrons (via the internal photoemission process) are generated by absorption of incident photon energy on the thin gold film. In the case where the Schottky energy barrier (E SB ) , hn , Eg, the predominant route to excite energetic electrons is internal photoemission in the metal film because the band-to-band excitation of the semiconductors can be negligible in the photon energy range 13,20 . The total photocurrent (A) measured on the tandem structure can be obtained by collecting the photocurrent from both subcells, as shown in Fig. 1c. Christine et al. developed models of the effect of the Schottky barrier on internal photoemission from the metal film into the semiconductor 21 . They presented the enhanced emission probability of hot carriers in the case of double Schottky barriers as a result of multiple hot carrier reflections within the metal film and emission over two Schottky barriers, compared with a single Schottky barrier. In our study, we combined these two mechanisms using a tandem-structured Schottky diode with double Schottky barriers. We found that the tandem-structured Au/Si diode capped with a TiO 2 layer as the second semiconductor exhibited improved solar energy conversion efficiency because of an increased incident photon to current conversion efficiency (IPCE) in the visible light region.
The effect of the double Schottky barriers on photocurrent was observed on the TiO 2 -deposited Au/Si diodes. Fig. 2a,b shows current-voltage curves of Au/Si and TiO 2 /Au/Si, respectively, revealing that the photocurrent increased when illuminated by a tungstenhalogen lamp and that the rectifying property of the diodes was well preserved after deposition of TiO 2 on the Au/Si. The photocurrent density measured under illumination by a halogen-tungsten lamp dramatically increased from 2.4 to 5.0 mA/cm 2 after deposition of TiO 2 on the Au/Si (Fig. 2c). While the Au/Si cell shows a rectangular shape, as expected, the TiO 2 /Au/Si tandem cell exhibited transient behavior (2% increase in current for 5 seconds). Such transient behavior was recently found in TiO 2 -based heterojunctions and was attributed to dielectric effects and oxide instability 22 .
The energy band diagram of the TiO 2 -deposited Au/Si diodes could be different from that of Au/Si, as proposed by Fig. 2d, because the second Schottky barrier was built at the interface between the TiO 2 and the thin gold layer. Fig. 2d shows the routes for internal photoemission from the thin gold film to both semiconductors through the double Schottky barriers. First, hot electrons with energy higher than each Schottky energy barrier can overcome both Schottky barriers toward the semiconductors (red lines). In addition, the emission probability of the hot electrons can be enhanced through multiple reflections (blue line) until the excess energy of the hot electrons is reduced to the lower Schottky energy barrier. If the photons with energy hn are absorbed uniformly and not emitted, the hot electrons are assumed to reflect elastically and diffusely from the metal/semiconductor interfaces. The number of trips taken by the hot electrons via multiple reflections within the double Schottky barriers can be given by where n is the total number of trips the hot electrons take via multiple reflections, L is the mean free path of the hot electrons, t is the thickness of the thin metal film, hn is the photon energy, and W B is the Schottky barrier 21 . In the case of a single Schottky barrier, the value of n max decreases by half because there is only one route for internal photoemission. At the same time, improved efficiency of the band-to-band excitation is expected by absorption of photon energy on the two semiconductors with different band gaps. In the tandemstructured Au/Si, the anti-reflecting effect can be negligible because the refractive index of the thin gold film is smaller than that of the TiO 2 film in the range of visible light incident on the tandem-structured Au/Si 23 .
The photocurrent density was measured as a function of TiO 2 layer thickness (Fig. 3a). The thickness of the TiO 2 was controlled by changing the concentration of the sol-gel solution, as shown in Fig. 3b-d. The photocurrent density increased as the TiO 2 layer thickness increased up to 44 nm thick; however, the photocurrent density decreased when the TiO 2 layers were thicker than 44 nm. An appropriate TiO 2 thickness is required to guarantee band bending at the interface between the TiO 2 and the Au. In the case of the 22 nm thick TiO 2 layer, the TiO 2 layer could be too thin for stable band bending at the interface between the TiO 2 and the Au. On the other hand, when the TiO 2 thickness exceeds 44 nm, absorption of incident light on the TiO 2 increases, leading to a decrease of incident light to the Au and Si. Therefore, there exists an optimal thickness (44 nm) of TiO 2 to maximize the photocurrent in the tandemstructured TiO 2 /Au/Si diode. Fig. 4a shows solar cell performance characteristics of the tandemstructured Au/Si. The short-circuit current density (J SC ) and opencircuit voltage (V OC ) were measured on the Au/Si (black line) and the tandem-structured TiO 2 /Au/Si (red line) diodes under illumination of air mass 1.5 global (AM 1.5 G). After TiO 2 deposition, the J SC increased from 22 to 41 mA/cm 2 , showing that the tandem-structured metal/semiconductor diode has an improved capability to harvest photon energy by utilizing the double Schottky barriers. The V OC increased from 0.33 V to 0.40 V. These V OC values are rather low, comparable to a diazole-based cell 24 . A possible explanation for the increased open-circuit voltage after the adsorption of TiO 2 on the Au/Si could be associated with a change in the Schottky barrier height. The change in the Schottky barrier of the TiO 2 -adsorbed Au/Si, compared with the Au/Si, was obtained by fitting I-V curves to the thermionic emission equation (Fig. S1). The Schottky barrier height was modified from 0.816 eV to 0.842 eV. In this approach, the Schottky barrier height, 0.842 eV, is representative of the overall value of the combined double Schottky barriers. For an ideal Schottky contact, the Schottky barrier height is expected to be W B 5 W m 2 x, where W m is the work function of the metal and x is the electron affinity of the semiconductor (n-type) 20 . In the Schottky diode, the V OC is linearly proportional to the Schottky barrier height 25 . In the double Schottky barrier, the overall Schottky barrier height can be affected by the formation of the second Schottky barrier because electrons overcome the potential barrier and travel into the semiconductors from the metal film in the thermionic emission process. Therefore, if a higher Schottky barrier is formed at the interface of the TiO 2 and the Au film rather than at the interface of the silicon and the Au film (0.816 eV), the overall Schottky barrier height can become higher (0.842 eV). In our previous work, the experimental Schottky barrier height of ,1.1 eV for Au/TiO 2 was reported, which is acceptable compared to the literature value 26,27 . As a result, it is probable that the newly formed higher Schottky barrier at the interface of the Au/TiO 2 could increase the overall Schottky barrier height, resulting in the increased V OC . Fig. 4b shows the IPCE obtained by measuring photocurrent as a function of wavelength. The IPCE plot of the tandem-structured TiO 2 /Au/Si is greater than that of the Au/Si across the entire range of wavelengths, whereas the  Au/TiO 2 didn't exhibit any photon response in the visible light spectrum. The improved ability for current generation of the tandem structure can originate from multiple reflections between the double Schottky barriers and enhanced band-to-band excitation. In addition, it is possible that the increase in the IPCE plot could be attributed to improved penetration of the incident light through the TiO 2 / Au, enhancing light absorption on the silicon substrate. The absorbance spectra were measured using a UV-Vis spectrometer on the TiO 2 -deposited Au film on quartz (Fig. S2). The deposition of the TiO 2 film on the Au film improved light penetration to the silicon substrate in the visible range, while the TiO 2 film on quartz absorbed nearly no incident light.
The current density of the Si/Au/TiO 2 tandem cell is 41 mA/cm 2 under AM 1.5 G with a light intensity of 100 mW/cm 2 , which results in a photovoltaic cell efficiency of 5.3% in the hot electron tandem cell. Compared with that of the Au/Si structure (3.1%), the tandem cell exhibited an increase in efficiency of ,70%. The V OC and J SC under AM 1.5 G improved on the tandem-structured TiO 2 /Au/Si; however, the fill factor (FF) decreased because the resistance factors of the cell increased (Fig. S3) 17 . The dielectric effect of the TiO 2 /Au/Si tandem cell can be at least partly associated with enhancement of the short-circuit current. However, as the change in current is not significant (i.e., less than 2% for 5 seconds), the dielectric effect or instability of the oxide might be minor effects.
To verify the concept of a tandem-structured hot electron solar cell, we constructed a TiO 2 /Au/TiO 2 tandem cell and compared the photocurrent of the tandem cell with that of a Au/TiO 2 single cell. Fig. 5a,b shows the scheme and scanning electron microscopy (SEM) image of the TiO 2 /Au/TiO 2 tandem cell, respectively. The top TiO 2 layer (50 nm thick) was prepared and coated on a Au/TiO 2 single cell using the sol-gel method. Fig. S4 shows the representative I-V curves of a Au/TiO 2 single cell and the TiO 2 /Au/TiO 2 tandem cell. After surface modification of the thin Au film and deposition of TiO 2 , the rectifying property was well preserved. Fig. 5c shows photocurrent measured on Au/TiO 2 , Au islands/TiO 2 , and TiO 2 /Au islands/TiO 2 . The higher photocurrent was obtained on the Au islands/TiO 2 , compared with the thin film Au/TiO 2 , as a result of surface plasmons 18 . A thin Au film was modified to a connected island structure to enhance hot electron generation via surface plasmons. The photocurrent of the TiO 2 /Au islands/TiO 2 tandem solar cell was higher than that of the Au islands/TiO 2 by a factor of three, which is beyond the range of the photocurrent caused by an antireflection layer. As shown in Fig. 5d, the peak in IPCE of the TiO 2 /Au/TiO 2 originating from hot electron flow is broader and higher than that of the Au/TiO 2 , which could be associated with enhanced hot electron flows because of the double barriers. We note that there is a slight contribution to photocurrent by thermionic emission in the range of 400-520 nm (IPCE , 0.5%); the exact contribution depends on the defect level of the TiO 2 17,18 . In addition, since the metallic layer is surrounded by semiconductor layers with different dielectric constant, the potential difference in the tandem cell can give additional complexities 28 .
This study suggests that the enhancement of hot electron injection by the use of double Schottky barriers is a general phenomenon. The improvement of the solar cell performance suggests that the concept of double Schottky barriers has significant potential as a novel strategy for light harvesting with high energy conversion efficiency.
In summary, hot electron-based tandem cells of TiO 2 /Au/Si and TiO 2 /Au/TiO 2 were fabricated for enhanced light harvesting. In the tandem-structured cell, the energy band diagram was modified to form double Schottky barriers at both interfaces between the Au film and the semiconductors. The double Schottky barriers can play a role as a reflector for an enhanced hot electron's trip and as an additional route for internal photoemission. In addition, light absorption is improved by the band-to-band excitation of both semiconductors  with different band gaps. The effect of the double Schottky barriers was exhibited through enhanced performance of the solar cell. After modification from Au/Si to the tandem-structured TiO 2 /Au/Si, the J SC and solar cell efficiency were enhanced by 86% and 70%, respectively. The photocurrent of TiO 2 /Au/TiO 2 exhibited a threefold increase, compared with that of the Au/TiO 2 single cell. A tandem hot electron photovoltaic device with double Schottky barriers may pave the way for novel methods for light harvesting.

Methods
For fabrication of a tandem-structured Schottky diode, vertically oriented Au/Si Schottky diodes were fabricated on an n-type (100) silicon wafer. The first step consists of depositing a 200 nm thick film of SiO 2 on a Si wafer via plasma-enhanced chemical vapor deposition through an aluminum shadow mask. A 50 nm thick film of aluminum and a 150 nm thick film of Au are then deposited through a second mask using electron beam evaporation, which constitutes the nanodiode's two ohmic electrodes. Finally, a thin gold film (10 6 2 nm thick) is deposited through a third mask. The short-circuit photocurrent was measured with a sourcemeter (Keithley Instrumentation, 2400) under illumination from a halogen-tungsten lamp. Only the active area of the diode was illuminated by the light source. We used a metal mask to block the other areas of the device. In addition, we measured the photocurrent using a laser light source (green, 532 nm 6 10, 5 mW) with a beam size of 0.5 mm. We found that when the laser source illuminated the active area, photocurrent was generated. Therefore, we can rule out the contribution of photocurrent from the bulk Si that is outside of the active region.
For electrical characterization of the diodes, current-voltage measurements were performed by sweeping the voltage between two electrodes and measuring the resulting current. By fitting the current-voltage curves of the diodes to the thermionic emission equation, the Schottky barrier heights, ideality factors, and series resistances for the nanodiodes were obtained 20,27 .
For preparation of a TiO 2 film, the sol-gel process was utilized. First, 0.5 ml of titanium tetraisopropoxide (Ti(OC 3 H 7 ) 4 ), or TTIP, was added to 9.5 ml of ethanol, which was then mixed with 5 ml of nitric acid 29,30 . The TTIP and ethanol were used for the precursor and solvent, respectively. The nitric acid assists the TiO 2 sols to form a gel and prevents precipitation of the TiO 2 particles. The TiO 2 was deposited using the spin coating method. Optical properties of the films were characterized using a UV-Vis spectrophotometer (U-4001, Hitachi). The spectral range capability of the instrument is 300-1100 nm. For measurement of the films, the thin gold layer and the sol-gel TiO 2 were deposited on a 1/160 thick quartz window, and the instrument was used in standard setup mode. A bare quartz window was used as a reference to remove the absorption of the quartz.