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Carrier Multiplication via Photocurrent Measurements in Dual-Gated MoTe_2

Although van der Waals layered transition metal dichalcogenides from transient absorption spectroscopy have successfully demonstrated an ideal carrier multiplication (CM) performance with an onset of nearly 2Eg,interpretation of the CM effect from the optical approach remains unresolved owing to the complexity of many-body electron-hole pairs. We demonstrate the CM effect through simple photocurrent measurements by fabricating the dual-gate P-N junction of a MoTe2 film on a transparent substrate. Electrons and holes were efficiently extracted by eliminating the Schottky barriers in the metal contact and minimizing multiple reflections. The photocurrent was elevated proportionately to the excitation energy. The boosted quantum efficiency confirms the multiple electron-hole pair generation of>2Eg, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.

One primary concern in single-junction solar cells for energy harvesting is the thermalization loss in semiconductors [1]. The excited electrons subject to incident light thermally relax to the conduction band edge, wasting excess energy above the bandgap as heat with a maximum conversion efficiency of ~34%, which is known as the Shockley-Queisser (SQ) limit [2]. Two approaches have been proposed to overcome the SQ limit: carrier multiplication (CM) by boosting the number of electronhole pairs [3] and hot-carrier generation by widening the open-circuit voltage [4]. During the CM process, the excess kinetic energy of photoexcited carriers above twice the bandgap can kick out an additional electron-hole pair through an inverse Auger process to multiply the number of electron-hole pairs assisted by a strong Coulomb interaction, which is the key to promoting the CM process in typically confined nano-materials [5][6][7][8][9][10].
Thus far, intense efforts have been devoted to determining the CM performance using pump-probe techniques. Quantum dots and carbon nanotubes exhibit a diverse CM onset energy of 2.0-3.0Eg and a CM conversion efficiency of 30%-90% [7,9,[11][12][13]. Recently, an ideal CM conversion efficiency of ~99% with an onset energy of nearly 2Eg has been reported in transition metal dichalcogenides (TMD) materials such as MoTe2 and WSe2 thin films [9,10]. However, the interpretation of the CM effect based on such an optical approach remains disputable owing to the complexity of the analysis and the possible many-body phenomena [14]. Therefore, it is of critical importance to verify the CM effect through an enhanced quantum yield of photocurrent as a function of photon energy. Solar cell is a typical device for evaluating the enhanced quantum yield through photocurrent measurement; however, it requires an optimized device architecture with several parameters including an energy band alignment, carrier transport, and choice of interfacial materials [15,16]. These are the major obstacles to the development of new potential CM materials. Although a two-terminal photocurrent can be simply measured under a strong source-drain voltage [17,18], this could be further complicated in the presence of a Schottky barrier at the metal contact, with a trap-induced photoconductive hysteresis and strong electric field-induced impact ionization. Therefore, a simple and efficient methodology to prevent such complexities is needed to evaluate the CM performance. In this report, we carefully designed a dual-gate p-n junction structure by stacking ultra-thin two-dimensional vdW layered materials and measured the excitation energy-dependent photocurrent without applying an external source-drain voltage. We are able to achieve a high quantum yield of over 100% with an onset energy of twice the bandgap in MoTe2 thin films.
To minimize the effect of the contact resistance and external electric field in a conventional photoconductive device [ Fig. 1(a)], we adopted a dual-gate p-n junction device [ Fig. 1 [19], Fig. S2], the Schottky barriers at the metal contacts are efficiently modulated from both gate biases such that the barrier width becomes narrow to allow tunneling to occur. The built-in potential constructed from dual-gate [see the Supplemental Material [19], Fig. S4] effectively extracts the photogenerated carriers in a channel. Our Spatially resolved photocurrent mapping was applied to confirm the zero Schottky barrier height and estimate the diffusion length of the photo-generated carriers with a scanned step of 200 nm at VDS = 0 V [ Fig. 2(b)]. The photocurrent is maximum at the center of the MoTe2 channel and minimum at the contact regions, in contrast with a previous report in which the highest photocurrent was observed with the Schottky barrier at near the contact regions [20]. This confirms the zero Schottky barrier height in the presence of a dual gating. The photocurrent was nearly negligible at the gate electrode region. The diffusion length of the photogenerated carriers is ~ 1 μm, similar to a previous report on few-layer WSe2 [21]. 4 The photocurrent of the dual-gate device (VG1 = −VG2 = 2.0 V) was measured at different light power densities at a fixed photon energy (Eex = 1.91 eV) [ Fig. 2(c)]. The dark I-V curve of the MoTe2 homojunction device shows an asymmetric diode behavior. The short-circuit current density at 0 V is elevated as the power density increases. The open-circuit voltage is shifted up to ~100 mV at a high power, which could be ascribed to the band filling of the accumulated photocarriers in the MoTe2 channel [21]. The photocurrent was further measured based on the power density and laser energy [  [23]. Note that the observed EQE feature with a transparent quartz substrate is entirely different from that on a silicon substrate because of multiple reflections, which is strongly influenced by the silicon reflection spectrum. (See the Supplemental Material [19], Fig. S10) The absorption spectrum (A) of a MoTe2 flake (thickness of ~18.1 nm) was determined from the experimentally measured transmission spectrum T and reflection spectrum R with Fig. 3(c) and see the Supplemental Material [19], Sec. 4, for the reflection and transmission measurements]. The measured transmission is almost constant with the increasing photon energy, whereas the reflection steeply doubles from ~25% to ~50%, probably owing to strong light reflection from the oxidized layers (MoOx and TeOx) on the surface of the MoTe2 flake [24,25].
Consequently, the absorption is dramatically reduced at high photon energies.
We next calculate the internal quantum efficiency (IQE) by normalizing the EQE spectrum with the absorption spectrum A, IQE = EQE/A. Figure 4(a) shows the IQE as a function of excitation energy (or normalized by the MoTe2 bandgap of 0.9 eV [23]). The IQE is gradually doubled from ~15% to ~30%, corresponding to ~100% to ~200% in the normalized IQE (quantum yield) when the excitation photon energy increases from 1.8 eV (~2Eg) to 2.6 eV (~2.8Eg). A similar change in IQE was observed with a film thickness of 22 nm. (See the Supplemental Material [19], Fig. S14) The doubly enhanced IQE with a photon energy from 2Eg to 3Eg is the fingerprint of CM in our MoTe2 p-n junction device, which is fully consistent with the CM results of a MoTe2 film investigated using optical approaches [9]. The slow upturn of the IQE near the CM onset energy can be attributed to locally induced traps and defects [26].
Finally, we compare the quantum yield of MoTe2 from the photocurrent measurement with the reported CM efficiency for MoTe2 as well as quantum dots using an optical approach. The onset CM energy from the photocurrent measurement is twice the bandgap, congruent with that of the photobleaching signal in the pump-probe optical approach [9]. The CM conversion efficiency resides within the range of 95%-99% between photo-bleaching and photo-induced absorption signals from MoTe2, which is still much higher than those of PbS [27] and PbSe quantum dots [28]. This high photocurrent CM efficiency in the MoTe2 p-n junction device is attributed to the small metal contact resistance and short channel length within the diffusion length, allowing for an efficient extraction of the photoexcited carriers. While several challenges remain to be resolved, such as defect passivation, metal contact, and band alignment for electron-hole extraction, this work represents a step forward toward a CM solar cell with 2D materials beyond the SQ limit.    Comparison of CM conversion efficiency of MoTe2 measured from photocurrent including previous reports on photo-bleaching (PB) and photo-induced absorption (PIA) in MoTe2 thin film [9] and PbS [27] and PbSe [28] quantum dots measured using optical approach.