arXiv subjects

Ji-Hee Kim

Publications and source records attributed to Ji-Hee Kim.

6 recordsLinked to original sources

arXiv2026

Hot carrier diffusion-assisted ideal carrier multiplication in monolayer MoSe2

Carrier multiplication (CM), the process of generating multiple charge carriers from a single photon, offers an opportunity to exceed the Shockley-Queisser limit in photovoltaic applications. Despite extensive research, no material has yet achieved ideal CM efficiency, primarily owing to significant energy losses from carrier-lattice scattering. In this study, we demonstrate that monolayer MoSe2 can attain the theoretical maximum CM efficiency permitted by energy-momentum conservation principle, using ultrafast transient absorption spectroscopy. By resolving the scatter-free ballistic transport of hot carriers and validating our findings with first-principles calculations, we identify the cornerstone of optimal CM in monolayer MoSe2: superior hot-carrier dynamics characterized by suppressed energy dissipation via minimized carrier-lattice scattering, and the availability of abundant CM pathways facilitated by 2Eg band nesting. Comparative analysis with bulk MoSe2 further emphasizes the enhanced CM efficiency in the monolayer, attributed by superior hot-carrier diffusion and access to additional CM pathways. These results position monolayer MoSe2 as a promising candidate for high-performance optoelectronic applications, providing a robust platform for next-generation energy conversion technologies.

cond-mat.mes-hall↗

arXiv2024

Cascade hot carriers via broad-band resonant tunneling

Extraction of hot carriers (HCs) over the band-edge is a key to harvest solar energy beyond Shockley-Queisser limit1. Graphene is known as a HC-layered material due to phonon bottleneck effect near Dirac point, but limited by low photocarrier density2. Graphene/transition metal dichalcogenide (TMD) heterostructures circumvent this issue by ultrafast carrier transfer from TMD to graphene2,3. Nevertheless, efficient extraction of photocurrent by means of HCs together with carrier multiplication (CM) is still missing. Here, we introduce an ultrathin broadband resonant tunneling (BRT) barrier, TiOX to efficiently extract photocurrent with simultaneous CM and HC measurements in MoS2/graphene/TiOX heterostructure. The BRT layer gives rise to boosting open circuit voltage which is linearly proportional to incident photon energy. Meanwhile, short circuit current rises rapidly over 2Eg with obvious CM feature. This was explained by defining the joint density of states between graphene and TiOX layer over positive and negative voltage. The broadband resonant tunneling states inherently constructed from oxidation states varying from Ti3+ to Ti4+ allow the ultrafast HCs to efficiently transfer from graphene to TiOX layer. We find that the number of available tunneling states is directly proportional to short circuit current, which is well corroborated with TiOX and MoS2 thickness variance. We obtained an optimum thickness of BRT layer of ~2.8 nm, yielding cascade open circuit voltage as high as ~0.7 V, two orders of magnitude higher than that without BRT layer to reach a record efficiency of 5.3% with improved fill factor owing to synergistic HC and CM conversion under 1-SUN with long-term stability.

physics.app-ph↗

arXiv2020

Bandgap renormalization in monolayer MoS_2 on CsPbBr_3 quantum dot via charge transfer at room temperature

Many-body effect and strong Coulomb interaction in monolayer transition metal dichalcogenides lead to shrink the intrinsic bandgap, originating from the renormalization of electrical/optical bandgap, exciton binding energy, and spin-orbit splitting. This renormalization phenomenon has been commonly observed at low temperature and requires high photon excitation density. Here, we present the augmented bandgap renormalization in monolayer MoS_2 anchored on CsPbBr_3 perovskite quantum dots at room temperature via charge transfer. The amount of electrons significantly transferred from perovskite gives rise to the large plasma screening in MoS_2. The bandgap in heterostructure is red-shifted by 84 meV with minimal pump fluence, the highest bandgap renormalization in monolayer MoS_2 at room temperature, which saturates with further increase of pump fluence. We further find that the magnitude of bandgap renormalization inversely relates to Thomas-Fermi screening length. This provides plenty of room to explore the bandgap renormalization within existing vast libraries of large bandgap van der Waals heterostructure towards practical devices such as solar cells, photodetectors and light-emitting-diodes.

cond-mat.mes-hall↗

arXiv2020

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.

cond-mat.mes-hall↗

arXiv2020

Band restructuring of ordered/disordered blue TiO2 for visible photocatalyst

Black TiO2 with/without noble metal has been proposed for visible photocatalyst, still leaving poor catalyst efficiency. Alternatively, phase-mixed TiO2 such as anatase and rutile has been commonly used for visible catalysts with the inevitable inclusion of noble metal. Here, we perform a noble metal-free visible photocatalyst blue TiO2 with type-II band-aligned ordered anatase/disordered rutile structure, via phase-selective reduction with alkali metals. The changed band alignment in this heterostructure was identified by absorption and ultraviolet photoemission spectroscopy, which was further confirmed by transient charge separation. The band alignment of type-I and type-II was clearly restructured by converting from ordered to disordered phase with a prolonged reduction period and as followed light absorbance enhancement also observed. Initiated type-I in a pristine sample, the type-II was organized from disordered rutile phase in 3-day Li-reduction. The type-II disordered rutile TiO2 heterostructure exhibits a remarkable photocatalytic performance by 55 times higher than conventional P25 TiO2 in solar-light driven hydrogen evolution reaction owing to an efficient electron and hole separation of type-II heterojunction. Furthermore, this restructured heterojunction type-II TiO2 demanded 10 times less Pt amount as a co-catalyst for the comparable photocatalytic performance, compared to Pt decorated type-I pristine anatase/rutile phase-mixed TiO2.

cond-mat.mtrl-sci↗

arXiv2018

Highly Efficient Carrier Multiplication in van der Waals layered Materials

Carrier multiplication (CM), a photo-physical process to generate multiple electron-hole pairs by exploiting excess energy of free carriers, is explored for efficient photovoltaic conversion of photons from the blue solar band, predominantly wasted as heat in standard solar cells. Current state-of-the-art approaches with nanomaterials have demonstrated improved CM but are not satisfactory due to high energy loss and inherent difficulties with carrier extraction. Here, we report ultra-efficient CM in van der Waals (vdW) layered materials that commences at the energy conservation limit and proceeds with nearly 100% conversion efficiency. A small threshold energy, as low as twice the bandgap, was achieved, marking an onset of quantum yield with enhanced carrier generation. Strong Coulomb interactions between electrons confined within vdW layers allow rapid electron-electron scattering to prevail over electron-phonon scattering. Additionally, the presence of electron pockets spread over momentum space could also contribute to the high CM efficiency. Combining with high conductivity and optimal bandgap, these superior CM characteristics identify vdW materials for third-generation solar cell.

cond-mat.mes-hall↗