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Author

Keye Sun

Bio: Keye Sun is an academic researcher from University of Virginia. The author has contributed to research in topics: Photodiode & Photodetector. The author has an hindex of 11, co-authored 62 publications receiving 458 citations.

Papers published on a yearly basis

Papers
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Journal ArticleDOI
TL;DR: In this article, temperature dependent energy levels of methylammonium lead iodide were investigated using a combination of ultraviolet photoemission spectroscopy and optical spectrography, and it was shown that the valence band maximum and conduction band minimum shift down in energy by 110 and 77
Abstract: Temperature dependent energy levels of methylammonium lead iodide are investigated using a combination of ultraviolet photoemission spectroscopy and optical spectroscopy. Our results show that the valence band maximum and conduction band minimum shift down in energy by 110 meV and 77 meV as temperature increases from 28 °C to 85 °C. Density functional theory calculations using slab structures show that the decreased orbital splitting due to thermal expansion is a major contribution to the experimentally observed shift in energy levels. Our results have implications for solar cell performance under operating conditions with continued sunlight exposure and increased temperature.

158 citations

Journal ArticleDOI
TL;DR: In this paper, a waveguide-coupled germanium photodiode with a 3-dB bandwidth of 265 GHz and 240 GHz at a photocurrent of 1 mA is presented.
Abstract: On a scalable silicon technology platform, we demonstrate photodetectors matching or even surpassing state-of-the-art III–V devices. As key components in high-speed optoelectronics, photodetectors with bandwidths greater than 100 GHz have been a topic of intense research for several decades. Solely InP-based detectors could satisfy the highest performance specifications. Devices based on other materials, such as germanium-on-silicon devices, used to lag behind in speed, but enabled complex photonic integrated circuits and co-integration with silicon electronics. Here we demonstrate waveguide-coupled germanium photodiodes with optoelectrical 3-dB bandwidths of 265 GHz and 240 GHz at a photocurrent of 1 mA. This outstanding performance is achieved by a novel device concept in which a germanium fin is sandwiched between complementary in situ-doped silicon layers. Our photodetectors show internal responsivities of 0.3 A W−1 (265 GHz) and 0.45 A W−1 (240 GHz) at a wavelength of 1,550 nm. The internal bandwidth–efficiency product of the latter device is 86 GHz. Low dark currents of 100–200 nA are obtained from these ultra-fast photodetectors. By sandwiching a germanium fin between complementary in situ-doped silicon layers, a waveguide-coupled germanium photodiode with a 3-dB bandwidth of 265 GHz, accompanied by high responsivity and low dark current, is realized.

101 citations

Journal ArticleDOI
Abstract: This paper reviews high-power photodiodes, waveguide photodetectors, and integrated photodiode-antenna emitters with bandwidths up to 150 GHz. Results from heterogeneous III-V photodiodes on silicon and Ge-on-Si photodiode arrays for analog applications are presented.

39 citations

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate high-speed and high-power evanescently coupled waveguide integrated modified unitraveling carrier photodiodes over 105 GHz bandwidth along with 2 and 1.3dBm RF output power at 100 and 105 GHz, respectively.
Abstract: We demonstrate high-speed and high-power evanescently coupled waveguide integrated modified unitraveling carrier photodiodes. Over 105-GHz bandwidth along with 2 and 1.3 dBm RF output power at 100 and 105 GHz, respectively, has been achieved.

37 citations

Journal ArticleDOI
TL;DR: In this paper, the effects of growth mechanisms on the opto-electrical performance of chemically deposited CdS were studied by means of spectroscopic ellipsometry, and coupled with structural, optical, and electrical characterization.

34 citations


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Journal ArticleDOI
TL;DR: In this article, the effect of alloying FA0.85Cs0.15PbI3 with CsPbIsI3 was investigated, and it was shown that the effective tolerance factor can be tuned and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1-xCsxPbisI3 is enhanced.
Abstract: Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–xCsxPbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 p...

1,483 citations

Journal ArticleDOI
TL;DR: In this paper, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed.
Abstract: There are only few semiconducting materials that have been shaping the progress of third generation photovoltaic cells as much as perovskites. Although they are deceivingly simple in structure, the archetypal AMX3-type perovskites have built-in potential for complex and surprising discoveries. Since 2009, a small and somewhat exotic class of perovskites, which are quite different from the common rock-solid oxide perovskite, have turned over a new leaf in solar cell research. Highlighted as one of the major scientific breakthroughs of the year 2013, the power conversion efficiency of the title compound hybrid organic–inorganic perovskite has now exceeded 18%, making it competitive with thin-film PV technology. In this minireview, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed. By analyzing the attainable photocurrent and photovoltage, realizing perovskite solar cells with 20% efficiency for a single junction, and 30% for a tandem configuration on a c-Si solar cell would be realistic.

1,033 citations

Journal ArticleDOI
TL;DR: Inorganic-organic lead-halide perovskite solar cells have reached efficiencies above 22% within a few years of research as discussed by the authors, and achieved photovoltages of > 1.2 V for a material with a bandgap of 1.6 eV.
Abstract: Inorganic-organic lead-halide perovskite solar cells have reached efficiencies above 22% within a few years of research. Achieved photovoltages of >1.2 V are outstanding for a material with a bandgap of 1.6 eV - in particular considering that it is solution processed. Such values demand for low non-radiative recombination rates and come along with high luminescence yields when the solar cell is operated as a light emitting diode. This progress report summarizes the developments on material composition and device architecture, which allowed for such high photovoltages. It critically assesses the term "lifetime", the theories and experiments behind it, and the different recombination mechanisms present. It attempts to condense reported explanations for the extraordinary optoelectronic properties of the material. Amongst those are an outstanding defect tolerance due to antibonding valence states and the capability of bandgap tuning, which might make the dream of low-cost highly efficient solution-processed thin film solar cells come true. Beyond that, the presence of photon recycling will open new opportunities for photonic device design.

425 citations

Journal ArticleDOI
TL;DR: In this article, temperature resolved UV-vis absorption and spectral photocurrent response measurements of MAPbI3 thin films and solar cells, together with ab initio simulations, were used to investigate the changes in material properties occurring across the tetragonal to cubic phase transition.
Abstract: We report temperature resolved UV-vis absorption and spectral photocurrent response measurements of MAPbI3 thin films and solar cells, together with ab initio simulations, to investigate the changes in material properties occurring across the tetragonal to cubic phase transition. We find that the MAPbI3 band-gap does not abruptly change when exceeding the tetragonal to cubic transition temperature, but it rather monotonically blue-shifts following the same temperature evolution observed within the tetragonal phase. Car–Parrinello molecular dynamics simulations demonstrate that the high temperature phase corresponds on average to the expected symmetric cubic structure assigned from XRD measurements, but that the system strongly deviates from such a structure in the sub-picosecond time scale. Thus, on the time scale of electronic transitions, the material seldom experiences a cubic environment, rather an increasingly distorted tetragonal one. This result explains the absence of dramatic changes in the optical of MAPbI3 across the explored temperature range of 270–420 K, which could have important consequences for the practical uptake of perovskite solar cells.

419 citations

Journal Article
TL;DR: In this article, the authors analyzed the electronic structure and optical properties of perovskite solar cells based on CH3NH3PbI3 with the quasiparticle self-consistent GW approximation.
Abstract: The performance of organometallic perovskite solar cells has rapidly surpassed those of both traditional dye-sensitized and organic photovoltaics, e.g. solar cells based on CH3NH3PbI3 have recently reached 18% conversion efficiency. We analyze its electronic structure and optical properties within the quasiparticle self-consistent GW approximation (QSGW ). Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. Several characteristics combine to make the electronic structure of this material unusual. First, there is a strong driving force for ferroelectricity, as a consequence the polar organic moiety CH3NH3. The moiety is only weakly coupled to the PbI3 cage; thus it can rotate give rise to ferroelectric domains. This in turn will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and may contribute to the current-voltage hysteresis found in perovskite solar cells. Second, spin orbit modifies both valence band and conduction band dispersions in a very unusual manner: both get split at the R point into two extrema nearby. This can be interpreted in terms of a large Dresselhaus term, which vanishes at R but for small excursions about R varies linearly in k. Conduction bands (Pb 6p character) and valence bands (I 5p) are affected differently; moreover the splittings vary with the orientation of the moiety. We will show how the splittings, and their dependence on the orientation of the moiety through the ferroelectric effect, have important consequences for both electronic transport and the optical properties of this material.

418 citations