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An 80-GHz DCO utilizing improved SC ladder and promoted DCTL-based hybrid tuning banks
Lu Tang, Yi Chen, Kui Wang
, Available online  

Abstract Full Text PDF

An 80-GHz DCO based on modified hybrid tuning banks is introduced in this paper. To achieve sub-MHz frequency resolution with reduced circuit complexity, the improved circuit topology replaces the conventional circuit topology with two binary-weighted SC cells, enabling eight SC-cell-based improved SC ladders to achieve the same fine-tuning steps as twelve SC-cell-based conventional SC ladders. To achieve lower phase noise and smaller chip size, the promoted binary-weighted digitally controlled transmission lines (DCTLs) are used to implement the coarse and medium tuning banks of the DCO. Compared to the conventional thermometer-coded DCTLs, control bits of the proposed DCTLs are reduced from 30 to 8, and the total length is reduced by 34.3% (from 122.76 to 80.66 μm). Fabricated in 40-nm CMOS, the DCO demonstrated in this work features a small fine-tuning step (483 kHz), a high oscillation frequency (79-85 GHz), and a smaller chip size (0.017 mm2). Compared to previous work, the modified DCO exhibits an excellent figure of merit with an area (FoMA) of -198 dBc/Hz.

An 80-GHz DCO based on modified hybrid tuning banks is introduced in this paper. To achieve sub-MHz frequency resolution with reduced circuit complexity, the improved circuit topology replaces the conventional circuit topology with two binary-weighted SC cells, enabling eight SC-cell-based improved SC ladders to achieve the same fine-tuning steps as twelve SC-cell-based conventional SC ladders. To achieve lower phase noise and smaller chip size, the promoted binary-weighted digitally controlled transmission lines (DCTLs) are used to implement the coarse and medium tuning banks of the DCO. Compared to the conventional thermometer-coded DCTLs, control bits of the proposed DCTLs are reduced from 30 to 8, and the total length is reduced by 34.3% (from 122.76 to 80.66 μm). Fabricated in 40-nm CMOS, the DCO demonstrated in this work features a small fine-tuning step (483 kHz), a high oscillation frequency (79-85 GHz), and a smaller chip size (0.017 mm2). Compared to previous work, the modified DCO exhibits an excellent figure of merit with an area (FoMA) of -198 dBc/Hz.
Exploration of high-speed 3.0 THz imaging with a 65 nm CMOS process
Min Liu, Ziteng Cai, Jian Liu, Nanjian Wu, Liyuan Liu
, Available online  

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This paper describes a promising route for the exploration and development of 3.0 THz sensing and imaging with FET-based power detectors in a standard 65 nm CMOS process. Based on the plasma-wave theory proposed by Dyakonov and Shur, we designed high-responsivity and low-noise multiple detectors for monitoring a pulse-mode 3.0 THz quantum cascade laser (QCL). Furthermore, we present a fully integrated high-speed 32×32-pixel 3.0 THz CMOS image sensor (CIS). The full CIS measures 2.81×5.39 mm2 and achieves a 423 V/W responsivity (Rv) and a 5.3 nW integral noise equivalent power (NEP) at room temperature. In experiments, we demonstrate a testing speed reaching 319 fps under continuous-wave (CW) illumination of a 3.0 THz QCL. The results indicate that our terahertz CIS has excellent potential in cost-effective and commercial THz imaging and material detection.

This paper describes a promising route for the exploration and development of 3.0 THz sensing and imaging with FET-based power detectors in a standard 65 nm CMOS process. Based on the plasma-wave theory proposed by Dyakonov and Shur, we designed high-responsivity and low-noise multiple detectors for monitoring a pulse-mode 3.0 THz quantum cascade laser (QCL). Furthermore, we present a fully integrated high-speed 32×32-pixel 3.0 THz CMOS image sensor (CIS). The full CIS measures 2.81×5.39 mm2 and achieves a 423 V/W responsivity (Rv) and a 5.3 nW integral noise equivalent power (NEP) at room temperature. In experiments, we demonstrate a testing speed reaching 319 fps under continuous-wave (CW) illumination of a 3.0 THz QCL. The results indicate that our terahertz CIS has excellent potential in cost-effective and commercial THz imaging and material detection.
Pressure manipulation of ultrafast carrier dynamics in monolayer WS2
Yao Li, Haiou Zhu, Zongpeng Song
, Available online  

Abstract Full Text PDF

Two-dimensional transition metal dichalcogenides (TMDs) have intriguing physic properties and offer an exciting platform to explore many features that are important for future devices. In this work, we synthesized monolayer WS2 as an example to study the optical response with hydrostatic pressure. The Raman results show a continuous tuning of the lattice vibrations that is induced by hydrostatic pressure. We further demonstrate an efficient pressure-induced change of the band structure and carrier dynamics via transient absorption measurements. We found that two time constants can be attributed to the capture process of two kinds of defect states, with the pressure increasing from 0.55 GPa to 2.91 GPa, both of capture processes were accelerated, and there is an inflection point within the pressure range of 1.56 GPa to 1.89 GPa. Our findings provide valuable information for the design of future optoelectronic devices.

Two-dimensional transition metal dichalcogenides (TMDs) have intriguing physic properties and offer an exciting platform to explore many features that are important for future devices. In this work, we synthesized monolayer WS2 as an example to study the optical response with hydrostatic pressure. The Raman results show a continuous tuning of the lattice vibrations that is induced by hydrostatic pressure. We further demonstrate an efficient pressure-induced change of the band structure and carrier dynamics via transient absorption measurements. We found that two time constants can be attributed to the capture process of two kinds of defect states, with the pressure increasing from 0.55 GPa to 2.91 GPa, both of capture processes were accelerated, and there is an inflection point within the pressure range of 1.56 GPa to 1.89 GPa. Our findings provide valuable information for the design of future optoelectronic devices.
Efficiency-loss analysis of monolithic perovskite/silicon tandem solar cells by identifying the patterns of a dual two-diode model’s current-voltage curves
Yuheng Zeng, Zetao Ding, Zunke Liu, Wei Liu, Mingdun Liao, Xi Yang, Zhiqin Ying, Jingsong Sun, Jiang Sheng, Baojie Yan, Haiyan He, Chunhui Shou, Zhenhai Yang, Jichun Ye
, Available online  

Abstract Full Text PDF

In this work, we developed a simple and direct circuit model with a dual two-diode model that can be solved by a SPICE numerical simulation to comprehensively describe the monolithic perovskite/crystalline silicon (PVS/c-Si) tandem solar cells. We are able to reveal the effects of different efficiency-loss mechanisms based on the illuminated current density-voltage (J-V), semi-log dark J-V, and local ideality factor (m-V) curves. The effects of the individual efficiency-loss mechanism on the tandem cell’s efficiency are discussed, including the exp(V/VT) and exp(V/2VT) recombination, the whole cell’s and subcell’s shunts, and the ohmic-contact or Schottky-contact of the intermediate junction. We can also fit a practical J-V curve and find a specific group of parameters by the trial-and-error method. Although the fitted parameters are not a unique solution, they are valuable clues for identifying the efficiency loss with the aid of the cell’s structure and experimental processes. This method can also serve as an open platform for analyzing other tandem solar cells by substituting the corresponding circuit models. In summary, we developed a simple and effective methodology to diagnose the efficiency-loss source of a monolithic PVS/c-Si tandem cell, which is helpful to researchers who wish to adopt the proper approaches to improve their solar cells.

In this work, we developed a simple and direct circuit model with a dual two-diode model that can be solved by a SPICE numerical simulation to comprehensively describe the monolithic perovskite/crystalline silicon (PVS/c-Si) tandem solar cells. We are able to reveal the effects of different efficiency-loss mechanisms based on the illuminated current density-voltage (J-V), semi-log dark J-V, and local ideality factor (m-V) curves. The effects of the individual efficiency-loss mechanism on the tandem cell’s efficiency are discussed, including the exp(V/VT) and exp(V/2VT) recombination, the whole cell’s and subcell’s shunts, and the ohmic-contact or Schottky-contact of the intermediate junction. We can also fit a practical J-V curve and find a specific group of parameters by the trial-and-error method. Although the fitted parameters are not a unique solution, they are valuable clues for identifying the efficiency loss with the aid of the cell’s structure and experimental processes. This method can also serve as an open platform for analyzing other tandem solar cells by substituting the corresponding circuit models. In summary, we developed a simple and effective methodology to diagnose the efficiency-loss source of a monolithic PVS/c-Si tandem cell, which is helpful to researchers who wish to adopt the proper approaches to improve their solar cells.
Highly efficient organic solar cells with improved stability enabled by ternary copolymers with antioxidant side chains
Ao Song, Qiri Huang, Chunyang Zhang, Haoran Tang, Kai Zhang, Chunchen Liu, Fei Huang, Yong Cao
, Available online  

Abstract Full Text PDF

The stability of organic solar cells (OSCs) remains a major concern for their ultimate industrialization due to the photo, oxygen, and water susceptibility of organic photoactive materials. Usually, antioxidant additives are blended as radical scavengers into the active layer. However, it will induce the intrinsic morphology instability and adversely affect the efficiency and long-term stability. Herein, the antioxidant dibutylhydroxytoluene (BHT) group has been covalently linked onto the side chain of benzothiadiazole (BT) unit, and a series of ternary copolymers D18-Cl-BTBHTx (x = 0, 0.05, 0.1, 0.2) with varied ratio of BHT-containing side chains have been synthesized. It was found that the introduction of BHT side chains would have a negligible effect on the photophysical properties and electronic levels, and the D18-Cl-BTBHT0.05: Y6-based OSC achieved the highest power conversion efficiency (PCE) of 17.6%, which is higher than those based active layer blended with BHT additives. More importantly, the unencapsulated device based on D18-Cl-BTBHTx (x = 0.05, 0.1, 0.2) retained approximately 50% of the initial PCE over 30 hours operation under ambient conditions, significantly outperforming the control device based on D18-Cl (90% degradation in PCE after 30 h). This work provides a new structural design strategy of copolymers for OSCs with simultaneously improved efficiency and stability.

The stability of organic solar cells (OSCs) remains a major concern for their ultimate industrialization due to the photo, oxygen, and water susceptibility of organic photoactive materials. Usually, antioxidant additives are blended as radical scavengers into the active layer. However, it will induce the intrinsic morphology instability and adversely affect the efficiency and long-term stability. Herein, the antioxidant dibutylhydroxytoluene (BHT) group has been covalently linked onto the side chain of benzothiadiazole (BT) unit, and a series of ternary copolymers D18-Cl-BTBHTx (x = 0, 0.05, 0.1, 0.2) with varied ratio of BHT-containing side chains have been synthesized. It was found that the introduction of BHT side chains would have a negligible effect on the photophysical properties and electronic levels, and the D18-Cl-BTBHT0.05: Y6-based OSC achieved the highest power conversion efficiency (PCE) of 17.6%, which is higher than those based active layer blended with BHT additives. More importantly, the unencapsulated device based on D18-Cl-BTBHTx (x = 0.05, 0.1, 0.2) retained approximately 50% of the initial PCE over 30 hours operation under ambient conditions, significantly outperforming the control device based on D18-Cl (90% degradation in PCE after 30 h). This work provides a new structural design strategy of copolymers for OSCs with simultaneously improved efficiency and stability.
Self-healing wearable self-powered deep ultraviolet photodetectors based on Ga2O3
Chao Wu, Huaile He, Haizheng Hu, Aiping Liu, Shunli Wang, Daoyou Guo, Fengmin Wu
, Available online  

Abstract Full Text PDF

Gallium oxide (Ga2O3) based flexible heterojunction type deep ultraviolet (UV) photodetectors show excellent solar-blind photoelectric performance, even when not powered, which makes them ideal for use in intelligent wearable devices. However, traditional flexible photodetectors are prone to damage during use due to poor toughness, which reduces the service life of these devices. Self-healing hydrogels have been demonstrated to have the ability to repair damage and their combination with Ga2O3 could potentially improve the lifetime of the flexible photodetectors while maintaining their performance. Herein, a novel self-healing and self-powered flexible photodetector has been constructed onto the hydrogel substrate, which exhibits an excellent responsivity of 0.24 mA/W under 254 nm UV light at zero bias due to the built-in electric field originating from the PEDOT: PSS/Ga2O3 heterojunction. The self-healing of the Ga2O3 based photodetector was enabled by the reversible property of the synthesis of agarose and polyvinyl alcohol double network, which allows the photodetector to recover its original configuration and function after damage. After self-healing, the photocurrent of the photodetector decreases from 1.23 to 1.21 μA, while the dark current rises from 0.95 to 0.97 μA, with a barely unchanged of photoresponse speed. Such a remarkable recovery capability and the photodetector’s superior photoelectric performance not only significantly enhance a device lifespan but also present new possibilities to develop wearable and intelligent electronics in the future.

Gallium oxide (Ga2O3) based flexible heterojunction type deep ultraviolet (UV) photodetectors show excellent solar-blind photoelectric performance, even when not powered, which makes them ideal for use in intelligent wearable devices. However, traditional flexible photodetectors are prone to damage during use due to poor toughness, which reduces the service life of these devices. Self-healing hydrogels have been demonstrated to have the ability to repair damage and their combination with Ga2O3 could potentially improve the lifetime of the flexible photodetectors while maintaining their performance. Herein, a novel self-healing and self-powered flexible photodetector has been constructed onto the hydrogel substrate, which exhibits an excellent responsivity of 0.24 mA/W under 254 nm UV light at zero bias due to the built-in electric field originating from the PEDOT: PSS/Ga2O3 heterojunction. The self-healing of the Ga2O3 based photodetector was enabled by the reversible property of the synthesis of agarose and polyvinyl alcohol double network, which allows the photodetector to recover its original configuration and function after damage. After self-healing, the photocurrent of the photodetector decreases from 1.23 to 1.21 μA, while the dark current rises from 0.95 to 0.97 μA, with a barely unchanged of photoresponse speed. Such a remarkable recovery capability and the photodetector’s superior photoelectric performance not only significantly enhance a device lifespan but also present new possibilities to develop wearable and intelligent electronics in the future.
Experimental investigation on the instability for NiO/β-Ga2O3 heterojunction-gate FETs under negative bias stress
Zhuolin Jiang, Xiangnan Li, Xuanze Zhou, Yuxi Wei, Jie Wei, Guangwei Xu, Shibing Long, Xiaorong Luo
, Available online  

Abstract Full Text PDF

A NiO/β-Ga2O3 heterojunction-gate field effect transistor (HJ-FET) is fabricated and its instability mechanisms are experimentally investigated under different gate stress voltage (VG,s) and stress times (ts). Two different degradation mechanisms of the devices under negative bias stress (NBS) are identified. At low VG,s for a short ts, NiO bulk traps trapping/de-trapping electrons are responsible for decrease/recovery of the leakage current, respectively. At higher VG,s or long ts, the device transfer characteristic curves and threshold voltage (VTH) are almost permanently negatively shifted. This is because the interface dipoles are almost permanently ionized and neutralize the ionized charges in the space charge region (SCR) across the heterojunction interface, resulting in a narrowing SCR. This provides an important theoretical guide to study the reliability of NiO/β-Ga2O3 heterojunction devices in power electronic applications.

A NiO/β-Ga2O3 heterojunction-gate field effect transistor (HJ-FET) is fabricated and its instability mechanisms are experimentally investigated under different gate stress voltage (VG,s) and stress times (ts). Two different degradation mechanisms of the devices under negative bias stress (NBS) are identified. At low VG,s for a short ts, NiO bulk traps trapping/de-trapping electrons are responsible for decrease/recovery of the leakage current, respectively. At higher VG,s or long ts, the device transfer characteristic curves and threshold voltage (VTH) are almost permanently negatively shifted. This is because the interface dipoles are almost permanently ionized and neutralize the ionized charges in the space charge region (SCR) across the heterojunction interface, resulting in a narrowing SCR. This provides an important theoretical guide to study the reliability of NiO/β-Ga2O3 heterojunction devices in power electronic applications.
Large-area β-Ga2O3 Schottky barrier diode and its application in DC–DC converters
Wei Guo, Zhao Han, Xiaolong Zhao, Guangwei Xu, Shibing Long
, Available online  

Abstract Full Text PDF

We demonstrate superb large-area vertical β-Ga2O3 SBDs with a Schottky contact area of 1 × 1 mm2 and obtain a high-efficiency DC–DC converter based on the device. The β-Ga2O3 SBD can obtain a forward current of 8 A with a forward voltage of 5 V, and has a reverse breakdown voltage of 612 V. The forward turn-on voltage (VF) and the on-resistance (Ron) are 1.17 V and 0.46 Ω, respectively. The conversion efficiency of the β-Ga2O3 SBD-based DC–DC converter is 95.81%. This work indicates the great potential of Ga2O3 SBDs and relevant circuits in power electronic applications.

We demonstrate superb large-area vertical β-Ga2O3 SBDs with a Schottky contact area of 1 × 1 mm2 and obtain a high-efficiency DC–DC converter based on the device. The β-Ga2O3 SBD can obtain a forward current of 8 A with a forward voltage of 5 V, and has a reverse breakdown voltage of 612 V. The forward turn-on voltage (VF) and the on-resistance (Ron) are 1.17 V and 0.46 Ω, respectively. The conversion efficiency of the β-Ga2O3 SBD-based DC–DC converter is 95.81%. This work indicates the great potential of Ga2O3 SBDs and relevant circuits in power electronic applications.