Band offset

About: Band offset is a research topic. Over the lifetime, 2446 publications have been published within this topic receiving 53450 citations.

Designing band offset of a-SiO:H solar cells for very high open-circuit voltage (1.06 V) by adjusting band gap of p–i–n junction

Abstract: We have matched the world’s highest open-circuit voltage (Voc: 1.06 V) achieved to date for a layered structure comprised of a glass/tin oxide (SnO2)/hydrogenated amorphous silicon oxide (a-SiO:H) (p–i–n)/back electrode. For the purposes of this study, we adjusted the band gaps of each layer (p–i–n) to improve overall film quality. Fine-tuning of band profiles with reference to activation energy and optical band gap allowed us to offset the conduction band and the valence band of each layer (p–i–n) and thus improve the built-in potential rather than the electron conductivity, Fourier transform infrared spectroscopy, transmittance or reflectance ratio, resulting in a high Voc. To fully exploit the characteristics of wide-band-gap materials and prevent problems with absorbance, we employed commercially available SnO2 in the front transparent conductive oxide instead of zinc oxide. Using our deposition and evaluation technologies to build a wide-band-gap single solar cell, we succeeded in matching the world’s highest Voc of 1.06 V (Eff: 5.38%, Jsc: 8.15 mA/cm2, FF: 0.624).

Electrical characterization of n-type a-SiGe:H/p-type crystalline-silicon heterojunctions

Abstract: Heterojunctions with an n-type hydrogenated amorphous silicon germanium (a-SiGe:H) on p-type crystalline-silicon heterojunctions were fabricated and electrically characterized. The electrical characterization was made by current density–voltage (J–V) and capacitance–voltage (C–V) measurements. The C–V results confirm the existence of an abrupt heterojunction. The temperature dependence of the J–V curves shows that in the forward conduction at low bias voltage (V 0.5 V), the space charge limited effect becomes the main transport mechanism. The conduction and valence band discontinuities of the heterojunction and the electron affinity of the n-type a-SiGe:H film were calculated using Anderson's model. Under reverse bias conditions the J–V curves suggest that the current density is limited by hopping through the localized states into the gap. One-dimensional (1D) simulations support the proposed transport mechanisms.

Quantum structures for multiband photon detection

Abstract: The work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Band Offset Reduction at Defect-Rich p/i Interface Through a Wide Bandgap a-SiO:H Buffer Layer

Abstract: In single junction p-i-n solar cells, the optical losses can be mitigated by inserting the wide band gap amorphous silicon oxide layer at the defect-rich p/i interface. In this paper, a simulation and experimental study on the performance of p-i-n solar cells by inserting the intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) buffer layer at the p/i interface is reported. The i-a-SiO:H film has been deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) (13.6 MHz) at a low substrate temperature of approximately 230°C. The p/i interface is crucial to solar cell performance because the first few nanometers of the intrinsic layer are a defect-rich layer, having the band gap discontinuity, resulting in band offset. Thus, the carrier recombination probability increases in the vicinity of p/i interface because of high defect density and short carrier lifetime. Aided by optically calibrated simulations and with the support of experimental results, this study shows that a wide band gap thin undoped a-SiO:H buffer layer with higher photoconductivity reduces the band-gap offset and minimizes the recombination of photogenerated charge carriers at the defect-rich p/i interface. It has also been found that the a-SiO:H buffer layer augmented the electric field inside the device. As a result, the overall performance of the a-Si:H-based single junction solar cell has significantly improved. By employing a ∼5 nm thick a-SiO:H buffer layer, the blue response of the cell has been improved, resulting in 7.34% and 18.62% enhancement in fill factor (FF) and power conversion efficiency ( η ), respectively, as compared to the buffer-less cell.