We investigate single-photon generation from individual self-assembled InGaAs quantum dots coupled to the guided optical mode of a GaAs photonic crystal waveguide. By performing confocal microscopy measurements on single dots positioned within the waveguide, we locate their positions with a precision better than 0:5 m. Time-resolved photoluminescence and photon autocorrelation measurements are used to prove the single-photon character of the emission into the propagating waveguide mode. The results obtained demonstrate that such nanostructures can be used to realize an on-chip, highly directed singlephoton source with single-mode spontaneous emission coupling efficiencies in excess of À $ 85% and the potential to reach maximum emission rates >1 GHz.
S-shaped current-voltage (I-V) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I-V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I-V curves have been treated separately. This review covers the main solar cell technologies-silicon, thin film, organic, hybrid-with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I-V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I-V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.Index Terms-Current-voltage characteristics, s-shape, roll over.
COMMUNICATIONshadow loss, and the sheet resistance and absorption losses associated with planar layers that facilitate lateral carrier transport to the grid fi ngers. [ 22,23 ] For high effi ciency silicon heterojunction (HIT) solar cells, contact design requires a trade-off between grid fi nger resistance and the sheet resistance and transmission losses of the transparent conducting oxide (TCO)/ amorphous silicon structures coating the cell front surface. [ 24 ] In this paper, we describe a new front contact design principle that overcomes both shadowing losses and parasitic absorption without reducing the conductivity. By redirecting the scattered light incident on the front contact to the solar cell active absorber layer surface, micrometer-scale triangular crosssection grid fi ngers can perform as effectively transparent and highly conductive front contacts. Previously, researchers have designed light harvesting strings that serve to obliquely refl ect light, which is then redirected into the cell by total internal refl ection from the encapsulation layers. [ 16 ] By contrast our front contact design does not require total internal refl ection at the encapsulation layer. Furthermore in our design, the contact fi ngers are micrometer sized and can be placed very close together such that a TCO with reduced thickness can be used-and in some cases the TCO layer might possibly be omitted completely. We demonstrate with simulations and experimental results that designs utilizing effectively transparent triangular cross-section grid fi ngers rather than conventional front contacts have the potential to provide 99.86% optical transparency while ensuring effi cient lateral transport corresponding to a sheet resistance of 4.8 Ω sq −1 due to their close spacing of only 40 µm. Thus effectively transparent contacts have potential as replacements for both the front grid and TCO layer used, e.g., in HIT solar cells. While related schemes for contacts were envisioned early in the development of photovoltaics technology, [ 25 ] they have not found application in current photovoltaic technology, which is increasingly dominated by high effi ciency silicon photovoltaics. Moreover, the effectively transparent front contact design is conceptually quite general and applicable to almost any other front-contacted solar cell or optoelectronic device. For example, we obtained similar experimental results when applying our structures to InGaP-based solar cells.Figure 1 a,b shows the steady-state electric fi eld magnitude distribution of a freestanding triangular contact and a fl at contact, respectively, with 550 nm monochromatic plane wave illumination normally incident at the top of the simulation cell. For planar grid fi ngers, part of the incident light is refl ected back toward the incidence direction, as is apparent from the high electric fi eld density above the contact plane. By contrast, the triangular cross-section grid fi nger does not exhibit a similar back refl ection, as indicated by the lack of an increased electric
We have investigated the influence of the spectral albedo on the power output of bifacial solar cells. We adapted the Shockley-Queisser radiative flux balance framework to account for a variation of the spectrum and intensity of the incoming light. We find that the ideal band gap and the maximum efficiency depend on the spectral albedo of the surroundings and that optimal cell performance cannot be assessed when only accounting for a spectrally independent albedo. With a spectral albedo model, we predict that the power output for a bifacial silicon solar cell surrounded by green grass is 3.1% higher than for a wavelength-independent albedo, and even 5.2% higher for white sand. We experimentally verify this trend for silicon heterojunction solar cells and we derive the ideal spectral albedo.
Thin, flexible, and efficient silicon solar cells would revolutionize the photovoltaic market and open up new opportunities for PV integration. However, as an indirect semiconductor, silicon exhibits weak absorption for infrared photons and the efficient absorption of the full above bandgap solar spectrum requires careful photon management. This review paper provides an overview on the fundamental physics of light trapping and explains known theoretical limits. Technologies that have been developed to improve light trapping will be discussed, and limitations will be addressed.
The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.
Enhanced current density and efficiency through microscale triangular cross-section effectively transparent contacts (ETCs).
We report on a computational study exploring the design of mesoscale metallic front contacts for solar cells. We investigated silver contact structures with circle, triangle and square cross-sections for various length scales and surface coverages. We found that for 'nanoscale' contacts with widths between 10 nm and 1000 nm, resonant coupling actually impairs light absorption in the semiconductor. Conversely, for 'mesoscale' contact widths > 1000 nm, the light interaction is determined by the geometric shadowing. We find that mesoscale silver contacts with triangular cross-section outperform other nanostructure morphologies in reducing shadow losses and yield contact transparency of >99% percent with sheet resistance <0.2 Ω/sq. Surprisingly, very densely spaced mesoscale silver triangular cross-section contacts can enhance the absorption of thin silicon/silver structures by up to 15% at a front contact coverage of 83%, due to light trapping by the front contact. Such structures can also maintain up to 100% absorption within the silicon, at a front contact coverage of 50%, relative to the same structure without metal.
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