Suppression Of Surface Recombination Research Articles

Photoluminescence properties of heavily Sb doped Ge1−x Sn x and heterostructure design favorable for n+-Ge1−x Sn x active layer

We investigated the photoluminescence (PL) properties of heavily Sb doped n+-Ge1−x Sn x layers and demonstrated the formation of a double heterostructure (DHS) for the n+-Ge1−x Sn x active layer. A single PL peak was observed for n+-Ge1−x Sn x layers thicker than 80 nm with increasing the Sb concentration up to 1020 cm−3, attributed to the superior crystallinity and pseudo-direct bandgap transition mechanism, while a 15 nm thick n+-Ge1−x Sn x layer did not exhibit PL signals. A favorable heterostructure for n+-Ge1−x Sn x is proposed from the viewpoint of the increased valence band offset (ΔE v) using n-Si y Ge1−y as the cladding layer. We demonstrated the formation of an n-Si y Ge1−y (15 nm)/n+-Ge1−x Sn x (15 nm)/n-Si y Ge1−y (15 nm) DHS with a superior crystallinity and high PL peak intensity comparable to that of a thick n+-Ge1−x Sn x . We discuss the reasons for the PL performance improvement by forming the DHS, including the sufficient carrier confinement and the suppression of surface recombination.

Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells.

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Investigation of effective near-infrared light-trapping structure with submicron diameter for crystalline silicon thin film solar cells

To obtain high efficiency in crystalline silicon thin-film solar cells, it is necessary to develop a novel light-trapping structure that not only has high light absorptance and can be fabricated with a small etching margin but also has a small surface area for a small effect of surface recombination. In this study, optical properties of silicon nanowires (SiNWs) with submicron diameters were investigated by the finite-difference time-domain (FDTD) method in the infrared wavelength region, in which the absorption coefficient of crystalline Si is quite low and the increase in optical path length is very important. To verify the simulation results, SiNWs with submicron diameters were successfully fabricated by the metal-assisted chemical etching method using two types of etching masks. The optical properties and minority carrier lifetime of the obtained structures were measured, and the results suggest that SiNWs with submicron diameters such as ∼700 nm are effective for both light trapping and the suppression of surface recombination.

Suppressing the electron–hole recombination rate in hematite photoanode with a rapid cooling treatment

The preparation of hematite photoanodes for photoelectrochemical water splitting reactions always involves a high-temperature annealing process. The cooling rate during the annealing process is discovered as an important factor deciding the anode performance. Rapid cooling of the hematite photoanode increased the photocurrent from 0.635mAcm−2 for the normally cooled sample to 0.856mAcm−2 at 1.23V vs. RHE. The rapidly cooled hematite nanowires showed decreased diameter, reduced surface disorder, Fe partial reduction, and decreased number of surface OH groups simultaneously with a red shift of light absorption. Both the charge separation and injection efficiencies are improved, confirming the simultaneous decrease of the bulk and surface electron–hole recombination rates. The bulk electron–hole recombination rate depends on the diameter of the nanowires, while the surface electron–hole recombination rate is lowered due to the reduced surface trap states. The photocurrent measurement results with H2O2 as the sacrificial agent indicate that the suppression of surface recombination is more important than that of bulk recombination. In contrast, slow cooling or rapid heating exhibits a negative effect on the performance of the hematite photoanode.

Nanoarchitectured Semiconducting Photoelectrodes for Enhanced Stability and Photon Conversion Efficiency

The characteristics of two dimensional (graphene), one dimensional (single walled carbon nanotube incorporated poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, composite) and zero dimensional (ZnO quantum dots) nanostructures have been systematically analyzed to elucidate the possibility of employing such nanostructures (transparent conducting corrosion resistant (TCCR)) for the passivation of the surface of a photoelectrode. Thin layer of graphene, carbon nanotubes embedded on a conductive polymer matrix (PEDOT:PSS) and ZnO quantum dots are incorporated on the surface of photoelectrode namely ‘In’ rich CuInS2 (ICIS) to act as corrosion resistant layers. The TCCR layers are found to be effective in inhibiting the surface recombination of charge carriers. Graphene exhibits a higher tendency towards the suppression of surface recombination across electrode-electrolyte interface, where an enhancement by a factor of 104 s is observed in the electron recombination life time. SWCNT/PEDOT:PSS composite films when employed as transparent conducting corrosion resistant layers enabled an enhancement of the cell stability by 95% when compared to unprotected ‘In’ rich CuInS2 electrode. The TCCR layers are found have a positive impact on the conversion efficiency of the photoelectrode and the maximum enhancement in the conversion efficiency achieved is 71% with ZnO quantum dot sensitized electrodes.

Suppression of surface recombination in CuInSe2 (CIS) thin films via Trioctylphosphine Sulfide (TOP:S) surface passivation

CuInSe2 (CIS) solar cells are promising candidates for thin film photovoltaic applications, one key limitation in their performance is surface recombination in these thin films. We demonstrate that passivating CIS films with Trioctylphosphine Sulfide (TOP:S) solution increases photoluminescence (PL) intensity by a factor of ∼30, which suggests that this passivation significantly reduces surface recombination. X-ray photoelectron spectroscopy (XPS) reveals that TOP:S forms both –S and –P bonds on the CIS film surface, which leads to a ∼4-fold increase in the surface Na peak intensity. This value is significantly higher than what would be expected from high temperature annealing alone, which has been linked to improvements in surface morphology and device efficiency in CIGS solar cells. We use Energy-Dispersive X-ray Spectroscopy (EDS) to measure the solid-state transport of Na within CIS films with and without passivation. EDS spectra on CIS film cross-sections reveals a saddle-shaped Na profile in the as-fabricated films and a concentration gradient towards the film surface in the passivated films, with 20% higher surface Na content compared with the unpassivated films. We employ Hybrid (B3PW91) Density Functional Theory (DFT) to gain insight into energetics of Na defects, which demonstrate a driving force for Na diffusion from bulk towards the surface. DFT Calculations with TOP:S-like molecules on the same surfaces reveal a ∼ 1eV lower formation energy for the NaCu defect. The experiments and computations in this work suggest that TOP:S passivation promotes Na diffusion towards CIS film surfaces and stabilizes surface Na defects, which leads to the observed substantial decrease in surface recombination.

Investigation of post-annealing enhancement effect of passivation quality of hydrogenated amorphous silicon

The excellent surface passivation scheme for suppression of surface recombination is a basic prerequisite to obtain high efficiency solar cells. Particularly, the HIT (heterojunction with intrinsic thin-layer) solar cell, which possesses an abrupt discontinuity of the crystal network at an interface between the crystalline silicon (c-Si) surface and the hydrogenated amorphous silicon (a-Si:H) thin film, usually causes a large density of defects in the bandgap due to a high density of dangling bonds, so it is very important for high energy conversion efficiency to obtain millisecond (ms) range of minority carrier lifetime (i. e. 2 ms). The a-Si:H, due to its excellent passivation properties obtained at low deposition temperatures and also mature processing, is still the best candidate materials for silicon HIT solar cell. Deposition of a transparent conductive oxide (TCO), such as indium tin oxide (ITO), has to be used to improve the carrier transport, since the lateral conductivity of a-Si:H is very poor. Usually, ITO is deposited by magnetron sputtering, but damage of a-Si:H layers by sputtering-induced ion bombardment inevitably occurs, thus triggering the serious degradation of the minority carrier lifetime, i. e., a loss in wafer passivation. Fortunately, this damage can be often recovered by some post-annealing. In this paper, however, the situation is different, and it is found that the minority carrier lifetime of ITO/a-Si:H/c-Si/a-Si:H heterojunction has been drastically enhanced by post-annealing after sputtering ITO on a- Si:H/c-Si/a-Si:H heterojunction (from 1.7 ms to 4.0 ms), not just recovering. It is very important to investigate how post-annealing enhances the lifetime and its physics nature. Combining the two experimental ways of HF treatment and vacuum annealing, three possible reasons for this enhancement effect (the field effect at the ITO/a-Si:H interface, the surface reaction-layer resulting from annealing in air, and the optimization of a-Si:H material itself) have been studied, suggesting this is irrelevant to the first two. The influence of post-annealing on a-Si:H/c-Si/a-Si:H heterojunction deposited at different temperatures has also been investigated. It is found that the remarkable enhancement effect of post-annealing is for low growth temperature(175 ℃) and not for high growth temperature(200 ℃), with the confirmation of an effective way for high quality passivation using growth at low temperature and then annealed at high temperature. Moreover, the configuration of a-Si:H at different growth temperatures between afore and after annealing has been discussed by an application of Fourier transform infrared (FTIR) spectroscopy. It is shown that the large increase of the lifetime of the heterojunction after annealing results from the improvement of microstructure of a-Si:H itself, which is essentially a competitive balance of the dominant role of some micro-factors, including hydrogen content, hydrogen bonding and network disorder in amorphous silicon film determined by the optimized matching between the growth temperature of a-Si:H materials and the annealing temperature of the heterojunction. An optimum control for this balance point is the essential cause of lifetime enhancement.

Suppression of surface recombination in surface plasmon coupling with an InGaN/GaN multiple quantum well sample

Temperature-dependent picosecond non-degenerate four-wave-mixing experiments were performed to explore the carrier dynamics in an InGaN/GaN multiple quantum well sample, in which light emission enhancement with surface plasmon (SP) coupling has been identified. In the time-resolved photoluminescence results, we can identify the faster carrier decay time of the sample with surface plasmon coupling. The faster decay time is due to this sample's ability to create additional channels for effective carrier recombination. In the four-wave-mixing results, a slower grating decay time of the sample with surface plasmon coupling was measured. The diffusion coefficients and surface recombination velocities of photo-created carriers were estimated by modeling the decay rate of transient grating signals. For the sample for which surface plasmon coupling exists, smaller diffusion coefficients and slower surface recombination velocities can be estimated when the temperatures are above 150 K. The carriers coupling with some SP modes is not the only mechanism contributing to emission enhancement. In the InGaN/GaN multiple quantum well sample, surface recombination suppressed by SP coupling is another factor for increased light emission efficiency.

Further Suppression of Surface-Recombination of an InGaP/GaAs HBT by Conformal Passivation

Conformal passivation on an InGaP/GaAs HBT with significant reduction in the base surface-recombination effect is demonstrated. Not only dc behaviors but also RF performances are remarkably improved compared with the conventional emitter-ledge structure. Based on the conformal passivation, i.e., the base surface is covered by the depleted InGaP ledge structure and sulfur ((NH4)2Sx ) treatment, lower base surface-recombination current density, lower specific contact resistance, lower sheet resistance, higher current gain, higher collector current, and higher maximum oscillation frequency are obtained

Ultrafast magnetophotoconductivity of semi-insulating gallium arsenide

The speed of opto-electronic switches is increased or decreased by the application of a magnetic field. This is achieved by inducing a carrier drift toward or away from the semiconductor surface, resulting in the enhancement or suppression of surface recombination. We establish that surface recombination plays a major role in determining the speed of the opto-electronic switch.