The simulation and optimization of the internal quantum efficiency of GaSb thermophotovoltaic cells with a box-shaped Zn diffusion profile (original) (raw)
Performance Improvement of the GaSb Thermophotovoltaic Cells With n-Type Emitters
GaSb cells are commonly fabricated using Zn diffusion into n-GaSb, and in this paper they have been designed inversely using Te diffusion into unintentionally p-doped GaSb. Numerical simulation is used to analyze the cell performance. We found that a GaSb cell with n-type emitters showed a significantly higher output power density compared with that of the cell with p-type emitters under 1500 K-blackbody radiation. The performance improvement is owing to the good matching of the diffusion length of minority carriers with the depth of their moving regions. Several parameters that affect the cell performance were analyzed, such as doping depth, substrate thickness, and surface recombination velocity. The cell performance was evaluated under low temperature radiations and found to have huge potentials for the use in low-temperature thermophotovoltaic systems.
Journal of Electronic Materials, 1999
The GaInSb material system is attractive for application in thermophotovoltaic (TPV) cells since its band gap can be tuned to match the radiation of the emitter. At present, most of the TPV cells are fabricated using epitaxial layers and hence are expensive. To reduce the cost, Zn diffusion using elemental vapors in a semiclosed diffusion system is being pursued by several laboratories. In this paper, we present studies carried out on Zn diffusion into n-type (Te-doped) GaSb substrates in an open tube diffusion furnace. The dopant precursor was a 2000Å thick, zinc doped spin-on glass. The diffusion was carried out at temperatures ranging from 550 to 600°C, for times from 1 to 10 h. The diffused layers were characterized by Hall measurements using step-and-repeat etching by anodic oxidation, secondary ion mass spectrometry measurements, and TPV device fabrication. For diffusion carried out at 600°C, the junction depth was 0.3 µm, and the hole concentration near the surface was 5 × 10 19 /cm 3 . The external quantum efficiency, measured without any anti-reflection coating of the TPV cells fabricated using mesa-etching had a maximum value of 38%. Masked diffusion was also carried out by opening windows in a Si 3 N 4 coated, GaSb wafer. TPV cells fabricated on these structures had similar quantum efficiency, but lower dark current.
Solar Energy Materials and Solar Cells, 2019
N-type vapor diffusions in p-GaSb wafers is investigated using the group-VI elements S, Se and Te as diffusion sources. The group-VI elements are found difficult to diffuse in bare p-GaSb wafers because they react with Ga atoms to form compounds, thus hindering the diffusion. Deposition of a SiO layer on the p-GaSb surface is found to be crucial for realizing n-type diffusion. With this SiO coating layer, the concentration of group-VI atoms decreased to a low value limiting the reaction with Ga atoms. Se atoms are found to be good n-type diffusion sources. GaSb thermophotovoltaic cells with n-on-p structures are fabricated using Se diffusion. Compared with a traditional GaSb cell with a p-on-n structure, the cell with the n-on-p structure had a higher quantum efficiency at wavelengths beyond 1000 nm. The output power density was also 1.42 times higher compared to the traditional cell when measured with a 1300°C-SiN ceramic IR emitter.
Journal of Electronic Materials, 2003
A single-step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion pro le appropriate for high-ef ciency GaSb thermophotovoltaic (TPV) cells. The junction depth was controlled through monitoring of light current-voltage (I-V) curves (photovoltaic response) during the post-diffusion emitter-etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum ef ciencies (QEs) and the open-circuit voltages in the fabricated devices. An optimum junction depth for obtaining the highest QE and open-circuit voltage is presented based on diffusion lengths (or minority carrier lifetimes), carrier mobility, and the typical diffused impurity pro le in GaSb.
Thermophotovoltaic GaSb Cells Fabrication and Characterisation
For production of highly effective photoconverters semiconductor materials with strictly determined parameters are required. For thermophotovoltaic (TPV) GaSb cells homogeneous Te-doping level of (2-7).1017 cm-3 in the bulk semiconductor is required to produce high efficient PV cells by the Zn diffusion process. In this paper we present data on investigation of the performance of the cells obtained on different GaSb:Te wafers of (100) and (211) orientation. Based on classical I/V measurements and external quantum efficiency (EQE) curves, we analyze cell performances in order to improve all fabrication stages like wafer surface preparation, p-type GaSb emitter elaboration by the zinc diffusion process, antireflection coating deposition and contact realization. Today good performances are obtained on both 3.5 × 3.5 mm2 (211) and 10×10 mm2 (100) GaSb cells. We obtained EQE of 70-76 % in the 800-1600 nm range for the first one and 80-88 % in the same spectrum for the second one. Electri...
Short interval open tube diffusion of Zn in GaAs at low temperatures
Semiconductor Science and Technology, 1999
Short interval open tube Zn diffusion in GaAs at low temperatures is studied for application to heterostructure lasers. The electrochemical capacitance voltage (ECV) profiling technique is used to obtain the carrier concentration versus depth profiles for Zn diffused samples. Diffusion rate is found to be somewhat different from the values obtained for longer durations employing similar techniques. Results are applied to improve the ohmic contact quality for Al-free semiconductor lasers grown in our laboratory. No deterioration is observed in the light versus current (L-I ) characteristics of these devices fabricated after Zn diffusion.
Evidence of enhanced Zn-diffusion observed during the growth of Inverted Metamorphic Solar Cells
2019 IEEE 46th Photovoltaic Specialists Conference (PVSC)
Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction solar cell. In this work, we show an important Zn-diffusion from the AlGaInP back-surface-field layer to the emitter of the GaInP top cell of an inverted multijunction solar cell. Through the analysis of different doping profiles, we provide strong evidence that the diffusion mechanism is (1) triggered by the growth of the tunnel junction cathode and (2) involves point defects. We analyze the implications of Zn-diffusion on the bandgap, the rear-passivation and the minority carrier quality of the GaInP solar subcell by relating the electrical performance of different samples to its corresponding doping profile.
Direct Auger recombination and density-dependent hole diffusion in InN
Scientific Reports, 2018
Indium nitride has a good potential for infrared optoelectronics, yet it suffers from fast nonradiative recombination, the true origin of which has not been established with certainty. The diffusion length of free carriers at high densities is not well investigated either. Here, we study carrier recombination and diffusion using the light-induced transient grating technique in InN epilayers grown by pulsed MOCVD on c-plane sapphire. We show that direct Auger recombination governs the lifetime of carriers at densities above ~10 18 cm −3. The measured Auger recombination coefficient is (8 ± 1) × 10 −29 cm −3. At carrier densities above ~5 × 10 19 cm −3 , we observe the saturation of Auger recombination rate due to phase space filling. The diffusion coefficient of holes scales linearly with carrier density, increasing from 1 cm 2 /s in low-doped layers at low excitations and up to ~40 cm 2 /s at highest carrier densities. The resulting carrier diffusion length remains within 100-300 nm range, which is comparable to the light absorption depth. This feature is required for efficient carrier extraction in bipolar devices, thus suggesting MOCVD-grown InN as the material fit for photovoltaic and photonic applications. Indium nitride with a direct band gap of 0.7 eV 1 is an attractive material for infrared optoelectronics. However, InN layers of high quality are difficult to obtain. In addition to structural problems, InN suffers from high residual electron density (n 0) caused by abundant point defects. n 0 can be diminished by growing thick InN layers using molecular beam epitaxy (MBE) 2 , but this is an expensive and hardly scalable approach. Other growth techniques were also employed, including metalorganic chemical vapor deposition (MOCVD) 3 , chemical vapor deposition 4 , sputtering 5 , or even sol-gel spin coating 6. The typical n 0 values, however, remain in the range from 10 18 cm −3 to mid-10 19 cm −3. It is likely that InN-based devices will have to operate at high electron densities, thus, it is essential to understand the impact of high carrier density on carrier dynamics. Carrier lifetime dependence on their density τ(n) is a powerful tool to reveal the dominating recombination mechanisms. Mainly linear or sublinear dependences were observed in InN layers by using the time-resolved photoluminescence, differential reflectance, or light-induced transient gratings (LITG) techniques. Based on these results, it was argued that Shockley-Read-Hall (SRH) 7-9 , Auger recombination in degenerate plasma 10 , or trap-assisted Auger recombination 11 were the dominant recombination mechanisms in InN. Carrier transport, especially that of minority holes, is less investigated. It was theoretically predicted that the room temperature hole mobility μ h can reach 220 cm 2 /Vs in low-doped InN, but should drop rapidly with n 0 above 10 17 cm −3 12. Experimentally, several techniques were used to measure μ h at fixed hole density. μ h = 17-36 cm 2 /Vs was estimated from sheet conductivity against sample thickness in Mg-doped layers at (1.4-3.0) × 10 18 cm −3 13. Variable magnetic field Hall measurements provided the mobility of heavy and light holes of 50 cm 2 /Vs and 600 cm 2 /Vs, respectively, in a sample with Mg doping at 3 × 10 20 cm −3 14. Hall measurements in InN layers Mg doped in a wide range from 10 18 to 10 20 cm −3 revealed p-type conductivity with similar μ h of 20-30 cm 2 /Vs 15. Application of LITG technique allowed for measuring the mobility of minority holes, which was ~40 cm 2 /Vs in high-quality MBE layers with n 0 in the mid-10 17 cm −3 16,17. This work is focused on the study of carrier dynamics in a wide range of carrier densities. Epilayers with different residual carrier densities were fabricated, while the increasing photoexcited carrier densities were generated using femtosecond laser pulses to ensure high time resolution. LITG technique is exploited to simultaneously extract the carrier lifetimes and their diffusion coefficients at different stages of the decay of nonequilibrium
Zinc Diffusion in GaAsSb from Spin-on Glass Dopant Sources
2004
The first study of p-type doping in GaAsSb lattice-matched to InP from Zn spinon glass sources is reported. Shallow diffusion profiles suitable for aggressively-scaled heterojunction devices and good electronic transport properties in the doped films have been observed. Potential applications for spin-on doping of p-type GaAsSb include the optimization of the extrinsic base resistance and the contact resistance to the base in GaAsSb/InP HBTs for ultra-high-speed applications, without the use of regrowth or other complex processing. Diffusions were carried out in a rapid thermal processor (RTP) using epitaxial heterostructures consisting of 2000 Å of p-type GaAs 0.51 Sb 0.49 grown on semiinsulating InP by MOCVD. The diffusions were performed at temperatures ranging from 350-625 • C and diffusion times of up to 30 minutes. SIMS depth profiling indicates that Zn starts to diffuse into GaAsSb layer at temperatures as low as 350 • C, forming very shallow diffusion profiles. For higher temperatures deep diffusion profiles extending up to 2500 Å were obtained. The Zn appears to remain largely electrically inactive for diffusions at temperatures below 500 • C, as indicated by Hall-effect measurements. The threshold temperature at which Zn starts to become electrically active is found to be in the range of 500-550 • C. Hall-effect measurements indicate that the average hole concentration increased by almost three Shishir K. Rai orders of magnitude for the lowest-doped (2.05 × 10 16 cm -3 ) test structure, rising from 2.05 × 10 16 cm -3 to 1.67 × 10 19 cm -3 for a 30 minute, 600 • C diffusion. The measured sheet resistance decreased from 280 kΩ/sq. to 910 Ω/sq., while the hole mobility was reduced from 59 cm 2 /Vs as-grown to 26 cm 2 /Vs due to increased impurity scattering. With increase in as-grown background doping concentration, the diffusion rate was found to increase. Zn distribution profiles measured by SIMS were modeled and a mechanism for Zn diffusion in undoped GaAs 0.51 Sb 0.49 is proposed. Zn diffuses by an interstitialsubstitutional mechanism in the region close to the surface but as it moves away from the surface and Zn concentration falls down, it diffuses by substitutional mechanism. Surface diffusivity of 1 × 10 -13 cm 2 /s at 600 • C for Zn interstitials is extracted from the diffusion profiles. Zn diffusivity away from the surface is found to be 1-2×10 -14 cm 2 /s at 600 • C. There is insufficient evidence to tell whether this diffusivity is that of substitutional Zn or of a complex that Zn atoms form with the surrounding defects. FIGURES .
IEEE Journal of Photovoltaics, 2021
We investigate the dynamics of Zn diffusion in MOVPE-grown AlGaInP/GaInP systems by the comparison of different structures that emulate the back-surface field (BSF) and base layers of a GaInP subcell integrated into an inverted multijunction solar cell structure. Through the analysis of secondary ion mass spectroscopy (SIMS), electrochemical capacitance-voltage (ECV) and spectrally resolved cathodoluminescence (CL) measurements, we provide experimental evidence that 1) the Zn diffusion is enhanced by point defects injected during the growth of the tunnel junction cathode layer; 2) the intensity of the process is determined by the cathode doping level and it happens for different cathode materials; 3) the mobile Zn is positively charged and 4) the diffusion mechanism reduces the CuPt ordering in GaInP. We demonstrate that using barrier layers the diffusion of point defects can be mitigated, so that they do not reach Zn-doped layers, preventing its diffusion. Finally, the impact of Zn diffusion on solar cells with different Zn-profiles is evaluated by comparing the electrical I-V curves at different concentrations. The results rule out the introduction of internal barriers in the BSF but illustrate how Zn diffusion under typical growth condition can reach the emitter and dramatically affect the series resistance, among other effects.
— An optimized design of a Heterojunction N+ on P GaSb thermophotovoltaic (TPV) cell with hydrogenated amorphous silicon interface passivation is presented. The N+ layer is a transparent conductive oxide (TCO). The interface recombination rate between the p-GaSb and a-Si:H(i) layers is found to have an important effect on cell performance. If this recombination rate can be reduced to 10 5 cm/s, the internal quantum efficiency in the wave range of 600~1700 nm surpasses 95% and the output power density reaches 2W/cm 2 under a given blackbody radiation of 1500K. The high minority carrier electron mobility and diffusion length in the p-GaSb leads to the high internal quantum efficiency. A potential advantage of this cell is its simple cell fabrication process for low cost in high volume manufacturing. Another advantage for this cell for TPV systems is a built in short pass plasma filter with a high reflectivity at longer wavelengths.
Diffusion of Pt in molecular beam epitaxy grown ZnSe
Diffusion of platinum in zinc selenide has been studied by the use of the 4 He and 12 C ion backscattering techniques. The samples were thin films grown by molecular beam epitaxy on GaAs 100 epitaxial layers followed by evaporation of platinum and annealing in the temperature range 500–800 °C. The diffusion coefficients were determined by the fitting of a concentration independent solution of the diffusion equation to the experimental depth profiles. The activation energy and the pre-exponential factor of the diffusion process were found to be 1.7 eV and 6.4 10 6 cm 2 /s, respectively. Zinc selenide as a base for blue light emitting diodes and blue semiconductor lasers has gained increased attention in the last few years. In order to make these devices work it is of great importance to be able to construct a thermally stable ohmic contact to the semiconductor. These contacts are often heterostructures consisting of many different metals, for example Pt, Au, Ti and Ni, 1–4 and to improve the contact properties they often have to be annealed at temperatures up to 350 °C for 45 min. 5 It is thus of prime importance to know the thermal stability of such a metal/semiconductor hetero-structure. Previously some research has been done on the electrical properties of Pt/ZnSe heterostructures 1,6 and interfacial reactions. 7 Some of these studies report indiffusion of platinum and outdiffusion of zinc, but they do not, however, report any quantitative data of the diffusion of platinum in ZnSe. In this letter we report on the activation energy and pre-exponential factor for the diffusion of platinum in ZnSe. Unintentionally doped n-type ZnSe was grown on an epitaxial p-GaAs buffer layer. The growth temperature and the beam pressure ratio Se:Zn of the layers were 290 °C and 2:1, respectively. The thickness of the ZnSe layer was 1.9 m. At this thickness the layer structure was relaxed, due to lattice mismatch between GaAs and ZnSe. The lattice relaxation was confirmed by x-ray diffraction measurement for each sample. After growth the sample sets were immediately transferred to an e-beam vacuum evaporator chamber, where a 10 nm thick platinum layer was deposited onto one of the sample sets labeled I and a 100 nm thick platinum layer onto a second one labeled II. The two thicknesses of platinum were deposited on the ZnSe in order to study the effect of the metal film thickness on the diffusion properties.
Boosting the Performance of Solar Cells with Intermediate Band Absorbers—The Case of ZnTe:O
Journal of Energy and Power Engineering, 2017
This work reports on modeling IB (intermediate band) solar cells based on ZnTe:O semiconductor and determination of their photovoltaic parameters using SCAPS (solar cell capacitance simulator) software. A comparative study between photovoltaic performance of ZnTe and ZnTe:O based solar cells has been carried out. It has been found that the energy conversion efficiency η, short-circuit current density J sc , EQE (external quantum efficiency) and FF (fill factor) increased with increasing oxygen doping concentration N t up to the shallow acceptor density N A and decreased when N t was higher than N A . The open circuit-voltage V oc remained constant for N t lower than the acceptor doping concentration N A and decreased for N t higher than N A . The increase of η, J sc and FF is due to the fact that IB is fully empted, so sub-bandgap photons can be absorbed by hole photoemission process from the VB (valence band) to the IB. The decrease of η, J sc , EQE and FF is attributed to overcompensation for the base doping N A making electron photoemission process from IB to the CB (conduction band) maximized. This indicates that there is a competition between oxygen doping and intrinsic acceptor defects. The optimal concentrations of oxygen and shallow acceptor carriers were found to be N t ≈ 10 15 cm -3 and N A ≈ 10 14 cm -3 . The corresponding photovoltaic parameters were η = 41.5%, J sc = 31.2 mA/cm 2 , V oc = 1.80 V and FF = 75.1%. Finally, the EQE spectra showed a blue shift of absorption edge indicating that the absorption process is extended to the sub-bandgap photons through IB.
2017
We use transient absorption spectroscopy to measure carrier lifetimes in the multiband semiconductor GaP y As 1−x−y N x. These measurements probe the electron populations in the conduction band, intermediate band, and valence band as a function of time after an excitation pulse. Following photoexcitation of GaP 0.32 As 0.67 N 0.01 , we find that the electron population in the conduction band decays exponentially with a time constant τ CB ¼ 23 ps. The electron population in the intermediate band exhibits bimolecular recombination with recombination constant r ¼ 2 × 10 −8 cm 3 =s. In our experiment, an optical pump pulse excites electrons from the valence band to the intermediate and conduction bands, and the change in interband absorption due to absorption saturation and induced absorption is probed with a delayed whitelight pulse. We model the optical properties of our samples using the band anticrossing model to extract carrier densities as a function of time. These results not only identify the short minority-carrier lifetime as a key factor affecting the performance of GaP y As 1−x−y N x-based intermediate-band solar cells but also provide guidance on ways to address this issue.
Carrier transport in high-efficiency ZnO/SiO 2/Si solar cells
Solar Energy Materials and Solar Cells, 2006
Carrier transport in ZnO/SiO 2 /n-Si solar cell has been theoretically analyzed with a consideration that the photo-carrier transport from silicon to ZnO layer through the barrier is dominated by quantum mechanical tunneling process of minority carrier. It was found that the highest efficiency of the cell could be achieved at SiO 2 layer thickness of around 20 Å . The efficiency of the cells decreases as the surface states density Q ss becomes higher. Moreover, the efficiency increases as the electron concentration of ZnO layer is increased due to the decrease of work function of ZnO. It was also found that the lower transmittance of the high carrier concentration ZnO due to the free-carrier absorption at infrared wavelength region does not give any significant effect to the cell performance. The efficiency of higher than 25% is achievable by optimizing the involved device parameters. r
External Quantum Efficiency of a Solar Cell Zno/Cdte: Effect of Emitter and Base Thicknesses
International Journal of Engineering Trends and Technoloy, 2015
In this work, a study is carried out in view to investigate the influence of base and emitter thicknesses on the quantum efficiency of a ZnO/CdTe solar cell. This quantum efficiency is a function of thicknesses of the ZnO transmitter and the CdTe base. This study allows to make choice on values of the thicknesses of ZnO and CdTe appropriate to optimize the performance of the solar cell.
Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se)2 solar cell
Journal of Physics and Chemistry of Solids, 2003
Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se) 2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200-320 K. The activation energy of the recombination under illumination follows the absorber band gap energy E g ¼ 1:07 eV of bulk Cu(In,Ga)(S,Se) 2 . The thermal dependence of the diode ideality factor is described by classical Shockley -Read -Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E 00 ¼ 18 meV. Two activation energies higher than E g ; namely 1.21 and 1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se) 2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se) 2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E 00 influenced by illumination. q