Analysis Of Internal Quantum Efficiency Research Articles

Analytical analysis of internal quantum efficiency with polarization fields in GaN-based light-emitting diodes

Abstract This paper presents the analytical model for the analysis of internal quantum efficiency as well as light output power of GaN-based light-emitting diodes by introducing the polarization factor, which accounts for the polarization fields in the active region, in the standard ABC model. With the increase of the polarization, effective volume decreases which causes strong localization of carriers at lower potentials causing the carrier density to increase. This effect leads to loss mechanisms such as carrier leakage and Auger loss. Our theoretical analysis for internal quantum efficiency and light output power shows good agreement with the experimental results for both the blue and green InGaN light-emitting diodes.

Extended spectral response analysis of conventional and front surface field solar cells

Historically, spectral analysis of internal quantum efficiency has played an important role in solar cell characterization. The applicability of this technique is reassessed in view of developments over recent years as a means of determining cell optical and electrical properties. The refined technique, applicable to both conventional and front surface field devices, highlights three distinct spectral regions, allowing additional front surface investigation by blue response analysis and more simplified analysis over a weakly absorbed region where photo-generation is uniform throughout the absorber and parasitic absorption remains sufficiently constant at these wavelengths. The approach is demonstrated by application to a modern front surface field device as a complement to previous work.

Internal quantum efficiency analysis of plasmonic textured silicon solar cells: surface plasmon resonance and off-resonance effects

Silver nanoparticles (Ag NPs) of various sizes and concentration were integrated on textured silicon solar cells for further confinement of incident light, generated photocurrent modifications were investigated using spectrally resolved short-circuit current measurements. Internal quantum efficiency (IQE) spectra were used for quantifying the effective minority carrier diffusion lengths (Leff) of plasmonic cells in the long wavelength region (850 < λ < 1020 nm). The Leff of an optimized plasmonic solar cell enhanced to 431 µm compared to 338 µm of the bare cell, which is due to interacting Ag NPs' scattered fields, leading to enhanced light absorption in the plasmonic cell. Despite the enhanced Leff values, the overall generated photocurrent reduced with Ag NPs which is due to the significant losses near the surface plasmon resonant region. Reduced IQE of plasmonic cells near and below the surface plasmon resonant region is due to size-dependent parasitic absorption and enhanced back scattering of Ag NPs, and a modified surface recombination process due to Ag NPs' strong near-fields.

Emission Wavelength Dependence of Internal Quantum Efficiency in InGaN Nanowires

The internal quantum efficiency (IQE) of InGaN nanowires with different emission wavelength of 485, 515, 555, and 580 nm has been studied by means of photoluminescence (PL) spectroscopy. It was found from the analysis of IQE as a function of excitation power density that the IQE was unchanged at about 100% under weak excitation conditions at low temperature. This indicated that the effects of nonradiative recombination processes were negligibly small at low temperature. Moreover, the IQE increased from 5 to 12% with increasing emission wavelength from 485 to 580 nm. Since the clear correlation between the IQE and the PL blue shift due to band filling effects of localized states was observed, the increase in the IQE reflected the increase in the effect of exciton localization with increasing indium composition.

Analysis of Internal Quantum Efficiency and Current Injection Efficiency in III-Nitride Light-Emitting Diodes

Current injection efficiency and internal quantum efficiency (IQE) in InGaN quantum well (QW) based light emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier systems. A self-consistent 6-band k ·p method is used to calculate the band structure for InGaN QW structure. Carrier-photon rate equations are utilized to describe radiative and non-radiative recombination in the QW and the barrier regions, carrier transport and capture time, and thermionic emission leading to carrier leakage out of the QW. Our model indicates that the IQE in the conventional 24-A In0.28Ga0.72 N -GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large thermionic carrier leakage. The efficiency droop phenomenon at high current density in III-nitride LEDs is thus consistent with the high-driving-current induced quenching in current injection efficiency predicted by our model. The effects of the monomolecular recombination coefficient, Auger recombination coefficient and GaN hole mobility on the current injection efficiency and IQE are studied. Structures combining InGaN QW with thin larger energy bandgap barriers such as AlxGa1-xN, lattice-matched AlxIn1-xN, and lattice-matched AlxInyGa1-x-y N have been analyzed to improve current injection efficiency and thus minimize droop at high current injection in III-nitride LEDs. Effect of the thickness of the larger energy bandgap barriers (AlGaN, AlInN and AlInGaN) on injection efficiency and IQE are investigated. The use of thin AlGaN barriers shows slight reduction of quenching of the injection efficiency as the current density increases. The use of thin lattice-matched AlInN or AlInGaN barriers shows significant suppression of efficiency-droop in nitride LEDs.

The analysis of light trapping and internal quantum efficiency of a solar cell with grating structure

A multiple light paths analysis of the internal quantum efficiency (IQE) of a silicon solar cell with back reflector using grating structure to improve the light trapping is presented and the contributions of diffusion length of base regions to IQEs are simulated. An optical model for the determination of generation profiles of the cell is adopted and for a refractive index n material up to 4n2 light paths are considered and compared with no light trapping structure. It is found that the spatial widths of the cell, the increase of diffusion length, the diffraction angle distribution, number of light trapping paths and transmitted angle can significantly affect the IQEb for lower absorption wavelength (i.e., 1000–1100nm). With 4nSi2 light trapping paths, the simulation results show that the best IQEs (≈IQEb) with transmitted angle can reach up to 73% which is 14% more than that with normal incident, the best achievable IQEb with grating structure is 49% at cell thickness wb=50μm, and the IQEb with diffraction angle θm=60° is 9% and 39% larger than that with transmitted angle θl=0° and without light trapping, respectively. For the case of normal transmitted angle, the IQEb with diffusion length 1000μm is about 81% and is 37% higher than that with diffusion length 25μm. The obtained results can provide essential information for designing a high-efficiency solar cell.

Analysis of local Al-doped back surface fields for high efficiency screen-printed solar cells

In this paper, we investigate the surface recombination of local screen-printed aluminum contacts applied to rear passivated solar cells. We measure the surface recombination velocity by microwave-detected photoconductance decay measurements on test wafers with various contact geometries and compare two different aluminum pastes. The aluminum paste which is optimized for local contacts shows a deep and uniform local back surface field that results in Smet = 600 cm/s on 1.5 Ωcm p-type silicon. In contrast, a standard Al paste for full-area metallization shows a nonuniform back surface field and a Smet of 2000 cm/s on the same material. We achieve an area-averaged rear surface recombination velocity Srear = (65 ± 20) cm/s for line contacts with a pitch of 2 mm. The application of the optimized paste to screen-printed solar cells with dielectric surface passivation results in efficiencies of up to 19.2 % with a Voc = 655 mV and a Jsc = 38.4 mA/cm² on 125×125 mm² p-type Cz silicon wafers. The internal quantum efficiency analysis reveals Srear = (70 ± 30) cm/s which is in agreement with our lifetime results. Applying fine line screenprinting, efficiencies up to 19.4 % are demonstrated.

Analysis of internal quantum efficiency in double-graded bandgap solar cells including sub-bandgap absorption

State of the art ZnO/CdS/Cu(In,Ga)Se 2 (CIGS) solar cells use bandgap grading, requiring special tools for the analysis of the experimentally obtained characteristic curves. We develop an analytical model for the photon flux and internal quantum efficiency in double-graded bandgap solar cells, considering the effects of sub-bandgap absorption and grading-dependent carrier collection properties. The short-circuit photocurrent density is calculated as a function of carrier diffusion length and front/back bandgaps, establishing optimum design criteria under solar operation. Even for a diffusion length of only 0.5 μm in a 3-μm-thick absorber, and no contribution from the CdS layer, an optimum back bandgap of 1.35 eV is found, yielding short-circuit current densities of 36.0 (33.5) mA cm −2 for a front bandgap of 1.05 (1.68) eV. Furthermore, simplifications to the model for specific energy ranges allow to extract the Urbach Energy E U and the minimum bandgap E g,min in the grading profile from experimental IQE curves. Finally, our model fits IQE measurements of 18% efficient CIGS solar cells, yielding values of E U between 31 and 41 meV, minimum bandgaps E g,min between 1.10 and 1.16 eV, and diffusion lengths close to 0.5 μm.

Internal quantum efficiency analysis of solar cell by genetic algorithm

To investigate factors limiting the performance of a GaAs solar cell, genetic algorithm is employed to fit the experimentally measured internal quantum efficiency (IQE) in the full spectra range. The device parameters such as diffusion lengths and surface recombination velocities are extracted. Electron beam induced current (EBIC) is performed in the base region of the cell with obtained diffusion length agreeing with the fit result. The advantage of genetic algorithm is illustrated.

Electrical characterisation of thin silicon layers by light beam induced current and internal quantum efficiency measurements

Thin crystalline silicon layers (50 μm) were grown by vapour phase epitaxy on monocrystalline substrates. Minority carrier diffusion length and surface recombination velocity were evaluated by light beam induced current experiment. Although it appeared difficult to apply existing analytical models to thin and high quality layers, multi-dimensional simulator DESSIS was used successfully to extract diffusion length of the order of 300 μm for p-type material and 80 μm for n-type material with surface recombination velocity of the order of 100–1000 cm s −1 when the surface was passivated by a thin silicon nitrite coating. Results were compared with the diffusion length evaluated from internal quantum efficiency analysis in fabricated photovoltaic cells made of the same material, using spectral response and reflectivity measurements.

Effective diffusion lengths for minority carriers in solar cells as determined from internal quantum efficiency analysis

We introduce a general relationship between the effective diffusion length LQ of solar cells derived from spectral quantum efficiency Q and the effective diffusion length LJ that determines the saturation current j0=qn0D/LJ of the diode in the dark. The general relation LQ⩾LJ holds in the presence of grain boundaries and of spatially nonhomogeneous doping profiles. We find the relation LQ=LJ for solar cells without potential fluctuations at the collecting junction.

Rapid thermal oxidation of porous silicon for surface passivation

Rapid thermal oxidation with dry oxygen has been carried out on porous silicon (PS) films formed by electrochemical etching. The purpose of the paper was to investigate the surface passivation capability of the oxidized PS layers and to understand the oxidation mechanism. Rutherford back scattering (RBS) and X-ray photoemission spectroscopy (XPS) analyses confirmed the formation of a stoichiometric quasi-silicon dioxide. Besides, elastic recoil diffusion analysis (ERDA) demonstrated that a high concentration of hydrogen is still present in the PS film even after oxidation. RTO resulted in a good surface passivation effect at high temperature (>1000°C) as seen by internal quantum efficiency analysis. However, lifetime in bulk silicon is affected by the RTO process.

Analysis of internal quantum efficiency and a new graphical evaluation scheme

The analysis of internal quantum efficiency data is extended by a new graphical evaluation scheme. On the one hand, our method allows one to estimate the minority carrier diffusion length L and the back surface recombination velocity S. On the other hand the limitations of the internal quantum efficiency method are studied. A mathematical treatment of the equations describing internal quantum efficiency demonstrates that already a single measurement of either the effective diffusion length L eff measured in the near infrared, or the collection efficiency η c measured in the near bandgap wavelength range, yields limits for L and S. More precise values can be obtained in specific cases, if the analysis of L eff and η c are combined. In those cases, information about the accuracy of L and S is obtained. We present a graphical solution for the evaluation and illustrate our method using quantum efficiency data from both thick and thin silicon solar cells.

Internal quantum efficiency of thin epitaxial silicon solar cells

A new theoretical expression is derived for the internal quantum efficiency of solar cells homoepitaxially grown on highly doped monocrystalline substrates. This expression is used to characterize our thin-layer silicon cells grown by chemical vapor deposition. These cells reach a confirmed efficiency of 17.3% although the active layer thickness is only 48 μm. The internal quantum efficiency analysis demonstrates that the open circuit voltage is limited by carrier injection into the highly doped substrate. Carrier generation in the substrates accounts for 0.7% of the short circuit current.

Comparative study of minority electron properties in p+-GaAs doped with beryllium and carbon

Minority electron properties in p+-GaAs doped with beryllium (Be) and with carbon (C) are reported. Measurements of essentially identical responses for structures differing only in dopant element demonstrate that the diffusivity (Dn) and the diffusion lengths (Ln) are the same in p+-GaAs doped to ∼1019 cm−3 with Be- and C-dopants. Zero-field time-of-flight analysis yields Dn=35 cm2/s and internal quantum efficiency analysis yields Ln=2.4 μm, which implies a lifetime that is approximately at the estimated radiative limit. In addition, the majority Hall mobility was also found to be identical for the Be- and C-doped material.