Minority Carrier Diffusion Length Research Articles

Effect of Sb and Na Incorporation in Cu2ZnSnS4 Solar Cells

Cu2ZnSnS4‐based solar cells suffer from limited power conversion efficiency (PCE) and relative small grain size compared to selenium containing absorbers. Introduction of Na in Cu2ZnSnS4 absorbers either during the synthesis or after this step is used to improve device performances and to determine whether its effect is based on structural properties improvement (grain size enhancement, better crystallization) or on opto‐electronic properties improvement (defect passivation). In both cases, presence of Na in the absorber notably improves current and voltage of the solar cells, but the effect is more pronounced when Na is present during synthesis. Quantum efficiency analysis shows that these improvements can be related to longer minority carrier diffusion length and reduced absorber/buffer interface recombination. Introducing Na in the process mostly leads to preferential (112) orientation of the crystal which is clearly correlated with better device performances. Otherwise, the performance limitation due to small grain size is discarded by the joint use of Sb and Na, which has a significant impact on grain size but does not affect solar cells efficiency.

Simulated Contrast of Two Dislocations

A three-dimensional Monte Carlo simulation algorithm is used to study the contrast of two dislocations perpendicular to the irradiated surface of an n-doped silicon sample in the electron beam induced current mode. The dislocations are positioned in the irradiation trajectory, and each of both is considered as a cylinder where the minority carrier diffusion length varies abruptly from a low inside dislocations up to a high value outside dislocations. The EBIC contrast was obtained by simulating the random diffusion of carriers generated at point-like sources randomly distributed within the generation volume. Results are analyzed on the basis of change in the generation volume in the bulk of the sample and of carrier trapping process inside dislocations. The EBIC contrast increases with the increase of the electron beam energy. It also increases when the minority diffusion length inside dislocations, or their separating distance decreases.

Porous ultrathin WO3 nanoflake arrays as highly efficient photoanode for water splitting

To overcome the limitation of minority carrier diffusion length and high recombination of electron-hole pairs, porous ultrathin tungsten trioxide (WO3) nanoplate arrays with amorphous layer were prepared by one-step hydrothermal method without pre-seeded which possessed the highest photocurrent density of 1.80 mA cm−2 at 1.23 V vs RHE and 100 mV cathodic shift of onset potential with 0.20 g dosage of (NH4)2C2O4. The remarkable photoelectrochemical performance mainly benefits from enhanced red-shift light absorption, cathodic shifted onset potential, lowest recombination of photoelectron-hole pairs and abundant active surface areas. These results confirm that engineering the thickness and surface state of oxide semiconductor nanoplate arrays are the promising ways to improve the photoelectrochemical performance for solar water splitting.

Nanoscale analysis of electrical junctions in InGaP nanowires grown by template-assisted selective epitaxy

We report the analysis of the electrical properties of Inx−1GaxP nanowires (NWs) grown by template-assisted selective epitaxy. The individual NW properties are investigated by means of electron beam induced current microscopy (EBIC) and current-voltage curves acquired on single nano-objects. First, a set of samples containing n-doped InGaP NWs grown on a p-doped Si substrate are investigated. The electrical activity of the hetero-junction between the NWs and the substrate is demonstrated and the material parameters are analyzed, namely, the n-doping level is determined in relation to the dopant flow used during the growth. These results were used to design and elaborate InGaP NWs containing a p-n homo-junction. The electrical activity of the homo-junction is evidenced using EBIC mapping on single NWs, and material parameters (namely, the doping and the minority carrier diffusion lengths) for the p- and n-doped InGaP segments are estimated. Finally, the first proof of a photovoltaic effect from the NW homo-junctions is obtained by photocurrent measurements of a contacted NW array under white light irradiation.

Surface photovoltage study of GalnNAs layers for photovoltaic applications

The optical and carrier transport properties of GaInNAs layers grown by molecular beam epitaxy on n-GaAs substrates are studied by surface photovoltage (SPV) spectroscopy combined with photoluminescence (PL) measurements data. The SPV spectra clearly show a red shift of the band gap energy with respect to the band gap of GaAs, which is in line with the PL results. The combined analysis of the SPV amplitude and phase spectra has allowed determining the residual doping type in the layer. The minority carrier diffusion length was assessed by means of the method called “constant SPV”.

Revealing lattice and photocarrier dynamics of high-quality MAPbBr3 single crystals by far infrared reflection and surface photovoltage spectroscopy

Hybrid organic-inorganic lead halide perovskite materials show great promise in a number of optoelectronic applications, including solar cells, light emitting diodes, and photodetectors. Understanding their intrinsic material properties is critical to enhancing device performance and enabling innovative material and device designs. Here, we study lattice dynamics using far-infrared (FIR) reflectance and photogenerated carrier dynamics using surface photovoltage (SPV) measurements on high-quality methylammonium lead bromide (MAPbBr3) single crystals. FIR reflectance shows three coherent infrared-active phonon modes between 40 and 200 cm−1 that result in reststrahlen bands with much higher peak reflectance than has been previously reported. The phonon mode strength and damping are comparable to classical oxide perovskite single crystals. However, the effects of defects on photogenerated carrier recombination are still evident in SPV measurements. By performing SPV over different spectral ranges, we are able to separate the effects of surface and bulk defects on the recombination dynamics of photogenerated charge carriers. We further apply SPV measurements to obtain the minority carrier (electron) diffusion length for the MAPbBr3 crystal. This study demonstrates that both FIR reflectance and SPV measurements provide useful information on the electromagnetic response properties of halide perovskite single crystals.

Simulated contrast of two dislocations -=SUP=-*-=/SUP=-

AbstractA three-dimensional Monte Carlo simulation algorithm is used to study the contrast of two dislocations perpendicular to the irradiated surface of an n -doped silicon sample in the electron beam induced current mode. The dislocations are positioned in the irradiation trajectory, and each of both is considered as a cylinder where the minority carrier diffusion length varies abruptly from a low inside dislocations up to a high value outside dislocations. The EBIC contrast was obtained by simulating the random diffusion of carriers generated at point-like sources randomly distributed within the generation volume. Results are analyzed on the basis of change in the generation volume in the bulk of the sample and of carrier trapping process inside dislocations. The EBIC contrast increases with the increase of the electron beam energy. It also increases when the minority diffusion length inside dislocations, or their separating distance decreases.

Binary metal zinc-lead perovskite built-in air ambient: Towards lead-less and stable perovskite materials

Organo-metal halide perovskites are promising new-generation photovoltaic materials owing to their efficient electrical/optical characteristics. Despite having high efficiency, perovskites are toxic due to the presence of lead and instable in air ambient. Therefore, the toxicity and instability call for more efforts in this area. Here, methylammonium-zinc-lead iodide (MAZn0.2Pb0.8I3) is synthesized in air ambient as lead-less and more stable perovskite material for applying as the active layer of solar cells. 1.65 eV optical band gap and 51 nm minority carrier diffusion length (Ldiff) have been obtained with the help of UV–Visible spectroscopy (UV–Vis) and steady-state photocarrier grating (SSPG) technique respectively. Structural, chemical bonding and surface studies are done by X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR) and Field Emission Scanning Electron Microscope (FESEM) respectively. The sample has been stored in dark air ambient with average relative humidity of 45% at room temperature (27 °C) for 60 days and the properties of 60 days aged sample is investigated and compared with characteristics of 60 days aged MAPbI3. The results confirm MAZn0.2Pb0.8I3 is more stable than MAPbI3.

Minority carrier lifetime and diffusion length in type II superlattice barrier devices

The minority carrier lifetime in p-type InAs/GaSb type II superlattices (T2SLs) is quite short, typically in the region of tens of nanoseconds. In spite of this, T2SLs are becoming a viable alternative to Mercury Cadmium Telluride as the sensing material of choice for high end MWIR and LWIR infrared detectors. For example, SCD now manufactures a 640 × 512 format, 15 μm pitch LWIR focal plane array detector, with a quantum efficiency close to 50%, a pixel operability of >99.5%, and a dark current only about one order of magnitude larger than the state of the art Rule 07 value. A key to the very high performance of this detector is the use of an XBp barrier architecture that both suppresses the G-R current and allows stable passivation to all steps of the fabrication process. Since both the dark-current and photo-current in the XBp structure are diffusion limited, measurements of these quantities as a function of the device dimension provide an excellent vehicle for estimating the minority carrier lifetime and diffusion length, when performed in conjunction with k⋅p calculations of the T2SL density of states. Typical lifetime results are presented, which are consistent with values found by others using direct measurements. Diffusion lengths are reported in the range 3–7 μm, although these are not necessary limiting values.

From EBIC images to qualitative minority carrier diffusion length maps

A novel method is presented with the aim to perform minority carrier diffusion length map on cross-sectional samples. The method is based on one Electron-Beam Induced Current (EBIC) acquisition and on the analyze of the EBIC signal slope variation on each scanned points. This method is applied on a pinned photodiode array realized on a low doped silicon epitaxy, and the electron diffusion length map which is extracted is in good accordance with our expectation taking into account the doping distribution of the device. A TCAD simulation also confirms quantitatively the measured diffusion length map. Advantages and drawbacks of this method are discussed in this study.

Comparative Study of ∼2.05 eV Lattice-Matched and Metamorphic (Al)GaInP Solar Cells Grown by MOCVD

This work investigates two conceptually different approaches, both based on the AlGaInP alloy family, to produce metal–organic chemical vapor deposition grown 2.05 eV direct bandgap materials for use in the top cell of AM0 multijunction solar cells. The first approach achieves the target bandgap via (Al0.32Ga0.68) 0.52In0.48P lattice-matched to GaAs. The second approach achieves the target bandgap via relaxed, lattice-mismatched Ga0.66In0.34P. Solar cell characterization and analytical device modeling reveal merits and challenges for both approaches. Both devices are found to possess comparable minority carrier diffusion lengths within the p-type base and an equal bandgap-voltage offset of 0.54 V, suggesting effectively similar base material quality. However, the metamorphic Ga0.66In0.34P cell exhibits a superior quantum efficiency across all wavelengths, particularly at short wavelengths. Modeling results attribute the enhanced short wavelength response to the wider direct bandgap of the internally lattice-matched Al0.69In0.31P window and a longer emitter diffusion length. As such, despite the non-negligible lattice mismatch, the enhanced short wavelength response for the metamorphic Ga0.66In0.34P approach proved significantly advantageous, at least with respect to the growth conditions used in this study. Assuming the use of a perfect antireflection coating, the lower parasitic optical absorption from the wider direct bandgap window alone provides as much as ∼0.72 mA/cm2 additional photocurrent. Combined with the higher quality emitter layer observed in this study, ∼1.1 mA/cm2 additional photocurrent was achieved for the metamorphic Ga0.66In 0.34P approach. These values correspond to ∼3.9% and ∼6.0% of the total available photocurrent for a 2.05 eV bandgap solar cell.

Analytical and numerical simulation of electron beam induced current profiles in p-n junctions

The electron beam induced current (EBIC) mode of a scanning electron microscope (SEM) is a widely used technique for the quantitative assessment of minority carrier diffusion length and surface recombination. Point source (one-dimensional) and extended source (two-dimensional) analytical models are two widely used approaches to assess this information in geometry where the electron beam (e-beam) is parallel to the p-n junction. In this article, a two-dimensional (2D) analytical model is evaluated and compared with 2D finite element numerical simulations, where the electron beam-solid interaction is modeled using a Monte Carlo simulation coupled with a drift-diffusion solver. The simulations are computed for both low and high level injection conditions. The effect of an e-beam injection level on the shape of EBIC profiles is analyzed to evaluate limitations of the analytical models.

Effect of surface texturization on minority carrier lifetime and photovoltaic performance of monocrystalline silicon solar cell

This paper presents the effect of surface texturization on the minority carrier lifetime and photoelectric conversion parameters of monocrystalline silicon solar cell. Two different wet-chemical texturization methods were employed to etch the monocrystalline silicon wafer surface. Morphology of the wafers was investigated by stylus surface profiler, field emission scanning electron microscope (FESEM) and surface reflection measurement (SRM) systems. Due to texturization pyramidal structures were formed on wafer surface as well as the surface roughness was increased by 3.07%. Untextured (S1) and textured (S2 & S3) samples were doped with phosphorus atoms with a constant flow rate of liquid phosphorus oxychloride (POCl3) in a high temperature diffusion furnace. Solar cell was fabricated using both untextured and textured silicon wafer. Maximum 1.66% decrease of optical reflectance was found in textured solar cell. The minority carrier lifetime and photoelectric parameters were investigated using surface photovoltage (SPV) and light-current-voltage (LIV) measurement system respectively. Minority carrier diffusion length and minority carrier lifetime were increased significantly in textured solar cell. Thus, surface texturization plays an important role on increasing the energy conversion efficiency of silicon solar cell.

Optimization design of betavoltaic battery based on titanium tritide and silicon using Monte Carlo code

This article presents the optimization design and simulation of a betavoltaic battery composed of a silicon p-n junction converter and a titanium tritide film as an isotope source. The self-absorption of β particles emitted from the tritium radioisotope in the titanium tritide film and the energy deposition of β particles in the silicon converter are investigated by the Monte Carlo simulation with the Geant4 radiation transport toolkit. The relationships between doping concentrations and basic parameters such as depletion region width, minority carrier diffusion length and leakage current of the PN junction are discussed through the calculation formulas. By optimizing the doping concentrations in the P-type and N-type regions, the optimized betavoltaic battery can maximize the output power and the conversion efficiency based on the energy deposition in the silicon. The results show that the optimal thickness of the titanium tritide film is about 0.7 µm and the optimal doping concentrations of the battery with a PN junction depth of 50 nm are Na=5.75×1019cm−3,Nd=2.95×1018cm−3. Under these parameters, the size 1mm×1mm proposed battery with 2.9 mCi/mm2 3H can achieve the output power 0.902 nW and the conversion efficiency 0.91%. The open circuit voltage, short circuit current and fill factor of the battery are 0.389 V, 3.03 nA and 0.766, respectively.

Optimizing two and four-terminal CuGaSe2/CuInGaSe2 tandem solar cells for achieving high efficiencies

Solar cells based on CuInGaSe2 (CIGS) have attained the highest record efficiency (22.9%) among thin film solar cells. CIGS is formed by the appropriate mixing of CuInSe2 (CIS) which has a bandgap around 1 eV and CuGaSe2 (CGS) which has a bandgap around 1.7 eV. These bandgaps are close to the optimal values for two-junction tandem solar cells. Therefore, CGS on CIGS or CGS on CIS tandem cells should be attractive and viable to achieve high efficiency and cost-effective thin film solar cells. In this work, a simple unified analytical model is used to design two-terminal and four-terminal CGS/CIGS tandem solar cells, establishing a realistic value for the expected efficiency in each case. Short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency, are determined as a function of the CGS and CIGS acceptor impurity concentrations, back surface recombination velocities, minority carrier diffusion lengths, and absorber layer thickness. It is shown that realistic efficiencies over 30% can be achieved under one sun AM1.5 spectrum for both of the terminal configurations, but the optimum composition for the CIGS sub-cell should be different depending on the respective tandem cell configuration.

Effective Removal of Surface Recombination Centers in Silicon Nanowires Fabricated by Metal-Assisted Chemical Etching

High-density silicon nanowires (SiNWs) were fabricated on p-Si substrates by metal-assisted chemical etching (MACE). A chemical cleaning with HF/piranha/HF was used to remove structural defects from the nanowire surface which are at the origin of surface recombination states. Surface photovoltage (SPV) spectroscopy, capacitance–voltage (C–V), and conductance measurements were used to study the SiNW properties and assess the effectiveness of the cleaning process. For the as-grown samples, the analysis of the SPV amplitude and phase spectra demonstrated high density of positively charged surface recombination centers. These centers increase the surface band bending and decrease the minority carrier lifetime and diffusion length. Si nanostructures at the SiNW surface introduce slow traps, as evidenced by the frequency dispersion at accumulation of the C–V curves and the increase of the diffusion length with frequency. All these features are effectively reduced when the SiNWs are subjected to HF/piranha/HF chemical cleaning. Thus, a cheap and effective way is demonstrated to reduce the surface state density and improve the electronic quality of the surface of SiNWs.

Influence of carrier diffusion on photo-Seebeck effect in zinc oxide

We measured the electrical conductivity and Seebeck coefficient under photo-illumination in single-crystalline ZnO and investigated the dependence of the correlation between the conductivity and Seebeck coefficient on the energy of applied ultraviolet light. The correlation was found to be completely independent of the light's energy, indicating that the photo-induced non-equilibrium state is unrelated to the light's energy, but is affected by the carrier diffusion. Furthermore, by assuming that the electron mobility is not changed by the photo-illumination, we estimated the ambipolar carrier diffusion length to be about 1 μm, which is roughly consistent with the minority carrier diffusion lengths in previous reports. Our work reveals that the photo-Seebeck effect is not only influenced by the majority-carrier mobility, but also by the minority-carrier mobility.

Surface Dielectric Tunnel Barrier Induced by Mn Doping in SnO2 Micro‐ and Nanostructures

Electrical properties of undoped and Mn doped SnO2 microplates and rods are studied by electron beam induced current (EBIC) in a scanning electron microscope (SEM), and I–V curves acquired at room temperature. AFM measurements reveal the formation of numerous terraces at the (−101) surface of the analyzed Mn‐doped SnO2 microplates, which also exhibit high carrier recombination processes at their central region, as confirmed by combined EBIC and cathodoluminescence (CL) measurements. A diffusion length for minority carriers about 205 nm is obtained by EBIC measurements. Different electrical conduction mechanisms, such as Fowler‐Nordheim, direct tunneling and Poole‐Frenkel, are evaluated in the electrical analysis of the samples. Mn doped microplates show lower conductivity than the undoped microds. Moreover the height of the surface tunnel barrier is increased by Mn doping, as confirmed by the analysis of the I–V curves acquired under transversal configuration. A value of the relative dielectric constant ϵr about 7.3 is estimated for the probed SnO2 microstructures.

Impact of temperature and gamma radiation on electron diffusion length and mobility in p-type InAs/GaSb superlattices

The minority carrier diffusion length was directly measured by the variable-temperature Electron Beam-Induced Current technique in InAs/GaSb type-II strain-layer-superlattice infrared-detector structures. The Molecular Beam Epitaxy-grown midwave infrared superlattices comprised 10 monolayers of InAs and 10 monolayers of GaSb to give a total absorber thickness of 4 μm. The diffusion length of minority electrons in the p-type absorber region of the p-type/barrier/n-type structure was found to increase from 1.08 to 2.24 μm with a thermal activation energy of 13.1 meV for temperatures ranging from 77 to 273 K. These lengths significantly exceed the individual 10-monolayer thicknesses of the InAs and GaSb, possibly indicating a low impact of interface scattering on the minority carrier diffusion length. The corresponding minority electron mobility varied from 48 to 65 cm2/V s. An absorbed gamma irradiation dose of 500 Gy halved the minority carrier diffusion length and increased the thermal activation energy to 18.6 meV, due to creation of radiation-induced defect recombination centers.

Control over Self‐Doping in High Band Gap Perovskite Films

AbstractIt is reported how differences in the composition of high bandgap Pb bromide‐based perovskites affect their carrier diffusion length and junction type. Pb‐based, APbX3, halide perovskite (HaP) films and devices are studied, where A can be a mixture of formamidinium, methylammonium (MA), and Cs, and X a mixture of Br and Cl, using a combination of dark‐ and photoconductivity and steady‐state photocarrier grating. The results show the cation and anion compositions affect both majority and minority carrier diffusion lengths. In particular, using electron beam‐induced current measurements, FTO\dTiO2mp‐TiO2HaP\PTAA (poly‐triarylamine)\Au devices are studied. The results enable identifying junction and built‐in voltage formation and track position and size of the space charge region width with changes in the HaP composition. As far as it is known, it is found for the first time that a mixed‐cation HaP forms a junction that has characteristics of a p‐i‐n one, with relatively long and comparable carrier diffusion lengths, while the single cation‐based bromide HaPs form clear p‐n junctions at the interface with the TiO2 [pure CsPbBr3 and MAPbBr3(Cl)] or a buried one (MAPbBr3) and shorter diffusion lengths. These differences are attributed to lower carrier density in MAPbBr3, and especially in the mixed cation HaP, which is comparable to iodide‐based HaP films.