Search

Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers have been intensively studied due to their high power conversion efficiencies. CIGS solar cells with Zn(O,S) buffer layers achieved record efficiencies due to their reduced parasitic absorption compared with the more commonly used CdS buffer. Accordingly, we have studied solution-grown Zn(O,S) buffer layers on polycrystalline CIGS absorber layers by complementary techniques. A bandgap energy Eg of 2.9 eV is detected by means of angle-resolved electroreflectance spectroscopy corresponding to Zn(O,S), whereas an additional Eg of 2.3 eV clearly appeared for a post-annealed CIGS solar cell (250 °C in air) compared with the as-grown state. To identify the chemical phase that contributes to the Eg of 2.3 eV, the microstructure and microchemistry of the Zn(O,S) buffer layers in the as-grown state and after annealing were analyzed by different transmission electron microscopic techniques on the submicrometer scale and energy-dispersive x-ray spectroscopy. We demonstrate that the combination of these methods facilitates a comprehensive analysis of the complex phase constitution of nanoscaled buffer layers. The results show that after annealing, the Cu concentration in Zn(O,S) is increased. This observation indicates the existence of an additional Cu-containing phase with Eg close to 2.3 eV, such as Cu2Se (2.23 eV) or CuS (2.36 eV), which could be one possible origin of the low power conversion efficiency and low fill factor of the solar cell under investigation.

Vacuum-based deposition techniques are a common route for the fabrication of high-quality optoelectronic devices on an industrialized scale at low cost and high yield. In the field of perovskite-based photovoltaics, however, vacuum deposition methods are less researched in the community today. Even though the fundamental concept of thermal evaporation of perovskite-based solar cells has been demonstrated, the number of reports about efficient upscalable all-evaporated approaches employing inexpensive raw materials is still limited. In this contribution, a novel architecture for efficient all-evaporated perovskite solar cells in pin -architecture based on a co-evaporated CH3NH3PbI3 absorber deposited on top of an electron-beam evaporated NiOx hole transport layer is reported. Stabilized power conversion efficiencies as high as 16.1% are achieved, resulting in the most efficient thermally evaporated perovskite solar cells employing a pin -architecture. Moreover, it is the first time in the literature that a co-evaporated perovskite absorber deposited directly on top of a metal oxide exceeeds a stable power conversion efficiency above 15%. Next to efficient devices, a remarkable stability against temperature variations up to 80 °C is demonstrated, highlighting the promising thermal stability of the employed charge extracting layers. Replacing the expensive gold rear electrode by copper reduces the material costs of the approach significantly while maintaining a good device performance and stability. The homogeneity and ease of upscaling of the all-evaporated approach toward industrial relevant areas is demonstrated by light-beam induced current mapping. Finally, a homogeneous deposition of the functional layers of the approach on top of a textured silicon wafer is shown.

In the development of kesterite copper zinc tin selenide (CZTSe) thin-film solar cells, the realization of absorber layers with large grains and suppression of unwanted secondary phases have always been important aspects. Incorporation of silver (Ag) in the CZTSe absorber can improve grain growth but also affects both the bulk and interface electronic properties. In this work, the Ag layer was incorporated at the back contact of vacuum-processed CZTSe solar cells and its impact on the absorber growth and overall device performance was investigated. The concentration of Ag in the absorber was varied by changing the thickness of the Ag layer at the bottom of the copper-zinc-tin (CZT) precursor. We found that the grain size in the absorber significantly increased with increasing Ag layer thickness. We further investigated the phase and morphology evolution during annealing of the CZT precursor in selenium atmosphere, both with and without the Ag layer present. Using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, we demonstrated that Ag incorporation induced a fast formation of larger grains in the early stages, which could be attributed to the formation of a silver selenide (Ag–Se) liquid phase. Time-of-flight secondary ion mass spectrometry depth profiles revealed a homogeneous distribution of Ag across the absorber. An improved power conversion efficiency was obtained for very thin (10 nm) Ag layers – due to an increase in the open-circuit voltage and fill-factor – while thicker layers deteriorate the device properties, mainly due to an increased series resistance. Therefore, the incorporation of small amounts of Ag is suggested to be useful to improve the absorber morphology of CZTSe solar cells while the electronic properties suffer from too high quantities.

We demonstrate that based on a post-sulphurization approach the mean sulphur concentration and bandgap of Cu2ZnSn(S,Se)4 (CZTSSe) solar cell absorber layers can be precisely controlled. However, X-ray diffraction and secondary ion mass spectrometry studies reveal an inhomogeneous sulphur incorporation at lower process temperatures resulting in a sulphur gradient in the absorber. Optimization of such gradients may enable further performance improvement of future devices. During the post-sulphurization process sulphur seems to accumulate at the back of the absorber first where there is already an enrichment of sulphur in the purely selenized absorbers. With higher temperatures more sulphur gets incorporated and the sulphur distribution gets evened out. Morphology studies show that the process of exchanging the chalcogenides leaves behind holes and results in a porous absorber layer. With higher process temperatures and therefore higher sulphur content the absorber gets more dense. The device performance of the post-sulphurized samples increases significantly with increasing process temperature and increasing sulphur content.

Solution-grown Zn(O,S) buffer layers in Cu(In,Ga)Se 2 (CIGS) solar cells are investigated by angle-resolved electroreflectance (ER) spectroscopy. We demonstrate that ER can be used directly to measure the bandgap energy of very thin Zn(O,S) buffer layers in the device. Furthermore, ER measurements on CIGS solar cells with different gallium concentrations before and after thermal annealing (TA) were conducted and show no significant influence of the gallium concentration and TA on the buffer’s bandgap energy, as determined in the range of 2.8 – 2.9 eV. Moreover, some ER spectra exhibit an additional contribution at 2.3 eV. This finding can be ascribed to a secondary phase at the interface between CIGS absorber and Zn(O,S) buffer layer.

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.

False-colour scanning electron microscopy cross-section picture of a passivated solar cell, with the front contact layers coloured in green, the 0.5 µm CIGS absorber in dark red, the MgF2 passivation layer in blue, and the Mo back contact in grey.Decreasing the absorber layer thickness of thin-film solar cells can be an effective solution for cost reduction of photovoltaic electricity generation. Unfortunately, this reduction leads to detrimental effects such as incomplete photon absorption and increased charge carrier recombination at the rear electrode. To tackle these losses in ultra-thin 0.5 µm Cu(In,Ga)Se2 (CIGS) solar cells, we developed different passivation structures made of MgF2 and Al2O3 at the molybdenum–CIGS interface, leading to localized back contacts. The influence of the distance between those contacts on the cell performance was studied by varying the periodicity of the applied 1D patterns from 6 μm to 30 μm. Thus, an increase in performance was measured for microstructured layers with a periodicity of up to 12 µm. More precisely, a MgF2 layer yielded an increase in power conversion efficiency (PCE) of up to 9%rel compared to an unpassivated cell design, and a passivation layer comprising Al2O3 led to up to a 5%rel increase in PCE. The gains were primarily attributed to an increased reflectivity of the back contact, while the formation of a negative backside field in the case of Al2O3 might have contributed to this increase by preventing electrons from recombining at the backside interface. Our findings indicate a high lateral conductivity for holes inside the multicrystalline CIGS compound over few tens of micrometres, which allows an independent design of future back contacts and light-trapping schemes.

In n-doped (Zn,Mn)Se, the (s,d) exchange influences the ground state level of the donor-bound electrons in an external B-field, depending on their local Mn environment. Due to the statistical Mn arrangement, this should induce a broadening of the donor energy distribution, inducing a magneto-resistance in transport, and also a broadening of the energy distribution of donor-bound excitons (D0,X). We have investigated this broadening for Zn0.94Mn0.06Se:Cl (n= 4.5×1017 cm-3) by electronic spin flip Raman spectroscopy (ESFRS) on the donor electrons. Utilizing the resonant enhancement of the ESFRS intensity upon resonance of the laser light with (D0,X), we actually observe a significant broadening of the ESFRS resonance profiles with increasing B-field, up to 11.5 meV (FWHM) for B = 5 T. Furthermore we detect a second resonance profile contribution, whose FWHM shows a much weaker B-field dependence. We assign this contribution to a resonance mediated by the free-exciton (X0). Our Raman results agree very well with magneto-transport results at the same sample. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Post-deposition treatment (PDT) of the absorber layer with alkali fluorides has led to a significant increase in the efficiency of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. In this contribution, we investigate the influence of alkali PDTs on the absorber's surface potential by means of Kelvin probe force microscopy (KPFM). To this end, we perform KPFM on cross sections of complete CIGS solar cells. To improve the reliability of the measurement procedure, we deposit a gold layer on top of the solar cell as a reference layer. Using this approach, we study the influence of RbF and KF PDT on the absorber's surface potential for CIGS solar cells with different absorber and buffer compositions. In all cases, an increased surface potential of the cross section of the absorber layer is measured for the cells with PDT.

With demonstrated power conversion efficiencies close to 23%, perovskite-based photovoltaics is already able to compete with established technologies like silicon, CdTe and CIGS. However, next to high efficiencies, the potential low-cost fabrication of devices with sufficient stability under real-world conditions is of key importance for the future economic prospects of the perovskite technology. In this contribution, we report on a novel inexpensive architecture for efficient and highly reproducible, all-evaporated perovskite solar cells. Our evaporated CH3NH3PbI3 absorber is sandwiched between nickel oxide as hole transport material and C60 as electron transport material. By replacing the highly expensive hole transport layer Spiro-MeOTAD by electron-beam deposited nickel oxide and the gold back electrode by copper, we reduce the cost of materials on the lab-scale to one third of the price of the common stack based on Spiro-MeOTAD. At the same time, extraordinary stable devices even at operating temperatures of 80°C are achieved. Stabilized power conversion efficiencies under standard test conditions exceed values of 14%. Moreover, the vacuum deposition combines the ease of controlled deposition and a simple upscaling, making it a favorable process for industry. A homogenous and reproducible deposition on substrates with an area of up to 8x8 cm² is demonstrated by light beam induced current mapping, which is a fundamental requirement for the fabrication of larger prototype modules. Finally, as an inverted architecture with the anode deposited on top of the substrate the investigated layer stack is a promising candidate for two-terminal tandem devices on top of CIGS or p-type silicon.

Temperature-dependent electromodulation spectroscopy is applied to perovskite solar cells for the determination of excitonic properties. Measurements have been performed on semitransparent (in transmission and reflection) and non-transparent devices (in reflection) as well as on cells with different absorber materials. The comparison with Elliott fits of standard absorption spectra enables a distinction between bound and continuum excitonic states and points towards bound electron-hole pairs as origin of the optical transitions observed in electromodulation spectra. The excitonic Sommerfeld enhancement leads to a significantly increased absorption in perovskites. Additionally, a comparison of multiple-cation mixed-halide perovskites with standard methylammonium lead iodide indicates a suppression of the phase transition at low temperatures.

Electroreflectance (ER) spectroscopy is utilized to study Cu(In,Ga)Se 2 solar cells with solution-grown CdS and Zn(O,S) buffer layers. Due to the weak ER signals of the thin buffer layers, interference effects can have a strong influence on the ER spectra. To suppress these interference effects, the thickness of the ZnO window layer, and hence the interference conditions, are changed by a step-by-step etching process while keeping the solar cell operational. This method was successfully applied to solar cells with CdS buffer. First results from transferring this approach to alternative Zn(O,S) buffer layers are also reported.

The replacement dynamics of Se by S is investigated in kesterite Cu 2 ZnSn(S,Se) 4 and Cu 2 ZnGe(S,Se) 4 . Two different post-sulphurization techniques yield the same results. In the initial stages of the replacement, the original selenium-rich phase coexists with a second mixed phase. This phase has an increased sulphur content of 30 %. In later stages of the reaction, the initial phase disappears in favor of the second phase. Thus, sulphur inclusions of less than 30% seem impossible while above 30% the sulphur amount is adjustable.

Multiple-cation mixed-halide perovskites show high power-conversion efficiencies and recently improved stability. But, even most advanced absorber materials still suffer from instabilities of the bandgap under illumination and applied bias. Here, we employ modulation spectroscopy as a highly sensitive electro-optical measurement technique to directly reveal such instabilities. We find a reversible decrease of the absorber bandgap of up to 70 meV in complete solar cells. In situ X-ray diffraction measurements under illumination and bias confirm structural changes of the perovskite and their reversibility, which are attributed to a segregation of the halides. These processes are most strongly activated when illumination and bias are combined, which leads to a 5 times increased shift compared to that with illumination only. Since this scenario is intrinsic to the solar cell's operation conditions, it can never be avoided completely. Furthermore, the bandgap decrease is strongly enhanced, but still reversible, by high relative humidity and oxygen content supporting the strict requirement of efficient encapsulation.

Based on solution-processed kesterite-type Cu2ZnSn(S1–x,Sex)4 solar cells, a detailed investigation of the source and effects of sodium in the absorber will be presented. In contrast to most investigations in literature so far, the main process for sodium supply during selenization does not seem to be the diffusion out of the sodium-containing glass substrate but a transfer from the environment into the absorber. Studies on the influence of sodium on the morphological properties of the absorbers let us assume that the formation of big grains is not necessarily coupled to the presence of sodium during selenization. However, a beneficial effect on the electrical properties can mainly be seen in the open-circuit voltage (Voc) and the fill factor (FF) which should be related to an increase in doping concentration (NA).

We report on the fabrication of epitaxial Cu2ZnSnSe4 films by a two-step fabrication approach. An epitaxial ZnSe(001) layer on GaAs(001) is grown by molecular-beam epitaxy followed by sequential deposition of Cu and Sn. The Sn/Cu/ZnSe(001) precursor is then thermally annealed in a selenium atmosphere. Raman spectroscopy confirms the presence of the kesterite phase. Electron microscopy shows that the films exhibit monocrystalline regions of several micrometers in size with inclusions of smaller grains with a different chemical composition. The latter is confirmed by electron backscatter diffraction measurements which prove the conservation of the crystal orientation defined by the cubic ZnSe/GaAs(001) precursor structure throughout the whole CZTSe film.

A post-sulphurization process for kesterite Cu 2 ZnSn(S,Se) 4 solar cell absorber layers is developed in order to control the sulphur concentration and thus the band gap of the resulting solar cell absorber layers. It is shown that at sulphurization temperatures above 450°C an efficient incorporation of S takes place. Varying temperature between 450°C and 600°C and hold time between 5 min and 1 hour the sulphur content can accurately be controlled, thus allowing band-gap tuning in the complete range between about 1 eV and 1.5 eV.

Changes in band gap energy of the buffer layer of kesterite Cu 2 ZnSn(S,Se) 4 solar cells were analyzed in dependence of different post-annealing temperatures and times using electroreflectance (ER). In order to measure the band gap of the buffer, the ER experiments were performed in “Brewster geometry” (angle of incidence and angle of detection equal Brewster's angle) allowing to reduce line shape distortions induced by interference effects and hence allowing reliable analysis of the ER spectra. Due to post-annealing, an irreversible reduction of the buffer layer's band gap is detected that increases for longer annealing times and higher temperatures. The decrease of band gap energy is attributed to an interdiffusion of atoms between the buffer and absorber layers.

Many plant surfaces, such as rose petals, display lens-like epidermal cells that are known to assist the collection and focusing of the sunlight. Those cells form an array with a high degree of structural irregularities including disorder in the height and orientation of the cells, and in their arrangement. In this study, we numerically analyze the influence of structural disorder on the optical properties of a 3D modeled epidermal cell array using ray tracing simulations. We conclude that the anti-reflection properties of such structures are almost unperturbed by disorder effects, although the latter can notably broaden the propagation angle distribution of the collected light. Those results also have a direct implication on the design of plant-inspired light management structures. This aspect is illustrated by introducing the example of a thin-film solar cell covered by a light harvesting epidermal cells replica and simulated for each of the three disorder types considered.

We explore the use of electroreflectance (ER) spectroscopy at room temperature to characterize simultaneously Cu(In,Ga)Se2 (CIGS) absorber and CdS/i-ZnO buffer layers of thin-film solar cells. Absorber layers are coevaporated from elemental sources. The CdS buffer layers are deposited by chemical bath deposition. We apply ER at different incidence angles to distinguish ER signals from interference fringes. Bandgaps, critical point energies, and broadening parameters that describe the band structure of absorber and buffer films are determined by fitting the ER spectra with a third-derivative functional form. Absorbers grown with a vertical gradient in the Ga/(Ga+In) (GGI) ratio are compared with cells without a GGI gradient but the same integral GGI compositions. We also measure samples with no gradient but different integral GGI ratios of 0 (CuInSe2), 0.3, 0.6, and 1 (CuGaSe2). The ER signal of CdS yields three excitonic transitions, consistent with theory.

Currently, the use of Zn(O,S) as buffer material for Cu(In,Ga) Se 2 (CIGS) solar cells is intensely studied in order to further boost the performance of these devices. In this context, nondestructive analytical tools are needed that enable the determination of buffer bandgap energies in the complete device. To this end, we developed a spectroscopic approach based on electroreflectance (ER). From a set of measured angle-resolved ER (ARER) spectra, an averaged modulus spectrum is numerically calculated. This method suppresses the commonly observed detrimental line-shape distortions due to interference effects in the layered device structure and thus enables the determination of bandgap energies even for thin buffer layers. To verify the working principle of ARER, we first apply it to CIGS absorber and CdS buffer layers. Then, we utilize it to investigate CIGS solar cells with Zn(O,S) buffers. All ARER results are compared to the results of diffuse ER, a technique previously developed for the suppression of interference fringes. We demonstrate that ARER is the superior ER method for nondestructive bandgap determination of thin buffer layers in complete CIGS solar cells. Moreover, a Cu containing compound was determined as a secondary phase in the Zn(O,S) buffer by combined ARER studies, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy.

Pressure dependent magnetotransport measurements have been carried out on a chlorine-doped Zn0.98Mn0.02Se epilayer with a carrier concentration below the critical concentration of the metal-insulator transition (MIT). The large positive magnetoresistance at low temperatures can qualitatively be related to the giant Zeeman splitting of the donor states. The increase of the resistivity and the magnetoresistance effects under hydrostatic pressure is mainly related to the increase of the electron effective mass in the conduction band. This increase of the effective mass is the reason for an increase of the donor depth and a corresponding shrinkage of the donor wavefunction, which in turn leads to a narrowing of the impurity band (IB). The splitting of this narrow IB, which results from an applied pressure, is the main reason for the enhancement of the large positive magnetoresistance effects under pressure.

Cu 2 ZnSn(S, Se) 4 (CZTSSe) shows broad and asymmetric photoluminescence spectra situated far below the absorption edge at low temperatures. The physical recombination paths for the observed transitions could not be assigned unambiguously yet in literature. Nevertheless we show in this contribution that the peak position of the photoluminescence at low temperatures can be used as an indirect measure of the degree of order in the Cu-Zn planes of the kesterite crystal structure. The degree of order can be changed easily by thermal annealing procedures. The photoluminescence for different degrees of order comprises an additional contribution which seems not to change its energetic position with the degree of order which is in contrast to the main radiative contribution and band parameters, i.e., the band gap. We attribute this transition to a secondary phase or some deep defect level which does not follow the CZTSSe band edge as determined by electroreflectance.

Optical cavities have been fabricated by overgrowth of truncated GaAs pyramids with a distributed Bragg reflector. The success of this overgrowth depends strongly on the crystallographic orientation of the pyramid facets and shows best results for { 1 1 4 } A facets. In order to fabricate mesas with precisely such facets, a wet-chemical etching process including several selective etching steps has been established. To determine the optical properties of these resonators, InAs quantum dots have been used as an internal broad-band light source. The quality factors for optical modes have been determined to range up to 8000 and show a dependency on cavity width.

In this study, we investigated the effect of in situ annealing on InAs quantum dots site-selectively grown on pre-structured GaAs substrates. A morphological transition is observed with original double dots merging into one single dot during annealing. This is accompanied by a reduction of quantum dots originally nucleating between defined sites. The photoluminescence intensity of annealed site-selective quantum dots is compared to annealed self-assembled dots with linewidths of single dot emission of about 170 and 81 μeV, respectively. UV-ozone cleaning is used to optimize the sample cleaning prior to quantum dot growth.

We investigate nuclear spin polarization in a single InGaAs quantum dot via electrical injection of spinpolarized electrons in a spin-injection light-emitting diode. The Overhauser shift in the single-dot electroluminescence is compared with results obtained for optical spin injection in the same structure. The results clearly demonstrate the possibility of efficient nuclear spin polarization by purely electrical means which might be useful for future spintronic applications. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to identify the impact of the degree of Cu–Zn order in kesterite Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails, we perform photoluminescence (PL), photoluminescence excitation, and time-resolved photoluminescence (TRPL) spectroscopy. The PL lineshape and further PL characteristics such as state filling are analysed as a function of Cu–Zn order. Furthermore, TRPL decays and band tails are quantified. No significant modification in the defect states is caused by changes in Cu–Zn order, meaning that the formation of the defect states is not mainly determined by disorder in the Cu–Zn plane. In regard to band tailing, a small tendency to a decrease in the tailing parameter for the states with a high degree of Cu–Zn order compared to states with a low degree of Cu–Zn order is obvious. However, this reduction is small compared to the reduction of the defect density accompanied by the increase in the degree of Cu–Zn order. Hence, band tails are not mainly formed due to disorder in the Cu–Zn planes.

Pyramidal micro-cavities represent a novel promising class of semiconductor optical cavities. In contrast to our previous approach based on pyramids sitting on distributed Bragg reflectors, we investigate reversed freestanding GaAs pyramids. The latter are achievable by a wet-chemical etching process where an AlAs sacrificial layer in the epitaxially grown layer structure is used. In freestanding GaAs pyramids, light is simply confined by total internal reflection at the interface of the high refractive index material GaAs to the surrounding. Due to strong optical confinement within the pyramidal shape, small mode volumes are expected. Quality factors up to 3000 were measured in first structures. However, simulations suggest the possibility of much higher values. Therefore, these freestanding pyramids are promising for an optimized ratio between quality factor and mode volume, which is crucial for quantum-optical applications.

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The spin of a single electron confined in a semiconductor quantum dot is a possible candidate for a qubit to realize quantum information processing. Here we discuss the initialization of the electron spins in single quantum dots using a semi-magnetic spin aligner layer. We have succeeded in the concurrent initialization of five quantum dots with near-unity fidelity. The electron spin is read-out by observing the polarization of the emitted photons. These originate from excitonic recombination of the electron with an unpolarized hole in the dot. We find dots emitting highly polarized photons over the whole inhomogeneously broadened quantum-dot ensemble emission spectrum. However, a pronounced dependency between the statistical average of the polarization degree and the quantum-dot ground-state energy is found. The influence of charged-exciton recombination due to donors and defects is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pyramidal microcavities are a new class of optical resonators with potentially small mode volume and high quality factor. Our GaAs pyramids with embedded InGaAs quantum dots are placed on top of GaAs/AlAs distributed Bragg reflectors to increase the optical confinement at the base of the pyramids. The pyramidal shape is achieved by a wet-chemical etching process using an AlAs sacrificial layer. Temperature-dependent micro-photoluminescence measurements are used to verify optical modes.

We investigate the dynamics of electrons injected into InAs/GaAs quantum dots by initializing and further observing the spin state of the electrons. For this purpose, we use spin polarized light-emitting diodes where the electron spin is set in a semimagnetic ZnMnSe layer. We find that the degree of optical polarization depends strongly on the ground state energy of the quantum dot. A dependence of polarization on dopant concentration in the spin aligner suggests an influence of residual electrons in the quantum dots.

We investigated InGaAs layers grown by molecular-beam epitaxy on GaAs (001) with transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 °C. Island formation is observed for the layer with the highest In-concentration. Inconcentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can well be fitted applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557] and are in excellent quantitative agreement with the photoluminescence peak positions. From our data we conclude that island formation occurs when the amount of Indium in the In-floating layer reaches 1.1±0.2 monolayers indium.

Electromodulated reflectance (ER) is a standard characterization method to determine critical points such as the band gap in the band structure of semiconductors. These critical points show up as spectrally narrow features in ER and are typically evaluated using Aspnes's third-derivative functional form. ER spectra of stratified semiconductor systems such as thin-film solar cells, however, are significantly distorted by optical interference due to their layered structure. Furthermore, strong built-in electric fields result in a deviation from the typically assumed low-field conditions. We present here simulations of ER spectra from stratified systems based on transfer matrices using the Franz-Keldysh theory in its general form. For realistic thin-film solar cell conditions, distortions of ER line shapes due to the above-mentioned interferences and strong electric fields appear in the simulations. Furthermore, the results show good agreement with measured ER spectra of a structurally well-characterized $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ (CIGS) solar cell. Our analysis points out the restrictions on the determination of energetic position and number of critical points from ER spectra of stratified systems.

We investigate the impact of order-disorder related band gap changes on finished Cu2ZnSn(S,Se)4 solar cells. Solution-processed solar cells are post-annealed in a tube furnace in order to modify the Cu-Zn disorder within the kesterite unit cell. A decrease in this disorder leads to an increased band gap of the absorber. The latter is detected using electroreflectance spectroscopy. In our experiments the change in the open-circuit voltage follows the change in the absorber band gap. However, the open-circuit voltage deficit of the solar cell remains unaltered. Still, power conversion efficiencies could be increased by 1%, mainly due to an increased open-circuit voltage.

Cu2ZnSn(S,Se)4 absorbers show disorder in the Cu-Zn lattice planes, which affects the band gap energy. The degree of disorder can be changed via post-annealing treatments. We report on the influence of this Cu-Zn disorder on the radiative recombination transitions in Cu2ZnSn(S,Se)4 solar cells. Temperature- and power-dependent photoluminescence meas-urements were performed to evaluate this influence. With increasing excitation power the samples show a large blue-shift of the photoluminescence peak energy while with increasing temperature at constant excitation power they show a red-shift. From an analysis of the shifting behavior as a function of the degree of Cu-Zn disorder we conclude that the degree of disorder does not alter the dominant recombination mechanism significantly.

The influence of the growth conditions during molecular-beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum-dot spin-injection light-emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate electrical spin injection from a semi-magnetic n-type ZnMnSe spin aligner into III–V p–i–n diodes with InGaAs quantum dots (QDs) in the active layer. Quantitative transmission electron microscopy techniques are applied to characterize the different dot types used. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values of more than 70% are obtained for the wetting layer and high-energy QD states. However, the CPD shows a strong spectral dependence due to spin relaxation at the stage, before the electrons are finally captured in the dots. This is, e.g., evidenced by an initial increase of the polarization degree with rising temperature, attributed to motional narrowing effects. As a prerequisite for more detailed studies, we also demonstrate electrical spin injection into single InGaAs QDs, which should provide the basis for future single spin manipulation experiments. Finally, we suggest GaInNAs as optically active material for the realization of spin-polarized light-emitting diodes with long-wavelength emission. First results indicate CPD values of up to 80% for λ = 1130 nm, suggesting this approach to be very promising. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the injection of electron spins into InGaAs quantum dots with an efficiency of up to 60%. This injection is observed in p-i-n light-emitting diode structures using the diluted magnetic semiconductor ZnMnSe as spin aligner (spin-LED). The degree of spin polarization is deduced from the circular polarization degree of the photons emitted when the injected electrons recombine in the quantum dots with unpolarized holes. We observe a strong energy dependence of the polarization degree with a strong increase starting from zero to a high value on the high energy side of the emission spectrum. To study the origin of this dependence, we compare results of two quantum-dot samples with emission peaks at 1.2 eV and 1.33 eV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present far-infrared (FIR) reflection studies for Zn, 1-x Mn x Se. It is found that a modified oscillator model provides improved agreement with the experimental data. Our results suggest an intermediate-mode behavior for the optical phonon modes in Zn 1-x Mn x Se. In addition to the known ZnSe-like and MnSe-like phonon resonances a weak defect-related feature 'D' is found below the MnSe-like phonon band. The frequency of this feature shows a temperature and Mn dependent red-shift. Furthermore, the observation of a drastic reduction in the free-electron concentration with increasing Mn content suggests a compensation effect related to the formation of Mn-related defect complexes.

The boundary conditions were investigated for the electron wavefunction in Ga1−xInxNyAs1−y-based heterostructures described by the band anticrossing model. Among other advantages, the utilisation of these boundary conditions simplifies the calculation of, for example, transition energies in quantum wells. The derived equations were applied to model the temperature-dependence of the effective bandgap in Ga1−xInxNyAs1−y/GaAs quantum well structures with high indium concentration. From a fit to the experimental photoreflectance data, evidence was found to show that the nitrogen level EN in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This extends similar results reported in the literature for low indium content epilayers.

Quantitative high-resolution transmission electron microscopy was applied to determine the indium and nitrogen distributions in a GaInNAs/GaAs heterostructure. Two almost identical samples grown by gas-source molecular beam epitaxy on GaAs (001) substrates were investigated. The samples contained InGaAs and GaInNAs wells with the same thickness and In concentration. According to high-resolution X-ray diffractometry the average In concentration is 27% in both samples, with an N concentration of 1.1% in the GaInNAs quantum well. The evaluation of the photoluminescence peak position of the InGaAs sample and additional photoreflectance measurements yielded an In concentration of 30%. In-concentration profiles were obtained with the composition evaluation by the lattice fringe analysis (CELFA) technique for the InGaAs well using the chemical sensitivity of the (002) reflection. The averaged In concentration in the InGaAs quantum well was determined to be (30±1)% in good agreement with the other measurement techniques. The N-concentration profiles in the GaInNAs wells were measured by the comparison of the CELFA results in the samples with and without nitrogen. The N distribution is characterised by two maxima close to the GaInNAs/GaAs interfaces. The measured N concentration in the central part of the quantum well is (2.5±1.0)%.

In this paper we derive and discuss the boundary conditions for the electron wavefunction in general Ga1−xInxNyAs1−y-based heterostructures described by the band anticrossing model. The use of these boundary conditions greatly simplifies the calculation of, for example, transition energies in quantum wells. We then apply the derived equations to model the temperature-dependent bandgap of Ga1−xInxNyAs1−y/GaAs quantum well structures with high indium concentrations. From a fit to our experimental photoreflectance data we find evidence that the effective nitrogen level EN in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This supports and extends similar results reported in the literature for low indium content epilayers.