Bias Dependent Photoluminescence Research Articles

Spatially resolved power conversion efficiency for perovskite solar cells via bias-dependent photoluminescence imaging

Spatially resolved power conversion efficiency for perovskite solar cells via bias-dependent photoluminescence imaging

Electric Field and Its Effect on Hot Carriers in InGaAs Valley Photovoltaic Devices

Power- and bias-dependent photoluminescence measurements are performed on devices with 25 and 100 nm InGaAs absorber layers to study the influence of the electric field on the maintenance and temperature of hot-carrier populations in valley photovoltaic III–V heterostructures. Photoluminescence and current density versus voltage measurements indicate that the electric field increases the hot-carrier temperature as a result of its role in accelerating low-energy carriers to the upper valleys. The net result of this behavior is a power-dependent hot phonon bottleneck at all fluences irrespective of absorbed power with a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nonzero</i> power-independent hot-carrier population.

Understanding of Photophysical Processes in DIO Additive-Treated PTB7:PC71BM Solar Cells

1,8-diiodooctane (DIO) additive is an important method for optimizing the morphology and device performance of polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7)-based polymer solar cells. However, the effect of DIO additive on charge photogeneration dynamics of PTB7-based polymer solar cells is still poorly understood. In this work, the effect of DIO additive on the carrier photogeneration dynamics, as well as device performance of PTB7: [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) solar cells was studied. Bias-dependent photoluminescence (PL) experiments of a neat PTB7 device show that the exciton cannot be dissociated by the electric field in the device within the operating voltage range, but it can be effectively dissociated by the high electric field. PL and time-resolved PL studies show that DIO additive reduces the phase size of PTB7 in the blend film, resulting in an increased exciton dissociation efficiency. The carrier recombination processes were studied by transient absorption, which shows geminate carrier recombination was suppressed in the DIO-treated PTB7:PC71BM device in ultrafast time scale. The increased exciton dissociation efficiency and suppressed carrier recombination in ultrafast time scale play an important role for DIO-treated PTB7:PC71BM solar cells to attain a higher power conversion efficiency.

Manipulation of parity and polarization through structural distortion in light-emitting halide double perovskites

Halide perovskite materials recently attracted wide attention for light-emitting applications. The intense white light emission and excited state lifetimes greater than 1 μs are the hallmarks of a good light-emitting material. Here, we provide a clear design strategy to achieve both of these aforementioned properties in a single material via the introduction of octahedral asymmetry in halide double perovskites Cs2AgMCl6 through iso-trivalent substitution at the M site. In the substituted Cs2AgMCl6, the presence of mixed M3+ sites distorts the [AgCl6]5- octahedra, affecting the parity of the valence and conduction band edges and thereby altering the optical transitions. The distortion also creates a local polarization that leads to an effective photogenerated carrier separation. Considering perovskite series with three M3+ cations, namely Bi3+, In3+ and Sb3+, the mixed trivalent cationic compounds with specific ratios of In3+ and Bi3+ show white light emission with intensity nearly 150 times larger than that of the parent compounds, and are characterised by excited state lifetimes nearing 1 μs. Using single crystal X-ray diffraction, far-infrared absorption, steady-state and time-resolved photoluminescence, bias-dependent photoluminescence, P-E loop traces and density-functional theory calculations, we hence demonstrate the role of octahedral distortion in enhancing white light emission and excited state lifetimes of halide double perovskites.

Bias-dependent time-resolved photoluminescence spectroscopy on 265 nm AlGaN-based LEDs on AlN substrates

Time-resolved photoluminescence spectroscopy under an external bias is performed on 265 nm AlGaN-based LEDs on AlN substrates. The bias dependences of the photoluminescence wavelength, intensity, and decay time are observed. Our experimental results indicate that the built-in electric field has the opposite sign as the polarization-induced electric field in the quantum-well layers. These results agree with the first-principles calculations but are contrary to a previous experimental study. Additionally, thermionic and tunneling escape processes from the quantum-well layers play a minor role in the non-unity current injection efficiency at room temperature under a low injection (non-droop) regime.

Photoelectrochemical response of GaN, InGaN, and GaNP nanowire ensembles

The photoelectrochemical responses of GaN, GaNP, and InGaN nanowire ensembles are investigated by the electrical bias dependent photoluminescence, photocurrent, and spin trapping experiments. The results are explained in the frame of the surface band bending model. The model is sufficient for InGaN nanowires, but for GaN nanowires the electrochemical etching processes in the anodic regime have to be considered additionally. These processes lead to oxygen rich surface (GaxOy) conditions as evident from energy dispersive X-ray fluorescence. For the GaNP nanowires, a bias dependence of the carrier transfer to the electrolyte is not reflected in the photoluminescence response, which is tentatively ascribed to a different origin of radiative recombination in this material as compared to (In)GaN. The corresponding consequences for the applications of the materials for water splitting or pH-sensing will be discussed.

Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells.

This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance-voltage (CV) signal is observed as a broad "shoulder" in the depletion region from -0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA+ with difficult-rotational formamidinium (FA+) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.

Determination of the radiative efficiency of GaN-based light-emitting diodes via bias dependent resonant photoluminescence

We report a method to determine the radiative efficiency (ηrad) of GaN-based light-emitting diodes using excitation density and bias dependent room temperature photoluminescence (PL) measurements selectively exciting the active region. Considering carrier escape by tunnelling out of the active region, we extrapolate the generation rate of charge carriers from photocurrent measurements under reverse bias. A model describing the recombination of carriers including phase-space filling is then fitted to excitation density dependent PL data obtained under forward bias to extract ηrad. Results show that ηrad vs. carrier density is asymmetric around its maximum due to phase-space filling.

Spectral Behavior of Bias-Dependent Photocurrent and Photoluminescence in Sputtered ZnO Layers

The bias-dependent behavior of the photocurrent (PC) and photoluminescence (PL) of sputtered ZnO layers has been investigated. Based on PC spectroscopy results, the PC intensity of the observed free exciton increased strongly up to electric field of 60 V/cm, after which its rate of increase slightly reduced due to disturbance of field-assisted dissociation of radical ion pairs, which leads to photocarrier generation. Thus, the energy of excitonic PC peaks showed a tendency to red-shift with increasing electric field, being attributed to the induced Stark effect. Therefore, it is concluded that the strong interaction between free excitons and photogenerated PC carriers leads to displacement or widening of the spectrum. In the PL measurements, near-band-edge (NBE) and violet emissions were observed. With increasing electric field, two PL emissions were progressively quenched. The combined PL/PC results reveal that the PL ions associated with the NBE and violet emissions readily interact with the PC carriers of photogenerated electrons and holes. This behavior reduces the recombination ratio and the lifetime of PL ions. So, the PL intensity quenching originates from a decrease in the number of carriers participating in recombination. Consequently, we find that the quenching mechanism of the NBE and violet emissions is strongly related to low external electric field.

Tilted dipole model for bias-dependent photoluminescence pattern

In a guest-host system containing elongated dyes and a nematic liquid crystal, both molecules are aligned to each other. An external bias tilts these molecules and the radiation pattern of the system is altered. A model is proposed to describe this bias-dependent photoluminescence patterns. It divides the liquid crystal/dye layer into sub-layers that contain electric dipoles with specific tilt angles. Each sub-layer emits linearly polarized light. Its radiation pattern is toroidal and is determined by the tilt angle. Its intensity is assumed to be proportional to the power of excitation light absorbed by the sub-layer. This is calculated by the Lambert-Beer's Law. The absorption coefficient is assumed to be proportional to the cross-section of the tilted dipole moment, in analogy to the ellipsoid of refractive index, to evaluate the cross-section for each polarized component of the excitation light. Contributions from all the sub-layers are added to give a final expression for the radiation pattern. Self-absorption is neglected. The model is simplified by reducing the number of sub-layers. Analytical expressions are derived for a simple case that consists of a single layer with tilted dipoles sandwiched by two layers with horizontally-aligned dipoles. All the parameters except for the tilt angle can be determined by measuring transmittance of the excitation light. The model roughly reproduces the bias-dependent photoluminescence patterns of a cell containing 0.5 wt. % coumarin 6. It breaks down at large emission angles. Measured spectral changes suggest that the discrepancy is due to self-absorption and re-emission.

Room temperature polariton light emitting diode with integrated tunnel junction

We present a diode incorporating a large number (12) of GaAs quantum wells that emits light from exciton-polariton states at room temperature. A reversely biased tunnel junction is placed in the cavity region to improve current injection into the device. Electroluminescence studies reveal two polariton branches which are spectrally separated by a Rabi splitting of 6.5 meV. We observe an anticrossing of the two branches when the temperature is lowered below room temperature as well as a Stark shift of both branches in a bias dependent photoluminescence measurement.

Electro-optic properties of GaInAsSb/GaAs quantum well for high-speed integrated optoelectronic devices

The electro-optic properties of strained GaInAsSb/GaAs quantum wells (QWs) are investigated. A single QW p-i-n sample was grown by molecular beam epitaxy with antimony (Sb) pre-deposition technique. We numerically predict and experimentally verify a strong quantum confined Stark shift of 40 nm. We also predict a fast absorption recovery times crucial of high-speed optoelectronic devices mainly due to strong electron tunneling and thermionic emission. Predicted recovery times are corroborated by bias and temperature dependent time-resolved photoluminescence measurements indicating (≤30 ps) recovery times. This makes GaInAsSb QW an attractive material particularly for electroabsorption modulators and saturable absorbers.

Effect of GaAs Step Layer Thickness in InGaAs/GaAsP Stepped Quantum-Well Solar Cell

A multiple-stepped quantum-well (MSQW) solar cell, in which GaAs step layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and the improved sub-GaAs-bandgap quantum efficiency has been demonstrated. The optical properties of MSQW solar cells with different GaAs step layer thickness are investigated. The recombination losses inside the QWs have been studied by bias-dependent photoluminescence. The recombination losses decrease with increasing the GaAs step layer thickness. Controlling the GaAs step layer thickness is a feasible way to increase short-circuit current without largely degrading open-circuit voltage.

Characterization of InGaN-based photovoltaic devices by varying the indium contents

InGaN-based photovoltaic structures with different indium contents in InGaN active layer were investigated. The peak wavelengths of the photoluminescence (PL) spectra were measured at 464.3nm and 525.4nm for the blue-photovoltaic device (B-PV) and the green-photovoltaic device (G-PV) structures, respectively. The flat-band voltage was observed at −8V through the lower temperature (10K) bias-dependent PL measurement in B-PV device, but the flat-band voltage cannot be observed in G-PV device when the reverse bias was larger than −15V. The piezoelectric field (PZ) of the G-PV device was larger than the B-PV device through the bias-dependent PL measurement that was caused by the large lattice mismatch between the GaN and InGaN layers with high indium content. The small open-circuit voltage and the larger short-circuit current density (Jsc) were measured in the G-PV device compared with the B-PV device. The large Jsc value of the G-PV device was caused by the large photocurrent escaped from the PZ-field induced tilted-band structure in InGaN active layer. The InGaN-based PV devices with a large indium content and a lager PZ field had potential for the InGaN photovoltaic device applications.

Enhanced Carrier Escape in MSQW Solar Cell and Its Impact on Photovoltaics Performance

A multiple stepped quantum wells (MSQWs) solar cell, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and improvements in short-circuit current and fill factor have been demonstrated. We have studied carrier recombination dynamics and carrier escape kinetics from the wells, comparing the MSQW with the normal multiple quantum well (MQW). Carrier recombination rate was examined by time-resolved photoluminescence (PL) and a longer carrier lifetime was observed for MSQWs than for MQWs. Carrier escape from the wells was also investigated in terms of temperature-dependent PL. Smaller thermal activation energy was observed for MSQWs than for MQWs. Carrier radiative recombination loss was investigated by bias-dependent PL, and it was found to be smaller for the MSQW than for the normal MQW. Such advantages of the MSQW allowed us to stack a sufficient number of quantum wells to increase short-circuit current without degrading fill factor. For normal MQW solar cells, degradation in fill factor is unavoidable.

Reduction of internal polarization fields in InGaN quantum wells by InGaN/AlGaN ultra-thin superlattice barriers with different indium composition

The internal well polarization field in InGaN quantum wells (QWs), surrounded by strain-compensated (InxGa1−xN)/(Al0.065Ga0.935N) ultra-thin superlattice (SC-SL) barriers with different indium composition, is investigated. The indium composition of InGaN constituent of superlattice barriers has been varied in the range from 0.04 to 0.18. It is observed that the increase of indium composition of InGaN into the barrier results in a strong blue-shift of the peak wavelength of the room-temperature photoluminescence (RT-PL) and the significant increase in the intensity of the luminescence emission until too much indium is added into InGaN layers of superlattice barriers. From the bias-dependent photoluminescence measurements, it is determined that the blue-shift and intensity increase of the emission are caused by the decrease of well polarization field as the indium composition in InGaN of SC-SL barrier increases. In case of In0.16Ga0.84N containing SC-SL barriers, the well internal polarization field is greatly reduced to −0.33 MV/cm from −1.5 MV/cm with respect to typical GaN barriers, indicating that the internal field reduction similar to that obtained in semi-polar InGaN/GaN quantum wells can be obtained by applying the strain-compensating barrier to polar substrates.

Effects of strain-compensated AlGaN/InGaN superlattice barriers on the optical properties of InGaN light-emitting diodes

In this article, metalorganic chemical vapor deposition (MOCVD)-grown InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with Al0.03Ga0.97N and Al0.03Ga0.97N/In0.01Ga0.99N superlattices-barrier layers on c-plane sapphire were studied for the influence of the strain-compensated barrier on the optical properties of the LEDs. High-resolution X-ray diffraction (HRXRD) analysis shows that the LEDs with a strain-compensated superlattice barrier (SC-SLB) have better interface quality than those using AlGaN. This difference in quality may result from the alleviation of strain relaxation in superlattice layers to improve the crystalline perfection of the epitaxial structures. It was also found that the degree of the exciton localization effect rises considerably as InGaN grows directly on the AlGaN barrier layers. However, the increase in the strength of the polarization fields within the MQWs (as evaluated from bias-dependent photoluminescence (PL) measurement) could reduce the radiative efficiency of the LEDs and shift their PL peaks toward long wavelengths. With suitable control of crystalline quality and the reduced quantum-confined Stark effect in the MQWs, the SC-SLB LEDs operating at 150-mA-current show a 22.3% increase in light output power as compared to their conventional counterparts.

Role of Morphology on Photoluminescence Quenching and Depletion Width Formed at the Interface of Aluminum and Poly(3-alkylthiophene)

The interface formed at aluminum (Al) and poly(3-alkylthiophene) (P3AT) has been studied using bias dependent photoluminescence (PL) quenching in indium tin oxide (ITO)/P3AT/Al sandwiched cells. Different types of bias dependent PL quenching patterns were obtained by varying the alkyl chain length and the regioregularity of the P3AT. Observed results indicate that the PL quenching and depletion layer are strongly correlated to the morphology in these P3AT films. Further, it has also indicated about the spatial charge carrier distribution in these films. Moreover by changing the alkyl chain length a complete trend reversal was obtained between the PL quenching and the Mott–Schottky plot of these cells. These results have been explained on the basis of Field induced PL quenching in these films.

Carrier Dynamics of Deep-Level States in InGaN/GaN Multiquantum Wells

InGaN/GaN (3 nm/10 nm) multiquantum wells (MQWs) were grown on (0001) sapphire substrates by metal–organic chemical vapor deposition. To characterize the effect of internal electric fields on deep-level states in InGaN MQWs, deep-level transient spectroscopy (DLTS) and bias-dependent photoluminescence (PL) measurements were performed. The thermal activation energy of MQWs obtained from DLTS spectra decreased to 0.15 eV with an increase in reverse measure bias from -2 to -10 V, which indicates partially cancelled internal fields. The blue shift of PL peak energy was about 0.05 eV when the reverse measure bias was changed from 0 to -5 V. This is explained well by carrier dynamics in the conduction band of InGaN/GaN MQWs, which affects carrier recombination energies corresponding to the band edge to edge transitions with changing reverse measure bias.

Voltage-controlled nuclear polarization switching in a singleInxGa1−xAsquantum dot

Sharp thresholdlike transitions between two stable nuclear-spin polarizations are observed in optically pumped individual ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ self-assembled quantum dots embedded in a Schottky diode when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the Overhauser field in the dot by up to 3 T. The bias-dependent photoluminescence measurements reveal the importance of the electron-tunneling-assisted nuclear-spin pumping. We also find evidence for the resonant LO-phonon-mediated electron cotunneling, the effect controlled by the applied bias and leading to the reduction of the nuclear-spin pumping rate.