Halide perovskites have emerged as a novel class of revolutionary semiconductors with wide tunability of energy bandgap, low cost, and simple solution process for optoelectronic devices such as LEDs, and photodetectors. In this talk, we will discuss the effects of ligands on low-dimensional perovskites including the defects passivation, phase distribution, carrier transportation, and confinement for improving the efficiency and stability of perovskite LEDs. Meanwhile, by introducing the new approaches of double-side crystallization and passivation, we realize high-performance perovskite photodetectors with a wide detection range from UV to NIR will be described. The work contributes to paving perovskites for practical optoelectronics.
Two-dimensional (2D) perovskites with organic spacer ligands are promising materials with superior stability and diversity for various photovoltaic devices. While traditional perovskite precursor solutions using high boiling point solvents easily cause poor uniformity, we demonstrate a new method for rapid crystallization of 2D perovskite by adopting low boiling point solvents. Our results show that 2D perovskite films have a high quality and their processing is simplified and shortened. Photoconductors are made based on the 2D perovskite films and applied in visible light and X-ray detection effectively. These findings suggest the easily processed 2D perovskites promising for practical optoelectronic applications.
Quasi-2D perovskite is considered as a promising candidate for blue perovskite light emitting diode (PeLEDs). However, the wide distribution of low-order phases, inefficient energy transfer, and defects/traps increase the non-radiative recombination, further aggravating PeLED external quantum efficiency (EQE). We demonstrate a unique quasi-2D perovskite with low-order phase suppression and defect passivation by incorporating a 2D perovskite and an excess ammonium salt into the quasi-2D perovskite. By optimizing the new quasi-2D perovskites, we achieve blue PeLEDs with brightness 1765 cd m-2, EQE 7.51%, low turn-on voltage 3.07 V, and long operation lifetime 3961 sec without any shift in electroluminescent spectra.
Hole transport layer (HTL) plays a critical role for achieving high performance solution-processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p-type inorganic semiconductor-based effective HTL on top of organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, the p-type top HTLs are mainly two-dimensional (2D) materials, which have vertical conduction limitation intrinsically and is too thin to function as practical HTL for large area optoelectronic applications. Here, we demonstrate a novel self-assembled quasi three-dimensional (3D) nanocomposite as a p-type top HTL [1]. Remarkably, the novel HTL achieves ~15 times enhanced conductivity and ~16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non-fullerene systems, device performance is significantly improved. The champion power conversion efficiency (PCE) reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance.
[1] J. Cheng, H. Zhang, Y. Zhao, J. Mao, C. Li, S. Zhang, K.S. Wong, J. Hou, W.C. H. Choy, "Self-assembled Quasi-3D Nanocomposite: A Novel p-Type Hole Transport Layer for High Performance Inverted Organic Solar Cells", Adv Funct. Mater., DOI:10.1002/adfm.201706403.
Recently, researchers have focused more to design highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. Here, we demonstrated that vacuum-assisted thermal annealing can be used to control the composition, morphology, and thus the quality of the perovskite films formed from the precursors of PbCl2 and CH3NH3I. Using our vacuum-assisted thermal annealing approach to completely remove the chlorine byproduct, pure, pore-free planar CH3NH3PbI3 films with enhanced morphology can be readily formed for high efficiency PVSCs with high stability and reproducibility. In addition, we will report new room temperature approaches for forming PVSCs. Regarding the hole transport layer (HTL), NiOx is a promising material for candidate for fabricating efficient PVSCs. Here, we demonstrate the flawless and surface-nanostructured NiOx film from a simple and controllable room-temperature solution process. Meanwhile, we will propose a new room temperature scheme formation of perovskite films with the features of PbI2 residue-free, large grain-sizes, and highly crystalline. We further layout the design rules for the broad, rational extension of our scheme to form high-quality perovskite films. Using our approach, a room-temperature processed PVSC is obtained with no hysteresis, high power conversion efficiency of about 18%, which is the best of the PVSCs fabricated by low-temperature techniques to date. Additionally, the device is very stable with performance maintance of 95% after 1000 hours. This work contributes to the large-sale and low-cost production of PVSCs with high device performances.
The lead halide-based perovskite solar cells have emerged as a promising candidate in photovoltaic applications. However, the precise control over the morphologiy of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, even though both will be necessary for further advancements. Here we introduce vacuum-assisted thermal annealing as a means of controlling the composition and morphology of the CH3NH3PbI3 films formed from PbCl2 and CH3NH3I as precursors. We identify the critical role that the CH3NH3Cl generated as a byproduct during the pervoskite synthesis plays for the photovoltaic performance of the perovskite film. Removing this byproduct through vacuum-assisted thermal annealing we succeeded in producing pure, pore-free planar CH3NH3PbI3 films showing high conversion efficiency (PCE) reaching 14.5%). Removal of CH3NH3Cl strongly attenuate the photocurrent hysteresis.
As the intrinsic electrostatic limit, space charge limit (SCL) for photocurrent is a universal phenomenon which is fundamental important for organic semiconductors. We will demonstrate SCL breaking by a new plasmonic-electrical concept. As a proof-ofconcept, organic solar cells (OSCs) comprising metallic planar and grating electrodes are studied. Interestingly, although strong plasmonic resonances induce abnormally dense photocarriers around a grating anode, the grating incorporated inverted OSC is exempt from space charge accumulation (limit) and degradation of electrical properties. The plasmonic-electrical concept will open up a new way to manipulate both optical and electrical properties of semiconductor devices simultaneously.
Bragg scattering by one dimensional periodic structures is investigated in order to enhance the outcoupling effciency of optically optimized planar top-emitting OLEDs. Using a soft imprint process, we fabricate extremely homogeneous gratings with sub- m period. These gratings are integrated beneath the bottom contact of topemitting OLEDs, without affecting the electrical device performance. The reflective contacts of the top emission geometry introduce pronounced micro-cavity effects for directly outcoupled and internally trapped light modes. Bragg scattering of the trapped waveguided and surface plasmon modes into the air cone, i.e. the forward direction, leads to interference with the directly outcoupled mode. As a result, constructive and destructive interference of the modes is detected and analyzed. Overall, we find that the introduction of shallow one dimensional sub- m periodic grating structures underneath top-emitting OLEDs leads to an EQE and luminous efficacy enhancement by up to 42%.
Although various optical designs and physical mechanisms have been studied both experimentally
and theoretically to improve the optical absorption of organic solar cells (OSCs) by incorporating
metallic nanostructures, the effects of plasmonic nanostructures on the electrical properties of OSCs
is still not fully understood. Hence, it is highly desirable to study the changes of electrical properties
induced by plasmonic structures and the corresponding physics for OSCs. In this work, we develop a
multiphysics model for plasmonic OSCs by solving the Maxwell’s equations and semiconductor
equations (Poisson, continuity, and drift-diffusion equations) with unified finite-difference method.
Both the optical and electrical properties of OSCs incorporating a 2D metallic grating anode are
investigated. For typical active polymer materials, low hole mobility, which is about one magnitude
smaller than electron mobility, dominates the electrical property of OSCs. Since surface plasmon
resonances excited by the metallic grating will produce concentrated near-field penetrated into the
active polymer layer and decayed exponentially away from the metal-polymer interface, a
significantly nonuniform and extremely high exciton generation rate is obtained near the grating.
Interestingly, the reduced recombination loss and the increased open-circuit voltage can be achieved
in plasmonic OSCs. The physical origin of the phenomena lies at direct hole collections to the
metallic grating anode with a short transport path. In comparison with the plasmonic OSC, the hole
transport in a multilayer planar OSC experiences a long transport path and time because the standard
planar OSC has a high exciton generation rate at the transparent front cathode. The unveiled
multiphysics is particularly helpful for designing high-performance plasmonic OSCs.
Typically, most low bandgap materials have low absorption with wavelength at around 500 nm.
Besides, the restrictions of active layer thickness of thin film organic solar cells (OSCs) make the
devices reduce to absorb light in long wavelength region (around 700 nm). As absorption would be a
joint effect of material band properties and optical structures, well-designed light-trapping strategies
for these low-bandgap PSCs will be more useful to further enhance efficiencies. We investigate the
change of optical properties and device performances of organic solar cells based on our newly
synthesized low-bandgap material with embedded poly-(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) PEDOT:PSS grating in the photoactive bulk heterojunction
layer.
In this work, we focus on introducing an alternative approach to realize transparent graphene
anodes. We report the use of very thin thermally evaporated gold (Au) nanoclusters with proper
UVO treatments to facilitate efficient hole collection at graphene electrodes, which significantly
benefits device performance while avoiding issues arising from PEDOT:PSS. We will investigate the
effects of Au thickness and UVO treatments for optimizing device performance. Ultraviolet photoemission spectroscopy (UPS) is conducted to further analyze the WF shift at the
graphene/polymer interface modified by UVO-treated Au.
To enhance the light trapping of organic solar cells (OSCs), metallic (e.g. Au, Ag) nanoparticles
(NPs) have been incorporated into the polymer layers conveniently in solution process. Although
power conversion efficiency (PCE) of OSCs has been shown to improve by incorporating metallic
NPs in either the buffer layer such as poly-(3,4-ethylenedioxythiophene) :poly(styrenesulfonate)
(PEDOT:PSS)[1] or the active layer[2], the understanding on the changes is still not quite clear.
Moreover, there are very limited studies on incorporating metallic NPs in more than one organic
layer and investigating their effects on the optical and electrical properties as well as the
performances of OSCs. In this work, monofunctional poly(ethylene glycol) (PEG)-capped Au NPs of
sizes 18 nm and 35 nm are doped in the PEDOT:PSS and poly(3-hexylthiophene) (P3HT):
phenyl-C61-butyric acid methyl ester (PCBM) layers respectively, leading to an improvement of
PCE by ~22% compared to the optimized control device. We will firstly identify the impact of NPs
in each polymer layer on OSC characteristics by doping Au NPs in either the PEDOT:PSS or
P3HT:PCBM layer. Then, we will investigate Au NPs incorporated in all polymer layers. We
demonstrate that the accumulated benefits of incorporating Au NPs in all organic layers of OSCs can
achieve larger improvements in OSC performances.
Bright and efficient stacked color-tunable organic light emitting devices (OLEDs) using intermediate Al/Au electrode
have been reported. The effects of the thickness of Al and Au layer on the luminance characteristics have been
comprehensively studied. After the optimization, After the optimization, the bottom-emission single-unit OLED of
4,4',4"-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine /N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-
4,4'-diamine /tris(8-hydroxyquinoline) aluminum has a maximum luminance efficiency (ηL) of 3.37 by using Al/Au as
the cathode and 2.92 cd/A by using Al/Au as the anode. By introducing the optimized intermediate Al/Au electrode into
the stacked color-tunable OLEDs, red unit with maximum ηL of 4.73 cd/A and blue unit with maximum ηL of 3.96 cd/A
have been obtained. The color can be tuned efficiently along a linear route from pure red with the Commission
Internationale de l'Eclairage (CIE) coordinates of (0.662, 0.330) to sky blue with the CIE coordinates of (0.155, 0.340).
This scheme can be a potential candidate for achieving high brightness and efficient stacked color-tunable OLEDs.
In this paper, the effects of hole injection layer (HIL) on the performance of typically used tris-(8-hydroxyquinoline) aluminum (Alq3) based OLEDs have been investigated. Three different HIL materials were used: copper phthalocyanine (CuPc), magnesium phthalocyanine (MgPc) and zinc phthalocyanine (ZnPc). The Metallophthalocyanines (MPcs) will be used to construct single hole injection layer (HIL) and double HIL (d-HIL). In the OLEDs, Alq3 acts as the emitting layer and electron transport layer. Although d-HIL structures show higher efficiency than that of the reference device, the highest current efficiency ~ 4.02 cd/A corresponds to the 15 nm ZnPc HIL device. Compared to an current efficiency of ~3.29 cd/A and a power efficiency of ~0.99 lm/W (at 100 cd/m2 luminance) of the reference device, an 15 nm ZnPc HIL device has ~22% higher current efficiency and ~67% higher power efficiency. The reasons for the improvements will be discussed.
Electroluminescent bipolar small molecules have been attracted with great interests recently. They are found to exhibit many interesting features such as (i) reducing the structural complexity of organic light emitting diodes (OLEDs) from multilayer heterojunction to monolayer homojunction devices; (ii) offering molecular p/n junction, and (iii) minimizing the formation of exciplexes. In this paper, the optical and electrical properties of novel oxadiazole-triphenylamine derivatives will be investigated. The derivatives are N-phenyl-N-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (POT) and N-phenyl-N-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (m-POT). The absolute absorption coefficient and refractive index have been investigated by ellipsometry and modeling. The electron mobility of POT at room temperature has been studied. The results show that the derivatives have bipolar characteristics. The electron-transporting properties of POT is better than that of m-POT. The EL emission peaks of POT and m-POT are the same at 435nm which match with their photoluminescent (PL) peaks.
By engineering a new cohosting system of tris(8-hydroxyquinoline) and 4,7-diphenyl-1,10-phenanthroline in the electron transport layer, the current efficiency of the organic light emitting diode is improved by more than 20% while the bias is reduced by ~40% as compared to the device with a single host of Alq3 as the electron transport layer. The maximum luminance is over 16000 cd/m2 at the bias of 22V and the current of 475mA/cm2, which is ~73% higher than the single host Alq3 device without optimizing the layer thickness. The lifetime under ambient environment is enhanced by a factor of ~1.8. The reasons for the improvement will be investigated. The results strongly indicate that the knowledge of bulk conductivity engineering of organic n-type transporters shows practical significance in OLED applications.
Diffused quantum wells (DFQW) optical devices have been widely investigated for use in optical electronics integrated circuits. In this paper, we report on the performance of five-period DFQW optical amplifiers and modulators. The result show that the QW amplifiers and modulators maintain at single guiding mode operation after the QW structure has been annealed. The running range of the operation l wavelength of QW optical amplifiers is 34 meV without a significant degradation in the modal gain peak by interdiffusing the QWs. The QW interdiffusion was accomplished by P+ ion implantation to the upper region of the top cladding layer of the multilayer structure and followed by rapid thermal annealing such that the implanted ions did not damage the QW structures. The I-V characteristics of the implanted QW are similar to that of the unimplanted. Concerning the TE electro-absorptive modulation, a large contrast ratio of 35dB can be obtained at (lambda) op equals 1.55 micrometers under a small bias of -1.5V fora 500 micrometers long modulator. For TM mode, a slightly higher CR Of 37dB can be obtained at the operation wavelength although the reverse bias voltage is double.
A theoretical study of the polarization independent quantum- well gain using interdiffusion is presented here. Group V sublattice interdiffusion in InGaAs/InP quantum wells is used to produce polarization independent optical gain. The reverse bias and carrier effects on the subband structures, transition energy and optical gain of the interdiffused quantum well are discussed. The interdiffused quantum well structures are optimized in terms of their subband structure, carrier density, structural parameters and properties of optical gain spectra. The results show that an optimized interdiffused quantum well structure can produce polarization independent optical gain over a range of operation wavelengths around 1.5 micrometer, although the differential gain and linewidth enhancement factor are slightly degraded. The required tensile strain for the polarization independence of a lattice-matched quantum well structure is generated here using interdiffusion. These results suggest that polarization independent optical devices can be fabricated using interdiffusion using a lattice-matched InGaAsP quantum well structure.
Electro-optic modulators using the interaction of surface acoustic waves (SAWs) with III-V semiconductor multiple quantum well structures have gained interests. A SAW induces potential field which provides the phase modulation. In order to improve the phase modulation, an AlGaAs/GaAs asymmetric double quantum well (DQW) optical phase modulator using SAWs is investigated theoretically. The optimization steps of the DQW structure are discussed. The optimized phase modulator structure is found to contain a five-period DQW active region. Analysis of the modulation characteristics show that by using the asymmetric DQW, the large change of the induced potential at the surface and thus large modification of the QW structure can be utilized. The modification of each QW structure is consistent, although this consistency is not always preserved in typical SAW devices. Consequently, the change of refractive index in each of the five DQWs is almost identical. Besides, the change of effective refractive index is 10 times larger here in comparison to a modulator with a five periods single QW as the active region and thus produces a larger phase modulation. In addition, a long wavelength and a low SAW power required here increase the size of the SAW transducer and simplify its fabrication.
Modeling is used to show that interdiffusion can generate a polarization independent parabolic-like quantum well. Criteria to achieve the parabolic-like quantum wells by interdiffusion are discussed. The results indicated that interdiffused quantum wells can produce equal eigen-state spacing, polarization insensitive Stark shift and modulation characteristics similar to an ideal parabolic quantum well. The design process to obtain polarization insensitive ON- and OFF-states in the parabolic-like interdiffused quantum wells is discussed. The predicted modulation depth is comparable to those measured using parabolic quantum wells. The diffused quantum wells have the advantage of using an as-grown rectangular quantum well with post-growth annealing to tailor its confinement profile. These features suggest that the interdiffused quantum well structure can be used to product polarization insensitive electro-absorptive modulation.
The characteristics of Al0.3Ga0.7As/GaAs QW acousto- absorption and acousto-optic modulators using the interaction between surface acoustic wave (SAW) and quantum well (QW) optical waveguide structures are analyzed here theoretically. The QW structures are optimized by maximizing the optical confinement of modal field in the active region and the piezoelectric effect of SAW and QWs. The electric field induced by SAW reduces non-uniformly in depth, which limits in the development of high efficiency modulators, especially for devices with a large number of QWs in the active region. For devices with thin active regions, the QW structures are designed so that at the top surface strong SAW effects can be obtained while for the 25 periods structure, the QWs located at a depth of 2/3 SAW wavelength in order to obtain an uniform SAW induced electric field. The results show that the single and five QW devices are suitable for absorptive modulation and optical modulation respectively while the 25-QW modulators can shorten the modulation interaction length and thus increase modulation bandwidth. The effective index change of these devices are at least 10 times larger than the conventional surface acoustic wave devices. These result make the quantum-well modulators more attractive for the development of acousto- optic device applications.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.