Exciton Generation Rate Research Articles

Perovskite Photovoltaic Cell with Hole Transport Layer Based on a Polyaniline Complex

Perovskite solar cells with photoactive layer of methylammonium lead iodide and hole transport layer based on a polyaniline complex with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) have been developed for the first time. The power conversion efficiency of obtained cells is comparable with that of known analogs. Results of simulation of the optical parameters of cells in the framework of the Maxwell–Garnet model showed that the experimentally observed weak dependence of the power conversion efficiency on the perovskite layer thickness within 350–500 nm is related to the absence of significant variation of both the energy absorbed by the photoactive layer and the exciton generation rate.

Перовскитный солнечный элемент с дырочным транспортным слоем на основе комплекса полианилина

A perovskite solar cells with a photoactive layer of lead methylammonium iodide and a hole transport layer based on a complex of polyaniline and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) were developed for the first time. The power conversion efficiency of the devices obtained corresponds to the known analogues. According to the results of modeling the optical parameters of the cell within the Maxwell-Garnet model, the experimentally observed weak dependence of the power conversion efficiency of the device on the perovskite layer thickness in the range of 350–500 nm is found to result from negligible change in the amount of energy absorbed by the photoactive layer and the exciton generation rate.

Synergy study on charge transport dynamics in hybrid organic solar cell: Photocurrent mapping and performance analysis under local spectrum

Charge transport dynamics in ZnO based inverted organic solar cell (IOSC) has been characterized with transient photocurrent spectroscopy and localised photocurrent mapping-atomic force microscopy. The value of maximum exciton generation rate was found to vary from 2.6 × 1027 m−3s−1 (Jsat = 79.7 A m−2) to 2.9 × 1027 m−3s−1 (Jsat = 90.8 A m−2) for devices with power conversion efficiency ranging from 2.03 to 2.51%. These results suggest that nanorods served as an excellent electron transporting layer that provides efficient charge transport and enhances IOSC device performance. The photovoltaic performance of OSCs with various growth times of ZnO nanorods have been analysed for a comparison between AM1.5G spectrum and local solar spectrum. The simulated PCE of all devices operating under local spectrum exhibited extensive improvement with the gain of 13.3–13.7% in which the ZnO nanorods grown at 15 min possess the highest PCE under local solar with the value of 2.82%.

Studies of Graphdiyne‐ZnO Nanocomposite Material and Application in Polymer Solar Cells

Graphdiyne‐ZnO (GDZO) composite material is prepared via a simple method and studied in detail for the first time. The transmission electron microscopy, Raman spectroscopy and X‐ray photoelectron spectroscopy (XPS) analyses confirm the formation of an adduct between GD and ZnO. Then the interaction between ZnO and GD is further investigated by first‐principles calculations. It is found that the Zn and O atom can coordinate bonding with GD, thus forming the CZn bond and CO bond, respectively. Polymer solar cells are fabricated based on the nanocomposites for the first time and an enhanced power conversion efficiency of 11.2%, compared with the devices with ZnO‐only (10%), is obtained. Simultaneously, the resultant devices show better stability, whether in glove box or in atmosphere, with humidity of 90%. The investigation of exciton generation rate, ideal current‐voltage model, and impedance spectra verify that the introduction of GDZO not only accelerates electron transfer but also reduces charge recombination occurring at the interface. The results indicate that GDZO is a promising electron transport material to enhance solar cell performance and presents a large potential for optoelectronic applications as well.

Profiling exciton generation and recombination in conventional and inverted bulk heterojunction organic solar cells

Applying the optical transfer matrix method and the drift-diffusion equations, the efficient light absorption, exciton generation, and recombination rate in bulk heterojunction (BHJ) organic solar cells (OSCs) with conventional and inverted configurations are studied. Analysing the influence of the electric field component of the electromagnetic radiation propagating through the layered structure of BHJ OSCs and using contour plots of normalized modulus squared of the electric field, the constructive interference points (CIPs) which represent the positions of maximum absorption of photons and hence generation of excitons within the active layer are investigated for both the conventional and inverted OSCs. Also, the influence of the thicknesses of other layers in both the inverted and conventional structures is investigated. It is found that except the thickness of MoO3 in the inverted structure the thicknesses of other layers do not have any significant influence on CIPs. The maximum CIP occurs at an active layer thickness of 190 nm, regardless of the thickness of the second layer, which is MoO3, Ag, or ITO in the inverted structure and PEDOT:PSS, Al, or ITO in the conventional structure. The results of 3D plots of the normalized modulus squared of the electric field reveal that the absorption of photons at the end of the active layer in the inverted structure is higher than that in the conventional structure for all the effective wavelengths and different active layer thicknesses. It is expected that this study provides a deeper understanding of exciton generation within the two structures.

Exploring photophysical processes in a ternary-blended polymer solar cell

Ternary bulk-heterojunction polymer solar cells have been fabricated using a polymer blend (PBDB-T: PBDTTPD) as a donor in combination with a small molecule ITIC as a non-fullerene acceptor. In addition to the similarity in their energy levels, both PBDB-T and PBDTTPD polymers demonstrated rather similar crystalline or lamellar packing structure to support an alloy-type blend model, which well-explained the progressively monotonic change in photovoltaic VOC against the compositions. In addition, photoinduced electron-transfer (PET) between the PBDB-T/PBDTTPD polymers forming an intermediate charge separation state between the donors. A synergistic improvement in exciton generation rate, enhancement in charge carrier transport with good balance, and charge separation rate due to PET all contribute to the achieve a photovoltaic device with high efficiency.

Fine Tuning the Light Distribution within the Photoactive Layer by Both Solution‐Processed Anode and Cathode Interlayers for High Performance Polymer Solar Cells

Light management is important for improving light absorption within active layers in polymer solar cells (PSCs). Electrode buffer layers play an important role in modulating the distribution of optical electric filed within the photoactive layer. Herein, the authors employ solution‐processed WOx or ReOx to substitute the acidic poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the anode buffer layer and ZrAcac or HfAcac to replace Mg as the cathode buffer layer. Optical transfer matrix formalism simulation is used to model the absorption spectra, exciton generation rate, and optical electric field distribution of devices. Simulated results shows that stronger absorption, quicker exciton generation rate, and more reasonable photoelectric field distribution can be achieved in the photoactive layer with solution‐processed buffer layer modification, which results in a higher short‐circuit current density (Jsc). Under the guidance of theoretical simulation, the device with architecture of ITO/WOx/PTB7‐Th:PC71BM/HfAcac/Al is optimized. Compared with the traditional PEDOT:PSS‐Mg based device, the Jsc is increased from 16.60 to 18.61 mA cm−2 and the best power conversion efficiency (PCE) is increased from 9.02% to 10.60% for the device with WOx‐HfAcac modification, which is among the best values reported for fullerene‐based PSCs. The good agreement between simulated and experimental results indicates that optical model is a useful tool for device design and optimization.

Valley dynamics of intravalley and intervalley multiexcitonic states in monolayer WS2

We present a comprehensive model comprising of a complete set of rate equations, which account for charge transfer among multiexcitonic channels including excitons, trions, and biexcitons, to investigate valley (locked with spin) dynamics in monolayer ${\mathrm{WS}}_{2}$. The steady-state photoluminescence (PL) spectra, underlying the laser power dependence of excitonic populations, are also determined. Our computed PL for all excitonic states agrees with the experimental data of Paradisanos et al. [Appl. Phys. Lett. 110, 193102 (2017)]. We find that the relative weight of PL, stemmed from different excitonic channels, strongly depends on the laser power even under dynamical conditions. Remarkably, the biexciton channel, having the weakest PL intensity at low laser powers, tends to prevail in PL over other excitonic states as the power strengthens. In addition, by accounting for intervalley scatterings, which enable transfer of excitonic states from one valley to the other, we determine the valley polarization, which strongly depends on intervalley scatterings and the exciton generation rates in the two valleys. On the other hand, the valley polarization for all excitonic channels is found almost independent of the laser power, consistent with experimental measurements as well. Finally, the valley dynamics involving both intra- and intervalley trions is discussed. Our model and numerical outcome should be beneficial to experiments especially featuring the interplay of multiexcitonic channels in, e.g., elucidating experimental data, estimating central excitonic quantities including recombination times and transition rates, and in widening possible new experimental scopes.

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS2 :hBN:MoS2 Heterostructures for Exciton Energy Transfer.

The 2D semiconductor monolayer transition metal dichalcogenides, WS2 and MoS2 , are grown by chemical vapor deposition (CVD) and assembled by sequential transfer into vertical layered heterostructures (VLHs). Insulating hBN, also produced by CVD, is utilized to control the separation between WS2 and MoS2 by adjusting the layer number, leading to fine-scale tuning of the interlayer interactions within the VLHs. The interlayer interactions are studied by photoluminescence (PL) spectroscopy and are demonstrated to be highly sensitive to the input excitation power. For thin hBN separators (one to two layers), the total PL emission switches from quenching to enhancement by increasing the laser power. Femtosecond broadband transient absorption measurements demonstrate that the increase in PL quantum yield results from Förster energy transfer from MoS2 to WS2 . The PL signal is further enhanced at cryogenic temperatures due to the suppressed nonradiative decay channels. It is shown that (4 ± 1) layers of hBN are optimum for obtaining PL enhancement in the VLHs. Increasing thickness beyond this causes the enhancement factor to diminish, with the WS2 and MoS2 then behaving as isolated noninteracting monolayers. These results indicate how controlling the exciton generation rate influences energy transfer and plays an important role in the properties of VLHs.

Tuning the spectral response of ultraviolet organic-inorganic hybrid photodetectors via charge trapping and charge collection narrowing.

Organic-inorganic hybrid ultraviolet photodetectors with tunable spectral response are desirable for many different applications. In this work, we blended poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) with ZnO nanoparticles in weight ratios of 1 : 1 and 2 : 1 to create charge traps within the active layers for devices with the conventional structure ITO/PEDOT : PSS/PTAA : ZnO/BCP/Al. Thin (150-200 nm) and thick (1400-1900 nm) active layers were employed to utilize charge collection narrowing (CCN). Both thickness and composition of the active layer impacted the spectral tunability of the photoresponse. A single narrow response peak centered at 420 nm (the PTAA absorption edge) with a full width at half maximum of 12 nm was achieved from the device with a 1900 nm active layer and PTAA : ZnO weight ratio of 1 : 1. Decreasing the active layer thickness to 150 nm resulted in a broad spectral response between 320-420 nm with an external quantum efficiency (EQE) value of 295% under 350 nm illumination and a -1 V bias, exhibiting photomultiplication via charge trapping and injection even at small reverse biases. Increasing the weight ratio of PTAA : ZnO to 2 : 1 lowered both the dark current and photocurrent, eliminated photomultiplication in the thin device, and diminished the efficacy of CCN to narrow the spectral photoresponse in the thick device. Transfer matrix method (TMM) and 3-dimensional finite-difference time-domain (3D-FDTD) simulations were performed to understand the impact of thickness and composition of the active layer on the spectral response of UV photodetectors in terms of exciton generation rate and electric field distribution within the devices.

Plasmonic organic bulk–heterojunction solar cells based on hydrophobic gold nanorod insertion into active layers

ABSTRACTIn this work, we fabricated plasmonic organic bulk–heterojunction solar cells by inserting hydrophobic gold nanorods (GNRs) into the active layers. Power conversion efficiency was improved from 7.43% to 8.22% because the plasmonic effect of GNRs improved the light harvesting efficiency. Maximum exciton generation rate was increased from 1.35 × 10−26 to 1.51× 10−26m−3 s−1, and the electron mobility was also increased from 8.6 × 10−5 to 1.5× 10−4cm−2 V−1 s−1. As a result, the short circuit current density was improved from 15.5 to 16.7 mA cm−2—the dominant reason for performance enhancement. The open circuit voltage and fill factor were improved simultaneously. The plasmonic device showed a highest PCE of 8.43%, indicating that doping GNRs into active layers is a simple and effective way to fabricate high‐performance organic solar cells. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45920.

Electrical and optical modeling of poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester P3HT–PCBM bulk heterojunction solar cells

In this work, we investigate a two-dimensional theoretical model for the photon conversion through an integration of the optical and electrical part of multilayer system in a bulk heterojunction solar cell based on poly(3-hexylthiophene) (P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) blend. The optical properties of the studied structure ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al, such as the exciton generation rate and the electrical field distribution, are predicted at vicinity of the active layer and have been used to solve Poisson and continuity, drift–diffusion equations of the electrical model which characterize the electrical behavior of semiconductor device using finite element method (FEM). The electrical parameters such as power conversion efficiency (PCE), open voltage circuit (V oc), short-circuit current density (J sc) and fill factor (FF) are extracted from the current–voltage (J–V) characteristics under illumination and in dark conditions. Highest external quantum efficiency (IPCE), up to 60%, is obtained around 520 nm, while a power conversion efficiency (PCE) value of 3.62% is found to be in good agreement with the literature results. Integration of such theoretical approach into technological applications dealing with optoelectrical material performance will rapidly provide to the user accurate data outputs required for efficient validation of proof-of-concepts.

FTO-free top-illuminated colloidal quantum dot photovoltaics: Enhanced electro-optics in devices

A solar cell device architecture with top-illumination, where the light does not pass through the substrate, is advantageous for many applications. It is also specifically useful for the construction of tandem or multiple junction photovoltaic devices, with illumination through the top solar cell. Here, a top-illuminated colloidal quantum dot solar cell (TI-CQDSC) is demonstrated and compared with a conventional colloidal quantum dot solar cell (C-CQDSC) constructed on a FTO (fluorine doped tin oxide) glass substrate both theoretically and experimentally. The optical electric field distribution in the solar cells with different configuration is simulated using transfer matrix formalism and a more intense optical electric field was observed in TI-CQDSC, leading to a higher exciton generation rate within the colloidal quantum dot solid. The TI-CQDSCs are constructed on both nonconductive glass and flexible substrates, and a maximum power conversion efficiency of 6.4% and 5.6% is achieved, respectively, comparing to that of 5.9% for the C-CQDSC. The improved performance of the top illuminated solar cell is attributed to a combination of enhanced optical electric field intensity in the colloidal quantum dot solid and superior conductivity of the transparent metal film electrode.

Plasmonic Effects of Metallic Nanoparticles on Enhancing Performance of Perovskite Solar Cells.

We report systematic design and formation of plasmonic perovskite solar cells (PSCs) by integrating Au@TiO2 core-shell nanoparticles (NPs) into porous TiO2 and/or perovskite semiconductor capping layers. The plasmonic effects in the formed PSCs are examined. The most efficient configuration is obtained by incorporating Au@TiO2 NPs into both the porous TiO2 and the perovskite capping layers, which increases the power conversion efficiency (PCE) from 12.59% to 18.24%, demonstrating over 44% enhancement, compared with the reference device without the metal NPs. The PCE enhancement is mainly attributed to short-circuit current improvement. The plasmonic enhancement effects of Au@TiO2 core-shell nanosphere photovoltaic composites are explored based on the combination of UV-vis absorption spectroscopy, external quantum efficiency (EQE), photocurrent properties, and photoluminescence (PL). The addition of Au@TiO2 nanospheres increased the rate of exciton generation and the probability of exciton dissociation, enhancing charge separation/transfer, reducing the recombination rate, and facilitating carrier transport in the device. This study contributes to understanding of plasmonic effects in perovskite solar cells and also provides a promising approach for simultaneous photon energy and electron management.

4P-NPD ultra-thin films as efficient exciton blocking layers in DBP/C70 based organic solar cells

Exciton blocking effects from ultra-thin layers of N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) were investigated in small molecule-based inverted organic solar cells (OSCs) using tetraphenyldibenzoperiflanthene as the electron donor material and fullerene (C70) as the electron acceptor material. The short-circuit current density (JSC) and power conversion efficiency (PCE) of the optimized OSCs with 0.7 nm thick 4P-NPD were approximately 16% and 24% higher, respectively, compared to reference devices without exciton blocking layers (EBLs). Drift diffusion-based device modeling was conducted to model the full current density–voltage (JV) characteristics and external quantum efficiency spectrum of the OSCs, and photoluminescence measurements were conducted to investigate the exciton blocking effects with increasing thicknesses of the 4P-NPD layer. Importantly, coupled optical and electrical modeling studies of the device behaviors and exciton generation rates and densities in the active layer for different 4P-NPD layer thicknesses were conducted, in order to gain a complete understanding of the observed increase in PCE for 4P-NPD layer thicknesses up to 1 nm, and the observed decrease in PCE for layer thicknesses beyond 1 nm. This work demonstrates a route for guiding the integration of EBLs in OSC devices.

Precisely control the morphology and crystallization of temperature-dependent aggregation bulk heterojunction by using co-solvent system for optimized light intensity distribution and its effect on thick active layer polymer solar cells

The effects of co-solvent on the morphology, crystallization and light intensity distribution of thick bulk heterojunction (BHJ) polymer solar cells (PSCs) based on polymer of [(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3000-di(2-octyldodecyl)-2,20,50,200,500,2000-quaterthiophen-5,5000-diyl)](PffBT4T-2OD):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) are studied. By adjusting different co-solvent systems in thick BHJ layer, it is found that the polymer crystallization, fullerene absorption and PffBT4T-2OD:PC71BM BHJ morphology are optimized by using a co-solvent of chlorobenzene (CB):dichlorobenzene (DCB):1,8-diiodooctanein (DIO). The optimized BHJ thickness of 250nm is analyzed by using transfer matrix theory, resulting in enhanced FF and PCE of 66.7% and 9.16%, respectively. This phenomenon is due to the active layer can absorb 90% of the incident light with a thickness of 250nm, which contributes to the light intensity distribution and exciton generation rate.

Study of external quantum efficiency of plasmonic coupled bilayer active device: influence of layer thickness and nanoparticle filling factor

To improve the efficiency of organic–inorganic photovoltaic devices, the inclusion of metal nanoparticles such as silver has been used to enhance photon absorption. Incorporation of silver nanoparticle into the active layer with various filling factor increases the exciton generation rate of active material. The exciton–plasmon coupling mechanism are analysed in terms of effective absorption coefficient via Maxwell Garnett theory including size effect. To obtain the exciton population of organic–inorganic heterojunction device with nanoparticle embedded into inorganic layer, we have solved one-dimensional steady-state exciton diffusion equation. On the basis of this, we have calculated the expression of photocurrent and external quantum efficiency with and without plasmonic effects under the influences of standard AM 1.5 solar spectrums. After embedding the silver nanoparticle (with filling factor 0.55) into inorganic active layer, optimum enhancement in the external quantum efficiency by 4.90% has been found as compared to reference device.

Optical Confinements in Correlated Spectral Characteristics of Vertically Aligned Silicon Nanometric Wires Followed by a Facile Fabrication Thereof

Nanometric wires, in particular, vertically aligned silicon nanowires, possess a huge prospect in current thin film solar cell technologies. A bottom-up process to acquire vertically aligned silicon nanowire (Si-NW) on c-Si wafer was demonstrated and characterized. A well coverage of Si-NWs on c-Si, ca. 6.5 × 108/cm2, was obtained. SEM micrograph confirmed a variation in Si-NW size in length as well as in diameter. On the other hand, inherent optical characteristics of nanometric wires define the performance of such nanowire-based thin film solar cell. Therefore, key factors, such as absorption profile, energy flow, electromagnetic (EM) field, and generation rate distribution in typical model, “Si-NW on c-Si slab,” have been observed at different wavelengths of solar spectrum. A single Si-NW of cylindrical shape was modelled on c-Si and optimized to elucidate the aforementioned characteristics. Light absorption at 700 nm was found to be the best in this scenario, and therefore, EM field and Poynting vector distribution were simulated at the same wavelength. It was revealed that at 700 nm, strongest sites of EM field and energy flow became more available and confined. Exciton generation rate was found to be distributed and confined all the way down to the bottom of the wire. A correlated phenomenon such as optical confinements in spectral characteristics in nanoscale is elucidated. Such a focused prediction in spectral optical characteristics and facile fabrication route is indispensable in optoelectronics as well as nanowire-based advanced electronic design.

Improve exciton generation and dissociation by increasing fullerene content in the mixed phase of P3HT/fullerene

Designing polymeric bulk heterojunction (BHJ) morphology with good exciton separation efficiency is one of the key factors in obtaining high-efficiency polymer solar cells (PSCs). Cascaded energy formed by the energy level difference (οE) between aggregated phase and the mixed phase is thought to encourage spatial separation of the geminate charge pair. However, how the composition of the mixed phase affects the exciton generation and dissociation process is still unknown. In this paper, different amount of fullerene in the mixed phase of poly(3-hexylthiophene) (P3HT)/fullerene was achieved by changing the ratio of two fullerenes ([6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and its bisadduct bis-PC71BM), which have different miscibility with P3HT. When the content of PC71BM increasing from 0% to about 60%, the mixed phase containing more fullerenes. More donor/acceptor (D/A) interface was formed, and the οE value also increased from 0.19eV to 0.23eV. As a result, the maximum exciton generation rate (Gmax) increased from 5.36ÿ1027m⿿3/s to 6.48ÿ1027m⿿3/s, and the exciton dissociation probabilities increased from 47% to 81%. Thus, perfect three-phase morphology for high performance solar cells was proposed, in which not only the donor and acceptor have good crystallinity to form interpenetrating network but also have good miscibility with each other to increase the fullerene content in mixed phase.

An Analytical Solution for Exciton Generation, Reaction, and Diffusion in Nanotube and Nanowire-Based Solar Cells.

Excitonic solar cells based on aligned or unaligned networks of nanotubes or nanowires offer advantages with respect of optical absorption, and control of excition and electrical carrier transport; however, there is a lack of predictive models of the optimal orientation and packing density of such devices to maximize efficiency. Here-in, we develop a concise analytical framework that describes the orientation and density trade-off on exciton collection computed from a deterministic model of a carbon nanotube (CNT) photovoltaic device under steady-state operation that incorporates single- and aggregate-nanotube photophysics published earlier (Energy Environ Sci, 2014, 7, 3769). We show that the maximal film efficiency is determined by a parameter grouping, α, representing the product of the network density and the effective exciton diffusion length, reflecting a cooperativity between the rate of exciton generation and the rate of exciton transport. This allows for a simple, master plot of EQE versus film thickness, parametric in α allowing for optimal design. This analysis extends to any excitonic solar cell with anisotropic transport elements, including polymer, nanowire, quantum dot, and nanocarbon photovoltaics.