Exciton Generation Rate Research Articles

Study of electrical properties of fullerene based all-small-molecule low donor organic devices with and without cathode buffer layers for photovoltaic application

This article examines electrical properties of fullerene (C70) based all-small-molecule organic photovoltaics with low donor concentration. Enhancements in the properties of devices are observed with the incorporation of lithium fluoride (LiF) and molybdenum trioxide (MoO3) as cathode buffer layers (CBLs). Samples are characterized using field emission-scanning electron microscopy (FESEM), optical absorption spectra, current-voltage (I-V) characteristics in dark and under illumination, and capacitance-frequency (C-ω) measurements. Device with single CBL (LiF) exhibits reduced photoabsorption leading to less saturation photocurrent density (Jsat) and lower maximum exciton generation rate (Gmax), due to non-uniform deposition of LiF in device. In contrast, device having two CBLs (LiF & MoO3) has enhanced the photoabsorption exhibiting high Jsat and Gmax attributed to uniform deposition of MoO3. Moreover, dielectric properties and AC conductivity of devices are also measured confirming the results and these are found to be dependent on the frequency and voltage.

Enhanced performance of PFO-ZnO nanorods nanocomposite photodiodes grown on ZnO NRs/ZnO/ITO-coated glass

An improved photodiode was fabricated using a nanocomposite of poly (9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and zinc oxide nanorods (ZnO NRs). The nanocomposite was prepared by scratching hydrothermally grown ZnO NRs on a glass substrate into the prepared PFO solution. The nanocomposite layer was deposited using the spin-coating method and silver (Ag) paste was applied as the electrode. The resulting photodiode configuration is Ag/PEDOT:PSS/PFO-ZnO NR/ZnO NRs/ZnO/ITO. Ultraviolet–visible (UV–Vis) spectroscopy showed a slight blue shift and a slight increase in the optical bandgap. The nanocomposite-based photodiode showed improved performance, due to increased active surface area for the separation of photogenerated electron-hole pairs between PFO and ZnO and also because of the higher current density of ZnO. Finite-Difference Time-Domain (FDTD) simulations showed ZnO NRs increased electric field strength and exciton generation rate throughout the PFO polymer in the nanocomposite. The photodiode with the nanocomposite showed the best performance at −5 V bias voltage under 385 nm illumination, obtaining responsivity of 1.45 A W−1, Detectivity of 2.19 x 1010 Jones, and External quantum efficiency (EQE %) of 468 %.

Vertical Phase Regulation with 1,3,5-Tribromobenzene Leads to 18.5% Efficiency Binary Organic Solar Cells.

The sequential deposition method assists the vertical phase distribution in the photoactive layer of organic solar cells, enhancing power conversion efficiencies. With this film coating approach, the morphology of both layers can be fine-tuned with high boiling solvent additives, as frequently applied in one-step casting films. However, introducing liquid additives can compromise the morphological stability of the devices due to the solvent residuals. Herein, 1,3,5-tribromobenzene (TBB) with high volatility and low cost, is used as a solid additive in the acceptor solution and combined thermal annealing to regulate the vertical phase in organic solar cells composed of D18-Cl/L8-BO. Compared to the control cells, the devices treated with TBB and those that underwent additional thermal processing exhibit increased exciton generation rate, charge carrier mobility, charge carrier lifetime, and reduced bimolecular charge recombination. As a result, the TBB-treated organic solar cells achieve a champion power conversion efficiency of 18.5% (18.1% averaged), one of the highest efficiencies in binary organic solar cells with open circuit voltage exceeding 900mV. This study ascribes the advanced device performance to the gradient-distributed donor-acceptor concentrations in the vertical direction. The findings provide guidelines for optimizing the morphology of the sequentially deposited top layer to achieve high-performance organic solar cells.

Thermal Activation of PEDOT:PSS/PM6:Y7 Based Films Leads to Unprecedent High Short‐Circuit Current Density in Nonfullerene Organic Photovoltaics

AbstractFinding an effective approach to suppress trap formation is a potential route for enhancing the performance of nonfullerene organic photovoltaic (NF‐OPVs) devices. Here, an extraordinary short‐circuit current density (JSC) value of 32.65 mA cm‐2 is achieved, higher than the state‐of‐the art NF‐OPVs reported, reaching a high power conversion efficiency (PCE) of 17.92%. This remarkable enhancement is exhibited through the fine‐tuning of PEDOT:PSS/PM6:Y7 films and interface morphologies via applying the prethermal treatment approach (Pre‐TT) to the devices, which exhibit JSC and PCE enhancement of 21% and 8%, respectively, compared to the pristine devices. Accordingly, the dependence of the JSC upon the Pre‐TT approach through a range of morphological, optical, electrical, and advanced transient measurements is investigated. The Pre‐TT‐based films are found to possess optimal smooth blend morphology with better dispersity owing to reduced domain size. Moreover, the measurements show that the optimized treated devices present higher exciton dissociation probabilities and generation rate of the free charge carriers, showing an ideal balanced electron/hole mobility that reveals the JSC and PCE enhancement. Hence, Pre‐TT approach provides a facile passivation strategy that reduces the trap state density of the blend film, improves interface charge transfer, allows balanced electron/hole mobility, and thus promotes device performance.

A simple doping strategy to improve PEDOT:PSS charge extraction capability in polymer solar cells

Black Phosphorus (BP) has shown great potential in optoelectronic devices due to its remarkable semiconductor properties. Herein, liquid-exfoliated BP nanosheets is doped into poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) to form a hybrid hole transport layers (HTLs). The incorporation of BP nanosheets in PEDOT:PSS dramatically changes the nanostructure and electrical conductivity of film. The BP doped PEDOT:PSS HTLs possess excellent charge selectivity and transport capability, which are proved in non-fullerene and fullerene blend system based polymer solar cells (PSCs). Furthermore, the nanostructure of PM6:Y6 light absorber changes a lot after introduction of BP nanosheets. Thus, weak charge recombination loss, high exciton generation rate and dissociation probability, and low transport resistance are observed in BP nanosheets doped PEDOT:PSS HTLs based cells, which result in improved power conversion efficiency (PCE). As a result, significant improvement of PCE (>18%) is observed in both non-fullerene and fullerene blend systems based cells. This result demonstrates a simple and effective strategy to form efficient HTLs in high-efficiency PSCs.

An improved performance of all polymer solar cells enabled by sequential processing via non-halogenated solvents

Polymer acceptors based on the polymerization of small-molecule acceptor (PSMAs) have developed rapidly in recent years. All-polymer solar cells (all-PSCs) composed of PSMAs and polymer donors not only retain the excellent mechanical properties, but also have better morphological properties than traditional polymer acceptors such as N2200. However, since PSMAs are generally not highly polymerized, their electron mobilities are generally lower than the hole mobilities of the high-molecular-weight polymer donors they are used with. In this work, we apply the sequential processing (SqP) method and use toluene as the processing solvent to prepare PSMA-based all-PSCs, which effectively improves the electron mobility and shows much higher efficiency (15.8 %) than the SqP device prepared from chloroform (13.9 %) – the conventional solvent used for high-performance PSMAs. Not only that, the efficiency of the toluene-processed SqP device is also higher than that of the device prepared by the traditional blending-casting (BC) method (14.2 %). In addition, for the first time, we reveal the film formation process of all-polymer active layers prepared by the SqP method using the in situ UV-Vis absorption technique, and investigate the vertical phase separation and the rate of exciton generation across the device using film-depth-dependent absorption spectroscopy, which systematically explained the effect of increased mobility on device parameters. This methodology is applied in two different all-polymer systems, namely, PM6:PYF-T-o and PM6:PY-IT, and both systems manifest the same conclusion.

Plasmonic enhancement in PTB7-Th:PC71BM organic photovoltaics

Organic photovoltaics struggles with limited exciton diffusion rate and low light absorption rates. The trade-off between these deficiencies limits the thickness of the active layer around 100–200 nm range. Plasmonic light management was introduced as one of the solutions to these problems in the last decade. The plasmonic metal nanostructures can manipulate light in subwavelength regime such as thin-film organic photovoltaics. In this work, we introduced a controllable and reproducible method to incorporate quasi-hexagonal arrays of Au nanostructure on the electron transport layer (ETL), which can directly facilitate light management in the active layer. The inverted PTB7-Th:PC71BM OPVs were selected as the model system, and Au nanostructure arrays were introduced in selected interfaces. In order to match the work function of Au nanostructures and the LUMO level of the PC71BM fullerene component of the bulk heterojunction, PEI was utilized to modify the work function of such surfaces. The plasmon-enhanced inverted OPV devices showed up to 22% higher power conversion efficiencies than reference devices. In order to elucidate the underlying reason for such high improvement, finite-difference time-domain simulations of whole OPV devices under AM1.5G solar illumination were performed. The results revealed up to 25 times higher local exciton generation rate in close proximity to the Au nanostructures and ETLs.

Efficient All-Polymer Solar Cells with Sequentially Processed Active Layers.

In this work, we apply the sequential processing (SqP) method to address the relatively low electron mobility in recent all-polymer solar cells (all-PSCs) based on the polymerized small-molecule acceptor (PSMA). Compared to the blend-casting (BC) method, all-PSCs composed of PM6/PY-IT via the SqP method show boosted electron mobility and a more balanced charge carrier transport, which increases the FF of the SqP device and compensates for the short-circuit current loss, rendering comparable overall performance with the BC device. Through film-depth-dependent light absorption spectroscopy, we analyze the sub-layer absorption and exciton generation rate in the vertical direction of the device, and discuss the effect of the increased electron mobility on device performance, accordingly.

Impacts of plasmonic nanoparticles incorporation and interface energy alignment for highly efficient carbon-based perovskite solar cells

This work utilizes a realistic electro-optical coupled simulation to study the (i) impact of mesoporous TiO2 removal; (ii) the embedding of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles; (iii) utilization of solution-processed inorganic p-type copper(I) thiocyanate (CuSCN) layer at the perovskite/carbon interface; and (iv) the increase of the work function of carbon electrodes (via incorporation of suitable additives/binders to the carbon ink) on the performance of carbon-based PSCs. Removal of mesoporous TiO2 increased the power conversion efficiency (PCE) of the device from 14.83 to 16.50% due to the increase in exciton generation rate and charge carriers’ mobility in the vicinity of the perovskite-compact TiO2 interface. Subsequently, variable mass ratios of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles are embedded in the vicinity of the perovskite-compact TiO2 interface. In the optimum cases, the PCE of the devices increased to 19.72% and 18.92%, respectively, due to light trapping, scattering, and strong plasmonic fields produced by the plasmonic nanoparticles. Furthermore, adding the CuSCN layer remarkably increased the PCE of the device with a 0.93% mass ratio of Ag@SiO2 nanoparticles from 19.72 to 26.58% by a significant improvement of Voc and FF, due to the proper interfacial energy band alignment and the reduction of the recombination current density. Similar results were obtained by increasing the carbon work function, and the cell PCE was enhanced up to 26% in the optimal scenario. Our results pave the way to achieve high efficiencies in remarkably stable printable carbon-based PSCs.

Investigation of conventional and inverted organic solar cell from optical aspect

As an interesting research topic, investigation on organic solar cells is increasing day by day. In this study, the performance of conventional P3HT: PCBM based organic solar cell (OSC) structure and inverted organic solar cell (IOSC) structure has been investigated through optical simulation using GPVDM software. As an active layer material, P3HT: PCBM is used. Al, ITO, PEDOT: PSS and TiO2 is used as cathode, anode, hole transporting layer (HTL) and electron transporting layer (ETL) respectively for OSC structure. In case of IOSC, Ag has been used as an anode. Investigations have been made to study the effect of active layer thickness on the exciton generation rate. It has been found that the exciton generation rate is oscillatory due to optical interference effect for both OSC and IOSC structure. Photon absorption reduces due to increase in active layer thickness. However, exciton generation increases due to creation of more peak into the active layer.

Design and fabrication of wafer-scale highly uniform silicon nanowire arrays by metal-assisted chemical etching for antireflection films

We present a novel and facile scheme for fabricating wafer-scale arrays of highly uniform vertically aligned silicon nanowires (Si-NWs) in the diameter range of 50 to 200 nm. The key idea is to fabricate thin gold (Au) nanostructures using magnetron sputtering followed by de-wetting of Au and subsequent realization of wafer-scale and highly uniform Si-NWs arrays by metal-assisted chemical etching in low temperature (2 °C) without resorting to complex photolithography. The excellent properties of these Si-NWs arrays were also demonstrated numerically. A maximum exciton generation rate of 1.76 × 1024 and a maximum energy generation rate of 28 can be obtained from Si-NWs arrays with the diameter of 200 nm. The reflectivity of the Si-NWs arrays is declined with decreasing annealing temperature and is below 10% over a wide wavelength range at the annealing temperature of 200 °C. Our work provides a promising approach for constructing Si-NWs arrays with good photoelectric conversion performance.

New BODIPY derivatives with triarylamine and truxene substituents as donors for organic bulk heterojunction photovoltaic cells

We have designed two new BODIPY derivatives, denoted as 6a and 6b, substituted with truxene moiety and triphenylamine (TPA) unit groups and have investigated their optical and electrochemical properties. Dyes 6a and 6b were employed as donor along with PC71BM or Y6 as acceptor for the fabrication of binary and ternary organic solar cells. After optimization of the binary and ternary active layers, we have achieved over all power conversion efficiency (PCE) of 11.37 % and 13.32% for 6a:PC71BM:Y6 and 6b:PC71BM:Y6 ternary organic solar cells, respectively, which are higher than the binary organic solar cells based on PC71BM or Y6 acceptor. The higher power conversion efficiency for ternary OSC is possibly attributed to the better exciton generation rate, exciton dissociation and charge collection.

Comparative Investigation of Fullerene PC71BM and Non-fullerene ITIC-Th Acceptors Blended With P3HT or PBDB-T Donor Polymers for PV Applications

Fundamentally, organic solar cells (OSCs) with a bulk-heterojunction active layer are made of at least two electronically dissimilar molecules, in which photoabsorption in one (donor) generates Frenkel excitons. The formation of free charge carriers emerge after exciton dissociation at the donor:acceptor interface. In the past decade, most of the progress in enhanced device performance has been steered by the rapid development of novel donor and acceptor materials and on device engineering. Among these donor materials, regioregular poly(3-hexylthiophene) (P3HT) produced better performance despite the mismatch of its absorption coefficient with the solar emission spectrum. Comparatively the donor PBDB-T exhibits an outstanding absorption coefficient with a deeper-lying highest occupied molecular orbital (HOMO) level. Previously most of the efficient acceptors were based on fullerene molecules characterized by limited photoabsorption and stability. In contrast, the recently developed non-fullerene OSCs have a tunable absorption spectrum and exhibit improved stability. In this work, we explore the fundamental sources of the differences in the device performance for different blend compositions made of fullerene derivative (PC71BM) and non-fullerene (ITIC-Th) when paired with the polymer donors P3HT and PBDB-T. The characteristic changes of the optical properties of these blends and their roles in device performance are also investigated. We also studied charge generation where PBDB-T:PC71BM showed the highest maximum exciton generation rate (Gmax) of 3.22 × 1028 s–1 while P3HT: ITIC-Th gave the lowest (0.96 × 1028 s–1). Also noted, PC71BM based counterparts gave better charge transfer capabilities as seen from the lower PL quenching and higher charge carrier dissociation plus collection probability P(E,T) derived from a plot of Jph/Jsat ratio under short-circuit conditions against the effective voltages.

A common optical approach to thickness optimization in polymer and perovskite solar cells

The structure of experimentally designed solar cells was optimized in terms of the photoactive layer thickness for both organic bulk heterojunction and hybrid perovskite solar cells. The photoactive layer thickness had a totally different behavior on the performance of the organic and hybrid solar cells. Analysis of the optical parameters using transfer matrix modeling within the Maxwell–Garnett effective refractive index model shows that light absorbance and exciton generation rate in the photoactive layer can be used to optimize the thickness range of the photoactive layer. Complete agreement between experimental and simulated data for solar cells with photoactive materials that have very different natures proves the validity of the proposed modeling method. The proposed simple method which is not time-consuming to implement permits to obtain a preliminary assessment of the reasonable range of layer thickness that will be needed for designing experimental samples.

Direct Visualization and Determination of the Multiple Exciton Generation Rate.

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

Fabrication and Spectral Characteristics of Silicon Nanowires for Efficient Solar Energy Harvesting

Solar cell utilizes a small portion of solar spectrum leaving higher energy (> band gap, Eg) as thermalization loss and lower energy (< band gap, Eg) as absorption loss. Wavelength-sensitive engineered absorbing layer such as nanometric absorber holds huge potential in this context. Here in this work, a simple and hands-on strategy was devised to grow silicon nanowires (Si-NWs) on silicon wafer. Nanoparticles were achieved in the first step and used as seeds to directed growth of Si-NWs. As-grown Si-NWs with coverage ca. 6.5 × 108/cm2 were characterized through scanning electron microscope. To realize such Si-NWs as nanometric absorber in nanowire solar cell, a three-dimensional finite-difference time-domain simulation has been carried out. Considering the possibility of Si-NWs of different diameters as observed in experimental investigations, Si-NW of 50-, 100-, and 150-nm diameters was chosen in simulation. Two specific wavelengths, 700 and 1100 nm, were in prime focus to understand the characteristics of exciton generation within Si-NW. Confinement in exciton generation rate distribution at 700-nm solar spectrum for Si-NW of 150 nm was found to be most effective, whereas at 1100-nm wavelength Si-NW of 100 nm showed higher exciton generation rate distribution with the nanowire. Exciton generation line profiles along the center and edge were extracted, and comparative analysis was carried out for different diameters of Si-NW at 700- and 1100-nm wavelengths. Such experimental and correlated simulation is indispensable not only to reduce costs but also to understand and improve the cell efficiency using the light-trapping technique.

Highly efficient, green-solvent processable, and stable non-fullerene polymer solar cells enabled by a random polymer donor

Abstract Rapid development of non-fullerene polymer solar cells (NF–PSCs) with high efficiency have gained wide research attention recently, however, high-performance large-area NF–PSCs are still lacking since most developed NF–PSCs use a thin active layer around 100 nm, which is not compatible with the fabrication of large-area devices. Herein, we fabricate high-performance and green-solvent processed NF–PSCs with thick active layer using a random copolymer (2TRA) and non-fullerene acceptors. Compared to its highly-crystallized analogue (2TRR), 2TRA with rearranged backbone enables better blend morphology, higher exciton generation rate, more efficient excitons dissociation, and less recombination in NF–PSCs. With O-IDTBR and IEICO-4F as non-fullerene electron acceptors, highly-efficient photovoltaic devices with PCE of 10.08% and 12.06% are achieved. Moreover, 2TRA-based NF–PSCs show good compatibility with multiple processing solvents, low sensitivity to the active layer thickness (up to 300 nm), and excellent photostability. The combined advantages of 2TRA-based solar cell devices indicate that 2TRA is a promising polymer donor for the application in large-area PSC modules through roll-to-roll technology, and the rearrangement design on polymer donors could be useful to design high-performance polymers for practical application.

2-D Modeling of Bulk Heterojunction Solar Cells With a Gaussian Shape in Donor-Acceptor Interface as Morphology

In this work, we presented numerical modeling for the bulk heterojunction polymer solar cells (BHJ) based on the finite difference method to obtaining the BHJ characteristics. Our model for the BHJ, where the electron donor and acceptor materials are mixed in a solution, is two separate domains with gaussian shape interface. In this model we consider a two-dimensional geometry of thickness L on y-axis and periodic along x-axis. We assume that the rate of exciton generation to be homogenous throughout device. Some of obtained characteristics are the density of electrons and the density of the holes, current density, open circuit voltage and short circuit current density.

Performance enhancement of conjugated polymer-small molecule-non fullerene ternary organic solar cells by tuning recombination kinetics and molecular ordering

We present our study of conjugated polymer-small molecule (SM)-non-fullerene ternary organic solar cells (OSCs), which employs conjugated polymer PTB7-Th and small molecule p-DTS(FBTTh2)2 as donors and non-fullerene molecule IEICO-4F as an acceptor. It is observed that the power conversion efficiency (PCE) of ∼10.9% for PTB7-Th: p-DTS(FBTTh2)2: IEICO-4F ternary OSCs with 15 wt% of p-DTS(FBTTh2)2 SM is higher than PCE of ∼9.8% for PTB7-Th: IEICO-4F OSCs. Morphological studies confirm that the addition of p-DTS(FBTTh2)2 SM in PTB7-Th: IEICO-4F binary blend improves molecular ordering and crystallinity of PTB7-Th due to the favorable interaction with p-DTS(FBTTh2)2 thereby providing 3-D textured structures consisting of a mixture of edge-on and face-on orientations. The improved molecular ordering is shown to enhance exciton generation rate, exciton dissociation, charge collection, and to reduce charge recombination, all of which boosts the PCE.

Optical Simulation of Planar CH3NH3PbI3 Perovskite Solar Cells

In this work, optical simulation results of planar CH3NH3PbI3 solar cells using a MATLAB script developed by McGehee’s group (Stanford University) will be presented. The device structure is FTO/HEL/AL/ETL/LiF/Al, where HEL is a hole-extraction layer, AL is an active layer (CH3NH3PbI3), and EEL is an electron-extraction layer. This MATLAB script uses the transfer matrix method, where transmission and reflection are calculated for each interface in the stack as well as attenuation in each layer. The wavelength-dependent optical constants (n and k) of each layer were measured by spectroscopic ellipsometry (SE). The exciton generation rates within the active layer were calculated and with an assumption that the Internal Quantum Efficiency (IQE) equals 100% at all wavelengths, the predicted short-circuit currents (JSC) were also estimated. These results show a good agreement with the experimental values of JSC measured on real devices.