External Quantum Efficiency Response Research Articles

Effects of Al2O3 Rear Interface Passivation on the Performance of Bifacial Kesterite‐Based Solar Cells with Fluorine‐Doped Tin Dioxide Back Contact

Herein, ultrathin Al2O3 is investigated as a rear interface passivation layer for kesterite solar cells with F:SnO2 (FTO) back contact for potential performance improvement. On the passivation layer, a thin Mo layer is deposited to improve the FTO's ohmicity. Further, NaF is evaporated on the copper zinc tin sulfide (CZTS) precursors to create openings at the passivation layer and achieve Na‐induced benefits in the absorber. The CZTS absorbers deposited directly on the Al2O3‐coated FTO peel off, while those with Mo interlayer do not. For pure sulfide kesterite devices, the Al2O3 layer reduces the short‐circuit current density (JSC), resulting in poor device efficiency. On the other hand, significantly higher JSC is realized for mixed sulfide and selenide kesterite devices with Al2O3 passivation layer, with the current density–voltage curve suggesting a reduced barrier height at the rear interface. As a result, the efficiency is improved from 1.5% for devices without Al2O3 to 4.6% for those with Al2O3. Likewise, improved external quantum efficiency response is observed in the devices with passivation layer for backside and frontside illumination. Therefore, contribution of Al2O3 passivation layer to the performance of kesterite‐based solar cells is evident from the results of this study.

Toward improving the performance of Cu2ZnSnS4-based solar cells with Zr, W or sulfurized layers at the SnO2:F/Cu2ZnSnS4 rear interface

The photovoltaic performance of Cu2ZnSnS4 (CZTS)-based thin film solar cells with transparent back contacts is mainly constrained by the ohmic loss due to degradation of the back contacts at high temperatures required for the growth of the CZTS absorbers. This work has attempted to use ultrathin Zr and W interlayers at the SnO2:F (FTO)/CZTS interface with the purpose of improving the ohmic properties of FTO and hence the performance of solar cells. 20 nm thick Zr and W layers were coated on FTO using direct current magnetron sputtering, followed by heating at 550 °C in a sulfur atmosphere for some of the samples. Inclusion of Zr and W interlayers compromised the transmittance of the FTO back contact, however heating of the samples in a sulfur atmosphere improved the transmittance to values comparable to or better than those of un-heated FTO. Furthermore, heated W/FTO showed a significant improvement in electrical conductivity as observed from Hall effect measurements. To make complete solar cells, CZTS absorber, buffer (CdS) and window (i-ZnO/ZnO:Al) layers were sequentially deposited on the un-heated FTO, Zr/FTO, W/FTO and Mo back contacts. Glow discharge optical emmision spectroscopy confirmed that Zr and W did not diffuse into the absorber and also prevented Na diffusion into the absorber. From the scanning electron microscopy cross sectional analysis, an impovement in the absober grain size and a clear junction between the absorber and FTO with W interlayer were observed. Grazing incident X-ray diffractometry confirmed the polycrystalline nature of the CZTS thin films and indicated the presence of other phases, particularly SnS2. On the resulting solar cell parameters, the inclusion of a W interlayer reduced the series resistance from 4.5 Ωcm to 3.7 Ωcm2. An improvement in short circuit current density (JSC) is also observed, enhancing the efficiency of the solar cells with CZTS/W/FTO to 3.1 % compared to that with CZTS/Mo at 2.7 % and CZTS/FTO at 3.0 %. W was found to improve the external quantum efficiency response and JSC of the solar cells for both backside and frontside light illumination. On the other hand, inclusion of a Zr interlayer (Zr/FTO) only slighlty improved the open circuit voltage, but compromised the JSC compared to FTO and W/FTO back contacts. Thus, the findings of this study demonstrate the prospect for improving the performance of the CZTS-based thin film solar cells through the inclusion of ultrathin Zr and W interlayers between the FTO back contact and CZTS absorber.

Enhanced performance of Cu2ZnSnS4 based bifacial solar cells with FTO and W/FTO back contacts through absorber air annealing and Na incorporation

This study reports on the influence of air annealing and Na incorporation into the absorber on the performance of Cu2ZnSnS4 (CZTS) bifacial solar cells with FTO and W/FTO back contacts. Na was incorporated by depositing ∼12 nm thick NaF on the CZTS precursors prior to the sulfurization process via thermal evaporation. After sulfurization, some of the samples were annealed in air at 300 °C for 90 s and subsequently at 200 °C for 600 s. Transmission electron microscopy confirmed sulfurization of the W interlayer to form WS2 which improves the FTO ohmicity. Na incorporation improved grain size of the absorber as revealed by scanning electron microscopy. Non-annealed samples had the unwanted SnS2 phase while the air annealed samples, particularly those with both W interlayer and Na incorporation, were exempt from SnS2 phase, as was confirmed through grazing incident X-ray diffraction and Raman spectroscopy. These results suggest that absorber air annealing and Na incorporation enhance absorber crystal growth which is advantageous in reducing bulk carrier recombination. As a result, the efficiency was significantly improved from 3.0% for solar cells fabricated directly on FTO to 5.2% for those whose absorbers were air annealed, incorporated with Na and made on W/FTO. The latter also exhibits the highest external quantum efficiency response and calculated short circuit current density for both sides illumination. This indicates that the air annealing, Na incorporation and W interlayer are enhancing the performance of bifacial CZTS solar cells with FTO back contact for both back side and front side illumination.

Ultraviolet-Visible-Short-Wavelength Infrared Broadband and Fast-Response Photodetectors Enabled by Individual Monocrystalline Perovskite Nanoplate.

Perovskite-based photodetectors exhibit potential applications in communication, neuromorphic chips, and biomedical imaging due to their outstanding photoelectric properties and facile manufacturability. However, few of perovskite-based photodetectors focus on ultraviolet-visible-short-wavelength infrared (UV-Vis-SWIR) broadband photodetection because of the relatively large bandgap. Moreover, such broadband photodetectors with individual nanocrystal channel featuring monolithic integration with functional electronic/optical components have hardly been explored. Herein, an individual monocrystalline MAPbBr3 nanoplate-based photodetector is demonstrated that simultaneously achieves efficient UV-Vis-SWIR detection and fast-response. Nanoplate photodetectors (NPDs) are prepared by assembling single nanoplate on adjacent gold electrodes. NPDs exhibit high external quantum efficiency (EQE) and detectivity of 1200% and 5.37 × 1012 Jones, as well as fast response with rise time of 80 µs. Notably, NPDs simultaneously achieve high EQE and fast response, exceeding most perovskite devices with multi-nanocrystal channel. Benefiting from the high specific surface area of nanoplate with surface-trap-assisted absorption, NPDs achieve high performance in the near-infrared and SWIR spectral region of 850-1450nm. Unencapsulated devices show outstanding UV-laser-irradiation endurance and decent periodicity and repeatability after 29-day-storage in atmospheric environment. Finally, imaging applications are demonstrated. This work verifies the potential of perovskite-based broadband photodetection, and stimulates the monolithic integration of various perovskite-based devices.

A comparative study of ZrS2-based thin film solar cells using the SCAPS solar cell capacitance simulator

Transition metal chalcogenides have been studied for their potential applications in optoelectronic devices such as light emitting diodes, solar cells, photodetectors, field-effect transistors, etc. Their unique structural and versatile electronic and optical properties, non-toxic chemical nature and abundance are some of the features that have attracted tremendous attention from researchers. In this study, optimised junctions formed between zirconium sulphide (ZrS2) and copper zinc tin sulphide (CZTS), copper indium sulphide, copper indium selenide and cadmium telluride absorber layers have been explored and compared using SCAPS (a solar cell capacitance simulator program) for photovoltaic applications. The impact of operating temperature, illumination intensity, series and shunt resistances on cell performance has been discussed in detail. Comparative study concluded that the Al-ZnO/ZrS2/CZTS structure presents the best efficiency of 9.72% at room temperature. Other performance parameters obtained are short circuit current density J sc = 25.16 mA cm−2, open circuit voltage V oc = 0.61 and fill factor FF = 68.86%. The external quantum efficiency response was examined under the AM1.5 spectrum for different device models. The simulation results suggest that n-ZrS2 can be used as an excellent buffer layer to fabricate environmentally friendly non-toxic solar cells.

Analysis of Se Co-evaporation and Post-selenization for Sb2Se3-Based Solar Cells

Sb2Se3 is a promising alternative absorber material for thin-film solar cells. However, by a thermal evaporation technique, it appears to partially decompose, leading to Se deficiency. In this work, we propose two alternative routes for the supply of selenium in the deposition of Sb2Se3 thin films. The first method is the co-evaporation of Se and Sb2Se3, while the second is the post-deposition selenization. Superstrate glass/FTO/TO/CdS/Sb2Se3/Au-configured thin-film cells are grown using thermal evaporation. X-ray diffraction patterns confirm the presence of CdS peaks along with the preferred (hk1) oriented grains, which are suppressed upon selenization. Modified surface morphology for the selenized samples is observed by atomic force microscopy. Enhanced current density is observed by J–V characterization and also confirmed by a remarkable external quantum efficiency (EQE) gain at long wavelengths. However, CdS deterioration reduces the EQE response in the short wavelength region, acting as a limiting factor for the efficiency improvement. The champion cell shows a power conversion efficiency of 3.6% with an open circuit voltage of 352 mV and a fill factor of 44.2%, which results in, to our knowledge, the highest value for thermally evaporated Sb2Se3 in superstrate configuration.

Light‐trapping and spectral modulation to increase the conversion efficiency of triple‐junction GaAs solar cells using antireflective coating comprising a periodic array of cone‐shaped TiO 2 structures

This study sought to increase the conversion efficiency of triple-junction GaAs solar cells via spectral light trapping through the application of a uniform TiO2 layer with an antireflective coating (ARC) comprising a periodic array of cone-shaped TiO2 structures. The cone-shaped TiO2 ARC structures were fabricated using a mask of polystyrene spheres (PS) of various diameters (600, 700, and 800 nm). This surface modification was meant to match external quantum efficiency (EQE) response and reflectance spectra in order to modulate the light-trapping effects. In experiments, forward light scattering and reduced reflectivity were shown to improve the EQE response of the cells to beyond that of the cell with a uniform layer of TiO2. The average weighted EQE (EQEW) values of the cell with the proposed ARC (PS: 600 nm) exceeded that of cells with larger surface structure (PS: 700 or 800 nm). These modifications were also shown to increase short-circuit current-density (JSC) and conversion efficiency (η), as confirmed by photovoltaic current-voltage measurements under AM 1.5G illumination. The cone-shaped ARC structures also increased the acceptance angle and electrical output under solar illumination of a given duration. Compared to the reference cell, the cell with 600-nm structures enhanced the EQEW by an impressive 40.26% (from 48.88% to 68.56%), JSC by 20.55% (from 9.73 to 11.73 mA/cm2), and η by 20.59% (from 20.98% to 25.30%).

New Solution-Processed Surface Treatment to Improve the Photovoltaic Properties of Electrodeposited Cu(In,Ga)Se2 (CIGSe) Solar Cells.

The surface Ga content for a CIGSe absorber was closely related to variation in the open-circuit voltage (VOC), while it was generally low on a CIGSe surface fabricated by two-step selenization. In this work, a solution-processed surface treatment based on spin-coating GaCl3 solution onto a CIGSe surface was applied to increase the Ga content on the surface. XPS, XRD, Raman spectroscopy, and band gap extraction based on the external quantum efficiency response demonstrated that GaCl3 post deposition treatment (GaCl3-PDT) can be used to enhance the Ga content on the surface of a CIGSe absorber. Meanwhile, a solution-processed surface treatment with KSCN (KSCN-PDT) was employed to form a transmission barrier for holes by moving the valence band maximum downward and decreasing the interface recombination between the CdS and CIGSe layers. Admittance spectroscopy results revealed that deep defects were passivated by GaCl3-PDT or KSCN-PDT. By applying the combination of GaCl3-PDT and KSCN-PDT, a champion device was realized that exhibited an efficiency of 13.5% with an improved VOC of 610 mV. Comparing the efficiency of the untreated CIGSe solar cells (11.7%), the CIGSe device efficiency with GaCl3-PDT and KSCN-PDT exhibited 15% enhancement.

Novel cost-effective approach to produce nano-sized contact openings in an aluminum oxide passivation layer up to 30 nm thick for CIGS solar cells

This work presents a novel method of local contact openings formation in an aluminum oxide (Al2O3) rear surface passivation layer by the selenization of the lithium fluoride (LiF) salt on top of the Al2O3 for ultra-thin copper indium gallium (di)selenide (CIGS) solar cells (SCs). This study introduces the potentially cost-effective, fast, industrially viable, and environmentally friendly way to create the nano-sized contact openings with the homogeneous distribution in the thick, i.e. up to 30 nm, Al2O3 passivation layer. The passivation layer is deposited by atomic layer deposition, while the LiF layer is spin-coated. Selenization is done in the H2Se atmosphere and the optimal process parameters are deduced to obtain nano-sized and uniformly allocated openings as confirmed by scanning electron microscopy images. The contact openings were produced in the different thicknesses of the alumina layer from 6 nm to 30 nm. Furthermore, the Al2O3 rear surface passivation layer with the contact openings was implemented into ultra-thin CIGS SC design, and one trial set was produced. We demonstrated that the created openings facilitate the effective current collection through the dielectric Al2O3 layer up to 30 nm thick. However, the upper limit of Al2O3 thickness in which the contact openings can be created by the described method is not established yet. The produced passivated CIGS SCs show increased external quantum efficiency response due to the optical enhancement of the passivated cells. However, the production of SCs on the Al2O3 passivation layer with the openings created by selenization of LiF is not optimized yet.

Enhancing luminescent down-shifting of Eu-doped phosphors by incorporating plasmonic silver nanoparticles for silicon solar cells

This study experimentally demonstrated enhanced fluorescence emissions from luminescent down-shifting (LDS) Eu-doped phosphors under the effects of localized surface plasmon resonance (LPSR) from silver nanoparticles (Ag NPs). The plasmonic effects of the Ag NPs was examined in terms of plasmon-enhanced Raman scattering and plasmonic absorption. The fluorescence emission of the Eu-doped phosphors was examined in terms of photoluminescence (PL). Overlap between the plasmonic absorption band of Ag NPs and the PL emission band of Eu-doped phosphors increased fluorescence emissions by enhancing excitation. This study also developed an optical process model describing LSPR (Ag NPs), LDS (Eu-doped phosphors), and plasmon-enhanced fluorescence (PEF; Eu-doped phosphors with Ag NPs) on silicon solar cells. PEF was experimentally confirmed in terms of PL, external quantum efficiency (EQE), and photovoltaic current–voltage measurements. Compared to the cell with only an anti-reflection layer of SiO2/SOG, the inclusion of Ag NPs with the Eu-doped phosphors enhanced the PL emission intensity by 1.1243 times at a wavelength of 514 nm and also improved the EQE response by 8.3% (from 57.28% to 62.05%) and conversion efficiency by 7.1% (from 12.04% to 12.89%).

Near-Infrared Electron Acceptors with Unfused Architecture for Efficient Organic Solar Cells.

The absorption of nonfullerene acceptors (NFAs) at near-infrared (NIR) regions is crucial for obtaining high current densities in organic solar cells (OSCs). Herein, two narrow-band gap NFAs with unfused backbones possessing broad (600-900 nm) and strong absorption are developed by the conjugation of a benzothiadiazole core to halogenated end groups through a cyclopentadithiophene bridge. Compared with the fluorinated counterpart BCDT-4F, the chlorinated NFA BCDT-4Cl shows stronger J-aggregation and closer molecular packing, leading to an optimized blend morphology when paired with the polymer donor, PBDB-T. Thus, an obvious improvement in external quantum efficiency response was obtained for BCDT-4Cl-based OSCs, presenting a higher efficiency of 12.10% than those (9.65%) based on BCDT-4F. This work provides a design strategy for NIR acceptors in the combination of electron-deficient core and halogenated terminal in unfused backbones, which results in not only fine-tuning the optoelectronic properties but also simplifying the synthetic complexities of molecules.

Near‐Infrared Nonfullerene Acceptors Based on Benzobis(thiazole) Unit for Efficient Organic Solar Cells with Low Energy Loss

AbstractThe simultaneous achievement of a low energy loss and high external quantum efficiency (EQE) response is the prerequisite for high power conversion efficiencies of organic solar cells (OSCs). Herein, this issue is examined through the design of two novel near‐infrared (NIR) nonfullerene acceptors (X‐PCIC and X1‐PCIC) with an absorption extending to 900 nm, which is realized by using benzobis(thiazole) unit as the core. This study reveals that benzobis(thiazole) unit with the quinoid‐resonance effect and electron‐withdrawing property is a good building block in extending absorption and maintaining suitable energy levels. Single crystal cultivation proves the function of S···N noncovalent interaction in locking molecular geometry. Besides, through adopting two different terminals (fluorinated terminal for X‐PCIC and thiophene‐fused terminal for X1‐PCIC), it is found that an increase in the J‐aggregation strength has a significant positive effect on the EQE response of the devices through the formation of more suitable domain sizes and crystallinity in the films, while the energy loss remains low (≈0.53 eV) and unaffected. Thus, a high efficiency of 11.50% is presented for OSC based on the X‐PCIC with stronger J‐aggregation strength, better than that (10.17%) based on the X1‐PCIC with weaker J‐aggregation strength. This work clearly demonstrates the application of benzobis(thiazole) unit in efficient small molecule acceptors and the importance of J‐aggregation modulation for the simultaneous achievement of low energy loss and high EQE response.

Improved photovoltaic performance of monocrystalline silicon solar cell through luminescent down‐converting Gd2O2S:Tb3+ phosphor

AbstractThis work reports on efforts to enhance the photovoltaic performance of standard p‐type monocrystalline silicon solar cell (mono‐Si) through the application of ultraviolet spectral down‐converting phosphors. Terbium‐doped gadolinium oxysulfide phosphor and undoped‐gadolinium oxysulfide precursor powders were prepared by a controlled hydrothermal decomposition of a urea homogeneous precipitation method. The resulting rare‐earth element hydroxycarbonate precursor powders were then converted to the oxysulfide by annealing at 900°C in a sulfur atmosphere. The as‐prepared phosphors were encapsulated in ethylene vinyl acetate co‐polymer resin and applied on the textured surface of solar cell using rotary screen printing. Comparative results from X‐ray powder diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy, and photoluminescence spectroscopy studies on the microstructure and luminescent properties of the materials are reported. We also compared the optical reflectance and external quantum efficiency response of the cells with and without a luminescent phosphor layer. The results obtained on the terbium‐doped gadolinium oxysulfide phosphor show clearly that the down‐conversion effect induced by the terbium dopant play a crucial role in enhancing the photovoltaic cells' performance. Under an empirical one‐sun illumination, the modified cells showed an optimum enhancement of 3.6% (from 16.43% to 17.02%) in conversion efficiency relative to bare cells. In the concentration range of 1 to 2.5 mg/mL, EVA/Gd2O2S (blank) composites also improve electrical efficiency, but not as much as EVA/Gd2O2S:Tb3+ composites.

Cascade Solar Cells Based on GaP/Si/Ge Nanoheterostructures

GaP/Si/Ge nanoheterostructures have been obtained using the method of pulsed laser deposition, and an energy band diagram of cascade solar cells based on these heterostructures were modeled. GaP and Ge nanolayers grown on Si substrates were studied by Raman spectroscopy and X-ray diffraction. Spectral dependences of the external quantum efficiency response of GaP/Si/Ge nanoheterostructures were determined.

Каскадные солнечные элементы на основе наногетероструктур GaP/Si/Ge

AbstractGaP/Si/Ge nanoheterostructures have been obtained using the method of pulsed laser deposition, and an energy band diagram of cascade solar cells based on these heterostructures were modeled. GaP and Ge nanolayers grown on Si substrates were studied by Raman spectroscopy and X-ray diffraction. Spectral dependences of the external quantum efficiency response of GaP/Si/Ge nanoheterostructures were determined.

Enhanced performance of a solar cell based on a layer-by-layer self-assembled luminescence down-shifting layer of core-shell quantum dots**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2017PF011), the National Natural Science Foundation of China (Grant No. E020701), and the Doctoral Scientific Research Foundation of Binzhou University, China (Grant No. 2014Y10).

In this paper, core–shell quantum dots (QDs) with two polar surface functional groups (ZnSe/ZnS–COOH QDs and ZnSe/ZnS–NH2 QDs) are synthesized in an aqueous phase. Photoluminescence (PL) and absorption spectra clearly indicate luminescence down-shifting (LDS) properties. On the basis of QDs, surface functional group multilayer LDS films (M-LDSs) are fabricated through an electrostatic layer-by-layer (LBL) self-assembly method. The PL intensity increases linearly with the number of bilayers, showing a regular and uniform film growth. When the M-LDS is placed on the surface of a Si-based solar cell as an optical conversion layer for the first time, the external quantum efficiency (EQE) and short-circuit current density (Jsc) notably increases for the LDS process. The EQE response improves in a wavelength region extending from the UV region to the blue region, and its maximum increase reaches more than 15% between 350 nm and 460 nm.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Fabrication of GaAs solar cells grown with InGaP layers by hydride vapor-phase epitaxy

The elevated manufacturing cost of highly efficient III–V multijunction devices limits them to space and high-concentration terrestrial applications. Hydride vapor-phase epitaxy (HVPE), in contrast to metal–organic vapor-phase epitaxy, can reduce the manufacturing costs due to the use of cheaper group III metal sources, capability to grow crystals under a low arsenic overpressure, and its low installation cost. In this study, we characterized the abruptness of the heterointerface between the InGaP and GaAs layers in GaAs solar cells grown by HVPE. InGaP passivation layers, such as the back-surface field (BSF) and window layers, were introduced in order to enhance the performance of the GaAs solar cells. Owing to the reduction of the surface recombination, the devices fabricated with the incorporated window layer showed a significant improvement in the short-wavelength range of the external quantum efficiency response, compared with that obtained for the unpassivated cells. This provided a cell efficiency improvement from 9.25 to 20.75%. However, the introduction of the BSF layer degrades the cell efficiency to 17.92%, owing to the formation of an anomalous interlayer at the GaAs-on-InGaP heterointerface.

Efficiency enhancement of single-junction GaAs solar cells coated with europium-doped silicate-phosphor luminescent-down-shifting layer

In this study, we investigated the electrical and optical performance of single-junction GaAs solar cells coated with an antireflective layer of indium tin oxide (ITO) via thermal sputtering deposition followed by a layer of SiO2 doped with 3 wt% europium-doped (Eu-doped) silicate phosphors via spin-on film technique. The chemical composition of the Eu-doped silicate phosphors was analyzed using energy-dispersive X-ray spectroscopy and the luminescent downshifting (LDS) characteristics were examined in terms of photoluminescence, optical reflectance, and external quantum efficiency (EQE) response. Reverse saturation-current and ideality factor were used to evaluate the passivation performance of ITO films thermal sputtered on GaAs solar cells. The antireflective performance of the ITO film and the LDS effects of the Eu-doped silicate phosphor coatings were respectively evaluated in terms of optical reflectance and EQE response. The enhancement of photovoltaic performance due to LDS effects was confirmed by photovoltaic current density–voltage characteristics of cells under one-sun air mass 1.5G solar simulations. The efficiency enhancement of the cell with only an ITO/SiO2 antireflective layer was 18.39%, whereas the cells coated a SiO2 layer that included various species of Eu-doped phosphors (species-A, species-B, or species-C) on ITO achieved efficiency enhancements of 19.83%, 20.29%, and 21.07%, respectively.

Synthesis and Characterization of a 2‐(1,1‐Dicyanomethylene) rhodanine‐based Nonfullerene Acceptor for OPVs

We synthesized an A–π–D–π–A‐type nonfullerene small molecule acceptor (Flu‐CNRH) based on fluorene and 2‐(1,1‐dicyanomethylene)rhodanine (CNRH) by a Knoevenagel condensation of the diformyl compound (Flu‐T‐CHO) and CNRH for the use as acceptors in organic photovoltaic cells (OPVs). Introduction of CNRH end groups effectively lowered the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the acceptor compared to those of the corresponding rhodanine‐based acceptor (Flu‐RH). The HOMO and LUMO levels of Flu‐CNRH are −5.71 and −3.60 eV, respectively. For comparison, Flu‐RH has HOMO and LUMO energy levels of −5.58 and −3.53 eV, respectively. This lowered LUMO energy level enables Flu‐CNRH to be combined with low bandgap polymer donors PTB7 and PTB7‐Th by providing proper LUMO offsets between donor and acceptor. OPV cells were fabricated with the device configuration of ITO/PEDOT:PSS/active layer/LiF/Al. The best power conversion efficiency of 1.47% was obtained in the P3HT:Flu‐CNRH devices at a D:A ratio of 1:1.5 and with annealing at 140°C. Moreover, Flu‐CNRH showed compatibility with low bandgap polymer donor PTB7 (1.22%) with external quantum efficiency response at a longer wavelength region up to 750 nm, while Flu‐RH had no photovoltaic behavior with PTB7 and PTB7‐Th.