External Quantum Efficiency Spectra Research Articles

Sub-bandgap Photocurrent Spectra of p-i-n Perovskite Solar Cells with n-Doped Fullerene Electron Transport Layers and Bias Illumination.

In p-i-n perovskite solar cells optical excitation of defect states at the interface between the perovskite and fullerene electron transport layer (ETL) creates a photocurrent responsible for a distinct sub-bandgap external quantum efficiency (EQE). The precise nature of these signals and their impact on cell performance are largely unknown. Here, the effect of n-doping the fullerene on the EQE spectra is studied. The n-doped fullerene is either deposited from solution or by coevaporation. The latter method is used to create undoped-doped fullerene bilayers and investigate the effect of the proximity of the doped region on the EQE spectra. The intensity of the sub-bandgap EQE increases when the ETL is n-doped and also when the device is biased with green light. Using these results, the sub-bandgap EQE signal is attributed to originate from electron trap states in the perovskite with an energy below the conduction band that are filled by excitation with low-energy photons. The trapped electrons give rise to photocurrent when they are collected at a nearby electrode. The enhanced sub-bandgap EQE observed when the ETL is n-doped or bias light is applied, is related to a higher probability to extract trapped electrons under these conditions.

The impact of interface recombination on the external quantum efficiency of silicon solar cells

In various types of organic/inorganic solar cells, optical response enhancement is consistently observed within the external quantum efficiency spectra owing to the improvement in interface passivation and the suppression of carrier recombination. In this study, we focused on crystalline silicon solar cells and systematically investigated the impact of interface recombination on the optical response upon dual-side illumination using numerical simulations. The results shed light on the interesting phenomenon that the surface recombination velocity has a significant impact on the external quantum efficiency, and it changes as the illumination direction changes. Moreover, from a practical perspective, the spectra of external quantum efficiency under dual-side illumination conditions can act as a powerful tool for the quick diagnosis of the passivation quality at the top and bottom interfaces.

Mitigating the Degradation of MAPbI3 Perovskite Solar Cells Under Continuous Light Soaking with ZnO@CdS:Epoxy Short‐Wavelength Sunlight‐Shielded Films

The stability of MAPbI3‐based (MA stands for methylammonium) perovskite solar cells (PVSCs) has been carefully examined by ZnO@CdS nanoparticle‐embedded epoxy films (EP‐ZC films), device encapsulation (of glass sheet and UV‐curing gel), and in different temperatures, 10 °C, room temperature (no temperature control), and 65 °C. EP‐ZC films are used for shielding the short wavelength of the sunlight (UV, near UV, violet, and deep blue) which causes the photodegradation of MAPbI3 and hence the decline of power conversion efficiency (PCE) of PVSCs. Some external quantum efficiency spectra of the device are recorded alongside of the 500 h continuous light‐soaking measurement of current density (J)−voltage (V) of the devices. The time‐of‐flight secondary‐ion mass spectroscopy analysis is performed for a quantitative comparison of the iodide derived from the degradation of MAPbI3 in the encapsulated devices with or without the light‐shielding EP‐ZC films at 10 °C and 65 °C, respectively. The EP‐ZC6 film shows the best protection for encapsulated PVSCs with 98% of the initial PCE at 10 °C after 500 h continuous light soaking. Herein, from the continuous light‐soaking results, one can prioritize the moisture, temperature, and light in the influence of the stability of PVSCs.

High detectivity, near-infrared organic printed photodiode

Sensitive detection of near-infrared (NIR) light could enable several important applications in optical communications, quality control, and sensing in industries, such as automotive and aerospace, and human–machine interface (HMI) biometrics. In this work, we demonstrate NIR photodetectors based on an organic bulk heterojunction absorbing layer with state-of-the-art performance across critical response metrics. The external quantum efficiency spectrum of the devices presented extends from 500 nm to approximately 900 nm covering both visible and NIR wavelengths. The organic photodiodes (OPDs) show specific detectivities of around 7.4×1011 Jones, which is comparable to the state of the art organic devices. Furthermore, the NIR OPD devices show a linear dynamic range (LDR) of ∼74 dB.

Boosting the efficiency of organic solar cells based on a highly planar π-conjugated solid additive working as the sensitizer

Solid additives have been demonstrated as a simple and effective approach to improve the performance of organic solar cells (OSCs). The exploitation of excellent solid additive is an important aspect to obtain efficient OSC. This research designs and synthesizes a novel solid additive IDT(BDT-IC)2 as the sensitizer for OSCs. The IDT(BDT-IC)2 is an A-π-d-π-A type molecule, which possesses a highly planar π-conjugated structure. This molecule exhibits a complementary light absorption with suitable energy levels, which meets the requirement of a qualified sensitizer. It helps to enhance the external quantum efficiency spectrum of the device. Thus, an increased photocurrent is achieved. The addition of IDT(BDT-IC)2 promotes the formation of a cascade energy level alignment and tunes the miscibility of the active layer. Both the exciton dissociation and electron transportation are improved, which enhances the fill factor. As a result, the average power conversion efficiency (PCE) of the PM6:Y6 OSC is increased from 15.75 % of the control device to 16.34 % by incorporating merely 0.25 wt% of IDT(BDT-IC)2. And the highest PCE with a value of 16.70 % is obtained. This research offers a facile and effective strategy for highly efficient OSCs by applying solid additive as the sensitizer.

Tailoring Fe3+‐Activated Broadband NIR Phosphors: Enhancing External Quantum Efficiency and Spectrum Adjustability Through Crystal Field Engineering in Double Perovskite Antimonate Structures

AbstractFor near‐infrared (NIR) luminescent materials, it is a challenge to develop the next generation materials with high external quantum efficiency (EQE), low toxicity, and adjustable spectrum. The transition metal ion Fe3+ can act as a good activator and emit NIR light in oxide host lattice, but there is limited research on this topic. Herein, authors have developed a series of Fe3+ activated A2BSbO6:yFe3+ (A = Ca, Sr, Ba; B = Sc, Y, Ga) phosphors with a double perovskite structure, which can efficiently convert the exciting light into NIR emission in the range of 750–1200 nm with a full‐width half‐maximum (FWHM) of 1372−1525 cm−1. The NIR luminescence originates from the 4T1 (4G)→6A1 (6S) transition of the Fe3+ ions situated in the octahedral sites. By employing crystal field engineering, the emission spectra peak can be adjusted within the range of 842–944 nm, and the excitation spectra can be tuned from 334 to 374 nm. These spectral adjustments enable a good match between the phosphors and commercially ultraviolet chips. When excited at 334 nm, the Sr2ScSbO6:0.1%Fe3+ phosphor demonstrates a remarkably high EQE of 54.2% and high luminescent thermostability. These characteristics make it suitable for applications in NIR spectral detection.

Defect analysis of 1-MeV electron irradiated flexible InGaAs solar cells by deep-level transient spectroscopy and photoluminescence

1-MeV electron irradiated InGaAs flexible solar cells were investigated based on solar cell I–V characteristics, external quantum efficiency, photoluminescence, and deep-level transient spectra (DLTS) mesurements. The electrical parameters of the solar cells, including Rs and Rsh, were extracted from the I–V curves. With an increase in irradiation fluence, the electrical and optical characteristics of the solar cells continuously degraded. The Voc, Isc, and Pmax values decreased to 83.16%, 81.96%, and 68.88% of their initial values, respectively, when the irradiation fluence was increased to 2 × 1015 e/cm2. Photoluminescence (PL) emission peak intensity decreased to 0.54% of its original value, and the minority carrier lifetimes derived from fitting PL emission spectra were reduced to 1.25%. DLTS technique was used to analyse the irradiation-induced defect properties. Similar fitting values for the minority carrier lifetimes were obtained from the PL and DLTS measurements.

Resonant perovskite solar cells with extended band edge

Tuning the composition of perovskites to approach the ideal bandgap raises the single-junction Shockley-Queisser efficiency limit of solar cells. The rapid development of narrow-bandgap formamidinium lead triiodide-based perovskites has brought perovskite single-junction solar cell efficiencies up to 26.1%. However, such compositional engineering route has reached the limit of the Goldschmidt tolerance factor. Here, we experimentally demonstrate a resonant perovskite solar cell that produces giant light absorption at the perovskite band edge with tiny absorption coefficients. We design multiple guide-mode resonances by momentum matching of waveguided modes and free-space light via Brillouin-zone folding, thus achieving an 18-nm band edge extension and 1.5 mA/cm2 improvement of the current. The external quantum efficiency spectrum reaches a plateau of above 93% across the spectral range of ~500 to 800 nm. This resonant nanophotonics strategy translates to a maximum EQE-integrated current of 26.0 mA/cm2 which is comparable to that of the champion single-crystal perovskite solar cell with a thickness of ~20 μm. Our findings break the ray-optics limit and open a new door to improve the efficiency of single-junction perovskite solar cells further when compositional engineering or other carrier managements are close to their limits.

Origin and Energy of Intra‐Gap States in Sensitive Near‐Infrared Organic Photodiodes

AbstractTrap states in organic semiconductors are notoriously detrimental to the performance of organic electronics. However, the origin and energetics of trap states remain largely elusive and under debate, especially for bulk‐heterojunction (BHJ) photodiodes consisting of electron donor and acceptor materials. Combining three sensitive techniques now enables locating the origin and energy of trap states in six state‐of‐the‐art polymer – non‐fullerene acceptor organic photodiodes (OPDs) with noise‐based specific detectivities exceeding 1013 Jones. Analyzing the temperature dependence of the reverse‐bias dark current density (Jd) identifies intra‐gap states in the polymers, lying 0.3−0.4 eV above the energy of the highest occupied molecular orbital, as being responsible for Jd. Sub‐bandgap external quantum efficiency spectra of donor‐only and acceptor‐only diodes confirm that intra‐gap states are much more abundant in the polymers. Likewise, responsivity measurements at ultra‐low light intensities (10−7 mW cm−2) show trap‐mediated charge recombination in BHJ and polymer‐only diodes, but not in acceptor‐only devices. The results imply that to further improve the specific detectivity of near‐infrared OPDs, the intra‐gap state energy, and density need to be reduced.

Applications of spectroscopic ellipsometry for multilayer analysis of CdTe solar cell structures incorporating Magnesium–Zinc oxide high resistivity transparent layers

An optical function parameterization of polycrystalline MgxZn1-xO (MZO) thin films as a function of bandgap Eg has been utilized in applications for the metrology of CdTe-based solar cell structures that incorporate MZO thin films as high resistivity transparent (HRT) layers. Parametric expressions are applied to facilitate mapping spectroscopic ellipsometry (M-SE) of device structures consisting of glass/SnO2:F/MZO. M-SE is shown to provide maps in the MZO effective thickness and bandgap within confidence limits of ± 1 nm and ± 0.003 eV, respectively. As a second application of these parametric expressions, an as-deposited glass/SnO2:F/MZO/CdS/CdTe device structure has been analyzed by through-the-glass spectroscopic ellipsometry (TG-SE). Such an analysis is also shown to provide the MZO effective thickness and bandgap. The outcome of the TG-SE analysis for this device structure enables simulations of the external quantum efficiency (EQE) spectrum of the resulting solar cell assuming different recombination losses within the individual layers of the structure. A comparison of these simulations with the experimental EQE spectrum reveals improved current collection from the front of the device incorporating an MZO HRT layer.

Experimental measurements of soiling impact on current and power output of photovoltaic panels

We have investigated the impact of three different soiling types (dust, leaf, rainfall) on the current–voltage and power–voltage characteristics of a solar panel located at different locations. The current and power losses were measured regularly for 50 days (10-day interval). The soiling ratio was calculated to be a reliable parameter for soiling impact assessment. The source of current and power losses due to soiling was rooted by measuring the transmittance loss and panel surface temperature increase, external quantum efficiency and electroluminescence spectrum recorded for the range of 950–1300 nm. The results confirm leaves as the most detrimental soling type with 38% power loss compared to dust and raindrop and the lowest loss in current density and power was related to raindrop (29%). This is confirmed by the significant decrease in the soiling ratio of the leaf-soiled panels from 0.9 to [Formula: see text] in 50 days. Electroluminescence spectra confirm the critical impact of leaf-soiling on defect generation in the materials and a reduced photocurrent generation. Leaf could reduce the current and power of the panel from 14[Formula: see text]A to 12[Formula: see text]A and from 190[Formula: see text]W to 100[Formula: see text]W, respectively.

Temperature dependent behavior of sub-monolayer quantum dot based solar cell

This work investigates the temperature dependence of the performance of In(Ga)As-based solar cells made from sub-monolayer (SML) quantum dots (QDs), quantum wells (QWs), and Stranski-Krastanov QDs (SK-QDs). Well-defined sub-bandgap peaks are observed in external quantum efficiency (EQE) spectra for all samples. Apparently, changes in minority carrier lifetime affect both the sub-bandgap and the above bandgap EQE. Lateral confinement of carriers in the SMLs and QDs also affects the temperature dependence of the EQE. Finally, a “U” shaped curve was observed from the temperature dependence of the short-circuit current (ISC) from these nanostructure-based solar cells. This is due to the competition between the decreasing bandgap and the increasing carrier loss to recombination within the nanostructures as the temperature increases. In contrast, the solar cell's open-circuit voltage and power efficiency decrease monotonically with temperature.

A highly sensitive filterless narrowband 4H-SiC photodetector employing a charge narrowing strategy

Silicon carbide (SiC) semiconductors with a wide bandgap have attracted much attention because they can endure harsh environments and high temperatures. SiC photodetectors based on conventional principles usually detect ultraviolet (UV) light without the ability to discriminate wavelength. Here, using the charge narrowing collection principle, we realize a highly sensitive filterless narrowband 4H-SiC photodetector. The 4H-SiC layer is sufficiently thick to facilitate charge collection narrowing of the device’s external quantum efficiency spectrum, inducing a full width at half-maximum of 14.5 nm at the peak wavelength of 355 nm. Thanks to the Fermi level pinning effect, the proposed photodetector can fully eliminate the injection current; thus it works as a photovoltaic type device with a remarkably low dark current. Consequently, the device has a photo-to-dark current ratio as high as 4 × 107, superior to the performance of most reported 4H-SiC UV photodetectors. In addition, the device can detect light signals with a power density as low as 96.8 pW cm−2, more than two orders of magnitude superior to that of the commercial product based on the photodiode principle. Moreover, it can endure high temperatures of 350 °C, demonstrating bright prospects in harsh industrial conditions.

Large‐Area Printed Display Based on Green Solvent Processable Blue Light‐Emitting Conjugated Polymer

AbstractGreen‐processable printed large‐area light‐emitting diode (LED) is the crucial component of the high quality solid lighting and flat‐panel display. Accelerating the replacement from fossil‐based solvents to sustainable ecofriendly solvent is desirable for solution‐based printed organic electronics. Herein, by simple introducing methyl to the benzene groups of polydiarylfluorene‐based polymer, this work realizes the solution processing of blue polymer light‐emitting diodes (PLEDs) with a bio‐based solvent of limonene toward large‐area printed display. Film processed from this solvent exhibits satisfactory morphology and remarkable photophysical performances, even though the polymer chains show more serious aggregation in solution. What's more, devices fabricated with spin‐coated films present higher external quantum efficiency (EQE) and better spectra repeatability. Encouraged by the good film‐forming ability of this polymer in limonene, this work realizes the fabrication of printed large‐area films (5 × 5 cm2) with uniform emission and surface morphology by blade coating. Furthermore, this work develops passive matrix (PM) PLEDs arrays with 20 × 20 pixels that can dynamically display monochrome letters and numbers, demonstrating the potential application of cost‐effective printing techniques in large‐area display.

1.5 eV GaInAsP Solar Cells Grown via Hydride Vapor‐Phase Epitaxy for Low‐Cost GaInP/GaInAsP//Si Triple‐Junction Structures

Multijunction solar cells combining III–V and Si materials can provide high photoelectric conversion efficiency. Two‐terminal III–V//Si triple‐junction solar cells with an efficiency of 35.9% have already been developed using metal–organic vapor‐phase epitaxy and the direct wafer bonding technique. This study, however, proposes the low‐cost fabrication of III–V solar cells using hydride vapor‐phase epitaxy (HVPE). GaInAsP solar cells are fabricated using HVPE to apply to middle cells in III–V//Si triple‐junction structures. By controlling the partial pressure of the precursors, the optimal bandgap energy of 1.5 eV is obtained for the HVPE‐grown GaInAsP quaternary alloys. The 1.5 eV GaInAsP single‐junction solar cells show higher open‐circuit voltage than the HVPE‐grown GaAs solar cells. The open‐circuit voltage of the GaInAsP solar cells fabricated with a GaInAsP growth rate of 77.6 μm h−1 reaches 1.1 V upon the formation of the rear‐heterojunction structure. In addition, the external quantum efficiency spectra of the HVPE‐grown GaInP/GaInAsP dual‐junction solar cells show that the 1.5 eV GaInAsP solar cells are superior to the GaAs solar cells in terms of current matching for subcells in the III–V//Si triple‐junction structures.

Influence of Cu-atomic ratio in the 3-stage deposition technique on the efficiency of CuIn1-xGaxSe2 solar cells

The 3-stage co-evaporation technique is one of the deposition processes used to fabricate photon absorber layer in high efficiency CuIn1-xGaxSe2 (CIGS) solar cells. For this technique, the [Cu]/[III] ratio (y), where [III] refers to group-III elements, evolves from Cu-poor (y < 1) to Cu-rich (y > 1) in the 2nd stage and finally ends with slightly Cu-poor (y ~ 0.9) in the 3rd stage of the 3-stage process. Here, the highest values of [Cu]/[III] in the 2nd stage are intentionally varied from 1.0 to 1.5 by setting the deposition time of the pre-calibrated Cu and Se fluxes in the 2nd stage. The [Ga]/[III] ratio (x) is set at 0.37 during the 1st and 3rd stages in all devices. The influences of the Cu-atomic ratio are examined for the crystal grain growth, elemental depth profiles of the CIGS absorbers as well as the photovoltaic parameters and external quantum efficiency (EQE) of the CIGS solar cells. The optimal value of y = 1.3 is found to provide the highest efficiency CIGS device. The double-grading depth profile in the [Ga]/[III] ratio has also been observed despite the constant fluxes of group-III elements set during the whole process. The performances of the CIGS solar cells are investigated under AM1.5 condition and found to have open-circuit voltage (V OC) of 670 mV, short-circuit current density (J SC) of 33.2 mA/cm2, fill factor (FF) of 75.5% and the power conversion efficiency of 16.8% for the best CIGS device. The J SC of the device with y = 1.3 is relatively higher than other devices due to the increase of photo-generated currents in the short wavelength region as seen in the EQE spectrum.

Large‐Area Wide Bandgap Indoor Organic Photovoltaics for Self‐Sustainable IoT Applications

Superior spectral responsivity, low open‐circuit voltage losses, and good scalability make organic photovoltaics (OPVs) potential power generating contenders for low‐power consumption electronic devices for indoor self‐sustainable applications. Herein, large‐area organic photovoltaic devices with PBTZT‐stat‐BDTT‐8: PC71PM photoactive layer are fabricated to develop indoor power sources for low‐power, portable, and wireless electronic devices. The better spectral matching of PBTZT‐stat‐BDTT‐8‐based photoactive absorber layer with light‐emitting diode (LED) emission spectra translates a power conversion efficiency (PCE) over 18% for single‐cell (0.38 cm2) device. Whereas, PCEs over 17% and 16% are observed for mini‐module (18.63 cm2) and submodule (40 cm2) under 1000 lux LED (2700 K) illumination, respectively. The emission powers of the LED lamps are carefully analyzed, and integral current densities of photovoltaics cells are calculated with the help of external quantum efficiency and photon flux spectrum to assure the reliability of photovoltaic measurements. Finally, the commercially available programmable Arduino boards and Bluetooth‐based Internet of Things devices integrated with OPVs to build a self‐sustainable communication system that can function well under indoor environment are demonstrated.

Performance improvement of blue TADF top-emission OLEDs by tuning hole injection barriers using a nickel-doped silicon dioxide buffer layer

The severe exciton self-quenching and twisted structure of typical thermally activated delayed fluorescence (TADF) emitters result in a low external quantum efficiency (EQE) and broad emission spectrum, especially in blue organic light-emitting diodes (OLEDs). These challenges have been overcome by employing complex device fabrication steps and complex molecular structures requiring fine stoichiometric adjustments. However, only a few attempts have been made to improve the performance of OLEDs through device structure engineering, particularly in top-emission OLEDs (TEOLEDs). Herein, we report blue TADF TEOLEDs fabricated by employing a second-order microcavity structure. These TEOLEDs simultaneously exhibit a high EQE (∼20.2 %) and narrow full width at half maximum (29 nm) owing to the incorporation of a Ni-doped SiO2 buffer layer that balances the injection charges. The wide bandgap of SiO2 prevents rapid hole injection from the anode and modifies the anode–organic layer interface, while tuning of the Ni doping concentration reduces the turn-on voltage of the device through co-sputtering. The charge balance mechanism is elucidated in detail by analyzing tunneling effects across the buffer layer and the bonding states of Ni atoms in the SiO2 film. This study can promote the development of TADF top-emission devices with high efficiency and high color purity.

Broadband Organic Ternary Bulk Heterojunctions Photodetector Based on Non-Fullerene Acceptor with Enhanced Flat-Spectrum Response Range from 200 to 1100 nm.

Both flat-spectrum responsivity and high external quantum efficiency (EQE) of bulk heterojunction organic photodetectors (BHJ OPDs) are greatly in demand and still challenging to realize from the ultraviolet (UV) to near-infrared (NIR) regions. In this article, conjugated polymer donor poly(3-hexylthiophene) (P3HT) and PTB7-Th are blended with a low band gap nonfullerene acceptor (NFA) IEICO-4F to form a ternary BHJ active layer, thereby forming a BHJ OPD with a broadband responsivity spectrum from UV to visible light to NIR region (200-1100 nm). Under 6 V voltage and in the range from 280 to 810 nm, the ternary BHJ OPD shows a relatively flat responsivity spectrum, and the highest responsivity is 1.348 A/W, which is 1.34 times that of the binary BHJ OPD. Specifically, the ternary BHJ OPD achieved the highest EQE at 285 nm and as high as 449.31%. In addition, the ternary OPD detectivity (D*) is 2.65 times that of the binary BHJ OPD. Therefore, ternary BHJ as an active layer provides an effective method to develop BHJ OPDs with an expanded response range, higher responsivity, improved EQE, and broadband spectrum with flat spectral response from the UV to NIR region.

Photomultiplication type organic photodetectors with different response characteristics under forward or reverse bias

Photomultiplication type organic photodetectors (PM-OPDs) with a sandwich structure of ITO/PNDIT-F3N/P3HT:[71]PCBM(100:1, wt/wt)/Al were successfully developed, exhibiting different response characteristics under forward or reverse bias. The rather less [71]PCBM can act as isolated electron traps to capture photogenerated electrons. The trapped electrons near ITO (Al) electrode can make interfacial band bending for hole tunneling injection from external circuit under forward (reverse) bias. The injected holes can be efficiently transported along the channel provided by P3HT and then collected by the corresponding electrode under bias. The external quantum efficiency (EQE) spectral shape of PM-OPDs is primarily determined by the trapped electron distributions near ITO or Al electrode under forward or reverse bias. The PM-OPDs exhibit flat EQE spectra under forward bias and ‘M-shaped’ EQE spectra under reverse bias, which is consistent with the curves of trapped electron distribution near ITO or Al electrode. The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias. The PM-OPDs exhibit distinct response speed under forward or reverse bias, which is primarily attributed to the different trapped electron density near ITO or Al electrode under incident light from ITO side. • The PNDIT-F3N interfacial layer makes PM-OPDs work well under forward or reverse bias. • The response speed of PM-OPDs was investigated under distinct polarity bias, resulting from trapped electrons near electrode. • The champion EQE values of PM-OPDs arrive to 2550% at 425 nm under 10 V bias and 2360% at 365 nm under −10 V bias.