External Quantum Efficiency Spectra Research Articles

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.

Double-Side Selective Spectral Response Organic Photodetectors for Demultiplexing Optical Signals.

Selective spectral response photodetectors (PDs) capable of accurately capturing the color of images have broad applications in industry and academia. Here, we demonstrate filter-free, wavelength-selective detection organic photodetectors (OPDs) with two distinct response ranges and having the planar bilayer heterojunction device structure by employing either a (semi-)transparent anode or a cathode as the incident light window. The resultant OPDs exhibit a responsivity of 51 mA W-1 in the range 300-650 nm and 11 mA W-1 in the range 650-850 nm under top illumination condition. Similarly, the devices show a responsivity of 2 mA W-1 for the short-wavelength region and 131 mA W-1 for the long-wavelength region when under bottom illumination condition, indicating a high responsivity ratio that meets the requirements for selective detection. Hence, our individual device not only works in either visible or NIR range but also provides narrowband detection with spectral widths down to 100 nm in the NIR range. The working mechanism of spectral selectivity is identified through quantitative analysis of the external quantum efficiency (EQE) spectra using optical modeling when compared to the OPDs with bulk heterojunction structure, thus clarifying the general validity of the device design concept. Finally, our OPDs can demultiplex intermixed optical signals from the light-communication system successfully. Our results should inspire new studies on the device design concept and new applications of OPDs.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Quantifying the Excitonic Static Disorder in Organic Semiconductors

AbstractOrganic semiconductors are disordered molecular solids, and as a result, their internal charge generation dynamics, charge transport dynamics, and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. This is particularly pertinent for emerging photovoltaic technology where the extractable power is directly dependent on these dynamics. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, it is shown that the static disorder of an organic semiconductor can be obtained from its photovoltaic external quantum efficiency spectrum at wavelengths near the absorption onset. A detailed methodology is presented, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the authors show that minimizing the limiting effects of optical interference is crucial for achieving high‐accuracy quantifications. Finally, transparent devices are employed to estimate the excitonic static disorder in several technologically relevant organic semiconductor donor–acceptor blends, including the high‐efficiency organic photovoltaic system PM6:Y6.

Effects of material properties of band‐gap‐graded Cu(In,Ga)Se2 thin films on the onset of the quantum efficiency spectra of corresponding solar cells

AbstractPolycrystalline Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge‐carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open‐circuit voltage (Voc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin‐film solar cells, represented by the standard deviation, 𝜎total, was evaluated to study the impacts of the effective band‐gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that 𝜎total can be disentangled into contributions of these material properties, in addition to a residual component 𝜎residual. Effectively, 𝜎total depends only on a contribution related to the Ga/In gradient as well as on 𝜎residual. The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component 𝜎residual. It is demonstrated that a flat band‐gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest 𝜎total values and thus result in lower recombination losses and gains in Voc.

Ultraviolet Narrowband Photomultiplication Type Organic Photodetectors with Fabry−Pérot Resonator Architecture

AbstractUltraviolet (UV) narrowband photodetectors play a critical role in missile detection, flame monitoring, optical communication, etc. It is a great challenge to realize UV narrowband organic photodetectors due to wide photo‐harvesting property of organic materials, especially for photomultiplication type organic photodetectors (PM‐OPDs). In this work, a smart strategy is proposed to achieve UV narrowband response by coupling Fabry−Pérot microcavity with PM‐OPDs. PM‐OPDs are realized by using poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C71‐butyric acid methyl ester (100:1, w/w) as active layers, exhibiting broadband response range covering from UV–vis region. Series of optical microcavities consisting of Ag/LiF/Ag with UV spectral selectivity are prepared, which are employed to couple with the PM‐OPDs for achieving UV narrowband response. The UV spectral selectivity of optical microcavity can be optimized by tuning the thickness of spacer layer and mirror layers, which can further regulate the photogenerated electron distribution near Al electrode to optimize the external quantum efficiency (EQE) spectra of the PM‐OPDs coupled with optical microcavity. The optimized PM‐OPDs coupled with optical microcavity exhibit EQE of 9300% at 350 nm and narrowband response with 33 nm full‐width at half‐maximum under −15 V bias. This work indicates that PM‐OPDs coupled with optical microcavity should be an efficient strategy for achieving UV narrowband response.

Nonradiative Recombination via Charge‐Transfer‐Exciton to Polaron Energy Transfer Limits Photocurrent in Organic Solar Cells

AbstractA recombination and exciton loss mechanism is reported in organic solar cells involving energy transfer between charge transfer (CT) excitons and polarons, impacting photocurrent generation, particularly in the near‐infrared where polaronic transitions typically reside. This process sets a low‐energy cut‐off in the external quantum efficiency spectrum of an excitonic donor/acceptor interface, determined by the low‐energy polaron absorption peak and the CT state reorganization energy. Furthermore, this process explains the deviation from unity and bias dependence of the CT state's internal quantum efficiency at low photon energies. This process is demonstrated in a variety of systems and it is hypothesized that CT state to polaron energy transfer recombination may be responsible for a share of nonradiative recombination in all organic photovoltaics and can explain numerous experimentally observed device trends regarding photocurrent generation and energy losses. Overall, this work enhances the understanding of photophysical processes in organic materials and allows the design of systems that can avoid this recombination pathway.

Relevant photovoltaic effect in N-doped CQDs/MoS2 (0D/2D) quantum dimensional heterostructure

In this work, a novel kind of solar cell based on the N-CQDs/MoS2 (0D/2D) quantum dimensional heterostructure has been fabricated using a two-dimensional (2D) MoS2 ultrathin layer and zero-dimensional (0D) Nitrogen-doped carbon quantum dots (N-CQDs). The combination of N-CQDs and MoS2 offers good optical and electrical characteristics. The current-voltage (I–V) characteristics of the quantum dimensional heterostructure have been recorded in the dark and under the illumination of solar radiations. A significant current rectification ratio has been observed in dark I–V characteristics. Furthermore, a remarkable shift of −13.49 mA in the I–V curve is observed on the exposure to solar radiation. The fabricated quantum dimensional heterostructure exhibits a significant power conversion efficiency (η) of 4.06% and an open-circuit voltage of 0.83 V. The external quantum efficiency (EQE) spectra support the recorded photovoltaic performance of the heterostructure. The aforementioned results could be ascribed to the effective separation of photogenerated electron-hole pairs at the junction contact. The N-CQDs serve as an electron blocking layer, preventing additional carrier recombination at the metal electrode. Our findings show that N-CQDs/MoS2 heterostructure has a great potential in futuristic low-cost, non-toxic, and highly efficient solar cells for next-generation energy harvesting applications.

Revealing defective interfaces in perovskite solar cells from highly sensitive sub-bandgap photocurrent spectroscopy using optical cavities

Defects in perovskite solar cells are known to affect the performance, but their precise nature, location, and role remain to be firmly established. Here, we present highly sensitive measurements of the sub-bandgap photocurrent to investigate defect states in perovskite solar cells. At least two defect states can be identified in p-i-n perovskite solar cells that employ a polytriarylamine hole transport layer and a fullerene electron transport layer. By comparing devices with opaque and semi-transparent back contacts, we demonstrate the large effect of optical interference on the magnitude and peak position in the sub-bandgap external quantum efficiency (EQE) in perovskite solar cells. Optical simulations reveal that defects localized near the interfaces are responsible for the measured photocurrents. Using optical spacers of different lengths and a mirror on top of a semi-transparent device, allows for the precise manipulation of the optical interference. By comparing experimental and simulated EQE spectra, we show that sub-bandgap defects in p-i-n devices are located near the perovskite-fullerene interface.

Voltage Loss Comparison in CdSe/CdTe Solar Cells and Polycrystalline CdSeTe Heterostructures

Cd(Se)Te solar cells have considerable headroom to increase voltage. Voltage losses occur due to incomplete absorption above bandgap <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_g</i> and band tail absorption below <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_g</i> ; such losses are quantified using radiative voltage. The largest voltage losses are attributed to nonradiative recombination, which is quantified via carrier lifetime and radiative efficiency. We compare radiative voltage, radiative efficiency, and carrier lifetime for Cu-doped CdSe/CdTe solar cells and for undoped polycrystalline CdSeTe heterostructures passivated with Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Using external quantum efficiency spectrum and a CdSeTe absorption spectrum obtained from absolute photoluminescence (PL), we show that the radiative voltage is greater than 1.1 V. Time-resolved PL experiments and modeling show that a major part of voltage losses can be attributed to recombination in the absorber bulk. The front interface recombination makes a larger impact within the first few nanoseconds after pulsed excitation, and the comparison of time-gated and time-integrated PL can be used to assess relative contributions of front interface and bulk recombination rates.

A comparative study of mixed halide perovskite thin film preparation by three- and two-step electrodeposition techniques

Nowadays, access to cheap, renewable and achievable energy sources is no longer just a dream with perovskite solar cells (PSCs). Reaching the record of ∼25% efficiency in 2019 has increasingly led researchers to address the challenges facing commercialization, including finding suitable large scale preparation techniques and increasing the lifetime of PSCs. According to various reports in the thin film solar cells fabrication based on electrodeposition (ED) routes, it is worth noting that the ED technique could be an interesting candidate for production of large-scale perovskite solar cells and low-cost modules in the future. Here, we compare the optoelectronic properties of the MAPb(IxBr1-x)3 perovskite layers (Eg = 1.9 eV) synthesized in three and two step based on ED-PbO2 layer on FTO (Fluorine-doped Tin Oxide) substrates. We show that the direct conversion of ED-PbO2 to perovskite, in addition to its cost-cutting and time-saving benefits, can lead to a uniform layer with significantly admirable structural properties compared to the conventional perovskite conversion using the 3-steps ED method. The measured external quantum efficiency (EQE) spectrum shows the photo-sensing capability of the 2-step ED perovskite layer in a wide range from ultraviolet to visible light region. The 2-step ED perovskite-based photodetector device exhibited superb performance, which resulted in high photo/dark current ratio of 107 at 1V under 660 nm (2000 μW power intensity) light illumination.

Efficiency of exciton splitting in organic photovoltaic cells within EQE spectrum

The paper presents a procedure of estimating the efficiency of exciton splitting at ED/EA interface. The procedure consists in evaluation of splitting of excitons into electron-hole pairs on the basis of the external quantum efficiency spectra of planar cells and spectra of absorbance of active organic layers. The fitting parameters are the exciton splitting probabilities at ED/EA interface. The presented procedure was applied to two different photovoltaic systems: ITO/MoO3/DBP/PTCBI/BCP/Ag and ITO/MoO3/DBP/F16ZnPc/BCP/Ag with quite similar energy diagrams at the ED/EA interface. The analysis performed led us to the conclusion that only the DBP/PTCBI interface can be considered attractive for organic photovoltaics and organic photodetection, while the DBP/F16ZnPc interface does not show any favourable properties for such applications.

High-temperature post-annealing effect on the surface morphology and photoresponse and electrical properties of B-doped BaSi2 films grown by molecular beam epitaxy under various Ba-to-Si deposition rate ratios

Semiconducting barium disilicide (BaSi2) is one of the emerging materials for light absorber layers in solar cell applications. We investigated the effect of post-annealing on various properties of lightly B-doped BaSi2 films by molecular beam epitaxy grown under various Ba-to-Si deposition rate ratios (RBa/RSi) in the range 0.4–4.0. Post-annealing was performed at 900 °C for 2 min in an Ar atmosphere. As-grown samples formed under Si-rich conditions (RBa/RSi = 0.4 and 1.2) showed p-type conductivity, while those formed under Ba-rich conditions (RBa/RSi = 2.2, 2.6, 3.0 and 4.0) showed n-type conductivity. The conductivity type and the carrier concentration remained almost unchanged after the post-annealing. In contrast, there was a significant difference in surface morphology and photoresponsivity. We compared the photocurrent density (Jph) obtained from integrating the product of each sample’s external quantum efficiency spectrum and AM 1.5 spectrum. For samples grown under Ba-rich conditions (RBa/RSi ≥ 2.2), cracks were generated on the surface, and the Jph decreased. The Jph particularly decreased by approximately two orders of magnitude for the sample formed at RBa/RSi = 2.6. On the other hand, the cracks were hardly observed by optical microscopy when grown under Si-rich conditions (RBa/RSi = 0.4 and 1.2), and the Jph increased. Transmission electron microscopy observation revealed the presence of crystalline Si islands around the BaSi2/Si interface caused by the aggregation of excess Si atoms in the sample formed at RBa/RSi = 1.2.

Lithium doped poly(3-hexylthiophene) for efficient hole transporter and sensitizer in metal free quaterthiophene dye treated hybrid solar cells

This work focuses on the role of Lithium doped Poly(3-hexylthiophene)(P3HT) in metal-free quaterthiophene (4T) dye treated Titanium dioxide (TiO2) based hybrid solar cells. The dye treated hybrid solar cells with Lithium doped P3HT showed efficiencies (3.95%) of nearly a factor of four times higher than the pristine P3HT based control TiO2/4T/P3HT devices (1.04%). The enhancement of the efficiency is mainly due to highly efficient charge collection attributed to enhanced charge transport and light harvesting properties of Lithium doped P3HT polymer. The optimized solar cells with Lithium doped P3HT showed a high short circuit current density over 13 mA/cm2, under simulated irradiation of intensity 100 mW/cm2 with AM 1.5 filter. This significant increase in current density in TiO2/4T/doped P3HT solar cell is also confirmed by both the broadened External Quantum Efficiency spectrum and significant photoluminescence quenching upon replacement of pristine P3HT with doped P3HT on 4T dye treated TiO2 electrode. With Lithium doped Spiro-OMeTAD instead of Lithium doped P3HT, similar devices showed efficiencies over 3.30% under simulated irradiation of 100 mW/cm2 with AM 1.5 filter.

Novel polymer donors based on simple A1-π-A2 structure for non-halogen solvent-processed organic solar cells

Two low band gap conjugated polymers based on acceptor1-π-acceptor2 (A1-π-A2) and donor-π-acceptor (D-π-A) type architecture were synthesized for organic solar cells. The novel A1-π-A2 type polymer contains a fluorine substituted benzotriazole (FTAZ) as acceptor1 block unit (A1), tetrazine (TTz) as acceptor2 unit (A2), and thiophene as the π-bridge, namely PBTZ-TTz. The compared D-π-A type polymer contains a benzodithiophene (BDT) as rich electronic unit, TTz as electron-deficient unit and thiophene as the π-bridge, namely PBDT-TTz. Although mounting more thiophene π bridge units for better solubility and backbone geometry, the novel “A1-π-A2” type copolymer PBTZ-TTz achieved a deep HOMO energy level of −5.55 eV, just slightly higher than that of D-π-A type polymer PBDT-TTz. Moreover, the polymer PBTZ-TTz obtained a wider and stronger UV–vis absorption, better coplanarity and π-π stacking than PBDT-TTz. After processed by non-halogen solvent 1,2,4- trimethylbenzene (TMB), the PBTZ-TTz:IT-4F based device achieved a decent open circuit voltage (Voc) of 0.88 V, a lower power conversion efficiency (PCE) of 5.9%, while PBDT-TTz based device reached the PCE of 7.4% with Voc of 0.95 V. The unexpected lower PCE of PBTZ-TTz:IT-4F device is mainly due to the holes cannot transfer efficiently from acceptor to donor, the lower external quantum efficiency (EQE) spectra responding, as well as the poor phase separation morphology of the blend film. It is worth noting that both the Voc achieved are in outstanding high levels among IT-4F based devices. These results reveal that the A1-π-A2 type copolymers can enrich non-halogen solvent-processed donor materials through simple synthesis, which is beneficial for future environmentally friendly processing.

Synthesis of a carboxylic acid-based ruthenium sensitizer and its applicability towards Dye-Sensitized Solar Cells

This work reports the synthesis of ruthenium based Ru(bpy)2(dcbpy)(ClO4)2[(bpy)2,2′-bipyridine;dcbpy = 4,4′-dicarboxy-2,2′-bipyridine] (RuC) dye and its application in solid and liquid state Dye-Sensitized Solar Cells (DSSCs). Synthesis resulted with high-pure orange coloured dye with a high yield percentage of 45%. The dye was characterized via Nuclear Magnetic Resonance (NMR) spectroscopy, Mass spectroscopy, Cyclic Voltammetry (CV) and UV–vis spectroscopy. The calculated bandgap (Eg) of 2.38 eV from absorbance spectra of pure RuC in ethanol solution showed best proximity with the data obtained from CV measurements. The RuC showed strong absorption in near UV region with the highest molar extinction coefficient (MEC) of 14,746 M−1cm−1 at 463 nm. Plateau of over 80% of External Quantum Efficiency (EQE) spectra reveals the efficient carrier generation of RuC in near UV region. Carboxylic acid groups of RuC provide the potential for enhanced electron transfer from TiO2 surface, and an increased electron density at the interface leads to higher current density. The RuC sensitized solid and liquid state DSSCs exhibited a short circuit current density (JSC) over 3.04 mA/cm2 and 5.82 mA/cm2, and power conversion efficiency (PCE) of 1.2% and 1.8% respectively under simulated Air Mass 1.5 irradiation (100 mWcm−2).

Au-doped mesoporous SiO2 scattering layer enhances light harvesting in quasi Solid-State dye-sensitized solar cells

This study investigates the morphological effect of different Au-doped SiO2 scattering layers on the performance of dye-sensitized solar cells (DSSCs). Particularly, the SiO2 sources were varied to yield different geometries, i.e., tetraethyl orthosilicate (TEOS) templated SiO2, Sidoarjo mud (LuSi) extracted SiO2, and commercial silica glass sphere. The microstructure, as well as physical, electronic, and optical properties of different Au-doped SiO2 particles, were characterized using SEM-EDX, TEM, BET, XRD, and various spectroscopy techniques. The photoelectrochemical performance of quasi-solid state DSSCs was indicated by current density–voltage (J-V) response, external quantum efficiency spectra, and the impedance response. The results indicate that the performance of TiO2-based DSSCs is enhanced quite significantly due to the improved photocurrent generation and fill factor. The short circuit current density is found up to 370% higher (and hence, the conversion efficiency) than the reference cell upon incorporating Au-doped crystalline SiO2 extracted from LuSi (Voc = 0.89 V, Jsc = 1.28 mA‧cm−2, FF = 0.65, and η = 0.75%). This substantial photocurrent enhancement stems from the combined effect of efficient light scattering by submicron SiO2 particles, surface plasmon resonance, and reduced interfacial recombination by SiO2 insulation. In addition, the optimum size of SiO2 particles is deduced as the results indicate the size-scattering dependency which controls the gain and loss of photocurrent due to the type of scattering.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells