Photogeneration Efficiency Research Articles

(Invited) Role of the Energy Offset in the Charge Separation at the Donor: Acceptor Interface

The trade-off between the short-circuit current density and open-circuit voltage has been one of the largest bottlenecks for further improving the device performance of organic solar cells (OSCs). Therefore, understanding the limit to which the energy offset can be reduced and the physics underlying the trade-off relationship is crucial. This study provides a threshold energy that can ensure high charge photogeneration efficiencies for Y-series nonfullerene acceptor-based OSCs and discusses the role of the energy offset in charge separation. In this study, we used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron-hole pairs generated at donor:acceptor interfaces. We found that an insufficient energy offset led to not only slow charge transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the charge transfer states. Our findings highlight the importance of overcoming the trade-off between fill factor and open-circuit voltage for further improving the power conversion efficiency.

Disorder and Photogeneration Efficiency in Organic Semiconductors.

An analytical description of the separation probability of a geminate pair in organic semiconductors is given. The initial diffusion of "hot" twins is anomalously strong due to energy disorder. This circumstance significantly increases the photogeneration quantum yield at low temperatures and weakens its temperature dependence relative to predictions of the Onsager model, in agreement with Monte Carlo and experimental results.

Role of the energy offset in the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

The quantum yield of long-range spatial dissociation of electron–hole pairs decreased with a decrease in the energy offset between the excited and charge-transfer states, leading to a threshold that can ensure high charge photogeneration efficiency.

Electrical hysteresis characteristics in photogenerated currents on laser-beam-derived in-plane lateral 1D MoS2-Schottky junctions

Atomically thin two-dimensional transition-metal dichalcogenide materials with van der Waals integration provide various interesting optoelectronic characteristics that can be used to realize highly efficient flexible solar cells and photosensors. We previously reported in-plane lateral one-dimensional Schottky junctions (SJs) on few-atom-layer 2H-phase semiconductor-molybdenum disulfide by forming a 1T′-metal phase using laser beam (LB) irradiation and clarified their unique optoelectronic properties. Although the LB-derived 1T′/2H phase SJs provided efficient photocurrent generation, they had a large number of defects owing to the excess heat accumulation caused by the LB. Here, we observe partial electric hysteresis properties in photogenerated currents (Iphoto) on the SJs under reverse bias voltage regions and reveal that they are very sensitive to the voltage sweep direction and its switching (holding) time. The properties persist under dark ambient conditions for a few minutes, even after photo-irradiation is complete. The temperature dependence reveals that a defect-derived deep carrier trap-center, which is unique to the present 1T′ phase, can be the cause of these phenomena. A larger Iphoto and an increase in photogeneration efficiency are obtained by eliminating this trap center through thermal annealing. In contrast, it is expected that these hysteresis properties lead to atomically thin photo-memristor devices for opto-neuromorphic systems.

Photoactive Red Fluorescent SiO2 Nanoparticles Based on Controlled Methylene Blue Aggregation in Reverse Microemulsions.

We present a reverse microemulsion synthesis procedure for incorporating methylene blue (MB), a known FDA-approved type-II red-absorbing photosensitizer and 1O2 generator, into the matrix of hydrophobic-core/hydrophilic-shell SiO2 nanoparticles. Different synthesis conditions were explored with the aim of controlling the entrapped-dye aggregation at high dye loadings in the hydrophobic protective core; minimizing dye aggregation ensured highly efficient photoactive nanoentities for 1O2 production. Monitoring the synthesis in real time using UV-vis absorption allowed tracking of the dye aggregation process. In particular, silica nanoparticles (MB@SiO2 NPs) of ∼50 nm diameter size with a high local entrapped-MB concentration (∼10-2 M, 1000 MB molecules per NP) and a moderate proportion of dye aggregation were obtained. The as-prepared MB@SiO2 NPs showed a high singlet oxygen photogeneration efficiency (ΦΔ = 0.30 ± 0.05), and they can be also considered as red fluorescent probes (ΦF ∼ 0.02, λmax ∼ 650 nm). The distinctive photophysical and photochemical characteristics of the synthesized NPs reveal that the reverse microemulsion synthesis procedure offers an interesting strategy for the development of complex theranostic nano-objects for photodynamic therapy.

Self-redox reaction driven in situ formation of Cu2O/Ti3C2Tx nanosheets boost the photocatalytic eradication of multi-drug resistant bacteria from infected wound

BackgroundMXenes with interesting optical and electrical properties have been attractive in biomedical applications such as antibacterial and anticancer agents, but their low photogeneration efficiency of reactive oxygen species (ROS) and poor stability are major concerns against microbial resistance.MethodsWater-dispersible single layer Ti3C2Tx-based MXene through etching tightly stacked MAX phase precursor using a minimally intensive layer delamination method. After addition of Cu(II) ions, the adsorbed Cu(II) ions underwent self-redox reactions with the surface oxygenated moieties of MXene, leading to in situ formation of Cu2O species to yield Cu2O/Ti3C2Tx nanosheets (heterostructures).ResultsUnder NIR irradiation, the Cu2O enhanced generation of electron–hole pairs, which boosted the photocatalytic production of superoxide and subsequent transformation into hydrogen peroxide. Broad-spectrum antimicrobial performance of Cu2O/Ti3C2Tx nanosheets with sharp edges is attributed to the direct contact-induced membrane disruption, localized photothermal therapy, and in situ generated cytotoxic free radicals. The minimum inhibitory concentration of Cu2O/Ti3C2Tx nanosheets reduced at least tenfold upon NIR laser irradiation compared to pristine Cu2O/Ti3C2Tx nanosheets. The Cu2O/Ti3C2Tx nanosheets were topically administrated on the methicillin-resistant Staphylococcus aureus (MRSA) infected wounds on diabetic mice.ConclusionUpon NIR illumination, Cu2O/Ti3C2Tx nanosheets eradicated MRSA and their associated biofilm to promote wound healing. The Cu2O/Ti3C2Tx nanosheets with superior catalytic and photothermal properties have a great scope as an effective antimicrobial modality for the treatment of infected wounds.Graphical

Giant Photoresponse Enhancement in Mixed‐Dimensional Van der Waals Heterostructure through Dielectric Engineering

AbstractThe reduced dielectric screening in the out of plane direction, makes 2D materials sensitive to the surrounding environment, offering a unique platform with greatly tunable optoelectronic properties. Large exciton binding energy in 2D materials limits their photogeneration efficiency. The strong electric field generated at a p–n junction will help in separating these strongly bound electron hole pairs. Here, the present study demonstrates how engineering the surrounding dielectric environment would facilitate a mixed dimensional van der Waals p–n junction to improve the photoresponse to a great extent. A 3D silicon‐2D monolayer MoS2 heterostructure is fabricated as a model system. Nearly three orders of magnitude enhancement in photoresponse is observed by modulating the surrounding dielectric environment. This huge enhancement is attributed to the easy separation of photogenerated carriers due to the screening of Coulomb interaction. The dielectric also screens the impurity potential, reducing the charge carrier scattering. In addition, there is a change in the overall bandgap of the heterostructure producing a lower energy barrier for the charge carriers. The findings lay a general pathway for improving the efficiency of 2D material based photodetectors through dielectric engineering.

(Invited) Optoelectronic Synapses with Two Dimensional Materials for Neuromorphic Computing

Optoelectronic synapses are devices that behave like photodetectors with analog memory. In neuromorphic computing, these devices have the potential to perform imaging and object identification using the same device. We first demonstrate that the strong photogeneration efficiency of methylammonium lead bromide perovskite quantum dots (PQDs) can be exploited by growing PQDs from the lattice of a single layer graphene by a defect-mediated process. The rationale for designing this hybrid superstructure stems from the ability of PQDs to absorb light and generate charge carriers. The charges generated are transferred to graphene, which transports the carriers across the active layer of the device. Through the implementation of this thin superstructure in a phototransistor geometry, we produce a photoresponsivity of 1.4×108 AW-1 at 430 nm and a specific detectivity (D*) of 4.72×1015 Jones, which is by far the best responsivity and detectivity across similar devices reported to date. This is promising for the development of highly efficient optoelectronic materials for high-speed communications, sensing, ultra-sensitive cameras and high-resolution imaging and displays. The graphene-PQD superstructure behaves as an optoelectronic synapse with low energy consumption of 36.75 pJ/ spike that mimics crucial characteristics of its biological equivalent, with unique optical potentiation and electrical habituation function. We also utilized the persistent photoconductivity of 2D MoS2 layers to demonstrate an optoelectronic synapse. A CVD-grown monolayer MoS2 FET exhibits substantial enhancement in drain current when illuminated with a laser of 450 nm wavelength as well as a negative shift in its threshold voltage due to the generation of photocarriers. The device undergoes optical potentiation and electrical depression. The device is “optically” potentiated by applying light pulses with ON/OFF interval of 5 s for each at 450 nm wavelength – measured at a constant drain bias of 1.0 V with VG = -2 V. Also, it is “electrically” depressed by applying 50 electrical pulses with ON/OFF interval of 1 s and 9 s, respectively – measured at a drain bias of -100 mV with VG = -2 V. These results confirm the active and simultaneous tunability of conductance essential for the electrical/optical training of optoelectronic synapse in neural network applications. The negative voltage pulses applied at the drain de-traps the holes from the SiO2/MoS2 interface, thus gradually depress the device to its initial low conductance state.

Turn-On Photocatalysis: Creating Lone-Pair Donor-Acceptor Bonds in Organic Photosensitizer to Enhance Intersystem Crossing.

There is growing interest in developing triplet photosensitizers in terms of implementing photochemical strategies in synthetic chemistry. However, synthesis of stable triplet organic photosensitizers is nontrivial and often requires the use of heavy atoms. Herein, an alternative strategy is demonstrated to enhance the triplet generation efficiency by implanting lone‐pair donor–acceptor bonds in the conjugated covalent organic frameworks (COFs). This powerful method is validated using COFs that host triazine, a moiety that has been extensively investigated in photocatalysis. Spectroscopic analysis and theoretical calculations reveal substantial improvements in the photoabsorptivity and triple‐state photogeneration efficiency, consistent with catalytic tests concerning industrially relevant sulfide oxidation. These systems represent a promising addition to the rapidly increasing arsenal of synthetic photocatalytic systems.

Photocatalytic Efficiency Tuning by the Surface Roughness of TiO2 Coatings on Glass Prepared by the Doctor Blade Method

A set of opaque films were prepared with Degussa P25® or Hombikat UV100® TiO2 powders by the doctor blade method on glass slides with different compositions of polyethylene glycol of 20kDa (PEG20), and they were characterized by spectroscopy, microscopy and photochemical kinetics measurements. After annealing treatment at 450 °C, about 5-7% C atom was incorporated into the films, as a consequence of the degradation of the organic complexing agents, inducing a small reduction of the energy band gap of TiO2 (i.e. 3.02≤Eg (eV) ≤ 3.08). All films were about 15±2μm thick but their micro-morphological characteristics depended on the content of PEG20, showing different patterns of cracks and aggregates that produce intense light scattering and retransmission phenomena with the result of a three-dimensional excitation of the TiO2 particles in the thick film. Back-face excitation with UVA light (365±42nm) of the opaque films in contact with an aqueous solution produced both surface-bound and free hydroxyl radicals (HO• ), as detected using a coumarin solution as a radical dosimeter. The photogeneration efficiency of HO• decreased with the surface roughness of the films, which varied between 135 and 439nm depending on the film's composition.

Perovskite Quantum Dot-Reduced Graphene Oxide Superstructure for Efficient Photodetection.

High-performance photodetectors require efficient photogeneration and charge transport. Perovskite quantum dots (PQDs) have received enormous interest for applications in optoelectronics due to their high photogeneration efficiency. However, they offer meager carrier transport. Reduced graphene oxide (RGO) exhibits inferior photoresponse compared to materials such as quantum dots. An effective synthesis protocol to grow PQDs from the RGO lattice may facilitate direct charge transfers from PQDs to RGO, which could not be accomplished by mixing individual PQDs with RGO or making a bilayer. At ambient condition, the photodetector fabricated with the PQD-RGO superstructure showed high responsivity of 1.07 × 103 A/W, detectivity of 1 × 1013 Jones as well as sharp switching in the visible wavelength. After 3 months in an unencapsulated sample, the photocurrent was decreased ∼10% of its initial value while preserving speed and cycle stability at ambient condition.

Theoretical maximum photogeneration efficiency and performance characterization of InxGa1−xN/Si tandem water-splitting photoelectrodes

InxGa1−xN is a promising material for flexible and efficient water-splitting photoelectrodes since the bandgap is tunable by modifying the indium content. We investigate the potential of an InxGa1−xN/Si tandem used as a water-splitting photoelectrode. We predict a maximum theoretical photogeneration efficiency of 27% for InxGa1−xN/Si tandem photoelectrodes by computing electromagnetic wave propagation and absorption. This maximum is obtained for an indium content between 50% and 60% (i.e., a bandgap between 1.4 eV and 1.2 eV, respectively) and a film thickness between 280 nm and 560 nm. We then experimentally assess InxGa1−xN photoanodes with the indium content varying between 9.5% and 41.4%. A Mott–Schottky analysis indicates doping concentrations (which effectively represent defect density, given there was no intentional doping) above 8.1 × 1020 cm−3 (with a maximum doping concentration of 1.9 × 1022 cm−3 for an indium content of 9.5%) and flatband potentials between −0.33 VRHE for x = 9.5% and −0.06 VRHE for x = 33.3%. Photocurrent–voltage curves of InxGa1−xN photoanodes are measured in 1M H2SO4 and 1M Na2SO4, and the incident photon-to-current efficiency spectra in 1M Na2SO4. The incident photon-to-current efficiency spectra are used to computationally determine the diffusion length, the diffusion optical number, as well as surface recombination and transfer currents. A maximum diffusion length of 262 nm is obtained for an indium content of 23.5%, in part resulting from the relatively low doping concentration (9.8 × 1020 cm−3 at x = 23.5%). Nevertheless, the relatively high surface roughness (rms of 7.2 nm) and low flatband potential (−0.1 VRHE) at x = 23.5% cause high surface recombination and affect negatively the overall photoelectrode performance. Thus, the performance of InxGa1−xN photoelectrodes appears to be a tradeoff between surface recombination (affected by surface roughness and flatband potential) and diffusion length (affected by doping concentration/defect density). The performance improvements of the InxGa1−xN photoanodes are most likely achieved through modification of the doping concentration (defect density) and reduction of the surface recombination (e.g., by the deposition of a passivation layer and co-catalysts). The investigations of the ability to reach high performance by nanostructuring indicate that reasonable improvements through nanostructuring might be very challenging.

Understanding the effect of the carbon on the photovoltaic properties of the Cu2ZnSnS4

In this work, crystalline structure, formation energy, electronic, optical and current-voltage properties of Cu2ZnSnS4 (CZTS) with the presence of carbon (C) impurity at various sites is studied using the first-principles density functional theory (DFT) based on the generalized gradient approximation (GGA). Here, we considered possible four substitutional configurations of carbon doped CZTS supercells: Cu by C (Ccu), Cu by Zn (Czn), Sn by C (Csn) and S by C (CS). It was found that the presence of C leads to the formation of localized polarons which have dual functionality; (i) cause an increase in the scattering-limited mobility which reduces the transport efficiency, (ii) create deep acceptor level which acts as a recombination center allowing the scattering of the generated charges carriers. An enhancement in the photo-absorption is observed due to an increase in the density of states at the valence bands after the incorporation of C. This results in the efficiency enhancement by 8%. In addition, the presence of C reduces the transport efficiency and improves the photogeneration efficiency of CZTS in solar cell applications.

Photoreduction of Methylviologen in Saponite Clay: Effect of Methylviologen Adsorption Density on the Reaction Efficiency

AbstractTo identify the mechanisms for and to estimate the photochemical reaction efficiency of molecules in solid-state host materials is difficult. The objective of the present research was to measure the photogeneration efficiency of the methylviologen cation radical (MV+•) hosted in a semi-transparent hybrid film composed of MV2+ and saponite, a 2:1 clay mineral. MV+• is the one-electron reduced species of MV2+. MV+• was generated by UV irradiation of these films. The fluorescence intensity of MV2+ and the photogeneration efficiency of MV+• depended on the loading level of MV2+. When the loading level of MV2+ was high (75% of the cation exchange capacity (abbreviated as % CEC) of saponite), its fluorescence was reduced considerably because of the self-fluorescence quenching reaction, and the photogeneration efficiency of MV+• was relatively high (quantum yield φ = 3.5×10–2) compared to that of films with low adsorption density (10% CEC, φ = 1.1×10–2). Furthermore, when the loading level of MV2+ was very low (0.13% CEC), a self-fluorescence quenching reaction was not observed and MV+• was not generated. From these observations, one of the principal mechanisms of the self-quenching reaction and MV+• formation in saponite is the electron transfer reaction between excited MV2+ and adjacent MV2+ molecules in the ground state.

A polaron approach to photorefractivity in Fe : LiNbO3

The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries in the temperature range between and room temperature. We show that the temperature and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length is assumed for all the samples and all the temperatures. The photo-generation efficiency ϕ under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10%–15% at room temperature to about 5% at 150 K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center.

Synthesis of Novel Intramolecular Charge-Transfer Molecules Bearing Triphenylamine and Benzothiadiazole Moieties, and Application to Single-Absorber Organic Solar Cells

Organic solar cells (OSCs) have been extensively studied due to their potential for flexible, printable, and low-cost devices. Recently, power conversion efficiency (PCE) has reached around 10% by using bulkhetero junction (BHJ) system which possesses miscible interface between two kinds of materials, electron donor and acceptor. In the BHJ system, LUMO offset is required to promote charge separation at donor-acceptor interface, causing serious photovoltage loss. In order to overcome this problem, we have pursued the possibility of single-absorber organic solar cells, whose active organic layer consists of one component. Generally, photogeneration efficiency in organic bulk film is quite low because of the large exciton binding energy (EBE). In previous work, we found that molecules with intramolecular charge transfer photoabsorption are suitable for the single-absorber OSC. A donor-acceptor (D-A) linked molecule DTDCPB (Fig. 1) was applied to the single absorber OSC, and comparatively good performance was obtained. In this study, we synthesized novel donor-acceptor linked molecules bearing triphenylamine and benzothiadiazole moieties by derivatizing DTDCPB (Fig. 1). DTDCPB was extended by insertion of thienothiophene at the donor-acceptor linkage part to form DTDCPB-TT. DTERPB and DTRCNPB bear an acceptor moiety of 3-ethyl rhodanine and 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile, respectively, instead of dicyanovinyl moiety. EBEs in the solid state were estimated with quantum chemical calculation to design a suitable molecule for lower EBE. The D-A molecules were applied to a single absorber OSC composed of ITO/PEDOT:PSS/organic/Ca/Al. In the DTDCPB-TT device, FF and hole mobility increased compared with those in the DTDCPB device because the film crystallinity was improved due to thienothiophene. In the DTERPB and DTRCNPB devices, the photovoltage loss was only 0.60 V, that was less than that of the typical BHJ system. The ratio of the voltage loss to the absorbed energy was 33% for DTERPB and 35% for DTRCNPB, which were rather small in comparison with the typical BHJ system, e.g. 65% for P3HT:PC61BM and 56% for PTB7:PC71BM. Figure 1

(Invited) Photocarrier Generation in Single-Absorber Organic Solar Cells

Power Conversion efficiency of organic photovoltaic devices has exceeded 10% by taking advantage of bulk heterojunction (BHJ) structure, where donor and acceptor molecules are microscopically mixed. A remained problem is photovoltage loss from absorbed photon energy. In the BHJ system, LUMO offset is required to promote photodissociation at the donor-acceptor interface, causing serious photovoltage loss. To solve this problem, we are approaching it from single absorber cells, where there is no donor-acceptor interface. Generally, photogeneration efficiency in organic film bulk is quite low because of large exciton binding energy of Frenkel exciton. On the other hand, photogeneration from CT excitons occurs almost spontaneously in the BHJ system. If we could generate charge-transfer (CT) excitons in the single-component film directly, photovoltage loss would be reduced. Recently, we found intramolecular charge-transfer molecules including donor and acceptor units within one molecule show considerable high performance in the single-absorber device. In particular, DTDCPB having triphenyl amine unit (donor) and benzothiadiazole unit (acceptor) was a promising material for the single-absorber system. In this presentation, photogeneration process in the single-component film of CT molecules was investigated from several viewpoints. Molecular design for intramolecular charge-transfer absorption is discussed from the viewpoint of quantum chemical calculation and organic synthesis. The photocarrier dynamics in the single-absorber solar cells is investigated by several techniques such as intensity-modulated photovoltage/photocurrent (IMVS/IMPS) and time delayed collection field (TDCF). Exciton binding energy, that is a key factor for photodissociation in the film bulk, was estimated by Onsager-Braun analysis by using a planar device. Figure 1

Hydrogen Photoassisted Generation by Visible Light and an Earth Abundant Photocatalyst: Pyrite (FeS2)

N-Type pyrite (FeS2) thin films, deposited on titanium substrates, have been synthesized and photoelectrochemically characterized. Its flat band potential has been estimated to be −0.75 ± 0.05V vs Ag/AgCl by electrochemical impedance spectroscopy (EIS). The corresponding energy level diagram of the FeS2/electrolyte interface has been established. Profuse hydrogen flows have been produced under visible light illumination of FeS2 photoanode in a photoelectrochemical cell (PEC) at different bias potentials. Their values, measured by mass spectrometry (MS), were higher than 5 μmol H2/min·cm2. Hydrogen photogeneration efficiencies of ∼8% have been reached.

Mechanisms of charge photogeneration in amorphous selenium under high electric fields

In this paper, we have investigated the physical mechanisms for excitation wavelength and electric field-dependent charge carrier photogeneration in amorphous selenium (a-Se) under high electric fields. Although the photogeneration efficiency at low to moderate electric field in a-Se has been most successfully described by the conventional Onsager theory of dissociation, this theory fails at very strong electric fields (Reznik et al. in Appl Phys Letts 100:132101, 2012). We have proposed a formulation for calculating the excitation wavelength and electric field-dependent initial separation of the photogenerated and thermalized geminate electron and hole, and applied it to explain the field-dependent photogeneration efficiency in a-Se. We have also adapted the exact extension of the Onsager model in order to explain the experimental results at very strong electric fields.

Understanding the effect of triplet sensitizers in organic photovoltaic devices

The effect of PtOEP as a dopant on the performance of MEH-PPV/C60 photovoltaic devices was studied. Bilayer heterojunction devices with various compositions and layer structures were used to determine the possible pathways by which the photogeneration efficiency is enhanced. A key finding is that photocurrent generation enhancement always occurs in the MEH-PPV absorption region, regardless of the PtOEP dopant concentration or the MEH-PPV layer thickness. This result suggests that the presence of PtOEP in the donor MEH-PPV layer is primarily responsible for increasing the triplet exciton diffusion length of MEH-PPV by acting as a triplet sensitizer, rather than as an additional absorber for direct photogeneration. Values obtained from simulation show that the enhancement of exciton diffusion length of MEH-PPV can be more than a factor of 2 with optimal PtOEP concentrations. Further support for the role of PtOEP as a triplet sensitizer in MEH-PPV was obtained in experiments incorporating a blocking layer between MEH-PPV and C60, whereby the various exciton transfer processes can be differentiated.