Photoelectron Spectroscopy In Air Research Articles

Surfactant Composition Dependent Evolution of Electronic/Structural Properties of P3HT Nanoparticle Colloids Synthesized via Mini-Emulsion Technique

: This study investigates the influence of surfactant (sodium dodecyl sulfate, SDS) concentration on the size and optoelectronic properties of poly(3-hexylthiophene) (P3HT) nanoparticles (NPs) synthesized via the mini-emulsion technique. P3HT NPs were fabricated with varying SDS concentrations. The size dependent optoelectronic properties, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels, were analyzed through photoelectron spectroscopy in air, UV-Vis absorption spectroscopy and cyclic voltammetry. A clear trend of decreasing NP size with increasing SDS concentration was observed. Higher SDS levels resulted in smaller NPs with enhanced pi-pi stacking and shorter conjugation lengths. This structural change led to a noticeable shift in HOMO and LUMO levels, indicating a direct correlation between surfactant concentration and the electronic properties of the P3HT NPs. The absorption spectra revealed a decrease in the A0-0/A0-1 ratio with smaller NP size, suggesting a transition towards more pronounced H-aggregate characteristics. As the size of the P3HT NPs decreases, there is a notable increase in H-aggregate formation. This increase can be attributed to enhanced interchain interactions between the polymer chains. In smaller NPs, the surface area-to-volume ratio is higher, leading to more significant interactions between adjacent polymer chains. These interactions promote the formation of H-aggregates, characterized by their interchain pi-pi stacking, which differs from the intrachain coupling observed in J-aggregates.

Pseudo‐Para‐Substituted [2.2]Paracyclophanes for Hole Transport in Perovskite Solar Cells

Abstract2,2′,7,7′‐Tetrakis(N,N‐di‐p‐methoxyphenylamine)−9,9′‐spirobifluorene (spiro‐OMeTAD) is the prevalent hole transport layer in perovskite solar cells (PSCs) with regular device architecture. Yet, its spirobifluorene core and multistep synthesis make it rather expensive. For the further technological success of PSCs, novel scalable and inexpensive alternative hole transport layers are needed. Herein, a study of the structure‐property relations of pseudo‐para‐substituted [2.2]paracyclophanes is presented. Eight different hole transport materials are synthesized via double CH activation, eliminating metal‐containing substituents for cross‐coupling reactions. The ionization potentials (IPs) of the disubstituted paracyclophanes (DiPCPs) are examined by photoelectron spectroscopy in air, cyclic voltammetry and theoretical calculations. Through variation of donor groups and π‐linkers, IPs that span a range from 5.14 to 5.86 eV are achieved, demonstrating high customizability. From the eight novel materials, five showed good solubility and are implemented into PSCs. The solar cells with a hole transport layer of undoped 4,16‐di(4‐(2‐thienyl)‐N,N‐bis(4‐methoxyphenyl)aniline)[2.2]paracyclophane (DiPCP‐2) exhibit a power conversion efficiency of 12.7% ± 0.4%. The facile synthesis of DiPCP‐2 enables an estimated cost reduction by two thirds compared to spiro‐OMeTAD.

A series of V-shaped small molecule non-fullerene electron acceptors for efficient bulk-heterojunction devices

Two simple semiconducting acceptor-acceptor1-donor-acceptor (A-A1-D-A) modular, small molecule non-fullerene electron acceptors, 2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-3-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (NDICz-5) and 2-(4-(3,3-dicyano-2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-1-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (NDICz-6), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of NDICz-5 and NDICz-6 were directly compared with those of a structural analogue, 7,7'-(((9-octyl-9H-carbazole-3,6-diyl)bis(ethyne-2,1-diyl))bis(4,1-phenylene))bis(2-hexadecylbenzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) (NDICz-4), which was designed based on an A-D-A format. All of these new materials were designed to be V-shaped and comprised an electron rich carbazole donor core (D) together with electron deficient naphthalene diimide terminal core (A). In the simple A-D-A system, naphthalene diimide was the terminal functionality, whereas in the A-A1-D-A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping D/A units constant. The inclusion of A1 was mainly done to induce cross-conjugation within the molecular backbone and hence to improve light-harvesting. The physical and optoelectronic properties were characterized by ultraviolet–visible, thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with NDICz-4, NDICz-5 and NDICz-6 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: NDI-Cz6-based device showed an impressive enhanced power conversion efficiency of 7.58%, an increase of around two-fold with respect to as-cast blend, after solvent vapor annealing using carbon disulfide. Not only are the results among the best in the current literature using the conventional donor polymer P3HT, but clearly demonstrate that A-A1-D-A type, V-shaped small molecules are promising non-fullerene acceptors in the research field of organic solar cells.

Self‐Assembled Hole Transporting Monolayer for Highly Efficient Perovskite Solar Cells

AbstractThe unprecedented emergence of perovskite‐based solar cells (PSCs) has been accompanied by an intensive search of suitable materials for charge‐selective contacts. For the first time a hole‐transporting self‐assembled monolayer (SAM) as the dopant‐free hole‐selective contact in p–i–n PSCs is used and a power conversion efficiency of up to 17.8% with average fill factor close to 80% and undetectable parasitic absorption is demonstrated. SAM formation is achieved by simply immersing the substrate into a solution of a novel molecule V1036 that binds to the indium tin oxide surface due to its phosphonic anchoring group. The SAM and its modifications are further characterized by Fourier‐transform infrared and vibrational sum‐frequency generation spectroscopy. In addition, photoelectron spectroscopy in air is used for measuring the ionization potential of the studied SAMs. This novel approach is also suitable for achieving a conformal coverage of large‐area and/or textured substrates with minimal material consumption and can potentially be extended to serve as a model system for substrate‐based perovskite nucleation and passivation control. Further gains in efficiency can be expected upon SAM optimization by means of molecular and compositional engineering.

Fluorescence-Lifetime-Sensitive Probes for Monitoring ATP Cleavage.

Adenosine triphosphate (ATP) probes modified with fluorescence dyes that change their fluorescence properties upon cleavage are an interesting tool for monitoring enzymatic ATP turnover. As a readout parameter, fluorescence lifetime is attractive because it is nearly independent of concentration. In our study, we synthesised and investigated fifteen different ATP analogues, in which the fluorophores were attached to the γ-phosphate of ATP. All analogues showed distinctly different fluorescence lifetimes compared to the corresponding values of the free fluorophores. Both increases and decreases in fluorescence lifetime were observed upon attachment to ATP. To shed light on the photophysical processes governing the lifetime changes, we performed photoelectron spectroscopy in air (PESA) to determine HOMO energy levels and time-resolved fluorescence spectroscopy to obtain rate constants. We present evidence that fluorescence quenching in the compounds tested is dynamic and attributed to photoinduced electron transfer (PET), whereas fluorescence lifetime increases are caused by stacking interactions between chromophore and the nucleobase reducing non-radiative relaxation. Finally, we demonstrate that enzymatic cleavage of the ATP analogues presented can be followed by continuous monitoring of fluorescence lifetime changes.

Interfacial energy band bending and carrier trapping at the vacuum-deposited MAPbI3 perovskite/gate dielectric interface

We report energy band bending of methylammonium lead halide (MAPbI3) perovskite film in contact with indium-tin-oxide (ITO) surface using photoelectron spectroscopy in air (PESA) and ultraviolet photoelectron spectroscopy (UPS) measurements. MAPbI3 perovskite films were vacuum-deposited using co-evaporation of methylammonium iodide (CH3NH3I, MAI) and lead iodide (PbI2) powders. Using PESA and UPS, the highest occupied molecular orbital levels were measured varying the thickness of the perovskite films. Substantial energy band bending close to the ITO surface was not observed. The vacuum-deposited perovskite films feature ambipolar carrier transport from field effect current measurements. Particularly, the threshold voltage for electron conduction was −20 and 25 V for the forward and reverse gate scans, exhibiting significant current hysteresis. The magnitude of the threshold voltage, far away from zero gate voltage, implies that interface trapping rather than energy band bending dominate the threshold voltage. In other words, perovskite/gate dielectric interface trap states for electrons modulate the magnitude of the threshold voltage. Our work provides insights into the origin of gate voltage dependent threshold voltage in perovskite FETs minimizing the contribution from ion migration based on the observation of ambipolar carrier transport at room temperature for vacuum deposited perovskite films.

Effect of ligand exchange on photocurrent enhancement in cadmium selenide (CdSe) quantum dot water splitting cells

We report photocurrent enhancement through ligand exchange in cadmium selenide (CdSe) photoelectrochemical (PEC) cells. CdSe quantum dot (QD) layers were used as a light absorption layer in combination with titanium dioxide (TiO2) layers. For a shorter ligand exchange, pristine CdSe QDs with oleic acid (OA) ligands were treated with 1,2-ethanedithiol (EDT) and CdSe QD PEC cells exhibited enhanced photocatalytic properties in comparison with pristine CdSe PEC cells. To articulate the origin of enhanced photocurrent, ligand dependent energy band bending at the CdSe/TiO2 interface was probed from thickness dependent highest occupied molecular orbital level measurements using photoelectron spectroscopy in air. Increase in the photocurrent also originates from enhancement in charge transport through ligand exchange to shorter EDT ligands in the bulk CdSe film with OA ligands suppressing back diffusion of carriers from TiO2 to CdSe.

Physico-chemical investigation of ZnS thin-film deposited from ligand-free nanocrystals synthesized by non-hydrolytic thio-sol–gel

Ultra-small and monodispersed zinc sulfide nanocrystals (NCs) (d ≤ 3 nm) have been prepared without the use of any surfactants by a synthetic route using benzyl mercaptan as a source of sulfur. The prepared NCs are dispersible in highly polar solvents and display the capability to closely pack-up in a bulky film. The NCs were characterized by TEM, XRD and UV–vis optical absorption as well as by steady-state and time-resolved photoluminescence (PL) spectroscopies. Uniform films of ZnS were spin-coated on glass and ITO-glass substrates using a NCs dispersion in N,N-dimethylformamide. The NCs and the resulting films were characterized by morphological and optoelectronic probing techniques such as AFM, SEM, diffuse reflectance, PL and photoelectron spectroscopy in air. These physical investigations confirmed that the chalcogenide NCs grown by this method have the potential to be utilized directly as photocatalysts and are potentially useful building-blocks/starting materials for the fabrication of semiconductor thin films for optoelectronic applications such as LED, luminescent screens, field effect transistor and solar cells. Insights on the chemistry involved in the NCs growth have been provided revealing that their formation proceeds through a mechanism involving a thioether elimination reaction.

Unraveling the mechanism of ultraviolet-induced optical gating in Zn1−xMgxO nanocrystal solid solution field effect transistors

We report ultraviolet (UV)-induced optical gating in a Zn1−xMgxO nanocrystal solid solution (NCSS) field effect transistor (FET) through a systematic study in which UV-induced charge transport properties are probed as a function of Mg composition. Change in the electrical properties of Zn1−xMgxO NCSS associated with electronic traps is investigated by field effect-modulated current–voltage characteristic curves in the dark and under illumination. Under UV illumination, significant threshold voltage shift to a more negative value in an n-channel Zn1−xMgxO NCSS FET is observed. Importantly, as the Mg composition increases, the effect of UV illumination on the threshold voltage shift is alleviated. We found that threshold voltage shift as a function of Mg composition in the dark and under illumination is due to difference in the deep trap density in the Zn1−xMgxO NCSS. This is supported by Mg composition dependent photoluminescence intensity in the visible range and reduced FET mobility with Mg addition. The presence of the deep traps and the corresponding trap energy levels in the Zn1−xMgxO NCSS are ensured by photoelectron spectroscopy in air.

2D black phosphorous nanosheets as a hole transporting material in perovskite solar cells

We demonstrate for the first-time liquid exfoliated few layers of 2D Black phosphorus (BP) nanosheets as a hole transporting material (HTM) for perovskite based solar cells. The photoelectron spectroscopy in air (PESA) measurements confirm the low laying valence band level of BP nanosheets (−5.2 eV) favourable for hole injection from CH3NH3PbI3 (MAPbI3). Our results show that ∼25% improvement in power conversion efficiency (PCE) of η = 16.4% for BP nanosheets + Spiro-OMeTAD as an HTM as compared to spiro-OMeTAD (η = 13.1%). When BP nanosheets are exclusively utilised as an HTM, a PCE of η = 7.88% is noted, an improvement over the 4% PCE values observed for HTM free devices. Photoluminescence (PL) quenching of MAPbI3 and impedance measurements further confirm the charge extraction ability of BP nanosheets. The structural and optical characterization of liquid exfoliated BP nanosheets is discussed in detail with the aid of transmission electron microscopy, Raman spectroscopy, absorption spectroscopy and photo-electron spectroscopy.

Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-one functionality for efficient bulk-heterojunction devices

Two simple semiconducting donor–acceptor–acceptor (D–A1–A) modular, small molecule, non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were directly compared with those of a structural analogue, 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was designed based on a D–A format. All of these new materials comprised an electron rich triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core (A). In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas in the D–A1–A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping the D/A units constant. The inclusion of A1 was primarily done to induce cross-conjugation within the molecular backbone and hence to generate low band gap targets. The physical and optoelectronic properties were characterized by ultraviolet–visible (UV–Vis), thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited broadened absorption spectra, for instance panchromatic absorbance in case of P3, excellent solubility and thermal stability, and energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: P3-based device showed an impressive enhanced power conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A1–A type small molecules are promising non-fullerene electron acceptors in the research field of organic solar cells.

Ionization potential and electron attenuation length of titanium dioxide deposited by atomic layer deposition determined by photoelectron spectroscopy in air

Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission threshold energy (also referred to as the “ionization potential”) of TiO2 thin films, avoiding inherent artifacts due to photoelectron emission from the substrate, which supplies misleading results. This has been achieved using PESA measurements performed as a function of TiO2 film thickness on two substrates with different work functions. We find that proper measurements of the photoemission threshold energy (including work function and ionization potential) of thin films by PESA require the use of films much thicker than their electron attenuation length (EAL). A photoemission threshold energy of 5.0±0.2eV is obtained for TiO2 and has been attributed to a trap level due to oxygen vacancies, which lie within the band gap of the TiO2. The analysis of the photoemission decay with film thickness also provides a method for determining a “practical” (or effective) EAL at excitation energy slightly above the photoemission threshold energy of the material. We extract an EAL for the deposited TiO2 of 0.65±0.02nm (at 0.5eV). The procedure can also be adopted for determining the thickness of extremely thin films, provided their thickness is smaller than their EAL.

Optical and electrical characterization of ZnO/CuO heterojunction solar cells

This paper reports the fabrication and characterization of nontoxic heterojunction of ZnO/CuO. The films were characterized through X-ray diffraction, UV spectroscopy and field emission scanning electron microscope (FESEM). The valence and conduction band edges were measured through Photo-Electron spectroscopy in air. The electrical parameter for FTO/ZnO/CuO/In heterojunction diode such as ideality factor, barrier height, and series resistance were calculated by using conventional forward bias I–V characteristics and also verified with the help of Cheung method. The high value of ideality factor was attributed to high series resistance, defects, other phases of CuO and recombination at interface and within bulk. The double logarithmic graph of current and voltage showed that the transport mechanism is due to the space charge limited current and trapped charge limited current. Photovoltaic parameters Voc of ∼190mV and Jsc of ∼0.40mA/cm2 of FTO/ZnO/CuO/In heterojunction were recorded.

Tuning Work Function of Multi-Walled Carbon Nanotubes By Defects Creation

Carbon nanotube(CNT) is a very attractive material to various electronic devices and optoelectronic applications owing to its promising structural and electronic properties comparing with other carbon materials.1CNT has been successfully applied on field emission displays, light emitting diodes, solar cells, etc. To these electronic or optoelectronic devices which require modification of the junction interface to achieve a well-defined electronic energy level leading to a desired performance, work function is a critical parameter. It is of importance to develop a systematic methodology to manipulate CNT’s work function to match diverse energy structures. Impacts of multi-walled carbon nanotube(MWCNT) surface functional groups on its work function has been poposed,2 which provides a potential strategy of tailoring an appropriate work function for different applications. In our previous study,3we found that creating defects on MWCNT benefits planting abundant functional groups on it, this would contribute to control work function in a wide range. In this work, a surface functional group decoration methods associated with coating, defects creation, surface functional groups decoration and doping on MWCNT for tuning work function has been designed and carried out. Inductively coupled plasma mass spectrometry(ICP-MS) are employed to distinguish the impact of impure metal elements, since the raw CNT always contains some metal impurities which may have negative effect on investigating the origin of work function changes. Annealing MWCNT samples in different temperatures to remove certain functional group is conducted to identify which certain functional group responsible for the work function tuning. Photo-electron spectroscopy in air (PESA)* and ultraviolet photoelectron spectroscopy (UPS) are conducted to measure modified CNT’s work function directly and also provide information of density of state (DOS) near Femi level. Temperature programed desorption(TPD) and X-ray photoelectron spectroscopy(XPS) analysis provides estimation of different functional group amount and compelling evidence of doping and coating. Field emission scanning electron microscopy(FESEM) and transmission electron microscopy(TEM) are adopted to observe the morphology of modified MWCNTs. Structure disorder information is supplied by Raman spectroscopy. It has been revealed that a 0.55 eV work function difference between different modified WMCNTs is achieved at current stage (Figure 1). The further modification parameters will be presented on the conference. PESA*: It is an instrument for Photoelectron Spectroscopy at atmospheric pressure. UV photons emitted from a deuterium lamp are monochromated by a grating spectrometer and focused on a sample. Then, photoelectrons emitted from the sample are counted by an open counter. In terms of metal, when the photon energy is smaller than work function, there is no electron reach counter. If the photon energy is larger than work function, the electron from sample starts to be counted by open counter which indicates the exact value of work function. After plotting data in a graph, the crossing point of the back ground and the yield line is a photoemission threshold energy, also called the work function or the ionization potential. Acknowledgement: This work partially was supported by COI STREAM from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Shin-Etsu Chemical Co., Ltd. Japan. and Riken Keiki Co., Ltd. Their contribution is greatly appreciated. Reference: 1 S. Iijima, Nature, 1991, 354, 561–58. 2 H. Ago, T. Kugler, F. Cacialli, W. R. Salaneck, M. Shaffer, A. H. Windle and R. H. Friend, J. Mater. Chem. B, 1999, 103, 8116–8121. 3 K, Waki, R. A. Wong, H. S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto and K. Yamada, Energy Environ. Sci., 2014,7, 1950-1958 Figure 1

Indium tin oxide as a semiconductor material in efficient p-type dye-sensitized solar cells

Indium tin oxide (ITO) is a well-known n-type degenerate semiconductor. Herein, mesoporous ITO is utilized as a photocathode material for p-type dye-sensitized solar cells in place of the commonly applied p-type semiconductors, such as nickel oxide. In conjunction with [Fe(acac)3]0/− as redox mediator and a new sensitizer, an impressive energy conversion efficiency of 1.96±0.12% was achieved. Photoelectron spectroscopy in air revealed that ITO exhibits a significant local density of states arising below −4.8 eV, which enables electron transfer to occur from ITO to the excited dye, giving rise to the sustained photocathodic current.

Mechanistic Investigation into the Light-Soaking Effect Observed in Inverted Polymer Solar Cells Using Indium Tin Oxide Electrodes Modified By Piperazine Derivatives

The well-controlled electrical double layer in polymer solar cells, which is formed between active layer and electrode, promotes both increase in built-in electric field and decrease in carrier injection barrier, resulting in effective carrier collection on the electrode. Previously, in ITO/P3HT:PCBM/PEDOT:PSS/Au inverted solar cells, we observed the improvement of power conversion efficiency according to decrease in ionization potential of ITO by modifying with amine compounds. However, the light-soaking effect which the open-circuit voltage (Voc) gradually increased with white light irradiation containing UV component was observed. In this work, we have investigated the mechanism of light-soaking effect by combining time-dependences of photovoltaic properties and electric capacity of the inverted polymer solar cells. To prepare the amine-modified ITO electrodes, we used ethanol as a solvent for piperazine, N’(2-aminoethyl)piperazine (NAP) and 1,4-bis(3-aminopropyl)piperazine (BAP). The dilute solutions containing the amines were spin-coated on the ITO substrates. The substrate was subsequently dried at 120 °C for 10 min on a hotplate. Additionally, to remove any extra amines from the ITO, the same operation was performed using a solvent instead of the amine solution. Their ionization potentials (I P) were measured by a photoelectron spectroscopy in air. To fabricate the inverted polymer solar cells, a P3HT:PCBM blend solution and a PEDOT:PSS dispersion solution were spin-coated on the modified ITO substrates sequentially. The Au rear electrode was vacuum-deposited on the PEDOT:PSS layer. Finally, the device was mechanically protected by laminating it with a film at 150°C for 5 min under a reduced pressure. The time-dependence of the current-voltage (I-V) curves of these solar cells was measured under AM 1.5G-100 mW cm-2 simulated sunlight condition. Using ac impedance spectroscopic measurement, both time-dependences of resistance and electric capacity (CPE) of these solar cells were obtained in the dark and under simulated sunlight irradiation. The cell fabrications and the measurements were carried out under an ambient atmosphere. The IP values of BAP-modified ITO electrodes decreased with increasing the concentration of the BAP solutions. The minimum IP was 4.39 eV, decreasing by ca.0.3 eV compared to that of bare ITO. In addition, the power conversion efficiency of the inverted cells increased by using the BAP-modified ITOs. This improvement is perhaps because of an efficient drift migration of photo-produced charge carriers by increase in the built-in potential. However, when white light was irradiated to the cells, the light-soaking effect was observed. The time taken to arrive at a constant Voc decreased as the IP of the amine-modified ITO decreased. That is, the light-soaking effect was small when the amine-modified ITO which had smaller IP was used. In addition, the CPE in an electric double layer formed in the ITO/active layer interface was estimated by ac impedance spectroscopy. This CPE gradually decreased by continuous white light irradiation. Furthermore, the degree of change of the CPE was small when the amine-modified ITO with smaller IP was used. The good correlation between both time-dependences of VOC and CPE was observed. The decrease in CPE and the increase in VOC were caused by decrease in IP . That is, the CPE corresponded to an electron injection barrier in the ITO/active layer interface, and the electron injection barrier became small according to decrease in IP . In summary, the improvement of VOC was caused by decreasing the electron injection barrier because of decrease in IP .

Controlling the Electronic Energy Structure of ZnS–AgInS2 Solid Solution Nanocrystals for Photoluminescence and Photocatalytic Hydrogen Evolution

Simultaneous control of the size and chemical composition is an advantageous strategy to obtain the desired photochemical properties of multinary semiconductor nanocrystals, ZnS-AgInS2 solid solution ((AgIn)xZn2(1–x)S2, ZAIS) nanocrystals (NCs), being different from conventional binary nanocrystals. The energy gap (Eg) of ZAIS NCs was enlarged with a decrease in particle size due to the quantum size effect or with an increase in ZnS content in the ZAIS solid solution. The levels of the conduction band edge and valence band edge, determined by photoelectron spectroscopy in air, were shifted more negatively and more positively, respectively, with an increase in Eg. A volcano-type dependence was observed between the PL quantum yield (QY) and the size of ZAIS NCs, in which the optimal PL QY for each x value was obtained at a similar particle size around 5–6 nm, except for x = 1.0, and maximum QY was recorded to be 79% for ZAIS NCs prepared with x = 0.5. The photocatalytic activity for H2 evolution was also gr...

Luminescent benzo- and naphthoquinolines: Synthesis and investigation of photophysical properties

The aim of this study was to synthesise new luminescent small molecular weight benzo- and naphthoquinoline derivatives (based on already known synthons) with different substituents (chlorine, tetrazole, iminophosphorane and amine), to characterize them by ultraviolet photoelectron spectroscopy in air (UPS), luminescence and absorption photospectrometry and to investigate the changes in these properties resulted by differences of molecular structure. Measured values of the HOMO, the LUMO energy levels and optical band gap (Eg_opt) were in range EHOMO=[−5,9;−5,5]eV, ELUMO=[−3,6;−3]eV, Eg_opt=[2;2,8]eV. Most of the examined materials showed high intensity fluorescence reaching quantum yields of 49%. Fluorescence peaks of thermally evaporated layers distributed in range of λLayer=[477; 596]nm – from blue to orange colour while in chloroform solution remained in blue-green interval λSolution=[471;537]nm. Primary study results of electroluminescence indicate that the materials have a potential to be used in organic optoelectronics.

(Invited) Tunable Electronic Energy Structure of ZnSe-AgInSe2 Solid Solution Nanoparticles for Solar Energy Conversion

Size-quantized semiconductor nanoparticles have attracted much attention as an efficient light absorbing material in quantum dot solar cells due to their tunable energy gap (Eg) for light energy conversion systems. Extending the optical absorption range of nanoparticles has been an important research subject to effectively utilize solar light, which contains light from the UV-visible to near-IR wavelength region. As for binary semiconductor nanoparticles (e.g. CdTe, PbS and PbSe), the control of their size has been a useful method to modify their optical properties. Recently, we have reported the syntheses of multinary semiconductor nanoparticles, such as AgInS2[1], ZnS-AgInS2[1], and Ag8SnS6[2], as an alternative to highly toxic binary particles such as CdTe and PbSe. The wide tunability of chemical composition also enabled to control the Eg of these particles. In this study, we report the fabrication and photoelectrochemical characterization of ZnSe-AgInSe2 (ZAISe) solid solution nanoparticles as a novel near-IR-responsive material for a solar energy conversion.[3] ZAISe(x) nanoparticles ((AgIn)xZn2(1-x)Se2; 0≦x≦1) were synthesized by a thermal decomposition of metal acetate and selenourea in oleylamine for 10 min at 250 oC under nitrogen atmosphere. Thus-obtained ZAISe nanoparticles were immobilized on TiO2 porous films deposited on FTO electrodes, in which 3-mercaptopropionic acid was used as a cross-linker between the surfaces of TiO2 and ZAISe(x) nanoparticles. These electrodes were finally surface-coated with a CdS thin layer via SILAR method to remove the surface defect sites on ZAISe particles (TiO2/ZAISe(x)/CdS). A quantum dot-sensitized solar cell (QDSSC) was prepared with the TiO2/ZAISe(x)/CdS photoelectrode and a counter electrode of Cu2S-deposited FTO electrode (Cu2S/FTO). The TiO2/ZAISe(x)/CdS was attached to the Cu2S/FTO with the use of a Himilan polymer film of 50 μm in thickness as a spacer. The cell was filled with an aqueous solution containing S2-/Sxredox species. Current density-voltage curves were measured with AM1.5G irradiation using a solar simulator. TEM observation revealed that spherical nanoparticles were formed with the size of around 4 nm, regardless of x value. As shown in Fig. 1a, the absorption onset of thus-obtained nanoparticles was blue-shifted from 900 nm to 400 nm with an increase in the content of ZnSe, indicating that the Eg of ZAISe(x) nanoparticles could be successfully controlled only by changing the value of x. Electronic energy levels of ZAISe(x) nanoparticles were measured by photoelectron spectroscopy in air. Figure 1b summarizes the electronic energy levels of ZAISe nanoparticles as a function of the value of x. Both levels of the conduction band edge (ECB) and the valence band edge (EVB) varied depending on the chemical composition of ZAISe particles: the EVB levels were slightly shifted from -5.38 to -5.04 eV with a decrease in the x value from 1.0 to 0.25, while the ECB levels were remarkably shifted from -3.94 to -2.97 eV, accompanied by an increase in Eg from 1.44 to 2.07 eV. The ECB levels of ZAISe(x) NCs were higher than that of bulk TiO2(-4.0 eV), regardless of their chemical composition. A QDSSC fabricated with TiO2/ZAISe(x)/CdS photoelectrodes exhibited different light-electricity conversion properties depending on the x value. With a decrease in the x value of ZAISe(x) from 1.0 to 0.5, both the short circuit current (JSC) the open circuit voltage (VOC) increased, resulting in a remarkable increment of power conversion efficiency (PCE). Further decrease of x from 0.5 to 0.25 slightly lessened the values of JSC, VOC, and PCE. The fill factor was almost constant at ca. 0.4 ~ 0.5, regardless of the composition of ZAISe(x) immobilized. Among the kinds of nanoparticles investigated, ZAISe(0.5) particles exhibited the optimal photovoltaic performance, JSC of 8.8 mA cm-2and PCE of 1.9%.

Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments

Colloidal quantum dots (CQDs) have received recent attention for low cost, solution processable, high efficiency solid-state photovoltaic devices due to the possibility of tailoring their optoelectronic properties by tuning size, composition, and surface chemistry. However, the device performance is limited by the diffusion length of charge carriers due to recombination. In this work, we show that band engineering of PbS QDs is achievable by changing the dipole moment of the passivating ligand molecules surrounding the QD. The valence band maximum and conduction band minimum of PbS QDs passivated with three different thiophenol ligands (4-nitrothiophenol, 4-fluorothiophenol, and 4-methylthiophenol) are determined by UV–visible absorption spectroscopy and photoelectron spectroscopy in air (PESA), and the experimental results are compared with DFT calculations. These band-engineered QDs have been used to fabricate heterojunction solar cells in both unidirectional and bidirectional configurations. The results show that proper band alignment can improve the directionality of charge carrier collection to benefit the photovoltaic performance.