Influence of organic material and sample parameters on the surface potential in Kelvin probe measurements

Financial support provided by ERDF 1.1.1.1 activity project Nr. 1.1.1.1/16/A/046 “Application assessment of novel organic materials by prototyping of photonic devices” as well as Scientific Research Project for Students and Young Researchers Nr. SJZ2016/20 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged.Scanning Kelvin probe is a method for material surface studies. It is used to determine the work function of metals. In the case of organic semiconductors, the measured surface potential is considered to be the Fermi level of the material which has been shown in some cases. But in most papers, the surface potential dependence on the metal electrode or film thickness was observed. Material properties and their influence on the measured surface potential and its relation to the Fermi level previously have not been systematically studied. In this work, the surface potential was measured for different materials—metal, organic dielectric material, and organic semiconductors. In most of the cases, the obtained surface potential was dependent on the metal electrode work function. This dependence decreased with the increase in electrical conductivity of the material. Several materials were chosen for studies where sample thickness was varied. Results showed that for most of the studied semiconductors the sample thickness of around 1.5–2 µm was required to obtain surface potential values which do not depend on the electrode work function.ERDF 1.1.1.1 activity project Nr. 1.1.1.1/16/A/046; ISSP UL Scientific Research Project for Students and Young Researchers No SJZ2016/20; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Energy level determination in bulk heterojunction systems using photoemission yield spectroscopy: case of P3HT:PCBM

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ2015/20 realized at the Institute of Solid State Physics, University of Latvia, is greatly acknowledged. This work has been supported by the Latvian State Research Program on Multifunctional Materials IMIS2. Jennifer Mann from Physical Electronics is greatly acknowledged for providing UPS data.Ultraviolet photoelectron spectroscopy (UPS) is commonly used method for energy level determination using planar heterojunction samples in either metal/organic or organic/organic systems. Only some attempts have been made in the study of bulk heterojunction systems. Photoemission yield spectroscopy (PYS) could be applied as a method for organic compound–organic compound interface studies in bulk heterojunction samples. Contrary to the UPS, PYS method does not require ultra-high vacuum, which simplifies experiment setup. Also, scanning depth of PYS is in the range of tens of nanometers, which allows studying deeper layers of the sample instead of only surface layer. In this work, poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films were studied as a model system. A mass ratio between P3HT and PCBM in the system was varied from 1:0 to 1:50. Ionization energy dependence on this ratio was studied using two methods: UPS and PYS. To study the influence of the sample morphology on the PYS measurements and obtainable results, phase-separated and homogeneously distributed samples were prepared for analyses. P3HT ionization energy shift of 0.40 eV was observed in the samples made from chloroform solution. Experiments showed the need for a low degree of phase separation between P3HT and PCBM to observe P3HT ionization energy shift using PYS. On the contrary, no ionization energy shift of P3HT was observed in the UPS measurements for the same systems.Latvian State Research Program IMIS2; ISSP UL Scientific Research Project for Students and Young Researchers No. SJZ2015/20; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Energy level determination in bulk heterojunction systems using photoemission yield spectroscopy: case of P3HT:PCBM

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ2015/20 realized at the Institute of Solid State Physics, University of Latvia, is greatly acknowledged. This work has been supported by the Latvian State Research Program on Multifunctional Materials IMIS2. Jennifer Mann from Physical Electronics is greatly acknowledged for providing UPS data.Ultraviolet photoelectron spectroscopy (UPS) is commonly used method for energy level determination using planar heterojunction samples in either metal/organic or organic/organic systems. Only some attempts have been made in the study of bulk heterojunction systems. Photoemission yield spectroscopy (PYS) could be applied as a method for organic compound–organic compound interface studies in bulk heterojunction samples. Contrary to the UPS, PYS method does not require ultra-high vacuum, which simplifies experiment setup. Also, scanning depth of PYS is in the range of tens of nanometers, which allows studying deeper layers of the sample instead of only surface layer. In this work, poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films were studied as a model system. A mass ratio between P3HT and PCBM in the system was varied from 1:0 to 1:50. Ionization energy dependence on this ratio was studied using two methods: UPS and PYS. To study the influence of the sample morphology on the PYS measurements and obtainable results, phase-separated and homogeneously distributed samples were prepared for analyses. P3HT ionization energy shift of 0.40 eV was observed in the samples made from chloroform solution. Experiments showed the need for a low degree of phase separation between P3HT and PCBM to observe P3HT ionization energy shift using PYS. On the contrary, no ionization energy shift of P3HT was observed in the UPS measurements for the same systems.Latvian State Research Program IMIS2; ISSP UL Scientific Research Project for Students and Young Researchers No. SJZ2015/20; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

The Relation between Photoconductivity Threshold and Open-Circuit Voltage in Organic Solar Cells

Financial support provided by Scientific Research Project for Students and Young Researchers No. SJZ/2020/08 implemented at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. Institute of Solid State Physics, University of Latvia as the Centre of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART².Most of the solar cell parameters (short-circuit current, fill factor, power conversion efficiency) can only be determined by creating and measuring the solar cell. However, there is an empirical relation that links energy level values of the materials in the active layer to an open-circuit voltage (Uoc) of the solar cell. Due to a variety of possible methods used to determine energy level values and the dispersion of obtained results, this estimate is not always correct. Even if correct energy level values are obtained for separate materials, energy level shift takes place at the interfaces when two materials are mixed. That is why a simple and reliable experimental method for Uoc estimation is required. Usually, photoconductivity is used to obtain the energy gap between molecule ionization energy and electron affinity of a single material. When two materials are mixed, direct charge transfer from donor to acceptor molecule can be observed. The threshold energy (ECT) shows the real difference between donor molecule ionization energy and acceptor molecule electron affinity. This difference should correspond to the Uoc. The present study makes the comparison between the open-circuit voltage estimated from material energy level values, the obtained ECT values for various donor:acceptor systems, and the real Uoc obtained from solar cell measurements. Strong correlation between ECT and Uoc is obtained and the photoconductivity measurements can be used in the estimation of Uoc. © 2022 R. Grzibovskis et al., published by Sciendo.--//-- This is an open access article R. Grzibovskis, A. Ruduss, A. Polaks The Relation Between Photoconductivity Threshold and Open-Circuit Voltage in Organic Solar Cells, Latvian Journal of Physics and Technical Sciences 59 (1), 2022; doi:10.2478/lpts-2022-0003; published under the CC BY 4.0 licence.Scientific Research Project for Students and Young Researchers No. SJZ/2020/08; Institute of Solid State Physics, University of Latvia as the Centre of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Photovoltaic effect in bulk heterojunction system with glass forming indandione derivative DMABI-6Ph

The aim of the work is to evaluate possible use of 2-[[4-(bis(2-trityloxyethyl)amino)phenyl]methylene]indane-1,3-dione (DMABI-6Ph) as light absorbing material for solar cells. DMABI-6Ph is a perspective material due to its good photoelectrical, thermal and chemical properties. The main advantage of DMABI-6Ph is its ability to form amorphous films by wet-casting methods thus allowing using the compound in organic solar cells made from solution. For now most popular materials for solution processable solar cells are polymer P3HT and fullerene derivative PCBM, but lot of investigations are in the field of new low molecular weight materials to replace the polymer. Photoelectrical measurements were made to determine molecule ionization and electron affinity levels of DMABI-6Ph. Difference of 2.06 eV between DMABI-6Ph ionization level and PCBM affinity level was obtained. Accordingly open circuit voltage of system DMABI-6Ph:PCBM was measured up to 0.78 V. The best power conversation efficiency was 0.11 % for the DMABI-6Ph:PCBM mass fraction 2:1. Limiting factor for high efficiency could be low charge carrier mobility which can be increase by additional DMABI-6Ph modification.European Social Fund Project No. 2013/0045/1DP/1.1.1.2.0/13/APIA/VIAA/018; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Low processing temperatures explored in Sb2S3 solar cells by close-spaced sublimation and analysis of bulk and interface related defects

This study was funded by the Estonian Research Council project PRG627 “Antimony chalcogenide thin films for next-generation semi-transparent solar cells applicable in electricity producing windows”, the Estonian Research Council project PSG689 “Bismuth Chalcogenide Thin-Film Disruptive Green Solar Technology for Next Generation Photovoltaics”, the Estonian Centre of Excellence project TK141 (TAR16016EK) “Advanced materials and high-technology devices for energy recuperation systems”, and the European Union's Horizon 2020 ERA Chair project 5GSOLAR (grant agreement No. 952509). The article is based upon work from COST Action Research and International Networking project "Emerging Inorganic Chalcogenides for Photovoltaics (RENEW-PV)," CA21148, supported by COST (European Cooperation in Science and Technology); Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD 01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.Antimony trisulfide (Sb2S3) is a promising photovoltaic absorber, which has so far been fabricated mainly by chemical deposition methods. Despite its aptness for congruent sublimation, less research efforts have been made on low-temperature Sb2S3 processing by physical methods. In this regard, recent studies show large variation in the processing temperature of Sb2S3 films, which overall brings into question the need for higher substrate temperatures (>350 °C). Furthermore, in-depth analysis of defect structure of Sb2S3 employing temperature-dependent admittance spectroscopy (TAS) and photoluminescence (PL) remains largely unexplored. In this work, we systematically study the effect of close-spaced sublimation (CSS) substrate temperature on Sb2S3 absorber growth, employing a wide temperature range of 240–400 °C. Temperatures above 320 °C caused cracking phenomena in the Sb2S3 absorber film, proving the unviability of higher processing temperatures. CSS processing temperature of 300 °C was found optimal, producing crack-free Sb2S3 films with increased presence of (hk1) planes, and achieving the best CdS/Sb2S3 device with photoconversion efficiency of 3.8%. TAS study revealed two deep defects with activation energies of 0.32 eV and 0.37 eV. Low-temperature PL measurement revealed a band-to-band emission at 1.72 eV and a broad band peaked at 1.40 eV, which was assigned to a donor-acceptor pair recombination. Temperature-dependent I-V analysis showed that recombination at CdS–Sb2S3 interface remains a large limitation for the device efficiency. --//-- R. Krautmann, N. Spalatu, R. Josepson, R. Nedzinskas, R. Kondrotas, R. Gržibovskis, A. Vembris, M. Krunks, I. Oja Acik, Low processing temperatures explored in Sb2S3 solar cells by close-spaced sublimation and analysis of bulk and interface related defects, Solar Energy Materials and Solar Cells, Volume 251, 2023, 112139, ISSN 0927-0248, https://doi.org/10.1016/j.solmat.2022.112139. (https://www.sciencedirect.com/science/article/pii/S0927024822005566) Published under the CC BY licence.Estonian Research Council project PRG627; Estonian Research Council project PSG689; Estonian Centre of Excellence project TK141 (TAR16016EK); European Union's Horizon 2020 ERA Chair project 5GSOLAR (grant agreement No. 952509; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD 01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

4.9% Efficient Sb2S3 Solar Cells from Semitransparent Absorbers with Fluorene-Based Thiophene-Terminated Hole Conductors

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Intrinsic Photoconductivity Spectral Dependence as a Tool for Prediction of Open-Circuit Voltage in Organic Solar Cells

Organic materials are known for their variety of molecules. Methods to predict the parameters of organic photovoltaic (OPV) cells are required to avoid the time- and resource-consuming processes of manufacturing and testing OPVs. Usually, the open-circuit voltage (Uoc) is estimated as the difference between the ionization energy level of the electron donor molecule (Id) and the electron affinity level of the electron acceptor molecule (EAa). Various measurement methods are used to determine the energy level values of pure materials, which, when combined with energy level shifts due to the donor:acceptor interactions, make these estimations less precise. In this work, photoconductivity measurements were applied to the donor:acceptor films. Near threshold energy, the electron can be directly transferred from the donor to the acceptor molecule. The obtained charge transfer energy (ECT) shows the difference between Id and EAa in the film. This difference was compared to the Uoc value of an OPV made of the same donor:acceptor combination. We show that this approach provides less scattered results and a higher correlation coefficient compared to the Uoc estimation using energy level values

Energy level determination of purine containing blue light emitting organic compounds

Organic light emitting diodes (OLED) have found their applications in the mobile and TV screens. Till now the commercially available diodes are made by expensive thermal evaporation in a vacuum. The costs of OLED fabrication could be decreased by applying low-cost wet casting methods, for example, spin-coating. In this work, we have studied a group of blue light emitting purine derivatives which could potentially be used in OLEDs. The advantage of these compounds is their ability to form amorphous thin films from solutions. All the thin films were prepared by the spincoating method from chloroform solution on ITO glass. The position of hole and electron transport energy levels is important for efficient OLED fabrication. Ionization energy was determined using photoelectron yield spectroscopy. The gap between ionization energy and electron affinity was determined using photoconductivity measurements. Electron affinity (Ea) then was calculated as a difference between ionization energy (I) and photoconductivity threshold value (Eth). Changes in the energy level values depending on the molecule structure were investigated. The position of electron acceptor group strongly affects the gap between ionization energy and electron affinity, while with the help of the attached substitute groups it is possible to alter the ionization energy. Fine “tuning” of the ionization energy values can be achieved by altering the length of the “tail” where the inactive bulky group is attached.Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART