Elektronide fotoindutseeritud väljaemissioon gaasikeskkonda

Käesolevas doktoritöös uuriti elektronide väljaemissiooni metall-dielektrikstruktuuridest gaasikeskkonda. Lisaks teaduslikule huvile on elektronide emissioonil tahkisest gaasikesk­konda ka praktiline väärtus (näiteks: plasmadispleid). Üheks lihtsamaks võimaluseks uurida väljaemissiooni on kasutada koroonalahendust, mis eksisteerib tugevasti mittehomogeenses elektriväljas. Varasematest töödest on teada, et elektrone emiteeriva elektroodi pinnal olevaid dielektrikulisandid mõjutavad oluliselt elektronide emissiooni. Selleks, et uurida elektronide emissiooni sõltuvust pinnal olevatest dielektrikulisanditest, kaeti emiteerivad elektroodid õhukeste TiO2 või MgO kiledega. Töö eksperimentaalses osas registreeriti koroonalahenduse volt-amper kõverad laias pingevahemikus allpool iseseisva lahenduse lävepinget (nõrga voolu lahendus). Mõõtmised viidi läbi N2 ja N2/O2 segus erinevatel rõhkudel. Lahendus initsieeriti UV kiirgusega. Osutus, et põhilised seaduspärasused ei sõltu elektroodi kattest. Eksperimendi tulemused on kirjeldatavad kaheastmelise väljameemissiooni mudeliga. Esimeses etapis liiguvad elektronid metallist dielektrikkattesse, kus nad akumuleeruvad lõksudes. Seejärel valguse toimel vabastatakse elektronid lõksudest ja emiteeritakse gaasikeskkonda. Erinevalt analoogilistest emissioonimudelitest vaakumisse, tuli käesolevas töös arvestada ka gaasikeskkonnas laviinpaljunemisel tekkivate positiivsete ioonide mõjuga. Tekkinud ioonid triivivad emiteeriva elektroodi pinna lähedusse, kus neil on kahene toime elektronide emissioonile. Ühelt poolt nad vähendavad elektronide emissiooni, sest ioonid rekombineeruvad dielektrikus või vahetult elektroodi pinna lähedal gaasikeskkonnas olevate elektronidega. Teisalt aga suurendab positiivsete ioonide ruumlaeng elektrivälja dielektrikus, millega kaasneb elektronide emissiooni kasv. Käesoleva töö tulemuseks oli mudel, mis kirjeldab koroona nõrga voolu lahenduse seaduspärasusi. In the present thesis field-assisted electron emission from the metal-dielectric structure to the gas medium was studied. The electron emission into the gas medium has besides scientific interest, also several applications (for example: plasma displays). The easiest way for the study of field-assisted electron emission into gases is to use corona discharge, which exists in highly divergent electric field. In earlier papers it was shown that dielectric inclusions at the surface of the emitting electrode influence the electron emission. To study the dielectric inclusions’ influence to the electron emission, the emitting electrodes were coated with thin films of TiO2 or MgO. In the experimental part of the present study, current-voltage curves of corona discharge were recorded in a wide range of voltages, below the onset potential of the self-sustained discharge (low current mode). The measurements were carried out in N2 and N2/O2 mixture at different gas pressures. The discharge was initiated by UV light. The study showed that the field-assisted two-step model of emission describes the experimental findings. During the first step of emission electrons are transferred from the metal into the dielectric. Then, under the influence of photons the electrons are transported from the traps of dielectric into the gas medium. The main difference between the emission into the gas and vacuum is that in gases are produced positive ions. The positive ions, which drift to the electrode influence the electron emission in two different ways. First, recombination processes decrease the number of electrons, which are emitted into the gas medium. On the other hand, ions accumulated near the electrode increase the electric field in the dielectric and thus the electron emission also increases. The model presented described properly the regularities of low current mode of corona discharge

Influence of alkali iodide fluxes on Cu2ZnSnS4 monograin powder properties and performance of solar cells

<h3>Abstract</h3><p>One promising and cost-effective method to produce flexible solar panels in the future is monograin layer (MGL) solar cell technology based on monograin powder (MGP) crystals. The results of the present study demonstrate the influence of different alkali salts (LiI, NaI, KI, RbI and CsI) on the properties of Cu2ZnSnS4 (CZTS) MGPs and their effect on the characteristics of MGL solar cells. SEM and EDX studies revealed that the morphology and composition of the formed crystals are influenced by the nature of the flux materials. Structural studies by XRD showed good crystallinities for all MGPs. However, CZTS crystals grown in LiI exhibited a shift of all diffraction peaks towards lower angles and larger lattice parameter values. In addition, powder grown in LiI exhibited the broadest main Raman peak (FWHM = 7.06 cm−1). When CsI was used, the Raman peaks were sharper and narrower with FWHM of 4.5 cm−1, showing a higher level of crystallinity compared to other produced powders. The estimated band gap energy values obtained from EQE measurements were ∼1.57 eV for NaI, KI, RbI, CsI and 1.65 eV for LiI grown CZTS MGPs. The analysis of temperature-dependent current–voltage characteristics indicated that tunneling enhanced interface recombination is the prevailing process in all materials. At low temperatures, the powder crystals grown in LiI, NaI, and RbI revealed the presence of recombination channels that were not observed at room temperature. The MGL solar cell based on CZTS powder grown in CsI resulted in the highest power conversion efficiency of 10.9%.</p&gt

Post deposition annealing effect on properties of CdS films and its impact on CdS/Sb2Se3 solar cells performance

This study was funded by the Estonian Research Council projects PRG627 “Antimony chalcogenide thin films for next-generation semi-transparent solar cells applicable in electricity producing windows,” 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 Horizon2020 programme under the ERA Chair project 5GSOLAR grant agreement No. 952509. The article/publication is based upon work from COST Action Research and International Networking on 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 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 selenide (Sb2Se3) is one of the emerging photovoltaic absorber materials possessing abundance and non-toxicity as the main attributes. Following CdTe technology, CdS is a widely used partner layer for Sb2Se3 solar cells. Related to CdS/Sb2Se3 device configuration, a number of studies reported findings and challenges regarding the intermixing phenomenon at the main interface and suitability of various annealing for CdS (and related interface) and still, significant room remains in developing strategies for interface optimization and understanding of the physiochemistry behind. In this perspective, this work provides a systematic investigation of the effect of vacuum and air annealing at temperatures between 200 and 400°C on the properties of CdS deposited by chemical bath deposition and combined with Sb2Se3 absorber obtained by close-spaced sublimation the direct impact of the CdS annealing on the device performance is illustrated. It is found that by varying the annealing temperature from 200 to 400°C in both, vacuum and air ambient, the morphology of CdS changes from highly dispersed small grain structure to sintered dense grains, the band gap decreases from 2.43 to 2.35 eV and the electron density drops from ∼1018 to ∼1011 cm−3. These changes were correlated with the changes in the CdS lattice and connected with the mobility of the OH group and the presence of secondary phases in CdS layers. 200°C air annealing of CdS was found as an optimal treatment resulting in 2.8% Sb2Se3/CdS cell efficiency - a 60% boost compared to the 1.8% performance of the device with as-deposited CdS. Material and device characterization analysis is performed, providing complementary insights on the interrelation between the physicochemical mechanism of the CdS annealing processes and device functionality. --//-- This is an open-access article adakkedath Gopi Sajeesh, Spalatu Nicolae, Basnayaka Madhawa, Krautmann Robert, Katerski Atanas, Josepson Raavo, Grzibovskis Raitis, Vembris Aivars, Krunks Malle, Oja Acik Ilona, Post deposition annealing effect on properties of CdS films and its impact on CdS/Sb2Se3 solar cells performance, Frontiers in Energy Research (11), 2023, https://www.frontiersin.org/articles/10.3389/fenrg.2023.1162576, Doi 10.3389/fenrg.2023.1162576 published under the CC BY 4.0 licence.Estonian Research Council projects PRG627, PSG689; Estonian Centre of Excellence project TK141 (TAR16016EK); EU Horizon2020 programme under the ERA Chair project 5GSOLAR grant agreement No. 952509; COST (RENEW-PV), CA21148; ithe nstitute of Solid-State Physics, University of Latvia 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

Cu2ZnSnS4 monograin layer solar cells for flexible photovoltaic applications

<h3>Abstract</h3><p>Monograin powder technology is one possible path to developing sustainable, lightweight, flexible, and semi-transparent solar cells, which might be ideal for integration with various building and product elements. In recent years, the main research focus of monograin technology has centered around understanding the synthesis and optoelectronic properties of kesterite-type absorber materials. Among these, Cu2ZnSnS4 (CZTS) stands out as a promising solar cell absorber due to its favorable optical and electrical characteristics. CZTS is particularly appealing as its constituent elements are abundant and non-toxic, and it currently holds the record for highest power conversion efficiency (PCE) among emerging inorganic thin-film PV candidates. Despite its advantages, kesterite solar cells' PCE still falls significantly behind the theoretical maximum efficiency due to the large <i>V</i>OC deficit. This review explores various strategies aimed at improving <i>V</i>OC losses to enhance the overall performance of CZTS monograin layer solar cells. It was found that low-temperature post-annealing of CZTS powders reduced Cu–Zn disordering, increasing <i>E</i>g by ∼100 meV and <i>V</i>OC values; however, achieving the optimal balance between ordered and disordered regions in kesterite materials is crucial for enhancing photovoltaic device performance due to the coexistence of ordered and disordered phases. CZTS alloying with Ag and Cd suppressed non-radiative recombination and increased short-circuit current density. Optimizing Ag content at 1% reduced CuZn antisite defects, but higher Ag levels compensated for acceptor defects, leading to reduced carrier density and decreased solar cell performance. Co-doping with Li and K resulted in an increased bandgap (1.57 eV) and improved <i>V</i>OC, but further optimization is required due to a relatively large difference between measured and theoretical <i>V</i>OC. Heterojunction modifications led to the most effective PCE improvement in CZTS-based solar cells, achieving an overall efficiency of 12.06%.</p&gt

Impact of Li and K co-doping on the optoelectronic properties of CZTS monograin powder

<p>Abstract</p><p>In this work, the impact of Li and K co-doping on the properties of Cu2ZnSnS4 (CZTS) monograin powders used as absorber materials in monograin layer (MGL) solar cells was investigated. CZTS powders were grown in the liquid phase of flux materials with different LiI-KI ratios by synthesis-growth method. According to Atomic Absorption Spectroscopy, the amount of K in the host material did not depend on the molar ratio of LiI to KI in the flux mixtures and remined constant at 0.01 at%. However, the Li concentration depended on the initial amount of LiI in the LiI-KI flux mixture and increased from 0.01 at% to 1.22 at% in the synthesized CZTS. X-ray diffraction, Raman analysis and photoluminescence (PL) analysis confirmed that Li content x = 0.2 and x = 0.4 in the input composition of (Cu1-xLix)1.84Zn1.09Sn0.99S4 resulted in formation of solid solution. The external quantum efficiency measurements showed that the effective bandgap energy of CZTS increased from 1.52 eV to 1.57 eV by increasing the Li content in CZTS from x = 0–0.02 to x = 0.4, respectively. The mean values of VOC of the corresponding monograin layer solar cells increased from 700 to 742 mV. The highest VOC of 784 mV was obtained with device based on solid solution containing 1.22 at% of Li (x = 0.4). The best performing solar cell with power conversion efficiency of 9.4% was obtained with Li and K co-doped CZTS powder (x = 0.002) showing output parameters: VOC = 718 mV, JSC = 20.9 mA/cm2 and FF = 62.5%.</p&gt

Sb2S3 Thin-Film Solar Cells Fabricated from an Antimony Ethyl Xanthate Based Precursor in Air.

The rapidly expanding demand for photovoltaics (PVs) requires stable, quick, and easy to manufacture solar cells based on socioeconomically and ecologically viable earth-abundant resources. Sb2S3 has been a potential candidate for solar PVs and the efficiency of planar Sb2S3 thin-film solar cells has witnessed a reasonable rise from 5.77% in 2014 to 8% in 2022. Herein, the aim is to bring new insight into Sb2S3 solar cell research by investigating how the bulk and surface properties of the Sb2S3 absorber and the current-voltage and deep-level defect characteristics of solar cells based on these films are affected by the ultrasonic spray pyrolysis deposition temperature and the molar ratio of thiourea to SbEX in solution. The properties of the Sb2S3 absorber are characterized by bulk- and surface-sensitive methods. Solar cells are characterized by temperature-dependent current-voltage, external quantum efficiency, and deep-level transient spectroscopy measurements. In this paper, the first thin-film solar cells based on a planar Sb2S3 absorber grown from antimony ethyl xanthate (SbEX) by ultrasonic spray pyrolysis in air are demonstrated. Devices based on the Sb2S3 absorber grown at 200 °C, especially from a solution of thiourea and SbEX in a molar ratio of 4.5, perform the best by virtue of suppressed surface oxidation of Sb2S3, favorable band alignment, Sb-vacancy concentration, a continuous film morphology, and a suitable film thickness of 75 nm, achieving up to 4.1% power conversion efficiency, which is the best efficiency to date for planar Sb2S3 solar cells grown from xanthate-based precursors. Our findings highlight the importance of developing synthesis conditions to achieve the best solar cell device performance for an Sb2S3 absorber layer pertaining to the chosen deposition method, experimental setup, and precursors