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On the Device Physics of High-Efficiency Ternary Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Electronic and photonic materials. Linköping University, Faculty of Science & Engineering. Centre of Advanced Materials, Heidelberg University, Heidelberg, Germany.
Linköping University, Department of Physics, Chemistry and Biology, Electronic and photonic materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Electronic and photonic materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-2582-1740
Linköping University, Department of Physics, Chemistry and Biology, Electronic and photonic materials. Linköping University, Faculty of Science & Engineering. Centre of Advanced Materials, Heidelberg University, Heidelberg, Germany.ORCID iD: 0000-0002-7104-7127
2022 (English)In: Solar RRL, E-ISSN 2367-198X, Vol. 6, no 11, article id 2200450Article in journal (Refereed) Published
Abstract [en]

Mixing a third compound into the active layer of an organic bulk heterojunction solar cell to form a ternary system has become an established way to improve performance. Various models, based on different assumptions regarding the active layer morphology and the energetics, have been proposed but there is neither consensus on the applicability of the various assumptions to different experimental systems, nor on the actual device physics of these, mostly qualitative, models. Kinetic Monte Carlo simulations are used to investigate the role of morphology and relative energy levels of the constituent materials. By comparing with experimental current–voltage characteristics, a consistent picture arises when the (minority) third compound is predominantly incorporated between the other (majority) compounds and has energy levels that are intermediate to those of the binary host. Even if morphologically imperfect, the resulting energy cascade promotes charge separation and reduces recombination, leading to higher fill factors and short-circuit current densities. The open-circuit voltage sits between that of the binary extremes, in agreement with data from an extensive literature review. This leads to selection criteria for third compounds in terms of energetics and miscibility that promote the formation of a cascade morphology in real and energy space.

Place, publisher, year, edition, pages
Wiley-V C H Verlag GMBH , 2022. Vol. 6, no 11, article id 2200450
Keywords [en]
Kinetic Monte Carlo, Modeling, Organic solar cells, Ternary systems
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-188235DOI: 10.1002/solr.202200450ISI: 000848250100001OAI: oai:DiVA.org:liu-188235DiVA, id: diva2:1693689
Note

Funding agencies: Swedish Research Council (grant number: OPV2.0), Carl Zeiss Foundation

Available from: 2022-09-07 Created: 2022-09-07 Last updated: 2023-08-17Bibliographically approved
In thesis
1. Kinetic Monte Carlo Modelling of Organic Photovoltaic Devices
Open this publication in new window or tab >>Kinetic Monte Carlo Modelling of Organic Photovoltaic Devices
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic photovoltaics (OPVs) is a rapidly growing low-cost PV technology sector that relies on multiple benefits of organic semiconductors viz. being environmental-friendly with a simplified processing/fabrication and tunable properties. While the performance of OPVs has gone up to over 19 % in the last 2 decades, it still lags behind the silicon-PV technology in terms of efficiency and stability. The performance of any solar cell is essentially a combination of three quantities: short circuit current density (jSC), fill- factor (FF), and open circuit voltage (VOC). Of these quantities, especially VOC still offers a scope of further improvement as it falls short of the theoretical achievable limit which requires a better understanding of the various loss channels in an actual device leading to a VOC reduction. Through this thesis, we have developed an understanding of VOC using non-equilibrium models and proposed ways leading to its enhancement.

The first step in this study was to develop an understanding of the underlying charge transport mechanism, which relates to the efficiency with which the charges can be extracted at the terminals, or electrodes. Charge transport in any disordered organic semiconductor occurs by a process of hopping, in which charge carriers jump by thermally activated tunneling between localized sites that are randomly distributed in the energy dependent density of states. Thus, among other hopping parameters, the mobility of a charge carrier is crucially dependent on the disorder. We implemented a new semi-analytical hopping model that allows for a consistent extraction of these parameters from the space charge limited conductivity (SCLC) experiments. The model was calibrated against a numerical kinetic Monte Carlo (kMC) model and was used to analyze temperature-dependent SCLC curves for multiple systems used frequently as an active layer in organic solar cells. We observed that there exists a critical ratio between the inter-site distance and the localization length that decides the applicability, or not, of the much-used extended Gaussian disorder model (EGDM). The improved hopping model functions well for both fullerene and non-fullerene-based systems and can also describe the charge transport in electron-only devices, which so far have not been described successfully using EGDM.

Having the charge dynamics and other hopping parameters in place, we subsequently developed and calibrated, by independent experiments, a robust and stochastic kMC model that can calculate a full j-V curve of a solar cell correctly. With the full calibration in place with respect to the morphology, recombination rate constant and injection barriers, the motivation was to have a model that can calculate both transients and steady-state j-V curves of a given device. So far, implementing kMC to analyze a full device has been a challenge, especially due to the numerical problems associated with the presence of Ohmic contacts. The calibrated model correctly predicts the device’s j-V and non-equilibrium hopping transport recombination dynamics.

A crucial approximation that stems from inorganic solar cells and that is commonly made for organic solar cells as well, is the fast and complete thermalization of charge carriers in the density of states. However, the relaxation of charge carriers in case of organic semiconductors is not as straightforward as in inorganics, but rather a complex two-step process consisting of a fast on-site relaxation followed by a slower global relaxation occurring via hopping to increasingly deep sites. We have shown that the second slow thermalization does not complete within the charge carrier lifetime in the device and leads to a VOC that is 0.1 - 0.2 V higher than the equilibrium value. This is found for both fullerene and high-performing non-fullerene OPV systems.

For a given OPV device, there is a significant difference between the upper limit for the efficiency set by theoretical considerations based on the assumption of near-equilibrium (the so-called Shockley-Queisser limit) and the actual measured efficiency. We numerically explored a new funnel-shaped morphology, which can lead to an impactful gain in VOC and efficiency of an organic solar cell. In contrast to the conventional blend morphology, which does not lead to a directed motion of the photogenerated charge carriers, the funnel morphology rectifies the otherwise undirected diffusive motion of ‘hot’ charges, which leads to a higher probability of extraction at the desired contact. We utilized the reciprocity analysis to calculate the gain in VOC and efficiency as compared to a hypothetical equilibrium system of the same material. We found that for an optimized funnel morphology, the efficiency can surpass the near-equilibrium limit.

Mixing materials to form a high-performing ternary OPV has emerged as a possible route to improve performance. We performed a review of literature data and deduced that the relative gain in VOC is too small to contribute to a large gain in the efficiency. Instead, the major contribution to the efficiency enhancement is due to gains in the FF and/or jSC. Also, the VOC of the ternary system is found to be tunable relative to the ratio of the added species in the host system. These experimental findings were consistently described by extensive numerical simulations in which the active layer morphology was assumed to give rise to an energetic cascade for at least one of the charge carriers. In contrast, our explicit calculations show that the commonly employed parallel junction model cannot explain the experimental findings.

Abstract [sv]

Termen "organisk" avser något som huvudsakligen består av kol, och "organisk solcell" är helt enkelt en anordning för energiutvinning av ett sådant organiskt material som omvandlar solljus till elektrisk energi. Det är en snabbt växande teknik för energiutvinning till låg kostnad med många fördelar som låg vikt, miljövänlighet, giftfrihet osv. Även om prestandan hos dessa anordningar har ökat avsevärt, är det fortfarande ett pågående arbete att göra dem jämförbara med annan teknik. En typisk solcells prestanda bestäms av dess elektriska egenskaper, särskilt strömmen och spänningen vid exponering för solljus. Denna avhandling fokuserar på strategier för att förbättra utgångsspänningen hos organiska solceller.

För att tillverka en organisk solcell används två typer av material; det ena fungerar som en donator, dvs. det kan lätt avge en elektron vid belysning, och det andra är en acceptor, dvs. ett material som kan ta upp denna elektron. Donatorn och acceptorn blandas för att bilda det aktiva skiktet i en cell, som deponeras mellan två strömuppsamlande elektroder. Dessa organiska material har inhomogeniteter i sin struktur, vilket leder till så kallad energetisk oordning, vilket innebär att inte alla platser där en laddning kan sitta har samma energi. Denna oordning påverkar hur lätt en elektron (eller ett hål - en positiv laddning) kan förflytta sig inom anordningen. Vi har studerat laddningstransportegenskaperna i en anordning med hjälp av en ny modell som ger en korrekt uppskattning av dessa egenskaper vid varje givet tillfälle. Vi har också utvecklat en numerisk modell som kan reproducera de experimentella ström- och spänningsegenskaperna hos en helt organisk solcell.

När laddningar rör sig i en organisk solcellsenhet förlorar de en betydande mängd energi på grund av den ovan nämnda oordningen. Denna energiförlust kan minskas om en mer "riktad" eller forcerad rörelse sker så att elektronerna och hålen endast går till den önskade elektroden. Detta är möjligt genom att skapa en ny mikroskopisk struktur (morfologi) som underlättar en sådan riktad laddningsrörelse. Vi har föreslagit en ny trattformad morfologi som bildas genom ett lämpligt arrangemang av donator- och acceptor materialen och som leder till en betydande ökning av utgångsspänningen och därmed till en övergripande förbättring av solcellsutrustningens prestanda.

Ett annat sätt att förbättra organiska solceller kan ske när en tredje organisk förening läggs till donator- och acceptor blandningen, som då kallas en ternär solcell. Det har visat sig att denna ytterligare art ger en förbättring av solcellens prestanda och gör det möjligt att göra ett tjockare aktivt skikt, vilket är fördelaktigt för att generera högre strömmar och därmed högre effekt. En litteraturgenomgång ger otillräcklig och icke-slutgiltig information om de ansvariga mikroskopiska strukturer som leder till högpresterande ternära enheter. Genom systematiska beräkningar och experiment har vi hittat den morfologi som konsekvent kan förklara dessa system. Författaren hoppas att detta arbete kommer att bidra till den forskning som är inriktad på att skapa förbättrade morfologier och system vilket kan leda till faktiska prestandaförbättringar i organiska solceller.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2022. p. 80
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2214
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-187966 (URN)10.3384/9789179292393 (DOI)9789179292386 (ISBN)9789179292393 (ISBN)
Public defence
2022-10-04, Online through Zoom (contact wendela.yonar@liu.se) and TEMCAS, Building T, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2022-09-01 Created: 2022-09-01 Last updated: 2022-09-07Bibliographically approved

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Upreti, TanviWang, YumingGao, FengKemerink, Martijn

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