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Impact of intentional photo-oxidation of a donor polymer and PC70BM on solar cell performance
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).ORCID-id: 0000-0002-7533-4860
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).ORCID-id: 0000-0002-4745-1074
Visa övriga samt affilieringar
2019 (Engelska)Ingår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, s. 22259-22271Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

A short lifetime is the main factor hindering the wider implementation of low-cost organic photovoltaics in large-area and outdoor applications. Ingress of oxygen and water vapour through non-ideal encapsulation layers is a known cause of degradation for polymer/fullerene based solar cells. To better understand the origin of this performance degradation, we study the effect of intentional exposure of the photo-active layer to simulated sunlight (AM1.5) in air both on the solar cell performance and on the molecular semiconductor materials. Cathode-free thin films of a blend of the electron donor polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and the electron acceptor fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) were exposed to simulated sunlight in air. Fourier-transform infrared spectra demonstrate the formation of carbonyl photo-oxidation products in the blend films, as well as in the pristine polymer and fullerene films. Solar cells prepared with photo-oxidized active layers show increasingly degraded electrical performance (lower short circuit current, open circuit voltage and fill factor) with increasing exposure time. The increased diode ideality factor indicates that trap-assisted recombination hinders device operation after exposure. The external quantum efficiency decreases drastically with increasing exposure time over the whole photon energy range, while the UV-vis absorption spectra of the blend films only show a mild photo-induced bleaching. This demonstrates that not only the photo-induced degradation of the solar cell performance is not predominantly caused by the loss in light absorption, but charge transport and collection are also hampered. This is explained by the fact that photo-oxidation of PC70BM causes bonds in its conjugated cage to break, as evidenced by the decreased ∏* intensity in C1s-NEXAFS spectra of PC70BM films. This degradation of unoccupied states of PC70BM will hinder the transport of photo-generated electrons to the electrode. Surface photovoltage spectroscopy gives direct evidence for gap states at the surface of a PC70BM film, formed after 2 hours of exposure and resulting in upward band bending at the PC70BM/air surface. These observations indicate that the photo-oxidation of PC70BM is likely to be the main cause of the performance degradation observed when the photoactive layer of a TQ1:PC70BM solar cell is intentionally exposed to light in air.

Ort, förlag, år, upplaga, sidor
Royal Society of Chemistry, 2019. Vol. 21, s. 22259-22271
Nationell ämneskategori
Fysik
Forskningsämne
Fysik
Identifikatorer
URN: urn:nbn:se:kau:diva-75110DOI: 10.1039/C9CP04384EISI: 000491079900005OAI: oai:DiVA.org:kau-75110DiVA, id: diva2:1358174
Forskningsfinansiär
Energimyndigheten, 38327-1Tillgänglig från: 2019-10-07 Skapad: 2019-10-07 Senast uppdaterad: 2019-11-14Bibliografiskt granskad
Ingår i avhandling
1. Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level
Öppna denna publikation i ny flik eller fönster >>Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Polymer solar cells (PSC) have reached record power conversion efficiencies of over 15%. The operational lifetime of PSCs, however, has to increase for their use in large area outdoor applications. In this work, a set of spectroscopic techniques (UV-vis, FTIR, NEXAFS, XPS) was used to study the impact of exposure to light and air (photo-oxidation) on the photoactive layer and its components. We focused on the electron acceptor components: the fullerene derivatives, PC60BM and PC70BM, and the polymer N2200. A comparative study of photo-oxidized PC60BM and PC70BM thin films by UV-vis and FTIR spectroscopy has shown that both materials undergo similar photochemical transformation, with the process being faster in PC60BM, due to the greater curvature of the C60 cage. Comparing experimental FTIR, XPS and NEXAFS spectra of the photo-oxidized PC60BM thin films with the calculated spectra for a large variety of photo-oxidation products, it was found that dicarbonyl and anhydride groups attach to the C60 cage during photo-oxidation. The study of photo-oxidized TQ1:PC70BM blend films by spectroscopic and J-V measurements shows that deterioration of the charge transport in PC70BM is the major contributor to the device performance degradation. Kelvin Probe measurements demonstrated that the charge transport deterioration was due to upward band bending and gap states being formed on the surface of photo-oxidized PC70BM. The TQ1:PC70BM blends films were further studied by AFM-IR in order to determine the lateral distribution of pristine components, as well as the photo-oxidation products. It was found that anhydride oxidation products of PC70BM are equally distributed over the blend film surface. The PC70BM is replaced with the polymer N2200 in the blend with TQ1. The photostability in air of the blend and its neat components was studied by UV-vis and FTIR spectroscopy. The spectra show that thermal annealing improves the photostability in air of both components.

Abstract [en]

Increase of the global energy demand and the climate change are two factors motivating the study and use of renewable energy sources, such as the solar energy. Organic photovoltaics (OPV) is a technology that uses organic molecules to convert solar energy into electricity. These organic molecules can be kept in ink form, allowing OPV device manufacture via coating, and ultimately roll-to-roll printing techniques, resulting in inexpensive, light weight, portable, and mechanically flexible sources of electricity. OPV devices have reached over 15% in power conversion efficiency, but their operational lifetime has to increase.

In this work, the photostability of the active layer in organic solar cells and its molecular components was studied by a variety of spectroscopy, microscopy and electrical characterization techniques, with focus on the chemical changes that these materials undergo during exposure to light and air. The aim was to determine the relation between materials’ degradation and the device performance degradation.

Ort, förlag, år, upplaga, sidor
Karlstads universitet, 2019. s. 59
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2019:30
Nyckelord
photovoltaics, polymer solar cell, conjugated polymers, fullerene, photo-oxidation, spectroscopy
Nationell ämneskategori
Fysik
Forskningsämne
Fysik
Identifikatorer
urn:nbn:se:kau:diva-75093 (URN)978-91-7867-054-3 (ISBN)978-91-7867-064-2 (ISBN)
Disputation
2019-11-29, 21A 342 Eva Erikssonsalen, 13:15 (Engelska)
Opponent
Handledare
Tillgänglig från: 2019-11-05 Skapad: 2019-10-08 Senast uppdaterad: 2019-11-05Bibliografiskt granskad

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Förlagets fulltexthttps://pubs.rsc.org/en/content/articlehtml/2019/cp/c9cp04384e

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Blazinic, VanjaEricsson, LeifLevine, IgalHansson, RickardOpitz, AndreasMoons, Ellen
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