Effect of co-adsorbate and hole transporting layer on the photoinduced charge separation at the TiO2-phthalocyanine interface

Understanding the primary processes of charge separation (CS) in solid-state dye-sensitized solar cells (DSSCs) and, in particular, analysis of the efficiency losses during these primary photoreactions is essential for designing new and efficient photosensitizers. Phthalocyanines (Pcs) are potentially interesting sensitizers having absorption in the red side of the optical spectrum and known to be efficient electron donors. However, the efficiencies of Pc-sensitized DSSCs are lower than that of the best DSSCs, which is commonly attributed to the aggregation tendency of Pcs. In this study, we employ ultrafast spectroscopy to discover why and how much does the aggregation affect the efficiency. The samples were prepared on a standard fluorine-doped tin oxide (FTO) substrates covered by a porous layer of TiO2nanoparticles, functionalized by a Pc sensitizer and filled by a hole transporting material (Spiro-MeOTAD). The study demonstrates that the aggregation can be suppressed gradually by using co-adsorbates, such as chenodeoxycholic acid (CDCA) and oleic acid, but rather high concentrations of co-adsorbate is required. Gradually, a few times improvement of quantum efficiency was observed at sensitizer/co-adsorbate ratio Pc/CDCA = 1:10 and higher. The time-resolved spectroscopy studies were complemented by standard photocurrent measurements of the same sample structures, which also confirmed gradual increase in photon-to-current conversion efficiency on mixing Pc with CDCAK.V. acknowledges the Doctoral Programme of Tampere University of Technology for the financial support. N.V.T. acknowledges NATO SPS project no. 985043. Financial support from Comunidad de Madrid, Spain (S2013/MIT2841, FOTOCARBON) and MINECO, Spain (CTQ2014- 52869-P and CTQ2017-85393-P) is acknowledged. IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, grant SEV-2016-0686)

Photoinduced Charge Transfer Processes at Organic-Semiconductor Interfaces

Ihmiskunnan energiankulutus kasvaa jatkuvasti, ja tähän saakka energiaa on tuotettu lähinnä fossiilisilla polttoaineilla. Öljyn, hiilen ja maakaasun poltto on nostanut sekä ilmakehän hiilidioksidipitoisuuden että maapallon lämpötilan vaarallisen korkeiksi. Siksi tarvitaan hiilineutraaleja energiantuotantomuotoja, kuten aurinkokennoja.Syvällinen ymmärrys aurinkokennoissa tapahtuvista valoindusoiduista kemiallisista reaktioista on tärkeää suunniteltaessa uusia, tehokkaampia aurinkokennoja. Ultranopea aikaerotteinen absorptiospektroskopia tarjoaa oivan välineen aurinkokennojen reaktiokinetiikan tutkimukseen kennojen hyötysuhteen optimoimiseksi.Tässä työssä tutkittiin valon aikaansaamia prosesseja orgaanisten yhdisteiden ja puolijohteiden rajapinnoilla. Tällaisia rakenteita voidaan käyttää esimerkiksi väriaineherkistetyissä aurinkokennoissa. Työssä tutkittiin kahta orgaanisen aineen ja puolijohteen hybridirakennetta: fullereeneja kvanttipisteiden pinnalla, ja ftalosyaniineja nanorakenteisen titaanidioksidin ja sinkkioksidin pinnalla. Kaikissa näytteissä havaittiin valon aikaansaamia elektroninsiirtoreaktioita, jotka johtavat sähkövirran syntymiseen aurinkokennonäytteissä. Reaktioiden nopeus vaihteli muutamasta pikosekunnista (ftalosyaniini titaanidioksidin pinnalla) noin 100 ps:iin (fullereenit kvanttipisteiden pinnalla).Ftalosyaniinijohdannaiset ovat aurinkokennokäyttöön hyvin soveltuvia väriaineita. Niillä on voimakas absorptio esityisesti punaisessa osassa spektriä, ja ne ovat hyvin stabiileja. Haittapuolena on niiden aggregoitumistaipumus. Aggregoituminen pienentää aurinkokennojen hyötysuhteita aggregaattien sisäisten häviöiden takia. Aggregoitumisen vähentämiseen on yleisesti käytössä kaksi menetelmää: seosaineet ja tilaa vievien sivuryhmien substituutio ftalosyaniinirakenteeseen. Kumpikin menetelmä vähensi aggregoitumista ftalosyaniinikerroksissa, mutta substituutiomekanismi oli tehokkaampi, kun tarkastellaan varausten erottumisen elinaikaa.Rakenteeseen lisättiin aukonkuljetusmateriaalia (spiro-MeOTAD) oikean aurinkokennon toiminnan simuloimiseksi. Aukonkuljetusmateriaalin vaikutusta valoindusoitujen reaktioiden kinetiikkaan tutkittiin. Havaittiin, että varausten erottuminen tapahtuu ensin orgaanisen aineen ja aukonkuljetusmateriaalin rajapinnassa, minkä jälkeen ftalosyaniini luovuttaa elektronin puolijohteeseen.Aikaerotteisten spektroskopiamittausten tulokset haluttiin myös linkittää todellisiin aurinkokennojen hyötysuhteisiin. Ftalosyaniinien aggregoitumisen ja aurinkokennojen tuottaman valovirran välillä havaittiin selkeä korrelaatio. Vähemmän aggregoituneet näytteet tuottavat suuremman sähkövirran absorboituneiden fotonien lukumäärää kohti. Työn tulokset auttoivat tunnistamaan modernien hybridiaurinkokennojen suunnittelussa esiintyviä pullonkauloja. Tulosten perusteella voidaan esittää keinoja kennojen hyötysuhteiden parantamiseksi.The growing energy demand of the mankind has lead to the extensive use of fossil fuels. The burning of oil, coal and natural gas has increased the global temperature and atmospheric carbon dioxide percentage to a dangerously high level. Therefore, carbon-neutral energy sources such as solar cells are needed.In order to design more effective solar cells, a deep understanding of the primary photochemical processes in the cells is needed. Ultrafast time-resolved spectroscopy, especially transient absorption methods, are a very useful tool for investigating the reaction kinetics in order to optimize the solar cell performance. In this thesis, kinetics of the photoinduced processes at the interface of an organic monomolecular layer and a semiconductor are studied. Such structures may be used as the active material e.g. in dye-sensitized solar cells. Two different types of organic–semiconductor hybrids were prepared: fullerenes (C60) immobilized on colloidal semiconductor quantum dots (QDs), and zinc phthalocyanine (ZnPc) derivatives on nanostructured titanium dioxide (TiO2) and zinc oxide (ZnO) surfaces. The driving force of photocurrent generation in these systems is a photonic excitation leading to an electron transfer reaction across the organic–semiconductor interface. The observed electron transfer rates vary from a few picoseconds in ZnPc monolayers on TiO2 to ca. 100 ps in QD–fullerene systems. Phthalocyanine derivatives are very attractive sensitizing dyes for solar cell applications because of their excellent stability and strong absorption in the red part of the spectrum. A drawback with these compounds is their tendency towards aggregation. It reduces the solar cell efficiencies due to intra-aggregate losses. There are two common methods for aggregation-reduction: the use of molecular co-adsorbates and substitution of the phthalocyanine core with bulky side groups. Both mechanisms were observed to lower the degree of aggregation in the ZnPc samples. The substitution method proved to be more efficient in terms of the lifetime of the charge-separated state. To more realistically mimic a solar cell, a hole-transporting material (HTM) was used. Its effect on the primary photoinduced reactions in the phthalocyanine–semiconductor samples was studied. With the chosen HTM, spiro-MeOTAD, the charge separation was observed to occur first at the phthalocyanine–HTM interface, followed by electron injection into the semiconductor material.Complete solar cell samples were prepared in order to link the ultrafast spectroscopy results to actual solar cell performance. A correlation between the degree of aggregation and the produced photocurrent was confirmed. The less aggregated samples produce a higher photocurrent per number of absorbed photons. This study indentifies bottlenecks in modern hybrid organic–semiconductor solar cell design and suggests solutions for improving the solar cell performance

Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot–Fullerene Hybrids

Photoinduced electron transfer (ET) in CdSe/ZnS core–shell quantum dot (QD) – fullerene (COOH–C₆₀) hybrids was studied by the means of time-resolved emission and absorption spectroscopy techniques. A series of four QDs with emission in the range 540–630 nm was employed to investigate the dependence of the electron transfer rate on the QD size. Emission of the QDs is quenched upon hybrid formation, and the quenching mechanism is identified as photoinduced electron transfer from the QD to the fullerene moiety due to the fullerene anion signature observed in transient absorption. In order to obtain quantitative information on the ET reaction, several kinetic data analysis techniques were used, including a conventional multiexponential fitting and a maximum entropy method for emission decay analysis, as well as a distributed decay model based on the Poisson distribution of fullerenes in the hybrids. The latter gradually simplifies the interpretation of the transient absorption spectra and indicates that the spectra of QD cations are essentially similar to those of neutral QDs, differing only by a minor decrease in the intensity and broadening. Furthermore, only a minor decrease in the ET rate with the increasing QD size was observed, the time constants being in the range 100–200 ps for all studied QDs. The charge recombination is extended to 10 ns or longer for all hybrids

Photoinduced Electron Injection from Zinc Phthalocyanines into Zinc Oxide Nanorods: Aggregation Effects

Phthalocyanines (Pc) are well-known light-harvesting compounds. However, despite the tremendous efforts on phthalocyanine synthesis, the achieved energy conversion efficiencies for Pc-based dye-sensitized solar cells are moderate. To cast light on the factors reducing the conversion efficiency, we have undertaken a time-resolved spectroscopy study of the primary photoinduced reactions at a semiconductor-Pc interface. ZnO nanorods were chosen as a model semiconductor substrate with enhanced specific surface area. The use of a nanostructured oxide surface allows to extend the semiconductor-dye interface with a hole transporting layer (spiro-MeOTAD) in a controlled way, making the studied system closer to a solid-state dye-sensitized solar cell. Four zinc phthalocyanines are compared in this study. The compounds are equipped with bulky peripheral groups designed to reduce the self-aggregation of the Pcs. Almost no signs of aggregation can be observed from the absorption spectra of the Pcs assembled on a ZnO surface. Nevertheless, the time-resolved spectroscopy indicates that there are inter-Pc charge separation–recombination processes in the time frame of 1–100 ps. This may reduce the electron injection efficiency into the ZnO by more than 50%, pointing out to a remaining aggregation effect. Surprisingly, the electron injection time does not correlate with the length of the linker connecting the Pc to ZnO. A correlation between the electron injection time and the ”bulkiness” of the peripheral groups was observed. This correlation is further discussed with the use of computational modeling of the Pc arrangements on the ZnO surface