How structural factors influence the performance of copper(I) bis(diimine) based DSCs

Abstract
This PhD thesis is based on the synthesis of new homoleptic copper(I) complexes and their applications in dye-sensitized-solar-cells (DSCs).
Chapter I: Is an evaluation of the anchoring ligands effect upon device performance containing ancillary ligands of 1st and 2nd generation hole transport triphenylamino-dendrons.
Chapter II: Describes the influence of six different substituents in the 6,6’-positions of the ancillary ligands on the device performance.
Chapter III: Is a short study of a more atom economic device assembling method, where the copper(I) complex is formed in situ on the TiO2 surface.
Chapter IV: Shows the influence of the dye conentration used during the dyeing process of the semi-conductor.
Chapter V: Is a study of how the enhanced photon absorption, achieved by extending the aromatic system of the ancillary ligand, affects the cell performance.
Chapter VI: Describes the use of different solvents during the dyeing process of the photoanode and their influence on DSC performance.
Chapter VII: Addresses issues concerning the TiO2 surface such as the aggregation of dye molecules and how the addition of co-adsorbants during the dyeing cycle may prohibit the formation of such aggregates.
Parts of this work have been published:
• B. Bozic-Weber, S. Y. Brauchli, E. C. Constable, S. O. Fürer, C. E. Housecroft and I. A. Wright, Phys. Chem. Chem. Phys., 2013, 13, 4500-4504.
• B. Bozic-Weber, S. Y. Brauchli, E. C. Constable, S. O. Fürer, C. E. Housecroft, F. J. Malzner, I. A.Wright and J. A. Zampese, Dalton Trans., 2013, 34, 12293-12308.
• S. Y. Brauchli, B. Bozic-Weber, E. C. Constable, N. Hostettler, C. E. Housecroft and J. A. Zampese, RSC Advances, 2014, 4, 34801-34815.
• S. Y. Brauchli, F. J. Malzner, E. C. Constable and C. E. Housecroft, RSC Advances, 2014, 4, 62728-62736.
• S. Y. Brauchli, E. C. Constable, C. E. Housecroft, Dyes and Pigments, 2015, 113, 447-450.
Summary
Within this study, 18 ligands (L1.1-L3.6) and their homoleptic copper(I) complexes [Cu(L1.1-L3.6)2][PF6] have been synthesized. They were fully characterized by 1H and 13C NMR, mass spectrometry, solution absorption spectrometry, melting point, elemental analysis and infrared spectrometry. Furthermore, all homoleptic Cu(I) complexes were electrochemically analysed by cyclic voltammetry and square-wave voltammetry.
By increasing the aromatic system in the ligands (Scheme 26), the light harvesting was effectively enhanced (e.g. going from L1.1 -> L2.1 -> L3.1). An increase in absorption by extending the aryl system was achieved in the homoleptic Cu(I) complexes, with an extinction of about twice that the free ligands.
Scheme 26: Representative ligands to illustrate the extension of the aryl system.
Furthermore, the substituents in the 6,6´-positions on the bipyridine were varied within each ligand generation (Scheme 26). All complexes were incorporated in DSCs.
In Chapter I, the first focus is on the influence of the anchoring ligand on the performance of a DSC. For this study, two representative capping ligands were introduced by treating an anchoring ligand covered photoanode with complexes [Cu(L2.1)2][PF6] and [Cu(L3.1)2][PF6]. By using these two example dyes, a set of four anchoring ligands was screened to identify the one that yielded the best conversion efficiency in the device.
Scheme 27: Set of anchoring ligands with phosphonic and carboxylic acids as anchoring groups.
It turned out that devices with anchoring ligands decorated with phosphonic acids (ALP and ALP1) generally achieve higher efficiencies than those with carboxylic acids (ALC and ALC1). Additionally, the influence of the extended aryl system on the ancillary ligands (L2.1 vs. L3.1) was examined in this set. Indeed, higher conversion efficiencies were obtained from devices incorporating the more conjugated ancillary ligand L3.1 compared to L2.1.
Scheme 28: Ancillary ligands L1.1-L1.6 examined in Chapter II.
In Chapter II, the influence of 6 different substituents in the 6,6’-positions of the bipyridine ancillary ligands (Scheme 28) in combination with anchoring ligands ALP and ALP1 was examined. It was found that DSCs incorporating anchoring ligands ALP1 reach much higher conversion efficiencies than those with ALP. Ancillary ligands L1.3 and L1.5 reached remarkably higher efficiencies, which was attributed to the reduced charge recombination rate.
Scheme 29: Two approaches to introduce a copper metal ion and an ancillary ligand on a TiO2 coated photoanode.
In Chapter III, a new strategy for incorporating heteroleptic complexes on the TiO2 surface was tested and compared with the state of the art methodology. The state of the art method works as followed. After an anchoring ligand has been adsorbed on a semiconductor surface, the photoanode is immersed in a solution of homoleptic Cu(I) complex. Due to the labile nature of Cu(I) complexes, a ligand exchange with the previously anchored ligand occurs, leaving with heteroleptic copper dye on the surface.
In the second methodology it becomes needless to prepare the homoleptic Cu(I) complex beforehand. By using the new method (stepwise methodology), an additional step during the dyeing process is required. Nevertheless, it is more economic than the conventional process. After the anchoring ligand is bound to the TiO2 surface, the anode is immersed in a solution of [Cu(MeCN)4][PF6]. At this stage the copper(I) binds to the anchoring ligand and it is assumed that a heteroleptic complex with two coordinating acetonitrile molecules is formed. In the last step, the anode with the intermediate heteroleptic complex on the surface is immersed in a solution of pure ligand, which replaces the acetonitrile molecules due to the chelating effect. The main outcome of this survey was that devices prepared by the state of the art method achieve a higher final conversion efficiency than those prepared from the stepwise assembly. However, using this new method, devices exhibited a higher initial efficiency than those prepared from the old method.
In Chapter IV, devices were prepared from four different concentrations of dye solutions ([Cu(L2.1)2][PF6] in CH2Cl2 at 2.0, 1.0, 0.5 and 0.1 mM). Their initial efficiencies and their development over several days were compared. The results showed that devices prepared from the least concentrated dye solutions reached their maximum efficiency immediately after assembling the cells and this efficiency was maintained over the whole measuring period. Additionally, it was found that DSCs prepared from the more dilute dye solutions reach a higher maximum conversion efficiency than those prepared from concentrated dye solutions.
In Chapter V, the focus was on the change in device performance by extending the aromatic systems of the ancillary ligands. Ligands L2.1-2.6 and L3.1-3.6 were introduced into the DSCs by applying the state of the art ligand exchange method using complexes [Cu(L2.1-2.6)2][PF6] and [Cu(L3.1-3.6)2][PF6]. Except for ancillary ligand L3.1, no increase in efficiency was recorded by extending the aromatic system and increasing the absorption. Although the solid state UV-vis absorption spectra of the photoanodes showed an increase in absorption intensity, no gain in Jsc was achieved.
Chapter VI addresses the use of two different solvents during the dyeing process of the photoanodes. The cells were prepared either from acetone or CH2Cl2 dye solutions of [Cu(L2.1-2.6)2][PF6] and [Cu(L3.1-3.6)2][PF6]. By measuring solid state absorption spectra of dye loaded photoanodes, it turned out that upon using acetone during the dyeing process a severe increase of dye adsorption on the TiO2 surface was achieved. Moreover, by using acetone dye solutions the devices incorporating the more conjugated ancillary ligands (L3.1-3.6) reach generally higher efficiencies than cells with ligands L2.1-L2.6. DSCs prepared from acetone dye solutions containing capping ligands L3.1-3.6 also exhibit higher efficiencies than those with the same ancillary ligands prepared from CH2Cl2 solutions. For devices with capping ligands L2.1-2.6, no clear trend could be discovered by comparing cells prepared from acetone and CH2Cl2 dye solutions.
In Chapter VII, the main attempt was to minimize the dye aggregation on the surface by adding a co-adsorbant (chenodeoxycholic acid) to the dye solution during the dyeing process of the photoanode. The homoleptic complexes [Cu(L3.1)2][PF6] and [Cu(L3.5)2][PF6] served as example dyes. Additionally, cells were prepared again from acetone and CH2Cl2 dye solutions. Interestingly, all devices prepared from CH2Cl2 in the presence of cheno showed a clear increase in efficiency compared to the control devices without co-adsorbant. Furthermore, the device with ancillary ligand L3.5 prepared from acetone dye solution with cheno showed a higher conversion efficiency than its control cell. The device with the capping ligand L3.1 obtained from an acetone dye solution with cheno did not show an increased performance.
Conclusion
It has been shown that by increasing the aromatic system of the ancillary ligand, a gain in absorption intensity and an increase in conversion efficiency was achieved under certain circumstances. The studies revealed the huge number of possible tuning sites of DSCs, such as structural properties of the dye, dye concentration and solvent used during the dyeing cycle, and aggregation issues concerning the molecular size of the dye. Nevertheless, this work showed that a dye that does not seem to yield a reasonable conversion efficiency at first might reveal its full potential after some time. Screening of dyes is quite delicate because it is simply impossible to know the optimal conditions for every dye and it is likely to miss a potentially good dye.
Outlook
For the future, one may want to think to test more solvents during the dyeing process of the photoanode in order to obtain even higher device performances. Additionally, it might be reasonable to add a co-adsorbant to all of the synthesized dyes during the dyeing process of the photoanode in order to reduce dye aggregation, reduce charge recombination and increase the efficiency. Furthermore, that it is also sensible to test new electrolytes in combination with these dyes in attempt to obtain, for example, a higher Voc.