Organizing Semiconductor Block Copolymers
1. Introduction: History and donor acceptor block copolymer architectures
2.3 Electronic and optical properties
2.3 Electronic and optical properties
In this section the materials are investigated in terms of their energy levels, their absorption profiles in film and their charge carrier mobilities. The HOMO and LUMO levels are estimated from cyclic voltammetry in solution in combination with UV‐vis spectroscopy and the charge carrier mobilities are extracted from measurements on organic field effect transistors (OFETs). As will be shown, the methoxy groups of PvDMTPA influence both, the position of the HOMO energy level as well as the charge carrier mobility of the donor block. Cyclic voltammetry (CV) measurements of PvTPA in solution feature an irreversible oxidation peak at 0.4 V vs ferrocene (Fc), giving a HOMO energy level of 5.2 eV (figure 4a,b).56 Irreversible oxidations are commonly observed for unsubstituted TPAs and the evolving new bands arise from dimerization products that are oxidized at lower potentials.57 PvDMTPA is oxidized at 0.2 V vs Fc, corresponding to a HOMO level of 5.0 eV. The oxidation peak remains constant after several cycles, demonstrating the reversibility of oxidation. Surprisingly, the first oxidation of PvDMTPD also occurs at 0.2 V vs Fc, resulting in the same HOMO level as PvDMTPA. The cyclic voltammogram of PPerAcr did not exhibit a clear oxidation peak, and therefore the monomer PerAcr was used for HOMO level determination. This compound showed an oxidation at 1.2 V vs Fc, corresponding to a HOMO level of 6.0 eV. The first reductions of PerAcr and PPerAcr both occured at ‐1.2 eV. Thus, the position of the LUMO level is 3.6 eV, and, as expected, is found to be independent of the molecular weight of PPerAcr. Generally, all events of oxidation and reduction were found to be independent of the molecular weight and the presence of the second block. The LUMO energy levels of the
amorphous donor blocks were estimated from the CV results and the UV‐vis absorbance in film. Figure 4c shows the absorption of the three block copolymers. The donor blocks exhibit the typical triarylamine absorption up to 400 nm and PPerAcr features three characteristic bands between 400 nm and 600 nm, corresponding to a highly aggreated state of the PBI moieties.58 Since the absorption profile is sensitive to changes in the relative orientation of the PBI chromophores59, we tentatively propose that stacking of backbone‐
neighbored PBI moieties occurs along columns with a rotational offset, in a similar fashion to what has been shown for low molecular weight PBIs.17 Figure 4d depicts the resulting energy levels of the three donor polymers with PPerAcr as the acceptor. As can be clearly seen, the resulting energy level offsets are sufficiently large for efficient charge separation in all the block copolymers, being 0.8‐ 1.0 eV for ΔHOMO and ~ 1.5 eV for ΔLUMO.60
5.2
300 400 500 600
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4 PvDMTPD-b-PPerAcr
PvDMTPA-b-PPerAcr
300 400 500 600
0,0
4 PvDMTPD-b-PPerAcr
PvDMTPA-b-PPerAcr
4 PvDMTPD-b-PPerAcr
PvDMTPA-b-PPerAcr were measured in o‐dichlorobenzene containing tetrabutyl ammonium hexafluorophosphate at 50 mV/s vs Fc (Fc: ferrocenene). c) Optical densities of films (70 nm) of PvTPA‐b‐PPerAcr (dotted line), PvDMTPA‐b‐PPerAcr (solid line) and PvDMTPD‐b‐PPerAcr (dashed line). Due to the similar content of PPerAcr in all block copolymers, the optical density between 400 and 600 nm is almost equal. d) Schematic drawing of the estimated energy levels in eV.
Organic field effect transistor measurements on the homopolymers were carried out for the determination of the charge carrier mobilities.61,62 Generally, TPDs are better hole conductors than TPAs and therefore, a higher charge carrier mobility can be expected for polymer PvDMTPD. The simplest polymer PvTPA showed a very weak performance, with high threshold voltages, small on‐off ratios and small charge carrier mobilities µ. Annealing the sample at approximately 15 °C above its Tg caused a significant increase in the performance leading to over ten times higher drain currents and a charge carrier mobility of around 3∙10‐5 cm²/Vs. The threshold voltage remained still quite high at ‐37 V. In contrast, the two polymers PvDMTPA and PvDMTPD showed a different behaviour. Right after spin casting, both the PvDMTPA and the PvDMTPD performed significantly better, but no significant improvement was seen after thermal annealing. Both materials show low threshold voltages between ‐4 to ‐5 V. The PvDMTPD is superior in terms of its charge carrier mobility and its on‐off ratio. The PvDMTPA has a mobility µsat= 5∙10‐5 cm²/Vs and an on‐off ratio of 10², whereby PvDMTPD has a mobility of µsat=1.2∙10‐4 cm²/Vs and an on‐off ratio of 10³. The transfer characteristics of the donor polymers after annealing are shown in figure 5a. In contrast, the electron mobility of PPerAcr depends strongly on the thermal history of the transistor device. Spin casting PPerAcr from chloroform yielded devices with weak performances and high threshold voltages around 20 V. This changes dramatically after annealing the samples at 210 °C for 60 min, which is above the melting temperature of PPerAcr of 190 °C. The threshold voltage drops significantly to 6.8 V, while the current and charge carrier mobility both increase by two orders of magnitude. Thus electron transport obilities of up to 1.2∙10‐3 cm²/Vs were achieved. Figure 5b shows the transfer plot of PPerAcr before and after annealing.
-60 -40 -20 0 20 40 60
Figure 5. a) Transfer plots of the homopolymers PvTPA, PvDMTPA and PvDMTPD after thermal annealing above the Tg for a drain voltage of 60 V (negative gate voltage) and transfer characteris‐tics of the PPerAcr homopolymer before and after annealing above the Tm (drain voltage 20 V, positive gate voltage).
Based on these results, we can derive several essential differences between PvTPA and PvDMTPA: Firstly, the HOMO level shifts from 5.2 eV to 5.0 eV. Secondly, PvDMTPA is electrochemically stable because the two para positions are blocked and can not give rise to dimerization reactions. Lastly, the charge carrier mobility of PvDMTPA is increased compared to PvTPA, due to the electron‐rich methoxy substituents. The HOMO level position of PvDMTPA is maintained in PvDMTPD, which carries tetraphenylbenzidine moieties, and the charge carrier mobility of PvDMTPD is highest among the three donor polymers. Thus, if incorporated into block copolymers with the acceptor polymer PPerAcr, this set of amorphous donor blocks is ideal for correlating the solar cell performance with important parameters such as energy levels, charge carrier mobility and morphology. By opposing these results with morphological informations from electron microscopy, this structure‐
property relationship is evaluated in the following chapter