The concept of introducing photocontrollable trapping states into a semiconducting matrix using DAEs was extended to n-type semiconductors. Therefore, DAEs possessing a strong modulation of their LUMO energies between the ring-open and ring-closed form are needed. Importantly, the absolute LUMO energies have to be very low in order to be compatible with common organic n-type semiconductors. Thus, structurally similar DAEs 2i and 4i were chosen, both possessing strongly electron accepting perfluorocyclopentene as bridging moiety and 3,5-bis(tri- fluoromethyl)phenyl substituents in the periphery (Scheme 43). From the results presented in section 4.5 can be deduced that these DAEs are amongst the structures possessing lowest LUMO energies of all compounds investigated in this work. While DAE 2i in its ring-closed form possesses a LUMO energy of -3.5 eV, the additional nitrogen of the thiazole rings of DAE
4i slightly shifts its LUMO energy further down to -3.7 eV (Table 21, Figure 50a). LUMO
levels of both ring-open isomers are more than 1 eV higher in energy.
Two fullerene based semiconductors, PCBM and ICBA (Scheme 43), were chosen as matrices for the construction of the OTFTs. Their LUMO energies perfectly align with that of the DAEs in order to fulfill the energetic requirements. Thereby, the LUMO of ICBA is positioned at slightly higher energies than both of the ring-closed DAEs while the LUMO of PCBM is positioned in-between the two ring-closed isomers (Table 21, Figure 50a). This results in the situation that the LUMO energy difference ΔE1 = 290 meV between ICBA and DAE 4i(c)
represents the largest driving force for electron trapping amongst the four possible fullerene/DAE blends. For the combinations of ICBA and 2i(c) as well as PCBM and 4i(c) a smaller driving force with ΔE2 = ΔE3 = 100 meV is predicted. Finally, for the combination of
PCBM with 2i(c) no driving force for electron trapping should exist.
Scheme 43. Semiconducting matrices and DAE derivatives for the construction of photocontrollable
n-type OTFTs. OMe O PCBM ICBA X S S X F3C F3C CF3 CF3 F F F F F F X = CH: X = N: 2i 4i
Table 21. Reduction potentials and derived LUMO energies of semiconducting matrices and DAE
derivatives, obtained from cyclic voltammetry. Potentials are given in reference to the ferrocene/ferrocenium redox couple (rev = reversible, qr = quasireversible, irr = irreversible).
comp. Epc1 / V ELUMO / eV 2i(o)a -2.56 (irr) -2.24 2i(c)a -1.27 (qr) -3.53 4i(o)a -2.11 (irr) -2.69 4i(c)a -1.08 (qr) -3.72 PCBMb -1.18 (rev) -3.62 ICBAb -1.37 (rev) -3.43 a) acetonitrile/0.1 M Bu4NPF6. b) toluene/acetonitrile 3:1/0.1 M Bu4NPF6.
OTFTs with active layers composed of blends of the four possible combinations between the two semiconductors and the ring-open DAE isomers were fabricated in the BG-BC geometry by spin-coating from chlorobenzene solutions. Thereby, all blends contained 20 wt% of the respective DAE derivatives. For PCBM based devices electron mobilities were essentially unaffected by the presence of the DAEs and amounted to 0.04 cm2 V-1 s-1. In contrast, electron
mobilities of the two blended ICBA devices were reduced by a factor of 6 compared to the pristine semiconductor, which possessed a mobility of 810-3 cm2 V-1 s-1. GIXD measurements
showed that all investigated films were completely amorphous. AFM measurements and Scanning Auger Microscopy indicated a higher degree of phase segregation for the PCBM blends, explaining that the presence of the DAEs did not reduce their electron mobility.
By irradiation of the transistors with UV (320nm) and visible (540 nm) light the expected photoresponse is observed. The pristine semiconductors show only negligible variation of the drain current upon irradiation. The same is observed for the PCBM/2i blend, which represents the energetically unfavorable combination for electron trapping in the ring-closed state of the DAE. Blends of PCBM with 4i as well as ICBA with 2i show a light induced current modulation of 14% and 11%, respectively. Notably, the ICBA/4i blend, possessing the largest driving force for electron trapping, shows the by far highest current modulation of 55%. Thus, the four fullerene/DAE combinations demonstrate that the photochemical current switching efficiency critically depends on the energy difference between the LUMO levels of the phohotchromic molecule and the semiconductor.
Both PCBM and ICBA transistors show a slow decrease of the absolute drain current measured over several switching cycles. This fatigue, which is more pronounced in the ICBA case, is attributed to degradation of the semiconductors and not to the degradation or by-product formation of the DAE molecules. Note that DAEs 2i and 4i have been characterized as highly fatigue resistant in section 4.3 of this work.
Figure 50. a) LUMO energy alignment between PCBM and ICBA as well as open (blue) and closed (red)
isomers of DAEs 2i and 4i, as obtained from cyclic voltammetry. b) Normalized drain current (VD = 40 V, VG = 80 V for ICBA, VG = 120 V for PCBM) during photoswitching of OTFT devices
containing blends of PCBM (top) and ICBA (bottom) with 20 wt% of DAEs 2i and 4i, respectively. Each switching cycle consisted of 30 s of UV irradiation (320 nm) and 10 min of visible light irradiation (540 nm). After each irradiation step the transistor was relaxed for 30 s in the dark before a transfer curve was recorded.
4.6.4 Summary
For the first time photocontrollable OTFTs were constructed by the light-induced reversible energy level modulation of DAEs blended with p-type and n-type semiconductors. The blending approach offers the advantage that each component can be chosen to fulfill specific optoelectronic functions. While the utilization of well-known polymer and small molecule semiconductors enables the electrical operation of the transistor devices with a high efficiency, the DAE component of the blend can be tuned by structural modifications in order to precisely align HOMO and LUMO energy levels with that of the semiconductor. Thereby, the energy difference between the semiconductor and charge carrier traps induced by the DAE directly correlates to the extent of photomodulation of the output current. Furthermore, the delicate interplay between phase segregation within the blended films and strong intermolecular interactions between the DAE and the semiconductor has been demonstrated. Strong phase segregation gives transistors with high charge carrier mobilities but photomodulation of the output current may be diminished. On the other hand, strong intermolecular interactions give large photomodulation but reduce the crystallinity of the semiconductor and thus alternate its electric properties. By proper substitution of the DAE components the morphology of the blended films can be tuned in order to meet the demands on the device. In addition to the compositional flexibility, the blending approach ensures easy and cheap fabrication processes as well as the applicability for large-area electronics.
For p-type transistors photomodulation of the drain current up to 90% using the small molecule BTBT and up to 80% using polymeric P3HT was achieved. In case of n-type transistors photomodulation efficiencies were significantly smaller and degradation of the fullerene matrices was observed.
To increase the transistor performance, DAE derivatives possessing LUMO levels lower in energy than -4.0 eV are of high interest for combining them with high-performing and air- stable n-type semiconductors such as PDIF-CN2[172] or P(NDI2OD-T2).[173] For the construction
of photocontrollable p-type OTFTs a broad library of DAE compounds can be accessed. However, it has to be noted that such electron-rich DAEs generally suffer from increased fatigue due to formation of the photochemical by-product (see section 4.3). It remains a challenge to design highly fatigue resistant DAEs possessing HOMO levels suited for charge trapping in p-type semiconducting matrices.
Further, currently ongoing studies are concerned with the immobilization of thiol functionalized DAEs 13b and 13c on gold electrodes comprising the source and drain within the transistor geometry. Thus, DAEs within a self-assembled monolayer comprise an interface between the electrode and the semiconductor. By photoinduced changes of energy levels the barriers for charge injection and thus the drain current shall be modulated.