Polycarbazoles as materials for organic solar cells

Carbazole presents an interesting unit for conjugated polymers for several reasons. On the one hand, 9H-carbazole is an inexpensive starting material. The completely aromatic configuration implicates a good stability. On the other hand, the nitrogen atom can be easily substituted with a multitude of functional groups. By this means, the solubility as well as the optical and electronical properties of the polymer can be influenced without causing steric interactions close to the polymer backbone. The bandgap of polycarbazole derivatives is lower than for polyphenylenes because of the bridged biphenyl unit. The carbazoles can be linked either at the positions 3 and 6 leading to poly(3,6-carbazole) or at the positions 2 and 7 resulting in poly(2,7-carbazole). The two polymers exhibit different properties and thus different fields of application.

33 Figure 23 shows the chemical structures of both polycarbazoles as well as the 9H-carbazole unit.^[59,172]

Figure 23: Chemical structures of 9H-carbazole (left), poly(3,6-carbazole) (middle), and poly(2,7-carbazole) (right).^[172]

Poly(3,6-carbazole)s possess a meta-linkage of the monomers units. In consequence, the conjugation length of dimeric units that can be considered as 4,4’-biphenyl building blocks is short.^[23,172] The materials are suited for the application in OFETs and OLEDs due to their high charge carrier mobilities as well as a blue luminescence arising from the short conjugation length.^[172,173] The para-linkage and thus the higher conjugation length enables the better migration of charge carriers along the polymer chain. For this reason, poly(2,7-carbazole) can be used as an efficient donor material for organic solar cells.^[23,172] Further factors are the low-lying HOMO level of the poly(2,7-carbazole)s that is important for the stability of the material in air and a high open-circuit voltage. With suitable structures and a good self-organization of the polymer chains, high hole transport mobilities can be achieved. Finally, the absorption spectrum can be adjusted to the solar spectrum by the copolymerisation with appropriate comonomers.^[23,59]

The synthetic route towards the 2,7-dibromocarbazole starting material requires several steps.^[174–176] The group of K. Müllen reported an efficient synthesis with only two steps in 2003 as depicted in Figure 24.^[177]

Figure 24: Synthetic strategy for the 2,7-dibromocarbazole unit.^[177]

The first step is the nitration of 4,4’-dibromobiphenyl using concentrated nitric acid yielding 4,4‘-dibromo-2-nitrobiphenyl. Subsequently, 2,7-dibromocarbazole was received by a reductive Cadogan ring closure in presence of triethyl phosphate.^[177–179]

K. Müllen et al. applied a soluble and thus well processable poly(2,7-carbazole) as a donor material in a BHJ solar cell for the first time. The polymer was equipped with a branched 2-decyltetradecyl substituent and was synthesized via Yamamoto coupling. Perylene tetracarboxydiimide was used as an acceptor. The donor polymer exhibited a low HOMO level of -5.6 eV yet a relatively high bandgap of 3.0 eV. The solar cell reached a high : value of 0.71 V, but just 0.6% efficiency could be achieved. This is attributed to the absorption spectrum of the

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active layer that only conforms badly to the solar spectrum.^[180] The chemical structures of the applied materials are presented in Figure 25.

Figure 25: Chemical structures of the materials of the first organic solar cell comprising a poly(2,7-carbazole) derivative.^[180]

For a better correlation of the absorption with the solar spectrum, poly(2,7-carbazolevinylene) derivatives exhibiting a low bandgap were synthesized by using electron-withdrawing comonomers and thus realizing D-A polymers. Mostly Horner-Emmons reactions were conducted as these lead to a very high purity of the materials but a further polymer was also synthesized via Stille coupling.^[23,59] The chemical structures of the polymers can be seen in Figure 26.

Figure 26: Chemical structures of different poly(2,7-carbazolevinylene)s.^[59]

The polymers show bandgaps between 2.3 eV and 1.7 eV while the HOMO levels lie between -5.6 eV and -5.5 eV.^[23] The bandgap decreases with increasing number of thiophene rings. The HOMO levels are also reduced with rising conjugation length except for the polymer with the non-aromatic thienyl dioxide unit. This building block possesses two localized carbon-carbon double bonds and two sulphur-oxygen bonds resulting in a higher electron affinity and thus a lower LUMO level.^[59] However, the efficiencies of solar cells comprising the presented donor polymers and PCBM as acceptor remain low between 0.2% and 0.4% for the first four materials. The best PCE of 0.8% was achieved with the thienyl dioxide containing polymer.

Furthermore, a high open-circuit voltage of 0.8 V was observed.^[23,59] In fact, the polymers exhibit

35 a low solubility and a low molecular weight that limits the performance of the solar cells. A further reason for the loss of efficiency is that the vinylene unit can also be damaged due to photooxidation.^[59]

Several poly(2,7-carbazole) derivatives including electron-withdrawing units were synthesized in the group of M. Leclerc.^[181,182] Figure 27 shows the chemical structures of these polymers.

Figure 27: Chemical structures of different alternating poly(2,7-carbazole) copolymers including PCDTBT.^[182]

Three of the electron-deficient units are symmetric and possess a benzene core. The other three building blocks include an asymmetric pyridine core. The HOMO levels of the polymers lie between -5.6 eV and -5.4 eV what is mostly determined by the carbazole part. The LUMO levels differ depending on the electron-withdrawing unit. The polymers containing pyridine are optimized with regard to the LUMO values that lie 0.25 eV lower than that of the benzene-based polymers. However, a better structural organisation of the symmetric polymers in the solid state leads to a higher charge carrier mobility and thus to a better performance of the solar cells in combination with PCBM as acceptor. The polymers with pyridine core show PCE values between 0.7% and 1.1% whereas the polymers with benzene core reach efficiencies between 1.8% and 3.6%. Using the symmetric polymers, also higher open-circuit voltages between 0.8 V and 1.0 V are achieved.^[23] The best results were received with the combination of carbazole and a 2,1,3-benzothiadiazole unit yielding poly-[(N-heptadecan-9’-yl)-2,7-carbazole-alt-5,5-(4′,7′-bis(thien-2-yl)-2′,1′,3′-benzothiadiazole)] (PCDTBT).^[181] High molecular weights, good film-forming properties and high hole mobilities up to 3 ∙ 10^-3 cm²V^-1s^-1 could be realized.^[59] In combination with different acceptor fullerenes, the hole mobilities of the blends lie in the range of 10^-4 cm²V^-1s^-1.^[183] The bandgap of PCDTBT is 1.9 eV with a HOMO level of -5.5 eV and a LUMO value of -3.6 eV. An open-circuit voltage of 0.9 V and a PCE of 3.6% were achieved.^[23] This conforms to the findings of the group of W. H. Jo about the evaluation of the effects of different acceptor units identifying benzothiadiazole as an optimal acceptor monomer. The performance of low bandgap polymers containing 2,1,3-benzothiadiazole BT, diketopyrrolopyrrole DPP, isoindigo I, thieno[3,4-c]pyrrole-4,6-dione TPD, and 3-fluorothieno[3,4-b]thiophene TT regarding the short-circuit current, the open-circuit voltage, the fill factor, and the power

PCDTBT

36

conversion efficiency was compared. Whereas DPP polymers show the highest Isc due to the low bandgaps, TPD-based polymers exhibit the highest Voc ascribed to the deep HOMO levels.

Polymers comprising BT and TT achieve intermediate values. Regarding the fill factor, DPP and isoindigo polymers reach lower values than the others. In the case of the PCE, the best results are attained with polymers based on BT and TT. The values of 9.55% and 9.30%, respectively, represent the average values of the top five devices that were reported for the polymer containing the corresponding acceptor unit.^[184]

The performance of PCDTBT were optimized by the group of A. J. Heeger. The use of a titanium dioxide electron transport layer and the incorporation of silver nanoparticles in blends composed of PCDTBT and PC70BM led to an increased open-circuit voltage, short-circuit current, fill factor, and EQE that can be attributed to an improved light absorption and charge transport.

In summary, an efficiency of 7.1% is reached.^[185] In addition, BHJ solar cells made from PCDTBT and PC70BM were modified with an electron transport layer of graphene oxide that was deposited via a stamping procedure. By combining the graphene oxide layer with titanium oxide, a high efficiency of 7.5% is achieved.^[186]