On the basis of theoretical models and calculations, several alternating polymeric structures have been investigated to develop optimized poly(2,7-carbazole) derivatives for solar cell applications. Selected low band gap alternating copolymers have been obtained via a Suzuki coupling reaction. A good correlation between DFT theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the corresponding polymers has been obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the molecular weight and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements have revealed hole mobilities of about 1 x 10(-3) cm2 x V(-1) x s(-1) and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new symmetric low band gap poly(2,7-carbazole) derivatives.

Harvesting energy directly from sunlight using photovoltaic cells (PCs) is a very important way to address growing global energy needs with a renewable resource while minimizing detrimental effects on the environment. For this purpose, the development of polymeric solar cells has received a great deal of attention from both academic and industrial laboratories. [1][2][3] Indeed, the utilization of semiconducting conjugated polymers as active components in bulk heterojunction photovoltaic devices offers significant potential advantages over existing inorganic materials in terms of ease of processing, formation of large surface areas, and costs. For example, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV), [4] regioregular poly(3-hexylthiophene) (P3HT), [5,6] and other polythiophene derivatives [7] have been extensively studied over the last decade, resulting in PCs with a power conversion efficiency between 3.0 and 5.0 %. However, the performances of these polymers are somehow restricted by their relatively large bandgap and the limited possibilities to modulate their physical properties by synthetic methods. New low-bandgap polymers have been developed over the years to better harvest the solar spectrum, especially in the 1.4-1.9 eV region. Promising copolymers based on fluorene units have been proposed by Andersson/Inganäs [8,9] and Cao, [10] with power conversion efficiencies between 2.0 and 2.8 %. Interestingly, the physical properties of polyfluorene derivatives can be easily modulated through the design of various alternating copolymers.[11] However, relatively low hole mobilities were reported for low-bandgap polyfluorene derivatives. Therefore, besides those recent advances, there is still a need for new polymeric materials to go beyond the 5 % efficiency of actual materials. [3,12,13] Along these lines, poly(N-vinylcarbazole) (PVK) is well known as an excellent photoconductor. [14,15] Furthermore, studies have demonstrated that PVK photoconduction increases when doped with sensitizers like 2,4,7-trinitrofluorenone (TNF) or C 60 . [15,16] In parallel, oligo-and poly(2,7-carbazole) derivatives have been successfully used in polymer lightemitting diodes (PLEDs) [17] and organic field-effect transistors (OFETs), [17,18] demonstrating good p-type transport properties. Recently, Müllen and co-workers [19] have reported solar cells with an efficiency of 0.6 % with poly(N-alkyl-2,7-carbazole), whereas Leclerc and co-workers [20] have shown an efficiency of 0.8 % with poly(2,7-carbazolenevinylene) derivatives. Moreover, in contrast with the fluorene unit the carbazole moiety is fully aromatic, providing a better chemical and environmental stability. Taking all of these results into account, the development of new low-bandgap copolymers based on carbazoles should therefore lead to interesting features for photovoltaic applications. However, poly(N-alkyl-2,7-carbazole)s generally exhibit poor solubilities and low molecular weights. [21] To solve these problems, bulky side chains are us...

Conjugated polymers combine the interesting optical and electrical properties of metals with the processing advantages and mechanical properties of traditional synthetic polymers. With clever use of a variety of synthetic tools, researchers have prepared highly pure polymers with optimized physical properties during the past 30 years. For example, the synthesis of well-defined polyacetylenes, polyphenylenes, polythiophenes, polyfluorenes, and other conjugated polymers have significantly improved the performance of these polymeric materials. However, one important class of conjugated polymers was missing from this chemical inventory: easy access to well-defined poly(2,7-carbazole)s and related polymers. This Account highlights advances in the synthesis of poly(2,7-carbazole) derivatives since they were first reported in 2001. Starting from 2-nitro-biphenyl derivatives, 2,7-functionalized carbazoles are typically obtained from Cadogan ring-closure reactions. In a second step, Yamamoto, Stille, Suzuki, or Horner-Emmons coupling polymerization leads to various poly(2,7-carbazole) derivatives. We discuss the characterization of their optical and electrical properties with a strong emphasis on the structure-property relationships. In addition, we carefully evaluate these polymers as active components in light-emitting diodes, transistors, and photovoltaic cells. In particular, several low band gap poly(2,7-carbazole) derivatives have revealed highly promising features for solar cell applications with hole mobilities of about 3 x 10(-3) cm2 V(-1) s(-1) and power conversion efficiencies up to 4.8%. Finally, we show how these new synthetic strategies have led to the preparation of novel poly(heterofluorene) derivatives and ladder-type conjugated polymers.

The technology behind a large area array of flexible solar cells with a unique design and semitransparent blue appearance is presented. These modules are implemented in a solar tree installation at the German pavilion in the EXPO2015 in Milan/IT. The modules show power conversion efficiencies of 4.5% and are produced exclusively using standard printing techniques for large‐scale production.

We have studied the utilization of PCDTBT, an alternating poly(2,7-carbazole) derivative, in organic bulk heterojunction solar cells. The effect of polymer molecular weight, PCDTBT:[60]PCBM ratio, and active layer thickness on the device performance is reported. The best performance was obtained when the number-average molecular weights (M n ) are around 20 kDa with a polydispersity index around 2.2. Both PCDTBT:[60]PCBM ratio and active layer thickness affect the light absorption and the charge transport properties. By optimizing these two parameters, power conversion efficiency (PCE) up to 4.35% was reached under calibrated AM1.5G illumination of 100 mW cm À2 . When blended with [70]PCBM, PCDTBT exhibited a PCE up to 4.6%.

The synthesis, characterization, and field-effect transistor (FET) properties of new indolo[3,2-b]carbazoles are described. In particular, an extensive characterization of their crystal structures has revealed the importance of the nature of the side chains (alkyl, phenyl, thienyl substituents) on their solid-state organization. These organic materials have exhibited p-type FET behavior with hole mobilities as high as 0.2 cm2 V(-1) s(-1) with an on/off current ratio higher than 10(6). Best results were obtained with phenyl-substituted indolo[3,2-b]carbazoles since the presence of phenyl substituents seems to allow efficient overlap between the oligomeric molecules. More importantly, FET properties were kept constant during several months in air.

A series of alternating poly(2,7-carbazole) derivatives have been synthesized. The evaluation of their thermoelectric properties in doped films revealed high electrical conductivity (up to 500 S/cm) and a relatively high Seebeck coefficient (up to 70 μV/K). The best compromise between these two thermoelectric parameters led to a maximum value of 19 μW m−1 K−2 as the power factor. As observed from X-ray analyses, it has been observed that the high electrical conductivity was obtained with structured polymers. Good air stability was also observed with these thermoelectric polymers.

While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...