Synthesis and characterization of polythiophenes functionalized with electron poor moieties for application in organic electronics

Harvesting the unlimited and renewable energy from sunlight to produce electricity is one of the major scientific and technological challenges of the 21st century. Among the available techniques, photovoltaic solar cells (PVCs) are very attractive because they can convert solar energy directly into electricity in a reasonable and economic appealing efficiency. The development of PVCs is therefore an attractive alternative to address global environmental issues. However, the current high cost for the devices based on inorganic semiconductors has limited their widespread application. Organic solar cells offer a compelling option for tomorrow’s PV devices, since they can be easily prepared using low-cost and efficient roll-to-roll manufacturing processes. During the last decade, Organic Photovoltaic (OPV) research has progressed remarkably both in terms of new materials and device performances. Particular interest have been devoted to bulk heterojunction (BHJ) devices in which the active layer consists of a blend of DONOR (p-type semiconductor) and ACCEPTOR (n-type semiconductor) materials. The active layer can be easily deposed through put techniques, facilitating the formation of large area, light weight, and potentially flexible devices. Fullerenes C60, C70 and in particular their soluble derivates (PCBM) are at the moment the most popular ACCEPTORS and only marginal research is devoted to the development of viable substitutes. On the other hand, the research has been focused on conjugated polymers, as DONOR material, due to their tunable properties by a structural design and the possibility to produce them at low costs. Over the past decade, research has focused on regioregular poly(3-hexylthiophene) (P3HT) as the standard electron-donating material in polymer BHJ solar cells, with important progresses having been made in understanding the device science and the associated improvements in device efficiency. However, P3HT is not the ideal polymer as it has a relatively large band gap (1.85 eV, and this means that it is not able to harvest the maximum of exploitable solar radiation) and its high-lying highest occupied molecular orbital (HOMO) (-5.1 eV) limits the open circuit voltage (Voc) of P3HT/PC61BM devices to 0.6 V, consequently limiting the efficiency to about 5%. To overcome these limitations, low band gap materials with broad absorption spectra, to enhance sunlight harvest for higher short circuit current (Jsc); appropriately lower HOMO energy levels, to maximize the open circuit voltage (Voc); higher hole mobilities, for higher Jsc; and higher fill factor (FF), have been proven to be an efficient strategy to improve device performance. Typically, a low band gap polymer is designed, via donor-acceptor (D-A) approach, by incorporating both electron-rich and electron-deficient moieties in the same conjugated backbone. Among a wide variety of donor material, new low band gap polymers based on thiophene, as the donor unit, and iso-DPP (iso-diketopyrrolo-pyrrole) or maleimide, as the acceptor moieties, were designed. These latter electron-withdrawing units combine a low HOMO level and a rigid planar core that permit π-conjugation length and charge transfer into the polymer backbone. The polymers and the molecules obtained by Stille condensations were characterized into the device as DONOR material or third compound to blend with the classical mix P3HT/PCBM. More into the detail, the Chapter 3 reports on the synthesis of a novel electron-deficient derivative, 1,4-dibutyl-3,6-di(thiophene-2-yl)pyrrolo[3,2-b]pyrrole-2,5-dione (iso-DPP). This new building block was copolymerized with bistannanes of thiophene and bithiophene by Stille polycondensation, affording the corresponding polymers (PDPPT and PDPPTT, respectively). These compounds exhibit small energy band gaps combined with low-lying HOMO energy levels. Energy band gaps of PPDPT and PPDPTT, calculated from absorption spectra, are 1.63 and 1.73 eV, respectively. The HOMO and LUMO energy levels of PDPPT and PDPPTT are -5.12, -3.50, -5.09 and -3.50 eV respectively, as determined by cyclic voltammetry. The power conversion efficiency of PDPPT:PC60BM-based photovoltaic cells illuminated by AM 1.5G was 1.24%, without optimization of materials, significantly higher than for PDPPTT:PC60BM, 0.33%. The results demonstrate that iso-DPP based polymers are promising materials for bulk heterojunction solar cell applications. A series of D-A polymers and oligomers based on N-alkyl-maleimide has been synthesized by a simple and efficient route explained in Chapter 4. The obtained low band-gap materials were applied into polymeric photovoltaic cells, to improve their efficiency by tuning their electronic properties. The introduction of small quantities (< 20% w/w) of polymers or oligomers containing N-alkyl-maleimide within active layer of P3HT/ PC61BM blends allowed to dramatically increase the efficiencies of BHJ solar cells (up to 80% of increase). This beneficial effect is attributed to improved charge photogeneration and transport. Poor photovoltaic results were obtained if the maleimide based polymer was employed alone as DONOR material, blended with PC61BM. In order to obtain good results in terms of device performances, not only a good chemical design of the DONOR polymer must be achieved, but also other parameters at the molecular and supramolecular levels should be carefully controlled. The full potential of any conjugated polymer for solar cells can only be realized with an optimized morphology. For this purpose the synthesis of random and diblock copolymers of poly(3-alkylthiophene)s bearing polar substituents was successfully developed by GRIM polymerization and the details are reported in Chapter 5. 3-Hexyl-thiophene was successfully copolymerised with a new derivative, 3-functionalised-thiophene (propyl 5-(2-(thiophen-3-yl)ethoxy) pentanoate), bearing an ester function. Under optimized conditions, this ester proved to be fully compatible with the Grignard metathesis polymerization. Saponification of the copolymer esters provided the corresponding polyacids. Photovoltaic properties of copolymers were investigated in bulk heterojunction devices with PC61BM as acceptor. Among all the amphiphilic copolymers, P3HT-b-P3AcidHT showed the best performance with a PCE of 4.2%, an open-circuit voltage (Voc) of 0.60 V, a short-circuit current density (Jsc) of 13.0 mAcm-2, and a fill factor (FF) of 0.60. All conjugated D-A molecule and polymers were characterized by chemical investigation and their optical, electrochemical, morphological and photovoltaic properties were investigated.