Organic solar cells have gained a large research interest over the last years as a potential alternative to silicon cells. However, organic solar cells still do not provide efficiencies and long-term stabilities high enough for commercialization. Some of the major unsolved problems in organic solar cells arise from their instability regarding different stress factors like heat or light exposure. Bulk heterojunction solar cells contain a mixture of donor and acceptor material. They often suffer from diffusion and subsequent aggregation of the low molecular weight acceptor upon device operation over a long time or at elevated temperatures. Hence, the device performance is decreased. The formation of multilayer solar cells is mainly restricted to the evaporation of low molar mass materials. Polymers are difficult to use in multilayer cells as they are solution processed. By spin coating a second polymer solution on top of the first polymer, the underlying layer is often dissolved or damaged. This is also the reason for the collapse of nanoimprinted patterns when applying the second active material.
Regarding these challenges, research on degradation mechanisms and device fabrication concerning a prolonged stability is of major importance. The dissolution problem can be avoided using orthogonal solvents for the different active materials. Furthermore, inorganic interlayers can be inserted which are not soluble in organic solvents. Tandem solar cells are a prominent example for that method. A versatile possibility to solve the problems associated with device stability is the application of crosslinkable materials. Such conjugated polymers bearing crosslinkable groups can be processed from solution. Upon crosslinking started by an initiator, exposure to UV light or heat, covalent bonds are formed between the polymer chains and the initial morphology is frozen. This results in a densely crosslinked polymer network which is insoluble. Using crosslinkable materials, three concepts for the stabilization of organic solar cells can be realized. First, a blend containing a crosslinkable donor polymer can be used for bulk heterojunction solar cells. Thus, the crosslinking of the donor prevents the diffusion and aggregation of the low molecular weight acceptor and the solar cell performance is retained.
The formation of multilayer devices from solution is a second aspect. Processing and subsequent crosslinking of a polymer results in an insoluble layer which allows the spin coating of a second polymer solution on top without dissolving the underlying layer. Nanoimprinted structures can also be stabilized. Therefore, a donor polymer is deposited and patterned by means of a stamp.
Crosslinking turns the structure totally insoluble and an acceptor can be spin coated or vacuum evaporated without damaging the pattern.
Furthermore, investigations of intrinsic mechanisms like charge carrier generation and recombination are essential for the basic understanding of the behaviour of different donor and acceptor materials in organic solar cells. For this purpose, polymers with properties that allow these specific studies are needed.
In this work, the synthesis of novel low bandgap polymers is described. These polymers are used in both device fabrication and fundamental studies. Chemical modifications of the low bandgap
63 polymers PCDTBT and PCPDTBT are realized. The chemical structures of the two polymers are shown in Figure 38.
Figure 38: Chemical structures of PCDTBT (left) and PCPDTBT (right).
The idea behind was not to invent totally new donor materials for organic solar cells, but to use well-known low bandgap polymers and modify them with respect to different properties taking advantage of the existing knowledge about PCDTBT and PCPDTBT.
The chemical modifications done on the polymers can be divided into two parts. On the one hand, crosslinkable derivatives of PCDTBT and PCPDTBT are synthesized. Oxetane is chosen as the crosslinking unit and is attached to the side chains of the donor units of the low bandgap polymers. This includes the synthesis of linear and branched aliphatic spacers bearing an oxetane unit as well as attaching the crosslinkable spacers to the donor cores carbazole and cyclopentadithiophene. The alternating copolymers are synthesized via palladium-catalysed Suzuki polycondensations. Additionally, the corresponding non-crosslinkable reference materials are also synthesized. On the other hand, comonomers are incorporated into the basic polymer structure of PCDTBT. Triphenyldiamine is selected as a comonomer due to its good hole transport characteristics. Therefore, a triphenyldiamine donor unit is prepared and polymerized in combination with the PCDTBT monomers via palladium-catalysed Suzuki couplings. In this work, these polymers are referred to as “low bandgap copolymers” to distinguish them from the polymers with only one donor and acceptor unit, which are known as low bandgap polymers but are actually also copolymers. Furthermore, the acceptor monomer is applied with and without aliphatic spacer. By means of this approach, a series of copolymers with varying properties is obtained. The corresponding reference polymers without the additional comonomer are synthesized as well.
The polymers are characterized regarding their chemical, thermal, optical, and electronic properties. Detailed analyses are performed with respect to the different polymer modifications. For the crosslinkable low bandgap polymers, the main focus lies on the crosslinking procedure. The mechanism behind the crosslinking of oxetane, as for other cyclic ethers, is a cationic ring-opening polymerization with high reaction rate. The influence of the crosslinking process and conditions on the polymer properties is examined. In the case of the low bandgap copolymers, the influence of the additional triphenyldiamine units as well as the spacers located at the acceptor monomers should be investigated in comparison to the reference polymers. This includes primarily the variation of the thermal properties due to the incorporation of the bulky comonomer as well as the solubilizing aliphatic spacers. Furthermore,
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the effect of the additional comonomer on the electronic properties of the polymers is examined by mobility measurements.
A possibility for the fabrication of multilayer devices from solution is the application of crosslinkable polymers. Thus, one aim of the thesis was to prepare a three-layer organic solar cell by making use of the insolubility obtained by crosslinking. This enables spin-coating of a second material on top.
Planar heterojunction solar cells are used as model systems for fundamental research, aiming at the detailed understanding of the processes at the donor acceptor interface. The low bandgap copolymers prepared in this work are used in basic studies concerning photogeneration and charge carrier recombination. Concerning prolonged device stability, the investigation of the diffusion of small acceptor molecules within the donor polymer is an important subject. A further aim of this work is the examination of the diffusion behaviour of fullerene in combination with different low bandgap polymers and a novel copolymer.
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