Advanced Functional Polymers for Increasing the Stability of Organic Photovoltaics

Abstract

The development of new advanced polymers for improving the stability of OPV is reviewed. Two main degradation pathways for the OPV active layer are identified: photochemically initiated reactions primarily starting in the side chains and morphological changes that degrade the important nanostructure. Chemical units can be introduced that impart an increased stability. Similarly, the morphological degradation of the optimal nanostructure can be reduced. Active polymers and blends with acceptor material are used to create nanoparticle links with controlled size. Most of these advanced polymers and processing methods have only been utilized in small‐scale devices prepared by standard techniques such as spin coating, but a few cases of roll‐to‐roll processed solar cells with heat‐cleaved side chains are discussed.

Procedures and Practices for Evaluating Thin‐Film Solar Cell Stability

“…[ 62b , 71,72 ] Crosslinking of the fullerene is also possible and in both cases thermal stability of the fi nal active layer is improved. [ 72,73 ] The metal and the transparent electrodes are also prone to degradation. Several materials within these two types of electrodes have been applied in OPVs: Ag, Al, Ca, Cu for the back metal electrode and indium and/or fl uorine-doped tin oxide for the transparent electrode.…”

Fused Thiophenes and Some Oligomers and Polymers Therefrom

“…Thus, reaction of thieno [3,4-b]thiophene-2-carbaldehyde (150) with heptadecafluorooctyl iodide in the presence of methyllithium to form 170 was followed by oxidation with MnO 2 giving ketone 171. Bromination with two equivalents of NBS furnished dibromide 172, and this was Stille cross-coupled with 173 in the presence of Pd 2 (dba) 3 to obtain the copolymer 174 [63].…”

“…Compounds with three annulated thiophene rings, known as dithienothiophenes (DTTs), have been utilized extensively to construct functional materials, such as p-type semiconductors for OFETs, owing to their planar, sulfur-rich, rigid, conjugated, and highly thermal-and photo-stable structures [3,[7][8][9]94]. Six isomers have been reported, namely dithieno [3,2- In 1971, the first synthesis of 242 used transmetallation of 3-bromothiophene (5) then reaction with the electrophilic sulfur source (PhSO 2 ) 2 S. The resulting 3, 3 0 -dithienyl sulfide (248) was α,α 0 -dilithiated and the carbon-carbon bond made by oxidation with CuCl 2 [95] …”

“…Thienothiophenes (TTs) are the simplest fused thiophenes, with four isomers, namely thieno [3,2-b]thiophene (1), thieno [3,4-b]thiophene (2), thieno [2,3-b]thiophene (3), and thieno [3,4-c]thiophene (4) [4,6,7,12,13]. Syntheses and polymerizations of all the TTs, except unstable thieno [3,4-c]thiophene (4), have been studied extensively.…”

“…Low conductivity in the off state, combined with high charge carrier mobility (>1 Â 10 À3 cm 2 V À1 s À1 ), is desirable for a good organic semiconducting material. Stability to oxidation is a necessary criterion as oxidation can diminish device performance [3]. Extending the π system increases π stacking, resulting in increased charge carrier mobility and decreased band gap, to which most investigations have been dedicated, to tune the band gap of organic conductive polymers [4][5][6].…”