Elucidating Batch-to-Batch Variation Caused by Homocoupled Side Products in Solution-Processable Organic Solar Cells

Abstract

Conjugated polymers and small molecules based on alternating electron donating (D) and electron accepting (A) building blocks have led to state-of-the-art organic solar cell materials governing efficiencies beyond 10%. Unfortunately, the connection of D and A building blocks via cross-coupling reactions does not always proceed as planned, which can result in the generation of side products containing D-D or A-A homocoupling motifs. Previous studies have reported a reduced performance in polymer and small molecule solar cells when such defect structures are present. A general consensus on the impact of homocouplings on device performance is, however, still lacking, as well as a profound understanding of the underlying causes of the device deterioration. To differentiate the combined effect of molecular weight and homocouplings in polymer solar cells, a systematic study on a small molecule system (DTS(FBBTh2)2) is presented here. The impact of homocouplings on the blend nanomorphology, thermal, and electrooptical properties is investigated. It is demonstrated that small quantities of homocouplings (<10%) already lead to sub-optimal device performance, as this strongly impacts the molecular packing and electronic properties of the photoactive layer. These results highlight the importance of material purity and pinpoint homocoupling defects as one of the most probable reasons for batch-to-batch variations.

Distinction between PTB7-Th samples prepared from Pd(PPh3)4 and Pd2(dba)3/P(o-tol)3 catalysed stille coupling polymerization and the resultant photovoltaic performance

“…Ideally, this synthetic approach yields perfectly alternating copolymers because the sp 2 carbonhalide bond can only react with an organotin bond (sp 2 C-Sn bond) and vice versa. 37 However, the alternation of D and A units does not always proceed as intended due to the existence of homocoupling side reactions when a specic palladium catalyst is employed, 38,39 let alone in the case of different catalysts. Moreover, the molecular weights and PDIs of the polymers with the same alternating D and A units could most likely be inconsistent as well with the utilization of different catalysts in preparation.…”

Phase Behavior in the Active Layer of Small Molecule Organic Photovoltaics: State Diagram of p-DTS(FBTTh2)2:PC71BM

“…These observations are in agreement with previous findings. 28 The as-cast as well as the slowly cooled mixtures display cold crystallization when DIO is used, even though the effect is somewhat less pronounced in the second heating. This is in contrast to the mixtures without DIO, where cold crystallization is absent after slow cooling at 1 K.min -1 (see 3.3.).…”

“…27 p-DTS(FBTTh2)2 has recently also been the subject of a study regarding the influence of homocoupling defects on photovoltaic performance and eutectic behaviour. 28,29 While the crystalline p-DTS(FBTTh2)2 phase is well studied, information on the amorphous phase and its influence on crystallinity is lacking, and therefore also a systematic understanding of the phase behavior of p-DTS(FBTTh2)2:PC71BM mixtures of different composition. The determination of the glass transition temperature (Tg) of the active layer is important with respect to device stability.…”

The Impact of Acceptor–Acceptor Homocoupling on the Optoelectronic Properties and Photovoltaic Performance of PDTSQxff Low Bandgap Polymers

“…Even upon applying “nearly perfect” conditions, e.g., an oxygen‐free atmosphere and a proper Pd‐ligand stoichiometry, homocoupling between either two organotin/boron or two arylhalide units can occur . The importance and possible abundance of these misscoupled structural units for the performance of the organic solar cells has only recently been recognized and a few reports on this have appeared …”

“…PDTSQx ff has a high solubility in the common organic solvents, allowing an easy characterization, and it affords a moderate solar cell efficiency of ≈5.5%, which allows proper detection of fluctuations of the photovoltaic performance. PDTSQx ff can also easily be analyzed by MALDI‐TOF up to molecular weights of 20 kDa to enable proper determination of the homocoupling content …”