Molecular Intercalation and Cohesion of Organic Bulk Heterojunction Photovoltaic Devices

Abstract

The phase separated bulk heterojunction (BHJ) layer in BHJ polymer:fullerene organic photovoltaic devices (OPV) are mechanically weak with low values of cohesion. Improved cohesion is important for OPV device thermomechanical reliability. BHJ devices are investigated and how fullerene intercalation within the active layer affects cohesive properties in the BHJ is shown. The intercalation of fullerenes between the side chains of the polymers poly(3,3″′‐didocecyl quaterthiophene) (PQT‐12) and poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (pBTTT) is shown to enhance BHJ layer cohesion. Cohesion values range from ≈1 to 5 J m−2, depending on the polymer:fullerene blend, processing conditions, and composition. Devices with non‐intercalated BHJ layers are found to have significantly reduced values of cohesion. The resulting device power conversion efficiencies (PCE) are also investigated and correlated with the device cohesion.

Structure and design of polymers for durable, stretchable organic electronics

“…If the PQT (7) was loaded at 1:4 ratio of PQT (7):PCBM (5), charge transport pathways were formed and PCE was dramatically improved (0.28% at 1:1, 1.63% at 1:4). 23 Likewise for PBTTT (6) (0.25% at 1:1, 2.51% at 1:4). 89 The downside is that adding significant amounts of PCBM (5) to the blend results in a highly brittle BHJ blend, regardless of the polymer it is blended with, resulting in decreased cohesion across every polymer.…”

“…[87][88][89] This co-crystallized material has an increased stiffness as compared with a non-intercalated blend. 23 This effect can be investigated by comparing a series of thiophenes with increasing spacing, from P3AT (2a) to PT2T (4), PBTTT (6) and PQT (7). P3HT does not allow for intercalation due to the narrow spacing between alkyl side chains, although the addition of PCBM still embrittles the blend as previously mentioned.…”

“…In the thin-film industry, it has been shown that improved mechanical properties are linked to higher production yield during mass production. 23 Flexibility relates to the ability of a material to be bent, such as rolling up a poster, whereas stretchability relates to the ability to pull on a material, such as pulling on a rubber band. This distinction is described pictorially in Figure 2.…”

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…[39] were validated by both comparing neat P3HT and poly(3,4-dihexyl-2,2'-bithiophene) PDHBT morphology against experiments ( Figure 1) and by comparing P3HT:PCBM and poly(2,2':5',2"-3,3"-dihexylterthiophene) (PTTT):PCBM mixture morphology against experimental and theoretical predictions from Ref. [60]. In the former case, lamellar spacing and chain spacing of P3HT and the hexagonal packing of PDHBT was facilitated with simulated diffraction analysis.…”

Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends

“…The current OPV literature remains predominantly concerned with the design of new molecules and polymers with well-defined electronic, optical, and redox properties as well as investigations of the intrinsic electronic processes underlying photocurrent generation. Although there is increasing interest in the mechanical properties of BHJ active layers, [1][2][3][4][5][6][7][8][9] and such studies are common for traditional polymers, [10][11] the efforts in the OPV arena remain limited when compared to performance efficiency studies. [12][13][14][15][16] Along with the electronic processes involved in photocurrent generation, the active-layer mechanical properties inherently depend on the multiphase morphology of the BHJ thin films [17][18][19][20][21][22] and therefore are contingent on the thin-film processing protocols and the nature and strength of non-covalent, solid-state intermolecular interactions.…”