Polythiophene:Perylene Diimide Solar Cells – the Impact of Alkyl‐Substitution on the Photovoltaic Performance

Abstract

The photovoltaic parameters, i.e., the short‐circuit current, open‐circuit voltage and device fill factor, of bulk heterojunction solar cells that use perylene diimide (PDI) derivatives as electron acceptors are often far below the theoretically expected values for reasons still not entirely understood. This article demonstrates that the photovoltaic characteristics of blend films of regioregular poly(3‐hexylthiophene) (rr‐P3HT) and PDI molecules are improved upon using a core‐alkylated PDI derivative instead of the often used N‐alkylated PDI molecules. A doubling of the power conversion efficiency of P3HT:PDI solar cells by using the core‐alkylated PDI derivative is observed leading to an unprecedented power conversion efficiency of 0.5% for a P3HT:PDI solar cell under AM1.5 solar illumination. Furthermore, the optical properties of the novel PDI derivative are compared to two standard exclusively N‐alkylated PDI derivatives by steady‐state and time‐resolved photoluminescence spectroscopy in solution and solid state. The experiments reveal that aggregation in the solid state determines the photophysics of all PDI derivatives. However, the emission energy and excited state lifetime of the aggregates are clearly influenced by the alkyl‐substitution pattern through its effect on the packing of the PDI molecules. X‐ray diffraction experiments before and after thermal annealing of PDI:polystyrene and PDI:P3HT blends reveal subtle differences in the packing characteristics of the different PDI derivatives and, problematically, that P3HT ordering is suppressed by all of the PDI derivatives.

Loss mechanisms in organic solar cells based on perylene diimide acceptors studied by time-resolved photoluminescence

“…The lower quenching efficiency of the PDI-1 blends can be explained in terms of an enhanced tendency of the PDI-1 molecules to aggregate, triggering the formation of more extended domains with higher diffusive losses. This is supported by the absorption curves of PDI-1 in PS and the PDI-1 blends (Figure 2), as they show a significant decrease of the 0-0 transition at 2.3 eV with respect to the 0-1 transition at 2.5 eV, which is typically observed in case of H-aggregates 12,24 . Furthermore, these stabilized intermolecular states are likely to act as traps for diffusive excitons.…”

“…Another reason for the absence of characteristic P3HT signatures could be substantial changes of the P3HT conformation in blends with PDI. In previous work, it has been pointed out that P3HT crystallization is strongly inhibited in P3HT:PDI-2 12 , as indicated by the absence of certain absorption bands around 2 eV in the blend spectra, which are assigned to P3HT aggregates. The absorption curves we measured for the P3HT blends corroborate these findings, also in case of P3HT:PDI-1 and P3HT:PDI-3.…”

“…For example, reduced aggregation has been demonstrated for a PDI derivative with core-alkylated side chains. 12 Furthermore, smaller domain sizes have been successfully realized with twisted dimers. [13][14][15][16][17][18] In this context also the rational selection of donor polymers and fine-tuning their properties have proven successful.…”

Solution processable graphene oxide hole transport layers and their application in P3HT:HHPER active layer based BHJSC

“…• C. Although there are higher PCE values reported with the use of bay-substituted PDI derivatives as n-type components of the active layer [11,15], this value is comparable with those of the P3HT:N-alkyl substituted PDI in the literature [12,15,39].…”

“…Although PCBM is widely used, because of its high cost and difficult synthetic requirements, alternative electron acceptor material investigations are carried out. Perylenediimides (PDIs) are considered as one of the potential material types due to their high absorption coefficients, high thermal stability, and relatively good electron acceptor properties [11][12][13][14][15]. In the first part of this study, a PDI derivative, i.e.…”