Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends

Abstract

A bulk-heterojunction (BHJ) structure of organic semiconductor blend is widely used in photon-to-electron converting devices such as organic photodetectors (OPD) and photovoltaics (OPV). However, the impact of the molecular structure on the interfacial electronic states and optoelectronic properties of the constituent organic semiconductors is still unclear, limiting further development of these devices for commercialization. Herein, the critical role of donor molecular structure on OPD performance is identified in highly intermixed BHJ blends containing a small-molecule donor and C60 acceptor. Blending introduces a twisted structure in the donor molecule and a strong coupling between donor and acceptor molecules. This results in ultrafast exciton separation (<1 ps), producing bound (binding energy ∼135 meV), localized (∼0.9 nm), and highly emissive interfacial charge transfer (CT) states. These interfacial CT states undergo efficient dissociation under an applied electric field, leading to highly efficient OPDs in reverse bias but poor OPVs. Further structural twisting and molecular-scale aggregation of the donor molecules occur in blends upon thermal annealing just above the transition temperature of 150 °C at which donor molecules start to reorganize themselves without any apparent macroscopic phase-segregation. These subtle structural changes lead to significant improvements in charge transport and OPD performance, yielding ultralow dark currents (∼10–10 A cm–2), 2-fold faster charge extraction (in μs), and nearly an order of magnitude increase in effective carrier mobility. Our results provide molecular insights into high-performance OPDs by identifying the role of subtle molecular structural changes on device performance and highlight key differences in the design of BHJ blends for OPD and OPV devices.

Light-intensity-dependent photoresponse time of organic photodetectors and its molecular origin

“…1b ). Both devices demonstrate a similar level of field-dependent responsivity, assigned to bias-dependent CT dissociation, similar to that previously reported for analogous IDDSe based OPDs 15 . Supplementary Figure 2 shows the photocurrent collection as a function of light intensity at different bias conditions.…”

“… 10 , while full details about MPTA synthesis are included in the Materials section of the Supplementary Methods . Both donors are comprised of electron-donating and electron-withdrawing units (D-A structure) based on fused heterocycle groups, yielding a compact structure suitable for the formation of highly-intermixed interpenetrated networks upon co-evaporation with C 60 10 , 11 , 15 , 16 . For NP-SA, the donor unit is constituted by a tertiary amine coordinating benzene and naphthalene units to the selenophene molecular core, while three fused rings form its acceptor unit.…”

“…Furthermore, molecular-structure induced intermolecular packing, orientation, and domain formation can also contribute to the energetic disorder and charge trapping. Even for sublimed OPDs, known to exhibit better device performance and reproducibility than solution-processed devices, the molecular origins determining these energetic disorder and charge trapping, and hence the OPD photoresponse times, remain largely unresolved 10 , 11 , 15 , 16 .…”

Identifying the Molecular Origins of Green BN‐TADF Material Degradation and Device Stability via in situ Raman Spectroscopy

“…To further explore the molecular origin of stability, these materials were illuminated at 633 nm excitation for 0.5 h via in situ Raman (see detail in Figure 5 and Supporting Information, Figure S16). In situ Raman spectroscopy is a superior technique to track both vibrational and luminescence changes [15b,c,16d] . As electronic excitations and molecular vibrations are strongly coupled in organic semiconductors, by simultaneously measuring the luminescence and vibrational transitions of the materials, the conformational change information and emission property of the organic semiconductors can be easily obtained [15b,21] .…”

Wavelength‐Selective Organic Photodetectors

“…[ 41 ] Further insights into the BHJ of NPIDDSe and C 60 can be found in the recent work by Limbu et al. [ 93 ] Lee and co‐workers compared this molecule with the naphthalenedione analog (NPBIDDTSe) and it was shown to perform better upon optimization of the ratio of C 60 (FWHM = 97 nm, D * = 4.37 × 10 13 Jones). [ 45 ] The influence of the donor moiety in these types of structures was investigated by Lim et al [ 43 ] A comparison was made between a non‐fused diphenylamine donor (DP) and the ring‐closed analogs with S (PS) or Se (PSe), both close to the cyanine limit (Table 1).…”