Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency.

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss

Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss.

Donor-acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g-E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state

Limits for Recombination in a Low Energy Loss Organic Heterojunction

Donor–acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (V$_{OC}$) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PC$_{61}$BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10$^{-2}$ cm$^{2}$ V$^{-1}$ s$^{-1}$), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the V$_{OC}$ is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.SMM, RHF, MKR, SAA, and JLB acknowledge support from the KAUST Competitive Research Grant Program. MKR, SAA, and JLB also acknowledge generous support of their work by KAUST and the Office of Naval Research Global (Award N62909­15­1­2003); they thank the KAUST IT Research Computing Team and Supercomputing Laboratory for providing computational and storage resources. NAR, MW, TQN, and GCB acknowledge support from the Department of the Navy, Office of Naval Research (Award Nos. N00014-14-1-0580 and N00014-16-1-25200. AS would like to acknowledge the funding and support from the India-UK APEX project. HLS acknowledges support from the Winton Programme for the Physics of Sustainability. MN and HS gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council though a Programme Grant (EP/M005141/1)

Fullerene derivative induced morphology of bulk heterojunction blends: PIPCP:PC61BM.

The performance of organic solar cells (OSCs) depends crucially on the morphology in bulk heterojunctions (BHJs), including the degree of crystallinity of the polymer and the amount of each material phase: aggregated donor, aggregated acceptor, and molecular mixed donor : acceptor phase. In this paper, we report the BHJ morphology of as-cast blend films incorporating the polymer PIPCP as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor. Tracking the scattering intensity of PC61BM as a function of PC61BM concentration shows that PC61BM aggregates into donor-rich domains and there is little to no phase where the PC61BM and PIPCP are intimately mixed. We further find that on blending the scattering peak due to PIPCP ordering along the backbone increases with decreasing PIPCP fraction, which is attributed to improved ordering of PIPCP due to the presence of PC61BM. Our results suggest that the improved ordering of PIPCP along the backbone (consistent with an increased conjugation length) with blending contributes to the observed low open-circuit voltage energy loss

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency.

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss

Enhancing Organic Semiconductor–Surface Plasmon Polariton Coupling with Molecular Orientation

Due to strong electric field enhancements, surface plasmon polaritons (SPPs) are capable of drastically increasing light-molecule coupling in organic optoelectronic devices. The electric field enhancement, however, is anisotropic, offering maximal functional benefits if molecules are oriented perpendicular to the interface. To provide a clear demonstration of this orientation dependence, we study SPP dispersion and SPP-mediated photoluminescence at a model Au/small-molecule interface where identical molecules can be deposited with two very different molecular backbone orientations depending on processing conditions. First, we demonstrate that thin films of <i>p</i>-SIDT­(FBTTh₂)₂ can be deposited with either all “in-plane” (parallel to substrate) or a 50/50 mix of in-plane/“out-of-plane” (perpendicular to substrate) optical transition dipoles by the absence or presence, respectively, of diiodooctane during spin-coating. In contrast to typical orientation control observed in organic thin films, for this particular molecule, this corresponds to films with conjugated backbones purely in-plane, or with a 50/50 mix of in-plane/out-of-plane backbones. Then, using momentum-resolved reflectometry and momentum-resolved photoluminescence, we study and quantify changes in SPP dispersion and photoluminescence intensity arising solely from changes in molecular orientation. We demonstrate increased SPP momentum and a 2-fold enhancement in photoluminescence for systems with out-of-plane oriented transition dipoles. These results agree well with theory and have direct implications for the design and analysis of organic optoelectronic devices