Acceptors In Bulk Heterojunction Solar Cells Research Articles

Donor-fullerene dyads for energy cascade organic solar cells

Organic bulk heterojunction (BHJ) solar cells require broad absorption of the incident solar spectrum by the donor and acceptor in the blend. Fullerene derivatives with covalently linked dyes, referred to as dyads, were designed to have strong optical absorption and appropriate electronic levels for electron transfer with common donor polymers in BHJs. Dyads with dyes based on diketopyrrolopyrrole and benzothiadiazole were synthesized using either a methanofullerene or a fulleropyrrolidine linkage. The performance of these dyad acceptors in BHJ solar cells with poly(3-hexylthiophene) (P3HT) and a low optical gap co-polymer of thiophene and diketopyrrolopyrrole (PDPP2FT) were examined. Although the solar power conversion efficiencies were low, charge generation from the molecular dye in the dyad could be observed in BHJs with PDPP2FT. The low power conversion efficiency was attributed to the morphology of the BHJs based on grazing incidence wide angle X-ray scattering patterns. Density functional theory was used to examine the charge transfer states between the donor and the dyad. The lowest energy charge-transfer state was found to be a transition from the donor polymer to the fullerene portion of the dyad.

N-Heteroacenes as a New Class of Non-Fullerene Electron Acceptors for Organic Bulk-Heterojunction Photovoltaic Devices

Herein, we present the first investigation of N-heteroacenes as acceptors in bulk-heterojunction solar cells. The optical and electronic properties of tetraazapentacene (TIPS-TAP), triptycenyl-tetraazapentacene (TIPS-TAP-1T), and bistriptycenyl-tetraazapentacene (TIPS-TAP-2T) compounds are characterized by UV-vis, photothermal deflection, and ultraviolet photoemission spectroscopies. We compare the photovoltaic performance of the N-heteroacenes and find that cells with TIPS-TAP-2T significantly outperform the other derivatives, achieving a power conversion efficiency of 2.5% without extensive optimization or processing additives. We characterize the morphology and order within the active layer by atomic force microscopy and grazing incidence wide-angle scattering measurements, and find that blends with TIPS-TAP result in a gross phase separation driven by its strong crystallization. The substitution with triptycenyl units suppresses this crystallization resulting in amorphous films with a finer intermixing and a smooth surface structure. Finally, we investigate the photophysics of charge separation at the donor/acceptor interface and find that it is fundamentally different from the “conventional” polymer-fullerene systems. In blends with the tetraazapentacene derivatives, exciton dissociation is relatively slow and charge separation is strongly field dependent. We observe improved charge generation and significantly reduced recombination for TIPS-TAP-2T as compared to the other derivatives, which in combination with the improved film microstructure is responsible for the enhanced photovoltaic performance.

Perylenediimides as non-fullerene acceptors in bulk-heterojunction solar cells (BHJSCs)

Perylenediimides are ideal candidates for the substitution of fullerene derivatives as electron acceptors in bulk heterojunction organic solar cells due to their extremely intense light absorbance, high electron mobility and excellent photochemical stability.

N-Type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells.

Knowledge of the critical factors that determine compatibility, blend morphology, and performance of bulk heterojunction (BHJ) solar cells composed of an electron-accepting polymer and an electron-donating polymer remains limited. To test the idea that bulk crystallinity is such a critical factor, we have designed a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. Pairing of the reference crystalline (crystalline domain size Lc = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (Lc = 9.15 nm) benzodithiophene-thieno[3,4-b]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends, whose BHJ solar cells have a power conversion efficiency (PCE) of 1.4%. However, pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm(2), external quantum efficiency = 91%). These photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the best so far for all-polymer solar cells, while the short-circuit current (Jsc) and external quantum efficiency are even higher than reported values for [70]-fullerene:PBDTTT-CT solar cells. The morphology and bulk carrier mobilities of the polymer/polymer blends varied substantially with crystallinity of the acceptor polymer component and thus with the NDI/PDI copolymer composition. These results demonstrate that the crystallinity of a polymer component and thus compatibility, blend morphology, and efficiency of polymer/polymer blend solar cells can be controlled by molecular design.

Fluorene-centered perylene monoimides as potential non-fullerene acceptor in organic solar cells

A fluorene-centered perylene monoimide dimer, PMI-F-PMI with a partly non-coplanar configuration has been developed as a potential non-fullerene acceptor for organic solar cells (OSCs). The optimum power conversion efficiency (PCE) of the OSC based on PMI-F-PMI as acceptor and poly (3-hexyl thiophene) (P3HT) as donor is up to 2.30% after annealing at 150°C. The PCE of 2.30% is the highest value for the OSCs based on P3HT donor and non-fullerene acceptor lies in that PMI-F-PMI’s lowest unoccupied molecular orbital (LUMO) level around −3.50eV matches well with the donor P3HT to produce higher open-circuit voltage (Voc) of 0.98V. Meanwhile, PMI-F-PMI makes remarkable contribution to devices’ light absorption as the maximum EQE (30%) of the devices is at 512nm, same to the maximum absorption wavelength of PMI-F-PMI. The other favorable characteristics of PMI-F-PMI in bulk heterojunction (BHJ) active layers is proved through the photo current density measures, the relatively balanced electron–hole transport, and the smooth morphology with root mean square (RMS) value of 1.86nm. For these advantages, PMI-F-PMI overwhelms its sister PMI-F and parent PMI as an acceptor in BHJ solar cells.

All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor

The lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer bulk heterojunction (BHJ) solar cells. Here, we report an evaluation of three naphthalene diimide (NDI) copolymers as electron acceptors in BHJ solar cells which finds that all-polymer solar cells based on an NDI-selenophene copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT) donor exhibit a record 3.3% power conversion efficiency. The observed short circuit current density of 7.78 mA/cm(2) and external quantum efficiency of 47% are also the best such photovoltaic parameters seen in all-polymer solar cells so far. This efficiency is comparable to the performance of similarly evaluated [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM)/PSEHTT devices. The lamellar crystalline morphology of PNDIS-HD, leading to balanced electron and hole transport in the polymer/polymer blend solar cells accounts for its good photovoltaic properties.