Origin of Hole Transport in Small Molecule Dilute Donor Solar Cells

Abstract

Dilute donor organic solar cells (OSCs) are a promising technology to circumvent the trade‐off between open‐circuit voltage (Voc) and short‐circuit current density (Jsc). The origin of hole transport in OSCs with donor concentrations below the percolation threshold is diversely discussed in the community. Herein, both hole back transfer and long‐range hopping (tunneling) are analyzed as possible mechanisms of photocurrent in small molecule dilute donor OSCs using kinetic Monte Carlo (kMC) simulations. In contrast to previous kMC studies, the driving force for exciton dissociation is accounted for. As a study system, nitrogen‐bridged terthiophene (NBTT) molecules in a [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC71BM) matrix are investigated. The simulations show that hole back transfer from the small molecule donor to the fullerene matrix explains the measured concentration dependences of the photocurrents as well as the Jsc dependence on the light intensity for donor concentrations below 5 wt%. For 5 wt%, distances between NBTT molecules decrease to reasonable ranges that long‐range hopping or tunneling cannot be discounted. Compared with polymer donors, larger hole localization is observed. The results emphasize that the barrier for hole back transfer is not only due to the highest occupied molecular orbital (HOMO) offset, but also by hole localization.

Re-Examining Open-Circuit Voltage in Dilute-Donor Organic Photovoltaics

“…Nitrogen-bridged terthiophene small molecules (NBTT-IDD and NBTT-Rho), , poly­(4,8-bis­(5-(2-ethylhexyl)­furan-2-yl)­benzo­[1,2- b :4,5- b 0]­difuran-alt-2,5-didodecyl-3,6-di­(furan-2-yl)­pyrrolo­[3,4- c ]­pyrrole-1,4­(2 H ,5 H )-dione) (PBDF-FDPP), and poly­(2,5-bis­(2-decyltetradecyl)-3,6-di­(thiophen-2-yl)­pyrrolo­[3,4- c ]­pyrrole-1,4­(2 H ,5 H )-dione- b -( E )-1,2-di­(furan-2-yl)­ethene) (PThDPP-FVF) polymers were synthesized according to published literatures. Poly­(3-hexylthiophene) (P3HT) was purchased from Rieke Metals, and poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl-3-fluoro)­thiophen-2-yl)-benzo­[1,2- b :4,5- b ′]­dithiophene))- alt -(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′- c :4′,5′- c ′]­dithiophene-4,8-dione)] (PM6), Y6, and PC 71 BM were purchased from Lumtec.…”

“…Materials. Nitrogen-bridged terthiophene small molecules (NBTT-IDD and NBTT-Rho), 34,35 poly(4,8-bis(5- 36 2.2. Ultraviolet−Visible Absorption Spectroscopy.…”

Role of the Interface and Extraction Layer Energetics in Organic Solar Cells

“…All calculations were performed using our kMC model, which has been used successfully to analyze the role of permittivity and energetics on the charge separation efficiency and dynamics, tuning the exciton dynamics using phosphorescent sensitizers, the role of interface energetics on the open-circuit voltage, and the origin of charge transport and the importance of the morphology in dilute donor organic solar cells. ,, Here, we briefly summarize the rate models used in the kMC simulations and the system setup. Details on the Monte Carlo algorithm based on the Gillespie method can be found elsewhere. , …”

Investigating the dielectric properties and exciton diffusion in C70 derivatives

“…We chose to study C 70 derivatives as they absorb more strongly in the visible range of the spectrum compared to C 60 due to the reduced symmetry of the higher fullerene which relaxes the symmetryforbidden nature of the electronic transitions. Furthermore, C 70 derivatives have been investigated as the main component in dilute-donor solar cells where they exhibit ambipolar chargetransport behaviour 9,10 and also remain important components of high efficiency ternary blend solar cells. 11 Although the dielectric properties of OSCs are a significant research topic, much remains to be understood about the effect of e r on the photophysics and optoelectronic properties of OSCs in the solid state.…”