Long-range exciton diffusion in molecular non-fullerene acceptors

Abstract

The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.

Ground- and excited-state characteristics in photovoltaic polymer N2200

“…43,44 In conjugated polymers, exciton-exciton annihilation can be employed to estimate the diffusion length of singlet exciton. 26,44,[48][49][50][51] The singlet exciton dynamics can be described as 44 dnðtÞ dt ¼ ÀknðtÞ À gn 2 ðtÞ (1)…”

Selective Extraction of Nonfullerene Acceptors from Bulk-Heterojunction Layer in Organic Solar Cells for Detailed Analysis of Microstructure

“…For BHJ 0.20 -based OSCs, the photocurrent was much lower than the ideal case due to the large grains/pore sizes of donor-acceptor materials. This could originate in the larger grain/pore radius than the exciton diffusion length of NFAs (for EH-IDTBR: 10–20 nm, for ITIC: 20–30 nm) [ 32 ] and donor polymers [ 33 ]. However, this did not affect FF.…”

A History and Perspective of Non‐Fullerene Electron Acceptors for Organic Solar Cells

“…A long exciton lifetime, implying low k f,ex , reduces the requirement for fast interfacial charge transfer (to achieve k d,ex > k f,ex ) and that would reduce the requirement for high driving force. Exceptionally long exciton diffusion lengths of 20–40 nm (implying long lifetimes) have been reported for a range of NFAs [ 191 ] and long exciton lifetimes observed in NFA based blends. [ 170 ] The latter study also argues that exciton lifetime is a key determinant of charge separation efficiency in organic photovoltaics.…”