Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells

Abstract

Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties, and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and FF, and therefore the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic bandgaps and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, with the conversion of ITIC to IT-4F reduces structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger p-p stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection towards anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encourage the intermixing of donor/acceptor for efficient charge generation.

Impact of Acceptor Quadrupole Moment on Charge Generation and Recombination in Blends of IDT‐Based Non‐Fullerene Acceptors with PCE10 as Donor Polymer

“…More generally, end group engineering is a common approach, that can lead to improved charge transport, as already highlighted in earlier reports. [7][8][9][10][11] Moreover, end-group modification such as fluorination/chlorination has led to highly-efficient systems due to enhanced electronic coupling between neighboring groups and concomitant suppression of charge recombination. [1,3,[12][13][14] Here, we focus on two NFAs, O-IDTBR, a rhodanineendcapped indacenodithiophene (IDT), [15] and O-IDTBCN, a structural analogue, in which the rhodanine end-groups have been replaced by dicyano-vinylene moieties, in other words an indacenodithiophene donor core, flanked on either side by electron-deficient benzothiadiazole (BT) and dicyanovinyl (DCV) moieties.…”

Mitigating the Undesirable Chemical Reaction between Organic Molecules for Highly Efficient Flexible Organic Photovoltaics

“…The recent reported highest power conversion efficiency (PCE) is 18.22% based on single‐junction NFA. [ 8 , 9 , 10 ] Howbeit, the development of the NF solar cells on flexible substrates is lagging compared to the rigid substrates. The high‐temperature processed and rigid cathode interfacial layers (CILs) retard the development of the NF flexible OPVs.…”

Matching electron transport layers with a non-halogenated and low synthetic complexity polymer:fullerene blend for efficient outdoor and indoor organic photovoltaics

“…Ideality factors close to unity indicate that bimolecular recombination is dominant in open-circuit conditions, with very limited monomolecular and trap-assisted recombination. 49 Similarly, by fitting J sc to a power law given by J sc ∝ Φ α we detect the occurrence of bimolecular recombination and space-charge in the device under short-circuit conditions. 50 In particular, α values approaching unity indicate the absence of bimolecular recombination, so that monomolecular recombination is the major recombination process limiting the performance.…”