Posted on 21 Mar 10:38
Being the first small-molecule electron acceptor to enable improved power conversion efficiencies (PCEs) over the best-performing fullerene acceptor (PC70BM), ITIC represents an exciting new advancement for organic photovoltaics (OPVs). Zhao et al. reported this in inverted-architecture PBDB-T (PCE12) devices, which achieved PCEs of 7.45% when PC70BM was used as the electron acceptor, and a staggering 10.68% when ITIC was used instead .
There are several reasons why scientists are seeking to replace fullerenes as the electron acceptor in OPV devices. These relate to the energy levels, absorption characteristics, and stability of fullerenes. It is difficult to tune the energy levels of fullerenes to align with donor polymers, and this limits their potential efficiency within solar cells (due to energy loss from sub-optimal energy level alignment). Fullerenes also generally have poor absorption in the range of the solar spectrum, with peak absorption towards ultraviolet where the photon flux is low. In terms of stability, fullerenes tend to crystallise over time, leading to reduced lifetimes for solar cells that utilise them.
Small molecules have the potential to address these issues, and indeed, ITIC addresses them all, improving the performance of polymer solar cells via three main mechanisms.
Firstly, the absorption spectrum of ITIC is much more complementary to PBDB-T than that of PC70BM, with significantly less overlap of the two spectra. This enables increased absorption for the device, particularly at the longer wavelengths (>650 nm) where the solar photon flux is high.
Secondly, ITIC offers an improvement of the energy level alignment of the materials. The molecular energy levels of ITIC are much closer to those of PBDB-T than PC70BM, resulting in reduced energy loss during exciton dissociation and charge transfer. Finally, whilst the electron mobility of ITIC is lower than that of PC70BM, this results in more balanced electron and hole transport within PBDB-T:ITIC devices, preventing a build-up of charge within the solar cell.
Showing excellent thermal stability, ITIC also addresses the instability issues caused by using fullerenes. Devices showed no performance loss after 250 hours at 100 °C. This is significantly better than that of devices using PC70BM, which lost nearly 50% of their PCE under the same conditions. This is due to the tendency of PC70BM to aggregate at high temperatures, causing phase separation and a loss of the efficient bulk heterojunction – a property which is not shared by ITIC.
Please note that Ossila has no formal connection to any of the authors or institutions in these references.
- Fullerene-Free Polymer Solar Cells with over 11% Efﬁciency and Excellent Thermal Stability, W. Zhao et al., Adv. Mater., 28, 4734–4739 (2016); DOI: 10.1002/adma.201600281.