The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices

Abstract

How 1,8-diiodooctane (DIO) enhances performance of polymer solar cells based on polymer HXS-1 and fullerene [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM) from 3.6% to 5.4% is scrutinized with several techniques by comparing devices or blend films spin-coated from dichlorobenzene (DCB) to those from DCB/DIO (97.5:2.5 v/v). Morphology of blend films is examined with atomic force microscopy (AFM), transmission electron microscopy (TEM) and electron tomography (3-D TEM), respectively. Charge generation and recombination is studied with photoluminescence, and charge transport with field effect transistors. The morphology with domain size in 10-20 nm and vertical elongated clusters formed in DIO system is supposed to facilitate charge transport and minimize charge carrier recombination, which are the main reasons for enhancing power conversion efficiency (PCE) from 3.6% (without DIO) to 5.4% (with DIO). Furthermore, a two year inspection shows no significant impact of DIO on the shelf-stability of the solar cells. No visible degradation in the second year indicates that the morphology of the active layers in the devices is relatively stable after initial relaxation in the first year.

Morphology of organic electronic materials imaged via electron tomography

“…The coarsened structure also correlates with improved performance in solar cell devices. Larger currents and higher fill factors are seen in devices made with HXS‐1 and PC 71 BM (Qin et al , 2009) with DIO as additive, compared to devices made with the same materials in the same stoichiometry, but without solvent additive (Li et al , 2011), in agreement with earlier results (Lee et al , 2008). The improved efficiency is achieved in correlation with a coarsening of the nanostructure, which may facilitate charge transport and charge separation.…”

Role of the Side Chain in the Phase Segregation of Polymer:Fullerene Bulk Heterojunction Composites

“…[ 10,11,23 ] As described above, the introduction of the DIO additive prevents large-scale phase segregation in BHJ composites. [ 11,24 ] However, other mechanisms, such as the polymer chain packing effect [ 25 ] and slow growth effect, [ 26 ] cannot be ruled out at this stage. Although the origin of this morphological change remains uncertain at this point, we speculate that the PC 71 BM molecules in the DIO additive can penetrate the polymer tangles during the fi lm casting process due to the relatively high boiling point of DIO (>332 °C) relative to that (>131 °C) of the chlorobenzene host solvent.…”

Fine-tuning of polymer photovoltaic properties by the length of alkyl side chains

“…Nanoscale morphology of different polymers/PCBM is critical to understand the interaction between polymers and PCBM as well as their solar cell performance, because both the charge generation and recombination depend strongly on the morphology of active layer [32] . The morphology was investigated by atomic force microscopy (AFM) using tapping mode.…”