Bis(tri-n-hexylsilyl oxide) Silicon Phthalocyanine: A Unique Additive in Ternary Bulk Heterojunction Organic Photovoltaic Devices

Abstract

Previous studies have shown that the use of bis(tri-n-hexylsilyl oxide) silicon phthalocyanine ((3HS)2-SiPc) as an additive in a P3HT:PC61BM cascade ternary bulk heterojunction organic photovoltaic (BHJ OPV) device results in an increase in the short circuit current (J(SC)) and efficiency (η(eff)) of up to 25% and 20%, respectively. The previous studies have attributed the increase in performance to the presence of (3HS)2-SiPc at the BHJ interface. In this study, we explored the molecular characteristics of (3HS)2-SiPc which makes it so effective in increasing the OPV device J(SC) and η(eff. Initially, we synthesized phthalocyanine-based additives using different core elements such as germanium and boron instead of silicon, each having similar frontier orbital energies compared to (3HS)2-SiPc and tested their effect on BHJ OPV device performance. We observed that addition of bis(tri-n-hexylsilyl oxide) germanium phthalocyanine ((3HS)2-GePc) or tri-n-hexylsilyl oxide boron subphthalocyanine (3HS-BsubPc) resulted in a nonstatistically significant increase in JSC and η(eff). Secondly, we kept the silicon phthalocyanine core and substituted the tri-n-hexylsilyl solubilizing groups with pentadecyl phenoxy groups and tested the resulting dye in a BHJ OPV. While an increase in JSC and η(eff) was observed at low (PDP)2-SiPc loadings, the increase was not as significant as (3HS)2-SiPc; therefore, (3HS)2-SiPc is a unique additive. During our study, we observed that (3HS)2-SiPc had an extraordinary tendency to crystallize compared to the other compounds in this study and our general experience. On the basis of this observation, we have offered a hypothesis that when (3HS)2-SiPc migrates to the P3HT:PC61BM interface the reason for its unique performance is not solely due to its frontier orbital energies but also might be due to a high driving force for crystallization.

Recent Advances in Bulk Heterojunction Polymer Solar Cells

“…Lessard and co-workers demonstrated the effects of heteroatom substitution and side chain modification by synthesizing a series of different phthalocyanine additives to improve the performance of P3HT:PCBM PSCs. 334 Despite the similar HOMO and LUMO energy levels, replacing silicon with boron and germanium leads to decreased device performance compared to the silicon version. In addition, they substituted tri-nhexylsilyl solubilizing groups with pentadecyl phenoxy groups.…”

Status and prospects for ternary organic photovoltaics

“…Similarly, replacing the silicon (Si) in SiPcs with boron (B) and germanium (Ge) resulted in decreased PCE despite the similar electronic properties (such as the HOMO and LUMO energy levels between SiPc, BPc or GePc), mainly due to the unfavourable locations of BPc or GePc in the blend. The unique behaviour of SiPc was ascribed to its high crystallinity, which might help to localize the dyes at the interface 50 . Introducing tert-butyl groups around the periphery of SiNc allowed for a much higher volume fraction (20 wt%) of the dye molecule to be incorporated before PCE started to drop 51 .…”

Boron Subphthalocyanines and Silicon Phthalocyanines for Use as Active Materials in Organic Photovoltaics

“…In 2014, Lessard et al . synthesized a series of phthalocyanine materials chemically similar to (3HS) 2 ‐SiPc to determine if they would perform in a similar manner, namely bis(3‐pentadecylphenoxy) silicon phthalocyanine ((PDP) 2 ‐SiPc) and bis(tri‐n‐hexylsilyl oxide) germanium phthalocyanine (3HS) 2 ‐GePc . Despite having high solubility and similar optoelectrical properties to (3HS) 2 ‐SiPc, both materials were detrimental to the efficiency of P3HT/PC 61 BM devices as ternary additives.…”