Self-assembled and cross-linked fullerene interlayer on titanium oxide for highly efficient inverted polymer solar cells

Yen-Ju Cheng*, Fong Yi Cao, Wei Cheng Lin, Chiu Hsiang Chen, Chao Hsiang Hsieh

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

120 Scopus citations


We have successfully designed and synthesized two oxetane-functionalized fullerene derivatives, [6,6]-phenyl-C61-butyric oxetane ester (PCBO) and [6,6]-phenyl-C61-butyric oxetane dendron ester (PCBOD). We demonstrated that the oxetane functionality with neutral nature can anchor onto the TiOx surface via cationic ring-opening reaction under thermal and UV treatment, as evidenced by contact angle measurement and X-ray photoelectron spectroscopy. The self-assembly of PCBO, functionalized with one oxetane group, on the TiOx surface forms an adhesive monolayer with intimate contact. Inverted bulk-heterojunction device B [ITO/TiOx/SA-PCBO/ P3HT:PCBM (1:1 w/w)/MoO3/Ag, where ITO is indium tin oxide, SA is self-assembled, P3HT is poly(3-hexylthiophene), and PCBM is [6,6]-phenyl-C 61-butyric acid methyl ester] with this self-assembled PCBO (SA-PCBO) modifier showed an impressive power conversion efficiency (PCE) of 4.1%, which outperforms the reference device A (PCE = 3.6%) without this monolayer [ITO/TiOx/P3HT:PCBM (1:1 w/w)/MoO3/Ag]. This SA-PCBO modifier exerts multipositive effects on the interface, including improvement of exciton dissociation efficiency, reduction of charge recombination, decrease of the interface contact resistance, and passivation of the surface electron traps at the interface of TiOx. Furthermore, PCBOD, containing two oxetane groups, is capable of self-assembling on the TiOx surface and simultaneously undergoing cross-linking, generating a dense, robust, and pinhole-free multimolecular interlayer to further strengthen the interface characteristics. Device C [ITO/TiOx/C-PCBOD/P3HT:PCBM(1:1, in wt%)/MoO3/Ag] incorporating this cross-linked PCBOD (C-PCBOD) interlayer delivered the highest PCE of 4.5% which represents 26% enhancement over device A. This simple and easy strategy smartly integrates the advantages of self-assembly and cross-linking in a single fullerene-based molecule, showing promise in producing highly efficient inverted PSCs.

Original languageEnglish
Pages (from-to)1512-1518
Number of pages7
JournalChemistry of Materials
Issue number6
StatePublished - 22 Mar 2011


  • cross-linking
  • inverted solar cells
  • oxetane
  • self-assembled monolayer
  • titanium oxide


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