Effect of regioregularity on recombination dynamics in inverted bulk heterojunction organic solar cells

Naresh Chandrasekaran, Amelia C. Y. Liu, Anil Kumar, Christopher R. McNeill, Dinesh Kabra

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The effect of polymer regioregularity on the charge transport properties and bimolecular recombination rates of polymer-based solar cells is studied in detail using transient photovoltaic techniques. We compare organic solar cells fabricated with an ITO/ZnO/PEIE/P3HT:PCBM/MoO3/Ag structure using either 100% regioregular poly(3-hexylthiophene) (DF-P3HT) yielding an average power conversion efficiency (PCE) of 3.8 ± 0.3% or 92% regioregular P3HT (rr-P3HT) that yields an average PCE of 3.28 ± 0.4%. Transient photocurrent measurements reveal the presence of less mobile photoinduced charges in rr-P3HT:PCBM cells when compared to DF-P3HT:PCBM solar cells. Transient photovoltage measurements are used to establish the relationship between regioregularity and bimolecular recombination rate constant (k) finding that under 1 Sun, devices with high regioregularity have a longer τ despite having a higher k. The high value of k for the DF-P3HT:PCBM system as compared to the rr-P3HT:PCBM system is attributed to enhanced mobility and better charge transport of mobile charges in the DF-P3HT:PCBM system, consistent with enhanced fibrillar order in DF-P3HT films observed with transmission electron microscopy. We also note a slight decrease in cell open circuit voltage with increase in polymer regioregularity, which is due to the increase in k. Other recombination mechanisms such as trap-assisted recombination are found to be important in the lower regioregular P3HT device compounded by the reduced mobility and poor inter-chain ordering.

Original languageEnglish
Article number015501
Number of pages10
JournalJournal of Physics D: Applied Physics
Issue number1
Publication statusPublished - 5 Dec 2017


  • bimolecular recombination
  • defects
  • organic solar cells
  • transient photovoltage

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