Critical light instability in CB/DIO processed PBDTTT-EFT:PC71BM organic photovoltaic devices

DOI: 10.1016/j.orgel.2015.12.024

Authors: Andrew J. Pearson (Cavendish Laboratory) , Paul E. Hopkinson (Cavendish Laboratory) , Elsa Couderc (Cavendish Laboratory) , Konrad Domanski (Cavendish Laboratory) , Mojtaba Abdi-jalebi (Cavendish Laboratory) , Neil C. Greenham (Cavendish Laboratory)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Organic Electronics , VOL 30 , PAGES 225 - 236

State: Published (Approved)
Published: March 2016

Abstract: Organic photovoltaic (OPV) devices often undergo ‘burn-in’ during the early stages of operation, this period describing the relatively rapid drop in power output before stabilising. For normal and inverted PBDTTT-EFT:PC71BM OPVs prepared according to current protocols, we identify a critical and severe light-induced burn-in phase that reduces power conversion efficiency by at least 60% after 24 hours simulated AM1.5 illumination. Such losses result primarily from a reduction in photocurrent, and for inverted devices we correlate this process in-situ with the simultaneous emergence of space-chare effects on the μs timescale. The effects of burn in are also found to reduce the lifetime of photogenerated charge carriers, as determine by in-situ transient photovoltage measurements. To identify the underlying mechanisms of this instability, a range of techniques are employed ex-situ to separate bulk- and electrode-specific degradation processes. We find that whilst the active layer nanostructure and kinetics of free charge generation remain unchanged, partial photobleaching (6% of film O.D.) of PBDTTT-EFT:PC71BM occurs alongside an increase in the ground state bleach decay time of PBDTTT-EFT. We hypothesise that this latter observation may reflect relaxation from excited states on PBDTTT-EFT that do not undergo dissociation into free charges. Owing to the poor lifetime of the reference PBDTTT-EFT:PC71BM OPVs, the fabrication protocol is modified to identify routes for stability enhancement in this initially promising solar cell blend.

Journal Keywords: PBDTTT-EFT; PC71BM; OPV; Lifetime; Solar; Instability

Subject Areas: Materials


Instruments: I07-Surface & interface diffraction