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Contact‐Induced Mechanisms in Organic Photovoltaics: A Steady‐State and Transient Study
Author(s) -
Cowan Sarah R.,
Li Jian V.,
Olson Dana C.,
Ratcliff Erin L.
Publication year - 2015
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201400549
Subject(s) - materials science , organic solar cell , photocurrent , optoelectronics , charge carrier , work function , indium tin oxide , heterojunction , carrier lifetime , polymer solar cell , acceptor , photoactive layer , thin film , chemical physics , energy conversion efficiency , nanotechnology , layer (electronics) , condensed matter physics , composite material , polymer , silicon , physics
The role of the contacts in thin‐film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole‐collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open‐circuit voltages ( V OC ) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting V OC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.