Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
Journal article, 2012

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (ED*) and the energy of the charge transfer state (ECT) will optimize the open-circuit voltage (Voc). In this work, this energy loss ED*-ECT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV1.5 eV), the optimized photovoltaic devices show large differences in Voc and internal quantum efficiency (IQE). For P3TI:PC71BM blends a ED*-ECT of similar to 0.1 eV, a Voc of 0.7 V and an IQE of 87% are found. For PTI-1:PC61BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher Voc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between ECT and qVoc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC61BM material system, Voc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (<0.1 eV) and a high Voc.

transfer state

charge transfer state

design

conjugated polymer

absorption

rules

conversion efficiency

polymer

charge-transfer excitons

fullerene

donor

organic solar cell

blend films

photogeneration

recombination

Author

K. Vandewal

Z. F. Ma

J. Bergqvist

Z. Tang

Ergang Wang

Chalmers, Chemical and Biological Engineering, Polymer Technology

Patrik Henriksson

Chalmers, Chemical and Biological Engineering, Polymer Technology

K. Tvingstedt

Mats Andersson

Chalmers, Chemical and Biological Engineering, Polymer Technology

Fengling Zhang

Olle Inganäs

Advanced Functional Materials

1616-301X (ISSN)

Vol. 22 16 3480-3490

Subject Categories

Chemical Sciences

DOI

10.1002/adfm.201200608

More information

Created

10/6/2017