Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
Artikel i vetenskaplig tidskrift, 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

Författare

K. Vandewal

Z. F. Ma

J. Bergqvist

Z. Tang

Ergang Wang

Chalmers, Kemi- och bioteknik, Polymerteknologi

Patrik Henriksson

Chalmers, Kemi- och bioteknik, Polymerteknologi

K. Tvingstedt

Mats Andersson

Chalmers, Kemi- och bioteknik, Polymerteknologi

Fengling Zhang

Olle Inganäs

Advanced Functional Materials

1616-301X (ISSN)

Vol. 22 3480-3490

Ämneskategorier

Kemi

DOI

10.1002/adfm.201200608