TY - JOUR
T1 - Bipolar charge transport in PCPDTBT-PCBM bulk-heterojunctions for photovoltaic applications
AU - Morana, Mauro
AU - Wegscheider, Matthias
AU - Bonanni, Alberta
AU - Kopidakis, Nikos
AU - Shaheen, Sean
AU - Scharber, Markus
AU - Zhu, Zhengguo
AU - Waller, David
AU - Gaudiana, Russell
AU - Brabec, Christoph
PY - 2008/6/24
Y1 - 2008/6/24
N2 - An experimental study of the transport properties of a low-bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (Eg-opt∼ 1-35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in-plane hole mobility, up to 1.5· × 10-2 cm2 V -1S-1 and quantify the electron mobility at 3 × ·10-5 cm2 V-1 s-1 on a SiO2 dielectric. In addition, singular contact behavior results in bipolar quasi-Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X-ray investigations revealed a degree of interchain π-stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]-PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]-PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8-octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]-PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]-PCBM clusters. Ionized and isolated [C60]-PCBM molecules can modify the local electric field in the solar cell by build-up of a space-charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical-field dependence of the hole transport.
AB - An experimental study of the transport properties of a low-bandgap conjugated polymer giving high photovoltaic quantum efficiencies in the near infrared spectral region (Eg-opt∼ 1-35 eV) is presented. Using a organic thin film transistor geometry, we demonstrate a relatively high in-plane hole mobility, up to 1.5· × 10-2 cm2 V -1S-1 and quantify the electron mobility at 3 × ·10-5 cm2 V-1 s-1 on a SiO2 dielectric. In addition, singular contact behavior results in bipolar quasi-Ohmic injection both from low and high workfunction metals like LiF/Al and Au. X-ray investigations revealed a degree of interchain π-stacking that is probably embedded in a disordered matrix. Disorder also manifests itself in a strong positive field dependence of the hole mobility from the electric field. In blends made with the electron acceptor methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM), the transistor characteristics suggest a relatively unfavorable intermixing of the two components for the application to photovoltaic devices. We attribute this to a too fine dispersion of [C60]-PCBM in the polymer matrix, that is also confirmed by the quenching of the photoluminescence signal measured in PCPDTBT [C60]-PCBM films with various composition. We show that a higher degree of phase separation can be induced during the film formation by using 1,8-octanedithiol (ODT), which leads to a more efficient electron percolation in the [C60]-PCBM. In addition, the experimental results, in combination with those of solar cells seem to support the correlation between the blend morphology and charge recombination. We tentatively propose that the drift length, and similarly the electrical fill factor, can be limited by the recombination of holes with electrons trapped on isolated [C60]-PCBM clusters. Ionized and isolated [C60]-PCBM molecules can modify the local electric field in the solar cell by build-up of a space-charge. The results also suggest that further improvements of the fill factor may also be limited by a strong electrical-field dependence of the hole transport.
UR - http://www.scopus.com/inward/record.url?scp=47349117184&partnerID=8YFLogxK
U2 - 10.1002/adfm.200701428
DO - 10.1002/adfm.200701428
M3 - Article
AN - SCOPUS:47349117184
SN - 1616-301X
VL - 18
SP - 1757
EP - 1766
JO - Advanced Functional Materials
JF - Advanced Functional Materials
IS - 12
ER -