Using a newly developed device model we have studied the effect of controlled thermal annealing on charge transport
and photogeneration in bulk-heterojunction solar cells (BHJ) made from blend films of regioregular poly(3-
hexylthiophene) (P3HT) and methanofullerene (PCBM). With respect to the charge transport, we demonstrate that the
hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases more than three
orders of magnitude, to reach a value up to ≈2×10-8 m2/Vs after the annealing process, as a result of an improved
crystallinity of the film. Slow drying leads to an additional 33-fold enhancement of the hole mobility even up to 5.0×10-
7 m2V-1s-1, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge
accumulation enables the use of thick films (~300 nm), absorbing most of the incoming photons, without losses in the
fill factor and short-circuit current of the device.
As a next step we performed model calculations to exploit the potential of polymer/fullerene bulk
heterojunction solar cells. Lowering the polymeric band will lead to a device efficiency exceeding 6%. Tuning the
electronic levels of PCBM in such a way that less energy is lost in the electron transfer process enhances the
efficiency to values in excess of 8%. Ultimately, with an optimized level tuning, band gap and balanced mobilities
polymeric solar cells can reach power conversion efficiencies approaching 11%.
The open-circuit voltage (Voc) of polymer/fullerene bulk heterojunction solar cells is investigated as a function of light intensity
for different temperatures. The observed photogenerated current and V oc are at variance with classical p-n junctionbased
models. The influence of light intensity and recombination strength on V oc is consistently explained by a model
based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of
charge carriers. The light intensity dependence of the short-circuit current density (J sc) is also addressed. A typical feature
of polymer/fullerene based solar cells is that Jsc does not scale exactly linearly with light intensity (I). Instead, a power
law relationship is found given by Jsc∝ Iα, where α ranges from 0.9 to 1. In a number of reports this deviation from unity
is attributed to the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in
bulk heterojunction solar cells is governed by the build-up of space charge in the device. The occurrence of space-charge
stems from the difference in charge carrier mobility of electrons and holes. In blends of poly(3-hexylthiophene) and 6,6-
phenyl C61-butyric acid methyl ester this mobility difference can be tuned in between one and three orders of magnitude,
depending on the annealing conditions. This allows us to experimentally verify the relation between space charge build-up
and intensity dependence of Jsc. Model calculations confirm that bimolecular recombination leads only to a typical loss
of 1% of all free charge carriers at Jsc for these devices. Therefore, bimolecular recombination plays only a minor role as
compared to the effect of space charge in the intensity dependence of J sc.
Bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3TH) and methanofullerene (PCBM) exhibit a ten-fold increase of the power conversion efficiency upon post production annealing. We have applied a numerical model to explain this dramatic enhancement of the efficiency. The mobilities of electrons and holes were selectively determined, and it was found that the hole mobility of P3HT in the blend increases by more than three orders of magnitude. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with the solar emission, which result in an increase of more than 60% in the generation rate of charge carriers. Our numerical model demonstrates that the unannealed devices suffer from the buildup of space-charge, as a consequence of the low hole mobility. Furthermore, at short-circuit the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, which explains the large quantum efficiencies measured in P3HT:PCBM blends.
One of the most studied systems in the field of organic photovoltaic devices are bulk heterojunction solar cells based on poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and methano[60] fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as electron acceptor. A striking feature of these types of solar cells is that at the optimal device thickness of typically 100 nm only 60% of the incident light is absorbed. It is evident that the absorption can be enhanced by increasing the thickness of the active layer. However, in spite of an increased absorption the overall power conversion efficiency shows a negative trend when increasing the device thickness beyond 100nm. From device point of view the reduced performance with increasing thickness mainly originates from a decrease of the fill factor. We demonstrate that this fill factor decrease is a result of the formation of space-charges in thick devices and charge recombination.
We have studied the photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. Two cases have been investigated: When only drift of charge carriers is taken into account, a voltage-independent photocurrent is expected, corresponding to the extraction of all generated charges. It is demonstrated that the experimental data are in disagreement with this prediction. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behavior: At low electric fields a linear behavior is predicted, which results from the diffusion of charges, followed by saturation at high fields. The agreement between the numerical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a charge carrier generation rate of G=1.6 × 1027 m-3s-1 is used, showing the importance of diffusion at low fields, i.e., near the open-circuit voltage.
KEYWORDS: Electrodes, Electron transport, Solar cells, Fullerenes, Silver, Heterojunctions, Temperature metrology, Organic photovoltaics, Polymers, Metals
We have measured the electron and hole mobility in blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) with varying MDMO-PPV/PCBM composition. It is shown that the electron mobility in the PCBM-rich phase gradually increases up to 80 wt.% PCBM, due to an increased number of percolated pathways from bottom to top electrode. In contrast to the expectations the hole mobility in the MDMO-PPV phase shows a similar behavior as a function of fullerene concentration; Starting at 40 wt.% with the value of pristine MDMO-PPV the hole mobility strongly increases and saturates beyond 67 wt.% at a value which is more than two order of magnitude higher. The large enhancement of the hole mobility and its saturation is related to recent findings on the film morphology of this material system.
Tuning the work functions of metals was demonstrated by chemically modifying the metal surface through the formation of chemisorbed self-assembled monolayers (SAMs) derived from 1H,1H,2H,2H-perfluorinated alkanethiols and hexadecanethiol. The ordering inherent in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Ag (ΦAg ~4.4 eV) to 5.5 eV (ΔΦ ~ 1.1 eV) for 1H,1H,2H,2H-perfluorinated alkanethiols. Hexadecanethiol on the other hand shifted ΦAg toward 3.8 eV (ΔΦ ~ 0.6 eV) and raised the energy barrier for hole injection. These SAMs on Au were less efficient. 1H,1H,2H,2H-perfluorodecanethiol raised ΦAu (4.9 eV) by 0.5 eV to 5.4 eV, whereas hexadecanethiol decreased ΦAu by only 0.1 eV. These chemically modified electrodes were applied in the fabrication of pLEDs and the hole conduction of MEH-PPV was investigated. An ohmic contact for hole injection between a silver electrode functionalized with the perfluorinated SAMs, and MEH-PPV with a HOMO of 5.2 eV was established. Conversely, a silver electrode modified with a SAM of hexadecanethiol lowered ΦAg to 3.8 eV, creating an efficient energy barrier for hole injection. This method demonstrates a simple and attractive approach to modify and improve metal/organic contacts in organic electronic devices like LEDs and photovoltaic cells.
Two models describing charge extraction from insulators have been used to interpret the experimental photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3`,7`-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. When only drift of charge carriers is taken into account, a square root dependence on voltage of the photocurrent is expected, governed by the difference between electron and hole mobility. It is demonstrated that both the magnitude and functional dependence of the predicted current are in disagreement with experimental data. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behaviour: At low electric fields a linear behaviour is predicted, which results from the diffusion of charges, folllowed by saturation at high fields. The agreement between the theoretical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a generation rate of G=1.46 × 1027 electron-hole pairs/m3s is used, showing the importance of diffusion at low fields, i.e., near the open-circuit voltage.
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