P-type doping of organic wide band gap materials by transition metal oxides: A case-study on Molybdenum trioxide

Kröger, Michael; Hamwi, Sami; Meyer, Jens; Riedl, Thomas; Kowalsky, Wolfgang; Kahn, Antoine

doi:10.1016/j.orgel.2009.05.007

Cited by 405 publications

(322 citation statements)

References 41 publications

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“…Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .…”

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confidence: 99%

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Dibb

Muth

Kirchartz

et al. 2013

Sci Rep

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While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping [12][13][14][15][16][17][18][19] and discussed the consequences for device behaviour 6,[20][21][22][23][24][25][26][27][28][29][30] . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8,[31][32][33] despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...

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mentioning

confidence: 99%

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Dibb

Muth

Kirchartz

et al. 2013

Sci Rep

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“…25 The working mechanism of MoO 3 as HIL is generally ascribed to the formation of an interfacial dipole and consequently a reduction of the injection barrier. 26,27 More specifically, Kröger et al 28 reported MoO 3 to be a n-type material with a high electron affinity ͑EA͒ of 6.7 eV. The hole injection of MoO 3 into the semiconductor was described to proceed by electron extraction from the HOMO of the semiconductor trough the conduction band of MoO 3 .…”

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Space-charge-limited hole current in poly(9,9-dioctylfluorene) diodes

Nicolai

Wetzelaer

Kuik

et al. 2010

Applied Physics Letters

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Characterization of the hole transport in blue-emitting polymers as poly(9,9-dioctylfluorene) (PFO) is strongly hindered by their large ionization potential of ∼6 eV. Using common anodes as poly(3,4-ethylenedioxythiophene)/poly(styrenesulphonic acid) leads to a strongly injection limited current. We demonstrate that molybdenum trioxide forms an Ohmic hole contact on PFO, enabling the observation of a space-charge-limited current(SCLC). This allows a direct determination of the hole mobility PFO of 1.3×10−9 m2/V s at room temperature, in good agreement with previously reported mobility values determined by time-of-flight measurements.

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“…Thus, significant values for the ionization efficiencies of electrons of about 10% were observed, though they were lower than the case of Si. In the case of MoO 3 , a doping efficiency of 3% was obtained under the assumption that MoO 3 forms the trimer (Mo 3 O 9 ) [53]. Though a similar mechanism to that in Figure 20b can also be applied in this case, in addition, the formation of larger MoO x clusters lowers the efficiency of CT-complex formation, which seems to lower the total doping efficiency.…”

Section: Band Mapping By Kelvin Probementioning

confidence: 80%

Bandgap Science for Organic Solar Cells

et al. 2014

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Abstract:The concept of bandgap science of organic semiconductor films for use in photovoltaic cells, namely, high-purification, pn-control by doping, and design of the built-in potential based on precisely-evaluated doping parameters, is summarized. The principle characteristics of organic solar cells, namely, the exciton, donor (D)/acceptor (A) sensitization, and p-i-n cells containing co-deposited and D/A molecular blended i-interlayers, are explained. 'Seven-nines' (7N) purification, together with phase-separation/cystallization induced by co-evaporant 3 rd molecules allowed us to fabricate 5.3% efficient cells based on 1 µm-thick fullerene:phthalocyanine (C 60 :H 2 Pc) co-deposited films.

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P-type doping of organic wide band gap materials by transition metal oxides: A case-study on Molybdenum trioxide

Cited by 405 publications

References 41 publications

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells

Space-charge-limited hole current in poly(9,9-dioctylfluorene) diodes

Bandgap Science for Organic Solar Cells

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