Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy

García-Belmonte, Germà; Munar, A.; Barea, Eva M.; Bisquert, Juan; Ugarte, Irati; Pacios, Roberto

doi:10.1016/j.orgel.2008.06.007

Cited by 542 publications

(404 citation statements)

References 25 publications

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“…We utilised capacitance-voltage measurements in the dark to determine the degree to which a depletion region is present in the active layer. Capacitance-voltage measurements are an established technique used to determine the doping density of a semiconductor 6,19,29,38,[41][42][43] . The width of the space-charge region is probed by a variation of the DC voltage and is detected by measuring the capacitance of the device as a function of the DC voltage.…”

Section: Resultsmentioning

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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Section: Resultsmentioning

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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“…(2) should be taken with caution because the series resistance potential drop is only a useful approximation of a rather complex picture concerning the Fermi level distribution across the active layer thickness [17]. In general terms transport mechanisms are coupled with charge storage and recombination (represented by  C and rec R ) in a transmission line circuit model [9], instead of the circuit comprising lumped elements as in Fig. 3(c).…”

Section: Resultsmentioning

confidence: 99%

“…It has been also proposed that the effect of mechanisms in series is observable at high frequencies through more featured impedance spectra, which were interpreted in connection with electronic transport processes [9]. Very recently impedance analysis has been employed by others groups identifying a RC parallel subcircuit governing the high frequency response as a combination of bulk transport resistance and geometrical capacitance [10,11].…”

Section: Usually Smentioning

confidence: 99%

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

Guerrero

Ripolles

Boix

et al. 2012

Organic Electronics

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Series resistance is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms arising from different structures within the solar cell can contribute to increasing it. We focus on the series resistance origin by altering the acceptor transport properties trough the incorporation of fullerene traps located at energies below the transporting electron levels. Indene-C 60 bisadduct as acceptor molecule blended with poly(3-hexylthiophene) forms the active layer in which small amounts of [6,6]-phenyl-C 61 -butyric acid methyl ester have been added as trapping sites. A complete analysis of the impedance response has allowed identifying bulk transport resistive circuit elements in the high-frequency part of the spectra. Series resistance is observed to be dependent on the concentration of fullerene traps, thus indicating a connection between bulk transport processes and resistive elements. By comparing different contacts it has been discarded that outer cathode interfaces influence the series resistance experimentally extracted from impedance spectroscopy.

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“…It is well known that P3HT is a conjugated polymer that in exposure to oxygen [3], or moisture [4] results p-doped exhibiting relatively high levels of free carries ( 17 15 10 10  cm -3 ) [5]. Under such conditions, it has been suggested that the P3HT-metal contact forms a Schottky barrier behavior: band bending with a corresponding majority-carrier depletion zone made up at the cathode contact [6,7], due to the presence of acceptor defects (see Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

Kinetics of occupancy of defect states in poly(3-hexylthiophene):fullerene solar cells

et al. 2012

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Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy

Cited by 542 publications

References 25 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

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

Kinetics of occupancy of defect states in poly(3-hexylthiophene):fullerene solar cells

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