Depletion-Layer Photoeffects in Semiconductors

Gärtner, Wolfgang W.

doi:10.1103/physrev.116.84

Cited by 1,000 publications

(466 citation statements)

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“…In principle, for solar cells that benefit from a sufficiently short dielectric relaxation time, there are two main mechanisms of charge extraction 14 . One is charge separation followed by drift through a depletion layer that is formed at a contact-semiconductor interface or a p-n junction.…”

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confidence: 99%

“…The successful operation of CQDs in an efficient solar cell is strongly dependent on the material-processing strategies used in film formation. When CQD are synthesized in solution, they are usually capped by organic molecules that employ long chains (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) carbons) to avoid agglomeration. Optimum photovoltaic operation of the CQD film requires the replacement of such ligands with short organic bidentate linkers, such as ethanedithiol or mercaptopropionic acid (MPA).…”

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confidence: 99%

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Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

et al. 2013

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Colloidal quantum dot solar cells achieve spectrally selective optical absorption in a thin layer of solution-processed, size-effect tuned, nanoparticles. The best devices built to date have relied heavily on drift-based transport due to the action of an electric field in a depletion region that extends throughout the thickness of the quantum dot layer. Here we study for the first time the behaviour of the best-performing class of colloidal quantum dot films in the absence of an electric field, by screening using an electrolyte. We find that the action of selective contacts on photovoltage sign and amplitude can be retained, implying that the contacts operate by kinetic preferences of charge transfer for either electrons or holes. We develop a theoretical model to explain these experimental findings. The work is the first to present a switch in the photovoltage in colloidal quantum dot solar cells by purposefully formed selective contacts, opening the way to new strategies in the engineering of colloidal quantum dot solar cells.

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Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

et al. 2013

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“…The electron-hole pair can still recombine and reduce the process efficiency 11 . On the other hand, this recombination could be avoided by means of the n-and p-type semiconductors coupling as ZnO and CuO respectively, generating a depletion zone in the interface, becoming more difficulty the charge return to the ground state 12 . Zinc oxide is non-stoichiometric (Zn 1+δ O) n-type semiconductor with the electrons excess that creates a donor band and favors the electron jump to the conduction band 13 .…”

Section: Introductionmentioning

confidence: 99%

PREPARATION AND CHARACTERIZATION OF ZnO/CuO SEMICONDUCTOR AND PHOTOCATALYTIC ACTIVITY ON THE DECOLORIZATION OF DIRECT RED 80 AZODYE

Silva

Takashima

2015

J. Chil. Chem. Soc.

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Zinc oxide, an n-type semiconductor, has been used as photocatalyst for contaminated water purification by organic compounds. The addition of a p-type semiconductor as copper oxide could increase the catalytic efficiency through the p-n junction in order to decrease the electron-hole recombination. In this work, zinc and copper oxides containing 1.0, 3.0, and 5.0 % of copper were prepared by manual grinding, followed by heat treatment at 300ºC during 5 h. The zinc oxide and copper oxide mixture as well as their precursors were characterized by x-ray diffraction, diffuse reflectance, scanning electron microscopy, and x-ray fluorescence. The photocatalytic efficiency was verified by the direct red 80 tetraazodye decolorization using UV radiation under pseudo-first order conditions at natural pH (7.5-8.0) and 30 °C. The highest photocatalytic activity occurred to the semiconductor containing 1.0 % copper oxide, whose decolorization rate constant was equivalent to 6.40×10 -2 min -1 , that is, 17.4 % larger in comparison to ZnO, 5.45×10 -2 min -1 .

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“…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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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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Depletion-Layer Photoeffects in Semiconductors

Cited by 1,000 publications

References 2 publications

Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

PREPARATION AND CHARACTERIZATION OF ZnO/CuO SEMICONDUCTOR AND PHOTOCATALYTIC ACTIVITY ON THE DECOLORIZATION OF DIRECT RED 80 AZODYE

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

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