Charge separation and transport in conjugated-polymer-semiconductor-nanocrystal

Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity

N.C.Greenham,*Xiaogang Peng,and A.P.Alivisatos

Department of Chemistry,University of California,and Molecular Design Institute,Lawrence Berkeley National Laboratory,

Berkeley,California94720

?Received19June1996?

We study the processes of charge separation and transport in composite materials formed by mixing

cadmium selenide or cadmium sul?de nanocrystals with the conjugated polymer

poly?2-methoxy,5-?2?-ethyl?-hexyloxy-p-phenylenevinylene??MEH-PPV?.When the surface of the nanocrys-

tals is treated so as to remove the surface ligand,we?nd that the polymer photoluminescence is quenched,

consistent with rapid charge separation at the polymer/nanocrystal interface.Transmission electron microscopy

of these quantum-dot/conjugated-polymer composites shows clear evidence for phase segregation with length

scales in the range10–200nm,providing a large area of interface for charge separation to occur.Thin-?lm

photovoltaic devices using the composite materials show quantum ef?ciencies that are signi?cantly improved

over those for pure polymer devices,consistent with improved charge separation.At high concentrations of

nanocrystals,where both the nanocrystal and polymer components provide continuous pathways to the elec-

trodes,we?nd quantum ef?ciencies of up to12%.We describe a simple model to explain the recombination

in these devices,and show how the absorption,charge separation,and transport properties of the composites

can be controlled by changing the size,material,and surface ligands of the nanocrystals.

?S0163-1829?96?03048-2?

I.INTRODUCTION

Composites of organic polymers and inorganic nanocrys-tals are particularly interesting materials in the study of elec-trical transport.The band gaps and offsets of typical semi-conducting polymers and nanocrystals are such that charges will separate across an interface between them.This paper explores the extent of such charge separation,and the nature of charge transport in polymer/nanocrystal blends.

Electronic processes in conjugated polymers are currently the subject of intensive study,both because of fundamental interest in the nature of the electronic excitations in these ‘‘one-dimensional’’semiconductors,and because they have potential applications in a range of electronic devices such as light-emitting diodes.1–3Conjugated polymers have the ad-vantage of being easy to process to form large-area devices, and their energy gap and ionization potential can readily be tuned by chemical modi?cation of the polymer chain.4,5 Large-area thin-?lm photovoltaic devices based on conju-gated polymers are also of interest,although devices fabri-cated using a single layer of polymer have been found to have low ef?ciencies of conversion of incident photons to electrons.6–9Ef?cient collection of charge carriers requires that the neutral excited states?singlet excitons?produced by photoexcitation be separated into free charge carriers,and that these carriers are then transported through the device to the electrodes without recombining with oppositely charged carriers.The possibility that conjugated-polymer/nanocrystal composites may have the desired attributes of charge sepa-ration and transport motivates the present work.

Charge separation in conjugated polymers has been found to be enhanced at the interface with a material of higher electron af?nity where it is energetically favorable for the electron to transfer onto the second material.Examples of

such materials include C60,10–13cyano-substituted conju-

gated polymers,14,15and various small organic molecules.16

Since the diffusion range of singlet excitons in conjugated

polymers is typically in the range5–15nm,13,17–19it is nec-

essary to have a large area of interface between the two

materials in order to achieve a high quantum ef?ciency for

charge separation.Furthermore,the charge separation pro-

cess must be fast compared to the radiative and nonradiative

decays of the singlet exciton,which typically occur with

time constants in the range100–1000ps.In composite ma-

terials where charge separation can occur,the photolumines-

cence is found to be strongly quenched,since the singlet

exciton is no longer able to decay radiatively to the ground

state.In the absence of an electric?eld to remove the sepa-

rated charges,there must exist a nonradiative process by

which recombination occurs between electron and hole on

adjacent materials.Although this process is not well under-

stood,it is likely to be much slower than the decay of the

singlet exciton.The lifetime of the charge-separated species

in composites of poly?2-methoxy,5-?2?-ethyl-hexyloxy?-p-phenylenevinylene??MEH-PPV?with C60has been esti-

mated to be of the order of milliseconds at80K.20The

problem of transport of carriers to the electrodes without

recombination is a more dif?cult one to solve,since it re-

quires that once the electrons and holes are separated onto

different materials,each carrier type has a pathway to the

appropriate electrode without needing to pass through a re-

gion of the other material.The transport must also be suf?-

ciently fast for the carriers to be removed from the device

before signi?cant nonradiative recombination can occur at

the interface between the two materials.Encouraging results

have been obtained using mixtures of polymers with differ-

ent electron af?nities that phase separate on a length scale

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VOLUME54,NUMBER24

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suitable to give effective charge separation whilst providing ef?cient charge transport to the electrodes.14,15Recently, high photovoltaic ef?ciencies have been reported in C60/MEH-PPV MEH-PPV composites with high C60 content.21In these composites,derivatization of the C60mol-ecule with a?exible alkyl group has been found to give optimum photovoltaic performance,although the detailed morphology of these composites has not yet been reported.

Nanometer-sized crystals of inorganic semiconductors are another interesting class of low-dimensional materials with useful optical and electronic properties.22,23When the size of the nanocrystal is smaller than that of the exciton in the bulk semiconductor,the lowest energy optical transition is signi?-cantly increased in energy due to quantum con?nement.The absorption and emission energy can thus be tuned by chang-ing the size of the nanocrystal.High-quality samples of nanocrystals of II-VI semiconductors such as CdS and CdSe can now be prepared by chemical methods.The surface of the nanocrystal is typically capped by an organic ligand which ensures solubility and passivates the surface electroni-cally.By changing the size from6to2nm,the energy gap can be tuned from2.6to3.1eV in CdS and from2.0to2.6

eV in CdSe.22,24The ability to tune the electronic structure of the nanocrystals makes them interesting optical materials; however,making contact to the nanocrystals for electrical measurements is more dif?cult.Nanocrystal/conjugated-polymer composites offer the prospect of allowing electrical access to the nanocrystals.The electron af?nity of CdS and CdSe nanocrystals is in the range3.8–4.7eV,hence they are suitable materials to act as electron acceptors when com-bined with conjugated polymers,where the electron af?nity is in the range2.5–3.0eV.In contrast to C60,the optical energy gap of these nanocrystals lies conveniently in the vis-ible region,and it is thus possible not only to study electron transfer from polymer to nanocrystal,but also to study hole transfer from nanocrystal to polymer after excitation in the nanocrystal.The nanocrystal surface ligand can be changed without altering the intrinsic electronic properties of the nanocrystal,hence it is possible to control the charge transfer between polymer and nanocrystal,and from nanocrystal to nanocrystal.The surface ligand is also important in deter-mining the morphology of the polymer/nanocrystal compos-ite.Charge separation at the interface between organic mol-ecules and nanocrystals is currently of great interest, particularly since the report by O’Regan and Gra¨tzel of ef?-cient photovoltaic devices based on organic dyes adsorbed on TiO2nanocrystalline?lms.25In these devices,the large area of TiO2surface allows a high optical density of dye molecules to be achieved,whilst maintaining ef?cient charge separation.An electrolyte solution is required to remove the holes from the organic dye after charge separation.Charge transfer between CdS nanocrystals and the hole-transporting polymer poly?N-vinylcarbazole?has previously been studied; however,the CdS nanocrystals were at low concentration, and acted primarily as a sensitizer,with charge transport oc-curring through the polymer,rather than from nanocrystal to nanocrystal.26Electroluminescence in blends of CdSe nanocrystals and poly?N-vinylcarbazole?has been studied by Dabbousi et al.27

In this paper,we study the photoluminescence and photo-conductivity of composite materials formed with MEH-PPV and either CdS or CdSe nanocrystals.The nanocrystals may either be in direct contact with the polymer,or,alternatively, the surface of each nanocrystal may be coated with a surfac-tant molecule,trioctylphosphineoxide?TOPO?,which forms a barrier of11?thickness between the nanocrystal core and the polymer.The structure of MEH-PPV,and schematic dia-grams of the TOPO-coated and noncoated nanocrystals are shown in Fig.1.In polymer/nanocrystal composites when the nanocrystal surface is not coated with TOPO,we?nd signi?cant quenching of the luminescence,indicating that charge transfer occurs at the polymer/nanocrystal interface. We also study the effect of nanocrystal concentration on the photoconductivity,and?nd that ef?cient photoconductivity can be achieved at high nanocrystal concentrations when electrons and holes can be transported,respectively,through the nanocrystal and polymer components of the composite material.Throughout the paper,we support our interpretation of the photophysical measurements with detailed transmis-sion electron microscope?TEM?studies of the morphology of the composite materials.

II.EXPERIMENTAL METHODS

MEH-PPV was synthesized by the base-induced polymer-ization of2,5-bis?chloromethyl?-1-methoxy-4-?2?-ethyl hexyloxy?benzene in tetrahydrofuran.The product was puri-?ed by precipitation with methanol,and was then dissolved in chloroform.CdS and CdSe nanocrystals were synthesized on a gram scale using a modi?cation of a previously reported synthesis.28,29A solution of dimethylcadmium and tribu-tylphosphineselenide or bis-trimethylsilanesul?de in tribu-tylphosphine was injected into tri-octylphosphineoxide ?TOPO?at360°C,followed by heating at300°C to give the required size.Full experimental details are given elsewhere.28,29The nanocrystals had a good size distribution, with??10%.In order to prepare samples where the surface TOPO was completely removed,after washing three times in methanol the nanocrystals were three times dissolved in

the FIG.1.Schematic diagram of MEH-PPV/nanocrystal compos-ites,showing the chemical structure of MEH-PPV and trioctylphos-phineoxide?TOPO?.?a?CdSe nanocrystals with surfaces coated by TOPO.?b?CdSe nanocrystals with naked surfaces.

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minimum quantity of pyridine and precipitated by hexanes.The ?nal precipitate was redissolved in chloroform without drying.Concentrations of the polymer and nanocrystal solu-tions were obtained by evaporating known volumes to dry-ness and weighing.Thin ?lms were obtained by spin coating from mixed polymer/nanocrystal solutions containing a known ratio of nanocrystal to polymer.Absorption spectra were obtained using a Hewlett Packard 8452A diode array spectrophotometer.Photoluminescence ef?ciencies of thin ?lms on glass substrates were measured using an integrating sphere,as described elsewhere.30Excitation was provided by an argon-ion laser at 476.1nm with a typical power of 0.1mW.Transmission electron microscopy ?TEM ?was per-formed on very thin ?lms ?typically 20nm ?using a JEOL 100-CX microscope operating at 80kV.TEM samples were prepared by spin coating from dilute solutions onto NaCl substrates.The ?lms were then ?oated off the NaCl onto the surface of a water bath,and transferred to holey carbon grids.Photovoltaic devices were prepared by spin coating a ?lm of nanocrystal/MEH-PPV composite onto a glass substrate coated with indium-tin oxide ?ITO ?.Film thicknesses were in the range 300–500nm.Aluminum electrodes were deposited by vacuum evaporation.The devices were then transferred to a vacuum of approximately 10?3mbar for measurement.Current-voltage curves were measured using a Keithley 928source-measure unit,both in the dark and under illumination.Monochromatic illumination was provided either by an argon-ion laser,or by the output of a tungsten lamp dispersed by a SPEX 270S monochromator.The spectral resolution of the monochromator system was 3nm.Spectral responses and quantum ef?ciencies were determined by normalizing with a calibrated silicon photodiode in the sample position.Quan-tum ef?ciencies ?electrons/photon ?were de?ned using the total incident ?ux;re?ection losses were neglected.

III.RESULTS A.Optical properties

Figure 2shows the absorption spectra of the various nanocrystal and polymer samples used in this work.The

emission spectrum of MEH-PPV is also shown.The photo-luminescence ef?ciency for pure MEH-PPV ?lms was found to be in the range 21–24%,slightly higher than previously reported for this material.30Figure 3shows the absorption spectra of composite ?lms containing 20,65,and 90wt %of CdSe.These spectra are simply the sum of the absorption spectra of the constituent parts of the composite ?lms,with no evidence of any additional absorption peaks in the spec-tral range measured ?350–820nm ?.These results indicate that there is negligible ground-state charge-transfer between the polymer and the nanocrystals.Also,there is no evidence for any absorption corresponding to a spatially indirect charge-transfer transition between polymer and nanocrystal.Since the absorption coef?cient of CdSe nanocrystals is much smaller than that of MEH-PPV,even at the highest concentration of CdSe the optical density of the nanocrystals at their lowest energy absorption peak is smaller than the optical density of the polymer at the ?-?*absorption peak.The ?lm-forming properties of the composite materials were good,and although a small amount of scattering was seen in the most concentrated samples,there was no evidence for gross phase separation on the scale of the wavelength of visible light.

B.Photoluminescence ef?ciencies

Figures 4?a ?and 4?b ?show the photoluminescence ?PL ?ef?ciency

of nanocrystal/MEH-PPV composites as a function of nanocrystal concentration for cadmium sul?de and cadmium selenide,respectively,with the surface either naked,or coated with TOPO.The easiest case to analyze is that of cadmium sul?de,where the energy gap of the nanocrystals is larger than that of the polymer.With excitation at 476.1nm,only the polymer is excited,and there is no possibility of

exciton transfer from polymer to nanocrystal by Fo

¨rster transfer.In the case where the nanocrystal surface is covered completely by TOPO,no quenching of the polymer lumines-cence is observed.This result indicates that electron transfer from polymer to nanocrystal through a layer of TOPO does not occur with rates which compete with the usual radiative and nonradiative decay processes in MEH-PPV.?Typical time constants for singlet exciton decay in MEH-PPV are

in

FIG.2.Absorption spectra of 4-nm-diameter CdS nanocrystals ?solid line ?,5-nm-diameter CdSe nanocrystals ?squares ?,and MEH-PPV ?dashed line ?.Also shown is the photoluminescence spectrum of MEH-PPV at room temperature,with excitation at 476.1nm ?circles ?.The nanocrystals were in toluene solution,and the MEH-PPV was a solid ?lm on a glass

substrate.

FIG.3.Absorption spectra of blends of MEH-PPV with 5-nm-diameter CdSe nanocrystals,containing 5%?solid line ?,65%?dashed line ?,and 90%CdSe ?circles ?by weight.

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N.C.GREENHAM,XIAOGANG PENG,AND A.P.ALIVISATOS

the range 200–300ps.31,32?Where the surface TOPO is re-moved by treatment with pyridine,however,there is a sig-ni?cant quenching of the PL,implying that the removal of TOPO allows electron transfer to occur,leading to the for-mation of separated electron-hole pairs that subsequently re-combine nonradiatively.We note that the quenching of the polymer luminescence,whilst substantial,is not complete,even at high nanocrystal concentrations.For pyridine-treated CdS nanocrystals,for example,the PL ef?ciency levels out at approximately 6%at high concentrations.This suggests the occurrence of phase segregation in the composite mate-rials,since if the nanocrystals were randomly dispersed throughout the sample,we would expect quenching to be much more complete.Similar partial quenching behavior was observed in phase segregated polymer/polymer blends.14The subject of phase segregation is discussed in more detail in Sec.III C.

Using CdSe nanocrystals,there is again substantial quenching of the luminescence in the composite materials ?Fig.4?b ??.Except for the case of 90wt %nanocrystals,excitation at 476.1nm produces excitation mostly in the polymer,due to the much higher absorption coef?cient of MEH-PPV compared with nanocrystals.In contrast to the situation with CdS,it can be seen that for CdSe,quenching occurs even with TOPO-coated nanocrystals,although it is not as effective as with nanocrystals which have been treated

with pyridine.With CdSe nanocrystals,there is a good overlap between the polymer emission spectrum and the nanocrystal absorption spectrum,and we therefore expect ef-?cient Fo

¨rster exciton transfer to occur from polymer to nanocrystal.In contrast to the process of charge transfer,Fo

¨rster transfer does not require wave-function overlap ?tun-neling ?between the two materials,and can therefore occur in the presence of a TOPO barrier which we have shown above to suppress electron transfer to CdS.The PL quenching ob-served with TOPO-coated CdSe can therefore be attributed

to Fo

¨rster transfer of the exciton to the nanocrystal,followed by decay with a radiative ef?ciency signi?cantly less than that of MEH-PPV.The PL spectrum of a composite contain-ing 90wt %of TOPO-coated CdSe was found to be similar to that of pristine MEH-PPV,indicating that the remaining luminescence comes predominantly from excitons which de-cay radiatively in the polymer,rather than from excitons which have been created on or transferred onto the nanocrystals.The PL ef?ciency for these nanocrystals in toluene solution was found to be less than 1%.This low value is consistent with the absence of signi?cant nanocrys-tal emission compared to the remaining polymer emission.With pyridine-treated CdSe nanocrystals,the quenching is more effective,suggesting that charge transfer is again tak-ing place.As with CdS,it is energetically favorable for the electron to transfer from polymer to nanocrystal.Alterna-tively,if the exciton is created on the nanocrystal,or trans-fers onto the nanocrystal by Fo

¨rster transfer,the hole can subsequently transfer to the polymer,again producing a charge-separated state with an electron on the nanocrystal and a hole on the polymer.The various electron and exciton transfer processes that can occur using pyridine-treated CdSe are illustrated in Fig.5.Even with 90wt %of nanocrystals,the polymer luminescence is not completely quenched.In this composite,65%of the radiation absorbed at 476.1nm will directly produce excitons in the nanocrystals,which will decay with low radiative ef?ciency.The measured PL ef?-ciency of 0.5%therefore corresponds to a radiative ef?-ciency of 1.5%for excitons produced in the polymer.Again,this value is consistent with the occurrence of phase separa-tion.Direct evidence for both electron and hole transfer pro-cesses in photovoltaic devices will be presented in Sec.III D.We note that dispersions of nanocrystals in semiconducting polymers,which generally give an alignment of energy lev-els as shown in Fig.5,are good for achieving charge sepa-ration for photovoltaic applications,but are not ideal for electroluminescence applications,since electrons and holes tend to remain separated on different materials,giving low ef?ciencies.27Devices containing a planar heterojunction be-tween polymer and nanocrystal components can,however,show ef?cient electroluminescence when the barriers to elec-tron and hole transport at the interface are such as to achieve a good balancing of electron and hole currents.33

C.Transmission electron microscopy

Figure 6?a ?shows TEM images of blends of MEH-PPV and pyridine-treated CdSe nanocrystals,for various concen-trations of nanocrystals.They show the total nanocrystal dis-tribution projected onto the plane of the ?lm.Since the ?lms are thin enough that only a maximum of three to four

layers

FIG.4.Photoluminescence ef?ciency of MEH-PPV/CdSe com-posites as a function of nanocrystal concentration,for TOPO-coated ?circles ?and pyridine-treated ?squares ?nanocrystals.?a ?4-nm-diameter CdS nanocrystals;?b ?5-nm-diameter CdSe nanocrystals.

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CHARGE SEPARATION AND TRANSPORT IN ...

of nanocrystal would be possible,it is easy to resolve the individual nanocrystals.It is clear that,even in dilute blends,the nanocrystals aggregate together to form regions of densely-packed nanocrystals surrounded by regions of poly-mer which are free from nanocrystals.As the concentration of nanocrystals is increased,the size of the nanocrystal do-mains increases,and at high concentrations the nanocrystals form a connected network in the plane of the ?lm.At 65wt %the size of the larger polymer regions is typically 70–120nm.It should be noted that these images are easy to interpret due to the large difference in scattering between CdSe and MEH-PPV.This contrasts with the situation in polymer/polymer or polymer/C 60blends,where it is neces-sary to dope one of the components selectively in order to achieve contrast in the TEM.14The observed morphology is consistent with the substantial but incomplete quenching of the polymer luminescence,given that exciton diffusion ranges in conjugated polymers are estimated to be in the range 5–15nm.13,17–19and that any exciton which reaches a polymer/nanocrystal interface will be quenched.A quantita-tive comparison is dif?cult without a more detailed knowl-edge of the three-dimensional morphology in the thicker ?lms used for the optical measurements.The driving force for phase segregation here is likely to be the difference in polarity between the pyridine-treated nanocrystals,which have polar faces,34and the polymer,which is relatively non-polar.

Figure 6?b ?shows TEM images of blends of MEH-PPV and TOPO-coated CdSe nanocrystals,at the same weight percentages of nanocrystals as for the pyridine-coated par-ticles.At the two highest concentrations,there is again clear evidence for aggregation of the nanocrystals,consistent with the PL quenching data.This aggregation is largely driven by van der Waals interactions between the nanocrystals.In this case,the nanocrystal cores within the aggregate are separated by the surface TOPO layers,which appear as light regions in the TEM images.At the lowest concentration,there is almost no aggregation,in contrast to the case where the nanocrystals were treated with pyridine.This improved solubility of the nanocrystals in the polymer is likely to be due to the reduced polarity of the TOPO-coated nanocrystals,and to improved mixing of the ?exible alkyl chains of the TOPO molecules with the alkoxy groups of MEH-PPV.

D.Photovoltaic devices

The devices studied comprised a layer of CdSe/MEH-PPV composite between electrodes of ITO and aluminum.For most of the devices,the diameter of the nanocrystals was 5nm,and the ?rst peak in their absorption spectrum was at 614nm.The nanocrystals had been treated with pyridine to remove the surface TOPO.Figure 7shows the current-voltage curve of such a device containing 90wt %of CdSe,both in the dark,and under monochromatic illumination at 514nm.The active area of the device was 7.3mm 2.

Figure 8shows the short-circuit quantum ef?ciency ?elec-trons per photon ?at 514nm as a function of nanocrystal concentration,at an excitation intensity of approximately 5W m ?2.For a pure polymer device,the ef?ciency was only 0.014%,consistent with results reported previously for this kind of device.14Adding 5wt %of nanocrystal gave a factor of 6improvement in ef?ciency.Increasing the nanocrystal concentration up to 90wt %further improved the quantum ef?ciency to a value of 12%.At ?3V and ?3V,this device gave quantum ef?ciencies of 52%and 71%,respectively.Figure 9shows the short-circuit current and open-circuit voltage of the device containing 90wt %of CdSe as a func-tion of incident light intensity.At intensities up to 10W m ?2,the short-circuit current I sc is approximately linear in intensity F with a dependence I sc ?F 0.9.At high intensities,the intensity dependence becomes more sublinear,with a de-pendence I sc ?F 0.65.The open circuit voltage shows a much weaker dependence on the intensity,increasing to a value of 0.53V at an intensity of 500W m ?2.

We have also studied devices containing TOPO-coated nanocrystals of the same core size used in the devices de-scribed above.These devices showed extremely low quan-tum ef?ciencies,with a value of 0.005%at a loading of 81wt %nanocrystals.

Figure 10shows the photoconductivity action spectrum for the same device described in Figs.7and 9,along with the action spectrum for a pure MEH-PPV device.The action spectrum of the composite device shows response in the re-gion 600–660nm where there is no absorption in the poly-mer.This response corresponds to light which is absorbed in the nanocrystals,with subsequent hole transfer onto the poly-mer.At wavelengths below 600nm,the response is due to a combination of absorption in the polymer and in the nanocrystals.The pure MEH-PPV device has a peak in

the

FIG. 5.Routes for exciton and charge transfer in MEH-PPV/CdSe blends.?a ?Absorption in the polymer,followed by electron transfer onto the nanocrystal.?b ?Absorption in the polymer,followed by exciton transfer onto the nanocrystal,fol-lowed by hole transfer onto the polymer.?c ?Absorption in the nanocrystal,followed by hole transfer onto the polymer.Note that for CdS nanocrystals,route ?b ?is not available since the nanocrystal energy gap is larger than that of the polymer,whereas it would be possible for excitons to transfer from nanocrystal to polymer,fol-lowed by electron transfer onto the polymer.In the presence of a TOPO barrier,the electron and hole transfer processes are sup-pressed,and only exciton transfer is possible.

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action spectrum at 560nm,on the rising edge of the polymer absorption spectrum,as has been described previously for this kind of device.7Due to the poor electron transport in the polymer,only those electrons generated close to the alumi-num electrode will contribute to the photocurrent by reach-ing the electrode without recombination.The peak photocur-rent therefore occurs where the optical density is low and light is absorbed throughout the thickness of the device,rather than at high optical densities where almost all the light is absorbed close to the ITO electrode.Similar behavior is seen in devices with low concentrations of CdSe.In contrast,at high concentrations of CdSe,the action spectrum closely follows the fraction of incident light absorbed in the device.This result indicates that both carriers are mobile within the composite material at high nanocrystal concentrations.

Figure 11shows the temperature dependence of the pho-tocurrent at 514nm,between room temperature and 10K.The heating and cooling curves show some hysteresis,which is repeatable on subsequent cycles.The photocurrent drops by only a factor of 5between room temperature and 10K,indicating that the limiting process in determining the pho-toconductivity is not strongly thermally activated.

IV.DISCUSSION

The photoluminescence data show that in blends of MEH-PPV and CdS or CdSe nanocrystals,excitons are dissociated at the polymer/nanocrystal interface,leaving the electron on the nanocrystal and the hole on the polymer.The morphol-ogy of the composite nanocrystal/polymer materials is such that a large fraction of the excitons produced are dissociated at an interface,even at relatively low nanocrystal concentra-tions.The presence of a TOPO surface layer on the nanocrystal,however,suppresses the charge transfer process.The photoconductivity action spectra show that both absorp-tion in the polymer and absorption in the nanocrystal lead to charge separation.

As discussed in the Introduction,ef?cient photoconduc-tivity requires not only ef?cient charge separation,but also ef?cient transport of the carriers to the electrodes without recombination.Recombination is minimized when the carri-ers are separated onto different materials,and both carriers have pathways to the appropriate electrode within the com-ponent of the ?lm onto which they have been separated.It can be seen from Fig.6?a ?that at low concentrations

of

FIG.6.TEM images of blends of MEH-PPV with 5-nm-diameter CdSe nanocrystals at concentrations of 5%,20%,and 65%by weight.?a ?Pyridine-treated nanocrystals.?b ?TOPO-coated nanocrystals.These images are taken on free-standing areas of the ?lms.The polymer background is clearly visible,showing that the ?lms are continuous.

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nanocrystals the size of the nanocrystal domains is small,so only the few electrons generated on the larger nanocrystal domains very close to the aluminum electrode will have a pathway to the electrode.At higher concentrations?see Fig.6?a?,with a concentration of65wt%nanocrystals?,the nanocrystals begin to form a connected network.In this case, it becomes much more likely for an electron generated on a nanocrystal at a general position within the device to be able to?nd its way through the thickness of the device.It should be noted that the direction of transport in the devices is of course perpendicular to the plane shown in the TEM images, and that the?lms used for the TEM images are much thinner than those used for devices.Nevertheless,there is clear evi-dence of stacking of the nanocrystals within the thin TEM ?lms in such a way as to allow perpendicular transport,and we believe that the morphology within the plane of the thin ?lms is a useful indication of the likely morphology affecting the transport in the perpendicular direction.

It is important to distinguish here between the effects of charge separation and the effects of transport.At40wt%of nanocrystals,for example,the PL is quenched by a factor of approximately10.In the presence of the internal?eld within the device,it is therefore likely that signi?cantly more

than FIG.8.Short-circuit quantum ef?ciency for devices containing

5-nm-diameter CdSe nanocrystals,as a function of CdSe concentra-

tion.Excitation was at514nm,at a power density of approximately

5W m?2

.

FIG.9.Short-circuit current?circles?and open-circuit voltage

?squares?as a function of incident light intensity for a device con-

taining90wt%of5-nm-diameter CdSe nanocrystals.Excitation

was from an argon ion laser at514nm.The beam pro?le was

approximately Gaussian,with a full width at half maximum of1.4

mm.The intensity shown is the approximate maximum intensity at

the center of the laser

spot.

FIG.10.Spectral response of the short circuit current for a

device containing90wt%5-nm-diameter CdSe nanocrystals?solid

line?,and for a pure MEH-PPV device?dashed line?.The data have

been normalized to?t on the same scale.

1763454 N.C.GREENHAM,XIAOGANG PENG,AND A.P.ALIVISATOS

90%of the excitons are dissociated at an interface.Although the short-circuit quantum ef?ciency of this device is much larger than that of a pure MEH-PPV device,it is still more than a factor of 10smaller than the maximum ef?ciency found in the device with 90wt %CdSe.We therefore con-clude that the improvement in ef?ciency between 40and 90wt %is largely due to improvement in electron transport,and that the improvement in ef?ciency at lower concentrations is due to improvements in both charge separation and transport.The peak in ef?ciency at 90wt %CdSe corresponds to a volume fraction of nanocrystal of approximately 65%,at which point there should be good percolation paths through both nanocrystal and polymer components.?The true volume percentages of nanocrystal are less than the fraction of dark area in the TEM images,since the TEM ?lms are thicker than one nanocrystal diameter.?

Even at the optimum concentration of nanocrystals,the photocurrent quantum ef?ciency is less than unity.We pro-pose that the loss of carriers is largely due to electrons which become trapped at ‘‘dead ends’’in the nanocrystal network,from where they cannot hop to another nanocrystal in the direction of the internal ?eld.These electrons will eventually recombine with holes in the neighboring polymer.This mechanism is consistent with the almost linear intensity de-pendence of the photocurrent observed at low intensities.A linear intensity dependence is characteristic of recombination at a ?xed number of recombination centers.In contrast,bi-molecular recombination of free electrons and holes would give an intensity dependence I sc ?F 0.5.The local ?eld gener-ated by an electron trapped at a dead end will tend to repel other electrons from taking this route,causing each dead end to act as a recombination center that can be either singly occupied or empty.This model is also consistent with the weak temperature dependence of the photocurrent,since the probability of recombination is determined largely by the morphology of the ?lm,hence the exact rates of transport and recombination at a polymer/nanocrystal interface only have a secondary effect on the measured current.

In order to study the effect of the nanocrystal size on the photoconductivity,we have also studied devices using

pyridine-treated CdSe nanocrystals of 2.4nm diameter ?ab-sorption peak at 516nm ?.These smaller nanocrystals also gave a signi?cant improvement in photoconductivity,al-though the peak ef?ciencies reached were not as high as with the larger nanocrystals.At 81wt %CdSe,for example,the quantum ef?ciency at 514nm was 0.9%,compared with 5.5%for the larger nanocrystals.TEM images show that the typical size of nanocrystal domains is smaller than for the larger nanocrystals,probably as a result of improved solubil-ity of the smaller particles.We attribute the lower ef?ciency to the reduced probability of electrons having a pathway to the aluminum electrode.

The low quantum ef?ciencies found using TOPO-coated nanocrystals are primarily a consequence of the fact that charge transfer is slow compared to the natural decay of excitons produced in either the polymer or the nanocrystals.The fact that the quantum ef?ciencies with TOPO-coated nanocrystals are even lower than those of pure MEH-PPV devices suggests that any free electrons which are produced by the internal ?eld in the device become trapped on the nanocrystals,and that the presence of the TOPO barrier in-hibits transport from nanocrystal to nanocrystal before re-combination occurs.This evidence for poor transport be-tween TOPO-coated nanocrystals may be relevant to explain the performance of light-emitting diodes fabricated from lay-ers of TOPO-coated CdSe nanocrystals.33

Figure 12shows a schematic energy level diagram for an ITO/MEH-PPV:CdSe/Al device,assuming that both semi-conductors act as intrinsic materials.The ionization potential and electron af?nity of 5-nm-diameter CdSe nanocrystals were estimated from the bulk values,35assuming that 75%of the shift due to quantum con?nement occurs in the conduc-tion band.The values for MEH-PPV are taken from Ref.36although a range of values are quoted in the literature.Since the work function of ITO is smaller than the ionization po-tential of MEH-PPV and the work function of Al close to the electron af?nity of CdSe nanocrystals,the expected maxi-mum open circuit voltage is given by the difference in work function between ITO ?4.7–4.9eV ?and Al ?4.3eV ?.The open-circuit voltages of the devices studied depended on the excitation intensity,and also showed some variation from device to device,but were typically in the range 0.5–0.6eV at intensities above 5W m ?2.These values are

consistent

17635

CHARGE SEPARATION AND TRANSPORT IN ...

with the difference in work functions between Al and ITO.

We do not see a signi?cant increase in the open-circuit volt-

age when using2.4-nm-diameter nanocrystals.This result is

as expected since the conduction band of these nanocrystals

lies above the work function of Al.We anticipate that larger

open circuit voltages could be achieved using small

nanocrystals with low work function electrodes.

Although the primary interest of this paper is to investi-

gate the physical processes of charge separation and trans-

port in composite polymer/nanocrystal materials,it is useful

for comparison to other work on photovoltaic materials to

calculate the energy conversion ef?ciency for the devices

studied here.The device described in Fig.7has a quantum

ef?ciency of12%at514nm.This value is signi?cantly

larger than values of2–6%reported for polymer/polymer

mixtures,14,15but not as large as the value of approximately

36%found at this excitation intensity in blends of deriva-

tized C60and MEH-PPV.21In the devices studied here,the

?ll factor?de?ned as?VI?max/V oc I sc,where(VI)max is the

area of the largest rectangle under the current-voltage curve

between0V and V oc?is0.26,and the open-circuit voltage is approximately0.5V,giving a power conversion ef?ciency

of0.6%at514nm.The device begins absorbing at650nm,

and thus covers a wider spectral range than devices where

absorption occurs solely in MEH-PPV.For the solar spec-

trum under AM1.5conditions?a typical solar spectrum?,the

device absorbs approximately37%of the incident solar en-

ergy,and the energy conversion ef?ciency for AM1.5con-

ditions at5W m?2is0.2%.Due to the sublinear intensity

dependence of the photocurrent,under one-sun conditions ?800W m?2?the solar power conversion ef?ciency will be approximately0.1%.For comparison,we estimate that the solar energy conversion ef?ciency for devices using blends of derivatized C60and MEH-PPV with calcium electrodes21 is no more than0.5%under the same conditions.We have identi?ed above the possibility of increasing the open-circuit voltage,and hence the power ef?ciency,by using smaller nanocrystals with lower work-function electrodes.Further understanding of the injection and transport of carriers of these devices may in the future allow improved?ll factors to be obtained.

V.CONCLUSIONS

We have demonstrated that the interface between a con-jugated polymer and a semiconductor nanocrystal can be used to provide ef?cient charge separation for neutral excited states produced either on the polymer or on the nanocrystal. We have also shown that both charge separation and charge transport from nanocrystal to nanocrystal are suppressed in the presence of a TOPO surface layer on the nanocrystal. The morphology of composite nanocrystal/polymer?lms shows phase separation of the polymer and nanocrystal com-ponents.In photovoltaic devices using the composite mate-rials,this phase segregation is crucial in providing paths for both electrons and holes to travel to the appropriate electrode without recombination.The use of nanocrystals allows great ?exibility in controlling the performance of photovoltaic de-vices,since both the electronic energy levels and the mor-phology of the composite materials may be altered by chang-ing the nanocrystal size,concentration,and surface ligand. We have shown how the absorption range of polymer de-vices can be widened by the incorporation of nanocrystals, and using transmission electron microscopy we have directly demonstrated the relationship between?lm morphology and photovoltaic performance.We have shown that large im-provements in photovoltaic ef?ciency can be achieved by using high concentrations of nanocrystals,and we anticipate that further improvements will be made in the future using the principles developed here to optimize the charge trans-port process.

ACKNOWLEDGMENTS

This work was supported by the U.S.Department of En-ergy under Contract No.DE-AC0376SF00098.N.C.G. thanks the Miller Institute for Basic Research in Science for support.We are grateful to M.Shtein for assistance with the experimental work.The TEM work was carried out in the Robert D.Ogg Electron Microscope Laboratory,University of California,Berkeley.

*Present address:Cavendish Laboratory,Madingley Road,Cam-bridge,CB3OHE,U.K.

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20L.Smilowitz,N.S.Sariciftci,R.Wu,C.Gettinger,A.J.Heeger, and F.Wudl,Phys.Rev.B47,13835?1993?.

21G.Yu,J.Gao,J.C.Hummelen,F.Wudl,and A.J.Heeger, Science270,1789?1995?.

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34J.J.Shiang,A.V.Kadavanich,R.K.Grubbs,and A.P.Alivisa-tos,J.Phys.Chem.99,17417?1994?.

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5417637

CHARGE SEPARATION AND TRANSPORT IN...

美国签证提交简历中英文样板---Sample-Resume-(使馆提供)

姓名(拼音和汉字): 性别: 出生日期: 出生地: 家庭住址: 单位地址: 家庭电话: 工作电话: 手机号码: 电子邮箱: 教育背景–请分别列出您取得的所有学位，从最高学位写起。 月，年—月，年大学名称 学位和专业 论文标题/研究焦点（只对硕士和博士学位）工作经历–请列举您的所有工作经历。 月，年—月，年单位名称 地址 职位或职称 工作职责 所获奖项及加入哪些团体组织（如果适用） 出版物—请列出您发表的所有出版物标题、合作者和年份（如果适用）

出国经历—请列举您到访过的所有国家及到访时间 国家（年） 例如：美国（2002，2003）；加拿大（2008，2009） 同行人–请写出和您一起赴美的所有同行人姓名及与您的关系

Name (in pinyin and Chinese): Gender: Date of Birth: Place of Birth: Home Address: Business Address: Home Phone: Work Phone: Mobile Phone: E-mail: Education – please list all degrees attained, beginning with the most recent Month, Year – Month, Year University Name Degree and Major Thesis Topic/Research Focus (for masters and phd degrees) Work Experience – please list all work experience Month, Year – Month, Year Employer Location Position or Title Duties Awards and Group Memberships – if any

中英文简历全套简历模板

诺和诺德 [请输入文档标题] [请输入文档副标题] [请输入作者]

Life is like riding a bicycle. To keep your balance you must keep moving. 生活就像骑单车，只有不断前进，才能保持平衡。 P E R S O N A L N a m e: H a n Zhongyu G e n d e r: Female A g e : 19 Health: Excellent Hobbies: Paint draw, Badminton ， Personality: Honest , Creative, Cooperative, Dutiful and Dedicated, St r o n g,p r o v e n c o m m u n i c a t i o n s a b i l i t i e s b o t h w r i t t e n a n d v e r b a l i n E n g l i s h OBJECTIVE ? ~ EDUCA TION ? ~ ? ~ ? ~ ? ~ WORKING E X P E R I E N C E ~ ? TIME ： ~ ? POSITION ：

EXPERIENCE ： ~ ? ~ ? TIME ： ~ ? POSITION ： ~ ? EXPERIENCE ： ~ KNO WLEDGE B A C K G R O U N D ? Hardware —— System s t u d i e d The C i r c u i t E l e m e n t s, The S i m u l a t i o n Circuit, The D i g i t a l Circuit , The Function o f Computer Hardware T echnology ? Software —— Expert in computer programming ,familiar with the software of MA TLAB , C , PRO/E , AutoCAD

美国签证需要用到的个人简历模板(中英文)

美国签证需要用到的个人简历模板（中英文） 这个模板是沈阳美领馆提供的简历模板，针对非移民签证申请人如果需要写个人简历的话，大家可以参考一下： Nonimmigrant Visa Resume T emplate Name: Date and country of birth: Gender: Name and date of birth of spouse: (if applicable) Names and dates of birth of children: (if applicable) Address and Contact information: Education List here all universities and higher education institutions you have attended, starting with the most recent. You should include the following information: Name of university Dates of study Degree level Degree major and minors Area of research Title of thesis Work experience List here all paid and voluntary work you have performed and positions held, starting with the most recent. You should include the following information: Name of company, organization or institutions Job titles Dates of jobs Detailed area of responsibility, research interests, project descriptions and applications of research Expertise in special software, machinery op equipment Awards and patents Have you received/won any awards related to your research or work at university or at work? Please list these. Do you hold any patents? List name, patent number and year registered.

海运进口货物报关委托合同示范文本

海运进口货物报关委托合 同示范文本 In order to solve or prevent disputes, through establishing certain legal relations and realizing some common interests and wishes, all parties to the cooperation reach an agreement after consultation, and all parties signing the agreement have legal effect and are bound. 某某管理中心 XX年XX月

海运进口货物报关委托合同示范文本使用指引：此协议资料应用在解决或防止纠纷，通过建立一定的法律关系并实现某些共同的利益和愿望，合作的所有方经协商后达成协议，签字的所有方具有法律效力并受约束。，文档经过下载可进行自定义修改，请根据实际需求进行调整与使用。 甲方：(托运人) 法定代表人： 法定地址：邮编: 经办人：联系电话：传真: 银行帐户： 乙方：中国外运公司(承运人) 法定代表人： 法定地址：邮编: 经办人：联系电话：传真: 银行帐户： 甲乙双方经过友好协商，就办理甲方货物海运进口代 理报关事宜达成如下合同：

1、乙方接受甲方委托为其办理下述海运进口货物的报关业务。 合同号：发票号： 运编号：提单号： 乙方的代理权限为： (1)代理甲方办理约定货物的报关、报验； (2)甲方其他的特别授权： 2、乙方作为甲方的报关代理人，应在甲方的授权范围内进行活动，认真履行职责，维护甲方的合法权益。乙方在代理权限内的任何责任和费用都应由甲方承担。乙方只对因自身的过失与疏忽给甲方造成的直接损失负有责任。 3、甲方必须保证报关货物不属于国家禁止或者限制进出境的物品。否则，甲方应对由此产生的一切后果承担责任。 4、甲方保证申报的内容均真实、准确、无欺诈，且与

Hey Jude 歌词流程图及英文歌词与翻译

Hey Jude, don't make it bad. 嘿！Jude，不要这样沮丧 Take a sad song and make it better 唱首悲伤的歌曲让事情好转Remember to let her into your heart 将她牢记在心底 Then you can start to make it better. 然后开始让事情好转 Hey Jude, don't be afraid 嘿Jude，不要害怕 You were made to go out and get her. 你生来就是要得到她 The minute you let her under your skin, 在你将她放在心上的时候

Then you begin to make it better. 你就开始做的更好 And anytime you feel the pain, 无论何时，当你感到痛苦 hey Jude, refrain, 嘿Jude 停下来 Don't carry the world upon your shoulders. 不要把全世界都扛在你肩膀上 For well you know that it's a fool who plays it cool 你应该很清楚谁耍酷谁就是笨蛋 By making his world a little colder. 这会使他世界更加冰冷 Hey Jude don't let me down 嘿Jude 不要让我失望 You have found her, now go and get her. 你已遇见她现在去赢的她芳心 Remember to let her into your heart, 记住将她牢记在你心中 Then you can start to make it better. 然后你就可以开始做的更好 So let it out and let it in, hey Jude, begin, 所以遇事要拿得起放得下嘿！jude ，振作起来 You're waiting for someone to perform with. 你一直期待的那个和你一起表演的人 And don't you know that it's just you, hey Jude, you''ll do 你不知道那个人就是你自己吗？嘿jude 你办得到的The movement you need is on your shoulder 下一步该怎么做就全看你自己 Hey Jude, don't make it bad. 嘿Jude 不要这样消沉 Take a sad song and make it better 唱首伤感的歌曲会使你振作一些 Remember to let her under your skin 记得心中常怀有她 Then you'll begin to make it better 然后你就会使它变得更好 Better better better better better better, Oh. 更好、更好、更好、更好、更好 Na na na, na na na na, na na na

美国英文简介-An Introduction to U.S.

The United States of America (also referred to as the United States, the U.S., the USA, or America) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Caribbean and Pacific. At 3.79 million square miles and with over 310 million people, the United States is the third or fourth largest country by total area. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The

hey,jude含义解析

Hey Jude The Beatles

?这首歌就是英国的难忘今宵！！！伦敦奥运会的压轴歌曲，我觉得很适合大合唱由麦卡特尼创作的，鼓励列农的儿子朱利安勇敢面对现实，在约翰列侬离婚后希望朱利安不要消沉其实这首歌的原名是Hey Julian，后来改为Hey Jules, 最终变成Hey Jude

?《Hey Jude》是Paul McCartney（保罗·麦卡特尼，The Beatles（披头士乐队，又称甲壳虫乐队）成员之一）为一个五岁的孩子写下的一首歌。这个男孩叫Julian，是John Lennon（约翰·列侬）与前妻Cynthia 的儿子。1968年夏天，John Lennon开始和Yoko Ono（小野洋子）同居了，他与前妻Cynthia的婚姻也到了崩溃的边缘

?Paul一直非常喜爱John Lennon的儿子Julian，他担心大人之间的婚姻变故会对一个小孩子带来心理上的阴影。（不过，当时Paul也正和相恋5年的未婚妻Jane Asher分手，开始与Linda Eastman 的感情）他曾说：“我总是为父母离异的孩子感到难过。大人们也许没什么，但是孩子……”同时，他也想要安慰一下Cynthia。于是有一天，他去了Cynthia的家里，还给她带了一枝红玫瑰，开玩笑的对她说：“Cyn，你说咱俩结婚怎么样？”说完两人同时大笑起来，Cynthia从他的玩笑中感受到了温暖和关心。

?Paul在车里为Julian写下了这首Hey Jude （Hey，Julian），可当时的Julian并不知道。直到二十年后，Julian才明白这首歌是写给自己的。他一直很喜爱爸爸的这个朋友，像一个叔叔一样的Paul。John Lennon也非常喜爱这一首歌。自从第一次 听到，他就觉得，“噢，这首歌是写给我 的！”Paul 说“Hey，John！去吧，离开我们和Yoko在一起吧。”他似乎又在说：“Hey，John！不要离开！来 自：”https://www.sodocs.net/doc/c04022689.html,/view/965993.htm

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heyjude中英文歌词

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