Organic Electronics 8 (2007) 601–605

www.elsevier.com/locate/orgel

Exciton diffusion length of tris (dibenzoylmethane)mono (phenanthroline) europium (III) measured

by photocurrent and absorption as a function of wavelength

M.R. Black *, C. Chavez, E. Brosha

Los Alamos National Laboratory, International, Space, and Response Division, P.O. Box 1663, Los Alamos, NM 87545, United States

Received 9 January 2006; received in revised form 23 April 2007; accepted 24 April 2007Available online 10 May 2007

Abstract

Ideally, the exciton diffusion length in an organic photovoltaic is comparable to the optical penetration depth, i.e., theinverse of the absorption coefficient. In most organic materials, the exciton diffusion length is up to several orders of mag-nitude smaller than the absorption penetration depth – resulting in lower energy conversion efficiencies. In metal-organicmaterials, the exciton lifetime has been predicted to be large due to strong spin orbit coupling. This increased lifetime maylead to increased exciton diffusion. One particular metal-organic material, tris (dibenzoylmethane) mono (phenanthroline)europium (III) or Eu-DM, is especially interesting for photovoltaic applications as it is soluble in toluene and therefore canbe spun-on and processed inexpensively. Using photocurrent and optical absorption as a function of wavelength, the exci-ton diffusion length of spun-on Eu-DM was measured to be 250 ± 8 A, for optical excitation wavelengths in the range350 nm < L < 450 nm. A long tail in the photocurrent and absorption spectra indicate a weakly allowed transition in thisenergy range. Possible implications of this tail are discussed.� 2007 Published by Elsevier B.V.

PACS: 72.40.+W; 73.20.Mf; 73.50.Pz; 78.20.Jq; 78.30.Jw; 78.66.Qn

Keywords: Exciton; Photovoltaics; Metal-organic; Diffusion length; Solar cell; Organo-metallic

1. Introduction

Third generation solar cells promise lower cost,greater efficiency, and/or increased functionality overfirst generation (single crystal silicon) and secondgeneration (polycrystalline, thin film, and microcrys-

1566-1199/$ - see front matter � 2007 Published by Elsevier B.V.

doi:10.1016/j.orgel.2007.04.011

* Corresponding author. Tel.: +1 505 667 9419; fax: +1 505 6061515.

E-mail address: marcie@alum.mit.edu (M.R. Black).

talline) photovoltaics [1]. The possible contenders forthird generation photovoltaics include photovoltaicsmade from organic materials. This type of solar cellwill likely have the advantage that it can be processedinexpensively, rapidly, and on flexible surfaces.Although many scientists are researching organicmaterials for LEDs and much of the acquired knowl-edge from their research can be applied to solar cells,solar cells have unique requirements separate fromthose of LEDs that must also be met.

Fig. 1. A molecule of tris (dibenzoylmethane) mono (phenan-throline) europium (III) (Eu-DM).

602 M.R. Black et al. / Organic Electronics 8 (2007) 601–605

Unlike inorganic materials, where an absorbedphoton creates an electron and a hole, withinorganic materials a bound electron hole pair or exci-ton is created. Since the exciton is charge neutral,the exciton can only diffuse (and not drift), through-out the material. Most excitons that disassociate doso at an interface, so the exciton must diffuse to aninterface and then disassociate into an electron anda hole. The electron and hole can then drift to oppo-site contacts producing photocurrent. As a result,material properties required for efficient organicexcitonic solar cells include: strong optical absorp-tion, large exciton diffusion length, and high carriermobility. This study focuses on measuring the exci-ton diffusion length within a particular organicmaterial.

In general, excitons are one of two types, singletsor triplets. Since the ground state is a singlet, tran-sitions to and from the excited singlet state conservespin and are therefore optically allowed. Hence,they tend to have large absorption coefficients, anadvantage for solar cells. At the same time, theyexhibit small exciton diffusion lengths, a disadvan-tage for excitonic solar cells. Transitions betweenthe ground state and the first excited triplet statedo not conserve spin and are therefore not opticallyallowed. Hence, triplet states have very large excitonlifetimes and therefore are likely to have long exci-ton diffusion lengths. On the other hand, tripletexcitons have very weak optical absorption.

In metal organic materials, the presence of themetal increases spin–orbit coupling and results inthe mixing of the singlet and triplet states (e.g.[2,3]). The spin–orbit coupling in metal organicmaterials can lead to singlet-like and triplet-likeexciton states that are both partially opticallyallowed and have long exciton lifetimes. Since theexciton diffusion length is proportional to thesquare root of the product of the diffusion coeffi-cient and the lifetime, the mixing of the triplet andsinglet exciton states may also lead to long excitondiffusion lengths.

Since metal organics are suspected to have largeexciton diffusion lengths, we choose a metal-organicfilm for this study. For solar cell applications, lowcost is imperative, and so spin coating has a distinctadvantage over evaporation. As a result, this studyconcentrated on metal-organic films which could bedissolved in a solvent and hence be spin coated ontoa surface. In particular, the exciton diffusion lengthof tris (dibenzoylmethane) mono (phenanthroline)europium (III) (Eu-DM) is investigated (see Fig. 1).

2. Experimental

Glass was used as a substrate for the devices, andwas first cleaned by a solvent rinse of acetone, tolu-ene, and IPA and then followed by a 5 min oxygenplasma ash. The glass substrates were subsequentlytransferred into a glove box filled with argon atatmospheric pressure and were not exposed to airfor the duration of the experiment. Next, a 150 Athin layer of aluminum (Al) was thermally evapo-rated onto the glass slide. The pressure during theevaporation was between 1 · 10�7 and 5 · 10�7 Torrand the rate was kept near 2 A s�1. The sample wasthen removed from the evaporator and Eu-DM wasspin coated onto the Al at 1000 rpm for 90 s froma solution of 25 mg of Eu-DM (H.W. SandsOPA9808) in 1 ml of toluene. Samples were also pre-pared by evaporating Eu-DM, and results were sim-ilar to the spin-coated samples. However, only thespun-on samples are presented. Both the evaporatedand spin coated Eu-DM were found to be amor-phous by X-ray diffraction. Prior to spin coating,in order to ensure full dissolution of the Eu-DM inthe toluene, the solution was heated to 50 �C foran hour and then cooled to room temperature. Fol-lowing spin coating, another layer of Al was evapo-rated onto the sample. Since the surface of the Eu-DM is not as smooth as the plain glass, a thickerlayer is needed for the back contact than the frontcontact to ensure a continuous film. Therefore, theback Al layer was made slightly thicker, 450 A. Toavoid the Al deposition throughout the Eu-DM filmand shorting out the top and bottom contacts, the Aldeposition was kept below 2 A s�1 and the pressurewas strictly kept in the low 10�7 Torr range. A sche-matic of the device is shown in the inset of Fig. 3.

To avoid exposing the sample to air, the photo-current was measured inside a cryostat at room

Fig. 2. A plot of the photocurrent and phase of an Al/Eu-DM/Aldevice with both front and backside illumination. The phase ofthe photocurrent is very similar for the two illuminationdirections, while the photocurrent intensity is slightly larger forbackside illumination.

M.R. Black et al. / Organic Electronics 8 (2007) 601–605 603

temperature. A 50 W tungsten lamp was used withan Acton Research Corporation Spectra Pro 2300imonochromator. The photocurrent was amplifiedusing a Stanford Research System model SR570current preamplifier, and detected using a StanfordResearch Systems SR830 lock-in amplifier at a fre-quency of 100 Hz. The photocurrent was normal-ized to the intensity of the incident light asmeasured by a calibrated photodector. Absorptionspectra were taken using a Hewlett Packard 8452diode array spectrometer in air. The same samplefor the photocurrent measurement was used forthe absorption measurements, but the absorptionspectrum was taken in an area of the sample thatwas masked off, and thus did not have a metal coat-ing. Glass is used as the background spectra.

3. Results

The most common method of measuring excitondiffusion lengths is to use photoluminescencequenching. Both glass and gold were tested asquenching surfaces for photoluminescence quench-ing. Glass did not quench the excitons in Eu-DM,and hence the quenching experiment on glass sub-strates did not yield useful information. Gold sur-faces acted as efficient quenching sites, but Eu-DMfilms on gold exhibited metal enhanced excitonquenching. As a result, the simple method of findingthe exciton diffusion length by plotting the photolu-minescence intensity as a function of sample thick-ness and finding the x-intercept could not be usedfor Eu-DM on gold. Metal enhanced excitonquenching is well understood (e.g. [4]), and so aset of samples with different thicknesses on goldcould be used to find the exciton diffusion length,but many samples with different thicknesses arerequired to accurately resolve the effects of metalenhanced exciton quenching. Hence, this study uti-lizes another method of determining the exciton dif-fusion length.

The exciton diffusion length also can be derivedfrom the photocurrent and absorption spectra as afunction of wavelength, provided several assump-tions are made [5,6]; the current is limited by excitondiffusion to contacts, the exciton diffusion is propor-tional to the space derivative of the exciton distribu-tion, and the metal surfaces are perfect excitonquenchers so that the exciton densities at the sur-faces are approximately zero. In order to helpensure that the current is limited by exciton diffu-sion, we choose aluminum as our metal contact,

since it is a poor electron and hole injector. Thisminimizes any photocurrent resulting from carrierinjection from the metal to the organic film. Theexciton distribution can be solved from the continu-ity equation using only the optical absorption coef-ficient and the exciton diffusion length. The excitondensity distribution n(E,z) is then

nðE; zÞ ¼ agI=D

b2 � a2

ebL � e�aL

e�bL � ebL

� �e�bz

�

� e�bL � e�aL

e�bL � ebL

� �ebz þ e�az

�; ð1Þ

where g is the exciton generation efficiency, a is theoptical absorption length at the incident photon en-ergy, 1/b is the exciton diffusion length, I is the inci-dent light flux, L is the sample thickness, z is thedistance into the device, and D is the exciton diffu-sion constant. The current is then found by the dif-fusion equation

JðE; zÞ ¼ �DdnðE; zÞ

dz

����z¼0;L

: ð2Þ

The calculation is greatly simplified if the photo-current is dominated by exciton dissociation at onlyone contact. In order to test if this is the case in ourdevices, the photocurrent was measured for bothfront and back side illumination (see Fig. 2). Itwas expected that front side illumination wouldyield larger photocurrent since the front aluminumis thinner (150 A) than the back side aluminum(450 A). Hence, more light should transmit throughthe front side than the back side. However, morephotocurrent was produced when the sample wasilluminated from the back side. In addition, since

604 M.R. Black et al. / Organic Electronics 8 (2007) 601–605

the phase of the photocurrent from front and backside illumination is very similar (see Fig. 2), wecan conclude that the same surface is dominatingthe photocurrent regardless of the direction of illu-mination. If the device were perfectly symmetric,switching the direction of illumination will switchthe direction of the current and hence result in a dif-ference in phase of 180� for the two directions ofillumination. Since more photocurrent was observedfrom backside illumination and since only one sidewas dominating the photocurrent, we concludedthat exciton disassociation at the back contact dom-inated the photocurrent. This was easily explainedby the larger surface roughness of the back Eu-DM/Al interface compared to the front Eu-DM/Al interface. The larger surface roughness provideda larger exciton disassociation area and thereforemore efficient exciton disassociation.

Fig. 3 compares the photocurrent and absorptionspectra of a device. The photocurrent follows theabsorption fairly closely. Interestingly, both have along tail from 420 to 550 nm. This tail could resultfrom disorder or could indicate the presence of aweakly allowed state which also contributes to thephotocurrent. One possible explanation for the tailin the absorption and photocurrent spectra is thatthe triplet-like excitons are directly being excitedby the incident light and contributing to the photo-current. However, more research into the absorp-tion mechanism in this energy range is neededbefore conclusions can be made as to the origin ofthe tail.

Since the backside dominates the photocurrent inour sample, we can simplify Eqs. (1) and (2) forfront side illumination if we can assume that theexciton diffusion length is smaller than the samplethickness (300 A). Thus,

Fig. 3. Photocurrent and absorption as a function of wavelengthfor an Eu-DM device and film respectively. The photocurrent andabsorption track each other and both exhibit a tail in the longwavelength side.

J / a expð�aLÞb� a

; ð3Þ

and the exciton diffusion length is determined byfinding the x-intercept on a graph of 1/[Jexp(aL)]vs. 1/a [7]. A similar calculation can be done forbackside illumination. However, in order to obtaina simple equation, the additional assumption thataL� 1 must be made. For our sample and the en-ergy range of interest, this assumption is not accu-rate; hence only front side illumination is shown.

Fig. 4 plots 1/[Jexp(aL)] as a function of 1/a forthe photocurrent measured using front side illumi-nation. Part of the graph is linear, as expected ifall the assumptions are met. The linear region ofthe graph results from data taken between 350 and420 nm. The exciton diffusion length derived fromthis data is 248 ± 8 A. For wavelengths above420 nm, the graph is non-linear, indicating thatone of the aforementioned assumptions is incorrectfor this energy range, or that the absorption or pho-tocurrent measurements are not accurate for thisenergy range. Many possible reasons could explainwhy the graph is non-linear in this regime. The mostprobable cause is that the absorption length islonger than the sample thickness and therefore themethod is not valid in this case. Theoretically, Eq.(3) could be rederived without any assumption ofthe exciton diffusion length relative to the samplethickness and then this new equation could be fittedto the data to obtain the exciton diffusion length.Using Eqs. (1) and (2), without making any assump-tions as to the exciton diffusion length relative to thesample thickness, the current from the backside canbe calculated as

J / a

b2 � a2�b

2� e�bL�aL � ebL�aL

e�bL � ebL

� �� ae�aL

� �:

Fig. 4. 1/[Jexp(aL)] as a function of 1/a. For the wavelengthrange between 350 and 420 nm, the x-intercept gives an excitondiffusion length of 248 A.

M.R. Black et al. / Organic Electronics 8 (2007) 601–605 605

When this new equation is derived and then fittedto the data, the data fits well. Unfortunately, evenwith the new equation, this method is expected tobe inaccurate when the absorption length is verysmall. However, ignoring the limits of this methodand fitting the data to find the exciton diffusionlength in the low energy range, an exciton diffusionlength much longer than the sample thickness isderived. Within this wavelength range, the absorp-tion and photocurrent have a low energy tail, so itis possible that the photocurrent and absorptionresult from direct coupling into a triplet-like statewhich should have a weak absorption and a verylarge exciton diffusion length. However, signifi-cantly more research needs to be conducted to accu-rately identify the absorption and photocurrentmechanisms in Eu-DM.

Table 1 compares the exciton diffusion length ofEu-DM, as determined from this study, with excitondiffusion lengths of other materials. The exciton dif-fusion length of Eu-DM is comparable to other smallmolecule systems which were evaporated or subli-mated, and the diffusion length is larger than the spincoated polymer films. The large exciton diffusionlengths expected for metal-organic films were notobserved in this study. However, the energy at whichthe large exciton diffusion length states exist inEu-DM could very likely be outside the energy rangeof this measurement and another method of measur-ing exciton diffusion would need to be employed.

Table 1Exciton diffusion lengths, the method of deposition, and thecorresponding references

Material Lex (A) Deposition method Refs.

Eu-DM 250 ± 8 Spin coated This workPTCBI 30 ± 3 Sublimation/evaporation [8]PTCDA 880 ± 60 Evaporation [5]PPEI 25,000 Evaporated [9]CuPc 100 ± 30 Sublimation/evaporation [8]

680 ± 200 Evaporation [10]C60 400 ± 50 Sublimation/evaporation [8]

141 Sublimation [11]Alq3 200 Evaporation [12]

210 Evaporation [13]PPV 70 ± 10 Spin coated [14]

120 ± 30 Spin coated [10]PEOPT 47 Spin coated [11]

50 Spin coated [15]

PTCBI = 3,4,9,10-perylenetetracarboxylic bis-benzimidazole,PTCDA = 3,4,9,10-perylenetetracarboxylic dianhydride, PPEI =Perylene bis(phenethylimide), CuPc = copper phthalocyanine,Alq3 = tris(8-hydroxyquinoline) aluminum, PPV = poly(pheny-lene–vinylene), PEOPT = poly (3-(40-(100,400,700-trioxaoctyl)-phenyl)thiophene).

4. Conclusion

The exciton diffusion length of spin coated Eu-DM films was measured by analyzing the photocur-rent and absorption as a function of wavelength.For exciton wavelengths between 350 and 420 nm,the exciton diffusion length is 250 ± 8 A. This iscomparable to other small molecule systems andlarger than spin coated polymers. The expectedlarge enhancement of the exciton diffusion lengthresulting from the spin–orbit coupling in Eu-DMwas not observed, but likely occurs at lower incidentphoton energies than those energies for which themethod employed in this paper is valid.

Acknowledgements

The authors thank Brian Crone for his feedbackon this work, help in setting up equipment, andsupervision of this project. We also acknowledgeIan Campbell, Marc Baldo, and Vladimir Bulovicfor useful discussions. This research was supportedby the Los Alamos National Laboratory DirectedResearch and Development Program.

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