Dicyanovinyl-quinquethiophenes with varying alkyl chain lengths: Investigation of theirperformance in organic devicesKerstin Schulze, Moritz Riede, Eduard Brier, Egon Reinold, Peter Bäuerle, and Karl Leo Citation: Journal of Applied Physics 104, 074511 (2008); doi: 10.1063/1.2990071 View online: http://dx.doi.org/10.1063/1.2990071 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/104/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrical response of highly ordered organic thin film metal-insulator-semiconductor devices J. Appl. Phys. 106, 114505 (2009); 10.1063/1.3267045 Interfacial electronic structure of a hybrid organic-inorganic optical upconverter device: The role of interfacestates J. Appl. Phys. 105, 083706 (2009); 10.1063/1.3110076 Effect of self-organization in polymer/fullerene bulk heterojunctions on solar cell performance Appl. Phys. Lett. 89, 063505 (2006); 10.1063/1.2335377 Fullerene-doped hole transport molecular films for organic light-emitting diodes Appl. Phys. Lett. 86, 143509 (2005); 10.1063/1.1899241 The effect of C60 doping on the device performance of organic light-emitting diodes Appl. Phys. Lett. 86, 063514 (2005); 10.1063/1.1861962

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Dicyanovinyl-quinquethiophenes with varying alkyl chain lengths:Investigation of their performance in organic devices

Kerstin Schulze,1,a� Moritz Riede,1 Eduard Brier,2 Egon Reinold,2 Peter Bäuerle,2 andKarl Leo1,b�

1Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1,01069 Dresden, Germany2Institut für Organische Chemie II und Neue Materialien, Universität Ulm, 89081 Ulm, Germany

�Received 3 June 2008; accepted 13 August 2008; published online 13 October 2008�

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl sidechain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells withfullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbitalenergy. However, differences in the current-voltage-characteristics, particularly in the fill factor, areobserved. We analyze both derivatives in hole-only devices and find a difference in the holeinjection between the doped hole transport layer and the oligothiophenes. Additionally, wedetermine the hole mobility of the two materials and explain the different behaviors of the twomaterials in solar cells. © 2008 American Institute of Physics. �DOI: 10.1063/1.2990071�

I. INTRODUCTION

In comparison to inorganic photovoltaics, organic solarcells have the potential to achieve lower cost: organic mate-rials usually exhibit high absorption coefficients, such thatthin layers of several tenths of nanometers of the compara-tively cheap and abundant organic compounds are sufficientfor photovoltaic devices. Additionally, the possibility of roll-to-roll deposition at room temperature reduces cost. Bothsmall molecule or polymer organic solar cells have alreadyachieved power conversion efficiencies of up to 5–6 %.1–3

Still, organic solar cells have to be improved further, espe-cially concerning efficiency and device stability.

One possibility of improving efficiency is the use of op-timized electron donor and acceptor materials to minimizeenergetic losses during exciton separation. We use here quin-quethiophenes with a low lying highest occupied molecularorbital �HOMO� �approximately −5.6 eV determined withultraviolet photoelectron spectroscopy �UPS�� instead of thefrequently used metal-phthalocyanines �e.g., HOMO of zincphthalocyanine: −5.2 eV4� as donor when using fullereneC60 as acceptor. We have shown before that derivatives ofthis material class are promising donors in combination withC60 in small molecule organic solar cells.5,6 These solar cellsreach an open circuit voltage Voc of approximately 1 V andpower efficiencies of up to 3.4 %. The approximately 0.4 Vhigher Voc compared to phthalocyanine based solar cells �inplanar heterojunction solar cells� is caused by the increase inthe ionization potential of the oligothiophene compared tothe phthalocyanine.

In our small molecule solar cells, the photoactive mate-rials are usually embedded between a hole transport layer�HTL� and a combination of a thin 4,7-diphenyl-1,10-phenanthroline �BPhen� layer, with aluminum serving as

cathode. Because of the high ionization potential of the oli-gothiophene, the contact to the HTL has a major influence onthe IV-curve of the device.6 When using HTLs with highionization potential, the difference between the built-in po-tential Vbi and the open circuit voltage Voc leads to a S-shapein the IV-curve. The Vbi is defined by the difference of theFermi levels at the contacts, in our case in the doped HTLand the BPhen/Al-contact. The Voc is defined by the differ-ence of the quasi-Fermi levels in the donor and acceptorunder illumination. It was shown that this problem can bereduced or avoided by using an HTL with nearly identicalHOMO values compared to the oligothiophene.5,7 As we willshow in this paper, the S-shape does not only depend on theenergetic difference between the HOMO-levels of the HTLand the oligothiophenes but additionally on the type of theoligothiophene.

We analyze the interface between HTLs and two oligoth-iophene derivatives in solar cells and hole-only devices. Thechemical structures of the two dicyanovinylquin-quethiophene �DCV5T� derivatives DCV5T-Bu andDCV5T-Et are shown in Fig. 1�a�. These molecules consistof five thiophene units and dicyanovinyl-groups as end-groups as well as two alkyl side chains at the second and

a�Electronic mail: schulze@iapp.de.b�Author to whom correspondence should be addressed. Electronic mail:

leo@iapp.de. FAX:��49-351-46337065.

FIG. 1. �Color online� �a� Chemical structure of the DCV5T with butyl- orethyl-sidechains �DCV5T-Bu and DCV5T-Et�. �b� Energetic alignment ofHOMO and LUMO of the donor and acceptor as well as the adjacent holetransport material MeO-TPD and �-NPD of the four devices.

JOURNAL OF APPLIED PHYSICS 104, 074511 �2008�

0021-8979/2008/104�7�/074511/5/$23.00 © 2008 American Institute of Physics104, 074511-1

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fourth thiophene. The only structural difference betweenDCV5T-Bu and DCV5T-Et is the length of the alkyl chains,which varies between C4H9 �Bu� and C2H5 �Et�.

II. EXPERIMENTAL

For the analysis of the behavior of these derivatives inorganic solar cells, two different hole transportmaterials N,N,N� ,N�-tetrakis�4-methoxyphenyl�-benzidine�MeO-TPD� �Sensient, Wolfen� and N,N�-di�naphthalen-1-yl�-N,N�-diphenyl-benzidine ��-NPD��Sensient, Wolfen� are used. These materials are p-doped us-ing the proprietary acceptor dopant NDP-2 from NovaledAG, Dresden �doping ratio of approximately 1.3 wt % inMeO-TPD and 10 wt % in �-NPD�. Between the p-dopedHTL and the oligothiophene, 5 nm of the intrinsic HTL isdeposited to avoid exciton quenching, which occurs by adirect contact between the dopant molecules and the photo-active materials, especially at a contact to a highly dopedHTL.8 In the hole-only devices, we use additionally ap-doped zinc phthalocyanine �ZnPc� �Alfa Aesar GmbH &Co. KG, Karlsruhe� layer in direct contact to Au to providean ohmic contact. The p-doping ratio is set to approximately2 wt %. The materials are cleaned at least twice by thermalgradient sublimation before evaporation, except the oligoth-iophenes and the acceptor dopant, which are used as re-ceived.

All solar cells use the following layer sequence: indiumtin oxide �ITO�/1 nm Au/HTL/donor/52 nm C60 /6 nmBPhen/100 nm Al. The materials and layer thicknesses of theHTL and donor are summarized in Table I. A scheme of theenergetic alignment of the HOMO and lowest unoccupiedmolecular orbital �LUMO� of the acceptor, donor, andadjacent HTLs is shown in Fig. 1�b�. The additionalp-doped 4,4� ,4�-tris�2-naphthylphenylamino�-triphenylamin�TNATA� layer in device B does not influence the IV-curve,but lowers the possibility of shortcuts when using relativelyrough ITO substrates. The structure of the hole-only devicesis: ITO/1 nm Au/30 nm p-doped �-NPD /x nmoligothiophene/20 nm p-doped �-NPD /11.4 nm p-dopedZnPc/50 nm Au using either DCV5T-Bu or DCV5T-Et withdifferent thicknesses �DCV5T-Bu: 45, 60, and 75 nm;DCV5T-Et: 75, 90, and 150 nm�.

The devices with a photoactive area of 4–7 mm2 aredeposited in a UHV multichamber system �typical pressureof 10−8–10−7 mbar during evaporation�. Current voltagemeasurements are carried out in a nitrogen glovebox using aSource Measurement Unit 236 �Keithley� and illumination isprovided by a sun simulator �Hoenle AG�. Intensity calibra-

tion of the sun simulator is done by using an outdoor refer-ence cell calibrated by the Fraunhofer Institute for Solar En-ergy Systems �Freiburg, Germany�. The external quantumefficiency �EQE� measurement is done by using a homemadesetup based on a xenon lamp and a grating monochromator.The setup is calibrated using a Newport powermeter. Absorp-tion and photoluminescence spectra are recorded with stan-dard spectrometers from Shimadzu and SPEX, respectively.

III. RESULTS AND DISCUSSION

The IV-curves of the devices A–D are presented in Fig.2�a�. The IV-curve of device B has already been shown inRef. 9 and the IV-curve of device D has been presented inRef. 6 but both solar cells were discussed in another context.Additionally, the absorbance of the photoactive materials C60

and DCV5T-Bu in thin film and the external quantum effi-ciency spectrum of device B are shown in Fig. 2�b�. As thecurrent density of this device scales linearly with light inten-sity, we integrate this spectrum with the AM 1.5 spectrum toestimate the spectral mismatch for this cell. For the othercells, we assume a comparable mismatch factor as that de-termined for device B. The characteristic parameters of the

TABLE I. Layer sequence of the HTL and donor in the four solar cells:ITO/1 nm Au/HTL/donor/52 nm C60 /6 nm BPhen/100 nm Al.

Device HTL Donor

A 40 nm p-doped MeO-TPD/5 nm MeO-TPD 9.8 nm DCV5T-BuB 30 nm p-doped TNATA/10 nm

p-doped �-NPD /5 nm �-NPD9.8 nm DCV5T-Bu

C 40 nm p-doped MeO-TPD/5 nm MeO-TPD 10 nm DCV5T-EtD 10 nm p-doped �-NPD /5 nm �-NPD 10 nm DCV5T-Et

FIG. 2. �a� IV-curves of the devices using either DCV5T-Bu or DCV5T-Etas donor and MeO-TPD or �-NPD as contacting HTL. �b� Absorption ofthin films of the materials C60 �dashed line� and DCV5T-Butyl �dotted line�and spectrum of the external quantum efficiency of device B �straight line�.

074511-2 Schulze et al. J. Appl. Phys. 104, 074511 �2008�

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photovoltaic cells are given in Table II. The current densitiesare the values determined under the sun simulator, whereasfor the estimated efficiencies, the spectral mismatch factorwas taken into account.

All devices reach a comparable open circuit voltages Voc

of approximately 1 V. This is due to the Voc being determinedby the difference of the quasi-Fermi levels in the donor andacceptor under illumination, which should be independent ofthe HOMO-value of the hole transport material. A compa-rable open circuit voltage of the devices means that the ion-ization potential of both derivatives has to be nearly identi-cal. By using UPS we determine the ionization potential ofapproximately 5.6 eV for both derivatives, which means thatthe ionization potential is independent of the length of thealkyl chains of these quinquethiophenes. However, a signifi-cant difference in the performance depending on the HTL isvisible, analogous to our previous results.5 Caused by thelow lying HOMO of the oligothiophene �−5.6 eV�, the dif-ference between Voc and Vbi when using MeO-TPD �HOMO:−5.1 eV, determined with UPS� as HTL is much highercompared to the devices with �-NPD �HOMO: −5.5 eV,determined with UPS�. In the voltage range between Vbi andVoc, the free charge carriers are only driven by diffusion tothe contacts. This causes a higher recombination rate, leadingto a S-shape in the IV-curve.

However, the two oligothiophene derivatives exhibit adifference in the shape of the IV-curves even for the sameHTL. This is especially obvious in solar cells B and D, bothusing �-NPD as adjacent hole transport material. UPS mea-surements indicate that these differences are not caused bydifferent energetic alignments; the data do not yield a differ-ence in energy level alignment between the HTL, the olig-othiophenes, and the fullerene. We determine the formationof an almost identical injection barrier between a p-doped�-NPD-layer and the oligothiophene derivatives of approxi-mately 0.22–0.25 eV. It seems that the S-shape in the deviceusing DCV5T-Bu is still pronounced but weaker in the de-vice with DCV5T-Et. This means that effects beyond theenergetic difference between the HOMO of the donor and theHTL must be responsible for the shape of the IV-curve. Wehave observed such behavior for many devices. In the fol-lowing, we give a possible explanation for the differences inthe solar cell performance depending on the oligothiophenederivative.

To further study these effects, we have prepared hole-only devices to analyze both the hole injection behavior be-tween p-doped �-NPD and the oligothiophenes and the hole

mobilities on DCV5T-Bu and DCV5T-Et. Two structureshave been prepared, each having the following layer se-quence: ITO/1 nm Au/30 nm p-doped �-NPD /75 nmoligothiophene/20 nm p-doped �-NPD /11.4 nm p-dopedZnPc/50 nm Au using either DCV5T-Bu or DCV5T-Et. TheIV-curves of these devices are shown in Fig. 3�a�. The holeinjection via ITO is given for negative applied voltages andvia the Au contact for positive applied voltages.

Two significant points are visible. First, the hole-onlydevice using DCV5T-Et reaches higher current densities atthe same applied voltages compared to the device withDCV5T-Bu. Second, for the device using DCV5T-Et, a sym-metrical IV-curve is observed, whereas the cell usingDCV5T-Bu shows an asymmetrical curve. For a better visu-alization, the absolute value of the ratio between forwardcurrent and backward current is shown in Fig. 3�b�. Thisindicates a better hole injection for p-doped �-NPD andDCV5T-Et via the ITO / p-doped �-NPD-contact than forDCV5T-Bu. The different behavior of the hole-only devicesseems to depend on the length of the alkyl chain. Interactionof the molecules is possible between the cyanogroups andthe hydrogenatoms �CN¯H� �Ref. 10� of the oligothiopheneand �-NPD molecules. The asymmetry of the device usingDCV5T-Bu shows that the problem in hole injection is morepronounced when DCV5T-Bu is deposited on top of thep-doped �-NPD layer, compared to a deposition of p-doped�-NPD on DCV5T-Bu. An amorphous growth of DCV5T-Bu, as it was observed using atomic force microscopy,7

might be responsible for the better hole injection on the sec-ond interface �according to the deposition sequence�,whereas the influence of the length of the alkyl chains isresponsible for the problems in hole injection on the firstinterface to p-doped �-NPD. An evaluation of the HOMOvalue of the oligothiophene and the �-NPD reveals that thefirst interface has an energetic barrier and thus seems respon-sible for the injection problem. This difference is not ob-served in the device using DCV5T-Et due to the shorter alkylchains, which might lead to a shorter distance betweenneighboring molecules. These findings might explain theS-shape of device B compared to device D, in which we donot observe such an effect. Problems in hole injection be-tween the donor and the HTL lead in this case to a higherconcentration of holes in the oligothiophene, which results ina higher recombination rate especially when the charge car-riers are mainly driven to the contacts by diffusion �which isthe case when voltages are applied between Vbi and Voc�.

To further elaborate the differences in the

TABLE II. Short circuit current density jsc �normalized at a sun intensity of the sun simulator of 100 mW /cm2

determined with the reference cell without spectral mismatch�, open circuit voltage Voc, fill factor �FF�, satu-ration factor, and estimated efficiency � �spectral mismatch of 1.34 as it was estimated for device B is taken intoaccount� of devices A, B, C, and D.

jsc Voc FF Saturation Estimate �

Device Donor HTL �mA /cm2� [V] [%] j−1 V / jsc [%]

A DCV5T-Bu MeO-TPD 9.7 1.00 28.2 1.16 2.0B DCV5T-Bu �-NPD 8.9 1.00 50.5 1.10 3.4C DCV5T-Et MeO-TPD 7.0 0.95 25.0 1.42 1.2D DCV5T-Et �-NPD 8.4 1.00 39.9 1.15 2.5

074511-3 Schulze et al. J. Appl. Phys. 104, 074511 �2008�

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IV-characteristics, we estimate the hole mobility � in theoligothiophenes. We use the following equation for space-charge limited currents11

j =9

8�0�r�

U2

d3 , �1�

where d is the thickness of the intrinsic layer. The voltagedrop in the p-doped layers is thereby neglected. Todetermine the hole mobility, we use the hole-only deviceshaving the following structure: ITO/1 nm Au/30 nm p-doped�-NPD /x nm oligothiophene/20 nm p-doped�-NPD /11.4 nm p-doped ZnPc/50 nm Au. We choose 45,60, and 75 nm thick DCV5T-Bu layers and 75, 90, and 150nm thick DCV5T-Et layers. Because of the higher hole cur-rents, thicker layers for the ethyl-type derivative are neces-sary. The IV-curves are all shown in Fig. 4. When fittingusing the relation j�da, we observe in both cases a value fora somewhat higher than the expected −3. Therefore, we as-sume that the current is not fully space charge limited andinjection limitation occurs additionally. We use the relation

j�Ux for the curves in Fig. 4 to prove the behavior ofj� f�U�. For the devices using DCV5T-Bu, we fit between0.25 and 3 V and for the devices using DCV5T-Et, a good fitwas found for applied voltages between 2.5 and 5 V. We findx�2 as expected from Eq. �1�. By neglecting the injectionlimitation and using an average value of 3.5 for �r, we esti-mate the hole mobility to 4.6�10−6 cm2 /Vs for DCV5T-Bu�extracted from the device using 60 nm DCV5T-Butyl� and1.5�10−5 cm2 /Vs for DCV5T-Et �for the device using 150nm DCV5T-Et�. The hole mobility in DCV5T-Et is roughlyone order of magnitude higher compared to DCV5T-Bu. Theshorter alkyl chains in DCV5T-Et might lead to a lower dis-tance between neighboring molecules, supporting the hop-ping transport to achieve higher hole mobility. Additionally,due to the CN¯H-interaction between the oligothiophenemolecules mentioned above, interactions between alkylchains are possible, which is more pronounced for longerchains.12 Unfortunately, we are not able to determine thedistance between the molecules, because the molecules tendto grow amorphous and measurements using transmissionelectron microscopy do not show any crystallinity.

FIG. 3. �a� IV-curves of the devices using the following layer sequence:ITO/1 nm Au/30 nm p-doped �-NPD /75 nm oligothiophene/20 nmp-doped �-NPD /11,4 nm p-doped ZnPc/50 nm Au. Using DCV5T-Et theIV-curve shows a symmetrical behavior, while using DCV5T-Bu an asym-metrical curve is observed. This is quantified in �b�, where the absolute ratiobetween forward to backward current is shown.

FIG. 4. IV-curves of hole-only devices using �a� 45, 60, and 75 nmDCV5T-Bu or �b� 75, 90, and 150 nm DCV5T-Et. In the inset, the parameterx according to the relation j�Ux is noted �used range: �a� 0.25–3 V and �b�2.5–5 V�.

074511-4 Schulze et al. J. Appl. Phys. 104, 074511 �2008�

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Besides the difference of the hole injection, the differ-ence in charge carrier mobility could be a possible explana-tion for the differences in the IV-curves of the solar cells Band D. This applies especially at voltages around Voc, wherethe free charge carriers are mainly driven by diffusion andnot supported by the internal electrical field. This leads to ahigher concentration of free charge carriers in the oligoth-iophene layer and this high concentration especially at theinterface between donor and acceptor leads to a higher re-combination. The diffusion coefficient is directly connectedwith the mobility of the charge carriers and this means thelower the charge carrier mobilities, the higher is the recom-bination rate at the interface in our devices.

IV. CONCLUSION

In conclusion, we present organic solar cells usingDCV5T derivatives with two different alkyl chain lengths,DCV5T-Bu and DCV5T-Et, as electron donor in planar het-erojunction devices. Both materials have an identical HOMOenergy, but show a different behavior of the IV-curves. Pos-sible explanations for the differences in the performance ofthe solar cells depending on the used oligothiophene deriva-tives are a difference in the hole injection between the HTLand the oligothiophene as well as a difference in hole mobil-ity of the two materials. This can be explained by the differ-ence in the distance between neighboring molecules, whichinfluences the packing. Therefore the molecule with shortersidechains, DCV5T-Et, exhibits less problems in hole injec-tion between the HTL and oligothiophene and a higher hole

mobility. This caused a lower free charge carrier density inDCV5T-Et, leading to lower recombination rates at appliedvoltages around Voc, so that an S-shape of the IV-curve doesnot appear.

ACKNOWLEDGMENTS

We thank Selina Olthof for the UPS measurements andthe Deutsche Forschungsgemeinschaft via the Leibniz-Preisand the Fonds der chemischen Industrie for financial support.

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