刘启洋，外⽂翻译原⽂
Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cells
Jingyu Zou,1Hin-Lap Yip,1,2Steven K.Hau,1and Alex K.-Y.Jen1,2,a?
1Department of Materials Science and Engineering,University of Washington,Seattle,
Washington98195,USA
2Institute of Advanced Materials and Technology,University of Washington,Seattle,
Washington98195,USA
transparent中文翻译Received19January2010;accepted23March2010;published online17May2010
A simple method was developed using metal grid/conducting polymer hybrid transparent electrode
to replace indium tin oxide?ITO?for the fabrication of inverted structure polymer solar cells.The
performance of the devices could be tuned easily by varying the width and separation of the metal
grids.By combining the appropriate metal grid geometry with a thin conductive polymer layer,
substrates with comparable transparency and sheet resistance to those of ITO could be achieved.
Polymer solar cells fabricated using this hybrid electrode show ef?ciencies as high as?3.2%.This
method provides a feasible way for fabricating low-cost,large-area organic solar cells.?2010
American Institute of Physics.?doi:10.1063/1.3394679?
Polymer solar cells?PSCs?are becoming as a viable
technology for low-cost power production.1Indium tin oxide ?ITO?is the most commonly used transparent electrode for PSCs because it offers good transparency in the visible range
of the solar spectrum as well as good electrical conductivity.
However,there are several de?ciencies that exist for using
ITO such as poor mechanical properties of ITO-coated plas-
tic substrates,2limited conductivity for fabricating large-area
solar cells,limited availability of indium,and complicated vacuum sputtering process tend to increase the cost for ITO. These limitations set a potential barrier for the commercial-ization of low-cost PSCs.To alleviate this problem,alterna-tive materials for transparent conducting electrodes are needed to replace ITO.There has been some research on exploring conductive polymers,2carbon nanotubes,3 graphenes,4and silver nanowires5as potential candidates to replace ITO.However,lower transparency and higher sheet resistance compared to ITO strongly hinder their use for transparent electrode.Metal grids have also been investi-gated as a promising alternative for transparent electrode.6,7 Utilizing micro?uidic deposition and nanoimprinting meth-ods,metal grids coated substrates have been used for fabri-cating conventional PSCs with PCE as high as2%.
Previously,inverted architecture PSC using ITO as cath-ode and evaporated silver?Ag?as anode has been proved to be more stable in ambient than the conventional devices us-ing sensitive metal as cathode.8Moreover,poly?3,4-ethylenedioxythiophene?:poly?styrenesulfonate??PEDOT-:PSS?has also been demonstrated by Hau et al.9as a potential replacement of ITO for fabricating inverted PSCs. However,the relatively high sheet resistance of PEDOT:PSS compared to ITO may limit the performance of PSCs The combination of conductive metal grids with PEDOT:PSS provides a good solution to solve this problem and obtain ITO-free and ambient stable PSCs.
Here,we report a simple method to fabricate high-ef?ciency ITO-free inverted structure PSCs using a metal grid/conducting polymer hybrid transparent electrode.By us-ing soft lithography and chemical etching,the metal grids
can be easily fabricated on substrates.The inverted device
architecture is used to fabricate PSCs with Ag as anode to
collect holes and zinc oxide?ZnO?as an electron selective
layer at the metal grid/conducting polymer interface to help
collect electrons.To prepare the metal grids,a1nm alumi-
num?Al??lm was deposited?rst followed by evaporating a
30nm thick silver?lm onto the glass substrates.It was found
that the very thin Al layer improves adhesion between the
substrate and the Ag?lm.
A micropatterned photoresist?SU-8,MicroChem??lm,
fabricated by standard photolithography,was used as a mas-
ter to replicate stamps for microcontact printing??CP?.A typical stamp was made by casting a10:1?v/v?mixture of
polydimethyl siloxane?PDMS?and curing agent against a
silanized master.The PDMS stamp was?rst soaked with an
“ink”containing1mM of mecaptoundecanoic acid?MUA?
in ethanol,then brought into contact with the surface of sil-
ver for60s.After the removal of the stamp,the patterned Ag
lm was developed by wet etching with aqueous
Fe3+/thiourea using the patterned SAM as resist.10MUA was chosen for both generating better wettability for the process-
ing of upper layer?lm.and facilitating better charge
collection.11
Three types of designed grids patterns were utilized
as width and separation with?1?5?m and50?m ?5?m/50?m?,?2?10?m and100?m ?10?m/100?m?,?3?20?m and200?m ? 20?m/200?m?,respectively.The optical transmittance of as-fabricated Ag grids was measured by UV-Vis spectros-copy? Fig.1?.The sheet resistance for metal grids with dif-ferent geometries was also measured.
The transmittance of ITO at550nm is85.7%.When the
transmittance of glass??93%?is taken into account,the maximum expected transmittance of the metal grids coated glass substrate is calculated to be?83–84%.It was found that the average transmittance of metal grid coated glass sub-strates in the range of250–1200nm is78%,80%,and82%, respectively,for grids of5?m/50?m,10?m/100?m, and20?m/200?m.The lower transmission may be due to
a?Electronic mail:ajen@www.doczj/doc/1d16844512.html
.
APPLIED PHYSICS LETTERS96,203301?2010?
0003-6951/2010/96?20?/203301/3/$30.00?2010American Institute of Physics
96,203301-1
some diffusion of MUA during contact printing on the metal surface.As the result,the actual Ag grid patterns expand 1–2?m in width.Figure 2?a ?shows the optical microscope images of an as-fabricated metal grid electrode on a glass substrate.
Another important parameter for transparent conducting electrodes is the sheet resistance.The sheet resistance of commercial available ITO substrates is 151,??while the Ag grid electrodes exhibited sheet resistances of
9.11,
146.31,and 254.11,for 5?m /50?m,10?m /100?m,and 20?m /200?m,re-spectively.Lower sheet resistances will
minimize the loss of photocurrent during charge transport due to the lowered lat-eral resistance of the electrode.In general,the transmittance and sheet resistance for thin conductive ?lms are related by the equation of
T =?
1+
188.5R s ?Op
DC
2
,
where ?Op is the optical conductivity ?here we quoted at ?=550nm ?and ?DC is the conductivity of the ?lm.?DC /?Op is a commonly used term to describe transparent conductors.12For ITO with R s of 151and T ?550nm ?of 85.7%,the ?DC /? Op is 156.7.The best results that have been achieved for the graphene-based ?lms 13and carbon nanotubes 14are 0.5and 25,respectively.Based on the best metal grid geometry used in this work ?5?m /50?m ?,R s =9.1?/?,and T ?550nm ?
=79.0%,a ?DC /?Op ratio as high as 165.6could be achieved.
To fabricate solar cells,the ITO substrate and the metal grids coated substrate were cleaned using standard cleaning procedures.A thin layer of ZnO nanoparticles ?ZnO NPs ?
was spin-coated onto these substrates.A C 60-based SAM ?C 60-SAM ?was deposited onto the ZnO surface using a spin-coating process as reported previously.15A 200nm bulk-heterojunction ?lm comprising of poly ?3-hexylthiophene ??P3HT Rieke Metals ?and ?6,6?phenyl C 61butyric acid methyl ester ?PCBM American Dye Source ?was then spin-coated in an argon-?lled glove box.
After depositing a 50nm of PEDOT:PSS ?lm ?H.C.Starck,CLEVIOS?P VP 4083?.8A layer of Ag was vacuum deposited on top of PEDOT:PSS as anode.The solar cells were tested under ambient using a Keithley 2400SMU and an Oriel Xenon lamp ?450W ?with an AM 1.5?lter.The light intensity was calibrated to 100mW /cm 2.
The device architectures are shown in Fig.2?b ?.The J -V characteristics under illumination and the solar cells perfor-mance are summarized in Fig.3?a ?and Table I ,respectively.The device with the 5?m /50?m Ag grid has the best performance with PCE of 2.97%.The lower ef?ciency of the metal grid substrate derived device is mainly due to lower J sc and ?ll factor.
An important parameter that needs to be considered for the design of metal grids is that the charges generated from the voids between the grid lines need to be ef?ciently col-lected.The inverted device structure utilizes a ZnO NP layer as an electron selective layer between the active layer and the metal grids to collect electrons.The inhomogeneous and poor charge collection in the voids due to high sheet resis-tance of ZnO decrease both J sc and ?ll factor.In
addition,
FIG.1.?Color online ?Transparency vs wavelength of different geometry Ag grids on glass as compared to transparency of ITO and glass,as refer-enced against
air.
FIG.2.?Color online ??a ?Optical microscope image of silver grid with 5?m width separated by a distance of 50?m.?b ? Device con?guration of the polymer solar cell using Ag grid as the transparent electrode with or without conductive PEDOT:PSS
layer.
FIG.3.?Color online ?The current density-voltage ?J ?V ?characteristics of polymer solar cells with ?a ?different Ag grid geometries ?b ?different Ag grid geometries combining 40nm PEDOT:PSS PH500?lm measures under AM1.5illumination from a calibrated solar simulator with a light intensity of 100mW cm ?2.
the lower transmittance of the5?m/50?m Ag grids ??78%?compared to ITO??85%?also contributes to the decrease in J sc.
Increasing the width and separation while maintaining the same aspect ratio dramatically reduces the J sc and?ll-factor.To alleviate the problem for poor charge collection,a PEDOT:PSS?H.C.Starck,CLEVIOS?PH500?conduct-ing polymer was inserted between the silver grids and the ZnO layer to form the hybrid electrode.
To demonstrate the function of the hybrid electrode,an inverted photovoltaic device with220nm thick PEDOT:PSS PH500?lm without metal grids as the bottom electrode was fabricated.The J-V characteri
stics under illumination and the solar cells performance are summarized in Fig.3and Table I, respectively.For the smaller separation Ag grids ?5?m/50?m?,the addition of the conducting PEDOT:PSS polymer layer improved the device performance to3.21% due to the reduced lateral resistance.For devices using larger separation grid lines?10?m/100?m and 20?m/200?m?,the addition of the PEDOT:PSS layer sig-ni?cantly improved the performance of the devices.For grids with separation distance of over10?m,additional layer of PEDOT:PSS is necessary to reduce the lateral resistance.All three silver grids electrodes can achieve near3%PCE using the additional PEDOT:PSS layer.The potential bene?t of using larger size grid patterns is the ease for device fabrica-tion especially for cost ef?cient industrial roll-to-roll pro-cessing.Without Ag grids,PEDOT:PSS PH500bottom elec-trode devices can only have ef?ciency of?2.2%which is due to the high sheet resistance.
In conclusion,we have demonstrated that silver metal grid electrodes fabricated by microcontact printing and wet chemical etching can replace conventional ITO electrodes for fabricating organic solar cells.The patterned metal elec-trodes on glass show high optical transmittance as well as good electrical www.doczj/doc/1d16844512.html
anic solar cells with opti-mized grid geometry show encouraging device performance. It was also found
that silver grid electrodes with smaller width and separation with the same aspect ratio facilitated better charge collection from the ZnO NP layers leading to increased FF,J sc,and PCE.By adding a PEDOT:PSS PH500 conducting polymer between Ag grid and ZnO,even devices with larger Ag grid spacing can achieve good performance. The use of inexpensive Ag grids compared to ITO allows the possibility of employing roll-to-roll process to realize low-cost,large-area organic solar cells.
This work is supported by the National Science Founda-tion’s NSF-STC Program under Grant No.DMR-0120967, the Department of Energy’s“Future Generation Photovoltaic Devices and Process”Program under Grant No.DE-FC36-
08GO18024/A000,and the Of?ce of Naval Research’s Pro-gram under Grant No.N00014-08-1-1129.A.K.-Y.J.thanks the Boeing-Johnson Foundation for?nancial support.
1G.Dennler,M.C.Scharber,and C.J.Brabec,Adv.Mater.?Weinheim, Ger.?21,1323?2009?.
2S.I.Na,S.S.Kim,J.Jo,and D.Y.Kim,Adv.Mater.?Weinheim,Ger.?20,4061?2008?.
3M.W.Rowell,M.A.Topinka,M.D.McGehee,H.J.Prall,G.Dennler,N. S.Sariciftci,L.Hu,and G.Gruner,Appl.Phys.Lett.88,233506? 2006?. 4G.Eda,Y.Y.Lin,www.doczj/doc/1d16844512.html
ler,C.W.Chen,W.F.Su,and M.Chhowalla, Appl.Phys.Lett.92,233305?2008?.
5J.Y.Lee,S.T.Connor,Y.Cui,and P.Peumans,Nano Lett.8,689?2008?. www.doczj/doc/1d16844512.html ingstedt and O.Inganas,Adv.Mater.?Weinheim,Ger.?19,2893?2007?.
7M.Kang,M.Kim,J.Kim,and L.J.Guo,Adv.Mater.?Weinheim,Ger.?20,4408?2008?.
8S.K.Hau,H.L.Yip,N.S.Baek,J.Zou,K.O’Malley,and A.K.-Y.Jen, Appl.Phys.Lett.92,253301?2008?.
9S.K.Hau,H.L.Yip,J.Zou,and A.K.-Y.Jen,Org.Electron.10,1401?2009?.
10J.M.McLellan,M.Geissler,and Y.Xia,J.Am.Chem.Soc.126,10830?2004?.
11H.L.Yip,S.K.Hau,N.S.Baek,and A.K.-Y.Jen,Appl.Phys.Lett.92, 193313?2008?.
12S.De,T.M.Higgins,P.E.Lyons,E.M.Doherty,P.N.Nirmalraj,W.J. Blau,J.J.Boland,and J.N.Coleman,ACS Nano3,1767?2009?.
13X.Wang,L.Zhi,and K.Mullen,Nano Lett.8,323?2008?.
14H.Z.Geng,D.S.Lee,K.K.Kim,G.H.Han,H.K.Park,and Y.H.Lee, Chem.Phys.Lett.455,275?2008?.
15S.K.Hau,H.L.Yip,H.Ma,and A.K.-Y.Jen,Appl.Phys.Lett.93, 233304?2008?.
TABLE I.Summary of PSCs performance with different Ag grids width and separation.
V oc ?V?
J sc
mA/cm2FF
PCE
%
R s
cm2?
R p
cm2?
ITO0.6110.720.66 4.350.9780.2 Ag Grid5?m/50?m0.609.570.52 2.97 2.81341.6 Ag Grid10?m/100?m0.59 6.620.42 1.65 3.0327.4 Ag Grid20?m/200?m0.58 4.330.49 1.00 3.1322.1 Ag Grid5?m/50?m-PEDOT0.609.390.57 3.21 2.81118.0 Ag Grid10?m/100?m-PEDOT0.609.140.58 2.93 2.91213.2 Ag Grid20?m/200?m-PEDOT0.608.950.53 2.85 3.0956.3 PEDOT0.628.910.40 2.2030.7365.1