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ApplPhysLett_99_063301

ApplPhysLett_99_063301
ApplPhysLett_99_063301

Air stable hybrid inverted tandem solar cell design

Feng Liu and Jean-Michel Nunzi

Citation: Appl. Phys. Lett. 99, 063301 (2011); doi: 10.1063/1.3622119

View online: https://www.sodocs.net/doc/bc18000263.html,/10.1063/1.3622119

View Table of Contents: https://www.sodocs.net/doc/bc18000263.html,/resource/1/APPLAB/v99/i6

Published by the American Institute of Physics.

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Air stable hybrid inverted tandem solar cell design

Feng Liu and Jean-Michel Nunzi a)

Department of Chemistry,Queen’s University,90Bader Lane,Kingston,Ontario K7L3N6,Canada

(Received13May2011;accepted14July2011;published online8August2011)

In order to get an air stable solar cell with high open circuit voltage(V oc),we fabricated an inverted tandem solar cell based on hybrid wet chemistry and vacuum thermal deposition techniques.A thin metallic interfacial layer was applied to improve charge recombination and maximize both the?ll factor and V oc of the tandem solar cell.A cationic dye doped electron transport layer was used to minimize space charge induced V oc loss.The tandem cell V oc reached

1.02V,which equals the sum of the two subcells’V oc.Increase of the metal nanoparticles’layer

thickness reduces the short circuit current density of the tandem owing to increasing light extinction.Our tandem cell design offers superior air stability due to additional encapsulation effect from top metal oxide layers.It retains about80%of its original ef?ciency after storage in air for three months.V C2011American Institute of Physics.[doi:10.1063/1.3622119]

Organic solar cells(OSC)promise incomparable low pro-duction cost in high volume.However,despite recent break-throughs,OSCs bear lower power conversion ef?ciency (PCE)and lower stability than inorganic ones,1–3which limits practical applications to date.In order to improve the ef?-ciency of OSC,tandem solar cells were developed.They exist in multijunctions of bulk heterojunction(BHJ)or bilayer cells. Tandem solar cells have a number of advantages compared to single cells:they deliver higher open circuit voltage V oc, which ideally equals the sum of subcells V oc when they are connected in series;they can be made of several subcells with different bandgaps which have complementary absorption and potentially cover the whole solar spectrum.So far,diverse types of tandem solar cells have been investigated,including small-molecule cells,polymer cells,and hybrid cells,which consist of a polymer subcell together with a small-molecule subcell.4–6Hybrid tandem cells can combine the merits of spin-coating and vacuum thermal deposition techniques and, consequently,bene?t from a larger panel of ef?cient small molecules and polymers.7,8

Although both tandem and inverted cells9–11have been investigated,only few researches were dedicated to the de-velopment of an inverted tandem cell with improved ef?-ciency and stability.12,13Moreover,existing inverted tandem cells were mostly fabricated from identical polymer subcells, which does not expand the absorption range;in addition,the top polymer cell can swallow the bottom cell during the spin coating process,unless a tight interfacial layer was employed,which might as well limit light transmission or in-hibit charge transport.That inspired us to develop an hybrid inverted tandem cell in which a small-molecule top cell fab-ricated by vacuum deposition would not disturb a polymer bottom cell,the two subcells having complementary absorp-tion.In this paper,we report on the development of an hybrid inverted tandem solar cell based on two well studied polymer and small-molecule subcells.We built a symmetri-cal n-i-p/Au/n-i-p tandem cell structure using a doped elec-tron transport layer(ETL),which brings signi?cant V oc improvement.We further improved V oc and?ll factor(FF) using metal nanoparticles,till the optimal addition of subcell performances.

Poly(3-hexylthiophene)(P3HT),molybdenum oxide (MoO3,99.99%),zinc oxide(ZnO,99.0%),copper phthalo-cyanine(CuPc,sublimated),and Rhodamine B(RhB)were purchased from Aldrich;[6,6]-phenyl C61butyric acid methyl ester(PCBM,99%),C60(99.9%),and C70(99.0%) were obtained from SES Research.All materials were used as received without further puri?cation.ZnO precursor solu-tion containing0.75M zinc acetate dihydrate and0.75M monoethanolamine in2-methoxyethanol was stirred over-night and aged another12h before use.10The precursor solu-tion was spin coated on pre-cleaned indium tin oxyde(ITO) glass with15X/h sheet resistance and then heated up on a hotplate at275 C for5min in air.The transparent ZnO?lm was then washed with distilled water,acetone,and isopropa-nol to remove residual organic materials from the surface. Then a250nm active layer consisting of P3HT and PCBM blend(20mg/ml P3HT in1,2-dichlorobenzeene,P3HT: PCBM?1:1by weight)was spin coated on top of ZnO.It was subsequently annealed at110 C in an N2atmosphere glove box for10min and transferred into the10à6mbar vac-uum chamber for further deposition.3nm MoO3,X nm gold (X?0.5,1,3,5),10nm C60,51nm CuPc/C60(1:0.7co-deposition weight ratio),and3nm MoO3were deposited in sequence.100nm Ag was?nally deposited as anode on top through a shadow mask.The obtained tandem cell has the con?guration shown in the insert of Fig.1.Photoactive area of each individual device is0.2cm2.We also fabricated a polymer cell with the structure ITO/ZnO/P3HT:PCBM/ MoO3/Ag and a small-molecule cell with the structure ITO/ ZnO/RhB doped C60/CuPc:C60/MoO3/Ag as reference cells. Current-voltage curves were measured with a Keithley4200-SCS semiconductor parameter analyzer,solar cells were irra-diated using an halogen lamp,and all the data were cali-brated afterwards with a standard AM1.5100mW/cm2solar simulator.All measurements were carried out in air.

Fig.1compares current/voltage characteristics of tan-dem cells built with different con?gurations.The pristine tandem cell without Au nanoparticles and doped ETL has

a)Author to whom correspondence should be addressed.Electronic mail:

nunzijm@queensu.ca.

0003-6951/2011/99(6)/063301/3/$30.00V C2011American Institute of Physics

99,063301-1

APPLIED PHYSICS LETTERS99,063301(2011)

the poorest performance:V oc ?0.4V,which is smaller than that of a single subcell,indicating a poor series connection,which can be ascribed to the C 60/MoO 3interface functioning as a counter-productive junction.14In order to create a charge recombination pathway at the interface,we deposited 1nm gold nanoparticle layer between the two subcells.How-ever,the tandem cell only shows slight improvement on V oc ,which implies that either holes are trapped in the MoO 3layer or electrons in the C 60layer,causing unbalanced hole/elec-tron recombination and accumulated space charge.It was previously reported that MoO 3could ef?ciently replace PEDOT:PSS in polymer solar cells and has good conductiv-ity.15It is thus more likely that electron transport slows down in C 60and charge accumulates at its boundary,thus reducing V oc .In order to improve C 60conductivity,we doped it with the cationic dye rhodamine B.If Cationic dyes were reported to form in situ a reduced neutral radical under thermal evaporation,16then electron transfer from the reduced cationic dye to the matrix can result in n-type dop-ing.As a result,the tandem cell with RhB doping of the ETL (RhB:C 60?0.7:1weight ratio)exhibits drastic V oc improve-ment from 0.4V to 0.95V.To con?rm the counter-junction effect at the interface between the two sub-cells,we built the tandem cell with only RhB doping;it still presents 7%V oc loss and exhibits the characteristic S-shaped J-V curve of a P-N junction connected in opposition to the desired solar cell,17which leads to poor FF ?35%.

We,therefore,combined both engineering techniques to fabricate a tandem cell with 1nm Au nanoparticle recombina-tion layer and RhB doped ETL.The tandem cell delivers fur-ther improved V oc ,without S-shape J-V curve and a decent FF ?45%.J-V curves comparison between this tandem cell and the two reference single cells is given in Fig.2.The small-molecule top cell delivers V oc ?0.47V and the polymer bottom cell a higher V oc ?0.55V,which are consistent with literature values.5,18The tandem cell delivers V oc ?1.02V,which is the exact addition of the two subcells’V oc ,showing that the two subcells are indeed well connected in series.In that situation,the current density from the tandem should be larger or equal to the smallest one from the two subcells.4,19In the present con?guration,the small-molecule subcell has lower J sc ?3.94mA/cm 2than the polymer cell J sc ?6.38mA/

cm 2.However,the tandem cell delivers J sc ?2.78mA/cm 2,which is only 70%of the limiting cell current,meanwhile the ?ll factor of the tandem is 45%,which is quite close to the 47%of the limiting cell.Above results suggest that our design with both metal nanoparticle layer and doped ETL was neces-sary to render the tandem functional,although the recombina-tion layer is the limiting factor of our device.

Fig.3shows the J-V curves of tandem cells with different Au nanoparticle layer thicknesses.All tandem cells have an RhB doped ETL.A 0.5nm gold nanoparticle layer is enough to suppress the S-shaped J-V characteristic.However,the thicker the gold layer,the lower the J sc .This tells us that the incident light is partly absorbed or back scattered by the metal-lic layer after crossing the bottom polymer cell.Gold nanopar-ticles are known to have a large scattering cross-section due to surface plasmon resonance,20thus even a few nanometer incre-ment on gold thickness can lead to substantial change of light extinction,which in turn impairs the light transmitted to the top limiting cell.As a result,J sc of the top subcell will decrease and so will J sc of the tandem.The 0.5nm gold nanoparticle layer delivers the best results in the present tandem design.

Fig.4shows V oc and J sc variations with RhB dopant concentration,with unchanged gold nanoparticle layer.The undoped n-i-p small-molecule top cell shows rather

low

FIG.1.(Color online)J-V curves of tandem solar cell with different cell struc-tures.Insert is the schematic diagram of the hybrid inverted tandem solar

cell.

FIG.2.(Color online)J-V curves of single top cell,single bottom cell,and tan-dem solar cell with 70%RhB doped ETL and 1nm Au recombination

layer.

FIG.3.(Color online)J-V curves of tandem solar cells with different gold nanoparticle layer thickness.

V oc ?0.4V,which might result from unbalanced charge car-rier transport between the p-type MoO 3hole transport layer (HTL)and the n-type C 60ETL.When 24%RhB is co-depos-ited with the ETL,V oc of the top reference cell increases to 0.43V.With further increase of the RhB concentration,top cell V oc increases to 0.47V and remains constant.Corre-spondingly,J sc of the top cell also increases with the RhB dopant and saturates when the doping concentration is above 30%.Simultaneous improvement of V oc and J sc with ETL doping relates to charge carrier extraction ef?ciency and con?rms conductivity improvement of the C 60layer.PCE of the optimized top cell is 1.9times larger than PCE of the undoped top cell.V oc of the tandem keeps increasing monot-onically with RhB concentration and reaches its maximum with 70%RhB in the ETL.

The two single reference cells in Fig.2show consider-able J sc difference.In consequence,excess holes generated from the bottom cell will not recombine with electrons from the top cell and thus,charge the bottom cell,which in turn will lead to reduced performance of the tandem.14In order to match currents from the subcells,we gradually decreased the P3HT thickness.An optimum is achieved with 75nm thick-ness.The structure of our optimized device is ITO/ZnO/P3HT:PCBM (75nm)/MoO 3(3nm)/Au (0.5nm)/RhB:C 60(0.7:1,17nm)/CuPc:C 60(1:0.7,51nm)/MoO 3(3nm)/Ag (100nm).Its characteristics are listed in Table I .The bottom cell J sc presents minor deviation from the top cell J sc .As a result,J sc of the tandem solar cell is signi?cantly improved up to 94%of the limiting cell J sc .Besides,null V oc loss is also achieved.(Note that V oc of the single polymer cell drops 0.01V and FF reduces from 52%to 43%,owing to the very thin active layer structure).As a result,PCE of the tandem reaches 84%of the sum of PCEs from the two subcells.

C 70with its larger absorbance around 500–700nm than C 60is considered as a candidate for improving PCE of

small-molecule solar cells.21Therefore,we attempted to replace C 60by C 70.However,the C 70tandem delivered slightly reduced V oc and constant J sc compared to the C 60tandem,which might come from unmatched absorption.

Interestingly,the P3HT and CuPc active layers were sandwiched between metal oxide ETL and HTL layers in our design.Metal oxide layers potentially prevent oxygen from the ambient atmosphere to penetrate into the cell via pinholes or grain boundaries at the electrodes.In consequence,in addi-tion to the high work function of the top electrode,9the designed structure promotes air stability of the tandem cell.Stability tests show that the tandem cell still keeps about 80%of its original PCE after three full months’storage in air.

An hybrid inverted tandem solar cell was designed.Incorporation of a metal recombination layer and a RhB dye doped ETL were essential for successful tandem cell per-formances.Tuning the Au nanoparticle layer thickness,RhB doping concentration and P3HT polymer thickness permitted optimization of the structure,delivering null V oc loss and high J sc recovery from the two subcells.Thanks to encapsu-lation from the metal oxide layers,our hybrid inverted tan-dem cell design shows prominent air stability.

Work was supported by the Photovoltaic Innovation Network of the Natural Sciences and Engineering Research Council.

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FIG.4.(Color online)V oc and J sc of single top cell and tandem cell vary with RhB doping concentration (RhB:C 60,mass ratio).

TABLE I.Optimal tandem cell and

corresponding

single

cell’s

performance.Cell J sc (mA/cm 2)

V oc (V)FF(%)PCE(%)Bottom 4.340.5443 1.00Top 3.940.47470.87Tandem

3.70

1.01

42

1.58

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