semitransparent polymer solar cells

The inner V2O5layer was introduced as a buffer layer to improve holes collection, while the outer V2O5layer served as a light coupling layer to enhance optical transmittance of the devic

Semitransparent polymer solar cells using V 2 O 5 /Ag/V 2 O 5

as transparent anodes

Liang Shen⇑, Yang Xu, Fanxu Meng, Fumin Li, Shengping Ruan, Weiyou Chen

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street,

Changchun 130012, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 12 December 2010

Received in revised form 23 March 2011

Accepted 23 March 2011

Available online 6 April 2011

Keywords:

Semitransparent

Inverted

Transmittance

Reflectance

a b s t r a c t

We demonstrate semi-transparent inverted polymer solar cells with transparent anodes, made of Vanadium pentoxide (V2O5)/silver (Ag)/V2O5 The inner V2O5layer was introduced

as a buffer layer to improve holes collection, while the outer V2O5layer served as a light coupling layer to enhance optical transmittance of the device The transmittance and reflectance of V2O5(10 nm)/Ag (13 nm)/V2O5(x = 20, 40, 60, 80 nm) electrode are mea-sured and compared, and the dependence of the device performances on the thickness of the outer V2O5layer was investigated The results show that the maximum transmittance

of 90%, which appears from 400 to 700 nm, is obtained when the thickness of outer V2O5

layer is 40 nm

Ó 2011 Elsevier B.V All rights reserved

1 Introduction

During the past decade, polymer solar cells (PSCs) that

convert solar light directly into electricity have been

exten-sively investigated due to their advantage of low-cost,

flex-ible, and large area electronic devices [1–4] The most

recent progress[5–7] has addressed these concerns well

and the manufacture of PSCs on an industrial scale is

pos-sible and the operational stability is sufficient to allow for

demonstrations and round robins The power conversion

efficiency (PCE) of PSCs has recently achieved 6% or more

[8,9]in bulk heterojunction (BHJ) structure, where a

pho-toactive layer is composed of a mixture solution of

poly-meric electron donors (D) and soluble fullerene-based

electron acceptor (A) The light absorption can be

strength-ened by either using a thick active layer or developing low

bandgap polymers However, the short exciton diffusion

length and relatively low charge mobility in organic

semi-conductor materials limit the thickness within 100 nm

around[10] The limited use of the solar spectrum is also

a difficult problem to improve efficiency[11] One promis-ing way to achieve this is to use variable bandgap polymers

in a tandem structure in which multiple subcells with dif-ferent energy gaps are stacked, which uses a semitranspar-ent electrode to connect the front and back solar cells

[12,13] A variety of semitransparent electrode structures have been reported, but low transmittance and high series resistance exist universally to reduce the PCE[14,15] Here,

we report semitransparent inverted PSCs with a multilayer anode structure of V2O5/Ag/V2O5 V2O5, which is a kind of transition metal oxides, has been reported to enhance the performance of the polymer solar cells as an anodic buffer layer by Shrotriya et al.[16] Norrman et al.[17]have re-cently shown the PEDOT: PSS is the reason that very long lifetime PSCs (based on PEDOT) is unlikely to be possible

as some fatal degradation paths are linked to the active layer and edot interface So we get rid of PEDOT: PSS and manufacture the inverted PSCs The V2O5/Ag/V2O5is both optically transparent and suitable for holes collecting The inner V2O5layer is inserted between the active layer and Ag to serves as an anodic buffer layer to enhance holes collection The outer V2O5layer is used as a top-capping layer to enhance light coupling It would also lower the series resistance of PSCs

1566-1199/$ - see front matter Ó 2011 Elsevier B.V All rights reserved.

⇑Corresponding author.

E-mail address: shenliang@jlu.edu.cn (L Shen).

Contents lists available atScienceDirect

Organic Electronics

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / o r g e l

2 Experimental

The photovoltaic device has a structure of ITO/nc-TiO2/

RR-P3HT: PCBM/V2O5/Ag/V2O5, as shown schematically in

Fig 1 The ITO-conducting glass substrate (a sheet

resis-tance of 15X/hwas precleaned by acetone, ethanol, and

de-ionized water, respectively, for 15 min Anatase TiO2

thin films were prepared as described in our previous

pa-pers[18,19] The thickness of TiO2is 25 nm P3HT (Lumtec

Corp., used as received) was dissolved in

1,2-dichloroben-zene to produce 18 mg/ml solution, followed by blending

with PCBM (Lumtec Corp, used as received) in 1:1 weight

ratio[20] The blend was stirred for 72 h in the air before

spin coating on top of TiO2film surface Then the samples

were baked in low vacuum (vacuum oven) at 160°C or

20 min The typical film thickness of P3HT: PCBM is about

200 nm Finally 10 nm V2O5, 13 nm Ag, and x (x = 20, 40,

60, 80 nm) V2O5were thermally evaporated in sequence

under a high vacuum (5  104Pa) without disrupting

the vacuum The deposition rate was about 0.02 nm/s,

which was monitored with a quartz-oscillating thickness

monitor (ULVAC, CRTM-9000) The active area of the

de-vice was about 6.4 mm2

Current density–voltage (J–V) characteristics were

mea-sured with a computer-programmed Keithley 2400 source/

meter under AM1.5G solar illuminations with an Oriel

300 W solar simulator The intensity of the solar simulator

was 100 mW/cm2 The light intensity was measured with a

photometer (International light, IL1400) which was

cor-rected by a standard silicon solar cell The transmission

spectra and the reflection spectra were measured by

means of ultraviolet/visible spectrometer (UV 1700,

Shimadzu)

3 Results and discussion

Fig 2a shows the J–V characteristics of semitransparent

inverted polymer solar cells with outer 40 nm (device II)

and without V2O5 (device I) under AM1.5G illumination

of 100 mW/cm2 The incidence light irradiate from the

ITO side (Bottom) The detailed results are given inTable 1

Device I shows short circuit current density (Jsc) of

5.31 mA/cm2, open circuit voltage (Voc) of 0.590 V, fill fac-tor (FF) of 0.540, and power conversion efficiency (PCE) of 1.69 DeviceP shows Jscof 4.83 mA/cm2, Vocof 0.581 V,

FF of 0.606, PCE of 1.70 It can be seen that the PCE of device

I is almost same as that of device II The Jscof deviceP de-creases, but FF increases dramatically compared to that of device I We attribute the decrease of Jscto lower reflec-tance of V2O5/Ag/V2O5 from 400 to 650 nm, which is shown inFig 3b It is evident that the resistivity of the thin

Fig 1 The schematic structure drawing of the semitransparent inverted

Fig 2 The J–V characteristics of device ITO/nc-TiO 2 /P3HT: PCBM/V 2 O 5 (10 nm)/Ag (13 nm)/V 2 O 5 (x = 0, 40 nm) when illuminated from (a) ITO side and (b) V 2 O 5 /Ag/V 2 O 5 side.

Table 1 Characteristic data of semitransparent inverted polymer solar cells with different thickness of the V 2 O 5 capping layer illuminated from ITO (bottom) and V 2 O 5 /Ag/V 2 O 5 (top) side.

Device (nm) Illumination J sc (mA/cm2) V oc (V) FF (%) PCE (%)

metal film (13 nm Ag here) is higher than that of the bulk

metal due to the scattering of the electrons from the

sur-face of the discontinuous film The outer V2O5decreases

the series resistance (Rsc), which is defined by the slope

of the J–V curve at J = 0 mA/cm2 The series resistance is

estimated to be 33.6X for device I and 23.3X for device

P The decrease of Rscresults in the increase of FF from

0.540 to 0.606 Fig 2b shows the J–V characteristics of

semitransparent inverted polymer solar cells (device I

andP) under AM1.5G illumination of 100 mW/cm2when

illuminated from V2O5/Ag/V2O5 side (Bottom) Here, the

mixture of P3HT and PCBM, which has main absorption

spectrum from 400 to 650 nm, is chosen as active layers

The photocurrent density is in direct proportion to light

absorption of the active layer Since devicePhas higher

transmittance from 400 to 650 nm, as shown inFig 3a, it

has bigger Jscthan device I

Fig 3a shows the transmittance spectra of the V2O5/Ag/

V2O5from 300 to 1000 nm It can be seen that the

trans-mittance becomes week with the increase of wavelength,

and high transmittance of 80% appears in short wavelength

range when the thickness of V2O5is zero When

introduc-ing outer VO (capping layer), the transmittance is

changed dramatically With the increase of the thickness

of V2O5 capping layer, the transmittance peaks are red-shifted The maximum transmittance of 90%, which ap-pears from 400 to 700 nm, is obtained when the thickness of V2O5 is 40 nm To continue increasing the thickness of V2O5, the transmittance gradually descends

It is evident that transmission can vary with the thickness

of the V2O5capping layer

Fig 3b shows the reflectance spectra of the V2O5/Ag/

V2O5from 300 to 1000 nm Since the absorption is little, the reflectance spectrum is nearly one to one complement with the transmittance spectrum It can be seen that reflectance peaks are redshifted gradually and would match better to the absorption spectra of the active layer (400–650 nm) when the thickness of the V2O5 capping layer increases

Fig 4a shows the the J–V characteristics of semitrans-parent inverted polymer solar cells under AM1.5G illumi-nation of 100 mW/cm2 when illuminated from ITO side (Bottom) The detailed results are given inTable 1 When the thickness of V2O5is 40 nm, the Jscis 4.83 mA/cm2, the

Vocis 0.581 V, the FF is 60.6%, and the PCE is 1.70% When

Fig 3 (a) The transmittance spectra of the transparent electrode V 2 O 5

(10 nm)/Ag (13 nm)/V 2 O 5 (x = 0, 20, 40, 60, 80 nm) (b) the reflectance

spectra of the transparent electrode V 2 O 5 (10 nm)/Ag (13 nm)/V 2 O 5 (x = 0,

20, 40, 60, 80 nm).

Fig 4 The J–V characteristics of device ITO/nc-TiO 2 /P3HT: PCBM/V 2 O 5 (10 nm)/Ag (13 nm)/V 2 O 5 (x = 20, 40, 60, 80 nm) dependent on the thickness of the V 2 O 5 capping layer when illuminated from (a) ITO side and (b) from V O /Ag/V O side.

the thickness of V2O5is 80 nm, the photovoltaic device has

a Jscof 5.96 mA/cm2, Vocof 0.594 V, FF of 59.8%, and PCE of

2.12% The Jscof the device with 40 nm V2O5is the smallest;

and the Jscof the device with 80 nm V2O5is the biggest It is

known that the reflectance of the top electrode plays an

important role in trapping light for the active layer to

reab-sorb The reflectance of V2O5/Ag/V2O5(80 nm) from 400 to

650 nm is the strongest, but the reflectance of V2O5/Ag/

V2O5(40 nm) is poorest inFig 3b

Fig 4b shows the the J–V characteristics of

semitrans-parent inverted polymer solar cells under AM1.5G

illumi-nation of 100 mW/cm2 when illuminated from V2O5/Ag/

V2O5side (Top) The detailed results are given inTable 1

When the thickness of V2O5 is 40 nm, the Jscis 3.79 mA/

cm2, the Voc is 0.565 V, the FF is 60.6%, and the PCE is

1.30% When the thickness of V2O5is 80 nm, the

photovol-taic device has a Jscof 2.67 mA/cm2, Vocof 0.551 V, FF of

59.8%, and PCE of 0.88% The Jscof the device with 40 nm

V2O5 is the biggest, and that for the device with 80 nm

V2O5is the smallest This is because the transmittance of

V2O5/Ag/V2O5(40 nm) is the highest, and the absorption

of active layer material is directly proportional to the

transmittance of the incidence electrode When

illumi-nated from V2O5/Ag/V2O5 side, the Voc becomes smaller

compared with that illuminated from the ITO side The

dependence of the Vocand the photocurrent (Iph) can be

generally expressed as follows[21]:

Voc¼kT

q Log

Iph

Is þ 1

ð1Þ

Where Isis reverse saturation current, k is Boltzman

constant, T is the temperature, and q is charge Under Eq

(1), the Vocis directly proportional to Iph The Iphfrom ITO

side is much bigger than that from V2O5/Ag/V2O5 side,

which is shown inTable 1

4 Conclusion

In summary, we present efficient semi-transparent

in-verted polymer solar cells with highly transparent V2O5/

Ag/V2O5anodes The inner V2O5layer was introduced as

a buffer layer to improve holes collection, while the outer

V2O5 served as a light coupling layer to enhance optical

transmittance The incident light transmittance changed

with the thickness of V2O5.When the thickness of V2O5is

40 nm, the highest transmittance is obtained The reflec-tance peaks are redshifted gradually and would match bet-ter to the absorption spectra of the P3HT: PCBM layer when the thickness of the V2O5capping layer increases Acknowledgements

The authors are grateful to Major Project of Science and Technology Development Plan of Jilin Provincial Science and Technology Department (Grant Nos 20070402,

20080330, 20100103), the China 863 Program (Grant No 2007AA03Z406, 2009AA032402), Scientific Frontier and Interdiscipline Innovative Projects of Jilin University (Grant Nos 200903087), National Natural Science Foundation of China (Grant Nos 60977031, 50977038, 61007022,

61077046, 61006013) and Doctoral Found of Ministry of Education of China Grant Nos 20090061110040,

20100061120045 for the support to the work

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