Chemical Management for Colorful, Ecient, and Stable InorganicOrganic Hybrid Nanostructured Solar CellsJun Hong Noh, Sang Hyuk Im, Jin Hyuck Heo, Tarak N. Mandal, and Sang Il Seok*,,

Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600,KoreaDepartment of Chemical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Youngin-si,Gyeonggi-do 446-701, Republic of KoreaDepartment of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea

*S Supporting Information

ABSTRACT: Chemically tuned inorganicorganic hybridmaterials, based on CH3NH3(MA)Pb(I1xBrx)3 perovskites,have been studied using UVvis absorption and X-raydiraction patterns and applied to nanostructured solar cells.The band gap engineering brought about by the chemicalmanagement of MAPb(I1xBrx)3 perovskites can be controllablytuned to cover almost the entire visible spectrum, enabling therealization of colorful solar cells. We demonstrate highlyecient solar cells exhibiting 12.3% in a power conversioneciency of under standard AM 1.5, for the most ecientdevice, as a result of tunable composition for the light harvesterin conjunction with a mesoporous TiO2 lm and a hole conducting polymer. We believe that the works highlighted in this paperrepresent one step toward the realization of low-cost, high-eciency, and long-term stability with colorful solar cells.

KEYWORDS: Inorganicorganic hybrid, nanostructured, colorful, ecient and stable solar cells

Solar energy is the largest noncarbon-based energy source.As opposed to more conventional resources such as coaland fossil fuels, solar energy can serve as a means to solving theserious issue of global warming that has arisen from theevolution of CO2. The use of solar cells is an eective methodof converting solar energy into electric energy, and it representsa very interesting research challenge. Although a good eciencyhas been achieved for these devices using single-crystallinematerials such as silicon and compound semiconductors,1 themanufacturing processes are still relatively expensive in terms ofboth materials and techniques. On the other hand, organic anddye-sensitized solar cells can be manufactured more inex-pensively due to the use of solution-processed materials,making them benecial. However, further enhancement of thepower conversion eciency (PCE) is still required for practicalapplications. In addition to the eorts to increase PCE andreduce fabrication cost, long-term operation without thedegradation of the active materials must be considered to bean equally important factor.Recently, methylammonium lead halide (CH3NH3PbX3, X =

halogen; CH3NH3: MA) and its mixed-halide crystals,corresponding to three-dimensional perovskite structures,have been used as light harvesters for solar cells.24 Theadvantages of these compounds have already been demon-strated as a noteworthy example of potentially useful physicalproperties such as nonlinear optical properties, electro-luminescence, organic-like mobility, magnetic properties,

conductivity, and so forth.58 Perovskite-based hybrids can besynthesized using simple and cheap techniques due to their self-assembling character.8 Thus, these hybrid materials have thepotential to be ideal light harvesters for low-cost and high-eciency solar cells. The cell architectures for solar cells consistof a porous layer of a nanocrystalline metal oxide covered withsunlight-absorbing MAPbX3 and a hole-transporting material.They are cost-eective due to their simple fabrication processand high PCE. As a result, these cells have attracted a great dealof attention among researchers in the eld. Furthermore, theseinorganicorganic hybrid solar cells may have another distinctadvantage: the band gap of MAPbX3 can easily be tuned bychemical management to produce an array of translucentcolors, and this feature can be used to create colorful solardesigns. As a result, these solar cells may have various buildingapplications, such as in replacing windows, roofs, and evenwalls. They can be designed to address functional requirementswhile at the same time producing electricity from the sun.Band-gap tuning of MAPbX3 has been achieved via

substitution of I with Br ions, which arises from a strongdependence of electronic energies on the eective excitonmass.9 We fabricated inorganicorganic heterojunction solar

Received: January 28, 2013Revised: March 7, 2013

Letter

pubs.acs.org/NanoLett

XXXX American Chemical Society A dx.doi.org/10.1021/nl400349b | Nano Lett. XXXX, XXX, XXXXXX

cells by using an entire range of MAPb(I1xBrx)3 as lightharvesters on mesoporous (mp) TiO2 and polytriarylamine(PTAA), which act as hole-transporting materials (HTMs).Interestingly, the cells fabricated with MAPb(I1xBrx)3performed stably at x = 0.2 compared with other compositions,and the substitution of I with Br also led to improved eciencywith a maximum power conversion eciency of 12.3% underair-mass 1.5 global (AM 1.5G) illumination of 100 mW cm2

intensity.MAPbI3 has a distorted three-dimensional perovskite

structure that crystallizes in the tetragonal I4/mcm spacegroup, while MAPbBr3 has a cubic perovskite structure phase ofthe Pm3 m space group at room temperature, as shown inFigure 1a and b.10 The structural dierence between the twoperovskites seems to originate from the dierences in the ionicradius of I and Br ions with 6-fold coordination, which are2.2 and 1.96 , respectively. The ionic radii relationship of A,M, and X in AMX3 perovskite structure has been widelyaccepted as a criterion for the distortion of the MX6octahedron; that is, the formation or stability of cubicperovskite structure and the smaller X ion radius is relativelyprotable for the formation of cubic structure.8,11 The bandgaps of MAPbI3 and MAPbBr3 were reported as 1.5 and 2.3 eV,respectively.3,4,12 As can be seen by the band alignment (Figure1c), these two materials have a suitable band position forheterojunction solar cells using a mesoscopic TiO2 photo-electrode since the conduction band energy (CBE) of bothMAPbI3 and MAPbBr3 is higher than that for TiO2. Here weexpect that band gap-tailored light harvesters can be simplydesigned by the alloying of MAPbI3 and MAPbBr3. Figure 1dshows the schematic device architecture of hybrid hetero-junction solar cell with a pillared MAPbX3 overlayer. Upon

illumination of light, the MAPbX3 perovskite absorbs the lightand generates electronhole pairs. The electrons are injectedinto TiO2, and simultaneously the hole is transported to PTAAand/or counter electrode through the perovskite itself.13,14 Thecross-sectional and tilted SEM image in Figure 1e conrms thatthe cell conguration is a three-dimensional bilayer structurewith a pillared MAPbX3 overlayer. In our previous work,

13 wereported that the MAPbX3 is densely inltrated into the poresof mp-TiO2 layer and interconnected with overlayer. PTAA wasmostly located on and near the surface of the pillared structure.Figure 2a exhibits X-ray diraction (XRD) patterns

monitored in the 2 range of 27.531 for MAPb(I1xBrx)3(0 x 1) with increasing Br ions. The XRD patterns ofMAPbI3 and MAPbBr3 in Supplemental Figure S1 indicate thatMAPbI3 and MAPbBr3 have tetragonal and cubic perovskitephases at room temperature, respectively, that are in agreementwith the reported structures.10 The MAPbI3 (x = 0) has twopeaks located at 28.11 and 28.36 which can be indexed to the(004) and (220) planes for the tetragonal I4/mcm phase.4 Thepeak diracted by the (004) plane gradually disappears uponincreasing x and nally merges into a single peak correspondingto (200) around x = 0.2 due to the increased symmetry. Inother words, the tetragonal phase of MAPbI3 is maintaineduntil x = 0.13 and then transits to a cubic phase around x = 0.2.Because the I4/mcm phase is transited from the Pm3 m phase inthe ideal MAPbX3 perovskite structure by the slight rotation ofthe PbX6 octahedrons along the 001 axis on the (00l) planewhile maintaining their corner-sharing connectivity, thetetragonal phase can be dened by a pseudocubic (pc) lattice,as shown in Figure 2b. Therefore, the (100)pc spacing in apseudocubic crystal system can match the (110)t spacing in atetragonal (t) crystal system in the MAPb(I1xBrx)3 phase. The

Figure 1. Crystal structures and energy levels of MAPbI3 and MAPbBr3 and hybrid heterojunction solar cell conguration. (a) Distorted tetragonalperovskite structure of MAPbI3 at room temperature. (b) Cubic perovskite structure of MAPbBr3 at room temperature. Red: polyhederon [PbX6/2]

(X = I, Br); green sphere: CH3NH3(= MA). (c) The conduction band minimum (CBM) and the valence band maximum (VBM) of MAPbI3,MAPbBr3, and TiO2 are represented in eV. (d) Device conguration of the hybrid heterojunction solar cell consisting of the 3D TiO2/MAPbX3bilayer nanocomposites, PTAA layer, and Au layer on the blocking TiO2 layer (bl-TiO2) coated FTO substrate. (e) FESEM images in cross sectionalview (left) and tilted surface view (right) of the real device.

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further introduction of Br ions into MAPbI3 instead of I ionscauses the systematic shift of the (200)c peak toward higher 2degrees because the gradual substitution of the larger I atomswith the smaller Br atoms decreases the lattice spacing. Figure2c exhibits the lattice parameter a of the MAPb(I1xBrx)3phases indexed by pseudocubic or cubic as a function of Brcontent (x). Although the slope is slightly changed at thetetragonal to cubic phase transition point between x = 0.13 and0.20, the lattice parameter of MAPb(I1xBrx)3 (0 x 1)phases exhibits a linear relationship with the Br content in eachregion. According to Vegards law, the variation of the latticeparameter in the alloy is linear with composition in the absenceof strong electronic eects.15,16 Therefore, the linear trendindicates the formation of the MAPb(I1xBrx)3 compound inthe entire range of MAPb(I1xBrx)3 (0 x 1) by a simplesolution-mixing process.To check the variation of optical properties in the alloyed

hybrid perovskite, we measured the UVvisible absorptionspectra of mp-TiO2/MAPb(I1xBrx)3 (0 x 1), as shown in

Figure 3a. The onset absorption band of MAPb(I1xBrx)3nanocomposites can be tuned from a 786 nm wavelength(1.58 eV) to 544 nm wavelength (2.28 eV), resulting in colortunability for colorful solar cells. Figure 3b shows thecorresponding device colors of mp-TiO2/MAPb(I1xBrx)3 (0 x 1). Through the compositional control of MAPb-(I1xBrx)3, we could tune the color from dark brown for mp-TiO2/MAPbI3 (x = 0) to brown/red for mp-TiO2/MAPb-(I1xBrx)3 and then to yellow for mp-TiO2/MAPbBr3 (x = 1)with increasing Br content. A systematic shift of the absorptionband edge to shorter wavelength with increasing Br content inMAPb(I1xBrx)3 (0 x 1) indicates that the energy band gap(Eg) can be tuned by the composition of the alloy. The Eg(estimated from the onset absorption band) variation with theBr content in MAPb(I1xBrx)3 was plotted in Figure 3c.Empirically, the nonlinear variation of Eg with a composition inthe alloy can be expressed by following quadratic eq 1:

Figure 2. X-ray diraction analysis of the MAPb(I1xBrx)3. (a) XRD patterns of MAPb(I1xBrx)3 (x = 0, 0.06, 0.13, 0.20, 0.29, 0.38, 0.47, 0.58, 0.71,0.84, 1.0), magnied in the region of the tetragonal (004)T and (220)T and cubic (200)C peaks (2 = 27.531.0). (b) Crystal structures and unitlattice vectors on the (00l) plane of the tetragonal (I4/mcm) (top) and cubic (Pm3m) (bottom) phases are represented. The tetragonal lattice can betaken by the pseudocubic (pc) lattice. (c) Lattice parameters of pseudocubic or cubic MAPb(I1xBrx)3 as a function of Br composition (x).

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= +

+

E

E E E b x

bx

[MAPb(I Br ) ]

[MAPbI ] ( [MAPbBr ] [MAPbI ] )

x xg 1 3

g 3 g 3 g 3

2 (1)

where b is the bowing parameter.17,18 A least-squares t (redline) of Eg in Figure 3c changes the eq 1 to eq 2 which yieldsthe bowing parameter of b = 0.33 eV.

= + +E x x x( ) 1.57 0.39 0.33g 2 (2)

The extent of bowing is a measure of the degree ofuctuations in the crystal eld or the nonlinear eect arisingfrom the anisotropic nature of binding.19 The quite smallbowing parameter (b = 0.33 eV) in MAPb(I1xBrx)3 comparedwith those of other alloys indicates that MAPbI3 and MAPbBr3have a great miscibility.19,20 This conrms again that MAPb-(I1xBrx)3 makes up the entire composition of the compoundand consequently enables convenient band gap tailoring by asimple mixing process.Figure 4a and b shows the trend of PCE as a function of x in

the cells fabricated with MAPb(I1xBrx)3 and the JVcharacteristics at x = 0, 0.06, 0.13, 0.20, 0.29, 0.58, and 1.0,respectively. The current density (Jsc) is decreased from 18 mAcm2 at x = 0 to 5 mA cm2 at x = 1 with increasing x, whereasopen circuit voltage (Voc) is increased from 0.87 to 1.13 V. Inaddition to the signicant improvement in Voc, an increase inthe ll factor (FF) from 0.66 to 0.74 was also observed upon

the incorporation of the Br ions. It is well-known that the seriesresistance (Rs) greatly aects the ll factor and eciency ofsolar cells. The main contribution to Rs comes from (1) theactive and interfacial layer resistances, (2) electrode resistances,and (3) the base contact resistances.21 In our work, Rs amongthe cells fabricated from MAPb(I1xBrx)3 will be mainly causedby the dierence of (1) the active layer resistance as lightharvester, because factors (2) and (3) will be similar due to theuse of same architecture and materials in the cell conguration.Based on these facts, we compared the slope obtained from therelationship between reciprocal series resistance and shortcircuit current density (Jsc) for undoped MAPbI3 and Br-dopedMAPb(I0.9Br0.1)3 as light harvesters. As can be seen in FigureS3, Br-doped cell has steeper slope than undoped cell,suggesting that the charge transport properties are substantiallyimproved.The reduction of Jsc with increasing x is directly related with

the blue-shift of absorption onset, which is responsible for the

Figure 3. Photographs and UVvis absorption spectra of MAPb-(I1xBrx)3. (a) UVvis absorption spectra of FTO/bl-TiO2/mp-TiO2/MAPb(I1xBrx)3/Au cells measured using an integral sphere. (b)Photographs of 3D TiO2/MAPb(I1xBrx)3 bilayer nanocomposites onFTO glass substrates. (c) A quadratic relationship of the band-gaps ofMAPb(I1xBrx)3 as a function of Br composition (x).

Figure 4. Photovoltaic characteristics of the MAPb(I1xBrx)3heterojunction solar cells. (a) Power conversion eciencies of theheterojunction solar cells as a function of Br composition (x). (b)Photocurrent densityvoltage (JV) characteristics of the hetero-junction solar cells based on MAPb(I1xBrx)3 (x = 0, 0.06, 0.13, 0.20,0.29, 0.58, 1.0). (c) Corresponding external quantum eciency (EQE)spectra.

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increased band gap, as explained previously. The improvementof Voc is attributed to the widening of the band gap withincreasing x in MAPb(I1xBrx)3. Therefore, the PCE of thesolar cells fabricated with MAPb(I1xBrx)3 in the range from x= 0 to 0.20 might exhibit similar average values over 10% at alight illumination of 100 mW cm2 AM 1.5 G due to thesimultaneous enhancement of Voc and FF against Jsc decreasedby adding Br ions because PCE is determined from theproducts of Jsc, Voc, and FF (Figure 4a). Nevertheless, an abruptreduction of Voc between x = 0.20 and x = 0.58 inMAPb(I1xBrx)3 was observed. This means that there areadditional factors inuencing the cell performance of MAPb-(I1xBrx)3 with increasing x except the band gap. We surmisethat charge transport and recombination behavior of MAPb-(I1xBrx)3 acting as both light harvester and hole transport wasgreatly changed near of x = 0.58 in MAPb(I1xBrx)3, eventhough the MAPb(I1xBrx)3 compound uniformly formed inthe entire range. The external quantum eciency (EQE)spectra of the devices illuminated by monochromatic light areshown in Figure 4c. An average EQE approaching 80% in therange of 450600 nm was observed for the cells fabricated withx = 0, 0.06, 0.13, 0.20, 0.29 in MAPb(I1xBrx)3, and theintegrated photocurrent density from EQE spectra were alsowell matched with the Jsc.It was known that perovskite-type hybrids are easily

decomposed in the presence of moisture because of thehygroscopic amine salts. This restricts the use of halidecompounds for many practical applications. In particular,MAPbI3 is more sensitive to moisture compared withMAPbBr3.

8 To investigate the stability of MAPb(I1xBrx)3hybrid solar cells with the composition, we measured thePCE of the unencapsulated MAPb(I1xBrx)3 (x = 0, 0.06, 0.20,and 0.29 as representative model device) hybrid solar cellsunder ambient conditions with controlled humidity along withthe storage time, as shown in Figure 5. Although the MAPbI3hybrid solar cell does not exhibit signicant degradation of PCEat low humidity (

and Technology of Korea, and by a grant from the KRICT2020 Program for Future Technology of the Korea ResearchInstitute of Chemical Technology (KRICT), Republic of Korea.

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Lett. 2010, 64, 27352737.(7) Patrin, G. S.; Volkov, N. V.; Prokhorova, I. V. Phys. Solid State2004, 46, 18911894.(8) Cheng, Z.; Lin, J. CrystEngComm 2010, 12, 26462662.(9) Tanaka, K.; Takahashi, T.; Ban, T.; Kondo, T.; Uchida, K.; Miura,N. Solid State Commun. 2003, 127, 619623.(10) Poglitsch, A.; Weber, D. J. Chem. Phys. 1987, 87, 63736378.(11) Galasso, F. S. Structure, Properties and Preparation of Perovskite-

type Compounds; Pergamon: New York, 1969.(12) Kitazawa, N.; Watanabe, Y.; Nakamura, Y. J. Mater. Sci. 2002,

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Supporting Information

Chemical Management for Colourful, Efficient, and Stable Inorganic-

Organic Hybrid Nanostructured Solar Cells

Jun Hong Noh, Sang Hyuk Im, Jin Hyuck Heo, Tarak N. Mandal, and Sang Il Seok

Experimental Section

Solar cell fabrication. A 60-nm-thick dense blocking TiO2 (bl-TiO2) was deposited onto a F-

doped SnO2 (FTO, Pilkington, TEC8) substrate by a spray pyrolysis deposition with a 20 mM

titanium diisopropoxide bis(acetylacetonate) (Aldrich) solution at 450 C to prevent direct

contact between FTO and the hole-conducting layer. A 600-nm-thick mesoporous TiO2 (mp-

TiO2) films were screen-printed onto the bl-TiO2/FTO substrate using the paste, which was

prepared according to a reported method,1 and the films were calcined at 500 C for 1 h to

remove the organic part. The films were then immersed in a 40 mM TiCl4 aqueous solution at

60 C for 1 h and then were heat-treated at 500 C for 30 min to improve interfacial contact

with the nanocrystalline TiO2. CH3NH3I and CH3NH3Br were synthesized from 30 mL

hydroiodic acid (57% in water, Aldrich) or 44 mL hydrobromide acid (48% in water, Aldrich),

respectively, by reacting 27.86 mL methylamine (40% in methanol, Junsei Chemical Co.,

Ltd.) in a 250-mL round-bottomed flask at 0 C for 2 h with stirring. The precipitates were

recovered by evaporation at 50 C for 1 h. The products were dissolved in ethanol,

recrystallized from diethyl ether, and finally dried at 60 C in a vacuum oven for 24 h. The

CH3NH3PbI3 solution of 40 wt% was made by reacting the synthesized CH3NH3I powder and

PbI2 (Aldrich) at a 1:1 mol ratio in -butyrolactone at 60 C for 12 h. The CH3NH3PbBr3

solution of 28.5 wt% was made by reacting the synthesized CH3NH3Br powder and PbBr2

(Aldrich) at a 1:1 mol ratio in dimethylformamide at 60 C for 12 h. The desired

CH3NH3Pb(I1-xBrx)3 solutions were made by stoichiometric mixing of synthesized

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CH3NH3PbI3 and CH3NH3PbBr3 solutions at 60 C for 1 h. The CH3NH3Pb(I1-xBrx)3 solution

was then coated onto the mp-TiO2/bl-TiO2/FTO substrate by consecutive spin coating at 2000

rpm for 60 s and at 3000 rpm for 60 s and then dried on a hot plate at 100 C for 5 min. A

poly-triarylamine (PTAA) (EM index, Mw = 17,500 g/mol)/toluene (15 mg/1ml) solution

with an additive of 13.6 l Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (28.3

mg/1 ml) and 6.8 l 4-tert-butylpyridine (TBP) was spin-coated on CH3NH3PbI3/mp-TiO2/bl-

TiO2/FTO substrate at 3000 rpm for 30 s. Finally, an Au counterelectrode was deposited by

thermal evaporation. The active area was fixed at 0.16 cm2.

Materials characterization. The morphology of the mp-TiO2/CH3NH3PbI3 bilayer

nanocomposites was observed with a field-emission scanning electron microscopy (FESEM,

Tescan Mira 3 LMU FEG). The structure analysis of CH3NH3Pb(I1-xBrx)3 deposited on mp-

TiO2 was conducted by X-ray diffraction (XRD, Rigaku D/Max II X-ray diffractometer) using

Cu K radiation (=0.1542 nm). The absorption spectra of the mp-TiO2/CH3NH3PbI3 bilayer

nanocomposites were measured using a UV-Vis spectrometer (Hitachi U-3300) with an

integrated sphere.

Device characterization. The IPCE was measured by a power source (Newport 300W Xenon

lamp, 66920) with a monochromator (Newport Cornerstone 260) and a multimeter (Keithley

2001). The current density-voltage (J-V) curves were measured by a solar simulator (Newport,

Oriel Class A, 91195A) with a source meter (Keithley 2420) at 100 mA cm-2

illumination AM

1.5G and a calibrated Si-reference cell certificated by NREL. The J-V curves of all devices

were measured by masking the active area with a metal mask of 0.096 cm2.

Reference

(1) Chang, J. A.; Rhee, J. H.; Im, S. H.; Lee, Y. H.; Kim, H.; Seok, S. I.; Nazeeruddin, Md.

K.; Grtzel, M. Nano Lett. 2010, 10, 26092612.

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10 20 30 40 50 60

* FTO substrate

*

*

*

*

(22

4)t

(31

0)t(

22

0)t

(11

0)t

(30

0)c

(22

0)c

(20

0)c

(11

0)c

Inte

nsity (

a.u

.)

2theta

(10

0)c

TiO

2 A

na

tase

*

Figure S1. XRD patterns of MAPbI3 (black line) and MAPbBr3 (red line) deposited on mp-

TiO2/FTO glass substrate. (hkl)t and Br(hkl)c indicate diffraction peak for (hkl) plane of

tetragonal MAPbI3 and cubic MAPbBr3, respectively.

10 20 30 40 50 60

(001) peak of PbI2

*

x=0.20

x=0.06

FTO substrate

*

**

Br(

21

0)

I(2

24

)I(3

10

)

I(2

20

)

I(1

10

)

Br(

30

0)

Br(

22

0)

Br(

20

0)

Br(

11

0)

Inte

nsity a

.u.)

2theta

Br(

10

0)

TiO

2 a

na

tase

MAPb(I1-xBr

x)3

*

x=0.0

*

Figure S2. XRD patterns of MAPb(I1-xBrx)3 (x = 0, 0.06, and 0.20) deposited on mp-

TiO2/FTO glass substrate after exposure to 55% humidity for 1 day. The peak around 12.5o at

x=0 and 0.06 appeared after the exposure which is indexed by (001) peak of PbI2. I(hkl) and

Br(hkl) indicate diffraction peak for (hkl) plane of MAPbI3 and MAPbBr3, respectively.

0 5 10 15 200.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

1/RS

(

-1)

JSC (mA/cm2)

Figure S3. The relationship between reciprocal series resistance and short circuit current

density (Jsc) for the cells fabricated from MAPbI3 and MAPb(I0.9Br0.1)3 as light harvesters. Jsc

was obtained as a function of light intensity.