Reduced ultraviolet light induced degradation and enhanced light harvesting usingYVO4:Eu3+ down-shifting nano-phosphor layer in organometal halide perovskite solarcellsNikhil Chander, A. F. Khan, P. S. Chandrasekhar, Eshwar Thouti, Sanjay Kumar Swami, Viresh Dutta, andVamsi K. Komarala Citation: Applied Physics Letters 105, 033904 (2014); doi: 10.1063/1.4891181 View online: http://dx.doi.org/10.1063/1.4891181 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Efficiency enhancement calculations of state-of-the-art solar cells by luminescent layers with spectral shifting,quantum cutting, and quantum tripling function J. Appl. Phys. 114, 084502 (2013); 10.1063/1.4819237 Performance enhancement of polymer solar cells with luminescent down-shifting sensitizer Appl. Phys. Lett. 103, 043302 (2013); 10.1063/1.4816383 LaPO4:Ce,Tb and YVO4:Eu nanophosphors: Luminescence studies in the vacuum ultraviolet spectral range J. Appl. Phys. 110, 053522 (2011); 10.1063/1.3634112 Spectral conversion for solar cell efficiency enhancement using YVO4:Bi3+,Ln3+ (Ln=Dy, Er, Ho, Eu, Sm, andYb) phosphors J. Appl. Phys. 109, 113526 (2011); 10.1063/1.3592889 Phosphor coatings to enhance Si photovoltaic cell performance J. Vac. Sci. Technol. A 25, 61 (2007); 10.1116/1.2393298

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Reduced ultraviolet light induced degradation and enhanced light harvestingusing YVO4:Eu31 down-shifting nano-phosphor layer in organometal halideperovskite solar cells

Nikhil Chander,1 A. F. Khan,1,2,a) P. S. Chandrasekhar,1 Eshwar Thouti,1

Sanjay Kumar Swami,1 Viresh Dutta,1 and Vamsi K. Komarala1

1Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi,New Delhi 110016, India2Department of Electronics and Information Technology, Ministry of Communications and InformationTechnology, Government of India, New Delhi 110003, India

(Received 21 March 2014; accepted 13 July 2014; published online 23 July 2014)

We report a simple method to mitigate ultra-violet (UV) degradation in TiO2 based perovskite solar

cells (PSC) using a transparent luminescent down-shifting (DS) YVO4:Eu3þ nano-phosphor

layer. The PSC coated with DS phosphor showed an improvement in stability under prolonged

illumination retaining more than 50% of its initial efficiency, whereas PSC without the phosphor

layer degraded to �35% of its initial value. The phosphor layer also provided �8.5% enhancement

in photocurrent due to DS of incident UV photons into additional red photons. YVO4:Eu3þ layer

thus served a bi-functional role in PSC by reducing photo-degradation as well as enhancing energy

conversion efficiency. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891181]

Inorganic–organic perovskite compounds (CH3NH3PbX3,

X¼ I, Br, and Cl) have recently been considered as light-

harvesting materials for hybrid solar cells because of their

high extinction coefficients and broader light-absorption.1–4

Sensitized solar cells (SSCs) have been extensively explored

ever since the first dye SSC (DSSC) came into existence in

the year 1991 following the work of O’Regan and Gratzel.5

Solid state perovskite solar cells (PSC) are present in develop-

mental stage and have the advantage that they do not suffer

from electrolyte leakage as in liquid electrolyte based DSSCs,

nor does the hole-transporting material (HTM) possess any

corrosive effect. Power conversion efficiencies (PCE) of

DSSCs sensitized with a porphyrin dye and PSCs have

reached up to 12.3% and 15%,6–8 respectively. The efficien-

cies of PSCs are expected to increase further, since research-

ers are aiming for better performance by utilizing different

fabrication methods, different structures of photoanodes,

improved sensitizers, and different kinds of HTMs. Also,

there are a few reports recently which have addressed the

problem of degradation of perovskite material due to moisture

and air.8,9 However, there is only one study on the degrada-

tion of PSCs due to ultra-violet (UV) light by Leijtens et al.10

which showed a loss of charge in PSCs due to UV exposure.

They proposed that the UV-degraded cells suffered from a

deep trapping of injected electrons within newly available

sites in the TiO2. This instability of PSC under UV exposure

can be rectified (1) by TiO2 surface states pacification, (2) by

completely removing the mesoporous TiO2 film, and (3) by

prevention of UV light reaching to the mesoporous TiO2 film.

Options (1) and (2) have been addressed by different research

groups.9,10 The third option where a suitable down-shifting

(DS) material absorbs UV light (k� 400 nm) falling on the

PSC and re-emits visible light which is then utilized by the

perovskite absorber can be an effective way of improving the

efficiency while reducing the UV light induced degradation.

Two important studies have appeared recently which

show that nano-structured TiO2 scaffold is essential for tri-

iodide PSCs as it increases the effective diffusion length of

electrons.11,12 Very recently, the fabrication of lead-free

PSCs based on tin has been reported which also make use of

meso-TiO2 layer.13,14 So by all accounts, it seems that TiO2

nano-structure in PSCs is here to stay and that is why we

have chosen meso-TiO2 to fabricate cells. Also, using a

nanostructured metal oxide framework allows researchers to

incorporate and study various types of nanomaterials like

metal nanoparticles (NPs) (for plasmonic enhancement),15

up-converting nanoparticles, and other dopants, which can

be mixed with TiO2 or Al2O3 in desired quantities.

Rare earth (RE) Europium (Eu3þ) doped Yttrium

Vanadate (YVO4) is an ideal candidate for making efficient

UV degradation resistant PSC devices since RE compounds

have specific 4f electronic structure and unique photo-,

electro-, and magneto-properties.16–18 To make the as pre-

pared NPs useful for PSCs, NPs should be uniformly distrib-

uted over the top surface of a PSC, i.e., non-conducting side

of fluorine doped tin oxide (FTO) glass. Most of the studies

on layer deposition employ self-assembly technique, where

the layer is formed via chemical interaction between the

substrate and the NPs.19–21 This self-assembly technique

requires chemically treated substrate surface and the layers

deposited suffer from enhanced defect susceptibility. The

above mentioned technique cannot be employed in the case

of PSCs as any chemical modification of the FTO substrate

may damage the perovskite layer. Spray deposition of

SHMP (sodium hexametaphosphate) capped YVO4:Eu3þ

nanophosphor layer on the top surface of PSC is a suitable

alternative to the above problem, and the role of this layer in

PSC is reported in this Letter. YVO4:Eu3þ red emitting

nanophosphor (5–8 nm) was synthesized by chemical

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

khanafk@gmail.com. Tel.: þ91-11-2659-6408.

0003-6951/2014/105(3)/033904/5/$30.00 VC 2014 AIP Publishing LLC105, 033904-1

APPLIED PHYSICS LETTERS 105, 033904 (2014)

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co-precipitation (CCP) method. The detailed synthesis

procedure is reported elsewhere.22

X-ray diffraction (XRD) pattern of YVO4:Eu3þ nanophos-

phor powder sample is shown in Figure 1(a). The size of phos-

phor particle roughly estimated (using Scherrer formula) comes

out to be �5 nm, which is in good agreement with transmission

electron micrograph (TEM) (Fig. 1(b)). Scanning electron

microscope (SEM) image of spray deposited DS YVO4:Eu3þ

NP film on a glass substrate is shown in Figure 1(c). The mac-

roscopic uniformity of spray deposited phosphor film (on

quartz) is evident from the digital image (Fig. 1(d)) recorded

under UV (k� 300 nm) illumination, which shows a uniform

red emission of nearly the same intensity over the entire area.

Spray deposition technique also provides the flexibility of vary-

ing the DS layer thickness for attaining maximum transmission

without compromising the photoluminescence (PL) emission.

Figure 2 shows PL emission spectra of as synthesized

YVO4:Eu3þ phosphor NPs with different Eu3þ concentra-

tions. Eu3þ concentration was varied according to the

formula Y1�xVO4:Eux, where x¼ 5–13 mol. % and PL emis-

sion intensity was found maximum for 11.0 mol. %. The PL

excitation (PLE) peak at 295 nm (Fig. 2(b)) arises due to

charge transfer from host (YVO4) to the dopant ion, i.e.,

Eu3þ. The other excitation peaks between 200 nm and

280 nm correspond to the different inter- and intra-molecular

transitions of VO43� in YVO4 which also contributes to the

energy transfer mechanism.23 Broad band excitation spectra

in UV range (200–350 nm) with peak at 295 nm and emis-

sion spectra in visible red region peaking at 614 nm (5D0–7F2

transition) make it suitable for use as a DS phosphor material

for solar cells. Time-resolved luminescence decay (TRLD)

was recorded at 615 nm (5D0–7F2 transition) emission with

an excitation wavelength of 295 nm by a time correlated

single photon counting technique (Fig. 2(b)). The results

demonstrate that the decay curve fitted well into mono-

exponential function as I¼AþB1 e(�t/s1) with 1/e decay

time s¼ 1.038 ms.

To fabricate PSCs, the sequential deposition method

reported by Burschka et al. was followed.8 For protecting PSCs

from high energy UV radiation, other PSC devices coupled

with DS YVO4:Eu3þ NPs (DS-PSC) were fabricated. Uniform

layers of DS YVO4:Eu3þ NPs were deposited using nearly 6 ml

of 1.5 mg/ml of as synthesized YVO4:Eu3þ NPs in propanol by

spray coating technique on top surface of the FTO substrate

(reverse side, see Scheme 1). This procedure was performed at

a temperature of �150 �C after the deposition of compact TiO2

layer. The dispersion of DS NPs was sprayed in very short

pulses of �2 s in order to achieve uniform film deposition and

also to maintain the substrate temperature at �150 �C.

The current density-voltage (J-V) plots and incident

photon to current conversion efficiency (IPCE) spectra for

the best devices of control PSC and DS-PSC are shown in

Figures 3(a) and 3(b). To obtain data of statistical signifi-

cance, four devices with three pixels each were fabricated

for control and DS-PSCs (total 24 devices). Table I shows

the average values of photovoltaic parameters of the twelve

devices along with the standard deviations. The best control

PSC has a photocurrent density of 16.7 mA/cm2 and PCE of

7.53%. DS-PSC shows an enhancement of 8.5% in photocur-

rent and �7.7% in efficiency (PCE �8.11%). The enhanced

photocurrent is primarily due to enhancement in the

350–450 nm wavelength region as seen from the IPCE spec-

tra (Fig. 3(b)), although a small enhancement is seen in the

FIG. 1. (a) XRD pattern and (b) TEM image of the YVO4:Eu3þ nanopar-

ticles. (c) SEM image of YVO4:Eu3þ nanoparticles spray deposited on a

glass substrate. (d) Digital photograph of a phosphor layer coated on quartz

substrate recorded under ultraviolet (k� 300 nm) illumination.

FIG. 2. The photoluminescence (a) emission and (b) excitation spectra of

YVO4:Eu3þ phosphor nanoparticles. (c) Time-resolved photoluminescence

decay curve of phosphor nanoparticles recorded at 615 nm.

SCHEME 1. Perovskite solar cell structure with down-shifting nano-phos-

phor layer spray coated on reverse of FTO glass. Absorbed UV light is

down-shifted to the red region.

033904-2 Chander et al. Appl. Phys. Lett. 105, 033904 (2014)

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entire spectrum. The use of down-shifting phosphor material

has been shown to result in the improvement of short-

wavelength spectral response because a part of the UV spec-

trum is converted to visible region, which is then absorbed

by the active material.24 The absorbance spectra of perov-

skite (CH3NH3PbI3) show dip in absorbance values above

500 nm (Fig. 3(b), inset, red dotted line). As synthesized DS

YVO4:Eu3þ nanophosphor layer emits strongly in red region

(Fig. 3(a), inset, black solid line), this matching of PL emis-

sion band of YVO4:Eu3þ NPs with the concavity of the per-

ovskite absorbance spectra in red region establishes that the

DS phosphor is providing additional red photons, which are

absorbed by the perovskite absorber material, giving rise to

the observed photocurrent enhancement (Fig. 3(a)).

Perovskite material has a large absorption coefficient of

the order of 105 cm�1 in the UV region, which is larger than

that of visible region.25 But the IPCE values in the UV-blue

region are relatively lower than that of green-yellow region

around 500 nm (Fig. 3(b)). Various mechanisms are respon-

sible for this less than expected spectral response in the short

wavelength region. One is the relatively high reflectance

losses in the short wavelength region (Fig. S2)26 arising

because of refractive index mismatch between air/vacuum

and the FTO substrate, which reduces the actual number of

photons going into the perovskite absorber layer. Second,

due to the high absorption coefficient of perovskite material

for short wavelengths, majority of the UV photons are

absorbed near the FTO-perovskite interface and hence pro-

duce charge carriers mainly near this interface. The typical

diffusion length of holes in CH3NH3PbI3 PSCs is �100 nm,

while the thickness of the device is �500 nm.27 So the holes

generated near the FTO-perovskite interface may not reach

the gold counter electrode and the charge collection effi-

ciency is reduced. Third, these charge carriers also have a

high probability of getting captured by trap sites and defects

present at the grain boundaries and FTO-perovskite

interface. A combined effect of all these processes lowers

the short wavelength spectral response of PSCs. The use of

DS phosphor does not address the first point, i.e., it has no

major anti-reflection property as evidenced by the reflectance

studies (Fig. S2). However, the down-shifting of UV light to

visible region provides a better spectral matching and more

number of charge carriers are generated away from the FTO-

perovskite interface. So the other two points are addressed

by the DS phosphor layer and an improvement in short-

wavelength spectral response is obtained (Fig. 3(b)).

Another interesting observation is the small enhancement

observed in the long wavelength region beyond 600 nm. To

understand this, we performed total as well as diffused trans-

mittance and reflectance measurements on bare and phosphor

coated FTO (supplementary material).26 A careful analysis of

these measurements revealed that the phosphor film primarily

absorbs light wavelengths up to 450 nm and scatters the long

wavelength light (k> 500 nm) mainly in the forward direc-

tion, thus increasing the effective path length of the long

wavelength radiation. This scattering phenomenon may be re-

sponsible for the small enhancement observed in the

500–800 nm region. However, the IPCE enhancement factor

is most pronounced for short-wavelengths, indicating that

down-shifting effects are dominant (Fig. 3(b), inset). The cur-

rent enhancement by DS NPs is an additional benefit consid-

ering the improved stability of PSC due to the absorption of

UV light and subsequent red emission.

The variations of the solar cell parameters with time,

under AM1.5G illumination provided by a solar simulator

(Sol3A, Newport Oriel, USA), of the two types of PSCs are

plotted in Figure 4. There was no control over temperature

and relative humidity levels and they varied between

25–31 �C and 25%–40%, respectively, during the measure-

ments. The prolonged absorption of UV light by TiO2 based

PSCs leads to a decrease in charge collection efficiency

because electrons get trapped in the UV generated deep trap

sites of TiO2 and recombine with holes in the HTM.10 This

process lowers the short-circuit current density (Jsc) and the

current values drop with time upon illumination leading to a

decrease in efficiency (Fig. 4(b)). DS-PSC shows better cur-

rent behaviour than the control PSC leading to comparatively

lesser degradation in efficiency (Fig. 4(b)). For DS-PSC, Jsc

at t¼ 2 h is �96% of its value at t¼ 0 h, indicating that there

is virtually no change in photocurrent; while the Jsc of con-

trol PSC drops to �85% of the initial value (Fig. 4(a)). This

is a clear indication of UV degradation effect and also

FIG. 3. (a) The current density-voltage

and (b) IPCE curves of best control

and DS phosphor coated PSC. Inset in

(a) shows the matching of absorbance

spectra of perovskite and photolumi-

nescence emission of down-shifting

phosphor. Inset in (b) shows the IPCE

enhancement factor as a function of

wavelength.

TABLE I. Photovoltaic parameters of control and DS phosphor coated per-

ovskite solar cells. Average values along with the standard deviations are

shown.

Device type

Short-circuit

current density,

Jsc (mA/cm2)

Open-circuit

voltage,

Voc (mV)

Fill factor

(%)

Efficiency

(%)

Control PSC 16.53 6 0.15 830 6 2.5 54 6 0.15 7.42 6 0.1

DS-PSC 17.77 6 0.31 834 6 3.5 53.5 6 0.4 7.93 6 0.17

033904-3 Chander et al. Appl. Phys. Lett. 105, 033904 (2014)

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130.113.86.233 On: Fri, 21 Nov 2014 18:26:37

demonstrates the ability of DS NPs to arrest this degradation.

The Jsc of control PSC degrades to �43% and that of DS-

PSC to �60% of their respective initial values after 12 h of

AM1.5G illumination. The open circuit voltages (Voc) and

fill factors (FFs) of the two cells do not show a significant

degradation (Fig. 4, insets). The small drop of FF may be

attributed to improper doping of the Spiro HTM.28 The over-

all drop in PCE of control PSC is more than 50%, PCE at

t¼ 12 h is �35% of the initial value. On the other hand, DS-

PSC registers a much better performance and the PCE at

t¼ 12 h is �52% of the initial value (see Fig. 4(b)).

Although the phosphor appears to delay degradation of

the device, the degradation rate becomes more or less identi-

cal after 6 h. This is an important observation and needs to

be understood. Under UV illumination and presence of oxy-

gen, TiO2 becomes more hydrophilic and accumulates more

moisture.29 Increased moisture near the perovskite acceler-

ates the degradation process. The phosphor only down-shifts

a relatively small portion of UV spectrum, so the degradation

is arrested only for a short duration of 6 h. After this, the

increased hydrophilicity of TiO2 and the UV-generated deep

trap sites accelerate the degradation of perovskite mate-

rial.10,29 A properly encapsulated device, sealed under very

low humidity levels, is required if the benefits of down-

shifting are to be fully realized.

We have optimized the spray volume (�6 ml) of DS

NPs dispersion for getting best performance. A smaller vol-

ume of DS material resulted in higher transmittance but pho-

tocurrent enhancement and device stability were not

improved because of insufficient down-shifting of UV light.

A larger spray volume resulted in a significant decrease in

transmittance value and the efficiency of the device got

reduced. The optimized value of spray volume represents a

balance between (1) good transmittance, (2) photocurrent

enhancement, and (3) UV protection.

In summary, the present work demonstrates the dual

benefits of using DS NPs: enhancement in photocurrent and

improved UV-stability of PSC. The method involved is sim-

ple: a spray coated transparent layer of DS NPs absorbs UV

light and reduces UV-induced degradation. The PCE of DS-

PSC is higher than control PSC due to down-shifting of UV

light to visible region and also due to path-length enhance-

ment of long-wavelength light because of scattering. A DS

material with a broad absorption band, which effectively

absorbs the 300–400 nm portion of the UV spectrum, would

be an ideal choice for improving photocurrent and UV

stability of PSCs. Our results may open up further avenues

for improving the performance of PSCs.

A.F.K. and V.D. would like to acknowledge the support

from the Department of Science and Technology (DST),

India under DST INSPIRE Faculty Award No. IFA-CH-27

and DST ESCORT research Project No. RP02499,

respectively. P.S.C. would like to acknowledge support under

DST INSPIRE fellowship (IF120755). S.K.S. acknowledges

the financial support from Ministry of New and Renewable

Energy (MNRE), Government of India. Authors thank Mr.

Piyush K. Parashar, and Mr. Sanjay K. Sardana for sputtering

of gold contacts for perovskite solar cells.

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FIG. 4. Normalized photovoltaic pa-

rameters of the fabricated perovskite

solar cells as a function of illumination

time.

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