Solar Energy Materials & Solar Cells 95 (2011) 2018–2022

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Spectroscopic study of Cu2 +/Cu+ doubly doped and highly transmittingglasses for solar spectral transformation

Susana Gomez a,n, Inigo Urra b, Rafael Valiente c, Fernando Rodrıguez a

a DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spainb Dpt. Construccion, Area de Energıa y medioambiente, CIDEMCO-Tecnalia, 20730 Azpeitia, Spainc Dpt. Fısica Aplicada, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain

a r t i c l e i n f o

Article history:

Received 28 August 2009

Received in revised form

12 July 2010

Accepted 22 July 2010Available online 8 August 2010

Keywords:

Glasses

Cu+

Cu2 +

Solar converter

Luminescence

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.07.022

esponding author. Tel.: +34 942201515; fax:

ail address: susana.gomez@unican.es (S. Gom

a b s t r a c t

This work investigates the formation of photoluminescence (PL) centres in high-transmission glasses

(HTG) doped with Cu2O and their capability to transform the solar spectrum by absorption/emission via

upconversion, downconversion and Stokes-shifted PL into a more efficient spectrum for photovoltaic

applications. Both the green PL Cu+ and the non-PL Cu2 + centres are formed in HTG, although their

relative concentration depends on the thermal treatment and the presence of other codopants. Given

that the absorption spectrum of Cu+ lies around the HTG band gap, measurement of the absorption

coefficient a(l) for these absorption bands is not easy due to corrections between the reflection

coefficient and chromatic dispersion. We present a procedure, named two-thickness method, to extract

the actual absorption coefficient for the spectrum of each formed centre. In addition it provides the

relative Cu+/Cu2 + concentration as well as their absolute values. Analysis of the spectra also provides

information on the absorption cross section, transition energy and bandwidth of each band, the

knowledge of which is essential to check the suitability of such centres for photovoltaic applications in

silicon solar cells.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

There is an increasing interest for investigating high-transmis-sion glass (HTG), which efficiently transforms the incoming solarspectrum enhancing the wavelength domain for photovoltaicapplications in silicon cells [1–4]. In particular, the focus is paidon transition-metal and rare-Earth oxides acting as HTG dopants,which are able to partially convert infrared (IR) or ultraviolet (UV)radiation into visible (VIS) light via upconversion (two IRphotons-one VIS photon), downconversion (one UV photon-two VIS photons) or by large Stokes-shifted photoluminescence(PL) processes [1–3]. Depending on whether the selected dopantsintroduced in HTG are capable to absorb inefficient UV or IRradiation into photovoltaic useful VIS light, those enriched HTGcan significantly improve the efficiency of the solar cells. In thiswork we investigate the absorption and emission properties ofCu2O-doped HTG and also Cu2O–CeO2-codoped HTG. The aim is toexplore the synthesis and subsequent thermal treatment yieldingthe formation of effective PL species for solar applications. In thisway, Cu2O-doped HTG contains both Cu+ and Cu2 + ions, therelative concentration of which strongly depends on the thermaltreatment and codopant species [5]. The knowledge of the

ll rights reserved.

+34 942201402.

ez).

absorption cross section associated with each band is funda-mental for a direct determination of the Cu+ and Cu2 +

concentrations by optical absorption measurements. Hence theabsorption spectrum can be used to probe the variation of therelative Cu+/Cu2 + concentration under different thermal treat-ments and compositional changes as well as to follow light-induced or doping-induced solid state reactions in the HTG.However there is a lack of information about the absorption crosssection of Cu+ and Cu2 + in spite of the numerous works devotedto investigate the optical properties of these ions in differentglasses [6,7]. Although Cu2 + is a major centre in Cu2O-doped HTGas it can be easily evidenced by its characteristic inhomogen-eously broadened absorption band at 790 nm [8] by the presenceof five-coordinate CuO5 centres [9,10], the Cu+ absorption bandhas not been detected yet. This is due to the impediments inmeasuring its associated optical absorption spectrum whosemain band associated with the interconfigurational transition3d104s0-3d94p1 lies at about 290 nm. In fact, this band overlapswith intrinsic absorption edge of the HTG band gap. In addition ithas been proved that the Cu+/Cu2 + ratio depends on differentfactors such as temperature, annealing time, composition, etc.[11,12], making their determination difficult from absorptionmeasurements alone.

In this work we investigate the PL properties of Cu+ and Ce3 +

centres formed in HTG by dissolving the corresponding oxidesinto fused HTG. We develop a procedure, named two-thickness

S. Gomez et al. / Solar Energy Materials & Solar Cells 95 (2011) 2018–2022 2019

method (TTM), to extract the actual absorption coefficients, a(l),of each absorbing centre as well as the real part of the refractiveindex and its chromatic dispersion. Through the TTM we are ableto measure the corresponding absorption cross sections of Cu+

and Cu2 + from which we obtain their relative concentrations. Thisinformation is crucial to eventually optimize the Cu+ content inHTG with respect to Cu2 + under different treatments and hence toexplore its feasibility as solar converters for photovoltaicapplications.

2. Experimental

We have synthesized Cu2O-doped HTG from two differentmethods: (i) graining and mixing commercial HTG together withcorresponding Cu and Ce oxides to produce fine homogeneouspowder; then it is placed in an alumina cast for a thermaltreatment at 1100 1C for 14 h and (ii) graining and mixinghomogeneously the raw materials (72%SiO2–2%Al2O3–15% Na2O)with dopants following a thermal treatment at 1400 1C for 3 h toproduce the HTG synthesis with dissolved dopants. Both theprocedures provided similar results regarding formation of Cu+

and Cu2 + centres. The real Cu and Ce content as well as the HTGcomposition were checked by X-ray fluorescence. The nominaland real concentrations of elements were identical within theinstrumental accuracy. For the spectroscopic studies we synthe-sized HTG samples with Cu2O concentrations between 0.05% and0.3% in weight. The casts employed to synthesize Cu2O-dopedHTG consist of alumina rings placed on an alumina plate, theinner part of which was loaded with the HTG powders andintroduced in an electrical oven for thermal treatment.The samples were prepared as discs of 2 cm diameter andtwo different thicknesses of 1.00 and 0.55 mm. After fine grinding,a final polishing was made using CeO2 powder as lustre

Fig. 1. Room-temperature transmittance spectra of 0.1% Cu2O-doped HTG and undoped

(b) effect of Cu2 + absorption at around 800 nm.

abrasive in order to maximize the optical transmittance of theHTG discs.

The absorption spectra were obtained by means of twospectrophotometers: (i) a CARY 6000i for high absorbancesensitivity measurements in the 200–1800 nm range and (ii) aPERKIN ELMER Lambda 9 for operating in the 200–3200 nm range.Each spectrum was taken in four different sample orientations inorder to minimize the polarization effects and suppress theabsorbance jumps upon changing the photodetector from photo-tube to IR semiconductor detector. This procedure allows us tomeasure the transmittances of 91.0% with accuracy better than0.1%.

For PL measurements, the excitation and emission spectrawere obtained by a JOBIN-YVON Fluoromax 2. The sampleconfiguration was 901 and forward scattering configurations.

3. Results and discussion

3.1. Optical transmittance and photoluminescence spectra

Fig. 1 shows the room-temperature transmittance spectrum ofCu2O (0.1%)-HTG in the 200–2500 nm range. The spectrum ofundoped HTG is also shown to emphasize the effect of Cu+

absorption on the total HTG transmittance. A clear absorptionband peaking at 790 nm appears in the NIR region and unravelsCu2 + formation. This absorption band, which is responsible for thelight blue colour of the HTG, increases with the Cu2Oconcentration. Besides, another absorption band seems toappear below 290 nm near the HTG band gap at 280 nm. Notethat the absorption threshold of the Cu2O-doped HTG is shifted tolonger wavelengths (lower energy) with respect to the absorptionthreshold of the undoped HTG. The presence of this band isnoteworthy since it unveils the formation of PL Cu+ centres.

HTG in the 200–2500 nm range. (a) Effect of Cu+ absorption in the UV region and

Fig. 2. Excitation and emission spectra of Cu2O-doped HTG. The emission

spectrum upon excitation at 4.5 eV (275 nm) is shown on the left side, whereas

the corresponding excitation spectrum taken at 2.5 eV (500 nm) is shown on the

right side.

60

50

40

30

20

10

0

Abs

orpt

ion

coef

fici

ent,

� (c

m-1

)

0 1 2 3 4

Photon energy (eV)

0.5 1 1.5 2 2.5 3

Cu2+

6

4

2

0

Undoped HTG0.1% Cu-doped HTG

Cu+

Fig. 3. Absorption coefficient for the Cu+ and Cu2 + bands (red lines) derived by

subtracting the HTG background contribution (blue lines) from the two-thickness

model. The Cu+ and Cu2+ bands peak at 4.43 eV (280 nm) and 1.57 eV (790 nm),

respectively. The corresponding absorption coefficients at the band maxima are 50

and 1.5 cm�1, respectively. Note that the absorption spectrum at around 4.43 eV

just corresponds to the Cu+ absorption plus an absorption background due to the

tail of the O2�-Cu2+ charge transfer band. (For interpretation of the references to

colour in this figure legend, the reader is referred to the web version of this article.)

S. Gomez et al. / Solar Energy Materials & Solar Cells 95 (2011) 2018–20222020

Its presence has been clearly demonstrated by the characteristicemission spectrum obtained under excitation at 290 nm. Theemission and corresponding excitation spectra are shown inFig. 2. The suitability of Cu+ emission (band maximum at 495 nm)for photovoltaic applications must be noted since radiation in the250–320 nm range, which is inefficient for photovoltaicapplications in silicon cells, is totally absorbed by Cu+ andemitted into light of 500 nm with an efficiency of about 50%. [13].Nevertheless the use of Cu2O is not adequate as solar spectrumconverter since, apart from Cu+, a similar amount of Cu2 + isformed. In fact, the balance between Cu2 + absorption and Cu+ PLis negative for such a purpose. However this balance can bereversed whatever the Cu+/Cu2 + ratio is enhanced. In this way thedevelopment of new methods for determining the actual Cu2 +

and Cu+ concentrations is crucial to achieve this goal.

3.2. Absorption coefficient: two-thickness model

Measurement of the absorption coefficient, a(l) or a(o), incircular samples from the transmission spectrum, T(l) or T(o) ofFig. 1, is a difficult task if the chromatic dispersion of the refractiveindex n(l) or n(o) is not known. So the absorption coefficient a(l)can be extracted from T(l) and n(l) through the equation [14]:

TðlÞ ¼ 1�RðlÞ� �2

e�aðlÞx

or, equivalently,

aðlÞ ¼ 1

xln 1�RðlÞ� �2

�lnTðlÞn o

ð1Þ

where x is the sample thickness and

RðlÞ ¼n�1

nþ1

��������2

¼ðn�1Þ2þk2

ðnþ1Þ2þk2

the reflection coefficient as a function of the complex refractiveindex n¼n+ik . The absorption coefficient is related to theimaginary part of n through a(l)¼2ok/c [14,15]. If n(l) is eitherconstant or slightly depends on l in the wavelength domain of theabsorption band, then a(l) can be easily derived from T(l) asa(l)¼�1/x ln[T(l)] plus a constant background Aback¼1/xln[1�R]2. This is the usual procedure to get a(l) from T(l) usingspectrophotometers. However the effect of chromatic dispersion inthe refractive index can affect significantly the shape of a(l) if n(l)varies in the spectral region of the absorption band. In addition,n(l) depends on the dopant concentration [Cu2O]. Cu+ and Cu2 +

are illustrative examples of these two behaviours, respectively. Itmust be noted that in contrast to Cu2+, a(l) for Cu+ cannot bedirectly derived from T(l) of the Cu2O-doped HTG and theundoped HTG as a(l)¼[1/x0 ln T0(l)–1/x ln T(l)] alone, where x0

and T0 refer to the sample thickness and transmission spectrum ofundoped HTG. This would be valid if refractive index does notdepend on the Cu2O concentration. But this is not the case withabsorption bands appearing near to or at the HTG band gap as theyoccur for Cu+. In fact, this procedure does not provide the resolvedCu+ band due to high chromatic dispersion at the band gap andthe different refractive index between doped and undoped HTG.However this problem can be solved using two HTG samples ofdifferent thicknesses with the same Cu2O concentration. In such acase, Eq. (1) is obeyed by two samples, which have the sameabsorption coefficient and the same reflection coefficient:aðlÞ ¼ 1=x1fln½1�RðlÞ�2�lnT1ðlÞg for sample 1 and a similarequation applies for sample 2, so that the absorption coefficientcan be directly derived from x1, x2, T1 and T2:

aðlÞ ¼ lnT2ðlÞ�lnT1ðlÞx1�x2

ð2Þ

Analogously, the reflection coefficient can be written as

RðlÞ ¼ 1�½Tx2

1 =Tx1

2 �1=2ðx2�x1Þ ð3Þ

Following this procedure we obtain the absorption coefficientsof the Cu2O-doped HTG [a(l)] and the undoped HTG [a0(l)].So the absorption coefficient for the Cu+ and Cu2 + bands can beobtained by subtracting the HTG background contribution asaCu(l)¼a(l)�a0(l). The results of applying the TTM are shown in

Table 1Spectroscopic parameters for Cu+ and Cu2+ corresponding to 0.1% Cu2O-doped HTG. The obtained relative concentrations from the absorption spectra are also included.

[Cu]¼2.1�1019 cm�3

Cu2O (0.1%)-doped HTG

Band maximum(eV–nm)

Bandwidth(eV–nm)

Oscillatorstrength (fabs)

Cross section at bandmax. (cm2)

Relativeconcentration (%)

Cu+ 4.50–275 0.50–30 0.08 2.5�10�17 5

Cu2+ 1.57–790 0.80–430 3�10�4 6�10�20 95

S. Gomez et al. / Solar Energy Materials & Solar Cells 95 (2011) 2018–2022 2021

Fig. 3. The so-obtained spectrum clearly indicates that the Cu+

band maximum is located at 280 nm (4.43 eV) with a bandwidthof 30 nm (0.5 eV) in agreement with the excitation spectrumwhose band peaks at 275 nm (4.51 eV) as shown in Fig. 2. Thediscrepancy is due to the self-absorption effects attained in theexcitation spectrum. Note that for Cu2 + the absorption coefficientand the band shape do not change significantly with respect to theusual procedure as a(l)¼[1/x0 ln T0(l)�1/x ln T(l)] using oneCu2O-doped HTG sample of thickness x, and an undoped HTGsample of thickness x0 as reference. The similitude between theoptical absorption spectra derived from the two methods ismainly attributed to small differences in the chromatic dispersionbetween Cu2O-doped HTG and undoped HTG around the Cu2 +

band wavelength range at 790 nm.

3.3. Absorption cross section and oscillator strength

From both Cu+ and Cu2 + absorption bands, we can derive theabsorption cross section for the two centres based on theassumption that only Cu+ and Cu2 + are formed in HTG, takingspectroscopic data of two different Cu2O concentrations. Table 1collects the spectroscopic parameters for Cu+ and Cu2 + and theirconcentrations are derived from the corresponding absorptionspectra. The so-obtained concentrations are similar to those givenin Refs. [8,11,13] for HTG and SiO2 doped with differenttransition-metal ions using redox methods and absorptionspectroscopy.

It is worth noting that the present absorption cross sectionsallow us a direct determination of the actual Cu+ and Cu2 +

concentrations in HTG from absorption measurements and thuscan be used to probe changes of their relative concentration underdifferent thermal treatments, annealing times and variations inHTG composition.

A relevant conclusion is the different relative concentrationobtained for Cu+ (5%) and Cu2 + (95%) in the investigated Cu2O(0.1%)-doped HTG following the synthesis process explained inthe experimental section. These concentrations slightly changewith the Cu2O concentration by reducing [Cu+] and increasing[Cu2 +]. From these values we obtain transition oscillatorstrengths: fabs¼0.08 for Cu+ band at 275 nm, and fabs¼3.0�10�4

for Cu2 + band at 790 nm. These values were obtained taking intoaccount the absorption background contribution due to the lowenergy side of the O2�-Cu2 + charge transfer band in the UVrange. This contribution represents about 15% of the totalabsorption band, the 75% remainder corresponding to the Cu+

band peaking at 4.50 eV.The oscillator strength for Cu2 + in the form of fivefold

coordinated CuO5 in HTG is stronger than that obtained for theintraconfigurational d–d transitions in centrosymmetric Cu2 +

centres, which are about 10�5 [16] due to Laporte’s rule [17].It means that fabs(Cu2+) in HTG is an order of magnitude higherthan that expected for vibrationally activated Cu2 + bands incentrosymmetric Cu2 +. Interestingly, fabs(Cu+) is higher thanfabs(Cu2 +) as the former is associated with the interconfigurationalparity-allowed 3d104s0-3d94p1 transition. Nevertheless, fabs(Cu+)

seems to be inconsistent with the corresponding PL lifetimemeasured at room-temperature: t¼46 ms. According to therelation between PL lifetime and associated electric-dipoleoscillator strength given by the following equation [17,18]:

fPL ¼1

t1:5� 104 9l2

0

ðn2þ2Þ2nð4Þ

where l0 in m is the wavelength at the band maximum and n therefractive index. Taking l0¼280 nm¼2.8�10�7 m and n¼1.52,we obtain a PL oscillator strength fPL¼8�10�6. This value is fourorders of magnitude smaller than the oscillator strength obtainedfrom the absorption spectrum (fPL¼0.08). This result clearly pointsout that the PL transition is probably related to the interconfigura-tional transition 3d94s1-3d10, which is, in contrast to ahypothetical 3d94p1-3d10 transition, parity forbidden in centro-symmetric centres like Cu+ in HTG [7] and thus weaker oscillatorstrengths, or equivalently longer PL lifetimes, should be expected.Therefore the present analysis provides evidence for assigning thePL transition of Cu+ in terms of 3d94s1-3d10 transition, whichdiffers from previous reported interpretations [6,13].

4. Conclusions

We have developed a procedure based on the two-thicknessmethod to obtain the absorption and reflection coefficients of Cu+

and Cu2 + formed in Cu2O-doped HTG. The method allows us todetermine their actual concentrations from optical spectroscopy,which makes it crucial for exploring efficient treatments of dopedHTG in order to enhance the presence of PL Cu+ with respect tonon-PL Cu2 +. Achievement of this goal is important to developnovel HTG with capabilities as solar spectrum converter forphotovoltaic applications. Based on absorption and PL lifetimedata we showed that the PL transition in Cu+ is mainly associatedwith the interconfigurational transition 3d94s1 rather than3d94p1. The latter transition (fabs¼0.08) would lead to a PLlifetime of 4.7 ns in contrast to the experimentally observedt¼46 ms.

Acknowledgements

This work has been done within the collaborative researchframework in photovoltaic glasses with the Research andInnovation Centre (CIDEMCO) in Azpeitia. Financial support fromthe Spanish Ministerio de Investigacion, former Ministerio deFomento (Project no. C81/2006) is acknowledged.

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