We report an optical fiber based spectral response measurement system for multijunction solar cells. We have also fabricated single junction GaAs cells on GaAS and Ge substrates and measured lighted I-V characteristics. Preliminary quantum efficiency measurements on these devices are also presented.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2200

Arora, B.M.

Bag, R.

Chandvankar, S.S.

Chaudhari, V.A.

Narasimhan, K.L.

Shah, A.P.

Tyagi, R.

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P. 662-666 Ó 2011 SumDU (Sumy State University)   662 PACS numbers: 42.81.Qb, 85.60.Bt  OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT SYSTEM FOR MULTI-JUNCTION SOLAR CELLS  A.P. Shah1, R. Bag2, R. Tyagi2, S.S. Chandvankar1, V.A. Chaudhari3, K.L. Narasimhan1, B.M. Arora3  1 Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, 400005, Mumbai, India E-mail: apshah@tifr.res.in 2 Solid State Physics Laboratory,   Timarpur, 110054, Delhi, India 3 Indian Institute of Technology Bombay,  Powai, 400076, Mumbai, India  We report an optical fiber based spectral response measurement system for multi-junction solar cells. We have also fabricated single junction GaAs cells on GaAS and Ge substrates and measured lighted I-V characteristics. Preliminary quantum efficiency measurements on these devices are also presented.  Keywords: MULT- JUNCTION SOLAR CELLS, QUANTUM EFFICIENCY, Ge-GaAs-InGaP, OPTICAL FIBER, BIAS LIGHT.  (Received 04 February 2011, in final form 11 October 2011)  1. INTRODUCTION  Triple junction solar cells (InGaP/GaAs/Ge) with efficiency ~ 40 % have been reported in the literature for operation at ~ 240 suns [1]. Design, fabrication and testing of these cells are challenging tasks. In optimally designed  cell,  base  layer  thickness  of  each  sub-cell  is  chosen  so  as  to  maximize the current flowing in all the sub-cells in series. To check the sub-cell limiting the current, quantum efficiency for each sub-cell has to be measured, with little influence of the other sub-cells. To accomplish this, say in a triple junction cell, two out of the three junctions in the cell are generally biased by using suitable light and voltage biasing [2], and short circuit current of the third junction is measured as function of wavelength by using chopped light. In this work, we report preliminary findings of a set up of spectral response measurements for a commercial triple-junction solar cell, as well as of some single-junction GaAs solar cells fabricated by us.   2. EXPERIMENTAL  We have designed a set up for the measurement of spectral response of sub-cells  in  a  triple  junction  solar  cell.  It  consists  of  a  fiber  cable  assembly,  which has three input legs and one output leg (Fig. 1). Each input leg is a bundle of 1mm diameter close pack of 14 fibers of 0.22 NA (200/220/245 mm) terminated in SMA 905 connector.      OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT… 663   Fig. 1 – Quantum efficiency measurement set up with 532 nm, 860 nm & 1310 nm bias light   The output leg combines 42 fibers (14 from each input leg) in a 2 mm diameter close pack nominal salt and pepper mapping, terminated in SMA 905 end connector. Three fiber coupled diode sources (0.53, 0.86 and 1.3 mm) are selected for bias light. The probe light for quantum efficiency measurement is a tungsten halogen lamp in combination with 1/8 m monochromator. Light from output slit of monochromator is chopped and is incident on one of the three legs of the input bundles. The other two legs carry the bias  light.  The output leg combines  the bias  light  and the probe light, which are coincident on the device under test. Spectral response measurements on the sub cells of a commercial triple junction cell consisting of  Ge  /  GaAs  /  InGaP  sub-cells  are  reported  with  and  without  bias  light.  Effect of dc voltage biasing is not investigated in this presentation.   We have initiated efforts to fabricate the multi-junction structures with single junction GaAs cells. This work is at initial stage. Single junction GaAs  cells  have   been  fabricated  from  structures  grown  by  us  using  (i)  liquid  phase  epitaxy  (LPE)  on  GaAs  substrates  (cell  A),  and  (ii)  organo-metallic  vapour  phase  epitaxy  (MOVPE)  on  Ge  substrates  (cell  B).  Cell  A  consists of n-GaAs substrate (2 ´  1017 cm – 3), 0.8 µm thick p-GaAs, 0.5 µm thick p+- Al0.75Ga0.25As window layer (~  1018 cm – 3) and 100 nm thick p+-GaAs contact layer (~ 3 ´ 1018 cm – 3);  while  Cell  B  consists  of  n+-Ge substrate (5 ´ 1018 cm – 3),  followed by 300 nm n+ -  GaAs layer  (1.5  ´  1018 cm – 3), 1.8 µm thick n-GaAs base layer (2 ´ 1017 cm-3), 170 nm thick p - GaAs emitter (5 ´ 1018 cm – 3) and 50 nm thick contact layer of p+ – GaAs (2 ´ 1019 cm – 3). No AlGaAs window layer was grown in cell B. Preliminary results of measurements on these devices are also presented.  3. RESULTS AND DISCUSSION  Fig. 2 shows the spectral response of triple junction cell without any bias light in the wavelength range 650 nm to 950 nm. From the nature of this measurement, it is clear that we observe a composite response of the triple junction cell with the band edges of GaAs and InGaP marked by the arrows. A small current is observed below the band edge of GaAs cell also, which is due to the Ge cell. However, it is relatively small. The above measurement is     664 A.P. SHAH, R. BAG, R. TYAGI, ET AL.   Fig. 2 – Spectral response of the triple-junction cell without any bias light. Arrows show the GaAs and InGaP band edges  far from the short circuit condition assumed for the spectral response measurement. Fig. 3 shows that shining bias light at 532 nm which is strongly absorbed by the InGaP sub-cell completely changes the shape of the response curve and it is more representative of the GaAs sub-cell. A small response  from  the  InGaP  sub-cell  is  still  present  and  further  tweaking  of  biasing parameters (light and voltage) is required to isolate the GaAs sub-cell response. We also see some current flowing below 900 nm, which must be due to the Ge sub-cell. To eliminate this, we show in Fig. 4, the response curve by shining both 532 nm and 1310 nm bias lights.   A dramatic reduction is seen in the response beyond 900 nm, completely eliminating the interference in the response of the GaAs sub-cell from the Ge sub-cell. By biasing with band gap dc light, both InGaP and Ge cells are forward biased and provide a reverse bias to the GaAs cell so that the ac current  due  to  chopped  light  is  representative  of  the  GaAs  sub  cell  short  Fig. 4 – Spectral response of GaAs sub-cell with 532 nm & 1310 nm bias light 700 800 900 Spectral Response (a.u.) Wavelength (nm)  Fig. 3 – Spectral response of GaAs sub-cell with 532 nm bias light 700 800 900 Spectral Respomse (a.u.)  Wavelength (nm)650 700 750 800 850 900 950G aA s band edgeInG aP band edge W avelength (nm )  Spectral Response (a.u.)    OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT… 665 circuit  current  response.  A  small  notch  observed  at  about  850  nm  (as  compared to Fig. 3) shows the effect of turning off the 1310 nm dc bias on the response of GaAs sub-cell within the regime of its sensitivity.  Fig. 5 shows the spectral response of the InGaP sub-cell in the wavelength range 350-700 nm. We see that shining of 860 nm dc bias light has elimi-nated the interference from the GaAs sub-cell in the wavelength range greater than 700 nm in comparison to the response curve of Fig. 2, where a substan-tial response of the GaAs sub-cell was seen. The response of Ge sub-cell ob-tained by shining 532 nm bias light bias light is presented in Fig. 6. As com-pared to  Fig.  2,  where the response from Ge sub-cell  was considerably sup-pressed, a significant improvement in the measured response of the Ge sub-cell is seen in the wavelength regime 900nm to 1700 nm. Work on optimizing the spectral response measurement of multi-junction solar cells is continuing.  Lighted I-V measurements of single junction GaAs cells show that the short circuit  current  of  cell  A  (grown  on  n+ – GaAs substrate with AlGaAs top window layer) is nearly double that of cell B (grown on n+ – Ge substrate without AlGaAs window but having TiO2 anti reflection coating). Spectral response measurements of these two cells are compared in Fig. 7. It is seen   Fig. 7 – Spectral response of single-junction GaAs solar cells: Cell A (a)& Cell B (b) 450 600 750 900Cell A  Spectral Response (a.u.)Wavelength (nm)450 600 750 900Cell B  Spectral Response (a.u.)Wavelength (nm)Fig. 5 – Spectral response of InGaP sub-cell with 860 nm bias light 400 500 600 700  Spectral Response (a.u.)Wavelength (nm)Fig. 6 – Spectral response of Ge sub-cell with 532 nm bias light 800 1000 1200 1400 1600  Spectral Response (a.u.)Wavelength (nm)   666 A.P. SHAH, R. BAG, R. TYAGI, ET AL.  that  cell  A  has  much  better  spectral  response  than  the  cell  B  in  the  wavelength range 450 to 600 nm because AlGaAs is much better window for surface passivation of GaAs than TiO2, the top coating used for cell B. TiO2 coating does improve the response, because it acts as antireflection coating but it is not as good as AlGaAs for interface passivation. Even response on the longer wavelength side in cell B is not as good as in the cell A. This may be related to back field reflection. Adding an AlGaAs hetero-layer between the  GaAs  base  layer  and  Ge  substrate  could  improve  the  long  wavelength  response for the cell B also. Further efforts to synthesize structures, incorporating high Al concentration AlGaAs window in the front and a low Al concentration back field reflector, on Ge substrates are currently underway. The next step then will be to move into two junction cells.  4. CONCLUSION  An optical fiber based spectral response measurement system for multi-junction solar cells is described. Typical spectral response curves for individual sub-cells in a triple junction (InGaP/GaAs/Ge) solar cells are obtained by using light biasing. Further refinement including voltage biasing is being tried. The refined measurements will be used to estimate the short circuit current expected and compared with the actual measurement in future.  Preliminary  results  on  GaAs  solar  cells  fabricated  on  GaAs  substrates by LPE and on Ge substrates by MOVPE are described.  REFERENCES 1. R.R. King, D.C. Law, K.M. Edmondson, C.M. Fetzer, G.S. Kinsey, H. Yoon, R.A. Sherif, N.H. Karam, Appl. Phys. Lett. 90, 183516 (2007). 2. D.L. King, B.R. Hansen, J.M. Moore, D.J. Aiken, Proc. 28th IEEE PV Specialists Conf. 1197 (2000). 

Optical fiber based spectral response measurement system for multi-junction solar cells

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P. 662-666 
Ó 2011 SumDU 
(Sumy State University) 
 
 
662 
PACS numbers: 42.81.Qb, 85.60.Bt 
 
OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT 
SYSTEM FOR MULTI-JUNCTION SOLAR CELLS 
 
A.P. Shah1, R. Bag2, R. Tyagi2, S.S. Chandvankar1, V.A. Chaudhari3, 
K.L. Narasimhan1, B.M. Arora3 
 
1 Tata Institute of Fundamental Research, 
Homi Bhabha Road, Colaba, 400005, Mumbai, India 
E-mail: apshah@tifr.res.in 
2 Solid State Physics Laboratory,  
 Timarpur, 110054, Delhi, India 
3 Indian Institute of Technology Bombay, 
 Powai, 400076, Mumbai, India 
 
We report an optical fiber based spectral response measurement system for multi-
junction solar cells. We have also fabricated single junction GaAs cells on GaAS and 
Ge substrates and measured lighted I-V characteristics. Preliminary quantum 
efficiency measurements on these devices are also presented. 
 
Keywords: MULT- JUNCTION SOLAR CELLS, QUANTUM EFFICIENCY, Ge-GaAs-
InGaP, OPTICAL FIBER, BIAS LIGHT. 
 
(Received 04 February 2011, in final form 11 October 2011) 
 
1. INTRODUCTION 
 
Triple junction solar cells (InGaP/GaAs/Ge) with efficiency ~ 40 % have 
been reported in the literature for operation at ~ 240 suns [1]. Design, 
fabrication and testing of these cells are challenging tasks. In optimally 
designed  cell,  base  layer  thickness  of  each  sub-cell  is  chosen  so  as  to  
maximize the current flowing in all the sub-cells in series. To check the sub-
cell limiting the current, quantum efficiency for each sub-cell has to be 
measured, with little influence of the other sub-cells. To accomplish this, 
say in a triple junction cell, two out of the three junctions in the cell are 
generally biased by using suitable light and voltage biasing [2], and short 
circuit current of the third junction is measured as function of wavelength 
by using chopped light. In this work, we report preliminary findings of a set 
up of spectral response measurements for a commercial triple-junction solar 
cell, as well as of some single-junction GaAs solar cells fabricated by us.  
 
2. EXPERIMENTAL 
 
We have designed a set up for the measurement of spectral response of sub-
cells  in  a  triple  junction  solar  cell.  It  consists  of  a  fiber  cable  assembly,  
which has three input legs and one output leg (Fig. 1). Each input leg is a 
bundle of 1mm diameter close pack of 14 fibers of 0.22 NA (200/220/245 
mm) terminated in SMA 905 connector.  
 
 
 
 OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT… 663 
 
 
Fig. 1 – Quantum efficiency measurement set up with 532 nm, 860 nm & 1310 nm 
bias light 
 
 The output leg combines 42 fibers (14 from each input leg) in a 2 mm 
diameter close pack nominal salt and pepper mapping, terminated in SMA 
905 end connector. Three fiber coupled diode sources (0.53, 0.86 and 
1.3 mm) are selected for bias light. The probe light for quantum efficiency 
measurement is a tungsten halogen lamp in combination with 1/8 m 
monochromator. Light from output slit of monochromator is chopped and is 
incident on one of the three legs of the input bundles. The other two legs 
carry the bias  light.  The output leg combines  the bias  light  and the probe 
light, which are coincident on the device under test. Spectral response 
measurements on the sub cells of a commercial triple junction cell consisting 
of  Ge  /  GaAs  /  InGaP  sub-cells  are  reported  with  and  without  bias  light.  
Effect of dc voltage biasing is not investigated in this presentation.  
 We have initiated efforts to fabricate the multi-junction structures with 
single junction GaAs cells. This work is at initial stage. Single junction 
GaAs  cells  have   been  fabricated  from  structures  grown  by  us  using  (i)  
liquid  phase  epitaxy  (LPE)  on  GaAs  substrates  (cell  A),  and  (ii)  organo-
metallic  vapour  phase  epitaxy  (MOVPE)  on  Ge  substrates  (cell  B).  Cell  A  
consists of n-GaAs substrate (2 ´  1017 cm – 3), 0.8 µm thick p-GaAs, 0.5 µm 
thick p+- Al0.75Ga0.25As window layer (~  1018 cm – 3) and 100 nm thick p+-
GaAs contact layer (~ 3 ´ 1018 cm – 3);  while  Cell  B  consists  of  n+-Ge 
substrate (5 ´ 1018 cm – 3),  followed by 300 nm n+ -  GaAs layer  (1.5  ´  1018 
cm – 3), 1.8 µm thick n-GaAs base layer (2 ´ 1017 cm-3), 170 nm thick p - 
GaAs emitter (5 ´ 1018 cm – 3) and 50 nm thick contact layer of p+ – GaAs 
(2 ´ 1019 cm – 3). No AlGaAs window layer was grown in cell B. Preliminary 
results of measurements on these devices are also presented. 
 
3. RESULTS AND DISCUSSION 
 
Fig. 2 shows the spectral response of triple junction cell without any bias 
light in the wavelength range 650 nm to 950 nm. From the nature of this 
measurement, it is clear that we observe a composite response of the triple 
junction cell with the band edges of GaAs and InGaP marked by the arrows. 
A small current is observed below the band edge of GaAs cell also, which is 
due to the Ge cell. However, it is relatively small. The above measurement is  
 
 
 
664 A.P. SHAH, R. BAG, R. TYAGI, ET AL.  
 
Fig. 2 – Spectral response of the triple-junction cell without any bias light. Arrows 
show the GaAs and InGaP band edges 
 
far from the short circuit condition assumed for the spectral response 
measurement. Fig. 3 shows that shining bias light at 532 nm which is 
strongly absorbed by the InGaP sub-cell completely changes the shape of the 
response curve and it is more representative of the GaAs sub-cell. A small 
response  from  the  InGaP  sub-cell  is  still  present  and  further  tweaking  of  
biasing parameters (light and voltage) is required to isolate the GaAs sub-
cell response. We also see some current flowing below 900 nm, which must 
be due to the Ge sub-cell. To eliminate this, we show in Fig. 4, the response 
curve by shining both 532 nm and 1310 nm bias lights.  
 A dramatic reduction is seen in the response beyond 900 nm, completely 
eliminating the interference in the response of the GaAs sub-cell from the 
Ge sub-cell. By biasing with band gap dc light, both InGaP and Ge cells are 
forward biased and provide a reverse bias to the GaAs cell so that the ac 
current  due  to  chopped  light  is  representative  of  the  GaAs  sub  cell  short  
Fig. 4 – Spectral response of GaAs 
sub-cell with 532 nm & 1310 nm bias 
light 
700 800 900
 S
pe
ct
ra
l R
es
po
ns
e 
(a
.u
.)
 Wavelength (nm)
 
 
Fig. 3 – Spectral response of GaAs 
sub-cell with 532 nm bias light 
700 800 900
 S
pe
ct
ra
l R
es
po
m
se
 (a
.u
.)
 
 
Wavelength (nm)
650 700 750 800 850 900 950
G aA s band edge
InG aP band edge
 W avelength (nm )
 
 
Sp
ec
tra
l R
es
po
ns
e 
(a
.u
.)
 
 
 
 OPTICAL FIBER BASED SPECTRAL RESPONSE MEASUREMENT… 665 
circuit  current  response.  A  small  notch  observed  at  about  850  nm  (as  
compared to Fig. 3) shows the effect of turning off the 1310 nm dc bias on 
the response of GaAs sub-cell within the regime of its sensitivity. 
 Fig. 5 shows the spectral response of the InGaP sub-cell in the wavelength 
range 350-700 nm. We see that shining of 860 nm dc bias light has elimi-
nated the interference from the GaAs sub-cell in the wavelength range greater 
than 700 nm in comparison to the response curve of Fig. 2, where a substan-
tial response of the GaAs sub-cell was seen. The response of Ge sub-cell ob-
tained by shining 532 nm bias light bias light is presented in Fig. 6. As com-
pared to  Fig.  2,  where the response from Ge sub-cell  was considerably sup-
pressed, a significant improvement in the measured response of the Ge sub-
cell is seen in the wavelength regime 900nm to 1700 nm. Work on optimizing 
the spectral response measurement of multi-junction solar cells is continuing. 
 
Lighted I-V measurements of single junction GaAs cells show that the short 
circuit  current  of  cell  A  (grown  on  n+ – GaAs substrate with AlGaAs top 
window layer) is nearly double that of cell B (grown on n+ – Ge substrate 
without AlGaAs window but having TiO2 anti reflection coating). Spectral 
response measurements of these two cells are compared in Fig. 7. It is seen 
 
 
Fig. 7 – Spectral response of single-junction GaAs solar cells: Cell A (a)& Cell B (b) 
450 600 750 900
Cell A
 
 
Sp
ec
tra
l R
es
po
ns
e 
(a
.u
.)
Wavelength (nm)
450 600 750 900
Cell B
 
 
Sp
ec
tra
l R
es
po
ns
e 
(a
.u
.)
Wavelength (nm)
Fig. 5 – Spectral response of InGaP 
sub-cell with 860 nm bias light 
400 500 600 700
 
 
Sp
ec
tra
l R
es
po
ns
e 
(a
.u
.)
Wavelength (nm)
Fig. 6 – Spectral response of Ge 
sub-cell with 532 nm bias light 
800 1000 1200 1400 1600
 
 
Sp
ec
tra
l R
es
po
ns
e 
(a
.u
.)
Wavelength (nm)
 
 
 
666 A.P. SHAH, R. BAG, R. TYAGI, ET AL.  
that  cell  A  has  much  better  spectral  response  than  the  cell  B  in  the  
wavelength range 450 to 600 nm because AlGaAs is much better window for 
surface passivation of GaAs than TiO2, the top coating used for cell B. TiO2 
coating does improve the response, because it acts as antireflection coating 
but it is not as good as AlGaAs for interface passivation. Even response on 
the longer wavelength side in cell B is not as good as in the cell A. This may 
be related to back field reflection. Adding an AlGaAs hetero-layer between 
the  GaAs  base  layer  and  Ge  substrate  could  improve  the  long  wavelength  
response for the cell B also. Further efforts to synthesize structures, 
incorporating high Al concentration AlGaAs window in the front and a low 
Al concentration back field reflector, on Ge substrates are currently 
underway. The next step then will be to move into two junction cells. 
 
4. CONCLUSION 
 
An optical fiber based spectral response measurement system for multi-
junction solar cells is described. Typical spectral response curves for 
individual sub-cells in a triple junction (InGaP/GaAs/Ge) solar cells are 
obtained by using light biasing. Further refinement including voltage 
biasing is being tried. The refined measurements will be used to estimate the 
short circuit current expected and compared with the actual measurement in 
future.  Preliminary  results  on  GaAs  solar  cells  fabricated  on  GaAs  
substrates by LPE and on Ge substrates by MOVPE are described. 
 
REFERENCES 
1. R.R. King, D.C. Law, K.M. Edmondson, C.M. Fetzer, G.S. Kinsey, H. Yoon, 
R.A. Sherif, N.H. Karam, Appl. Phys. Lett. 90, 183516 (2007). 
2. D.L. King, B.R. Hansen, J.M. Moore, D.J. Aiken, Proc. 28th IEEE PV Specialists 
Conf. 1197 (2000). 


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Optical fiber based spectral response measurement system for multi-junction solar cells

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