Effect of oxygen ambient during phosphorous diffusion on silicon solar cellDinesh Kumar, S. Saravanan, and Prakash Suratkar Citation: J. Renewable Sustainable Energy 4, 033105 (2012); doi: 10.1063/1.4717513 View online: http://dx.doi.org/10.1063/1.4717513 View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v4/i3 Published by the American Institute of Physics. Related ArticlesHighly efficient crystalline silicon/Zonyl fluorosurfactant-treated organic heterojunction solar cells Appl. Phys. Lett. 100, 183901 (2012) Selective emitters design and optimization for thermophotovoltaic applications J. Appl. Phys. 111, 084316 (2012) Effect of Gaussian doping on the performance of a n+-p thin film polycrystalline solar cell under illumination J. Renewable Sustainable Energy 4, 023118 (2012) A scattering model for nano-textured interfaces and its application in opto-electrical simulations of thin-film siliconsolar cells J. Appl. Phys. 111, 083108 (2012) Light scattering at textured back contacts for n-i-p thin-film silicon solar cells J. Appl. Phys. 111, 083101 (2012) Additional information on J. Renewable Sustainable EnergyJournal Homepage: http://jrse.aip.org/ Journal Information: http://jrse.aip.org/about/about_the_journal Top downloads: http://jrse.aip.org/features/most_downloaded Information for Authors: http://jrse.aip.org/authors

Effect of oxygen ambient during phosphorous diffusionon silicon solar cell

Dinesh Kumar,a) S. Saravanan,b) and Prakash Suratkarc)

Cell Technology and Process Engineering, TATA BP Solar India Ltd.,Bangalore 560100, India

(Received 11 January 2012; accepted 23 April 2012; published online 16 May 2012)

Phosphorous (P) diffusion is the most important and crucial process in the

fabrication of silicon (Si) solar cells from p-type Si substrates. P-diffusion using

phosphorous-oxycholoride (POCl3) as a precursor in a tube furnace had shown the

best cell performance over the belt diffusion because of uniform dopant

concentration all over the Si surface and gettering of metallic impurities present in

the substrate. The emitter formation by using POCl3 is a complex and advanced

process which provides the gettering and forming the unwanted dead layer on the

front surface due to inactive phosphorous. Along with temperature, the ambient

conditions during the diffusion process, such as gas flow rates and their composition,

flow kinetics also have an impact on the emitter properties. In the present paper, the

impact of oxygen (O2) flow during the diffusion process on the emitter formation

and the solar cell performance were studied. It has been found that, the presence of

oxygen during the diffusion process influences the concentration of inactive

phosphorous over the surface and the gettering process as well. The optimized

oxygen flow shows an improvement in the effective minority carrier lifetime of

�24 ls after diffusion and an absolute efficiency gain of 0.2% at pilot production.VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4717513]

INTRODUCTION

Diffusion process for realizing P-N junction in the solar cell is a complex process which

always has been a centre of research interest in photovoltaic industries. Diffusion process finds

its importance in being a key process that converts a wafer into device. At present, the starting

material of the industrial silicon solar cells is p-type silicon which needs phosphorous (P) diffu-

sion for the formation of n-type emitter.1 P-diffusion has an great impact on current density and

voltage of solar cell through emitter formation and also affects the subsequent solar cell proc-

esses, such as passivation properties of antireflective coating (ARC), contact formation, etc. At

industrial production level, either phosphorous-oxycholoride (POCl3) or ortho-phosphoric acid

(H3PO4) is used as a source for phosphorous diffusion.2 Among these two, doping of phospho-

rous by POCl3 in tube diffusion is the most preferable method of emitter formation due to bet-

ter quality and uniformity.3 Besides this, P-diffusion by POCl3 provides gettering of metallic

impurities present in the substrate.4 In such a gettering process, the recombination active tran-

sient metallic impurities move out of the bulk and are captured in the heavily doped P-layer

(Si-P matrix) near the top surface.5 Several reports have been devoted on emitter formation and

optimization of POCl3 diffusion.6 Most of the reports reported that the doping profile of P in

emitter was optimized to achieve the best conversion efficiency. For optimum doping profile,

the diffusion process is optimized by varying the diffusion temperature and temperature profiles

and very little attention is paid on the oxygen (O2) flow kinetics.7 In literature, very few reports

are available on the effect of doping ambient on the final emitter formation and cell

a)Electronic mail: dinesh.kumar@tatabp.com.b)Electronic mail: saravanan.s@tatabp.com.c)Electronic mail: prakash.suratkar@tatabp.com.

1941-7012/2012/4(3)/033105/8/$30.00 VC 2012 American Institute of Physics4, 033105-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 4, 033105 (2012)

performance. During diffusion process, mainly POCl3 and O2 gases are used along with the

nitrogen (N2) as a carrier or flushing gas. It is very well known that the presence of O2 during

P-diffusion is very essential as POCl3 reacts with O2 at higher temperatures and forms P2O5

from which further P gets diffused in the silicon.8,9

During diffusion process, the presence of excess P on the surface provides the gettering

effect to trap the undesired metallic impurities from the bulk silicon and immobilizes near to

the top emitter surface.10 However, excess phosphorus, greater than the solid solubility in the

silicon, ends up as a layer of inactive phosphorus.3 This inactive P is termed as dead layer

which acts as the recombination centre for charge carriers and thus affects the solar cell per-

formance.11 Therefore, it is always desired to avoid the formation of dead layer, and at the

same time gettering of impurities is also highly desired. Hence it is necessary to trade-off

between the two should be done for the optimum results. Alternatively, the impurity gettering

should be carried out followed by removing the dead layer using controlled etching of silicon.12

However, the later method is not industry friendly and involves high cost of cell fabrication.

Therefore, the diffusion process should be optimized to have gettering effect with minimal

inactive-P. In order to achieve this, the role of ambient gases especially O2 should be investi-

gated as it helps to control the P-concentration available for diffusion through in pre-deposition

phase while during drive-in phase it helps to trap the excess inactive P in SiO2 layer over the

silicon surface. Hence, it is important to study the effect of oxygen ambient and its flow rates

on the formation of emitter and cell performance. In the present work, the effect of O2 ambient

during the drive in phase on the electrical parameters of the solar cells was studied.

EXPERIMENTAL DETAILS

Boron doped, CZ Si wafers of size 125 mm� 125 mm with thickness 200 6 10 lm and

bulk resistivity in the range of 0.5-3 X cm were used as the starting material for the solar cell

fabrication. Solar cells were fabricated by the conventional silicon solar cell process with screen

print technology. The saw damages and the residual contamination were removed by alkaline

chemicals followed by alkaline texturization. Consecutively, P-diffusion was carried out by

using POCl3 in a tube furnace. The phosphosilicate glass formed during diffusion process was

removed in buffered HF followed by edge isolation. Silicon nitride (Si3N4) ARC of thickness

�75 nm and refractive index �2.0 was deposited in tube plasma enhanced chemical vapour

deposition (PECVD) system. The front and back contacts were made by using conventional

screen print technology and co-fired to realize the ohmic contacts as well as aluminium (Al)

back surface field (BSF). In the present study, solar cells fabricated with oxygen flow rate of

0.95 ltr/m3s during the drive in phase were taken as control cells for the optimizing the diffu-

sion process. Oxygen flow rates were changed during the drive-in phase to understand the effect

of oxygen ambient on the diffusion process and solar cell performance. However, oxygen flow

rate was kept constant at 0.1 ltr/m3s during the pre-deposition phase during all the experimental

as well as control cells. I-V characteristics of the finished solar cell were measured under the

AM1.5 G simulated solar radiation at 25 �C. Spectral response and diffused reflectivity of the

solar cells were measured by PV Measurements, USA. The emitter doping profile was measured

by using Electrochemical C-V measurements done at WEP, Germany. The minority carrier life-

time was measured using a lifetime tester (Sinton WCT-100) in the quasi steady state.13 The

emitter sheet resistance was measured using four probe method.

RESULTS AND DISCUSSIONS

The P-diffusion by using liquid POCl3 as a precursor can be understood from the following

equations:

4POCl3ðgÞþ3O2ðgÞ ! 2P2O5ðlÞ þ 6Cl2ðgÞ; (1)

2P2O5ðlÞ þ 5SiðsÞ ! 5SiO2ðsÞ þ 4PðsÞ: (2)

033105-2 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

From Eqs. (1) and (2), it can be understood that the oxygen reacts with POCl3 forms P2O5 on

the surface of the silicon. Simultaneously, the top surface of the silicon gets oxidized in the

presence of oxygen. As a result, complex of silicon oxide and P2O5 called as phosphor-silicate

glass (PSG) forms over the wafer surface. Oxygen ambient during the diffusion helps in creat-

ing a thin layer of liquid P2O5 over the silicon surface which further provides elemental phos-

phorous for the diffusion by reacting with Si. During this process, a minimum quantity or flow

of oxygen is always required to form a uniform emitter (doping) over the silicon surface by cre-

ating a uniform layer of P2O5. Similarly, an optimum flow of POCl3 is required to have getter-

ing effect but with minimal dead layer formation.

Figure 1 shows the sheet resistance scan for three different groups of diffused wafers with

different oxygen flow rates. From Figure 1 it can be observed that initially with increased oxy-

gen flow, sheet resistance uniformity throughout the wafer was increased where as for higher

flow rates it becomes non uniform. Here, it is interesting to see that the increase of oxygen

flow rates during the diffusion created high sheet resistance region in the centre. For in depth

understanding of this behaviour, further investigation is required. Table I shows the variation in

average bulk minority carrier lifetime of the wafers after diffusion and after removal of PSG

with different oxygen flow rates. From Table I, it is observed that the minority carrier lifetime

in diffused wafers improves with increasing oxygen flow rates up to 1.3 ltr/m3s. However for

the O2 flow rate of 1.51 ltr/m3s, a degradation of the minority carrier lifetime from the maxi-

mum value is observed. Also the observations of Table I can be interpreted in a way that, from

Eq. (1), it is clear that a minimum amount of oxygen is required to deposit a thin layer of P2O5

on the Si surface during the pre-deposition phase. During drive in phase, the oxygen consumes

the excess inactive phosphorous by oxidising the silicon surface. From the minority carrier, life-

time after diffusion has increased from 76 ls to 100 ls when oxygen flow rate was increased

from 0.95 ltr/m3s to 1.3 ltr/m3s. However, further increase in oxygen flow rate results in degra-

dation of minority carrier lifetime to 85 ls. By increasing the flow rate of oxygen, the dead

layer thickness might have been controlled in a better way and simultaneously gettering effect

might have been achieved. Also the increase in oxygen flow rate probably may cause the getter-

ing process due to excess consumption of surface P in fast grown silicon oxide. This results in

degradation of the minority carrier lifetime in diffused wafers with 1.51 ltr/m3s oxygen flow

rate during drive in process which has been clearly seen from Table I. Similar trend was

FIG. 1. Sheet resistance distribution across the phosphorous diffused wafers for three different groups of oxygen flow rates

during diffusion (drive-in) process, i.e., 0.95 ltr/m3s (control), 1.3 ltr/m3s, and 1.51 ltr/m3s.

TABLE I. Life-time for three different groups of 0.95 ltr/m3s, 1.3 ltr/m3s, and 1.51 ltr/m3s after

diffusion and after removal of PSG.

Oxygen flow

rate (ltr/m3s)

Minority carrier lifetime

after diffusion (ls)

Minority carrier

lifetime after PSG removal (ls)

0.95 76 37

1.3 100 60

1.51 85 53

033105-3 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

observed after removing the PSG and neutralizing the temporary HF passivation effect. The mi-

nority carrier lifetime trend after removal of PSG clearly indicates that with 1.3 ltr/m3s oxygen

flow rate gettering has improved. Hence, it is important to maintain the optimized oxygen flow

rates (which is 1.3 ltr/m3s in present case) during the diffusion process in order to eliminate the

possibilities of dead layer formation and effective gettering of metallic impurities.

Figure 2 shows the I-V characteristics of the solar cells with respect to the oxygen flow rates

whereas inset table shows the electrical parameters of the cells. The variation of cell performance

is similar to the minority carrier lifetime trend observed with O2 flow. From I-V characteristics,

it can be seen that the current and fill factor are improved with increased oxygen flow. In the

present study, the optimized value of oxygen flow rate during the drive in phase is 1.3 ltr/m3s.

Compared to 0.95 ltr/m3s, an improvement of 0.2% absolute in the solar cell efficiency has been

observed with optimized oxygen ambient (1.3 ltr/m3s) during the diffusion. With optimized oxy-

gen ambient, Voc is improved by 1 mV while there is no change in Jsc as compared to control

cell. Further, the series resistance of the cell with optimized oxygen ambient decreases by more

than 1 X similarly an improvement in the shunt resistance was also observed. However, for

1.51 ltr/m3s flow rate again FF and Voc gets reduced near to values that for 0.95 ltr/m3s. From

Figure 2, it is clear that the improvement in cell efficiency is mainly due to improved ohmic con-

tacts especially due to decreased series resistance. After observing a change in fill factor, contact

resistance scan (CoRRescan) has been carried for comparative study for control and optimized

1.3 ltr/m3s group solar cells (figure not shown here). It was found that the 0.95 ltr/m3s cells pos-

sess high contact resistance regions whereas 1.3 ltr/m3s cells possess uniform and low contact re-

sistance areas. These high contact resistance regions in 0.95 ltr/m3s cells could be due to the non

uniform emitter and dead layer where as the higher and optimized oxygen ambient helps in uni-

form emitter formation with minimal dead layer formation. This has been interpreted with the

lower series resistance values in 1.3 ltr/m3s cells compared to 0.95 ltr/m3s cells. Similarly,

improved uniformity in light beam induced current (LBIC) scans was observed for 1.3 ltr/m3s

cells compared to 0.95 ltr/m3s cells (figure not shown here).

Figure 3 shows the internal quantum efficiency (IQE) of three experimental groups with

different oxygen flow kinetics. The reflectivity of the samples after ARC coating is also shown

FIG. 2. Illuminated I-V characteristics of the solar cells fabricated for three different groups of oxygen flow rates during

diffusion (drive-in) process, 0.95 ltr/m3s (control), 1.3 ltr/m3s, and 1.51 ltr/m3s. Electrical parameters are also shown in the

table in inset.

033105-4 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

in Figure 3. From Figure 3, it has been observed that short wavelength response of the solar

cell has improved when oxygen flow rate is increased from 0.95 ltr/m3s to 1.3 ltr/m3s. It was

observed that the short wavelength response for 1.51 ltr/m3s cells is reduced when compared to

0.95 ltr/m3s cells. These observations could be understood from the phosphorous piling up phe-

nomenon at the SiO2/Si interface because it has segregation coefficient >1 for Si/SiO2 interfa-

ces.14 It results in rejection of phosphorous from the SiO2 to wards the Si during oxidation.

Similar phenomenon might have happened in the present case too, however, it requires further

in depth investigations. However, it should be noted that the piled up phosphorous could have

been trapped in the oxide layer and later on gets removed during the PSG etch process which

might have helped in control of dead layer formation. During low oxygen flow rates, there was

no high P-concentration region formation which results in reduced gettering, also the excess

phosphorous ends up as inactive dopant inside the emitter. It is well known that the near

surface vacancies in the silicon depends on the oxidizing ambient and generation of these

vacancies increases with oxygen flow rates.15–17 This might have resulted in increased active

phosphorous concentration near the top region of emitter with increased oxygen flow rates. Fur-

ther increase in oxygen flow rates to 1.51 ltr/m3s results in reduced response at short wave-

lengths. It could be due to complex dependence of the P-segregation coefficient on the oxygen

flow rates and temperatures which might have resulted in higher concentration of inactive P in

emitter.18 In depth investigations should be carried out to further understanding the exact effect

of oxygen. It has been found from the IQE measurements that, response of solar cell has

improved for longer wavelengths with increased oxygen flow to 1.3 ltr/m3s rates which could

be due to the improved impurity gettering from the bulk. Fast out diffusion of metallic impur-

ities from the bulk due to gettering near the top surface might have resulted in this improve-

ment. Also, the increased generation rate of self interstitials near the top surface due to

increased oxygen flow rate might have moved inside the bulk. However, it should be investi-

gated further. In conclusion, we suspect improved bulk gettering with higher oxygen flow rates

(1.3 ltr/m3s) as compared with 0.95 ltr/m3s group.

Figure 4 shows the doping profile of active phosphorous in diffused wafers with different

oxygen flow rates. The doping profiles reveal that the active phosphorus concentration near the

FIG. 3. IQE of control (oxygen flow during diffusion: 0.95 ltr/m3s) and experimental (oxygen flow during diffusion: 1.3 ltr/

m3s and 1.51 ltr/m3s) solar cells. Reflectance after Si3N4 ARC coating is also plotted.

033105-5 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

top surface is increased with the increase of oxygen flow rates. This observation might be an

indication of effect of higher oxygen flow to reduce the dead layer formation. It is interesting

to see that oxygen ambient is affecting emitter thickness and overall doping profile as well.

Also, it has been observed that the junction depth has increased with increase of oxygen flow.

As we discussed earlier, the increased oxidation ambient enhances the self generation rate of

self-interstitials.17,18 Therefore, the increased interstitials in the silicon might have helped in

increased active phosphorous concentration near the top emitter as confirmed from ECV meas-

urements. Also, the P diffusion inside the silicon could increase due the above said increased

generation rate of self interstitials. This might have resulted in the increased junction depth for

higher oxygen flow rate ambient. The modified doping profile might have affected the charge

carrier’s collection efficiency of emitter and thus short circuit current in the solar cells which

first increases with oxygen flow rates and then decreases. Further, to verify the effect of

increased oxygen flow rates, solar cells have been fabricated in a pilot production (1000

wafers/group) for control and 1.3 ltr/m3s flow rate. Figure 4 shows the distribution of electrical

parameters for control (0.95 ltr/m3s) and 1.3 ltr/m3s flow rates. From Figure 5, it has been noted

that desired results of pilot production are in conformity with the results of small scale. A gain

in Voc �1 mV was observed which is in accordance to the initial experiments. It was observed

that the fill factor has been improved due to high oxygen flow rate. This change in fill factor

can be well understood from the distribution plot of series resistance (see Figure 5) in which

the change in oxygen flow rates reduced the series resistance by �1 X. However, average short

circuit current was found to be decreased by small amount of �10 mA which contradicts our

results observed at small scale experiments. This could be due to different emitter profiles of

the solar cells, need of process optimization, i.e., PSG removal, ARC deposition etc. However,

this observation needs further in depth investigations at laboratory scale as well as at industrial

scale. Effect of different oxygen ambient on maximum power output was also studied and the

distribution for the same at is plotted in Figure 5. Overall an average absolute gain of 0.2% in

efficiency and an average gain of �3 mW in maximum power output were observed. In conclu-

sion, the pilot production results confirmed that oxygen has a crucial role to play during the

thermal diffusion of phosphorous and thus solar cell fabrication.

FIG. 4. Emitter doping profile (measured by EVC method) for the active phosphorus concentration for three different cate-

gories of oxygen flow rates during diffusion (drive-in) process, i.e., control (0.95 ltr/m3s) and experimental (1.3 ltr/m3s and

1.51 ltr/m3s).

033105-6 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

CONCLUSIONS

The presence of different oxygen flow rates during the POCL3 diffusion process was stud-

ied. By optimizing the oxygen flow conditions during the diffusion process, an absolute

improvement of 0.2% in the power conversion efficiency was observed. Further investigations

should be carried out to understand the effect of oxygen ambient during diffusion process.

ACKNOWLEDGMENTS

Authors wish to thank Mr. K. Subramanya, CEO, TATA BP Solar India Ltd. for the support

and encouragement in carrying out this work. Authors are also indebted to Mr. C. Nagesh, TATA

BP Solar India Ltd., Mr. Thomas Wolff, WEP Germany, and Mr. Orri Jonsson, PV measurements

for the sample preparation and characterization.

1J. S. Kang, “Gettering in silicon,” J. Appl. Phys. 65(8), 2974 (1989).2S. M. Sze, Semiconductor Devices: Physics and Technology, 2nd ed. (Wiley, USA, 2002), p. 453.3P. Kittidachachan, T. Markvart, G. J. Ensell, R. Greef, and D. M. Bagnall, “An analysis of a “dead layer” in the emitterof nþppþ solar cells,” in Proceedings IEEE-Photovoltaic Specialists Conference (IEEE, 2005), p. 1103.

4A. Bentzen, A. Holt, R. Kopecek, G. Stokkan, J. S. Christensen, and B. G. Svensson, “Gettering of transition metalimpurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing,” J. Appl. Phys. 99(9)93509 (2006).

5A. Schneider, R. Kopecek, G. Hahn, S. Noel and P. Fath, “Comparison of gettering effects during phosphorus diffusionfor one- and double-sided emitters,” IEEE Photovoltaic Specialists Conference (IEEE, 2005), pp. 1051–1054.

6S. Graf, J. Junge, S. Seren, and G. Hahn, “Emitter optimization for mono and multicrystalline silicon: A study of emittersaturation currents,” in Proceedings 25th European Photovoltaic Solar Energy Conference (EU-PVSEC, 2010), pp.1770–1773 (2010).

7L. C. Siu, N. S. Abdul Ghani, M. Y. Khairy, and W. S. Anizan, “Optimization on junction formation by three-stack fur-nace POCl3 diffusion and analysis on solar cell performance,” ECS Trans. Silicon Technol. Electron. Photovoltaics27(1), 1053 (2010).

8D. Holger Neuhaus and A. Munzer, “Industrial silicon wafer solar cells,” Adv. Opt. Electron. 2007, 1–15, doi:10.1155/2007/24521.

FIG. 5. Percentage distribution of the electrical parameters, open circuit voltage (Voc), short circuit current (Isc), series re-

sistance (Rseries), fill factor (FF), efficiency (EFF), and maximum power (Pmax), of solar cells fabricated at pilot run for

two different oxygen flow rates during diffusion, i.e., 0.95 ltr/m3s(control) and 1.3 ltr/m3s (optimized). 1000 solar cells for

each group were fabricated.

033105-7 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)

9C. S. Yoo, Semicond. Manuf. Technol. 13, 310–316 (2008).10H. J. Moller, C. Funke, M. Rinio, and S. Scholz, “Multicrystalline silicon for solar cells,” Thin Solid Films 487, 179

(2005).11P. Ostoja, S. Guerri, P. Negrini, and S. Solmi, “The effects of phosphorus precipitation on the open-circuit voltage in

Nþ/P silicon solar cells,” Solar Cells 11(1), 1 (1984).12N. P. Singh, S. N. Singh, N. K. Arora, R. K. Kotnala, and B. K. Das, “Diffused junction optimization in silicon solar cells

by a photochemical method,” Solar Cells 11(3), 293 (1984).13R. A. Sinton and A. Cuevas, “Contact less determination of current–voltage characteristics and minority carrier lifetimes

in semiconductors from quasi steady state photoconductance data,” Appl. Phys. Lett. 69(17), 2510 (1996).14F. Lau, L. Mader, C. Mazure, Ch. Werner, and M. Orlowski, “A Model for phosphorus segregation at the silicon-silicon

dioxide interface,” Appl. Phys. A 49, 671–675 (1989).15M. M. Atalla and E. Tennenbaum, “Impurity redistribution and junction formation in silicon by thermal oxidation,”

BELL Syst. Tech. J. 39, 933–946 (1960).16J. Middelhoek and J Holleman, “Low phosphorus concentrations in Si by diffusion from doped oxide layers,” J. Electro-

chem. Soc.: Solid State Sci. Technol. 121, 132–137 (1974).17J. P. John and M. E. Law, “Oxidation enhanced diffusion of phosphorus in silicon in heavily doped background concen-

trations,” J. Electrochem. Soc. 140 1489–1491 (1993).18V. Aleksandrov and N. N. Afonin, “Effect of thermal oxidation on the segregation of phosphorus implanted into silicon,”

Inorg. Mater. 41, 972–980 (2005).

033105-8 Kumar, Saravanan, and Suratkar J. Renewable Sustainable Energy 4, 033105 (2012)