Cu(In, Ga)Se2 microcells: High efficiency and low ...

Cu(In, Ga)Se2 microcells: High efficiency and low material consumption Myriam Paire, Laurent Lombez, Frédérique Donsanti, Marie Jubault, Stéphane Collin et al. Citation: J. Renewable Sustainable Energy 5, 011202 (2013); doi: 10.1063/1.4791778 View online: http://dx.doi.org/10.1063/1.4791778 View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v5/i1 Published by the AIP Publishing LLC.

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JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 5, 011202 (2013)

Cu(In, Ga)Se2 microcells: High efficiency and low material consumption de rique Donsanti,1,2,3 Myriam Paire,1,2,3,4 Laurent Lombez,1,2,3 Fre 1,2,3 5 phane Collin, Jean-Luc Pelouard,5 Marie Jubault, Ste Jean-François Guillemoles,1,2,3 and Daniel Lincot1,2,3 1

EDF R&D, Institut de Recherche et D eveloppement sur l’Energie Photovolta€ıque (IRDEP), 6 quai Watier, 78401 Chatou, France 2 CNRS, IRDEP, UMR 7174, 78401 Chatou, France 3 Chimie ParisTech, IRDEP, 75005 Paris, France 4 UPMC, 75005 Paris, France 5 CNRS, Laboratoire de Photonique et de Nanostructures (LPN), Route de Nozay, 91460 Marcoussis, France (Received 13 September 2012; accepted 26 November 2012; published online 27 February 2013)

Using solar cells under concentrated illumination is known to improve the conversion efficiency while diminishing the active area and thus material consumption. Recent concentrator cell designs tend to go miniaturized devices, in the 0.5–1 mm range, enabling a better thermal evacuation due to higher surface to volume ratio. If the cell size is further reduced to the micrometric range, spreading resistance losses can be made vanishingly small. This is particularly interesting for the thin film technology which has been limited up to now to very low concentration systems, from 1 to 10, due to excessive resistive losses in the window layer and difficult thermal management of the cells, grown on glass substrates. A new solar cell architecture, based on polycrystalline Cu(In,Ga)Se2 (CIGS) absorber, is studied: microscale thin film solar cells. Due to the reduced lateral dimension of the microcells (5 to 500 lm in diameter), the resistive and thermal losses are drastically decreased, enabling the use of high concentration (>100). This results in a breakthrough for concentration on this type of devices, which were previously limited to the low concentration range (about 10). Due to light concentration, the open circuit voltage increases up to several thousand suns equivalent, to reach over 900 mV. The temperature increase is limited to less than 20 C over the ambient at concentration around 1000. A 5% absolute efficiency increase on microcells at 475 is observed and a 21.3% 6 0.2% C 2013 American equivalent efficient microcell of 50 lm of diameter is measured. V Institute of Physics. [http://dx.doi.org/10.1063/1.4791778]

INTRODUCTION

The sustainability of photovoltaics is becoming an increasingly important axis of research. Indeed, most photovoltaic technologies will be faced with material shortages in the short to long term3 in the absence of technological evolution. In the thin film Cu(In,Ga)Se2 (CIGS) communities, In scarcity is the main concern.4 Several approaches are envisioned to diminish the indium consumption of Cu(In,Ga)Se2 cells. First, the absorbers layer can be thinned,5,6 with a material gain limited to 20. Second, In-free compounds are developed but their efficiencies7 are still lagging behind their Cu(In,Ga)Se2 counterparts.8 We proposed recently the use of concentration on microcells. In concentrating photovoltaic, the material gain is roughly proportional to the optical concentration ratio and can thus attain factors of the order of 1000. Thin films were traditionally limited to the low concentration ratio,9–12 but previous work from our group showed that the miniaturization of thin film solar cells results in negligible spreading resistance and low heating.1,2,13 Thus, high concentrations can be employed without damaging the microcells. Efficiency gains can thus be important and material consumption drastically decreased. In 1941-7012/2013/5(1)/011202/5/$30.00

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this paper, we report the fabrication of Cu(In,Ga)Se2 microcell with Cu(In,Ga)Se2 deposited by three-stage co-evaporation process, yielding an equivalent efficiency of 21.3% 6 0.2% at 475. EXPERIMENTAL Solar cell fabrication

These microcells are based on the standard CIGS stack (soda-lime glass/Mo/CIGS/CdS/ ZnO:i/ZnO:Al). The fabrication process was presented in detailed in previous works,1,2,14 and the main steps are summarized here. A Mo layer (1 lm thick) is sputtered on soda lime glass. The absorber is then deposited by a standard three-stage process15 in our laboratory. The first copper-poor phase lasts 60 min, and the resulting absorber thickness is 2.9 lm. Then a 50 nm thick CdS layer is deposited by chemical bath deposition from a thiourea aqueous solution at 65 C. An intrinsic ZnO layer (50 nm) is deposited by r.f. magnetron sputtering. UV-photolithography is used to protect disk-shaped areas from the deposition of a 400 nm thick SiO2 and 20/300 nm thick Ti/Au bilayer. The photoresist is lifted-off in acetone and 400 nm thick ZnO:Al is deposited by r.f. magnetron sputtering. The microcells are then electrically isolated to enable individual testing. Test of the microcells under concentration

CIGS microcells are first characterized under AM1.5G spectrum using a class AAA solar simulator (Newport). The external quantum efficiency is measured on a homemade spectral response setup. The microcells are then characterized under concentrated illumination with a laser light. Previous studies have shown that laser lights are representative of measurements under real sunlight.16 The lasers used in the present work are a 532 nm, 644 nm, and 1064 nm wavelength lasers. The power incident on the cell is varied by neutral density filters and monitored by a power-meter that measures in-situ a fixed percentage of the incident flux, reflected by a beamsplitter. Due to the observed linearity of short-circuit current density with incident power, the short-circuit current density measured under AM1.5G illumination is taken as the reference photocurrent. For a measurement at X suns, the laser power is adjusted so that the short-circuit current density is X times the reference photocurrent density. No temperature control system is implemented. RESULTS AND DISCUSSION Material properties

The absorbers are characterized before microcells fabrication. Scanning electron microscope observation shows well crystallized Cu(In,Ga)Se2 layers, with grain size in the 1–2 lm range (Figure 1 left). The Mo layer presents a characteristic rice grain-like structure. A secondary ion mass spectroscopy measurement enables to detect the composition gradient in the absorber. A V-shape Ga gradient is observed. The average composition of the absorber, measured by X-Ray fluorescence spectroscopy, gives a Ga/(In þ Ga) ratio of 0.32. External quantum efficiency measurement gives an optical bandgap of 1.16 eV. Photovoltaic performance

The microcells are measured under AM1.5G illumination. The open-circuit voltages for the different microcells vary between 600 and 650 mV for microcells of area larger than 5 10 5 cm2 (Figure 2 left). For cells smaller than 5 105 cm2, the mesa becomes a significant shunt path and the open-circuit voltages decrease. In the absence of shunts, the opencircuit voltage is a function of dark saturation currents according to the two-diodes model:14,17

Voc; theo

2kT J02 þ ln ¼ q

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! J02 2 þ 4J01 Jsc ; 2J01

(1)


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FIG. 1. (Left) SEM image of the bare Cu(In,Ga)Se2 absorber. (Right) SIMS profile of the x ¼ Ga/(In þ Ga) ratio. A Vshape grading is found. The calibration of the elemental signals is done by comparison with X-Ray fluorescence spectroscopy results.

where kT=q is the thermal voltage, J01 and J02 are the saturation current densities related to ideality factor 1 and 2, respectively. We do not observe a size-dependence of J01 and J02 in our samples. The correlation between the measured open-circuit voltages and the open-circuit voltage deduced from the saturation current densities (Eq. (1)) is very good, confirming the low influence of shunts for the largest devices. The coefficient of correlation worsens on the smallest devices, indicating a higher influence of shunts and more uncertainty on the dark current fit. The average efficiency of the microcells is 14.5% 6 1.3%. Indeed due to open-circuit voltage variations, the efficiency of the microcells can vary significantly between micro-devices. This may reflect partly material homogeneity variations. Behavior under concentrated illumination

Under concentrated illumination, the open-circuit voltage increases, according to the theoretical predictions and previously observed behavior.1,17 The sample exhibiting the highest open-circuit voltage in this work is a microcell of diameter 25 lm. It shows a Voc of 905 mV at 4750 (Figure 3). This corresponds to less than 230 mV difference with the Voc of an ideal cell of the same bandgap (1.16 eV) according to the Shockley-Queisser limit. For the best Cu(In,Ga)Se2 solar cells to date, this difference under AM1.5 is around 200 mV.8,18 The open-circuit voltage measurement is independent on the laser wavelength, as shown in Figure 3 (right).

FIG. 2. (Left) Open-circuit voltage under AM1.5G illumination as a function of the microcell area. (Right) Comparison of measured and calculated open-circuit voltages. Dashed lines: guides for the eye.


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FIG. 3. (Left) Open-circuit voltage as a function of short-circuit current density or concentration ratio for a microcell of 25 lm diameter for a 532 nm illumination. The theoretical Shockley-Queisser limit is plotted as a dotted line. (Right) Open-circuit voltage as a function of short-circuit current density or concentration ratio for the record microcell of 50 lm diameter for three illumination wavelengths (532 nm, 644 nm, and 1034 nm).

As a consequence of the improved open-circuit voltage, the efficiency of the microcells increases under concentration. Due to the miniaturization, resistive losses are drastically reduced,2,13 and high current-densities can be measured without detrimental resistive losses. In Figure 4, the efficiency versus concentration for the cell that presents the highest efficiency (50 lm diameter) is plotted. An equivalent efficiency of 21.3% 6 0.2% is recorded at 475, compared to a 16.3% efficiency under AM1.5G. This result is a record for thin film solar cells. If the efficiency is still slightly below the record cell of the National Renewable Energy Laboratory (NREL),10 which started from an AM1.5 efficiency of 17.9%, the optimum concentration ratio at 475 is a breakthrough for this type of solar cells, very close to what is achieved on crystalline III-V solar cells19 and far above previous experiments on polycrystalline thin films (2-20).9–11 An absolute 5% efficiency increase is thus obtained.

FIG. 4. Efficiency as a function of short-circuit current density or concentration ratio for the champion microcell of diameter 50 lm. Measurements under 532 nm laser light. The orange star corresponds to AM1.5G measurement.


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CONCLUSIONS

We have presented the fabrication and characterization of thin film Cu(In,Ga)Se2 microcells. A maximum open-circuit voltage of 905 mV is measured on a 25 lm Cu(In,Ga)Se2 microcell. The champion cell shows a maximum equivalent efficiency of 21.3% 6 0.2% at 475. This is the highest reported value of optimum concentration ratio on a polycrystalline thin film solar cell to date. Our work is focused on the Cu(In,Ga)Se2 technology, but similar results can be expected from CdTe microcells. Miniaturization is a trend that emerges in the concentrated community.20 Contrary to microcells fabricated from crystalline materials,21–23 Cu(In,Ga)Se2 microcells reported here have performances comparable or superior to their macro-counterparts, showing that miniaturization is not counterbalanced by efficiency losses. Micro-concentration thin film photovoltaic systems could appear in the near future as useful tools to meet the needs for high efficiency and low material usage devices in the thin film community. ACKNOWLEDGMENTS

The authors would like to thank Nicolas Pere-Laperne for his help in the Cu(In,Ga)Se2 microcell project. 1

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