IEEE JOURNAL OF PHOTOVOLTAICS

Accelerated Aging and Contact Degradation

of CIGS Solar Cells Thomas Ott, Thomas Walter, Dimitrios Hariskos, Oliver Kiowski, and Raymund Schaffier

Ahstract-The long-term stability of solar cells is a crucial factor for the competitiveness of a technology. In this study, the acceler­ated aging of CIGS solar cells was studied, and the influence of an applied bias during the endurance test on the open-circuit volt­age v;,c and fill factor (FF) was investigated. Time constants for parameter drifts of the open-circuit voltage and the associated ac­tivation energy were determined. The observed parameter drifts will be discussed, and a model will be proposed based on SCAPS simulations, explaining the observed behavior of the electrical char­acteristics of the solar cells. Therefore, cells were dark annealed under dry conditions at two different temperatures and different voltage biases were applied to the cells. Our study revealed that the application of a positive bias, which is similar to light soaking, first leads to an improvement and stabilization of the open-circuit volt­age and FF followed by a slow decrease of these parameters. This long-term decrease can be explained in terms of a back barrier or phototransistor, as simulated with SCAPS. However, applying a positive bias enhances the long-term stability of these devices. The appearance of a back barrier is associated with a time constant exceeding 30 years. Therefore, this degradation mechanism is not critical.

Index Terms-Accelerated aging, activation energy, contact degradation, Cu(In, Ga)Se2 (CIGS).

I. INTRODUCTION

THE importance of renewable energies is rapidly increasing

since the confidence in nuclear power has declined. Terres­

trial photovoltaics (PV) plays an important role in the renewable

energy mixture. A crucial point to all PV technologies is long­

term stability, due to life-time guarantees of at least 20 years,

which can be assessed by accelerated aging [1]. CIGS thin­

film solar cells reached efficiencies over 20% [2], and therefore,

their importance for the future is steadily increasing. Investiga­

tion of their reliability and long-term stability is an important

subject.

Accelerated aging can be addressed by a dark anneal at ele­

vated temperatures. Previous studies have investigated the be-

Manuscript received May 21, 2012; revised September 18, 2012; accepted October 14, 2012. This work was supported in part by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety under Project RECIS.

T. Ott and T. Walter are with the Department of Mechatronics and Medical Engineering, Ulm University of Applied Sciences, 89081 Ulm, Germany (e­mail: th.ott@hs-ulm.de;walter.th@hs-ulm.de).

D. Hariskos and O. Kiowski are with the Department of Photo­voltaics, Stuttgart Centre for Solar Energy and Hydrogen Research Baden­Wlirttemberg, 70565 Stuttgart, Germany (e-mail: oliver.kiowski@zsw-bw.de; Dimitrios.hariskos@zsw-bw.de).

R. Schiiffler is with Manz CIGS Technology GmbH, 74523 Schwabisch Hall, Germany (e-mail: RSchaeffier@manz.com).

Digital Object Identifier 10.1 109/JPHOTOV.2012.2226141

havior of CIGS solar cells for such endurance tests under unbi­

ased conditions in the dark, e.g. , in [3]. Furthermore, activation

energies for parameter drifts of open-circuit voltage and fill

factor (FF) at elevated temperatures were determined and the

mean time to failure (MTTF), well-known from the reliability

of microelectronics circuits, was calculated [4] in order to assess

and extrapolate the lifetime of the devices under normal opera­

tion conditions (NOCT). Results of these life-time predictions

seemed not to be critical for parameters such as open-circuit

voltage and FF. In addition, the influence of an applied positive

or negative bias during the endurance tests was investigated by

several studies, e.g. , see [5]. It was observed that the level of the

open-circuit voltage remains constant after a first fast increase or

decrease affected by an applied positive or negative bias. It could

be also shown that the application of a positive bias corresponds

to the situation of a light soak. Furthermore, these experiments

exhibited that a positive bias enhances and stabilizes parame­

ters such as the open-circuit voltage and the FF. A negative bias

leads to a decrease of these parameters. Capacitance-voltage

(C-V) measurements led to the conclusion that a change of the

net acceptor density is responsible for this behavior. An applied

positive bias increases the capacitance as well as the net doping

concentration, leading to an initial increase of the open-circuit

voltage under the assumption of a dominant recombination in

the space charge region.

In this study, we investigate the long-term behavior of these

parameters by determination of the activation energy. The test

duration was expanded by a factor of 10 compared with previ­

ous experiments. During the endurance test, different positive

and negative biases were applied to the cells. A parameter drift

of the open-circuit voltage and the FF was observed and will be

discussed in this contribution. Based on these parameter drifts,

a life-time prediction with the help of the activation energy was

carried out. Measurements of the current-voltage (I -V) charac­

teristics, after the endurance test, exhibit a behavior known from

low-temperature studies, e.g. , see [6]. We compare these mea­

surement results with SCAPS [7] simulations and will present

a model that explains the observed parameter drifts and charac­

teristics of the solar cells after the accelerated aging.

This contribution is focused on the case of applying a positive

bias to the cell, close to the maximum power point (MPP),

which is relevant for the situation in the field. However, we will

also discuss the general influence of an applied bias during the

accelerated aging.

II. EXPERIMENTAL DETAILS AND RESULTS

In this study, hot dry tests in the dark were performed for

3500 h under different bias conditions. Small highly efficient

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2

0,70

0,68

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0,62

0,60 0

1100 105 C

x +400 m V @ 105 C /:; +400 m V @ 145 C

•••••••• initial value

1100 145 C

3000 6000

Time in hours 9000

Fig. I. Voe versus time of two devices (endurance test at 105°C and 145°C). A positive bias of 400 mY was applied to the cells.

cells were dark annealed in two convection ovens at two differ­ent temperatures (105 °C and 145°C). During the endurance test, a positive bias of 400 m V or a negative bias of -100 m V was applied to the highly efficient cells (size 0.5 cm2). Addi­tional devices were kept under unbiased conditions. At regular intervals, I-V characteristics, in the dark and under illumina­tion at 1 sun, and C-V characteristics were measured at room temperature with an Agilent 4155 C Semiconductor Parameter Analyzer and an HP 4192A LF Impedance Analyzer. It should be noted that the measurement duration of the illuminated I-V characteristics can be considered as long compared with time constants of the "blue metastability." Thus, a "red kink" indicat­ing negative charges in the vicinity of the interface could not be expected and observed during these white light measurements. On the other hand, the intervals between the measurements were long compared with the duration of the measurement. However, it cannot be completely excluded that this white light illumina­tion affected the charge distribution in the heterojunction and, therefore, influenced the dark anneal to a certain degree. The activation energy for the parameter drifts was determined and extrapolated to NOCT. After the endurance test over 3500 h, I-V characteristics were measured at different temperatures between 80 and 330 K, with a Microstat He2 short working dis­tance liquid helium optical cryostat from Oxford Instruments. A Keithley 4200-SCS Semiconductor Characterization System was used for measurements of I-V characteristics at different illumination intensities. Simulations were realized with SCAPS from the University of Gent.

A. Activation Energy

Fig. 1 shows the Vac versus time characteristics of two sam­ples dark annealed under an applied bias of +400 m V. After 120 h, the open-circuit voltage of the device dark annealed at 105°C increased from the initial state (640 mY) to a maximum of 672 m V. The device at 145°C had an initial value of 641 m V and increased in 40 h to a maximum of 672 m V followed by a slow decrease. Using a linear extrapolation of the values, the time constant tlOO before Vac returns to the initial value was determined. This approach is similar to the MTTF in micro-

IEEE JOURNAL OF PHOTOVOLTAICS

1,OE+05

1,OE+04

.r:. .5 1,OE+03 0 :1

1,OE+02

1,OE+01 2 2,2 2,4 2,6

1000/T(1/K)

Fig. 2. Arrhenius plot oftloo (MTTF).

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", --", . - . .... . -....... .... 0,60 iiIIiM-.+.&.. --+�<:>� ..........

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> 0,55 ".....--- �iI :-.i:. ':" . __ -a- - -0-0-: .. c: -•. _ . ..... --g 0,50 .... �-.. unbiased @ 105 C > --A-- +400 mV @ 105 C

0,45

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- 8 - -100 mV@ 105 C ......- unbiased @ 145 C ----,l- - +400 mV @ 145 C � . -100mV@145C

3

0,35 +------.-------.-----.-----, o 1000 2000

Time in h 3000 4000

Fig. 3. Voe versus time for devices that were dark annealed at temperatures of 105°C and 145°C. During the endurance test, the cells were under unbiased conditions, or a positive bias of 400 mY was applied.

electronics, which can be described by Black's equation (1) in case of electromigration. In our investigations, tlOO was taken asMTTF

MTTF=Ae� (1)

where MTTF is the mean time to failure, A is the prefactor, and EA is the activation energy. As shown in Fig. 2 an EA of 568 meV and a prefactor A of 2.2 x 1O�4 h were deduced. Fig. 2 shows the Arrhenius plot of MTTF (tlOO) for the Voc parameter drift. Assuming an operation temperature of 40°C, the equation

4 568 rneV tloo = 2.2 x 1O� h x e----;;r- (2)

leads to a lifetime of about 35 years. Thus, this reliability pa­rameter seems not to be critical for dry conditions.

B. Bias-Affected Parameter Drifts

In this section, the influence of the bias on the open-circuit voltage and FF is discussed. The bias was chosen to be close to the MPP that is relevant for the situation in the field.

1) Open-Circuit Voltage: In Fig. 3, Vac is plotted versus time. In the first 24 h, Vac drifts to a value that depends on the applied voltage. For an applied bias of +400 m V, Vac in­creases by about 20-30 mV with a time constant depending on temperature. However, the maximum Vac does not depend on temperature. After having reached its maximum, Vac de­creases slowly. The dynamics of this process were described in

OT T el al.: ACCELERATED AGING AND CONTACT DEGRADATION OF CIGS SOL AR CELLS

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.... � ... unbiased @ 105 C

--A-- +400 mV @ 105 C

--+- unbiased @ 145 C

---.,.. - +400 mV @ 145 C

0 2000 4000 X intitial state

Time in h value

Fig. 4. FF versus time for devices that were dark annealed at temperatures of 105°C and 145°C. During the endurance test, the cells were under unbiased conditions, or a positive bias of 400 mY was applied.

0,06

0,04

c:r: c: 0,02

0,00

-0,02 0,0 0,2 0,4

,

0,6 0,8

Voltage in V

-- initial state

......... 20h

_ . - 470h

- -870h

_ . . 1700h

-----3600h

1,0

Fig. 5. Evolution of the illuminated I -V characteristics for an endurance test under 400 mY at 145°C.

the preceding section. Under unbiased conditions, a fast initial decrease of Vac is followed by stabilization, relatively indepen­dent of temperature (regarding the stabilized value). In the case of an applied bias of -100 m V, the behavior is similar to the unbiased case; however, the saturation occurs at a lower level. C-V measurements exhibited that the change of Voc is corre­lated with a change of the net doping density. For a positive bias, the net doping density increases, whereas for the unbiased and the -100 m V case, the value decreases.

2) Fill Factor: Fig. 4 shows the FF over time characteristics. The FF shows a similar behavior as the open-circuit voltage. Under an applied bias of +400 mY, during the endurance test, the FF increases initially followed by a decrease predominantly at the highest temperature. Under unbiased conditions, the FF shows a slow continuous decrease. In the following section, a model for this behavior will be proposed.

C. Back-Barrier Model

Fig. 5 shows the evolution of the illuminated I -V character­istics under an applied bias of +400 m V during the endurance test at 145°C. The curve exhibits a kink getting stronger with increasing time. As can be deduced from Fig. 5, a blocking be­havior of the forward currents evolves for long test durations.

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. ....... ·1%

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_ . . 50%

o +---.--- .--- .-_ ........ ..,:..:�. �----� _____ 80% "-"-' i

------------;," - --- ----0,02

0,00 0,50

Voltage in V

- - -100%

1,00

Fig. 6. I-V characteristics for different illumination intensities for a device after 3500 h dark anneal at 145°C.

1 200

1 000

> 800

E c: 600 � initial state " >0

400 ---i\.- +0,4 V @ 145 C

200 --9- - unbiased @ 145 C 200 K to 300K extrapo.

0 0 1 00 200 300 400

Temperature in K

Fig. 7. Open-circuit voltage for temperatures from 80 to 330 K for cells in their initial state and after 3500 h at 145 °C under different bias conditions during accelerated aging.

This blocking behavior also affects the FF and seems to be cor­related with the slow decrease of the Vac, as discussed earlier in Section II-B.

The dependence of the illuminated I-V characteristics on the illumination intensity in the final stage of the endurance test (unbiased case) is illustrated in Fig. 6. Two signatures should be noticed from Fig. 6: The blocking behavior of the forward current depends strongly on the illumination intensity and Vac is rather independent of the illumination intensity. Such a sat­uration of Voc has been observed at low temperatures [6] and associated with the existence of a back diode at the CIGS-Mo interface. A discussion and extension of this model will be given in the following.

Fig. 7 shows the open-circuit voltage at different tempera­tures from 80 to 330 K. Cells in the initial state and after an endurance test of 3500 h at 145 °C under different bias con­ditions (+400 m V and unbiased) during the accelerated aging were measured. For the devices in the initial state, Vac extrap­olates to the bandgap energy at 0 K for temperatures above 200 K. However, the measurements show a flattening of the curves for temperatures below 200 K. Such a saturation of Vac is well known from literature, e.g., see [6]. After the endurance testing, this saturation of Vac already appears at room temper­ature for both bias conditions. An important conclusion of our

4

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w -2

-2,5 0 0,5 1,5

X in Ilm

Fig. 8. Band diagram for a standard CIGS thin-film solar cell with a barrier of 300 me Y at the back contact. The dotted line displays the equilibrium Fermi level.

80,00

60,00

40,00 c:t E 20,00 s:::

0,00

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-40,00 0

--100%

- - - 50%

-----10%

-' - 0%

---------------------- �

---- - -- - - - --

0,2 0,4

Voltage in V 0,6 0,8

Fig. 9. I-V characteristic simulated for a device with a barrier of 300 mY at the back contact for different illumination levels.

observations can be condensed in the statement that the re­sult of the long-term endurance tests coincides with the low­temperature behavior of CIGS-based devices, as reported in the literature [8].

1) Model and Simulations: Fig. 8 shows a band diagram with a barrier of 300 me V at the back contact, as simulated with SCAPS developed by the University of Gent [7]. Such a band diagram can be interpreted in terms analogous to a transistor with an n-ZnO emitter, a p-CIGS base, and a Schottky collector. However, the base is not contacted. Therefore, in the dark, no collector current can flow, as no base current is present explain­ing the blocking behavior in the dark. This situation changes under illumination. Photogenerated holes provide a base cur­rent, bringing the transistor into an active state. Consequently, a "collector" current will flow explaining the experimentally observed crossover of dark and illuminated I-V characteris­tics. This "collector" current consists of injected electrons that are collected in the field of the back barrier. Fig. 9 shows the simulated I-V characteristics for different illumination levels for a cell with a barrier of 300 meV at the back contact. As can be clearly seen from Fig. 9, the signatures, which were described previously for Figs. 5 and 6, are reproduced by this phototransistor model. The dependence of the forward current

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IEEE JOURNAL OF PHOTOVOLTAICS

...... , . ...,. .... .... ., ...... .... ..... , ..... ,,� .. , . .... ";;::: ......... 300 K to 500 K 1 % extrapo.

." ..... : .... 100 200 300 400 500

Temperature in K

Fig. 10. Voe over temperature for different illumination intensities, as simu­lated by SCAPS.

on the illumination and the Vac independence of the illumination correspond to the measurement results after the endurance test. However, some discrepancies between the measurements and the simulations should be noted. The kink in the measurement (see Fig. 6) seems to vanish at higher illumination intensities, whereas in the simulation, the slope increases but the kink still remains visible (see Fig. 9). In the transistor model, this slope depends on the extension of the space charge regions and on the diffusion length (early effect). Therefore, adjustment of these parameters could improve the match of the measured and sim­ulated characteristics.

The proposed phototransistor model is an extension of a pure back diode model and is valid under certain conditions. One of these conditions is a diffusion length in the order of the field-free region between the main junction and the back barrier so that injected electrons can diffuse into the field region of the col­lector. Our proposed phototransistor model can explain the ob­served features after endurance testing such as crossover of the dark and illuminated I -V characteristics, the saturation of Vac with illumination intensity, dependence of the forward current on illumination intensity, and saturation of Vac with decreas­ing temperature. Fig. 10 contains the simulated dependence of Vac on temperature for different illumination intensities. This figure clearly reveals the experimentally observed saturation of Vac with respect to lowering the temperature or increasing the illumination level. A more detailed discussion of the under­lying physics was presented in [9]. The basic parameter that determines the saturated value of Vac is the difference between the barrier height of the principal and back barrier. Vac can­not exceed this value neither by lowering the temperature nor by increasing the illumination level. Such a situation can be considered as a kind of effective f1atband condition where the previously described difference of the barrier height determines the effective flatband condition.

III. DISCUSSION

Appling a bias to the cells during a dark anneal at elevated temperatures influences parameters such as the open-circuit voltage and the FF. A positive bias stabilizes or enhances these parameters, whereas a negative bias leads to a decrease. After

OT T el al.: ACCELERATED AGING AND CONTACT DEGRADATION OF CIGS SOL AR CELLS

long terms, the parameters show a slow decline for the cells with an applied bias of +400 m V, corresponding to the situation in the field. The activation energy was determined, and lifetime predictions were undertaken. However, due to extremely long time constants, this degradation seems to be not critical under dry heat conditions.

I -V characteristics after the endurance test over 3500 h show a different behavior compared with the initial state including a blocking behavior of the forward current, which is increasing with the duration of the accelerated aging. The blocking behav­ior of the forward current strongly depends on the illumination intensity, whereas in contrast, Vac does not depend on the il­lumination intensity. Furthermore, after the endurance test, the saturation of Vac with decreasing temperature already occurs at room temperature. Such a behavior is well known for devices prior to degradation, however, at temperatures below 200 K.

It seems as if the endurance test shifts the I-V characteris­tics with the associated non idealities to room temperature. The observed non idealities, including the blocking behavior of the dark current, the crossover of the dark and illuminated charac­teristics, and, especially, the saturation of Voc with temperature and illumination intensity, can be explained in terms of a photo­transistor model as deduced from SCAPS simulations. The key parameter for the occurrence of these effects is the difference between the barrier heights of the principal junction and the back barrier. Vac cannot exceed this limit neither by lowering the temperature nor by increasing the illumination level. These non idealities occur at room temperature if Voc touches this limit already at temperatures above 300 K. A signature for such a sit­uation is the aforementioned Vac saturation. The saturation with the illumination intensity is expressed in very low diode ideality factors (even lower than unity) for Isc versus Vac measurements in contrast with a high diode ideality factor in the dark due to the blocking behavior.

The origin of the postulated back barrier might be a Schottky diode at the back contact or a very low doped layer at the back contact in combination with a high doping level at the heteroint­erface. Such a charge distribution has been suggested as a result of amphoteric defects and the associated charge states [10]. An­other model that can explain the observed crossover of dark and illuminated I-V characteristics involves negative charges in the vicinity of the heterointerface. These negative charges form a barrier at the interface, leading to a reduced photocurrent col­lection and to a blocking-like behavior of the forward current in the dark. Under illumination, photogenerated holes in the CdS buffer layer can compensate the negative charges, leading to the observed crossover. Thus, this so-called "blue metasta­bility" can explain "red kinks" and crossovers. However, it is not straightforward to account for the observed saturation of Vac with temperature and illumination intensity without assum­ing varying transport mechanisms at different temperatures. In contrast, the assumption of a back barrier can account for the signatures described previously by using a single parameter set for all intensities and the complete temperature range. However, it has to be mentioned that a p+ layer at the interface (which is often postulated in the context of the "blue metastability") also forms a back barrier with an assumed low doped or even

intrinsic CIGS at the back contact. Therefore, these two mod­els are not fundamentally different. In addition, both models explain the crossover by a light-induced barrier reduction as a consequence of the compensation of negative charges by pho­togenerated holes [9].

IV. CONCLUSION

This paper has demonstrated that a positive bias during the accelerated aging enhances Vac and the FF. Time constants for a subsequent decline are fairly long, leading to the conclusion that this degradation is not critical for dry conditions. The ob­served characteristics of the devices after an endurance test at high temperatures could be explained best by a barrier at the back contact of the device. A phototransistor seems to be an ap­propriate model for such behavior. A signature for the existence of a back barrier is the saturation of Vac with temperature and illumination intensity. The measurements and simulations lead to the conclusion that the barrier at the back contact increases with progressing time, leading to a steady decrease of Vac and FF.

REFERENCES

[1] K. Morita, S. Kera, and S. Kiehn, "Performance change from light-soak and annealing effects of thin-film PY modules," in Proc. 26th Eur. Ph% ­voltaic Solar Energy Conf, 2011, pp. 3392-3396.

[2] M. Powalla, D. Hariskos, P. Jackson, F. Kessler, S. Paetel, W. Wischmann, W. Witte, and R. Wurz, "CIGS solar cells with efficiencies> 20% Current status and new developments," in Proc. 26th Eur. Photo voltaic Solar Energy Conf, 2011, pp. 2416-2420.

[3] P. Mack, T. Ott, T. Walter, D. Hariskos, and R. Schiiffler, "Optimization of reliability and metastability of CIGS solar cell parameters," in Proc. 25th Eur. Ph% voltaic Solar Energy Cont, 2010, pp. 3337-3340.

[4] P. Mack, T. Ott, T. Walter, D. Hariskos, and R. Schiiffler, "Parameter drifts and accelerated ageing of CIGS solar cells," presented at the Eur. Mater. Res. Soc. Spring Meet., Strasbourg, France, Jun. 7-11, 2010.

[5] T. Ott, P. Mack, F. Trudel, Y. Schulz, T. Walter, D. Hariskos, O. Kiowski, and R. Schiiffler, "Accelerated ageing and luminescence of CIGS solar cells," in Proc. 26th Eur. Ph% voltaic Solar Energy Cont, 2011, pp. 2421-2424.

[6] T. Eisenbarth, R. Caballero, M. Nichterwitz, C. A. Kaufmann, H. W. Schock, and T. Unold, "Characterization of metastabilities in Cu(ln,Ga) Se2 thin-film solar cells by capacitance and current-voltage spectroscopy," 1. Appl. Phys., vol. 110, pp. 094506-1 094506-l3, 2011.

[7] K. Decock, P. Zabierowski, and M. Burgelman, "Modeling metastabili­ties in chalcopyrite-based thin film solar cells," 1. Appl. Phys., vol. Ill, pp. 043703-1-043703-7, 2012.

[8] T. Eisenbarth, T. Unold, R. Caballero, C. A. Kaufmann, and H. W. Schock, "Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(ln,Ga)Se2 thin film solar cells," 1. Appl. Phys., vol. 107, pp. 034509-1-034509-12, 2010.

[9] T. Ott, T. Walter, and T. Unold, "Phototransistor effects in Cu(In,Ga)Se2 solar cells," presented at the Eur. Mater. Res. Soc. Spring Meet., Stras­bourg, France, May 14-18 2012.

[10] U. Rau, K. Weiner, Q. Nguyen, M. Mamor, G. Hanna, A. Jasenek, and H. W. Schock, "Device analysis of Cu(ln,Ga)Se2 heterojunction solar cells-some open questions," presented at the Mater. Res. Soc. Symp., San Francisco, CA, 200 I.

Thomas Ott received the Diploma degree in mecha­tronics engineering from the Ulm University of Ap­plied Sciences Ulm, Germany.

Since 2009, he has been a Research Assistant with the Ulm University of Applied Sciences in the project RECIS. His research is focused on the long-term sta­bility of CIGS thin-film solar cells.

6

Thomas Walter received the Diploma and Ph.D. de­grees in electrical engineering from Stuttgart Univer­sity, Stuttgart, Germany, in 1990 and 1995, respec­tively. His Ph.D. dissertation focused on the charac­terization of CIGS-based thin-film solar cells.

He then joined the corporate research of Bosch GmbH, Stuttgart, where he was involved in research on microsystem technology and optical communi­cation systems and in the business unit "driver as­sistance systems," where he was responsible for the introduction of SiGe millimeter-wave integrated cir­

cuits into automotive radar sensors. Since 2005, he has been a Professor of microelectronics and microsystem technology with the Ulm University of Ap­plied Sciences, Ulm, Germany. His main research interests include the long-term stability of thin-film solar ceUs, luminescence characterization, and electrical characterization/simulation of CIGS-based devices.

Dimitrios Hariskos studied chemistry with RWTH Aachen University, Aachen, Germany. He received the Ph.D. degree from the University of Stuttgart, Stuttgart, Germany, and the Technical University of Darmstadt, Darmstadt, Germany.

In 1998, he joined the Centre for Solar Energy and Hydrogen Research, Stuttgart, where he is currently involved in research on the upscaling of ClGS thin­film technology from a small-area cell to a full-size commercial module.

IEEE JOURNAL OF PHOTOVOLTAICS

Oliver Kiowski studied chemistry with the Karlsruhe Institute of Technology, Karlsruhe, Germany, and the University of Massachusetts, Amherst. He received the Ph.D. degree for research on carbon nanotubes.

He is currently with the Centre for Solar Energy and Hydrogen Research, Stuttgart, Germany, where he is involved in research on thin-film photovoltaics using optical and electrical metrology.

Raymund Schiiffler received the Diploma degree in electrical engineering from Stuttgart University, Stuttgart, Germany.

From 1989 to 1996, he was with Stuttgart Univer­sity, where he was involved in the field of analytics, transparent conductive oxide, and CIGS. In 1996, he joined ZSW Stuttgart, where his research was fo­cused on module technology and encapsulation. From 1999 to 2011, he was with Wlirth Solar. Since 2012, he has been with Manz ClGS Technology GmbH, Schwabisch Hall, Germany, as a Scientific Associate.

Ql. Author: What does RECIS stand for?

Q2. Author: What does SCAPS stand for?

QUERIES

Q3. Author: Please provide the name of the institution where the author "Oliver Kiowski" received the Ph.D. degree.