Radiation Resistance of Super-Straight Type Amorphous Silicon Germanium Alloy Solar Cells

Shin-ichiro Satol, Tomomi Meguro2, Takashi Suezaki2, Kenji Yamamoto2 and Takeshi Ohshima1

I Japan Atomic Energy Agency (JAEA), Takasaki, Gunma 370-1292, Japan,

2 Kaneka Corporation, Settsu, Osaka 566-0072, Japan

Abstract - Performance degradation of super-straight type amorphous silicon germanium alloy (a-SiGe) solar cells due to proton irradiation are investigated using an in-situ current-voltage (I-V) measurement system. The results show that a-Si solar cells have higher radiation resistance than a-SiGe solar cells. Also, the room temperature annealing effects immediately after irradiation are investigated. It is shown that the recovery of the short-circuit current is especially prominent in all the parameters and is more remarkable as the Ge concentration is lower. The variations of dark 1- V characteristics are also analyzed. Based on the obtained results, we propose a radiation hardened design of amorphous silicon alloy multi-junction solar cells.

Index Terms - amorphous semiconductors, photovoltaic cells, radiation effects, particle beams, radiation hardening

I. INTRODUCTION

Thin film amorphous silicon multi-junction (a-Si MJ) solar cells are one of the major candidates for flexible space solar cells, since they have many distinct advantages: high specific power, high flexibility, ruggedness, and tight rollup feature for stowage [1]. Additionally, they also have the potential for reductions of both cost and stowage volume. Recently, the specific power of 1,200 W/kg has been attained by a-Si alloy triple junction (TJ) solar cells [2]. In response to this, radiation degradation of many types of a-Si solar cells (a-Si single­junction (SJ), a-Si/a-SiGe, a-Si/flc-Si, and a-Si/flc-Si/a-SiGe etc.) and their thermal recovery at post-irradiation have been investigated [3-5]. It has been shown that a-Si solar cells have strong radiation resistance compared to crystalline silicon and III-V compounds solar cells. However, the radiation degradation mechanism of a-Si solar cells has not been completely clarified yet. Also, the radiation effects on amorphous silicon germanium alloy (a-SiGe) solar cells, which are used as the middle or bottom subcell in a-Si TJ solar cells, are less well known than that on a-Si SJ solar cells. In particular, difference between radiation resistance of a-Si solar cells and a-SiGe solar cells, and the effects of germanium (Ge) concentration on radiation degradation remain to be elucidated.

In the previous conferences (PVSC38 and PVSC39) [6,7], Sato et al. reported the degradation behavior of a-Si/a-SiGe/a­SiGe TJ solar cells irradiated with various energy protons, electrons, and heavy ions. These papers clarified the relationship between the performance degradation and the energy deposition process of incident radiation. The radiation degradation mechanism was discussed on the basis of the obtained results. However, the radiation degradation behavior

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of each subcell should be clarified in greater detail in order to improve the radiation resistance of a-Si MJ solar cells. In this paper, we report the proton irradiation effects on a-SiGe SJ solar cells and compare the degradation behaviors. We also report the room temperature recovery from the radiation degradation which has been often observed in a-Si solar cells [8].

II. EXPERIMENTAL

The samples used in this study were four kinds of super­straight type a-SiGe solar cells fabricated at Kaneka Corporation, which are listed in Table I. The sample name was given according to Ge concentration and thickness of i-layer. For example, "a-SiGeHIOO" denotes that the concentration of Ge is High and the thickness of i-layer is 100 nm. Also, "a­Si300" denotes Ge-free a-Si solar cells of which i-layer is 300 nm. These samples were irradiated with 350 keY protons, and the degradation as a function of proton tluence and room temperature recovery immediately after irradiation were investigated.

The experimental setup is illustrated in Fig. 1. Proton irradiation was performed at the Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency (JAEA Takasaki). Current-Voltage (J- V) characteristics under both AMO, 1 sun light illumination and dark conditions were measured in-situ in an irradiation chamber, being represented by light J- V and dark J- V, respectively. As shown in Fig. 1 , the incident direction of protons was opposite to that of AMO light. All the J- V measurement was done 1 minute after irradiation was stopped, and the proton irradiation was resumed after J- V measurement. The irradiation was performed until the short­circuit current was reduced to less than 50%. Also, the annealing (recovery) effect of the cell performance after

TABLE I SAMPLE LIST

Cell Name Ge Concentration Thickness of i-Layer (nm)

a-Si300 None 300

a-SiGeL300 Low 300

a-SiGeM100 Middle 100

a-SiGeH100 High 100

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K-Type

Thermocouple

Proton Beam

to /-VTester

to /-V Tester

.. AMO Light

Glass Substrate

Fig. 1. Schematic drawing of in-situ radiation testing system for super-straight type solar cell.

irradiation was investigated for 60 minutes at room temperature (305 K for light I- Vand 299 K for dark J- V measurements). The cells were kept under dark conditions except when the light J- V measurement was performed. No light induced degradation due to the in-situ light J- V measurement was observed in the irradiation experiments.

III. RESULTS

Figure 2 shows variations of the light J- V characteristics of the cells irradiated with 350 keY protons. Protons with energies of 350 keV pass through the active layer of the cell and provide the radiation damage almost uniformly, according to the SRIM calculation [9]. All the cell parameters; short-circuit current (Isc), open-circuit voltage (Voc), maximum output (Pmax), and fill factor (FF) decreased with increasing proton fluence. In particular, the degradation of FF was more significant than the other parameters.

Figure 3 shows the degradation curves of a-Si300 and a­SiGeL300. All the parameters of a-SiGeL300 decreased with increasing fluence more rapidly than that of a-Si300. This indicates that the radiation resistance of a-Si cells was higher than that of a-SiGe cells. This result corresponds to the results which were reported previously by Sato et al. [ 10]. Figure 4 shows the degradation curves of a-SiGeM I 00 and a-SiGeH 100. The degradation of a-SiGeHI 00 was slightly more significant than that of a-SiGeMI00. This result may indicate that the radiation resistance of a-SiGe cells is lower as the Ge concentration is higher, although the difference between them is small.

When the remaining factor of Isc was reduced to less than 50 %, the proton irradiation was terminated and the performance recovery immediately after irradiation was investigated in the irradiation chamber. Figure 5 shows the performance recovery of a-Si300 and a-SiGeL300 after

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30 a-Si300 a-SiGeL300

20

350 keV Protons

� 10 -- Initial

<r: -- lxlO'2crn-2

5 --lx1013 cm-2 ,.... 0 " OJ t: ;::l u 20

10

00 0.2 0.8 0 0.2 0.4

Voltage (V) 0.8 1.0

Fig. 2. Degradation behavior of light J- V characteristics due to 350 ke V proton irradiation. Lines in black, red, and green represent the data before irradiation, after irradiation of LOx 1012 cm-2 and LOx 1013 cm-2, respectively.

1.0

0.2 -

0.0

��� �\ 350 keV Proton InadlatlOn

Closed a-S1300

Open a-SiGeL300

-.-lsc ___ Voc

-.a.-Pmax -T-FF

I " " o 1012 10"

Fluence (cm-2)

\\ . ......... .. �\.

Fig. 3. Degradation curves of a-Si300 (closed symbols) and a-SiGeL300 (open symbols) irradiated with 350 keV protons. All the values are normalized by the values before irradiation. Square, circle, triangle, and inverted triangle symbols represent the values of Isc, Voc, Pm ax, and FF, respectively. The same is true in Figs. 4 and 5.

1.0

.8 0.8

iil .... OJ) 0.6 c

·S ·8 S 0.4

� 0.2

0.0

-

-

<:>- ���s:8� � � \\§

350 keY PlOton TnadmtlOTI

�\� Closed a-SlGeMIOO

Open a-SlGeHIOO

-.-Tsc -.-Voc '\ . . . . • -.6-Pmax ----...-FF 'fJ>.

I " " o 1012 1013

Fluence (cm-2) Fig. 4. Degradation curves of a-SiGeM 1 00 and a-SiGeH 100 irradiated with 350 keV protons. Closed: middle Ge concentration, open: high Ge concentration.

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irradiation. All the parameters were significantly recovered with time and in particular, the recovery of Isc was the most significant in both cases (+ 12. I % for a-Si300 and + I 0.7 % for a-SiGeL300, 60 minutes later). The similar results were also obtained in the cases of a-SiGeMlOO and a-SiGeHIOO.

Figure 6 shows the variation of dark /-V characteristics of a­SiGeL300 due to 350 keY proton irradiation. The current at around 0.4 V increased up to the tluence of 5.0x 1 012 cm-2 (see arrow A in Fig.6) and after that, the current at around 0.8 V rather decreased (arrow B). When the variation of the dark I-V

characteristics is compared to the degradation of cell performance, the degradation of FF occurs in the former fluence regime (less than 5.0x 1012 cm-2) and the drastic degradation of Isc occurs in the latter tluence regime (above 5.0x 1 012 cm-2). The almost same results were obtained in the other samples (a­Si300, a-SiGeM 1 00, and a-SiGeH 100).

IV. DISCUSSION

A. Recovery after Irradiation

The thermal recovery from radiation degradation occurs in amorphous materials even at room temperature. Thus, the recovery of Isc immediately after irradiation is thought to be due to the increase in carrier lifetime and drift length based on annihilation of radiation defects. Assuming the variation of Isc is proportional to the variation of defect density in active layer, the following equation is obtained [8]:

(1)

where Is(t) denotes the short-circuit current when the time tis elapsed, and /s(O) denotes that immediately after irradiation. r' and fJ are the characteristic time and the stretched exponent. In the case of amorphous materials which have a dispersive transport mechanism, the value of fJ varies between 0 and I. The value of fJ depends on temperature and becomes 0.5 at around 300�330 K [11, 12].

The characteristic times obtained using (1) are shown in Fig. 7. The abscissa shows the four kinds of samples and the Ge concentration of samples gets higher on the right side of graph. The results clearly shows that the characteristic time is higher as the Ge concentration is higher, indicating the recovery rate is higher as the Ge concentration is lower. This is in good agreement with the results obtained from the recovery behavior of a-Si/a-SiGe/a-SiGe TJ solar cells degraded due to proton irradiation [ 10]. The information about characteristic time plays a key role in the establishment of radiation degradation prediction method of amorphous-type solar cells in actual space, since the recovery effect should be taken into account.

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350 keY Proton Irradiation 1.0 ....... ............. ... .. ... ... ........ - ............ .

Closed a-Si300, 1.0x101-1- em-:

: : Open a-SiGeL300, 5.0/1013 cm-2

�=i=�·····i···· i=�=�===�===�===� • VOC .

o 10 20 30 40 50 60

Elapsed Time after In-adiation (min)

Fig. 5. Performance recovery of a-Si300 and a-SiGeL300 immediately after 350 keY proton irradiation at room temperature. The irradiation fluences were l.Ox 1014 cm-z for a-Si300 and 5.0x 1013 cm-2 for a­SiGeL300.

350 keY Proton TlTadiation

10-2 a-SiGeL300

-- Initial

-- 5.0x 1 all cm-2

-- 5.0XlO12 cm-2

--2.0xlO13 cm-2

--S.OxlO13 cm-2

10-10 '--�--'--�--'--�--'--�--'--�--'--�----' -0.2 0.0 0.2 OA 0.6 0.8 1.0

Yoltage (V)

Fig. 6. Dark /- V characteristics variations of a-SiGeL300 due to 350 keY proton irradiation.

2.0,...-------------------, After 350 keY Proton Irradiation

a-Si300 a-SiGeL300 a-SiGeM 1 00 a-SiGeH 1 00

Sample Fig. 7. Characteristic time of Isc recovery at room temperature after irradiation. The error bars represent the standard error.

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B. Analysis of Dark 1-V Characteristics

The dark 1-V characteristics of single junction a-Si solar cell is represented by the following equation [ 13]:

1= 10 [exp (q(Vn�;

Rs)) - 1]

V -IR + R

s + a(V -I Rs)m sh (2)

where 10, Rs, n, and Rsh are the reverse saturation current, the diode ideality factor, the series resistance, and the shunt resistance, respectively. 1 and V denote the actual current and voltage. q, k, and T are the elementary charge, the Boltzmann's constant, and the temperature. a and m are the constant values to represent the electric field and depletion effects in the intrinsic layer, although the last term was less important to variation of dark 1-V characteristics of the samples used in this study. The values of 10, n, and Rsh changed significantly due to 350 keY proton irradiation.

The variation of three parameters are shown in Fig. 8. The similar variations were obtained in both a-Si300 and a­SiGeL300. The values of 10 and n increased with increasing fluence whereas the value of Rsh decreased. At the high fluence regime, however, the reciprocal changes were observed in all the parameters. This fluence regime roughly corresponds to that in which the variation shown as arrow B in Fig. 6 was observed. In order to explain the mechanism of this phenomenon, the effects of radiation defects on these parameters should be investigated by device simulation studies.

V. SUMMARY

In this study, we investigated the degradation behavior of a­SiGe alloy solar cells due to 350 keY proton irradiation, room temperature recovery immediately after irradiation, and the variation of dark 1-V characteristics. Based on the obtained results, we discussed the Ge concentration dependence of radiation resistance and concluded that a-Si solar cells have higher radiation resistance than a-SiGe solar cells. Therefore, when a-Si MJ solar cells (e.g. a-Si/a-SiGe/a-SiGe TJ solar cells) are designed for space use, the top (a-Si) subcell should be thinner and the middle/bottom (a-SiGe) subcell should be thicker than that for terrestrial use. This is because the current matching among subcells should be performed in the condition of end-of-life (EOL).

We also discussed the room temperature annealing of radiation degradation in terms of the characteristic time of recovery, and clarified that the recovery from degradation was more remarkable as the Ge concentration was lower. We suggest that the information about characteristic time of recovery is practically important for the radiation degradation prediction in actual space, since the significant recovery is expected during a long-term mission.

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)

10-11

3.0

2.5

:;: 2.0

1.5

0.8

0.6

0.2

o

350 ke V Proton Irradiation -.- a-Si300 -0- a-SiGeL300

10 Reverse Saturation Current

n Diode Quality Factor Rsh Shunt Resistance

(a) o�o

0"""-0/

0/ 0"""-0- _0-0'---

\ ./ .-.---

. - -

(b)

0--0-

•

/X\ /6/ /0

......-0 / • 0..--0 0--

./ ./

.--.-.--. ..--

(c) ._-.-.-.

---,

�� / 0" _/

0.0

° ____ 0_0

1012 1013

Fluence (cm')

Fig. 8. Variations of (a) reverse satnration current, (b) diode factor, and (c) shunt resistance due to 350 keY proton irradiation. All the values are normalized by the values before irradiation.

ACKNOWLEDGEMENT

Work at Kaneka Corporation was funded by New Energy and Industrial Technology Development (NEDO) of Japan.

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REFERENCES

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[8] S.-i. Sato, H. Sai, T. Ohshima, M. Imaizumi, K. Shimazaki, and K. Kondo, "Temperature Influence on Performance Degradation of Hydrogenated Amorphous Silicon Solar Cells Irradiated with Protons," Prog. Photovolot: Res. Appl., vol. 21, pp. 1499-1506, 2013.

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[II] J. Kakalios, R. A. Street, and W. B. Jackson, "Stretched Exponential Relaxation Arising from Dispersive Diffusion of Hydrogen in Amorphous Ailicon," Phys. Rev. Let., vol. 59 pp. 1037-1040,1987.

[12] A. Scholz and B. Schroder, "Interpretation of the Saturation Behaviour of the Metastable Defect Density Created in Intrinsic a-Si:H by keY-Electron Irradiation," 1. Non-Cryst. Sol., vol. 137&138, pp. 259-262, 1991.

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