TEMPERATURE DEPENDENCE OF Voc IN CdTe AND Cu(lnGa)(SeSb-BASED SOLAR CELLS

Christopher P. Thompson, Steven Hegedus, William Shafarman, and Darshini Desai

Institute of Energy Conversion, University of Delaware, Newark, DE 19716

ABSTRACT

The temperature and intensity dependence of Voe in CdTe and Cu(lnGa)(SeS)2 polycrystalline thin film solar cells was examined. Voe was measured from 100-330K and from 0.1 to 1 sun illumination. Two distinct regimes of temperature dependence are commonly observed: a linear regime at higher temperatures with slope -0.5 to -3 mV/K and a logarithmic intensity dependence; and a saturation regime at lower temperatures, with little intensity or temperature dependence. The T=O K intercept extrapolated from the linear regime around 300K is related to the activation energy of the dominant recombination mechanism and is equal to the absorber bandgap for Shockley-Read-Hall recombination, or in some cases, from heterojunction interface recombination, which is less than the absorber bandgap. In this work, the temperature dependence of Voe will be characterized for CdTe and Cu(lnGa)Se2 devices with differences in composition and processing conditions. Analysis will focus on the activation energy of the recombination mechanism and saturation at lower temperatures which indicates a maximum separation of the quasi Fermi levels as thermally activated SRH recombination is frozen out. The saturation voltage is -1 V for a typical CdTe device (E g=1.45 eV), -1V for low bandgap Cu(lnGa)Se2 (Eg=1.15), and -1.1 V for wider bandgap Cu(lnGa)Se2 (E g=1.38 eV).

INTRODUCTION

Analysis of open circuit voltage (Voe) is a valuable tool for characterization of the recombination mechanisms which limit the performance of solar cells [1). Since there is no current through the device at open circuit, there are no effects of parasitic circuit elements such as non-ohmic contacts or blocking barriers. By characterizing the temperature and illumination dependence, different mechanisms can be distinguished [1-6). From the standard diode equation:

(1)

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and defining the diode current (Jo) as:

J o = J oo ex{ -JtTJ; (2)

we can write an equation for Voe:

The temperature dependence of Voe in CdTe and Cu(lnGa)(SeS)2 solar cells yields the activation energy of the dominant recombination mechanism (EA), from the T=OK intercept, and a measure of the diode ideality factor (A), and saturation current density prefactor (Joo) from the slope. EA, A, and Joo all depend on the recombination !'1echanism responsible for the injected current [1, 3, 5). It IS assumed that A and Joo are constants with respect to temperature and illumination. The linearity of the Voe(T) curve in the fitting region supports this assumption. An A factor greater than 1 and less than 2, and EA=EG are consistent with Shockley-Read-Hall (SRH) recombination dominating in the absorber layer. Voe typically saturates below a certain temperature as the SRH recombination is frozen out, indicating an upper limit to Voe for that particular device. EA is not always equal to EG, if other recombination mechanisms dominate the device, such as ~n interface recombination mechanism [2,4), The Interface recombination mechanism is the energy gap between the fermi level in the CdS and the valence band at the interface [3,4).

In this work, we will show that voltage loss in Cu(lnGa)Se2 devices with low Na is due to recombination at the CdS/Cu(lnGa)Se2 hetero-interface, as well as present an overview of the EA and Voe 8AT of variations of Cu(lnGa)Se2 and CdTe solar cells. The temperature and illumination dependent measurements of Voe for CdTe and C~(lnGa)(SeS)2 devices with different absorber layer properties and processing variations will be presented and results discussed in terms of their Voe-limiting behavior. ~pecifically, concerning Cu(lnGa)(SeS)2 devices, the Influence of composition, sodium, and substrate temp'erature (T 88) on low temperature voltage saturation (Voe 8AT) and EA will be examined; for CdTe devices the influence of Cu, interface quality, and accele;ated degradation ("stress") will be examined.

EXPERIMENTS

Polycrystalline Cu(lnGa)(SeS)2 films were fabricated using elemental co-evaporation from individual elemental sources[7]. Cu(lnGa)(SeS)2 cells had a structure of SL glass/Mo/Cu(lnGa)(SeS)2/CdS/ZnO/ITO/(Ni/AI grid). Cu(lnGa)(SeS)2 devices included a baseline Cu(lnGa)Se2 device with [Ga]/[ln+Ga] = 0.32 and Eg = 1.18 eV, a wide bandgap Cu(lnGa)Se2 device with [Ga]/[ln+Ga] = 0.67 and Eg = 1.4 eV, a CulnS2 device with Eg = 1.53 eV, a low sodium Cu(lnGa)Se2 device with [Ga]/[ln+Ga]=0.28, and Eg=1.16 eV, and a low temperature Cu(lnGa)Se2 device with [Ga]/[ln+Ga]=0.31, EG=1.17eV, with Cu(lnGa)Se~ deposited at Tss=400°C. All other Cu(lnGa)(SeS)2 devices were deposited at T ss=550°C. The low sodium device was deposited on a substrate with a Si02 barrier between the glass and Mo which reduced the Na diffusion by an order of magnitude. The CulnS2 device was deposited with excess Cu which was etched off before the CdS deposition.

Polycrystalline CdTe films were fabricated using a high throughput vapor transport method [8). CdTe cells had a structure of glass/Sn02/CdS/CdTe/Cu/metal or graphite contact unless otherwise noted. CdTe devices included baseline devices (before and after thermal stressing), a Cu-free device, and a Schottky device made without a CdS window layer. The inclusion of a thin Cu contact layer on a CdTe device is important to obtain high Voc as well as device stability [6).

Table 1 contains the performance of the devices studied here under standard test conditions (AM1.5, 100mW/cm 2, 25°C).

For the temperature dependent Voc measurements the devices were mounted and contacted inside of a CTI~ Cryostat and cooled with liquid helium. An ELH lamp was used to illuminate the device. Neutral density filters were used to vary the intensity of the illumination while the temperature was swept from 300K to 100K in 20K steps. A piece of Schott KG-4 heat glass was used to block infr~red illumination to prevent unnecessary heating of the device from the lamp. The relative intensity is indicated on each figure.

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Table 1 Device Performance under simulated AM1.5 at 25°C

Device type Voc J sc FF Eff EG (\/) mA/cm 2 (%) (%) (eV)

Cu(lnGa)Se2 0.670 31.0 74.4 15.4 1.18 Baseline Cu(lnGa)Se2 0.555 26.8 66.9 9.8 1.18 Tss=400 Cu(lnGa)Se2 0.769 21.4 56.8 9.4 1.41 Wide Eo Cu(lnGa)Se2 0.480 32.6 68.3 10.7 1.16 10wNa

CulnS2 0.620 20.8 63.3 8.5 1.53 CdTe 0.817 19.4 65.9 Baseline 10.41

CdTe no CdS 0.183 18.4 38.4 1.29 CdTe no Cu 0.727 17.0 60.5 7.4

1.5

CdTe 0.774 17.8 61.0 Stressed 8.6

RESULTS AND DISCUSSION

The .results ~iII be broken up into sections describing specific behavior, followed by a discussion of the results. Results from baseline devices will be examined first Cu(lnGa)Se2 EG=1.2eV, Cu(lnGa)Se2 EG=1.4eV, and CdTe baseline. We will then examine devices that show signs of interface recombination. The last devices we will examine are a Cu(lnGa)Se2 device deposited at low temperature and a thermally stressed CdTe device.

Baseline Cell Results

Typical behavior for the Voc-T dependence for a baseline Cu(lnGa)Se2 and CdTe device is characterized by two regimes of temperature dependence (see Figs. 1 & 2) .. The fir~t is a high temperature (above -200K) linear regime, which extrapolates to T=O to give EA=EG, and the second is a lower temperature saturation regime. In the linear regime, Voc shows a logarithmic dependence on light intensity as predicted by equation 1. In addition to the high temperature linear region of the Voc - T curve there is a low temperature region, in which the primary recombination mechanism is frozen out. Voc saturates with respect to light intensity and temperature.

The CdTe device saturates with respect to temperature and light intensity around 200K. The Cu(lnGa)Se2 device saturates below 150K. The Cu(lnGa)Se2 device has a VOCSAT =0.95V, EA=1.21eV, and EG=1.18eV. The wide-bandgap Cu(lnGa)Se2 device has VOC~AT=1.15V, ~~=1.42eV, and EG=1.41eV. The CdTe device has Voc =1.0V EA=1.55eV, and EG=1.5eV. (see Fig. 1)

1.6 • Baseline 10%

EA=1.42eV • Baseline 20% 1.4 ...... • Baseline 100%

" .. • Wide-Bandgap 1 0% "ot~. • Wide-Bandgap 20% ... : ....

1.2 ... : .. • Wide-Bandgap1000 ... ..

~

~ 0.8

0.6

0.4 ~---:~~~-~-...J.--...J...-...J o 50 100 150 200 250 300

T(K) Fig 1. Baseline Cu(lnGa)Se2 EG=1.22eV, and wide­bandgap Cu(lnGa)Se2, EG=1.41 eV, measured at 3 intensities.

1.6 r---~--r--"""T'""---r---"----'

1.4

1.2

i 0.8

0.6

0.4

0.2

0 0

• 100% • 60% • 40% x 6%

50 100 150 200 250 T(K)

Fig. 2. Baseline CdTe EG=1.45eV measured intensities.

Interface Recombination

300

at 4

For a CdTe device without a CdS window layer, Voe(T) extrapolated yields an EA=0.56eV, well below the bandgap of 1.5eV (see Fig. 3). Voe(T) was onl~ measured down to 220K, so no information on Voe SA could be obtained. The room temperature Voe was substantially lower than a baseline CdTe device with a CdS layer, 0.18V compared to 0.82V (see Table 1.)

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1.6

1.4 0 100% • 48%

1.2 • 24% • 6%

1

> EA=0.55eV - 0.8 ()

~ 0.6

0.4

0.2

50 1 00 150 200 250 300

T(K) Fig 3. CdTe with no CdS EG=1.45 measured at 4 intensities.

A CdTe device without CdS has a Schottky barrier established at the Sn02/CdTe interface. The Sn02/CdTe junction exhibits a higher Jo compared to a baseline CdTe device, with a nominal change in the A factor. Since the bulk of the device is unchanged, we can assert that the interface is the source of the increased recombination. This interface recombination results in a lower EA [2,3).

1.6 r----r---r--"""T'""---r----r----,

1.4

1.2

1

0.8

0.6

--e-10%, EA=0.81eV ..... - 20%, EA=0.83eV

--.--100%, EA=0.86eV

0.4 , o 50 100 150 200 250 300

T(K) Fig. 4. Low Na Cu(lnGa)Se2 EG=1.2eV compare to Fig. 1 measured at 3 intensities. EA is dependent upon intensity in this case.

The low Na device has EA=0.83eV and EG=1.18eV, a lower Voe at baseline temperature, 0.48V compared to 0.67V, (see Table 2 & Fig. 4) and A=1.4. Similarly, a Na

free device, grown on a titanium foil substrate, showed a EA=0.9V.

It is reasonable to conclude that these devices are limited by interface recombination, rather than SRH recombination in the absorber layer. Also of interest is the fact that there is no appreciable difference between decreasing the amount of Na in the device by an order of magnitude, and completely removing the Na from the device. There is evidence to suggest that Na at the surface of Cu(InGa)Se2 grains is responsible for the decrease in device performance [9]

CulnS2 devices also had EA<EG, with EA=1.1eV, and EG=1.5eV. Voe(T) curve did not saturate, with respect to light intensity, or temperature, but rather changed slope.

It has been suggested that the Cu-poor surface layer in Cu(InGa)Se2 devices is responsible for preventing recombination by forming a barrier to hole transport. The stoichiometric composition of the CulnS2 surface eliminates this barrier and therefore recombination at the CdS/CulnS2 interface controls the device [2].

Increased Bulk Recombination/Defect Density

Cu(InGa)Se2 films deposited at low temperature, Tss=400°C, have decreased room temperature open circuit voltage. A low temperature device with EG=1.18eV has Voe(300k)=0.54V, and EA=1.18eV. The device saturates with respect to intensity and temperature at Voe SAT =0.76V.

Lowering the T ss during deposition of Cu(InGa)Se2 films has an effect on the electrical properties of the films reducing open circuit voltage [10], and increasing mid-gap defects [11]. Low temperature films (T ss=400°C) have similar voltage losses to films grown with low or no Na content, but identical composition otherwise. Comparing Voe(T) of a baseline Cu(InGa)Se2 device, and one grown at low temperature, but with the same composition, we can see that in both cases EA=EG. The Voe(T) curve, (see Fig. 5) of the low temperature device has a higher slope than the baseline curve. An increase in defect states decreases Voe at room temperature, but the device is still dominated by bulk SRH recombination, in contrast to the low/no Na case [9].

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~ u ~

1.6

1.4

1.2 .... 1

0.8

0.6

o 10% A 20% • 100%

101.1.1.. EA=1.17eV ... .... ':..:. .. "'",- . "",,- .. '" ... "", ..

'''~~ ... • • • , \'l.:'.

, ·'''~i:·. '" .~ . . ,. ...

.~~ .. .. ~ .... ·t.·

OA ~ o 50 1 00 150 200 2 300

T(K) Fig. 5. Cu(InGa)Se2 Tss=400°C, EG=1.18eV.

Thermally stressed CdTe devices have a EA=1.45eV, and Voe SAT =0.95V. The curve is similar to the baseline results, but shifted down by 50mV.

Low Measurement Temperature-Freezeout Regime

The low temperature regime in the baseline Cu(InGa)Se2 and CdTe cases (see Fig. 1 & Fig. 2) show a saturation voltage Voe SAT independent of intensity and temperature. We found Voe SAT = 1V and EA=1.5 eV for the baseline CdTe device (Eg=1.45 eV). These were not significantly changed by thermal stressing despite a decrease in Voe(300k) of 0.15V [6]. Baseline Cu(InGa)Se2 with Eg = 1.2 eV had Voe SAT = 0.95 Vs't!rhile wider bandgap Cu(InGa)Se2 (E g = 1.4 eV) had V9;9., = 1.15 V. In the Na-free Cu(InGa)Se2 device, Voe T=700mV. The low temperature Cu(InGa)Se2 device had a VoeSAT=760~V, which is significantly lower than that of baseline Cu(InGa)Se2 devices.

Voe SAT is attributed to the freezing out of the recombination mechanism that dominates in the higher temperature linear regime. Voe SAT indicates a maximum splitting of the quasi-fermi levels and the limit of Voe for that device and is shown in Table 2 for devices studied here.

It has been suggested the lower carrier concentration of the un-doped (Cu free) CdTe caused Voe to freeze out earlier [[6]. However, in Cu(InGa)Se2 devices, the low Na device experiences freeze out at the lowest temperature of all the devices (100K). Reduction of Na in a baseline Cu(InGa)Se2 device causes the average carrier density to drop an order of magnitude [9].

The low Na case does not see a saturation of the intensity dependence. Low temperature (T ss=400)

Cu(lnGa)Se2 saturates earlier than wide bandgap and baseline devices, but has similar free carrier concentrations. Relative saturation of voltage to band~p, qVoeSAT/EG, can be used as a metric to compare Voe s in devices with varying bandgaps.

Table 2. Voe at -300K, absorber bandgap, Activation E dV St f nergles an oe a ura Ion

Device type Voe Eg EA Voe"a qv:T

(V) (eV) (eV) (V) EG Cu(lnGa)Se2 0.670 1.18 1.21 0.95 80%

Baseline Cu(lnGa)Se2 0.540 1.18 1.18 0.76 64%

Tss=400 Cu(lnGa)Se2 0.769 1.41 1.42 1.15 81%

Wide Eo Cu(lnGa)Se2 0.480 1.16 0.83 740 63%

10wNa CulnS2 0.620 1.53 1.10 0.88 57% CdTe 0.817 1.55 1.0 68%

Baseline CdTe no 0.183 0.56 ~ ~ CdS 1.5

CdTe no Cu 0.727 1.34 0.85 58% CdTe 0.774 1.45 0.95 65%

Stressed

1.6 ,--...-----,-----,----,--,-------,

5> -

1.4

1.2

0.8

0.6

• Baseline • LowTemp • NoNa .... " ..

" ... ".: ...... .. ....... ".. " .. . . .... ~

". .. ... """ -.. _.. ... •• e.... . .... .... . .... '. .,-k..... • t... ' ..

.......... . ... ..... .. ........ : .•. ".-:

0.4 '--_--L-_--'-_---'- __ -'--_-'-_---l

o 50 100 150 200 250 300 T(K)

Fig. 6. Comparison of variously processed Cu(lnGa)(SeS)2 devices, EG=1.2eV 100% light intensity.

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5> -

1.6 ,--...-----,-----,----,--,-------,

1.4

1.2

1

0.8

0.6

0.4

0.2

..

..... :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:::::::: .. ......... e=.~ ... ••• •• · ... t~·:t.

... ·:..·.&.2I.o~"t .... ...",,;

' •• r--------,

• Baseline • no CdS ... NoCu • Stressed

........ ..... ....... O~~==~==~---L--~~ o 50 100 150 200 250 300

T(K) Fig. 7. Comparison of variously processed CdTe device, 100% light intensity.

CONCLUSIONS

Two regimes in the temperature dependence of Voe are characterized to determine the recombination mechanisms and limits of the devices. Baseline Cu(lnGa)(SeS)2 and CdTe devices are dominated by SRH recombination at high temperatures (T>250K) characterized by EA=EG. Some devices with lower Voe at room temperature have EA < Eg, indicating that the voltage is reduced by a different mechanism such as interface recombination. Notably, this includes Na-free Cu(lnGa)Se2, CulnS2, and a CdTe Schottky cell with no CdS layer. At lower temperatures, the SRH recombination mechanism is frozen out, and a recombination with a negligible temperature and intensity dependence becomes dominant, indicating that the quasi-fermi levels have become pinned by a bulk or interface defect. In general, lower device efficiency correlates with the relative saturation voltage. Cu(lnGa)(SeS)2 devices have a higher relative saturation compared to CdTe devices, i.e . Voe is a higher percentage of Eg as shown in the last column of Table 2.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the technical support of Josh Cadoret, Kevin Hart, Jim Wilson, Shiro Nishiwaki, John Allen, and the financial support of NREL through the Thin Film Partnership ADJ-1-30630-12.

REFERENCES

[1] S. S. Hegedus and W. N. Shafarman, "Thin-Film Solar Cells: Device Measurements and Analysis," Prog. Photovolt: Res. Appl. 12, pp.155-176 (2004).

[2] M. Turcu, O. Pakama, and U. Rau, "Interdependence of absorber composition and recombination mechanism in Cu(ln,Ga)(Se,S)2 heterojunction solar cells," Appl. Phys. 80, p. 2598 (2002).

[3] U. Rau, H.w. Shock, "Electronic properties of Cu(ln,Ga)Se2 heterojunction solar cells-recent achievements, current understanding, and future challenges," Appl. Phys. 69, 131 (1999).

[4] V. Nadenau, U. Rau, A Jasenek, and H.w. Shock, "Electronic properties of CuGaSe2-based heterojunction solar cells. Part I. Transport analysis," Appl. Phys. 87, 584 (2000).

[5] J.E. Phillips, et ai, "Polycrystalline Heterojunction Solar Cells: A Device Perspective," Phys. Stat. Sol 194, 31 (1996).

[6] S. S. Hegedus, B. E. McCandless, "CdTe contacts for CdTe/CdS solar cells: effect of Cu thickness, surface preparation and recontacting on device performance and stability," Solar Energy Materials & Solar Cells 88, (2005) pp. 75-95.

[7] W.N. Shafarman, R. Klenk, and B.E. McCandless, "Device and material characterization of Cu(inGa)Se2 solar cells with increasing band gap," J. Appl. Phys, 79 (9), 1996

[8] B. E. McCandless, W. A. Buchanan, and R. W. Birkmire, "High Throughput Processing of CdTe/CdS Solar Cells," 31 st IEEE PVSC, p. 295-298 (2005)

[9] P.T. Erslev, A. Halverson, W. Shafarman, and J.D. Cohen, "Study of the Electronic Properties of Matched Na-Containing and Reduced-Na CuinGaSe2 Samples Using Junction Capacitance Methods," Mater. Res. Soc. Symp. Proc. 1012, pp. 445-450 (2007).

[10] W.N. Shafarman, J. Zhu, "Effect of substrate temperature and deposition profile on evaporated Cu(lnGa)Se2 films and devices," Thin Solid Films 361-362, 2000, pp. 473-477.

[11] J.T. Heath, J.D. Cohen, and W.N. Shafarman, "Correlation Between Deep Defect States and Device Parameters In Culn1-xGa xSe2 Photovoltaic Devices," 2002, 29 th IEEE PVSC, pp. 596-599.

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