Light-Bias-Dependent External Quantum Efficiency of Kesterite

Article

Light bias-dependent External Quantum Efficiency of Kesterite Cu2ZnSnS4 Solar Cells Fangyang Liu, Chang Yan, Kaiwen Sun, Fangzhou Zhou, Xiaojing Hao, and Martin A. Green ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Light bias-dependent External Quantum Efficiency of Kesterite Cu2ZnSnS4 Solar Cells Fangyang Liu, Chang Yan, Kaiwen Sun, Fangzhou Zhou, Xiaojing Hao* and Martin A. Green School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia E-mail: [email protected].

ABSTRACT

The light bias-dependent behavior of external quantum efficiency (EQE) in kesterite Cu2ZnSnS4 solar cells utilizing different buffers has been reported. Under red light bias, the blue EQE can be enhanced significantly exceeding unity in magnitude due to photoconductivity of buffers increasing depletion region width and thereby increasing collection efficiency. Under blue light bias, the red EQE increases only in devices with a hybrid buffer. It stays constant with CdS buffer due to saturated photoconductivity and even decreases with In2S3 buffer due to optical injection of blue photons through the higher transparent In2S3 into the absorber. Under white illumination, the enhancement can be observed only with unsaturated buffers such as In2S3 and the hybrid buffer, while the red EQE reduces due to optical injection. This light bias dependent behavior in EQE (including EQE exceeding unity) can be attributed to two key factors: photoconductivity of the buffer layers combined with low minority carrier lifetime of absorber.

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The effect leads to disagreement between Jsc measured under a simulator that calculated from the integral of the EQE, revealing the necessity for light bias dependence investigation when verifying the consistency between them. Improving the minority carrier lifetime in absorber and majority carrier concentration in buffer to boost the device electrical property and performance is suggested based on this investigation.

KEYWORDS: kesterite, Cu2ZnSnS4, light bias, external quantum efficiency, photoconductivity

Kesterite Cu2ZnSnS4 (CZTS) semiconductor has attracted worldwide attention due to its excellent optical and electronic properties comparable to traditional Cu(In, Ga)Se2 (CIGS) and CdTe materials for thin film solar cells while consisting of only earth-abundant and non-toxic constituent elements. Good progress has been made over the past few years and the highest power conversion efficiency (PCE) of 9.1% for CZTS

1

and 12.6% for Se-incorporated CZTS

(CZTSSe) 2 solar cells have been achieved showing substantial commercial promise. Since this efficiency is still below half of its theoretical limit (28-30%), work to understand the fundamentals to improve the efficiency of CZTS based solar cells is extremely imperative. Currently, the improvement of the electrical properties of devices without straying from low-cost techniques that are amenable to industry is believed to be the biggest challenge faced by CZTS based cells. In the last several years, intensive research toward absorber quality improvement 3-4, defect passivation

5-6

, interface optimization

7-9

, hetero-junction designing

10

etc. have been

carried out in order to improve the electrical properties of CZTS devices. In this paper, we will focus on using quantum-efficiency (QE) measurements to reveal some aspects of the device


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physics in CZTS solar cells, with the aim of facilitating the improvement of the electrical properties and efficiencies. The quantum efficiency of a solar cell gives valuable information about the spectral composition of the cells current and is widely used to deduce the loss mechanisms responsible for reducing the measured JSC from the maximum achievable photocurrent. Conventionally, it is measured at zero electrical bias and under illumination bias to keep the cell at a reasonable injection level11-13. External QE (EQE) is the parameter more commonly measured, which can be affected by factors ‘external’ to the diode, such as top surface reflections and rear metal absorption. The photocurrent can be evaluated by integrating the measured EQE over the whole illumination spectrum, typically the AM1.5 spectrum for terrestrial solar cells. Thus, it is usually a good verification for performance measurements when the JSC obtained from J-V test is consistent with the that calculated from the integration of EQE with the AM1.5 spectrum14. Good agreement between the integrated EQE and the JSC will typically be found in well-behaved devices such as crystalline Si solar cells. However, dramatic differences can also occur in some solar cells where QEs measured with applied voltage bias and different injection levels can vary significantly. For kesterite solar cells, Wan-Ching Hsu et al.

15

have reported a blue-photon

sensitive non-linear effect in reverse voltage-biased EQE in Cu2ZnSnSe4 (CZTSe) solar cells. Reverse voltage-biased EQE also has often been used to identify the carrier collection loss mechanisms.16-18 In this work, the EQE performance of CZTS solar cells with various buffer layers under a variety of light bias conditions is investigated. Both the light bias dependent behavior in EQE and EQE exceeding unity in CZTS solar cells have been observed. The reasons for these features are explored and some guidance on how to improve electrical properties and efficiencies of CZTS solar cells are proposed.


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Experimental Section A typical CZTS solar cell configuration Mo/CZTS/buffer/i-ZnO/ITO/Ag as shown in Figure S1 was used in this study. CZTS absorbers were prepared by sulfurization of co-sputtered Cu/ZnS/SnS precursors by a magnetron sputtering system (AJA International, Inc., model ATC2200) on Mo-coated soda lime glass substrates. The sulfurization process was performed at a temperature of 560 oC in a sulfur containing atmosphere. Buffer layers of CdS with thickness of 60 nm and In2S3 with thickness of 80 nm were prepared by chemical bath deposition (CBD) method. Specifically, cadmium sulfate (3CdSO4 8H2O), excess thiourea (H2NCSNH2) and ammonium hydroxide were mixed with a base environment for CdS deposition, with details shown elsewhere.19 In terms of In2S3 deposition, the solution system consists of thioacetamide (CH3CSNH2), indium chlorite (InCl3), and ethylic acid in an acid environment.20 Hybrid buffer was prepared by depositing In2S3 and CdS successively. The intrinsic ZnO (i-ZnO) and ITO films were deposited by RF sputtering. Ag grids were used as the top contact. The total area of the final cells is 0.3~0.4 cm2 defined by mechanical scribing. The J-V curves of CZTS solar cells with different buffers were measured using a solar simulator (Darkstar) calibrated with a standard Si reference cell. Capacitance spectroscopy was carried out using an impedance analyzer at a frequency of 100 kHz with a DC voltage bias sweeping from -1.5 to 0.5 V. External quantum efficiency (EQE) data were collected by a QEX10 spectral response system (PV measurements, Inc.) calibrated by the National Institute of Standards and Technology (NIST)-certified reference Si and Ge photodiodes. This system uses monochromatic light chopped at a frequency of 120 Hz unless otherwise specified and equipped with a DC white light bias with intensity variables from 0 to 100 mW/cm2. Various optical filters were employed to get the required spectra of the light bias. “Blue” light bias was obtained with a


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filter passing wavelengths below 600 nm and “red” light bias was obtained with a filter passing wavelengths above 630 nm. The intensity of the blue and red light bias is roughly the same. The terms of blue and red in this paper are used broadly to denote the short or long wavelength portion of the spectrum, respectively.

Results and Discussion Before EQE investigation, device characteristics from J-V and C-V tests (Figure 1) for CZTS solar cells with different buffer layers are presented in Table 1 to ensure the effectiveness of the devices. Compared to the device with the commonly used CdS buffer (Eg of ~ 2.4 eV device with the In2S3 buffer (Eg of ~ 2.1 eV

21

19

), the

) yields higher Voc, but much lower Jsc and FF;

while the device with theIn2S3/CdS hybrid buffer shows both higher Voc and Jsc. Among these three different types of buffer, the CZTS device with hybrid buffer yields the highest efficiency of 7.24%. The depletion region width Wd in the dark at short-circuit, is about 93 nm, 61 nm and 81 nm for CdS, In2S3 and In2S3/CdS hybrid buffer, respectively as deduced from C-V measurements. The detailed explanation from materials, interface and electrical characterization will be reported elsewhere. Here we mainly investigate the features of these devices from the perspective of EQE measurements.


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Figure 1. (a) Light J-V and (b) C-V characteristics of CZTS devices with various buffer layers.

Table 1. Device characteristics of the CZTS solar cells with various buffer layers. Voc

Jsc

FF

Efficiency

Wd

(mV)

(mA/cm2)

(%)

(%)

(nm)

CdS

659.9

16.2

60.2

6.44

93.3

In2S3

714.9

12.4

35.3

3.13

61.1

In2S3/CdS

737.9

17.6

55.5

7.24

81.4

Buffer


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Figure 2(a) shows the EQE results under various illumination conditions for the CZTS device with the CdS buffer. It is obvious that the red light bias greatly increases the blue EQE with values exceeding unity at λ < 520 nm. This effect is believed from CdS photoconductivity. The n-type CdS is known highly compensated with comparable densities of shallow donor and deep acceptor levels present 11, 22. Under illumination containing photons with energies above the CdS band gap energy, for example blue light, some of these light-generated holes are trapped in acceptor states, leaving behind light-generated electrons and effectively increasing the n-type character of the CdS layer. This is the source of the CdS photoconductivity and can be regarded as “photon-doping” of the CdS 23. For instance, from the Hall measurement of CdS by CBD on non-conductive glass substrate, the resistivity decreases one order of magnitude from 105 Ω cm under dark to 104 Ω cm under illumination, mainly from the one order of magnitude increase of carrier concentration from ~1013 cm-3 under dark to ~1014 cm-3 level under white light illumination (about 1020 photons/cm2/secend). The higher effective doping density in the CdS widens the depletion region in the CZTS absorber in order to balance the increased positive space charge in CdS layer, maintaining space charge neutrality, thereby improving the collection of carriers generated deeper into CZTS by red light. Besides, the “photon-doping” effect by blue light actually increases effective donor concentration (ND) of CdS and decreases its work function (the increased electron concentration leads to higher Femi level). This will cause more band bending of CZTS (as shown from the schematic conduction band diagram in Figure S2) and increase the fraction of "built-in" electric field (Vbi) falling across CZTS thereby facilitating the collection of electrons which have been generated in the absorber layer by incident light. Consequently, the modulation of CdS conductivity and CZTS space charge width and collection efficiency (and/or junction field) by AC chopped blue probing beam is responsible for the


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increase in blue EQE with red light bias. During EQE measurement using red light bias, CdS conductivity is low when the chopped blue beam is off, while both CdS conductivity and CZTS depletion width increase when the chopped blue beam is on. This enables enhanced collection of light bias generated carriers from the CZTS in synchronization with the chopped blue light. Thus, carriers from the red light bias exit the device in phase with those from the chopped blue light, resulting in the current to be a summation of a standard small ‘‘blue’’ current (generated by the chopped blue probing beam) plus an increase in the large ‘‘red light bias’’ current (generated by the red light bias and modulated by the chopped blue probing beam with the same frequency). The additional red current can be of comparable magnitude, or much greater than the blue current because the intensity of the probing beam (monochromatic light) is much weaker than that of the light bias. Therefore, even though the change in the CdS photoconductivty by the blue probing beam is very small leading to a slight increase in the depletion width and collection efficiency, blue EQE can be significantly enhanced due to the small modulation of the large DC red light bias current.


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Figure 2. EQE of CZTS device with (a) CdS buffer with various illumination bias conditions; (b) CdS buffer without light bias or with red light bias at various voltage bias; (c) In2S3 buffer under various light bias; (d) In2S3/CdS hybrid buffer with various light bias.

On the other hand, the red light bias suppresses red EQE i.e. EQE is higher in the dark than in the light in the wavelength range of 550 -800 nm. This is because CZTS becomes less p-type under red light bias due to the population of photo-generated electrons in the conduction band, decreasing the effective work function of this region and leading to worse collection efficiency. This is also so-called optical injection, similar to minority carrier electrical injection by forward voltage bias, shrinking the depletion region width.


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Under blue light bias, there is no obvious difference in EQE compared with in the dark. According to the discussion above, the blue light bias should enhance red EQE due to the more heavily n-doped CdS (photoconductive nature) widening the depletion width in the CZTS absorber, which enables enhanced collection from red photons absorbed deep in the CZTS. This effect has been reported in early CdS/Cu2S24 or CdS/CuInSe2

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devices where the CdS was

several times thicker than CdS adopted in present CdS/CZTS devices producing a much greater change in field or depletion width in the p-type absorber. However, in present CdS/CZTS device with a CdS layer as thin as about only 60 nm thickness, the depletion width in CZTS layer would widen by a very small amount, and the collection efficiency for red photons from the probing beam absorbed in the CZTS layer, i.e. red EQE, would be enhanced very slightly or even undetectably. And the ‘‘blue light bias’’ current is not added up with the red EQE from the red probing beam because the chopped red probing beam with photon energy smaller than the band gap of CdS cannot modulate the conductivity of CdS and depletion region width. Besides, there is no blue EQE enhancement under blue light bias. This is because the blue probing beam is of much lower intensity than the blue light bias, and only marginally modulate the CdS conductivity, i.e. the light bias effect on CdS conductivity have saturated. In this case, the effects described for the aforementioned red light bias are negligible. Under white light bias, no difference in blue EQE was observed from in the dark, except that in the wavelength range of 500 nm to 850 nm is lower than that in the dark. White light bias containing both blue and red photons shows a similar effect on blue EQE to the blue light bias. This is because the blue probing beam is of much lower intensity than the blue light component of white illumination preventing the photon from causing a measurable EQE increase in CdS response region although additional red photons are present in the white bias light. However, red


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component of the white light bias again shrinks the depletion (decreases from 93 nm to about 60 nm deduced from CV measurement under white light bias) by increasing the bulk carrier concentration. It should be noted that it is impossible to quantitatively determine the change of carrier concentration of CdS from Hall measurements by blue probing beam or bias light because of the semi-conductive CZTS substrate for CBD CdS layer and interaction/intermixing between them 26. Since optical injection shrinks the depletion width and thereby deteriorates the collection efficiency, reverse voltage bias should recoup the loss in collection efficiency to some content while the applied forward voltage bias should further worsen the collection efficiency. Figure 2(b) shows the EQE of CZTS device with CdS buffer under red light bias at various voltage bias conditions. Compared to the EQE in the dark, under red light bias, the enhancement in blue EQE and reduction in red EQE is observed again. The difference in the height of the blue EQE with that in Figure 1(a) is because of the weaker intensity (only 100 mW/cm2, lower than that of 300 mW/cm2 in Figure 1(a)) of the white light before filter. Under the same light bias conditions, the red EQE increases clearly with applied -0.5 V reverse voltage bias, higher than that with zero voltage bias. This is because the enhanced depletion width in CZTS under reverse voltage bias improves the collection efficiency of red photons, which also implies that the collection efficiencies are most likely limited by very low carrier lifetime (only ~ 10 ns according to our previous report 26, obviously shorter than that of CIGS with hundreds of ns

27-28

). However, this

red EQE is still lower than that in the dark, suggesting that the increase in collection efficiency from wider depletion region width under reverse voltage bias of -0.5V does not compensate for the loss caused by the red light bias. Besides, it is observed that the blue EQE with reversed voltage bias is lower than that with the zero or forward voltage bias, which should be due to the


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less effective change in depletion region width caused by blue probing beam under reverse voltage bias. At the same light bias conditions, the EQE decreases across the whole spectrum range with applied +0.5 V forward voltage bias compared with zero voltage bias. This is because the depletion width in CZTS shrinks significantly with forward voltage bias thereby reducing the collection efficiency of red photons, although the depletion region width change caused by the blue probing beam becomes more effective, causing greater current relative increases compared to zero bias voltage. From the above discussion, one finds that the photoconductive buffer layer and low bulk minority carrier lifetime are two key factors determining the light bias-dependent behavior of the EQE. In order to further reveal the effects of voltage bias on collection efficiency, the EQE tests were performed in the dark at forward, zero and reverse voltage bias, as shown in Figure S3(a). Compared to EQE with zero voltage bias, the EQE is much lower with +0.5 V forward voltage bias in the whole spectrum range, and higher with -0.5 V forward voltage bias, especially in red region. From the EQE ratio at reverse bias (-0.5 V) and zero bias in Figure S3(b), it is indicated that a larger depletion width via applying reverse bias improves the collection efficiency. This observation reveals low minority carrier diffusion length from the low lifetime and mobility of the absorber. 17 For an In2S3 buffer, under red light bias, the enhancement in blue EQE and reduction in red EQE is similar to the case of CdS buffer, as shown in Figure 2(c). But the reduction in red EQE in the case of In2S3 buffer is much more significant than the CdS buffer one, revealing much worse collection efficiency under red light bias compared to the CdS buffer. As the depletion region for CZTS device with the In2S3 buffer (61.1 nm) is much narrower than CdS buffer (93.3 nm at zero bias from the CV measurements), the collection efficiency is easier to be


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suppressed by light bias and/or forward voltage bias considering the low minority carriers lifetime of CZTS. Under white light bias, it is notable that the blue EQE is higher than in the dark but lower than in red light bias. This feature is obviously different from the devices with CdS buffer. The higher blue EQE under white light bias than that under dark indicates In2S3 conductivity still can be modulated under white light bias and the current from light bias is collected, which suggests nonsaturated effects of the light bias on the conductivity of In2S3 with larger thickness than CdS. However, the increasing in the blue EQE under white light bias is much less than under red light bias, which is because the depletion region width can only be adjusted in a smaller range by blue probing beam via slight change in In2S3 conductivity, collecting less current generated by red photons in the absorber layer than under red bias light without blue photons. The red EQE under white light bias is somewhat higher than under red light bias indicating that the increase in In2S3 conductivity by blue photons in white illumination retrieves a small part of loss from the shrinkage of depletion region width by red photons. For the CZTS device with In2S3 buffer, under blue light bias, the red EQE is relatively lower while the blue EQE is somewhat higher compared to the EQE in the dark. This is also different from the CZTS device with CdS buffer whose EQE is almost unchanged from that in the dark. From the optical transmission spectra (Figure S4) of In2S3 and CdS deposited on glass substrates by the same deposition conditions as for CZTS substrates, the transmittance of In2S3 in the wavelength range between 450 nm and 630 nm is much higher than that of CdS, indicating a considerable number of photons in this wavelength range will pass through the In2S3 buffer and be absorbed by the underneath CZTS layer. This means a not negligible light injection into absorber is still occurring, which narrows the depletion region width and deteriorates the


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collection efficiency in the long wavelength region. On the other hand, the blue EQE under blue light bias is higher than that in the dark, which can be due to the adjustable photoconductivity (non-saturated) by the blue probing beam making the collection of current generated by the transmitted blue light bias (450 nm to 630 nm) in CZTS possible. The voltage bias-dependent behavior in EQE of CZTS device with In2S3 buffer is also examined. As shown in Figure S5, this trend is quite similar with that observed in the device with CdS buffer, again revealing the low minority carrier lifetime. The narrower depletion region width (45nm) of the device with In2S3 buffer leads to EQE being more easily affected by forward voltage bias compared to the CdS buffer case with wider depletion region width (82 nm) as well as low minority carrier lifetime, and red EQE is very low and even close to zero under +0.5V forward voltage bias. For hybrid In2S3/CdS buffer with total thickness of ~140 nm (~80 nm In2S3 + ~60 nm CdS), the red light bias boosts blue EQE dramatically as demonstrated in Figure 2(d). The peak (about 400nm) EQE value is close to 400%, far higher than that in devices with either CdS or In2S3 buffer. The blue light bias causes slightly higher red EQE compared to without light bias, which is also different from the cases with either CdS or In2S3 buffer where the red EQE in blue light bias is similar to or lower than in the dark. These differences from the case of a single buffer layer can be attributed to more remarkable photoconductivity effects in the much thicker hybrid buffer layer. For a hybrid buffer with larger thickness, the stronger increase in photoconductivity by the blue probing beam will cause greater broadening in depletion region in CZTS side compared to either CdS or In2S3 buffer, enabling the collection of electrons more deeply generated in CZTS absorber by red light bias. Meanwhile, under blue light bias, the more significant increase in photoconductivity of the hybrid buffer makes the depletion region width


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in the CZTS region wider than a device using single CdS or In2S3 buffer, enhancing collection from the red probing beam and thereby causing an observable increase in the red EQE. The white light bias also enhances the blue EQE but by a relatively smaller factor than red light bias. As discussed previously, this is because the adjustability of depletion region width by the blue probing beam is weaker under white light bias containing blue photos than under red light bias without blue photons, although the photoconductivity effect has not saturated yet making the blue EQE in both white and red light bias still higher than in the dark. It is observed again that the red EQE is suppressed to some extent under red and white light bias. According to the discussion before, this is because of the shrinkage of depletion region width by light injection of red photons in the red light bias or white light bias. Since the EQE enhancement occurs with the modulation of the conductivity of buffer layer at the chopped probing beam frequency, causing carriers generated in absorber by light bias (not only probing beam) to be extracted and measured by the lock-in amplifier, this enhancement effect will be limited to the chopping frequency range where carrier kinetics can follow the chopped probing beam light, and decrease gradually at higher chopping frequencies.11, 29 Under the assumption that the photoconductivity of the buffer is caused by trapping of holes in deep states, the enhancement effect will be limited to the chopping frequency range where traps can follow the chopped probing beam light. At higher chopping frequencies, the enhancements should decrease gradually. In this work, EQE measurements were performed with chopping frequencies from 32 Hz to 190 Hz. As shown in Figure 3(a) a steady decrease in EQE is demonstrated with increasing chopping frequency in the selected range. In order to further confirm the EQE enhancement results from light bias only, but not from other mechanisms such as multiple exciton generation (MEG), the light bias was shut down during the EQE test


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processing. As shown in Figure 3(b), it is found that the EQE drops back to the same value as in the dark.

Figure 3. (a) EQE of CZTS device with an In2S3/CdS hybrid buffer with white bias illumination at various frequencies; (b) EQE of CZTS device with an In2S3/CdS hybrid buffer with interrupted white light bias.

The influence of light intensity of white light bias on EQE is also further investigated. It is shown from Figure 4(a) that the blue EQE is boosted obviously with the increasing intensity of the white light bias, reaching a peak value of about 115% at 0.3 sun intensity, and then staying steady independent of light intensity, while red EQE decreases slowly with the increase in intensity of light bias. The integral value from EQE curve shows a rapid growth followed by a gradual decrease after arriving at a peak value, as illustrated in Figure 4(b). It also should be remarked that the Jsc from integrated EQE curve under dark in Figure 4(b) as well as in Figure 2(d) (12.1 mA/cm2) is substantially lower than that tested from J-V curve for a device with In2S3/CdS hybrid buffer. However, for CdS and In2S3 buffer, the Jsc from EQE curves under dark (16.4 mA/cm2 in Figure 2(a) and 13.5 mA /cm2 in Figure 2(c), respectively) is consistent with


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the Jsc measured from the J–V curves. This phenomenon should be related to the much larger thickness of the hybrid buffer with more significant photoconductivity effects, and has been observed in our recent report. 30

Figure 4. (a) EQE of CZTS device with In2S3/CdS hybrid buffer with white bias with increasing light intensity; (b) The current calculated by integrating of EQE of the CZTS device with In2S3/CdS hybrid buffer.

From above discussion, the photoconductivity effect could provide an explanation to higher Jsc and Voc in the device using hybrid buffer layer with depletion width of only 60 nm than in device using single CdS buffer with wider depletion width of 80 nm. Under standard illumination with white light (AM 1.5, 1000 W/m2), the stronger photoconductivity effect in the hybrid buffer will produce a much greater enhancement in depletion region width (possibly in the built-in electric field as well) in the CZTS absorber, enabling better collection efficiency than for a single CdS or In2S3 buffer. These results also reveal two directions to improve the electrical properties and performance of the CZTS devices: increasing the minority carrier lifetime in CZTS absorber and increasing the majority carrier concentration of the buffer. Note that although majority carrier


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concentration in the buffer can be increased by photo-doping under illumination, such a device may show poor performance under weak light condition or illumination with a low concentration of blue photons. The photoconductivity effects also suggest that the Jsc from a solar simulator tested could disagree with the integration of EQE curve for thin film solar cells with low minority carrier lifetime absorber and a photoconductive emitter. In this case, in order to understand the relationship of J-V curve and EQE, further investigation on the light bias and spectral dependence of the EQE and electrical (such as C-V and J-V) characteristics is necessary.

Conclusions In summary, the light bias-dependent external quantum efficiency of kesterite Cu2ZnSnS4 solar cells has been reported. CZTS devices with different buffers (CdS, In2S3 and their hybrid buffers) have been used to in-depth illustrate this light bias EQE behavior. Under red light bias, the blue EQE can be enhanced significantly, leading to EQE value exceeding unity due to photoconductivity of buffers increasing depletion region width and thereby giving higher collection efficiencies. Under blue light bias, the red EQE increases only in a device with a hybrid buffer, while remains similar to that in the dark for a CdS buffer due to saturated photoconductivity. It even decreases with In2S3 buffer due to optical injection of blue photons through higher transparent In2S3 layer into the absorber. Under white bias illumination, the enhancement can be observed only with unsaturated buffer such as In2S3 and hybrid buffer, while the red EQE reduces due to optical injection. These light bias dependent EQE including EQE exceeding unity can be attributed to two key factors: photoconductivity of the buffer and low minority lifetime of the absorber. This leads to the disagreement between Jsc from J-V curves measured using a simulator and deduced from the integral of the EQE curves.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:. Acknowledgements This Program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) and Australian Research Council (ARC) and Guodian New Energy Technology Research Institute. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. The authors declare no competing financial interest. References 1. Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar cell efficiency tables (version 46). Prog. Photovoltaics 2015, 23, 805-812. 2. Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465. 3. Duan, H.-S.; Yang, W.; Bob, B.; Hsu, C.-J.; Lei, B.; Yang, Y. The Role of Sulfur in Solution-Processed Cu2ZnSn(S,Se)4 and its Effect on Defect Properties. Adv. Funct. Mater. 2013, 23, 1466-1471. 4. Mousel, M.; Schwarz, T.; Djemour, R.; Weiss, T. P.; Sendler, J.; Malaquias, J. C.; Redinger, A.; Cojocaru-Miredin, O.; Choi, P. P.; Siebentritt, S. Cu-Rich Precursors Improve Kesterite Solar Cells. Adv. Energy. Mater. 2014, 4, 1300543. 5. Werner, M.; Keller, D.; Haass, S. G.; Gretener, C.; Bissig, B.; Fuchs, P.; La Mattina, F.; Erni, R.; Romanyuk, Y. E.; Tiwari, A. N. Enhanced Carrier Collection from CdS Passivated Grains in Solution-Processed Cu2ZnSn(S,Se)4 Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 12141-12146. 6. Li, J. V.; Kuciauskas, D.; Young, M. R.; Repins, I. L. Effects of sodium incorporation in Co-evaporated Cu2ZnSnSe4 thin-film solar cells. Appl. Phys. Lett. 2013, 102, 163905.


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7. Scragg, J. J.; Kubart, T.; Wätjen, J. T.; Ericson, T.; Linnarsson, M. K.; Platzer-Björkman, C. Effects of back contact instability on Cu2ZnSnS4 devices & processes. Chem. Mater. 2013, 25, 3162-3171. 8. Shin, B.; Zhu, Y.; Bojarczuk, N. A.; Chey, S. J.; Guha, S. Control of an interfacial MoSe2 layer in Cu2ZnSnSe4 thin film solar cells: 8.9% power conversion efficiency with a TiN diffusion barrier. Appl. Phys. Lett. 2012, 101, 053903. 9. Liu, F.; Sun, K.; Li, W.; Yan, C.; Cui, H.; Jiang, L.; Hao, X.; Green, M. A. Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface. Appl. Phys. Lett. 2014, 104, 051105. 10. Kim, J.; Hiroi, H.; Todorov, T. K.; Gunawan, O.; Kuwahara, M.; Gokmen, T.; Nair, D.; Hopstaken, M.; Shin, B.; Lee, Y. S.; Wang, W.; Sugimoto, H.; Mitzi, D. B. High Efficiency Cu2ZnSn(S,Se)4 Solar Cells by Applying a Double In2S3/CdS Emitter. Adv. Mater. 2014, 26, 7427-7431. 11. Gloeckler, M.; Sites, J. R. Apparent quantum efficiency effects in CdTe solar cells. J. Appl. Phys. 2004, 95 (8), 4438-4445. 12. Bätzner, D. L.; Agostinelli, G.; Romeo, A.; Zogg, H.; Tiwari, A. N. Voltage Dependent Carrier Collection in CdTe Solar Cells. MRS Online Proceedings Library 2001, 668, H5.17.1H5.17.6. 13. K. Emery, D. D., H. Field, and T. Moriarty. Photovoltaic Spectral Responsivity Measurements. In Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 1998; Vol. 18656, pp 2298-2301. 14. Hegedus, S. S.; Shafarman, W. N. Thin-film solar cells: device measurements and analysis. Prog. Photovoltaics 2004, 12, 155-176. 15. Hsu, W.-C.; Repins, I.; Beall, C.; DeHart, C.; Teeter, G.; To, B.; Yang, Y.; Noufi, R. The effect of Zn excess on kesterite solar cells. Sol. Energy Mater. Sol. Cells 2013, 113, 160-164. 16. Xin, H.; Katahara, J. K.; Braly, I. L.; Hillhouse, H. W. 8% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Redox Equilibrated Simple Precursors in DMSO. Adv. Energy. Mater. 2014, 4, 1301823. 17. Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. The path towards a highperformance solution-processed kesterite solar cell. Sol. Energy Mater. Sol. Cells 2011, 95, 14211436. 18. Li, J.; Wang, H.; Luo, M.; Tang, J.; Chen, C.; Liu, W.; Liu, F.; Sun, Y.; Han, J.; Zhang, Y. 10% Efficiency Cu2ZnSn(S,Se)4 thin film solar cells fabricated by magnetron sputtering with enlarged depletion region width. Sol. Energy Mater. Sol. Cells 2016, 149, 242-249. 19. Liu, F.; Lai, Y.; Liu, J.; Wang, B.; Kuang, S.; Zhang, Z.; Li, J.; Liu, Y. Characterization of chemical bath deposited CdS thin films at different deposition temperature. J. Alloy. Compd. 2010, 493, 305-308. 20. Hariskos, D.; Ruckh, M.; Ruhle, U.; Walter, T.; Schock, H. W. In A novel cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells, Photovoltaic Energy Conversion, 1994. Conference Record of the Twenty Fourth IEEE Photovoltaic Specialists Conference, 1994. 1994 IEEE First World Conference on 5-9 Dec 1994. vol.1, pp 91-94. 21. Jiang, F.; Ozaki, C.; Gunawan; Harada, T.; Tang, Z.; Minemoto, T.; Nose, Y.; Ikeda, S. Effect of Indium Doping on Surface Optoelectrical Properties of Cu2ZnSnS4 Photoabsorber and Interfacial/Photovoltaic Performance of Cadmium Free In2S3/Cu2ZnSnS4 Heterojunction Thin Film Solar Cell. Chem. Mater. 2016, 28, 3283-3291.


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22. Cowache, P.; Vedel, J. CdS solar cells with electrochemically formed Cu2S: Improvement of the current collect. Sol. Cells 1985, 15, 359-364. 23. Hegedus, S.; Ryan, D.; Dobson, K.; McCandless, B.; Desai, D. Photoconductive CdS: how does it Affect CdTe/CdS Solar Cell Performance? MRS Online Proceedings Library 2003, 763, B.9.5.1-B.9.5.6. 24. Rothwarf, A. The CdS/Cu2S solar cell: Basic operation and anomalous effects. Sol. Cells 1980, 2, 115-140. 25. Hegedus, S. S. The photoresponse of CdS/CuInSe2 thin-film heterojunction solar cells. IEEE Trans. on Electron Devices 1984, 31, 629-633. 26. Liu, F.; Yan, C.; Huang, J.; Sun, K.; Zhou, F.; Stride, J. A.; Green, M. A.; Hao, X. Nanoscale Microstructure and Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells. Adv. Energy. Mater. 2016, 6, 1600706. 27. Metzger, W. K.; Repins, I. L.; Contreras, M. A. Long lifetimes in high-efficiency Cu(In,Ga)Se2 solar cells. Appl. Phys. Lett. 2008, 93, 022110. 28. Werner, J. H.; Mattheis, J.; Rau, U. Efficiency limitations of polycrystalline thin film solar cells: case of Cu(In,Ga)Se2. Thin Solid Films 2005, 480–481, 399-409. 29. Stolle, C. J.; Harvey, T. B.; Pernik, D. R.; Hibbert, J. I.; Du, J.; Rhee, D. J.; Akhavan, V. A.; Schaller, R. D.; Korgel, B. A. Multiexciton Solar Cells of CuInSe2 Nanocrystals. J. Phys. Chem. Lett. 2013, 5, 304-309. 30. Yan, C.; Liu, F.; Sun, K.; Song, N.; Stride, J. A.; Zhou, F.; Hao, X.; Green, M. Boosting the efficiency of pure sulfide CZTS solar cells using the In/Cd-based hybrid buffers. Sol. Energy Mater. Sol. Cells 2016, 144, 700-706.


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Light bias-dependent External Quantum Efficiency of Kesterite Cu2ZnSnS4 Solar Cells Fangyang Liu, Chang Yan, Kaiwen Sun, Fangzhou Zhou, Xiaojing Hao* and Martin Green Abstract figure

Accompanying text The light bias dependent behavior in EQE and EQE exceeding unity in kesterite Cu2ZnSnS4 solar cells have been observed. This light bias dependent behavior in EQE (including EQE exceeding unity) can be attributed to two key factors: photoconductivity of the buffer layers combined with low minority carrier lifetime of absorber. Improving the minority carrier lifetime in absorber and majority carrier concentration of buffer to boost the device electrical property and performance is suggested based on this investigation.

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Light-Bias-Dependent External Quantum Efficiency of Kesterite

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