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Device characteristics of high performance Cu2ZnSnS4 solar cell

Oki Gunawan, Tayfun Gokmen, Byungha S. Shin, Supratik Guha IBM T. J. Watson Research Center, PO Box 218, Yorktown Heights, NY 10598 USA

ABSTRACT

Recently we reported a record efficiency of 8.4% for Cu2ZnSnS4 (CZTS) solar cell made by vacuum process [1]. We present a more comprehensive characterization of this CZTS cell employing various techniques such as temperature dependent I-V and biased quantum efficiency. By comparing the cell with the data from a reference champion CdTe cell that has similar bandgap, we identify that the biggest problem is the VOC deficit followed by JSC deficit and low fill factor issue. We present analyses of various characterization results to elucidate the underlying factors in the performance bottleneck issues in this present generation of CZTS cell.

INTRODUCTION Cu2ZnSnS4 (CZTS) thin film solar cell has attracted intense research efforts recently as a candidate for large scale photovoltaics technology utilizing all earth abundant and non-toxic materials compared to other leading thin film technologies, such as Cu(In1-xGax)(S1-ySey)2 (CIGSSe) and CdTe. CZTS absorber layer has many desirable characteristics such as direct bandgap, with values near the optimum single junction bandgap value (1.45 eV) which is also close to the bandgap of CdTe, high absorption coefficient (~10

4 /cm) and the ability to be

deposited by a variety of processes [2]. There has been promising progress in CZTS development recently starting from power conversion efficiency (PCE) of 6.7% in 2008 by Katagiri's group [3] and the latest record of 8.4% synthesized by vacuum processing by our group [1]. Furthermore, CZTS sub-module (5x5 cm

2) with

7.2% PCE has also been demonstrated by Solar Frontier [4], asserting its potential as a commercially viable technology in the near future. Here we present a more detailed characterization of our 8.4% champion cell utilizing various characterization techniques such as temperature dependence I-V characterization and biased quantum efficiency. We describe in more detail the performance bottlenecks issues in our present generation of CZTS cells. We also compare the performance of our CZTS cell with a reference champion CdTe cell [5] due to its close bandgap to CZTS (Eg~1.45 eV), to elucidate various performance bottlenecks issues in our cell.

Cell PCE FF Voc Jsc/JscMAX

sc Eg Voc/Eg% % V eV

CdTe 16.7 75.5 0.845 26.1 ~1.45 0.583 0.846

CZTS 8.4 65.8 0.661 19.5 1.50 0.449 0.673

50.3 87.2 77.0 79.6Ratio CZTS/CdTe (%)

J

mA/cm2

Table I. Device characteristic comparison between the champion CZTS [1] and a reference champion CdTe [5].

RESULTS AND DISCUSSIONS The photovoltaics device characteristics of the champion CZTS and CIGS is presented in Table I. For comparison purpose we also show the ratio of the CZTS device parameters with respect to CdTe. For Voc and Jsc comparison we compare the values “normalized to bandgap” using VOC/Eg and JSC/JSC,MAX respectively where JSC,MAX is the maximum theoretical JSC at that bandgap for AM1.5G illumination and 100% EQE. Here we clearly observe that the number one issue (among FF, VOC and JSC) in the performance of CZTS solar cell is the VOC deficit followed by JSC and FF. We will address these issues one by one in the following analyses.

Further insight in the VOC deficit issues could be obtained from the temperature dependence data of the VOC as shown in Fig. 1 to reveal the dominant recombination process in the CZTS. The VOC can be given as [6]:

Figure 1. Temperature dependence of VOC and JSC of the champion CZTS. The collapse of JSC at low T is due to the divergence in the series resistance.

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00/ / ln /

A LVoc E q AkT q J J= − × (1)

where EA, A, k, J00 and JL are the activation energy of the dominant recombination mechanism, diode ideality factor, Boltzmann constant, reverse saturation current prefactor and the photocurrent, respectively. The VOC vs. T data at high temperature typically yields a straight line and extrapolate to the activation energy EA/q at T = 0 K. The VOC vs. T data for our CZTS cell has intercepts that is significantly lower than its bandgap value. This kind of VOC deficit problem could arise from dominant interface recombination. In contrast, the VOC vs T data for high performance CIGS and CdTe cells have T=0 K intercept that agree very well with their bandgap values [7] indicating that the dominant recombination mechanism is in the space charge region (SCR).

Figure 2. Temperature dependence of the ideality factor n (from JSC-VOC curves) and the inverse ideality factor (1/n). More information regarding the recombination process could be obtained from the temperature dependence of the ideality factor n (and its reciprocal 1/n) as shown in Fig. 2 obtained from JSC-VOC data. The JSC-VOC curve is obtained by attenuating the solar simulator light intensity from 1 sun to ~10

-4 sun using neutral density filters. The

ideality factor is obtained from the slope of the ln(JSC)-VOC curves near 1 sun. We notice a strong increase in n at lower temperature. Higher ideality factor represents a higher recombination in the p-n junction transport, and this increase could be attributed to higher tunneling-assisted recombination process [8]. Such an enhanced recombination could partly explain the low maximum EQE in our cell (Fig. 3). The second issue is the JSC deficit which is apparent in the EQE curve of our champion CZTS cell as shown in Fig. 3. Note that the JSC estimated from the EQE curve by integrating it with AM1.5G solar spectrum yields JSC = 20.3 mA/cm

2, consistent with the I-V measurement (JSC = 19.5

mA/cm2, Table I). We observe that the EQE is still lacking

Figure 3. External quantum efficiency and its biased-ratio at -1 V and 0V bias (upper panel). The bandgap (Eg=1.50

eV) derived from the peak of dEQE/dλ curve.

at all wavelengths notably at long wavelength regime. If we focus on this regime, we notice that the ratio of the EQE at -1V to 0V (Fig. 1, top inset) shows an increase towards longer wavelength. This indicates that our CZTS has limited collection efficiency due to short depletion width and/or short diffusion length (Ld) [9]. In such situation an increase in the negative reverse bias will increase the depletion width and if Ld is comparable, it will increase the

EQE. Since dL Dτ= where τ is the minority (electron)

carrier lifetime and D is the diffusion coefficient which is related to the electron mobility through Einstein equation D

= kTµ/q. The time-resolved photoluminescence done in

our cell [1] indicates a moderate minority carrier lifetime τ~ 8 ns, thus this low Ld could be due to poor electron mobility. Thus one route to increase the JSC in our cell is to investigate the mobility limiting mechanism. Furthermore we could gain JSC improvement by increasing the front

side absorption (λ < 600 nm) by using thinner CdS or alternative buffer with higher bandgap.

We also notice a rather low EQE (~ 85%) at the mid band (~500-700 nm) which suggests a further evidence of the high interface recombination which is also made worse by assisted tunneling process across the CdS/CZTS interface as have been discussed above. Finally we investigate the CZTS fill factor issue, already at 87% of the CdTe fill factor, which is the least problem for CZTS. There is a dramatic difference exhibited by the CZTS temperature dependence of the FF compared to CIGSSe [11]. CZTS fill factor (and thus the efficiency and JSC, see Fig. 1) collapses at low temperature which is attributed to the divergence of the series resistance (both in the dark and in the light) as shown in Fig. 4. This behavior could arise from the presence of a non-ohmic

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Figure 4. Temperature dependence of the series resistance under dark and under light condition (from the difference between JSC-VOC and light J-V data [10]). back contact or also most likely due carrier freeze out effect, where the free carrier density is quenched at very low temperature due to lack of shallow acceptor in CZTS. This phenomenon have been recently observed in admittance spectroscopy study of the CZTSSe cells [12]. Note that several theoretical studies [13,14] have suggested that CuZn antisite defects with deep energy level of 0.12 eV is the dominant acceptor impurity in CZTS, not the usual copper vacancies (VCu) that has shallow energy level as usually found in CIGSSe system. Such a deep acceptor defect and the absence of shallow acceptor give rise to the divergence of series resistance and collapse of the FF, JSC and efficiency at low temperature in CZTS. Note that poor (majority) carrier mobility as have been suspected from the EQE data could also limit the FF at room temperature.

CONCLUSION In conclusion, we have performed temperature dependent analysis of our CZTS cell with record PCE of 8.4%. By benchmarking its performance with a reference champion CdTe cell, we gain insights into the degree of performance limiting factors in our device. We found that the VOC deficit is the number one problem, followed by JSC and then the FF. The VOC deficit issue is suspected to be dominated by severe recombination process as supported by the VOC vs. T, ideality factor vs. T and EQE at the middle band data. The low JSC problem could be attributed to high recombination losses everywhere and notably low minority carrier diffusion length with suspected very poor mobility in CZTS. The low FF problem which collapses at very low temperature is suspected due to the carrier freeze-out effect due to the lack of shallow acceptor in CZTS.

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[2] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, and S. Guha, The path towards a high-performance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cells 95, 1421 (2011).

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[12] O. Gunawan et al., Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods, Appl. Phys. Lett. in-press (2012).

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