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Ultrafast Carrier Dynamics of CdTe: The Surface Effects Xing He, Napat Punpongjareorn, Chengyi Wu, Ilya A. Davydov, and Ding-Shyue Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02771 • Publication Date (Web): 09 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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The Journal of Physical Chemistry

J. Phys. Chem. C

Revised 04/07/2016

Ultrafast Carrier Dynamics of CdTe: The Surface Effects

Xing He, Napat Punpongjareorn, Chengyi Wu, Ilya A. Davydov, and Ding-Shyue Yang* Department of Chemistry, University of Houston, Houston, Texas 77204 United States

* To whom correspondence should be address. Email: [email protected]. Tel: +1 713-743-6022.

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Abstract Cadmium telluride has been an important absorber material used in solar cells for decades given its near-optimum bandgap and lower cost for device fabrication. However, the overall efficiency has been low compared to the theoretical limit. One major contributor to the problem of the relatively low open-circuit voltage is the high surface recombination of photogenerated carriers. In this contribution, time-resolved pump-probe reflectivity was used to study the carrier dynamics of CdTe(111) under the influence of selected surface conditions. It was found that surfaces after the processes of chemical etching and thermal oxidation for short durations exhibit bulk-like transient reflectivity and a clearly reduced surface recombination velocity, according to a model calculation. The comparative study indicates that the origins of carrier traps include the midgap defect states that are associated with the uncapped Te surface atoms, namely the Te antisites and Te interstitials, and subsurface damages resulting from polishing and ion sputtering. The present results suggest that the formation of a thin oxide layer through annealing after chemical etching may be beneficial for surface improvement for CdTe-based solar cells.


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1. INTRODUCTION Cadmium telluride (CdTe) has been regarded as one of the most promising absorber materials for cost-effective thin-film photovoltaic devices and thus continues to attract attention and research endeavors for decades.1-2 After nearly 20 years of the rather stagnant progress,3 remarkable improvements in the efficiency of CdTe-based solar cells were achieved recently, reaching 21.5%.4 A breakthrough in the open-circuit voltage (Voc) was finally reported earlier this year, from the decades-old records of 800~900 mV to over 1 V.5 All of these efforts aimed to enhance the overall performance of CdTe-based photovoltaic devices, and there is still a large room for improvements before reaching the theoretical limits.6-7 For the persistent issue of low Voc, it has been demonstrated that the excessive bulk and surface carrier recombination has major contributions.5, 8-9 A clear understanding of the carrier dynamics, dissipation pathways, and loss mechanisms is therefore critical. Experimentally, for direct bandgap materials, time-resolved photoluminescence has been the method of choice8, 10-11 to examine the minority-carrier lifetimes and also the influence of various surface conditions and preparation methods.9, 12 Due to the different penetration depths and photoexcitation conditions, one-photon measurements10 provide results that may include the effects of the materials surfaces, whereas two-photon experiments with the laser beam focused at deeper regions essentially give the bulk carrier lifetimes.8, 11 A growing body of literature is seen mainly for the goals of finding the ideal methods and elucidating the mechanisms that can resolve the issues of defects8, 13 and grain boundaries,14-16 using surface passivation and treatments.9, 11-12, 17-19 New opportunities may exist for further insights on the surface/defect states and dynamics at materials surfaces by using scanning ultrafast electron microscopy20-21 (for real-space surface imaging) and time-resolved xray photoelectron spectroscopy22 (for energy-resolved photoelectrochemical dynamics), given



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the escape depths of secondary and photoelectrons in few nanometers for surface-specific probing. In this report, time-resolved pump-probe reflectivity is used to examine the surface effects on the dynamics of photogenerated carriers in CdTe(111) single crystals with selected surface treatments. This study was motivated by the need to understand the dynamics of photogenerated carriers at intermediate excitation levels before recording of the structural dynamics using time-resolved electron diffraction methods, and by the observations that singlecrystalline CdTe surfaces show photoexcitation damages at much lower laser fluences compared to other semiconductors such as Si and GaAs. Here, together with a theoretical calculation, photocarriers are shown to exhibit similar behaviors as in the bulk for surfaces that undergo the procedures of etching and oxidization briefly. The negative impacts of subsurface damages and exposed surface defects on the free-carrier density are prominent. These results suggest the necessity of combining etching and annealing for encapsulation to improve the surface condition and increase the carrier lifetimes, which may have implications for device fabrication.

2. EXPERIMENTAL METHODS 2.1. Spectroscopic Measurements. Time-resolved pump-probe reflectivity measurements were conducted in ultrahigh vacuum (UHV) with a base pressure of the order of 10‒10 torr to study the influence of different surface conditions on the dynamics of photogenerated carriers of CdTe, at an injection level that is orders of magnitude higher than9, 12, 19 or similar to11 the highest used in time-resolved photoluminescence measurements. The surface sensitivity of transient reflectivity (TR) originates from the effective observation depth of the probe beam given by =

/(4 ||), where is the wavelength and is the index of refraction of the material, instead of

the penetration depth given by /(4 ) with being the imaginary part of the refractive 4 ACS Paragon Plus Environment

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index.23-24 In our experiments, the laser is a Yb:KGW regeneratively amplified system (PharosSP, Light Conversion) delivering infrared (IR) pulses whose wavelength centers at 1030 nm (1.20 eV) with a temporal width of ~170 fs. To photoexcite CdTe, which has a direct bandgap of ~ 1.5 eV at room temperature, the fundamental output of the laser was frequency-doubled, then mechanically chopped, directed through a viewport, and loosely focused on the specimen inside the UHV chamber at near normal incidence, with a full-width-at-half-maximum (FWHM) spot size of 360 µm. The penetration depth of the 515-nm excitation beam (ℎ = 2.41 eV) is about 120 nm25-26 and the averaged apparent fluence in the probed region was typically F = 77 µJ/cm2. The wavelength of the p-polarized probe beam was either 1030 nm for two-color experiments (dprobe ~ 29 nm) or 515 nm for one-color experiments (dprobe ~ 14 nm),25 with a FWHM size of 90 µm on the specimen at an incidence angle of = 35° and a low fluence of ~1 µJ/cm2. The reflected probe pulses passed through a long-pass filter (1030 nm) or a linear polarizer (515 nm) to suppress scattered light from the pump beam. The intensity change of the reflected beam following photoexcitation, hence the transient reflectivity (Δ#$ /#$ where #$ is the reflectivity), was measured by a silicon photodiode coupled to a lock-in amplifier. A computer-controlled optical delay line was used to adjust the relative arrival times between the pump and probe pulses to define the time axis. 2.2. Surface Treatments and Characterization. Single crystals of undoped CdTe(111) were acquired from MTI Corporation, which had a carrier density of determined from the TR data. For carrier relaxation, it is found that the initial population decay within 100 ps is mainly due to surface recombination, based on the model of one-dimensional carrier diffusion with an effective carrier recombination rate together with the surface boundary conditions.38-40 Using the ambipolar diffusion coefficient of Da = 3.0 cm2/s measured at lower carrier densities,41 a surface recombination velocity S below 105 cm/s can be deduced from Fig. 5B for the present excitation level; S ~ 7.5×104 cm/s if a slightly larger Da value is used.12, 42 Radiative recombination further reduces the carrier density on the time scale of hundreds of ps to nanoseconds (ns), as manifested more clearly in the one-color TR results (Fig. 3). A time constant of 380 ps (with a biexponential decay model) to 500 ps (through single exponential decay with a constant offset) is obtained for the faster component of the fit, which is in a good agreement with the estimate of


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the radiative lifetime, OP = (Q%)LR ~ 0.3 ns, with the radiative recombination coefficient12, 43 B ~ 3×10‒9 cm3/s and a carrier density N at the 1018-cm‒3 level. 3.2 Surface Effects on Carrier Dynamics. The analysis in the previous section confirms that the procedure used for etching followed by thermal oxidization produces a CdTe crystal surface that resembles the bulk in its photoexcitation response. Hence, the etched-oxidized surface is a desired improvement, with a relative low density of carrier-quenching centers.9 Using this photodynamics as a reference we then have the following observations. First, the as-received surface contains a very high density of defects that trap the photocarriers soon after they are generated, as evidenced by the much reduced TR minimum that is directly related to the free-carrier density (Fig. 2A). Second, the surface recombination velocity of the as-received surface is also much greater than desired, based on the fast decay of the TR signal in just few ps (Fig. 2B). These are the traits that are detrimental to the performance of CdTe photovoltaic devices and need to be resolved. According to the XPS results, the origins of the defects should include chemisorbed/physisorbed contamination and nonstoichiometric surface species. Argon ion sputtering is effective in removing these species and restoring the surface stoichiometry.44 However, it is clear that sputtering alone does not resolve much of the issues of free-carrier depletion and surface recombination; only ~10% of the initial theoretical free-carrier density was recovered. At longer times after 100 ps, whether a higher or depleted density as a result of the surface effects, the remaining free carriers exhibit similar recombination behaviors as a function of time (Fig. 3). Further improvement of the surface condition was seen when the procedure of etching or thermal oxidation was used. Each method reduces a certain type of defects. We found that ~20% or more of the theoretical free-carrier density was further recovered through surface etching



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compared to the sputtered surface. A clear reduction of the surface recombination was also observed (see the much slower TR recovery in Fig. 2B). Given the different amount of CdTe removed by sputtering and by etching, the experimental results indicate that substantial subsurface damages exist in the sub-µm to µm range even after very fine polishing and cannot be removed by limited sputtering. These damages, though difficult to detect, play a major role in carrier quenching9 and here an estimate was obtained for the extent of the improvement of a bare surface after chemical etching. In contrast, thermal oxidation improves the surface by forming TeO2 to remove the dangling bonds of surface atoms and nonstoichiometric species/defects that form the midgap states.8, 45 Reduction of the surface recombination was prominent (Figs. 2B and 3, sputtered-oxidized), despite the rather limited increase of the initial free-carrier density. Overall, the combination of etching and thermal oxidation, both processed for a short duration, was found essential to reduce the negative surface effects on the carrier dynamics of CdTe(111).

Figure 6. Comparison of the two-color transient reflectivity results before and after sputtering of the thin oxide layer on CdTe(111)A. We note that the thin thermal oxide layer is critical to the lifetime of photocarriers. Shown in Fig. 6 is the comparison of the dynamics before and after the oxide layer of the etched-


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oxidized surface is removed by sputtering. It is clear that the initial free-carrier density was reduced, which is attributed to the reappearance of the midgap states for carrier trapping after the removal of the oxide layer. Given the surface stoichiometry and sputter-induced changes, our observation strongly supports that the detrimental defect states are most likely the Te antisites and Te interstitials.8, 46 Furthermore, a faster TR recovery compared to that of the etched surface signifies that ion sputtering reintroduces surface damages to the material and thus may not be an ideal surface treatment method for CdTe in the final step. Such a result is consistent with the recent report that lowest surface recombination velocities were seen for treatments that involve thermal processes, instead of sputtering, in the final step.9 Therefore, the present study suggests that annealing and chemical encapsulation are essential steps to consider for the fabrication of CdTe-based photovoltaic cells.17 Due to the low conductivity and potential issues of causing poor back contact,1,

47-48

oxides have not been

commonly considered in research and device fabrication. Historically, oxygen was sometimes deliberately incorporated in a small fraction of the ambient atmosphere during the crystal growth or deposition stage.49-51 However, benefits of brief annealing in an oxygen-containing ambient, including the air, have been reported.52-53 Similar observations were also made in actual devices about the formation of Te(IV)‒O species to reduce the density of recombination centers and improve minority carrier lifetime.52 Here, in light of the TR data from the as-received surface, which also shows extensive oxidation, we argue that beneficial oxides need to be formed in a controlled environment of annealing at an elevated temperature; completion of the valence shells of Te and O in TeO2 better eliminates the defects of the Te antisites and Te interstitials. In contrast, oxide species formed on aged surfaces may be of various compositions and conformations and still contain detrimental defect states.



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4. CONCLUSION Time-resolved pump-probe reflectivity was used to study the dynamics of photogenerated carriers of CdTe and the influence of the surface conditions. The surface treatments performed on single-crystalline CdTe(111) include argon ion sputtering, chemical etching, thermal oxidation, and their combinations. Transient reflectivity results showed that the surfaces that undergo the processes of chemical etching and thermal oxidation for brief durations exhibit bulklike carrier dynamics and largely reduced surface recombination. Surfaces without encapsulation by thermally formed oxides (e.g., the as-received and stoichiometric bare surfaces) showed clearly larger surface recombination velocities and reduction of the free-carrier density, which supports the model of Te antisites and Te interstitials as the major midgap defect states to quench the photocarriers. Subsurface damages induced by polishing and sputtering were also found to be major free-carrier traps and hence should be eliminated. The present study provides a consistent picture with previous reports that the combination of etching and thermal annealing may be a crucial step to improve the performance of CdTe-based solar cells, where the treatments that incorporate a thin thermal oxide layer are worth further investigation.

■ ASSOCIATED CONTENT Supporting Information Description of the methods and parameters used to calculate the change of the refractive index, and two-color transient reflectivity results of CdTe(111)B with different surface conditions. This information is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Notes The authors declare no competing financial interests.

■ ACKNOWLEDGEMENT This research was supported by the R. A. Welch Foundation (Grant No. E-1860) and the University of Houston (UH). X.H. acknowledges Dr. Herman Suit and Dr. Joan Suit for the Eby Nell McElrath Postdoctoral Fellowship.

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46. Gessert, T. A.; Burst, J. M.; Wei, S.-H.; Ma, J.; Kuciauskas, D.; Rance, W. L.; Barnes, T. M.; Duenow, J. N.; Reese, M. O.; Li, J. V., et al., Pathways toward Higher Performance CdS/CdTe Devices: Te Exposure of CdTe Surface before ZnTe:Cu/Ti Contacting. Thin Solid Films 2013, 535, 237-240. 47. Singh, V. P.; Erickson, O. M.; Chao, J. H., Analysis of Contact Degradation at the CdTeElectrode Interface in Thin Film CdTe-CdS Solar Cells. J. Appl. Phys. 1995, 78, 4538-4542. 48. Visoly-Fisher, I.; Dobson, K. D.; Nair, J.; Bezalel, E.; Hodes, G.; Cahen, D., Factors Affecting the Stability of CdTe/CdS Solar Cells Deduced from Stress Tests at Elevated Temperature. Adv. Funct. Mater. 2003, 13, 289-299. 49. Tang, C. W.; Vazan, F., Effect of Oxygen on the Photoluminescence of CdS/CdTe ThinFilms. J. Appl. Phys. 1984, 55, 3886-3888. 50. Metzger, W. K.; Albin, D.; Levi, D.; Sheldon, P.; Li, X.; Keyes, B. M.; Ahrenkiel, R. K., Time-Resolved Photoluminescence Studies of CdTe Solar Cells. J. Appl. Phys. 2003, 94, 3549-3555. 51. Corwine, C. R.; Sites, J. R.; Gessert, T. A.; Metzger, W. K.; Dippo, P.; Li, J.; Duda, A.; Teeter, G., CdTe Photoluminescence: Comparison of Solar-Cell Material with SurfaceModified Single Crystals. Appl. Phys. Lett. 2005, 86, 221909. 52. Rugen-Hankey, S. L.; Clayton, A. J.; Barrioz, V.; Kartopu, G.; Irvine, S. J. C.; McGettrick, J. D.; Hammond, D., Improvement to Thin Film CdTe Solar Cells with Controlled Back Surface Oxidation. Sol. Energy Mater. Sol. Cells 2015, 136, 213-217. 53. Wang, F. F.; Fahrenbruch, A. L.; Bube, R. H., Properties of Metal-Semiconductor and MetalInsulator-Semiconductor Junctions on CdTe Single Crystals. J. Appl. Phys. 1989, 65, 35523559.



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Ultrafast Carrier Dynamics of CdTe: Surface Effects - The Journal of

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