Improved CdTe Solar-Cell Performance by Plasma ... - IEEE Xplore

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IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 3, NO. 2, APRIL 2013

Improved CdTe Solar-Cell Performance by Plasma Cleaning the TCO Layer Drew E. Swanson, Russell M. Geisthardt, J. Tyler McGoffin, John D. Williams, and James R. Sites

Abstract—A hollow-cathode plasma-cleaning source, designed for uniformity, was added to the load-lock region of an existing single-vacuum CdTe-cell fabrication system. This plasma source cleans the transparent-conductive-oxide layer of the cell prior to the deposition of the CdS and CdTe layers. This plasma exposure enables both thinner CdS layers and enhanced cell voltage. The net result is a reduction in CdS thickness by approximately 20 nm, while maintaining the same cell voltage or, equivalently, an increase in voltage of as much as 80 mV for the same thickness of CdS. Maps that are generated by electroluminescence and lightbeam-induced current show modest uniformity improvement with plasma-cleaning treatment. Index Terms—Cadmium telluride (CdTe), cleaning transparent conductive oxide (TCO), photovoltaic (PV) cells, plasma cleaning, thin films.

I. INTRODUCTION HE growing photovoltaic (PV) market requires technologies that utilize economic large-scale manufacturing with high device efficiency. Cadmium telluride (CdTe) has been expanding in the U.S. PV market over the past because of a bandgap that is near optimum and a large absorption coefficient that allows nearly complete absorption in a 1-μm layer. Companies such as First Solar and Abound Solar have demonstrated the ability to manufacture CdTe thin-film solar cells at competitive costs and have developed small-area CdTe cells with efficiencies up to 17.3% [1]. Advanced, scalable technologies are needed to further improve upon this technology. The highest efficiency CdTe solar cells are produced in a superstrate configuration [2], where the light enters the cell through the supporting glass. The first film deposited onto the glass is a transparent conductive oxide (TCO), which forms the front contact of the solar cell. The condition of the TCO surface has an important impact on the quality of the other films comprising the cell. In particular, sputter treatments have been

T

Fig. 1.

Structure of CdTe device and TEC glass.

shown to reduce carbon contamination and modify the resulting interface between SnO2 and CdS [3]. Poor growth regions, also known as pin holes, can form in the n-type CdS layer, allowing a shunt path between the front contact and the p-type CdTe [4]. These pinholes are particularly troublesome when a thin CdS layer is deposited following industry standard-cleaning techniques [5]. Scanning-white-lightinterferometry (SWLI) images of a CdS film that is deposited on TCO with standard-cleaning display the formation of these pinholes [6]. Even a small number of pin holes can cause a substantial decrease in fill factor (FF) and open-circuit voltage VOC [7]. To minimize these pinhole effects, relatively thick (100– 200 nm) CdS is often deposited. This results in a current density loss from the absorption of light above the CdS bandgap of 2.4 eV, and a current density reduction in the cell by as much as 4.6 mA/cm2 [8]. It has been shown that with SWLI images of a CdS film on a TCO plasma treated substrate, evidence of pinholes is removed [6]. This paper will expand upon research performed in [6] with improved plasma-cleaning source hardware, an empirically optimized plasma-cleaning current density, and an increased range of CdS thicknesses studied. The research will concentrate on the positive impact in cell performance from plasma cleaning. II. MATERIALS AND METHODS A. Fabrication and Preparation of CdS/CdTe Cells

Manuscript received October 16, 2012; accepted January 17, 2013. Date of current version March 18, 2013. This work was supported by the U.S. Department of Energy F-PACE under Award DE-EE0005399. D. E. Swanson and J. D. Williams are with the Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523 USA (e-mail: [email protected]; [email protected]). R. M. Geisthardt, J. T. McGoffin, and J. R. Sites are with the Department of Physics, Colorado State University, Fort Collins, CO 80523 USA (e-mail: [email protected]; [email protected]; [email protected]. edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2013.2244163

The CdS/CdTe solar cells that are studied here are manufactured by sublimating films using heated-pocket deposition sources integrated into a single-vacuum deposition system [9], [10]. The cells are manufactured on 3.6-in × 3.1-in Pilkington TEC 10 superstrates that consist of soda lime silicate glass, followed by a thin intrinsic SnO2 layer, a thin SiO2 film, and finally the thick SnO2 :F layer as shown in Fig. 1 [11]. Prior to plasma cleaning or film deposition, the TCO film is cleaned using a standard ultrasonic detergent rinse, an ultrasonic deionized water rinse, and finally an isopropyl alcohol wash. Plasma treatment can be done at this stage before the cell proceeds through the

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SWANSON et al.: IMPROVED CDTE SOLAR-CELL PERFORMANCE BY PLASMA CLEANING THE TCO LAYER

Fig. 2.

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Single-vacuum deposition system for CdTe cells at CSU. TABLE I CELL MANUFACTURING SETPOINTS

Fig. 4. Open-circuit voltage and blue photon short-circuit current density versus CdS thickness with industry-standard cleaning.

Fig. 5.

QE at varying plasma cleaning treatment current densities.

Fig. 3. Plate anode, hollow-cathode glow discharge integrated into a graphite source for plasma cleaning TCO.

fabrication system that is shown in Fig. 2 for the deposition of the CdS and CdTe films. Similar to [6], the CdTe cells receive posttreatments with cadmium chloride, but in this study they will receive an additional copper doping at the back contact. Specific cell fabrication details are outlined in Table I. The plasma cleaning and film deposition are performed without breaking vacuum, and the samples are removed and carbon nickel layers are sprayed on for the back contact. B. Plasma Cleaner Hardware and Treatment Initial plasma-cleaning results [6] used a hollow-cathode, wire-anode, plasma discharge integrated into a graphite pocket. The wire anode has now been replaced (see Fig. 3) by a solidplate anodes embedded within the walls of the source. This change allows for improved reliability and uniformity of the plasma discharge [14] and a more robust source for industrial applications. The plasma discharge is ignited by biasing the plates positive with respect to the plasma source walls and ground (see Fig. 3).

Fig. 6. J−V curves of standard and plasma cleaned TCO at a CdS thickness of 65 nm.

When the TCO is placed over the plasma source, the SnO2 :F layer is held at ground and thus allows ion bombardment for plasma cleaning. The plasma-cleaning current density for this paper was empirically optimized to 0.11 mA/cm2 (400 V) for 30 s, and is held at 200 mTorr N2 O2 environment (2% O2 ) for uniformity. For experimental control, the plasma cleaning is applied to only half of each 3.6-in × 3.1-in superstrate, leaving the other half untreated. Current–voltage (J–V), quantum efficiency (QE), electroluminescence (EL), and light-beam-induced-current (LBIC) measurements were used to characterize the differences in the plasma-cleaned and standard-cleaned devices. QE was used to calculate CdS thickness from the Beer–Lambert Law. The EL images were taken by a silicon charge-coupled device camera that was cooled to –25 ◦ C. The forward current for EL was 20 mA/cm2 : a similar magnitude to Jsc [12]. The light for the

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TABLE II PARAMETER COMPARISON FOR VARYING CDS THICKNESS

LBIC measurements was provided by an electrically modulated laser diode at 638 nm wavelength focused to a 100-μm spot size. To produce a map, the samples were moved under the laser spot using stepper motors. The LBIC mapping was done under short-circuit conditions and with a voltage bias applied [13]. III. RESULTS AND DISCUSSION CdTe cells typically maintain a nearly constant Vo c when CdS thickness is greater than a given value, but significant decreases and loss of cell efficiency below that value. Fig. 4 shows the amount of blue current (from photons with energy above the CdS bandgap) and the corresponding VOC from a set of cells with varying CdS thickness. At CdS thicknesses below approximately 90–110 nm, the cells experience a reduction in Vo c that dominates the cells efficiency. This results in an optimum CdS thickness where Vo c is not significantly decreased. The current density used for plasma cleaning can affect the CdS thickness (see Fig. 5). When a low plasma current density (dashed red line) is applied, the thickness of the CdS becomes less than on the adjacent cell without plasma cleaning, as seen with increased QE blue response. With larger plasma-cleaning current, the CdS thickness increases gradually (dotted red line) and eventually surpasses the standard treatment thickness (solid red line). Using the different plasma geometry, there are similar trends in changing CdS thickness with plasma treatment power as reported in [6]. There is not a good explanation for the variation in CdS thickness with the aggressiveness of the predeposition plasma cleaning. What is clear is that even with thinner CdS following plasma cleaning, the cell voltage is increased. For a direct comparison of cells with and without plasma cleaning, Fig. 6 shows two cells where the CdS deposition time was adjusted so that both CdS thicknesses are very close to 65 nm, as shown in Table II Thinnest (which is in the region of decreasing Vo c shown in Fig. 4). The two cells had a nearly identical Jsc of 24 mA/cm2 . The voltage of the plasma-treated device, however, was 80 mV greater than the standard-treated device at this CdS thickness. There are cell-to-cell variations in how much current and voltage increase when low-current (0.11 mA/cm2 ) plasma cleaning is employed. In all side-by-side comparisons, however, the plasma-cleaned cells had both higher current and higher voltage. Fig. 7 demonstrates voltage and efficiency for a broad range of cells with and without plasma-cleaned TCO. The standardcleaned TCO cells begin to degrade in Vo c at around 90–100 nm,

Fig. 7. Open-circuit voltage and device efficiency versus CdS thickness under plasma-cleaned and standard-cleaned conditions.

Fig. 8. TCO.

Temperature dependence of J−V for plasma- and standard-cleaned

while the plasma-cleaned TCO cells maintain Vo c down to 60– 70 nm. The efficiency difference near 65 nm was 1.6% absolute. At larger CdS thicknesses above 100 nm, the plasma-cleaning treatment shows only marginal increases in cell performance compared with standard cleaning. Further J−V investigation of the cells in Fig. 6 reveals a constant offset of Vo c over a range of temperatures. Fig. 8 shows that the y-intercept, or built-in voltage of the plasma-cleaned cell, is improved to a value approximately equal to the CdTe bandgap. This improvement can be attributed to the removal of pin holes between the TCO and p-type CdTe. The parallel slopes imply no other significant effects.

SWANSON et al.: IMPROVED CDTE SOLAR-CELL PERFORMANCE BY PLASMA CLEANING THE TCO LAYER

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Fig. 10. LBIC spatial maps of standard- and plasma-cleaned cells from Fig. 6 with and without forward voltage bias. The histogram demonstrates the spatial uniformity under forward bias Red is with plasma cleaning; blue without. Fig. 9. EL of plasma- and standard-cleaned TCO cells at varying CdS thicknesses. The CdS thickness and V o c are noted within each EL image.

more clearly in the histogram. The histogram in Fig. 10 shows the percent QE change from the average QE value of the cell. IV. CONCLUSION

Table II gives a summary of representative cells with three different CdS thicknesses, comparing standard cleaning and plasma cleaning that are near the same CdS thickness. In each case, the current density is also nearly the same, the FF is increased after plasma cleaning, and the voltage is significantly increased for thinner CdS after plasma cleaning. EL intensity correlates well with cell Vo c for CdTe [12]. Fig. 9 shows a side-by-side comparison of the EL performed on cells from the same substrate with standard-cleaned and plasmacleaned TCO. Data support that the plasma-cleaned cells have a larger and more uniform EL signal; this agrees with the higher Vo c measured by J−V and suggests a more uniform growth of the device. The histogram data show a small increase in uniformity for the thin CdS condition (C&D) but no noticeable increase for thicker CdS (A&B). Note that the axis in Fig. 9 (log of the EL pixel value) is proportional to Vo c ; subsequently, the histogram represents a voltage distribution of the cell. LBIC results also demonstrate an improvement in uniformity of plasma-cleaned cells. Fig. 10 compares the two thinnest CdS samples from Table II at short circuit and with a forward voltage bias approaching Vm p . At zero voltage bias, both cells are relatively uniform over the whole area, with a few extra localized defects on the standard-cleaned sample. At forward bias, however, the nonuniformity is enhanced, accentuating the weak diodes. The plasma-cleaned samples show better uniformity than the standard-cleaned sample, both in the map and

Plasma cleaning of the TCO-coated glass prior to the deposition of CdS/CdTe solar cell led to improved PV efficiency by as much as 1.5% absolute through increases in both current and voltage. The current increases resulted from thinner CdS following the cleaning. Both voltage cliffs for the standard-cleaned and plasma-cleaned samples are shown with the widened CdS range, resulting in a 20–40 nm reduction in CdS thickness at no Vo c loss for the plasma-cleaned TCO samples. This reduced voltage loss under thin CdS conditions comes from pinhole reduction from the plasma-cleaning treatment. The voltage increases were constant with temperature and consistent with increasing average EL intensity. Modest improvement in cell uniformity from plasma cleaning was demonstrated with EL and maps of LBIC in forward bias. ACKNOWLEDGMENT The authors thank W. S. Sampath, P. Kobyakov, J. Kephart, G. Metz, J. Raguse, K. Cameron, K. Barricklow, J. Clark, P. McCurdy, and L. Bowers for helpful conversations and suggestions in this research. REFERENCES [1] Nat. Renewable Energy Lab. Nat. Cen. Photovoltaics, Golden, CO, USA. (2012). Research Cell Efficiency Records [Online]. Available: http://www.nrel.gov/ncpv

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[2] X. Wu, “High-efficiency polycrystalline CdTe thin-film solar cells,” Sol. Energy, vol. 77, pp. 803–814, 2004. [3] J. Fritsche, S. Gunst, A. Thiben, R. Gegenwart, A. Klein, and W. Jaegermann, “CdTe thin film solar cells: The CdS/SnO2 front contact,” Mater. Res. Soc., vol. 668, 2001. [4] A. R. Davies, “Effects of contact-based non-uniformities in CdS/CdTe thin film solar cells,” Ph.D. dissertation, Dept. Phys., Colorado State Univ., Fort Collins, CO, USA, 2008. [5] M. A. Tashkandi and W. S. Sampath, “Eliminating pinholes in CSS deposited CdS films,” in Proc. 38th IEEE Photovotaics Spec. Conf., Jun. 2012, pp. 143–146. [6] D. E. Swanson, R. M. Lutze, W. S. Sampath, and J. D. Williams, “Plasma cleaning of TCO surfaces prior to CdS/CdTe deposition,” in Proc. 38th IEEE Photovoltaics Spec. Conf., Jun. 2012, pp. 859–863. [7] G. T. Koishiyev and J. R. Sites, “Effect of shunts on thin-film CdTe module performance,” presented at the Mater. Res. Soc. Symp., Warrendale, PA, USA, 2009. [8] S. H. Demtsu and J. R. Sites, “Quantification of losses in thin-film CdS/CdTe solar cells,” in Proc. 31st IEEE Photovoltaics Spec. Conf., Jan. 2005, pp. 347–350. [9] K. L. Barth, R. A. Enzenroth, and W. S. Sampath, US Patent 6 423 565, Jul. 2002. [10] K. E. Walters, “Computational fluid dynamics (CFD) modeling for CdTe solar cell manufacturing,” M.S. thesis, Dept. Mech. Eng., Colorado State Univ., Fort Collins, CO, USA, 2011. [11] K. Von Rottkay and M. Rubin, “Optical indices of pyrolytic tin-oxide glass,” in Proc. Mater. Res. Symp., 1996. [12] J. Raguse, J. T. McGoffin, and J. R. Sites, “Electroluminescence system for analysis of defects in CdTe cells and modules,” in Proc. 38th IEEE Photovoltaics Spec. Conf., 2012, pp. 448–451. [13] J. F. Hiltner, “Investigation of spatial variation in collection efficiency of solar cells” Ph.D. dissertation, Dept. Phys., Colorado State Univ., Fort Collins, CO, USA, 2001. [14] G. E. Metz, “Characterization of a plasma reactor device for photovoltaic applications” M.S. thesis, Dept. Mech. Eng., Colorado State Univ., Fort Collins, CO, USA, 2012.

Russell M. Geisthardt received the B.A. degree in physics from Lawrence University, Appleton, WI, USA, in 2008. He received the M.S. degree in physics from Colorado State University, Fort Collins, CO, USA, in 2011, where he is currently working toward the Ph.D. degree.

Drew E. Swanson received the B.S. degree in mechanical engineering and the M.S. degree from Colorado State University, Fort Collins, CO, USA, in 2011 and 2012, respectively, where he is currently working toward the Ph.D. degree.

James R. Sites received the B.S. degree from Duke University, Durham, NC, USA, in 1965. He received the M.S. and Ph.D. degrees from Cornell University, Ithaca, NY, USA, in 1968 and 1969, respectively. Since 1971, he has been with Colorado State University (CSU), Fort Collins, CO, USA, where he is currently the Senior Associate Dean for Research with the College of Natural Sciences. From 1990 to 2000, he served as the Department Chair in physics with CSU.

J. Tyler McGoffin received the B.S. degree in physics and mathematics from Colorado State University, Fort Collins, CO, USA, in 2012, where he is currently working toward the M.S. degree.

John D. Williams received the B.S. and Ph.D. degrees from Colorado State University, Fort Collins, CO, USA, in 1986 and 1991, respectively. Since 2002, he has been with Colorado State University, where he is currently a tenured Professor with the Department of Mechanical Engineering.