CdCl2 Treatment-Induced Enhanced Conductivity ... - ACS Publications

Subscriber access provided by Fudan University

Letter 2

CdCl Treatment-Induced Enhanced Conductivity in CdTe Solar Cells Observed Using Conductive-Atomic Force Microscopy Mohit Tuteja, Antonio B. Mei, Vasilios Palekis, Allen Hall, Scott MacLaren, Chris S. Ferekides, and Angus A Rockett J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02399 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

CdCl2 Treatment-Induced Enhanced Conductivity in CdTe Solar Cells Observed Using Conductive-Atomic Force Microscopy Mohit Tuteja1, Antonio B. Mei1, Vasilios Palekis2, Allen Hall1, Scott MacLaren3, Chris S. Ferekides2, Angus A. Rockett1,* 1

Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801 2

3

Department of Electrical Engineering, University of South Florida, Tampa, FL 33620

Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, IL 61801

*Corresponding author. Contact Information: [email protected], +1 (217) 333 0417

Abstract The rear surfaces of CdTe photovoltaic devices without back contacts, grown by closedspace sublimation (CSS) were analyzed using conductive – atomic force microscopy (CAFM). As-deposited and CdCl2 treated CdTe samples were compared to clarify the effect of the treatment on charge flow through grains and grain boundaries. The CdCl2 treated samples exhibit a more homogenous and enhanced current flow across the grains as compared to the as-deposited samples. The grain boundaries show variable current. Under high bias, grain boundaries dominate current flow when the main junction is reverse biased and conducting current in reverse breakdown. Under the opposite bias conditions, where the contact of the conductive tip to the surface is reverse biased and under breakdown conditions the current flow is uniform with little contrast between grains and grain boundaries. The results are interpreted as resulting from the improved crystallinity of the CdTe with reduced p-type doping along the grain boundaries.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 18

TOC Graphic


2

Page 3 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recently, 22.1% device efficiencies have been achieved in CdTe-based solar cells1. These high efficiency devices use CdTe in polycrystalline thin-film form as the active absorber layer. The electrical conductivity of polycrystalline materials is degraded by the presence of grain boundaries, which usually act as recombination centers for photo-generated carriers and give rise to localized grain-boundary states that cause carrier scattering2. Despite these effects, polycrystalline thin-film CdTe solar cells outperform their single crystal counterparts, in terms of their photo-current collection properties3. This unexpected behavior has prompted many investigations3–9. Some studies suggest that grain boundaries provide electrically active, low resistance conduction channels to the front contact4. CdCl2 annealing is an important step in CdTe solar cell fabrication that leads to improved open circuit voltage, short circuit current, and fill factor of the device10,11. Atomic force microscopy, scanning electron microscopy, x-ray diffraction, and timeresolved photoluminescence measurements11–13 have demonstrated that CdCl2 annealing treatment induces recrystallization of CdTe films and promotes grain growth, which lead to enhanced minority carrier lifetime. CdTe is intrinsically p-type14 but recently it has been demonstrated that after a CdCl2 anneal, Cl is incorporated in the CdTe grain boundaries5,7,15. The presence of Cl, an n-dopant for CdTe, results in local doping, type inversion and carrier depletion, and results in p-doped CdTe grains separated by n-type or depleted p-type grain boundaries3,7–9. Investigating individual grain boundary properties is challenging since interaction volumes probed by conventional characterization techniques span multiple grains. Individual grain boundary properties have been investigated via electron beam induced current (EBIC)5,7,16,17; combinations of scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), and scanning Kelvin probe force microscopy3,18,19; near-field optical beam induced current (OBIC)17,20; and scanning microwave impedance microscopy8,9. However, the electrical conductivity of the grain boundaries as a function of CdCl2-anneal treatment remains unclear. In order to clarify the effect of Cl incorporation, we compare close-spaced sublimation (CSS) deposited thin-film CdTe, with and without CdCl2 annealing. This was accomplished using conductive atomic-force microscopy (C-AFM), which is a scanning probe technique that


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 18

enables high-resolution nanoscale mapping of the current from the tip to the sample. The CdTe samples were fabricated on Corning EagleXG glass at the University of South Florida. Indium tin oxide deposited by sputtering was used as the transparent conducting oxide (TCO) front contact. CdS and CdTe were then deposited by chemical bath deposition (CBD) and closed-space sublimation (CSS), respectively. During CSS growth, the substrate temperatures were in the range of 550-600°C and source temperatures were 650-680°C. One sample was removed at this stage and the other was subjected to CdCl2 deposition by evaporation followed by annealing in a He and O2 ambient (20% partial pressure of O2) at 4000 C for 30 min. Additional details of the growth process can be found in Ref. 21. Representative device performance parameters for the finished CdTe solar cells fabricated using this method, with and without the CdCl2 annealing, are listed in Table 1.

as-deposited

VOC (mV) 700

JSC (mA/cm2) 19

FF (%) 53

η (%) ~7

CdCl2 annealed

840

23

73

~14

Sample

Table 1: Device performance parameters of CdTe solar cells with- and without CdCl2 anneal treatment.

C-AFM measurements were performed on the surfaces of the CdTe films at the University of Illinois using an Asylum Research MFP-3D AFM with commercially available solid-Platinum cantilevers from Rocky Mountain Nanotechnology, LLC. CAFM utilizes a conductive probe in an AFM instrument. The typical radius of curvature of the tip is 20 nm. The AFM system was housed in a lightproof enclosure such that there was no unintentional light source apart from the AFM tracking laser illuminating the sample. The AFM tracking laser wavelength is ~850 nm (1.45 eV), which is slightly lower in energy than the band gap of CdTe (1.45-1.53 eV, depending on film stress)21–23, thus no appreciable number of photo-carriers were generated by that laser. A voltage bias was applied between the tip and the sample, and the conductive probe was rastered across


4

Page 5 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the sample in contact mode. During each scan the current flowing between the tip and the sample was measured at each scan pixel, enabling generation of a nanoscale-resolution map of electrical current through the sample. In all of the experiments reported here, the tip was grounded and the voltage bias was applied to the TCO layer underlying the junction. We have also made measurements with blanket sample illumination using an illuminator lamp (white light) that is part of the AFM instrument. Figures 1(a) and 1(b) show representative C-AFM current vs. sample voltage bias (I-V) plots along the grain bulk and the grain boundary regions for the as-deposited and CdCl2 annealed CdTe samples, respectively. These curves have been obtained from sample locations as marked in Figures 2(a) and 3(a). It can be seen that it is only possible to pass relatively high currents through the CdTe samples, both CdCl2 annealed and unannealed, for relatively high (compared to device operating conditions) sample biases. Supplementary Figure S1 and the associated discussion provide a more detailed view of the I-V curves measured from these samples.

Figure 1: C-AFM current vs. sample bias curves for the (a) as-deposited and (b) CdCl2 annealed CdTe sample. The data for grains and grain boundaries are taken at spots 1 and 2 as indicated in Figures 2(a) and 3(a), respectively. The estimates of the valence band energy in CdTe (6eV, from vacuum level to valence band maximum)24 and the work function of Pt (5.1-5.5 eV)25 indicate that we can


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 18

anticipate creation of a high Schottky barrier between the CdTe film and the Pt-probe. The p-n junction diode (CdS/CdTe) and the Schottky diode (CdTe/Pt) are oriented in opposite directions. Therefore one diode is always reverse biased and would be expected to determine the resistance of the device overall. This is indeed what we observe. With positive voltage applied to the sample, the back Schottky contact is forward biased and the reverse biased junction is the main diode. Thus, the behavior illustrated in Figure 1 for both samples is dominated by reverse breakdown in the main CdS/CdTe junction in the positive sample bias regime. This reverse current flowing through the p-n junction diode increases exponentially with voltage and is referred to as pre-breakdown reverse current. This has been a subject of many investigations in the past26–29 and some studies have concluded that such reverse currents primarily arise due to material defects/imperfections in the case of polycrystalline solar cells.28,29 We also observe that the current density in the light is much higher than in the dark for the untreated sample, even though the illumination is weak and no significant short circuit current is found. Figure 2(a) shows a deflection (derivative of topography) image for the asdeposited CdTe sample. We assume here that the grain boundaries are defined primarily by topography such that grains are delimited by the large consistent surface protrusions and the grain boundaries are the regions between these features. This is consistent with electron backscatter diffraction mapping measurements (not shown). The corresponding C-AFM current images acquired at +2V of sample bias on this as-deposited CdTe sample, without and with external illumination by the optical microscope light, are shown in Figures 2(b) and 2(c), respectively. Figure 2(d) is a superposition of areas of Figures 2(b) and 2(c), corresponding to the area indicated in Figure 2(a), after color-coding them red and green, respectively. Figures with an alternate color-coding scheme are presented in the supplementary material [Figure S2] to assist people with red-green color blindness. The areas of Figure 2(d) that appear red indicate greater current in the dark, while areas that appear green indicate greater current under light. The areas that appear yellow in Figure 2(d) indicate areas of higher current flow in the sample regardless of the illumination. The green colored appearance of several areas corresponding to grains in Figure 2(d) indicates that external illumination of the sample results in current flow along areas of the grains that did not conduct well in the absence of illumination [see also


6

Page 7 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1(a)].

Figure 2: C-AFM analysis of an as-deposited CdTe sample. (a) Deflection (derivative of topography) image for the scan area to highlight the topographical features. C-AFM current maps of the sample in (b) the absence, and (c) the presence of weak external sample illumination. (d) Superposition of red color-coded area of image (b) with green color-coded area of image (c) [corresponding to the dotted region in (a)]. Note: RMS roughness of the scan area is 165 nm. All images represent the same scan area and the scale bars represent 2 µm in length. The sample voltage bias is +2 V.

Figure 3(a) shows a deflection image for the CdCl2 annealed CdTe sample. The corresponding C-AFM current images obtained at +2V sample bias, without and with external illumination, are shown in Figures 3(b) and 3(c), respectively. Figure 3(d) is a


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 18

superposition of areas of Figures 3(b) and 3(c), corresponding to the area indicated in Figure 3(a), after color-coding them red and green, respectively. Figures 3(b) and 3(c) show some inter-grain current variations but most prominently reveal that the current conduction is much more uniform across all of the grains in the scan area as compared to the case for the as-deposited sample. The darker horizontal lines at the bottom of Figure 3(c) are scan artifacts which are likely a result of minor tip contamination during the repeated contact mode scans. Most grains in Figure 3(d) have a yellow appearance indicating that these correspond to areas of high C-AFM current flow in the sample regardless of external illumination. The absolute enhancement of the maximum C-AFM current in the treated sample is similar to that in the untreated sample, although the relative increase is smaller. As can be seen from Figure 3(d), several grain boundary areas are dark, which indicates that regardless of illumination there is little or no current flow along those grain boundaries in the reverse breakdown condition for the main junction. The fact that in Figures 3(b) and 3(c) the concave areas of the surface (along the grain boundaries) do not correlate with high current, rules out the possibility that sample topography artifacts (high current due to increased tip-sample contact area along the grain boundaries) dominate the C-AFM measurements.


8

Page 9 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: C-AFM analysis of a CdCl2 annealed CdTe sample. (a) Deflection (derivative of topography) image for the scan area to highlight the topographical features. C-AFM current maps of the sample in (b) the absence, and (c) the presence of weak external sample illumination. (d) Superposition of red color-coded area of image (b) with green color-coded area of image (c) [corresponding to the dotted region in (a)]. Note: The RMS roughness of scan area is 135 nm. All images represent the same scan area and the scale bars represent 2 µm in length. The sample voltage bias is +2 V.

We have also performed measurements under more extreme bias conditions, which are far from the conventional operation conditions of a CdTe solar cell. Figures 4(a) and 4(b) show the dark C-AFM current map on the as-deposited CdTe sample along the scan area as shown in Figure 2(a) at sample bias conditions of +6V and -6V, respectively. Under +6V sample bias [Figure 4(a)] the current is uniformly distributed across the scan area with both the grains and the grain boundaries acting as current


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 18

conduction paths. Under -6V sample bias [Figure 4(b)] the C-AFM currents observed are much lower due to the reverse biased Schottky contact at the CdTe/Pt junction. Figures 4(c) and 4(d) show the dark C-AFM current maps on the CdCl2 annealed CdTe sample along the scan area as shown in Figure 3(a) at sample bias conditions of -6V and +6V, respectively. Similar to the case of Figure 4(b), Figure 4(c) shows that a large part of the scan area has low levels of C-AFM current, due to reverse biased Schottky contact. Figure 4(d) shows the C-AFM current map for +6V sample bias, where the main junction is heavily reverse biased (as compared to operation conditions of the solar cell). It can be seen that under this measurement condition the largest C-AFM current flows along the grain boundary areas as compared with the grain interiors.

Figure 4: C-AFM current maps acquired at +6V [(a) and (d)] and -6V [(b) and (c)] sample bias for the as-deposited [(a) and (b)] and CdCl2 annealed [(c) and (d)] CdTe samples.


10

Page 11 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


When a Schottky contact is forward biased, it leads to a flow of majority carriers from the semiconductor into the metal.30 For the p-doped CdTe, this means that the CAFM current observed in the positive sample bias regime is normally composed of holes flowing from the CdTe layer into the Pt-tip. When the main CdS/CdTe junction is under sufficient reverse bias to be in a breakdown condition, it looses its rectifying characteristics and allows hole current to pass though the p-n junction relatively freely. The C-AFM current variations in this case would normally be expected to represent the variation in microscale electrical conductivity of the CdTe film for the flow of the majority charge carriers (holes) from the p-n junction to the Schottky contact. Recently, we demonstrated that the CdCl2 annealed CdTe samples exhibit carrier compensation and a reduced doping density along the grain boundaries due to incorporation of Cl. This produces a field in the grains along the grain boundaries that promotes exciton separation8. The Schottky diode is primarily a majority carrier device with a very small fraction of minority carrier injection current. However, at large forward voltage biases to the Schottky diode, a low acceptor doping in CdTe (for example in the grain boundaries) and a large Schottky barrier height for majority carrier flow, we could expect electron injection into CdTe along the grain boundaries to be more favorable than hole injection into the Pt-tip. These electrons would be collected spontaneously at the reverse-biased main junction to the grain boundary, since some grain boundaries run through the thickness of the CdTe31. We conclude that when the tip is positioned over the p-doped CdTe grains, the charge motion is holes into the tip from the grains at all positive sample biases and current is controlled by the main junction breakdown current. However, along the grain boundaries in the CdCl2 annealed sample, electron injection from the Pt-tip into the CdTe is likely and increases as the positive voltage bias is increased. This current may be collected directly by the reverse-biased main junction. In the negative sample bias regime, the p-n junction is forward biased and current flow should occur by injection of electrons into the p-type CdTe by the p-n junction. This current is limited due to either electron collection by the sample tip or by recombination with the reverse hole current in the Schottky contact.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 18

It can be seen from Figure 2(b) (for the as-deposited sample) that under positive sample bias conditions, in the absence of external illumination, the current flows from the TCO layer to the Pt-tip primarily along the grain interiors, presumably as holes. Further, there is large inhomogeneity in electrical conductivity from one grain to the other. This point is further illustrated by Supplementary Figures S1 (a) and (b), both displaying high variability in the current levels and “turn-on” voltages (the positive voltages at which the current starts to flow) among the various grains and the grain boundaries. It is also interesting to note the hysteresis in some of the I-V curves in Figures 1 and S1. We attribute this is to charge trapping in defects resulting from currents in the high sample bias regimes. The enhancement of the current in several grains [green areas in Figure 2(d)] demonstrates the photoconductivity effect, near the surface, of the CdTe film. Upon comparing Figures 1(a) and 1(b), and Figures 2(b) and 3(b) we can see that, in the +2V sample bias regime, the currents flowing through the CdTe film are a factor of ~8 greater after the CdCl2 anneal treatment as compared with the as-deposited samples. This indicates that the electrical conductivity of the CdTe film has improved upon the CdCl2 anneal treatment. By comparison of Figures 2(b) and 3(b) we can also see that after the CdCl2 anneal treatment the current distribution along the grains in the sample becomes much more homogenous as compared to the as-deposited sample. This point is also illustrated by Supplementary Figures S1(c) and S1(d), where we see that the I-V curves collected at various grain and grain boundary locations on the CdCl2 annealed sample are more well-behaved showing a more consistent “turn-on” voltage and resistance. Hence, the CdCl2 treatment makes the electrical properties of the grains in the CdTe film more homogenous and increases their electrical conductivity for the flow of holes. In Figures 3 (low bias) there is considerable variation in grain boundary conductivity relative to the surrounding grains including both significantly higher and significantly lower currents as shown in the supplementary Figure S3 where a line scan across part of the area in Figure 3(c) is shown and the topography and C-AFM currents are compared. This is consistent with our previous findings that not all grain boundaries show similar amounts of carrier depletion/compensation following CdCl2 treatment.8,9 We observe that at higher +6V bias [Figure 4(d)] there is strong contrast between the grains and grain boundaries with several boundaries conducting much higher current. In


12

Page 13 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


this case the current flow along the grain boundaries is due to electron injection from the Pt-tip. A key point here is that the holes injected into the grain interiors by the p-n junction under reverse breakdown do not simply flow through the highly conductive grain boundaries. They are blocked from the grain boundaries by the junction between the more p-type grains and the less p-type or somewhat n-type Cl-doped grain boundaries. In all cases where the grain boundaries are conductive the adjacent grains do not appear to transfer holes to those grain boundary conductive channels. Finally, we can see that some grain boundaries in Figure 4(d) are at similar current levels to the adjacent grains [one example marked by arrows in Figures 3(a) and 4(d)]. This further proves that the incorporation of Cl along the boundaries and the resultant doping effects are not uniform across all the grain boundaries. There is no major current contrast observed between the grains and the grain boundaries for the as-deposited sample at +6V bias, as shown in Figure 4(a). The variation in charge flow in the CdCl2 annealed sample along the grains and the grain boundaries under positive and negative sample bias conditions has been summarized in Figure 5. Lastly, it can be seen that the current values for the grain in Figure 1(b) undergo a change from super-linear to linear voltage dependence around +3V. The precise reason for this effect remains unclear and needs to be further investigated.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

Figure 5: Schematic energy band diagrams along the grains and grain boundaries for the CdCl2 annealed CdTe device at various applied sample bias conditions. The CdS film is not shown in the energy band diagram schematics under positive sample bias as the CdS/CdTe junction is under reverse breakdown, which is not the case under negative sample bias.

We conclude that CdCl2 treatment homogenizes and increases the electrical conductivity of CdTe films. Doping of the grain boundaries promotes charge separation, collection of electrons by the grain boundary, and their conduction to the front contact of the device.

This is supported by the observation here of high current in the grain


14

Page 15 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


boundary under conditions where electrons are expected to be the dominant charge carrier. In the CdCl2 annealed sample, the lack of conduction of holes injected into the grains through the grain boundaries to the back contact shows why recombination at the grain boundaries does not occur in the photovoltaic device under its normal operating conditions. In the complete device the p-type doping of the back of the CdTe by the back contact prevents shunting through the conductive grain boundaries. Finally, the photoconductivity effect observed here shows that exposure of CdTe film to light leads to promotion of current flow in both the grains and grain boundaries.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 18

Acknowledgements The authors are thankful for the financial support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (contract DE-EE0005405). This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois.

Supplementary Information Additional information on the current-voltage data, and C-AFM images. References (1) Press Release. First Solar Achieves yet Another Cell Conversion Efficiency World Record. February 23, 2016. (2) Fahrenbruch, A. L.; Bube, R. H., Chapter 9 - Polycrystalline Thin Films For Solar Cells. In Fundamentals of Solar Cells; Fahrenbruch, A. L., Bube, R. H., Eds.; Academic Press, 1983; pp 330–416. (3) Visoly-Fisher, I.; Cohen, S. R.; Ruzin, A.; Cahen, D. How Polycrystalline Devices Can Outperform Single-Crystal Ones: Thin Film CdTe/CdS Solar Cells. Adv. Mater. 2004, 16 , 879–883. (4) Sutter, P.; Sutter, E.; Ohno, T. R. High-Resolution Mapping of Nonuniform Carrier Transport at Contacts to Polycrystalline CdTe/CdS Solar Cells. Appl. Phys. Lett. 2004, 84, 2100–2102. (5) Poplawsky, J. D.; Paudel, N. R.; Li, C.; Parish, C. M.; Leonard, D.; Yan, Y.; Pennycook, S. J. Direct Imaging of Cl- and Cu-Induced Short-Circuit Efficiency Changes in CdTe Solar Cells. Adv. Energy Mater. 2014, 4, 1400454. (6) Li, H.; Liu, X. X.; Lin, Y. S.; Yang, B.; Du, Z. M. Enhanced Electrical Properties at Boundaries Including Twin Boundaries of Polycrystalline CdTe Thin-Film Solar Cells. Phys. Chem. Chem. Phys. 2015, 17, 11150–11155. (7) Li, C.; Wu, Y.; Poplawsky, J.; Pennycook, T. J.; Paudel, N.; Yin, W.; Haigh, S. J.; Oxley, M. P.; Lupini, A. R.; Al-Jassim, M.; et al. Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells. Phys. Rev. Lett. 2014, 112, 156103. (8) Tuteja, M.; Koirala, P.; MacLaren, S.; Collins, R.; Rockett, A. Direct Observation of Electrical Properties of Grain Boundaries in Sputter-Deposited CdTe Using ScanProbe Microwave Reflectivity Based Capacitance Measurements. Appl. Phys. Lett. 2015, 107, 142106. (9) Tuteja, M.; Koirala, P.; Palekis, V.; MacLaren, S.; Ferekides, C. S.; Collins, R. W.; Rockett, A. A. Direct Observation of CdCl2 Treatment Induced Grain Boundary Carrier Depletion in CdTe Solar Cells Using Scanning Probe Microwave Reflectivity Based Capacitance Measurements. J. Phys. Chem. C 2016, 120, 7020– 7024.


16

Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Ferekides, C. S.; Balasubramanian, U.; Mamazza, R.; Viswanathan, V.; Zhao, H.; Morel, D. L. CdTe Thin Film Solar Cells: Device and Technology Issues. Sol. Energy 2004, 77, 823–830. (11) McCandless, B. E.; Youm, I.; Birkmire, R. W. Optimization of Vapor PostDeposition Processing for Evaporated CdS/CdTe Solar Cells. Prog. Photovolt. Res. Appl. 1999, 7, 21–30. (12) Moutinho, H. R.; Al-Jassim, M. M.; Levi, D. H.; Dippo, P. C.; Kazmerski, L. L. Effects of CdCl2 Treatment on the Recrystallization and Electro-Optical Properties of CdTe Thin Films. J. Vac. Sci. Technol. A 1998, 16, 1251–1257. (13) Moutinho, H. R.; Hasoon, F. S.; Abulfotuh, F.; Kazmerski, L. L. Investigation of Polycrystalline CdTe Thin Films Deposited by Physical Vapor Deposition, Close spaced Sublimation, and Sputtering. J. Vac. Sci. Technol. A 1995, 13, 2877–2883. (14) McCandless, B. E.; Sites, J. R. Cadmium Telluride Solar Cells. In Handbook of Photovoltaic Science and Engineering; Luque, A., Hegedus, S., Eds.; John Wiley & Sons, Ltd, 2011; pp 600–641. (15) Harvey, S. P.; Teeter, G.; Moutinho, H.; Al-Jassim, M. M. Direct Evidence of Enhanced Chlorine Segregation at Grain Boundaries in Polycrystalline CdTe Thin Films via Three-Dimensional TOF-SIMS Imaging. Prog. Photovolt. Res. Appl. 2015, 23, 838–846. (16) Edwards, P. R.; Galloway, S. A.; Durose, K. EBIC and Luminescence Mapping of CdTe/CdS Solar Cells. Thin Solid Films 2000, 361–362, 364–370. (17) Galloway, S. A.; Edwards, P. R.; Durose, K. Characterisation of Thin Film CdS/CdTe Solar Cells Using Electron and Optical Beam Induced Current. Sol. Energy Mater. Sol. Cells 1999, 57, 61–74. (18) Visoly-Fisher, I.; Cohen, S. R.; Cahen, D. Direct Evidence for Grain-Boundary Depletion in Polycrystalline CdTe from Nanoscale-Resolved Measurements. Appl. Phys. Lett. 2003, 82, 556–558. (19) Brooks, W. S. M.; Irvine, S. J. C.; Taylor, D. M. Scanning Kelvin Probe Measurements on As-Doped CdTe Solar Cells. Semicond. Sci. Technol. 2013, 28, 105024. (20) Smith, S.; Zhang, P.; Gessert, T.; Mascarenhas, A. Near-Field Optical BeamInduced Currents in CdTe⁄CdS Solar Cells: Direct Measurement of Enhanced Photoresponse at Grain Boundaries. Appl. Phys. Lett. 2004, 85, 3854–3856. (21) Ferekides, C. S.; Marinskiy, D.; Viswanathan, V.; Tetali, B.; Palekis, V.; Selvaraj, P.; Morel, D. L. High Efficiency CSS CdTe Solar Cells. Thin Solid Films 2000, 361–362, 520–526. (22) Green, M. A. Radiative Efficiency of State-of-the-Art Photovoltaic Cells. Prog. Photovolt. Res. Appl. 2012, 20, 472–476. (23) Koirala, P.; Tan, X.; Li, J.; Podraza, N. J.; Marsillac, S.; Rockett, A. A.; Collins, R. W. Mapping Spectroscopic Ellipsometry of CdTe Solar Cells for PropertyPerformance Correlations. In Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th; 2014; pp 0674–0679. (24) Kumar, S. G.; Rao, K. S. R. K. Physics and Chemistry of CdTe/CdS Thin Film Heterojunction Photovoltaic Devices: Fundamental and Critical Aspects. Energy Environ. Sci. 2013, 7, 45–102.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 18

(25) Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48, 4729–4733. (26) Alonso-García, M. C.; Ruíz, J. M. Analysis and Modelling the Reverse Characteristic of Photovoltaic Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 1105– 1120. (27) Breitenstein, O.; Bauer, J.; Wagner, J.-M.; Lotnyk, A. Imaging Physical Parameters of Pre-Breakdown Sites by Lock-in Thermography Techniques. Prog. Photovolt. Res. Appl. 2008, 16, 679–685. (28) Lausch, D.; Petter, K.; Bakowskie, R.; Czekalla, C.; Lenzner, J.; Wenckstern, H. von; Grundmann, M. Identification of Pre-Breakdown Mechanism of Silicon Solar Cells at Low Reverse Voltages. Appl. Phys. Lett. 2010, 97, 73506. (29) Breitenstein, O.; Bauer, J.; Lotnyk, A.; Wagner, J.-M. Defect Induced Non-Ideal Dark – Characteristics of Solar Cells. Superlattices Microstruct. 2009, 45, 182– 189. (30) Sze, S. m.; Ng, K. K. Metal-Semiconductor Contacts. In Physics of Semiconductor Devices; John Wiley & Sons, Inc., 2006; pp 134–196. (31) Koirala, P.; Li, J.; Yoon, H. P.; Aryal, P.; Marsillac, S.; Rockett, A. A.; Podraza, N. J.; Collins, R. W. Through-the-Glass Spectroscopic Ellipsometry for Analysis of CdTe Thin-Film Solar Cells in the Superstrate Configuration. Prog. Photovolt. Res. Appl. 2016, 24, 1055–1067.


18

CdCl2 Treatment-Induced Enhanced Conductivity ... - ACS Publications

Recommend Documents