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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 11150
Enhanced electrical properties at boundaries including twin boundaries of polycrystalline CdTe thin-film solar cells†
Received 29th January 2015, Accepted 31st March 2015
H. Li,ab X. X. Liu,*ab Y. S. Lin,b B. Yangab and Z. M. Duab
DOI: 10.1039/c5cp00564g www.rsc.org/pccp
The effect of grain boundaries (GBs), in particular twin boundaries (TBs), on CdTe polycrystalline thin films is studied by conductive atomic force microscopy (C-AFM), electron-beam-induced current (EBIC), scanning Kelvin probe microscopy (SKPM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). Four types of CdTe grains with various densities of {111} R3 twin boundaries (TBs) are found in Cl-treated CdTe polycrystalline thin films: (1) grains having multiple {111} R3 TBs with a low angle to the film surface; (2) grains having multiple {111} R3 TBs parallel to the film surfaces; (3) small grains on a scale of not more than 500 nm, composed of Cd, Cl, Te, and O; and (4) CdTe grains with not more than two {111} R3 TBs. Grain boundaries (including TBs) exhibit enhanced current transport phenomena. However, the {111} R3 TB is much more beneficial to micro-current transport. The enhanced current transport can be explained by the lower electron potential at GBs (including TBs) than the grain interiors (GIs). Our results open new opportunities for enhancing solar cell performances by controlling the grain boundaries, and in particular TBs.
Introduction The widespread implementation of solar-cell technology is hindered by its relatively high cost compared to conventional energy sources, such as petroleum and natural gas. Polycrystalline thin-film photovoltaic (PV) devices, such as cadmium telluride (CdTe),1 copper indium gallium selenide (CIGS),2 and perovskite,3,4 show great potential to significantly reduce the cost of solar electricity. In particular, CdTe solar cells have the shortest energy payback time among all PV technologies, and have the second largest market a
The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected] b Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China † Electronic supplementary information (ESI) available: Details of CdTe solar cell fabrication, characterization, EBSD, EBIC, C-AFM, and SKPFM results, Table S1 and Fig. S1–S13. See DOI: 10.1039/c5cp00564g
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share after the conventional crystalline silicon technology.5,6 Recently, substantial progress has been made in the efficiency of CdTe cells, with the best laboratory and module efficiencies being 21% and 17.5%, respectively.7 However, the currently available commercial modules are still not competitive with the efficiencies of single-crystal Si modules (16–17%). Therefore, improving the energy conversion efficiency of CdTe solar cells will further reduce their cost, making their widespread application more feasible. The energy conversion efficiency of a solar cell is closely related to the carriers’ generation, separation, transport, collection, and recombination processes. Two-dimensional defects, such as grain boundaries (GBs) and grain surfaces in polycrystalline thin films, have critical effects on these processes in thin film solar cells such as CdTe and CIGS.8–13 However, understanding the true effect of GBs on micro-electrical processes in these thin film solar cells still remains a challenge, especially in the case of CdTe solar cells. GBs are rich in a high density of defects, including dangling bonds, dislocations, and impurities, leading to deep level centers (i.e. trap and recombination centers for carriers).15,16 Therefore, GBs are usually considered to be trapping and recombination centers for carriers, thereby limiting the device’s performance.14–16 GBs can also form potential barriers, hindering carrier conduction.15,16 Additionally, some researchers have found that GBs exhibit higher recombination rates (Sx B 104 cm s 1) than those of the grain interiors.17 Contrastingly, other researchers have found that after Cl treatment (a key step to improve the CdTe efficiency),18 the GBs no longer act as recombination centers, but actively contribute to carrier collection.8,9,19–22 The increased carrier collection at the GBs is explained by the presence of a local space charge region with a width of about 100–350 nm, which is beneficial to electron–hole pair separation.8,9 It is worth noting, however, that these studies focused on GBs without consideration of twin boundaries (TBs).8,9,22 In CdTe polycrystalline thin films, TBs, such as the S3 coincident site lattice (CSL), are quite common.23–27 A 18.6%-efficient CdTe solar cell made by GE also shows high density of TBs.27
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Therefore, the effect of GBs (and, in particular, TBs) on carrier behavior requires further exploration. In this communication, conductive atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM), and electron beam induced current (EBIC) analyses were conducted to investigate the micro-electrical properties of CdTe solar cells. The GBs, including TBs, were found to enhance current transport phenomena in CdTe solar cells. These results are beneficial to device engineering and the improvement of the performance of CdTe polycrystalline thin film solar cells, and therefore other chalcogenide solar cells, such as CIGS.
Results and discussion Experimentally, the CdTe solar cells were deposited by the low temperature radio frequency magnetron sputtering method (see ESI† for details). The sample preparation for EBSD, CAFM, and other methods can also be found in the ESI.† X-ray diffraction (XRD) patterns reveal that Cl-treated CdTe exhibits a zincblende structure.28 The GB type and grain orientation of Cl-treated thin films with Cu incorporation (Cu incorporation is important for fabrication of low-resistance back contact) were characterized by electron backscatter diffraction (EBSD), as shown in Fig. 1a, b and Fig. S1 (ESI†). The chaotic assortment of colors (Fig. 1b) indicates that the CdTe grains show random growth after the Cl treatment. There are three main types of GBs: i-type: GBs between neighboring lattices differing in orientation by a high angle (i.e., more than 151, high angle boundaries, about 98%), (Fig. 1a and Fig. S1a, b, ESI†); ii-type: boundaries between
Fig. 1 (a) Electron backscatter diffraction (EBSD) grain boundary map of a Cl-treated and Cu-incorporated CdTe film. Blue color: random grain boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801); red color: S3 CSL boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801); green color: boundaries between neighboring lattices differing in orientation by an angle of (5–151). (b) EBSD map of a Cl-treated and Cu-incorporated CdTe film. The colors are based on the crystallographic orientation according to the key on the right side. (c) High-resolution transmission electron microscopy (HRTEM) image of CdTe solar cells. (d) HRTEM image of the red dotted rectangular region in (c).
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neighboring lattices differing in orientation by a low angle (i.e., 2–51); and iii-type: other boundaries. Importantly, around 53.2% of grain boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801) are CSL boundaries (Fig. S1c and d, ESI†), 47% of which are S3 TBs (Fig. S1c and d, ESI†). Moreover, almost all the grains with a grain size larger than 1 mm contain at least one TB. Many grains contain even more than ten TBs (Fig. 1a and Fig. S1, ESI†). High-resolution transmission electron microscopy (HRTEM) was further conducted to characterize the cross-section of CdTe solar cells. It is seen from the HRTEM images (Fig. 1c and d) that many TBs exist in the Cl-treated CdTe thin films. The calculated d value is 0.38 nm, which is consistent with the {111} space of CdTe (d = 0.37 nm). This demonstrates that the CSL boundaries are in fact {111} TBs. The ratio of coincidence sites to the total number of sites of the two grains is 1/3 (Fig. 1c and d), indicating that the CSL boundaries are S3 CSL TBs.29 Therefore, the EBSD and HRTEM results show that the CSL TBs are most {111} S3 CSL TBs. We speculate that the formation of {111} S3 CSL TBs is the result of low stacking fault formation energy (9.0 1 mJ m 2).23 In the Cl-treated CdTe, {111} S3 CSL TBs were always observed.23–27,30 We then employed C-AFM measurements to study the local electric properties of CdTe polycrystalline thin films at the nanoscale. The CdTe films were scanned using a Pt/Ir-coated conductive probe (SCM-PIT, parameters are shown in the ESI†) in a contact mode for morphology, and with a different DC voltage applied to the tip for current mapping analysis. SKPM measurements were also conducted, for which the morphology image was obtained in a contact mode, after which the tip was raised to 100 nm from the sample surface for the surface potential scan (Fig. S2, ESI†). The roughness of the sputtered CdTe surface was in the range of 20–30 nm,7 which was small enough to conduct C-AFM and SKPM measurements directly on the surface without any etching. SnO2:F (FTO) was grounded during the C-AFM current mapping and SKPM potential scans (Fig. S2, ESI†). We first performed C-AFM measurements on the surface of CdTe with Cl treatment and Cu incorporation, and the result is shown in Fig. 2. The deflection-error image is sensitive to the height difference (Fig. S3, ESI†). Therefore, the GBs are quite clear in the deflection-error image. According to the S3 CSL TB type, four types of grains were observed in the height-sensor images of the CdTe films with Cl treatment and Cu incorporation (Fig. 2a and b, Fig. S3, ESI†): Type-1 grains with multiple {111} S3 TBs with a low angle to the CdTe film surface, Type-2 grains with multiple {111} S3 TBs parallel to the CdTe surfaces, more clearly seen in Fig. S4 (ESI†), Type-3 (small) grains with a grain size of B500 nm or less, composed of Cd, Cl, Te, and O, (see our published paper18 for further information and Fig. S5, ESI†), and Type-4 CdTe grains with not more than two {111} S3 CSL TBs, are possibly perpendicular to the film’s surface. Typical grains of each type have been labeled with the corresponding number in Fig. 2a, c–f. The four types of grains are clearly seen from the SEM images (Fig. S4, ESI†). Type-3 grains
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Fig. 3 (a) Atomic force microscopy (AFM) height-sensor image. (b) Conductive AFM (C-AFM) current mapping image of the Cl-treated and Cu-incorporated CdTe surface with multiple Type-1 S3 TBs with distances of only B100 nm between each other. The C-AFM current mapping image was obtained with 1.0 V DC tip voltage and no specially applied illumination. (c) Corresponding AFM height-sensor image of CdTe shown in the rectangular box of (a). (d) Corresponding C-AFM current mapping image of CdTe shown in the rectangular box of (b). The scale bar in (a and b) is 1 mm. The scale bar in (c and d) is 0.1 mm. Fig. 2 Atomic force microscopy (AFM) results for CdTe with Cl treatment and Cu incorporation. (a) AFM deflection-error image of Cl-treated CdTe with Cu incorporation. (b) Sketch of the four types of CdTe grains according to the S3 CSL TB varieties. (c–f) Corresponding conductive AFM (C-AFM) current mapping images with different DC tip voltages (TVs) of 0–0.8 V. The maximum current value of the (g) Type-1, (h) Type-2, and (i) Type-3 CdTe grains under different DC TVs. The scale bar is 1 mm.
are not found in the EBSD image because of the fact that it is only covered on the CdTe surfaces. I–V characteristics of these four types of grains are quite different. For Type-1 CdTe grains, the TBs are clearly seen in the deflection-error image (Fig. 2a, labeled by number 1). It is seen that, in such types of grains, current transport is enhanced at TBs according to the C-AFM current mapping (Fig. 2c–f). The current value is actually the largest along the TBs (Fig. 2c, Fig. S6, ESI†), even with neither intentional illumination (diffused light only) nor with DC tip bias voltage. The largest current value is 7.96 nA, with a 0 V tip DC bias. As the tip DC voltage increases from 0 V to 0.8 V, the current value along TBs gradually increases, as shown in Fig. 2. The largest current value can reach 77.65 nA (Fig. 2g) with a tip DC voltage of 0.8 V. However, the conductive area along the TBs remains unchanged. For some area away from TBs, such as the area marked by the rectangle in Fig. 3a and b, it appears that the grain interiors show a larger current value (Fig. 3b). However, this is caused by the overlapping multiple {111} S3 CSL TBs, as revealed under a high-resolution scan. This area is actually composed of highdensity TBs with distances of only B100 nm between each other, as clearly seen in the higher magnification AFM height-sensor image in Fig. 3c. In fact, the current values are still larger at TBs than those at the grain interiors (Fig. 3d). Thus, it is seen that the {111} S3 TBs are major carrier transport paths with a tip DC voltage of 0–1.0 V.
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For the Type-2 CdTe grains, the current value is also relatively large at the {111} S3 TBs. However, at 0 V tip DC bias, the current value along the {111} S3 TB is only 5.905 nA (Fig. 2h), which is smaller than that of Type-1 grains. The current value in the {111} S3 TBs gradually increases (Fig. 2c–f, h) as the tip DC bias increases to 0.8 V. The highest current value is 40.9 nA with a 0.8 V tip DC bias (Fig. 2h). The conductive area along the TBs enlarges with the increase of tip DC bias. For example, the conductive region of the ‘‘2’’ CdTe grain shown in Fig. 2c–f along the horizontal direction increases from 0.588 mm to 0.627 mm as the tip DC voltage increases from 0 V to 0.8 V. The TB-enhanced current transport phenomenon is quite common in our CdTe polycrystalline films with Cl treatment and Cu incorporation. Another typical example is shown in Fig. S7 (ESI†), in which the Type-2 grains are quite clearly seen. Type-3 grains show an enhanced current transport around the grain boundaries (Fig. 2, Fig. S6a and b, ESI†). However, the current value is much smaller than that of Type-1 and Type-2 grains. The maximum current value is only 1.725 nA at 0 V tip DC bias (Fig. 2i). As the tip DC voltage increases from 0 V to 0.8 V, the maximum current increases from 1.725 nA to 4.505 nA (Fig. 2i), and the length of the conductive region along the horizontal direction also increases. The I–V curves for the region with obvious current in the Type-1, -2, and -3 grains were measured by varying the voltage from 2 to 2 V. The typical I–V curve is shown in Fig. S8 (ESI†). Accordingly, conductive regions for Type-1, -2, and -3 grains exhibit clear diode characteristics (Fig. S8, ESI†). For the Type-4 grains with not more than two {111} S3 TBs, in which the random high-angle GBs are the main boundaries, very low current was collected with the tip DC bias between 0 V and 0.8 V when no additional light was applied (except for diffused light from the laser diode of the AFM system), as shown in Fig. 2c–i.
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Fig. 4 (a) Atomic force microscopy (AFM) height-sensor image of Cl-treated and Cu-incorporated CdTe thin films. (b) Scanning Kelvin probe microscopy (SKPM) image of Cl-treated and Cu-incorporated CdTe thin films. (c) Section of the AFM height-sensor curve along the line in (a). (d) Corresponding SKPM curve along the line in (a).
This may be due to the lower volume for photocarrier generation in the TB-produced local junction on the light path. As shown in Fig. 4d, the lateral local junction produced by TBs only extends to a scale of about a couple of hundred nanometers. When the TBs oriented at a high angle to the film surface, as in Type-4 grains, only the end of the TBs on the shallow surface (around 200 nm below the film surface for the 633 nm red laser light) could absorb light, due to the high absorption coefficient of CdTe.5 This could lead to a current collected by the local junction of TBs lower than the sensitivity of the AFM equipment, which is 100 pA. While, due to the low-angle orientation to the film surface, the volume for photocarrier generation in a single TB of Type-1 and Type-2 grains is much larger than that in Type-4 grains. However, the random high-angle GB enhanced current transport is observed in Type-4 grains when illuminated by homemade LED arrays through the glass side (Fig. S9 and S10, ESI†). Therefore, in CdTe solar cells, {111} S3 CSL TBs are much more electrically beneficial than random high-angle GBs and other CSL TBs. This is the first direct observation that {111} S3 CSL TBs are electrically beneficial, which has been theoretically predicted by Yoo et al.31 To understand the transport difference between the grain interior and GBs especially TBs, SKPM was conducted on the surface of Cl-treated and Cu-incorporated CdTe films. The distance between the CdTe surface and the tip was kept at 100 nm during
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the SKPM scan. The SKPM result is shown in Fig. 4b and d. A potential difference is observed in the GBs including TBs, and grain interiors (Fig. 4). From SKPM mapping and the corresponding cross-sectional images (Fig. 4) of Cl-treated CdTe, the boundaries have a larger work function than that of grain interiors, leading to a potential gradient between the boundaries and grain interiors along the lateral direction.32,35 The potential shows a close relationship with the surface roughness. In order to exclude the distance-induced potential difference, we can turn our attention to the grains circled in red and green in Fig. 4a and b. For the CdTe grains circled in green, the distance between the tips is smaller than that of the GBs circled in red. However, the potential is higher than that of the GBs circled in red (Fig. 4b). Thus, the potential difference is mainly due to localized material properties, rather than surface morphology. Combined with C-AFM with different tip DC voltages, EBIC (Fig. S11, ESI†), and the potential measurements, the current difference between the boundaries and grain interiors is due to a potential difference. The potential difference leads to a local electric field between the grain interior and boundaries, which aids in electron and hole separation in the lateral direction. The lower potential at GBs and TBs attracts electrons into the boundaries and moves holes into the grain interiors.33 This lateral potential difference is helpful in separating and transporting free electrons and holes in CdTe, which reduces the recombination rate and is electrically beneficial to the CdTe solar cell performance. Therefore, in CdTe devices, boundaries (especially TBs) are electrically favorable to the carrier’s separation, transport, and collection. Fig. 5 schematically shows the local band-diagram between the grain interior and boundaries including TBs (Fig. 5a) and the electron and hole flowing paths in the CdTe film composed of Type-1, 2, 3, and 4 grains (Fig. 5c) during SKPM acquisition. It is worth noting that the illustration of Type-1 and 2 grains in Fig. 5c is according to our direct observation from the scanningTEM (STEM) cross-sectional image of our devices, as shown in Fig. 5d. From the aspect of electrical properties, the top bending of the band-diagram at grain interiors (Fig. 4d and 5a) could lead to local tunneling contacts to the metal back electrode in the complete device, as shown in Fig. 5b. Since holes converge in the grain-interiors, the surfaces of grain interiors could work as the best channels for the transport of majority carriers into the back electrode with a smaller work function than CdTe, such as Au. Meanwhile, from the aspect of surface morphology, the back electrode has a better chance to contact the surface of grain-interiors than GBs, due to larger height than that at GBs (Fig. 4a and 5c). These two aspects could explain the feasibility of flexible materials as a back electrode to CdTe cells, such as graphite paste,6 graphene,6 and carbon single wall nanotubes34 when contact with the valleys of the CdTe film surface is less possible than vacuum deposited metal layers. The hole-rich surfaces of CdTe grain-interiors can also explain the formation of good ohmic contact though the less conductive Type 3 grains exist on the film surface, which mostly locate at the boundaries (Fig. S5, ESI†).28 Type-1 and Type-2 grains may be the same if we only consider the TB compositions. However, the electron and hole transport
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but only one order of magnitude higher than that of Te at 550 1C (CSS). The potential of grain boundaries of as-grown CdTe is lower than that of grain interiors (Fig. S12, ESI†), but similar to that of Cl-treated and Cu-incorporated CdTe. The results show that the potential difference between boundaries and grain interiors is not completely due to the Cl treatment in sputtered CdTe films. Therefore, the origin of the potential difference between boundaries and grain interiors of CdTe should be further investigated.
Conclusions Fig. 5 (a) A sketch showing the band diagram of the lateral local electric field between the grain interior and boundaries including TBs. (b) The band diagram of the CdTe surface and the back electrode with a work function smaller than CdTe. The dotted lines represent the Schottky barrier in CdTe without heavy doping while the solid lines represent the tunneling contact due to top bending of the band diagram. (c) A sketch showing the electron and hole flow direction in the CdTe solar cell composed of Type-1, 2, 3, and 4 grains. The holes move towards the back electrode along the CdTe surface. The electrons move towards the front electrode mainly along the TBs and converge at boundaries. (d) Bright-field STEM image of the CdTe solar cell.
path is different in Type-1 and Type-2 grains. For Type-1 grains (TBs oriented at shallow angles to the film surface), electrons could reach the main junction along the TBs without the assistance of GBs, which is understood from the STEM cross-sectional image of the device (Fig. 5d). However, for Type-2 grains, electrons and holes are separated near TBs and transport in different sides of random large-angle GBs. In this case, random large-angle GBs may be necessary for conducting channels for electron and hole transport in Type-2 grains. Some researchers found that the Cl substitution of Te at GBs is the reason for the potential difference between GBs and grain interiors.21 GB-enhanced carrier transport was not found in as-grown CdTe. However, these studies were focused on hightemperature CdTe thin films. For low-temperature CdTe films, no such studies have been conducted. Accordingly, we conducted SKPM (Fig. S12, ESI†) and C-AFM (Fig. S13, ESI†) measurements on as-grown CdTe thin films grown by magnetron sputtering. Even though no current can be detected when the tip DC bias was less than 1 V, the enhanced current at GBs was observed (Fig. S13, ESI†) with a tip bias of 3 V. From height-sensor and C-AFM current mapping images (Fig. S13a and b, ESI†), it is seen that as-grown CdTe shows GB-enhanced current transport, although the current value is quite small (100 picoamps). This result is quite different from that of as-grown CdTe polycrystalline thin films obtained by the close space sublimation (CSS) method.8,9,21 CdTe obtained by the CSS method shows enhanced current transport at the grain interior.8,9,21 This may be due to the higher density of cadmium vacancies in as-grown CdTe films deposited by sputtering, as compared to those by CSS. This can be explained by the fact that the saturation pressure of Cd is two orders of magnitude higher than that of Te at 270 1C (sputtering),
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We have found that the {111} S3 CSL TBs were the most electrically beneficial type of boundary to the performance of CdTe cells. GB, including TB, enhanced current transport was directly observed in sputtered CdTe films after Cl treatment and Cu incorporation. SKPM and EBIC measurements confirm that the enhanced current transport can be explained by the local lateral field between the grain boundaries and grain interiors. This local lateral field is beneficial to the separation, transport, and collection of carriers. We speculate that these results may open new opportunities for enhancing solar cell performances by controlling the grain boundaries, especially by controlling TBs.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51472239, and 61274060), Youth Innovation Promotion Association, CAS (ARP No.: Y410421C41), the 100 Talents Program of IEE CAS (ARP No: Y010411C41), the 100 Talents Preferred Support Plan of the CAS (ARP No: Y210431C41), Innovative and Interdisciplinary Team Award (project: Highefficiency Utilization of Renewable Energy Resources Innovative and Interdisciplinary Team).
References 1 J. D. Major, R. E. Treharne and K. Durose, Nature, 2014, 511, 334–337. 2 J. R. Tuttle, M. A. Contreras, T. J. Gillespie, K. R. Ramanathan, A. L. Tennant, J. Keane, A. M. Gabor and R. Noufi, Prog. Photovoltaics, 1995, 3, 235–250. 3 G. Hodes, Science, 2013, 342, 317–318. 4 R. F. Service, Science, 2014, 344, 458. 5 A. Luque and S. Hegedus, Handbook of Photovoltaic Science and Engineering, 2nd edn, Wiley, 2011, p. 600. 6 S. G. Kumar and K. S. R. K. Rao, Energy Environ. Sci., 2014, 7, 45–102. 7 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2015, 23, 1–9. 8 I. Visoly-Fisher, S. R. Cohen, A. Ruzin and D. Cahen, Adv. Mater., 2004, 16, 879–883. 9 I. Visoly-Fisher, S. R. Cohen, K. Gartsman, A. Ruzin and D. Cahen, Adv. Funct. Mater., 2006, 16, 649–660. 10 M. Takihara, T. Minemoto, Y. Wakisaka and T. Takahashi, Prog. Photovoltaics, 2013, 21, 595–599.
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11 K. Durose, S. E. Asher, W. Jaegermann, D. Levi, B. E. McCandless, W. Metzger, H. Moutinho, P. D. Paulson, C. L. Perkins and J. R. Sites, Prog. Photovoltaics, 2004, 12, 177–217. 12 K. L. Chopra, P. D. Paulson and V. Dutta, Prog. Photovoltaics, 2004, 12, 69–92. 13 M. Hafemeister, S. Siebentritt, J. Albert, M. C. Lux-Steiner and S. Sadewasser, Phys. Rev. Lett., 2010, 104, 196602. 14 S. K. Das and G. C. Morris, J. Appl. Phys., 1992, 72, 4940. 15 J. D. Major, Y. Y. Proskuryakov, K. Durose, G. Zoppi and I. Forbes, Sol. Energy Mater. Sol. Cells, 2010, 94, 1107–1112. 16 B. G. Mendis, L. Bowen and Q. Z. Jiang, Appl. Phys. Lett., 2010, 97, 092112. 17 J. Moseley, M. M. Al-Jassim, H. R. Moutinho, H. L. Guthrey, W. K. Metzger and R. K. Ahrenkiel, Appl. Phys. Lett., 2013, 103, 233103. 18 H. Li, Y. Biao, X. X. Liu and P. J. Wang, RSC Adv., 2014, 4, 5046–5054. 19 P. R. Edwards, S. A. Galloway and K. Durose, Thin Solid Films, 2000, 361, 364–370. 20 P. R. Edwards, S. A. Galloway and K. Durose, Thin Solid Films, 2000, 372, 284–291. 21 C. Li, Y. Wu, J. Poplawsky, T. J. Pennycook, N. Paudel, W. Yin, S. J. Haigh, M. P. Oxley, A. R. Lupini, M. Al-Jassim, S. J. Pennycook and Y. Yan, Phys. Rev. Lett., 2014, 112, 156103. 22 S. L. Marina, A. Maxim, J. L. Henri, G. Anthony, A. A. Talin and B. Z. Nikolai, ACS Nano, 2014, 8, 11883–11890. 23 T. Paulauskas, C. Buurma, E. Colegrove, Z. Guo, S. Sivananthan, M. K. Y. Chan and R. F. Klie, Appl. Phys. Lett., 2014, 105, 071910.
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24 H. R. Moutinho, R. G. Dhere, M. J. Romero, C. S. Jiang, B. To and M. M. Al-Jassim, J. Vac. Sci. Technol., A, 2008, 26, 1068. 25 D. J. Smith, S.-C. Y. Tsen, Y. P. Chen, J.-P. Faurie and S. Sivananthan, Appl. Phys. Lett., 1996, 67, 1591. 26 Y. Xin, N. D. Browning, R. Rujirawat, S. Sivananth, Y.-P. Chen, P. D. Nellist and S. J. Pennycook, J. Appl. Phys., 1998, 84, 4292. 27 B. A. Korevaar, J. R. Cournoyer, O. Sulima, A. Yakimov and J. N. Johnson, Prog. Photovoltaics, 2014, 22, 1040–1049. 28 H. Li and X. X. Liu, Sol. Energy, 2015, 115, 603–612. 29 V. Consonni, N. Baier, O. Robach, C. Cayron, F. Donatini and G. Feuillet, Phys. Rev. B: Condens. Matter Mater. Phys., 2014, 89, 035310. 30 C. Li, J. Poplawsky, Y. Wu, A. R. Lupini, A. Mouti, D. N. Leonard, N. Paudel, K. Jones, W. Yin, M. Al-Jassim, Y. Yan and S. J. Pennycook, Ultramicroscopy, 2013, 134, 113–125. 31 S. H. Yoo, K. T. Butler, A. Soon, A. Abbas, J. M. Walls and A. Walsh, Appl. Phys. Lett., 2014, 105, 062104. 32 C. S. Jiang, M. A. Contreras, I. Repins, H. R. Moutinho, Y. Yan, M. J. Romero, L. M. Mansfield, R. Noufi and M. M. Al-Jassim, Appl. Phys. Lett., 2012, 101, 033903. 33 M. Nonnenmacher, M. P. O’Boyle and H. K. Wickramasinghe, Appl. Phys. Lett., 1991, 58, 2921. 34 A. B. Phillips, R. R. Khanal, Z. Song, R. M. Zartman, J. L. DeWitt, J. M. Stone, P. J. Roland, V. V. Plotnikov, C. W. Carter, J. M. Stayancho, R. J. Ellingson, A. D. Compaan and M. J. Heben, Nano Lett., 2013, 13(11), 5224. 35 S. Sadewasser and T. Glatzel, Kelvin Probe Force Microscopy, Springer, 2012, p. 13.
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Cite this: Phys. Chem. Chem. Phys., 2015, 17, 11150
Enhanced electrical properties at boundaries including twin boundaries of polycrystalline CdTe thin-film solar cells†
Received 29th January 2015, Accepted 31st March 2015
H. Li,ab X. X. Liu,*ab Y. S. Lin,b B. Yangab and Z. M. Duab
DOI: 10.1039/c5cp00564g www.rsc.org/pccp
The effect of grain boundaries (GBs), in particular twin boundaries (TBs), on CdTe polycrystalline thin films is studied by conductive atomic force microscopy (C-AFM), electron-beam-induced current (EBIC), scanning Kelvin probe microscopy (SKPM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). Four types of CdTe grains with various densities of {111} R3 twin boundaries (TBs) are found in Cl-treated CdTe polycrystalline thin films: (1) grains having multiple {111} R3 TBs with a low angle to the film surface; (2) grains having multiple {111} R3 TBs parallel to the film surfaces; (3) small grains on a scale of not more than 500 nm, composed of Cd, Cl, Te, and O; and (4) CdTe grains with not more than two {111} R3 TBs. Grain boundaries (including TBs) exhibit enhanced current transport phenomena. However, the {111} R3 TB is much more beneficial to micro-current transport. The enhanced current transport can be explained by the lower electron potential at GBs (including TBs) than the grain interiors (GIs). Our results open new opportunities for enhancing solar cell performances by controlling the grain boundaries, and in particular TBs.
Introduction The widespread implementation of solar-cell technology is hindered by its relatively high cost compared to conventional energy sources, such as petroleum and natural gas. Polycrystalline thin-film photovoltaic (PV) devices, such as cadmium telluride (CdTe),1 copper indium gallium selenide (CIGS),2 and perovskite,3,4 show great potential to significantly reduce the cost of solar electricity. In particular, CdTe solar cells have the shortest energy payback time among all PV technologies, and have the second largest market a
The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected] b Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China † Electronic supplementary information (ESI) available: Details of CdTe solar cell fabrication, characterization, EBSD, EBIC, C-AFM, and SKPFM results, Table S1 and Fig. S1–S13. See DOI: 10.1039/c5cp00564g
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share after the conventional crystalline silicon technology.5,6 Recently, substantial progress has been made in the efficiency of CdTe cells, with the best laboratory and module efficiencies being 21% and 17.5%, respectively.7 However, the currently available commercial modules are still not competitive with the efficiencies of single-crystal Si modules (16–17%). Therefore, improving the energy conversion efficiency of CdTe solar cells will further reduce their cost, making their widespread application more feasible. The energy conversion efficiency of a solar cell is closely related to the carriers’ generation, separation, transport, collection, and recombination processes. Two-dimensional defects, such as grain boundaries (GBs) and grain surfaces in polycrystalline thin films, have critical effects on these processes in thin film solar cells such as CdTe and CIGS.8–13 However, understanding the true effect of GBs on micro-electrical processes in these thin film solar cells still remains a challenge, especially in the case of CdTe solar cells. GBs are rich in a high density of defects, including dangling bonds, dislocations, and impurities, leading to deep level centers (i.e. trap and recombination centers for carriers).15,16 Therefore, GBs are usually considered to be trapping and recombination centers for carriers, thereby limiting the device’s performance.14–16 GBs can also form potential barriers, hindering carrier conduction.15,16 Additionally, some researchers have found that GBs exhibit higher recombination rates (Sx B 104 cm s 1) than those of the grain interiors.17 Contrastingly, other researchers have found that after Cl treatment (a key step to improve the CdTe efficiency),18 the GBs no longer act as recombination centers, but actively contribute to carrier collection.8,9,19–22 The increased carrier collection at the GBs is explained by the presence of a local space charge region with a width of about 100–350 nm, which is beneficial to electron–hole pair separation.8,9 It is worth noting, however, that these studies focused on GBs without consideration of twin boundaries (TBs).8,9,22 In CdTe polycrystalline thin films, TBs, such as the S3 coincident site lattice (CSL), are quite common.23–27 A 18.6%-efficient CdTe solar cell made by GE also shows high density of TBs.27
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Therefore, the effect of GBs (and, in particular, TBs) on carrier behavior requires further exploration. In this communication, conductive atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM), and electron beam induced current (EBIC) analyses were conducted to investigate the micro-electrical properties of CdTe solar cells. The GBs, including TBs, were found to enhance current transport phenomena in CdTe solar cells. These results are beneficial to device engineering and the improvement of the performance of CdTe polycrystalline thin film solar cells, and therefore other chalcogenide solar cells, such as CIGS.
Results and discussion Experimentally, the CdTe solar cells were deposited by the low temperature radio frequency magnetron sputtering method (see ESI† for details). The sample preparation for EBSD, CAFM, and other methods can also be found in the ESI.† X-ray diffraction (XRD) patterns reveal that Cl-treated CdTe exhibits a zincblende structure.28 The GB type and grain orientation of Cl-treated thin films with Cu incorporation (Cu incorporation is important for fabrication of low-resistance back contact) were characterized by electron backscatter diffraction (EBSD), as shown in Fig. 1a, b and Fig. S1 (ESI†). The chaotic assortment of colors (Fig. 1b) indicates that the CdTe grains show random growth after the Cl treatment. There are three main types of GBs: i-type: GBs between neighboring lattices differing in orientation by a high angle (i.e., more than 151, high angle boundaries, about 98%), (Fig. 1a and Fig. S1a, b, ESI†); ii-type: boundaries between
Fig. 1 (a) Electron backscatter diffraction (EBSD) grain boundary map of a Cl-treated and Cu-incorporated CdTe film. Blue color: random grain boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801); red color: S3 CSL boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801); green color: boundaries between neighboring lattices differing in orientation by an angle of (5–151). (b) EBSD map of a Cl-treated and Cu-incorporated CdTe film. The colors are based on the crystallographic orientation according to the key on the right side. (c) High-resolution transmission electron microscopy (HRTEM) image of CdTe solar cells. (d) HRTEM image of the red dotted rectangular region in (c).
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neighboring lattices differing in orientation by a low angle (i.e., 2–51); and iii-type: other boundaries. Importantly, around 53.2% of grain boundaries between neighboring lattices differing in orientation by a high angle (i.e., 15–1801) are CSL boundaries (Fig. S1c and d, ESI†), 47% of which are S3 TBs (Fig. S1c and d, ESI†). Moreover, almost all the grains with a grain size larger than 1 mm contain at least one TB. Many grains contain even more than ten TBs (Fig. 1a and Fig. S1, ESI†). High-resolution transmission electron microscopy (HRTEM) was further conducted to characterize the cross-section of CdTe solar cells. It is seen from the HRTEM images (Fig. 1c and d) that many TBs exist in the Cl-treated CdTe thin films. The calculated d value is 0.38 nm, which is consistent with the {111} space of CdTe (d = 0.37 nm). This demonstrates that the CSL boundaries are in fact {111} TBs. The ratio of coincidence sites to the total number of sites of the two grains is 1/3 (Fig. 1c and d), indicating that the CSL boundaries are S3 CSL TBs.29 Therefore, the EBSD and HRTEM results show that the CSL TBs are most {111} S3 CSL TBs. We speculate that the formation of {111} S3 CSL TBs is the result of low stacking fault formation energy (9.0 1 mJ m 2).23 In the Cl-treated CdTe, {111} S3 CSL TBs were always observed.23–27,30 We then employed C-AFM measurements to study the local electric properties of CdTe polycrystalline thin films at the nanoscale. The CdTe films were scanned using a Pt/Ir-coated conductive probe (SCM-PIT, parameters are shown in the ESI†) in a contact mode for morphology, and with a different DC voltage applied to the tip for current mapping analysis. SKPM measurements were also conducted, for which the morphology image was obtained in a contact mode, after which the tip was raised to 100 nm from the sample surface for the surface potential scan (Fig. S2, ESI†). The roughness of the sputtered CdTe surface was in the range of 20–30 nm,7 which was small enough to conduct C-AFM and SKPM measurements directly on the surface without any etching. SnO2:F (FTO) was grounded during the C-AFM current mapping and SKPM potential scans (Fig. S2, ESI†). We first performed C-AFM measurements on the surface of CdTe with Cl treatment and Cu incorporation, and the result is shown in Fig. 2. The deflection-error image is sensitive to the height difference (Fig. S3, ESI†). Therefore, the GBs are quite clear in the deflection-error image. According to the S3 CSL TB type, four types of grains were observed in the height-sensor images of the CdTe films with Cl treatment and Cu incorporation (Fig. 2a and b, Fig. S3, ESI†): Type-1 grains with multiple {111} S3 TBs with a low angle to the CdTe film surface, Type-2 grains with multiple {111} S3 TBs parallel to the CdTe surfaces, more clearly seen in Fig. S4 (ESI†), Type-3 (small) grains with a grain size of B500 nm or less, composed of Cd, Cl, Te, and O, (see our published paper18 for further information and Fig. S5, ESI†), and Type-4 CdTe grains with not more than two {111} S3 CSL TBs, are possibly perpendicular to the film’s surface. Typical grains of each type have been labeled with the corresponding number in Fig. 2a, c–f. The four types of grains are clearly seen from the SEM images (Fig. S4, ESI†). Type-3 grains
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Fig. 3 (a) Atomic force microscopy (AFM) height-sensor image. (b) Conductive AFM (C-AFM) current mapping image of the Cl-treated and Cu-incorporated CdTe surface with multiple Type-1 S3 TBs with distances of only B100 nm between each other. The C-AFM current mapping image was obtained with 1.0 V DC tip voltage and no specially applied illumination. (c) Corresponding AFM height-sensor image of CdTe shown in the rectangular box of (a). (d) Corresponding C-AFM current mapping image of CdTe shown in the rectangular box of (b). The scale bar in (a and b) is 1 mm. The scale bar in (c and d) is 0.1 mm. Fig. 2 Atomic force microscopy (AFM) results for CdTe with Cl treatment and Cu incorporation. (a) AFM deflection-error image of Cl-treated CdTe with Cu incorporation. (b) Sketch of the four types of CdTe grains according to the S3 CSL TB varieties. (c–f) Corresponding conductive AFM (C-AFM) current mapping images with different DC tip voltages (TVs) of 0–0.8 V. The maximum current value of the (g) Type-1, (h) Type-2, and (i) Type-3 CdTe grains under different DC TVs. The scale bar is 1 mm.
are not found in the EBSD image because of the fact that it is only covered on the CdTe surfaces. I–V characteristics of these four types of grains are quite different. For Type-1 CdTe grains, the TBs are clearly seen in the deflection-error image (Fig. 2a, labeled by number 1). It is seen that, in such types of grains, current transport is enhanced at TBs according to the C-AFM current mapping (Fig. 2c–f). The current value is actually the largest along the TBs (Fig. 2c, Fig. S6, ESI†), even with neither intentional illumination (diffused light only) nor with DC tip bias voltage. The largest current value is 7.96 nA, with a 0 V tip DC bias. As the tip DC voltage increases from 0 V to 0.8 V, the current value along TBs gradually increases, as shown in Fig. 2. The largest current value can reach 77.65 nA (Fig. 2g) with a tip DC voltage of 0.8 V. However, the conductive area along the TBs remains unchanged. For some area away from TBs, such as the area marked by the rectangle in Fig. 3a and b, it appears that the grain interiors show a larger current value (Fig. 3b). However, this is caused by the overlapping multiple {111} S3 CSL TBs, as revealed under a high-resolution scan. This area is actually composed of highdensity TBs with distances of only B100 nm between each other, as clearly seen in the higher magnification AFM height-sensor image in Fig. 3c. In fact, the current values are still larger at TBs than those at the grain interiors (Fig. 3d). Thus, it is seen that the {111} S3 TBs are major carrier transport paths with a tip DC voltage of 0–1.0 V.
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For the Type-2 CdTe grains, the current value is also relatively large at the {111} S3 TBs. However, at 0 V tip DC bias, the current value along the {111} S3 TB is only 5.905 nA (Fig. 2h), which is smaller than that of Type-1 grains. The current value in the {111} S3 TBs gradually increases (Fig. 2c–f, h) as the tip DC bias increases to 0.8 V. The highest current value is 40.9 nA with a 0.8 V tip DC bias (Fig. 2h). The conductive area along the TBs enlarges with the increase of tip DC bias. For example, the conductive region of the ‘‘2’’ CdTe grain shown in Fig. 2c–f along the horizontal direction increases from 0.588 mm to 0.627 mm as the tip DC voltage increases from 0 V to 0.8 V. The TB-enhanced current transport phenomenon is quite common in our CdTe polycrystalline films with Cl treatment and Cu incorporation. Another typical example is shown in Fig. S7 (ESI†), in which the Type-2 grains are quite clearly seen. Type-3 grains show an enhanced current transport around the grain boundaries (Fig. 2, Fig. S6a and b, ESI†). However, the current value is much smaller than that of Type-1 and Type-2 grains. The maximum current value is only 1.725 nA at 0 V tip DC bias (Fig. 2i). As the tip DC voltage increases from 0 V to 0.8 V, the maximum current increases from 1.725 nA to 4.505 nA (Fig. 2i), and the length of the conductive region along the horizontal direction also increases. The I–V curves for the region with obvious current in the Type-1, -2, and -3 grains were measured by varying the voltage from 2 to 2 V. The typical I–V curve is shown in Fig. S8 (ESI†). Accordingly, conductive regions for Type-1, -2, and -3 grains exhibit clear diode characteristics (Fig. S8, ESI†). For the Type-4 grains with not more than two {111} S3 TBs, in which the random high-angle GBs are the main boundaries, very low current was collected with the tip DC bias between 0 V and 0.8 V when no additional light was applied (except for diffused light from the laser diode of the AFM system), as shown in Fig. 2c–i.
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Fig. 4 (a) Atomic force microscopy (AFM) height-sensor image of Cl-treated and Cu-incorporated CdTe thin films. (b) Scanning Kelvin probe microscopy (SKPM) image of Cl-treated and Cu-incorporated CdTe thin films. (c) Section of the AFM height-sensor curve along the line in (a). (d) Corresponding SKPM curve along the line in (a).
This may be due to the lower volume for photocarrier generation in the TB-produced local junction on the light path. As shown in Fig. 4d, the lateral local junction produced by TBs only extends to a scale of about a couple of hundred nanometers. When the TBs oriented at a high angle to the film surface, as in Type-4 grains, only the end of the TBs on the shallow surface (around 200 nm below the film surface for the 633 nm red laser light) could absorb light, due to the high absorption coefficient of CdTe.5 This could lead to a current collected by the local junction of TBs lower than the sensitivity of the AFM equipment, which is 100 pA. While, due to the low-angle orientation to the film surface, the volume for photocarrier generation in a single TB of Type-1 and Type-2 grains is much larger than that in Type-4 grains. However, the random high-angle GB enhanced current transport is observed in Type-4 grains when illuminated by homemade LED arrays through the glass side (Fig. S9 and S10, ESI†). Therefore, in CdTe solar cells, {111} S3 CSL TBs are much more electrically beneficial than random high-angle GBs and other CSL TBs. This is the first direct observation that {111} S3 CSL TBs are electrically beneficial, which has been theoretically predicted by Yoo et al.31 To understand the transport difference between the grain interior and GBs especially TBs, SKPM was conducted on the surface of Cl-treated and Cu-incorporated CdTe films. The distance between the CdTe surface and the tip was kept at 100 nm during
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the SKPM scan. The SKPM result is shown in Fig. 4b and d. A potential difference is observed in the GBs including TBs, and grain interiors (Fig. 4). From SKPM mapping and the corresponding cross-sectional images (Fig. 4) of Cl-treated CdTe, the boundaries have a larger work function than that of grain interiors, leading to a potential gradient between the boundaries and grain interiors along the lateral direction.32,35 The potential shows a close relationship with the surface roughness. In order to exclude the distance-induced potential difference, we can turn our attention to the grains circled in red and green in Fig. 4a and b. For the CdTe grains circled in green, the distance between the tips is smaller than that of the GBs circled in red. However, the potential is higher than that of the GBs circled in red (Fig. 4b). Thus, the potential difference is mainly due to localized material properties, rather than surface morphology. Combined with C-AFM with different tip DC voltages, EBIC (Fig. S11, ESI†), and the potential measurements, the current difference between the boundaries and grain interiors is due to a potential difference. The potential difference leads to a local electric field between the grain interior and boundaries, which aids in electron and hole separation in the lateral direction. The lower potential at GBs and TBs attracts electrons into the boundaries and moves holes into the grain interiors.33 This lateral potential difference is helpful in separating and transporting free electrons and holes in CdTe, which reduces the recombination rate and is electrically beneficial to the CdTe solar cell performance. Therefore, in CdTe devices, boundaries (especially TBs) are electrically favorable to the carrier’s separation, transport, and collection. Fig. 5 schematically shows the local band-diagram between the grain interior and boundaries including TBs (Fig. 5a) and the electron and hole flowing paths in the CdTe film composed of Type-1, 2, 3, and 4 grains (Fig. 5c) during SKPM acquisition. It is worth noting that the illustration of Type-1 and 2 grains in Fig. 5c is according to our direct observation from the scanningTEM (STEM) cross-sectional image of our devices, as shown in Fig. 5d. From the aspect of electrical properties, the top bending of the band-diagram at grain interiors (Fig. 4d and 5a) could lead to local tunneling contacts to the metal back electrode in the complete device, as shown in Fig. 5b. Since holes converge in the grain-interiors, the surfaces of grain interiors could work as the best channels for the transport of majority carriers into the back electrode with a smaller work function than CdTe, such as Au. Meanwhile, from the aspect of surface morphology, the back electrode has a better chance to contact the surface of grain-interiors than GBs, due to larger height than that at GBs (Fig. 4a and 5c). These two aspects could explain the feasibility of flexible materials as a back electrode to CdTe cells, such as graphite paste,6 graphene,6 and carbon single wall nanotubes34 when contact with the valleys of the CdTe film surface is less possible than vacuum deposited metal layers. The hole-rich surfaces of CdTe grain-interiors can also explain the formation of good ohmic contact though the less conductive Type 3 grains exist on the film surface, which mostly locate at the boundaries (Fig. S5, ESI†).28 Type-1 and Type-2 grains may be the same if we only consider the TB compositions. However, the electron and hole transport
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but only one order of magnitude higher than that of Te at 550 1C (CSS). The potential of grain boundaries of as-grown CdTe is lower than that of grain interiors (Fig. S12, ESI†), but similar to that of Cl-treated and Cu-incorporated CdTe. The results show that the potential difference between boundaries and grain interiors is not completely due to the Cl treatment in sputtered CdTe films. Therefore, the origin of the potential difference between boundaries and grain interiors of CdTe should be further investigated.
Conclusions Fig. 5 (a) A sketch showing the band diagram of the lateral local electric field between the grain interior and boundaries including TBs. (b) The band diagram of the CdTe surface and the back electrode with a work function smaller than CdTe. The dotted lines represent the Schottky barrier in CdTe without heavy doping while the solid lines represent the tunneling contact due to top bending of the band diagram. (c) A sketch showing the electron and hole flow direction in the CdTe solar cell composed of Type-1, 2, 3, and 4 grains. The holes move towards the back electrode along the CdTe surface. The electrons move towards the front electrode mainly along the TBs and converge at boundaries. (d) Bright-field STEM image of the CdTe solar cell.
path is different in Type-1 and Type-2 grains. For Type-1 grains (TBs oriented at shallow angles to the film surface), electrons could reach the main junction along the TBs without the assistance of GBs, which is understood from the STEM cross-sectional image of the device (Fig. 5d). However, for Type-2 grains, electrons and holes are separated near TBs and transport in different sides of random large-angle GBs. In this case, random large-angle GBs may be necessary for conducting channels for electron and hole transport in Type-2 grains. Some researchers found that the Cl substitution of Te at GBs is the reason for the potential difference between GBs and grain interiors.21 GB-enhanced carrier transport was not found in as-grown CdTe. However, these studies were focused on hightemperature CdTe thin films. For low-temperature CdTe films, no such studies have been conducted. Accordingly, we conducted SKPM (Fig. S12, ESI†) and C-AFM (Fig. S13, ESI†) measurements on as-grown CdTe thin films grown by magnetron sputtering. Even though no current can be detected when the tip DC bias was less than 1 V, the enhanced current at GBs was observed (Fig. S13, ESI†) with a tip bias of 3 V. From height-sensor and C-AFM current mapping images (Fig. S13a and b, ESI†), it is seen that as-grown CdTe shows GB-enhanced current transport, although the current value is quite small (100 picoamps). This result is quite different from that of as-grown CdTe polycrystalline thin films obtained by the close space sublimation (CSS) method.8,9,21 CdTe obtained by the CSS method shows enhanced current transport at the grain interior.8,9,21 This may be due to the higher density of cadmium vacancies in as-grown CdTe films deposited by sputtering, as compared to those by CSS. This can be explained by the fact that the saturation pressure of Cd is two orders of magnitude higher than that of Te at 270 1C (sputtering),
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We have found that the {111} S3 CSL TBs were the most electrically beneficial type of boundary to the performance of CdTe cells. GB, including TB, enhanced current transport was directly observed in sputtered CdTe films after Cl treatment and Cu incorporation. SKPM and EBIC measurements confirm that the enhanced current transport can be explained by the local lateral field between the grain boundaries and grain interiors. This local lateral field is beneficial to the separation, transport, and collection of carriers. We speculate that these results may open new opportunities for enhancing solar cell performances by controlling the grain boundaries, especially by controlling TBs.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51472239, and 61274060), Youth Innovation Promotion Association, CAS (ARP No.: Y410421C41), the 100 Talents Program of IEE CAS (ARP No: Y010411C41), the 100 Talents Preferred Support Plan of the CAS (ARP No: Y210431C41), Innovative and Interdisciplinary Team Award (project: Highefficiency Utilization of Renewable Energy Resources Innovative and Interdisciplinary Team).
References 1 J. D. Major, R. E. Treharne and K. Durose, Nature, 2014, 511, 334–337. 2 J. R. Tuttle, M. A. Contreras, T. J. Gillespie, K. R. Ramanathan, A. L. Tennant, J. Keane, A. M. Gabor and R. Noufi, Prog. Photovoltaics, 1995, 3, 235–250. 3 G. Hodes, Science, 2013, 342, 317–318. 4 R. F. Service, Science, 2014, 344, 458. 5 A. Luque and S. Hegedus, Handbook of Photovoltaic Science and Engineering, 2nd edn, Wiley, 2011, p. 600. 6 S. G. Kumar and K. S. R. K. Rao, Energy Environ. Sci., 2014, 7, 45–102. 7 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2015, 23, 1–9. 8 I. Visoly-Fisher, S. R. Cohen, A. Ruzin and D. Cahen, Adv. Mater., 2004, 16, 879–883. 9 I. Visoly-Fisher, S. R. Cohen, K. Gartsman, A. Ruzin and D. Cahen, Adv. Funct. Mater., 2006, 16, 649–660. 10 M. Takihara, T. Minemoto, Y. Wakisaka and T. Takahashi, Prog. Photovoltaics, 2013, 21, 595–599.
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Published on 10 April 2015. Downloaded by Carleton University on 13/06/2015 11:03:28.
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11 K. Durose, S. E. Asher, W. Jaegermann, D. Levi, B. E. McCandless, W. Metzger, H. Moutinho, P. D. Paulson, C. L. Perkins and J. R. Sites, Prog. Photovoltaics, 2004, 12, 177–217. 12 K. L. Chopra, P. D. Paulson and V. Dutta, Prog. Photovoltaics, 2004, 12, 69–92. 13 M. Hafemeister, S. Siebentritt, J. Albert, M. C. Lux-Steiner and S. Sadewasser, Phys. Rev. Lett., 2010, 104, 196602. 14 S. K. Das and G. C. Morris, J. Appl. Phys., 1992, 72, 4940. 15 J. D. Major, Y. Y. Proskuryakov, K. Durose, G. Zoppi and I. Forbes, Sol. Energy Mater. Sol. Cells, 2010, 94, 1107–1112. 16 B. G. Mendis, L. Bowen and Q. Z. Jiang, Appl. Phys. Lett., 2010, 97, 092112. 17 J. Moseley, M. M. Al-Jassim, H. R. Moutinho, H. L. Guthrey, W. K. Metzger and R. K. Ahrenkiel, Appl. Phys. Lett., 2013, 103, 233103. 18 H. Li, Y. Biao, X. X. Liu and P. J. Wang, RSC Adv., 2014, 4, 5046–5054. 19 P. R. Edwards, S. A. Galloway and K. Durose, Thin Solid Films, 2000, 361, 364–370. 20 P. R. Edwards, S. A. Galloway and K. Durose, Thin Solid Films, 2000, 372, 284–291. 21 C. Li, Y. Wu, J. Poplawsky, T. J. Pennycook, N. Paudel, W. Yin, S. J. Haigh, M. P. Oxley, A. R. Lupini, M. Al-Jassim, S. J. Pennycook and Y. Yan, Phys. Rev. Lett., 2014, 112, 156103. 22 S. L. Marina, A. Maxim, J. L. Henri, G. Anthony, A. A. Talin and B. Z. Nikolai, ACS Nano, 2014, 8, 11883–11890. 23 T. Paulauskas, C. Buurma, E. Colegrove, Z. Guo, S. Sivananthan, M. K. Y. Chan and R. F. Klie, Appl. Phys. Lett., 2014, 105, 071910.
This journal is © the Owner Societies 2015
PCCP
24 H. R. Moutinho, R. G. Dhere, M. J. Romero, C. S. Jiang, B. To and M. M. Al-Jassim, J. Vac. Sci. Technol., A, 2008, 26, 1068. 25 D. J. Smith, S.-C. Y. Tsen, Y. P. Chen, J.-P. Faurie and S. Sivananthan, Appl. Phys. Lett., 1996, 67, 1591. 26 Y. Xin, N. D. Browning, R. Rujirawat, S. Sivananth, Y.-P. Chen, P. D. Nellist and S. J. Pennycook, J. Appl. Phys., 1998, 84, 4292. 27 B. A. Korevaar, J. R. Cournoyer, O. Sulima, A. Yakimov and J. N. Johnson, Prog. Photovoltaics, 2014, 22, 1040–1049. 28 H. Li and X. X. Liu, Sol. Energy, 2015, 115, 603–612. 29 V. Consonni, N. Baier, O. Robach, C. Cayron, F. Donatini and G. Feuillet, Phys. Rev. B: Condens. Matter Mater. Phys., 2014, 89, 035310. 30 C. Li, J. Poplawsky, Y. Wu, A. R. Lupini, A. Mouti, D. N. Leonard, N. Paudel, K. Jones, W. Yin, M. Al-Jassim, Y. Yan and S. J. Pennycook, Ultramicroscopy, 2013, 134, 113–125. 31 S. H. Yoo, K. T. Butler, A. Soon, A. Abbas, J. M. Walls and A. Walsh, Appl. Phys. Lett., 2014, 105, 062104. 32 C. S. Jiang, M. A. Contreras, I. Repins, H. R. Moutinho, Y. Yan, M. J. Romero, L. M. Mansfield, R. Noufi and M. M. Al-Jassim, Appl. Phys. Lett., 2012, 101, 033903. 33 M. Nonnenmacher, M. P. O’Boyle and H. K. Wickramasinghe, Appl. Phys. Lett., 1991, 58, 2921. 34 A. B. Phillips, R. R. Khanal, Z. Song, R. M. Zartman, J. L. DeWitt, J. M. Stone, P. J. Roland, V. V. Plotnikov, C. W. Carter, J. M. Stayancho, R. J. Ellingson, A. D. Compaan and M. J. Heben, Nano Lett., 2013, 13(11), 5224. 35 S. Sadewasser and T. Glatzel, Kelvin Probe Force Microscopy, Springer, 2012, p. 13.
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