Si-Doping Effects in Cu(In,Ga)Se2

Subscriber access provided by PEPPERDINE UNIV

Article 2

Si-Doping Effects in Cu(In,Ga)Se Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells Shogo Ishizuka, Takashi Koida, Noboru Taguchi, Shingo Tanaka, Paul Fons, and Hajime Shibata ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09019 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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.

ACS Applied Materials & Interfaces 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 35

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

ACS Applied Materials & Interfaces

Si-Doping Effects in Cu(In,Ga)Se2 Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells Shogo Ishizuka1*, Takashi Koida1, Noboru Taguchi2, Shingo Tanaka2, Paul Fons3, and Hajime Shibata1 1

Research Center for Photovoltaics, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 2

Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science

and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan 3

Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan KEYWORDS: solar cells, grain interface, Cu(In,Ga)Se2, Si-doping, buffer-free configuration

ABSTRACT We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure bufferfree solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results

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 35

obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance.

1. INTRODUCTION Thin film photovoltaic technologies based on polycrystalline chalcopyrite materials are expected to expand the application of solar energy due to the production of flexible and lightweight solar modules as well as conventional rigid type solar panels with high energy conversion efficiencies.1-4 Since the world’s first Cu(In,Ga)Se2 (CIGS) solar cell was made in the early 1970s, the basic concept of the CIGS solar cell device structure has been invariant and the pCIGS photoabsorber layer/n-buffer layer configuration has been commonly used.1,5-8 Typical CIGS solar cells employ one or two buffer layers, such as n-type CdS, ZnS(O,OH), or In2S3 (first buffer layer) and highly resistive intrinsic ZnO or ZnMgO (second buffer layer).1,5-8 The first CIGS solar cell was fabricated using a hetero-structure of p-CuInSe2/n-CdS, that is, a single crystalline CuInSe2 wafer with a 5-10 µm thick CdS layer deposited on it.5 Subsequently, a homo-junction model based upon a buried p-n junction formed with a type-converted CIGS film surface due to doping with group II elements such as Cd or Zn diffused from buffer layers into the CIGS film surface was suggested.9-12 To date, the roles played by buffer layers are believed to be diverse. For instance, a buffer layer may serve as an n-type material to form a p-n junction with p-CIGS or literally act as a buffer to reduce sputtering damage to the CIGS surface during the deposition of the transparent conductive oxide (TCO) layer or prevent the formation of electrical shunt-paths.13-16 Buffer layers are often deposited by a wet solution process, though most parts of the CIGS device yielding high cell efficiencies are generally grown using dry processes.7 A reduction in the number of process steps and the use of all dry processes in


2

Page 3 of 35

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


addition to the use of a simplified device structure is expected to contribute to significant cost reductions in the final product. Also, simplification of the CIGS device structure is expected to be useful for the understanding of CIGS device mechanisms and the underlying physics because both CIGS material and conventional device structures are relatively complicated and a multitude of poorly understood areas still remain in contrast to the success of high-efficiency practical solar module production. To date, buffer layers have been considered to be an indispensable component necessary to demonstrate high cell efficiencies from CIGS solar cells. Although there have been many attempts to eliminate buffer layers from the CIGS device structure so far, cell efficiencies demonstrated without buffer layers have remained low. For example, modification of the TCO layers using Zn(O,S):Al or ZnMgO:Al as alternatives to conventional ZnO:Al to tune the conduction band offset between the CIGS and TCO layers in a buffer-free configuration was attempted and cell efficiencies of 11.3% with Zn(O,S):Al and 7.96% with ZnMgO:Al were demonstrated.17,18 As for CIGS surface modifications, ion implantation with elemental Cd or Zn has been reported to be effective in enhancing buffer-free cell efficiencies and 10.2% efficiency was demonstrated with Cd ion implantation.19 Post deposition treatments of the CIGS surface with elemental Zn has also been reported to be effective and a cell efficiency of 11.5% has been reported.20 Among these attempts, Cd partial electrolyte treatments (Cd-PE) of the CIGS surface appear effective in enhancing buffer-free CIGS cell efficiencies. Using the Cd-PE method, relatively high cell efficiencies over 15% have been reported, though this technique is a wet solution process that requires the use of the heavy metal element Cd and the cell devices employed an intrinsic ZnO second buffer layer.21,22 Incidentally, similar levels of cell efficiencies of around 10% have been reported for buffer-free CdTe solar cells as well.23


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 35

In this work, a breakthrough in the cell efficiencies of buffer-free CIGS solar cells without the use of heavy metals such as Cd, no additional group II elemental dopants, and no highly resistive second buffer layer consisting of materials such as intrinsic ZnO is achieved. To realize this accomplishment, elemental Si-doping of CIGS was employed. Successful control of structural and electrical properties by group IV element doping has been reported for other polycrystalline Cu-related compound thin films.24,25 Specifically, Si and Ge have been suggested to be effective in controlling polycrystalline Cu2O film properties such as carrier concentration and growth orientation possibly due to the modification of grain boundary properties rather than a mechanism based on substitutional defect formation.24 Here, this concept is applied to control the CIGS film and device properties and to develop a new technique to passivate polycrystalline CIGS grain interfaces. There have been very few reports regarding the effect of group IV doping in CIGS in contrast to a relatively large number of studies reporting on that of group II elements. This fact can serve as the motivation for our attempt to use Si-doping to explore new materials science in this study. The distinctive characteristics of Si-doped CIGS thin films and some points of similarity to CIGS films and devices grown using KF or RbF post CIGS film deposition treatments (KF-PDT or RbF-PDT) are presented.

2. RESULTS Thin film properties of Si-doped CIGS. Figure 1 compares cross sectional scanning electron microscopy (SEM) and electron beam induced current (EBIC) images of CIGS photoabsorber layers grown on Mo-coated soda-lime glass (SLG) substrates with and without Sidoping. Here, CIGS layers were grown by the three-stage coevaporation process26 and Si-doping was carried out during the third stage. Details about the Si-doping can be found in the Methods


4

Page 5 of 35

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


section. The Si-doped CIGS films show a distinctive grain morphology due to the formation of grain boundary layers as shown in Figure 1a, whereas a control (non Si-doped) CIGS film, made for reference, shows a typical CIGS grain morphology conventionally observed without the presence of such boundary layers (Figure 1b). In a sense, the shape of grain boundary layers can also be viewed as ‘clinging ivy’ coiling around CIGS grains rather than a ‘layer’ as can be seen in the other SEM images (see Figure S1, Supporting Information). Hereafter, however, the term ‘boundary layer’ is nominally used to indicate the deposits at Si-doped CIGS grain boundaries. The formation and thickness of grain boundary layers depend on the Si-doping level. We found that CIGS films grown with a Si concentration of about 3×1019 cm-3, which is equivalent to about 0.1 atomic %, or more led to the prominent emergence of grain boundary layers and a larger amount of Si led to thicker boundary layers, though excessive Si-doping resulted in the delamination of the CIGS films from the Mo back contact layer. Figure 1d,e shows cross sectional EBIC images obtained from identical devices to those shown in Figure 1a,b. The device structure employs B-doped ZnO (ZnO:B) as the TCO layer as shown in Figure 1c. The EBIC image of the control (non Si-doped) CIGS device shown in Figure 1e is quite similar to typical EBIC images observed from conventional CIGS solar cells fabricated with the use of a CdS buffer layer27,28 and both bright and dark grain regions are present. On the other hand, the Sidoped CIGS device (Figure 1d) exhibits bright regions throughout the cross sectional surface and the EBIC signal at the grain boundaries is stronger than that in the grain interior. This distinctive EBIC image is attributable to the modified CIGS grain boundaries in the film. The control CIGS device (Figure 1e) shows a relatively strong EBIC signal near the CIGS/ZnO:B interface region. This indicates that the active area of the solar cell device is in the p-CIGS/n-ZnO:B heterostructure and the concomitant electric field is present there. Similar EBIC profiles can be


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 35

observed from conventional buffer-based CIGS devices.27,28 On the other hand, the strong EBIC signals observed at the grain boundaries shown in Figure 1d indicate that the effective electric field and electron-hole pair generation (separation) centers are present at the grain boundaries in the Si-doped CIGS film, in addition to the presence of an electric field formed at the p-CIGS/nZnO:B heterostructure. This EBIC profile suggests that Si-doping leads to an increase in the upward band bending of the conduction band minimum (Ec) at the CIGS grain boundaries,28,29 similar to the case of KF-PDT CIGS films reported in the literature.30 To date, downward band bending of Ec at CIGS grain boundaries has been generally believed to occur31,32 and thus the upward band bending model has not been widely accepted. However, there are a variety of CIGS grain boundary types33 which are expected to lead to various types of energy band diagrams at the grain boundaries in a CIGS film. The presence of upward band bending is, therefore, not contradicted and Si-doping or KF-PDT may have an effect of the modification of such energy band diagrams at CIGS grain boundaries. Analysis of the grain boundary layers formed in Si-doped CIGS films was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) measurements (Figure 2a-c). Cross sectional TEM images of a boundary layer observed in a Si-doped CIGS film prepared with a focused ion beam (FIB) are shown in Figure 2a. A well-ordered lattice structure was observed in both the CIGS grain interior and the grain boundary layer, though prominent disordering in the lattice structure was observed at the interface. We found that there was no significant difference in the elemental composition of the CIGS grain interior and the boundary layer as determined by a TEM-EDX point analysis (not shown), though traces of elemental Si were detected at the grain boundary (Figure S2, Supporting Information). The analysis of CIGS grain boundaries by TEM


6

Page 7 of 35

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


measurements is, however, very complicated due to the ambiguous results often observed, for example, some grain boundaries show a Cu-poor composition but other regions show a Cu-rich trend as reported in the literature.29,33 We have also found this trend (Figure S3, Supporting Information). This is attributable to the presence of various types of CIGS grain boundaries as well as to issues of sample preparation and the effect of the relatively high energy electron beam irradiation used for TEM measurements. It should be noted here that the boundary layer may consists of relatively unstable material phases. Figure 2b shows cross sectional SEM images of an as-grown (without TCO layer deposition) Si-doped CIGS film preserved in a desiccator for 10 months after film growth. As can be seen, this film exhibits degraded boundary layers. Such degradation was not observed once TCO layers were deposited on Si-doped CIGS films after growth, suggesting that the degradation is due to a reaction with oxygen or residual moisture by exposure to air for long periods. This result implies that the boundary layer (or the grain interface) is relatively unstable possibly due to the formation of unstable Si-related compound phases. Thus, TEM measurements of FIB processed Si-doped CIGS specimens using the relatively high acceleration voltages of 200-300 kV are likely lead to the degradation of the sample and generate concomitant artifacts in the analysis. To avoid this issue, SEM-EDX measurements were carried out using a freshly cleaved cross section of a Si-doped CIGS film (Figure 2c). SEM-EDX point and line analyses clearly indicate the presence of a higher concentration of elemental Si in the grain boundary (points 3 and 4) rather than in the CIGS interior (points 1 and 2). This result suggests that the elemental Si supplied during the third stage of CIGS film growth diffuses along CIGS grain boundaries rather than within CIGS grain interiors due to the presence of liquid Cu-Se phases on/in the growing CIGS film and leads to the formation of Si-doped CIGS or Si-related compounds in the CIGS grain boundary.


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 35

Secondary ion mass spectrometry (SIMS) measurements revealed that Si-doping of CIGS affected the distribution profiles of alkali metals in the CIGS layers (Figure 3a-c). Alkali metals, specifically Na and K, are known to diffuse from SLG/Mo substrates into CIGS layers during growth.34 KF-PDT is known to have the effect of reducing the Na concentration leading to an increase in K concentration in the CIGS layers.35 This trend can be observed in Figure 3b,c. On the other hand, the Si-doping of CIGS carried out during the third stage was found to have no significant effect on the Na concentration in the CIGS layer, though the Na concentration near the surface region was slightly less than that in the control CIGS film (Figure 3b). However, Sidoping was found to affect the K distribution profile in the CIGS layers and led to a decrease in the K concentration near the surface region and an increase in the K concentration in the lower part of CIGS film as shown in Figure 3c. This result suggests that the presence of elemental Si during CIGS film growth interferes with the diffusion of elemental K from the substrate to the CIGS layer surface. Consequently, in Si-doped CIGS films, a large amount of K accumulates in the lower region of the CIGS film without a corresponding decrease in the Na concentration. These alkali metals are reported to accumulate at grain boundaries.30,36 We have also observed the presence of K in CIGS grain boundaries using TEM-electron energy-loss spectroscopy (EELS) measurements (Figure S4, Supporting Information). Thus, it is suggested that an increased K concentration is likely present at the CIGS grain boundary, specifically in the disordered lattice region visible in Figure 2a, and affects the electrical properties of CIGS films as can be seen in the EBIC profile (Figure 1d).

Solar cell device properties. The current-voltage (J-V), power-voltage (P-V), and external quantum efficiency (EQE) curves acquired for a Si-doped CIGS solar cell fabricated using a


8

Page 9 of 35

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


buffer-free configuration are shown in Figure 4a,b (the original measurement report of the certified efficiency issued by Japan Electrical Safety and Environment Technology Laboratories can be found in Figure S5, Supporting Information). The Si-doped CIGS photoabsorber layer used for this device was grown in an identical growth batch to the CIGS film used for the SIMS measurements shown in Figure 3a-c. A cell efficiency exceeding 15% is a remarkably high value among the buffer-free CIGS solar cells reported to date.17,18,37 The EQE curve implies that the solar cell absorbs light well over a wide wavelength region. The relatively low open circuit voltage (Voc) value is attributable to residual recombination centers at the CIGS/TCO interface. Figure 5a-d compares the solar cell parameters obtained from buffer-free devices fabricated using control (non Si-doped), Si-doped, and KF- or RbF-PDT CIGS layers. All devices employ the common structure shown in Figure 1c. Si-doped and PDT CIGS devices were found to demonstrate enhanced cell efficiencies with improved cell parameters, when compared to the control CIGS devices. Although Si-doped and PDT CIGS devices showed comparable cell parameter values, a clear difference can be found in the J-V curve shape of these devices. Figure 5e-g shows typical J-V curves measured under dark and light conditions with bias voltages in both forward and reverse directions. Buffer-free devices fabricated with control (non Si-doped) CIGS layers showed a prominent avalanche current in the second quadrant of Figure 5e attributable to an insufficient tolerance to the increasing reverse bias voltage. In general, this phenomenon can be suppressed with the use of buffer layers. The use of Si-doped or PDT CIGS layers has been found to be effective in suppressing the avalanche phenomenon in buffer-free devices as shown in Figure 5f,g. The use of KF- or RbF-PDT CIGS layers, however, often leads to cross-over and roll-over J-V curve shapes as can be seen in the fourth quadrant of Figure 5g. We have also found that the use of a CdS buffer layer can suppress the emergence of these cross-


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 35

over and roll-over shapes in the J-V curves of PDT CIGS devices, indicating the phenomenon is not due to the Mo/CIGS interface but due to a CIGS/TCO interface issue. We found that the surface of KF- or RbF-PDT CIGS films commonly exhibited a rough surface morphology filled with ‘wormholes’ as shown in Figure 5j. The formation of such a rough surface morphology with KF-PDT has also been reported in the literature,38 however, such a drastic morphology variation was not observed for the control and Si-doped CIGS grain surfaces (Figure 5h,i). The KF- or RbF-PDT CIGS surface may be easily oxidized during the MOCVD TCO layer deposition process, resulting in a double-diode effect which leads to the deformation of the J-V curve shape.39 Consequently, we found that the use of a Si-doped CIGS layer was effective in suppressing both the avalanche and double-diode effects as shown in Figure 5f. Capacitance-voltage

(C-V) and

scanning

spreading resistance

microscopy (SSRM)

measurements were carried out to analyze the electrical properties of Si-doped CIGS layers. Space charge density (NCV)-depletion width profiles calculated from the C-V results obtained from control, Si-doped, and PDT CIGS devices are shown in Figure 6a. The value of NCV at a bias voltage of 0 V, that is, the nominal carrier density in CIGS, was found to increase with Sidoping. On the other hand, the NCV values obtained from KF- or RbF-PDT CIGS devices were almost the same as the values for a control CIGS device, though the depletion width tended to increase for these PDT devices. Two possible mechanisms behind the increase in the value of NCV with Si-doping are suggested. One is a true increase in hole concentration and the other is an artifact caused by an increase in the effective p-n junction interface area due to the formation of locally spreading p-CIGS/n-CIGS:Si junctions in the CIGS film. There have been very few reports on group IV element doping of CIGS to date. However, some reports on the Si or Ge doping of ternary CuGaSe2 in an attempt to obtain n-type material can be found in the


10

Page 11 of 35

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


literature.40,41 Group IV element doping of CuGaSe2 (or CIGS) is expected to form potential donor-type defects by forming IVCu or IVGa(In) substitutions. Although theoretical studies using first principle calculations on the formation energies of various types of Si-related substitutional defects in CIGS are needed for a rigorous discussion, elemental Si is expected to occupy cation sites and form donor-type defects in CIGS taking into account the fact that the electronegativity of Si (1.8) is almost the same as that of Cu (1.9), In (1.7) and Ga (1.6) and is smaller than that of Se (2.4) as well as the features of the Si cations present in typical Si-chalcogenide materials such as Cu2SiSe342,43 and SiSe2.44,45 It has been reported, however, that a Si concentration of 2×1018 cm-3 is effective to reduce the hole carrier concentration in CuGaSe2 but further increases in the Si concentration lead to an increase in the hole concentration.41 In addition to this, a similar trend of increasing hole concentration with group IV element doping has been observed in other II-VI derivative compounds. For example, in polycrystalline Cu2O, elemental Si is also expected to act as a donor if SiCu substitutional defects are formed, but in practice an increase in the hole carrier concentration has been observed with Si-doping, possibly due to the effect of grain boundary passivation rather than donor-type defect formation in the bulk crystal.24,25 In this sense, an increase in the effective hole carrier concentration with Si-doping does not contradict the results reported in the literature. However, the resistance of Si-doped CIGS is relatively high when compared with the control and KF-PDT CIGS layers. It should be noted here that the value of resistance measured by SSRM significantly depends on sample placement and thus the data obtained should be treated as relative results for each sample but not as absolute values. Nevertheless, it should be also noted that we have reproducibly observed a trend of higher resistance in Si-doped CIGS layers compared to control CIGS layers using SSRM measurements. Taking into account the SSRM result, the SiCu and SiGa(In) donor-type defect formation model in


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 35

CIGS, and the EBIC results, the reason for the increase in the value of NCV with Si-doping is more likely to be due to an increase in the effective p-n junction interface area in the CIGS film rather than a true increase in the hole concentration. This leads to an increase in the nominal capacitance resulting in an increase in the NCV value and a decrease in the depletion layer width even if the actual hole concentration in bulk decreases with the formation of Si-related donortype defects.

3. DISCUSSION The modification of CIGS grain boundaries with Si-doping is expected to occur at the beginning of the third stage where a liquid Cu-Se phase is present at the growing film surface.46 Elemental Si is known to have good solid solubility with Cu and Se and forms compounds such as Cu2SiSe3,42,43 SiSe2,44,45 and CuInSe2-SiSe2 and CuGaSe2-SiSe2 alloy systems.47 Elemental Si supplied to the growing film surface during the third stage may react with Cu-Se and lead to the changes in the nature of the elemental diffusion of Cu and the group III elements In and Ga in the growing CIGS film. The formation of boundary layers implies that film growth during the third stage depends on elemental migration along CIGS grain boundaries. The high Si concentration observed at CIGS grain boundaries suggests that Si doped in CIGS films chiefly affects CIGS grain boundaries and the resulting electrical properties of the CIGS film. The formation of the boundary layers and a concomitant unique EBIC profile are manifestations of this. The improved device performance demonstrated using Si-doped CIGS photoabsorber layers can be attributable to the modified CIGS grain boundaries and film surface. In the Si-doped CIGS layer, improved carrier transport due to the presence of electron-hole pair generation centers formed at grain boundaries as demonstrated by the EBIC profiles shown in Figure 1d and


12

Page 13 of 35

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 concomitant effective carrier diffusion length being longer in Si-doped CIGS layers are expected. Also, the presence of a large amount of elemental K with Si-doping as shown in Figure 3c leads to an enhancement in this effect because K at CIGS grain boundaries is known to have the effect of widening the band-gap energy (Eg), possibly due to the formation of Cu- and/or Gadepleted grain surfaces and the concomitant creation of In2Se3 or KInSe2 compounds as suggested in the literature,48 resulting in an increase in the upward conduction band bending and an enhancement of the effects of electron carrier repulsion.30 In addition to the modified grain boundaries, an additional mechanism behind the improved device performance with Si-doping due to the modification of the CIGS film surface, that is, the modified conduction band offset of the CIGS/TCO (ZnO:B) junction can be suggested. Not only at the grain boundaries in a CIGS layer, but also the presence of a large amount of elemental Si is observed in the near surface region of Si-doped CIGS layers (Figure 3a) because Si is supplied during the third stage of CIGS growth. Also, it should be noted here that alkali Na and K concentrations in the surface region of a Si-doped CIGS layer are remarkably low in comparison with those observed from a control (non Si-doped) CIGS surface (Figure 3b,c). The elemental composition ratio [Ga]/[III] ([III] = [In] + [Ga]) in the surface region of the CIGS layers used in this study is 0.20 as determined by an electron probe micro analyzer (EPMA) using an acceleration voltage of 5 kV. The value of [Ga]/[III] ~ 0.20 is equivalent to the CIGS band-gap energy of 1.10 eV and results in a higher conduction band minimum value by 0.16 eV49 than that of ZnO:B and thus a schematic of the band diagram of the CIGS/TCO junction can be drawn as shown in Figure 7, taking into account the Fermi level (EF) alignment. It has been reported in the literature that a reduction of the conduction band offset present between the CIGS/TCO junction is effective in enhancing the Voc and FF values and the consequent energy conversion efficiency for this type of (Type II)


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 35

heterojunction.49,50 For the case of the KF-PDT CIGS/CdS structure, the formation of CdCu donor defects at the CIGS surface is expected to push the value of EC towards EF resulting in inactivating interface recombination centers due to the defect levels being located below EF and therefore occupied.51 For the case of a Si-doped CIGS surface, similar effects can be expected due to the formation of SiCu or SiIII donor defects leading to the modification of the band structure drawn with dotted lines in Figure 7, resulting in enhanced Voc and FF values. On the other hand, the improved device performance observed from KF-/RbF-PDT CIGS devices fabricated using a buffer-free structure may have a different mechanism from that of the Sidoped CIGS surface. For example, the KF-PDT has been suggested to lead to homogeneous electronic properties such as reducing fluctuations of the work function at the CIGS surface.30 Such an effect is expected to contribute to improvements in device performance. The newly found effects of Si-doping in CIGS can be summarized as: i) Si-doping leads to the formation of grain boundary layers, ii) it affects alkali metal diffusion profiles in CIGS films and the modified grain boundaries are expected to improve carrier transport, iii) a modification of the energy band diagram at the Si-doped CIGS film surface is also expected to contribute to the enhancement of the buffer-free device performance. More detailed studies on the effects of Si-doping in CIGS on topics such as band diagram variation and the detailed composition of boundary layers carried out using, for example, photoemission and inverse photoemission spectroscopies (PES/IPES) and Atom probe tomography (APT) measurements, respectively, are expected to add further important information to distinguish among Si-doping effects in future work.

4. CONCLUSION


14

Page 15 of 35

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


In summary, the use of Si-doped CIGS films as the photoabsorber layer of simplified structure buffer-free solar cell devices resulted in high cell efficiencies of over 15%, record setting values for this type of device configuration. Si-doping of CIGS films was found to be effective in controlling polycrystalline grain boundaries and alkali metal distribution profiles in the film. Elemental Si doped during the third stage of CIGS film growth led to the formation of grain boundary layers several tens of nanometers thick and thus exhibited an unique CIGS film morphology. Interestingly, the presence of boundary layers was found to be beneficial rather than harmful in the enhancement of device performance. Although there is room for further investigation about the effect of Si in CIGS in greater detail, the results obtained in this study offer a new approach and concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials.

EXPERIMENTAL SECTION CIGS Thin Film Growth and Si-Doping. CIGS photoabsorber layers were grown by the three-stage process26 using elemental Cu, In, Ga, and Se Knudsen cell sources in a vacuum chamber with a base pressure in the order of 10-7 Pa. The substrate temperature was 350℃ during the first stage and 550℃ during the second and third stages. Elemental In, Ga, and Se were supplied during the first and third stages and Cu and Se were supplied during the second stage. Si-doping was carried out during the third stage using an elemental Si Knudsen cell source. Si source material with 10-N purity was evaporated using a tantalum crucible with a Knudsen cell temperature ranging from 1500-1600℃. Note that we have attempted various timings for the Si-doping, namely, adding Si to growing CIGS films was carried out during the first stage, second stage, or the third stage before and after the [Cu]/[III] composition ratio became less than


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 35

unity, or post deposition treatments with Si after the third stage. At present, we have found that Si-doping performed during the third stage is most effective in leading to high cell efficiencies using buffer-free configuration device structures. The typical CIGS film composition ratios [Cu]/[III] and [Ga]/[III] were 0.9 and 0.2-0.3, respectively, and the film thickness of the CIGS layers were 2.0-2.5 µm. All CIGS films were grown on 0.8-µm thick Mo coated SLG substrates. KF- and RbF-PDT CIGS films grown for a comparative study were prepared using KF or RbF Knudsen cell sources. These PDTs were carried out after CIGS film growth in an identical growth chamber used for the film growth at a substrate temperature of 350℃ for 10 min. Solar Cell Device Preparation. Solar cell devices were fabricated using a 2-µm thick TCO layer consisting of ZnO:B deposited directly on the CIGS layer by metal organic chemical vapor deposition (MOCVD) at a substrate temperature of 160℃. Diethylzinc, water, and diborane were used for the source materials. The flow rates of diethylzinc, water, and diborane were 100, 200, and 0.2 µmol/min, respectively. The electrical properties of the ZnO:B layers used were typically; Resistivity ~ 4×10-3 Ω cm, mobility ~ 30 cm2/Vs, carrier concentration ~ 5×1019 cm3

, and sheet resistance ~ 20 Ω/□. More details about the deposition conditions and properties of

the ZnO:B layer can be found in the literature.52 CIGS thin films were washed in a dilute ammonia solution before TCO deposition. Buffer layers consisting of materials such as CdS, ZnS(O,OH) or In2S3 (first buffer layer), and highly resistive layers consisting of materials such as intrinsic ZnO or ZnMgO (second buffer layer) were not used in this study. Grid contacts made of Al or Ni/Al metals were formed on the TCO layer. Selected cells were coated with an MgF2 anti-reflection coating. The designated device area was 0.52 cm2. CIGS film and device characterizations. SEM measurements were performed to observe the CIGS film morphologies using acceleration voltages ranging from 3-5 kV. EBIC measurements


16

Page 17 of 35

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


were performed with an acceleration voltage of 5 kV using Ar-ion milled cross section of the CIGS films. Note that ion milled samples may lead to the formation of artifacts in the EBIC image due to possible ion damage. We have observed, however, similar bright grain boundary images from just cleaved cross sections as well. TEM measurements were carried out with acceleration voltages ranging from 200-300 kV using Ga-ion FIB processed CIGS samples. SIMS measurements were carried out using Cs+ as the primary ion with an acceleration voltage of 5 kV. Na, K, and Si concentrations in the CIGS layers were calculated using standard CIGS samples prepared by ion implantation. J-V curves were measured in-house under AM 1.5 G (100 mW/cm2) illumination at 25℃. Also, independently certified J-V and EQE measurements were carried out at JET. C-V measurements were carried out with a 50 mV and 100 kHz AC signal within the bias voltage range from 0.6 to -3 V and successive NCV values were calculated from the data. SSRM measurements were performed using a conductive diamond-coated Si probe with a bias voltage of 1.5 V. Cross sectional surfaces of CIGS samples used for the measurements were prepared by mechanical polishing with diamond and colloidal silica abrasives.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM, TEM-EDX and TEM-EELS images of CIGS thin films, the original data sheet of the independently efficiency measured by JET (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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 35

ORCID Shogo Ishizuka: 0000-0002-4404-5257 Author contributions S.I. conceived the idea, designed the experiments, and wrote the manuscript. S.I. fabricated and processed all treated CIGS thin films and solar cell devices and performed SEM, EBIC, J-V, and C-V measurements at AIST. S.I. also directed SEM, EBIC, TEM, SIMS and SSRM measurements and analyses carried out at the Foundation for Promotion of Material Science and Technology of Japan (MST) and directed J-V and EQE measurements carried out at JET. T.K. deposited ZnO:B TCO layers for device fabrication. N.T. and S.T. performed TEM-EDX and TEM-EELS measurements at AIST and analyzed the relevant data. S.I. and P.J.F. collaborated on the paper. All authors contributed to discussions and reviewed the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank H. Higuchi, M. Iioka, Y. Ueno, and H. Takahashi for their help with the experiments and technical support. We also acknowledge J. Nishinaga and Y. Kamikawa for helping in the experiments. This work was supported by JSPS KAKENHI Grant Number 16K04969, also supported in part by AIST internal funds, the Department of Energy and Environment Innovation Program and the Research Center for Photovoltaics Step-Up Program.

REFERENCES


18

Page 19 of 35

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


(1) Kato, T. Cu(In,Ga)(Se,S)2 Solar Cell Research in Solar Frontier: Progress and Current Status. Jpn. J. Appl. Phys. 2017, 56, 04CA02. (2) Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of Heavy Alkali Elements in Cu(In,Ga)Se2 Solar Cells with Efficiencies Up to 22.6%. Phys. Stat. Sol. RRL 2016, 10, 583-586. (3) Kranz, L.; Gretener, C.; Perrenoud, J.; Schmitt, R.; Pianezzi, F.; La Mattina, F.; Blösch, P.; Cheah, E.; Chirilá, A.; Fella, C. M.; Hagendorfer, H.; Jäger, T.; Nishiwaki, S.; Uhl, A. R.; Buecheler, S.; Tiwari, A. N. Doping of Polycrystalline CdTe for High-Efficiency Solar Cells on Flexible Metal Foil. Nat. Commun. 2013, 4, 2306; doi: 10.1038/ncomms3306. (4) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Ho-Baillie, A. W. Y. Solar Cell Efficiency Tables (Version 49). Prog. Photovolt. Res. Appl. 2017, 25, 313. (5) Wagner, Sigurd; Shay, J. L.; Migliorato, P.; Kasper, H. M. CuInSe2/CdS Heterojunction Photovoltaic Detectors. Appl. Phys. Lett. 1974, 25, 434-435. (6) Shay, J. L.; Wagner, Sigurd; Kasper, H. M. Efficient CuInSe2/CdS Solar Cells. Appl. Phys. Lett. 1975, 27, 89-90. (7) Niki, S.; Contreras, M.; Repins, I.; Powalla, M.; Kushiya, K.; Ishizuka, S.; Matsubara, K. CIGS Absorbers and Processes. Prog. Photovolt. Res. Appl. 2010, 18, 453-466. (8) Matsunaga, K.; Komaru, T.; Nakayama, Y.; Kume, T.; Suzuki, Y. Mass-Production Technology for CIGS Modules. Sol. Energy Mater. Sol. Cells 2009, 93, 1134-1138. (9) Wada, T.; Hayashi, S.; Hashimoto, Y.; Nishiwaki, S.; Sato, T.; Negami, T.; Nishitani, M. High Efficiency Cu(In,Ga)Se2 (CIGS) Solar Cells with Improved CIGS Surface. Proc. of the


19


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 20 of 35

2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion 1998, 403408. (10)

Nakada, T.; Kunioka, A. Direct Evidence of Cd Diffusion into Cu(In,Ga)Se2 Thin Films

During Chemical-Bath Deposition Process of CdS Films. Appl. Phys. Lett. 1999, 74, 24442446. (11)

Jiang, C.-S.; Hasoon, F. S.; Moutinho, H. R.; Al-Thani, H. A.; Romero, M. J. Direct

Evidence of a Buried Homojunction in Cu(In,Ga)Se2 Solar Cells. Appl. Phys. Lett. 2003, 82, 127-129. (12)

Hunger, R.; Fons, P.; Iwata, K.; Yamada, A.; Matsubara K.; Niki, S.; Nakahara K.;

Takasu, H. Observation of Interdiffusion in ZnO/CuInSe2 Heterostructures and Its Effect on Film Properties. Mat. Res. Soc. Symp. Proc. 2001, 668, H8.21.1. (13)

Orgassa, K.; Rau, U.; Nguyen, Q.; Schock, H.-W.; Werner, J. H. Role of the CdS Buffer

Layer as an Active Optical Element in Cu(In,Ga)Se2 Thin-Film Solar Cells. Prog. Photovolt. Res. Appl. 2002, 10, 457-463. (14)

Hultqvist, A.; Li, J. V.; Kuciauskas, D.; Dippo, P.; Contreras, M. A.; Levi, D. H.; Bent, S.

F. Reducing Interface Recombination for Cu(In,Ga)Se2 by Atomic Layer Deposited Buffer Layers. Appl. Phys. Lett. 2015, 107, 033906. (15)

Ishizuka, S.; Sakurai, K.; Yamada, A.; Matsubara, K.; Fons, P.; Iwata, K.; Nakamura, S.;

Kimura, Y.; Baba, T.; Nakanishi, H.; Kojima, T.; Niki, S. Fabrication of Wide-Gap Cu(In1xGax)Se2

Thin Film Solar Cells: a Study on the Correlation of Cell Performance with Highly

Resistive i-ZnO Layer Thickness. Sol. Energy Mater. Sol. Cells 2005, 87, 541-548. (16)

Scheer, R.; Messmann-Vera, L.; Klenk, R.; Schock, H.-W. On the Role of Non-Doped

ZnO in CIGSe Solar Cells. Prog. Photovolt. Res. Appl. 2012, 20, 619-624.


20

Page 21 of 35

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


(17)

Mizumoto, Y.; Chantana, J.; Hironiwa, D.; Yamamoto, A.; Yabuki, K.; Nakaue, A.;

Minemoto, T. Evaluation of Junction Quality of Buffer-Free Zn(O,S):Al/Cu(In,Ga)Se2 ThinFilm Solar Cells. Appl. Phys. Express 2014, 7, 125503. (18)

Kuwahara, Y.; Minemoto, T. Impact of Zn1-xMgxO:Al Transparent Electrode for Buffer-

Less Cu(In,Ga)Se2 Solar Cells. Renewable Energy 2014, 65, 113-116. (19)

Haarstrich, J.; Teichmann, M.; Metzner, H.; Gnauck, M.; Ronning, C.; Wesch, W.;

Rissom, T.; Kaufmann, C. A.; Schock, H.-W.; Scheumann, W.; Mannstadt, W. Buffer-Free Cu(In,Ga)Se2-Solar Cells by Near-Surface Ion Implantation. Sol. Energy Mater. Sol. Cells 2013, 116, 43-48. (20)

Sugiyama, T.; Chaisitsak, S.; Yamada, A.; Konagai, M.; Kudriavtsev, Y.; Godines, A.;

Villegas, A.; Asomoza, R. Formation of PN Homojunction in Cu(In,Ga)Se2 Thin Film Solar Cells by Zn Doping. Jpn. J. Appl. Phys. 2000, 39, 4816-4819. (21)

Ramanathan, K.; Hasoon, F. S.; Smith, S.; Young, D. L.; Contreras, M. A.; Johnson, P.

K.; Pudov, A. O.; Sites, J. R. Surface Treatment of CuInGaSe2 Thin Films and Its Effect on the Photovoltaic Properties of Solar Cells. J. Phys. Chem. Solids 2003, 64, 1495-1498. (22)

Hiraga, H.; Nakagawa, N.; Shibasaki, S.; Yamazaki, M.; Yamamoto, K.; Sakurada, S.

Photovoltaic Properties of Homojunction CIGS Solar Cells Using Various Divalent N-Type Dopants. Proc. of the 28th European Photovoltaic Solar Energy Conference and Exhibition 2013, 2299-2301. (23)

Paudel, N. R.; Yan, Y. Enhancing the Photo-Currents of CdTe Thin-Film Solar Cells in

Both Short and Long Wavelength Regions. Appl. Phys. Lett. 2014, 105, 183510. (24)

Ishizuka, S.; Kato, S.; Okamoto, Y.; Akimoto, K. Control of Hole Carrier Density of

Polycrystalline Cu2O Thin Films by Si Doping. Appl. Phys. Lett. 2002, 80, 950-952.


21


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

(25)

Page 22 of 35

Ishizuka, S.; Akimoto, K. Control of the Growth Orientation and Electrical Properties of

Polycrystalline Cu2O Thin Films by Group-IV Elements Doping. Appl. Phys. Lett. 2004, 85, 4920-4922. (26)

Gabor, A. M.; Tuttle, J. R.; Albin, D. S.; Contreras, M. A.; Noufi, R.; Hermann, A. M.

High-Efficiency CuInxGa1-xSe2 Solar Cells Made From (Inx,Ga1-x)2Se3 Precursor Films. Appl. Phys. Lett. 1994, 65, 198. (27)

Contreras, M. A.; Mansfield, L. M.; Egaas, B.; Li, J.; Romero, M.; Noufi, R.; Rudiger-

Voigt, E.; Mannstadt, W. Wide Bandgap Cu(In,Ga)Se2 Solar Cells with Improved Energy Conversion Efficiency. Prog. Photovolt. Res. Appl. 2012, 20, 843-850. (28)

Kawamura, M.; Yamada, T.; Suyama, N.; Yamada, A.; Konagai, M. Grain Boundary

Evaluation of Cu(In1-xGax)Se2 Solar Cells. Jpn. J. Appl. Phys. 2010, 49, 062301. (29)

Keller, D.; Buecheler, S.; Reinhard, P.; Pianezzi, F.; Bissig, B.; Carron, R.; Hage, F.;

Ramasse, Q.; Erni, R.; Tiwari, A. N. Band Gap Widening at Random CIGS Grain Boundary Detected by Valence Electron Energy Loss Spectroscopy. Appl. Phys. Lett. 2016, 109, 153103. (30)

Nicoara, N.; Lepetit, Th.; Arzel, L.; Harel, S.; Barreau, N.; Sadewasser, S. Effect of the

KF Post-Deposition Treatment on Grain Boundary Properties in Cu(In,Ga)Se2 Thin Films. Sci. Rep. 2017, 7, 41361; doi: 10.1038/srep41361. (31)

Jiang, C.-S.; Noufi, R.; AbuShama, J. A.; Ramanathan, K.; Moutinho, H. R.; Pankow, J.;

Al-Jassim, M. M. Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films. Appl. Phys. Lett. 2004, 84, 3477-3479.


22

Page 23 of 35

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


(32)

Leendertz, C.; Streicher, F.; Lux-Steiner, M. Ch.; Sadewasser, S. Evaluation of Kelvin

Probe Force Microscopy for Imaging Grain Boundaries in Chalcopyrite Thin Films. Appl. Phys. Lett. 2006, 89, 113120. (33)

Abou-Ras, D.; Schaffer, B.; Schaffer, M.; Schmidt, S. S.; Caballero, R.; Unold, T. Direct

Insight into Grain Boundary Reconstruction in Polycrystalline Cu(In,Ga)Se2 with Atomic Resolution. Phys. Rev. Lett. 2012, 108, 075502. (34)

Ishizuka, S.; Yamada, A.; Fons, P.; Niki, S. Flexible Cu(In,Ga)Se2 Solar Cells Fabricated

Using Alkali-Silicate Glass Thin Layers as an Alkali Source Material. J. Renewable Sustainable Energy 2009, 1, 013102. (35)

Chirilá, A.; Reinhard, P.; Pianezzi, F.; Bloesch, P.; Uhl, A. R.; Fella, C.; Kranz, L.; Keller,

D.; Gretener, C.; Hagendorfer, H.; Jaeger, D.; Erni, R.; Nishiwaki, S.; Buecheler, S.; Tiwari, A. N. Potassium-Induced Surface Modification of Cu(In,Ga)Se2 Thin Films for HighEfficiency Solar Cells. Nat. Mater. 2013, 12, 1107-1111. (36)

Raghuwanshi, M.; Cadel, E.; Duguay, S.; Arzel, L.; Barreau, N.; Pareige, P. Influence of

Na on Grain Boundary and Properties of Cu(In,Ga)Se2 Solar Cells. Prog. Photovolt. Res. Appl. 2017, 25, 367-375. (37)

Minemoto, T.; Julayhi, J. Buffer-Less Cu(In,Ga)Se2 Solar Cells by Band Offset Control

Using Novel Transparent Electrode. Curr. Appl. Phys. 2013, 13, 103-106. (38)

Reinhard, P.; Bissig, B.; Pianezzi, F.; Avancini, E.; Hagendorfer, H.; Keller, D.; Fuchs,

P.; Döbeli, M.; Vigo, C.; Crivelli, P.; Nishiwaki, S.; Buecheler, S.; Tiwari, A. N. Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se2 Absorbers for High Efficiency Thin Film Solar Cells. Chem Mater. 2015, 27, 5755-5764.


23


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

(39)

Page 24 of 35

Ishizuka, S.; Fons, P. J.; Yamada, A.; Kamikawa-Shimizu, Y.; Shibata, H. Interface

Oxygen and Heat Sensitivity of Cu(In,Ga)Se2 and CuGaSe2 Solar Cells. Appl. Phys. Lett. 2016, 108, 203902. (40)

Rusu, M.; Wiesner, S.; Würz, R.; Lehmann, S.; Doka-Yamigno, S.; Meeder, A.; Fuertes

Marrón, D.; Bär, M.; Koteski, V.; Mahnke, H.-E.; Arushanov, E.; Beckmann, J.; Höhn, K.; Fritsch, W.; Bohne, W.; Schubert-Bischoff, P.; Heuken, M.; Jäger-Waldau, A.; Rumberg, A.; Schedel-Niedrig, Th. CuGaxSey Chalcopyrite-Related Thin Films Grown by Chemical CloseSpaced Vapor Transport (CCSVT) for Photovoltaic Application: Surface- and Bulk Material Properties, Oxidation and Surface Ge-Doping. Sol. Energy Mater. Sol. Cells 2011, 95, 15551580. (41)

Thiru, S.; Fujita, M.; Kawaharazuka, A.; Horikoshi, Y. Photoluminescence Study of Si

Doped and Undoped Chalcopyrite CuGaSe2 Thin Films. Appl. Phys. A 2013, 113, 257-261. (42)

Chen, X.-a.; Wada, H; Sato, A.; Nozaki, H. Synthesis, Structure, and Electronic

Properties of Cu2SiQ3 (Q=S, Se). J. Alloys Compounds 1999, 290, 91-96. (43)

Shigemi, A.; Maeda, T.; Wada, T. First-Principles Calculation of Cu2SnS3 and Related

Compounds. Phys. Stat. Sol. B 2015, 252, 1230-1234. (44)

Tenhover, M.; Hazle, M. A.; Grasselli, R. K. Atomic Structure of SiS2 and SiSe2 Glasses.

Phys. Rev. Lett. 1983, 51, 404-406. (45)

Celino, M.; Massobrio, C. Origin of Network Connectivity and Structural Units in

Amorphous SiSe2. Phys. Rev. Lett. 2003, 90, 125502. (46)

Fearheiley, M. L. The Phase Relations in the Cu,In,Se System and the Growth of CuInSe2

Single Crystals. Solar Cells 1986, 16, 91-100.


24

Page 25 of 35

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


(47)

Sachanyuk, V. P.; Olekseyuk, I. D.; Parasyuk, O. V. Phase Diagram of the CuGaSe2-

SiSe2 and CuInSe2-SiSe2 Systems. J. Alloys Compounds 2006, 420, 54-57. (48)

Handick, E.; Reinhard, P.; Alsmeier, J.-H.; Köhler, L.; Pianezzi, F.; Krause, S.; Gorgoi,

M.; Ikenaga, E.; Koch, N.; Wilks, R. G.; Buecheler, S.; Tiwari, A. N.; Bär, M. Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se2 Surfaces – Reason for Performance Leap? ACS Appl. Mater. Interfaces 2015, 7, 27414-27420. (49)

Minemoto, T.; Hashimoto, Y.; Satoh, T.; Negami T.; Takakura, H.; Hamakawa, Y.

Cu(In,Ga)Se2 Solar Cells with Controlled Conduction Band Offset of Window/Cu(In,Ga)Se2 Layer. J. Appl. Phys. 2001, 89, 8327-8330. (50)

Minemoto, T.; Matsui, T.; Takakura, H.; Hamakawa, Y.; Negami, T.; Hashimoto, Y.;

Uenoyama, T.; Kitagawa, M. Theoretical Analysis of the Effect of Conduction Band Offset of Window/CIS Layers on Performance of CIS Solar Cells Using Device Simulation. Sol. Energy Mater. Sol. Cells 2001, 67, 83-88. (51)

Pinanezzi, F.; Reinhard, P.; Chirilá, A.; Bissig, B.; Nishiwaki, S.; Buecheler, S.; Tiwari,

A. N. Unveiling the Effects of Post-Deposition Treatment with Different Alkaline Elements on the Electronic Properties of CIGS Thin Film Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 8843-8851. (52)

Koida, T.; Nishinaga, J.; Higuchi, H.; Kurokawa, A.; Iioka, M.; Kamikawa-Shimizu, Y.;

Yamada, A.; Shibata, H.; Niki, S. Comparison of ZnO:B and ZnO:Al Layers for Cu(In,Ga)Se2 Submodules. Thin Solid Films 2016, 614, 79-83.


25


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 26 of 35

Figure Captions Figure 1. Cross sectional SEM images of Mo/CIGS/ZnO:B devices fabricated with (a) a Sidoped CIGS photoabsorber layer and (b) a control (non Si-doped) CIGS layer for reference. (c) The solar cell device structure used in this study. (d) A cross sectional SEM image overlapped with a color EBIC map image and EBIC raw data measured using an identical Si-doped CIGS device to that shown in (a). (e) A cross sectional SEM image overlapped with a color EBIC map image and EBIC raw data measured using a non Si-doped CIGS device identical to that shown in (b). Figure 2. (a) Cross sectional TEM images of a Si-doped CIGS film. (b) Cross sectional SEM images of an as grown Si-doped CIGS film which was 10 months after growth. (c) SEM-EDX results obtained from the cross section of a Si-doped CIGS film. Points 1 and 2 indicate the CIGS grain interior (GI) profiles and points 3 and 4 indicate grain boundary (GB) region profiles. Line 5 indicates elemental line profiles from the cross to the circle markers. Figure 3. (a) Elemental Si distribution profiles in the CIGS layers. (b,c) elemental Na and K distribution profiles in the CIGS layers. Elemental Zn signal tails observed in the CIGS surface region are artifacts due to the uneven CIGS surface morphology. Also, the elemental Si signal observed near the CIGS/ZnO:B interface is an artifact attributable to interference from ions such as 16O + 17O + 64Zn + 64Zn (= 28Si + 133Cs = 161). Figure 4. (a) J-V, P-V and (b) EQE curves obtained from a Si-doped CIGS solar cell fabricated with a buffer-free Mo/CIGS/ZnO:B configuration. An MgF2 anti-reflection coating was used for the measurements. The solar cell efficiency was independently certified by the Japan Electrical Safety and Environment Technology Laboratories (JET), Yokohama, Japan. Eff., Jsc, FF are efficiency, short circuit current density, fill factor, respectively. Figure 5. (a-d) Solar cell parameters obtained from a Mo/CIGS/ZnO:B configuration devices using control (non Si-doped), Si-doped, and KF- or RbF-PDT CIGS photoabsorber layers. No anti-reflection coating was used and no light soaking treatments were performed. Red circles and green triangles indicate KF- and RbF-PDT CIGS devices, respectively. (e-g) Typical J-V curve shapes measured under dark and light (AM 1.5 G, 100 mW/cm2, 25℃) conditions in both forward (black solid lines) and reverse (red dotted lines) applied bias directions with a measurement step and a delay time of 0.01 V and 1 ms, respectively. (h-j) CIGS surface SEM images of (h) control (non Si-doped), (i) Si-doped, (j) RbF-PDT CIGS thin films after washing in a dilute ammonia solution. Jsc, short circuit current density. FF, fill factor. Figure 6. (a) Typical space charge density (NCV)-depletion width profiles calculated from C-V measurements for control, Si-doped, and KF- or RbF-PDT CIGS devices. The control, Si-doped and KF-PDT CIGS films used for the C-V measurements were grown in an identical growth batch to the CIGS films used for the SIMS measurements shown in Figure 3a-c. (b) Cross sectional SSRM maps obtained from control, Si-doped, and KF-PDT CIGS devices. Figure 7. Schematic band diagrams of the CIGS/TCO (ZnO:B) junction for control (straight lines) and Si-doped (dotted lines) CIGS devices. EC and EV are the conduction band minimum


26

Page 27 of 35

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


and valence band maximum, respectively. The scale of the energy levels is not accurate and is exaggerated to emphasis the variation.


27


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. Cross sectional SEM images of Mo/CIGS/ZnO:B devices fabricated with (a) a Si-doped CIGS photoabsorber layer and (b) a control (non Si-doped) CIGS layer for reference. (c) The solar cell device structure used in this study. (d) A cross sectional SEM image overlapped with a color EBIC map image and EBIC raw data measured using an identical Si-doped CIGS device to that shown in (a). (e) A cross sectional SEM image overlapped with a color EBIC map image and EBIC raw data measured using a non Si-doped CIGS device identical to that shown in (b). 150x121mm (300 x 300 DPI)


Page 28 of 35

Page 29 of 35

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 2. (a) Cross sectional TEM images of a Si-doped CIGS film. (b) Cross sectional SEM images of an as grown Si-doped CIGS film which was 10 months after growth. (c) SEM-EDX results obtained from the cross section of a Si-doped CIGS film. Points 1 and 2 indicate the CIGS grain interior (GI) profiles and points 3 and 4 indicate grain boundary (GB) region profiles. Line 5 indicates elemental line profiles from the cross to the circle markers. 170x193mm (300 x 300 DPI)



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. (a) Elemental Si distribution profiles in the CIGS layers. (b,c) elemental Na and K distribution profiles in the CIGS layers. Elemental Zn signal tails observed in the CIGS surface region are artifacts due to the uneven CIGS surface morphology. Also, the elemental Si signal observed near the CIGS/ZnO:B interface is an artifact attributable to interference from ions such as 16O + 17O + 64Zn + 64Zn (= 28Si + 133Cs = 161). 150x330mm (300 x 300 DPI)


Page 30 of 35

Page 31 of 35

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


Caption : Figure 4. (a) J-V, P-V and (b) EQE curves obtained from a Si-doped CIGS solar cell fabricated with a buffer-free Mo/CIGS/ZnO:B configuration. An MgF2 anti-reflection coating was used for the measurements. The solar cell efficiency was independently certified by the Japan Electrical Safety and Environment Technology Laboratories (JET), Yokohama, Japan. Eff., Jsc, FF are efficiency, short circuit current density, fill factor, respectively. 85x130mm (300 x 300 DPI)



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 5. (a-d) Solar cell parameters obtained from a Mo/CIGS/ZnO:B configuration devices using control (non Si-doped), Si-doped, and KF- or RbF-PDT CIGS photoabsorber layers. No anti-reflection coating was used and no light soaking treatments were performed. Red circles and green triangles indicate KF- and RbFPDT CIGS devices, respectively. (e-g) Typical J-V curve shapes measured under dark and light (AM 1.5 G, 100 mW/cm2, 25℃) conditions in both forward (black solid lines) and reverse (red dotted lines) applied bias directions with a measurement step and a delay time of 0.01 V and 1 ms, respectively. (h-j) CIGS surface SEM images of (h) control (non Si-doped), (i) Si-doped, (j) RbF-PDT CIGS thin films after washing in a dilute ammonia solution. Jsc, short circuit current density. FF, fill factor. 170x159mm (300 x 300 DPI)


Page 32 of 35

Page 33 of 35

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 6. (a) Typical space charge density (NCV)-depletion width profiles calculated from C-V measurements for control, Si-doped, and KF- or RbF-PDT CIGS devices. The control, Si-doped and KF-PDT CIGS films used for the C-V measurements were grown in an identical growth batch to the CIGS films used for the SIMS measurements shown in Figure 3a-c. (b) Cross sectional SSRM maps obtained from control, Si-doped, and KF-PDT CIGS devices. 85x132mm (300 x 300 DPI)



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 7. Schematic band diagrams of the CIGS/TCO (ZnO:B) junction for control (straight lines) and Sidoped (dotted lines) CIGS devices. EC and EV are the conduction band minimum and valence band maximum, respectively. The scale of the energy levels is not accurate and is exaggerated to emphasis the variation. 85x55mm (300 x 300 DPI)


Page 34 of 35

Page 35 of 35

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


Abstract Figure 85x39mm (300 x 300 DPI)


Si-Doping Effects in Cu(In,Ga)Se2

Recommend Documents