Abstract:
A process described herein provides an economical means for producing the oxide-based buffer layers using a wet chemical CSD process wherein the desired buffer layer material results from the evaporation of a chemical already containing the material in solution. Thus, no residual liquid chemical elements remain after deposition, and as there is no reaction to create the buffer material, as is the case with CdS CBD, the liquid elements in CSD have sufficiently long shelf life after mixing to as to improve manufacturability and further reduce waste. Furthermore, as there is no in-chamber reaction to create the buffer material solution, there are many options for delivering said solution to the CIGS absorber layer. Finally, as the oxide films for the CdS replacement have inherently better transmission in the blue spectrum, aggressive thinning of films to improve current generation is unnecessary.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority of Provisional Application Ser. No. 61/380,994 filed Sep. 8, 2010, which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to Cd-free, oxide buffers layers for thin film copper indium gallium di(selenide) (CIGs) solar cells and processes for making and using the same. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    A solar cell, or photovoltaic cell, is a device that converts solar energy into electrical energy. Solar cells generate voltage, or electrical current, upon irradiation with electromagnetic radiation, such as sunlight. Traditional solar cells are fabricated from silicon-based semiconducting materials. Other solar cells contain polycrystalline material comprising copper indium gallium (di)selenide (CIGS). CIGS is a semiconductor material utilized as a light absorber for photovoltaic cells and is typically present as a polycrystalline thin film. 
         [0004]    CIGS-based solar cells operate by absorbing light within the CIGS layer and subsequently generating free electrons with the capacity for movement throughout the CIGS material. Free electrons may diffuse within the CIGS material until reaching an electric field at a junction region. For CIGS devices, junction regions are often formed between the semiconductors CIGS and zinc oxide (ZnO) and may also contain a thin buffer layer containing materials such as cadmium sulfide (CdS) and intrinsic ZnO (i-ZnO). CdS is typically used as a heterojunction partner to CIGS due to certain electrical properties, as well as the synergistic surface effects with a chemical bath deposition (CBD). 
         [0005]    CIGS material demonstrates a variety of advantages for solar cells such as displaying a high extinction coefficient that facilitates the fabrication of thin solar cells. For example, reports indicate absorption of about 99% of incident within approximately 1 μm of a CIGS layer. CIGS-based solar cells also show superior performance properties as compared to other semiconducting materials. For example, CIGS material displays one of the highest current densities of known semiconductor material, thereby offering the possibility to produce high current outputs. Moreover, CIGS material shows superior solar-to electrical energy conversion efficiencies with reports indicating greater than 20% conversion efficiencies for CIGS-based solar cells. 
         [0006]    Buffer layers for CIGS-based solar cells may also include zinc oxide (ZnO), tin dioxide (SnO 2 ), and (SnO,S) 2 . These buffer layers, however, exhibit low efficiencies, typically between 9%-12%. Moreover, these buffer layers are typically deposited via chemical bath or physical vapor deposition, which are slow, complex, and require multiple chemicals, (R. Mikami et. al, 3 rd  World Conference of PV Energy Conversion, p. 5198 (2003); D. Hariskos et. al, Proc. 13 th  European PV-Solar Energy Conference, p. 1995 (1995). 
         [0007]    Traditional construction of copper-indium-gallium-(di)selenide (CIGS) solar cells consists of a suitably smooth substrate, a first electrical conductor, a CIGS-based absorber layer, a cadmium sulfide (CdS) buffer layer, and a combination of transparent intrinsic and conductive oxide films that serve as a top electrical contact. While CdS historically has been the buffer layer of choice, there are several compelling reasons for its substitution in the stack, including, but not limited to, blue-spectrum attenuation, limited market acceptance for cadmium (Cd)-containing materials, and additional cost to produce the device in an environmentally benign manner. The nominal chemical bath deposition (CBD) process includes a temperature-sensitive reaction between chemical elements that results in the deposition of the desired CdS material during which the solution is exhausted and must be removed and sequestered from the process chamber. Additional expense for sequestering and filtering Cd from waste streams in the factory associated with wet-chemical CdS processing that is the most common method for CdS deposition also adds to the cost of the final product. CIGS-based solar cells with Cd-free, oxide buffer layers with a chemical solution deposition (CSD) process facilitates inexpensive, facile, and non-toxic buffer layer deposition. 
       SUMMARY 
       [0008]    The presently disclosed instrumentalities advance the art by providing improved Cd-free, oxide buffer layers for CIGS solar cells having increased energy conversion efficiencies. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1A  shows a conventional CIGS-based device that contains a CdS buffer layer with  FIG. 1B  showing expanded detail with respect to a selected area of  FIG. 1A . 
           [0010]      FIG. 2  shows an example of a CIGS-based device that contains a ZTO buffer layer with  FIG. 2A  showing expanded detail with respect to a selected area of  FIG. 2 . 
           [0011]      FIG. 3  shows the performance of a CIGS-based device with a CdS layer and a CIGS-based device with a ZTO layer deposited via CSD. 
           [0012]      FIG. 4  shows the performance of a CIGS-based device without an i-ZnO layer. 
           [0013]      FIG. 5  shows a series of micrographs of ZTO films on CIGS devices. 
           [0014]      FIG. 6  shows a CIGS-based device with a ZTO layer and without an i-ZnO layer with  FIG. 6A  showing expanded detail with respect to a selected area of  FIG. 6 . 
           [0015]      FIG. 7  shows the effects of pre-washing CIGS samples prior to ZTO deposition. 
           [0016]      FIG. 8  shows the temperature dependence of a CIGS sample with CdS and a CIGS sample with ZTO. 
           [0017]      FIG. 9  shows the performance of a CIGS sample with a ZTO layer and a CIGS sample with a CdS layer for shorter duration deposition dwell time. 
           [0018]      FIG. 10  shows various coating systems including a chemical spray ( FIG. 10A ), slot die coating system ( FIG. 10B ) and a gravure coating system ( FIG. 10C ) suitable for ZTO deposition. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present disclosure provides improved CIGS-based solar cells with Cd-free, oxide buffer layers that facilitate inexpensive, facile, and non-toxic buffer layer deposition. In a preferred embodiment, Cd-free, oxide buffer layers for CIGS-based solar cells may comprise zinc-tin-oxide (ZTO) material. In one embodiment, a CIGS-based solar cell may contain multiple functional layers including a substrate layer, a electrical contact layer, a buffer layer, a CIGS layer and a transparent, conductive layer. In one embodiment, the electrical contact layer contains molybdenum, molybdenum alloy or multilayer metallic films. In various embodiments, the substrate contains glass, polymer such as polyimide, molybdenum, aluminum, copper, and/or stainless steel. In one embodiment, the transparent, conductive layer contains indium tin oxide (ITO), which alternatively may include CuAlO 2 , SnO—F, and/or Ag. 
         [0020]      FIG. 1  shows a conventional CIGS device  100  positioned atop a polyimide substrate  112  and a back-side layer  114 . In one embodiment, the back-side layer  114  serves to balance film stresses from the device  100  and to provide the substrate  112  with basic environmental protection prior to encapsulation. In one embodiment, this back-side layer contains molybdenum, other metals, or metal oxides.  FIG. 1A  is an expanded view of the CIGS device  100  at an increased proportional scale.  FIG. 1A  contains multiple layers comprising transparent, conductive layer  102 , i-ZnO  104 , CdS  106 , CIGS  108 , and electrical contact layer  110 . In one embodiment, the transparent, conductive layer contains indium tin oxide (ITO), which alternatively may include CuAlO2, SnO—F, and/or Ag. Electrical contact layer  110  serves as a base electrode due to its electrical conductivity and work function. In one embodiment, the electrical contact layer  110  contains molybdenum, molybdenum alloys, or multilayer films that function as an electrical conductor as a whole. Where sub-bandgap light transmission through the CIGS film  108  is desired, electrical contact layer  110  may also contain metallic oxides that are transparent in the desired portions of the spectrum. The CdS buffer layer  106  and the CIGS absorber layer  108  comprise the n and p-type semiconductors needed to generate the requisite electrical field necessary for proper photovoltaic operation. In this embodiment, the transparent conductive layer  102  and the electrical contact layer  110  function as the negative and positive terminals, respectively, of the resultant photovoltaic device. The transparent conductive layer may be made of, for example, indium tin oxide, CuAlO 2 , SnO—F, and/or Ag. 
         [0021]    In the embodiment illustrated in  FIG. 1A , light hitting the transparent conductive layer  102  is transmitted through to the underlying i-ZnO film  104 . This light is then transmitted through the CdS buffer layer  106  to the CIGS absorber layer  108 . Energy from the light reaching the absorber layer  108  is either converted to electricity, reflected, or is converted to waste energy (heat). Each layer through which the light passes,  102 ,  104 , and  106 , each absorb some portion of the spectrum, although  102  and  104  are ideally highly transparent in the visible spectrum to which the CIGS absorber layer  108  is sensitive. Depending upon the buffer layer composition and/or thickness, a portion of the spectrum passing through buffer layer  106  may be attenuated and thus not available for conversion in the absorber layer  108 . 
         [0022]    Another complication of the embodiment shown in  FIG. 1A  is the nature of the CdS buffer layer. While layers  102  and  104  are relatively transparent within the light spectrum to which the absorber layer  108  is responsive, most embodiments of the CdS buffer layer  106  are not. Attenuation in the blue portion of the spectrum restricts the light available in this portion of the spectrum from reaching the absorber layer  108 ; thus, CdS layers are often kept as thin as possible (approximately 200 Å-300 Å) to attenuate the least amount of light while providing the requisite electrical function to the device. Consequently, the intrinsic layer  104  is often employed to aid in preventing electrical shorting, or shunts through pinholes that may be present in excessively-thinned CdS buffer layer  106 . While the presence of the i-ZnO does not necessarily provide a mechanism for performance reduction in as-fabricated devices, the hygroscopic nature of the zinc oxide system and its links to moisture-related degradation in CIGS devices can provide a mechanism for long-duration failures of CIGS photovoltaic devices  100  in the field. Thus, reduction in the intrinsic ZnO components  104  in the device can be advantageous as well, but not at the expense of attenuation from a thicker CdS buffer layer  106 . 
         [0023]    Advantageously, ZTO material does not attenuate light within the blue portion of the spectrum, thereby providing benefits such as film thickness independence. For example, when using ZTO the light attenuation within the spectrum to which CIGS absorber layer  108  is sensitive is not an issue and, as such, a thicker buffer layer comprised of ZTO will not be an issue similar to the CdS buffer film  106  noted earlier. Thus, a ZTO buffer film may be sufficiently thick so as to eliminate potential pinholes and other poor deposition coverage issues that may be related to device shunting. 
         [0024]    In some embodiments, utilizing thick ZTO layers within a Cd-free CIGS-based solar cell may permit elimination of i-ZnO layer. In one nonlimiting example, a highly efficient CIGS solar cell is generated by using a thick ZTO film and eliminating an i-ZnO layer, thereby eliminating the possibility of water vapor-related environmental failures. Elimination of water vapor-related environmental failures may provide a robust device with increased longevity. In one embodiment, eliminating an i-ZnO layer and using a ZTO layer permits use of Cd-free CIGS-based solar cells for building-integrated photovoltaic. In another embodiment, using a ZTO layer facilitates the replacement of an i-ZnO cathode with an ITO cathode to improve deposition speed and throughput. 
         [0025]    In some embodiments, the application of Cd-free, oxide buffer to CIGS absorbers may occur through chemical solution deposition, spin-coating, or roll-to-roll (R2R) coating system employing a chemical spray, slot-die or gravure printing approach. In a preferred embodiment, the zinc-tin-oxide material is applied to CIGS material via spin-coating in non-commercial applications. These application methods are facile and rapid without producing excessive chemical waste. In one embodiment, the application of Cd-free, oxide buffer to CIGS absorbers occurs at temperatures less than 300° C. 
         [0026]    In one example, a zinc-tin based material is applied to a CIGS absorber with a formula: 
         [0000]      Zn (1-x) Sn (x)    
         [0000]    Where x may range from 0.0 to 0.75 and more preferably ranges from 0.25 to 0.5. In various embodiments, x may range from: 0.0&lt;x≦0.25, 0.25≦x≦0.33, 0.33≦x≦0.5, 0.5≦x≦0.75. In one embodiment, x=0.25. Alternately, zinc-tin based material may also be doped with cadmium-based material, such as cadmium sulfide. Other nonlimiting examples of dopants for zinc-tin based material include Ga, In, Mg, F, and Cl. In one example, dopants may be present within the zinc-tin based material between 0 weight % and 3 weight %. 
         [0027]    The following descriptions will show and describe, by way of non-limiting examples, improved CIGS solar cells with Cd-free, oxide buffer layers. The following nonlimiting examples describe preparation and characterization of CIGS solar cells with Cd-free, oxide buffer layers. It is to be understood that these examples are provided by way of illustration and should not be unduly construed to limit the scope of what is disclosed herein. 
       Example  1   
     Preparation and Characterization of Cigs Solar Cells with Cd-Free, Oxide Buffer Layers 
       [0028]    This example teaches by way of illustration, not by limitation, preparation and characterization of CIGS solar cells with Cd-free, oxide buffer layers. A metal-organic solution of Zn-Acetate and Sn-chloride is made in Methanol/Tri-Fluoro Acetic Acid solvent system. The concentration of the metal-organic solution of Zn-Acetate and Sn-chloride is varied from 0.1 M to 0.5 M. Also, composition x, in Zn(1−x)Sn(x), is varied from 0.0, 0.25, 0.33, 0.5 and 0.75. These solutions were applied to CIGS absorbers by spin-coating at various revolutions per minute (RPMs) for 30 seconds. The deposited films were first dried at approximately 150° C. and subsequently processed at approximately 200°-300° C. and at ambient conditions for a duration between 3-30 minutes. The samples were then cooled to room temperature. Additional iZnO and ITO layers were subsequently deposited by physical vapor deposition (PVD) under a base pressure ˜1.5×10 −5  Torr, deposition pressure ˜3.5×10 −5  Torr in 25% Ar/O 2  mixture under a flow rate of ˜10-50 sccm. For ITO, the depositions conditions also included water under a flow rate of 0.0-1.0 sccm methods ( FIG. 2 ), followed by Ag-grids either by e-beam or screen-printing. The image in  FIG. 2  is provided for purposes of illustration and may not be true to scale.  FIG. 2A  is an expanded view of the CIGS device  200  at an increased proportional scale.  FIG. 2A  contains consecutive layers comprising conductive, transparent layer  202 , i-ZnO  204 , ZTO  206 , CIGS  208 , and electrical contact layer  210 . In one embodiment, conductive, transparent layer  202  is ITO, which alternatively may include CuAlO2, SnO—F, and/or Ag. Electrical contact layer  110  serves as a base electrode due to its electrical conductivity and work function. In one embodiment, the electrical contact layer  110  contains molybdenum, molybdenum alloys, or multilayer films that function as an electrical conductor as a whole. The CIGS device of  FIG. 2  is positioned atop a polyimide layer  212  and an back-side layer  214 . In one embodiment, the back-side layer  214  serves to balance film stresses from the device  200  and to provide the substrate  212  with basic environmental protection prior to encapsulation. In one embodiment, this back-side layer contains molybdenum, other metals, or metal oxides. After fabrication, these devices were tested for current-voltage characteristics (I-V test) under 1.5 AM. 
         [0029]      FIG. 3  shows comparison between a CIGS-based device containing a CdS layer and a CIGS-based device containing a ZTO layer, with both CIGS-based devices containing an i-ZnO layer and a ITO layer. The graphs in  FIG. 3  show the open circuit voltage (VOC) (units of Volts (V)), the fill factor, the percent solar-to electrical energy conversion efficiencies (% eff), and the current density (units mA/cm 3 ). VOC refers to the difference in electrical potential between two terminals within a device without an external load. Fill factor refers to the ratio of actual maximum obtainable power to actual power. 
         [0030]    Curve  300 , curve  302 , curve  304 , and curve  306  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for CIGS-based device containing a CdS layer. Curve  308 , curve  310 , curve  312 , and curve  314  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing ZTO layer (with composition Zn (1-x) Sn (x)  and x=0.25) applied via chemical solution deposition (CSD), processed at 250° C. at ambient conditions for 15 minutes and post-annealed at 200° C. for 15 minutes. Curve  316 , curve  318 , curve  320 , and curve  322  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing ZTO layer (with composition Zn (1-x) Sn (x)  and x=0.25) applied via CSD, processed at between 200-250° C. at ambient conditions for 15 minutes and post-annealed at 200° C. for 15 minutes. Curve  324 , curve  326 , curve  328 , and curve  330  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing ZTO layer (with composition Zn (1-x) Sn (x)  and x=0.25) applied via CSD, processed at 250° C. at ambient conditions for 15 minutes and post-annealed at 200° C. for 30 minutes. Curve  332 , curve  334 , curve  336 , and curve  338  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing ZTO layer (with composition Zn (1-x) Sn (x)  and x=0.25) applied via CSD, processed between 200-250° C. at ambient conditions for 15 minutes and post-annealed at 200° C. for 15 minutes. Overall, the curves for the CIGS-based device containing a ZTO layer compare favorable to the curves for the CIGS-based device containing a CdS layer. Overall, the devices show performance parity with chemical bath deposited CdS layers after post-annealing at approximately 200° C. for 15-30 minutes ( FIG. 3 ). 
       Example 2 
     Preparation and Characterization of CIGS Solar Cells without an i-ZnO Buffer Layer 
       [0031]    This example teaches by way of illustration, not by limitation, preparation and characterization of CIGS solar cells with a Cd-free, oxide buffer layer and without an i-ZnO buffer layer. Elimination of the i-ZnO buffer layer eliminates moisture sensitivity of the CIG device. The need for an additional i-ZnO layer is eliminated by utilizing thicker ZTO layers, as shown in  FIG. 4 .  FIG. 4  compared a CIGS-based device with a CdS layer, a CIGS-based devices with ZTO layers. Curve  400 , curve  402 , curve  404 , and curve  406  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a CdS layer. Curve  408 , curve  410 , curve  412 , and curve  414  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a ZTO layer (with composition Zn 0.75 Sn 0.25 O y ), an i-ZnO layer and an ITO layer. Curve  416 , curve  418 , curve  420 , and curve  422  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a ZTO layer (with composition Zn 0.75 Sn 0.25 O y ) and an ITO layer.  FIG. 5  shows a series of micrographs of ZTO films on CIGS devices. 
         [0032]    The image in  FIG. 6  is provided for purposes of illustration and may not be true to scale. The CIGS device  600  of  FIG. 6  is positioned atop a polyimide layer  610  and a back-side layer  612 . In one embodiment, the back-side layer  612  serves to balance film stresses from the device  600  and to provide the substrate  610  with basic environmental protection prior to encapsulation. In one embodiment, this back-side layer contains molybdenum, other metals, or metal oxides.  FIG. 6A  is an expanded view of the CIGS device  600  at an increased proportional scale.  FIG. 6A  contains consecutive layers comprising transparent conductive layer  602 , ZTO  604 , CIGS  606 , and electrical contact layer  608 . In one embodiment, transparent conductive layer  602  contains ITO, which alternatively may include CuAlO 2 , SnO—F, and/or Ag. Electrical contact layer  608  serves as a base electrode due to its electrical conductivity and work function. In one embodiment, the electrical contact layer  110  contains molybdenum, molybdenum alloys, or multilayer films that function as an electrical conductor as a whole. 
       Example  3   
     Characterization of CIGS Solar Cells with Cd-Free, Oxide Buffer Layers 
       [0033]    This example teaches by way of illustration, not by limitation, additional characterizations of CIGS solar cells with a Cd-free, oxide buffer layer and without an i-ZnO buffer layer.  FIG. 7  shows the effects of pre-washing CIGS samples prior to ZTO deposition. Pre-washing samples consisted of washing in warm water by immersion for 1-2 minutes in order to achieve a consistent surface condition prior to ZTO deposition by removing excess materials, such as sodium, that result from the CIGS fabrication process. Curve  700 , curve  702 , curve  704 , and curve  706  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a CdS layer. Curve  708 , curve  710 , curve  712 , and curve  714  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device pre-washed at 250° C. for 10 minutes prior to ZTO deposition. Curve  716 , curve  718 , curve  720 , and curve  722  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device pre-washed at 250° C. for 5 minutes prior to ZTO deposition. Pre-washing the CIGS samples prior to ZTO deposition affects the current density, the fill factor, the VOC, and the conversion efficiencies, thereby indicating sensitivity to surface conditions. 
         [0034]      FIG. 8  shows the temperature dependence on a CIGS sample with a CdS layer and a CIGS sample with a ZTO layer. The curves in  FIG. 8  show similar hysteresis effects between CIGS samples containing CdS and CIGS samples containing ZTO. Curve  800 , curve  802 , curve  804 , and curve  806  display the current density, the fill factor, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a ZTO layer. Curve  808 , curve  810 , curve  812 , and curve  814  display the fill factor, current density, the VOC, and the conversion efficiencies, respectively, for a CIGS-based device containing a layer. 
         [0035]      FIG. 9  shows the performance of a CIGS sample with a ZTO layer and a CIGS sample with a CdS layer for shorter duration deposition dwell time. As shown in  FIG. 9 , a pre-washed sample of CIGS/ZTO at 10 minutes had similar, if not better, properties to the baseline CIGS/CdS.  FIG. 9  demonstrates that CIGS samples with ZTO permits shorter dwell time as compared to CdS, thereby improving the throughput capabilities. Curve  900 , curve  902 , curve  904 , and curve  906  display the fill factor, the VOC, the current density, and the conversion efficiencies, respectively, for a CIGS-based device containing a CdS layer. Curve  908 , curve  910 , curve  912 , and curve  914  display the fill factor, the VOC, the current density, and the conversion efficiencies, respectively, for a CIGS-based device containing a ZTO layer without a pre-wash. Curve  916 , curve  918 , curve  920 , and curve  922  display the fill factor, the VOC, the current density, and the conversion efficiencies, respectively, for a CIGS-based device containing a ZTO layer with a pre-wash. 
         [0036]    The present disclosure permits ZTO deposition using existing chemical bath deposition (CBD) equipment already in place in FAB1 and FAB2. This equipment utilizes a framed step-and-repeat (e.g. not continuous) deposition process. ZTO is also deposited using a continuous process, such as, but not limited to, ‘Spray’ using a fixed wide spray head or rastering a more focused delivery system, ‘Slot-Die’ using slot die equipment, or ‘Gravure’ using gravure printing apparatus, as shown in  FIG. 10 . All three processes permit a continuous precision deposition of material can be made with nearly full utilization of materials. This provides a high-yield alternative to chemical bath deposition. Spray, Slot Die and Gravure equipment are commercially available in all scales from R &amp; D to Production. 
         [0037]    It will be appreciated that the foregoing embodiments may be adapted for use in cells that are connected by monolithic integration, for example, as described in U.S. Pat. No. 7,994,418 issued to Tandon et al., which is hereby incorporated by reference to the same extent as though fully replicated herein. Other methods of monolithic incorporation are known in the art. 
         [0038]    Changes may be made in the above methods and systems without departing from the scope hereof. It should be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system and reasonable variations thereof, which, as a matter of language, might be said to fall therebetween.