1 • SOLAR CELL AND MANUFACTURING METHOD THEREOF 5 1. Field of the invention [0001] The described technology relates generally to a solar cell and a manufacturing method thereof. BACKGROUND 2. Description of the Related Art 10 [0002] A solar cell is a photoelectric conversion device that converts light energy, such as solar light energy, into electrical energy. The solar cell includes a rear-surface electrode layer formed on a substrate, a light absorbing layer located thereon, and a transparent electrode layer. [0003] A solar cell may be, for example, a silicon solar cell using silicon as a light 15 absorption layer (or a photoelectric conversion layer), a compound semiconductor solar cell using compounds such as CIS (Cu, In, Se) or CIGS (Cu, In, Ga, Se), or the like. Among them, in a compound semiconductor solar cell, an alkali metal (e.g., sodium) may be included in the light absorbing layer to increase efficiency of the light absorbing layer, and studies relating thereto have been undertaken. For example, a compound including 20 an alkali metal may be directly added, or an alkali metal included in the substrate may be spread to the light absorbing layer. [0004] Methods for spreading the alkali metal in the substrate to the light absorbing layer through the rear electrode layer have been studied, as such methods may cause problems, such as adherence of the rear electrode layer to the substrate, complexity of 25 the manufacturing process, and control of the concentration of the alkali metal. [0005] The above information disclosed in this Background section is only for enhancement of understanding of the background of the described technology, and 2 , • therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 5 SUMMARY [0006] Embodiments of the present invention provide a solar cell for controlling an alkali metal spread through a rear electrode layer for providing excellent adherence to a substrate through a relatively simple process, and a manufacturing method thereof. 10 [0007] According to an embodiment of the present invention, a method for manufacturing a solar cell includes placing oxygen atoms in the rear electrode layer through a relatively simple and easy process, and allows the alkali metal to be efficiently spread into the light absorbing layer by using the oxygen atoms. Also, according to embodiments of the present invention, a solar cell with excellent adherence between the 15 substrate and the rear electrode layer may be realized. [0008] According to one embodiment of the present invention, there is provided a solar cell including a substrate, a rear electrode layer on the substrate, the rear electrode layer including a plurality of metal columnar grain layers, a light absorbing layer on the rear electrode layer, and a transparent electrode layer on the light absorbing layer. 20 [0009] Each of the metal columnar grain layers may include molybdenum. [0010] A thickness of each of the metal columnar grain layers may be between about 20 nm and about 500 nm. [0011] The thickness of each of the metal columnar grain layers may be between about 50 nm and about 100 nm. 25 [0012] The solar cell may further include an interface between an adjacent pair of the metal columnar grain layers, the interface including oxygen atoms. [0013] An amount of the oxygen atoms may be between about 1 atomic % and about 70 atomic % of a total amount of atoms of the rear electrode layer. 3 [0014] The amount of the oxygen atoms may be between about 1 atomic % and about 20 atomic % of the total amount of atoms of the rear electrode layer. 5 [0015] The rear electrode layer may include no more than 9 metal columnar grain layers. [0016] The light absorbing layer may include at least one of Cu, In, Ga, or Se. [0017] According to another embodiment of the present invention, there is provided a method of forming a solar cell, the method including placing a substrate in a deposition 10 chamber, forming a rear electrode layer including a plurality of metal columnar grain layers, forming a light absorbing layer on the rear electrode layer, and forming a transparent electrode layer on the light absorbing layer. [0018] Forming the rear electrode layer may include forming one of the metal columnar grain layers by depositing molybdenum on the substrate or on a previous one 15 of the metal columnar grain layers, and forming a next one of the metal columnar grain layers by depositing molybdenum on the one of the metal columnar grain layers following a break time after forming the one of the metal columnar grain layers. [0019] The break time may be between about 1 second and about 1 hour. [0020] Oxygen atoms may be placed in the rear electrode layer during the break time. 20 [0021] An amount of the oxygen atoms placed in the rear electrode layer may correspond to at least one of a length of the break time or a number of break times. [0022] According to another embodiment of the present invention, there is provided a method of forming a solar cell, the method including placing a substrate in a deposition chamber, forming a rear electrode layer by depositing molybdenum on the substrate to 2 5 form a first metal columnar grain layer, and depositing molybdenum on the first metal columnar grain layer following a break time after forming the first metal columnar grain layer to form a second metal columnar grain layer on the first metal columnar grain layer. [0023] Oxygen atoms may be placed in the rear electrode layer during the break time. 4 [0024] An amount of oxygen atoms may correspond to at least one of a length of the break time or a number of break times. 5 [0025] The break time may be between about 1 second and about 1 hour. [0026] The molybdenum may be deposited under a pressure of about 0.05 Pa to about 5 Pa. [0027] The molybdenum may be deposited by sputtering. 10 BRIEF DESCRIPTION OF THE DRAWINGS [0028] FIG. 1 shows a cross-sectional view of a solar cell according to an exemplary embodiment of the present invention. [0029] FIG. 2 shows an enlarged view of the area II of the solar cell of the embodiment shown in FIG. 1. 15 [0030] FIGS. 3A to 3C show a method for manufacturing a rear electrode layer according to an exemplary embodiment of the present invention. [0031] FIG. 4 shows a graph showing an atom ratio with respect to a thickness of the rear electrode layer as measured by X-ray photoelectron spectroscopy (XPS) according to an exemplary embodiment of the present invention. 20 [0032] FIGS. 5A and 5B respectively show photographs of a cross-section of a rear electrode layer, as captured by a scanning electron microscope (SEM) of an exemplary embodiment and of a comparative example. [0033] FIG. 6 shows a graph of results of a peel strength test according to an exemplary embodiment and according to a comparative example. 25 DETAILED DESCRIPTION [0034] Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the 5 1 • invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the 5 spirit or scope of the present invention. [0035] FIG. 1 shows a cross-sectional view of a solar cell according to an exemplary embodiment, and FIG. 2 shows an enlarged view of the area II of the solar cell of the embodiment shown in FIG. 1. [0036] Referring to FIG. 1 and FIG. 2, the solar cell 100 includes a substrate 10, a 10 rear electrode layer 20, a light absorbing layer 30, a buffer layer 40, and a transparent electrode layer 50. [0037] The solar cell 100 may be, for example, a silicon solar cell using silicon for the light absorbing layer 30, or a compound semiconductor solar cell including CIS (Cu, In, Se) or CIGS (Cu, In, Ga, Se) for the light absorbing layer 30. The light absorbing layer 15 30 including the CIS or the CIGS will be exemplified hereinafter. [0038] The substrate 10 is at an outermost side of the solar cell 100. That is, the substrate 10 is farthest from the side (e.g., surface) on which light is applied. The substrate 10 may be formed with various materials including, for example, plate-type glass, ceramic, stainless steel, metal, or film-type polymers. 20 [0039] The rear electrode layer 20 is located on the substrate 10, and is made of a metal with excellent optical reflective efficiency and with excellent adhesion to the substrate 10. For example, the rear electrode layer 20 may include molybdenum (Mo). Molybdenum (Mo) has high electrical conductivity, may form an ohmic contact with the light absorbing layer 30, and realizes great stability during a high temperature heat 25 treatment for forming the light absorbing layer 30. An embodiment in which the rear electrode layer 20 is made of molybdenum (Mo) will be exemplified hereinafter. [0040] As shown in FIG. 2, the rear electrode layer 20 has a multi-layered structure including a plurality of metal columnar grain layers 20-1-200+1 • Here, n represents a 6 1 * number of break times during the metal deposition process during a process for forming the rear electrode layer 20 (to be described), and is an integer defined by 1