A solar cell apparatus comprising: a substrate; a back electrode layer on the substrate; a light absorbing layer on the back electrode layer; a buffer layer on the light absorbing layer; a high resistance buffer layer on the buffer layer, a window layer on the high resistance buffer layer; and a connection part extending from the window layer and connected to the back electrode layer of an adjacent solar cell thereby connecting two adjacent solar cells in series; wherein the window layer includes a first window layer and a second window layer; wherein the first window layer including a first oxide on the light absorbing layer; wherein the second window layer provided on the first window layer and including a second oxide having a composition ratio of oxygen higher than a composition ratio of oxygen contained in the first oxide; wherein the first window layer directly making contact with the high resistance buffer layer; wherein the second window layer directly making contact with the first window layer; wherein the connection part comprises: a first conductive layer integrally formed with the first window layer and directly making contact with the back electrode layer; and a second conductive layer integrally formed with the second window layer and provided on the first conductive layer; wherein the first conductive layer has a thickness smaller than a thickness of the second conductive layer; wherein the first conductive layer has an oxygen composition ratio lower than an oxygen composition ratio of the second conductive layer. a substrate; a back electrode layer on the substrate; a light absorbing layer on the back electrode layer; a buffer layer on the light absorbing layer; a high resistance buffer layer on the buffer layer, a window layer on the high resistance buffer layer; and a connection part extending from the window layer and connected to the back electrode layer of an adjacent solar cell thereby connecting two adjacent solar cells in series; wherein the window layer includes a first window layer and a second window layer; wherein the first window layer including a first oxide on the light absorbing layer; wherein the second window layer provided on the first window layer and including a second oxide having a composition ratio of oxygen higher than a composition ratio of oxygen contained in the first oxide; wherein the first window layer directly making contact with the high resistance buffer layer; wherein the second window layer directly making contact with the first window layer; wherein the connection part comprises: a first conductive layer integrally formed with the first window layer and directly making contact with the back electrode layer; and a second conductive layer integrally formed with the second window layer and provided on the first conductive layer; a first conductive layer integrally formed with the first window layer and directly making contact with the back electrode layer; and a second conductive layer integrally formed with the second window layer and provided on the first conductive layer; wherein the first conductive layer has a thickness smaller than a thickness of the second conductive layer; wherein the first conductive layer has an oxygen composition ratio lower than an oxygen composition ratio of the second conductive layer.