A solar cell comprising: a photovoltaic substrate having a light-receiving surface and a back surface opposite to said light-receiving surface; a front electrode disposed on said light-receiving surface of said photovoltaic substrate, and a back electrode disposed on said back surface of said photovoltaic substrate, and including a collector layer disposed on said back surface of said photovoltaic substrate and having at least one collector opening which extends along a first direction, and a bus electrode disposed on said back surface of said photovoltaic substrate and including at least one bus electrode segment which extends along the first direction and which corresponds in position to said collector opening, wherein said back surface of said photovoltaic substrate has an uncovered area which is not covered by said bus electrode and said collector layer, said solar cell characterized by: said at least one collector opening including: a first end portion, a second end portion opposite to said first end portion, a main opening portion between said first and second end portions (611, 612), and a first expansive opening portion formed at said first end portion, said first expansive opening portion having a first outer expansive edge which is distal from said main opening portion and which is at least partially arcuate, said first outer expansive edge extending along a second direction transverse to the first direction, said first expansive opening portion having a width larger than a width of said main opening portion, wherein said at least one bus electrode segment is exposed from said at least one collector opening, and has a first end portion and a second end portion, said first end portion being spaced apart from said first outer expansive edge and being exposed from said first expansive opening portion, said second end portion being opposite to said first end portion of said at least one bus electrode segment in the first direction, said uncovered area corresponding in position to said first expansive opening portion of said at least one collector opening and underlying a spacing between, said first end portion of said at least one bus electrode segment and said first outer expansive edge of said first expansive opening portion. a photovoltaic substrate having a light-receiving surface and a back surface opposite to said light-receiving surface; a front electrode disposed on said light-receiving surface of said photovoltaic substrate, and a back electrode disposed on said back surface of said photovoltaic substrate, and including a collector layer disposed on said back surface of said photovoltaic substrate and having at least one collector opening which extends along a first direction, and a bus electrode disposed on said back surface of said photovoltaic substrate and including at least one bus electrode segment which extends along the first direction and which corresponds in position to said collector opening, wherein said back surface of said photovoltaic substrate has an uncovered area which is not covered by said bus electrode and said collector layer, said solar cell characterized by: said at least one collector opening including: a first end portion, a second end portion opposite to said first end portion, a main opening portion between said first and second end portions (611, 612), and a first expansive opening portion formed at said first end portion, said first expansive opening portion having a first outer expansive edge which is distal from said main opening portion and which is at least partially arcuate, said first outer expansive edge extending along a second direction transverse to the first direction, said first expansive opening portion having a width larger than a width of said main opening portion, said at least one collector opening including: a first end portion, a second end portion opposite to said first end portion, a main opening portion between said first and second end portions (611, 612), and a first expansive opening portion formed at said first end portion, a first end portion, a second end portion opposite to said first end portion, a main opening portion between said first and second end portions (611, 612), and a first expansive opening portion formed at said first end portion, said first expansive opening portion having a first outer expansive edge which is distal from said main opening portion and which is at least partially arcuate, said first outer expansive edge extending along a second direction transverse to the first direction, said first expansive opening portion having a width larger than a width of said main opening portion, wherein said at least one bus electrode segment is exposed from said at least one collector opening, and has a first end portion and a second end portion, said first end portion being spaced apart from said first outer expansive edge and being exposed from said first expansive opening portion, said second end portion being opposite to said first end portion of said at least one bus electrode segment in the first direction, said uncovered area corresponding in position to said first expansive opening portion of said at least one collector