Our goal is the improvement of the data collection in solar-powered wireless sensor networks. One of the methods to improve the data collection is cooperative transmission by which a node relays data from a source node to a base station. However, the cooperative transmission is not enough to prevent the data collection performance from the decline in low power generation, such as in a cloudy day and at night. We tackled this problem by introducing a repeater role to same nodes, which only relays data from the other nodes without transmitting its own data. Our proposal improves the data collection performance in the low power generation. Numerical examples compare and show the effectiveness of the conventional method and three assignment methods of the repeater role: one is to assign the repeater role to some nodes fixed in place around the farthest nodes, one is to assign the repeater role uniformly randomly to some nodes, and another is to assign the repeater role to some nodes that do not have sufficient battery levels.