Detailed Instructions: In this task, you are given a paragraph, event and an entity. The event is part of the given paragraph and it changes the state of the entity. Your task is to classify the state of the entity into three classes: 1) not exist, 2) unknown location and 3) known location. "not exist" means the entity doesn't exist in that state anymore. "unknown location" means the entity exists in that state but location is unknown. "known location" means the entity exists and location is known.
Q: "process paragraph : : A photovoltaic cell is created with two layers of semiconductor material. The top layer are "doped" with impurities to allow a negative charge. The bottom layer are "doped" to allow for a positive charge. This produces an electric field that allows electrons to flow from negative to positve. Sunlight enters the cell. Energy from sunlight knocks electrons loose in both fields. This prevents the flow of electrons from negative to positive. Adding an external circuit allows the exchange to continue. Give the cells owner a flow of electricity. ", "event : The top layer are "doped" with impurities to allow a negative charge.", "entity : photovoltaic cell"
A:
unknown location