Basically the Church, east and west, used leavened bread from the earliest times. The East has, and continues to do so.
"In the West, various ordinances appeared from the ninth century on, all demanding the exclusive use of unleavened bread for the Eucharist. A growing solicitude for the Blessed Sacrament and a desire to employ only the best and whitest bread, along with various scriptural considerations -- all favored this development. Still, the new custom did not come into exclusive vogue until the middle of the eleventh century. Particularly in Rome it was not universally accepted till after the general infiltration of various usages from the North..."(Jungman, The Mass of the Roman Rite, volume II, pp 33-34)
"Another change introduced into the Roman Rite in France and Germany at the time [i.e., 8th - 9th century] was the use of unleavened bread and of thin white wafers or hosts instead of the loaves of leavened bread used hitherto" (O'Shea, The Worship of the Church, p 128)
"The Eucharistic bread has been unleavened in the Latin rite since the 8th century -- that is, it is prepared simply from flour and water, without the addition of leaven or yeast..in the first millennium of the Church's history, both in East and West, the bread normally used for the Eucharist was ordinary 'daily bread,' that is, leavened bread, and the Eastern Church uses it still today; for the most part, they strictly forbid the use of unleavened bread. The Latin Church, by contrast, has not considered this question very important." (Emminghaus, The Eucharist: Essence, Form, Celebration, page 162)
"But they [i.e. the ancients] many times speak of leavened bread, and sometimes the Eucharist is called 'Fermentum', 'Leaven', upon that account. As appears from the Pontifical [i.e. Liber Pontificalis] in the Lives of Melchiades, and Siricius, and a Letter of Pope Innocent, where he says, it was the custom at Rome to Consecrate the Fermentum, (that is, the Eucharist) in the Mother Church and send it thence on the Lord's Day to the Presbyters in the Tituli, or lesser churches, that they might not think themselves separated from the Bishop's Communion. But they did not send it to any country parishes, because the Sacraments were not to be carried to places at any great distance. What is here called the Consecrated Fermentum, is by Baronius and other, who tread in the track of the Schoolmen [i.e. Scholastics], interpreted of the Eulogia, or Panis Benedictus, the bread that was blessed for such as did not communicate [i.e. the antidoron]. But Innocent plainly says, he meant it of the Sacrament, which was consecrated by the Bishop, and sent to the presbyters for the use of lesser church. Which shews, that at the time, even in Rome itself, the Eucharist was consecrated in common or leavened bread. It is observable, that neither Photius, nor any other Greek writer, before Michael Cerularius, A.D. 1051, ever objected to the use of unleavened bread to the Roman Church: which argues, that the use of it did not prevail till about that time; else there is no doubt but Photius would among other things, have objected this to them. These arguments put the matter beyond all dispute, that the Church for a thousand years use no other but common or leavened bread in the Eucharist; and how the change was made or the time exactly when, is not easy to determine." (Bingham, Antiquities of the Christian Church, Volume 1, Book XV, pg. 738)
