Solar cells themselves do loose efficiency very slowly (less than 1% per year for most modern monocrystaline silicon cells) due to defects collecting in the sem

Solar cells themselves do loose efficiency very slowly (less than 1% per year for most modern monocrystaline silicon cells) due to defects collecting in the semiconductor material. It is the same reason that LEDs slowly loose power over time. Due to the extreme conditions in the semiconductor material (high electric fields, often moderate to high temperatures) the dopants can diffuse into places they do not belong, or cluster together both of which hurt the performance. That said, most panels die before the cells themselves go bad due to mechanical failure of the cells (cracking, etc) or failures in the electrical wiring in the panel due to corrosion. It is the same problem as with LED lighting. I have not seen a single LED bulb which lasted long enough for the actual semiconductor devices to age noticeably, but I have seen dozens of bulbs where thermal cycling caused the electrical contacts to the chip fail or the power supply capacitors have gone bad. Also, due to the rapidly increasing efficiency and decreasing costs of solar cells, it can often be economically viable to replace your array with fresh cells just for the added power output you get. If you took cells form 20 years ago and replace them with current cells you can quadruple your output power for the same area!