Thin Film Coatings Turning Black After Annealing

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Reason for tin oxide thin film coatings turning black after annealing?
As can be seen in the linked image (https://i.postimg.cc/TYXkrvZB/blacklayers.jpg), the tin oxide thin film coatings have turned black after annealing at 500C for 1 hour, the solution obtained following the experimental procedure (with slight concentration variation) in this
Article Properties of fluorine-doped SnO2 thin films by a green sol-gel method
article can be seen as clear in the first picture. For quick reference, it is Methanol, Ammonium hydroxide (1.5mol ratio), Monoethanolamine (1 mol ratio), 0.5M Tin Chloride Pentahydrate.
Solution is spin coated at 3500rpm for 40 seconds, then dried at 200C for 5 mins, 10 layers prepared. The layers are transparent (but dark) after drying, but turn completely black after annealing. I don't know what could be responsible for this as all organic materials should evaporate in the drying step and tin oxide should be highly transparent.
Edit: I anneal only once after all 10 layers have been deposited and dried individually, it is with this annealing step that the layers turn black, they are transparent before it.
As can be seen in the linked image (https://i.postimg.cc/TYXkrvZB/blacklayers.jpg), the tin oxide thin film coatings have turned black after annealing at 500C for 1 hour, the solution obtained following the experimental procedure (with slight concentration variation) in this
Article Properties of fluorine-doped SnO2 thin films by a green sol-gel method
article can be seen as clear in the first picture. For quick reference, it is Methanol, Ammonium hydroxide (1.5mol ratio), Monoethanolamine (1 mol ratio), 0.5M Tin Chloride Pentahydrate.
Solution is spin coated at 3500rpm for 40 seconds, then dried at 200C for 5 mins, 10 layers prepared. The layers are transparent (but dark) after drying, but turn completely black after annealing. I don't know what could be responsible for this as all organic materials should evaporate in the drying step and tin oxide should be highly transparent.
Edit: I anneal only once after all 10 layers have been deposited and dried individually, it is with this annealing step that the layers turn black, they are transparent before it.