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Long-term changes in the responsivity of unbiased silicon photodiodes were observed when operating at the temperature of 90 K. The change manifested as a monotonic increase in current output for a fixed optical power input. The current output has been observed to increase by 20% or more relative to room-temperature output. In all trials, the effect was fully reversed upon returning to room temperature. Analysis of spectrally resolved data showed that the effect is due to formation of a contaminant thin film, of as-yet-unknown composition.