The term is explained in Srimad Bhagavatam.

Shuddha sattva is a technical term to explain the fact that paramatman has only good qualities without any defects. 

Here is a beautiful verse from [Bhagavatam][1] explaining this

>viśuddha-sattvaṁ tava dhāma śāntaṁ
tapo-mayaṁ dhvasta-rajas-tamaskam
māyā-mayo ’yaṁ guṇa-sampravāho
na vidyate te grahaṇānubandham

Translation:
>King Indra said: Your transcendental form, a manifestation of pure goodness, is undisturbed by change, shining with knowledge and devoid of passion and ignorance. In You does not exist the mighty flow of the modes of material nature, which is based on illusion and ignorance.


[1]:https://www.vedabase.com/en/sb/10/27/4