Accounts receivable, net - Allowance for doubtful accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable, net.
Opening allowance for doubtful accounts	$ 142,236	$ 158,902
Provision for doubtful accounts	129,107	0
Accounts written off during the year	(1,233)	(16,666)
Ending allowance for doubtful accounts	$ 270,110	$ 142,236