Accounts receivable, net - Allowance for doubtful accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable, net
Opening allowance for doubtful accounts	$ 158,902	$ 158,902
Provision for doubtful accounts	0	0
Accounts written off during the year	(16,666)	0
Ending allowance for doubtful accounts	$ 142,236	$ 158,902