Unfortunately, you cannot get a VOA at IKA or other airports. You should apply for a tourist visa which takes about 2 weeks. After you get the reference code, you can pick up your visa at the embassy in one single day.

Here is the most recent list of nation who are not eligible to get airport visa. Make sure you are eligible before you travel to Iran.

Nationalities from these countries cannot get a visa upon arrival;

Afghanistan

Bangladesh

Canada

Colombia

England (UK)

Iraq

Jordan

Pakistan

Somalia

USA