There is also a paid variant "Contact QR" of this app which contains no advertisement.

Want to quickly share your contact? Want to tell the finder of your lost device whom to bring it back to? Even when you have an active Passcode Lock? Put a QR code with your contact information on your Lock Screen! This app generates an appropriate picture with the QR code that you can put to your Lock Screen. Just select the contact information from your address book, edit it so that it contains everything you want, and generate the picture!

Privacy: The app requires access to your address book and photos. None of your data is sent out or used for any purpose other than generating the QR code. The address book is accessed to import a contact you select and to temporarily store the contact to generate the QR code from. The generated picture is stored to your photos.