They 'work' by delivering a shock, noise or smell that the dog finds deeply unpleasant. It is not for sure that the dog ever associates the shock with barking. He could just as easily associate it with the doorbell going - making his fear much worse. They are a lazy gamble with the dog's welfare.
Sarah Whitehead talks about a similar case in her book, Clever Dog, in which a beep and electric shocks were delivered when a dog roamed outside the yard. The dog never associated the shock with roaming. Instead it associated all beeping with being hurt, couldn't figure out what caused it, suffered with anxiety and wouldn't leave the bed. Ever. They couldn't help the dog and rehomed it in the end to give it a shot at being happy in a new environment.
Firstly, cover that window up or use physical barriers to stop him sitting there looking. Allowing him to sit on guard is habit forming and he needs to break the habit. Window film, such as Purlfrost, is cheap, easy to apply and very effective at obscuring the view without cutting down the light.
The doorbell rings, have him sit somewhere safe and give him a treat. Repeat, repeat, repeat. Practise by ringing the bell yourself. Repeat some more. Assume it will take multiple attempts. If the dog cannot sit quietly because the bell is too exciting then start lower, such as just knocking on the door. Change the sound of the doorbell so he gets a chance to learn with a fresh, new sound if you think he is too reactive to the current sound.