Is Plan B the best solution after my boyfriend ejaculated in me?

The emergency contraceptive drugs are Levonorgestrel & Ulipristal Acetate. They come in many different brand names. It's those 2 ingredients you need to look for. One or the other.

Levonorgestrel is most commonly known as Plan B, but has many other names, such as Take Action, for example.
Levonorgestrel works by delaying your ovulation until the sperm leaves your body. If you're already ovulating then it can't do that. You'd need the other emergency pill; Ulipristal Acetate, most commonly known as Ella.
You have 72 hours/3 days from the time of the unprotected sex, to use Levonorgestrel. It is more likely to be effective the sooner you take it, as long as you aren't close to ovulation.

Ella (Ulipristal Acetate) has been shown to work in more ways than Plan B. It's therefore more likely to protect you if you are already ovulating. Not guaranteed, but there's more chance. You have 120 hours/5 days from the time of unprotected sex to use Ulipristal Acetate.
Emergency contraception is not as effective as regular hormonal birth control. It's simply worth a try in an emergency. Better than leaving it to chance.
Emergency contraceptives are huge doses of artificial hormone for your body to deal with. They can therefore cause lots of unpleasant hormonal symptoms, such as irregular periods, for weeks or months after using the drugs, & other symptoms you might get with a period or pregnancy, or other hormonal change in your body.
They are meant for total emergencies.

Another common form of emergency contraception is to have an IUD fitted by a doctor. That may prevent pregnancy if inserted within 5 days of the unprotected sex.