How to reduce heat passing through walls?

good day. i would like to seek an utmost advice. our bedroom's wall if faced directly to sunlight in the afternoon. Because of this, our rooms gets really hot, but we do not prefer putting on aircon due to some allergies. Is there a way to reduce the heat passing thru our wall? when we touch the wall, heat can be felt.

Hope you can give advice for ang DIY that could help. thank you.
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