Is working in the marketing of solar energy products permissible (halal), given that customers will acquire them through loans, and my role is limited to introducing the product and encouraging people to use it, without direct selling? And is this considered indirect encouragement to take out loans?

There is no harm in your work with the aforementioned company. The company's sale of its product to a customer who desires it on credit is not considered a loan; rather, it is a sale with a deferred price, which is permissible. Even if the customer needs an interest-based loan to buy it, that is an independent transaction. The important thing is that you do not help him with the interest-based loan or encourage him to take it. Let your work remain within the framework of promoting the product.