In a volatile market where not all stocks are participating in the rally, there are some hidden gems that offer attractive dividend yields. These three high-yield dividend stocks could potentially provide significant passive income for investors in the long run.

First on the list is AT&T (T), a telecommunications business that recently divested its unpredictable media assets. Despite lowering its dividend payout, the stock still offers a hefty 6.4% dividend yield at current prices. With AT&T being one of the three major 5G wireless network providers in America, investors can anticipate steady growth in cash flows in the coming years. The company has also launched a fixed wireless broadband service, which is expected to drive total broadband revenue up by over 7% this year.

Ares Capital (ARCC), the largest publicly traded business development company (BDC), is the second stock to consider. As a lender to mid-sized businesses that often struggle to secure financing from larger banks, Ares Capital holds a diverse portfolio valued at approximately $23 billion. With a 9% dividend yield currently, there's potential for the company to raise its dividend in the future. The majority of Ares Capital's assets consist of first- and second-lien senior secured loans, ensuring a measure of security for investors.

The third stock worth considering is Hercules Capital (HTGC), another BDC with a different approach. Hercules provides loans in the range of $25 million to $100 million to early-stage businesses with disruptive technology. While some investments may not pay off, the ones that succeed can offset the losses significantly. Hercules Capital offers a regular dividend of $0.40 per quarter, steadily increasing since 2010, and a supplemental dividend that can vary from year to year. With a current supplemental quarterly dividend of $0.08 per share, investors who purchase the stock at recent prices can enjoy an impressive 9.9% yield.

These high-yield dividend stocks provide investors with the opportunity to boost their passive income streams. While their dividend payouts may fluctuate over time, the potential for future dividend increases is promising. AT&T's predictable cash flows as a pure telecommunications business, Ares Capital's focus on middle-market companies, and Hercules Capital's investments in promising technology companies make these stocks stand out in terms of potential long-term returns.

It's important for investors to conduct thorough research and consider their risk tolerance before investing in any stock. However, adding these three high-yield dividend stocks to a portfolio could potentially result in significant passive income by the time investors are ready to retire.

Disclaimer: The mentioned author holds positions in Ares Capital. The Motley Fool recommends T-Mobile US and Verizon Communications. The Motley Fool has a disclosure policy.