Dividend stocks can aptly be described as the heart of any successful investment portfolio. While investors can succeed by purchasing companies that don't pay a dividend, the odds are most certainly in your favor if you load your portfolio with dividend stocks.

According to a 2013 report from J.P. Morgan Asset Management, the average annualized total return of non-dividend-paying stocks between 1972 and 2012 was a meager 1.6%. Comparatively, companies that initiated and grew their dividends over this same 40-year period delivered average annualized returns of 9.5%. 

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These figures really shouldn't come as a shock, because dividend stocks typically have time-tested business models and relatively predictable cash flow. After all, they'd be unlikely to continue rewarding shareholders if they didn't have decent operational visibility.

Perhaps best of all, dividend stocks allow investors to utilize dividend reinvestment plans, or DRIPs, to really supercharge their wealth creation. A DRIP allows payouts to be automatically reinvested into more shares of dividend-paying companies. This leads to a compounding effect of investors owning more shares of dividend-paying stock, and therefore receiving a larger eventual payout.

While there are hundreds upon hundreds of dividend stocks for investors to choose from, we'd prefer those with the highest yields possible. I mean, who doesn't want the most income possible? But the thing is, risk tends to rise with yield. Thus, high-yield dividend stocks -- those with a payout of at least 4% per year -- tend to be especially risky for income seekers.

However, the following three high-yield dividend stocks could easily be considered among the safest for income investors to buy.

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AT&T

Among high-yield dividend stocks, you'd struggle to find a more boring operating model than that offered by AT&T (NYSE:T). Thankfully, boring is beautiful when you're looking for a rock-solid dividend stock, and AT&T certainly delivers with its 5.3% yield and 36-year streak of increased annual payouts. 

For AT&T, its wireless division is what makes it tick. With smartphones a virtual necessity and the company's wireless segment built on subscriptions, we've seen very little churn and highly predictable cash flow from wireless for a long time.

The company's key growth driver looks to be the ongoing upgrade of its wireless infrastructure to 5G. While this will prove costly in the near term, AT&T should benefit from a substantive increase in data usage and smartphone upgrades by consumers. Let's not overlook that data is where AT&T makes the bulk of its wireless profit.

Additionally, this is a company that should be able to slowly but surely monetize its streaming assets. After acquiring Time Warner and its TBS, TNT, and CNN networks last year, AT&T can utilize these assets, as well as DIRECTV (another owned asset), to lure new customers and potentially even steal them from other streaming services. Despite a modest single-digit growth rate, AT&T represents one of the safest high-yield stocks you can buy.

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Innovative Industrial Properties

Generally speaking, when you think of marijuana stocks, you probably aren't considering income or safety. This year we've witnessed pot stocks get absolutely taken to the woodshed as supply problems in Canada and high tax rates in the U.S. have given rise to the black market. And yet, one name continues to thrive: Innovative Industrial Properties (NYSE:IIPR).

Innovative Industrial Properties is a cannabis real estate investment trust (REIT). It acquires medical marijuana cultivation and processing facilities, then leases out these properties for an extended period of time (usually 10 to 20 years). Because marijuana remains illegal at the federal level in the U.S., access to capital remains dicey for cannabis stocks. This makes the sale-leaseback model particularly intriguing -- and lucrative -- for Innovative Industrial Properties.

Since the beginning of the year, IIP has seen its property count nearly quadruple from 11 to 42, with the company having a presence in 13 states. More importantly, its weighted-average lease length is 15.5 years, with an average return on invested capital of 13.6%. In other words, Innovative Industrial Properties will see a complete payback on its investments in a little over five years, but should have steady and predictable cash flow for more than a decade.

Having announced a 28% sequential quarterly dividend increase to $1 per share last week, IIP is now on track to match AT&T with a 5.3% yield. This may be a pot stock, but it's also an exceptionally safe high-yield source of income.

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Duke Energy

Last but not least, high-yield income seekers could do a lot worse than electric and gas utility giant Duke Energy (NYSE:DUK), which is currently paying out 4.2% annually, or about double the yield of the average S&P 500-listed company.

The beauty of utility stocks is that they supply a basic-need service. Whether you own a home or rent, you need electricity or gas to heat your home or cook your food. This means consumption habits of gas and electricity don't change much, whether the economy is firing on all cylinders or is in complete shambles. It also helps that electric and gas utilities tend to operate as monopolies or oligopolies.

To build on this point, Duke Energy's utility services are mostly regulated. In essence, Duke can't simply pass along rate hikes whenever it chooses. Instead, it has to request rate increases from public utility commissions in each state. While this might sound like a nuisance, it's actually great news, because it means the company isn't exposed to potentially wild price fluctuations of the wholesale market. Again, it's all about the predictability of cash flow.

It's worth mentioning that Duke is also investing heavily in renewable energy. Not only are these investments good for the environment, but they'll be excellent for Duke Energy's long-term growth as they lower energy-generation costs.

Utilities might appear boring on paper, but companies like Duke Energy provide predictability that's hard to match in any industry. That's what makes this high-yield dividend stock such a safe bet.