Not one probe that has or will leave the Solar System (Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons) has a Solar Power System. This leads to the question, is there a reason behind this? And secondly if a probe were to be launched out of the Solar System with a solar power system couldn't it theoretically communicate with earth indefinitely? The reasoning behind that is even billions of miles out they could turn off all systems for say 3 months, let the batteries recharge and then after the 3 months turn the power back on,communicate with it for a few days and then repeat the process.
There isn't enough solar energy to power a spacecraft once you get a certain distance from the sun, even if you had 100% power conversion efficiency. The intensity of sunlight diminishes inversely proportional to the square of the distance from the Sun. The intensity of sunlight at Mars is half what it is at Earth, and at Jupiter it's only 5%. At the Earth's orbit the sun's intensity is about 1400 W/m2 (watts per meter squared), but at Uranus it's only 4 W/m2. Juno's solar array, which is about 70 square meters and 10%-ish efficiency, generates almost 500 Watts of power at Jupiter, but would only generate 28 Watts at Uranus, which is nowhere near what it would need to function. Pluto averages 1 W/m2, which equates to about 7 Watts. Even if you had 100% efficiency a 70m2 array would generate 70 Watts at that distance.
In addition to a few hundred W of electrical power, an RTG produces a few kW of thermal power which is used to keep the spacecraft warm. Without heaters, the spacecraft's temperature would drop to -200 °C. It needs to be warmed up for the electronics to work well, and the thermal cycling would soon break stuff.
The primary missions of the Voyagers, Pioneers, and New Horizons was exploration of the planets of our solar system, not beyond it. Continuous power during their approaches to our system's planets was much more important than their ability to tell us about interplanetary space.
RTGs provide more continuous power per unit mass than solar panels at Jupiter distance from the sun or further. Any mass dedicated to solar panels would be mass that couldn't be spent on scientific instruments.
A simple estimate, at a distance of 50 AU (1 AU is the distance earth to sun) the solar power is 1/2500 of that at earths orbit. Collecting energy for three months and use it in three days is only a factor of 30, but we need more than a factor of 2500. 3*2500 is 7500 days or more than 20 years. As others wrote, the self discharge of the batteries would be much faster than this slow charge.
The Kuiper belt goes from some 40 to 75 AU, I used 50 AU for the estimate.