During motion of a planet from perihelion to aphelion, the work done by gravitational force of sun on it is : 

-ve

As per Kepler's of law of areas

$v_A<v_P$

where,
$v_A=$ speed of planet at aphelion
$v_P=$ speed of planet at perihelion

Now, apply work-energy theorem on planet,

$W_{ext}=\Delta KE$

Considering motion of planet from perihelion to aphelion

$\Rightarrow W_{sun}=\frac{1}{2}m{v}_A^2-\frac{1}{2}m{v}_P^2$

$\because v_A<v_P$

$\therefore W_{sun}\to \text{negative}$