Semiconductor quantum dots are bright, on-demand single photon sources to realise quantum communication devices. An important requirement to realise such devices is the high-fidelity preparation of the biexciton state since the subsequent radiative decay produces entangled photon pairs. Although resonant excitation to the biexciton state has been successful, its sensitivity to the excitation intensity fluctuations and transition dipole moment calls for a robust preparation scheme. Here we demonstrate adiabatic rapid passage with chirped picosecond laser pulses in GaAs/AlGaAs quantum dots at 4K, and show the high fidelity generation of biexcitons, supported by theoretical simulations. Our results pave the way for utilizing quantum dots as sources of deterministic time-bin entangled photons.