Traditional inorganic photovoltaics (PV), such as silicon (Si), III-V multijunction, Cu(In,Ga)Se₂ (CIGS), and Gallium arsenide (GeAs), have dominated for aerospace applications. However, Si-based PV is nonflexible and delivers specific power (SP) (power to mass ratio) of 0.38 W/g, and with the CIGS PV, the SP could be achieved up to 3 W/g.[1] The low SP and the material and fabrication cost of inorganic PV also are critical concerns. Recently the development of solution-processed organic photovoltaic (OPV) has shown advantages such as high absorption coefficient, compatibility with a flexible substrate, lightweight, low-cost, etc. [2] Moreover, OPV demonstrated a high SP up to 10-14 W/g.[3], [4] Although, these SP were calculated for low power conversion efficiency (PCE) material systems. Currently, OPV achieved PCE of over 20% with the incorporation of the non-fullerene-based small molecule acceptor, and high SP is believed to be obtained. To enter the market, a high-altitude platform station (HAPS) is the first place to start with. The HAPS environmental condition includes high temperature variation (-20 to 10 ^oC on average daytime, and up to -85 ^oC in night-time), ultra-violet (UV) rich solar radiations (AM0, 1366.1 W/m²), and low pressure (1 to 250 mbar).[5] In this work, we explore and compare the in-situ performance of two high-performing OPV, using the same donor (PM6) but different acceptors (IT-4F and Y6), in a mimic HAPS environment where the pressure, temperature and illumination conditions are controlled for the first time. It is found that PM6: Y6 device performed well, and the PCE dropped between -20 to 10 ^oC is negligible. Whereas, for PM6: IT-4F device, the PCE dropped almost 12% at -20 ^oC compared to its peak value (at 20 ^oC). Moreover, at -80 ^oC the PM6: Y6 device could still hold up to 68% of its initial value (at 20 ^oC). Whereas PM6: IT-4F device remained 50% of PCE under similar conditions. The major drops came into J_sc and FF under the low temperature of PM6: IT-4F device. We found that the use of an acceptor can result in a substantial difference in performance at low temperatures.  In conclusion, the PM6: Y6 device performed well and showed the promising performance under HAPS temperature environmental conditions. This work provides an in-strength study on a benchmark OPV based on PM6: Y6 system and highlights its great potential for HAPS or even space applications.