Solution-processable 2D metal-halide perovskites are highly promising for cost-effective optoelectronic applications due to their intrinsic multiquantum well structure. However, the lack of stability is still a major obstacle in the use of this class of materials in practical devices. Here, the authors demonstrate the stable optoelectronic properties using formamidinium (FA)-based centimeter-long 2D perovskite (BA)2FAPb2I7 high-quality single-crystal controlled by the thickness of two perovskite layers. The large area single-crystal exhibits good crystallinity, phase purity, and spectral uniformity. Moreover, the (BA)2FAPb2I7 single-crystal shows excellent stability at open atmospheric conditions when compared to methylammonium (MA)-based (BA)2MAPb2I7 counterparts. The photodetectors fabricated using 2D perovskite single-crystal on the rigid Si/SiO2 substrate reveal high photoresponsivity (Rλ)(≈5 A W−1), the fast response time (<20 ms), specific detectivity (D*) (≈3.5 × 1011 Jones), and excellent durability under 488 nm laser illumination. The Rλ and D* values are obtained from the (BA)2FAPb2I7 single-crystal 25 times and three orders magnitudes, respectively, higher than the (BA)2MAPb2I7 single-crystal. Additionally, the perovskite material on flexible polymer substrate reveals good photo-sensing properties in both bending and nonbending states.