The quantum efficiency and the crosstalk of a photodiode in a long wavelength IR (LWIR) 2-D diode array are studied by numerically solving the 2-D diffusion equation of photocarriers in an array environment. The quantum efficiency depends strongly on both the diode size and the junction depth, and can be expressed, to good accuracy, as a quadratic function of the two variables. The results, corresponding to HgCdTe n+-on-p backside illuminated photovoltaic (PV) diodes, are compared with published Monte Carlo results and with analytical (1-D) special cases. For a given pitch, smaller diodes have lower crosstalk, but also lower quantum efficiency. This implies an optimal diode size. The highest quantum efficiency is obtained for low junction depths (? 1 ?m), and the optimum diode size, maximizing the quantum efficiency and minimizing the crosstalk, lies in the range of 15 to 30 ?m for a pitch of 50 ?m over a wide range of diffusion lengths. The quantum efficiency depends weakly on the pitch, for a given ratio of diode size to the pitch.