The performance of visible light communication (VLC) systems are limited by the modulation bandwidth of light emitting diodes (LEDs), despite the availability of the wide optical spectrum. A multiple-input, multiple-output (MIMO) scheme can be employed to further improve the data rate by means of harnessing the spatial multiplexing gain. Nevertheless, it is known that the MIMO VLC channel can be rank-deficient, and consequently its performance can be degraded significantly. The rank of the MIMO VLC channel is highly affected by the geometries of light emitting diodes and photodiodes (PDs). In this paper, the effects of PD configurations on the MIMO performances are investigated. It will be shown that certain PD configurations can lead to a rank-deficient channel. We propose Irregular PD configurations (IPC) to overcome this issue. Moreover, bi-objective problems are formulized to obtain the optimal IPC. A significant gain in terms of the condition number of the channel is shown with respect to regular PD configurations, which further suggests a benefit of IPC in MIMO VLC.