Perovskite materials have been extensively explored for use in a variety of optoelectronic applications, including light-emitting diodes, neuromorphic systems, photodetectors, X-ray sensors, lasers, and photovoltaic devices, owing to their outstanding physical and chemical properties. In this work, mathematical simulation of a vertical photodetector was implemented out, based on cesium tin germanium triiodide (CsSnGeI3) as primary light photoactive layer to improve device performance. The device architecture was composed of PC61BM serving as electron transport material (ETM) and Cz-N functioning as hole transport material (HTM), with CsSnGeI3 placed between them. The effect of absorber layer thickness and work function on electrical characteristics of photodetector was analyzed through simulation. The SCAPS-1D software was utilized for this purpose. By boosting the thickness and doping level of active layer, a peak responsivity of 6.5 × 10−1 A/W and a detectivity of 1.1 × 1014 Jones were achieved.