In this work we perform comprehensive characterization on a 6.9% pure sulphide Cu₂ZnSnS₄ (CZTS) device and construct a 2D device model in Sentaurus TCAD to identify possible roots of performance bottlenecks. Electrical and optical parameters of the absorber from comprehensive electrical and optical characterization, together with other necessary parameters from literature, we successfully reproduce measured current density-voltage (J-V) and external quantum efficiency (EQE) curves. Absorption coefficient is extracted by fitting measured internal quantum efficiency (IQE). At the buffer/absorber interface, a surface recombination velocity of 1×10⁵ cm/s is identified to adequately describe measured spectral response at the near-interface region. Furthermore, a bulk lifetime of ~29 ns is extracted from effective lifetime modelling in conjunction with time resolved photoluminescence (TRPL) results. The key underlying V_oc deficit, ~200 mV, is pinpointed to bandgap fluctuation, which is backed by the simulation study on both QE tail and steady state photoluminescence.