Photon counting detectors (PCD) with energy discrimination capabilities have the potential for improved detector performance over conventional energy integrating detectors. Additionally, PCDs are capable of advanced imaging techniques such as material decomposition with a single exposure, which may have significant impact in breast imaging applications. Our goal is to develop a large area amorphous Selenium (a-Se) photon counting detector. By using our novel direct conversion field-Shaping multi-Well Avalanche Detector (SWAD) structure, the inherent limitations of low charge conversion gain and low carrier mobility of a-Se can be overcome. In this work we developed a spatio-temporal charge transport model to investigate the effects of charge sharing, energy loss and pulse pileup for SWAD. Using a monoenergetic 20 keV source we found that 32% of primary interactions have K-fluorescence emissions that escape the target pixel, 62.5% of which are reabsorbed in neighboring pixels, while 37.5% escape the detector entirely for a 100 μm × 100 μm pixel size. Simulated pulse height spectra for an input count rate of 50,000 counts/s/pixel with a 2 μs dead time was also generated, showing a photopeak FWHM = 2.6 keV with ~10% pulse pileup. Additionally we present the first time-of-flight (TOF) measurements from prototype SWAD samples, showing successful unipolar time differential (UTD) charge sensing. Our simulation and initial experimental results show that SWAD has potential towards making a large area a-Se based PCD for breast imaging applications.