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In this work, an amorphous silicon (a-Si:H) thin-film transistor (TFT) circuit designed for charging of intermediary energy storage devices using photovoltaic (PV) solar cell arrays is demonstrated and analyzed. The proposed circuit combines the functionality of a linear DC-DC voltage regulator and a maximum power point tracking unit (MPPT). The circuit reduces the dependence of the charging voltage on light intensity and offers relatively stable operation when electrically stressed. The effects of light intensity and the PV array size on the system and circuit efficiencies as well as on PV utilization are investigated. A peak system efficiency of 18% is demonstrated. Although this is significantly lower than state-of-the-art switch mode DC-DC voltage regulator and MPPTs, the combined use of thin-film technology and low fabrication temperatures (below 150 °C) allow its integration within a wide range of mobile devices, making it an attractive solution for energy harvesting systems.