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In order to reduce the cost of silicon photovoltaics, new low cost methods of silicon purification and crystal growth are being developed including Upgraded Metallurgical Grade (UMG) silicon. These new materials are likely to contain significantly higher concentrations of impurities and crystallographic defects than current high purity silicon. One of the most common approaches to deactivation of these defects is the use of hydrogen passivation, which has been an area of significant research for the photovoltaic community. This thesis explores the importance of the charge states of hydrogen for effective passivation of defects in silicon, and how these charge states may be manipulated. The main areas of work are modelling of hydrogen charge states, hydrogen-dopant interaction in silicon and manipulation of hydrogen charge states through illumination to greatly enhance defect passivation in solar cells. Modelling of hydrogen charge states explores the factors that govern the fractional concentrations of the three charge states of interstitial hydrogen in silicon, H+, H0 and H-. The importance of the silicon doping concentration and excess carrier density is established. The properties of the various charge states are also explored, and it is identified that, while H+ is dominant in p-type silicon under almost all conditions, there are reasons to expect that H0 and H- may be important for the passivation of some defects in silicon. Hydrogen-dopant interaction focuses upon the use of hydrogen to electrically deactivate dopant impurities. Dopant impurities, and in particular boron, are difficult to remove from silicon using metallurgical processes due to their low segregation coefficients, and so high concentrations are likely to exist in UMG silicon. It is demonstrated how hydrogen may be used to deactivate boron in the near-surface region of a heavily boron doped silicon wafer, and how this may improve the performance of a photovoltaic device fabricated on such a substrate. Finally this thesis will examine how manipulation of the hydrogen charge states through illumination can increase the effectiveness of hydrogen passivation in UMG silicon. Results are presented showing through this method implied open circuit voltages of over 710 mV may be achieved, making the materials attractive as substrates for photovoltaic devices.