You can choose to increase specific carbon free technologies, or choose to reduce emissions, but you cannot do both.
With a carbon tax or a good cap and trade you directly choose how much carbon emissions will be reduced by. But you do not have control on how much of the reduction will be from efficiency, reduced consumption, or substitutes.
With a subsidy you have direct control over how much of a technology will be increased and which technology will be used. But you sacrifice long term control on how much of this will be substituting existing polluting methods, or just increasing consumption of electricity.
That is the question facing policy makers around the world regarding climate change.