How does the voltage drop across each junction in tandem solar cell play their role? and why the band gap should be in decreasing order in these? How the Voc and Isc depends in these cells?
You can just consider two solar cells. If at least one of them is partly transparent, then the other one can be placed in the shadow of the first one and collect the power of light in this shadow. This is already a tandem.
Next step is to simplify the system by removing or merging duplicated elements.
Simplest is to have neither electrodes nor glass/plastic walls in between the cells in a tandem. This way, you force the same current in both parts of the tandem, and overall voltage is a sum of the voltages on those parts. (Having the same current makes it difficult to impossible to operate both cells in an optimal point of the IV curve, but simplification is huge.)
It's more common that a material is transparent for wavelengths longer than the wavelengths useful for generating the photo current, than the other way around. This is not a Law, but really quite general. And this gives rise to the rule of decreasing order of band gaps.
Hope this picture helps to grasp the idea of a tandem cell...
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