Use of first person in a PhD Thesis
I generally avoid "I" in scientific texts altogether, though some authors are in fact using it if they are the sole author. I can't remember seeing it in a thesis though. In texts with a sole author, I usually understand "we" as meaning the author and the reader, and I'd suggest that it's fine to use it in places where it can have that meaning. For example something like "When substituting a by b, we get ..."
A generally useful advice would be to read into some of the theses written in your group, department, and university (in decreasing relevance), and see whether there is a common pattern.
Summary: Think about the habits and traditions in your field, think about the nature of your field and do not hesitate to take responsibility for your own (possibly not that great) ideas.
Now, let me elaborate more:
The question encourages personal opinions for a good reason. Various sources on writing research papers differ vastly, though it seems majority does not favor the first person "I" form. For one of the more serious in computer science not in favour of "I", see e.g., Knuth's Mathematical Writing (pg.4) - although later on, the material also discusses the opposite (pg.62 and 113).
Now to a personal position. I do make use of "I" in some contexts. Namely, when I write a paper as a single author and I did so in my PhD thesis. At the same time, you should have clear rules when to mix it with "we" and how. For the dissertation, I explained those rules very early on in the preface: I use "I" whenever the text speaks about my own decisions and choices I made and is the default voice. It means, that it's me who is to blame for whatever incorrect decisions exposed in the thesis. Only if I can show that there is an external force which would push anybody on my place to take the same route, I would use "we" to mean the (research) community, or humankind. I use "we", whenever the discourse is explanatory, such as an exposition of a proof. Therein, "we" stands for "me and the reader". I also strictly use "we", whenever I speak about an insight, or a result which was produced in a collaboration, such as developed in a joint research paper with somebody else. As a side-effect, since this voice is not the default one, occurrence of such "we" always enforces a citation to the joint work, which is a Good Thing.
My personal opinion also is that third person is very bad writing style, since it offloads responsibility for the presented results to some external entity. As if it wasn't me who made the stupid decision to push that other guy from the cliff, but the guy was (somehow) pushed from the cliff. In my opinion, "we" solves that problem only a little bit, because now the writer admits a bit of responsibility for the act, but still dilutes it by taking into the game somebody else (either the reader, or the abstract research community). Saying "I did this and that and by doing it I personally found this and that" for me is fully taking responsibility for my results. It's not about bragging, or so. Now in some fields, this might be inappropriate, e.g., in pure mathematics, one studies a problem and is not pushed into any arbitrary decisions (e.g., regarding experimental setup), so a style "we" = "the two of us, you, the reader, and me, the writer" is more appropriate.
It is interesting to see what Charles Darwin did in his scientific writing.
According to Serendip Studio:
Darwin usually speaks in the first person plural when analyzing empirical evidence he has collected and only uses the first person singular when he is specifically speaking about his own actions, such as, "..many special facts which I have collected," or when he is speaking about his own qualms, such as "I am well aware that there are on, on this view, many cases of difficulty, some of which I am trying to investigate." However, when analyzing his evidence, he always uses "we", such as "we notice", or "we understand"(2). Darwin's change in footing when he is explaining his theory places himself and the reader on the same level and makes him a more "humble" presenter, allowing us to suspend disbelief for at least the time being and trust him.
When Darwin is speaking as the scientist, he uses "we", and when he is speaking as the human being, he uses "I". I really like that distinction.
I find the forced use of "we" when you mean "I" misplaced. It is important to sound as natural as possible in your writing - just look at Richard Feynman. You don't have to use convoluted language to win a Nobel prize. Clarity is king.