How to establish inter-rater reliability as a sole PhD researcher
The measurement of an inter-rater reliability premises multiple raters.
I therefore see three possible solutions for you to thematize this issue:

First, aim to get another rater. The best approach for you would be to apply for and obtain funding in order to hire one. I say that this is the 'best' approach because it would demonstrate (a) your ability to get grants for research purposes and (b) your accuracy and honesty with regards to this methodical task. But if that does not work, there are other options; perhaps you can ask among your friends and colleagues to volunteer their time for you (this is what I once did, and my wife volunteered for a small rating task). Or perhaps your supervisor can arrange a student or research assistant for this task.

Second, you can disclose that you were unable to obtain a second rater (due to resource constraints). Your paper's discussion section would highlight this as a limitation of your research, and it would identify the measurement of your approach's reliability as a gap to be closed through 'future research'. This is not shameful; in every scientific publication there should be a discussion section that identifies limitations and further research paths anyway.

Third, you could split yourself into two 'raters' by assuming that you were rater number one at time t1 and rater number two at time t2. However, I have never seen this approach, and it is merely a bold speculation from my side; I am sure that this would be accompanied with grave methodical concerns. Thus, I should also add: Don't follow this advice, just to be sure.