  Frequently, trends in record AM1.5 power-conversion efficiencies versus time,
such as the NREL efficiency chart, are used to analyze the relative merits of
different photovoltaic material technologies. However, this approach belies the
effort expended in achieving these levels of performance. We introduce
cumulative publications as a proxy for total R&D efforts and find surprisingly
that silicon, Cu(In,Ga)Se2 (CIGSe), CdTe, and halide perovskite technologies
have each followed essentially the same learning curve of 20-24% efficiency
within 10,000 publications and a consistent marginal rate of 5% efficiency
increase per factor of 10 in publications. While learning spillover from non-PV
technologies, cross-pollination from other PV technologies, and hidden
commercial effort are not accounted for by this metric, this analysis still
yields useful and novel insights into PV technology trajectories. Trajectories
below this learning curve have required more total effort per performance and
plateaus of efficiency stagnation at large numbers of publications may indicate
(but do not guarantee) the existence of fundamental barriers to commercially
relevant performance. Lastly, examples to watch are identified for technologies
currently exhibiting higher marginal slopes, including some that appeared
dormant by this metric in past years.
