Environmental regulation, technology density, and green technology innovation efficiency.

From the perspective of the innovation value chain, this study divides the innovation efficiency of green technology into two stages: R&D efficiency and achievement transformation efficiency. Technology density is introduced as a threshold variable to examine the influence of environmental regulation on the efficiency of green technology innovation at both stages. The findings reveal that China's overall green technology innovation efficiency (GTIE) is improving. R&D efficiency initially declined, then increased, while the efficiency of achievement transformation experienced a three-stage pattern: rise-fall-rise. The GTIE distribution across the two stages progressively increases from the northwest to the southeast, resulting in a concentrated, contiguous "line-shaped" and "block-shaped" pattern. High-efficiency areas are primarily found in the eastern coastal regions. Nationally, Environmental regulation and R&D efficiency share an inverted U-shaped relationship, with a double threshold effect of technology density. Environmental regulation does not significantly affect achievement transformation efficiency, but there is a single threshold effect due to technology density.