High-Efficiency Polymer Homo-Tandem Solar Cells with Carbon Quantum-Dot-Doped Tunnel Junction Intermediate Layer

The tunnel junction (TJ) intermediate connection layer (ICL), which is the most critical component for high-efficient tandem solar cell, generally consists of hole conducting layer and polyethyleneimine (PEI) polyelectrolyte. However, because of the nonconducting feature of pristine PEI, photocurrent is open-restricted in ICL even with a little thick PEI layer. Here, high-efficiency homo-tandem solar cells are demonstrated with enhanced efficiency by introducing carbon quantum dot (CQD)-doped PEI on TJ–ICL. The CQD-doped PEI provides substantial dynamic advantages in the operation of both single-junction solar cells and homo-tandem solar cells. The inclusion of CQDs in the PEI layer leads to improved electron extraction property in single-junction solar cells and better series connection in tandem solar cells. The highest efficient solar cell with CQD-doped PEI layer in between indium tin oxide (ITO) and photoactive layer exhibits a maximum power conversion efficiency (PCE) of 9.49%, which represents a value nearly 10% higher than those of solar cells with pristine PEI layer. In the case of tandem solar cells, the highest performing tandem solar cell fabricated with C-dot-doped PEI layer in ICL yields a PCE of 12.13%; this value represents an ≈15% increase in the efficiency compared with tandem solar cells with a pristine PEI layer.