Role of Hole Conductors in Quantum Dot and Organometal Perovskite Based Solid State Solar Cells

Semiconductor quantum dots (QDs) have the potential to design high efficiency solid-state QDSCs and ETA (Extremely Thin Absorber) solar cells. Tunable band gap and band edge through size quantization, large intrinsic dipole moments, and high extinction coefficients make them suitable light absorbing materials.  A number of semiconductors have been explored in the design of solid state solar cells. These include CdS, CdSe, In₂S₃, and Sb₂S₃.  Of particular interest is Sb₂S₃ which has a band gap of 1.7 − 1.8 eV allowing for light absorption throughout the visible range. Organometal perovskites such as CH₃NH₃PbI₃ are the new semiconducting materials that have led to advances in thin film solar cells.  However, hole transport remains a limiting factor in maximizing power conversion efficiency.

Using Sb₂S₃ (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. We have employed transient absorption spectroscopy to probe the hole transfer between Sb₂S₃ and CuSCN. In the Sb₂S₃ absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S^−•) species. This trapped hole is transferred from the Sb₂S₃ absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. Similarly the hole transport properties in (CH₃NH₃PbI₃) and hole conductor copper iodide has been probed by impedence spectroscopy. Charge separation in Sb₂S₃ and (CH₃NH₃PbI₃) based solar cells which provide new insight into the design of new architectures for higher efficiency devices will be discussed.

ACKNOWLEDGMENT.  The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533.