A dopant-free organic hole transport material for efficient planar heterojunction perovskite solar cells

We demonstrate efficient planar perovskite solar cells using a dopant-free donor-acceptor (D-A) conjugated small molecule as a hole transport material. The photovoltaic cell reaches a power conversion efficiency (PCE) of 14.9%, which is comparable to or even better than that of the devices using the traditional doped 2,2',7,7'-tetrakis(N, N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) hole transport material under equivalent conditions. We ascribe the high performance to the excellent charge transporting properties of the D-A conjugated small molecule. Time-resolved photoluminescence (PL), transient photocurrent response, and impedance spectroscopy characterization indicate that this D-A conjugated small molecule plays a key role in hole collection and extraction in perovskite based photovoltaic devices. The dopant-free D-A small molecule hole transport material used here not only improves the efficiency, but also facilitates the fabrication process and thus potentially reduces the fabrication cost of perovskite solar cells.