Development of Dopant-Free Donor-Acceptor-type Hole Transporting Material for Highly Efficient and Stable Perovskite Solar Cells

In perovskite solar cells (PSCs), overlying hole transporting materials (HTMs) are important for achieving high efficiencies as well as protecting perovskite active layers from degradation factors. This study reports the synthesis of a dopant-free HTM based on a D′-A-D-A-D-A-D′ (D, D′: electron donor, A: electron acceptor) conjugated structure and incorporation of the HTM into a PSC. The resulting PSC exhibits a high efficiency of 17.3%, which is comparable to that of the device based on doped spiro-OMeTAD HTM, and exhibits much improved stability: without encapsulation, the PSC based on the new HTM was found to retain 80% of its initial performance over 500 h under the conditions of 60% relative humidity/1 sun light-soaking without encapsulation. The high performance is attributed to efficient hole-extraction/collection and hole transport. We demonstrate that the extended π-structure of the D′-A-D-A-D-A-D′-type HTM slows moisture intrusion and protects the perovskite layer better than smaller D-A-type molecules. The improved stability is primarily due to the hydrophobic nature of the HTM; the relatively large π-conjugated molecule forms denser films, which effectively decrease the spaces between the molecules and retard water intrusion. The dopant-free D-A-type HTM with an extended π-structure is effective not only in improving device efficiency, but also device stability.