Solvent management and Li+/Mg2+ co-doping enable efficient n-i-p NiOx-based perovskite solar cells

Spin-coating pre-fabricated NiO_x nanocrystals using an annealing-free process is a compatible technique for fabricating the hole transport layer in n-i-p structured perovskite solar cells. The solvothermal method, assisted by long-chain fatty acids or amines such as oleic acid and oleylamine, has demonstrated the applicability of fabricating NiO_x nanoparticles with uniform morphology and size. However, the long-chain fatty acids or amines covering the NiO_x nanoparticles cannot be removed at low temperature, obstructing the transport of photo-generated holes due to their insulating characteristics. Herein, triphenylphosphine is employed to replace a portion of oleylamine in the solvothermal reaction solution. Experimental results demonstrate that the introduction of triphenylphosphine does not affect the dispersion of the synthesized NiO_x nanoparticles in toluene. The as-fabricated n-i-p structured device receives an efficiency of 12.63 %. Thereafter, Li⁺ doping and Li⁺-Mg²⁺ co-doping strategies are used to further enhance the devices' performance. The best-behaved device with Li⁺-Mg²⁺ co-doping NiO_x hole transport layer acquires an efficiency of 16.20 %. This research provides a practical approach for fabricating efficient NiO_x hole transport materials for n-i-p structured perovskite solar cells.