Reduced energy offset via substitutional doping for efficient organic/inorganic hybrid solar cells

Abstract

Charge carrier transport in bulk heterojunction that is central to the device performance of solar cells is sensitively dependent on the energy level alignment of acceptor and donor. However, the effect of energy level regulation induced by nickel ions on the primary photoexcited electron transfer and the performance of P3HT/TiO₂ hybrid solar cells remains being poorly understood and rarely studied. Here we demonstrate that the introduction of the versatile nickel ions into TiO₂ nanocrystals can significantly elevate the conduction and valence band energy levels of the acceptor, thus resulting in a remarkable reduction of energy level offset between the conduction band of acceptor and lowest unoccupied molecular orbital of donor. By applying transient photoluminescence and femtosecond transient absorption spectroscopies, we demonstrate that the electron transfer becomes more competitive after incorporating nickel ions. In particular, the electron transfer life time is shortened from 30.2 to 16.7 ps, i.e., more than 44% faster than pure TiO₂ acceptor, thus leading to a notable increase of power conversion efficiency in organic/inorganic hybrid solar cells. This work underscores the promising virtue of engineering the reduction of 'excess' energy offset to accelerate electron transport and demonstrates the potential of nickel ions in applications of solar energy conversion and photon detectors.

New Insight into Sulfurized and Selenized Kesterite–Titania Nanostructures for CdS-free and HTM-free Photovoltaic and Voltage-Modulated Photodetecting Applications

“…However, in order to further improve the photoconversion efficiency of this device, it is important to first address the issue of band misalignment of these two materials as indicated by our XPS measurements. Although a sufficient CBM energy offset (typically in the range of 0.1−0.3 eV) 34 is necessary to allow fast charge injection and transfer, 35 the 0.6−1.0 eV CBM offset measured in the case of kesterite−titania renders charge transfer very inefficient. To achieve a high power output as required by a high efficiency solar cell, future strategies should be directed toward raising the E CBM of TiO 2 (e.g., by extrinsic element doping) or employing new electron acceptors with higher E CBM .…”