Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
Thin-film solution-processable materials are attractive for efficient and low-cost implementation of photovoltaic and other optoelectronic devices. The figure of merit that characterizes the relevant material transport properties is the charge carrier diffusion length which, in general, depends on the mobility and recombination lifetime. In the Shockley-Read-Hall recombination regime, however, the lifetime becomes inversely proportional to mobility, rendering diffusion length mobility-invariant and determined solely by the density of deep traps.
Photoluminescence-based techniques were successfully employed to measure diffusion lengths on the order of 80 nm for colloidal quantum dot films and exceeding 1 um for lead halide perovskites. An effective trap density can be extracted from diffusion length, however, the exact amount of traps, their capture cross-section and depth, and thus the origin of traps, remain unknown, providing no input for further optimization of the material.
This work focuses on the temperature dependence of the diffusion length, measured using J-V characteristics and photoluminescence lifetime. We find diffusion lengths decreasing significantly at lower temperatures and explain this with shallow (~ 0.1 eV) nature of traps. Comparing different quantum dot and perovskite processing techniques we delineate whether recent advances in photovoltaic performance were achieved due to reduced trap densities vs. trap depths.
Obtained results suggest that in the shallow trap regime, improved mobility can enhance the carrier escape rate from traps and thus improve the diffusion length. Using DFT simulations, we have studied the factors affecting the inter-dot coupling in quantum dot films and propose several chemical routes to improving mobility for both electrons and holes. Our new experimental results demonstrate four-fold improvement of diffusion length in colloidal quantum dot films, achieving record-high short-circuit currents in solution-processed thin-film photovoltaics.
Measured and simulated temperature dependence of short-circuit current for different trap depths.
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International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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