Quantum Dot Photovoltaics

 

Energy and environmental problems have now become major issues to be solved. There is an urgent need for renewable and environmentally-friendly energy generation system. Solar energy is enormous renewable and environmentally-friendly energy source which has potential to substitute fossil fuels. For an hour, solar energy is illuminated to Earth more than the energy consumed by humans for a year. Because of its infinite energy source, solar cell has great potential to become next-generation energy generation system. There are lots of efforts to commercialize the solar cell. The global solar energy market is growing at more than 30% per year. Silicon solar cells has already reached 20% efficiency and many companies successfully commercialized them. However, the actual portion of solar energy supply is insignificant. The high price of the solar cells makes them incompetent compared to fossil fuels. To solve this problem, second-generation solar cells such as CIGS and CdTe solar cells have been developed. They achieve reduction in the cost of materials, but they still have high processing cost in vapor deposition. Due to Schockley-Queisser limit in efficiency, we should reduce manufacturing cost to compete with fossil fuels.

 

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Colloidal quantum dots are of great interest in application of photovoltaic devices, due to their cost-effectiveness and solution-based processability. They also have excellent size-tunable optical properties which are highly desirable for optoelectronic devices. Lead(Ⅱ) sulfide colloidal quantum dots are promising material for photovoltaic devices due to their strong quantum confinement and high dielectric constant.

 

 

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 1. Mild Oxidation of QD

 

Our laboratory introduced oxidized layer into the cell which showed great improvement in performance and stability. Oxidized layer blocked dark current which leads to better open circuit voltage and removes undesirable trap-states.

 

 

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 2. Depletion width Extension

 

Depletion region width extension in QDSCs

Although there has been rapid progress i­­­n QDSCs, there are still remaining challenges to be solved for further improvement of energy conversion efficiency. One of the critical issues is the relatively short minority carrier diffusion length (typically several tens of nm) in a QD layer due to their high trap state densities and low carrier mobility. This issue seriously limits the optimum QD thickness of QDSCs, which is only slightly larger than the depletion region width of QD film because increasing thickness of QD film beyond the depletion region width significantly reduces the charge collection efficiency. The small QD thickness is insufficient to fully absorb incident light, considering the absorption coefficient (~104 cm-1, near the band edge) of QD thin films, which limits the short-circuit current density (Jsc) and the overall performance of QDSCs. Therefore, increasing the depletion region width of QDSCs is very significant for the utilization of a thicker QD layer without sacrificing charge collection efficiency.

 

 

We report that increasing the doping-level of metal oxide can significantly boost the depletion region width in the QD layer and improve the PCE of metal oxide/QD heterojunction solar cells. The performances of ZnO/PbS QDSCs with a lightly doped ZnO (n-ZnO, n ~ 1016 cm-3) film or a heavily doped ZnO (n+-ZnO, n ~ 1019 cm-3) layer were systematically characterized to elucidate the effect of metal oxide doping level. The introduction of n+-ZnO instead of n-ZnO achieves an approximately 30% increase in the depletion region width in the QD layer from ~186 nm to ~242 nm, which was quantitatively estimated on the basis of current density-voltage (J-V) characteristics and capacitance-voltage (C-V) measurements. Therefore, the optimum thickness of QD layer also increases from ~220 nm (n-ZnO device) to ~300 nm (n+-ZnO device). As a result, n+-ZnO/PbS QDSCs demonstrate the maximum PCE of 7.55% and the Jsc of 23.5 mA/cm2, which are significantly higher than those of n-ZnO/PbS QDSCs (PCE = 5.52%, and Jsc = 16.7 mA/cm2).

 

 

[REFERENCE]

■Min-Jae Choi, Sunchuel Kim, Hunhee Lim, Jaesuk Choi, Dong Min Sim, Soonmin Yim, Byung Tae Ahn, Jin Young Kim* and Yeon Sik Jung*, ”Highly Asymmetric n+-p Heterojunction Quantum Dot Solar Cells with Significantly Improved Charge Collection Efficiencies” Advanced Materials, 2016, 28, 1780-1787  [PDF file]

■Min-Jae Choi, Jihun Oh, Jung-Keun Yoo, Jaesuk Choi, Dong Min Sim, and Yeon Sik Jung,*, ”Tailoring the PbS/metal interface in colloidal quantum dot solar cells for improvements of performance and air stability” Energy & Environmental Science, 2014, 7, 3052-3060  [PDF file]