R Lopez-Delgado

Enhancing the power conversion efficiency of solar cells employing down-shifting silicon quantum dots

We report the synthesis and characterization of silicon quantum dots that exhibit down-shifting, photo luminescent characteristics. We also discuss the fabrication and characterization of single crystal Silicon (c-Si) Solar cells with and without the influence of the previously mentioned QDs. The incorporation of these nanostructures triggers improvements in the performance of the fabricated photovoltaic devices, especially in the open circuit voltage (Voc) and short circuit current density (Jsc). Specifically, the experimental results showed increments in the Voc from 532.6 to 536.2 mV and in the Jsc from 33.4 to 38.3 mA/cm2. The combined effect of those improved Voc and Jsc values led to an increment in the power conversion efficiency (PCE) from 11.90 to 13.37%. This increment represents an improvement of the order of 12.4% on the power conversion efficiency of this type of solar cells. The observed results could be conducive to promoting the proliferation of photovoltaic structures.

Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots

Silicon solar cells have captured a large portion of the total market of photovoltaic devices mostly due to their relatively high efficiency. However, Silicon exhibits limitations in ultraviolet absorption because high-energy photons are absorbed at the surface of the solar cell, in the heavily doped region, and the photo-generated electron-hole pairs need to diffuse into the junction region, resulting in significant carrier recombination. One of the alternatives to improve the absorption range involves the use of down-shifting nano-structures able to interact with the aforementioned high energy photons. Here, as a proof of concept, we use downshifting CdSe/CdS quantum dots to improve the performance of a silicon solar cell. The incorporation of these nanostructures triggered improvements in the short circuit current density (Jsc, from 32.5 to 37.0 mA/cm2). This improvement led to a ∼13% increase in the power conversion efficiency (PCE), from 12.0 to 13.5%. Our results demonstrate that the application of down-shifting materials is a viable strategy to improve the efficiency of Silicon solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

ZnO photoluminescent quantum dots with down-shifting effect applied in solar cells.

We report the synthesis of Zinc Oxide (ZnO) quantum dots (QDs) and their influence on the power conversion efficiency of photovoltaic devices. With an excitation wavelength of 340 nm the down-shifted emission peaks were observed to be located at 510 and 540 nm for colloidal solutions with pH values of 10 and 12, respectively. The largest PCE variation was observed to increase from 14.60% to 15.49% when dispersing the QDs extracted from a 4 mL colloidal solution that were subsequently dispersed in PMMA. This represents an improvement of ~6.1%.

Red-shift of the photoluminescent emission peaks of CdTe quantum dots due to the synergistic interaction with carbon quantum dot mixtures

We report the relatively large red-shift effect observed in down-shifting carbon quantum dots (CQDs) that is anticipated to have a positive impact on the power conversion efficiency of solar cells. Specifically, with an excitation wavelength of 390 nm, CQDs of different sizes, exhibited down-shifted emission peaks centered around 425 nm. However, a solution comprised of a mixture of CQDs of different sizes, was observed to have an emission peak red-shifted to 515 nm. The effect could arise when larger carbon quantum dots capture the photons emitted by their smaller counterparts followed by the subsequent re-emission at longer wavelengths. Furthermore, the red-shift effect was also observed in CdTe QDs when added to a solution with the aforementioned mixture of Carbon QDs. Thus, whereas a solution solely comprised of a collection of CdTe QDs of different sizes, exhibited a down-shifted photoluminescence centered around 555 nm, the peak was observed to be further red-shifted to 580 nm when combined with the solution of CQDs of different sizes. The quantum dot characterization included crystal structure analysis as well as photon absorption and photoluminescence wavelengths. Subsequently, the synthesized QDs were dispersed in a polymeric layer of poly-methyl-methacrylate (PMMA) and incorporated on functional and previously characterized solar cells, to quantify their influence in the electrical performance of the photovoltaic structures. We discuss the synthesis and characterization of the produced Carbon and CdTe QDs, as well as the observed improvement in the power conversion efficiency of the fabricated photovoltaic devices.

Silicon solar cell efficiency improvement employing the photoluminescent, downshifting effects of carbon quantum dots

There is a generalized trend to demonstrate higher solar cell efficiency with more affordable devices to promote their widespread utilization. One of the options explored involves the utilization of photoluminescent, down-shifting quantum structures to enable the capture of high-energy photons that tend to interact with lattice phonons rather than generate electron-hole pairs. In order to address the aforementioned perceived need, we report the synthesis and characterization of Carbon quantum dots, as well as the observed improvement in the power conversion efficiency of the produced photovoltaic devices. The quantum dot characterization included crystal structure analysis and optical properties such as: photon absorption, photon excitation and photoluminescence. The synthesized carbon quantum dots of different sizes were dispersed in a polymeric layer of PMMA 100 nm thick and incorporated on functional and previously characterized solar cells, to quantify their influence in the electrical performance of the aforementioned photovoltaic devices. We discuss the synthesis approach employed as well as the measurements collected on the quantum dots themselves and the influence on the solar cells involved.

Influence of photo-luminescent CdSe/CdS core shell quantum dots in solar cell efficiency

We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) that exhibit absorption in the UV range of the solar spectrum and emit photons with wavelengths centered around 625 nm, a wavelength that is well suited for silicon absorption and electron-hole pair generation. We also report the fabrication and characterization of single crystal silicon (c-Si) solar cells with and without the aforementioned photo luminescent, down-shifting CdSe/CdS- QDs. The incorporation of these nanostructures triggered improvements in the performance of the devices, particularly in the open circuit voltage (Voc) and short circuit current density (Jsc) for which the measured values showed an increase from 543 to 546 mV and from 32.5 to 37.0 mA/cm2, respectively. The combined effect of the improved values led to an increment in the power conversion efficiency (PCE) from 12.01 to 13.54%. This increase represents a 12.7% improvement in the PCE of the fabricated devices. The effort described herein is considered a good fit to the generalized trend to improve the efficiency of solar cells with mass-compatible techniques that could serve to promote their widespread utilization.