Maykel Courel

GaAs/GaInNAs quantum well and superlattice solar cell

A theoretical study of GaAs/GaInNAs solar cells based on multiple-quantum well solar cells (MQWSCs) and superlattice solar cell (SLSC) configuration is presented. The conversion efficiency as a function of the quantum well width and depth is modeled for MQWSC, reaching high values. A study of the SLSC viability is also presented. The influence of the cluster width on the conversion efficiency is researched showing a better performance when width and the cluster number are increased. The SLSC conversion efficiency is compared with the maximum conversion efficiency obtained for the MQWSC showing that it is reached an amazing increment of 4%.

Toward a high Cu2ZnSnS4 solar cell efficiency processed by spray pyrolysis method

In this work, a review about the influence of the growth parameters on the chemical and physical properties of Cu2ZnSnS4 (CZTS) deposited by pneumatic spray pyrolysis technique and its impact on the thin film solar cells is presented and analyzed in order to identify the major drawbacks of this technique and the possibility to improve the device efficiency. Our best solar cell using sprayed CZTS shows an open-circuit voltage of 361u2009mV, a short-circuit current density of 7.5u2009mA/cm2, a fill factor of 0.37, and an efficiency of 1% under irradiation of AM 1.5 and 100 mW/cm2. Some of the key mechanisms related to the properties of sprayed CZTS layers, as well as those concerning the solar cells mechanisms that limit the cell performance, are also analyzed.

Towards a CdS/Cu2ZnSnS4 solar cell efficiency improvement: A theoretical approach

In this work, a device model for Cu2ZnSnS4 (CZTS) solar cell with certified world record efficiency is presented. A study of the most important loss mechanisms and its effect on solar cell performance was carried out. The trap-assisted tunneling and CdS/CZTS interface recombination are introduced as the most important loss mechanisms. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations (quantum efficiency, efficiency, Jsc, FF, and Voc) reported under normal operating conditions. Finally, a discussion about a further solar cell efficiency improvement is addressed.

Route towards low cost-high efficiency second generation solar cells: current status and perspectives

The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe compounds, known as second generation polycrystalline thin films. The challenge of these materials is to reduce the cost per watt of solar energy conversion, but they are actually formed by expensive and/or scanty elements in the earth’s crust such as In, Ga, Te and other that present toxicity issues like Cd. In the last years, new materials with properties of interest for photovoltaic applications and formed by non toxic and relatively abundant elements, have been suggested as alternatives to the main second generation solar cells based on CdTe and CIGSSe. Semiconductor compounds with kesterite structure (Cu2ZnSn(SxSe1−x)4, Cu2ZnSnS4, Cu2ZnSnSe4) and other like In2S3, all of them Cadmium-free have been proposed as new candidates for thin film solar cells. However, reported solar cell efficiencies for these compounds have not yet reached the expected values. In this work, we present a review of the limiting factors for achieving high efficiency in thin film solar cells, related to deposition methods as well as the different mechanisms that limit cell performance. Significant results in the processing of solar cells using some of these compounds and preliminary results of the In2S3 deposition with an overview to its use as buffer layer are presented.

Trap and recombination centers study in sprayed Cu2ZnSnS4 thin films

In this work, a study of trap and recombination center properties in polycrystalline Cu2ZnSnS4 thin films is carried out in order to understand the poor performance in Cu2ZnSnS4 thin film solar cells. Thermally stimulated current has been studied in Cu2ZnSnS4 deposited by pneumatic spray pyrolysis method using various heating rates, in order to gain information about trap centers and/or deep levels present within the band-gap of this material. A set of temperature-dependent current curves revealed three levels with activation energy of 126u2009±u200910, 476u2009±u200925, and 1100u2009±u2009100u2009meV. The possible nature of the three levels found is presented, in which the first one is likely to be related to CuZn antisites, while second and third to Sn vacancies and SnCu antisites, respectively. The values of frequency factor, capture cross section, and trap concentration have been determined for each center.

An approach to high efficiencies using GaAs/GaInNAs multiple quantum well and superlattice solar cell

A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice (SL) are inserted in the i-region of a GaAs p-i-n solar cell (SC) is presented. The results suggest the device can reach record efficiencies for single-junction solar cells. A theoretical model is developed to study the performance of this device. The conversion efficiency as a function of wells width and depth is modeled for MQW solar cells. It is shown that the MQW solar cells reach high conversion efficiency values. A study of the SL solar cell viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficient for SL structure are calculated in order to determinate the J-V characteristic. The influence of superlattice length on the conversion efficiency is researched, showing a better performance when width and cluster numbers are increased. The ...

The role of buffer/kesterite interface recombination and minority carrier lifetime on kesterite thin film solar cells

This paper presents for the first time a theoretical study of the impact of kesterite/buffer interface recombination and kesterite minority carrier lifetime on both CZTS and CZTSe solar cells. It demonstrates that only an 11% efficiency can be reached in CZTS solar cells by improving absorber crystalline quality, pointing out the need for an improved CdS/CZTS interface. It further demonstrates that a CZTS solar cell efficiency enhancement of up to 18%, with an open-circuit voltage value of up to 918 mV, can be achieved depending on CZTS minority carrier lifetime and CdS/CZTS interface recombination speed values. Moreover, this paper shows that by improving CZTSe crystalline quality, a record efficiency value of 17% could be achieved without focusing on improving CdS/CZTSe interface quality. Consequently, CZTSe is presented as a better candidate for solar cell applications. Conditions under which CdS/kesterite interface recombination and trap-assisted tunneling recombination become dominant are provided. In particular, we find that CdS/CZTS interface recombination is the dominant transport mechanism for CZTS minority carrier lifetime values higher than 5 ns, while for CZTSe minority carrier lifetime values lower than 0.1 μs, CdS/CZTSe interface losses are negligible.

CdTe:Bi films deposited by closed space vapor transport under variable pressure and doping levels: evidences of the possible formation of an intermediate band

In this work, the results about the properties of CdTe:Bi thin films grown by the Closed Space Vapor Transport (CSVT) method are presented. Two procedures were developed for the CdTe:Bi films deposition: (a) using powders with different Bi concentrations at a constant pressure in the CSVT chamber and (b) varying the total final Ar pressure in the CSVT chamber. The CdTe powders used in our experimental conditions were obtained by using CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi-dopant concentration in the 1.0xa0×xa01017 to 4xa0×xa01019xa0at/cm3 range. Finally, the possible influence of both parameters on the existence of an intermediate band in CdTe:Bi thin films and CdS/CdTe solar cell characteristics is analyzed.

Different Approaches for Thin Film Solar Cell Simulation

This book chapter summarizes different approaches for modeling thin film solar cells. Particular attention will be given to kesterite solar cells as recently have emerged as promising candidates for replacing CdTe and CIGS technology. So far, only few theoretical works have been proposed to understand high Voc deficit. However, most of these, consider transport mechanisms such as diffusion, radiative and non-radiative recombinations which are not able to explain experimental data reported for solar cells with the highest efficiencies. Currently, only two approaches have been able to reproduce experimental data accurately, one is by considering the effect of potential fluctuations (band-tailing) and the other is by means of tunneling mechanisms assisted by defects along with losses at kesterite/buffer interface. Advantages and disadvantages of each modeling method are presented and discussed.

Cu content dependence of Cu 2 Zn(SnGe)Se 4 solar cells prepared by using sequential thermal evaporation technique of Cu/Sn/Cu/Zn/Ge stacked layers

The doping of Cu2ZnSnSe4 semiconductor with Ge element has demonstrated improvements to kesterite solar cell efficiency. However, the impact of different Cu concentrations on Cu2ZnSnGeSe4/CdS solar cell performance has been poorly studied. In this work, Cu2ZnSnGeSe4 thin films with different Cu contents were synthesized by selenization of sequential thermal evaporation precursors. Solar cells based on kesterite-type Cu2ZnSnGeSe4 (CZTGSe) were fabricated and the influence of the Cu thickness on the chemical composition and morphology of the layers and electro-optical properties of solar cells was studied. The stacking process was performed at room substrate temperature. Efficiency values in the range of 2.0–6.8% are reported as a function of Cu concentration. The highest efficiency of 6.8%, was achieved for solar cell with glass/Mo/CZTGSe/CdS/i-ZnO/ITO structure using the stacking of Cu (3xa0nm)/Sn (248xa0nm)/Cu (112xa0nm)/Zn (174xa0nm)/Ge (20xa0nm).