Papers by Maykel Courel Piedrahita
In this work, SnS thin films were deposited by employing the Close Spaced Vapor Transport (CSVT) ... more In this work, SnS thin films were deposited by employing the Close Spaced Vapor Transport (CSVT) technique
under air atmosphere. Single-phase, p-type SnS thin films were synthesized by varying the final pressure in
the chamber and its effect on the properties of SnS were studied. The pressure impact on the directional preferred
orientation (DPO) of grains is presented for the first time. The analysis of different pressure values on deposited
film properties was performed by X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy,
atomic force microscopy, optical measurements and electric characterization techniques.
While the standard interpretation treats CdS/CdTe, CdS/Cu(In,Ga)Se2(CIGSe) and CdS/kesterites as ... more While the standard interpretation treats CdS/CdTe, CdS/Cu(In,Ga)Se2(CIGSe) and CdS/kesterites as p-n junctions, some recent reports suggest these solar cells operate rather as a Metal-Insulator-Semiconductor (MIS) structure. In this paper, we report results about the dependence of electrical, optical, morphological and structural properties of CBD-CdS on growth conditions in order to obtain the best layers to be used in solar cells according to the MIS model. In particular, CBD-CdS thin films were synthesized by changing the complexing agents in the bath solution, the Cd source and the nominal S/Cd ratio. In order to study the impact of the different CdS deposition approaches, CIGSe and CZTSe solar cells were processed. The influence of CdS:Cu as well as Cd sources such as Cd(NO3)2 and CdSO4 on solar cell electrical parameters is studied.
Low open circuit voltage (Voc) values have been widely reported in kesterite Cu2ZnSnSe4 (CZTSe)-b... more Low open circuit voltage (Voc) values have been widely reported in kesterite Cu2ZnSnSe4 (CZTSe)-based thin film solar cells. So far, a complete understanding of the main sources of these low performances is far from clear. In this work, a theoretical model for CZTSe solar cell with record efficiency is presented. Among the different device loss mechanisms, trap-assisted tunneling recombination is introduced as the major hurdle to boost Voc values. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations. Finally, it is found that a further solar cell efficiency enhancement of up to 19.4% with an open circuit voltage close to 708 mV can be achieved by using more resistive CdS layers which is in contradiction to p-n junction behavior. In this way, a MIS performance is proposed to promote Voc and efficiency values. As a result, this approach could help to solve at least one of the main issues of this technology.
In this work, the results about the properties of CdTe:Bi thin films grown by the Closed Space Va... more 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 1.0 9 10 17 to 4 9 10 19 at/cm 3 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.
Despite Cu 2 ZnSnS 4 (CZTS) is a potential candidate for solar cell applications, so far, low eff... more Despite Cu 2 ZnSnS 4 (CZTS) is a potential candidate for solar cell applications, so far, low efficiency values have been reported. In particular, for spray-deposited CZTS, efficiencies lower than 2% are commonly achieved. It is well known that one of the most important parameters governing solar cell performance is minority carrier diffusion length (L n). In this work, CZTS thin film solar cells with different composi-tional ratios are fabricated in order to study its impact on L n values. The L n parameter is calculated for sprayed-CZTS layers using external quantum efficiency measurements in conjunction with optical absorption coefficient versus wavelength measurements – for the first time. Values in the range of 0.11–0.17 lm are obtained emphasizing the need for improving sprayed-CZTS crystalline quality.
One of the most important issues in kesterite Cu2ZnSnS4 (CZTS)-based thin film solar cells is low... more One of the most important issues in kesterite Cu2ZnSnS4 (CZTS)-based thin film solar cells is low open circuit voltage, which is mainly related to loss mechanisms that take place in both CZTS bulk material and CdS/CZTS interface. A device model for CZTS/CdS solar cell which takes into account loss mechanisms influence on solar cell performance is presented. The simulation results showed that our model is able to
reproduce experimental observations reported for CZTS/CdS based solar cells with the highest conversion efficiencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms,
trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mechanism, a solar cell efficiency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efficiency improvement is discussed.
In this work, a device model for Cu2ZnSnS4 (CZTS) solar cell with certified world record efficien... more 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.
New types of thin film solar cells made from
earth-abundant, non-toxic materials and with adequat... more New types of thin film solar cells made from
earth-abundant, non-toxic materials and with adequate
physical properties such as band-gap energy, large absorption coefficient and p-type conductivity are needed in
order to replace the current technology based on
CuInGaSe2 and CdTe absorber materials, which contain
scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). The constituent
elements of the SnS film are abundant in the earth’s crust,
and non-toxic. If this compound is used as the absorber
layer in solar cells, high efficient devices should be fabricated with relative low cost technologies. Despite these
properties, low efficiency SnS-based solar cells have been
reported up to now. In this work, we present a review about
the state of the art of SnS films and devices. Finally, an
analysis about different factors that are limiting high efficiency solar cells is presented.
The most commonly used commercial conducting glass is the coated SnO2:F glass substrate. However,... more The most commonly used commercial conducting glass is the coated SnO2:F glass substrate. However, it usually presents poor morphological properties and therefore is not adequate for solar cell applications. In this work, we report results about the morphological properties improvement of commercial conductive glass by the deposition of a SnO2 buffer layer by spray pyrolysis technique. The use of a high
resistivity transparent layer (buffer) onto low resistivity transparent oxide has been found to improve the solar cell performance. The methodology for improving film morphology through the deposition of SnO2 buffer layers by spray pyrolysis technique, with appropriate electro-optical properties, is presented. The solar cell results related to the use of an automatized spray pyrolysis system are presented.
Cu2ZnSnS4 (CZTS) thin films were grown onto glass substrates using pneumatic spray pyrolysis meth... more Cu2ZnSnS4 (CZTS) thin films were grown onto glass substrates using pneumatic spray pyrolysis method under different growth conditions.The electrical properties have been investigated by means of dark and illuminated conductivity measurements in the 293–353 K temperature range and Hall measurements at
room temperature. The influence of growth parameters on the grain boundary barrier height and other associated grain boundary parameters of the CZTS thin films was determined from electrical, optical, and morphological characterization.The CZTS electrical properties relation with secondary phases formation as well as with transport mechanisms is presented. Finally, CZTS electrical parameters impact on the properties of polycrystalline thin film solar cells is discussed.
In this work, a study of trap and recombination center properties in polycrystalline Cu2ZnSnS4 th... more 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 126610, 476625, and 11006100 meV. 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.
The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe compound... more 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.
We present a review of our works in quantum well and superlattice solar cells. In this work, the ... more We present a review of our works in quantum well and superlattice solar cells. In this work, the quantum efficiency for AlGaAs/GaAs multi-quantum well solar cell has been calculated and compared with available data from the group at the Imperial College, London. Quantum efficiency calculations is presented and compared with experimental data for several AlGaAs/GaAs multi-quantum well solar cell, obtaining good agreement. The photocurrent is calculated from the quantum
efficiency calculations and included in the J(V) relation to optimize the efficiency. It also shows that for a range of quantum well widths and barrier bandgaps the conversion
efficiencies of the quantum well solar cell are higher than the corresponding homogeneous p-i-n solar cell. Our results give a broad representation of quantum well solar cell operation, and provide a profitable guide for designing and interpreting
the performance characteristics of AlGaAs/GaAs QWSCs. Also, a theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cell. Using the Transfer Matrix Method, the conditions for resonant tunneling are established for particular superlattice geometry with variably spaced quantum wells. The effective density of states and the absorption coefficients are calculated to determinate the J-V characteristic. Radiative, non radiative and interface recombination were evaluated from a modeled superlattice solar cell and their values are compared with a multi-quantum well solar cell of a same aluminum composition. A discussion about the conditions where superlattice solar cell performance overcomes that of a multi-quantum well solar cell is addressed.
The use of a high resistivity transparent oxide (buffer) onto low resistivity transparent oxide o... more The use of a high resistivity transparent oxide (buffer) onto low resistivity transparent oxide or TCO has been found to improve the solar cells performance. The most commonly used commercial conducting glass is the coated SnO2:F glass substrates due that offers the highest properties/cost ratio; however it usually presents poor morphological properties and therefore is not adequate for solar cells applications. The methodology for improving the film morphology with appropriate electro-optical properties on commercial conducting glass, through the deposition of buffer layers of SnO2, by using spray pyrolysis system is presented. The solar cells results related to the use of an automated spray pyrolysis system are presented.
Thermally stimulated current has been studied in polycrystalline Cu2ZnSnS4 thin films deposited b... more Thermally stimulated current has been studied in polycrystalline Cu2ZnSnS4 thin films deposited by pneumatic spray pyrolysis method using various heating rates, in order to gain information about the trapping centers and/or deep levels present within the band-gap of this material. A set of temperature-dependent current curves revealed three trap levels with activation energies of 0.12, 0.50 and 1.05 eV. The values of the frequency factor, the capture cross-section and the trap concentration have been determined for each center. A discussion about the possible nature of each trap is also added.
In this work, a review about the influence of the growth parameters on the chemical and physical ... more 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 361 mV, a short-circuit current density of 7.5 mA/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.
In this work we report the relation between different secondary phases formation and the values o... more In this work we report the relation between different secondary phases formation and the values of Cu/(Zn+Sn) and Zn/Sn ratios of Cu2ZnSnS4 (CZTS) thin films deposited by pneumatic spray pyrolysis using air as carrier gas. The films were grown on soda lime glass substrates via (CH3COO)2 Zn 2H2O, CuCl2 2H2O, SnCl2 2H2O and thiourea as precursors salts in the solution. The secondary phases present in the films were determined from Raman spectroscopy and the chemical composition of the elements from X-ray fluorescence (XRF) measurements. Varying the growth parameters and post-deposition thermal and chemical KCN treatments we have studied the different secondary phases formation dependence on the values of the Cu/(Zn+Sn) and Zn/Sn ratios and its impact on the properties of polycrystalline thin films solar cells where CZTS is used as the absorber layer.
A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice... more 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 SL solar cell conversion efficiency is compared with the maximum conversion efficiency obtained for the MQW solar cell and shows an efficiency enhancement.
A theoretical study of GaAs/GaInNAs solar cells based on multiple-quantum well solar cells (MQWSC... more 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%.
A theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cel... more A theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cell (SLSC). Using the Transfer
Matrix Method, the conditions for resonant tunneling are established for a particular SL geometry with variably spaced
quantum wells. The effective density of states and the absorption coefficient are calculated to determinate the J–V characteristic. Radiative, non-radiative, and interface recombination were evaluated from a modeled SLSC, and their values were compared with a multiple quantum well solar cell of the same aluminum composition. A discussion about the conditions, where SLSC performance overcomes that of a multiple quantum well solar cell, is addressed.
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Papers by Maykel Courel Piedrahita
under air atmosphere. Single-phase, p-type SnS thin films were synthesized by varying the final pressure in
the chamber and its effect on the properties of SnS were studied. The pressure impact on the directional preferred
orientation (DPO) of grains is presented for the first time. The analysis of different pressure values on deposited
film properties was performed by X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy,
atomic force microscopy, optical measurements and electric characterization techniques.
reproduce experimental observations reported for CZTS/CdS based solar cells with the highest conversion efficiencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms,
trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mechanism, a solar cell efficiency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efficiency improvement is discussed.
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.
earth-abundant, non-toxic materials and with adequate
physical properties such as band-gap energy, large absorption coefficient and p-type conductivity are needed in
order to replace the current technology based on
CuInGaSe2 and CdTe absorber materials, which contain
scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). The constituent
elements of the SnS film are abundant in the earth’s crust,
and non-toxic. If this compound is used as the absorber
layer in solar cells, high efficient devices should be fabricated with relative low cost technologies. Despite these
properties, low efficiency SnS-based solar cells have been
reported up to now. In this work, we present a review about
the state of the art of SnS films and devices. Finally, an
analysis about different factors that are limiting high efficiency solar cells is presented.
resistivity transparent layer (buffer) onto low resistivity transparent oxide has been found to improve the solar cell performance. The methodology for improving film morphology through the deposition of SnO2 buffer layers by spray pyrolysis technique, with appropriate electro-optical properties, is presented. The solar cell results related to the use of an automatized spray pyrolysis system are presented.
room temperature. The influence of growth parameters on the grain boundary barrier height and other associated grain boundary parameters of the CZTS thin films was determined from electrical, optical, and morphological characterization.The CZTS electrical properties relation with secondary phases formation as well as with transport mechanisms is presented. Finally, CZTS electrical parameters impact on the properties of polycrystalline thin film solar cells is discussed.
efficiency calculations and included in the J(V) relation to optimize the efficiency. It also shows that for a range of quantum well widths and barrier bandgaps the conversion
efficiencies of the quantum well solar cell are higher than the corresponding homogeneous p-i-n solar cell. Our results give a broad representation of quantum well solar cell operation, and provide a profitable guide for designing and interpreting
the performance characteristics of AlGaAs/GaAs QWSCs. Also, a theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cell. Using the Transfer Matrix Method, the conditions for resonant tunneling are established for particular superlattice geometry with variably spaced quantum wells. The effective density of states and the absorption coefficients are calculated to determinate the J-V characteristic. Radiative, non radiative and interface recombination were evaluated from a modeled superlattice solar cell and their values are compared with a multi-quantum well solar cell of a same aluminum composition. A discussion about the conditions where superlattice solar cell performance overcomes that of a multi-quantum well solar cell is addressed.
Matrix Method, the conditions for resonant tunneling are established for a particular SL geometry with variably spaced
quantum wells. The effective density of states and the absorption coefficient are calculated to determinate the J–V characteristic. Radiative, non-radiative, and interface recombination were evaluated from a modeled SLSC, and their values were compared with a multiple quantum well solar cell of the same aluminum composition. A discussion about the conditions, where SLSC performance overcomes that of a multiple quantum well solar cell, is addressed.
under air atmosphere. Single-phase, p-type SnS thin films were synthesized by varying the final pressure in
the chamber and its effect on the properties of SnS were studied. The pressure impact on the directional preferred
orientation (DPO) of grains is presented for the first time. The analysis of different pressure values on deposited
film properties was performed by X-ray diffraction analysis, Raman spectroscopy, scanning electron microscopy,
atomic force microscopy, optical measurements and electric characterization techniques.
reproduce experimental observations reported for CZTS/CdS based solar cells with the highest conversion efficiencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms,
trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mechanism, a solar cell efficiency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efficiency improvement is discussed.
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.
earth-abundant, non-toxic materials and with adequate
physical properties such as band-gap energy, large absorption coefficient and p-type conductivity are needed in
order to replace the current technology based on
CuInGaSe2 and CdTe absorber materials, which contain
scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). The constituent
elements of the SnS film are abundant in the earth’s crust,
and non-toxic. If this compound is used as the absorber
layer in solar cells, high efficient devices should be fabricated with relative low cost technologies. Despite these
properties, low efficiency SnS-based solar cells have been
reported up to now. In this work, we present a review about
the state of the art of SnS films and devices. Finally, an
analysis about different factors that are limiting high efficiency solar cells is presented.
resistivity transparent layer (buffer) onto low resistivity transparent oxide has been found to improve the solar cell performance. The methodology for improving film morphology through the deposition of SnO2 buffer layers by spray pyrolysis technique, with appropriate electro-optical properties, is presented. The solar cell results related to the use of an automatized spray pyrolysis system are presented.
room temperature. The influence of growth parameters on the grain boundary barrier height and other associated grain boundary parameters of the CZTS thin films was determined from electrical, optical, and morphological characterization.The CZTS electrical properties relation with secondary phases formation as well as with transport mechanisms is presented. Finally, CZTS electrical parameters impact on the properties of polycrystalline thin film solar cells is discussed.
efficiency calculations and included in the J(V) relation to optimize the efficiency. It also shows that for a range of quantum well widths and barrier bandgaps the conversion
efficiencies of the quantum well solar cell are higher than the corresponding homogeneous p-i-n solar cell. Our results give a broad representation of quantum well solar cell operation, and provide a profitable guide for designing and interpreting
the performance characteristics of AlGaAs/GaAs QWSCs. Also, a theoretical model is performed to study the viability of the AlGaAs/GaAs superlattice solar cell. Using the Transfer Matrix Method, the conditions for resonant tunneling are established for particular superlattice geometry with variably spaced quantum wells. The effective density of states and the absorption coefficients are calculated to determinate the J-V characteristic. Radiative, non radiative and interface recombination were evaluated from a modeled superlattice solar cell and their values are compared with a multi-quantum well solar cell of a same aluminum composition. A discussion about the conditions where superlattice solar cell performance overcomes that of a multi-quantum well solar cell is addressed.
Matrix Method, the conditions for resonant tunneling are established for a particular SL geometry with variably spaced
quantum wells. The effective density of states and the absorption coefficient are calculated to determinate the J–V characteristic. Radiative, non-radiative, and interface recombination were evaluated from a modeled SLSC, and their values were compared with a multiple quantum well solar cell of the same aluminum composition. A discussion about the conditions, where SLSC performance overcomes that of a multiple quantum well solar cell, is addressed.