Periodic Reporting for period 1 - TFQD (Thin film light-trapping enhanced quantum dot photovoltaic cells: an enabling technology for high power-to-weight ratio space solar arrays.)
Berichtszeitraum: 2016-01-01 bis 2018-12-31
Being the primary power source in spacecraft systems, solar arrays impose tight constraints to novel satellite designs, which should be more versatile and efficient to meet the needs of future space applications.
The TFQD project was conceived to develop high-efficiency, flexible and lightweight space solar cells combining a thin film approach with the integration of quantum dots and photonic gratings. The TFQD solar cells target efficiency higher than 26% under AM0, an eightfold increase of power-to-weight ratio vs. commercial triple junction solar cells and a very low bending radius (< 3cm), allowing for the development of rollable or inflatable solar arrays.
To accomplish its objectives, TFQD exploited cost-effective and scalable fabrication processes such as Nano Imprint Lithography (NIL) for gratings fabrication and Epitaxial Lift-Off (ELO) for thin-film processing, Molecular Beam Epitaxy (MBE) and Metalorganic Chemical Vapor Deposition (MOCVD) for material growth. Advanced electrical and electromagnetic simulation tools were used to support the device development.
The research activities were carried out by a consortium made by 4 universities, one SME and one large enterprise:
• Modeling and design by Politecnico di Torino (POLITO) with the cooperation of Tampere University of Technology (TUT);
• ELO process and development of high efficiency thin-film solar cells, included the MOCVD growth of GaAs cells, by Radboud University (RBU) and the SME tf2 devices (tf2);
• MBE growth of QD InAs/GaAs solar cells by TUT and University College London (UCL);
• Nanoimprinting of photonic gratings by TUT;
• Device space requirements and space testing by Thales-Italia Space (TAS-I).
The TFQD project was conceived to develop high-efficiency, flexible and lightweight space solar cells combining a thin film approach with the integration of quantum dots and photonic gratings. The TFQD solar cells target efficiency higher than 26% under AM0, an eightfold increase of power-to-weight ratio vs. commercial triple junction solar cells and a very low bending radius (< 3cm), allowing for the development of rollable or inflatable solar arrays.
To accomplish its objectives, TFQD exploited cost-effective and scalable fabrication processes such as Nano Imprint Lithography (NIL) for gratings fabrication and Epitaxial Lift-Off (ELO) for thin-film processing, Molecular Beam Epitaxy (MBE) and Metalorganic Chemical Vapor Deposition (MOCVD) for material growth. Advanced electrical and electromagnetic simulation tools were used to support the device development.
The research activities were carried out by a consortium made by 4 universities, one SME and one large enterprise:
• Modeling and design by Politecnico di Torino (POLITO) with the cooperation of Tampere University of Technology (TUT);
• ELO process and development of high efficiency thin-film solar cells, included the MOCVD growth of GaAs cells, by Radboud University (RBU) and the SME tf2 devices (tf2);
• MBE growth of QD InAs/GaAs solar cells by TUT and University College London (UCL);
• Nanoimprinting of photonic gratings by TUT;
• Device space requirements and space testing by Thales-Italia Space (TAS-I).
TFQD tackled with the two main issues in QD solar cells (QDSC), namely the large reduction of the open circuit voltage (Voc) compared to single-junction cells and the small QD photogeneration. TFQD has demonstrated based on theory and experiments that the Voc reduction is due to intrinsic and extrinsic mechanisms. Intrinsic loss causes a linear dependence of the Voc with the QD emission wavelength as proven experimentally by TUT fabricated samples and theoretically by the quantum-corrected drift-diffusion simulator developed by POLITO. MBE grown 10x InAs/GaAs QD solar cells with high in-plane density QDs fabricated at TUT demonstrated Voc higher than 0.95 V, the highest Voc ever reported for MBE grown QDSCs, in line with record MOCVD grown QDSCs.
Light management was shown to drastically improve the QD absorption and the cell efficiency. Towards this goal, broadband nanostructured antireflection coatings and micro-scale patterned reflectors for the cell rear-side were designed and developed. Structured metal/polymer back reflectors with different shapes were fabricated using commercial polymers demonstrating efficient diffraction efficiency.
Photon management in thin-film cells was investigated as a means to optimize photon-recycling. A selective patterning technique of the bottom contact layer implemented in GaAs MOCVD grown cells by RBU, yielded a clear increase of Voc well correlated to the increase of reflectance as the contact layer coverage is reduced from 100% to 10%. Cells with different placement of the junction were studied, confirming that beginning-of-life (BOL) optimization demands to place the junction as close as possible to the rear contact.
Successful ELO of MBE grown layer structure from 3-in wafers was demonstrated. ELO QDSCs show a two-fold increase of the QD photocurrent. A high Voc voltage of 0.88 V was attained, the highest ever reported for thin-film III-V QD cells. As the quality of the QD solar cell structures approaches that of regular solar cells more closely, ELO QD cells can achieve higher Voc than their substrate based counterparts, as already demonstrated in non-QD solar cells.
More than 500 solar cells were subjected to various elements of BOL and end-of-life (EOL) testing upon electron and proton irradiation. The results demonstrate that thin-film cells perform better than wafer based cells; deep-junction cells, although better at BOL, suffer more than shallow-junction cells; thinner active layers are more resilient and combined with light trapping offer the potential for improved trade-off between BOL and EOL efficiency. In this respect, an important aspect to be optimized is the radiation resilience of the solar cell hetero-interfaces. QD cells showed radiation hardness equivalent to that one of GaAs cells, with a similar damage mechanism. Polymer samples subjected to electron irradiation showed no remarkable changes in their properties (optical and mechanical).
Light management was shown to drastically improve the QD absorption and the cell efficiency. Towards this goal, broadband nanostructured antireflection coatings and micro-scale patterned reflectors for the cell rear-side were designed and developed. Structured metal/polymer back reflectors with different shapes were fabricated using commercial polymers demonstrating efficient diffraction efficiency.
Photon management in thin-film cells was investigated as a means to optimize photon-recycling. A selective patterning technique of the bottom contact layer implemented in GaAs MOCVD grown cells by RBU, yielded a clear increase of Voc well correlated to the increase of reflectance as the contact layer coverage is reduced from 100% to 10%. Cells with different placement of the junction were studied, confirming that beginning-of-life (BOL) optimization demands to place the junction as close as possible to the rear contact.
Successful ELO of MBE grown layer structure from 3-in wafers was demonstrated. ELO QDSCs show a two-fold increase of the QD photocurrent. A high Voc voltage of 0.88 V was attained, the highest ever reported for thin-film III-V QD cells. As the quality of the QD solar cell structures approaches that of regular solar cells more closely, ELO QD cells can achieve higher Voc than their substrate based counterparts, as already demonstrated in non-QD solar cells.
More than 500 solar cells were subjected to various elements of BOL and end-of-life (EOL) testing upon electron and proton irradiation. The results demonstrate that thin-film cells perform better than wafer based cells; deep-junction cells, although better at BOL, suffer more than shallow-junction cells; thinner active layers are more resilient and combined with light trapping offer the potential for improved trade-off between BOL and EOL efficiency. In this respect, an important aspect to be optimized is the radiation resilience of the solar cell hetero-interfaces. QD cells showed radiation hardness equivalent to that one of GaAs cells, with a similar damage mechanism. Polymer samples subjected to electron irradiation showed no remarkable changes in their properties (optical and mechanical).
TFQD has provided advances beyond the state of the art in several areas: ELO thin film technology; QD solar cells, with new insight on the Voc issue and the demonstration of record Voc cells both in substrate and thin-film configuration; effective light trapping approaches; novel strategies for photon recycling. Many of the achieved results are attractive for application in other solar cell technologies or material systems. The results attained with the QD cells demonstrate that high efficiency QDSCs are feasible, paving the way in the short-term to the use of QDs for the development of multijunction cells when a suitable bulk material is not found. Moreover, the scientific and technological know-how gained in TFQD will be essential to develop a variety of next generation concepts based on QDs.
The development of thin-film solar cells with high efficiencies represents a breakthrough for satellites design and performance, since flexibility and light weight may push the development of new missions and new architectures. Demonstration of the TF(QD) cells would represent an innovation boost for space solar panel manufacturers, since their implementation in space solar panels can improve the panel’s power-to-weight ratio by a staggering factor of four.
The successful penetration of thin-film III-V cells in the space solar cell market, would lead to a remarkable growth for companies in the field of lift-off of III-V films and thin-film III-V solar cell processing. Furthermore, since the ELO technique enables a significant cost reduction in view of wafer reuse – which is a key-requirement for the environmental sustainability of large scale production of III-V cells - the wafer reclaim industry will receive a significant boost.
The building blocks developed in TFQD have themselves the potential to strengthen the competiveness and growth of companies: besides lift-off and thin-film processing, high quality epitaxial material grown by MBE and production of photonic structures through an industrialize-able process such as NIL are two areas of strong potential impact.
On account of wafer reuse and simplicity of the epitaxial structures, the TF(QD) solar cells are less expensive than commercially available multi-junction solar cells, thus also important potential impact in terrestrial applications is foreseen, starting with concentrating photovoltaic (CPV) systems. CPV exploitation in turn could lead to a significant reduction of the Levelized Cost Of Electricity of this technology, opening up other markets such as those of unmanned aerial vehicles and stratospheric platforms, automotive, portable electronics and building integrated photovoltaics.
The development of thin-film solar cells with high efficiencies represents a breakthrough for satellites design and performance, since flexibility and light weight may push the development of new missions and new architectures. Demonstration of the TF(QD) cells would represent an innovation boost for space solar panel manufacturers, since their implementation in space solar panels can improve the panel’s power-to-weight ratio by a staggering factor of four.
The successful penetration of thin-film III-V cells in the space solar cell market, would lead to a remarkable growth for companies in the field of lift-off of III-V films and thin-film III-V solar cell processing. Furthermore, since the ELO technique enables a significant cost reduction in view of wafer reuse – which is a key-requirement for the environmental sustainability of large scale production of III-V cells - the wafer reclaim industry will receive a significant boost.
The building blocks developed in TFQD have themselves the potential to strengthen the competiveness and growth of companies: besides lift-off and thin-film processing, high quality epitaxial material grown by MBE and production of photonic structures through an industrialize-able process such as NIL are two areas of strong potential impact.
On account of wafer reuse and simplicity of the epitaxial structures, the TF(QD) solar cells are less expensive than commercially available multi-junction solar cells, thus also important potential impact in terrestrial applications is foreseen, starting with concentrating photovoltaic (CPV) systems. CPV exploitation in turn could lead to a significant reduction of the Levelized Cost Of Electricity of this technology, opening up other markets such as those of unmanned aerial vehicles and stratospheric platforms, automotive, portable electronics and building integrated photovoltaics.