Current photovoltaic technologies harvest only a fraction of incoming solar energy since they are unable to utilize photons with energies below the cell band gap. Placed behind a solar cell, the upconverter converts transmitted low-energy photons to photons with energies higher than the cell band gap. The higher energy photons are absorbed by the solar cell and contribute to the photocurrent. We developed optical models of several state-of-the-art commercial and research thin-film solar cells incorporating the upconversion layer. We present both analytical models based on published EQE data as well as detailed finite difference time domain (FDTD) models that incorporate absorption in all cell layers. We model the improvement in absorption and overall cell performance of amorphous Si, CIGS, GaAs, CdTe, and Cu2O cells with upconverting layers. We incorporate and discuss the effect of interface texture and different cell layers on the absorption of upconverted photons and make suggestions for improving the overall cell design to get the maximum benefit from upconversion. We estimate that the cell efficiency enhancement can range from 0.5% to up to 5% absolute depending on the cell type and upconversion efficiency. This work connects to the fundamental efficiency limit analysis of narrow-bandwidth solar upconversion by our collaborators [1], but presents concrete optical models of current solar cells and discusses the promise of upconversion for particular applications.

Nano-scale surface textures have been developed as photon management schemes for crystalline silicon (c-Si) solar cells with very thin absorber layers to compensate for light absorption losses. This paper investigates the optical properties of periodic “nano-muffin” and inverted nano-pyramid surface textures, simulated using the Rigorous Coupled Wave Analysis (RCWA) method. Obtained results are compared against those of a planar silicon film with equal thickness. The simulation results demonstrate that “nano-muffin” and inverted nano-pyramid surface textures with a small aspect ratio are able to achieve substantial absorption enhancement over a broadband wavelength range. Further investigation indicates that “nano-muffin” surface textures could trap light by concentrating light within a volume close to the texture (micro-lensing effect). With such nano-scale textures, light trapping similar to that of much larger scale textures can be achieved.

The heterojunctions formed between solution phase grown Cu2ZnSn(SxSe1- x)4 (CZTS,Se) and a number of important buffer materials including CdS, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission spectroscopy (fs-UPS) and photovoltage spectroscopy. With this approach we extract the magnitude and direction of the CZTS,Se band bending, locate the Fermi level within the band gaps of absorber and buffer and measure the absorber/buffer band offsets under flatband conditions. We will also discuss two-color pump/probe experiments in which the band bending in the buffer layer can be independently determined. Finally, studies of the bare CZTS,Se surface will be discussed including our observation of mid-gap Fermi level pinning and its relation to Voc limitations and bulk defects.

Core-shell quantum dots (QDs) with enhanced photostability compared with bare QDs are promising light absorbers for solar cell applications. In this work, electron injection from excited CdSe/ZnS QDs to Zinc Oxide (ZnO) nanowires (NWs) prepared by two techniques were demonstrated. Arrays of ZnO NWs were fabricated by hydrothermal growth and etching. ZnO NWs were sensitized with hydrophobically ligated colloidal CdSe/ZnS QDs. The electron transfer dynamic in QD/ZnO NW architecture was examined using photoluminescence (PL) and decay lifetime analyses. The quenching of the QD emission peak and lowered average lifetime in QD/ZnO NW architecture confirms the deactivation of the excited QDs via electron transfer to ZnO NWs. Electron transfer was enhanced by using smaller QDs. This study provides insight on charge transfer dynamics at the QD/ZnO NW interface in order to engineer high performance quantum dot sensitized solar cells (QDSSCs).

Understanding the effect of chlorine-related defects on the CdTe electric properties is important both for obtaining high resistivity CdTe-based detectors and for high efficiency CdTe-based thin-film solar cells. The actual mechanism of the effect of Cl on electric properties of CdTe is not clear and different sometimes contradictory hypotheses appear. For example ClTeVCd shallow acceptor complex defect was proposed both as a reason of increased carrier concentration in CdTe thin film and also as a reason of high resistivity of CdTe:Cl thin films. In the present work we are trying to clarify the effect of Cl on CdTe electric properties and to find the reason of high resistivity of CdTe:Cl crystals using first principles calculations and defect chemistry modeling. For the first time we are trying to develop a model capable to describe experimental data on both high temperature and room temperature conductivity of CdTe:Cl.

In this work, sulfurizing metal precursors prepared by magnetron sputtering was applied in Cu2ZnSnS4 (CZTS) thin film fabrication. Three precursor structures, namely substrate/ Zn/(Cu&Sn), substrate/Zn/Cu/Sn/Cu and substrate/Zn/Sn/Cu, were compared for their synthesized CZTS film quality. It is notable that CZTS film made of the precursor structure of substrate/Zn/(Cu&Sn) has the best film quality with no obvious voids and biggest average grain size. When applying this precursor structure into device fabrication, a working CZTS device with an efficiency of 2.26% was made. The impact of metal precursors on the structural property of CZTS film were characterised by SEM, XRD, Raman and TEM. Thick MoS2 interfacial layer (∼200nm) between absorber and back Mo contact and ZnS formed in the front and back absorber regions are the possible reasons limiting short-circuit current and fill factor of the cell.

III-V on Si multijunction solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs/Si tandem solar cells is performed to assess the performance potential and sensitivity to spectral variations. Recorded time-dependent spectral irradiance data in two locations (Singapore and Denver) were used. We found that a 4-terminal contact scheme with thick top cell confers distinctive advantages over a 2-terminal scheme, giving a yield potential 21% higher than the 2-terminal scheme in Singapore and 17% higher in Denver. The theoretical energy yield benefit of a 4-terminal device emphasizes the need for further technology development in this design space.

In this paper, the effect of phosphorus diffusion and hydrogen passivation on the material properties of laser crystallised silicon on glass is investigated. Photoluminescence imaging, as well as Hall effect and Suns-Voc techniques are applied for the characterisation of laser crystallized silicon thin-film material properties. Hall effect as well as Suns-Voc measurements supports the photoluminescence imaging results; phosphorus diffusion and hydrogen passivation of laser crystallized films improves the overall material quality. Hydrogen passivation is more effective at improving the electronic properties of the laser crystallized films than phosphorus diffusion. Hydrogen passivated samples improved the photoluminescence intensity even further by a factor of 3. In addition, a correlation between photoluminescence intensity and open-circuit voltage is demonstrated: samples with highest photoluminescence intensity (1678 counts/s), gave the highest voltage (530 mV). Hall effect measurement shows a significant improvement in the bulk material, with carrier mobility increasing from 208 cm2/Vs to 488 cm2/Vs.

We review recent progresses on in-situ observation of lattice relaxation of III-V lattice-mismatched system and analyses of defect properties in III-V-N solar cell materials. We found that there were five phases during the InGaAs growth on GaAs substrate. The transition point of the dominant dislocation behavior could be determined precisely. We also found that compositionally step-graded InGaAs/GaAs(001) buffers with overshooting (OS) layers were effective to control the strain of the top layer from tensile to compression. To understand the defect properties that dominate the electrical property of CBE-grown GaAsN films, we characterized deep levels in CBE-grown GaAsN films by DLTS. In this characterization, a well-known electron trap E1 (Ec-0.33eV) center in n-GaAsN and p-GaAsN was confirmed to be non-radiative recombination center by using double-carrier pulse DLTS.

Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not well understood. In this investigation we apply advanced micro-structural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis and XPS for composition-depth profiling. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the cadmium telluride layer.

CZTS monograin powder samples were synthesized in CdI2 as flux material. The obtained materials were analysed by EDX, SEM, and Raman methods. It was found that Cd from flux was incorporated into the formed compound leading to the formation of solid solution Cu2Zn1-xCdxSnS4. The content of Cd in the compound was studied in the dependence of synthesis temperature and time. It was found that Cd content in the formed Cu2Zn1-xCdxSnS4 did not depend on synthesis duration at constant temperature and increased with temperature. The activation energy of the Cd incorporation process was estimated as 17.5 ± 2 kJ/mol.

Quantum dots (QDs) and nanoparticles (NPs) with tunable optoelectronic properties are actively researched for photovoltaic (PV) fabrication and will enter mainstream manufacturing in the future. The toxicology, health and safety of these new materials are not fully explored yet. In this work, the toxicological potencies of nanomaterials in PV fabrication, study needs, and metrology requirements are presented. Practical processes involving QDs and NPs developed for PV fabrication are presented. Experimental evidence on the presence of airborne nanomaterials in the condensates collected from process environment underlines the need for in-depth toxicity studies before these technologies scale up to the PV manufacturing stage. Required technical capabilities for the metrology tools to accurately detect, identify, and quantify QDs and NPs in PV manufacturing requirements are also presented based on the potential range of nanomaterials to be used in PV technology. These studies are key to develop safe techniques and processing environments, and to establish safety guidelines for PV fabrication with nanomaterials.

The focus of this work is on back contact improvement for sputtered CZTS thin film solar cells. Three methods have been investigated including a thin Ag coating, a thin ZnO coating on the Mo back contact and rapid thermal annealing of the back contact. All of these methods have been found to reduce defects such as voids as well as secondary phases at the back contact region and inhibit the formation of MoS2. Consequently all the mothods effectively enhances Voc, Jsc, FF and therefore efficiency significantly.