Thin GaAs photovoltaic heterostructures are grown by MOCVD with various p-GaAs base thicknesses. The total n/p absorbing thickness is varied systematically. Output voltages up to ∼1.155V were obtained for individual n/p junctions at an average illumination intensity of ∼8W/cm2. Novel phototransducer devices are then achieved with a vertical epitaxial heterostructure architecture, monolithically integrating 5 or more such thin n/p junctions. Around the design wavelength, the stacked heterostructure design is yielding an optimal external quantum efficiency approaching unity divided by the number of junctions. The modeled and measured conversion efficiencies are exceeding 60%. The photocarrier extraction properties are simulated for different junction thicknesses using a model based on a 3-dimensional (3D) radially-symmetric TCAD implementation of the heterostructures. The study clearly demonstrates that for such thin n/p junctions the photocarrier extraction can still be efficient due to the operation at reduced current densities and higher voltages in heterostructures enhancing electrical power extraction. With the supplementary add-on of a window layer with a reduced sheet resistance for the stacked structure, we demonstrate the possible efficient operation of phototransducers for optical inputs exceeding 150 W/cm2, even for the case of devices designed without gridlines.

Nanostructured quantum well and quantum dot solar cells are being widely investigated as a means of extending infrared absorption and enhancing photovoltaic device performance. In this work, we describe the impact of nanostructured layer number on the performance of flexible, highvoltage InGaAs/GaAs quantum well solar cells. Multiple quantum well structures are observed to have a higher short circuit current but a lower open circuit voltage than similar single quantum well structures. Analysis of the underlying dark diode characteristics indicate that these highvoltage structures are limited by radiative recombination at high bias levels. The results of this study suggest that future development efforts should focus on maximizing the current generating capability of a limited number of nanostructured layers and minimizing recombination within the nanostructured absorber.

Recently, metal-halide perovskites have demonstrated an extraordinarily rapid advance in single junction cell efficiency to over 20%, while still offering potentially low costs. Since the bandgap is larger than the ideal single-junction value, perovskite-based tandem cells can theoretically offer even higher efficiencies. Instead, however, the record tandem cell performance in experiments to date has come in slightly below that of record single junctions, although slightly higher than the same single junctions. In this work, we consider both how this disconnect can be explained quantitatively, and then devise experimentally feasible, variance-aware approaches to address them. The first stage of our approach is based on reconfiguring dielectric front coatings to help reduce net reflected power and balance junction currents by reshaping the reflection peaks. This method could be applied to post-fabrication stage of perovskite/c-Si tandem cells, and also applicable to cell and module level structures. In the second stage of our approach, we can almost entirely eliminate Fresnel reflection by applying a conformal periodic light trapping structure. In the best case, a short circuit current (Jsc) of 18.0 mA/cm2 was achieved, after accounting for 4.8 mA/cm2 of parasitic loss and 1.6 mA/cm2 reflection loss. Further improvements may require a change in the baseline materials used in perovskite cells.

The influence of deep level defects (DLs) on the conversion efficiency of multicrystalline Si-based standard solar cells (SCs) is investigated. Multicrystalline p-type Si wafers with 156×156 mm dimensions and 200 μm thickness were used for SCs preparation. Three types of SCs with conversion efficiency 10%, 16.8% and 20.4% were studied using capacitance voltage characteristics method (C-V) and by current deep level transient spectroscopy (I-DLTS). The correlation between the total concentration of DLs and the values of the SCs conversion efficiency is found.

A new high rate deposition method has been used to fabricate thin film CdTe photovoltaic devices using pulsed dc magnetron sputtering. The devices have been deposited in superstrate configuration on to a commercial fluorine doped tin oxide transparent conductor on soda lime glass. The cadmium sulphide and cadmium telluride thin films were deposited from compound targets. The magnetrons were mounted vertically around a cylindrical chamber and the substrate carrier rotates so that the layers can be deposited sequentially. The substrates were held at 200°C during deposition, a process condition previously found to minimize the stress in the coatings. Optimization of the process involved a number of parameters including control of pulse frequency, power and working gas pressure. The devices deposited using the process are exceptionally uniform enabling the CdTe absorber thickness to be reduced to ∼1um. The as-deposited material is dense and columnar. The cadmium chloride treatment increases the grain size and removes planar defects. The microstructure of the films before and after activation has been characterized using a number of techniques including transmission electron microscopy, Energy Dispersive mapping and these measurements have been correlated to device performance. The deposition rate is much higher than can be obtained with radio-frequency sputtering and is comparable with methods currently used in thin film CdTe module manufacturing such as Vapour Transport Deposition and Close Space Sublimation.

Methylammonium lead halide perovskites have been developed as highly promising materials to fabricate efficient solar cells in the past few years. We have investigated degradation of co-evaporated CH3NH3PbI3 films in ambient air, oxygen and water respectively using x-ray photoelectron spectroscopy (XPS), small angle x-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The CH3NH3PbI3 film has an excellent atomic ratio and crystallinity. XPS results indicate that the film is not sensitive to oxygen and dry air, while ambient and water exposures achieve similar effects. XRD further indicates a structural conversion to PbI2 and a drastic morphology change from smooth to rough is revealed by AFM and SEM. The experiment indicated that H2O plays a dominated role in the degradation of CH3NH3PbI3 films. The degradation can be characterized by almost complete removal of N, substantial reduction of I, residual of PbI2, C, O, and I compounds on the surface.

Polymer solar cell (PSC) has great spectrum matching with the room lights and PSC has great photovoltaic performance under the low to high illuminance compared to other types of solar cells. So, we fabricate the PSC module for the indoor use and investigate the electrical characteristics under the various light sources. The PSC photovoltaic performance is 43.5µW/cm2 under the fluorescent light 1000lux (0.1mW/cm2) and PCE=6.0% under the sunlight (1SUN-100mW/cm2).

This PSC module has practical durability and stability against the heat and light exposure. So, PSC is used as the self-charging batteries for mobile devices and wireless sensor networks.

We considered modification of the defect density of states in CdTe as influenced by a buffer layer in ZnO(ZnS, SnSe)/CdS/CdTe solar cells. Compared to the solar cells employing ZnO buffer layers, implementation of ZnSe and ZnS resulted in the lower net ionized acceptor concentration and the energy shift of the dominant deep trap levels to the midgap of CdTe. The results clearly indicated that the same defect was responsible for the inefficient doping and the formation of recombination centers in CdTe. This observation can be explained taking into account the effect of strain on the electronic properties of the grain boundary interface states in polycrystalline CdTe. In the conditions of strain, interaction of chlorine with the grain boundary point defects can be altered.

Here, we expose planar and plasmonic Ag surfaces to a low-power O2/Ar plasma to form an ultrathin surface oxide layer. We study the chemical state and morphology of the plasma-treated Ag surfaces using X-ray photoelectron spectroscopy, scanning electron microscopy, and dark-field microscopy. We observe the formation of an ultrathin layer (< 10 nm) composed of both AgOx and Ag2CO3 for a plasma exposure time of 1 s by investigating shifts in the Ag3d, O1s, and C1s core level binding energies. For an exposure time of 1 s, the surface structure of the planar and plasmonic Ag surfaces remains unchanged. For exposure times of 5 - 30 s, the planar Ag surfaces become porous and exhibit increased surface roughness. We demonstrate that the plasma-treated planar and plasmonic Ag surfaces lead to improvements in the excited-state population of a polymer:fullerene coating through ultrafast pump-probe reflectometry.

We report the experimental demonstration of a low-cost paradigm for photovoltaic power generation that utilizes a prismatic Fresnel-like lens to simultaneously concentrate and separate sunlight into laterally spaced spectral bands. The optical element is designed using geometric optics and optical dispersion and its performance is simulated with a ray-tracing software. The device, fabricated by injection molding, suitable for large-scale mass production, is experimentally characterized. We report an average optical transmittance above 85% over the VIS-IR range and spectral separation in excellent agreement with our simulations. Finally, the system is tested with a pair of copper indium gallium selenide based solar cells. We demonstrate an increase in peak electrical power output of 160% under outdoor sunlight illumination, corresponding to an increase in power conversion efficiency of 15% relative to single-junction full-spectrum one-sun illumination. Given the ease of manufacturability and the potential of the proposed solution, we project that our design can provide a cost-effective alternative to multi-junction solar cells ready for mass production.

Anatase and rutile titanium dioxide thin films grown by a low temperature process are investigated for their use as a single layer antireflection coating for GaAs solar cells. The thin films are obtained by spin coating a layer from the TiO2 sol-gel and subsequently annealing at 150 °C. The sol-gel is synthesized by the hydrolysis of titanium isopropoxide in the presence of an acid or a base. By controlling the pH of the sol-gel during growth, pure anatase and rutile phases are obtained. A pH of around 3.0 yields anatase phase while a pH of 9.0 yields pure rutile phase TiO2. The two different phases of TiO2 are characterized by measuring the Raman scattering spectra. The optical constants, thickness and reflectance of the thin films on GaAs are obtained using a spectroscopic ellipsometer. The sol-gel is spin coated on GaAs based solar cells and annealed at 150 °C to form the anti-reflective layer. The performance of the solar cells is evaluated before and after coating with the TiO2 films. The anatase TiO2 anti-reflective films performed better than the rutile with a maximum power conversion efficiency enhancement of 50%. Quantum efficiency enhancement of 58% and 25% are obtained with anatase and rutile phase films respectively. The performance enhancement of the solar cells using these thin films can be attributed to the destructive interference of light associated with a single layer coating on the solar cell surface.

This article describes the electrical and physical properties of polysilicon doped with novel N+ and P+ screen printed inks using a thermally activated process. Unique ink formulations for N and P type doping of silicon are evaluated in volume production in order to enable a low cost, high throughput process. Inks can be used with multiple substrate types and form factors. The concentrated doping source combined with thermal drive in and activation results in degenerately doped layers of polysilicon. Inks are semiconductor grade which is demonstrated by their use in fabricating high mobility, low leakage Thin Film Transistor (TFT) devices on 300 mm stainless steel substrates. Reproducible sheet resistance values (700 A polysilicon) can be engineered from levels typically ranging from 200 - 2000 ohm/sq. The additive approach substitutes the use of high capital cost ion implantation and lithography processes. The ink formulation results in screen printed widths capable of ranging from 100-300 um. As both N and P type layers can be printed adjacent to each other, it is critical to prevent cross doping using surface preparation techniques. Post doping cleaning of surfaces can be achieved in-situ or by plasma removal depending on process integration and product considerations. Reproducibility and uniformity data to demonstrate manufacturability in a production environment is shown. In summary, a simple, low cost, high throughput additive process based on proprietary inks that can be used in multiple product flows (CMOS TFT, Solar etc.) is demonstrated.

Macroscopic and microscopic photovoltage characteristics of detonation nanodiamonds (DNDs) with distinct surface terminations are presented. Organic photodiodes are fabricated based on P3HT+DNDs mixture (50 wt%). We compare effect of hydrogen and oxygen termination of DNDs. Compared to photodiodes without DNDs the current-voltage characteristics of photodiodes with O-DNDs in dark and under AM 1.5 illumination show reduced dark current, and higher photocurrent and open circuit voltage. H-DNDs shunt the photodiodes, which is attributed to their surface conductivity. Kelvin probe force microscopy detects a reproducible photovoltage of around 5 mV generated by a green laser (532 nm) on both types of pristine DNDs. Thus although conductivity of H-DNDs may represent a problem for photodiodes, both types of DNDs alone can function as miniature energy conversion devices.

An experimental investigation to verify the suitability of MoOx as the hole collection layer for a-Si:H based thin film photovoltaic cell is carried out. The photovoltaic cell investigated has the structure of MoOx (hole collection layer) / intrinsic a-Si:H (photoactive layer) / phosphorus doped a-Si:H (electron collection layer) / Ag (back reflector electrode); all deposited in that order onto an Asahi glass (type U) substrate, which is also acting as the transparent front electrode for the cell. The effects of different post deposition annealing temperatures are investigated. The highest efficiency values are obtained for the cells annealed at 120°C. For the photovoltaic cell with 100 nm thick photoactive layer, the highest efficiency is measured to be 6.46 % with an open current voltage (Voc) of 827 mV and a short current density of (Jsc) of 10.44 mA/cm2. For the photovoltaic cell with 300 nm thick photoactive layer, the highest efficiency is measured to be 7.93 % with Voc of 818 mV and Jsc of 13.24 mA/cm2. The efficiency measurements are carried out under AM1.5 test conditions. Jsc values are corrected according to the external quantum efficiency measurements of the cells in the AM1.5 photovoltaic spectrum region between 270 nm and 800 nm. Compared to the reference cell with boron doped μ-SiOx layer acting as the hole collection layer, the cell with MoOx hole collection layer has similar FF, lower Voc, higher Jsc for wavelength up to the green light region of the AM1.5 spectrum and lower Jsc for the longer wavelengths.

Substrate geometry CdTe solar cells have been modified with the addition of metal-catalysed nano-structures in order to influence their efficiency. Conditions for the growth of Au- and Bi-catalysed nanostructures were explored. The substrate devices themselves comprised indium tin oxide/CdS/CdTe/Mo foil and were developed using the MgCl2 alternative to the usual CdCl2 processing – this yielded open circuit voltages of up to 740 mV. It was demonstrated that the addition of Au-catalysed nanowires to 200 nm thick CdTe films on glass substrates decreased their optical transmission by 10%, this being significantly higher than for thick films. However, reproducibility issues with forming Bi nanostructures limited the device modification tests to the use of Au-catalysed wires, and these always acted to depress photovoltaic performance.

We employed ultrasonic spray deposition method for production of high quality FTO thin film TCOs to be employed in a silver embedded grid type and monolithic type dye sensitized solar modules. Produced films exhibited dense and crystalline structure with homogeneous coverage on solar glass substrates. Obtained resistivity and light transmission values of FTO are comparable to commercially available FTO coated glasses used widely in the industry. After optimization of the chemistry and deposition conditions, 10x10 cm sized glass substrates could be produced for large area photovoltaic modules. Produced FTO films were used to construct monolithic type and parallel type dye sensitized solar modules. Monolithic modules yielded 1.61% active area efficiency value. In order to enhance the active area of the parallel type modules, silver grid lines were embedded in glass substrate and FTO coating was deposited on the lines. Due to this effective design, we achieved 2.42% efficiency on the total area of the 55x55 mm sized module compared to 2.90% active area efficiency, proving that this design is suitable for enhancing efficiency values of parallel type dye sensitized solar modules.