Thin GaAs photovoltaic heterostructures are grown by MOCVD with various p-GaAsbase thicknesses. The total n/p absorbing thickness is varied systematically.Output voltages up to ∼1.155V were obtained for individual n/pjunctions at an average illumination intensity of ∼8W/cm2.Novel phototransducer devices are then achieved with a vertical epitaxialheterostructure architecture, monolithically integrating 5 or more such thin n/pjunctions. Around the design wavelength, the stacked heterostructure design isyielding an optimal external quantum efficiency approaching unity divided by thenumber of junctions. The modeled and measured conversion efficiencies areexceeding 60%. The photocarrier extraction properties are simulated fordifferent junction thicknesses using a model based on a 3-dimensional (3D)radially-symmetric TCAD implementation of the heterostructures. The studyclearly demonstrates that for such thin n/p junctions the photocarrierextraction can still be efficient due to the operation at reduced currentdensities and higher voltages in heterostructures enhancing electrical powerextraction. With the supplementary add-on of a window layer with a reduced sheetresistance for the stacked structure, we demonstrate the possible efficientoperation 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 widelyinvestigated as a means of extending infrared absorption and enhancingphotovoltaic device performance. In this work, we describe the impact ofnanostructured layer number on the performance of flexible, highvoltageInGaAs/GaAs quantum well solar cells. Multiple quantum well structures areobserved to have a higher short circuit current but a lower open circuit voltagethan similar single quantum well structures. Analysis of the underlying darkdiode characteristics indicate that these highvoltage structures are limited byradiative recombination at high bias levels. The results of this study suggestthat future development efforts should focus on maximizing the currentgenerating capability of a limited number of nanostructured layers andminimizing recombination within the nanostructured absorber.

Recently, metal-halide perovskites have demonstrated an extraordinarily rapidadvance in single junction cell efficiency to over 20%, while still offeringpotentially low costs. Since the bandgap is larger than the idealsingle-junction value, perovskite-based tandem cells can theoretically offereven higher efficiencies. Instead, however, the record tandem cell performancein experiments to date has come in slightly below that of record singlejunctions, although slightly higher than the same single junctions. In thiswork, we consider both how this disconnect can be explained quantitatively, andthen devise experimentally feasible, variance-aware approaches to address them.The first stage of our approach is based on reconfiguring dielectric frontcoatings to help reduce net reflected power and balance junction currents byreshaping the reflection peaks. This method could be applied to post-fabricationstage of perovskite/c-Si tandem cells, and also applicable to cell and modulelevel structures. In the second stage of our approach, we can almost entirelyeliminate Fresnel reflection by applying a conformal periodic light trappingstructure. In the best case, a short circuit current (Jsc) of 18.0mA/cm2 was achieved, after accounting for 4.8 mA/cm2of parasitic loss and 1.6 mA/cm2 reflection loss. Furtherimprovements may require a change in the baseline materials used in perovskitecells.

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

A new high rate deposition method has been used to fabricate thin film CdTephotovoltaic devices using pulsed dc magnetron sputtering. The devices have beendeposited in superstrate configuration on to a commercial fluorine doped tinoxide transparent conductor on soda lime glass. The cadmium sulphide and cadmiumtelluride thin films were deposited from compound targets. The magnetrons weremounted vertically around a cylindrical chamber and the substrate carrierrotates so that the layers can be deposited sequentially. The substrates wereheld at 200°C during deposition, a process condition previously foundto minimize the stress in the coatings. Optimization of the process involved anumber of parameters including control of pulse frequency, power and working gaspressure. The devices deposited using the process are exceptionally uniformenabling the CdTe absorber thickness to be reduced to ∼1um. Theas-deposited material is dense and columnar. The cadmium chloride treatmentincreases the grain size and removes planar defects. The microstructure of thefilms before and after activation has been characterized using a number oftechniques including transmission electron microscopy, Energy Dispersive mappingand these measurements have been correlated to device performance. Thedeposition rate is much higher than can be obtained with radio-frequencysputtering and is comparable with methods currently used in thin film CdTemodule manufacturing such as Vapour Transport Deposition and Close SpaceSublimation.

Methylammonium lead halide perovskites have been developed as highly promisingmaterials to fabricate efficient solar cells in the past few years. We haveinvestigated degradation of co-evaporatedCH3NH3PbI3 films in ambient air, oxygen andwater respectively using x-ray photoelectron spectroscopy (XPS), small anglex-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electronmicroscopy (SEM). The CH3NH3PbI3 film has anexcellent atomic ratio and crystallinity. XPS results indicate that the film isnot sensitive to oxygen and dry air, while ambient and water exposures achievesimilar effects. XRD further indicates a structural conversion toPbI2 and a drastic morphology change from smooth to rough is revealedby AFM and SEM. The experiment indicated that H2O plays a dominatedrole 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 compoundson the surface.

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

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

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

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

We report the experimental demonstration of a low-cost paradigm for photovoltaicpower generation that utilizes a prismatic Fresnel-like lens to simultaneouslyconcentrate and separate sunlight into laterally spaced spectral bands. Theoptical element is designed using geometric optics and optical dispersion andits performance is simulated with a ray-tracing software. The device, fabricatedby injection molding, suitable for large-scale mass production, isexperimentally characterized. We report an average optical transmittance above85% over the VIS-IR range and spectral separation in excellent agreement withour simulations. Finally, the system is tested with a pair of copper indiumgallium selenide based solar cells. We demonstrate an increase in peakelectrical power output of 160% under outdoor sunlight illumination,corresponding to an increase in power conversion efficiency of 15% relative tosingle-junction full-spectrum one-sun illumination. Given the ease ofmanufacturability and the potential of the proposed solution, we project thatour design can provide a cost-effective alternative to multi-junction solarcells ready for mass production.

Anatase and rutile titanium dioxide thin films grown by a low temperature processare investigated for their use as a single layer antireflection coating for GaAssolar cells. The thin films are obtained by spin coating a layer from theTiO2 sol-gel and subsequently annealing at 150 °C. Thesol-gel is synthesized by the hydrolysis of titanium isopropoxide in thepresence of an acid or a base. By controlling the pH of the sol-gel duringgrowth, pure anatase and rutile phases are obtained. A pH of around 3.0 yieldsanatase phase while a pH of 9.0 yields pure rutile phase TiO2. Thetwo different phases of TiO2 are characterized by measuring the Ramanscattering spectra. The optical constants, thickness and reflectance of the thinfilms on GaAs are obtained using a spectroscopic ellipsometer. The sol-gel isspin coated on GaAs based solar cells and annealed at 150 °C to formthe anti-reflective layer. The performance of the solar cells is evaluatedbefore and after coating with the TiO2 films. The anataseTiO2 anti-reflective films performed better than the rutile with amaximum power conversion efficiency enhancement of 50%. Quantum efficiencyenhancement of 58% and 25% are obtained with anatase and rutile phase filmsrespectively. The performance enhancement of the solar cells using these thinfilms can be attributed to the destructive interference of light associated witha single layer coating on the solar cell surface.

This article describes the electrical and physical properties of polysilicondoped with novel N+ and P+ screen printed inks using athermally activated process. Unique ink formulations for N and P type doping ofsilicon are evaluated in volume production in order to enable a low cost, highthroughput process. Inks can be used with multiple substrate types and formfactors. The concentrated doping source combined with thermal drive in andactivation results in degenerately doped layers of polysilicon. Inks aresemiconductor grade which is demonstrated by their use in fabricating highmobility, low leakage Thin Film Transistor (TFT) devices on 300 mm stainlesssteel substrates. Reproducible sheet resistance values (700 A polysilicon) canbe engineered from levels typically ranging from 200 - 2000 ohm/sq. The additiveapproach substitutes the use of high capital cost ion implantation andlithography processes. The ink formulation results in screen printed widthscapable of ranging from 100-300 um. As both N and P type layers can be printedadjacent to each other, it is critical to prevent cross doping using surfacepreparation techniques. Post doping cleaning of surfaces can be achieved in-situor by plasma removal depending on process integration and productconsiderations. Reproducibility and uniformity data to demonstratemanufacturability in a production environment is shown. In summary, a simple,low cost, high throughput additive process based on proprietary inks that can beused in multiple product flows (CMOS TFT, Solar etc.) is demonstrated.

Macroscopic and microscopic photovoltage characteristics of detonationnanodiamonds (DNDs) with distinct surface terminations are presented. Organicphotodiodes are fabricated based on P3HT+DNDs mixture (50 wt%). Wecompare effect of hydrogen and oxygen termination of DNDs. Compared tophotodiodes without DNDs the current-voltage characteristics of photodiodes withO-DNDs in dark and under AM 1.5 illumination show reduced dark current, andhigher photocurrent and open circuit voltage. H-DNDs shunt the photodiodes,which is attributed to their surface conductivity. Kelvin probe force microscopydetects 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-DNDsmay represent a problem for photodiodes, both types of DNDs alone can functionas miniature energy conversion devices.

An experimental investigation to verify the suitability of MoOx as thehole collection layer for a-Si:H based thin film photovoltaic cell is carriedout. The photovoltaic cell investigated has the structure of MoOx(hole collection layer) / intrinsic a-Si:H (photoactive layer) / phosphorusdoped a-Si:H (electron collection layer) / Ag (back reflector electrode); alldeposited in that order onto an Asahi glass (type U) substrate, which is alsoacting as the transparent front electrode for the cell. The effects of differentpost deposition annealing temperatures are investigated. The highest efficiencyvalues are obtained for the cells annealed at 120°C. For thephotovoltaic cell with 100 nm thick photoactive layer, the highest efficiency ismeasured to be 6.46 % with an open current voltage (Voc) of 827 mVand a short current density of (Jsc) of 10.44 mA/cm2. Forthe photovoltaic cell with 300 nm thick photoactive layer, the highestefficiency is measured to be 7.93 % with Voc of 818 mV andJsc of 13.24 mA/cm2. The efficiency measurements arecarried out under AM1.5 test conditions. Jsc values are correctedaccording to the external quantum efficiency measurements of the cells in theAM1.5 photovoltaic spectrum region between 270 nm and 800 nm. Compared to thereference cell with boron doped μ-SiOx layer acting as thehole collection layer, the cell with MoOx hole collection layer hassimilar FF, lower Voc, higher Jsc for wavelength up to thegreen light region of the AM1.5 spectrum and lower Jsc for the longerwavelengths.

Substrate geometry CdTe solar cells have been modified with the addition ofmetal-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 andwere developed using the MgCl2 alternative to the usualCdCl2 processing – this yielded open circuit voltages ofup to 740 mV. It was demonstrated that the addition of Au-catalysed nanowires to200 nm thick CdTe films on glass substrates decreased their optical transmissionby 10%, this being significantly higher than for thick films. However,reproducibility issues with forming Bi nanostructures limited the devicemodification tests to the use of Au-catalysed wires, and these always acted todepress photovoltaic performance.

We employed ultrasonic spray deposition method for production of high quality FTOthin film TCOs to be employed in a silver embedded grid type and monolithic typedye sensitized solar modules. Produced films exhibited dense and crystallinestructure with homogeneous coverage on solar glass substrates. Obtainedresistivity and light transmission values of FTO are comparable to commerciallyavailable FTO coated glasses used widely in the industry. After optimization ofthe chemistry and deposition conditions, 10x10 cm sized glass substrates couldbe produced for large area photovoltaic modules. Produced FTO films were used toconstruct monolithic type and parallel type dye sensitized solar modules.Monolithic modules yielded 1.61% active area efficiency value. In order toenhance the active area of the parallel type modules, silver grid lines wereembedded in glass substrate and FTO coating was deposited on the lines. Due tothis effective design, we achieved 2.42% efficiency on the total area of the55x55 mm sized module compared to 2.90% active area efficiency, proving thatthis design is suitable for enhancing efficiency values of parallel type dyesensitized solar modules.