Solar Energy Materials and Solar Cells

A three-stage process starting with the deposition of (In,Ga)₂Se₃ precursor films has been successful in the fabrication of graded band-gap Cu(In,Ga)Se₂ thin films. In this work we examine (1) the reaction of Cu+Se with (In,Ga)₂Se₃, which leads to a spontaneous grading in the Ga content as a function of depth through the film, and (2) modification of the Ga content in the surface region of the film through a final deposition of In+Ga+Se. We show how band-gap grading can be enhanced by the formation of nonuniform precursors, how counterdiffusion limits the degree of grading possible in the surface region, and how the Cu_xSe secondary phase acts to homogenize the film composition

Several factors that may affect the net discoloration rate of the ethylene-vinyl acetate (EVA) copolymer encapsulants used in crystalline-Si photovoltaic (c-Si PV) modules upon accelerated exposure have been investigated by employing UV-visible spectrophotometry, spectrocolorimetry, and fluorescence analysis. A number of laminated films, including the two typical EVA formulations, A9918 and 15295, were studied. The results indicate that the rate of EVA discoloration is affected by the: (1) curing agent and curing conditions; (2) presence and concentration of curing-generated, UV-excitable chromophores; (3) UV light intensity; (4) loss rate of the UV absorber, Cyasorb UV 5311; (5) lamination; (6) film thickness; and (7) photobleaching rate due to the diffusion of air into the laminated films. In general, the loss rate of the UV absorber and the rate of discoloration from light yellow to brown follow a sigmoidal pattern. A reasonable correlation for net changes in transmittance at 420 nm, yellowness index, and fluorescence peak area (or intensity) ratio is obtained as the extent of EVA discoloration progressed

The actual efficiency of photovoltaic generators is often lower than predicted by standard test conditions (STC) or standard operating conditions (SOC). This is caused mainly by an underestimation of reflection losses and solar cell temperature in the module. The authors describe how, in order to obtain more accurate results in predicting the performance of PV-modules, the parameters influencing incoming (optical parameters) and outgoing power flow (electrical and thermal parameters) were investigated by simulation and some verifying experiments at the University of New South Wales and the Australian desert

A high-efficiency AlGaAs/Si tandem solar cell is fabricated by metal-organic chemical vapor deposition (MOCVD). It consists of a Al_0.15Ga_0.85As top cell and a Si bottom cell. The crystalline quality of the Al_0.15Ga_0.85As heteroepitaxial layer grown on Si is improved using a high-temperature growth process (800°C) and thermal cycle annealings (300~900°C). The quantum efficiency of the Si bottom cell in the long wavelength region is improved by back surface field. The conversion efficiencies of the tandem solar cell under AMO and 1 sun measurement conditions with 4-terminal and 2-terminal configuration are 20.0% and 19.0%, respectively. The conversion efficiencies of the tandem solar cell with graded-band-gap emitter Al_xGa_1-xAs layer achieved 20.6% and 19.9% under same condition with 4-terminal and 2-terminal configuration, respectively

Optimised thicknesses of the individual layers in any thin film a-Si:H solar cell can be obtained using optical analysis methods. By minimising the reflectance using an optimal antireflection coating layer and selecting a suitable rear contact material, a significant increase in the photon collection efficiency at long wavelengths can be achieved. The effect of the variation in thicknesses of different layers on the performance characteristics of a cell is discussed. The calculated results and analyses show that the present method can be used directly to design any thin film solar cell with an optimised structure for a desired possible high efficiency

Silicon solar cell efficiencies of 17.1%, 16.4%, 14.8%, and 14.9% have been achieved on FZ, Cz, multicrystalline (mc-Si), and dendritic web (DW) silicon, respectively, using simplified, cost-effective rapid thermal processing (RTP). These represent the highest reported efficiencies for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. Appropriate diffusion temperature coupled with the added in-situ anneal resulted in suitable minority-carrier lifetime and diffusion profiles for high-efficiency cells. The cooling rate associated with the in-situ anneal can improve the lifetime and lower the reverse saturation current density (J_o), however, this effect is material and base resistivity specific. PECVD antireflection (AR) coatings provided low reflectance and efficient front surface and bulk defect passivation. Conventional cells fabricated on FZ silicon by furnace diffusions and oxidations gave an efficiency of 18.8% due to greater short wavelength response and lower J_o

Improvements in the stabilized efficiency of single-junction and multi-junction a-Si solar cells are studied. It has been found that the stabilized efficiency can be improved by suppressing the oxygen and nitrogen content in the i-layer. The defect density and conversion efficiency are strongly correlated with the content of hydrogen having an SiH₂ configuration rather than with the total hydrogen content in a-Si i-layers. A practical estimation method, which considers the variation of the I-V characteristics of each component cell, is also developed in order to optimize multi-junction cells. The world's highest stabilized efficiency of 8.8% for a single-junction a-Si solar cell and 10.6% for an a-Si/a-SiGe double-junction solar cell are achieved for 1 cm² cells

Solar cells based on Cu(In,Ga)Se₂ were prepared replacing the `standard buffer layer' CdS with a In_x(OH,S) _y thin film. The film is deposited in a chemical bath (CBD) process using an aqueous solution containing InCl₃ and thioacetamide. X-ray photoemission spectroscopy measurements were performed in order to characterize the growth kinetics and the chemical composition. The influence of different concentrations of InCl₃ and thioacetamide in the solution on the electrical properties of the solar cells was studied by measuring the j-V characteristics and the spectral quantum efficiencies. Capacitance-voltage (C-V) measurements indicate that the high V_oc values of devices with the novel buffer layer are correlated with narrower space charge widths and higher effective carrier concentrations in the absorber materials. The achieved conversion efficiency of approximately 15% using the cadmium free In_x(OH,S)_y buffer demonstrates the potential of this process as an alternative to the standard chemical bath deposition of CdS

The alignment of energy bands in a Mo/CIS/CdS/ZnO solar cell structure is presented. Special attention is paid to the surface chemistry of the molybdenum coated substrate. In this study we have performed in situ analyses on polycrystalline thin films starting from the molybdenum coated soda lime glass. The different films have been deposited sequentially under ultra high vacuum conditions and analyzed with photoelectron spectroscopy techniques. We have found that Mo surface is significantly oxidized. Air annealing at 200°C leads to a diffusion of sodium through the Mo layer. Mo-O and Mo-Se compounds are formed and present during the evolution of the Mo-CIS interface. Band lineups have been determined for the CIS/CdS/ZnO interfaces of the solar cell structure. We have found no indication for band discontinuities deteriorating the solar cell device performance

This paper explores the possibility of measuring the open-circuit voltage as a function of wavelength as a tool for device characterization. Computer simulations show that the spectral response of the open-circuit voltage exhibits a similar dependence to the spectral response of the short-circuit current. Experimental studies on silicon solar cells confirmed the strong spectral dependence of the open-circuit voltage. The spectral measurements have been performed using a quasi-steady state open-circuit voltage method, which also allows to determine the spectral response of the maximum power voltage. The advantages of this new technique over conventional spectral response measurements include its applicability directly after junction formation and its simple apparatus.

This paper addresses the problem of preparing a good p-layer contact with zinc oxide as TCO (transparent conducting oxide). Our approach was to deposit pin cells with different p-layer recipes on ZnO coated SnO₂:F and on uncoated SnO₂:F in one run. The pin cells prepared on the ZnO surface exhibit a lower fill factor (FF). Our experiments demonstrate that the hydrogen interaction with the ZnO surface plays the most decisive role for the ZnO/p contact. We explain the observed effects using a band diagram of the ZnO/p interface and show that the accumulation layer at the ZnO surface-caused by atomic hydrogen in the plasma-is responsible for the low FF in pin cells. Based on this model the contact problem is solved by introducing a μc-n-Si intralayer between ZnO and p layer resulting in an identical high FF on both ZnO and SnO₂ substrates

Preliminary research into the development of single-junction Ga _xIn_1-xAs thermophotovoltaic (TPV) power converters is reviewed. The devices structures are grown epitaxially on single-crystal InP substrates. Converter band gaps of 0.50 -0.74 eV have been considered in accordance with modeling calculations. A 1-sun, AMO efficiency of 12.8% is reported for a lattice-matched, 0.74-eV converter. Converters with lower band gaps are fabricated using lattice-mismatched, compositionally graded structures. Functional TPV converters with good performance characteristics have been demonstrated for band gaps as low as 0.5 eV

Thin films based on Cu(In,Ga)Se₂ prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se-O, In-O and Ga-O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher on soda lime glass and solar cells prepared on molybdenum-coated substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se₂ when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed

The thermal oxidation of CuInSe₂ single crystals and bulk polycrystalline plates of n- and p-type conductivity with polished facets have been carried out at elevated temperatures 400 to 610°C in dry oxygen/air atmosphere. All the crystals demonstrated an enhancement in hole conductivity near the surface. Additionally, transparent layers of In₂O₃ chemical composition have been grown on the surface. Electron-probe analysis, Rutherford backscattering, X-ray diffraction, optical and photoelectric experimental data have provided a basis to develop a physico-chemical model thermal oxidation of CuInSe₂ crystals. The model includes the In₂O₃ compound appearance, V_Se, V_Cu vacancy absorption, Se_i, Cu_i extra atom and Cu_xSe generation. The chemical reactions are accompanied by arising dislocations and mechanical stresses at the In₂O₃/CuInSe₂ interface

This paper reports the effect of solvent and dopant impurities on the performance of LPE silicon solar cells. For LPE layers grown from Sn and In based solutions and having similar surface morphology and resistivity, the performance of solar cells made on LPE layers grown from In was always higher than that of cells made on LPE layers grown using Sn as solvent. Consistently higher performance was also obtained from solar cells fabricated upon Ga-doped LPE layers than from cells made on Al-doped LPE silicon. The best cell was fabricated upon a Ga-doped LPE layer grown from In solution and had a total area efficiency of 16.4% confirmed by Sandia measurements. The observed phenomena are explained on the basis of Hall mobilities and minority carrier lifetimes of LPE layers grown from different solutions, and also the oxidation difference of these layers during cell processing

Presents results of the role of the oxygen partial pressure (p₀₂) used on the properties exhibited by doped μc silicon oxycarbide films produced by a two consecutive decomposition and deposition chamber (TCDDC) system, where a spatial separation between the plasma and the growth regions is achieved. The films produced are highly conductive and transparent with suitable properties for optoelectronic applications. The film's structure, composition, morphology and optoelectronic properties were analysed by means of grazing X-ray diffraction, electron secondary chemical analysis (ESCA), secondary ion mass spectroscopy (SIMS) as a function of temperature (T) and optical absorption (ranging from 300 to 80 K) measurements

For the first time the Si/C interface in poly-Si photovoltaic thin films on graphite substrate is described. The isostatically pressed graphite is covered with a 3-5 μm thick amorphous silicon layer which is recrystallized by the ZMR method (zone melting recrystallization by means of a line electron beam). During ZMR the molten silicon penetrates into the graphite pores, while at the interface β-SiC particles with a size of 50-1000 nm were formed (TEM, SEM analysis). Furthermore there exists a “reaction zone” where Si, SiC and C are found by electron diffraction. As a consequence the poly-Si layer shows an excellent adhesion to the substrate. Since there is no continuous electrically insulating SiC layer formed, highly boron doped seed layers show an ohmic contact to the graphite. The investigated poly-Si thin films on graphite substrate offer a great potential for photovoltaic application after epitaxy up to 50 μm by LPE or CVD

Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10^-7 cm^-1 have been determined, revealing structure due to 3- and 4-phonon assisted absorption. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross-section for electron photoemission from the indium level, a crucial parameter for modelling indium's IPV effect, is determined

A new method for the fabrication of a columnar, multicrystalline silicon layer on a graphite substrate is presented. This method basically involves three process steps: (1) deposition of a thin (3-5 μm) silicon layer; (2) zone melting recrystallization of this layer with a line electron beam as the heat source to form a multicrystalline seed layer; and (3) thickening of the seed layer by high temperature, epitaxial chemical vapour deposition (CVD) to a thickness of 20-40 μm. The recrystallization leads to (110)[112]-textured silicon seed layers if sufficiently high scan velocities are applied. The degree of deviation from the ideal (110)[112]-texture increases with decreasing scan velocity. The doping level of the seed layer is found to be only weakly affected by the zone melting recrystallization. The epitaxial layer grown on top of the seed layer exhibits a columnar grain structure

Thin film crystalline silicon solar cells can only achieve high efficiencies if light trapping can be used to give a long optical path length, while simultaneously achieving near unity collection probabilities for all generated carriers. This necessitates a supporting substrate of a foreign material, with refractive index compatible with light trapping schemes for the silicon. The resulting inability to nucleate growth of crystalline silicon films of good crystallographic quality on such foreign substrates, prevents the achievement of high efficiency devices using conventional single junction solar cell structures. The parallel multijunction solar cell provides a new approach for achieving high efficiencies from very poor quality material, with near unity collection probabilities for all generated carriers achieved through appropriate junction spacing. Heavy doping is used to minimise the dark saturation current contribution from the layers, therefore allowing respectable voltages. The design strategy, corresponding advantages, theoretical predictions and experimental results are presented

We used a 10-kW high-flux solar furnace (HFSF) to diffuse the front-surface n⁺-p junction and the back-surface p-p⁺ junction of single-crystal silicon solar cells in one processing step. We found that all of the HFSF-processed cells have better conversion efficiencies than control cells of identical structures fabricated by conventional furnace diffusion methods. HFSF processing offers several advantages that may contribute to improved solar cell efficiency: (1) it provides a cold-wall process, which reduces contamination; (2) temperature versus time profiles can be precisely controlled; (3) wavelength, intensity, and spatial distribution of the incident solar flux can be controlled and changed rapidly, (4) a number of high-temperature processing steps can be performed simultaneously; and (5) combined quantum and thermal effects may benefit overall cell performance. The HFSF has also been successfully used to texture the surface of silicon wafers and to crystallize a-Si:H thin films on glass

The transitory and erratic nature of shunt currents, whether caused by light-soaking or electrical biasing, in amorphous Si (a-Si) single- and triple-junction solar cells has been a real puzzle and made the study of these cells difficult. The authors present a careful study of the time/voltage dependence of these current transients in several different a-Si solar cell structures and find they reveal more about the basic shunt mechanism. In single-junction cells, they see stepwise current changes that increase in size and number with reverse bias and can be removed with forward bias. This stepwise, on and off switching suggests a discrete shunt path conduction mechanism. The kinetics of these metastable shunt paths show that both the “on-state” and “off-state” possess memory. Cells without (Al)ZnO show no metastable switching. The authors associate the stepwise features with the textured substrate and the switching metastability with contact to (Al)ZnO. Switching in triple-junction cells occurs with hundreds of oscillations at each step

4,7-Bis(2,3-dihydrothieno [3,4-b] [1,4] dioxin-5-yl)-2-dodecyl-2H-benzo [1,2,3] triazole (BEBT) was polymerized both electrochemically (ePBEBT) and chemically (cPBEBT). Since chemical polymerization enabled a soluble polymer in common organic solvents, a single layer electrochromic device of ePBEBT was constructed. The polymer cPBEBT was also used in bulk heterojunction (BHJ) solar cells as the active layer in combination with a soluble fullerene derivative, 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 PCBM.

A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D–A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D–A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm2. The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH2 and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH2 and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO3 hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO3 structure, and a high Voc of 1.27 V has been achieved.Highlights► Organic solar cells utilizing phthalocyanine derivative (C6PcH2) have been investigated. ► The photovoltaic properties demonstrated the strong dependence of active layer thickness. ► By inserting MoO3 buffer layer the FF and energy conversion efficiency were improved. ► The tandem solar cell has been investigated by utilizing C6PcH2 and poly(3-hexylthiophene). ► The optimized thickness was 120 nm, and a high Voc of 1.27 V has been achieved in the tandem cell.