doc. Mgr. Jakub Holovský, Ph.D.

Numerous parameters are regulated in the wet chemical oxidation process for TOPCon/POLO solar cell technology to improve silicon oxide passivation (SiO2). Understanding the electronic properties, particularly the lifetime of the carriers and their thickness, requires knowledge of the properties of the surface of crystalline silicon (c-Si), which is subjected to native oxide etching followed by wet chemical oxidation, such as nitric acid or hot water oxidation and various hydrogenation methods. This is tracked with lifetime measurement equipment, and spectral ellipsometry is used to measure the thickness of the oxide layer by using the single sided polished wafers with surface orientation (1 1 1). In addition to the actual values, their time stability is also tracked. Before the hydrogenation step was introduced, the lifetime of the samples was in the order of approximately 0.001 ms, which is less than the bulk lifetime. With the hydrogenation , lifetime increased by more than order of magnitude for relatively long time with no difference between (1 1 1) and (1 0 0) wafers indicating that hydrogenation of the Si/SiO2 interface is performed.

Mixed-halide perovskites are highly promising materials for tandem solar cells. The phenomenon of phase segregation, however hinders their application. Here, we combine Fourier-Transform photocurrent spectroscopy with photoluminescence and current density–voltage (J–V) measurements to study the effect of light soaking on such materials and devices. At first, we observe a gradual formation of an I-rich phase, which correlates with an increase in deep defect level concentration. We attribute these deep defects to charged iodide interstitials and associate phase segregation with iodide migration through interstitial positions. Upon further light soaking, the second less I-rich phase forms, while the deep level concentration simultaneously decreases. An empirical model describing the phase segregation mechanism is proposed to rationalize these observations. Further, we point to an important role of grain size in determining the degree and terminal phase of segregation.

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material’s minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley–Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Heterojunction carrier selective contacts for solar cells have gained great attention because of the ability of these contacts to efficiently collect majority carriers while hindering the recombination of minority carriers, thus resulting in the highest reported voltage among crystalline silicon technologies. The electrode work-function, doping and thickness of the doped layer remain the key parameters for governing the cell performance. Recently, we have studied the requirements for carrier selective contacts under various illumination levels and reported logarithmic dependence of these parameters. In this work, we define a new metric for describing the ability of a contact to collect the carriers and named it as contact strength. We have developed an analytical approach for contact strength of hole selective contacts which represents the requirements on doping, thickness of the contact layer, and electrode work-function for a given illumination. First, the numerical model is calibrated with the experimental data for a wide range of illumination (0.01 Sun – 1.0 Sun) and then simulations have been fitted with analytical model. For the selective contact layer, we observe that only the total charge, rather than thickness and doping individually, matters. The work-function of the top electrode also contributes strongly to contact strength and can in principle substitute doped layer. This insightful metric will guide the solar cell technologists to better understand the carrier selective contacts and to maximize the cell performance not only at standard test conditions (STC), but also at low light illuminations.

We use periodic density functional theory (periodic DFT) to rigorously assign vibrational spectra measured on nanorough wet-processed hydrogenated Si(111) surfaces. We compare Si(111)-(1×1) surfaces etched by dilute HF and NH4F, featuring two vibrational patterns that systematically appear together. They are attributed to vibrations observed on vicinal surfaces featuring 112̅ and 1̅1̅2 steps terminated with monohydrides and dihydrides, respectively. For the first time, we fully assign vibration patterns of realistic silicon surfaces with variable nanoroughness directly by periodic DFT simulations involving contributions from isolated species but also contributions from highly coupled species forming standing waves. This work opens the path to a better quantitative characterization of imperfect and nanorough Si(111) surfaces from vibrational spectra.

The periodic density functional theory (periodic DFT) method was employed for the interpretation of infrared radiation spectra (IR spectra) measured for the hydrogen-covered H/Si(100) surface after the standard step of native oxide removal by brief etching in 40% NH4F. The IR employed the attenuated total reflectance (ATR) method. The periodic DFT calculations of IR spectra focused on reconstructions of H/Si(100) that involved combinations of surface-terminating Si-H groups including the double-occupied dimer (DOD), dihydride (DH), and trihydride (TH). The IR spectra calculated with periodic DFT for H/Si(100) surfaces were compared with the IR spectra calculated by means of DFT in Si-H clusters. The periodic DFT provided considerably better and more reliable theoretical description of the IR spectra by keeping the periodicity of the silicon material that guaranteed proper spatial distribution of the Si-H species within H/Si(100).

Molybdenum oxide is an intensively studied material, thanks to its high bandgap, high work function, and potentially also photochromism, plasmonic properties, and layered structure. In this contribution, we employ Pulsed Laser Deposition (PLD) from stoichiometric MoO3 and metal Mo target at temperature range of 25 °C – 500 °C and oxygen pressure variation of 0.1 mbar – 0.4 mbar to deposit high transparency MoO3 layers. The combination of Photothermal Manuscript File Click here to view linked ReferencesDeflection Spectroscopy (PDS) and Spectral Ellipsometry is applied to accurately track all the optical properties. The X-ray diffraction and Scanning Electron Microscopy (SEM) are used to monitor crystallinity and surface morphology.

A method for improving mesoporous TiO2/perovskite interfacial characteristics in perovskite solar cells (PSCs) is demonstrated by modifying mesoporous TiO2 with potassium (K) treatment. It is found that the modification of mesoporous TiO2 with K treatment enhances the perovskite crystallization process, producing a perovskite film with higher crystallinity and larger grains. K treatment passivates trap sites in mesoporous TiO2 and reduces sub-band-gap deep defect states at the interface of the perovskite, thereby suppressing the nonradiative recombination and improving Voc of PSCs. Stronger photoluminescence quenching and shorter carrier lifetime are also observed for the perovskite on K-treated mesoporous TiO2, indicating more efficient charge collection across the interface. As a result of these advancements, PSC based on K-treated mesoporous TiO2 shows a high power conversion efficiency of 20.60% compared to PSC without K treatment and improves the environmental stability in air

Ultrathin layers of hydrogenated amorphous silicon (a‐Si:H), passivating the surface of crystalline silicon (c‐Si), are key enablers for high‐efficiency silicon heterojunction solar cells. In this work, the authors apply highly sensitive attenuated total reflectance Fourier‐transform infrared spectroscopy, combined with carrier‐lifetime measurements and carrier‐lifetime imaging. To gain insight, the a‐Si:H/c‐Si interfacial morphology is intentionally manipulated by applying different surface, annealing and ageing treatments. Changes are observed in the vibrational modes of hydrides (SiHX), oxides (SiHX(SiYOZ)) together with hydroxyl and hydrocarbon surface groups. The effect of unintentional oxidation and contamination is considered as well. Electronic interfacial properties are reviewed and discussed from the point of hydrogen mono‐layer passivation of the c‐Si surface and from the perspectives of a‐Si:H bulk properties.

Crystalline large grains with uniform morphologies of the perovskite films are important for achieving stable, high-performance perovskite solar cells. Herein, a strategy to control the growth of grains in CH3NH3PbI3 films is demonstrated by modifying the perovskite film deposition process through forming an intermediate CH3NH3PbI3·methylammonium chloride (MACl)·xCH3NH2 liquid phase induced by CH3NH2 gas treatment in combination with MACl additive. By tuning the incorporation of MACl additive to the perovskite precursor solution, this intermediate phase enables the controlled growth of grains up to 3 μm, uniform morphology, and high crystallinity in the films. The high-quality CH3NH3PbI3 film leads to enhanced carrier lifetime and reduced charge-trap density and nonradiative recombination of perovskite films. Solar cells made via CH3NH3PbI3·MACl·xCH3NH2 phase exhibit high power conversion efficiency of 18.4%, higher than solar cells made without MACl (15.8%).

The ideal electron transport layer of a high performance perovskite solar cell should have good optical transparency, high electron mobility, and an energy level alignment well-matched with the perovskite material. In this work, we investigate the role of TiCl(4)post-treatment of the mesoporous TiO(2)electron transport layer by varying the concentration of TiCl(4)and characterizing optical and electrical properties, charge carrier dynamics, and photovoltaic performance of mesoscopic CH(3)NH(3)PbI(3)solar cells. It is found that the TiCl(4)treatment provides an additional interconnection between the TiO(2)particles, leading to better percolation as evident from high resolution cross-section images and chemical maps. This enhances effective electron mobility in the material as well as significantly reduces average sub-bandgap absorption due to defects and electronic disorder determined by photothermal deflection spectroscopy.

High efficiency silicon solar cells generally feature carrier-selective contacts, for which there is interest in using a wide range of materials. The electrical and optical requirements that these layers must fulfill have been investigated previously for standard test conditions. Here, we investigate how the required work functions and layer thickness differ under other illumination conditions. The differences will be important for the optimization of tandem device subcells, and for devices which are intended for use in low-light conditions or under low-level concentration. Heterojunction cells are fabricated and the effect of reduced contact thickness and doping at different illumination levels is experimentally demonstrated. Simulations of a-Si/c-Si heterojunctions and ideal metal-semiconductor junctions reveal a logarithmic variation with illumination level of 0.1–10 suns in the electrode work function, and the heterojunction contact layer work function and thickness required.

Metal-halide perovskites feature very low deep-defect densities, thereby enabling high operating voltages at the solar cell level. Here, by precise extraction of their absorption spectra, we find that the low deep-defect density is unaffected when cations such as Cs+ and Rb+ are added during the perovskite synthesis. By comparing single crystals and polycrystalline thin films of methylammonium lead iodide/bromide, we find these defects to be predominantly localized at surfaces and grain boundaries. Furthermore, generally, for the most important photovoltaic materials, we demonstrate a strong correlation between their Urbach energy and open-circuit voltage deficiency at the solar cell level. Through external quantum yield photoluminescence efficiency measurements, we explain these results as a consequence of nonradiative open-circuit voltage losses in the solar cell. Finally, we define practical power conversion efficiency limits of solar cells by taking into account the Urbach energy

Transition metal oxides are materials combining properties of electrical conductivity, optical transparency, and catalytical function. They are widely used in applications including solar cells, flat panel displays, and detectors. In particular, high work function oxides such as MoO3, WO3, and V2O5 have become popular. In many applications, low deposition temperatures are required, leading to amorphous structure. In this study, thin films of amorphous MoOX, WOX, and VOX were prepared by pulsed laser deposition, and their optical properties and work function were determined. Samples of polycrystalline ZnO were also prepared for comparison. Substrate temperature was varied in the range of 25 °C–100 °C and oxygen pressure was varied in the range of 10–20 Pa during the process. Effect of pressure during sample cool-down and chamber venting was also observed.. Optical characterization was based on photothermal deflection spectroscopy, which is a non-contact and non-destructive method for measuring directly absorptance spectra with sensitivity down to 10–4. Absorptance in the band gap serves as an indication of the presence of defects such as oxygen vacancies or metallic phases. Our optimized films achieved a sub-bandgap absorption coefficient as low as 103 cm−1 for MoOX, VOX, and 102 cm−1 in the case of the WOX. From the gradient of the absorption edge, Urbach energy was obtained, evaluating disorder in the semiconductor material. The work function of each material was obtained by Kelvin probe, and a slight correlation with Urbach energy was found. X-ray photoelectron spectroscopy indicated successful stochiometric transfer mainly for the lowest pressure and highest temperature samples.

Near-infrared absorption in transparent conducting oxides (TCOs) is usually caused by electronic intraband transition at high doping levels. Improved infrared transparency is commonly explained by enhanced drift mobility in these TCOs. Here, an alternative cause behind the high infrared transparency of La-doped barium stannate (LBSO) transparent electrodes is presented. Following the Drude model formalism, we reconstructed spectrally resolved dielectric permittivity for a set of thin films with different free electron concentrations. A comparison of optical properties of LBSO with the tin-doped indium oxide thin films with identical carrier concentrations suggests that the redshift of the screened plasma wavelength for LBSO originates from its large high-frequency dielectric constant of 4.4, one of the highest reported for the s-orbital-based TCOs. Moreover, our measurements confirm an optical mobility significantly higher (>300 cm(2)/V s) than the drift mobility.

Organic-inorganic halide perovskites provide new opportunities for improvement of optoelectronic device performance, especially the efficiency of solar cells. To evaluate the quality of a new material many parameters has to be taken into account. Here, we discuss one of the often overlooked semiconductor’s parameters, Urbach energy, which is an easily accessible measure of material disorder. Moreover, we present its importance on the example of organic-inorganic halide perovskites.

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy is reviewed. As new materials appear, a prediction of potentially achievable open-circuit voltage is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. We analyzed a series of hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking. Combining empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage for a given defect density and absorber thickness. We observed that, rather than defect density or thickness, it is the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives also useful upper bound to the open-circuit voltage.

TiO2 is commonly employed as an electron transport layer (ETL) in mesoscopic n−i−p perovskite solar cells (PSCs). However, the low electron mobility, low electrical conductivity, and high electronic trap states of TiO2 have negative impacts on further enhancement of PSC performance. Metal doping is an efficient way to improve the electronic properties of TiO2 films. In this work, we investigate the concentration-dependent impact of alkali lithium metal doping of the mesoporous TiO2 ETL on the performance of mesoscopic CH3NH3PbI3 PSCs. It was found that Li doping results in improvement in electrical conductivity and electron mobility and reduces the number of electronic trap states arising due to the oxygen vacancies within TiO2 lattice. The device performance relies on the concentration of Li doping, and the power conversion efficiency (PCE) of the PSC was improved from 13.64% to 17.59% for a Li doped mesoporous TiO2 layer with an optimized concentration of 30 mg/mL.

The effect of PECVD deposition of amorphous Silicon on perovskite layers, and perovskite layers prepared on pre-deposited a-Si are investigated in the interest of determining the interaction between the two materials and developing a-Si as a material for direct integration into perovskite cells. This may include as selective contacts or carrier transport layers, an encapsulation material, or in other ways. Doped and intrinsic a-Si layers are deposited at a range of temperatures, with the resulting structures characterized by methods including photoluminescence, SEM imaging, absorption, resistivity measurements, and degradation rate. Comparisons are made to layers deposited on glass without any PECVD deposition carried out and layers subjected to heating without PECVD deposition. The implications for use of a-Si in perovskite cells are explored.

Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.

To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH3NH3PbI3), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH3NH3PbI3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Low-light applications often bring amorphous silicon and other thin-film cell technologies to mind. While there has been work to improve the low-light efficiency of crystalline silicon solar cells, their potential has not been fully described. The description of fundamental limits, and much of the work to realize those limits, has been limited to one-sun illumination levels and higher. This work aims to establish the fundamental Auger limit for waferbased silicon solar cells at illuminations lower than one-sun, the additional technological limits imposed by surface recombination, shunt resistance, and recombination in the space charge region, and the degree to which low-light potential of these devices had been realized. High efficiencies demonstrate that in choosing technologies for low illuminations, crystalline silicon solar cells should not be overlooked.

Broadband transparent and highly conducting electrodes are key to avoid parasitic absorption and electrical losses in solar cells. Here, we propose zirconium-doped indium oxide (IO:Zr) as the front electrode in silicon heterojunction (SHJ) solar cells. The exceptional properties of this material rely on the combination of high-doping and high electron mobilities, achieving with this a wide optical band gap (3.5–4 eV), low free carrier absorption, and high lateral conductivity. A single film of IO:Zr has an electron mobility of 100 cm^2/Vs with a carrier density of 2.5–3x10^20 cm^-3, resulting in a sheet resistance of around 25 Ω/sq for 100-nm-thick films. Their implementation as a front electrode in SHJ solar cells results in an important gain in current density as compared to the standardly used Sn-doped indium oxide. SHJ devices with the optimized IO:Zr front electrode, resulting in current densities of 40 mA/cm2, a fill factor of 80%, and a conversion efficiency of 23.4%.

We have elaborated contactless method of measurement and evaluation of doping profiles in silicon polished wafers based on infrared reflectance under high angle of incidence. We have found higher angle of incidence increases sensitivity, however approaching Brewster angle increases also experimental error, therefore 65 angle has been chosen. Moreover, to increase reproducibility we divide the measured spectra by reference spectra taken on an undoped sample, and further we rescale the spectra to fixed value in the region of 4000 cm-1–7000 cm-1. To reduce number of evaluated parameter, the carrier profile in boron-doped samples was parametrized by 3 parameters and that in phosphorous-doped samples was parametrized by 4 parameters, using additional empirically determined assumption that the first part of the profile is a constant plateau and that the following two exponential tails are joined at a value of 3x10^19 cm-3.

The grazing angle infrared reflectance method of the measurement and evaluation of charge carrier profiles in polished wafers was developed. Experimental errors were minimized by division by reference spectra taken on an undoped sample and further by normalization to a fixed value in the region of 4000/cm to 7000/cm. The carrier profile in boron-doped samples was parametrized by 3 parameters and that in phosphorous-doped samples was parametrized by 4 parameters, using additional empirically determined assumptions. As a physical model, the Drude equation is used with two parameters assumed to be concentration-dependent: relaxation time and contribution from band-to-band excitations. The model parameters were calibrated independently by infrared ellipsometry. The presented method gives results in satisfactory agreement with the profiles measured by the electrochemical capacitance-voltage method.

Organicko-anorganické perovskity se nedávno ukázaly jako nadějný materiál pro výrobu levných tenkovrstvých slunečních článků s vysokou účinností. Příspěvek představuje strukturu perovskitů, jednoduché metody jejich přípravy a slibnou perspektivu tandemového slunečního článku perovskitu s křemíkem. Zmíněny jsou též dvě slabiny, které zatím brání komerční výrobě perovskitových slunečních článků.

In this work, the photoinduced degradation of CH3NH3PbI3 perovskite films under illumination in ambient conditions (relative humidity 30-50%) was studied by Kelvin Probe and Photoluminescence techniques. Using Kelvin Probe techniques, we investigated the effects on the work function of the CH3NH3PbI3 film on fluorine-doped SnO2 (FTO) at various photoinduced degradation states in the dark and under illumination. It was found that the work function of CH3NH3PbI3 film on FTO was gradually increased in the dark after every 10 min illumination step until the film was completely degraded. The gradual increase in work function due to the degradation can be ascribed to modulation doping of the CH3NH3PbI3 by PbI2 phase. It was also found that the contact potential difference (CPD) of CH3NH3PbI3 on FTO was increased under illumination as a result of a positive surface photovoltage relative to the FTO.

We show that two-terminal multijunction cells interconnected by tunnel junctions are fairly immune to individual local shunts, thanks to the shunt quenching. Interestingly, they may still suffer from global shunts. We revise the paradigm of a multijunction cell as a simple serial connection of component cells. This paradigm remains valid only for multijunction cells with laterally conductive interlayers. Instead, a new equivalent circuit is proposed and verified by measurement and simulations. As a main approach, selective illumination is applied and the voltage is measured at the end terminals. The global shunt is seen as a shift from logarithmic to linear intensity response. The presence of tunnel junction is important for an optimum configuration of tandem structures such as metal-halide perovskite with crystalline silicon solar cell.

The link between sub-bandgap states and optoelectronic properties is investigated for amorphous zinc tin oxide (a-ZTO) thin films deposited by RF sputtering. a-ZTO samples were annealed up to 500 °C in oxidizing, neutral, and reducing atmospheres before characterizing their structural and optoelectronic properties by photothermal deflection spectroscopy, near-infrared-visible UV spectrophotometry, Hall effect, Rutherford backscattering, hydrogen forward scattering and transmission electron microscopy. By combining the experimental results with density functional theory calculations, oxygen deficiencies and resulting metal atoms clusters are identified as the source of subgap states, some of which act as electron donors but also as free electron scattering centers. The role of hydrogen on the optoelectronic properties is also discussed. An amorphous indium-free transparent conductive oxide, with a high thermal stability and an electron mobility up to 35cm^2/V/s, is demonstrated.

The concept of light scattering and light trapping used to be inevitable for maintaining the technology of amorphous silicon profitable for long time. This strong light trapping concept will again rise in importance for current technologies, but fast methods of simulations will have priority in more comprehensive optimization over the time-consuming methods such as finite element or FDTD method. In this paper we present a novel approach of optical simulation of multilayer with accounting surface scattering that is based on complex transfer matrix concept, similarly as the non-scattered Fresnel component. This allows treatment of light trapping and effects of evanescent waves also for the scattered light. The only condition is to include phase randomization present in the case of light scattering.

Optical absorptance spectroscopy of polycrystalline CH3NH3PbI3 films usually indicates the presence of a PbI2 phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI2 phase restricts charge-carrier transport, suggesting that PbI2 encapsulates CH3NH3PbI3 grains. We also find that PbI2 injects holes into the CH3NH3PbI3 grains, increasing the apparent photosensitivity of PbI2.

We use photothermal deflection spectroscopy (PDS) to robustly characterize the absorption edge of lead sulfide (PbS) QD films for different bandgaps, ligands, and processing conditions used in leading devices. We also present a comprehensive overview of band tailing in many commercial and emerging PV technologies—including c-Si, GaAs, a-Si:H, CdTe, CIGS, and perovskites—then calculate detailed-balance efficiency limits incorporating Urbach band tailing for each technology. Our PDS measurements on PbS QDs show sharp exponential band tails, with Urbach energies of 22±1 meV for iodide-treated films and 24±1 meV for ethanedithiol-treated films, comparable to polycrystalline CdTe and CIGS films. From these results, we calculate a maximum efficiency of 31%—close to the ideal limit without band tailing. This finding suggests that disorder does not constrain the long-term potential of QD solar cells.

This work compares structural and optical properties of silicon nanocrystals prepared by two fundamentally different methods, namely, electrochemical etching of Si wafers and low-pressure plasma synthesis, completed with a mechano-photo-chemical treatment. This treatment leads to surface passivation of the nanoparticles by methyl groups. Plasma synthesis unlike electrochemical etching allows selecting of the particle sizes. Measured sizes of the nanoparticles by dynamic light scattering show 3 and 20 nm for electrochemically etched and plasma-synthetized samples, respectively. Electrochemically etched sample exhibits dramatic changes in photoluminescence during the mechano-photo-chemical treatment while no photoluminescence is observed for the plasma-synthetized one. We also used the Fourier transform infrared spectroscopy for comparison of the chemical changes happened during the treatment.

The thin film technology of photovoltaic modules is perspective from the low cost point of view and with the respect of special, for example flexible modules. However, within a range of allowed temperature it is hard to prepare a high quality semiconductor material. On the other hand the technology can easily adopt a complex multi-layer structure, particularly a tandem structure, in which the cell with the narrower band gap is covered by the one with the wider band gap. For the characteristics and diagnostics of tandem cells it is necessary to distinguish the contributions of the component sub-cells to the total performance. One of the tasks is the determination of the component contributions to the total voltage, which is strongly influenced by the amount and spatial distribution of shunts in each sub-cell. We show that the total voltage is not a simple sum of component voltages. The experimental results are presented in this paper.

The fundamental sheet conductance of graphene can be directly related to the product of its absorption coefficient, thickness and refractive index. The same can be done for graphene’s fundamental opacity if the so-called thin-film limit is considered. Here, we test mathematically and experimentally the validity of this limit on graphene, as well as on thin metal and semiconductor layers. Notably, within this limit, all measurable properties depend only on the product of the absorption coefficient, thickness, and refractive index. As a direct consequence, the absorptance of graphene depends on the refractive indices of the surrounding media. This explains the difficulty in determining separately the optical constants of graphene and their widely varying values found in literature so far. Finally, our results allow an accurate estimation of the potential optical losses or gains when graphene is used for various optoelectronic applications.

Parasitic absorption in the transparent conductive oxide (TCO) front electrode is one of the limitations of silicon heterojunction (SHJ) solar cells efficiency. To avoid such absorption while retaining high conductivity, TCOs with high electron mobility are preferred over those with high carrier density. Here, we demonstrate improved SHJ solar cell efficiencies by applying high-mobility amorphous indium zinc oxide (a-IZO) as the front TCO. We sputtered a-IZO at low substrate temperature and low power density and investigated the optical and electrical properties, as well as subband tail formation—quantified by the Urbach energy–as a function of the sputtering oxygen partial pressure.

The thin-film limit is derived by a nonconventional approach and equations for transmittance, reflectance and absorptance are presented in highly versatile and accurate form. In the thin-film limit the optical properties do not depend on the absorption coefficient, thickness and refractive index individually, but only on their product. We show that this formalism is applicable to the problem of ultrathin defective layer e.g. on a top of a layer of amorphous silicon. We develop a new method of direct evaluation of the surface defective layer and the bulk defects. Applying this method to amorphous silicon on glass, we show that the surface defective layer differs from bulk amorphous silicon in terms of light soaking.