Multi-junction Thin-film Solar Cells Research Articles

Low-temperature growth of narrow optical gap highly conducting nc-Ge thin films with superior crystallinity involving dominant orientation

The nc-Ge thin films are grown at a moderately low temperature (∼220 °C) in rf PECVD via optimizing the GeH4 plasma with H2-dilution, within 97.5 ≤ D(H2)(%) ≤ 99.5. At elevated D(H2), while the bonded H-content in the matrix reduces, the film's crystallinity and dc conductivity increase. However, the optical band gap widens for 97.5 ≤ D(H2)(%) ≤ 98.5, via the elevated quantum confinement effect from the increased number of Ge-ncs of dimension below Bohr radius. Conversely, for D(H2) ≥ 98.5%, switching in the quantum effect in larger-sized Ge-ncs induces the band gap narrowing. Significantly reduced ultra-nanocrystalline-Ge (unc-Ge) component occupying the grain boundary zone, minimizes the contribution of grain-boundary defects. However, the persistent amorphous Ge matrix becomes highly defective and less dense, and those induce enhanced O-absorption from the ambient in the dominant Ge-Ox (x < 2) configuration. On increased crystallinity, the activation energies of dipole relaxation (ΔEτ) and resistance (ΔE) in the impedance data reduce dominantly in the a-Ge component than its nc-Ge counterpart, which occurs identically for the dc conductivity when the amorphous component in the matrix sharply disappears at increased D(H2) > 98.5%. A narrow band gap (∼0.90 eV), conducting (σd ∼3.0 ×10−2 S cm−1), and photosensitive nc-Ge network, involving dominant <111>-oriented Ge-ncs (∼35.2 nm) of enriched volume fraction (XC ∼90.2%), obtained at D(H2) ∼99.5%, seems extremely useful for need-based applications as a narrow band gap absorber layer in the bottom sub-cell of flexible high-efficiency cost-effective multijunction thin film solar cells.

Influence on the Haze Effect of Si Thin-Film Solar Cell on Multi-Surface Textures of Periodic Honeycomb Glass

Double textures were applied to use for multi-junction silicon thin film (Si-TF) solar cells. To improve the longer wavelength, we were formed the microscale periodic honeycomb textured surface on a glass substrate. We achieved the transmittance of 94.26% and found the approximately 52% for haze value. Etched AZO films on the textured glass is crucial role for improving the short-wavelength in Si-TF solar cells. Textured AZO film using a 1% HF solution is suitable for the growth of microcrystalline Si and better light trapping. The transmittance was recorded 80.8% that was 3% higher than that of reference fluorine doped tin oxide (FTO, Asahi-U type) glass. We also achieved for high haze value of 59.8% to enable the light trapping across the entire wavelength region. Using a finite-difference time-domain (FDTD) simulation, we designed a multi-junction Si-TF solar cells with high short-circuit current density of 12.5 mA/cm2. To reduce the plasma damage and surface stress on the μc-Si:H layer during growth, we fabricated an a-Si:H/µc-Si:H tandem cell on a multi-surface textures with a µc-Si:H layer of only 1.5 µm thick and obtained a conversion efficiency of 13.3%. The developed double textures structure and fabrication technique are expected to improve the performance of various multi-junction Si thin-film solar cells.

Design and Simulation of Single-Junction and Multi-junction Thin-Film Solar Cells Based on Copper Tin Sulfide

In this paper, single- and multi-junction thin-film solar cells based on copper tin sulfide (CTS) are proposed. The proposed single-junction cell consists of copper zinc tin sulfide (CZTS) and CTS, as a bilayer absorber. The double-junction structure is made of CZTS and CTS absorber layers for the top and bottom cells, respectively. Furthermore, a CZTS/(CZTSe)/CTS triple-junction cell, which is based on earth-abundant and non-toxic elements, is investigated. The performance of the proposed cells in the presence of tin sulfide layers as the back surface field to reduce recombination is also examined. In order to reach the maximum efficiency, the thickness of layers is varied and optimized. The simulated efficiency of the triple-junction structure, including CZTS, CZTSe, and CTS with the optimal thickness of 0.4 μm, 0.7 μm, and 0.7 μm, respectively, was as high as 34.1%.

Narrow band gap high conducting nc-Si1-xGex:H absorber layers for tandem structure nc-Si solar cells

Advantages of spectral sensitivity in the infrared range and good stability against light exposure together have made nanocrystalline silicon–germanium alloy (nc-Si1-xGex) as an ideal absorber material at the bottom-cell in multi-junction Si thin film solar cells. Evolution of good quality SiGe thin films with sustained nanocrystallinity, particularly at low growth temperature (TS), is a challenging task in view of very different chemical reaction behaviors controlled via different energy requirements of two frequently used source gases, e.g., SiH4 and GeH4. Growth of nc-SiGe:H thin films have been pursued at TS ∼220 °C, using H2-diluted (by ∼96%) (SiH4 + GeH4) plasma in 13.56 MHz RF-PECVD, by varying the RF power applied to the parallel plate capacitively coupled electrodes of the reactor. At lower RF power Ge-dominated low band gap SiGe network has been produced with Si component in mostly amorphous configuration that leads to a low electrical conductivity. At high RF power Si–Si network becomes highly crystalline; however, Ge content reduces drastically. Breaking of weaker Ge–Ge bonds creates defects via dangling bonds and increased grain boundary defects around Si nanocrystals together produce relatively wider band gap SiGe network with degraded electrical conductivity. The novelty and significance of the present investigation remains in realizing the narrowing of the optical band gap (Eg ∼1.499 eV) even at lower Ge content, through moderate crystallization in the SiGe network and a simultaneous high electrical conductivity of the nc-SiGe:H films (σD ∼2.38 × 10−3 S cm−1) obtained at a high growth rate (∼9.5 nm/min), via optimization of the applied RF power at a moderate level (∼100 W). In such circumstances nanocrystallization of the network plays leading role in narrowing the optical band gap, overriding the effect arising out of the restricted presence of Ge.

Single-graded CIGS with narrow bandgap for tandem solar cells

Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se2 with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe2 base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.

Advanced light trapping scheme in decoupled front and rear textured thin-film silicon solar cells

We present the study of an advanced light trapping scheme applied to thin-film silicon-based solar cells, overcoming the broadband Green absorption limit, that is the generalized case of the 4n2 classical absorption limit for all wavelengths. This result is achieved by the 3-dimensional optical modelling of a fully functional thin-film hydrogenated nano-crystalline silicon (nc-Si:H) solar cell endowed with decoupled front and back textures. Our results stem from rigorously characterized optical properties of state-of-the-art materials, optimized geometric nano-features on the front and rear surfaces of the solar cell, and thickness optimization of the front transparent oxide. The simulated improvements derive from a gain in light absorption, especially in the near-infrared part of the spectrum close to the band gap of nc-Si:H. In this wavelength region, the material is weakly absorbing, whereas we now find significant absorptance peaks that can only be explained by the concurrent excitation of guided resonances by front and rear textures. This insight indicates the need to modify the temporal coupled-mode theory, which fails to predict the absorption enhancement achieved in this work, extending its validity to the case of decoupled front/back texturing. Our approach results in substantially high photocurrent density (>36 mA/cm2), creating a platform suitable for high efficiency single and multi-junction thin-film solar cells based either on typical silicon alloys or on the novel and promising barium (di)silicide (BaSi2) absorber. In the latter case, using the same advanced light trapping employed for nc-Si:H, we demonstrate a very high implied photocurrent density of 41.1 mA/cm2, for a device endowed with 2-μm thick absorber.

Spectral Response of CuGaSe2/Cu(In,Ga)Se2 Monolithic Tandem Solar Cell With Open-Circuit Voltage Over 1 V

Thin-film multijunction solar cells are considered to be the most promising structure for next-generation photovoltaic devices. We fabricated CuGaSe2 (CGS)/Cu(In,Ga)Se2 (CIGS) monolithic tandem solar cells. The intermediate AZO film was used as a recombination layer between the top cell and the bottom cell, and its thickness was varied from 50 to 200 nm. The best tandem cell parameters with a 50 nm thick Al-doped ZnO (AZO) layer showed a V OC = 1.03 V, J SC = 10.24 mA/cm2, FF = 41.5%, and efficiency = 4.32%. We showed the V OC of monolithic tandem cell to be over 1 V under illumination. We also observed the current continuity between the CGS cell and the CIGS cell which were connected in series as subcells. As the AZO thickness increased, the spectral response of the top cell decreased and the bottom cell was not completely saturated. The 50 nm thick AZO layer leads the CIGS bottom cell to be current-limiting, whereas the 200 nm thick AZO layer shifts the limitation to the CGS top cell. The results also showed that the In element diffusion into the CGS top absorber enhanced the electrical and optical properties of the top cell, whereas the Zn element diffusion into CIGS bottom absorber tended to degrade the bottom cell simultaneously.

Versatility of doped nanocrystalline silicon oxide for applications in silicon thin-film and heterojunction solar cells

To optimize the optical response of a solar cell, specifically designed materials with appropriate optoelectronic properties are needed. Owing to the unique microstructure of doped nanocrystalline silicon oxide, nc-SiOx:H, this material is able to cover an extensive range of optical and electrical properties. However, applying nc-SiOx:H thin-films in photovoltaic devices necessitates an individual adaptation of the material properties according to the specific functions in the device. In this study, we investigated the detailed microstructure of doped nc-SiOx:H films via atom probe tomography at the sub-nm scale, thereby, for the first time, revealing the three-dimensional distribution of the nc-Si network. Furthermore, n- and p-type nc-SiOx:H layers with various optical and electrical properties were implemented as a window, back contact, and an intermediate reflector layer in silicon heterojunction and multi-junction thin-film solar cells with a focus on the key aspects for adapting the material properties to the specific functions. Here, nc-SiOx:H effectively reduced the parasitic absorption and opened new possibilities for the photon management in the solar cells, thereby, demonstrating the versatility of this material. Remarkably, using our adapted nc-SiOx:H layers in distinct functions enabled us to achieve a combined short circuit current density of 15.1mAcm−2 for the two a-Si:H sub-cells in an a-Si:H/a-Si:H/µc-Si:H triple-junction thin-film solar cell and an active area efficiency of 21.4% was realized for a silicon heterojunction solar cell.

Fabrication of Cu2SnS3 thin films by ethanol-ammonium solution process by doctor-blade technique

In the present study, a low-cost and simple method is applied to fabricate Cu2SnS3 (CTS) thin films. Namely CTS thin films are prepared by a doctor-blade method with a slurry dissolving the Cu2O and SnS powders obtained from CBD reaction solution into ethanol-ammonium solvents. Series of characterization methods including XRD, Raman spectra, SEM and UV-Vis analyses are introduced to investigate the phase structure, morphology and optical properties of CTS thin films. As a result, monoclinic CTS films have been obtained with the disappearance of binary phases CuS and SnS2 while increasing the annealing temperature and time, high quality monoclinic CTS thin films consisting of compact and large grains have been successfully prepared by this ethanol-ammonium method. Moreover, the secondary phase Cu2Sn3S7 is also observed during the annealing process. In addition, the post-annealed CTS film with a band-gap about 0.89 eV shows excellent absorbance between 400 and 1200 nm, which is proper for the bottom layer in multi-junction thin film solar cells.

Monolithic CIGS–Perovskite Tandem Cell for Optimal Light Harvesting without Current Matching

We present a novel monolithic architecture for optimal light harvesting in multijunction thin film solar cells. In the configuration we consider, formed by a perovskite (PVK) cell overlying a CIGS cell, the current extracted from the two different junctions is decoupled by the insertion of a dielectric nonperiodic photonic multilayer structure. This photonic multilayer is designed by an inverse integration approach to confine the incident sunlight above the PVK band gap in the PVK absorber layer, while increasing the transparency for sunlight below the PVK band gap for an efficient coupling into the CIGS bottom cell. To match the maximum power point voltages in a parallel connection of the PVK and CIGS cells, the latter is divided into two subcells by means of a standard three-laser scribing connection. Using realistic parameters for all the layers in the multijunction architecture we predict power conversion efficiencies of 28%. This represents an improvement of 24% and 26% over the best CIGS and PVK sin...

Thin-film amorphous silicon germanium solar cells with p- and n-type hydrogenated silicon oxide layers

Mixed-phase hydrogenated silicon oxide (SiOx:H) is applied to thin-film hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells serving as both p-doped and n-doped layers. The bandgap of p-SiOx:H is adjusted to achieve a highly-transparent window layer while also providing a strong electric field. Bandgap grading of n-SiOx:H is designed to obtain a smooth transition of the energy band edge from the intrinsic to n-doped layer, without the need of an amorphous buffer layer. With the optimized optical and electrical structure, a high conversion efficiency of 9.41% has been achieved. Having eliminated other doped materials without sacrificing performance, the sole use of SiOx:H in the doped layers of a-SiGe:H cells opens up great flexibility in the design of high-efficiency multi-junction thin-film silicon-based solar cells.

Design of periodic nano- and macro-scale textures for high-performance thin-film multi-junction solar cells

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/μc-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si:H/μc-Si:H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 μm and vertical peak-to-peak heights around or above 0.3 μm are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 μm and heights around 0.6 μm lead to a >6% gain in short-circuit current of the bottom cell.

Nano-composite microstructure model for the classification of hydrogenated nanocrystalline silicon oxide thin films

Abstract The unique microstructure of nanocrystalline silicon oxide (nc-SiO X :H) thin films results in excellent optoelectronic properties that can be tuned in a wide range to fulfill the requirements of the specific application. For photovoltaic applications, this material is used as doped layers in silicon heterojunction solar cells and intermediate reflectors in multijunction thin-film solar cell. In this paper, we present a microstructure model based on a large number of n- and p-doped nc-SiO X :H films that were deposited under various deposition pressures, plasma powers, plasma frequencies and gas mixtures. This model is meant to provide guidelines for the systematic classification of the complex material system nc-SiO X :H by establishing a link between the structure of the deposited films and the optoelectronic performance of nc-SiO X :H. Based on this model, the deposition of nc-SiO X :H films can be divided into four characteristic regions: (i) fully amorphous region, (ii) onset of nc-Si formation, (iii) oxygen and nc-Si enrichment region, and (iv) deterioration of nc-Si. According to our microstructure model, an optimal phase composition with respect to the optoelectronic performance can be achieved with a high amount of highly conductive nc-Si percolation paths embedded in an oxygen rich a-SiO X :H matrix.

Electrical properties of hydrogenated microcrystalline silicon carbon alloys: effect of deposition parameters and light soaking

New materials with electrical properties similar to those of hydrogenated amorphous silicon (a-Si:H) or hydrogenated amorphous silicon germanium alloys, but that are stable under light soaking, could be a breakthrough for multi-junction thin-film silicon solar cell technology. With this aim, hydrogenated microcrystalline silicon-carbon alloy (c-:H) thin films seem like promising candidates as the variation of the carbon in the alloy composition can be used to tune the effective bandgap, keeping a high crystalline fraction. In this study, a highly hydrogen-diluted silane and methane gas mixture was used to deposit c-:H thin films by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The effect of the independent variation of three deposition parameters (variation of methane flow rate, addition of a small amount of silicon tetrafluoride and variation of the RF-power density) on the electronic transport and defect properties was investigated. Selected samples were studied in their ‘as deposited’ state, after light soaking until saturation, and after a high temperature annealing. Particular care was paid to comparing the behaviour of this new material system to that of a reference a-Si:H sample, deposited by standard RF-PECVD.

Characterisation of intrinsic silicon oxide absorber layers for use in silicon thin film solar cells

The use of a wide bandgap absorber layer in the top cell of a multi-junction silicon thin film solar cell is necessary to achieve a high-conversion efficiency. A higher bandgap of the absorber results in a higher open-circuit voltage (Voc) of the cell. In this work, intrinsic hydrogenated amorphous silicon oxide (i)a-SiO:H films have been prepared by using 13.56 MHz radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) at a substrate temperature of 195 °C. The carbon dioxide (CO2) to silane (SiH4) ratio rc was varied and the influence of the ratio rc on the optoelectronic film properties was investigated. These thin films have been studied in detail in terms of their dark (σd) and photo (σph) conductivity and photoresponse PR (σph/σd). The defect density Nd and Urbach energy EU were determined by constant photocurrent method (CPM). The optical bandgap Eg,Tauc was derived from Tauc plots. Optical constants were determined by spectroscopic ellipsometry measurements in the range between 300 and 1000 nm. It was found that the increase in the CO2 to SiH4 ratio rc not only leads to a higher optical bandgap, but also higher defect density Nd and Urbach energy EU and on the other hand to a lower photoresponse PR. A suitable photoresponse PR = 8.64 × 105 at a high bandgap of Eg,Tauc = 1.91 eV and a defect density of Nd = 1.7 × 1016 cm−3 was achieved. The refraction index and extinction coefficient were lowered with increasing rc. The analysis of light-induced degradation in the (i)a-SiO:H layers showed a smaller increase of deep defects Nd and a higher increase of the Urbach energy EU in terms of the light soaking time with respect to the (i)a-Si:H reference layer.

Low-temperature growth of single-crystal Cu(In,Ga)Se2 films by pulsed electron deposition technique

High quality epitaxial crystalline Cu(In,Ga)Se2 (CIGS) films were grown on n-type (100)—Germanium (Ge) substrates using pulsed electron deposition (PED) technique at a remarkably low substrate temperature of 300°C, thanks to the high-energy of adatoms arriving to the substrate. The crystalline quality was confirmed by X-ray diffraction techniques and from Transmission Electron Microscopy and the only defects found were twin boundaries along the (112) direction in these CIGS films; surprisingly neither misfit dislocations nor Kinkerdall voids were observed. A 100meV optical band located below the band edge was observed by Photoluminescence technique. Current–voltage and capacitance–voltage measurements confirm an intrinsic p-type conductivity of CIGS films, with a free carrier concentration of ≈3.5×1016cm−3. These characteristics of crystalline CIGS films are crucial for a variety of potential applications, such as more efficient absorber layers in single-junction and as an integral component of multi-junction thin-film solar cells.

Microcrystalline SiGe Absorber Layers in Thin-film Silicon Solar Cells

We report on physical properties of microcrystalline silicon-germanium (μc-SiGe:H) absorber layers for the use as a bottom structure in silicon based multijunction thin-film solar cells. Due to incorporation of Ge the absorption of the film is enhanced compared to pure μc-Si:H films. This provides the opportunity to significantly reduce the absorber layer thickness. The experiments were carried out in a 13.56MHz PECVD reactor using germane, silane and hydrogen as process gases. Single layers were characterized for their optical and electrical properties. Results from single and multijunction solar cells using a μc- SiGe:H absorbers will be shown. In tandem solar cells a reduction of about 60% of the absorber layer thickness could be reached by using SiGe alloys compared to pristine silicon tandem cells.

(Invited) New Paradigms for Cost-Effective III-V Photovoltaic Technology

In this paper, we discuss two new paradigms for enabling cost-effective III-V solar cell photovoltaics using (i) our novel layer transfer technology, called controlled spalling and (ii) new solar cell device structures. First, we present the application of the controlled spalling for making high-efficiency thin-film multi-junction solar cells. Finally, a novel GaAs heterojunction solar cell structure is described, consisting of thin hydrogenated amorphous silicon stack deposited directly on the GaAs surface at 200ºC, aiming to eliminate the III-V epitaxy. This feature in conjunction with a kerf-less removal of GaAs using the controlled spalling technique offers a viable pathway for realizing low cost high-efficiency III-V photovoltaic technology.

CuAlxGa1−xSe2 thin films for photovoltaic applications: Optical and compositional analysis

Wide-band gap chalcopyrite semiconductors have a great interest due to their potential application in multi-junction thin film solar cells or as window layers. Polycrystalline CuAlxGa1−xSe2 (CAGS) thin films have been prepared by selenization of evaporated metallic precursor layers on bare and Mo-coated soda lime glass substrates. The optical properties of CAGS films of 2 thicknesses have been analyzed by spectrophotometry in the visible-infrared (VIS-IR) and the compositional characteristics have been studied by energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS). The optical transmission increases and the band gap energy shifts toward higher values as the Al content increases, which indicates the partial substitution of Ga by Al. The dependence of the band gap with the composition has resulted to be nonlinear and a bowing parameter of b=0.62 and b=0.54 for 0.6μm and 1.1μm-CAGS samples, respectively, has been obtained. XPS data have shown an Al, Ga and Se composition gradient in depth and a surface strongly oxidized. However, XPS reveals that the Cu composition remains constant in depth and the oxidation is relatively low in bulk increasing slightly in the interface with Mo/SLG. Moreover, samples with high Al content reveal a higher contribution of CuO in depth.

Light Induced Changes in PIN Solar Cells: Beyond the Staebler-Wronski Effect

ABSTRACTHydrogenated polymorphous silicon (pm-Si:H) is one of the most promising candidates for a stable top cell material in multi-junction thin film solar cells. Solar cells using pm-Si:H as their absorbing layer show very interesting degradation kinetics when compared to hydrogenated amorphous silicon (a-Si:H), summarized by macroscopic structural changes and irreversible changes in solar cell characteristics, while nevertheless preserving a higher stabilized efficiency. Notably, pm-Si:H solar cells, once degraded, respond to neither annealing nor further light-soaking. Such results suggest a device degradation mechanism including structural changes, active hydrogen motion, and interface delamination mediated by fast hydrogen diffusion and accumulation at the interface. Interestingly, a similar behavior was reported for a-Si:H solar cells under severe light soaking conditions (at 350 °C or under 50 suns) while pm-Si:H solar cells show such behavior under 1 sun at 40 °C.