Single Junction Thin Film Research Articles

Metasurface absorber based single junction thin film solar cell exceeding 30% efficiency.

In this article, we report, as per our knowledge, for the first time, a thin film single junction solar cell with a metasurface absorber layer directly incorporated. We have used an interconnected dual inverted split ring resonator pattern in the InAsP absorber layer. The structure eliminated patterns of conventional metals, such as silver, aluminum, and gold, from the active layer, a common drawback in conventional solar absorbers, hindering their direct integration into solar cells. Optical simulation results show a peak ideal short circuit current density of 76.23mA/cm2 for the meta-absorber structure under solar illumination. This current is the highest among previously reported absorbers based on Group IV materials and III-V compounds, overcoming the low solar absorption of such metasurfaces. The final proposed solar cell structure combines this meta-absorber layer with traditional efficiency enhancement methods namely anti-reflecting coating, textured back reflector, and transparent top electrode. This novel single junction structure shows a solar absorption efficiency of 97.86% and a power conversion efficiency of 30.87%, the highest for III-V solar cells. Our device proves the ability of metasurface absorber layers to produce high-efficiency solar cells and is expected to pave the way for integrating novel meta-devices into state-of-the-art photovoltaic devices, aiding the global transition towards clean energy sources.

Interplay of Photons and Charge Carriers in Thin-Film Devices

Thin films are gaining ground in photonics and optoelectronics, promising improvements in their efficiency and functionality as well as decreased material usage as compared to bulk technologies. However, proliferation of thin films would benefit not only from continuous improvements in their fabrication, but also from a unified and accurate theoretical framework of the interplay of photons and charge carriers. In particular, such a framework would need to account quantitatively and self-consistently for photon recycling and interference effects. To this end, here we combine the drift-diffusion formalism of charge carrier dynamics and the fluctuational electrodynamics of photon transport self-consistently using the recently introduced interference-extended radiative transfer equations. The resulting equation system can be solved numerically using standard simulation tools, and as an example, here we apply it to study well-known GaAs thin-film solar cells. In addition to obtaining the expected device characteristics, we analyze the underlying complex photon transport and recombination-generation processes, demonstrating the physical insight provided for unevenly excited structures through the direct and self-consistent description of photons and charge carriers. The methodology proposed in this work is general and can be used to obtain accurate physical insight into a wide range of planar optoelectronic devices, of which the thin-film single-junction solar cells studied here are only one example.

Investigations on Optical, Material and Electrical Properties of aSi:H and aSiGe:H in Making Proposed n+aSi:H/i-aSi:H/p+aSiGe:H Graded Bandgap Single-junction Solar Cell

Objective: This work identifies materials that satisfy refractive index, optical band gap, composition profile, conductivity, hall mobility, carrier type and carrier concentration to utilize them in making thin film photovoltaic cells. Methods: We fabricated phosphorous doped amorphous silicon (n+ aSi:H), boron doped amorphous silicon germanium(p+ aSiGe:H) and intrinsic amorphous silicon (i-aSi:H). A detailed and systematic characterization of the fabricated layers was done. The phosphorous doped amorphous silicon (n+ aSi:H) showed an optical band gap of 1.842 eV and an electron mobility of 295.45 cm2V-1s-1. The boron doped amorphous silicon germanium (p+ aSiGe:H) exhibited an optical band gap of 1.74 eV and a hole mobility of 158.353 cm2V-1s-1. The intrinsic amorphous silicon (i-aSi:H) has an optical band gap of 1.801 eV. The films of n+ aSi:H, i-aSi:H and p+ aSiGe:H can be utilized for fabricating graded band gap single junction thin film solar cells, as they are semiconducting materials with varying band gaps in the range of 1.74 eV to 1.84 eV. The tailoring of band gap achieved by the proposed material combination has been presented using its energy band diagram. Results: In this work, we are proposing a single junction graded band gap solar cell with aSi:H and aSi- Ge:H alloys of varying doping to achieve grading of band gap, which improves the efficiency while keeping the cell compact and light. Conclusion: As a first step in the validation, we have simulated a thin film solar cell using SCAPS1D simulation software with the measured parameters for each of the layers and found that it successfully performs as solar cell with an efficiency of 14.5%.

Overall analysis of change in power generation with outdoor exposure of photovoltaic modules installed at AIST Kyushu Center

In this paper, the current status of the power generation capacity of installed photovoltaic modules, which include monocrystalline Si, multicrystalline Si, thin-film multijunctions consisting of hydrogenated amorphous Si (a-Si:H) and hydrogenated microcrystalline Si, copper indium gallium selenide, thin-film single-junction a-Si:H, passivated emitter and rear cell (PERC) monocrystalline Si, Si heterojunction (SHJ), and interdigitated back contact (IBC) modules, at the National Institute of Advanced Industrial Science and Technology is reported. The degradation of PERC modules was confirmed by indoor measurement and a light soaking test. In the case of SHJ modules installed in 2016, the output power was unstable under the initial light irradiation conditions. In the case of IBC modules, potential-induced degradation was observed for one type within only 6 months of outdoor exposure, and for the other type, it was observed within four years of outdoor exposure. A formula for estimating the amount of power generation from the degradation rate was proposed.

Computer analysis of microcrystalline silicon hetero-junction solar cell with lumerical FDTD/DEVICE

The computer analysis of tandem solar cell, c-Si/a-Si:H/[Formula: see text]c-SiGe, is studied within Lumerical FDTD/Device 4.6. The optical characterization is performed in FDTD and then total generation rate is transported into DEVICE for electrical characterization. The electrical characterization of the solar cell is carried out in DEVICE. The design is implemented by staking three sub cells with band gap of 1.12[Formula: see text]eV, 1.50[Formula: see text]eV and 1.70[Formula: see text]eV, respectively. First, single junction solar cell with both a-Si and [Formula: see text]c-SiGe absorbing layers are designed and compared. The thickness for both layers are kept the same. In a single junction, solar cell with a-Si absorbing layer, the fill factor and the efficiency are noticed as [Formula: see text], and [Formula: see text]. For [Formula: see text]c-SiGe absorbing layer, the efficiency and fill factor are increased as [Formula: see text] and [Formula: see text], respectively. Second, for tandem thin film solar cell c-Si/a-Si:H/[Formula: see text]c-SiGe, the fill factor [Formula: see text] and efficiency [Formula: see text] have been noticed. The maximum efficiency for both single junction thin film solar cell c-Si/[Formula: see text]c-SiGe and tandem solar cell c-Si/a-Si:H/[Formula: see text]c-SiGe are improved with check board surface design for light trapping.

Electrodeposition of germanium-containing precursors for Cu2(Sn,Ge)S3 thin film solar cells

Cu2(Sn,Ge)S3 has recently emerged as an absorber layer for single junction thin film solar cells, already achieving power conversion efficiencies of up to 6.7%. Electrodeposition of metallic precursors and their subsequent annealing is an attractive synthesis method for such solar cell absorber layers, since it does not require vacuum conditions and little energy is consumed. The aqueous electrodeposition of metallic germanium based on current knowledge is limited to very thin layers, which do not provide the stoichiometric amount of material required for thick semiconductor layers. Therefore we introduce an electrodeposition method for the growth of germanium-containing precursors for Cu2(Sn,Ge)S3, using propylene-glycol-based electrolytes. These baths do not require additives, give rates of germanium electrodeposition higher than other plating baths and are cheap. The electrochemical behaviour of copper, tin and germanium in propylene glycol was studied and the corresponding deposits were characterised. Smooth and compact layers of each of the pure elements were deposited. Additionally, the co-electrodeposition of copper and germanium was studied as well and thin films containing the alloy Cu3Ge and metallic copper were obtained. A constant [Cu]/[Ge] ratio of around 3 was found over a wide potential range, with a higher plating efficiency than that of pure germanium. For these reasons, copper and germanium were incorporated into the precursor via the referred co-deposition method. After thermal annealing in the presence of elemental sulphur, the semiconductor Cu2(Sn,Ge)S3 was successfully formed and it was incorporated in a working solar cell structure with efficiency of 0.7%

Development of Improved n-μc-SiO$_x$ :H Films and Its Innovative Application in Silicon-Based Single Junction Thin Film Solar Cells

In this paper, we have discussed about the development of high-quality n-μc-SiO:H films by a seeding technique working as a potential back reflector layer (BRL). Highly crystalline and conducting n-μc-Si:H films are used as a seed layer. Phosphorous-doped SiO $_x$ :H film with suitable optoelectronics properties has been deposited by a radio frequency plasma enhanced chemical vapor deposition technique using parallel plate reactors (capacititively coupled). Optoelectronics properties have been controlled and optimized varying the deposition parameters such as process pressure, power density, partial pressure of CO2, etc. We have also innovatively replaced the n-layer, i.e., n-a-Si:H in case of the a-Si:H cell and n-μc-Si:H for μc-Si:H cells by this optimized seed + n-μc-SiO:H films. The performance of this n-μc-SiO:H film grown on the n-μc-Si:H seed layer as n-layer as well as BRL of single junction a-Si:H and μc-Si:H solar cells has been evaluated and better Photovoltaic (PV) characteristics of the solar cell are realized as compared to those of the films developed without any seed layer. a-Si:H solar cells with initial efficiency of 9.38% and μc-Si:H solar cell of 8.50% have been successfully fabricated by using improved doped SiO $_x$ :H based BRL replacing conventional n-layer of the single junction cells.

Ultra-thin LiF Layer As The Electron Collector For a-Si:H Based Photovoltaic Cell

ABSTRACTAn ultra-thin LiF layer in conjunction with an Al layer is employed as the electron collector for the a-Si:H based single-junction thin film photovoltaic cell. The cell has the structure of boron doped μ-SiOx (hole collector) - intrinsic a-Si:H (photoactive layer) - LiF / Al (electron collector and back electrode). The substrate used is U type Asahi glass, which is also acting as the transparent front electrode. For the cell with the 1.5 nm thick LiF layer, annealed at 120°C, the open current voltage (VOC) of 0.936 V, the short current density (JSC) of 13.598 mA/cm2, and the fill factor (FF) of 0.690 are achieved. The JSC and VOC values are comparable to the values measured for the a-Si:H based p-i-n reference cell, but the FF value is found to be lower, which is attributed to the losses due to recombination at the intrinsic a-Si:H / LiF / Al junction. The current versus voltage measurements are carried out under the standard test conditions. The JSC values are corrected according to the external quantum efficiency measurements of the cells in the AM1.5 spectrum region between 270 nm and 800 nm.

Novel double-stage high-concentrated solar hybrid photovoltaic/thermal (PV/T) collector with nonimaging optics and GaAs solar cells reflector

A novel double stage high-concentration hybrid solar photovoltaic thermal (PV/T) collector using nonimaging optics and world record thin film single-junction gallium arsenide (GaAs) solar cells has been developed. We present a detailed design and simulation of the system, experimental setup, prototype, system performance, and economic analysis. The system uses a parabolic trough (primary concentrator) to focus sunlight towards a secondary nonimaging compound parabolic concentrator (CPC) to simultaneously generate electricity from single junction GaAs solar cells, as well as high temperature dispatchable heat. This study is novel in that (a) the solar cells inside the vacuum tube act as spectrally selective mirrors for lower energy photons to maximize the system exergy, and (b) secondary concentrator allows the thermal component to reach a concentration ratio ∼60×, which is significantly higher than conventional PV/T concentration ratios. The maximum outlet temperature reached was 365°C, and on average the thermal efficiency of the experiment was around 37%. The maximum electrical efficiency was around 8%. The total system electricity generation is around 25% of incoming DNI, by assuming the high temperature stream is used to power a steam turbine. The installed system cost per unit of parabolic trough aperture area is $283.10per m2.

Behavioral data of thin-film single junction amorphous silicon (a-Si) photovoltaic modules under outdoor long term exposure.

Four years׳ behavioral data of thin-film single junction amorphous silicon (a-Si) photovoltaic (PV) modules installed in a relatively dry and sunny inland site with a Continental-Mediterranean climate (in the city of Jaén, Spain) are presented in this article. The shared data contributes to clarify how the Light Induced Degradation (LID) impacts the output power generated by the PV array, especially in the first days of exposure under outdoor conditions. Furthermore, a valuable methodology is provided in this data article permitting the assessment of the degradation rate and the stabilization period of the PV modules.Further discussions and interpretations concerning the data shared in this article can be found in the research paper “Characterization of degradation and evaluation of model parameters of amorphous silicon photovoltaic modules under outdoor long term exposure” (Kichou et al., 2016) [1].

Characterization of degradation and evaluation of model parameters of amorphous silicon photovoltaic modules under outdoor long term exposure

The analysis of the degradation of thin-film single junction a-Si PV (photovoltaic) modules and its impact on the output power of a PV array under outdoor long term exposure located in Jaén (Spain), a relatively dry and sunny inland site with a Continental-Mediterranean climate is addressed in this paper. Furthermore, a new procedure of solar cell model parameters extraction experimentally validated is presented. The parameter extraction procedure allows obtaining main model parameters of the solar cells forming the PV array from monitored data of the PV system in real operation of work. Results obtained of the evolution of each one of the solar cell model parameters along the PV system outdoor long term exposure campaign are analysed in order to achieve a better understanding of the performance changes of the PV modules and the behaviour of the output power of the PV array.

Impact of photon recycling and luminescence coupling on III–V single and dual junction photovoltaic devices

Modeling single junction solar cells composed of III–V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon recycling indicate that Shockley–Read–Hall (SRH) lifetimes of electrons and holes must be longer than 3 and 1 μs, respectively, in a 2-μm thin active region, and that the native substrate must be removed such that the cell is coupled to a highly reflective rear-side mirror. The model is generalized to account for luminescence coupling in tandem devices, which yields direct insight into the top cell’s nonradiative lifetimes. A heavily current mismatched GaAs/GaAs tandem device is simulated and measured experimentally as a function of concentration between 3 and 100 suns. The luminescence coupling increases from 14% to 33% experimentally, whereas the model requires increasing electron and hole SRH lifetimes to explain these results. This could be an indication of the saturating defects which mediate the SRH process. However, intermediate GaAs layers between the two subcells may also contribute to the luminescence coupling as a function of concentration.

Tuning of the open-circuit voltage by wide band-gap absorber and doped layers in thin film silicon solar cells

We present an experimental study combined with computer simulations on the effects of wide band-gap absorber and window layers on the open-circuit voltage (Voc) in single junction thin film silicon solar cells. The quantity ΔEp, taking as the difference between the band gap and the activation energy in ⟨p⟩ layer, is treated as a measure of the p-layer properties and shows a linear relation with Voc over a range of 100 mV with a positive slope of around 430 mV/eV. Two limiting mechanisms of Voc are identified: the built-in potential at lower ΔEp and the band gap of the absorber layer at higher ΔEp. The results of the experimental findings are confirmed by computer simulations. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Fabrication of single junction amorphous silicon solar cell/mini module using novel n-type nanocrystalline SiOx:F:H back reflector

A novel n-type nanocrystalline hydrogenated and fluorinated silicon oxide (nc-SiOx:F:H) based back reflector layer (BRL) has been successfully developed for the fabrication of amorphous silicon based single junction solar cells by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique. Optoelectronic properties, particularly the refractive index (n) suitable for BRL were controlled by varying the oxygen content within the film without compromising the essential nanocrystalline growth. Surface morphology, structural and optical analysis of nc-SiOx:F:H were investigated by using AFM, HRTEM, Raman, FTIR and Ellipsometer. Advantage of this nanocrystalline material as BRL has been demonstrated by incorporating it into a single junction amorphous silicon solar cell and compared its performance with conventional aluminium doped zinc oxide (ZnO:Al) and n doped hydrogenated microcrystalline silicon oxide (n-µc-SiOx:H) based BRL. Single junction thin film a-Si solar cells with initial efficiency ~9.66 % have been successfully fabricated. It is found that the solar cell fabricated using newly developed BRL shows ~6 % more efficiency compared to n-µc-SiOx:H based BRL. Spectral response studies revealed that the improvement was mainly due to enhanced response at visible as well as higher wavelength region.

Development of n-μc-SiOx:H as cost effective back reflector and its application to thin film amorphous silicon solar cells

Development of doped silicon oxide based microcrystalline material as a potential candidate for cost-effective and reliable back reflector layer (BRL) for single junction solar cells is discussed in this article. Phosphorus doped μc-SiOx:H layers with a refractive index ∼2 and with suitable electrical properties were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, using the conventional capacitively coupled reactors. Optoelectronic properties of these layers were controlled by varying the oxygen content within the film. The performance of these layers as BRL have been investigated by incorporating them in a single junction amorphous silicon solar cell and compared with the conventional ZnO:Al based reflector layer. Single junction thin film a-Si solar cells with efficiency ∼9.12% have been successfully demonstrated by using doped SiO:H based material as a back reflector. It is found that the oxide based back reflector shows analogous performance to that of conventional ZnO:Al BRL layer. The main advantage with this technology is that, it can avoid the ex-situ deposition of ZnO:Al, by using doped μc-SiO:H based material grown in the same reactor and with the same process gases as used for thin-film silicon solar cells.

Synthesis of Group IV Clathrates for Photovoltaics

Although Si dominates the photovoltaics market, only two forms of Si have been thoroughly considered: amorphous Si and Si in the diamond structure ( d-Si). Silicon can also form in other allotropes, including clathrate structures. Silicon clathrates are inclusion compounds, which consist of an Si framework surrounding templating guest atoms (e.g., Na). After formation of the type II Na <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">24</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">136</sub> clathrate, the guest atoms can be removed (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">136</sub> ), and the material transitions from degenerate to semiconducting behavior with a 1.9 eV direct band gap. This band gap is tunable in the range of 1.9-0.6 eV by alloying the host framework with Ge, enabling a variety of photovoltaic applications that include thin-film single-junction devices, Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">136</sub> top cells on d-Si for all-Si tandem cells, and multijunction cells with varying Si/Ge ratios. In this study, we present electronic structure calculations that show the evolution of the direct transition as a function of Si/Ge ratio across the alloy range. We demonstrate the synthesis of type II Si/Ge clathrates spanning the whole alloy range. We also demonstrate a technique for forming Si clathrate films on d-Si wafers and sapphire substrates.

Simulation and Experimental Study of Photogeneration and Recombination in Amorphous-Like Silicon Thin Films Deposited by 27.12 MHz Plasma-Enhanced Chemical Vapor Deposition

Amorphous-like silicon (a-Si:H-like) thin films are prepared by 27.12 MHz plasma-enhanced chemical vapor deposition technique. The films are applied to p-i-n single junction thin film solar cells with varying i-layer thickness to observe the effects on the short-circuit current density, as well as the open-circuit voltage, fill factor, and conversion efficiency. The most significant experimental result is thatJschas two different behaviors with increasing the i-layer thickness, which can be related to carrier collection efficiency in the long wavelength region. Furthermore, technology computer-aided design simulation software is used to gain better insight into carrier generation and recombination of the solar cells, showing that for the i-layer thickness of 200 to 300 nm the generation dominates the carrier density and thusJsc, whereas for the i-layer thickness of 300 to 400 nm the recombination becomes the leading factor. The simulation results of cell performances are in good agreement with experimental data, indicating that our simulation has great reliability. In addition, the a-Si:H-like solar cells have low light-induced degradation, which in turn can have a great potential to be used for stable and high-efficiency solar cells.

A study of ZnO:B films for thin film silicon solar cells

Boron doped zinc oxide (ZnO:B) films with different thicknesses were prepared with low pressure chemical vapor deposition (LPCVD) technique and implemented in thin film silicon solar cells as front and back electrodes. It is found that thick back ZnO:B film electrode in thin film silicon solar cells leads to a high fill factors (FF), which is attributed to an improvement of the electrical properties of the thick ZnO:B films, and in the meanwhile a slightly low short circuit currents (Jsc) due to a high light absorption in the thick back ZnO:B films. Differently, the thicker front ZnO:B film electrodes result in a high Jsc but a low FF of solar cells compared to the thinner ones. The low FF of the solar cells may be caused by the local shunt originated from the pinholes or by the cracks (zones of non-dense material) formed in particular in microcrystalline silicon materials deposited on rough front ZnO:B films. As to the high Jsc, it is expected to be due to a good light trapping effect inside solar cells grown on rough front ZnO:B films. Moreover, the application of high reflective polyvinyl butyral (PVB) foils effectively enhances the utilization of incident light in solar cells. By optimizing deposition process of the ZnO:B films, high efficiencies of 8.8% and 10% for single junction thin film amorphous silicon solar cells (a-Si:H, intrinsic layer thickness<200nm) and amorphous/microcrystalline silicon tandem solar cells (a-Si:H/μc-Si:H, intrinsic amorphous silicon layer thickness<220nm), respectively, are achieved.

Highly conductive GaN anti-reflection layer at transparent conducting oxide/Si interface for silicon thin film solar cells

Highly conductive GaN film was prepared by magnetron sputtering and this was applied as an anti-reflection layer (ARL) between a transparent conducting oxide and microcrystalline silicon (μc-Si:H) in order to decrease optical reflection. The efficiency (8.81%) of μc-Si:H single junction thin film solar cell with the proposed GaN ARL exceeded that of the cell (8.36%) with the widely used TiO2/ZnO bilayer ARL. Moreover, the proposed GaN ARL requires no protection layer against hydrogen plasma such as ZnO overcoating (∼10nm) in case of the TiO2/ZnO bilayer. GaN ARL can replace the TiO2/ZnO bilayer ARL in terms of high performance and simple fabrication process.

Silicon thin film solar cells on commercial tiles

Nanostructured silicon single junction thin film solar cells were deposited on commercial red clay roof tiles with engobe surfaces and earthenware wall tiles with glazed surfaces, with a test area of 24 mm2. We studied the influence of the type of substrate tile, back contact, buffer layer and SiOx passivation layer on the optoelectronic performance of the solar cells. Despite the fact that typical micrometre-sized defects on the surfaces of the tiles and the porous nature of the ceramic substrates make deposition of homogeneous thin films on them quite challenging, we have been able to achieve a cell efficiency of 5% and a quantum efficiency of 80% on non-fully optimized cells on commercial tiles. The method is industrially employable utilizing pre-existing plasma-enhanced chemical vapour deposition technologies. The cost-effectiveness and industrial feasibility of the technique are discussed. Our study shows that photovoltaic tiles can combine energy generation with architectural aesthetics leading to significant implications for advancement in building integrated photovoltaics.