P-type Absorber Research Articles

Design and simulation of highly efficient CZTS/CZTSSe based thin-film solar cell

Thin-film solar cells are a substitute for more common crystalline silicon solar cells, which consist of thin semiconductor layers. Thin-film materials comprise direct bandgap and can absorb sunlight more efficiently than silicon. In this article, a double-absorber-based thin-film solar cell comprising CZTS/CZTSSe is designed and optimized through numerical simulation. The proposed solar cell structure consists of a transparent window layer made of aluminum-doped zinc oxide, followed by an intrinsic zinc oxide layer, an n-type cadmium sulfide layer, and a p-type combined absorber layer of copper zinc tin sulfide (Cu2ZnSnS4) (CZTS) and copper zinc tin sulfur-selenium alloy (Cu2ZnSn(S,Se4)) (CZTSSe). The structure is further optimized by introducing two interfacial layers between CZTSSe/CZTS and CZTS/CdS. The highest conversion efficiency is achieved by adjusting the thicknesses of the layers, the doping densities in different layers, and the defect densities in the two absorber layers. The optimized model, with a total thickness of 2.01 μm, demonstrates an open-circuit voltage (Voc) of 0.7669 V, a short-circuit current (Jsc) of 48.57740 mA/cm2, a fill factor (FF) of 70.61%, and efficiency (η) of 26.31%. These results suggest that CZTS is a promising candidate for replacing other thin-film photovoltaic materials, such as CdTe. The proposed double-absorber thin-film solar cell optimizes doping concentration, thickness, and defect density to enhance performance metrics and efficiency while utilizing non-toxic materials to promote cost-effective, environmentally friendly energy solutions.

Improved Efficiency in WSe2 Solar Cells Using Amorphous InOx Heterocontacts.

van der Waals (vdW) layered materials have been shown to have excellent optoelectronic properties relevant to photovoltaics. Despite their promise, the demonstrated efficiencies of vdW material solar cells remain low and are seldom supported by statistics or spectral quantum efficiency analysis. In this study, we utilize a p-type WSe2 absorber, forming a solar cell with a transparent front InOx electron contact, and a rear Pd reflector/hole contact. We fabricate multiple devices providing statistics for 10 devices with an average 1 sun conversion efficiency above 5%, among which a champion efficiency of 6.37% is achieved. This is the highest AM 1.5G 1 sun efficiency reported for a vdW material solar cell, with a current density supported by external quantum efficiency analysis. This cell is also shown to have near unity quantum efficiency around λ = 600 nm. This work provides support to vdW materials being considered as serious candidates for future thin-film solar cells.

Comparison of type II superlattice InAs/InAsSb barrier detectors operating in the mid-wave infrared range

Four types of barrier detectors based on a type II InAs/InAsSb superlattice with a wide-gap barrier made of a solid AlInAsSb lattice matched to the GaSb buffer were compared. The tested detectors differed in the type of doping of the active layer and the level and type of doping of the contact layer at the barrier. The epitaxial layers were deposited on GaAs (100) substrates using the molecular beam epitaxy method. The spectral and current–voltage characteristics of the analyzed detectors were compared. The highest current responsivities were observed in the structure with a p-type absorber (p+BpN+). Detectors with an n-type absorber (p+Bnn+, n+Bnn+, and nBnn+) show an increase in the current responsivity with an increase in the reverse bias voltage due to the reduction in the undesirable barrier in the valence band. Arrhenius characteristics for the dark current show that only in nBnn+ detectors, it was possible to limit the generation–recombination current. These detectors at 150 K were characterized by the highest normalized detectivity of approximately 3 × 1011 cm · Hz1/2/W. The obtained results were compared with literature data, showing that the parameters of type II superlattice photodetectors are close to those of HgCdTe photodiodes according to the “Rule 07” and “Rule 22” principles.

Low-temperature aluminum doped and induced polysilicon and its application as partial rear contacts on p-type silicon solar cells

Low-temperature contact techniques offer benefits for various solar cells, including tandem solar cells, which risk degradation from high-temperature contact fabrication for top cell materials, such as in III-V/Si multijunction cells. This paper presents aluminum (Al) doped and induced poly-Si (formed through annealing at 190 °C for 5 min) as localized contacts on p-type silicon absorbers. This approach avoids the high-temperature processes required for poly-Si formation and the toxic gases involved in in-situ doping through chemical vapor deposition. Raman spectroscopy confirms poly-Si formation due to Al-induced recrystallization of a-Si:H(i), with electrochemical capacitance-voltage (ECV) measurements validating Al doping in the newly-formed poly-Si. The contact stack of p-Si/a-Si:H(i)/Al exhibits ohmic behavior after annealing at 190 °C for 5 min, achieving a contact resistivity of ∼13.2 mΩ⋅cm2. Finally, a cell featuring a localized Al-doped poly-Si contact structure is fabricated, realizing a 3% absolute efficiency improvement compared to a full-area Al back contact cell.

Exploration of Cd1−xZnxSe as a window layer for CIGS based solar cell with PEDOT: PSS as back surface field layer

This research investigates the potential of Cd1−xZnxSe thin film for photovoltaic applications. The electrical behavior of CIGS based solar cell is examined with the novel Cd1−xZnxSe as buffer layer material by Solar Cell Capacitance Simulator (SCAPS). The tunability of Cd1−xZnxSe facilities to reduce the defects between absorber and buffer layer by determining the ideal conduction band offset. It is revealed that cross-over occurs between the p-type absorber and the metal back contact if the metal work function is below 4.6 eV. In this research, a thin PEDOT: PSS back surface (BSF) layer was integrated which enhances the device efficiency from 22.5 percent to 28.32% while retaining the metal work function at 5.1 eV. The trade-off between the use of metal having higher work function and inclusion of heavily doped BSF layer is one of the important findings of this research. These findings pave the way for Cd1−xZnxSe to be commercially used as a buffer layer material for CIGS solar cell.

Role of CdTe Interface Structure on CdS/CdTe Photovoltaic Device Performance.

Glancing angle deposition (GLAD) of CdTe can produce a cubic, hexagonal, or mixed phase crystal structure depending upon the oblique deposition angles (Φ) and substrate temperature. GLAD CdTe films are prepared at different Φ at room temperature (RT) and a high temperature (HT) of 250 °C and used as interlayers between the n-type hexagonal CdS window layer and the p-type cubic CdTe absorber layer to investigate the role of interfacial tailoring at the CdS/CdTe heterojunction in photovoltaic (PV) device performance. The Φ = 80° RT GLAD CdTe interlayer and CdS both have the hexagonal structure, which reduces lattice mismatch at the CdS/CdTe interface and improves electronic quality at the heterojunction for device performance optimization. The device performance of HT CdS/CdTe solar cells with Φ = 80° RT with 50 to 350 nm thick GLAD CdTe interlayers is evaluated in which a 250 nm interlayer device shows the best device performance with a 0.53 V increase in open-circuit voltage and fill-factor product and a 0.73% increase in absolute efficiency compared to the HT baseline PV device without an interlayer.

DLTS Study of Defects in HgCdTe Heterostructure Photodiode

Deep-level transient spectroscopy (DLTS) measurements were performed on HgCdTe heterostructure photodiode grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrate. In order to extract defects from individual layers of the heterostructure, three consecutive etchings were performed. In the first experiment, the N+/T/p/T/P+/n+ structure was chemically etched to the N+ bottom contact to obtain a mesa-type detector. Six localized defects were extracted across the entire photodiode. In the second experiment, the bottom contact was made to the p-type absorber. Two localized defects were found in the p/T/P+/n+ structure. In the third experiment, the top layers were removed and N+/T/p type detector was made—the cap contact was made to the p-type absorber, the bottom to the N+ layer. Five defect levels were identified, three of which overlap with the first experiment. A deep-trap level located at 183 meV above the top of the valence band was identified within the absorber bandgap. This energy coincides with the activation energy determined from the Arrhenius plot for the dark currents. A defect at the level of ∼0.5Eg suggests that the dark current at low reverse bias voltage is dominated by the Shockley–Read–Hall (SRH) mechanism.

High Speed 1550 nm Indium Gallium Arsenide-Indium Phosphide Photodetector

This paper presents preliminary results of a high speed 1550 nm indium gallium arsenide (InGaAs)-based mesa-type modified uni-traveling carrier photodiode (M-UTC-PD) structure. Conventional UTC-PD refers to P-I-N type photodiodes which selectively use electrons as active carriers. Photons absorbed in the relatively thin P-type absorber create minority carriers which are field accelerated toward a depleted collector thereby establishing high velocity ballistic transport, making these structures applicable for high speed applications. The M-UTC-PD structure presented uses spatially tailored P-type absorber regions to limit minority carrier generation both in the lateral and axial dimensions. Utilizing an otherwise conventional UTC-PD epitaxial structure where the top P-type layers are undoped, the spatially tailored P-type regions are defined by closed ampoule Zinc diffusion techniques. The M-UTC-PD structure presented utilizes a series of nested p-doped rings within a mesa structure to limit dark current and reduce overall capacitance to improve high speed operation. Two photodiode structures will be investigated for this research project, a conventional UTC-PD structure and a modified structure, utilizing similar device designs, epitaxial designs and fabrication processes. The conventional structure will be utilized for fabrication process development, verification of epi quality and development of rapid prototyping approach toward chip-based testing and subsequent high speed RF testing procedures. Conventional UTC-PD device results will be used as a comparison to quantify the performance of the M-UTC-PD structure utilizing Zn-doped defined p-type absorber regions. Results are given for chip tests of UTC-PD chips verifying epitaxial quality and fabrication process, subsequent testing of packaged devices and RF analysis remains. Process development of the Zn-doped devices is underway, once completed, these devices will be compared to the base design to quantify performance enhancement associated with the modified design.

Photoluminescence Study of As Doped p-type HgCdTe Absorber for Infrared Detectors Operating in the Range up to 8 µm

A HgCdTe photodiode grown by chemical vapor deposition (MOCVD) on a GaAs substrate operating in the long-wave infrared (LWIR) range was characterized using photoluminescence (PL) measurements. At high temperatures, the PL spectrum originates from a free-carrier emission and might be fitted by a theoretical expression being the product of the density of states and the Fermi–Dirac distribution. At low temperatures, the PL spectrum consists of multiple emission peaks that do not originate solely from the energy gap. Such spectra are not unambiguous to interpret due to the prominence of different optical transitions. Spectral response (SR) measurements were used to determine the energy gap (Eg) and extract the band-to-band transition from the PL spectra. PL peaks visible within the band gap were fitted by a Gaussian distribution. To identify the sources of individual emission peaks, excitation power dependence analysis was conducted. Band-to-band, free-to-bound, acceptor-bound exciton, and defect-bound exciton transitions were identified. At low temperatures, transitions are mainly impurity-related, with shallow impurity levels estimated to be 6 meV and 16 meV for the donor and acceptor, respectively, while deep-level impurities were associated with VHg. The latter transition with an energy of about 78 meV does not vary with temperature. Its relative positions with respect to the energy gap is 0.8 Eg at 18 K and 0.67 Eg at 80 K.

Transformations and Environmental Impacts of Copper Zinc Tin Sulfide Nanoparticles and Thin Films.

Quaternary chalcogenide copper zinc tin sulfide (CZTS) nanoparticles are used to make the p-type absorber layer in CZTS solar cells, which are considered more benign alternatives to those based on cadmium telluride (CdTe) and less expensive than copper indium gallium selenide. CZTS has an ideal band gap and a high absorption coefficient for solar radiation, making the nanoparticles an attractive option for photovoltaic cells. In this work, we explore the toxicity of CZTS nanoparticles using an environmentally relevant bacterial model Shewanella oneidensis MR-1. This study also focuses on understanding the stability of CZTS-based thin films and their direct interaction with bacterial cells. Bacterial cell viability, stability of nanoparticles and thin films, as well as mechanisms of toxicity were evaluated using various analytical tools. The CZTS nanoparticle suspensions show significant acute toxic effects on bacterial cells, but long-term (72 h) exposure of bacterial cells to CZTS-based thin films (made from nanoparticles) do not exhibit similar detrimental impacts on bacterial viability. This result is compelling because it suggests that CZTS nanomaterials will have minimal unintended toxicity as long as they are incorporated into a stable film structure.

High-speed and low dark current InGaAs/InAlAs avalanche photodiodes with P-type absorption layers

High-speed and low dark current InGaAs/InAlAs avalanche photodiodes with P-type absorption layers

Defect engineering enabling p-type Mo(S,Se)2:TM (TM = V, Nb, Ta) towards high-efficiency kesterite solar cells

One important issue limiting the development of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the severe recombination loss at CZTSSe/Mo(S,Se)2/Mo back contact, primarily arising from the non-matched potential barrier and inferior electric contact between absorber and weak n-type Mo(S,Se)2 interfacial layer. It is expected that a p-type Mo(S,Se)2 can effectively improve the hole extraction and enhance the device performance. In this work, we propose a plausible direction to build up an electrically benign back surface field by in-situ doping Mo(S,Se)2 with group VB elements (TM = V, Nb and Ta). It is turned out that the generated VMo, NbMo, or TaMo shallow acceptors could convert Mo(S,Se)2 from weak n-type to p-type conductivity and increase its work function (WF) and hole concentration. With the contact of p-type absorber (WF,absorber < WF,Mo(S,Se)₂), a benign ohmic contact with favorable downward band bending is formed, thereby accelerating the holes extraction and minimizing interface recombination loss. With significant gains in open-circuit voltage (Voc) and fill factor (FF), the Mo(S,Se)2:Ta device finally increases the efficiency from 10.82 % to 12.72 %. This convenient in-situ Mo(S,Se)2 doping strategy is different from previous complicated or unstable cases and should serve as a basis for high-quality back contact engineering in kesterite photovoltaics.

The effect of polyvinyl alcohol (PVA) on the optical, electrical and solid state properties of copper zinc tin sulfide (CZTS) deposited by sensitive spray pyrolysis (SSP)

Recently, the use of CZTS as the basis for other generation of low cost thin films solar cells has stimulated further researches. Its excellent p-type absorber nature, relatively high absorption coefficient and ideal energy band-gap of 1.5eV motivated these efforts. Additionally, CZTS consist of earth-abundant, cheap and non-toxic elements with very low manufacturing cost. Initially, copper indium gallium selenide (CIGS) solar cell device emerged but suffered limitations in further development because of rare indium and gallium in the device structure therefore, CZTS is recently preferred as an alternative to CIGS commercial solar cell absorber layer. In this work, solution mixture of CZTS and PVA was deposited on a substrate at temperature of 150 °C. Sensitive spray pyrolysis was used to grow the thin films where calculated amount of the precursor mixture was allowed to fall and be deposited on a heated substrate to form CZTS/PVA thin films. Subsequently, the thin film samples were annealed at a temperature of 200oCfor 1 h to achieving pure crystalline thin film formation. SEM, XRD analysis, Optical, Solid State properties and Raman analysis were studied. The XRD analysis showed that the thin films fell into the pure kesterite structure of CZTS. Results show that produced thin films exhibited higher absorption coefficient and optical conductivity than pure CZTS, 106 m−1 and 1014(S−1) against 104cm−1 and 1012(S−1) respectively. The band-gap is between 1.53eV and 1.73eV. Using a PVA concentration of 0.05 M yielded highest absorbance and optical conductivity with lowest real dielectric constant and transmittance. These improved optical, electrical and solid state properties suitably qualify these thin films as absorber layer material for solar cell applications.

Numerical Simulation of Non-toxic ZnSe Buffer Layer to Enhance Sb2S3 Solar Cell Efficiency Using SCAPS-1D Software

The use of renewable energy, especially solar photovoltaic, has grown more and more necessary in the context of the diversification of the use of natural resources. Sb2S3 is emerged as an attractive candidate for today's thin-film solar cells due to its band gap of 1.65 eV and high absorption coefficient greater than 105 cm-1. Cadmium Sulfide is the most commonly used buffer layer material in thin film solar cells, but cadmium is a metal that causes severe toxicity in humans and the environment. This article tried to avoid cadmium for solar cell generation. This paper presents the findings of a computer simulation analysis of a thin film solar cell based on a p-type Sb2S3 absorber layer and an n-type ZnSe buffer layer in a structure of (Sb2S3/ZnSe/i-ZnO/ZnO: Al) utilizing simulation software (SCAPS-1D). The simulation included detailed configuration optimization for the thickness of the absorber layer, buffer layer, defect density, temperature, and series-shunt resistance. In this work, the Efficiency (η), Fill Factor (FF), Open-circuit Voltage (Voc), and short-circuit current (Jsc) have been measured by varying thickness of absorber layer in the range of 0.5µm to 4 µm and by varying thickness of buffer layer in the range of 0.05 µm to 0.1µm. The optimized solar cell shows an efficiency of 20.03% when the absorber layer thickness is 4µm and the buffer layer thickness is 0.08µm.

Design of a High-Speed PIN Photodiode With a Gradually-Doped Composite Absorption Layer for 100 Gbit/s Optical Receivers

In this paper, we propose a high-speed PIN photodiode with a composite absorption layer. It supports a 3-dB bandwidth of 43 GHz and can be used in 100 Gbit/s optical receivers. The composite absorption layer consists of a gradually-doped p-type absorption layer and an undoped absorption layer. It has different carrier transport mechanisms from the traditional PIN-PD with a single intrinsic absorption layer. The gradually-doped p-type absorption layer accelerates electron diffusion in the p-type absorption layer. Comparison of three kinds of PIN photodiodes with different absorption layers shows that the proposed PIN-PD provides the highest bandwidth while ensuring satisfactory responsivity. The simulation used a numerical model, calculated by Crosslight APSYS software. The results were compared with the analytical method and obtained a good agreement. This work offers a novel structure and good guidance for the design and optimization of photodiodes.

Spectroscopic ellipsometry and photovoltaic characteristics for n-CdS/p-Cu2ZnSnS4 heterojunction by annealing for solar cells

Owing to its direct bandgap in the range to be used as an absorbent material and due to its high absorption rate, kesterite Cu2ZnSnS4 (CZTS) is a p-type prospective absorber material (with thickness 500 nm) for solar cell applications. Kesterite Cu2ZnSnS4 (CZTS) thin films were prepared using the thermal evaporation technique. The (CZTS) thin films were annealed at different annealing temperatures chosen according to TGA analysis in the range of (400o C, 450o C, 475o C, and 500o C). The influence of annealing temperatures on the structural, morphology and optical properties of the CZTS films was investigated. The XRD patterns and Raman spectra have revealed the formation of CZTS thin with a high-quality crystal structure. The optical constants refractive index n, and extinction coefficient, k consequently band gap are estimated from SE via construction an optical model. The refractive index n of the CZTS /glass films received from SE model increases with the annealing temperature that is credited to the rise of the size of the crystal. It was also found that when the annealing temperature of the CZTS layer increases, the general behavior of the extinction coefficient k of the CZTS /glass film increases. In addition, it is found that the direct optical transition with energy band gap is compact from 1.75 eV at RT to 1.49 eV at maximum crystallization 500 o C. The Ni/n-CdSe/p-CdTe/Pt heterojunction has been successfully assembled. The dark and illumination (currentvoltage) behavior of fabricated heterojunctions had been suggested at distinctive different annealing of CZTS layer, as well as for voltages ranging from -2 to 2 volts.

Copper-Arsenic-Sulfide Thin-Films from Local Raw Materials Deposited via RF Co-Sputtering for Photovoltaics.

The inexorable increase of energy demand and the efficiency bottleneck of monocrystalline silicon solar cell technology is promoting the research and development of alternative photovoltaic materials. Copper-arsenic-sulfide (CAS) compounds are still rather unexplored in the literature, yet they have been regarded as promising candidates for use as p-type absorber in solar cells, owing to their broad raw material availability, suitable bandgap and high absorption coefficient. Here, a comprehensive study is presented on the structural and optoelectronic properties of CAS thin-films deposited via radio-frequency magnetron co-sputtering, using a commercial Cu target together with a Cu-As-S target with material obtained from local resources, specifically from mines in the Portuguese region of the Iberian Pyrite Belt. Raman and X-ray diffraction analysis confirm that the use of two targets results in films with pronounced stoichiometry gradients, suggesting a transition from amorphous CAS compounds to crystalline djurleite (Cu31S16), with the increasing proximity to the Cu target. Resistivity values from 4.7 mΩ·cm to 17.4 Ω·cm are obtained, being the lowest resistive films, those with pronounced sub-bandgap free-carrier absorption. The bandgap values range from 2.20 to 2.65 eV, indicating promising application as wide-bandgap semiconductors in third-generation (e.g., multi-junction) photovoltaic devices.

Insights into post-growth doping and proposals for CdTe:In photovoltaic devices

This paper is motivated by the potential advantages of higher doping and lower contact barriers in CdTe photovoltaic devices that may be realized by using n-type rather than the conventional p-type solar absorber layers. We present post-growth doping trials for indium in thin polycrystalline CdTe films using the diffusion of indium metal with indium chloride. Chemical concentrations of indium up to 1019 cm−3 were achieved and the films were verified as n-type by hard x-ray photoemission. Post-growth chlorine treatment (or InCl3) was found to compensate the n-doping. Trial structures comprising CdS/CdTe:In verified that the doped absorber structures performed as expected both before and after chloride treatment, but it is recognized that this is not an optimum combination. Hence, in order to identify how the advantages of n-type absorbers might be fully realized in future work, we also report simulations of a range of p–n junction combinations with n-CdTe, a number of which have the potential for high V oc.

Mid-Infrared HgCdTe Heterostructure and Its Potential Application to APD Operation

Mid-wave infrared HgCdTe photodiode grown by metal-organic chemical vapor deposition was characterized for its use in avalanche operation. N^&#x002B;/N/p/P/P^&#x002B;/n^&#x002B; heterostructure meets the assumptions of the separated absorption and multiplication design. The p-type absorber, with an assumed Cd molar composition of 0.305, was optimized to operate up to <inline-formula> <tex-math notation="LaTeX">$4.4 ~\mu \text{m}$ </tex-math></inline-formula> at 230 K. The multiplication region is contained within the N layer with a wider energy gap (<inline-formula> <tex-math notation="LaTeX">$\text{x}_{\textit {Cd}}=0.33$ </tex-math></inline-formula>). Satisfactory gain of &#x007E;50 was achieved for a moderate reverse bias of &#x2212;3.5 V and a temperature of 160 K.

Long-wavelength InAs/GaSb superlattice double heterojunction infrared detectors using InPSb/InAs superlattice hole barrier

We demonstrate two double heterojunction long-wavelength infrared detectors based on InAs/GaSb superlattice on InAs substrates grown by metal-organic chemical vapor deposition. In the two structures, the hole barrier employs a novel InPSb/InAs superlattice to achieve conduction-band alignment, while the electron barrier is InAs/GaSb superlattice to achieve valence-band alignment. Two devices with n-type absorber layer and p-type absorber layer exhibit cut-off wavelengths of ∼10.4 μm and ∼12.2 μm, dark current densities of 9 × 10−4 A cm−2 and 2 × 10−2 A cm−2, and specific detectivities of ∼1.7 × 1010 cm Hz1/2 W−1 and ∼1.5 × 1010 cm Hz1/2 W−1, respectively. The device with n-type absorber has a lower dark current due to the natural valence-band alignment, but it has a low quantum efficiency (QE) resulting from the use of n-type absorber layer. In contrast, the device with p-type absorber has a higher dark current that can be possibly attributed to the conduction-band misalignment, but it achieves a higher QE due to the benefits from the p-type absorber.