Table of contents

A selective fabrication method for monoclinic-scheelite (m-s) BiVO₄ and tetragonal-zircon (t-z) BiVO₄ thin films using radio fRequency (RF) sputtering from a single target was developed. The kinetic energy of the sputtered atoms was controlled by varying the sputtering power to obtain BiVO₄ films with m-s and t-z crystalline phases. Although the band gap of the t-z BiVO₄ phase (3.0 eV) was larger than that of m-s BiVO₄ (2.5 eV), the deposited t-z BiVO₄ films showed a comparable photocurrent density (1.5 mA cm⁻²) at 1.23 V versus the reversible hydrogen electrode (400 W Xe lamp). This was mainly because of the reduced sputtering damage in the t-z BiVO₄ crystal, which originated from the low sputtering power as well as the deep valence-band position in t-z BiVO₄ that enabled the efficient utilization of the photocarriers. This work provides insights into crystalline phase control using the particle kinetic energy in sputtering.

In electrochemical CO₂ reduction reactors, polymer electrolyte membrane (PEM) reactors, also known as zero-gap cells, have great potential for achieving significant CO₂ reduction. Because these cells have a thin reactor core with a thickness of several hundred micrometers, it is difficult to determine their internal voltage distribution. To determine the anode voltage, ohmic loss in the membrane, and cathode voltage in the PEM reactors, we set three reference electrodes in the reactor and investigated the voltage values obtained from each reference electrode. We demonstrated that the reference electrode in contact with the anion exchange membrane extending to the outside of the cell provides the most reliable voltage. The voltage measured by this reference, combined with the resistance of the exchange membrane obtained through electrochemical impedance spectroscopy, provides a breakdown of the voltage inside the cell.

We applied hot-carrier extraction to solar cells and photocatalysts used for artificial photosynthesis including water splitting and CO₂ reduction, and elucidated the differences between these two applications: hot-carrier solar cells (HC-SCs) and hot-carrier photocatalysts (HC-PCs) by detailed balance calculations. The hot-carrier effect in the photocatalysts is less significant than that in the solar cells, because of the larger bandgaps required for generating sufficiently high-energy carriers consumed for the reactions. On the other hand, impact ionization and Auger recombination (IA) improves the efficiency of the HC-PCs more notably, because the IA functions like photon upconverters and hence narrows the optimal bandgap. Furthermore, the IA improves the spectral robustness by eliminating the constraint of the particle-number conservation for both the HC-SCs and HC-PCs. These benefits of the IA are contrasting with the well-recognized fact that the IA only reduces the carrier number and consequently lowers the efficiency of the conventional counterparts.

In this work, the effects of He ion and electron beam irradiation on substrate-type CdTe solar cells that are suitable for the detection of alpha and beta radiation are investigated. He ion irradiation revealed that the induced current proportionally increased with the increase of He ion current and that it is possible to detect alpha radiation using a substrate-type CdTe solar cell dosimeter. Next, the electron irradiation effects were investigated. The induced current proportional to the electron flux was also observed, and the sensitivity depended on the electron energy. In addition, the degradation of a substrate-type CdTe solar cell dosimeter by electron beam irradiation was investigated. No significant degradation in short-circuit current was observed at the electron irradiation of 3 × 10¹⁶ cm⁻² at 200 keV and 1 × 10¹⁶ cm⁻² at 400 keV. This result suggests that the substrate-type CdTe solar cell dosimeter is sufficiently resistant to electron irradiation.

We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films composed of sol–gel glass with Zn-based nanoparticles (NPs) dispersed on the characteristics of Si solar cells. Their internal quantum efficiencies (IQEs) are successfully measured by separating the contributions of downshifted photons in measuring reflectance for 300–400 nm, wavelengths of incident photons absorbed by the NPs. We find that IQEs for this wavelength range are more enhanced by employing thicker LDS films, i.e. LDS films with higher optical densities. We also discuss the relationship between the number density of NPs in LDS films, their optical properties, and the IQEs of cells. We observe a discrepancy between the measured and calculated IQEs and note that this is the result of downshifted photons escaping across the sides of the LDS films.

The additive effect of a gadolinium ion into a formamidinium lead iodide (FAPbI₃) perovskite crystal on electronic structures and molecular dynamics was investigated for improving photovoltaic performance with stability. The electronic structures, band structure, partial density of state, and molecular dynamics were determined by first-principles calculation. The band distribution and charge transfer between the 5d orbital of the gadolinium atom, the 5p orbital of the iodine atom, and the 6p orbital of the lead atom promoted the carrier generation and diffusion related to short-circuit current density. The enthalpy and kinetic energy prompted stabilization of the gadolinium-doped crystal with a slight distortion of coordination structure, as compared with the decomposition of the FAPbI₃ crystal. Diffusion coefficients of iodine and lead ions in the FAPbI₃ crystal with defect were increased, predicting decomposition. The gadolinium-doped FAPbI₃ perovskite crystal has great potential for applications in photovoltaic devices by improving photovoltaic performance.

It has been attempted to preferentially orientate Pb-I layers in two-dimensional (2D) organic-inorganic hybrid perovskite thin films (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)Pb₂I₇ perpendicular to substrates only by thermal annealing after spin coating of a reagent solution for improvements in the energy-conversion-efficiency of solar cells. It is found from X-ray diffraction measurements that the ratio of diffraction intensity from the (202) plane to that from the (060) plane becomes larger in thermally annealed (50 °C–135 °C) samples. This indicates that the Pb-I layer tends to grow perpendicular to the surface of the substrate. In particular, the ratio has reached 8.2, which is larger compared with the ratio of 2.7 for the randomly oriented powder sample, for the sample prepared on SnO₂ substrates. Such (202) oriented films seem to contribute to improvements in the energy-conversion-efficiency of tandem-type solar cells utilizing the 2D perovskite thin films as an active layer of the top cell.

GaAs:N dilute nitride intermediate band solar cells (IBSCs) with different barrier heights for blocking electron transport from the intermediate band (IB) were prepared and the role of such an electron blocking layer (EBL) was investigated. It is found that the EBL effectively confines electrons in the IB states, and the two-step photocurrent generation process is strongly affected by its barrier height. A rate equation analysis well reproduced experimental observations of the applied voltage dependence of the two-step photocurrent generation process. It revealed that the electron extraction both from the conduction band and the IB through the EBL is governed by the thermionic emission process. To meet the requirements of an ideal IBSC, optimized device structures including the barrier height of the EBL have to be designed properly taking into account the material parameters and carrier dynamics under the actual operating condition.

The use of graphite substrates has been demonstrated in thin-film Cu₂ZnSnS₄ (CZTS) solar cells and can serve as alternative electrodes for next-generation, thin-film solar cells. From the early stages of CZTS development, Mo-coated substrates composed of copper (Cu), zinc (Zn), tin (Sn), and sulfur (S) have been employed for stability at high temperature. However, Mo has become a rare metal in recent years; correspondingly, there are potential risk of supply shortages and depletion problems. We focus herein on graphite as an electrode and substrate owing to its versatility and low cost. The CZTS precursor was prepared by radiofrequency sputtering. Subsequently, NaF was deposited using the E-B vapor deposition method to control the Na composition ratio. CZTS films were obtained by gas-phase sulfurization at 898 K for 1 h. CZTS solar cells with MgF₂/Al/AZO/CdS/CZTS/graphite structure were prepared, and their characteristics were evaluated. J–V measurement of the precursor with a 20 nm thick NaF yielded η = 2.37%, V_oc = 543 mV, J_sc = 10.7 mA cm⁻², and FF = 40.8%. These results suggest that CZTS on graphite with NaF control has the potential for realizing the low cost CZTS solar cells.

A color-control technique based on optical thin films on glass is investigated by means of numerical simulation and experiment, with a particular focus on building-integrated photovoltaic (BIPV) applications. We show that white-colored BIPV modules can be realized with a minimum optical loss by applying optical thin films that have multiple reflection peaks in a complementary color relationship, like the emission spectrum of white LEDs. The angular dependence of hue, which is an inherent drawback of optical thin film systems, is suppressed by modifying the design of the film to maintain the complementary color relationship of multiple reflection peaks. The simulated thin film design is confirmed by an experiment with a silicon solar cell, resulting in a white color and a low short-circuit current density loss (ΔJ_SC) of 5.9%. These results indicate that our approach is a promising way to realize fascinating colors and a high energy yield simultaneously in BIPV modules.

A composition ratio prediction model for BaSi₂ thin films deposited by thermal evaporation was constructed using machine learning. BaSi₂ was prepared by thermal evaporation in a vacuum chamber, and the composition ratio was measured by energy-dispersive X-ray spectroscopy. The results show that the composition ratio is affected by various experimental parameters. To consider these parameters, kernel ridge regression was performed with Si/Ba ratio as the objective variable, and with experimental parameters as explanatory variables. A good fitting result was obtained by kernel ridge regression. The next step was to select a kernel function. We evaluated four types of kernel functions, and confirmed that two of them, the polynomial kernel and the sigmoid kernel, have relatively high prediction accuracy. Then we investigated different combinations of explanatory variables and found the best combination with the highest generalization performance. From the above, a composition ratio prediction model with a mean absolute error of less than 0.2 was obtained.

Herein, we have studied the exciton dynamics of a novel fused ring π-conjugated molecule (YS3) in solution and film states by spectroscopic measurements. This molecule incorporates dithienonaphthobisthiadiazole as a core unit that is a two-dimensionally π-extended fused ring. As a result, we found a long exciton lifetime in YS3 films originating from reduced radiative and nonradiative transitions. This is partly because radiative deactivation is effectively suppressed because of the dipole-forbidden transition in H-aggregates and partly because rotational deactivation is effectively suppressed in the crystalline film state.

Perovskite solar cells are expected to be applied as photoreceivers for high-efficiency optical wireless power transfer for electric vehicles. The use of aluminum gallium nitride (AlGaN) as an electron transport layer (ETL) for wide-gap perovskite solar cells is hereby proposed in this paper. The electrical properties and energy-band alignment of AlGaN deposited by either hydride vapor phase epitaxy or metal-organic CVD are investigated. AlGaN shows a higher conduction band level than conventional ETL materials. Simulation of the performance of a perovskite solar cell with CH₃NH₃PbBr₃ as the absorbing layer and AlGaN as the ETL was performed using a solar-cell capacitance simulator. The results suggest that AlGaN increases the power conversion efficiency of the solar cell by improving the conduction band offset between the perovskite layer and the ETL.

Radiation tolerance of Cu(In,Ga)Se₂ (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 10¹⁶ MeVn_eq cm⁻², however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

To avoid formation of the photo-inactive δ-phase of formamidinium-cesium lead triiodide, copper or germanium was added to the perovskite compounds to stabilize the photoactive α-phase. It was found that the substitution of lead by germanium (Ge) or copper (Cu) provided the stabilization of the α-phase in the present work. The first-principles molecular dynamics calculations indicated that displacements of formamidinium molecules were suppressed by the Ge doping. X-ray diffraction results indicated that the Ge or Cu doping of the perovskite compounds could be effective for suppression the phase transition from α- to δ-phase.

Copper iodide (CuI) is under extensive research due to its low cost, easy fabrication process, and wide bandgap. This research includes the fabrication of perovskite solar cells using the p–i–n structure (inverted structure) with a focus on the hole transport layer (HTL) layer. In this paper, we demonstrate the applicability of using CuI as a HTL in perovskite solar cells using the iodine/ethanol solution method. Using the iodine/ethanol solution for preparing the CuI, a power conversion efficiency of 0.76%, a short-circuit current density of 4.56 mA cm⁻², an open-circuit voltage of 0.494 V as well as a fill factor of 0.34 were obtained. The overall performance of the solar cell still requires much improvement. We have successfully deposited the CuI using RF magnetron sputtering and the iodine/ethanol solution method and understand that the low performance of the device is mainly due to the voids and gaps present within the CuI layer.

Electron beam induced current (EBIC) measurements have been widely used to investigate charge carrier collection in Cu(In,Ga)Se₂ solar cells. However, we found that this electron beam irradiation could significantly change the EBIC signal intensity during the measurement. In this study, the charge state variation of the V_Se–V_Cu divacancy proposed by Lany et al. was introduced into the device simulator to explain the phenomenon. In the simulation, the defects take on three different charged states, i.e. positive, neutral, and negative states, where their transitions are affected by the quasi-Fermi level position in the bandgap. The transient response of the EBIC signal was successfully explained by incorporating these complex state defects.

We clarified the design guides for H₂- and CO-producing artificial photosynthetic devices. The combination of a voltage-matched (VM) tandem solar-cell (SC) module and an electrochemical (EC) module was adopted. The parallel-connected top and bottom SC modules, in which multiple organic–inorganic hybrid perovskite (PVK) SCs with a bandgap of 1.7 eV and crystalline-silicon SCs were connected in series, respectively, powered the EC module consisting of series-connected multiple EC reactors. It was found that the design parameters of the series connection numbers must be optimized under slightly greater solar intensity and higher temperature than the average values to minimize the mismatch between the device operating voltage and SC maximal power voltage. This is in contrast to that the annual electricity production of the VM SC module coupled with a power conditioner is not sensitive to the optimization conditions. Increases in the bandgaps of the PVK SCs do not affect the annual production significantly.

The degradation of surface passivation performance by metallization is a challenge in realizing highly efficient crystalline Si solar cells that use novel carrier-selective contacts. Here, we report on a simple method to study the effect of metallization on passivation of titanium oxide (TiO_x)/Si heterostructures. We investigated the relationship between the implied open-circuit voltage (iV_OC) and the photoluminescence (PL) intensity imaging of solar cell precursors before metallization. Based on the relationship obtained, the change of the iV_OC before and after metallization on the TiO_x was evaluated quantitatively. The results showed that the iV_OC predicted by the PL measurement decreases by 23–104 mV after metal deposition and shows a good agreement with the measured V_OC in the finished solar cells. These results demonstrate that the iV_OC evaluation by PL measurement provides a good prediction of the V_OC after metallization, which is useful in analyzing the passivation degradation induced by metallization.

Cu₂ZnSnS₄ (CZTS) is an attractive material for thin film solar cells because all its constituents are Earth-abundant elements, and it's a direct transition semiconductor with a band gap energy of 1.5 eV that is suitable for absorbing solar light spectrum effectively. CZTS is generally formed by precursor formation followed by heat treatment at 500 °C–600 °C to enhance the growth of crystal grain. In this work, a novel CZTS crystal grain re-growth process using post-laser annealing was investigated. 445 nm wavelength laser irradiation was performed on the Al-doped ZnO/CdS/CZTS/Mo/substrate stacked structure. X-ray diffraction and scanning microscope showed the CZTS crystal grain enlargement. Solar cells were fabricated on those structures and the external quantum efficiency was found to be improved especially at 500–1000 nm wavelength light absorption. That resulted in a short circuit current improvement.

29.2%-conversion efficiency of a two-terminal (2T) perovskite/crystalline Si heterojunction tandem solar cell using 145 μm thick industrial Czochralski (CZ) Si wafer is obtained. The structural optimization, such as surface passivation of the perovskite layer and better light management techniques, improved power conversion efficiency (PCE). To our knowledge, this PCE is the best in 2T-tandem solar cells using CZ wafers. Towards industrialization, crucial issues with the 2T tandem solar cells with crystalline Si bottom cell are discussed. Four-terminal (4T) tandem solar cells are evaluated as an approach to avoid the crucial issues. Examining our base technologies which realize 22.2%-conversion efficiency perovskite single junction solar cell module and 26%-heterojunction back-contact solar cells, we clarified that the based technologies were ready to realize 30%-conversion efficiency 4T perovskite/heterojunction crystalline Si tandem solar cells with approximately quarter size of an industrial crystalline Si solar cell (∼64 cm²).

We report on the effective passivation of cut edges of n-type (100) crystalline silicon by forming thin oxide layers achieved by heat treatment in liquid water at 90 °C for 2 h followed by heating in an air atmosphere at 300 °C for 1 h. The mechanical cut with the (110) oriented cleaved edge markedly decreased the photo-induced effective minority carrier lifetime τ_eff to 6.9 × 10⁻⁴ s, which was 0.22 times the initial value of 3.2 × 10⁻³ s, and which was maintained by the region 0.5 cm away from the edge. The present passivation treatment resulted in the reduction of τ_eff to 0.43, with τ_eff values of 4.0 × 10⁻⁴ s at the edge and 9.4 × 10⁻⁴ s at 0.2 cm from the edge. The analysis with a simple model of carrier diffusion in the lateral direction resulted in the recombination velocity at the cut edge, which was initially higher than 2000 cm s⁻¹, being decreased to 50 cm s⁻¹ by the present treatment, while the recombination velocity at the sample surface was increased from 8 (initial) to 46 cm s⁻¹, probably due to the field-induced depletion effect.

The stability and performance of photovoltaic (PV) modules can be assessed by outdoor testing where external conditions such as illumination and module temperature are measured at regular time intervals along with the jV-curve of the module. However, the fluctuation and seasonal variation of external conditions can make it difficult to trace changes such as degradation in PV-module properties (at e.g. standard test conditions). This contribution demonstrates the use of multiple linear regressions (MLR) to overcome these difficulties. The data gathered over large periods is condensed into a set of few predictors, that reproduce the jV parameters at infrequently encountered conditions that are required for comparison. Furthermore, the parameters of a physical device model are calculated directly from MLR-predictors, validating our procedure two-fold, by applying the MLR-method to simulated data, replicating the original input parameters, and comparing monthly parameter averages between the MLR-method and a known parameter extraction method.

Tunnel oxide passivated contact (TOPCon) structures using highly doped n-type polycrystalline silicon were fabricated using facing target sputtering and ion implantation techniques for a SiH₄-free fabrication process of high-efficiency silicon solar cells. We investigated the structural and electrical properties of the highly doped n-type poly-Si layers to optimize the ion implantation process. We also investigated the surface passivation quality of our TOPCon structure. An effective carrier lifetime of 2.01 ms and an implied open circuit voltage of 704 mV were obtained for our sample annealed at 950 °C. The sample also exhibits a low contact resistance of 3.22 × 10⁻³ Ω cm⁻². Our results open the way for SiH₄-free fabrication of silicon solar cells with a TOPCon structure.

A novel concept of unencapsulated modules was developed to avoid many degradation phenomena originating from encapsulants, reduce material costs, and also allow for both easy cell repair and easy recycling of modules. The reliability and durability of the novel concept modules were investigated using damp-heat (DH) testing, thermal-cycle (TC) testing, and sequential testing including DH and TC testing. No large reduction in maximum power after DH testing for 2700 h or TC testing for 1000 cycles was found for unencapsulated modules, irrespective of cell-connection method, cell spacing, or the existence of intentional microcracks. However, because of thermomechanical stress, unstable contact between interconnector ribbons and busbar electrodes was found after TC testing. Superiority of shingling connections was found for this novel concept of unencapsulated modules.

We investigated the effect of the B₂H₆ plasma treatment on p-type hydrogenated amorphous silicon (p-a-Si:H) surfaces for high-performance silicon heterojunction (SHJ) solar cells. Secondary ion mass spectroscopy measurements revealed that the boron concentration at the p-a-Si:H surface is increased by employing the B₂H₆ plasma treatment. Furthermore, specific contact resistance is decreased by about one-third after the B₂H₆ plasma treatment. No degradation of passivation performance is induced by the B₂H₆ plasma treatment. The power conversion efficiency of the SHJ solar cells with the B₂H₆ plasma treatment is improved by the increase in fill factor (FF) due to decreased series resistance and increased shunt resistance. From numerical simulations, the upward band bending is enhanced at the heterointerface between transparent conductive oxide (TCO) and p-a-Si:H by the B₂H₆ plasma treatment, which is responsible for the improved FF owing to facilitated tunneling holes from c-Si to p-a-Si:H layers and the TCO/p-a-Si:H heterointerface.

A Zn(O, S) thin-film is deposited utilizing an open-air CVD method by evaporating zinc-diethyldithiocarbamate, which is a non-vacuum and dry process. In an X-ray diffraction measurement, it is revealed that the films have a wurtzite structure and an [O]/([O] + [S]) ratio of 10%. A bandgap energy of 3.1 eV is estimated from the transmittance and reflectance spectra. By applying the Zn(O, S) as an n-type buffer layer, Cu(In, Ga)Se₂ solar cells are fabricated. In the current density–voltage characteristics, distortion is observed at the bias voltages above the open-circuit voltage. It is implied that a large conduction band offset exists at a Zn(O, S)/CIGS interface. A quantum efficiency spectrum in the wavelength region of 380–512 nm is improved compared to a traditional CdS buffer layer. Finally, a 9.2%-efficient CIGS solar cell is demonstrated utilizing the Zn(O, S) buffer layer through an all-dry process.

Cu₂SnS₃ (CTS), obtained by depositing Au on an Sn/Cu metal stacked precursor fabricated by electron beam deposition and sulfurization, was investigated. In thin films obtained by sulfurization at 560 °C of the precursor with SLG/Mo/Sn/Cu/Au/NaF structures fabricated on Soda lime glass substrates containing alkali metals, a significant increase in the CTS grain size was observed in the Au deposition thickness range of 5–25 nm. By contrast, no crystal growth was observed in thin films with a precursor without an NaF layer fabricated using alkali-free glass (EAGLE XG), regardless of the thickness of the Au-deposited film. Therefore, appropriate amounts of Au and Na promote the crystal growth of CTS. In addition, at the sulfurization temperature of 570 °C, the crystal grains were larger than those of the thin film fabricated at 560 °C. In the fabricated CTS thin-film solar cells, with a sulfurization temperature of 570 °C and an Au deposition thickness of 10 nm, open circuit voltage of 0.261 V, short circuit current density of 25.4 mA cm⁻², fill factor of 0.425, and a power conversion efficiency of 2.82% were obtained.

Experiments and first-principles calculations were performed to investigate the effects of Cu substitution in CH₃NH₃PbI₃ perovskite crystals. The first-principles calculations indicated that the energy level of the Cu d orbital formed above the VB maximum would be an acceptor or defect level. The effect of Cu addition on device properties was investigated, and the device with added 2% Cu provided higher efficiencies than the standard device. On the other hand, the decrease in short-circuit current density with increasing Cu content would be attributed to the defect level of the Cu d orbitals. First-principles calculations and experimental results provided insight into the function of Cu in CH₃NH₃-based perovskite crystals.

Bismuth vanadate (BiVO₄) is a promising semiconductor for O₂ production as a photocatalyst/photoanode due to its suitable band gap (2.4 eV) for absorption of the solar spectrum. Nevertheless, it is challenging to develop an applicable preparation process for size and crystallinity-controllable BiVO₄ photocatalysts/photoanodes. Here, we report an innovative method of introducing an aqueous metal-chelate solution containing Bi³⁺ and V⁵⁺, appropriate chelators, and a water-soluble polymer to obtain nanoparticulate BiVO₄ photocatalysts/photoanodes with efficient photo-oxidation performances under visible-light irradiation. The structural characteristics and photocatalytic performances of the particles/photoelectrodes obtained were changed by the kind of polymer, even prepared under the same process.

P-doped ZnTe thin films were grown by MBE on ZnTe (100) substrates using InP as the P source under various InP fluxes. Secondary ion mass spectroscopy (SIMS) analyses showed that the P concentration in ZnTe thin films increased with increasing InP flux, although In atoms were also incorporated in the films. To suppress In incorporation, the outlet of the InP cell was modified by mounting a cap and a plate with small holes. As a result, In incorporation was significantly suppressed, resulting in an In concentration three orders of magnitude lower, as confirmed by SIMS, although the P concentration also decreased by almost one order of magnitude compared with the case without a cap. An acceptor-bound exciton (I_a) peak was observed at around 2.36 eV in the P-doped ZnTe thin film grown with a cap, and the I_a intensity increased after annealing, indicating the activation of P acceptors.

In this article, CuInS₂ (CIS) and Cu(In,Ga)S₂ (CIGS) absorbers are prepared via sulfurization by a sulfur powder source for co-evaporated Cu–In(–Ga) metal precursors without toxic KCN and H₂S. The CIS and CIGS growth mechanism during sulfurization and their application to solar cells are discussed. X-ray diffraction and Raman spectroscopy analyses indicate that CuS and (In,Ga)₂S₃ exist at the frontside and the backside, respectively, in the CIGS films at the temperature between 250 °C and 350 °C. Then, these intermediate phases react at 400 °C or higher forming CIGS. Finally, CIS and CIGS solar cells with efficiencies of 3.7% and 7.2% are achieved, utilizing an optimum temperature of 600 °C.

We investigate the second-stage potential-induced degradation (PID) of n-type front-emitter (n-FE) crystalline silicon (c-Si) photovoltaic (PV) modules. The PID of n-FE c-Si PV modules is known to occur in three stages under negative bias stress. The second-stage PID is characterized by a reduction in fill factor (FF), due to the invasion of sodium (Na) into the depletion region of a p⁺–n junction and the resulting increase in recombination current. The second-stage PID shows a curious independence from a negative bias voltage for the PID stress. This may indicate that the Na inducing the FF reduction comes not from the cover glass but originally existed on and/or near the cell surface. The FF reduction is recovered quite rapidly, within a few seconds, by applying a positive bias to the degraded cell. The recovered n-FE c-Si PV modules show more rapid degradation if they receive the negative bias stress again, which can be explained by Na remaining in the p⁺ emitter.

The potential-induced degradation (PID) is one of the significant issues in realizing low-cost electricity from photovoltaic (PV) power generation plants. In this paper, we have investigated PID in crystalline Si (c-Si) PV modules with conventional p-type multicrystalline Si solar cells after the application of lightning impulse strikes. Lightning impulses with a voltage of −40 kV were applied to the module between the shorted electrodes of the c-Si cell and the mimic aluminum frame. It is confirmed that no degradation in the electrical characteristics of the c-Si cell occurs by applying the impulse only. We have found that the PID of c-Si PV modules was accelerated by applying the impulses between a c-Si cell and a metal frame. The acceleration of PID in the module applied with a lightning impulse might be caused by the migration of Na⁺ ions easily toward the c-Si cell owing to damage to the ethylene-vinyl acetate encapsulant by impulses.

Nanocrystalline gallium nitride (nc-GaN) layers were deposited by RF magnetron sputtering for the electron transport layer of the cesium lead bromide (CsPbBr₃) photovoltaic power converter. We investigated the structural and electrical properties of the nc-GaN layers and found that substrate heater temperature is a key factor to determine the electrical conductivity of the nc-GaN layers. CsPbBr₃ photovoltaic power converters with nc-GaN electron transport layers show good photovoltaic performance. The best performance was obtained at the substrate heater temperature of 550 °C and a conversion efficiency of 5.56% (V_OC = 1.24 V, J_SC = 6.68 mA cm⁻², FF = 0.66) under AM1.5 G illumination with a light intensity of 100 mW cm⁻². The estimated conversion efficiency under blue light with a wavelength of 450 nm is 28.8%.

Annual trends of indoor output measurement (P_max(stc)) results from photovoltaic modules exposed outdoors in Tosu city from 2012 to 2022 were investigated. The P_max(stc) of mono-Si (E-1A), as conventional Si modules, was almost unchanged from 2012 to 2022; however, that of mono-Si (E-1B), as conventional Si modules, decreased after 2019. In the case of Si heterojunction modules, a moderate degradation rate is expected with prolonged exposure. In the case of passivated emitter and rear cell modules, it was found that characteristics due to light and elevated-temperature induced degradation were observed with good reproducibility in 2021 and 2022.

In this study, we produced thin-film solar cells using co-evaporated Ge–Sn–S thin film as the light-absorbing layer. The thin films were prepared at different concentrations of Ge and substrate temperatures. We characterized the solar cells and compared their physical properties with those of an SnS thin film fabricated using only Sn and S. The Ge_xSn_1−xS (x = 0.27) thin film solar cell exhibited the best performance, with short circuit current density J_sc = 0.66 mA cm⁻², curve factor FF = 0.324, power conversion efficiency PCE = 0.036%, and open circuit voltage V_oc = 0.169 V. The band gap of the Ge_xSn_1−xS (x = 0.27) thin film estimated by extrapolating the absorption edge of the external quantum efficiency was 1.57 eV, which is larger than that of the SnS thin film. This suggests that Sn (in SnS) is partially replaced by Ge to form a solid solution, thus widening the band gap.

This study presents two-wavelength excited photocurrent (TWEPC) measurements in GaP_1−xN_x grown by metalorganic vapor phase epitaxy. TWEPC measurements reveal that photocurrent generation is significantly enhanced when above gap excitation and below gap excitation (BGE) sources are applied simultaneously. With increasing BGE photon energy, a large increase in photocurrent is observed. The external quantum efficiency measurements show that the effect of BGE light is higher with a higher density of tail states present. The extended numerical study by rate equations reproduced the results in a good manner. Furthermore, the simulation results showed that the addition of the BGE light affects the electron occupancy as well as the electron lifetime, which is found to be 0.1 ns in this study.

We investigated the long-term durability of our newly developed encapsulant-less p-type crystalline silicon (c-Si) photovoltaic (PV) modules, with a base made of polycarbonate (PC), against potential-induced degradation (PID) in dry and damp-heat (DH) environments. Encapsulant-less modules were found to have high PID resistance compared to conventionally encapsulated c-Si PV modules in both PID conditions. We observed a slight PID for the encapsulant-less modules in which the cover glass was in contact with the solar cell. The slight PID can be suppressed by using a base with a deeper groove so that a sufficient gap between the cover glass and the cell is prepared. Yellow precipitates were formed in the encapsulant-less modules in the DH environment. This is probably due to the hydrolysis of the PC, and proper measures to prevent the precipitate formation should be applied for the industrialization of the encapsulant-less modules.

Ultrathin Al-doped Si oxide (SiO_x) layers were formed by a simple wet chemical treatment, and their hole-selective passivating contact and electrical properties were investigated. From the evaluated contact resistivity (ρ_c) and saturation current density (J₀), carrier selectivity (S₁₀) was estimated to be 13.3. Moreover, in Si nitride (SiN_y)/Al-doped SiO_x stacks, negative values of fixed charge density (Q_f) were obtained, despite a high positive Q_f existing in the single SiN_y layer. This result implies that Al-doped SiO_x has high negative fixed charges and overcompensates the charge polarity in the stacks, which forms an inversion layer and accumulates holes on the Si surface. Furthermore, the negative fixed charges realize excellent carrier separation by the induced upward band bending. In addition, we proposed a novel device architecture named Al-induced charged oxide inversion layer solar cells and confirmed device operation in a simple device configuration.

A mixed paste of aluminum (Al) and germanium (Ge) (7:3) was prepared and screen-printed on silicon (Si) substrates, followed by annealing at a peak temperature of 1000 °C in an IR rapid thermal annealing furnace to investigate the liquid-phase growth of silicon–germanium (SiGe) epitaxial layers. The gas ambient during annealing was changed to investigate the effect on SiGe layer quality and physical properties. The SiGe formed samples were observed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Oxygen-containing atmosphere suppressed the SiGe layer formation by oxidizing the Al particle surface, limiting the reaction of the particle to the Si surface. On the other hand, annealing in an argon atmosphere without oxygen resulted in the formation of SiGe layers with a thickness of over 30 μm.

MicroLink Devices manufactures triple-junction inverted metamorphic solar cells on GaAs wafers with a selectively etched release layer in an epitaxial lift-off (ELO) process to create ultra-thin foils of semiconductor with a metal backing. To improve radiation tolerance, each absorber layer is thinned and doping levels are reduced. The bottom subcell employs a mirror structure to maintain current, while the middle subcell requires a distributed Bragg reflector (DBR) to increase the absorption path length and maintain current. MicroLink utilizes a non-conventional In_x(Al_yGa_1−y)_1−xP/GaAs DBR that is compatible with ELO. The DBR has a bandwidth of ∼110 nm and a peak reflectance in air near 95%. Average cell efficiencies of 28% (1-Sun AM0) are achieved with a power retention of 84% after 1E15 cm⁻² electron dosing (1 MeV energy). Large-area cells and coupons underwent internal thermal stress testing and showed no degradation. Clear pathways to achieve >29% efficient cells with >85% power retention are discussed.

The recycling method for thermal decomposition of photovoltaic modules is a recycling method that can completely remove EVA, which is a sealing material, and can neatly separate the cells and glass. However, most back sheets are made of PET, and if they are simply thermally decomposed, a large amount of complex bonded carbides ("soot") is generated. We succeeded in suppressing the generation of soot by heating resin components such as PET and EVA, melting and dropping them into a ceramic filter supported by a catalyst, promoting the oxidation reaction, and separating the glass and the cells cleanly. It was confirmed that the thermal decomposed PV glass can be recycled into flat glass by a float glass manufacturing company.

Newly-developed polyvinyl butyral (PVB) encapsulants were employed for Si-related photovoltaic modules. Modules of amorphous Si with a glass–glass structure and those of crystalline Si with glass and a back-sheet structure were subjected to hygrothermal stresses by a damp-heat (DH) test and high-voltage stress by a potential-induced degradation (PID) test, respectively. Even after a quite long DH test at 85 °C and 85% relative humidity for 49 000 h, retention of output power of about 85% was obtained without edge sealant. No decrease in output power was also observed after a PID test with −1000 V at 60 °C and 85% relative humidity for 168 h. These facts suggest that newly-developed PVB is well applicable to encapsulant materials for highly reliable photovoltaic modules.

Moisture absorption and TbCl₃ doping of CsPbBr₃ thin films were investigated to improve the carrier transport properties. We found that post-deposition moisture-absorbing treatment improved the carrier diffusion length of CsPbBr₃ thin films. The moisture-absorbing treatments under a relative humidity of about 20%–40% were effective to improve the carrier diffusion length. TbCl₃ doping during the thermal evaporation of CsPbBr₃ affected the structure of the deposited films. An excessive amount of TbCl₃ doping leads to the formation of CsPb₂Br₅ additional phase, but a small amount of TbCl₃ doping (1%) can improve the carrier diffusion length. The moisture-absorbing treatment and TbCl₃ doping are promising techniques to improve the optoelectronic properties of CsPbBr₃.

In this work, we developed an efficient inverse design approach for optimal intermediate band solar cells (IBSC) device design given a target performance by using a joint drift-diffusion simulator and deep reinforcement learning scheme. The drift-diffusion simulator for IBSC simulation was constructed by using the semiconductor module and wave optics module of COMSOL MultiPhysics^®. The deep deterministic policy gradient (DDPG) algorithm was chosen as the learning algorithm to optimize the specified device structure. A GaAs quantum well-embedded $\mathrm{GaAs}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ IBSC was used as the test candidate to verify the performance of the DDPG-based inverse design approach. A maximum efficiency of $\eta =33.42 \%$ was reached for the device with optimal structure parameters searched by the DDPG agent, which exceeds the target efficiency of 30%. The subsequent optical analysis revealed that the electric field enhancement due to light absorption at the IB region with a wavelength between 450 nm and 600 nm is mainly contributing to the significantly increased short-circuit current for the optimized device. Meanwhile, a parameters correlation with target conversion efficiency evaluated by topological data analysis successfully identified all the positive and negative parameters with respect to the target parameter, indicating the physical soundness of the optimized structure parameters. Our work presented here demonstrates that a well-trained AI agent can fulfill the target efficiency by searching the optimal parameters for solar cell devices. The AI-based inverse design approach shows promising potential to serve as an efficient device design tool by greatly reducing the number of trial-and-error experiment demonstrations and replacing laborious human-guided device design workload.

This paper discusses the hypervelocity impacts of micrometeoroids and orbital debris (MMODs) on inverted metamorphic triple-junction (IMM3J) and perovskite solar cells, which are much thinner than conventional triple-junction (3J) solar cells. We experimentally found that IMM3J solar cells can suffer from short-circuit faults due to the hypervelocity impacts of MMODs, unlike conventional 3J cells, and determined the projectile diameters and velocities that could cause them using a model proposed by Burt. No short-circuit mode was identified in perovskite solar cells, but they had open-circuit faults several days after the hypervelocity impact experiment, which are possibly attributed to the decomposition of the perovskite crystal by moisture in the air due to the broken seal.

ZnS and CdZnS (a mixed crystal phase of ZnS and CdS) were formed using the open-air CVD method. Cadmium diethyldithiocarbamate (C₁₀H₂₀CdN₂S₄) and zinc diethyldithiocarbamate (C₁₀H₂₀ZnN₂S₄) were used as the source materials for CdS and ZnS, respectively. By changing the ratio of source materials, it was found that the bandgap and the lattice constant of the CdZnS film were continuously changing without a miscibility gap. Furthermore, the bandgap of the obtained ZnS films was less than the reported bandgap of ZnS (3.68 eV) due to incorporation of oxygen. X-ray diffraction analysis revealed that the increase of Zn in CdZnS film generated a crystalline disorder. When the substrate temperature was changed from 421 °C to 464 °C, the deposition rate increased fourfold for the CdS and ZnS films. The impact of substrate temperature on the bandgap and lattice constant was found to be less pronounced.

To clarify the mechanism of thermal runaway in solar cells, our study included experiments and simulations that focused on changes in the size of the shunt spot where thermal runaway occurs. Our analysis suggests that the rapid temperature increase of the shunt spot leading to failure is caused by the positive feedback of increased backflow current, increased heat generation, and reduced shunt spot size (i.e. increased combined thermal resistance between the shunt spot and normal cell area). The thermal runaway tolerance can be improved by designing solar cells with negative feedback that prevents positive feedback from progressing.

Prediction of photovoltaic (PV) system generations is a powerful tool for managing the electric grids with multiple PV systems for reducing the instability of their electricity supply. For the prediction, solar irradiance, which is directly related to PV generation is forecasting using a weather forecasting model in this study. The target country, Thailand, is in the tropical zone. In the tropics, cumulus and cumulonimbus appear frequently, and their behavior makes weather forecasting difficult. The correlations of the forecasted irradiances with the observations are about 0.8 or more in the intra-day, next-day, and 2-day ahead forecastings. It shows that the validity of solar irradiance forecasting in the tropics for the prediction of PV output. On cloudy days, the tendency of the fluctuation of solar irradiance is reproduced well in the forecasting, but a phase of the fluctuation is shifted in the forecasting.

BiVO₄ thin films doped with various concentrations of sulfur were fabricated using RF sputtering followed by post-deposition sulfurization. The incorporation of sulfur in the samples was calculated to be approximately 8–11 at% from the S2s peak in their X-ray photoelectron spectra. The optical bandgap of sulfur-doped BiVO₄ was generally smaller than that of the undoped sample. BiVO₄ films doped with ∼8 at% sulfur showed the highest photoelectrochemical performance compared to the undoped sample. Almost similar minority-carrier lifetimes in undoped and low sulfur-doped BiVO₄, measured by time resolve photoluminescence, suggest that the crystal qualities in terms of the recombination properties are roughly the same for both cases. Thus, although further investigation may be necessary, the improved photocurrent in 8 at% sulfur-doped BiVO₄ in our study can roughly be attributed to the decrease in the bandgap, which facilitates more photoexcited carriers to contribute to the photoelectrochemical reaction. A further increase in sulfur doping above 10 at% distorted the BiVO₄ local crystal structure, inducing defects, thus resulting in a lower photocurrent.

This work proposes a simple simulation method for the optimization of n-i-p perovskite solar cell (PSCs) via SCAPS-1D and aims to achieve high-performance devices. Nowadays, the carrier recombination induced by heavy defects in bulk and interfaces is one of the main obstacles which restricts PSC efficiency and is also harmful to device stability. Here we modify the MAPbI₃ device through a series of structural and basic optimizations, including the thickness of each layer, carrier diffusion length, interface recombination, doping concentration and overall series resistance. Through the modified simulation, a high-performance MAPbI₃ device with suppressed recombination and optimized structure is realized, resulting in an encouraging power conversion efficiency of 20.09%, an enhanced V_oc of 1.087 V, J_sc of 22.56 mA cm⁻² and an FF of 78.5%. These findings unveil the critical effect of defect suppression on PSCs and offer a simple method to achieve high-performance devices.

Multijunction solar cells (MJSCs) are capable of converting sunlight to electricity more efficiently than single-junction solar cells. The intermediate scattering layers between the individual junctions contribute to high efficiency by impacting the generated currents, photon recycling (PR), as well as luminescent coupling (LC) in the device. The MJSC efficiency can be simulated using expressions that involve a simplified and idealized intermediate layer structure but cannot accurately reflect its actual performance. This work, however, aims to establish a systematic optical model for MJSCs with complicated intermediate layers. It begins with incorporating the LC and PR effects into the developed model, emphasizing requirements for the cut-off wavelength and long-wavelength transmission of the intermediate layer. Furthermore, a three-dimensional metallic nanocylinder array is designed as the intermediate layer to improve device performance. With the model, high-performance MJSCs can be designed and optimised by quantifying the impact of PR and LC on device parameters.

The modification of the sputtered NiO_x (x ≧ 1)/CH₃NH₃PbI₃ interface by 2-(3,6-dimethoxy-9H-carbazol-9-yl) ethyl] phosphonic acid (MeO-2PACz) considerably enhances the power conversion efficiency of perovskite solar cells whose structure is ITO/NiO_x/CH₃NH₃PbI₃/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/Ag. In devices without MeO-2PACz, the internal quantum efficiency (IQE) above 450 nm increases with the increase in NiO_x thickness from 4 to 53 nm, although even in the thickest case, the IQE never reaches 90%. On the other hand, devices with MeO-2PACz modified NiO_x show thickness-insensitive IQE of ca. 90%. We propose that (1) MeO-2PACz effectively fills the pinholes in thinner NiO_x and (2) it passivates the carrier trapping/recombination defects at the NiO_x/perovskite interface.

A three-dimensional (3D) band-gap energy (E_g) map was constructed for a (Cu_1−yAg_y)(Ga_xIn_1−x)Se₂ (CAGISe) system. This system's E_g increases monotonically from CuInSe₂ (CISe) as the ratios of both Ga/(Ga+In) [GGI], x, and Ag/(Cu+Ag) [ACA], y, increase. Furthermore, the energy levels of the VB maximum (VBM) and the conduction band minimum (CBM) were also mapped in 3D. In this CAGISe system, there is no significant change in VBM, whereas CBM does show an increase as the GGI ratio increases. However, as the ACA ratio increases, there is a decrease in VBM level but no significant change in CBM. The substitution effects of Ga for In and Ag for Cu in CISe are discussed on the basis of "principles of orbital interaction."

Many photovoltaic (PV) systems are connected to electrical power grids, and the grids are at risk of instability due to fluctuation of PV output. Numerical weather prediction (NWP) models are used to forecast solar irradiance and proper grid management. NWP usually has many physical parameterization options, and appropriate schemes of these options should be selected for accurate forecasting. The options should be determined by regional and climatic conditions and other factors. The target country is Thailand, which is in the tropics. In Thailand, cumulus and cumulonimbus clouds frequently appear, and their behavior makes weather forecasting difficult. The optimal combination of schemes in the tropics is determined through a sensitivity analysis of the options. By the optimization the forecasting accuracy increases from 0.773 to 0.814 of the correlation coefficient. It is also found that surface layer and PBL processes make a significant contribution to the improvement of accuracy.

As photovoltaic (PV) power generation systems become more widespread, the instability of electric power grids with PV connection is becoming an issue. For appropriate management of the grids, probability prediction of solar irradiance is proposed. The lagged average forecasting method is used for ensemble forecasting. The 72 h ahead forecasting of solar irradiance is operated in Thailand once a day, and it contains intraday, next-day, and 2-day ahead forecasts. Ensemble forecasting has three ensemble members. The accuracy of intraday forecasting is higher than that of the other members, and it is employed as the most probable value of the forecast. The relation between spreads and forecasting errors is analyzed. From the result, the confidence intervals of the predictions are derived for an arbitrary confidence level. The probability prediction is performed with the most probable value and the confidence intervals. The interval changes its width due to spread changes and captures the observation in it.