Development of back‐junction back‐contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back‐junction back‐contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open‐circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co‐diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm² by using screen printing and co‐firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm². The present work has demonstrated that the commercialization of low‐cost and high‐efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.     

GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

Impact of a Short-Pulse High-Intense Proton Irradiation on High-Performance Perovskite Solar Cells

This work investigates the radiation resistance of high-performance multi-component perovskite solar cells (PSCs) for the first time under extreme short-pulse proton irradiation conditions. The devices are subjected to high-intensity 170 keV pulsed (150 ns) proton irradiation, with a fluence of up to 10¹³ p cm^−2, corresponding to ≈30 years of operation at low Earth orbit. A complex material characterization of the perovskite active layer and device physics analysis of the PSCs before and after short-pulse proton irradiation is conducted. The obtained results indicate that the photovoltaic performance of the solar cells experiences a slight deterioration up to 20 % and 50 % following the low 2 × 10¹² p cm⁻² and high 1 × 10¹³ p cm⁻² proton fluences, respectively, due to increased non-radiative recombination losses. The findings reveal that multi-component PSCs are immune even to extreme high-intense short-pulse proton irradiation, which exceeds harsh space conditions, including intense coronal ejection events usually associated with solar flares.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Voltage dependent photocurrent collection in CdTe/CdS solar cells

The voltage dependence of the photocurrent J_L(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. J_L(V) reduces the fill factor (FF) of typical cells by 10–15 points, the open circuit voltage (V_OC) by 20–50 mV, and the efficiency by 2–4 points. Eliminating the effect of J_L(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p‐i‐n solar cells in which fitting yields a parameter consistent with lifetimes of 10⁻⁹ s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht‐like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1·5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1·8 to 7·0 µm, in initial and stressed states. Accelerated thermal and bias stressing increases J_L(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright © 2007 John Wiley & Sons, Ltd.     

Operation of 4H-SiC high voltage normally-OFF V-JFET in radiation hard conditions: Simulations and experiment

Operation of high-voltage 4H-SiC vertical-JFET in radiation hard environment was investigated by simulation and experiment. Commercial 1700 V normally-OFF SiC JFETs in TO-247 package were irradiated with fast neutrons to fluences of 4.0 × 10¹⁴ cm^− 2 (1 MeV Si equivalent) and the effect of radiation on their characteristics was then thoroughly analyzed. Four degradation mechanisms were identified, of which the most important is the increase of JFETs ON-state resistance due to the mobility degradation and removal of carriers from transistor's light doped channel and drift regions. As a result, the JFET ON-state losses grow and, at fluences higher than 4 × 10¹⁴ cm^− 2, the low doped n-regions are fully compensated and transistor loses its functionality. On the contrary, irradiation slightly improves JFET's switching characteristics. The effect of neutron irradiation on operation of SiC V-JFET in a real application was then investigated on the step-UP 15 V/60 V DC-DC converter where the SiC JFET was used as an active switch. Converter characteristics were analyzed by means of the mixed-mode simulation using the developed 2D model of the neutron irradiated transistor. Results showed that the duty cycle of the PWM regulator is growing due to the increase in the voltage drop on the switching JFET. This effect, which is caused by the abovementioned increase the JFET's ON-state resistance, increases power dissipation and deteriorates converter efficiency. Finally, the effect of neutron irradiation on operation SiC V-JFET in the 850 V/24 V auxiliary flyback switching mode power supply was analyzed. We showed that the growth of the ON-state resistance increases transistor's conduction losses and decreases converter efficiency. Exceeding the fluence of 3.3 × 10¹⁴ cm^− 2 neutrons then causes JFET overheating and subsequent destruction.     

Short‐circuit current of solar cells under artificial light

A model to estimate the short‐circuit current of a solar cell under artificial light from the short‐circuit current of the same solar cell under AM1.5 1 kW/m² is described. The results may help designers of solar‐powered portable equipment and consumer products working indoors or under a mixture of artificial and sunlight. It is concluded that the ratio of the short‐circuit currents of the same solar cell generated under fluorescent light of 1 lux illuminance divided by the short‐circuit current generated under standard 1 Sun AM1.5 conditions is around 3 × 10⁻⁶ for typical crystalline silicon and CIS solar cells. This value is one order of magnitude greater if the light source considered is an incandescent lamp. In the case of amorphous silicon solar cells the value of the ratio is close to 8 × 10⁻⁶ either for fluorescent or incandescent lamps. CdTe solar cells are also considered, and this factor is about 4 × 10⁻⁶ under fluorescent light, and four times bigger when an incandescent lamp is used. Some measurements performed validate the figures obtained. Copyright © 2002 John Wiley & Sons, Ltd.     

Graphene-oxide doped PEDOT:PSS as a superior hole transport material for high-efficiency perovskite solar cell

Inverted perovskite solar cells have attracted a great deal of attention due to its high power conversion efficiency, simple configuration, and low-cost processing. The hole transport material (HTM) is a crucial factor in high performance inverted perovskite solar cell. However, the hole mobility for most common of HTM is too low to matching perovskite materials. Herein, we report a superior HTM with high hole mobility to significantly improve solar cell efficiency. Upon doing the commonly used PEDOT:PSS HTM by graphene oxide (GO), its hole mobility is increased from 5.55 × 10⁻⁵ to 1.57 × 10⁻⁴ cm² V⁻¹ s⁻¹, leading to efficient hole extraction and low current leakage, therefore 20% higher power conversion efficiency comparing to the control device without the GO doping. The development open the opportunities for efficient HTMs based on the two-dimensional materials in the perovskite solar cells.     

Analysis of perimeter recombination in the subcells of GaInP/GaAs/Ge triple‐junction solar cells

This paper studies the recombination at the perimeter in the subcells that constitute a GaInP/GaAs/Ge lattice‐matched triple‐junction solar cell. For that, diodes of different sizes and consequently different perimeter/area ratios have been manufactured in single‐junction solar cells resembling the subcells in a triple‐junction solar cell. It has been found that neither in GaInP nor in Ge solar cells the recombination at the perimeter is significant in devices as small as 500 μm × 500μm(2.5 ⋅ 10 ^{− 3} cm²) in GaInP and 250μm × 250μm (6.25 ⋅ 10 ^{− 4}cm²) in Ge. However, in GaAs, the recombination at the perimeter is not negligible at low voltages even in devices as large as 1cm², and it is the main limiting recombination factor in the open circuit voltage even at high concentrations in solar cells of 250 μm × 250μm (6.25 ⋅ 10 ^{− 4} cm²) or smaller. Therefore, the recombination at the perimeter in GaAs should be taken into account when optimizing triple‐junction solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Two‐dimensional simulation of thin‐film silicon solar cells with innovative device structures

We investigate the optical and electrical properties of thin‐film silicon solar cells by means of numerical simulations. The optical design under investigation is the encapsulated‐V texture, which is capable of absorbing sunlight corresponding to a maximum short‐circuit current density of 35 mA cm⁻². Because the layer thickness can be restricted to only 4 μm, the encapsulated‐V structure also provides a good collection efficiency for photogenerated charge carriers. The results for our simulations suggest that practical efficiencies above 12% can be expected for Si material with a minority carrier lifetime as low as 10 ns. Increased lifetimes of 100 ns allow for about 14% efficiency. The benefit of multiplejunctions within the device structure strongly depends on surface recombination. The efficiency of a single‐junction cell can be improved by more the 3% absolute with a multi‐junction device if the surface combination velocity is as high as 10⁵ cm s⁻¹. For moderate surface recombination, the gain is only 1%. Copyright © 1999 John Wiley & Sons, Ltd.     

The influence of high energy electron irradiation on the Schottky barrier height and the Richardson constant of Ni/4H-SiC Schottky diodes

The influence of high energy electron (HEE) irradiation from a Sr-90 radio-nuclide on n-type Ni/4H–SiC samples of doping density 7.1×10¹⁵ cm⁻³ has been investigated over the temperature range 40–300 K. Current–voltage (I–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) were used to characterize the devices before and after irradiation at a fluence of 6×10¹⁴ electrons-cm⁻². For both devices, the I–V characteristics were well described by thermionic emission (TE) in the temperature range 120–300 K, but deviated from TE theory at temperature below 120 K. The current flowing through the interface at a bias of 2.0 V from pure thermionic emission to thermionic field emission within the depletion region with the free carrier concentrations of the devices decreased from 7.8×10¹⁵ to 6.8×10¹⁵ cm⁻³ after HEE irradiation. The modified Richardson constants were determined from the Gaussian distribution of the barrier height across the contact and found to be 133 and 163 A cm⁻² K⁻² for as-deposited and irradiated diodes, respectively. Three new defects with energies 0.22, 0.40 and 0.71 eV appeared after HEE irradiation. Richardson constants were significantly less than the theoretical value which was ascribed to a small active device area.     

Control of Crystal Growth toward Scalable Fabrication of Perovskite Solar Cells

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10^?2% cm^?2, which is 82 and 3.5 times higher than crystalline Si (1.7 × 10^?4% cm^?2) and thin film technologies (≈4 × 10^?3% cm^?2) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.     

Hexa-peri-hexabenzocoronene and diketopyrrolopyrrole based D-A conjugated copolymers for organic field effect transistor and polymer solar cells

Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped conjugated molecule with strong π-π stacking property, high intrinsic charge mobility and good self-assembly property. But for a long time, the organic photovoltaic (OPV) solar cells based on HBC small organic molecules demonstrated low power conversion efficiencies (PCEs). In this study, a series of polymers named as PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT were designed and synthesized through copolymerization of HBC with bulky mesityl substituents and strong electron-withdrawing diketopyrrolopyrrole (DPP) with different alkyl side chains and various π-bridges. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowered the band gaps. Bulky mesityl substituents attached to periphery of HBC prevented polymers from self-aggregating into too large domain size in the blend films of photovoltaic devices. The different π-bridges have significant effect on the structure conformation of the polymers. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated the broadest absorption for its extensive π-conjugation and more coplanar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave field-effect hole mobilities of 1.35 × 10⁻³, 2.31 × 10⁻⁴, 2.79 × 10⁻⁴ and 8.60 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. The solar cells based on these polymers displayed PCEs of 2.12%, 2.85%, 1.89% and 2.74%. To our knowledge, 2.85% is the highest PCE for the HBC-based photovoltaic materials till now.     

New Family of Ruthenium‐Dye‐ Sensitized Nanocrystalline TiO2 Solar Cells with a High Solar‐Energy‐Conversion Efficiency

A new type of ruthenium complexes 6 – 8 with tridentate bipyridine–pyrazolate ancillary ligands has been synthesized in an attempt to elongate the π‐conjugated system as well as to increase the optical extinction coefficient, possible dye uptake on TiO₂, and photostability. Structural characterization, photophysical studies, and corresponding theoretical approaches have been made to ensure their fundamental basis. As for dye‐sensitized solar cell applications, it was found that 6 – 8 possess a larger dye uptake of 2.4 × 10^–7 mol cm^–2, 1.5 × 10^–7 mol cm^–2, and 1.3 × 10^–7 mol cm^–2, respectively, on TiO₂ than that of the commercial N3 dye (1.1 × 10^–7 mol cm^–2). Compound 8 works as a highly efficient photosensitizer for the dye‐sensitized nanocrystalline TiO₂ solar cell, producing a 5.65 % solar‐light‐to‐electricity conversion efficiency (compare with 6.01 % for N3 in this study), a short‐circuit current density of 15.6 mA cm^–2, an open‐circuit photovoltage of 0.64 V, and a fill factor of 0.57 under standard AM 1.5 irradiation (100 mW cm^–2). These, in combination with its superior thermal and light‐soaking stability, lead to the conclusion that the concomitant tridentate binding properties offered by the bipyridine‐pyrazolate ligand render a more stable complexation, such that extended life spans of DSSCs may be expected.     

Benzodithiophene based ternary copolymer containing covalently bonded pyrrolo[3,4-c]pyrrole-1,3-dione and benzothiadiazole for efficient polymer solar cells utilizing high energy sunlight

A new electron deficient monomer (PDBT) unit incorporating a weak electron accepting unit, pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD), and a strong electron accepting unit, 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BT), with a configuration of DPPD-BT-DPPD was prepared, and copolymerized with a benzodithiophene (BDTT) derivative to afford a new alternating ternary copolymer P(BDTT-PDBT). The estimated optical band gap (E_g) and highest occupied/lowest unoccupied (HOMO/LUMO) molecular orbital energy levels of P(BDTT-PDBT) were 1.72 eV and −5.45 eV/−3.62 eV, respectively. A field effect transistor made from P(BDTT-PDBT) exhibited a hole mobility of 3.2 × 10⁻³ cm² V⁻¹ s⁻¹. Conventional single layer polymer solar cells (PSCs) prepared using P(BDTT-PDBT) with an additive offered a maximum power conversion efficiency (PCE) of 5.73% with an open-circuit voltage (V_oc) of 0.90 V, a short-circuit current (J_sc) of 9.73 mA/cm², and a fill factor (FF) of 0.66. Interestingly, the PSC device prepared without an additive also showed a similar PCE of 4.84% (V_oc ∼ 0.92 V, J_sc ∼ 8.08 mA/cm², and FF ∼ 0.65). This paper reports the preparation of an efficient ternary copolymer for polymer solar cells utilizing high energy sunlight, as well as the property modulation of an alternating binary copolymer, P(BDTT-DPPD), containing BDTT and DPPD derivatives via the insertion of a strong electron accepting BT unit on the P(BDTT-DPPD) backbone.     

Investigations on the annealing behavior of high-energy carbon irradiated Au/n-GaAs Schottky barrier diodes

Au Schottky barrier diodes (SBDs) have been irradiated using high-energy carbon ion fluences of 1×10¹¹, 1×10¹² and 1×10¹³ cm⁻². Current–voltage characteristics of unirradiated and irradiated diodes have been analyzed. The change in reverse leakage current increases with increasing ion fluence due to the irradiation-induced defects at the interface. The diodes were annealed at 523 and 623 K to study the effect of annealing. The rectifying behavior of the irradiated SBDs improves at 523 K. But at 623 K, the diode behavior deteriorates irrespective of the fluences. Better enhancement in the barrier height and also improvement in the ideality factor of the diodes has been observed at the annealing temperature of 523 K. Scanning Electron Microscopic analysis was carried out on the irradiated samples to delineate the projected range of the defects by high-energy carbon ion irradiation.     

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2 MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1 × 10⁹ cm^?2 to 1 × 10¹³ cm^?2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5 × 10¹⁰ cm^?2. Remarkable changes in device characteristics were seen at a fluence of 1 × 10¹² cm^?2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1 × 10¹² cm^?2. The results found on devices and on the GaN layer were compared and correlated.     

Electrical and photovoltaic properties of SnSe/Si heterojunction

Thin film of SnSe is deposited on n-Si single crystal to fabricate a p-SnSe/n-Si heterojunction photovoltaic cell. Electrical and photoelectrical properties have been studied by the current density–voltage (J–V) and capacitance–voltage (C–V) measurements at different temperatures. The fabricated cell exhibited rectifying characteristics with a rectification ratio of 131 at ±1 V. At low voltages (V<0.55 V), the dark forward current density is controlled by the multi-step tunneling mechanism. While at a relatively high voltage (V>0.55 V), a space charge-limited-conduction mechanism is observed with trap concentration of 2.3×10²¹ cm⁻³. The C–V measurements showed that the junction is of abrupt nature with built-in voltage of 0.62 V which decreases with temperature by a gradient of 2.83×10⁻³ V/K. The cell also exhibited strong photovoltaic characteristics with an open-circuit voltage of 425 mV, a short-circuit current density of 17.23 mA cm⁻² and a power conversion efficiency of 6.44%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50 mW cm⁻².     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

We report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10⁻⁴ to mid 10⁻⁵ Ohm cm² range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.