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Hot carrier solar cells are a concept of photovoltaic devices, which offers the opportunity to harvest solar energy beyond the Shockley-Queisser limit. Unlike conventional photovoltaic devices, hot carrier solar cells convert excess kinetic energy into useful electrical power rather than losing it through thermalisation mechanisms. To extract the carriers while they are still “hot”, efficient energy-selective contacts must be developed. In previous studies, the presence of the hot carrier population in a p-i-n solar cell based on a single InGaAsP quantum well on InP substrate at room temperature has been demonstrated by means of complementary optical and electrical measurements, leading to an operating condition for this device beyond the limit for classical device operation. This result allows to design a new generation of devices to increase the hot carrier conversion contribution. In this work, we study InGaAs/AlInAs type II heterojunction as a selective contact for a future hot carrier solar cell device epitaxially grown on (001) oriented InP substrate. Two p-i-n solar cells have been grown by molecular beam epitaxy on InP. The absorber is a 50 nm-thick InGaAs layer surrounded by AlInAs barriers, all lattice-matched to InP. Two architectures are compared, the first with two symmetrical AlInAs barriers and the second with a single InGaAs quantum well in the center of the n-side barrier to allow electron tunneling across the barrier. Electrical characteristics under laser illumination with two different wavelengths have been measured to investigate the effect of the selective contact compared to the barrier. This preliminary study of InGaAs/AlInAs-based selective contacts show that such III–V combination is adapted for a future hot carrier solar cell in the InP technology.

The possibility of using admittance spectroscopy to characterize the quality of ITO/MoOx/n-Si structures is shown. It has been demonstrated that magnetron evaporation of ITO layer at room temperature leads to the formation of radiation defects in the near-surface region of Si near the MoOx/Si interface with a depth of 0.13 and 0.26 eV below the conduction band with a capture cross-section of (1–5) × 10–19 and (5–10) × 10–19 cm–2, respectively. An increase in the deposition temperature of the ITO layer to 130°C allows reducing the concentration below the sensitivity leading to a significant improvement of solar cells characteristics. [ABSTRACT FROM AUTHOR]

• Light soaking effect is observed in ZnS/p-Si solar cells for the first time. • Different light soaking conditions were systematically explored. • A model is proposed to qualitatively explain the light soaking effect. • Deep-level transient spectroscopy was applied to verify the proposed model. An innovative dopant-free ZnS/p-Si heterojunction solar cell features hole selective WO 3 contact were fabricated by thermal evaporation at room temperature. Light soaking effect was observed in the ZnS/p-Si heterojunction solar cells for the first time. The device performance is found to increase with the increasing light soaking times and finally reaches saturation. Besides, when the illumination stops, the power conversion efficiency (PCE) would decrease slightly. Device performance under different light soaking time and preservation time in the dark were investigated. Moreover, different monochromatic illuminations were conducted to further specify the active layer that is responsible for the light soaking effect. The result shows the ZnS layer and the short-wavelength light absorbed in ZnS layer are supposed to be the main reason for the effect. Based on these results, a model was proposed from the point of view of defect states variation in the energy band of ZnS to explain the effect. At last, Deep Level Transient Spectroscopy (DLTS) measurements with different monochromatic illumination were conducted to further verify the proposed model. [ABSTRACT FROM AUTHOR]

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO2 deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO2 film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm2 and a high power conversion efficiency of 19.2%.

One of the remaining obstacles to achieving the theoretical efficiency limit of crystalline silicon (c-Si) solar cells is high interface recombination loss for minority carriers at the Ohmic contacts. The contact recombination loss of the ultra-thin-film c-Si solar cells is more severe than that of the state-of-art thick cells due to the smaller volume and higher minority carrier concentration. This paper presents a design of an electron passing (Ohmic) contact for n-type Si that is hole-blocking with significantly reduced hole recombination. By depositing a thin titanium dioxide (TiO2) layer, we form a metal-insulator-semiconductor (MIS) contact for a 2 μm-thick Si cell to achieve an open circuit voltage (Voc) of 645 mV, which is 10 mV higher than that of an ultra-thin cell with a traditional metal contact. This TiO2 MIS contact constitutes a step towards high-efficiency ultra-thin-film c-Si solar cells.

It is shown that the degree of damage of the near-surface layer of p-silicon can be estimated with the aid of a MoOx/p-Si selective contact, the current–voltage characteristics of which are highly sensitive to states on the silicon surface formed during the deposition of silicon oxide by method of magnetron sputtering. [ABSTRACT FROM AUTHOR]

One of the remaining obstacles to achieving the theoretical efficiency limit of crystalline silicon (c-Si) solar cells is high interface recombination loss for minority carriers at the Ohmic contacts. The contact recombination loss of the ultra-thin-film c-Si solar cells is more severe than that of the state-of-art thick cells due to the smaller volume and higher minority carrier concentration. This paper presents a design of an electron passing (Ohmic) contact for n-type Si that is hole-blocking with significantly reduced hole recombination. By depositing a thin titanium dioxide (TiO2) layer, we form a metal-insulator-semiconductor (MIS) contact for a 2 μm-thick Si cell to achieve an open circuit voltage ($V_{oc}$) of 645 mV, which is 10 mV higher than that of an ultra-thin cell with a traditional metal contact. This TiO2 MIS contact constitutes a step towards high-efficiency ultra-thin-film c-Si solar cells. [ABSTRACT FROM AUTHOR]

Stabilized power output at maximum power point (mpp) has been considered as one of the most reliable parameters as it provides a key performance indicator for perovskite solar cells (PSCs) revealing the operational stability of the photovoltaic device. Here, we show the effect of selective contact on the power output change under mpp tracking, which closely correlates with the charge recombination dynamics with a time scale of minutes. The normal n-i-p cell architecture comprising cp-TiO 2 /mp-TiO 2 /perovskite/ spiro -MeOTAD (doped by either Li-TFSI or Zn-TFSI 2) and the inverted p-i-n structure, NiO x /perovskite/PCBM, are examined to investigate the specific effect of the nature of the interface on operational stability. The normal structure with Li-TFSI shows a gradual performance decrease at mpp owing to the enhanced recombination at the interface between the perovskite and the spiro -MeOTAD, becoming the dominant recombination process, although the bulk-related recombination is suppressed. On the other hand, the inverted structure demonstrates an improved photocurrent at mpp due to the effectively suppressed recombination both in bulk and at the interface. Remarkably, the deteriorating performance of the normal structure with Li-TFSI at mpp is successfully avoided by replacing Li-TFSI with Zn-TFSI 2 , leading even to an increased power output with stable performance at mpp. Image 1 • The normal structure with Li-TFSI as a dopant for spiro-MeOTAD shows a pronounced recombination at the interface at mpp. • This is responsible for a gradual performance decrease although the bulk-related recombination is suppressed. • The unstable performance of the normal structure is resolved by replacing Li-TFSI with Zn-TFSI 2. • The inverted structure shows a stable power output owing to the suppressed recombination both in bulk and at the interface. [ABSTRACT FROM AUTHOR]

A tunneling atomic layer deposited (ALD) AlO x layer was inserted in a Si|SiO x |AlO x |MoO x |metal tunnel diode structure to investigate the impact of fixed interface charge on Schottky barrier height. ALD AlO x provides a synthesis-tunable fixed charge and can provide excellent field effect passivation at the SiO x |AlO x interface. A large (>1 × 1012 q.cm−2) negative fixed charge density was expected to decrease Schottky barrier height on p-type silicon and increase Schottky barrier height on n-type silicon by up to 0.4 eV based on an electrostatic model. Fixed charge density and interface trap state density were measured using a thick layer of AlO x in metal oxide semiconductor capacitor structures (MOSCAPs); Schottky barrier height and specific contact resistivity were measured using a tunneling-active layer of AlO x in contacted structures with MoO x. In some samples, HfO x interface layers were inserted between AlO x and silicon to quench the fixed negative charge. The Schottky barrier height was found to vary with processing conditions but did not have a strong correlation with expected fixed charge density. Thus, while fixed charge may play a role in determining the Schottky barrier height, other parameters also have significant affect. Further work is needed to minimize contact resistivity and elucidate other factors impacting the Schottky barrier height. • Effect of SiO x.|ALD AlO x fixed charge on ALD MoOx-based selective contact studied with varied processing. • Schottky barrier height found not to vary solely with Nf; other factors contribute to changes. • Schottky barrier height and fixed charge density affected by silicon surface preparation, AlO x deposition temperature, annealing, and the insertion of ALD HfO x at the SiO x.|AlO x interface. • Achieved Schottky barrier height range between <0.2 eV and 1.0 eV with p-type and n-type Si.|SiO x |AlO x |MoO x |Al structures. • Contact resistivity shown indicated to be highly dependent on SiO x layer and/or total oxide thickness. [ABSTRACT FROM AUTHOR]

It is shown for the first time that thin boron phosphide (BP) layers on silicon substrates can be formed by low-temperature plasma-enhanced atomic layer deposition at 250°C. Experiments demonstrated the possibility of using these BP/Si interfaces as selective hole contacts to silicon in solar cells. [ABSTRACT FROM AUTHOR]

Abstract: Silicon (Si)‐based dopant‐free heterojunction solar cells (SCs) featuring carrier‐selective contacts (CSCs) have attracted considerable interest due to the extreme simplifications in their device structure and manufacturing procedure. However, these SCs are limited by the unsatisfactory contact properties on both sides of the junction, and their efficiencies are not comparable with those of commercially available Si SCs. In this report, a high‐performance silicon‐oxide/magnesium (SiOx/Mg) electron‐selective contact (ESC) design is described. Combining an ultrathin SiOx and a low work function Mg layer, the novel ESC simultaneously yields low recombinative and resistive losses. In addition, deposition of Mg on SiOx relaxes the restriction on the threshold thickness of the SiOx for electron tunneling and therefore broadens the optimization space for rear‐sided passivation. Meanwhile, hole‐selective contact with boosted light harvesting and suppressed interfacial recombination is achieved by forming a fully conformal contact between the conducting poly(3,4‐ethylene dioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) and periodic Si pyramid arrays. With the double‐sided carrier‐selective contact designs, PEDOT: PSS/Si/SiOx/Mg SCs with efficiency of 15% are finally obtained via a totally dopant‐free processing. Subsequent calculations further indicate a pathway for the improvement of these contacts toward an efficiency that is competitive with conventionally diffused pn junction SCs. [ABSTRACT FROM AUTHOR]

N-Type crystalline silicon (c-Si) heterojunctions with V 2 O 5 /SiO x stacks as passivating/hole-selective contacts are reported. SiO x interlayers were prepared by low temperature (T<110 ºC) oxidation processes, followed by thermal evaporation of V 2 O 5 at ambient temperature. A high surface passivation, with an implied open-circuit voltage (i-V OC ) of 675 mV, is obtained for a 2.1 nm thick SiO x interlayer ‘naturally’ formed during V 2 O 5 evaporation. X-ray photoelectron spectroscopy analyses indicate a sub-stoichiometric configuration (SiO x~1.4 ), whereas hydrogen-containing species like Si–OH and Si–(O 3 H) are identified by secondary ion-mass spectroscopy as possible contributors to dangling bond passivation. A conversion efficiency of 16.5% (V OC = 642 mV) is attained, confirming the potential of these dopant-free novel structures for c-Si photovoltaics. [ABSTRACT FROM AUTHOR]

Screening for potential new materials with experimental and theoretical methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early experimental stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estimated band gap. We discuss this using copper-deficient CuCrO2 as an example, as it was already shown to be a good p-type TCO grown at low temperatures. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment experimental methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells. [ABSTRACT FROM AUTHOR]

We present efficient perovskite solar cells using a mesoscopic TiO 2 /Al 2 O 3 /NiO/carbon structure as framework. The CH 3 NH 3 PbI 3 -based device with quadruple-layer architecture achieves a power conversion efficiency of 15.03% under AM 1.5G illumination. Detailed investigations show an increased charge collection and reduced charge recombination in this device structure compared to that without NiO interlayer. It is found that these perovskite solar cells exhibit good stability both in dark and under illumination. [ABSTRACT FROM AUTHOR]

A series of conductive polymers, including poly(p-phenylene) (PPP), polythiophene (PT) and poly(4,4′-bis(N-carbazolyl)-1,1′-biphenyl) (PPN), were prepared via the electrochemical polymerization and their application as hole-transporting layer in CH 3 NH 3 PbI 3 perovskite solar cells was explored. The open circuit voltage of the corresponding devices with an ITO/conductive-polymer/CH 3 NH 3 PbI 3 /C 60 /BCP/Ag construction was correlated directly with the work functions of the conductive polymers that served as the hole-selective contact materials to the perovskite layer, as well as the recombination resistances of the devices. Highest open circuit voltage of 1.05 V was obtained on the devices with PPP that has a work function of −5.31 eV as the hole-transporting layer. Its power conversion efficiency reached about 16.5% with a high fill factor of 0.75, good open voltage of 1.03 V and short circuit current-density of 21.6 mA cm −2 . The results indicated that high conductive hole-transporting material with a work function about −5.3 eV is more compatible with this device construction to afford desired hole-selective contact. Furthermore, this simple, prompt, controllable and economic method to prepare hole-transporting materials would be favorable for large-scale production of hole-transporting layer for perovskite solar cells. [ABSTRACT FROM AUTHOR]

Near stoichiometric molybdenum oxide (MoO 3-x) film has attracted extensive interest as superior hole-selecting material in optoelectronic devices. However, the existence of multi-oxidation states significantly affects its work function and carrier transport behaviors. In this work, we have proposed a facile way to modulate the oxidation states of solution-processed MoO 3-x with vanadium ions (V5+) doping (MoO 3-x :V), beneficial for improved hole-selecting contact performance with silicon. As the doping concentration of V5+ increases, oxygen vacancies and reduced Mo5+ ions reduce, resulting in the increase of work function of MoO 3-x film. The effective carrier lifetime of MoO 3-x deposited Czochralski silicon has been largely improved from 60.1 to 153.0 μs (Δn = 1015 cm−3) at the optimized doping concentration of 5%, and the contact resistivity is reduced from 9.1 to 2.1 Ω cm2 simultaneously. The finished solar cells with the scheme of Ag/MoO 3-x :V/n-Si have exhibited significantly improved conversion efficiency. Our results have demonstrated a very promising way to modulate the stoichiometry and work function of MoO 3-x film, which has great potential in solar cell and light emitting diode applications. [ABSTRACT FROM AUTHOR]

Silicon heterojunction (SHJ) solar cell, as one of the promising technologies for next-generation passivating contact solar cells, employs an undiffused and n-type mono-crystalline silicon (c-Si) substrate and two amorphous-silicon-based selective contacts with opposite polarities. In this work, a numerical model based on Richter's theory has been developed to simulate the performances of a 25.11 % efficiency SHJ solar cell obtained recently. Analyses on series resistivity (R s) explicit that the upper bound for the sum of contact resistivities for p-type (ρ c,p) and n-type (ρ c,n) contacts is 0.073 Ω cm2. With the updated contact resistivities, the theoretical limiting efficiency estimated by Brendel's formulation is therefore 28.5 % for SHJ solar cells, which is comparable to 28.7 % for bi-facial tunneling-oxide passivating contact (TOPCon) solar cells. A hybrid structure consisting of p-type contact from SHJ and n-type contact from TOPCon is feasible to reach 28.9 % limiting efficiency in principle, showing potential of solar cells with hybrid structures. • A numerical model has been developed for SHJ solar cells to simulate the performances of Hanergy-25.11% record solar cell. • Breakdown of series resistivity explicits that the upper bound for the sum of n/p-type contact resistivities is 0.073 Ω⋅cm2. • Following Brendel's theory limiting efficiency for SHJ is thus updated to 28.5%, comparable to 28.7% for bi-facial TOPCon. • Limiting efficiency of 28.9% is feasible for a hybrid structure with p-type contact from SHJ and n-type contact from TOPCon. [ABSTRACT FROM AUTHOR]

In this paper, a record-breaking efficiency of 12.3% is experimentally demonstrated for a flexible free-standing, 2.7-μm-thick ultrathin crystalline silicon (c-Si) solar cell, which is the highest ever-reported sub 3 μm c-Si solar cells. The first breakthrough of this study is achieving an exceptionally small exposure feature size of 350 nm using photolithography stepper without any additional stringent requirements on wafer flatness or depth of focus (DOF) of the projection system. This small exposure feature size enables the fabrication of a light-trapping scheme: a design of close-packed 700-nm-period touching inverted-pyramid nanostructures for enhanced light absorption. Furthermore, to reduce contact recombination, this work is also first to report the integration of nickel oxide (NiO x) as hole-selective contacts in conjunction with the nano-structure light trapping scheme on a free-standing ultrathin cell. The efficacy of these two mechanisms to enhance ultrathin c-Si cell efficiency is characterized in terms of the short circuit current density and open circuit voltage. The free-standing cell also shows good mechanical flexibility and compliance. In addition to demonstrating silicon-compatible fabrication, this work shows the potential of ultrathin c-Si technology to not only reduce material cost, but also maintain competitive device performance. Image 1 • Free-standing sub 3 μm ultrathin c-Si cell with high efficiency above 12.0%. • Close-packed 700 nm feature size nanostructure light trapping by photolithography. • Integration of both NiO x hole-selective contact and light trapping. [ABSTRACT FROM AUTHOR]

Currently, a significant amount of photovoltaic device cost is related to its requirement of high-quality absorber materials, especially in the case of III-V solar cells. Therefore, a technology that can transform a low-cost, low minority carrier lifetime material into an efficient solar cell can be beneficial for future applications. Here, we transform an inefficient p-type InP substrate with a minority carrier lifetime less than 100 ps into an efficient solar cell by utilizing a radial p-n junction nanowire architecture. We fabricate a p-InP/n-ZnO/AZO radial heterojunction nanowire solar cell to achieve a photovoltaic conversion efficiency of 17.1%, the best reported value for radial junction nanowire solar cells. The quantum efficiency of ∼95% (between 550 and 750 nm) and the short-circuit current density of 31.3 mA/cm 2 are among the best for InP solar cells. In addition, we also perform an advanced loss analysis of the proposed solar cell to assess different loss mechanisms in the solar cell.

Hot-wire chemical vapor deposition was used to deposit in situ-doped amorphous silicon layers for poly-Si/SiO x passivating contacts at a high deposition rate of 42 nm/min. We investigated the influence of a varied phosphine gas (PH 3 ) concentration during deposition on (i) the silicon film properties and (ii) the passivating contact performances. The microstructural film properties were characterized before and after a high-temperature crystallization step to transform amorphous silicon films into polycrystalline silicon films. Before crystallization, the silicon layers become less dense as the PH 3 concentrations increase. After crystallization, an increasing domain size is derived for higher PH 3 concentrations. Sheet resistance is found to decrease as domain size increased, and the correlation between mobility and domain size was discussed. The performances of the passivating contact were measured, and a firing stable open circuit voltage of 732 mV, a contact resistivity of 8.1 mΩ·cm 2 , and a sheet resistance of 142 Ω/□ could be achieved with the optimized PH 3 concentration. In addition, phosphorous doping tails into the crystalline silicon were extracted to evaluate the Auger recombination of the passivating contact.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages ( V OC ) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

Transparent passivated contacts (TPCs) using a wide band gap microcrystalline silicon carbide (μc-SiC:H(n)), silicon tunnel oxide (SiO 2 ) stack are an alternative to amorphous silicon-based contacts for the front side of silicon heterojunction solar cells. In a systematic study of the μc-SiC:H(n)/SiO 2 /c-Si contact, we investigated selected wet-chemical oxidation methods for the formation of ultrathin SiO 2 , in order to passivate the silicon surface while ensuring a low contact resistivity. By tuning the SiO 2 properties, implied open-circuit voltages of 714 mV and contact resistivities of 32 mΩ cm 2 were achieved using μc-SiC:H(n)/SiO 2 /c-Si as transparent passivated contacts.

In this paper, the integration of metal oxides as carrier-selective contacts for ultrathin crystalline silicon (c-Si) solar cells is demonstrated which results in an ∼13% relative improvement in efficiency. The improvement in efficiency originates from the suppression of the contact recombination current due to the band offset asymmetry of these oxides with Si. First, an ultrathin c-Si solar cell having a total thickness of 2 μm is shown to have >10% efficiency without any light-trapping scheme. This is achieved by the integration of nickel oxide (NiO x ) as a hole-selective contact interlayer material, which has a low valence band offset and high conduction band offset with Si. Second, we show a champion cell efficiency of 10.8% with the additional integration of titanium oxide (TiO x ), a well-known material for an electron-selective contact interlayer. Key parameters including V oc and J sc also show different degrees of enhancement if single (NiO x only) or double (both NiO x and TiO x ) carrier-selective contacts are integrated. The fabrication process for TiO x and NiO x layer integration is scalable and shows good compatibility with the device.

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO 2 deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO 2 film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm 2 and a high power conversion efficiency of 19.2%.