Impurity Photovoltaic Research Articles

Numerical simulation on the impurity photovoltaic (IPV) effect in c-Si wafer-based dual-heterojunction solar cell

The impurity photovoltaic effect (IPV) with thallium (TI) impurity has theoretically been investigated in crystalline silicon (c-Si) wafer reigned dual-heterojunction (DH) solar cell that ensures an extended absorption of longer wavelength light with zinc selenide (ZnSe) window and aluminium antimonide (AlSb) back surface layers. The results imply that the incorporation of impurity enhances the photocurrent while the dual-heterojunction ensures the higher V OC of the proposed solar cell structure. The pristine Si (150 µm wafer) solar cell exhibits a power conversion efficiency (PCE) of 35.04% with J SC = 38.85 mA/cm 2 and V OC = 1.05 V, respectively. On the contrary, the introduction of Tl impurity within the bandgap of absorber enhances the PCE to 44.47%, with a J SC = 48.95 mA/cm 2 , V OC = 1.063 V and FF= 85.46%, respectively. This augmentations of the V OC and J SC and hence PCE is originated from the DH structure and longer wavelength light absorption due to impurity-assisted two-step photon upconversion phenomena in the solar cell. • IPV effect in an n + -p-p + Si DH solar cell with Tl impurity has been simulated. • ZnSe and AlSb have been used as window and BSF layers, respectively. • The optimized metal contacts Ti (cathode) and Ni (anode) has been established. • A maximum J SC of 48.95 mA/cm 2 and PCE= 44.47% with V OC = 1.063 V has been achieved. • n + - ZnSe /p- Si /p + - AlSb DH solar cell with Tl impurity shows potential in future.

High efficiency silicon solar cells with back ZnTe layer hosting IPV effect: a numerical case study

To get enhanced long-wavelength absorption, an impurity photovoltaic (IPV) mechanism was implemented within a transparent conducting oxide (TCO) at the rear of a solar cell. The numerical simulation of the N+/P (silicon)/IPV-TCO device was carried out by using SCAPS-1D program which allows the inclusion of optically active defects. In the proposed heterostructure configuration, ZnTe is a suitable material as back surface reflector. In analyzing the Si/ZnTe interface, lattice mismatch, energy band alignment and defects density were considered with appropriate treatment. In particular, to cure the detrimental 12% lattice mismatch at the interface, a thin silicon amorphous layer was inserted in-between, allowing 22.98% conversion efficiency. With adapted ZnTe Lucovsky's model for the optical capture cross sections and introduction of an oxygen radiative IPV defect (O2 level at 0.4 eV below the conduction band), a conversion efficiency of 27.15% was ultimately achieved. The experimental feasibility of the high-efficiency heterostructure device is evaluated.

Photoemission cross section: A critical parameter in the impurity photovoltaic effect**Project supported by the National Natural Science Foundation of China (Grant Nos. 61464007, 61306084, 11664025, and 51561022), the Postdoctoral Science Foundation of Jiangxi Province of China (Grant Nos. 2014KY32, 2013RC08, and 2015KY12), the Natural Science Foundation of Jiangxi Province of China (Grant Nos. 20151BAB207055 and 20161BAB201012), and the Postdoctoral Science

A numerical study has been conducted to explore the role of photoemission cross sections in the impurity photovoltaic (IPV) effect for silicon solar cells doped with indium. The photovoltaic parameters (short-circuit current density, open-circuit voltage, and conversion efficiency) of the IPV solar cell were calculated as functions of variable electron and hole photoemission cross sections. The presented results show that the electron and hole photoemission cross sections play critical roles in the IPV effect. When the electron photoemission cross section is cm , the conversion efficiency η of the IPV cell always has a negative gain () if the IPV impurity is introduced. A large hole photoemission cross section can adversely impact IPV solar cell performance. The combination of a small hole photoemission cross section and a large electron photoemission cross section can achieve higher conversion efficiency for the IPV solar cell since a large electron photoemission cross section can enhance the necessary electron transition from the impurity level to the conduction band and a small hole photoemission cross section can reduce the needless sub-bandgap absorption. It is concluded that those impurities with small (large) hole photoemission cross section and large (small) electron photoemission cross section, whose energy levels are near the valence (or conduction) band edge, may be suitable for use in IPV solar cells. These results may help in judging whether or not an impurity is appropriate for use in IPV solar cells according to its electron and hole photoemission cross sections.

Indium as a p-type dopant of thin film silicon solar cells

The effect of using indium, a deep-level acceptor in silicon, as a dopant for an ultra-thin film (2.5μm thick) single-crystalline silicon solar cell has been investigated. Indium acts as a p-type dopant in silicon with an energy level 0.157eV above the valence band and the use of this deep-level acceptor has been proposed as a method to enable sub-bandgap transitions via the impurity photovoltaic (IPV) effect. In the current work, a maximum conversion efficiency under 1 sun illumination, air mass 1.5, of 4.74±0.2% is measured for a solar cell using indium as a p-type dopant. An equivalently processed cell using boron in place of indium has a maximum efficiency of 4.16±0.2%. Similar relative increases in efficiency have been observed for nine individual devices. The area of the cells is 0.5×0.5cm2 with 7% covered by the metal contacts. A dual-junction analytical model is used to show that for all indium-doped cells in this study there exists an enhancement in the generation of charge carriers compared to boron-doped cells. External-Quantum-Efficiency measurements of both boron and indium doped cells shows an enhancement in optical to electrical conversion for wavelengths of ~470–1000nm when indium is used as a dopant. No significant conversion is observed for wavelengths >1100nm, in contradiction of an IPV effect existing in these devices.

Enhanced near-infrared responsivity of silicon photodetector by the impurity photovoltaic effect**Project supported by the National Natural Science Foundation of China (Grant Nos. 61464007, 61306084, and 51361022), the Postdoctoral Science Foundation of Jiangxi Province, China (Grant No. 2014KY32), and the Natural Science Foundation of Jiangxi Province, China (Grant No. 20122BAB202002).

The near-infrared responsivity of a silicon photodetector employing the impurity photovoltaic (IPV) effect is investigated with a numerical method. The improvement of the responsivity can reach 0.358 A/W at a wavelength of about 1200 nm, and its corresponding quantum efficiency is 41.1%. The origin of the enhanced responsivity is attributed to the absorption of sub-bandgap photons, which results in the carrier transition from the impurity energy level to the conduction band. The results indicate that the IPV effect may provide a general approach to enhancing the responsivity of photodetectors.

Impurity photovoltaic effect in silicon solar cells doped with two impurities

In this work, a numerical study has been carried out to investigate the impurity photovoltaic (IPV) effect for silicon solar cells doped with two impurities (indium and thallium). It is found that the conversion efficiency \(\eta \) of the IPV solar cell doped with two impurities can improve by 2.21 % absolute, which is greater than that of the IPV solar cell doped with indium (\(\Delta \eta =1.63\,\%\)), but less than that of the one doped with thallium (\(\Delta \eta =2.69\,\%\)). It is concluded that introducing two IPV impurities may not be a good selection for implementing the IPV effect since one impurity with poorer IPV effect can absorb some sub-bandgap photons while contributing fewer currents. The location of impurity energy level is critical to the IPV cell performance. For an acceptor-type IPV impurity, the optimized location of the IPV impurity energy level locates at 0.20–0.26 eV above the valence band edge. Our results may help to make better use of the IPV effect for improving solar cell efficiency.

Effective harvesting of photons for improvement of solar energy conversion by graded bandgap multilayer solar cells

Various ways to harvest photons in the solar spectrum and heat energy from the surroundings have been considered by utilizing solid state physics principles. A graded bandgap multi-layer solar cell has been designed to absorb UV, visible and infrared radiation, and to combine impact-ionization and impurity photovoltaic (PV) effect to enhance photo-generation of charge carriers and the collection. The above theoretical concepts were experimentally tested using the well researched material system, GaAs/AlxGa(1-x)As. The highest reported Voc ~ 1175 mV together with the highest possible fill factors (FFs) in the mid 80 %’s, have been achieved for the initial devices. The overall efficiency achieved using only two growths is ~ 20 %. As expected from the initial design, the impurity PV effect is experimentally observed producing Voc values in the range of 650-900 mV under complete darkness for the GaAs/AlxGa(1-x)As based devices. Both the theoretical concepts and experimental results are presented in this paper. DOI: http://dx.doi.org/10.4038/jnsfsr.v41i2.5702 J.Natn.Sci.Foundation Sri Lanka 2013 41 (2):73-80

Improving Silicon Solar Cell Efficiency by Using the Impurity Photovoltaic Effect

The necessity to find new forms of renewable energy is very important and urgent nowadays. The renewable sources of energy derived from the sun are one of the promising options. The photovoltaic cells as one of renewable energy sources have been largely studied in order to obtain cheap, efficient and secure PV cells. The conversion efficiency is the most important property in the PV domain. The most important aim of PV manufacturers is to reduce the price of the solar cells and increase their efficiencies above the Shockley Queisser limit. Third generation concepts have been studied recently in an attempt to improve solar cell efficiency above this limit. The impurity photovoltaic (IPV) effect is one of these concepts used to augment cell infrared response and therefore enhance cell conversion efficiency. The idea of the IPV effect is based on the insertion of deep defects in the solar cell. These defects provide a multistep absorption mechanism for sub-band gap photons to create new electron-hole pairs. In this paper we study numerically the potential of the IPV effect in crystalline silicon solar cell doped with a new IPV impurity. We investigate the effect of certain impurity and structure parameters on silicon solar cell characteristics such as short circuit current density Jsc, open circuit voltage Voc, conversion efficiency and quantum efficiency QE using SCAPS simulator. We find that the incorporation of the IPV impurities into silicon solar cells can enhance spectral response, short circuit current density and conversion efficiency only under some conditions

A Vertical PN Junction Utilizing the Impurity Photovoltaic Effect for the Enhancement of Ultra-thin Film Silicon Solar Cells

ABSTRACTWe report a preliminary study on the influence of indium doping on ultra-thin film silicon solar cells. The design of the cell reported here is such that it should elucidate the impact of the indium dopant, which is concentrated in the thin film. Indium, a deep level in silicon (0.157 eV above the valence band), acts as a p-type dopant and a sensitizer. Absorption through sub-bandgap transitions is expected based on the previously reported Impurity PhotoVoltaic (IPV) Effect [1]. It is proposed that the implementation of a novel vertical PN junction configuration together with the IPV effect enhances the efficiency of ultra-thin solar cells. The most efficient cell fabricated to date, in our research group, has a conversion efficiency of 4.3 % (active silicon thickness of 2.5 μm), a short-circuit current density of 14.9 mA/cm2 and an open-circuit voltage of 0.51 V under 1 sun illumination. The cell has not been optimized with any type of light trapping technique and 11.24 % of the cell surface is covered by the metal contacts. Numerical simulation indicates that for the geometry used, the maximum efficiency that may be expected is 9.8 % (compared to the 4.3 % measured).

Impurity Photovoltaic Effect in Multijunction Solar Cells

A study has been carried out to investigate the Impurity Photovoltaic (IPV) effect on the performance of a double junction solar cell, which is yet, to the best of the authors’ knowledge, the first attempt at incorporating IPV effect in multijunction solar cell. In this work, impurity has been introduced in a double junction solar cell - separately and simultaneously in the two sub-cells which have been found to improve the cell performance. However, the effect of impurity becomes prominent when it is applied to the sub-cell with lower current. Furthermore, impact of impurity concentration has been obtained on short-circuit current density, open-circuit voltage and overall efficiency which have been explained through compensation by deep-level impurity dopant, conversion of conductivity type, carrier lifetime and total equilibrium carrier density. This improvement demonstrates the potential of the combination of IPV effect in multijunction solar cell.

Impurity photovoltaic effect in magnesium‐doped silicon solar cells with two energy levels

AbstractIn this paper, we have carried out a numerical study to evaluate the potential of the impurity photovoltaic (IPV) effect in magnesium‐doped silicon solar cells with two energy levels. We found that the cell efficiency can realize an absolute increase of above 2% due to the IPV effect. The efficiency improvement results from the extension of the infrared response of the solar cell. These results indicate that the IPV effect in silicon solar cells doped with magnesium is a potential means for improving cell efficiency.

Positive or negative gain: Role of thermal capture cross sections in impurity photovoltaic effect

In this work, we carried out a numerical study on the role of thermal capture cross sections in impurity photovoltaic (IPV) effect for silicon solar cells doped with indium. The short-circuit current density, the open-circuit voltage and the conversion efficiency of the IPV cell were calculated in dependence of variable electron and hole thermal capture cross sections of indium in silicon. We found that the thermal capture cross section of electron is crucial to the device performance and that of hole has few influence on the cell property for this IPV cell since acceptor-type indium impurity level is near the valence band edge. If the electron thermal capture cross section is less than 10−20 cm2, a positive gain of conversion efficiency for the IPV cell would be presented. We concluded that those impurities with small electron (or hole) thermal capture cross sections may be suitable for use in the IPV cell with acceptor-type (or donor-type) impurity level near the valence (or conduction) band edge. These results may help to evaluate the potential of the IPV effect for improving cell efficiency according to the thermal capture cross sections of the impurity in host semiconductor.

Numerical simulation of the impurity photovoltaic effect in silicon solar cells doped with thallium

Many attempts have been made to increase the efficiency of solar cells by introducing a deep impurity level in the semiconductor band gap. Since Tl may be the most suitable impurity for crystalline Si solar cells, the impurity photovoltaic (IPV) effect in silicon solar cell doped with thallium as impurity was investigated by the numerical solar cell simulator SCAPS. Results show that the IPV effect of thallium extends the spectral sensitivity in the sub-band gap range from 1000 to about 1400 nm. When the Tl concentration ( N t ) is lower than the base doping density ( N D ), the short-circuit current density and efficiency increase with increasing N t . But they decrease rapidly as the impurity density exceeds the shallow base doping density ( N t > N D ). The optimum Tl concentration is about equal to the base doping density. For the Si solar cells with high internal reflection coefficients, the IPV effect becomes appreciable (Δ J sc≈9 mA/cm 2 and Δ η≈2%).

Impurity photovoltaic effect in GaAs solar cell with two deep impurity levels

The impurity photovoltaic (IPV) effect has been extensively investigated for silicon solar cells with indium impurities. A small positive effect on the efficiency was theoretically predicted, provided that efficient light trapping schemes are applied. Since IPV effects are predicted to be more pronounced in wider band gap materials and possibly also by introducing more than one impurity level, we carried out a numerical study on GaAs solar cells with copper impurities. Indeed, copper introduces multiple acceptor type mid-gap levels in GaAs, two of which are investigated here: one at 0.14 eV and another at 0.40 eV above the valence band edge. We used a solar cell device simulator (SCAPS) specially adapted to include the IPV effect. We varied the impurity concentration and the light trapping parameters of the device, and calculated the occupation probability of the impurity levels, the generation and recombination through these levels (thermal and optical) and the solar cell characteristics: quantum efficiency, QE( λ), short-circuit current J sc, open-circuit voltage V oc and efficiency η. A significant IPV effect with extended response in the infrared is calculated when efficient light trapping is present. When an internal reflection coefficient R=0.99 can be reached at both the front and the back surface of the solar cell, an increase of the short-circuit current densities 2 mA/cm 2 can be obtained. The consequences of two levels instead of one are calculated and discussed, leading to a trade-off between J sc improvement and a V oc decrease.

Impurity photovoltaic effect: Fundamental energy conversion efficiency limits

In the past, minimal improvements have been predicted for efficiency enhancement of solar cells using the impurity photovoltaic (IPV) effect, where optical excitation through midgap defect levels allows the use of long wavelength photons to increase the conversion efficiency of sunlight to electricity. In the present work, the principle of detailed balance is used to calculate the limiting efficiency of solar cells with the inclusion of the impurity photovoltaic effect, in the idealized case when all transitions are assumed to be radiative. Based on these calculations, the limiting efficiency of the IPV device with a large number of different defect species is determined to be 77.2%. The terrestrial performance of the IPV device is also investigated by comparing its spectral sensitivity with that of tandem solar cell designs.

The impurity photovoltaic (IPV) effect in wide‐bandgap semiconductors: an opportunity for very‐high‐efficiency solar cells?

AbstractFollowing recent progress in the study of limiting efficiencies of photovoltaic devices with multiple energy levels, we suggest using the impurity photovoltaic (IPV) effect in wide‐bandgap semiconductors as a means to achieve very‐high‐efficiency solar cells. We discuss the requirements for a high‐efficiency IPV device and review some of the material systems that could be used. As a case study, we investigate theoretically β‐SiC IPV solar cells with a model based on a modified Shockley–Read–Hall theory. The high‐efficiency potential is confirmed and the important issues for implementation are presented and discussed. Copyright © 2002 John Wiley &; Sons, Ltd.

Impurity photovoltaic effect in c-Si solar cells. A numerical study

A numerical study is made of the impurity photovoltaic (IPV) effect in crystalline Si p-n junction solar cells. Application is made to In impurities whose energy level is at 0.157 eV above the valence band edge. The numerical resolution of Poisson’s equation and the continuity equations for electrons, holes and defect levels yields the current–voltage curves for different impurity concentrations Nt, from which the short-circuit current density Jsc, the open-circuit voltage Voc, and the energy conversion efficiency η is obtained. It is shown that increasing the defect concentration Nt with respect to the background donor concentration leads to an increase of Jsc but decreases Voc and η. A modified defect concentration profile in a p-n-n+ structure is suggested to extract the maximum benefit from the IPV effect.

Study for improvement of solar cell efficiency by impurity photovoltaic effect

This paper is to study for efficiency improvement of solar cells by utilizing impurity traps introduced in the band gap of semiconductor, that is, impurity photovoltaic (IPV) effect. It is revealed theoretically that there is a certain energy range where impurity-traps act as stepping stones in two-step excitation of electrons from the valence band to the conduction band under suppression of carrier recombination through such traps. Indium is selected as one of proper impurities that satisfy this condition in crystalline silicon, and theoretical prediction is experimentally verified. A good agreement between theory and experiment is obtained concerned with photoconductive properties. It is concluded that the IPV effect is useful to improve the cell efficiency.

Extended infrared response of silicon solar cells and the impurity photovoltaic effect

Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10 −7 cm −1 have been determined, revealing structure due to 3- and 4-phonon assisted absorption. These values are compared with a more recent determination of the absorption edge based on photoluminescence measurements. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross section for electron photoemission from the indium level, a crucial parameter for modelling indium's IPV effect, is determined.

Efficiency improvements of silicon solar cells by the impurity photovoltaic effect

A theoretical investigation of the impurity photovoltaic (IPV) effect for improving silicon solar-cell efficiency is presented. The approach is better than previous analyses because of the improved treatment of generation and recombination via impurities, and because it includes the effects of optical competition and light trapping. The approach is applied to the nonmidgap, deep-level impurity indium as the IPV effect impurity incorporated into an idealized silicon solar cell. The analysis is based on experimentally determined parameters for indium. Improvements of cell current, subgap spectral response, and energy conversion efficiency are quantified. The analysis reveals the importance of light trapping and proper selection of indium and dopant concentrations. The impurity photovoltaic effect is predicted to improve solar-cell efficiency.