Reduction In Open-circuit Voltage Research Articles

Charge carrier-selective contacts for nanowire solar cells

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.

Long-term performance and degradation rate analyses of photovoltaic modules exposed in the Gobi Desert, Mongolia

Two types of crystalline silicon (c-Si) photovoltaic (PV) modules have been tested in the cold-dry climate of the Gobi Desert of Mongolia, from 2002 to 2012, to verify the preliminary estimation of PV power generation. This study mainly focuses on the analysis of the long-term performance of the PV modules using indices such as energy yield, reference yield, performance ratio (PR), and characterization indices of the main electrical parameters as well as degradation rate. Average annual energy yields were 1880 h for Module 1 and 1789 h for Module 2. Thus, PRs were 0.85 and 0.80, while the average annual reference yield was 2223 h. On the other hand, degradation rates of Modules 1 and 2 were −1.28 and −0.86%/year, respectively. The electrical degradation observed in both modules was due to the loss of short-circuit current as open-circuit voltage reduction was not considered in this field test.

Trade-off between Photon Management Efficacy and Material Quality in Thin-Film Solar Cells on Nanostructured Substrates of High Aspect Ratio Structures

Although texturing of the transparent electrode of thin-film solar cells has long been used to enhance light absorption via light trapping, such texturing has involved low aspect ratio features. With the recent development of nanotechnology, nanostructured substrates enable improved light trapping and enhanced optical absorption via resonances, a process known as photon management, in thin-film solar cells. Despite the progress made in the development of photon management in thin-film solar cells using nanostructures substrates, the structural integrity of the thin-film solar cells deposited onto such nanostructured substrates is rarely considered. Here, we report the observation of the reduction in the open circuit voltage of amorphous silicon solar cells deposited onto a nanostructured substrate with increasing areal number density of high aspect ratio structures. For a nanostructured substrate with the areal number density of such nanostructures increasing in correlation with the distance from one edge of the substrate, a correlation between the open circuit voltage reduction and the increase of the areal number density of high aspect ratio nanostructures of the front electrode of the small-size amorphous silicon solar cells deposited onto different regions of the substrate with graded nanostructure density indicates the effect of the surface morphology on the material quality, i.e., a trade-off between photon management efficacy and material quality. This observed trade-off highlights the importance of optimizing the morphology of the nanostructured substrate to ensure conformal deposition of the thin-film solar cell.

InAs thermophotovoltaic cells with high quantum efficiency for waste heat recovery applications below 1000 °C

InAs thermophotovoltaic (TPV) cells with external quantum efficiency at the peak wavelengths reaching 71% at low temperature and 55% at room temperature are reported, which are the highest values to date for InAs. The TPV exhibited 10% power conversion efficiency at 100K cell temperature. The dark and light current-voltage characteristics were measured at different cell temperatures (100–340K) in response to heat sources in the range 500–800°C. The resulting dependences of the output voltage and current as well as the spectral response of the InAs TPV have been extensively characterized for waste heat recovery applications. The performance of these cells is strongly determined by the dark current which increases rapidly with increasing cell temperature originating from bandgap narrowing, which resulted in a reduction of open circuit voltage and output power.

Performance Evaluation of PV Panel Under Dusty Condition

The performance of PV panel depends on the incoming sunlight on its surface. The accumulated airborne dust particles on panel surface creates a barrier in the path of sunlight and panel surface, which significantly reduces the amount of solar radiation falling on the panel surface. The present study shows a significant reduction in short circuit current and power output of PV panel due to dust deposition on its surface, whereas the reduction in open circuit voltage is not much prominent. This study has been carried in the field as well as in the laboratory. The reduction in maximum power output of PV panel for both the studies ensures a linear relation with the dust deposition on its surface. In the field study, the reduction in the power output due to 12.86gm of dust deposition on the panel surface was 43.18%, whereas in the laboratory study it was 44.75% due to 11gm of dust depositionArticle History: Received July 10th 2017; Received in revised form Sept 15th 2017x; Accepted 1st Oct 2017; Available onlineHow to Cite This Article: Tripathi, A.K., Aruna, M. and Murthy, Ch.,S.N. (2017). Performance Evaluation of PV Panel Under Dusty Condition. International Journal of Renewable Energy Develeopment, 6(3), 225-233.https://doi.org/10.14710/ijred.6.3.225-233

Effect of KF absorber treatment on the functionality of different transparent conductive oxide layers in CIGSe solar cells

AbstractThis contribution studies the impact of the KF‐induced Cu(In,Ga)Se2 (CIGSe) absorber modification on the suitability of different transparent conductive oxide (TCO) layers in solar cells. The TCO material was varied between ZnO:Al (AZO), ZnO:B (BZO), and In2O3:H (IOH). It is shown that the thermal stress needed for optimized TCO properties can establish a transport barrier for charge carriers, which results in severe losses in fill factor (FF) for temperatures >150°C. The FF losses are accompanied by a reduction in open circuit voltage (Voc) that might originate from a decreased apparent doping density (Nd,app) after annealing. Thermally activated redistributions of K and Na in the vicinity of the CdS/(Cu,K)‐In‐Se interface are suggested to be the reason for the observed degradation in solar cell performance. The highest efficiency was measured for a solar cell where the absorber surface modification was removed and a BZO TCO layer was deposited at a temperature of 165°C. The presented results highlight the importance of well‐designed TCO and buffer layer processes for CIGSe solar cells when a KF post deposition treatment (KF‐PDT) was applied.

Deleterious Effect of Negative Capacitance on the Performance of Halide Perovskite Solar Cells

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current–voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance.

High efficiency organic solar cells based on amorphous electron-donating polymer and modified fullerene acceptor

The current manuscript examines the structure and property relationship of amorphous conjugated polymer based bulk heterojunction solar cells. Solvent vapor annealing (SVA) is used to optimize the morphology. It is seen that the structure order of the blended thin film does not change but phase separation becomes prominent. The space charge limited current and admittance spectroscopy reveals that the charge transport ability of the SVA-treated device is significantly strengthened. Furthermore, both the current-voltage analysis and impedance spectroscopy illustrates that SVA can effectively promote the fill factor (FF) and short circuit current density (Jsc) due to the enhanced charge transport properties. However, the increased mobility after the SVA treatment causes non-geminate charge carrier recombination and reduces geminate recombinations. In particular, the reduction in open circuit voltage (Voc) of the solvent vapor annealed devices is found to originate from the charge recombination induced by the quasi-Fermi level variation between the donor and acceptor materials.

Textured and micropillar silicon heterojunction solar cells with hot-wire deposited passivation layers

The passivating quality of hydrogenated amorphous silicon thin films is essential for achieving high open-circuit voltages in silicon heterojunction solar cells. This work reports our progress towards the use of hot wire chemical vapor deposition for fabricating these passivation layers. We have achieved 19.4% energy conversion efficiency for a solar cell with a conventional alkali-etched pyramidal texture. Next, we tested radial heterojunction devices that consist of up to 20-μm high micropillar arrays. Proper passivation of these devices is challenging, since the structures feature an enlarged surface area and vertical walls. We have made use of the ability of hot wire chemical vapor deposition to deposit conformal thin films at high deposition rate. The micropillar solar cells show only limited reduction of open-circuit voltage in comparison to a flat reference solar cell and have an improved overall performance. We created additional stacked multijunction silicon-based junctions on top of the micropillars to increase the open-circuit voltage of the solar cells for future applications in which a higher potential is required such as water splitting devices.

UV-induced degradation of PERC solar modules with UV-transparent encapsulation materials

In this paper, we report on the stability of p-type passivated emitter and rear cells under ultraviolet (UV) exposure with various silicone nitride passivation coatings and embedded in different encapsulation polymers. Our results reveal that UV transparent polymers can result in a module power loss of up to 6% under a UV irradiation dose of 497 kWh/m2. We show that the degradation in power is caused by a reduction in open circuit voltage. This loss is related to an increased recombination in the cell, which we ascribe to a degradation of the surface passivation. With ray tracing simulations, we determine the number of photons reaching the passivation interface. Assuming that all photons with energies above 3.5 eV de-passivate the interface is in agreement with our experimental results. Copyright © 2017 John Wiley & Sons, Ltd.

InGaAs quantum well-dots based GaAs subcell with enhanced photocurrent for multijunction GaInP/GaAs/Ge solar cells

MOCVD-grown GaAs single-junction solar cells (SC) with quantum well-dots (QWD) were fabricated and tested. The QWD were formed by the deposition of In0.4Ga0.6As layers separated with GaAs spacers. A remarkable improvement of photocurrent was achieved and the reduction of open-circuit voltage was partly suppressed by decreasing the spacers’ growth rate as well as increasing their thickness up to 40 nm. Based on the experimentally obtained characteristics of these single-junction SCs we estimated that using QWD media in the middle (GaAs-based) subcell can provide 1 abs. %, increasing the efficiency of the triple-junction GaInP/GaAs/Ge SCs.

Recent progress on quantum dot solar cells: a review

Semiconductor quantum dots (QDs) have a potential to increase the power conversion efficiency in photovoltaic operation because of the enhancement of photoexcitation. Recent advances in self-assembled QD solar cells (QDSCs) and colloidal QDSCs are reviewed, with a focus on understanding carrier dynamics. For intermediate-band solar cells using self-assembled QDs, suppression of a reduction of open circuit voltage presents challenges for further efficiency improvement. This reduction mechanism is discussed based on recent reports. In QD sensitized cells and QD heterojunction cells using colloidal QDs well-controlled heterointerface and surface passivation are key issues for enhancement of photovoltaic performances. The improved performances of colloidal QDSCs are presented.

Numerical simulation of silicon heterojunction solar cells with Si/Si1-xGex quantum wells

Heterojunction with intrinsic thin-layer (HIT) solar cells attract attention due to their high open circuit voltage and stable performance. However, short circuit current density is difficult to improve due to light losses of transparent conductive oxide and hydrogenated amorphous silicon passivation (a-Si:H) layer and low absorption coefficient of crystalline silicon (c-Si). Silicon germanium alloy (Si/Si1-xGex) quantum wells and quantum dots are capable of improving low light utilization by strong optical absorption in the infrared region. In this article, opto-MoS2of the HIT solar cells integrated with Si/Si1-xGex quantum wells (HIT-QW) as a surface absorber are investigated by numerical simulation with Technology Computer Aided Design (TCAD). The influences of germanium content on the MoS2of HIT solar cells with long carrier lifetimes of Si1-xGex layers (p*) and defect-free a-Si:H/c-Si interface are investigated at first. The simulation results indicate that optical utilization in the infrared region is enhanced with the increase of germanium fraction, while open circuit voltage degrades due to the decreasing of the energy band gap of Si1-xGex, radiative recombination and auger recombination mechanism in the Si/Si1-xGex quantum wells. And the conversion efficiency reaches a maximum value at a germanium fraction of 0.25 then drops distinctly. When the germanium fraction increases from 0 to 0.25, the short circuit current density increases from 34.3 mA/cm2 to 34.8 mA/cm2, while the open circuit voltage declines from 749 mV to 733 mV. Hence, the conversion efficiency increases from 21.5% to 21.7% due to the fact that the enhancement of short circuit current density compensates for the reduction of open circuit voltage. When the germanium content increases to more than 50%, a serious open circuit voltage loss of more than 130 mV associated with the energy band gap loss of Si1-xGex arises in the HIT-QW solar cells, which indicates that the dominating carrier transport mechanism changes from shockley diffusion to recombination in the Si/Si1-xGex quantum wells. Subsequently, the influences of interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells are discussed. Both interface holes at a-Si:H/c-Si interface and bulk holes in Si1-xGex quantum wells can be recombined through the interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells, respectively, which restricts the position of hole fermi level in the open circuit condition. When the germanium fraction increases, the influence of interface defects at a-Si:H/c-Si interface becomes weak on the degradation of open circuit voltage compared with the significant influence of the bulk trap centers. Moreover, p* of longer than 510-5 s is necessary for the retention of electrical performance of HIT-QW solar cells by the simulation. Based on this research, high-efficiency HIT solar cells can be achieved by incorporating high-quality Si/Si0.75Ge0.25 quantum wells, which also requires the impactful passivation of a-Si:H/c-Si interface.

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of Jsc, Voc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in Jsc and Voc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.

Analysis of Losses in Open Circuit Voltage for an 18-μm Silicon Solar Cell

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.

Low irradiance loss quantification in c-Si panels for photovoltaic systems

Performance of bulk Si based solar photovoltaic (PV) panels deteriorate in weak light conditions. This generally affects the efficiency of associated power electronic components and compounds the overall loss in the yield of a PV system. It is, therefore, imperative to carefully plan the PV sizing and quantify this inherent loss in the efficiency of PV panels. In this work, the impact of varying solar irradiation on the efficiency of crystalline Si based solar panels has been quantified through a novel mathematical formulation, which models the deterioration in performance based on actual insolation and temperature profiles. This loss phenomenon is attributed mainly to a reduction in open circuit voltage and fill factor of a panel which in turn lowers the conversion efficiency. An empirical relationship is also derived which relates the loss in energy harvested to the actual peak-sunlight hours of a region. Results indicate that this low irradiance loss has approximately a linear dependence on equivalent peak sunlight hours (EPSH). For instance, the loss of 3.8%, 4%, 4.5% for Austin, Washington and Seattle was observed for EPSH of 4.4, 4, and 3.4, respectively. Moreover, the loss variation from one year to another for a region has been approximately constant. Finally, a comparison was performed for predicted energy harvested to the measured yield and a good fit has been observed between predicted and actual values.

Enhanced photovoltaic performance of a cadmium sulfide/cadmium selenide-sensitized solar cell using an aluminum-doped zinc oxide electrode

Al-doped ZnO nanorods were used as the photoanode of quantum-dot-sensitized solar cells (QDSSCs). Solar cell with CdS/CdSe quantum dots co-sensitized 0.5% Al-doped ZnO nanorods yielded a photovoltaic conversion efficiency of 3.16%, which is higher than that of the cell based on undoped ZnO nanorods (2.30%). This energy conversion efficiency improvement might be attributed to the increased light harvesting efficiency induced by the large amount of quantum dots adsorbed on the large surface area of ZnO NRs, and reduction of transport resistance in the photoanode of QDSSCs with introduction of Al into ZnO structure, which leads to the increasing short-circuit current density. The results also show that recombination rate increases with introduction of Al into ZnO, resulting in the reduction of open-circuit voltage.

Minimized open-circuit voltage reduction in GaAs/InGaAs quantum well solar cells with bandgap-engineered graded quantum well depths

The use of InGaAs quantum wells with composition graded across the intrinsic region to increase open-circuit voltage in p-i-n GaAs/InGaAs quantum well solar cells is demonstrated and analyzed. By engineering the band-edge energy profile to reduce photo-generated carrier concentration in the quantum wells at high forward bias, simultaneous increases in both open-circuit voltage and short-circuit current density are achieved, compared to those for a structure with the same average In concentration, but constant rather than graded quantum well composition across the intrinsic region. This approach is combined with light trapping to further increase short-circuit current density.

Phosphor-doping enhanced efficiency in bilayer organic solar cells due to longer exciton diffusion length

We fabricated bilayer organic solar cells (OSCs) in the structure glass/ITO/PEDOT:PSS/PtOEP:MEH-PPV/C70/Al, where MEH-PPV was doped with platinum octaethylporphyrin (PtOEP). Enhanced exciton diffusion length (LD) is realized via converting generated singlet excitons to triplet excitons. Investigation based on transfer matrix simulations reveals that it is the extended exciton LD of the doping donor layer that leads to the short-circuit current density (Jsc) and power conversion efficiency (PCE) improvement, when compared with those of the OSCs with a non-doping donor layer. As a result of the increased LD, Jsc and PCE increase by 30% and 42% respectively for a device with 5wt% PtOEP-doped 25nm-thick donor layer. Meanwhile, by doping with phosphorescent bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III), the reduction in open-circuit voltage and the comparable Jsc are shown due to its higher HOMO level and higher LUMO level, leading to the decrease of PCE. It demonstrates that doping a polymer with a suitable phosphorescent molecule is an important approach to be considered to increase the exciton LD.

Plasmonic Nanoparticle Films for Solar Cell Applications Fabricated by Size-selective Aerosol Deposition

A soft deposition method for incorporating surface plasmon resonant metal nanoparticles within photovoltaic devices was studied. This self-assembly method provides excellent control over both nanoparticle size and surface coverage. Films of spherical Ag nanoparticles with diameter of ∼100nm were fabricated by depositing size-selected aerosols on various substrates using electrophoresis. This novel deposition method opens the route to embed plasmonic nanoparticles in the intermediate reflector of a micromorph silicon tandem PV cell. We have for the first time fabricated working tandem cells of this type. Compared to a flat reference device the Ag particles enhanced the short-circuit current density due to improved light trapping. The enhancement is, however, limited by the sulfidation on the surface of Ag nanoparticles and a further optimization of the cell fabrication method is required to prevent a reduction of open-circuit voltage and fill factor.