Absolute Efficiency Increase Research Articles

The Effects of Absorption and Recombination on Quantum Dot Multijunction Solar Cell Efficiency

The key characteristics of quantum dot (QD)-enhanced multijunction solar cells (MJSC) are explored theoretically by focusing on the generation and recombination rates throughout the QD layers in the middle subcell. The quantum dots are modeled using an effective medium to describe light absorption, confinement, and recombination properties. We report an 8% increase in the short-circuit current density accompanied by a 3% drop in an open-circuit voltage for a QD- enhanced MJSC at 1 sun illumination (1 kW/m2) compared with a control MJSC without QD. The drop in an open-circuit voltage is due in part to the increased recombination rates in the depletion region, decreased carrier lifetimes in the QDs, and the increased recombination rates resulting from carrier escape and capture. Overall, these contribute to an absolute increase in efficiency of over 1% for the studied QD-enhanced MJSC design for a QD density of 125 QD/μm2.

Lightly Doped Emitters for High Efficiency Silicon Wafer Solar Cells

Tube diffusion is the de facto industry standard for emitter formation for silicon wafer solar cells. In this paper, we report on optimised dopant profiles to produce a lightly doped emitter (LDE) with a lower surface dopant concentration. Solar cells are fabricated with a LDE and achieve a significant gain in open-circuit voltage (Voc) due to a reduced front surface and emitter recombination. The POCl3 tube diffusion process is optimised by introducing a pre-oxidation and multi-plateau drive-in during the diffusion process. Using the optimised process, the surface dopant concentration of our tube diffused n+ emitter is reduced from 4.5x1020 to 2.5x1020 atoms/cm3 for a ∼70 Ohms/sq emitter. An increase of 4mV in Voc, 0.2mA/cm2 in short-circuit current Jsc and 0.2% absolute increase in efficiency is achieved for solar cells using a LDE compared to solar cells with our standard emitter profile. The average fill factors for the standard and LDE batches are above 79%, suggesting the Ag paste (DuPont Solamet PV17F) used in this work had no issues in contacting the lower surface concentration of the LDE.

Cu(In, Ga)Se2 microcells: High efficiency and low material consumption

Using solar cells under concentrated illumination is known to improve the conversion efficiency while diminishing the active area and thus material consumption. Recent concentrator cell designs tend to go miniaturized devices, in the 0.5–1 mm range, enabling a better thermal evacuation due to higher surface to volume ratio. If the cell size is further reduced to the micrometric range, spreading resistance losses can be made vanishingly small. This is particularly interesting for the thin film technology which has been limited up to now to very low concentration systems, from ×1 to ×10, due to excessive resistive losses in the window layer and difficult thermal management of the cells, grown on glass substrates. A new solar cell architecture, based on polycrystalline Cu(In,Ga)Se2 (CIGS) absorber, is studied: microscale thin film solar cells. Due to the reduced lateral dimension of the microcells (5 to 500 μm in diameter), the resistive and thermal losses are drastically decreased, enabling the use of high concentration (>×100). This results in a breakthrough for concentration on this type of devices, which were previously limited to the low concentration range (about ×10). Due to light concentration, the open circuit voltage increases up to several thousand suns equivalent, to reach over 900 mV. The temperature increase is limited to less than 20 °C over the ambient at concentration around ×1000. A 5% absolute efficiency increase on microcells at ×475 is observed and a 21.3% ± 0.2% equivalent efficient microcell of 50 μm of diameter is measured.

Water injection in a micro gas turbine – Assessment of the performance using a black box method

Microturbines offer new perspectives in small-scale heat and power production however their profitability depends strongly on the yearly amount of running hours. The non-continuous heat demand often leads to a reduction in running hours. This paper proposes an alternative by recuperating the lost thermal power through the injection of heated water in the micro Gas Turbine (mGT). Water injection is considered a successful way to increase power and efficiency in industrial gas turbines and similar effects are expected for microturbines. This paper reports on a series of simulations of water injection performed on a Turbec T100 mGT. The goal of this study was to investigate the potential of water injection in the mGT cycle using an adiabatic black box method in the Aspen® process simulation tool. Past experiments with steam injection on the T100 demonstrated the potential of introducing steam/water in the microturbine cycle.Calculations revealed that the key parameters for maximum heat recuperation are stack and pinch temperature. Simulations showed that most of the exhaust heat can be recovered through injection of heated water after the compressor, resulting in an 18% decrease in fuel consumption and an absolute increase in electrical efficiency of 7%.

Record Efficiency on Large Area P-Type Czochralski Silicon Substrates

In this work we report a world record independently confirmed efficiency of 19.4% for a large area p-type Czochralski grown solar cell fabricated with a full area aluminium back surface field. This is achieved using the laser doped selective emitter solar cell technology on an industrial screen print production line with the addition of laser doping and light induced plating equipment. The use of a modified diffusion process is explored in which the emitter is diffused to a sheet resistance of 90 Ω/□ and subsequent etch back of the emitter to 120 Ω/□. This results in a lower surface concentration of phosphorus compared to that of emitters diffused directly to 120 Ω/□. This modified diffusion process subsequently reduces the conductivity of the surface in relation to that of the heavily diffused laser doped contacts and avoids parasitic plating, resulting an average absolute increase in efficiency of 0.4% compared to cells fabricated without an emitter etch back process.

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

Light trapping in pyramidally textured crystalline silicon solar cells using back‐side diffractive gratings

ABSTRACTBack‐side diffractive gratings enhance a solar cell's efficiency by trapping light inside the cell and increasing the probability of absorption. We introduce a three‐dimensional, polarization‐sensitive optical model combining ray tracing and rigorous coupled‐wave analysis to investigate silicon solar cells with pyramidal front‐side texturing and back‐side gratings. Parameter optimization is performed to increase the short‐circuit current density for a linear binary grating with grating period p and height h. For the investigated 180‐µm‐thick pyramidally textured silicon solar cells, the simulation yields a maximum enhancement of the short‐circuit current density by ΔJSC = 1.79 mA/cm2 corresponding to an absolute efficiency increase of Δη = 0.90%. Furthermore, we report on fabrication and reflectance measurements of solar cells with gratings and key challenges in achieving efficiency gains using back‐side diffractive gratings. Copyright © 2012 John Wiley & Sons, Ltd.

Towards ultrathin copper indium gallium diselenide solar cells: proof of concept study by chemical etching and gold back contact engineering

ABSTRACTAn innovative approach combining chemical etching and a “lift‐off” process, which allows back contact processing after CIGSe deposition, permitted to use Au as a highly reflective back contact in ultrathin CIGSe solar cells. The Au back contact does not degrade the other parameters of the cell, as good ohmicity on CIGSe is achieved. An important photocurrent increase compared with regular Mo back contact solar cells is achieved by the enhanced light trapping effect due to the back reflector, leading to an absolute efficiency increase of +2.5% for a CIGSe thickness of 0.4 µm. This approach could be used for further investigations in improving the back side of ultrathin CIGSe solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

Nickel Silicide Formation Using Excimer Laser Annealing

In this work, we report on a self-aligned nickel silicide formation technique based on excimer laser annealing (ELA). We evaluate this process for the front contact formation of industrial PERC type solar cells on random pyramid textured Si surfaces where damage to surface texture, emitter passivation, or to the shallow junction should be avoided or minimized. PERC type solar cells obtained by POCl3 diffusion were processed on large area (12.5x12.5cm2) CZ-Si. Self-aligned litho-free Ni/Cu contacts defined by ps-laser ablation of the SiO2/SiNx anti-reflective coating (ARC) and subsequent ELA of the Ni layer were compared to conventional Ag screen printed contacts.The novel ELA process results in an absolute gain in Jsc of 0.8mA/cm2 as well as a drop of 0.3Ω.cm2 in series resistance (Rs) compared to SP Ag contacts due to reduced shading and resistance losses. This leads to 0.5% absolute increase in efficiency from 19.3% to 19.7% since other characteristics (Voc, pFF) could be maintained to the same level. In this work, the best performing cell with the ELA process reached an outstanding 20.0% energy conversion efficiency with Jsc=39.3mA/cm2, Voc=649.8mV, and FF=78.3%.

Microscale solar cells for high concentration on polycrystalline Cu(In,Ga)Se2 thin films

We report high concentration experiments on polycrystalline thin film solar cells. High level regime is reached, thanks to the micrometric scale of the Cu(In,Ga)Se2 cells, which strongly decreases resistive losses. A 4% absolute efficiency increase is obtained at a concentration of ×120, and current densities as high as 100 A/cm2 can be measured. These results show that the use of polycrystalline thin films under high concentration is possible, with important technological consequences.

Resistive and thermal scale effects for Cu(In, Ga)Se2 polycrystalline thin film microcells under concentration

Using solar cells under concentrated illumination is known to improve the conversion efficiency while diminishing the active area, and thus material consumption. Recent concentrator cell designs tend to go to smaller devices, in the 0.5–1mm lateral range, enabling a better thermal evacuation due to higher surface to volume ratio. If the cell size is further reduced to the micrometric range, spreading resistance losses can be made vanishingly small. This is particularly interesting for thin film technology which has been limited up to now to very low concentrations, 1–10 suns, due to excessive resistive losses of the window layer and difficult thermal management of the cells, grown on glass substrates. In order to prove that high injection regime can be implemented on polycrystalline thin film solar cells, we fabricated Cu(In, Ga)Se2 (CIGS) thin film microcells with diameter from 7 μm to 150 μm, and characterized them under concentrated illumination. A 4% absolute efficiency increase is obtained at 120 suns, and current densities as high as 100 A cm−2 can be measured, without affecting the cell performances. The temperature increase under high fluxes is drastically reduced in microcells: less than 20 K at 1000 suns for microcells under 50 μm in diameter. These results show that the use of polycrystalline thin films under high concentration is indeed possible, with important technological consequences.

Using Design of Experiments Approach to Optimize Custom Emitter Clean Process used in PV Manufacturing

Abstract We describe a ‘Design of Experiment’ (DOE) technique used to optimize the process parameters of a new commercially available emitter clean solution aimed to enhance the output efficiency of the cell. Emitter etch solutions are known to substantially improve efficiency by reducing surface recombination of charge carriers and increasing the short circuit current of the cell. We have used a full factorial design of experiment with three variables kept at two levels. Appropriate randomization of the run order and three centre points were chosen in order to reduce experimental errors. Adjacent multicrystalline wafers chosen for this experiment were subjected to acid texturization, phosphorus doping, diffusion and phosphosilicate glass etching before the emitter clean step. We chose mean emitter sheet resistance (Rsheet) (average over 25 positions on each wafer) as the response of this experiment. The aim was to achieve an increment of 10% on Rsheet after emitter clean. Voc scan of the wafers were done to validate the Rsheet increments observed. The results showed that H2O2 concentration and temperature had statistically significant effects on change of Rsheet. Regression analysis was used to predict the optimized parameters that may achieve a 10% increase of Rsheet. A validation run was carried out with this recipe in an automated production line and solar cells fabricated from this run showed a 0.18% (absolute) increase in efficiency and 15% (relative) reduction of recombination current density (Jo2) compared to standard reference cells processed without emitter clean.

An opening for solar cells

Collaborating researchers from Taiwan and the US have developed a laser opening technique for selective emitter (SE) formation of multi-crystalline Si solar cells on 6x6 inch wafers, and have achieved up to nearly a 1% absolute increase in efficiency. This research forms part of an initiative driven by the Industrial Technology Research Institute (ITRI) in Taiwan. Professor Jack (Jyh-Jier) Ho, one of the collaborators in this work from the National Taiwan Ocean University, explained their motivations: “ITRI has three mission statements: one, to expedite the development of new industrial technology; two, to aid in the process of upgrading industrial technology techniques; and three, to establish future industrial technology. In last few years, we have recognised the necessity for eco-sustainable industry development and global environmental protection, and therefore have devoted more resources to developing eco-friendly and sustainable energy technologies. We also aim to break down research bottlenecks and develop cutting-edge technology that would encourage domestic suppliers to invest in emerging energy solutions and environmental protection actions.”

Simulation and implementation of a porous silicon reflector for epitaxial silicon solar cells

AbstractOne of the main challenges in the ongoing development of thin film crystalline silicon solar cells on a supporting silicon substrate is the implementation of a long‐wavelength reflector at the interface between the epitaxial layer and the substrate. IMEC has developed such a reflector based on electrochemical anodization of silicon to create a multi‐layer porous silicon stack with alternating high and low porosity layers. This innovation results in a 1–2% absolute increase in efficiency for screenprinted epitaxial cells with a record of 13·8%. To reach a better understanding of the reflector and to aid in its continued optimization, several extensive optical simulations have been performed using an in‐house‐developed optical software programme. This software is written as a Microsoft Excel workbook to make use of its user‐friendliness and modular structure. It can handle up to 15 individual dielectric layers and is used to determine the influence of the number and the sequence of the layers on the internal reflection. A sensitivity analysis is also presented. A study of the angle at which the light strikes the reflector shows separate regions in the physical working of the reflector which include a region where the Bragg effect is dominant as well as a region where total internal reflection plays the largest role. The existence of these regions is proved using reflection measurements. Based on these findings, an estimate is made for the achievable current gain with an ideal reflector and the potential of epitaxial silicon solar cells is determined. Copyright © 2008 John Wiley & Sons, Ltd.

Development of Screen-Printed Silicon Solar Cells With High Fill Factors on 100<tex>$Omega$</tex>/sq Emitters

High-quality DuPont screen-printed Ag contacts were achieved on high sheet-resistance emitters (100 /spl Omega//sq) by rapid alloying of PV168 Ag paste. Excellent specific contact resistance (/spl sim/1 m/spl Omega/-cm/sup 2/) in conjunction with high fill factor (FF) (0.775) were obtained on 100 /spl Omega//sq emitters by a 900/spl deg/C spike firing of the PV168 paste in a belt furnace. The combination of the alloying characteristics of the PV168 Ag paste and optimized single-step rapid low-thermal budget firing resulted in a cost-effective manufacturable process for high-efficiency Si solar cells. In addition, the co-fired 100 /spl Omega//sq cell showed a noticeable improvement of /spl sim/0.5% in absolute efficiency over a conventional co-fired 45 /spl Omega//sq emitter cell. Lighter doping in the 100 /spl Omega//sq emitter cell resulted in better blue-response compared to the conventional cell, contributing to /spl sim/1.3 mA/cm/sup 2/ improvement in short-circuit current. Improved surface passivation on 100 /spl Omega//sq emitter cell resulted in additional 0.6 mA/cm/sup 2/ increase in J/sub sc/, 15-mV higher V/sub oc/, and a 0.6% increase in absolute cell efficiency. Front grid design optimization resulted in a FF of 0.780, and a further improvement in cell efficiency to reach 17.4%.

Enhanced silicon solar cell performance by rapid thermal firing of screen-printed metals

Rapid thermal processing (RTP) of screen-printed (SP) Al on the back and silver (Ag) grid on the front produced significant improvement in back surface field (BSF) of n/sup +/-p-p/sup +/ float-zone (FZ) Si solar cells. Two-step firing was found to form more effective BSF than co-firing, resulting in 0.6-1.0% increase in absolute cell efficiency. In addition, RTP was found to be more effective than the beltline processing (BLP), resulting in 0.5-1.0% increase in absolute cell efficiency. Although the Al-BSF formed by the BLP was inferior to the RTP, the difference between the two is virtually eliminated during the subsequent RTP contact firing. Internal quantum efficiency (IQE) analysis of the solar cells gave effective back surface recombination velocities (S/sub eff/) of >5000 cm/s and /spl sim/1500 cm/s for co-firing in the BLP and the RTP, respectively. Two-step firing produced S/sub eff/ of /spl sim/1500 cm/s and /spl sim/700 cm/s in the BLP and the RTP, respectively. However, S/sub eff/ for the two-step firing, involving BLP BSF formation followed by RTP contact firing, was found to be /spl sim/700 cm/s, which indicates that RTP contact firing with a faster ramp-up (100/spl deg/C/s) restores the poor-quality BLP BSF. On the other hand, BLP contact firing with a slow ramp-up (<10/spl deg/C/s) degrades the high-quality RTP BSF, increasing S/sub eff/ from /spl sim/700 cm/s to /spl sim/1500 cm/s.

Gettering and hydrogen passivation of edge-defined film-fed grown multicrystalline silicon solar cells by Al diffusion and forming gas anneal

This study shows for the first time that a combination of Al treatment on the back, oxide passivation on the front, and 400 °C forming gas anneal in the presence of Al, raised the double-layer antireflection-coated edge-defined film-fed grown (EFG) silicon cell efficiency from 7.8% to 14.1%. Front oxide passivation contributed an ∼0.8% increase in absolute cell efficiency, Al diffusion on the back increased the efficiency by 1.4% (absolute), and the forming gas anneal (FGA) after the Al diffusion improved the cell efficiency by an additional 4.1% (absolute). A combination of the above three steps improved the EFG cell efficiency by 6.3%, indicating that the above three effects are complimentary. Oxide passivation reduced front surface recombination velocity and Al diffusion, while FGA improved diffusion length via gettering. We propose that the large increase in cell efficiency produced by the forming gas anneal results from bulk defect passivation by atomic hydrogen generated in the processing.

Design, fabrication, and analysis of 17-18-percent efficient surface-passivated silicon solar cells

A simple analytical model has been developed which provides useful guidelines for fabricating high-efficiency silicon solar cells. Consistent with the model calculations, both surfaces of n+-p-p+solar cells were passivated by a thin of thermally grown SiO 2 . Oxide passivation resulted in 17.2-percent efficient solar cells on 4 Ω . cm base material. Passivated cells show about 3 mA/cm2increases in J SC , ∼20 mV improvement in V OC , and ∼2-percent increase in absolute cell efficiency compared to the counterpart 15.2-percent efficient unpassivated cells. The majority of improvement in V OC came from the emitter surface passivation, while both front- and back-surface passivation contributed to the increase in J SC . The emitter region should not be regarded as a dead layer because emitter surface passivation can increase the quantum efficiency at short wavelengths from 40 percent to greater than 75 percent.