Dark Saturation Current Density Research Articles

Development and characterization of multifunctional PassDop layers for local p+-laser doping

We present the development of aluminum oxide (AlOx) and boron-doped silicon nitride (SiNx:B) layer stacks for application on the back side of monocrystalline p-type silicon wafers. Two deposition techniques are used for the deposition of the AlOx/SiNx:B layer stacks, atomic layer deposition and plasma-enhanced chemical vapor deposition. Both techniques enable excellent surface passivation with surface recombination velocities of 4 cm/s after firing. Also, heavy local doping with sheet resistances down to 20 Ω/sq is possible by laser processing. We call this concept the PassDop approach. For the laser processed area where the silicon surface is locally boron-doped and the AlOx/SiNx:B passivation layer stack is locally removed, a quite low dark saturation current density of about 900 fA/cm2 is determined. The PassDop approach can be a solution to realize passivated emitter and rear locally doped PERL solar cells by improving their rear side properties while maintaining industrial applicability.

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer.

In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitates charge extraction from the BHJ active layer but also minimizes the energy loss within the charge transport processes. In addition, the organic-inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturation current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy, but effective, approach to reach high performance inverted PSCs.

Investigation of surface passivation schemes for p-type monocrystalline silicon solar cell

This paper represents an experiment to analyze the dark saturation current densities of passivated surfaces for monocrystalline silicon solar cells. The samples are diffused at peak temperatures of 800–950 °C. Basically, symmetrical lifetime samples with different doping profiles are prepared with alkaline textured and saw damage etched (planar) surfaces. After POCl3 diffusion, the phosphorous silicate glass layers are removed in a wet chemical etching step. Several designs are chosen for the determination of the sheet resistance (Rsh), the concentration profile for excess charge carrier and the minority carrier effective lifetime of the diffused surfaces. The dark saturation current densities (Jo) and the doping profiles are determined accordingly via quasi-steady state photoconductance decay measurement and electrochemical capacitance–voltage measurement. Three different passivation schemes are investigated as follows: silicon nitride (SiNx) deposited by plasma-enhanced chemical vapor deposition (PECVD) technique, silicon-rich oxynitride (SiriOxNy) capped with a PECVD SiNx layer, and thin thermally grown oxide, capped with a PECVD SiNx layer.

RETRACTED: Characterization of recombination properties at diffused surfaces for industrial silicon solar cell concepts

Abstract This paper describes an experiment to evaluate the surface recombination properties of phosphorous diffused surfaces for crystalline silicon solar cells. In this experiment the analysis of surface recombination properties for the phosphoryl chloride (POCl 3 ) diffusion is carried out. Investigation of recombination properties on diffused surfaces is crucial for the conversion efficiency of silicon solar cells. Hence, the dark saturation current densities ( J o ) are determined via quasi steady state photoconductance (QSSPC) decay measurement and the doping profiles by electrochemical-capacitance voltage (ECV) measurement. All the diffusion processes are performed in a quartz tube furnace. The samples are diffused at peak temperatures of 800–950 °C. Therefore; the influence of several parameters for example the extent of surface concentration, the diffusion temperature and the dark saturation current density ( J o ) are evaluated.

Improved empirical method for calculating short circuit current density images of silicon solar cells from saturation current density images and vice versa

An empirical dependence of the short circuit current density Jsc as a function of the dark saturation current density J01 is proposed, which describes this dependence down to a bulk lifetime of 1ns. This method avoids artifacts, which appear when applying the previously proposed quadratic dependence. The parameters of the new dependence are fitted to PC1D simulations and to experimental LBIC results for various wavelengths and AM 1.5 for a typical industrial BSF-type solar cell and a PERC cell. This dependence can also be used to calculate J01 images from LBIC-based Jsc images. It turns out that this method is more reliable in BSF than in PERC cells.

Insight into the energy loss in organic solar cells based on benzotrithiophene copolymers: A dark current analysis at low temperature

Owing to the formation of the charge transfer (CT) state, the open-circuit voltage (Voc) of organic photovoltaic (OPV) devices commonly suffers an energy loss of 0.8–1.3 eV from the effective bandgap. Benzotrithiophene (BTT)-based low-bandgap polymers that we have recently reported showed deep HOMO levels (−5.4 to −5.6 eV) and moderate optical bandgaps of 1.7–1.8 eV, which resulted in high Voc’s of 0.78–0.98 V and relatively low energy losses when blended with methano[60]fullerene (PCBM). Here, we report the temperature-dependent dark current analysis of organic solar cells of BTT copolymers:PCBM blends. Shockley diode analyses revealed the dominant contribution of CT energy and concomitant pre-exponential factor of dark saturation current density associated with charge recombination. The findings could establish a fundamental aspect to draw a design rule in BTT-based polymers towards their evolutions in OPV devices.

Towards understanding the characteristics of Ag–Al spiking on boron-doped silicon for solar cells

With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver–aluminum (Ag–Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j0,met and give detailed insight to which degree a solar cell’s performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j0,met until their tips reach depths where boron doping concentration is lower than approximately 1018cm−3. Deeper spikes then lead to an exponential increase in j0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j0-results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.

Void Formation on PERC Solar Cells and Their Impact on the Electrical Cell Parameters Verified by Luminescence and Scanning Acoustic Microscope Measurements

Ideally formed local Al-contacts of passivated emitter and rear contact solar (PERC) cells feature an eutectic and an uniform local back surface field (LBSF). Under certain conditions the eutectic is missing after the co-firing process, referring to the well-known voids. So far light beam induced current (LBIC) measurements are used to obtain information concerning the passivation quality of the LBSF in local contacts in general. In addition, the destructive technique of scanning electron microscopy (SEM) is established for distinguishing whether a void features a sufficiently thick BSF-layer or a very thin/no BSF-layer. However, both methods are very time consuming.This paper shows a non-destructive and fast characterization of solar cells by applying electroluminescence (EL) and photoluminescence (PL) measurements to investigate the effect on the electrical parameters after locating the voids by scanning acoustic microscopy (SAM). For filled contacts EL and PL measurements correlate well with the resulting values for series resistance (RS) and dark saturation current density (j0): the formed LBSF leads to a good surface passivation (high PL signal intensity, low value for j0) and the eutectic layer ensures a good electrical contact (high EL signal intensity, low value for RS). Voids with a sufficiently thick LBSF show a high PL signal intensity whereas the intensity is significantly reduced for a very thin or completely missing LBSF. Increased values for RS can be explained by the missing eutectic layer. In addition, the electrical connection of the LBSF to the paste can be derived from the value of RS.

Codiffusion Sources and Barriers for the Assembly of Back-Contact Back-Junction Solar Cells

In this study, several diffusion sources are investigated, aiming at codiffusion for the fabrication of back-contact back-junction (BC-BJ) silicon solar cell. As a gaseous diffusion source, a POCl3-diffusion process is investigated, and for solid diffusion sources, phosphorus- and boron-doped silicate glass (PSG and BSG) deposited by the means of plasma-enhanced chemical vapor deposition are considered. The n+-doped areas diffused from a solid PSG layer allow for a precise adjustment of the sheet resistance $(R_{{\rm sh}})$ in the range of 40–400 Ω/sq, along with a dark saturation current density $(J_{o})$ of 55 fA/cm $^{2}$ . Subsequently, boron diffusion from solid BSG layer leads to p+-doped areas with high doping levels ( $R_{\rm sh}=$ 50 Ω/sq). However, gaseous POCl3 diffusion in combination with solid boron diffusion from the BSG layer can only be successfully performed if the BSG layer is protected with an SiO x layer. Furthermore, by adjusting the gas flows during POCl3 diffusion, n+-doped areas with $R_{{\rm sh}}$ in the range of 150– 300 Ω/sq are achieved. The corresponding surfaces feature $J_{o}$ values of 30 fA/cm $^{2}$ . The result of this study is a flexible codiffusion setup allowing for the efficient integration in advanced process chains of BC-BJ solar cells which results in the cell efficiencies well above above $\eta =$ 20%.

Impact of implanted phosphorus on the diffusivity of boron and its applicability to silicon solar cells

Boron diffusivity reduction in extrinsically doped silicon was investigated in the context of a process combination consisting of BBr3 furnace diffusion and preceding Phosphorus ion implantation. The implantation of Phosphorus leads to a substantial blocking of Boron during the subsequent Boron diffusion. First, the influences of ion implantation induced point defects as well as the initial P doping on B diffusivity were studied independently. Here, it was found that not the defects created during ion implantation but the P doping itself results in the observed B diffusion retardation. The influence of the initial P concentration was investigated in more detail by varying the P implantation dose. A secondary ion mass spectrometry (SIMS) analysis of the BSG layer after the B diffusion revealed that the B diffusion retardation is not due to potential P content in the BSG layer but rather caused by the n-type doping of the crystalline silicon itself. Based on the observations the B diffusion retardation was classified into three groups: (i) no reduction of B diffusivity, (ii) reduced B diffusivity, and (iii) blocking of the B diffusion. The retardation of B diffusion can well be explained by the phosphorus doping level resulting in a Fermi level shift and pairing of B and P ions, both reducing the B diffusivity. Besides these main influences, there are probably additional transient phenomena responsible for the blocking of boron. Those might be an interstitial transport mechanism caused by P diffusion that reduces interstitial concentration at the surface or the silicon/BSG interface shift due to oxidation during the BBr3 diffusion process. Lifetime measurements revealed that the residual (non-blocked) B leads to an increased dark saturation current density in the P doped region. Nevertheless, electrical quality is on a high level and was further increased by reducing the B dose as well as by removing the first few nanometers of the silicon surface after the BBr3 diffusion.

Heterojunction Diodes and Solar Cells Fabricated by Sputtering of GaAs on Single Crystalline Si

This work reports fabrication details of heterojunction diodes and solar cells obtained by sputter deposition of amorphous GaAs on p-doped single crystalline Si. The effects of two additional process steps were investigated: A hydrofluoric acid (HF) etching treatment of the Si substrate prior to the GaAs sputter deposition and a subsequent annealing treatment of the complete layered system. A transmission electron microscopy (TEM) exploration of the interface reveals the formation of a few nanometer thick SiO2 interface layer and some crystallinity degree of the GaAs layer close to the interface. It was shown that an additional HF etching treatment of the Si substrate improves the short circuit current and degrades the open circuit voltage of the solar cells. Furthermore, an additional thermal annealing step was performed on some selected samples before and after the deposition of an indium tin oxide (ITO) film on top of the a-GaAs layer. It was found that the occurrence of surface related defects is reduced in case of a heat treatment performed after the deposition of the ITO layer, which also results in a reduction of the dark saturation current density and resistive losses.

Short-Circuit Current Density Imaging Via PL Image Evaluation Based on Implied Voltage Distribution

Luminescence imaging has found wide application for the characterization of silicon solar cells and wafers over the past decade. One special application is based on a combination of electroluminescence and photoluminescence imaging. Images of a single solar cell at different operating conditions are taken. With suitable methods, it is possible to evaluate the image series and extract spatially resolved solar cell parameters. In the past, methods have been introduced focusing on the extraction of local dark saturation current density and local series resistance. Past methods usually assumed a laterally homogeneous short-circuit current density corresponding to laterally homogeneous external quantum efficiency. In this study, we give a step-by-step description of a newly developed method, which does not rely on the assumption of homogeneous short-circuit current density. The evaluation method instead additionally yields an image of the local short-circuit current density or of the external quantum efficiency. We apply the method to different solar cell types, and we give a detailed comparison to its predecessor the “coupled determination of dark saturation current density and series resistance” method. We compare the short-circuit current density images with images obtained from the “light beam-induced current” technique.

Correlation of Interfacial Transportation Properties of CdS/CdTe Heterojunction and Performance of CdTe Polycrystalline Thin-Film Solar Cells

The light and dark output performances of CdS/CdTe solar cells made by close-spaced sublimation (CSS) were investigated to elucidate the transportation properties of carriers at CdS/CdTe heterojunction interface. It has been found that the interfacial transportation properties were relatively sensitive to variations of the characteristics of heterojunction due to the series resistance and shunting effects. For the high quality cell with 12.1% efficiency, narrow depletion region of ~1.1 microns and large electric field intensity of ~1.3 V/μm allow the sufficient energy-band bending close to CdS layer at CdS/CdTe heterojunction, which changes the carrier transportation mechanism from emission to diffusion and leads to the optimal rectifying characteristics with small dark saturation current density ~6.4 × 10−10 A/cm2. As a result, the schematic diagram of heterojunction band structure corresponding to various performances of solar cells has also been presented.

Characterization of POCl$_{3}$-Based Codiffusion Processes for Bifacial N-Type Solar Cells

Bifacial n-type silicon solar cells typically feature two highly doped areas, namely, a boron-doped emitter and phosphorus-doped back-surface field (BSF). Complexity of the process sequence for forming these highly doped areas is one of the major obstacles for industrial application. This study investigates a POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -based codiffusion process that allows for forming boron-doped emitter and phosphorus-doped BSF in one single high-temperature step. As a boron source, we use a borosilicate-glass (BSG) layer deposited before the diffusion process using atmospheric pressure chemical vapor deposition. We discuss the influence of the POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> concentration in the process atmosphere with respect to recombination and contact formation of the BSF. By tuning the POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> concentration, we achieve specific contact resistances of screen printed contacts below 5 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and dark saturation current densities below 160 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on a textured surface. We present solar cell results on 156-mm n-type Cz wafers with peak efficiencies of 19.6%.

Metallization‒induced recombination losses of bifacial silicon solar cells

AbstractIn this study, we investigate the metallization‒induced recombination losses of high efficiency bifacial n‒type and p‒type crystalline Si solar cells. From the experimental data, we found that the most efficiency limiting parameter by the screen‒printed metallization is the open‒circuit voltage (VOC) of the cells. We investigated the mechanism responsible for this loss by varying the metallization fraction on either side of the cell and determined the local enhancement in the dark saturation current density beneath the metal contacts (J0(met)). Under optimum fabrication conditions, the J0(met) at metal‒p+ (boron) emitter interfaces was found to be significantly higher compared with the values obtained for metal‒n+ emitters. A two‒dimensional simulation model was used to get further insight into the recombination mechanism leading to these VOC losses. The model assumes that metal contacts penetrate (or etch) into the diffused region following the firing process and depassivate the interface. Applying this model to our n‒type solar cells with a boron p+ emitter, we demonstrated that the simple loss of passivated area beneath the metal contact cannot explain the degradation observed in the VOC of the cell without considering a significant etching or metal penetration into the emitter region. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Codiffused Bifacial n-type Solar Cells (CoBiN)

We present codiffused bifacial n-type (CoBiN) solar cells on 156 mm Czochralski grown (Cz) Si wafers with peak efficiencies of 19.6 % fabricated using a lean industrial process. Simultaneous diffusion of phosphorus back surface field (BSF) and boron emitter in one single tube furnace process, the so called codiffusion, leads to a significant process simplification. Manipulation of the borosilicate glass (BSG) layer, deposited by atmospheric pressure chemical vapor deposition (APCVD) prior to the POCl3 based codiffusion process, allows for emitter profile tuning, without influencing the phosphorus doped BSF. Analytical simulations concerning the BSF identify the dark saturation current density of the passivated part of the BSF J0pass, BSF as the parameter that allows for maximum improvement of cell efficiency.

Surface passivation of n-type c-Si wafers by a-Si/SiO2/SiNx stack with &amp;lt;1 cm/s effective surface recombination velocity

The passivation quality of an a-Si/SiO2/SiNx (aSON) stack deposited by conventional PECVD at &amp;lt;250 °C with and without additional corona charging of SiNx is presented. &amp;lt;2 fA/cm2 surface dark saturation current density and &amp;lt;1 cm/s effective surface recombination velocity were demonstrated on both planar and textured n-type Czochralski (CZ) substrates. It was shown that very good passivation can be achieved using &amp;lt;5 nm a-Si layers to provide low parasitic absorption. We also report effective minority carrier lifetimes &amp;gt;60 ms on 5000 Ω-cm and 20.9 ms on 1.7 Ω-cm mirror polished float zone (FZ) material passivated with aSON stacks.

Co‐diffusion from solid sources for bifacial n‐type solar cells

AbstractWe present a simplified process sequence for the fabrication of large area n‐type silicon solar cells. The boron emitter and full area phosphorus back surface field are formed in one single high temperature step using doped glasses deposited by plasma enhanced chemical vapour deposition (PECVD) as diffusion sources. By optimizing the gas composition during the PECVD process, we not only prevent the formation of a boron rich layer (BRL), but also achieve doping profiles that exhibit a low dark saturation current density while allowing for contact formation by screen printing. The presented co‐diffusion process allows for major process simplification compared to the state of the art diffusion process relying on multiple high temperature processes, masking and wet chemistry steps.magnified imageSolar cell based on n‐type silicon featuring a co‐diffused boron emitter and phosphorus back‐surface field (BSF).(© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Simplified Front Surface Field Formation for Back Contacted Silicon Solar Cells

We investigate the formation of deep Phosphorus diffusion profiles with low surface concentrations in one single diffusion step. Such processes are suited for front surface field formation for n-type back-contact back-junction (BC- BJ) silicon solar cells or p-type silicon solar cells with alternative metallization techniques. The deposition temperature allows accurate control of the sheet resistance whereas the temperature of the in-situ oxidation strongly influences the profile-depth and surface concentration. The newly developed process leads to low dark saturation current densities well below 30 fA/cm2 on alkaline-textured surfaces and low short circuit current loss at the front side. Due to their depth and adjustable surface concentration, the obtained profiles are promising for front-surface- fields (FSF) and novel n-type emitters. A first implementation in BC-BJ solar cells with aluminum-alloyed emitter shows a good blue response of the solar cells and an improved efficiency compared to the reference process by 0.6%abs.

Transfer of the HIP-MWT Solar Cell Concept to n-type Silicon

First n-type Czochralski silicon (Cz-Si) metal wrap through (MWT) solar cells based on the high-performance MWT (HIP-MWT) concept are presented. Reference n-type H-pattern solar cells were processed in parallel. The MWT as well as the H-pattern cells with an edge length of 125mm are completely metallized by screen printing. The fabricated n-type Cz-Si MWT solar cells achieved conversion efficiencies up to 17.7%. One of the main challenges for further development is to decrease the dark saturation current density j02 and to increase the shunt resistance RShunt above the current value of about 1 kΩcm2 The H-pattern reference cells also show similarly low RShunt as well as high j02 values and reach conversion efficiencies up to 17.1%. Thus the low RShunt and high j02 values cannot be attributed to the MWT concept. Specific contact resistances of about 8 mΩcm2 for front and rear metallization show the successful electrical contacting of the differently doped surfaces using sequential tube furnace diffusions. The advantage of less metallized front surface for the MWT cells compared to the H-pattern references is clearly visible in increased values for short-circuit current density and open-circuit voltage. With the results obtained, the feasibility of the transfer from p- to n-type Cz-Si is demonstrated.