Query: a new approach to high efficiency solar cells by band gap grading in Cu(In,Ga)(S,Se2) chalcopyrite semiconductors

Candidate papers:

abstract: High efficiencies in Cu(In,Ga)(S,Se)2 solar cells result from alloying CuInSe2 base material with the corresponding Ga or S containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu depleted absorber surface is essential. We consider this Cu/In grading non intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.
author_names: Thorsten Dullweber

Giordana Anna

Uwe Rau

Hans W Schock
title: A new approach to high efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors
year: 2001

abstract: Wide band gap Cu(In,Ga)Se2 ZnSe (CIGZSe) thin films have been synthesized using a sequential process, with the objective to demonstrate the possibility to tune the band gap and introduce double gradients playing with the content of Ga at the back and Zn at the front. In a first approach, we start by varying the 3 valent cationic composition of the system, and then we modify the reactive annealing conditions in order to understand and control the elemental gradients. Structural, compositional, and morphological properties of the corresponding absorbers were analysed by X ray diffraction, Raman spectroscopy, energy dispersive spectrometer, Auger spectroscopy, and scanning electron microscopy. Solar cells were fabricated and characterized, focusing on the identification of the most promising cationic composition. The compounds from the complex Cu(In,Ga)Se2 ZnSe system can adopt either the chalcopyrite or sphalerite phases depending on the [Zn][metals] and [Ga][Ga] [In] ratios. We demonstrate that Ga naturally diffuses towards the back region forming a Ga rich, wide band gap chalcopyrite phase at the rear contact, as is commonly observed for a Cu(In,Ga)Se2 synthesized via selenization process. On the contrary, Zn is preferably accumulated at the surface, forming wide band gap sphalerite Cu(In,Ga)ZnSe3 phases with high Zn and very low Ga contents at the surface. This opens an additional way to control the surface's band gap. With this approach, the formation of a doubly graded band gap profile with Ga rich layers at the back and Zn rich layers at the front is demonstrated in a single selenization step, showing promising efficiency and open circuit voltage values (up to 6.7% and 709 mV, respectively)
author_names: Rokas Kondrotas

Florian Oliva

Xavier Alcobe

Victor Izquierdo-Roca

Alejandro Perez-Rodriguez

Edgardo Saucedo

Paul Pistor
title: Double band gap gradients in sequentially processed photovoltaic absorbers from the Cu(In,Ga)Se2 ZnSe pseudobinary system
year: 2018

abstract: ZSW reported record conversion efficiency of 22.6% for the Cu(In,Ga)Se2 (CIGSe) solar cell by postalkali metal treatment [1] Alkali metals, specifically Na, which are incorporated into the CIGSe absorber layer, are widely known to have significantly beneficial effects that enhance photovoltaic performance. The high efficiency CIGSe solar cells reported by EMPA and ZSW were fabricated by evaporation process with the post deposition of alkali metal fluorides, such as NaF, KF, RbF, and CsF. Consequently, the effect of post alkali metal treatment has been actively studied in the past few years. Recently, we reported the first principles study on the alkali metal effect of Li, Na, and K in CuInSe2 (CISe) [2] The substitution energies of Li, Na, and K atoms in chalcopyrite type CISe and CuGaSe2 (CGSe) were calculated for two different cationic atom positions of Cu and In/Ga in the chalcopyrite unit cell. The migration energies of the alkali metal atoms of Li, Na, and K in CISe and CGSe were calculated using a combination of the linear and quadratic synchronous transit (LST/QST) methods and the nudged elastic band (NEB) method. Figure 1 shows a schematic illustration of calculated substitution energy (DEsub) and migration energy (DEm) of Li, Na, and K atoms in CISe [2] In CISe and CGSe, the substitution energies of Na atom for Cu atom (NaCu) were much lower than those of NaIn, NaGa and the interstitial Nai at the tetrahedral site and Nai at the octahedral site. The theoretical migration energy of the Na atom at the Cu site to the nearest Cu vacancy (NaCu VCu) was much lower than that of (NaIn VCu) in CISe and CGSe. The substitution energies of K atom for Cu atom (KCu) in CISe and CGSe were much higher than those of NaCu. We considered that K atoms cannot easily substitute for the Cu or In atom and most of the K atom will exist near the CIGSe surface or be segregated as K2Se or K Se compounds at the CIGSe grain boundaries. On the other hand, the substitution energies of Li atom for Cu atom (LiCu) were considerably lower than that of NaCu. Therefore, we considered that Li atoms can easily substitute for the Cu atom in CISe and CGSe. Recently, we studied crystallographic, optical, and electronic properties of the (Cu,Li)InS2 system [3] We successfully obtained the single phase chalcopyrite type (Cu1 xLix)InS2 solid solution with 0.0 x 0.10 by a mechano chemical process and post heating at 550oC in 5% H2S gas. The band gap energy of the (Cu1xLix)InS2 solid solution linearly increased from 1.45 eV of CuInS2 (x 0.0) to 1.53 eV of (Cu0.9Li0.1) InS2 (x 0.10) The valence band maximum (VBM) of the (Cu1 xLix)InS2 solid solution from the vacuum level, which was measured by photoemission yield spectroscopy (PYS) became deeper from the vacuum level with increasing Li content. In addition, most recently, we prepared (Cu1 xLix)In(S1 ySey)2 samples, and the singlephase (Cu1 xLix)In(S0.5Se0.5)2 powders were successfully obtained for the sample with 0.0 x 0.20 [4] In the present study, we calculated the DEsub and theDEm of light and heavy alkali elements in stannitetype CuIn5Se8 and CuGa5Se8 and compared them with those in chalcopyrite type CuInSe2 and CuGaSe2. We will discuss the effects of light and heavy alkali elements such as Li, Na, K, Rb, and Cs in CIGSe solar cells.
author_names: Tsuyoshi Maeda

Takahiro Wada
title: Area: 2 Thin Film Compound Semiconductor PV FIRST PRINCIPLES STUDIES ON EFFECTS OF LIGHT AND HEAVY ALKALI ELEMENTS IN Cu(In,Ga)Se2 SOLAR CELLS
year: 2017

abstract: Abstract Cu In Ga Se 2 Solar cells with graded band gap and efficiencies up to 13 have been fabricated on transparent ZnO:Al back contacts. The back contact structure includes a transparent 10 nm thin Mo interlayer with NaF precursor between the ZnO:Al and the Cu In Ga Se 2 absorber that transforms the blocking ZnO:Al/ Cu In Ga Se 2 interface into an Ohmic back contact. To investigate the electronic quality of the back contact, the cells are analyzed by internal quantum efficiency measurements under illumination from front and back side. A new semianalytical model for the quantum efficiency of graded band gap absorbers yields quantitative information about the back contact recombination velocity as well as optical and electronic material parameters of the absorber layer. Band gap grading significantly increases carrier collection. However, in the immediate vicinity of the back contact carrier collection is limited by a high ratio of back contact recombination velocity and diffusion constant S n D n 10 7 cm 1
author_names: Julian Mattheis

P J Rostan

Uwe Rau

Jurgen H Werner
title: Carrier collection in Cu(In,Ga)Se2 solar cells with graded band gaps and transparent ZnO:Al back contacts
year: 2007

abstract: Abstract The effect of Auger generation on the thermodynamic efficiency of a reverse graded band gap Cu(In,Ga)Se 2 thin film solar cell is studied using a detailed balance approach. For the graded profile (1.5 1 eV) a wider range of photons can be considered as high energy photons to produce hot electrons which can pump an additional electron from the valence band to the conduction band. The effects of the carrier multiplication probability, P and grading strength of the front band gap on the variation of efficiency are studied. Auger mechanism is more effective at lower band gaps due to higher multiplication probability. A thermodynamic efficiency of about 41% is obtained, which is higher than the maximum efficiency of 31% for a graded band gap cell without Auger generation. Moreover, we study the effect of Auger generation on the current voltage characteristics of the cell. The carrier multiplication increases the short circuit current due to the increased photogenerated electrons and almost does not change the open circuit voltage. However, the band gap grading enhances the open circuit voltage by reducing the recombination rate. Also, a known result is the red shift in the optimum band gap towards a lower band gap (1.15 eV) with respect to the Shockley Queisser limit.
author_names: Nima E Gorji

Ugo Reggiani

Leonardo Sandrolini
title: Auger generation effect on the thermodynamic efficiency of Cu(In,Ga)Se2 thin film solar cells
year: 2013

abstract: The development of solution processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S 2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap wider than their selenide counterparts which can potentially increase the attainable photovoltage. A straightforward and all solution processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S 2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg) For optimized electrodes, the sequential deposition of thin layers of n type CdS and TiO2 by solution based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm( 2) and onsetting at similar to 0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state of the art performance for these materials.
author_names: Nestor Guijarro

Mathieu S Prevot

Xiaoyun Yu

Xavier A Jeanbourquin

Pauline Bornoz

Wiktor S Bouree

Melissa Johnson

Florian Le Formal

Kevin Sivula
title: A Bottom Up Approach toward All Solution Processed High Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting
year: 2016

abstract: Cu (In,Ga)Se2 (CIGS) compound is a p type semiconductor that has been used as light absorber layer in high efficiency thin film solar cell. The CIGS compound can be adjusted the band gap energy by varying the ratio of [Ga][In][Ga] ratio (x) From theoretical and simulation, it was found that band gap grading in CIGS thin films showed the advantages to increase the efficiency of solar cells. Generally, the band gap grading can be done by the growth of non homogeneous x ratio in depth of CIGS thin films. In this work, we develop two approaches to create band gap grading in CIGS thin films; (1) modifying the growth profile and (2) using Na incorporation in the growth process. The effects of Ga graded would be revealed and compared with homogeneous CIGS thin films. CIGS thin films were grown on soda lime glass and Al2O3 coated soda lime glass substrates by molecular beam deposition method. The growth process was based on 2 stage and 3 stage growth profiles. The as grown films were characterized for their structural property, chemical composition and optical transmission as well as solar cell performance. The Auger electron spectroscopy in depth profiles revealed the variation of x ratio increasing from the surface toward the back contact in CIGS films with our modified growth profile and Na incorporation. This result indicated Ga graded in CIGS thin films. The structural property of Ga graded CIGS films showed the (112) preferred orientation of the chalcopyrite structure with a broad asymmetric spectrum related to the inhomogeneous structure. The optical transmission measurements of the Ga graded CIGS film showed the broad transition near the absorption edge indicating the effect of the band gap grading as a result of the variation in depth of the Ga content. From I V measurements, the solar cell efficiencies significantly increase due to the advantages of Ga graded constitute.
author_names: Rachsak Sakdanuphab

Sojiphong Chatraphorn

Kajornyod Yoodee
title: The Advantages of Ga Graded Obtained by Growth Profile Modification and Na Incorporation on Cu(In,Ga)Se2 Solar Cells
year: 2014

abstract: Three different issues are discussed in this thesis, which aim for a deeper understanding of Cu(In,Ga)Se2 material properties and solar cell functionality: (1) Near surface ion implantation in Cu(In,Ga)Se2 absorber layers is studied as a method for the fabrication of buffer free solar cells. Simulations show the beneficial effect of an n type surface layer (buried junction) To obtain an inversion by means of ion implantation, an annealing procedure is developed that avoids annealing induced degradation of the cell, minimizes the diffusion of the ions, and recovers the implantation damage. Buffer free solar cells made from implanted absorbers show strongly improved diode characteristics comparable to the ones of cells with a CdS buffer and a maximum efficiency of 10.2% (11.2% active area) (2) A model is developed for the application of luminescence techniques on band gap graded semiconductor thin films. Several DA emissions are detected in the cathodoluminescence (CL) and photoluminescence (PL) spectra of Ga graded in Cu(In,Ga)Se2 and related to the local Ga contents in the layer. The depth distribution of the luminescence signal is correlated to the band gap profile. Drift of excited charge carriers towards the band gap minimum is caused by the grading induced quasi electric field, which must be considered when explaining the spectra obtained in plan view measurements. Monochromatic CL imaging on cross sections is capable to determine the minority carrier mobility. (3) An increase of the substrate temperature from the standard value of 530degC to 610degC, which was possible only using specially developed Na containing high temperature resistant glass, is shown to strongly influence the formation of the Cu(In,Ga)Se2 absorber layers. The higher temperature leads to an increased vertical and lateral homogeneity, an increase of the grain size, a decrease of the losses at the p n junction and, as a consequence, a considerable increase of the solar cell efficiency up to 19.4%
author_names: J Haarstrich
title: Interface modification by ion implantation and optical characterization of high efficiency Cu(In,Ga)Se2 solar cells
year: 2012

abstract: In recent years, the energy crisis makes people give great concern over thin film solar cells. As a promising absorber layer material, chalcopyrite semiconductor Cu2ZnSnS4(CZTS) are of great interest due to a suitable band gap of around 1.5eV and high optical absorption coefficient beyond 10 4 cm 1 [1] Compared with other chalcopyrite semiconductors such as Cu(In, Ga)Se2, CuSbSe2, CdTe etc, CZTS has good application potential owing to the composition of earth abundant and non toxic elements. The power conversion efficiency of solar cells based on pure Cu2ZnSnS4 as high as 8.4% has been reported [2] But the power conversion efficiency can be improved further because it is far behind the physical Shockley Queisser limit of conversion efficiency ~31%[3] Various methods have been employed to synthesize CZTS thin films, such as sputtering, thermal evaporation, spray pyrolysis, electrodeposition, sol gel, chemical bath deposition, successive ionic layer absorption and reaction, solvothermal synthesis[4 19] Among them, the solvothermal method has a number of advantages. Firstly, the chemical compositions can be readily controlled in a closed environment. Secondly, the size uniformity and structural homogeneity can be improved. To date, the solvothermal method has been mainly employed to prepare CZTS nanocrystals. There is some literature on synthesizing CZTS thin films directly via a solvothermal method. As far as we know, no reports have been found to synthesize doped type CZTS thin films directly by solvothermal method. The optical and electrical properties can be optimized by varying stoichiometry and structure of CZTS. Chen revealed that the band gap of Cu2ZnSn(S, Se)4 is tuned from 1.0eV to 1.5eV by partly substituting S with Se[20] Barkhouse fabricated Cu2ZnSn(S, Se)4 solar cells, exhibiting a power efficiencies as high as 10.1%[21] The power efficiencies as high as 8.4% has been obtained by partly Ge replacing Sn[22] Unfortunately, the Se and Ge are still unfriendly and rare. Therefore, other elements such as Fe, Mn, Co, Ni attract more and more attention because of their similar structures of Cu2ZnSnS4, Cu2FeSnS4, Cu2MnSnS4, Cu2CoSnS4 and Cu2NiSnS4. A few
author_names: Hao Guan

H L Shen

Li J
title: SYNTHESIS AND OPTICAL PROPERTIES OF Cu 2 Zn 1x Mn x SnS 4 THIN FILMS WITH TUNABLE BAND GAP
year: 2017

abstract: Abstract The efficient photon harvesting in near infrared wavelength range is still a challenging problem for high performance Cu(In1 x, Gax)Se2 (CIGS) solar cell. Herein, adjusting the energy band distribution of CIGS solar cell could provide significant academic guidance for devices with superior output electric power. To understand the role of each functional layer, the optimal 3000 nm CIGS absorber layer with 1.3 eV bandgap and 30 nm CdS buffer layer were firstly obtained via simulating the uniform band gap structures. By introducing CIGS absorber layer with a double grading Ga/(Ga+In) profile, the power conversion efficiency of the double gradient band gap cell is superior to that of uniform band gap cell through extending absorption of near infrared wavelength range. Upon optimization, the best power conversion efficiency of CIGS with a double gradient band gap solar cell is improved significantly to 24.90% among the best values reported in literatures, which is an 8.17% relative increase compared with that of the uniform band gap cell. Our findings provide a theoretical guide toward the design of high performance solar cells and enrich the understandings of the energy band engineering for developing of novel semiconductor devices.
author_names: Lingling Yan

Yiming Bai

Boran Yang

Nuofu Chen

Zhan'ao Tan

Tasawar Hayat

Ahmed Alsaedi
title: Extending absorption of near infrared wavelength range for high efficiency CIGS solar cell via adjusting energy band
year: 2018