Mike Scarpulla, University of Utah
Charles Hages, University of Florida
Byungha Shin, Korea Advanced Institute of Science and Technology
Mirjam Theelen, TNO
Angstrom Engineering Inc.
EN07.01/EN06.01: Joint Session I: When Perovskite Meets Chalcogenide
Sunday AM, April 18, 2021
8:00 AM - *EN07.01/EN06.01.01
Charge Carrier Dynamics and Non-Radiative Recombination in Chalcogenide and Halide Perovskite Photovoltaic Materials
Helmholtz Zentrum Berlin für Materialien und Energie GmbH1Show Abstract
Application of semiconductor thin film materials in photovoltaic devices requires optimized optoelectronic properties and thus understanding of the underlying physical mechanisms and limits governing carrier transport and recombination. Although material parameters such as doping density, recombination lifetimes and mobilities belong to the most basic properties eventually defining the solar cell operation, they can be quite elusive or ambiguous in their characterization. Here, chalcogenides (e.g. chalcopyrite and kesterite) and halide perovskites share some similarities, but also a number of differences. Considerable discussion prevails about the role of interface versus bulk recombination in devices, the values of carrier mobilities, doping levels and minority carrier lifetimes. Combining different characterization techniques such as time-resolved luminescence, photoluminescence quantum yield as well as pump-probe terahertz spectroscopy we attempt to draw a relatively consistent picture of material properties and device function, exposing current limits and paths to improvement.
8:25 AM - *EN07.01/EN06.01.02
Towards Perovskite-CIGS Large Area Tandem Architectures
Valerio Zardetto1,Marcel Simor1,Pieter Jan Bolt1,Tom Aernouts2,Gianluca Coletti1,Mariadriana Creatore3,René Janssen3,Veronique Gevaerts1,Sjoerd Veenstra1,Hans Linden1,Ronn Andriessen1
TNO partner in Solliance1,IMEC partner in Solliance, Thin Film PV2,Eindhoven University of Technology3Show Abstract
The fabrication of cost-effective multi-junction (MJ) device architectures is presently considered the strategy to increase the Power Conversion Efficiency (PCE) of the solar cells in order to lower the levelized cost of energy (LCOE), decreasing therefore the price of an installed photovoltaic (PV) system. The outstanding advancement of metal halide perovskite absorber (PSC) technologies in the last years motivates the PV community to investigate tandem hybrid architectures, consisting of a perovskite based top cell with a commercial bottom PV device such as crystalline silicon (c-Si) or copper indium gallium selenide (CIGS). The combination with the latter, in particular, offers unique advantages: i) compatibility with lightweight flexible plastic or metal foils as substrates, in view of a roll to roll (R2R) manufacturing line, further reducing the manufacturing and the installation costs, as well as the environmental footprint of the KWh generated; ii) large scale integration of PV devices in, amongst others, buildings (BIPV and vehicles (VIPV).
This contribution will provide an overview of the activities within the Solliance consortium on 4 terminal tandem and preliminary work on 2 terminal architectures, on rigid, as well as flexible substrates. In order to increase the overall PCE of the tandem hybrid configuration, the optimization of the sub-cells’ performance with respect to their spectral responses is required. Several combinations of suitable complementary band gaps of the two absorbers are explored, as well as the optimization of the near infrared transparency of the top perovskite cell by tuning the free carrier absorption of the TCO adopted. Furthermore, in the case of the 2-terminal architecture, the development of an efficient interconnected layer among the two sub-cells is utmost crucial to provide the monolithic series interconnection and, at the same time, to minimize the parasitic absorption.
Optimized TCO, transport layers and anti-reflective properties lead to a near-infrared transmittance of about 90% (an average taken from the wavelength range of 700-800 nm to 1200 nm) on glass or plastic based top PSC optimizing therefore the response of the filtered CIGS. As a result, coupling a semitransparent glass-based PSC and a flexible CIGS we calculate a 4T tandem PCE of 25.2% on small area devices (<0.1cm2). When using a flexible semi-transparent perovskite solar cell, a record-breaking power conversion efficiency of 23.0% is calculated. We also evaluated the PSC-CIGS combination on a larger area (> 80 cm2), currently adopting a no-optimized (in the NIR region) large area perovskite module. We calculated already a 1.9% absolute increase in the efficiency with respect to the single CIGS device, with a further improvement that can be achieved by improving the light management and fine tuning the perovskite band gap. Whereas the 4T architecture offers already a significant increment in performance with respect to the single junction devices, the development of the 2T tandem PSC-CIGS exhibits more challenges. As observed by the first results, the efficiency of the flexible 2T tandem PSC-CIGS, still lags behind the power conversion efficiency of the single CIGS. New upscalable perovskite formulations as well as the optimization of the recombination layer will also be addressed as crucial topics to boost the overall performance.
8:50 AM - EN07.01/EN06.01
9:05 AM - *EN07.01/EN06.01.03
Chalcogenide Perovskite Thin Films for Energy Conversion—Prospects, Challenges and Routes Forward
Jonathan Scragg1,Corrado Comparotto1,Kostiantyn Sopiha1
Uppsala University1Show Abstract
Great progress in perovskite photovoltaics has been enabled by the excellent intrinsic material properties of lead-halide perovskites. These relatively newly-available materials are already opening paths to higher performance in mass-market PV technologies, including tandem PV modules. Nonetheless, reaching competitive levelized energy costs will hinge on achieving comparable robustness to existing silicon-based technology. This is a high and rising benchmark, which has not yet been approached by halide perovskite technologies. This makes it less likely that investors will support the upscaling and deployment of perovskite-based solar technology.
Chalcogenide perovskites, in which halide anions are replaced by chalcogenides (O, S, Se) could offer a new path forward. Examples include BaZrS3 and SrHfS3, among several others. The solar cell-relevant properties of these materials are projected to be comparable to halide perovskites. More critically though, their chemistry provides them with excellent stability under conditions for which halide perovskites would rapidly degrade, e.g. in water, humid air or at high temperatures.
The major challenge with chalcogenide perovskites has been to synthesise them in thin film form with good quality. Therefore, despite impressive measurements from bulk materials, it has not been possible to validate their application in thin film solar cells. There are several reasons for the synthetic difficulties, connecting to intrinsic material properties as well as to the use of inappropriate synthesis methods. To date, we still have incomplete understanding of the growth processes of these materials.
In this presentation, we briefly summarise the potential of the chalcogenide perovskites based on prior literature. Then, our own progress toward thin films suitable for solar-cell integration will be presented. A number of sputter-based growth routes to BaZrS3 have been investigated and evaluated using combinatorial approaches. Materials have been characterised using XRD, STEM-EDS, SEM/TEM imaging, XAFS, Photoluminescence mapping and Ion-beam techniques including RBS and ERDA, to construct a picture of the growth sequence and final compositional, microstructural and optoelectronic characteristics of the films. This is a means to establish the first “cookbook” for chalcogenide perovskite films suitable for solar cells, i.e. the optimal growth conditions for high quality material. Film formation will be discussed in the context of thermodynamic and kinetic aspects, applicable to any type of growth method. The aim of this talk is to promote and support research into these highly interesting materials in other labs, and stimulate collaboration on this exciting topic.
9:30 AM - EN07.01/EN06.01.04
Effect of the HTM/TCO Interface on the Electrical Properties of Semi-Transparent Perovskite Solar Cells for Tandem Applications
Emilie Raoult1,2,3,Marion Provost2,Romain Bodeux1,2,Sébastien Jutteau1,2,Marie Legrand1,2,Samuel Rives1,2,Armelle Yaiche1,2,Damien Coutancier2,4,Jean Rousset1,2,Stéphane Collin2,3
Electricité De France (EDF)1,Institut Photovoltaïque d'Ile-de-France (IPVF)2,Centre for Nanoscience and Nanotechnology (C2N)3,CNRS4Show Abstract
Spiro-OMeTAD, CuSCN and PTAA are attractive hole transport materials (HTM) in conventional n-i-p architectures for perovskite cells1. In this work, we combine these three HTMs on semi-transparent (ST) solar cells with different sputtered electrodes made of In2O3-SnO2 (ITO) or In2O3-ZnO (IZO) to study their impact on the optical and electrical properties of the cells.
First, thanks to an optical model based on the Transfer Matrix Method and careful material characterizations2,3, a good agreement has been obtained between the transmission, reflection and absorption spectra of the complete structure of the ST cell with Spiro and ITO. This model allows to simulate the efficiency of a tandem cell with a silicon bottom cell, to identify optical losses and to optimize the thickness and properties of the each layer. It is shown that the Spiro and ITO layers have a non-negligible parasitic infrared absorption in the IR, resulting in a lower efficiency for the filtered silicon cell. The gain in efficiency enabled by the use of PTAA, CuSCN and IZO, which are almost transparent in the IR region, is assessed.
Second, triple cation perovskite solar cells with these three HTMs are synthetized and characterized in detail using scanning electron microscopy, ellipsometry spectroscopy, glow discharge emission optical spectroscopy, X-ray diffraction and transmission spectroscopy. Regarding the electrical properties, the Spiro is a layer known for its sensitivity, and the poor reproducibility obtained on ST cells is often associated with degradation due to energetic sputtering4. Investigations using CuSCN as HTM showed a slightly better reproducibility than Spiro but strong light soaking effects also appeared, and the resulting efficiencies are low. The replacement of the Spiro by PTAA has improved the reproducibility of cells, while maintaining good performances. A 17.4% efficiency perovskite semi-transparent cell with PTAA was obtained and which is significantly above our previous results with Spiro (16.4%)5.
After electrode sputtering deposition, a characteristic S-shape appears in IV characteristics. By regularly following the cells over time while keeping them under vacuum atmosphere and in the dark, IV curves recovered until the S-shape disappeared completely. For example, a cell with Spiro and ITO have 8.7% efficiency and 43% Fill Factor (FF) the first day and reach 16.8% with FF=70% after 66 days. When the Spiro is replaced by PTAA, the S-shape is less pronounced and the recovery time markedly reduced to 15 days. These effects are not visible on cells with CuSCN.
Lee et al.6 obtained a similar S-shaped curve on perovskite cell with a Spiro/silver electrode, with a short recovery time of about one day. A local reaction between the dopant Li-SIF and the silver could cause the deoxygenation of the Spiro at the surface, which would create a temporary electrical barrier. It is interesting to note that this dopant is common to PTAA and Spiro but not to CuSCN. Preliminary results obtained by time resolved photoluminescence (TRPL) shows a decrease of the PL signal over time. Experiments are underway to determine if the cause of this decrease is due to improved extraction at the HTM/TCO barrier as suggested by Lee et al.6, or if it comes from a reduction in radiative recombination related to defect healing. We will discuss the reaction mechanisms for these three HTM.
 Z. Shariatinia, Renew. Sustain. Energy Rev., 119, 109608, 2020
 E. Raoult et al., 36th Eur. Photovolt. Sol. Energy Conf. Exhib. 757, 2019
 E. Raoult et al., SPIE: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices IX, 2020
 H. Kanda et al., J. Phys. Chem. C, 120, 50, 28441, 2016
 F. J. Ramos et al., Sci. Rep., 8, 1, 2018
 D. G. Lee et al.,, ACS Appl. Mater. Interfaces, 11, 51, 48497, 2019
9:45 AM - EN07.01/EN06.01.05
Late News: A Transparent Polymer Impregnated with Conductive Particles for Tandem Photovoltaic Devices
Joshua Crunk1,Kai Outlaw-Spruell1,Nicolas Gaillard1
Hawaii Natural Energy Institute1Show Abstract
Tandem photovoltaic (PV) devices offer great advantages over individual photovoltaic cells including the potential to reach power conversion efficiencies (PCEs) greater than that of the composing individual cells. The current record PCE for a PV tandem cell is over 29%, set by a perovskite/Si tandem. Perovskite solar cells (PSCs) are ideal for tandem devices because they have proven to be highly efficient and the bandgap can be tuned. However, PSCs are extremely sensitive to air and light. Another great candidate for tandem devices are chalcopyrite thin-film solar cells. Chalcopyrite cells also have tunable bandgaps and high efficiencies. The manufacturing process for chalcopyrite cells includes parameters that would degrade the performance of the bottom cell in a monolithic tandem device and the architecture of the cell prohibits a sufficient amount of energy below the bandgap from reaching the bottom cell. Thus, we present a strategy for fabricating a semi-monolithic chalcopyrite/perovskite tandem device by employing a transparent polymer impregnated with conductive particles (TPCP).
The TPCP was designed for electrically coupling solar cells into tandem devices. The TPCP can be employed in three ways: (i) fully-bonded, (ii) semi-bonded , and (iii) freestanding. When fully-bonded, the TPCP permanently bonds two surface allowing for passage of light and electrical current. TPCPs have been used to fabricate a CuGaSe2/Si tandem device with an open circuit voltage of 1.04 V and PCE of 2.45%. Fully-bonded TPCPs were used to exfoliate the CuGaSe2 cell at the MoSe2 layer with an FTO-coated glass handle. The exfoliated CuGaSe2 cell was then fully bonded with the TPCP to a Si bottom cell. The semi-bonded TPCP passes light and electrical current while also providing the protective features of a polymer over a surface. The semi-bonded TPCP has successfully been employed as an encapsulation technique for preserving PSCs. The TPCP encapsulation did not affect the performance of the PSC. An encapsulated PSC exposed to air and light for 14 days did not show any visible signs of degradation. The freestanding TPCP enables the coupling of PV cells without permanently bonding the cells. Ultraviolet-visible spectroscopy measured over 90% transmittance across the visible spectrum. The resistance of the TPCP measured by linear sweep voltammetry yielded a series resistance to the order of 10-1 Ω●cm2.
Based on the successful fabrication of the CuGaSe2/Si tandem device and encapsulation of the PSC, we will investigate the fabrication of a chalcopyrite/perovskite tandem device composing of a wide bandgap chalcopyrite top cell and a narrow bandgap PSC. Additionally, the application of the freestanding TPCP to fabricate a tunable tandem device with exchangeable cells will be attempted.
EN07.02: Advances in CdTe-Based Solar Cells I
Sunday PM, April 18, 2021
10:30 AM - *EN07.02.01
Thin Film Solar Cells Using n-CdTe Absorber Layers
Ken Durose1,Luke Thomas1,K Cheetham1,2,M Isaccs3,Theo Hobson1,N Tarbuck1,L Phillips1,Jon Major1
University of Liverpool1,STFC Daresbury Laboratory2,Research Complex at Harwell3Show Abstract
There are good reasons to abandon the usual p-type absorbers in CdTe-based solar cells in favour of n-type ones. Despite recent advances in CdTe PV (e.g. eliminating CdS, the use of CdSeTe and intentional p-type doping using CdTe:As), the basic design of ‘superstrate’ CdTe-based devices remains unchanged since the 1960s: light enters through the glass/transparent conducting electrode/window layer and generation takes place in a shallow homojunction in the CdTe-based absorber layer - which is otherwise p-type. These cells have limited Voc. The use of n-type CdTe absorbers sidesteps two fundamental physical limitations of p-CdTe and replaces them with technological challenges: For p-type CdTe it is well known that formation of Ohmic contacts requires that a barrier be overcome (this being the value of the electron affinity plus a large fraction of the bandgap). Also, wide- and medium-gap semiconductors cannot stably be doped p-type at a high density due to self-compensation limitations imposed by native defects. Such factors limit the Voc achievable from CdTe-based devices. For n-type CdTe, doping with indium can achieve close to 100% activation and allows higher stable doping densities to be achieved than with CdTe:As for example. Ohmic contacts to it may be formed without issue. Both the n-doping and Ohmic contacts are well-known for single crystal n-CdTe.
Hence the challenge for n-type devices, lies elsewhere, notably in the choice of partner layers and their electrodes, the selection of a device architecture and the control of n-doping in the context of thin film growth and processing. There are essentially four choices of device architecture: A) ‘substrate’ devices in which the light enters through the glass/TCO combination and then first encounters either a) n-CdTe or b) a p-type partner layer or B) ‘superstrate’ devices in which the light enters through a transparent electrode and then first encounters either a) n-CdTe or b) its p-type partner layer. To sum them up in other words: there is either an n- or a p-type window layer. We ruled out using p-type TCOs immediately - since they have low conductivity - and instead we considered other options. In a preliminary simulation study (using SCAPS) we identified a range of materials choices and architectures for n-CdTe devices that gave Voc ≥ 1V. This indicated that experimentation would be worthwhile.
For the CdTe:In doping we have been trialling post-growth introduction of indium into CSS-grown CdTe using elemental indium (with and without pre- or post-treatment using MgCl2), both aqueous and methanolic InCl3 and also treatments with and without nitric/phosphoric etching. We will report conductivity measurements, transfer length measurements (contacts), SIMS and XPS results. There is some indication that diffusion can be blocked by surface oxides. As an alternative we are trialling intentionally pre-doped CdTe:In source material for use in close space sublimation film growth directly. At the time of writing device work is in progress using a variety of wafer (test structure) and thin film partner layers including organics. This part of the work is at an early stage and we will report on progress at the conference.
10:55 AM - EN07.02.02
Reconciliation of Theory with Experiment for Defects in CdTe—The Case of the Cadmium Vacancy
Seán Kavanagh1,2,Aron Walsh2,David Scanlon1
University College London1,Imperial College London2Show Abstract
The ability to accurately model, understand and predict the behaviour of crystalline defects would constitute a significant step towards improving photovoltaic device efficiencies and semiconductor doping control, accelerating materials discovery and design. In this work, we apply state-of-the-art ab initio techniques - hybrid Density Functional Theory (DFT) including spin-orbit coupling - to accurately model the atomistic behaviour of the cadmium vacancy (VCd) in cadmium telluride (CdTe). In doing so, we resolve several longstanding discrepancies in the extensive literature on this subject.
CdTe is a champion thin-film absorber for which defects, through facilitation of non-radiative recombination, significantly impact photovoltaic (PV) performance, contributing to a reduction in efficiency from an ideal (Shockley-Queisser) value of 32% to a current record of 22.1%. Despite over 70 years of experimental and theoretical research, many of the relevant defects in CdTe are still not well understood, with the definitive identification of the atomistic origins of experimentally-observed defect levels remaining elusive.1–3
In this work, through identification of a tellurium dimer ground-state structure for the neutral Cd vacancy, we obtain a single negative-U defect level for VCd at 0.35 eV above the VBM, finally reconciling theoretical predictions with experimental observations. Moreover, we reproduce the polaronic, optical and magnetic behaviour of VCd-1 in excellent agreement with previous Electron Paramagnetic Resonance (EPR) characterisation.4
The origins of previous discrepancies between theory and experiment, namely incomplete mapping of the defect potential energy surface (PES) and inherent qualitative errors in lower levels of electronic structure theory, are analysed in detail. Accordingly, this work helps to establish robust procedures for accurate and reliable modelling of defect processes in emerging materials, informing future investigations and enabling the acceleration of materials discovery and design procedures.
1 A. Lindström, S. Mirbt, B. Sanyal and M. Klintenberg, J. Phys. D: Appl. Phys., 2015, 49, 035101.
2 J.-H. Yang, W.-J. Yin, J.-S. Park, J. Ma and S.-H. Wei, Semicond. Sci. Technol., 2016, 31, 083002.
3 A. Shepidchenko, B. Sanyal, M. Klintenberg and S. Mirbt, Scientific Reports, 2015, 5, 1–6.
4 P. Emanuelsson, P. Omling, B. K. Meyer, M. Wienecke and M. Schenk, Phys. Rev. B, 1993, 47, 15578–15580.
11:10 AM - EN07.02.03
WITHDRAWN 4/16/2021 EN07.02.03 N-Type CdTe Thin-Film Solar Cells
Vasilios Palekis1,Wei Wang1,Sheikh Tawsif Elahi1,Md Zahangir Alom1,Chris Ferekides1
University of South Florida1Show Abstract
Recent developments in cadmium telluride (CdTe)-based thin film technology has demonstrated record efficiencies for both CdTe cells (22.1 %) and modules (18.6 %). As a result thin film CdTe solar cells have become a competitive and an important alternative to silicon based devices. Even with this progress, record efficiency CdTe solar cells have open circuit voltage VOC = 880 mV, which is considerable lower than the theoretical limit of VOC = 1.15V. Much of the focus on CdTe has been associated with increasing its net p-type doping concentration in order to increase the device VOC. High p-type doping in CdTe has been challenging, while various studies have shown 1016 - 1019 cm-3 n-type doping using In grown by the Bridgman or molecular beam epitaxy (MBE) methods. Recent studies of In doped monocrystalline CdTe have shown lifetimes exceeding 2 µs and solar cells fabricated with this material demonstrated VOC greater than 1 V.
In this paper the effect of indium (In) doping on CdTe thin film solar cells was investigated. CdTe thin films were deposited using the elemental vapor transport (EVT) technique under various Cd/Te gas phase ratios and In vapor concentrations. Solar cells of the superstrate configuration (glass/TCO/CdS/n-CdTe/p-ZnTe/BC) have been fabricated and characterized. There was a correlation between the concetration of In in the vapor phase and net n-type doping for CdTe devices fabricated near Cd/Te stoichiometric ratio; increasing the amount of indium resulted in higher n-type doping in CdTe. From C-V measurements doping levels >1016cm-3 were achieved. Devices were also fabricated at various Cd/Te vapor ratios. Films deposited at lower Cd/Te vapor ratios (i.e. Te-rich) exhibited higher n-type doping. Lower Cd/Te ratios favor the creation of Cd-vacancies which are needed for substitutional In doping, which can explain why the net doping increases at lower Cd/Te ratios. To-date n-type CdTe solar cells yielded devices with VOC= 730 mV, JSC= 20 mA/cm2, FF= 62 % and efficiency near 9 %. Device physics simulation tools - SCAPS and AMPS – were used to guide the development of n-type CdTe solar cells. The effect of n-type doping, carrier lifetimes, and interface states of the n-CdTe-absorber and p-ZnTe window layer are being investigated.
11:15 AM - EN07.02.04
In Situ Doping for CSS Growth of n-CdTe Hetero-Structures
Theo Hobson1,Luke Thomas1,Laurie Philips1,Jon Major1,Ken Durose1
University of Liverpool1Show Abstract
In this work we investigate the use of pre-doped n-type CdTe source material to fabricate PV devices via close-space sublimation (CSS). The more readily dopable n-type CdTe offers the potential advantages of easier Ohmic contacting and higher Voc than are achieved with conventional p-type CdTe material.
Metallic indium was incorporated into bulk CdTe source material (Sigma Aldrich 6N) in quartz capsules sealed under vacuum and heated above the melting point of 1091°C for 3 days. In order to achieve doping in the range 1018 – 1016 cm-3 successive dilution steps were required. The resulting material was qualified by chemical analysis using EDX (to determine homogeneity for the highly doped compositions) and by ICP-OES to determine the absolute indium concentrations in the bulk material more generally. XRD was used to confirm the phase purity.
Thin films of CdTe:In deposited using CSS onto NSG TEC-AB substrates (highly resistive SnO2 on glass) were examined. Incorporation of the indium during the CSS deposition was investigated by quantitative SIMS of the films, and by scraping off the material for ICP-OES analysis. Growth on the resistive TEC-AB substrates also allowed for in-plane conductivity and contact resistance measurements using the transmission line method (TLM). C-V measurements were carried out on glass/Mo/CdTe:In/In structures and were used to give estimates of the carrier concentration. The carrier type was confirmed using the hot probe method.
In addition to the characterisation of the films, we also investigated the use of n-CdTe in test-structures and devices based on p-type semiconductor wafers. Simple evaluations of the band line-ups between n-CdTe and p-type wafers of Si, GaAs, and InP, using the Anderson model, indicated favourable junction properties. We therefore grew these structures by CSS transport of CdTe:In onto (001) oriented wafers. Initial deposition tests indicated that the films adhered well to their substrates and were crystalline in appearance. The junction properties and PV performances of these structures will be reported.
11:25 AM - EN07.02.05
Post-Growth Doping of N-Type CdTe for Thin-Film Devices
Luke Thomas1,M Isaccs2,K Cheetham3,Theo Hobson1,Jon Major1,Ken Durose1
Stephenson Institute for Renewable Energy1,HarwellXPS2,STFC, Daresbury Laboratory3Show Abstract
The motivation of this work is to deploy intentionally doped n-type CdTe in place of the more conventional p-type: this offers the possibility of high, stable doping in the absorber layer which has the potential to give high Voc in photovoltaic devices. In this work we explore the use of post-growth doping of CSS-grown CdTe films, both to investigate the feasibility of doping and in preliminary devices.
Several approaches to post-growth doping with indium were trialled, including in-diffusion of evaporated indium metal, and the annealing of films that had been sprayed with InCl3 in either aqueous or methanolic solutions. Annealing was conducted either in air or under flowing nitrogen.
The surface chemistry of the treated samples was investigated using XPS. The instrument used was a Kratos Axis Ultra-DLD photoelectron spectrometer with a monochromatic AlKα x-ray source. The elements/peaks screened for were Cd3d, Te3d, In3d, Cl2p and O1s. The In MNN Auger line was also measured since it exhibits larger chemical shifts than the In3d XPS line. A pass energy of 20 eV was used for the XPS lines and 80 eV for the Auger.
Chemical incorporation of the indium in the CdTe films was evaluated by quantitative SIMS. Implantation standards were created using single crystal THM-grown B surfaces of CdTe (Japan Energy) with xxx-In and yyy-Cl. SIMS measurements were done by Loughborough Surface Analysis using a Cameca IMS 7F-auto SIMS instrument using O2+ and Cs+ as the primary ion species at an energy of 10 keV. Profiles were obtained for Cd, Te, In, Cl, O and Sn. It was found that indium could be incorporated throughout an approx. 5 µm thick film at a density of 6 x 1018 cm-3. The conductivity of the films and contact resistances were measured using the transmission line method (TLM) and the hot probe method was used to verify the carrier type.
A variety of ‘superstrate’ device structures were considered, for example glass/FTO/n-CdTe/ZnTe/P3HT/Au and SCAPS simulations indicated that many were capable of achieving Vocs of > 1 V in principle. Preliminary devices were fabricated on NSG TEC15 FTO/glass. ZnTe was evaporated from a powder source while the CSS-grown CdTe films were post-growth doped using the indium metal route. Device and diode characterisation results will be presented.
11:30 AM - EN07.02.06
Combinatorial Co-Optimization of Elemental Compositions in Thin-Film Solar Cells with CdSeyTe1-y Absorbers and MgxZn1-xO Contacts
Imran Khan1,Tursun Ablekim1,Wyatt Metzger1,Andriy Zakutayev1
National Renewable Energy Laboratory1Show Abstract
CdTe is one of the major commercial thin-film photovoltaic (PV) technologies with over 20 GW installation and record laboratory efficiency of 22.1%. CdTe-based PV technology has been improved in the past decade due to two factors: (1) integration of MgxZn1-xO (MZO) as the contact and window material improving the carrier collection in the short wavelengths. (2) Employing CdSeyTe1-y (CdSeTe) as the absorber material allowed more carrier collection in the longer wavelengths. However, both Mg in MZO and Se in CdSeTe changes the conduction band positions, which can detrimentally impact the open-circuit voltage due to the conduction band offset in an unoptimized MZO/CdSeTe interface.
Here, we investigated the effect of the elemental compositions in the MZO and CdSeTe alloys in determining the PV device performance . Combinatorial libraries of PV devices were fabricated with orthogonal composition gradients in MZO and CdSeTe. MZO was deposited by combinatorial RF sputtering. CdSeTe was implemented by sequential evaporation of CdSe/CdTe and CdCl2 heat treatment, leading to a graded bandgap with Se diffusion. Solar cell performance parameters were a direct function of both the elemental compositions. The device data facilitated identifying the optimal composition range for MZO (x = 0.10 - 0.15) and CdSeTe (y = 0.20 - 0.25), which resulted in PV device efficiency up to 17.7% with VOC = 831 mV, fill factor = 71% and JSC = 29.8 mA/cm2. We also recently studied and optimized MZO contact integration to CdTe  and CuGa3Se5  absorbers, and the results will be mentioned in this presentation.
 manuscript in preparation.
 arXiv preprint arXiv:2010.01635
11:45 AM - EN07.02.07
Combinatorial Development of MZO Emitters for Emerging CdTe Based Solar Cells
Gavin Yeung1,Carey Reich2,Adam Danielson2,Walajbad Sampath2,Colin Wolden1
Colorado School of Mines1,Colorado State University2Show Abstract
Due to the low material and manufacturing costs of polycrystalline CdTe-based solar cells, it is the leading thin-film photovoltaic technology. Magnesium zinc oxide (MgxZn1-xO) has proven to be a highly effective emitter layer due to its transparency and ability to tune the conduction band alignment with CdTe. State-of-the-art devices employ graded CdSeyTe1-y (CST) layer at the interface, and emerging absorbers employ group V doping with significantly higher carrier concentrations. The goal of this work is to engineer MZO to adapt to the evolving nature of the absorber. Combinatorial libraries of MZO were fabricated by reactive sputtering and characterized by ellipsometry, XRD, UV-visible spectroscopy, 4-point probe, and Hall effect. After screening to identify promising compositions, uniform MZO films are used to optimize device performance. For standard CdTe the optimum Mg content was found to be x~0.15, producing >16% devices. With CST based devices the optimum Mg content increased to x~0.25, producing >19% devices. The as-deposited MZO is highly insulating, so Ga was added to increase carrier concentration. GMZO libraries are fabricated with band gaps spanning 700 mV and whose electrical resistivity may be systematically varied >6 orders of magnitude by controlling the rate of Ga deposition. We plan to report on the efficacy of these promising GMZO libraries to optimize band gap and carrier concentration for both CST and CST:GrV based devices.
12:00 PM - EN07.02.08
Optical and Structural Properties of CdSSe Thin-Film Deposition by Pulsed Laser Technique
Devendra Kumar1,Pawan Kumar1,Arvind Kumar2,Ram Katiyar3
Gurukul Kangri Vishwavidyalaya1,Kalindi College, DU2,University of Puerto Rico, Rio Piedras Campus3Show Abstract
We have investigated the comparative studies on structural and optical properties of Cadmium Sulfoselenide thin films (thickness around 200nm) deposited on three different substrate (Glass, Indium Tin Oxide (ITO) and Si-p type) for sample and work with three different annealing temperature (150OC-350OC) for each substrate. The sample were prepared by Pulsed Laser Deposition (PLD) technique for deposition of thin films. Semiconductor CdSSe core shell nano film exhibit very intense and narrow absorption and photoluminescence spectra in visible range, which makes it extraordinary for sowing many applications in optoelectronic devices. The samples were characterized by X-ray diffraction (XRD) for Structural analysis, UV-Vis with help of Transmittance spectra in the range 300nm to 800nm at and Photo Luminescence (PL)(300nm to 800nm) have done for optical studies and for Surface morphology Atomic Force Microscopy (AFM) were carried Out.
12:05 PM - *EN07.02.09
What Limits the Voltage of CdSeTe Solar Cells—Material Quality, Surface Passivation or Contact Selectivity?
Arthur Onno1,Adam Danielson2,Carey Reich2,Siming Li3,William Weigand1,Darius Kuciauskas3,Walajbad Sampath2,Zachary Holman1
Arizona State University1,Colorado State University2,National Renewable Energy Laboratory3Show Abstract
Polycrystalline cadmium telluride (CdTe) and cadmium selenide telluride (CdSeTe) photovoltaic devices suffer from a sizable voltage deficit when compared with competing technologies (e.g., crystalline silicon, perovskite, III-V). Record-efficiency devices have not yet broken the 900-mV open-circuit voltage limit, even though, given the wide direct bandgap of CdSeTe absorbers (1.4–1.5 eV depending on the selenium content), a voltage above 1100 mV could be theoretically expected. Yet, a precise understanding of the origin of this voltage deficit and, more importantly, a clear pathway to overcome it are currently lacking.
The historically poor material quality of polycrystalline CdTe absorbers with, until recently, minority carrier lifetimes τ in the nanoseconds at best, has long been held responsible for these poor Voc performances. To remedy this, extensive work has focused on increasing both the lifetime and the acceptor (i.e. activated dopant) concentration NA, as the quasi-Fermi-level separation is a direct function of the NA×τ product. Double heterostructure CdSeTe samples, with both surface passivated with either magnesium zinc oxide (MZO) or Al2O3, have reached lifetimes in the hundreds of nanoseconds, indicating that excellent material quality was achievable with CdSeTe absorbers and suggesting that poor surface passivation—i.e., passivation of the absorber front and back interfaces, as opposed to passivation of defects at the grain boundaries—was an important contributor to the limited voltages measured on finished devices. Similarly, arsenic doping of the absorber has led to acceptor concentrations in excess of 1016 cm-3.
Yet, these improvements in both lifetime and acceptor concentration have not translated into meaningful increases in Voc. Two questions thus arise:
1) What limits the voltage of Cd(Se)Te devices?
2) How can we easily and systematically decouple the sources of voltage loss in finished devices?
In this work, we answer these two questions by comparing the thermodynamic voltage limit Voc,ideal of finished solar cells with their quasi-Fermi-level splitting (QFLS)—also referred to as the internal or implied open-circuit voltage iVoc (QFLS=q×iVoc with q the elementary charge)—and their external or terminal voltage Voc. We calculate Voc,ideal from the external quantum efficiency (EQE) of the device and access iVoc through measurement of the device external radiative efficiency (ERE) using a calibrated photoluminescence tool, while Voc is measured through standard J–V curve tracing.
We show that As-doped cells with Al2O3-passivated contacts can exhibit EREs in excess of 1%—surpassing the best crystalline silicon devices and on par with the best perovskite devices—but, concurrently, band tails limit their thermodynamic voltage limit. Nevertheless, our best device achieves an internal voltage iVoc>980 mV. This is close to 100 mV higher than the external open-circuit voltage (Voc) of the record-efficiency polycrystalline CdSeTe solar cell. Thus, in these highly luminescent devices, the voltage is limited by the contact selectivity rather than by the material quality of the absorber. We then propose a novel passivated and selective back contact structure—based on an Al2O3 passivating layer followed by a p-type boron-doped hydrogenated amorphous silicon (a-Si:H) hole selective layer—with the goal of translating these high iVoc values into high Voc values and achieving Voc>1 .
EN07.03: Advances in CdTe-Based Solar Cells II
Sunday PM, April 18, 2021
1:00 PM - *EN07.03.01
On the Fundamental Aspects of Performance and Stability of CdTe Solar Cells
First Solar1Show Abstract
First Solar is the largest solar manufacturer in Western Hemisphere. While possessing CdTe cell and module efficiency records of 22.1% and 18.2%, respectively, we continue research to further push the boundaries of CdTe cell technology. In particular, we conduct fundamental studies of losses and instabilities within active layers of CdTe cells.
The current Cu-doped CdTe technology suffers from insufficient absorber doping caused by atomic interactions between Cu and Cl. Besides limiting the maximum doping achievable in practical fabrication process, defect reactions between Cu and Cl cause further reduction in absorber doping during field operation, which contributes to slow degradation of Cu-doped devices.
High and stable absorber doping of 1e16 cm-2 and above could be achieved in chlorinated CdTe films by replacing Cu with arsenic acceptors. However, taking the full advantage of heavily-doped CdTe absorbers requires high quality front interface with small conduction band offset. Our atomic simulations predict strong conduction band offset at CdTe/SnO2 interface, which prevents achieving high Voc values. Additional performance improvement of arsenic-doped cells could be achieved by further increase in doping activation, which is currently limited by formation of kinetically-stabilized deep donors that not only reduce the built-in potential, but may also limit the bulk lifetime.
Another important aspect of CdTe cell performance is reversible short-range metastabilities present both in Cu- and As-doped devices. Our kinetic device modeling suggests that observed metastable phenomena are caused by the defect reactions at the front interface.
1:25 PM - EN07.03.02
Investigating Carrier Dynamics in CdTe Full Device Studied with Time-Resolved Terahertz and Photoluminescence Spectroscopies
Mohammad Mehdi Taheri1,Triet Truong1,Shannon Fields2,William Shafarman2,Brian McCandless2,Jason Baxter1
Drexel University1,University of Delaware2Show Abstract
Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for higher Voc and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but not all materials are strongly emissive or otherwise suitable. Pump – probe spectroscopy presents valuable complementary information, wherein an optical pump pulse photoexcites carriers within the absorber and the transient photoconductivity is probed with a terahertz (THz) pulse. Until now, experimental constraints have limited the use of THz probes to interrogate full device stacks, and measurements were instead made on exfoliated films or films grown on unconventional substrates. However, interfaces are critical to the behavior of photoexcited carriers in solar cells, and the substrates themselves often influence the film growth and bulk properties. Therefore, it is important to probe the behavior of PV absorbers as close as possible to their normal operating conditions.
Here we report on cross-cutting metrology tools based on time-resolved THz spectroscopy (TRTS) and TRPL that enable in operando characterization of carrier dynamics and recombination mechanisms in working PV devices. Through a combination of experiments and modeling, we can obtain key parameters including Shockley-Read-Hall (SRH) lifetime, minority carrier mobility and diffusion length, and interface recombination velocity using non-contact probes. By varying pulsed photoexcitation conditions, the contributions of interface and bulk recombination can be determined as a function of applied bias.
TRTS has been applied to thin film absorbers but has not been used for full devices because the conductive contacts absorb or reflect the THz radiation. Here we create wire-grid patterns using conventional laser scribing to increase the signal-to-noise ratio by at least 5-fold compared to unscribed samples. The TRTS signal corresponds to transient photoconductivity, while TRPL arises from radiative recombination. We can simulate both of these by numerical solution of the continuity equations and Poisson’s equation. Best-fit values of the SRH lifetime and surface and interface recombination velocities are then determined by global fitting of multiple TRTS and TRPL data sets having different photoexcitation conditions.
We have developed and validated this approach using CdTe thin film PVs. For instance, we found a bulk lifetime of 1.6 ns, CdTe/CdS interface recombination velocity of less than 100 cm/s, and back surface recombination velocity of ~2x103 cm/s for a CdTe device treated with CdCl2. In contrast, as-deposited films without CdCl2 treatment had bulk lifetime of only ~50 ps and higher interface and back-surface recombination velocities of ~2x104 cm/s. We are now applying this approach to understand how dynamics change depending on processing conditions designed to enable high doping densities and on choice of buffer material.
By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin film PVs with high higher Voc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, CIGS, CZTSSe, and other emerging technologies.
1:40 PM - EN07.03.03
Towards Large-Grain Epitaxial CdTe Solar Cells Made by Close-Spaced Sublimation (CSS) or Vapor Transport Deposition (VTD)
Mahisha Amarasinghe1,2,David Albin2,Matthew Reese2,John Moseley2,Helio Moutinho2,Wyatt Metzger2
University of Illinois at Chicago1,National Renewable Energy Laboratory2Show Abstract
Polycrystalline CdTe thin films for solar energy applications are deposited by fast high throughput deposition techniques on glass coated with nanocrystalline transparent conducting oxides. While this approach makes polycrystalline CdTe solar panels a cost leader, the deposition has resulted in small grain sizes on the order of microns for decades.
Grain boundaries (GBs) are active recombination centers that reduce overall carrier lifetime and hence the device performance of CdTe solar cells [1,2]. Therefore, increasing grain size and reducing GB density, especially at the critical junction region, can improve carrier lifetime and solar cell performance [3-5].
The CdTe community relies on CdCl2 treatments to facilitate grain growth and passivate GBs, increasing the aggregate lifetime from ps to tens of ns. This passivation combined with CdSeTe bandgap engineering has improved CdTe record efficiency to 22.1%. Material challenges are that CdTe grain growth during CdCl2 anneals is driven by Ostwald ripening, which has historically been limited to the order of the film thickness (i.e. a few microns). Furthermore, CdCl2 treatments do not completely passivate GBs . For example, correlated GB recombination velocities have been measured to be as high as 105–106 cm/s . Finally, Cl compensation may reduce activation ratios in Group-V doped CdTe devices resulting in microscopic potential fluctuations and lower VOC .
Colossal grain growth (CGG) was recently reported in micron-thick Cd(Se,Te) thin films deposited on glass . The resulting CdTe structures had extremely large ~ 1-mm lateral size grains that were much larger than anything previously reported and a layer structure relevant for CdTe solar cells.
In this work, we will illustrate that the resulting, near single-crystal thin films can be used as a growth template for epitaxy of subsequent CdTe layers by techniques appropriate for making devices. Starting device structures have been made using Corning 7059/75-nm Cd2SnO4/60-nm MgZnO substrates upon which a CdSe0.10Te0.9 layer was evaporated for CGG. Because of its lower bulk resistivity, Cd2SnO4 films could be made thin enough for devices and smooth for CGG of the Cd(Se,Te) layer. After the Cd(Se,Te) was deposited, CdCl2 treatments with temperatures ranging from 420 °C to 500 °C (with and without oxygen) were carried out before and after the CGG anneal, and subsequent CdTe was deposited epitaxially by using either close-spaced sublimation or vapor transport deposition. Lifetime, device performance, and cathodoluminescence results will be presented for both glass/Cd2SnO4/MgZnO/CGG-Cd(Se,Te)/CdTe and glass/Cd2SnO4/MgZnO/CGG-Cd(Se,Te) structures at the conference.
1. Moseley, J. et al., IEEE J. Photovolt. 4, 1671–1679 (2014)
2. Moseley J. et al., J. Appl. Phys, 118, 025702 (2015)
3. Kanevce A. et al., J. Appl. Phys, 121, 214506 (2017)
4. Amarasinghe M. et al., Adv. Energy Mater. 2018, 8, 1702666
5. Amarasinghe M. et al., IEEE J. Photovolt. 8 (2), 600-603 (2018)
6. Moseley J. et al., J. Appl. Phys, 124, 113104 (2018)
7. Moseley J. et al., Journal of Applied Physics, 128, 103105 (2020).
8. Albin D. et al., J. Phys Energy
1:55 PM - EN07.03.04
Direct Microscopy Imaging of Nonuniform Carrier Transport in Polycrystalline Cadmium Telluride
National Renewable Energy Laboratory1Show Abstract
Inhomogeneous microscopic carrier transport is difficult to study but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.
2:10 PM - *EN07.03.05
N-Type CdTe Thin Films and Solar Cells
Chris Ferekides1,Vasilios Palekis1,Wei Wang1,Sheikh Tawsif Elahi1,Md Zahangir Alom1
University of South Florida1Show Abstract
Recent developments in cadmium telluride (CdTe)-based thin film technology has demonstrated record efficiencies for both CdTe cells (22.1 %) and modules (18.6 %). Even with this progress, record-efficiency CdTe solar cells have open-circuit voltage VOC ~ 880 mV, which is considerably lower than the theoretical limit of VOC ~ 1.15V. Much of the focus on CdTe has been associated with increasing its net p-type doping concentration in order to increase the device VOC. High p-type doping in CdTe has been challenging, while various studies have shown 1016 - 1019 cm-3 n-type doping in crystalline CdTe using In grown by the Bridgman or molecular beam epitaxy (MBE) methods. Recent studies of In doped crystalline CdTe have shown lifetimes exceeding 2 µs and solar cells fabricated with this material demonstrated VOC greater than 1 V.
In this paper the effect of indium (In) doping on CdTe thin film solar cells will be discussed. CdTe thin films were deposited using the elemental vapor transport (EVT) technique under various Cd/Te gas phase ratios and In vapor concentrations. Solar cells of the superstrate configuration (glass/TCO/CdS/n-CdTe/p-ZnTe/BC) have been fabricated and characterized. There was a correlation between the concetration of In in the vapor phase and net n-type doping for CdTe devices fabricated near Cd/Te stoichiometric ratio. Increasing the amount of In vapors resulted in higher n-type doping in CdTe. From C-V measurements doping levels >1016cm-3 were achieved. Devices were also fabricated at various Cd/Te vapor ratios at a constant In vapor concentration. Films deposited under Cd-rich conditions exhibited higher lifetimes than those deposited under Te-rich conditions. On the other hand, films deposited at under Te-rich conditions exhibited higher n-type doping. To-date n-type CdTe solar cells yielded devices with VOC= 730 mV, JSC= 20 mA/cm2, FF= 62 % and efficiency near 9 %. SCAPS and AMPS solar cell simulation tools were used to better understand and aid the development of n-type CdTe solar cells. Doping, carrier lifetimes, and interface states of the CdTe-absorber and ZnTe window layer are being investigated.
2:35 PM - *EN07.03.06
Progress and Challenges in Doping Polycrystalline CdTe-Based Alloys with Arsenic
Amit Munshi1,Akash Shah1,Ramesh Pandey1,Pascal Jundt1,Carey Reich1,Adam Danielson1,Walajbad Sampath1
Colorado State University1Show Abstract
Improvement in polycrystalline CdTe photovoltaics efficiency is primarily limited by the deficit in open-circuit voltage. The deficit can be overcome by reducing the carrier recombination while improving carrier concentration in the film. Traditionally, CdTe-based devices have been doped with copper. However, increasing carrier concentration using copper while reducing recombination is fundamentally limited by various complexes copper may form. To overcome this issue, doping with group V elements has been of interest to the research community for decades and more recently it has been one of the primary focuses of the CdTe research community.
Using sublimation as the deposition method, our research has shown that arsenic can be effectively used to improve carrier concentration while simultaneously reducing recombination. Doping with arsenic is certainly not devoid of challenges as various defects such as the presence of dimers, tetramers, AX-centers, and arsenic clusters in the device pose challenges. However, using an advanced fabrication approach guided by state-of-the-art predictive computational simulation models, we have been able to overcome several of these limitations. Using the method that will be discussed, we have been able to demonstrate carrier lifetimes greater than 2 µs, carrier concentration over 5x1015 cm-3, and surface recombination velocity less than 100 cm/s. Following fabrication of films with characteristics suitable for high performing devices, several other factors in the film fabrication process and device structure had to be refined to employ the advantages of doping CdTe-based devices with arsenic as shown by various numerical simulations.
In addition to improvement in device performance, using arsenic as the doping element for CdTe-based photovoltaics may have advantages in terms of longevity of the modules in the field as well. With optimized doping in CdTe with copper, large volumes of modules have been commercially manufactured and installed. However, arsenic doping in polycrystalline films is found to have superior metastability. This is one of the critical factors to be taken into consideration as the elimination of copper may improve projected life of CdTe-based photovoltaic panels from 25 years currently to over 50 years.
EN07.04: Novel Absorber Materials I
Sunday PM, April 18, 2021
4:00 PM - EN07.04.01
Optoelectronic and Material Properties of Solution-Processed Earth-Abundant
Cu2BaSn(S, Se)4 Films for Solar Cell Applications
Betul Teymur1,Sergiu Levcenko2,Hannes Hempel2,Eric Bergmann3,Jose Marquez Prieto2,Thomas Unold2,Ian Hill3,David Mitzi1
Duke University1,Helmholtz-Zentrum Berlin für Materialien und Energie GmbH2,Dalhousie University3Show Abstract
Cu2BaSn(S,Se)4 (CBTSSe) has been gaining attention as a prospective solar absorber, since it employs low-toxicity and abundant metals, while offering low-cost manufacturing options, controllable stoichiometry, high absorption coefficient (>104 cm-1) and bandgap (Eg) tunability (1.5-2.0 eV). Besides these suitable optoelectronic properties, CBTSSe does not suffer from anti-site disorder in contrast with Cu2ZnSn(S,Se)4, as confirmed by Shin et al. with vacuum-deposited films . The current study focuses on solution-deposited nominally stoichiometric CBTSSe films  with a bandgap of 1.59 eV and explores the fundamental film properties using several spectroscopic techniques. Temperature- and excitation-dependent photoluminescence studies reveal a dominant defect emission at ~1.5 eV and a second deep defect feature at 1.15 eV. Time-resolved terahertz spectroscopy measurements show few tens of picoseconds (~50 ps) surface and few nanoseconds (~3 ns) bulk lifetimes, as well as ~140 cm2/Vs mobility, the latter of which is in the range of reported values for CZTSSe. The solution-processed CBTSSe films suffer from exacerbated surface recombination at the junction of CdS and CBTSSe, due to cliff-like band alignment with 0.63 eV conduction band offset (as measured by ultraviolet photoemission spectroscopy). Employing these films in an Al-Ni/ITO/ZnO/CdS/CBTSSe/Mo photovoltaic device structure, we report open-circuit voltage (VOC), short-circuit current density, fill factor, and efficiency of 470 mV, 14.3 mA/cm2, 43.6%, and 2.93%, a record performance level among solution-processed CBTSSe devices. Device performance in this study may be mainly limited by interfacial properties and inadequately selected device structure (e.g., poorly matched buffer layer), which both enhance recombination-associated losses in VOC. The physical measurements provided for the nominally stoichiometric solution-processed CBTSSe absorber point to critical areas for future improvement of CBTSSe and related photovoltaic cells in the quest for higher efficiency devices based on earth-abundant metals.
 D. Shin, B. Saparov, T. Zhu, W. P. Huhn, V. Blum, D. B. Mitzi, BaCu2Sn(S,Se)4: Earth-Abundant Chalcogenides for Thin-Film Photovoltaics, Chem. Mater., 28 (2016), 4771-4780.
 B. Teymur, Y. Zhou, E. Ngaboyamahina, J. T. Glass, D. B. Mitzi, Solution-Processed Earth-Abundant Cu2BaSn(S,Se)4Solar Absorber Using a Low-Toxicity Solvent, Chem. Mater., 30 (2018), 6116-6123.
4:15 PM - EN07.04.02
Silver Doping of Enargite (Cu3AsS4) Absorbers Using Amine-Thiol Chemistry for the Fabrication of Solar Cells
Apurva Pradhan1,Scott McClary1,Joe Andler1,Rakesh Agrawal1
Purdue University1Show Abstract
Enargite (Cu3AsS4) is an emerging candidate for photovoltaic applications with a direct band gap of 1.41 eV, consisting of earth abundant elements with differing ionic radii and consequently lower likelihood of cation disorder, ideal predicted and experimentally verified optoelectronic properties, and demonstrated use in working thin-film solar cells. Previous experimental work on this material from our group has demonstrated that enargite films have a Shockley-Read-Hall, or defect assisted, recombination lifetime on the timescale of nanoseconds, relatively shallow defect energies, and carrier concentrations on the order of 1015 cm-3. However, carbonaceous fine grain layers, secondary phases, pinholes in the film, and poor band alignment with n-type CdS may be part of the reason that power conversion efficiencies for enargite-based solar cells remained below 0.5%. While investigating suitable n-type junction partners is a subject of a parallel research effort, in this work, we present a novel solution-processed route to create enargite films using an amine-thiol based molecular precursor approach that eliminates carbonaceous fine grain layers, creates phase-pure films, and improves film morphology.
A mixture of amine and thiol has shown to be a very versatile solvent system for dissolving a variety of metal chalcogenides for the fabrication of thin films and nanoparticles. Arsenic sulfides and copper sulfides can undergo reactive dissolution in amine-thiol solvents to form complexes that can decompose at relatively low temperatures. These molecular precursor solutions can be combined into an ink that can be directly coated onto conductive substrates to create tennantite (Cu12As4S13) films that transform and coarsen into micron-sized enargite grains with low surface roughness and no detectable secondary phases after heating in an arsenic-sulfide atmosphere at 425°C. Unfortunately, shunt resistances on devices made using this approach remained low and SEM has confirmed the presence of pin-holes in the film. To improve film morphology and increase shunt resistances, inspiration was taken from silver alloyed Cu(In,Ga)Se2 thin films.
Silver alloying in Cu(In,Ga)Se2 (CIGSe) solar cells has been an area of active research over the past decade. In CIGSe, Ag alloying has been shown to lower the valence band edge, increase the bandgap, increase grain size, and reduce structural defects, partially due to the lower melting temperature of these alloyed films. The amine-thiol solvent system allowed for facile incorporation of varying concentrations of silver sulfide into coating inks for the creation of silver doped enargite films. With silver incorporation, we observed significant improvements in film morphology that fused adjacent enargite grains in the film together and eliminated pinholes. Preliminary optoelectronic characterization on these films have revealed a shift to higher energy bandgaps with increasing silver content. Devices created with silver-doped enargite show increased shunt resistance and produced champion device efficiencies exceeding 0.6%. This is higher than past demonstrated efficiencies for this material and shows that silver alloying has the potential to improve film properties and device efficiencies. Further optimization of film growth conditions and silver content is expected to allow for further increases in device efficiencies.
4:30 PM - EN07.04.03
Embedded Au and Ag Nanoparticles in Pulsed Laser Deposited Photovoltaic Thin Films
Mehmet Sahiner1,Jasmyne Emerson1,Faith Akinlade1,Matthew Herington1,Venise Castillon1
Seton Hall University1Show Abstract
We have used pulsed laser deposition to deposit nanoparticles (Ag, Au) to investigate
the effects of these impurities on the photovoltaic properties of the CdS/CdTe based thin films. The main
objective was to investigate how the inclusion of nanoparticles will affect light scattering in the at the interfaces
and whether the different size and shape of nanoparticles will have a positive effect on the overall electrical
performance of these thin film solar cells. In our previous studies, we have investigated the effects of the
embedded Ag nanoparticles on the photoelectric conversion efficiency on CdS/CdTe based thin film solar cells
as synthesized by Pulsed Laser Deposition (PLD). Silver was shown to enhance the photovoltaic performance
by almost doubling the photovoltaic conversion efficiency of the conventional CdS/CdTe films . A careful
comparison of photovoltaic performance of Au/Si versus Ag embedded thin films of CdS/CdTe on indium tin
oxide coated glass substrates have been performed. Our results on the Ag case revealed electrical
performance of these cells have correlates with the particles density and the particle size on the CdS/CdTe
interface. This study concentrates on the Au and Ag nanoparticle deposition on the CdS/CdTe interface with
varying particle size and distributions. Structural and compositional characterization were performed using
XRD, AFM, and SEM/EDX. Photovoltaic properties were measured using a LabView assisted Keithley
Sourcemeter set-up. The comparison of Ag vs Au nanoparticles on the structure and photovoltaic
conversion efficiency will be presented. Ag and Au nanoparticles have contrasting effects on the
photovoltaic conversion efficiency in terms of their relative coverage at the interface, This will be discussed in
the light of plasmonic resonances and effective light scattering for Ag and Au particles.
 Olivia Rodgers, Anthony Viscovich, Yunis Yilmaz, Mehmet Sahiner, “The Effect of Embedded Ag
Nanoparticle on the Photovoltaic Conversion Efficiency in CdTe/CdS Thin Films”, American Physical Society
Bulletin, X17.13 (2018).
This work is supported by NSF Award #:DMI-0420952
4:35 PM - EN07.04.04
Synthesis and Characterization of Selenium-Alloyed Bournonite—A Prospective Semiconductor for Optoelectronic Applications
Eric Chang1,Gabrielle Koknat1,Volker Blum1,David Mitzi1
Duke University1Show Abstract
In the search for solar absorber materials for thin-film photovoltaics (PV), the materials class of complex chalcogenides has maintained consistent interest. In the past two decades, the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has proven promising as an earth-abundant, nontoxic solar absorber. However, with an efficiency plateauing at 12.6%, a necessary search for novel earth-abundant and nontoxic chalcogenide PV candidates continues. Recently, bournonite (CuPbSbS3) has been identified as a potential ferroelectric photovoltaic material with a bandgap (1.2-1.3 eV) appropriate for single junction photovoltaic devices (1-1.6 eV). Bournonite is an appealing candidate for a number of reasons, including its structural and electronic three dimensionality, absorption coefficients comparable to that of MAPbI3 and GaAs, an optical dielectric constant similar to that of hybrid perovskites, high predicted defect tolerance, and the potential for Rashba/Dresselhaus splitting, a phenomenon that has been linked to slow electron hole recombination in lead-halide perovskites.1 Additionally, as a naturally occurring mineral, bournonite is predicted to be stable, avoiding the instability challenges that face the highly popular perovskites. In the past year, progress in bournonite’s application for PV has accelerated with two studies reporting thin-film processing of bournonite.2,3 One such study also reported the first PV device to employ bournonite as the absorber layer (PCE of 2.23%).3 In an effort to explore band gap engineering of bournonite, we report the solid state bulk synthesis of selenium-alloyed bournonite (CuPbSb(S1-xSex)3) across the full range of selenium concentrations (x = 0.0 - 1.0). We characterize the crystal structure and band gap of the samples using x-ray diffraction and diffuse reflectance spectroscopy, reporting a band gap that approaches 1.1 eV near x = 0.5. We compare these results with computational band structure predictions.
1. Wallace, S. K.; Svane, K. L.; Huhn, W. P.; Zhu, T.; Mitzi, D. B.; Blum, V.; Walsh, A., Candidate photoferroic absorber materials for thin-film solar cells from naturally occurring minerals: enargite, stephanite, and bournonite. Sustainable Energy & Fuels 2017, 1 (6), 1339-1350.
2. Koskela, K. M.; Melot, B. C.; Brutchey, R. L., Solution Deposition of a Bournonite CuPbSbS3 Semiconductor Thin Film from the Dissolution of Bulk Materials with a Thiol-Amine Solvent Mixture. Journal of the American Chemical Society 2020, 142 (13), 6173-6179.
3. Liu, Y.; Yang, B.; Zhang, M.; Xia, B.; Chen, C.; Liu, X.; Zhong, J.; Xiao, Z.; Tang, J., Bournonite CuPbSbS3: An electronically-3D, defect-tolerant, and solution-processable semiconductor for efficient solar cells. Nano Energy 2020, 71, 104574.
4:40 PM - EN07.04.05
Fabrication of pn Homojunction of SnS and Its Photovoltaic Properties
Issei Suzuki1,Sakiko Kawanishi1,Sage Bauers2,Andriy Zakutayev2,Hiroyuki Shibata1,Hiroshi Yanagi3,Takahisa Omata1
Tohoku University1,National Renewable Energy Laboratory2,University of Yamanashi3Show Abstract
Tin sulfide (SnS) is a promising ligth-absorbing material for next-generation solar cells due to its safe and abundant constituent elements and suitable optical properties. SnS homojunction structure is expected to overcome the limitation of conversion efficiency (η) of the conventional SnS solar cells based on the heterojunction between p-type SnS and n-type CdS or Zn(O,S) with η of 4-5% at maximum. However, SnS homojunction has not been reported due to difficulties in realizing n-type SnS. In recent years, we have demonstrated the n-type conductive SnS single crystals with halogen (Cl or Br) doping, and growth of large single crystal in centimeter-scale. In this study, a p-type SnS thin film was deposited on the large size n-type crystal to form a pn homojunction, and its photovoltaic properties were evaluated.
Cl-doped large n-type SnS single crystals were prepared by the flux method using Sn as the main solvent. A p-type SnS thin film with a diameter of 1 mm and a thickness of 270 nm was deposited at 340° C on the (100) cleavage plane of the n-type single crystal by RF sputtering. The device structure was GaIn/n-type SnS single crystal/p-type SnS thin film/ZnO/ITO. Structural and electrical properties of the film and homojunction were evaluated. Photovoltaic properties were studied by I-V and C-V measurements.
The p-type SnS thin film deposited on the single crystal was <100>-oriented in the growth direction and was also gently oriented in the growth plane. In addition, there was no void at the n-type single crystal/p-type thin film interface, and the change in Cl concentration was very abrupt. The open circuit voltage (VOC) was 360 mV, which is comparable to the highest value of the previously reported SnS homojunction cells (372 mV). Since the built-in potential of the homojunction was 920 mV and much larger than the previous heterojunction (~700 mV), it should be possible to improve the VOC further. The conversion efficiency was 1.4%, which is lower than the record η of heterojunction (4-5%), mainly due to its low closed circuit current density (JSC) of 7.5 mA/cm2. Once the device structure of the homojunction cell is optimized to efficiently collect the photogenerated carriers and achieve a comparable JSC as the conventional heterojunction cells (~25 mAcm-2), conversion efficiency exceeding 4-5% would be realized with homojunction with improving the VOC.
 P. Sinsermsuksakul et al., Adv. Energy Mater., 4, 1 (2014).
 H. Yanagi et al., Appl. Phys. Express, 9, 051201 (2016); Y. Iguchi et al., Inorg. Chem., 57, 6769 (2018).
 S. Kawanishi et al., Cryst. Growth Des. 20, 5931 (2020).
5:00 PM - EN07.04.06
Synthesis of Cu2ZnSnS4 Using Low-Temperature, Water-Based Binary Sulfide Nanoparticle Inks for Sustainable, Low Cost Photovoltaics
Han Wang1,Nathaniel Quitoriano1,George Demopoulos1
McGill University1Show Abstract
Kesterite Cu2ZnSnS4 (CZTS) is a promising semiconductor material for next-generation thin film photovoltaics since it has earth-abundant and nontoxic elements, a stable structure, and strong light absorption properties. Nanoparticle, solution-based methods have been demonstrated as a potential route for the deposition of CZTS thin films, but many of these methods incorporate organic solvents or additives which can leave a carbon residue that affects device performance. In addition, these organic solvents can create problems with scalability and sustainability. The presented work demonstrates the capability of an environmentally friendly, aqueous-based nanoparticle ink to create the CZTS absorber thin film. Metal chalcogenide complexes formed with ammonium sulfide and tin sulfide are used to stabilize a mixture of component binary sulfides in water without other additives, which are deposited as a compact absorber film with spin coating. It is shown that the concentration of the ammonium sulfide content greatly affects the ink stability, the thin film morphology, and optoelectronic properties of the absorber layer. Furthermore, the incorporation of the tin sulfide complex enables a low temperature transition from binary sulfide nanoparticles to the CZTS phase.
5:05 PM - EN07.04.07
Towards Hybrid III-V/SiGeSn Solar Cells Integrated on Silicon
Simone Assali1,Anis Attiaoui1,Sebastian Koelling1,Mahmoud Atalla1,Mohammad Chowdhury1,Oussama Moutanabbir1
Polytechnique Montréal1Show Abstract
While the development of III-V multi-junction solar cells is mainly driven by space applications, a large potential market for this technology could rely on terrestrial applications. However, the integration of III-V solar cells in large-scale consumer applications is strongly dependent on the development of scalable, high efficiency technologies at a lower material cost. Currently, bulk gallium arsenide (GaAs) or germanium (Ge) substrates are required for the growth of InGaP/InGaAs/Ge triple-junction solar cells, yielding to prohibitive costs. The limited world supply of Ge and the increasing demand for military, microelectronics applications will further increase costs in the coming years. A key strategy in developing a new family of multi-junction solar cells is to introduce epitaxial silicon-germanium-tin (SiGeSn) in the fabrication of III-V solar cells. The bandgap of SiGeSn can be tuned to 1.0 eV while being lattice-matched to Ge and GaAs, which will enable optimal absorption of the solar spectrum and further enhance device efficiency. In addition, high-crystalline quality SiGeSn and Ge layers can be grown on Si wafers,[2-5] thus lifting the requirement for Ge wafers in favor of the mechanically robust, inexpensive Si substrates. This is key to achieve the cost-effectiveness crucial for large-scale, terrestrial applications of hybrid III-V/IV solar cells.
In this talk, we will discuss the epitaxial growth and opto-electronic properties of SiGeSn semiconductors that are grown on a Ge virtual substrate on Si (Ge-VS/Si) using a chemical-vapor-deposition (CVD) reactor. High crystalline quality, uniform composition, and absence of short-range ordering effects and Sn clusters are estimated down to the atomic level by combining TEM, XRD, and atom probe tomography (APT) measurements. A tunable SiGeSn direct band gap above 0.8 eV is obtained by controlling the Si/Sn incorporation during growth and the optical properties of the material will be addressed using ellipsometry and absorption measurements. Photoconductive devices (PD) fabricated using a 1.5-μm thick Si0.06Ge0.90Sn0.04 epilayer show a maximum responsivity of ~1.1 A/W at ~1.0 eV at room temperature. The measured responsivity is slightly higher than the reference Ge-VS PD with a similar total thickness and it indicates that the high epilayer quality of the substrate is preserved during the lattice-matched SiGeSn epitaxy on top. This also results in a comparable dark current in both materials PDs.
To enable the development of hybrid multi-junction solar cells tunneling junction layers between the different sub-cells are required. To this end, SiGeSn p-i-n heterostructures were developed by incorporating Boron (p-SiGeSn) and Arsenic (n-SiGeSn). Active carrier concentrations in the 2x1017-5x1019 cm-3 range were obtained for both p- and n-layers by controlling the supply of the gas precursors during growth. The effect of dopant incorporation on material and device properties will be discussed by combining atomistic and opto-electronic characterization methods. Single- and multi-junction solar cells made of Sn-rich group IV semiconductor alloys will be discussed. Moreover, the epitaxial growth of III-V sub-cells in the SiGeSn/Ge-VS/Si heterostructure will be discussed and it will pave the way for the fabrication of hybrid III-V/IV multi-junction solar cells.
 A. Attiaoui and O. Moutanabbir, J. Appl. Phys. 116, 063712 (2014)
 J.-H. Fournier-Lupien, et al., Appl. Phys. Lett. 103, 263103 (2013)
 S. Assali, et al., Appl. Phys. Lett. 112, 251903 (2018)
 S. Assali, et al. J. Appl. Phys. 125, 025304 (2019)
 S. Assali, et al., "Opto-electronic properties of a 1.5 µm-thick SiGeSn epilayer on Si", submitted.
5:10 PM - EN07.04.08
Tio2-Cuo Heterostructure for Cost-Effective and Better Optoelectronic Properties of Solar Cell
Sajal Islam1,Ariful Haque2,M.F.N. Taufique3,Kartik Ghosh1
Missouri State University1,North Carolina State University2,Pacific Northwest National Laboratory3Show Abstract
Oxide heterostructures have drawn great attention recently, because of its environment-friendly properties and potential applications in optoelectronic devices like LEDs and Solar Cells. In this work, we have performed a simulation study of the heterojunction solar cell with SCAPS (a Solar Cell Capacitance Simulator) using Ti02 as an n-type layer and CuO as a p-type layer. The thickness and the dopant dependent simulation studies have shown that the solar cell operates at a maximum efficiency of 19.15% when the thickness of the TiO2/CuO layers are chosen 1.4µm/1.2µm. We carried out an indium doped tin oxide (ITO) vs Fluorine doped tin oxide (FTO) comparison study as well where FTO worked better as the anode in this structure. We chose gold as the cathode from the simulation study of a range of elements and observed that efficiency increases with better metal work function. Based on these simulation results we have fabricated the oxide-based heterojunction solar cell on FTO substrates using pulsed laser deposition (PLD) for the TiO2 layer and spin coating for the CuO layer. We have conducted structural-property correlations of individual layers using x-ray diffraction (XRD), Raman analysis, photoluminescence spectroscopy (PL), scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), and different electrical measurements e.g. hall measurements, and sheet resistance measurements. Using specific PLD parameters, we have successfully grown both the rutile phase and anatase phase of TiO2 separately on several FTO substrates. The results and analysis from the characterizations confirm the successful growth of high-quality oxide thin film layers of TiO2 on FTO and CuO on TiO2. Finally, to make the cathode layer of gold, the sputtering technique is used. The solar cell characterization is performed by the I-V measurement using a standard solar simulator and UV-VIS. We have used Origin, Vesta, and MATLAB software for data analysis. The analysis of the solar cell characterization results will be presented at the conference. This facile and cost-effective process of fabrication of all oxide-based heterojunction solar cells will reduce the overall cost of solar cells and increase efficiency with a better optoelectronic performance.
5:15 PM - EN07.04.09
Solar Cell Effect on Ferromagnetic Metal-Based Molecular Spintronics Devices
Pawan Tyagi1,Christopher Riso1
University of the District of Columbia1Show Abstract
Can spin property of electron lead to a novel form of solar? This question is critical because almost all the solar cells created so far are based on electronic charge. This paper reports the solar cell phenomenon based on the spin property of electrons. The spin-based solar cell effect was observed on magnetic tunnel junction based molecular spintronics devices (MTJMSD). MTJMSDs were produced by covalently bonding organometallic molecular clusters (OMCs) between the top and bottom ferromagnetic electrodes along the exposed side edges of magnetic tunnel junctions with Co/NiFe/AlOx/NiFe thin film configurations. The MTJMSD configuration showing the photovoltaic effect also exhibited OMC induced strong antiferromagnetic coupling and room temperature current suppression by >five orders of magnitude. Our MTJMSD was fabricated below 100-degree C temperature and employed earth-abundant transition metals like nickel and iron. This paper shows that MTJMSD’s photovoltaic effect was susceptible to the magnetic field, temperature, and light intensity. MTJMSDs exhibited three different current states termed as high (µA), medium (nA), and low (pA). In each state, light radiation produced the photovoltaic effect. OMC molecule appears to create robust exchange coupling between the two ferromagnetic electrodes of the magnetic tunnel junctions leading to significant changes in the electrical, magnetic, and optical properties of the ferromagnetic electrodes. OMC induced changes in the ferromagnetic electrodes also propagated outside the MTJ’s perimeter. Magnetic studies, KPAFM, and Raman studies suggested that OMC transformed a ferromagnetic film into photoresponsive material and produced a built-in potential in the MTJMSD. MTJMSD’s ability to absorb white light radiation and the ability to separate opposite spins in the three different current states lead to the net photovoltaic effect. MTJMSD’s photovoltaic response responded to the magnetic field. This paper mainly reports the experimental observations. Further investigation about the deeper understanding of the spin-photovoltaic effect is needed. We were also not able to provide an exact estimate of the energy conversion efficiency. It was experimentally challenging to determine the exact area responding to light radiation. Future work may focus on simultaneous KPAFM, MFM, and I-V measurements under dark and light for further understanding and new insights.
5:20 PM - EN07.04.10
Late News: Stability, Electronic and Optical Properties in Ternary Nitride Phases of MgSnN2—A First Principles Study
Bishal Dumre1,Daniel Gall2,Sanjay Khare1
The University of Toledo1,Rensselaer Polytechnic Institute2Show Abstract
We studied the disordered-rocksalt, orthorhombic and disordered-wurtzite phases of the ternary nitride semiconductor MgSnN2 from first principles methods using density functional theory (DFT). We find that MgSnN2 is mechanically and dynamically stable in all three phases. However, COHP analysis suggests that the disordered rocksalt structure has anti-bonding states below the Fermi level between -5 eV to -2 eV as compared to the bonding states in other two phases, indicative thermodynamic metastability. Computed lattice constant and electronic band gap values of 4.56 Å and 2.69 eV of MgSnN2 in disordered rocksalt structure compare well with experimentally reported values of 4.48 Å and 2.3 eV respectively. Furthermore, band gaps were computed in MgSnN2-xOx, with x = 0.5, 1.0, 1.5, 2.0, to elucidate the role of possible oxygen impurity. Of the three phases, disordered-rocksalt structure shows the lowest charge carrier effective masses. Moreover, this phase has promising absorption coefficient and reflectivity to be used as the absorber layer of tandem solar cells in the higher energy region of the visible portion of the solar spectrum. The other two phases can be utilized as a window layer of solar cells owing to their larger band gap values of 4.36 eV and 4.86 eV respectively.
5:25 PM - EN07.04.11
Late News: Photocurrent Transient Characteristics of Chemically Synthesized Lateral and Vertical PbS Photovoltaics for Application as High Performance Infrared Photodetectors
Emmanuel Ampadu1,Eunsoon Oh1,Jungdong Kim1,DeaKwan Noe1,KeunSoo Kim2,DongYun Lee2
Chungnam Naitonal University1,Department of Physics and Graphene Research Institute-Texas Photonics Center International Research Center (GRI-TPC IRC)2Show Abstract
Infrared (IR) photodetectors play a major role in numerous applications, including communications, environmental monitoring and security . Various materials have been used for the purposes of infrared photo detection. Notable among them include Ge, InGaAs and InAs . Research on lead based chalcogenide (PbTe, PbS, and PbSe) for infrared applications have been extensively explored by the scientific community due of the possibility to tune the bandgap in the infrared wavelength range even up to the Terahertz (THz) region . Various photodetectors have been commercialized; photoconduction type which employ the use of an external bias and photovoltaic type detectors which operate with no external bias.
Owing to high transmission in the infrared region, high electrical conductivity, excellent atomic lattice structure and the broad applications for industrial purposes, great attention from the scientific community has been focused on graphene. Graphene is also a very good material for the purposes of transparent contacts in optoelectronic devices operating in the mid - to - far - infrared range, where other transparent oxides such as ITO are no longer desirable due to their free carrier absorption . The increasing interest in the need for bendable, flexible and high performance optoelectronic devices makes graphene a good candidate and has therefore led to various functional materials/graphene hybridized materials [5-6]. Great efforts on the achievement of high-performance graphene-based photodetectors have been concentrated on the development of graphene (G) hybrid structure such as G-semiconductors, G-quantum dots (QD) and G-polymer. Studies have also been conducted to form epitaxial junctions by directly depositing various functional materials on large area CVD-graphene .
We fabricated vertical photovoltaic type G/PbS/Ti device by taking advantage of Ti/PbS Schottky junction and discussed the photocurrent transient characteristics. Lead sulfide (PbS), which has a bandgap of 0.42 eV at room temperature, was deposited directly on post-annealed large-area CVD (Chemical vapor deposition) graphene by CBD (Chemical bath deposition). Using a metal mask and an e-beam evaporator, we deposited Ti on glass/G/PbS. Temperature dependent photocurrent spectra of our G/PbS/Ti photovoltaic devices were measured by a Fourier transformed infrared (FTIR) set-up. Post-annealing was found to be an important activity for the adhesion of PbS films on G/glass. As the bandgap energy of PbS decreases with decreasing temperature, the cut-off wavelength increases to longer wavelength, as temperature decreases.
We also fabricated a lateral type device which was comprised of two (2) Ti electrodes separated by a gap of 7 mm and a PbS layer and discussed the laser power dependent photocurrent transient. Because we could obtain fast photo-response from the lateral type device, photocurrent spectrum at room temperature using a FTIR equipment was measured. The cut off wavelength of the detector was found to be ~3.2 μm at room temperature (300 K) which is typical for PbS devices. Our photovoltaic PbS devices can detect not only short - to – mid - infrared but also terahertz radiation at room temperature, which is highly applicable in many fields.
5:30 PM - EN07.04.12
N-Type SnS Thin Films Applicable for Homojunction Solar Cells
Issei Suzuki1,Sakiko Kawanishi1,Sage Bauers2,Andriy Zakutayev2,Hiroyuki Shibata1,Minesok Kim3,Hiroshi Yanagi3,Takahisa Omata1
Tohoku University1,National Renewable Energy Laboratory2,University of Yamanashi3Show Abstract
Since tin (II) sulfide (SnS) is a semiconductor composed of abundant and safe elements and possesses a bandgap (1.1-1.3 eV) suitable for solar cells, it is expected as a light absorber for the next-generation thin film solar cells. The acceptor-type defects, such as Sn antisite (SnS) and Sn vacancy (VSn), are easy to be formed in SnS due to their low formation enthalpy, SnS typically exhibits p-type conduction and it is difficult to convert it into n-type . SnS solar cells, therefore, have adopted a heterojunction structure with p-type SnS and n-type semiconductors such as CdS, however, the conversion efficiency is still 4-5% at maximum . Such low efficiencies are attributed to an unfavorable band offset and interface defects at the hetero interfaces. Homojunction structure is expected to exhibit higher conversion efficiency and therefore many efforts have been made to realizing n-type SnS. In recent years, it has been reported that n-type SnS sintered compact and single crystals are obtained by halogen (Cl or Br) doping, which paved the way for SnS homojunction . However, the n-type SnS thin films have not been reported so far and it remains as a barrier towards realizing homojunction devices. In this study, we succeeded in obtaining n-type Cl-doped SnS thin films by RF sputtering deposition under a use of RF atomic sulfur supplier (sulfur cracker).
Cl-doped SnS thin films were deposited by RF puttering on a SiO2 glass substrate heated at 220-340 °C. Atomic sulfur was supplied to the deposition field of the thin film at the same time from the sulfur cracker installed next to the sputtering cathode. The electrical conductivity, Hall coefficient, chemical compositions, and optical absorption spectra of the obtained thin films were studied.
All of the obtained SnS thin films were p-type conductive without using sulfur cracker and n-type conductive with sulfur cracker, which indicates that supply of the atomic sulfur to the deposition suppresses the formation of acceptor-type defects. Whereas the n-type films exhibited sharp absorption due to the direct band gap around 1.3 eV, p-type films additionally exhibited a broad absorption in the range of 0.9 to 1.2 eV. This broad absorption indicates that there are defect levels with a large density of states within the band gap, which supports the above speculation that the acceptor-type defects were suppressed by performing the deposition with the sulfur cracker.
J. Vidal et al., Appl. Phys. Lett., 2012, 100, 032104.
P. Sinsermsuksakul et al., Adv. Energy Mater. 2014, 4, 1400496.
H. Yanagi et al., Appl. Phys. Express, 2016, 9, 051201.
EN07.05: Characterization and Passivation I
Sunday PM, April 18, 2021
6:30 PM - EN07.05.01
First Principles Prediction of Incoherent Interfaces for CdTe/SnO2
Stephan Lany1,Abhishek Sharan1
National Renewable Energy Laboratory1Show Abstract
The electronic structure properties of interfaces are of central importance to photovoltaics. Computational approaches can provide information complementary to experimental characterization. In particular, first principles methods can establish an unambiguous link between atomic structures features and their electronic consequences. This strength is also a challenge, because realistic atomic structure models must be available for such calculations. Only for highly idealized situations, such as the epitaxial interface between two reasonably lattice matched materials with the same crystal structure (coherent interface), can the atomic interface structure be trivially constructed. The situation becomes much more complicated when the interface must accommodate incommensurate surface unit cells for the two materials. In this case, the interface becomes incoherent, i.e., exhibiting a reduced periodicity requiring a larger surface unit cell or even forming an essentially (2-dimensionally) amorphous non-periodic structure.
In this work, we adopted the kinetically limited minimization (KLM) approach, previously utilized for unconstrained crystal structure prediction, to sample atomic structures for SnO2/CdTe interfaces without and with the presence of CdCl2. The simulations treat SnO2 as a substrate, and CdTe as a thin film, with a variable number of atomic layers. No initial assumptions are made about the CdTe atomic structure. Both the correct zinc-blende bulk structure and the known surface structures of the (110) and (001) back-surfaces are reproduced over the course of the sampling. The inclusion of CdCl2 results in improved bonding and periodicity, and considerably lowers the total interface energy. While the full sampling of the compositional and configurational space remains challenging, this work provides a proof of concept for first principles atomic structure prediction for incoherent interfaces.
6:45 PM - EN07.05.02
High Throughput, Single-Source Scribing Mechanism for Optimal Interconnections in Thin–Film Photovoltaic Modules
Austin Flick1,Reinhold Dauskardt1
Stanford University1Show Abstract
The advancement of thin film photovoltaic devices requires the successful implementation of a series of high-performing, low-cost scribes for efficient module interconnections. These scribes, which serve to electrically isolate the front electrode (P1) and rear electrode (P3), and provide the electrical interconnection (P2) between adjacent cells, have been explored using a wide variety of scribing options, primarily laser-based mechanisms. Laser scribing offers the benefits of high tunability and alignment accuracy, allowing for the selective removal of specific layers while mitigating dead area losses. Different laser wavelengths and process parameters are often employed to achieve this selectivity; however, the demand for multiple unique laser systems for complete module fabrication results in high equipment costs and increased process complexity.
Our work demonstrates the successful implementation of a single-source scribing mechanism capable of performing the P1, P2, and P3 scribes, enabling further simplification and cost reduction over other laser scribing methods. This mechanism utilizes one of the most commercially available laser wavelengths (1064nm) without the need for expensive yet commonly employed ultrashort laser pulsed systems, relying exclusively on interactions between the laser and front electrode material and thus demonstrating broad applicability across a variety of materials and architectures. This unique scribing mechanism is compatible with extremely high throughputs on the order of several meters per second, an order of magnitude increase over the speeds of other laser scribing mechanisms. An improvement at this scale further enables reductions in cost of over 90% for the laser scribing steps in a MW-scale photovoltaic system.
6:50 PM - *EN07.05.03
New Analysis Methods for Dopant Fluctuations in Thin-Film Solar Cells
National Renewable Energy Laboratory1Show Abstract
Quantifying the performance impacts of band tails on CdTe, CIGSe, and CZTS thin-film solar cells remains a significant challenge to further development of these technologies. Band tails are prevalent in CdTe, CIGSe, and CZTS absorption, quantum efficiency, and luminescence data and are believed to reduce the open-circuit voltage (VOC). However, quantifying band-tail related performance impacts is not straightforward. Tailing present in macroscopic optoelectronic data may arise from inhomogeneous impurity/matrix-element distributions, from structural defects such as dislocations and grain boundaries, or from intentional compositional grading [e.g., Cu(InxGa1-x)Se2]. Furthermore, inhomogeneities may span from nm to mm in scale. Thus, quantifying performance impacts includes the challenges of separating tailing due to inhomogeneities and structural defects from tailing due to compositional grading; determining the characteristic length scale of inhomogeneity; and reconciling macroscopic and microscopic data and models.
The thin-film solar cell community knows well that fluctuations in dopant densities produce electrostatic potential fluctuations. However, the community still relies heavily on early theoretical work (from Shklovskii and Efros and others) to estimate potential fluctuation magnitudes. Commonly used expressions were only meant to provide first-order estimates and are potentially inadequate for describing fluctuations occurring in thin-film solar cell materials. In this talk, I will summarize our recent efforts to develop realistic estimates of fluctuation impacts on performance, using Cd(Se)Te solar cells as an example.
Optoelectronic data on Cd(Se)Te cells doped with arsenic show an increasing tail below the bandgap that is correlated with increasing arsenic incorporation level. These cells have As-dopant activation levels of only 1-5%, and an inverse relationship exists between dopant activation and incorporation, consistent with self-compensation mechanisms. Macroscopic photoluminescence and microscopic cathodoluminescence measurements were conducted at room and low temperature. The data indicate that, to a large extent, the tailing observed in macroscopic measurements, and the inhomogeneity that causes it, is present within individual µm-size grains.
Consistent with this picture, we consider that tails are created by randomly distributed As donor and acceptor (shallow) defects. The densities of donor and acceptor defects are based on incorporation (secondary-ion mass spectrometry) and carrier-density (capacitance-voltage) data. Donor and acceptor defects are randomly distributed in a cube and each is assigned a screened coulomb potential. Fluctuations in the potential can be clearly seen and we confirm that this numerical approach reproduces results from early semiclassical band-tail theory. Furthermore, we apply semiclassical theory to estimate VOC losses in Cd(Se)Te solar cells.
Addressing the weaknesses of semiclassical theory, we develop self-consistent numerical methods to analyze random dopant fluctuations. We distribute donor and acceptor defects randomly within a 2D domain and solve Poisson’s equation with density-gradient corrections to the electron and hole densities. The results show how compensation can enhance potential fluctuation amplitudes and induce large carrier-density fluctuations. Interestingly, we find that the effective band gap energy is not constant but has a distribution with a low energy tail. Finally, we use simple formulas to show how the effective bandgap tail can produce tails in absorption and luminescence spectra. These methods can potentially be used to form realistic models of fluctuation impacts on thin-film solar cell performance and facilitate progress.
7:30 PM - *EN07.05.04
Impact of Se Alloying on Metastable Defect Properties in Cd-Rich Group-V Doped CdTe Single Crystal
Akira Nagaoka1,Kensuke Nishioka1,Kenji Yoshino1,Koji Kimura2,Kouichi Hayashi2,Darius Kuciauskas3,Mike Scarpulla4
University of Miyazaki1,Nagoya Institute of Technology2,National Renewable Energy Laboratory (NREL)3,University of Utah4Show Abstract
Cadmium telluride (CdTe) photovoltaic (PV) modules are relatively easy to produce at a low cost, making them one of the most competitive commercially available PV technologies. Recently, the world-record PV cell has a power conversion efficiency of 22.1%, which is well below the theoretical limits. The improvement of efficiency without increasing the production costs will make PV generation more competitive with fossil fuels. The practical strategy for higher efficiency is improvements of open-circuit voltage (VOC) and short-circuit current (JSC).
Recently, it was discovered that group-V element doping (P, As, Sb, or Bi) and Cd-rich composition are key technologies for increasing VOC, and these results were attributed to high p-type doping and long minority carrier lifetime. PV devices fabricated with P-doped CdTe single crystals as absorbing layer and with Cd-rich composition could result in hole concentration of ~1017 cm-3, minority carrier lifetime of several hundreds of nano second, and VOC exceeding 1 V. However, it is difficult to obtain group-V doping higher than 1017 cm-3 because doping efficiency typically becomes lower with higher dopant incorporation (above ~1016 cm-3). The origin of this doping limit has not been established, but theoretical studies suggest that it may be caused by the formation of self-compensating AX center or defect complex. The AX center is formed due to a large lattice relaxation of the substitutional dopant which results in conversion to a donor state. Recently, with in situ As-doping in polycrystalline thin film CdTe-based PV devices larger than 20% power conversion efficiency was be achieved, but dopant activation remained low (2-4%).
On the other hand, CdTe alloying with Se (CdTeSe) has attracted strong interest for improving JSC because lower bandgap leads to increased absorption in the long wavelength part of the spectrum. Bandgap tuning through alloying is widely used in PV. For example, high-efficiency Cu(InxGa1-x)Se2 (CIGS) absorbers employ bandgap grading by controlled variation of In/Ga composition.
We combine both approaches to develop more efficient CdTe solar cells: group V doping and alloying with Se. We have grown high-quality As-doped Cd-rich CdTeSe single crystals from metal Cd solvent near thermodynamic equilibrium using the traveling heater method (THM). In this presentation, we will report growth and characterization of group-V doped Cd-rich CdTeSe single crystals from THM. It is not clear how the formation of CdTeSe alloy affects the activation of group-V doping, defect compensation mechanism and PV performance, and we propose some possible answers to these questions.
7:55 PM - EN07.05.05
Late News: Shape-Effect on Quantum Confinement in Alloy Quantum Dots and Their Applications for Photovoltaic Green Energy
Nicholas Ulizio1,Zubaer Hossain1
University of Delaware1Show Abstract
Alloy quantum dots (aQD) represent a new class of low-dimensional materials whose electron confinement is determined not exclusively by size, but also by composition field. While homogeneous quantum dots have been more widely explored for applications in photonics, optoelectronics, and photovoltaics, aQDs remain less investigated. The differences in both size and composition field across aQDs lead to differences in material properties. These variations in material properties present compelling potential applications, specifically in the field of energy generation and photovoltaics. This is primarily because of the complexity in characterizing nonuniform fields and establishing the correlation between inhomogeneity and confinement. In order to effectively analyze the varied electron confinement of these materials, we employ a combination of density functional theory based quantum simulations, and finite element analysis based k.p calculations to determine the electron energy levels for a number of uniform and nonuniform physical variables. As a part of the analysis, composition maps were constructed for each of the different aQD shapes, namely, the steep cone, normal cone, hut, and dome. Each of these compositions maps was fitted with an analytical function that was then used to analyze the electronic properties of the aQDs. Results show that a generic expression for all four of the aQDs exists such that modification of this equation allows each of the disparate aQDs to be modeled.
The steep cone shaped aQD with a 15° sidewall angle corresponds to a composition map where spatial decay occurs from the tip of the cone to the bottom. The normal cone shaped aQD exhibits a similar behaviour, but the sidewall angle is instead 30°, and the fitting function is modified to a product of a quadratic and a linear function. For the hut shaped aQD, the composition map takes the shape of a “hut” where decay occurs from the top edges towards the bottom edges in a non-linear fashion. For the final shape, the dome, spatial decay emanates from the top tip of the dome. The dome is fitted by two sidewalls per side that have angles of 11° and 30° respectively.
For each of the quantum dots above, composition maps obtained are independent of size to determine the singular effects of the compositional variation. Our results suggest that inhomogeneity in composition field has a consequential role in controlling the confinement energy levels. Thus, the shape of aQDs can play a critical role in tailoring the effective electronic behavior. Furthermore, a larger alloy quantum dot, can behave electronically like a smaller quantum dot, if the composition field is inhomogeneous. By engineering quantum dots with certain composition maps, the photon absorption properties of the respective quantum dot can thus be tailored substantially. In particular, the most apparent application of this is to solar cells and spintronics; engineering the properties of the quantum dot allows us to increase the absorption efficiency of the photovoltaic cell, which directly leads to greater potential for green energy generation.
8:00 PM - EN07.05.06
Semi-Transparent and Ultra-Thin Silicon Solar Cells Fabricated by the Surface-Bulk Micromachining
Erfan Pourshaban1,Aishwaryadev Banerjee1,Chayanjit Ghosh1,Adwait Deshpande1,Hanseup Kim1,Carlos Mastrangelo1
The University of Utah1Show Abstract
Widespread research in the field of wearable electronics, has ignited interest in stand-alone and efficient power generators. Photovoltaic devices are promising candidates for such applications. Due to their high stability and biocompatibility, Si solar cells are an attractive choice. However, the absorption coefficient of Si is highest in the visible and NIR region of the EM spectrum. Thus, efficient Si cells are opaque, which makes them undesirable for a wide range of applications that require the on-board electronics to be transparent. Additionally, the thickness of the conventional Si solar cells is ~200-300 μm which makes them rigid and unsuitable for applications that require the system components to be thin and flexible. Here, we present the design, fabrication, and performance evaluation of semi-transparent (T ~ 30% and 60%) and ultra-thin (15 µm) Si solar cells fabricated by surface-bulk micromachining (SBM) process.
A <111> boron-doped Si wafer with an average bulk resistivity of 5-10 Ω.cm was used as the substrate and base region. 400 nm of SiO2 was thermally grown which acts as the diffusion barrier for subsequent doping steps. Phosphorous doping was performed at 1000 °C for 60 minutes to define the emitter region (sheet resistance = 8 Ω.sq-1). Micro holes (with a diameter of 170 µm) were then lithographically patterned in a hexagonal arrangement to achieve different levels of transparency in the fabricated Si solar cell. Next, deep reactive ion etching (DRIE) was performed on these patterns to realize trenches of 15 μm depth. This defines the final thickness of the solar cell. After this, 300 nm of LPCVD low-stress Si nitride was deposited on the sidewalls of the trenches as an etch resistant layer. This was followed by patterning and a second DRIE (DRIE 2) to dig deeper trenches than those achieved previously. The final transparency and effective surface area of the Si solar cell is determined during this step. The diameter of DRIE 2 micro holes was designed to be ~150 µm with a filling ratio of 25% and 50% that corresponds to an inter-hole distance (s) of 138 µm and 54 µm, respectively. This inter-hole spacing determines the required minimum depth of DRIE 2 trenches (h), which is crucial to release the ultra-thin device from the underlying substrate. Since (111) family planes have a negligible etch rate in KOH/TMAH, they act as an effective etch-stop layer. Therefore, to ensure that the etchant completely undercuts the device before reaching the etch-stop layer, the DRIE-2 depth (h) must be higher than s . tan (19.47°). The DRIE-2 trench depth for 25% and 50% filling ratio devices were designed to be 394 µm and 154 µm, respectively. Finally, the samples were dipped into a 10 wt% tetramethylammonium hydroxide (TMAH) solution at 100 °C for completely releasing the structures. The time taken to release devices with 25% filling ratio and 50% filling ratio was 10 hours and 4 hours, respectively. After fishing out the released devices from the TMAH solution and removing the remaining Si nitride by CF4/O2 RIE dry-etching, 20 nm of SiO2 was deposited as a passivation layer on the bottom of the solar cell and the sidewalls of DRIE trenches.
15 µm thick Si solar cells with a transparency of ~ 30% and ~ 60% were fabricated using the proposed method. Photocurrent-voltage measurements were conducted under indoor white light condition with an input power density of 13 mW.cm-2. I-V results demonstrate the open-circuit voltage of 300, 120 mV and the short circuit current density of 34, 1.4 µA.cm-2 for the 30% and 60% transparent silicon solar cells, respectively.
8:05 PM - *EN07.05.07
Controlling the Defects of Kesterite Solar Cells
University of New South Wales1Show Abstract
The Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) semiconductor is a compelling emerging light-harvesting material for low-cost, environment-benign, and high-efficiency thin-film photovoltaics. The highest power conversion efficiency so far for kesterites are 11% and 12.6 % for pure sulfide kesterite and Se-containing kesterites, respectively. The current state-of-the-art kesterite devices suffer from the challenge of controlling defects (e.g. cation—disordering defects, defect clusters), which generally results in severe potential fluctuation, low minority carrier lifetime, and thereby unsatisfactory device performance. Identifying effective ways of controlling these defects is of vital importance for further efficiency breakthroughs. In our recent work, we demonstrated that some of these defects can be suppressed by either controlling the local chemical environment during the chalcogenisation synthesis of kesterite, or lithium post-deposition-treatment. By controlling the local chemical environment in time when the kesteirte phase starts to form, we demonstrated an independently confirmed 12.5% efficiency for CZTSe solar cells with high VOC of 491 mV, showing the promise of these defect control strategies.
Mike Scarpulla, University of Utah
Charles Hages, University of Florida
Byungha Shin, Korea Advanced Institute of Science and Technology
Mirjam Theelen, TNO
Angstrom Engineering Inc.
EN07.06: Advances in CIGS Solar Cells I
Monday AM, April 19, 2021
8:00 AM - *EN07.06.01
Ag-Alloying in Cu(In,Ga)Se2
Marika Edoff1,Jes Larsen1,Jan Keller1,Kostyantin Sopiha1,Tobias Törndahl1,Lars Stolt1
Uppsala University1Show Abstract
The chalcopyrite family has emerged as a material with many possibilities and has yielded high efficiencies above 23 %. One of the additions to this family is alloying with silver to form (Ag,Cu)(In,Ga)Se2 (ACIGS). Among the features of Ag alloying as compared to Cu(In,Ga)Se2 (CIGS) without Ag, are a lower melting temperature, that leads to large grains also for relatively low deposition temperatures, but also lower voltage losses as compared to the Shockley Queisser limit, and a slight increase in bandgap, due to Ag, accompanied by lowering of both the conduction band minimum and of the valence band maximum energy. These advantages open up interesting applications for ACIGS as e.g. top cells in tandem devices, either in combination with low bandgap CuInSe2 (with or withot Ag) or with Si as bottom cells. Due to its electronic band structure, ACIGS with high bandgap up to 1.5 eV yields high efficiencies with a conventional buffer layer like CdS, but also with ZnSnO and Zn(O,S). In addition to high efficiency, also long-term performance is essential for all applications and there are some questions related to phase stability for the ACIGS material, that need to be investigated further, but where we have obtained new insights with the help of first principle calculations. Among the studies made in our group during the last few years are ACIGS with Ag/(Ag+Cu) ratios varying from 10 to 100 % and with varying Ga/(Ga+In) from 0 to 100 %. We have also applied ACIGS to solar cells with submicron thick absorber layers, as well as investigated the impact of post deposition treatments to this type of materials. In the talk, a short review of the state-of-the art of ACIGS will be given together with some of our insights from this line of research.
 Nakamure et al, IEEE Journal of Photovoltaics, 2019, 9 (6), 1863-1867
 Edoff et al, IEEE Journal of Photovoltaics, 2017, 7 (6), 1789-1794
 Boyle et al, Journal of Applied Physics, 2014, 115 (22), 223504
 Keller et al, Progress in Photovoltaics, 2020, 28 (4), 237-250
 Sopiha et al, Journal of Materials Chemistry A, 2020, 8 (17), 8740-8751
8:25 AM - EN07.06.02
The Effect of Damp Heat—Illumination Exposure on CIGS Solar Cells—A Combined XRD and Electrical Characterization Study
Mirjam Theelen1,Ruud Hendrikx2,Nicolas Barreau3,Henk Steijvers1,Amarante Bottger2
TNO Solliance1,Delft University of Technology2,Institute des Materieaux Jean Rouxel (IMN)-UMR3Show Abstract
In order to obtain long-term stable and low-cost photovoltaic modules, it is important to know possible degradation mechanisms. In order to identify long term behavior, unpackaged Cu(In,Ga)Se2 solar cells were simultaneously exposed to damp heat and illumination (settings: 85oC/85% relative humidity and 1000 W/m2 BAA illumination) . In-situ monitoring of their electrical parameters demonstrated a rapid decrease of the efficiency, mainly driven by changes in the series and shunt resistances. Moreover, non-degraded and degraded solar cells were studied by in-depth XRD and SIMS to investigate the material changes leading to efficiency loss.
SIMS revealed the migration of sodium and potassium, likely leading to a severe decrease in the shunt resistance (from 500±100Ω to 20-30Ω within 200 hours) and voltage. It also displayed the ingression of hydroxide, especially in the ZnO:Al film. Extensive XRD measurements showed that molybdenum oxide was formed, which likely affected the Ohmic contact between Mo and CIGS. Moreover, an in-plane stress increase in the ZnO:Al film from -183±29 to -450±40 MPa was observed. This stress increase is most likely due to the incorporation of species like hydroxides and carbonates in the grain boundaries of the ZnO:Al film. These phenomena could lead to the observed increased series resistance in the solar cells .
 M. Theelen et al., JoVE 140 (2018)
 M. Theelen et al., Sol. Mat. Sol. Cells 157 (2016) 943–952
8:40 AM - EN07.06.03
Late News: Investigation of the Aptness of Pulsed Laser Deposition for the Sequential Fabrication of Cu(In,Ga)Se2-Based Thin-Film Solar Cells
Evripides Kyriakides1,Panagiotis Ioannou1,Christiana Nicolaou1,Vasiliki Paraskeva1,Maria Hadjipanayi1,George Georghiou1,Paris Papagiorgis1,Grigorios Itskos1,John Giapintzakis1
University of Cyprus1Show Abstract
Thin-film photovoltaic (PV) technologies, and Cu(In,Ga)Se2 (CIGS) thin-film solar cells in particular, have been the subject of increasingly rigorous study of late. There are several motivating factors for the development of thin-film photovoltaics, such as the reduction in raw material usage, the decrease in solar cell weight, and the possibility of deposition on flexible substrates. Furthermore, CIGS-based thin-film solar cells hold several advantages compared to rival solar cell technologies. Specifically, they have the highest conversion efficiencies among chalcogenide thin-film PV technologies (23.35% for cells and 17.5% for modules), high radiation resistance, and outstanding stability.
The typical CIGS-based solar cell consists of a soda-lime glass (SLG) substrate, a Mo back contact, CIGS as the p-type absorber layer, CdS as the n-type buffer layer, and i-ZnO/ZnO:Al as the decoupling and transporting window layers, respectively. However, in the state-of-the-art CIGS-based solar cells, each of these layers is deposited with a different method; co-evaporation for the CIGS layer, chemical bath deposition for the CdS layer, and reactive sputtering for the i-ZnO/ZnO:Al bilayer.
This work reports on the utilization of pulsed laser deposition (PLD) as a single technique for the preparation of the aforementioned layers of a complete CIGS-based solar cell stack. Employing a single deposition technique greatly reduces manufacturing complexity. Furthermore, it potentially decreases processing time and associated fabrication costs through streamlined production lines.
The presented results discuss the challenges faced in the completion of this task. The properties of the PLD-grown thin films with respect to structure, composition and morphology are parametrically investigated through X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Hence, the influence of PLD process parameters on film growth is evaluated. TCAD simulations realized in Synopsys Sentaurus Device are utilized for the optimization of the function of the cell. Electrical and optical measurements carried out in a solar simulator are used to assess the photovoltaic behavior of the complete structure and, critically, the CdS/CIGS junction heterointerface. The solutions implemented, involving modifications to the typical PLD process, are discussed. Finally, the resulting improvement in conversion efficiency is demonstrated.
9:05 AM - *EN07.06.05
WITHDRAWN EN07.06.05 4/19/2021 Influence of Sodium and Potassium on Electro-Optical Properties of Cu(In,Ga)Se2 Thin Films and Solar Cells
Pawel Zabierowski1,Malgorzata Igalson1,Aniela Czudek1,Aleksander Urbaniak1,Konrad Wisniewski1,Marek Pawlowski1,Roland Wurz2,Alexander Eslam2
Warsaw University of Technology1,ZSW – Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg2Show Abstract
The influence of alkali elements on CIGSe solar cells has been investigated already for decades, and despite an impressive achievements in understanding of structural and compositional effects, there is still an ongoing debate on detailed mechanisms leading to efficiency improvements. The controversies pertain even to the role Na and K play in the increase of the doping level, not mentioning more complex issues such as e.g. the passivation of grain boundary defect states or the influence on secondary barriers. The relatively slow progress in understanding the mechanisms behind the impact of Na and K on opto-electronic CIGSe-devices’ properties is mainly due to quite sophisticated deposition procedures and a complex structure of top-efficiency CIGSe devices. Therefore, in this work we investigate carrier transport mechanisms in CIGSe devices fabricated in single-stage process to ensure that the absorber material was uniform in composition. The amount of Na and K was controlled by changing the annealing temperature during NaF and KF post-deposition treatment and quantified by SIMS measurements. In accordance with the literature, we observed a gain in Voc and FF as Na and K concentrations increased. In order to elucidate the mechanisms responsible for this improvement a wide range of experimental techniques was used: dark and light current-voltage curves, Suns-Voc, capacitance profiling, conductivity, photoluminescence, admittance and deep level transient spectroscopy, both in relaxed and light-induced metastable states. Thanks to extensive numerical modelling of experimental results we were able to separate the effects occuring at heterointerfaces, in the CIGSe bulk and at grain boundaries. Concerning sodium, the main results can be summarized as follows: (1) The transport mechanism changes from interface to space charge recombination as Na content increases from a low level of 20-80 ppm to a high level of 200-480 ppm, respectively. Interestingly, the interface recombination could better be understood as being limited by light -dependent potential barriers at grain boundaries. (2) A very important finding is that Na influences not only the free hole concentration but also reduces the concentration of bulk/GB recombination centers.(3) Since secondary barriers (cross-over and red-kink) decrease gradually at the same pace with increasing Na content, we conclude that Na-PDT influences also the window/buffer/absorber interface region. (4) The introduction of sodium caused the signature of a well-defined deep level to be replaced by a signal which, due to its specific properties, was assigned to a process related to the transport over grain boundary barriers. (5) The metastable increase in hole concentration due to light soaking is proportional to the Na concentration in the absorber layer. Hole concentration in cells and conductivity in thin films follow a similar trend, indicating that the effect originates from the absorber layer and is related to sodium concentration. Our results suggest that the large lattice relaxation model of an amphoteric VSe–VCu vacancy complex being responsible for PPC cannot itself explain the Na-PPC relation. Therefore, we propose that either the passivation of the grain boundary barriers by sodium or a sodium-related modification of a defect complex needs to be included in the model. For KF-treated absorbers we also observed similar improvement of the main junction quality with increasing potassium concentration. However, because of much more pronounced parasitic barriers caused by K accumulation at the front interface, the observed changes were more abrupt and a clear separation of transport mechanisms was hindered.
9:30 AM - *EN07.06.06
High Efficiency Solar Cells Based on Co-Evaporated CuIn0.7Ga0.3S2—Correlation Between Growth Process and Absorber Characteristics
Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN1Show Abstract
CuIn0.7Ga0.3S2 semi-conductor has a bandgap energy of 1.7 eV, which is well adapted to applications as top cell in tandem structures with c-silicon bottom cell. Recently, we have shown that solar cells with standard structure, namely Glass/Mo/CuIn0.7Ga0.3S2/CdS/ZnO/AZO, can reach efficiencies above 14 %; the CuIn0.7Ga0.3S2 layer being grown by co-evaporation following a 3-stage process. Determination of compositional depth profile of these absorbers revealed that the distribution of group III elements (i.e. In and Ga) is not homogeneous throughout the layers, as expected for films grown by means of sequential process. However, in contrast to V-shaped GGI evolution commonly observed in selenide absorbers, these sulfide films appear composed of two distinct layers, a first with GGI ~ 0.7/0.8 near the back contact and a second with GGI ~ 0.2/0.3 close to the surface; note that the abrupt interface between these layers is located at about half of total film thickness. A large part is the present contribution will focus on the origins for such bilayered compositional structure, which will be discussed accounting relative phase stability while copper content is increased during the second stage. In addition, although optimal bulk material quality could be achieved, device performance still appears limited by dominant interface recombination. To further explore that issue, the first results on epitaxial CuIn0.7Ga0.3S2 layers will be presented.
EN07.07: Advances in CIGS Solar Cells II
Monday PM, April 19, 2021
10:30 AM - *EN07.07.01
Development of Ultrathin Cu(In,Ga)Se2 Solar Cells
Jessica de Wild1,Thierry Kohl1,Gizem Birant1,Dilara Budu1,Guy Brammertz1,Marc Meuris1,Jef Poortmans2,Bart Vermang1
Thin film solar cells have the possibility to be made flexible, semi-transparent and/or may be applied for tandem structures or building integrated. Having multiple usages, it is of interest to make these thin film solar cells as fast and cheap as possible. From the available thin films, Cu(In,Ga)Se2 (CIGS) has one of the best solar cell performance . There are concerns about the usage of indium though when CIGS will be widely applied. Therefore, making the CIGS layer thinner and thereby reducing the amount of In, is an interesting option to explore. When the absorber material becomes thin, interfaces are generally limiting the performance and the path length to absorb all the incoming light may be too short to absorb the longer wavelengths. There are various approaches to tackle these problems. Passivation of the back contact by applying a dielectric is the main route investigated. Not only reduces it the interface recombination at the Mo/CIGS back contact but it also increases the reflection. We will present here various approaches which are aimed to be industrial viable and easy to make to reduce the losses in ultrathin CIGS solar cells. This also holds for the absorber layers which are made by a single stage coevaporation process. In this case, the Ga gradient at the back is replaced by a passivation layer and no copper rich stage is applied. The effect of the simplified growth is mostly visible in the grain boundaries . As passivation layer, AlOx is often used. As this layer blocks the current flow, it has to be either sufficiently thin to allow tunneling or requires holes allowing for the current to flow to the Mo back contact. We will show that the latter approach can be applied for layers up to 6 nm without lithography steps by simply adding NaF before CIGS growth . When applying a thin Ag layer under the AlOx the cells also show optical improvement. This is likely due to formation of scattering particles, making the CIGS rougher and thereby increasing the optical path length. To improve the absorber layer itself alkali treatments are applied . At the front, the CIGS/buffer interface may also need to be adapted. Application of a passivation layer at the front, either with holes or thin enough for tunneling, is investigated. Before applying a passivation layer on the CIGS surface, cleaning treatments need to be applied . To analyze the effect of the various treatments a combinatorial approach based on bias dependent admittance spectroscopy and (time resolved (TR)) photoluminescence (PL) is developed. Generally, with (TR)-PL a quick analysis whether the treatments are improving the absorber/interface quality before finishing into a device is possible. Features like interference and blue shifts of the spectra may be observed as well and can be related to the emission profile and scale of the potential fluctuations in the absorber layer . However, improvement due the various treatments observed in the (TR)-PL does not necessarily translates into better solar cell performance. This can indicate that the treatments are sensitive to the processing conditions of the window layers for instance and/or that barriers are formed. With bias-dependent admittance spectroscopy losses at interfaces and bulk can be found by introducing a so-called CVf loss map . These maps visualize the losses in the solar cell, which makes it possible to distinguish defects in the bulk from the interface and barriers at front or back interfaces. The bottlenecks observed when developing ultrathin CIGS solar cells will be discussed and possible options to mitigate them are investigated.
 M.A. Green, DOI: 10.1002/pip.3303
 T. Kohl, DOI: 10.1021/acsaem.0c00610
 G. Birant, DOI: 10.1016/j.solener.2020.07.038
 J. de Wild, DOI: 10.1021/acsaem.9b01370
 D. G. Buldu, DOI:10.1002/pssa.202000307
 J. de Wild, DOI: 10.1063/5.0024840
 G. Brammertz, DOI: 10.1109/JPHOTOV.2020.2992350
10:55 AM - EN07.07.02
Sputtered Gallium Oxide Applied to the Front Side of Cu(In,Ga)Se2 Thin-Film Solar Cells
Wolfram Witte1,Wolfram Hempel1,Stefan Paetel1,Richard Menner1,Dimitrios Hariskos1
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW)1Show Abstract
Oxide-based materials could be candidates for buffer, passivation, or high resistive (HR) layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. In recent years, oxide materials like (Zn,Mg)O, Al2O3, ZnxTiyO , (In,Ga)2O3  and Sn1-xGaxOy  were under investigation. Most of them were deposited by atomic layer deposition. However, from an industrial point of view a very fast method, with deposition times <1 min for a complete layer, would be preferable for buffer, passivation, or HR layer growth.
In this study, we present our results with rf-magnetron sputtered Ga2O3 buffer layers as a substitute for our solution-grown CdS buffer in combination with i-ZnO as HR layer and ZnO:Al front contact. So far, best cells achieved efficiencies up to 14% without post-annealing compared to the CdS-buffered reference cells, which exhibit efficiencies up to 17%. As a result of the very high bandgap energy around 4.7 eV of Ga2O3, as determined on quartz glass by optical transmittance measurements, and reduced parasitic absorption even higher Jsc values could be achieved compared to the CdS-buffered reference cells. Nevertheless, the difference in efficiency is mainly due to reduced Voc and FF values of the CIGS cells with Ga2O3.
The solar cell performance of cells with Ga2O3 buffers strongly depends on the deposition conditions as well as on the thickness of the sputtered Ga2O3 layers. We observed poor cell efficiencies for cells with Ga2O3 sputtered at room temperature and good values for films grown at elevated substrate temperatures (120 – 200 °C). This result is similar to the behavior we observed for CIGS cells with sputtered Zn(O,S) and InxSy buffers . The Ga2O3 layers deposited in the relevant temperature range for solar cell applications are X-ray amorphous.
When using CIGS absorber layers with a RbF post-deposition treatment, a wet chemical treatment of the CIGS surface before the Ga2O3 deposition has a significant positive impact to achieve decent efficiencies. Non-rinsed samples could exhibit poor efficiencies, mainly due to bad FF values.
In addition, we could achieve a comparable good efficiency of 19.4% (with anti-reflective coating) with Ga2O3 applied as an HR layer instead of i-ZnO (19.7%) in combination with CdS as a buffer and ZnO:Al as front contact.
 J. Löckinger et al., ACS Appl. Mater. Interfaces 10 (2018) 43603
 T. Koida et al., IEEE J. Photovolt. 5 (2015) 956
 F. Larsson et al., J. Vac. Sci. Technol. A 37 (2019) 030906
 D. Hariskos et al., Appl. Sci. 10 (2020) 1052
11:10 AM - EN07.07.03
Chemical and Electronic Structure of GaOx/Cu(In,Ga)Se2 Interfaces in Thin-Film Solar Cells with RbF Post-Deposition Treatment
Dirk Hauschild1,2,Elizaveta Pyatenko1,Vladyslav Mikhnych1,Ralph Steininger1,Mary Blankenship2,Dimitrios Hariskos3,Wolfram Witte3,Michael Powalla3,Clemens Heske1,2,Lothar Weinhardt1,2
Karlsruhe Institute of Technology (KIT)1,University of Nevada, Las Vegas (UNLV)2,Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW)3Show Abstract
The introduction of post-deposition treatments (PDTs) has increased the Cu(In,Ga)Se2-based (CIGSe) thin-film solar cell efficiencies to >23 % on a laboratory scale. Such high-efficiency devices are often processed with a solution-deposited CdS buffer layer. However, the CdS band gap leads to absorption losses in the buffer layer, reducing the solar-cell efficiency. Hence, there is a strong interest to replace CdS with an alternative wide(r) band gap material. In addition to merely increasing the buffer layer band gap, using such an alternative material would also lead to various changes at the buffer/absorber interface, including the electronic and chemical structure. A detailed knowledge of these changes is crucial for further device optimization.
In this contribution, we report on industrially relevant CIGSe absorbers with and without an NH3-based rinsing step applied after the RbF PDT. Their interface with a wide band-gap GaOx buffer layer is studied using x-ray photoelectron spectroscopy (XPS), hard XPS (HAXPES) at the synchrotron, and a combination of UV photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The results paint a comprehensive picture of the GaOx/CIGSe interface and will be discussed in view of the performance of the corresponding CIGSe solar cells.
11:35 AM - EN07.07.04
Late News: Atomic Layer Deposition of Zn1-xMgxO as Transparent Conducting Films for Chalcopyrite Solar Cells
Poorani Gnanasambandan1,2,Mohit Sood2,Noureddine Adjeroud1,Renaud Leturcq1,Susanne Siebentritt2
Luxembourg Institute of Science and Technology1,University of Luxembourg2Show Abstract
We investigate atomic layer deposited zinc magnesium oxide films with varying Mg content as transparent conducting films and as electron transport layers for chalcopyrite solar cells. Optimizing a ternary process by mixing two binary ALD process has its challenges . We achieve high degree of control on composition by optimizing the growth conditions with varying deposition temperatures and supercycle parameters such as pulse ratios and bilayer period. We examine the effect of these films on the performance of high-bandgap solar cells based on Cu(In,Ga)S2 absorbers.
Previous studies on the impact of Zn1-xMgxO:Al as transparent electrodes and Mg doped ZnO thin films for the window layer of CIGS Cu(In,Ga)(S,Se)2 solar cells employed co-sputtering , electrodeposition and ALD respectively . With the advantage of low temperature and highly conformal thin film growth, we study ALD grown Zn1-xMgxO with x varying from 0.1 to 0.4 and elucidate the effect of doping on the band alignment, electrical and optical properties. With variation in Mg content we were able to achieve 11% efficient Cu(In,Ga)S2 solar cell with an open-circuit voltage of 941 mV.
. Mackus, Adriaan JM, et al. "Synthesis of doped, ternary, and quaternary materials by atomic layer deposition: a review." Chemistry of Materials 31.4 (2018): 1142-1183.
. Kuwahata, Yoshihiro, and Takashi Minemoto. "Impact of Zn1-xMgxO: Al transparent electrode for buffer-less Cu (In, Ga) Se2 solar cells." Renewable energy 65 (2014): 113-116.
. Wang, Mang, et al. "Electrodeposition of Mg doped ZnO thin film for the window layer of CIGS solar cell." Applied Surface Science 382 (2016): 217-224.
. Inoue, Yukari and Hala, Matej et al. "Optimization of buffer layer/i-layer band alignment" in 42nd IEEE Photovoltaic Specialist Conference (IEEE, New Orleans, 2015), pp. 1
. Hiroi H, Iwata Y, Adachi S, Sugimoto H, Yamada A. New World-Record Efficiency for Pure-Sulfide Cu(In,Ga)S2; Thin-Film Solar Cell With Cd-Free Buffer Layer via KCN-Free Process. IEEE J Photovolt. 2016;6(3):760-763.
11:50 AM - EN07.07.05
Solution Processed Bismuth Halide and Chalcohalide Thin-Film Solar Cells
David Fermin1,Devendra Tiwari2
University of Bristol1,Northumbria University2Show Abstract
Solution processing of inorganic thin-film solar cells is a key challenge in the growing area of system integrated photovoltaics. The ability of processing high quality materials at temperatures below 200 °C enables the fabrication of devices onto flexible composite materials. Semiconductor compounds with high degree of defect tolerance are an exciting class of materials particularly well-suited to these applications. Indeed, compounds based on Bi3+ can lead to attractive opto-electronic properties similar to those observed in Pb2+ hybrid perovskites, such as large spin-orbit coupling, dielectric constant and band dispersion.1
In this contribution, we will discuss the structure and opto-electronic properties of phase-pure BiI3 and BiSI obtained by solution based method. BiI3 is prepared by spontaneous gas-phase iodination of Bi2S3 films at 200 °C,2 while BiSI is generated by thermolysis of a precursor solution composed of Bi(NO3)2, thiourea and NH4I.3 In addition to accurate structure refinement from XRD data of the thin-films, we find excellent correlation between experimental Raman spectra and Raman modes calculated by density functional perturbation theory. Interestingly, BiI3 exhibits p-type conductivity (acceptor density of the order of 1015 cm-3) and a band gap of 1.7 eV, while BiSI is n-type with a donor density in the range of 1019 cm-3. Quasi-particle G0W0 calculations of both materials show that the conduction band is more dispersed than the valance band due to spin-orbit coupling promoted by Bi3+. Band edge energy values are estimated by electrochemical impedance spectroscopy, enabling the design of PV devices with appropriate band alignment. Devices with the structure Glass/FTO/TiO2/BiI3/F8/Au and Glass/FTO/SnO2/BiI3/F8/Au , where F8 is Poly(9,9-di-n-octylfluorenyl-2,7-diyl), display open-circuit voltage above as high as 600 mV and record power conversion efficiency of 1.32% under AM 1.5G illumination. We will expand the discussion to other complex Bi chalcogenides,4 addressing the key limiting factor in the PV performance of these materials, i.e. their short carrier lifetimes.
1- A.M. Ganose et al. Chem. Commun. 2017, 53, 20
2- D. Tiwari and D.J. Fermin, ACS Energy Lett. 2018, 3, 1882
3- D. Tiwari et al. ACS Appl. Energy Mater. 2019, 2, 3878
4- D. Tiwari et al. Chem. Mater. 2020, 32, 1235
12:05 PM - *EN07.07.06
Development and Application of Transparent Back Contacts in CIGSe Solar Cells
Roland Scheer1,Thomas Schneider1,Johanna Troendle1,Bodo Fuhrmann1,Frank Syrowatka1,Heiko Kempa1,Torsten Hoelscher1,Marcel Placidi2,3,Alejandro Perez-Rodriguez2,4
Martin-Luther-Universität-Halle-Wittenberg1,IREC2,Polytechnic University of Catalonia3,Universitat de Barcelona4Show Abstract
Replacing the established Molybdenum back contact in Cu(In,Ga)Se2 solar cells by a transparent conducting layer would enable a variety of new applications: Bifacial solar modules, semitransparent building integration without back side mirroring, and ultrathin solar cells with scattering back reflector. In this work, we review the status of alternative back contacts for CIGSe solar cells and present own work using ITO. It is shown that for low temperature CIGSe deposition, the ITO back contact is on par with the Mo standard regarding PCE, and may even surpass Mo for ultrathin devices. Admittance measurements on ITO back contacts indicate the absence of a back contact barrier and thus the avoidance of the wellknown reach-through effect. Using bifacial illumination, the back contact recombination velocity is determined to around 105 cm/s for the ITO/CIGSe interface formed at around 480°C by co-evaporation of CIGSe on sputtered ITO. Cells of 300 nm CIGSe thickness demonstrate quantum efficiency of 55% if illuminated from the back side. If the ITO layer is grown on top of a reflective back contact of 100 nm Aluminum, ultrathin devices can be constructed. Here, the Aluminum acts as the optical reflector and the ITO serves as a diffusion barrier and electronic back contact. This back contact can be further functionalized by light scattering elements. In experiment, a 300 nm structure element height below a 500 nm CIGS film gives the highest current density, which amounts to 88% of that of a 2.8 µm reference solar cell on Molybdenum.
EN07.08: Novel Absorber Materials II
Monday PM, April 19, 2021
1:00 PM - *EN07.08.01
Bismuth Oxyiodide Solar Cells—Defect Tolerance, Device Engineering and Indoor Light Harvesting
Imperial College London1Show Abstract
Lead-halide perovskites have emerged as a leading thin film solar absorber over the past decade. One of the key enabling properties is their tolerance to point defects, which enables them to achieve long charge-carrier lifetimes despite high defect densities when grown by low-temperature, simple fabrication methods. However, the toxicity of the water-soluble lead content may limit their widescale adoption. This talk examines bismuth-based semiconductors as a low-toxicity and stable alternative to lead-halide perovskites, with particular focus on bismuth oxyiodide (BiOI). We show that BiOI replicates the tolerance of lead-halide perovskites to point defects using experimental and computational methods [1,2]. An all-inorganic device structure is developed, and we achieve photovoltaic devices with external quantum efficiencies of up to 80% at 450 nm wavelength. Both the BiOI absorber and devices are stable in air. Although the 1.9 eV band gap is too wide for single-junction devices under 1-sun illumination, this band gap is ideal for indoor light harvesting. We demonstrate BiOI devices with comparable performance to hydrogenated amorphous silicon (the industry standard for indoor photovoltaics) under fluorescent and white LED lighting. We show that these devices, with lab-scale millimetre-squared areas, are sufficient to power carbon nanotube-based inverters . Finally, we discuss the key limiting factors that need to be overcome to achieve further improvements in performance [1,3,4], as well as the potential of the broader family of bismuth-based perovskite-inspired materials for applications in solar energy and indoor light harvesting .
 R. L. Z. Hoye, et al. Adv. Mater. 2017, 29,1702176
 T. N. Huq, L. C. Lee, …, R. L. Z. Hoye, Adv. Funct. Mater. 2020, 30, 1909983
 Y. Peng, T. N. Huq, J. Mei, …, R. L. Z. Hoye, V. Pecunia, Adv. Energy Mater. 2020, Under Revision
 R. A. Jagt, …, R. L. Z. Hoye, J. Mater. Chem. C 2020, 8, 10791
1:25 PM - EN07.08.02
Defect Identification and Full Lattice Dynamics of Zinc Phosphide, an Earth-Abundant Semiconductor for Photovoltaics
Elias Stutz1,Diego Sandoval Salaiza1,Alexander Litvinchuk2,Mahdi Zamani1,Simon Escobar Steinvall1,Rajrupa Paul1,Jean-Baptiste Leran1,Anna Fontcuberta i Morral1,Mirjana Dimitrievska1
École Polytechnique Fédérale de Lausanne1,University of Houston2Show Abstract
Zinc phosphide (Zn3P2) is an earth-abundant semiconductor material capable of addressing the rising demand for low-cost and efficient optoelectronic devices. Its 1.5 eV direct bandgap, long minority carrier diffusion length (5-10 μm), and high absorption coefficient make it very promising for applications as an absorber in thin films solar cells. Furthermore, it has the added benefit of being a binary phase over more complex multinary materials.
Two crucial challenges need to be overcome to produce high-efficiency devices. First, poor crystal quality when grown on commercially available substrates due to high interface defect densities and high coefficient of thermal expansion. This challenge has been recently solved in our laboratory, as we have demonstrated different fabrication methods of high-quality reproducible thin films, with the possibility of facile transfer onto commercially available substrates.  The second challenge is the control of bulk defects and doping, which is crucial for optoelectronic performance. Usually, uncontrolled growth results in p-doped material due to the formation of intrinsic acceptor defects.
To solve the challenge of defect control, we have investigated structural defects in the room-temperature-stable phase of zinc phosphide (α-Zn3P2, space group P42/nmc) using Raman spectroscopy. For that purpose, we have first fully characterized the lattice dynamics of zinc phosphide by combining density functional theory (DFT) calculation with polarized micro-Raman measurements on single-crystalline zinc phosphide. This is the first complete analysis of phonons in Zn3P2, essentially providing a reference for further characterization of this material. We identified 33 of the 39 expected Raman lines and have calculated the atomic displacement in all vibrational modes.
Then, using factorial and combinatorial studies of Zn3P2, along with the micro-Raman spectroscopy and previously mentioned results, has allowed us to probe different structural defects within Zn3P2. Systematic changes in the peak position, intensity, and widths in correlation with the presence of Zn and P interstitials and vacancies are presented and discussed. This work demonstrates the possibility of defect engineering in Zn3P2, which is of the foremost importance for the improvement of solar cell performance, as well as the potential of Raman scattering for point defect assessment in this system.
 R. Paul et al., “Van der Waals Epitaxy of Earth-Abundant Zn3P2 on Graphene for Photovoltaics,” Cryst. Growth Des., vol. 20, no. 6, pp. 3816–3825, 2020.
 S. Escobar Steinvall et al., “Multiple morphologies and functionality of nanowires made from earth-abundant zinc phosphide,” Nanoscale Horizons, vol. 5, pp. 274–282, 2020.
 E. Z. Stutz et al., “Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (Zn3P2) nanowires,” Nanotechnology (accepted)
1:40 PM - EN07.08.03
Novel Photovoltaic Materials—Multinary Adamantine Phases Derived from Ternary Chalcopyrites
Yvonne Tomm1,Susan Schorr1,2
Helmholtz-Zentrum Berlin für Materialien und Energie1,Freie Universität Berlin2Show Abstract
Adamantine-type compounds, including kesterites, are currently the most promising material for a fully inorganic thin film photovoltaic technology that is free of critical raw materials and thus provides sustainable solutions.
Adamantines are compounds crystallizing in a structure, in which every atom is tetrahedrally bonded to four nearest neighbours . To find new compounds for absorber layers as well as window materials for thin film solar cells, the ΑIBIIIXVI2 chalcopyrite compound family, belonging to the Adamantine compounds, can be extended by chemical substitution. Coming from the chalcopyrite-type structure (space group I-42d), one A+1 (occupying the structural site 4a) and one B+3 cation (occupying the structural site 4b) are replaced by one four-valent cation (A+1 + B+3 ↔ C+4). This substitution will consequently lead to the formation of cation vacancies. In this way a quaternary “defect adamantine”  such as I--III-IV-VI4 is formed.
Here we report on the growth of CuBCX4 single crystals, with B = Ga, In, C = Ge, Sn and X = S, Se such as CuGaGeS4, CuInGeS4, and CuGaSnS4, and the analysis of their structural as well as optoelectronic properties.
The single crystals were grown by chemical vapor transport using iodine as transport agent. The evolved material and the grown crystals were characterized by X-ray fluorescence spectroscopy (XRF) for chemical composition as well as by X-ray diffraction (XRD) at 298K. The obtained powder patterns were analyzed by LeBail refinement to determine the lattice parameter. The chalcopyrite-like crystal structure was used as a structural model in the refinement.
The band gap energy of the material was revealed by solid-state UV/Vis reflectance spectroscopy. The obtained data were analyzed using the Kubelka−Munk pseudo-absorption function and the Tauc-plot method. The structure-property relations of CuGaGeS4 and CuGaSnS4 will be discussed in detail in the presentation.
We present first results on the growth and characterization of the novel compound semiconductor CuGaGeS4 as well as CuGaSnS4. These indicating a potential for future applications.
 B.R. Pamplin, Prog. Crystal Growth Charact. vol.3, pp. 179-192 (1981)
2:05 PM - EN07.08.04
Towards Defect-Free Thin Films of the Earth-Abundant Absorber Zinc Phosphide Through Nano-Patterning
Simon Escobar Steinvall1,Elias Stutz1,Rajrupa Paul1,Mahdi Zamani1,Nelson Y. Dzade2,Valerio Piazza1,Martin Friedl1,Virginie de Mestral1,Jean-Baptiste Leran1,Reza R. Zamani3,Anna Fontcuberta i Morral1,4
Laboratory of Semiconductor Materials, Institute of Materials, Ecole Polytechnique Fédérale de Lausanne1,School of Chemistry, Cardiff University2,Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne3,Institute of Physics, Ecole Polytechnique Fédérale de Lausanne4Show Abstract
Zinc phosphide (Zn3P2) is an earth-abundant semiconductor with optoelectronic properties suitable for photovoltaic applications, such as a direct bandgap (1.50 eV), long minority carrier diffusion lengths (up to 10 μm), and high absorption in the visible range.1,2 However, applications of the material has been limited due to challenges introduced by the lack of substrate with a matching lattice constant (for epitaxial growth) and thermal expansion coefficient, and the doping due to off-stoichiometry composition resulting in the formation of self-interstitials. Molecular beam epitaxy overcomes this by providing a low-temperature growth route with precise control over the composition, facilitating the fabrication of high-quality zinc phosphide.
A technique which can be used to further improve the material quality is selective area epitaxy (SAE).3,4 Using a nano-patterned oxide mask on indium phosphide substrates we demonstrate the selective growth in the holes, limiting the interface area to improve its quality.5 The SAE grown zinc phosphide first grows as nanopyramids. By controlling the growth time and pitch they can be made to overgrow the oxide and coalesce, forming a thin film. The structural properties of the material and the epitaxial relationship were investigated through transmission electron microscopy, which also showed the influence of hole size on the composition of the zinc phosphide and the formation of rotational core-shell structures. Furthermore, using conductive atomic force microscopy (CAFM) and photoluminescence spectroscopy (PL) we evaluated the functional properties of the material. The CAFM measurements showed a diode-like behavior when the zinc phosphide is grown on an n-type substrate, whilst the PL showed clear bandgap emission at 1.53 eV, ideal for photovoltaic applications. This approach has shown great potential in producing high-quality zinc phosphide, and upcoming studies will use it to transfer the growth to an earth-abundant substrate (silicon), and making prototype photovoltaic devices based on SAE grown zinc phosphide.
1. G. M. Kimball et al. Appl. Phys. Lett., 95, 112103 (2009).
2. M. Y. Swinkels et al. Phys. Rev. Applied, 14, 024045 (2020).
3. M. Friedl et al. Nano Lett., 18, 2666–2671 (2018).
4. C. –Y. Chi et al. Nano Lett., 13, 2506–2515 (2013).
5. F. Glas Phys. Rev. B, 74, 121302 (2006).
2:20 PM - EN07.08.05
Growth and Characterization of Earth-Abundant Zinc Phosphide Thin Films for Photovoltaics Applications
Mahdi Zamani1,Simon Escobar Steinvall1,Elias Stutz1,Rajrupa Paul1,Jean-Baptiste Leran1,Mirjana Dimitrievska1,Anna Fontcuberta i Morral1,2
École Polytechnique Fédérale de Lausanne1,Ecole Polytechnique Fédérale de Lausanne2Show Abstract
Thanks to the abundancy of zinc and phosphorous, zinc phosphide has great potential for large-scale thin film photovoltaics. In particular, it exhibits high carrier mobility, diffusion length, absorption coefficient, and furthermore, its direct bandgap of 1.5 eV is close to optimal value of the Shockley-Queisser limit. In addition, compared to complex earth abundant semiconductors such as Kesterite, it benefits from thermodynamic stability and the simplicity of phase diagram. Separate source Molecular Beam Epitaxy (MBE) is an ideal platform for optimizing the growth of thin films as it provides precise control over the molecular/atomic fluxes and substrate temperature. In particular, growth of this material at lower temperatures has been recently achieved in our group, which addresses the long standing issue of abnormally high thermal expansion coefficient, which results in cracking upon cooling down from high growth temperatures , .
Here, we present the growth of zinc phosphide thin films on InP(100) substrates. While there had been previous attempts to grow this material using compound  and other techniques , this substrate has been largely neglected for the growth of this material. We provide an overview of the thin film morphology and crystalline quality as a function of the zinc and phosphorous fluxes as well as V/II ratio and growth temperature. The thin films are probed by a combination of different characterization methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), (scanning) transmission electron microscopy (STEM/ TEM), electron energy loss spectroscopy (EELS) and Raman spectroscopy and the conditions for selectively changing the thin film type from amorphous to polycrystalline and monocrystalline are acquired. In particular, it is observed that high growth temperature and V/II ratios result is amorphous thin films. The role of interface oxide on the crystalline quality of the thin films is established by STEM and EELS. It is observed that in-situ degassing of the substrates at high temperature, which results in removal of native substrate oxide, could result in formation of monocrystalline thin films. The functionality of the different types of thin films is further evaluated by photoluminescence (PL) spectroscopy, demonstrating the superior properties of monocrystalline films. This study provides information about the growth of zinc phosphide thin films that could be valuable for implementation of this material for photovoltaics applications.
 R. Paul et al., “Van der Waals Epitaxy of Earth-Abundant Zn3P2on Graphene for Photovoltaics,” Cryst. Growth Des., vol. 20, no. 6, pp. 3816–3825, Jun. 2020, doi: 10.1021/acs.cgd.0c00125.
 S. Escobar Steinvall et al., “Multiple morphologies and functionality of nanowires made from earth-abundant zinc phosphide,” Nanoscale Horizons, vol. 5, no. 2, pp. 274–282, Feb. 2020, doi: 10.1039/c9nh00398c.
 J. P. Bosco, D. O. Scanlon, G. W. Watson, N. S. Lewis, and H. A. Atwater, “Energy-band alignment of II-VI/Zn 3 P 2 heterojunctions from x-ray photoemission spectroscopy,” J. Appl. Phys., vol. 113, no. 20, p. 203705, May 2013, doi: 10.1063/1.4807646.
 M. Bhushan and A. Catalano, “Polycrystalline Zn3P2 Schottky barrier solar cells,” Appl. Phys. Lett., vol. 38, no. 1, pp. 39–41, Jan. 1981, doi: 10.1063/1.92124.
EN07.09: Advances in CIGS/CZTS Solar Cells
Monday PM, April 19, 2021
4:00 PM - *EN07.09.01
Metastability in CIGS Solar Cells with Na and RbF Alkali Treatments with Varitaion in Buffer Layers
University of Nevada, Las Vegas1Show Abstract
The effects of Na and RbF alkali treatment on the metastability behavior of Cu(In,Ga)Se2 solar cells with two different buffers have been investigated with stress factors of heat, junction bias and illumination.
For CdS buffer, four device types with and without Na or RbF treatments have been subjected to heat- and light-soaking under open- and short-circuit (OC, SC) junction bias. Low-Na devices show a higher bandgap due to increased minimum Ga content, higher recombination current and lower open-circuit voltage (VOC). Devices with RbF post-deposition treatment (PDT) show an improvement in net doping density ~ 1016 cm-3, VOC and efficiency. Heat- and light-soaking under OC junction bias provokes an increase in net carrier concentration and VOC irrespective of the alkali treatments. After SC stress, a decrease in VOC and net carrier concentration are observed which can be stabilized by RbF-PDT. ToF-SIMS measurements reveal an increase in Na and O concentration in CIGS for baseline and reduced-Na devices, respectively, after OC stress. Oxygen concentration in CdS decreases after heat- and light-soaking for devices without RbF-PDT, whereas it remains unchanged for devices with RbF-PDT. The atomic concentration profiles in CIGS significantly stabilize as a function of stress with the addition of RbF-PDT.
Devices under open-circuit (OC) light soaking show an increase in VOC with CdS buffer, however, VOC decreases for ZnOS buffer devices. All devices show an increase in net carrier concentration after OC light soak irrespective of RbF-PDT or the buffer layers, even though ZnOS buffer devices show degradation. Under short-circuit (SC) light soak CdS buffer devices with RbF-PDT show stabilization of VOC attributed to the increase in ionized acceptors in ordered vacancy compound (OVC) or buffer donor density. ZnOS buffer devices, on the other hand, show a decrease in VOC under both OC and SC light soak irrespective of RbF treatment. A comprehensive discussion of effect of interface defects, conduction band offset and near interface doping density will be presented for ZnOS based CIGS solar cells.
4:25 PM - EN07.09.02
Tailoring the Amine-Thiol Solvent System for the Deposition of High Quality Metal Selenide Films to Fabricate Cu(In,Ga)(S,Se)2 Solar Cells
Jonathan Turnley1,Swapnil Deshmukh1,Rakesh Agrawal1
Purdue University1Show Abstract
The solution processing of thin-film solar cells is of great interest due to the potential of this method to decrease costs and dramatically increase manufacturing throughput compared to the vacuum analogue. By utilizing hydrazine as a solvent, researchers have fabricated solution-processed Cu(In,Ga)(S,Se)2 devices with efficiencies greater than 17%. However, hydrazine is highly toxic and explosive, potentially limiting its use is large-scale manufacturing. Several alternative solution processing methods are under investigation, though none have matched hydrazine in efficiency. Because of the difficulty in producing a soluble selenium species, these methods will often deposit a nanocrystalline sulfide precursor film. The precursor film is then converted to a large-grain selenide film by heating in a selenium atmosphere. During this conversion, grain growth is achieved as the nanocrystalline sulfide material dissolves in a liquid selenium front and then recrystallizes into micron-scale selenide grains. This process often results in a distinct “fine-grain” layer, particularly as film thickness increases, limiting researchers to absorber layer of 1-1.5μm in thickness.
The amine-thiol solvent system has emerged as one of the most promising alternatives to hydrazine, producing Cu(In,Ga)(S,Se)2 devices with efficiencies above 15%. With its ability to dissolve metals and metal chalcogenides, the amine-thiol solvent system is able to avoid potential anionic impurities that arise when using other metal salts. Additionally, because selenium and many metal selenides can be dissolved in amine-thiol, this system has the potential to tune the sulfur to selenium ratio of the precursor film, as is done in the hydrazine system. One challenge, however, is that the metal-thiolates produced in this dissolution can easily decompose into metal sulfides upon heating, often leading to significant sulfur content in the precursor film, along with some residual carbon.
In this research, we will discuss investigations into amine-thiol dissolutions of metal chalcogenide species through the use of 1H-NMR, Raman, and ESI-MS. By developing a deeper understanding of the chemistry in this reactive dissolution, we can then make carefully controlled modification to the resulting metal complexes. We will present the various methods we have developed to alter the metal-chalcogenide bonding in the soluble precursor species, thereby changing the decomposition products and allowing for additional control over precursor film fabrication. We will also discuss the implications of the precursor film chalcogen composition on the further processing of the absorber layer, particularly as it relates to grain growth and will show how it can help reduce the carbon content in the film. To this end, we have utilized this new chemistry to produce the first ever 2μm-thick, solution-processed absorber films with no fine-grain layer. Finally, we will discuss how these strategies can be implemented to further the ultimate goal of high efficiency, solution-processed solar cells.
In conclusion, this research focuses on developing an understanding of the fundamental chemistry of the metal complexes created with the amine-thiol solvent system and connecting this knowledge with the practical application of solution-processed thin-film PV.
4:40 PM - EN07.09.03
Chemical and Electronic Properties of CdS/Cu(In,Ga)Se2 Interfaces with High Ga/(Ga+In) Ratio and Post-Deposition Treatment
Mary Blankenship1,Dirk Hauschild1,2,Elizaveta Pyatenko2,Wolfram Witte3,Dimitrios Hariskos3,Wanli Yang4,Nan Jiang1,4,Monika Blum4,Michael Powalla3,Lothar Weinhardt1,2,Clemens Heske1,2
University of Nevada, Las Vegas (UNLV)1,Karlsruhe Institute of Technology (KIT)2,Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW)3,Lawrence Berkeley National Laboratory (LBNL)4Show Abstract
The conversion efficiency of Cu(In,Ga)Se2-based (CIGSe) thin-film solar cells has reached values above 23% on a laboratory scale. To further increase the solar cell performance, higher open-circuit voltages can be achieved by increasing the Ga/(Ga+In) (GGI) ratio to create a larger absorber band gap and/or by applying a post-deposition treatment (PDT) to the absorber surface. Consequently, it is crucial to understand how a high GGI and a PDT affects the chemical structure and band alignment at the buffer/absorber interface.
We have investigated in-line deposited and industrially relevant CIGSe absorbers with high bulk and surface GGI ratios. Two sample series, one with and one without RbF PDT, were prepared, and their interfaces with a chemical bath deposited CdS buffer layer were characterized with a combination of electron and soft x-ray spectroscopies. For this purpose, laboratory-based x-ray and UV photoelectron spectroscopy (XPS and UPS, respectively), inverse photoemission (IPES), and x-ray-excited Auger electron spectroscopy (XAES) were combined with synchrotron-based soft x-ray emission spectroscopy (XES). A detailed picture of the electronic and chemical structure at and near the interface can be painted, which will be discussed in relation to the performance of the corresponding CIGSe solar cells.
5:00 PM - *EN07.09.04
Cell-Level Reliability Starring Cu(In,Ga)Se2 Thin-Film Photovoltaics
Lorelle Mansfield1,Ingrid Repins1,Stephen Glynn1,Christopher Muzzillo1,Bart Stevens1,Peter Hacke1,Kent Terwilliger1,Steve Johnston1,Steven Harvey1,Matthew Young1,Helio Moutinho1,C.S. Jiang1,Chuanxiao Xiao1,Darius Kuciauskas1,Timothy Silverman1
National Renewable Energy Laboratory1Show Abstract
The reliability of photovoltaics is commonly studied at the module level. Many reliability problems do originate from module attributes, such as metal interconnections to cells, junction boxes, etc. However, significant work in reliability can also be done before an entire module is designed. In this talk I summarize how we investigated three reliability concerns in Cu(In,Ga)Se2 (CIGS) photovoltaics at the cell level. The first is metastability, which in CIGS presents as an increase in VOC over time during prolonged light exposure. Although increasing VOC is not detrimental to module performance, it does complicate accurately measuring a module’s performance and subsequent rating. The second is shading-induced hot spots where cells are irreversibly damaged by reverse voltage. This can happen in less than 1 second of partial shading, and the damage is often visible to the eye. Finally, we looked at potential-induced degradation (PID) or a decrease in performance caused by voltage differentials which drive ion diffusion. PID is a growing concern as the number of modules wired in series rises, hence increasing voltage differentials. Examining these perceived problems required developing robust measurement protocols including the fabrication of novel testing structures. These efforts ensured that we were incorporating the right metrics for investigating the phenomena and provided insights for improvement strategies.
NREL is in a unique position to test reliability at the cell level as we are making improvements to PV technology. If we look at reliability earlier in the research cycle, we have the potential to avoid commonly known module reliability problems before cell changes are implemented on a large scale. Cell-level reliability studies could thus lower the rates of module failures in the field and provide confidence to investors that new technologies will perform as advertised.
5:25 PM - EN07.09.05
Late News: Enhanced Efficiency of Solution-Processed CuIn(S,Se)2 Solar Cells by In Situ Incorporation of Al2O3
Wilman Septina1,Christopher Muzzillo2,Craig Perkins2,Anne Curtis Giovanelli1,Thomas West1,Kenta Ohtaki1,Hope Ishii1,John Bradley1,Kai Zhu2,Nicolas Gaillard1
University of Hawaii1,National Renewable Energy Laboratory2Show Abstract
In recent years, passivation of Cu(In,Ga)Se2 (CIGSe) surface and/or interfaces with insulator materials has attained lots of attention to further increase efficiency. Such passivation is usually achieved with a thin layer of Al2O3 (<50 nm) deposited either before or after fabrication of CIGSe, most predominantly by atomic layer deposition method. The application of Al2O3 layer has been shown to reduce the number of electrically active defects at the semiconductor surface, which in turn lowers surface recombination velocities. This led to a decrease in interface recombination and resulted in improved efficiency. Passivation with Al2O3 has also been applied successfully to other thin film absorbers such as Cu2ZnSn(S,Se)4 and CdTe.
In this report, we present a novel approach to improve solution-processed CuIn(S,Se)2 (CISSe) solar cell efficiency by in-situ incorporation of Al2O3 . Specifically, AlNO3 was added to inks containing CuCl, InCl3, and thiourea dissolved in methanol. After spin coating of these solutions performed in air, samples were subjected to a selenization process. Our study showed that the Al formed amorphous nanosized-Al2O3 covering parts of the film top and bottom surfaces as well as within bulk and grain boundaries. Power conversion efficiency (PCE) as high as 11.6% was measured on cells using CISSe incorporated with Al2O3, a value higher than that measured on CISSe fabricated from Al-free ink (maximum PCE: 8.3%). This efficiency boost stemmed primarily from an increase in both in open-circuit voltages and fill factors. Defect passivation via this in-situ formed Al2O3 is thought to play a major role in the improvement of the solar cell performance through defect passivation, while permitting Cd diffusion to take place at the absorber sub-surface. Details of the fabrication process, materials, and photovoltaic characterization will be discussed.
5:30 PM - EN07.09.06
Silver Indium Diselenide—A High Mobility, Low-Defect Chalcopyrite Material with Potential for Thin-Film Photovoltaics
David Rokke1,Kyle Weideman1,Anna Murray1,Rakesh Agrawal1
Purdue University1Show Abstract
An area of interest in the thin film photovoltaics community that has grown in recent years is the alloying of Cu(In,Ga)(S,Se)2 (CIGS) with silver to create (Ag,Cu)(In,Ga)(S,Se)2 (ACIGS). Alloying of silver in CIGS has resulted in favorable device characteristics such as increasing the bandgap to optimize devices for top cells in tandem architectures, reduced structural disorder (observed by lower Urbach energies) and lower open circuit voltage losses. However, we anticipate that the alloying of silver may only complicate the fabrication of CIGS devices by introducing a sixth element that must be controlled in the material. Indeed, a recent report1 showed that high bandgap ACIGS devices have a low tolerance to off-stoichiometry, and another recent study2 has suggested that the ACIGS system may be prone to phase segregation at certain temperatures and compositions. These observations foreshadow possible complications in the controlled scale-up of ACIGS devices.
In this work, we propose that some advantages of silver incorporation in chalcopyrite materials could be harnessed differently, by moving away from the complex stoichiometry of ACIGS to the far simpler AgInSe2 (AIS). AIS has been studied only briefly in the literature, but initial findings suggest that the material is worth investigating further as a photovoltaic absorber material. By eliminating copper and gallium, the possibility for phase segregation is greatly reduced and the challenges presented by ACIGS are avoided. In addition, AIS has a direct bandgap in the ideal range for single-junction cells at 1.24eV, has been shown to have a carrier mobility much higher than CIGS, and has been predicted to have higher defect formation energy than CIGS.
To explore the potential of this material, we have fabricated thin films of AgInS2 through the amine-thiol solution processing route by dissolving silver sulfide and metallic indium in a monoamine-dithiol solvent mixture, creating a solution of bis(1,2-ethanedithiolate) indium (III) and oligomeric Ag(I) thiolates. These solutions were cast on to molybdenum-coated glass substrates and annealed to form orthorhombic AgInS2 films. These were then selenized in an elemental selenium atmosphere to create chalcopyrite thin films.
Our preliminary electronic characterization of AgIn(S,Se)2 thin films suggest that the carrier concentration is roughly 1 x 1013 cm-3. In previous literature3 it has been shown that the carrier concentration can be controlled by changing the concentration of selenium in the annealing atmosphere. We find that carrier mobilities and photoluminescence characteristics are superior to similarly fabricated CIS thin films. From these initial results, we expect AIS to have superior electronic properties and defect characteristics when compared to CIS/CIGS films.
A side effect of the use of silver instead of copper is a change of the majority carriers from holes (p-type) to electrons (n-type). Taking full advantage of AIS will require a redesign in the optimal device architecture, as incorporation in to a heterojunction with CdS or Zn(O,S,OH) (as is done with CIGS) is no longer a viable approach. Our ongoing work is exploring possible junction partners for AIS to propose viable device architectures for further investigation.
In this work, we have developed a solution processing route for the synthesis of AgIn(S,Se)2 thin films and demonstrated exciting electrical properties of this under-studied material. Along with our results, we point out the promise that this material holds for the thin film PV community.
(1) Keller, J.; Stolt, L.; Sopiha, K. V.; Larsen, J. K.; Riekehr, L.; Edoff, M. Sol. RRL 2020, 2000508 (1), 2000508
(2) Sopiha, K. V.; Larsen, J. K.; Donzel-Gargand, O.; Khavari, F.; Keller, J.; Edoff, M.; Platzer-Björkman, C.; Persson, C.; Scragg, J. J. S. J. Mater. Chem. A 2020, 8 (17), 8740
(3) Tell, B.; Shay, J. L.; Kasper, H. M. J. Appl. Phys. 1972, 43 (5), 2469
5:35 PM - *EN07.09.07
The High Efficiency CZTSSE Solar Cells on Flexible Metal Foils
Jin-Kyu Kang1,2,Dae-Ho Son1,Seung-Hyun Kim1,Se-Yun Kim1,2,Dae-Kue Hwang1,Shi-Joon Sung1,Kee-Jeong Yang1,Dae-Hwan Kim1
Daegu Gyeongbuk Institute of Science and Technology (DGIST)1,Kyungnam University2Show Abstract
The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as building integrated photovoltaics (BIPV) and mobile applications. In addition, low-cost flexible substrates can be used to lower manufacturing costs, which can contribute to the expansion of the renewable energy market by shortening the energy payback time. Solar cells with the CZTS-based (Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnSn(S,Se)4) absorbers are advantageous for lowing the cost, but the development of high efficiency solar cells based on flexible substrates has been relatively unexplored. In this work, flexible CZTSSe solar cells applying the Mo and SUS foil substrate were developed. When we used the metal precursor, double CZTSSe layer, voids and ZnSSe secondary phase were observed. The ZnSSe layer formed during soft annealing plays important role as a layer that blocks the volatilization of Zn. Large void can cause the formation of unwanted secondary phase and non-uniform composition. Applying SnS precursor is a good strategy as a way to suppress void formation. To optimize the processes of the flexible CZTSSe thin film solar cells, Na doping, back contact modifying, blister formation suppression were studied. Applying NaF decreased the defect level and Voc deficit(Eg – qVOC). This improvement can be explained by the associated decrease in defects, which are considered the recombination centers in the CZTSSe absorber layer. The weak adhesion between CZTSSe and Mo layers and the high stress of CZTSSe layer can form blisters. The increase of adhesion between CZTSSe and Mo layers can prevent the formation of blister. The best power conversion efficiency of the prepared CZTSSe solar cell on flexible Mo foil is 11.59%, which is world top efficiency record on flexible substrate.
Acknowledge: This work was supported by the DGIST R&D Program of the Ministry of Science and ICT (19-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).
EN07.10/EN06.07: Joint Session II: When Perovskite Meets Chalcogenide
Monday PM, April 19, 2021
7:45 PM - *EN07.10/EN06.07.01
Compositional and Defect Engineering in Halide Perovskite and Multinary Chalcogenide Solar Absorber Materials
Duke University1Show Abstract
Chalcogenide CdTe and Cu(In,Ga)(S,Se)2 (CIGS) and halide-perovskite-related (e.g. CH3NH3PbI3) semiconductors represent the fastest growing commercial and the most vigorously studied thin-film photovoltaic (PV) technologies, respectively. Alternatives to these semiconductors are also being sought to mitigate toxicity and elemental abundance concerns, for improved compatibility with pervasive scaling of the technologies. Inherent to these pursuits, is the need to understand materials and device limitations, especially as they relate to defects. In this talk, we will consider additive engineering and doping in CH3NH3PbI3-related semiconductors [1,2], as well as proposed defect-resistant Cu2Ba(Sn,Ge)(S,Se)4 materials [3,4], and employ a range of characterization techniques, including carrier-resolved photoHall, conventional Hall effect, terahertz spectroscopy and other time-resolved spectroscopies, to understand associated properties and performance-limiting or -enhancing mechanisms.
 O. Gunawan, S. R. Pae, D. M. Bishop, Y. Virgus, J. H. Noh, N. J. Jeon, Y. S. Lee, X. Shao,
T. Todorov, D. B. Mitzi, B. Shin, Nature 575, 151 (2019).
 J. Euvrard, O. Gunawan, D. B. Mitzi, Adv. Energy Mater. 9, 1902706 (2019).
 D. Shin, B. Saparov, D. B. Mitzi, Adv. Energy Mater. 7, 1602366 (2017).
 G. C. Wessler, T. Zhu, J.-P. Sun, A. Harrell, W. P. Huhn, V. Blum, D. B. Mitzi, Chem. Mater. 30, 6566 (2018).
8:10 PM - *EN07.10/EN06.07.02
Searching Stable Perovskite Solar Cell Materials Using Materials Genome Techniques and High-Throughput Calculations
Beijing Computational Science Research Center1Show Abstract
The solar cells based on the emerging organic-inorganic hybrid halide perovskite is progressing rapidly in the last decade, but the commercialization of the perovskite solar cells still faces significant challenges. The major issues are the poor long-term stability and toxicity. In light of this, materials design and screening of novel stable perovskites is becoming an important research direction. In contrast to conventional trail-and-error processes, materials genome techniques and high-throughput calculations have played important roles in this area and accelerated materials discovery. In this talk, I will present a review that summarizes our recent progress in this field, mainly focusing on four classes of perovskites including AM2+X3 halide single perovskites, AM4+Y3 chalcogenide single perovskites, A2M+M3+X6 halide double perovskites, and A2M3+M5+Y6 chalcogenide double perovskites. The stability, electronic, optical, and defect properties of these materials in terms of their applications as solar cell absorbers are discussed.
8:35 PM - *EN07.10/EN06.07.03
Carrier-Resolved Photo-Hall Effect in High Performance Perovskite Solar Absorbers
Oki Gunawan1,Seong Ryul Pae2,Byungha Shin2,Douglas M. Bishop1,David Mitzi3
IBM T.J. Watson Research Center1,Korea Advanced Institute of Science and Technology2,Duke University3Show Abstract
Majority and minority carrier properties such as type, density and mobility represent fundamental yet difficult to access parameters governing semiconductor device performance, most notably solar cells. Obtaining this information simultaneously under light illumination would unlock many critical parameters such as recombination lifetime, recombination coefficient, and diffusion length, however this goal has remained elusive. We demonstrate here a new carrier-resolved photo-Hall technique that rests on a new equation relating hole-electron mobility difference (△μ), Hall coefficient (H), and conductivity (σ), and a rotating parallel dipole line ac-field Hall system with Fourier/lock-in detection for clean Hall signal measurement (Nature 575, 151, 2019). We successfully apply this technique to a world-record-quality perovskite film and map the results against varying light intensities, demonstrating unprecedented simultaneous access to the above-mentioned parameters.
9:00 PM - EN07.10/EN06.07
9:15 PM - *EN07.10/EN06.07.04
Advances and Challenges for Commercialization of Perovskite Solar Cells
National Taiwan University1Show Abstract
The interest in perovskite photovoltaics has significantly increased over the last few years. High power conversion efficiencies (PCE) and low-cost manufacturing make perovskite PVs a very promising candidate for future applications. Despite the very advantageous features of perovskite materials, several issues still need to be solved before the commercialization of perovskite solar cells (PSCs). The main challenges of bringing perovskite technologies to the market are (i) scaling-up of the cells and modules dimension, (i) usage of lead in the perovskite solar panels and (iii) stability of the PSC modules. These three challenging topics will be discussed in the presentation, and possible solutions to overcome these issues will be proposed. The implementation of the proposed measures will help to demonstrate the feasibility of high-volume production. It is an important milestone towards the industrial manufacturing of perovskite photovoltaics and their future commercialization.
9:40 PM - EN07.10/EN06.07.05
Analysis for Efficiency Potential of II-VI Compound, Chalcopyrite and Kesterite Based Tandem Solar Cells
Masafumi Yamaguchi1,Hitoshi Tampo2,Hajime Shibata2,Nobuaki Kojima1,Yoshio Ohshita1
Toyota Tech. Inst.1,National Institute of Advanced Industrial Science and Technology2Show Abstract