Zr‐Doped Indium Oxide (IZRO) Transparent Electrodes for Perovskite‐Based Tandem Solar Cells

Parasitic absorption in transparent electrodes is one of the main roadblocks to enabling power conversion efficiencies (PCEs) for perovskite‐based tandem solar cells beyond 30%. To reduce such losses and maximize light coupling, the broadband transparency of such electrodes should be improved, especially at the front of the device. Here, the excellent properties of Zr‐doped indium oxide (IZRO) transparent electrodes for such applications, with improved near‐infrared (NIR) response, compared to conventional tin‐doped indium oxide (ITO) electrodes, are shown. Optimized IZRO films feature a very high electron mobility (up to ≈77 cm² V^?1 s^?1), enabling highly infrared transparent films with a very low sheet resistance (≈18 Ω □^?1 for annealed 100 nm films). For devices, this translates in a parasitic absorption of only ≈5% for IZRO within the solar spectrum (250–2500 nm range), to be compared with ≈10% for commercial ITO. Fundamentally, it is found that the high conductivity of annealed IZRO films is directly linked to promoted crystallinity of the indium oxide (In₂O₃) films due to Zr‐doping. Overall, on a four‐terminal perovskite/silicon tandem device level, an absolute 3.5 mA cm^?2 short‐circuit current improvement in silicon bottom cells is obtained by replacing commercial ITO electrodes with IZRO, resulting in improving the PCE from 23.3% to 26.2%.     

Na‐induced efficiency boost for Se‐deficient Cu(In,Ga)Se2 solar cells

Among different process routes for Cu(In,Ga)Se₂ (CIGS) solar cells, sufficient Se supply is commonly required to obtain high‐quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se‐deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se‐deficient CIGS device without extra NaF reveals a roll‐over I–V curve at room temperature as well as significantly reduced J_sc and fill factor at low temperatures. The electrical characteristics of Se‐deficient CIGS films are well explained and modeled by the low p‐type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti‐bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na‐induced passivation of Se vacancies and the increased p‐type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se‐deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se‐rich atmosphere. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural tuning of wide‐gap chalcopyrite CuGaSe2 thin films and highly efficient solar cells: differences from narrow‐gap Cu(In,Ga)Se2

Simultaneous realization of high values of open circuit voltage (V_oc), fill factor (FF), and energy conversion efficiency (η) in wide‐gap CuGaSe₂ (CGS) solar cells has long been one of the most challenging issues in the realm of chalcopyrite photovoltaics. In this communication, structural tuning of CGS thin films by means of controlling the amount of Se flux used during CGS film growth and improvements in solar cell performance (V_oc > 0.9 V, FF > 0.7, and η > 10%) are demonstrated. Systematic variations in CGS film properties with the Se flux and correlation with device properties are shown. The unique CGS thin‐film growth kinetics, which are different from narrow‐gap Cu(In,Ga)Se₂, are also presented and discussed. This development of double digit efficiency for CGS solar cells opens a new frontier for the broad application of a new class of chalcopyrite‐based devices. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of combined additional indium deposition and potassium fluoride post‐deposition treatments on Cu(In,Ga)Se2 thin film solar cells

The effect of additional indium on copper indium gallium selenide (CIGS) thin films and solar cells was investigated with respect to potassium fluoride post‐deposition treatment (KF‐PDT) using current‐voltage, external quantum efficiency, scanning electron microscopy, X‐ray photoelectron spectroscopy, time‐resolved photoluminescence and capacitance‐voltage measurements. The cell performance, particularly open‐circuit voltage (V _oc) improved drastically by the combined treatments of additional indium deposition after CIGS growth and subsequent KF‐PDT. A Cu deficient layer at the CIGS surface increased after both treatments rather than only KF‐PDT. Photoluminescence intensity, lifetime and net carrier concentration of KF‐untreated CIGS solar cells did not change significantly by only additional indium deposition. However, they improved because of the combined treatments. Copyright © 2017 John Wiley & Sons, Ltd.     

Performance enhancement of III–V compound multijunction solar cell incorporating transparent electrode and surface treatment

To enhance the performance of tandem‐type III–V compound multijunction solar cells, the transparent indium‐tin‐oxide (ITO) film was used to replace conventional metal electrode for increasing the incident light area. For performing ohmic contact between the n‐AlInP window layer and the ITO film, a transition layer of Au/AuGeNi thin metals was used and investigated. Besides, to improve ohmic performance and to passivate the surface states, (NH₄)₂S_x surface treatment was used. The conversion efficiency of the (NH₄)₂S_x‐treated triple‐junction solar cells was increased more than 3.09%. Furthermore, an improved oblique SiO₂/SiO₂/ITO triple antireflection structure was designed to reduce the reflectivity of illuminating sunlight. The conversion efficiency of the (NH₄)₂S_x‐treated triple‐junction solar cell with improved antireflection structure could be improved more than 4.23%. Simple and effective approaches were designed to improve the performances of tandem‐type III–V triple‐junction solar cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of Se flux on CuIn1‐xGaxSe2 film in reactive sputtering process

CuIn_1‐xGa_xSe₂ (CIGS) thin films were grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker was used to deliver reactive Se molecules. The Cu_0·6 Ga_0·4 and Cu_0·4In_0·6 targets were simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on CIGS film deposition were investigated. The CIGS film growth rate decreased, and the surface roughness of a film increased as the Se flux increased. The [112] crystal orientation was dominant, and metallic crystal phases such as Cu₉Ga₄ and Cu₁₆In₉ in a film were disappearing with increasing Se flux. A solar cell fabricated using a 900‐nm CIGS film showed the power conversion efficiency of 8·6%, the highest value found in a sub‐micron thick CIGS solar cell related to a reactive sputtering process with metallic targets. Copyright © 2011 John Wiley & Sons, Ltd.     

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Gallium gradients in Cu(In,Ga)Se2 thin‐film solar cells

The gallium gradient in Cu(In,Ga)Se₂ (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe₂ and CuGaSe₂ were calculated using density functional theory. The migration barrier for the In_Cu antisite in CuGaSe₂ is significantly lower compared with the Ga_Cu antisite in CuInSe₂, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Bandgap optimization of submicron‐thick Cu(In,Ga)Se2 solar cells

Reducing Cu(In,Ga)Se₂ (CIGS) absorber thickness into submicron regime provides an opportunity for reducing CIGS solar cell manufacturing time and cost. However, CIGS with submicron‐thick absorber would suffer strong absorption loss in the long‐wavelength region. In this paper, we report a new fabrication route for CIGS solar cells on soda‐lime glass substrates with different Ga content (0.3 < [Ga]/([Ga] + [In]) < 0.6), all with absorber thicknesses around 0.9 µm. Efficiency of 17.52% has been achieved for cells with high Ga content of [Ga]/([Ga] + [In]) = 41%, which is currently the best reported efficiency for submicron‐thick CIGS solar cells. Unlike the normal‐thickness absorber (2–3 µm) that has an optimal [Ga]/([Ga] + [In]) of ~32%, the increased value of optimal [Ga]/([Ga] + [In]) in submicron‐thick absorber greatly enhances the open‐circuit voltage, by nearly 15% compared with that of samples with Ga content optimized at normal absorber thickness. This large gain in V_OC well compensates the absorption loss in the long‐wavelength region and contributes to the enhancement of final solar cell efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

Cu-In-Ga-Se nanoparticle colloids as spray deposition precursors for Cu(In, Ga)Se2 solar cell materials

The use of nanoparticle colloids for spray deposition of Cu(In,Ga)Se₂ (CIGS) precursor films and subsequent fabrication of CIGS solar cells has been investigated. According to this approach, amorphous Cu-In-Ga-Se nanoparticle colloids were first prepared by reacting a mixture of CuI, InI₃, and GaI₃ in pyridine with Na₂Se in methanol at reduced temperature. Purified colloid was sprayed onto heated molybdenum-coated sodalime glass substrates to form Cu-In-Ga-Se precursor films. After thermal processing of the precursor films under a selenium ambient, CIGS solar cells were fabricated. Cu-In-Ga-Se colloids and films were characterized by inductively coupled plasma atomic emission spectroscopy, thermogravimetric analysis, transmission electron microscopy, x-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. Standard current-voltage characterization was performed on the CIGS solar cell devices with the best film exhibiting a solar conversion efficiency of 4.6%.     

Parameterized complex dielectric functions of CuIn1−xGaxSe2: applications in optical characterization of compositional non‐uniformities and depth profiles in materials and solar cells

In‐situ spectroscopic ellipsometry (SE) was employed to extract the complex dielectric functions ε = ε₁ + iε₂ over the spectral range of 0.75–6.5 eV for a set of polycrystalline CuIn_1−xGa_xSe₂ (CIGS) thin films with different alloy compositions x = [Ga]/{[In] + [Ga]}. For highest possible accuracy in ε for each CIGS thin film, specialized SE procedures were adopted including (i) deposition to a thickness of ~600 Å on smooth native oxide covered crystal silicon wafers, which minimizes the surface roughness on the film and thus the required corrections in data analysis, and (ii) measurement in‐situ, which minimizes ambient contamination and oxidation of the film surface. Assuming an analytical form for each of the ε spectra for these CIGS films, oscillator parameters were obtained in best fits, and these parameters were fit in turn to polynomials in x. With the resulting database of polynomial coefficients, the ε spectra for any composition of CIGS can be generated from the single parameter, x. In addition to enabling accurate contactless determination of bulk and surface roughness layer thicknesses of CIGS films by high speed multichannel SE, the database enables characterization of the composition and its profile with depth into these films, and even how the depth profile varies spatially within the plane of the films. In this study, depth profile parameters were found to correlate spatially with solar cell performance parameters. As a result, SE provides the capability of contactless compositional analysis of production‐scale CIGS photovoltaic modules at high speed. Copyright © 2016 John Wiley & Sons, Ltd.     

Flexible Cu(In,Ga)Se2 solar cell on stainless steel substrate deposited by multi‐layer precursor method: its photovoltaic performance and deep‐level defects

Cu(In,Ga)Se₂ (CIGS) films on soda‐lime glass and stainless steel (SUS) substrates with several [Ga]/([Ga] + [In]), GGI, and Fe concentrations are fabricated by so‐called “multi‐layer precursor method”. From optical deep‐level transient spectroscopy, deep‐level defect located at 0.8 eV from valence band maximum (E_V) is observed. This defect becomes recombination center when GGI is over 0.4, thereby decreasing cell performances. Fe‐related deep‐level defect is moreover detected in CIGS film on SUS substrate situated at 0.45 eV from E_V. Its density is consistent with Fe concentration in CIGS films. According to SCAPS simulation and experimental results, Fe concentration of above threshold (1.0 × 10¹⁶ atom/cm³) decreases carrier lifetime and carrier density and has more harmful influence on cell performances with GGI of above 0.4. On the other hand, Fe concentration of below threshold (1.0 × 10¹⁶ atom/cm³) has no detrimental impact on cell performances. Namely, conversion efficiency (η) is slightly changed by below 2%. CIGS solar cell on SUS substrate with η of 17.5% is fabricated by decreasing Fe concentration to approximately 5.2 × 10¹⁶ atom/cm³ although higher than the threshold value. Copyright © 2016 John Wiley & Sons, Ltd.     

CIGS Thin Films for Cd-Free Solar Cells by One-Step Sputtering Process

Cu(In_1?x Ga _x )Se₂ (CIGS) thin films were deposited by a one-step radio frequency (RF) magnetron sputtering process using a quaternary CIGS target. The influence of substrate temperature on the composition, structure, and optical properties of the CIGS films was investigated. All the CIGS films exhibited the chalcopyrite structure with a preferential orientation along the (112) direction. The CIGS film deposited at 623 K showed significant improvement in film crystallinity and surface morphology compared to films deposited at 523 and 573 K. To simplify the manufacturing procedure of solar cells and avoid the use of the toxic element Cd, the properties of ZnS films prepared by RF sputtering were also investigated. The results revealed that the sputtered ZnS film exhibits good lattice matching with the sputtered CIGS film with significantly lower optical absorption loss. Finally, all-sputtered Cd-free CIGS-based heterojunction solar cells with the structure SLG/Mo/CIGS/ZnS/AZO/Al grids were fabricated without post-selenization. Furthermore, the results demonstrated the feasibility of using a full sputtering process for the fabrication of Cd-free CIGS-based solar cell.     

Reactive Sputtering Process for CuIn1‐xGaxSe2 Thin Film Solar Cells

CuIn_1‐xGa_xSe₂ (CIGS) thin films are grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker is used to deliver reactive Se molecules. The Cu and (In_0.7Ga_0.3)₂Se₃ targets are simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on film composition are investigated. The Cu/(In+Ga) composition ratio increases as the Se flux increases at a plasma power of less than 30 W for the Cu target. The (112) crystal orientation becomes dominant, and crystal grain size is larger with Se flux. The power conversion efficiency of a solar cell fabricated using an 800‐nm CIGS film is 8.5%.     

Growth of Cu(In,Ga)Se2 thin films by a novel single‐stage route based on pulsed electron deposition

We report a novel route for growing Cu(In,Ga)Se₂ (CIGS) thin films, based upon the Pulsed Electron Deposition (PED) technique. Unlike other well‐known deposition techniques, PED process allows the stoichiometric deposition of CIGS layers in a single stage, without requiring any further treatments for Cu/(In + Ga) ratio adjustment nor selenization. The structural properties of polycrystalline CIGS films strongly depend on the growth temperature, whereas post‐deposition annealing enhances the grain size and the <112> out‐of‐plane preferred orientation of the chalcopyrite structure, without affecting the film composition. Preliminary measurements of the performances of solar cells based on these films confirm the great potentiality of PED‐grown CIGS as absorber layers. Copyright © 2011 John Wiley & Sons, Ltd.     

A Novel Photocathode Material for Sunlight‐Driven Overall Water Splitting: Solid Solution of ZnSe and Cu(In,Ga)Se2

Thin films of a solid solution of ZnSe and CuIn_0.7Ga_0.3Se₂ ((ZnSe) _x (CIGS) _1–x ) are prepared by co‐evaporation. Structural characterization reveals that the ZnSe and CIGS form a solid solution with no phase separation. (ZnSe)_0.85(CIGS)_0.15‐based photocathodes modified with Pt, Mo, Ti, and CdS exhibit a photocurrent of 7.1 mA cm^?2 at 0 V_RHE, and a relatively high onset potential of 0.89 V_RHE under simulated sunlight. A two‐electrode cell containing a (ZnSe)_0.85(CIGS)_0.15 photocathode and a BiVO₄‐based photoanode has an initial solar‐to‐hydrogen conversion efficiency of 0.91%, which is one of the highest values reported for a photoanode–photocathode combination. Thus, (ZnSe)_0.85(CIGS)_0.15 is a promising photocathode material for efficient photoelectrochemical water splitting.     

Photovoltaic Devices: Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells (Adv. Funct. Mater. 6/2011)

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Fabrication of Cu(In,Ga)Se2 thin films by sputtering from a single quaternary chalcogenide target

Single‐layered Cu‐In‐Ga‐Se precursors were fabricated by one‐step sputtering of a single quaternary Cu(In,Ga)Se₂ (CIGS) chalcogenide target at room temperature, followed by post selenization using Se vapor obtained from elemental Se pellets. The morphological and structural properties of both as‐deposited and selenized films were characterized by X‐ray diffraction (XRD), Raman spectroscope and scanning electron microscope (SEM). The precursor films exhibited a chalcopyrite structure with a preferential orientation in the (112) direction. The post‐selenization process at high‐temperature significantly improved the quality of the chalcopyrite CIGS. The CIGS layers after post‐selenization were used to fabricate solar cells. The solar cell had an open‐circuit voltage Voc of 0.422 V, a short‐circuit current density J = 24.75 mA, a fill factor of 53.29%, and an efficiency of 7.95%. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication and characterization of thin-film Cu (In,Ga) Se2 solar cells on a PET plastic substrate using screen printing

Chalcopyrite copper indium gallium diselenide (CIGS) ink was prepared by dissolving copper, indium, gallium acetylacetonate and Se powder in oleylamine using the hot injection methods. CIGS films were deposited on a PET plastic substrate by a screen-printing technique using CIGS ink with a Ga content ranging from 0.3 to 0.6. X-ray diffraction patterns reveal that the films exhibit a chalcopyrite-type structure. The crystalline grain sizes of the films decrease with increasing Ga content. AFM data shows that the root mean square (RMS) surface roughness of the CIGS film decreases with increasing Ga content. The effects of the Ga content in the CIGS absorber layer on the optical properties of the corresponding thin films and solar cells were studied. The band-gap energies of the CIGS thin films increased with an increasing Ga/(In+Ga) ratio. The short-circuit current (I_SC) of the solar cell decreased linearly with the Ga/(In+Ga) ratio, while the open-circuit voltage (V_OC) increased with this ratio. The solar cell exhibited its highest efficiency of 4.122% at a Ga/(In+Ga) ratio of 0.3.     

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

Wide‐bandgap perovskite solar cells (PSCs) with optimal bandgap (E_g) and high power conversion efficiency (PCE) are key to high‐performance perovskite‐based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E_g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open‐circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four‐terminal tandem configuration with respect to variations in the perovskite bandgap for two state‐of‐the‐art bottom solar cells is experimentally validated.