Transparent Front Contact Research Articles

In Situ IR Spectroscopy Studies of Atomic Layer-Deposited SnO2 on Fullerenes for Perovskite Photovoltaics.

In recent years, atomic layer deposition (ALD) has established itself as the state-of-the-art technique for the deposition of SnO2 buffer layers grown between the fullerene electron transport layer (ETL) and the ITO top electrode in metal halide perovskite-based photovoltaics. The SnO2 layer shields the underlying layers, i.e., the fullerene-derivative materials such as C60 and PCBM, as well as the perovskite absorber, from water ingress and damage induced by the sputtering of the transparent front contact. Our study undertakes a comprehensive investigation of the impact of SnO2 ALD processing on fullerenes by means of in situ spectroscopic ellipsometry (SE) and transmission infrared spectroscopy (FTIR). While no difference in SnO2 bulk properties is observed and the perovskite absorber degradation is nearly entirely avoided during exposure to heat and vacuum, when the absorber is introduced beneath the organic ETLs, a SnO2 growth delay of about 50 ALD cycles is measured on PCBM, whereas the delay is limited to 10 cycles in the case of growth on C60. Notably, FTIR measurements show that while C60 remains chemically unaffected during SnO2 ALD growth, PCBM undergoes chemical modification, specifically of its ester groups. The onset of these modifications corresponds with the detection of the onset, after the initial delay, of ALD SnO2 growth. It is expected that the modification that the PCBM layer undergoes upon ALD SnO2 processing is responsible for the systematic lower photovoltaic device performance in the case of PCBM-based devices, with respect to C60-based devices.

Development of n-Type, Passivating Nanocrystalline Silicon Oxide Films via Plasma-Enhanced Chemical Vapor Deposition

Nanocrystalline silicon oxide (nc-SiOx:H) is a multipurpose material with varied applications in solar cells as a transparent front contact, intermediate reflector, back reflector layer, and even tunnel layer for passivating contacts, owing to the easy tailoring of its optical properties. In this work, we systematically investigate the influence of the gas mixture (SiH4, CO2, PH3, and H2), RF power, and process pressure on the optical, structural, and passivation properties of thin n-type nc-SiOx:H films prepared in an industrial, high-throughput, plasma-enhanced chemical vapor deposition (PECVD) reactor. We provide a detailed description of the n-type nc-SiOx:H material development using various structural and optical characterization techniques (scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Raman spectroscopy, and spectroscopic ellipsometry) with a focus on the relationship between the material properties and the passivation they provide to n-type c-Si wafers characterized by their effective carrier lifetime (τeff). Furthermore, we also outline the parameters to be kept in mind while developing different n-type nc-SiOx:H layers for different solar cell applications. We report a tunable optical gap (1.8–2.3 eV) for our n-type nc-SiOx:H films as well as excellent passivation properties with a τeff of up to 4.1 ms (implied open-circuit voltage (iVoc)~715 mV) before annealing. Oxygen content plays an important role in determining the crystallinity and hence passivation quality of the deposited nanocrystalline silicon oxide films.

Interface Engineering for Highly Efficient Perovskite Photovoltaics

In the past decade, major progress has taken place in the field of metal halide perovskite photovoltaics (PV), as witnessed by the recently achieved conversion efficiency of 25.7% [1] and the concrete opportunity to go beyond the thermodynamic limit of single junction PV by means of perovskite/ crystalline silicon tandems, with the very recent record of 33.2% [2]. Among the most recent efforts towards further development of perovskite PV and subsequent commercialization, strategies ranging from perovskite surface passivation and compositional engineering to thin film encapsulation, contribute to suppress the perovskite absorber and device instability to moisture ingress. In parallel, research efforts presently investigate the role of inorganic charge transport layers (CTLs) next to the one of the most commonly used organic CTLs, such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or C60. The inorganic layers are acknowledged to impart thermal and mechanical stability to the device, as well as to improve device efficiency yield.In this contribution, our recent research on atomic layer deposited (ALD) selective CTLs for metal halide perovskite PV [3] is reviewed by addressing two case studies, namely, SnO2 thin films, selective towards electron transport, and NiO thin films, selective towards hole transport. ALD is selected as deposition technology because of its merit of ultimate control over film thickness and conformality.ALD SnO2 [4]. based on cycles of tetrakis(dimethylamido)-tin and H2O as precursor and co-reactant, respectively, is the state-of-the art in both p-i-n perovskite single junction as well as tandem perovskite/crystalline silicon and perovskite/CIGS PV. Its presence is key to the device thermal and mechanical stability, to prevent humidity ingress in the device and suppress damage to the fullerene (PCBM or C60) during the sputtering of the transparent front contact. Moreover, SnO2 plays a key role in perovskite/perovskite tandem PV, where it serves as solvent barrier to prevent damage of the wide-band gap perovskite sub-cell during solution processing of the narrow-band gap perovskite top cell. In parallel, our in situ studies by means of IR spectroscopy during ALD of SnO2 on perovskite/fullerene disclose the reason behind the difference in device conversion efficiency when SnO2 is processed either on PCBM or C60, i.e. the devices based on PCBM underperform those based on C60 by 3% absolute difference. Specifically, the Sn-precursor is responsible for the modification of the ester group in PCBM thereby affecting the PCBM/perovskite interface and device efficiency.ALD NiO thin (7 nm) films [5], based on cycles of bis(methylcyclopentadienyl)nickel and H2O as precursor and co-reactant, respectively, are found to impart stability to the perovskite device under acceleration test at 85°C, with 80% retention of the initial conversion efficiency after 300 hours at 1 sun illumination. At the same time, the presence of the phosphonic acid-based self-assembled monolayer (SAM), is essential to engineer an almost lossless (i.e., in terms of suppression of charge recombination) NiO/SAM/perovskite interface. Also, the SAM homogeneity and surface coverage on NiO improve with respect to direct SAM processing on ITO (see figure), as witnessed by transmission electron microscopy (TEM) and conductive atomic force microscopy. This result is explained in terms of chemisorption reactions between SAM phosphonic acid groups and NiO hydroxyl groups. The SAM homogeneity on NiO leads to higher shunt resistance in the device with respect to the one with SAM directly processed on ITO. Finally, the combination of NiO and SAM results in a narrower distribution of device performance reaching more than 20% efficient champion device. In parallel, two-terminal perovskite/crystalline silicon tandem devices with an ITO/NiO/SAM tunnel recombination junction (TRJ) exhibit a better device yield with respect to tandem devices with an ITO/SAM TRJ, i.e. the standard deviation decreases from 4.6% with ITO/SAM to 2.0% with ITO/NiO/SAM.[1] Green et al., Prog. Photovolt. Res. Appl. 30, 687 (2022)[2] https://www.kaust.edu.sa/news/kaust-team-sets-world-record-for-tandem-solar-cell-efficiency[3] Zardetto et al., Sustainable Energy & Fuels 1, 30 (2017)[4] Bracesco et al., J. Vac. Sci. Technol. A, 38(6), 063206-1 (2020)[5] Phung et al., ACS Applied Materials and Interfaces. 14(1), 2166 (2022)[6] Phung et al., submitted to Sol. Mat. (2023) Figure 1

Fabrication and characterization of ZnS and ZnSe absorber layers for UV-selective transparent photovoltaics

We have fabricated ZnS, ZnSe and mixed Zn(S,Se) thin film layers for application as solar cell absorbers in ultraviolet (UV)-selective solar cells. We fabricated the thin film layers using a two-step process involving evaporation of the Zn layer followed by an anneal in an H2S and/or H2Se containing atmosphere. A study of the annealing conditions was performed to minimize hole formation in the thin film layer. A mixed Sulfur and Selenium containing layer was also developed in order to reach a 3 eV band gap, such that the layer absorbs all the UV light and leaves the visible light passing through. Next, we tried to increase the conductivity of the layer by adding extrinsic doping elements. Cu was found to increase the conductivity by more than three orders of magnitude, while leaving the transparency of the layer mostly unchanged. Finally, solar cell devices were fabricated using transparent back and front contacts and a NiO buffer layer. We could measure a very small photocurrent of about 100 μA/cm2 and an open circuit voltage of about 500 mV under AM1.5 G illumination.

Design and Device Modeling of Lead Free CsSnI3 Perovskite Solar Cell

Research of lead-free Perovskite based solar cells has gained speedy and growing attention with urgent intent to eliminate toxic lead in Perovskite materials. The main purpose of this work is to supplement the research progress with comparative analysis of different lead-free Perovskite based solar cells by numerical simulation method using solar cell capacitance simulator (SCAPS-1D) software. The environmental friendliness and excellent thermal stability proves Cesium Tin Iodide (CsSnI3) as one of the promising materials for the commercialization of the Perovskite solar cells. However, CsSnI3 solar cells suffer from poor efficiency due to having low open-circuit voltage, VOC attributed to poor absorber film quality as well as energy level mismatch at the interfaces between different layers like transparent front contact. The architecture of the solar cell is n-i-p device structure acts as light CsSnI3 absorber active layer, TiO2 as electron transport layer and Spiro-OMeTAD as hole transport layer with device structure FTO/ TiO2/CsSnI3 / Spiro-OMeTAD /Au. The open circuit voltage Voc, short circuit current density Isc, fill factor and power conversion efficiency Voc=1.09V, Jsc=28.85mA/cm2, FF=88.65%, eta=28.09%, V_MPP=0.99V, J_MPP=28.15 mA/cm2 respectively.

Ultra-thin passivation layers in Cu(In,Ga)Se2 thin-film solar cells: full-area passivated front contacts and their impact on bulk doping

In the search for highly transparent and non-toxic alternative front layers replacing state-of-the-art CdS in Cu(In,Ga)Se2 thin-film solar cells, alternatives rarely exceed reference devices in terms of efficiency. Full-area ultra-thin aluminium oxide tunnelling layers do not require any contact patterning and thus overcome the main drawback of insulating passivation layers. Even a few monolayers of aluminium oxide can be deposited in a controlled manner by atomic layer deposition, they show excellent interface passivation properties, low absorption, and suitable current transport characteristics on test devices. Depositing a ZnO-based transparent front contact, however, results in extremely poor solar cell performance. The issue is not necessarily a low quality of the alternative front layer, but rather the intricate relation between front layer processing and electronic bulk properties in the absorber layer. We identify three challenges critical for the development of novel front passivation approaches: (i) both Cd and Zn impurities beneficially reduce the high native net dopant concentration in the space charge region, (ii) sputter deposition of ZnO damages the passivation layer resulting in increased interface recombination, (iii) thermal treatments of devices with ZnO layer result in substantial Zn diffusion, which can penetrate the full absorber thickness already at moderate temperatures.

Enhancement of the thermal properties of heterojunction perovskite solar cells by nanostructured contacts design

A perovskite solar cell design is structured and analyzed to investigate the thermal behavior of the cell. In order to perform large scale industrial devices, it is required to understand the effect of varying light intensity, ambient temperature and other sources such as joule heating and non-radiative recombination on the module. Subsequently, the enhancement of thermal stability is accomplished by selecting layers that can contribute to the reduction of operating temperature, such as transparent front electrodes and back electrodes. COMSOL Multiphysics is used to structure and simulate a regular planar heterojunction perovskite solar cell with ITO as a transparent front contact and Au as the back contact. These conventional contacts selection resulted in a maximum temperature of 79 °C for all sources of thermalization. Moreover, nine structures combinations are investigated by using FTO, AZO, Ag, and RGO electrodes. It is found that the best thermal structure is the one that utilizes AZO as a transparent contact and RGO as a back contact. This design achieves a huge reduction in the maximum temperature to ~32 °C; with a total of 59.5% thermal reduction when compared to conventional ITO/Au contacts structure.

Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells

A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.

Multifunctional metal oxide electrodes: Colour for thin film solar cells

Building integrated photovoltaics (BIPV) is an essential part to reduce the CO2 footprint of metropolitan areas. Currently, full integration of photovoltaic elements in a building is a very costly and complex undertaking, as it usually requires expensive custom modules. In order to increase the market share of BIPV in the residential mass market, a low-cost, flexible technical process to change the appearance of solar elements is required. Transparent conductive electrodes consisting of an oxide-metal-oxide (OMO) stack of thin layers have been optimized for application in thin film solar cells. Here the OMO stack is multifunctional: It provides the transparent front contact electrode and at the same time allows tuning of the module colour. This has several advantages compared to other colouring techniques, i.e. coloured glass or additional interlayers. The OMO colouring does not require an additional process step, and with sputtering an already existing deposition method is used. By varying the thickness of the oxide layers it is possible to change the reflected spectrum of the stack and with it the module colour. In this publication, we present how the optical model of the OMO stack, that is necessary for precise tuning of the colour, is first developed for OMO/glass samples and then report the changes necessary to adapt the OMOs for use on Cu(In,Ga)Se2 thin film solar cells.

All‐solution‐processed transparent front contact for monograin layer kesterite solar cells

AbstractThe paper presents a fully solution‐processed transparent front contact for kesterite monograin layer solar cells, which is composed of a silver nanowire network above a ZnO‐based window bilayer. This window bilayer comprises a layer of ZnO nanoparticles and a second layer of chemical‐bath‐deposited doped ZnO. The optimized front contact can be processed within minutes and performs equally to a reference front contact with a sputtered iZnO/ZnO:Al window bilayer, both on solar cell and module level with sizes up to 20 × 20 cm2. To achieve this, it was necessary to obtain a compact morphology of the chemical‐bath‐deposited, doped ZnO film even for very short deposition times. It is found that, firstly, this depends on the homogeneity of the seed layer, which is the prerequisite for thin film growth by chemical bath deposition (CBD) on the substrate. It is shown that the ZnO nanoparticle layer covers the rough monograin substrate in a continuous and unperturbed manner. The second aspect of importance was the dopant type used in the CBD process (Al, Ga, or In). Optimal combination of photovoltaic performance and process speed was obtained for Ga‐doped ZnO, yielding compact films for deposition times as low as 1 minute. The solution‐processed window layers are susceptible to a light‐soaking effect, which impairs the cell performance upon storage in dark conditions. While this effect can be easily overcome in the laboratory, the effect in a real application scenario was investigated under outdoor conditions for a period of 4 weeks.

Utilization of 2D gahnite nanosheets as highly conductive, transparent and light trapping front contact for silicon solar cells

The current scenario illustrates distinct interest in developing renewable energy sources for power generation. In this regard, several researches are performed in enhancing the power conversion efficiency of solar cells. The present work focuses on utilizing ZnAl2O4 (gahnite) spinel as antireflection coating material to improve the power conversion efficiency of silicon solar cells. Gahnite was synthesized using two precursors namely zinc nitrate hexahydrate and aluminum nitrate nonahydrate through sol–gel technique. The thickness of the prepared gahnite sheets measured through atomic force microscopy was around 50 nm. Single to quintuple layers of gahnite was deposited on silicon solar substrate using spin coating technique. The influence of gahnite coating on the structural, optical, electrical properties and cell temperature of silicon solar cells are analyzed. The synthesized gahnite bears spinel crystal structure in the form of two dimensional nanosheet. Increment in layer thickness proves the deposition of single to quintuple layer on silicon substrate. A maximum of 93% transmittance and 20.72% power conversion efficiency at a low cell temperature (39.4 °C) has been achieved for triple layer deposition proving diffusion of more photons on the substrate. The obtained results prove gahnite as suitable anti-reflection coating material for enhancing the power conversion efficiency of silicon solar cells.

Enhancement of the photocurrent and efficiency of CdTe solar cells suppressing the front contact reflection using a highly-resistive ZnO buffer layer

We report on the effects of using an atomic layer deposited ZnO transparent buffer layer with > 106 Ω cm resistivity on the performance of CdZnS/CdTe solar cells grown by metalorganic chemical vapour deposition (MOCVD). The buffer film thickness is adjusted by optical modelling to suppress the reflection losses at the front contact. A clear improvement, up to 1.8% in conversion efficiency, was obtained in comparison to reference devices without the ZnO buffer layer, thanks to the enhancement of the current density (Jsc) and fill factor (FF). Device spectral response showed improved collection for most of the visible region. Reflectance measurements confirmed that the ZnO film reduced the optical reflectance around the transparent front contact. This effect permitted light management through the front contact leading to an improvement of the Jsc and hence the photovoltaic conversion efficiency. These results are intriguing since the literature on CdTe solar cells did not previously report improvement to the photocurrent and device response through controlling the highly-resistive transparent buffer layer.

Numerical Modelling of Front Contact Alignment for High Efficiency Cd1-xZnxTe and Cd1-xMgxTe Solar Cells for Tandem Devices

AbstractWide bandgap Cd1-xZnxTe (CZT) and Cd1-xMgxTe (CMT) have drawn attention as top cells in tandem devices. These materials allow tuning of the band gap over a wide range by controlling the Zn or Mg concentration with little alteration to the base CdTe properties. Historically, CdS has been used as a heterojunction partner for CZT or CMT devices. However, these devices show a significant lower open circuit voltage (VOC) than expected for wide bandgap absorbers. Recent modelling work suggests that poor band alignment between the CdS emitter and absorber results in a high concentration of holes at the interface, which increased recombination and limits the VOC. This recombination should be exacerbated for wider bandgap absorbers such as CZT and CMT. In this study, we use numerical simulations with SCAPS-1D software to investigate the band alignment in the front contacts for wider bandgap CdTe based absorbers. Results show that by replacing the CdS with a wide bandgap emitter layer, the VOC can be greatly improved, though under certain conditions, the fill factor remains sensitive to the location of the emitter conduction band. As a result, different transparent front contacts were also investigated to determine a device structure required to produce a high performance CZT or CMT top-cell for tandems devices.

Transparent conducting Al:ZnO thin films on large area by efficient cylindrical rotating DC magnetron sputtering

Al: ZnO (AZO) thin film provides non-toxic, cost effective visible light transparent conductive front contact for various photonic and electronic applications. Development of AZO thin films on large area soda lime glass substrates using direct current magnetron sputter coater with cylindrical rotating ceramic targets is investigated and is presented in this work. Design of experiments and Investigation on detailed structural, electrical, optical characterization was carried out to fine tuning of various sputter process parameters such as sputtering power, pressure and substrate temperature to obtain uniform electrical conductivity and transmittance over a large area. The best resistivity of 4.07 × 10−4 Ω cm and transmittance of 84% with thickness uniformity better than 97% on 300 mm × 300 mm area glass substrate was achieved on the AZO thin films sputtered using optimized process conditions.

Si‐Doped Cu(In,Ga)Se2 Photovoltaic Devices with Energy Conversion Efficiencies Exceeding 16.5% without a Buffer Layer

AbstractIn this communication, novel and simplified structure Cu(In,Ga)Se2 (CIGS) solar cells, which nominally consist of only a CIGS photoabsorber layer sandwiched between back and front contact layers but yet demonstrate high photovoltaic efficiencies, are reported. To realize this accomplishment, Si‐doped CIGS films grown by the three‐stage coevaporation method, B‐doped ZnO transparent conductive oxide front contact layers deposited by chemical vapor deposition, and heat–light soaking treatments are used. Si‐doping of CIGS films is found to modify the film surfaces and grain boundary properties and also affect the alkali metal distribution profiles in CIGS films. These effects are expected to contribute to improvements in buffer‐free CIGS device performance. Heat–light soaking treatments, which are occasionally performed to improve conventional buffer‐based CIGS device performance, are found to be also effective in enhancing buffer‐free CIGS photovoltaic efficiencies. This result suggests that the mechanism behind the beneficial effects of heat–light soaking treatments originates from CIGS bulk issues and is independent of the buffer materials. Consequently, over 16.5% efficiencies, including an independently certified value, are demonstrated from completely buffer‐free CIGS photovoltaic devices.

Optical Design of Textured Thin-Film CIGS Solar Cells with Nearly-Invisible Nanowire Assisted Front Contacts.

The conductivity of transparent front contacts can be improved by patterned metallic nanowires, albeit at the cost of optical loss. The associated optical penalty can be strongly reduced by texturization of the cell stack. Remarkably, the nanowires themselves are not textured and not covered in our design. This was shown by optical modeling where the width of the nanowire, the texture height and the texture period were varied in order to obtain a good insight into the general trends. The optical performance can be improved dramatically as the reflection, which is the largest optical loss, can be reduced by 95% of the original value. The spectra reveal absorption in the Cu(In,Ga)Se2 (CIGS) layer of 95% and reflection below 2% over a large part of the spectrum. In essence, a virtually black CIGS cell stack can be achieved for textured cells with a metal nanogrid. Moreover, it turned out that the ratio between the width of the nanowire and the height of the texture is a critical parameter for optical losses.

Silicon heterojunction solar cells with effectively transparent front contacts

Enhanced current density and efficiency through microscale triangular cross-section effectively transparent contacts (ETCs).

Effectively Transparent Front Contacts for Optoelectronic Devices

Effectively transparent front contacts for optoelectronic devices achieve a measured transparency of up to 99.9% and a measured sheet resistance of 4.8 Ω sq^(−1). The 3D microscale triangular cross-section grid fingers redirect incoming photons efficiently to the active semiconductor area and can replace standard grid fingers as well as transparent conductive oxide layers in optoelectronic devices.

Synthesis and some surface studies of laminated ZnO/TiO2 transparent bilayer by two-step growth

A laminated bilayer was prepared by first depositing titanium dioxide (TiO2) nanocrystals on indium tin oxide (ITO) coated glass by a two-electrode cell. Zinc oxide (ZnO) thin film was thereafter deposited on the TiO2 by two different techniques: electrochemical deposition and vacuum evaporation. The films were characterized by some surface probing techniques. Morphological study revealed that particle size of the TiO2 underlayer increases between 110 and 138nm with increase in deposition voltage. It also showed that ZnO thin film (overlayer) completely covered the underlying TiO2 without chemical interaction between constituents of both layers. Cross-sectional FESEM study gave values of layered film thickness below 55µm. Exhibition of strong diffraction peak at plane (121) indicated preference of TiO2 film's growth orientation. It also suggested a feature of phase-pure brookite. Optical studies showed that each film exhibited strong absorption edge at λ=~330nm and transmitted fairly across visible light region. Energy band gap lied between 3.24 and 3.43eV. This study demonstrated successive layer deposition of transparent metal oxide structures from inorganic reagents. It also reaffirmed TiO2 as a recipe for barrier layer that can hinder transition of holes from absorber to transparent front contact of nanostructured photonic devices.

Improved open-circuit voltage in Cu(In,Ga)Se2 solar cells with high work function transparent electrodes

Hydrogenated indium oxide (IOH) is implemented as transparent front contact in Cu(In,Ga)Se2 (CIGS) solar cells, leading to an open circuit voltage VOC enhanced by ∼20 mV as compared to reference devices with ZnO:Al (AZO) electrodes. This effect is reproducible in a wide range of contact sheet resistances corresponding to various IOH thicknesses. We present the detailed electrical characterization of glass/Mo/CIGS/CdS/intrinsic ZnO (i-ZnO)/transparent conductive oxide (TCO) with different IOH/AZO ratios in the front TCO contact in order to identify possible reasons for the enhanced VOC. Temperature and illumination intensity-dependent current-voltage measurements indicate that the dominant recombination path does not change when AZO is replaced by IOH, and it is mainly limited to recombination in the space charge region and at the junction interface of the solar cell. The main finding is that the introduction of even a 5 nm-thin IOH layer at the i-ZnO/TCO interface already results in a step-like increase in VOC. Two possible explanations are proposed and verified by one-dimensional simulations using the SCAPS software. First, a higher work function of IOH as compared to AZO is simulated to yield an VOC increase by 21 mV. Second, a lower defect density in the i-ZnO layer as a result of the reduced sputter damage during milder sputter-deposition of IOH can also add to a maximum enhanced VOC of 25 mV. Our results demonstrate that the proper choice of the front TCO contact can reduce the parasitic recombination and boost the efficiency of CIGS cells with improved corrosion stability.