Transparent Back Contact Research Articles

Bifacial and Angular‐Resolved Performance Characterization of Ultrathin Cu(In,Ga)Se2 Solar Cells Including Nanostructures

Bifacial solar cells experience growing interest not just for crystalline silicon photovoltaic modules. Thin‐film solar cells deposited on a transparent back contact bring inherent semitransparency, making them ideally suited for bifacial applications. Herein, a systematic investigation of bifacial measurement procedures is performed on semitransparent ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide, including nanostructures. The measurements are further extended by angular‐resolved performance studies. The bifaciality of the samples is determined to be ≈80% in current and ≈65% in power, and enables the calculation of an equivalent irradiance for solar cell testing under >1 sun front illumination only. The results are compared to bifacial operation, i.e., simultaneous front and rear irradiance, and to the summation of individual front and rear performance measurements up to 1 sun. It is revealed that highly similar results can be obtained for these approaches and that the integration of nanostructures supports device stabilization. Particularly, the higher (75 nm) SiO2 nanomeshes can enable performance enhancement. Furthermore, the angular‐dependent behavior follows the expected trend of reduced illumination intensity according to the cosine of the incident angle. In these findings, the suitability of semitransparent ultrathin CIGSe solar cells for bifacial operation and the benefit of integrated nanostructures is confirmed.

Bifacial Wide‐Gap (Ag,Cu)(In,Ga)Se2 Solar Cell with 13.6% Efficiency Using In2O3:W as a Back Contact Material

This study evaluates In2O3:W as a transparent back contact material in wide‐gap (bandgap range = 1.44–1.52 eV) (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells for potential application as a top cell in a tandem device. High silver concentrations and close‐stoichiometric absorber compositions result in a complete depletion of free charge carriers, allowing for decent electron collection, despite the low diffusion length. Remarkable efficiencies of 13.6% and 7.5% are reached using 1 μm‐ and 400 nm‐thick absorbers, respectively. At rear illumination (i.e., superstrate backwall), the best cell shows an efficiency of 8.7%. For each of the four analyzed samples, the short‐circuit current at rear illumination reaches at least 60% of the value at front illumination. Losses arise from recombination at the back contact and a too low drift/diffusion length. The parasitic absorption by the transparent electrodes for photon energies close to the bandgap of a potential Si bottom cell (1.1 eV) is close to 15%. Strategies to reduce this value and to further increase the efficiency are discussed.

Four‐Terminal Perovskite–CdSeTe Tandem Solar Cells: From 25% toward 30% Power Conversion Efficiency and Beyond

Thin‐film tandem photovoltaic (PV) technology has emerged as a promising avenue to enhance power conversion efficiency beyond the radiative efficiency limit of single‐junction devices. Combining a tunable wide‐bandgap perovskite cell with a commercially established narrow‐bandgap cadmium selenium telluride (CdSeTe) cell in a comparatively easy‐to‐fabricate four‐terminal (4‐T) arrangement is a great step in that direction. Herein, the impact of the transparent back contact and the perovskite absorber bandgap on the performance of 4‐T perovskite–CdSeTe tandem solar cells is investigated. 4‐T perovskite–CdSeTe tandem device architecture with ≈25% efficiency is demonstrated and a feasible pathway is shown to improve the tandem efficiency to more than 30%. The results show that the integration of CdSeTe with perovskite in 4‐T tandem PV configurations represents a significant advancement toward achieving higher efficiency and low‐cost tandem PVs.

Hierarchical Transparent Back Contacts for Bifacial CdTe PV.

A hierarchical transparent back contact leveraging an AlGaOx passivating layer, Ti3C2Tx MXene with a high work function, and a transparent cracked film lithography (CFL) templated nanogrid is demonstrated on copper-free cadmium telluride (CdTe) devices. AlGaOx improves device open-circuit voltage but reduces the fill factor when using a CFL-templated metal contact. Including a Ti3C2Tx interlayer improves the fill factor, lowers detrimental Schottky barriers, and enables metallization with CFL by providing transverse conduction into the nanogrid. The bifacial performance of an AlGaOx/Ti3C2Tx/CFL gold contact is evaluated, reaching 19.5% frontside efficiency and 2.8% backside efficiency under 1-sun illumination for a copper-free, group-V doped CdTe device. Under dual illumination, device power generation reached 200 W/m2 with 0.1 sun backside illumination.

Impact of Band‐gap Gradient in Semi‐Transparent and Bifacial Ultra‐Thin Cu(In,Ga)Se2 Solar Cells

AbstractUltra‐thin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide back contact reduce the material consumption of rare indium and gallium and simultaneously exhibit great potential for semi‐transparent bifacial application. For highly efficient CIGSe solar cells, a steep back Ga grading and Na treatment are expected. However, Na will promote the formation of highly resistive GaOx at the rear interface owing to Ga accumulation. In this work, the three‐stage co‐evaporation process is renewed and the effect of the deposition sequence in the first stage on the Ga distribution as well as the cross‐correlated influence of Na is explored. In particular, the standard deposition sequence of Ga+In is altered to start with In. When a thin In layer is pre‐deposited on the back contact, the fill factor and efficiency increase. The deposition of In+Ga+In in the first stage of CIGSe growth leads to efficiencies 28% (on average) higher than for the standard deposition sequence of Ga+In. Additionally, 2.74% efficiency is reached under rear and 9.32% under simultaneous front and rear illumination. Therefore, adapting the deposition sequence in the first stage of CIGSe growth is identified as a key to improving the device performance on transparent back contact.

Toward improving the performance of Cu2ZnSnS4-based solar cells with Zr, W or sulfurized layers at the SnO2:F/Cu2ZnSnS4 rear interface

The photovoltaic performance of Cu2ZnSnS4 (CZTS)-based thin film solar cells with transparent back contacts is mainly constrained by the ohmic loss due to degradation of the back contacts at high temperatures required for the growth of the CZTS absorbers. This work has attempted to use ultrathin Zr and W interlayers at the SnO2:F (FTO)/CZTS interface with the purpose of improving the ohmic properties of FTO and hence the performance of solar cells. 20 nm thick Zr and W layers were coated on FTO using direct current magnetron sputtering, followed by heating at 550 °C in a sulfur atmosphere for some of the samples. Inclusion of Zr and W interlayers compromised the transmittance of the FTO back contact, however heating of the samples in a sulfur atmosphere improved the transmittance to values comparable to or better than those of un-heated FTO. Furthermore, heated W/FTO showed a significant improvement in electrical conductivity as observed from Hall effect measurements. To make complete solar cells, CZTS absorber, buffer (CdS) and window (i-ZnO/ZnO:Al) layers were sequentially deposited on the un-heated FTO, Zr/FTO, W/FTO and Mo back contacts. Glow discharge optical emmision spectroscopy confirmed that Zr and W did not diffuse into the absorber and also prevented Na diffusion into the absorber. From the scanning electron microscopy cross sectional analysis, an impovement in the absober grain size and a clear junction between the absorber and FTO with W interlayer were observed. Grazing incident X-ray diffractometry confirmed the polycrystalline nature of the CZTS thin films and indicated the presence of other phases, particularly SnS2. On the resulting solar cell parameters, the inclusion of a W interlayer reduced the series resistance from 4.5 Ωcm to 3.7 Ωcm2. An improvement in short circuit current density (JSC) is also observed, enhancing the efficiency of the solar cells with CZTS/W/FTO to 3.1 % compared to that with CZTS/Mo at 2.7 % and CZTS/FTO at 3.0 %. W was found to improve the external quantum efficiency response and JSC of the solar cells for both backside and frontside light illumination. On the other hand, inclusion of a Zr interlayer (Zr/FTO) only slighlty improved the open circuit voltage, but compromised the JSC compared to FTO and W/FTO back contacts. Thus, the findings of this study demonstrate the prospect for improving the performance of the CZTS-based thin film solar cells through the inclusion of ultrathin Zr and W interlayers between the FTO back contact and CZTS absorber.

Radio-frequency magnetron sputtering deposition process for In2O3:H transparent conductive oxide films for application in Cu(In,Ga)Se2 solar cells

Typical Cu(In,Ga)Se2 solar cells are deposited on an opaque molybdenum back contact. However, for applications such as bifacial, semi-transparent or tandem solar cells a transparent back contact is required, for which various transparent conductive oxides have been tested, such as indium- or fluorine-doped tin oxide, and hydrogen-doped indium oxide. Here, a radio-frequency magnetron sputtering deposition process for In2O3:H (IOH) is investigated, where H is supplied from a Ar/H2 (5%) mixed gas and oxygen is pulsed during the entire deposition at room temperature. After deposition, the films are post-annealed in vacuum to optimize their optoelectronic properties. The oxygen plays an important role for the optoelectronic properties, where a high content of oxygen allows higher transparency but also increases the film sheet resistance. Optimum oxygen and Ar/H2 partial pressures of 3.2 × 10−2 Pa and 13×10−2 Pa, respectively, were found, producing IOH films with average visible transparency of 87% and sheet resistance after annealing of 19 Ohm/sq.

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.

2‐step process for 5.4% CuGaSe2 solar cell using fluorine doped tin oxide transparent back contacts

AbstractAs single‐junction solar cells are approaching theoretical limits, multijunction solar cells are becoming increasingly relevant, and low‐cost wider bandgap light harvesters in tandem with silicon are the next frontier in thin film photovoltaic research. Cu‐based chalcogenide compounds have achieved great success as standard absorbers, but performance for bandgaps above 1.5 eV is still lacking. Additionally, the use of transparent back contacts remains challenging for this class of materials. In this work, we report on the fabrication of wide bandgap CuGaSe2 absorbers by a combination of metallic sputtering and reactive thermal annealing grown on transparent fluorine‐doped tin oxide‐coated glass substrate. The annealing temperature is carefully tuned in regard to material and photovoltaic device properties. The introduction of an ultrathin Mo interlayer at the CuGaSe2/back interface favors a higher contact's ohmicity and results in an important improvement of all figures of merit. A record conversion efficiency of 5.4% is obtained, which is the highest value reported for this class of absorber on transparent back contact. Fundamental material characterization of the as‐grown CuGaSe2 films reveals a better homogeneity in Cu distribution throughout the absorber's thickness when using a Mo interlayer, along with an enhanced crystalline quality. The sub‐bandgap transparency of the final device remains perfectible, and improvement pathways are proposed using transfer matrix‐based optical modeling, suggesting to use more specular interfaces to enhance optical transmission.

Wide‐Gap Chalcopyrite Solar Cells with Indium Oxide–Based Transparent Back Contacts

Herein, the performance of wide‐gap Cu(In,Ga)Se2 (CIGS) and (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with In2O3:Sn (ITO) and In2O3:H (IOH) as transparent back contact (TBC) materials is evaluated. Since both TBCs restrict sodium in‐diffusion from the glass substrate, fine‐tuning of a NaF precursor layer is crucial. It is found that the optimum Na supply is lower for ACIGS than for CIGS samples. An excessive sodium amount deteriorates the solar cell performance, presumably by facilitating GaO x growth at the TBC/absorber interface. The efficiency (η) further depends on the absorber stoichiometry, with highest fill factors (and η) reached for close‐stoichiometric compositions. An ACIGS solar cell with η = 12% at a bandgap of 1.44 eV is processed, using IOH as a TBC. The best CIGS device reaches η = 11.2% on ITO. Due to its very high infrared transparency, IOH is judged superior to ITO for implementation in a top cell of a tandem device. However, while ITO layers maintain their conductivity, IOH films show an increased sheet resistance after absorber deposition. Chemical investigations indicate that incorporation of Se during the initial stage of absorber processing may be responsible for the deteriorated conductivity of the IOH back contact in the final device.

Bifacial semi-transparent ultra-thin Cu(In,Ga)Se2 solar cells on ITO substrate: How ITO thickness and Na doping influence the performance

Ultra-thin Cu(In,Ga)Se2 (CIGSe) is a promising absorber for thin-film solar cells, as it combines the advantages of low raw material consumption and high conversion efficiency. In addition, ultra-thin absorbers on transparent back contacts bring the advantage of semitransparency, which is essential for e.g. tandem or bifacial solar cells. This work optimizes ultra-thin CIGSe on In2O3:Sn (ITO) for application in bifacial semi-transparent ultra-thin (BSTUT) CIGSe solar cells. Firstly, 100–400 nm ITO were coated onto glass substrates, and it was revealed that the thickness of ITO influences its optical bandgap Eg due to the Burstein-Moss (B-M) shift. The band gap of 400 nm ITO increased by 0.14 eV compared to the 100 nm thick ITO, and the Voc of the related BSTUT CIGSe solar cells raised by 0.043 V as a result of the diminished Schottky barrier Φb at the ITO/CIGSe interface. Secondly, 0–8 mg of NaF used for post deposition treatment (PDT) of the CIGSe were applied to the BSTUT solar cells. Compared to the reference without NaF, 8 mg NaF PDT enhanced the carrier density NA from 2 × 1015cm−3 to 1.2 × 1016cm−3 and diminished the ITO/CIGSe Schottky barrier Φb by 0.21 eV. In conclusion, we found that NaF PDT can tune the carrier density of ultra-thin CIGSe on ITO, and both thicker ITO and higher NaF PDT dose can reduce the ITO/CIGSe Schottky barrier. These discoveries enable future optimization of BSTUT CIGSe solar cells.

Sodium control in Ultrathin Cu(In,Ga)Se2 solar cells on transparent back contact for efficiencies beyond 12%

Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide (TCO) back contacts combine advantages of ultrathin cells for reducing material consumption of rare indium and gallium and TCO-transparency benefited applications in tandems, bifacial configurations etc. However, their efficiencies are still limited and the back barrier potential is a primary reason from an electrical perspective. In this work, we explore the effects of Nadoping by post deposition treatment (PDT) on the performance of ultrathin CIGSe solar cells on ITO (Sn:In2O3)-coated Na-free glass substrates. Na doping enhances not only the open circuit-voltage (Voc) by increasing the doping level, but also the fill factor (FF) by switching the Schottky contact to an Ohmic contact at the CIGSe/ITO interface, which we propose is due to the increased recombination at the back interface. The optimum performance is achieved at a NaF dose of 2 mg with a top efficiency of 12.9%, which exhibits an enhancement by nearly 48% relative compared to the references without Na doping. To our best knowledge, this is the highest efficiency achieved for ultrathin cells (<500 nm absorber thickness) on TCO without additional antireflection or back reflecting layer. Therefore, the results show that sodium control offers a solid basis for the development of ultrathin CIGSe cells on TCO in above-mentioned promising applications.

The path towards efficient wide band gap thin-film kesterite solar cells with transparent back contact for viable tandem application

Wide band gap thin-film kesterite solar cell based on non-toxic and earth-abundant materials might be a suitable candidate as a top cell for tandem configuration in combination with crystalline silicon as a bottom solar cell. For this purpose and based on parameters we have extracted from electrical and optical characterization techniques of Cu2ZnGeSe4 absorbers and solar cells, a model has been developed to describe the kesterite top cell efficiency limitations and to investigate the different possible configurations with transparent back contact for four-terminal tandem solar cell application. Furthermore, we have studied the tandem solar cell performance in view of the band gap and the transparency of the kesterite top cell and back contact engineering. Our detailed analysis shows that a kesterite top cell with efficiency > 14%, a band gap in the range of 1.5–1.7 eV and transparency above 80% at the sub-band gaps photons energies are required to achieve a tandem cell with higher efficiency than with a single silicon solar cell.

Semitransparent CdTe solar cell with over 70% near-infrared transmittance

CdTe solar cell is a promising alternative to conventional silicon solar cells and it is quite potential to be utilized as a top sub-cell for tandem solar cells to improve the power conversion efficiency of the existing single-junction devices, whose efficiency has been approaching their practical efficiency limit. Thus, the research and development of semitransparent CdTe solar cells are urgently required. In this work, semitransparent CdTe solar cells were prepared by utilizing 900-nm-thick ultrathin CdTe absorber and CuCl/ITO transparent back contacts. The ultrathin CdTe in the present work was prepared by magnetron sputtering and the effects of substrate temperature and deposition ambient were extensively investigated. CdTe thin films with minimal dislocation density and lowest internal strain were fabricated at a substrate temperature of 235 °C with pure Ar atmosphere. The improved optical–electrical properties of CdTe absorber were then contributing on device performance, and as a result, we successfully fabricated a semitransparent CdTe solar cell with an optimal efficiency of 8.60%. It is worth noting that the transmittance around 1000 nm in the near-infrared region (NIR) is over 70%, which is the highest value among the as-reported semitransparent CdTe solar cells. The present work paves a way to optimize the performance and improve the optical transmittance of semitransparent CdTe solar cells so that it strongly supports their potential applications in bifacial and/or tandem configurations, building integrated photovoltaics, and so on.

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se)2 films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se)2 can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd1−xZnxS or Zn1−xSnxOy, result in higher efficiencies than devices with CdS buffers when sulfur-rich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells.

Antimony‐Doped Tin Oxide as Transparent Back Contact in Cu2ZnSnS4 Thin‐Film Solar Cells

Antimony‐doped tin oxide (Sn2O3:Sb, ATO) is investigated as a transparent back contact for Cu2ZnSnS4 (CZTS) thin‐film solar cells. The stability of the ATO under different anneal conditions and the effect from ATO on CZTS absorber growth are studied. It is found that ATO directly exposed to sulfurizing anneal atmosphere reacts with S, but when covered by CZTS, it does not deteriorate when annealed at T &lt; 550 °C. The electrical properties of ATO are even found to improve when CZTS is annealed at T = 534 °C. At T = 580 °C, it is found that ATO reacts with S and degrades. Analysis shows repeatedly that ATO affects the absorber growth as large amounts of Sn−S secondary compounds are found on the absorber surfaces. Time‐resolved anneal series show that these compounds form early during anneal and evaporate with time to leave pinholes behind. Device performance can be improved by addition of Na prior to annealing. The best CZTS device on ATO back contact herein has an efficiency of 2.6%. As compared with a reference on a Mo back contact, a similar open‐circuit voltage and short‐circuit current density are achieved, but a lower fill factor is measured.

Development of Back and Front Contacts for CdTe Layer in Tandem Flexible Photoelectric Converters on Basis of CdTe/CuInSe2

By the method of nonreactive high-frequency magnetron sputtering on Upilex polyimide films, transparent and conductive layers of ITO were obtained. These layers, after high-temperature annealing, at temperatures typical for the solar cell formation, had a resistance of 11 ohm/□ and a transmittance of up to 72%. The use of such an ITO layer with the addition of a 100 nm thick layer of undoped zinc oxide, as the front contact, and Cu/ITO composition, as the back contact, made it possible to obtain a flexible solar cell polyimide/ITO/CdS/CdTe/Cu/ITO with an efficiency of 10.4%. With a thickness of the base layer of cadmium telluride 2.5 μm, the average transmittance of the SC in the 850-1100 nm wavelength range is 46.8%. The developed design of a flexible solar cell based on cadmium telluride due to the use of a transparent back contact with a comb metal electrode is easily interfaced with existing designs of flexible solar cells based on copper and indium diselenide, which allow the formation of flexible tandem photoelectric converters CdTe/CuInSe2.

Self-Integrated Hybrid Ultraviolet Photodetectors Based on the Vertically Aligned InGaN Nanorod Array Assembly on Graphene.

Integration of one-dimensional (1D) semiconductors with two-dimensional (2D) materials into hybrid systems is identified as promising applications for new optoelectronic and photodetection devices. Herein, a self-integrated hybrid ultraviolet (UV) photodetector based on InGaN nanorod arrays (NRAs) sandwiched between transparent top and back graphene contacts forming a Schottky junction has been demonstrated for the first time. The controlled van der Waals epitaxy of the vertically aligned InGaN NRA assembly on graphene-on-Si substrates is achieved by plasma-assisted molecular beam epitaxy. Moreover, the self-assembly formation mechanisms of InGaN NRAs on graphene are clarified by theoretical calculations with first-principles calculations based on density functional theory. The peculiar 1D/2D heterostructure hybrid system-based integrated UV photodetector simultaneously exhibits ultrafast response time (∼50 μs) and superhigh photosensitivity (∼105 A/W). It is highly believed that the concept proposed in this work has a great potential and can be widely applied for the next-generation integrated 1D/2D nano-based optoelectronic and photodetection devices.

Control of Structural and Electrical Properties of Indium Tin Oxide (ITO)/Cu(In,Ga)Se2 Interface for Transparent Back-Contact Applications

Development of transparent-conducting oxide (TCO) back contact for Cu(In,Ga)Se2 (CIGS) absorber is crucial for bifacial CIGS photovoltaics. However, inherent GaOx formation at the TCO/CIGS interface has hampered the photocarrier extraction. Here, by controlling the Na doping scheme, we show that the hole transporting properties at the indium–tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaOx formation. Na incorporation from the glass substrate during the GaOx forming phase created defective states at the interface, which allowed efficient hole extraction from CIGS, while post Na treatment after GaOx formation did not play such a role. Furthermore, we discovered that an almost GaOx-free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaOx-free condition, post-Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the i...

Development of reflective back contacts for high-efficiency ultrathin Cu(In,Ga)Se2 solar cells

Because of poor light absorption, Cu(In,Ga)Se2-based (CIGS) solar cells with an ultrathin absorber layer (<500 nm) require the development of reflective back contacts. To enhance rear reflectance in CIGS ultrathin devices, we investigate novel back contact architectures based on a silver metallic mirror covered with a thin layer of In2O3:Sn (ITO), which is fully compatible with nanopatterning for further light trapping improvements. First, numerical electromagnetic simulations of complete solar cells have been performed for a 490 nm thick CIGS absorber with various back contacts. We predict a short-circuit current density of JSC = 34.0 mA/cm2 for a 490 nm thick CIGS absorber with a silver nanostructured mirror. Second, we have fabricated and characterized 490 nm thick CIGS solar cells with transparent back contacts made of ITO, and reflective back contacts made of silver covered with ITO. Solar cells with a transparent ITO back contact exhibit an average efficiency of 10.0%, compared to 9.3% for standard molybdenum back contacts. A 5 nm thick Ga2O3 layer is revealed at the ITO/CIGS interface by transmission electron microscopy and energy dispersive X-ray spectroscopy. When silver is added, the reflective back mirror leads to a JSC improvement of 4.6 mA/cm2 (from 22.4 to 27.0 mA/cm2). These results pave the way for efficient ultrathin CIGS solar cells on reflective back contacts.