Influence of Ga back grading on voltage loss in low-temperature co-evaporated Cu(In,Ga)Se2 thin film solar cells

The performance of Cu(In,Ga)Se2 (CIGS) solar cells is limited by the presence of the highly recombinative CIGS/Mo interface. The recombination at the CIGS/Mo interface is influential for the open circuit voltage (VOC) in high quality CIGS absorbers with increased charge carriers diffusion length. A quantitative understanding of the role of the Ga back grading height (ΔGGI) in suppressing back interface recombination is needed. In this work, we take advantage of a low temperature process to modify the ΔGGI while keeping the composition in the notch and front regions almost unchanged. Improvement in both VOC deficit and time-resolved photoluminescence lifetime are observed with increasing ΔGGI. With a combination of back surface modification experiments and numerical simulations, we quantify a voltage loss in ungraded devices of approximately 100 mV solely from the back interface recombination. Nice agreement between simulation and experimental data is reached while constraining the values of possible diffusion lengths. Our results suggest that a ΔGGI of about 0.50 is required to effectively suppress the back interface recombination, highlighting the importance of grading control in high-performance CIGS solar cells and devices.Bundesamt für Energie, Grant/Award Number: SI/501614-01; Horizon 2020 Framework Programme, Grant/Award Number: EMPIR project HyMet; Swiss State Secretary for Education, Research and Innovation (SERI), Grant/Award Number: 17.00105 (EMPIR project HyMet

Silver-promoted high-performance (Ag,Cu)(In,Ga)Se2 thin-film solar cells grown at very low temperature

Achieving high power conversion efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells grown at low temperature is challenging because of insufficient thermal energy for grain growth and defect annihilation, resulting in poor crystallinity, higher defect concentration, and degraded device performance. Herein, the possibilities for high-performing devices produced at very low temperatures (≤450 °C) are explored. By alloying CIGS with Ag by the precursor layer method, (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells grown at about 450 °C reach an efficiency of 20.1%. Only a small efficiency degradation (0.5% and 1.6% absolute) is observed for ACIGS absorbers deposited at 60 and 110 °C lower substrate temperature. CIGS devices exhibit a stronger efficiency degradation, driven by a decrease in the open-circuit voltage (Voc). The root cause of the Voc difference between ACIGS and CIGS devices is investigated by advanced characterization techniques, which show improved morphology, reduced tail states, and higher doping density in ACIGS absorbers. The proposed approach offers several benefits in view of depositions on temperature-sensitive substrates. Increased Cu diffusion promoted by Ag allows end-point detection in the three-stage process at the substrate temperatures below 300 °C. The modified process requires minimal modification of existing processes and equipment and shows the potential for the use of different flexible substrates and device architectures.This work received funding from the Swiss Federal Office of Energy (SFOE) under ImproCIS project (Contract no.: SI/501614-01) and from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the participating States and by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 850937. X.S. acknowledges funding from the ETH Zurich Postdoctoral Fellowship. M.K. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 850937. J.S. acknowledges funding from the Swiss National Science Foundation (grant number 200021_172764)

High-mobility In2O3:H electrodes for four-terminal perovskite/CuInSe2 tandem solar cells

Four-terminal (4-T) tandem solar cells (e.g., perovskite/CuInSe2 (CIS)) rely on three transparent conductive oxide electrodes with high mobility and low free carrier absorption in the near-infrared (NIR) region. In this work, a reproducible In2O3:H (IO:H) film deposition process is developed by independently controlling H2 and O2 gas flows during magnetron sputtering, yielding a high mobility value up to 129 cm2 V–1 s–1 in highly crystallized IO:H films annealed at 230 °C. Optimization of H2 and O2 partial pressures further decreases the crystallization temperature to 130 °C. By using a highly crystallized IO:H film as the front electrode in NIR-transparent perovskite solar cell (PSC), a 17.3% steady-state power conversion efficiency and an 82% average transmittance between 820 and 1300 nm are achieved. In combination with an 18.1% CIS solar cell, a 24.6% perovskite/CIS tandem device in 4-T configuration is demonstrated. Optical analysis suggests that an amorphous IO:H film (without postannealing) and a partially crystallized IO:H film (postannealed at 150 °C), when used as a rear electrode in a NIR-transparent PSC and a front electrode in a CIS solar cell, respectively, can outperform the widely used indium-doped zinc oxide (IZO) electrodes, leading to a 1.38 mA/cm2 short-circuit current (Jsc) gain in the bottom CIS cell of 4-T tandems.This work was supported by funding from the Swiss Federal Office of Energy (SFOE)-BFE (project no. SI/501805-01), Swiss National Science Foundation (SNF)-Bridge (project no. 20B2-1_176552/1), and the European Research Council (ERC) under EU’s Horizon 2020 Research and Innovation Program (grant agreement no. 681312). We thank Dr. Yi Hou for the supply of the antireflection foil

Investigation of interface and device properties in Cd-free CIGS thin film solar cells with vapor and plasma deposited ZnMgO buffer layers

Environmentally safe, sustainable and affordable generation of energy is able to solve the majority of humanity‘s energy related problems. With abundant and available power, energy intensive work like desalination of seawater, extraction of greenhouse gases from the atmosphere, recycling of valuable trace metals and transportation of any kind becomes feasible. Currently photovoltaics provide the most accessible and cheapest way of generating electricity. Solar cells based on Si are the dominant available technology, but alternatives promise faster energy payback times, less material consumption and new use-case scenarios. Thin film photovoltaic (TF-PV) systems based on the absorber material Cu (In,Ga)Se2 (CIGS) show high efficiencies of over 23 % and promise low cost manufacturing by using roll-to-roll fabrication methods. A major contributor to the complexity of the fabrication process and therefore cost is the hazardous chemical bath deposited cadmium sulfide (CBD-CdS) buffer layer. The wet deposition method breaks the vacuum and has low material yield, which leads to waste treatment and increases cost. Additionally, the low bandgap of CdS leads to parasitic absorbtion and therefore reduced current of the solar cell during operation. In this work, a substitute for CBD-CdS is investigated in both material and deposition method. Zn1-xMgxO is chosen as substitute buffer material due to its high bandgap, chemical similarity to CdS and the ability to modify the conduction band position. First, atomic layer deposition (ALD) is used to fabricate CIGS devices with Zn1-xMgxO buffer layer since this deposition method allows a high degree of control and ensures conformal coverage of the deposited film. The buffer layer stoichiometry, thickness and growth mode are investigated. Absorber surface treatments are implemented, to imitate vii viii Abstract the surface conditions present during the initial stages of CBD-CdS growth. In this proof of concept, the best solar cell with ALD Zn1-xMgxO reaches an efficiency of 18 % compared to 20.1 % of the CdS reference device. For industrial applications sputter deposition is preferred, since sputtering is faster, easier to scale and requires lower initial investment. Second, the feasibility of sputtering Zn1-xMgxO buffer layers is investigated. CIGS devices with sputtered Zn1-xMgxO exhibit device performances below 5 %, which is traced to sputter damage. Sputter damage in CIGS is an insufficiently studied phenomenon. Here, the impact of sputter damage on the carrier lifetime is quantified using time resolved photoluminescence (TRPL) before the completion of the device. The introduction of TRPL to investigate sputter damage marks a substantial improvement to the impedance based methods used before. Different possible mechanisms are discussed, the interaction of the absorber surface with Ar+ and O- ions being the most likely. Last, the nature of sputter damage is investigated and the involvement of Ar+ and O- are confirmed. Based on a physical, chemical and structural analysis of the sputter generated defects using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and capacitance frequency analysis (Cf), strategies to mitigate sputter damage are proposed and tested. Sputtering at lower powers and without oxygen result in a better device performance. Additionally, absorber surface sulfurisation was used to create a barrier in the valence band, which increase the device performance by reducing interface recombination and ultimately makes the solar cell more resistant against sputter damage. The results of this study are the proof of concept of ALD Zn1-xMgxO buffer layers in high efficiency CIGS solar cells, which allows to substitute CBD-CdS on laboratory scale. Further, with the introduction of TRPL as a method to quantify sputter damage and the consequent analysis thereof, the underlying mechanism could be identified to be the interaction of the absorber surface with Ar+ and O-. This resulted in the creation of several methods to increase device performance when sputtered buffer layers are used, the implementation of which doubled the initial 5 % device efficiency to 10 %

Modification of the Schottky Barrier Height at the RuO2 Cathode During Resistance Degradation of Fe-doped SrTiO3

The long‐term stability of electronic devices at high temperatures and electric fields might be strongly influenced by the electronic properties of interfaces. A modification of Schottky barrier heights at electrode interfaces of functional oxides upon changes of the external oxygen partial pressure is well documented in literature. In this work, an experimental approach using X‐ray photoelectron spectroscopy is presented, which enables to study transient changes in the Schottky barrier height induced by electrical degradation. A rise of the Fermi level at the RuO2 cathode interface of Fe‐doped SrTiO3 single crystals by 0.6 eV is observed in the course of resistance degradation. The change of the effective barrier height is associated to the migration of oxygen vacancies towards the cathode and accompanied by the observed reduction of Ti. Different scenarios are discussed to explain the origin of barrier modification and the localization of the reduced Ti

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

High efficiency chalcopyrite thin film solar cells generally use chemical bath deposited CdS as buffer layer. The transition to Cd-free buffer layers, ideally by dry deposition methods is required to decrease Cd waste, enable all vacuum processing and circumvent optical parasitic absorption losses. In this study, Zn1−xMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers. A specific composition range was identified for a suitable conduction band alignment with the absorber surface. We elucidate the critical role of the CIGS surface preparation prior to the dry ALD process. Wet chemical surface treatments with potassium cyanide, ammonium hydroxide and thiourea prior to buffer layer deposition improved the device performances. Additional in-situ surface reducing treatments conducted immediately prior to Zn1−xMgxO deposition improved device performance and reproducibility. Devices were characterised by (temperature dependant) current-voltage and quantum efficiency measurements with and without light soaking treatment. The highest efficiency was measured to be 18%