Heat-light Soaking Research Articles

Comparison of polycrystalline and epitaxial Cu(In, Ga)Se2 solar cells with conversion efficiencies of more than 21%

Polycrystalline Cu(In,Ga)Se2 (CIGS) solar cells are widely studied for their application in thin-film photovoltaics, but analysis of their crystalline properties is difficult owing to the presence of grain boundaries and defects. Epitaxial CIGS (epi-CIGS) solar cells can potentially address the issues of polycrystalline CIGS (poly-CIGS) solar cells, but their conversion efficiency has not been widely studied. We fabricated epi-CIGS layers using techniques developed for high-efficiency ploy-CIGS solar cells such as Ga grading, Na doping, and heat-light soaking. Here, scanning transmission electron microscopy and secondary ion mass spectrometry were used to compare the structural characteristics of poly- and epi-CIGS solar cells with conversion efficiencies of more than 21%. Both types of solar cells had similar Ga gradient profiles, and abrupt interfaces were observed between the CIGS and Mo, CIGS and GaAs, and CdS and CIGS layers. The poly-CIGS layer had a higher Na concentration than the epi-CIGS layer because the grain boundaries of the former included high concentrations of Na ions. Meanwhile, the Na ions in the epi-CIGS layer promoted the interdiffusion of In and Ga. For both types of solar cells, heat-light soaking increased the carrier concentration by more than 1 × 1017 cm−3. These observations suggest that grain boundaries are not the main factor limiting the conversion efficiency of poly-CIGS solar cells.

Radiation resistant chalcopyrite CIGS solar cells: proton damage shielding with Cs treatment and defect healing via heat-light soaking

Cu(In, Ga)Se2 (CIGS) solar cells are recognized as next-generation space technology due to their flexibility, lightweight nature, and excellent environmental stability.

Analysis of the combined effect of long-term heat light soaking and KF/NaF post-deposition treatment on the open-circuit voltage loss in CIGS solar cells

Heat light soaking (HLS) has been known to impact the photovoltaic parameters of Cu(In,Ga)Se2 (CIGS) solar cells for a long time. Recently, the focus shifted to the effect of the procedure on alkali fluoride-treated CIGS. Here, we investigate the impact of long-term HLS on the open-circuit (V OC) loss in high-efficiency CIGS with potassium fluoride (KF) and sodium fluoride (NaF) post-deposition treatment (PDT). HLS is shown to increase the net doping density, however, the subsequent improvement of the V OC is lower than expected. Using an analysis based on the SQ theory, we show that HLS reduces the nonradiative recombination rate in the bulk but increases the one at the interface. We present a model to explain the increase of interface recombination. We further demonstrate that a combination of HLS and KF/NaF-PDT is necessary to enhance the positive impacts of HLS and mitigate the detrimental ones leading to high-efficiency CIGS devices (22%).

Effects of alkali-metal incorporation into epitaxial Cu(In,Ga)Se2 solar cells prepared by molecular beam epitaxy

Cu(In,Ga)Se2 (CIGS) solar cells with energy conversion efficiencies of more than 20% have been reported by several groups; most studies that reported such high efficiencies employed alkali-metal fluoride postdeposition treatments (PDTs) and Na doping. In this study, we investigated the effects of KF-PDT and NaF doping on epitaxial CIGS solar cells grown on GaAs substrates. We fabricated epitaxial CIGS layers without grain boundaries by molecular beam epitaxy methods at a substrate temperature of 520 °C. NaF doping and KF-PDT had positive effects on conversion efficiencies, even when using Cu stoichiometric CIGS layers. A conversion efficiency of more than 20% was achieved using Cu poor CIGS with NaF doping and KF-PDT after heat light soaking. Time-resolved photoluminescence was used to compare the minority carrier lifetimes of CdS-capped epitaxial CIGS layers with and without alkali-metal doping. NaF doping and KF-PDT increased minority carrier lifetimes similarly as in polycrystalline CIGS solar cells.

Effect of cesium fluoride treatment in bifacial Cu(In1-x,Gax)Se2 solar cell

Bifacial Cu(In1-x,Gax)Se2 (CIGS) thin film solar cells were fabricated on titanium-doped indium oxide (ITiO) coated soda-lime glass substrate, and the effect of cesium fluoride (CsF) treatment on solar cell performance was investigated. CsF treatment improved the photovoltaic properties of CIGS/ITiO-based solar cells not only under front-side illumination but also under rear-side illumination. The quantum efficiency for rear-side illumination improved significantly in CsF-treated CIGS solar cells. As a result, the bifaciality improved in the CsF-treated CIGS solar cell. ITiO exhibited good contact with the CIGS thin film, regardless of Cs incorporation. In addition, the effect of heat-light soaking treatment was also investigated in CIGS/ITiO-based solar cells.

Temperature‐dependent current–voltage and admittance spectroscopy analysis on cesium‐treated Cu (In1 − x,Gax)Se2 solar cell before and after heat‐light soaking and subsequent heat‐soaking treatments

AbstractRecently, we demonstrated the positive effects of heat‐light soaking (HLS) and subsequent heat‐soaking (HS) on cesium fluoride (CsF) treated Cu(In1‐x,Gax)Se2 (CIGS) solar cells. However, the role of defects formation and its influence on the electronic properties have not been analyzed. With this motivation, here, we analyzed the electronic properties of CsF‐free and CsF‐treated CIGS solar cells before and after HLS and subsequent HS treatments using temperature‐dependent current–voltage (J–V–T), admittance and low‐temperature capacitance–voltage (C–V) measurements. We noticed that CsF‐treated CIGS solar cells form a minority carrier trap level after HLS. The subsequent HS treatment was found to be beneficial to compensate this defect level. The admittance measurement showed a shift of the shallow energy position to a higher value after HLS and subsequent HS treatments, irrespective of Cs incorporation. This is expected to be due to the formation of a secondary diode toward the CIGS/molybdenum contact. The positive and negative effects of HLS and subsequent HS treatments on CsF‐treated CIGS solar cell are discussed using low‐temperature C–V measurements. By optimizing the HLS and HS processes, CsF‐treated CIGS solar cells yielded total efficiencies of over 20%.

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

Metastable behavior of cesium fluoride (CsF)‐treated Cu(In1–x,Gax)Se2 (CIGS) solar cells is investigated under heat‐light soaking (HLS) and heat‐soaking (HS) treatments. HLS increases open‐circuit voltage, fill factor, efficiency, and net carrier concentration and decreases short‐circuit current density, whereas heat‐soaking treatment acts oppositely. The performance of a CsF postdeposition treatment to CIGS thin film in selenium vapor, and closer to stoichiometry copper content, did not mitigate the open‐circuit voltage improvement after HLS. These results argue the traditional concept of the VSe–VCu divacancy complex for the total beneficial effect of HLS in alkali‐treated CIGS solar cells. The metastable behavior observed in the CsF‐treated devices due to the HLS and HS treatments is explained by the specific behavior of alkali‐containing new compounds at the surface and/or the migration of alkali metals at the surface and bulk regions.

Impact of heat‐light soaking and heat‐bias soaking on NaF‐treated CIGS thin film solar cells

AbstractThe effects of heat‐light soaking (HLS) and heat‐bias soaking (HBS) were investigated on NaF‐treated Cu (In,Ga)Se2 (CIGS) solar cells. The HLS improved the cell efficiency with increased open‐circuit voltage (VOC) for NaF‐treated CIGS solar cells, whereas the short‐circuit current density (JSC) slightly decreased. The reduced JSC was caused by the significantly high carrier concentration due to the HLS, leading to a fairly narrow space charge region. However, the reduced JSC could be successfully recovered by the subsequent moderate heat soaking (HS) in the dark, thereby improving cell efficiency. On the other hand, no beneficial effect was observed for NaF‐free solar cells. Secondary ion mass spectrometry (SIMS) measurements revealed that the Na and K ions increased especially near the CIGS surface region after the HLS process. This result suggests that these ions were shifted by the photo‐induced electric field. The thermally and electrically activated alkali ions may eliminate the donor‐like defects in CIGS absorber, resulting in an increased acceptor concentration. Furthermore, we found that an external forward bias without light irradiation (HBS) resulted in the same effect as HLS. Based on these results, a possible mechanism of HLS and HBS effects was proposed on NaF‐treated CIGS solar cells.

I2 vapor-induced degradation of formamidinium lead iodide based perovskite solar cells under heat–light soaking conditions

We reveal an iodine vapor-induced degradation mechanism in formamidinium–lead-iodide-based perovskite solar cells stressed under combined heat and light illumination.

Effects of RbF postdeposition treatment and heat-light soaking on the metastable acceptor activation of CuInSe2 thin film photovoltaic devices

Granular particles formed on a CuInSe2 (CIS) thin film surface with alkali-halide RbF post-deposition treatment (RbF-PDT) are found to concentrate preferably on other than smooth (112)-planes present on the CIS film surface. As a consequence, a large number of distinctive pores are selectively formed at relatively rough facets. Although the open circuit voltage and fill factor values of CIS solar cells improve with either RbF-PDT or heat-light soaking (HLS) treatment, the effects of these treatments are distinguishable. The ratio of carrier recombination rates at the interface (Ri) to that in the bulk (Rb), namely, Ri/Rb, decreases with RbF-PDT. HLS treatment performed on RbF-PDT CIS devices leads to a further decrease in the value of Ri/Rb, whereas the value of Ri/Rb of CIS devices fabricated without RbF-PDT increases with HLS treatment. Ternary CIS solar cells grown with RbF-PDT and HLS treatment reproducibly demonstrate enhanced photovoltaic efficiencies.

Single-crystal Cu(In,Ga)Se2 solar cells grown on GaAs substrates

Single-crystal Cu(In,Ga)Se2 (CIGS) solar cells were produced with techniques developed for high-efficiency polycrystalline CIGS solar cells. The CIGS layers of a lattice match with GaAs were grown on GaAs(001) substrates by co-evaporation. The presence of a single-crystal CIGS layer without dislocations was confirmed by transmission electron microscopy. Alkaline metal incorporations were achieved by doping and postdeposition treatments. Ga grading structures were fabricated by two-layer deposition with different Ga contents. The Ga grading significantly increased the fill factor and open-circuit voltage. The best efficiency of 20% was achieved after heat–light soaking.

Enhancement of photovoltaic performances of Cu (In,Ga)(S,Se)2 solar cell through combination of heat‐light soaking and light soaking processes

AbstractPotassium‐treated Cu (In,Ga)(S,Se)2 (CIGSSe)‐based solar cell with power conversion efficiency (η) of 19.4% is obtained using high transparent Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al layers to minimize optical loss at short wavelength (~520 nm) and to control total conduction band minimum alignment. To further enhance η, the post treatment named HLS + LS process, including heat‐light soaking (HLS) at 110°C under AM 1.5G illumination followed by light soaking (LS) under AM 1.5G illumination, is conducted successively on the as‐fabricated solar cell. It is revealed that HLS in the HLS + LS process mainly yields the increase in open‐circuit voltage. On the other hand, LS in the HLS + LS process primarily leads to the increase in fill factor, attributable to the decrease in sheet resistance of Zn0.88Mg0.12O:Al. The HLS + LS process consequently gives rise to not only the enhancement of carrier concentration but also the decrease in the recombination rate at the buffer/absorber interface through passivating the recombination centers. As a result, 21.2%‐efficient CIGSSe solar cell with the Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al layers is attained after the HLS + LS process, which is an effective process to enhance photovoltaic performances.

An over 18%-efficient completely buffer-free Cu(In,Ga)Se2 solar cell

In this letter, an independently certified photovoltaic efficiency of 18.4% demonstrated from a completely buffer-layer-free Cu(In,Ga)Se2 (CIGS) solar cell is reported. A Si-doped CIGS thin film was used as the photoabsorber layer and a conductive B-doped ZnO (BZO) front electrode layer was directly deposited on the CIGS layer. Metastable acceptor activation by heat-light soaking treatment was performed to maximize the efficiency. The results presented here are expected to serve as a benchmark for simplified-structure CIGS devices as well as a reference for discussions on the role of buffer layers used in conventional CIGS solar cells.

20% Efficient Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/Cu(In,Ga)(S,Se)2 Solar Cell Prepared by All-Dry Process through a Combination of Heat-Light-Soaking and Light-Soaking Processes.

Development of Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe)-based thin-film solar cells fabricated by an all-dry process is intriguing to minimize optical loss at a wavelength shorter than 520 nm owing to absorption of the CdS buffer layer and to be easily integrated into an in-line process for cost reduction. Cd-free CIGSSe solar cells are therefore prepared by the all-dry process with a structure of Zn0.9Mg0.1O:Al/Zn0.8Mg0.2O/CIGSSe/Mo/glass. It is demonstrated that Zn0.8Mg0.2O and Zn0.9Mg0.1O:Al are appropriate as buffer and transparent conductive oxide layers with large optical band gap energy values of 3.75 and 3.80 eV, respectively. The conversion efficiency (η) of the Cd-free CIGSSe solar cell without K-treatment is consequently increased to 18.1%. To further increase the η, the Cd-free CIGSSe solar cell with K-treatment is next fabricated and followed by posttreatment called the heat-light-soaking (HLS) + light-soaking (LS) process, including HLS at 110 °C followed by LS under AM 1.5G illumination. It is disclosed that the HLS + LS process gives rise to not only the enhancement of carrier density but also the decrease in the carrier recombination rate at the buffer/absorber interface. Ultimately, the η of the Cd-free CIGSSe solar cell with K-treatment prepared by the all-dry process is enhanced to the level of 20.0%.

Tunable Carrier Concentration of NaF‐Treated CIGS Solar Cells Using Heat–Light Soaking and Subsequent Heating

The combined effect of the heat–light soaking (HLS) and subsequent heat‐soaking (HS) processes is investigated on NaF‐free and NaF‐treated CIGS solar cells with CdS buffer layer. The HLS treatment improves cell efficiency slightly with increased open‐circuit voltage for NaF‐treated CIGS solar cells, whereas the cell efficiency deteriorates for NaF‐free CIGS solar cells. The different behaviors between both types of solar cells may be due to a Na‐containing new layer, which is formed at the CIGS surface region by NaF‐treatment. On the other hand, the short‐circuit current density (JSC) decreases for both types of solar cells after HLS treatment, which is attributed to the extremely high carrier concentration (NCV), leading to a narrower space charge region. However, it is found that the large NCV can be tuned to an appropriate value using the subsequent HS technique. As a result, the JSC loss is successfully reduced, therefore improving cell efficiency. The details of this improved efficiency are discussed with respect to the change of NCV in CIGS solar cells by the HLS and HS processes.

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.

Impact of heat‐light soaking on potassium fluoride treated CIGS solar cells with CdS buffer layer

AbstractIn this study, the effects of light‐soaking (LS), heat‐soaking (HS), and combined LS and HS, that is, heat‐light soaking (HLS) on potassium fluoride (KF)‐treated and KF‐free copper indium gallium selenide (CIGS) solar cells with CBD‐CdS buffer layer were investigated. LS and HS did not change the basic solar cell parameters of CIGS solar cells when they were performed separately. In contrast, HLS improved cell efficiency with increased open‐circuit‐voltage for KF‐treated CIGS solar cells, whereas it reduced cell performance for KF‐free CIGS cells. Capacitance‐voltage measurements confirmed a significantly increased carrier concentration in KF‐treated CIGS solar cells, as compared to KF‐free cells by HLS. X‐ray photoelectron spectroscopy measurement revealed that the HLS did not change the atomic concentration of Cd, S, and O in CBD‐CdS buffer layer. However, the concentration of Na atoms slightly increased at the CIGS surface region, as confirmed from SIMS measurement. It implies a possible reason for increased carrier concentration in KF‐treated CIGS solar cells after HLS. Temperature‐dependent current‐voltage measurements suggests that HLS modify a K‐containing new layer and affects cell performance.

Effects of long-term heat-light soaking on Cu(In,Ga)Se2 solar cells with KF postdeposition treatment

The effects of long-term heat-light soaking on Cu(In,Ga)Se2 (CIGS) solar cells with KF postdeposition treatment (KF-PDT) have been investigated. CIGS solar cells with KF-PDT frequently deteriorate after storage in the dark because of decreasing hole concentrations in the CIGS layers. Although light soaking improves hole concentrations, the resulting increase in conversion efficiency is not significant. In contrast, we found that long-term heat-light soaking effectively increases conversion efficiency, since the saturation current density and ideality factor are clearly improved by passivating the recombination centers at CdS/CIGS interfaces.

A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

AbstractThis paper reports a comparative study of Cu(In,Ga)(S,Se)2 (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm2. The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N1 defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley & Sons, Ltd.

Temperature dependent current–voltage and admittance spectroscopy on heat-light soaking effects of Cu(In,Ga)Se2 solar cells with ALD-Zn(O,S) and CBD-ZnS(O,OH) buffer layers

Heat-light soaking effects on Cu(In,Ga)Se2 (CIGS) solar cells with atomic layer deposition (ALD)-Zn(O,S) and chemical bath deposition (CBD)-ZnS(O,OH) buffer layers were investigated using temperature-dependent current–voltage and admittance spectroscopy measurements. Both CBD-ZnS(O,OH)/CIGS and ALD-Zn(O,S)/CIGS solar cells showed a significant increase in the cell performance after the combined heat and light soaking (HLS) post-treatment. Temperature-dependent current–voltage measurements showed a reduced roll-over of current density–voltage curve after the HLS post-treatment, suggesting a reduced recombination at the absorber/buffer interface. Admittance spectroscopy measurement revealed a remarkable shift towards shallower energy positions for the defect N1 after HLS post-treatment in both CIGS solar cells fabricated using CBD and ALD methods. By optimizing the HLS and the deposition conditions for each buffer layer, CBD-ZnS(O,OH)/CIGS and ALD-Zn(O,S)/CIGS solar cells yielded total efficiencies of 18.8 and 18.7%, respectively.