arXiv subjects

Susanne Siebentritt

Publications and source records attributed to Susanne Siebentritt.

At least 19 recordsLinked to original sources

arXiv2026

What enables GaOx as hole transport layer for a 16 percent 1.0 eV CuInSe2 Bottom Cells with VOC above 550 mV?

Among the highly efficient photovoltaic technologies, that do not rely on epitaxy, only chalcopyrites have a bandgap tunable down to 1.00 eV, the ideal for tandem applications. This is obtained with a pure CuInSe2 absorber without Ga. GaOx has been shown to be an efficient hole transport layer that prevents recombination at the metallic back contact. On the other hand, GaOx has proven detrimental, when it forms on In containing transparent back contacts in bifacial solar cells. Here, we investigate the conditions that make the GaOx layer conductive. We employ a GaOx hole transport layer that is formed through ion exchange during co-evaporation of the low band gap absorber layer. We find that no additional Cu is needed, and that Na is not necessary for a conductive GaOx. Nor did we find a systematic influence of oxygen flow during the sputtering process of the oxide layer. The GaOx layer is partly crystalline. The optimized passivating hole transport layer enables a CuInSe2 bottom solar cell, without any addition of Ag or heavy alkalis, with an active area efficiency above 16% and a record-certified open-circuit voltage VOC of 552meV

cond-mat.mtrl-sci↗

arXiv2026

Near 13% efficient semitransparent Cu(In,Ga)S2 solar cells with band gap of 1.6 eV on transparent back contact

Wide-gap Cu(In,Ga)S2 solar cells with In2O3:Sn (ITO) as transparent back contact are evaluated for the application as top cells in tandem devices. The effect of Na on the solar cell performance is investigated by supplying additional Na by NaF co-evaporation or exclusively by Na diffusion from glass. An efficiency of 12.7% is achieved for a semitransparent solar cell with a band gap of 1.6 eV, with sufficient Na diffusion from glass only, allowed by a thin ITO layer. Absorber grown with additional NaF co-evaporation during Cu(In,Ga)S2 growth on thicker ITO show a comparable efficiency of 12%. High temperature growth at Tsub = 630°C enhances overall absorber quality and results in wide-gap absorbers, with photoluminescence quantum yield improved to 1.5 x 10-5, two orders of magnitude higher than absorber grown at low temperature. NaF co-evaporation is effective in suppressing deep defects, thereby reducing non-radiative recombination and enhancing photoluminescence quantum yield further. A GaOx interfacial layer is formed at the rear contact, likely contributing to the passivation of the back contact. With the presence of thick GaOx layer, current blocking effects are visible in the current-voltage curves. On the contrary, a thinner ITO tends to result in thinner GaOx layer and no current blocking is observed.

cond-mat.mtrl-sci↗

arXiv2026

Loss analysis of Low Bandgap (Ag,Cu)(In,Ga)Se2 Solar Cells for Tandem Applications

Tandem solar cells can better harness the energy of the solar spectrum. Chalcopyrite solar cells have drawn attention, being the only highly efficient devices with bandgap around 1.0 eV, suitable for bottom cells. In the quest for better efficiencies, we conduct a complete loss analysis of 1.0 eV bandgap (Ag,Cu)(In,Ga)Se2 cells with efficiencies around 18.5%. We perform absolute photoluminescence, electroluminescence, JV and EQE measurements on the absorber and the finished cells to analyze losses of short-circuit current, open-circuit voltage and fill factor. The relevant losses in current are due to absorption losses in the absorber and could only be mitigated by light management structures. But the most significant losses are found in the voltage, due to non-radiative recombination in the absorber, and the fill factor, due to a high diode factor. The diode factor of the cells is significantly higher than in the absorber alone, indicating a strong influence of recombination in the space charge region.

cond-mat.mtrl-sci↗

arXiv2026

ALD Zinc Tin Oxide Buffers for Chalcopyrite Solar Cells: Electrical Barriers and Conduction Band Cliffs

Sulfide chalcopyrite, Cu(In,Ga)S2, having wide bandgap (larger than 1.5 eV), favorable optoelectronic properties, and high stability, is a promising top-cell absorber for tandem applications. Adapting device structures optimized for 1.0 - 1.2 eV absorbers to wide bandgap absorbers requires modification of the buffer layer. This work investigates atomic layer deposition of ZnSnO as an alternative buffer layer to conventional CdS. A critical parameter for bufferperformance is the conduction band offsets on both sides of the buffer. To investigate these buffers we electrically characterize solar cells utilizing different compositions of ZnSnO. The Sn/(Sn+Zn) atomic ratio is controlled by the ratio of ZnO to SnO cycles during atomic layer deposition. Solar cells were fabricated utilizing CuInSe2, Cu(In,Ga)Se2, and Cu(In,Ga)S2 absorbers, allowing cross-comparison with a variety of conduction band minimum energies. Buffer variation has two primary effects on cell performance: 1. Low tin buffers decrease the activation energy of interface recombination, reducing open circuit voltage. These observations indicates a cliff, a decrease of the conduction band minimum from absorber to buffer. 2. High tin buffers reduce the fill factor for all measured cells, and reduce the short circuit current under certain conditions. This observation indicates an electron transport barrier, conduction band offsets which limit the transport of electrons across the buffer, in either direction. We conclude that tin content correlates positively with the conduction band minimum of these buffers. Comparing different absorbers, cliffs occurs at lower Sn contents and the effects of barriers are more dramatic for absorbers with lower conduction band minima.

cond-mat.mtrl-sci↗

arXiv2025

Silver Alloyed Wide Bandgap (Ag,Cu)(In,Ga)S2 Thin Film Solar Cells With 15.5% Efficiency

Sulfide chalcopyrite Cu(In,Ga)S2 (CIGS) is a wide bandgap semiconductor suitable for the top cell of a tandem solar device. Here we demonstrate significant improvements in absorber quality by alloying with Ag to form (Ag,Cu)(In,Ga)S2 (ACIGS) absorbers. We report the Ag alloying effect on compositional, structural, and optoelectronic properties of absorbers. We demonstrate suppressed bulk recombination and improved carrier lifetime in ACIGS, as a result of improved grain size, porosity reduction and defect passivation. We also show that Ag alloying flattens the Ga gradient. Consideration of this impact of Ag will be necessary in future engineering of the Ga profile to maximize charge carrier collection and avoid interface recombination. Exploiting the beneficial effects of Ag alloying, we report a wide bandgap (1.58 eV) ACIGS solar cell with a high power conversion efficiency of 15.5% and a large open-circuit voltage (VOC) of 948 mV, improving on the reference pure CIGS solar cell, with an 11.2% efficiency and an 821 mV VOC. Ag alloying is a useful route to further increase the efficiency of CIGS solar cells and future tandem devices.

cond-mat.mtrl-sci↗

arXiv2025

Sodium induced beneficial effects in wide bandgap Cu(In,Ga)S2 solar cell with 15.7% efficiency

This study underscores the pivotal role of sodium (Na) supply in optimizing the optoelectronic properties of wide bandgap (~1.6 eV) Cu(In,Ga)S2 (CIGS) thin film absorbers for high efficiency solar cells. Our findings demonstrate that the synergistic use of Na from the glass substrate, in conjunction with in-situ sodium fluoride (NaF) co-evaporation, significantly enhances the structural and optoelectronic properties of the CIGS. CIGS grown under either Na-deficient or excess conditions exhibits inferior microstructural and optoelectronic properties, whereas an optimal Na supply leads to enhanced photovoltaic performance. Optimal Na incorporation minimizes vertical gallium fluctuations and improves the grain size and crystallinity. An absolute 1 sun calibrated photoluminescence (PL) measurement reveals a substantial suppression of bulk defects and a reduction in non-radiative losses, resulting in a high quasi-fermi level splitting (ΔEF) of 1.07 eV, 93 meV higher than in Na-deficient CIGS with the same bandgap. Optimal Na supply further increases excited carrier decay time, as revealed from time-resolved PL, and hole doping density. Cross-sectional hyperspectral cathodoluminescence mapping reveals that optimal Na supply significantly reduces defect density near the surface, thereby effectively translating ΔEF to open-circuit voltage (VOC). As a result, a champion wide bandgap CIGS solar cell with a cadmium-free ZnSnO buffer layer achieved an impressive VOC of 971 meV and an active area power conversion efficiency of 15.7%, highlighting its potential for advancing tandem photovoltaic technologies with stable inorganic top cell.

cond-mat.mtrl-sci↗

arXiv2025

Sub-micron Cu(In,Ga)Se2 solar cell with efficiency of 18.2% enabled by a hole transport layer

Reducing the thickness of Cu(In,Ga)Se2 solar cells is a key objective in order to reduce production cost and to improve sustainability. The major challenge for sub-micron Cu(In,Ga)Se2 cells is the recombination at the backside. In standard Cu(In,Ga)Se2 backside recombination is suppressed by a bandgap gradient, acting as a back surface field. This gradient is difficult to maintain in sub-micron thick absorbers. In this study, a hole transport layer passivates the back contact and enables efficient sub-micron Cu(In,Ga)Se2 solar cells without the need of a Ga gradient. The backside passivation by the hole transport layer is as effective as an optimized Ga gradient, resulting in a significant increase in open-circuit voltage by 80 mV in comparison to the reference sample without passivation. Moreover, the hole transport layer exhibits good transport properties, leading to a fill factor as high as 77%. Photoluminescence quantum yield of 0.15% and solar cell efficiency above 18% are demonstrated in sub-micron Cu(In,Ga)Se2 absorbers.

cond-mat.mtrl-sci↗

arXiv2025

The effect of a band gap gradient on the radiative losses in the open circuit voltage of solar cells

The radiative open circuit voltage loss in a solar cell occurs because the absorptance spectrum near the band gap shows gradual increase rather than sharp step function like transition. This broadening effect has been attributed to band gap fluctuations and or to Urbach tails. In this report, we use modelling based on Planck s generalized law to distinguish between these two effects. Our results demonstrate that Urbach tails have only a minimal effect on the absorptance edge broadening and clarify that even an ideal direct semiconductor with no band gap fluctuations shows broadening at the absorptance onset. Furthermore, state of the art inorganic thin film solar cells often incorporate a band gap gradient across their thickness, which can further contribute to absorptance broadening. Using Cu(In,Ga)Se2 (CIGSe) absorbers as a case study, we perform a comprehensive analysis of voltage losses through absolute photoluminescence and electroluminescence spectroscopy, combined with photospectrometry and high-spatial-resolution cathodoluminescence measurements. We find that the loss analysis based on the combination of radiative, generation and non-radiative losses is complete. Samples with a graded band gap profile show more pronounced broadening of the absorptance onset and up to 16 mV higher radiative losses compared to the samples with uniform band gap. There is indication, that band gap-graded samples also have larger lateral band gap inhomogeneity.

cond-mat.mtrl-sci↗

arXiv2024

Improved sequentially processed Cu(In,Ga)(S,Se)2 by Ag alloying

Alloying small quantities of silver into Cu(In,Ga)Se2 was shown to improve the efficiency for wide and low band gap solar cells. We study low band gap industrial Cu(In,Ga)(S,Se)2 absorbers, substituting less than 10% of the copper with silver, using absolute photoluminescence and cathodoluminescence spectroscopy. Silver improves the grain size and promotes the interdiffusion of Ga and In across the depth of the absorber, resulting in a smoother band gap gradient. However, a certain lateral inhomogeneity is observed near the front and back sides. The non-radiative losses in the bare absorbers are reduced by up to 30 meV.

cond-mat.mtrl-sci↗

arXiv2023

Composition dependence of electronic defects in CuGaS2

CuGaS2 films grown by physical vapour deposition have been studied by photoluminescence (PL) spectroscopy, using excitation intensity and temperature dependent analyses. We observe free and bound exciton recombinations, three donor-to-acceptor (DA) transitions, and deep-level transitions. The DA transitions at ~ 2.41 eV, 2.398 eV and ~ 2.29 eV are attributed to a common donor level ~ 38+-5 meV and three shallow acceptors at ~ 76 meV, ~ 90 meV and 210 meV above the valence band. This electronic structure is very similar to other chalcopyrite materials. The donor-acceptor transitions are accompanied by phonon replicas. Cu-rich and near-stoichiometric material is dominated by the transitions due to the acceptor at 210 meV. All films show deep-level transitions at ~ 2.15 eV and 1.85 eV due to broad deep defect bands. Slightly Cu-deficient films are dominated by intense transitions at ~ 2.45 eV, attributed to excitonic transitions and the broad defect transition at 2.15 eV.

cond-mat.mtrl-sci↗

arXiv2023

CuIn(Se,Te)2 absorbers with bandgaps < 1 eV for bottom cells in tandem applications

Thin-film solar cells reach high efficiencies and have a low carbon footprint in production. Tandem solar cells have the potential to significantly increase the efficiency of this technology, where the bottom-cell is generally composed of a Cu(In,Ga)Se2 absorber layer with bandgaps around 1 eV or higher. Here, we investigate CuIn(Se1-xTex)2 absorber layers and solar cells with bandgaps below 1 eV, which will bring the benefit of an additional degree of freedom for designing current-matched 2-terminal tandem devices. We report that CuIn(Se1-xTex)2 thin films can be grown single phase by co-evaporation and that the bandgap can be reduced to the optimum range for a bottom cell (0.92 - 0.95 eV). From photoluminescence spectroscopy it is found that no additional non-radiative losses are introduced to the absorber. However, Voc losses occur in the final solar cell due to non-optimised interfaces. Nevertheless, a record device with 9 % power conversion efficiency is demonstrated with a bandgap of 0.96 eV and x=0.15. Interface recombination is identified as a major recombination channel for larger Te contents. Thus, further efficiency improvements are anticipated for improved absorber/buffer interfaces.

physics.app-ph↗

arXiv2023

Composition variations in Cu(In,Ga)(S,Se)2 solar cells: not a gradient, but an interlaced network of two phases

Record efficiency in chalcopyrite-based solar cells Cu(In,Ga)(S,Se)2 is achieved using a gallium gradient to increase the band gap of the absorber towards the back side. Although this structure has successfully reduced recombination at the back contact, we demonstrate that in industrial absorbers grown in the pilot line of Avancis, the back part is a source of non-radiative recombination. Depth-resolved photoluminescence (PL) measurements reveal two main radiative recombination paths at 1.04 eV and 1.5-1.6 eV, attributed to two phases of low and high band gap material, respectively. Instead of a continuous change in the band gap throughout the thickness of the absorber, we propose a model where discrete band gap phases interlace, creating an apparent gradient. Cathodoluminescence and Raman scattering spectroscopy confirm this result. Additionally, deep defects associated to the high gap phase reduce the absorber performance. Etching away the back part of the absorber leads to an increase of one order of magnitude in the PL intensity, i.e., 60 meV in quasi Fermi level splitting. Non-radiative voltage losses correlate linearly with the relative contribution of the high energy PL peak, suggesting that reducing the high gap phase could increase the open circuit voltage by up to 180 mV.

cond-mat.mtrl-sci↗

arXiv2022

On the origin of tail states and VOC losses in Cu(In,Ga)Se2

The detrimental effect of tail states on the radiative and non-radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single crystals. The results demonstrate that alkalis decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of alkali incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high-efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.

cond-mat.mtrl-sci↗

arXiv2022

How much gallium do we need for a p-type Cu(In,Ga)Se2?

Doping in the chalcopyrite Cu(In,Ga)Se2 is determined by intrinsic point defects. In the ternary CuInSe2, both N-type and P-type conductivity can be obtained depending on the growth conditions and stoichiometry: N-type is obtained when grown Cu-poor, Se-poor and alkali-free. CuGaSe2, on the other hand, is found to be always a P-type semiconductor that seems to resist all kinds of N-type doping no matter whether it comes from native defects or extrinsic impurities. In this contribution, we study the N-to-P transition in Cu-poor Cu(In,Ga)Se2 single crystals in dependence of the gallium content. Our results show that Cu(In,Ga)Se2 can still be grown as an N-type semiconductor until the gallium content reaches the critical concentration of 15-19%, where the N-to-P transition occurs. Furthermore, trends in the Seebeck coefficient and activation energies extracted from temperature-dependent conductivity measurements, demonstrate that the carrier concentration drops by around two orders of magnitude near the transition concentration. Our proposed model explains the N-to-P transition based on the differences in formation energies of donor and acceptor defects caused by the addition of gallium.

cond-mat.mtrl-sci↗

arXiv2022

Metastable defects decrease the fill factor of solar cells

Cu(In,Ga)Se2 based solar cells exceed power conversion efficiencies of 23 %. Yet, the fill factor of these solar cells, with best values around 80 %, is relatively low (Si reaches 84.9%) mostly due to diode factors greater than one. Recently, we proposed metastable defects, a general feature of the Cu(In,Ga)Se2 alloy, to be the origin of the increased diode factor. We measure the diode factor of the bare absorber layers by excitation-dependent photoluminescence. For high quality and thus high luminescent polycrystalline absorbers, we evaluate the diode factor excitation dependence over four orders of magnitude. Using simulations and the model of metastable defects, we can well describe the experimental findings on n- and p-type epitaxial films as well as the polycrystalline absorbers, providing additional evidence for this model. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0 % is obtained by Ag alloying, i.e. an (Ag,Cu)(In,Ga)Se2 absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for Cu(In,Ga)Se2-based solar cells.

cond-mat.mtrl-sci↗

arXiv2022

Post-deposition annealing and interfacial ALD buffer layers of Sb$_2$Se$_3$/CdS stacks for reduced interface recombination and increased open-circuit voltages

Currently, Sb$_2$Se$_3$ thin films receive considerable research interest as a solar cell absorber material. When completed into a device stack, the major bottleneck for further device improvement is the open circuit voltage, which is the focus of the work presented here. Polycrystalline thin film Sb$_2$Se$_3$ absorbers and solar cells are prepared in substrate configuration and the dominant recombination path is studied using photoluminescence spectroscopy and temperature dependent current-voltage characteristics. It is found that a post-deposition annealing after the CdS buffer layer deposition can effectively remove interface recombination since the activation energy of the dominant recombination path becomes equal to the bandgap of the Sb$_2$Se$_3$ absorber. The increased activation energy is accompanied by an increased photoluminescence yield, i.e. reduced non-radiative recombination. Finished Sb$_2$Se$_3$ solar cell devices reach open circuit voltages as high as 485 mV. Contrarily, the short-circuit current density of these devices is limiting the efficiency after the post-deposition annealing. It is shown that atomic layer deposited intermediate buffer layers such as TiO$_2$ or Sb$_2$Se$_3$ can pave the way for overcoming this limitation.

cond-mat.mtrl-sci↗

arXiv2022

Electrical barriers and their elimination by tuning (Zn,Mg)O composition in Cu(In,Ga)S2: Systematic approach to achieve over 14% power conversion efficiency

Traditional CdS buffer layer in selenium-free Cu(In,Ga)S2 solar cells leads to reduced open-circuit voltage because of a negative conduction band offset at the Cu(In,Ga)S2/CdS interface. Reducing this loss necessitates the substitution of CdS by an alternative buffer layer. However, the substitute buffer layer may introduce electrical barriers in the device due to unfavorable band alignment at the other interfaces such as between buffer/ZnO i-layer. This study aims to reduce interface recombinations and eliminate electrical barriers in Cu(In,Ga)S2 solar cells using a combination of Zn1-xMgxO and Al-doped Zn1-xMgxO buffer and i-layer combination deposited using atomic layer deposition and magnetron sputtering, respectively. The devices prepared with these layers are characterized by current-voltage and photoluminescence measurements. Numerical simulations are performed to comprehend the influence of electrical barriers on the device characteristics. An optimal composition of Zn1 xMgxO x = 0.27 is identified for a suitable conduction band alignment with Cu(In,Ga)S2 with a bandgap of ~1.6 eV, suppressing interface recombination and avoiding barriers. Optimized buffer composition together with a suitable i-layer led to a device with 14 % efficiency and an open-circuit voltage of 943 mV. A comparison of optoelectronic measurements for devices prepared with ZnO and Al:(Zn,Mg)O shows the necessity to replace the ZnO i-layer with Al:(Zn,Mg)O i-layer for a high-efficiency device.

cond-mat.mtrl-sci↗

arXiv2021

Origin of interface limitation in CuInS$_2$ based solar cells

Copper indium disulfide (CuInS$_2$) grown under Cu-rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination resulting in poor solar cell performance. An unfavorable cliff like conduction band alignment at the buffer/CuInS$_2$ interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in Zn(O,S)/CuInS$_2$ devices. Temperature-dependent current-voltage analysis indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bandgap, confirming the dominant recombination channel to be present at the Zn(O,S)/CuInS2 interface. However, photoelectron spectroscopy measurements indicate a small spike like conduction band offset of 0.1 eV at the Zn(O,S) CuInS$_2$ interface, excluding an unfavorable energy level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also rules out Fermi level pinning as the main reason, leaving near-interface defects (as recently observed in Cu-rich CuInSe2)1 as the likely reason for the performance limiting interface recombination.

physics.app-ph↗