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Interface recombination in a complex multilayered thin-film solar structure causes a disparity between the internal open-circuit voltage (VOC,in), measured by photoluminescence, and the external open-circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near-surface defect model is developed for copper indium di-selenide solar cells grown under Cu-excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi-level pinning. The model presented is based on experimental analysis of admittance and deep-level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near-surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi-level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

Currently, Sb2Se3 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 Sb2Se3 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 Sb2Se3 absorber. The increased activation energy is accompanied by an increased photoluminescence yield, that is, reduced non‐radiative recombination. Finished Sb2Se3 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 TiO2 or Sb2S3 can pave the way for overcoming this limitation. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of solar cells strongly depends on the open‐circuit voltage VOC which, in turn, depends on the recombination activity within the device. A possible source of detrimental charge carrier recombination is band tails. An empirical linear relationship between VOC loss and the Urbach energy of the band tails has been shown in the past. Here we discuss how band tails influence the radiative recombination and the nonradiative recombination in the bulk of the absorber. First, we show through photoluminescence that the band tails can be willfully tuned in state‐of‐the‐art thin‐film Cu (In,Ga)Se2 (CIGSe) absorbers and solar cells on a 20% efficiency level and beyond through the incorporation of alkali atoms. In the second part, we compare our CIGSe results to published results from other solar cell technologies. This comparison reveals that CIGS solar cells follow the previously described empirical trend: an increase in the open‐circuit voltage with decreasing band tails. Finally, we model the influence of tail states on the radiative and nonradiative recombination losses: Radiative recombination is increased because carriers thermalize into the tail states and nonradiative recombination of free carriers in the bands is increased because of Shockley–Read–Hall recombination through the tail states. The comparison with experimental data shows that the influence of tail states is even worse than the increase in radiative and SRH recombination predicted by our model. Our results thus suggest that band tails act as one of the main remaining voltage limitations in the majority of state‐of‐the‐art solar cells. [ABSTRACT FROM AUTHOR]

Reported growth processes for kesterite absorber layers generally rely on a sequential process including a final high temperature annealing step. However, the impact and details for this annealing process vary among literature reports and little is known on its impact on electrical properties of the absorber. We used kesterite absorber layers prepared by a high temperature coevaporation process to explicitly study the impact of two different annealing processes. From electrical characterization it is found that the annealing process incorporates a detrimental deep defect distribution. On the other hand, the doping density could be reduced leading to a better collection and a higher short circuit current density. The activation energy of the doping acceptor was studied with admittance spectroscopy and showed Meyer-Neldel behaviour. This indicates that the entropy significantly contributes to the activation energy. [ABSTRACT FROM AUTHOR]

Thin‐film Cu(In,Ga)Se2 solar cells reach power conversion efficiencies exceeding 23% and nonradiative recombination in the bulk is reported to limit device performance. The diode factor has not received much attention, although it limits the fill factor, and therefore the efficiency, for state‐of‐the‐art solar cells. Herein, the diode factor of Cu(In,Ga)Se2 absorbers, measured by photoluminescence spectroscopy, and of solar cells, measured by current–voltage and capacitance–voltage characteristics, are compared, supported by simulations using rate equations of generation and recombination. It is found that the diode factor is already increased in the neutral zone of the absorber due to metastable defects, such as the VSe–VCu defect found in Cu(In,Ga)Se2, because of an increased net acceptor density upon minority‐carrier injection. The metastable and persistent increase of the net acceptor density has a detrimental effect on the device performance. Diode factors of 1 and efficiencies exceeding 24% are expected when, in current state‐of‐the‐art Cu(In,Ga)Se2 solar cells, the formation of metastable defects is suppressed. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley–Read–Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation. [ABSTRACT FROM AUTHOR]

We study the impact of different alkali post‐deposition treatments by thermal admittance spectroscopy and temperature‐dependent current‐voltage (IVT) characteristics of high‐efficiency Cu(In,Ga)Se2 thin‐film solar cells fabricated from low‐temperature and high‐temperature co‐evaporated absorbers. Capacitance steps observed by admittance spectroscopy for all samples agree with the widely observed N1 signature and show a clear correlation to a transport barrier evident from IVT characteristics measured in the dark, indicating that defects are likely not responsible for these capacitance steps. Activation energies extracted from capacitance spectra and IVT characteristics vary considerably between different samples but show no concise correlation to the alkali species used in the post‐deposition treatments. Numerical device simulations show that the transport barrier in our devices might be related to conduction band offsets in the absorber/buffer/window stack. We study the impact of different alkali postdeposition treatments of high‐efficiency Cu(In,Ga)Se2 thin‐film solar cells. Capacitance steps observed by admittance spectroscopy agree with the widely observed N1 signature and show a clear correlation to a transport barrier evident from temperature‐dependent current‐voltage characteristics, indicating that defects are likely not responsible for these capacitance steps. Numerical device simulations show that the transport barrier in our devices might be related to conduction band offsets in the absorber/buffer/window stack. [ABSTRACT FROM AUTHOR]

Among the thin-film solar cell technologies, Cu(In,Ga)Se2-based solar cells demonstrate the highest efficiencies, where the recent boost in efficiency is triggered by a KF postdeposition treatment (PDT). In this contribution, Cu(In,Ga)Se2-based solar cells are fabricated using RbF PDTs after absorber layer growth with varying substrate and RbF source temperature. The electronic charge transport properties of the solar cell devices are investigated using temperature-dependent current-voltage analysis and admittance spectroscopy. To investigate the observed transport barriers, a novel concept based on the differential series resistance is proposed. This approach is supported by simulations of current-voltage curves, which reproduce qualitatively experimental data. Experimentally, two parallel conduction paths are found, which act as barriers with different activation energies and impede the charge carrier transport. Both the thickness and height of these barriers increase with an increasing amount of incorporated Rb and can lead to losses in the fill factor and power conversion efficiency at room temperature. Etching in HCl prior to CdS buffer layer deposition reduces the barrier width and can recover these losses. [ABSTRACT FROM AUTHOR]

This review summarizes the current status of Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS-based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se 2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley-Read-Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se 2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation.