Igor Sankin

CdTe Solar Cells at the Threshold to 20% Efficiency

After a long period of stagnancy for record cell efficiency and several years of growth of the industry, CdTe solar cell efficiency has been rapidly increasing recently. First Solar (FSLR) fabricated an 18.7% NREL certified cell at the end of 2012 and reports an increase to a certified 19.0% in this paper. Although the improvements were dominated by increases in short-circuit current and fill factor, there is now evidence that the open-circuit voltage of polycrystalline CdTe is not fundamentally limited to ~850 mV, and first devices exceeding 900 mV have been demonstrated. In-depth device characterization indicates that the responsible third-level metric is the increase of carrier lifetime, which can be characterized by transient photoluminescence. The progress at hand suggests that the near-term achievable target for CdTe solar cells should be raised from 19% to 22%. A detailed numerical model is used to translate cell results into predicted module efficiency. These simulations are in agreement with FSLRs recently certified 16.1% total-area module efficiency record. Forward-looking simulations show that with already demonstrated technology components, a module efficiency exceeding 17% is attainable. Disruptive changes and implementation of new device architectures can provide further room for improvement for cell efficiency beyond 22%.

Defect interactions and the role of complexes in the CdTe solar cell absorber

While the electrical and optical properties of most crystalline materials are determined by the point defects, the association of these defects into complexes may further alter material properties, introducing new important phenomena. The properties of isolated point defects in CdTe have attracted significant research efforts, yet understanding of the complex defects in this material remains insufficient. This paper investigates the thermodynamic aspects of defect association in chlorinated copper-doped CdTe absorbers from first principles, using a supercell approach with the range-separated hybrid exchange–correlation functional. Based on the complex association energies calculated for 76 defect reactions, we propose the most favorable pair complexes formed in Cl- and Cu-doped CdTe absorbers. Most of the complexes studied in this work appear to be harmful for p-doping and may be responsible for the performance instabilities observed in CdTe devices. We also discuss a plausible passivation mechanism that mitigates TeCd recombination centers during Cl treatment and consider the formation of larger defect clusters and segregation of the point defects on extended defects.

Extracting Cu diffusion parameters in polycrystalline CdTe

It is well known that Cu plays an important role in CdTe solar cell performance as a dopant. In this work, a finite-difference method is developed and used to simulate Cu diffusion in CdTe solar cells. In the simulations, which are done on a two-dimensional (2D) domain, the CdTe is assumed to be polycrystal-line, with the individual grains separated by grain boundaries. When used to fit experimental Cu concentration data, bulk and grain boundary diffusion coefficients and activation energies for CdTe can be extracted. In the past, diffusion coefficients have been typically obtained by fitting data to simple functional forms of limited validity. By doing full simulations, the simplifying assumptions used in those analytical models are avoided and diffusion parameters can thus be determined more accurately.

Cu migration and its impact on the metastable behavior of CdTe solar cells

In this work we report on development of a 1D reaction-diffusion simulator of Cu kinetics in CdTe solar cells to investigate its role in the observed device performance changes. Evolution of intrinsic and Cu-related defects in CdTe solar cell has been studied in time-space domain self-consistently with free carrier transport. The simulation successfully predicts decelerating reductions of device performance for open-circuit stress and steadier reductions for short-circuit stress under elevated temperature, in agreement with experimental findings. The simulation results indicate that the movement of Cu interstitials could be responsible for such changes. Although 1D simulation has intrinsic limitations when applied to polycrystalline films, the results presented here imply that the 1D approach is still suitable in better understanding of the performance and metastabilities of CdTe photovoltaic device.

Self-consistent simulation of CdTe solar cells with active defects

We demonstrate a self-consistent numerical scheme for simulating an electronic device which contains active defects. As a specific case, we consider copper defects in cadmium telluride solar cells. The presence of copper has been shown experimentally to play a crucial role in predicting device performance. The primary source of this copper is migration away from the back contact during annealing, which likely occurs predominantly along grain boundaries. We introduce a mathematical scheme for simulating this effect in 2D and explain the numerical implementation of the system. Finally, we will give numerical results comparing our results to known 1D simulations to demonstrate the accuracy of the solver and then show results unique to the 2D case.

Numerical Simulation of Copper Migration in Single Crystal CdTe

Absorber material with high and stable p-type doping that does not impede free carrier lifetime is the key component enabling efficiency and stability improvements in thin-film CdTe technology. To better understand the compensation mechanism and the metastable effects related to Cu acceptors, the most common p-type dopant in CdTe, i.e., a detailed kinetic model describing the behavior of intrinsic and Cu-related defects in this material, has been developed and applied for the first time. Migration and reactions of these point defects in single crystal CdTe have been investigated by solving diffusion-reaction equations in the time-space domain self-consistently with free carrier transport. The simulation results supported by reasonable match to experimental data have shed light on the nature of limited Cu incorporation (also known as the Cu solubility limits) and Cu self-compensation during the annealing and cooling processes.

One-dimensional reaction-diffusion simulation of Cu migration in polycrystalline CdTe solar cells

In this work, we report on developing 1D reaction-diffusion solver to understand the kinetics of p-type doping formation in CdTe absorbers and to shine some light on underlying causes of metastabilities observed in CdTe PV devices. Evolution of intrinsic and Cu-related defects in CdTe solar cell has been studied in time-space domain self-consistently with free carrier transport and Poisson equation. Resulting device performance was simulated as a function of Cu diffusion anneal time showing pronounced effect the evolution of associated acceptor and donor states can cause on device characteristics. Although 1D simulation has intrinsic limitations when applied to poly-crystalline films, the results suggest strong potential of the approach in better understanding of the performance and metastabilities of CdTe photovoltaic device.

Using diffusion-reaction simulation to study the formation and self-compensation mechanism of Cu doping in CdTe

An improved model of copper p-type doping in CdTe absorbers is proposed that accounts for the mechanisms related to tightly bound Cu(i)-Cu(Cd) and Cd(i)-Cu(Cd) complexes that both limit diffusion and cause self-compensation of Cu species. The new model explains apparent discrepancy between DFT-calculated and fitted diffusion parameters of Cu reported in our previous work, and allows for better understanding of performance and metastabilities in CdTe PV devices.

Modeling Metastability in CdTe Solar Cells Due to Cu Migration

Thin-film modules of all technologies often suffer from performance degradation over time. Some of the performance changes are reversible and some are not, which makes deployment, testing, and energy-yield prediction more challenging. Manufacturers devote significant empirical efforts to study these phenomena and to improve semiconductor device stability. Still, understanding the underlying reasons of these instabilities remains clouded due to the lack of ability to characterize materials at atomistic levels and the lack of interpretation from the most fundamental material science. The most commonly alleged causes of metastability in CdTe device, such as “migration of Cu,” have been investigated rigorously over the past fifteen years. Still, the discussion often ended prematurely with stating observed correlations between stress conditions and changes in atomic profiles of impurities or CV doping concentration. Multiple hypotheses suggesting degradation of CdTe solar cell devices due to interaction and evolution of point defects and complexes were proposed, and none of them received strong theoretical or experimental confirmation. It should be noted that atomic impurity profiles in CdTe provide very little intelligence on active doping concentrations. The same elements could form different energy states, which could be either donors or acceptors, depending on their position in crystalline lattice. Defects interact with other extrinsic and intrinsic defects; for example, changing the state of an impurity from an interstitial donor to a substitutional acceptor often is accompanied by generation of a compensating intrinsic interstitial donor defect. Moreover, all defects, intrinsic and extrinsic, interact with the electrical potential and free carriers so that charged defects may drift in the electric field and the local electrical potential affects the formation energy of the point defects. Such complexity of interactions in CdTe makes understanding of temporal changes in device performance even more challenging and a closed solution that can treat the entire system and its interactions is required. In this book chapter we first present validation of the tool that is used to analyze Cu migration in single crystal (sx) CdTe bulk. Since the usual diffusion analysis has limited validity, our simulation approach presented here provides more accurate concentration profiles of different Cu defects that lead to better understanding of the limited incorporation and self-compensation mechanisms of Cu in CdTe. Finally, simulations are presented that study Cu ion’s role in light soaking experiments of CdTe solar cells under zero-bias and forward-bias stress conditions.

Diffusion-reaction modeling of Cu migration in CdTe solar devices

In this work, we report on development of one-dimensional (1D) finite-difference and two-dimensional (2D) finite-element diffusion-reaction simulators to investigate mechanisms behind Cu-related metastabilities observed in CdTe solar cells [1]. The evolution of CdTe solar cells performance has been studied as a function of stress time in response to the evolution of associated acceptor and donor states. To achieve such capability, the simulators solve reaction-diffusion equations for the defect states in time-space domain self-consistently with the free carrier transport. Results of 1-D and 2-D simulations have been compared to verify the accuracy of solutions.