David Regesch

Degradation and passivation of CuInSe2

The degradation of CuInSe2 absorbers in ambient air is observed by the decay of the quasi-Fermi level splitting under well defined illumination with time. The decay is faster and stronger in absorb ...

Influence of copper excess on the absorber quality of CuInSe2

The compositional dependence of the optoelectronic quality of CuInSe2 thin film absorbers is investigated on single- and poly-crystalline films with varying [Cu]/[In]-ratios. We quantify the quality of the absorbers by the splitting of quasi-Fermi levels, determined by spectral photoluminescence. This quantity determines the maximum achievable open circuit voltage by an absorber. Our results indicate a significant increase of this value for growth under Cu-excess, indicating a decrease of recombination losses. By comparison of the predicted achievable open circuit voltage and the actually measured ones of finished solar cells, we find a huge “un-utilized potential” for the Cu-rich devices.The compositional dependence of the optoelectronic quality of CuInSe2 thin film absorbers is investigated on single- and poly-crystalline films with varying [Cu]/[In]-ratios. We quantify the quality of the absorbers by the splitting of quasi-Fermi levels, determined by spectral photoluminescence. This quantity determines the maximum achievable open circuit voltage by an absorber. Our results indicate a significant increase of this value for growth under Cu-excess, indicating a decrease of recombination losses. By comparison of the predicted achievable open circuit voltage and the actually measured ones of finished solar cells, we find a huge “un-utilized potential” for the Cu-rich devices.

Direct Evaluation of Defect Distributions From Admittance Spectroscopy

Evaluating interfering capacitance steps in admittance spectroscopy for solar cell defect analysis is still a problem which needs to be solved. While the common analysis developed by Walter et al.[1] is capable of extracting defect distributions from the capacitance data, it results in erroneous defect densities in the presence of overlapping capacitance steps. We derive an expression for the capacitance step caused by defects with a density of states distributed in energy. By adding several of these defect distributions, interfering capacitance steps can be described. Thus, it is possible to fit the entire capacitance spectrum simultaneously for all temperatures. We apply the presented method to Cu2ZnSnSe4 -based solar cells with power conversion efficiencies between 5% and 7%. Comparing the obtained defect parameters with the ones obtained by the method from Walter et al. reveals that the Walter method overestimates the defect densities in the case of overlapping capacitance steps.

Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas-phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source.

Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10 K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined fr...

The influence of Se pressure on the electronic properties of CuInSe2 grown under Cu-excess

Standard Cu-poor Cu(In,Ga)Se2 solar cell absorbers are usually prepared under high Se excess since the electronic properties of the absorbers are better if prepared under high Se pressure. However, in CuInSe2, grown under Cu-excess, it was found that solar cell properties improve with lowering the Se pressure, mostly because of reduced tunnel contribution to the recombination path. Lower Se pressure during Cu-rich growth leads to increased (112) texture of the absorber films, to better optical film quality, as seen by increased excitonic luminescence and to lower net doping levels, which explains the reduced tunnelling effect. These findings show an opposite trend from the one observed in Cu-poor Cu(In,Ga)Se2.

The importance of Se partial pressure in the laser annealing of CuInSe 2 electrodeposited precursors

One method for producing CuInSe2 (CISe) absorber layers is electrodeposition followed by annealing. Replacing the commonly used furnace annealing step with a laser can reduce annealing times by 2-3 orders of magnitude: from 30 minutes to 1 s. However, laser processing has, to date, not resulted in absorber layers which can form functioning final devices. One reason is due to Se loss during annealing even on these short timescales. We show how this Se loss is reduced by using a background partial pressure of Se (PSe) during annealing. Higher PSe results in increased grain size and drastically increased photoluminescence yield. The introduction of an elevated PSe in the laser annealing chamber enabled the fabrication of the first known CuInSe2 photovoltaic device using electrodeposition followed by laser annealing which gave 1.6% efficiency.