M. Gloeckler

Grain-boundary recombination in Cu(In,Ga)Se2 solar cells

Two-dimensional simulations are performed to investigate the impact of grain boundaries (GBs) on Cu(In,Ga)Se2 (CIGS) solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1μm, the effective grain-boundary recombination velocity must be less than 104cm∕s to allow for record-efficiency devices. The majority-hole repulsion (additional donors at the GB) and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion (additional acceptors at the GB) will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block ...

The impact of charged grain boundaries on thin-film solar cells and characterization

We use two-dimensional computer simulations to examine how charged columnar grain boundaries (GBs) affect transport, recombination, characterization, and performance in polycrystalline Cu(In,Ga)Se2 solar cells. Although the simulations show that charged GBs can increase photocurrent by forming minority-carrier collection channels, this generally occurs at the expense of overall efficiency. Carrier dynamics induced by the GBs significantly alter time-resolved photoluminescence, near-field scanning optical microscopy, electron-beam-induced current microscopy, and quantum efficiency spectra. Consequently, these experiments can place bounds on the role and strength of GB charge in polycrystalline materials. Simulations of these experiments indicate that GB charge sufficient to significantly increase photocurrent collection is generally inconsistent with the actual observations for Cu(In,Ga)Se2 solar cells.

Potential of submicrometer thickness Cu(In,Ga)Se2 solar cells

Thin-film solar cells based on Cu(In,Ga)Se2 absorbers with thicknesses similar to or below their optical absorption length are investigated with numerical simulations. The key issue for cells with thicknesses below 1.0μm is to limit back-contact recombination, which can be accomplished by the choice of back-contact material, surface modifications, or inclusion of Ga∕(Ga+In) grading. Unlike in thicker cells, the benefit of grading is maximized if it is limited to a narrow region at the back contact, where it acts as an electron reflector. The potential for optical improvement is evaluated considering variations in back-contact reflectivity and light trapping. Back-contact passivation combined with improved back-contact reflectivity should allow for thinning of the absorber material from 3 to 0.3μm with efficiencies above 17%. A sensitivity analysis with respect to material parameters illustrates that very thin cells suffer substantially stronger from nonuniformities.

Apparent quantum efficiency effects in CdTe solar cells

Quantum efficiency measurements of n-CdS/p-CdTe solar cells performed under nonstandard illumination, voltage bias, or both can be severely distorted by photogeneration and contact-barrier effects. In this work we will discuss the effects that are typically observed, the requirements needed to reproduce these effects with modeling tools, and the potential applications of apparent quantum efficiency analysis. Recently published experimental results are interpreted and reproduced using numerical simulation tools. The suggested model explains large negative apparent quantum efficiencies (≫100%) seen in the spectral range of 350–550 nm, modestly large negative apparent quantum efficiencies (>100%) in the spectral range of 800–850 nm, enhanced positive or negative response observed under red, blue, and white light bias, and photocurrent gain significantly different from unity. Some of these effects originate from the photogeneration in the highly compensated CdS window layer, some from photogeneration within t...

Hole current impedance and electron current enhancement by back-contact barriers in CdTe thin film solar cells

The combined effects of a significant back-contact barrier and a low absorber carrier density frequently alter the current-voltage (J-V) characteristics of CdTe solar cells. This combination leads to two competing mechanisms that can alter the J-V characteristics in two different ways. One is a majority-carrier (hole) limitation on current in forward bias that reduces the fill factor and efficiency of the solar cell. The second is a high minority-carrier (electron) contribution to the forward diode current that results in a reduced open-circuit voltage. CdTe solar cells are particularly prone to the latter, since the combination of a wide depletion region and impedance of light-generated holes at the back contact increases the electron injection at the front diode. The overlap of front and back space-charge regions will generally enhance the electron current, but is not a requirement for substantially increased forward current. The simulated J-V curves, illustrating the two major effects, are in good agre...

Effect of back-contact copper concentration on CdTe cell operation

CdTe solar cells were fabricated with five different concentrations of copper, including zero, used in back-contact formation. Room-temperature J-V curves showed progressive deterioration in fill factor with reduced copper. J/sub SC/ and QE were similar for all Cu-levels. Capacitance measurement suggested enhanced intermixing at the back contact with copper present. Photocurrent mapping was much less uniform for reduced-Cu cells. Elevated-temperature stress induced very little change in J-V when sufficient Cu was used in the contact.

Explanation of Light/Dark Superposition Failure in CIGS Solar Cells

CIGS solar cells in many cases show a failure of light/dark superposition of their currentvoltage (J-V) curves. Such failure generally becomes more pronounced at lower temperatures. J-V measurements under red light may also show an additional distortion, known historically as the “red kink”. The proposed explanation is that a secondary barrier results from the conduction band offset between CIGS and the commonly employed CdS window layer. This barrier produces a second diode with the same polarity and in series with the primary photodiode. The secondary-diode barrier height is modified by photoinduced changes of trap occupancy in the CdS layer, hence creating a voltage shift between dark and light conditions. Numerical modeling of the proposed explanation, including a band offset consistent with experimental and theoretical values, gives a very good fit to measured light and dark J-V curves over a wide temperature range. It also predicts the observed difference between illuminated J-V curves with photon energy above the CdS band gap, and those with sub-band-gap illumination.

Conduction-Band-Offset Rule Governing J-V Distortion in CdS/CI(G)S Solar Cells

A type-I (“spike”) conduction-band offset (CBO) greater than a few tenths of an eV at the n/p interface of a solar cell can lead to significant distortion of the current-voltage (J-V) curve. Such distortion has been observed in CdS/CIS cells, low-gallium CdS/CIGS cells, and CIGS cells with alternative windows that increase the CBO. The basic feature is reduced current collection in forward bias. The distortion is mitigated by photoconductivity in the CdS or other window layer, and it is therefore more severe if the illumination contains no photons with energies greater than the band gap of the window layer. The device-physics analysis of such distortion is numerical simulation incorporating a three-layer [TCO/CdS/CI(G)S] approximation for the solar cell. The parameters that influence the barrier height, and hence the distortion, are the magnitude of the CBO, the doping of the p- and n- layers, the defect density of the CdS, and the thicknesses of the CdS and TCO layers. The key value, however, is the energy difference between the quasi-Fermi level for electrons and the conduction band at the CdS/CIS interface. Thermionic emission across the interface will limit the current collection, if the difference exceeds approximately 0.48 eV at 300 K and one-sun illumination. This constraint is consistent with experiment, and strategies to satisfy the 0.48-eV rule when designing solar cells are enumerated.

Simulation of Polycrystalline Cu(In,Ga)Se2 Solar Cells in Two Dimensions

The extent to which grain boundaries (GBs) in polycrystalline materials may be detrimental, benign, or even beneficial is explored with numerical simulations in two dimensions. We focus on the e ects of GB recombination in Cu(In,Ga)Se2 (CIGS) solar cells and its e ects on solar-cell performance. The simulations predict that (1) for device e ciency exceeding 17%, the e ective GB recombination velocity must be less than 10 4 cm/s; (2) grain boundaries within the space-charge region (SCR) lower the open-circuit voltage, whereas the short-circuit current is reduced by grain boundaries in the bulk material; and (3) horizontal GBs are relatively benign unless they are located in the SCR. Modifications to the electronic structure near grain boundaries show that charge-induced band-bending at grain boundaries will most likely have a negative e ect on device performance, whereas a down-shift in the valence-band energy at the grain surface can e ectively passivate the GBs and reduce the e ective recombination velocity. For the models considered, GBs generally have a deleterious e ect on e ciency, and GBs alone can not explain the apparent superiority of polycrystalline over single-crystalline CIGS materials.