A.O. Pudov

Surface treatment of CuInGaSe2 thin films and its effect on the photovoltaic properties of solar cells

Solar cells have been fabricated with partial electrolyte treatments of CuInGaSe2 (CIGS) thin film absorbers in lieu of a CdS layer. Treatment of the absorbers in a Cd or Zn containing solution is shown to produce conditions under which efficient solar cells can be fabricated. A similar effect is also observed in CuInGaSSe2 (CIGSS) graded band gap absorbers. These observations can be explained by the ability of Cd and Zn to produce n-type doping or inversion in the surface region. We also provide a brief review of similar work done elsewhere and identify directions for future investigations.

Performance and Loss Analyses of High-Efficiency Chemical Bath Deposition (CBD)-ZnS/Cu(In1-xGax)Se2 Thin-Film Solar Cells.

Chemically deposited ZnS has been investigated as a buffer layer alternative to cadmium sulfide (CdS) in polycrystalline thin-film Cu(In1-xGax)Se2 (CIGS) solar cells. Cells with efficiency of up to 18.1% based on chemical bath deposition (CBD)-ZnS/CIGS heterostructures have been fabricated. This paper presents the performance and loss analyses of these cells based on the current–voltage (J–V) and spectral response curves, as well as comparisons with high efficiency CBD-CdS/CIGS and crystalline silicon counterparts. The CBD-ZnS/CIGS devices have effectively reached the efficiency of the current record CBD-CdS/CIGS cell. The effects of the superior current of the CBD-ZnS/CIGS cell and the superior junction quality of the CBD-CdS/CIGS cell on overall performance nearly cancel each other.

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.

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.

Interface properties of CIGS(s)/buffer layers formed by the Cd-partial electrolyte process

A chemical-bath treatment that does not form a CdS layer has been used on CIGS absorbers made at the National Renewable Energy Laboratory (NREL). The resultant cells have moderate to high efficiency, with improved current collection at shorter wavelengths. Room temperature quantum efficiency (QE) and capacitance-voltage (CV) results indicate that the different surface treatments yield electro-optical differences in the bulk of the absorber. Room temperature current density-voltage (JV) and AMPS modeling results are used to compare and contrast the results of the surface treatments, primarily from the NREL devices.