Attempts to fabricate new CuIn1-xBxSe2 (CIBS) and CuBSe2 (CBS) thin-film materials have been complicated by the formation of interfering crystallites and by the loss of boron from the magnetron sputtered precursor alloys during the selenization and annealing processes. Raman and Auger spectroscopic analysis as well as x-ray diffraction studies show that the formation of boron selenide may be contributing to the difficulty in creating these new materials.

Using scanning transmission electron microscopy and nanoprobe X-ray energy-dispersive spectroscopy, we studied the locations of Cu in CdTe solar cells and test interfaces with intentionally introduced Cu sources. We found three primary locations of Cu: 1) back-contact region, 2) CdTe layer, and 3) CdTe/CdS junction areas. In the back-contact region, Cu diffused from back-contact can convert the Te-rich layer into Cu-Te compounds. In the CdTe layer, a higher concentration of Cu is found to distribute along grain boundaries, but not in twin boundaries and stacking faults. In the CdTe/CdS junction area, Cu is found uniformly in the CdS layer. However, significant segregation of Cu into CdS/TCO interfaces is also found.

Unlike other photoactive layers (e.g. Si) polycrystalline chalcopyrite layers achieve higher efficiency than monocrystalline ones. Although grain boundaries may play an important role for this phenomenon, there is currently no commonly accepted model for the electronic structure of grain boundaries. First experimental results on CuGaSe2 absorbers indicated a small neutral barrier for majority carriers (20-40meV) at sigma 3 grain boundaries, which is the predominant grain boundary in polycrystalline chalcopyrite absorbers [1]. These results are in discrepancy with theory, which predicts a 10 times higher barrier; we suspect that the copper excess might reduce the barrier height.

Here we present a study using Hall-effect and Kelvin Probe Force Microscopy (KPFM) measurements to investigate the composition dependence of the barrier height of epitaxially grown CuGaSe2 layers containing a sigma 3 grain boundary as a function of the Cu/Ga ratio. First results show that the barrier height is independent of the copper content. In addition, we present initial results on sigma 9 grain boundaries which are observed to occur in high efficiency absorbers.

CdS and PbS are well known semiconductor materials. Starting in 1969 and into 1970's CdS-PbS cells were reported with open circuit voltage (Voc) up to 450 mV and short circuit current density (Jsc) < 1 mA/cm2. However, further reports are scarce. These two materials are also the most investigated by chemical deposition technique. In this work we revisit this type of photovoltaic junctions and present the photovoltaic behavior of distinct type of cell structures prepared by chemical deposition: glass/CdS/PbS/Ag, SnO2:F/CdS/PbS/Ag, and SnO2:F/CdS/(Bi2S3 or/and CdSe)/PbS/Ag. Depending on the cell type, Voc of > 500 mV or Jsc of > 3 mA/cm2 could be obtained under illumination of 1-3 kW/m2. This work opens up possibilities for developing simple solar cell structures by sequential chemical deposition of semiconductors.

Solar-cell performance degradation due to physical nonuniformities becomes more significant as the thickness of polycrystalline absorbers is reduced. “Voltage” nonuniformities such as those due to band-gap fluctuations, variations in the back-contact proximity, and areas where the absorber is completely depleted can have very significant impact on cell performance. Similarly local shunts can seriously degrade the efficiency. “Current” nonuniformities such as optical defects have generally much less impact. The analysis presented is based on Cu(In,Ga)Se2 cells, but the qualitative results should be applicable to thin-absorber devices in general. For lateral nonuniformity studies, the solar cell is simulated by a two dimensional network of parallel diodes separated by resistors. The nonuniformities are approximated by small regions of reduced photovoltage, often referred to as “weak diodes”, and by isolated shunt resistors. The weak-diode approach allows investigation of device performance as a function of the weak-diode voltage deficit, the ratio of weak-to strong-diode area, and the weak diodes' spatial distribution. Increased TCO resistance can isolate weak diodes, thus limiting the voltage loss due to nonuniformities, but increasing fill-factor losses.

Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin-film solar cells were fabricated by an in-line, close-space-sublimation (CSS) process at Colorado State University. Source temperature control was used to reduce the deposited CdS thickness. Quantum efficiency (QE) showed CdS thicknesses that varied over a range from 250 to 10 nm. Current-Voltage (J-V) measurements showed increased Jsc as CdS was thinned. Thin CdS resulted in reduced voltage (800 mV to 350 mV) and fill factor, which offset gains in current, and caused efficiencies to drop from 12.6% for thick CdS layers to 4.5% for devices with the thinnest CdS. These performance trends are consistent with calculations assuming parallel junctions of CdS/CdTe and SnO2/CdTe. Localized weak-junction formation was characterized by high-resolution laser-beam-induced current (LBIC) mapping. Greater incidence of spatial non-uniformities in photocurrent response accompanied thinning of the CdS layer, with 638-nm spectral response varying spatially by 4.5% for thin CdS devices compared to variations less than 1% for devices with thicker CdS. Non-uniformities of cells with thin CdS are highly sensitive to voltage bias and are likely indicative of parallel p-n and Schottky-type junctions.

Stoichiometric Cu-doped ZnSe crystalline films were prepared on glass substrates from ZnSe and Cu2Se powders using the evaporation method without intentional heating. Post-annealing treatment at 400°C in a vacuum improved the crystallinity, and no secondary phase was observed using X-ray diffraction for a film with a Cu concentration of Cu/(Zn + Cu) = 0.10, which had a conductivity of 28 Scm-1 and good transparency at green and red wavelengths. The thermal probe test indicated p-type carrier polarity, and the effects of annealing were confirmed by irradiating the as-deposited films with a YAG laser (ë = 355 nm). Possibility of producing transparent p-type conducting films on polymer substrates is demonstrated.

Molybdenum thin films were deposited by r.f. magnetron sputtering. The strain in the films was mostly tensile and varied, with the Argon pressure and the r.f. power, along a bell curve, reaching a maximum at 8 mTorr (80 W) and 6 mTorr (100 W). The grain size deduced from the X-Ray diffraction measurements (FWHM) decreased with increasing Argon pressure and, to a lower extent, with decreasing r.f power. The change in grain size, as well as in morphology, was confirmed by AFM imaging. Finally, the resistivity of the films was found to increase with increasing Argon pressure, while the films maintained good adhesion to the soda-lime glass substrates for all Argon pressures and r.f. power.

We compare the electroluminescence (EL) of three polycrystalline ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells with similar bandgaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent electroluminescence measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperatures to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. The thus broadened room temperature luminescence is one effect which reduces the open circuit voltage of the devices below the Shockley-Queisser-limit. The other effect is the existence of non-radiative recombination currents, which determine the efficiency of the device as light emitting diode. To quantify the open circuit voltage losses, we use reciprocity relations between electroluminescent and photovoltaic action of solar cells, which allow us to predict the light emitting diode efficiency. Measurements support the theory and show that Cu(In,Ga)Se2 solar cells reach external LED efficiencies approaching.

Two-dimensional numerical device simulations investigate the influence of grain boundaries on the performance of Cu(In,Ga)Se2 solar cells focussing on the question whether or not grain boundaries can improve the efficiency of those devices. The results unveil the following statements: (i) The mere introduction of a grain boundary by adding localized defects into a device that has a high performance from the beginning is not beneficial. (ii) Polycrystalline solar cells can outperform monocrystalline ones, if the total number of defects is equal in both devices. I.e. a given number of recombination centers is better dealt with if these defects are concentrated at the grain boundary rather than homogeneously distributed in the bulk. (iii) A significant improvement of carrier collection via the grain boundaries is found if the bulk of the devices is assumed as relatively poor. In this situation, addition of defects that are not much recombination ac-tive but provide a large charge density at the grain boundaries can improve the device performance. (iv) Passivation of grain boundaries by an internal band offset in the valence band is effective only if the internal barrier amounts at least to 300 meV.

For understanding the origin of the improvements of properties in the CIGS-based cells, of which the CIGS absorber has been fabricated by H2O-introduced co-evaporation [CIGS-H2O], band alignment at the interfaces between chemical bath deposited CdS and CIGS-H2O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H2O showed an identical electronic structure with that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and a location of conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was 0.2 ~ 0.3 eV. XPS measurements of the core-level signals over the interface showed a small upward bend bending of 0.1 ~ 0.2 eV. Consequently, the conduction band offset (CBO) and valence bad offset (VBO) at the CBD-interface over the CIGS-H2O (Ga~40%) are about +0.1, and 0.9 ~ 1.1 eV, respectively. This positive CBO is contrast with a slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS with Ga ~ 40 % measured previously. These results indicate that the H2O introduction is effective to extend the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of Voc and efficiency in the CIGS-H2O based cells.

Chemical vapor transport (CVT) using carbon as a transporting agent is studied for homoepitaxial growth on O-polar ZnO substrates. To increase growth rate at high temperatures, we keep a substrate close to ZnO source powder. Surface smoothness and crystal quality of epilayers are remarkably improved by increasing a substrate temperature. Smooth surfaces are observed on the epilayer grown at substrate temperatures above 920°C.