Study of Cu(In,Al)Se2 thin films prepared by selenisation of sputtered metallic precursors for application in solar cells

Cu-In, Cu-Al and Cu-In-Al metallic precursor layers were deposited using radio-frequency magnetron sputtering and selenised to produce thin films of CuInSe2 (CIS), CuAlSe2 (CAS) and CuIn1-xAlxSe2 (CIAS), respectively. The selenisation stage of this 2-stage process was carried out in a tube furnace (TF) or a rapid thermal processor (RTP) in the presence of elemental Se, either deposited on top of the precursor film or provided from an external source in the chamber, in order to fabricate the chalcopyrite material. The aim was to produce single phase, device quality CIS, CAS and CIAS for use as an absorber layer material in thin film photovoltaic solar cells. Profilometry performed on the as-deposited Cu-In-Al metallic precursors showed an important increase in surface roughness compared to the Cu-In and Cu-Al precursors. This was found to be due to the preferential formation of Cu9(In,Al)4, which stoichiometry led the excess In to form island-shaped In phases at the surface of the bulk, while only Cu2In and CuIn2 formed in Cu-In precursors. Regarding the selenisation, temperatures ranging from 250°C to 550°C were used, and the resulting samples were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and glow-discharge optical emission spectroscopy (GD-OES). Thin films of single phase CIS and CAS were successfully produced with energy band gaps of 0.99 eV and 2.68 eV, respectively. However the incorporation of Al proved to be difficult. The results showed that no incorporation of the Al into the chalcopyrite lattice was achieved in the samples selenised in the RTP, which was believed to be due to the oxidation of the element Al into amorphous Al2O3. In the tube furnace, possibly due to lower levels of oxidation, incorporation occurred more readily but Al and In segregated towards the back and front of the layer, respectively. The causes of the segregation were studied and solutions to avoid it developed, resulting under certain conditions in successful production of CuIn1-xAlxSe2. Samples were tested in a photoelectrochemical cell and showed (apparent) external quantum efficiency values comparable to a CuInSe2 (CIS) sample used as a standard

Formation of Cu(In1-xAlx)Se2 by selenising RF magnetron sputtered Cu/Al/In precursor layers

The fabrication of Cu(In1-xAlx)Se2 using a multi-step process is reported. This process consists of depositing layers of Cu/Al/In, using magnetron sputtering, to form a metallic precursor layer, capping the stack with a layer of selenium, and then annealing in selenium vapour to synthesise the compound. The effects of annealing conditions on the chemical and physical properties of the converted layers were investigated. Rapid thermal processing at different temperatures indicated the formation of Cu(In1-xAlx)Se2 but only for annealing temperatures<360?C; a CuInSe2 phase was found to be present in all the layers fabricated. Annealing processes at higher temperatures (530-550?C) in a large tube furnace did succeed in producing Cu(In1-xAlx)Se2 without CIS, but only when a copper capping layer was included on the precursor stack

Study of the Al-grading effect in the crystallisation of chalcopyrite Cu(In,Al)Se2 thin films selenised at different temperatures

Chalcopyrite CuIn1−xAlxSe2 (CIAS) thin films with an atomic ratio of Al/(In + Al) = 0.4 were grown by a two-stage process onto soda-lime glass substrates. The selenisation was carried out at different temperatures, ranging from 400 °C to 550 °C, for metallic precursors layers evaporated with two different sequences. The first sequence, C1, was evaporated with the Al as the last layer, while in the second one, C2, the In was the last evaporated element. The optical, structural and morphological characterisations led to the conclusion that the precursors sequence determines the crystallisation pathway, resulting in C1 the best option due to the homogeneity of the depth distribution of the elements. The influence of the selenisation temperature was also studied, finding 540 °C as the optimum one, since it allows to achieve the highest band gap value for the C1 sequence and for the given composition

Crystallographic properties and elemental migration in two-stage prepared CuIn1−xAlxSe2 thin films for photovoltaic applications

Two-stage fabrication of CuIn1−xAlxSe2 thin films for photovoltaic absorbers using sputtered Cu–In–Al metallic precursors has been investigated. Precursors containing different relative amounts of Al were selenised and their structural and chemical properties characterised. X-ray diffraction (XRD) analyses revealed that the Al was only incorporated into the chalcopyrite structure for precursor composition ratios x = [Al]/([Al] + [In]) ⩾ 0.38, while chemical analysis of the cross-section indicated partial segregation of Al near the back of the film. Precursor films in the range of compositions that yielded no Al incorporation were then selenised at four different temperatures. Glow discharge optical emission spectroscopy, plasma profiling time-of-flight mass spectrometry and XRD analyses provided an insight into the diffusion processes and reactions taking place during the selenisation stage. The effect of post-selenisation annealing without additional Se was also investigated, and led to partial incorporation of the Al into the CuInSe2 lattice but no rediffusion

Rapid thermal processing of CuAISe2

CuAl thin film metallic precursors were selenised using a tube furnace or a rapid thermal processor (RTP). A comparison is made between the two processes for slightly Cu rich films and best crystallographic and elemental properties are obtained for films selenised by RTP: it was found that ternary compound could only be formed using the RTP. In both cases a large amount of CuxSey grains are found to develop at the surface of the films. Only samples processed in the RTP showed cathodoluminscence excitation at 2.68 eV characteristics of the electronic bandgap. Al rich samples were used to study the effect of etching the CuxSey phases from the surface in order to reveal the underlying CuAlSe2 material

Study of crystallographic properties and elemental migration in two-stage prepared Cu(In,Al) Se2

CuInAl metallic precursor films were selenised at different temperatures and the migration of the elements investigated. GD-OES was used to determine the elemental depth profiles, and XRD analysis gave an insight into the phase transformations taking place. These combined techniques made it possible to study the diffusion and reaction processes taking place during the selenisation stage. Post selenisation annealing was also investigated, which led to partial incorporation of the Al into the CuInSe2 lattice

Post-mortem analysis of a commercial Copper Indium Gallium Diselenide (CIGS) photovoltaic module after potential induced degradation

An extensive post-mortem analysis was conducted on a commercial copper-(indium-gallium)-diselenide (CIGS) photovoltaic module that degraded after exposure to the high voltage stress of a standardized potential induced degradation (PID) test. We employed a custom-developed coring technique to extract samples from the full-size field module, which showed degraded and nondegraded areas (regarded as reference) in electroluminescence after the PID test. The resulting solar cell samples were compared based on their electrical properties and sodium profiles using a wide range of laboratory-based analysis techniques including photoluminescence and lock-in thermography imaging, current–voltage measurements, and glow discharge optical emission spectroscopy. The samples that were extracted from the degraded areas of the module showed lower photoluminescence intensity and had significantly lower open-circuit voltage V(oc) and fill factor (FF) values in comparison with reference samples. An increased content of sodium within the absorber layer was also observed for these samples, linking sodium migration to PID. These observations at the module level are consistent with earlier reports on PID-stressed CIGS cells and mini-modules. This is to our knowledge the first reported study of a microscopic investigation on a real-life full-scale CIGS module after PID