CdTe/CdS solar cells of ∼10% efficiency, developed with a vacuum deposition method were irradiated with high-energy protons of different fluences. The Voc and f.f. of irradiated cells increase or decrease depending on the fluence. The normal soda lime glass substrate darkens under the irradiation; therefore low Isc is measured. Measurements suggest that CdTe solar cells are highly stable under proton flux. Flexible and lightweight solar cells were developed in a superstrate configuration on polymer substrates. 8.6 % efficiency cells with Voc∼770 mV and Isc of 20.3 mA/cm2 were achieved.

The CuInS2 thin film formation from a Cu/In precursor stack in the presence of elemental sulfur using a rapid thermal process under Cu-poor conditions has been studied. The process has been aborted at appropriate stages and the corresponding samples were investigated by XRD, Raman spectroscopy and SEM. The sulfurisation starts from elemental Cu and CuIn2. Elemental In and the binary phases Cu11In9 and Cu7In3 appear as intermediate phases. At the end of the sulfurisation the sample contains the ternary phases CuInS2 and CuIn5S8. CuS and β-In2S3 are detected by Raman spectroscopy at the sample surface and at distinct stages of the sulfurisation only. A difference in CuInS2 crystal quality is observed between the surface and the bottom of the samples.

To improve the efficiency of heterostructure solar cells based on chalcopyrite semiconductors a good understanding of the interface properties is crucial. By Kelvin Probe Force Microscopy it is possible to obtain laterally resolved images of the work function of semiconductor surfaces in addition to the topographical information usually obtained by noncontact atomic force microscopy. We studied the CuGaSe2/ZnSe interface prepared by growth of CuGaSe2 onto the (110) face of freshly cleaved ZnSe single crystals using chemical vapor deposition. We observed different work function values for different crystal facets on single CuGaSe2 grains. From the obtained work function data and surface photovoltage measurements a schematic band diagram for the CuGaSe2/ZnSe heterostructure is proposed.

The influence of the evaporation rate profiles on the microstructure of co-evaporated Cu(In,Ga)Se2, (CIGS), is discussed. The influence of Cu excess in the beginning of the CIGS growth has been investigated. In addition, the Ga rate has been varied in order to create bandgap grading in the CIGS film. By studying CIS and CGS films separately and as CGS/CIS stacks results on interdiffusion of In and Ga interdiffusion have been obtained. The resulting thin films are investigated mainly using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Solar cell devices were prepared and IV measurements performed on samples with varying CIGS deposition parameters.

Growth and properties of indium sulfide layers (< 300 nm) prepared by atomic layer deposition (ALD) have been studied. Growth rate of about 0.6 A per cycle has been measured for films deposited at 160°C from indium acetylacetonate and hydrogen sulfide precursors. The films are crystalline with the β modification. They possess high band gap values (2.7-2.8 eV) which are related to small grain sizes (3-4 nm) through quantum size effects. Electrical properties have been addressed using the semiconductor electrolyte junction. They are n type with a doping level around 1016 cm−3 and possess a good blocking behavior under reverse bias. The flat band potential is close to -1 V vs MSE. These figures are close to those measured under similar conditions with CdS CBD buffer layers and could explain the good cell performances obtained with ALD In2S3.

Hydrogen with an energy of 300 eV was implanted into single crystalline CuInSe2 samples at temperatures of 200 °C and 300 °C during implantation. We found that the hydrogen is not limited to the expected implantation depth but diffuses into the bulk of the sample. The hydrogen concentration ranges from 1019 H/cm3 in a depth of about 300 nm up to some 1021 H/cm3 next to the surface and resembles a diffusion profile. The hydrogen induced change of composition was not only at the surface, but also up to a depth of about 200 nm similar to that of the hydrogen profile. Mainly a Cu deficiency after hydrogen implantation could be observed and is explained as the passivation of VCu by hydrogen and the additional production of VCu by the induced band-bending.

The laser-assisted chemical vapor deposition (LCVD) technique has proved to be an effective means of depositing cadmium sulfide layers in a well-controlled manner over select small areas. Observations of favorable conditions for deposition and typical pitfalls in the process are discussed. Analysis of the LCVD CdS layers is presented in conjunction with comparisons to conventionally grown CdS layers. The stoichiometry, thickness, and grain structure of the LCVD CdS layers have been suitably optimized for use in solar-cell applications.

For solar cells based on CuGaSe2(CGS) absorber layers physical vapour deposition (PVD) was employed to deposit polycrystalline CGS thin films. Efficiencies up to 7.9% were achieved by applying a two-stage deposition process for absorber preparation. The high structural quality of the absorbers is shown. For highest cell efficiencies our standard recipe for the CdS buffer deposition had to be re-adapted. Voltage dependent spectral response measurements of the device in conjunction with absorption measurements of the absorber layers allow calculation of the minority carrier diffusion length, space charge region (SCR) width, absorber carrier concentration and the built-in voltage. The defect state density at the buffer/absorber interface is estimated. It can be shown that the benefical effect of the modified buffer deposition is mainly due to a reduced acceptor concentration at the absorber/buffer interface.

Thin film solar cells of CuInS2/Zn(Se,O)/ZnO configuration have been studied as a function of the Zn(Se,O) buffer layer deposition parameters. Deposition of the buffer films was carried out by the chemical bath deposition (CBD) method, at different bath temperatures and compositions, and followed in situ with a quartz crystal microbalance. The CBD conditions were chosen to grow Zn(Se,O) buffer layers under different kinetic regimes but maintaining the same buffer thickness. The cells have been characterised with current-voltage and quantum efficiency measurements. Light soaking effects and medium term stability have been checked. It is found that Zn(Se,O) grown under predominant electroless kinetics gives rise to buffer films richer in oxygen, which allow for higher fill factors, higher efficiencies (around 10%) and stability of the cells. These cells show however lower open circuit potential. On the other hand, Zn(Se,O) buffers grown under chemical regime become richer in selenium, which gives rise to cells with higher open circuit potential, but lower fill factor, conversion efficiency and stability. Light soaking effects are also more important with the chemically grown buffers.

The processing of large area thin films solar cells represents a challenge, mainly due to the need for optimal setting of the critical growth parameters for scaling-up of the processes that guarantee homogeneous properties in the sub-minimodule architecture. We have been working with a Radio-Frequency Planar Magnetron Sputtering (RF-PMS) system, processing 450 cm2 areas of CdS and CdTe thin films grown on soda lime glasses and conducting glasses (SnO2:F-7ω/ ) for CdS, using for it a 6” balanced gun. The films have been processed with substrate temperatures (Ts) of 250°C, Ar chamber-pressure of 20 mTorr, and radio frequency power of 300 watts. The films were characterized by morphological, optical and electrical techniques, presenting in this work the corresponding measurements. Our results show that the better polycrystalline perfection, adherence and pinhole free films are obtained at substrate temperatures of 250°C.

We investigate radiation-induced defects in high-efficiency Cu(In,Ga)Se2/CdS/ZnO heterojunction solar cells after 1-MeV electron and 4-MeV proton irradiation. We use electron and proton fluences of more than 1018 cm−2 and up to 1014 cm−2, respectively. The irradiation experiments performed at three independent electron irradiation facilities consistently prove the superior radiation resistance of these Cu(In,Ga)Se2 devices compared to other types of solar cells. The reduction of the solar cell efficiency in all experiments is predominantly caused by a loss ΔVOC of the open circuit voltage VOC. An analytical model describes ΔVOC in terms of radiation-induced defects enhancing recombination in the Cu(In,Ga)Se2 absorber material. From our model we extract the defect introduction rates for recombination centers in Cu(In,Ga)Se2 for the respective particles and energies. Isochronal annealing steps fully recover VOC of the irradiated Cu(In,Ga)Se2 solar cells. Exposure to temperatures of approx. 400 K are sufficient to restore the initial VOC within less than 5 %, even after excessive irradiation. The annealing process displays an activation energy of EA = 1.1 eV. Admittance spectroscopy directly reveals the generation and the annealing of radiation-induced defects.

Atmospheric pressure chemical vapor deposition (APCVD) is being studied as an alternative for large-area manufacturing of CdTe thin films. High efficiency research cells have been constructed, but the fundamental materials properties and limitations have not been fully explored. APCVD material is examined with several techniques and compared with close-space sublimation (CSS). Transmission and scanning electron microscopy studies show a similar morphology to CSS CdTe. However high resolution TEM scans show the formation of a disordered layer between the CdTe and CdS, and the removal of defects within some grain structures upon annealing. Cathodoluminescence shows electronic defect states localized to grain boundaries. A large concentration of trap states was also observed with deep-level transient spectroscopy that may correspond to hole traps found in lower amounts in other materials. The presence of traps was also indicated in impedance spectroscopy measurements. The latter studies indicate a high grain boundary resistance contributes to transport.

We have ion implanted a series of dopants into single crystal CdTe. The influence of the impurities on the CdTe has been studied by low temperature photoluminescence spectroscopy. The impurities studied are: O, Cl, Cu, S, Na and Sb, and were selected because of their potential importance in thin film CdTe/CdS photovoltaic devices. Each impurity was implanted to give a smooth gaussian distribution in the first few microns of material with a doping density of 1×1015cm−3. Low temperature photoluminescence spectra were recorded from each sample and compared with an undoped sample. The PL features observed in each sample are discussed and the data is compared to an earlier photoluminescence study of thin film CdTe/CdS photovoltaic device structures.

Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) devices are subject to stress under various biases. Striking differences are observed with the Current-Voltage, and Capacitance- Voltage measurements for cells degraded at 100°C in dark under forward (FB), open circuit (OC), and reverse (RB) biases. RB stress provides the greatest degradation, and the apparent doping density profile shows anomalous behavior at the zero bias depletion width. Thin films of CdS, both doped and undoped, with Cu are characterized with photoluminescence (PL). The PL spectra from the CdS films are correlated with the CdS spectra from stressed devices, revealing that Cu signatures in the CdS layer of stressed devices are a function of stress biasing. Device modeling using AMPS-1D produces IV curves similar to that in RB degraded devices, by only varying the trap level concentration in the CdS layer.

One of the most promising strategies for lowering the cost of photovoltaics (PV) is the use of thin-film technologies in which thin films are formed from particulate precursor materials deposited by non-vacuum techniques. The quality of particle-based PV thin films and in turn the sunlight-to-electricity conversion efficiencies and cost structures of devices based these films are strongly affected by the planarity and packing density of the layers of particles from which the films are formed and by the materials use efficiency of the processes used to deposit the layers. This paper reports progress made in depositing planar, well-packed layers of particles using non-vacuum spray deposition techniques.

Solar cells based on Cu-In-S-phase absorber layers on a quasi-endless copper tape were made in a series of consecutive roll-to-roll processes, for which a base line has recently been set up. The so called “as grown cell absorber”, which consists of a stack of CuIn5S8 and CuInS2 layers, is completed to a solar cell by a CuI buffer layer of about 70 nm thickness and a 1 μm thick ZnO-TCO window layer. The design of the whole cell including the phase constitution of the absorber layer and the actual standard electrical characteristics will be presented. Considering the first stability tests and the experimental possibilities of the base line, an outlook will be given concerning the ability and perspectives of using these cell tapes for module production.

CuIn1−xGaxS2 (CIGS2) thin-film solar cells are of interest for space power applications because of the near optimum bandgap for AM0 solar radiation in space. CuIn1−xGaxSe2−ySy (CIGS) and CIGS2 solar cells are expected to be superior to Si and GaAs solar cells for the space missions especially in terms of the performance at the end of low earth orbit (LEO) mission. Ultra-lightweight thin-film solar cells deposited on flexible stainless steel (SS) foils have a potential for achieving high specific power.

Magnetron-sputter-deposition parameters of molybdenum back-contact layer were optimized so as to minimize residual stress. Cu-rich Cu-Ga/In layers were sputter-deposited on unheated Mo-coated SS foils from CuGa(22%) and In targets. Well-adherent, large (3 μm), compact-grain Cu-rich CIGS2 films were obtained by sulfurization in a Ar:H2S 1:0.04 mixture and argon flow rate of 650 sccm, at the maximum temperature of 475° C for 60 minutes with intermediate 30 minute annealing step at 120° C. p-type CIGS2 thin films were obtained by etching away the Cu- rich layer segregated at the surface in a dilute KCN solution. XRD analysis of a CIGS2 film on SS foil revealed growth of chalcopyrite CIGS2 phase having ao= 5.519 Å and co= 11.125 Å and {112} preferred orientation. Positive SIMS depth profile of CIGS2 film showed gallium concentration increasing toward the back contact.

Solar cells were completed by deposition of CdS heterojunction partner layer by chemical bath deposition, transparent-conducting ZnO/ZnO:Al window bilayer by RF sputtering, and vacuum deposition of Ni/Al contact fingers through metal mask. PV parameters of a CIGS2 solar cell on SS flexible foil measured under AM 0 conditions at the NASA GRC were: Voc = 802.9 mV, Jsc = 25.07 mA/cm2, FF = 60.06%, and η = 8.84%. For this cell, AM 1.5 PV parameters measured at NREL were: Voc = 788 mV, Jsc = 19.78 mA/cm2, FF = 59.44%, η = 9.26%. Quantum efficiency curve showed a sharp QE cutoff equivalent to CIGS2 bandgap of ∼1.50 eV, fairly close to the optimum value for efficient AM0 PV conversion in the space.

CdTe-rich and CdS-rich alloy films were deposited at temperatures less than 300C using an rf sputtering technique. Transmission-reflection and photoluminescence measurements were conducted to study the near-band edge properties of this ternary system for films of different compositions at room temperature and at 10K. Results were compared to data on defect states introduced in CdTe single crystal by annealing at several different overpressure conditions, including CdS and CdCl2 overpressures. A below-band-gap photoluminescence excitation technique was used to study systematically the effect of different steps in the production of the complete solar cell on band gap states of CdSxTe1−x alloys as well as pure binary constituents.

This paper presents a detailed analysis of the spatially resolved evolution of External Quantum Efficiency of electrodeposited CdS/CdTe solar cells as a function of the duration of the post-deposition annealing process which promotes n to p-type conversion of electrodeposited CdTe. Strips of 30×4 cm were cut from deposited plates at various stages of the fabrication process and processed into cells. Annealing treatments were carried out on these strips at 400°C for times ranging from 1 to 120 minutes. 130 spectral response curves (in the range of 300 to 900 nm) have been measured to trace temporal evolution and spatial non-uniformity of the materials and extrapolate parameters such as effective diffusion lengths, evolution of the collection profiles, and junction depth. Correlation between these parameters suggests that recrystallisation does not take place uniformly but progresses through the film. The analysis of structural vs. optical parameters along the cells provides evidence for spatial non-uniformities of the state of crystallisation of as-deposited and annealed material. Poor performance has been observed near the edges of the cells where material is resistant to recrystallisation.

We examined the Cu(In,Ga)Se2 thin-film growth from (In,Ga)2Se3 precursors and found that the evolution of the microstructure and intrinsic native defects depends on the compositional changes that occur as the film transitions from being Cu rich to In(Ga) rich.