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Na plays an important role in the electrical performance of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells. Traditionally, Na has been introduced during the growth of CIGS by thermal diffusion from the soda-lime glass (SLG) substrate; however, better control of the amount of Na is needed to have a more precise control of growth conditions. The introduction of Na into CIGS was studied in three different ways: from the SLG, from a NaF precursor, and from a Na-doped Mo (MoNa) back contact. The most successful approaches were obtained by using the conventional SLG and the NaF precursor. Different growth temperatures of CIGS were tested in an attempt to diffuse more Na from the MoNa layer.

NaF precursor layers used for providing Na to Cu(In,Ga)Se2 (CIGS) grown on Na-free substrates have been studied. The NaF layers were deposited on top of the Mo back contact prior to the CIGS co-evaporation process. The co-evaporation process was interrupted after the preheating steps, and after part of the CIGS layer was grown. Completed samples were also studied. After the preheating, the NaF layers were analyzed with X-ray Photoelectron Spectroscopy and after growing part and all of the CIGS film, the Mo/NaF/CIGS stack was characterized using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). The NaF layers were found to be stable in thickness and composition during the pre-heating in selenium containing atmosphere before the CIGS process. The TEM analyses on the partly grown samples show a layer at the CIGS/Mo interface, which we interpret as a partly consumed NaF layer. This is corroborated by the SIMS analysis. In finalized samples the results are less clear, but TEM images show an increased porosity at the position of the NaF layer.

The sun provides us with a surplus of energy convertible to electricity using solar cells. This thesis focuses on solar cells based on chalcopyrite (CIGSe) as well as kesterite (CZTS(e)) absorber layers. These materials yield record efficiencies of 20.4 % and 11.1 %, respectively. Especially for CZTS(e), the absorber layers often do not consist of one single desired phase but can exhibit areas with deviating material properties, referred to as secondary phases. Furthermore, several material layers are required for a working solar cell, each exhibiting interfaces. Even though secondary phases and interfaces represent a very small fraction of the solar cell they can have a profound influence on the over-all electrical solar cell characteristics. As such, it is crucial to understand how secondary phases and interfaces influence the local electrical characteristics.Characterising secondary phases and interfaces is challenging due to their small sample volume and relatively small differences in composition amongst others. This is where electronmicroscopy, especially transmission electron microscopy, offers valuable insight to material properties on the microscopic scale. The main challenge is, however, to link these material properties to the corresponding electrical characteristics of a solar cell.This thesis uses electron beam induced current imaging and introduces a new method for JV characterisation of solar cells on the micron scale. Combining microscopic structural and electrical characterisation techniques allowed identifying and characterising local defects found in the absorber layer of CIGS solar cells after thermal treatment. Furthermore, CZTSe solar cells in this thesis exhibited a low photo-current density which is traced to the formation of a current blocking ZnSe secondary phase at the front contact interface. The electron microscopy work has contributed to an understanding of the chemical stability of CZTS and has shown the need for an optimised back contact interface in order to avoid chemical decomposition reactions and formation of detrimental secondary phases. With this additional knowledge, a comprehensive picture of the material properties from the macroscopic down to the microscopic level can be attained throughout all required material layers.

Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu(2)ZnSn(S,Se)(4) (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS(2) and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.

Na-containing surface compounds is likely to form during the formation of CZTS absorber. Still, the understanding of any potential influence on buffer layer growth and device performance is limited. In this work, we observed that NaxS compound can possibly form on the CZTS surface after annealing, and negatively affect the growth of the subsequent CdS buffer. The Na x S compound is oxidized to Na 2 SO 4 via air exposing the annealed CZTS surface, which allows greatly improved quality of the CdS layer. This provides new insights for improving the CdS/CZTS interface during the fabrication of CZTS solar cells.

Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2(CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2(Zn,Sn)(S,Se)4(CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (VOC; +17%rel.), short-circuit current (JSC ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

The influence of the thickness of atomic layer deposited Zn1−xSnxOy buffer layers and the presence of an intrinsic ZnO layer on the performance of Cu(In,Ga)Se2 solar cells are investigated. The amo ...

Laser micro-welding is a technique that can be used for interconnective (P2) patterning of Cu ( In , Ga ) Se 2 (CIGS) thin film solar cells after deposition of the ZnO:Al window layer. The micro-welding process transforms CIGS into a conductive compound that forms a contact between the ZnO:Al front contact and the Mo back contact. In this work we present a study on the influence of process parameters on the interconnectivity of the laser micro-weld as well as an investigation of the morphology of the micro-weld zone. We found that the current method results in an overlap of conditions where electrical connection is achieved and where Mo/glass damage is caused by laser action. Stability testing shows that interconnections processed without appreciable damage are stable. The connecting region contained metal rich zones and segregated CuxSe that we believe account for its conductivity.

Cu2ZnSnS4 (CZTS) thin films are made using sulfurisation of co-sputtered metallic and sulfur-containing precursor films. The CZTS grain size is larger for metallic precursors than for sulfur-containing precursors while more uniform films with fewer voids are obtained in the latter case. During sulfurisation of precursors with tin-excess in closed quartz ampoules, tin is lost from the films with greater losses from metallic precursors. We suggest that the reduced grain size and the reduced tin-loss for sulfur-containing precursors can be explained by a larger number of CZTS nuclei being formed early in the sulfurisation process. In sulfur containing precursors with large tin excess, SnS2 is observed together with CZTS, and a tin-rich bottom layer segregates. This indicates that tin-diffusion in CZTS is relatively slow. Solar cell devices made for a range of compositions at and around stoichiometric CZTS show highest efficiencies in two compositional groups; Zn-rich and Cu-poor/Sn-rich, while close to stoichiometric material gives poor devices. Devices including the tin-rich bottom layer show efficiencies of up to 3.2%. The role of secondary phases such as ZnS and SnS2 on device performance is discussed.

We present a monitoring system for the field test of photovoltaic modules. The system is designed for the monitoring of individual modules under maximum power point (MPP) conditions that allow the extraction of the energy yield of different modules under optimum conditions. It can also be used to monitor the performance and long-term stability of modules under realistic field conditions. The monitoring system consists of an individual MPP tracker attached to each module under test. The measurement data are transmitted to a central multichannel data logger by means of analog voltages proportional to the current at MPP, i.e., Imp, and voltage at MPP, i.e., Vmp, of the modules.

A deeper understanding of Cu(In,Ga)Se2 (CIGS) solar cells is important for the further improvement of these devices. This thesis is focused on the use of electrical modelling as a tool for pursuing this aim. Finished devices and individual layers are characterized and the acquired data are used as input in the simulations. Band gap gradients are accounted for when modelling the devices. The thesis is divided into two main parts. One part that treats the influence of cadmium free buffer layers, mainly atomic layer deposited (Zn,Mg)O, on devices and another part in which the result of CIGS absorber layer modifications is studied. Recombination analysis indicates that interface recombination is limitting the open circuit voltage (Voc) in cells with ZnO buffer layers. This recombination path becomes less important when magnesium is introduced into the ZnO giving a positive conduction band offset (CBO) towards the CIGS absorber layer. Light induced persistent photoconductivity (PPC) is demonstrated in (Zn,Mg)O thin films. Device modelling shows that the measured PPC, coupled with a high density of acceptors in the buffer-absorber interface region, can explain light induced metastable efficiency improvement in CIGS solar cells with (Zn,Mg)O buffer layers. It is shown that a thin indium rich layer closest to the buffer does not give any significant impact on the performance of devices dominated by recombination in the CIGS layer. In our cells with CdS buffer the diffusion length in the CIGS layer is the main limitting factor. A thinner CIGS layer improves Voc by reducing recombination. However, for thin enough absorber layers Voc deteriorates due to recombination at the back contact. Interface recombination is a problem in thin devices with Zn(O,S) buffer layers. This recombination path is overshadowed in cells of standard thickness by recombination in the CIGS bulk. Thin cells with Zn(O,S) buffer layers have a higher efficiency than CdS cells with the same absorber thickness.

A new atomic layer deposition process was developed for deposition of Zn–Sn–O buffer layers for Cu(In,Ga)Se2 solar cells with tetrakis(dimethylamino) tin, Sn(N(CH3)2)4, diethyl zinc, Zn(C2H5)2, and water, H2O. The new processgives good control of thickness and [Sn]/([Sn]+[Zn]) content of the films. The Zn–Sn–O films are amorphous as foundby grazing incidence X-ray diffraction, have a high resistivity, show a lower density compared with ZnO and SnOx, andhave a transmittance loss that is smeared out over a wide wavelength interval. Good solar cell performance was achievedfor a [Sn]/([Sn]+[Zn]) content determined to be 0.15–0.21 by Rutherford backscattering. The champion solar cell with aZn–Sn–O buffer layer had an efficiency of 15.3% (Voc=653mV, Jsc(QE)=31.8mA/cm2, and FF=73.8%) compared with15.1% (Voc=663mV, Jsc(QE)=30.1mA/cm2, and FF=75.8%) of the best reference solar cell with a CdS buffer layer. Thereis a strong light-soaking effect that saturates after a few minutes for solar cells with Zn–Sn–O buffer layers after storage in thedark. Stability was tested by 1000h of dry heat storage in darkness at 85°C, where Zn–Sn–O buffer layers with a thicknessof 76nm retained their initial value after a few minutes of light soaking.

Cu(2)ZnSnS(4) (CZTS) shows great potential for cheap, efficient photovoltaic devices. However, one problem during synthesis of CZTS films is the loss of Sn as a result of decomposition and evaporat ...

The influence of the average Se-to-metal overpressure during three-stage co-evaporation of Cu(InxGa1−x)Se2 solar absorber layers has been investigated. Solar cell devices were fabricated using a baseline process consisting of chemical bath deposited CdS, magnetron sputtered intrinsic and Al-doped ZnO, and e-beam evaporated Ni/Al/Ni current collection grid. For the higher Se-to-metal rate ratios studied, an increased short-circuit current in combination with a decreased fill factor is observed, while the open-circuit voltage stays fairly constant. Based on quantum efficiency measurements, fitting of IV data to a one-diode model, and simulations, we suggest the observed effects to be due to a decreased effective doping in combination with a decreased bulk recombination with higher average Se-to-metal overpressures. This could e.g. be explained by a decreasing number of Se-vacancies or VSe–VCu divacancies with higher Se rate. For the range of lower average Se-to-metal rate ratios studied, device performance drops due to a decreased open-circuit voltage and, for the lowest Se rate investigated, fill factor. In addition to electrical characterization, the effects on absorber microstructure are discussed based on results obtained from X-ray diffraction and scanning electron microscopy.

Photoconductivity and photocapacitance of Cu(In,Ga)Se2 and CuGaSe2 thin films and devices induced by sub-bandgap illumination are investigated. Both effects have been attributed to the optical transition from valence band to the same empty levels situated around 0.8–0.9 eV above the valence band. The influence of the metastable states created by illumination and voltage bias on the sub-bandgap response has been studied. The experimental results are discussed in the framework of a model based on negative-U property of a native defect in chalcopyrites, i.e. V Se –V Cu divacancy. The arguments are presented that the levels involved in the optical transition observed in photoconductivity and photocapacitance might be antibonding levels of the acceptor configuration of this defect.

A baseline parameter set for electrical modelling of Cu(In,Ga)Se2 solar cells with compositionally graded absorber and CdS buffer layer is established. The cases with and without Fermi level pinnin ...

Reducing the Cu(In,Ga)Se2 (CIGS) thickness is one way of improving the throughput and capacity in existing production, provided that the efficiency can be kept at a high level. Our experimental res ...

We use secondary-ion mass spectrometry, X-ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se2 thin films grown in th ...

Laser patterning of thin film solar cells has proven technically feasible for all layers but still remains a challengingtopic for research and development. We present a method where P2 laser pattern ...

We investigate Cu(In,Ga)Se2 thin films grown in multi-stage coevaporation processes and solar cells fabricated from such absorbers. Despite some interdiffusion during film growth, Ga/(Ga+In) gradients defined via evaporation-profile variations in the process are to a good part retained in the finished film. This indicates that the bandgap can be engineered in this type of process by varying the evaporation profiles, and consequently also that unintended profile variations should be noted and avoided. With front-side gradients the topmost part of many grains seems to be affected by a higher density of lattice defects due to the strong change of gallium content under copper-poor growth conditions. Electrically, both back-side gradients and moderate front-side gradients are shown to yield an improvement of device efficiency. If a front-side gradient is too wide, though, it causes strong voltage-dependent collection and the fill factor is severely reduced.

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS. In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer. This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells. Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices. While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell.

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS. In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer. This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells. Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices. While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell.

The use of solar cells for energy production has indeed a bright future. Reduction of cost for fabrication along with increased efficiency are key features for a market boom, both achieved as a result of increased knowledge of the technology. Especially the thin film solar cell technology with absorbers made of Cu(In,Ga)Se2 (CIGS) is promising since it has proven high power conversion efficiency in combination with a true potential for low cost fabrication. In this thesis different recipes for fabrication of the Cu(In,Ga)Se2 absorber layer have been studied. The deposition technique used has been co-evaporation from elemental sources. For all depositions the substrate has been heated to a constant temperature of 500 oC in order for the growing absorber to form a chalcopyrite phase, necessary for the photovoltaic functionality. The selenium has been evaporated such to always be in excess during depositions whereas the metal ratio Cu/(In+Ga) has been varied according to different recipes but always to be less than one at the end of the process. In the work emphasis has been on the radiative properties of the CIGS film during growth. The substrate heater has been temperature controlled to maintain the constant set temperature of the substrate, regardless of varying emitted power caused by changing surface emissivity. Depending on the growth conditions the emissivity of the growing film is changing, leading to a readable variation in the electrical power to the substrate heater. Since the thermal radiation from the substrate during growth has been of central focus, this has been studied in detail. For this reason the substrate has been treated as an optical stack composed of glass/Mo/Cu(In,Ga)Se2/CuxSe which determine the thermally radiated power by its emissivity. An optical model has been adopted to simulate the emissivity of the stack. In order to use the model, the optical constants for Cu(In,Ga)Se2 and CuxSe have been derived for the wavelength interval 2 μm to 20 μm. The simulation of the emissivity of the stack during CIGS growth agreed well with what has been seen for actual growth. Features of the OP-signal could hereby be explained as a result of film thickness of Cu(In,Ga)Se2 and CuxSe respectively. This is an important knowledge for an efficient fabrication in large scale.

A surface treatment by evaporated selenium on Cu(In,Ga)Se, (CIGS) is shown to improve open circuit voltage, V and in some cases fill factor, FF, in solar cells with CdS, (Zn,Mg)O or Zn(O,S) buffer layers. V increases with increasing amount of crystalline Se, while FF improves only for small amounts. The improvements are counteracted by a decreasing short circuit current assigned to absorption in hexagonal Se. Improved efficiency is shown for device structures with (Zn,Mg)0 and Zn(O,S) buffer layers by atomic layer deposition. Analysis by grazing incidence X-ray diffraction and photoelectron spectroscopy show partial coverage of the CIGS surface by hexagonal selenium. The effects on device performance from replacing part of the CIGS/buffer interface area by a Se/buffer junction are discussed.

In this paper, lightsoaking and temperature-dependent current-voltage (JVT) measurements on Cu(In,Ga)Se2 solar cells with atomic layer deposited Zn1-xMgxO buffer layers are presented. A range of Mg ...

Thin-film solar cells based around the absorber material CuIn1-xGaxSe2 (CIGS) are studied with respect to their stability characteristics, and different ways of modelling device operation are investigated. Two ways of modelling spatial inhomogeneities are detailed, one fully numerical and one hybrid model. In the numerical model, thin-film solar cells with randomized parameter variations are simulated showing how the voltage decreases with increasing material inhomogeneities.With the hybrid model, an analytical model for the p-n junction action is used as a boundary condition to a numerical model of the steady state electrical conduction in the front contact layers. This also allows for input of inhomogeneous material parameters, but on a macroscopic scale. The simpler approach, compared to the numerical model, enables simulations of complete cells. Effects of material inhomogeneities, shunt defects and grid geometry are simulated.The stability of CIGS solar cells with varying absorber thickness, varying buffer layer material and CIGS from two different deposition systems are subjected to damp heat treatment. During this accelerated ageing test the cells are monitored using characterization methods including J-V, QE, C-V and J(V)T. The degradation studies show that the typical VOC decrease experienced by CIGS cells subjected to damp heat is most likely an effect in the bulk of the absorber material.When cells encapsulated with EVA are subjected to the same damp heat treatment, the effect on the voltage is considerably reduced. In this situation the EVA is saturated with moisture, representing a worst case scenario for a module in operation. Consequently, real-life modules will not suffer extensively from the VOC degradation effect, common in unprotected CIGS devices.

Many electrical characteristics of Cu(In,Ga)Se 2 -based solar cells, including current–voltage characteristics, are affected by specific properties of negative-U defects in the absorber. We present these characteristics and discuss them in the framework of a V Se – V Cu defect model proposed by Lany and Zunger. We show how these defects influence photocarrier transport and the dominant recombination mechanism, and hence also the photovoltaic parameters of the cells. Numerical simulations validating our approach will also be presented.

Thin-film heterojunction solar cell devices are modelled in two dimensions, from fundamental material parameters, using the finite element method. The electrostatic potential is solved for, togethe ...

The band gap of Zn(O,S) and (Zn,Mg)O buffer layers are varied with the objective of changing the conduction band alignment at the buffer layer/CuGaSe2 interface. To achieve this, alternative buffer layers are deposited using atomic layer deposition. The optimal compositions for CuGaSe2 solar cells are found to be close to the same for (Zn,Mg)O and the same for Zn(O,S) as in the CuIn0.7Ga0.3Se2 solar cell case. At the optimal compositions the solar cell conversion efficiency for (Zn,Mg)O buffer layers is 6.2% and for Zn(O,S) buffer layers it is 3.9% compared to the CdS reference cells which have 5–8% efficiency.

In this work we make an attempt to clarify ambiguities and to present our present understanding of defects and defect-related phenomena affecting the capacitance characteristics of Cu(In,Ga)Se 2 -based solar cells. We discuss deep defect levels derived from admittance and deep level transient spectroscopy, as well as shallow levels affecting the charge distributions by capacitance–voltage profiling. The discussion includes two types of metastable effects affecting capacitance characteristics: one induced at room temperature by light or voltage bias, and one created at low temperature by red illumination of reverse-biased junction (ROB effect). Recent theoretical achievements on negative-U properties of such intrinsic defects as selenium vacancies and In Cu antisites are used to explain the experimental data. We show that the most prominent level in the admittance spectra is due to the response of interface states combined with contribution of deep V Se –V Cu −/2− acceptor level. We attribute the ROB metastability to the relaxation of In Cu defects upon electron capture. Finally we discuss the influence of these defects on the device efficiency.

This work reports on processing, analysis and characterization of copper indium gallium diselenide (CIGS) used as a photosensitive layer for sensors such as sun angle detectors in space applications. CIGS-based solar cell devices with different CIGS layer thicknesses and the pn-junction located on the opposite side of the incidence of light were illuminated through their ultra-thin transparent molybdenum back contacts. The results from the current density versus voltage and quantum efficiency measurement indicate that the CIGS absorber layer may not exceed 750 nm at backside illumination, due to the limited CIGS diffusion length.

CdS is conventionally used as a buffer layer in Cu(In,Ga)Se2, CIGS, solar cells. The aim of this thesis is to substitute CdS with cadmium-free, more transparent and environmentally benign alternative buffer layers and to analyze how the material properties of alternative layers affect the solar cell performance. The alternative buffer layers have been deposited using Atomic Layer Deposition, ALD. A theoretical explanation for the success of CdS is that its conduction band, Ec, forms a small positive offset with that of CIGS. In one of the studies in this thesis the theory is tested experimentally by changing both the Ec position of the CIGS and of Zn(O,S) buffer layers through changing their gallium and sulfur contents respectively. Surprisingly, the top performing solar cells for all gallium contents have Zn(O,S) buffer layers with the same sulfur content and properties in spite of predicted unfavorable Ec offsets. An explanation is proposed based on observed non-homogenous composition in the buffer layer. This thesis also shows that the solar cell performance is strongly related to the resistivity of alternative buffer layers made of (Zn,Mg)O. A tentative explanation is that a high resistivity reduces the influence of shunt paths at the buffer layer/absorber interface. For devices in operation however, it seems beneficial to induce persistent photoconductivity, by light soaking, which can reduce the effective Ec barrier at the interface and thereby improve the fill factor of the solar cells. Zn-Sn-O is introduced as a new buffer layer in this thesis. The initial studies show that solar cells with Zn-Sn-O buffer layers have comparable performance to the CdS reference devices. While an intrinsic ZnO layer is required for a high reproducibility and performance of solar cells with CdS buffer layers it is shown in this thesis that it can be thinned if Zn(O,S) or omitted if (Zn,Mg)O buffer layers are used instead. As a result, a top conversion efficiency of 18.1 % was achieved with an (Zn,Mg)O buffer layer, a record for a cadmium and sulfur free CIGS solar cell.

Presented here is an examination of unstructured and structured (by anisotropic etching), monocrystalline silicon wafers coated with sputter deposited aluminum and chemical vapor deposited silicon dioxide for high solar reflectance and high thermal emittance, respectively. The topography of the samples was characterized with optical and scanning electron microscopy. Optical properties were examined with reflectance and transmittance spectroscopy, partly by usage of an integrating sphere. The measurement results were used to estimate the equilibrium temperature of the surfaces in space. The suitability of the surfaces with high solar reflectance and high thermal emittance to aid in the thermal control of miniaturized, highly integrated components for space applications is discussed. A silicon dioxide layer on a metal layer results in a slightly lower reflectance when compared to surfaces with only a metal layer, but might be beneficial for miniaturized space components and modules that have to dissipate internally generated heat into open space. Additionally, it is an advantage to microstructure the emitting surface for enhanced radiation of excess heat.

Performance comparison of Cu(In,Ga)Se2 solar cells fabricated using RF and DC sputtered ZnO:Al transparent conducting oxides

Thin-film solar cells based around the absorber material CuIn1-xGaxSe2 (CIGS) are studied with respect to their stability characteristics, and different ways of modelling device operation are investigated. Two ways of modelling spatial inhomogeneities are detailed, one fully numerical and one hybrid model. In the numerical model, thin-film solar cells with randomized parameter variations are simulated showing how the voltage decreases with increasing material inhomogeneities.With the hybrid model, an analytical model for the p-n junction action is used as a boundary condition to a numerical model of the steady state electrical conduction in the front contact layers. This also allows for input of inhomogeneous material parameters, but on a macroscopic scale. The simpler approach, compared to the numerical model, enables simulations of complete cells. Effects of material inhomogeneities, shunt defects and grid geometry are simulated.The stability of CIGS solar cells with varying absorber thickness, varying buffer layer material and CIGS from two different deposition systems are subjected to damp heat treatment. During this accelerated ageing test the cells are monitored using characterization methods including J-V, QE, C-V and J(V)T. The degradation studies show that the typical VOC decrease experienced by CIGS cells subjected to damp heat is most likely an effect in the bulk of the absorber material.When cells encapsulated with EVA are subjected to the same damp heat treatment, the effect on the voltage is considerably reduced. In this situation the EVA is saturated with moisture, representing a worst case scenario for a module in operation. Consequently, real-life modules will not suffer extensively from the VOC degradation effect, common in unprotected CIGS devices.

Photoluminescence and electroluminescence spectra of the absorber layer in ZnO/CdS/Cu(In,Ga)Se-2 solar cells were measured. Their dependence on temperature, excitation intensity and applied voltage were studied. Electroluminescence measurements were used to investigate light- and bias-induced metastabilities in the absorber of the cells. We showed that metastable changes of defect distributions, which produce an effect on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se2 material, affect also the luminescence yield. The dependence of the intensity and shape of the electroluminescence spectra on the state of the sample is observed. These results fit well into the theoretical calculations of Lany and Zunger model showing that divacancy complex (V-Sc-V-Cu) is responsible for metastable changes observed in ZnO/CdS/Cu (In, Ga) Se-2-based solar cells. We conclude that during light soaking or/and forward bias the probability of nonradiative recombination is decreased.

Recent theoretical calculations have traced an origin of light- and voltage bias-induced metastabilities in Cu(In,Ga)Se2-based solar cells to negative-U properties of the VSe–VCu complex. In this paper we compare experimental findings with theoretically predicted properties of these defects and calculated values of parameters characteristic for transitions between their different states. Profiles of net acceptor concentrations in the relaxed and metastable states obtained by capacitance profiling have been studied, as well as annealing kinetics of the persistent defect distributions by thermally stimulated capacitance and conductivity. Good qualitative and quantitative agreement are found between theory of VSe-related defects and experimental results. The consequences from the point of view of photovoltaic efficiency of the Cu(In,Ga)Se2-based solar cells are discussed.

(Zn,Mg)O films, fabricated by atomic layer deposition, ALD, are investigated as buffer layers in Cu(In,Ga)Se2-based thin film solar cells. Optimization of the buffer layer is performed in terms of thickness, deposition temperature and composition. High efficiency devices are obtained for deposition at 105–135 °C, whereas losses in open circuit voltage are observed at higher deposition temperatures. The optimal compositional region for (Zn,Mg)O buffer layers in this study is for Mg/(Zn + Mg) contents of about 0.1–0.2, giving band gap values of 3.5–3.7 eV. These devices appear insensitive to thickness variations between 80 and 600 nm. Efficiencies of up to 16.2% are obtained for completely Cd- and S-free devices with (Zn,Mg)O buffer layers deposited with 1000 cycles at 120 °C and having a band gap of 3.6 eV.

In this work numerical modeling of Cu(In,Ga)Se-2 (CIGS) thin-film solar cells is presented. The main features of the optical simulator SunShine and the electrical simulator Aspin that are used for ...

Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS. The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing. ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer. For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.

Gas flow sputtering of Cu(In,Ga)Se2 (CIGS) from two opposing Cu(In,Ga)Se2 targets with slightly Cu-poor stoichiometry was performed, using i) selenium only provided by the target and ii) using additional selenium from an elemental source inside the sputtering system. In both cases the composition of the sputtered CIGS film was similar to the target. A sputter process without additional selenium supply led to poor cell results at about 2 % efficiency. After introducing a post-treatment in selenium atmosphere immediately after the sputter deposition, the cell results were dramatically improved to 12 %. With selenium added during the sputtering process, 13.7 % conversion efficiency was obtained without any post treatment. Gas flow sputtering uses a high gas flow to transport the material from the plasma to the growing film, thereby the atoms will be thermalized, similarly to in an evaporation process. Reactant gases can be supplied close to the substrate, outside the plasma, thereby reducing the risk for sputter damage.

Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2(Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage ( $V_{{\rm OC}}$ ; +17%rel.), short-circuit current ( $J_{{\rm SC}}$ ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

Cu(In,Ga)Se-2 thin-film solar cells with Ga-graded absorber layers and a [Cu]/([In] + [Ga]) ratio varying between 0.5 and 1.0 were prepared by coevaporation and investigated. Except for the sample with a final [Cu]/([In] + [Ga]) ratio of 1.0, the samples were Cu-poor at all times during the evaporation. The variation in copper was found to influence the material properties in several ways: 1) Changing the Cu content had a strong impact on In and Ga interdiffusion, resulting in decreased Ga gradients in samples with large Cu deficiency; 2) the Cu-poor Cu(In, Ga)(3)Se-5 phase was detected in absorbers with [Cu]/([In] + [Ga]) ratios of 0.65 and below; and 3) the grain size changed significantly with the Cu variation. We observe a trend of reduced solar cell efficiencies for [Cu]/([In] + [Ga]) ratios of 0.65 and below, with an efficiency of 13.4% for the sample with a [Cu]/([In] + [Ga]) ratio of only 0.5, i.e., far from stoichiometry. We tentatively attribute the efficiency loss to a high concentration of point defects caused by the Cu deficiency.

Al2O3 rear surface passivated ultra-thin Cu(In,Ga)Se-2 (CIGS) solar cells with Mo nano-particles (NPs) as local rear contacts are developed to demonstrate their potential to improve optical confinement in ultra-thin CIGS solar cells. The CIGS absorber layer is 380 nm thick and the Mo NPs are deposited uniformly by an up-scalable technique and have typical diameters of 150 to 200 nm. The Al2O3 layer passivates the CIGS rear surface between the Mo NPs, while the rear CIGS interface in contact with the Mo NP is passivated by [Ga]/([Ga] + [In]) (GGI) grading. It is shown that photon scattering due to the Mo NP contributes to an absolute increase in short circuit current density of 3.4 mA/cm(2); as compared to equivalent CIGS solar cells with a standard back contact. (C) 2014 The Authors. Published by Elsevier B.V. This work is funded by the European Commission via FP7 MarieCurie IEF 2011 Action No. 300998, the Knut and Alice Wallenberg Foun-dation through Grant No. 2012.0083 and the Swedish Research Councilunder Grant No. 2008-6572 via the Linköping Linnaeus EnvironmentLiLi-NFM. Lastly, F.H.would like to thankthe European and Wallonia Re-gion FEDER Grant ECP12020011678F (MINATIS project) for thefinan-cial support

This paper presents the development of a miniaturized, thin film sun sensor for sun angle detection in space applications. The overall design, different thin film layer sequences, processes, materials used and system integration are described. The sun sensor has a field of view (FoV) of greater than 2π sr (±90°) and shall have a resolution of approximately 1° in elevation and azimuth angle. This sun sensor is therefore a coarse sun sensor with the advantage of having a large field of view. Key elements are the curved shape, the photosensitive layer consisting of copper indium gallium diselenide (CIGS), the transparent conductive layer consisting of thin molybdenum or aluminum-doped zinc oxide, and its integrated design. Different possible layer sequences are presented and discussed. The sun sensor will be one of the sensors in the attitude determination and control system (ADCS) of a nano-satellite.

In this paper, we summarize our investigations of the minority carrier traps in Cu(In,Ga)Se2 devices by use of the Laplace-Deep Level Transient Spectroscopy (Laplace-DLTS) method. We explain the nonlinearities of the Arrhenius plots in terms of the thermally assisted tunneling (TAT) effect and indicate that the standard analysis by use of admittance spectroscopy (AS) may lead to significant errors in trap parameters. We show that the position of the peak in the DLTS and admittance spectra depends on activation energy as well as on electric field. The values of the activation energies after correction for the electric field agree very well with the transition energies predicted by theoretical calculations for InCu++, InCu+, (InCu+VCu) and VSe++ defects. Thus, we conclude that the minority carrier signal is not only due to a continuous distribution of traps at the CdS/Cu(In,Ga)Se2 interface but also to the close-to-interface bulk donor states. Basing on these findings and using the concept of the defect layer, we propose a model which explains the metastabilities in the minority carrier trap spectra induced by illumination and reverse bias.

In this paper the effect of an in depth variation of the Ga/(In+Ga) ratio in CIGS based thin film solar cells is presented. The conclusions made are based on a review of earlier publications, theoretical considerations and results from a large set of new devices. For standard devices with normally thick CIGS films (1.5–2 μm) deposited at a relatively long deposition time (60 min) an improved efficiency of around 0.4% units for the devices with an increased Ga/(In+Ga) ratio towards the back contact is observed. This improvement is due to a field assisted carrier collection resulting in an improved QE response at long wavelengths. When the CIGS thickness is reduced the importance of the increased Ga/(In+Ga) ratio towards the back contact is enhanced and at a CIGS thickness of 0.5 μm a gain of 2.5% units is obtained. The gain is due to an improved Voc and FF. The main reason for the improvement is passivation of the back contact, which becomes increasingly detrimental for the device performance as the CIGS thickness is reduced. Also for pure CIS a significant improvement of the device performance is obtained by introducing an increased Ga concentration towards the back contact. This improvement is, however, more related to the introduction of Ga itself than the gradient of the Ga-concentration. From many simulations the largest gain is predicted for an increased Ga/(In+Ga) ratio towards the CIGS surface. However, neither in the literature nor from our own experiments we can find evidence for an improved device performance due to an increased Ga-concentration towards the CIGS surface.

For solar cells with absorbers made of co-evaporated Cu(In,Ga)Se 2 (CIGS), the two-stage process originating from Boeing is a familiar recipe. A key issue for this process is that the film during the first stage is grown Cu-rich, after which the composition in the second stage evolves to be Cu-poor. In our extreme version of the Boeing recipe, which we denote CURO, the second stage is performed completely without Cu flux. The CURO films have relatively large grains, but a rough surface with crevices between the grains. In this paper we examine a reversed two-stage process, INRO, where the metal fluxes are constant in the fist stage and yield a Cu-poor film. In the second stage, the In and the Ga sources are turned off, and the film evolves gradually less Cu-poor until the final composition is reached. For both processes, end-point detection (EPD) is used to achieve a predetermined composition of the CIGS. Comparing these films with the ones from CURO, the grains from the reversed process are smaller and the top surface is smoother. This is true for CIGS films with Cu-poor stoichiometry with a Cu content sufficiently low to fabricate solar cell devices. If the process continues, the grains undergo a transformation and grow to grain sizes usually seen in CIGS films grown under Cu-rich conditions and, finally, the stoichiometry becomes Cu-rich. The crevices from the CURO process have been explained by transport of Cu from Cu x Se, which during the first stage is segregated in the grain boundaries and subsequently consumed during the second stage. For the reversed process, no similar material transport is observed. Devices that originated from the INRO process did not perform well, but the process is an interesting key for understanding the grain restructuring observed close to Cu-poor/Cu-rich transitions.

The diffusion of indium and gallium in polycrystalline thin film Cu(In,Ga)Se2 layers has been investigated. Bilayer structures of CuInSe2 on top of CuGaSe2 and vice versa have been fabricated in both a Cu-rich and Cu-poor process (in relation to the ideal stoichiometry). In each process molybdenum coated soda-lime glass with and without a sodium barrier was used. These bilayers were analyzed with secondary ion mass spectrometry, X-ray diffraction, scanning electron microscope and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. It was found that the grain boundary diffusion was not significantly higher than the diffusion inside the grains, also for Cu-rich layers. The diffusion is suggested to mainly proceed via vacant metal sites in the lattice structure. In sodium free films a higher diffusion into the bottom layers, compared to films with sodium, was seen in all cases. This observation was explained with a larger number of vacancies, that facilitates indium and gallium diffusion, in the sodium free films. The difference in diffusion between indium in CGS layers and gallium in CIS layers, in both Cu-rich and Cu-poor processes, was small for layers with sodium. q 2003 Elsevier Ltd. All rights reserved.