An increase in conversion efficiency of CIGS thin film solar cells has been reached at Ångström Solar Center (ÅSC) due to an introduction of a post-deposition-treatment (PDT) with potassium fluoride (KF) on the CIGS absorber layer. The PDT will however affect the growth of the cadmium sulfide (CdS) buffer layer normally deposited on the CIGS layer. The purpose of this study has therefore been to increase the conversion efficiency of the CIGS solar cells by optimizing the process parameters when growing CdS with a chemical bath deposition process (CBD) on CIGS with KF-PDT. The purpose has also been to understand how CdS grows on CIGS with KF-PDT. CdS has been deposited with CBD on both CIGS with KF-PDT and on soda lime glass samples by varying process parameters such as time, bath temperature and concentrations. The solar cells were characterized using current-voltage measurements as well as external quantum efficiency measurements. Further methods of analysis were profilometry, x-ray fluorescence spectroscopy and scanning electron microscopy. It was found that the solar cells with the best performance were obtained at a bath temperature of 70 °C, process time of 3 min after preheating the cadmium acetate and ammonia solutions for 6 min and using the concentrations of the baseline process for CBD of CdS at ÅSC. The best cell of this sample resulted in a conversion efficiency of 19.1 % without antireflective coating.

En quelques années, l'efficacité des cellules solaires à base de Cu(In,Ga)Se2 (CIGS) est passée de 20% à 22.6%. La rapidité de ce développement montre que le CIGS est un matériaux idéal pour les technologies solaires en couches minces. Pourtant, le coût de production cette technologie doit encore être abaissé pour une meilleure compétitivité. La fabrication d'un module avec une couche CIGS plus fine permettrait d'augmenter la production d'une usine et de réduire sa consommation en métaux. Ce travail de thèse vise à réduire l'épaisseur du CIGS d'un standard de 2.0-2.5 µm à une épaisseur inférieure à 500 nm sans altérer les performances des cellules. Cependant, comme rapporté dans la littérature, nous avons observé une diminution des rendements, ce que nous avons analysé en détail en comparant simulations et caractérisations d'échantillons. Celle-ci est causée à la fois par une faible absorption de la lumière dans la couche de CIGS et par un impact important du contact arrière (fortes recombinaisons et faible réflectivité). Pour dépasser ces limites, nous démontrons à la fois théoriquement et expérimentalement que le contact arrière en molybdène peut être remplacé par un oxyde transparent conducteur couplé à un miroir métallique. Nous obtenons de cette manière de meilleurs rendements de cellules. Pour atteindre ce résultat, une optimisation du dépôt de CIGS a été nécessaire. De plus, nous prouvons qu'une couche d'oxyde perforée, insérée entre le CIGS et le contact arrière, limite les recombinaisons des porteurs de charges et réduit l'influence des courants parallèles. Au final, nous avons fabriqué une cellule avec un rendement de 10.7% sur SnO2:F passivé par Al2O3. / In the past three years, record efficiency of Cu(In,Ga)Se2 (CIGS) based solar cells has improved from 20% up to 22.6%. These results show that CIGS absorber is ideal for thin-film solar cells, even if this technology could be more competitive with a lower manufacture cost. The fabrication of devices with thinner CIGS absorbers is a way to increase the throughput of a factory and to reduce material consumption. This PhD thesis aims to develop cells with a CIGS thickness below 500 nm instead of the conventional 2.0-2.5 µm. However, as reported in the literature, we observed a decrease in cell performance. We carefully analyzed this effect by the comparison between simulations and sample characterizations: it is attributed, on one hand, to a lack of light absorption in the CIGS layer and, on the other hand, to an increased impact of the back-contact (high recombination and low reflectivity). To resolve these problems, we demonstrated theoretically and experimentally that the use of an alternative back-contact, other than molybdenum, such as a transparent conducting oxide coupled with a light reflector, improves the cell efficiency. To achieve this result, an optimization of the CIGS deposition was necessary. Moreover, we proved that a porous oxide layer inserted between the CIGS and the back-contact limits the charge-carrier recombination and removes some parasitic resistance. Finally, an efficiency of 10.7% was achieved for a 480-nm-thick CIGS solar cell with a SnO2:F back-contact passivated with a porous Al2O3 layer.

The improvement of low cost, efficient photovoltaic devices is a leading technological challenge in the recent decade. There is a need to develop scalable and high-throughput manufacturing techniques that could reduce costs and improve manufacturing of chalcogenide solar cells. Copper, indium, gallium, and selenium (CIGS) Thin films polycrystalline heterojunction solar cells appear to be most appropriate with to cost and ease of manufacture. Currently Cu (In,Ga) (Se, S)2 materials hold the highest record cell efficiency of 22.3% in laboratory scale for thin films solar cells and the efficiency still be boosted by improving the different layers of the photovoltaic devices. CIGS chalcogenide absorber layers has been a leading candidate material in photovoltaic devices for thin films solar cells and space applications due to its unique optical-electronic properties as well as its radiation resistance. In the present work, thin films of Cu (In, Ga) (Se, S)2 were deposited at room temperature on glass substrates coated with ITO and Mo by electrodeposition techniques. The obtained polycrystalline thin films were characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analysis. Thin films of Cu (In, Ga) (Se, S)2 grown by electrodeposition were subsequently processed into several sets of conditions including vacuum heat treatment, heat treatment in the presence of selenium or sulfur, heat treatment in nitrous gas atmosphere (N2H2) at different temperature and processing times. To improve the composition and the crystalline structure of the thin layers and to optimize the electro-optical properties a heat treatment of the thin films was developed in two stages after the electrodeposition. It was observed that the first annealing step (heating treatment at 450 °C in a selenium atmosphere 40 minutes) produced an appreciable improvement in the crystalline structure in the thin layer composition. In a second stage a sulfurization of the CuGaSe2 films was performed at 400 °C for 10 min in the presence of molecular sulfur and under the forming gas atmosphere. The effect of sulfurization was the complete conversion of selenium to sulfur and, therefore, the transformation of CuGaSe2 into CuGaS2. The formation of CuGaS2 thin films was evidenced by the by the displacement of the diffraction peaks of the CuGaSe2 towards higher angles to which makes the X-Ray diffraction 18 pattern which makes it coincide with the diffraction pattern of the CuGaSe2 films, and by the shift towards the blue (higher energies) of the optical gap. The optical gap found for the CuGaSe2 layer was 1.66 eV, while the optical gap for the CuGaS2 was raised up to 2.2 eV. CdS thin films have been widely used as buffer layer in CIGS solar cells. However, when alloyed with Zn, ZnCdS can still improve its performance as buffer layer. ZnCdS can be used as buffer and as window material in photoconductive devices and in heterojunction thin film solar cells due the possibility to tune the bandgap with the content of Zn. The band spacing of this ternary material can be from 2.42 to 3.50 eV, depending on the Cd/Zn ratio. / La obtención de dispositivos fotovoltaicos más eficientes y de bajo coste es uno de los desafíos tecnológicos más importantes de las últimas décadas. Existe la necesidad de desarrollar técnicas de fabricación escalables y de alto rendimiento que puedan reducir los costos y mejorar la fabricación de células solares de capa fina. Las células solares de heterounión de capas finas de seleniuro (o sulfuro) de cobre, indio y galio (CIGS) parecen estar bien adaptadas lograr este reto debido a su bajo costo, facilidad de fabricación y elevado rendimiento de los dispositivos. En la actualidad, Cu(In, Ga)Se2 ostenta el record de eficiencia de células solares con 22,3% a escala de laboratorio y esta eficiencia todavía puede ser acrecentada si se mejoran las diferentes capas de los dispositivos fotovoltaicos. Además, las capas absorbedoras de calcogenuros CIGS son un material candidato importante en dispositivos fotovoltaicos para capas delgadas celdas solares para aplicaciones espaciales debido a sus propiedades electrónicas, así como a su resistencia a la radiación. En el presente trabajo, las películas delgadas de Cu(In, Ga)(Se, S)2 se depositaron a temperatura ambiente sobre sustratos de vidrio recubiertos con ITO y Mo mediante técnicas electroquímicas. Las películas finas policristalinas obtenidas se caracterizaron por espectroscopia óptica UV-Vis, difracción de rayos X (XRD), microscopía electrónica de barrido (SEM), microscopía de fuerza atómica (AFM), microscopía electrónica de transmisión (TEM) y espectroscopia de energía dispersiva (EDS). Las películas finas de Cu(In, Ga)(Se, S)2 crecidas por electrodeposición se procesaron posteriormente en varios conjuntos de condiciones que incluían tratamiento térmico en vacío, tratamiento térmico en presencia de selenio o de azufre, tratamiento térmico en atmósfera gas nidrón (N2H2) a diferentes temperaturas y tiempos de procesado. Para mejorar la composición y la estructura cristalina de las capas finas y para optimizar las propiedades electro-ópticas se desarrolló un tratamiento térmico de las películas finas en dos etapas posterior a la electrodeposición. Se observó que la primera etapa de recocido (tratamiento térmico a 450 ºC en una atmósfera de selenio durante 40 minutos) producía una mejora apreciable en la estructura cristalina y en la composición de la capa fina. 20 En una segunda etapa se realizó una sulfuración de las películas de CuGaSe2 se realizó a 400 °C durante 10 min en presencia de azufre molecular y bajo la atmósfera reductora de gas nidrón. El efecto de la sulfuración fue la completa conversión del selenio en azufre y, por tanto, la transformación de CuGaSe2 en CuGaS2. La formación de películas delgadas de CuGaS2 se evidenció por el desplazamiento de los picos de difracción de las capas de CuGaSe2 hacia ángulos más altos hasta lo que hace que el patrón de difracción de rayos X lo que hace que coincida con el patrón de difracción del CuGaS2 y por el desplazamiento hacia el azul (energías más altas) del gap óptico. El gap óptico encontrado para las capas de CuGaSe2 era de 1,66 eV, mientras que el gap óptico para las capas de CuGaS2 se elevó hasta 2,2 eV. Las películas delgadas de CdS se han utilizado ampliamente como capa tampón en células solares CIGS. Sin embargo, cuando se alea con Zn, para formar el ternario ZnCdS, todavía puede mejorar su rendimiento como capa buffer. ZnCdS puede utilizarse como tampón y como ventana óptica en dispositivos fotoconductores y en células solares de capa fina de heterounión debido a la posibilidad de ajustar el bandgap con el contenido de Zn. / L'obtenció de dispositius fotovoltaics més eficients i més barats és un dels reptes tecnològics més importants de les últimes dècades. Hi ha la necessitat de desenvolupar tècniques de fabricació que siguen escalables i d'alt rendiment i que permeten reduir els costos de fabricació i millorar el rendiment de les cèl·lules solars de capa fina. Les cèl·lules solars de heterounió de capes fines de seleniur (o sulfur) de coure, indi i gal·li (CIGS) semblen estar ben adaptades per assolir aquest repte degut a del seu baix cost, facilitat de fabricació i elevat rendiment dels dispositius. En l'actualitat, el Cu(In, Ga)Se2 ostenta el rècord d'eficiència de cèl·lules solars amb 22,3% a escala de laboratori i aquesta eficiència encara pot ser augmentada si es milloren les característiques de les diferents capes dels dispositius fotovoltaics. Les capes absorbidores de calcogenurs CIGS són un candidat important per dispositius fotovoltaics per a pel·lícules primes en cel·les solars i aplicacions espacialles degut a les seues propietats electròniques així com a la seua resistència a la radiació. En el present treball, les pel·lícules primes de Cu(In, Ga)(Se, S)2 es van dipositar a temperatura ambient sobre substrats de vidre recoberts amb ITO i Mo mitjançant tècniques electroquímiques. Les pel·lícules fines policristal·lines obtingudes es van caracteritzar per espectroscòpia òptica UV-Vis, difracció de raigs X (XRD), microscòpia electrònica de rastreig (SEM), microscòpia de força atòmica (AFM), microscòpia electrònica de transmissió (TEM) i espectroscòpia d'energia dispersiva (EDS). Les pel·lícules fines de Cu(In, Ga)(Se, S)2 crescudes per electrodeposició es van processar posteriorment en diversos conjunts de condicions que incloïen tractament tèrmic en buit, tractament tèrmic en presència de seleni o de sofre, tractament tèrmic en atmosfera reductora de gas nidró (N2H2) a diferents temperatures i temps de processat. Per millorar la composició i l'estructura cristal·lina de les capes fines i per optimitzar les propietats electro-òptiques es va desenvolupar un tractament tèrmic de les pel·lícules fines en dues etapes posterior a la electrodeposició. Es va observar que la primera etapa de recuit (tractament tèrmic a 450 º C en una atmosfera de seleni durant 40 minuts) produïa una millora apreciable en l'estructura cristal·lina i en la composició de la capa fina. 24 En una segona etapa es va dur a terme una sulfuració de les pel·lícules de CuGaSe2 que es va realitzar a 400 °C durant 10 min en presència de sofre molecular i sota l'atmosfera reductora de gas nidró. L'efecte de la sulfuració va ser la completa conversió seleni en sofre i, per tant, la transformació de CuGaSe2 a CuGaS2. La formació de pel·lícules primes de CuGaS2 es va evidenciar pel desplaçament dels pics de difracció de les capes de CuGaSe2 cap angles més alts fins el que fa que el patró de difracció de raigs X el que fa que coincideixi amb el patró de difracció del CuGaS2 i pel desplaçament cap al blau (energies més altes) del gap òptic. El gap òptic trobat per a les capes de CuGaSe2 era de 1,66 eV, mentre que el gap òptic per a les capes de CuGaS2 es va elevar fins a 2,2 eV. Les pel·lícules primes de CdS s'han utilitzat àmpliament com a capa amortidora en cèl·lules solars de CIGS. No obstant això, quan s'alea amb Zn per formar ZnCdS encara pot millorar el seu rendiment com a capa d'amortiment. ZnCdS pot utilitzar-se com capa tampó i com a finestra òptica en dispositius fotoconductors i en cèl·lules solars de pel·lícula fina d'heterounió degut a la possibilitat d'ajustar el seu bandgap que depoen del contingut de Zn. / Ullah, S. (2017). THIN FILM SOLAR CELLS BASED ON COPPER-INDIUMGALIUM SELENIDE (CIGS) MATERIALS DEPOSITED BY ELECTROCHEMICAL TECHNIQUES [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86290 / TESIS

Ma thèse a permis d’initier l’activité de recherche sur les cellules en couche mince à base de matériaux chalcopyrite et kësterite (CIGS et CZTS) et d’entamer leur réalisation technologique. En intégrant des critères économiques et environnementaux, nous avons défini une stratégie en deux points (i) utilisation de la pulvérisation cathodique comme unique procédé de dépôt pour tous les matériaux et utilisation de cibles uniques pour les matériaux composés. (ii) remplacement des matériaux toxiques et rares. J’ai alors développé le contact arrière en molybdène déposé sur verre sodocalcique, ayant les propriétés électriques requises et une bonne adhésion au substrat. Nous avons vérifié que le sodium présent dans le substrat migre jusqu’à la surface du molybdène. Après une étude bibliographique du dépôt de CIGS par cible unique, j’évalue les avantages et difficultés afférentes à cette méthode. Après une étude bibliographique des méthodes de formation du CZTS et après une étude de pulvérisation de CZTS à partir d’une cible unique, je justifie le choix d’une méthode originale de pulvérisation à partir de trois sources. Je présente ensuite une étude bibliographique évaluant la possibilité de déposer une couche tampon en Zn(O,S) par pulvérisation pour remplacer le CdS déposé par bain chimique. Enfin, après une étude montrant que notre équipement ne permet pas d’obtenir des couches d’AZO ayant les propriétés électriques requises, une étude matériau des couches d’ITO déposées par pulvérisation RF est réalisée. Des couches d’ITO amorphe ayant d’excellentes propriétés électriques et optiques sont obtenues et leur utilisation en tant que fenêtre optique des cellules est proposée. / Thin film photovoltaic cells based on CIGS and CZTS materials has been initiated in this work. Environmental and economic issues have been taken into account to define an original strategy. We aim to substitute all the toxic and rare materials by abundant and non-toxic materials. In order to simplify the fabrication process, we also decide to deposit all layers using sputtering technique. The molybdenum back contact has been developed on a soda lime glass (SLG) substrate, with adequate electrical properties and good adhesion to the substrate even after thermal treatments similar to those used during the absorber formation. We have verified the required sodium migration from the SLG substrate to the molybdenum surface. A bibliographic study has been done to evaluate a single-target sputtering method to form CIGS and CZTS films. CZTS thin film deposition from a single target has been studied, with unsatisfactory results. We finally suggest an original multi-target method. Then, a bibliographic study has been done to evaluate the relevance of a sputtered Zn(S,O) buffer layer to replace the CBD-CdS conventional buffer layer. A study of RF-sputtered AZO films has been carried out, but we didn’t obtain the required electrical conductivity. We finally study RF-sputtering of ITO films. We developed amorphous ITO thin films with excellent electrical and optical properties. We suggest using this material as the window layer of solar cells.

De nos jours, et après plus de quatre décennies de recherche et développement de la technologie CIS, une concurrence directe avec la technologie silicium est toujours loin d'être gagnée; néanmoins, il existe certains marchés de niche où la technologie silicium ne peut pas être utilisée ou est moins appropriée. Les procédés de fabrication actuels présentent certains inconvénients: (i) certain(e)s matériaux et/ou techniques utilisé(e)s ont une forte empreinte environnementale, (ii) certains matériaux alternatifs développés sont coûteux, (iii) certaines techniques utilisées ne sont pas facilement industrialisables ou basées sur l'utilisation d'atmosphères toxiques. Ce travail de thèse présente une stratégie de transition vers un processus basé exclusivement sur l’utilisation à la fois de la pulvérisation cathodique et de matériaux respectueux de l'environnement. Dans ce cadre, nous avons utilisé la pulvérisation cathodique pour déposer le contact arrière en molybdène. Nous avons développé un nouveau procédé de dépôt de la couche absorbante basé sur l’utilisation de la pulvérisation, à température ambiante et à partir d'une seule cible quaternaire sans apport supplémentaire de sélénium. Nous avons également développé des procédés de pulvérisation à température ambiante pour déposer la couche tampon (ZnSxO1-x) et la couche fenêtre (i-ZnO/AZO). Plusieurs techniques de caractérisation (XRD, SEM, FIB-SEM, EDX, Raman, SIMS, Effet Hall, GDOES, UV-Vis, et IV) ont été utilisées pour étudier l'effet des conditions de dépôt sur les propriétés des couches minces ainsi que pour caractériser les cellules solaires finales dont le meilleur résultat obtenu sur l'efficacité est proche de 12%. / Nowadays, and after more than four decades of research and development of the CIS based technology, a direct competition with silicon technology is still far from being won; however there exists some niche markets where the silicon technology cannot be used (flexible photovoltaic) or less favourable (BIPV). The current fabrication processes are still suffering from some drawbacks: (i) some of the used materials and/or techniques have a large environmental footprint (CBD-CdS/CBD-ZnSxO1-x), (ii) some developed alternatives are expensive (evaporated InxSy), (iii) the used techniques are not easily up scalable (evaporation) or based on the use of toxic atmospheres (Se based). This PhD work presents our strategy in moving towards a full sputtering process and in using only environmentally friendly materials. In this framework, we kept using standard material and process for the deposition of the back contact layer. We developed a new process for the deposition of the absorber layer which is based on pulsed DC-magnetron sputtering at room temperature from a single quaternary target without any additional selenium supply, followed by an annealing under inert atmosphere. We developed a room temperature sputtering process for the deposition of the ZnSxO1-x buffer layer. We also developed our appropriate pulsed DC and RF sputtering processes for the deposition of the window layer. Several characterisation techniques (XRD, SEM, FIB-SEM, EDX, Raman, SIMS, Hall Effect, GDOES, UV-Vis, and I-V) have been used to investigate the effect of deposition conditions on thin films properties as well as to characterize the final solar cells which best efficiency result is slightly under 12%.

The goal of this research project was to contribute to the understanding of CuGaSe2/CdS photovoltaic devices, and to improve the performance of these devices. The initial part of the research dealt with the optimization of a Sequential Deposition process for CuIn(Ga)Se2 absorber formation. As an extension of this, a recipe (Type I Process) for CuGaSe2 absorber layer fabrication was developed, and the deposition parameters were optimized. Electrical characterization of the thin films and completed devices was carried out using techniques such as Two-Probe and Three-Probe Current-Voltage, Capacitance-Frequency, Capacitance-Voltage, and Spectral Response measurements. Structural/chemical characterization was done using XRD and EDS analysis. Current densities of up to 15.2 mA/cm2, and Fill Factors of up to 58% were obtained using the Type I CuGaSe2 Process. VOC's, however, were limited to less than 700 mV. Several process variations, such as changes in the rate/order/temperature of depositions and changes in the thickness of layers, resulted in little improvement. With the aim of breaking through this VOC performance ceiling, a new absorber recipe (Type II Process) was developed. VOC's of up to 735 mV without annealing, and those of up to 775 mV after annealing, were observed. Fill Factors were comparable to those obtained with Type I Process, whereas the Current Densities were found to be reduced (typically, 10-12 mA/cm2, with the best value of 12.6 mA/cm2). This performance of Type II devices was correlated to a better intermixing of the elements during the absorber formation. To gain an understanding of the performance limitations, two simulation techniques, viz. SCAPS and AMPS, were used to model our devices. Several processing experiments and SCAPS modeling indicate that a defective interface between CuGaSe2 and CdS, and perhaps a defective absorber layer, are the cause of the VOC limitation. AMPS simulation studies, on the other hand, suggest that the back contact is limiting the performance. Attempts to change the physical back contact, by changes in the absorber processing, were unsuccessful. Processing experiments and simulations also suggest that the CuGaSe2/CdS solar cell involves a true heterojunction between these two layers.

No description available.

Following the concept utilize binary selenides as precursors with rapid thermal process (RTP) to fabricate CuInSe2 (CIS) thin film. In order to find the most promise process to get high quality CIS, several precursor stacking sequences have been tested which including SLG/In-Se/Cu-Se/Se, SLG/Cu-Se/In-Se/Se, SLG/0.1In-Se/Cu-Se/0.9/In-Se/Se, and SLG/0.5In-Se/Cu-Se/0.5/In-Se/Se, and the experiment result shows SLG/In-Se/Cu-Se/Se is the most suitable stacking sequence. Subsequently, varying Se flux to obtain several kinds copper selenides (Cu7Se4, Cu3Se2, CuSe, CuSe2) and indium selenides, try to find the suitable pairs through these binary selenides in SLG/In-Se/Cu-Se/Se structure. The suitable combination phase in Cu-Se precursor layer is CuSe blend with CuSe2. Large grain size CIS, about 1£gm, can be prepared in such precursor phase with film thickness between 700nm to 1£gm, strong (112) prefer-orientation vertical with substrate as well as good adhesion. Films were characterized through scanning election microscopy (SEM) to obtain grain size, surface morphology as well as film thickness. The X-ray diffractometer (XRD) was used to identify phase contained in whole film, and the phase constitution near surface layer was examined by Raman spectroscopy. If there are some second phases remaining in the thin film, combining the phase examination result of XRD and Raman spectroscopy, it can be estimate the second phase exist in the surface layers or internal film area.

This study is using selenization of stacked elemental layers to form Cu(In,Ga)Se2(CIGS). In the process, use Cu/Sb/In/Ga/Se precursor to heat to 550 oC at Se vapor in vacuum chamber. From the result of XRD¡BRaman and EPMA, that show of the precursor do not form to CIGS. After that, The result of using different layers precursor to form CIGS show that only Cu/In/GaSe/Se reach to form CIGS, but it still has second phase. According to the literature¡Athe reason for the formation of CIGS selenide process due to interdiffusion caused the formation of ternary solid phase, the solid phase diffusion reaction could be hampered.And then change to use rapid thermal selenization to form CIGS with two step of heating (hold at 300 oC and 650 oC) at N2 atmosphere. The laminated follow the best results in the selenide process Cu/In/GaSe/Se precursors in Se atmosphere, the (112) preferred orientation is 26.8o-26.9o in the XRD results of the fixed process conditions. EPMA composition analysis and comparison of Ga actual amount will increase with the estimated value of the amount of increase(Estimated value 4atom% actual value 2atom%¡FEstimated value 9.2tom% actual value 10atom%¡AGa/¢»=0.32), but the composition has yet to amend. Then will join Sb on CIGS observed from the SEM results Sb does improve the CIGS thin film flatness as well as to help grain growth in rapid thermal selenization, grain size of about 1 to 3£gm.

Colloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light. / text