The Inverse Temperature Crystallization (ITC) and seed-induced method were used to grow high-quality cubic samples within hours using a 1M solution of methylammonium lead bromide (MAPbBr3) samples. The current-voltage (I-V) hysteresis observed within the MAPbBr3 perovskite sample demonstrates anomalous dependence on scan rate and various preconditioning pulses. We investigate this dependence and the relationship of current-transient with slow migrating ions. The current transients fitted using a bi-exponential decay model produced two distinct time constants t1 = 38.4 s and t2 = 6.49 s associated with migrating ionic species. From the Arrhenius plot an activation energy of Ea = 0.410 associated with migrating Br ions was extracted. Future research is required towards the understanding of I-V hysteresis and the link to ion migration in MAPbBr3 perovskite. / Dissertation (MSc (Physics))--University of Pretoria, 2020. / NRF Funding / Physics / MSc (Physics) / Restricted

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Submitted by SILVIA LETICIA FERNANDES null (sy.fernandes@hotmail.com) on 2016-05-31T17:30:17Z No. of bitstreams: 1 Tese_Silvia Leticia Fernandes.pdf: 5734164 bytes, checksum: 5485bc4d14d63bd72484d78e0e3a1907 (MD5) / Rejected by Ana Paula Grisoto (grisotoana@reitoria.unesp.br), reason: Solicitamos que realize uma nova submissão seguindo a orientação abaixo: O arquivo submetido está sem a ficha catalográfica. A versão submetida por você é considerada a versão final da dissertação/tese, portanto não poderá ocorrer qualquer alteração em seu conteúdo após a aprovação. Corrija esta informação e realize uma nova submissão contendo o arquivo correto. Agradecemos a compreensão. on 2016-05-31T20:24:02Z (GMT) / Submitted by SILVIA LETICIA FERNANDES null (sy.fernandes@hotmail.com) on 2016-05-31T22:31:21Z No. of bitstreams: 1 Tese_Silvia Leticia Fernandes.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-06-01T14:17:17Z (GMT) No. of bitstreams: 1 fernandes_sl_dr_bauru.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5) / Made available in DSpace on 2016-06-01T14:17:17Z (GMT). No. of bitstreams: 1 fernandes_sl_dr_bauru.pdf: 5362328 bytes, checksum: 3ad1eb46dfedf8840bc516a67f927a4f (MD5) Previous issue date: 2016-04-27 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O desenvolvimento das células solares de perovskita foi acompanhado por uma revolução no campo dos dispositivos fotovoltaicos. Células solares de perovskita atingiram eficiências de conversão de energia maiores que 21% em apenas 5 anos após sua descoberta, colocando-as em competição com as células solares comerciais de silício. Apesar de promissores, os dispositivos de perovskita enfrentam desafios que impedem sua comercialização, sendo o maior deles o problema de estabilidade. Nesse âmbito, a presente tese teve como principal foco o desenvolvimento de células solares de perovskita baseadas em filmes nanoestruturados de Nb2O5 e TiO2, visando melhor compreensão do funcionamento desses dispositivos afim de se obter a solução dos problemas hoje enfrentados. Os resultados obtidos mostram eficiências maiores que 13% para o sistema: filme compacto de Nb2O5/ filme mesoporoso de TiO2/ CH3NH3PbI3; e eficiências tão elevadas quanto 15% para sistema usando filme compacto e mesoporoso de TiO2/ CH3NH3PbI3. As melhores células solares montadas com filmes compactos de Nb2O5 apresentaram correntes de curto circuito de 19 mA/cm2, tensão de circuito aberto de 960 mV, fator de preenchimento de 75% e eficiências de 13%. Para as células formadas com filmes de compactos de TiO2 foram obtidas correntes de curto circuito de 20 mA/cm2, tensão de circuito aberto de 1V, fator de preenchimento de 70% e eficiências de 15%. A estabilidade dos dispositivos e a presença de histerese nas curvas de tensão-corrente foram estudadas variando parâmetros como a composição da camada compacta (TiO2 versus Nb2O5), a espessura dessa camada; assim como o método de síntese utilizado para preparar os filmes de perovskitas (método de deposição sequencial versus método de engenharia dos solventes). De maneira geral, os resultados mostraram que células solares preparadas com filmes de Nb2O5 de 50 nm em conjunto com perovskitas preparada pelo método de deposição sequencial resultaram em dispositivos sem histerese e com maior estabilidade do que os preparados com filmes de TiO2 ou mais espessos que 50nm. / The development of perovskite solar cells was accompanied by a revolution in the photovoltaics field. Perovskite solar cells have reached higher energy conversion efficiencies of 21% in just 5 years after its discovery, putting them in competition with commercial silicon solar cells. Although promising, the perovskite devices face some challenges which delay their commercialization, and one of most important is the stability. In this context, the present thesis intended the development of perovskite solar cells based on nanostructured films of Nb2O5 and TiO2, in order to better understand the functioning of these devices. Efficiencies up to 13% were obtained for the system composed of: compact Nb2O5 / mesoporous TiO2/ CH3NH3PbI3 and efficiencies as high as 15% for compact system using compact TiO2/ mesoporous TiO2/ CH3NH3PbI3. The best solar cells prepared using compact Nb2O5 films showed a short circuit current of 19 mA/cm 2 , open circuit voltage of 900 mV, fill factor of 75% and 13% of efficiency. Devices prepared using compact TiO2 films reached short circuit current of 20 mA/cm2 , open circuit voltage of 1V, fill factor of 70% and 15% of efficiency. The stability of the devices and the presence of current-voltage hysteresis were studied by changing parameters such as the composition and the thickness of the compact layer (TiO2 vs. Nb2O5), as well as the synthesis method used to prepare the perovskite films (sequential deposition method vs solvent-engineering method). Overall, the results showed that solar cells prepared with 50 nm Nb2O5 film in combination with perovskite prepared by sequential deposition method have resulted in devices without hysteresis and greater stability than those prepared with TiO2 films or thicker than 50nm. / FAPESP: 2012-07745-9

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The work deals with the theory of preparing perovskite solar cells. How about basic structures and the specific types of training opportunities and reproducibility of results. In the third part describes the complete preparation of the article, which reached the highest efficiency and the procedure for subsequent repetition of the experiment

This thesis studies the degradation of perovskite photovoltaic cells placed in atmospheres of different moisture. Samples with inverse structure: ITO/ PEDOT:PSS/ CH3NH3PbI3–XClX / PC70BM/ Ca/ Al were prepared. Electrical characteristics were measured for 2 months and similar degradation trend was observed for all the samples. Perovskite cell efficiency PCE decreased to 20 % of the initial value in t80= 46±3 days in laboratory, t80=23±1$ days in nitrogen atmosphere and t80=25,7±0,6 dní days in dry atmosphere. For the initial 27 days of the experiment, a faster degradation linked with the decrease of ISC, FF and VOC was observed. After this period, the value of FF has stabilised at 0,777±0,009 % and the value of VOC at 0,70±0,02 % of their original value (in the laboratory atmosphere). Additional fall of PCE resulted only from the decrease of ISC and was slower than in the initial period. From the results acquired, it has emerged that during the experiment, the cell encapsulation provided a sufficient barrier against outer moisture. Residual moisture present in the sctructure, was labeled as the source of the degradation. The moisture is believed to enter with the hygroscopic material PEDOT:PSS during the samples preparation procedure. During the degradation, absorption measurements of photovoltaic cells were executed. The absorption spectra didn't change. That indicates, that the decrease of ISC is not caused by the reduction of light absorption. The thesis also studied the degradation of perovskite solar cells under illumination. The samples were exposed to UV radiation for 55,5 hours. The PCE time of decrease to 20 % of the initial value was t80 = 6±2 days. It was revealed, that UV radiation significantly accelerates the decrease of ISC.

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Der Schwerpunkt der vorliegenden Arbeit liegt in der Charakterisierung der elektronischen Eigenschaften von hybriden organisch-anorganischen Perowskit (HOIP)-Schichten während der Schichtbildung und in verschiedenen Umgebungen mittels Photoelektronenspektroskopie (PES). Insbesondere wird der Methylammonium-Blei-Iodid-Chlorid-Perowskit (MAPbI3-xClx) untersucht. Als erstes werden Änderungen in den elektronischen Eigenschaften, der Zusammensetzung, sowie der Kristallstruktur mittels PES, Flugzeit-Sekundärionenmassenspektrometrie, sowie Röntgendiffraktometrie mit streifendem Einfall analysiert. Die Resultate weisen auf die entscheidende Rolle von Chlor im texturierten Wachstum der Perowskitschicht hin. Die auskristallisierte Perowskitschicht weist eine stärkere n-Typ Eigenschaft auf, welche auf die Änderung der Zusammensetzung während der Schichtbildung zurückgeführt werden kann. Außerdem beweisen die Ergebnisse eindeutig die Ablagerung von Chlor an der Grenzfläche zwischen der Perowskitschicht und dem Substrat. Zweitens werden die separaten Einflüsse von Wasser, Sauerstoff, und Umgebungsluft auf die elektronischen Eigenschaften von MAPbI3-xClx-Schichtoberflächen untersucht. Bereits geringste Wassermengen ähnlich wie im Hochvakuum oder in inerter Umgebung können eine reversible Reduzierung der Austrittsarbeit hervorrufen. Höherer Wasserdampf-Partialdruck führt zu einer Verschiebung des Valenzbandmaximums (VBM) weit vom Fermi-Niveau, sowie zu einer Reduzierung der Austrittsarbeit. Im Gegensatz dazu führt eine Sauerstoffexposition zu einer Verschiebung des VBM in Richtung des Fermi-Niveaus und zu einer Steigerung der Austrittsarbeit. Analog kommt es zu einer Verschiebung von bis zu 0.6 eV bei einer Exposition gegenüber Umgebungsluft, was den vorwiegenden Einfluss von Sauerstoff demonstriert. Die vorliegenden Untersuchungen betonen den kritischen Einfluss der Schichtbildung, der Zusammensetzung, sowie der Umgebungsbedingungen auf die elektronischen Eigenschaften von HOIP. / The present thesis aims at characterizing the electronic properties of solution-processed hybrid organic-inorganic perovskites (HOIPs) in general, and the HOIP methyl ammonium (MA) lead iodide-chloride (MAPbI3-xClx) films, in particular, at different stages, namely from its formation to its degradation, by means of photoelectron spectroscopy (PES). Firstly, the formation of MAPbI3-xClx films upon thermal annealing is monitored by a combination of PES, time-of-flight secondary ion mass spectrometry, and grazing incidence X-ray diffraction for disclosing changes in electronic properties, film composition, and crystal structure, respectively. Overall, the results point to the essential mediating role of chlorine in the formation of a highly textured perovskite film. The film formation is accompanied by a change of composition which leads to the film becoming more n-type. The accumulation of chlorine at the interface between perovskite and the underlying substrate is also unambiguously revealed. Secondly, the separate effects of water and oxygen on the electronic properties of MAPbI3-xClx film surfaces are investigated by PES. Already low water exposure – as encountered in high vacuum or inert conditions – appears to reversibly impact the work function of the film surfaces. Water vapor in the mbar range induces a shift of the valence band maximum (VBM) away from the Fermi level accompanied by a decrease of the work function. In contrast, oxygen leads to a VBM shift towards the Fermi level and a concomitant increase of the work function. The effect of oxygen is found to predominate in ambient air with an associated shift of the energy levels by up to 0.6 eV. Overall, the findings contribute to an improved understanding of the structure-property relationships of HOIPs and emphasize the impact of least variation in the environmental conditions on the reproducibility of the electronic properties of perovskite materials.