Titanium dioxide was extensively used in solar cell industry and currently has been studied to produce passivated contacts in PERC/PERT and TOPCon solar cells. The aim of this paper was to analyze the impact of the thickness of TiO2 thin films deposited by electro-beam evaporation on the weighted reflectance and the surface passivation on silicon solar cells. Thin films with different thicknesses were deposited to produce PERT solar cells, varying from 50 to 90 nm. The surface passivation was enhanced as the thickness was increased. For instance, at 400 nm, the internal quantum efficiency increased from 71% to 76% when the thickness of the TiO2 was augmented from 50 nm to 90 nm. The lowest weighted reflectance was obtained in samples with 80 nm thick TiO2 films. Considering the compromise between antireflection properties and surface passivation, the highest efficiency solar cells were produced with 80 nm thick TiO2.

Abstract The silicon solar cells achieved relatively low prices in the last years and to introduce a new structure in the PV industry, the amount of silicon per watt has to be reduced, requiring a cost-effective manufacturing process. The use of n-type solar grade silicon has the advantages of presenting higher minority carrier lifetime than p-type one and the absence of the boron-oxygen defects. The aim of this paper is to present the development of 100 μm thick n+np+ silicon solar cells with a selective p+ rear emitter formed by boron deposited by spin-on and an Al/Ag grid deposited by screen-printing. The firing temperature of Ag/Al (rear face) e Ag (front face) was optimized and the temperature of 840 °C produced the devices with higher efficiency. The solar cells presented efficiencies of 16%, achieving a low silicon consumption of 1.6 g/W, 40% lower than thick p-type devices produced by the same process.

The diffusion of phosphorus and aluminum in a single-step thermal process may reduce the production cost of multicrystalline silicon (mc-Si) solar cells. The goal of this paper is to analyze the influence of phosphorus diffusion parameters on the electrical characteristics of p-type mc-Si solar cells processed with a single-step diffusion of phosphorus and aluminum. To avoid the wafer bowing during aluminum paste firing, the aluminum was deposited by e-beam and co-diffused in the same thermal step that the phosphorus diffusion. First, the aluminum diffusion was performed and, then, the POCl3 was introduced into the quartz tube. The steps of the phosphorus diffusion were experimentally optimized. The best result was found with the temperature of 875 ºC. The POCl3 concentration of 0.15 % and the optimized phosphorus diffusion parameters lead to the efficiency of 14.1%. The reduction of the oxygen flow during ramp temperature and aluminum diffusion improved the efficiency.

RESUMO A passivação das superfícies das células solares de silício é importante para obter alta eficiência, pois reduz a recombinação dos portadores de carga minoritários. O crescimento de SiO2 é a técnica mais efetiva para passivar lâminas de silício cristalino. O objetivo deste trabalho é apresentar o desenvolvimento e a análise da passivação na face posterior e frontal com diferentes espessuras de SiO2, crescido por oxidação seca. Células solares foram processadas com campo retrodifusor seletivo em lâminas de silício Czochralski, tipo p, grau solar. O emissor frontal foi produzido pela difusão de fósforo e o campo retrodifusor seletivo foi formado pela difusão de boro e alumínio, para possibilitar a passivação também na face posterior. Variou-se a temperatura (Toxi) e o tempo (toxi) de oxidação para formar camadas de SiO2 com diferentes espessuras. Com base em trabalhos anteriores, fixou-se toxi em 7 minutos e variou-se Toxi de 770 °C a 920 °C. A seguir, fixou-se Toxi em 800 °C e variou-se toxi de 2 a 90 minutos. As espessuras das camadas de SiO2 produzidas variaram entre 7 e 80 nm no emissor frontal e entre 1 e 14 nm na face posterior. Para avaliar a passivação, comparou-se a tensão de circuito aberto (Voc) e constatou-se que aumentou até toxi = 45 min e até Toxi = 860 °C. Com base nestes resultados, variou-se o tempo de oxidação para Toxi = 860 °C. A maior Voc, de 604 mV, foi obtida para: a) Toxi = 860 °C e toxi = 30 min e b) Toxi = 800 °C e toxi = 45 min, resultando em uma camada de SiO2 da ordem de 50 - 60 nm e de 10 - 14 nm na face dopada com fósforo e com boro, respectivamente. A maior eficiência alcançada foi de 16,0 %.

ABSTRACT The silicon solar cell passivation is essential to achieve high efficiency, because reduces the minority charge carrier recombination. The SiO2 growth is the most effective technique to passivate crystalline silicon wafers. The goal of this paper is to present the development and analysis of the rear and front passivation with different thicknesses of SiO2, grown by dry oxidation. Solar cells were processed with selective back surface field (BSF) in solar grade p-type Czochralski-grown wafers. The front emitter was produced by phosphorus diffusion and the selective BSF was formed by boron and aluminum diffusion to allow the passivation also in the rear face. The oxidation temperature (Toxi) and time (toxi) were ranged to form SiO2 layer with different thicknesses. Based on previous studies, the Toxi was kept at 800 °C and the toxi was ranged from 2 minutes to 90 minutes. In the next step, the Toxi was ranged from 770 °C to 920 °C for the toxi = 7 minutes. The thicknesses of the SiO2 layer ranged from 7 nm to 80 nm in the front emitter and from 1 nm to 14 nm in the rear face. The open circuit voltage (Voc) was compared to evaluate the passivation and this parameter increased up to toxi = 45 minutes and up to Toxi = 860 °C. Based on these results, the oxidation time was ranged for Toxi = 860 °C. The higher Voc, of 604 mV, was obtained for: Toxi = 860 °C and toxi = 30 minutes and b) Toxi = 800 °C and toxi = 45 min, resulting in a SiO2 layer of around 50 - 60 nm and of 10 - 14 nm in the phosphorus and boron doped face, respectively. The highest achieved efficiency was of 16.0 %.

RESUMO No custo de produção de uma célula solar, a lâmina de silício representa da ordem de 50 % do valor final do dispositivo. Para reduzir os custos, vem sendo proposta a diminuição das espessuras atuais de 180-200 µm para 100-120 µm. Embora o silício tipo p seja o mais usado na fabricação de células solares, este material é suscetível à degradação devido à interação boro-oxigênio. Os dispositivos fabricados em lâminas de Si tipo n, dopadas com fósforo não apresentam tal degradação e têm potencial de obtenção de altas eficiências devido ao maior tempo de vida dos portadores de carga minoritários. O objetivo deste trabalho é apresentar o desenvolvimento de células solares n+np+ fabricadas sobre lâminas de silício Cz (Czochralski) com 100 μm de espessura. Foram fabricados dispositivos finos com a estrutura n+np+ e comparados com os dispositivos n+pp+ fabricados em lâminas convencionais. A eficiência média alcançada foi de (14,6 ± 0,4) % e o dispositivo mais eficiente atingiu 15,3 %. O maior valor de eficiência ficou abaixo do obtido com estruturas n+pp+ de 200 μm de espessura, quando atingiu-se uma eficiência de 16,2 %. O principal parâmetro que reduziu a eficiência das células finas foi o fator de forma, que alcançou um valor máximo de 0,748. Embora a eficiência seja menor com as lâminas finas, obteve-se uma baixa relação massa/potência, de 1,5 g/W, valor que é aproximadamente 53 % inferior ao obtido quando foram usadas lâminas espessas tipo p.

ABSTRACT In the solar cell production cost, the silicon wafer represents roughly 50 % of the final value of the device. To lower the costs, the reduction of the current wafer thickness from 180-200 µm to 100-120 µm has been proposed. Although the p-type silicon is used to manufacture solar cells, it is susceptible to degradation due to boron-oxygen interaction. The devices manufactured in n-type silicon wafers doped with phosphorus do not present such degradation and has the potential of achieving higher efficiencies due to the larger minority charge carrier lifetime. The aim of this work is to present the development of n+np+ solar cells fabricated in Cz silicon wafers 100 μm thick. Thin devices were manufactured with the n+np+ structure and compared to n+pp+ devices manufactured in standard thick wafers. The average efficiency of (14.6 ± 0.4) % was achieved and the most efficient device reached 15.3%. The efficiency was lower than obtained with n+pp+ thick devices, which presented 16.2 %. The main parameter that reduced the efficiency of thin cells was the fill factor, which reached a maximum value of 0.748. Although the efficiency reached by thin n-type devices was worse than thick p-type ones, the former achieved a low mass/power ratio of 1.5 g/W, which is approximately 53 % lower than that obtained from thick p-type devices.

The aim of this work is to present the development and comparison of thin n+pp+ industrial bifacial silicon solar cells produced with the local screen-printed Al back surface field (BSF) to those with the selective BSF doped with aluminum-boron. To produce solar cells with selective BSF, the boron diffusion based on spin-on dopant was introduced in the process sequence. The thin SiO2 layer (10 nm) thermally grown did not produce good passivation on the rear face and wafers were contaminated during aluminum diffusion in the belt furnace. The implementation of selectively doped BSF improved the efficiency by reflecting minority charge carriers and the wafer contamination by belt furnace was compensated by boron diffusion. The bifacial solar cells with B-Al selective BSF achieved an efficiency of 13.7% / 8.9% (front / rear illumination) and presented lower sensitivity to the belt furnace processing and to the quality of the rear surface passivation.

The goal of this paper is to present the development and comparison of p-type multicrystalline silicon solar cells with screen printed metallization. Industrial processes were developed to manufacture the n+pn+and n+pp+ solar cells. The n+pn+ structure was obtained with single phosphorus diffusion. The n+pp+ solar cell was developed and the p+ region was optimized experimentally by using one single high temperature step for phosphorus and aluminum diffusion. The results showed that the average efficiency of n+pn+ solar cells with no texturing was 12.3%. The efficiency increased 1.5% (absolute) with the implementation of acidic solution texturing. The highest efficiency achieved for solar cell without (n+pn+) and with (n+pp+) back surface field (BSF) was 13.8% and 14.1%, respectively. The BSF does not result in a significant improvement in the efficiency when the diffusion is carried out in a single step thermal diffusion in industrial process with screen printed metallization.

A redução no custo de fabricação de células solares, mantendo a eficiência de conversão de energia solar em elétrica, é uma das principais metas de pesquisa, na área de energia solar fotovoltaica. O objetivo desse trabalho está centrado no desenvolvimento e na análise de um processo de fabricação de células solares industriais de silício, com região de campo retrodifusor, formada pela deposição de pasta de Al, por serigrafia e difusão em forno de esteira. A queima das pastas de prata e a queima de prata-alumínio, usadas para formar a malha metálica frontal e posterior das células solares e a difusão de Al foram realizadas simultaneamente. A melhor célula foi fabricada com processo de difusão/queima a temperatura de 860ºC e velocidade de esteira de 150 cm/min, resultando em dispositivos de 12,8% de eficiência. A região de campo retrodifusor obtida não foi tão efetiva quanto à obtida por meio da deposição de alumínio em alto vácuo e por meio da difusão em forno convencional ou forno de esteira.

Cost reduction of solar cells while maintaining high solar energy conversion efficiencies is one of the main research goals in photovoltaic solar energy. The aim of this work is centered on the development of a fabrication process of industrial silicon solar cells with a back surface field region obtained by Al deposited by screen-printing and diffused into silicon wafer in a belt furnace. Ag and Ag-Al pastes were fired to establish the metallic contacts simultaneously to the Al diffusion. The best cell was processed at a temperature of 860°C and belt speed of 150 cm/min, achieving an efficiency of 12.8%. The BSF region was not as effective as that obtained from Al deposited at high vacuum and diffused in standard tube furnaces or belt furnaces.

O crescimento do mercado de células solares e a necessidade de substratos de menor custo tornam o silício multicristalino a principal alternativa para a fabricação de tais células. O objetivo desse artigo é apresentar o desenvolvimento de um processo industrial para fabricação de células solares em silício multicristalino com somente uma difusão de fósforo e metalização por serigrafia. Por meio de simulações, foram otimizados o emissor n+ e a malha de metalização. Também foram analisadas a influência da largura das trilhas e a influência do tempo de vida dos minoritários. Experimentalmente, foi avaliada a influência do processo de queima das pastas de metalização. De acordo com as simulações, o valor da resistência de folha deve ser maior que 50 Ω/□, para que a eficiência não diminua. Também se concluiu que a eficiência diminui de 0,3% a 0,5%, quando a largura das trilhas da malha de metalização é dobrada. Dos resultados experimentais, constatou-se que a temperatura de queima das pastas de metalização afeta, concretamente, a eficiência. A maior eficiência alcançada foi de 11,5%, para a temperatura de queima de 860°C.

The market growth and the need of low cost materials make multicrystalline silicon the main alternative for the solar cell industry. The goal of this article is to present the development of an industrial process for manufacturing multicrystalline silicon solar cells by using only one phosphorus diffusion and screen printing metallization. The n+ emitter and the metallic grid were optimized through simulations and the influence of the width of the metal fingers and the bulk minority carrier lifetime were also analyzed. The influence of the firing conditions of the metal pastes was experimentally studied. According to the results obtained from the simulations, the value of the sheet resistance must be greater than 50 Ω/□ in order to maintain the efficiency of the solar cells. We also concluded that the efficiency decreases from 0.3% to 0.5% when the width of the metal fingers is doubled. From experimental results, it was found that the firing temperature of metallization pastes significantly affects efficiency. The highest efficiency achieved was 11.5% for a firing temperature of 860°C.

Brazil is a country that receives a large amount of solar radiation. Moreover, high quality quartz mines has been found in its territory and the country has well-established metallurgical grade silicon industries. However, PV technology is not used in large scale and there are no active solar cell fabs. In this context, a pilot plant to develop industrial PV technology was carried out in a Brazilian university combining education, research and industrial development. Silicon solar cell processes based on low cost chemicals and gases as well as gettering mechanisms were developed and more than 12,000 solar cells were fabricated, achieving efficiencies up to 16%, and two hundred photovoltaic modules were produced and characterized.