Over 14% Efficiency of Directly Sputtered Cu(In,Ga)Se₂ Absorbers without Postselenization by Post‐Treatment of Alkali Metals

Abstract

junction quality, thus increasing open circuit voltage (V oc ) and fill factor (FF). It has also been observed that KF-PDT allows a steeper Ga gradient [8] and a thinner CdS buffer layer [6,8] to be employed without having a negative impact on device performance; therefore, the short circuit current (J sc ) can be improved after optimizing Ga gradient and CdS thickness.Though the improvement in device parameters has been generally observed, the underlying mechanism of KF-PDT is still unclear. One possible reason for the improved junction quality is the formation of a Cu-depleted surface layer, which may enlarge the surface bandgap [9] and promote the in-diffusion of Cd [6] during buffer layer deposition. However, the formation of a Cu-depleted surface does not seem to be always observed. For example, Mansfield et al. have claimed that no Cudepleted surface formed on the selenized CIGS after KF-PDT. [10] Another interesting observation is the reduction of Na content in CIGS after KF-PDT, nevertheless the consequences of which have not yet been discussed in literature. Further investigation is still required to fully understand the effects of KF-PDT.The research into CIGS deposited by three-stage coevaporation or two-step processes (metal deposition and postselenization) continues to advance in the pursuit of mass production and low-cost processing. In addition, various fabrication processes of CIGS thin films have been demonstrated and continue to be developed, for example, fabrication from electrodeposited [11] or nanoparticle-based [12] precursor layers. In these processes, however, selenization is indispensable. The need for a postselenization process inevitably increases production costs. Some researchers have attempted to eliminate this unfavorable and time-consuming process. For example, in a hydrazine-based process, device-quality CIGS absorbers were obtained by annealing the spin-coated precursors in an inert atmosphere, yielding cell efficiency of over 15%. [13] One of the key factors of eliminating selenization in the hydrazine-based process is that extra Se can be added into the precursor ink so extra Se in the atmosphere is not required during annealing. [14] However, the main disadvantage of the hydrazine-based process is its toxicity and instability.Directly sputtering from a single CIGS target is another promising process that could produce efficient CIGS absorbers Direct sputtering of a single quaternary Cu(In,Ga)Se 2 (CIGS) target without postselenization is a promising approach to fabricating CIGS absorbers. However, the device efficiency of the quaternary-sputtered CIGS is limited to 10%-11% due to the low and uncontrollable Se supply during the quaternary sputtering process. Here, an enhanced efficiency of 14.1% is reported by directly sputtering from a CIGS target without extra Se supply followed by sequential postdeposition treatments (PDT) of NaF and KF. The effects of different post-treatments of alkali metals on quaternary-sputtered CIGS thin films are discussed in detail. A Cu-depleted su...

Effects of Ammonia‐Induced Surface Modification of Cu(In,Ga)Se2 on High‐Efficiency Zn(O,S)‐Based Cu(In,Ga)Se2 Solar Cells

“…Combing the results of CGI, GGI, Se, and Na, we can speculate the following scenario: the CIGS sample grown by the three‐stage fabrication procedure inevitably consists of a non‐uniformity in the thickness direction, which not only has a Ga‐grading but also has a Cu deficiency. While such structure is globally beneficial to the device performance, imperfection still occurs. With the co‐deposition of In, Ga, and Se in the third stage of the three‐stage process, the very surface seems to lack of Cu in a high degree.…”

Caracterização de células solares de filmes finos de CIGS

“…Deve-se destacar também a dopagem com elementos alcalinos, notadamente o sódio, que aumenta substancialmente a concentração de portadores positivos do filme [8,9]. Esta dopagem é feita via tratamento pós-deposição, uma etapa do processo que é hoje implementada na quase totalidade das rotas de produção da célula.…”

“…O grande desafio na produção da camada de CIGS continua sendo o controle da composição. Uma camada deficiente em Se, por exemplo, introduz defeitos doadores, tais como V Se e In Cu , os quais prejudicam o desempenho da célula [9]. Sabe-se, no entanto, que uma pequena modificação na composição da superfície do filme pode contribuir para reduzir a recombinação superficial, com impactos positivos no fator de forma (FF) da célula.…”

“…A eletrodeposição também tem sido usada para crescimento do CIGS, tal como no fizeram Duchatelet e colaboradores [14], que depositaram CIGS em dois estágios, a partir da eletrodeposição sequencial das camadas metálicas e posterior selenização (porém, sem o tratamento com metais alcalinos), obtendo dispositivos com 8,7% de eficiência. A pulverização catódica [15] é uma téc-nica adequada para uso em linhas de produção e, por isso, vários autores vêm obtendo sucesso na deposição de filmes de CIGS por este processo [7,9]. Recentemente, Hsu e colaboradores [9] divulgaram uma célula com 14,1% de eficiência a partir da pulverização catódica de um alvo estequiométrico de CIGS sem tratamento de selenização, fazendo uso somente do pós-tratamento com metais alcalinos.…”

“…A pulverização catódica [15] é uma téc-nica adequada para uso em linhas de produção e, por isso, vários autores vêm obtendo sucesso na deposição de filmes de CIGS por este processo [7,9]. Recentemente, Hsu e colaboradores [9] divulgaram uma célula com 14,1% de eficiência a partir da pulverização catódica de um alvo estequiométrico de CIGS sem tratamento de selenização, fazendo uso somente do pós-tratamento com metais alcalinos.…”