CuInS₂ Thin Films And Solar Cells Based On Metal Inks

Photovoltaic technology is considered as one of effective approaches to solve the energy problems, for it is a green energy with little pollution, long service time and no geographical limitations. Recent years have witnessed the booming development of world PV market, steady increment of installed capacity and vigorous stage of PV industry. However, there's no doult that the cost of PV power is relatively so high that policy subsidies are needed to support it. Cost cutting plays the key role in the large-scale commercial application of PV power.It depends on technological innovations and developments to cut the cost. The thin film soalr cell technologies provide this potential. Compared with custom technologies for crystal silicon solar cells, the thin film soalr cell technologies have the following advantages:low consumption of both raw materials and manufacturing energies, rapid production on large scale, etc. Among the familiar materials for thin film solar cells, CuInS2 is endowed with nearly optimal bandgap (1.5 eV) for solar spectrum, high absorption coefficient and theoretical light-to-electricity conversion efficiency. Therefore, it can become a very promising material for thin film solar cells. Unfortunately, preparation methods for CuInS2 films are mostly based on vacuum, which certainly will increase the manufacturing cost.This thesis researched on the preparation of CuInS2 films and soalr cells based on metal inks. Firstly Cu, In and Cu-In nanoparticles were synthesized by the polyol-reduction method. Then metal inks and precursor films were fabricated from these nanoparticles. Sulfurization processes were investigated and CuInS2 film solar cells were prepared finally. The primary significant results were summarized as follows:(1) Cu, In and Cu-In nanoparticles were synthesized by the polyol-reduction method. It was found that the reaction temperature and rate are two dominant points in the synthesis of In nanoparticles, the decomposition rate of NaBH4 determined the growth time, and polymer additives acted as steric hindrance and influence the uniformity of In nanoparticles as well. Based on thes knowledge, In nanoparticle with diameter of 10 nm or even smaller could be synthesized at 140℃by combining the hot-injection method and using triethanolamine (TEA) as additives. The particle size was controlled by the reaction temperature and the amount of TEA. The hot-injection method were able to form numerous nuclei simultaneously and growth rapidly. However, the high reaction temperature could speed up the decomposition of NaBH4, shorten the growth time, disrupt the reductive environment, lead to the passivation of In nanoparticles and limit their further growth. By employing the similar hot-injection method, Cu-In nanoparticles with size of below 100 nm and narrow distributions could be synthesized. The reaction temperature and time had little effects on the morphology of Cu-In nanoparticles, but could enhance their crystallinty.(2) The Cu-In and In nanoparticles were ready to be oxidized enven in high vacuum CVD chamber filled with H2, indicating their instability at high temperature. By morphological and structural investigation, after high temperature annealing the Cu-In nanoparticles were turned to In2O3 nanowires, whereas the In nanoparticles were inclined to hollow spheres. By comparing the morphological differences of the products of In nanoprticle on glass and Cu substrates after annealing, it was proved that Cu acted as catalyst during the growth of In2O3 nanowires.(3) Metal nanoparticles were used to prepare precursor films and sulfurized to form CuInS2 films. By using the porous CuxS and InxSy films by Chemical Bath deposition (CBD), In or Cu nanoparticles were supplemented on copper or indium sulfide films respectively, then sulfurized to form compact CuInS2 films. CuInS2 film solar cells with an efficiency of about 2% were fabricated on CuInS2 films by In supplement on CuxS films. Metal precursor films with Cu/In or In/Cu stacking layers were prepared with Cu and In nanoparticle inks, those with Cu-In mixing layers were directly prepared with Cu-In nanoparticles. After sulfurization, these two kinds of precursor films were transformed to compact CuInS2 films with large grains. CuInS2 films from mixing layers had fewer impurity phases and solar cells from these films acquired an efficiency of 0.7%.(4) The sulfurization processes of Cu-In nanoparticles were investigated. With the increasing of Cu/In ratios in metal precursor films, the grain sizes of CuInS2 films were larger and crystallinity better. Na played a more significant role in the In-rich precursor films than in Cu-rich precursor films:For precursor films with a Cu/In ratio of 0.6, when they were sulfurized on Na-containing glass substrates, the impurity phases were Na2In2S4, on Mo substrates without Na, the impurity phases were CuIn5S8; for precursor films with Cu/In ratios of 0.9 and 1.3, Na had little influences on the structures of the sulfurized films, no matter Na were from the substrates or doped in metal inks. The intermediate phase of CuIn5S8 took part in the sulfurization process. This was due to the different diffusion rates and sulfurization rates of Cu and In, which resulting in the temporary phase segregation of Cu-rich phase (CuS) and In-rich phase (CuIn5S8). However, CuS and CuIn5S8 phases could react with each other to form CuInS2 phase. The cross section image of CuInS2 films were displayed as two-layer structures:the upper layer with large grains and bottom layer with small grains, which were probably sulfurized from active and inactive Cu-In nanparticles respectively.(5) CuInS2 thin film solar cells were fabricated and the effects of some key processes on the performances of solar cells were investigated. Appropriate thickness of CdS buffer layer and i-ZnO barrier layer could suppress dark current and increase open voltage of the devices. Device performances were able to be effectively improved by a 200℃air-annealing. The open voltage could be increased a bit with light-soaking. Based on the above-mentioned knowledge on devices, CuInS2 thin film solar cells with an efficiency of 1.43% were fabricated based on Cu-In metal inks.