| This thesis is concerned with the application of silicon nanocrystals (Si NCs) in photovoltaic devices. Two types of novel solar cells, hybrid solar cells and Si NCs-only thin-film photovoltaic devices, have been developed. Hybrid solar cells are made from polymers and Si NCs. The first hybrid solar cells were fabricated by using poly-3(hexylthiophene) (P3HT) which has a good hole mobility and matches the energy band alignment of Si NCs. The solar cell performance of Si NCs/P3HT devices was studied as a function of the weight ratio of Si NCs/P3HT and Si NC size. Three groups of Si NCs were used in this study: Si NCs 3-5 nm in diameter, 5-9 nm in diameter, and 10-20 nm in diameter. The open-circuit voltage and short-circuit current increased by using the smallest size NCs due to the high surface-area-to-volume ratio and quantum confinement effect. Those results indicate that Si NCs are a good candidate as an electron acceptor in hybrid solar cell application.;To improve the efficiency of Si NCs/P3HT hybrid solar cells, we started to optimize the fabrication conditions by modification of the polymer concentration, usage of post-production heat treatment, and application of different metal electrodes. After optimization, a hybrid solar cell from 50wt% (weight ratio) Si NCs/P3HT annealed at 150°C for 2 hours with aluminum (Al) electrodes had a power conversion efficiency of 1.47% with a fill factor of 0.47, short-circuit current of 3.8 mA/cm2, and open-circuit voltage of 0.8 V under air mass 1.5 direct (AM 1.5D) one sun illumination. To understand the hole mobility of P3HT before and after post-production heat treatment, a hole-only device was fabricated by depositing gold (Au) electrodes, which block electron injection from the electrodes to Si NCs. The results suggest that the hole mobility of 50wt% Si NCs/P3HT film increases one order of magnitude after heat treatment, due to improved crystallinity in the P3HT, which can enhance hybrid solar cell efficiency.;Literature has reported that the compatibility of polymers and nanocrystals plays an important role in hybrid solar cell efficiency. Although P3HT is a good hole conductor and light absorber in solar cell applications, other polymers should be tested to find the best compatibility for Si NCs. Knowing this, P3HT was replaced by poly [2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in 3-5 nm Si NCs/MDMO-PPV hybrid solar cells. Although Si NCs/MDMO-PPV devices have a higher open-circuit voltage than Si NCs/P3HT devices, the power conversion efficiency of Si NCs/MDMO-PPV devices is not as high as that of Si NCs/P3HT devices. To understand the reasons for the low efficiency from Si NCs/MDMO-PPV devices, the hole mobility of MDMO-PPV, energy band alignment between MDMO-PPV and Si NCs, and absorption spectrum of MDMO-PPV were studied and compared to those of P3HT. To measure the hole mobility of MDMO-PPV, Au electrodes were again utilized to block electron injection into the Si NCs. The results show that the hole mobility of MDMO-PPV is lower than that of P3HT. The absorption spectrum of MDMO-PPV (400-600 nm) is narrower than that of P3HT (400-650 nm) so that exciton generation in P3HT is more efficient than in MDMO-PPV under AM 1.5 conditions. Additionally, MDMO-PPV has a lower highest occupied molecular orbital level than P3HT so the efficiency of hole injection from Si NCs into MDMO-PPV may not be as efficient as for P3HT. These reasons explain why the efficiency of Si NCs/MDMO-PPV devices is not as good as Si NCs/P3HT devices.;From Si NC solution processing, we found that 10-20 nm bare Si NCs without any surface modification can form a stable cloudy colloid with 1,2-dichlorobenzene. This colloid can be spin-cast onto an ITO substrate to form a continuous and dense thin film. A Schottky photovoltaic device consisting of a single layer of intrinsic Si NCs was fabricated in a glove box to verify that films can be cast from colloid Si NCs. This photovoltaic device has a sandwich structure with a 250 nm Si NC layer between ITO and Al electrodes. Under AM 1.5D one sun illumination, the Si NC Schottky device showed a significant photovoltaic response with a power conversion efficiency of 0.02%, a fill factor of 0.26, short circuit-current density of 0.148 mA/cm2, and open-circuit voltage of 0.51 V. This result suggests that the solution processing of bare Si NCs can be a new way to manufacture low-cost and high-quality silicon-based thin films. |
