Device Physics of Solution Processable Solar Cells

This Ph.D work reports the studies of photovoltaic devices produced by solution processable methods. Two material systems are of interest: one is based on organic semiconductors, and another on organic/inorganic hybrid composites. Specifically, organic photovoltaic (OPV) devices are made using photoactive materials consisted of a p-conjugated polymer [Poly(3-hexylthiophene), or P3HT] and fullerene derivative [phenyl-C60-butric acid methyl ester, or PCBM] in a bulk heterojunction (BHJ) structure of donor/acceptor network. On the other hand, hybrid photovoltaic (HPV) devices are made from blend of quantum dots and p-conjugated polymers. The QD material presented here are of the lead sulfide (PbS), and lead selenide (PbSe), whereas the polymers are either P3HT or Poly(3-dodecyl thienylene vinylene) (PTV)with controlled regio-regularity. For OPV devices, two different device geometries are investigated, namely, the conventional or normal structure where indium tin oxide (ITO) is used an anode, and a metal cathode is fabricated by thermal vapor deposition (TVD). In this geometry, thin layer (about 30~35nm) of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited from aqueous solution onto ITO as hole transport layer (HTL). The second geometry, called the inverted structure, uses ITO as the cathode of the device. A thin layer of cesium carbonate (Cs2CO3) (about 1~2nm) is applied over the ITO and functions as electron transport layer (ETL), thereby decreasing the work function of ITO and allowing it to function as the cathode. In this case, PEDOT:PSS is mixed with 5vol.% of dimethylsulfoxide to increase conductivity for serving as anode. Two solution processable methods, spin-coating and spray processes were investigated, and a detailed study of nanomorphology influence under different annealing conditions, different solvents and thickness are reported. The main contribution of this Ph.D. work was the development and implementation of a layer-by-layer (LBL) all-spray solutionprocessable technique to fabricate large-scale OPV arrays, with more than 30% transmission in the visible to near IR range. Comparing with traditional laboratory OPV fabrication based on spin-coating and using metal as cathode contact, which greatly limits transparency of solar cells and posts difficulty for large scale manufacturing, this LBL spray process solves these two problems simultaneously. This technology eliminates the need for high-vacuum, high temperature, low rate and high-cost manufacturing associated with current silicon and in-organic thin film photovoltaic products. Furthermore, this technology could be used on any type of substrate including cloth and plastic. Single cell OPV with active area of 4mm2 was used as preliminary test device to obtain fabrication parameters for multi-cell OPV arrays. Three different sizes of OPV arrays were fabricated and tested under various illumination conditions. Starting from a 4” x 4” array with 50 cells in series connection 4” x 4” substrate consisting of 50 cells with total active area of 30cm2 , a scaled up 1’ x 1’ array was fabricated as a proof of concept, and whose results are reported. Scaled down arrays, called micro arrays, are also presented in this work. OPV micro array has the potential application in DC power supplies for electrostatic Microelectromechanical systems (MEMS) devices. The first generation micro array consists of 20 small (1mm2 ) solar cells connected in series for a total device area of approximately 2.2cm2 . The 2nd generation micro array with 60 cells shares the same size substrates and single cell active area as the first generation. However, the 2nd generation micro array cell has a new design with reduced series resistance and improved cell occupancy by 3 fold. Infrared quantum dots (QD) such as PbS and PbSe have potential in photovoltaic applications. These solution processable quantum dots with tunable electronic properties offer very attractive approach for expanding spectral sensitivity of p-conjugated polymers to infrared region of solar spectrum. However, these QDs often have defects originated from either incomplete surface passivation or imperfections in the quantum Dots. The electronic levels of defects often are within the bandgap of the semiconductor. These ingap states are of great importance since they affect the final destiny of excitons. Continuous wave photoinduced absorption spectroscopy has proven to be a convenient and successful technique to study long-lived photoexcitations of in-gap states. Part of this Ph.D work was the investigation of a peculiar gap state found in films of PbS QDs. This gap state bears confinement dependence, with a lifetime about 2µs. A detailed analysis of the Stokes shift, temperature dependence of PL, absorption and photoinduced absorption reveals the unconventional GS is a new state of a trapped exciton in a QD film. This gap state is directly relevant to exciton dissociation and carrier extraction in this class of semiconductor quantum dots. As synthesized PbSe and PbS quantum dots usually have bulky ligands such as oleic acids or TOPO (trioctylphosphine oxide). This capping layer is necessary to prevent nanocrystals from coalescence, however, the bulky ligands hinder charge extraction from and charge transport through the nanocrystals, as well as exciton dissociation at the nanocrystal/polymer interface. Common ways to manipulate ligands include ligand wash and ligand exchange in solution, and ligand removal on films. Through this Ph.D. work, a novel method using electric field to manipulate quantum dots ligands for interface of quantum dots and polymer, which possibly could facilitate charge extraction from the quantum dots and charge transfer between quantum dots and polymers, without the need of harmful chemicals. Over four orders improvement of photoconductivity at zero bias and more than six orders improvement at 5V reverse bias in a sandwich structure quantum dots photovoltaic device, and more than 5x improve in film smoothness. After thorough fundamental study on QD optoelectronic properties, hybrid photovoltaic (HPV) device was fabricated using a blend solution of PbS QDs and P3HT. Two different solution processes are used to form the QD/polymer active layer, one is the traditional spin coating method, and another is the spray technique developed in this Ph.D. Work. Different film morphology was observed with these two methods. Although the film is slightly rougher in the case with sprayed QD/polymer active layer, the phase segregation is more distinct and with smaller domain, which is beneficial for charge transport