Investigation On Preparation And Performance Of Novel Structure Polymer Solar Cells And Photodetectors

Recently, solar cells have been one of the top research fileds. Bulk heterojunction (BHJ)polymer solar cells (PSCs) based on conjugated polymers and fullerene derivatives in recentyears have attracted great attention due to their postential low-cost, easy fabrication andfrexible devices. Performance of PSCs have been developed continuously since thedevelopment of photoinduced electron transfer from a conducting polymer tobuckminsterfullerene and the built-up of bulk herterojunction concept between conjugatedpolymer and fullerene derivative. So far, over9.31%of power conversion efficiency (PCE)has been reported for single-junction PSCs and approaching11%of PCE has also beenreported for tandem PSCs. It is not-far distance for PSCs to be used in commercial application.However, except efficiency which limits PSCs for application, stability is another key factorwhich has been problemic to date for PSCs. Therefore, to solve these problems, novelstrcutres especially inverted structure PSCs have received great intesets since it has potentialhigher device stability than that of devices based on a conventional structure. Nevertheless,the efficiency of inverted PSCs which were affected by the development of conjugatedpolymers and complicated interfaces was low. In the last two years, inverted PSCs developedfast due to the development of efficient polymers and the improvement of novel interfaciallayers, making it to be candidate for futural application.Similar to PSCs, Polymer photodetectors (PDs) have attracted much attention due to thelow-cost processing and high performance, which lead to the potential for a big variety ofapplications. Particularly, with the development of new narrow bandgap conjugated polymersand better control of the nanoscale morphology of the interpenetrating electron donor/acceptornetworks, the detectivity of solution-processed polymer PDs with the spectral response fromultraviolet (UV) to infrared (IR) region have reached1013cm·H1/2/W (1Jones=1cm·H1/2/W).However, the same problem to PSCs is also stability for polymer PDs.This thesis is concentrated on the preparation and performance of novel structurepolymer solar cells and polymer photodetectors.In Chapter3, we report high performance PSCs with an inverted device structure using solution-processed ZnO thin film as a buffer layer. PCEs of3.8%were observed from PSCswith an inverted device structure with ZnO as a buffer layer. Without ZnO buffer layer, PSCsonly show PCEs of1.67%, which is less than half value from PSCs with ZnO buffer layer. Wealso found that PSCs with ZnO layer have very good stabilities. Operated at room temperature,there is no obvious degradation from the PSCs with ZnO layer after continuously illuminatingthe devices for4hours. However, PSCs without ZnO buffer layer degraded significantly afterilluminating the devices only for1hour. Furthermore, PSCs with ZnO buffer layer also showvery good shelf stability; only5%degradation observed in PCEs after47days. All theseresults demonstrate that ZnO buffer layer plays an important role in enhancement of PSCsperformance with an inverted device structure. We further investigated the role of ZnO ininverted PSCs based on poly(3-hexylthiophene)(P3HT) mixed with [6,6]-phenyl C61-butyricacid methyl ester (PCBM). Power conversion efficiencies of PSCs made by P3HT and PCBMwere3.50%and1.21%for PSCs with and without the ZnO thin film, respectively. The lightintensity dependence of the photocurrent and the capacitance-voltage measurementdemonstrated that the increased PCEs were due to the restriction of strong bimolecularrecombination when a thin ZnO layer was inserted between the polymer active layer and theITO electrode.In Chapter4, through a conventional active-layer processing methodology, efficient BHJPSCs with the inverted device structure based on PBDT-DTNT:PC71BM composites havebeen fabricated. Power conversion efficiency of8.4%under AM1.5G illumination wasachieved from the inverted PSCs by using a PFN-Br interfacial layer to engineering ZnOelectron extraction layer. Without PFN-Br interfacial layer, the inverted PSCs only showed anefficiency of6.1%under the same condition. The overall enhanced short-circuit currentdensity, open-circuit voltage, fill factor and corresponding high efficiency in the invertedPSCs with a thin conjugated polyelectrolyte interfacial layer, were attributed to the goodcontact between ZnO electron extraction layer and PBDT-DTNT:PC71BM BHJ active layer,good interface adhesion between the electron extraction layer and PBDT-DTNT:PC71BM BHJactive layer, and enhanced charge transport via suppressed bimolecular recombination. Theseresults provided an important progress for solution-processed PSCs, and demonstrated thatPSCs with an inverted device structure are comparable with PSCs with the conventional device structure. Enhanced performance was also observed from inverted PSCs with PFN-Brwhen using PFO-DBT35: PCBM as the active layer. We also demonstrated that PFN-Br canbe used as a hole blocking layer in PSCs. A novel fullerene derivative PCBM-G2was used asa buffer layer in inverted PSCs and enhanced performance was observed from the deviceswith PCBM-G2than that without PCBM-G2. This demonstrated that PCBM-G2can be usedas a buffer layer in inverted PSCs.In Chapter5, low temperature-annealed sol-gel-derived MoOx(S-MoOx) thin film as ahole extraction layer in BHJ PSCs was successfully made and demonstrated. Thecharacterization of S-MoOxthin films by absorption spectrum, x-ray photoelectron spectrum,transmitted electron microscopy, atomic force microscopy (AFM) and peak force tappingtunneling AFM (PFTUNA) module indicated that S-MoOxfilms can be used as a buffer layerfor BHJ PSCs. The PCEs of5.86%observed from BHJ PSCs with a device structure ofITO/S-MoOx/PBDT-DTNT:PC71BM/Al are comparable with the PCEs of5.89%obtainedfrom BHJ PSCs with a device structure of ITO/PEDOT:PSS/PBDT-DTNT:PC71BM/Al. Theseresults demonstrated that low temperature-annealed sol-gel-derived MoOxthin film can beused as a hole extraction layer in BHJ PSCs for approaching high performance. We canfurther decrease the annealing temperature of sol-gel derived MoOxto120oC by UV lightprocess. The optimized time of UV process on S-MoOx is20min.In Chapter6, novel low bandgap fluorinated copolymers have been designed andsynthesized. The studies of the effect of fluorine units on the electronic properties of, and thethin film morphologies of fluorinated copolymers demonstrated that an optimal window of20-40%coupled fluorine units with thieno[3,4-b]thiopehene exits for maximum hole mobilitythat correlated well with a dispersed interconnected morphology of the fluorinated polymer.Higher fluorine units coupled with thieno[3,4-b]thiopehene adversely affected the phasemorphology to coarsen that in turn reduced device performance. Nevertheless, the PCE deviceperformance of the highest fluorinated content polymer was twice as much as thenon-fluorinated polymer. These results indicate that the electronic properties of as well as thethin film morphologies of electroactive and photoactive polymers are collectively veryimportant for organic photovoltaic devices.In Chapter7, we report the fabrication of high performance broadband polymer photodetectors based on narrow bandgap conjugated polymers with an inverted devicestructure, where electrons and holes are collected on ITO and metal contact with high workfunction. High quality wide bandgap vertically aligned ZnO nanowire array offers anenhanced surface area and is used as the cathode buffer layer in this device for effectivelyextracting electrons and blocking holes from the active polymer layer. The room temperaturedetectivity of polymer photodetectors with such an inverted device structure is greater than1010Jones with the spectral response from400nm to1450nm. Our results define a promisingpathway for fabrication of high sensitivity polymer photodetectors with an inverted devicestructure using ZnO nanowire array cathode buffer layer for a wide range of applications.