Research On Semi-transparent Polymer Solar Cells

Polymer solar cells （PSCs） have great potential as the future energy candidate dueto its advantage of low cost, being flexible and large area fabrication. During the pastdecades, there has been an extensive research for polymer solar cells, which arefabricated by conjugated polymer and fullerene derivative. However, powerconversion effciency （PCE） is still limited by a few factors, such as narrowabsorption range, short exciton diffusion length, and low charge carrier mobility.Increasing the thickness of the active layer cannot harvest a broader spectrum andthen improve the effciency. How to utilize the solar spectrum more effectively is stillan enormous challenge for PSCs. One promising way to solve this problem is toexploit variable bandgap polymers in a tandem structure in which multiple subcellswith different energy gaps are stacked.Semitransparent polymer solar cells （PSCs） have attracted more and moreattentions due to their application in energy harvesting windows and the tandem solarcells. It can be integrated onto window panes in homes, skyscrapers, and automobilesto enhance the functionality of already utilized transparent surfaces. Thissemitransparent PSCs technology can maintain the glass transparency and transformthe non transmitted light to electricity, simultaneously.The thesis focus on the interface modificaton between active layer and electrodesaffect the performance of solar cells and semitransparent inverted polymer solar cellsand semitransparent polymer solar cells with one dimensional photonic crystals. Moredetails are now listed below:1. First of all, the important steps during the development history of solar cells, andwork principle, characterization and related measure instruments of the polymer solarcells are described, and we also reviewed the recent work focusing on semitransparent polymer solar cells and related processing and so on.2. For the conventional device architecture, solar cells based on glass/indium tinoxide （ITO）/poly （3,4ethylenedioxythiophene）:poly （styrene sulfonate）（PEDOT:PSS）/RR P3HT:PCBM/LiF/Al, PCE in the range of4%–5%have beenreported. However, PEDOT:PSS is hygroscopic so extensive exposure to moisture canreduce its conductivity. In addition, recent research has shown that PEDOT:PSSdegraded the performance of device due to its corrosion to ITO. The ITO/PEDOT:PSSinterface is known to be rather unstable, as well as the usual Al top electrode. The lowwork function metals that are typically employed as cathodes in conventional polymersolar cells are susceptible to oxidation. For these reasons, inverted polymer solar cellsare employed to replace PEDOT:PSS and Al. The inverted device structure is usuallyITO/TiO₂/RR P3HT:PCBM/buffer layers/Ag. TiO₂acts as an electron transport layerwith a lower work function （4.2eV）. In this paper, we will demonstrate anenhancement in polymer solar cell performance by just evaporating three kinds ofmetal oxide such as CoO, NiO, and V₂O₅as anode buffer layers between the activelayer and the top anode. The anode buffer layer flm can module the Schottky barrierand form an ohmic contact at the organic/metal interface, which make it a great holestransport layer. The device performance with CoO, NiO, and V₂O₅of differentthicknesses are investigated and compared.We found that the short circuit current density of the polymer solar cells with10nmCoO interlayer was substantially enhanced, resulting in a47.2%enhancement inpower conversion effciency. The impedance spectra are measured to show that theintroduction of CoO decreases the series resistance and improve the contact betweenactive layer and top Ag electrode. The maximum power conversion effciency with10nm CoO is1.37%under under AM1.5G illumination of100mW/cm2.The resultsshow that the power conversion effciency （PCE） increases by fvefold with2nm NiOcompared to the control cell without NiO. The optimum device performance with2nm NiO shows the Jscof7.92mA/cm2, Vocof0.46V, FF of39.8%, and thus the PCEof1.45%under AM1.5G illumination of100mW/cm2. The performance ofphotovoltaic device with V₂O₅layer is dramatically improved compared with thatwithout V₂O₅. The power conversion efficiency is improved from0.79%to2.58%under under AM1.5G illumination of100mW/cm2.3. In tandem PSCs, a semitransparent subcell is indispensable.SemitransparentPSCs can be designed to transmit light in a specifc spectral range. We demonstrated semitransparent inverted polymer solar cells by employing a transparentWO₃/Ag/MoO₃anode, which exhibited high transmittance in the visible region. Theinner WO₃layer was introduced as a buffer layer to improve the hole collection, whilethe outer MoO₃layer served as a light coupling layer to enhance the opticaltransmittance of the device. The dependence of the device performances on thethickness of the outer MoO₃layer was investigated, and the transmittance andreflectance of the WO₃（10nm）/Ag （13nm）/MoO₃（0,20,40,60, and80nm） electrodewere compared. The results show that a high transmission of80%with a high powerconversion effciency of1.24%was achieved when the thickness of the MoO₃layerwas40nm under AM1.5G illumination of100mW/cm2from the WO₃/Ag/MoO₃side.The structure of the anode is MoO₃/Ag/V₂O₅. The inner MoO₃layer is introduced asa buffer layer to improve holes collection, while the outer V₂O₅layer served as a lightcoupling layer to enhance optical transmittance of the photovoltaic device. Themaximum transmittance of90%from400nm to700nm is obtained when thethickness of V₂O₅is40nm. When the thickness of V₂O₅is40nm, under AM1.5Gillumination of100mW/cm2illuminated from ITO side, the Jscis5.01mA/cm2, theVocis0.591V, the FF is61.8%, and the PCE is1.83%; from MoO₃/Ag/V₂O₅, the Jscis4.28mA/cm2,the Vocis0.585V, the FF is61.9%, and the PCE is1.55%. Thetrend of the optical electric feld distribution is highly consistent with the Jsc, which isdependent on the thickness of the V₂O₅top capping layer.4. A well semitransparent subcell should have a high absorption in its ownabsorption wavelength range and a high transparency in the complementarywavelength range in the meantime. But this selectivity characteristic for wavelengthcannot be obtained easily due to the contradiction between the effciency and thetransparency of the device. The thickness of the active layer is usually thin tomaintain high transmittance through the devices. As a result, the weaker absorptioninduces the smaller photocurrent, which results in the lower effciency than that ofconventional PSCs. So, the investigation of semitransparent PSCs is aimed to obtain ahigh effciency and transmittance at the same time. Here, we introduce a light trappingstructure to inverted semitransparent PSCs to improve light absorption andtransmittance. The structure is a photonic crystals （PCs） reflector, which usesone dimensional photonic crystals （1DPCs） as a distributed Bragg reflector （DBR）.The PCs have been applied in PSCs previously, such as the highly ordered arrays ofnanoscale active layer fabrication, the semitransparent ITO electrode in theory. Until now, the semitransparent PSCs/1DPCs have not been reported to study in experiment.In this paper,（WO₃/LiF）Nis used as1DPCs and is capped on the Ag anode of thesemitransparent PSCs to improve the device effciency and transparencysimultaneously. The1DPCs are composed of multiple pairs of WO₃and LiF layers.The layer thicknesses of the WO₃and LiF are calculated with d1=λ/4n1andd₂=λ/4n₂, whereλis the central light wavelength, n1and n2are the refractiveindexes of WO₃and LiF, respectively. Current density–voltage （J–V） test, incidentphoton to electron conversion effciency （IPCE） test, and transferred matrixsimulation （TMM） are employed to investigate the device performance. The1DPCswith8pair of WO₃/LiF act as distributed reflectors within the photonic bandgap. Then,power conversion effciency of2.58%is achieved and there is an improvement of26.3%in the effciency when compared with that of the conventional device withoutthe1DPCs. The average transmittance of the device with8pair of WO₃/LiF is almostzero in400–600nm wavelength range. It means that the light is absorbed suffcientlyin the active layer. The enhanced light absorption results in effciency mprovementremarkably.