High-performance Polymer Solar Cells Enhanced By Light Management At Surface And Interface Layers

With the increasing population of China and serious problem of the environmental pollution,natural gas,coal,oil and other non-renewable energy have been unable to meet the needs of the rapid development of economy and increasing demand for"beautiful scenery"ecological environment.Solar energy has become the most ideal energy due to the advantages of extensive sources without regional restrictions,pollution-free and inexhaustible.Along with the continuous development of science and technology,the state-of-the-art solution-processed polymer solar cells（PSCs）based on adjustable bandgap of polymer and fullerene derivative are considered vigorous candidates to convert clean solar energy into electrical power with the advantages of being low cost,being lightweight,and easily processing large area fabrication.To date,the power conversion efficiency（PCE）of single-junction and tandem PSCs have been reported as high as 14%.However,despite the high PCE,further improvement was impeded by the intrinsic disadvantage of organic semiconductor materials,such as relatively narrow absorption spectra,low carrier mobility,and short exciton diffusion length due to the limited hopping transport mechanism.As a result,the optimal active layer thickness of bulk-heterojunction PSCs is limited to subseveral hundred nanometers in order to obtain as much as charge collection while minimizing the degree of recombination loss.Nevertheless,despite organic materials generally possessing a high absorption coefficient,active layers in such thin thickness cannot achieve complete absorption above their band gaps.Therefore,the most critical challenge to develop high-performance PSCs is to address the fundamental tradeoff between sufficient light absorption（which needs thick films）and efficient photogenerated charge collection（which needs thin films）.Benefiting from the light trapping methods they can facilitate light absorption enhancement under no increasing thickness of the active layer by engineering the optical structure.Herein,we introduce Cu nanoparticles（NPs）into the buffer layer by the thermal evaporation method,the results reveals that the localized surface plasmon resonances（LSPRs）effect of Cu NPs can further boost absorption of the active layer.At the same time,in order to expand the commercial application range of PSCs,the semitransparent polymer solar cells（ST-PSCs）is also studied,which can be used as integrated building windows.In order to solve the contradiction between the transmittance and the efficiency of ST-PSCs,the surface O-P structure is designed combining with the optical adjusting layer（OAL）and photonic crystals（PCs）.At last,the ST-PSCs of high efficiency and high color index were prepared by combining two kinds of light management structure units at interface and surface layers.The results are listed as following:Firstly,a light management structure unit has been designed by thermally evaporating diameter-controlled Cu NPs into the buffer layer.Benefiting from the large gap of the surface energy of Cu and WO₃,Cu NPs can be uniformly distributed in the mode of isolated islands and trigger the LSPRs effect notably.The steady state photoluminescence（PL）spectra and transmittance spectra identically provide direct evidence that the light excitation rate and absorption enhancement are enabled by Cu NPs.The complex impedance spectra for devices demonstrate that the introduction of Cu NPs into the buffer layer can enhance the charge transport property and decrease the devices resistance.The results of short circuit current density（Jsc）and incident photoconversion efficiency（IPCE）measurement reveal that Cu NPs improve significantly in a broader wavelength range of 380-610 nm,yielding the highest PCE of 6.38±0.18%of the device with 3 nm thick Cu NPs compared with 4.65±0.14%of the control device.Secondly,an O-P structure consisting of a OAL and a PCs has been designed on the surface of ST-PSCs.According to the location of main absorption peak of P3HT:ICBA,510 nm is used forthe centre wavelength of PCs.Here,5 pairs of PCs are chosen instead of more pairs in order to reduce relatively tedious fabrication process and improve transmittance of devices.It is important to highlight that the peak value of IPCE increases remarkably and appears red-shift to the region of red light with the increase of OAL thickness.The optimum device with the thickness of80 nm of MoO₃ layer showed a notable PCE enhancement from 4.03±0.14%to 4.93±0.14%compared to the one without OAL.Additionally,an excellent average visible transmission（AVT）of 26.10%was obtained simultaneously,which meets the building windows application requirement well.Furthermore,the theory simulation by transfer matrix modeling（TMM）reveals that the OAL can effectively adjust the reflection phase and enable a redistribution of optical electric field.This dramatical enhancement of light absorption in the active layer is in perfect accordance with the experimental results.We believe the performance improvement caused by new light management method of O-P structure can pave the way towards further ST-PSCs.Finally,the color characteristic of the light source is combined with ST-PSCs to explore the efficiency and color rendering index（CRI）of PTB7-th:PC₇₁BM device.First,the Au/Ag alloy NPs are successfully introduced to decrease the resistance and enhance the efficiency of ST-PSCs via the LSPRs effect.The PCE improved dramatically from 5.50±0.15 to 7.15±0.17%with 17±1.2 nm Ag and 8±0.5 nm Au NPs.Then,two pairs of PCs covered on the device with optimized Ag/Au alloy NPs can effectively flat and improve the transmittance spectra.By adjusting the center wavelength of PCs,we obtained a device with excellent optical properties of a CRI of95,chromaticity difference（DC）of 0.0025,correlated color temperature（CCT）of5340 K,AVT of 20.38%and the electrical characteristics of a PCE as high as 7.07±0.16%.This suggests the method of combining ST-PSCs with multiple light management methods will give a guideline for future commercial applications of integrated building windows.