Photovoltaic Cells And Its Stability Based On Cross-linkable Conjugated Polymers

Polymer solar cells have attracted considerable attention due to the ligeht-weight, low-cost, solution processability and large-area manufacture on the flexible substractes. Currently, the power conversation of the polymer solar cells is relatively low, and the stability is not so good, which limit its application and development to a great extent. In this thesis, we have synthesized defferent donor and acceptor materials. The relationship between molecular structures, morphology of the active layer and photovoltaic device performace were also studied. The worls are divided as follows:Firstly, a novel stable and photocrosslinkable electron-donor material, liquid-crystalline polythiophene containing cyano-biphenyl mesogenic pendant, namely, poly{3-[6-(4’-cyano-biphenyloxy)hexyl]thiopheneylenethiophene-alt-3-(6-bromohexyl)thiophene}(PTcbpTT), was designed and synthesized. The structural anisotropy originating from cyano-biphenyl mesogens can induce the PTcbpTT to assemble into a well ordered morphology and consequently lead to the red-shift absorption, enhance photoluminescence. The thermal treatment drives further development of the morphology of the copolymer and its blend films mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), towards a state of microphase separation in the nanometer scale. Furthermore, the bulk heterojunction devices based on the PTcbpTT/PCBM (1:1wt.%) active layer have been constructed. The LC annealing device yields an enhancement of power conversion efficiency from0.5%to1.2%, showing a significantly increased Jsc and FF with respect to its untreated counterpart, thanks to the ordered microphase separation channels for charge transportation. Unlike devices prepared from PTcbpTT/PCBM blend without UV treatment, photocrosslinked PTcbpTT/PCBM devices are stable even when annealed for two days at the elevated temperature of150℃, implying that the photocrosslinked structure dramatically suppresses largescale phase segregation.Then, a novel approach for thermally stable bulk heterojunction (BHJ) solar cells based on the inter-crosslinking of a vinyl-functionalized low band-gap copolymer donor poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b’]dithiophene-alt-4,6-thieno[3,4-b]thiophen-2-yl-2-ethylhexan-1-one-alt-4,8-bis(oct-7-en-1-yloxy)benzo[1,2-b:4,5-b ’]dithiophene}(PBDTTT-V), and vinyl-functionalized fullerene derivatives acceptor [6,6]-phenyl C61butyric acid dec-9-en-1-yl ester (PCBD), is developed. The crosslinking at the vinyl groups of the molecules upon the thermal treatment was confirmed by the insolubility of the films in organic solvents detected by UV-vis absorption spectra. Compared with non-crosslinked corresponding heteroj unction and mono-crosslinked corresponding heterojunction, inter-crosslinked heterojunctions with and without addition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) showed well-maintained and stable morphology after thermal annealing at150℃for8hours, in which the large-scale phase separation caused by the aggregation of fullerenes was dramatically suppressed. Although mono-crosslinked PBDTTT-V/PCBM blend efficiently improved the stability of device with respect to non-crosslinked corresponding heterojunction device, the power conversion efficiency (PCE) still decreased to an half of its initial efficiency value after annealing at150℃for8h. On the contrary, a remarkably enhanced stability was obtained in inter-crosslinked devices with performance maintained by60%for PBDTTT-V/PCBM/PCBD and by79%for PBDTTT-V/PCBD. It is worthy to note that the device based on PBDTTT-V/PCBM/PCBD showed comparable performance to the non-crosslinked one, indicating that the inter-crosslinking of both donor and acceptor with unsaturated bonds is a promising approach to ensure the device with the long-term thermal stability, and maintain the good performance at the same time.