High-performance Photomultiplication-type Organic Photodetector Based on Double-donor Ternary Bulk Heterojunction
As photoelectric conversion devices,organic photodetectors have the advantages of a wide range of material options,relatively simple preparation process,low production cost,lighter quality,flexibility,adjustable response spectral range,etc.,and are widely used in high-tech fields,such as optical communications,medical detection,image sensors,and so on.In order to expand the spectral response range to the near-infrared,P3HT and PTB7-Th,which have complementary absorption spectra,were selected as the double donors,and non-fullerene IEICO-4F was used as the acceptor,and a ternary bulk heterojunction strategy of dual donors and single acceptor was adopted to prepare photomultiplication-type organic photodetectors with the basic structure of ITO/PEDOT:PSS/Active Layer/Al based on the solution method,with the active layer made of P3HT∶PTB7-Th∶IEICO-4F(100-x∶x∶1,wt/wt/wt).The effect of different mass ratios of PTB7-Th on the optoelectronic properties of the devices was investigated by keeping the content of the acceptor IEICO-4F in the active layer(1 wt%)constant,and x denotes the content of PTB7-Th in the donor,which was 0,20 wt%,40 wt%and 50 wt%,respectively.The exciton dissociation efficiency can be improved by adopting the ternary bulk heterojunction structure,in which a large proportion of donors(P3HT and PTB7-Th)surrounds a small proportion of non-fullerene acceptors(IEICO-4F),and the lowest unoccupied molecular orbitals(LUMO)energy levels of IEICO-4F differ from those of P3HT and PTB7-Th by 0.99 and 0.53 eV,respectively,and the small proportion of IEICO-4F can be used as an electron trap to assist in inducing the bending of the energy band at the cathode(Al)/active layer interface thereby realizing the tunnelling injection of holes into the external circuit and hence photomultiplication.The absorption coefficients in the spectral range of 650～850 nm increase with the increasing mass ratio of PTB7-Th,which is attributed to the good light absorption of PTB7-Th in this region.Since the HOMO of Al to PTB7-Th has a larger hole injection barrier,the hole injection barrier increases with the increasing mass ratio of PTB7-Th in the double donor,and the dark current density then decreases gradually.When the mass ratio of P3HT∶PTB7-Th∶IEICO-4F is 50∶50∶1,the dark current density is the smallest,and it is as low as 1.51×10-4 A/cm2 at-15 V.The dark current density of P3HT∶PTB7-Th∶IEICO-4F is the lowest.Upon illumination,a large number of photogenerated electron-hole pairs(excitons)move to the P3HT/IEICO-4F,PTB7-Th/IEICO-4F and P3HT/PTB7-Th interfaces and dissociate,and the electron traps in the vicinity of the cathode trap more electrons,which build up at the active layer/Al interface,thus bending the interfacial energy bands,which in turn induces the injection of hole tunneling from the external circuitry into the active layer.As a result,the photocurrent density increases significantly under reverse bias voltage,and the photocurrent density of the device at-15 V is two to three orders of magnitude larger than the dark current density.The photocurrent density and external quantum efficiency of the device under-15 V bias voltage with the same wavelength illumination firstly increase and then decrease with the increase of PTB7-Th mass ratio.Compared with the binary system P3HT∶IEICO-4F,which has only one exciton dissociation interface(P3HT/IEICO-4F),the ternary system P3HT∶PTB7-Th∶IEICO-4F has three exciton dissociation interfaces(P3HT/IEICO-4F,PTB7-Th/IEICO-4F and P3HT/PTB7-Th).Therefore,when the mass ratio of PTB7-Th is increased from 0 to 40%,the exciton dissociation efficiency of the active layer is continuously enhanced,and the optical current density and external quantum efficiency of the device are increased.The ternary system P3HT∶PTB7-Th∶IEICO-4F has three types of electronic traps(deep trap P3HT/IEICO-4F/P3HT,medium trap P3HT/IEICO-4F/PTB7-Th,and shallow trap PTB7-Th/IEICO-4F/PTB7-Th),while the binary system P3HT:IEICO-4F has only one electron trap(P3HT/IEICO-4F/P3HT).When the PTB7-Th mass ratio exceeds 40%to 50%,the number of medium traps and shallow traps in the active layer increases,along with the increasing hole injection barrier,leading to the decrease of the photocurrent density and external quantum efficiency.The mass ratio of PTB7-Th in the active layer of the best mass ratio device is 40%,and the external quantum efficiencies of the best mass ratio device under-15 V biasvoltage at 450,520,655 and 850 nm illumination are 2 666.40%,1 752.11%,1 894.26%,and 938.22%,respectively,and the responsivity is 965.80,733.35,998.68,and 641.91 A/W,and the specific detectivities are all over 1013 Jones.Under 850 nm illumination,the device's responsivity and specific detectivity are 2.23 and 7.08 times higher than those of the two-component device,P3HT∶IEICO-4F(100∶1,wt/wt),at the same conditions,respectively.The linear dynamic range of the best mass ratio device is 69.81 dB under-15 V bias voltage and 520 nm laser source.The results show that doping the binary system P3HT:IEICO-4F with an appropriate amount of PTB7-Th not only extends the spectral response range to the near-infrared,but also modifies the exciton dissociation interface,the type of electron traps,hole injection barrier height,and optimizing the electrical performance of the device.
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