Zhi Yuan Wang

High-performance broadband photodetectors based on low-bandgap conjugated polymers are scarce due to a trade-off between the photoresponse range and device performance. In this work, the polymers with structural features responsible for both broad response and high detectivity are realized by using the combined electron deficient moieties at the molecular level. Based on the electron-deficient diketopyrrolopyrrole (DPP) and benzothiadiazole (BT), the combined units of DPP–DPP and DPP–BT–DPP are deemed more electron deficient and were successfully incorporated into the corresponding polymers. The arrangement of electron deficient moieties in the polymer main chain was found to be able to greatly affect the absorption profile, energy level, molecular stacking, and blend film morphology. Consequently, the photodetector based on the polymer with a DPP–BT–DPP acceptor exhibited a D* value of over 10¹² Jones in the spectral region of 320–930 nm under −0.1 V bias.

Three acceptor polymers with different LUMO energy levels were synthesized. High-gain all-polymer photodiodes based on low-LUMO acceptor polymers exhibit a high specific detectivity of 3.5 × 10¹⁴ Jones and responsivity over 110 A W⁻¹. This is the first work of high-gain all-polymer photodiodes, which provides a novel way for obtaining all-polymer photodiodes.

Three strong electron‐withdrawing monomers and one electron‐donating monomer were chosen by design to impart some desirable properties to the target terpolymers (P1‐P3) for use in the photodiodes, such as strong donor‐acceptor charge transfer, low bandgap, high mobility and good film morphology. Photodiodes with a device structure of ITO/ZnO/active layer/BCP/Al exhibited a significant increase of EQE only under forward bias. In particular, the P2‐based device had the specific detectivity greater than 10 ¹³ Jones from 330 nm to 1060 nm and 10 ¹¹ Jones from 300 nm to 1600 nm under 0.5 V and linear dynamic range over 100 dB under 2.0 V. In comparison, after the UV light treatment to the ZnO layer, the P2‐based photodiodes exhibited a high gain in photocurrent under both forward and reverse bias and had specific detectivity above 10 ¹³ Jones at 320–1140 nm, 10 ¹² Jones at 300–1460 nm and 10 ¹¹ Jones at 300–1600 nm under 0.5 V. Our work has firstly demonstrated that high gain and high detectivity in polymer photodetector could be readily achieved under forward bias without the UV light treatment.

Polymer photodiodes with broad spectral response above 1500 nm can be used for imaging in the near‐infrared window of the atmosphere. Three low‐bandgap polymers based on 3,6‐dithiophen‐2‐yl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP), [1,2,5]thiadiazolo[3,4‐g]quinoxaline (TQ), benzobisthiadiazole (BBT), and dithienopyrrole (DTP) are designed and synthesized. No‐gain and gain polymer photodiodes with polymers:PC₇₁BM as active layer materials and with aluminum doped ZnO nanoparticles:2,9‐bis(3‐(dimethylamino)propyl)anthra[2,1,9‐def:6,5,10‐d′e′f′]diisoquinoline‐1,3,8,10(2H,9H)‐tetraone (AZO:PDIN), MoO₃, and 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP) as interlayer materials are prepared. No‐gain photodiodes based on polymer P1 exhibit the external quantum efficiency (EQE) of 7.8% at 1200 nm, which is among the best values of polymer photodiodes known to date. Gain photodiodes based on P1 and P3 also exhibit a maximal EQE near 1200 nm. In addition, gain photodiodes based on P1 have a specific detectivity over 10¹³ Jones in 300–1360 nm and spectral response in the region of 300–1700 nm.

A series of n-type semiconducting conjugated polymers based on naphthalene diimide (NDI) having three different side chains, 5-decylpentadecyl, 2-octyldodecyl and 4-(2-octyldodecyloxy)phenyl, were synthesized. Experimental results showed that the subtle structural changes at the side chains can influence the molecular packing, electron mobility, blend film morphology and thus performance of bulk-heterojunction polymer photodetectors using these NDI-based polymers as acceptors. The optimized all-polymer photodetector exhibited a specific detectivity (D*) of over 10¹³ Jones in the spectral region of 300–800 nm under −0.1 V bias, which is among the best D* values of the reported UV–vis–NIR all-polymer photodetectors and comparable to the best fullerene-based photodetectors.

We report the photoresponse characteristics of a pyrimido[4,5-g]quinazoline-4,9-dione (PQ) based polymer, PPQ2T-BT-24, which served as an active channel layer in organic phototransistors (OPTs). OPTs using this polymer showed very short rise time of 3 ms and fall time of 59 ms, photoresponsivity (R) of 0.24 and external quantum efficiency (EQE) of 50%. When PC₆₁BM was incorporated into PPQ2T-BT-24, a significantly shortened fall time of 8 ms along with a much-improved rise time of 1 ms were realized. These response times are among the best values reported for OPTs. The R and EQE of the devices using this polymer blend also increased to 0.88 A W⁻¹ and 189%, respectively.

A series of four low-bandgap polymers containing diketopyrrolopyrrole (DPP) and dithienopyrrole (DTP) functionalized with four different side chains of 4-phenyl, 4-octylphenyl, 4-(octyloxy)phenyl and 4-(octylthio)phenyl were synthesized. Subtle changes in the side chains of polymers are found to effectively affect the molecular stacking and crystallinity, film morphology, and photodetector performance. The best polymer photodetector displays an average external quantum efficiency (EQE) of 40% and a specific detectivity (D*) of over 10¹³ Jones in the spectral region of 350–960 nm under −0.1 V bias, which is among the best EQE and D* values of all the known UV–vis–NIR polymer photodetectors reported to date.

A series of low bandgap polymers containing thiophene and diketopyrrolopyrrole (DPP) units were synthesized and their electrochromic (EC) properties were investigated. It was found that the ratio (R_da) of thiophene to DPP units correlates to the EC properties, in particular optical contrast (ΔT) and coloration efficiency (CE) in the visible and near-infrared spectral regions (e.g., 1310 nm). At the wavelength of 1310 nm, the CE of polymer P2 (R_da = 3 : 1) and ΔT of polymer P4 (R_da = 4 : 1) reached the high values of 1383 cm² C⁻¹ and 81.5%, respectively. Polymer P3 (R_da = 3.5 : 1) had a relatively short switching time of 1.24 s from the doped state to neutral state. Colors of polymer films changed reversibly between green or blue (neutral state) and grey (doped state) upon the redox cycling.

Polymer photodetector based on a novel low-bandgap polymer showed a detectivity of greater than 10 ¹² Jones in the spectral region of 300–980 nm and over 10 ¹⁰ Jones at 300–1600 nm under 0.1 V.

A series of random copolymers containing the two electron-withdrawing units naphthalene diimide (NDI) and diketopyrrolopyrrole (DPP) and a thiophene (T) spacer were designed and synthesized as non-fullerene acceptors for all-polymer photodetectors. Increasing the number of DPP–T segments in the polymer backbone leads to a significant red shift in maximal absorption, bandgap narrowing, better film morphology and lower dark current density. A specific detectivity over 10¹² Jones in a broad spectral region of 340–960 nm under −0.1 V bias was achieved with a photodetector using a blend of a donor polymer based on dithienopyrrole and diketopyrrolopyrrole (PDTP–DPP) and a copolymer (PNDI–DPP10) as an acceptor. In comparison, the device performance was found to be better than for the one based on PDTP–DPP and the PNDI homopolymer.