Inorganic semiconductor‐based broadband photodetectors are ubiquitous in imaging technologies such as digital cameras and photometers. Herein a broadband organic photodiode (OPD) that has performance metrics comparable or superior to inorganic photodiodes over the same spectral range is reported. The photodiode with an active layer comprised of a poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:[6,6]‐phenyl‐C71‐butyric acid methyl ester bulk heterojunction blend had a dark current < 1 nA/cm2, specific detectivity of ∼1013 Jones, reverse bias −3 dB frequency response of 100 kHz to 1 MHz, and state‐of‐the‐art Linear Dynamic Range for organic photodiodes of nine orders of magnitude (180 dB). The key to these performance metrics was the use of a thick junction (700 nm), which flattened the spectral response, reduced the dark current and decreased performance variations. The strategy also provides a route to large area defect free “monolithic” structures for low noise integrated photo‐sensing, position determination, or contact, non‐focal imaging.

A solution-processable dibromoindigo with an alkyoxyphenyl solubilizing group is developed and used as a new electron acceptor in organic photodiodes. The solution-processed fullerene-free organic photodiodes show an almost spectrally flat response with a high responsivity (0.4 A W(-1)) and a high detectivity (1 × 10(12) Jones). These values are comparable to silicon-based photodiodes.

COMMUNICATION metrology, photometry and position sensing. However, these devices involve the deposition of multiple layers (up to 32 layers) and require the use of the expensive molecular beam technique. [ 4,5 ] Recently, solution-processed small molecular or non-polymeric OPDs have also been demonstrated. [23][24][25][26] However, the performance thus far has fallen behind their polymeric and/or evaporated small-molecular counterparts, especially in terms of detectivity, broad spectral response, and responsivity.In this paper, we describe how the key OPD device characteristics can be optimized and demonstrate high-performance solution-processed broadband OPDs based on a non-polymeric organic semiconductor including a near spectrally fl at spectral response from 350 nm to 700 nm, low dark current density (0.1 nA/cm 2 ), high responsivity (0.4 A/W), high detectivity (9.2 × 10 12 Jones) and a linear dynamic range of 140 dB. These performance parameters are several orders of magnitude better compared to previously reported solution based non-polymeric OPDs, and comparable to polymer and inorganic silicon based photodiodes. Furthermore, we have studied the dark current, noise current and EQE as a function of the active-layer thickness, electrode work function, annealing and hole-blocking layer. The manuscript provides full analysis of the noise current and operating mechanism of the devices.The basic device architectures used were ITO/PEDOT:PSS/ active layer/C 60 /Al or Ca/Al (see Figure 1 ). The active layer was comprised of 7,7′-[4,4-bis(2-ethylhexyl)-4 H -silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]bis[6-fl uoro-4-(5′n -hexyl-{2,2′-bithiophen}-5-yl)benzo[ c ][1,2,5]thiadiazole] ( p-DTS(FBTTh 2 ) 2 ) [ 27 ] and [6,6]phenyl-C 71 -butyric acid methyl ester (PC 71 BM) blend, with their chemical structures also shown in Figure 1 . In the following discussion we describe how each of the device parameters (dark current, external quantum effi ciency, detectivity, noise equivalent power, and linear dynamic range) can be optimised. The molecular blend of the p -type and n -type organic semiconductors forms what is termed a bulk heterojunction (BHJ).To achieve optimum OPD performance, minimization of the dark current is crucial, and hence its origins need to be identifi ed. Figures 2 a and b show the dark current density at −0.5 V as a function of the active layer thickness for as-formed and annealed OPDs. The dark current densities as a function of voltage bias are shown in Figures 2 c and d and S1-2. Under reverse bias, the dark current density for both the as-formed and annealed devices increased with increasing applied voltage and decreased with increasing thickness of the active layer. That is, as expected the thicker junction devices exhibited a much lower dark current than the thinner junctions. For the devices with 320 nm thick junctions the use of Al electrodes gave a much lower dark current (0.17 nA/cm 2 ) compared with (11 nA/cm 2 )

SummaryPhotovoltaic characteristics of dye-sensitized solar cells (DSSCs) using TiO2 nanotube (TNT) arrays as photoanodes were investigated. The TNT arrays were 3.3, 11.5, and 20.6 μm long with the pore diameters of 50, 78.6, and 98.7 nm, respectively. The longest TNT array of 20.6 μm in length showed enhanced photovoltaic performances of 3.87% with significantly increased photocurrent density of 8.26 mA·cm−2. This improvement is attributed to the increased amount of the adsorbed dyes and the improved electron transport property with an increase in TNT length. The initial charge generation rate was improved from 4 × 1021 s−1·cm−3 to 7 × 1021 s−1·cm−3 in DSSCs based on optical modelling analysis. The modelling analysis of optical processes inside TNT-based DSSCs using generalized transfer matrix method (GTMM) revealed that the amount of dye and TNT lengths were critical factors influencing the performance of DSSCs, which is consistent with the experimental results.

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