Photocurrent/dark current ratio

Pig. 7. Relative carrier yield versus excitation wavelength. All data are normalized to the same absorbed photon flux. The electric field was t300 V/cm oriented approximately in the polymer chain direction in a standard surface cell configuration. The peak photocurrent-to-dark current ratio was 300 at 300 °K and >600 at 120 °K. Also shown is the absorption spectrum of PTS. The solid portion of the curve refers to the polymer chain direction. The dashed portion is an extrapolation derived from absorption measurements for a dispersion of PTS in a KBr pellet24 ... 

In derivatives of 9-dicyanomethylene-2-nitrofluorene 7 the ratios of photocurrent to dark current range between 100 and 20.000 depending on the position of the introduced nitro groups 82). 

PPX-composite films containing semiconductor nanocrystals of PbO (formed by oxidation of Pb nanocrystals) and PbS show photoconductivity [71, 89]. In the PbO-PPX films the photocurrent value (/ph) in the wavelength range 250-350 nm does not depend on wavelength and the ratio between 7ph and dark current is close to 104. Then /ph is gradually reduced to zero at increase of wavelength to 450 nm [89],... 

The ratio of peak photocurrent to dark current was as high as 300 at 300 °K and 600 at 120 °K. The action spectrum of photoconductivity of this and other studied polydiacetylenes24 is significantly shifted to shorter wavelengths from the absorption spectrum. 

Another group of polymers and oligomers with extended conjugation in the chain, polystyrylpyrimidines35 with the structure suggested as follows were identified as fairly efficient photoconductors. The ratio of photocurrent to dark current... 

The ratio of photo- to dark current was as high as 200—300 with panchromatic light intensity of 20-30 W/cm2 the photocurrents reached 10 7 A/cm2 at electric fields of 4—5 x 10s V/cm. In these polymers the absorption spectra coincide with the photoconductivity action spectra. 

An anodic plateau current is measured on -Si in the dark thus indicating electron injection into the conduction band. This plateau current is only slightly affected by HF concentration but is proportional to Cr concentration. Illumination of the n-Si causes an increase of the anodic plateau current with a quantum efficiency of about 2.8 due to electron injection into the conduction band by the silicon dissolution intermediates. The quantum efficiency for the anodic photocurrent depends on the HF/CrOs ratio, about 2.8 at a high HF/CrOs ratio (>20) and about 1 at very low ratios. 

The photocurrents observed are usually in the range of 1 to 100 pA depending on the power of the light source. A special purification effort is required in order to reduce the dark conductivity below these values (see Section 1.8). Usually, photo current spectra are taken at a fixed voltage. An example is shown in Figure 4. These data must be normalized with respect to the spectral distribution in order to estimate the photoconductivity energy threshold. The ratio of photocurrent to light intensity... 

Photovoltaic cells were prepared with a polymer layer sandwiched between electrodes with different work functions, as in polymer-based LEDs (Fig. 29.2). In the dark, the device exhibits a rectification ratio of 10 at 3.5 V. Under illumination, devices of this type show a strong photoresponse, with open-circuit voltages of 0.6 V and short-circuit currents that correspond to quantum efficiencies of up to 6%. Under forward or reverse biases, the quantum efficiencies rise rapidly, reaching 15% at a reverse bias of 3.5 V, 40% at 10 V. and considerably higher values under forward bias. These performance figures are very much better than those for devices made with aluminum electrodes and either MEH-PPV or CN-PPV alone. Thus, Halls et al. report values for the quantum yield of the short-circuit photocurrent in the MEH-PPV of 0.04% at the peak response wavelength (2.2 eV) and on the order of 10 %... 

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