WO2008038324A1

WO2008038324A1 - Color sensing device using organic semiconductor-based photodiodes

Info

Publication number: WO2008038324A1
Application number: PCT/IT2007/000657
Authority: WO
Inventors: Guglielmo Lanzani; Maria Rosa Antognazza; Paolo Monti
Original assignee: Politecnico Di Milano; Fondazione Istituto Italiano Di Tecnologia
Priority date: 2006-09-29
Filing date: 2007-09-21
Publication date: 2008-04-03
Also published as: ITMI20061866A1

Abstract

The invention relates to a color sensing device comprising three arrays of organic semiconductor-based photodiodes, wherein each of the three arrays is sensitive over a wavelength range and has a spectral responsivity curve within said wavelength range which is substantially close to a specific CIE standard colorimetric function. In the preferred embodiment of the invention, the color sensing device has three organic semiconductor-based photodiodes, one for each array, operating over the wavelength ranges from 360 nm to 780 nm, from 360 nm to 700 nin and from 360 nm to 600 nm respectively (dashed lines), and having a respective spectral responsivity curve substantially close to a specific colorimetric function x(λ), y(λ) or z(λ) of the CIE 1931 system (solid lines).

Description

Color sensing device using organic semiconductor-based photodiodes

The present invention relates to a color sensing device using organic semiconductor-based photodiodes. Colorimetry is a science that measures color and systematically identifies it through optical measures, and represents the confluence of several branches, such as optics, physiology, psychology and engineering. Methods have been designed for color measurement, which try to simulate the human eye response to color. Three different types of photoreceptors are present in human retina; their activation generates the so-called "tristimulus space", within which an infinite number of standard systems may be selected. The Commission Internationale d'Eclairage (CIE) is an international body dealing with the selection of the most appropriate standard systems, the standardization of color measurements and the definition of a standard (or ideal) observer, through the definition of suitable standard colorimetric functions, which are three in number to describe the tristumulus space. Particularly, the CIE 1931 standard observer was defined under foveal vision conditions, as characterized by three colorimetric functions, traditionally denoted by the symbols x(λ), y(λ) and z(λ), and known as color matching functions or CIE 1931 colorimetric functions. Their domains extend in the wavelength ranges of 400 run - 700 mm, with a main maximum in the red region (600 nm), 440 run - 680 nm, with a single maximum in the green region (560 nm), and 360 nm - 480 nm, with a single maximum in the blue region (440 nm), respectively. CIE 1964 colorimetric functions have been further defined for an observer under extra-foveal vision conditions, i.e. the x'(λ), y'(λ) and z'(λ) colorimetric functions for the so-called Vos observer, and the fundamental standard system was finally defined, known as LMS natural system, which accounts for the real activation of the three types of human photoreceptors, i.e. the L, M and S cones. The CIE 1931, CIE 1964 systems and other systems derived therefrom are mostly used in the colorimetric practice, whereas the natural system finds interest in scientific studies on color vision. In any case, it is known that one can move from one standard system to any other by appropriate linear transformations, i.e. appropriate calculation means.

Various color measuring devices are known, more commonly referred to as colorimeters, which are used in colorimetry studies, in chemical labs or in factories such as paint factories.

In industrial applications, the sensitivity, and hence the measuring accuracy, of the devices is an essential feature for the use thereof. High sensitivity requirements accordingly exclude the selection of visual colorimeters, even being either color comparators or visual tristumulus colorimeters, which base on the comparison between a known color and the color to be specified by an operator.

Photoelectric colorimeters ensure higher sensitivity and adaptability to industrial applications. Among these, filter colorimeters are known in which the response of the photocell to incident light, coming from the colored object, is modified by suitable red, green and blue filters, which respectively transmit light at such wavelengths that the responses of photodetectors in combination with the transmittances of the three filters reproduce the colorimetric functions of a CIE standard observer. The degree of tuning between such CIE standard functions and the responsitivities produced by each "photodetector + filter" pair determines the accuracy of the colorimeter. In these devices, the maximum efficiency of the detector generally does not match with the spectral region transmitted by the filter.

Although the design of primary filters exactly corresponding to an ideal is theoretically possible, it is impossible in practice to obtain primary filters with transmission factors exactly corresponding to the ideal. Therefore, the performances of these devices are limited by the accuracy of the light transmission properties of the filters, and further long-term performances are limited by the long-term stability of the filters. These limitations concerning the use of the filters are further aggravated by the poor availability of the materials for making them, which cause the final colorimeters to have a high cost.

Therefore, an object of the present invention is to provide a device having a good sensitivity and measuring accuracy, while being more cost-effective than prior art well-performing colorimeters.

A further object is to provide a device having such a good spectral tunability with color standard colorimetric functions and simple construction peculiarities, to provide a colorimeter that is easily adaptable to the environment in which color has to be detected. The above objects have been achieved through a color sensing device comprising three arrays of organic semiconductor-based photodiodes having the features as defined in claim 1.

The color sensing device of the invention comprises three arrays of organic semiconductor-based photodiodes, each of which is sensitive over a wavelength range and has a spectral responsivity curve within said wavelength range which is substantially close to a specific standard colorimetric function.

The invention will be now described in greater detail with reference to the accompanying drawings, wherein: Figure 1 is a spectral responsivity to wavelength diagram, which shows the three colorimetric functions x(λ), y(λ), and z(λ) of the CIE 1931 system;

Figure 2 is a schematic view of the apparatus for characterizing a photodiode in the device according to the invention;

Figures 3 a, 3 b, 3 c show the spectral responsivities of three photodiodes according to the invention over the wavelengths of the visible solar spectrum;

Figure 4a shows the comparison between the CIE 1931 colorimetric functions and the experimental curves (spectral responsivity to wavelength) of the color sensing device according to the invention;

Figure 4a shows the comparison between the CIE 1931 colorimetric functions and the experimental curves (responsivity to wavelength) of the color sensing device according to the invention after calibration; and

Figure 5 shows the three experimental signals of the evaluation example 5 on the device of the invention.

According to the invention, the term "organic semiconductor-based photodiode" is meant to be an optical device that can recognize a predetermined wavelength in a predetermined spectral domain and can transform this event into an electrical current signal, where the material capable of such transformation is an organic material, thus including polymers and oligomers.

The term "spectral responsivity curve" of an array of photodiodes or of a photodiode is meant to be the combined photocurrent spectrum of the array or the photocurrent spectrum of the photodiode, respectively.

The term "visible solar spectrum" is meant to be the electromagnetic radiation within the wavelengths from about 360 nm to about 780 run. A color sensation corresponds to each wavelength of the visible solar spectrum. The red (around 650 nm), green (around 530 nm) and blue (around 400 nm) regions are identified within the solar spectrum. According to the invention, when an array of photodiodes or a photodiode is deemed to be "sensitive" in a wavelength range, it is meant that an array or the a photodiode can operate and thus has a spectral domain in those wavelengths.

The term "standard colorimetric function" is meant to be a theoretical mathematical spectral responsivity function as defined by CIE and used in a standard color determination system. In the CIE space, the colorimetric functions are three.

When reference is made to the CIE 1931 colorimetric functions, the colorimetric functions are meant traditionally referred to as x(λ), y(λ), and z(λ) and shown in Figure 1. According to the CIE 1931 color determination system, from the colorimetric functions x(λ), y(λ), and z(λ), the tristimulus coordinates X, Y and Z can be extracted, from which the chromaticity coordinates (x, y) of the CIE system chromaticity diagram can be obtained, which univocally define a color for a predetermined luminance value.

According to the invention, the term "substantially close", when referred to the spectral responsivity curve of an array or a photodiode, is meant to be a combined spectral responsivity curve or a spectral responsivity curve of the photodiode with a maximum total deviation not higher than 40% with respect to a specific standard colorimetric function.

According to the invention, three arrays of organic semiconductor-based photodiodes are provided. Therefore, the invention provides a device comprising three arrays of photodiodes, wherein each array may contain one or more photodiodes, selected so that the overall responsivity curve of the array or the single photodiode is substantially close to a specific standard colorimetric curve.

Preferably, the device according to the invention has three arrays of organic semiconductor-based photodiodes, more preferably one photodiode for each array, operating over the wavelength ranges from 360 nm to 780 nm, from 360 nm to 700 nm and from 360 nm to 600 nm respectively, and having a respective spectral responsivity curve substantially close to a specific CIE standard colorimetric function.

Advantageously, the invention comprises three arrays of organic semiconductor-based photodiodes, more preferably one photodiode for each array, operating in the wavelengths from 400 to 780 nm, with a maximum in the red region, from 420 to 700 nm, with a maximum in the green region, and from 360 to 600 nm, with a maximum in the blue region, respectively.

More advantageously, in the device according to the invention, each of the three arrays of organic semiconductor-based photodiodes has a spectral responsivity curve substantially close to a specific colorimetric function x(λ), y(λ), or z(λ). In the preferred embodiment of the invention, the color sensing device comprises three organic semiconductor-based photodiodes, one for each array, operating over the wavelength ranges from 360 nm to 780 nm, from 360 nm to 700 nm and from 360 nm to 600 nm respectively, and having a respective spectral responsivity curve substantially close to a specific colorimetric function x(λ), y(λ) or z(λ) of the CIE 1931 system.

The device of the invention comprises three arrays of organic semiconductor- based photodiodes.

The organic semiconductor-based photodiodes of the invention comprise a negative electrode, i.e. a cathode, a positive electrode, i.e. an anode, and an organic material acting as a semiconductor.

Preferably, the anode comprises a layer of indium and tin oxide, known as ITO, more preferably having a thickness of 30 nm to 500 nm, most preferably of about 100 nm.

The cathode preferably comprises a layer of aluminum (Al) having a thickness of 50 nm to 300 nm, preferably of about 150 nm.

The organic semiconductor according to the invention is an organic material, including polymers and/or oligomers. Suitable polymers and/or oligomers include all polymers or oligomers or mixtures thereof operating over a wavelength range corresponding to the wavelength range of the standard colorimetric curve and whose corresponding photodiode has a spectral responsivity curve substantially close to a CIE standard colorimetric function.

According to the invention, photodiodes may be each provided with its own anode and cathode or alternatively the anode and the cathode may be the same for all the photodiodes or arrays of photodiodes involved.

One or more photodiodes, independently of each other, used in the invention may comprise, as a polymer material, one of: - the regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) of formula:

where R is a hexyl group, preferably having a molecular weight of about 87000;

- the polymer 16,17-bis(octyloxy)anthra-[9,l,2-cde]benzo[rst]-pentaphene- 5,10-dione, commonly known as 16,17-bis(octyloxy)-violanthrone of formula:

preferably having a molecular weight of about 712.91;

- the polymer known as poly(paraphenylene) of the methyl-substituted ladder type of formula:

preferably having a molecular weight of about 122500.

The photodiode comprising the regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) according to the invention as an organic semiconductor material has a spectral responsivity curve in the wavelengths from 360 nm to 750 nm; the photodiode comprising the 16,17-bis(octyloxy)anthra-[9,l,2-cde]benzo[rst]- pentaphene-5,10-dione polymer according to the invention as an organic semiconductor material has a spectral responsivity curve in the wavelengths from 360 nm to 680 nm; and the photodiode comprising the poly(paraphenylene) polymer of the methyl-substituted ladder type according to the invention as an organic semiconductor material has a spectral responsivity curve in the wavelengths from 360 nm to 500 nm.

In the preferred embodiment of the invention comprising three photodiodes with spectral responsivity curves close to the CIE 1931 colorimetric functions, the three photodiodes comprise the above mentioned polymers respectively. Such a device may be used for color sensing in the CIE 1931 standard system, as better shown below.

In an alternative embodiment, the device according to the invention comprises three arrays of photodiodes, wherein each array may contain more than one photodiode, so that each array has a combined spectral responsivity curve substantially close to a specific CIE colorimetric function.

In another aspect, the invention relates to a process for preparing the device of the invention, as defined in claim 18.

Specifically, the process of the invention includes the step of preparing three arrays of organic semiconductor-based photodiodes, each array is sensitive over a wavelength range and has a spectral responsivity curve substantially close to a specific standard colorimetric function.

In the preferred embodiment of the device according to the invention, the process will include the step of preparing three photodiodes, one for each array. In this case, each photodiode will have a spectral responsivity curve substantially close to a specific standard colorimetric function. In a more preferred embodiment of the invention, the three photodiodes are prepared with the following organic semiconductor materials, respectively: regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), 16, 17-bis(octyloxy)anthra-[9, 1 ,2-cde]benzo[rst]-pentaphene-5, 10-dione and poly(paraphenylene) of the methyl-substituted ladder type.

The photodiodes according to the invention may be prepared, for instance, by using the spin coating technique for the polymer deposition, i.e. by coating technique through centrifugation. According to this technique, a film is deposited from a polymer solution on an anode, and the metal cathode is deposited by vacuum evaporation of suitable metals.

According to the invention, once the materials for preparing the photodiode have been defined on the basis of the spectral responsivity curve of the array of photodiodes or the photodiode, which has to be substantially close to a CIE colorimetric function in a color standard system, the device of the invention may be calibrated by comparison with the colorimetric functions themselves to improve the tunability with standard curves and hence the measurement reliability. The calibration may be performed, for instance, by a standard method based on the minimization of the mean-square deviation of standard colorimetric functions and a linear combination of the spectral responsivities of the device when the latter is hit by an incident calibration radiation.

The calibration procedure according to the invention further allows to obtain a device that can be used in a standard system different from the one initially selected. For instance, once a color sensing device has been prepared through the approximability of spectral responsivity curves to the colorimetric curves x(λ), y(λ) and z(λ) of the CIE 1931 system, it is possible, by way of suitable calibration means consisting of appropriate processing and calculation means, to move to a standard system other than CIE 1931, thereby obtaining a color sensing device in the new system selected.

According to the invention and as it will be clear by the experimental part set forth below, when the device according to the invention is hit by an incident light of unknown color, it can produce three electric signals, which are processed through appropriate processing means into three tristimulus coordinates, which allow to univocally determine a color in the standard system used for the preparation of the device.

Therefore, in another aspect, the invention relates to a colorimeter comprising the color sensing device according to the invention and a suitable lighting source standardized in the CIE reference system as defined in claim 22. Suitable lighting sources may include, for instance, a tungsten lamp operating at a color temperature of 2856 K (standard source A) or fluorescent lamps (standard sources F2, F7, Fl 1). Therefore, in order to overcome the drawbacks associated to the use of filters in the design of prior art colorimeters, the inventors of the present invention have surprisingly found that the use of arrays of organic semiconductor-based photodiodes having spectral responsivity curves substantially close to specific standard colorimetric functions, which univocally define a color in the different color determination systems, allows to obtain a reliable color sensing device, as shown in the annexed examples. Without wishing to be bound by any theory, the inventors of the present invention believe that the removal of filters and the detection of a signal directly produced by the photodiode allow to improve the measurement accuracy and therefore to reliably take a color. Furthermore, Ms approach, together with an appropriate selection of materials, allows a direct tuning of the spectral response of the devices to any desired colorimetric function. A specific example of such a flexibility is the production of photodiodes having a spectral response close to that of the natural photoreceptors in the retina. An example of preparation of a device according to the invention comprising three photodiodes according to the preferred embodiment of the invention and an example of use of the device so obtained for color detection in the CIE 1931 standard system will be now described by way of non-limiting examples. Example 1 Preparation of the device according to the invention

A) Preparation of the photodiode comprising mLPPP

A 2 cm x 2 cm x 1.1 mm glass sheet coated with a 100 run ITO layer having a resistance of 20 Ohm/q was subjected to etching. Specifically, a vertical portion having a width of about 1 cm and a length equal to the depth of the sheet (2 cm) was coated with a protective paint and the sheet with the painted strip was dipped in a solution comprising three parts of double-distilled water and a part of 37% fuming hydrochloric acid, so that ITO is removed from the edges. Once the removal of oxide from the desired parts was observed by using an ohmmeter, the paint was removed by using acetone and the substrate so obtained was accurately cleaned in several cleaning cycles in an ultrasound bath.

20 mg mLPPP with a molecular weight of about 122500, supplied by Bergische Universitat Wuppertal, were weighed under an aspiration hood and solubilized in 1 ml toluene, while heating with an electric stove in order to obtain the complete solubilization of the polymer.

The polymer solution was then deposited on the sheet by spin-coating, i.e. by deposit through centrifugation, using the spin-coater p6700 by Speedline Technologies (rotation speed = rpm 2000; ramp time τ= Is with an overall rotation time of 30 s). The thickness of the polymer so deposited was of about 130 nm.

The cathode consisted of 99.9% pure aluminum was finally deposited by evaporation on the polymer layer, by using the commercial evaporator EDWARDS Auto 306, consisted of a system of vacuum pumps (a rotary pump and a diffusive pump) with an evaporation chamber of up to 10^"6 mbar.

B) Preparation of the photodiode comprising 16J7-bis(octyloxy)- violanthrone

A photodiode with a semiconductor of 16,17~bis(octyloxy) violanthrone was obtained by repeating the Example IA) with the same equipment and materials, with the addition of 20 mg 16,17-bis(octyloxy) violanthrone having a molecular weight of about 712.90, supplied by Sigma-Aldrich, in 1 ml toluene, in place of the mLPPP polymer.

C) Preparation of the photodiode comprising regioregular P3HT

A photodiode with a semiconductor of regioregular P3HT was obtained by repeating the Example IA) with the same equipment and materials, with the addition of 20 mg regioregular P3HT having a molecular weight of about 87000, supplied by Sigma-Aldrich, in 1 ml chloroform, in place of the mLPPP polymer.

The three photodiodes so obtained were thus equipped with electric contacts for electric current measurement. Specifically, the electric contact was made by connecting metal wires to the electrodes through silver paste acting as conductive adhesive.

Example 2

Characterization of the three photodiodes

The photodiodes obtained as described in Example 1 were characterized by using the apparatus shown in Figure 2.

Referring to Figure 2, the photodiode was illuminated by a halogen lamp (model ASB-W-030 by Spectral Product), characterized by emissions in the wavelength spectrum from 300 nm to 2600 nm, which collimated the light beam owing to an inner focusing optical system; a monochromator (model CMI lO 1/8 meter by Spectral Product) with entrance and exit slits of 0.6 mm was provided in direct contact with the lamp and interfaced with a computer, thus allowing the selection of the desired gratings and the wavelength adjustment. The light of predetermined wavelength, emitted by the lamp and passing through the collimator, then impinged upon a mechanical light-modulating chopper, a collimating lens and finally a focusing lens, which focused light on the photodiode to be characterized, which was placed on a micrometer adjustment support displacing in three directions. The modulated photocurrent was then detected by a lock-in amplifier which transmitted data through a computer interface to a software that could instantaneously produce the measured values. The Keithley (model 236 Source- Measure Unit (SMU)) as shown in Figure 2 acted as a voltage generator and was used in order to increase the efficiency of the devices, without meanwhile causing changes in the line shape of their spectral responsivity. The measures so obtained, as indicated below, related in any case to operation under zero voltage conditions.

The apparatus of Figure 2 as described above was used to obtain photocurrent measures in the wavelengths from 360 to 780 nm (i.e. from 3.45 to 1.59 eV), and therefore to obtain the curve of spectral responsivities (SR) for each photodiode prepared in the Example 1. mLPPP-based photodiode:

All measures for the mLPPP-based photodiode were made with a voltage of 0 V (photovoltaic regime), a frequency of 80 Hz, a pitch of 2 nm and a time constant between data readings of 300 ms. Photocurrent had a peak of 87.3 pA at 456 nm, whereas the spectral responsivity curve had a maximum peak at 454 nm. The diagram of Figure 3 a was thus obtained. 16, 17-bis{octyloxy)-violanthrone-based photodiode:

All measures for the 16,17-bis(octyloxy)-violanthrone-based photodiode were made with a voltage of 0 V (photovoltaic regime), a frequency of 80 Hz, a pitch of 2 nm and a time constant between data readings of 1 s. Photocurrent had a peak of 8.79 pA at 570 nm, whereas the spectral responsivity curve had a maximum peak at 570 nm. The diagram of Figure 3 b was thus obtained.

Regioregular P3HT-based photodiode:

All measures for the regioregular P3HT-based photodiode were made with a voltage of 0 V (photovoltaic regime), a frequency of 80 Hz, a pitch of 3 nm and a time constant between data readings of 1 s. Photocurrent had a peak of 462 pA at 611 am, whereas the spectral responsivity curve had a maximum peak at 611 nm and a second peak at 437 nm.

The diagram of Figure 3 c was thus obtained.

Therefore by comparing the spectral responsitivities as shown in Figures 3 a, 3b and 3 c and the colorimetric functions (x(λ) y(λ) and z(λ)) of the CIE 1931 system as indicated in Figure 1, the photodiodes comprised in the device of the present invention were found to be suitable for manufacturing the device for use in the CIE

1931 color sensing system. Indeed, the spectral responsivity curves of the three organic semiconductor-based photodiodes were substantially close to the colorimetric functions of the CIE 1931 system. For an easier comparison, the curves have been drawn in a single diagram, as shown in Figure 4a. The overall maximum deviation was estimated not to exceed 10%.

As clearly shown in Figure 4a, the spectral responsivity cure of the photodiode comprising mLPPP was substantially close to the colorimetric function z(λ) of the blue region, the spectral responsivity curve of the photodiode comprising 16,17-bis(octyloxy)-violanthrone was substantially close to the colorimetric function y(λ) of the green region, the spectral responsivity curve of the photodiode comprising regioregular P3HT was substantially close to the colorimetric function x(λ) of the red region. Example 3:

Experimental evaluation of the color sensing device of the invention

The device according to the invention, comprising the three photodiodes, was then calibrated in order to improve tunability of the spectral responsivity curves obtained by using the photodiodes of the invention with the colorimetric functions of the CIE 1931 system, and the calibrated curves of the device according to the invention as shown in Figure 4b were obtained. As easily shown by the comparison of the two Figures 4a and 4b, the two sets of curves are not particularly different from each other, with an overall maximum deviation again not exceeding 10%.

Specifically, a standard method was used for calibration, based on the minimization of the mean-square deviation of colorimetric functions of the CIE 1931 system and a linear combination of the spectral responsivities of the device as obtained by each photodiode. The tristimulus coordinates X, Y and Z were thus obtained, extending over the whole range from 360 nm to 780 nm, which allowed to obtain the colorimetric coordinates (x, y) of the chromaticity diagram of the CIE system, by using the following known mathematical formulas:

X x ^{= ■}

X + Y + Z

Y y - X + Y + Z

Then, the device was tested on the basis of three monochromatic signals, one of which corresponding to the measure obtained with laser light at 405 nm ad the other two being obtained by imposing two deltas at particular wavelengths (at 555 nm and 610 nm). The three signals used for the test are shown in Figure 5.

Then, the colorimetric coordinates X_COL, Y_COL for these signals, as obtained by the device according to the invention, were compared with the known coordinates of the CIE 1931 system, X_CIE and ycm-

The following values were obtained by the device according to the invention:

X₄₀₅ = 0.1988 ± 0.0083 y₄₀₅ = 0.0322 ± 0.0050

X₅₅₅ = 0.279 ± 0.11 y₅₅₅ = 0.612 ± 0.14

X₆₁₀ = 0.683 ± 0.17 y₆₁₀ = 0.317 ± 0.09

These were compared with the CIE values, as shown in the following Table 1:

Table 1: Values of the chromaticity coordinates x, y obtained with the device according to the invention and the CIE chromaticity coordinates As shown in Table I₃ the chromaticity coordinates obtained with the device of the invention are similar to standard coordinates, with a greater measurement error at short wavelengths, owing to a greater line shape difference in this spectral region between the standard colorimetric function and the spectral responsivity curve of the photodiode.

Then, the device according to the invention was tested with lights at other wavelengths and the values so obtained are indicated in Table 2.

Table 2: Values of the chromaticity coordinates and the CIE coordinates for the same wavelength As proved by the above results, the device according to the invention has coordinate values similar to the values of the CIE 1931 standard chromaticity coordinates, thereby demonstrating the effectiveness of the device itself.

Therefore, the device according to the invention has the advantage of being easily manufactured with lower cost materials as compared with prior art colorimeters, while ensuring equal reliability and flexibility, as extensively shown in the above Examples.

The device has been described with reference to the CIE 1931 color measurement system, but a different color determination reference system may be certainly used, having different CIE standard colorimetric functions, in which case a spectral responsivity similar to the reference curves will be obtained by appropriate selection of the materials for preparing the photodiodes to be used in the device, or alternatively the coordinates obtained in the CIE 1931 system may be processed by a suitable calibration system into coordinates for use in a different system, without departing from the scope of the invention.

Claims

1. A color sensing device comprising three arrays of organic semiconductor- based photodiodes, wherein each of the three arrays of organic semiconductor-based photodiodes is sensitive over a wavelength range and has a spectral responsivity curve within said wavelength range which is substantially close to a specific standard colorimetric function.

2. The device according to claim 1, wherein each array comprises one or more organic semiconductor-based photodiodes, wherein the photodiode or the array of photodiodes has a responsivity curve substantially close to a specific standard colorimetric function.

3. The device according to claim 2, wherein each array comprises an organic semiconductor-based photodiode having a responsivity curve substantially close to a specific standard colorimetric function.

4. The device according to any one of claims 1 to 3, wherein the first of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 360 nm to 780 nm, the second of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 360 nm to 700 nm and the third of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 360 nm to 600 nm.

5. The device according to claim 4, wherein the first of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 400 nm to 780 nm with a maximum in the red region, the second of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 420 nm to 700 nm with a maximum in the green region and the third of the three arrays of organic semiconductor-based photodiodes is sensitive over the wavelength range from 360 nm to 600 nm with a maximum in the blue region.

6. The device according to claim 4 or 5, wherein the first, second and third arrays of organic semiconductor-based photodiodes comprise a single organic semiconductor-based photodiode respectively.

7. The device according to any one of claims 1 to 6, wherein each of the three arrays of organic semiconductor-based photodiodes has a spectral responsivity curve substantially close to a specific colorimetric function x(λ), y(λ), or z(λ) of the CIE 1931 system.

8. The color sensing device according to claim 1, wherein:

- the first of the three arrays of organic semiconductor-based photodiodes has a single photodiode sensitive over the wavelength range from 360 nm to 780 nm and has a spectral responsivity curve substantially close to the colorimetric function x(λ) of the CIE 1931 system;

- the second of the three arrays of organic semiconductor-based photodiodes has a single photodiode sensitive over the wavelength range from 360 nm to 700 nm and has a spectral responsivity curve substantially close to the colorimetric function y(λ) of the CIE 1931 system; and

- the third of the three arrays of organic semiconductor-based photodiodes has a single photodiode sensitive over the wavelength range from 360 nm to 600 nm and has a spectral responsivity curve substantially close to the colorimetric function z(λ) of the CIE 1931 system.

9. The device according to any one of the preceding claims 1 to 8, wherein at least one photodiode of the three arrays of organic semiconductor-based photodiodes comprises an anode comprising a layer of indium and tin oxide.

10. The device according to claim 9, wherein the layer of indium and tin oxide has a thickness in a range from 30 nm to 500 nm, preferably of about 100 nm.

11. The device according to any one of the preceding claims 1 to 10, wherein at least one photodiode of the three arrays of organic semiconductor-based photodiodes comprises a cathode comprising a layer of aluminum.

12. The device according to claim 11, wherein the aluminum layer has a thickness in a range from 50 nm to 300 nm, preferably of about 150 nm.

13. The device according to any one of the preceding claims 1 to 12, wherein each of the three arrays of organic semiconductor-based photodiodes comprises an organic material as a semiconductor so that the spectral responsivity curve of each array is substantially close to a specific standard colorimetric curve.

14. The device according to claim 13, wherein the organic material is the polymer known as regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) of formula:

where R is a hexyl group.

15. The device according to claim 13 or 14, wherein the organic material is the polymer known as 16,17-bis(octyloxy)anthra-[9,l,2-cde]benzo[rst]-pentaphene- 5, 10-dione of formula:

16. The device according to any one of the preceding claims 13 to 15, wherein the organic material is the polymer known as poly(paraphenylene) of the methyl-substituted ladder type, of formula:

17. The device according to claim 13, wherein the first array of organic semiconductor-based photodiodes comprises a single photodiode having as organic material the polymer regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) with a spectral responsivity curve in the wavelengths from 360 nm to 750 nm; the second array of organic semiconductor-based photodiodes comprises a single photodiode having as organic material the polymer 16,17-bis(octyloxy)anthra-[9,l,2- cde]benzo[rst]-pentaphene-5, 10-dione with a spectral responsivity curve in the wavelengths from 360 nm to 680 nm; and the third array of organic semiconductor- based photodiodes comprises a single photodiode having as organic material the polymer poly(paraphenylene) of the methyl-substituted ladder type with a spectral responsivity curve in the wavelengths from 360 nm to 500 run.

18. Process for preparing the device according to any one of claims 1 to 17, comprising the step of preparing three arrays of organic semiconductor-based photodiodes, each array of the three arrays of organic semiconductor-based photodiodes being sensitive over a wavelength range and having a spectral responsivity curve substantially close to a specific standard colorimetric function.

19. The process according to claim 18 comprising the step of preparing three organic semiconductor-based photodiodes, one for each array, by spin-coating of a layer of organic material from the solution of the organic material on an anode and subsequent evaporation of a metal cathode on the layer of organic material so obtained.

20. The process according to claim 18 or 19, wherein the first array is prepared by using the polymer regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), the second array of photodiodes is prepared by using the polymer 16,17- bis(octyloxy)anthra-[9,l,2-cde]benzo[rst]-pentaphene-5,10-dione and the third array of photodiodes is prepared by using the polymer poly(paraphenylene) of the methyl- substituted ladder type.

21. The process according to any one of claims 18 to 20, wherein the step of preparing the three arrays of photodiodes is followed by a step of calibration of the device on the basis of the comparison with standard colorimetric functions.

22. A colorimeter comprising a device according to any one of claims 1 to 17, and a CIE standardized lighting source.

23. A use of the color sensing device according to any one of claims 1 to 17, for sensing color in a selected CIE standard system.

24. The use according to claim 23, wherein the device senses color in the CIE 1931 standard system.