Photodiodes embedded within electronic textiles

A novel photodiode-embedded yarn has been presented and characterized for the first time, offering new possibilities for applications including monitoring body vital signs (including heart rate, blood oxygen and skin temperature) and environmental conditions (light, humidity and ultraviolet radiation). To create an E-Textile integrated with electronic devices that is comfortable, conformal, aesthetically pleasing and washable, electronic components are best integrated within the structure of a textile fabric in yarn form. The device is first encapsulated within a protective clear resin micro-pod before being covered in a fibrous sheath. The resin micro-pod and covering fibres have a significant effect on the nature of light received by the photoactive region of the device. This work characterised the effects of both encapsulating photodiodes within resin micro-pods and covering the micro-pod with a fibrous sheath on the opto-electronic parameters. A theoretical model is presented to provide an estimate for these effects and validated experimentally using two photodiode types and a range of different resin micro-pods. This knowledge may have wider applications to other devices with small-scale opto-electronic components. Wash tests confirmed that the yarns could survive multiple machine wash and drying cycles without deterioration in performance.

Section 1: Detailed specifications, IV curves, and variation analysis of the photodiodes used in the experiment. Fig. S1(a-c) show images and a cross-sectional depiction of the two photodiodes tested. Fig. S1 (d,e) depict typical characteristic curves (IV and power) for the two PD types, generated under the baseline test settings, along with the corresponding fill factors (FF) of and ideality factors (n), estimated for PD1 and PD2 respectively. PD2 had a larger area to perimeter ratio, and showed a ~13% higher fill factor and lower ideality factor compared to PD1, possibly due to lower edge recombination of charge carriers. Table S1 and S2 provide full specifications of the PDs. Table S1 -TEMD 7000x01 photodiode basic characteristics Figure S1 -Two photodiode (PD) types employed in the experiments (a) Image of the TEMD 7000x01 PD (b) Image of the VEMD 6060x01 PD (c) Front view schematic of the PDs (d) Characteristic curves, fill factor (FF) and ideality factor (n) for TEMD 7000x01 PD (e) Characteristic curves, fill factor (FF) and ideality factor (n) for VEMD 6060x01 PD . (IV curves given in blue lines and power curves given in orange lines).
Variation analysis for TEMD 7000x01 before and after encapsulating inside resin micro-pods.
Thirty samples created using PD1 were tested under identical conditions (202.22 W/m 2 with no light filter; hence referred to as the baseline test setting) using the optical test rig, with ISC and VOC recorded. The soldered PDs showed an average, standard deviation (SD) and co-efficient of variation (CV) of 12.87µA, 1.14 and 8.86% for ISC ( Fig. S2.a) and 0.381V, 0.0045 and 1.18% for VOC values ( Fig. S2.b).
Twenty of the thirty soldered PDs were randomly selected and encapsulated inside cylindrical RMPs with ~2 .7mm diameter using the Dymax 9001E-V3.5 acrylated urethane resin. The PDs were encapsulated at the bottom of the resultant RMP. The encapsulated samples had average, SD and CV values of 23.20 µA, 1.88 and 8.12 % for ISC ( Fig. S2.a) and 0.4101 V, 0.0040 and 0.97 % for VOC ( Fig. S2.b) values. From these results, it was clear that there was a significant increase in ISC and VOC values after the encapsulation. There existed a variation in ISC values before the encapsulation process, which remained unaltered after the encapsulation, indicating that no measurable variation was introduced to the ISC values by the encapsulation process. These variations were within the manufacturer's specifications.   proposed.

Generalized Ray Tracing Model
In its simplest terms, a single ray incident of light can be considered. The ray had an intensity and angle γ to the vertical axis, which met the boundary of the cylindrical micro-pod defined by = ( ) at co-ordinates 0 , 0 , as illustrated in Fig. S3. A fraction of the incident ray was reflected ( ) at the boundary surface and the remaining fraction ( ) was refracted into the RMP. The refracted ray was attenuated during its travel inside the RMP before reaching the plane of measurement ( ). A fraction of the ray ( ) was partially reflected at the photocell surface, and the residual ray ( ) was transmitted to the semiconductor.
Ei-Intensity of incident ray Erm-Intensity of reflected ray at air micro-pod boundary Erp-Intensity of reflected ray at the plane of measurements Ep-Intensity of the ray penetrated through the micro-pod Et -Intensity of transmitted ray at the plane of measurement Ec -Intensity of transmitted to the photocell γ-Angle of incident light to the vertical axis g(x) -Boundary surface function of the material nr -Refractive index of the resin material relative to air np -Refractive index of the photocell material relative to air µ -decadic attenuation coefficient of the material θ -Angle of incident light to the normal of the boundary surface α -Angle of refracted light to the normal of the boundary surface β -Angle of transmitted light to the horizontal axis Ei Erm Et Ep Erp Ec Figure S3 -Generalized ray-tracing model depicting the cross-sectional view of the cylindrical micro-pod.

= − (I)
Based on the geometry: From Snell's law of refraction: * sin = sin From Fresnel equation for partial reflection of non-polarized light at the boundaries of non-magnetic material: Based on the theory of absorption of electromagnetic radiation inside a homogeneous material: Using the Fresnel equation for partial reflection of non-polarized light at the boundaries of nonmagnetic material, the reflection at the micropod-photocell boundary can be given as: The intensity of the ray transmitted to the photocell:

= −
The average Intensity between two points ( , ℎ) and ( , ℎ) on the horizontal plane along (x1,h) are given by: Simplification of the generalized model for a circular cross section In order to generate comparative values with the experimental data, the generalized mathematical model was simplified to a RMP with a circular bases.
Incident light is uniform and parallel to the vertical axis. The boundary surface is circular and the width of the section is equal to the diameter.
Estimating short circuit current and open circuit voltage for a crystalline photocell encapsulated inside of a RMP The intensity estimates were converted into estimated ISC and VOC values using the fundamentals of semiconductor photovoltaics. For a crystalline silicon photocell embedded inside a RMP, with a rectangular photo-active area and photoactive width matching the measurement plane, and measurement width discussed in the above ray tracing model, the irradiance intensity on the photocell can be given as the: = Based on the theory the relationships between short-circuit current ( )and irradiance intensity for a photocell can be given as:

= *
Where is a constant that characterizes the relative variation of short circuit current as a function of irradiance intensity.
The nominal values for short circuit current ( ) and open circuit voltage ( ) of the photocell are determined experimentally under nominal irradiance intensity ( ) before encapsulating inside a RMP.
Using the above equations the short circuit current for an irradiance intensity can be estimated as: Also the open circuit voltage for a given irradiance intensity can be given as: Here -Boltzmann Constant -Ideality factor -Electron charge -Absolute temperature The ideality factor ( ) for the photocell is determined using the IV curve of the photocell under nominal irradiance condition . The IV curve is fitted to an exponential function in the below form:

= −
Where is the current is the voltage and α, and are constants. According to theory the ideality factor is given by:

Comparison of optical behaviour of actual and model defined photodiode
When the effects of optical reflection, refraction and absorption were considered, it was estimated that the epoxy layer (on the un-encapsulated PD) made an insignificant difference on the irradiance intensity at the photoactive plane for all the scenarios evaluated with these particular PDs.

Before encapsulation
When the actual PD and model defined PD configuration was compared before encapsulation, the loss in transmission due to Fresnel reflection was the same due to the same acrylic-epoxy material interfaces. The drop in intensity due to absorption of light by the epoxy material for actual PD configuration is given as: % ℎ ℎ 0.5 ℎ = (1 − 10 −(.5 0.0001) ) * 100 = 0.0115% It is clear that the model-defined configuration has only a fraction of the above loss due to substantially small thickness of the epoxy layer.

After encapsulation
Let's consider a ray which passes the air-acrylic resin interface, enters the acrylic material and reaches an edge of the photoactive material. Based on the calculations in this case the ray reaches the acrylic material at an angle no more than 10 o to the vertical axis, for all the scenarios discussed in this paper for both PD types. Both the rays undergo the same material interfaces therefore; the reduction in transmittance due to Fresnel reflectance (partial reflectance at material interfaces) is the same. Since the epoxy layer in the mathematically modelled configuration is very thin, the effect of refraction can be neglected.
The from the Snell's law the angle of transmitted ray into the epoxy material is given by θ as below.
1.51 * (10 ) = 1.55 * = 9.74 The deflection ( ) of the incident ray due to refraction in the actual PD configuration can be calculated as below for an epoxy layer thickness of 0.5mm. The actual epoxy layer thickness of PDs are less than that. This deflection is negligible compared to the width (0.92mm) of the photoactive material (less than 0.25%). Additionally, similar attenuation coefficients of the acrylic and epoxy materials result in a negligible difference in loss due to optical absorption.
Based on the above calculations, it can be concluded that the effect on light intensity due to the physical differences between the actual PD configuration and mathematical model is negligible.
Section 3 -Depth of encapsulation of photodiodes for different micro-pod diameters in different configurations; theoretical and practical limits and experimental results.  In the first method, the test specimens were prepared by attaching the PD1 embedded yarns (1.5mm RMP, 2mm yarn diameter) to onto cotton fabric swatches using a zig-zag embroidery stitch (See Fig.   S11(a)), with the photoactive side of the yarn fully exposed. Each cotton fabric swatch was attached with five electronic yarns and was affixed onto white cotton T-shirt using male/female press-stud snap attachments as depicted in Fig. S11  Kingdom) were employed for the testing. Wash and tumble dry programmes with approximately similar settings to the wash and tumble dry cycles prescribed in the wash standard were selected as mentioned in the Table S3.
The second wash test was conducted on PD1 embedded yarn in woven fabric form ( Fig. S12(a)).
The fabric was constructed with a basket weave structure. Cotton yarns were used as the warp and PD embedded yarns were inserted as the weft with the photoactive side of the PD yarn fully exposed.
Between two adjacent PD embedded yarns, five knit braided yarns without a core were inserted along with cotton weft yarns in an alternating order as shown in Fig. S12(b). In this case the fabrics were machine washed (similar to the previous case) inside a washbag and line-dried under room temperature.