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Article Thickness Optimization of Thin-Film Tandem Organic

Kamran Ali Bangash 1, Syed Asfandyar Ali Kazmi 1, Waqas Farooq 1 , Saba Ayub 2, Muhammad Ali Musarat 3,* , Wesam Salah Alaloul 3 , Muhammad Faisal Javed 4 and Amir Mosavi 5,6,7,8,*

1 Department of Electrical Engineering, Sarhad University of Science and Information technology, Peshawar 25000, Pakistan; [email protected] (K.A.B.); [email protected] (S.A.A.K.); [email protected] (W.F.) 2 Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh 32610, Perak, Malaysia; [email protected] 3 Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh 32610, Perak, Malaysia; [email protected] 4 Department of Civil Engineering, COMSATS University Islamabad Abbottabad Campus, Abbottabad 22060, Pakistan; [email protected] 5 Faculty of Civil Engineering, Technische Universität Dresden, 01069 Dresden, Germany 6 Department of Informatics, J. Selye University, 94501 Komarno, Slovakia 7 Information Systems, University of Siegen, Kohlbettstraße 15, 57072 Siegen, Germany 8 John von Neumann Faculty of Informatics, Obuda University, 1034 Budapest, Hungary * Correspondence: [email protected] (M.A.M.); [email protected] (A.M.)

 Abstract: The solar cells also known as organic solar cells (OSCs) have drawn attention  due to their cynosure in industrial manufacturing because of their promising properties such as low Citation: Bangash, K.A.; Kazmi, weight, highly flexible, and low-cost production. However, low η restricts the utilization of OSCs for S.A.A.; Farooq, W.; Ayub, S.; Musarat, potential applications such as low-cost harvesting devices. In this paper, OSCs structure based M.A.; Alaloul, W.S.; Javed, M.F.; on a triple-junction tandem scheme is reported with three different absorber materials to enhance Mosavi, A. Thickness Optimization of the absorption of which in turn improves the η, as well as its correlating performance Thin-Film Tandem . 2 parameters. The investigated structure gives the higher value of η = 14.33% with Jsc = 16.87 (mA/m ), Micromachines 2021, 12, 518. https:// Voc = 1.0 (V), and FF = 84.97% by utilizing a stack of three different absorber layers with different doi.org/10.3390/mi12050518 band . The proposed structure was tested under 1.5 (AM) with 1 (W/m2). The impact of

Academic Editors: Javier the top, middle, and bottom subcells’ thickness on η was analyzed with a terse to find the optimum Martinez Rodrigo and thickness for three subcells to extract high η. The optimized structure was then tested with different Nam-Trung Nguyen combinations, and the highest η was recorded with FTO/Ag. Moreover, the effect of upsurge temperature was also demonstrated on the investigated schematic, and it was observed Received: 11 March 2021 that the upsurge temperature affects the photovoltaic (PV) parameters of the optimized cell and η Accepted: 28 April 2021 decreases from 14.33% to 11.40% when the temperature of the device rises from 300 to 400 K. Published: 5 May 2021 Keywords: organic solar cells; triple junction; ; tandem; ; energy Publisher’s Note: MDPI stays neutral harvesting; temperature; photovoltaic; micromachines; thin-film with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Organic solar cells (OSCs) have been a point of attention and main consideration for many research communities over the past few decades due to their countless advantages Copyright: © 2021 by the authors. such as light weight, thin size, low manufacturing cost, and semi-transparency. To date, Licensee MDPI, Basel, Switzerland. the extracted conversion efficiency for multi- solar cells based on absorption material This article is an open access article has not much exceeded as collating to inorganic solar cells [1]. The recorded η is, however, distributed under the terms and slightly more than the single-layer-based OSCs [2], which indicates that there is still a conditions of the Creative Commons great chance for improving the schematic of tandem organic solar cells (TOSCs). For Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ better configuration of TOSCs, the optimal feature of bandgap energies should be the 4.0/). main consideration that needs to be optimized for extracting high conversion efficiency.

Micromachines 2021, 12, 518. https://doi.org/10.3390/mi12050518 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 518 2 of 10

The theoretical view for TOSCs cells has the potential to counter the double junction drawbacks in terms of improved PV parameters, i.e., Js, FF, Voc, and η [3]. For TOSCs, the differences in the bandgap Eg values of the top, middle, and bottom subcells absorber layers would be adequate and cause maximum chances for each subcell to generate a large number of carriers [4]. For example, the multi-junction thin-film solar cells have an optimal configuration to obtain high current which improves the overall η of the cell, and thus tandem features top absorber layer with wide bandgap, medium absorber layer with intermediate bandgap, and bottom absorber layer with a low bandgap. Therefore, the configuration of the tandem cells formed on bandgap becomes Eg1 > Eg2 > Eg3 as suggested by [5–7]. This optimal scheme of bandgap cannot be implemented directly to OSCs due to a lack of donor materials having band energies as small as 1 eV [8]. However, when the material has a low bandgap such as 1 eV, it will give rise to several issues such as the low value of (Voc) and lowest unoccupied molecular orbital (LUMO) which results in low efficiency of charge separation [9]. To cover a broad spectrum of radiation with enhanced absorption, absorber materials with a large band need to be produced and designed first or various narrow or low bandgap absorbers have to be stacked in tandem multi-junction [9]. In TOSCs, when two non-overlapping absorption spectra are stacked, a broad range of absorption spectra including visible and IR range can be utilized [10]. Depending upon the absorber materials used for absorption different approaches have been demonstrated. Generally, tandem solar cells can be categorized into three types as described by Peumans et al. [10]. Here, it is important to mention that structures like these are complex to realize via solution processing due to the dissolution of orthogonal which cannot be circumvented [11]. In context to the thin-film solar cell, thickness optimization is one of the key parameters to extract high conversion efficiency because the thicker the layer, the higher the cost. Thus, to reduce the cost of the cell, the optimization of the active layer is the hotspot point. Moreover, if the optimization is not performed, the recombination losses occur and result in the low performance of the cell. Here, it is important to mention that utilization of the optimized material along with the combination of the compatible layer is needed to obtain high-performance electrical photovoltaic parameters. Structures without optimization and with unmatched stacking layers result in low absorption of photons, which in turn delivers low performance because such structure suffers from dominant losses. The main objective of this study was to analyze the impact of thickness optimization on the tandem organic solar cell to obtain improved efficiency. Herein, we investigated a tri-layer tandem OSC at an optimized temperature of 300 K, which is engineered in such a way to maximize the output photocurrent incorporating with bandgap energies. The schematic of the investigated structure consists of the three absorber layers in which top material has wide bandgap Eg1, middle absorber layer has medium bandgap Eg2, and bottom absorber layer has low bandgap Eg3, such as Eg1 > Eg2 > Eg3. The investigated structure is Glass/FTO/TiO2/first absorber/MoO3/TiO2/second absorber/WO3/MoO3/TiO2/third absorber/WO3/Ag.

2. Device Modeling and Working In this investigation, the presented schematic was framed with thirteen effective layers in which the top electrode is composed of transparent conductive oxide, i.e., FTO as a front electrode because it has dual nature as it has high transparency in the visible spectrum and also fetches carriers when used as an electrode [12]. In addition to this, it has low sheet resistance and high . After the FTO, TiO2 is introduced as a window layer (WL) because of its high transmission, low resistance, and lower cost as compared to other window layer materials. After WL, the first top absorber material (P3HT: PCBM) with a high bandgap is stacked with variable thickness. After the first absorber layer, MoO3 is introduced as a hole transporting layer (HTL) [13] and is stacked below the P3HT: PCBM layer. Next, again, TiO2 is placed below the MoO3 layer, thus making a tunnel junction and connecting layers with the next subcells. After the tunnel junction, Micromachines 2021, 12, 518 3 of 10

a second absorber layer (PTB7: PCBM) with an intermediate bandgap is introduced for capturing those photons that were left unabsorbed in the first absorber layer. Right below the second absorber material, an excellent hole extracting layer (HEL), i.e., tungsten trioxide (WO3), is introduced because of its matchless properties such as hole transport carrier, non-toxicity, easy , and low cost [14,15]. After HEL, again MoO3 and TiO2 are placed for making the second tunnel junction for the next subcell. After the second tunnel junction, a low bandgap absorber material PDTS-DFTTBT: PCBM is introduced. After the third absorber layer, again tungsten trioxide (WO3) is placed because it has a high work function and can boost the hole transport and collection at the anode interference (WO3/Ag). Moreover, it reduces the series and contact resistance. All the electrical simulations were performed in General-Purpose Photovoltaic Device Model (GPVDM) software. In this study, the electrical model of the cell is considered. The Jsc of the device is obtained by the photocurrent densities. The incident radiation that produces photocurrent density can be written as [16,17].

Z ∞ Jphl(V) = Jp(λ, V) + Jn(λ, V) dλ (1) 0

where for holes and photocurrent density is represented as Jp(λ, V) and Jn(λ, V) in Equation (1). The photocurrent density for solving is expressed as:

 Z W Z W  e ∂δn Jn(λ, V) = µnF δndx − Dn dx (2) W 0 0 ∂x

Similarly, for electrons, the photocurrent densities become:   e Z W Z W ∂δp Jp(λ, V) = µpF δpdx − Dp dx (3) W 0 0 ∂x

The continuity equation is described as:

2 ∂ ∂ ∂ δn −Xα(λ) δn (δn) = µnF (δn) + Dn + Ge − = 0 (4) ∂t ∂x ∂x τn

where δn is the concentration of generated electrons, µn and Dn are the electron mobility and diffusion, G is the optical generation rate of the carrier, and F in the equation shows the electric field in the cell. The Voc is given by:   nkT Jpki Voc = × ln + 1 (5) q J0 where k is the Boltzmann constant, n is the ideality factor, q is the electron charge density, J0 is the saturation current density, and T is the temperature. The Fill factor (FF) of the optimized cell can be calculated as:

JmpVmp FF = (6) JscVoc

300 K is the optimized temperature, which is kept constant throughout the simulation.

3. Results and Discussion In this investigation, tandem structures are analyzed based on three junctions as shown in Figure1, the follow-up strategy for configuring the three junctions is to absorb the non-absorbed photons from single- and double-layer junctions. Thickness is optimized for the top, middle, and bottom subcell layers to extract high η. The influence of top, middle, and bottom subcell thickness on the performance parameters, i.e., Voc, Jsc, FF, and η, are studied. Three different absorber layers were utilized in such a way that P3HT: PCBM is Micromachines 2021, 12, x FOR PEER REVIEW 4 of 10

the non-absorbed photons from single- and double-layer junctions. Thickness is opti- Micromachines 2021, 12, 518 4 of 10 mized for the top, middle, and bottom subcell layers to extract high η. The influence of top, middle, and bottom subcell thickness on the performance parameters, i.e., Voc, Jsc, FF, and η, are studied. Three different absorber layers were utilized in such a way that P3HT: PCBMused as is aused top activeas a top layer active having layer coefficient having coefficient of absorption of absorptionα = 52,925 α cm = −52,9251, PTB7: cm PCBM−1, PTB7: for PCBMsecond for active second layer active having layer coefficient having coefficient absorption absorptionα = 67,237 α cm= 67,237−1 [18 ]cm and−1 [18] PDTS-DFTTBT: and PDTS- DFTTBT:PCBM absorber PCBM absorber material for material bottom for subcell bottom because subcell its because bandgap its is bandgap narrower is thannarrow P3HT:er thanPCBM P3HT: and PCBM PTB7:PCBM. and PTB7:PCBM.

FigureFigure 1. 1. ConfigurationConfiguration of of a aproposed proposed structure. structure.

3.1.3.1. Investigation Investigation on on Top Top Subcell Subcell TheThe proposed proposed structure isis firstlyfirstly analyzed analyzed by by altering altering the the thickness thickness of theof topthe absorbertop ab- sorberlayer, aslayer, these as layersthese performlayers perform a key role a key in therole performance in the performance of the device. of the Thedevice. optimization The op- timizationof the cell of thickness the cell thickness is the critical is the parameter critical parameter as there isas a there mismatch is a mismatch of the current of the whencur- rentdealing when with dealing tandem with structures. tandem structures. The thickness The thickness of the top of layerthe top is alteredlayer is between altered be- 145 tweenand 165 145 nmand while165 nm keeping while keeping the middle the middle and bottom and bottom subcell subcell thickness thickness fixed fixed at 120 at 120 and and90 nm.90 nm. By By growing growing the the thickness thickness of theof the cell cell from from 145 14 to5 165to 165 nm, nm the, theJsc Jrisessc rises from from 15.23 15.2 to3 2 J to16.49 16.49 (mA/cm (mA/cm)2) as as shown shown in in Figure Figure2a. 2 Thisa. This rise rise in insc Jissc is because because when when the the thickness thickness of theof thecell cell height, height, the the area area of theof the absorber, absorber increases,, increases, a large a large amount amount of of sunlight which which consists con- of photons is absorbed and results in a greater number of , and thus the J increases. sists of photons is absorbed and results in a greater number of excitons, and thussc the Jsc Here, the Voc is not much affected while varying the absorber thickness. Figure2b shows increases. Here, the Voc is not much affected while varying the absorber thickness. Figure the influence of absorber layer thickness on FF and η. The Jsc and Voc are directly linked to 2b shows the influence of absorber layer thickness on FF and η. The Jsc and Voc are directly FF FF linkedand toη FF, as and a result η, as whena result the when thickness the thickness increases, increases the ,and the ηFFalso and increase. η also increase. The highest The recorded η while varying the top layer thickness is 14.06% at 165 nm and FF = 85.24%. highest recorded η while varying the top layer thickness is 14.06% at 165 nm and FF = 85.24%.

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V J Figure 2.Figure Effect 2.of Effecttop active of top absorber active absorber layer thickness layer thickness on photovoltaic on photovoltaic parameters parameters.. (a) Voc and (a) Jscoc; (andb) FFsc and;(b )ղ. Figure 2. Effect of topFF activeand absorberη. layer thickness on photovoltaic parameters. (a) Voc and Jsc; (b) FF and ղ. 3.2. Investigation on Middle Subcell 3.2.3.2. Investigation Investigation on on Middle Middle Subcell Subcell After analyzing the top subcell thickness, the middle subcell thickness is altered from After analyzing the top subcell thickness, the middle subcell thickness is altered from After analyzing120 to 140 the nm top while subcell keeping thickness, the top the subcell middle and subcell bottom thickness subcell is thickness altered from fixed at 165 and 120 to 140 nm while keeping the top subcell and bottom subcell thickness fixed fixed at 165 and 90 nm. It is90 shown nm. inIt is Figure shown3a in that Figure the middle 3a that subcellthe middle thickness subcell is foundthickness to be is optimizedfound to be optimized 90 nm. It is shownat 130 nmin Figure because 3a whenthat the the middle thickness subcell of the thickness subcell isincreases found to from be optimized 130 nm, the Jsc tends at 130 nm because when the thickness of the subcell increases from 130 nm, the Jsc tends to sc at 130 nm becauseto decrease when, and the thethickness decrease of thein the subcell Jsc value increases is because from of 130 unmatched nm, the J carrier tends generation. decrease, and the decrease in the Jsc value is because of unmatched carrier generation. At to decrease, and the decrease in the Jsc value is because of unmatched carrier generation. 130 nm, theAt highest 130 nmη ,of the 14.33% highest is recordedη of 14.33% and is FFrecordedis found and to FF be atis found 84.97% to as be shown at 84.97% in as shown At 130 nm, the highest η of 14.33% is recorded and FF is found to be at 84.97% as shown Figure3b. Thisin Figure suggests 3b. that This there suggests is a need that to there optimize is a theneed thickness to optimize of the the subcell thick becauseness of the subcell in Figure 3b. This suggests that there is a need to optimize the thickness of the subcell by modifyingbecause the thickness by modifying of the subcell,the thickness thicknesses of the subcell, can redistribute thicknesses the can uneven redistribute the uneven because by modifying the thickness of the subcell, thicknesses can redistribute the uneven absorptionsphoton and cause absorptions matched and photocurrents cause matched [19,20 photocurrents]. [19,20]. photon absorptions and cause matched photocurrents [19,20].

Figure 3. Effect of second absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc; (b) FF and ղ. Figure 3. Effect of second absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc;(b) FF and η. Figure 3. Effect of second absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc; (b) FF and ղ. 3.3. Investigation3.3. Investigation on Bottom Subcell on Bottom Subcell 3.3. Investigation on Bottom Subcell Next, the secondNext, absorberthe second thickness absorber of thickness the subcell of isthe altered subcell from is altered 90 to 110 from nm 90 while to 110 nm while keepingNext, the thekeeping remaining second the absorber tworemaining subcell thickness two thickness subcell of the fixed thicknesssubcell at 165 is fixed andaltered 130 at from165 nm, and and90 to130 it 110 is nm found nm, and while that it is found that keepingthe adequate the theremaining height adequate for two the height subcell third for absorber thickness the third layer fixed absorber (bottom at 165 layer and subcell) (bottom130 nm is 90, subcell)and nm. it Asis isfound shown90 nm. that in As shown in theFigure adequate4a, theFigure hJeightsc at 4a 90for, the nm the Jsc was thirdat 90 found nmabsorber was to befound layer highest to(bottom be among highest subcell) the among varied is 90 the nm. thickness, varied As shownthickness and in at , and at this Figurethis thickness, 4a, thethickness J thesc at η90recorded, nmthe was η recorded tofound be at to 14.33%to be be highest at which 14.33% among is thewhich the highest variedis the among thicknesshighest the among other, and variedat the this other varied thicknessthicknesses., thethicknesses. The η recorded reported The to results reportedbe at provide 14.33% results awhich smooth provide is agreementthe a smoothhighest with agreementamong Lambert–Beer’s the with other Lambert varied law –Beer’s law thicknesses.that can be statedthat The can reported as: be stated results as: provide a smooth agreement with Lambert–Beer’s law that can be stated as: A(λ) = e(λ)lc (7) 퐴(휆) = е(휆)푙푐 (7) 퐴(휆) = е(휆)푙푐 (7)

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Figure 4. Effect of third absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc; (b) FF and ղ. Figure 4. Effect of third absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc;(b) FF and η. Figure 4. Effect of third absorber layer thickness on photovoltaic parameters. (a) Voc and Jsc; (b) FF and ղ. TheThe coefficient coefficient of of absorbance absorbance wavelength andand carrierscarriers path path length length in in (7) (7) is is symbolized symbol- The coefficient of absorbance wavelength and carriers path length in (7) is symbol- izedas e as(λ )еand(휆) land. Thus, 푙. Thus, from from the obtained the obtained results, results, the thickness the thickness of the of top, the middle, top, middle, and bottom and ized as е(휆) and 푙. Thus, from the obtained results, the thickness of the top, middle, and bottomsubcell subcell was optimized was optimized to be atto 165be at nm, 165 130nm, nm, 130 andnm, 90and nm 90 asnm summarized as summarized in Figure in Fig-5. bottom subcell was optimized to be at 165 nm, 130 nm, and 90 nm as summarized in Fig- ureFurther, 5. Further, an increase an increase in the in thickness the thickness results results in a decrease in a decrease in performance in performance parameters parameters because ure 5. Further, an increase in the thickness results in a decrease in performance parameters becauseof inefficient of inefficient charges. charges. After optimizing After optimizing the thickness the thickness for the top, for middle, the top, and middle, bottom and subcell, bot- because of inefficient charges. After optimizing the thickness for the top, middle, and bot- tomwe subcell, tested different we tested electrode different configurations electrode configurations with the proposed with the structure proposed as structure depicted as in tom subcell, we tested different electrode configurations with the proposed structure as depictedFigure6 ,in and Figure their 6 extracted, and their parameters extracted areparameters summarized are summarized in Table1. The in differentTable 1. The scheme dif- depicted in Figure 6, and their extracted parameters are summarized in Table 1. The dif- ferentfor electrode scheme configurationfor electrode withconfiguration the optimized with cellthe helpsoptimized in selecting cell helps the in best selecting material the for ferent scheme for electrode configuration with the optimized cell helps in selecting the bestthe material top and bottomfor the top electrode. and bottom The investigationelectrode. The on inve thestigation electrode on configuration the electrode suggestsconfig- best material for the top and bottom electrode. The investigation on the electrode config- urationthat the suggests selection that of FTO/Ag the selection for the of top FTO/Ag and bottom for the electrode top and holds bottom 0.32%, electrode 0.45%, holds 1.02%, uration suggests that the selection of FTO/Ag for the top and bottom electrode holds 0.32%,1.3%, and0.45%, 1.35% 1.02%, more 1.3%, efficiency and 1.35% as compared more efficiency to FTO/Al, as compared FTO/Au, ITO/Al,to FTO/Al, ITO/Ag, FTO/Au, and 0.32%, 0.45%, 1.02%, 1.3%, and 1.35% more efficiency as compared to FTO/Al, FTO/Au, ITO/Al,ITO/Au, ITO/Ag respectively., and ITO/Au, respectively. ITO/Al, ITO/Ag, and ITO/Au, respectively.

FigureFigure 5. 5. OptimizedOptimized thickness thickness for for top, top, middle middle,, and and bottom bottom S Subcell.ubcell. Figure 5. Optimized thickness for top, middle, and bottom Subcell.

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Table 1. Extracted performance parameters with different electrode configurations.

Electrodes Jsc (mA/cm2) FF% η% Combination FTO/Ag 16.87 84.97 14.33 FTO/Al 15.64 81.02 14.01 FTO/Au 15.55 79.23 13.88 ITO/Al 14.84 78.90 13.31 Micromachines 2021, 12, 518 7 of 10 ITO/Ag 14.61 77.02 13.03 ITO/Au 14.57 76.95 12.98

Figure 6. DifferentFigure 6. investigatedDifferent investigated electrode configuration electrode configuration:: (a) FTO/Ag, (b (a) )FTO/Al FTO/Ag,, (c) (FTO/Aub) FTO/Al,, (d) ITO/Ag, (c) FTO/Au, (e) ITO/Al (d) , and (f) ITO/Au.ITO/Ag, (e) ITO/Al, and (f) ITO/Au.

The absorption spectrum range of the investigated cell is displayed in Figure 7, which Table 1. Extracted performance parameters with different electrode configurations. suggests that the utilization of the three absorber layers helps to cover a broad spectrum Electrodesrange of wavelength and covers ~80% of the absorption. Jsc (mA/cm2) FF% η% Combination FTO/Ag 16.87 84.97 14.33 FTO/Al 15.64 81.02 14.01 FTO/Au 15.55 79.23 13.88 ITO/Al 14.84 78.90 13.31 ITO/Ag 14.61 77.02 13.03 ITO/Au 14.57 76.95 12.98

The absorption spectrum range of the investigated cell is displayed in Figure7, which suggests that the utilization of the three absorber layers helps to cover a broad spectrum range of wavelength and covers ~80% of the absorption. MicromachinesMicromachines 20212021, ,1212, ,x 518 FOR PEER REVIEW 8 8of of 10 10

FigureFigure 7. 7. AbsorptionAbsorption spectrum spectrum coverage coverage against against the the wavelength wavelength..

3.4.3.4. High High-Temperature-Temperature Analysis Analysis HighHigh temperature temperature degrades degrades the the stability stability and and performance performance of of the the cell. cell. Here Here in in this this section,section, the the influence influence of ofhigh high temperature temperature on the on theinvestigated investigated structure structure is tested. is tested. The tem- The temperature dependence with the bandgap (Eg) of the absorber layer by the Varshni perature dependence with the bandgap (Eg) of the absorber layer by the Varshni equation canequation be presented can be presentedas [17]: as [17]: ∝∝ T푇22 E퐸g(T(푇))==E퐸g((00))−− (8(8)) 𝑔 𝑔 ((T푇++ β훽))

TheThe generated generated JscJsc relyrely on on the the spectrum spectrum of of given given solar irradiance,, and and at at each each temper- tempera- ature,ture, the JJscsc isis calculated calculated by by (9) (9) and is given by [17]: [17]: ∞ 푑푁푝ℎ 퐽 Z∫∞ dNph 푑(ℎ푣) J 푠푐 푑ℎ푣 d(hv) (9) sc ℎ푣=퐸푔 (9) hv=Eg dhv In Figure 8a, it is visible that the Voc is greatly dropped from 1.00 (V) to 0.45(V). The In Figure8a, it is visible that the Voc is greatly dropped from 1.00 (V) to 0.45(V). The observed drop in the Voc values is due to the reverse saturation current which changes observed drop in the Voc values is due to the reverse saturation current which changes with with the temperature [21]. There is a slight increase in the Jsc values, which does not con- the temperature [21]. There is a slight increase in the Jsc values, which does not contribute tribute to the improvement. The small rise in Jsc is due to reduction in the size of the to the improvement. The small rise in Jsc is due to reduction in the size of the bandgap Eg bandgap Eg which reduces with high temperature, and as a result, more photons get ab- which reduces with high temperature, and as a result, more photons get absorbed and a sorbed and a slight increase occurs [21]. The FF and η also drop with the rise in tempera- slight increase occurs [21]. The FF and η also drop with the rise in temperature intensity. In ture intensity. In this investigation, the η drops from 14.33% to 11.40% when the temper- this investigation, the η drops from 14.33% to 11.40% when the temperature of the device ature of the device increased from 300–400 K as depicted in Figure 8b. increased from 300–400 K as depicted in Figure8b.

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FigureFigure 8. 8. InfluenceInfluence of of temperature temperature on on photovoltaic photovoltaic parameters parameters.. ( (aa)) Vococ andand JscJsc; (;(bb) )FFFF andand ղ.η . 4. Conclusions 4. Conclusions In this work, we optimized OSCS based on the tandem design with three different In this work, we optimized OSCS based on the tandem design with three different absorber materials (P3HT: PCMB, PTB7: PCMB, and PDTS-DFTTBT: PCBM) in which absorber materials (P3HT: PCMB, PTB7: PCMB, and PDTS-DFTTBT: PCBM) in which transition metal oxide layer tungsten trioxide (WO3) was also utilized as a hole transporting transition metal oxide layer tungsten trioxide (WO3) was also utilized as a hole transport- layer. By optimizing the thickness of the active layers, we obtained a η of 14.33% at 300 K. ing layer. By optimizing the thickness of the active layers, we obtained a η of 14.33% at Moreover, the influence of high temperature was also tested, and it was observed that the 300 K. Moreover, the influence of high temperature was also tested, and it was observed upsurge temperature has greatly affected the PV parameters. These results revive a new that the upsurge temperature has greatly affected the PV parameters. These results revive avenue for many researchers in the field of photovoltaic notably in the field of thin-film a new avenue for many researchers in the field of photovoltaic notably in the field of thin- solar cell technologies. film solar cell technologies. Author Contributions: Conceptualization, K.A.B., S.A.A.K., W.F. and S.A.; Formal analysis, K.A.B., AuthorS.A.A.K., Contribution W.F. and S.A.;s: Conceptualization, Methodology, K.A.B., K.A.B., S.A.A.K., S.A.A.K., W.F., W.F. S.A.,, and M.A.M. S.A.; Formal and W.S.A.;analysis, Writing— K.A.B., S.A.A.K., W.F., and S.A.; Methodology, K.A.B., S.A.A.K., W.F., S.A., M.A.M., and W.S.A.; Writing— original draft, K.A.B., S.A.A.K., W.F. and S.A.; Data curation, M.A.M., W.S.A., M.F.J. and A.M.; original draft, K.A.B., S.A.A.K., W.F., and S.A.; Data curation, M.A.M., W.S.A., M.F.J., and A.M.; Investigation, M.A.M., W.S.A., M.F.J. and A.M.; Project administration, M.A.M., W.S.A., M.F.J. and Investigation, M.A.M., W.S.A., M.F.J., and A.M.; Project administration, M.A.M., W.S.A., M.F.J., and A.M.; Writing—review and editing, M.A.M. and W.S.A.; Resources, M.F.J. and A.M. All authors have A.M.; Writing—review and editing, M.A.M. and W.S.A.; Resources, M.F.J. and A.M. All authors read and agreed to the published version of the manuscript. have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Funding: This research received no external funding. Data Availability Statement: All the data are available within this manuscript. Data Availability Statement: All the data are available within this manuscript. Acknowledgments: The authors would like to thank Sarhad University of Science and Informa- Acknowledgments: The authors would like to thank Sarhad University of Science and Information tion Technology (SUIT) and Universiti Teknologi PETRONAS (UTP) for the support provided for Technology (SUIT) and Universiti Teknologi PETRONAS (UTP) for the support provided for this this research. research. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Ramanujam, J.; Bishop, D.M.; Todorov, T.K.; Gunawan, O.; Rath, J.; Nekovei, R.; Artegiani, E.; Romeo, A. Flexible CIGS, CdTe and 1. Ramanujam,a-Si: H based J.; thin Bishop, film solarD.M.;cells: Todorov, A review. T.K.; Prog.Gunawan, Mater. O.; Sci. 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