Improvement in the photovoltaic properties of hybrid solar cells by incorporating a QD-composite in the hole transport layer

Abstract

A hybrid solar cell (HSC) based on a ZnSe and CdSe QDs-composite with improved conversion efficiency has been demonstrated. A novel approach of incorporating a QDs-composite (CdSe and ZnSe QDs simultaneously), in the poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate) (PEDOT : PSS) matrix by a simple cost effective solution processing technique, has been adopted. The combination of the QDs produced a 33% increase in the photo-conversion efficiency with a corresponding 67% enhancement in the fill factor (FF) when compared with the reference device. The micro-Raman analysis revealed effective strong coupling between both ZnSe and CdSe QDs, which promotes smooth charge transfer. This improved efficiency due to enhanced FF was achieved through interfacial engineering of the solution-processed hole transport layer, leading to facilitated charge transport and restrained bimolecular recombination. The present approach, outdoing the need of a cascaded layered structure, is compatible with the state-of-the-art hybrid solar cells, thus offering better throughput and a low cost manufacturing process for an improved-performance device.

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Introduction

The emergence of nanotechnology is believed to revolutionize the technological market which is expected to usher enormous economic growth in terms of low cost and increased efficiencies,¹ mainly through the proper manipulation of semiconducting materials. For instance, the physical alteration of these materials down into nano-size can remarkably incite new and fascinating properties of the devices, whilst persevering the bulk properties at the nano-scale, which appear as a consequence of quantum confinement.^2,3 Semiconductor quantum dots (QDs) have been reported to own multi-dimensional sizes and shapes with highly tunable band gap, cheap production cost, enhanced absorption coefficient and multiple exciton generation (MEG); most of which are crucial for developing highly effective light absorbing materials.^4,5 The fabrication process of semiconductor QDs or thin layers is significantly less costly as compared to the conventional bulk solar cells due to their less complex synthesis approaches.¹ Henceforth, this material type would serve as an excellent candidate for the development of hybrid solar cells (HSCs).

Colloidal quantum dots (QDs) thus seem very promising for the applications of the third generation HSCs as they have the capacity to achieve high power conversion efficiency (PCE) apart from their simplistic and inexpensive processing methods for solar cell fabrication.⁶ HSCs are therefore very much favorable to battle with Quantum Dot Sensitized Solar Cells (QDSSCs) and organic photovoltaic (OPV) solar cells. Applications of semi-conductive QD materials, which include CdS, CdSe, PbS, PbSe, CdTe, ZnSe, ZnS and InP as light harnessing component of solar cells, have been largely investigated mainly due to their outstanding unbeatable characteristics.^7–9 Most of the metal chalcogenide QDs have been employed as sensitizers in the QDSSCs with their up to date reported efficiencies of 6–8%.^10–12 As QD-sensitizers, rejuvenated CdSe has become a potentially viable material for photovoltaic devices. Due to its enhanced light harvesting capacity and effective electron injection, CdSe with absorption range extended to higher wavelengths has been used in QDSSCs.^7,13 ZnSe, due to its wide band gap, possesses several limitations particularly in the application of QDSSCs such as preventing absorption of sunlight within both visible and infrared ranges but, on the contrary, it plays an important role in inhibiting recombination of electrons.⁷ A superlative energy conversion efficiency of 3.46% has been reported in the case of CdSe QDSSC treated with ZnSe.¹⁴ Albeit, HSCs offer many advantages but their (PCE) are still considerably lower as compared to the present QDSSCs.¹⁵ Nevertheless, rigorous research on HSCs has provided insight in understanding and developing nanostructured solar cells for enhanced efficiencies. Hence, numerous approaches such as tailoring the structure and exploiting state-of-the-art fabrication techniques have to be adopted to bring them at par with existing highly efficient solar cell applications.

Mixing of QDs, which has demonstrated higher efficiencies of solar cells,^16–19 offers many advantages such as charge carrier dispersion by preventing charge recombination and appropriate alignment of energy band levels enhancing charge mechanism.¹⁹ For this reason we have combined two QDs, namely, CdSe and ZnSe (termed as QDs-composite), in a poly(3,4-ethylenedioxythiophene) : poly(styrene sulphonate) (PEDOT : PSS) matrix, which is used as a hole transport layer (HTL) having the advantages of high electrical conductivity, exceptional stability and decent optical transparency, factors that are considerably critical for developing a high performance organic photovoltaic devices.²⁰ Apart from the fact that PEDOT : PSS is used as a hole transport/buffer layer in the organic solar cells (OSCs), it has also been restructured by doping nano-crystals in the polymer to enhance the conductivity and consequently the efficiency of OSCs.^20,21 In the present work, a simple novel approach of combining ZnSe and CdSe QDs, capped with mercaptocarboxylic acids, in the aqueous solution of PEDOT : PSS and then depositing the HTL by solution processing technique, has been adopted. The QDs-composite has resulted in more efficient solar cell than the ones incorporating individual QDs. In addition, till date, the QDs have been combined in the core/shell arrangement which requires multilayered deposition of QDs to form photo-anodes for QDSSCs.^7,13 The approach adopted in the present work offers an advantage of outdoing the need of layered configuration and has demonstrated the efficiency higher than the one reported for CdSe/ZnSe in the multilayered arrangement.²² To the best of our knowledge, the above simple-to-adopt procedure has been presented for the first time, which supports present trends and suggests an easy method for the fabrication of HSCs.

Experimental section

Materials

Zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O], 3-mercaptopropionic acid [C₃H₆O₂S] (3MPA), mercaptoacetic acid (MAA), sodium seleno sulfate (Na₂SeSO₃), regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) were purchased from Sigma Aldrich. The aqueous solution of CLEVIOSTM PH-1000 PEDOT : PSS [poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate)] was received from H.C. Starck (Goslar, Germany). All the chemicals used in this study were of analytical grade and were used as received.

Synthesis of zinc selenide (ZnSe) QDs

The preparation of ZnSe QDs was done by colloidal method.²³ Firstly 2.8975 g of zinc acetate was dissolved in de-ionized (DI) water, which was followed by addition of 1.3398 g of 3-mercaptopropionic acid (3MPA) under constant stirring and under nitrogen atmosphere. Once the 3MPA is added, the solution turned turbid white in color. Then the pH was adjusted to 11.8 by drop-wise addition of sodium hydroxide (NaOH) to the prepared solution. The solution became clear and transparent soon after this step. After this the freshly prepared sodium seleno sulfate (14.5790 g) was added to the formed mixture. The temperature was then raised to 40 °C for the growth of ZnSe QDs and refluxed under N₂ atmosphere for nearly four hours. The ZnSe QDs capped with 3MPA were precipitated by adding 2-propanol to crude solution and were further separated and purified by repeated centrifugation.

Synthesis of cadmium selenide (CdSe) QDs

CdSe QDs were prepared in aqueous solution in the presence of mercaptoacetic acid (MAA), which acted as an effective stabilizing agent according to the method reported elsewhere.^24,25 After this, prepared sodium seleno sulfate solution was added to nitrogen saturated cadmium acetate dehydrate solution at pH 11 in the presence of MAA. The molar ratio of Cd²⁺ : Se²⁻ : MAA was chosen as 1 : 0.65 : 2.52. Both particle size and chemical composition were administered by heat treatment sand relative ratio, through post-preparative size-selective precipitation. The synthesis mechanism is presented in the flow chart of Fig. 1.

Fig. 1 Flow chart for the synthesis of QDs using 3MPA and MAA capping molecules.

Deposition of CdSe : ZnSe nano-composite HTL on ITO electrode

A pre-patterned ITO glass substrate (Ossila, 20 Ω sq.⁻¹), with dimensions of 20 mm × 15 mm, has been used as anode electrode for the fabrication of HSC. The substrate was cleaned according to the standard procedures in the following manner: prior to the deposition of HTL, the ITO substrate were sequentially washed with acetone, isopropanol, ethanol and DI water using sonication process. The cleaned substrate was then blown dry with a stream of nitrogen. The QDs-composite doped PEDOT : PSS was prepared by introducing CdSe and ZnSe QDs in the filtered aqueous solution of PEDOT : PSS. The 0.2 mg mL⁻¹ concentrated solutions for both types of QDs were prepared in PEDOT : PSS, separately. The employed concentration of QDs was optimized by PL spectra and was selected for maximum PL quenching. The prepared solutions were, then, sonicated for about 5 min to ensure a homogeneous dispersion of QDs in the PEDOT : PSS matrix. The final blend was obtained by mixing the two (CdSe and ZnSe) solutions in 1 : 1 stoichiometry by volume. The prepared solution was stirred for two hours using a magnetic stirrer. The QDs-composite HTL layer was spin coated, using WS-650 MZ-23NPP spin coater, at a speed of 4000 rpm for one minute resulting in the thickness of 40 nm. The deposited layer was further annealed at 100 °C for 30 min.

Fabrication of hybrid solar cell

The organic donor–acceptor (D/A) composite blend of P3HT : PC₇₁BM was prepared with a stoichiometry of interest as 1 : 1 in chloroform, which was deposited on the QDs-composite loaded HTL using a simple and cost effective spin coating technique. The standardized deposition of P3HT : PC₇₁BM thin film was attained at a rotational speed of 2500 rpm for 30 s resulting in a 70 nm thick photoactive layer, which was, subsequently, annealed at a temperature of 120 °C for 30 min. Finally, aluminum (Al) cathode was thermally deposited, using thermal evaporator (Edwards Auto 306), on top of the photoactive thin film using a shadow mask with an active area ∼4.5 mm² and thickness ∼100 nm. The device was exposed to post-fabrication annealing at 100 °C for 30 min. The fabricated organic solar cell was then kept in a glove box under N₂ atm. The HSC having a cell architecture of ITO/PEDOT : PSS : CdSe : ZnSe/P3HT : PC₇₁BM/Al was developed as a final product. For comparison purposes, two controlled devices with individual QDs were also fabricated. The device architecture is presented in Fig. 2(a), while Fig. 2(b) shows molecular structure of PEDOT : PSS, P3HT and PC₇₁BM.

Fig. 2 (a) Proposed device internal architecture of CdSe–ZnSe QDs-composite embedded PEDOT : PSS hole transport layer based hybrid solar cell; (b) chemical structure for PEDOT : PSS, P3HT and PC₇₁BM.

Methods

The sizes of QDs were estimated by transmission electronic microscope (TEM, HT-7700, Hitachi) operated at 120 kV. The scale bars are re-drawn using Image-J software. The surface morphology and elemental compositions of the QDs were investigated using a field emission scanning electron microscope (FE-SEM, SU-8000, Hitachi) equipped with an energy-dispersive X-ray spectrometer (EDX) operated at 10 kV. The micro-Raman scattering and photoluminescence (PL) measurements were accomplished by Renishaw inVia Raman microscope using He–Cd laser. PL spectra of QDs were recorded by using 325 nm laser wavelength in 300–1000 nm range. An excitation wavelength of 514 nm with the laser power of 50 mW was used for Raman spectra acquisition. Perkin Elmer Lambda 750 spectrophotometer was used to obtain UV-visible absorption spectra. The surface morphologies of the thin films of PEDOT : PSS without and with the inlay of QDs were characterized by atomic force microscopy (AFM) using Agilent Technologies 5500 Scanning Probe Microscope in tapping mode with a scan rate of 1 Hz. Photo-current density–voltage (J–V) features of the photodiode in dark and under illumination were logged by programmable Keithley 236 source measuring unit (SMU). For photo-current response measurement a Xe lamp was used as a light source with an output power of (100 mW cm⁻²) and was controlled by NEWPORT 69907 Oriel Digital Arc Lamp Power Supply. All the measurements were conducted under ambient conditions at 300 K.

Results and discussion

Transmission electron microscopy (TEM) allows us to qualitatively determine the average size of CdSe and ZnSe QDs. TEM micrographs of CdSe, ZnSe and QDs-composite are presented in Fig. 3. These images have been obtained by dispersing QDs in methyl alcohol then sonicating them for 20 min. The colloids were drop casted on carbon coated copper grids. Fig. 3(a) and (b) shows morphologies of neat CdSe and ZnSe QDs with average sizes of about ∼5 and ∼7 nm, respectively. The images of CdSe and ZnSe QDs in PEDOT : PSS are shown in the insets of Fig. 3(a) and (b), respectively. Furthermore, the image of QDs-composite in Fig. 3(c) demonstrates that the QDs are almost evenly dispersed in the matrix of PEDOT : PSS. The improved efficiency of the solar cell may be attributed to the smooth dispersion of both CdSe and ZnSe in the PEDOT : PSS domain.

Fig. 3 TEM images of (a) neat CdSe QDs, (inset: PEDOT : PSS : CdSe); (b) neat ZnSe QDs (inset: PEDOT : PSS : ZnSe); (c) PEDOT : PSS : CdSe : ZnSe (PPCZ).

To investigate the effect of stand-alone QDs and their colloidal mixture on the hole transport layer, the AFM measurements were probed on the neat PEDOT : PSS and QDs loaded PEDOT : PSS thin films prepared on the glass surface. Fig. 4 shows AFM topographic images of the thin films of neat PEDOT : PSS (PP), PEDOT : PSS : CdSe (PPC), PEDOT : PSS : ZnSe (PPZ) and PEDOT : PSS : CdSe : ZnSe (PPCZ). From the AFM images, the morphologies of the neat and QDs embedded PEDOT : PSS layers are quite distinguishable. Fig. 4(a) and (b) reveal adequately rougher surfaces of PEDOT : PSS and CdSe QDs with root mean square (rms) values of 2.0 and 2.36 nm, respectively. Fig. 4(c) shows relatively smooth surface of ZnSe QDs with rms roughness of 0.5 nm. The combined colloid further adds impact to the roughness of PEDOT : PSS and increases it to about 2.68 nm, as shown in Fig. 4(d). Since the roughness is thought as a signature of phase separation and grain formation in an active layer, the improved roughness of QDs-composite sensitized PEDOT : PSS thin film leads to an enhancement in the charge mobility on their regions.²⁰ It can be inferred that the increased surface roughness might contribute to a greater light absorption as the rough surfaces reflect more light between the metal electrode and the photoactive layer.²⁶ Moreover, the improved surface roughness offers a better interaction between the QDs-composite doped HTL and the photoactive layers.²⁷ The comparison of the PP, PPC, PPZ and PPCZ thin films reveal that the increased roughness leads to enhanced interfacial adhesion between hole transport layer and the active bulk heterojunction layer phases of the device due to the filling of interstitial space by QDs, which consequently give rise to enhanced charge transport and efficiency.

Fig. 4 3-D AFM micrographs of (a) neat PEDOT : PSS; (b) PEDOT : PSS : CdSe (PPC); (c) PEDOT : PSS : ZnSe (PPZ); (d) PEDOT : PSS : CdSe : ZnSe (PPCZ).

For identification of elemental composition of the specimen, the EDX analysis was performed which is shown in Fig. 5, the selected areas of the QDs were found with Zn, Se, Cd, C, O and S elements revealing successful synthesis of QDs and formation of their composite. The S element was introduced by sodium sulfite during the synthesis process of CdSe and ZnSe QDs.

Fig. 5 EDX analysis for CdSe, ZnSe and QDs-composite.

The micro-Raman scattering has been employed to study the effect of combining CdSe and ZnSe QDs in the composite form. Raman spectra were recorded by laser beam at a wavelength of 514 nm. The results presented in Fig. 6(a) demonstrate substantial variation of the Raman scattering spectra. The Raman fingerprints of CdSe and ZnSe have been previously investigated elsewhere.^28–30 The peak at 200 cm⁻¹ is attributed to a longitudinal optical phonon mode (LO) of CdSe. However, the LO phonon peak of ZnSe appeared at 250 cm⁻¹. It is known that the LO phonon of bulk CdSe and ZnSe are located at 211 and 252 cm⁻¹, respectively.^31,32 It is clear that the LO phonon of CdSe QDs is highly shifted toward the lower energy in comparison with ZnSe from their bulk vibration. These results indicate the smaller nano-size of CdSe as compared to ZnSe, which is in agreement with TEM analysis. These two peaks (200 and 250 cm⁻¹) appear in the QDs-composite, which confirm the presence of CdSe and ZnSe in the composite. The low intensity peaks of CdSe, in comparison with ZnSe, observed at 440, 584, 988, 1250 and 1440 cm⁻¹ indicate a higher ZnSe content in the QDs-composite structure, which is consistent with the EDX results of Fig. 5. It can be clearly seen that the observed peaks with enhanced intensity, due to both types of QDs, in the composite product reveal a strong coupling between these semiconductor species.²² In principle, this strong coupling paves the way towards smooth charge transfer, thus highlighting the effectiveness of the composite semiconductors.

Fig. 6 (a) Raman spectra of neat CdSe, neat ZnSe and QDs-composite in PEDOT : PSS HTL; (b) UV-vis absorption spectra of PEDOT : PSS/P3HT : PC₇₁BM and PEDOT : PSS/P3HT : PC₇₁BM embedded with ZnSe and CdSe QDs in stand-alone and composite form, (inset: UV-vis absorption spectra of neat CdSe, neat ZnSe and QDs-composite); PL spectra of (c) CdSe and (d) ZnSe QDs in PEDOT : PSS layer with varying concentration of 0.2 mg, 0.4 mg and 0.6 mg.

In order to observe changes in the optical domain upon the addition of QDs, four types of thin films (neat and QDs embedded HTL samples) were prepared in the following manner: PEDOT : PSS/P3HT : PC₇₁BM (PPPP), PEDOT : PSS : CdSe/P3HT : PC₇₁BM (PPCPP), PEDOT : PSS : ZnSe/P3HT : PC₇₁BM (PPZPP) and PEDOT : PSS : CdSe : ZnSe/P3HT : PC₇₁BM (PPCZPP). The absorption spectra of all the prepared samples are presented in Fig. 6(b). The samples, PPPP, PPCPP, PPZPP and PPCZPP, show almost similar absorption pattern. It can be seen from the figure that by introducing QDs-composite, the absorption intensity has increased. Moreover, red shift has also been observed, which is evident from the shoulders at 550 and 600 nm. The contribution of QDs, especially the combined colloid (CdSe and ZnSe QDs-composite), in the sample PPCZPP is characterized by not only a pronounced increase in the absorption intensity but also by a well-defined absorption band ranging from 350 to 730 nm in the visible region and a feeble absorption tail extending into the near infrared region up to 800 nm.³³ The absorption spectra of neat QDs and QDs-composite shown in the inset of Fig. 6(b), demonstrate that the enhanced photo-absorption of the PPCZPP is primarily due to high intensity of the QDs-composite. Hence, it would be reasonable to conjecture that the augmented absorption in the sample containing QDs-composite, is associated with enhanced light trapping effect due to scattering within the polymeric network.³⁴ The incorporation of combined colloid of CdSe : ZnSe QDs leads to a pronounced increase in the absorption intensity which may be the reason for improved electrical properties and hence the efficiency of the fabricated device. A comparison of the surface morphologies of neat PEDOT : PSS and combined colloid doped PEDOT : PSS, PPCZ, thin film samples (see Fig. 4(a) and (d)) supports our interpretation, since a higher light scattering is expected from rougher surface.³⁵

Fig. 6(c) and (d) shows photoluminescence of PEDOT : PSS loaded with different concentrations, namely 0.2, 0.4 and 0.6 mg mL⁻¹ of CdSe and ZnSe QDs, respectively. The PL spectra of the QDs doped polymer are obtained by an excitation wavelength of 325 nm. It can be analyzed from the figure that characteristic broad emission peaks lie at ∼700 nm for both CdSe and ZnSe QDs. These peaks are dominated by deep level emission which may be ascribed to the presence of traps, faults and interstitial and surface states.³⁶ The deep level emissions are highly undesirable when semiconductor QDs are employed for novel semiconductor devices as they severely attenuate excitonic emissions.³⁷ Philipose and co-workers have lately reported that the deep level emissions are primarily due to the recombination of a donor–acceptor pair, where Zn states serve as acceptor while Zn interstitial states or Se states act as the acceptor species.³⁸ Similarly, the intra-band-gap states in CdSe, which give rise to optical recombination in deep level emission region, are known as donor–acceptor pairs that can have various origins i.e. interstitial and surface vacancies.³⁹ The PL studies have been employed to quench undesirable deep level emissions by optimizing the stoichiometric composition of QDs in the PEDOT : PSS matrix. Both PL spectra of CdSe and ZnSe in Fig. 6(c) and (d) reveal that PEDOT : PSS, embedded with lowest concentration of QDs i.e. 0.2 mg mL⁻¹, shows maximum quenching effect up to 18 and 42% for CdSe and ZnSe QDs, respectively. The quenching, which is a substantial dropping of deep-level PL, is expected to lower the deep-level to excitonic emission ratio, thereby, generating more charge carriers.⁴⁰ Hence, lowest colloid concentration, with maximum quenching, has been employed for device fabrication.

Fig. 7(a) shows the current density versus voltage (J–V) characteristics of hybrid solar cells using QDs embedded HTL. For fair comparison, a reference OSC device that consists of neat PEDOT : PSS hole transport layer, has also been fabricated. Device parameters such as short circuit current density (J_SC), open circuit voltage (V_OC), fill factor (FF), and power conversion efficiency (PCE) are deduced from the J–V characteristics. For the reference device, containing neat PEDOT : PSS as an HTL, J_SC of 8.95 mA cm⁻², V_OC of 0.62 V and FF of 0.3 were obtained, which led to an overall PCE of 1.8%. The obtained parameters are in accordance with the values reported elsewhere.⁴¹ The device using QDs-composite incorporated PEDOT : PSS HTL, yielded J_SC, V_OC and FF of 8.55 mA cm⁻², 0.60 V and 50%, respectively, which led to a value of 2.4% of PCE. The mixing of QDs in PEDOT : PSS, has shown significant increment in FF value which has led to an appreciable increase in the PCE. Effectively, this combination has made FF to increase from 0.3 to 0.5 which is about 67% increase when compared with the reference cell, as can be observed from the shaded areas, highlighting the FF of both devices, in Fig. 7(a). The augmented FF of QDs-composite doped HTL produced 33% increase in PCE of the HSC as compared to the reference OSC. Fig. 7(b) demonstrates that the FF has significantly incremented as compared to reference cell, which clearly indicates that the increased efficiency of the QDs-composite based solar cell is ascribed, mainly, to the fill factor. There are several factors that influence the FF of the OSCs, which include the series resistance of the interfacial layers, the leakage current and the diodic behavior of the cell.⁴² Often, in the fabrication of OSCs, choice of proper buffer layers can help reduce the contact resistance or the leakage current^43–45 and, moreover, improve the morphology,^46,47 which is advantageous for improving the FF and ultimately the efficiency. The enhancement in the FF, here, is mainly because of the better interfacial adhesion between QDs-composite loaded PEDOT : PSS HTL and the active layer and ITO substrate.⁴⁸ So it can be concluded that a good interfacial morphology can prevent current leakage and bimolecular recombination of photo-generated carriers produced by augmented light scattering, thereby, resulting in amplified FF and improved efficiency.⁴² Here, it is also observed that the V_OC for reference device and QDs incorporated device exhibit almost the same value of V_OC i.e. 0.62 and 0.60 V, respectively. However, the FF of the HSC, with QDs loaded HTL, is far better than the reference device, which may be ascribed to the improved diodic behavior of the former device.⁴²

Fig. 7 (a) Photocurrent density–voltage (J–V) characteristics of pristine organic solar cell and solar cell doped with QDs-composite; (b) efficiency/fill factor versus HTL plot; energy band diagrams of proposed hybrid solar cell demonstrating energy levels (c) before and (d) after the combination of CdSe and ZnSe QDs. The values related to energy band structure have been obtained from the previously reported data.^22,51,52

Table 1 has listed comparison of several parameters of solar cells. Significant change in FF has been observed as compared to other parameters as it experiences large increase. The FF has been obtained by simply dividing maximum output power (shown by blue and red shaded area in Fig. 7(a)) by the product of short circuit current (J_SC) and open circuit voltage (V_OC). Then, the PCE was calculated accordingly as follow: η (%) = (J_SC × V_OC/input power) × fill factor × 100. Where, Fill Factor (FF) can be expressed as FF = (J_max × V_max)/(J_SC × V_OC). J_max and V_max represent the current and voltage at maximum output power, respectively.

Table 1 Comparison of photovoltaic parameters of QDs-composite incorporated solar cells

	JSC (mA cm^−2)	VOC (V)	FF	PCE (%)	Ref.
PEDOT : PSS : ZnSe : CdSe	8.55	0.60	0.50	2.4	(Present work)
CdSe/CdS : ZnS	7.3	0.43	0.39	1.2	16
CdS/ZnSe : CdSe	15.1	0.49	0.30	2.3	22

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The present work demonstrates the impact of the combination of semiconductor QDs on the efficiency of hybrid solar cells. Indeed, the J–V plots of Fig. 7(a) and the data of Table 1 show that an efficiency of around 2.4% is easily achieved by mixing these materials. Table 1 presents a comparison of the electrical characteristics of the present cell with previously reported devices made by using anode engineered HTL and electrodes bearing composite QDs. Improvement of efficiency was previously observed by investigating QDSSCs of multilayered structure employing a photo-anode made of CdSe/ZnS–CdS and CdS/CdSe : ZnSe QDs deposited on nano-crystalline titania.^16,22 In these cells, multilayered photo-anode layout has been adopted with the former cell, further, carrying a liquid electrolyte with all sealing and back-charge-transfer problems.²² Here, it is important to underline that the present approach of HSCs surpasses the need of multilayered formation, thereby, giving better efficiency. Fig. 7(b) suggests that when neat CdSe and ZnSe QDs were used, the efficiency and fill factor dropped in comparison with the composite QDs.

Fig. 7(c) shows energy band diagram of the components involved in the fabrication of hybrid solar cells. In this kind of device structure, type II energy alignment has been exploited. In type II layout, the conduction band (E_CB) and valence band (E_VB) of one type of QDs (ZnSe) are higher than those of the second type of QDs (CdSe), which is beneficial in terms of cascaded charge transfer for improved photovoltaic performance.⁷ CdSe mainly absorbs light in the visible region and, hence, generates photo-excited electron–hole pairs upon illumination. These electrons are, then, effectively transported to Al electrode through the active layer. ZnSe due to its wide band gap has proven to be a better selection for reduced electron recombination, which leads to more efficient electron hole pair separation.⁷ The alignment of energy levels for improved efficiency of solar cells has been the objective of many researchers which has led to the enhanced power conversion efficiency by combining different QDs.^22,49 When QDs are combined they offer a variety of interesting features such as reduction of recombination by eradicating defects and evenly distributing photo-generated charge carriers.²² Moreover, combination of QDs enables modification of energy band levels in the composite colloid which produces more favorable sites for energy levels (between the hole transport layer and the bulk heterojunction photoactive layer, in the present case).⁵⁰ Fig. 7(c) demonstrates energy levels of individual components of the hybrid solar cell which are redrawn in Fig. 7(d) for the composite species. It is expected that some intermediate energy levels are introduced due to the formation of CdSe–ZnSe nano-composite which further assists smooth flow of charge carriers in the cascaded order from the hole transport layer to the photoactive layer with reduced probability of recombination.²² The values related to energy band structure have been obtained from the previously published data.^22,51,52 The energy band gaps of independent CdSe and ZnSe QDs are 2.7 and 2.1 eV, respectively.^50,53 Utilizing the optical absorption data, Sfyri and co-authors have obtained the energy gap of 2.2 eV by merging the CdSe and ZnSe QDs together,²² which supports the carrier flow in the proper ladder step mechanism. Overall, CdSe and ZnSe QDs, collectively, play an important role in improving the photovoltaic performance of the QDs incorporated hybrid solar cells. CdSe, having smaller QDs-size and known for greater multiple excitons generation (MEG),^54,55 contributes towards some recombination and leakage current which might be the reason for decreased photocurrent of QDs-composite based hybrid solar cell. Most importantly, combination of ZnSe and CdSe QDs has produced significant impact on the performance of OSCs based on P3HT : PC₇₁BM blend by giving rise to the improved FF and PCE.

Conclusions

This work demonstrates that the incorporation of QDs-composite into the hole transport layer of hybrid solar cells has two main effects: (1) a modification of the surface morphology resulting in an augmented light scattering, thus increasing light absorption, (2) a reduction in recombination due to the incorporation of QDs-composite HTL. The overall effect is an increase of 67% in FF and 33% in the photo-conversion efficiency when the combined colloid QDs are used in the HTL under the configuration ITO/PEDOT : PSS : CdSe : ZnSe/P3HT : PC₇₁BM/Al. These results suggest that the introduction of QDs, especially QDs-composite, in the HTL could be useful to improve efficiency of the HSCs. We have successfully synthesized a novel nano-composite PEDOT : PSS, embedded with QDs-composite (ZnSe and CdSe QDs simultaneously), with a high interface contact surface area, which has been used as the buffer layer on the ITO surface. The results reveal that the higher power conversion efficiency in comparison with the reference cell is achieved, mainly, due to improved FF. The increased efficiency has been obtained by improved morphology, less leakage current, reduced bimolecular recombination and improved diodic behavior after the incorporation of QDs-composite in the HTL of HSC. This economic and facile approach provides a new promising and efficient pathway for improved device performance in the hybrid solar cell applications.

Acknowledgements

This research was supported by High Impact Research MoE Grant UM.S/625/3/HIR/MoE/SC/26 with account number UM.0000080/HIR.C3, UMRG grant of RP007A/13AFR, IPPP research grant PG089-2012B and FP046-2015A from University of Malaya. Fakhra Aziz (PhD) is highly thankful to Higher Education Commission (HEC) of Pakistan for providing Post-Doctoral Fellowship.

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