A review of thin film solar cell technologies and challenges

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• references:
  • https://zhuanlan.zhihu.com/p/88875239
    Abstract

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• The three major thin film solar cell technologies
  • amorphous silicon (α-Si)
    • is plagued with low efficiency and light-induced degradation
    • extinct in terrestrial applications
  • copper indium gallium selenide (CIGS)
    • dominant
  • cadmium telluride (CdTe)
    • dominant

tags: thin film solar cell technologies

Introduction

• Sun's energy to produce electricity.

  • reliable and cost effective.
  • Several solar technologies:
    • wafer
    • thin film
    • organic
  • crystalline silicon, 90% of the global PV market [1].
  • Cost effectiveness
    • less material
      • thin film can satisfy minimum material usage
    • energy conversion efficiency
      • wafer technology is capable of meeting the high efficiency goal

• α-Si, CdTe and CIGS

  • Common:
    • direct band gap (Table 1)
      • enables the use of very thin material [3].
    • low temperature coefficient

    In semiconductor physics, the band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are different, the material has an "indirect gap". The band gap is called "direct" if the crystal momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.

  • wafer technologies and their performance are not impeded by low light intensity.
  • all the three technologies can be incorporated into building integrated photovoltaics (BIPV).
  • The amorphous silicon(α-Si) solar cell finds its use mainly in consumer electronics such as calculators, watches, etc. [2,6].

• compared to CdTe and CIGS, α-Si
  • requires a lower amount of silicon
  • less toxic.
    • CdTe's usage of cadmium proves to be harmful
      • slightly limiting its commercial applications [3].
  • α-Si has had the longest time in the commercial arena
  • reliable power source of watches, clocks, and calculators in the late 1980s [2,6].
  • CIGS and CdTe are relatively new technologies, more energy conversion efficiency than α-Si
• Despite this advantage, CIGS and CdTe technologies still lag behind crystalline silicon solar cell counterparts in efficiency and reliability.

paper structure

• Section 2: evolution of each technology
• Section 3: discussion of thin film solar cells in commercial applications
• Section 4: explains the market share of three technologies in comparison to crystalline silicon technologies
• Section 5: discusses the reliability, more specifically a comparison between fill factor and temperature coefficients, and the materials availability of thin film technologies.
• Section 6: highlights emerging next generation thin film technologies such as Perovskite materials, Copper zinc tin sulfide (CZTS), and quantum dots (QD).
• Section 7: draw conclusions and highlight major accomplishments and developments based on the review.

2.Evolution of thin film solar cell

• 2.1. α-Si solar cell

  • direct band gap material (emit light, note: comare to indirect band gap): allows a significant fraction of sunlight to be absorbed within a thin layer of a few micrometers [1].
  • short orders in amorphous material and the dangling bonds: short minority carrier diffusion lengths and abnormal electrical behavior.
  • Hydrogen passivation, also designated as α-Si: H, can

    • Reduce dangling, thus improve the minority carrier length. However, this hydrogenation is responsible for the Staebler-Wronski light degradation effect.

      • hydrogenation -> the Staebler-Wronski light degradation effect

      The defect density of hydrogenated amorphous silicon (a-Si:H) increases with light exposure, causing an increase in the recombination current and reducing the efficiency of the conversion of sunlight into electricity.

  Hydrogen Passivation

  Hydrogen passivation refers to the stabilization of silicon material surfaces from chemical reactions through the creation of hydrogen silicon bonds.

  • The optical absorption spectrum of hydrogenated α-Si: H 1.7 eV~2 eV. These initial properties have
    • led to a wide spread
    • low manufacturing cost
    • shorter energy payback time.
  • Fig. 1 shows the first α-Si: H solar cell
    • energy conversion efficiency: 2.4%
    • Carlson and Wronski, 1976 at RCA Laboratory [4].
    • This is a p-i-n α-Si: H structure deposited at 250– 4000C onto a glass substrate coated with indium tin oxide (ITO).
    • thick intrinsic layer between thinner p-type and n- type layers
      • allows for large diffusion lengths for both minority and majority carriers.
      • 250 and 500 nm in thickness, often contains small amounts of boron (0.1– 1 ppm) to ensure nearly intrinsic behavior under illumination.
    • The absorber intrinsic layer possesses a generated electric field, creating the electric charge separation and enabling collection [5].
    • The p- and n-type doped regions are very thin.
      • The p-layer is usually a hydrogenated amorphous-silicon carbon alloy doped with boron
      • The n-layer is either phosphorus doped α-Si: H or phosphorus doped microcrystalline α-Si: H and is about 20–30 nm thick.
    • The rear contact is evaporated or sputtered aluminum.

  • Fig. 2 shows the evolution of laboratory α-Si solar cells

    • 1976, Carlson and Wronski [4] fabricated their first p-i-n α-Si solar cell with efficiency of 2.4%. Based on this structure
    • 1978, Wilson [6] used the MIS or Schottky diode structure (Fig. 3) instead of the p-i-n, which led to 4.8% efficiency. The Schottky barrier α-Si solar cell is constructed with a metal-to-N junction rather than a p-n semiconductor junction [6]. Schottky barrier α-Si solar cells, thin but highly doped, p-type α-Si. This increases the Voc and Jsc of the Schottky barrier α-Si solar cell [6].
    • 1980, Carlson [7], efficiency of 6% on the p-i-n structure and 6.1% efficiency for the Schottky counterpart.

  • Hydrogenated α-Si: H

    • 1982, by Tawada et al. [8,9], -> 8.04% efficiency.
    • 1986, Yamazaki et al. [10] reported 9.3% efficiency, light trapping features developed by Yablonovitch and Cody in 1982 [16] and hydrogenated α-Si: H.
    • early 1990s, multi-junction cells and modules (Figs. 4 and 5), which have multiple bandgaps to allow response at multiple wave- lengths.
      • The solar spectrum encompasses a variety of photon energies, and photons with energies less than the semiconductor bandgap are often not absorbed.
      • In multijunction technologies, bandgaps can be adjusted via varied alloying
        • but it is required that the top junction has a higher bandgap than the bottom junction.
        • Some of the energy that would normally be lost in single junction solar cells can be captured and converted [11].
        • Additionally, multijunction technologies exhibit less light-induced degradation,
        • yet it has also been shown that wiring two mechanically different α-Si cells optically in series may yield higher performances as well [17].
        • With the multi-junction concept, Guha et al. [11] demonstrated ~11.0% α-Si: H solar cell.

  • Yang et al., conversion efficiencies of 11.8%, 1996 [12] and 13% [13], “spectrum splitting, triple-junction structure”.

    • through a dual junction and low band-gap α-Si: H/α-Si: Ge alloyed cell (Fig. 4).
    • more efficient TCO, also known as the top conducting oxide, was created along with a p-n tunnel junction between the component cell, better transfer of current and electric generation within the cell [12,13].

  • 2013, Kim et al. [14], 13.4% stabilized efficiency α-Si: H solar cell using:

    • top cell, α-Si: H
    • middle cell, α-SiGe: H
    • bottom cell, hydrogenated microcrystalline silicon(μc-Si: H)
    • Similar triple junction technologies were explored by Sai et al. [15] with an α-Si: H/μc-Si: H//μc-Si: H cell, stabilized efficiency of 13.6%.
    • The improvement in PCE was attributed to the introduction of textured substrates with a hexagonal dimple array (also known as “honeycomb-textured substrates”) into multi-junction tech- nologies, better light trapping [15].
    • μc-Si: H possesses a higher tolerance to light soaking than pure amorphous materials [18]. The usage of multiple semiconducting materials has allowed for a broader absorption of wavelengths, thus improving the cell's power conversion efficiency.

• CIGS solar cell
  

  
  

2.2. CIGS solar cell
Fig. 6 shows the evolution of the CIGS solar cell technology efficiency. Kazmerski et al. [19], in 1976, created the first thin film CIGS solar cell having a conversion efficiency of 4.5%. The structure of the CIGS is given in Fig. 7, with soda lime glass as the substrate. On top of the glass is the molybdenum, which contacts the p-type Cu(InGa) Se2. The p-type Cu(InGa)Se2 forms the main junction with n-type CdS, which serves as the buffer layer. An intrinsic zinc oxide layer lies on top of the CdS and finally, the n-type ZnO:Al layer functions as the front contact. Altogether, the semiconductor used in this solar cell is 1.2–4.04 μm, which is very thin compared to the crystalline silicon at 170– 200 μm.

n-type ZnO:Al: front contact

---

i-ZnO(intrinsic zinc oxide layer)

---

n-type CdS

---

p-type Cu(InGa) Se2: 跟 n-type CdS 行成 main junction
(main junction: the buffer layer)

---

molybdenum

---

soda lime glass: substrate

Further work by Kazmerski et al. [20] included using an In fingered contact as a top electrode, which allowed passage of ~75% of the incident radiation and improved the efficiency to 5.7%.

Kazmerski 用 In fingered contact 當作 top 電子, 這個讓大約 75% 的入射射線進入, 也讓效能達到 5.7%

This improvement was attributed to better absorption of carriers through the CdS window and improved junction characteristics.

這項改進歸功於載體更好的吸收, 這吸收是透過 Cds window 跟 改良的 junction

Additionally, the lattice parameters of InP and CuInSe2 proved to match with CdS, further minimizing interfacial states.

除此之外 InP 跟 CuInSe2 的晶格參數證明了與 CdS 符合, 進一步地最小化界面狀態

However, the low efficiency of below 10% was due to high series resistance and ideality factor, and the low JSC was attributed to the grain-boundary recombination.

然而, 因為 阻抗跟理想因子造成 低於 10% 的效能, 還有, 因為 grain-boundary 的再結合導致 低 JSC

In 1980, Mickelsen and Chen [21] fabricated a CdS/CuInSe2 heterojunction solar cell with 5.7% efficiency using a simultaneous elemental evaporation technique to deposit CuInSe2 film.

做了一個 異質 junction 的 solar cell (5.7% 效能). 用 simultaneous elemental evaporation technique 去沉澱 CuInSe2 片

This cell showed a remarkably high short circuit current density (JSC) of ~31 mA/cm2 for a 1-cm2 cell, even without an antireflection coating (ARC).

這個 cell 展現了很高的短路電流密度, 約在 31 mA/cm2, 甚至沒有 antireflection coating (反反射 coating)

The high JSC was attributed to combination of Fermi position and band bending in the high-resistivity CuInSe2, further lowering open circuit voltage (VOC).

高短路電流密度歸因於 Fermi position 和 band bending 在 高阻抗 CuInSe2 的結合, 更進一步地降低開路電壓

By using n-type and p-type CuInSe2 in conjunction with CdS, Chen and Mickelsen [22] improved efficiencies to 7.5%. Peculiar to this structure were the adjustments of the two selenide layers and addition of 10% CdSe in the CdS layer.

透過在 conjunction 搭配 CdS 使用 n-type and p-type CuInSe2, Chen 和 Mickelsen 使效能達到 7.5%. Peculiar 對於這個架構作為兩個 selenide layers 的調整和而額外的 10% 的 CdSe 在 CdS layer

Based on this result, they proposed a 10% efficiency cell through optimization of selenide resistivity and minimization of photon losses. In 1981, Mickelsen and Chen [23] demonstrated a 9.4% efficient thin-film CuInSe2/CdS solar cell. The efficiency improvement was due to the difference in the method of evaporating the two selenide layers.

根據這個結果, 他們提出了一個 10% 效能的 cell (透過 對於 selenide 阻抗 和 光損最小化的優化). 81 年, Mickelsen and Chen 演示了一個 9.4% 效能的 thin-film CuInSe2/CdS solar cell. 這個效能的改善歸因於對於 evaporating 兩個 selenide layers 的方法的不同

The films were deposited with fixed In and Se deposition rates, and the Cu rate was adjusted to achieve the desired composition and resistivity. The transport property of the heterojunction is dominated by interface state recombination, and the low VOC was primarily blamed on high resistivity of Se.

films 跟 固定的 In 和 Se 的 deposition 率有關.
透過調整 Cu rate 來達到我們想要的 composition 跟 resistivity.
heterojunction 的 轉移特性主要被 interface state recombination 主宰.
而 低的 open circuit voltage 主要是因為高阻抗的 Se.

To understand the limitations of the cell performance, the cells were annealed in a combination of H2/Ar and then subsequently annealed in pure oxygen.

為了瞭解 cell 的效能限制, cell 會被 在 H2/Ar 結合中慢慢冷卻, 然後繼而在純氧中退火 (cooled slowly)

The JSC improved in all the cases due to thermal effect independent of
ambient but VOC and fill factor (FF) only improved in an oxygencontaining environment. Thus, a significant improvement in the cell
efficiency can be achieved by subjecting the cells to a post deposition
bake at 200 °C in air or an oxygen containing environment.

短路電流密度 會在 熱影響下 被改善, 而 開路電壓跟fill factor 只有在含氧的環境下被改善.
在 200 °C baking, 含氧環境下 cell 效能 有明顯的改善

In 1982,
Mickelsen and Chen [24] used a mixed ZnxCd1−xS to improve the VOC
and JSC and thus resulted in a record efficiency of 10.6%. The excellent
crystallinity features between the selenide/sulfide interfaces in the
columns without unwanted planar grain boundaries was responsible
for the high JSC. The addition of Zn to the sulfide layer increased the
VOC because of the improved match in electron affinity between the
sulfide and selenide layers.

82 年, Mickelsen and Chen 用 混和 ZnxCd1−xS 來改善 開路電壓 跟 短路電流密度, 達到了 10.6% 的效能.
這個傑出的, 介於 selenide/sulfide 介面的 crystallinity features 是高短路電流密度的原因.
額外的 Zn 對於 sulfide layer 提升了 VOC, 因為介於 sulfide 跟 selenide layers 的電子親和性的吻合

Potter et al. in 1985 [25] introduced ZnO in conjunction with p-type
CuInSe2 and thin undoped (Cu, Zn)S or CdS to achieve an efficiency of
11.2%. A thin film of CdS allowed the efficient capture of photons with
< 520 nm wavelength, while the ZnO antireflection coating improved
JSC by 25%.

85 年, Potter 在 conjunction 中引入了 ZnO, 還有 p-type
CuInSe2, thin undoped (Cu, Zn)S 或 CdS, 藉此達到 11.2% 的效能.
thin film of CdS 可以有效率的捕捉 < 520nm 波長的光子. Zno antireflection coating 讓 JSC 進步 25%

In 1988, Mitchell and Liu [26] further improved the
efficiency by an additional 1%, resulting in a 12.2% efficiency using the
same structure as Potter. By texturing the ZnO layer, optical reflection
was significantly reduced to ~6%. A combination of the direct band gap
of ~1.0 eV, which enhanced the photocurrent, and annealing, which
reduced the resistivity of the CIS film, led to a 1% absolute improvement in efficiency.

88 年, Mitchell and Liu 進一步的改良了額外的 1% 效能. 用跟 Potter 一樣的架構, 達到了 12.2%.
靠著 texturing the ZnO layer, 光的反射率下降了大約 6%. ~1.0eV 的 direct band gap (加強了光電流) and annealing (降低了 CIS film 的阻抗), 導致了 1% 的絕對效能改善

In 1990, Devany et al. [27] replaced the ternary
CuInSe2 with quaternary CuInGaSe2, whose band gap can be adjusted
by the percentage of In incorporation. By exploiting the higher bandgap
of the quaternary CuInGaSe2 layer, the VOC was increased, and the
infrared absorption losses in ZnO were minimized, thus achieving a
12.5% efficiency.

90年, Devany 用 quaternary CuInGaSe2 (它 band gap 可以透過調整 In 的比例來調整) 替換了 三重 CuInSe2.
透過更高的 band gap 的 quaternary CuInGaSe2 layer, VOC 可以上升, 在 Zno 的紅外線吸收損失被最小化, 因此達到了 12.5% 的效能

In 1993, Chen et al. [28] reported 13.7% efficiency with the
quaternary (CIGS) layer as the p-type absorber, CdZnS mixed alloys
as the n-type layer and an n-type overlayer of ZnO transparent
conducting oxide for majority carrier collection. The efficiency improvement was due to an improved CIGS grain structure resulting from
an increase in the substrate temperature ( > 500 °C). Therefore, by
reducing the reflection loss, decreasing the ZnO absorption losses, and
increasing the Ga content in the selenide, a total efficiency of 13.7%
was reached.

93年, Chen 聲稱透過 把 the quaternary (CIGS) layer 當作 the p-type absorber, CdZnS 合金當作 n-type layer 還有 Zno 的 n-type overlayer當作主要載體 達到了 13.7% 的效能.
這個改進是因為改良了 CIGS grain structure, 這提高了 substrate 的溫度 ( > 500 °C ).
因此, 透過降低反射損失, Zno 吸收損失, 跟提升了在 selenide 的 Ga content.

Gabor et al. [29] in 1994 reported a 15.9% efficient CuInxGa(1−x)Se2 solar cell made from (Inx, Ga1−x)2Se3 precursor films.

94年 Gabor 聲稱達到了 15.9% 的效能.

In 1996, Tuttle and others [30] used Mo back contact as a conduit for
impurity migration from the glass to absorber and sputtered ZnO
emitter along with MgF2 antireflection coating, which led to efficiency
of 17.7%. The enhanced VOC can be attributed to a slight shift in the
absorber band edge to higher energies, which resulted from the
reduced recombination in the conduction band of CdS and the reduced
thickness of the CdS layer.

96年, Tuttle and others 用 Mo back contact 當作給 不純的 migration 的導管 (Zno + MgF2 coating). 結果讓效能來到了 17.7%.
VOC 的強化有賴於 absorber 的 band edge 偏移至更高的能階, 這是因為更少的 CdS 導通再結合, 還有 更薄的 CdS layer

By optimizing the ZnO window layer, improving the interface
between CIGS absorber and CdS buffer layer, enhancing diffusion
length of the minority carriers and reducing recombination in the
space-charge region, Contreras et al. [31] reported an efficiency of
18.8% in 1999.

透過優化 ZnO window layer, 這可以連動地改善 CIGS absorber 跟 CdS buffer layer 的介面, 強化 minority carriers 的 diffusion length 跟 減少再 space-charge 區的再結合.
Contreras 聲稱達到 18.8% 的效能, 99 年

Ramanathan et al. [32] in 2003 used bandgap grading
of different concentrations of Ga and In to increase VOC, which led to
efficiency of 19.2% ZnO/CdS/CuInGaSe2 thin film cells.

03 年, Ramanathan 用 不同濃度的 Ga 跟 In 的 bandgap grading 來增加 VOC, 這讓效能來到了 19.2% 

What is bandgap grading

In 2005, Contreras et al. [33] reported a 19.5% efficient CIGS solar cell. The improvement in their cell resulted from the engineered bandgap of the
absorber to ~1.14 eV, low values of the diode saturation current density
(J02) of ~3×10−8 mA/cm2 , and an ideality factor (n-factor) of 1.30 < n
< 1.35 which led to reduced space-charge region recombination.

05 年, Contreras 聲稱她的 CIGS solar cell 達到 19.5%, 這項改進是因為 absorber 的 engineered bandgap 到了 ~1.14 eV.
diode 的 飽和電流密度 (J02) 變低到 ~3x10-8 mA/cm2.
ideality factor (n-factor) 到了 1.30 < n < 1.35, 這降低了 space-charge region recombination

Bandgap engineering
ideality factor:
The ideality factor of a diode is a measure of how closely the diode follows the ideal diode equation. 這是用來衡量 diode 多接近 diode 理想方程式的指標.

Repins et al. in 2008 [34] reported an efficiency of 19.9% as a result
of decreased recombination from lowering the bandgap in a portion of
the space charge region. Thus, the VOC stayed the same as reported by
Contreras [33] but the FF soared to 81.2% with ideality factor of 1.14
and Jsc value of 2.1×10−9 mA/cm2.

08年, Repins 聲稱她的效能達到 19.9%, 因為透過降低部分的 space charge region 的 bandgap, 減少了 recombination.
因此 VOC 跟 Contreras 的一樣, 但 FF 飆升至 81.2%, ideality factor 是 1.14,
Jsc value 2.1×10−9 mA/cm2.

the depletion region, also called depletion layer

The major goals of manufacturers and researchers worldwide are
low-cost and high efficiency solar energy production. Researchers from
companies, research institutions, national laboratories and universities
are investigating to improve the efficiency by any means while making the device low-cost compatible.

大家的目標是 成本低, 效能高的太陽能產量 (不然勒...

The Center for Solar Energy and
Hydrogen Research, ZSW [35,36], in Germany in 2010 reported a
record 20.3% efficient CIGS cell with a total thickness of 4 µm
including the metal contacts.

德國的太陽能中心 https://www.zsw-bw.de/en/about-us.html.
10 年 聲稱 CIGS cell 達到了 20.3% 效能, 總體厚度是 4 µm (包括 metal contacts)

In 2013, the Swiss Federal Laboratories
for Materials Science and Technology (EMPA) [37] engineered a thin
film CIGS solar cell on a flexible polymer substrate with an efficiency of
20.4%. The thin CIGS layer is mounted onto a polymer substrate,
permitting roll-to-roll continuous production of the cells.

13 年, (EMPA)[https://en.wikipedia.org/wiki/Swiss_Federal_Laboratories_for_Materials_Science_and_Technology] 在 polymer(https://www.thoughtco.com/what-is-a-polymer-820536) substrate 上搞 CIGS solar cell, 效能達到 20.4%.
CIGS layer 在 polymer substrate 的這種東西, 讓 roll-to-roll production of the cells 可以 continuous 持續

(roll-to-roll processing)[https://www.energy.gov/sites/prod/files/2015/02/f19/QTR Ch8 - Roll To Roll Processing TA Feb-13-2015.pdf]

Powalla et al.
[38] also reported a 20.4% efficient cell using a static co-evaporation
process. Peculiar to their cell was the use of Zn(O,S) buffer layer
instead of the conventional CdS between the light absorbing CIGS layer
and the transparent ZnO front electrode. The heart of their technology
lies in the high-vacuum cluster deposition system that allows the static
co-evaporation of the CIGS absorber's constituent elements and the
sputter deposition of i-ZnO and ZnO:Al as window materials.

Powalla 也最到了 20.4%, 使用了 static co-evaporation process 這東西.
差別 (peculiar) 在於 使用了 Zn(O,S) buffer layer, 而不是 傳統的 在 吸收光的 CIGS layer 跟 transparent ZnO front electrode 放 CdS.
關鍵技術在於, high-vacuum cluster deposition system(Cluster & Custom Deposition Systems. A cluster tool utilizes a central robotic distribution chamber to connect multiple process (PVD, ALD) and metrology chambers while enabling substrate transfer under vacuum).
這讓 static co-evaporation 可行.

Powella
et al. [39] with continued investigation reported an improved efficiency
of 20.8%, which resulted from intentional potassium doping of the
Cu(In, Ga)Se2 layer. The new doping process allowed a shift in the
CIGS absorber composition towards higher gallium content while
maintaining the efficiency. The novel deposition procedure also enabled partial overcoming of the saturation of Voc and allowed a shift in
CIGS absorber composition at higher gallium content. The saturation
of Voc for higher Ga content previously had prevented the progression
for higher-band gap CIGS cells.

Powella 又來到 20.8%, 因為 對於 Cu(In, Ga)Se2 layer 做 intentional potassium doping.
這個新的 doping 程序 讓 CIGS absorber composition 有更多 Ga(鎵) 的成分, 同時不失效能.
這個嶄新的 deposition 程序 解決了 VOC 的飽和, 也 讓CIGS absorber composition 有更多 Ga(鎵) 的成分.
因此, 更高的 VOC

In 2014, Herrmann et al. [40] reported
a 21% efficient CIGS cell. The improvement in efficiency was attributed
to increased CIGS deposition rates. Additionally, a 21.7% efficiency in
2014 was reported by Jackson et al. [41]. The improved efficiency
stems from the optimization of the alkali post deposition treatment of
the cell [43]. Finally, an efficiency of 22.3% was achieved by Solar
Frontier through improvements to the CIS absorber layer and junction
formation process [42].9*

14 年, Herrmann 的 CIGS cell 來到 21%.
因為增加了 deposition rate.
更進一步地
14 年, Jackson 來到 21.7%.
因為 alkali post deposition treatment 的優化.
最後 Solar Frontier 透過改進 CIS absorber layer 跟 junction formation process 讓效能來到 22.3%

One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer. (https://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp00614c#!divAbstract)
要加入 alkaline 材料的一個方法是 post deposition treatment (PDT), 一層薄的 NaF and/or KF 在 CIGS layer

Formation and Properties of Junction Diode - Physics

2.3. CdTe solar cell
CdTe serves as a prime candidate for all thin film solar cells. It is a
direct band gap material like CIGS with a large absorption coefficient,
and it is a stable compound which can be produced from a wide variety
of methods.

CdTe 是最主要的 thin film solar cells 候選人。
他跟 CIGS 一樣是 direect band gap 材料 但她 absorption coefficient 比較大，而且有很多方法可以穩定的生產其

A thin film of CdTe is adequate for producing high
efficiency cells if both bulk and surface recombination are curbed.
The first significant laboratory CdTe cell was reported in 1972 by
Bonnet and Rabnehorst (Fig. 8) who developed a thin film graded gap
CdTe-CdS p-n heterojunction solar cell with 6% efficiency. This cell
(Fig. 9) was created in a three-step process involving high temperature
vapor phase deposition of CdTe and high vacuum evaporation of CdS
[44]. One of the features of this includes a graded gap junction to
improve the transport of the photogenerated carriers to enhance the
Jsc. Bonnet and Rabnehorst encountered several problems involving
the back contact between Mo and CdTe, because using pure Mo as a
substrate lead to considerably high series resistances and.
low FF.

如果 bulk 跟 surface 有好好限制，那要生產 高效能的 CdTe thin film cells 很OK。
72 年 Bonnet 跟 Rabnehorst 做出第一塊， 6% 效能 CdTe-CdS p-n heterojunction.
做出他的三步驟牽扯到 (1) CdTe 的 高溫蒸氣沉澱 (2) 高真空 CdS.
增強載體有助於改善 Jsc.
問題: Mo 跟 CdTe 的接觸, 因為 pure Mo 作為 substrate 會導致 高阻抗、低 FF

Solutions to making a good non-rectifying contact to wide
band gap p-type semiconductors include using a high work function
metal, and generating a heavily-doped layer in the semiconductor
adjacent to the contact metal. This creates a resulting depletion layer so
thin that charge carriers can efficiently tunnel through.

要做出好的 non-rectifying 接觸 p-type 半導體可以: 高能材料、高 doped 層，讓電荷載體可以有效穿梭隧道

Rapid interest in the CdTe field resulted in a variety of different
fabrication methods. Previous work by Bonnet involved vapor growth
and vacuum deposition, but in 1976, Nakayama et al. [45] explored
screen-printing methods in CdTe solar cells. The In2O3 film and the
CdS ceramic film were coated on top of a glass substrate. The CdS
ceramic layer served as an ohmic contact transparent electrode to the
CdTe layer, reducing both the series resistance and the surface
recombination at the n-CdTe layer. This resulted in an 8.1% efficient
CdTe solar cell [40].

剛剛說到 Bonnet 的方法牽扯到 vapor.
但 76 年, Nakayama 用 screen-printing 的方法, 在 In2O3 跟 CdS film 上鍍上一層 glass substrate. 
CdS 主要擔任 ohmic contact, 降低 阻抗 跟 在n-CdTe 層的 recombination.
這讓效能來到 8.1%

What are ohmic and non-ohmic devices?

Bube and others [46] further explored CdS/CdTe
heterojunction solar cell with 8.4% efficiency. In this structure, indiffusion of donors from the n - -CdS into p-CdTe during junction
formation were believed to have created n-CdS/n-CdTe/p-CdTe junctions, in which the CdS film served as a contact to the CdTe homojunction.
It is noted that increasing the effective acceptor doping
concentration in the CdTe to 1017 cm−3 would be significant in
minimizing the effects of the interface recombination velocity.

Bube 跟他的同夥 在 CdS/CdTe heterojunction 架構上 來到 8.4%.
值得注意的是, doping 濃度 在 CdTe 的提升可以小化界面recombination 速率的影響

CdS/CdTe solar cells have a distinct cost advantage over singlecrystal silicon solar cells. Efforts to establish techniques for the fabrication of cells with improved conversion efficiencies are shown
through the inventions of Eastman Kodak Company [47,48], where
polycrystalline CdS and polycrystalline CdTe in contiguous layers
resulted in a conversion efficiency of 8.9%.

CdS/CdTe solar cells 價格上有優勢.
polycrystalline CdS and polycrystalline CdTe in contiguous layers 讓效能來到 8.9%

Depositing and sublimating
semiconductive layers in an oxygen-containing atmosphere allowed for
the incorporation of oxygen atoms into the semiconductor layer,
resulting in a more efficient device.

半導層在 含氧大氣層 讓 氧原子可以進到半導層 這讓裝置更有效率

The same technique of introducing
oxygen to the semiconductor layers achieved an efficiency of 10.5%
later. The oxygen enhances the p-type character of the CdTe film and
ensures the shallow-junction behavior of the device.

所以引入氧原子很重要 讓效能來到 10.5%.
**氧氣讓 CdTe 的 p-type 特性加強 並 確保了 shallow-junction 行為**

Tyan and others
[49] in 1982 at Eastman Kodak Company, reported a 10% efficiency by
integrating a layer comprising tellurium between the metal contact and
the layer of p-type CdTe. The surface portion of the CdTe in contact
with the tellurium layer is cadmium deficient and the CdTe layer has
intact grain boundaries.

Tyan 跟同夥在 82 年搞到了 10%.
他是靠著在金屬接觸跟 p-type CdTe 中整合 tellurium(Te).

Increasing experience and previous work from literature showed
junction preparation technique often controlled the final cell properties. Vacuum evaporation of CdS results in heterojunctions, whereas
chemical vapor deposition techniques of CdS produce buried homojunctions.

真空 evaporation 導致 heterojunction.
化學性的 vapor deposition 產出 buried homojunction.

In 1983, Werthen and others [50] demonstrated that using
low-doped CdTe greatly improved VOC.
Although low-doped CdTe
resulted in depletion region recombination but significantly reduced
the recombination at the interface. Werthen and others concluded that
an additional heat treatment of the semiconductor layers may have
been important in improving efficiencies, and recognized low-doped
CdTe to be a viable approach for all CdTe solar cells.

83 年 Werthen 用 low-doped CdTe 改善 VOC.
雖然 low-doped CdTe 導致 region recombination 不足, 但明顯地降低了介面的 recombination.
他們覺得是因為額外的熱對效能很重要. 覺得他們的方法對所有 CdTe soloar cells 都適用.

Kuribayashi and others [51,52] with the introduction of carbon
electrode and oxygen during deposition of the CdS and CdTe layer to
enhance the p-type character of the CdTe film and ensure the shallowjunction behavior of the device, improved the efficiency to 12.8% in 1984.

Kuribayashi 在 CdS 跟 CdTe 層之間的 deposition 引入 碳電子 跟 氧氣.
藉此加強 p-type 特性, 確保 shallowjunction.
效能達到 12.8%.

• 重點
  Appropriate amounts of Cu (50–100 ppm) in the carbon paste
  during the diffusion of the carbon into the CdTe layer during heat
  treatment helped make the CdTe layer p+ type and additionally
  contributed to further reduce the contact resistance between the
  CdTe and carbon electrode as shown in Fig. 10.

適當的 Cu, 對於降低 CdTe 跟 carbon electrode 的阻抗很有用.
怎麼做呢? 在 碳 diffusion 進 CdTe 層時, 加熱.

1987 revolutionized the CdTe solar cell industry. Previous efforts
mentioned in this review focused on the optimization of processes such
as electrodeposition, screen printing, and close-space sublimation.

Closed space sublimation offers the advantages of simple deposition
apparatus and high transport efficiency done with low vacuum conditions at moderate temperatures. Works of previous researchers involved the creation of p-CdTe based heterojunctions, however, these
heterojunctions led to problems with making low resistivity contacts
and low minority-carrier lifetimes.

Meyers and others at Ametek [53] introduced a novel n-i-p design
(Fig. 11) that possesses many advantages over the state-of-the-art CdTe
heterojunction devices, designed to accommodate its innate properties.
High efficiency cells were often constructed of high resistivity CdTe,
which were generally better quality than low resistivity films.

Meyers 搞出了一個新穎的 n-i-p design.
這個設計有許多當時做強的 CdTe 的優點.

Additionally, rectification rather than low resistance contacts was
utilized, and the field in the i-layer assisted in the collection of charge
carriers. CdS and ZnTe are naturally n-type and p-type materials as
well. The minimal discontinuity in the valence band edge at the CdTe/
ZnTe interface permits the free flow of holes from the CdTe into the
ZnTe.

Such rapid improvements led to an efficiency of 10.4% [54].
Creating a successful ohmic contact to CdTe is generally very hard
because of CdTe's large work function. Thus, a buffer layer of high
conductivity p-type semiconductor such as p-ZnTe was used.

rectification 這種改良導致效率到 10.4%.
造一個成功的 ohmic contact 很難, 這是因為 CdTe 工作很大.
因此使用 p-ZnTe 當 buffer layer.

With these rapidly increasing efficiencies, cost became a major
factor in the commercialization of the CdTe solar cells. The goals of
many researchers involved achieving high efficiency CdTe solar cells
using low-cost simple high throughput processing. Unlike previous
attempts of developing a CdS/CdTe junction, Bottenberg and others at
Arco Solar [54] focused on the creation of a three-layer device without
CdS that utilized a front conductive layer (a wide band-gap transparent conductor to eliminate the need for a CdS layer). Their work achieved
an efficiency of 10.5%.

跟其他人不一樣 Bottenberg 致力於 三層裝置, 不用 CdS. 效能來到 10.5%

In 1989, 12.3% efficiency was achieved by Chamberlin and others at
Photon Energy [55]. Suitable doping applied to the CdTe made the
material more conductive with hole concentrations as high as
2.8×1016 cm−3. Additionally, the use of various chemical rinses and
etches on the CdTe surface helped to reduce contact resistance, leading
to higher device efficiencies.

89 年 Chamberlin 來到 12.3%.
靠著適當的 doping, 讓 材料導度更好 電洞濃度更高.
然後用一堆化學材料 rinses 跟 etches 在 CdTe 表面上, 讓 接觸阻抗更小.

Tottszer et al. [56] further improved the
material band gap at the CdS/CdTe interface and reported efficiencies
of 13.1%. Their findings were promising- a reduced electric field at the
junction caused an increased trapping of the light generated electrons
by midgap recombination centers.

Tottszer 進一步改進 材料 band gap 效能 13.1%.
在 junction 降低電場導致 light trapping 升高 產生電子

However, the difficulty of producing low-resistivity p-CdTe films,
along with forming stable low resistance contacts to the p-CdTe films
became a challenge, which generated a lot of research interest.

困難點在於 生產低阻抗的 p-CdTe films

Chu et al. pushed efficiencies to 13.4% [57]. Peculiar to their method was
the preparation of CdS films from an ammonical solution of Cd-salt,
ammonium salt, and thiourea. Previous efforts in the deposition of CdS
formed discontinuity, but Chu's methods showed highly adherent,
smooth, reflective CdS surfaces.

Chu 效能到 13.4%.

Chu continued this rapid improvement
through the development of a device measuring 14.6% efficiency [58].
The high efficiency was attributed to the interface reaction between CdS
and CdTe during the deposition of CdTe at high temperatures. It was
observed that CdSxTe1−x formed at the interface, shifting the electrical
junction from the metallurgical interface into CdTe, improving photovoltaic characteristics of the junction. Thus, the control of CdS/CdTe
interfacial chemistry became extremely important to achieving high
solar conversion efficiencies.

這是因為 CdS 跟 CdTe 在 deposition 高溫 的介面反應.
CdSxTe1−x 在介面形成, 電子 junction 從  metallurgical interface 到 CdTe.
這改善了 photovoltaic 特性.
因此對 CdS/CdTe 的控制很重要

In 1993 and 1994, Britt and others [59–
61] at the University of South Florida improved upon Chu's work,
reporting efficiencies of 15.8%.

93~94 年 Britt 沿著 Chu 的方法來到 15.8%

Fig. 12 shows their structure.
The high temperature used during the deposition of CdTe by close
spaced sublimation (CSS) was hypothesized to have caused the
formation of an interdiffused region between CdS and CdTe.
CdxS1−xTe region shifted the electrical junction away from the metallurgical junction. Additionally, the use of graphite paste containing
mercury proved to serve as an adequate electrical contact in this work.

Britt et al. [59–61] found further discoveries by annealing CdS layers in
H2 prior to deposition, resulting in improved photovoltaic characteristics, including an increase in bandgap energy of 0.12 eV, reduction in
interface states, and increase in grain size. Annealing has also left the
surface of the films Cd-rich, and also resulted in thinner CdS films to
sufficiently reduce and avoid the formation of shunting paths as shown
in Fig. 13. Thinner films also resulted in the enhancement of the blue
response of CdTe/CdS solar cells.

Britt 發現 在 deposition 之前 annealing CdS layers in H2.
導致 photovoltaic 特性的改善.
包括 bandgap energy 提升 0.12 eV, interface state 降低, grain size 提升.
圖13說明更薄的films導致blue response的增強.

1997 marked a year of a new world record efficiency of 16% set by
researchers at the Matsushita PV Research and Development Center . The cell was prepared from a CdS layer that was deposited by
metal organic chemical vapor deposition, a low-temperature and lowcost technique, and from a CdTe layer deposited by CSS.

97年是重要的一年.
靠著 用 metal organic 化學 vapor deposition 在低溫, 低成本的技術 效能來到了 16%

The increase
in the conversion efficiency is due to the decrease of high-quality CdTe
thickness, which is closely related to the series resistance of solar cells.
The fill factor approached 73% as well. Fig. 14 shows the structure of
their cell.
Wu et al. at NREL [63,64] took a unique approach by modifying the
conventional SnO2/CdS/CdTe device structure. A new device using a
modified CTO/ZTO/CdS/CdTe device structure was developed. Four
approaches were taken: firstly, a novel process was developed to

prepare high-quality cadmium stannate (Cd2SnO4, CTO) transparent
conductive oxide (CTO films, which have two to six times lower
resistivity, higher transmittance and smoother surfaces than conventional SnO2 TCO films. By replacing the SnO2 transparent conductive
oxide with Cd2SnO4, Jsc and FF of CdTe cells was improved. The low
resistivity of CTO films allows for the increase in the width of the subcell in modules, thereby increasing total-area module efficiency.
Additionally, a Zn2SnO4 (ZTO) novel buffer layer was used in these
devices, along with a modified CdS film with a higher optical bandgap,
resulting in 16.5% efficiency.
Focus in research had now shifted to the improvement of the
transparent conductive oxide layer. Because CdTe cells operate best in
a superstrate configuration in which light enters the active junction
through the glass, the transparent front contact is deposited first and
must survive all subsequent deposition steps. Three characteristics are
necessary for a TCO to be used as a front contact: high transparency of
better than 85% in the wavelength of interest, low resistivity of
2×10−4 Ω cm−1
, and good stability at a range of temperatures. No
diffusion from the TCO into layers deposited is allowed.
Wu et al. at NREL continued their work [65] and reported
efficiencies of 16.5%. The same Cd2SnO4 (CTO) replaced the SnO2
TCO, and similar technologies of the high resistivity ZnSnOx (ZTO)
buffer layer were placed between the TCO and the CdS film as shown in
Fig. 15. However, this time, in order to improve the FF and the
efficiency, Wu et al. [65] developed an oxygenated nano-crystalline
CdS:O window layer with a higher optical bandgap (2.5–3.1 eV) than
the poly-CdS film. The bandgap increased with an increase in oxygen
content and a decrease in grain size. The oxygen present in the nanocrystalline CdS:O films suppressed the Te inter-diffusion from the CdTe
to the CdS film and the formation of a CdS1−yTey alloy, resulting in
higher quantum efficiency. Controlling the Te inter-diffusion is important because small changes in the Te contact in the CdS can result in
a decrease in bandgap. This led to the achievement of 75.51% FF. 16.5% efficiency was additionally achieved by Acevedo in 2005 [66].
Their results let to the speculation that there is a presence of a CdSTe
alloy film due to the diffusion of sulfide into CdTe and Te into CdS. This
interlayer helps to improve the efficiency of the solar cell by reducing
the interface carrier recombination, further shown in Fig. 16.
Additionally, the thermal annealing after the deposition of the CdCl2
on top of the CdTe layer resulted in a better organized CdTe matrix.
First Solar Research and Development has dominated the past
decade, with reported efficiencies of 18.7%, 19.6%, 20.4%, and 21%
[67–73] with a heavier focus on commercialization. First Solar utilizes
a continuous manufacturing process, creating a complete solar module
shown in Fig. 17 in less than 2.5 h.
It is reported that the window layer, CdS, is deposited by VTD
(vapor transport deposition), and then an absorber layer (CdTe) is
deposited. First Solar continues to create record-setting research cells
that demonstrate the highest single junction thin film on record.
(Figs. 18 and 19).